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+52 55 56270220
Avenida Revolucion 333 Col. Tacubaya Mexico City 11870 Mexico
Location This hotel is located on one of the most important avenues in the city, just a few minutes from shopping centres, tourist attractions and financial centres. Guests can reach the historical city centre and the Fine Arts Museum in 35 minutes using public transport, Chapultepec forest is just 10 minutes away, the Zona Rosa is 20 minutes and Xochimilco is 35 minutes away. Toluca is approximately 85 km from the hotel and Internacional Benito Juarez airport is around 15 km away. Facilities The hotel was built in 1985. Guests are welcomed at the accommodation, which has a total of 252 rooms. In the air-conditioned establishment, travellers are welcomed at the reception area with 24-hour reception and a 24-hour check-in/check-out service. The upper floors of the hotel can be accessed by lift. Amenities include a cloakroom, a safe and a currency exchange service. Wireless internet access in public areas allows guests to stay connected. Among the culinary options available at the accommodation are a restaurant, a dining area, a breakfast room, a café and a bar. Shopping facilities are also available. Additional facilities at the establishment include a playroom. Travellers arriving by car can leave their vehicles in the garage or in the car park. Available services and facilities include a babysitting service, a childcare service, medical assistance, room service and a laundry service. Rooms Each of the rooms is appointed with air conditioning, central heating and a bathroom. The rooms have a king-sized bed and a sofa bed. Separate bedrooms are also available. Extra beds can be requested. A safe and a minibar are also available. Guests will also find a tea/coffee station included as standard. An ironing set is also available for guests' convenience. Little extras, including internet access, a telephone, a TV and WiFi, contribute to a great stay. Some of the rooms are adapted for wheelchair users. The bathrooms are fitted with a shower, a bathtub and a hot tub. A hairdryer can also be found in each of the bathrooms. Bathrooms with wheelchair access can also be booked. Sports/Entertainment The hotel offers a pool, a children's pool and an indoor pool. Sun loungers on the sun terrace offer an inviting place to relax. Leisure options at the accommodation include a gym. For a fee, a sauna, a steam bath and massage treatments are also available. Fun and entertainment are offered by a nightclub. Meals Travellers can choose from a range of snacks and meals including breakfast, lunch or dinner. Staff are also happy to provide special dietary cuisine. The establishment also offers special catering options. Payment The following credit cards are accepted at the hotel: American Express, VISA and MasterCard.


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MMA Forum - UFC Forums - UFC Results - MMA Videos (http://www.mmaforum.com/forumindex.php)
- UFC (http://www.mmaforum.com/ufc/)
- - 152 is done with, we all know what's next! (Bandwagon inside) (http://www.mmaforum.com/ufc/105439-152-done-we-all-know-whats-next-bandwagon-inside.html)
152 is done with, we all know what's next! (Bandwagon inside)
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So who's with me?!!!
Anyone believe in long-shots? :D
I do but i think Anderson is too much of a pro not to take bonnar seriously, which should be enough
Be nice if he pushes him abit though. also he doesnt have to do to much to avoid being one of andersons easiest fights
thats probably an interesting question. rather than who has done the best, who has done the worst - and in what order
1 forrest
2 leben
3 irving
4 okami
5 vitor
i guess you could say leites aswell but atleast he lasted longer
I don't see why no one is giving Bonnar a chance. He's a big dude with decent grappling. He could use his size to dry hump Anderson against the cage for all we know.
Bonnar isn't going to win this but I'll be damned if I'm not on the Bonnar band wagon for this!
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Oh I'm under no disillusions here, but for old times sake I'm Bonnar all the way!
I'm on for the ride. Planning to be derailed in epic fashion, but it'll be fun!
Great thread btw Liza.
this has FOTY potential, bonnar has to hang in there and leave it all out. I believe in longshot and fairytales
bonnar in the 3rd or 4th by sub :D
I go all in on Bonnar to become the richest fella on this forum xD
No seriously i hope he can pull it off. He has a better chance than people give him credit.
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Channel that inner P4P champ that lives inside you Mr Bonnar! :D
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Great cruise!
Sail Date: March 01, 2013
Reviewed: 4 years ago
Traveled As: Singles/Friends
Room Type: Oceanview
Pros: The lap pool was a great way to work off the tasty meals and endless buffets.
Cons: The production shows were not as good as on other cruise lines.
Recommends Caribbean Princess? Yes!
Category Ratings
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|Ship Quality
|Food and Dining
|Only one bad meal (Beef Wellington) which was sent back. Otherwise the food was great and there were many sugar free options.
|Cabin / Stateroom
|Service and Staff
The variety of activities was great. However\, the shows in the main theater were not all that great. The comedian was good and so was the first show but after that it was downhill.
There were lots of activities on days at sea. However\,\"The Sanctuary\" needs to change. You have to line up at about 7 in the morning to pay to get in for the day and they do not take reservations even a day in advance. Clearly not worth the wait to use a facility that you have to pay extra to use.


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We use our MacBook Pro while on the go. Despite it has fantastic battery life, you might want to turn off the display quickly to save the battery, for suppose you went for one minute's work out of your MacBook Pro.
Easy as Mac is always easy:Advertisement
Hold the SHIFT + CONTROL key and hit the Eject key (top right corner of your MacBook Pro's keyboard and it will be turned off. Hit Esc key to activate it again.


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The BEST Drill you Can do to Improve Your Carbine Skills
Prove your Skill Set in 8 rounds. We utilize the 1-2-3 Drill to test our on-demand performance capability to transition to our secondary weapon system when our rifle has run dry. This drill has the added value of requiring the shooter to then reload their rifle after engaging the target with their pistol, and then…read more


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An unacceptable number of adolescent athletes are returning to play before fully recovering from concussion. These athletes are at risk of negative outcomes including serious lifelong physical and mental health problems or even death. Given the potentially catastrophic consequences of returning to play before being fully recovered following concussion, there is need to improve education on appropriate return-to-play guidelines and to find ways to ensure these guidelines are being applied. Sports medicine professionals, parents, coaches and sports administrators must work together to ensure proper recovery following concussion.
Press Release: Concussions May Lead to Lingering Symptoms that Could Impair a Child's Daily Functioning - March 5, 2012
PubMed Abstract: Reliable Change in Postconcussive Symptoms and Its Functional Consequences among Children With Mild Traumatic Brain Injury - March 5, 2012
Press Release: Study Details the Effects of Computerized Neurocognitive Testing for High School Athletes with Concussions - November 30, 2011
Press Release: New National Study Examines Concussion Management in High School Sports - December 1, 2010
PubMed Abstract: High School Concussions in the 2008-2009 Academic Year: Mechanism, Symptoms, and Management - December 2010
Press Release: New Study Shows Long-Term Dangers of Severe Concussions - March 2, 2009
PubMed Abstract: Hospitalizations for Sport-Related Concussions in US Children aged 5 to 18 Years During 2000-2004 - August 2008.
Press Release: Player-to-Player Contact Source for High School Soccer Concussions - July 23, 2008
Sports & Injury Safety
Concussion Research In the News


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The Best Make Up, In Stick Form
A solid routine.
From cushion compacts to rubber masks, K-beauty brands can't stop, won't stop churning out innovations destined to inspire companies closer to home. Last year, Alicia Yoon of Peach & Lily noted that stick skincare was big in Korea, so it's not surprising that this new format is finally finding its way onto our shelves. (Not only is this a cool way to care for your complexion, these solids are TSA-friendly―no need to jam them into that clear plastic Ziploc.) Here are six new sticks that can pretty much take care of your entire regimen.


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Укр | Рус | Eng
Prolongation of domain names
"Imena.UA" Internet Service offers its customers the service for prolonging the domain names (which have been registered through our service) after expiring the registration period.
At the moment of expiring the registration period for domain name, it will be blocked and the appropriate activities will be ceased. Therefore, your web site will discontinue the operations to its address and you will not be able to send e-mails by means of already invalid address in domain, your addressees will not be able to answer you. You will be separated from the world.
In addition, in one month your domain may be caught by your competitor or simply cyber-squatter - and only God knows how he will begin to use it.
In order to prolong the domain name it will be sufficient to pay for next registration period. The prolongation is possible for period from 1 till 10 years depending on domain area:
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While requesting for invoicing, please, indicate the period for which you need to prolong the domain.
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Lagosians experienced the most violent robbery operation last Sunday, after a 12-man-robbery gang in a convoy of three vehicles held the entire state hostage. The heavily armed gang in a commando-like manner, fired sporadically as they moved from one area to another, leaving in their trail the death of three policemen and several others.
Although the 'evil day' has come and gone. But it left in its wake anguish, vacuum hard to be filled and emotional wreckage in the hearts of many families whose loved ones were sent to their early graves. Some of them were Christian faithfuls who were either on their way to or from their respective places of worship.
Among them was a woman around Gbagada area simply identified as Mrs Gbadebo, who was reportedly hit by a stray bullet from the robbers who fired indiscriminately as they were leaving the Bureau-de-Change office, where over N200 million was reportedly carted away.
The dare-devils who threw caution to the dogs, engaged a team of policemen attached to the Rapid Response Squad in a fierce gun battle in Ikeja area, leaving three of them dead. Even in the face of death, the policemen braced up to the challenge and fought like gallant men. But unfortunately, they were overpowered by the number of the bandits.
*Late Corporal Gift
There were however indications that one of the slain policemen, Corporal Gift Ogbor, had a premonition of the unexpected. Crime Guard gathered that the widower reported at the office that same day, after being on sick leave for a week.
Even on that day, he was reportedly hesitant to go to work, as he told some of his friends that he would manage to report at work and come back early. Perhaps, if he had listened to his intuition, he would have probably escaped the bell of death that was jingling . But no! He rather choose to obey his constitutional clarion call, only to be shattered by the bullets of those he was trained to fight.
When Crime Guard visited the 39-year-old Corporal's residence at 10, Edosa Street, Ijaniki,Thursday, sympathizers were seen discussing the tragedy that had befallen the Delta State-born Gift. Out of curiosity, these reporters asked after his widow, only to be told that she died about seven years ago, leaving behind their two children; a boy and a girl. Unable to bear the burden of looking after the children who were then eight and five years respectively, Gift , resorted to taking them to his aged mother in Isoko, Delta State.
The deceased elder brother, Mr EjiroOgbor, who spoke with Crime Guard, lamented that " If he had known, he would have remained at home. those who saw him last Saturday said they asked what he was doing at home by that time and he told them he had been sick and was instructed to go home for treatment.
He said he was supposed to resume the previous day, which was on Friday but that he called his boss to inform him he was not yet okay. He then stated that he would manage to go to work the next day being Sunday, since he had already taken an extra day off. I never knew the lingering illness was a warning sign for him not to go to work that day".
Meanwhile, back in Delta State,the news of Corporal Gift's demise has left members of his family shattered. His mother whose age was put over 60 years, seemed to be the worst hit as she currently finds it difficult to come to terms with the rude shock of her son's death.
A relative who spoke on condition of anonymity stated that, " he was mama's favorite. He was always sending money to her on monthly basis for his children's upkeep and other needs", he stated, quoting the old woman as asking questions on why such fate should befall her, wishing she had died in her son's stead.
Describing the late Gift as a generous and caring man, his elder brother, Ejiro said, " He touched virtually every life here. He was caring and generous in his own little way. He never fought nor quarreled with any of his co-tenants during his stay in this compound. His death has dealt us with a heavy blow. My worry now, is my mother .
He paused for a while, bent his head and when he lifted it, it was obvious he was trying hard not to betray his emotion. But he managed to continue, " My major worry are the children. Who will continue from where their father stopped?. You know what? Gift was everything to them and my mother.
His first child, Onorode is 15 years old while the second one, a girl, is 12 years. He had dreams of sending them to the university and refused to re-marry after his wife's death seven years ago. Even when we urged him to re-marry, he would say he did not want any woman to deprive him of taking care of his children.
He said he would only re-marry after the children would have graduated from the university, pointing out that by then, they would have been old enough to fend for themselves. He always prayed for our mother to be alive till then , so as to inculcate good morals in the children . But little did he realise he would not live to see his dreams come through", he stated..
Crime Guard was reliably informed that the late Corporal's children who have been rendered orphans by their father's demise might not be able to continue their education in private school, as their was making arrangement on how to send the children's 1st term school fees at the end of September, after he must have received his salary, only to be deprived by the bullets from men of the underworld.
1-yr-old victim of stray bullet undergoes surgery .
Meanwhile, several other persons who also sustained bullet woulds are nursing them in different hospitals. Among them is one -year-old Taiwo Lawal, who is currently lying at the Intensive Care Unit of the Lagos University Teaching Hospital(LUTH) Idiaraba, Lagos. Baby Taiwo was hit by the robbers stray bullet while playing outside his father's compound at Gbagada. The bullet as gathered, pierced through his skull.
Explaining, his father, Wasiu Lawal, said, "Immediately the incident happened, we rushed him to Gbagada General Hospital from where he was placed on oxygen and drip. Because of the severity of the injury, he was refereed to LUTH, where a surgery was carried on him.
A scan was done where it was discovered that the bullet pierced through one end of his skull to the other end but it did not exit through the other end. And it also missed his brain. We were told that had the bullet exited, he would have bled to death.
He lamented on Wednesday that the family had spent all they had on their son and appealed to well meaning Nigerians to come to their aid financially with aview to getting baby Taiwo back on his feet again.
Responding to the appeal, Telecommunication giant, MTN, donated the sum of N500,000 for the treatment of the baby Thursday, a gesture Mr Lawal described as a practical display of humanitarian service.[subscribe2]


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Shannon Cannings: Trigger Happy
The paintings in Shannon Canningss show Trigger Happy pay homage to a forbidden fruit of her youth: toy guns. The artist says her parents wouldnt allow anything resembling the weapon into the house, and this, you could say, triggered some rebelliousness in her adulthood. On a recent hunt for subject matter in the toy aisle (Cannings tends to paint relics of childhood), she was drawn to a pack of squirt guns in rainbow colors. They just looked like a pack of giant Jolly Ranchers, she says. I was just thrilled with them.
Cannings makes realistic paintings of the toy guns using a mix of glaze and oil to render the toys shiny and translucent qualities. Trigger Happy features Canningss tiara-crowned, cherub-faced daughter Madeline aiming a plastic pistol at the viewer, and in Friendly Fire, two squirt guns are aimed at each other.
The names of the pieces are things that I think about being a happy veneer for something that carries a great consequence, she says. Like Friendly Fire, that sounds like a game you play with a hose in your backyard; it doesnt sound as horrible as it really is. It seems Canningss upbringing had lasting effects. I didnt tell my mother that I bought those plastic guns for a long time Im an adult, you know, Im allowed to own plastic guns! she says and laughs.
She wont let her kids play with the toy weapons either. She says she felt uneasy about letting Madeline hold the squirt gun even for arts sake because shes worried about sending mixed messages. Before this, I worked a lot with some candy as well, and at one point she picked up a lollipop and said, Is this food or art? 11 a.m. to 6 p.m. Tuesdays through Saturdays. Through August 9. Anya Tish Gallery, 4411 Montrose. For information, call 713-524-2299 or visit www.anyatishgallery.com. Free.
Tuesdays-Saturdays. Starts: July 12. Continues through Aug. 9, 2008
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Due to scheduling conflicts next year, the NFL is moving the draft to another location. Mike Florio says this is yet another brilliant strategy by the NFL to cash in on America's obsession with football.
PFT Live: Draft leaving Radio City
Posted by Scott Dargis on June 6, 2014, 1:52 PM EDT
1 Response to "PFT Live: Draft leaving Radio City"


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Your Relaxing Music Mix
On Now Eamonn Kelly
Text 65588Listen Live
The music industry would be a more boring place without divas - we're just glad we've never been on the receiving end of their rants... Here, we choose our 7 favourite divas ever:
Picture 2 of 7
New Yorker Mariah Carey is arguably the Queen of the Divas. She is never seen without her large entourage, whether it be at award shows, for performances or as guests on television shows. These days, many of the new kids on the block try to emulate the idea of being a diva, but Carey is the original.
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Last month, a feature length documentary produced and directed by Andrew Hevia for WLRN-TV premiered.
Rising Tide is a story about six Miami artists during the weeks surrounding the upcoming Art Basel Miami Beach, the largest contemporary art fair in the world. The film explores Miami's growing art scene using a combination of styles that have impacted the art fair culture and what it means to be an art city. The six Miami artists include: TM Sisters, Justin H. Long and Robert Lorie, Jen Stark, Brookhart Jonquil, Venessa Monokian.
More Info: Click Here


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WELCOME to the Fiqh Council Birmingham (FCB). The FCB was formed to help resolve the matrimonial problems of Muslims living in the United Kingdom in the light of sacred Islamic law. The services offered by the FCB include Islamic marital arbitration covering Talaq, Faskh of Nikah and Khula. read more >
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|Bryan Pendleton <bpendle...@amberpoint.com>
|Re: DERBY-800, lock scheduling issue?
|Sat, 11 Feb 2006 16:49:34 GMT
> When a thread tries to create a table, it will first get a shared lock > on the dictionary (DataDictionaryImpl.startReading). This is released > before it tries to lock the dictionary exclusively. The way > DataDictionaryImpl.startwriting works is that it first checks whether > someone is holding a lock on the dictionary. If so, it will sleep for a > while a then try again. This goes on for a while until it gets > impatient and actually requests an exclusive lock and enters the lock > queue. In the mean time, a lot more threads have acquired a shared lock > and the updating thread will have to wait for all of them to release it. This sounds like there is a lock scheduling fairness issue. Your description makes it sounds like this: - One or more transactions get shared locks on a resource - A transaction requests an exclusive lock on a resource, and blocks - Additional transactions arrive, requesting shared locks, and their locks are granted. It is possible to implement a lock scheduling policy such that those later locks, even though they are compatible with all currently granted locks, are not immediately granted, but instead are placed onto the resource queue behind the pending exclusive request. Perhaps we should consider whether there is an opportunity to enhance the locking policies to be more fair to writers? thanks, bryan
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A very big...and loud...ship
Sail Date: October 01, 2016
Reviewed: 8 months ago
Traveled As: Couple
Room Type: Oceanview
First time on the Oasis and probably our last. Had several small issues but enough to make our time in the Customer Service line a real pain. Had a beautiful room overlooking Central Park but all we could see was the top of trees. Dinner at Chops was great, well worth the extra expense.
We like the idea of sitting with the same people nightly for dinner. We had dinner reservations nightly and that was chaos. Each line, with and without reservations, was pretty long. Bad thing is people were walking by the table throughout the meal.
The noise on the ship was amazing. I'm sure there were places to sit and have a relaxing drink but we didn't find them.
Category Ratings
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|Cabin / Stateroom
|Embarkation and Disembarkation
|Ship Quality
|Destinations and Excursions
|Onboard Activities
|Children's Programs
|Service and Staff
|Food and Dining
Port Ratings
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|Ft. Lauderdale (Port Everglades), Florida
|Nassau, Bahamas
|Charlotte Amalie, St. Thomas
|Philipsburg, St. Maarten
Prepare to walk.
5 Comments
( to make sure we didn't miss our shows) and no lines. The ship can be noisy in some areas but we found quiet places all throughout the ship. Schooner bar was a fav for people watching on promenade
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is back-ordered. We will ship it separately in 10 to 15 days.
The perfect companion to your Steelux Pans, our Steelux Premium Glass Lids are innovative as well as stylish. Developed with the same design principles of our Clippix spatter shield, the glass lids can be clipped right to the frypan handle in an upright position, so condensation drips into the pan, not on your counter. Fissler's Steelux Frying System is based on the 2-pan philosophy-Crispy & Protect. A Crispy stainless steel pan is perfect for fast, low-fat frying at high temperatures, while a Protect, non-stick pan is ideal for easy, gentle cooking and is PFOA free. No kitchen is complete without the two. Our crispy 18/10 stainless steel frypans feature the internationally patented NOVOGRILL bottom, a unique honeycomb texture perfect for non-stick grilling and frying without the use of any oils, a healthy way to grill indoors. Protectal Plus is the Stiftung Warentest winner among other European nonstick coatings. All our pans are tested for chip-resistance, durability, longevity, evenness, heat distribution, safety, ease of cleaning, and quality of coating after abrasion tests. The ergonomic safety handle lies perfectly in your hand and the innovative shape of the heat shield offers effective protection because it stops fingers from slipping. The CookStar all-stove base, which is comprised of a pure aluminum core and high-quality 18/10 stainless steel, ensures that the base that will never separate or warp. The energy-saving base also provides quick optimal heat distribution, diffusion, and storage without any hot spots. Safe for induction stoves. The attractive mirrored hanging eyelet allows for space-saving solutions in the kitchen. The pouring rims on all the pans allow for precise and dripless pouring. Limited lifetime warranty.
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Do you have active kids who are ready for a new adventure? Girls and boys have fun, gain self-confidence, get in better shape, and make friends in our Parkour/Freerunning class! Parkour teaches kids a safe way to overcome obstacles, as … [Read more...]
Study the way of the sword! Learn the Olympic sport of fencing. Drills, games and competition are used to teach skills needed to enjoy a lifetime of fencing. This great sport has been described as an athletic form of chess. Both beginning and … [Read more...]
Job Readiness
Granite Peaks has teamed up with Vocational Rehabilitation to provide Job Readiness. This is a series of workshops to prepare individuals to transition from school to employment. Each week will be a new topic including resume building, interview … [Read more...]
Relief printmaking
In this 8 week introduction to relief printmaking, you will learn the skills to design, carve, and print at least three separate projects. In this class you will start with simple black and white lines and work your way up to reduction prints. You … [Read more...]
Quilting for Beginners
Whether you have never touched a sewing machine or have been using one for years, this class is for those interested in learning the ins and outs of quilting. Make an entire strip quilt in just two classes. Pattern and sewing machines included. … [Read more...]
Basics of Making a Will
This class is a 2 part class taught by a licensed attorney. The first day we'll discuss the basics of wills, including what documentation you need to prepare, what happens if you don't have a will, what you can put in your will, what scenario … [Read more...]
Reindeer Sign
Won't you guide my sleigh tonight? Paint your own rustic reindeer sign and top it off with a red nose! All materials are provided. Come create a stand out piece for your holiday decorations with us during our Reindeer sign class this November … [Read more...]
Zentangle through the Holiday Season
Zentangle your way to a Zen-like Holiday season. Prepare for the Holiday stress and chaos by learning this simple and meditative art form. class starts November 15th, 2016. Start your Holiday season off right and join us for our … [Read more...]
Creative Writing
Creative Writing Writing is the best way to become a better writer. In this class we explore aspects of creative writing from the starting process to in-class peer reviews. We examine writing as a daily habit, and will use writing … [Read more...]
Yoga for Stress Relief
Yoga for Stress Relief Integrate your mind, body, and spirit through Yoga. This is option 1 of a 6 week session in which you will Incorporate physical postures, breath control, and meditative techniques to lower stress levels, reduce pain, alleviate … [Read more...]
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|Publication number
|Publication type
|Application number
|Publication date
|Aug 25, 2009
|Filing date
|Jan 18, 2002
|Priority date
|Apr 1, 1994
|Fee status
|Also published as
|Publication number
|Gregg D. Sucha, Martin E. Fermann, Donald J. Harter
|Original Assignee
|Imra America, Inc.
|Export Citation
|BiBTeX, EndNote, RefMan
|Patent Citations (84), Non-Patent Citations (49), Referenced by (19), Classifications (33), Legal Events (2)
|External Links: USPTO, USPTO Assignment, Espacenet
This is a divisional of application Ser. No. 09/169,577 filed Mar. 27, 1998 now U.S. Pat. No. 6,396,856, which is a Divisional Application of application Ser. No. 08/602,457, filed Feb. 16, 1996 now U.S. Pat. No. 5,778,016, which is a Continuation-In-Part of application Ser. No. 08/221,516, filed on Apr. 1, 1994 now U.S. Pat. No. 5,585,913; the disclosure of which is incorporated herein by reference.
1. Field of the Invention
This invention is related to the field of ultra-short pulsewidth lasers, and particularly to apparatuses and methods for performing temporal scanning with minimal (i.e., micron-scale) mechanical movement. The invention also relates to methods used for obtaining high-accuracy (i.e., sub-picosecond) timing calibration, applicable to the above-mentioned temporal scanning methods or to conventional temporal scanning methods. In particular, the invention eliminates the need for a mechanical scanning delay arm in a correlator or other type of pump-probe device, including ranging, 3-D imaging, contouring, tomography, and optical time-domain reflectometry (OTDR).
2. Description of the Related Art
Ultrafast laser oscillators are presently known which are capable of generating pulsewidths of the order of tens of femtoseconds with nanojoule-level pulse energies, at repetition rates ranging from 5 MHz to as high as 1 GHz. Such short pulses are used for many applications including measurements by time gating, including metrology. Many applications of such short optical pulses require that one set of optical pulses be delayed with respect to another set of optical pulses, in which temporal delays must be known to a very high accuracy, such as on the order of 10 femtoseconds. Temporal delays for short pulses find many uses in such applications as biological and medical imaging, fast photodetection and optical sampling, optical time domain reflectometers, and metrology.
The conventional method for delaying and scanning optical pulses is to reflect the pulses from a mirror and to physically move the mirror, using some mechanical means, by a distance D, which is defined by the product of the time delay, ΔT, and the speed of light in vacuum c=3.0�108 meters/sec. Thus:
D=c/2�ΔT or D(cm)=15�ΔT(nsec).
This type of delay will be termed here a physical delay. Also, scanning, as that term is used here, refers to the systematic changing of the difference in time of arrival between two optical pulses. Various methods and devices have been developed to provide the accurate positioning and scanning of the mirror, involving:
The physical delay methods suffer from a number of disadvantages, the chief one being the large space required if large delays are desired. For example, a delay of 10 nsec requires a mirror displacement of 5 feet. There are other physical limitations and disadvantages as well. Misalignment and defocusing can distort measurements when large delays are used. Using corner-cube retroreflectors reduces the problem of misalignment, but not defocusing. The defocusing effect can occur when the scan amplitude is an appreciable fraction of the confocal parameter of the beam. A time delay of 10 nsec entails a change in free-space propagation of 10 feet (˜3 meters). Thus, to minimize the effects of defocusing, the confocal parameter (ZR) must be approximately 10-times this number or ZR=30 meters. At a wavelength of 1550 nm, this requires a beam radius of wo=12 mm. This is impractically large for many situations.
The need for large mirror displacement can be reduced by multi-passing the delay line (e.g., double-passing the delay line cuts the required mirror displacement in half), however, this does not alleviate the defocusing problem. Multipassing causes its own set of problems in that alignment procedures are more complicated, and the optical losses are increased.
Yet another limitation has to do with the scanning rates and scanning frequencies which can be achieved simultaneously. It is often desirable to signal average while scanning rapidly (>30 Hz) in order to provide "real-time" displays of the measurement in progress. However, the scanning range is limited at such high scan frequencies. The best achieved scan range is 100 psec at a rate of 100 Hz using the scanning FPS method (Kwong et al.). Any further increase of the scanning range and/or frequency with such reciprocating devices would cause high levels of vibration, which can be disruptive to laser operation. Rotating glass blocks avert the vibration problem, and are capable of higher scan speed, but lack any adjustability of scan range, and they introduce variable group velocity dispersion which makes them inappropriate for use with pulses having widths shorter than 100 fsec.
In addition to the physical delays, methods have been introduced which provide temporal scanning without the need for any mechanical motion. These include:
The non-mechanical methods, in particular, are capable of high speed scanning. The free-scanning lasers produce a scan range which spans the entire repetition period of the laser. For example, a known free-scanning laser system is shown in
The chief drawback to this technique is that it is highly wasteful of data acquisition time for two main reasons:
Kafka et al. address these limitations and suggest that this can be partially circumvented by using lasers with higher repetition rates, (e.g., νo=1 GHz). However, this solution is unacceptable for many applications which require a large variety of scan ranges. For example, pump-probe measurements of semiconductors are frequently conducted over a large variety of time ranges. Lifetimes of carriers (i.e., electrons and holes) of the semiconductor are on the order of several nanoseconds which makes a 1 GHz laser completely unacceptable, since residual carriers from the previous laser pulse would still be present when the next pulse arrives. Yet at the same time, it is often desirable to zoom-in on a much narrower time scale (e.g., 50 psec) to look at extremely fast dynamics. Thus, the free-scanning laser technique lacks the versatility of scan range selection which is required in many applications. The way to get a large temporal dynamic range without having extremely long acquisition times is to have the flexibility of a coarse and fine timing adjustment.
In related work, several methods have been used to stabilize the timing between two modelocked lasers, in cases where the lasers were actively mode locked, passively modelocked, and regeneratively modelocked, or with combinations of passively and actively mode-locked lasers. The methods used for synchronization can be divided into two main types: (1) passive optical methods; (2) electronic stabilization. The highest synchronization accuracy is achieved by passive optical methods in which the two lasers interact via optical effects (J. M. Evans, D. E. Spence, D. Bums, and W. Sibbett; "Dual-wavelength selfmode-locked Ti:sapphire lasers." Opt. Lett., 13, pp. 1074-7, Jul. 1, 1993; M. R. X. de Barros and P. C. Becker; "Two-color synchronously mode-locked femtosecond Ti:sapphire laser." Opt. Lett., 18, pp. 631-3, Apr. 15, 1993; D. R. Dykaar, and S. B. Darak, "Sticky pulses: two-color cross-mode-locked femtosecond operation of a single Ti:sapphire laser," Opt. Lett., 18, pp. 634-7, Apr. 15, 1993 (hereinafter Dykaar et al.); Z. Zhang and T. Yagi, "Dual-wavelength synchronous operation of a mode-locked Ti:sapphire laser based on self-spectrum splitting." Opt. Lett., 18, pp. 2126-8, Dec. 15, 1993). These optical effects, (e.g., cross-phase modulation) cause a rigid locking effect between the two lasers which become synchronized to within less than one pulse width (<100 fsec). Although these give the most accurate synchronization, the time delay between the lasers is rigidly fixed; so that in order to scan the time delay between them, one must use the conventional physical scanning delay methods.
Electronic stabilization using simple RF phase detection gives the most flexibility in terms of adjusting the relative time delay, but at the present time these systems can maintain timing accuracy of no better than a few picoseconds (˜3 psec). Such a system is commercially available for stabilizing a Ti:sapphire laser to an external frequency reference, or for synchronizing two mode-locked Ti:sapphire lasers. (Spectra Physics Lok-to-Clock™ system). Stabilization of better than 100 fsec has been achieved by use of a pulsed optical phase locked loop (POPLL). This is a hybrid opto-electronic method, as disclosed in S. P. Dijaili, J. S. Smith, and A. Dienes, "Timing synchronization of a passively mode-locked dye laser using a pulsed optical phase locked loop." Appl Phys. Lett., 55, pp. 418-420, July 1989 (hereinafter Dijaili et al.), in which the electronic stabilizer circuit derives the timing error signal from an optical cross-correlator. However, this method suffers from the same lack of timing adjustability as the passive optical methods. The timing can only be adjusted by less than one pulse width. Thus, using the POPLL method, it would be necessary to insert some sort of physical delay line into one laser beam if it were desired to change the relative pulse timing by anything more than one pulsewidth.
The performance of timing stabilization by RF methods could improve if the intrinsic timing jitter of the laser can be reduced. Some reduction of intrinsic laser jitter can be achieved by insuring that the two lasers are exposed to identical environmental conditions as much as possible. The Sticky-Pulse laser, which is disclosed by Dykaar et al., employs a spatially split pump laser beam to pump two spatially separate regions of a Ti:sapphire laser crystal. This is essentially two separate lasers which share the same pump laser, laser crystal, air space, and most other intracavity elements except for the two end mirrors. In this way, the two lasers experience the same thermal fluctuations, pump laser noise, and turbulence, thus minimizing the difference in repetition rate jitter. This allows even a weak optical interaction between the two lasers to lock the pulses together. This general principle of "environmental coupling" can be applied to other types of lasers including mode-locked fiber lasers. However, it should be noted the object of Dykarr et al. is to lock the two lasers together, which is undesirable for the purposes of the present invention, because then the time delay cannot be scanned; i.e., the timing of the pulses from the two coupled lasers of the Sticky-Pulse laser discussed above, are locked together through the optical coupling and they cannot be independently controlled.
It is an object of the present invention to provide a method and apparatus for rapidly scanning the time delay between two modelocked lasers, e.g., "master" and "slave" lasers, without the need for large mechanical movements of optical elements, which would allow scanning of any selected, contiguous subinterval of the pulse repetition period TR. This is accomplished by making appropriate adjustments, on the order of microns, to the cavity length of one laser (e.g., the slave laser), while using an electronic feedback circuit to continually monitor the average timing (phase) between the two lasers. Additionally, it is desired to modify the RF stabilization of the two lasers in such a way that, unlike the free-scanning laser method, the relative time delay can be scanned over some sub-interval of the repetition period, thus greatly improving the duty cycle of data acquisition.
It is another object of the present invention to minimize the timing jitter caused by fluctuations in environmental conditions, such as vibration, air turbulence, and varying temperature. This is accomplished by constructing both lasers with identical elements within the same enclosure, and pumping them with the same pump laser, yet allowing them to be independently controlled. In particular, in the case of fiber lasers, it can be accomplished by co-wrapping the fibers together on the same spool.
It is yet another object of the present invention to provide methods for calibrating the scanning time scale with subpicosecond accuracy, to be used during and in conjunction with the scanning method mentioned above. For example, the present invention can be used to cross-correlate the pulses from one laser (e.g., a master laser) with pulse sequences obtained by passing pulses from the other laser (e.g. a slave laser) through a series of partially reflective optical elements.
The above and other objects and advantages of the invention will becomes more apparent and more readily appreciated from the following detailed description of the presently preferred embodiments of the invention taken in conjunction with the accompanying drawings, of which:
FIGS. 3(A)-(C) show time delay TD(t) and waveforms applied to the PZT in the laser cavity;
FIGS. 7(A)-(D) show plots of two full scan cycles including the cross-correlation signals between a single laser pulse and the pulse train transmitted through an air-spaced FP etalon;
The embodiments of the present invention are described below in more detail with reference to the above drawings, in which like reference numerals refer to the same or similar elements.
Principle of Fast Scanning Method
The present invention, like the free-scanning laser system, consists of two lasers, a master laser 110 and a slave laser 120, which have nearly identical repetition rates as shown, for example, in
Again, it is necessary for the dithering frequency to exceed the bandwidth of the PLL circuit in order that the PLL not counteract the time scanning.
As an example of how the cavity length scanning operates, a square wave is applied to slave laser PZT 121 at a scanning frequency of fs. Then the cavity length mismatch is a rapidly varying function of time:
ΔL(t)=ΔL 0 �Sq(f s t) Eq. 1
where ΔL0 is the amplitude of the square wave displacement, and Sq(x) is the square wave function. This would give positive and negative linear scanning delays for one half of the scan cycle (a triangle wave). In the time stationary case, a constant cavity length mismatch of ΔL produces an offset frequency of:
which can be rewritten as
However, in the fast scanning method, the cavity length is dithered at a frequency high enough so that the pulses never have a chance to completely walk through each other. That is, the scan frequency and amplitude satisfy the condition
In this case, the time-varying time delay Td(t) would be proportional to the time integral of the cavity length mismatch:
where c is the speed of light in vacuum, and the period of integration is on the order of one scan cycle.
or, in terms of the offset frequency:
The scan rate, in units of (msec/msec) is given by:
or, in more convenient laboratory units:
The "sampling grid" is given by the temporal pulse advance per round trip:
δt g=2ΔL/c Eq. 7.
A scanning velocity parameter can be defined, which is the amount of spatial pulse advance per second. This is given by
Note that these concepts of scan rate, sampling grid, and scan velocity apply equally well to free-scanning lasers.
As an example, a pair of lasers with a nominal cavity length of L=1.5 meters gives ν1=100 MHz. If a scan amplitude of ΔL=15 μm is assumed, an offset frequency of Δν=1 kHz, a scan rate of Rscan=104 psec/msec, and a sampling grid of δtg=100 fsec result. Full pulse walkoff occurs in 1 msec if the cavity length is not dithered. Thus, a scan frequency of fs>1 kHz is required to prevent total walkoff if a 15 μm scan amplitude is used.
Tables 1, 2, and 3 show some possible scan ranges, scan rates, and sampling grid intervals as a function of various scan parameters, for modelocked lasers with repetition rates ν of 10 MHz, 100 MHz, and 1 GHz. It can be seen from Table 3 that for a 1 GHz laser, most practical scan conditions produce total walkoff, hence, the time delay cannot exceed 1 nsec. Thus, for lasers with repetition rates under 1 GHz, the dithering method of the present invention is preferable to the free-scanning laser methods.
Scan Parameters for a 10 MHz Laser (L = 15 m).
Offset Freq.
Scan rate
Scan Vel.
fs (Hz)
Δν (Hz)
Scan Parameters for a 100 Mhz Laser (L = 1.5 m).
Offset Freq.
Scan rate
Scan Vel.
fs (kHz)
Δν (Hz)
Scan Parameters for a 1 Ghz Laser (L = .15 m)
Offset Freq.
Scan rate
Scan Vel.
fs (kHz)
Δν (Hz)
*Total walkoff (1 nsec maximum)
Symetric square wave dithering is not the only desirable method.
The abrupt voltage changes on the leading and falling edges of the rectangular and square waves can cause mechanical shock and ringing in the PZT, thus distorting the otherwise linear time scale of scanning. Some type of signal conditioning (e.g., smoothing of transitions) which would reduce these effects is desirable. Alternatively, a sinusoidal voltage can be applied to the PZT, as shown in
The scan parameters given in Tables 1-3 were provided based on symmetric square wave modulation of the PZT. For sinusoidal scanning, the scan range and scan rate would be modified somewhat. The time-dependent delay is still given by equation 4, however, the cavity length modulation is given by:
ΔL(t)=ΔL 0�cos(2πf s t) Eq. 9.
Then the delay is:
This will give sinusoidal scanning, although the time delay will be 90 degrees out of phase with the PZT position. Note that the scanning characteristic is not linear, and some type of scale correction is needed.
It should be noted that although the laser scanning technique described here was demonstrated using a pair of mode-locked fiber lasers, this technique is not limited to the embodiments described above, but is applicable to many types of mode-locked lasers including solid-state, diode lasers, and dye lasers.
Scanning Fiber Laser System
Both lasers are pumped by the same laser diode LD 205, whose power is divided between the two lasers by a splitter SPL 206. Master laser 210 is terminated by an integrated Faraday rotator mirror FRM 215, while slave laser 220 is terminated by an optical assembly PZT-FRM 225 which is equivalent to FRM 215, but which also contains a mirror mounted on a PZT. The two fiber lasers contain identical modelocking optics (i.e., waveplates λ/4, λ/2, Faraday rotators (FR), and polarizing beamsplitters PBS). Laser mode-locking is initiated by saturable absorbers SA. The output pulses from the low-noise output ports of the lasers are detected by photodiodes PD-1 and PD-2 which are used to drive stabilizer 130.
Both fiber lasers have a nominal repetition rate of ν0=4.629 MHz. They are based on an environmentally stable design disclosed in M. E. Fermann, L. M. Yang, M. L. Stock, and M. J. Andrejco, "Environmentally stable Kerr-type mode-locked erbium fiber laser producing 360-fs pulses." Opt. Lett., 19, pp. 43-5, January 1994, which uses nonlinear polarization evolution (NPE) as the mode-locking mechanism. The lasers have identical components except for the Faraday rotator mirrors; master laser 210 is terminated with an integrated Faraday rotator mirror FRM 215 which is a single package, while slave laser 220 is terminated by assembly PZT-FRM 225 of discrete components which is equivalent to a FRM, but which also contains a mirror mounted on a PZT. The PZT used here had a total travel range of 40 microns. The lasers are thermally and mechanically coupled by co-wrapping them on the same fiber spool 280. Additionally, both lasers are pumped by the same pump laser diode LD 205 so that the pump noise in the two lasers is correlated. The relative timing between the lasers is set and stabilized by stabilizer 130, which includes a PLL circuit. Once stabilizer 130 has been enabled and the lasers are set at the proper delay, a dithering signal is applied to the PZT of slave laser 220 to make it scan. The dithering signal is applied by a signal generator 135, which is added to a stabilizing signal output from stabilizer 130 to produce a signal for driving the PZT. Alternatively, stabilizer 130 can generate the dithering signal, so that the output of stabilizer 130 includes both the stabilizing and dithering signals. Each laser has two output ports associated with its intracavity polarizer. These two output ports are shown in
Sinusoidal time scanning has been accomplished by the inventors by using the twin fiber laser system.
A scanning mechanical delay is used in one arm of a known correlator. However, in the correlator of the present invention, no mechanical delay was used. All scanning was done using the laser dithering method described above. Also,
This cross-correlation method was used to measure the timing jitter between the two lasers. The timing calibration was provided by inserting a glass etalon of having a thickness of 2 mm into one arm of the correlator, which creates a sequence of pulses separated by 20 psec. These sequence of pulses are clearly visible on the scans shown in
The measured jitter demonstrates the limitations of the precision of the electronic PLL circuit of stabilizer 130, but it also demonstrates how precise timing information can be obtained in spite of this jitter. If the scan is done rapidly enough, the relative timing jitter within the scan time can be quite small. And if a stable series of timing pulses is provided, e.g., by passing laser pulses through an etalon, then even the jitter during the scan time can be known accurately. In this way, the scan characteristic can be known to subpicosecond accuracy even though the lasers are jittering by several picoseconds.
The beneficial effects of co-packaging the lasers is important. In a previous, and less successful dual laser system built by the inventors, the same pair of lasers were constructed separately on different breadboards and were pumped by different lasers. With the stabilizer enabled, the slave laser would track the master laser for only about 30 minutes before the cavity length mismatch would exceed the 40 micron travel range of the PZT. At this point, tracking became impossible. Thus, even under a normal room temperature environment, the cavity length mismatch between a pair of 5 MHz lasers can drift by more than 40 microns, which is beyond the range of most PZTs. On the other hand, the copackaged system of the present invention is able to track indefinitely, indicating that the cavity length mismatch stays well within the 40 micron limit of the PZT under normal room conditions. Measurements of the absolute and relative frequency drifts show that the relative frequency drift between the two lasers is 7-times less than the absolute drift of one laser. The relative drift can be improved still further by making the lasers truly identical in construction. This can be accomplished by terminating both lasers with identical PZT-FRM assemblies, or by terminating them both with identical FRM packages and changing the slave laser cavity length with a fiber stretcher.
Further reduction of both the relative and absolute timing drifts can be accomplished by acoustic damping of fiber spool 280 and other components, and building the lasers in the same enclosure, which could itself be acoustically damped and temperature controlled. Eventually, with all of the above methods employed, relative laser jitter could reach the quantum limit (H. A. Haus and A. Mecozzi, "Noise of mode-locked lasers," IEEE J. Quantum Electron., QE-29, pp. 983-996, March 1993). Since the quantum-limited timing jitter increases with increasing dispersion, the jitter could be further reduced by operating the mode-locked lasers near the zero-dispersion wavelength.
Dithering the end mirror of a mode-locked laser can induce amplitude fluctuations at the scan frequency due to mirror misalignment and defocusing. The misalignment effect can be minimized by focusing on the PZT-mirror to reduce angular sensitivity, and by using a 3-point mirror-scanning PZT to preserve alignment, as is done with commercial scanning Fabry Perot interferometers. The defocusing effect can occur when the scan amplitude is an appreciable fraction of the confocal parameter of the beam waist incident on the PZT-mirror. In a fiber laser, this defocusing causes a reduction in the coupling efficiency of the beam back into the fiber, which in turn, causes power fluctuations. Thus, tight focusing on the PZT mirror is undesirable. A judicious choice of beam collimation can reduce the defocusing effect. For example, if a PZT is used having a scan range of 40 microns, a confocal parameter of at least a few millimeters would be required. Then, the quantity (ΔL/ZR)2˜10−4 is quite small, where ZR is the confocal parameter of the beam waist at the PZT mirror. When this quantity is small, the amplitude modulation of the laser is correspondingly small.
It should be pointed out that even if slight mirror misalignments cause some small amplitude fluctuations during the scanning, the beam pointing stability is not degraded in any way, due to the guiding properties of the fiber. However, with a solid-state modelocked laser, some deviation of the output beam could occur if measures are not taken to prevent it.
An alternative laser system might have both lasers terminated by identical FRM, and the cavity lengths could be adjusted by a PZT fiber stretcher, such as a piezoceramic tube actuator PiT 40�18�1 made by Piezomechanik GmbH of Germany.
Timing Calibration
Even with the PLL circuit of stabilizer 130 stabilizing the average relative timing delay, this position can fluctuate by several picoseconds so that it is necessary to insure that the data acquisition unit 50 is properly triggered with a timing signal which has the desired accuracy, i.e. 100 fsec. Such a signal can be obtained from a cross-correlator employing nonlinear optical mixing in a nonlinear crystal such as BBO, as was done in the demonstration mentioned above, and by others, such as Kafka et al. This type of triggering has also been shown to be essential in obtaining high accuracy in metrology measurements, even when using a mirror shaker for the scanning time delay. However, while this is necessary, it may not be sufficient. Jitter measurements show that the timing can fluctuate not only from scan to scan, but even during a single scan cycle. That is, the scan rate Rscan, changes from scan to scan, and even during the scan cycle. It is therefore essential that time scaling operations occur before signal averaging. Thus, it is necessary to have at least two timing pulses per scan from the correlator—one for triggering, and the other for time scale information. To do this, the present invention fills the scanning interval with a sequence of pulses, either uniform or nonuniform (in both amplitude and time). In this way, timing information can be obtained about the scanning interval on each and every scan cycle.
Timing Scale Generation
An important key point of the time scale calibration is selecting the optical methods used to generate the timing information. In a preferred embodiment, a sequence of pulses can be produced by reflecting a single pulse from a high-finesse Fabry-Perot (FP) etalon to produce a temporally uniform sequence of many pulses, as shown in
T max≈(17 pulses)�(6.7 psec/pulse)=114 psec Eq. 11.
The data in
The reflected pulse train is identical to the transmitted pulse train, which are each shown in
Of course, etalons with lower finesse can also be used, but then the transmitted pulse train decays much more quickly, and the correlator's dynamic range becomes the limiting factor. For example, an etalon with surface reflectivity of R=30% produces a rapidly decaying series of pulses in which each pulse is 10-times weaker than the previous pulse. Thus, only about 3 pulses can be used for calibration in real time, since most data acquisition has a single shot dynamic range of only three decades. This was the case for the correlation shown in
In yet another embodiment, combinations of FP etalons can be used. For example, a thin etalon (such as a cover-slip) would produce a pair of closely spaced pulses (˜1 psec) which would then be sent to a higher finesse FP with a larger spacing (e.g., 20 psec). The result would be a sequence of pulse pairs spaced by 20 psec. This gives local derivative information about the timing characteristic.
The FP etalons could be either solid or air-spaced. A solid etalon is more rugged and compact, but an air-spaced etalon is adjustable and avoids the problem of pulse broadening due to group velocity dispersion which would occur for those pulses which have made many round-trips through the etalon. For high accuracy, it is necessary to control the temperature of the etalon. For example, the fractional change in group delay through a piece of fused silica is approximately Δl/l˜10−6/� C. Thus, if 1 micron (6 fsec) of accuracy is required over an entire range of 1 meter (6 nsec), then the etalon temperature should be constant to within 1� C. Air-spaced etalons have an advantage in that they can be constructed using thermally compensating mounting techniques. Thus, the temperature sensitivity can be reduced over that of solid etalons, and temperature control may not be required for normal room-temperature operation.
Another possible implementation is to use a series of photorefractively grown fiber gratings 310, as shown in
Additionally, if it is desired that the pulses traverse substantial lengths of fiber (i.e., >0.5 meters) the pulses will broaden due to group velocity dispersion (GVD) in the fiber. Then, a series of chirped gratings could be grown in such a way that the GVD of the fiber being traversed is properly compensated. In fact, chirped gratings are necessary for high time resolution due to the fact that normal unchirped gratings have reflectance bandwidths of only about 2 nm making them unsuitable for subpicosecond pulses. At a center wavelength of 800 nm, a 100 fsec pulse has a full-width half-maximum (FWHM) bandwidth of 8 nm; and at a wavelength of 1500 nm, the bandwidth is about 30 nm. Only chirped gratings have sufficiently broad bandwidths for these pulses. It is known that reflecting femtosecond pulses from chirped gratings causes the pulses themselves to become broadened and chirped. In this case, it is necessary to sequentially reflect the pulses from two chirped gratings with opposite senses of chirp, as is schematically illustrated in
Two fiber gratings can also be used to form a fiber FP etalon, which could be used either in transmission or in reflection. Alternatively, a passive optical fiber loop could be used. Other structures which can produce reflections are poor splices between optical fibers and microbends in a fiber. Temperature control of the fiber would be necessary in these cases, or variations in timing calibration would be known beforehand, and compensated for mathematically.
Pulse sequences can also be produced by a series of partially reflecting mirrors, with the reflectivity and spacing of each mirror being carefully chosen so as to give the desired sequence of pulses, if it is physically possible. Algorithms have been developed for calculating the required mirror parameters to produce the desired pulse sequences from a single pulse (V. Narayan et al., "Design of multimirror structures for high-frequency bursts and codes of ultrashort pulses," IEEE J. Quantum Electron. QE-30, pp. 1671-1680, July 1994). There are a number of other types of optical devices, composed of partial reflectors, which can be used as well to generate pulse trains from a single pulse.
Arbitrary, programmable pulse sequences can be produced by passing a laser pulse through a pulse-shaping dispersive delay line (DDL). This gives much greater flexibility than the methods mentioned above, but is limited in terms of the maximum achievable pulse spacing. Practical limits are about 100-200 psec. Any larger pulse spacing requires a physically larger setup, which can become prohibitive.
Another embodiment, most useful for low repetition rate (≦30 MHz) lasers at wavelengths near 1550 nm, would be to inject a sequence of pulses into a regenerative soliton storage ring consisting of a fiber loop with a gain section (e.g. Er-doped fiber). The pulse sequence injected into the ring may be obtained in any number of ways including the methods discussed here (i.e., Fabry-Perot etalon, fiber gratings, pulse shaper, etc.). Because the loop is regenerative, it is necessary to dump it and re-inject it on every laser pulse. This would occur every 200 nsec for a 5 MHz laser. The dumping and injection can be accomplished by some type of optical switch, either AO or EO, which are commercially available.
Yet, another embodiment would be to inject short pulses from one laser into a laser diode biased at or near threshold. The laser diode facets have a reflectivity of 30%, forming a low-finesse FP etalon. However, the gain of the laser diode would retard or prevent the "ringdown" of the optical pulses. In this way, it should be possible to obtain a train of several tens of pulses. Eventually, group velocity dispersion and gain narrowing in the diode would broaden pulses which make many round trips through the structure, and thus limit the number of pulses which are actually usable. This device could be used either in reflection or transmission.
Extremely fine (sub-micron) calibration can be obtained by inserting a birefringent crystal (e.g. quartz) into one beam of the measurement system and comparing the pulse arrival times when the pulses propagate along the ordinary and extraordinary axes of the crystal. Typical plate thicknesses of 1 mm give a few wavelengths of retardation, which produces a readily discernable difference in pulse arrival times if pulses of 100 fsec duration or less are used.
Cross-Correlator Optics
The sequence of pulse signals generated according to the above-described methods are cross-correlated with the other laser pulses using some sort of nonlinear element. Various nonlinear processes can be used for detection of pulse coincidence or relative pulse timing. Some optical nonlinearities which could be used include, but are not limited to: optical mixing via second harmonic generation (SHG), sum frequency generation, gain saturation, absorption saturation, four-wave mixing (FWM), and nonlinear response of photocurrents. A likely choice for the nonlinear element is a SHG crystal. However, using a SHG crystal has the disadvantage of only providing a signal when there is overlap between the laser pulses. Thus, it can serve only as a coincidence detector. This requires the use of the various pulse train generators, pulse shapers, etalons, . . . etc., discussed above. In some situations, the extremely high accuracy which can be obtained using SHG crystals is not required. In some cases it may be more desirable to use an element which has a noninstantaneous response, and thus, can give information about time delay via amplitude information. There are a number of devices or materials which might be suitable, such as traveling-wave laser diode amplifiers (TWAs); saturable absorbers; photodetectors, which rely on saturation effects, such as PIN diodes and avalanche photodiodes (APDs); or perhaps even SEED devices. For example, TWA's have been used for optical clock recovery in optical communications. They have the advantage that when they are driven far into saturation, they become relatively immune to amplitude fluctuations. Here, the accuracy and range are determined by the recovery time of the device. TWA devices are known to measure phase accuracy to about 10−3 radians. In such optoelectonic devices, nonlinearity of optical absorption is not the only method which can provide timing information. Pulse timing information can also be obtained by monitoring electrical properties such as photocurrent, voltage, capacitance, etc., of the device as pulses propagate through them. This would be a significant simplification over detecting the change in optical properties.
These timing calibration techniques, although intended for use with the fast-scanning laser method, are expected to be applicable not only to the laser scanning systems described here, but also to free-scanning lasers, dual-wavelength mode-locked lasers, and even to the more conventional scanning systems employing physical delays. An example of this is the calibration data shown in
Laser Stabilization
Because any cavity length error is constantly accumulated on each round trip of the laser cavity, very small cavity length fluctuations can cause large timing errors. Thus, it is necessary to use a servo loop to hold the time-averaged repetition rate to ν1; or equivalently, the time-averaged cavity length mismatch, ΔL, must be held to zero. The feedback signal which can be used to control the average cavity mismatch is derived from a pair of photodetectors PD-1 and PD-2 shown in
A detailed diagram of the preferred embodiment of the laser stabilization and dithering system, including stabilizer 130, is shown in
In a preferred embodiment of the stabilizer 130, the phase detector 133 for the PLL can be a standard RF phase detector, or mixer, or a logic gate, such as an XOR gate, which has improved linearity. The linearity is very important if it is desired to perform scanning of any subinterval of the round trip time. Alternatively, a time-to-amplitude converter (TAC) can be used as the phase detector. This is also very linear, and would be especially appropriate at low repetition rates, such as 5 MHz. One limitation to highly accurate phase stability is the AM to FM conversion which can occur in a simple RF mixer. That is, laser amplitude fluctuations are converted to timing fluctuations by the mixer. It is thus desirable to perform signal conditioning on the electrical pulses which are produced by the photodiodes PD-1 and PD-2. This can be accomplished most readily by using timing discriminators 131 and 132 before phase detector 133, as shown in
It is also important that the scanning frequency be greater than the bandwidth of the stabilizer system. In this way, the PLL would maintain the average time position properly, but would not counteract the applied mirror scanning. The twin fiber lasers described earlier were stabilized using a PLL circuit which had a bandwidth of only 30 Hz. This rather low bandwidth is advantageous for allowing scan rates ranging from 30 Hz up to several kHz. However, in order to be able to use a 30 Hz stabilizer bandwidth and still maintain synchronization with <10 psec accuracy, the intrinsic relative timing jitter of the laser pair must be very low. This is achieved using the methods of construction described earlier, which ensure that the two lasers are subjected to the same environmental conditions to the greatest extent possible.
It is also possible that a simple correlator could provide the necessary feedback signal for stabilization. While the laser is scanning, the peak position, as measured by the correlator, could be used as an error signal to feed back to stabilizer 130. Note that the simple stationary cross-correlator technique of Dijaili et al. will not work here, since in the present invention the lasers are constantly scanning.
The fast scanning system of the present invention has many advantages over conventional scanning physical delays and free-scanning lasers. In particular, unlike the conventional scanning physical delays, which have moving arms, no misalignment or defocusing of the laser beams occurs in the present invention even when scanning large delays of several nsec. With physical delays, very careful alignment must be performed, and the confocal parameter must be >1 meter for even a 1 nsec delay line. Furthermore, high scanning speed, and even hypersonic scan velocities are possible with the present invention. However, with a physical delay, scanning even 1 nsec at 100 Hz (100 feet/sec) is not practical. Although, with the present invention, large (adjustable) scanning ranges are possible, for example, (from ˜50 psec up to 200 nsec for a 5 MHz fiber laser). To achieve such a scanning range in a conventional scanning system, 200 feet of delay line would be required. Also, there is no need to employ a high repetition rate laser for better duty cycle, as in conventional free scanning systems. Further, large temporal dynamic range is possible, which would be useful for scanning of remote targets or OTDR. For example, a total scan range of TR=200 nsec with time resolution less than 1 fsec gives a temporal dynamic range of more than 108. Also, an improved duty cycle with greatly reduced dead-time is achieved, unlike the conventional free scanning system. Also, extremely simple and compact cross-correlator design is now possible with the present invention, without any moving parts. For example, the correlator could be the size of a Game-Boy. Moreover, experimental setups are greatly simplified because pathlengths do not need to be matched in the present invention.
The fast scanning and timing calibration methods discussed above can be used to perform many types of measurements and experiments. A few of the applications in which the methods and apparatuses of the present invention can be used, are discussed below. However, a person skilled in the art will readily understand that the present invention is applicable to many applications and is not limited to the following applications described below.
1. On-the-fly (OTF) time scale correction—In this technique, the timing pulses form a time scale. For example, if sinusoidal scanning is used, then even a uniform pulse sequence will appear non-uniform in time. Fast processors could use this time scale information on each scan to appropriately adjust the scanning data (e.g., by using interpolation) with the correct time scale before signal averaging.
In other words, each scanning point is corrected by the fast processor based on the time scale formed by the timing pulses. For example,
2. Scan rejection (smart triggering)—This technique uses a "scan-selector" which looks for a number of timing pulses (at least two) to occur within certain well-defined time slots relative to the trigger pulse. The selector then makes a GO/NO-GO decision on whether to sum the scan data into the signal averager. If the timing pulses from the correlator fall within the time slots, the scan is summed into the existing data buffer; if one of the timing pulses "misses" its timing slot, the scan is rejected. After signal averaging is completed, any nonlinearity in the time scale can be properly compensated.
The smart triggering method is the simplest to implement; however, many scans would be wasted. The on-the-fly scale correction is more sophisticated and more time efficient in terms of wasted scans, but is more computationally intensive. Accordingly, the constraints of the application will likely determine which technique to use.
Metrology System
As an example of a more specific application,
Alternatively, this lateral displacement effect could be used to some advantage in modifying the shape of the pulse train envelope to be something other than a decaying exponential. This could occur via aperturing of the beams, or by the angular selectivity of the phase matching condition of the nonlinear mixing crystal in the correlator.
OTDR System
Other configurations of the laser, timing units and correlators are possible. For example, the roles of the master and slave lasers 710 and 720, could be interchanged. The pulse shaper could also consist of a fiber FP etalon, or a fiber loop (either passive or with gain).
EO Sampling Oscilloscope
Pulses from master laser 810 are split into two beams. One beam is sent to a timing unit 841; the other pulse is sent to generate electrical pulses on the device under test (DUT), which in this case is mounted on an integrated circuit (IC) 890, via photo-conductive (PC) switch 891. Pulses from slave laser 820 are also split into two beams. One beam is sent to the electro-optic (EO) probe tip 892, and the other beam is sent to the pulse shaper 880 which produces a sequence of pulses, which in turn are sent to timing unit 841 for time scale calibration. The pulses returned from the EO probe tip 892 are modulated by the interaction between voltage from the DUT and the EO probe tip 892. These pulses are detected by polarizing optics 893 and sent to a Y-channel of DAQ 850. Accordingly, precision timing calibration is obtained in a similar manner as described for the OTDR of
The timing delay, scanning interval, and scanning frequency are set to the desired values by the stabilizer 830. For example, if it is desired to increase the scanning frequency by a factor of two (but not the scan range), then it is necessary to double the modulation frequency of the PZT in slave laser 820, and to increase the PZT scanning voltage by a corresponding amount. On the other hand, to increase the scan range without changing the scan frequency, one simply increases the PZT scanning voltage. The relative delay of the time sweep range is adjusted by the phase control of stabilizer 830. This flexibility of time scale adjustment is similar to that afforded by the delaying time base of a conventional oscilloscope.
One could equally well use photoconductive sampling for this purpose. This has much higher sensitivity, however, the time resolution is limited to ˜2 psec.
The most widely used applications of short laser pulses and scanning delays have been in pump-probe measurements of physical, chemical, & electronic systems. However, the range of applications for this technology is increasing rapidly, and their introduction into commercial products beyond the scientific market, is imminent. The rapid scanning and time calibration techniques described here can be used in almost any application which utilizes ultrafast laser pulses, because virtually all of these applications require some type of adjustable time delay between laser pulses. These applications include, but are not limited to, electo-optic testing of ultrafast electronic and optoelectronic devices characterization of charge dynamics in semiconductor materials and devices, all-optical signal processing photoconductive sampling, and various forms of time-resolved, nanometer probing. Additionally, there are a number of new applications being developed for terahertz beams to which these scanning methods could be applied including terahertz imaging, such as that disclosed by B. B. Hu et al., "Imaging with Terahertz Waves", Optics Letters, Vol. 20, No. 16, Aug. 15, 1995, pp. 1716-1719. Recently, ultrafast photodetectors have become commercially available, whose speed (50-GHz, 10 psec FWHM) far outstrips the measurement capability of commercial sampling oscilloscopes, such as Newport Corp. Model #PX-D7, manufactured by Picometrix. Subpicosecond lasers and scanning delays are required to take full advantage of the speed of these detectors. Furthermore, the scanning method of the present invention, because of its unusual versatility of scanning range selection, is especially appropriate for submillimeter-resolution laser radar, and profiling of remote targets. The flexibility of the scan range adjustments makes the fast-scanning system analogous to the adjustable time base on an oscilloscope.
The very high scan rate achievable with the fast-scanning methods described here, makes possible many new potential applications. The scan velocity parameter (see Tables 1-3) is a figure of merit which is particularly useful. Note that the scan velocities in Tables 1-3 range from 3 m/sec to 30,000 m/sec. Thus, supersonic scan velocities are possible with the free-scanning or fast scanning techniques, making them potentially useful for studies of, or applications involving, laser-induced acoustic and photoelastic effects in solids and liquids, for example. Such applications are not practical using conventional scanning methods because it would be necessary to move the scanning mirror at sonic velocities in a physical delay system, which is unrealistic.
The present invention has been described in connection with the preferred embodiments, and is not intended to be limited only to the above-described embodiments. Other modifications and variations to the invention will be apparent to those skilled in the art from the foregoing disclosure and teachings. Thus, while only certain embodiments of the invention have been specifically described herein, it will be apparent that numerous modifications may be made thereto without departing from the spirit and scope of the invention.
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|U.S. Classification
|372/30, 372/6
|International Classification
|Cooperative Classification
|European Classification
|Jan 30, 2013
|Fee payment
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|Apr 7, 2017
|Maintenance fee reminder mailed


text:
MMA Forum - UFC Forums - UFC Results - MMA Videos (http://www.mmaforum.com/forumindex.php)
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joined just to say...
Just want to say Bisping is a straight bitch, PERIOD..!
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Welcome to the forum. You definatly found the right section to post this thread in. :thumbsup:
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