Collimators may be used with laser diodes and CO cutting lasers. For pulsed lasers and invisible wavelengths, other power limits apply. Visible and infrared femtosecond lasers were also applied for lithography. The first diode lasers were homojunction diodes. They have potential uses in diode lasers, amplifiers, and biological sensors. Another method of powering some diode lasers is the use of optical pumping. The latest research developments include the use of fibre optics, lasers and interferometers. Modern instruments usually have multiple lasers and fluorescence detectors. LPE afforded the technology of making heterojunction diode lasers. Coumarin dyes are extensively used as gain media in blue-green tunable organic dye lasers. The range of applications of tunable lasers is extremely wide. DBR lasers can be edge emitting lasers or VCSELs. This effect only becomes significant with very intense beams such as those from lasers. This was accomplished by combining the output beams of several 100-W fiber lasers. Vertical external-cavity surface-emitting lasers, or VECSELs, are similar to VCSELs.