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Highest peak power from a normal-dispersion Yb-fiber oscillator with anomalous dispersion higher-order-mode fiber

Mode-locked (ML) Yb-doped fiber lasers (YFL) are very attractive to supplant solid state lasers in many applications offering compact size, reliability, and eventually freedom from misalignment. Easy to integrate fiber-based anomalous dispersion control is critical for the development of broadband monolithic YFL. Currently intracavity-dispersion compensation is mostly done either by a grating arrangement or a prism-grating sequence. In this contribution we present a passively ML Yb-fiber oscillator that uses a higher-order mode (HOM) fiber [1] for dispersion compensation and operates in the net normal dispersion regime. The HOM fiber allows reducing the repetition rate of the system without compromising pulse duration and pulse quality. Pulse energies of up to 6 nJ were achieved and pulses as short as 150 fs were generated. This represents the highest peak power ever reported from an Yb ML fiber oscillator utilizing an anomalous dispersion HOM fiber (see [2] for comparison). We compare the pulse characteristics of a ML ring oscillator with a HOM fiber with an all normal dispersion (ANDi) oscillator with the same repetition rate of 24 MHz. ML-operation is ensured by nonlinear polarization rotation, and both the oscillator with the HOM fiber and the ANDi oscillator are self starting. The HOM fiber is spliced on both sides to a short length of SMF, which allows for an easy integration in our oscillator: the HOM fiber replaces the equivalent length of SMF in the ANDi oscillator. The output couplers are free space polarizing beam splitters. Behind the second output port an interference filter is installed in the oscillator, this is required to reliably start ML operation. Although the birefringence of the HOM fiber is almost negligible, stable ML operation requires strict control of the polarization state at its input and output, hence inline polarization controllers are installed directly before and after the HOM fiber. Oscillator configurations with one and two output ports were investigated. The latter is known to improve the output pulse quality of ANDi oscillators [3], with the first output port acting as an intracavity pulse cleaner, and the second port act as main output coupler. We compress the output pulses with a grating pair, and retrieve the spectral and temporal characteristics with frequency resolved optical gating (FROG). Excessive TOD is known to destabilize the pulses of oscillators with net normal intracavity dispersion. With this in mind, one may expect that the output pulses of the oscillator with the HOM fiber are more compressible, and a better pulse quality may be expected. Indeed, our measurements show (see Fig. 2 a–c) that the pulses from the oscillator with the HOM fiber exhibit little structure away from the pulse peak at a pulse energy of 6 nJ while the the pulses from the ANDi oscillator show a very strong pedestal and structure away from the pulse peak at the maximum pulse energy of 5 nJ. We observe a substantial improvement of the pulse quality for the two output port configuration, both in the case of the ANDi oscillator an in the case of the oscillator with the HOM fiber (see Fig. 2 d–f). The pulse pedestal is greatly reduced in both cases. The compressed pulse duration of 150 fs at a pulse energy of 2.3 nJ from the oscillator with the HOM shows that in the case of the oscillator with the HOM fiber pulse compressibility is significantly improved as well.