Abstract
In this paper we present the stabilization of the pulse repetition rate of dual-comb lasers using an intracavity semiconductor saturable absorber mirror (SESAM) for passive modelocking and an intracavity birefringent crystal for polarization-duplexing to obtain simultaneous emission of two modelocked beams from the same linear cavity sharing all components. Initially surprising was the observation that the cavity length adjustments to stabilize one polarization did not significantly affect the pulse repetition rate of the other. We gained insight in the underlying physics using both a semiconductor and Nd:YAG laser gain material with the conclusion that the pulse arrival timing jitter of the two beams is decoupled by the uncorrelated time delay from the saturated SESAM and becomes locked with sufficient but not too much pulse overlap. Noise stabilization is in all cases still possible for both combs. The dual-comb modelocked laser is particularly interesting for the semiconductor laser enabling the integration of gain and absorber layers within one wafer (referred to as the modelocked integrated external-cavity surface emitting laser--MIXSEL).
Highlights
A modelocked laser can support very short pulses but at the same time very precise frequency metrology for the most accurate clocks
We have introduced the concept of dual-comb lasers [19] to generate two modelocked beams with slightly different pulse repetition rates using only a single gain medium in a shared cavity
We have successfully demonstrated dual-comb lasers for both optically pumped semiconductor and Nd:YAG gain materials
Summary
A modelocked laser can support very short pulses but at the same time very precise frequency metrology for the most accurate clocks. A dual-comb MIXSEL (Fig. 1 and 2), revealed an initially very surprising result: when the cavity length was corrected by the error signal of one beam only, the other beam was not affected significantly (i.e. the timing jitter noise was uncorrelated) This means that the pulse repetition rates of the two cross-polarized beams seem to be decoupled from each other even though they share all components within the linear cavity (Fig. 2). We test our observation for two lasers, an optically pumped MIXSEL and Nd:YAG laser, using the same pump laser and the same SESAM for modelocking These lasers operate in different regimes of noise properties [35,38,39] with strong relaxation oscillations, long upper state lifetimes and small gain cross sections in the latter case, but show very similar behavior with regards to the uncorrelated timing jitter
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