Abstract
We present a new method for intra-cavity control of the carrier-envelope offset (CEO) frequency of ultrafast lasers that combines high feedback bandwidth with low loss, low nonlinearity, and low dispersion. A semiconductor saturable-absorber mirror (SESAM) inside a modelocked laser is optically pumped with a continuous-wave (cw) laser. In this way, the SESAM acts as intra-cavity opto-optical modulator (OOM): the optical power of the cw-laser corresponds to a high-bandwidth modulation channel for CEO frequency control. We experimentally verified this method for a femtosecond Er:Yb:glass oscillator (ERGO), in which one SESAM is in parallel used for modelocking and as intra-cavity OOM for achieving a tight CEO lock. This laser can also be CEO-stabilized in the usual scheme, in which the laser pump current is modulated, i.e., the gain element acts as intra-cavity OOM. We compare the performance with gain and SESAM OOM measuring CEO transfer function, frequency noise power spectral density (PSD), and Allan deviation for integration times up to 1000 s. In the case of the gain OOM, the millisecond upper-state lifetime of the Er:Yb:glass limits the achievable CEO-control bandwidth to <10 kHz. The feedback bandwidth of the SESAM OOM was more than a factor of 10 higher than the gain OOM bandwidth and was mainly limited by the used current driver. The residual integrated phase noise (1 Hz - 100 kHz) of the ~20-MHz CEO beat was improved by more than an order of magnitude (from 720 mrad to less than 65 mrad), and the fractional frequency stability by a factor of 4 (from 1∙10<su⁻⁸ to 2.5∙10⁻⁹ at 1 s).
Highlights
Optical frequency combs from modelocked lasers have revolutionized numerous research areas in time-frequency and optical metrology in the last decade
As a rule-of-thumb, the required bandwidth is approximated by the crossing point of the frequency noise power spectral density (PSD) of the free-running carrier-envelope offset (CEO) beat with the β-separation line introduced by Di Domenico et al [7]
We show significant improvements in terms of integrated phase noise, feedback-loop bandwidth, as well as Allan deviation of the stabilized CEO frequency using opto-optical modulator (OOM) of the semiconductor saturable-absorber mirror (SESAM), even though the overall performance of our laser stabilization was limited by the bandwidth of the current driver used for SESAM OOM
Summary
Optical frequency combs from modelocked lasers have revolutionized numerous research areas in time-frequency and optical metrology in the last decade. While a large feedback loop bandwidth of >1 MHz was demonstrated using this technique, the intra-cavity graphene EOM does not seem to be suitable for the stabilization of high-power lasers, due to the relatively high non-saturable losses (
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