Optical repetition rate control of an Erbium-doped all-fiber laser

Summary form only given. We present the optical repetition rate stabilization of an Erbium-doped femtosecond all-fiber laser utilizing an additional intracavity Ytterbium-doped fiber. The pump-dependent refractive index change of optically pumped active fibers is well-described [1-3] and already used, e.g., for coherent combining of continuous-wave Erbium amplifiers by Ytterbium fibers [3] and vice versa [2]. However, the obvious step to apply this technique for the stabilization of the repetition rate of a mode-locked laser can now be reported for the first time.We implemented an Ytterbium fiber module that is capable of tuning the repetition rate of the 31.4 MHz Erbium fiber (see Fig. 1a) laser by up to 28 Hz corresponding to a change in optical path length of about 8.0 μm [5]. Although, this change in repetition rate is sufficient for an all-optical control on short time scales an additional thermal control of the local temperature of the Ytterbium fiber was added to account for long-term drifts of the room temperature, which extended the tuning range of the whole module to an optical path length difference of about 80 μm.The stabilization was realized by an analog proportional-integral-derivative (PID) controller acting on a current sink adjusting the current of the laser diode pumping the Ytterbium-doped fiber. The error signal for the stabilization was generated by a phase detector working at the 176th harmonic of the lasers repetition rate [4]. Additionally, the controller output served as an error signal for a second proportional controller, which adjusted the temperature of the Ytterbium fiber module compensating the long-term environmental temperature drift. In free-running mode the repetition rate of the laser was significantly subjected to this drift (see Fig. 1). With stabilization the laser repetition rate fluctuations were reduced to only small residual fluctuations with a standard deviation of 22 mHz over 12 hours. Compared to mechanical methods (e.g. fiber stretching) the presented optical control of the repetition rate in an all-fiber setup provides advantages to applications where for instance mechanical abrasion is a problem or high reliability is asked for and finally, it reveals a new opportunity for the control of light by light. Currently we investigate the possibility for full optical control of the frequency comb by controlling the CEO frequency through additional pump modulation of the Erbium gain fiber.