1. Introduction

Over the past years, mode-locked lasers have revolutionized frequency metrology and optical clocks [1]. Stabilized mode-locked lasers now allow scientists to perform synthesis of few-cycle optical pulses [2]. Diode pumped solid-state lasers currently dominate the area of ultrashort pulsed light sources for frequency comb generation offering not only extremely short optical pulses comprising several optical cycles, but also broad tunability [2–4].

Fiber lasers are being developed in many laboratories around the world as alternatives to bulky and expensive solid-state lasers. Recent unprecedented growth of telecom industry had resulted in development of mature fiber technology, and reliable and cost effective components which make suitably designed fiber lasers real contenders to conventional solid state lasers. The fiber laser technology is to replace bulk-optic ultrafast systems with simpler and hands-off alternatives. The broad fluorescence spectrum makes different fiber gain media attractive for tunable and ultrashort pulse sources. Additionally, fiber lasers offer attractive, new means of controlling the frequency comb, particularly the offset frequency [5].

The heart of any optical system used in frequency metrology is a stabilized mode-locked oscillator. Typical stabilization technique employs servo loop control feedback with piezoelectric actuator for active control of the cavity length and therefore of the repetition rate [3–5]. Using optical signal for controlling a passively mode-locked fiber laser has recently been demonstrated [7–11]. In this technique, repetition rate stabilization and timing jitter reduction were achieved with modulating the loss of the saturable absorber mirror by pumping it optically at the cavity fundamental frequency or its harmonic. Improved stability of the mode-locked pulse train owing to significant reduction of the unwanted harmonics has been demonstrated.

Here, we demonstrate a new and simple technique for mode-locked laser stabilization based on cross-phase modulation (XPM) in an optical fiber. Seed pulses are produced by a telecom DFB laser diode operating at 1540 nm. The stabilization technique we demonstrate here should be applicable to different types of mode-locked fiber lasers used in those systems that require low jitter ultrashort pulse oscillators.

2. Laser system

The master-slave geometry used for synchronization of the passively mode-locked laser to the external clock is shown in Fig 1.

 

Fig. 1. Schematic of 1050 nm-fiber laser synchronized to the pulse train produced by a 1.54 μm laser diode.

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The seed source, which comprises a distributed-feedback (DFB) laser diode and standard telecom components, emits optical pulses with tunable repetition rate. The repetition rate was determined by the frequency synthesizer that acts as a clock generator. After passing through an Er-doped fiber amplifier, the pulses are launched into the slave laser cavity. The shortest pulse duration obtained here was limited by the bandwidth of the components and drive electronics available for these experiments. The state-of-the-art telecom devices, however, allow the generation of much shorter pulses and may result in an improved performance of the technique described in this paper.

As a slave laser we used a passively mode-locked ytterbium fiber laser with an intra-cavity 1540/1050 nm dichroic combiner for launching the seeding signal from the master source (Fig 1). The Yb-laser comprises a linear cavity terminated by the SESAM used as a cavity end mirror. Another cavity mirror was a dichroic fiber loop mirror providing ~90%-reflectivity at 1050 nm and high transmission at 1540 nm. Optical gain is provided by a 70 cm Yb-doped fiber with NA=0.13 and cutoff wavelength of ~910 nm, pumped with a 150 mW, 980 nm semiconductor laser through a 980/1060 WDM combiner. The gain fiber has pump absorption of 434 dB/m at 980 nm, a core diameter of 6.2 μm and normal group velocity dispersion of -30 ps/nm-km at 1050 nm. The large normal chromatic dispersion of the fiber in mode-locked laser is offset by a 1200 mm^-1 grating pair placed in the free space section of the cavity that allows changing the overall cavity dispersion over a broad range, from normal to anomalous. The overall cavity dispersion of the Yb slave laser was 0.7 ps/nm, as inferred from the soliton side-bands in the optical spectrum of the mode-locked pulses, shown in Fig. 3 [12]. The SESAM-lens assembly can be precisely translated axially providing us with a means of controlling the cavity length and thus the laser repetition rate. SESAM-enforced self-starting mode-locking was obtained in the Yb fiber laser. The slave laser operated at a fundamental repetition rate of 24 MHz.

The locking signal from the seed source was coupled into the mode-locked slave laser cavity through the 1.05 μm/1.54 μm dichroic fiber combiner. The 2m span of Corning HI 1060 fiber between the combiner and the output coupler acts as a nonlinear interaction medium for seed master pulses and slave pulses. The interaction length also included the loop mirror, however because of its short length of 0.1 m, nonlinear interaction inside the loop can be ignored. The XPM interaction fiber has a MFD of 6.2 μm and normal dispersion of -29 ps/nm∙km at 1.05 μm [13]. To avoid any optical coupling between seed and slave oscillators, an optical isolator was placed at the input of the master/slave combiner which resulted in oneway coupling from the seed to the mode-locked laser. The fiber loop-mirror acts as an output coupler which supplies the two-color pulse train from the system. All fiber components were made from Corning HI 1060 fiber with NA=0.14 and cutoff wavelength of 939 nm using computer-controlled fusing technology that allows the required spectral characteristics to be achieved. The combined output of the system was detected and fed to the input of an electrical spectrum analyzer with 10 Hz resolution for simultaneous monitoring of the seed and mode-locked lasers repetition rates.

3. Results

First, by changing the frequency of the 1.54 μm seed pulsed signal, its repetition rate was brought close to the fundamental repetition rate of the 1.05 μm mode-locked laser. Then, the length of the free-space section of the slave laser cavity was precisely adjusted by moving the SESAM-assembly. The repetition rate was detected with a fast photodiode and analyzed by monitoring its radio-frequency (RF) spectrum. The cavity length of the mode-locked slave laser was tuned until the two pulse trains became locked owing to XPM-interaction in the common optical fiber path [10]. Monitoring the slave laser pulse train with the aid of an oscilloscope triggered by the clock signal allows the mode-locked pulse train to be viewed (stopped) in the synchronized state. In free-running regime, the pulse train could not be clearly displayed on the oscilloscope. It should be noted that mode-locked operation of the slave laser was sustained during adjustment of the cavity length and with variations in the seed signal.

The master laser pulse spectrum and temporal shape are shown in Fig 2, revealing an optical pulse FWHM of 100 ps. The master pulse appears inverted on the oscilloscope screen due to the electronic amplifier employed in the fast photo-detection scheme.

 

Fig. 2. (a) Optical spectrum of the seed pulses and (b) pulse shape obtained with a fast

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The pulse spectrum for the 1.05 μm mode-locked laser operating in the synchronized mode is shown in Fig. 3. The corresponding autocorrelation trace in the insert reveals a clean pulse with 2 ps FWHM.

 

Fig. 3. Optical spectrum of the mode-locked pulses. Inset shows intensity autocorrelation corresponding to pulse duration of 2ps.

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Figure 4 shows the change in repetition rate of the slave laser in free-running and synchronized regimes. The free-running laser repetition rate exhibits large environmental variation of ±30 Hz. When synchronized, the repetition rate of the mode-locked slave laser remained constant and equal to the repetition rate of the clock within the analyzer bandwidth, for a measurement time of 30 minutes without any external cavity-length stabilization. In these measurements, the average power of the seed signal was 130 mW, whereas the 1.05 μm laser produced about 2 mW at the output.

By changing the cavity length of the slave laser until locking of the lasers was lost, we could estimate that the maximum tolerable cavity-length mismatch is about 15 μm, corresponding to a locking bandwidth of 60 Hz.

 

Fig. 5. Temporal evolution of the slave laser central frequency when the seed signal is switched periodically off and on.

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To further study the strength of the locking mechanism based on the DFB diode-induced XPM in optical fiber, we periodically turned off the seed signal and monitored the slave laser transition from locked to free-running state and back. Fig. 5 shows a fragment of these actions. As observed from the scope and RF analyzer, when the seed signal is turned on, the slave Yb-fiber laser returns to the locked state in nearly instantaneous manner; the actual transition time, however, could not be measured.

4. Conclusion

We demonstrated optical synchronization of the repetition rate of a passively mode-locked fiber laser to a pulse train generated by a DFB laser diode-based seed source. The locking scheme used here to synchronize mode-locked laser to the external clock differs from conventional approaches in that the seed pulsed signal with tunable repetition rate is produced by modulating a laser diode with an external clock signal. The cross-phase modulation in the fiber cavity of the slave mode-locked oscillator enables the generation of pulse trains with highly stable repetition rate locked to the external clock. We have found that the phase modulation produced by the seed master pulses from the laser diode creates a sufficiently strong locking mechanism for interpulse spacing stabilization. Although shorter pulses would improve the strength of the locking mechanism, we demonstrate that tight control of the repetition rate can be achieved with seed pulse duration above 100 ps. The seed source and electronics components used in the synchronization scheme employ only standard communication components. Using state-of the art high-frequency telecom devices that allow ps-duration pulses generation will greatly improve the locking bandwidth and make this method applicable to different mode-locked laser systems in the picosecond and femtosecond regimes.