Using a master - slave configuration, a robust synchronization was achieved for two-color Erbium and Ytterbium mode-locked fiber lasers. Due to enhanced nonlinear interaction in the fiber, noise-free pulse-locking was achieved allowing for a cavity mismatch tolerance of 140 μm. This is the highest tolerable cavity-length difference ever been obtained for synchronized mode-locked oscillators.
©2004 Optical Society of America
The technology of Semiconductor Saturable Absorber Mirror (SESAM) has greatly pushed the development of mode-locked fiber laser. The main features of fiber-based devices - high efficiency, reliability and small footprint - make them promising for applications traditionally occupied by ultrafast solid-state lasers. The broad fluorescence spectrum makes different fiber gain media attractive for tunable and ultrashort pulse sources. Recently pulse trains covering the vast wavelength range from 895 to 1560 nm were reported using neodymium, ytterbium and erbium doped mode-locked fiber lasers [1–3]. Different fields of science and technology have a strong need to pursue research using ultrafast lasers in an arrangement called two-color experiment. The two-color source appears to be a valuable instrument for ultrafast research including difference-, harmonic- and sum-frequency generation, coherent anti-Stokes Raman scattering microscopy and two-color pump-probe investigations. For the majority of the applications, the relative jitter between the pulses is a crucial factor severely limiting the performance of the system. Compared to the active synchronization of two-color mode-locked lasers with an electrical feedback scheme [4–5], the passive technique using cross-phase modulation (XPM) should result in a very small timing jitter [6–9]. It should also be noted that a two-color source using a single gain medium suffers from limited bandwidths, gain competition, poor tunability and close optical frequencies.
Using coupled-cavity lasers is shown to be a powerful technique for generation of a synchronized two-color mode-locked pulse train. In this approach, the two laser cavities share the same medium with Kerr-type nonlinearity that ensures synchronous mode-locking of the coupled oscillators. This technique is particularly attractive with fiber lasers because of long interaction length that can be conveniently constructed for coupled-cavity fiber lasers . However, integrated two-color coupled-cavities usually require complicated dichroic components and should be carefully designed to avoid excessive noise due to laser coupling.
In this Letter we demonstrate an alternative approach for synchronized two-color mode-locked system based on master-slave configuration. In this set-up the highly stable master mode-locked erbium-fiber oscillator is isolated from the slave ytterbium-fiber laser and, therefore, it is free from any disturbing influences that may occur in the lasers sharing joint cavity. Synchronization is achieved by the cross-phase modulation (XPM) in the slave oscillator induced by the master laser. We demonstrate that the slave mode-locked laser can be tightly synchronized by the pulse train injected into its cavity from independent master source with a record cavity-length detuning exceeding 140 μm. We have found that the performance of the master-slave type technique is similar to the coupled-cavity set-up otherwise it is simpler, more robust and well suited for use in practical systems.
The experimental setup is shown in Fig. 1. The output of the erbium-fiber master-laser operating at 1550 nm was injected into the 1060 nm slave laser cavity. The slave laser comprises a linear cavity terminated by the SESAM on one side and a fiber loop mirror on the other. The loop mirror acts as a dichroic output coupler providing ~90%-reflectivity at 1060 nm and high transmission at 1550 nm. Optical gain medium is a 70 cm of Yb-doped fiber with NA=0.13 and cutoff wavelength ~910 nm pumped by a 150 mW single-mode 980 nm semiconductor laser via a 980/1060 WDM coupler. 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 1060 nm. The large normal chromatic dispersion of the fiber in slave laser cavity 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 SESAM-lens assembly can be precisely translated axially providing us with a means of controlling the cavity length and thus the slave laser repetition rate.
The master laser is a stable, linear cavity laser passively mode-locked by a SESAM [2, 11]. Master laser is coupled to the slave laser cavity through a 1060/1550 dichroic fiber combiner. The fiber span between the combiner and the loop mirror acts as an interaction medium of the master and slave laser pulses. The segment of the slave laser cavity, where XPM occurs, has anomalous dispersion of the fiber at 1550 nm. The fiber dispersion was measured to be of 7.8 ps/nm·km. The low value of dispersion and short length of interaction fiber (~1.5 m) allowed concluding that the distortion of the 1.55-μm control pulse in a laser cavity was negligible. To ensure master laser protection against back reflections from the slave cavity, an optical isolator was placed at the input of the master/slave combiner to achieve one-way coupling from the master to the slave laser. The loop mirror supplies a two-color output from the master-slave system. The pump WDM, master-slave beam combiner and the loop mirror were made from Corning HI1060 fiber (NA=0.14, cutoff wavelength 939 nm) using computer-controlled fusing technology to achieve the required spectral characteristics.
The master laser repetition rate was measured to be around 40 MHz. By changing the fiber length and shifting the SESAM-lens holder, the cavity length of the slave laser was made to be essentially identical to cavity length of the master oscillator. The locking behavior was then investigated by only fine-tuning the slave cavity-length using the translation stage. Movement of the translation stage was monitored using an inductive transducer gauge with a reading precision of 1 μm. SESAM-enforced self starting mode-locking was obtained with both master and slave lasers. The pulse widths of 1 ps and 2 ps with the assumption of a sech2 shape were deduced from the autocorrelation measurements for the master and slave oscillators, respectively, as shown in Fig. 2(a) and Fig. 2(b).
The combined output of the master and slave lasers was detected and fed to the input of an electrical spectrum analyzer with a resolution of 100 Hz. This allows for repetition rate monitoring of the master and slave lasers, simultaneously. By aligning the slave laser cavity-length while recording the cavity elongation together with repetition rate for both lasers, the slave laser pulse train could be easily set to be locked to the master pulses. Figure 3 shows the repetition rate of the slave laser as a function of the SESAM assembly displacement. As shown in Fig. 3, a slave cavity-length mismatch over 140 μm is tolerable without losing locking mode with the repetition rate identical to the master oscillator. The stable synchronization was maintained within the locking range even under continuous changing the slave cavity-length. In these measurements, the average output power of the master laser was 40 mW, whereas the slave laser average power was about 2 mW. The synchronized mode of the master-slave system was independently confirmed by monitoring pulse trains on an oscilloscope. Triggering the scope with the master pulse train allowed the slave laser train to be clearly visualized. Otherwise, the slave laser could not be displayed on the scope without synchronization. In the synchronized mode, when master and slave pulses coincide temporally, strong nonlinear effect causes the slave pulse spectrum to be affected by the control master train.
Figure 4 shows the wavelength shift in the slave laser spectrum recorded simultaneously with repetition rate monitoring, shown in Fig. 3. This feature is due to the XPM induced by the Er-fiber master laser in the Yb-fiber cavity [8, 9]. The wavelength shift of the slave laser pulses generated through the XPM allows compensating for cavity-length variations owing to adequate change in the pulse group velocity . The synchronized operation was also possible with a spectral filter inserted in the slave laser cavity. However, with slave laser wavelength firmly set by the filter, the locking range vanished, as expected from the above discussions.
The average dispersion in the slave laser cavity obviously plays a crucial role in the locking procedure. The repetition rate of the slave laser was initially higher than that of the master oscillator (Fig. 3). Since the slave laser had anomalous cavity dispersion, the pulses experience abrupt red-shift to reach the locking state with an increase in the cavity-length (Fig. 4). With subsequent increase in the slave cavity-length in the synchronized mode, gradual blue-shift was detected, as seen from Fig. 4. As the cavity-mismatch reaches the locking limit, the slave laser wavelength returns to the free-running value.
With a decrease in the cavity dispersion, the same locking range can in principle be expected provided that a larger spectral shift is imposed on the slave laser pulses to compensate for the master-slave velocity mismatch. However, we found that the locking range for arbitrarily low cavity dispersion was ultimately limited by the spectral broadening generated by the master laser pulses through cross-phase modulation. This conclusion was derived from the behavior of the cavity mismatch tolerance with increasing master pulse energy, shown Fig. 5. The higher the master pulse energy, the stronger the phase modulation seen by the slave pulse and, consequently, the larger the spectral shift they acquire. Therefore, the locking range increases with increasing master pulse energy since larger mismatch tolerances can be compensated for a given dispersion of the cavity. From this observation, we conclude that the master pulse energy is a key parameter in our system limiting the locking range by setting a maximum value for wavelength variation.
The average dispersion of the slave laser cavity could be changed by altering the gratings separation. Figure 6 shows the change in slave laser cavity tolerance for several dispersion values of cavity. The decrease in the locking range with the decrease in the cavity dispersion originates from the limit in the wavelength shift capability set by the amount of the spectral broadening generated through the XPM. Alternatively, higher value of the chromatic dispersion of the slave laser cavity allows for larger change in the pulse group velocity for a given spectral shift, and thus leads to increased cavity mismatch tolerance.
Regardless the pulse energy limitation in our system, the strong mode overlap provided by the fiber as well as the long interaction length in the laser cavity make XPM-based synchronization very strong and, consequently, the locking range in a fiber system is shown to be approximately an order of magnitude larger than in bulk solid-state counterpart. With a large cavity tolerance, it was possible to maintain the synchronization state for long time without cavity stabilization. Figure 7 shows RF power variations in the slave laser output in the synchronized mode at the frequency corresponding to the repetition rate of the master oscillator. It should be noted that this enhancement in the locking mechanism is obtained with pulse energy of ~1 nJ that is at least by the 2–3 orders of magnitude lower than in typical solid-state lasers.
In conclusion, we demonstrated synchronized 1.55 and 1.06 μm mode-locked fiber lasers using master-slave configuration. Using optically isolated noise-free master oscillator, we have achieved a record value of the locking tolerance for the slave laser cavity mismatch of 140 μm. This result has been achieved owing to fiber-cavity advantages and can be exploited in practical two-color synchronized ultrafast sources. Contrary to the solid-state lasers, where two laser beams need to be carefully adjusted to overlap spatially in the nonlinear gain medium, the waveguide properties of the single-mode fiber ensure automatically large overlap volume of the master-slave beams. Another benefit of the master-slave fiber system is that even with very large interaction volume no gain competition is expected owing to the passive nature of the nonlinear fiber.
References and links
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