We report on generation of 850fs mode-locked pulses at 1065nm in a completely fiber integrated format. The figure-of-eight, fiber laser source incorporates anomalous dispersion compensation by using a holey fiber and a short length, high gain, double-clad Yb-fiber amplifier.
©2003 Optical Society of America
The figure-of-eight geometry (F8L) incorporating a nonlinear amplifying loop mirror (NALM) for soliton pulse generation and additive-pulse mode-locking has been widely explored at telecom wavelengths where anomalous, or near-zero fiber-cavity dispersion can be provided within Er- or Pr-doped fiber gain bands [1,2]. In the situation where some of the fiber in the cavity is normally dispersive, it is still possible through dispersion management to achieve an overall net anomalous dispersion and maintain operation in the average  or guiding center soliton regime . However, this approach has never been used around the 1µm wavelength region where the gain of Yb-doped or Nd-doped fibers can be exploited. Until recently, at this wavelength, zero or anomalous dispersion could only be achieved with bulk dispersion compensating elements, primarily gratings or prisms. For example, 42fs mode-locked pulses were obtained at 1µm with a Nd gain fiber in a bulk Fabry-Perot configuration . In such geometry, passive mode-locking exploiting the wide gain bandwidth can be initiated either by using active modulators , semiconductor antiresonant saturable absorbers  or mechanical perturbations . In a ring cavity geometry, non-linear polarization evolution in an isotropic fiber due to the Kerr non-linearly of the silica can be used to produce mode-locked pulses and similar dispersion compensation techniques applied, both bulk and fiber. For femtosecond generation, grating-based dispersive delay lines (DDL) for intracavity positive dispersion compensation [8–10] have normally been used and formation of pulses as short as 65 fs has been demonstrated .
The advent of holey fibers (HF) has led to few attempts to utilize their anomalous dispersion characteristics in mode-locking configurations were a length of actively doped Yb holey fiber was bulk-coupled in a cavity incorporating an acousto optic element and pulse durations were inferred from the mode-locked bandwidth .
The fiber integration of the femtosecond sources presents a challenge not only because of the technological difficulties of interfacing holey and standard single mode fibers with low loss, but also due to the inherent anisotropy of holey fibers which makes the use of the non-linear polarization rotation mode-locking difficult in Fabry-Perot and loop-cavity configurations. It has been shown  that in conditions of strong birefringence of the HF, the non-linear polarization evolution can be exploited for femtosecond stretched soliton modelocking only if a strict control on near single-polarization-mode excitation in the HF is imposed. The challenge of the full-fibre integration of femtosecond sources with holey fibres also relates to the problem of minimizing the normal dispersion of the cavity at 1µm by using short lengths of gain fibers.
Most of these limitations can be overcome by utilizing a figure-of-eight geometry at 1µm. Here we report on a totally fiber integrated femtosecond source at one micron where a F8L with an asymmetrical NALM based on a short length, double clad Yb amplifier and dispersion compensation in un-doped, anomalously dispersive HF were used. We have analyzed the evolution of the mode-locked pulses at different points in the laser, examine working regimes of the F8L and also discuss alternative geometries for sub-picosecond generation at 1µm by using multi-mode-diode pumped Yb fiber amplifiers.
2. Experimental set-up and results
The experimental setup of the femtosecond F8L is shown in Fig. 1. A specially designed, IPG Photonics, single-mode, 33dBm, double-clad Yb fiber amplifier with 1.4m of active fiber was employed in the asymmetrical NALM. To provide an overall anomalous dispersion we compensated the normal dispersion of the amplifier fiber, polarization controllers, isolator and couplers (a total length of 7.3m, normal dispersion of ~ -25ps/nm·km) by either a 20m, 2.6µm core HF with dispersion of about 45ps/nm·km or 100m of 2.3 µm HF with dispersion of 33ps/nm·km at 1 micron. By using a filament splicer we developed a technique which allowed direct splicing of the HF into the NALM with losses per splice not exceeding 0.8dB. The total loss of the 20m long HF and two splices at 1µm was 2.8 dB.
Two main regimes of pulsed operation were obtained. Firstly, the ellipticity of the core, and consequent anisotropy of the holey fiber led to polarization sensitive discrimination of the noise perturbations in the NALM and resulted in a mode-locked regime of laser. As in mode-locking due to non-linear polarization switching , nanosecond scale, square-shape pulses could be obtained. Figure 2 shows the output power and mode-locked pulse duration versus the pump current of the Yb amplifier. In this instance 100m of holey fiber was spliced in the NALM. A representative pulse shape (in this case a pulse of ~2ns) and spectrum are shown as inserts.
With a 20m length of HF spliced into the NALM and by varying the NALM reflection through changing the polarization state, the pulses could be shortened to 400ps and on optimization of the polarization state long pulse operation was entirely eliminated with simultaneous formation of single short, soliton-like pulses at the round trip repetition rate. A minimum pulse duration of 1.1ps (a sech profile is assumed throughout this work) supported by a 6.7nm bandwidth was obtained (Fig. 3).
In a second regime, typically of a F8L close to Q-switched operation, the formation of sub-picosecond pulses was also controllably obtained. After the reduction of the length of the normally dispersive fiber in the NALM by 3m and in the feedback loop by 1m (7.3m total remaining length of the standard fiber), a mode-locked train of 850fs pulses (3.1nm spectral width) was obtained at the cavity round trip repetition rate (Fig. 4). The mode-locking was initiated by a perturbation in the NALM. Similar to that which has been previously observed in Er-based systems, multipulsing and pulse bunching, was also observed through variation of cavity polarization and gain increase.
As in additive pulse mode-locked lasers, because of the lumped dispersion compensation using the HF, the clockwise and counter-clockwise propagating pulses exiting the alternate ends of the output coupler had positive and negative chirp. This was confirmed by the compression of the counter-clockwise propagating pulses in an external standard normally dispersive fiber (Fig. 5).
Operating with lumped dispersion compensation in the laser to achieve an average anomalous dispersion, gives rise to soliton-like performance. In this situation, a non-uniform soliton-like formation occurs and the mode-locked pulse parameters vary along the length of the NALM and the laser system. Under these circumstances, the output pulse duration τ can be estimated by taking into account the average dispersion value and by using the standard soliton propagation equations in the anomalously dispersive F8L [14,15]:
Here λ is the lasing wavelength, c is the speed of light, g is the linear gain of the amplifier, and |D|L is the average dispersion of the NALM which can be estimated as |DHF·LHF - DSF·LSF|, where DHF and DSF are the dispersion coefficients, and LHF=20m and LSF=4.8m are the lengths of the holey and standard fiber in the NALM. Taking into account the total losses in the NALM, we estimated the Yb amplifier's gain g to be around 10.4dB. Hence, the estimated soliton width is 1.5ps, which, despite the assumptions on average anomalous dispersion, correlates well with the experimentally observed pulse duration.
It should be noted that in comparison with the ring geometries, the F8L mode locked operation in soliton-like regime is based on the non-linear, phase dependent transmission of the NALM, and therefore can be realized in a polarization preserving format. This gives a particular advantage to the use of the holey fibers in the F8L fully fiber integrated scheme. The stability of the mode-locking can be further improved by employing the resonant, active amplitude modulation in the laser.
The generation of femtosecond pulses at 1065nm in a Yb-doped figure-of eight configuration has been demonstrated in an all-fiber integrated format. The average anomalous dispersion in the NALM allows soliton shaping, with a holey fiber providing the required anomalous dispersion. The dynamics of the output pulse formation and output chirp dependence on the direction of propagation in the NALM has been shown. Minimum pulse durations of 850 fs have been achieved without optimization of the overall cavity dispersion. Through optimization, sub-100 fs pulses should be achieved from such a laser or variations of it that have been well established for operation in alternative wavelength ranges.
References and links
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