1. Introduction

Mode-locked fiber lasers are attractive as a source of femtosecond pulses because they are compact, and the production cost can be much lower than that of bulk solid-state lasers. Substantial progresses in high-energy ytterbium-doped fiber lasers have been reported recently [1] [2] [3] [4]. The widespread use of these lasers beyond laboratory environments is hampered by the fact that mode-locked operation can be disturbed by external perturbations. The design of an environmentally-stable fiber laser that generates femtosecond pulses at 1 µm encounters a two-fold challenge. First, femtosecond lasers require anomalous-dispersion elements for soliton-like pulse shaping [5] [6], and at this wavelength, grating- or prism- pairs have been necessary. Such bulk components reduce integrability, and so are not desirable in environmentally-stable lasers. To begin to address this issue, Lim et al. demonstrated the use of microstructured fibers [7] [8] such as photonic crystal fiber (PCF) [9] and photonic bandgap fiber (PBF) [10] for dispersion control in 1-µm fiber lasers. The laser with hollow-core fiber is of particular interest in high-energy fiber laser development, because a segment with anomalous group velocity dispersion (GVD) that has negligible nonlinearity is a prerequisite to operation of femtosecond fiber lasers in regimes that avoid wave-breaking at high pulse energy [3] [4]. Second, mode-locking should be sustained in the presence of thermal and mechanical perturbations when the induced random birefringence in fiber could disrupt pulse-shaping. Ideally, a polarization-maintaining (PM) fiber laser would be robust against such environmental disturbances, but in practice it is difficult to construct a laser completely with PM fiber components. Fermann et al. demonstrated an erbium-doped fiber laser in which Faraday rotators in a Fabry-Perot cavity cancel the birefringence fluctuations in the non-PM fiber [11]. Jones et al. and Carruthers et al. demonstrated environmentally-stable pulse generation with a so-called sigma cavity (which contains linear and ring segments, as illustrated in Fig. 1), at 1.55 µm wavelength [12] [13]. The hybrid design includes PM fibers, and the pulse-shaping remains unaffected by external perturbations. However environmentally-stable fiber lasers at other wavelengths have not been demonstrated to date.

The polarization properties of PBF have been studied theoretically and experimentally [14] [15] [16]. Slight asymmetry of the core structure removes degeneracy of modes in PBF, and measurements of polarization have shown that the birefringence could be comparable to, or exceed that, of conventional PM fibers. The large form birefringence of PBF will be an essential feature of the fiber in the development of an environmentally-stable, all-fiber laser.

Here we present an environmentally-stable femtosecond ytterbium fiber laser, the first environmentally-stable mode-locked laser at 1 µm wavelength to our knowledge. A highly-birefringent PBF controls polarization while compensating dispersion in the cavity. The laser generates 1-nJ pulses, which can be dechirped to 70 fs duration outside the cavity. The mode-locked operation is self-sustaining, and robust under mechanical disturbances. The results reported here represent a significant step toward a short-pulse laser at 1 µm that consists only of PM fiber.

2. Experimental

The experimental configuration is illustrated in Fig. 1, and is referred to as a sigma-type cavity [12] [13]. A single polarization is maintained throughout the unidirectional ring segment of the laser, which contains only birefringent or polarization-dependent components. We confirmed the birefringence of the PBF with a method similar to that described in Ref. 15. The transmission of linearly-polarized broadband light through the PBF and a polarizer (Fig. 2, right) exhibits spectral oscillations owing to the group delay between two orthogonally-polarized beams. The observed polarization beat length L_B is wavelength-dependent (~14 mm at 1060 nm), and varies drastically near the edge of the photonic bandgap. Two half-waveplates adjacent to the PBF align the linear polarization of the incident beam along the axes of the fiber. In the linear part of the cavity, non-PM fiber is placed between Faraday rotators, which compensate changes in the fiber birefringence. Nonlinear polarization evolution (NPE), which occurs in the linear segment of the cavity and the subsequent polarizer, initializes and stabilizes the mode-locked operation. The propagation through the PM ring segment does not produce self-amplitude modulation (SAM); an entirely-PM fiber laser will require SAM from a process other than NPE. Unlike prisms and gratings, the bulk elements employed in the laser are also available as fiber-pigtailed versions. The pulse evolution is monitored in situ with outputs from three locations in the cavity: Output 1 represents a supported mode of the laser, transmitted by NPE. After propagation through the PBF, a faction of the energy is obtained as output 2, before entering into the linear segment. Therefore, the effect of birefringence of the PBF appears as changes on the pulse between outputs 1 and 2. Finally output 3 is a part of the pulse rejected by NPE, which does not complete the round trip but rather is eliminated in the isolator. Both outputs 2 and 3 pass through the PBF, but their paths in the PM segment are separated by the NPE action. Hence the role of NPE can be inferred from the difference between outputs 2 and 3.

 

Fig. 1. Experimental configuration. FR, Faraday rotator, AOM, acousto-optical modulator, and WDM, wavelength-division multiplexer.

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A short ytterbium-doped gain fiber (25 cm long, 23,600 ppm doping, NA=0.12, core diameter 6 µm) is pumped in-core by a diode laser at 980 nm capable of supplying up to 500 mW. The GVD of the PBF is nominally -0.050 ps²/m at 1030 nm, and this value was separately verified by measuring the temporal broadening of ~100-fs pulses after propagation through the fiber. The total length of single-mode fiber (SMF) is 2 m, and the 3-m segment of PBF (air-filling fraction >90%, 7-cell core, 3-µm pitch, supplied by Blazephotonics, Inc.) has a bandgap in 1000–1150 nm range (Fig. 2, left). The repetition rate of the laser is 30 MHz. The SMF and PBF segments provide GVD of 0.10 ps², -0.15 ps², respectively, so the net (path-averaged) GVD is approximately -0.05 ps². With this value we expect the laser to operate in a stretched-pulse regime [17].

 

Fig. 2. Left, dispersion (dashed) and attenuation (solid) spectra of the PBF (data supplied by Blazephotonics, Ltd). Right, the measured birefringence of the 3-m PBF.

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3. Results

Mode-locked operation is obtained with assistance from an acousto-optical modulator (AOM), which is then turned off while the outputs are characterized. Once initiated, mode-locking is sustained for days without the AOM, nor further alignment. The operation is robust and the laser remains mode-locked when the SMF or the PBF is moved, picked up, or twisted. We find that it is possible to modelock the laser even when there is considerable angular offset between the beam polarization and the axes of the PBF. The strong SAM evidently overcomes the detrimental effect of polarization mode-splitting in the PBF.

We verified the presence of a single pulse in the cavity by observing i) the long-range autocorrelation, ii) the spectrum, and iii) the pulse train with a fast photodetector with ~300 ps response time (Fig. 3, inset). Positively-chirped, 1-nJ pulses are obtained from output 1 (the average power is 30 mW) at 400 mW pump power, while negatively-chirped pulses with much lower average power (~6 mW) are obtained from outputs 2 and 3. We choose output 1 as the primary output, because of the ease of variable power extraction and the fact that it can be conveniently dechirped with an external grating pair. Furthermore, NPE enhances the pulse quality at this point of the cavity. Directly from the laser, 1.8-ps pulses are obtained. The dechirped pulse duration is 70 fs (Fig. 3), assuming a Gaussian pulse profile. This implies a stretching ratio of ~20, as expected given the net cavity GVD. The autocorrelation of a dechirped pulse is 60% broader than expected from the zero-phase Fourier transform of the measured spectrum (gray lines in Fig. 3). We attribute the deviation from transform limit to the uncompensated third-order dispersion in the cavity, which is predominantly that of the PBF. The generation of shorter pulses in the present laser configuration will also be limited by the components’ band-width; for example, the dips in the spectrum near 1010 nm (Fig. 4, left) arise from cross-talk within the coupler for the pump laser.

 

Fig. 3. Interferometric autocorrelation of a dechirped pulse from output 1, and the zero-phase Fourier transform of the measured power spectrum (gray line). Inset, pulse train.

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Fig. 4. Spectra of output 1 on logarithmic and linear scales, and of output 2, 3 on linear scales.

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Spectra of the outputs are illustrated in Fig. 4. The modulation on the spectrum of output 2 (S ₂(λ)) is consistent with the measured birefringence of the PBF (Fig. 2, right). In addition to the polarization mode beating, the complicated structure on the spectrum of output 3 (S ₃(λ), right panel of Fig. 4) indicates that the NPE action is strong. Stable trains of mode-locked pulses are generated despite the spectral modulation arising from the PBF.

4. Conclusion

We have demonstrated what we believe is the first environmentally-stable femtosecond fiber laser at 1 µm wavelength. This development relies on the birefringence of a hollow-core PBF, which implements a segment of the dispersion map in the polarization-maintaining portion of the laser cavity. The laser is a robust source of 70-fs and 0.5-nJ pulses, even in the presence of perturbations. We anticipate that self-starting operation of the laser will be achieved in the near future with a semiconductor saturable absorber in the cavity. Finally, this study opens the possibility to high-energy all-fiber and environmentally-stable lasers through self-similar mode-locking.

This work was supported by the National Institutes of Health, National Science Foundation, and DARPA. H. Lim is currently affiliated with theWellman Center for Photomedicine, Boston, MA 02114.