We demonstrate the use of photonic bandgap fiber for dispersion compensation in a short-pulse fiber laser. The anomalous dispersion provided by the photonic bandgap fiber enables us to construct a femtosecond fiber laser at 1 micron wavelength without prisms or diffraction gratings. The laser is self-starting and produces 160-fs pulses with 1-nJ energy, and represents a significant step toward all-fiber devices capable of much higher pulse energies.
©2004 Optical Society of America
Fiber lasers have attracted much attention for the advantages they offer, such as freedom from alignment, superior thermal handling, and compact size. The development of short-pulse fiber lasers with emission wavelength near 1 μm is motivated by biological and medical applications, among others. Recent work with short-pulse Yb fiber amplifiers has produced impressive results , which further motivate the development of integrated fiber sources for seed pulses.
Significant advances in Yb fiber lasers have occurred in the last two years. A practical ultra-fast fiber laser with Yb fiber as a gain medium and grating pairs for dispersion control generated pulses as short as 50 fs, and pulse energies of 2 nJ . A number of advances have built on this laser, with higher pulse energies  and shorter pulse duration  perhaps the most remarkable. The latest oscillators routinely generate stable pulses with nanojoules of pulse energy and tens of femtoseconds in pulse duration. The demonstration of self-similar pulse evolution in a laser implies that pulse energies from femtosecond fiber lasers may exceed those of solid-state lasers in the future .
A segment with anomalous group velocity dispersion (GVD) is needed whenever pulse shaping in a laser is based on the interplay of nonlinearity and GVD  . The grating pairs used in the cavities of prior lasers     require alignment and reduce the integrability of the laser, so all-fiber designs are desired. Anomalous GVD is not available from conventional silica single-mode fiber (SMF) at wavelengths below 1.3 microns, and this fact naturally hampers the development of all-fiber short-pulse lasers. Microstructured fibers can provide anomalous dispersion at wavelengths well below the zero-dispersion wavelength of fused silica. Lim et. al. demonstrated a Yb fiber laser in which anomalous GVD was provided by a solid core photonic crystal fiber (PCF) . The anomalous GVD is a consequence of strong waveguide dispersion, which arises from the small core. Large birefringence of the PCF may be relevant to the construction of an environmentally-stable version of the laser. The laser generated 100-fs pulses with 1-nJ energy. However, modelocking was not self-starting, and produced small secondary pulses through polarization mode dispersion of the PCF. Avodokhin et al. reported 850-fs pulse generation from a figure-of-eight laser that exploits the PCF dispersion in a similar fashion . The combination of large effective nonlinearity (through the small core size) and anomalous GVD in a PCF will restrict its use to low-energy (~1 nJ and below) lasers.
Another candidate for a waveguide with anomalous GVD is hollow-core microstructured fiber, also called photonic bandgap fiber (PBF) . In contrast to solid-core PCF, in which guidance is by step-index total internal reflection, light is confined in hollow core due to a photonic bandgap that arises from a regular 2-dimensional array of air holes in the cladding. The lowest attenuation level reported is only 10 times higher than that of SMF , and several prototypes with bandgaps at different wavelengths are commercially available. The GVD of a PBF derives not so much from a material property as from the photonic bandgap itself, and is anomalous at the longer wavelengths of the bandgap. The production of high-power solitons in PBF confirms that the nonlinearity is that of air, i.e. 1000 times smaller than that of conventional SMFs . Such a PBF was recently used to dechirp pulses from an amplifier  . An anomalous-GVD segment with negligible nonlinearity is a prerequisite to wave-breaking-free  or self-similar  operation of femtosecond fiber lasers.
Here we report the use of photonic bandgap fiber to provide anomalous GVD in a femtosecond fiber laser. Stable, good-quality pulses with energy as large as 2 nJ are obtained, and these can be dechirped to 160 fs duration and 1-nJ energy external to the laser cavity. These results demonstrate that PBF can replace diffraction gratings in fiber lasers, without apparent sacrifice in performance. To our knowledge, this is the first laser of any kind based on PBF.
The properties of PBF relevant to construction of a modelocked fiber laser deserve some discussion. Owing to the drastically different cross-sections, PBF is not as compatible with standard SMF as one would desire; splicing together the sections of the laser is currently not practical. As a consequence, the laser we report herein is not entirely all-fiber, nor addresses lack of the integrability with intracavity gratings completely. On the other hand, hollow-core fiber appreciably reduces Fresnel reflections from the fiber ends. Since even small reflections can inhibit modelocking, this is a potential advantage of PBF in this application. Multimode guidance within the bandgap is a serious concern in a modelocked laser. However, excitation of primarily the fundamental mode is possible, and a segment of SMF following the PBF can effectively filter higher-order modes before they disrupt pulse shaping. Surface or interface modes could have strong impact on the attenuation and dispersion of the PBF  . The lossy modes tend to develop within the bandgap and thus limit the range of transmission. It was suggested recently that a 7-cell core PBF with a low density of surface modes is more suitable for femtosecond applications . Finally, birefringence (or more generally, polarization-dependence) of the PBF offers potential complications as well as benefits. We observe wavelength-dependent polarization at the output when linearly-polarized broadband light propagates through a PBF. However, we cannot accurately assess the extent to which multimode guiding of the PBF affects the result, so it is difficult to quantify the polarization dependence.
The laser cavity is similar to that described in Ref. , with the diffraction gratings replaced by a segment of PBF (Fig. 1). A unidirectional ring cavity helps to achieve self-starting operation. A highly-doped, 20-cm ytterbium gain fiber (NA = 0.12, core diameter 6 μm, 23,600 ppm doping) is pumped in-core by a 980-nm diode that can supply power up to 500 mW. The total length of SMF is 4.5 m, and a 3-m PBF (air-filling fraction > 90%, 7-cell core, 3-μm pitch, supplied by Blazephotonics, Ltd.) with a bandgap in range 1000 – 1150 nm is placed in the cavity. Careful mode-matching between the SMF and PBF allows high coupling efficiency into the PBF (typically > 75% with a 20X, 0.40-NA microscope objective) and selective launching of the fundamental mode. The PBF was chosen to have anomalous GVD at 1030 nm, which is the peak of the gain of the short Yb gain fiber. The GVD of the PBF from the manufacturer’s catalog is -0.050 ps2/m at 1030 nm, and this value was separately confirmed by measuring the broadening of ~100-fs pulses after propagation through the fiber. The net cavity GVD is -0.05 ps2, so the laser operates in the weakly-stretched soliton regime. The positively-chirped output pulse from the laser is dechirped with an external grating pair. Nonlinear polarization evolution (NPE) initiates and stabilizes the modelocking. Linear polarization is maintained in the PBF, and the polarization axis is adjusted with the half-waveplate prior to the PBF.
Self-starting modelocked operation is obtained by adjustment of the waveplates for pump powers above 400 mW. The laser is found to reach a stably-modelocked regime only for two orthogonal orientations of the polarization incident on the PBF, which presumably correspond to the principal axes. By direct observation of the pulse train (Fig. 2) and measurement of the autocorrelation over a 0.1-ns delay range we verified that a single pulse is in the cavity. With a repetition rate of 30 MHz, the pulse energy is 2 nJ. This pulse energy is obtained with 300 mW pump power; at higher pump powers the laser exhibits doule-pulsing. The spectrum is shown in Fig. 3(a). The spectral sidebands  imply that the operation is soliton-like, with weak pulse-stretching, as expected. The GVD inferred from the positions of the spectral sidebands agrees with the nominal value, given that the experimental bandwidth is adequate to support a pulse duration of ~120 fs. The positively chirped output pulse is ~800 fs in duration. After dechirping, the pulse duration is 160 fs (Fig. 3(b)), so the stretching ratio is approximately 5. In contrast to the laser that employed PCF for dispersion control , we observe no sign of pulse distortion from birefringence, in either the spectrum or the temporal profile. The envelopes plotted in Fig. 3(b) are calculated from the measured power spectrum assuming a constant phase. The pulse duration is ~40% greater than the transform-limited value (116 fs). We attribute the deviation to the uncompensated third-order dispersion (TOD) since the TOD of the PBF is an order of magnitude larger than that of SMF.
The laser is already quite stable and typically remains modelocked for days without adjustments. In particular, the modelocked operation is robust against mechanical and thermal perturbations of the PBF. Bending the fiber has little effect beyond increasing the coupling to higher-order modes, and the modelocking is sustained until the resulting loss exceeds a threshold.
We have demonstrated a femtosecond fiber laser with the anomalous dispersion provided by a hollow-core photonic bandgap fiber. The performance of the laser is comparable to that of femtosecond lasers with diffraction gratings for anomalous dispersion, with the obvious benefit of being a step closer to an all-fiber, integrated device. With further development, we expect that lasers based on this approach will find numerous applications. Finally, this work allows us to consider the design of all-fiber lasers based on self-similar pulse propagation. Such lasers should combine the performance of solid-state lasers such as Ti:sapphire with the practical benefits of fiber.
This work was supported by the National Institutes of Health under grant EB002019 and Clark-MXR, Inc. The authors thank J.C. Knight, D. G. Ouzounov and F.O. Ilday for valuable discussions.
References and links
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