1. Introduction

Recently, mode-locked fiber lasers have gained an enormous interest owing to their excellent beam and pulse quality, small footprint and user-friendly operation [1, 2]. The semiconductor saturable absorber mirror (SESAM) technology has dramatically improved the performance of mode-locked fiber lasers by providing both robust self-starting and strong pulse shaping mechanisms [3–5]. Despite excellent performance reported to date, Yb-doped fiber laser development remains hindered by the issue of chromatic dispersion compensation. Owing to both doped and standard single mode fibers exhibiting normal chromatic dispersion around 1 µm, fiber lasers without proper dispersion management tend to operate at these wavelengths in a stretched pulse regime associated with long pulse width and problematic start-up of mode-locked operation [6]. To overcome these problems, normal dispersion of the laser cavity should be compensated by an appropriate cavity element with anomalous dispersion. Regularly, dispersion compensation is achieved with bulk optical components such as prisms [7] and diffraction gratings [8]. These solutions, however, suffer from disadvantages that limit their applicability to fiber lasers: prisms exhibit too low dispersion and require impractically large separation, while diffraction gratings are strongly polarization dependent and lossy. Furthermore, the use of intracavity bulk optical elements violates the “all-fiber” nature of the laser. Recently, the photonic crystal technology has been used to build dispersion compensators which allow for robust and all-fiber compact systems [9]. Unlike conventional optical fibers, photonic bandgap (PBG) fibers do not guide light by total internal reflection but rely on a photonic bandgap in the fiber’s cladding [10]. A hollow-core PBG fiber consists of a hollow core surrounded by a cladding whose periodicity creates a bandgap for the photons guided in the fiber’s core. Using hollow-core fibers should be useful for high peak power ultrashort pulse applications [11].

Whereas hollow-core PBG fibers are attractive alternative to bulk elements, they suffer from poor matching with the standard fibers and, consequently, may generate high intracavity loss. Particularly in a mode-locked laser cavity, a hollow-core PBG fiber spliced with standard fiber could provide Fresnel back-reflection which affects badly the starting capability and quality of the pulse operation. Better matching with the standard fiber could be achieved by using solid-core photonic crystal fibers. These fibers with light guided by total internal reflection typically have a solid silica-based core surrounded by a silica-air photonic crystal cladding [10]. Anomalous dispersion in this type of crystal fiber could be generated with rather small core diameters. Because of high nonlinearity, these fibers are widely used in supercontinuum generation, but poor matching with standard fibers prevents using them inside the laser cavity. An alternative solution is based on a solid-core photonic bandgap fiber (SC-PBG) made of a pure silica core and an array of higher index (e.g. Ge-doped) strands in the cladding. Such an all-solid approach has the advantage of easy splicing to a standard fiber. Furthermore, the structure exhibits no surface modes [12] and allows for high anomalous dispersion with less critical non-linear thresholds than index guiding photonic crystal fibers.

In this Letter, we report on the use of a solid-core photonic bandgap fiber to compensate the dispersion of an ytterbium mode-locked laser. We demonstrate that using semiconductor saturable absorber mirror together with photonic bandgap fiber allows for self-starting allfiber mode-locked laser around 1-µm wavelength range.

2. Experimental

The experimental setup of the mode-locked laser is illustrated in Fig. 1. The fiber cavity is comprised of 1.0 m of ytterbium-doped fiber, 2.7 m of standard single mode fiber and a 2.8-m long segment of SC-PBG fiber. The Yb-doped fiber has an unpumped loss of 414 dB/m at 976 nm and normal group velocity dispersion (GVD) of +0.071 ps²/m at 1.04 µm. The SC-PBG fiber has the mode field diameter of 9 µm and exhibits anomalous GVD of −0.054 ps²/m at 1.04 µm. Figure 2 shows the transmission band and dispersion of the SC-PBG fiber. The inset in Fig. 2 shows the microscope image of the fiber cross-section displaying 10 rings of Ge-doped rods around the undoped silica core. The transmission loss of the SC-PBG fiber was estimated from the measurement as 68.4 dB/km and the group birefringence as~10^-5 at 1.04 µm. The 10% output coupler and the dichroic pump coupler were made of single-mode fiber with normal GVD of+0.023 ps²/m and mode-field diameter of 6.4 µm. The round-trip dispersion of this cavity was estimated from measurements to be in the anomalous dispersion regime for wavelengths longer than 1040 nm. The mode size in the SC-PBG fiber is slightly larger than the diameter of the mode field in the core of a standard fiber. It is expected, therefore, that optical nonlinearity of the SC-PBG fiber would not dominate the total nonlinearity in the laser cavity. Despite some mode mismatch, the guided mode of the bandgap fiber fits quite well the mode of the normal fiber resulting in ~1 dB splice loss with standard fiber. The other end of the SC-PBG fiber was butt-coupled to a high reflective mirror in order to avoid further splice loss and any unwanted reflections from the fiber end.

 

Fig. 1. Laser setup with SC-PBG fiber for cavity dispersion compensation. HR mirror-high reflectivity mirror.

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Fig. 2. Transmission (black curve) and dispersion (red curve) of the SC-PBG fiber used for dispersion compensation. The inset shows the cross-sectional view of the fiber.

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3. Results

The laser was pumped with a single-mode grating-stabilized laser diode, which provided a power of 120 mW at 980 nm. An antireflection-coated aspheric lens with a focal length of 2.0 mm was used to focus the beam on the absorber mirror. Proper SESAM alignment resulted in self-starting mode-locking with a fundamental repetition rate of ~15 MHz. The laser delivered an average output power of ~2 mW corresponding to the pulse peak power of 250 W.

Although the SC-PBG fiber shows a good performance in GVD compensation, it was concluded that a major limitation to the pulse width and quality arises from the third-order dispersion (TOD) generated in the SC-PBG fiber used in this study. The TOD of the photonic bandgap fiber estimated from the measurements gives the value of 1.4 ps3/km at 1.04 µm. The effect of the higher-order dispersion was further studied using tunable pulsed operation by inserting a birefringent filter at the Brewster angle into the air-segment of the cavity as shown in Fig. 1. Without the filter, the laser operates at ~1035 nm where the SC-PBG fiber has the transmission maximum, as seen in Fig. 2. Figure 3 shows the pulse spectra and intensity autocorrelations when the laser was tuned to different wavelengths. From Figs. 3(a)-(d), it is clear that owing to low value of anomalous dispersion generated by SC-PBG fiber at the short wavelengths (<1035 nm), the laser operates with net normal cavity dispersion resulting in long pulses. This feature is expected from the dispersion curve shown in Fig. 2. In contrast, operation at longer wavelengths [Figs. 3(e)-3(h)] shows substantial soliton pulse compression indicating the total anomalous dispersion of the laser cavity.

 

Fig. 3. The intensity autocorrelation traces (right) and corresponding spectra (left) obtained for tunable mode-locked operation. Plots (a)-(d) correspond to net normal cavity dispersion, (e)-(h) to total anomalous cavity dispersion.

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Fig. 4. Mode-locked pulse spectra obtained with 0.55-m long ytterbium fiber. The total cavity dispersion calculated from the soliton sidebands equals-0.13 ps2 at 1040 nm and-0.52 ps² at 1055 nm.

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Next, to further illustrate strong wavelength dependence of the SC-PBG fiber dispersion and, consequently, high value of TOD, the length of an ytterbium fiber was decreased down to 0.55 m. The spectral sidebands in the pulse spectra, shown in Fig. 4, give a clear signature of the soliton regime. The average cavity dispersion estimated from the soliton sidebands is -0.13 ps² at 1040 nm and -0.52 ps² at 1055 nm. The group-velocity dispersion derived from the spectral location of the sidebands agrees with total cavity GVD calculated from measured dispersion of the fibers comprised in the cavity.

In order to optimize the pulse duration generated by the laser, we changed both the length of the active fiber and the operation wavelength. The results of this study are summarized in Fig. 5. The shortest pulse we obtained by dispersion compensation with SC-PBG fiber has the width of 0.46 ps. With the spectral width of 4.6 nm, the time-bandwidth product becomes 0.59 remaining still above the transform-limited value. Since the laser comprises sections of fiber with both normal and anomalous dispersion, the pulse duration and chirp evolve gradually when pulse propagates inside the laser cavity. The nearly transform-limited pulse is expected to appear by placing the output coupler at the location within the cavity that corresponds to the highest pulse compression factor. The output fiber pigtail may also add certain chirp to the pulse. Although further pulse reduction is expected, it is obvious that third-order dispersion in the photonic bandgap fiber would eventually limit the dechirped pulse duration. These aspects remain a subject for future study. Also, we expect that the performance of this laser can be improved with advances in photonic bandgap structures.

 

Fig. 5. Interferometric autocorrelations and spectra for cavities with different lengths of ytterbium-doped fiber: (a), (b) 1.15 m, (c), (d) 1.7 m and (e), (f) 1.7 m. Pulse durations and corresponding optical pulse bandwidths are shown.

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4. Summary

We demonstrated an environmentally stable all-fiber soliton ytterbium laser using solid-core photonic bandgap fiber for dispersion compensation at 1 µm. The self-starting mode-locked operation of the subpicosecond soliton laser is achieved by the semiconductor saturable absorber. This approach may constitute an important step towards novel generation of ultrafast fiber oscillators.