Sub-200 fs Polarization-Maintaining All-Fiber Thulium-Doped Dissipative Soliton Fiber Laser System at 1920 nm

Abstract

A polarization-maintaining all-fiber laser source based on a nonlinear amplifying loop mirror with broadband operation (64 nm) around 1920 nm is demonstrated. The oscillator can generate 66 pJ up-chirped dissipative soliton pulses at a repetition rate of 22.8 MHz with a high polarization extinction ratio of 17 dB. By adding a polarization controller to the polarization-maintaining dispersion-compensating fiber, the filter behavior can be adjusted allowing for the tuning of the emission to a center wavelength of 1878 nm, 1907 nm, and 1926 nm. Using an all-polarization-maintaining single-mode fiber amplifier with anomalous dispersion, the pulses are amplified to 0.9 nJ and compressed to a near Fourier-limited pulse duration of 170 fs with a peak power of 4.3 kW. Such all-fiber-based sources are attractive due to their compact size, high beam quality, and good environment stability.

1. Introduction

Ultrafast femtosecond sources have gained significant interest due to their high peak powers and broadband operation that can enable many different applications in nonlinear wavelength conversion, laser processing, and medicine [1,2,3]. Due to the relatively large gain bandwidth of thulium (Tm)-doped fibers from 1.7 to 2.0 µm [4], ultrashort pulses on the order of sub-100 fs can be supported.

To achieve ultrashort pulses, saturable absorbers with an intensity dependent transmission are used for mode locking. Material-based saturable absorbers, such as semiconductor saturable absorber mirrors [5] or carbon nanotubes [6,7], allow for a straightforward integration with a polarization-maintaining (PM) system; however, their low damage threshold and degradation over time can limit the long-term stability [8]. Furthermore, material-based saturable absorbers with a small modulation depth (e.g., single-walled carbon nanotubes) can lead to longer build-up times of a few seconds for self-starting [9]. Artificial saturable absorbers based on various optical effects such as nonlinear polarization evolution [10,11] or a nonlinear amplifying loop mirror (NALM) [12] can have a faster response time than material-based saturable absorbers, which has been shown to result in lower intrinsic noise and timing jitter [13]. Furthermore, artificial saturable absorbers can be very robust and support long-term operation with a minimal amount of maintenance and high environmental stability.

To date, only selective femtosecond-level Tm-doped fiber oscillators with an all-PM configuration have been demonstrated. Most of the Tm-doped PM fiber lasers generate conventional soliton pulses [14,15] or dispersion-managed solitons [13,16,17] with the largest reported spectral 3 dB bandwidth spanning 39.4 nm [18]. In a non-PM fiber configuration incorporating bulk optics, solitons with a pulse duration down to 58 fs at 1.9 µm were produced in a Tm-doped fiber laser cavity with a net dispersion of −0.017 ps² [19]. Furthermore, by using a special Tm-doped ZBLAN fiber with bulk optics, broadband soliton pulses were generated from 1730 to 2050 nm, compressed down to 45 fs [20]. In an all-PM fiber system, relying on a nonlinear wavelength conversion stage from an erbium-doped oscillator with self-frequency shifted pulses and a subsequent Tm-doped amplifier, compressed pulses of 99 fs in the gain window of thulium [21] were demonstrated.

While dissipative soliton operation has led to the generation of broader optical spectra and higher pulse energies [22,23], only a handful of oscillators have been demonstrated in an all-PM configuration. Due to the highly up-chirped nature of dissipative solitons, the energy scalability of these pulses is less susceptible to wave-breaking, allowing for high pulse energies [23]. Due to the limited availability of all-PM normal dispersion fiber components, the development of an oscillator with a net-normal cavity dispersion requires detailed optimization and designs. Furthermore, differences in core sizes and polarization rod location between normal dispersion and standard PM fibers can often result in more lossy splices, making it more challenging to achieve stable mode locking when these fibers are spliced together [16,24,25].

To date, only a few demonstrations of dissipative soliton PM fiber oscillators have been reported. In an oscillator using a custom-fabricated Tm-doped silicate fiber with normal dispersion, an optical spectrum with a 3 dB bandwidth of 24.6 nm at a center wavelength of 1996 nm was generated, corresponding to a pulse duration of 340 fs after a single-stage amplifier [26]. In a NALM oscillator, by varying the net dispersion, dissipative soliton pulses with a spectral bandwidth of 4.95 nm at a center wavelength of 1950 nm at 3 dB were demonstrated; however, a strong spectral modulation was observed, so the pulse duration after amplification was limited to 640 fs [27]. A PM dissipative soliton all-fiber ring oscillator was mode-locked with a single-walled carbon nanotube as the saturable absorber, relying on a Lyot filter to tune the center wavelength to 1876 nm [28]. To minimize splicing losses, a bridge fiber between PM1550 and the normal dispersion fiber was incorporated, which overall led to a pulse duration of 390 fs with an edge-to-edge spectral bandwidth of 26 nm after amplification and compression in a free-space grating pair [28].

Shorter-pulse-duration dissipative solitons have been generated in all-fiber Tm-doped oscillator, but with standard non-PM fibers, down to 140 fs, after pulse compression in SMF-28e [29]. Using a custom-fabricated Tm-doped silicate fiber, the corresponding spectrum featured a center wavelength around 1930 nm and a spectral 3 dB bandwidth of 52.8 nm. Using only commercially available fibers, dissipative soliton pulses from the ring laser with a center wavelength of 1915 nm with an edge-to-edge spectral bandwidth of 52.8 nm were generated by optimizing the net intracavity dispersion and polarization state. These output pulses could be compressed to 152 fs in single-mode fiber [30]. A few other all-fiber Tm-doped oscillators demonstrated standard non-PM reporter pulse durations around 200 fs after compression [31,32,33].

Here, we address how to generate dissipative solitons from an all-fiber Tm-doped oscillator, whose pulse durations rival the shortest pulse durations reported, but with the benefit of an all-PM fiber cavity implementation. By minimizing the spectral filtering effect from the dispersion-compensating fiber and optimizing the small net-normal cavity dispersion, an oscillator with an edge-to-edge spectral bandwidth of 64 nm (20 dB spectral width) at a center wavelength of 1920 nm is demonstrated. By adding a polarization controller (PC) to the dispersion-compensating fiber, a spectral Lyot filter is formed, allowing for three additional dissipative soliton pulses centered at 1878 nm, 1907 nm, and 1926 nm with an edge-to-edge spectral bandwidth of 40.9 nm, 52.6 nm, and 62.7 nm, respectively. Using a PM fiber amplifier, the output broadband pulses at a repetition rate of 22.8 MHz were amplified and compressed to an average output power of 20.4 mW and a close to transform-limit pulse duration of 170 fs. To the best of our knowledge, this is the shortest pulse duration reported from a PM dissipative soliton system, reaching a peak power of 4.3 kW.

2. Materials and Methods

The schematic of the all-fiber PM laser system is illustrated in Figure 1. The system has two major sections: a dissipative soliton oscillator and an amplifier, which also serves to compress the pulses externally to the cavity. The PM all-fiber design of the system ensures compactness and high environmental stability.

The oscillator is mode-locked with a nonlinear amplifying loop mirror in a figure 9 configuration formed by a 1 × 2 70:30 fast-axis-blocking PM fiber coupler (DKphotonics, Shenzhen, China) and a fiber mirror (DKphotonics, Shenzhen, China) in the linear arm. The asymmetric coupling ratio between the counter- and clockwise pulse-propagation direction lowers the required pump power necessary to achieve a nonlinear phase difference to initiate mode locking.

To promote a sufficient nonlinear phase bias between the counterclockwise and clockwise pulse-propagation direction, a short segment (11 cm) of highly doped Tm gain fiber (Coherent, Saxonburg, PA, USA, PM-TSF-5/125) is connected closer to the 70% port and a 2 × 2 60:40 fast-axis-blocking PM coupler (DKphotonics, Shenzhen, China) closer to the 30% port. A nonreciprocal π/2 phase shifter (Advanced Fiber Resources, Zhuhai, China) is introduced in the nonlinear fiber loop to lower the required pump threshold for self-starting mode locking [8,34]. For a coupled pump power of 545 mW (at a wavelength of 1565 nm), reliable self-starting multi-pulsing mode locking is achieved. Lowering the coupled pump power to 440 mW, stable single pulsing with up-chirped dissipative solitons can be generated. At the 60% port of the output coupler, an average output power of 1.5 mW was measured, corresponding to an output pulse energy of 66 pJ.

The combination of a 4 m segment of PM dispersion-compensating fiber (DCF, Coherent, Saxonburg, PA, USA, PM2000D, β₂ = 107 ps²/km around a wavelength of 1920 nm) with PM1550 fiber (β₂ = −82 ps²/km around a wavelength of 1920 nm) results in an overall slightly normal net cavity dispersion of 0.008 ps², enabling broadband dissipative soliton mode-locking operation. The PM1550 (with a core diameter of d = 8.5 μm) and PM2000D (DCF, d = 2.1 μm) fibers feature a mismatch in their mode-field diameter and the polarization rod location, which can lead to polarization crosstalk at the splicing points and hinder the mode-locking capability of all-PM Tm-doped oscillators [24,25]. Here, with an optimized splicing recipe with a long arc duration, an adiabatic tapering of the PM2000D is created, resulting in a splicing loss as low as 0.2 dB. For individual splices, a tested polarization extinction ratio (PER) degradation from 20 dB to ~12 dB was measured. For splices on both ends of the DCF/PM2000D fiber, the polarization extinction ratio only degrades to ~10 dB. The PER degradation is most likely due to the difference in polarization rod location causing a small non-PM section at the splicing point. The splice was tested outside of the cavity using a broadband linearly polarized source (PER = 20 dB) around 1.9 µm. After each splice, the fiber output was measured with a half-wave plate and a polarizing beam-splitter cube to measure the change in PER. The half-wave plate was used to align the fiber output pulses to match the polarization orientation of the polarizing beam-splitter cube.

Due to the induced polarization cross-talk at the splicing points, a Lyot filter is created based on the birefringence of the dispersion-compensating fiber, which can limit the spectral bandwidth. A Lyot filter is formed based on the fiber birefringence of the PM DCF, since the fast and slow axes experience a difference in optical path length and linear phase, causing an interference effect after passing through a polarization dependent component. This interference leads to a wavelength-dependent transmission filter. The effect of the Lyot filter is shown in the orange spectrum in Figure 2a, with an edge-to-edge spectral width of 23 nm and a periodic spectral modulation with a period of ~7 nm (highlighted on the figure by the gray arrows). It has been shown that the Lyot filter can be suppressed by introducing a 90° splice between PM fibers to allow for birefringence compensation [35]. Here, two 90° angle-splices are introduced to divide the 4 m length of DCF into three segments (1 m, 2 m, 1 m), with the two 90° DCF splice losses being negligible. These splice points are chosen to avoid 90° splices between the DCF and PM1550 fiber, which can otherwise lead to a PER degradation of ~2.5 dB for a single splice in comparison to a 0° splice. In comparison, implementing a 90° splice in the DCF fiber did not lead to any noticeable changes in PER most likely due to the much shorter splicing arc time (~1/8 times shorter) required. With this splicing arrangement, an equal optical propagation path along both fiber polarization axes is designed so that both polarization components encounter similar optical-phase differences based on birefringence. For a good comparison, the performance of the birefringence-compensated cavity with the 90° splices is compared to the same cavity without the additional splices. In both instances, the same net cavity dispersion is maintained with and without the 90° splices, making sure that dispersion management does not affect performance changes due to potentially different spectral broadening or narrowing.

3. Results

With the 90° splices and the optical spectrum of the seed pulses—see Figure 2a—the blue curve has an edge-to-edge spectral bandwidth of 64 nm (Yogokawa AQ6375 with a resolution of 0.05 nm), which overall can support a transform-limited pulse duration of 196 fs. This is significantly broader than the optical spectrum without any additional splices with an edge-to-edge spectral bandwidth of 23 nm. Both spectra feature steeper spectral edges, characteristic of dissipative solitons. The RF spectrum, cf. Figure 3a, features a high signal-to-noise ratio of 70 dB for the fundamental repetition rate at 22.8 MHz (resolution bandwidth of 100 Hz). The flat long-range RF spectrum is indicative of stable operation in a single-pulsing regime, as shown in Figure 3b. The oscilloscope trace in Figure 3c presents a uniform pulse train with pulses separated by a roundtrip time of 44 ns, which corresponds to the cavity length of 9.1 m and the repetition rate of 22.8 MHz. The pulse train and RF spectra are measured by a 20-GHz digital oscilloscope (Tektronix, Beaverton, OR, USA MSO72004C) and a radio-frequency analyzer (R&S, Columbia, MD, USA, FSUP50) together with a 12.5-GHz InGaAs photodetector (EOT, Saxonburg, PA, USA, ET-5000F), respectively. Due to the fast-axis-blocking output couplers, the output pulses are highly linearly polarized with a PER of 17 dB. The long-term stability of the oscillator was confirmed through continuous output power measurements over 15 h, with a root mean square fluctuation of <0.57% and 30 min root mean square fluctuation of <0.33%. This measurement was performed on a free-running laser system, so that with external packaging and better environmental room temperature control this can be further improved.

By incorporating an inline polarization controller (Thorlabs, Newton, NJ, CPC250) on the DCF fiber section, the polarization state can be modified, which leads to a different spectral shape and center wavelength of the oscillator, cf. yellow, red, and green spectra in Figure 2b. In these cases, the center wavelength can be shifted to 1878 nm, 1907 nm, and 1926 nm with an edge-to-edge spectral bandwidth of 40.9 nm, 52.6 nm, and 62.7 nm, respectively. The output power of the states is 1.3 mW, 1.4 mW, and 1 mW, respectively. On the other hand, placing a polarization controller on the other passive fiber, PM1550, at various positions (e.g., in the NALM loop between the DCF and the 60:40 coupler, between the WDM and gain fiber, or between the gain fiber and DCF) has no effect on the output spectrum. This can be either due to the relatively small birefringence of the PM2000D DCF fiber ($~2\times {10}^{-5}$), which is an order of magnitude lower compared to the birefringence of PM1550 ($~3\times {10}^{-4}$) [36], or due to detuning of the polarization from the optimized birefringence compensation from the 90° splices in the DCF. This can allow for a weak spectral Lyot filter to be formed in the DCF segment, leading to some wavelength tunability.

Due to the up-chirp nature of dissipative soliton pulses, a PM all-fiber amplifier built with anomalous dispersion fiber is optimized for amplification and pulse compression. A PM fiber isolator (DKphotonics, Shenzhen, China) is placed after the oscillator to prevent unwanted back reflections into the laser cavity. A forward pumping scheme for the Tm-doped amplifier (with a 2 m long Tm-doped single-mode PM gain fiber from Coherent, PM-TSF9/125) is chosen to limit the nonlinear-phase accumulation after amplification. The total length of PM1550 passive fiber of the isolator and amplifier is optimized to ~2.25 m (2.05 m before the gain fiber and 0.2 m after the gain fiber) to allow for optimal dispersion compensation and subsequent pulse compression for the broadband oscillator state with the edge-to-edge spectral bandwidth of 64 nm. The length of the PM1550 fiber after the gain fiber is kept short to minimize excess nonlinear-phase accumulation that otherwise can lead to temporal pulse distortions. The output pulses after the oscillator with 1.5 mW of power are amplified to a maximum value of 12.6 mW, corresponding to a pulse energy of 0.55 nJ for a coupled pump power of 0.53 W at a wavelength of 1565 nm at the amplifier.

The amplified spectrum (orange) is shown in Figure 4a, which shows no sign of nonlinear spectral broadening. Using a commercially available autocorrelator (APE, Berlin, Germany, Pulsecheck NX 150), the intensity autocorrelation trace (AC) of the amplified pulses, shown in Figure 4b as a solid black curve, is measured with a pulse duration of 220 fs (transform-limited pulse duration of 196 fs) assuming a Gaussian fit (dashed orange). Slight pedestals to the main pulse are visible. Based on the ratio between the area under the Gaussian fit and the autocorrelation trace, ~85% of the power resides within the main pulse, corresponding to a peak power of 2.4 kW. The pedestal in the autocorrelation trace is most likely due to uncompensated third-order dispersion in the amplifier or some residual nonlinear chirp. At a higher pump power of 0.88 W for the amplifier, the amplified spectrum, shown in Figure 4c, does show some signs of nonlinear spectral broadening due to self-phase modulation. This leads to a measured pulse duration of 170 fs, c.f. Figure 4d, with ~81% of power in the main pulse. Furthermore, the pulses have an average power of 20.4 mW (corresponding to a pulse energy of 0.9 nJ), leading to a peak power of 4.3 kW. Based on the Fourier transform of the amplified spectrum, shown in Figure 4d as a solid blue line, the compressed pulses are close to the transform-limited (TL) pulse duration of 164 fs. Further amplification can be pursued with optical components that can support higher pump power.

4. Discussion

In conclusion, we reported a 1920 nm PM all-fiber Tm-doped oscillator generating broadband dissipative soliton pulses with an edge-to-edge spectral bandwidth of 64 nm. It operates at a repetition rate of 22.8 MHz and can support transform-limited pulses of 196 fs. The broadband spectrum generated by the thulium-doped oscillator system in the dissipative soliton regime is supported by the small net-normal cavity dispersion and by reducing the spectral modulation that is otherwise formed by the DCF splice points. Furthermore, by adding a PC to the DCF in the oscillator, the center wavelength can be tuned to 1898 nm, 1907 nm, and 1926 nm with an edge-to-edge spectral bandwidth of 40.9 nm, 52.6 nm, and 62.7 nm, respectively. By optimizing the passive fiber length of a PM all-fiber amplifier for simultaneous pulse compression and amplification, the pulse duration can be compressed down to 170 fs for an amplified average power of 20.4 mW. These amplified pulses are close to the transform-limited pulse duration of 164 fs, which marks the shortest pulse duration for a polarization-maintaining all-fiber dissipative soliton source, to the best of our knowledge. Due to the polarization-maintaining all-fiber design, the laser system is compact and environmentally stable, making it an attractive source. Furthermore, with all fibers supporting single-mode operation at a center wavelength around 1.9 µm and the gain fiber being core-pumped, this system benefits from the high spatial beam quality offered by fiber. Overall, this broadband polarization-maintaining all-fiber thulium-doped source operating in the dissipative soliton regime can be of interest for various applications, such as supercontinuum generation or nonlinear frequency conversion. Due to the short transform limit of 220 fs and highly up-chirped nature of dissipative solitons, the oscillator can be an attractive seed for chirped-pulse amplification for high-energy pulses.