Sub-60 fs, 1300 nm Laser Pulses Generation from Soliton Self-Frequency Shift Pumped by Femtosecond Yb-Doped Fiber Laser

Abstract

We report on the generation of 1300 nm ultrashort laser pulses via the soliton self-frequency shift in a high-nonlinearity fiber, pumped by the 41.9 MHz, 67.9 fs, 1073 nm femtosecond laser emitted from an Yb-doped fiber laser system. A numerical simulation was applied to investigate the spectral broadening process driven by the soliton self-frequency shift with increased pump power. The experimental results are in good agreement with the numerical results, delivering a 33 mW, 57.8 fs 1300 nm Raman soliton filtered by a longpass filter. The impact of the polarization direction of the injected pump laser on the soliton self-frequency shift process was also further investigated. The root means squares of the Yb-doped fiber laser and the nonlinearly spectral broadened laser were 0.19%@1h and 0.23%@1h, respectively.

1. Introduction

Ultrafast laser sources with a central wavelength located at 1300 nm make great contributions in the field of three-photon microscopy (3PM), especially for deep tissue imaging such as deep brain imaging in freely behaving rodents [1,2,3,4,5]. Utilizing a high-energy, 1300 nm femtosecond pulse as the illumination source for the 3PM allows for deep penetration depth (>1 mm) with high signal-to-background contrast and negligible out-of-focus fluorescence [1,2,3,4,5]. A. Klioutchnikov et al. reported a head-mounted three-photon microscope (3PM) for imaging deep cortical layer neuronal activity in a freely moving rat by using a 55.7 fs, 200 nJ, 1320 nm ultrafast laser source, enabling an imaging depth of >1.1 mm below the cortical surface [1]. C. Zhao et al. demonstrated a miniature 3PM to maximize fluorescence collection by using a <100 fs, 126 nJ, 1300 nm laser source, which was capable of imaging calcium activity throughout the entire cortex and dorsal hippocampal CA1 with an imaging depth up to 1.2 mm [2].

A high-energy, ultrafast 1300 nm illumination source for the 3PM can be realized with the optical parametric amplification (OPA) process [6,7], delivering a 1300 nm laser as the amplified idler laser, with the signal laser realized by white-light generation. However, the potential long-term instability of the white-light generation process and the potential spatial chirp of the amplified idler laser during the OPA process seriously affect the operational performance of the 3PM. Directly utilizing the 1300 nm sub-hundred femtosecond laser as the seed laser for the OPA process is an ideal approach in realizing a high-energy, ultrafast 1300 nm laser source with better long-term systematic stability and significantly inhibited spatial chirp [8].

Nowadays, a 1300 nm femtosecond seed laser can be generated directly either from the mode-locked fiber lasers [9,10,11,12,13] or nonlinear frequency conversion systems [14,15,16]. Bismuth-doped (Bi-doped) fibers are reported to be the appropriate gain medium in generating the 1300 nm mode-locked laser pulses [9,10]. However, the relatively low small-signal gain of ~0.12 dB/m at 1.32 μm seriously restricts its further applications in generating a sub-hundred femtosecond seed laser [9,17]. The nonlinear frequency conversion based on the self-phase modulation (SPM) or the Raman soliton self-frequency shift (SSFS) can be utilized in generating sub-hundred femtosecond 1300 nm seed lasers [14,15,16]. H. Chung achieved 97 fs, 15.8 nJ, 1300 nm laser pulses employing on the SPM effect, after propagating a 290 fs, 160 nJ, 1560 nm laser through the dispersion-shifted fiber [14]. No further amplification results realized by the OPA process were reported, limited by the power scaling capability of the reported 1560 nm fiber laser source.

The SSFS driven by the stimulated Raman scattering (SRS) in a negative dispersion region at 1 μm can be utilized to generate sub-hundred femtosecond 1300 nm laser pulses, which was realized with large mode area (LMA) fiber and noble gas-filled hollow-core fiber. L. Rishøj et al. reported the generation of 30 nJ, 120 kHz, 53.6 fs, 1317 nm femtosecond laser in an LMA fiber with an effective mode area of 660 μm² [18]. Due to the LMA property of the fiber, the transverse mode of the generated 1317 nm laser was LP₀₉, which requires extra optical beam conversion construction to reconvert the beam back to the Gaussian-like beam. Y. Eisenberg et al. realized a 500 nJ, 200 kHz, 80 fs, 1310 nm ultrafast laser in a hollow-core fiber filled with N₂ gas [19]. However, the utilized hollow-core fiber with a 30 μm core diameter can also introduce transverse mode problems with increased systematic complexity and higher costs [20]. In comparison, based on the development of the photonic crystal fiber (PCF), the single-mode (SM) 1300 nm Raman soliton laser with sub-hundred femtosecond pulse duration can be generated with much simpler and more robust systematic construction by utilizing high-nonlinearity PCF (HNPCF). Based on the modified air holes of the cladding, the waveguide properties of the HNPCF, such as dispersion, transverse mode, and mode field diameter, can be carefully designed by adjusting the air hole diameter and the distance between the adjacent air holes. Typically, the zero dispersion of the pure silica fiber can be shifted from ~1310 nm to ~1040 nm, with the mode field diameter decreased to ~4 μm [21]. These properties enable the generation of a sub-hundred femtosecond, SM 1300 nm soliton laser with the SSFS process, pumped by a femtosecond Yb-doped fiber laser with high power-scaling capability. Further, the input polarization can also be optimized to ensure the optical power is mainly distributed inside the fundamental Raman soliton before and after the Raman soliton contents shift to 1300 nm, since the birefringent properties of the HNPCF can introduce a polarization-dependent SSFS process [22,23]. Therefore, an ideal 1300 nm Raman soliton seed laser can be generated with promising optical power for the OPA process, fulfilling the urgent requirements of the 3PM applications. To the best of our knowledge, this kind of 1300 nm seed laser generation system has not been reported yet.

In this paper, we report the generation of a 33 mW, 57.8 fs, 1300 nm seed laser, realized with the SSFS process pumped by a 136 mW, 67.9 fs, 1073 nm femtosecond Yb-doped fiber laser. Numerical simulation based on solving the nonlinear Schrödinger equation was conducted to predict the spectral evolution under different injected pump power in the HNPCF. Further experiment achieved a 33 mW, 57.8 fs, 1300 nm femtosecond laser with high pulse quality. The impact of the polarization direction of the pump laser on the SSFS process was also investigated, indicating a polarization-dependent SSFS process. The output power stabilities of the Yb-doped fiber amplifier and the spectral broadened laser were measured with root mean squares (RMSs) of 0.19%@1 h and 0.23%@1 h, respectively.

2. Experimental Setup

The schematic construction of the 1300 nm femtosecond laser system is shown in Figure 1. The system consists of a polarization-maintaining (PM) mode-locking Yb-doped fiber oscillator, a PM SM Yb-doped fiber pre-amplifier, a grating-pair pre-chirper, a PM double-cladding (DC) Yb-doped fiber power amplifier, a grating-pair compressor, and an SSFS stage. The home-built PM mode-locking Yb-doped fiber oscillator based on the nonlinear amplifying loop mirror (NALM) consists of a nonlinear loop, a reflection linear arm, and a transmission port. A PM fiber coupler with a power splitting ratio of 45:55 was utilized to connect the nonlinear loop with the linear arm and the transmission port. About 50 cm PM Yb-doped fiber (Yb401-PM, INO, Quebec, QC, Canada) was utilized in the nonlinear loop as the gain medium, pumped with a SM 976 nm laser diode (LD) through a PM fiber WDM. A PM fiber phase bias with a phase shift of −π/2 was included in the nonlinear loop to ensure the self-starting mode-locking. The linear arm consists of a bandpass spectral filter (FLH1030-10, Thorlabs, Newton, USA) for spectral management and a transmission grating pair (T-1000-1040, LightSmyth, Eugene, OR, USA) for intracavity dispersion management.

The mode-locked laser pulses output from the fiber oscillator were delivered from the transmission port and subsequently injected into the PM fiber pre-amplifier. The PM fiber pre-amplifier consists of a PM WDM, a SM 976 nm LD, and an about 50 cm Yb-doped fiber (Yb401-PM, INO, Quebec, QC, Canada). The pre-amplified mode-locking laser pulses were then modulated with a transmission grating-pair pre-chirper which provided negative group velocity dispersion. After that, the mode-locking laser pulses were coupled into the fiber power amplifier composed of a 976 nm multi-mode (MM) LD, a PM fiber combiner, a PM Yb-doped fiber (PM-YDF-5/130, Coherent, Saxonburg, PA, USA), and a PM fiber collimator. The amplified laser pulses were then compressed by a transmission grating-pair compressor.

The compressed laser pulses were coupled from free space into an about 7.5 cm HNPCF (SC-5.0-1040-PM, NKT photonics, Birkerød, Denmark) to pump the SSFS process. The input pump power was controlled with a variable attenuator consisting of a half-wave plate (HWP) and a polarizing beam splitter (PBS). The polarization direction of the input pump laser can be controlled with the HWP before the HNPCF. The output optical spectrum can be shifted to 1300 nm with the SSFS process. Finally, a longpass spectral filter (LPF, FELH1250, Thorlabs, Newton, NJ, USA) was utilized to filter out the sub-hundred femtosecond 1300 nm laser pulses.

3. Experimental and Numerical Results

The NALM fiber oscillator operated in a dispersion-managed regime, delivering 1 mW, 41.9 MHz mode-locking laser pulses with a pulse to pulse interval of 23.87 ns, as shown in Figure 2b. The optical spectrum of the mode-locking laser pulses is illustrated in Figure 2a, corresponding to a central wavelength of 1030 nm and a 3 dB spectral bandwidth of 16.6 nm. The average power of the mode-locked laser pulses was boosted to 52 mW by the PM fiber pre-amplifier. The pre-chirper provided a negative dispersion of −0.517 ps² at 1030 nm, enhancing the nonlinear spectral broadening process in the subsequent nonlinear fiber power amplifier. An about 7 m PM DC Yb-doped fiber was utilized inside the fiber power amplifier, ensuring the gain-managed nonlinear amplification process can be realized [24]. The optical spectrum of the amplified laser pulses was broadened to 54.3 nm with the central wavelength shifted to 1072.7 nm, as illustrated in Figure 2c. During the nonlinear amplification process, the SPM introduces linear chirp to the propagating laser pulses. Therefore, the amplified laser pulses can be compressed to the nearly transformed limited pulse duration of 67.9 fs, which is 1.48 times the Fourier transform limited pulse duration. The corresponding measured auto-correlation trace is exhibited in Figure 2d, the residual pedestal may be attributed to uncompensated high-order dispersion. The optical average power of the compressed laser pulses was 407 mW, corresponding to an optical compression efficiency of ~80% and a polarization extinction ratio of 21.7 dB.

Figure 3 illustrates the dispersion curve of the HNPCF obtained from the NKT photonics. The corresponding photograph of the cross section is shown as the inset. The zero-dispersion wavelength is located at ~1040 nm, indicating a negative group velocity dispersion region with the optical wavelength above 1040 nm. The nonlinearity coefficient of the HNPCF is 11 (W·km)⁻¹@1060 nm with a mode-field diameter of 4 μm. Based on the aforementioned optical characteristics, a numerical model was implemented to simulate the SSFS process with the 1072.7 nm pump laser propagating through the 7.5 cm PCF. The numerical model was built by solving the generalized nonlinear Schrödinger equation with the split-step algorithm [25,26]. The experimentally measured optical parameters of the 1072.7 nm pump laser were directly employed as the input parameters for the numerical simulation. The optical dispersion, SPM, SRS and the self-steepening effects in the fiber were included for the numerical simulation. The crucial parameters of the Raman response of the fiber such as the fractional contribution and characteristic times were obtained from Ref. [27].

The numerical simulated optical spectral evolution of the 1072.7 nm pump laser propagating through the 7.5 cm HNPCF with different average power is illustrated in Figure 4. Figure 4a depicts the two-dimensional optical spectral evolution with increased pump power. The pure SPM dominated spectral broadening process can be observed with an injected 1072.7 nm pump power lower than 14 mW. The first-order Raman spectral contents were generated under the pump power of 19 mW, while the optical power located in the spectral range of 1050–1106 nm was clearly transferred to the Raman spectral contents. With the injected pump power increasing to 136 mW, the Raman spectral contents transformed to the Raman soliton and shifted from 1113 nm to 1300 nm with the anomalous group velocity dispersion of the HNPCF. The corresponding dispersive wave was also generated around 750 nm, located in the normal group velocity dispersion region. Further higher-order Raman spectral contents can also be generated with higher injected pump power, as shown in Figure 4a,b. The calculated average power of the filtered 1300 nm Raman soliton is 32.5 mW. The calculated pulse duration of the corresponding transform limited auto-correlation trace is 66.2 fs, based on the Sech² assumption.

Based on the numerical results, the SSFS process was experimentally investigated with the linearly polarized pump laser propagating in the HNPCF. The output optical spectral evolution under different input pump powers is illustrated in Figure 5a, which is highly consistent with the numerical results. The SPM-dominated spectral broadening process under the pump power of 19 mW can be observed. Further increasing the pump power can result in the formation and frequency shift of Raman soliton in the negative group velocity dispersion region, along with the dispersive wave in the normal group velocity dispersion region. The measured output spectrum under the pump power of 136 mW is depicted in Figure 5b. The reddest Raman soliton component was successfully shifted to 1300 nm, which can be separated from the broadened optical spectrum with the LPF. Figure 5c shows the optical spectrum of the filtered 1300 nm soliton laser pulses, centered at ~1300 nm with a 3 dB spectral bandwidth of 46.6 nm. The corresponding measured auto-correlation trace is shown in Figure 5d, indicating a pulse duration of 57.8 fs based on the Sech² assumption, which is 1.38 times that of the Fourier transform limited pulse duration. The optical power of the sub-hundred femtosecond 1300 nm soliton laser was 33 mW, indicating a nonlinear frequency conversion efficiency of 24.3%.

The impact of the input polarization on the SSFS process was further investigated by incrementally rotating the input polarization direction of the pump laser. The generated broadened two-dimensional optical spectra under the pump power of 136 mW is depicted in Figure 6. Due to the high birefringent properties of the HNPCF (birefringence > 1.7 × 10⁻⁴@1060 nm), the Raman scattering induced the SSFS process in the negative group velocity dispersion region can be highly affected with different input polarizations [22,23]. As illustrated in Figure 6, the output spectrum shows evident variation with the rotating polarization direction of the input pump laser. The broadest nonlinear spectral broadening process induced by the SSFS can be realized with the polarization direction of the linearly polarized input pump laser rotated to be parallel with the optical axes of the HNPCF. As shown in Figure 6, with the generated 750 nm dispersive wave, the broadest nonlinearly broadened spectra can be achieved with HWP rotation angles of 38.7°, 83.7°, 128.7°, 173.7°, 218.7°, 263.7°, 308.7°, and 353.7°, and periodically increased with the rotation angle of 45°. The realized broadest nonlinearly broadened spectrum matches well with the simulated results shown in Figure 4a, indicating a perfect numerical model and reliable simulation results. Further modifying the input polarization direction can lead to the input pump laser being decomposed into two orthogonal polarized optical components propagating through the fast axis and slow axis of the HNPCF. Therefore, the nonlinear spectral broadening process can be inhibited with the initial coupled pump power being distributed into these two orthogonal polarized laser components, which can be further utilized to suppress the generation of high-order Raman solitons and concentrate more optical power in the fundamental Raman soliton. The narrowest spectral broadening results were achieved with HWP rotation angles of 14.5°, 59.5°, 104.5°, 149.5°, 194.5°, 239.5°, 284.5°, and 329.5°, and periodically increased with the rotation angle of 45°. The cross-phase modulation was also introduced into the nonlinear spectral broadening process, leading to the further modulated spectral structures. Based on the aforementioned investigations, the HWP rotation angle was finally tuned to be 150° to modify the input polarization direction of the 136 mW pump laser to generate the 33 mW, 57.8 fs, 1300 nm seed laser for the OPA, as shown in Figure 5.

To evaluate the stability of this system, the average power stabilities within 1 h of the laser pulses output from the Yb-doped fiber laser and the HNPCF are both shown in Figure 7, of which the root mean squared (RMS) stability are 0.19%@1 h and 0.23%@1 h, respectively, indicating the high stability of the fiber laser system.

4. Conclusions

In conclusion, a 33 mW, 57.8 fs, 1300 nm femtosecond laser was realized through SSFS in an HNPCF pumped by an Yb-doped fiber laser. The Yb-doped fiber laser can deliver 41.9 MHz, 67.9 fs, 1072.7 nm femtosecond laser pulses with a maximum average power of 407 mW, and with RMS power stability within 1 h of 0.19%. The spectral evolution of the 1 μm pump laser in the HNPCF under different input powers was investigated with numerical simulation and experiments. The nonlinearly broadened spectrum perfectly covered the 1300 nm soliton waveband under the input pump power of 136 mW, with RMS power stability within 1 h of 0.23%. This work provides an ideal sub-hundred femtosecond 1300 nm seed laser for the OPA source of the 3PM applications [28].