1. Introduction

As a key technology to empower global navigation systems, precise inter-satellite ranging requires lasers with qualities such as ultrashort pulse width, low timing jitter, compact size and lightweight design [1–3]. Compared with conventional Ti-sapphire mode-locked lasers, ultrafast fiber lasers can better meet the above-mentioned requirements due to their all-fiber and bulk-free configurations.

Wavelength is essential parameter that could limit the applications of ultrafast fiber lasers in precise ranging. Ytterbium is the most successful doping platform in generating fiber lasers with either high average power (up to tens of kilowatt) [4–6] or short pulse width (down to few cycles) [7–9]. Yet, the emission spectrum of ytterbium is restricted within the wavelength range from 970-1100 nm. Silicon-based detectors are an ideal choice for space applications due to their low cost and high photo sensitivity. The peak response wavelength of the silicon-based detector is around 900 nm, which cannot be covered by Yb-doped fiber lasers. Indeed, the photo response at 900 nm is 4.43 times as much as the one at 1064 nm [10]. Therefore, for space applications which are sensitive to power consumption, the development of compact and reliable picosecond laser at ∼ 900 nm is highly demanded.

There are two main fiber-based techniques can be adopted to generate ultrafast pulses around 900 nm. One is nonlinear frequency conversion, utilizing high-power Yb-doped or Er-doped ultrafast fiber lasers as excitation sources [11–13]. These lasers induce nonlinear effects in fibers (such as self-phase modulation, dissipative wave, etc.) to produce spectral branches near 900 nm, and then ∼900 nm ultrafast fiber lasers can be obtained by spectral filtering.

In contrast, another more direct and efficient method is mode-locked neodymium-doped fiber lasers, which utilize the ⁴F_3/2→⁴I_9/2 energy level transition of neodymium ions to directly generate ∼900 nm laser pulses. Material-based saturable absorber (SA) [14–16], nonlinear polarization rotation (NPR) [17,18] and nonlinear amplifying loop mirror (NALM) [19,20] are three commonly used mechanisms to initialize and stabilize mode-locked operation in Nd-doped fiber lasers. Lasers mode-locked by material-based SA have the advantage of simple structure. Nevertheless, the characteristics of material-based SA will gradually deteriorate over time, which limits its practical applications, especially for space scenarios. NPR technology has advantages of wavelength flexibility, deep modulation depth, and easy implementation. However, NPR suffers from poor environmental stability due to its inherent dependence on polarization changes [21].

Compared with the above mechanisms, NALM is more engaging with long-term stability. Wang et al. utilized this technique to achieve a femtosecond all-polarization-maintaining (PM) 920 nm fiber laser by introducing a pair of gratings into the cavity to adjust the dispersion [20]. Long-term stability is ensured by the all-PM configuration and the immunity for material deterioration. However, for this laser, the introduction of bulk optical devices will increase its volume and reduce the reliability, limiting its applications. Therefore, developing an NALM mode-locked Nd-doped fiber laser with an all-PM and all-fiber structure has important research significance for space applications.

Besides the wavelength, another issue is the repetition rate. The repetition rate of conventional mode-locked fiber lasers is commonly in the range from tens of to hundreds of MHz. Due to the high dispersion and nonlinearity of single-mode fibers, it is difficult to achieve single pulse operation in a long fiber cavity. Yet, low repetition rate picosecond fiber laser is highly demanded for optical ranging, since it can effectively reduce the range ambiguity in time-of-flight (TOF) measurements while maintaining a millimeter accuracy [22]. Although the repetition rate can be down-converted by applying a pulse picker outside the cavity, the involvement of extra acousto-optic modulator would increase the laser complexity.

In this letter, we present an all-fiber, all-PM picosecond NALM laser operating at 915 nm with a repetition rate of 3.05 MHz. A figure-9 laser structure is used to provide artificial saturable absorption effect to achieve mode locking. In the experiment, a chirp-free fiber Bragg grating (FBG) is employed as the end mirror of the figure-9 structure, serving the dual purpose of reflection and spectral filtering simultaneously. Centered at the wavelength of 915 nm, a stable laser output with a pulse width of 15.2 ps and a single pulse energy of 4.65 nJ was obtained. This all-fiber, all-PM 915 nm picosecond fiber laser could be a promising light source for applications like precise inter-satellite ranging.

2. Experimental setup

The schematic of the Nd-doped mode-locked fiber laser is illustrated in Fig. 1, which is all constructed by PM fiber components. The laser has a figure-9 structure, which consists of an NALM and a linear fiber arm. They are connected by a 2 × 2 PM fiber coupler with a 30:70 splitting ratio. The NALM is based on a Sagnac fiber interferometer, which serves as an artificial saturable absorber for mode locking. It includes an 808/915 nm PM wavelength division multiplexer (WDM), a piece of 1.2 m gain fiber (Coractive, Nd103-PM, 40 dB/m absorption at 808 nm), a fiber output coupler to extract laser power from the cavity, and a nonreciprocal phase shifter (PS). The gain fiber is pumped by a single-mode 808 nm laser diode (LD) with a maximum power of 250 mW. The 808/915 nm WDM has a reflection band of 800-840 nm and a transmission band of 900-1100 nm, The linear arm contains a PM FBG with a center wavelength of 915 nm, a bandwidth of 0.25 nm, and a reflectivity of 99%. It is worth mentioning that the gain fiber is placed asymmetrically in the NALM in order to induce proper nonlinear phase shift difference between the clockwise and counterclockwise propagated light to enable mode-locking. The overall cavity length is ∼69.2 m, which gives a net cavity dispersion of 2.5 ps² at 915 nm.

 

Fig. 1. Schematic diagram of the Nd-doped mode-locked fiber laser, which consists of a figure-9 seed and a fiber amplifier. WDM: wavelength division multiplexer; LD: laser diode; NDF: Nd-doped fiber; ISO: isolator; FBG: fiber Bragg grating.

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The nonreciprocal PS is essential for achieving pulsed operation in a figure-9 laser cavity. Figure 2 shows the reflection curve of a nonlinear optical loop mirror with respect to nonlinear phase shift difference (Δφ_NL) under a coupling ratio of 30:70. It can be seen from the solid line that when the figure-9 cavity has no linear phase shift, the reflectance gradually decreases with the accumulation of Δφ_NL, which makes the figure-9 laser favor continuous wave (CW) operation instead of pulsed one (point #1 in Fig. 2). After biasing a -π/2 linear phase shift via a nonreciprocal PS, the reflectance (dashed line in Fig. 2) could grow rapidly with the increase of the Δφ_NL (from point #2 to point #3), which promotes the self-starting of the mode-locked pulses [23].

 

Fig. 2. The reflection curve of the figure-9 cavity with respect to the nonlinear phase shift difference Δφ_NL under a coupling ratio of 30:70. Δφ₀ represents the linear phase shift difference in the ring.

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A fiber amplifier is implemented following the figure-9 seed laser to boost the pulse energy to several nJ. The schematic of the amplifier is also illustrated in Fig. 1. A PM fiber isolator (ISO) is placed between the seed laser and the amplifier to protect the seed from back reflections. A 915/1064 nm WDM is positioned after the seed to filter out ASE noise at ∼1.0 µm in the seed pulse. In the fiber amplifier, the same type of PM-NDF as used in the seed laser, with a length of 1.2 m, is core-pumped by another single-mode 808 nm LD through an 808/915 WDM. The output light is filtered again by a 915/1064 WDM to remove most of the 1064 nm ASE generated in the amplifier stage.

In the experiment, the output power was measured by thermal power meter. The temporal pulse train was detected by an InGaAs photodetector (Thorlabs, DET01CFC, 1.2 GHz bandwidth) and a 2.5 GHz oscilloscope (Keysight, DSO-S 254A). The optical spectrum of the laser was measured using an optical spectrum analyzer (Yokogawa, AQ6370D) with a resolution of 0.02 nm. The radio-frequency (RF) spectrum was recorded by an RF spectrum analyzer with a bandwidth of 20 GHz (Keysight, N9020A), and the pulse width was measured by a commercial autocorrelator (APE, Pulsecheck USB 150).

3. Results and discussion

3.1 Figure-9 Nd-doped fiber oscillator

For mode-locked fiber lasers, a delicate balance between gain, loss, dispersion and nonlinearity needs to be achieved in order to generate stable solitons [24]. Since single-mode-fibers have strong dispersion and nonlinearity characteristics, accomplishing this balance is rather difficult in a long fiber cavity, such as the 69.2 m long cavity in this experiment. This difficulty is further enlarged in a figure-9 cavity, which depends on controlling the nonlinear phase shift to trigger mode-locking. Therefore, the figure-9 fiber cavity needs to be optimized to generate single pulse soliton with high pulse energy. Two cavity parameters have been changed and studied for the laser optimization. One is the length difference between the passive fibers on each side of the gain fiber; the other is the output coupling ratio of the coupler inside the NALM.

Assuming that the length between the 70% port of the 2 × 2 coupler and the gain fiber is L₁ and the one between the 30% port and the gain fiber is L₂. We started up with a L₁-L₂ value of -12 m and an output coupling ratio of 30%, as shown in the configuration #1 of Fig. 3. Since the nonlinear phase shift difference is large and the loss of the cavity is small, mode-locked operation would be easily self-started with a low pump power of 105.3 mW. Yet, the laser functioned in a multi-pulse state after being self-started. Gradual reduction of pump power to 65.9 mW facilitated stable single-pulse mode-locking, which gave an output power of 0.38 mW at 915 nm.

 

Fig. 3. Optimization of the laser with three different configurations. Configuration #1: L₁-L₂= -12 m, 30% output coupling ratio; Configuration #2: L₁-L₂= 30 m, 30% output coupling ratio; Configuration #3: L₁-L₂= -12 m, 70% output coupling ratio.

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As shown in Fig. 2, certain amount of Δφ_NL is required to realize mode-locking operation. Since nonlinear phase is determined by intensity times fiber length, modifying the length difference could help to increase the pulse’s power to reach the Δφ_NL for single pulse operation. Therefore, to achieve higher output power, we changed the L₁-L₂ value to 30 m, as shown in the configuration #2 of Fig. 3. Compared to the one with a value of -12 m, the NALM has a smaller Δφ_NL at certain pump power level. Therefore, higher pump power is required to reach the working point of mode-locking, which would eventually lead to higher power of the 915 nm pulses. Experimentally, the pump power for self-starting is increased to 250 mW and the one for single-pulse operation is 79.3 mW. The corresponding output power of the 915 nm pulses is enlarged to 0.85 mW, which is more than twice as much as the one of configuration #1. Further increase the L₁-L₂ value should result in higher output power. Yet, limited by the 250-mW maximum pump power of the 808 nm LD, the mode-locking would be difficult to self-start in this case.

Apart from changing fiber length bias, another method for power optimization is to increase the output coupling ratio. As shown in the configuration #3 of Fig. 3, we switched output coupling ratio from 30% to 70% and kept the fiber length unchanged. In this case, the pump power for single pulse operation has been increased to 121 mW and the corresponding output power is scaled up to 1.56 mW, which is further doubled compared with the one in configuration #2.

The laser characteristics corresponding to the three laser configurations are summarized in Table 1. In the experiment, the output power at 915 nm was derived by spectral integration. According to the experimental results, increasing the output coupling ratio is more effective than changing the fiber length bias for the purpose of scaling up the single pulse energy of a figure-9 fiber laser. This is because that increasing the output coupling ratio not only scales up the output power directly, but also reduces the nonlinear phase shift difference in the NALM. While changing the fiber length bias can only achieve the effect of the latter.

Table 1. Laser characteristics corresponding to the three laser configurations

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The RF spectrum of the 915 nm pulses is shown in Fig. 4(a). The fundamental frequency spectrum confirms a peak at ∼3.05 MHz. The high stability of the mode-locked laser is confirmed by a signal-to-noise ratio (SNR) higher than 95 dB, benefitted from the all-PM fiber structure. The temporal pulse train is shown in Fig. 4(b), which showed a uniform intensity. The temporal spacing of the measured pulse train is 328 ns, which corresponds to the pulse round-trip time of the 69.2 m long cavity. The pulse width was measured to be 13.7 ps and the result is depicted in Fig. 4(c). The bottom of the autocorrelation curve does not fit well with the sech² curve, which is due to the fact that the 1.56 mW output power is not adequate for high SNR autocorrelation test.

 

Fig. 4. Output parameters of the figure-9 seed laser. (a) RF spectrum of the fundamental repetition frequency (Inset: the one showing harmonic repetition frequencies); (b) pulse train; (c) autocorrelation signal of the 915 nm pulse; (d) global output spectrum from 750 to 1200 nm and (e) global output spectrum filtered by 915/1064 WDM (Inset: local output spectrum at 915 nm).

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Figure 4(d) shows the optical spectrum measurement of the output pulses from 700-1200 nm with a spectral resolution of 0.02 nm. The result confirms that the total 2.3 mW output contained some amount of 808 nm pump and 1064 nm ASE apart from the 915 nm laser. Through spectral integration, the powers of the 808 nm pump, the 1064 nm ASE and the 915 nm laser are calculated to be 0.64 mW, 0.1 mW and 1.56 mW, respectively. For ASE suppression in subsequent fiber amplifier, the 1064 nm ASE in the seed has to be removed. It is proved that the 1064 nm ASE can be effectively filtered out by a 915/1064 nm WDM, and the filtered spectrum is shown in Fig. 4(e). The insert figure gives an enlarged spectrum of the 915 nm mode-locked laser, which has a 10-dB width of 0.27 nm. It is worth noticing that ASE around 915 nm is also observed (40 dB lower than the main peak), which can be further removed by a narrowband filter if necessary.

3.2 Nd-doped fiber amplifier

Many applications have requirements for 915 nm laser pulses with pulse energy over several nJ. Therefore, in order to meet this demand, a Nd-doped fiber amplifier is applied after the figure-9 seed to boost the output pulse energy. A 915/1064 nm WDM is deliberately inserted between the seed and the amplifier to suppress the 1064 nm ASE before laser amplification. We designed a comparative experiment to prove the necessity of this WDM. Figure 5(a) shows the output laser spectrum without the WDM at three different pump powers of the amplifier. The results indicate that 1064 nm ASE would grow dramatically with the increase of pump power. The SNR of the 915 nm laser with respect to the 1064 nm ASE deteriorated to 15.9 dB at the pump power of 91.4 mW. Further increasement of the pump power would lead to parasitic lasing at 1064 nm, which is harmful to the stable operation of the Nd-doped laser. Figure 5(b) shows the output laser spectrum with the WDM at the same three pump power levels. In this case, the SNR is increased to a value of 34.9 dB at the pump power of 91.4 mW. The results clearly indicate that the ASE could be effectively suppressed during the amplification, as long as the ASE in the seed laser is removed.

 

Fig. 5. Laser output characteristics with and without ASE filtering by a 915/1064 nm WDM. Output laser spectrum (a) with and (b) without WDM at three pump powers; (c) output power and (d) power ratio of the 915 nm laser as a function of the pump power with and without WDM.

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Apart from the output spectrum, the output powers of the 915 nm laser and their ratio in the total output power in the two sets of experiments are summarized in Fig. 5(c) and 5(d), with respect to the pump power range from 0 to 91.4 mW. The 915 nm power ratio is defined as the ratio of the power at 915 nm to the total power of the output laser. At the pump power of 91.4 mW, the 915 nm laser powers reached 6.42 mW and 5.55 mW respectively in the case with and without ASE filtering. Although the difference in output power is not significant, the power ratio of the 915 nm laser would decrease rapidly with the increase of the pump power when the WDM is absent, as shown in Fig. 5(d). The 915 nm laser power ratio are 95.5% and 55.3% respectively at the pump power of 91.4 mW. These results confirm the importance of ASE filtering before amplification.

Since the ASE-induced parasitic lasing has been suppressed by the 915/1064 nm WDM, we further increased the pump power to 169.2 mW and achieved a 915 nm laser output of 14.16 mW, corresponding to a single pulse energy of 4.65 nJ and a 9.5 dB gain with respect to the seed. As shown in Fig. 1, another 915/1064 nm WDM was applied after the fiber amplifier to remove the 1064 nm ASE generated during the amplification and further increase the 915 nm output power ratio. The output power long-term stability test was conducted and the result is shown in Fig. 6(a). The laser exhibited outstanding power stability with a root-mean-square (RMS) power fluctuation of 0.29% over a time span of 7 hours. Figure 6(b) gives a 2 hours spectral evolution of the 915 nm pulses, which further confirms the excellent stability of our laser. The stability is resulted from the all-fiber and all-PM structure used in both the figure-9 seed and the fiber amplifier. Figure 6(c) shows the sampled spectrum at the time slot of 100 min in the spectral range from 700 to 1200 nm, and the inset shows the local enlarged spectrum at the central wavelength of 915 nm. It is worth noting that the 10 dB bandwidth of the laser spectrum is 0.51 nm, which is nearly twice as much as the seed. The spectral broadening is resulted from self-phase modulation during the amplification, which also leads to a clear spectral dip at 915 nm. By spectral integration, we determined that the powers at 808 nm and 915 nm are 0.37 mW and 14.16 mW respectively, which gives a 915 nm power ratio of 97.4%. As displayed in Fig. 6(d), the measured autocorrelation trace of the 915 nm laser exhibits a sech²-shaped profile with a full width at half maximum (FWHM) of 23.4 ps, which corresponds to a pulse width of 15.2 ps with a deconvolution factor of 1.54. Such a compact and stable 915 nm picosecond laser could be an ideal light source for the application of precise inter-satellite ranging.

 

Fig. 6. (a) Long-term stability test of the output power over 7 hours; (b) temporal evolution of the optical spectrum over 2 hours; the color bar shows the optical power spectral density; (c) global output spectrum from 900 to 1100 nm (Inset: local enlarged spectrum at 915 nm) and (d) autocorrelation signal of the output pulse, taken at the 100 min time slot.

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4. Conclusions

In summary, we demonstrate an all-fiber, all-PM figure-9 laser operating at 915 nm. The figure-9 cavity is formed by an FBG and an NALM to serve as two end mirrors. To meet requirements for ranging applications, figure-9 laser with 69.2 m long cavity length is designed and optimized, so that stable single pulse operation could be realized with high pulse energy and low repetition rate. The seed can produce 915 nm laser pulses with a pulse energy of 0.51 nJ, a pulse duration of 13.7 ps under the repetition rate of 3.05 MHz. The pulse energy is further boosted to 4.65 nJ by a Nd-doped fiber amplifier. The 1064 nm ASE is deliberately filtered out by 915/1064 nm WDMs before and after the fiber amplifier, in order to prevent parasitic lasing and increase the spectral proportion of the 915 nm laser. Output power and spectral stability testing have confirmed the laser's excellent mode-locking performance and environmental robustness. This all-PM-fiber 915 nm figure-9 picosecond laser with low repetition rate could be a promising light source for next-generation global navigation satellite systems, in which precise inter-satellite ranging with high accuracy and reduced range ambiguity is highly demanded.