1 Introduction

In recent years, lots of work has been dedicated to scaling the average and peak powers of laser pulses from all-solid-state and fiber lasers [1–5], especially for laser pulses characterized by ns pulse duration, high pulse repetition frequency (PRF), good beam quality and narrow spectral line width, which enabled many scientific and industrial applications such as nonlinear frequency conversion, remote sensing and material processing [6, 7].

The main technical approach utilized for achieving the above-mentioned laser pulse characteristics is the master oscillator power amplifier (MOPA) structure laser. It refers to a configuration consisting of a seed laser (low power but good performance) and an optical amplifier to boost the output power. However, for the actively Q-switched all-solid-state MOPA laser, it is very hard to maintain stable seed laser pulses at high repetition rate since the gain is not high enough to sustain stable Q-switching operation [8, 9]. Liu et al. reported a high-power, high-repetition-rate TEM00 mode all-solid-state MOPA laser using acousto-optical Q-switching. A seed laser from a dual end-pumped Nd:YVO4 oscillator was scaled up to 183.5-W average power at 850 kHz after being amplified by four-stage power amplifiers. However, at the typical pulse repetition rate of 850 kHz, the standard deviation jitter of the pulse energy was degraded to 12 %, and some pulses was found to be missing in the pulse train at 900 kHz [8].

The semiconductor diode seeded fiber MOPA laser avoids the problem of unstable seed laser pulses. But, restricted by nonlinear effects, such as the stimulated Brillouin scattering (SBS) phenomenon especially for narrow line width pulsed lasers, it is difficult to magnify the seed laser to very high power [10–13]. Ye et al. presented a comprehensive observation of the SBS phenomenon in a high-power linearly polarized Yb-doped double-clad fiber amplifier. Due to the high-peak-power giant pulses caused by the cascaded SBS, the damage originated from the core of the fiber and broke down the whole silica fiber around the core [10]. Khitrov et al. reported a linearly polarized pulsed fiber laser with tunable 2 ns–0.2 μs pulse durations, 50 kHz–50 MHz repetition rates and available high peak and average powers (>10 kW and 50 W, respectively). The extremely high peak power at the rising edge of the pulse caused detrimental nonlinear effects in the fiber and limited pulse average power and energy at lower repetition rates [11].

In this paper, we developed a high average and peak power, high-beam-quality, high-pulse-repetition-rate, linear-polarization, narrow line width pulsed fiber laser, based on a direct-modulated semiconductor diode and four cascaded fiber amplifier stages. The maximum average power of a 1063-nm TEM00 laser was scaled up to 88 W from 400 kHz to 1.5 MHz with the optical–optical conversion efficiency as high as 60.2 %. No SBS pulse or optical damage was observed although the maximum pulse peak power has exceeded 112 kW. To the best of our knowledge, both the peak and average powers are the highest values ever reported for a pulsed fiber laser with ns pulse duration and narrow line width. The beam quality factors M 2 were measured as 1.24 and 1.18 in two orthogonal directions and the stabilities of pulse duration and peak power were both better than 1.6 % (RMS). In addition, we obtained good signal-to-noise ratio (SNR) of ∼20 dB at the maximum output power with 0.065-nm spectral line width (FWHM) and 18.4 dB polarization extinction ratio (PER).

2 Experimental setup

As shown in Fig. 1, the whole experimental system is composed of a 1063-nm seed laser, pre-amplifiers (I and II), a mid-amplifier and a power amplifier. The seed laser is a distributed feedback (DFB) type semiconductor diode characterized by high beam quality, narrow line width, linear polarization and stable pulse train with maximum peak power of 100 mW. In low duty cycle, the average power of the seed laser is too weak to effectively extract pump power. Thus, we utilize two stages of core-pumped pre-amplifiers to magnify the seed laser with estimated gains of 20 dB and 13 dB, respectively. To fulfill the sufficient absorption of pump power, single-mode Yb-doped fibers with 2 m in length and high absorption coefficient ∼10 dB/m at 976 nm are used in each stage of the pre-amplifiers. The forward pumping structure is adopted to prevent possible damage of the pump source by the amplified laser pulses. The seed and pump lasers are concurrently coupled into the fiber core by wavelength division multiplexing (WDM) devices. The maximum 450-mW pump power is afforded by the pump laser diode (LD) in pre-amplifier II, which is 1.5 times higher than that in the first stage.

Fig. 1
figure 1

Holistic experimental setup of 88-W high-repetition-rate narrow line width pulsed fiber laser

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To scale the average and peak powers and meanwhile maintain the excellent seed laser performance, all fibers through which the laser pulses propagate are Panda-type polarization-maintaining (PM) fibers with successively larger core diameters. In the stage of the mid-amplifier, we use a large mode area (LMA) double-cladding Yb-doped fiber with 15-μm core diameter and 5-dB/m absorption coefficient. A long stretch of fiber about 4 m in length is employed for adequate absorption of pump power. The PM (2+1)×1 combiner concurrently couples laser pulses and maximum 25 W/port pump power into the active fiber with low loss of ∼0.5 dB.

In order to prevent potential damage of the seed laser and fiber amplifiers by the backward light, we added fiber optic isolators in front of each stage of the amplifiers. Meanwhile, each stage of the amplifiers is followed by band-pass filters to suppress the amplified spontaneous emission (ASE). All the components in pre- and mid-amplifier stages are fusion spliced by a Nytran FFS-2000 splicer, enabling a monolithic and compact system.

For a pulsed fiber laser with narrow line width, the main restriction that prevents further scaling of the peak power is the stimulated Brillouin scattering (SBS) caused by nonlinearity of the fiber medium and generally related to acoustic phonons. When the peak power of the laser pulses is higher than a certain threshold power, namely the SBS threshold, SBS can reflect most of the power of the laser pulses, thus leading to a strong nonlinear optical gain of backward light. For CW or quasi-CW cases, the SBS threshold occurring in passive fibers can be approximated as [14]

$$ P_{\mathrm{SBS}} = \frac{A_{\mathrm{eff}}}{kL_{\mathrm{eff}}g_{\mathrm{B}}}, $$
(1)

where A eff is the effective mode field area of the fiber, L eff is the effective fiber length, g B is the Brillouin gain (g B=5×10−11 m/W for fused silica) and k is the polarization factor (k=1 for a linear polarization laser).

In the SBS process, the response time of the transmission material is governed by the phonon lifetime of ∼10 ns. The above equation is valid for pulse durations of 100 ns or more, but not applicable for the <5 ns pulse duration regime. However, the model still gives qualitative guidance for the effect of short pulses on the SBS threshold.

In the experiment, we introduce a Yb-doped photonic crystal fiber (PCF, manufactured by NKT Photonics) as the active fiber in the power amplifier stage [15]. The PCF has a large core diameter of ∼40 μm with unique waveguide structure, in which only the TEM00 mode can propagate with low loss. Compared with traditional LMA double cladding fibers, the PCF has larger A eff and shorter L eff, thus ensuring high SBS threshold.

The detailed experimental setup of the backward-pumped power amplifier is shown in Fig. 2. The laser pulses output from the mid-amplifier are coupled into the PCF through a free-space coupling system with an optimized ratio of focus lengths of the coupling lens. We insert the high-power isolator for protecting the former fiber amplifiers from the backward light and observing the potential SBS pulse. The high-power isolator consists of a 45° Faraday rotator, a 45° quartz rotator and two pieces of polarization beam splitters (PBSs). The laser-injecting fiber is oriented in a certain direction for the best transmission efficiency of laser pulses on PBS 1 and PBS 2. The half-wave plate is used to adjust the polarization orientation of laser pulses to be parallel to the slow-axis direction of the PCF to maintain linear polarization.

Fig. 2
figure 2

Detailed experimental setup of power amplifier stage based on photonic crystal fiber. OSA: optical spectrum analyzer; PM: power meter; BPA: beam propagation analyzer; OSC: oscilloscope with high-speed photo-detector

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The maximum 150-W pump power is provided by six pieces of 25-W LDs combined with the 7×1 combiner and coupled into the PCF through another free-space coupling system. One piece of dichroic mirror (HR at 1064 nm and AR at 976 nm) is inserted between the coupling lens. In virtue of the high-brightness pump light supplied by the 7×1 combiner, a sufficient coupling efficiency of >95 % is ensured.

The SBS pulse generally propagates in the backward direction, and its polarization orientation will be rotated by 90° under the combined effect of quartz and Faraday rotators. Therefore, when the SBS pulse occurs, it will be reflected by PBS 1 and captured by the digital oscilloscope (Agilent 54845A, 1.5 GHz) with a high-speed photo-detector (Thorlabs DET210, 350 MHz).

The final output laser pulses (forward direction) are systematically measured by the Ophir FL250A-LP1-DIF power meter, the Spiricon M2-200s beam propagation analyzer, the optical spectrum analyzer (Agilent 86140B, 0.06-nm resolution) and the same digital oscilloscope mentioned above.

3 Experimental results and discussion

The gains of the pre- and mid-amplifiers were measured as 20 dB, 13.5 dB and 15 dB, respectively, and the maximum average power of laser pulses output from the mid-amplifier was around 2.5 W. Through the free-space coupling system and high-power isolator, a 2.1-W laser was coupled into the PCF with ∼1.8 W confined in the fiber core. Some laser pulses dispersed into the inner cladding due to mode mismatching and the single-polarization property of PCF. In the fiber core, only the laser pulses with a certain polarization direction (parallel to the slow axis) can transmit, whereas most of the laser pulses with another or random polarization direction will disperse into the inner cladding. As demonstrated in Figs. 3a and b, we divided the laser pulses by a rotating polarizer and observed them by the M2-200s beam propagation analyzer (BPA) with different attenuation rates. With high attenuation rate of 36 dB, the laser pulses polarized along the slow axis are obviously brighter than those polarized along the fast axis with low attenuation rate of 20 dB.

Fig. 3
figure 3

(a) Laser pulses polarized along the slow axis with high attenuation rate of 36 dB and (b) laser pulses polarized along the fast axis with low attenuation rate of 20 dB

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The average power of laser pulses output from the power amplifier varying with the pump power is shown in Fig. 4. At a typical repetition rate of 822 kHz, the average power increased as the pump power rose and the maximum power of 88 W was achieved at the total pump power of 143 W, corresponding to an optical–optical conversion efficiency as high as 60.2 %. The pulse repetition rate had little influence on the output average power, since it was high enough, and from 400 kHz to 1.5 MHz, the average power was maintained at a stable value of 88±0.5 W.

Fig. 4
figure 4

Output power and optical–optical efficiency versus pump power

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We systematically measured the output laser at the typical pulse repetition rate of 822 kHz and the maximum power of 88 W. As evident in Fig. 5, the central wavelength was maintained at 1063.08 nm with ∼20 dB signal-to-noise ratio (SNR), and no ASE hump occurred. The average value of 3 dB line width was 0.065 nm during the 20-min test with maximum fluctuation of ±0.002 nm.

Fig. 5
figure 5

Signal-to-noise ratio (SNR) with 0.5-nm resolution bandwidth (RBW) and spectral line width with 0.06-nm RBW

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The polarization extinction ratio (PER) was computed by comparing the maximum and minimum powers passing through a rotating polarizer. In the test duration of 20 min, the average PER value was 18.4 dB (>69:1).

The beam quality factors were measured as \(M^{2}_{x} = 1.24\) and \(M^{2}_{y} = 1.18\) in two orthogonal directions with the standard 4σ knife-edge simulation method (see Fig. 6a), i.e. a near-diffraction-limit laser was obtained. The Gaussian-like spatial form in the far field is shown in Fig. 6b.

Fig. 6
figure 6

(a) Beam quality factors in two orthogonal directions and (b) laser spatial form in far field

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The laser pulse characteristics are shown in Fig. 7. At the typical PRF of 822 kHz, the pulse duration was measured as 1.96 ns. The standard deviation jitters of pulse duration and peak power were measured as less than 1.3 % and 1.6 % (RMS), respectively. At a lower pulse repetition rate of 400 kHz, the pulse peak power increased to an outstanding 112 kW. Although the power density in the fiber core has approached 17 GW/cm2, no SBS or optical damage was evident in the final output test.

Fig. 7
figure 7

Laser pulse characteristics at 822 kHz and maximum output power of 88 W

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4 Conclusion

We demonstrated a high-power narrow line width pulsed fiber laser, and scaled the average and peak powers up to 88 W and 112 kW, respectively, for the first time in the ns duration range. The four-cascaded fiber amplifiers provided a total amplification of 60 dB in a wide PRF operating range from 400 kHz to 1.5 MHz. The output laser exhibited a stable pulse duration of 2 ns with good signal-to-noise ratio of ∼20 dB, narrow spectral line width of 0.065 nm, high polarization extinction ratio of 18.4 dB and excellent pulse stabilities of better than 1.6 % (RMS). Besides, the beam quality factors were measured in two orthogonal directions as \(M^{2}_{x} = 1.24\) and \(M^{2}_{y} = 1.18\), respectively.

This novel pulsed fiber laser has high output power and outstanding temporal and spatial properties. The integrated advantages make it an attractive source for enhancing the conversion efficiency of nonlinear frequency conversion or extremely long-distance range finding and navigation.