1 Introduction

In recent years, there has been growing interest in single-frequency linearly polarized high-power pulse lasers driven by a range of applications, such as optical parametric oscillators (OPO), coherent beam combining (CBC), spectroscopy, communication, and lidar. Same applications require the lasers working at 1,083 nm. For example, 1,083 nm laser sources were designed for helium optical pumping [1, 2], and the pulse laser of a temperature lidar was also working at 1,083 nm [3]. Using a single-frequency 1,083 nm wavelength laser diode (LD) as the booster, an 1.2 W continuous wave (CW) laser was sent out from a Yb-doped fiber (YDF) amplifier by P. Cancio et. al [4], and S. Huang et al. [5] obtained single-frequency 1,083 nm CW fiber laser by employing loop-mirror filter and polarization controller (PC) in their linear laser cavity. It is possible to achieve a single-frequency pulsed laser seed by means of Q-switching [6, 7] and external modulation [8, 9], etc.

Master-oscillator power amplifier (MOPA) architecture [10–12], which typically consists of a low or medium power master-oscillator followed by high-power fiber amplifiers, seems to be an attractive solution to high-power output and becomes active areas of research. A single-mode single-frequency pulsed fiber laser with an average power of 1.08 W at a repetition rate of 20 kHz was delivered from a MOPA architecture [13], and a kW-peak-power single-frequency pulses have been demonstrated in a Tm-doped fiber MOPA system, the average power is about 200 mW [14].

In this article, an all fiber 1,083 nm single-frequency linearly polarized YDF MOPA laser with 61.6 W average power is reported. A home-made single-frequency CW laser is connected to pass two pre-amplifiers and a PM isolator (ISO) and then transmits a single-frequency linearly polarized power of 50 mW. The 1.5 mW pulse seed, which is generated by modulating the CW laser using an electro-optic modulator (EOM), is amplified to 61.6 W using three-stage amplifier. In this way, single-frequency linearly polarized pulse laser with pulse energy of 6.16 μJ, repetition rate of 10 MHz, and duration time of 16 ns is obtained.

2 Experimental setup

The MOPA experimental setup is illustrated in Fig. 1. It comprises a pulse laser seed and three-stage amplifiers. The first PM ISO is used to transmit a linearly polarized laser, and more ISOs are used to prevent back-reflected light such as amplified spontaneous emission (ASE).

Fig. 1
figure 1

Schematic of pulsed fiber laser

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The pulse seed laser is obtained by means of external modulation. A 10 mW single-frequency CW laser is delivered from a ring cavity. A single-mode YDF, which is pumped by a 976 nm LD through a wavelength division multiplexer (WDM), is used as a gain medium. The SM YDF has a core diameter of 6 μm and a length of 25 cm (Core Absorption @ 975 nm: 250 dB/m).The fiber Bragg grating (FBG), which has 99 % reflectivity at 1,083 nm, coarsely selects the wavelength of the laser, and the saturable absorber (SA) forms a tracking filter with narrower bandwidth. The SA is an un-pumped YDF with the same parameters as the gain medium and a length of 1.5 m [15]. PCs are used to optimize the polarization coupling for a round-trip in the cavity and suppress-mode hopping. Stable single-frequency operation is obtained by the cooperation of the SA and PCs [16]. A 20/80 SM coupler is used for the output of the ring laser. The laser delivered from the 20 % port of the coupler is first amplified to 100 mW using two SM YDF pre-amplifiers pumped by 976 nm LDs through WDMs. The SM YDF has a core diameter of 6 μm and a length of 1 m (Core Absorption @ 975 nm: 250 dB/m). The first PM ISO, which is an important element in this system, enhances the polarization extinction of the laser and transmits about a 50 mW linearly polarized laser. Then, a EOM drove by a function generator (FG) modulates the CW laser to generate pulse laser with average power of 1.5 mW and repetition of 10 MHz.

The pulses are first amplified to about 50 mW of average power with a commercial PM amplifier (IPG PM AMP). The pulse laser is then amplified to about 1.5 W by the second-stage amplifier, which bases on a PM large mold area double clad YDF (PM LMA YDF) with core diameter of 15 μm, cladding diameter of 130 μm (NA = 0.46) and length of 5 m (Cladding Absorption @ 975 nm: 6.0 dB/m). The PM LMA YDF is clad pumped by a double-clad fiber pigtailed 976 nm LD via a PM (1 + 1) × 1 signal/pump combiner.

The third-stage amplifier, whose gain fiber is also a PM LMA YDF with core diameter of 25 μm, cladding diameter of 250 μm (NA >0.46) and length of 3 m (Cladding Absorption @ 975 nm : 6.6 dB/m), is the main amplifier. Two 40 W double clad fiber pigtailed 976 nm LDs are used to pump the gain fiber through a PM (6 + 1) × 1 signal/pump combiner. A detector being connected to one port of the combiner is used to monitor the nonlinear increasing of backscattering light, which is always induced by nonlinear effects such as simulation brillouin scattering (SBS). Devices can be protected by turning off electrical source of LDs when nonlinear increasing of the backscattering is observed. A ~ 8° angle fiber end cap is arranged at the output port to avoid signal feedback and prevent end facet damage.

3 Experimental result and discussion

The single-frequency oscillation in the ring laser is sensitive to the polarization state of the laser wave. A stable single-frequency oscillation without mode hopping is constructed by slowly changing the state of the two PCs. Figure 2 shows spectrum of the single-frequency operation CW/pulse laser detected by a scanning Fabry–Perot interferometer over the free spectral range (FSR) of 4 GHz. One can find that the spectra of the pulse laser broaden because of modulation [17].

Fig. 2
figure 2

Spectrum of the CW laser and pulse seed over a Fabry–Perot interferometer

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The duration and repetition rate of the pulse laser seed can be changed by changing the EOM’s signal generated from the FG, however, the average power of the seed changes at the same time. In order to provide enough power for the amplifiers, the repetition rate is set to be 10 MHz. Figure 3 is the pulse shape of the pulse laser seed. There have a great deal of ringing in the pulses, and the reason is that the FG has a relative low-bandwidth and cannot provide smooth pulse signals with 10 MHz repetition rate.

Fig. 3
figure 3

Pulse shape of the pulse laser seed

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It is well-known, relatively low-seed power will cause saturation effect and ASE, and this is one of the reasons why high-power MOPA laser always contains lots of amplifier stages [18]. In our experiment, Two stages before the main amplifier are used to amplify the average power from 1.5 to 50 mW, and then to 1.5 W. The 1.5 W pulse laser is used as the signal of the main amplifier. The output power of the main amplifier increases monotonously with the increasing of pump power. However, the backscattering power increases at the same time. Figure 4a shows the amplified output power with the pump power. A maximum power of 61.6 W is obtained when pump power is 78.3 W. The overall slope efficiency is 74.3 %. Figure 4 b shows the increasing of the backscattering power. No nonlinear increasing is observed. The nonlinear increasing is always caused by SBS, which has the lowest threshold among nonlinear effects in the single-frequency fiber amplifiers [19]. It can be concluded that the power is only limited by the available LDs for the time-being. Further scaling the laser power can be obtained by increasing the pump power.

Fig. 4
figure 4

Output power (a) and backscattering power (b) with pump power in the main amplifier

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The optical spectrum of the main amplifier is measured using an optical spectrum analyzer (86142B produced by Agilent) with spectral resolution of 0.06 nm. No ASE is observed when the pump power is low. However, it appears as continue to increase pump power of the amplifier. Figure 5a shows the spectrum from 950 nm to 1,150 nm when output power is 61.6 W. One can find that ASE induced optical-signal-to-noise ratio (OSNR) is better than 23 dB. Figure 5a shows the spectrum from 1,082 to 1,084 nm with different pump power, the spectrum almost does not broad as pump power increases. It is revealed that no non-linear effect such as the self-phase modulation occurs.

Fig. 5
figure 5

Spectrum of the main amplifier. a Large scale when output power is 61.6 W. b Small-scale with different pump power

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The pulse shape of the fiber amplifier is shown in Fig. 6. The duration time is about 16 ns, which is almost the same as the pulse laser seed. The pulse energy is about 61.6 μJ. The output power can be enhanced by increasing the number of LDs in main amplifier. When the pump power is sufficient, increasing the stages of the amplifier can support higher signal power and compress the saturation effect and ASE.

Fig. 6
figure 6

Pulse shape of the output laser and the seed

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4 Conclusion

In conclusion, we have demonstrated an single-frequency linearly polarized 1,083 nm all fiber pulse laser with the 10 MHz repetition rate, 16 ns pulse duration, 61.6 W average power, and 61.6 μJ pulse energy. The nonlinear increase of the backscattering power, which is caused by nonlinear effects such as SBS, is not observed. Higher output power can be obtained by increasing the LDs of the main amplifier, as our system is not SBS threshold limited.