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We present a high power ultra-narrowband pulsed fiber amplifier at 2 μm. A single frequency fiber laser was modulated by a phase modulator and an intensity modulator to serve as the ultra-narrowband pulsed seed laser with a bandwidth of 307 MHz. The pulsed seed laser was amplified by a monolithic Tm-doped fiber master oscillator power amplifier (MOPA). The average output power reaches 105 W with a slope efficiency of 0.41. The output pulse train has a repetition rate of 1 MHz and a pulse width of 66 ns. The output power is limited by the onset of stimulated Brillouin scattering. Higher output power can be achieved by further broadening the linewidth or narrowing the pulse width to several nanoseconds. To the best of our knowledge, this is the first demonstration on a monolithic ultra-narrowband nanosecond pulsed MOPA at 2 μm with an average power exceeding 100 W.
© 2015 Optical Society of America
1. Introduction
Lasers employing Tm-doped fibers (TDFs) have a rather wide emission bandwidth ranging from ~1700-2200 nm, and cover the absorption lines of various atmospheric gases and liquid water [1, 2]. Thus, Tm-doped fiber lasers (TDFLs) are important sources in applications such as eye safe coherent Doppler LIDAR, remote sensing, laser communication, nonlinear frequency conversion, and material processing [2–4]. In many of the above applications, pulsed lasers have unique advantages such as high peak power and high energy intensity. So researchers have paid much attention to pulsed TDFLs in recent years [5–11]. However, the output power of the pulsed TDFLs is usually comparatively low, and power amplification using Tm-doped fiber amplifier (TDFA) is needed to make the pulsed laser sources available in high power and high energy applications [12, 13]. In 2010, for example, Xu et al. reported a 120 W nanosecond pulsed TDFA with a linewidth of 0.8 nm employing bulk-optics based amplifier [14]. In fact, due to the advantages of robustness, flexible installation, and reliability, all-fiber configuration is also widely employed in pulsed TDFAs. Liu et al. reported a 120 W picosecond pulsed TDFA for supercontinuum generation employing all-fiber master oscillator power amplifier (MOPA) configuration in 2013 [15]. Sims et al. demonstrated a femotosecond pulsed TDFA with a peak power exceeding 1 MW [16], Wan et al. also reported high power and high energy femtosecond pulsed TDFAs [17, 18]. In 2014, Tang et al. demonstrated a 50 W nanosecond pulsed TDFA with a linewidth of 1.4 nm [19].
In various applications, such as nonlinear frequency conversion and coherent Doppler LIDAR, single frequency (SF) or narrow bandwidth pulsed lasers are more favorable due to their high spectral intensity and long coherent length. There are two typical methods to generate SF or narrow bandwidth pulsed fiber lasers at 2 μm. One is to directly Q-switch a short-cavity fiber laser [20, 21], and the other one is to modulate a continuous wave SF fiber laser via intensity modulator (IM) [22]. Both of the methods above need further power amplification to increase the average power, pulse energy and peak power. In 2011, Geng et al. presented a SF pulsed TDFA, the peak power reached 1 kW and the pulse width was ~7 ns [20]. In the same year, Shi et al. demonstrated a SF pulsed TDFA with a pulse energy of 220 μJ and a peak power of 2.75 kW employing Tm-doped germanate fibers [21]. In 2012, Fang et al. reported a high power and high energy monolithic SF TDFA at 2 μm based on large core germanate fibers, the peak power can reach 78.1 kW, the pulse energy can reach ~1 mJ, and the average power was 16 W with a linewidth of 277 MHz [22]. So far, the average output power of SF or ultra-narrowband pulsed TDF MOPAs is limited to ~20 W, and it is worthwhile and meaningful to further increase the output power to enhance the practicability of ultra-narrowband pulsed TDF MOPAs.
In this paper, we present a high power ultra-narrowband nanosecond pulsed TDFA based on a monolithic MOPA configuration. The linewidth of the pulsed MOPA seed is about 307 MHz, and the output pulse train’s repetition rate is 1 MHz with a pulse width of 66 ns. By using all-fiber MOPA configuration, the output power reaches 105 W with a slope efficiency of 0.41. The central wavelength locates at 1971 nm with an optical signal-to-noise ratio (OSNR) more than 45 dB. This is the first demonstration, as far as we know, on a high power ultra-narrowband nanosecond pulsed TDFA with an average output power exceeding 100 W.
2. Experimental setup
The setup of the ultra-narrowband pulsed seed is shown in Fig. 1. A SF fiber laser at 1971 nm based on ultra-short cavity was used as the seed laser, which has a linewidth of less than 100 kHz and an output power of 40 mW [23]. The SF seed laser was modulated by an electro-optic phase modulator (PM) to broaden the linewidth slightly in order to raise the SBS threshold [24]. The insertion loss of the PM is about 3 dB, the bandwidth is 100 MHz, and the half-wave voltage is 5 V. The signal on the PM was sine wave with a repetition rate of 20 MHz and an amplitude of 5 V. The signal light was then amplified to 640 mW by a TDFA, which consisted of a home-made 1550 nm fiber laser, a 1550/2000 nm wavelength division multiplexer (WDM), and a length of 3 m single cladding TDF with a core diameter of 9 μm and a cladding diameter of 125 μm. Polarization-insensitive isolators (ISOs) were used to protect the seed laser and the amplifier from backward lasing. An acousto-optic modulator with a rise-time of ~70 ns was used as the IM to modulate the continuous wave laser. The repetition rate of the seed pulse train was 1 MHz, and the pulse width was 156 ns. Both of the signals on the PM and IM were provided by a double channel digital function generator. The final output power of the pulsed seed laser after the IM was about 10 mW.
Fig. 1 Schematic sketch of the ultra-narrowband pulsed seed. ISO: isolator; PM: phase modulator; WDM: wavelength division multiplexer; TDF: Tm-doped fiber; IM: intensity modulator.
The pulsed seed laser was then pre-amplified via a three-stage TDFA, as depicts in Fig. 2. A 90/10 fiber coupler was used to extract 10% of the signal light for monitoring. In the first two stages of the pre-amplifier, 1550 nm fiber lasers, WDMs, 3 m long single cladding TDFs (core diameter of 9 μm and cladding diameter of 125 μm), polarization-insensitive ISOs, and band pass filters (BPFs), were employed. The BPFs were used to filter the amplified spontaneous emission (ASE), and the bandwidth of the BPFs is about 10 nm. The third stage of the pre-amplifier was composed of two 793 nm multimode laser diodes (LDs), a (2 + 1) × 1 signal-pump combiner, a piece of 8 m double cladding TDF (DC TDF), and a high power ISO. The cladding absorption efficiency of the DC TDF at 793 nm was about 3 dB/m. The power of the pulsed seed laser was scaled up to 110 mW, 550 mW and 6 W at the three amplifying stages, respectively.
Fig. 2 Schematic sketch of the pre-amplifier. Pump laser: 1550 nm fiber laser; WDM: wavelength division multiplexer; TDF: Tm-doped fiber; ISO: isolator; BPF: band pass filter; LD: laser diode; DC TDF: double cladding Tm-doped fiber.
Figure 3 shows the main amplifier. A high power 90/10 fiber coupler was used to monitor the forward and backward lasing. Then the signal laser was launched into the gain fiber via a (6 + 1) × 1 signal-pump combiner with matched fibers. Four commercial 793 nm multimode LDs were used as the pump lasers. The gain fiber was a piece of 2.9 m DC TDF with a core diameter of 25 μm. The peak cladding absorption efficiency of the DC TDF at 793 nm was about 9 dB/m. The unabsorbed 793 nm pump light was dumped out at the fusion spliced joint between the DC TDF and a piece of 0.5 m matched passive fiber with high refractivity gel. The output end of the passive fiber was angle cleaved with an angle of 8 degrees to reduce the feedback from Fresnel reflection. All the DC TDF and the corresponding fusion spliced joints were placed on a water-cooled conductive heat sink to remove the waste heat and protect the fiber system. The output spectrum was measured by an optical spectrum analyzer (OSA) with a resolution of 0.05 nm.
3. Experimental results and analysis
Initially, we applied no signal on the PM, so the pulsed seed can be regarded as SF pulsed laser. The pulse train’s repetition rate was 1 MHz and the pulse width was 156 ns as aforementioned, which were kept unchanged in our experiment for the convenience of contrastive investigation. The output power data of the SF pulsed laser were shown in Fig. 4. The output power reaches 37 W with a slope efficiency of 0.38. The peak power can be calculated to be about 490 W [25], and the pulse energy was about 37 μJ. Further scaling-up of the output power was stopped by the onset of SBS. The efficiency of the MOPA also decreases when the output power reaches about 35 W.
The backward power data monitored from the coupler in Fig. 3 are plotted in Fig. 5. It is shown that the backward power increases nonlinearly when the output power reaches about 35 W, which indicates that the SBS may occur. From the backward spectra at the maximum output power, we can find a small peak at 1971.54 nm, which is 0.1 nm longer than the wavelength of Rayleigh scattering of the signal laser. Thus, we can confirm that the SBS process occurs and the average output power is limited.
There are two common methods to heighten the threshold of SBS process. One is to employ pulses with width less than ~10 ns, since the Brillouin gain is substantially reduced in pump pulses with widths that are shorter than the lifetime of the acoustic phonon [26]. The other is to broaden the spectral linewidth of the pulsed laser [24]. The rise-time of the IM was too long (~70 ns) to generate pulses with width of several nanoseconds, so we broadened the linewidth of the pulsed seed laser via phase modulation. A scanning Fabry-Perot interferometer (SFPI) was used to measure the spectral linewidth of the pulsed seed laser. The finesse of the SFPI is ~300, and the free spectrum range is 1 GHz, thus the resolution of the SFPI is about 3 MHz. The measured data are shown in Fig. 6 and replotted in Fig. 7. The intensities of the two groups of peaks in one scanning period in Fig. 6 are different, which may come from the improper adjustment of the SFPI, but the results of linewidth measurement are not degraded. From the time domain profiles in the digital oscilloscope (shown in Fig. 6), one can calculate the spectral linewidth of the pulsed laser according to the proportion of the time domain widths. We can find out that the linewidth is increased from ~24 MHz to ~307 MHz via phase modulation. Although it is still an ultra-narrowband linewidth for pulsed laser, the SBS threshold may be increased.
Fig. 6 Spectral linewidth data measured by a SFPI. Top: no phase modulation signals; Bottom: phase modulation signals are on.
The power of the ultra-narrowband pulsed seed was then amplified by the monolithic TDF MOPA, and the results are shown in Fig. 8. The output power increases linearly with the launched pump power, and the maximum power reaches 105 W when the pump power was 250 W. The slope efficiency is 0.41. No power saturation or roll-over phenomenon is observed.
The backward power data are plotted in Fig. 9, which show no evidently nonlinear increase, so the SBS threshold is not reached yet. The monitored backward spectrum also shows no evidence of SBS process. However, at the maximum output power, the backward power increases slightly more, which may indicate that the SBS threshold might be approached if the output power is further scaled up. Thus, the SBS threshold of this pulsed MOPA has been improved by a factor of ~3 via phase modulation.
The spectra of the MOPA at the maximum output power are shown in Fig. 10. Neither ASE nor parasitic lasing is observed in the spectra. The central wavelength locates at 1971 nm, and the optical signal-to-noise ratio can reach more than 45 dB. Since the SFPI is fiber-coupled and can only endure several milliwatts laser power, direct measuring the linewidth of output laser after the main-amplifier is not feasible due to the lack of picking, collimating and focalizing optical devices in our team for the time being. However, we have measured the output spectrum of the signal laser via the fiber coupler after the third-stage of the pre-amplifier. The results show no evident broadening of the linewidth, except slight deformation of the spectral intensity profile. Thus, we think that the linewidth is not evidently broadened in the amplification process. Further investigation of the deformation of the spectral intensity profile will be performed in our future endeavors.
The pulse shape of the ultra-narrowband pulsed MOPA was measured by a high speed photodetector with a rise time of 25 ns and a digital oscilloscope with a bandwidth of 500 MHz. The measured data were redrawn in Fig. 11 and Fig. 12. The pulse train is stable and the fluctuation of the peak power is less than 5%. From Fig. 12 one can find out that the output pulse is narrowed to 66 ns due to the insufficient stored energy in the DC TDF. The peak power can be calculated to be 1.42 kW, and the pulse energy is about 105 μJ.
The SBS threshold of continuous wave laser can be approximately estimated using the following formula [27]:
where ΔυP is the linewidth of the pump laser, and ΔυB is the spontaneous Brillouin linewidth. For TDFA, ΔυB is ~10-13 MHz [28]. In the experiment, the phase modulation have increased the linewidth of the laser to ~307 MHz, i.e., the linewidth of the pump laser ΔυP in formula (1) is ~307 MHz, thus the SBS threshold can be increased by a factor of ~30 according to the formula. However, the discreteness of the modulated spectrum in Fig. 7 means that formula (1) might not be applied directly to estimate the SBS threshold [29–31], and comprehensive model should be setup to estimate the threshold.Furthermore, as for pulsed laser amplifier, the SBS threshold can also be influenced by the pulse width, the repetition rate and the pulse shape [32]. Thus, the SBS threshold of pulsed TDFA can be further improved by narrowing the pulse width, which can mitigate the interaction between the signal laser and the SBS light by reducing the interaction length and time [22, 33]. Meanwhile, the peak power can also be improved. Actually, when the pulse width is less than the lifetime of the acoustic phonon (usually ~10 nanoseconds), the SBS threshold will be largely increased. Although the rise-time of the AOM in our experiment is limited to ~70 ns, the pulse width can be narrowed by employing high speed electro-optic intensity modulator instead. So high power output with higher peak power and higher pulse energy can be achieved in this system, which will be our future work if the electro-optic intensity modulator is employed.
4. Conclusion
In conclusion, we present a high average power ultra-narrowband pulsed laser at 1.971 μm employing all-fiber TDF MOPA configuration. The average output power of the MOPA reaches 105 W with a slope efficiency of 0.41. The repetition rate of the pulsed laser is 1 MHz, and the output pulse width is 66 ns, corresponding to a peak power of 1.42 kW. The linewidth of the pulsed laser is modulated by a PM to ~307 MHz in order to increase the SBS threshold. Higher output power can be achieved by further scaling up the SBS threshold via using pulse with width of several nanoseconds, or increasing the linewidth of the seed laser. This is the first demonstration, as far as we known, on an ultra-narrowband nanosecond pulsed monolithic TDF MOPA with an average power exceeding 100 W.
Acknowledgment
This work was supported by the Innovation Foundation for Graduates of National University of Defense Technology (Grant No. B130704), National Natural Science Foundation of China (Grant No. 11274386) and the Program for New Century Excellent Talents in University.
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