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We demonstrate a high-power, high signal-to-noise ratio single-frequency Brillouin all-fiber laser with high slope efficiency at 1 μm wavelength. The laser is pumped by an amplified single-longitudinal-mode distributed Bragg reflector fiber laser with a linewidth of 33 kHz. By optimizing the length of the Brillouin ring cavity to 10 m, stable single-frequency Brillouin fiber laser is obtained with 3 kHz linewidth owing to the linewidth narrowing effect. At the launched pump power of 2.15 W, the Brillouin fiber laser generates maximum output power of 1.4 W with a slope efficiency of 79% and the optical signal-to-noise ratio of 77 dB.
© 2015 Optical Society of America
1. Introduction
High optical signal-to-noise ratio (OSNR) single-frequency fiber lasers with high output power are of huge interest because of their potential applications, such as high-precision sensing, coherent detection, and laser LIDAR [1–3]. The conventional methods of achieving single-frequency fiber lasers mainly include distributed Bragg reflector (DBR) and distributed-feedback (DFB) configuration, both approaches normally being limited to output power of a few tens of milliwatt and linewidth in the tens of kHz range. Using different rare earth ions doped fibers, single-frequency DBR and DFB fiber lasers have been broadly explored at wavelength ranges around 1 μm, 1.5 μm, and 2 μm [4–6]. Guan et al. reported the realization of a single-polarization, single-frequency Yb-doped DBR fiber laser with 35 mW output power [4]. Bernier et al. demonstrated an all-fiber DFB laser operating at 2.8 μm with 12 mW output power and less than 20 kHz linewidth [6]. However, the direct output power of these lasers is relatively low. By using a master-oscillator power-amplifier (MOPA) scheme, the output power can be increased [7,8], with the drawback that the amplified spontaneous emission (ASE) noise maybe degrade the OSNR and broaden the laser linewidth. In addition, achieving a high output power is hindered by backward stimulated Brillouin scattering (SBS). However, by utilizing the narrow gain bandwidth of SBS [9,10], the Brillouin fiber laser (BFL) can realize high-power, high OSNR single-frequency operation with a narrow laser linewidth.
The SBS process can be described classically as a nonlinear interaction between the pump and Stokes wave mediated by the acoustic wave. Due to the stronger damping of the acoustic field relative to the optical field, the noise of the BFL is greatly suppressed compared to the pump laser noise [11,12]. Additionally, due to the linewidth narrowing effect [13], the Stokes wave has a very narrow linewidth, potentially several orders of magnitude narrower than the pump wave. A BFL achieving a few kHz linewidth has been demonstrated for ultrahigh resolution spectral analysis of continuous optical wave sources [14].
In the past several years, single-longitudinal-mode (SLM) BFLs based on single-mode fibers (SMF) [15], highly nonlinear fibers (HNLF) [16], as well as chalcogenide fibers [17,18] have been widely explored. Guan et al. reported a single-frequency 1 μm hybrid Brillouin/ytterbium fiber laser with the output power of 1 W and an OSNR of greater than 55dB [19]. However, hybrid Brillouin fiber lasers include active medium in the cavity, inevitably introducing ASE noise from the active fiber. This degrades the laser’s OSNR and broadens its linewidth. Unlike hybrid BFLs, BFLs without active fiber in the cavity have a higher OSNR and a narrower linewidth. Wang et al. reported a high-power BFL with output power of 1.04 W using a single-pass Brillouin ring cavity configuration at 1.5 μm wavelength [20]. The OSNR was 75 dB and the measured linewidth was less than 6 kHz. Another paper studied a high OSNR, single-frequency 2 μm BFL, which achieved 1.08 W output power, 62 dB OSNR, and 8 kHz linewidth [21].
In this paper, we report a stable high-power, high OSNR single-frequency 1 μm Brillouin all-fiber laser without active fiber in the cavity. The laser directly generates 1.4 W output power at the launched pump power of 2.15 W. The slope efficiency of the BFL is 79% and the OSNR is 77 dB. By optimizing the length of the Brillouin ring cavity to 10 m, stable single-frequency Brillouin fiber laser is obtained with the linewidth of 3 kHz owing to the linewidth narrowing effect.
2. Experimental setup
The setup of the single-frequency Brillouin fiber laser is illustrated in Fig. 1, which consists of a high-power single-frequency fiber laser and a single-pass Brillouin ring cavity. The high-power single-frequency fiber laser comprises a continuous-wave (CW) Yb-doped single-frequency fiber laser as a seed source and one-stage Yb-doped all-fiber amplifier, as shown in Fig. 1(a). The seed source is a home-made short-linear-cavity (SLC) single-longitudinal-mode DBR fiber laser with an output power of 35 mW and a linewidth of 33 kHz. In the Yb-doped fiber amplifier (YDFA), a segment of 6.4 m double-clad single-mode Yb-doped fiber (YDF) with a 10/130 μm core/cladding diameter and corresponding NA of 0.075/0.46 (Nufern, Inc., cladding absorption of ~3.9 dB/m at 975 nm) is used as gain medium. The YDF is cladding pumped by a 976 nm CW multimode laser diode (LD) with a maximum output power of 29.5 W. A (2 + 1) × 1 pump combiner is used to deliver forward pump light to the gain fiber from the LD. The output end of the YDF is fusion-spliced to a matching 10/130 μm passive fiber (0.5 m) in order to strip off residual pump laser. Subsequently, a polarization-independent isolator suppresses backward propagation of the laser. After one-stage of YDFA, the single-frequency seed laser is amplified to 2.61 W (limited by the damage threshold of the optical isolator). The amplified laser is then split into two parts with a 99/1 fiber coupler. The main output with 2.58 W output power is then clockwise injected into the Brillouin ring cavity by an optical circulator. Taking into account the loss of the circulator, only 2.15 W output laser serves as the launched pump laser for the BFL.
Fig. 1 Experimental setup of the all-fiber single-frequency Brillouin fiber laser. ISO: isolator, LD: laser diode, YDF: Yb-doped fiber, OC: optical circulator, PC: polarization controller.
The Brillouin fiber ring cavity consists of an optical three-port circulator, a polarization controller (PC), and a 70/30 optical coupler, as shown in Fig. 1(b). The launched pump laser passes through the cavity in one roundtrip, and there is no resonant condition for the pump. The Brillouin Stokes laser propagates counterclockwise in the cavity, and forms the resonance for efficient lasing. At the 70/30 optical coupler, 70% of the Brillouin Stokes laser is coupled out of the cavity. The PC is used to change the state of polarization to adjust the threshold of the SBS and the output power of the BFL. The length of the Brillouin single-pass ring cavity is optimized to 10 m by changing the length of the 1060-XP fiber, corresponding to a free spectral range (FSR) of 20 MHz. This ensures that the laser is operating in a stable SLM due to the narrow Brillouin gain bandwidth (~32 MHz at 1063 nm).
3. Experimental results and discussion
The output power of the BFL monitored by a power meter is depicted in Fig. 2(a) as a function of the launched pump power. The intra-cavity lasing threshold is determined to be 373 mW. We estimate the theoretical lasing threshold of the resonant BFL, which is given by [20]:
Here, denotes the fraction of the Stokes power fed back after each roundtrip, is the peak value of the Brillouin gain, and are the effective length and mode area of the fiber, respectively, and = 1.5 dB/km is the attenuation constant of the fiber. Thus, we obtain mW for the theoretical lasing threshold. In an all-fiber ring resonator, the intra-cavity pump power is enhanced over the pump power input to the circulator by the factor [9], where is the fractional coupler power loss, and is the resonant coupling constant. The fiber power transmission for a ring resonator of perimeter L is given by . Thus, the theoretical threshold in the Brillouin single-pass cavity equals 346.4 mW, which corresponds well to the result of our measurement (373 mW).Fig. 2 (a) BFL output power as the function of the launched pump power; (b) Power stability measurement of the Brillouin fiber laser over 60 min.
When the pump power surmounts the threshold power, the SBS effect is initiated and the output power grows almost linearly with the increase of the pump power. The maximum output power reaches 1.4 W at 2.15 W launched pump power, and the more output power is expected if a higher power pump laser is launched. Thus, we obtain the conversion efficiency of the BFL of 68% and the slope efficiency of 79% with respect to the launched pump power (Fig. 2(a)). In addition, with the laser output power of 1.4 W, the power stability of the BFL is measured at the wavelength of 1063 nm over one hour. As shown in Fig. 2(b), the peak-to-peak power fluctuation of the BFL is about 2.9%, indicating that a stable high-power Brillouin fiber laser output is obtained.
Figure 3 depicts the output spectra of the BFL, which is measured by an optical spectral analyzer (Yokogawa, AQ6373) with a 0.02 nm resolution. The Stokes central wavelength is 1062.86 nm, upshifted by 0.06 nm compared to the central wavelength of the pump laser. The OSNR of the BFL operating at 1.4 W is 77 dB, which is improved by 27 dB compared to that of the pump source due to the noise reduction of the SBS process [22]. As pump and Stokes laser propagate in opposite directions, the noise of the pump laser is naturally separated from that of the Stokes laser. Meanwhile, A. Debut et al. observed that the phase noise of the pump wave is transferred to the emitted Stokes wave after being strongly reduced under the combined influence of the acoustic damping and the cavity feedback [11].
The length of the Brillouin single-pass ring cavity is 10 m, and the Brillouin gain profile provides enough discrimination to make the laser operate in SLM. The single-frequency output is verified with a scanning Fabry-Perot interferometer (FPI) with a cavity length of 50 mm, corresponding to a FSR of 1.5 GHz. With a finesse of 200, the FPI has a resolution of 7.5 MHz. This is sufficient to resolve the cavity modes of the laser, which has a mode spacing of 20 MHz confirming. Figure 4 confirms the single-frequency operation of the setup.
Fig. 4 Laser output longitudinal mode characteristics measured by a scanning Fabry-Perot interferometer.
In order to further investigate the spectral characteristics of the laser, we analyze the linewidth of the Brillouin fiber laser. The linewidth of the Stokes wave strongly depends on the linewidth of the pump wave, and is usually narrower than that of the pump wave as a result of the linewidth narrowing effect [11,13,23]. In our experiment, the linewidths of the pump and Stokes laser are measured by the beat method [14]. The Brillouin Stokes laser is combined with the pump laser to produce a beat signal in the 15-16 GHz range, depending on the source wavelength. A 25 GHz wideband photodetector and a signal analyzer (Agilent N9030A PXA) are used to monitor the spectrum of the beat signal. Because the linewidth of the Stokes laser is much narrower than that of the pump laser, the linewidth of the pump laser can be retrieved from the width of the beat frequency.
The linewidths of Brillouin Stokes and pump laser are successfully measured using this system, as shown in Fig. 5. Firstly, we measure the beat signal of Stokes and pump laser. The beat frequency is centered at 15.80 GHz, and the linewidth of the pump laser is shown in Fig. 5(a). In the next step, the linewidth of the Stokes laser is measured by constructing another BFL, using the Brillouin Stokes as pump laser in order to generate the next order Stokes laser, and then beat them to measure the linewidth of the Brillouin Stokes laser. The beat frequency is centered at 15.81 GHz and the linewidth of the Stokes laser is depicted in Fig. 5(b). The 20-dB down linewidths retrieved from the spectrum [24] are 30 kHz and 330 kHz for the Brillouin Stokes and pump laser, respectively. This corresponds to a full-width at half-maximum (FWHM) linewidth of 3 kHz for the Stokes laser and 33 kHz for the pump laser, respectively. The linewidth of the Brillouin laser is reduced by ~11x compared to the pump laser, which indicates the narrower linewidth single-frequency laser operation.
Fig. 5 Spectrum of the beat frequency for the linewidth measurements of (a) pump laser and (b) Brillouin Stokes laser.
Theoretically, the linewidth of a Brillouin fiber laser can be estimated via the following equations [11]:
Here and are the linewidths of the Stokes and pump wave, respectively. Furthermore, and , represent the damping rate of the acoustic wave and cavity loss rate, respectively, with denoting the FWHM of the Brillouin gain curve. is the speed of light in the fiber of length , and is the amplitude feedback parameter characterizing the ring cavity. represents the combined influence of the acoustic damping and of the cavity feedback. Using the values of our experiment, is calculated to be 19.2, and can be estimated as 1.72 kHz at a pump linewidth of 33 kHz. The theoretical estimate of the Stokes linewidth is smaller than that of our measurement (3 kHz). This could be due to temperature fluctuations and acoustic noise, broadening the experimentally derived linewidth. Likewise, also the polarization controller may suffer from environmental instabilities, resulting in the laser that is sensitive to mechanical perturbations and temperature variation. One effective method to reduce this instability would be to use polarization-maintaining (PM) fiber instead of the PC and single-mode fiber [14,25]. We also expect a BFL with narrower linewidth can be realized when further methods to improve stability are be applied.4. Conclusion
We have demonstrated a high-power, high OSNR single-frequency 1 μm Brillouin all-fiber laser, which pumped with an amplified single-longitudinal-mode DBR fiber laser with the linewidth of 33 kHz. The laser generates a maximum output power of 1.4 W at launched pump power of 2.15 W with a slope efficiency of 79%. Furthermore, the Brillouin fiber laser features a high OSNR of 77 dB and a narrow linewidth of 3 kHz. The scheme turns out to be appropriate for high power single-frequency fiber lasers as a result of the simplicity and stability.
Acknowledgments
The authors acknowledge the financial support from the National Natural Science Foundation of China (NSFC) (Nos. 61235010, 61177048 and 61307054), and the Beijing University of Technology, China.
References and links
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