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A narrow-linewidth Q-switched random fiber laser (RFL) based on a half-opened cavity, which is realized by narrow-linewidth fiber Bragg grating (FBG) and a section of 3 km passive fiber, has been proposed and experimentally investigated. The narrow-linewidth lasing is generated by the spectral filtering of three FBGs with linewidth of 1.21 nm, 0.56 nm, and 0.12 nm, respectively. The Q switching of the distributed cavity is achieved by placing an acousto-optical modulator (AOM) between the FBG and the passive fiber. The maximal output powers of the narrow-linewidth RFLs with the three different FBGs are 0.54 W, 0.27 W, and 0.08 W, respectively. Furthermore, the repetition rates of the output pulses are 500 kHz, and the pulse durations are about 500 ns. The corresponding pulse energies are about 1.08 μJ, 0.54 μJ, and 0.16 μJ, accordingly. The linewidth of FBG can influence the output characteristics in full scale. The narrower the FBG, the higher the pump threshold; the lower the output power at the same pump level, the more serious the linewidth broadening; and thus the higher the proportion of the CW-ground exists in the output pulse trains. Thanks to the assistance of the band-pass filter (BPF), the proportion of the CW-ground of narrow-linewidth Q-switched RFL under the relative high-pump—low-output condition can be reduced effectively. The experimental results indicate that it is challenging to demonstrate a narrow-linewidth Q-switched RFL with high quality output. But further power scaling and linewidth narrowing is possible in the case of operating parameters, optimization efforts, and a more powerful pump source. To the best of our knowledge, this is the first demonstration of narrow-linewidth generation in a Q-switched RFL.
© 2016 Optical Society of America
1. Introduction
Random fiber lasers (RFLs) have drawn a great deal of attention for their attractive features, such as mode-less, mirror-less, low coherence length and simplicity of structure [1,2]. Hence, the RFLs based on the extremely weak random scattering in a piece of fiber are of great importance in many applications, such as medical and telecom applications, illumination sources, sensing technology and spectroscopic monitoring [3–5]. Recently, intensity investigations of RFL have been reported on the output characteristic of high power [6–8], spectral tunable [9], multi-wavelength operation [10], and so on [11].
Especially, most of the reported RFLs operate in continuous-wave (CW) mode, and the RFLs can also provide pulsing outputs, which have a wide range of potential applications, such as sources for communication and interrogating fiber optic sensors [11]. Unlike traditional rare-earth (RE) doped fiber lasers and Raman fiber lasers with common resonant cavity, in which pulsing operation can be obtained via well-known routes, such as mode-locking and spatial interference, pulsing operation of RFL has fundamental scientific challenge as they are cavity less [12,13]. Recently, many efforts have been carried out to obtain pulsing operation of RFL via internal modulation or Q switching. M. Bravo et al. [12] demonstrated an internal modulated RFL by utilizing an electro-optical modulator (EOM), and no distortion of the modulating frequency or self-mode locking effects were measured. But the maximal output power is just about 0.05 mW. Y. L. Tang et al. [14] and S. M. Wang et al. [15] reported stimulated Brillouin scattering (SBS) assisted passive Q-switched RFL based on thulium-doped fiber and erbium-doped fiber, respectively. The repetition rates of output pulses were typically on the kilohertz scale, nevertheless the controlling of pulse durations were difficult. Comparing with the passive Q switching, the advantage of active Q switching is the easy control of the pulse repetition rate, and consequently, the pulse duration [16]. A. G. Kuznetsov et al. [16] presented an actively Q-switched RFL with a repetition range of about 30 kHz and pulse duration of 1-2μs. Furthermore, some other schemes are also employed to obtain pulsing operation of RFL. B. C. Yao et al. [13] demonstrated a graphene based 900 ps and widely-tunable pulse generation, with up to 41 dB polarization extinction and about milliwatt level output power for random fiber lasers. H. Wu et al. [17] proposed and experimentally constructed a polarization-modulated RFL by inserting a polarization switch (PSW) in the loop mirror with a repetition rate of 20 Hz, maximum output power of about 120 mW and extinction ratio of about 15 dB. However, the laser radiation of most of the reported non-filtered pulsing RFL is rather broad having a typical spectral width of several nanometers, and almost no control over the spectral properties of the emission. The narrow-linewidth RFL could be a good candidate to investigate temporal and statistical properties of RFL [18]. The narrow-linewidth generation from RFL can be obtained by means of spectral filter, such as fiber Bragg gratting (FBG), Fabry-Perot filter, high finesse narrow-band Fabry-Perot interferometer (FPI), and so on [18–22]. And the applications of these spectral filtering units to Q-switched RFLs are significant.
In this manuscript, we propose and experimentally explore a narrow-linewidth Q-switched RFL. The challenges of the demonstration of narrow-linewidth Q-switched RFL are analyzed, the influences of linewidth of FBG on the output characteristics are investigated, and the optimization of the laser scheme is attempted. The proposed scheme is a simple all-fiber setup that employs a combination of narrow-linewidth FBG as the spectrum filter and acousto-optical modulator (AOM) as the Q-value modulator, accordingly. This actively spectral and temporal managing scheme allows us to control the temporal and spectral properties of output light more feasible and conveniently. To the best of our knowledge, this is the first demonstration of narrow-linewidth generation in a Q-switched RFL.
2. Experimental setup
The experimental setup of the narrow-linewidth Q-switched RFL system is plotted in Fig. 1. To decrease the threshold of the RFL, half-opened cavity [6,23] is employed, which is realized by narrow-linewidth FBG and a section of 3 km long passive fiber (G. 652, few-mode fiber with core diameter of 8.2 µm and numerical aperture of 0.14). The passive fiber provides Raman gain and random distributed feedback via backward Rayleigh scattering. Narrow-linewidth lasing output can be obtained by the FBG with the selection of broad-linewidth radiation section of initial spontaneous Raman emission. The residual pump laser and broadband initial spontaneous Raman emission is leaked from the other port of FBG. For comparison, three FBGs centered at about 1080 nm with different linewidth (1.21 nm, 0.56 nm and 0.12nm) are utilized to investigate the difference of output characteristics induced by the linewidth factor. The Q-value of the random laser is modulated by an AOM. As the insertion loss of the AOM for single passion is as high as 2.87 dB for operating wavelength, the AOM is not inserted between the FBG and passive fiber directly. Two fiber circulators (Cir 1 and 2) are utilized and the AOM is fused between the third port of circulator 1 and first port of circulator 2. The broad-linewidth initial spontaneous Raman emission from the passive fiber can deliver to the FBG and narrow-linewidth feedback from the FBG can inject into the passive fiber via the two circulators. The total insertion loss induced by the two circulators is 1.48 dB. The pump laser we employed is a homemade main-oscillator-power-amplifier (MOPA) structured fiber laser centered at 1032.8 nm with maximal output power of 5.95 W. The pump light is injected into the passive fiber via the first port of fiber circulator 3 (Cir 3). And the narrow-linewidth pulsed random laser is coupled out from the random laser cavity via the third port of circulator 3. A selectable band-pass filter (BPF) centered at operation wavelength with a stop bandwidth of 11.50 nm is fused after the third port of circulator to purify the output spectrum. To suppress the point reflection, the end facets of the output port of FBG and BPF are angle cleaved with 8°.
3. Experimental results and discussions
3.1 Narrow-linewidth Q-switched operation without BPF
Firstly, the performances of the regular half-opened Q-switched RFL without BPF are investigated. By changing the Q-factor of the distributed cavity via the AOM, which is driven by rectangular electric pulse with 500 kHz repetition rate and 1 μs pulse duration, narrow-linewidth pulsed lasing can be obtained. The maximal output powers of the narrow-linewidth Q-switched RFL with three different linewidth are 0.54 W, 0.27 W and 0.12 W, respectively. And the maximal output powers are limited by the available pump power. The output spectra at maximum output powers are measured by a spectrum analyzer with 0.02 nm optical resolution and depicted in Fig. 2(a). The full width at half-maximum (FWHM) linewidths are about 1.13 nm, 0.67 nm and 0.16 nm, respectively. The optical signal-to-noise ratio (OSNR) of the output light is limited by the spontaneous Raman emission around central wavelength for this narrow-linewidth Q-switched RFL, and the OSNR are 20.55 dB, 19.53 dB and 17.06 dB, respectively. The broad-linewidth spontaneous Raman emission covering 1060~1090 nm wavelength range can be induced by the backward spontaneous Raman emission of the pump laser and the distributed reflection of the forward spontaneous Raman emission as the 3 km passive fiber can active as distributed mirror [24]. What’s more, the optical peak of residual pumping light for the output are about 24 dB lower than the signal light. This may be caused by the distributed reflection of the pump light by the passive fiber, and the cross talk between the ports of fiber circulator. Figure 2(b) shows the output pulses of the narrow-linewidth RFL at maximal output power. The repetition rates of the output pulses are 500 kHz, which agree well with the repetition rate of the electrical driving pulse. But the pulse durations of the output pulses are about 500 ns, which is far narrower than the pulse duration of the electrical driving pulse. The narrowing of pulse duration is caused by the pump depletion [16] as the front of pulse experiences much higher amplification than the tail. As depicted in the insertion graph of Fig. 2(b), the durations of RFL with FBG3 are slightly narrower than the values of RFLs with FBG1 and FBG2 for the little difference in the pump depletion process. Additionally, small-scale spikes can be observed in the generated pulse. The potential causes mainly lie on the influence from the environment and the transferring of self-pulsing instability component in the pumping ytterbium-doped fiber laser [25,26]. What’s more, obvious CW-ground can be found in the output pulse trains, which indicates the containing of CW-component in the output light. The proportions of CW-ground for the narrow-linewidth RFL with FBG 1, 2 and 3 are 2.92%, 5.77% and 17.50%, respectively. As mentioned above, the output light consists of narrow-linewidth signal light, broad-linewidth spontaneous Raman emission around central wavelength and little pump light. For the continuous operation of pump source, the spontaneous Raman emission and pump laser in the output light always exist thus forming the CW-ground of output pulse train. And the narrower the FBG, the higher the proportion of spontaneous Raman emission and pump laser of the output light, and the stronger the CW-ground of the output pulse trains. In a sense, the quality of output pulse can be influenced by the operating linewidth directly. And the demonstration of narrow-linewidth Q-switched RFL with high quality output is full of challenge. Hence, the optimization of laser system is significant to improve the performance of narrow-linewidth Q-switched RFL.
Fig. 2 Output characteristics of the narrow-linewidth Q-switched RFL without BPF at maximal output power (a) Output spectra; (b) output pulses of RFL.
3.2 Narrow-linewidth Q-switched operation with BPF
The fiber BPF is added after the third port of circulator 3 to strip the pump laser and broad-linewidth spontaneous Raman emission from the output light. The employing of BPF can help us to confirm the above inferring of CW-ground generation in the output pulse trains and makes the method for further output performance improvement clear cut. Figure 3 shows the characteristics of the output light. Thanks to the assistance of BPF, no optical peak at pump wavelength can be observed at maximal pump power, and the broad-linewidth spontaneous Raman emission covers 1077~1087 nm wavelength range, which is much narrower than the spectra before BPF. But the optical OSNR of the output light cannot be improved for the limitation of the filtering bandwidth of BPF as the operation bandwidth is broader than the output linewidth of random lasing. Additionally, the weak fluctuation of the spectrum wings may be induced by the filtering process. For the employing of FBGs, the output spectra of the RFL with the three different FBGs at maximal power level have single spectral peak consistently, in contrast to that in the reported result in Ref [16], where dual peaks can be observed in the output spectrum at maximal pump power. Output pulse trains of the narrow-linewidth Q-switched RFL are depicted in Fig. 3(b). The repetition rates and pulse durations maintain well at maximal pump power. What’s more, the proportions of CW-ground for narrow-linewidth Q-switched RFL with the three different FBGs are only 1.42%, 2.18% and 7.97%, respectively. The values are about half of the proportions of former RFL system without BPF. This indicates that the filtration of output light can improve the performance of narrow-linewidth Q-switched RFL.
Fig. 3 Output characteristics of the narrow-linewidth Q-switched RFL with BPF at maximal output power (a) Output spectra; (b) output pulses.
Figure 4 shows the evolutions of output characteristics of the narrow-linewidth Q-switched RFL. The thresholds of RFL with FBG1, FBG2, and FBG3 are 1.24 W, 1.69 W and 2.13 W, respectively. With 5.95 W pump light injected into the passive fiber, the maximal output powers are 0.54 W, 0.27 W and 0.08 W, as depicted in Fig. 4(a). The corresponding pulse energies are about 1.08 μJ, 0.54 μJ and 0.16 μJ, accordingly. These can indicate that the radiation of pulsing random laser is challenging with the narrowing of linewidth of FBG. It may be caused by the effective feedback of the FBG. Although the reflectivities of the three FBGs are all higher than 99.9%, the reducing of FBG linewidth to relative low level will weak the feedback power as the narrow-linewidth random laser is generated from the broad-linewidth spontaneous Raman emission by the reflection of FBG. Figure 4(b) plots the evolution of linewidth versus the injected pump power, which is described by the absolute FWHM linewidth and linewidth broadening factor. Here, the factor is defined by the formula Δλout/ΔλFBG, whereΔλout andΔλFBG are the FWHM linewidth of the output light and FBG, respectively. The FWHM linewidth for the narrow-linewidth Q-switched RFL with FBG1 at maximal pump power level is about 1.13 nm and the linewidth broadening factor is about 1 during the power scaling process, which indicates the nice maintaining of narrow-linewidth characteristics. Oppositely, slightly linewidth broadening can be observed for the RFL with FBG1 and FBG2 with the enhancement of pump power. The FWHM linewidth for the RFL with FBG2 and FBG3 at maximum pump power are 0.67 nm and 0.16 nm, respectively. The corresponding linewidth broadening factors are 1.20 and 1.38, accordingly. The linewidth broadening may be lies on the nonlinear effects, such as self-phase modulation (SPM) and cross-phase modulation (XPM) [27]. What’s more, the narrower the FBG, the easier the linewidth broadening. Comparing with the obtained results in Ref [16], the values of output average power and linewidth of narrow-linewidth Q-switched RFL with FBG1 at maximal pump power are almost identical. The evolution of proportion of signal light in band (<20 dB lower than the signal peak) from the spectral integration and proportion of CW-ground from the temporal calculation (ratio of vale-peak) are depicted in Figs. 4(c) and 4(d). With the increment of pump power from thresholds to maximal pump level, the proportion of signal light of the RFL with FBG3 decreases from 79.26% to 67.92%, while the proportions of signal light of the RFL with FBG1 and FBG2 are relatively high and stable. Simultaneously, the proportion of CW-ground of the RFL with FBG3 increases obviously from 4.17% to 7.97%, while the variation of proportions of RFL with FBG1 and FBG2 are relatively small. Although the variation tendency of the proportions of signal light (integrated from output spectra) and proportions of CW-ground (calculated from pulsing trains) are almost complementary, the value of the proportion of signal light is not exactly equal to the proportion of CW-ground for the narrow-linewidth Q-switched RFL with special FBG at same pump level. It may be explained as follows. As the proportion of signal light is roughly calculated by integrating the light which is below the signal peak with 0~20 dB range, the sideband of the reflection shape of FBG is not considered strictly, and the feedback from the sideband of FBG with relative low reflectivity can also generate pulsing random radiation. Consequently, the proportion of pulsing composition will be underestimated by this simply spectral integration although this method can be utilized to investigate the evolution tendency of output pulse trains.
Fig. 4 Evolutions of output light as functions of pump power (a) Output power; (b) linewidth; (c) proportion of signal light; (d) proportion of CW-ground.
3.3 Discussion on the operating challenges of narrow-linewidth Q-switched RFL
The experimental results indicate that there are many operating challenges of narrow-linewidth Q-switched RFL. Firstly, the performance of narrow-linewidth Q-switched RFL can be influenced by the operating linewidth directly. For narrow-linewidth Q-switched RFL, the optical-to-optical conversion efficiency is limited at a relative level for the influence of operation loss induced by the Q-switching of AOM and spectral selection of narrow-linewidth FBG. Under this relative high-pump low-output condition, the CW components in the output light, such as spontaneous Raman emission and pump light, cannot be neglected. What’s more, the narrower the FBG, the lower the output power at the same CW pump level. In contrast, the powers of the CW components induced by the CW pump light are relative consistent at special pump level with different FBGs. Therefore, the narrower the FBG, the higher the proportion of CW-ground of the output light. The experimental results indicate that it’s full of challenge to demonstrate narrow-linewidth Q-switched RFL as the quality of output pulse can be influenced by the operating linewidth directly. Fortunately, the quality of output pulse of narrow-linewidth Q-switched RFL can be improved via spectral filtration. In our experiment, the CW-ground of the output pulse with a pump power of 5.95 W and FBG linewidth of 0.12 nm can be decreased from 17.50% to 7.97% by the assistance of BPF with a stop bandwidth of 11.50 nm. And further improvement can be realized by a BPF with narrower stop bandwidth. Secondly, the linewidth broadening of the output light is obvious for the narrow-linewidth operation. As the linewidth broadening is induced by the nonlinear effects mainly [23], the optimization of the operating parameters, such as length of passive fiber and operating power level, can weaken the linewidth broadening of narrow-linewidth Q-switched RFL. Generally speaking, the demonstration of narrow-linewidth Q-switched RFL is full of challenge, and further power scaling and linewidth narrowing is possible in case of operating parameters optimization to suppress the nonlinear effects induced linewidth broadening and accurate spectral filtration to eliminate the CW-ground of the output pulse train with the aid of narrowband BPF.
4. Summary
To conclude, we propose and experimentally construct a narrow-linewidth Q-switched RFL. Half-opened cavity, which is realized by narrow-linewidth FBG and a section of passive fiber, is employed to decrease the threshold of lasing. The narrow-linewidth radiation is provided by the filtering process of narrow-linewidth FBG, and the Q-factor modulation of the distributed cavity is realized by the AOM placed between the FBG and passive fiber. Three different FBGs with linewidth of 1.21 nm, 0.56 nm, and 0.12 nm are utilized to investigate the influence of linewidth to the output characteristics. The maximal output powers of the RFL with the three different FBGs are 0.54 W, 0.27 W and 0.08 W, respectively. The corresponding pulse energies are about 1.08 μJ, 0.54 μJ and 0.16 μJ, accordingly. Furthermore, the repetition rates of the output pulses are 500 kHz, and the pulse durations are about 500 ns. The linewidth of FBG can influence the output characteristics in full scale. The narrower the FBG, the higher the pump threshold, the lower the output power at the same pump level, the more serious the linewidth broadening, and the higher the proportion of CW-ground exist in the output pulse trains. The experimental results indicate that it’s full of challenge to demonstrate narrow-linewidth Q-switched RFL with high quality pulse output. But further power scaling and linewidth narrowing is possible in the case of operating parameters optimization efforts and more powerful pump source. To the best of our knowledge, this is the first demonstration of narrow-linewidth generation in a Q-switched RFL.
Funding
National Natural Science Foundation of China (NSFC) (61322505) and Program of China for the New Century Excellent Talents in University.
Acknowledgments
We are particularly grateful to Man Jiang, Haiyang Xu and Rongtao Su for their supports on this work.
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