1. Introduction

Over recent years, single-frequency fiber lasers have been recognized as an attractive laser source with narrow linewidth, low noise and compact all-fiber structure, all of which make this type of laser competetive in many applications such as remote optical sensing, coherent beam combining, and spectroscopy [1–3 ]. For example, coherent LIDAR is a reliable and accurate apparatus in measurement of atmosphere, especially the thermal structure in thermosphere, which is important for the research on space weather and long-distance wireless communication [4,5 ]. To implement such a system, a probing signal operating at 1083 nm is indispensable [6,7 ]. In addition, the coherent LIDAR system can be supported with strong anti-interference capability and high-precision measurement when the probing signal is operated in linearly frequency-modulated single-frequency regime [8,9 ]. Moreover, a coherent LIDAR operated in pulsed mode is more robust for long range measurement as considering significant attenuation in atmosphere.

Due to the uncertainty of interactive effects between different modulating devices in a single laser cavity, it is rather complicated to realize single-frequency lasing with pulsing and linearly frequency-modulating performance simultaneously. The traditional way trends to obtain linearly frequency-modulating and pulsing operation of single-frequency fiber laser separately. With regard to the pulsing operation, the Q-switching is an attractive method to obtain narrow pulse width and considerably high power in output pulse [10,11 ]. Besides, to utilize piezoelectric tranducer (PZT) is an effective method to achieve frequency-modulating operation in a laser cavity. Chen et al have proposed a fast-tuning compact Brillouin fiber laser by employing a PZT into the fiber ring cavity and realized a frequency-tuning range of ~60 MHz [12]. But the frequency excursion is limited due to the relative narrow Brillouin gain bandwidth, moreover, the pulsing operation haven’t been adressed in the experiment. In our previous work, a frequency-modulated Q-switched fiber laser with single-frequency performance was reported [13]. However, linearly frequency-modulating performance is difficult to achieve as the complex effects on the properties of fiber Bragg grating (FBG) by PZT modulation, i.e., the uncertainty varies of center wavelength, bandwidth and reflectivity. Consequently, a robust single-frequency Q-switched fiber laser operated with linearly and widely frequency-modulating performance is demanded in terms of intergration and application.

In this letter, we demonstrate a linearly frequency-modulated Q-switched ring fiber laser with single-frequency operation on the basis of an all-fiber-structure acousto-optic modulator (AOM) and injection seeding from a single-longitudinal-mode (SLM) short linear cavity, which has been realized in our previously published works [14–17 ]. By modulating the PZT clamped onto the middle of the ultra-short cavity, a linearly and widely frequency-modulating performance of the ring cavity is achieved after injection seeding. In our experiment, over 1.05-GHz frequency-tuning range is obtained and the output pulse has a maximum peak power of > 3.83 W.

2. Experimental setup

The schematic drawing of the laser is shown in Fig. 1 . A 3-m-long Yb³⁺-doped silicon fiber (YSF) is used as the gain fiber and pumped by a single-mode 976-nm laser diode (LD 1) thought a 980/1083 nm wavelength division multiplexer (WDM 1). The output port of the WDM 1 is coupled to an isolator (ISO 1) employing to force the laser to propagate in one direction and thus avoid the standing wave formation. The other end of the ISO 1 is connected to a 10/90 coupler which is employed to split 10% of laser power as the output laser, while the remainder is coupled to a band pass filter (BPF) centered near 1083 nm with 8-nm bandwidth to stabilize the cavity and suppress the amplified spontaneous emission (ASE). A fiber coupled AOM (T-M300-0.1C2G-3-F2P Gooch & Housego Ltd.) which has an extinction ratio of 50 dB and a rise time of 6 ns is utilized to be the Q-switcher in the cavity and is trigged by a pulsed wave from a function generator. And the AOM is connected to the YSF via another 50:50 coupler to obtain a closed ring cavity.

 

Fig. 1 Schematic drawing of linearly frequency-modulated pulsed single-frequency fiber laser at 1083 nm. NB-FBG: narrow band fiber Bragg grating, WB-FBG: wide band fiber Bragg grating.

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The ultra-short cavity is constructed by using two FBGs which are fused splicing with a 1.8-cm-long Yb³⁺-doped phosphate fiber and also pumped by the LD 2 through the WDM 2. The ISO 2 is used to avoid the detrimentally reflected laser from the output fiber. The SLM short-linear-cavity fiber laser with an output power of 10 mW is injected into the ring cavity through the 50:50 coupler to realize single-frequency laser output in the ring cavity. To achieve frequency-modulating performance, a PZT clamped on the middle of the ultra-short cavity is driven by a modulating signal to obtain an elongation and then cause a significant frequency excursion in the injection-seeding laser [15]. Moreover, as frequency excursion is proportional to longitudinal strain of the cavity, a linearly frequency-tuning response can be easily achieved. The modulating signal is supplied by a function generator and then amplified before driving the PZT. By injecting the frequency-modulated laser to the Q-switched ring cavity, the output pulsed laser with a frequency-modulating performance could be realized.

3. Results and discussion

The frequency-tuning range of the output laser versus modulating frequency is shown in Fig. 2(a) . The applied modulating signal was a triangle wave with an amplitude of 20 V. The frequency-modulating performance of the laser was measured by a scanning Fabry-Parot interferometer with a resolution of 7.5 MHz and a free spectral range of 1.5 GHz. From Fig. 2(a), the frequency response of the laser is approximately flat when the modulating frequency is < 1 kHz whilst decreases with the enhancement of the modulating frequency owing to the capacitance properties of the PZT. It is observed that the maximum tuning range is up to 1.05 GHz at the modulating frequency of 500 Hz and the maximum modulating frequency reaches 50 kHz with a tuning range of > 250 MHz. Figure 2(b) shows the frequency-tuning response corresponding to the applied voltage at modulating frequencies of 500 Hz, 1 kHz, 5 kHz and 10 kHz, respectively. The frequency-tuning range has a roughly linear enhancement with the increasing of the applied voltage, and indicates a good proportional property in the process of frequency modulation. It is believed that a wider tuning range of the laser can be realized by further increasing the applied voltage.

 

Fig. 2 (a) Frequency-tuning range versus modulating frequency of the PZT with a modulating signal voltage of 20 V. (b) Frequency-tuning response against the applied voltage at four different modulating frequencies

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The spectra of the output laser from the ring cavity with and without injection are shown in Fig. 3(a) , where the pump power in the ring cavity is 350 mW. The spectra were measured by an optical spectrum analyzer (OSA) with resolution of 0.02 nm and span of 15 nm. From Fig. 3(a), it can be observed that the central wavelength of the injection seeded ring cavity laser is 1083.19 nm, which is 3.72 nm longer than that of the laser without seeding. Besides, the spectrum of the laser without injection seeding has a ~8-nm span of multiple peaks which indicates that the laser operates in multi-longitudinal-mode regime. After injection of the single-frequency fiber laser, the peaks were completely suppressed and an optical signal-to-noise ratio (OSNR) of > 58.4 dB was measured. Figure 3(b) shows the single-frequency performance of the ring cavity with injection seeding, and SLM operation was confirmed.

 

Fig. 3 (a) Spectra of output laser with and without injection seeding. (b) Single-frequency performance of the injection seeded ring laser measured by FPI.

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The parameters of output pulse are also very important for applications. A maximum tunable range of the repetition rates from 1 kHz to 40 kHz was achieved. Figure 4(a) demonstrates the average power and peak power of the laser pulse at different repetition rates under the pump power of 350 mW while the modulating signal was applied with a modulating frequency of 500 Hz and an amplitude of 20 V. The average power increases with the enhancement of repetition rate, and after the repetition rate of 32 kHz, it reaches saturation, which results from the saturation of usable inversion at high repetition rate. And a maximum average power reached 2.18 mW at the repetition rate of 32 kHz. The peak power of the output laser exhibits a monotonously decrease trend with the increasing of the repetition rate, and it indicates less accumulated energy at higher repetition rate. And the maximum peak power of 3.83 W was obtained at the repetition rate of 1 kHz. In the experiment, the narrowest pulse width of 304 ns was also realized at the repetition rate of 1 kHz, as shown in the inset of Fig. 4(a). It can be observed a multi-peak structure on the pulse shape, this characteristic is attributed to the very short rise time of AOM (6 ns), which breaks the shape of single pulse. And it is believed that an output pulse with a smoothly single-peak structure could be generated by utilizing an AOM with a long enough rise time [18]. In addition, the trace of the Q-switched pulse trains and the corresponding driving signals of the AOM at repetition rate of 1 kHz are shown in Fig. 4(b), no parasitic or multiple optical pulse is observed during the frequency-modulating process. It indicates that the stability of the output optical pulse suffers from no deterioration with a widely frequency-tuning range obtained after injection-seeding.

 

Fig. 4 (a) Average power and peak power of the output pulse versus repetition rate with a fixed pump power. Inset: Pulsing shape of the laser at repetition rate of 1 kHz. (b) Trace of the pulse trains (red) and the corresponding driving signals (blue) of AOM at repetition rate of 1 kHz.

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4. Conclusion

In conclusion, an actively Q-switched ring cavity laser at 1083.19 nm has been presented to obtain a linearly frequency-modulated single-frequency laser pulse based on the injection of a frequency-modulated SLM ultra-short laser and the modulation of a fiber coupled AOM. The achieved OSNR was more than 58.4 dB. Over 1.05-GHz maximum frequency excursion and the maximum modulating frequency of > 50 kHz were obtained. By utilizing the AOM, the pulsing operation was realized with the maximum peak power of 3.83 W at the repetition rate of 1 kHz. Besides, no instability was observed in the output pulse of the ring cavity while the frequency-modulating signal was implemented.