1. Introduction

Tunable lasers have been widely used in the field of optical sensing and communication [1–8 ]. A large tuning range, fast response time and low cost are expected in practical applications. The kernel part of a tunable laser with a fast response time is a fast tunable filter, in which acousto-optic (AO) structure plays an important role. AO structures can conveniently realize a large tuning range covering the whole C band, and the response time can be smaller than 1 ms, which fulfills most practical applications. AO structures in all-fiber configurations especially based on the commercial single mode fiber (SMF) are preferred because of small insertion loss and low cost compared to on-chip ones. The first all-fiber AO structure was reported in 1986, which realized the mode switching in a piece of two-mode fiber [9]. After years of development, all-fiber AO structures have been used as band-rejection filters, band-pass filters, gain flatting of erbium-doped fiber amplifier (EDFA), and to control slow and fast light in optical fiber, and so on [10–14 ]. To construct band-pass filters which are needed in tunable lasers, most available configurations are mainly based on special fiber, such as two-mode fiber [9], high-numerical-aperture fiber [12], high-birefringent fiber [15, 16]. Configurations based on SMF have also been exploited. A band-pass filter based on a null coupler driven by the acoustic wave was demonstrated in 1994, and its validity in a tunable laser with large tuning range was proved in 1995 [17, 18]. Yet, the null-coupler needs precisely manufacturing, and it is more likely to be damaged in the process of integration with the acoustic transducer, because the heated and stretched SMF is of a very small diameter (several micrometers) and becomes very fragile. Moreover, the band-pass filter based on the null-coupler usually introduces strong side bands which are only about 3 dB smaller than the main resonance passband that one actually needs [17, 18]. Therefore, gain flattening is necessary to suppress the laser output within the side bands, when a null-coupler is exploited to construct a tunable ring laser. The problem would be even worse in another configuration based on dual AO mode coupling to construct a bandpass filter [19]. Because the passband is generated at the bottom of a relatively wide rejection band, it is not suitable for lasers of large tuning range.With an acoustic wave modulated fiber Bragg grating, a tunable ring laser was also realized in 2005 [20]. However, the tuning range was relatively small, and an additional circulator was necessary in such a configuration.

In 2012, we demonstrated an add/drop acousto-optic tunable coupler (AOTC) based on an acousto-optic tunable filter (AOTF) and a tapered fiber (TF) [21]. In 2013, the configuration of two parallel AOTFs was proposed to improve the tuning process with a uniform insertion loss [22]. Note that both structures are based on propagating flexural acoustic wave in the SMF and can serve as AO tunable band-pass filters. The same as the configuration based on a null coupler, both of the demonstrated AOTCs can provide a output port for a tunable laser. At the same time, the side bands of the band-pass spectrum are much smaller, which are more than 10 dB smaller than the main passband. With these features, it would be promising to realize a fast tunable laser with better performance.

In this paper, we reported a tunable ring laser based on an AOTC which consisted of an AOTF and a TF. In the experiment, the tuning range of the ring laser was from 1532.1 nm to 1570.4 nm, which covered the whole C band. The device could work stably, and the wavelength drift was within 0.1 nm while the power fluctuation was smaller than 0.33 dB in one hour. The phenomenon of pulse output similar to a sliding frequency soliton laser was also observed. The configuration gains advantages of simple structure, a large tuning range, a fast tuning speed and low cost. Compared to a null coupler driven by the acoustic wave, the proposed configuration can simplify the process of manufacture and achieve a higher signal-to-noise ratio.

2. Configuration and experimental results

The configuration of the proposed all-fiber tunable ring laser is shown in Fig. 1. The kernel part is an AOTC [21], which consists of an AOTF and a TF. In the AOTF, the acoustic wave produced by the acoustic transducer propagates along the unjacketed SMF in the acousto-optic interaction region with a length L _AO and induces dynamic micro-bending gratings, which results in the mode coupling between the core fundamental mode ( ${\text{LP}}_{01}^{\text{co}}$) and the co-propagating cladding mode ( ${\text{LP}}_{1u}^{\text{cl}}$) when the phase matching condition [23]

 

Fig. 1 The experimental configuration of the tunable fiber ring laser based on an AOTF and a TF. EDFA: erbium doped fiber amplifier. FI:fiber isolator. PC: polarization controller. AOTC: acousto-optic tunable coupler. OSA: optical spectrum analyzer. RF: radio frequency generator. SMF: single mode fiber. TF: tapered fiber.

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Besides an AOTC, the ring laser in the experiment also included an erbium doped fiber amplifier (EDFA) as the gain media as the gain media pumped by a laser at 980 nm, a fiber isolator (FI) for unidirectional lasing, and a polarization controller (PC) to adjust the polarization state of the laser. These components were linked clockwisely to build a typical ring laser. The total fiber length of the ring laser was about 30 m, including a 20-meter erbium doped fiber in the EDFA. Because Port 3 of the AOTC showed band-pass characteristic, it was used as the wavelength selector in the ring laser and connected to the EDFA; Port 2 was used as the laser output port for its band-rejection characteristic and was connected to an optical spectrum analyzer (OSA).

In the experiment, the diameter of the unjacketed SMF between Port 1 and Port 2 was etched down to 29 μm by the hydrofluoric acid to enhance the AO interaction, and L _AO was 16 cm. The TF was fabricated by heating and pulling a piece of SMF and had a waist diameter of 18 μm and a waist length of 20 mm, respectively. The TF was placed parallel to the AOTF with an evanescent wave coupling length of L _C = 15 mm. To tune the laser wavelength effectively, the AOTC was fabricated with a 3-dB bandwidth of 8 nm, which was smaller than that in our previous work [21], while the insertion loss and sidebands were −3.5 dB and −10 dB, respectively.

By tuning the RF frequency applied to the acoustic transducer, the tunability of the laser was measured. During the tuning process, the pump current of the EDFA was set to be 75 mA. The wavelength of the output laser could be tuned from 1570.4 nm to 1532.1 nm when the frequency of the RF signal was tuned from 0.888 MHz to 0.920 MHz, as shown in Fig. 2(a). The uniform spectral peak power of the laser output at different wavelengths was obtained by tuning the power of the RF signal applied to the acoustic transducer, which would tune the output coupling ratio to flatten the gain profile of the EDFA. Figure 2(b) shows the tuning relationship between the wavelength of the laser and the frequency of the driving RF signal. The black squares are the experimental results and the red line is a a linear fit with a tuning slope of −1.22 nm/kHz. During the tuning process, the 3-dB bandwidth of the output laser remained to be about 0.2 nm.

 

Fig. 2 (a) The wavelength tuning spectra of the output laser. (b) The tuning relationship between the output laser wavelength and the RF signal frequency with a tuning slope of −1.22 nm /kHz.

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The output efficiency and temporal stability of the laser were also measured, as shown in Fig. 3. The laser power was measured by a power meter connected to Port 2. The maximal output power of 13.8 mW was achieved, and the threshold current of the EDFA was 39 mA, as shown in Fig. 3(a). The temporal stability of the laser was monitored for 1 hour in the OSA and is shown in Fig. 3(b). The wavelength drift was smaller than 0.1 nm, and the peak power fluctuation was smaller than 0.33 dB.

 

Fig. 3 (a) The laser output power versus the pump current. The threshold current was 39 mA. (b) The temporal stability of the output wavelength and power. The wavelength drift was smaller than 0.1 nm and power fluctuation was smaller than 0.33 dB in one hour.

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Note that the AOTC in the ring laser configuration is based on the gratings induced by the travelling acoustic wave, and a frequency down-shift occurs when the light transmits from Port 1 to Port 3 [9, 24]. Such a laser has been studied intensively and is called a sliding-frequency soliton laser [18,25], which tends to output pulse. However, the self-pulsing behavior requires enough pump power to stimulate the nonlinearity, and the total group velocity dispersion (GVD) of the laser cavity should be anomalous [25]. In our ring laser configuration, the GVD of the erbium doped fiber in the EDFA is normal, so that SMF with an anomalous GVD has to be long enough to support the frequency-sliding soliton. With a 10-m SMF in the ring laser cavity, pulse output was not obtained, which indicated that the total GVD was still normal. With a piece of 400-meter SMF whose GVD was −18 ps²/km inserted between the PC and the AOTC to increase the anomalous GVD of the laser cavity and the pump current in the EDFA increased to 200 mA, the laser pulse output could be observed in the experiment under an optimized polarization. The laser outputs measured by an oscilloscope and an optical spectrum analyzer are shown in Fig. 4. Figure 4(a) shows the pulse laser spectrum with sidebands, which is similar to that of a mode locked pulse laser [26–28 ]. The 3-dB bandwidth of the output spectrum was 0.4 nm, which was twice of that of the continuous wave output. Figure 4(b) shows the pulse train profile which is almost uniform, and the details of the laser pulses are shown in Fig. 4(c). The interval between adjacent pulses was 2.1 μs which was equal to the round trip time of the laser cavity, and the full-width at half-maximum (FWHM) was 220 ns. With a larger frequency shift and broader 3-dB bandwidth of the AOTC, the pulses could be shorter [25].

 

Fig. 4 (a) The pulse laser spectrum. (b) The pulse train profile of the output laser. (c) The details of the laser pulses.

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3. Conclusions

In summary, a new configuration for a tunable all-fiber ring laser was presented. The wavelength and the output coupling ratio could be dynamically tuned by an all-fiber acousto-optic tunable coupler, which was constructed with an acousto-optic tunable filter and a tapered fiber. The wavelength of the fiber laser could be tuned from 1532.1 nm to 1570.4 nm with a 3-dB bandwidth of around 0.2 nm by tuning the frequency of the RF driving signal. The wavelength drift in the experiment was smaller than 0.1 nm, and the power fluctuation was smaller than 0.33 dB within 60 minutes. Our scheme gains advantages of fast tuning, simple structure, and low cost, which are convenient for practical applications.