1. Introduction

Continuous-wave (CW) wavelength-tunable OPOs are attractive sources for MIR radiation, which have a wide range of applications in gas trace sensing, environmental monitoring, and spectroscopy [1–3]. In the CW regime, quasi-phase-matched (QPM) nonlinear materials are mostly adopted in OPOs in order to exploit the highest nonlinear tensor coefficients combined with long interaction lengths under noncritical phase matching [4]. Periodically poled lithium niobate (PPLN) crystal as a kind of QPM nonlinear material is frequently employed in OPOs to generate wavelength-tunable 1.4~4 μm infrared emission [5–10].

Generally, there are several approaches to obtain tunable MIR emission from an OPO system, including changing the angle [11] or the temperature [12–14] of the PPLN crystal, exploiting a fast tuning spectral filter [15] and adopting wavelength-tunable pump sources [7, 16–19]. Tunable OPO systems based on temperature adjustment are difficult to achieve fast tuning, since that the temperature could not change rapidly and it may make take several minutes to stabilize [7]. While tunable OPO systems achieved by changing the angle of the PPLN crystal cannot obtain continuous tuning wavelength emission. Compared with these approaches, OPO pumped by tunable laser source holds the advantage of compactness and capability of fast and continuous tuning, which make it gradually become one of the research focuses. In 2003, Klein et al. demonstrated a CW tunable OPO pumped by a tunable ytterbium fiber laser. By tuning of the fiber-laser wavelength over 33 nm through an intracavity acousto-optic tunable filter, the OPO idler wavelength can be tuned from 3160 to 3500 nm [16]. In 2005, Lindsay et al. proposed a tunable OPO pumped by a fiber-amplified DBR diode laser. Rapid, continuous tuning of the pump allowed the idler output of the OPO to be tuned by 110 GHz in 29 ms without mode hops, while discontinuous tuning of the pump allowed the idler output of the OPO to be tuned with mode-hops over 20 nm [17]. In 2006, Henderson et al. demonstrated the first single frequency, CW singly resonant OPO pumped by an all-fiber pump source. MIR emission between 2650 and 3200 nm was achieved by tuning the fiber-amplified, distributed feedback fiber laser as pump source [18]. In 2012, Hong et al. reported a CW, broadly tunable MIR MgO:PPLN OPO pumped by a fiber amplifier. Using pump tuning with synchronized temperature optimization, mode hopping free tuning range of 900 GHz was achieved [19]. Very recently, Zhao et al. reported a single-frequency CW difference-frequency generation (DFG) source based on PPMgLN crystal. Single-frequency wavelength-tunable ytterbium- and erbium-doped fiber master oscillator–power amplifiers are used as the pump and signal source, respectively. Based on this configuration, idler light reached a wavelength-tuning range from ~3117.2 to ~3598.8 nm, by tuning the launched pump and signal wavelengths [7].

Tunable random fiber laser (RFL), as a kind of novel fiber laser that utilizes Raman gain and random distributed feedback, has the advantage of modeless property, which makes it free from mode competition and ensure the stationary narrow-band continuous modeless spectrum [20-21]. There are already some different schematics of tunable RFLs. In 2011, Babin et al. demonstrated a tunable RFL that can be continuously tuned from 1535 to 1570 nm by exploiting a tunable filter [21]. In 2013, Zhu et al. reported a tunable multiwavelength RFL that utilizes a Fabry-Perot cavity combined with a long-period fiber gratings based Mach-Zehnder interferometer, which can be tuned from 1553.9 to 1565.4 nm [22]. In 2014, Wang et al. proposed a tunable RFL with the help of a tunable fiber Fabry-Perot interferometer, the wavelength of which can be tuned from 1525 to 1565 nm [23]. In 2015, Du et al. demonstrated a tunable RFL, where 1-km-long single mode fiber is used to provide random distributed feedback and a piece of ytterbium-doped fiber is used to provide active gain, and the wavelength is tunable from 1040 to 1090 nm [24]. Most recently, Zhang et al. proposed a RFL with ultra-broad tunable range from 1 to 1.9 m by continuously adjusting the pump laser wavelength and increasing the pump power [25]. Previous study has proved that random fiber laser has the capability of achieving stationary, high power output and extraordinary wavelength tunability [25–28], and has been applied to pump OPO [29]. Exploiting tunable random fiber laser in OPO could be a promising way of obtaining stable, high power, MIR output with wavelength-tunability.

In this paper, we demonstrate a CW tunable OPO pumped by a tunable random fiber laser. While the pump wavelength is tuned from 1089.78 to 1103.25 nm, the idler light can be continuously tuned from 3977.34 to 4059.65 nm, spanning a spectral range of ~82.31 nm, with highest output power ~2 W. Moreover, dual-wavelength MIR emission at 3659.73 and 4059.65 nm is achieved by amplifying the residual 1045 nm pump laser and random laser (1103.25 nm) at the same time. To the best of our knowledge, this is the first time for achieving a CW tunable OPO system by exploiting a tunable random fiber laser, which provide a new approach for achieving tunable OPO system.

2. Experimental setup

The experimental schematic of the tunable OPO system is shown in Fig. 1. The pump source is provided by a tunable RFL, the experimental setup of which is depicted in Fig. 1 (a). This tunable RFL is pumped by a polarization maintaining ytterbium-doped fiber amplifier (YDFA), whose central wavelength is 1045 nm, with a maximum output power ~50 W. The output of the pump source is injected into a piece of 450 m long polarization-maintaining germanium-doped fiber (GDF, 10/125 m) through a 1070/1120 nm wavelength division multiplexer (WDM). This piece of GDF is used to provide Raman gain as well as random distributed feedback. A 50:50 coupler working at 1120 nm together with an optical tunable filter (OTF) is used to serve as a cavity mirror. The 50:50 coupler with two ports spliced together works as a Sagnac fiber loop mirror to provide broadband reflection while the OTF in the loop can select the exact working wavelength. The output end of the GDF is cleaved with 8-degree angle to avoid unwanted backward reflection. Based on this configuration, wavelength tunable operation can be achieved within the Raman gain spectrum range.

 

Fig. 1 (a) Experimental setup of the tunable random fiber laser (b) Experimental setup of the OPO system.

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In order to meet with the power requirement of OPO system, an amplifier is adopted to boost the output power of the tunable RFL. Between the tunable RFL and the amplifier, a polarization maintaining isolator (PM ISO) is inserted to avoid any backward light that would destroy the tunable RFL. The core and cladding diameter of the pigtail fiber of the PM ISO is 10 and 125 μm, respectively. The amplifier is composed of a piece of 4 m long ytterbium-doped fiber (YDF, 10/125 m) and two multimode laser diodes (LDs), whose central wavelength is 976 nm. Then, the output power from the amplifier is collimated and delivered into the singly resonant oscillator (SRO), which is a typical four-mirror ring cavity composed of two concave mirrors (M1 and M2 with the radius of curvature 150 mm), two plane mirrors (M3 and M4) and a MgO:PPLN crystal. All these four cavity mirrors are highly reflective for the pump light over 1–1.1 μm and the idler light over 3–4 μm. The mirrors M1, M3 and M4 have high reflecting coating for the signal over 1.4–1.7 μm and the mirror M2 is the output coupling mirror. The length between the mirrors M1 and M2 is ~130 mm, and the length between the mirrors M1 and M4 is ~184.5 mm and the incident angle is ~8 degree. The beam size is about 3 mm after the collimator. The beam focuses in PPLN crystal after the focusing lens with beam size ~80 μm at the focusing point. Mirror M5 is highly reflective over 1.0-1.1 μm to extract the pump light, while M6 is highly reflective over 1.4–1.7 μm to extract the signal light. Therefore, idler light can be obtained at the output end. The MgO:PPLN crystal with a single grating period of ᴧ = 29.5 μm is used as the nonlinear medium, the size of which is 50 mm × 10 mm × 1 mm. An isolator (ISO) is inserted between the SRO and the amplifier to avoid any unexpected backward reflection.

3. Results and discussion

The output characteristics of the tunable random fiber laser is firstly investigated. Different from other reported tunable RFLs, which mostly adopts a tunable pump source to change their emitting wavelength, this tunable RFL exploits the Raman gain spectrum at a fixed pump wavelength. And tunable random laser emission is efficiently achieved within the Raman gain spectrum simply by changing the working wavelength of the OTF, as shown in Fig. 2 (a). The output wavelength of this RFL can be continuously tuned from 1089.78 to 1103.25 nm. The pump power is not fully depleted and the residual pump wavelength (1045 nm) can be distinguished in the output spectra.

 

Fig. 2 (a) The output spectra at different wavelengths. The output power evolution when the OTF is tuned at (b)1089.78 nm (c)1095.56 nm and (d) 1099.64 nm.

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First of all, the characteristics of tunable random fiber laser is explored within the tunable range of 1089.78 to 1099.64 nm. The output power as a function of the pump power is depicted in Fig. 2 (b), (c) and (d) when the OTF is tuned at 1089.78, 1095.56, and 1099.64 nm, respectively. The maximum output powers can reach 24.0, 23.9 and 24.2 W at the pump power of ~40 W at the above three wavelengths, with output powers of the 1st order Stokes light reaching 18.3, 20.32, and 21.54 W accordingly and linewidths around 1.21, 1.43 and 0.77 nm, respectively. The polarization degree is calculated by $PER=10\lg (a/b)$, where a and b are values of the major and minor axes of the polarization eclipse. The polarization degree is ~13dB within the wavelength tunable range. The maximum output power of the 1st order Stokes light increases as the wavelength becomes longer, which indicates the effective Raman gain is getting larger towards longer wavelength.

Then, the output power of the RFL is further scaled to ~50 W by the power amplifier. The output power of the random laser reaches 52, 55 and 51 W at 1089.78, 1095.56, and 1099.64 nm, respectively. The normalized spectra are plotted in a linear coordinate, as shown in Fig. 3. Although the residual pump wavelength still can be seen, the 1st order Stokes light is amplified more efficiently than the residual pump light (1045.0 nm) by the amplifier. The linewidths of the three random laser wavelengths are 1.37, 1.67, and 0.88 nm, respectively.

 

Fig. 3 The output spectra of the pump wavelength after amplifier at 1089.78, 1095.56 and 1099.64 nm.

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Thereafter, the output power of this tunable random fiber laser is injected into the SRO cavity. The output spectra of the OPO system under the maximum pump power are measured with the help of an optical spectrum analyzer (ANDO, AQ6370D) and a Bristol infrared spectrum (Bristol, 721B). Figure 4 presents the output spectra of the signal lights, the central wavelengths of which are 1501.08, 1506.88, and 1511.42 nm, corresponding to the pump wavelength of 1089.78, 1095.56 and 1099.64 nm. The linewidths of the three signal lights are 0.08, 0.08 and 0.07 nm accordingly. There are also other wavelength components, which are located at 1490.52, 1511.82, 1496.42 and 1517.7 nm. The first two wavelength components are the Raman wavelengths stimulated by 1501.08 nm signal light, while the rest two are Raman wavelengths stimulated by 1506.88 nm signal light. The occurrence of these Raman components are resulted from the strong signal lights.

 

Fig. 4 The output spectra of the signal light pumped by different pump wavelengths.

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The central wavelengths of the idler light are 3977.34, 4007.66 and 4035.32 nm, spanning a spectral range of 57.98 nm, as shown in Fig. 5, corresponding to the pump wavelength of 1089.78, 1095.56 and 1099.64 nm. The linewidths of the three idler lights are 19.11, 15.67 and 11.02 nm accordingly. The optical frequency of the pump wavelength (${\text{ω}}_{\text{p}}$), signal wavelength (${\text{ω}}_{\text{s}}$) and the idler wavelength (${\text{ω}}_{\text{i}}$) fits the relation: ${\text{ω}}_{\text{p}}={\text{ω}}_{\text{s}}+{\text{ω}}_{\text{i}}$.

 

Fig. 5 The output spectra of the idler light pumped by different pump wavelengths.

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The linewidth of the pump light, signal light and the idle light are different. Actually, the linewidths of the signal light and the idler light are affected by different factors. The linewidth of the signal light is determined by the resonator. Only the longitudinal modes that satisfies the eigenmode condition can form stable oscillation in the resonator and outputs as signal light. When the linewidth of the signal light is fixed, the linewidth of the idler light can be affected by the strong pump power as well as the linewidth of the pump light. In our experiment, the pump power is ~50 W, which is not very strong, and the linewidth is mainly affected by the linewidth of pump light. As a result, the linewidth of the signal light is about ten times of the light of the pump light. Taking account of some other nonlinear effects (e.g. self-phase modulation), the linewidth of the signal light reaches more than ten nanometres in our work.

The output powers of the three idler wavelengths are also measured. The output power of the idler light with respect to the pump wavelength is depicted in Fig. 6. The lasing threshold is ~25 W at different wavelengths and the maximum output power is ~2 W, with a slope efficiency ~7%. The output powers achieve 2.05, 2.07 and 1.7 W at 3977.34, 4007.66 and 4035.32 nm, respectively. The high lasing threshold and low efficiency can be ascribed to the low transparency of the PPLN crystal over 3.8 μm. Moreover, the poor polarization degree of the pump light may also account for that.

 

Fig. 6 Output power of the idler light as a function of the pump power.

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In order to test the fluctuation of power property of this OPO system, we record the maximum output power at 4007.66 nm with a time duration of 15 seconds, the result of which is shown in Fig. 7. The power fluctuates around 2.05 W, ranging from 2.02 to 2.07 W, with an overall fluctuation of ~2.4%.

 

Fig. 7 Fluctuation of the output power of OPO system at 4007.6 nm.

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The wavelength of the random fiber laser can be further tuned to 1103.25 nm, the spectrum of which is depicted in Fig. 8(a). However, this wavelength is hard to be amplified efficiently in the current amplifier, since that the gain coefficient of pump wavelength is larger than that of the random laser and the residual pump power can be amplified at the mean time. The maximum output power that can be obtained after the amplifier is ~56 W, where the power of the pump light is ~24 W and the power of the random laser is ~31 W. The pump wavelength as well as the 1st order Stokes wavelength can be clearly distinguished, as shown in Fig. 8(b).

 

Fig. 8 Output spectrum of the random laser (1103.25 nm) (a) before amplification and (b) after amplification. (c) Spectrum of the signal lights of the OPO system (d) Spectrum of the idler lights of the OPO system.

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Consequently, two signal wavelengths and two idler wavelengths can be achieved when this pump source is applied in the OPO system, as shown in Fig. 8 (c) and (d). The central wavelengths of the signal light and idler light locate at 1461.94 and 3659.73 nm, respectively, corresponding to the pump wavelength of 1045 nm. The signal light and idler light pumped by 1103.2 nm center at 1514.92 and 4059.65 nm correspondingly. The maximum output power of the idler light reaches 0.65 W. This part of work provides a new idea of achieving dual-wavelength MIR emission with the help of random fiber lasers. By adjusting the pump power of the random laser, the power portion of the residual pump power and the 1st order Stokes light output from the random laser can be controlled. Therefore, the power portion of the residual pump and the 1st order Stokes light after power amplification can be varied indirectly, further controlling the dual wavelength output spectrum obtained from the OPO system.

We have also calculated the theoretical curve of the signal light and idler light using Sellmeier equations and made a comparison with the experimental results. As shown in Fig. 9, the experimental results generally fit well with the theoretical value, but there is small deviation. The wavelength of the signal light is a bit longer than the theoretical value while the wavelength of the idler light is a little shorter than the theoretical calculation. The deviation between the experimental results and the theoretical calculation is resulted from the thermal effect caused by the absorption of idler light of the PPLN crystal and shows the phenomena of temperature tuning. Further controlling the temperature of PPLN crystal can hopefully make the experimental results fit better with the theoretical values.

 

Fig. 9 Theoretical results and experimental results of the signal light and idler light of the OPO system.

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4. Conclusion

In summary, we report a wavelength tunable PPLN-based MIR OPO system pumped by a tunable random fiber laser, to the best of our knowledge, for the first time. The wavelength of the MIR output can be continuously tuned from 3977.34 to 4059.65 nm, with maximal output power around 2 W. The tunable random laser is accomplished by adopting an OTF to select the operation wavelength within the Raman gain spectrum. Moreover, dual-wavelength MIR emission at 3659.73 and 4059.65 nm is achieved by amplifying the residual pump (1045 nm) and 1st order Stokes light (1103.25 nm) at the same time to provide pump power. The dual-wavelength spans almost 400 nm, which provides a new scheme of achieving dual-wavelength MIR emission with the help of a random fiber laser.

This work not only offers an effective approach in the research of tunable optical parametric processes, but also broadens the application scenarios of random fiber lasers. Further performance improvement can be achieved by adopting a tunable pump source or a broadband super fluorescence pump source to extend the available effective Raman gain spectral range for broader wavelength-tunable operation. Moreover, more accurate wavelength tuning can be expected by controlling the temperature of the PPLN crystal to eliminate temperature tuning. Higher output power is also foreseeable by providing much more pump power.