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Abstract
Owing to the special power distribution property, a random distributed feedback Raman fiber laser can achieve a high power spectrally flexible output with a low power spectrally tuning device. Here, an all-fiberized linearly polarized dual-wavelength random distributed feedback Raman laser with wavelength, linewidth, and power ratio tunability is demonstrated. By adopting two watt-level bandwidth adjustable optical filters, a spectrum-manipulable dual-wavelength output with nearly a 10 W output power is achieved. The wavelength separation can be tuned from 2.5 to 13 nm, and the 3 dB linewidth of the output can be doubled by increasing the bandwidth of the optical filter. The power ratio of each laser line can be tuned from 0 to nearly 100% with the help of two variable optical attenuators. A maximum output power of 9.46 W is realized, with a polarization extinction ratio up to 20.5 dB. The proposed dual-wavelength fiber laser can be employed as a pump source in frequency tunable, bandwidth adjustable terahertz microwave generation, and mid-infrared optical parametric oscillators.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Multi-wavelength fiber lasers have been widely investigated owing to their great applications in wavelength division multiplexing communication, optical sensors, optical testing instruments, and precise spectroscopy [1–4]. And dual-wavelength fiber laser, as a special kind of multi-wavelength fiber laser, is attracting great attention thanks to its unique applications in terahertz microwave generation [5–9]. In recent years, dual-wavelength fiber lasers have seen great progress in linewidth narrowing [10–12], pulse generation [13–16], and wavelength tuning [17–20]. Conventional dual-wavelength fiber lasers are usually based on rare earth ions doped fibers, such as ytterbium-doped, erbium-doped and thulium-doped fibers, which could provide high gains over a wide wavelength band. However, at a room temperature, the homogeneous gain broadening in these rare ions doped fibers may lead to a strong mode competition, which can result in unstable lasing. Lots of methods have been adopted to solve this problem, including adding a piece of unpumped erbium-doped fiber in the cavity as saturable absorber [21–23], enhancing polarization hole burning effect through polarization controlling [24–26], using gain mediums with high nonlinearity to induce four-wave-mixing effect [27–29].
Proposed by Turitsyn et al. in 2010 [30], random Raman fiber lasers have attracted great attention due to their special characteristics, such as simple configuration, low coherence, modeless emission and high stability. And the special power distribution property of random fiber laser enable high power spectrally flexible laser output with low-power spectrally tuning device [31]. In recent years, random Raman fiber lasers have advanced considerably in many aspects such as high power [32,33], wavelength tunable [34,35], and linearly-polarized operation [36,37]. Moreover, random fiber laser has been proven to be a good choice for dual-wavelength laser generation thanks to its high stability [38]. By using two fiber Bragg gratings with different reflection wavelengths, El-Taher et al. firstly demonstrated dual-wavelength lasing output in a random fiber laser [38]. In 2016, Aporta et al. reported a tunable and switchable dual-wavelength random fiber laser with bi-directional pumping source, the wavelength tunability is achieved through applying different strains on the fiber Bragg grating, while the wavelength switching property is realized with variable optical attenuator (VOA) [39]. Recently, based on the special phosphosilicate fiber, Song et al. demonstrated a dual-wavelength random fiber laser with over 23 W output power [40]. Furthermore, independently wavelength and linewidth tunability are important for fiber lasers in lots of applications. For example, tunable dual-wavelength fiber lasers can be adopted for tunable terahertz microwave generation [8,41]. And linewidth tunability is important in noise or coherence optimizing for fiber lasers [42,43]. In 2019, Supradeepa et al. achieved hundred-watt-level single wavelength laser output with wavelength and linewidth tunability in a conventional ytterbium-doped fiber laser, the wavelength of which can be tuned from 1050 to 1100 nm while the linewidth can be tuned from 0.4 to 1 nm [44].
In this paper, we demonstrated an all-fiberized linearly polarized dual-wavelength random distributed feedback Raman laser with wavelength, linewidth, and power ratio tunability. A linearly polarized output with a maximum power of 9.46 W and a polarization extinction ratio (PER) above 20.5 dB is achieved. By using two bandwidth adjustable optical filters (BA-TOF), both the central wavelengths and linewidths of the two Stokes lines can be tuned independently. As a result, the wavelength spacing of the two Stokes lines can be tuned from 2.5 to 13 nm, and the 3 dB linewidth of the Stokes line can be tuned from 1.01 to 2.20 nm. The power ratio of the each Stokes line can be tuned from 0 to nearly 100% with the help of two VOAs. Moreover, this dual-wavelength random distributed feedback Raman fiber laser exhibits high stability.
2. Experimental setup
The configuration of the proposed dual-wavelength random distributed feedback Raman fiber laser is shown in Fig. 1. A half-open cavity structure is employed to provide dual-wavelength feedback and reduce the lasing threshold. A homemade master oscillator power amplifier structure based linearly polarized laser source, which is seeded by a linearly polarized ytterbium-doped fiber laser operating at 1057 nm and boosted by two stage amplifiers, is employed as pump source. After the two stage amplifiers, a maximum output power of 22.7 W can be achieved at 1057 nm at the output port of the pump source. A section of 800 m polarization maintaining germanium-doped silica fiber with a numerical aperture of 0.12 is utilized to provide Raman gain as well as random distributed feedback. The core and cladding diameters of the polarization maintaining passive fiber fiber are 5.5 µm and 125 µm, respectively. A 1070/1120 nm wavelength division multiplexer (WDM) is adopted to separate the pump light and the first Stokes light. The output of the pump source is coupled into the passive fiber through the 1070 nm port of the WDM, while the 1120 nm port of the WDM is connected with the feedback part. The feedback part consists of a 3 dB coupler and two individual light controlling parts, each of which is composed of a circulator, a BA-TOF, and a VOA. The back scattering light is divided into two beams through the coupler, both beams are injected into the light controlling parts through the port 2 of the circulator, while a VOA and a BA-TOF is spliced between the port 1 and port 3 of the circulator to control the feedback light. The transmission loss of the VOA can be tuned over a range of 30 dB, with a minimum transmission loss of 1 dB, and it is insensitive to the wavelength [40]. The central wavelength of the BA-TOF can be tuned from 1055 to 1115 nm, while the bandwidth of which can be tuned from 0.6 to 40 nm. With the help of the VOA and BA-TOF, the central wavelengths, linewidths and the power ratio of the two Stokes lines can be controlled. To get a linearly polarized radiation, all the components we used are polarization maintained. Moreover, the output port of the passive fiber is cleaved at an angel of 8° to minimize the backward reflection. The optical spectra of the laser’s output are monitored with an optical spectra analyzer with a resolution of 0.02 nm.
Fig. 1. The experimental setup of the dual-wavelength random distributed feedback Raman fiber laser. PM VOA, polarization maintaining variable optical attenuator; PM Cir, polarization maintaining circulator; PM WDM, polarization maintaining wavelength division multiplexer;
3. Result and discussion
3.1 Dual-wavelength operation
Firstly, we tuned the wavelengths of the two BA-TOFs at 1110 nm and 1115 nm. The frequency shifts between the wavelength of pump source (1057 nm) and the wavelengths of the two BA-TOFs are 13.6 THz and 14.7 THz respectively, where the Raman gains are relatively high. Figure 2(a) shows the power evolution of the dual-wavelength output. As we can see, with the increasing of the pump power, the two Stokes lines are generated asynchronously. Once the pump power exceeds 8 W, the output power of the first Stokes line increases rapidly, and until the pump power reaches 10.9 W, the second Stokes line at 1115 nm is generated. The threshold power difference of the two Stokes lines may be contributed to the difference in Raman gains and transmission loss at the two wavelengths. As the Raman gains at the two wavelengths are different (gain at the first laser line is lower), in order to equalize the output power of the two laser lines under maximum pump power, the transmission losses of the two VOAs are optimized and the transmission loss of the first VOA is set at a lower value. Thus the intensity of the feedback light at the first laser line is higher, as a result of which, the threshold of the first Stokes line is lower. When the pump power reaches a maximum of 22.7 W, a highest output power of 9.46 W is achieved. The output power of first Stokes line and second Stokes line are 4.77 and 4.60 W respectively, and the power of the residual pump is 0.09 W. The corresponding output spectrum is shown in Fig. 2(b). Restricted by the available pump power, the Raman conversion is incomplete and the output power is limited. In our experimental setup, the VOA has a smallest optical power handling capacity of 2 W. In the half-open cavity based forward pumped random fiber laser, the backward power intensity is generally several dozen times lower than the output power, so we infer that nearly hundred-watt level output can be realized in this laser system with sufficient pump power [31]. But nonlinear optical effect such as four-wave-mixing induced spectral broadening is strengthened under higher power, and the wavelength, linewidth tunability may be limited under such high power, for example, the available smallest linewidth and wavelength separation may be wider.
Fig. 2. (a) The power of total output, residual pump, first line, and second line under different pump powers. (b) The optical spectra of dual-wavelength output at maximum pump power (c) The output optical spectra and (d) The power intensities and central wavelengths of the dual-wavelength output at an interval of 1 minute during 7 minutes (triangle black dashed line: wavelength of the first laser line, square black solid line: intensity of the first laser line, triangle black dashed line: wavelength of the second laser line, square black solid line: intensity of the second laser line).
The temporal stability of this dual-wavelength output at maximum power is tested with an interval of 1 minute during 7 minutes. The corresponding output spectra are shown in Fig. 2(c). And the central wavelengths and power intensities of the two laser lines are shown in Fig. 2(d). The wavelength shifts of the two Stokes lines are within 0.08 nm. At the first laser line (1110 nm), the power intensity fluctuation is within 0.74 dB, while the power intensity fluctuation at second laser line (1115 nm) is within 1.2 dB.
3.2 Wavelength and linewidth tunability
The spectra manipulability of our dual-wavelength random distributed feedback Raman fiber laser is achieved through two BA-TOFs, which have always been used for wavelength tuning in fiber lasers [43,45]. By adjusting the central wavelengths and the bandwidths of the two BA-TOFs, the central wavelengths and linewidths of the two Stokes lines can be tuned independently.
Firstly, we fixed the transmission bandwidths of the two BA-TOFs at 0.6 nm (minimal bandwidth of the BA-TOF) and the central wavelength of BA-TOF1 at 1115 nm (upper limit of BA-TOF’s tuning range), by tuning the wavelength of BA-TOF2, dual-wavelength output with tunable wavelength separation is achieved. As shown in Fig. 3(a), the wavelength separation of the two Stokes lines can be tuned from 2.5 to 13 nm. The optical signal to noise ratio (OSNR) is above 28 dB, the peak-to-valley contrast of the dual-wavelength output is more than 10 dB over this tuning range. Then, the central wavelengths of the two Stokes lines are tuned simultaneously with a fixed wavelength separation of 4.5 nm, corresponding to a peak-to-valley contrast of 19 dB. As shown in Fig. 3(b), the wavelengths both Stokes lines can be tuned independently, with a tuning range of about 8 nm. And the OSNR is about 28.5 dB over this tuning range. The wavelength tuning range of this dual-wavelength random distributed feedback Raman fiber laser is restricted by the spontaneous Raman scattering emission. Like other conventional germanium-doped silica fiber, the fiber we used has two Raman gain peaks at frequency shifts of 13.2 THz and 14.7 THz. As shown in Figs. 3(a) and 3(b), with the further decreasing of the wavelength, the Raman gains at the corresponding wavelength is not high enough to suppress the spontaneous Raman scattering, as a result of which, Raman Stokes lines at 1109 nm (corresponding to the frequency shift of 13.2 THz) or 1115 nm (corresponding to the frequency shift of 14.7 THz) are generated. The lower Raman gain at shorter wavelength is considered to be the reason for this phenomenon.
Fig. 3. (a) The optical spectra of the dual-wavelength output with different wavelength separations. (b) The dual-wavelength output optical spectra at different wavelengths with a fixed wavelength separation.
The linewidth tunability of this laser system is demonstrated under both dual-wavelength operation and single wavelength operation. Firstly, we fixed the central wavelengths of the two BA-TOFs at 1105 nm and 1111 nm respectively, and adjusted the transmission losses of the two VOAs to appropriate values to get a dual-wavelength output. By tuning the bandwidths of the two BA-TOFs, the linewidths of the two Stokes lines can be adjusted independently. The spectra of the dual-wavelength output under different bandwidths is shown in Fig. 4(a). By increasing the bandwidths of the two BA-TOFs from 0.6 to 2 nm, the 3 dB linewidth of the first Stokes line is increased from 0.84 to 1.05 nm, and the second Stokes is increased from 0.70 to 1.16 nm. Restricted by the wavelength separation of the two Stokes lines, the further increasing of the bandwidths of BA-TOFs may result in the merging of two Stokes lines. Thus the linewidth tuning ranges of the two Stokes line are limited. Figure 4(b) shows the linewidth tunability under single wavelength operation. The transmission loss of the VOA2 is set at maximum for single wavelength operation, and the central wavelength of the BA-TOF1 is fixed at 1109 nm, corresponding to the first Raman gain peak at a frequency shift of 13.2 THz. By increasing the bandwidth of the BA-TOF1 from 0.6 to 20 nm, the 3 dB linewidth of the output can be tuned from 1.01 to 2.20 nm. The Raman gain difference at different wavelengths as well as nonlinear effects (such as four-wave-mixing effect) induced spectral broadening of the output may contribute to the nonlinearly property between the 3 dB linewidth of the output and the bandwidth of BA-TOF. Here, the smallest available linewidth is limited by the bandwidth of the filter and nonlinear effects induced spectral broadening, while the largest available linewidth is limited by gain competition. Moreover, under single wavelength operation, output power at the single Stokes line is higher than that of dual-wavelength output, as a result of which, the 3 dB linewidth of the output is larger than that of dual-wavelength output.
Fig. 4. (a) The output optical spectra at different bandwidths of BA-TOFs under dual-wavelength operation. (b) The optical spectra and 3 dB linewidth of the output at different bandwidths of BA-TOF under single wavelength operation.
3.3 Power ratio tunability
With the help of the two VOAs, the power ratio of the two Stokes lines can be tuned flexibly. To measure the power ratio tunability, the central wavelengths of the two BA-TOFs are set at 1109 nm and 1115 nm respectively, corresponding to the two spontaneous Raman Stokes lines.
In the experiment, we keep the transmission loss of the VOA1 unchanged and increase the transmission loss of VOA2 (corresponding to the loss at 1115 nm) gradually, as a result of which, the power ratio between the two Stokes lines can be tuned flexibly. As shown in Fig. 5, the power ratio between the two Stokes lines can be tuned from −33.83 dB to 26.27 dB, which could be limited by the spontaneous Raman noise. Correspondingly, the power ratio of each Stokes lines can be tuned continuously from 0 to nearly 100%.
The PER of the output is measured under single wavelength operation at 1115 nm with a system consisted of a collimator, a dichroic mirror, a half-wave plate and a polarization beam splitter [37]. The measured PER and the corresponding output power at 1115 nm under different pump power is shown in Fig. 6. The PER of the output keeps above 20.5 dB and slight improvement of PER is observed with the increasing of the output power. As the stimulated Raman scattering is a polarization sensitive process, the Raman gain coefficient under orthogonally polarized situation is much lower than that of the co-polarized case, which could be responsible for the slight improvement of PER [37]. The result indicates that linearly polarized random distributed feedback Raman fiber lasers have excellent polarization characteristic.
Fig. 6. The output power and polarization extinction ratio (PER) of the Stokes light at 1115 nm as functions of pump power.
4. Conclusion
In this paper, we demonstrate a linearly polarized dual-wavelength random distributed feedback Raman fiber laser. By adopting two BA-TOFs, the wavelengths of the two Stokes lines can be tuned independently. As a result, the wavelength separation can be tuned from 2.5 to 13 nm, and the 3 dB linewidth of the output can be doubled, from 1.01 to 2.20 nm. The power ratio of the each Stokes lines can be tuned from 0 to nearly 100% with the help of two VOAs. A maximum output power of 9.46 W is realized under dual-wavelength operation, corresponding to a maximum pump power of 22.7 W. The PER of both Stokes lines keep above 20.5 dB and no apparent degradation is observed. The power of the output can be further increased with a more powerful pump source. For the first time, the central wavelengths, linewidths, and power ratio of the two Stokes lines can be tuned simultaneously, and the multiple tunability of a laser system can improve its versatility and adapt it for wider applications. Owing to its flexible wavelength and linewidth tunability, the proposed dual-wavelength fiber laser can be employed as pump source in frequency tunable, bandwidth adjustable terahertz microwave generation and mid-infrared optical parametric oscillators.
Funding
National Natural Science Foundation of China (61635005, 61905284); Natural Science Foundation of Hunan Province (2018JJ03588); Hunan Innovative Province Construction Project (2019RS3017); Hunan Provincial Innovation Foundation for Postgraduate.
Acknowledgments
We are grateful to Sen Guo, Tao Wang, and Dr. Pengfei Ma for their help on this work.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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