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Abstract
We demonstrated a linearly-polarized supercontinuum (SC) directly generated in an all-polarization-maintaining random fiber laser (RFL) structure. Owing to the comparatively high Raman gain of the polarization-maintaining germanium doped fiber (GDF), the spectrum of the output SC shows an enhanced bandwidth and improved spectral flatness compared to the unpolarized counterpart. The output SC has an average output power of 4.43 W with a spectrum covering from 600 nm to 1900nm. The polarization extinction ratio (PER) is measured to be greater than 18 dB from 800 nm to 1700nm at the highest output power level. To the best of our knowledge, this is the first demonstration of a linearly-polarized SC generated directly from a RFL. This work is meaningful to help further expand the bandwidth of SC generated from a RFL and provides a simple and cost-effective method of generating linearly-polarized SC for practical applications.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The random fiber laser (RFL) has attracted considerable attention due to its unique properties of modeless emission, low coherence and simple structure [1–3]. In recent years, a large number of researches on RFL have been reported in terms of linewidth narrowing [4,5], power scaling and efficiency improving [6–8], pulsed operation [9–11], spectrum tuning [12,13], linearly polarized operating [14–16], and so on. Furthermore, explorations of the application of RFL have also shown in the fields of telecommunication [17], nonlinear frequency conversion [18], speckle free imaging [19], remote sensing [20], biotechnology [21,22], pumping in mid-infrared lasers [23] and supercontinuum (SC) generation [24–27].
SC generation directly from a RFL was first demonstrated by Ma et al. [24], and a SC with 20-dB bandwidth about 250 nm was achieved. In 2019, our group reported a RFL structure that generates visible to near-infrared SC in a 1 km passive fiber [26]. Notably, this SC generation method has been proven to be competitive and very promising because its spectral properties is comparable with that of the conventional SC generation method which using a high peak power pulsed laser to pump a piece of photonic crystal fiber (PCF) [27]. The above mentioned SC generated in a RFL structure is generally unpolarized. However, many practical applications require linearly-polarized SC [28–32]. Some researches about the linearly-polarized SC output have been reported in recent years [33–35]. In 2015, Zhang et al. reported a high power linearly-polarized SC source generated in a four-stage polarization-maintaining (PM) master-oscillation power-amplifier structure (MOPA) [33]. Tao et al. realized a linearly-polarized SC covering from 480 to 2100 nm [34]. A linearly-polarized seed laser is indispensable in this method and the complicated MOPA configuration will increase the complexity, instability and cost of the system. Moreover, there is no report so far on the linearly-polarized SC output directly from a RFL. Especially, SC generation by the RFL structure has the merits of simple operation, low-cost and perfect robustness compared with the commonly used method of using a high peak power pulsed fiber laser to pump a piece of PCF. With the improvement of spectral width and flatness of the generated SC, this SC generation method shows powerful vitality and has a wide application prospect.
In this manuscript, we report a linearly-polarized SC output directly from an all-polarization-maintaining RFL structure. Comparing with the unpolarized counterpart, the linearly-polarized RFL shows an enhanced output spectrum which covering 600 nm to 1900nm. Improved spectral flatness are obvious even with a shorter polarization-maintaining germanium doped fiber (PM-GDF). The measured PER at all wavelengths from 800 nm to 1700nm is greater than 18 dB, which verifies the SC is well linearly-polarized. To the best of our knowledge, this is the first all-fiber configuration linearly-polarized SC source generated directly from a RFL ever reported.
2. Experimental setup
Figure 1 shows the experimental setup. Two RFL structures are constructed for SC generation comparison: a linearly-polarized RFL in Fig. 1(a) and an unpolarized RFL in Fig. 1(b). In these two structures, a commercially 976 nm laser diode (LD) is used as the pump source and it is launched into the cavity through a (2 + 1) × 1 pump and signal combiner. A piece of 5-m-long 10/125 µm polarization-maintaining Ytterbium-doped fiber (PM-YDF) is spliced with the common port of the PM combiner to provide active gain and it has a nominal absorption coefficient of 5 dB/m at 976 nm. The core and cladding numerical apertures (NA) are 0.08 and 0.46 respectively. A polarization-maintaining optical fiber mirror (PM-OFM) with a reflective bandwidth of 40 nm is spliced to the PM combiner to form a half-open cavity with broadband reflection, and its operating center wavelength is 1064 nm. A spool of 100 meters PM-GDF with a core/cladding diameter of 10/125 µm is fusion spliced after the PM-YDF to provide both the random distributed feedback and Raman gain. The birefringence of the PM-GDF is 3×10−4 at 1060 nm and the calculated dispersion curve is shown in Fig. 2. The zero-dispersion wavelength (ZDW) is located at 1300 nm. In order to prevent the parasitic light reflection, the free pump end of the PM combiner and the other end of the PM-GDF are angle cleaved. Apart from the birefringence and polarization-maintaining properties of the PM-OFM and PM combiner, the OFM and combiner employed in Fig. 1(b) have the same characteristic parameters as that in Fig. 1(a). The lengths of the YDF and the GDF are the same with that of the PM-YDF and the PM-GDF respectively, and they have the same core and cladding diameters. The outputs are monitored at the right end of the GDF and PM-GDF by a power meter (Ophir, 30A-BB-18) and three optical spectrum analyzers (Yokogawa, AQ6370, AQ6456 and AQ6375).
Fig. 1. The experimental schemes of the RFL for SC generation: (a) linearly-polarized RFL; (b) unpolarized RFL. LD, laser diode; PM, polarization maintaining; OFM, optical fiber mirror; YDF, ytterbium-doped fiber; GDF, germanium-doped fiber.
3. Experimental results and discussion
At first, the output power performances of the above two structures are investigated. The diagram in Fig. 3 illustrates the output power of the linearly-polarized RFL and the unpolarized RFL versus the 976 nm pump power. The output power for both structures have an increasing tendency with the pump power increasing at first. When the pump power reaches near 3.72 W, there is a sudden drop in the output power for both structures. The emergence of this phenomenon is induced by the sharp broadening of the spectrum which can be verified later in Fig. 4 and Fig. 5. The 3.72 W pump power is the threshold of SC generation and these two structures have nearly the same threshold. Then, these two structures show a little different slope efficiencies, even though the output power of both structures have a nearly linear increase with the increasing of the pump power. The slope efficiencies of the unpolarized RFL and the linearly-polarized RFL are 24% and 20.4%, respectively. The main reason for their difference comes from the different final output spectrum. As can be seen in Fig. 6, the two distinct spectral bandwidth of the generated SC will result in the unequal quantum defects during the spectrum broadening and energy transfer which will lead to different conversion efficiencies. When the pump power reaches 22.9 W, the output power of the unpolarized RFL and the linearly-polarized RFL are 5.38 W and 4.43 W respectively.
Fig. 3. The output power of the linearly polarized RFL and the unpolarized RFL versus 976 nm pump power.
Fig. 6. The spectral comparison between the unpolarized RFL and the linearly-polarized RFL under the maximum pump power
The evolutions of the output spectrum of the linearly-polarized RFL and the unpolarized RFL with different pump powers are shown in Fig. 4 and Fig. 5 respectively. It can be seen that the spectral evolutions of the unpolarized RFL and the linearly-polarized RFL are identical except the bandwidth and flatness. When the pump power is 1.03 W, the amplified spontaneous emission (ASE) exhibits a broadband spectrum spanning a wavelength range over 170 nm from 1030 nm to 1200 nm. Owing to the combined effects of OFM, Rayleigh scattering and stimulated Raman scattering, a remarkable spike which is located near 1064 nm is superimposed on a broadband spectrum base. In this process, a new broadband pumping source is formed and it will play a significant role in further spectrum broadening. Substantial stochastic spectral spikes with different bandwidth and different locations can be easily noticed from the blue line in both Fig. 4 and Fig. 5. The reason for this phenomenon is the interaction between the stimulated Brillouin scattering (SBS) and the RS effects, which is an typical characteristic of RFL near threshold [1,26]. With the pump power further increasing, SC with different spectral bandwidth can be obtained from the linearly-polarized RFL and unpolarized RFL respectively. When the pump power is above 14.5 W, both spectrums don’t show obvious further spectral broadening which can be seen from the three curves plotted in orange, green and red color in Fig. 4 and Fig. 5. It is also important to note that although similar common cavity RFL structure and longer fiber length has been used in Ref. [36], the output spectra is quite different compared with the results illustrated in our manuscript where broadband SC is generated rather than narrowband random lasing. The important and determinant device is the OFM, broadband feedback provided by the OFM can help the lights with different wavelength oscillate effectively in the half-opened RFL structure and enhance the energy transfer to the whole spectrum of SC. However, the narrowband fiber Bragg grating (FBG) with high reflection can only support a few wavelength (near the central wavelength of the FBG) to oscillate, which result in the large difference between the output spectra.
Figure 6 is the comparison of the final output spectrum between the unpolarized RFL and the linearly-polarized RFL under the maximum pump power. There is a significant difference between the two structures in terms of the output spectral characteristics. The bandwidth of the SC are spanning from 600 to 1900 nm and 700 to 1700 nm for the linearly-polarized RFL and the unpolarized RFL respectively, and the output SC from the linearly-polarized RFL shows a better spectral flatness especially in the short wavelength band. The reason for the spectral difference of the two structures will be discussed in detail later. Meanwhile, we also compare the realized output parameters with the currently published works. The widest spectral bandwidth of the SC by using a 1 km GDF in the RFL structure is about 1100 nm [26]. Even a relatively short 100 m PM-GDF is used in the linearly-polarized RFL structure, a wider spectral bandwidth of SC is obtained which is about 1300 nm. The efficiency of the RFL is also improved from 7.7% to 19.3% (linearly-polarized case) with the shorter fiber length.
The main peak wavelength ω (1064 nm) is identical with the central wavelength of the OFM. As the 1064 nm pump peak is located in the normal-dispersion region of the PM-GDF, so the stimulated Raman scattering (SRS) effect dominants the initial spectrum broadening. The bandwidth of the 1064 nm pump is very wide due to the combination effects of the wide bandwidth of the OFM, the half-open RFL structure and the ytterbium ion gain, which has a broad base just as the yellow line shows in Fig. 4 and Fig. 5. Then, the broad bandwidth 1064 nm pump results in the Stokes wave with broad bandwidth base too. The base of these Stokes waves will merge together with the increase of the pump power. The third order Stokes wave is centered at 1245 nm, but this broadband Stokes wave spanning a wavelength range over 70 nm from 1210 nm to 1280 nm. With further increasing of the pump power, the cascaded SRS still happens but with no obvious Stokes wave peak. In this process, both the Raman gain in the GDF and PM-GDF as well as the ytterbium ion gain in the YDF and PM-YDF contribute to the spectrum broadening and a total of three orders of Stokes waves are generated which makes the spectrum broaden to around 1.3 µm. However, no obvious Stokes peaks can be observed in the anomalous dispersion region of the fiber. This is because the generated higher order Stokes waves have broad bandwidth but weaker intensity so they can be easily merged together and with good flatness. As the pump power is transferred to the anomalous dispersion region of the fiber, modulation instability (MI) and soliton related effects (e.g. soliton fission and soliton self-frequency shift) will keep the spectrum further broadening to the long wavelength direction. Dispersive wave generation and four-wave mixing (FWM) effects are the main reasons to extend the spectrum to the short wavelength side. These combined nonlinear effects contribute to the generation of the final broadband SC. The output spectral differences between the two structures mainly result from the preserved polarization which makes the most of the power concentrate on one axis, so the nonlinear interactions can be effectively enhanced for the linearly-polarized RFL even at the same pumping condition [35]. As a result, soliton related effects and dispersive wave generation are strengthened which can broaden the spectra in both the longer and shorter wavelength regions. Meanwhile, as the Raman gain is polarization sensitive [37,38], it is found that when the pump light and Stokes waves are polarized parallelly to each other, they have a much higher Raman gain than the randomly polarized case. Thus, the enhancement of SRS effect will be achieved by the employment of PM-GDF which is beneficial for the spectrum broadening to the long wavelength side. Besides, the long distance fiber length in an RFL structure further enhanced these effects which makes the output spectrum of the linearly-polarized RFL extend to both sides smoothly. In addition, an obvious spectral peak ω (685 nm) is emerged at the short wavelength side. This is probably induced by the FMW effect, where phase matching and group velocity matching can easily meet in these processes [26].
Significantly, we also notice that the spectral characteristics of the linearly-polarized RFL with 100 m PM-GDF is better than our previous experimental results using unpolarized RFL with 1000 m GDF [26]. The comparative experimental results are illustrated in Fig. 7. In Ref. [39], we have also investigated the influence of the GDF length on the final output SC. By increasing the length of GDF to 1000 m, the spectral characteristics of SC show obvious improvement in spectral flatness and the short wavelength side of the spectrum is extended to 600 nm. But in this paper, we find that the spectral characteristics of the linearly-polarized RFL shows better performance even with a much shorter fiber length of only 100 m. The 20 dB bandwidth of the output SC is about 1000 nm which is much wider than the unpolarized RFL with 1000 m GDF where the 20 dB bandwidth is only 660 nm, and the long wavelength edge of the output SC is also enhanced from 1700nm to 1900nm. Usually, to excite SC generation in the RFL structure, higher nonlinear fiber and longer length fiber are used for the accumulation of nonlinear effects [24–27]. However, the preserved polarization and high Raman gain of the PM-GDF can enhance the nonlinear interactions, so there’s no need for a longer distance fiber or higher nonlinear fiber. The results indicate that the linearly-polarized RFL can greatly shorten the cavity length compared with unpolalrized RFL and shows a better spectral characteristics in terms of bandwidth and flatness, which is very meaningful to further simplify the RFL structure and improve the output SC performance.
Fig. 7. The spectral comparison between the unpolarized RFL and the linearly-polarized RFL while 1000 m GDF and 100 m PM-GDF are used respectively.
The polarization characteristics of the output spectrum under different pump power is also investigated. The polarization extinction ratio (PER) of the linearly-polarized RFL is monitored by an extinction ration meter (ERM) and its operating wavelength range is 800 to 1700 nm. Figure 8 shows the PER of the output SC at different pump power. The PERs of the output SC are all above 18 dB and it reduces gradually and finally reaches a stable state as the pump power increases. The deterioration of the PER is probably induced by the coupling of the two orthogonal polarization states [33]. Comparing with the results at high pump power, the PER shows a better performance at low pump power. We believe the reason is that the SC power from a polarization state will be transferred to the orthogonal state when the pump power in the PM-GDF reaches a certain level. As the pump power keeps increasing, the SC power transferring is gradually enhanced and finally reach a stable state, which narrows the gap between the two polarization states. Though the PER shows a little decreasing trend with the increasing of the pump power, however, the generated SC can still be treated as a well linearly-polarized SC source based on the theoretical analysis illustrated in Ref. [29].
4. Conclusion
In this paper, an all-polarization-maintaining RFL structure is constructed for the first time for linearly-polarized SC generation. An average output power of 4.43 W with a spectral range from 600 nm to 1900nm is obtained, and the measured PER is greater than 18 dB at the highest output power level. To the best of our knowledge, this is an all-fiber linearly-polarized SC output directly from a RFL ever reported. The comparison experiment shows that the linearly-polarized RFL has an enhanced bandwidth and improved spectral flatness compared with the unpolarzied counterpart, and can greatly shorten the cavity length which is very meaningful to further simplifying the RFL structure. Compared with other linearly-polarized SC generation method, the method applying in this paper not only inherit the simplicity of the RFL structure, but also show the potential of further improving the SC bandwidth and flatness, which is very meaningful to provide a simple, low cost and reliable method of generating linearly-polarized SC for many practical applications such as hyperspectral imaging and remote sensing.
Funding
State Key Laboratory of Pulsed Power Laser Technology (SKL2019ZR02).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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