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Abstract
An all-fiberized random distributed feedback Raman fiber laser (RRFL) with mode-modulation-induced wavelength manipulation and dual-wavelength generation has been demonstrated, where an electrically controlled intra-cavity acoustically-induced fiber grating (AIFG) is employed to adjust the input modal content at the signal wavelength. The wavelength agility of both the Raman effect and the Rayleigh backscattering in RRFL benefits on broadband laser output in case of broadband pumping. The feedback modal content at different wavelengths can be adjusted by AIFG, and then the output spectral manipulation can be ultimately manifested through the mode competition in RRFL. Under the efficient mode modulation, the output spectrum can be continuously tuned from 1124.3 nm to 1133.8 nm with single wavelength, while ulteriorly the dual-wavelength spectrum can be formed at 1124.1 nm and 1134.7 nm with a signal-noise-ratio of 45 dB. Throughout, the power is beyond 47 W with good stability and repeatability. To the best of our knowledge, this is the first dual-wavelength fiber laser based on mode modulation and the highest output power ever reported for an all-fiberized continuous wave dual-wavelength fiber laser.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The dual-wavelength fiber lasers (DWFLs) have become a research hotspot and caused enormous interest and attention due to their humorous applications in the fields of microwave generation [1,2], optical sensors [3,4], and step-height measurement [5]. For example, the sensing system based on a DWFL generating two quadrature phase-shifted signals could detect temperature changes [4]. Up to now, DWFLs based on rare-earth-doped fibers have been investigated broadly. The key problems are the inevitable intense mode competition and mode hopping existing in the homogeneously broadening gain medium, which may cause lasing instability [6]. To deal with those problems, researchers tried to cool the fiber with liquid nitrogen [7,8], which proved to be beneficial but costly and complex. In addition, other methods to release the temperature condition have also been employed, such as using the frequency shift feedback technique [9], adding wavelength or intensively-dependent loss structures [10], inserting filter devices [11], and using mediums with high nonlinearity [12].
In the past decades, the random distributed feedback Raman fiber laser (RRFL) proposed by Turitsyn et al. has made considerable progress [13–23]. Although the backward reflection provided by the random Rayleigh backscattering is extremely small (∼0.1), the lasing threshold may be achieved when enough distributed Raman gain is provided. Compared with the conventional Raman fiber lasers, the RRFL possesses advanced features in lower coherence and simpler structure. Moreover, the broadband nature of both Raman gain and Rayleigh backscattering in the RRFL is attractive for spectral manipulation [24–27] and dual-wavelength generation [28–31], opening a novel possibility to tune the generated wavelength in a wide range. Besides, it has been shown that the spectral and power performances of multiwavelength RRFL overpass those of conventional lasers based on end reflection [32]. A. E. El-Taher et al. demonstrated the first dual-wavelength RRFL by inserting two fiber Bragg gratings (FBGs) at 1551 nm and 1550 nm at both sides of the same fiber span [33], but the long fiber (as long as 200 km) and the losses of FBGs made the efficiency less than 5%. In 2019, J. Song et al. proposed a dual-wavelength RRFL with over 100 nm wavelength interval based on phosphosilicate fiber owing to the two Raman gain peaks at the frequency shifts of 13.2 THz (silica-related) and 39.9 THz (phosphorus-related) [28]. The output wavelengths were 1120 nm and 1237 nm respectively, with maximum output power exceeding 23 W and total conversion efficiency of ∼60%, which was limited by the high loss of phosphosilicate fiber and second-order Stokes light. Furthermore, Y. Zhang et al. achieved a spectra-manipulable and dual-wavelength output around 1110 nm by adopting two bandwidth-adjustable optical filters with nearly a 10 W output power and signal-noise-ratio (SNR) of 29 dB [34].
In this paper, we proposed a novel method to accomplish an all-fiberized RRFL with mode-modulation-induced dual-wavelength generation as well as single-wavelength shift by inserting an acoustically-induced fiber grating (AIFG) to regulate the feedback modal content near the high-reflective port. By switching the modes between the LP01 mode and the LP11 mode at a specific wavelength based on the AIFG, the single central wavelength shifted from 1124.3 nm to 1133.8 nm with a maximum power over 47 W. Moreover, at the opportune intermediate frequency of 785.5 kHz, a typical dual-wavelength spectrum was formed whose maximum power was 47.7 W with a wavelength interval and SNR of 10.6 nm and 45 dB, respectively. To the best of our knowledge, this is the first DWFL based on mode modulation and the highest output power ever reported for a continuous wave all-fiberized DWFL.
2. Experimental setup
To investigate the dependence of feedback modal content and output spectrum, the RRFLwith mode-switchable feedback is demonstrated, as shown in Fig. 1. An amplified spontaneous emission (ASE) source including two amplification stages is utilized as the pump source because of its advantages in high temporal stability [35–38]. As shown in Fig. 2, the pump wavelength is 1074.5 nm with a 3-dB linewidth of 2.6 nm at the maximum power level. The pump source is then coupled into the half-open cavity of RRFL by wavelength division multiplexing (WDM). The half-open cavity is formed by a high-reflective (HR) optical fiber mirror which is attached to the WDM, a piece of few mode fiber (FMF), and a homemade fiber endcap. The reflectance of the HR mirror is more than 99.5% at 1-2 µm and the endcap is anti-reflection coated to evade unwanted end feedback. All fusion splices are handled carefully in order not to generate extra reflection in the RRFL. The FMF is commercial graded-index (GRIN) fiber with the parabolic profile of refractive index. The core diameter and numerical aperture (NA) of the FMF are 20 µm and 0.14 separately with a length of 400 m.
The AIFG is the key component to modulate feedback mode in RRFL, which is located between the WDM and the GRIN fiber. The AIFG consists of a piezoceramic transducer (PZT), a piece of few-mode fiber (FMF), and a radio frequency (RF) source attached to provide the electrical signal acting on the PZT. When the periodic electrical signal loads, the PZT starts to vibrate, and then the acoustic wave caused by vibration further modulates the refractive index of the fiber in AIFG periodically. Once the phase matching condition [39] is matched, which is shown as the frequency is appropriate for a certain wavelength, the AIFG could convert LP01 mode to LP11 mode effectively with a switching time of less than 1 ms. Compared with other kinds of mode converters, the AIFG possesses the advantages of wavelength tunability and high power tolerability. There is a one-to-one correspondence between the frequency of electrical signal and the operating wavelength, and the modal conversion could shift to longer wavelengths when the frequency increases. More characteristics of AIFG are introduced in Reference [40–42].
3. Results and discussion
3.1 Spectrum and power of RRFL without AIFG working
Figure 3(a) exhibits the output signal power as a function of the pump power when there is no signal loaded on AIFG. It is worth noting that the growth curve of output signal power is not straight. When the pump power exceeds the threshold of around 34 W, but below 50 W, the signal power grows rapidly and the calculated slope efficiency η1 is up to 169.2%, which is far higher than formal quantum efficiency η=λp/λs. This phenomenon has been explained by numerical calculation in detail in Ref. [32]. Indeed, in the RRFL with a forward-pumped configuration, the highest power and efficiency are achieved in a short fiber, despite the generation threshold being comparatively high. Thus, a small increase in the pump power near the threshold can cause a large amount of energy to be transferred to the signal wavelength, resulting in the slope efficiency greatly exceeding the quantum efficiency. The output power curve is sufficiently nonlinear which can be divided into two different regimes: a high slope efficiency just above the lasing threshold and a lower slope efficiency and higher absolute efficiency at higher pump power. Therefore, the output power grows slower and the calculated slope efficiency η2 is 77.4% when the pump power exceeds 50 W. And the overall light-to-light conversion efficiency is 69.4%.
Fig. 3. (a) The output signal power as a function of pump power. (b) The spectrum at different output power levels.
Figure 3(b) demonstrates the spectra at six output power levels. It is evident that in the first two lower power levels, the 1st-order Raman Stokes light is quite unstable, which appears as many burrs on the spectra, and even 2nd-order and 3rd-order Raman Stokes light occurred. When the pump is near the threshold, the signal spectrum is broad and corresponds to the spontaneous Raman scattering spectrum generated spontaneously in the RRFL. Nevertheless, the spectrum becomes much narrower for higher pump power over the threshold, indicating that longitudinal mode competition is completed and the laser output becomes stable. At the maximum output power, the central wavelength and 3 dB linewidth of the signal light is 1130.24 nm and 1.6 nm, respectively, where the intensity of 2nd-order Stokes is 48 dB lower than that of the signal light. The further increase of pump power and the optimization of fiber length are expected to achieve higher output power of the RRFL.
3.2 Single-wavelength shift and dual-wavelength generation
To investigate the dependence of feedback modal content and output spectrum, electrical signals at different frequencies are loaded on the AIFG. Figure 4(a) shows the spectra at different loaded signal frequencies with corresponding output beam spots inserted at the maximum pump power. When the loaded frequency changes from 776 kHz to 795 kHz, the central wavelength continuously shifts from 1124.3 nm to 1133.8 nm, with a typical dual-wavelength spectrum at 785.5 kHz. The red arrows refer to the modulated wavelength corresponding to the loaded frequency of the electrical signal. Indeed, it seems that most of the energy is always concentrated outside the regulated wavelength of AIFG in all spectra. Figure 4(b) provides more details about the dual-wavelength spectrum at 785.5 kHz, where the two peak wavelengths are 1124.1 and 1134.7 nm independently with a wavelength interval of 10.6 nm and the SNR of 45 dB. There is a trough at 1129.5 nm between the two peaks which is precisely the modulated wavelength corresponding to 785.5 kHz with a peak-to-peak intensity of 22 dB. Moreover, there are two lower peaks located on both sides of the dual-wavelength, whose wavelengths are 1112.5 nm and 1147.3 nm, respectively, which are caused by the four-wave mixing effect since there are two main optical frequency components propagate simultaneously in the fiber. Figure 4(c) illustrates the output power and the purity of the LP01 mode as a function of the loaded frequency, from which we can see that when the frequency increases, the output power decreases first and then increases, with the lowest power of 47.1 W at 785 kHz, which is near the dual-wavelength performance. Moreover, during the frequency-increasing process, the output beam profiles remained in the Gaussian shape, as the beam profiles inserted in Fig. 4(a). To know more about the mode component of the output laser, we conduct the mode decomposition based on the stochastic parallel gradient descent (SPGD) algorithm [43]. As shown in Fig. 4(c), the mode purity of the LP01 mode is beyond 95% in the whole process, implying that the AIFG could no longer accomplish the mode conversion when it was employed near the high-reflective port.
Fig. 4. (a). The spectra at different frequencies with corresponding output beam spots. (b) The dual-wavelength spectra at 785.5 kHz. (c) The output power and the purity of the LP01 mode as a function of the loaded frequency.
During the power-increasing process of both single-wavelength and dual-wavelength output, the instability of spectra near the threshold has been observed, and the output temporal performances and statistical properties are measured and characterized seriously, which are recorded in a time frame of 4 seconds using a 2 GHz oscilloscope combined with a 5 GHz photodetector. Figure 5 shows the temporal traces at the output power of 4 W, 20 W, and 49 W of the single-wavelength output and the output power of 20 W and 48 W of the dual-wavelength output. As shown in Fig. 5, in the single-wavelength case, the temporal traces at 4 W are much more unstable than that of 49 W, and the calculated standard deviation (Std) of the former is about six times of the latter. The diversity of the results is connected to the unstable lasing caused by the spontaneous Raman scattering near the threshold, as mentioned in 3.1. In the dual-wavelength case, the obvious dual-wavelength spectra caused by AIFG are observed only after the power level exceeded 20 W, which was considered as the dual-wavelength threshold. Not surprisingly, the temporal traces at the highest power of 48 W are more stable than that of the threshold in the dual-wavelength case, which is similar to the single-wavelength condition. Moreover, the temporal traces for the single-wavelength output is more stable than that of the dual-wavelength at the same power level of 20 W, and the calculated Std of the latter was twice of the former since the single-wavelength output has become stabilized while the dual-wavelength is just near the threshold. According to the principle of the AIFG, the dual-wavelength generation is related to the mode evolution and competition along the fiber, which inevitably increases irregularity and causes degraded results in temporal stability when the dual-wavelength output comes into being. Nevertheless, the degradation is no longer observed at the highest power levels (∼49 W for single-wavelength and ∼48 W for dual-wavelength) since the mode-modulation-induced dual-wavelength generation has been stabilized. The measured temporal performances indicate that the mode-modulation-induced dual-wavelength generation doesn’t increase the instability of RRFL.
Fig. 5. The temporal traces of single-wavelength output at the output power of 4 W, 20 W, and 49 W, and dual-wavelength output at the output power of 20 W and 48 W.
For potential practical applications, the spectral and power stability are also checked. Figure 6(a) presents the single-wavelength output spectra of the RRFL, taking the central wavelength of 1132.24 nm as an example, which are recorded every 4 minutes within a total time frame of 20 minutes. As shown in Fig. 6(b), the maximum peak-power fluctuation measured is about 0.6 dB, and the maximum fluctuation of central wavelength is less than 0.12 nm. As for the dual-wavelength situation, as shown in Fig. 6(c) and Fig. 6(d), at the first lasing line (1124.1 nm), the maximum peak-power fluctuation is within 0.95 dB, while the maximum peak-power fluctuation at the second lasing line (1134.7 nm) is within 1.6 dB. The insignificant wavelength variation and power fluctuation indicate that the laser possesses good stability.
Fig. 6. (a) Spectrum stability of 1132.24 nm. (b) Wavelength and peak-power fluctuation of the laser line versus time. (c) Spectrum stability of dual-wavelength spectrum. (d) Wavelength and peak-power fluctuation of the two laser lines versus time.
3.3 Further discussion
By inserting the wavelength-tunable AIFG, the mode-modulation-induced dual-wavelength generation, as well as single-wavelength shift, have been achieved, and the dynamics of the mechanism should be studied. According to the characteristics of mode conversion based on AIFG, we consider that the modulated spectra are related to the mode competition in the RRFL. There are two laser gain mechanisms in RRFL, one is the stimulated Raman scattering (SRS) effect which provides a frequency shift of 13.2 THz in silica-based optical fiber [44], and the other is random-distributed feedback scheme provided by Rayleigh backscattering on a natural disorder of the fiber core’s refractive index [13]. To explain the phenomenon, the power evolution of RRFL with different modes propagated is simulated by a set of temporal-spatial-coupled cascaded Raman equations taking into account the distributed Rayleigh backward scattering [45]:
Based on the equations above, the power evolution in the RRFL with different modes propagated can be simulated. In the GRIN fiber, the calculated effective mode field area of LP01 mode and LP11 mode are 76.5 µm2 and 102.5 µm2 around the pump wavelength. Figure 7 shows the calculated power evolution as a function of fiber length within 200 m at 100 W pump power, where R01 and R11 represent the relative mode components of the LP01 mode and LP11 mode at the input port, respectively, which was controlled by the AIFG.
Fig. 7. The calculated power evolution as a function of fiber length with different reflectivities of LP01 mode and LP11 mode. (a) R01 = 1. (b) R01 = 0.1, R11 = 0.9.
As shown in Fig. 7(a), when the AIFG is not working, there is only LP01 mode propagated along the fiber (R01 = 1). It is no doubt that the output power of the LP01 mode grows rapidly with the increase in fiber length. When the AIFG is working, the propagating LP01 mode could be converted to LP11 mode. Taking the mode conversion efficiency of the AIFG into account, which is set as 90%, the main mode propagated in the RRFL is the LP11 mode generated by the AIFG (R01 = 0.1, R11 = 0.9). As shown in Fig. 7(b), it is worth noting that even though the component of the LP11 mode is considerably higher than that of the LP01 mode, it is still the LP01 mode that dominates the RRFL output ultimately. The decrease in component of the LP01 mode only reduces the absorption rate of the pump light, making the signal light increases slower along the fiber. Based on these simulations, it can be concluded that only the LP01 mode can be efficiently amplified in the RRFL with this configuration, which could explain why the purity of the LP01 mode is more than 95% in the output laser and the lower intensity at the modulated wavelength in Fig. 4(a).
In a typical RRFL configuration, both Raman gain and Rayleigh backscattering are possible to generate wavelength in a wide range. Ulteriorly, the broadband pump ASE source also improves the possibility of broad bandwidth in the setup. However, according to the principle of AIFG, there is a one-to-one correspondence between the loaded signal frequency and the modulated wavelength. In other words, when the AIFG is applied to a laser with broadband output, it can only regulate a specific single wavelength but does not affect other wavelengths. Assuming the LP01 mode is converted to LP11 mode at a specific signal wavelength λi, the absorption rate of the pump and the Raman gain at λi are slower than that of the other wavelengths, thus other wavelengths of LP01 mode grow first and resulting in lower intensity at λi. A schematic diagram of this process is shown in Fig. 8, from which one can see that after the transmission along the passive fiber, the wavelength transmitting LP11 mode is suppressed and the mode competition in different wavelengths ultimately manifests as wavelength manipulation. It is worth noting that there is a special intermediate modulated wavelength, in which the energy could be evenly concentrated on both sides, causing a unique dual-wavelength phenomenon to come into being. As a result, the intensity at the wavelength of LP11 mode is lower consequently, causing the spectrum “cupped”, which looks like the energy has been “squeezed” to other wavelengths. The experimental results also prove that the mode-modulation-induced dual-wavelength generation is practical and maneuverable.
4. Conclusion
In this paper, we demonstrate an RRFL with wavelength manipulation and dual-wavelength output. By adjusting the electrical frequency loaded on the intra-cavity AIFG, the output spectrum can be tuned from 1134.3 nm to 1133.8 nm continuously. Moreover, a typical dual-wavelength spectrum appeared at the frequency of 785.5 kHz, whose wavelength interval and signal-noise-ratio are 10.6 nm and 45 dB, respectively. Furthermore, the power evolution of RRFL with different modes propagated is simulated, and the dynamics of mode-modulation-induced spectral manipulation has been explained. Thanks to the wavelength-tunable and high-power-tolerable mode conversion property of AIFG, the RRFL possesses agile spectra manipulation capability with high power output. The output power is beyond 47 W in the whole process, and to the best of our knowledge, this is the first DWFL based on mode modulation and the highest output power ever reported for a continuous wave all-fiberized DWFL. Owing to the flexible spectra and dual-wavelength output, the proposal RRFL can be far utilized as the pump source in terahertz microwave generation, optical sensors, and optical parametric oscillators.
Funding
National Natural Science Foundation of China (12174445, 62061136013).
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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