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Abstract
In this paper, we make a comprehensive study on tailoring the spectrum and transverse mode of random fiber lasers (RFLs). By simply temperature tuning, the mode gain profile of RFL can be flexibly and precisely manipulated. The spectrum of laser output can be easily tailored in single-wavelength, dual-wavelength, and three-wavelength, respectively. Meanwhile, the operating transverse mode is also optional among LP01 mode, LP11 mode, and hybrid mode. The slope efficiency of 17.9% and 27.3% are obtained for LP11 mode and LP01 mode operation, respectively. Besides, the coherence control can be confirmed by making speckle contrast measurements. This high-efficiency RFL with the customizable spectrum and spatial mode would have unique applications in wavelength or mode division multiplexing systems, speckle-free imaging, secure communication, and information encryption.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Random fiber lasers (RFLs) with the feedback provided by the weak distributed Rayleigh scattering have attracted considerable attention since the first demonstration in 2010 [1]. Due to the special advantages of simple configuration, modeless emission, and low spatial coherence, RFLs have various applications in communication [2], sensing [3–5], imaging [6–8]. RFLs also present rich physical properties in the spectral, spatial, and temporal domains compared to conventional fiber lasers with a well-defined cavity structure. In the past decades, the development of RFLs has experienced a process from complete disorder to controllable output. Meanwhile, the manipulations of RFLs in the spectral, temporal, and spatial domains have attracted increasing attention to meet the requirements for specific applications. The controllable output of RFLs in the spectral domain has been well developed, such as multi-wavelength output [9–11], wavelength tuning [12–14], narrow linewidth [15,16], super-continuous spectrum generation [17,18]. Besides, RFLs with pulse operation could be realized based on acoustic-optical modulator [19], saturable absorber [20,21], stimulated Brillouin scattering effect [22], and spatiotemporal gain-modulation [23]. It is worth noting that most of the reported random fiber lasers are composed of single-mode fiber, resulting in only operating in fundamental mode, which means that those RFLs are unable to realize the direct manipulation of the spatial modes. Thus, to solve this problem, some methods are proposed to obtain high order mode output in RFLs such as splicing a length of the multimode fiber after the output of RFL [24], using few-mode fiber Bragg grating (FMFBG), long-period fiber gratings, mode selective couplers, mode injection locking, and spatial light modulator [25–29]. However, those proposed RFLs usually have a high threshold and low efficiency, essentially oscillating in the fundamental mode regime and lacking the flexible manipulation of the spatial mode, which impedes the practical applications of RFL.
Particularly, random fiber lasers show unique application prospects in spectral and spatial domains, such as super-resolution spectroscopy [30], speckle-free imaging [31,32], reducing the scintillation induced by atmospheric turbulence in the free space optical communication [33,34], secure communication, and information encryption [35]. However, most of the previously reported random fiber lasers are composed of single-mode fiber and only demonstrate single manipulation. Synchronous manipulation on the spectrum and spatial mode in random fiber laser has not yet been reported.
In this letter, we propose and demonstrate a few-mode random fiber laser that can achieve spectrum and spatial mode customization simultaneously. A homemade Lyot filter with a free spectral range of about 4 nm is used to manipulate the mode gain of RFL. The filtered spectrum of the Lyot filter can be quantitatively shifted through a temperature control system. By properly setting the temperature, single-, dual-, triple-wavelength outputs can be tailored, respectively. Meanwhile, in this all few-mode fiber structure, random fiber laser can directly oscillate in hybrid mode, LP01 mode, and LP11 mode respectively. Moreover, the controllability of the coherence is characterized by the speckle contrast measurements in different operating states.
2. Experimental setup and operating principle
The schematic for tailoring the spectrum and spatial mode of random fiber laser via temperature controlling is presented in Fig. 1. A segment of 1.5 m homemade ring core doped fiber (RC-YDF) (doping concentration of 5000 ppm) is used as gain fiber, which is pumped by a 980 nm laser diode via a 980/1060 nm wavelength division multiplexer (WDM). To reduce the lasing threshold, the RFL employs a half-open structure, which is composed of a 2 km long SMF-28e providing randomly distributed feedback and a few-mode fiber Bragg grating (FMFBG) serves as a highly reflective mirror. Noting that SMF-28e fiber (core diameter of 8.2 μm and numerical aperture of 0.14) is used as a few-mode fiber (FMF), which can support LP01 and LP11 mode at 1060 nm. The FMFBG is written on SMF-28e (reflectivity of 98% around 1056 nm). It is worth noting that the randomly distributed feedback is provided by Rayleigh scattering resulting from randomly distributed refractive index inhomogeneities of the fiber. Although Rayleigh scattering in the fiber is extremely weak, the effect can be accumulated in a 2 km long fiber. Only a fraction of the Rayleigh scattered light can be recaptured and amplified, which gives rise to the necessary feedback process for lasing action. A homemade all-fiber Lyot filter is used to modulate the gain profile of the mode in the cavity. A temperature control system with a resolution of 0.1℃ is adopted to tune precisely the transmission spectrum of the Lyot filter. An isolator with FMF pigtails is placed behind a 2 km-long fiber to eliminate unwanted Fresnel reflection at the output end facets of the fiber and works as an output coupler supporting LP01 and LP11 mode. In addition, to ensure random lasing and avoid parasitic oscillation, all the free end facets are cleaved at an angle of 8°. The characteristics of the laser output are analyzed by an optical spectrum analyzer, a CCD camera, a power meter, and a radio-frequency (RF) spectrum analyzer, respectively.
Fig. 1. Experimental setup for tailoring the spectrum and spatial mode of random fiber laser via temperature control. LD, laser diode; WDM, wavelength division multiplexer; FMFBG, few-mode fiber Bragg grating; RC-YDF, ring-core doping Yb-doped fiber; ISO, isolator; PBS, polarization beam splitter; TCS, temperature control system. FMF, few-mode fiber; SMF, single-mode fiber.
In our experiment, the Lyot filter is specially designed to realize multidimensional manipulation in the RFL, which is consisted of two polarization beam splitters (PBSs) with few-mode polarization-maintaining fiber (PMF) pigtails and a piece of 62-cm-long PM1550 fiber. The two PBSs (one works as a polarizer, and the other is used as the analyzer) are spliced with PM1550 fiber at an angle of 45° to couple light into both axes of the PMF, where port 2 is the output port of the Lyot filter, as shown in Fig. 1. The accumulated phase difference between the slow and fast axes of the PMF is given by $\textrm{2}\pi BL\textrm{/}\lambda $, where λ is the wavelength, L is the length of the PMF, and B represents the birefringence of the PMF. The Lyot filter has almost the same transmission functions for different transverse modes due to approximate birefringence. The transmittance of this Lyot filter is written as [36]:
The free spectral range (FSR) is calculated to be
Thus, the FSR can be tailored by changing the length of the PMF. The operation principle of the controllable RFL is illustrated in Fig. 2. The red line in Fig. 2(a) represents the reflection spectrum of the FMFBG. The three peaks in the reflection spectrum correspond to the reflection from LP11 to LP11 (1054.0 nm), LP01 to LP11 (1055.0 nm), and LP01 to LP01 (1056.0 nm), respectively. The blue line in Fig. 2(a) represents the simulated transmission spectrum of the Lyot filter with 62 cm-long PMF, and the wavelength spacing of the adjacent peak-valley is about 2 nm, which is equal to the spacing between reflection peaks of LP01-LP01 mode and LP11 to LP11 mode of the FMFBG. The peak (1054 nm) of the filtering spectrum locates at the left reflection peak of the FMFBG, which means that LP11 mode can get the highest gain, and LP01 mode can be well suppressed. Thus, this RFL can directly oscillate in LP11 mode. Moreover, due to the temperature dependence of birefringence B, the transmission spectrum of this filter can be tuned precisely by changing the temperature of the PMF. The temperature T and phase φ satisfy the following relationship.
Fig. 2. Operation principle of the controllable RFL. (a) The measured reflection spectrum of the FMFBG (red) and the simulated transmission spectrum of the Lyot filter (blue). (b) The measured temperature dependence of the transmission spectrum of the Lyot filter.
where LH is the length of heated PMF. Moreover, the effect of temperature on PMF length is negligible, that is dLH/dT = 0. The temperature dependence of the transmission spectrum is measured, as shown in Fig. 2(b). As the temperature increases, the minima of the transmission spectrum shifts toward a shorter wavelength at a rate of about 0.13 nm/℃. The three reflection peaks of the FMFBG will experience different gains and losses in different temperatures, which results in multiple operation states. Thus, the spectra and spatial modes of the random fiber laser can be customized by simply adjusting the temperature.
3. Results and discussion
3.1 Operation without the Lyot filter
Firstly, we investigate the spectral performance of random fiber laser without employing the Loyt filter. The output spectra with different pump power are plotted in Fig. 3. Random fiber laser operates at 1056 nm and stochastic spectral behavior can be observed at 1055 nm when the pump power is below 384.6 mW. As the pump power increases to maximum pump power of 580.4 mW, the spectra located at 1055 nm and 1056 nm broaden and stochastic spectral appear in 1054 nm. Taking the gain of specially designed RC-YDF and the total loss for LP11 (1054nm) and LP01 mode (1056 nm) into consideration, the net gain of LP01 mode is slightly higher than that of LP11 mode. Thus, the operating state of the laser is actually unstable due to strong mode competition, as presented in Fig. 3. In this case, the spectrum of the random fiber laser can not be flexibly manipulated and the oscillating spatial mode is also not tailored to LP11 mode.
3.2 Operation with the Lyot filter
By contrast, as the Lyot filter is used, spectral and spatial modes of the random fiber laser can be flexibly tailored. The transmission spectrum of this filter can be tuned precisely by changing the temperature. When the temperature is set to 24℃, the peak (1054 nm) and valley (1056 nm) of the filtering spectrum of the Lyot filter just locate at the FMFBG’s reflection peaks of LP11-LP11 mode and LP01-LP01 mode, respectively. It means that LP11 mode can obtain the highest gain, and LP01 mode will be effectively suppressed. Thus, the RFL can directly oscillate in LP11 mode, as shown in Fig. 4(a) (red line). The operating wavelength is 1054.01 nm corresponding to the left reflection peaks of the FMFBG. The 3 dB bandwidth is measured to be 0.05 nm, and the side-mode suppression ratio (SMSR) is about 40 dB. Moreover, the ratio between the signal peak and non-resonant peak of LP01-LP01 mode is more than 55 dB, which indicates that the RFL is of high mode purity. The doughnut-shaped intensity profile is shown in the inset of Fig. 4(a) (left), which is the typical characteristic of the second-order mode. Using the bending method proposed in Ref. [37], when random fiber laser operates at LP11 mode, the fiber of the output end is bent to a circle with a radius of 1.2 cm, and the output power drops from 23.58 mW to 0.42 mW, which gives a 98.2% loss. To eliminate the contribution of the fundamental mode, we measure the power change from 20.3 mW to 18.84 mW when the fundamental mode propagates in the same conditions, which gives a 5.8% loss. Thus, the purity of LP11 mode is estimated to be 98.1%.
Fig. 4. Output results when the temperature is set to 24℃ and 39℃. (a) The spectrum of the laser oscillating in LP11 mode (red) and LP01 mode (blue). The inset shows the intensity distribution of the output beam. (b) Output power varies with the pump power in two different operation states. The stability measurement of the output spectrum in 30 minutes for (c) LP11 mode operation and (d) LP01 mode operation; The corresponding intensity (blue) and central wavelength (black) variation for (e) LP11 mode operation and (f) LP01 mode operation.
As the temperature is set to 39℃, the filtering spectrum can be easily tailored into another special case. The filtering spectrum blueshifts with the temperature increasing. In this case, the output spectrum is monitored, as presented in Fig. 4(a) (blue line). The peak (1056 nm) of the filtering spectrum locates at the right reflection peak of the FMFBG, and the left reflection peak of the FMFBG lies in the valley (1054 nm). That is to say, the wavelength at 1056 nm experiences low loss and gets higher gain, and the wavelength at 1054 nm will be completely suppressed due to the filtering effect. Thus, the RFL operates at 1056 nm. Owing to the particular transverse mode-wavelength association characteristics of the FMFBG [38], only LP01 mode can be reflected into the cavity and directly oscillate above the laser threshold. The output intensity distribution is presented in the inset of Fig. 4(a) (right), which is the typical feature of the fundamental mode (LP01). The 3 dB linewidth of the output spectrum is 0.09 nm, which is limited by the bandwidth of the FMFBG and available pump power. The SMSR is as high as 47 dB, and the ratio between the signal peak and non-resonant peak of LP11-LP11 mode is more than 60 dB, which verifies that LP11 mode has been effectively suppressed.
Figure 4(b) shows the variation curve of the output power with the pump power increasing for the above two operation states. The threshold of the RFL operating in LP11 mode is about 153 mW and the slope efficiency is measured to be 17.9%, which is higher than that of reported RFLs with LP11 mode output [25–28]. The relatively high efficiency may be attributed to the LP11 mode direct oscillation and the special design of the RC-YDF benefiting for LP11 mode competition [39]. Similarly, the output power increases with a slope efficiency of 27.3% once the pump power is over the lasing threshold of 103.6 mW for LP01 mode operation when the temperature is set to 39℃. At the maximum pump power of 580.4 mW, the maximum output power of LP11 mode and LP01 mode operation are 81.6 mW, 132.4 mW, respectively.
Moreover, to confirm the long-term stability of the proposed RFL, the output spectrum and corresponding intensity are monitored in two different operation states for 30 minutes. The spectra in Fig. 4(c) and 4(d) are repeatedly scanned at a 5-min interval in 30 minutes. The variation of the intensity and central wavelength are summarized in Figs. 4(e) and 4(f). For LP11 mode operation, the central wavelength maintains at 1054.0 nm and the fluctuation of the output power is less than 0.5 dB, as shown in Fig. 4(e). Similarly, for LP01 mode operation, the central wavelength nearly keeps at 1056.02 nm and the intensity fluctuation is within 0.5 dB. These results indicate that the proposed RFL has good stability when operating in LP11 mode and LP01 mode.
Except for the two special outputs mentioned above corresponding to single wavelength LP11 mode operation and single wavelength LP01 mode operation, respectively, dual-wavelength, three-wavelength hybrid mode outputs can be also tailored by controlling the temperature. Figure 5 shows the output spectra of this RFL at different temperatures. Dual-wavelength output is shown in Fig. 5(a) when the temperature increases from 24℃ to 31.5℃. The oscillating wavelengths are 1054 nm and 1056 nm, respectively. It is worth noting that the valley of the filter spectrum just locates at the peak of the mutual coupling between the LP01 and LP11 mode, which causes the wavelength at 1055 nm to be suppressed. An approximate flattop profile is obtained, as presented in the inset, which can be considered as an incoherent superposition of a donut-shaped hollow beam and a solid center beam (LP01 mode) [40]. The dual-wavelength operation can be flexibly switched as the temperature is set to 45.5℃ and 49℃, as shown in Fig. 5(b) and Fig. 5(d). In different operation states, the intensity profile of the laser output is also variable due to the change of component content for spatial modes, as shown in the inset. When the temperature is set to 47℃, the RFL can operate at triple wavelengths state, as shown in Fig. 5(c). The oscillating wavelengths are 1054 nm, 1055 nm, and 1056 nm, which corresponds to the three reflection peaks of the FMFBG, respectively. Moreover, to intuitively understand the tuning behavior of the proposed RFL, the evolution of the output spectrum with the temperature changing from 24℃ to 52℃ is investigated, as shown in Fig. 6. It can be seen that the filtering spectrum blueshifts as the temperature increases and the intensity of the wavelength at 1054 nm first decreases and then increases. On the contrary, the intensity of the wavelength at 1055 nm and 1056 nm first increases and then decreases. The spectrum of the random fiber laser can be quantitatively controlled by an appropriate temperature setting.
Fig. 5. The spectra of the laser output in different temperatures: (a) 31.5℃. (b) 45.5℃. (c) 47℃. (d) 49℃. The intensity profile of the corresponding output beam is shown in the insets.
To confirm the controllability of the coherence and the effectiveness of the designed RFL in reducing laser speckle, the speckle patterns of the laser output beam in different operating states are measured through a ground glass diffuser (grit number of 400 which provides moderate scattering strength), as shown in Fig. 7. Figure 7(a) shows speckle patterns of the conventional fiber laser operating in single-wavelength LP01 mode. Figures 7(b)-(e) show speckle patterns of the proposed random fiber laser operating in single-wavelength LP01 mode (39℃), single-wavelength LP11 mode (24℃), dual-wavelength hybrid mode (31.5℃), and three-wavelength hybrid mode (47℃), respectively. The quantitative analysis of speckle contrast C = σ/〈I〉 (where σ is the standard deviation of the intensities, and 〈I〉 is the average intensity) is presented. The speckle contrast for conventional fiber laser is calculated to be 0.502. The speckle contrasts for random fiber laser are calculated to be 0.482, 0.436, 0.358, and 0.322 correspondings to the operating temperature of 39℃, 24℃, 31.5℃, and 47℃, respectively. It can be seen that RFL can reduce the influence of laser speckle better than that of conventional fiber lasers. Besides, in terms of designed random fiber laser, the speckle contrast with single-wavelength LP11 mode output (24℃) reduces by 9% compared with single-wavelength LP01 mode output (39℃), and with three-wavelength and multi-spatial mode excited simultaneously as the temperature is set to 47℃, the speckle contrast further reduces by 33%. The three-wavelength regime has a smaller speckle contrast than that of the two-wavelength regime, which contributes to a wider effective bandwidth for the three-wavelength regime. These results show the proposed RFL can reduce the laser speckle, and confirm that coherence can be flexibly controlled just by changing the temperature.
Fig. 7. Speckle patterns formed passing through a ground glass diffuser in different operating states: (a) conventional fiber laser. (b) Single-wavelength LP01 mode, (c) Single-wavelength LP11 mode, (d) Dual-wavelength hybrid mode and (e) Three-wavelength hybrid mode generated by the proposed random fiber laser.
Finally, to further verify that the proposed laser operates as random lasing, radio-frequency (RF) beat spectra were analyzed in different operation states, as shown in Fig. 8. Random fiber laser without a well-defined cavity length presents a modeless spectrum comprised of random frequency components due to random distributed feedback provided by Rayleigh scattering along a long single-mode fiber. Figures 8(a) show RF spectra of the laser output when the laser operates at LP01 mode. It can be seen that there is no obvious longitudinal mode beating signal corresponding to the cavity length ($\varDelta \mathrm{\nu =\ }\,c/2nL \approx 50$ kHz) [41]. However, a conventional fiber laser that has a fixed cavity length shows the distinct longitudinal mode beating signal when the isolator of the output end is removed and the output end is cleaved to be flat to provide Fresnel reflection of 4%. The laser oscillates in the LP01 mode regime, and the corresponding RF spectrum is shown in Fig. 8(b). The frequency interval is about 50 kHz corresponding to the cavity length of 2 km. These results further verify that the laser we proposed operates as a random fiber laser.
Fig. 8. Measured RF spectra within a 1 MHz span range with a resolution of 30 Hz in different operation states. (a) RFL with an isolator operating in the LP01 mode state. (b) Conventional fiber laser operating in the LP01 mode state when the output end is cleaved to flat.
4. Conclusion
In summary, we experimentally demonstrate tailoring the spectrum and spatial mode of few-mode random fiber laser by temperature tuning. A homemade all-fiber Lyot filter is employed to manipulate the mode gain profile of random fiber laser. By setting appropriate temperature, the filtering spectrum of the filter can be precisely controlled, single-, dual-, three-wavelength output can be easily realized, respectively. Meanwhile, owing to the particular transverse mode-wavelength association characteristics of the FMFBG, the oscillating transverse mode of RFL is controllable among LP11 mode, hybrid mode, and LP01 mode. High efficiency of 17.9% and 27.3% are obtained as RFL directly oscillates in LP11 mode and LP01 mode regime, respectively. Besides, speckle contrast measurements are made to verify the controllability of coherence in different operation states, which means that coherence can be tailored by manipulating spectrum and spatial mode. This few-mode random fiber laser with high efficiency and simple structure may have a promising application in wavelength or mode division multiplexing systems, secure communication, information encryption, and speckle-free imaging. Moreover, our work may a valuable reference on the controllable emission of multimode random fiber laser.
Funding
National Key Research and Development Program of China (2021YFF0307804).
Disclosures
The authors declare no conflicts of interest.
Data availability
The 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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