We experimentally demonstrate a method to obtain central wavelength tunable orbital angular momentum beams with switchable topological charges (+1 or -1) in a stimulated Brillouin scattering erbium-doped fiber laser. Multiwavelength operation is achieved through cascaded stimulated Brillouin scattering in a single-mode fiber with a length of 6 km initiated by an external Brillouin pump. High-efficiency mode conversion between the fundamental mode and the orbital angular momentum modes is realized through a broadband two-mode long-period fiber grating. High-purity orbital angular momentum beams with up to 10 stable wavelength channels with a tuning range of 35 nm are achieved, which is the highest number of operating wavelengths and tuning range in an all-fiber laser for orbital angular momentum beam emission to the best of our knowledge. Both the operational central wavelength and number of operating wavelengths can be tuned by adjusting the primary pump power and the center wavelength of the tunable bandpass filter in conjunction with changing the Brillouin pump wavelength.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Light beams carrying orbital angular momentum (OAM), also known as vortex beams, owing to their special properties such as donut intensity distributions and the helical phase front, have found considerable applications in optical communication [1–3], optical manipulation , and high-resolution imaging . In free-space optical systems, many approaches, including applying helical phase plates , Q-plates  and spatial light modulators , have been demonstrated to convert Gaussian beams to OAM beams.
In recent years, generating OAM beams in an optical fiber has attained increasing attention due to their high level of integration, excellent stability and low cost. Since OAM modes can be expressed as the superposition of high-order modes with a 90° phase difference (i.e., HE21even±iHE21odd modes) [9,10], it is essential to make use of few-mode fibers (FMFs) supporting high-order modes. Several structures have been applied to convert the fundamental mode to high-order modes, such as offset-fused fibers [11,12], mode selective couplers (MSCs) [13–15] and few-mode long-period fiber gratings (LPFGs) [16–18]. All fiber lasers producing OAM beams based on the above structures have been demonstrated recently [19–21].
Multiwavelength fiber lasers (MWFLs) have been extensively studied for their potential to be cost-effective multipurpose sources for many demanding applications, including optical sensing , wavelength division multiplexing (WDM) and radiofrequency signal generation . Numerous methods have been demonstrated to obtain MWFLs, for instance, by employing various fiber-based filters [24–28] and stimulated Brillouin scattering (SBS)  in ultralong single mode fibers (SMFs). SBS in SMFs has the advantages of low threshold power, narrow linewidth Brillouin gain spectrum and high nonlinear coefficient, which makes it an ideal technique for multiwavelength generation [30–32]. Utilizing SBS, multiwavelength lasing with narrow-linewidth spectral lines was achieved in an erbium-doped fiber laser (EDFL), which is called hybrid Brillouin–erbium fiber laser (BEFL). In optical communications, narrow-linewidth light sources are often desired in dense WDM systems. In addition to their simple configuration, BEFLs possess attractive features such as narrow linewidth and rigid frequency spacing, which makes them promising candidates for dense WDM systems [33–36]. A multiwavelength BEFL generating OAM beams may offer a low-cost and compact light source for a wavelength-OAM multiplexing system to increase the communication transmission capacity . This may also have relevant applications in remote sensing, high-resolution imaging and optical manipulation.
Until now, few reports have paid attention to the generation of OAM beams in a switchable MWFL. Yao et al. presented a MWFL with OAM beams emission by using a MSC and a few-mode fiber Bragg grating (FM-FBG) . Limited by the FM-FBG, the maximum number of operational channels was three, and the tuning range was only 1.95 nm. Zheng et al. proposed wavelength-switchable MWFL generating tunable OAM beams with a MSC and a microknot resonator . Up to four channels and a tuning range of 16 nm were realized with this laser. Recently, Tang et al. employed a MSC and a Mach–Zehnder interferometer to generate multiwavelength OAM beams in an EDFL . The maximum tuning range was increased to 22.64 nm, and four output channels were obtained with this laser. However, because of the intensive mode competition, the tuning range decreased to 8.67 nm, and stable output was not realized when the fiber laser operated with a quadruple wavelength. In the multiwavelength OAM fiber lasers mentioned above, both wavelength tuning and OAM generation were achieved by adjusting the polarization states via polarization controllers (PCs). This complex and uncertain adjustment process led to difficulty in adjusting the channel number and continuously tuning the center wavelength of OAM beams. In addition, the existence of strong intracavity mode competition also hindered the generation of multiwavelength OAM beams with a large channel number in fiber lasers. Noting that no more than four channels are generated in the fiber lasers mentioned above and the tuning range is still relatively narrow, these lasers have limited practical applications in some systems, such as optical communication.
In this paper, we demonstrate a multiwavelength BEFL generating OAM beams at tunable wavelengths. A two-mode LPFG (TM-LPFG) is used as the transverse mode convertor to produce OAM beams. An external tunable laser is applied to seed Brillouin multiwavelength comb generation in an SMF with a length of 6 km. Multiwavelength OAM beams with switchable topological charges (+1 or -1) are generated by adjusting the PCs. The generation of 10 stable channels with a 35-nm tuning range is achieved by optimizing the primary pump power and the Brillouin pump (BP) power, as well as altering the center wavelength of the tunable bandpass filter (TBF) in combination with adjusting the BP wavelength. Multiwavelength OAM beams generated from the proposed fiber laser possess attractive features, including narrow linewidth and rigid frequency spacing provided by SBS, which makes them a promising source for OAM-based applications, such as optical communications and remote sensing.
2. Experimental configuration
The structure of the designed fiber laser is depicted in Fig. 1. We use a 974-nm laser diode (LD) as the primary pump for the 75-cm-long high-concentration erbium-doped fiber (EDF, Liekki Er80-8/125). The Brillouin gain is offered by the SMF with a length of 6 km. A TM-LPFG is used to realize mode conversion between the fundamental mode and the high-order LP11 mode with high efficiency. The optical fiber end faces of output 1 are cut vertically and coated with reflective film with a reflectivity of 60% to form a wideband fiber partial reflector (FPR), which acts as the output mirror. The two ports of circulator (Cir) 1 are connected to form another cavity mirror. A TBF with a 3 dB bandwidth of 1.5 nm is inserted in the circulator to fix the operating wavelength range of the fiber laser. We use a tunable laser (TL) with a linewidth of 200 kHz as the external BP. The narrow linewidth pump light of the TL is injected into the cavity through a 3-dB coupler spliced between the 6-km-long SMF and the TM-LPFG. Port 1 of circulator Cir 2 is used to input BP power, and port 3 of Cir 2 acts as the output of the reference Gaussian beam for detecting OAM characteristics. The output spectra were measured by an optical spectrum analyzer (OSA, ANDO AQ6317B). A fiber collimator (Col) is installed behind the FPR to adjust the size of the output beams. The images of the output beams are captured through a CCD camera placed after the collimator.
In our experiment, the LPFG is written in a two-mode fiber where only the fundamental mode and the second higher-order modes are supported. The grating period satisfies the following equation to satisfy the phase matching condition:
The grating will efficiently convert power between the fundamental mode and the LP11 mode when phase matching is achieved. The TM-LPFG in our experiment is fabricated with a high-reliability, low-loss electric arc writing technique with a central operational wavelength of 1550 nm. To obtain the wavelength response characteristic of the TM-LPFG, we measure the transmission spectrum of the fundamental mode. In the measurement, a broadband ASE source is injected into one port of the TM-LPFG, and the spectrum of another port is measured by the OSA. We insert two mode strippers at the input side and the output side of the TM-LPFG. The mode stripper inserted on the input side is used to eliminate the LP11 mode to purify the input fundamental mode, and another mode stripper inserted on the output side is used to deplete the LP11 mode converted by the TM-LPFG to ensure that only the untransformed fundamental mode is detected. Figure 2 shows the measured transmission spectrum of the fundamental mode of the TM-LPFG. The result indicates that TM-LPFG can achieve a conversion efficiency between the fundamental mode and the LP11 mode higher than 99% from 1545 nm to 1555 nm and 90% from 1535 nm to 1570 nm. The OAM±1 modes are high-order modes supported by the FMF; thus, the conversion efficiency of TM-LPFG is essential for the generation of high-purity OAM beams. Based on this fact, the conversion bandwidth of the TM-LPFG determines the bandwidth of the generation of high-purity OAM beams. Balancing the tuning range and purity of the OAM beams, we chose to generate multiwavelength OAM beams in the range of 1535-1570 nm.
In a two-mode fiber, circularly polarized OAM±1 modes can be obtained through superposition of the higher order degenerate HE21 (even and odd) vector modes with a phase difference of π/2 (HE21even±iHE21odd). By adjusting PC1 placed at the input port of the TM-LPFG to control the polarization of the input fundamental mode, HE21 (even and odd) vector modes can be excited . The OAM±1 modes can be generated by introducing a phase difference of π/2 between two HE21 modes via PC2 placed on the output port of the TM-LPFG. By monitoring the polarization state and phase characteristic of output beams via polarization analysis and interference during the process of adjusting PCs, the OAM states of output beams can be confirmed.
3. Results and discussion
The operation mechanism of the proposed fiber laser is described as follows. The TL injects the BP signal into the cavity through the 3-dB coupler. The BP signal experiences double-pass amplification of the EDF before returning back to the 6-km-long SMF. Once the power of the amplified BP signal exceeds the SBS threshold in the ultralong SMF, the first-order Stokes component downshifted by a Brillouin frequency shift from the BP wavelength will be generated and propagate backward. The signal light without experiencing Brillouin gain will propagate forward and then be converted to the LP11 mode with high efficiency as it passes through the TM-LPFG. Similarly, the LP11 mode will be efficiently converted to the fundamental mode when it is reflected by the FPR and passes through the TM-LPFG. Then, the first-order Stokes component will also be generated as the power of the fundamental mode converted by the TM-LPFG is above the threshold of the SBS in the SMF with a length of 6 km. Thus, the Stokes component can be generated in both directions in a round trip. Then, the first-order Stokes component will act as the BP signal for the generation of the second-order Stokes component. The high-order Stokes component will continue to be generated through the cascaded SBS effect until the cavity loss is equal to the total gain offered by the EDF and the ultralong SMF, leading to multiwavelength comb generation in the fiber laser.
Without BP, the structure functions as the linear cavity EDFL. The self-oscillating cavity modes are mainly dependent on the center wavelength of the TBF since the bandwidth of the TBF is relatively narrow. Figure 3 shows the self-oscillating modes with wavelength tuning from 1535 nm to 1570 nm by adjusting the TBF. Once the BP is launched, the EDFL self-oscillating modes and the Stokes components generated by SBS will share the erbium gain. With a proper BP wavelength and sufficient BP power, the EDFL self-oscillation cavity modes can be suppressed sufficiently, leading to stable multiwavelength emission. Figure 4 exhibits the evolution of the number of wavelength channels with increasing primary pump power at a fixed BP wavelength of 1549.75 nm. The figure shows that the number of wavelength channels grows gradually when the pump power increases from 75 mW to 170 mW with the BP power tuned at 0.35 mW or 0.43 mW, and relatively more channels are obtained with the BP power of 0.35 mW. This is because a higher pump power will provide more gain for high-order Brillouin Stokes signals, and a higher BP power means that the gain will be exhausted earlier during the cascaded SBS process . Figure 5 shows the power of output 1 versus the primary pump power when the power and wavelength of BP are fixed at 0.35 mW and 1549.75 nm. The slope efficiency is calculated to be approximately 1.9%. The relatively low slope efficiency may be attributed to the high insertion loss of the TBF (∼2 dB) and the 6-km-long SMF (∼1.2 dB). Besides, the reflection of the wideband FPR may also influence the slope efficiency . By optimizing the reflectivity of the wideband FPR, improvement of the slope efficiency is expected. By optimizing the primary pump power and the BP power, as well as adjusting the TBF in conjunction with modifying the BP wavelength, the laser operation of 10 channels with wavelength tuning from 1535 nm to 1570 nm is achieved. The upper limit of the channel number is theoretically determined by the bandwidth of the TBF. By further optimizing the primary pump power, BP power and BP wavelength, a laser operation with more than 10 channels is obtained. However, more channels also mean a more complex and time-consuming optimization process. Thus, we made a tradeoff between the ease of multiwavelength generation and the channel number, where we found that the generation of 10 channels is optimal. Moreover, the bandwidth of the TBF affects the stability of the self-oscillating cavity modes and further affects the stability of the multiwavelength output. The use of a TBF with a larger bandwidth will increase the channel number, but simultaneously decrease the stability of the multiwavelength output. Balancing the channel number and the output stability, we choose to use the TBF with 3 dB bandwidth of 1.5 nm.
Figure 6(a) shows the output spectra of single-channel laser operation with corresponding BP wavelengths of 1534.4 nm, 1539.38 nm, 1544.36 nm, 1549.75 nm, 1554.94 nm, 1559.78 nm, 1564.5 nm and 1569.45 nm at relatively low primary pump power levels. To better show the details of the output spectra, the wavelength coordinate axis does not use the absolute coordinate scale, and the scale interval is 0.2 nm. The start-point and end-point wavelength coordinates of each curve are marked in Fig. 6, and the central wavelength of the first channel (the leftmost peak) corresponds to the BP wavelength. The signal-to-noise ratio (SNR) of each channel in Fig. 6(a) is approximately 43 dB. By increasing the primary pump power, laser operation with 4, 7 and 10 channels is obtained at each BP wavelength, as depicted in Figs. 6(b)-(d). The wavelength spacing between two adjacent channels is approximately 0.088 nm, which is in good agreement with the Brillouin frequency shift in the SMF at approximately 1550 nm. To confirm the stability of multiwavelength laser operation, the output spectra are monitored every three minutes with a total time of 27 minutes when the laser operates with 4, 7 and 10 channels with a BP wavelength fixed at 1549.75 nm, as illustrated in Fig. 7.
The stabilization of the center wavelength and power of each channel is further analyzed when the fiber laser operates with 10 channels. The wavelength deviation of each channel is measured to be no higher than 0.01 nm, which is limited by the performance of the OSA used in the experiment. Figures 8(a)-(b) show the power and power fluctuation of each channel during the measurement. “CH1-CH10” denotes the first to the tenth channel, and the channel order increases in the direction of longer wavelengths. Overall, the power fluctuation of each channel is no higher than 1.92 dB, as illustrated in Fig. 8(b). The result shows the high stability of the laser operation. Note that the power fluctuations of the first eight peaks are no higher than 0.85 dB. The last two channels, corresponding to the two highest-order Stokes components, are produced at the last stage of the cascaded Brillouin process, which are more sensitive to the power in the cavity. As a result, the power fluctuations of “CH9” and “CH10” are relatively high, which are 1.2 dB and 1.92 dB, respectively. Similar results regarding to output stabilities have been observed when the center wavelength of the multiwavelength laser operation is tuned to other wavelengths. By improving the output stability of the TL and the LD, the stability of the high-order channels can be further enhanced.
In the process of generating multiwavelength OAM beams, the TBF and BP are first tuned at the expected center wavelength. By adjusting the intracavity PCs, OAM beams are obtained at output 1. The output beams are monitored by the CCD camera placed behind the collimator. Figures 9(a)-(h) show the intensity distributions of the donut-shaped OAM beams at different BP wavelengths from 1535 nm to 1570 nm when the fiber laser operates with 10 channels, where the BP wavelength is marked in the lower left corner of each figure. According to the technique used in Ref. , the mode purities are calculated to be higher than 96%, which means that the fiber laser is able to emit high purity OAM beams during wavelength tuning. Furthermore, we measure the helical phase by interfering with the output beam of output 1 with the reference Gaussian beam obtained from output 2. The fork-shaped interference patterns with opposite directions depicted in Figs. 9(i)-(x) indicate that the fiber laser can output OAM beams with a topological charge of 1 or -1 (OAM±1 beams). By simply changing the primary pump power, the number of output channels can be tuned from 1 to 10. Figure 10 illustrates the intensity profiles of the obtained OAM beams and corresponding interferograms when the fiber laser operates with 1, 4, 7 and 10 channels under the condition that the wavelength and power of BP are tuned to 1549.75 nm and 0.35 mW. It is clear that the donut shape of the obtained OAM beams and fork shape of interferograms are well maintained during the tuning of the number of operation channels, indicating that the OAM beam of each channel has consistent helical phase characteristics.
In conclusion, a multiwavelength BEFL generating OAM beams with tunable central wavelength and channel number is demonstrated experimentally. TM-LPFG is employed in the laser cavity as a highly efficient LP11 mode convertor from 1535 nm to 1570 nm. A multiwavelength comb is generated through the cascaded SBS in a 6 km-long SMF with an external TL acting as the seed. By optimizing the BP power and the primary pump power, the fiber laser is able to operate with a maximum of 10 channels. The number of operation channels can be adjusted by modifying the primary pump power. The tuning range can reach 35 nm by changing the center wavelength of the TBF and the BP wavelength. High-purity OAM beams with switchable topological charges (+1 or -1) are obtained by adjusting the PCs placed in the cavity. The demonstrated fiber laser will find promising applications in many fields, including optical communication, remote sensing and high-resolution imaging.
National Key Research and Development Program of China (2020YFB2205802); National Natural Science Foundation of China (92050202).
The authors declare no conflicts of interest.
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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