1. Introduction

Multi-wavelength single longitudinal mode (SLM) fiber lasers have attracted great attention in recent years for their wide applications in high-speed optical communications, fiber optic sensors, high resolution medical instruments, and microwave photonics systems. Particularly, a narrow linewidth dual-wavelength SLM fiber laser is a good candidate for highly pure microwave or terahertz source since narrower linewidth leads to lower high-frequency phase noise for the beating signal [1–4]. Since Erbium-doped fiber is the primary homogeneous gain medium at room temperature which leads to strong mode competition and unstable lasing, and a fiber ring laser usually has a long cavity, thus it is difficult to obtain simultaneous dual-wavelength oscillations in erbium-doped fiber lasers (EDFLs). To eliminate multi-longitudinal-mode oscillation and mode hopping, ultra-narrow band-pass filters (BPF) with two transmission peaks is necessary. The ultra-narrow BPF can be realized by many means, such as intra-cavity 3-stage all fiber Lyot filter [5], structured PM-CFBG and polarization-maintaining chirped moiré fiber Bragg grating (PM-CMFBG) filter [6], phase-shifted fiber Bragg grating (PS-FBG) [7, 8], a Fabry–Pérot (F-P) filter [9], and superimposed fiber Bragg gratings [10]. In order to compress laser linewidth, anew compression mechanism based on the Rayleigh scattering in fiber is proposed and demonstrated, and a fiber ring laser with a linewidth as low as 200 Hz is achieved [11–14].

In this paper, we demonstrated a tunable dual-wavelength fiber laser with ultra-narrow linewidth based on Rayleigh backscattering in fiber. A typical ring cavity is adopted in the laser, two cascade fiber Bragg gratings (FBGs) is used as the filter to select the two lasing wavelength, eventually stable dual-wavelength single longitudinal mode lasing is achieved by adjusting the variable optical attenuator (VOA) and the polarization controller (PC). In the proposed system, the 110 m tapered single mode fiber(SMF-28e)is used to generate the stimulated Rayleigh backscattering which acts as the key element to compress the linewidth of the two lasing wavelengths. An ultra-narrow linewidth of ~700 Hz is achieved for each wavelength by the delayed self-heterodyne detection method. By exerting micro-strain on the 1550 nm FBG, the 1550 nm wavelength laser can be tunable in a range of about 3nm while the linewidth still remains to be ~700 Hz. The results show that multi-wavelength laser linewidth can be compressed simultaneously based on the Rayleigh backscattering compression mechanism.

2. Experimental setup

The configuration of the proposed dual-wavelength ultra-narrow linewidth fiber laser is shown in Fig. 1. The 7 m erbium doped fiber (EDF) pumped by a 980 nm laser provides a broadband gain spectrum from 1525 nm to 1565nm. Two FBGs centered at 1530.082 nm and 1550.308 nm with a bandwidth of 0.2nm act as filters to select the two lasing wavelength. The light from port 2 of the optical circulator (OC) is launched into the 110m tapered fiber whose parameters are follows: the taper diameter is 112.09 μm, the taper length is ~2 cm, the distance between two adjacent taper sections is about 5m, the taper number is 21, and the total insertion loss is about 0.05 dB [11]. In theory, long tapered fiber is good for the collection of the Rayleigh backscattering which intensifies the linewidth compression effect, however, too long tapered fiber results in too long laser cavity which is bad for the stability of laser. Hence the length of the tapered fiber should be determined by making a compromise between the linewidth and the stability. Then the light is reflected by the two FBGs to form two resonant cavities respectively for each wavelength. The variable optical attenuator (VOA) is used to adjust the cavity loss to obtain dual-wavelength lasing, and by adjusting the polarization controller (PC), stable dual-wavelength single longitudinal mode operating can be achieved. In order to monitor the optical spectrum and the electrical spectrum in the same time, the output is divided by a 9:1 coupler, 10% output is monitored by an optical spectrum analyzer (OSA) with a resolution of 0.01 nm (ADVANTEST, Q8384), and 90% output is divided by a coarse wavelength division multiplexer (CWDM), then the two wavelengths can be detected with the self-heterodyne method respectively. An acoustic optical modulator (AOM) with a frequency shift of 100 MHz is inserted into the lower arm of the Mach Zehnder interferometer (MZI), and 50 km normal single-mode fiber is used to generate ~250 μs delay in the upper arm. The beating signal of the two arms is detected by a photoelectric detector (PD) with a frequency response range of DC-350 MHz. Finally, the linewidth of the laser can be measured by the electrical spectrum analyzer (ESA). The testing resolution of the 50 km delay fiber is 10 kHz based on the self-heterodyne system, and the measured linewidth is prone to be wider than its actual value if the delay fiber is not long enough due to the 1/f noise. However, too long SMF introduces great loss which is hard to overcome without any additional amplifier in the actual detection. For practical purposes, 50 km delay fiber is used in the detection. And the laser linewidth is verified to be indeed hundreds of hertz by phase noise detection in our experiment.

 

Fig. 1 The experimental setup. WDM: wavelength division multiplexing device; EDF: erbium-doped fiber; FBG1: fiber Bragg grating centered at 1530nm; FBG2: fiber Bragg grating centered at 1550nm; OC: optical circulator; PC: polarization controller; VOA: variable optical attenuator; C1, C2, C3: optical couplers; CWDM: coarse wavelength division multiplexing device; AOM: acoustic optic modulator.

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3. Experimental results

Through carefully tuning the VOA and PC to adjust the gain and loss of the two cavities, stable dual-wavelength lasing is finally achieved, Fig. 2 shows the optical spectrum of the dual-wavelength laser measured by an optical spectrum analyzer (OSA) with a resolution of 0.01nm when the pump power is 100 mW. The lasing wavelengths are 1530.082 nm and 1550.308 nm respectively, which are consistent with the peak wavelength of the two FBGs.

 

Fig. 2 Optical spectrum of the dual-wavelength laser.

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To measure the linewidth of each wavelength, the two lasers are separated by the CWDM and then imported into the delayed self-heterodyne detect system. Figures 3(a) and 3(b) shows the electrical spectrum of 1530 nm laser and 1550 nm laser respectively, 20 times average is applied in the measurement due to the interference induced by the environmental vibration, temperature fluctuation and other ambient noise. The cavity length of both fiber ring lasers are approximately 240 m, corresponding to free spectrum range (FSR) of about 0.8 MHz, and the scanning range is 4 MHz which is much larger than the FSR. It can be seen from the spectrum of 1550 nm laser that the measured FSR is in consistence with the computation. We can see that both lasers are in the SLM operation, and the side mode suppression ratio gets higher along with the increase of the pump power due to the enhancement of the Rayleigh backscattering in the tapered fiber, the maximum side mode suppression ratio (SMR) is higherthan 60 dB and 50 dB for 1530 nm and 1550 nm laser respectively. The key mechanism of successful SLM operation can be explained qualitatively as follows. Firstly, the RBS occurs at multiple scattering centers along the tapered fiber. The multiple random reflections act as a distributed mirror resulting in a variable ring cavity length and the strongest variable mode separation at the front end of the fiber position. Secondly, the EDF exhibits a fluorescence relaxation time in the millisecond-scale. Since the RBS light travels within nanoseconds for a single round trip of the ring laser, the EDF will allow the RBS light to travel several million times to create high gain and long coherence length, and hence modifies the laser’s performance and establishes SLM operation. Finally, as the Rayleigh bandwidth is relatively narrow, the pump bandwidth in the cavity could become narrower and narrower after several million times resonance, and a single longitudinal mode output can be formed. The different SMRs can be explained as follows: the resonant cavity length of the two lasers is not the same, and the cavity length of 1530 nm laser gets optimized cavity length, therefore the SMR of the 1530 nm laser is higher than that of the 1550nm.When the pump power is 200 mW, the narrowest linewidth namely 700 Hz is achieved. Figures 3(c) and 3(d) show the Lorentz fitting results of 1530 nm and 1550 nm laser respectively when the pump power is 200 mW. It shows that the 3 dB linewidth is ~700 Hz for both the two wavelength. To verify the compression that Rayleigh backscattering introduced, the electric spectrum of the none-Rayleigh backscattering dual-wavelength fiber laser is also measured after moved off the 110m tapered fiber. By carefully adjusting the PC and VOA, stable dual-wavelength lasing is achieved, and the linewidths of the two wavelength are detected in the circumstance of SLM operation. Figures 4(a) and 4(b) show that the two wavelengths are in SLM operation, and Fig. 4 (c) shows the linewidth of the two wavelength lasers changed along with the pump power. The linewidth is about 13 kHz for both the two lasers when there’s no 110 m tapered fiber, clearly it is much larger compared to the linewidth measured when there is tapered fiber in the cavity. In the experiment, only the PC and the VOA is adjusted, and the tuning of the two devices cannot lead to great compression effects. The results support that it is the Rayleigh backscattering in fiber that compresses the linewidth of the dual-wavelength laser simultaneously.

 

Fig. 3 The electrical spectrum and their Lorentz fit of each wavelength with Rayleigh compression. (a) The electrical spectrum of 1530 nm laser. (b) The electrical spectrum of 1550 nm laser. (c) The Lorentz fit of the 1530 nm laser. (d) The Lorentz fit of the 1550 nm laser.

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Fig. 4 The electrical spectrum and their Lorentz fit of each wavelength without Rayleigh compression. (a) The electrical spectrum of 1530 nm laser. (b) The electrical spectrum of 1550 nm laser. (c) The linewidth of the two wavelength lasers changed along with the pump power.

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To analyze the linewidth fluctuations along with the change of pump power, the pump power increases from 50 to 450 mW while the cavity loss is fixed. The linewidth range is shown in Fig. 5(a). We can see that the linewidths of the two laser are the narrowest when the pump power is 200 mW, and then the linewidth is becoming wider with the increase of the pump power. This can be explained by the laser linewidth compression mechanism based on Rayleigh backscattering. To get good linewidth compression effect, it is critical to carefully adjust the specific gravity of the feedback Rayleigh backscattering while an excellent laser cavity is guaranteed. Particularly, the specific gravity should meet the following conditions: firstly, the cavity loss and the loss induced by the Rayleigh scattering should be compensated by the amplified feedback Rayleigh backscattering after resonance; secondly, proper specific gravities of the feedback Rayleigh backscattering and the original laser generated by the pump power should be reached; then the laser linewidth would be compressed efficiently. In our experiment, efficient linewidth compression is achieved when the pump power is 200 mW with the certain length of EDF and coupler1. In order to study the stability of the dual-wavelength fiber laser, the optical spectrum of the dual-wavelength fiber laser is measured repeatedly for 10 times repeated scans with a 3 minute interval, as shown in Fig. 5(b), the measured power fluctuation and wavelength fluctuation are less than 0.1 dB and 0.01 nm for each wavelength, which means the dual-wavelength fiber laser is relatively stable.

 

Fig. 5 (a). The linewidth of the two wavelength lasers changed along with the pump power. (b) The optical spectrum with repeated scans at 3 minutes intervals.

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To further study the feasibility of multi-wavelength laser linewidth compression simultaneously based on Rayleigh scattering mechanism, an experiment of tunable dual-wavelength fiber laser is conducted. The wavelength tuning is realized by the following method. The fiber is fixed on two micro-positioners, and a micro-strain is applied on the 1550 nm FBG by tuning the distance between the two micro-positioners, so the center wavelength of the FBG is changed, resulting in the change of the lasing wavelength. And during tuning the center wavelength of the 1550 nm FBG, the optical spectrum and electronic spectrum are detected while the pump power is maintained at 200 mW. Figure 6(a) is the optical spectrum of the tunable dual-wavelength fiber laser, and Fig. 6(b) shows the optical spectrum of the tunable wavelength, we can see that the wavelength can be tuned in a range of about 3 nm, namely from 1550.308 nm to 1553.362 nm along with the micro-strain change from 0 to 2770 με. Figure 6(c) shows that the tunable laser keeps in SLM operation while changing the micro-strain, and Fig. 6(d) is the linewidth of the two wavelengths while changing the micro-stain on the 1550 nm FBG, we can see both the two linewidths keep being about 700 Hz while changing the peak wavelength of the 1550 nm FBG, the fluctuation range is less than 30 Hz. The results show that the linewidth compression based on the Rayleigh backscattering is relatively stable while changing the wavelength, and further indicate that the Rayleigh backscattering can be used in multi-wavelength linewidth compress in a wide range of spectrum. The central wavelength of Rayleigh backscattering is the same as that of the input laser while the linewidth of the Rayleigh backscattering is ultra-narrow, and for different wavelength lasers, the Rayleigh backscattering is centered at different wavelength which means there is no interference between different wavelengths. Therefore, the Rayleigh backscattering can be used to simultaneously compress the linewidth of a multi-wavelength laser provided that the wavelength space is wider than that of the bandwidth of Rayleigh backscattering. To get excellent stability, the cavity loss should be adjusted very carefully while low ambient disturbances and noise are also necessary.

 

Fig. 6 (a) The optical spectrum of the tunable dual-wavelength fiber laser. (b) The optical spectrum of the tunable laser. (c) The electric spectrum of the tunable laser. (d) The linewidth of the two wavelength while changing the micro-strain on the 1550 nm FBG.

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4. Conclusion

In conclusion, we have achieved a tunable dual-wavelength single longitudinal mode fiber laser with ultra-narrow linewidth based on the RBS in a tapered fiber. The 3 dB linewidth is ~700 Hz for each wavelength, and the SNR is 60 dB. By exerting micro-strain on one of the two FBGs, the wavelength can be tuned in a range of 3nm. It can be seen that the laser linewidth compression mechanism based on Rayleigh backscattering is able to compress multi-wavelength lasers at the same time. In this experiment, the laser wavelength is determined by the two FBGs, hence, the wavelength can be expanded from the ultraviolet to the far infrared by utilizing proper filters and gain medium. Furthermore, a pure microwave signal can be generated by beating the two ultra-narrow linewidth lasers achieved in our experiment in the future.