
- Chinese Optics Letters
- Vol. 19, Issue 9, 091203 (2021)
Abstract
1. Introduction
Tunable fiber lasers with single-longitudinal-mode operation are of considerable interest for many applications, such as optical precision metrology, high-resolution spectroscopy, high-precision sensors, and communication systems[
Most wavelength-tunable (WT)-RFLs use single-mode fibers (SMFs) to provide backward Rayleigh scattering (RS) and use Raman gain or Brillouin gain to support random lasing. However, the backward RS coefficient and the gain coefficient of SMFs are relatively lower. WT-RFLs based on SMFs are not typically compact with high threshold due to using tens of kilometers SMF to provide efficient gain and random feedback[
The WT-RFL based on an RBGA has been realized by using a tunable optical band-pass filter (TOBPF)[
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In this Letter, we propose a compact and low-threshold WT-RFL with a full-open cavity formed by two RBGAs. A transfer matrix model including spectral regimes for both RBGAs and
2. Theoretical Analysis and Numerical Results
The configuration of the proposed WT-RFL is shown in Fig. 1. To analyze and predict the characteristics of the WT-RFL output spectra, we analyze the reflection spectra of the RBGA[
Figure 1.Schematic of the experimental setup for the WT-RFL. OC, optical coupler.
The complex reflection coefficient of the RBGA,
The transmission coefficient of the
The simulation results are shown in Fig. 2. Figure 2(a) is calculated by Eqs. (1)–(4), where many random intervals in the RBGA cause multiple randomly scattered lights to overlap each other, resulting in narrow chaotic peaks in the reflection bandwidth. Random longitudinal modes of RFL are determined by these random intervals and are superimposed on each other to form a continuous longitudinal mode structure, which is different from a traditional fixed cavity structure. Figure 2(b) is calculated by Eqs. (5) and (6). The transmission window of the
Figure 2.Numerical simulation spectra of the RBGA and
3. Experiments and Results
Two RBGAs form a compact full-open-cavity structure, which shortens the length of the random resonant cavity and reduces the lasing threshold due to its strong random feedback efficiency. Two RBGAs are fabricated in a common SMF (Corning-28), where 30 FBGs are fabricated by 248 nm ultraviolet (UV) exposure and the phase mask method. The length, center wavelength, and reflectivity of each FBG are controlled to 3 mm, 1537.45 nm, and
We use an OSA with a resolution of 1.12 pm to measure the reflection spectra and the transmission spectra of two RBGAs. Figure 3(a) shows that RBGA1 has a 3 dB bandwidth of 0.393 nm and a center wavelength of 1537.454 nm, and Fig. 3(b) shows that RBGA2 has a 3 dB bandwidth of 0.352 nm and a center wavelength of 1537.458 nm. The transmittance (
Figure 3.Reflection spectra and transmission spectra of (a) RBGA1 and (b) RBGA2.
The
Figure 4.(a) Reflection and transmission spectra of two RBGAs and π-FBG. (b) The wavelength of π-FBG as a function of the strain.
When the WT-RFL operates at a stable single wavelength of 1537.452 nm, we measure the change of output power as a function of the injected pump power. As shown in Fig. 5, the pump threshold is
Figure 5.Output power versus different pump power.
We fix the lasing wavelength at about 1537.452 nm and measure the lasing spectra from the output port at different pump powers ranging from 22 mW to 220 mW. As shown in Fig. 6, single-wavelength lasing is caused by photon localization in the two RBGAs when the pump power is above the threshold. As the pump power increases, the output wavelength does not vary with the pump power, and the center wavelength of the laser remains at 1537.452 nm, which indicates that the proposed WT-RFL has good wavelength stability. Besides, the 3 dB linewidth of the lasing spectrum is expected to be much less than 1 pm at the maximum pump power of 220 mW.
Figure 6.Output spectra measured under different pump power at 1537.452 nm.
To control and select different lasing wavelengths, the
Figure 7.Laser spectra at different measured wavelengths.
It is also important to check the long-term stability of the WT-RFL. Figure 8 shows the fluctuation of lasing wavelengths and peak power of the WT-RFL within 16 min of three selected wavelengths when the pump power is 180 mW. We can record every 2 min for each case and obtain stable output at different wavelengths due to the short random distributed feedback fiber and high random feedback efficiency of two RBGAs. The peak-power fluctuations recorded in Fig. 8 are 0.42 dB, 0.55 dB, and 0.36 dB, respectively; meanwhile, we observe that the maximum variation of wavelength is less 1 pm for three selected lasing wavelengths. These experimental results indicate that we propose a compact WT-RFL, which has stable single-wavelength operation at our selected wavelengths.
Figure 8.Wavelength and peak-power fluctuations of selected wavelengths versus time.
4. Conclusion
In conclusion, we have proposed and demonstrated a compact and stable WT-RFL with a full-open cavity formed by two RBGAs. The design of two RBGAs combined with a
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