One KHz Order Narrow Linewidth Fiber Laser Using Rayleigh Backscattering Mechanism in an Additional Piece Optical Fiber

Abstract

A simple, low-cost, single-longitudinal mode, C-band narrow-linewidth optical fiber laser is presented based on the methodology of the Rayleigh backscattering (RBS). In this paper, a 1551 nm fiber ring laser is developed, and single-mode fiber (SMF) is added to compress the line width. When the SMF length of the RBS cavity is 120 m, the laser has better performance than that in other SMF lengths with a laser line width of 1.46 KHz with housing shield. The optical signal-to-noise ratio (OSNR) is 59.86 dB, and its maximum output power is 9.4 mW. It can quickly achieve the single longitude-mode operation by controlling the variable optical attenuator (VOA). The bit error rate at 10 Gb/s PRBS NRZ modulation is measured to be 10⁻⁹ when the optical receiving power is −16.2 dBm.

1. Introduction

With the development of technology, high precision measurement has become the mainstream. With low phase noise and long coherent length, narrow linewidth laser [1,2,3,4] has been widely used in high precision detection fields, such as high precision spectrum, Lidar [5,6], remote sensing, etc. It provides an excellent laser source for optical communication [7,8], distributed sensing [9], and microwave generation [10]. The characteristics of the laser line width play a decisive role in the detection distance, precision [11], sensitivity, and noise characteristics of the systems. It becomes important to precisely measure the laser linewidth. Narrow linewidth fiber laser has a long service life, high conversion efficiency, comprehensive wavelength coverage, slight wavelength drift, high-quality light source, low noise, and long coherence length; thus, it is widely used in many fields.

In optical communication systems, the linewidth directly affects phase information extraction. With the advancement of signal modulation formats, the line width has gradually reduced from 1 MHz to 10 kHz or even 100 Hz. In optical fiber sensing, the frequency modulated continuous wave technology and the principle of lightwave coherence can also be used to measure ultra-fine, ultra-long distance, and weak signals. In addition, narrow-linewidth lasers play important roles in oil and gas exploration systems, pipeline monitoring, gas leakage detection, cold atomic physics, and microwave photonics. In recent years, some groups had investigated KHz−order narrow linewidth. For example, Kaijun Zhou et al. [12] established a kHz-order linewidth controllable 1550 nm single-frequency fiber laser using a low-pass filtered white Gaussian noise (WGN) signal applied on a fiber stretcher in an optical feedback loop. By using WGN signals with different signal amplitudes and different cutoff frequencies, we control the linewidth in the range of 0.8 to 353 kHz. The result indicates that optical signal-to-noise ratio is significantly higher than 72.0 dB. However, the structure they designed is expensive, which requires special FBG. On the other hand, Zhou et al. [13] proposed an efficient single-frequency fiber laser by using a newly developed Er3+/Yb3+ co-doped single-mode phosphate glass fiber. The laser power is 300 mW, and the linewidth is less than 2 kHz. Their laser requires two pump lasers, and the stretcher is strongly influenced by temperature. Yu et al. [14] proposed that a widely tunable distributed Bragg reflector (DBR) laser with kilohertz linewidth output can be demonstrated by using a dual-loop self-injection scheme. The linewidth is less than 10 kHz. Their design also requires additional unique optical fiber. Moreover, Ou Zh et al. [15] proposed a Brillouin fiber laser with an ultra-narrow linewidth of 30 Hz; however, the structure is complex. It requires strong pump power and a long fiber length, which induces colossal loss. Jiang et al. [16] demonstrated a switchable single longitudinal mode (SLM) narrow linewidth fiber laser with cylindrical vector beam (CVB) output. At 1543.35 nm and 1544.38 nm, the OSNR is higher than 60 dB, and the laser linewidth is 5.7 KHz and 6.8 khZ, respectively. However, the design is complex including multiple cavities. which need to be controlled the grating and polarization state of the laser cavity. On the other hand, in this proposal, an extra-narrow laser linewidth can be obtained by using a simple structure. Pan Fu et al. [17] demonstrated a switchable dual-wavelength single-longitudinal-mode (SLM) narrow-linewidth Ytterbium-doped fiber laser (YDFL) based on a non-linear amplifying optical loop mirror (NALM). The OSNRs of both laser wavelengths are better than 50 dB. Their laser linewidth of them are 17.07 kHz and 18.64 kHz, respectively. However, the lasers linewidths are wide and the laser characteristic is unstable and susceptible to external influences. Based on the Vernier effect for mode suppression, the effective free space range (FSR) becomes the least common multiple number of all FSR’s. Qiao et al. [18] used a main ring cavity incorporated with a sub-ring cavity which included two fiber couplers and two optical circulators. The result shows that it could achieve an ultra−narrow linewidth of 459 Hz. Similar to [18], Yang [19] replaced the sub−ring cavity of above structure with one fiber Fabry−Perot tunable filter and one fiber Fabry−Perot interferometer. The laser linewidth was less than 310 Hz. However, the method should take into account the FSR of filters, which means narrow linewidth of fiber laser could only be achieved by using an appropriate optical bandpass filter. Compared to [18] and [19], our proposed fiber laser has simpler structure and less cost.

This paper presents the development of a narrow linewidth single-longitudinal mode (SLM) optical fiber laser based on the Rayleigh backscattering (RBS). RBS feedback is optimized by building a structure of linewidth measurement to maximize the quality of the light source and improve its performance. With a simple architecture, we achieve a low-cost narrow-line-width SLM fiber laser.

2. Experimental Setup

The experimental setup is shown in Figure 1. First, a 980 nm semiconductor laser is used as a pump laser source for Erbium-doped fiber (EDF). Wavelength selection is performed by using a fiber Bragg grating (FBG) with a central wavelength of 1551.95 nm and a reflectivity of 98.34% on port 2 of optical circulator 1. To accumulate sufficient feedback of backscattered Rayleigh light and increase the threshold value of Brillouin scattering, single-mode optical fiber is added to not only stimulate the next cycle of backscattered Rayleigh light under a Faraday rotating mirror (FRM), but also adjust the light polarization state. This also controls the RBS feedback and suppresses the side mode through a variable optical attenuator (VOA). This optical structure utilizing backscattering can be seen as an infinite number of cascaded scattering cross-sections. The scattered light is accumulated as weak feedback to compress the linewidth. Backscattered light enters the main cavity from port 3 of optical circulator (OC) 2. Therefore, the signal light and backscattered Rayleigh light share the same light path and simplify the resonator structure. In addition, adjusting the VOA can effectively and efficiently make the resonant cavity become a SLM lasing output. Finally, an optical power meter and optical spectrum analyzer (OSA) can be used to monitor the output of the 90/10 fiber ratio coupler.

The laser linewidth is measured using the fiber delayed self-heterodyne detection method [20,21,22], as shown in Figure 2. This method is simple in structure and can meet the parameter requirements measured in this experiment. The laser output launches to a 50:50 fiber coupler accompanied by a VOA to adjust the laser power at each router. The upper router uses an acousto-optic modulator to induce a frequency shift of 80 MHz, while the lower branch uses 75 Km SMF as a delay line. Two lights enter a 50:50 fiber ratio splitter. Finally, the beat signal is detected by a photodetector, and its linewidth is displayed on an electrical spectrum analyzer. Since the required fiber delay line must be longer than the laser coherence time, the time delay between two lights is enough to destroy the coherence between two lasers. It can be considered that two incoherent lasers with central closing wavelengths create a beat frequency. Double the laser linewidth can be observed on a high-resolution electron spectroscopy analyzer. Therefore, the full width at half maximum (FWHM) shown on the spectrum analyzer should be divided by two to obtain the correct laser line width.

Under the scheme of linewidth measurement in Figure 2, the signal light is accurately displaced at 80 MHz through the Acousto-optic modulator (AOM). It enables the stable output of the laser power to a particular range, but the laser drifts slightly due to external factors. There is a mode jump phenomenon. The average power intensity is taken during linewidth measurement, and it is found that external factors directly affect the line width value. Therefore, it is necessary to minimize environmental factors, such as vibration and temperature changes.

3. Experimental Results and Discussion

3.1. Determination of EDF and SMF Lengths

In this experiment, we test the influence of different lengths of EDF on the performance of the system laser, as shown in Figure 3. In Figure 3a, with 15 m EDF, the laser has the lasing power of 3.7 mW @400 mA; with 11 m EDF, the laser has the best pump slope efficient of 5 mW @400 mA. In Figure 3b, with 12 m EDF, the laser has the highest optical-signal-to-noise ratio (OSNR) of 59.86 dB, and the output power is 2.8 mW. To achieve a better single longitudinal mode state, 12 m EDF is selected in the subsequent experiments.

When the SMF lengths are 30-, 70-, 100-, 120-, 200-, and 280 m, respectively, the corresponding OSNR is as shown in Figure 4a. Since the reflectivity of the FBG is the same, the variation of OSNR variation is slight. When the pump current is 400 mA, the output of the proposed laser is 2.8 mW, and an OSNR at multiple-longitudinal modes is 59.86 dB. At this point, there are many side modes in the output spectrum. The transverse mode affects the distribution of the field intensity when the laser propagates and determines the laser spatial coherence. In comparison, the longitudinal mode affects the laser linewidth and determines the time coherence of the laser. By adjusting the VOA near the FRM from a multiple-longitudinal modes state to single-longitudinal modes state, the optical power is attenuated from 2.8 mW to 1.4 mW. Nevertheless, the OSNR still maintains 59.86 dB. The SLM laser output in 0 to 1 GHz spectrum range is obtained after fine-tuning, only the 80 MHz frequency is observed as shown in Figure 4b.

Next, the impact of SMF with different lengths on laser linewidth is measured and compared. As shown in Figure 5a, when the length of SMF is 30-, 70-, 100-, 120-, 200-, and 280 m, the corresponding laser linewidths are 8.668-, 10.087-, 6.184-, 4.176-, 6.080- and 5.816 kHz, respectively. From Figure 5b, it is found that the smallest linewidth of optical laser is obtained when the SMF length is 120 m. The corresponding laser linewidth is 4.176 kHz.

3.2. Fiber Laser Shielding

The environmental air vibration and acoustics wave may affect both the fiber laser power and linewidth. In order to determine the optimum length of EDF and SMF, various length of two kinds of fibers were tested by trial and error, as described in Section 3.1. Many fiber connectors were used to connect discontinuous points of the fiber laser module. These fiber connectors may induce loss and reflection in the fiber laser cavity. Once the lengths of EDF and SMF had been determined, all the connection points were replaced by splicing method to reduce the optical loss and reflection effect. Thus, the laser power could be increased to 56.3 mW. In this measurement, all connection points are spliced to reduce the fiber-to-fiber interface reflection and connection loss. By adjusting the VOA near the FRM to obtain single longitude mode from multiple longitude modes, the optical power is attenuated from 56.3 mW to 9.3 mW. As shown in Figure 6a, to investigate the impact of air perturbation and acoustic vibration, we set up a transparent shielding box to partially un-shield or fully shield the fiber laser. The red arrow indicates the laser traveling direction along the fiber spool. As shown in Figure 6b, when the fiber laser is fully shielded, the linewidth is stable at 1.46 KHz after one min measurement. The output power is 9.3 ± 0.3 mW with 3.3% power variation. In the unshielded case, the average measured laser linewidth is 4.97 KHz during one min measurement. The laser output power has larger variation and is unstable. The output power has larger variation so not easy to measure. Figure 7 shows that the frequency and linewidth are stable in 80 MHz and 1.46 KHz. respectively, for one minute when the fiber laser is fully shielded.

3.3. The Bit Error Rate (BER) Measurement

The experimental setup is shown in Figure 8. The transmission quality of the proposed fiber laser is verified by 10 Gb/s, 2³¹-1 pseudorandom binary sequence (PRBS), non-return-to-zero (NRZ) format. The home made fiber laser with 59.86 dB OSNR and original launched power of 1.4 mW was used. It was then amplified by an EDFA located after the EOM. All the fibers used in the transmission system are SMF. The extinction ratio (ER) of the EOM is 40 dB. Figure 9a shows the blue, red and green curves of the original fiber laser, fiber laser with filter and fiber laser being amplified, respectively. The measured BER is 10⁻⁹ for 10 Gb/s modulation speed when the receiving power is −16.2 dBm, as is shown in Figure 9b.

4. Conclusions

In this paper, a simple, low-cost, narrow linewidth, single longitudinal mode fiber ring laser is designed. The effect of different lengths of EDF on the laser performance is investigated. Using a 12 m EDF, 59.86 dB of OSNR is obtained. The central wavelength is 1551 nm, and the maximum output power is 2.8 mW. To optimize the laser linewidth, the feedback of RBS is tested by adjusting the SMF length of the RBS cavity. We find that a 120 m SMF inside the RBS cavity has the best laser performance. The single longitudinal mode output can be achieved by adjusting the VOA with a laser line width of 1.46 kHz and laser power of 9.4 mW. In a receiving power of −16.2 dBm, the BER performance is 10⁻⁹ at 10 Gb/s NRZ modulation speed. Future work will design a thermally controlled methodology and package the fiber laser to isolate it from the environmental conditions.