1550nm High Efficiency Narrow Linewidth Fiber Laser

Abstract: A high efficient narrow linewidth fiber laser based on fiber Bragg grating Fabry- Perot( FBG F- P) cavity was demonstrted. The spatial hole burning effect was restrained by fiber Faraday rotator( FR) . Two short FBG F- P cavities as narrow bandw idth filters discriminated and selected the laser longitudinal modes efficiently. Stable single frequency 1550nm laser was acquired. Pumped by two 976nm LD, the fiber laer exhib ited a 11 mW threshold. The 73mW output power was obtained upon the maximum 145mW pump power. The optica-l optical  efficciency was 50% and the slope efficiency was 55% . T he 3 dB linewidth of laser was less than 10 kHz, measured by the delayed sel-f heterodyne method with 10 km mono- mode fiber. T he high power narrow linewidth fiber lasr can be used in high resolution fiber sensor system.

1 Introduction

As a fiber laser sensor source, the narrow linewidth fiber laser has the characteristics of interference to the electromagnetic field, safety, small size and remote control. Currently, there are three options for obtaining a single longitudinal mode narrow linewidth fiber laser. 1) Eliminate the incoherent technique of spatial hole burning caused by the standing wave effect by controlling the polarization state of the light waves encountered in the cavity; 2) Adding unpumped doped fiber to the laser cavity to select the frequency, and suppressing the saturated absorption of the mode hopping 3) Short cavity fiber lasers, including DFB fiber lasers and short cavity DBR fiber lasers. Comparing the three schemes, schemes 1 and 2 need to use multiple polarization controllers, and most of them are annular cavity structures, which are difficult to control, low conversion efficiency, and extremely low output power. Scheme 3 is simple in structure and has an output power exceeding 200 mW. With a slope efficiency of 24%, the difficulty lies in the pumping method to achieve high output power on short gain fibers and how to implement special packages. Ultra-short cavity DBR structure fiber lasers have also been studied in China, but the laser efficiency is low, the output power is only 11 mW at maximum, and the line width is limited to the MHz range.

In this paper, a high-doped Er 3+ linear cavity narrow linewidth fiber laser with dual fiber grating Brier-Perot (FBG F-P) cavity mode selection is developed. The fiber laser combines non-coherent technology, high output power, high energy conversion efficiency, narrow line width, simple structure, full fiber and high signal-to-noise ratio, which can be applied to high-precision fiber sensing systems.

2 Narrow Linewidth Fiber Laser Experiment Results

The fiber laser is mainly composed of two FBG F-P cavities and a highly doped Er3+ fiber linear cavity. The experimental device is shown in Figure 1. The gain medium of the laser is a highly doped Er 3+ fiber with a length of 3 m, a peak absorption coefficient of 17 dB/m at 978 nm, and a peak absorption coefficient of 30 dB/m at 1 550 nm. In the experiment, a two-way pumping method was adopted. The pumping source was an LD with a center wavelength of 976 nm, and the maximum pumping power of LD 1 and LD 2 was 76 mW and 69 mW, respectively. Due to the space hole burning effect in the linear cavity structure, causing multi-longitudinal mode oscillation, the Faraday rotator (FR) of the all-fiber structure can change the polarization state of the reciprocating beam passing through it by 90b, thereby destroying the station in the fiber laser. Wave formation conditions suppress spatial hole burning effects.

The structure of the FBG F-P cavity is shown in Figure 2. The FBG F-P cavity is etched on a common single-mode fiber with a distance of 1 cm between the gratings and an overall cavity length of no more than 5 cm. The FBG F-P cavity reflection spectrum measured with the bandwidth A SE source is shown in Figure 3. FBG F-P I consists of two FBGs with 50% reflectivity and a total reflectivity of 80%. FBG F- P II consists of two FBBs with 80% reflectivity and a total reflectance of 99%. In the experiment, the FBG F-P cavity is the function of the cavity mirror and the F-P cavity selection as the insertion, which is beneficial to realize the full fiberization of the laser. The number of output lines of the FBG FP cavity is determined by the cavity length of the FP cavity and the reflection bandwidth of the FBG. The shorter the cavity length of the FBG FP cavity, the larger the output line spacing, the narrower the reflection bandwidth of the FBG, and the FP cavity can accommodate The fewer the number of modes, the better the frequency selection characteristics than the normal FP cavity [13, 14]. Finally, due to the gain saturation, in the mode competition of a few longitudinal modes, the single longitudinal mode of the center frequency dominates, and finally a single longitudinal mode laser output is obtained.

Ando6319 spectrum analyzer using test recording, maximum precision of the spectrometer 0. 01 nm. The output optical power is accurately measured using an optical power meter. When the pumping power is 11 mW, the laser starts to oscillate. As the pump power increases, a stable spectral line begins to be obtained, and the 3 dB linewidth of the line is not affected by the increase in pumping power. Figure 4 is an output spectrum with a scan range of 5 nm and an accuracy of 0.01 nm. The output of the spectrometer has a laser center wavelength of 1 550 nm, a 3 dB linewidth of less than 0.01 nm, and a signal-to-noise ratio greater than 50 dB. 01 nm。 The continuous observation of 1 h, the laser has no mode-hopping phenomenon, the wavelength drift range is less than 0. 01 nm. When the pumping power is 145 mW maximum, the output power is 73 mW, the light-to-light conversion efficiency is 50%, and the slope efficiency is 55%. Figure 5 shows the output power as a function of pump power. As can be seen from the figure, the output power varies linearly with the increase of pumping power.

3  Self-Heterodyne Line Width Measurement Experiment Results

Currently measuring the linewidth of kHz magnitude lasers with delayed self-heterodyne/zero difference spectroscopy. The self-zero difference method, compared to the self-heterodyne method, does not require the use of a frequency shifter, but cannot be measured directly using a standard RF spectrum analyzer. The improved self-zero difference measurement system [15] requires the addition of a phase modulator and a local RF oscillator, which is more complex than the structure of the heterodyne measurement system. In order to obtain an accurate laser linewidth, a delay self-heterodyne method is selected to measure the fiber laser linewidth.

Delayed self-heterodyne method experimental system, as shown in Figure 6. The entire experimental system consists of a 10 km single mode fiber delay line, an acousto-optic frequency shifter with a center frequency of 70 MHz, two 1 @ 2 type 3 dB fiber couplers, a photodetector, and an AD-VANTEST R3267 RF spectrum analyzer. . The measurement accuracy of the delay self-heterodyne measurement method is related to the length of the fiber delay line [16]. According to the calculation, the measurement accuracy of the 10 km long fiber delay line is 10 kHz. The measurement results of the laser linewidth are shown in Figure 7. The 3 dB bandwidth of the spectrum is 5 kHz. The 3 dB linewidth of the fiber laser is considered to be less than 10 kHz due to measurement accuracy.

4 Conclusion

Combined with non-coherent technology, a single longitudinal mode narrow linewidth fiber laser was developed by using two short FBG F-P cavity mode selection by using all-fiber FR to suppress spatial hole burning effect. The power output characteristics are given. The threshold pumping power is 11 mW, the output signal optical power is 73 mW, and the slope efficiency is 55%. The laser output center wavelength is 1550 nm, the spectrum is stable, and the signal-to-noise ratio is high. The delay self-heterodyne linewidth measurement was performed using a 10 km single-mode fiber delay line. Due to measurement accuracy limitations, the 3 dB linewidth of the fiber laser was finally less than 10 kHz.