Characteristics of a distributed feedback diode laser with feedback from a fiber-Bragg-grating-based long external cavity
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摘要:
窄线宽激光器是光谱学和精密计量学等实验的基本组成部分。由于半导体激光器对外部光学反馈十分敏感，所以可以利用光反馈的高带宽抑制半导体激光器的相位噪声，进而压窄线宽。本文采用光纤布拉格光栅作为反馈元件，搭建了长外腔反馈回路。为了降低外界环境温度起伏和气流扰动的影响，对反馈光路的光纤控温，使得1小时内最大温度起伏从0.039 °C降低到0.003 °C。此外，测试了反馈带宽对激光线宽的影响，尽管实验所用光纤布拉格光栅的带宽远大于自由运转的激光线宽，但仍然可以观察到激光线宽被压窄，且光纤光栅的带宽越小，激光线宽越窄。对于此现象，分析认为在反馈回路中应该存在一种负反馈机制，可以将激光线宽稳定到反馈光谱的某个斜率处，所以光纤光栅的反馈带宽越窄，反馈光谱的斜率越大，反馈越灵敏。通过调整光纤光栅的反馈功率在0~1 mW范围内改变，观察到当反射功率为0.8 mW时，光反馈将激光线宽从自由运转的100.5 kHz压窄到最窄的11.5 kHz，0.2 kHz~2 MHz范围内的相位噪声降低约20 dB。
Abstract:Narrow linewidth lasers are the basic components of spectroscopy, precision metrology, and other experiments. Because diode lasers are very sensitive to external optical feedback, the phase noise of diode lasers, can be suppressed by using a high optical feedback bandwidth, enabling the narrowing of the linewidth. In the paper, we use fiber Bragg grating (FBG) as a feedback element and builds a long external cavity feedback loop. In order to reduce the influence of temperature fluctuation of external environment and air flow disturbance, the temperature of the optical fiber is controlled. Then, the maximum temperature fluctuation within 1 hour is reduced from 0.039 °C to 0.003 °C. In addition, the effect of feedback bandwidth on laser linewidth is also tested. Although the bandwidth of the FBG used in the experiment is much larger than the free-running laser linewidth, a narrowing of the laser linewidth is still observed. The smaller the FBG bandwidth, the narrower the laser linewidth. For this phenomenon, we believe that there should be a negative feedback mechanism in the feedback loop, which can stabilize the laser linewidth to a certain slope of the feedback spectrum. Therefore the narrower the feedback bandwidth of the fiber grating, the larger the slope of the feedback spectrum, and the more sensitive the feedback. In addition, by changing the feedback power of FBG in the range of 0~1 mW, it is observed that at the reflected power of 0.8 mW, the optical feedback narrows the laser linewidth from the free-running 100.5 kHz to the narrowest 11.5 kHz, and phase noise in the range of 0.2 kHz to 2 MHz is reduced by about 20 dB.
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图 2 具有光反馈的完整激光系统示意图
Figure 2. Schematic diagram of complete laser system with optical feedback
图 3 两种条件下两段光纤的温度起伏
Figure 3. Temperature fluctuations of two optical fibers under the conditions with and without temperature control
图 5 0.130-nm带宽 FBG在反射功率为0.8 mW时的拍频谱
Figure 5. The beat note spectrum of 0.130-nm bandwidth FBG at reflected power of 0.8 mW
图 7 激光线宽随两个不同带宽的FBG的反射功率的变化情况(0.130 nm和0.195 nm)
Figure 7. The laser linewidth varies with the reflected power of two FBGs with different bandwidths (0.130 nm and 0.195 nm)
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