Investigation of characteristics of a DFB laser diode 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。于是我们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。为了降低了外界环境的温度起伏和气流扰动的影响，我们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，并且温度起伏的方差降低了两个数量级。此外我们也测试了反馈带宽对激光线宽的影响，尽管我们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，但我们仍然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，我们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。另外我们通过在0~1 mW范围内改变光纤光栅的反馈功率，观察到在反射功率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11.5 kHz，在反射功率1 mW时，0.2 kHz~2 MHz范围内的相位噪声得到了大概22 dB的抑制。
Abstract:Narrow linewidth lasers are the basic components of spectroscopy and precision metrology and other experiments. Because semiconductor laser is very sensitive to external optical feedback, the phase noise of semiconductor laser can be suppressed by using the high bandwidth of optical feedback, and then the linewidth can be narrowed. So we use fiber Bragg grating as feedback element and build a long external cavity feedback loop. In order to reduce the influence of external environment temperature fluctuation and air flow disturbance, we control the temperature of the fiber of the feedback optical path. Then the maximum temperature fluctuation within 1 hour is reduced from 0.039 °C to 0.003 °C, and the variance of temperature fluctuation is reduced by two orders of magnitude. In addition, we also test the effect of feedback bandwidth on laser linewidth. Although the bandwidth of the fiber Bragg grating used in our experiment is much larger than the free-running laser linewidth, we still observe that the laser linewidth is narrowed, and the smaller the bandwidth of the fiber Bragg grating, 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, so the narrower the feedback bandwidth of the fiber grating, the larger the slope of the feedback spectrum, the more sensitive the feedback. In addition, by changing the feedback power of FBG in the range of 0~1 mW, we observed that at the reflected power of 0.8 mW, the optical feedback narrowed the laser linewidth from the free-running 100.5 kHz to the narrowest 11.5 kHz, and at the reflected power of 1 mW, phase noise in the range of 0.2 kHz to 2 MHz is suppressed by about 22 dB.
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Key words:
- diode laser /
- long external cavity /
- light feedback /
- phase noise suppression /
- linewidth narrowing
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图 4 两种条件下两段光纤的温度起伏
Figure 4. Temperature fluctuation of two optical fibers under two conditions
图 6 0.13 nm带宽 FBG的反射功率0.8 mW时的拍频谱
Figure 6. The beat spectrum of 0.13 nm bandwidth FBG at 0.8 mW reflected power
图 8 激光线宽随两个不同带宽的FBG的反射功率的变化(0.130 nm and 0.195 nm)
Figure 8. The laser linewidth varies with the reflected power from two FBGs with different bandwidth (0.130 nm and 0.195 nm)
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