Experimental Research on the Noise Characteristics of the Output Field of the Optical Filter Cavity

Abstract

Precision measurement is an important direction of today's frontier scientific research. Using lasers to achieve high-precision target measurement has become an important way to improve measurement accuracy, which can be applied in various fields. However, for a certain application, the measurement accuracy will directly depend on the noise level of the laser source. Most applications require that the measurement frequency band is concentrated in the audio frequency band. In order to obtain a low-noise laser source with shot noise limited in the applied frequency band, active and/or passive noise reduction are the usual choice, i.e., active feedback control and filter cavity technique, and so on. Therefore, noise analysis and suppression techniques are the main concern of the precision measurement. The optical filter cavity acts as an optical low-pass filter, which can effectively suppress high-frequency noise beyond its linewidth. In this work, we found that the intensity noise of the output field of an optical filter cavity is higher than the noise floor of the laser. The main sources of noise are analyzed through experiments:(1) excess noise introduced by cavity length locking; (2) laser phase and pointing noises coupled to the intensity one by the cavity. To cancel the excess noise as much as possible, we optimize the feedback control loop by measuring the open-loop and closed-loop transfer functions of the MC, combined with the critical proportionality method. All the control loop are homemade, and the PID is designed with a FPGA board for expediently achieving a noise reduction up to 30 dB at the audio frequency. Then the control loop is optimized as the best condition without introducing the excess noise. Compared with the free-running laser, MC filters out the high-frequency noise, meanwhile converts the phase noise and pointing noise of input field into the intensity noise of the output field. Therefore, the power noise spectrum in the audio segment is still higher than that of the input optical field itself. In the future, an active control loop will be applied to suppress the noise power. The experimental results provide the basic means for applied research such as feedback control loop noise analysis, which will promote the development of precision measurement to higher measurement accuracy.