Experimental study on noise characteristics of audio frequency band in output field of 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 used in various fields. However, for a certain application, the measurement accuracy will directly depend on the noise level of the laser source. Most of 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 is usually an option, i.e. active feedback control or filter cavity technique, etc. 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 find 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 experimentally, showing that 1) excess noise is introduced by cavity length locking, and 2) laser phase and pointing noises are 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 mode cleaner (MC), combined with the critical proportionality method. All the control loops are homemade, and the proportional-integral-derivative (PID) is designed with a field programmable gate array board for expediently achieving a noise reduction up to 30 dB at the audio frequency. Then the control loop is optimized to 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 band is still higher than that of the input optical field itself. In the future, an active control loop will be used to suppress the noise power. The experimental results provide the basic means for application research such as feedback control loop noise analysis, which will promote the development of precision measurement toward higher measurement accuracy.