Study Of Suppression Technique Of Laser Noise Based On Homodyne Coherent Optical Communication

Homodyne coherent optical communication system is an important way to solve the bandwidth bottleneck of satellite communication in future, this system has high performance requirements for intensity noise, linewidth and output power of laser source. Solid state laser tends to obtain narrow linewidth and high power output, and that make it to be a suitable alternative laser source for homodyne coherent optical communication system. Presently, homodyne coherent optical communication choose the 1.06μm Nd: YAG NPRO as the laser source. Its performance in noise, linewidth and power is more and more perfect. Compared with the wavelength of 1.06μm, 1.55μm laser widely applied in laser ranging and optical communication, because it located in the third communication window,which can better compatible with existing optical communication devices. Therefore, study on noise suppression technology for 1.55μm Er: Glass laser is necessary. In this paper, the intensity noise of the Nd:YAG NPRO(non-planar ring oscillator) laser and Er:Glass single-frequency laser is suppressed by optoelectronic feedback loop. This paper consists of four aspects as follwing:(1) NPRO is designed based on medium with low refraction index, and enough loss difference of eigenpolarization would be achieved, so that the unidirectional traveling wave would operate stably. The intensity noise spectrum is theoretically simulated, the gain and loss of the laser cavity are optimized to reduce the noise output of the free-running laser.(2) The quantum theory analysis model of three-level system laser intensity noise is applied to analyze the characteristics of noise, it can overcome the inaccurate of quantitative analysis which used the traditional rate equations theory. The analytical expressions for the output noise spectrum with and without feedback are derived.(3) In order to compensate for the phase delay in high frequency field, noise suppression effect of different order feedback control loop are analyzed and compared. Feedback control loop with higher-order phase-advancing circuit is designed and made for improve noise suppression effect.(4) In the experiment, Nd:YAG NPRO single frequency laser was built, and output beam collimation, polarization control and fiber coupling also completed. The RIN around the peak of Nd:YAG NPRO laser is reduced by 24 dB,under the feedback control with one-order phase advance circuit, in addition, the peak of RIN moves to higher frequency field; the RIN is reduced about 32 dB under feedback control with two-order phase advancing; under feedback control with three-order phase advancing circuit, the RIN is suppressed about 42 dB, the RIN level is below-145dB/Hz. So the experiment has verified noise suppression effect of the high-order phase-advance circuit for the four-level laser. A Plane-Concave cavity single frequency lasers with Er:Glass was built, and a low finesse intra-cavity etalon used for single longitudinal mode selection. Then the laser noise suppression experiment was carried out, the result show that, no obvious noise suppression effect under feedback control with first-order phase advancing. But RIN peak is suppressed by 12.2dB, when the laser under feedback control with three-order phase advancing. This experiment has proved that feedback control with high-order phase-advancing circuit also improve the noise reduction effect for three level laser.