HIGH-EFFICIENCY DIRECT FREQUENCY MODULATION OF EXTERNAL-RESONATOR SEMICONDUCTOR LASER DIODE

本文介绍了一种提高外腔半导体激光二极管（LD）调频调制效率并降低调频噪声的理论和实验验证，方法是通过同步调制LD驱动电流与由外腔组成的反射镜反射的返回光的相位。​而同步调频的调制效率随着外腔长度L（=18cm）调制幅度Lm的增加而单调增加。而当<DELTA>Lm =(< λ >/2) （<DELTA>fm/fex）时，该值增加到孤立LD的1.6 GHz/mA，是传统直接调频效率的（1+ <xi>）倍。通过进一步增加<DELTA>Lm，调频调制效率提高到13 GHz/mA，是孤立LD的8倍；其中<lambda>为光波长，<DELTA>fm为正弦电流调制的孤立LD的最大频率偏差，外腔频率为fex。另一方面，在与APC同步调制的情况下，FM噪声降低2.4 dB， FM调制效率提高5倍，当<xi>设置为0.8时，FM调制效率为7.9 GHz/mA。此外，通过APC配合40分钟，返回光的相位波动被抑制了约二十分之一到14度，使调频信号幅度变化稳定在0.2 dB以内，调频噪声变化稳定在0.8 dB以内。(2) 11 MHz正弦波同步调频实验：利用LiNbO_3光相位调制器在11 MHz调制返回光的相位，实现了调频降噪和调频效率的提高。少

Described are a theory and experimental verification of improving the FM modulation efficiency together with the FM noise reduction of an external-resonator semiconductor laser diode (LD), by means of synchronously modulating LD drive current with phase of the returned light, reflected from the mirror comprising the external resonator.(1) Experiment on synchronous FM modulation with 110kHz sinusoidal wave : In case of modulation without automatic phase control(APC) for the returned light, FM noise has been reduced approximately proportioning to 1/(1+<xi>)^2 as feedback parameter <xi> is increased to 5, except for<xi> 1; the reduction reached 1/27 times the value of solitary LD. While modulation efficiency for the synchronous FM increased monotonously with increase of amplitude Lm of modulation of the external-resonator length L(=18cm). And when <DELTA>Lm =(<lambda>/2) (<DELTA>fm/fex), that increased to the value for solitary LD, 1.6 GHz/mA, which is (1+ <xi>) times conventional direct … More FM modulation efficiency. By further increase of the <DELTA>Lm, FM modulation efficiency increased as high as 13 GHz/mA, which is 8 times the value for solitary LD; where <lambda> is light wavelength, <DELTA>fm maximum frequency deviation for solitary LD modulated with sinusoidal current, and fex external resonator frequency. On the other hand,in case of synchronous modulation with APC, FM noise reduction was 2.4 dB, and FM modulation efficiency improvement 5 times, resulting 7.9 GHz/mA, when <xi> was set 0.8. Furthermore, by cooperating of the APC for 40 minutes, phase fluctuation of the returned light has been suppressed by about a twentieth to 14 degrees, which stabilized FM signal amplitude variation within 0.2 dB, and FM noise variation within 0.8 dB.(2) Experiment on synchronous FM modulation with 11 MHz sinusoidal wave: FM noise reduction and FM modulation efficiency improvement are also achieved by modulating phase of the returned light using LiNbO_3 optical phase modulator at 11 MHz. Less