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

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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