| 摘要 |
<p>An external modulator controlling device for modulating the output light from a light source with an input signal comprises a low-frequency signal generating means for generating a low-frequency signal having a predetermined frequency and outputting it, signal superimposing means for superimposing a first modulating signal which is the input signal on the low-frequency signal to generate a second modulating signal and applying the second modulating signal to the external modulator, low-frequency signal detecting means for detecting the low-frequency signal component from the light output from the external modulator, comparing the phase of the detected low-frequency signal with the phase of the low-frequency signal outputted from the low-frequency signal generating means, and outputting a signal corresponding to the phase difference, an operating point control means for making a control to compensate the operating point drift of the external modulator according to the output signal from the low-frequency signal detecting means, light power level detecting means for detecting an average of the light output from the external modulator, variable gain amplifying means whose gain is set according to the detection output from the light power level detecting means and which amplifies the output from the low-frequency signal detecting means and outputs the amplified output to the operating point control means. The gain of the variable gain amplifying means is set so that the closed loop gain of the loop from the output side of the external modulator through the operating point control means back to the external modulator is constant.</p> |
