| 摘要 |
PURPOSE:To reduce coherency of a laser diode and to enhance yield strength of reflected light, by a method wherein the magnitude of at least one of both a bias current and pulse current which are to be supplied to the laser diode is varied with time. CONSTITUTION:An OFF mode is set with a bias current 18 flowing while an ON mode is set with both bias current IB and pulse current IP flowing, and thereby a laser diode 6 is directly modulated. In this connection, the magnitude of at least one of both the bias current IB and pulse current IP is varied with time. For example, the ratio of the bias current IB to the pulse current IP is varied with time, while the sum of the bias current IB and pulse current IP is made constant. As a result, only the frequency of an optical output of the laser diode 6 can be varied in the order of GHo to dozens GHo without the level of the optical output thereof being varied. Accordingly, when a reflected light is returned, the difference between the frequency of the reflected light and the oscillation frequency of an emitted light becomes more than several times as large as the used bit rate. As a result, there is no noise in the optical output waveform and there is no flare in code error rate characteristics, and therefore large yield strength of reflected light can be provided. |
