Auto-power control circuit to maintain extinction ratio of optical output from laser diode

Abstract

The present invention provides an auto-power control (APC) circuit and a method to stabilize the extinction ratio of an optical output from a laser diode (LD) in an optical transmitter. The APC circuit according to the invention includes two feedback loops for the modulation I M and the bias current I B each having variable loop gain. The extinction ratio of the optical output from the LD is kept constant by setting the ratio of the loop gains of respective APC circuits to be ER−1.

1 - 10 . (canceled) 
   
   
       11 . A method to set an extinction ratio of light output from a laser diode constant by use of an auto-power control circuit with first and second feedback loops for a bias current and a modulation current, respectively, said method comprising steps of
 setting closed loop gain of said first feedback loop to a first value; and   setting closed loop gain of said second feedback loop to a second value,   wherein said loop gain of said first feedback loop and said loop gain of said second feedback loop have a ratio substantially equal to ER−1 and are independent of temperature.   
   
   
       12 . The method according to  claim 11 , wherein
 said first feedback loop and said second feedback loop include a photodiode and a current-to-voltage converter each common to said first feedback loop and said second feedback loop, and   said photodiode monitors said light emitted from said laser diode and generates a photocurrent with conversion efficiency, and said current-to-voltage converter converts said photocurrent into a voltage signal with a current conversion gain.   
   
   
       13 . An optical transmitter, comprising:
 a semiconductor laser diode to emit light with a preset extinction ratio ER provided with a bias current and a modulation current; and   an auto-power control circuit including:   a first feedback loop with a first loop gain for adjusting said bias current; and   a second feedback loop with a second loop gain for adjusting said modulation current,   wherein said first loop gain and said second loop gain have a ratio substantially equal to ER−1 and are independent of temperature.   
   
   
       14 . The optical transmitter according to  claim 13 , further including a photodiode and a current-to-voltage converter each common to said first feedback loop and said second feedback loop,
 wherein said photodiode monitors said light emitted from said laser diode and generates a photocurrent with a conversion efficiency, and said current-to-voltage converter converts said photocurrent into a voltage signal with a current conversion gain.