Laser power control circuit

Abstract

A laser power control circuit that is constituted by CMOS transistors reduces variations in a laser power that is emitted from a semiconductor laser, which are caused by a mismatch of the transistors. An offset amount of a differential amplifier is digitally calculated using an A/D converter that is located on the same chip, and a voltage value of a variable voltage source is controlled for applying a voltage in a direction opposite to the offset voltage of the differential amplifier to correct the offset voltage of the laser power control circuit, thereby reducing the variations in the laser power emitted from the semiconductor laser.

What is claimed is:  
     
         1 . A laser power control circuit in which an electric signal that is obtained by performing a photoelectric conversion to a part of light which is applied from a semiconductor laser is connected to one of input terminals of a differential amplifier, a reference voltage is connected to the other input terminal, and an output of the laser power control circuit is connected to a driving circuit of the semiconductor laser, 
 said laser power control circuit obtaining a constant laser power by constituting a negative feedback circuit so that a photoelectric-converted voltage and the reference voltage become equal to each other,    said laser power control circuit,    including a unit for generating a difference voltage between the input terminals of the differential amplifier; and an A/D converter,    converting the reference voltage and the photoelectric-converted voltage into digital signals by the A/D converter, and    controlling the voltages of the input terminals of the differential amplifier so as to eliminate a voltage difference between the reference voltage and the photoelectric-converted voltage, which have been converted into the digital signals.    
     
     
         2 . The laser power control circuit of  claim 1  including: 
 reading an offset voltage of the differential amplifier at power-on, and  
 correcting the offset voltage of the differential amplifier by supplying voltages corresponding to the offset voltage to the input terminals of the differential amplifier.  
 
     
     
         3 . The laser power control circuit of  claim 1  including: 
 changing a reference voltage of the A/D converter at reading an offset voltage of the differential amplifier, thereby increasing a resolution.  
 
     
     
         4 . The laser power control circuit of  claim 2  including: 
 changing the reference voltage of the laser power control circuit so as to be within a dynamic range of the A/D converter at reading the offset voltage of the differential amplifier.  
 
     
     
         5 . The laser power control circuit of  claim 1  wherein 
 the differential amplifier comprises amplifiers, and  
 at correcting the offset voltage of the differential amplifier, the voltages of the input terminals of the differential amplifier are controlled so that a potential of the input and a potential of the output of the differential amplifier have the same value when the driving circuit of the semiconductor laser and the control circuit are electrically disconnected and the input of the differential amplifier is short-circuited.  
 
     
     
         6 . The laser power control circuit of  claim 5  wherein 
 the amplifiers of the differential amplifier comprise a first amplifier and a second amplifier, and  
 at reading the offset voltage of the differential amplifier, an offset voltage of the first amplifier by itself and an offset voltage at a time of connecting the first amplifier and the second amplifier to each other are read, respectively, thereby deciding a correction amount.  
 
     
     
         7 . The laser power control circuit of  claim 6  wherein 
 the correction amount is certain correction which is performed according to the reference voltage of the laser power control circuit and an output voltage.  
 
     
     
         8 . The laser power control circuit of  claim 6  wherein 
 the differential amplifier comprises a first feedback amplifier and a second feedback amplifier, and  
 the feedback amplifier that forms a feedback loop is changed between at the offset voltage reading and at the normal operation.  
 
     
     
         9 . The laser power control circuit of  claim 1  wherein 
 the photoelectric-converted voltage and the reference voltage are interchangeably connected to the respective input terminals of the differential amplifier.