Ultra-linear laser drive circuit with feedback loop

Abstract

The ultra-linear laser drive circuit of the present invention includes a transconductance operational amplifier circuit integrated with a low-pass filter. The transconductance operational amplifier circuit linearizes the output power of the laser drive circuit. The low-pass filter is a second-order low-pass filter which reduces the noise of the laser drive circuit.

What is claimed is:  
     
         1 . A system for linearizing the output of a laser drive circuit, comprising: 
 a transconductance operational amplifier circuit operable to linearize the output power of said laser drive circuit; and    a low-pass filter integrated with said transconductance operational amplifier circuit operable to reduce the noise of said laser drive circuit.    
     
     
         2 . The system for linearizing said laser drive circuit of  claim 1 , wherein said transconductance operational amplifier circuit comprises: 
 a MOSFET, said MOSFET having a source, a drain, and a gate;    a transconductance operational amplifier, said transconductance operational amplifier having a non-inverting input, an inverting input, a positive voltage supply, a negative voltage supply, and a current output, said inverting input connected to said source of said MOSFET;    a laser diode;    a negative voltage source;    a first resistor connecting said current output of said transconductance operational amplifier to said gate of said MOSFET;    a second resistor connecting said drain of said MOSFET to said laser diode; and    a third resistor connecting said source of said MOSFET to said negative voltage source.    
     
     
         3 . The system for linearizing said laser drive circuit of  claim 2 , wherein said low-pass filter integrated with said transconductance operational amplifier circuit comprises: 
 a fourth resistor;    a fifth resistor;    an input voltage source operable to provide an input voltage to said low-pass filter, said input voltage source having a negative terminal connected to ground and a positive terminal connected to a first side of said fourth resistor;    a first capacitor wherein a first plate of said first capacitor is connected to said inverting input of said transconductance operational amplifier circuit and a second plate of said first capacitor is connected to a second side of said fourth resistor and a first side of said fifth resistor; and    a second capacitor wherein a first plate of said second capacitor is connected to a second side of said fifth resistor and said non-inverting input of said transconductance operational amplifier; further wherein a second plate of said second capacitor is connected to said negative voltage source.    
     
     
         4 . The system for linearizing said laser drive circuit of  claim 1 , wherein said laser drive circuit is operable to pass any tone above 20 kilohertz on an optical data channel.  
     
     
         5 . The system for linearizing said laser drive circuit of  claim 4 , wherein said laser drive circuit reads said tone using synchronous detection via a synchronous loop.  
     
     
         6 . The system for linearizing said laser drive circuit of  claim 5 , wherein a rear-beam detector provides power to said synchronous loop.  
     
     
         7 . The system for linearizing said laser drive circuit of  claim 6 , wherein said rear-beam detector is not used to linearize said laser drive circuit.  
     
     
         8 . The system for linearizing said laser drive circuit of  claim 2 , wherein said transconductance operational amplifier further comprises a bipolar transistor, said bipolar transistor having a base, an emitter, and a collector.  
     
     
         9 . The system for linearizing said laser drive circuit of  claim 3 , wherein said low-pass filter is a second-order low-pass filter.  
     
     
         10 . The system for linearizing said laser drive circuit of  claim 8 , wherein both said MOSFET and said bipolar transistor may either be N-channel or P-channel.  
     
     
         11 . The system for linearizing said laser drive circuit of  claim 9 , wherein said second-order low-pass contains two poles which may be symmetrical.  
     
     
         12 . The system for linearizing said laser drive circuit of  claim 1 , wherein said laser drive circuit is linear over a dynamic range of −30 dBm to +5 dBm.  
     
     
         13 . The system for linearizing said laser drive circuit of  claim 1 , wherein said laser drive circuit's output drive characteristics are constant over a plurality of different types and brands of said laser diode.  
     
     
         14 . The system for linearizing said laser drive circuit of  claim 1 , wherein said laser drive circuit reduces ground bounce by 30-60 dB.  
     
     
         15 . A system for linearizing the output of a laser drive circuit, comprising: 
 a transconductance operational amplifier circuit operable to linearize the output power of said laser drive circuit, said transconductance operational amplifier comprising: 
 a MOSFET, said MOSFET having a source, a drain, and a gate;  
 a transconductance operational amplifier, said transconductance operational amplifier having a non-inverting input, an inverting input, a positive voltage supply, a negative voltage supply, and a current output, said inverting input connected to said source of said MOSFET;  
 a laser diode;  
 a negative voltage source;  
 a first resistor connecting said current output of said transconductance operational amplifier to said gate of said MOSFET;  
 a second resistor connecting said drain of said MOSFET to said laser diode; and  
 a third resistor connecting said source of said MOSFET to said negative voltage source; and  
   a low-pass filter integrated with said transconductance operational amplifier circuit operable to reduce the noise of said laser drive circuit, said low-pass filter comprising: 
 a fourth resistor;  
 a fifth resistor;  
   an input voltage source operable to provide an input voltage to said low-pass filter, said input voltage source having a negative terminal connected to ground and a positive terminal connected to a first side of said fourth resistor;    a first capacitor wherein a first plate of said first capacitor is connected to said inverting input of said transconductance operational amplifier circuit and a second plate of said first capacitor is connected to a second side of said fourth resistor and a first side of said fifth resistor; and    a second capacitor wherein a first plate of said second capacitor is connected to a second side of said fifth resistor and said non-inverting input of said transconductance amplifier; further wherein a second plate of said second capacitor is connected to said negative voltage source.    
     
     
         16 . A method for linearizing the output of a laser drive circuit comprising the steps of: 
 integrating a transconductance operational amplifier circuit with a low-pass filter circuit comprising the steps of: 
 connecting a current output of a transconductance operational amplifier to a gate of a MOSFET using a first resistor;  
 connecting a drain of said MOSFET to a laser diode using a second resistor;  
 connecting a source of said MOSFET to a first side of a third resistor, said first side of said third resistor also connected to an inverting input of said transconductance operational amplifier;  
 connecting a negative voltage source to a second side of said third resistor;  
 connecting a negative terminal of an input voltage source to ground;  
 connecting a positive terminal of said input voltage source to a first side of a fourth resistor;  
   connecting a first plate of a first capacitor to said inverting input of said transconductance operational amplifier;    connecting a second plate of said first capacitor to a second side of said fourth resistor and a first side of a fifth resistor;    connecting a first plate of a second capacitor to a second side of said fifth resistor and a non-inverting input of said transconductance operational amplifier; and    connecting a second side of said second capacitor to said negative voltage source.    
     
     
         17 . The method for linearizing the output of said laser drive circuit of  claim 16 , further comprising the steps of: 
 injecting a tone on said laser drive circuit;    passing said tone on an optical data channel;    using synchronous detection to read said tone via a synchronous loop; and    using a rear-beam detector to provide power to said synchronous loop.    
     
     
         18 . The method for linearizing the output of said laser drive circuit of  claim 17 , wherein said rear-beam detector is not used to linearize said laser drive circuit.  
     
     
         19 . The method for linearizing the output of said laser drive circuit of  claim 16 , wherein said transconductance operational amplifier further comprises a bipolar transistor, said bipolar transistor having a base, an emitter, and a collector.  
     
     
         20 . The method for linearizing the output of said laser drive circuit of  claim 16 , wherein said low-pass filter is a second-order low-pass filter.  
     
     
         21 . A method for linearizing the output of said laser drive circuit of  claim 19 , wherein both said MOSFET and said bipolar transistor may either be N-channel or P-channel.  
     
     
         22 . The method for linearizing the output of said laser drive circuit of  claim 16 , wherein said second-order low-pass filter contains two poles which may be symmetrical.  
     
     
         23 . The method for linearizing the output of said laser drive circuit of  claim 16 , wherein said laser drive circuit is linear over a dynamic range of −30 dBm to +5 dBm.  
     
     
         24 . The method for linearizing the output of said laser drive circuit of  claim 16 , wherein said laser drive circuit's output drive characteristics are constant over a plurality of different types and brands of said laser diode.  
     
     
         25 . The method for linearizing the output of said laser drive circuit of  claim 16 , wherein said laser drive circuit reduces ground bounce by 30-60 dB.  
     
     
         26 . A method for linearizing the output of a laser drive circuit comprising the steps of: 
 linearizing the output power of said laser drive circuit using a transconductance operational amplifier circuit; and    reducing the noise of said laser drive circuit using a low-pass filter integrated with said transconductance operational amplifier circuit.