Laser control circuit

Claim 2

Apparatus for controlling the operating current level of an injection laser so that said injection laser operates at the point of maximum linearity comprising: means for modulating the current through said injection laser with a test signal at a predetermined frequency; and means for detecting the harmonic distortion component in the light output of said laser at twice said predetermined frequency and for automatically adjusting said operating current level for the minimum harmonic distortion component.

Claim 3

Apparatus as defined in claim 1 wherein said means for detecting the harmonic distortion component includes means for generating a dc signal having a first phase when the operating current level is above the point of maximum linearity and a second phase when said operating current level is below the point of maximum linearity.

Claim 4

Apparatus as defined in claim 2 wherein said means for detecting the harmonic distortion includes a lock-in amplifier phase-synchronized to said test signal.

Claim 5

Apparatus for maintaining the operating current level of an injection laser at the point of maximum linearity comprising: means for modulating the current through said injection laser with a test signal having a predetermined frequency; means for detecting the light generated by said laser in response to the modulated current through said laser; and means for detecting the second order harmonic distortion component of said test signal in said detected light and for automatically adjusting said operating current level for the minimum second order harmonic distortion component.

Claim 6

Apparatus for maintaining the operating current level of an injection laser at the point of maximum linearity comprising: means for modulating the current through said injection laser with a test signal having a predetermined frequency; means for detecting the light generated by said laser in response to the modulated current through said laser; and means for detecting the second order harmonic distortion component of said test signal in said detected light and for automatically adjusting said operating current level for the minimum second order harmonic distortion component.

Claim 7

Apparatus as defined in claim 4 wherein said means for detecting the light generated by said laser is a PIN photodetecting diode.

Claim 8

Apparatus as defined in claim 4 wherein said means for detecting the second order harmonic distortion component and for automatically adjusting said operating current level includes means for generating a dc signal having a first phase when the operating current level is above the point of maximum linearity and a second phase when said operating current level is below the point of maximum linearity.

Claim 9

Apparatus as defined in claim 6 wherein said means for detecting the second order harmonic distortion component and for automatically adjusting said operating current level is a lock-in amplifier phase-synchronized to said test signal.

Claim 10

Apparatus for maintaining the operating current level of an injection laser at the point of maximum linearity; means for modulating the current through said injection laser with a test signal having a predetermined frequency, the current through said injection laser being at said operating current level for a zero test signal magnitude; means for detecting the laser voltage in response to the modulated current through said laser; and means for detecting the second order harmonic distortion component in said voltage and for automatically adjusting said operating current level for the minimum second order harmonic distortion component.

Claim 11

Apparatus for maintaining the operating current level of an injection laser at the point of maximum linearity; means for modulating the current through said injection laser with a test signal having a predetermined frequency, the current through said injection laser being at said operating current level for a zero test signal magnitude; means for detecting the laser voltage in response to the modulated current through said laser; and means for detecting the second order harmonic distortion component in said voltage and for automatically adjusting said operating current level for the minimum second order harmonic distortion component.

Claim 12

Apparatus as defined in claim 8 wherein said means for detecting the second order harmonic distortion component and for automatically adjusting said operating current level includes means for generating a dc signal having a first phase when the operating current level is above the point of maximum linearity and a second phase when said operating current level is below the point of maximum linearity.

Claim 13

Apparatus as defined in claim 9 wherein said means for detecting the second order harmonic distortion component and for automatically adjusting said operating current level is a lock-in amplifier phase synchronized to said test signal.

Claim 14

In combination an injection laser, means for biasing said laser, a plurality of channel banks connected to said laser for modulating the current through said laser in response to a plurality of frequency multiplexed information signals, an audio oscillator for modulating the current through said laser at a predetermined test frequency, a PIN photodetector diode for detecting the light output of said laser, a lock-in amplifier phase-synchronized with the output of said audio oscillator and frequency-synchronized at twice said predetermined test frequency for measuring the second harmonic distortion component in the laser light detected by said PIN diode generated in response to the modulation of the current through said laser by said audio oscillator, said lock-in amplifier generating a dc voltage in response to said second harmonic distortion component for automatically stabilizing the operating current level of said laser at the point of inflection in the laser current-versus-light power output characteristic of said laser, said dc voltage having a first phase or polarity when the operating current level of said laser is above the said point of inflection and a second phase or polarity when the operating current level is below said point of inflection.

Claim 15

In combination an injection laser, means for biasing said laser, a plurality of channel banks connected to said laser for modulating the current through said laser in response to a plurality of frequency multiplexed information signals, an audio oscillator for modulating the current through said laser at a predetermined test frequency, a PIN photodetector diode for detecting the light output of said laser, a lock-in amplifier phase-synchronized with the output of said audio oscillator and frequency-synchronized at twice said predetermined test frequency for measuring the second harmonic distortion component in the laser light detected by said PIN diode generated in response to the modulation of the current through said laser by said audio oscillator, said lock-in amplifier generating a dc voltage in response to said second harmonic distortion component for automatically stabilizing the operating current level of said laser at the point of inflection in the laser current-versus-light power output characteristic of said laser, said dc voltage having a first phase or polarity when the operating current level of said laser is above the said point of inflection and a second phase or polarity when the operating current level is below said point of inflection.