Method and apparatus for controlling and protecting pulsed high power fiber amplifier systems

Abstract

An electronic circuit for controlling a laser system consisting of a pulse source and high power fiber amplifier is disclosed. The circuit is used to control the gain of the high power fiber amplifier system so that the amplified output pulses have predetermined pulse energy as the pulse width and repetition rate of the oscillator are varied. This includes keeping the pulse energy constant when the pulse train is turned on. The circuitry is also used to control the temperature of the high power fiber amplifier pump diode such that the wavelength of the pump diode is held at the optimum absorption wavelength of the fiber amplifier as the diode current is varied. The circuitry also provides a means of protecting the high power fiber amplifier from damage due to a loss of signal from the pulse source or from a pulse-source signal of insufficient injection energy.

1 . A high power fiber amplifier system comprising:
 a high power fiber amplifier;   a controller operable to control a diode current of a pump diode in said high power fiber amplifier, wherein the pump diode is operable to generate output pulses;   means for setting the pump diode current or power to a desired value;   means for monitoring the pump diode current or power; and   means for maintaining the pump diode current or power at the desired value,   wherein a corresponding energy of the pump diode is maintained in accordance with the desired value as the pulse width and repetition rate are varied.   
     
     
         2 . A high power fiber amplifier system as claimed in  claim 1 , further comprising:
 means for initiating operation of the pump diode sufficiently in advance of first pulses; and   means for ramping up the pump diode current to produce equal pulse energy for the first pulses when the system is turned on.   
     
     
         3 . A high power fiber amplifier system as claimed in  claim 1 , further comprising:
 means for storing the desired pump diode current setting as a function of system pulse width and repetition rate, wherein the energy of the output pulse is maintained at the desired value as the pulse width and repetition rate are varied.   
     
     
         4 . A high power fiber amplifier system as claimed in  claim 1 , further comprising:
 means for calculating the desired pump diode current setting as a function of system pulse width and repetition rate.   
     
     
         5 . A high power fiber amplifier system as claimed in  claim 1 , further comprising:
 means for calculating an appropriate pump diode temperature as a function of the pump diode current setting, wherein the emission wavelength of the pump diode is maintained at a wavelength that provides maximum absorption of the pump diode energy by the fiber amplifier medium as the pump diode current is varied.   
     
     
         6 . A high power fiber amplifier system as claimed in  claim 1 , further comprising:
 means for storing an appropriate pump diode temperature setting as a function of the pump diode current setting, wherein the emission wavelength of the pump diode is maintained at a wavelength that provides maximum absorption of the pump diode energy by the fiber amplifier medium as the pump diode current is varied.   
     
     
         7 . A high power fiber amplifier system as claimed in  claim 1 , further comprising:
 means for monitoring a repetition rate of pulses injected into the amplifier system;   means for comparing the repetition rate of the injected pulses to a predetermined minimum repetition rate; and   means for either disabling or reducing the current to the amplifier pump diode if the injected pulses have a lower repetition rate than the minimum repetition rate.   
     
     
         8 . A high power fiber amplifier system as claimed in  claim 1 , further comprising:
 means for measuring an amplitude of a pulse injected into the amplifier system;   means for comparing the amplitude of the pulse being injected into the fiber amplifier with a predetermined minimum amplitude value; and   means for disabling or reducing the current to the amplifier pump diode if the amplitude of the pulse being injected into the fiber amplifier is lower than the predetermined minimum.   
     
     
         9 . A high power fiber amplifier system comprising:
 means for monitoring a repetition rate of an oscillator; and   means for calculating a required down counter divide ratio needed to obtain a lower repetition rate,   wherein pulses with a desired repetition rate are output even if the oscillator repetition rate varies as a function of time or temperature.   
     
     
         10 . A high power fiber amplifier system as claimed in  claim 9 , further comprising:
 means for synchronizing an oscillator with an external reference signal; and   means for varying a frequency of the external reference signal,   wherein a repetition rate of the oscillator varies in accordance with the external reference signal.   
     
     
         11 . A high power fiber amplifier system as claimed in  claim 10 , wherein
 a down counted repetition rate is varied in accordance with the external reference signal.   
     
     
         12 . A method of operating a high power fiber amplifier system, the method comprising:
 varying one or more of a pulse width and a repetition rate of output pulses; and   controlling a pump diode current to dynamically control a gain of the power amplifier to maintain uniform pulse energy as one or more of the pulse width and the repetition rate of the output pulses is varied.   
     
     
         13 . A method as claimed in  claim 12 , further comprising:
 maintaining the output pulse energy at a constant value during a turn-on phase of the pulse train.   
     
     
         14 . A method as claimed in  claim 12 , further comprising:
 varying a current of the pump diode; and   controlling a temperature of the fiber amplifier pump diode such that a wavelength of the pump diode is maintained at a fixed value while the current of the pump diode changes.   
     
     
         15 . A method as claimed in  claim 12 , further comprising:
 measuring a repetition rate of injected oscillator pulses or an external signal;   shutting off or reducing the pump diode current if the repetition rate falls below a predetermined minimum repetition rate,   
     
     
         16 . A method as claimed in  claim 12 , further comprising:
 measuring an amplitude of oscillator pulses; and   shutting off or reducing the pump diode if the amplitude of the oscillator falls below a predetermined threshold.   
     
     
         17 . A method of operating a high power fiber amplifier system, the method comprising:
 controlling a diode current of a pump diode in the high power fiber amplifier system;   setting one or both of the pump diode current and power to a desired value;   measuring one or both of the pump diode current and power; and   maintaining one or both of the pump diode current and power at the desired value,   wherein uniform output energy is achieved as one or more of a pulse width and a repetition rate of output pulses.   
     
     
         18 . A method as claimed in  claim 17 , further comprising:
 initiating operation of a pump diode sufficiently in advance of first pulses; and   ramping up the pump diode current to produce equal power for the first pulses when the system is turned on.   
     
     
         19 . A method as claimed in  claim 17 , further comprising:
 storing the desired pump diode current setting as a function of system pulse width and repetition rate, wherein the energy of the output pulse is maintained at the desired value as the pulse width and repetition rate are varied.   
     
     
         20 . A method as claimed in  claim 17 , further comprising:
 calculating the desired pump diode current setting as a function of system pulse width and repetition rate, wherein the energy of the output pulse is maintained at the desired value as the pulse width and repetition rate are varied.   
     
     
         21 . A method as claimed in  claim 17 , further comprising:
 calculating an appropriate pump diode temperature as a function of the pump diode current setting;   varying the pump diode current; and   maintaining an emission wavelength of the pump diode, whereby the maximum absorption of the pump diode energy by the fiber amplifier medium is achieved as the pump diode current is varied.   
     
     
         22 . A method as claimed in  claim 17 , further comprising:
 storing an appropriate pump diode temperature as a function of the pump diode current setting;   varying the pump diode current; and   maintaining an emission wavelength of the pump diode, whereby the maximum absorption of the pump diode energy by the fiber amplifier medium is achieved as the pump diode current is varied.   
     
     
         23 . A method as claimed in  claim 17 , further comprising:
 measuring a repetition rate of pulses injected into the amplifier system;   comparing the repetition rate of the injected pulses to a predetermined minimum repetition rate; and   disabling or reducing the current to the amplifier pump diode if the injected pulses have a lower repetition rate than the minimum repetition rate.   
     
     
         24 . A method as claimed in  claim 17 , further comprising:
 measuring an amplitude of a pulse injected into the amplifier system;   comparing the amplitude of the pulse being injected into the fiber amplifier with a predetermined minimum amplitude value; and   disabling or reducing the current to the amplifier pump diode if the amplitude of the pulse being injected into the fiber amplifier is lower than the predetermined minimum.   
     
     
         25 . A method as claimed in  claim 17 , further comprising:
 measuring a repetition rate of an oscillator of the system; and   calculating a required down counter divide ratio needed to obtain a lower repetition rate.   
     
     
         26 . A method as claimed in  claim 17 , further comprising:
 varying the repetition rate of an oscillator of the system as a function of time or temperature;   outputting pulses with a desired repetition rate while the repetition rate of the oscillator is varied.   
     
     
         27 . A method as claimed in  claim 17 , further comprising:
 synchronizing an output of an oscillator of the system with an external reference signal;   varying a frequency of the external reference signal; and   automatically varying a repetition rate of the oscillator in accordance with the variations of the external reference signal.   
     
     
         28 . A method as claimed in  claim 27 , further comprising:
 varying a down counted repetition rate in accordance with the variations of the external reference signal.   
     
     
         29 . A high power fiber amplifier system comprising:
 a first controller operable to control an electric current of a pump diode and further control a gain of a fiber amplifier, wherein during operation of the system, an output pulse energy is controlled to be constant as a pulse width and repetition rate of system output pulses are adjusted.   
     
     
         30 . A high power fiber amplifier system as claimed in  claim 29 , wherein the output pulse energy is maintained at a constant value during a turn-on phase of a pulse train. 
     
     
         31 . A high power fiber amplifier system as claimed in  claim 30 , further comprising:
 a second controller operable to control a temperature of the fiber amplifier pump diode such that a wavelength of the pump diode is maintained at a fixed value while a current of the pump diode changes.   
     
     
         32 . A high power fiber amplifier system as claimed in  claim 31 , further comprising:
 a first measuring device operable to measure the repetition rate of injected oscillator pulses or an external signal; and   a shut-off device operable to shut-off or reduce the pump diode current if the repetition rate falls below a predetermined repetition rate threshold.   
     
     
         33 . A high power fiber amplifier system as claimed in  claim 32 , further comprising:
 a second measuring device operable to measure the amplitude of the oscillator pulses, wherein said shut-off device is further operable to shut-off or reduce the pump diode if the amplitude falls below a predetermined amplitude threshold.