Method and apparatus to suppress gain-switched spikes in fiber laser amplifier systems

1. An apparatus comprising:

an optically-pumped optical-fiber gain medium operatively coupled to receive a first pulsed seed signal and amplify the seed signal to form amplified laser pulses;

an optical pump source operatively coupled to the optical-fiber gain medium to deliver optical pump energy that is stored in the optical-fiber gain medium and builds up over time;

a distributed Bragg-reflector (DBR) laser-diode optical-seed source operably coupled to deliver the pulsed seed signal to the gain medium; and

an electrical-drive-current source operatively coupled to drive a pulsed current signal to the DBR laser diode, which outputs the first pulsed laser seed signal based on the pulsed current signal, wherein the electrical-drive-current source is operated to deliver a peak-drive-current value to operate the DBR laser diode during the pulses at a level above lasing threshold but below a level that causes mode hopping.

2. The apparatus of claim 1 , wherein the electrical-drive-current source is configured to apply a bias current to the DBR laser diode between pulses, wherein the bias current is set as high as possible without exceeding a lasing-threshold current for the DBR laser diode, in order to reduce a heat-load change during the pulses.

3. The apparatus of claim 1 , wherein each pulse of the first pulsed laser seed signal has a frequency chirp, the apparatus further comprising:

an optical switch operably coupled between the DBR laser diode and the gain medium, wherein the optical switch is configured to receive the first pulsed laser seed signal and to select a predetermined non-zero temporal subportion of the first chirped laser pulse, in order to limit a linewidth of the seed signal as delivered to the optical-fiber gain medium.

4. The apparatus of claim 1 , wherein each pulse of the first pulsed laser seed signal has a frequency chirp, the apparatus further comprising:

an optical switch operably coupled between the DBR laser diode and the gain medium, wherein the optical switch is configured to gate a temporal subportion of the first pulsed laser seed signal that has a variable selected duration in order to control a duration of each of the amplified laser pulses and that has a variable selected delay from a start of each pulse of the first pulsed laser seed signal to control a bandwidth of the temporal subportion selected.

5. The apparatus of claim 1 , wherein the electrical-drive-current source is controlled to suppress gain-switched spikes in the amplified laser pulses.

6. The apparatus of claim 1 , wherein the electrical-drive-current source is controlled such that the amplified laser pulses have a single longitudinal optical mode with greater than 20 dB side-mode suppression.

7. The apparatus of claim 1 , wherein the peak-drive-current value is about 75 milliamps (mA).

8. A method comprising:

driving a distributed Bragg-reflector (DBR) laser diode with a pulsed electrical-current signal, in order to generate a first pulsed laser seed signal from the DBR laser diode, wherein the electrical-drive-current signal has a value between pulses that is below a lasing threshold of the DBR laser diode and a peak-drive-current value during the pulses that operates the DBR laser diode at a level above lasing threshold but below a level that causes mode hopping;

pumping an optically-pumped optical-fiber gain medium with optical pump energy that is stored in the optical-fiber gain medium and builds up over time;

coupling the first pulsed laser seed signal into the optically-pumped optical-fiber gain medium; and

amplifying the seed signal to form amplified laser pulses.

9. The method of claim 8 , wherein the electrical-drive-current signal supplies a bias current to the DBR laser diode between pulses, wherein the bias current is set as high as possible without exceeding a lasing-threshold current for the DBR laser diode, in order to reduce a heat-load change during the pulses.

10. The method of claim 8 , wherein each pulse of the first pulsed laser seed signal has a frequency chirp, the method further comprising:

optically gating the first pulsed laser seed signal to select a predetermined non-zero temporal subportion of the first chirped laser pulse, in order to limit a linewidth of the seed signal as delivered to the optical-fiber gain medium.

11. The method of claim 8 , wherein each pulse of the first pulsed laser seed signal has a frequency chirp, the method further comprising:

optically gating the first pulsed laser seed signal to pass a temporal subportion of the first pulsed laser seed signal that has a variable selected duration in order to control a duration of each of the amplified laser pulses and that has a variable selected delay from a start of each pulse of the first pulsed laser seed signal to control a bandwidth of the temporal subportion selected.

12. The method of claim 8 , wherein the electrical-drive-current signal is controlled to suppress gain-switched spikes in the amplified laser pulses.

13. The method of claim 8 , wherein the electrical-drive-current signal is controlled such that the amplified laser pulses have a single longitudinal optical mode with greater than 20 dB side-mode suppression.

14. The method of claim 8 , wherein the peak-drive-current value is about 75 milliamps (mA).

15. An apparatus comprising:

means for driving a distributed Bragg-reflector (DBR) laser diode with a pulsed electrical-current signal, in order to generate a first pulsed laser seed signal from the DBR laser diode, wherein the electrical-drive-current signal has a value between pulses that is below a lasing threshold of the DBR laser diode and a peak-drive-current value during the pulses that operates the DBR laser diode at a level above lasing threshold but below a level that causes mode hopping;

means for pumping an optically-pumped optical-fiber gain medium with optical pump energy that is stored in the optical-fiber gain medium and builds up over time;

means for coupling the first pulsed laser seed signal into the optically-pumped optical-fiber gain medium, in order to amplify the seed signal to form amplified laser pulses.

16. The apparatus of claim 15 , wherein the means for driving the DBR laser diode supplies a bias current to the DBR laser diode between pulses, wherein the bias current is set as high as possible without exceeding a lasing-threshold current for the DBR laser diode, in order to reduce a heat-load change during the pulses.

17. The apparatus of claim 15 , wherein each pulse of the first pulsed laser seed signal has a frequency chirp, the method further comprising:

optically gating the first pulsed laser seed signal to select a predetermined non-zero temporal subportion of the first chirped laser pulse, in order to limit a linewidth of the seed signal as delivered to the optical-fiber gain medium.

18. The apparatus of claim 15 , wherein each pulse of the first pulsed laser seed signal has a frequency chirp, the apparatus further comprising:

means for optically gating the first pulsed laser seed signal to pass a temporal subportion of the first pulsed laser seed signal that has a variable selected duration in order to control a duration of each of the amplified laser pulses and that has a variable selected delay from a start of each pulse of the first pulsed laser seed signal to control a bandwidth of the temporal subportion selected.

19. The apparatus of claim 15 , wherein the means for driving the DBR laser diode is controlled to suppress gain-switched spikes in the amplified laser pulses.

20. The apparatus of claim 15 , wherein the means for driving the DBR laser diode is controlled such that the amplified laser pulses have a single longitudinal optical mode with greater than 20 dB side-mode suppression.