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An apparatus, method and associated fiber-laser architectures for high-power pulsed operation and pumping wavelength-conversion devices. Some embodiments generate blue laser light by frequency quadrupling infrared (IR) light from Tm-doped gain fiber using non-linear wavelength conversion. Some embod

An apparatus, method and associated fiber-laser architectures for high-power pulsed operation and pumping wavelength-conversion devices. Some embodiments generate blue laser light by frequency quadrupling infrared (IR) light from Tm-doped gain fiber using non-linear wavelength conversion. Some embodiments use a fiber MOPA configuration to amplify a seed signal from a semiconductor laser or ring fiber laser. Some embodiments use the frequency-quadrupled blue light for underwater communications, imaging, and/or object and anomaly detection. Some embodiments amplitude modulate the IR seed signal to encode communication data sent to or from a submarine once the modulated light has its wavelength quartered. Other embodiments transmit blue-light pulses in a scanned pattern and detect scattered light to measure distances to objects in a raster-scanned underwater volume, which in turn are used to generate a data structure representing a three-dimensional rendition of the underwater scene being imaged for viewing by a person or for other software analysis.

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1. A method comprising: providing a fiber gain medium, wherein the fiber gain medium is arranged as a ring laser;optically pumping the fiber gain medium;forcing a majority of a signal beam to travel in a first direction around the ring laser;forming a free-space version of the signal beam in the rin

1. A method comprising: providing a fiber gain medium, wherein the fiber gain medium is arranged as a ring laser;optically pumping the fiber gain medium;forcing a majority of a signal beam to travel in a first direction around the ring laser;forming a free-space version of the signal beam in the ring laser;filtering the signal beam to limit a linewidth of the signal beam in the ring laser;at a timing independently controlled by an electrical signal for each of a plurality of pulses, amplitude-modulating the signal beam to form the plurality of pulses;polarizing the signal beam to a linear polarization;extracting the signal beam as an intermediate optical signal output beam having a first wavelength between 1800 nm and 2000 nm from the free-space version of the signal beam using a polarizing beam splitter;rotating a direction of polarization on both of two sides of the polarizing beam splitter, wherein the forcing of the majority of the signal beam is done between the two rotatings of the direction of polarization;frequency quadrupling the intermediate optical signal output beam to form a frequency-quadrupled optical signal at a wavelength different from the first wavelength of the intermediate optical signal output beam;transmitting the frequency-quadrupled optical signal through water;providing an optical parametric oscillator;optically coupling the frequency-quadrupled optical signal into the optical parametric oscillator;outputting, from the optical parametric oscillator, an idler wavelength and a signal wavelength; andfrequency doubling the idler wavelength and the signal wavelength to form a frequency-doubled idler wavelength and a frequency-doubled signal wavelength, wherein the transmitting of the frequency-quadrupled optical signal includes transmitting at least one of the frequency-doubled idler wavelength and the frequency-doubled signal wavelength through the water. 2. The method of claim 1, further comprising: detecting a light signal caused by light interaction of the frequency-quadrupled signal with a thermocline in the water; andprocessing the detected light signal to derive image information. 3. An apparatus comprising: an infrared laser outputting a Q-switched laser signal having a first wavelength between 1800 nm and 2000 nm as a Q-switched intermediate optical signal output beam, wherein the infrared laser includes a Q-switch operatively coupled to the laser and configured to Q-switch the laser signal, wherein an electrical signal controls a timing of the Q-switch;a frequency quadrupler operably coupled to receive the Q-switched intermediate optical signal output beam and to output a frequency-quadrupled optical signal;a beam transmitter operably coupled to transmit the frequency-quadrupled optical signal through water;an optical parametric oscillator coupled to receive the frequency-quadrupled optical signal and configured to output an idler wavelength and a signal wavelength; anda frequency doubler configured to frequency double the idler wavelength and the signal wavelength to form a frequency-doubled idler wavelength and a frequency-doubled signal wavelength, wherein the beam transmitter is further operably coupled to transmit at least one of the frequency-doubled idler wavelength and the frequency-doubled signal wavelength through the water. 4. The apparatus of claim 3, further comprising: a data encoder operably coupled to encode data on at least one of the intermediate optical signal output beam and the frequency-quadrupled optical signal such that the frequency-quadrupled optical signal has encoded data. 5. The apparatus of claim 3, further comprising: a light detector and processor configured to detect and process reflections from the transmitted frequency-quadrupled optical signal to generate 3D image data. 6. The apparatus of claim 3, wherein the infrared laser includes a large-mode-area (LMA) fiber. 7. An apparatus comprising: a fiber gain medium;means for optically pumping the fiber gain medium;means for Q-switching a laser seed signal, at a timing controlled by an electrical signal, to form a Q-switched laser seed signal;means for optically coupling, into the fiber gain medium, the Q-switched laser seed signal;means for outputting, from the fiber gain medium, an amplified version of the Q-switched laser seed signal as an intermediate optical signal output beam having a first wavelength between 1800 nm and 2000 nm;means for frequency quadrupling the intermediate optical signal output beam to form a frequency-quadrupled optical signal;means for transmitting the frequency-quadrupled optical signal through seawater;means for receiving the frequency-quadrupled optical signal and outputting an idler wavelength and a signal wavelength; andmeans for frequency doubling the idler wavelength and the signal wavelength to form a frequency-doubled idler wavelength and a frequency-doubled signal wavelength, wherein the means for transmitting further transmits at least one of the frequency-doubled idler wavelength and the frequency-doubled signal wavelength through seawater. 8. The apparatus of claim 7, further comprising: means for encoding the laser signal with data to be communicated through water. 9. The apparatus of claim 8, wherein the means for transmitting the frequency-quadrupled optical signal is arranged to communicate data between two ships, at least one of which is a submarine. 10. The apparatus of claim 7, further comprising: means for detecting, from water, a light signal caused by light interaction of the frequency-quadrupled signal; andmeans for processing the detected light signal to derive image information. 11. The apparatus of claim 10, wherein the means for transmitting the frequency-quadrupled signal further includes means for scanning the transmitted frequency-quadrupled signal across a range of angles in order to detect three-dimensional (3D) image information. 12. The apparatus of claim 7, wherein the means for frequency quadrupling the intermediate optical signal output beam further includes: means for frequency doubling the intermediate optical signal output beam to form a second optical signal output beam having a second wavelength that is one-half of the first wavelength of the intermediate optical signal output beam; andmeans for frequency doubling the second optical signal output beam to form the frequency-quadrupled optical signal having a third wavelength that is one-half of the second wavelength of the second optical signal output beam. 13. The apparatus of claim 7, further comprising means for communicating through seawater using the frequency-quadrupled optical signal. 14. The apparatus of claim 13, wherein the means for transmitting the frequency-quadrupled optical signal operates from a surface vehicle. 15. The apparatus of claim 13, wherein the means for transmitting the frequency-quadrupled optical signal operates from an aircraft. 16. The apparatus of claim 7, further comprising means for imaging through seawater using the frequency-quadrupled optical signal. 17. The apparatus of claim 7, further comprising: means for illuminating underwater features using the frequency-quadrupled optical signal; andmeans for detecting and processing reflected light from the frequency-quadrupled optical signal to form an image. 18. The apparatus of claim 7, further comprising means for detection and ranging of underwater bodies using the frequency-quadrupled optical signal. 19. The apparatus of claim 7, further comprising means for imaging disturbances in a thermocline using the frequency-quadrupled optical signal. 20. The apparatus of claim 7, wherein the fiber gain medium includes a large-mode-area (LMA) fiber. 21. A method comprising: providing a fiber gain medium;optically pumping the fiber gain medium;Q-switching a laser seed signal, at a timing controlled by an electrical signal, to form a Q-switched laser seed signal;optically coupling, into the fiber gain medium, the Q-switched laser seed signal;outputting, from the fiber gain medium, an amplified version of the Q-switched laser seed signal as an intermediate optical signal output beam having a first wavelength between 1800 nm and 2000 nm;frequency quadrupling the intermediate optical signal output beam to form a frequency-quadrupled optical signal;transmitting the frequency-quadrupled optical signal through water;providing an optical parametric oscillator;optically coupling the frequency-quadrupled optical signal into the optical parametric oscillator;outputting, from the optical parametric oscillator, an idler wavelength and a signal wavelength; andfrequency doubling the idler wavelength and the signal wavelength to form a frequency-doubled idler wavelength and a frequency-doubled signal wavelength, wherein the transmitting of the frequency-quadrupled optical signal includes transmitting at least one of the frequency-doubled idler wavelength and the frequency-doubled signal wavelength through water. 22. The method of claim 21, further comprising: encoding the laser signal with data to be communicated through the water. 23. The method of claim 22, wherein the transmitting of the signal is between two ships, at least one of which is a submarine. 24. The method of claim 21, further comprising: detecting a light signal caused by light interaction of the frequency-quadrupled signal with a thermocline in the water; andprocessing the detected light signal to derive image information. 25. The method of claim 21, further comprising: detecting a light signal caused by light interaction of the frequency-quadrupled signal with an anomaly in the water; andprocessing the detected light signal to derive image information. 26. The method of claim 25, wherein the transmitting of the frequency-quadrupled signal further includes scanning the transmitted frequency-quadrupled signal across a range of angles in order to detect three-dimensional (3D) image information. 27. The method of claim 21, wherein the frequency quadrupling of the intermediate optical signal output beam further includes: frequency doubling the intermediate optical signal output beam to form a second optical signal output beam having a second wavelength that is one-half of the first wavelength of the intermediate optical signal output beam; andfrequency doubling the second optical signal output beam to form the frequency-quadrupled optical signal beam having a third wavelength that is one-half of the second wavelength of the second optical signal output beam. 28. The method of claim 21, further comprising using the frequency-quadrupled optical signal for communications through seawater. 29. The method of claim 21, further comprising using the frequency-quadrupled optical signal for imaging through seawater. 30. The method of claim 21, wherein the fiber gain medium includes a large-mode-area (LMA) fiber. 31. The method of claim 21, wherein the transmitting of the frequency-quadrupled optical signal includes steering a majority of the frequency-quadrupled optical signal toward a desired target receiver. 32. The method of claim 21, wherein the frequency-quadrupled optical signal has a wavelength that is set to a wavelength of Fraunhofer feature F.