Master oscillator power amplifier with interference isolated oscillator

• US 5,126,876 A
• Filed: 02/28/1991
• Issued: 06/30/1992
• Est. Priority Date: 11/20/1989
• Status: Expired due to Term

• Current Assignee: Hughes Electronics Corporation (AT&T, Inc.)
• Original Assignee: Hughes Aircraft Company (Rtx Corporation)
• Inventors: O'Meara, Thomas R.
• Primary Examiner(s): Moskowitz, Nelson
• Application Number:
  US07/663,277
• Time in Patent Office: 488 Days
• Field of Search: 359/338, 359/334
• US Class Current: 359/338
• CPC Class Codes: H01S 3/005 Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping shaping laser beam for working metal or other materials B23K26/06; optical elements, systems or apparatus in general G02B
• CPC Class Codes: H01S 3/0064 Anti-reflection devices, e.g. optical isolaters absorbing layers for marking or protecting purposes in laser working B23K26/50; magneto-optical non-reciprocal devices G02F1/093, G02F1/0955
• CPC Class Codes: H01S 3/10076 using optical phase conjugation, e.g. phase conjugate reflection
• CPC Class Codes: H01S 3/2333 Double-pass amplifiers
• CPC Class Codes: H01S 3/2383 Parallel arrangements

Abstract

A master oscillator power amplifier (MOPA) system which isolates the master oscillator (MO) from the return amplified beam by dividing the return beam into two components, introducing a phase shift between the components, and recombining the phase shifted beam components through constructive interference into an output beam directed away from the MO. A destructive interference return output is directed back towards the MO, but is held to a negligible level by making the phase shift approximately equal to an odd number of half-wavelengths at the return beam wavelength. In the preferred embodiment a stimulated Brillouin scattering phase conjugate mirror (PCM) is used to direct the amplified beam for a second amplification pass, while a Mach-Zender interferometer transmits the beam both before and after double amplification. The original beam is replicated after its first path through the interferometer by having the interferometer phase shift also equal an integral number of wavelengths at the input beam wavelength, prior to frequency shifting by the PCM.

Claims

1. A master oscillator power amplifier (MOPA) system, comprising:

a master oscillator (MO) for generating an input coherent optical beam along an input path,

an optical amplifier positioned to amplify said beam in a first amplifying pass,

a phase conjugate mirror (PCM) positioned to receive the amplified beam and to direct a phase conjugate of the input beam back to said amplifier for a second amplifying pass, said phase conjugate beam being shifted from the input beam by a predetermined frequency shift,

interferometer means positioned in the return beam path between said optical amplifier and said MO, said interferometer means producing constructive and destructive interference return beam outputs with said destructive return beam output directed back towards the MO and said constructive return beam output directed along an output path, wherein said interferometer means is also positioned along said input beam path and produces constructive and destructive interference input beam outputs, with the constructive input beam output directed into said optical amplifier for said first amplifying pass, a said interferometer having a path length differential approximately equal to (a) an integral number of wavelengths at the MO wavelength, and (b) an odd number of half-wavelengths at the phase conjugated return beam wavelength, and including servo control means for controlling effective path length differential, wherein said servo control means comprises;

at least a first cooled diffraction grating beam splitter for dividing the input coherent optical beam from said master oscillator into two components,

at least a second cooled diffraction grating beam splitter,

a first reflecting means which is fixed for reflecting one of said two components onto said second cooled diffraction grating beam splitter,

a second reflecting means which is moveable for reflecting the other of said two components onto said second cooled diffraction grating beam splitter for combining with said first component,

detector means for detecting destructive interference output from said second cooled diffraction grating and generating a signal, and

control means for adjusting the position of said second reflecting means in response to the detector signal.

2. The MOPA of claim 1, further comprising trimming means for adjusting the input beam frequency to trim the number of wavelengths in said unbalance path.

3. The MOPA system of claim 1, said PCM comprising a stimulated Brillouin scattering PCM.

4. The MOPA system of claim 1, said optical amplifier comprising a spatial array of a plurality of optical amplifier sections, and further comprising means for expanding the beam from the MO so that it is incident upon said plurality of optical amplifier sections, and means for contracting the beam after the first amplifying pass onto said PCM.

5. A master oscillator power amplifier (MOPA) system, comprising:

a master oscillator (MO) for generating an input coherent optical beam along an input path,

a first beam splitter positioned in said input path for splitting the input beam into first and second components,

a second beam splitter,

beam directing means directing said first and second input beam components between said first and second beam splitters along respective first and second beam paths that differ in effective length by approximately an integral number of wavelengths at the MO wavelength, whereby said first and second beam components interact at the second beam splitter to produce a constructive interference beam,

an optical amplifier positioned in the path of said constructive interference beam to amplify said beam in a first amplifying pass,

a phase conjugate mirror (PCM) positioned to receive the amplified beam and to direct a phase conjugate of the amplified beam back to said amplifier for a second amplifying pass, said phase conjugate beam being shifted from the input beam by a predetermined frequency shift and being directed from said amplifier back to said second beam splitter, said second beam splitter splitting the phase conjugate beam into first and second components directed by said beam directing means back to said first beam splitter along said first and second beam paths, respectively, said first and second beam paths differing in effective length by approximately an odd number of half-wavelengths at the phase conjugate beam wavelength, whereby said phase conjugate beam components undergo destructive interference at said first beam splitter back along said input path towards the MO, and constructive interference to produce an output beam along an output path which does not enter the MO, and

trimming means for adjusting the effective path length differential between first and second beam paths,

wherein said beam directing means comprises;

a dielectric block formed from a transparent material positioned in said input path so that one of said first or second input beam components is transmitted through said dielectric block,

a plurality of mirrors positioned for reflecting said input beam component transmitted through said dielectric block towards said second beam splitter.

6. The power amplifier of claim 5, said PCM comprising a stimulated Brillouin scattering PCM.

7. The power amplifier of claim 5, said optical amplifier comprising a spatial array of a plurality of optical amplifier sections, and further comprising means for expanding the input beam so that it is incident upon said plurality of optical amplifier sections, and means for contracting the beam after the first amplifying pass onto said PCM.

8. The power amplifier of claim 5, further comprising means for polarizing the input beam prior to said first beam splitter, and means located between said second beam splitter and said PCM for rotating the beam polarization to a cross-polarization orientation, said second beam splitter being polarization sensitive to direct components of said phase conjugate beam having said cross-polarization along said output path.