Optical: systems and elements – Optical amplifier – Raman or brillouin process
Reexamination Certificate
2000-04-26
2001-10-30
Tarcza, Thomas H. (Department: 3662)
Optical: systems and elements
Optical amplifier
Raman or brillouin process
C372S006000, C372S018000
Reexamination Certificate
active
06310715
ABSTRACT:
STATEMENT OF GOVERNMENT INTEREST
The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty hereon.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is in the field of laser multiplexing, and in particular relates to the coherent beam combination of the output of multiple phase-locked optical amplifiers or lasers to produce a high-power diffraction-limited laser source.
2. Description of the Related Art
The peak far-field intensity produced by M phase-locked optical amplifiers or lasers of equal power can ideally be M times as great as the intensity produced by the same sources if their relative phases are random. This fact has motivated a great deal of work on achieving coherently phased optical sources. An example is a device which phases the output of 19 phase-locked diode lasers by sending this light through single-mode optical fibers. [L. Bartelt-Berger, U. Brauch, A. Giesen, H. Huegel, and H. Opower, “Power-scaleable system of phase-locked single-mode diode lasers,” Appl. Opt. 38, 5752-5760 (1999).] Piezoelectric transducers are used to stretch the fibers so as to shift the optical phases. The optimal phases are maintained by means of an electronic feedback circuit.
Phase control is only one aspect of the problem of coherent beam combination. Generally the far-field of a phased array of emitters has power distributed partially into side lobes, so that the central peak contains only a fraction of the total power. To some extent this problem can be reduced by the use of microlens arrays which collimate the light from the individual sources when it has diffractively spread almost to the point of overlapping. [J. R. Leger, M. L. Scott, and W. B. Veldkamp, “Coherent addition of AlGaAs lasers using microlenses and diffractive coupling,” Appl. Phys. Lett. 52, 1771-1773 (1988).] This increases the filling factor of the array, which is the ratio of the sub-beam diameters at their waists to the distance between beams. A large filling factor results in a greater fraction of far-field power in the central lobe. However, the microlens array does not produce a Gaussian beam, and the beam quality is not diffraction limited.
It is an object of the present invention to produce a high-power polarized Gaussian beam that is diffraction limited by multiplexing narrow-bandwidth phase-locked beams from multiple lasers or laser amplifiers. It is a further object to demultiplex a common optically encoded signal, such as a cable television signal, into multiple optical pathways.
SUMMARY OF THE INVENTION
A configuration of stacked optically contacted birefringent crystal plates has been conceived for the purpose of coherently combining (multiplexing) multiple phase-locked laser beams into a single diffraction-limited beam. Conversely, a beam of light propagating in the reverse direction at normal incidence through a stack consisting of N crystals is split (demultiplexed) into 2
N
linearly polarized beams. In the direction of multiplexing the thickness of successive crystal plates increases by a factor of {square root over (2)}, and the transverse direction of extraordinary-wave walk-off in successive plates flips back and forth by 45°. The relative phases of the 2
N
beams to be combined must be controlled in order for beam combination to occur. Otherwise the number of emerging beams increases to [2
(N+1)/2
−1]
2
if N is odd and (2
N/2
−1)(2
1+N/2
−1) if N is even. By monitoring the power emitted into certain of the unwanted emerging beams and minimizing this power by means of an electronic feedback circuit controlling optical phase adjusters, most of the emerging power can be concentrated into the single desired beam.
The beams to be combined must also have the correct spatial positions and alignment. This is greatly simplified by using a device where these beams are produced by phase-conjugate reflection of a demultiplexed beam from a front-end master oscillator (FMO) which has passed through the stack in the opposite direction. Stimulated Brillouin scattering (SBS) in multimode fibers can produce the required phase conjugation and can reverse the effects of aberrations and depolarization that occur on the forward pass through the device. Laser amplifiers, such as Yb-doped multimode fibers pumped by a diode-laser array, can be used to amplify both the incoming radiation and the reflected Stokes radiation. A back-end laser master oscillator (BMO) at the Stokes frequency can be used to phase-lock the beams emitted by the different amplifiers, and piezoelectric fiber stretchers can be used to adjust the beam phases to achieve multiplexing in the crystal stack.
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L. Bartelt-Berger, U. Brauch, A. Giesen, H. Huegel, and H. Opower, “Power-scaleable system of phase-locked single-mode diode lasers,” Appl. Opt. 38, 5752-5760 (1999).
J. R. Leger, M. L. Scott, and W. B. Veldkamp, “Coherent addition of AlGaAs lasers using microlenses and diffractive coupling,” Appl. Phys. Lett. 52, 1771-1773 (1988).
B. Ya. Zeldovich, N. F. Pilipetsky, and V. V. Shkunov,Principles of Phase Conjugation(Springer, Berlin, 1985) p. 46.
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V. I. Kovalev and R. G. Harrison, “Diffraction limited output from a CW Nd:YAG master oscillator/power amplifier with fibre phase conjugate SBS mirror,” Opt. Commun. 166, 89-93 (1999).
T. R. Loree, D. E. Watkins, T. M. Johnson, N. A. Kurnit, and R.A. Fisher, “Phase locking two beams by means of seeded Brillouin scattering,” Opt. Lett. 12, 178-180 (1987).
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Callahan Kenneth E.
Hughes Deandra
Tarcza Thomas H.
The United States of America as represented by the United States
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