High brightness optical parametric amplifier array

A high brightness optical parametric amplifier array uses energy--scaleable optical parametric amplifiers that provide high brightness output. The leability in energy is achieved by using an array of parallel crystal amplifiers to handle high laser energies. High brightness is obtained by using an optical phase conjugator to keep the phase front of the array coherent.

GOVERNMENTAL INTEREST 
The information described herein may be manufactured, used and licensed by 
or for the United States Government for governmental purposes. 
BACKGROUND OF THE INVENTION 
Laser radiation is presently used in various devices by industry, commerce, 
medicine and armed services. In certain applications, the need exists for 
wavelength--tunable laser radiation having both high brightness and high 
energy capability. In the past optical parametric amplifiers (OPAS) and 
optical parametric oscillators (OPOs) have been used as devices for 
generating wavelength--tunable laser radiation with limited energy output. 
Prior art OPAs usually comprised an appropriately cut nonlinear 
birefringent crystal which amplified a weak "signal" beam by channeling 
energy from a strong "pump" beam of shorter wavelength. Two or more 
crystals were sometimes used in series to increase the OPA gain. In prior 
art devices the signal beam and pump beam usually arrive at the OPA 
colinearly. The difference of the photonenergies between the pump and 
signal beams is emitted in the form of an "idler" beam. 
The parametric process is most effective when the "phase matching 
condition" is satisfied among the pump, signal and idler waves. The phase 
matching is satisfied by choosing appropriate angles between the crystal 
axes and the beam directions. 
The optical parametric oscillator (OPO) is essentially the same as an OPA 
with a resonator cavity. In the case of an OPO, only a pump beam needs to 
be provided. The signal and idler beams are both generated within the 
resonator cavity. The wavelength of the generated signal idler pair are 
determined by the phase matching condition. The output wave-lengths may be 
tuned simply by changing the angle of the pump incidence, which is usually 
done by turning the crystal in the cavity. 
One of the problems with the aforementioned is that all crystals, including 
those used for parametric conversion, have some laser intensity limit 
beyond which they damage. Since the size of the crystals that can be grown 
is limited, there is a limit on how much energy may be converted by a 
single aperature optical parametric array. 
In the past when there was a need to frequency convert large amounts of 
pump laser energies one might have thought to use an array of many 
parallel OPAS. By using such a parallel arrangement one would scale up the 
energy conversion, but at the cost of ruining the beam phase fronts. The 
phase front degradation in a parallel OPA arrangement is caused by the 
fact that each individual OPA has slightly different lengths. The 
different lengths, when measured to a fraction of the laser wavelength, 
causes different parts of the original phase front to experience differing 
optical path lengths which leads to distortions of the phase front. The 
aforementioned phenomena of phase front degradation caused by differing 
optical path lengths is called "piston error". Phase front distortions 
results in an increase in beam divergence and lower brightness. This 
condition is generally unacceptable because for most laser applications 
the beam brightness is of central importance. A similar problem occurs in 
scaling up laser amplifiers. 
SUMMARY OF THE INVENTION 
The present invention relates to energy--scaleable optical parametric 
amplifiers that provide a high brightness output. 
An object of the invention is to provide energy scaleable parametric 
amplifiers with high brightness capability using a number of parallel 
optical parametric amplifiers arranged in a matrix using appropriate 
optical phase conjugate mirror to prevent phase front distortion. 
A further object of the present invention is to provide a high-energy high 
brightness optical parametric amplifier wherein the scaleability in energy 
is achieved by using an array of parallel crystal amplifiers whose numbers 
can be increased to handle high laser energies.

Throughout the following description, like numerals are used to denote like 
parts of the drawings. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the generic high-brightness OPA array of FIGS. 1 and 2, a 
low power and high-quality low divergence signal beam 10 is directed 
toward a first optical isolator 12 and passes through it and proceeds on 
to a beam combiner/splitter 14. A high energy pump laser beam 16 is 
directed toward a second optical isolator 17 and passes through it and 
proceeds on to the beam combiner/splitter 14. Signal beam 10 which is 
equal in diameter to that of the pump beam 16 is reflected by beam 
combiner/splitter 14 and combines with the high energy pump laser beam 16. 
The combined signal and pump beams 18 arrive together at parallel OPAs 
arranged in a matrix 20 which amplifies the signal beam 10 as it depletes 
the pump beam 16 and generates an idler beam. Because of "piston errors", 
aforementioned, among the individual OPAS, the uniform phase front of the 
signal beam gets broken up upon exiting the OPA array 20. The depleted 
pump beam and idler beam phase fronts also become similarly broken up. The 
broken up phase fronts 22 of the signal, pump and idle beams are directed 
toward a phase conjugator 24 which reverses the phases and the directions 
of propagation of each phase front. 
The phase conjugate mirror (PCM) may consist of a lens and a stimulated 
Brillouin scattering (SBS) cell. Common SBS materials include liquids such 
as carbon tetrachloride or freon and pressurized gases like nitrogen and 
methane. Alternatively, the PCM can be an electromechanical deformable 
adaptive optics mirror. The reversed phase conjugated signal, pump and 
idler beams 26 travel backwards and for a second time through the OPA 
array 20 where further parametric amplification takes place. A prism or 
filter member 28 is operatively positioned intermediate the OPA array 20 
and phase conjugator 24. The prism or filter member function will be 
described hereinafter. 
The backward travel of the phase reversed phase fronts results in a 
restoration of the uniform phase fronts through the well known properties 
of optical phase conjugation. In this manner, one is able to achieve 
energy amplification by an array of OPAs without ruining the phase front 
and sacrificing beam brightness. The residual pump beam passes through the 
combiner/splitter 14 and is dumped by the second optical isolator 17. The 
amplified phase conjugated signal and idler beams 30 are reflected by beam 
splitter 14 in the direction of signal optical isolator 12 and idler beam 
separator 32. The signal beam arriving back at the first isolator 12 is 
reflected off as the amplified signal output 34. The idler beam arriving 
at idler separator 32 is reflected off as an amplified idler output 36, 
which generally is a useful output. 
Referring now to FIG. 3, in this first embodiment, the optical isolators 12 
and 17 of FIG. 1 are shown as first and second Faraday isolators 12 and 17 
respectively. The Faraday isolator is a one-way device that passes the 
incident beam in one direction only and reflects to one side a beam 
propagating in the reverse direction. 
In operation, this embodiment of FIG. 3 uses a short-wavelength passing 
mirror 38 which allows the pump beam 16 to pass through but is fully 
reflective of longer wavelength signal and idler radiation. The pump and 
signal beams, 16 and 10 respectively, arriving at the short pass mirror 38 
combine and then propagate together as a combined beam 18 to the OPA array 
20 where the signal beam 10 is amplified and the idler wave is generated. 
As previously stated, the OPA array introduces distortions into all the 
phase fronts. The distorted pump, signal and idler waves 22 are then 
focused with a lens 40 into a stimulated Brillouin scattering (SBS) cell 
42 which reverses the phase of each of the waves. The phase reversed waves 
26 then travel backward through the OPA array 20 where they undergo a 
phase restoration to form nearly uniform phase fronts. The second backward 
passage through the OPA array 20 results in further parametric 
amplification of the signal and idler beams 30 and further depletion of 
the pump beam. The waves next arrive back at the short-pass mirror 38 
which transmits the residual pump beam and reflects the vastly amplified 
signal and idler beams toward the Faraday isolator 12'. 
If the signal and idler beams are of the same polarization, as is the case 
in Type I phase matched parametric amplification, and are nearly equal in 
wavelength, then Faraday isolator 12' will reflect off to one side both 
waves at the output. If the signal and the idler beams differ 
significantly in wavelength, a dichroic mirror 44 is used to split off the 
amplified idler beam 36 prior to the signal reaching the Faraday isolator 
12'. When the idler beam is orthogonally polarized with respect to the 
signal beam, as in a type II phase matching, a polarizer is used to 
separate out the idler from the signal beam. 
Referring now to FIG. 4, when there is a significant difference of 
wavelengths of the pump, idler and signal beams causing interference in 
conjugating from a common focal volume, a dispersive prism 46 is inserted 
in the beam path in front of lens 40. The prism 46 will cause each wave 
length to come to a focus in a separate volume. 
Finally, it should be noted that it is not essential to conjugate all of 
the waves. At the cost of some conversion efficiency one may conjugate 
just the signal beam alone or the signal and the pump beams and dump the 
remaining waves with a filter 28 positioned as shown in FIG. 1. 
While specific embodiments of the invention have been shown and described 
in detail to illustrate the application of the principles of the 
invention, it will be understood that the invention may be embodied 
otherwise without departing from such principles.