Birefringence compensated laser architecture

Apparatus, and a related method, for compensating for birefringence introduced in a birefringent medium, such as a solid-state amplifier. The invention includes the combination of a quarter-wave plate, a Faraday rotator and a mirror, which may be a phase conjugation cell. Light passing through the quarter-wave plate is substantially circularly polarized, which is advantageous if the mirror is a phase conjugation cell using stimulated Brillouin scattering (SBS). A second pass through the quarter-wave plate provides a linearly polarized beam of which the polarization angle is orthogonally related to that of the original beam, to facilitate out-coupling of energy from the apparatus. The Faraday rotator effects a total polarization angle rotation of 90.degree. in two passes and helps compensate for birefringence when the beam is passed through the birefringent medium again on the return pass. The combination of the quarter-wave plate and the Faraday rotator provides better birefringence cancellation than either element acting alone.

BACKGROUND OF THE INVENTION 
This invention relates generally to high-power solid-state lasers and, more 
particularly, to techniques for reducing birefringence in solid-state 
lasers. Solid-state lasers with an average power up to 100 W (watts), and 
even higher powers, are needed in a variety of military, industrial and 
commercial applications, including X-ray photolithography, laser machining 
and drilling, space and underwater communication, and medical 
applications. 
The brightness of a laser beam is proportional to the average power and is 
inversely proportional to the square of the beam quality, where the beam 
quality is in turn defined in relation to a diffraction-limited beam, 
i.e., a diffraction-limited beam has an ideal beam quality of 1.0. A worse 
beam quality of, say, 1.5 results in a brightness of 1/(1.5).sup.2 or 
44.4% of the brightness of the diffraction limited beam. Since the 
brightness falls off in proportion to the square of the beam quality, it 
is extremely important to control the beam quality if high brightness is a 
design goal. The parent application cross-referenced above was principally 
concerned with various structural features that improved beam quality and 
brightness. Another aspect of the same problem is birefringence in the 
optical components used to generate the beam. Birefringence can be 
optically filtered from the output beam, and discarded to avoid equipment 
damage, but the removal of the birefringence component results in a lower 
output power and brightness. Therefore, it is more desirable to compensate 
for birefringence rather than to remove it from the output beam. 
A number of laser architectures disclosed in various prior patents use a 
phase conjugated master oscillator power amplifier (PC MOPA) 
configuration, but still fail to produce a desirably bright beam, or have 
other drawbacks. The parent application cross-referenced above discloses 
and claims a high brightness solid-state laser source that includes a 
master oscillator, a solid-state amplifier and a phase conjugation cell 
positioned to receive the amplified beam from the solid-state amplifier 
and to reflect the beam in phase conjugated form back into the solid-state 
amplifier for a second pass. Aberrations introduced in the amplifier 
during the first pass are practically canceled during the second pass and 
the amplified beam has both high power and good beam quality. Although 
this laser source operates satisfactorily in many applications, it still 
suffers from birefringence as the amplifier heats up in operation. At 
higher temperatures, the amplifier crystal is thermally stressed and 
becomes anisotropic, exhibiting different indices of refraction along its 
different axes. Consequently, light propagates through the crystal at 
different speeds along the different axes, resulting in birefringence. 
Light emerging from the amplifier is no longer linearly polarized, but in 
general is elliptically polarized to some degree. 
As described in the cross-referenced patent application, birefringence 
components in the amplified beam subject the master oscillator to possibly 
serious damage if they are reflected back into the master oscillator. One 
way to avoid this problem is to install a Faraday rotator, referred to in 
the prior description as a Faraday isolator, next to the master 
oscillator. The Faraday rotator protects the master oscillator from energy 
leaking through a polarizer used to couple an output light beam from the 
laser source. In theory, the polarizer reflects the light beam returning 
from the amplifier and thereby couples it out of the laser source. 
However, any birefringence components in the return beam will pass through 
the polarizer and back into the master oscillator, which can be seriously 
damaged as a result. The Faraday rotator rotates the polarization 
direction by 45.degree. on each pass, with the result that the 
birefringence components are effectively dumped out of the system by the 
polarizer. This is a known technique for removing birefringence 
components. Since the components are effectively discarded, they represent 
a loss in the power of the output laser beam. 
The foregoing is only one example of an optical system in which unwanted 
birefringence arises. More generally, there is a need for birefringence 
compensation in a variety of optical systems. 
It has been proposed by I. D. Carr and D. C. Hanna, in a paper entitled 
Performance of a Nd:YAG Oscillator/Amplifier with Phase-Conjugation via 
Stimulated Brillouin Scattering, Appl. Phys. B 36, 83-92 (1985), that a 
Faraday rotator may be positioned between an amplifier and a mirror in a 
master oscillator power amplifier (MOPA) system, to reduce birefringence 
effects. Birefringence effects manifest themselves in the form of two 
light beams that, because of anisotropic crystalline properties of the 
amplifier as the temperature increases, travel at different velocities. 
The amplifier crystal exhibits a lower index of refraction, and a 
correspondingly higher speed of transmission in one direction, as compared 
with an orthogonal direction in which the index is higher and the speed of 
transmission lower. If light from the amplifier is rotated 45.degree. by 
the Faraday rotator and then an additional 45.degree. during a return pass 
through the Faraday rotator, the resulting light beam has its "fast" and 
"slow" components interchanged. A second pass through the amplifier 
effectively nullifies the birefringence. Intuitively, one can appreciate 
this effect by considering that the "fast" component of the beam takes the 
"slow" path through the amplifier crystal on the return pass. Likewise, 
the "slow component of the beam takes the" "fast" path. The net effect, in 
theory, is to nullify the birefringence components in the amplified beam. 
A practical difficulty with this approach to compensating for birefringence 
is that it cannot be used to advantage with one of the most commonly used 
phase conjugating mirrors, the stimulated Brillouin scattering (SBS) cell. 
In an SBS cell, containing a suitable SBS medium, such as liquid freon or 
gaseous nitrogen, the SBS process reverses the wavefront of an input beam. 
(Portions of the wavefront that were lagging become leading, and vice 
versa.) Aberrations impressed on the wavefront during the first pass 
through the amplifier are, therefore, negated and virtually removed during 
the second pass after reflection from the SBS cell. The SBS cell operates 
most effectively when the incident light is circularly polarized and the 
SBS medium is subject to optical breakdown if linearly polarized light is 
used. Therefore, use of a Faraday rotator to compensate for birefringence 
limits the effectiveness of the SBS cell because the incident beam is 
predominantly linearly polarized. 
Another practical difficulty with Faraday rotators is that they do not 
always provide a desired angle of rotation of the direction of 
polarization. If the nominal rotation angle is 45.degree., it is not 
uncommon for the actual rotation angle to be in error by a few degrees, 
and for spatial variations to occur over the aperture of the rotator. 
After two passes through the rotator, the expected rotation angle of 
90.degree. may be in error by as much as .+-.5.degree. or more. Clearly, 
this inaccuracy results in less than complete birefringence compensation. 
In the laser source described in the cross-referenced application, a 
quarterwave plate located next to the SBS cell serves to produce 
circularly polarized light. More specifically, on the first pass through 
the quarter-wave plate the linear polarization of the beam is converted to 
circular polarization. On the return pass, the circularly polarized beam 
is converted back to linearly polarized light, but with a polarization 
direction orthogonal to that of the original beam. The orthogonal 
relationship between the forward and return beams is used to outcouple 
light by means of a polarizer. 
Although a quarter-wave plate produces circularly polarized light, which is 
desirable for operation of an SBS phase conjugating mirror, the plate does 
not provide birefringence compensation. A significant birefringence 
component finds its way back to the master oscillator, where it must be 
removed to avoid equipment damage. Therefore, it will be appreciated that 
there is still a need for improvement in techniques for birefringence 
compensation in laser sources having medium to high power and good beam 
quality. The present invention satisfies this need. 
SUMMARY OF THE INVENTION 
The present invention resides in a laser architecture for compensating for 
optical birefringence. Briefly, and in general terms, the laser 
architecture of the invention comprises a quarter-wave plate disposed in 
the path of a laser light beam that is substantially linearly polarized 
but may contain a birefringence component as a result of passing through a 
birefringent medium; a polarization angle rotator, preferably based on the 
Faraday effect, disposed in the path of light emerging from the 
quarter-wave plate, and selected to provide a polarization angle rotation 
of approximately 45.degree.; and a mirror disposed in the path of light 
emerging from the polarization angle rotator, to provide a reflected light 
beam passing back through the polarization angle rotator and the 
quarter-wave plate. The reflected light beam is subject to another 
45.degree. polarization angle rotation in the polarization angle rotator, 
and any birefringence component is nullified when the reflected beam is 
passed back through the birefringent medium. 
In a disclosed embodiment of the invention, the mirror is a stimulated 
Brillouin scattering (SBS) phase conjugation cell, which operates more 
efficiently when receiving circularly polarized light as a result of the 
presence of the quarter-wave plate. In the disclosed embodiment, the 
birefringent medium is an optical amplifier and the architecture further 
comprises a type II frequency doubler for providing light output at double 
the normal frequency. The polarization angle rotator is a conventional 
Faraday rotator. 
The invention may also be defined as a phase conjugated master oscillator 
power amplifier (PC MOPA) with birefringence compensation. The PC MOPA 
comprises a master oscillator generating a pulsed input beam having a 
nearly diffraction limited beam quality; a solid-state amplifier 
positioned to receive and amplify the beam from the master oscillator, 
during a first pass through the amplifier, wherein the solid-state 
amplifier may introduce a birefringent component into the light beam; a 
phase conjugation cell positioned to receive the amplified input beam from 
the solid-state amplifier and to reflect the beam in phase conjugated form 
back into the solid-state amplifier for a second pass, whereby aberrations 
introduced in the solid-state amplifier during the first pass are 
practically canceled during the second pass; a quarter-wave plate 
positioned to receive light from the solid-state amplifier in the first 
pass; a Faraday rotator positioned between the quarter-wave plate and the 
phase conjugation cell, to effect a polarization angle rotation of 
approximately 90.degree. as a result of the first and second passes 
through the rotator; and a polarizer for extracting an output beam from 
the laser architecture. Light beams in the first and second passes have 
orthogonally related polarization angles and can, therefore, be separated 
by the polarizer for extraction of an output beam. The combination of the 
quarter-wave plate and the Faraday rotator effectively nullify any 
birefringence component when a second pass is made through the solid-state 
amplifier, which thermally induces birefringence in the first pass. 
In terms of a method of birefringence compensation, the invention comprises 
the steps of passing a primary light beam through a quarter-wave plate, 
the primary light beam being substantially linearly polarized but 
containing a possible birefringence component as a result of having passed 
through a birefringent medium; passing the primary light beam output from 
the quarter-wave plate through a Faraday rotator, to effect a rotation in 
polarization by approximately 45.degree.; reflecting light output from the 
Faraday rotator back into the Faraday rotator and thence through the 
quarterwave plate again; and passing the reflected light beam emerging 
from the quarter-wave plate back through the birefringent medium to 
nullify any birefringent components. 
It will be appreciated from the foregoing that the present invention 
provides a significant advance over existing techniques for birefringence 
compensation. In particular, the combination of a quarter-wave plate and a 
Faraday rotator together provide compensation that is approximately ten 
times better than can be obtained using the quarter-wave plate alone, or 
approximately two and a half times better than can be obtained using the 
Faraday rotator alone. Other aspects and advantages of the invention will 
become apparent from the following more detailed description, taken in 
conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in the drawings for purposes of illustration, the present 
invention pertains to birefringence compensation, especially as applied to 
laser light sources of medium to high power. Although the phase-conjugated 
master oscillator power amplifier (PC MOPA) configuration has been used in 
many variations, all have to date suffered from less than outstanding beam 
quality and, therefore, diminished brightness levels. The configuration 
described and claimed in the cross-referenced application overcomes many 
of the difficulties of the prior art but still suffers from significant 
birefringence. 
The present invention may be used in a variety of applications in which it 
is desired to compensate for or nullify birefringence effects. An example 
of such an application is birefringence compensation in a PC MOPA, such as 
the one shown diagrammatically in FIG. 1. This configuration includes a 
master oscillator, indicated by reference numeral 10, a polarizer 12, an 
amplifier 14, a quarter-wave plate 16, and a phase conjugation mirror 18. 
The MOPA architecture may also include additional components, some of 
which are described in more detail in the cross-referenced application, 
but which are not directly pertinent to the present invention. The master 
oscillator 10 produces a high quality, low energy optical beam of 
insufficient power for many purposes. The master oscillator beam may be 
first passed through a beam shaping telescope (not shown), to further 
condition the beam, before it enters the amplifier 14. It will be 
understood that the amplifier 14 is mentioned as one example of a source 
of birefringence. In the more general case, birefringence may be 
introduced by some other birefringence medium, or by another component in 
the optical train, and the birefringence may be statically or thermally 
induced. The amplifier 14, which may consist of a string of amplifiers, 
amplifies the beam on a first pass and the beam then is transmitted onto 
the phase conjugation mirror 18. Again, phase conjugation is not an 
essential element of the present invention, but is an important component 
of the PC MOPA configuration. Almost certainly, the amplifier 14 causes 
phase aberrations in optical wavefronts of the beam as it passes through 
the amplifier medium. However, as is well known, phase conjugation may be 
used to cancel these aberrations by passing a phase-conjugated form of the 
beam back through the amplifier 14. 
Extraction of an output beam from the architecture shown in FIG. 1 is 
effected by means of the Faraday rotator 22 and the polarizer 12. The 
Faraday rotator 22 rotates the polarization angle of the beam as a result 
of two passes through the rotator. More specifically, on the first pass 
through the rotator beam polarization is rotated 45.degree. and on the 
return pass the polarization is rotated another 45.degree., resulting in a 
polarization direction orthogonal to that of the original beam. Since the 
amplified beam leaving the amplifier 14 on the return pass has an 
orthogonal polarization with respect to the input beam, the polarizer 12 
can be used to extract the output beam. As described in the 
cross-referenced patent application, an alternative extraction scheme uses 
only a quarter wave plate 16 to rotate the polarization angle of the beam 
as a result of two passes through the plate. More specifically, on the 
first pass through the quarter-wave plate, the linear polarization of the 
beam is converted to circular polarization due to birefringence in the 
plate. On the return pass, the circularly polarized beam is converted back 
to linearly polarized light but with a polarization direction orthogonal 
to that of the original beam. In the present invention, however, the 
quarter-wave plate 16 is positioned in front of the Faraday rotator 22, 
and the quarter-wave plate only serves to provide circularly polarized 
light in the rotator and the SBS cell. 
In accordance with the present invention, birefringence is substantially 
nullified by a combination of the quarter-wave plate 16 and a Faraday 
rotator 22 positioned between the quarter-wave plate and the mirror 18. 
Although the Faraday rotator 22 alone should, in theory, operate to 
compensate for birefringence, in practice, functioning alone, it does not 
completely achieve the desired result, as shown by Carr and Hanna in the 
paper referenced above. The Faraday rotator 22 alone effects an overall 
rotation of approximately 90.degree. in the polarization angle, but 
because the rotation angle is typically not exactly 90.degree. over its 
entire aperture, birefringence is not completely nullified. The 
quarter-wave plate 16 positioned in front of the Faraday rotator 22 
provides circularly polarized light in the rotator, which serves to reduce 
the sensitivity of birefringence compensation to the Faraday rotation 
angle, and substantially improves compensation. Further, use of the 
Faraday rotator alone does not produce circularly polarized light, which 
is desired for use in an SBS phase conjugation mirror to avoid breakdown 
in the SBS medium. 
The combined effect of the quarter-wave plate 16 and the Faraday rotator 22 
is to compensate for birefringence almost completely. By way of example, 
the graph of FIG. 2 shows the percentage birefringence plotted against the 
energy extracted from the laser light source. Using the quarter-wave plate 
16 alone (curve A), there is still about 6-7% birefringence in the return 
beam from the amplifier 14. With the quarter-wave plate 16 and the Faraday 
rotator 22 operating in tandem, birefringence is reduced to below 1%. More 
precise measurements show that birefringence is reduced by approximately a 
factor of ten using the combination of the present invention. 
Successful operation of the combination is dependent upon the correct 
angular orientation of the quarter-wave plate 16. Therefore, initial 
adjustment of the orientation of the quarter-wave plate is needed to 
maximize birefringence cancellation. 
It should also be noted the quarter-wave plate 16 and the Faraday rotator 
22 cannot be interchanged. That is, the Faraday rotator 22 must be 
positioned between the quarter-wave plate 16 and the mirror 18. The 
invention simply will not work with the two components reversed in 
position and they will substantially nullify each other. 
Although the effectiveness of the combination is easily demonstrated, its 
theory of operation is not intuitive. An important aspect of the theory is 
that the combination of the quarter-wave plate 16 and Faraday rotator 22 
has a reduced sensitivity to the angle of rotation induced by the Faraday 
rotator. Although this insensitivity to the Faraday rotation angle can be 
demonstrated mathematically, it is not intuitively obvious that such a 
result would follow from a combination of the quarter-wave plate and the 
Faraday rotator. In the experimental observations mentioned above, in 
which a ten-fold reduction in birefringence was obtained, the Faraday 
rotator employed had a measured average angle of rotation of 42.degree. 
and exhibited spatial non-uniformities in its magnetic field. These 
spatial non-uniformities produce spatial variations in the amount of 
polarization rotation effected by the device. The quarter-wave plate 16 
used in the combination of the invention compensates to a very large 
degree for these departures from the desired theoretical rotation angle of 
45.degree.. 
The Faraday rotator 22 corrects for depolarization caused by elements of 
the optical train, by introducing a rotation of approximately 90.degree. 
in the polarization angle of the beam returning from the mirror 18. 
A further advantage of the invention is that, when used in conjunction with 
a phase conjugation mirror that relies on stimulated Brillouin scattering 
(SBS) for its operation, the combination provides circularly polarized 
light to the SBS cell. As is well known, the use of circularly polarized 
light in the SBS cell increases its dynamic range by increasing the 
threshold for breakdown. 
Another advantage of the invention is that, when used in conjunction with a 
phase conjugation mirror, the invention allows the use of a type II 
doubler in the phase conjugation path. Type II doublers have a random 
residual birefringence that passes through 0.degree.-180.degree. cycles as 
the doubler crystal is tuned for phase matching. The quarter-wave plate 
and Faraday rotator combination eliminates the sensitivity of the optical 
train to this type of birefringence. 
It will be appreciated from the foregoing that the present invention 
represents an important advance in techniques for birefringence 
compensation. In particular, the invention achieves a substantial 
reduction in birefringence induced thermally or otherwise in optical 
components such as amplifiers in laser light sources. It will also be 
appreciated that various modifications may be made without departing from 
the spirit and scope of the present invention. Accordingly the invention 
should not be limited except as by the following claims.