Apparatus for aligning the optic axis of an intra-cavity birefringent element, and tunable laser using same

Alignment of a birefringent device characterized by a first face and a second face cut at Brewster's angle so that a plane of polarization is defined. The optical device is secured in the optical path by a rotatable member having an axis of rotation that is normal to the Brewster faces of the optical device. Coupled with the rotatable member on the axis of rotation is a support allowing adjustment of the angular position about the axis of rotation of the rotatable member. An operator adjusts the angular position of the rotatable member until the C-axis lies in the plane of polarization. Even though the C-axis may not be parallel to the direction of polarization, the depolarization effects of the misalignment are eliminated by proper adjustment of the angular position relative to the plane of polarization. A tunable, solid state laser using the alignment apparatus with a birefringent filter achieves greater linear tuning range.

CROSS-REFERENCE TO RELATED APPLICATIONS 
The following U.S. patent application is incorporated by reference as if 
fully set forth herein, and is related to the present application. 
1. Woodward et al., TUNABLE DYE LASER WITH THIN BIREFRINGENT FILTER FOR 
IMPROVED TUNING, Ser. No. 07/260,980; Filed Oct. 21, 1988. 
The above listed related applications was owned at the time of invention 
and is currently owned by the same assignee as the present invention. 
BACKGROUND OF THE INVENTION 
1 Field of the Invention 
The present invention relates to the alignment of a crystalline axis of an 
optical device having Brewster faces; and, more particularly, to aligning 
a crystalline axis of a birefringent element so that it lies in the plane 
of the polarization defined by Brewster faces of the birefringent element. 
2. Description of Related Art 
Optical devices having Brewster cut faces are in widespread use. Brewster 
faces define a plane of polarization for light propagating though the 
optical device. Many such devices are fabricated from crystalline material 
with well-defined crystalline axes. Such optical devices are manufactured 
so that one such crystalline axis is as close to the direction of 
polarization as possible. However, with current manufacturing techniques, 
it is difficult to ensure that the selected crystalline axis is closely 
aligned with the direction of polarization. 
Where the optical device is birefringent and defines a long path between 
the Brewster faces, this misalignment of the crystalline axis can result 
in significant depolarization effects. When such an optical device is used 
in a laser cavity that includes another element which is sensitive to 
polarization, the depolarization effects can have an effect on the 
performance of the sensitive element. For instance, birefringent filters 
in widespread use in tunable lasers are very sensitive to the polarization 
of the beam in the cavity. Background concerning birefringent filters can 
be found in Bloom, "Modes of a Laser Resonator Containing Tilted 
Birefringent Plates", JOURNAL OF THE OPTICAL SOCIETY OF AMERICA, Vo., 64, 
No. 4, April 1974; Preuss et al., "Three-Stage Birefringent Filter Tuning 
Smoothly Over the Visible Region: Theoretic Treatment and Experimental 
Design", APPLIED OPTICS, Vol. 19, No. 5, 1 Mar. 1980; Holtom et al., 
"Design of a Birefringent Filter for High-Power Dye Lasers", IEEE JOURNAL 
OF QUANTUM ELECTRONICS, Vol. QE-10, No. 8, August 1974; Mudare et al., 
"Simple Alignment Procedure for the Assembly of Three-Plate Birefringent 
Filters for Tunable Dye Lasers", APPLIED OPTICS, Vol. 22, No. 5, 1 Mar. 
1983; and November et al., "Derivation of the Universal Wavelength Tuning 
Formula for a Lyot Birefringent Filter", APPLIED OPTICS, Vol. 23, No. 14, 
15 July 1984. 
One of the factors which limits the smooth tuning range of tunable solid 
state lasers, such as those using a Ti:sapphire or a cobalt magnesium 
fluoride gain medium, is the misalignment of the selected crystalline axis 
of the gain medium with resulting depolarization, affecting the operation 
of the birefringent tuning filter. As recognized in Schulz, 
"Single-Frequency Ti:A1.sub.2 0.sub.3 Ring Laser", IEEE JOURNAL OF QUANTUM 
ELECTRONICS, Vol. 24, No. 6, June 1988, it is very important to ensure 
that the crystallographic orientation of the Brewster faces aligns the 
C-axis of the Ti:sapphire crystal as close as possible to the direction of 
polarization of the laser beam. However, due to the relatively long 
optical path through the gain medium, the effect of a small misalignment 
can be significant. 
An adjustment of the C-axis might be achieved in the prior art by simply 
rotating the rod about an axis collinear with the axis of propagation of 
the laser light within the rod. However, such a rotation would change the 
orientation of the Brewster faces of the rod with respect to the optical 
path, resulting in a misalignment of the laser cavity. Such an adjustment 
would then require the re-alignment of the remaining cavity components 
with each adjustment of the rod. 
Accordingly, it is desirable to have an apparatus for minimizing the 
depolarizing effect of misalignment of the optic axis of birefringent 
elements with respect to Brewster cut faces of those elements. Further, it 
is desirable to make such an alignment adjustment without disturbing the 
operation of the laser cavity. 
SUMMARY OF THE INVENTION 
The present invention provides an apparatus for aligning an optical device 
in an optical path. The optical device is characterized by a first face 
and a second face cut at Brewster's angle so that a plane of polarization 
is defined, and by having a direction of propagation extending between the 
first and second faces. The optical device is secured in the optical path 
by a rotatable member having an axis of rotation that is normal to the 
Brewster faces of the optical device. 
Coupled with the rotatable member on the axis of rotation is a support 
connected on the axis of rotation allowing adjustment of the angular 
position about the axis of rotation of the rotatable member. Using an 
adjuster, connected to the support and the rotatable member, an operator 
adjusts the angular position of the rotatable member until a selected 
crystalline axis (i.e. C-axis for Ti:Sapphire) lies in the plane of 
polarization. Even though the crystalline axis may not be parallel to the 
direction of polarization, it is sufficient that the crystalline axis be 
aligned to lie within the plane of polarization to eliminate 
depolarization effects. 
According to another aspect, the present invention is a tunable laser 
comprising a laser cavity defining an optical path. A birefringent filter, 
or other means for tuning the laser, is mounted within the laser cavity 
along an optical path, allowing for tuning of the output wavelength of the 
laser. A birefringent gain medium is mounted within the optical path. The 
gain medium has a first face and a second face cut at Brewster's angle so 
that a plane of polarization is defined and has a direction of propagation 
extending between the first and second faces. The birefringent gain medium 
is mounted in a rotatable member having an axis of rotation which is 
normal to the first face of the gain medium. The rotatable member is 
supported on its axis of rotation in the optical path by an element 
allowing for adjustment of the angular position about the axis of rotation 
of the rotatable member to minimize the effect of the birefringent gain 
medium on the operation of the birefringent filter. 
According to yet another aspect, the rotatable member is formed of heat 
conducting material in heat flow communication with the gain medium, or 
other optical device. In one embodiment, channels are cut in the rotatable 
member to provide a path for flow of a heat transfer medium in contact 
with the rotatable member to control the temperature of the optical 
device. 
Other aspects, features and advantages of the present invention can be seen 
upon review of the figures, detailed description and claims which follow.

DETAILED DESCRIPTION 
With reference to the figures, a detailed description of a preferred 
embodiment of the present invention is provided. 
FIG. 1 is a schematic diagram of a laser system in which a preferred 
embodiment of the present invention is applied. The laser system 
illustrated in FIG. 1 is described in detail in U.S. Pat. No. 4,894,831 
entitled LONGITUDINALLY PUMPED LASER OSCILLATOR. FIGS. 2 and 3 illustrate 
misalignment of a crystalline axis (e.g. C-axis) in a titanium:sapphire 
rod with respect to Brewster faces of the rod. 
FIGS. 4 and 5 set forth a preferred embodiment of the means for aligning 
the gain medium or another optical element according to the present 
invention. 
FIG. 1 illustrates a longitudinally pumped folded cavity Ti:sapphire laser 
resonator. The resonator consists of a flat output coupler M1 with a 
transmission T equal to 3.5 percent, spherical concave mirror M2 with a 
radius of 10 cm, spherical concave mirror M3 with a radius of 10 cm, and a 
flat high reflector M4. Mirrors M2 and M3 are high reflectors at the laser 
wavelength .lambda..sub.L and transparent at the pump wavelength 
.lambda..sub.p. A Ti:sapphire rod Brewster cut to a path length of 2 cm 
with a crystal C-axis cut to be as closely parallel to the optical 
electric field as possible is utilized. This optical electric field 
defines a plane of polarization for the cavity mode within the rod, and 
its direction is determined by the orientation of the Brewster faces of 
the rod. The geometry of the C-axis, which is a crystalline axis of the 
Ti:sapphire crystal, is described in more detail with reference to FIGS. 2 
and 3. 
In the laser resonator of FIG. 1, a longitudinal pump beam is supplied from 
an argon ion laser. The pump beam is guided off a first flat reflector M6 
to spherical concave mirror M5. Mirror M5 is a high reflector which guides 
the pump beam through mirror M2 collinearly with the cavity mode in the 
Ti:sapphire rod 10. 
The resonator is tuned through a range of 700-1000 nanometers with a 
birefringent filter 11 such as is described in the above cross-referenced 
U.S. patent application entitled TUNABLE DYE LASER WITH THIN BIREFRINGENT 
FILTER FOR IMPROVED TUNING. 
An alignment apparatus 15 secures the Ti:sapphire rod within the optical 
path of the laser resonator. The alignment apparatus is described in 
detail with reference to FIGS. 4-5. 
A heat transfer medium, such as water, is flowed in contact with the 
alignment apparatus 15 through tubes 11, 12 from a heat transfer system 13 
to control the temperature of the gain medium 10. 
FIG. 2 is a top view of the Ti:sapphire gain medium used in the laser 
resonator of FIG. 1 in a rotatable holder 25. The gain medium 10 includes 
a first face 20, and a second face 21 which are cut optically parallel and 
at Brewster's angle so that a beam along optical path 22 entering first 
face at Brewster's angle .theta..sub.B will be transmitted with a plane of 
polarization defined by the Brewster angle through the Ti:sapphire crystal 
10. 
The Ti:sapphire crystal 10 is cut so that the crystalline axis 23 is 
oriented with respect to the Brewster faces so that .theta..sub.c is 
parallel as practical to the optical electric field normal to the optical 
path. However, due to manufacturing tolerances of a couple of degrees, the 
crystalline axis 23 will be non-parallel. It is difficult using present 
techniques to ensure that the crystalline axis is within better than 1/2 
of one degree of the direction of the optical electric field. 
According to the present invention, the crystal 10 is mounted in a 
rotatable holder 25 having an axis of rotation 26 that is perpendicular to 
the Brewster faces. The axis 26 is shown in FIG. 2 centered from the top 
view, but it need not be so centered. 
FIG. 3 illustrates another characteristic of the misalignment of the optic 
axis. In particular, the Brewster faces 20 and 21 define a plane of 
polarization 30. FIG. 3 is an end view of the rod showing a plane of 
polarization 30. The line 31 represents a crystalline axis on the plane of 
the paper. The plane of the paper is normal to the plane of polarization. 
As can be seen, the crystalline axis has an angular position .theta..sub.A 
with respect to the plane of polarization 30. 
Because of the misalignment of the crystalline axis as illustrated in FIGS. 
2 and 3, and because of the birefringence of a Ti:sapphire rod, slight 
depolarization of the beam passing along the optical path 22 occurs. This 
is particularly problematic if the gain medium 10 has substantial length. 
As mentioned above with respect to the Schulz article, this misalignment of 
the crystalline axis and the resulting depolarization, has an effect on 
the operation of the birefringent filter in the laser resonator. 
Therefore, the present invention includes an alignment apparatus (referred 
to schematically by reference manual 15 in FIG. for aligning the 
crystalline axis in the plane of polarization 30. Even though the 
crystalline axis remains non-parallel to the optical electric field, it 
lies essentially within the plane of polarization 30 and the birefringence 
effects of the rod are not seen in terms of depolarization. Rather they 
occur merely as a slight loss in gain in the rod due to misalignment 
reflected in .theta..sub.c. 
The alignment apparatus 15 according to the present invention is 
illustrated in FIGS. 4 and 5. FIG. 4 is an exploded view of the alignment 
apparatus according to the present invention. The alignment apparatus 
includes a rotatable member 40, a mount 41 allowing adjustment of the 
angular position of the rotatable member 40, an adjustment mechanism 
(block 64, screw 63, spring 69) and a translation member 42 on which the 
mount 41 is secured. 
The rotatable member 40 includes a first part 43 and a second part 44. 
First and second parts 43, 44 are connected by screws 45. The first part 
43 has a channel 46 machined on the surface between the first part 43 and 
the second part 44. This channel is adapted to receive a Brewster cut rod 
of a laser gain medium with the Brewster faces exposed at either end of 
the channel 46. Likewise, the second part 44 has a channel 47 machined in 
the surface between the second part 44 and the first part 43. The channel 
47 matches the channel 46 to form an enclosure for supporting the laser 
gain medium. The channels 46, 47 are lined with a soft metal foil, such as 
indium, (not shown) so that the fit of the gain medium within the channels 
46, 47 is snug and provides good heat flow communication between the 
rotatable member 40 and the gain medium. The foil may not be necessary in 
some embodiments. 
Fittings 48, 49 are adapted to be screwed into the second part 44 to 
provide a flow of heat transfer medium such as water through tubes 
machined in the part 44. This is more clearly illustrated in FIG. 5. 
The support 41 includes an adapter plate 50 upon which bearing posts 51 and 
52 are mounted. Bearings 53 and 54 are inserted through bearing posts 51 
and 52 respectively and screwed into the second part 44 of the rotatable 
member 40 at respective mating passages on the axis of rotation, e.g. 
passage 55. A lock washer 56 secures the bearings 53 and 54 into the 
rotatable member 40. Washers 57 and 58 couple with the bearing posts 52 to 
allow rotation of the bearing 54 and the rotatable member 40 through the 
bearing post 52. Likewise, washers 59 and 60 allow rotation of the bearing 
53 through bearing posts 51 with the rotatable member 40. Screws 61 and 62 
are used to secure the adapte plate 50 of the support 41 onto the 
translation member 42. 
Adjustment of the angular position of the rotatable member 40 is 
accomplished by bearings 53 and 54 connected at the axis of rotation of 
the rotatable member 40 in combination with adjustment screw 63. The 
adjustment screw 63 is connected through block 64 machined into the second 
part 44 of the rotatable member 40. This block 64 has a threaded passage 
65 which receives the adjustment screw 63. The adjustment screw 63 
contacts pad 66 which is bonded to the adapter plate 50 at a position 67 
to one side of the axis of rotation. A lock screw 68 is used to secure the 
adjustment screw, and thereby the position of the rotatable member 40 
after adjustment. 
Spring 69 is connected through roll pin 70 which is secured to the 
underside of the adapter plate 50 beneath passage 71 and through roll pin 
72 which is secured to the side of the second part 44 of the rotatable 
member 40 through passage 73. The passage 73 is formed on block 75 which 
is bonded to the second part 44. The spring 69 tends to hold the position 
of the rotatable member against the position set by the adjustment screw 
63. 
Ball 76 is bonded to the side of the adapter plate 50 at position 77 to 
provide a contact point for translation positioning of the gain medium 
alignment apparatus. 
The translation member 42 is a commercially available apparatus providing 
precision translation movement of the apparatus. Such members are 
commercially available from Daedal, Inc. in Harrison City, Pa. 
The rotatable member 40 is manufactured using a material with good heat 
transfer characteristics and that is suitable for precision manufacturing, 
such as a tellurium/copper alloy (UNS C14500). The support structure 41 is 
manufactured using aluminum, but can be made of any of a variety of 
materials that allow for precision adjustment of the position. 
FIG. 5 shows a side view of the second part 44 of the rotatable member 40. 
This illustrates schematically the cooling passages formed in the second 
part 44 of rotatable member. These passages are formed by boring a passage 
90 horizontally through the center of the part below the axis of rotation 
91. Likewise, two vertical passages 92, 93 are drilled to intersect with 
horizontal passage 90. Passages 90, 92 and 93 are sealed with plugs 94, 95 
and 96, respectively. Threaded holes 97 and 98 are then drilled into the 
vertical passages 92, 93 and threaded to mate with the nozzles 48, 49 
(FIG. 4) to provide a passage for flowing a heat transfer medium through 
the block 44. FIG. 5 also shows the position of holes 99 drilled to 
receive the screws 45 (FIG. 4) to secure the first part to the second part 
44 of the rotatable member 40. 
As can be seen, the axis of rotation 91 is formed below the passage 100 
which is machined to receive the gain medium. Likewise, the axis for 
rotation 91 does not intersect the gain medium in the preferred 
embodiment. However, this axis for rotation could be positioned anywhere 
that allows the mechanical free range of motion of a few degrees rotation 
for adjustment of the alignment of the gain medium as discussed above. 
For the titanium:sapphire embodiment discussed above, the heat transfer 
mechanism will be used to cool the gain medium. For some embodiments, 
temperature control of the gain medium, or other optical device mounted in 
this alignment member, may be desired other than cooling. For instance 
certain gain media require elevated temperature for efficient operation. 
The heat transfer medium could be used to maintain the temperature of the 
gain medium or other optical device at a preferred level. 
The water cooled member shown in FIG. 5 is representative of a range of 
temperature control mechanisms available, including thermo-electric 
cooling, air cooling fins with active or passive airflow mechanisms, 
liquid cooling in direct contact with the gain medium, and others. 
It can be seen that by mounting the optical device, such as the Ti:sapphire 
gain medium, in the rotatable member 40, so that the axis of rotation of 
the rotatable member is normal to the Brewster face of the optical device, 
the rotatable member acquires an adjustable angular position about the 
axis of rotation relative to the plane of polarization through the optical 
device. The adjustment screw 63 provides a means for adjusting the angular 
position about the axis of rotation of the rotatable member so that a 
crystalline axis of the optical device can be brought within the plane of 
the polarization. 
It will be appreciated by those in the art that the rod shaped gain medium 
is not the only optical device that could be aligned using an apparatus 
according to the present invention. Different shaped optical devices would 
result in different shaped passages through the rotatable member 40. 
A key feature of the invention is that the rod adjustment may be undertaken 
during the operation of the laser without causing misalignment of the 
optical path with respect to the other components of the laser cavity 
(e.g. mirrors, filters). 
The foregoing description of a preferred embodiment of the present 
invention has been provided for the purposes of illustration and 
description. It is not intended to be exhaustive or to limit the invention 
to the precise form disclosed. Obviously, many modifications and 
variations will be apparent to practitioners skilled in this art. The 
embodiment was chosen and described in order to best explain the 
principles of the invention and its practical application, thereby 
enabling others skilled in the art to understand the invention for various 
embodiments and with various modifications as are suited to the particular 
use contemplated. It is intended that the scope of the invention be 
defined by the following claims and their equivalents.