Bilithic unidirectional ring laser

A bilithic laser resonator having a first block separated from a second block by a gap that can be adjusted to tune the wavelength of the laser. At least one of these blocks consists of a material that is capable of lasing and at least one of these blocks consists of a material that is capable of producing Faraday rotation of an optical beam.

FIELD OF THE INVENTION 
This invention relates in general to lasers and relates more particularly 
to ring lasers. 
DEFINITIONS 
A "bilithic" device is a device consisting of a pair of discrete 
components. 
A "ring" is a closed path traversed by the laser beam within the ring 
laser. 
A "supported mode" of a ring laser is a mode in which the net amplification 
for a single pass around the ring is equal to or greater than one. 
An "unsupported mode" of a ring laser is a mode in which the net 
amplification for a single pass around the ring is less than one. 
A "minor angle between a pair of planes" is the smaller of two angles 
formed between a pair of intersecting planes. Parallel planes, by 
definition, have a minor angle of intersection of zero degrees. 
BACKGROUND OF THE INVENTION 
In conventional lasers, a laser cavity establishes standing wave modes that 
function to select the frequencies at which the laser operates. In many 
applications, including optical communications utilizing optical fibers, 
it is advantageous for the laser to provide a single-mode beam of light. 
For optical fiber communication, the wavelength of this single mode should 
be within the wavelength range in which the optical fiber has minimal 
attenuation and/or minimum dispersion. At present, this wavelength is on 
the order of 1.3 microns. 
To be commercially viable, the laser should have low power requirements, 
high reliability, modest size and reasonable cost. To accomodate 100 
Megabaud data rates, the linewidth of this single laser mode should be 
less than 200 kilohertz using heterodyne detection and phase-shift keying 
data encoding and the laser frequency should be equally stable. Ring 
lasers have proven to meet these requirements. 
In a ring laser, a travelling mode is utilized in place of the standing 
modes of conventional lasers. An optical source provides light that 
travels around a closed path (the "ring") within the laser. A typical ring 
laser (See, for example, U.S. Pat. No. 3,824,492 by Michael J. Brienza, et 
al, entitled "Solid State Single Frequency Laser", issued July 16, 1974), 
includes three or more reflective elements to direct a travelling wave 
around the ring. Since waves can travel in either direction around this 
ring, such a ring could support modes travelling in opposite directions 
around the ring. 
To ensure single mode oscillation of the laser, the ring laser design 
should support only one of these two travelling wave modes. The reason for 
this is that, if modes in both directions of travel are supported, then 
these modes will interfere spatially to produce spatial variation of the 
beam intensity around the ring. There is a periodic pattern of nodes and 
antinodes separated by 1/4 of a wavelength. Because of gain saturation, 
the gain is reduced where the optical intensity is maximum. Therefore, the 
gain is also spatially modulated with the maximum gain regions lying at 
the nodes of the optical interference pattern. This phenomenon is known as 
"spatial hole burning". Because the wavelengths of adjacent modes are 
slightly different, the spatial interference patterns generated by the two 
modes will not coincide. Therefore, each mode will extract gain from 
spatial regions not saturated by the adjacent mode. For this reason, 
bidirectional rings and linear ring resonators tend to oscillate in more 
than one mode. 
Support of only one of these two travelling modes has an additional 
advantage. Many laser applications result in some of the light emitted 
from the laser reflecting back into the laser. Such reflected light will 
destabilize the operation of the laser. In a ring laser, such reflected 
light is in the direction of the unsupported travelling wave mode and 
therefore is attenuated before it can significantly affect laser 
operation. 
In a typical discrete component ring laser (see, for example, U.S. Pat. No. 
3,824,492 by Brienza et al entitled "Solid State Single Frequency Laser" 
issued July 16, 1974), the polarization of the travelling wave beam is 
rotated by a wave plate and is also rotated by a Faraday rotator. For the 
supported mode, these two rotations cancel so that the polarization is 
unchange by a complete traversal of the ring. For the unsupported mode, 
these two rotations add to produce a net rotation around the ring. A 
polarizer is located within the ring to attenuate the unsupported mode and 
to transmit substantially all of the supported mode. This selective 
attenuation assures that only one of these two modes is supported. 
In the monolithic unidirectional ring laser presented in U.S. Pat. No. 
4,747,111 by Trutna, Jr. et al. entitled "Quali-Planar Monolithic 
Unidirectional Ring Laser" issued May 24, 1988, a single block of material 
is shaped to direct the a travelling wave beam around a ring and to rotate 
the polarization of the beam, thereby avoiding the need for a wave plate. 
This beam reflects off of four sides of this block. This block is formed 
out of a material that is selected to lase at the wavelength of this 
travelling wave beam and is also selected to act as a Faraday rotator in 
the presence of an applied magnetic field. For one direction around the 
ring, there is a geometrical polarization rotation induced by the 
out-of-plane reflections that is cancelled by the Faraday rotation 
allowing a low loss reflection from the polarizing output mirror. In the 
other direction, the two rotations add leading to an attenuation of the 
beam by the lossy polarizer. Unfortunately, this laser is not tunable so 
that it is limited to applications in which only a single beam frequency 
is needed. 
SUMMARY OF THE INVENTION 
In accordance with the illustrated preferred embodiments, a tunable ring 
laser is presented. This laser includes an optical source and two blocks 
of material separated by a gap. At least one of the blocks is capable of 
lasing at a selected laser wavelength and at least one of these blocks, in 
the presence of an applied magnetic field, produces Faraday rotation of 
the laser beam as it travels around the ring. 
These two blocks contain at least four reflecting surfaces that are 
oriented to reflect the beam around a closed path (the "ring"). For one 
direction of travel around the ring (the "support mode direction"), the 
laser beam experiences a greater amplification than the opposite direction 
around the ring (the "unsupported mode direction"). The net amplification 
for a complete traversal around the loop is greater than one for the 
supported mode and is less than one for the unsupported mode. 
The two blocks are mounted on a translation stage that can translate one 
block relative to the other. Such translation varies the width of a gap 
between the blocks to vary the frequency of the supported mode. 
In a first class of embodiments, the ring is contained within the region 
consisting of one of these two blocks plus the gap between these two 
blocks. In a second class of embodiments, the ring is contained within the 
region consisting of both blocks and the gap between these two blocks. In 
a third class of embodiments, one block is glass and the other block is a 
material, such as a Neodymium doped Yttrium-Aluminum-Garnet (denoted 
Nd:YAG) or Neodymium doped gadolinium gallium garnet (denoted Nd:GGG), 
capable of lasing and introducing Faraday rotation. In a fourth class of 
embodiments, one of the blocks is of a material capable of lasing and the 
other block is of a material capable of producing Faraday rotation in an 
applied magnetic field. This latter class of embodiments has the advantage 
of enabling separate selection of the material capable of lasing and the 
material capable of Faraday rotation, thereby enabling each of these 
choices to be separately optimized.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1A-1D illustrate a tunable ring laser consisting of a bilithic laser 
resonator 11 and an optical pump source 12. Bilithic laser resonator 11 
includes a first block 14 and a second block 15, separated by a gap 115. 
Block 15 is bounded by a first end surface 16, a second end surface 17, a 
pair of side surfaces 18 and 19, a pair of bevel surfaces 110 and 111, a 
top surface 112 and a bottom surface 113. Block 14 directs an input beam 
13 from optical source 12 through a reflection vertex A on a surface 114 
of block 14. This beam then reflects off of surfaces 110, 17, 111 and 114 
to form a ring ABCDA. For the supported mode travelling around ring ABCDA 
in the counterclockwise direction, the reflections at reflection vertices 
B, C and D are total internal reflections. The beam enters and exits 
surface 16 at Brewster's angle so that this surface acts as a partial 
polarizer. Light polarized in the plane of FIG. 1B suffers no reflection 
loss on transmission, while light polarized perpendicular to the plane of 
FIG. 1B is largely reflected (approximately 30% for YAG). Surface 114 is 
coated with a multilayer dielectric film so that, at reflection vertex A, 
most of the laser beam is reflected, but part of the laser beam exits 
through block 14 as laser output beam 116. Surface 114 is curved to focus 
the optical beam as it reflects off of this surface. Surface 117 also has 
a curvature relative to surface 114 such that block 14 corrects some of 
the optical astigmatism in output beam 116 introduced by the off-axis 
reflections and refractions from surface 114. 
The primary functions of block 14 are to let beam 13 pass into block 15, to 
let a portion of the laser beam exit as output beam 116 and to reflect 
most of the beam incident from reflection vertex D onto reflection vertex 
A. Thus, a wide selection of materials can be utilized for block 14, 
including glasses and quartz. 
Block 15 functions to amplify the laser beam and to rotate the polarization 
of the laser beam. Thus, block 15 consists of a material, such as a 
neodymium doped yttrium-iron-garnet (denoted Nd:YAG), that can amplify the 
laser beam by stimulated emission and that can rotate the polarization of 
the laser beam by Faraday rotation. 
Input beam 13 is polarized parallel to the plane of FIG. 1B so that the 
fraction of this beam transmitted into block 14 is maximized as a function 
of this polarization and so that this same polarization is present at 
reflection vertex A. In the presence of a magnetic field H, this 
polarization rotates as the beam travels within block 15. The directions 
of the normals to surfaces 16, 110, 17 and 111 and the amount of Faraday 
rotation are selected so that, for the supported mode, the geometrical 
polarization rotation introduced by the nonplanar geometry and the Faraday 
rotation cancel. In this case, low loss linear polarization circulates 
through the Brewster angle polarizer (surface 16) unchanged on each round 
trip. In other words, linear polarized light is a polarization mode of the 
resonator. However, for the nonsupported mode, the two polarization 
rotations add, leading to a higher loss elliptically polarized 
polarization mode. 
In one particular embodiment, minimum attenuation for the supported mode 
occurs for the following choice of parameters: width W=4.91 mm, length 
L=4.70 mm, thickness t=1 mm for element 14, a gap width w.sub.g =0.7 mm, 
an applied magnetic field of about 2,000 Gauss, a pump threshold of 10 mW, 
and a Nd dopant concentration of 1.1%, a 4 mm radius of curvature of 
surface 114, approximately 0.45 degree out-of-plane normals to surfaces 
110, and 111, and an approximately 1 degree out-of-plane normal to surface 
17. The directions of the normals are selected to produce a minor angle 
.phi. between plane ABD (i.e., the plane containing reflection vertices A, 
B and D) and plane CBD (i.e., the plane containing reflection vertices C, 
B and D) less than 45 degrees. Preferrably, angle .phi. is in the range 
0.5-2.0 degrees. Such a choice of angle .phi. means that the amount of 
Faraday rotation of the beam is equally small. This enables the use of 
relatively small magnetic field (on the order of 100 Gauss), thereby 
enabling the use of an inexpensive magnet 118 to produce magnetic field H. 
Such small angle .phi. means that the geometrical polarization rotation 
will be sufficiently small that it can be canceled by the weak Faraday 
rotation in the YAG crystal. If .phi. is too large, then there will be a 
net polarization rotation on one round trip which leads to a lossy 
elliptically polarized mode. If .phi. is too large, the intracavity loss 
will be too large to sustain laser action. 
After a complete traversal of ring ABCDA, the beam returns to reflection 
vertex A with zero net change in polarization. The pathlength of ring 
ABCDA is an integral multiple of the wavelength of the supported mode. As 
the width W.sub.g of gap 115 is varied, the wavelength of the supported 
mode is varied to keep constant the number of wavelengths around the ring. 
Thus, the wavelength of the laser beam is tuned by varying the width of 
gap 115. 
In other embodiments, the input pumping beam 13 can be injected at some 
point in the ring other than at point A. For example, the normal to 
surface 210 can be selected so that the light is not totally internally 
reflected, but instead reflects at an angle that enables light to be 
injected at point B and to also be extracted at that point. A dielectric 
film mirror would also be deposited on surface 210 to enhance the 
reflectivity for the lasing mode. 
FIGS. 2A-2D illustrate a bilithic laser resonator in which the ring extends 
into both components of the laser resonator and is contained within the 
region consisting of both components of the laser resonator plus the gap 
between these two sections. This embodiment corresponds closely to the 
embodiment of FIGS. 1A-1D. Corresponding elements in these two embodiments 
are indicated by reference numerals that differ only that in FIG. 1 all 
reference numerals begin with a 1 and in FIG. 2 all reference numerals 
begin with a 2. In this embodiment, reflection vertex A is located on 
surface 216, whereas in the embodiment of FIGS. 1A-1D, reflection vertex A 
is located on surface 114. As a result of this difference, part of the 
ring is contained in block 24, whereas none of the ring in the embodiment 
of FIGS. 1A-1D is contained in block 14. 
This difference between these two embodiments produces the following 
advantage of the embodiment of FIGS. 2A-2D. All of the amplification can 
be provided by block 24 and all of the Faraday rotation can be provided by 
block 25. This enables the choice of material for block 24 to be made 
without any constraint that such material must also be able of provide 
Faraday rotation, and enables the choice of material for block 25 to be 
made without any constraint that such material must also be able to lase. 
Thus, each of these choices can be separately optimized. Preferred choices 
for block 24 are rare earth doped crystals such as Nd doped yttrium 
aluminum garnet (Nd:YAG) crystals and Nd doped gadolinium gallium garnet 
(Nd:GGG) crystals. In particular, Nd doped YAG crystals are preferred 
because they presently are one of the best lasing materials. Preferred 
choices for block 25 are any materials having a large Verdet constant 
(i.e., exhibit a strong Faraday rotation) and having low absorption at 1.3 
microns. For example, M16 Faraday Rotation Glass is available from Kigre, 
Inc., 100 Marshland Road, Hilton Head, S.C. 29928. Other choices for block 
25 are zinc selenide crystals, as well as SF-6 and SF-57 available from 
Schott Glass Technologies, Inc., 400 York Avenue, Duryea, Pa. 18642. 
As illustrated above, use of a bilithic laser resonator has the advantages 
of: (a) providing tunability; (b) limiting the number of components to the 
smallest number (two) that allows such tunability via variation of the gap 
between these components; and (c) enabling embodiments in which the lasing 
material and Faraday rotating material can be separately selected. By 
limiting the number of components to two, the amount of alignment required 
is minimized subject to the constraint that the laser be tunable. 
There are a number of existing devices that can be utilized to vary the 
width W.sub.g of gap the gap between the two blocks. For example, roller 
bearing stages and flexure pivot stages are particular examples. In the 
preferred embodiment illustrated in FIG. 2A, blocks 24 and 25 are mounted 
on piezoelectric shear plates that consist of piezoelectric crystals 221 
and 223 plus electrodes 221, 222, 224, and 225. In this particular 
embodiment, the polarity between plates 221 and 222 is opposite to that 
between plates 224 and 225 so that these two shear plates operate in a 
push-pull arrangement--namely, when shear plate 220 moves block 25 in the 
-y direction, shear plate 223 moves block 24 in the +y direction. In 
alternate embodiments, one or the other of these two shear plates is 
replaced by a rigid mount for its associated block. Alternatively, an 
expansion mode piezoelectric crystal attached to an end of one of these 
blocks can be used to translate that block. These embodiments have the 
advantage of simplicity and of providing a rigid support for both blocks. 
Because piezoelectric materials exhibit hysteresis, there is no 1--1 
relationship between the applied voltage and the amount of translation of 
the associated block. Therefore, a mechanism is included for determining 
the amount of variation of the gap W.sub.g. In one embodiment, optical 
encoders are included on one or both of the blocks and sensing means 
measures the amount that block is translated. In another embodiment, the 
laser frequency is measured and this value is utilized to adjust W.sub.g 
until the laser frequency is equal to the selected frequency.