Single mode laser and method suitable for use in frequency multiplied

A single mode laser implementation and associated method are disclosed. The laser may accommodate a frequency multiplying material to provide an intracavity doubled single frequency laser. The laser implementation includes an input mirror and an output mirror defining a resonant cavity and a light path within the resonant cavity and between the mirrors. A lasant material is positioned in the light path for lasing at a desired fundamental wavelength and possibly other, unwanted wavelengths. A first birefringent member is also positioned in the light path for refracting in different directions the different wavelengths of light which are present along the path. Also positioned in the light path is a second birefringent member which cooperates with the first birefringent member such that the first and second birefringent members together discriminate between the desired fundamental wavelength and the unwanted wavelengths so that one polarization of the desired fundamental wavelength is refracted in one direction which causes it to lase while certain portions of the unwanted wavelengths are refracted in other directions which cause all polarizations of the unwanted wavelengths to be extinguished.

BACKGROUND OF THE INVENTION 
The present invention relates generally to single mode lasers and more 
particularly to a single mode laser implementation which may accommodate a 
frequency multiplying material to provide an intracavity doubled single 
frequency laser. 
In the prior art, a variety of single longitudinal mode (hereinafter SLM) 
lasers have been developed. One particular reason for the interest in SLM 
lasers resides in the ease with which an SLM laser can be converted to a 
frequency doubled configuration through the addition of a non-linear 
material within the laser cavity. In addition, certain problems have been 
encountered when attempts have been made to convert types of laser other 
than SLM lasers to the intracavity doubled frequency configuration, as 
will be seen immediately hereinafter. 
The non-linear frequency doubling technique of the prior art has often been 
used to produce coherent radiation in the visible and ultraviolet spectral 
region. Acceptable optical conversion efficiency has been achieved in this 
manner. However, many of these frequency doubled lasers suffer a so called 
"green noise" problem which limits their usefulness in a number of 
applications. More specifically, the green noise problem introduces 
amplitude noise (i.e. variation in the intensity of the output beam at the 
doubled frequency) which is believed to due to gain competition introduced 
by the presence of additional modes other than one longitudinal 
fundamental mode in the laser's resonant cavity in combination with the 
phenomenon of longitudinal mode coupling through a nonlinear doubling 
process between the various modes which are present. One popular approach 
to solving the "green noise" problem is to eliminate the additional modes 
in the laser light (i.e., use an SLM laser) which excites the non-linear 
material and thereby eliminate longitudinal mode coupling so as to obtain 
a single doubled output frequency. 
A variety of intracavity doubled single longitudinal mode (SLM) laser 
systems have been developed in the prior art. One approach in achieving 
SLM operation is through the use of a ring laser geometry. In a ring laser 
geometry, spatial holeburning is eliminated by a unidirectional traveling 
wave. SLM operation is thus achieved in a homogeneous broadened laser 
system. One example of an intracavity doubled SLM laser is disclosed in 
U.S. Pat. No. 5,052,815, issued Oct. 1, 1991 to Nightingale et al. One of 
the principal drawbacks in using a ring laser geometry is that it is 
difficult to align and operate. Also, a ring laser is generally more 
complicated than a simple linear cavity because of the optical diode and 
reciprocal retardation compensator used. Further, beam pointing stability 
of a ring laser is usually not as good as that obtained using a linear 
cavity. Nevertheless, a ring laser is generally believed to be more 
efficient than a standing wave linear cavity since the traveling wave 
extracts all the available gain uniformly. However, in an intracavity 
doubled laser, a ring laser is not necessarily more efficient than a 
linear cavity simply because more intracavity elements are required in a 
ring cavity for unidirectional operation. These additional elements yield 
more intracavity losses in the doubled frequency ring geometry as compared 
with those in a doubled frequency linear geometry since intracavity 
doubled laser systems are extremely sensitive to cavity losses. Obviously, 
more losses result in less doubled power. 
Another technique for producing an SLM laser is disclosed by Lukas et al in 
U.S. Pat. No. 5,164,947, issued on Nov. 17, 1992. In this patent 
disclosure, a twisted-mode technique is employed to eliminate spatial 
holeburning so as to obtain SLM operation. The laser cavity comprises an 
input mirror and an output coupler which define a linear laser cavity. 
Inside the laser cavity, a lasant rod is inserted between two quarter-wave 
plates. A polarizer and a nonlinear optical crystal are also included in 
the laser cavity to define the polarization direction of the fundamental 
wave and to generate output radiation at twice the frequency of the 
fundamental wave. The laser mode is circularly polarized in the laser rod, 
resulting in a standing wave in which the electric field vector rotates 
through the gain medium and in which there are no standing wave nodes 
within the gain medium. Spatial holeburning is thus eliminated. However, 
this approach has its own limitations and is often difficult to implement. 
First, it relies on having two precise quarter-wave plates inside the 
cavity. Second, the laser rod has to be non-birefrigent. This requirement 
restricts the laser to a limited number of laser hosts. Further, as the 
laser crystal is optically excited, the thermally and stress induced 
birefringence will introduce spatial holeburning in the gain medium, again 
resulting in multiple mode operation. From the standpoint of 
implementation, the complexities of this laser design make it difficult to 
scale and operate. 
A recent patent disclosure, U.S. Pat. No. 5,381,421, issued to Wedekind et 
al. on Jan. 10, 1995, describes another way to achieve SLM laser operation 
in a linear laser cavity. In this approach, a Brewster polarizer and a 
birefrigent material form a Lyot filter which narrows the frequency 
bandwidth for single longitudinal mode operation. The major inconvenience 
of this approach resides in its use of a Brewster polarizer. As we know, 
the Brewster angle is usually greater than 45 degrees and, thus, is not 
convenient to work with. Also, a Brewster polarizer is not a perfect 
polarizer in that it only has about a 16% loss for the polarization which 
is discriminated against. For this reason the Brewster polarizer may not 
provide loss at levels sufficient to suppress longitudinal modes other 
than the fundamental. Further, any slight deviation from the Brewster 
angle will introduce additional insertion loss which may harm frequency 
doubling and selection. Similarly, another patent disclosure U.S. Pat. No. 
5,430,754, issued to Suzuki et al. on Jul. 4, 1995 discloses a Lyot filter 
formed by an off-axially cut Nd:YVO.sub.4 and a birefrigent material such 
as KTP to generate SLM radiation. The limitation of this approach is its 
reliance on strong birefringence and long crystal length to enhance the 
Poynting vector walkoff. Also, in the case of Nd:YVO.sub.4, which is the 
preferred mode of operation, the Nd:YVO.sub.4 is cut 43 deg off of the 
cleavage plane. The fabrication of such an off-axially cut crystal is not 
trivial and, typically, is accompanied by a low yield. Moreover, in both 
of these Lyot filter approaches, a plurality of intracavity elements have 
surfaces substantially normal to the cavity axis. Residual reflections 
from these surfaces can lead to intracavity etalon and coupled cavity 
effects resulting in mode-hopping. 
The present invention provides a heretofore unseen approach and associated 
method for producing an SLM laser which eliminates the problems described 
above and which is suitable for use in intracavity frequency doubled 
applications. 
SUMMARY OF THE INVENTION 
As will be described in more detail hereinafter, there is disclosed herein 
a single mode laser implementation which may accommodate a frequency 
doubling material to provide an intracavity doubled single frequency. An 
associated method is also disclosed. The laser implementation includes an 
input mirror and an output mirror defining a resonant cavity and a light 
path within the resonant cavity and between the mirrors. A lasant material 
is positioned in the light path for lasing at a desired fundamental 
wavelength and possibly other, unwanted wavelengths. A first birefringent 
means is also positioned in the light path for refracting in different 
directions the different wavelengths of light which are present along the 
path. Also positioned in the light path is a second birefringent means 
which cooperates with the first birefringent means such that the first and 
second birefringent means together discriminate between the desired 
fundamental wavelength and the unwanted wavelengths so that the desired 
fundamental wavelength is refracted in one direction which causes it to 
lase while the unwanted wavelengths are refracted in other directions 
which cause them to be extinguished. 
In accordance with the method of the present invention, light which is 
produced by a lasant material or other such suitable light source passes 
through a specifically configured birefringent means such that the 
polarization of a component of the light at a desired, single wavelength 
is affected in a predetermined way and so that the polarization of certain 
components of the light at other wavelengths are affected in other ways. 
Thereafter, the light is refracted such that these certain other 
components of light are refracted at angles which cause the other 
wavelengths to be rejected and so that the single wavelength is refracted 
at an angle which causes the single wavelength to retrace itself and 
thereby lase within the laser's cavity. 
In one aspect of the invention, the laser can output a single longitudinal 
mode of light. 
In another aspect of the invention, a non-linear material may be exposed to 
a single fundamental light mode within the laser such that a particular 
harmonic of the fundamental is generated and output from the laser.

DETAILED DESCRIPTION OF THE INVENTION 
Turning to the drawings, wherein like components are designated by like 
reference numerals throughout the various figures, attention is initially 
directed to FIG. 1 which illustrates one embodiment of a laser 
manufactured in accordance with the present invention and generally 
indicated by the reference numeral 10. Laser 10 includes a pumping light 
source 12 which emits light 14 at a predetermined wavelength. Light 14 
passes through a lens 16 and, thereafter, into a resonant cavity 18 that 
is defined between an input mirror 20 and an output mirror 22. In 
accordance with the present invention, a lasant material 24, a 
birefringent wedge 26 and a birefringent crystal 28 are positioned within 
cavity 18, as will be described in further detail at appropriate points 
hereinafter. 
Continuing to refer to FIG. 1, a face 30 of input mirror 20 is coated in a 
known manner to be highly transmissive to light 14 while, at the same 
time, being highly reflective to a fundamental lasing wavelength (i.e., 
the light emitted by lasant material 24). A face 32 of output mirror 22 is 
also coated such that a lasing light path 33 is defined between the two 
mirrors based upon the intended application of laser 10. For example, if 
the laser is used as an SLM laser, a partially reflective coating is 
applied to face 32 of the output mirror such that a relatively small 
percentage of light (indicated as an arrow 34) at the fundamental lasing 
wavelength which is incident upon the output mirror passes therethrough. 
As another example, if laser 10 is intended to produce an output wavelength 
which requires a frequency multiplying configuration (through a 
modification to be described below), face 32 of the output mirror is 
coated so as to be highly reflective to the fundamental lasing wavelength 
and highly transmissive to a desired harmonic, for example, the second 
harmonic of the fundamental wavelength, as will be described at an 
appropriate point below. Although Nd:YVO.sub.4 is the preferred lasant 
material, other lasant materials may also be used. These other materials 
include, but are not limited to, Nd:YAG, Nd:YLF, Nd:SFAP, Nd:YALO.sub.3, 
Cr:LiSrF.sub.4, Cr:LiCaF.sub.4, Cr:BeAlO.sub.3 and Cr:LiSrF.sub.4. 
Therefore, the fundamental wavelength and second harmonic wavelength can 
vary depending upon the specific lasant host and cooperating coatings on 
faces 30 and 32. For purposes of simplicity, the remaining discussions 
will consider the use of Nd:YVO.sub.4 with a fundamental wavelength of 
1064 nm and second harmonic wavelength of 532 nm. However, with the 
application of the principles described below, the use of lasant materials 
other than Nd:YVO.sub.4 will be clear to those of skill in the art in view 
of this overall disclosure. 
Still referring to FIG. 1, pumping light source 12 is arranged such that 
pumping light beam 14 emitted from source 12 is optically coupled to and 
directed through input mirror 16 into lasant material 24. Pumping light 
source 12 may be provided in a wide variety of different forms so long as 
it produces or emits light beam 14 at a predetermined wavelength that is 
keyed to the lasant material, as is well known. Pumping beam 14, when 
directed into lasant material 24, is absorbed by the lasant material and 
causes the material to lase thereby injecting a predetermined fundamental 
wave as well as unwanted wavelengths into cavity 18. In one embodiment, 
pumping light source 12 is a laser diode which emits light at a wavelength 
at or about 809 nm. As is known in the art, laser diodes of this type are 
readily available. 
In accordance with the present invention, birefringent wedge 26 and 
birefringent member 28 may be formed from birefringent materials such as, 
for example, calcite, orthovanadate, quartz or any other suitable such 
material. As is known in the art, birefringent materials include an 
ordinary axis and an extraordinary axis. In FIG. 1, the extraordinary axis 
of wedge 26 is indicated along the direction of an arrow 36 as E.sub.W 
while the ordinary axis, O.sub.W, of wedge 26 is oriented in a direction 
which is perpendicular to the plane of the figure, as indicated at 
reference numeral 38 by a symbol denoting an arrow which is perpendicular 
to the plane of the figure. It is noted that the positive and negative 
directions along each of the O.sub.W and E.sub.W axes is not critical in 
accordance with the teachings herein. In addition, wedge 26 includes one 
surface 40 which resides in the plane of O.sub.W and E.sub.W and which is 
generally normal to light path 33. Generally opposing surface 40 is a 
wedge plane surface 42. For descriptive purposes, a line 44 is shown as 
normal to wedge plane 42. Another line 46 is shown normal to the plane 
defined by O.sub.W and E.sub.W such that an angle .theta. is formed 
between the two lines. 
Referring now to FIG. 2 in conjunction with FIG. 1, birefrigent crystalline 
member 28 is placed along light path 33 with its extraordinary axis, 
E.sub.M oriented at an angle .beta. which is preferred to be 45 degrees 
with respect to E.sub.W. Additionally, the plane defined by O.sub.M and 
E.sub.M is generally normal to light path 33. It should be noted with 
regard to FIG. 2 that the various axes are illustrated with respect to the 
length of light path 33 as if looking from lasant material 20 towards 
output mirror 22. Light path 33 is assumed to be straight in FIG. 2 for 
purposes of clarity. However, one of skill in the art will recognize that 
light path 33 is, in fact, not straight (see FIG. 1) as a result of 
passing through the various components along its length. Moreover, as 
mentioned previously, the positive and negative directions along these 
axes are not critical and, therefore, are not illustrated in FIG. 2. While 
a .beta. of 45.degree. is preferred, other angles may be found to be 
suitable as will be described in further detail at an appropriate point 
below. The configuration of wedge 26 in conjunction with the overall 
relationship between wedge 26 and birefringent member 28 along light path 
33 result in highly advantageous frequency/wavelength discrimination along 
light path 33 in a way which has not been seen heretofore and which will 
be described hereinafter in conjunction with a description of the 
operation of laser 10. 
Now that the basic configuration of laser 10 has been described and still 
referring to FIGS. 1 and 2, the operation of the laser in accordance with 
the present invention will be described in detail with regard to a 
particular implementation. In this implementation, Nd:YVO.sub.4 is used as 
lasant material 24 with 1% Nd dopant concentration and having a 1 mm 
length along light path 33. The fundamental wavelength is 1064 nm. 
Operation of this laser is best understood by considering the polarization 
of the fundamental wavelength during its round-trip travel in cavity 18. 
First, it is assumed that the fundamental is polarized along the E.sub.W 
axis of wedge 26 starting from lasant material 24. As the fundamental wave 
goes through wedge plane 42 on light path 33, it experiences a refraction 
at the wedge plane which is governed by Snell's law. It then travels 
through birefrigent member 28 and is retra-reflected back by output mirror 
22 along light path 33. Since E.sub.M of birefringent member 28 is at 
.beta.=45.degree. with respect to the E.sub.W axis of the wedge, the 
returned fundamental wave will have two polarization components at the 
fundamental frequency, one along the E.sub.W axis and the other along the 
O.sub.W axis. The relative amplitude of each component depends on the 
retardation of birefringent member 28. For example, when member 28 
comprises a half-wave or full-wave plate along light path 33, the returned 
fundamental wave is linearly polarized along the E.sub.W axis of wedge 26 
resulting in a particular polarization orientation. On the other hand, 
when member 28 comprises a quarter-wave plate, the retro-reflected 
fundamental wave is still linearly polarized, but along the O.sub.W axis 
of wedge 26 resulting in a different polarization orientation. 
As the fundamental wave re-enters wedge 26, because of the difference in 
the index of refraction along the O.sub.W and E.sub.W axes, the two 
polarization components will refract differently according to Snell's law 
and subsequently arrive at different positions on surface 30 of input 
mirror 20. If input mirror 20 is aligned to retro-reflect the E.sub.W 
polarized component, the O.sub.W polarized component will be misaligned 
and rejected outside light path 33 for a 100% loss. Such a rejected path 
is indicated by the reference number 47 in FIG. 1. Stated in a slightly 
different way, one polarization of a fundamental wave that experiences a 
half-wave or full wave retardation through member 28 will experience no 
change in its polarization as a result of one round-trip through 
birefringent member 28 while the other polarization will experience a 
change. With the cooperation of refraction at wedge plane 42 and alignment 
of input mirror 30, such a fundamental polarization (the unchanged one) 
then retraces itself along light path 33 so as to experience a minimal 
loss over one round-trip. It is to be understood that the O.sub.W 
polarization component may be selected simply by adjusting input mirror 
20. Thus, the output polarization of laser 10 may readily be modified to 
suit a particular application. 
Having described the operation of laser 10 with regard to the polarization 
of the fundamental wavelength, unwanted wavelength modes are now 
considered which are different than that of the fundamental, but which 
nonetheless are capable of resonating within cavity 18 in the absence of 
some sort of frequency discrimination. In this regard, it should be 
appreciated that birefringent member 28 is normally selected to provide a 
full or half-wave retardation at a selected fundamental wavelength. Any 
unwanted (unselected) mode present within cavity 18 will have a wavelength 
such that the unwanted mode will not experience a half-wave or full-wave 
retardation through birefringent member 28. Therefore, a round-trip 
through member 28 will result in polarization changes for these other 
unwanted modes and differing degrees/directions of refraction upon passing 
through wedge 26. These modes will, consequently, be reflected by input 
mirror 30 in such a way that that a component (i.e., a particular 
polarization) of them will not retrace thereby reducing the overall 
amplitude of each unwanted mode. Thus, the unwanted modes will be 
extinguished as a result of substantial cumulative losses which are higher 
than the gain provided by the lasant material over each round-trip. In 
other words, one polarization of the unwanted modes will ultimately "walk 
off" of mirror 20 with sufficient round trip travel while one polarization 
of one particular frequency may be selected to lase using the frequency 
selection or filter configuration of the present invention. Therefore, 
single mode operation is achieved in a cost effective, relatively simple 
and highly advantageous way. 
Referring to FIG. 1, in order to discriminate against unwanted frequencies, 
wedge angle .theta. is chosen such that after one round trip, the 
displacement between polarization components of the fundamental frequency 
on surface 30, as refracted by birefringent wedge 26, is greater than the 
diameter of the pump beam waist. This condition can be approximately 
represented by the expression: 
##EQU1## 
where .DELTA.n.sub.W is the birefringence of wedge 26, L is the length of 
the laser cavity and .omega. is the pump beam waist at lasant material 24. 
For example, .theta. is calculated for a yttrium orthovanadate wedge using 
a cavity length of 10 mm, a pump beam waist of 50 .mu.m and .DELTA.n.sub.W 
=0.21, as being greater than approximately 1.4.degree.. It should be 
appreciated that larger wedge angles are equally effective. However, from 
the standpoint of manufacturing costs, the smaller wedge angle 
advantageously reduces the overall amount of relatively expensive material 
which is required to form birefringent wedge member 26. 
Referring now to FIG. 3, the loss introduced by the frequency filter 
configuration disclosed herein (the combination of wedge 26 and member 28) 
can be expressed as: 
EQU Loss=Sin(2.beta.)Cos.sup.2 (2.pi..DELTA.n.sub.M L.sub.M v/c), (2) 
where .DELTA.n.sub.M is the birefringence of birefringent member 28, 
L.sub.M is the length of the birefringent member along light path 33, v is 
the frequency of the fundamental wave and c is the speed of light. At or 
near the fundamental wave frequency and for a 5 mm KTP birefringent member 
length, the percentage loss obtained from equation 2 is plotted against 
frequency. As can be seen, the percentage loss varies vertically in the 
figure from zero to 100% over the depicted frequency range for 
.beta.=45.degree.. The free spectral range (hereinafter FSR) of the 
frequency filtering configuration of the present invention is determined 
as: 
##EQU2## 
Using the values given above, an FSR of approximately 330 Ghz is obtained 
for a 5 mm KTP length. The gain bandwidth of Nd:YVO.sub.4 is between 
approximately 250 Ghz to 300 Ghz (.about.50% gain points) and is shown as 
"w" in FIG. 3 between dashed lines 48. In order to achieve single mode 
operation, it is desirable that only one loss mimimum is present within 
gain bandwidth w of the lasant material. The frequency filtering 
configuration of the present invention permits a particular loss minimum, 
for example, the minimum indicated by reference number 49 at a frequency f 
to be shifted (not shown) to within gain bandwidth range w based upon the 
frequency spacing between minima as established by the FSR. Shifting loss 
minimum 49 may be accomplished, for example, by temperature tuning 
birefringent member 28 or by supporting the member so as to be tiltable 
against light path 33 using suitable means. 
Once loss minimum 49 has been shifted to within gain bandwidth w, its 
effect is to limit the range of frequencies which may resonate within 
cavity 18 to a relatively small .DELTA.f (not shown) at either side of f. 
At the same time, the cavity can only support a discrete number of 
longitudinal modes within its gain bandwidth. For instance, in the case of 
a 10 mm cavity length which includes 1 mm Nd:YVO.sub.4 and 5 mm KTP 
lengths, the mode spacing between each longitudinal mode is about 11 Ghz. 
Thus, .DELTA.f should be less than 11 Ghz, in the present example, in 
order to facilitate the selection of a single mode for resonance. With the 
proper tilt or temperature adjustment applied to member 28, minimum 49 can 
be shifted to the frequency of a selected mode, the nearest one of which 
is indicated by reference number 50, such that only one longitudinal mode 
will experience minimum loss thereby remaining above lasing threshold, as 
is required for single mode operation. According to equation 1, an 
adjacent longitudinal mode (11 Ghz away and not shown) will experience a 
1.1% loss, the 2nd neighboring mode (22 Ghz away and not shown) will 
experience a 4.3% loss and the 3rd neighboring mode (33 Ghz away and not 
shown) will experience a 9.5% loss. Even though the loss for the 1st 
adjacent mode appears to be small, it is sufficient to limit laser 
oscillation to an SLM. Additional losses may be provided, for example, by 
spatial hole-burning and other such known design considerations. In 
essence, the aforedescribed procedure overlaps the loss minimum of the 
frequency filtering configuration with the gain peak of the lasant 
material and also with a selected longitudinal mode of the resonant cavity 
such that only the selected longitudinal mode will lase, the remaining 
modes being extinguished due to losses. 
Referring to FIGS. 1-3, it was noted earlier that .beta. is preferred to be 
45.degree.. The result of changing .beta. is readily understood by 
considering its influence on the loss curve of FIG. 3 which is plotted for 
.beta.=45.degree.. At this latter angle, a 100% loss is experienced at the 
peaks of the curve. As .beta. is reduced towards 0.degree., the peaks of 
the curve correspondingly drop due to the sin(2.beta.) term appearing in 
equation 2 such that unwanted modes experience less loss. At a .beta. of 
0.degree., the curve becomes a straight line at 0% loss such that no 
frequency discrimination is provided. Therefore, it is evident that a 
.beta. of 45.degree. is advantageous. As another note, it is to be 
understood that .beta. defines the relationship between the ordinary and 
extraordinary axes of birefringent member 28 and wedge 26. The actual 
directions in which these axes may point can be varied (i.e., rotated 
about light path 33 in FIG. 2) in an unlimited number of ways so long as 
their relative directions define a suitable .beta. in accordance with the 
teachings herein. 
Referring to FIG. 1, it should be mentioned that lasant material 24 is 
placed adjacent input mirror 20. In this way, spatial holeburning will 
provide additional suppression for neighboring modes. Although it is 
preferred to place the lasant material close to mirror 20 to take 
advantage of spatial holeburning for frequency selection, such placement 
is not a requirement since the frequency filtering configuration of the 
present invention typically provides for levels of frequency selection 
which result in single mode operation when configured in accordance with 
the teachings herein. 
As mentioned previously, laser 10 may readily be modified in a way which 
produces light at a harmonic of any particular fundamental wavelength at 
which laser 10 lases. More specifically, a non-linear material 51 
(indicated as a dashed line) is placed in light path 33. Typically, it is 
desired to produce light at double the lasing frequency of the 
fundamental. For example, when the lasant material is Nd:YVO.sub.4 and the 
wavelength of pumping light source 12 is close to 809 nm, the fundamental 
wavelength is 1064 nm and the second harmonic wavelength is 532 nm in the 
green. As another example, the Nd:YVO.sub.4 has a fundamental wavelength 
at 916 nm and a second harmonic wavelength at 458 nm in the blue. Thus, 
depending upon the chosen fundamental wavelength and the specific coating 
designs of the input mirror on surface 30 and the output mirror on surface 
32, the laser can produce either 532 nm in the green or 458 nm in the 
blue. In the cases of other lasant materials, the fundamental and second 
harmonic wavelengths may be different as will be appreciated by those of 
skill in the art. 
In one embodiment of a frequency doubled configuration, nonlinear material 
51 includes potassium titanate phosphate (KTP) which, in response to 
fundamental wave input 14, produces second harmonic light 52 indicated by 
arrows. The mirror on surface 32 of the output mirror may be designed as 
transparent to light 52 such that the latter passes therethrough, as 
indicated by arrows 54. It is noted that, in this instance, the output 
mirror is designed to be totally reflective at the fundamental wavelength 
such that the small percentage of light which is denoted by arrow 34 in 
FIG. 1 is not passed through the output mirror. Although the nonlinear 
material is described as KTP, this is not a requirement. The invention 
equally applies to other nonlinear materials such as, for example, 
KN.sub.b O.sub.3, LBO, BBO, MgO:LiN.sub.b O.sub.3, KDP and PPLN. 
Turning now to FIG. 4, another embodiment of a frequency doubled laser 
manufactured in accordance with the frequency filtering concept of the 
present invention is generally indicated by the reference numeral 60. 
Laser 60 is essentially identical to previously described laser 10 in a 
frequency doubled configuration with two exceptions. First, lasant 
material 24 and wedge 26 have been replaced with a birefringent, lasant 
wedge 62 and, second, birefringent member 28 and non-linear material 51 
have been replaced with a birefringent doubling element 64 which is cut 
for Type II phase matching. For example, if element 64 is formed from KTP, 
the latter may include a length of 5 mm along light path 33. The KTP is 
cut for type II phase matching (.theta.=90.degree., .phi.=23.5.degree.) at 
1064 nm and its z axis is formed at a 45.degree. angle with respect to the 
E.sub.W axis of lasant wedge 62 and 90.degree. with respect to line 46 in 
a manner which is known in the art. Lasant wedge 62 is configured with 
wedge angle .theta. determined by the previously described expression of 
equation 1. The overall orientation of the ordinary and extraordinary axes 
of lasant wedge 62 and birefringent doubling element 64 remain unchanged 
as compared with laser 10 and as shown previously in FIG. 2. For purposes 
of brevity, these descriptions will not be repeated. Moreover, the 
operation of laser 60 is essentially identical in spirit with the 
operation of laser 10 in its doubled frequency configuration. The reader 
is, therefore, referred to previous descriptions of the operation of 
system 10. It is noted, however, that laser 60 is advantageous in the 
sense that it includes fewer components than system 10. One of skill in 
the art will appreciate that laser 60 includes all the advantages of 
previously described laser 10 since the frequency filtering configuration 
of the present invention is employed. In addition, it should also be 
appreciated that the specific multi-functional components (i.e., lasant 
wedge 62 and birefringent nonlinear element 64) of system 60 and their 
arrangement in this implementation have not been seen heretofore. 
Attention is now directed to FIG. 5 which illustrates still another 
embodiment of a frequency doubled laser manufactured in accordance with 
the frequency filtering concept of the present invention and which is 
generally indicated by the reference numeral 70. Laser 70 is similar to 
previously described laser 60 except that the need for separate input and 
output mirrors has been eliminated through the use of an input mirror 
coating 72 applied to a surface 74 of lasant wedge 62 and an output mirror 
coating 76 which is applied to a surface 78 of birefringent doubling 
element 64. Of course, input mirror coating 72 is highly reflective at the 
fundamental wavelength while output mirror coating 76 is transparent at 
the doubled frequency as well as being highly reflective at the 
fundamental frequency such that light 54 passes through the output coating 
at the second harmonic wavelength. As to its operation and design in 
accordance with the concepts taught herein, laser 70 is essentially 
identical to laser 60. 
Although the embodiments above have been described with their various 
components having particular respective orientations, it should be 
understood that the present invention may take on a wide variety of 
specific configurations in which various components are located in a wide 
variety of positions and mutual orientations while still remaining within 
the scope of the present invention. For example, it is to be understood 
that any of the frequency doubled configurations disclosed above may 
readily be converted to SLM laser configurations by removing their 
nonlinear component and modifying their mirror transmission coatings 
accordingly. As another example, although the laser of the present 
invention has been illustrated in the embodiments above as being arranged 
with its components aligned along a central axis, this is not a 
requirement, as will be seen immediately hereinafter. 
Referring to FIG. 6, one frequency doubled, alternative embodiment of a 
laser manufactured in accordance with the present invention is indicated 
by the reference numeral 80. In this embodiment, a third mirror 82 is 
added which is coated to be highly reflective at both the fundamental and 
second harmonic wavelengths, 1064 nm and 532 nm, respectively. A light 
path 84 is defined within the cavity of laser 80 which includes an acute 
angle .alpha.. It should be appreciated that non-linear material 51 
produces one portion 52a of doubled frequency light in response to the 
fundamental wavelength in one direction and produces another portion 52b 
of doubled frequency light in response to the fundamental wavelength in 
the opposite direction. In this configuration, mirror 82 advantageously 
reflects light portion 52b back toward output mirror 22 such that the 
forward and backward second harmonic light radiation with the same phase 
are combined and output as light 86 from the output mirror. The optical 
path of laser 80 may include a shape other than shown such as, for 
example, an "L" shape or any other suitable shape. In one variation, 
mirror 82 may be replaced with a highly reflective coating (not shown) on 
a surface 88 of non-linear member 51. 
As compared with the prior art, the frequency discrimination configuration 
and associated method of the present invention are highly advantageous in 
a number of different ways. For example, the use of a Brewster plate is 
avoided. As described above, the birefringent wedge or lasant wedge member 
of the present invention can be fabricated using a minimal amount of 
material since .theta. can be held to a value which is dramatically less 
than the typical 45.degree. angle required in a Brewster plate. The exact 
value of .theta. is also not critical as long as it is greater than the 
predetermined value established by equation (1). Moreover, this advantage 
may in and by itself result in an overall assembly which is more compact. 
As another example, the light output of the present invention is 
polarization selective at a fundamental wavelength in an SLM laser 
configuration or at a harmonic wavelength in a frequency multiplied 
configuration. That is, the output polarization may readily be modified in 
accordance with the foregoing teachings. 
It is noted that all intracavity optical surfaces are preferred to be 
inclined at a small angle off normal to the cavity axis so as to minimize 
coupled cavity and intracavity etalon effects. 
In that the laser assemblies disclosed herein may be provided in a variety 
of different configurations and produced using a number of different 
methods, it should be understood that the present invention may be 
embodied in many other specific forms without departing from the spirit or 
scope of the invention. Therefore, the present examples and methods are to 
be considered as illustrative and not restrictive, and the invention is 
not to be limited to the details given herein, but may be modified within 
the scope of the appended claims.