High efficiency decoupled tuning configuration intracavity doubled laser and method

As will be described in more detail hereinafter, an intracavity doubled single longitudinal mode laser is disclosed which provides stable (i.e., low amplitude variation in its output beam intensity), efficient operation in conjunction with a highly advantageous decoupled alignment configuration. An associated method is also disclosed. A lasant material is positioned in the light path of the laser for producing light at a desired fundamental wavelength and possibly other, unwanted wavelengths. Polarizing means is also positioned in the light path for polarizing the desired fundamental wavelength and the unwanted wavelengths of light. In addition to the polarizing means, birefringent means is positioned in the light path on one side of the polarizing means for selectively subjecting the unwanted wavelengths to losses in cooperation with the polarizing means such that the unwanted wavelengths are extinguished while the desired fundamental wavelength is subjected to a level of loss above a lasing threshold so that the desired fundamental wavelength lases. A type I doubling material is adjustably arranged in the light path for producing the doubled frequency light at twice the frequency of the desired fundamental frequency passing therethrough and for adjustment of phase matching which establishes an efficiency at which the doubled light frequency is produced. The birefringent means and the type I doubling material are arranged on opposite sides of the polarizing means along the light path so as to substantially decouple wavelength selection performed by adjusting the birefringent means from the phase matching adjustment performed by adjusting the type I doubling material.

BACKGROUND OF THE INVENTION 
The present invention relates generally to intracavity doubled single mode 
lasers and more particularly to an intracavity doubled single mode laser 
implementation which exhibits high efficiency in converting light from a 
fundamental frequency to a doubled, visible frequency and which provides 
for highly advantageous decoupled alignment adjustments. The invention is 
particularly well suited for efficiently producing visible light in the 
blue frequency range. 
In the prior art, a variety of intracavity doubled lasers have been 
developed. 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 be due to gain competition between 
multiple modes which lase in the laser's resonant cavity in combination 
with a longitudinal mode coupling phenomenon between the lasing modes 
which is a consequence of a nonlinear doubling process. 
One popular approach in solving the green noise problem is to limit the 
lasing modes in the laser to a single longitudinal mode. The single mode 
then excites the nonlinear material to produce a single, doubled output 
frequency. However, as will be seen hereinafter, certain problems have 
been encountered with regard to use of a single longitudinal mode laser 
(hereinafter SLM) in efficiently producing light at particular frequencies 
such as, for example, that of blue. 
Referring to FIG. 1, as mentioned, one approach for producing a stable 
output beam at a doubled frequency in accordance with the prior art is to 
excite a nonlinear material with a single longitudinal mode. In a specific 
implementation (the physical elements of which are not shown), a type II 
birefringent nonlinear material cooperates with a polarizing element so as 
to reject all but a desired fundamental wavelength. FIG. 1 illustrates, in 
this implementation, the well known orientation of the polarizing 
element's polarization axis 10 with respect to the ordinary axis 12 and 
extraordinary axis 14 of the birefringent type II nonlinear material. 
Specifically, ordinary axis 12 and extraordinary axis 14 are each oriented 
at an angle of 45.degree. with respect to polarization axis 10. In this 
way, a Lyot filter is formed in the laser's cavity which serves to 
discriminate against all but one desired fundamental frequency while 
providing low insertion loss for the desired mode. It should be 
appreciated that, in order to achieve useful output power, this 
implementation is most appropriate in applications where a reasonably 
efficient nonlinear type II doubling material is available for use at the 
wavelengths of concern. For example, green output light at 532 nm may be 
produced with relative efficiency from a 1064 nm fundamental wavelength 
using type II KTP. Unfortunately, however, the efficient production of 
blue output light using type II configured materials is complicated by the 
fact that the effective nonlinear doubling coefficient is much less in 
comparison with that in type II KTP for the green at 532 nm. 
In order to compensate for this lower optical gain, it is desirable to use 
much more efficient doubling materials. At first appearance, it would seem 
that certain doubling materials such as, for example, potassium niobate 
(KNbO.sub.3) would serve well in a high efficiency conversion role since 
this material possesses an effective doubling efficiency for blue 
interactions which is greater than thirty times that of KTP in green 
interactions. However, KNbO.sub.3 will only phase match blue interactions 
in a type I configuration such that it is not useful in the orientation of 
FIG. 1. Therefore, type I doubling materials must be used in other 
geometries, as will be described immediately hereinafter. These other 
geometries are typically inefficient as a result of additional intracavity 
components which constrain the laser to SLM operation. As an example, 
intracavity etalons are currently the component of choice for defining SLM 
operation. In most instances, etalons introduce excessive and undesirable 
insertion losses which significantly reduce the laser's visible light 
output power. Moreover, intracavity etalons do not constrain the 
polarization state of the laser in a precise way, thereby reducing the 
etalon's effectiveness in cases where the gain material being used does 
not exhibit sufficient gain anisotropy. 
Other implementations may also employ type I doubling materials. Examples 
include "twisted mode" cavity designs, in which a combination of 
intracavity waveplates is used, and ring lasers. These implementations 
normally embody complex design considerations and geometries. Ring lasers, 
in particular, are difficult to align. 
As will be seen hereinafter, the present invention provides a heretofore 
unseen and highly advantageous intracavity geometry and associated method 
which utilizes a type I doubling material in a way that provides stable 
SLM operation, high efficiency and predictable tuning behavior. 
SUMMARY OF THE INVENTION 
As will be described in more detail hereinafter, an intracavity doubled 
single longitudinal mode laser is disclosed which provides stable (i.e., 
low amplitude variation in its output beam intensity), efficient operation 
in conjunction with a highly advantageous decoupled alignment 
configuration. An associated method is also disclosed. The laser includes 
an input mirror and an output mirror defining a resonant cavity and a 
light path within the resonant cavity and between the mirrors with the 
output mirror being substantially transparent to a doubled output 
frequency produced within the cavity. A lasant material is positioned in 
the light path for producing light at a desired fundamental wavelength and 
possibly other, unwanted wavelengths. Polarizing means is also positioned 
in the light path for polarizing the desired fundamental wavelength and 
the unwanted wavelengths of light. In addition to the polarizing means, 
birefringent means is positioned in the light path on one side of the 
polarizing means for selectively subjecting the unwanted frequencies to 
certain losses while subjecting the desired fundamental frequency to other 
losses such that the unwanted frequencies are extinguished as a result of 
the certain losses while the other losses to which the desired fundamental 
frequency is subjected permit the desired fundamental frequency to remain 
above a lasing threshold so that the desired fundamental frequency lases. 
A type I doubling material is adjustably arranged in the light path for 
producing the doubled frequency light at twice the frequency of the 
desired fundamental frequency passing therethrough and for adjustment of 
phase matching which establishes an efficiency at which the doubled light 
frequency is produced. The birefringent means and the type I doubling 
material are arranged on opposite sides of the polarizing means along the 
light path such that any light passing through the doubling material at 
the fundamental wavelength includes a known polarization so as to 
substantially decouple wavelength selection performed by adjusting the 
birefringent means from the phase matching adjustment performed by 
adjusting the type I doubling material.

DETAILED DESCRIPTION OF THE INVENTION 
Attention is immediately directed to FIG. 2, which diagrammatically 
illustrates one embodiment of an intracavity doubled single mode laser 
manufactured in accordance with the present invention and generally 
indicated by the reference numeral 20. Laser 20 includes a pumping light 
source 22 which emits a pump beam 24 at a predetermined frequency. Pump 
beam 24 is directed into a lens 26 and emerges therefrom to form a beam 
28. Beam 28 then passes into a resonant cavity 30 through an input mirror 
32 which is designed to be highly transmissive at the predetermined 
frequency of the pumping source while being highly reflective to a 
fundamental lasing wavelength. An opposing end of the resonant cavity is 
defined by an output mirror 34. Resonant cavity 30 defines a lasant light 
path L between the input and output mirrors. It is mentioned that light 
path L is depicted as being straight for purposes of clarity, however, the 
light path is not typically straight as a result of the various optical 
elements disposed along its length. 
Continuing to refer to FIG. 2, output mirror 34 is designed to be highly 
reflective to the fundamental lasing wavelength while being highly 
transmissive to a frequency at or near double the frequency of the 
fundamental wavelength. After passing through the input mirror, light beam 
28 is incident upon a laser gain medium 36. 
Still referring to FIG. 2, pumping light source 22 may be provided in a 
variety of different forms so long as it produces or emits pump beam 24 at 
the predetermined frequency which is matched to the absorption of the 
material used as the laser gain medium, as is well known. In one 
embodiment, a laser diode which emits light at a wavelength at or about 
808 nm is used as pumping light source 22. As is well known in the art, 
laser diodes of this type are readily available. When directed into laser 
gain medium 36, pump beam 24 excites atomic laser transitions within the 
laser gain medium which cause the material to produce a range of 
frequencies/wavelengths in resonant cavity 21 including a desired 
fundamental frequency as well as other, unwanted frequencies. The 
fundamental and output frequencies of laser 20 can vary depending upon the 
specific materials used as the laser gain medium. For purposes of 
simplicity, the remaining discussion will consider the use of Nd:YAG 
(neodymium doped YAG) as the laser gain medium with a central fundamental 
wavelength of 946 nm. However, with the application of the principles 
described herein, the use of other materials for the laser gain medium 
will be clear to those of skill in the art in view of this overall 
disclosure. 
Referring to FIG. 3 in conjunction with FIG. 2, a polarizing element 38 
such as, for example, a brewster plate or a prism is positioned along 
light path L such that the fundamental frequencies emitted by the gain 
material are polarized along a polarization axis 40. It should be 
understood that any suitable form of polarizing element may be utilized in 
accordance with the teachings herein. In accordance with the present 
invention, a type I birefringent nonlinear material 42 is also positioned 
along light path L. Material 42 includes an ordinary/primary axis 44 and 
an extraordinary axis 46. As seen in FIG. 3, ordinary axis 44 is oriented 
so as to be generally transverse to light path L (extending perpendicular 
to the plane of the figure) while forming an angle .beta. relative to 
polarization axis 40. Remarkably, it has been empirically discovered that 
with a relatively small value for .beta., for example, in the range of 
1.degree. to 5.degree., polarizing element 38 and material 42 cooperate in 
a way which forms a Lyot filter within resonant cavity 30. It is to be 
understood that the specified approximate range of .beta. is intended only 
as exemplary, rather than limiting and that any angle is suitable for use 
provided only that the desired Lyot effect results. In practicing the 
present invention, .beta. should be gradually increased only to a point at 
which sufficient birefringence is introduced in order to establish Lyot 
filter action. Thereafter, tuning techniques such as, for example 
temperature tuning are used to select the desired fundamental lasing 
frequency which, in turn, establishes the doubled output frequency. It is 
to be understood that the above described arrangement may be varied in any 
suitable way in accordance with the teachings herein so long as a Lyot 
filter is defined. For example, ordinary axis 44 may readily be positioned 
on the opposite side of polarization axis 40 at angle .beta.. Moreover, it 
should be emphasized that .beta. defines only the relative association 
between the polarization axis and the ordinary axis of the type I 
material. Additionally, ordinary axis 44 is preferred to form a slight 
angle (not shown) off normal with light path L in a known manner so as to 
minimize coupled cavity and intracavity etalon effects. It is noted that 
all intracavity optical surfaces should be arranged slightly off normal 
with respect to light path L for the same reason. 
In conventional lasers, type I birefringent materials are typically 
oriented for maximum conversion efficiency. That is, the ordinary or 
extraordinary axis of the type I material is arranged parallel with a 
polarization axis defined within the cavity. In such an orientation, no 
frequency discrimination is provided by these elements via the Lyot 
effect. That is, such an implementation will provide a multi-mode, 
generally unstable output. One might assume that frequency discrimination 
could be provided in such a parallel axis orientation through the simple 
expedient of adding an intracavity waveplating element adjacent to the 
type I material as a way of forming a Lyot filter to simulate the above 
described type II doubling regime. Unfortunately, however, it is submitted 
that the birefringence of the type I material, in such a configuration, 
may serve to cancel out the frequency selective depolarization behavior of 
the type II waveplate. The result will be a very complex interaction 
between the type I and waveplate materials in which SLM operation will not 
be achieved. As will be seen below, the configuration of the present 
invention permits the use of a combination of type I doubling material and 
a birefringent crystal, serving as a waveplate, while avoiding the problem 
of setting up a complex interaction between these two elements. 
In one aspect, the present invention recognizes that detuning the ordinary 
axis of Type I material 42 slightly off of polarization axis 40 results in 
a slight degradation in conversion efficiency (which is minimized by 
reducing .beta., as described above), but causes the range of fundamental 
frequencies possible within the laser cavity to be subjected to the 
birefringence of the type I material. In this way, the birefringence of 
the type I material cooperates with polarizing element 38 to provide Lyot 
filter action with its associated and highly advantageous low insertion 
loss. As an example, since loss in conversion efficiency due solely to 
detuning the Type I material is roughly proportional the fourth power of 
the cosine of .beta., a loss of less than 2% is experienced for a 
.beta.=5.degree.. Thus, an intracavity doubled SLM laser is provided at 
wavelengths such as, for example, that of blue light with an efficiency 
which has not been seen heretofore at such wavelengths. 
It should be appreciated that the operation of laser 20 may be described in 
essentially the same terms as a prior art laser which uses a type II 
material in defining a Lyot filter. Specifically, only one desired 
fundamental wavelength remains unchanged in polarization (retaining 
polarization along polarization axis 40 of polarizing element 38) after 
having passed through type I material 42. Precise adjustment of the 
desired wavelength may be obtained in a known manner, for example, by 
temperature tuning. Therefore, the desired fundamental wavelength 
experiences a level of loss in combination with gain provided by gain 
material 36 which permits the desired wavelength to lase along light path 
L by remaining above a lasing threshold. Of course, as the desired 
fundamental passes through type I material 42, frequency doubled light 48 
is generated with a high conversion efficiency in an overall efficient and 
heretofore unseen intracavity geometry, as described above. Since output 
mirror 34 is highly transmissive at the doubled frequency, frequency 
doubled light 48 passes through the output mirror as output light 50. 
With regard to the unwanted fundamental wavelengths/modes, it should be 
appreciated that type I material 42 is selected to provide full or 
half-wave retardation at the desired fundamental wavelength. Any unwanted 
(unselected) mode present within cavity 30 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 material 42 will result in polarization changes for these unwanted 
modes. Upon passing through polarizing element 38, the unwanted modes will 
experience substantial losses as a result of the polarization changes. 
These losses, in spite of gain provided by gain material 36, will result 
in power levels for the unwanted modes which place them below lasing 
threshold such that they are ultimately extinguished over a number of 
roundtrips within cavity 30. 
It should be appreciated that laser 20 of FIG. 2 is particularly 
advantageous with regard to polarization of light along light path L. 
Specifically, polarization of the fundamental wavelength is established by 
polarizing element 38. For this reason, the polarization of light at the 
lasing, fundamental wavelength passing through non-linear material 42 in 
either direction is known. Moreover, no other components are present along 
light path L (i.e., birefringent components) which would disturb this 
known polarization. The advantage obtained as a result of this known 
polarization will become even more evident below with the description of 
another embodiment of the present invention. In this regard, many prior 
art configurations utilize a birefringent element which serves as a 
waveplate for rejection of unwanted wavelengths. However, as noted above, 
the presence of such a birefringent element immediately adjacent the 
non-linear material generally introduces a complex interaction between the 
type I doubling material and the birefringent material. This interaction 
results in undesirable and unpredictable changes in polarization which 
adversely reduce the amplitude of the laser's output wavelength and which 
result in difficulties when aligning the laser during the manufacturing 
process. Remarkably, however, the problem has been overcome using a 
particular geometry to be described immediately hereinafter. 
Attention is now directed to FIG. 4 which diagrammatically illustrates 
another embodiment of an intracavity doubled single mode laser 
manufactured in accordance with the present invention and generally 
indicated by the reference numeral 60. Because laser 60 includes certain 
components which are also used by previously described laser 20, like 
reference numbers have been applied wherever possible and detailed 
descriptions of such like components have not been repeated for purposes 
of brevity. However, laser 60 is unlike laser 20 in two important ways. 
First, type I doubling material 42 is oriented for maximum doubling 
efficiency. That is, previously described angle .beta. (FIG. 3) is equal 
to zero, in this instance, such that light which passes through the 
crystal at the selected polarization, as determined by polarizing element 
38, is not subjected to the birefringence of the doubling crystal. Second, 
a birefringent crystal 62 is positioned on the opposite side of polarizing 
means 38 with respect to doubling crystal 42. Birefringent crystal 62 may 
be formed, for example, from calcite, orthovanadate, quartz or any other 
suitable such material. 
It should be appreciated that, in view of not subjecting the unwanted 
wavelengths of light to the bierfringence of nonlinear material 42, an 
alternative means must be provided to facilitate rejection of the unwanted 
wavelengths. Birefringent crystal 62 cooperates with polarizing element 38 
in a way which provides the desired wavelength selection. Specifically, 
crystal 62 includes a predetermined length along light path L which 
provides full or half wave retardation at a desired fundamental 
wavelength. Crystal 62 also includes an ordinary axis and an extraordinary 
axis (neither of which is shown). In the configuration of laser 60, the 
ordinary and extraordinary axis of crystal 62 should be arranged at an 
angle of approximately 45.degree. with respect to the polarization axis of 
polarizing element 38. In this manner, Lyot filter action will occur 
whereby discrimination is provided against all but one desired wavelength 
while, at the same, time the insertion loss associated with the 
birefringent crystal is low for the desired wavelength. Since only one 
polarization component of that wavelength is able to pass through 
polarizing element 38, that particular polarization component will retrace 
along light path L, experience gain greater than losses resulting in an 
overall gain above a lasing threshold and, therefore, lase. All unwanted 
wavelengths will be subjected to levels of loss through the cooperation of 
the polarizing element and crystal 62 which will cause the unwanted 
wavelengths to be extinguished. While the arrangement of FIG. 1 is similar 
as to orientation of the various axes, it should be remembered that FIG. 1 
is associated with the use of a type II nonlinear doubling material in 
configuration which is completely different than that of the present 
invention. 
Still referring to FIG. 4, it should be appreciated that the configuration 
of laser 60 is highly advantageous with respect to the alignment process 
which is required in the manufacture of any laser. In particular, the 
configuration of laser 60 serves to decouple phase matching performance 
and adjustments from wavelength selection adjustments. The decoupled 
effect is achieved due to positioning birefringent crystal 62 and doubling 
crystal 42 on opposite sides of polarizing element 38. As a result, the 
polarization of light at the desired fundamental, as defined by the 
polarization axis of polarizing element 38, is constant as it passes 
through doubling crystal 42. Therefore, phase matching adjustments 
performed, for example, by rotating or tilting doubling material 42 are 
essentially unaffected by other adjustments made using birefringent 
crystal 62 for purposes of wavelength selection. Hence, ease of 
manufacture is facilitated by the decoupled alignment adjustment 
configuration of the present invention in that a high degree of 
predictability is present in the behavior of the laser during the 
alignment procedure. 
It should be mentioned that configurations (not shown) placing a 
birefringent crystal immediately adjacent to the non-linear doubling 
material are considered as disadvantageous in accordance with the present 
invention since alignment adjustments may readily act in an unpredictable 
manner. That is, an interaction is present between the non-linear material 
and the birefringent crystal in which phase matching adjustments may also 
affect wavelength selection and vice versa. In such an environment, an 
alignment procedure may become, in essence, a frustrating and time 
consuming guessing game. In accordance with the teachings herein, the 
interaction problem has been resolved with a highly advantageous and 
heretofore unseen configuration. 
Since 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.