Dual laser resonator and beam combiner

Apparatus for providing beams of laser radiation at wavelengths of 1.44 .mu.m and 1.064 .mu.m on demand. Two Nd:YAG lasers are arranged in a side-by-side configuration and operated to provide laser output at wavelengths of 1.44 .mu.m and 1.064 .mu.m, respectively. In the preferred embodiment, a reflective spectral filter comprised of two reflectors reduces the amount of 1.064 .mu.m radiation in the output beam from the 1.44 .mu.m laser by a factor of 100 to 1000, but only reduces the amount of 1.44 .mu.m radiation in the output beam from the laser by less than 2%. The apparatus also provides collinear addition of the output from the 1.064 .mu.m laser. This collinear addition enables output radiation from both lasers to be coupled together into a single optical fiber. In fact, a user can choose to operate either laser separately or to operate both lasers simultaneously.

TECHNICAL FIELD OF THE INVENTION 
The present invention pertains to a dual laser resonator and beam combinet. 
BACKGROUND OF THE INVENTION 
It is known in the art that a standard neodymium doped YAG (Nd:YAG) laser 
crystal can be used to build a laser that emits radiation at a wavelength 
of 1.44 .mu.m with reasonable efficiency. This wavelength is useful in a 
variety of applications. For example, the high absorption coefficient of 
water for radiation at a wavelength of 1.44 .mu.m (.sup.- 26 cm.sup.-1) 
allows efficient coupling of radiation at this wavelength to biological 
tissue in surgical applications, and the radiation at a wavelength of 1.44 
.mu.m is also classified as "eye-safe." Although these same 
characteristics can be associated with Cr,Tm, Ho:YAG lasers operating near 
2.1 .mu.m, the opportunity to base a laser system upon the standard Nd:YAG 
technology provides several technological advantages. For example, laser 
efficiency drops significantly with increasing temperature in the 
Cr,Tm,Ho:YAG materials, thereby limiting the efficiency of such lasers in 
high average power applications. In particular, it is known that the 
average power of a pulsed Ho:YAG laser does not increase linearly as 
repetition rate is increased. This occurs as a result of the small energy 
difference between the lower level of laser transitions and the ground 
level in Cr,Tm,Ho:YAG. Such a characteristic does not apply to Nd:YAG and 
the problems associated with elevated temperature are, therefore, much 
less severe in Nd:YAO than in Cr,Tm,Ho:YAG. 
The strongest emission line in Nd:YAG is at 1.064 .mu.m and efficient 
lasers at this wavelength have been widely used in a variety of 
applications. Indeed, in order to obtain reasonably efficient oscillation 
at 1.44 .mu.m with Nd:YAG it is required: (1) to suppress oscillation at 
wavelengths other than 1.44 .mu.m which can deplete the upper level of the 
1.44 .mu.m transition and (2) to operate the 1.44 .mu.m laser well above 
threshold. One consequence of this is that it is essential to suppress 
oscillation at 1.064 .mu.m. The first requirement above arises from the 
fact that the gain cross section at 1.44 .mu.m is low and, in particular, 
it is approximately ten times less than that at 1.064 .mu.m in this 
material. In addition, oscillation at 1.32-1.36 .mu.m must also be 
suppressed. As a result, feedback of all of these wavelengths into the 
excited laser crystal must be kept much lower than that at 1.44 .mu.m. 
The second requirement above arises from the fact that the low gain cross 
section at 1.44 .mu.m also leads to a high threshold for laser oscillation 
at 1.44 .mu.m. 
In spite of taking the above requirements into account, a significant 
fraction of the output power from a multimode 1.44 .mu.m Nd:YAG laser may 
be radiation at a wavelength of 1.064 .mu.m. In fact, if a 1.44 .mu.m 
resonator is misaligned, the relative ratio between output at 1.064 .mu.m 
and 1.44 .mu.m can become very large, for example, the power at 1.44 .mu.m 
can go to zero while the 1.064 .mu.m output rises significantly. Such a 
happenstance, i.e., the emission of radiation at a wavelength of 1.064 
.mu.m at a time when radiation at a wavelength of 1.44 .mu.m is expected 
by a user, is undesirable and possibly dangerous in some applications 
since the interaction between biological tissue and laser radiation 
differs significantly for radiation at these two wavelengths. However, it 
would be desirable for the user to be able to choose either wavelength on 
demand since both are known to be useful. For example, 1.44 .mu.m 
radiation is useful for incising and ablating biological tissue whereas 
1.064 .mu.m radiation is useful for coagulating blood and, therefore, for 
providing hemostasis. 
In light of the above, there is a need for a method of reducing the amount 
of spurious radiation from a 1.44 .mu.m laser and for a laser system for 
providing beams of radiation at wavelengths of 1.44 .mu.m and 1.064 .mu.m 
on demand. 
SUMMARY OF THE INVENTION 
Embodiments of the present invention advantageously satisfy the 
above-identified need in the art and provide a laser system for providing 
beams of radiation at wavelengths of 1.44 .mu.m and 1.064 .mu.m on demand. 
In particular, a preferred embodiment of the present invention is apparatus 
comprised of two Nd:YAG lasers which are arranged in a side-by-side 
configuration and which are operated to provide laser output at 
wavelengths of 1.44 .mu.m and 1.064 .mu.m, respectively. In the preferred 
embodiment, a reflective spectral filter comprised of two reflectors 
reduces the amount of 1.064 .mu.m radiation in the output beam from the 
1.44 .mu.m laser by a factor of 100 to 1000, but only reduces the amount 
of 1.44 .mu.m radiation in the output beam from the laser by less than 2%. 
The apparatus also provides collinear addition of the 1.44 .mu.m output 
with output from the 1.064 .mu.m laser. This collinear addition enables 
output radiation from both lasers to be coupled together into a single 
optical fiber. 
In particular, embodiments of the present invention, comprise: a first 
radiation source which provides a first beam comprised of radiation at a 
first wavelength and a second wavelength; a second radiation source which 
provides a second beam comprised of radiation at a third wavelength; a 
first optical means, disposed in the path of the first beam, for 
reflecting a substantial portion of the radiation at the first wavelength 
and for transmitting a substantial portion of the radiation at the second 
wavelength; and a second optical means, disposed in the path of the second 
beam and the reflected radiation at the first wavelength, for reflecting a 
substantial portion of the radiation at the first wavelength and for 
transmitting a substantial portion of the radiation at the third 
wavelength.

DETAILED DESCRIPTION 
FIG. 1 shows, in pictorial form, preferred embodiment 50 of the present 
invention which is a laser system for providing beams at wavelengths of 
1.44 .mu.m and 1,064 .mu.m on demand. 
In the preferred embodiment of the present invention shown in FIG. 1, 
lasers 100 and 200 are arranged in a side-by-side configuration and 
provide laser output at wavelengths of 1.44 .mu.m and 1,064 .mu.m, 
respectively. Since the most efficient operation for most lasers is 
achieved well above threshold, laser 100 is driven with pulses of high 
peak power to provide efficient operation at 1.44 .mu.m in Nd:YAG crystal 
130. In the preferred embodiment, flashlamps 140 and 150 are used to pump 
laser 100 and energy is delivered to each lamp in a manner which is well 
known to those of ordinary skill in the art by discharging a capacitor in 
a simple pulse forming network (not shown). As a result, lamp-driven laser 
100 operates most efficiently in a pulsed mode to produce multimode output 
energies at a wavelength of 1.44 .mu.m of as high as 4J in energy. 
Further, we have determined that laser 100 produces output radiation at 
1.44 .mu.m and also produces some amount of output radiation at 1,064 
.mu.m. A reflective spectral filter comprised of reflectors 160 and 170 
which is fabricated in accordance with the present invention reduces the 
amount of 1.064 .mu.m radiation in output beam 730 by a factor of 100 to 
1000 with respect to beam 700 from laser 100, but only reduces the amount 
of 1.44 .mu.m radiation in output beam 730 by less than 2% with respect to 
beam 700 from laser 100. 
Advantageously, the preferred embodiment of the present invention shown in 
FIG. 1 also provides collinear addition of the beam of laser 200 which 
produces output radiation at a wavelength of 1,064 .mu.m. This collinear 
addition allows output beams 700 and 720 from lasers 100 and 200, 
respectively, to be coupled together into single optical fiber 190. In 
fact, by utilizing switches (not shown), a user can choose to operate 
either laser 100 or laser 200, or to operate both lasers 100 and 200 
simultaneously. Further, the outputs from lasers 100 and 200 can be 
delivered to a location for application, for example, in a surgical 
procedure, with an optical fiber. In the preferred embodiment of the 
present invention for use in surgical procedures, 1.44 .mu.m laser 100 is 
driven in a pulsed mode in a manner which is well known to those of 
ordinary skill in the art while 1.064 .mu.m laser 200 is driven in a 
continuous wave mode in a manner which is well known to those of ordinary 
skill in the art. Thus, the user may choose to obtain: (a) pulsed 1.44 
.mu.m radiation for use in incision and ablation; (b) continuous 1.064 
.mu.m radiation for use in coagulation and hemostasis; or (c) simultaneous 
pulsed 1.44 .mu.m radiation and continuous 1.064 .mu.m radiation for a 
superposition of the effects produced by such radiation. 
FIG. 1 shows, in pictorial form, lasers 100 and 200 which provide laser 
output radiation at wavelengths of 1.44 .mu.m and 1,064 .mu.m, 
respectively. As shown in FIG. 1, a resonator for laser 100 is comprised 
of cavity mirrors 110 and 120 and ND:YAG crystal 130 is disposed within 
the laser resonator formed by cavity mirrors 110 and 120. Further, Nd:YAG 
crystal 130 is pumped by flashlamps 140 and 150 in a manner which is well 
known to those of ordinary skill in the art. In the preferred embodiment, 
flashlamps 140 and 150 are driven by pulse forming networks (not shown), 
the capacitors of which are charged by a switching power supply (not 
shown) which is well known to those of ordinary sill in the art. We have 
determined that, as a result of pumping, laser 100 produces laser output 
radiation at 1.44 .mu.m and 1.064 .mu.m and that the ratio of radiation at 
these wavelengths depends on pump power. Radiation which is emitted from 
laser 100 as beam 700 impinges upon first surface 161 of reflector 160 and 
is reflected as beam 710 towards reflector 170. Beam 710 is then reflected 
by first surface 171 of reflector 170. In accordance with the present 
invention, first surface 161 of reflector 160 has a reflectivity of 
greater than approximately 99% at a wavelength of 1.44 .mu.m and a 
reflectivity of less than approximately 10% at a wavelength of 1.064 
.mu.m. Further in accordance with the present invention, first surface 171 
of reflector 170 has a reflectivity of greater than approximately 99% at a 
wavelength of 1.44 .mu.m and a reflectivity of less than approximately 10% 
at a wavelength of 1.064 .mu.m. Of course those of ordinary skill in the 
art will readily appreciate that the combination of reflectors 160 and 170 
can be achieved for either s or p polarized light or for randomly 
polarized light. Lastly, the radiation obtained after reflection by 
reflector 170 may be focused by lens system 180 for coupling to optical 
fiber 190. 
FIG. 1 also shows, in pictorial form, laser 200 which produces laser output 
radiation at 1.064 .mu.m. As shown in FIG. 1, a resonator for laser 200 is 
comprised of cavity mirrors 210 and 220 and ND:YAG crystal 230 is disposed 
within the laser resonator formed by cavity mirrors 210 and 220. Further, 
Nd:YAG crystal 230 is pumped by arc lamp 240 in a manner which is well 
known to those of ordinary skill in the art. In the preferred embodiment, 
arc lamp 240 is driven in a continuous mode by a switching power supply 
(not shown) which is well known to those of ordinary skill in the art, 
which switching power supply is configured as a current source in a manner 
which is well known to those of ordinary skill in the art. Radiation which 
is emitted from laser 200 as beam 720 impinges upon reflector 170 and is 
mostly transmitted to lens system 180. 
In the preferred embodiment of the present invention, cavity mirror 110 of 
laser 100 has the following properties for approximately 0.degree. degrees 
incidence: high reflectance at 1.44 .mu.m (reflectance at 1.44 
.mu.m&gt;approximately 99%); high transmittance at 1.064 .mu.m (reflectance 
at 1.064 .mu.m&lt; approximately 10%); and high transmittance at 1.310-1.360 
.mu.m (reflectance at 1.310-1.360 .mu.m&lt;approximately 40%). In the 
preferred embodiment of the present invention, cavity output mirror 120 of 
laser 100 has the following properties for approximately 0.degree. degrees 
incidence: approximately 75% reflectivity output coupler at 1.44 .mu.m 
(reflectance at 1.44 .mu.m=75%.+-.2%); high transmittance at 1,064 .mu.m 
(reflectance at 1.064 .mu.m&lt;10%); and high transmittance at 1,310-1.360 
.mu.m: (reflectance at 1.310-1.360 .mu.m&lt;40%). As is well known to those 
of ordinary skill in the art, the radii of curvature of the cavity mirrors 
are chosen in accordance with thermal lens parameters and desired output 
beam characteristics. 
The properties of suitable resonator mirrors for 1.064 .mu.m laser 200 are 
well known to those of ordinary skill in the art. Further, those of 
ordinary skill in the art readily appreciate that although we are 
utilizing the terms 1.44 .mu.m radiation and 1,064 .mu.m radiation these 
terms refer to radiation substantially at these wavelengths and in 
reasonably suitable ranges thereabout. 
In the preferred embodiment of the present invention, reflectors 160 and 
170 comprise dichroic mirrors 160 and 170 which are mounted at 
approximately 45.degree. degree angles to the optic axes of lasers 100 and 
200, respectively. Dichroic mirrors 160 and 170 have the following 
properties. The substrates of reflectors 160 and 170 are fabricated of a 
material that is transparent to radiation at wavelengths substantially 
equal to 1.06 .mu.m. For first surfaces 161 and 171 of dichroic mirrors 
160 and 170, respectively, at approximately 45.degree. degrees incidence: 
high reflectance at 1.44 .mu.m, for polarizations p and s (reflectance at 
1.44 .mu.m, polarization p&gt;99.0% and reflectance at 1.44 .mu.m, 
polarization s&gt;99.7%) and high transmittance at 1,064 .mu.m, for 
polarizations p and s (reflectance at 1,064 .mu.m, polarization p&lt;10.0% 
and reflectance at 1,064 .mu.m, polarization s&lt;10.0%). For second surfaces 
162 and 172 of dichroic mirrors 160 and 170, respectively, at 
approximately 45.degree. degrees incidence: narrow band antireflection 
coating at 1.064 .mu.m (reflectance at 1,064 .mu.m, polarization p&lt;0.5% 
and reflectance at 1,064 .mu.m, polarization s&lt;0.5%). As a result of this, 
advantageously, most of the 1.44 .mu.m wavelength radiation in beam 700 
output from laser 100 is reflected from mirror 160 and most of the 1,064 
.mu.m wavelength radiation in beam 700 is transmitted through mirror 160 
while most of the 1.44 .mu.m radiation in beam 710 is reflected from 
mirror 170 and most of the remaining 1,064 .mu.m radiation in beam 710 is 
transmitted through mirror 170. Those of ordinary skill in the art will 
readily appreciate that mirrors 160 and 170 may also be designed for use 
at angles other than at approximately 45.degree. degrees incidence. 
In the preferred embodiment of the present invention, beam 700 emitted by 
1.44 .mu.m laser 100 is reflected from mirror 160 at .about.45.degree. 
degrees incidence as beam 710 with less than 1% loss at 1.44 .mu.m. 
Further, beam 710 is then reflected from mirror 170 at .about.45.degree. 
degrees incidence with less than 1% additional loss at 1.44 .mu.m. Thus, 
the overall loss in the 1.44 .mu.m portion of beam 700 is typically less 
than 2% after these two reflections. Further, any undesired 1,064 .mu.m 
radiation in beam 700 emitted from laser 100 is reduced by a factor of 100 
to 1000. As a result, the use of mirrors 160 and 170 has advantageously 
removed 1.064 .mu.m radiation from the output of laser 100. The 
reflectance of each of combining mirrors 160 and 170 is less than 10% at 
1,064 .mu.m. The low reflectance at 1,064 .mu.m for mirror 170 is also 
advantageously used for transmission of beam 720 emitted by 1,064 .mu.m 
laser 200. In particular, beam 720 is transmitted through mirror 170 with 
less than 10% power loss. Lastly, mirrors 160 and 170 are aligned in a 
manner which is well known to those of ordinary skill in the art to 
provide that the 1.44 .mu.m beam 710 reflected from mirror 160 and 1,064 
.mu.m beam 720 emitted by laser 200 emerge collinearly from system 50. 
Advantageously, both beams, i.e., 1.44 .mu.m wavelength radiation in beam 
710 and 1,064 .mu.m beam 720, are coupled to the same optical fiber, i.e., 
optical fiber 190. This advantageously allows a user to select either 
wavelength for delivery through the same optical fiber delivery system. 
Further, it should be clear to those of ordinary skill in the art that one 
may utilize two independent power supplies for driving lasers 100 and 200 
so as to provide individual or simultaneous excitation thereof. In 
addition, one may utilize a single power supply for driving lasers 100 and 
200 independently or simultaneously. 
As shown in FIG. 1, lasers 100 and 200 are built, side-by-side, each with 
its own laser rod, laser rods 130 and 230, respectively- This is 
advantageous in that it permits one to optimize each laser independently 
in a manner which is well known to those of ordinary skill in the art with 
respect to laser rods 130 and 230; cavity mirrors (110, 120) and (210, 
220), respectively; lamps (140, 150) and (240), respectively; lamp driving 
characteristics; and pump cavities. In the preferred embodiment of the 
present invention, the pump cavities of lasers 100 and 200 are disposed in 
the same water-cooled housing. 
FIG. 2 shows in pictorial form a first alternative embodiment of the 
present invention. As shown in FIG. 2, prism 400 is inserted in the cavity 
of laser 100. Prism 400 is designed in a manner which is well known to 
those of ordinary skill in the art, so that 1.064 .mu.m radiation which is 
excited in the cavity of laser 100 is directed out of the cavity. This 
provides a benefit in that it further reduces 1.064 .mu.m radiation in the 
mainly 1.44 .mu.m output from laser 100. 
FIG. 3 shows, in pictorial form, a second alternative embodiment of the 
present invention. As shown in FIG. 3, lasers 100 and 200 are not aligned 
substantially parallel to each other as was the case for the embodiment 
shown in FIG. 1. In FIG. 3, the output from lasers 100 and 200 emerge at 
an angle and impinge upon prism 500. Prism 500 is designed in a manner 
which is well known to those of ordinary skill in the art so that 1.44 
.mu.m radiation impinging thereon from laser 100 is directed to lens 510 
and, in turn, thereby to optical fiber 520. Further, prism 500 is designed 
so that 1,064 .mu.m radiation impinging thereon from laser 100 is directed 
away from lens 520. Finally, prism 520 is designed so that 1.064 .mu.m 
radiation impinging thereon from laser 200 is directed to lens 510 and, in 
turn, thereby to optical fiber 520. 
We have described the present invention in an embodiment wherein laser 100 
outputs radiation at a first wavelength substantially equal to 1.44 .mu.m 
and radiation at a second wavelength substantially equal to 1.064 .mu.m. 
However, it should be understood that the radiation at a second wavelength 
may also comprise radiation of several wavelengths. For example, the 
radiation at the second wavelength may comprise radiation at the following 
wavelengths: 1.32 .mu.m, 1.34 .mu.m, and/or 1.36 .mu.m. Of course, in the 
preferred embodiment described above, the radiation at the second 
wavelength is predominantly radiation at a wavelength substantially equal 
to 1.064 .mu.m. 
It is to be appreciated and understood that the specific embodiments of the 
invention described hereinbefore are merely illustrative of the general 
principles of the invention. Various modifications may be made by those 
skilled in the art consistent with the principles set forth hereinbefore. 
For example, although embodiments have been described with references to 
FIGS. 1 and 2 wherein lasers 100 and 200 have been aligned in a 
side-by-side arrangement, embodiments of the present invention are not 
limited to such an arrangement. It is considered within the scope of the 
present invention to fabricate embodiments wherein lasers 100 and 200 have 
other alignments and wherein the radiation output therefrom is directed by 
optical means which are well known to those of ordinary skill in the art, 
along with other alignments of mirrors 160 and 170, to achieve the 
advantageous function achieved above with respect to the embodiments shown 
in FIGS. 1 and 2. As a further example, it is considered within the scope 
of the present invention to fabricate embodiments wherein reflector 170 is 
mounted on a translation stage that provides removal of reflector 170 from 
the path of beam 720 from laser 200 whenever beam 720 is desired by a 
user. Such an arrangement advantageously avoids the imposition of loss on 
beam 720 before coupling to fiber 190 or any other application. In 
addition, reflector 160 may be similarly mounted on the same translation 
stage as reflector 170 to reduce alignment problems.