Method and apparatus for optimizing the output of a multi-peaked frequency harmonic generator

A method and devices for obtaining optimized output harmonic frequency light at a plurality of peak frequencies from a nonlinear ferroelectric generator optical device by controlling the overall temperature of sections along the length of the optical structure and differentially applying external energy to the sections, with greater energy input near the input end than near the output end each section.

FIELD OF THE INVENTION 
The present invention relates to methods and apparatus for optimizing the 
output and controllability of nonlinear harmonic generators that receive 
light of a plurality at a plurality of primary frequencies and emits light 
at a plurality of frequencies that are harmonic frequencies of the primary 
frequencies. 
BACKGROUND OF THE INVENTION 
Bierlein U.S. Pat. No. 3,949,323 discloses the preparation of crystals, 
such as KTiO(PO.sub.4) crystals (KTP crystals), that are useful as second 
harmonic generators (SHG). 
Miyazuka et al. U.S. Pat. Nos. 4,953,931 and 4,953,943 describe nonlinear 
devices having a LiNbO.sub.3 thin film waveguide layer formed on 
LiTaO.sub.3 substrate. 
Tamada et al. U.S. Pat. No. 5,022,729 discloses a SHG having a Ta.sub.2 
O.sub.3 TiO.sub.2 system amorphous thin film optical waveguide on a 
substrate. The substrate may be a nonlinear optical crystal material. The 
waveguide can be made more effective by forming periodically poled regions 
of selected period and depth in the nonlinear substrate. 
Yamamoto et al. U.S. Pat. No. 4,591,291 discloses a semiconductor laser and 
an optical nonlinear device positioned on a submount with the laser's 
active layer and the nonlinear device's surface waveguide facing the 
submount so that the fundamental light from the laser is directly applied 
to the nonlinear device and doubled in frequency producing a visible 
light. 
Yamamota et al. U.S. Pat. No. 5,253,259 discloses a frequency doubler 
comprising a nonlinear crystal having domain inverted regions and a 
waveguide coupled to a semiconductor diode laser. The device includes a 
means for heating the frequency doubler to tune it to the desired 
frequency regardless of the ambient temperature. 
Endo et al. U.S. Pat. No. 5,546,220 discloses an optical structure that is 
clad with a metal coating to conduct electricity and uniformly heat an 
optical structure to tune it. 
Welch U.S. Pat. No. 5,185,752 discloses a diode laser having a reflective 
back end coupled to a SHG. The SHG comprises a periodically poled 
waveguide having ferroelectric domains and a periodic reflector, 
particularly a distributed Bragg reflector (DBR) grating. This arrangement 
forms an optically resonant chamber feedback system that stabilizes the 
frequency output of the diode laser and efficiently couples the diode 
laser to the SHG. 
By the techniques described above, and numerous others, significant 
improvement has been made in the efficiency, in terms of power output, 
currently being obtained from laser/SHG systems. Nevertheless, the 
efficiencies obtainable by the currently available devices are still very 
low. There is a need for achieving far greater efficiency, and so output 
power, because higher power laser/SHG units would be useful in a number of 
areas where current devices are unsatisfactory, such as optical data 
storage, remote sensing, and therapeutic medical applications. Practicing 
the present invention, using a KTP crystal optical structure, increases 
the output power from 2-5 times that from a similar KTP optical structure 
used in accordance with the prior art. Also the narrower wavelength of the 
output light from the devices of the present invention makes them more 
accurate in devices for detecting specific materials, avoiding false 
readings and making it unnecessary to have expensive devices to avoid 
false readings. 
It has been observed that the second harmonic efficiency of nonlinear 
harmonic generator crystals of identical dimensions and apparent 
compositions often have different second harmonic generation efficiencies. 
It is also believed that various locations along the length of a single 
crystal may have different generation efficiencies. Also it has been 
observed that during second harmonic generation the generator tends to 
heat differentially along the length of the generator, with the greatest 
heating occurring in the zone near the output end of he generator. 
Additionally it has been observed that the internal conversion 
efficiencies of longer crystal generators tends to be less than for 
shorter length generators. 
The method of the present invention increases the internal conversion 
efficiency of nonlinear optical structures. 
SUMMARY OF THE INVENTION 
The present invention relates to a device that emits light comprising a 
nonlinear harmonic generator optical structure that receives light of a 
plurality of primary frequencies at an input end thereof and emits at an 
output end thereof light of a plurality of second frequencies that are 
harmonics of primary frequencies, and at least two heat exchange means, 
each of which is adapted to exchange thermal energy between the heat 
exchange means and a section of the optical structure. Each heat exchange 
means is also independently controllable, so that more thermal energy is 
applied near the input end of each such section than near the output end 
of each such section of the optical structure. 
The present invention also relates to the method of obtaining and 
optimizing output light at a plurality of harmonic frequencies from a 
nonlinear optical structure comprising introducing light at a plurality of 
primary frequencies into an input end of the optical structure; applying 
heat to a first section of the optical structure and modifying the 
application of heat to obtain output light at a desired first harmonic 
frequency, applying heat differentially along the length of the first 
section at a plurality of separate locations, with more heat being applied 
near the input end than near the output end of the first section of the 
optical structure to optimize the first desired harmonic light output; 
applying heat to a second section of said optical structure and modifying 
the application of heat to obtain output harmonic light at a desired 
second harmonic frequency; and applying heat differentially along the 
length of the second section of the optical structure, with more heat 
being applied near the input end than near the output end of the second 
section to optimize the output of harmonic light at the desired second 
frequency. The first and second desired harmonic lights can be obtained 
simultaneously or sequentially one at a time.

DEFINITIONS 
Definitions of some terms, as used herein, are as follows: 
"Light" means electromagnetic radiation of any wavelength. "Light" can be 
of constant intensity, modulating intensity, or pulsed. 
"Primary frequency" means the frequency of the input fundamental light beam 
that produces the desired harmonic frequency output frequencies. 
"Higher" harmonics frequencies means a numerical multiple of the 
fundamental frequency, as an example the second harmonic is double the 
frequency of the input fundamental light beam. 
"Sections", when referring to the optical structure, means, separate, 
specific independent zones along the length of the optical structure. 
Sections have no borders, and to some extent separate sections may 
overlap. A section "near the input end" is readily distinguishable from a 
section "near the output end", although in a small optical structure these 
two sections might be adjacent to each other with no clear border between. 
In some situations the term "section" refers to the entire optical 
structure, in which cases the terms "first section" and "second section" 
both refer to the entire optical structure (always true?) 
"Heat exchange" means the flow of thermal energy between two locations. 
"Applying heat" and "applying thermal energy" mean transferring heat or 
thermal energy to some location. The terms "applying external energy (or 
heat) at a greater rate", "greater application of energy (or heat)", and 
"applying more energy (or heat)", when used in reference to energy 
transfer to or from the optical structure, are used to define net energy 
transfer. These terms include removing thermal energy as well as adding 
thermal energy, i.e. heating to add heat and cooling to remove heat from 
the optical structure. Thus the term "applying more thermal energy or heat 
near the input end than near the output end of the optical structure" 
includes the application of more energy, i.e. heat, to segments located 
near the input end than to segments located near the output end of the 
optical structure; it also includes removing less energy (i.e. cooling) 
from segments near the input end than from segments near the output end of 
the optical structure; and it also includes heating near the input end 
while cooling near the output end. 
"Optimizing", as used in describing the desired output light includes not 
only maximizing the output power, but also includes producing the same 
output power while minimizing the required input power, i.e. using a lower 
power less expensive diode laser, and also improving the spectral quality. 
"Near the end of the optical structure" means near the end of that part of 
the overall optical structure that generates harmonics. For example, if 
the optical structure has a Bragg reflector along it's length at one end, 
the term "near the end of the optical structure", referring to the Bragg 
reflector end, means near that section of the optical structure that 
terminates where the Bragg reflector begins, since the Bragg reflector 
section usually does not generate harmonic light. 
The term "plurality" when referring to light frequencies means more than 
one frequency, which can occur simultaneously or sequentially. When used 
in referring to sections of an optical structure, "plurality" means more 
than one at the same time (simultaneously), and also includes only a 
single section when used sequentially more than one time. 
The term "substantially different" means that the difference is sufficient 
to have a negative impact on performance and/or utility. The term 
"substantially the same" means that the difference is insufficient to have 
a negative impact on performance or use. 
The term "spectral quality" as used in describing a light includes the 
characteristics of frequency bandwidth, mode structure (what's this mean), 
and the shape of the power versus frequency graph. 
The term "peak harmonic frequency" as used in reference to the output light 
means a frequency of major output power. 
The term "overall temperature" when used in reference to an optical 
structure means the operational temperature before application of 
differential heating. 
DETAILED DESCRIPTION OF THE PRESENT INVENTION 
The principle of the present invention is the discovery that by controlling 
the heat distribution of an optical device having independently 
controllable heat exchange means at a plurality of locations along the 
length of the optical structure, it is possible to generate output light 
at a plurality of peak harmonic frequencies and optimize each peak 
harmonic frequency output and/or control the spectral quality of each 
peak. The individual harmonic frequency outputs can be generated 
simultaneously or sequentially. 
In the practice of the method of the present invention, the application of 
heat to the optical structure is done in two phases. In the first heating 
phase, a first section of the optical structure, which may be the entire 
optical structure, is heated to the appropriate overall temperature for 
the optical structure to produce the first desired harmonic frequency 
output light. Then in the second phase of the application of heat, this 
section of the optical structure is heated differentially along its 
length, with more heat applied near the input end of the section that is 
applied near the output end of the section. This differential heating is 
adjusted to optimize the output of the first desired harmonic frequency 
light. 
To obtain the desired second harmonic frequency output light, a second 
section of the optical structure is heated overall to the temperature that 
enables the optical structure to produce the desired second harmonic 
frequency output light. Then the second section is heated differentially 
along its length, with more heat applied near the input end of the section 
than is applied near the output end of the section and the differential 
heating is adjusted to optimize the second desired harmonic frequency 
output light. 
If the first and second heated sections are the same section, such as the 
entire optical structure, the first and second desired harmonic frequency 
output lights are produced sequentially, each output light being optimized 
sequentially by adjusting the differential heating after the overall 
temperature of the optical structure has been fixed to produce the desired 
peak frequency output light. If the first and second sections are 
different locations along the length of the optical structure, and the two 
phases of heat controlling are done simultaneously on both sections with 
the overall temperatures of each section being different, output light 
with the two desired optical peak harmonic frequencies optimized will be 
produced simultaneously. 
To produce output light having a plurality of desired harmonic frequencies, 
it is necessary to have the input light at a plurality of frequencies that 
are appropriate for producing the desired output harmonic frequencies. The 
plurality of frequencies can be supplied simultaneously from a single 
input laser light that has a wide band of input frequencies. Alternatively 
the plurality of input light frequencies can be supplied sequentially from 
a narrow frequency band input laser light, the frequency of which can be 
adjusted such as by adjusting the operating temperature of the laser. 
The devices of the present invention for producing light at a plurality of 
harmonic frequencies comprise a nonlinear harmonic optical structure that 
receives coherent light of a plurality of primary frequencies at an input 
end thereof and emits light at an output end thereof that is a harmonic of 
more than one of the primary frequencies; and a plurality of external 
independently controllable heat exchange means located adjacent to the 
optical structure. The heat exchange means are structured to independently 
control the overall temperature of sections of the optical structure along 
the length of the optical structure, and also to independently control the 
energy exchange applied to the individual segments. The heat exchange 
means are therefore structured to independently set and maintain upon 
command the segments of the optical structure at the appropriate overall 
temperature to generate light at the desired harmonic frequencies of one 
of said primary light frequencies, and to apply more heat near the input 
end than to the output end of each section to optimize the output power of 
harmonic frequency generated by each section. 
The segments may be a single segment, such as the entire optical structure. 
In such a device the heat exchange means are structured to maintain the 
segment sequentially at a plurality of appropriate overall temperatures 
and to optimize the power output of harmonic generated at each overall 
temperature. Such a device sequentially generates light at the desired 
plurality of harmonic frequencies. 
Alternatively, the segments may be separate distinct segments, each of 
which is a distinct harmonic frequency generating zone. Such an optical 
device has an optical structure that is divided into two or more distinct 
harmonic generating sections, an input end generating section, an output 
end generating section and, if desired, one or more intermediate 
generating sections along the length of the optical structure. In such a 
device the heat exchange means are structured to maintain each section at 
an appropriate overall temperature to generate and to optimize one of the 
desired harmonic frequency outputs. 
In each of these devices the heat exchange means are structured to tune 
each section by applying more heat near the input end than near the output 
end of each section to optimize the output light produced by that section. 
Referring to FIG. 1, a simple form of the preferred device of the present 
invention comprises a nonlinear harmonic generator optical structure 11, 
such as a bulk KTP crystal of the type disclosed in Bierlein U.S. Pat. No. 
3,949,323, or KTP waveguide of a type disclosed in Bierlein U.S. Pat. No. 
5,028,107. Optical structure 11 is capable of receiving an input light of 
a plurality of primary frequencies into an input end 12 and converting 
part of the primary frequency light to output light to any harmonics of 
the primary frequencies input light, along with unconverted primary 
frequency light. The plurality of harmonic frequency lights are emitted 
from the output end 13 of the optical structure. 
To practice the method of the present invention using the device of FIG. 1, 
at least two heat exchange means at different locations along the length 
of the optical structure are required. FIG. 1 shows two heat exchangers 14 
and 15, positioned adjacent to and along the length of the optical 
structure 11, heat exchanger 14 to heat the section of the optical 
structure near input end 12 and heat exchanger 15 to heat the section of 
optical structure 11 near the output 13. Heat exchangers 14 and 15 must be 
independently controllable so that each can apply (or remove) heat at the 
appropriate rates to these two separate sections along the length of 
optical structure 11, more heat being applied near the input end 12 than 
near the output end 13 of optical structure 11. Heat exchange controller 
16 regulates the heat applied by each heat exchanger 14 and 15. Heat 
exchange controller 16 also monitors the temperatures at the interfaces 
between optical structure 11 and heat exchangers 14 and 15. The power of 
the output light emitted from optical structure 11 is monitored by 
separate instrumentation which is commercially available. Heat exchange 
controller 16 is adjustable, in response to the power output, to apply the 
appropriate heat inputs to optical structure 11 by heat exchangers 14 and 
15 to obtain maximum power output (efficiency) from optical structure 11. 
The device of FIG. 1, having only two heat exchange locations along the 
length of the optical structure, can be used in practicing the method of 
the present invention to obtain sequentially a plurality of harmonic 
frequency output lights. This is done by first setting the overall 
temperature of the optical structure by adjusting both heat exchangers 14 
and 15 so that the optical structure produces the first desired harmonic 
frequency. Then, while monitoring the power output, this frequency output 
is optimized by modifying the heat applied by each of these heat exchanger 
until maximum power output is obtained. When this is achieved, more heat 
will be applied by heat exchanger 14 near the input end of this section of 
the optical structure (the entire optical structure) than is applied by 
heat exchanger 15 near the output end of this section. To obtain the 
second desired harmonic frequency output, the overall temperature is 
reset, by adjusting heat exchangers, so that the optical structure is 
producing the second desired harmonic frequency output. This output is 
optimized by adjusting heat exchangers 14 and 15. 
When using the device of FIG. 1, for obtaining maximum power for each of 
the desired frequency sequential outputs, heat exchanger 14 applies more 
heat to optical structure 11 than heat exchanger 15 during each of the 
output sequences. In an optical structure having substantial uniform 
birefringence index along its entire length, the difference in heat energy 
introduced by heat exchangers 14 and 15 will be substantially equal to the 
difference between the amount of heat generated near the input end 12 and 
the output end 13 of the optical structure 11 by the conversion of the 
input light to the higher harmonic frequency light in optical structure 
11. 
In a device for obtaining sequential peak harmonic frequency outputs, when 
using an optical structure having a waveguide, the two heat application 
locations shown in FIG. 1 is particularly suitable, because optical 
structures having a waveguide have the ability to generate harmonics along 
the length of the waveguide. However, when using a bulk crystal optical 
structure as the optical structure in a sequential multi-peak output 
device like FIG. 1, for optimizing the output light it is desirable to 
also have a heat exchange means located near the middle segment of the 
crystal. Normally this heat exchange means will apply to the optical 
structure less heat than is applied to any other section, because the 
middle segment tends to be heated more than the ends by the harmonic 
generation. On the other hand, optical structures that have a waveguide 
and also a Bragg reflector at the output end of the optical structure 
perform best with a heat exchange means located adjacent the reflector to 
independently control the application of heat to the Bragg reflector, and 
also a heat exchange means upstream (toward the input end of the optical 
structure) from the Bragg reflector, nearer the middle of the optical 
structure, that applies heat to that harmonic generating section of the 
waveguide upstream from the Bragg reflector. Such a device is otherwise 
operated above described in the practice of the present invention. 
FIG. 2 is a detailed drawing of a device of the type schematically shown in 
FIG. 1. In FIG. 2 optical structure 11 has a waveguide 22 positioned in 
direct contact with two aluminum heat conducting aluminum blocks 29 near 
the input end 12 of optical structure 11 and 42 near output end 13. A 
Ti-sapphire laser (Spectra Physics Model 3900) directs light of a first 
frequency into waveguide 22. Harmonic light and unconverted light of the 
first frequency is emitted through output end 13 to power meter 41, which 
monitors the output power at the various output frequencies. Adjacent 
aluminum blocks 29 and 42 are Peltier TEC heat exchangers 31 near input 
end 12 and 32 near output end 13 of optical structure 11. Thermister 
temperature sensors 33 and 34 detect the temperature of aluminum blocks 29 
and 42 respectively, and feed this information through wires (sets of two 
wires) 36 and 37 to heat exchange controllers 39 and 40. Heat exchangers 
31 and 32 are connected to controllers 39 and 40, respectively by wires 
(two wire sets) 35 and 38. Controllers 39 and 40 are independently 
controllable; they are used to set the overall temperature of optical 
structure 11 to produce the desired frequency harmonic output light, and 
to optimize this output light. 
Referring to FIG. 3, this device has three thermister heat exchangers 30 
located along the length of optical structure 11. These are adjacent heat 
conducting aluminum plate 24, to differentially control the application of 
heat to optical structure 11 at the three locations. Each heat exchanger 
25 is connected to heat exchange controller 16 by wires 26. The 
temperature of aluminum plate 24 is detected by temperature sensors 25 and 
fed back to monitor 16 through wires 27. Input light, generated by laser 
51, is directed into optical structure 11 at input end 12; output light is 
emitted at output end 13, which is monitored by power meter 41. Optical 
structure 11 has a waveguide 22. 
This device of FIG. 3 can also be used for obtaining sequential desired 
output light of differing peak frequencies. However, this device is 
particularly suited for simultaneously producing output light having two 
optimized desired harmonic frequency peaks. This is done by adjusting the 
input end and middle heat exchangers to set the segment at the input end 
of optical structure 11 at the overall temperature that produces the first 
desired harmonic frequency and then optimizing its output, shown by a peak 
output on monitor 16. Then by adjusting the output end heat exchanger 30 
the segment at the output end of optical structure 11 is set at the 
temperature that produces the desired second harmonic frequency output. 
Since the device of FIG. 2 has only three heat exchangers, the middle heat 
exchanger will be applying less heat than the input end heat exchange, and 
the output end heat exchanger will be applying less heat than the middle 
heat exchanger. Therefore the output segment of the optical structure will 
be operating at a lower overall temperature than the input segment, 
producing a second desired peak output harmonic frequency that is of lower 
frequency than the first desired harmonic frequency output generated by 
the input end segment of the optical structure. To have greater 
flexibility in producing two peak out frequency harmonics, it is often 
desired to have four independently controlled heat exchangers, two to 
modify the temperature of each separate segment of the optical structure. 
Six heat exchangers, controlling three separate segments of the optical 
structure, are preferred when producing output harmonic light having three 
desired peak frequencies. 
The optical structures used in the devices of the present invention can be 
any of the prior art optical devices, exemplified by those in the above 
mentioned patents. They are made by conventional techniques; many such 
optical structures are commercially available. Also a number of suitable 
bulk crystal optical structures, and optical structures with various types 
of waveguides and reflectors, are commercially available. The preferred 
optical structures are crystals of KTP, LiNbO.sub.3, and LiTaO.sub.3. As a 
generalization any nonlinear optical structure, preferably a ferroelectric 
crystal, that is suitable for use as a harmonic generator can be used in 
the devices of the present invention. 
Any suitable heating and/or cooling exchange means can be used in the 
devices of the present invention. They must be capable of controllable 
heating and/or cooling the specific section of the optical structure. At 
least two independently controllable heat exchange means are used, 
preferably thermoelectric or resistive heat transfer elements. 
Independently controlled heat exchange means are usually positioned near 
the input end and near the output ends of the optical structure. In 
devices that have a Bragg reflector, it may be desirable to have a 
separately controlled heat exchange means that can exchange thermal energy 
with the Bragg reflector, thereby heating or cooling it as necessary to 
obtain the desired Bragg operating frequency. 
The preferred heat exchange units are thermoelectric devices that can 
effect thermal energy flow in and out of the optical structures. 
Preferably these are affixed onto the face of the optical structure that 
is nearest to the light path through the optical structure. Thus if the 
optical structure has a waveguide adjacent to one surface of the optical 
structure, the preferred positioning of the heat exchangers is adjacent to 
the waveguide surface. 
To control the differential heat transfer along the length of the optical 
structure by the heat exchange units, any control means can be used that 
is capable of reaction to the temperature at the areas of heat transfer 
and that is capable of independently modifying the heat transfer by each 
heat exchange unit in response to an automatic or operator's command. If 
the optical structure is to be tuned to produce the desired output 
frequency, either the heater controllers or some other means must be 
capable of modifying the overall temperature of the optical structure to 
produce the desired output harmonic frequencies. A useful preferred range 
of input light wavelengths is from about 400 nm to 10,000 nm. 
Absolute maximum efficiency of the devices of the present invention require 
nonlinear and changeable heating along the length of the optical structure 
because the harmonic generation process is nonlinear, generating 
nonlinearly amounts of absorbed energy from the harmonic generation 
increasing toward the output end of the optical structure. In addition, 
manufactured irregularities in properties of the optical structure, 
irregularities between apparently similar crystals, irregularities 
introduced by etching, and by use can also result in not linear 
conditions. Consequently no specific predetermined setting for the 
applications of heat transfer will be the optimum efficiency settings 
between energy inputs at the various locations along the length of the 
optical structure. Consequently the optimum heat transfer settings are not 
completely predictable, and must be optimized by trial that is well within 
the skill of an operator or automated controller, using the output power 
as the reference, and are likely to require resetting as changes are 
induced in the optical structure due to use. 
The practice of the invention will become further apparent from the 
following non-limiting Examples. 
EXAMPLE 1 
The device of FIG. 2 is used to sequentially produce two desired harmonic 
frequency output lights. To produce the first desired harmonic frequency 
output light, the optical structure is operated at room temperature of 
21.degree. C. The optical structure is a nonlinear KTP waveguide harmonic 
generator optical structure 6.77 mm in length, 2.35 mm in width, and 0.77 
mm in thickness. This optical structure has a periodic waveguide and is 
thermally controlled by two "Peltier" TEC thermoelectric coolers (MELCOR 
FC0.7-18-05L), one applying heat to the segment of the optical structure 
near the input end and the other applying heat to the segment near the 
output end of the optical structure. The TECs are affixed with 
cyanoacylate (super glue) to two conductive thin aluminum plate 4.0 mm 
long, 6.0 mm wide and 0.8 mm thick located under the input and output ends 
of the optical structure. Temperature sensor YSI model #44016 thermisters 
are affixed inside each aluminum plate with :Lucite: delta bond thermally 
conductive adhesive. The thermisters are connected by wires to two 
independent ILXLDT model 5910 manually adjustable temperature controllers 
that show the temperatures at the end interfaces of the aluminum plates 
with the optical structure. The optical structure is placed (not glued) on 
top of the two aluminum plates. Initially each TEC is set at 21.degree. C. 
by the heater controller connected to each TEC. The input light, directed 
to optical structure, is supplied by a titanium sapphire laser that scans 
a predetermined range of wavelengths from 849 nm to 854 nm with a power 
input of 48 mW. 
The device is activated and after it has reached stable conditions, the 
output light is analyzed by a United Detector Technology power meter model 
390 (UDT 390) calibrated in accordance with standards set forth by the 
National Institute of Standards and Technology (NIST) and a Burleigh 
wavemeter model WA-2000. This initial output light has its major peak 
wavelength at 425.76 nm, the first desired harmonic frequency output 
light. The heat being applied near the two ends of the optical structure 
by the two TECs is then adjusted to give the maximum power output of this 
harmonic frequency light, with more heat being applied to the optical 
structure by the TEC near the input end of the optical structure than is 
applied by the TEC near the output end of the optical structure. 
To obtain the second desired harmonic frequency output light, having its 
major peak wavelength at 426.16 nm, the two heater controllers are used to 
set the overall temperature (define) of the optical structure at 
approximately 31.degree. C. While monitoring the output power, the heat 
being applied to the two segments of the optical structure by the two TECs 
are adjusted to give maximum power output of this second desired harmonic 
frequency output light, which is at a power level approximately the same 
as the optimized output level of the first desired harmonic frequency 
light. Again the heat applied near the input end of the optical structure 
is greater than the heat applied near the output end. 
The device of this Example, producing two sequential desired harmonic 
frequency outputs, is used in laser spectroscopy equipment and other 
measurement equipment. 
EXAMPLE 2 
This Example uses a device similar to the device of FIG. 2, the only 
difference being that the optical structure is heated by four TEC heaters, 
one at each end and two near the middle of the optical structure. The 
optical structure is the same type KTP crystal with a waveguide as in 
Example 1, except that the optical structure is 11 mm in length. It rests 
on four aluminum plates identical to those of Example 1, each of which is 
hooked up to individual temperature controllers of the type described in 
Example 1. The four TECs are identical to those described in Example 1. 
Thus the optical structure is divided into two segments, one extend from 
the input end to almost the middle of the optical structure and the second 
segment extending from just past the middle to the output end of the 
optical structure. Each segment has independently controllable heat 
exchange means located to apply heat near the input and near the output 
end of the segment. The same Ti-sapphire laser is used to produce the 
input light, and the same output power meter is used as in Example 1. 
The device is activated and after it has reached stable conditions, the 
overall temperature of the segment near the input end of the optical 
structure is set at 21.degree. C. The output light is analyzed by the 
power meter. This output light has it's major power peak at 426.76 nm 
wavelength. The power of this first desired harmonic frequency output is 
then maximized by adjusting the heat being applied at each end of this 
segment by the TECs located near the input and output ends of this segment 
of the optical structure. 
To obtain a second desired harmonic frequency peak power output, the 
overall temperature of the second segment of the optical structure, 
located from the middle to the output end of the optical structure, is 
then set at about 41.degree. C. At this temperature the optical structure 
emits a second major peak harmonic light having a wavelength of 427.56 nm. 
The power of this second harmonic output is then maximized by adjusting 
the heat being applied by the TECs located near the input and output end 
of this segment of the waveguide. At this time the optical structure is 
simultaneously emitting output light having two peak output wavelengths, 
one at 426.76 nm and a second at 427.56 nm, both of which are at about the 
same power level. 
This device is useful in laser spectroscopy equipment. 
Particular embodiments of the invention are included in the examples. Other 
embodiments will become apparent to those skilled in the art from a 
consideration of the specification or practice of the invention disclosed 
herein. It is understood that modifications and variations may be 
practiced without departing from the spirit and scope of the novel concept 
of this invention. It is further understood that the invention is not 
confined to the particular modifications and examples herein illustrated, 
but it embraces such modified forms thereof as come within the scope of 
the claims.