U.S. patent number RE48,028 [Application Number 15/147,816] was granted by the patent office on 2020-06-02 for laser power and energy sensor utilizing anisotropic thermoelectric material.
This patent grant is currently assigned to Coherent, Inc.. The grantee listed for this patent is Coherent, Inc.. Invention is credited to Erik Krous, James Schloss, Robert Semerad.
United States Patent |
RE48,028 |
Semerad , et al. |
June 2, 2020 |
Laser power and energy sensor utilizing anisotropic thermoelectric
material
Abstract
A laser-radiation sensor includes a copper substrate on which is
grown an oriented polycrystalline buffer layer surmounted by an
oriented polycrystalline sensor-element of an anisotropic
transverse thermoelectric material. An absorber layer, thermally
connected to the sensor-element, is heated by laser-radiation to be
measured and communicates the heat to the sensor-element, causing a
thermal gradient across the sensor-element. Spaced-apart electrodes
in electrical contact with the sensor-element sense a voltage
corresponding to the thermal gradient as a measure of the incident
laser-radiation power.
Inventors: |
Semerad; Robert (St. Wolfgang,
DE), Krous; Erik (Wilsonville, OR), Schloss;
James (Tigard, OR) |
Applicant: |
Name |
City |
State |
Country |
Type |
Coherent, Inc. |
Santa Clara |
CA |
US |
|
|
Assignee: |
Coherent, Inc. (Santa Clara,
CA)
|
Family
ID: |
50384323 |
Appl.
No.: |
15/147,816 |
Filed: |
May 5, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61709060 |
Oct 2, 2012 |
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Reissue of: |
13944830 |
Jul 17, 2013 |
9012848 |
Apr 21, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01J
1/4257 (20130101); G01K 17/003 (20130101); G01J
5/046 (20130101); H01L 31/0368 (20130101); G01K
17/003 (20130101); G01J 5/12 (20130101); G01J
5/12 (20130101); H01L 31/0368 (20130101); G01J
1/4257 (20130101); G01J 5/046 (20130101) |
Current International
Class: |
G01J
5/00 (20060101); H01L 31/0368 (20060101); G01J
5/12 (20060101); G01J 5/04 (20060101); G01K
17/00 (20060101); G01J 1/42 (20060101) |
Field of
Search: |
;250/338.3 ;257/467 |
References Cited
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Feb 1997 |
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Aug 1999 |
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Sep 1996 |
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JP |
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2002-289931 |
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WO |
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2010/058559 |
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May 2010 |
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WO |
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2011/148425 |
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Dec 2011 |
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WO |
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2014/055374 |
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Apr 2014 |
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WO |
|
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|
Primary Examiner: Gagliardi; Albert J
Attorney, Agent or Firm: Morrison & Foerster LLP
Parent Case Text
PRIORITY CLAIM
This application claims priority of U.S. Provisional Application
No. 61/709,060, filed Oct. 2, 2012, the complete disclosure of
which is hereby incorporated herein by reference.
Claims
What is claimed is:
1. A laser-radiation sensor, comprising: a copper substrate; an
oriented polycrystalline buffer-layer deposited on a surface of the
substrate, the buffer-layer having a crystal-orientation at a first
angle between about 10 degrees and about 45 degrees relative to a
normal to the surface of the substrate; an oriented polycrystalline
sensor-element of a thermoelectric material selected from the group
of thermoelectric materials consisting of dysprosium barium
cuprate, strontium sodium cobaltate, and strontium cobaltate
deposited on the buffer layer, the sensor-element having a
crystalline c-axis orientation at a second angle between about 10
degrees and about 45 degrees relative to the normal to the surface
of the substrate; a radiation-absorber layer in thermal
communication with the sensor-element; and first and second
elongated electrodes spaced apart in electrical contact with the
sensor-element.
2. The laser-radiation sensor of claim 1, wherein the
sensor-element is a continuous layer of the oriented
polycrystalline sensor-material extending between the first and
second electrodes.
3. The laser-radiation sensor of claim 1, wherein the
sensor-element includes a plurality of strips of the oriented
polycrystalline sensor-material spaced apart, parallel to each
other, and extending between the first and second electrodes.
4. The laser-radiation sensor of claim 3, wherein the strips of the
sensor-element are aligned parallel to the crystalline c-axis of
the oriented polycrystalline sensor-material.
5. The laser-radiation sensor of claim 1, further including a
protection layer between the sensor-element and the
radiation-absorber layer.
6. The laser-radiation sensor of claim 5, wherein the protection
layer is a layer of one of magnesium oxide, and silicon
dioxide.
7. The laser-radiation sensor of claim 6, wherein the
radiation-absorber layer is a layer of a radiation-absorbing
material selected from a group of radiation-absorbing materials
consisting of boron carbide, titanium nitride, chromium oxide, gold
black, and carbon.
8. The laser-radiation sensor of claim 1, wherein the electrodes
include a metal selected from a group of metals consisting of gold,
platinum, silver, and palladium.
9. The laser- radiation sensor of claim 1, wherein the buffer layer
is a layer of material selected from a group of materials
consisting of magnesium oxide, yttrium stabilized zirconia, and
cerium oxide.
10. The laser-radiation sensor of claim 1, wherein the first and
second angles are about the same.
11. A laser-radiation sensor, comprising: a .Iadd.copper
.Iaddend.substrate .[.of a highly thermally conductive material.].;
an oriented polycrystalline buffer-layer deposited on a surface of
the substrate, the buffer-layer having a crystal-orientation at a
first angle between about 10 degrees and about 45 degrees relative
to a normal to the surface of the substrate; an oriented
polycrystalline sensor-element of a thermoelectric material
selected from the group of thermoelectric materials consisting of
dysprosium barium cuprate, strontium sodium cobaltate, and
strontium cobaltate deposited on the buffer layer, the
sensor-element having a crystalline c-axis orientation at a second
angle between about 10 degrees and about 45 degrees relative to the
normal to the surface of the substrate; a protection layer
deposited on the sensor-element; a radiation-absorber layer
deposited on the protection layer; first and second elongated
electrodes spaced apart in electrical contact with the
sensor-element; and wherein the sensor-element includes a plurality
of strips of the oriented polycrystalline sensor-material spaced
apart, parallel to each other, and extending between the first and
second electrodes, with each of the strips in electrical contact
with the first and second electrodes.
.[.12. The laser-radiation sensor of claim 11, wherein the
substrate is a copper substrate..].
13. The laser-radiation sensor of claim 11, wherein the buffer
layer has a thickness between about 0.5 micrometers and about 3.0
micrometers, and is a layer of material selected from a group of
materials consisting of magnesium oxide, yttrium stabilized
zirconia, and cerium oxide.
14. The laser-radiation sensor of claim 11, wherein the strips of
sensor material have a thickness between about 5 nanometers and
about 500 nanometers.
15. The laser-radiation sensor of claim 11, wherein the protection
layer has a thickness of between about 0.2 micrometers and about
2.0 micrometers, and is a layer of one of magnesium oxide, and
silicon dioxide.
16. The laser-radiation sensor of claim .[.12.]. .Iadd.11.Iaddend.,
wherein the absorber layer has a thickness of between about 0.5
micrometers and about 5.0 micrometers, and is a layer of a
radiation-absorbing material selected from a group of
radiation-absorbing materials consisting of boron carbide, titanium
nitride, chromium oxide, gold black, and carbon.
.Iadd.17. A laser-radiation sensor, comprising: a copper substrate;
an oriented polycrystalline buffer-layer deposited on the
substrate, the buffer-layer having a crystal-orientation at a first
angle between about 10 degrees and about 45 degrees relative to a
normal to the surface of the substrate; an oriented polycrystalline
sensor-element of a thermoelectric material selected from the group
of thermoelectric materials consisting of dysprosium barium
cuprate, strontium sodium cobaltate, and strontium cobaltate
deposited on the buffer layer, the sensor-element having a
crystalline c-axis orientation at a second angle between about 10
degrees and about 45 degrees relative to the normal to the surface
of the substrate; a radiation-absorber layer in thermal
communication with the sensor-element; and first and second
elongated electrodes spaced apart in electrical contact with the
sensor-element..Iaddend.
.Iadd.18. The laser-radiation sensor of claim 17, wherein the
sensor-element is a continuous layer of the oriented
polycrystalline sensor-material extending between the first and
second electrodes..Iaddend.
.Iadd.19. The laser-radiation sensor of claim 17, wherein the
sensor-element includes a plurality of strips of the oriented
polycrystalline sensor-material spaced apart, parallel to each
other, and extending between the first and second
electrodes..Iaddend.
.Iadd.20. The laser-radiation sensor of claim 19, wherein the
strips of the sensor-element are aligned parallel to the
crystalline c-axis of the oriented polycrystalline
sensor-material..Iaddend.
.Iadd.21. The laser-radiation sensor of claim 17, further including
a protection layer between the sensor-element and the
radiation-absorber layer..Iaddend.
.Iadd.22. The laser- radiation sensor of claim 17, wherein the
buffer layer is a layer of material selected from a group of
materials consisting of magnesium oxide, yttrium stabilized
zirconia, and cerium oxide..Iaddend.
.Iadd.23. The laser-radiation sensor of claim 17, wherein the first
and second angles are about the same..Iaddend.
.Iadd.24. A laser-radiation sensor, comprising: a copper substrate;
an oriented polycrystalline buffer-layer deposited over the
substrate, the buffer-layer having a crystal-orientation at a first
angle between about 10 degrees and about 45 degrees relative to a
normal to the surface of the substrate; an oriented polycrystalline
sensor-element of a thermoelectric material selected from the group
of thermoelectric materials consisting of dysprosium barium
cuprate, strontium sodium cobaltate, and strontium cobaltate
deposited over the buffer layer, the sensor-element having a
crystalline c-axis orientation at a second angle between about 10
degrees and about 45 degrees relative to the normal to the surface
of the substrate; a radiation-absorber layer in thermal
communication with the sensor-element; and first and second
elongated electrodes spaced apart in electrical contact with the
sensor-element..Iaddend.
.Iadd.25. The laser-radiation sensor of claim 24, wherein the
sensor-element is a continuous layer of the oriented
polycrystalline sensor-material extending between the first and
second electrodes..Iaddend.
.Iadd.26. The laser-radiation sensor of claim 24, wherein the
sensor-element includes a plurality of strips of the oriented
polycrystalline sensor-material spaced apart, parallel to each
other, and extending between the first and second
electrodes..Iaddend.
.Iadd.27. The laser-radiation sensor of claim 26, wherein the
strips of the sensor-element are aligned parallel to the
crystalline c-axis of the oriented polycrystalline
sensor-material..Iaddend.
.Iadd.28. The laser-radiation sensor of claim 24, further including
a protection layer between the sensor-element and the
radiation-absorber layer..Iaddend.
.Iadd.29. The laser-radiation sensor of claim 24, wherein the
buffer layer is a layer of material selected from a group of
materials consisting of magnesium oxide, yttrium stabilized
zirconia, and cerium oxide..Iaddend.
.Iadd.30. The laser-radiation sensor of claim 24, wherein the first
and second angles are about the same..Iaddend.
Description
TECHNICAL FIELD OF THE INVENTION
The present invention relates in general to laser-radiation
detectors. The invention relates in particular to a laser-radiation
detector that utilizes a transverse thermoelectric effect.
DISCUSSION OF BACKGROUND ART
Laser-radiation detectors (sensors) are used in laser applications
wherein laser-radiation power needs to be measured or monitored.
The power measurement may be required from simple record-keeping or
as part of some closed loop control arrangement. Commonly used
radiation detectors are based on either photodiodes or
thermopiles.
The photodiode-based sensors detect laser-radiation by converting
photon energy of radiation to be measured into an electron-hole
pairs in the photo-diode thereby generating a corresponding
current, which is used a measure of laser-radiation power.
Photodiodes sensors have a relatively fast temporal response, with
rise times typically less than 1 microsecond (.mu.s). A
disadvantage of photodiode detectors is a limited spectral
response. This spectral response is determined by the particular
semiconductor materials used for forming the photodiode. By way of
example, photodiode sensors based on silicon have a spectral
acceptance bandwidth between about 0.2 micrometers (.mu.m) and
about 2.0 .mu.m. A second limitation of a photodiode is relatively
low optical power saturation. Photodiodes are typically limited to
direct measurement of laser powers of less than 100 milliwatts
(mW).
Thermopile sensors include a solid element which absorbs the
radiation, thereby heating the element. One or more thermocouples
in contact with the element create a current or voltage
representative of the laser-radiation power incident on the
element. Thermopile sensors have a slow response time relative to
photodiode detectors. The response time is dependent on the size of
the sensor-element. By way of example radial thermopiles with
apertures of 19 millimeters (mm) and 200 mm have response times of
approximately 1 second and 30 seconds respectively. Spectral
response of the thermopile sensors depends on the absorption
spectrum of the sensor. With a suitable choice and configuration of
the sensor, the spectral response can extend from ultraviolet (UV)
wavelengths to far infrared wavelengths. With a sufficient heat
sink, thermopile sensors can measure lasers power up to about 10
kilowatts (kW).
One relatively new detector type which has been proposed to offer a
temporal response comparable to a photodiode detector and a
spectral response comparable with a thermopile detector is based on
using a layer of an anisotropic transverse thermoelectric material
as a detector element. Such an anisotropic layer is formed by
growing the material in an oriented polycrystalline crystalline
form, with crystals inclined non-orthogonally to the plane of the
layer.
The anisotropic layer absorbs radiation to be measured thereby
heating the layer. This creates a thermal gradient through the
anisotropic material in a direction perpendicular to the layer.
This thermal gradient, in turn, creates an electric field
orthogonal to the thermal gradient. The electric field is
proportional to the intensity of incident radiation absorbed. Such
a detector may be referred to as a transverse thermoelectric effect
detector. If the anisotropic layer is made sufficiently thin, for
example only a few micrometers thick, the response time of the
detector will be comparable with that of a photodiode detector.
Spectral response is limited only by the absorbance of the
anisotropic material. A disadvantage is that the transverse
thermoelectric effect is relatively weak compared to the response
of a photodiode.
One transverse-thermoelectric-effect detector is described in U.S.
Pat. No. 8,129,689, granted to Takahashi et al. (hereinafter
Takahashi). Takahashi attempts to offset the weakness of the
transverse thermoelectric effect by providing first and second
anisotropic material layers which are grown on opposite sides of a
transparent crystalline substrate. In the Takahashi detector,
radiation not absorbed by the first layer of anisotropic material
is potentially absorbed by the second layer. It is proposed that a
reflective coating can be added to the second layer to reflect any
radiation not absorbed by the second layer to make a second pass
through both layers.
Oriented polycrystalline layers can be deposited by a well-known
inclined substrate deposition (ISD) process. This process is
described in detail in U.S. Pat. No. 6,265,353 and in U.S. Pat. No.
6,638,598. Oriented polycrystalline layers have also been grown by
a (somewhat less versatile) ion-beam assisted deposition (IBAD)
process. One description of this process is provided in a paper
"Deposition of in-plane textured MgO on amorphous Si.sub.3N.sub.4
substrates by ion-beam-assisted deposition and comparisons with
ion-beam assisted deposited yttria-stabilized-zirconia" by C. P.
Wang et.al, Applied Physics Letters, Vol 71, 20, pp 2955, 1997.
The above described Takahashi detector allows the anisotropic
material layers to remain thin, while increasing the amount of
light absorbed, but requires a transparent crystalline substrate
polished on both sides, at costs potentially prohibitive for most
commercial applications. Further, the Takashi detector arrangement
isolates the crystalline substrate limiting the ability to
heat-sink the substrate. This limits the power-handling capability
of the detector to a maximum power of less than about 10 Watts (W),
and may lead to a non-linear response.
SUMMARY OF THE INVENTION
In one aspect, a radiation detector sensor in accordance with the
present invention comprises a substrate of a highly thermally
conductive material. An oriented polycrystalline buffer-layer is
deposited on a surface of the substrate. The buffer-layer has a
crystal-orientation at a first angle between about 10 degrees and
about 45 degrees. Formed on top of the buffer is an oriented
polycrystalline sensor element of a thermoelectric material
selected from the group of thermoelectric materials consisting of
dysprosium barium cuprate, strontium sodium cobaltate, and
strontium cobaltate is deposited on the buffer layer. The
sensor-element has a crystalline c-axis orientation at a second
angle between about 10-degrees and about 45-degrees relative to the
normal to the surface of the substrate. A radiation-absorber layer
is provided, the radiation-absorber absorber layer being in thermal
communication with the sensor layer. First and second electrodes
are spaced apart in electrical contact with the sensor-layer.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and constitute
a part of the specification, schematically illustrate a preferred
embodiment of the present invention, and together with the general
description given above and the detailed description of the
preferred embodiment given below, serve to explain principles of
the present invention.
FIG. 1 is a cross-section view schematically illustrating a
preferred embodiment of a transverse thermoelectric detector in
accordance with the present invention, including a copper
substrate, a buffer layer on the substrate a sensor layer on the
buffer layer, a protective layer on the sensor layer, and absorber
layer on the protective layer, with spaced apart electrodes in
electrical contact with the sensor layer.
FIG. 2 is a plan-view from above schematically illustrating a
preferred arrangement of electrodes and patterned sensor layer
material for the detector of FIG. 1.
FIG. 3 is a graph schematically illustrating measured transverse
thermoelectric signal as a function of incident CW laser-radiation
power for an example of the detector of FIG. 2 wherein the sensor
layer is a layer of dysprosium barium cuprate.
FIG. 4 is a graph schematically illustrating measured peak
thermoelectric voltage and reflected energy as a function of
incident 10-nanosecond pulsed-energy for the detector example of
FIG. 3.
FIG. 5 is a graph schematically illustrating a transverse
thermoelectric voltage signal as a function of time for the
detector example of FIG. 4 in response to irradiation by single
10-nanosecond pulse.
FIG. 6 is a contour plot schematically illustrating normalized
spatial uniformity of efficiency in the detector example of FIG.
3.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to the drawings, wherein like components are
designated by like reference numerals, FIG. 1 schematically
illustrates a preferred embodiment 30 of a transverse
thermoelectric sensor in accordance with the present invention.
Sensor 30 includes a substrate 32 of a highly thermally conductive
material. A preferred material for substrate 32 is copper (Cu).
Copper is a preferred material due to its high thermal conductivity
and relatively low cost. Substrate 32 has a polished surface 32A,
preferably having a RMS roughness less than about 0.5 .mu.m. The
substrate is optionally in contact with a heat-sink 48, which can
be passively or actively cooled.
An oriented polycrystalline buffer-layer 34 is deposited on a
surface 32A of the substrate. A preferred material for buffer layer
34 is magnesium oxide (MgO). Other suitable buffer layer materials
include yttrium stabilized zirconia (YSZ), cerium oxide
(CeO.sub.2). Buffer layer 34 has a columnar grain structure with
crystal-axis (the c-axis) 46 thereof tilted at an angle .alpha. in
the direction by between about 10-degrees and about 45-degrees
relative to a normal 47 to substrate surface 32A. In the drawing,
the a-c plane of the crystal axes is in the plane of the drawing
with the crystalline b-axis perpendicular to the plane of the
drawing. A preferred thickness for the buffer layer is between
about 0.5 .mu.m and about 3.0 .mu.m.
A layer 36 of sensor-material 36 is deposited on buffer layer 32.
The inclined oriented crystal structure of the buffer layer causes
the layer of sensor-material to grow in the inclined
polycrystalline form necessary for providing the desired transient
thermoelectric effect. The tilted crystalline structure is
indicated I the drawing by long-dashed lines.
The use of the buffer eliminates a need for the substrate to be
crystalline, allowing the use of the preferred copper substrate.
The crystalline orientation of the sensor layer (c-axis
orientation) is comparable to that of the buffer layer, i.e.,
between about 10 degrees and about 45 degrees but more probably
between about 15-degrees and about 40-degrees. The inclination
angles for the buffer and sensor layers can be about the same or
somewhat different angles within the stated ranges.
The material of the sensor-layer is a material selected from the
group of thermoelectric materials consisting of dysprosium barium
cuprate (DyBa.sub.2Cu.sub.3O.sub.7-d, often abbreviated to DyBCO),
strontium sodium cobaltate (Sr.sub.0.3Na.sub.0.2CoO.sub.2), and
strontium cobaltate (Sr.sub.3Co.sub.4O.sub.9). Dysprosium barium
cuprate is most preferred. A preferred thickness for sensor layer
36 is between about 5 nanometers (nm) and about 500 nm. This
thickness is less than that of the buffer layer and is required for
creating a high thermal gradient across the sensor layer.
Optionally, a layer 50 is deposited for protecting the sensor layer
from environmental degradation. Such a protection layer is critical
when DyBCO is used for sensor layer 36. Preferred materials for the
protection layer include MgO, and silicon dioxide (SiO.sub.2). In
the absence of a protective layer, the thermoelectric properties of
DyBCO will degrade over a relatively quick time with exposure to
ambient oxygen and elevated temperatures. Similarly, strontium
cobaltate and strontium sodium cobaltate are degraded by exposure
to atmospheric humidity. A preferred thickness for protective layer
50 is between about 0.2 .mu.m and about 2.0 .mu.m.
An optically black radiation-absorbing layer 42 is grown on
protective layer 50. The absorption spectrum of this layer
essentially determines the spectral response of the inventive
transverse thermoelectric radiation sensor. Suitable materials for
layer 42 include boron carbide, titanium nitride, chromium oxide,
gold black, or carbon. The absorption layer preferably has a
thickness between about 0.5 .mu.m and about 5.0 .mu.m. Whatever the
selected material, layer 42 is preferably made sufficiently thick
such that about 95% or greater of radiation is absorbed and
converted to heat within the absorption layer. Incomplete
absorption in layer 42 results in less than optimum thermoelectric
response signal, and can result in a non-linear response.
When the radiation-absorber layer is heated by incident radiation a
thermal gradient is formed across sensor layer 36 between the
radiation-absorber layer and copper substrate 32. Because of a high
anisotropy of the thermoelectric properties of sensor layer 36
resulting from the tilted crystal-axis, heat flow across the
thickness of the sensor layer, generates an electric field in the
sensor layer perpendicular (transverse to) to the heat-flow
(thermal-gradient) direction. This transverse electric field
results from significantly different values of Seebeck coefficients
in the crystalline a-b and c directions for the sensor-layer
material.
Elongated electrodes 38 and 40, parallel to each other and spaced
apart, are deposited on sensor layer 36 in electrical contact
therewith. Suitable materials for the electrodes include gold (Au),
platinum (Pt), silver (Ag), and palladium (Pd). The transverse
electric field between the electrodes results in a voltage between,
the electrodes, linearly proportional to the incident radiation
power on the absorbing layer. This voltage can be approximated by
an equation:
.times..times..DELTA..times..times..function..times..function..times..tim-
es..alpha. ##EQU00001## where V.sub.x is the voltage produced
between the first electrode 38 and the second electrode 40; t is
the thickness of sensor-layer 36, .DELTA.T.sub.z is the temperature
differential across sensor layer 36; .alpha. is the tilt angle of
the crystalline c-axis of layer 36; S.sub.ab and S.sub.c are the
Seebeck coefficients in respectively the a-b and c crystal
directions of the sensor layer; and L is the diameter of the
incident beam of laser radiation.
FIG. 2 is plan-view from above schematically a preferred
arrangement of sensor layer 36 in which the sensor layer is
patterned into a plurality of strips 36A, each thereof extending
between electrodes 38 and 40. The width of the strips is designated
as W.sub.1 and the width of the gaps between the strips is
designated W.sub.2. Here, the strips are aligned parallel to the
c-axis direction of the sensor layer. The strips can be formed by
photolithography and wet-etching of a continuous layer of
thermoelectric material. Layer 36 can be defined for purposes of
this description and the appended claims as a sensor-element, which
term applies to continuous sensor-layer and or a layer patterned
into the parallel strips of FIG. 2 or some other pattern.
In one example of the inventive detector, strips (c-axis aligned)
of DyBCO having a width W.sub.1 of about 300 .mu.m, with gaps
W.sub.2 of about 50 .mu.m therebetween, with a length between
electrodes of about 33 mm and a width of about 32 mm across the
pattern of strips, provided a thermoelectric signal of about 100
microvolts (.mu.V) when the detector was irradiated by carbon
dioxide (CO.sub.2) laser-radiation having a power of about 100
Watts (W). Without patterning, i.e., with sensor-element 36 as a
continuous sheet between the electrodes, the thermoelectric signal
voltage was about 35 .mu.V.
In another example of the inventive detector, with dimensions as in
the above example, but with strips 36A aligned at 45-degrees to the
c-axis direction, the thermoelectric signal was about 60 .mu.V. In
yet another example, with 45-degree aligned strips, but with
W.sub.1 and W.sub.2 each about 100 .mu.m, the thermoelectric signal
was about 61 .mu.V. These exemplary results indicate that, for a
given active area of the detector, the thermoelectric signal is
dependent on the alignment of sensor-material strips with the
crystalline c-axis of the thermoelectric material, but may not be
sensitive to the width of the strips and gaps therebetween. Indeed,
strip-width to gap ratios from 1 to 6 were tested with no
significant change observed in thermoelectric response.
FIG. 3 is a graph schematically illustrating thermoelectric signal
voltage as a function of incident CW CO.sub.2 laser power for an
example of the inventive detector having a DyBCO sensor-element
patterned as depicted in FIG. 2. Again, the active area is 33
mm.times.32 mm. It can be seen by comparing individual data points
(circles) with the best-fit straight line that the sensor response
is very linear.
FIG. 4 is a graph schematically illustrating peak thermoelectric
voltage (circles) and reflected energy (diamond) as a function of
incident pulse-energy for the detector example of FIG. 3 responsive
to incident 10 nanosecond (ns) pulses from a 1064-nm solid state
laser. The solid straight line in the graph of FIG. 4 is a best-fit
to the circle (peak-voltage) data-points, indicating the same high
degree of linearity of response experienced with CW radiation as in
the graph of FIG. 3.
FIG. 5 is a graph schematically illustrating thermoelectric signal
as a function of time for one of the pulses of the graph of FIG. 4.
The response-time (rise-time) of the signal is about 640
nanoseconds, which is comparable to the response of a photodiode
detector.
The above-described patterning of sensor layer 36 not only improves
sensitivity of the inventive detector but also the spatial
uniformity of the sensitivity. Normalized spatial distribution of
sensitivity of the detector of FIGS. 3 and 4 is schematically
depicted in FIG. 6. It can be seen that the spatial uniformity over
most of the useful area of the detector is about .+-.5%. The
spatial uniformity for the same detector without a patterned sensor
layer was about .+-.20% over the same region.
Regarding power-handling capability of the inventive detector, for
any particular substrate and buffer layer, this will be determined
by the selection of the sensor-layer material. By way of example,
cuprates, such as dysprosium barium cuprate, have a maximum service
temperature of .ltoreq.350.degree. C. Based on heat Transfer
calculations it is estimated that a detector using dysprosium
barium cuprate as a sensor-material will be limited to measuring
radiation power up to about 2 kilowatts (kW). Cobaltate transverse
thermoelectric materials, such as strontium cobaltate, in principle
have service temperatures .gtoreq.350.degree. C. and should allow
measurement of laser power greater than 2 kW.
In summary, the present invention is described in terms of a
preferred and other embodiments. The invention is not limited,
however, to the embodiments described and depicted herein. Rather
the invention is limited only by the claims appended hereto.
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