U.S. patent application number 14/936275 was filed with the patent office on 2017-01-05 for apparatus for real-time non-contact non-destructive thickness measurement using terahertz wave.
The applicant listed for this patent is KOREA RESEARCH INSTITUTE OF STANDARDS AND SCIENCE. Invention is credited to Ji Sang Yahng, Dae-Su Yee.
Application Number | 20170003116 14/936275 |
Document ID | / |
Family ID | 57419892 |
Filed Date | 2017-01-05 |
United States Patent
Application |
20170003116 |
Kind Code |
A1 |
Yee; Dae-Su ; et
al. |
January 5, 2017 |
APPARATUS FOR REAL-TIME NON-CONTACT NON-DESTRUCTIVE THICKNESS
MEASUREMENT USING TERAHERTZ WAVE
Abstract
Provided is an apparatus for real-time non-contact
non-destructive thickness measurement using a terahertz wave, and
more particularly, an apparatus for real-time non-contact
non-destructive thickness measurement using a terahertz wave, which
is capable of measuring a thickness of a sample by irradiating a
terahertz continuous wave, which is generated from a
wavelength-fixed laser and a wavelength-swept laser and of which
the frequency is changed at a high speed, to the sample and
measuring the terahertz wave transmitting or reflected from the
sample.
Inventors: |
Yee; Dae-Su; (Daejeon,
KR) ; Yahng; Ji Sang; (Daejeon, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KOREA RESEARCH INSTITUTE OF STANDARDS AND SCIENCE |
Daejeon |
|
KR |
|
|
Family ID: |
57419892 |
Appl. No.: |
14/936275 |
Filed: |
November 9, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01S 17/34 20200101;
G01B 9/02003 20130101; G01B 11/06 20130101; G01B 9/02084 20130101;
G01S 7/4802 20130101; G01B 9/02004 20130101 |
International
Class: |
G01B 11/06 20060101
G01B011/06 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 30, 2015 |
KR |
10-2015-0093296 |
Claims
1. An apparatus for real-time non-contact non-destructive thickness
measurement using a terahertz wave, comprising: a wavelength-fixed
laser generating first laser light having a first fixed wavelength
.lamda..sub.1; a wavelength-swept laser generating second laser
light having a second wavelength .lamda..sub.2 changed from a
preset minimum wavelength to a preset maximum wavelength at a high
speed for one period; a driver applying a voltage modulated at the
same frequency as a wavelength sweep rate to the wavelength-swept
laser to change the second wavelength from the minimum wavelength
to the maximum wavelength for the one period; a coupler coupling
the first laser light with the second laser light to form mixed
light and splitting the mixed light into first mixed light and
second mixed light; an emitter receiving the first mixed light
split from the coupler to output a terahertz wave having a
frequency f.sub.THz=|f.sub.1-f.sub.2| corresponding to a difference
between a frequency f.sub.1=c/.lamda..sub.1 (c is the speed of
light in vacuum) corresponding to the first wavelength
.lamda..sub.1 and a frequency f.sub.2=c/.lamda..sub.2 corresponding
to the second wavelength .lamda..sub.2; a sample irradiated with
the terahertz wave output from the emitter; a detector receiving
the second mixed light split from the coupler and the terahertz
wave transmitting or reflected from a sample to generate a
photocurrent; a data acquisition unit converting the photocurrent
into digital data to acquire and output the digital data; and a
calculator generating frequency-domain data from the output digital
data, performing fast Fourier transform on the frequency-domain
data to generate time-domain data, and calculating a thickness of
the sample on the basis of the time-domain data.
2. The apparatus of claim 1, wherein the one period is an inverse
number of the wavelength sweep rate which is equal to or more than
100 Hz.
3. The apparatus of claim 1, wherein the calculator uses d = c 2 (
.DELTA..tau. 2 - 2 .DELTA..tau. 1 ) ##EQU00006## as Equation
calculating the thickness of the sample, in case that the second
mixed light and the terahertz wave transmitting the sample are
input to the detector, wherein d: Thickness of the sample, c: Speed
of light in vacuum, .DELTA..tau..sub.1: Difference between time
taken to propagate the terahertz wave without transmitting the
sample and time taken to propagate the terahertz wave by once
transmitting the sample, and .DELTA..tau..sub.2: Time taken for the
terahertz wave to once reciprocate in the sample.
4. The apparatus of claim 3, wherein the calculator finds out a
time delay when the terahertz wave is propagated without
transmitting the sample, in the time-domain data measured without
transmitting the sample, and a time delay when the terahertz wave
is propagated after once transmitting the sample and a time delay
when the terahertz wave is propagated by once transmitting the
sample after the terahertz wave once reciprocates in the sample, in
the time-domain data measured with transmitting the sample, in case
that the second mixed light and the terahertz wave transmitting the
sample are input to the detector to use the .DELTA..tau..sub.1
which is a value obtained by subtracting the time delay when the
terahertz wave is propagated without transmitting the sample from
the time delay when the terahertz wave is propagated after once
transmitting the sample and the .DELTA..tau..sub.2 which is a value
obtained by subtracting the time delay when the terahertz wave is
propagated after once transmitting the sample from the delay time
when the terahertz wave is propagated by once transmitting the
sample after the terahertz wave once reciprocates in the
sample.
5. The apparatus of claim 1, wherein the calculator uses d = c 2 n
g .DELTA..tau. 2 ##EQU00007## as Equation calculating the thickness
of the sample, in case that the second mixed light and the
terahertz wave reflected from the sample are input to the detector,
wherein d: Thickness of the sample, n.sub.g: Group refractive index
of the sample, c: Speed of light in vacuum, and .DELTA..tau..sub.2:
Time taken for the terahertz wave to once reciprocate in the
sample.
6. The apparatus of claim 5, wherein the calculator finds out a
time delay when the terahertz wave is reflected from a surface of
the sample and a time delay when the terahertz wave is reflected
from a surface opposite to the surface after once transmitting the
sample in the time-domain data measured after the terahertz wave is
reflected from the sample, in case that the second mixed light and
the terahertz wave reflected from the sample are input to the
detector to use the .DELTA..tau..sub.2 which is a value obtained by
subtracting the time delay when the terahertz wave is reflected
from the surface of the sample from the time delay when the
terahertz wave is reflected from the surface opposite to the
surface after once transmitting the sample.
7. The apparatus of claim 1, wherein the first wavelength of the
first laser light generated by the wavelength-fixed laser is fixed
to 1545 nm, the minimum wavelength and the maximum wavelength of
the second wavelength of the second laser light generated by the
wavelength-swept laser are each 1544 nm and 1558 nm, and the one
period for which the second wavelength is changed from the minimum
wavelength to the maximum wavelength is 1 ms.
8. The apparatus of claim 1, further comprising: a variable time
delay tool disposed between the coupler and the detector to
variably time-delay the second mixed light split from the coupler
and input the variably time-delayed second mixed light to the
detector.
9. The apparatus of claim 1, further comprising: a variable time
delay tool disposed between the coupler and the emitter to variably
time-delay the first mixed light split from the coupler and input
the variably time-delayed first mixed light to the emitter.
10. The apparatus of claim 1, further comprising: an amplifier
disposed between the detector and the data acquisition unit to
amplify the photocurrent generated from the detector and transfer
the amplified photocurrent to the data acquisition unit.
11. The apparatus of claim 1, wherein the data acquisition unit
converts the photocurrent into the digital data to acquire the
digital data for the one period (inverse number of the wavelength
sweep rate) while being triggered by a synchronous signal of the
same frequency as the wavelength sweep rate provided from the
driver.
12. The apparatus of claim 1, wherein the data acquisition unit
repeats the acquisition of the digital data for the one period by a
preset number of averaged traces and provides the repeatedly
acquired digital data traces to the calculator and the calculator
averages the repeatedly acquired digital data traces to improve a
signal to noise ratio.
13. The apparatus of claim 1, wherein the wavelength-fixed laser is
a DFB-LD.
14. The apparatus of claim 1, wherein the wavelength-swept laser or
the wavelength-fixed laser further includes an optical fiber
amplifier disposed at an output terminal to amplify an optical
power of the first laser light or the second laser light.
15. The apparatus of claim 1, further comprising: an off-axis
parabolic mirror or a lens disposed on a path of the terahertz wave
from the emitter to the detector.
16. The apparatus of claim 1, further comprising: a beam splitter
disposed on a path of the terahertz wave from the emitter to the
detector.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority under 35 U.S.C. .sctn.119
to Korean Patent Application No. 10-2015-0093296, filed on Jun. 30,
2015, in the Korean Intellectual Property Office, the disclosure of
which is incorporated herein by reference in its entirety.
TECHNICAL FIELD
[0002] The following disclosure relates to an apparatus for
thickness measurement using a terahertz wave, and more
particularly, to an apparatus for real-time non-contact
non-destructive thickness measurement using a terahertz wave, which
is capable of measuring a thickness of a sample by irradiating a
terahertz continuous wave, which is generated from a
wavelength-fixed laser and a wavelength-swept laser and of which
the frequency is changed at a high speed, to the sample and
measuring the terahertz wave transmitting or reflected from the
sample.
BACKGROUND
[0003] A terahertz wave band (0.1 THz to 10 THz) transmits a
non-metallic material and a non-conductive material and has an
electromagnetic wave of very low energy as much as several meV and
therefore little affects a human body.
[0004] An example of the typical non-contact method for measuring a
thickness of non-metallic material may include an optical method,
which may not measure a thickness of material through which light
is not transmitted. On the other hand, the terahertz wave may
transmit the non-metallic material, and therefore a method for
measuring a thickness of a non-metallic material through which
light is not transmitted may use a terahertz wave.
[0005] Further, a resonance frequency of very various molecules is
distributed in the terahertz wave band. In this case, these
molecules are identified in real-time by a non-destructive,
non-opened, non-contact method. As a result, it is expected that a
new-conceptual analysis technology which does not exist now in
medical treatment, medical science, agricultural food,
environmental measurement, bio, advanced material evaluation, etc.,
may be implemented.
[0006] Therefore, researches to develop and use wave sources
operated in a frequency band of 0.1 to 10 THz which is called a THz
gap region have been conducted.
[0007] As the related art document disclosing a method for
generating a terahertz continuous wave of which the frequency is
changed at a high speed using a wavelength-fixed laser and a
wavelength-swept laser, there is Korean Patent No. 10-1453472.
[0008] However, the method suggests only a technology of merely
generating a terahertz wave but does not suggest a technology of
measuring a thickness of a sample using the generated terahertz
wave.
RELATED ART DOCUMENT
Patent Document
[0009] 1. Korean Patent No. 10-1453472 (Published on Oct. 21,
2014)
SUMMARY
[0010] An embodiment of the present invention is directed to
providing an apparatus for real-time non-contact non-destructive
thickness measurement using a terahertz wave, which is capable of
measuring a thickness of a sample by irradiating a terahertz
continuous wave, which is generated from a wavelength-fixed laser
and a wavelength-swept laser and of which the frequency is changed
at a high speed, to the sample and measuring the terahertz wave
transmitting or reflected from the sample.
[0011] Other objects of the present invention may be easily
understood based on the following description of embodiments.
[0012] In one general aspect, an apparatus for real-time
non-contact non-destructive thickness measurement using a terahertz
wave includes: a wavelength-fixed laser generating first laser
light having a first fixed wavelength .lamda.1; a wavelength-swept
laser generating second laser light having a second wavelength
.lamda.2 changed from a preset minimum wavelength to a preset
maximum wavelength at a high speed for one period; a driver
applying a voltage modulated at the same frequency as a wavelength
sweep rate to the wavelength-swept laser to change the second
wavelength from the minimum wavelength to the maximum wavelength
for the one period; a coupler coupling the first laser light with
the second laser light to form mixed light and splitting the mixed
light into first mixed light and second mixed light; an emitter
receiving the first mixed light split from the coupler to output
the terahertz wave having a frequency f.sub.THz=|f.sub.1-f.sub.2|
corresponding to a difference between a frequency
f.sub.1=c/.lamda..sub.1 (c is the speed of light in vacuum)
corresponding to the first wavelength .lamda..sub.1 and a frequency
f.sub.2=c/.lamda..sub.2 corresponding to the second wavelength
.lamda..sub.2; a sample irradiated with the terahertz wave output
from the emitter; a detector receiving the second mixed light split
from the coupler and the terahertz wave transmitting or reflected
from a sample to generate a photocurrent; a data acquisition unit
converting the photocurrent into digital data to acquire and output
the digital data; and a calculator generating frequency-domain data
from the output digital data, performing fast Fourier transform on
the frequency-domain data to generate time-domain data, and
calculating a thickness of the sample on the basis of the
time-domain data.
[0013] The one period may be an inverse number of the wavelength
sweep rate which is equal to or more than 100 Hz.
[0014] The calculator may use
d = c 2 ( .DELTA..tau. 2 - 2 .DELTA..tau. 1 ) ##EQU00001##
as Equation calculating the thickness of the sample, in case that
the second mixed light and the terahertz wave transmitting the
sample are input to the detector.
[0015] d: Thickness of the sample
[0016] c: Speed of light in vacuum
[0017] .DELTA..tau..sub.1: Difference between time taken to
propagate the terahertz wave without transmitting the sample and
time taken to propagate the terahertz wave by once transmitting the
sample
[0018] .DELTA..tau..sub.2: Time taken for the terahertz wave to
once reciprocate in the sample
[0019] The calculator may find out a time delay when the terahertz
wave is propagated without transmitting the sample, in the
time-domain data measured without transmitting the sample, and a
time delay when the terahertz wave is propagated after once
transmitting the sample, and a time delay when the terahertz wave
is propagated by once transmitting the sample after the terahertz
wave once reciprocates in the sample, in the time-domain data
measured with transmitting the sample, in case that the second
mixed light and the terahertz wave transmitting the sample are
input to the detector to use the .DELTA..tau..sub.1 which is a
value obtained by subtracting the time delay when the terahertz
wave is propagated without transmitting the sample from the time
delay when the terahertz wave is propagated after once transmitting
the sample and the .DELTA..tau..sub.2 which is a value obtained by
subtracting the time delay when the terahertz wave is propagated
after once transmitting the sample from the delay time when the
terahertz wave is propagated by once transmitting the sample after
the terahertz wave once reciprocates in the sample.
[0020] The calculator may use
d = c 2 n g .DELTA..tau. 2 ##EQU00002##
as Equation calculating the thickness of the sample, in case that
the second mixed light and the terahertz wave reflected from the
sample are input to the detector.
[0021] d: Thickness of the sample
[0022] n.sub.g: Group refractive index of the sample
[0023] c: Speed of light in vacuum
[0024] .DELTA..tau..sub.2: Time taken for the terahertz wave to
once reciprocate in the sample
[0025] The calculator may find out a time delay when the terahertz
wave is reflected from a surface of the sample and a time delay
when the terahertz wave is reflected from a surface opposite to the
surface after once transmitting the sample in the time-domain data
measured after the terahertz wave is reflected from the sample, in
case that the second mixed light and the terahertz wave reflected
from the sample are input to the detector to use the
.DELTA..tau..sub.2 which is a value obtained by subtracting the
time delay when the terahertz wave is reflected from the surface of
the sample from the time delay when the terahertz wave is reflected
from the surface opposite to the surface after once transmitting
the sample.
[0026] The first wavelength of the first laser light generated by
the wavelength-fixed laser may be fixed to 1545 nm, the minimum
wavelength and the maximum wavelength of the second wavelength of
the second laser light generated by the wavelength-swept laser may
be each 1544 nm and 1558 nm, and the one period for which the
second wavelength is changed from the minimum wavelength to the
maximum wavelength may be 1 ms.
[0027] The apparatus may further include: a variable time delay
tool disposed between the coupler and the detector to variably
time-delay the second mixed light split from the coupler and input
the variably time-delayed second mixed light to the detector.
[0028] The apparatus may further include: a variable time delay
tool disposed between the coupler and the emitter to variably
time-delay the first mixed light split from the coupler and input
the variably time-delayed first mixed light to the emitter.
[0029] The apparatus may further include: an amplifier disposed
between the detector and the data acquisition unit to amplify the
photocurrent generated from the detector and transfer the amplified
photocurrent to the data acquisition unit.
[0030] The data acquisition unit may convert the photocurrent into
the digital data to acquire the digital data for the one period
(inverse number of the wavelength sweep rate) while being triggered
by a synchronous signal of the same frequency as the wavelength
sweep rate provided from the driver.
[0031] The data acquisition unit may repeat the acquisition of the
digital data for the one period by a preset number of averaged
traces and provide the repeatedly acquired digital data traces to
the calculator and the calculator may average the repeatedly
acquired digital data traces to improve a signal to noise
ratio.
[0032] The wavelength-fixed laser may be a DFB-LD.
[0033] The wavelength-swept laser or the wavelength-fixed laser may
further include: an optical fiber amplifier disposed at an output
terminal to amplify an optical power of the first laser light or
the second laser light.
[0034] The apparatus may further include: an off-axis parabolic
mirror or a lens disposed on a path of the terahertz wave from the
emitter to the detector.
[0035] The apparatus may further include: a beam splitter disposed
on a path of the terahertz wave from the emitter to the
detector.
[0036] Other features and aspects will be apparent from the
following detailed description, the drawings, and the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] FIG. 1 is a block diagram of an apparatus for real-time
non-contact non-destructive thickness measurement using a terahertz
wave according to an exemplary embodiment of the present
invention.
[0038] FIG. 2 is a graph illustrating the improvement result in
signal to noise ratio by repeatedly measuring data by the apparatus
for real-time non-contact non-destructive thickness measurement
using a terahertz wave according to the exemplary embodiment of the
present invention.
[0039] FIG. 3A is a graph illustrating frequency-domain data
measured for various time delays by the apparatus for real-time
non-contact non-destructive thickness measurement using a terahertz
wave according to the exemplary embodiment of the present
invention.
[0040] FIG. 3B is a graph illustrating time-domain data generated
by performing fast Fourier transform on the frequency-domain data
measured for various time delays by the apparatus for real-time
non-contact non-destructive thickness measurement using a terahertz
wave according to the exemplary embodiment of the present
invention.
[0041] FIG. 4A is a graph illustrating the time-domain data
measured by the apparatus for real-time non-contact non-destructive
thickness measurement using a terahertz wave according to the
exemplary embodiment of the present invention.
[0042] FIG. 4B is a diagram for describing a method for calculating
a thickness of a sample by the apparatus for real-time non-contact
non-destructive thickness measurement using a terahertz wave
according to the exemplary embodiment of the present invention.
[0043] FIG. 5A is a block diagram of an apparatus for real-time
non-contact non-destructive thickness measurement using a terahertz
wave according to another exemplary embodiments of the present
invention.
[0044] FIG. 5B is a diagram for describing a method for calculating
a thickness of a sample by the apparatus for real-time non-contact
non-destructive thickness measurement using a terahertz wave
according to another exemplary embodiment of the present
invention.
DETAILED DESCRIPTION OF MAIN ELEMENTS
[0045] 100: Wavelength-fixed laser [0046] 200: Wavelength-swept
laser [0047] 300: Coupler [0048] 400: Emitter [0049] 500: Sample
[0050] 510: Off-axis parabolic mirror [0051] 511: Lens [0052] 520:
Beam splitter [0053] 600: Detector [0054] 610: Variable time delay
tool [0055] 620: Amplifier [0056] 700: Data acquisition unit [0057]
710: Time delay when terahertz wave is propagated without
transmitting sample [0058] 711: Terahertz wave propagated without
transmitting sample [0059] 720: Time delay when terahertz wave is
propagated after once transmitting sample [0060] 721: Terahertz
wave propagated after once transmitting sample [0061] 730: Time
delay when terahertz wave is propagated by once transmitting sample
after once reciprocating in sample [0062] 731: Terahertz wave
propagated by once transmitting sample after once reciprocating in
sample [0063] 741: Terahertz wave reflected from surface of sample
[0064] 751: Terahertz wave reflected from surface opposite to
surface after once transmitting sample [0065] 800: Calculator
[0066] 900: Driver
DETAILED DESCRIPTION OF EMBODIMENTS
[0067] Since the present invention may be variously modified and
have several exemplary embodiments, specific exemplary embodiments
will be shown in the accompanying drawings and be described in
detail in a detailed description. However, it is to be understood
that the present invention is not limited to the specific exemplary
embodiments, but includes all modifications, equivalents, and
substitutions included in the spirit and the scope of the present
invention.
[0068] Throughout the accompanying drawings, the same reference
numerals will be used to describe the same components. Further,
when it is determined that the detailed description of the known
art related to the present invention may obscure the gist of the
present invention, the detailed description thereof will be
omitted.
[0069] Hereinafter, exemplary embodiments of the present invention
will be described in detail with reference to the accompanying
drawings.
[0070] In the case of using a terahertz pulse wave to measure a
thickness, an expensive femtosecond laser needs to be used. In the
case of using a terahertz continuous wave to measure a thickness,
costs are reduced, but data need to be measured while changing a
frequency of the terahertz continuous wave and therefore
measurement time may be long. To overcome the above problem, as one
of the two lasers used to generate the terahertz continuous wave, a
wavelength-swept laser is used. That is, frequency-domain data are
measured at a high speed while a frequency of the terahertz
continuous wave is changed at a high speed, and thus a thickness
may be measured by a real-time non-contact non-destructive manner
based on signal processing and calculation.
[0071] FIG. 1 is a block diagram of an apparatus for real-time
non-contact non-destructive thickness measurement using a terahertz
wave according to an exemplary embodiment of the present invention.
Referring to FIG. 1, an apparatus for real-time non-contact
non-destructive thickness measurement using a terahertz wave
according to an exemplary embodiment of the present invention may
include a wavelength-fixed laser 100, a wavelength-swept laser 200,
a coupler 300, an emitter 400, a sample 500, an off-axis parabolic
mirror 510, a detector 600, a variable time delay tool 610, an
amplifier 620, a data acquisition unit 700, a calculator 800, and a
driver 900.
[0072] The wavelength-fixed laser 100 generates first laser light
having a first fixed wavelength. The wavelength-fixed laser may be
a distributed feedback laser diode (DFB-LD). To obtain the
terahertz continuous wave of a wide band frequency, it is
preferable to fix the first wavelength to be close to a threshold
value (minimum value or maximum value) in a varying section of a
second wavelength. In detail, when the varying section of the
second wavelength ranges from 1544 nm to 1558 nm, the first
wavelength may be 1545 nm.
[0073] The wavelength-swept laser 200 generates second laser light
having the second wavelength changed at a high speed.
[0074] The driver 900 applies a voltage modulated at the same
frequency as a wavelength sweep rate to the wavelength-swept laser
to change the second wavelength from a preset minimum wavelength to
a preset maximum wavelength based on an inverse number of the
wavelength sweep rate as a period. When optical power of the
wavelength-swept laser and the wavelength-fixed laser is small,
optical fiber amplifiers may be disposed at each output terminal to
amplify the optical power.
[0075] The coupler 300 may couple the first laser light with the
second laser light to form mixed light and split the mixed light
into first mixed light and second mixed light.
[0076] The emitter 400 may transform the first mixed light split
from the coupler into the terahertz wave. When the emitter 400 is a
photomixer, the emitter 400 may include a photoconductor and an
antenna. The photoconductor transforms the mixed light into a
photocurrent which may emit the terahertz wave through the antenna.
The emitter 400 may use a beating phenomenon to generate the
terahertz wave having a frequency f.sub.THz=|f.sub.1-f.sub.2|
corresponding to a difference between a frequency
f.sub.1=c/.lamda..sub.1 (c is the speed of light in vacuum)
corresponding to the first wavelength .lamda.1 and a frequency
f.sub.2=c/.lamda..sub.2 corresponding to the second wavelength
.lamda..sub.2. Therefore, the frequency of the terahertz wave
generated by the first fixed wavelength and the second wavelength
changed at a high speed may be changed at a high speed. A frequency
sweep rate of the terahertz wave is equal to the wavelength sweep
rate of the wavelength-swept laser 320 and may rely on a wavelength
sweep period which is the inverse number of the wavelength sweep
rate. The sweep rate may range from hundreds of Hz to several kHz.
When a sweep period of the second wavelength is 1 ms, the sweep
rate may be 1 kHz.
[0077] The sample 500 is an object of which the thickness is to be
measured, and preferably, may be a non-metallic material and a
non-conductive material.
[0078] The off-axis parabolic mirror 510 changes a direction of the
terahertz wave generated from the emitter and may let the terahertz
wave reach the detector through collimation and focusing. When
there is no need to change an optical path of the terahertz wave,
as illustrated in FIG. 5, a lens 511 may be used instead of the
off-axis parabolic mirror 510.
[0079] The detector 600 is input with the second mixed light split
from the coupler and the terahertz wave transmitting the sample and
a photocarrier excited by the second mixed light in the detector
600 is biased by an electric field of the terahertz wave to
generate a photocurrent. When the time delay is excessively
increased due to a difference between lengths of the two optical
paths from the coupler to the detector, coherence between the
terahertz wave and the second mixed light may be reduced in the
detector. Therefore, the time delay may be appropriately controlled
by using the variable time delay tool 610 which may time-delay the
first mixed light or the second mixed light to keep the coherence
between the terahertz wave and the second mixed light in the
detector 600.
[0080] Here, the apparatus for real-time non-contact
non-destructive thickness measurement using a terahertz wave may
include an amplifier 620 which amplifies the photocurrent output
from the detector 600 and transfers the amplified photocurrent to
the data acquisition unit 700.
[0081] The data acquisition unit 700 converts the photocurrent into
the digital data to acquire the digital data for one period
(inverse number of the wavelength sweep rate) while being triggered
by a synchronous signal of the same frequency as the wavelength
sweep rate provided from the driver 900. The data acquisition unit
700 provides the acquired data to the calculator 800. Further, the
data acquisition unit 700 may repeatedly acquire the digital data
for one period by a preset number of averaged traces and provide
the repeatedly acquired digital data traces to the calculator 800
and the calculator 800 may average the repeatedly acquired digital
data traces to improve a signal to noise ratio and use the
resulting digital data to calculate a thickness of the sample.
[0082] FIG. 2 is a graph illustrating the improvement result in
signal to noise ratio by repeatedly measuring digital data by the
apparatus for real-time non-contact non-destructive thickness
measurement using a terahertz wave according to the exemplary
embodiment of the present invention. That is, in the apparatus for
real-time non-contact non-destructive thickness measurement using a
terahertz wave according to an exemplary embodiment of the present
invention, the signal to noise ratio is improved when the
calculator 800 averages the digital data traces repeatedly acquired
by the data acquisition unit 700. Referring to FIG. 2, when the
number of averaged traces is increased, it may be appreciated that
the signal to noise ratio of the resulting digital data is
improved. However, a measurement time may be increased in
proportion to the increasing number of averaged traces.
[0083] Referring back to FIG. 1, the calculator 800 may transform
the digital data acquired by the data acquisition unit 700 into the
frequency-domain data. For this purpose, the change in the second
wavelength of the wavelength-swept laser 200 over time for one
period needs to be measured in advance. The change .lamda..sub.2(t)
in the second wavelength of the wavelength-swept laser over time
for one period may be measured by a Fabry-Perot interferometer or a
Mach-Zehnder interferometer. The change f.sub.THz
(t)=|c/.lamda..sub.1-c/.lamda..sub.2 (t)| in the frequency of the
terahertz wave over time for one period may be found by the
previously measured change in the second wavelength over time for
one period. The calculator 800 may transform the digital data y(t)
into the frequency-domain data y (f.sub.THz) by using the change in
the frequency of the terahertz wave over time for one period.
Further, the calculator may perform the fast Fourier transform on
the frequency-domain data to generate the time-domain data Y(.tau.)
and calculate the thickness of the sample on the basis of the
time-domain data.
[0084] Hereinafter, a principle of calculating the thickness using
the time-domain data will be described.
[0085] FIG. 3A is a graph illustrating the frequency-domain data
measured for various time delays by the apparatus for real-time
non-contact non-destructive thickness measurement using a terahertz
wave according to the exemplary embodiment of the present invention
and FIG. 3B is a graph illustrating the time-domain data measured
for various time delays by the apparatus for real-time non-contact
non-destructive thickness measurement using a terahertz wave
according to the exemplary embodiment of the present invention.
That is, in the apparatus for real-time non-contact non-destructive
thickness measurement using a terahertz wave according to the
exemplary embodiment of the present invention, when a terahertz
wave reaches the detector 600 without transmitting the sample with
time delays of 10 ps, 20 ps, 40 ps, and 80 ps set by the variable
time delay tool 610, the graph (FIG. 3A) illustrates the
frequency-domain data generated by allowing the calculator 800 to
transform the digital data acquired by the data acquisition unit
700 and the graph (FIG. 3B) illustrates the time-domain data
generated by performing the fast Fourier transform on the
frequency-domain data. Unlike FIG. 3A illustrating the
frequency-domain data measured by a transmission-type thickness
measurement apparatus according to the exemplary embodiment of the
present invention, it may be appreciated from FIG. 3B illustrating
the time-domain data generated by performing the fast Fourier
transform on the frequency-domain data that a peak is generated at
a position of the delayed time and thus the delay time information
may be easily extracted. The delay time occurs due to the
difference between the lengths of the two optical paths from the
coupler to the detector. The time delay may depend on the thickness
of the sample when the sample is present in the path from the
emitter 400 to the detector 600.
[0086] FIG. 4A is a graph illustrating the time-domain data
measured by the apparatus for real-time non-contact non-destructive
thickness measurement using a terahertz wave according to the
exemplary embodiment of the present invention and FIG. 4B is a
diagram for describing a method for calculating the thickness of
the sample by the apparatus for real-time non-contact
non-destructive thickness measurement using a terahertz wave
according to the exemplary embodiment of the present invention.
[0087] Referring to FIG. 4A, in the apparatus for real-time
non-contact non-destructive thickness measurement using a terahertz
wave according to the exemplary embodiment of the present
invention, when the sample 500 is not present in the path from the
emitter 400 to the detector 600, the time-domain data generated by
the calculator 800 is indicated by a red line. Referring to FIG.
4B, this condition may be a condition in which a terahertz wave 711
propagated without transmitting the sample reaches the detector
600. Referring back to FIG. 4A, in the apparatus for real-time
non-contact non-destructive thickness measurement using a terahertz
wave according to the exemplary embodiment of the present
invention, when the sample 500 is disposed in the path from the
emitter 400 to the detector 600, the time-domain data generated by
the calculator 800 is indicated by a blue line. This condition may
include the condition that in FIG. 4B, a terahertz wave 721
propagated after once transmitting the sample and a terahertz wave
731 propagated by once transmitting the sample after once
reciprocating in the sample reach the detector 600 at a time
difference. Referring to FIG. 4A, information on a time delay 710
when the terahertz wave is propagated without transmitting the
sample, a time delay 720 when the terahertz wave is propagated
after once transmitting the sample, and a time delay 730 when the
terahertz wave is propagated by once transmitting the sample after
once reciprocating in the sample may be obtained from the
time-domain data illustrated by the graph. That is, when in the
graph, amplitude of the time-domain data is set to be a y axis and
time is set to be an x axis, each time delay 710, 720, or 730 may
be obtained from an x coordinate of a point at which the amplitude
has a local maximum.
[0088] A method for calculating the thickness of the sample from
the time-domain data measured by using the apparatus for real-time
non-contact non-destructive thickness measurement using a terahertz
wave according to the exemplary embodiment of the present invention
includes obtaining the time delay 710 when the terahertz wave is
propagated without transmitting the sample, the time delay 720 when
the terahertz wave is propagated after once transmitting the
sample, and the time delay 730 when the terahertz wave is
propagated by once transmitting the sample after once reciprocating
in the sample from the time-domain data and obtaining the thickness
of the sample from the time delays 710, 720, and 730.
[0089] First, the finding out of the time delays from the
time-domain data will be described. First, the time-domain data are
obtained without disposing the sample 500 in the path from the
emitter 400 to the detector 600 and then local maxima are
investigated to find out the local maximum having the largest
value, and the time delay 710 when the terahertz wave is propagated
without transmitting the sample is determined. Further, the sample
500 is disposed in the path from the emitter 400 to the detector
600, the time-domain data are obtained, local maxima are
investigated to find out the local maximum having the largest value
among the local maxima, the time delay 720 when the terahertz wave
is propagated after once transmitting the sample is determined, the
local maximum having the second largest value among the local
maxima is found, and the time delay 730 when the terahertz wave is
propagated by once transmitting the sample after once reciprocating
in the sample is determined.
[0090] Next, the obtaining of the thickness of the sample from the
found times delays will be described. When .DELTA..sub..tau.1 is
the difference between the time taken to propagate the terahertz
wave without transmitting the sample and the time taken to
propagate the terahertz wave by once transmitting the sample and
.DELTA..tau..sub.2 is the time taken for the terahertz wave to once
reciprocate in the sample, .DELTA..sub..tau.1 is a value obtained
by subtracting the time delay 710 when the terahertz wave is
propagated without transmitting the sample from the time delay 720
when the terahertz wave is propagated after once transmitting the
sample and .DELTA..tau..sub.2 is a value obtained by subtracting
the time delay 720 when the terahertz wave is propagated after once
transmitting the sample from the time delay 730 when the terahertz
wave is propagated by once transmitting the sample after once
reciprocating in the sample.
[0091] Referring to FIG. 4B, .DELTA..tau..sub.1 and
.DELTA..tau..sub.2 may be each represented by the following
Equations 1 and 2.
.DELTA..tau. 1 = ( n g - 1 ) d c [ Equation 1 ] .DELTA..tau. 2 = 2
n g d c [ Equation 2 ] ##EQU00003##
[0092] In the above Equations 1 and 2, d is the thickness of the
sample, c is the speed of light in vacuum, and n.sub.g is a group
refractive index of the sample. From the above Equations 1 and 2,
the thickness d of the sample may be represented by the following
Equation 3.
d = c 2 ( .DELTA..tau. 2 - 2 .DELTA..tau. 1 ) [ Equation 3 ]
##EQU00004##
[0093] Therefore, the thickness of the sample may be obtained by
the above Equation 3 and .DELTA..tau..sub.1 and
.DELTA..tau..sub.2.
[0094] As described above, the thickness may be measured in the
transmission mode where a terahertz wave transmits a sample, by
using the apparatus for real-time non-contact non-destructive
thickness measurement using a terahertz wave according to the
exemplary embodiment of the present invention.
[0095] Next, a reflection-type apparatus for real-time non-contact
non-destructive thickness measurement using a terahertz wave
according to another exemplary embodiment of the present invention
will be described.
[0096] In describing FIG. 5A, the same components as FIG. 1 have
the same configuration and operation principle and therefore the
description thereof will be omitted. Referring to FIG. 5A, the
terahertz wave emitted from the emitter 400 transmits the beam
splitter 520 to be reflected from the sample 500 and then is again
reflected from the beam splitter 520 to be input to the detector
600.
[0097] In this case, the lens 511 may be added between the beam
splitter 520 and the detector 600, which may serve to focus the
terahertz wave on the detector 600 similar to the off-axis
parabolic mirror 510.
[0098] Even in an apparatus for real-time non-contact
non-destructive thickness measurement using a terahertz wave
according to another exemplary embodiment of the present invention
of FIG. 5A, the calculator 800 may generate the time-domain data by
the same method as described above. Only the difference is that the
terahertz wave input to the detector may include the terahertz wave
reflected from the surface of the sample and the terahertz wave
reflected from the surface opposite to the surface by once
transmitting the sample as illustrated in FIG. 5B. The method for
calculating the thickness of the sample from the time-domain data
measured by using the apparatus for real-time non-contact
non-destructive thickness measurement using a terahertz wave
according to another exemplary embodiment of the present invention
includes obtaining a time delay 741 when the terahertz wave is
reflected from the surface of the sample and a time delay 751 when
the terahertz wave is reflected from the surface opposite to the
surface after once transmitting the sample from the generated
time-domain data and obtaining the thickness of the sample from the
time delays.
[0099] First, the finding out of the time delays from the
time-domain data will be described. Time-domain data are obtained
by the calculator 800 of the apparatus for real-time non-contact
non-destructive thickness measurement using a terahertz wave
according to another exemplary embodiment of the present invention
as illustrated in FIG. 5. In the obtained time-domain data, local
maxima are investigated to find out the local maximum having the
largest value among the local maxima, the time delay 741 when the
terahertz wave is reflected from the surface of the sample is
determined, the local maximum having the second largest value among
the local maxima is found, and the time delay 751 when the
terahertz wave is reflected from the surface opposite to the
surface after once transmitting the sample is determined.
[0100] Next, the obtaining of the thickness of the sample from the
found time delays will be described. When .DELTA..tau..sub.2 is the
time taken for the terahertz wave to once reciprocate in the
sample, .DELTA..tau..sub.2 is a value obtained by subtracting the
time delay when the terahertz wave is reflected from the surface of
the sample from the time delay when the terahertz wave is reflected
from the surface opposite to the surface after once transmitting
the sample.
[0101] .DELTA..tau..sub.2 may be represented by the above Equation
2, and therefore the thickness of the sample may be represented by
the following Equation 4.
d = c 2 n g .DELTA..tau. 2 [ Equation 4 ] ##EQU00005##
[0102] Therefore, when the group refractive index of the sample is
known in advance, the thickness of the sample may be obtained by
the .DELTA..tau..sub.2 found by the apparatus for real-time
non-contact non-destructive thickness measurement using a terahertz
wave according to another exemplary embodiment of the present
invention.
[0103] It may be appreciated from the foregoing description that
the thickness of the sample may be measured by using a
reflection-type thickness measurement apparatus.
[0104] As described above, according to the exemplary embodiments
of the present invention, it is possible to provide the apparatus
for real-time non-contact non-destructive thickness measurement
using a terahertz wave, which is capable of measuring a thickness
of a sample by irradiating a terahertz continuous wave, which is
generated from a wavelength-fixed laser and a wavelength-swept
laser and of which the frequency is changed at a high speed, to the
sample and measuring the terahertz wave transmitting or reflected
from the sample.
[0105] The apparatus for real-time non-contact non-destructive
thickness measurement using a terahertz wave according to the
exemplary embodiments of the present invention may measure the
thickness of the non-conductive material in the transmission mode
or the reflection mode, and preferably, may be expected to be
usefully used to measure the thickness of the paint coat on the
metal substrate made of an iron material, etc.
[0106] The foregoing description relates to the embodiments of the
present invention, but the claims of the present invention are not
limited to specific embodiments illustrated and described in the
present specification but various modified embodiments which may be
practiced by those skilled in the art without departing from the
subject described in the appending claims may be construed to be
included in the scope of the present invention.
* * * * *