- Email: [email protected]
Application of LCTV to nonlinear speckle correlator
Optics Communications 86 ( 1991 ) 513-522 North-Holland
OPTICS COMMUNICATIONS
Full length article
Application of LCTV to nonlinear speckle correlator Akifumi Ogiwara Graduate School of Electronic Science and Technology, Shizuoka University, 3-5-1Johoku, Hamamalsu 432, Japan
Hiro-aki Sakai and Junji Ohtsubo Faculty of Engineering, Shizuoka University, 3-5-1Johoku, Hamamatsu 432, Japan
Received 14 May 1991
The intensity and phase modulation characteristics of a liquid crystal television (LCTV) and its diffraction efficiency as a spatial light modulator are investigated. A nonlinear optical correlator using the phase modulation of an LCTV and the hard clipping property of an Optic RAM detector is proposed. The velocity measurement of a light scattering object is conducted by a real-time speckle clipping correlator based on the proposed method.
1. Introduction In many applications o f laser speckles, for example, such as the displacement and velocity measurements of a light scattering object, one o f the recent concerns is the realization o f the system for a fast calculation of the speckle correlation function. Bates et al. have applied a liquid crystal television (LCTV) to a real-time speckle metrology as a spatial light intensity modulator [ 1,2]. We have also proposed a real-time optical correlator using an LCTV and an Optic RAM detector as a binary phase spatial light modulator [ 3,4 ]. The LCTV has been recently used in the field o f optical information processing because of the low cost of the device and the ease o f the commercial availability. Though we can expect a high diffraction efficiency of light by using a phase-only spatial light modulator, it is difficult to obtain a perfect phase-only modulation in a usual twisted nematic LCTV device. Both the intensity and phase modulation ratios through the LCTV panel are affected by the polarization o f the input light and become a function o f the applied voltage. However, we can attain approximate phase-only modulation for a twisted nematic LCTV, especially for a binary signal. In this paper, we discuss the intensity and phase modulation properties o f a twisted nematic type
LCTV. A nonlinear optical correlator is proposed by using the phase modulation property of the LCTV. To obtain the high diffraction efficiency, the video signal corresponding to 0 and n phase values is written on the LCTV panel by the binarization o f the input pattern. In section 2, we evaluate the intensity and phase modulation ratios through the LCTV and the importance of its phase modulation is pointed out. In section 3, we experimentally investigate the performance o f the LCTV by using electronically composed non-clipped and clipped specklegrams when we use it as a phase dominant spatial light modulator. The effects o f the binarization for the fringe patterns produced by the Fourier transform o f the specklegrams on the correlation functions are also studied. In section 4, we propose a nonlinear optical correlator using the phase modulation o f the LCTV and the hard clipping property o f an Optic R A M detector. To demonstrate the usefulness o f the system, the velocity measurement of a rotating ground glass plate is conducted in real-time.
2. Intensity and phase modulation properties of I~TV The LCTV used in this experiment is a projection
0030-4018/91/$03.50 © 1991 Elsevier Science Publishers B.V. All fights reserved.
513
Volume 86, number 6
15 December 1991
OPTICS COMMUNICATIONS
TV panel of VPJ-700 (Seiko Epson) which is a 90 ° twisted nematic device with a thin film transistor (TFT). The panel consists of 220× 320 pixels each having a size of 80 × 90 Mm. As is shown in fig. 1, the liquid crystal molecule director is vertically aligned at the front of the panel while it is twisted by the right angle at the exit face. After removing the original plastic polarizers from the LCTV panel, we evaluate the intensity transmittance and phase shift of the LCTV as almost the same manner that in ref. [5]. The extreme cases of the results are shown in fig. 2. The intensity transmittance and phase shift are plotted against the input composite video signal level having a 8-bit gray scale. In fig. 2a, the orientation of the polarizer in fig. 1 is aligned to be parallel to the liquid crystal molecule director at the front panel and that of the analyzer is rotated by 90 ° , i.e. corresponding to the configuration of (~v~, ~v2)= (0, 90) in fig. 1. The phase shift from 0 to 1.2rr radians and the normalized intensity transmittance of 0.3 ~ 1 are obtained in this configuration. Fig. 2b is the result for the case of (~Ul, ~v~)= (90, 0). The phase modulation is not observed in this configuration, while the intensity transmittance is changed by the variation of the video signal [ 5,6 ]. Though we can ob-
serve no distinct phase change, the intensity modulation has the same tendency as that for the case in fig. 2a. This configuration may be used as an intensity modulation filter; however, it is shown in the following that the diffraction efficiency obtained by the intensity modulation filter is very low. Next, we investigate the diffraction efficiency for the LCTV spatial light modulator. In the experimental setup shown in fig. 3, a grating pattern having a rectangular wave is used to evaluate the diffraction efficiency. The LCTV panel is illuminated by a collimated light. The transmitted light is Fourier transformed by a lens behind of the panel and detected by a CCD camera at the focal plane of the lens. Fig. 4a shows the result of the intensity distribution of the Fourier power spectrum when the two levels of the grating pattern correspond to the phase values of 0 and 7r, respectively, under the configuration of (~v~, ~v2)= (0, 90). The + 1 order diffraction peaks are clearly seen while the zero order term is almost suppressed. The phase modulation effect is dominant in this configuration. Fig. 4b shows the diffraction pattern under the configuration of (~vl, ~u2)= (90, 0) when the minimum and maximum intensity transmittance levels in fig. 2b are used as the binary grat
y'""" .-- ....-'J"""
0 0
0 0
0 0
0
0
0
.<:~ 0
0
0
0
0
0
0
I
.i
,°
.........
........... ........ TWISTED NEMATIC LCTV PANEL
POLARIZER
Fig. 1. Configurationof the LCTVwith polarization filters. 514
Volume 86, number 6
15 December 1991
OPTICS COMMUNICATIONS 1.0
2.0[I 0 PHASE
•
INTENSITY TRANSMITTANCE
09 -< "4
T 13-
Z
o
• l. O I
o •
o
o
•
0
0.5 3>
0 0
0
m
0
~-"---0 0 0
(a)
O I
50
I i00
I 150
I
I
200 SIGNAL
250 LEVEL
(a)
2.011
1.0 z
co -< -q
E
z
1.0]~
0.5 3> Ftq
(b)
[ • 50
t
I
I
I
I00
150
200
250
SIGNAL
LEVEL
(b)
Fig. 2. Phase and intensity modulation properties of the LCTV at (a) (~, ~u2)= (90, 0) and (b) (~, ~u2)= (90,0). ing pattern. The strong DC peak has appeared and the _+ 1 order diffraction peaks are reduced compared with the pattern in fig. 4a. The LCTV panel behaves as an intensity spatial light modulator in this configuration. In the investigation in this section, it is shown that
the phase modulation property o f the LCTV rather than the intensity modulation property plays an important role to attain the high diffraction efficiency. Especially, the configuration o f (~1, ~'2) --- (0, 90) is suited for the application o f the LCTV as a spatial light phase modulator, for example, the application 515
FULL LENGTH ARTICLE
Volume 86, number 6
OPTICS COMMUNICATIONS
He-Ne Laserl
I
I "~, A I.. "
15 December 1991
P('~'I ..........
Colllmater
Lens Polarizer
Analyzer
\
Microcomputer
Flame memory"
I<:
Fig. 3. Optical system to evaluate the diffraction efficiency of the rectangular wave grating pattern written on the LCTV panel.
Fig. 4. Intensity distributions in the Fourier plane produced by the rectangular wave grating, (a) (~1, ~u2)= (0, 90) and (b) ( ~ , Yr) = (90, 0). 516
Volume 86, number 6
OPTICS COMMUNICATIONS
Fig. 5. (a) Non-clipped specklegram and (b) clipped specklegram. to an optical correlator. In the following experiment, the L C T V panel is used as a phase m o d u l a t i o n filter at this configuration.
3. Nonlinear optical speckle correlation In the previous papers [3,4], to c o m p e n s a t e the weak points o f a LCTV panel such as a low contrast ratio a n d a low space b a n d w i d t h product, clipped speckle intensity signals detected by an Optic R A M detector are used for the calculation o f the speckle
15 December 1991
Fig. 6. Fringe patterns produced by the Fourier transform corresponding to fig. 5a and b. correlation. As the result, a clear fringe pattern has been o b t a i n e d at the F o u r i e r plane. Binarizing the fringe pattern, a further i m p r o v e m e n t o f the correlation signal is expected due to the nonlinear effect [7]; for example, the higher peak value and narrower width o f the correlation spots. In this section, we investigate the effects o f the binarizations both o f the input specklegram and its F o u r i e r fringe on the diffraction efficiency in the correlation plane when we use the LCTV as a spatial light modulator. At first, we c o m p a r e the fringe patterns in the F o u r i e r plane 517
r
FULL LENG] H ARTICLE
Volume 86, number 6
OPTICS COMMUNICATIONS
15 December 1991
!
z w
~o.5
,I 1.0
2.0
3.0
4.0
5.0
4.0
5.0
(a)
Fig. 7. Correlation patterns calculated from fig. 6b, (a) for nonclipped fringe pattern and (b) for clipped fringe pattern.
6.O (~)
~0.5 z o
produced by electronically composed non-clipped and clipped specklegrams. Next, for the case of the clipped specklegram, we show the optically calculated correlation patterns with and without the binarization of the fringe pattern. Fig. 5a shows a specklegram consisting of 256 × 256 pixels to be written onto the LCTV panel. Each speckle pattern is separated 15 pixels along the horizontal direction. The process to make the specklegram is as follows. At first, a speckle pattern of a 8-bit gray scale having 512 × 512 pixels is experimentally detected by a CCD camera and stored in a microcomputer. Then, the bias level of the video signal is subtracted from the original pattern. From this 512 × 512 points speckle pattern, two speckle patterns each having the area of 256 X 256 pixels separated by the offset are chosen and added by the microcomputer, thus forming the specklegram. In fig. 5a, the normalized intensities of the specklegram are distributed over the phase values from 0 to x on the LCTV according to the phase shift in fig. 2a. On the other hand, fig. 5b shows the clipped specklegram obtained by the binarization of the pattern a t the average intensity. The two levels in this figure correspond to the phase values of 0 and :t, respectively. 518
,I 1.0
2.0
3.0
6.O (ram)
{b)
Fig. 8. One dimensional intensity distributions of the correlation functions of fig. 7a and b alongthe line to the +_l order diffraction spots. These specklegrams are used to calculate the Fourier power spectra, i.e. the Young's fringes produced by them, by the experimental setup shown in fig. 3. Fig. 6a is the fringe pattern obtained from the nonclipped specklegram corresponding to fig. 5a, while fig. 6b corresponds to that for the clipped specklegram of fig. 5b. Comparing fig. 6b with fig. 6a, the DC term in the fringe pattern for the clipped specklegram is suppressed and the fringe is clearly seen to the higher diffraction orders. In the following discussion, we use only the clipped specklegram to calculate the correlation function because of the advantage of the high diffraction efficiency. The fringe pattern shown in fig. 6b is again dis-
15 December 1991
OPTICS COMMUNICATIONS
Volume 86, number 6
Lens
Rotating ground glass
Pinhole
Liill>E
Lena
LCTV X-~
I
"
' "',V Collimator
I
I
/ Beam
] ens L
spliter
Microcomputer
Fig. 9. Speckledetection system and implementation of the nonlinear optical correlator.
played on the LCTV in the experimental setup in fig. 3. By the optical Fourier transform of the fringe pattern, i.e. the Fourier transform of the power spectrum, we obtain the correlation pattern. Fig. 7a shows the correlation function calculated directly from the fringe pattern shown in fig. 6b; the signal levels of the fringe pattern to be fed to the LCTV are normalized and distributed over the phase values from 0 to ft. The two correlation spots are observed along the horizontal direction, but they are not distinct because of the strong DC term. Next, the fringe patterns are binarized at the average intensity level and displayed on the LCTV as the binary phase values of 0 and 7r radians. Fig. 7b shows the Fourier transform of the binary fringe pattern. By the Fourier transform of the binary fringe pattem, the correlation spots are clearly seen more than that in fig. 7a. To observe the intensity distribution more precisely, the CCD camera at the focal plane of the lens in fig. 3 is replaced by a PIN photodiode which has a wide dynamic range of light detection. The PIN photodiode is mounted on a linearly moving X-table driven by a stepping motor and scanned to detect the intensity
distribution of the correlation function under the control of the microcomputer. Fig. 8 shows one dimensional intensity distributions across the direction of the diffraction spots shown in fig. 7. The correlation peak obtained from the binary fringe pattern is five times higher than that for the non-clipped case.
4. Implementation of optical correlator using nonlinear detector I n section 3, it is shown that the performance of the optical correlator using the LCTV is improved by the binarization of signals not only for the input specklegram but also for the fringe pattern obtained by its Fourier transform. Based on the result of the previous section, we propose the system for calculating the real-time correlation function using Optic RAM detectors which enable the nonlinear detection of the signals. The Optic RAM detector (Micron Technology) used in the experiment consists of 256X256 elements each measuring 6.45<6.4 ~tm [ 3,4,8 ]. Since the operation of this detector is based 519
FULL LENGTH ARTICLE
Volume 86, number 6
OPTICS COMMUNICATIONS
15 December 1991
on the soft error effect of a dynamic random access memory ( D ~ M ) , the Optic ~ detector has hard clipping property of incoming light intensity and can detect a clipped image at an arbitrary threshold level. These properties are suited for the detector in the nonlinear optical correlator. The detection of speckle patterns and the realization of the nonlinear optical correlator are shown in fig. 9. A rotating ground glass plate of ~400 mounted on a two-dimensional X - Z state is illuminated by a plane wave of a 5 mW laser diode. The object is imaged by a two-lens imaging system together with a pinhole. A time dependent clipped specklegram (the logical OR pattern between before and after the displacements of the clipped speckle patterns) is detected by the O p t i c RAM detector. After the allocation of the binary pattern to the two' signal values corresponding to the phase shifts 0 and
Fig. 10. (a) Clipped specklegram detected by the Optic RAM detector, (b) binary fringe pattern of the specklegram, and (c) correlation output by the Fourier transform of (b).
520
through a frame memory. Illuminating a collimated light from a 5 mW HeNe laser, the phase modulated signal on the LCTV is Fourier transformed by a lens behind of the panel. Another Optic RAM located at the focal plane of the lens detects the Fourier transformed pattern as a binary signal. By changing the exposure time or the threshold voltage to the Optic RAM detector from the microcomputer, we can appropriately adjust the threshold level of the detection for the fringe intensity. After storing the binary fringe pattern in the microcomputer, it is again displayed on the LCTV panel as a 0 or n phase pattern. Then, it is Fourier transformed b y the same optical system and the correlation pattern is detected by a CCD camera in fig. 9. Fig. 10a shows the binary specklegram taken by the system. We can recognize the clipped speckle pairs in fig. 10a. Fig. 10b is the power spectrum calculated by the Fourier transform of fig. 10a which is detected by the Optic RAM detector as a binary signal. The five fringes can be seen in this figure. Fig. 10c is the output pattern of the optical correlator obtained by the Fourier transform of fig. 10b. We can see the clear correlation spots in the diagonal direction. By using this system, we conducted an in-plane velocity measurement at several positions on the glass plate by shifting the X - Z stage to the line perpendicular to the optical axis. The result is shown in fig. 11. The horizontal axis is the radius from the rotat-
15 December 1991
OPTICS COMMUNICATIONS
Volume 86, number 6 (pixel)
60
LU ~D
0
4O
0
u)
0
7, 0 0 0
2O
(_#
,
t
i
I
50
i
l
,
i
I
I
I
i00 RADIUS
(mm)
Fig. 11. Result of the velocitymeasurementby the proposed system.
ing center of the plate while the vertical axis is the distance between the +_ 1 order diffraction peaks of the correlation output. The DC term as shown in fig. 10c always appears in the correlation pattern and affects the precise detection of the correlation signals. However, we can easily eliminate the DC term from the correlation pattern by masking the pattern optically or electronically since the term is stationary during the measurement. From the good linear relation between the correlation distance and the radius of the rotating center in the figure, it is shown that the system can be used for the practical velocity measurement of a light scattering object. However, the information concerning the direction of the velocity can not be obtained in this system. The measurement of vector velocity can be implemented by the approach of a joint transform correlator [4 ].
5. Conclusions
The phase and intensity modulation properties of an LCTV have been investigated. The phase shift from 0 to 1.2n radians and intensity transmittance
variation of 0.3-1.0 of the LCTV panel have been obtained. It is shown that the phase modulation of the LCTV is more efficient for the realization of the optical correlator than the intensity modulation. As the LCTV panel used in the experiment is a twisted nematic type, we could not realize a perfect phaseonly modulation. A liquid crystal device such as a homogeneous alignment type will be suited for an ideal phase-only spatial light modulator [9 ]. However, we can attain approximate phase-only modulation for the twisted nematic LCTV for a certain configuration of polarization filters. The effects of the binarization of the signals both for the input specklegram and the fringe patterns (its Fourier transform pattern), especially the effect of it on the diffraction efficiency of light, have been discussed. It is found that the correlation signal is enhanced by the binarizations and, thus, we can expect the high performance of the optical correlator. Based on these results, a real-time nonlinear optical correlator is proposed by using the phase modulation property of the LCTV together with the combination of nonlinear Optic RAM light detectors and applied to the velocity measurement of a light scattering ob521
Volume 86, number 6
OPTICS COMMUNICATIONS
ject. T h e system d e s c r i b e d here i m p l e m e n t s a correlation architecture consisting o f a single spatial light m o d u l a t o r [ 10 ].
Acknowledgement We wish to t h a n k T. S o n e h a r a for s u p p l y i n g the L C T V panel. T h e w o r k is s u p p o r t e d by T h e Tateisi Science a n d T e c h n o l o g y F o u n d a t i o n .
References [ 1] B. Bates and P.C. Miller, Appl. Optics 27 (1988) 2816.
522
15 December 1991
[2] B. Bates, P.C. Miller and W. Luchuan, J. Mod. Opt. 36 (1989) 317. [3] A. Ogiwara, H. Sakai and J. Ohtsubo, Optics Comm. 78 (1990) 213. [4] A. Ogiwara, H. Sakai and J. Ohtsubo, Optics Comm. 78 (1990) 322. [5] K. Lu and B.E.A. Saleh, Opt. Eng. 29 (t990) 240. [6] T.H. Barnes, T. Eiju, K. Matsuda and N. Ooyama, Appl. Optics 28 (1989) 4845. [7] B. Javidi, Appl. Optics 28 (1989) 2358. [8] J. Marron and G.M. Morris, Appl. Optics 25 (1986) 789. [9] D.A. Yocky, T.H. Barnes, K. Matusmoto, N. Ooyama and K. Matsuda, Optik 84 (1990) 140. [10] B. Javidi z D.A. Gregory and J.L. Homer~ Appl. Optics 28 (1989) 411.
OPTICS COMMUNICATIONS
Full length article
Application of LCTV to nonlinear speckle correlator Akifumi Ogiwara Graduate School of Electronic Science and Technology, Shizuoka University, 3-5-1Johoku, Hamamalsu 432, Japan
Hiro-aki Sakai and Junji Ohtsubo Faculty of Engineering, Shizuoka University, 3-5-1Johoku, Hamamatsu 432, Japan
Received 14 May 1991
The intensity and phase modulation characteristics of a liquid crystal television (LCTV) and its diffraction efficiency as a spatial light modulator are investigated. A nonlinear optical correlator using the phase modulation of an LCTV and the hard clipping property of an Optic RAM detector is proposed. The velocity measurement of a light scattering object is conducted by a real-time speckle clipping correlator based on the proposed method.
1. Introduction In many applications o f laser speckles, for example, such as the displacement and velocity measurements of a light scattering object, one o f the recent concerns is the realization o f the system for a fast calculation of the speckle correlation function. Bates et al. have applied a liquid crystal television (LCTV) to a real-time speckle metrology as a spatial light intensity modulator [ 1,2]. We have also proposed a real-time optical correlator using an LCTV and an Optic RAM detector as a binary phase spatial light modulator [ 3,4 ]. The LCTV has been recently used in the field o f optical information processing because of the low cost of the device and the ease o f the commercial availability. Though we can expect a high diffraction efficiency of light by using a phase-only spatial light modulator, it is difficult to obtain a perfect phase-only modulation in a usual twisted nematic LCTV device. Both the intensity and phase modulation ratios through the LCTV panel are affected by the polarization o f the input light and become a function o f the applied voltage. However, we can attain approximate phase-only modulation for a twisted nematic LCTV, especially for a binary signal. In this paper, we discuss the intensity and phase modulation properties o f a twisted nematic type
LCTV. A nonlinear optical correlator is proposed by using the phase modulation property of the LCTV. To obtain the high diffraction efficiency, the video signal corresponding to 0 and n phase values is written on the LCTV panel by the binarization o f the input pattern. In section 2, we evaluate the intensity and phase modulation ratios through the LCTV and the importance of its phase modulation is pointed out. In section 3, we experimentally investigate the performance o f the LCTV by using electronically composed non-clipped and clipped specklegrams when we use it as a phase dominant spatial light modulator. The effects o f the binarization for the fringe patterns produced by the Fourier transform o f the specklegrams on the correlation functions are also studied. In section 4, we propose a nonlinear optical correlator using the phase modulation o f the LCTV and the hard clipping property o f an Optic R A M detector. To demonstrate the usefulness o f the system, the velocity measurement of a rotating ground glass plate is conducted in real-time.
2. Intensity and phase modulation properties of I~TV The LCTV used in this experiment is a projection
0030-4018/91/$03.50 © 1991 Elsevier Science Publishers B.V. All fights reserved.
513
Volume 86, number 6
15 December 1991
OPTICS COMMUNICATIONS
TV panel of VPJ-700 (Seiko Epson) which is a 90 ° twisted nematic device with a thin film transistor (TFT). The panel consists of 220× 320 pixels each having a size of 80 × 90 Mm. As is shown in fig. 1, the liquid crystal molecule director is vertically aligned at the front of the panel while it is twisted by the right angle at the exit face. After removing the original plastic polarizers from the LCTV panel, we evaluate the intensity transmittance and phase shift of the LCTV as almost the same manner that in ref. [5]. The extreme cases of the results are shown in fig. 2. The intensity transmittance and phase shift are plotted against the input composite video signal level having a 8-bit gray scale. In fig. 2a, the orientation of the polarizer in fig. 1 is aligned to be parallel to the liquid crystal molecule director at the front panel and that of the analyzer is rotated by 90 ° , i.e. corresponding to the configuration of (~v~, ~v2)= (0, 90) in fig. 1. The phase shift from 0 to 1.2rr radians and the normalized intensity transmittance of 0.3 ~ 1 are obtained in this configuration. Fig. 2b is the result for the case of (~Ul, ~v~)= (90, 0). The phase modulation is not observed in this configuration, while the intensity transmittance is changed by the variation of the video signal [ 5,6 ]. Though we can ob-
serve no distinct phase change, the intensity modulation has the same tendency as that for the case in fig. 2a. This configuration may be used as an intensity modulation filter; however, it is shown in the following that the diffraction efficiency obtained by the intensity modulation filter is very low. Next, we investigate the diffraction efficiency for the LCTV spatial light modulator. In the experimental setup shown in fig. 3, a grating pattern having a rectangular wave is used to evaluate the diffraction efficiency. The LCTV panel is illuminated by a collimated light. The transmitted light is Fourier transformed by a lens behind of the panel and detected by a CCD camera at the focal plane of the lens. Fig. 4a shows the result of the intensity distribution of the Fourier power spectrum when the two levels of the grating pattern correspond to the phase values of 0 and 7r, respectively, under the configuration of (~v~, ~v2)= (0, 90). The + 1 order diffraction peaks are clearly seen while the zero order term is almost suppressed. The phase modulation effect is dominant in this configuration. Fig. 4b shows the diffraction pattern under the configuration of (~vl, ~u2)= (90, 0) when the minimum and maximum intensity transmittance levels in fig. 2b are used as the binary grat
y'""" .-- ....-'J"""
0 0
0 0
0 0
0
0
0
.<:~ 0
0
0
0
0
0
0
I
.i
,°
.........
........... ........ TWISTED NEMATIC LCTV PANEL
POLARIZER
Fig. 1. Configurationof the LCTVwith polarization filters. 514
Volume 86, number 6
15 December 1991
OPTICS COMMUNICATIONS 1.0
2.0[I 0 PHASE
•
INTENSITY TRANSMITTANCE
09 -< "4
T 13-
Z
o
• l. O I
o •
o
o
•
0
0.5 3>
0 0
0
m
0
~-"---0 0 0
(a)
O I
50
I i00
I 150
I
I
200 SIGNAL
250 LEVEL
(a)
2.011
1.0 z
co -< -q
E
z
1.0]~
0.5 3> Ftq
(b)
[ • 50
t
I
I
I
I00
150
200
250
SIGNAL
LEVEL
(b)
Fig. 2. Phase and intensity modulation properties of the LCTV at (a) (~, ~u2)= (90, 0) and (b) (~, ~u2)= (90,0). ing pattern. The strong DC peak has appeared and the _+ 1 order diffraction peaks are reduced compared with the pattern in fig. 4a. The LCTV panel behaves as an intensity spatial light modulator in this configuration. In the investigation in this section, it is shown that
the phase modulation property o f the LCTV rather than the intensity modulation property plays an important role to attain the high diffraction efficiency. Especially, the configuration o f (~1, ~'2) --- (0, 90) is suited for the application o f the LCTV as a spatial light phase modulator, for example, the application 515
FULL LENGTH ARTICLE
Volume 86, number 6
OPTICS COMMUNICATIONS
He-Ne Laserl
I
I "~, A I.. "
15 December 1991
P('~'I ..........
Colllmater
Lens Polarizer
Analyzer
\
Microcomputer
Flame memory"
I<:
Fig. 3. Optical system to evaluate the diffraction efficiency of the rectangular wave grating pattern written on the LCTV panel.
Fig. 4. Intensity distributions in the Fourier plane produced by the rectangular wave grating, (a) (~1, ~u2)= (0, 90) and (b) ( ~ , Yr) = (90, 0). 516
Volume 86, number 6
OPTICS COMMUNICATIONS
Fig. 5. (a) Non-clipped specklegram and (b) clipped specklegram. to an optical correlator. In the following experiment, the L C T V panel is used as a phase m o d u l a t i o n filter at this configuration.
3. Nonlinear optical speckle correlation In the previous papers [3,4], to c o m p e n s a t e the weak points o f a LCTV panel such as a low contrast ratio a n d a low space b a n d w i d t h product, clipped speckle intensity signals detected by an Optic R A M detector are used for the calculation o f the speckle
15 December 1991
Fig. 6. Fringe patterns produced by the Fourier transform corresponding to fig. 5a and b. correlation. As the result, a clear fringe pattern has been o b t a i n e d at the F o u r i e r plane. Binarizing the fringe pattern, a further i m p r o v e m e n t o f the correlation signal is expected due to the nonlinear effect [7]; for example, the higher peak value and narrower width o f the correlation spots. In this section, we investigate the effects o f the binarizations both o f the input specklegram and its F o u r i e r fringe on the diffraction efficiency in the correlation plane when we use the LCTV as a spatial light modulator. At first, we c o m p a r e the fringe patterns in the F o u r i e r plane 517
r
FULL LENG] H ARTICLE
Volume 86, number 6
OPTICS COMMUNICATIONS
15 December 1991
!
z w
~o.5
,I 1.0
2.0
3.0
4.0
5.0
4.0
5.0
(a)
Fig. 7. Correlation patterns calculated from fig. 6b, (a) for nonclipped fringe pattern and (b) for clipped fringe pattern.
6.O (~)
~0.5 z o
produced by electronically composed non-clipped and clipped specklegrams. Next, for the case of the clipped specklegram, we show the optically calculated correlation patterns with and without the binarization of the fringe pattern. Fig. 5a shows a specklegram consisting of 256 × 256 pixels to be written onto the LCTV panel. Each speckle pattern is separated 15 pixels along the horizontal direction. The process to make the specklegram is as follows. At first, a speckle pattern of a 8-bit gray scale having 512 × 512 pixels is experimentally detected by a CCD camera and stored in a microcomputer. Then, the bias level of the video signal is subtracted from the original pattern. From this 512 × 512 points speckle pattern, two speckle patterns each having the area of 256 X 256 pixels separated by the offset are chosen and added by the microcomputer, thus forming the specklegram. In fig. 5a, the normalized intensities of the specklegram are distributed over the phase values from 0 to x on the LCTV according to the phase shift in fig. 2a. On the other hand, fig. 5b shows the clipped specklegram obtained by the binarization of the pattern a t the average intensity. The two levels in this figure correspond to the phase values of 0 and :t, respectively. 518
,I 1.0
2.0
3.0
6.O (ram)
{b)
Fig. 8. One dimensional intensity distributions of the correlation functions of fig. 7a and b alongthe line to the +_l order diffraction spots. These specklegrams are used to calculate the Fourier power spectra, i.e. the Young's fringes produced by them, by the experimental setup shown in fig. 3. Fig. 6a is the fringe pattern obtained from the nonclipped specklegram corresponding to fig. 5a, while fig. 6b corresponds to that for the clipped specklegram of fig. 5b. Comparing fig. 6b with fig. 6a, the DC term in the fringe pattern for the clipped specklegram is suppressed and the fringe is clearly seen to the higher diffraction orders. In the following discussion, we use only the clipped specklegram to calculate the correlation function because of the advantage of the high diffraction efficiency. The fringe pattern shown in fig. 6b is again dis-
15 December 1991
OPTICS COMMUNICATIONS
Volume 86, number 6
Lens
Rotating ground glass
Pinhole
Liill>E
Lena
LCTV X-~
I
"
' "',V Collimator
I
I
/ Beam
] ens L
spliter
Microcomputer
Fig. 9. Speckledetection system and implementation of the nonlinear optical correlator.
played on the LCTV in the experimental setup in fig. 3. By the optical Fourier transform of the fringe pattern, i.e. the Fourier transform of the power spectrum, we obtain the correlation pattern. Fig. 7a shows the correlation function calculated directly from the fringe pattern shown in fig. 6b; the signal levels of the fringe pattern to be fed to the LCTV are normalized and distributed over the phase values from 0 to ft. The two correlation spots are observed along the horizontal direction, but they are not distinct because of the strong DC term. Next, the fringe patterns are binarized at the average intensity level and displayed on the LCTV as the binary phase values of 0 and 7r radians. Fig. 7b shows the Fourier transform of the binary fringe pattern. By the Fourier transform of the binary fringe pattem, the correlation spots are clearly seen more than that in fig. 7a. To observe the intensity distribution more precisely, the CCD camera at the focal plane of the lens in fig. 3 is replaced by a PIN photodiode which has a wide dynamic range of light detection. The PIN photodiode is mounted on a linearly moving X-table driven by a stepping motor and scanned to detect the intensity
distribution of the correlation function under the control of the microcomputer. Fig. 8 shows one dimensional intensity distributions across the direction of the diffraction spots shown in fig. 7. The correlation peak obtained from the binary fringe pattern is five times higher than that for the non-clipped case.
4. Implementation of optical correlator using nonlinear detector I n section 3, it is shown that the performance of the optical correlator using the LCTV is improved by the binarization of signals not only for the input specklegram but also for the fringe pattern obtained by its Fourier transform. Based on the result of the previous section, we propose the system for calculating the real-time correlation function using Optic RAM detectors which enable the nonlinear detection of the signals. The Optic RAM detector (Micron Technology) used in the experiment consists of 256X256 elements each measuring 6.45<6.4 ~tm [ 3,4,8 ]. Since the operation of this detector is based 519
FULL LENGTH ARTICLE
Volume 86, number 6
OPTICS COMMUNICATIONS
15 December 1991
on the soft error effect of a dynamic random access memory ( D ~ M ) , the Optic ~ detector has hard clipping property of incoming light intensity and can detect a clipped image at an arbitrary threshold level. These properties are suited for the detector in the nonlinear optical correlator. The detection of speckle patterns and the realization of the nonlinear optical correlator are shown in fig. 9. A rotating ground glass plate of ~400 mounted on a two-dimensional X - Z state is illuminated by a plane wave of a 5 mW laser diode. The object is imaged by a two-lens imaging system together with a pinhole. A time dependent clipped specklegram (the logical OR pattern between before and after the displacements of the clipped speckle patterns) is detected by the O p t i c RAM detector. After the allocation of the binary pattern to the two' signal values corresponding to the phase shifts 0 and
Fig. 10. (a) Clipped specklegram detected by the Optic RAM detector, (b) binary fringe pattern of the specklegram, and (c) correlation output by the Fourier transform of (b).
520
through a frame memory. Illuminating a collimated light from a 5 mW HeNe laser, the phase modulated signal on the LCTV is Fourier transformed by a lens behind of the panel. Another Optic RAM located at the focal plane of the lens detects the Fourier transformed pattern as a binary signal. By changing the exposure time or the threshold voltage to the Optic RAM detector from the microcomputer, we can appropriately adjust the threshold level of the detection for the fringe intensity. After storing the binary fringe pattern in the microcomputer, it is again displayed on the LCTV panel as a 0 or n phase pattern. Then, it is Fourier transformed b y the same optical system and the correlation pattern is detected by a CCD camera in fig. 9. Fig. 10a shows the binary specklegram taken by the system. We can recognize the clipped speckle pairs in fig. 10a. Fig. 10b is the power spectrum calculated by the Fourier transform of fig. 10a which is detected by the Optic RAM detector as a binary signal. The five fringes can be seen in this figure. Fig. 10c is the output pattern of the optical correlator obtained by the Fourier transform of fig. 10b. We can see the clear correlation spots in the diagonal direction. By using this system, we conducted an in-plane velocity measurement at several positions on the glass plate by shifting the X - Z stage to the line perpendicular to the optical axis. The result is shown in fig. 11. The horizontal axis is the radius from the rotat-
15 December 1991
OPTICS COMMUNICATIONS
Volume 86, number 6 (pixel)
60
LU ~D
0
4O
0
u)
0
7, 0 0 0
2O
(_#
,
t
i
I
50
i
l
,
i
I
I
I
i00 RADIUS
(mm)
Fig. 11. Result of the velocitymeasurementby the proposed system.
ing center of the plate while the vertical axis is the distance between the +_ 1 order diffraction peaks of the correlation output. The DC term as shown in fig. 10c always appears in the correlation pattern and affects the precise detection of the correlation signals. However, we can easily eliminate the DC term from the correlation pattern by masking the pattern optically or electronically since the term is stationary during the measurement. From the good linear relation between the correlation distance and the radius of the rotating center in the figure, it is shown that the system can be used for the practical velocity measurement of a light scattering object. However, the information concerning the direction of the velocity can not be obtained in this system. The measurement of vector velocity can be implemented by the approach of a joint transform correlator [4 ].
5. Conclusions
The phase and intensity modulation properties of an LCTV have been investigated. The phase shift from 0 to 1.2n radians and intensity transmittance
variation of 0.3-1.0 of the LCTV panel have been obtained. It is shown that the phase modulation of the LCTV is more efficient for the realization of the optical correlator than the intensity modulation. As the LCTV panel used in the experiment is a twisted nematic type, we could not realize a perfect phaseonly modulation. A liquid crystal device such as a homogeneous alignment type will be suited for an ideal phase-only spatial light modulator [9 ]. However, we can attain approximate phase-only modulation for the twisted nematic LCTV for a certain configuration of polarization filters. The effects of the binarization of the signals both for the input specklegram and the fringe patterns (its Fourier transform pattern), especially the effect of it on the diffraction efficiency of light, have been discussed. It is found that the correlation signal is enhanced by the binarizations and, thus, we can expect the high performance of the optical correlator. Based on these results, a real-time nonlinear optical correlator is proposed by using the phase modulation property of the LCTV together with the combination of nonlinear Optic RAM light detectors and applied to the velocity measurement of a light scattering ob521
Volume 86, number 6
OPTICS COMMUNICATIONS
ject. T h e system d e s c r i b e d here i m p l e m e n t s a correlation architecture consisting o f a single spatial light m o d u l a t o r [ 10 ].
Acknowledgement We wish to t h a n k T. S o n e h a r a for s u p p l y i n g the L C T V panel. T h e w o r k is s u p p o r t e d by T h e Tateisi Science a n d T e c h n o l o g y F o u n d a t i o n .
References [ 1] B. Bates and P.C. Miller, Appl. Optics 27 (1988) 2816.
522
15 December 1991
[2] B. Bates, P.C. Miller and W. Luchuan, J. Mod. Opt. 36 (1989) 317. [3] A. Ogiwara, H. Sakai and J. Ohtsubo, Optics Comm. 78 (1990) 213. [4] A. Ogiwara, H. Sakai and J. Ohtsubo, Optics Comm. 78 (1990) 322. [5] K. Lu and B.E.A. Saleh, Opt. Eng. 29 (t990) 240. [6] T.H. Barnes, T. Eiju, K. Matsuda and N. Ooyama, Appl. Optics 28 (1989) 4845. [7] B. Javidi, Appl. Optics 28 (1989) 2358. [8] J. Marron and G.M. Morris, Appl. Optics 25 (1986) 789. [9] D.A. Yocky, T.H. Barnes, K. Matusmoto, N. Ooyama and K. Matsuda, Optik 84 (1990) 140. [10] B. Javidi z D.A. Gregory and J.L. Homer~ Appl. Optics 28 (1989) 411.