- Email: [email protected]
Pyroelectric Vidicons for Submillimeter Wavelengths
ADVANCES IN ELECTRONICS AND ELECTRON PHYSICS.VOL. 648
Pyroelectric Vidicons for Submillimeter Wavelengths W. M. WREATHALL English Electric Valve Company Limited, Chelmsford, Essex, England
INTRODUCTION Until quite recently little use has been made of the submillimeter region of the electromagnetic spectrum, compared with optical and infrared wavelengths and microwaves. One reason for the neglect of the spectrum between 20 p m and 1 mm has been a lack of sufficiently powerful sources, but this has changed since there are now numerous lasers that fill the gap.' These offer potential applications for imaging, most obviously for mapping the mode patterns of the lasers, but also for radar modeling2u3and for detection of concealed object^.^ The pyroelectric vidicon, a television pick-up tube with bolometric action, can exploit these needs. Standard pyroelectric vidicons are optimized for operation with thermal radiation in the 8- to 14-pm band wavelengths. When used at much longer wavelengths these tubes have significant limitations. First, they are relatively insensitive. Second, particularly when used with coherent radiation, they produce spurious images. These arise principally from radiation that is reflected back to the target by the mesh and, to a lesser extent, by radiation that is reflected between the target and the surfaces of the faceplate. This article describes how these limitations have been overcome to produce tubes that are optimized for imaging using wavelengths around one-third of a millimeter, in particular 337 pm, the emission from an HCN laser. ASSESSMENT OF STANDARD TUBES The transmission and reflection spectra of sample components have been measured (as a function of frequency), at the National Physical Laboratory, by Fourier transform spectroscopy. 425 Copyright 8 198s by Academic Press. Inc. (London) Ltd. All rights of reproduction in any form reserved. ISBN 0-12-014724-6
426
W. M. WREATHALL
The DTGS Target
The transmission curves (Fig. 1) relate to a standard target of deuterated triglycine sulfate (DTGS), 18 p m thick. The two curves correspond to mutually perpendicular polarizations of the incident radiation; the target is orientated for maximum dichroism. At 30 cm-I (333 pm) transmission ranges from 42 to 65%. Absorption will be less than the complement of the transmission, because of reflection losses. The Germanium Faceplate
The transmission spectrum (Fig. 2) of the germanium faceplate is dominated by channel fringes due to interference between waves partially
0 9--
08
--
Wavenumber (cm-')
FIG.I . Far-infrared transmission spectrum of standard DTGS target.
427
PYROELECTRIC VIDICONS
reflected at the faces. The transmission at 30 cm-I therefore depends upon the precise thickness of the faceplate, and also on the angle of incidence; however, it would typically be 25%. These measurements show that no more than 20% and possibly less than 10% of the incident radiation at 337 p m would be used by a standard tube. COMPONENT DEVELOPMENT Analysis of Target
Since DTGS is transparent in the far infrared, resort must be made to surface layers to provide absorption. The thermal capacity of these must be low compared with that of the DTGS. This rules out bulk absorbers Wavelength (,urn)
tom
0.40
800
600500
Lm
L
0.30-.
O.ZS-t
.-
In
.-
E In
0.20
..
0.15
-.
0.10
-'
e
c
z3
a"
i1 . : ; ; ; ; ; ; ; ; t
Oo5
I
5
; : : : : ; : : : : I : : : : ; : : : : ; : : : : I ! : :
10
1s
P
25
30
Wavenurnber (crn-'1
FIG.2. Transmission spectrum of germanium faceplate.
35
t
428
W. M. WREATHALL
and favors metal films. The behavior of the latter has been in terms of the thickness of the dielectric and the conductivities of the metal films. These analyses show that total absorption is possible in a composite with the following properties: (1) a dielectric film with a thickness equal to an odd number of quarter wavelengths of the radiation; (2) the conductivity of the metallization on the front, radiation incident side should be 377 fl 0 - I ; (3) the rear side should be highly conductive. The front metallization serves conveniently as the signal plate, but the conductive rear side would short out the pattern of signal charge. This can be circumvented by making the surface in the form of an array of closely spaced metallic islands separated by insulating strips. Structures of this type are used as filters and beam-splitters in the far infrared. Analysis of their optical behavior7s8shows that maximum reflectivity is obtained when the pitch of the islands is equal to the wavelength of the radiation in the dielectric. Wavelength ( p m )
FIG.3. Transmission spectra of metallized DTGS target.
429
PYROELECTRIC VIDICONS
Target Fabrication
The DTGS target has a thickness rather less than 35 pm. Both surfaces have vacuum-deposited metal layers. The islands on the rear side are defined by evaporating aluminum through a mesh mask in contact with the DTGS. The meshes are made by plating on to a master pattern ruled on glass, with a pitch of 150 lines per inch. Spectral Measurements
Reflection and transmission spectra recorded for sample-metallized targets are shown in Figs. 3 and 4. The dependence of the spectra on polarization, due to birefringence in DTGS, is demonstrated by the pairs of spectra. Transmission and reflection coefficients are given in Table I for wavelengths of 337 and 400 pm; at the latter wavelength this particular target absorbs over 87% of the radiation incident on it, independently of polarization. Wavelength (pm)
4 : : : : : 5:
:::l::::f::::l::::I::::!:::: 10
15
20
25
30
r : : : :Lo: : : :LS: i : : :50: l
35
Wavenumber (cm-’1
FIG.4. Reflection spectra from front surface of metallized DTGS target.
430
W . M . WREATH ALL
TABLEI Transmission and reflection coefficients of metallized DTGS targets Wavelength
Transmission Reflection Front (parallel) Front (perpendicular) Rear
337 pm
400 pm
0.028
0.035
0.044
0.09 0.09 0.80
0.27 0.80
Similar high efficiency is obtained at 337 pm from a thinner target. The high reflectivity of the rear surface ensures that little of the small fraction of transmitted radiation can interfere with the primary image. Reflection by the vidicon mesh is therefore of no consequence. Wavelength ( p m ) XKK)
10 4
E
800
M)O
500
400
3w
250
200
a
e
-
05/ LOW HIGH
RESLUTION RESOLUTMN
:
2m-l
:
0 Scm-'
a
O0 23 I
f::.:
, !
5
: : :
I::::
10
i : : : : :
15
20
::::;::!!I:::' I : : ! ! I : ::: I : !:: I : : 25
30
35
LO
Wavenumber (cm-'1
FIG.5. Transmission spectrum of crystal quartz faceplate.
L5
50
55
1
PY ROELECTRIC VIDICONS
43 1
The Faceplate Crystal quartz is highly transparent in the far infrared. Fused quartz might be preferred on economic grounds but is substantially more absorbent. Faceplates are therefore made from crystal quartz, z-cut to avoid birefringence. The refractive index is quite high, 2.11, so that reflection losses and channel fringes are intrusive as demonstrated by the measured transmission spectrum (Fig. 5 ) . The reflections can be eliminated by using a quarter wavelength of material with a refractive index close to 1.45. It is impractical to achieve the required thickness (nearly 60 ym) by vacuum evaporation. However, polythene has a close match to the required refractive index9and can be obtained in the form of film. Such films have been successfully bonded to crystal quartz. Transmission and reflection spectra of a faceplate with polythene antireflection films are shown in Figs. 6 and 7. These show that the reflections have been almost completely eliminated at 337 pm.
Wavelength
LOW
RESOLUTION
HIGH
RESOLIJTON
(pm)
2cm-1
o
scm-1
FIG.6. Transmission spectrum of polythene bonded crystal quartz faceplate.
432
W . M. WREATHALL
Wavenumber (cm-'1
FIG.7. Reflection spectrum of polythene bonded crystal quartz faceplate
TUBESA N D PERFORMANCE Tubes of 25 mm incorporating 20-mm-diameter metallized targets as described above have been assembled and pumped. Indium rings were used to seal the quartz faceplates to the glass bulb. The tubes were run in a pyroelectric camera operating at CCIR TV scan standards. In focus, the metal islands on the scanned side of the target were well defined and it was found preferable to defocus the reading beam to avoid large signal spikes arising from instantaneous discharge of the islands.
PYROELECTRIC VIDICONS
433
Modulation Transfer Function
MTFs, measured in the near infrared, using a bar pattern in front of a radiant source, imaged by a germanium lens at fll.4, were as follows: 50 TV lines per picture height (1.4 lp mm-I) 0.49; 75 TV lines per picture height (2.1 lp mm-I) 0.20; 100 TV lines per picture height (2.8 lp mm-I) 0.10. These measurements were made with the beam defocused to eliminate modulation by the island structure. The performance achieved is well matched to the diffraction limit of lenses in the far infrared; for example, the cut-off frequency at f/2 is less than 1.5 Ip mm-' at a wavelength of 337 pm. Responsivitj
Responsivity, measured using an HCN laser as a source of known power at 337 pm, exceeds 4 pA W-'. The pyroelectric efficiency, characterized by the responsivity times thickness product, is somewhat greater than that of standard targets operated in the 8- to 14-pm band. APPLICATION TO RADAR MODELING Because radars use coherent radiation, the returns are confounded by interference between reflections from various parts of the target and are sensitive to changes of orientation and of polarization. Any object therefore has a set of radar signatures that can only be compiled by experiment, and, since radars and targets are both unwieldy and expensive, the experiments are most economically conducted with models. As both model and wavelength have to be scaled equally, optical wavelengths demand impossibly precise modeling, hence the preference for submillimeter wavelengths. A typical model is illustrated in Fig. 8a. When this vehicle is illuminated by radiation from an HCN laser, with a wavelength of 337 pm, a typical return is as shown in Fig. 8b. This was recorded from a pyroelectric camera fitted with the long-wave length tube and with a 100-mm focal length lens made of TPX, a plastic that has a low loss in the far infrared. The object is unrecognizable, although with experience it might be possible to memorize the signature. In order to identify the reflecting areas, it is desirable to superimpose them on an outline of the vehicle. As the TPX lens and quartz faceplate are both transparent to visible light, this can be done by placing a lamp behind the model, with the result shown in Fig. 8c. It is found that large changes in signature can result from small shifts of the target vehicle.
434
W . M . WREATHALL
FIG.8. (a) Model road roller; (b) returns from (a); (c) returns against silhouette.
CONCLUSION Pyroelectric vidicons have been designed and manufactured which are highly efficient in the far infrared and demonstrate useful imagery with HCN laser illumination. The same techniques could be applied to produce efficient tubes for any part of the submillimeter spectrum. ACKNOWLEDGMENTS Acknowledgment is due to E.M.I. Ltd. and the National Physical Laboratory (NPL) for the use of HCN lasers and to NPL also for spectral measurements. This work was supported by the Ministry of Defence, Royal Signals and Radar Establishment through the Procurement Executive. D.C.V.D.
REFERENCES 1. Waniek, R. W., Laser Focus 19, 79 (1983). 2. Cram, L. A. and Woolcock, S. C., Radio & Electron. Eng. 49, 381 (1979).
PYROELECTRIC VIDICONS
3. 4. 5. 6. 7. 8. 9.
Waldman, J., Proc. S. P. I . E. 259, 152 (1980). Barker, D.H., Proc. S.P. I. E . 67, 27 (1975). Hilsum, C., J . Opr. SOC.Am. 44, 188 (1954). Stilberg, P. A., J. Opt. SOC.Am. 47, 575 (1957). Ulrich, R., Infrared Phys. 7, 37 (1%7). Durschlag, M. S. and DeTemple, T. A., Appl. Opr 20, 1245 (1981). Birch, J. R., Infrared Phys. 21, 225 (1981).
435
Pyroelectric Vidicons for Submillimeter Wavelengths W. M. WREATHALL English Electric Valve Company Limited, Chelmsford, Essex, England
INTRODUCTION Until quite recently little use has been made of the submillimeter region of the electromagnetic spectrum, compared with optical and infrared wavelengths and microwaves. One reason for the neglect of the spectrum between 20 p m and 1 mm has been a lack of sufficiently powerful sources, but this has changed since there are now numerous lasers that fill the gap.' These offer potential applications for imaging, most obviously for mapping the mode patterns of the lasers, but also for radar modeling2u3and for detection of concealed object^.^ The pyroelectric vidicon, a television pick-up tube with bolometric action, can exploit these needs. Standard pyroelectric vidicons are optimized for operation with thermal radiation in the 8- to 14-pm band wavelengths. When used at much longer wavelengths these tubes have significant limitations. First, they are relatively insensitive. Second, particularly when used with coherent radiation, they produce spurious images. These arise principally from radiation that is reflected back to the target by the mesh and, to a lesser extent, by radiation that is reflected between the target and the surfaces of the faceplate. This article describes how these limitations have been overcome to produce tubes that are optimized for imaging using wavelengths around one-third of a millimeter, in particular 337 pm, the emission from an HCN laser. ASSESSMENT OF STANDARD TUBES The transmission and reflection spectra of sample components have been measured (as a function of frequency), at the National Physical Laboratory, by Fourier transform spectroscopy. 425 Copyright 8 198s by Academic Press. Inc. (London) Ltd. All rights of reproduction in any form reserved. ISBN 0-12-014724-6
426
W. M. WREATHALL
The DTGS Target
The transmission curves (Fig. 1) relate to a standard target of deuterated triglycine sulfate (DTGS), 18 p m thick. The two curves correspond to mutually perpendicular polarizations of the incident radiation; the target is orientated for maximum dichroism. At 30 cm-I (333 pm) transmission ranges from 42 to 65%. Absorption will be less than the complement of the transmission, because of reflection losses. The Germanium Faceplate
The transmission spectrum (Fig. 2) of the germanium faceplate is dominated by channel fringes due to interference between waves partially
0 9--
08
--
Wavenumber (cm-')
FIG.I . Far-infrared transmission spectrum of standard DTGS target.
427
PYROELECTRIC VIDICONS
reflected at the faces. The transmission at 30 cm-I therefore depends upon the precise thickness of the faceplate, and also on the angle of incidence; however, it would typically be 25%. These measurements show that no more than 20% and possibly less than 10% of the incident radiation at 337 p m would be used by a standard tube. COMPONENT DEVELOPMENT Analysis of Target
Since DTGS is transparent in the far infrared, resort must be made to surface layers to provide absorption. The thermal capacity of these must be low compared with that of the DTGS. This rules out bulk absorbers Wavelength (,urn)
tom
0.40
800
600500
Lm
L
0.30-.
O.ZS-t
.-
In
.-
E In
0.20
..
0.15
-.
0.10
-'
e
c
z3
a"
i1 . : ; ; ; ; ; ; ; ; t
Oo5
I
5
; : : : : ; : : : : I : : : : ; : : : : ; : : : : I ! : :
10
1s
P
25
30
Wavenurnber (crn-'1
FIG.2. Transmission spectrum of germanium faceplate.
35
t
428
W. M. WREATHALL
and favors metal films. The behavior of the latter has been in terms of the thickness of the dielectric and the conductivities of the metal films. These analyses show that total absorption is possible in a composite with the following properties: (1) a dielectric film with a thickness equal to an odd number of quarter wavelengths of the radiation; (2) the conductivity of the metallization on the front, radiation incident side should be 377 fl 0 - I ; (3) the rear side should be highly conductive. The front metallization serves conveniently as the signal plate, but the conductive rear side would short out the pattern of signal charge. This can be circumvented by making the surface in the form of an array of closely spaced metallic islands separated by insulating strips. Structures of this type are used as filters and beam-splitters in the far infrared. Analysis of their optical behavior7s8shows that maximum reflectivity is obtained when the pitch of the islands is equal to the wavelength of the radiation in the dielectric. Wavelength ( p m )
FIG.3. Transmission spectra of metallized DTGS target.
429
PYROELECTRIC VIDICONS
Target Fabrication
The DTGS target has a thickness rather less than 35 pm. Both surfaces have vacuum-deposited metal layers. The islands on the rear side are defined by evaporating aluminum through a mesh mask in contact with the DTGS. The meshes are made by plating on to a master pattern ruled on glass, with a pitch of 150 lines per inch. Spectral Measurements
Reflection and transmission spectra recorded for sample-metallized targets are shown in Figs. 3 and 4. The dependence of the spectra on polarization, due to birefringence in DTGS, is demonstrated by the pairs of spectra. Transmission and reflection coefficients are given in Table I for wavelengths of 337 and 400 pm; at the latter wavelength this particular target absorbs over 87% of the radiation incident on it, independently of polarization. Wavelength (pm)
4 : : : : : 5:
:::l::::f::::l::::I::::!:::: 10
15
20
25
30
r : : : :Lo: : : :LS: i : : :50: l
35
Wavenumber (cm-’1
FIG.4. Reflection spectra from front surface of metallized DTGS target.
430
W . M . WREATH ALL
TABLEI Transmission and reflection coefficients of metallized DTGS targets Wavelength
Transmission Reflection Front (parallel) Front (perpendicular) Rear
337 pm
400 pm
0.028
0.035
0.044
0.09 0.09 0.80
0.27 0.80
Similar high efficiency is obtained at 337 pm from a thinner target. The high reflectivity of the rear surface ensures that little of the small fraction of transmitted radiation can interfere with the primary image. Reflection by the vidicon mesh is therefore of no consequence. Wavelength ( p m ) XKK)
10 4
E
800
M)O
500
400
3w
250
200
a
e
-
05/ LOW HIGH
RESLUTION RESOLUTMN
:
2m-l
:
0 Scm-'
a
O0 23 I
f::.:
, !
5
: : :
I::::
10
i : : : : :
15
20
::::;::!!I:::' I : : ! ! I : ::: I : !:: I : : 25
30
35
LO
Wavenumber (cm-'1
FIG.5. Transmission spectrum of crystal quartz faceplate.
L5
50
55
1
PY ROELECTRIC VIDICONS
43 1
The Faceplate Crystal quartz is highly transparent in the far infrared. Fused quartz might be preferred on economic grounds but is substantially more absorbent. Faceplates are therefore made from crystal quartz, z-cut to avoid birefringence. The refractive index is quite high, 2.11, so that reflection losses and channel fringes are intrusive as demonstrated by the measured transmission spectrum (Fig. 5 ) . The reflections can be eliminated by using a quarter wavelength of material with a refractive index close to 1.45. It is impractical to achieve the required thickness (nearly 60 ym) by vacuum evaporation. However, polythene has a close match to the required refractive index9and can be obtained in the form of film. Such films have been successfully bonded to crystal quartz. Transmission and reflection spectra of a faceplate with polythene antireflection films are shown in Figs. 6 and 7. These show that the reflections have been almost completely eliminated at 337 pm.
Wavelength
LOW
RESOLUTION
HIGH
RESOLIJTON
(pm)
2cm-1
o
scm-1
FIG.6. Transmission spectrum of polythene bonded crystal quartz faceplate.
432
W . M. WREATHALL
Wavenumber (cm-'1
FIG.7. Reflection spectrum of polythene bonded crystal quartz faceplate
TUBESA N D PERFORMANCE Tubes of 25 mm incorporating 20-mm-diameter metallized targets as described above have been assembled and pumped. Indium rings were used to seal the quartz faceplates to the glass bulb. The tubes were run in a pyroelectric camera operating at CCIR TV scan standards. In focus, the metal islands on the scanned side of the target were well defined and it was found preferable to defocus the reading beam to avoid large signal spikes arising from instantaneous discharge of the islands.
PYROELECTRIC VIDICONS
433
Modulation Transfer Function
MTFs, measured in the near infrared, using a bar pattern in front of a radiant source, imaged by a germanium lens at fll.4, were as follows: 50 TV lines per picture height (1.4 lp mm-I) 0.49; 75 TV lines per picture height (2.1 lp mm-I) 0.20; 100 TV lines per picture height (2.8 lp mm-I) 0.10. These measurements were made with the beam defocused to eliminate modulation by the island structure. The performance achieved is well matched to the diffraction limit of lenses in the far infrared; for example, the cut-off frequency at f/2 is less than 1.5 Ip mm-' at a wavelength of 337 pm. Responsivitj
Responsivity, measured using an HCN laser as a source of known power at 337 pm, exceeds 4 pA W-'. The pyroelectric efficiency, characterized by the responsivity times thickness product, is somewhat greater than that of standard targets operated in the 8- to 14-pm band. APPLICATION TO RADAR MODELING Because radars use coherent radiation, the returns are confounded by interference between reflections from various parts of the target and are sensitive to changes of orientation and of polarization. Any object therefore has a set of radar signatures that can only be compiled by experiment, and, since radars and targets are both unwieldy and expensive, the experiments are most economically conducted with models. As both model and wavelength have to be scaled equally, optical wavelengths demand impossibly precise modeling, hence the preference for submillimeter wavelengths. A typical model is illustrated in Fig. 8a. When this vehicle is illuminated by radiation from an HCN laser, with a wavelength of 337 pm, a typical return is as shown in Fig. 8b. This was recorded from a pyroelectric camera fitted with the long-wave length tube and with a 100-mm focal length lens made of TPX, a plastic that has a low loss in the far infrared. The object is unrecognizable, although with experience it might be possible to memorize the signature. In order to identify the reflecting areas, it is desirable to superimpose them on an outline of the vehicle. As the TPX lens and quartz faceplate are both transparent to visible light, this can be done by placing a lamp behind the model, with the result shown in Fig. 8c. It is found that large changes in signature can result from small shifts of the target vehicle.
434
W . M . WREATHALL
FIG.8. (a) Model road roller; (b) returns from (a); (c) returns against silhouette.
CONCLUSION Pyroelectric vidicons have been designed and manufactured which are highly efficient in the far infrared and demonstrate useful imagery with HCN laser illumination. The same techniques could be applied to produce efficient tubes for any part of the submillimeter spectrum. ACKNOWLEDGMENTS Acknowledgment is due to E.M.I. Ltd. and the National Physical Laboratory (NPL) for the use of HCN lasers and to NPL also for spectral measurements. This work was supported by the Ministry of Defence, Royal Signals and Radar Establishment through the Procurement Executive. D.C.V.D.
REFERENCES 1. Waniek, R. W., Laser Focus 19, 79 (1983). 2. Cram, L. A. and Woolcock, S. C., Radio & Electron. Eng. 49, 381 (1979).
PYROELECTRIC VIDICONS
3. 4. 5. 6. 7. 8. 9.
Waldman, J., Proc. S. P. I . E. 259, 152 (1980). Barker, D.H., Proc. S.P. I. E . 67, 27 (1975). Hilsum, C., J . Opr. SOC.Am. 44, 188 (1954). Stilberg, P. A., J. Opt. SOC.Am. 47, 575 (1957). Ulrich, R., Infrared Phys. 7, 37 (1%7). Durschlag, M. S. and DeTemple, T. A., Appl. Opr 20, 1245 (1981). Birch, J. R., Infrared Phys. 21, 225 (1981).
435