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Electronic speckle pattern interferometry using optical fibers
Volume 38. number 3
OPTICS COMMUNICATIONS
ELECTRONIC SPECKLE PATTERN INTERFEROMETRY
1 August 1981
USING OPTICAL FIBERS
Ole J. L@KBERG and K&ireKRAKHELLA Physics Department, The Norwegian Institute of Technology, N- 7034, NTH, Trondheim, Norway
Received 6 February 1981
Initial experiments using electronic speckle pattern interferometry with fiberoptic imaging and illumination are described.
1. Introduction
It has been shown earlier that the Electronic Speckle Pattern Interferometer - ESPI - can be used to measure the vibrations of very unstable objects and also to reveal the movements of living biological objects [ 11. Combining the properties of ESPI with the flexibility of fiberoptic illumination and observation would open up some new application areas. In general, we could do measurements on objects situated in difficult accessible places, in hostile environments or in strongly absorbing liquids. As a specific example, this could enable us to observe and measure in real time the movements of interior human organs. In this paper we will describe some initial experiments undertaken to investigate how well fiberoptics can be adapted to an ESPI-system.
2. The set-up In principle, fiberoptics could be used in different ways in the ESPI-system. We could use a coherent fiberbundle to convey the image-reference waves interferogram directly onto the TV-target. (Note: in this context coherent fiber is used to indicate an im,age-carrying fiberbundle). However, we would still have the bulky part of the optical set-up close to the object and very little would be gained for some of the applications outlined above. In addition, optical noise 0 030-4018/8
l/0000-OOOO/% 02.50 0 North-Holland
from the fiber transmission would reduce the uniformity of the reference wave and result in interference patterns of low quality. We chose instead to use fiberoptics to illuminate the object and to relay the object wave close to the TV-target where it is mixed with the reference wave in a conventional way. The experimental set-up is shown on fig. 1. The Ar-laser is used at A = 0.5 146 pm with a single mode output of 0.25 W. The single mode operation is advantageous as we do not have to match the effective optical pathlength of the object - and reference waves through glass and air in the interferometer. The chopper, a slotted disk which can be rotated in synchronism with the TV-frame rate, is used whenever shortened exposures are necessary to increase the interferometric stability [I]. The main part of the laser light is reflected from the variable beamsplitter BS and directed by mirror M, into an incoherent fiber. The diverging beam emerging from this fiber is used to illuminate the object. When necessary we use an extra lens (not shown on fig. 1) to increase the illuminated object area. The object is imaged by the lens 4 onto the entrance face of the coherent fiberbundle in the fiberscope. The image is then transmitted to the exit face of the fibers, where it is imaged by lens L3 to the TV-target via mirror M,. The aperture of this mirror acts as the exit aperture of the imaging system. The beam transmitted through BS is first reflected from mirror Mm,. This mirror is vibrated whenever we want to get phase-modulation effects in the ESPI Publishing Company
155
Volume 38, number 3
1 August 1981
OPTICS COMMUNICATIONS
AUX. ELECTR.
WATER TANK
TV-
(PrOpdIE =
=d
CAM ERA
FILTER RECT.
-
SINE WAWE GEN.
Fig. 1. Fiberoptic ESPI set-up,
[2-41. The beam is then reflected by mirror M3 into a lens-pinhole combination LP. The pinhole is placed in a small hole drilled at an angle through mirror M,. In this way the reference wave can be adjusted to emerge from the center of the exit aperture in-line with the object wave.The object and reference wave combine to form an image-interferogram on the TV-target. The photoelectric action of the TV-camera converts the intensity of this image-interferogram into a corresponding video signal, which is electronically processed before it is converted to an ilnage on the TV-monitor. This video recording and processing action can be considered wholly equivalent to the recording and reconstruction processes in ordinary holography [5 J. Therefore, we will observe the image of a moving object covered with the same interference fringes as in holographic interferometry. For example, a sinusoidally vibrating object will be displayed with J$fringes while a two-step motion (within a TV-frame) results in cos2-fringes. The only difference will be that the ESPI-image has a coarser speckle structure due to the low resolution of the TV-system compared to holographic film. During the experiments two different coherent fiberbundles or fiberscopes were used. Fiberscope 1 was an Olympus Castro fiberscope used in medicin for system
156
gastric investigation and biopsy sampling. The total length of the fiber was 100 cm. The diameter of the image-carrying part of this fiberscope was approximately 5 mm. Its resolution was 30 l/mm, which gave an equivalent TV-resolution of less than 15 l/mm when the image was enlarged to cover the area of the TV-target. The viewing ocular of the fiberscope was retained and a negative lens was used to compensate for the ocular. A separate fiberbundle for object illumination was encapsulated in this fiberscope. The illumination and observation direction was set at right angles to the fiber direction by means of mirrors. The observed area could be remotely varied by bending of the fiberscope’s endsection in two orthogonal directions. Fiberscope 3 was a 2 m long Bendik Imagescope IS 8 10 originally bought for high-quality viewing of TV-monitors. No illumination fiberbundle was included and we had to fasten a fiber onto the outside of the cable or use conventional illumination of the object. The I8 mm-f: 1.4 imaging lens on the fiberscope was left on, but the viewing ocular at the exit was removed to obtain more flexible imaging onto the TVtarget. The active fiber area in this case was 8 X 12 mm2 and the equivalent image resolution on the TV-target was approximately 30 l/mm.
Volume 38, number 3
OPTICS COMMUNICATIONS
1 August 1981
3. Experiments As testing objects we used two metal plates, one rectangular 6 X 9 cm2 and the other circular, diameter 10 cm. The fiberoptic ESPI system was used only in its time average mode of operation as we did not have access to a videostcre which is necessary for two-step interferometry. The plates could be vibrated by piezo-electric transducers excited by a sine-wave generator. This generator was also used to vibrate the modulating mirror. The phasemodulation was controlled by the box labelled “AUX. ELECTR.” in fig. 1 which contained amplifier, phaseshifter and frequency translator (see refs. [2-41). The experiments were done with the objects in air and on the bottom of a water tank, depth 60 cm. To simulate realistic conditions, water turbulencc_could be induced by playing a small rotating propeller in the tank. The resolution of the enlarged image from fiberscope 1 was 213 of the resolution of our TV-target [TOSHIBA CHALNICON] . We must therefore expect a coarser speckle interferogram. [The system’s overall resolution using flberscope 1 is comparable to ordinary ESPI using TV-camera with Si-target] . A typical recording of the rectangular plate vibrating in the water tank is shown in fig. 2a. More than 4 fringe orders were difficult to observe, although phase modulation could increase this range. Using fiberscope 2 we obtained good ESPI fringequality as witnessed by fig. 2b, where the circular object is recorded while vibrating on the bottom of the water tank. This result was to be expected as the resolution of this fiberscope was higher than the resolution of our TV-target. Using the ESPI system in its ordinary time-average mode of operation up to 8 Ji -fringe orders could be detected, depending on the complexity of the vibration pattern. Moreover, the amplitude measuring range could be extended upwards by use of phase modulation techniques. In this way vibration amplitudes corresponding to more than 20 Ji -fringe orders were measured. The stability of the interferometer was satisfactory due to the combination of short exposure and high framing rate of the TV-system. During the experiments in the water tank we could put the rotating propeller close to the object and still detect the vibration pattern at normal TV-exposure (25 ms by European TV-
Fig. 2. Time-average ESPI recordings of vibrating objects; a) in air, fiberscope 1; b) in water, fl%erscope2.
standard). Reducing the exposure to 10 ms gave fringe patterns with full contrast. We did, however, experience stability problems if the fiberscopes were supported only at their endfaces. In this case, vibrations were introduced in the freely hanging cables. This would tilt the endfaces of the cable unless they were clamped very tightly. The really negative result of the experiments was the light loss suffered in the transmission of the illumination and object waves. With the largest object the full power of the Ar-laser (0.25 W) had to be used to get optimal fringe quality, although detectable fringe patterns could be obtained at far less power (25-30 n;W). We measured light losses in the fiber transmission up to 95% compared to a directly illuminated and observed object in the ESPI-system.
157
Volume 38, number 3
OPTICS COMMUNICATIONS
4. Discussion As the possibility of measuring movements inside a biological body represents the most difficult and chanlenging application of this technique, we shall comment upon the experimental results with this application in mind. The stability of the system itself is good enough for such an experiment where we have to use exposure of short durations anyway to stop the object’s movement. The resolution of the ESPI fringe patterns should also be adequate even when using medical fiberscopes of moderate resolution. The main problem will be to get a sufficiently high objectwave intensity back to the TV-target from the uncoated tissue. To this end, it seems unlikely that the combination of chopped Ar-laser light and ordinary vidicon will be sensitive
158
1 August 1981
enough to record a reasonable large area of observation. However, if we turn to low light detection devices like e.g. SIT camera we get the necessary increase in sensitivity without noticeable resolution loss [6].
References 111 O.J. LQkberg, Appl. Optics 18 (1979) 2377. [21 O.J. Ldkberg and K. H#gmoen, J. Phys. E. 9 (1976) 847. [31 O.J. L$kberg and K. H$gmoen, Appl. Optics 15 (1976) 2701. [41 K. HQgmoen and O.J. LQkberg, Appl. Optics 16 (1977) 1869. I51 O.J. LQkberg, Phys. in Technol. 11 (1980) 16. [61 G.A. Slettemoen, Proc. Norw. ElectroGptics Meeting 1 (1977) 75.
OPTICS COMMUNICATIONS
ELECTRONIC SPECKLE PATTERN INTERFEROMETRY
1 August 1981
USING OPTICAL FIBERS
Ole J. L@KBERG and K&ireKRAKHELLA Physics Department, The Norwegian Institute of Technology, N- 7034, NTH, Trondheim, Norway
Received 6 February 1981
Initial experiments using electronic speckle pattern interferometry with fiberoptic imaging and illumination are described.
1. Introduction
It has been shown earlier that the Electronic Speckle Pattern Interferometer - ESPI - can be used to measure the vibrations of very unstable objects and also to reveal the movements of living biological objects [ 11. Combining the properties of ESPI with the flexibility of fiberoptic illumination and observation would open up some new application areas. In general, we could do measurements on objects situated in difficult accessible places, in hostile environments or in strongly absorbing liquids. As a specific example, this could enable us to observe and measure in real time the movements of interior human organs. In this paper we will describe some initial experiments undertaken to investigate how well fiberoptics can be adapted to an ESPI-system.
2. The set-up In principle, fiberoptics could be used in different ways in the ESPI-system. We could use a coherent fiberbundle to convey the image-reference waves interferogram directly onto the TV-target. (Note: in this context coherent fiber is used to indicate an im,age-carrying fiberbundle). However, we would still have the bulky part of the optical set-up close to the object and very little would be gained for some of the applications outlined above. In addition, optical noise 0 030-4018/8
l/0000-OOOO/% 02.50 0 North-Holland
from the fiber transmission would reduce the uniformity of the reference wave and result in interference patterns of low quality. We chose instead to use fiberoptics to illuminate the object and to relay the object wave close to the TV-target where it is mixed with the reference wave in a conventional way. The experimental set-up is shown on fig. 1. The Ar-laser is used at A = 0.5 146 pm with a single mode output of 0.25 W. The single mode operation is advantageous as we do not have to match the effective optical pathlength of the object - and reference waves through glass and air in the interferometer. The chopper, a slotted disk which can be rotated in synchronism with the TV-frame rate, is used whenever shortened exposures are necessary to increase the interferometric stability [I]. The main part of the laser light is reflected from the variable beamsplitter BS and directed by mirror M, into an incoherent fiber. The diverging beam emerging from this fiber is used to illuminate the object. When necessary we use an extra lens (not shown on fig. 1) to increase the illuminated object area. The object is imaged by the lens 4 onto the entrance face of the coherent fiberbundle in the fiberscope. The image is then transmitted to the exit face of the fibers, where it is imaged by lens L3 to the TV-target via mirror M,. The aperture of this mirror acts as the exit aperture of the imaging system. The beam transmitted through BS is first reflected from mirror Mm,. This mirror is vibrated whenever we want to get phase-modulation effects in the ESPI Publishing Company
155
Volume 38, number 3
1 August 1981
OPTICS COMMUNICATIONS
AUX. ELECTR.
WATER TANK
TV-
(PrOpdIE =
=d
CAM ERA
FILTER RECT.
-
SINE WAWE GEN.
Fig. 1. Fiberoptic ESPI set-up,
[2-41. The beam is then reflected by mirror M3 into a lens-pinhole combination LP. The pinhole is placed in a small hole drilled at an angle through mirror M,. In this way the reference wave can be adjusted to emerge from the center of the exit aperture in-line with the object wave.The object and reference wave combine to form an image-interferogram on the TV-target. The photoelectric action of the TV-camera converts the intensity of this image-interferogram into a corresponding video signal, which is electronically processed before it is converted to an ilnage on the TV-monitor. This video recording and processing action can be considered wholly equivalent to the recording and reconstruction processes in ordinary holography [5 J. Therefore, we will observe the image of a moving object covered with the same interference fringes as in holographic interferometry. For example, a sinusoidally vibrating object will be displayed with J$fringes while a two-step motion (within a TV-frame) results in cos2-fringes. The only difference will be that the ESPI-image has a coarser speckle structure due to the low resolution of the TV-system compared to holographic film. During the experiments two different coherent fiberbundles or fiberscopes were used. Fiberscope 1 was an Olympus Castro fiberscope used in medicin for system
156
gastric investigation and biopsy sampling. The total length of the fiber was 100 cm. The diameter of the image-carrying part of this fiberscope was approximately 5 mm. Its resolution was 30 l/mm, which gave an equivalent TV-resolution of less than 15 l/mm when the image was enlarged to cover the area of the TV-target. The viewing ocular of the fiberscope was retained and a negative lens was used to compensate for the ocular. A separate fiberbundle for object illumination was encapsulated in this fiberscope. The illumination and observation direction was set at right angles to the fiber direction by means of mirrors. The observed area could be remotely varied by bending of the fiberscope’s endsection in two orthogonal directions. Fiberscope 3 was a 2 m long Bendik Imagescope IS 8 10 originally bought for high-quality viewing of TV-monitors. No illumination fiberbundle was included and we had to fasten a fiber onto the outside of the cable or use conventional illumination of the object. The I8 mm-f: 1.4 imaging lens on the fiberscope was left on, but the viewing ocular at the exit was removed to obtain more flexible imaging onto the TVtarget. The active fiber area in this case was 8 X 12 mm2 and the equivalent image resolution on the TV-target was approximately 30 l/mm.
Volume 38, number 3
OPTICS COMMUNICATIONS
1 August 1981
3. Experiments As testing objects we used two metal plates, one rectangular 6 X 9 cm2 and the other circular, diameter 10 cm. The fiberoptic ESPI system was used only in its time average mode of operation as we did not have access to a videostcre which is necessary for two-step interferometry. The plates could be vibrated by piezo-electric transducers excited by a sine-wave generator. This generator was also used to vibrate the modulating mirror. The phasemodulation was controlled by the box labelled “AUX. ELECTR.” in fig. 1 which contained amplifier, phaseshifter and frequency translator (see refs. [2-41). The experiments were done with the objects in air and on the bottom of a water tank, depth 60 cm. To simulate realistic conditions, water turbulencc_could be induced by playing a small rotating propeller in the tank. The resolution of the enlarged image from fiberscope 1 was 213 of the resolution of our TV-target [TOSHIBA CHALNICON] . We must therefore expect a coarser speckle interferogram. [The system’s overall resolution using flberscope 1 is comparable to ordinary ESPI using TV-camera with Si-target] . A typical recording of the rectangular plate vibrating in the water tank is shown in fig. 2a. More than 4 fringe orders were difficult to observe, although phase modulation could increase this range. Using fiberscope 2 we obtained good ESPI fringequality as witnessed by fig. 2b, where the circular object is recorded while vibrating on the bottom of the water tank. This result was to be expected as the resolution of this fiberscope was higher than the resolution of our TV-target. Using the ESPI system in its ordinary time-average mode of operation up to 8 Ji -fringe orders could be detected, depending on the complexity of the vibration pattern. Moreover, the amplitude measuring range could be extended upwards by use of phase modulation techniques. In this way vibration amplitudes corresponding to more than 20 Ji -fringe orders were measured. The stability of the interferometer was satisfactory due to the combination of short exposure and high framing rate of the TV-system. During the experiments in the water tank we could put the rotating propeller close to the object and still detect the vibration pattern at normal TV-exposure (25 ms by European TV-
Fig. 2. Time-average ESPI recordings of vibrating objects; a) in air, fiberscope 1; b) in water, fl%erscope2.
standard). Reducing the exposure to 10 ms gave fringe patterns with full contrast. We did, however, experience stability problems if the fiberscopes were supported only at their endfaces. In this case, vibrations were introduced in the freely hanging cables. This would tilt the endfaces of the cable unless they were clamped very tightly. The really negative result of the experiments was the light loss suffered in the transmission of the illumination and object waves. With the largest object the full power of the Ar-laser (0.25 W) had to be used to get optimal fringe quality, although detectable fringe patterns could be obtained at far less power (25-30 n;W). We measured light losses in the fiber transmission up to 95% compared to a directly illuminated and observed object in the ESPI-system.
157
Volume 38, number 3
OPTICS COMMUNICATIONS
4. Discussion As the possibility of measuring movements inside a biological body represents the most difficult and chanlenging application of this technique, we shall comment upon the experimental results with this application in mind. The stability of the system itself is good enough for such an experiment where we have to use exposure of short durations anyway to stop the object’s movement. The resolution of the ESPI fringe patterns should also be adequate even when using medical fiberscopes of moderate resolution. The main problem will be to get a sufficiently high objectwave intensity back to the TV-target from the uncoated tissue. To this end, it seems unlikely that the combination of chopped Ar-laser light and ordinary vidicon will be sensitive
158
1 August 1981
enough to record a reasonable large area of observation. However, if we turn to low light detection devices like e.g. SIT camera we get the necessary increase in sensitivity without noticeable resolution loss [6].
References 111 O.J. LQkberg, Appl. Optics 18 (1979) 2377. [21 O.J. Ldkberg and K. H#gmoen, J. Phys. E. 9 (1976) 847. [31 O.J. L$kberg and K. H$gmoen, Appl. Optics 15 (1976) 2701. [41 K. HQgmoen and O.J. LQkberg, Appl. Optics 16 (1977) 1869. I51 O.J. LQkberg, Phys. in Technol. 11 (1980) 16. [61 G.A. Slettemoen, Proc. Norw. ElectroGptics Meeting 1 (1977) 75.