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An ESPI apparatus with multichannel data acquisition
Volume 30, number 1
OPTICS COMMUNICATIONS
July 1979
AN ESPI APPARATUS WITH MULTICHANNEL DATA ACQUISITION P. OTTONELLO, C. PONTIGGIA Istituto Scienze Fisiche, Universitd di Genova, Genova, Italy
and L. ROSSI Istituto Nazionale Fisica Nucleare, Sezione di Genova, Genova, Italy
Received 3 January 1979 Revised manuscript received 22 March 1979 An ESP1 (Electronic Speckle Pattern Interferometry) apparatus employing a linear photodiodes array for multichannel data acquisition is descried. The technique is applied for measurements of normal displacements of surfaces when time and space resolutions are required.
The simplest concept o f a speckle interferometer to detect normal-plane vibrations and longitudinal movements o f a scattering surface is a Michelson interferometer, one o f whose mirrors has been replaced b y the surface under examination. When the surface translates in the direction of the incident light, each speckle will vary in brightness cyclically from light to dark.
A lot o f works in the field o f measurements o f the displacements o f a diffuse object has been published [1,2] and b y some techniques displacement amplitudes o f the order or less than one angstr6m are recorded [3,4]. We are employing a linear photodiode array, allowing multichannel, parallel data acquisition, for two applications in which the down amplitude limit required is o f the order o f some ratio of ~ (light
s I I
P
M
L = I 5 mW laser
!~-I
~ --
~
-IP"
.
.
.
.
.
I i
P,
I
photodiode array
"=
Is.,.c,,onl 1 '°g'c I Fig. 1. Block diagram of the digital ESPI apparatus (S: moving surface; M: mirror; L: lens 5 cm focus; P: pupil; photodiode array: Reticon RL-64P); P1 and P2 are rectilinear polarizers for balancing the scattered and reference light intensities. 20
Volume
30, number
1
OPTICS COMMUNICATIONS
wavelength). Our measurements concern the thermal coefficient of expansion of very small metal samples (about 5 X 5 X 5 mm3) and deformations of different zones in a power diode surface induced by very high and fast current pulses. The aim of this note is to illustrate the employed technique which is very useful when it is necessary to follow the surface motion with time and space resolution. A block diagram of the apparatus is shown in fig. 1. Scattered light from the moving surface S is focused by L to the detector plane; in the actual experiment a silicon photodiode camera (64 photodiodes arranged in a linear array) is used. Provided that the imaging lens is sufficiently stopped down and the illumination of the surface correspondingly increased, only one speckle is image onto each photodiode, corresponding at a single observation point [S] . When the surface is translating with velocity u the single photodiode’s signal is the alternating current following the Doppler shift of the backscattered light relative to the superimposed reference beam:
July 1979
cy v, depends from the product of amplitude A, and frequency vv. In oui case (data acquisition rate = lo4 words/set) we have: VJ,
= 2.5 x 102 x.
A factor of about 100 in data acquisition
speed can
i(t) a cos (47r/h) ut. The array’s diodes are continuously active and are discharged by the photocurrent at a rate proportional to the local light intensity. They are serially accessed and recharged through an output common line; thus the output signal is a train of 64 pulses (per scan) with magnitude proportional to the recharging current supplied to each diode i.e. proportional to the corresponding diode’s light exposure. To fully utilize this multichannel sensor a fast data acquisition system is required. Data are analog to digital converted at a rate of lo4 words/set and stored, via a CAMAC interface, in the main memory of a NORD 10 minicomputer for further processing. Data from a single selectable photodiode of the array are sampled-hold for continuous display on a CRT for visual inspection. Referring to a vibrating surface the number of light maxima in a period l/v, of the mechanical vibration is a measure of the displacement amplitude A, while time between two adjacent maxima gives a measure of the instantaneous velocity. The number of light maxima per one mechanical cycle is larger for larger vibration amplitudes (see fig. 2). Therefore for a complete reconstruction of the vibration displacements of an imaged zone of the surface the array scanning frequen-
Fig. 2. CRT output from the computer main memory of a stored run; a), b) represent one mechanical cycle of a forced vibration with frequency vv = 55 Hz and amplitudeA, = 1.5 h and 2.5 h respectively; c) is a low pass filtered version of b).
21
Volume 30, number 1
OPTICS COMMUNICATIONS
easily be gained with the addition of a DMA (Direct Memory Access) module to the CAMAC interface. The ultimate amplitude-frequency limit of this technique for real time measurements therefore is:
July 1979
very long time intervals. Correlations and detailed calculations can be worked out as soon as data are written into the NORD 10, thus fully exploiting the capabilities of the computer.
%A” = 2 x 104 x. The limit imposed by the optoelectronic device sensitivity is consistent with the above mentioned one, in the hypothesis of diode matrice programmed for reading less than 64 elements. The computer memory size allows < 64 Kwords to be transferred, thus limiting the time interval of the observation with increasing acquisition frequency. At the actual (lo4 words/set) data acquisition rate, continuous transfer is possible to a magnetic tape, whose size is about lo7 words, extending the observation to
22
References [II A.E. Ennos, Topics in Applied Physics: Laser and related phenomena (Springer Verlag, Berlin, Heidelberg, New York, 1975). PI R.A. Bruce and G.L. Fitzpatrick, Appl. Opt. 14 (1975) 1621. [31 D. Joyeux, Appl. Opt. 15 (1976) 1241, 1248. [41K. Hogmoen and O.J. Lokberg, Appl. Opt. 16 (1977) 1869. [51 K.J. Ebeling, OpticsComm. 24 (1978) 125.
OPTICS COMMUNICATIONS
July 1979
AN ESPI APPARATUS WITH MULTICHANNEL DATA ACQUISITION P. OTTONELLO, C. PONTIGGIA Istituto Scienze Fisiche, Universitd di Genova, Genova, Italy
and L. ROSSI Istituto Nazionale Fisica Nucleare, Sezione di Genova, Genova, Italy
Received 3 January 1979 Revised manuscript received 22 March 1979 An ESP1 (Electronic Speckle Pattern Interferometry) apparatus employing a linear photodiodes array for multichannel data acquisition is descried. The technique is applied for measurements of normal displacements of surfaces when time and space resolutions are required.
The simplest concept o f a speckle interferometer to detect normal-plane vibrations and longitudinal movements o f a scattering surface is a Michelson interferometer, one o f whose mirrors has been replaced b y the surface under examination. When the surface translates in the direction of the incident light, each speckle will vary in brightness cyclically from light to dark.
A lot o f works in the field o f measurements o f the displacements o f a diffuse object has been published [1,2] and b y some techniques displacement amplitudes o f the order or less than one angstr6m are recorded [3,4]. We are employing a linear photodiode array, allowing multichannel, parallel data acquisition, for two applications in which the down amplitude limit required is o f the order o f some ratio of ~ (light
s I I
P
M
L = I 5 mW laser
!~-I
~ --
~
-IP"
.
.
.
.
.
I i
P,
I
photodiode array
"=
Is.,.c,,onl 1 '°g'c I Fig. 1. Block diagram of the digital ESPI apparatus (S: moving surface; M: mirror; L: lens 5 cm focus; P: pupil; photodiode array: Reticon RL-64P); P1 and P2 are rectilinear polarizers for balancing the scattered and reference light intensities. 20
Volume
30, number
1
OPTICS COMMUNICATIONS
wavelength). Our measurements concern the thermal coefficient of expansion of very small metal samples (about 5 X 5 X 5 mm3) and deformations of different zones in a power diode surface induced by very high and fast current pulses. The aim of this note is to illustrate the employed technique which is very useful when it is necessary to follow the surface motion with time and space resolution. A block diagram of the apparatus is shown in fig. 1. Scattered light from the moving surface S is focused by L to the detector plane; in the actual experiment a silicon photodiode camera (64 photodiodes arranged in a linear array) is used. Provided that the imaging lens is sufficiently stopped down and the illumination of the surface correspondingly increased, only one speckle is image onto each photodiode, corresponding at a single observation point [S] . When the surface is translating with velocity u the single photodiode’s signal is the alternating current following the Doppler shift of the backscattered light relative to the superimposed reference beam:
July 1979
cy v, depends from the product of amplitude A, and frequency vv. In oui case (data acquisition rate = lo4 words/set) we have: VJ,
= 2.5 x 102 x.
A factor of about 100 in data acquisition
speed can
i(t) a cos (47r/h) ut. The array’s diodes are continuously active and are discharged by the photocurrent at a rate proportional to the local light intensity. They are serially accessed and recharged through an output common line; thus the output signal is a train of 64 pulses (per scan) with magnitude proportional to the recharging current supplied to each diode i.e. proportional to the corresponding diode’s light exposure. To fully utilize this multichannel sensor a fast data acquisition system is required. Data are analog to digital converted at a rate of lo4 words/set and stored, via a CAMAC interface, in the main memory of a NORD 10 minicomputer for further processing. Data from a single selectable photodiode of the array are sampled-hold for continuous display on a CRT for visual inspection. Referring to a vibrating surface the number of light maxima in a period l/v, of the mechanical vibration is a measure of the displacement amplitude A, while time between two adjacent maxima gives a measure of the instantaneous velocity. The number of light maxima per one mechanical cycle is larger for larger vibration amplitudes (see fig. 2). Therefore for a complete reconstruction of the vibration displacements of an imaged zone of the surface the array scanning frequen-
Fig. 2. CRT output from the computer main memory of a stored run; a), b) represent one mechanical cycle of a forced vibration with frequency vv = 55 Hz and amplitudeA, = 1.5 h and 2.5 h respectively; c) is a low pass filtered version of b).
21
Volume 30, number 1
OPTICS COMMUNICATIONS
easily be gained with the addition of a DMA (Direct Memory Access) module to the CAMAC interface. The ultimate amplitude-frequency limit of this technique for real time measurements therefore is:
July 1979
very long time intervals. Correlations and detailed calculations can be worked out as soon as data are written into the NORD 10, thus fully exploiting the capabilities of the computer.
%A” = 2 x 104 x. The limit imposed by the optoelectronic device sensitivity is consistent with the above mentioned one, in the hypothesis of diode matrice programmed for reading less than 64 elements. The computer memory size allows < 64 Kwords to be transferred, thus limiting the time interval of the observation with increasing acquisition frequency. At the actual (lo4 words/set) data acquisition rate, continuous transfer is possible to a magnetic tape, whose size is about lo7 words, extending the observation to
22
References [II A.E. Ennos, Topics in Applied Physics: Laser and related phenomena (Springer Verlag, Berlin, Heidelberg, New York, 1975). PI R.A. Bruce and G.L. Fitzpatrick, Appl. Opt. 14 (1975) 1621. [31 D. Joyeux, Appl. Opt. 15 (1976) 1241, 1248. [41K. Hogmoen and O.J. Lokberg, Appl. Opt. 16 (1977) 1869. [51 K.J. Ebeling, OpticsComm. 24 (1978) 125.