Pulsed sandwich holography

Hans Bjelkhagen

A new method is introduced which utilizes sandwich holography combined with a pulsed laser. In a specialrotating hologram holder two plates sandwiched together are exposed through a slit during the double pulseoperation of a laser. The two pulses are separated by less than 1 msec. Afterward, in an evaluating holder,one plate is rotated in relation to the other so that the images from the two pulses are overlapped. Then thetwo plates are glued together. If the emulsions are separated, fringes caused by object tilt between the twopulses can be eliminated by an analogous, but much larger, tilt of the sandwich hologram during reconstruc-tion. Even the direction of tilt, forward or backward, is found this way.

Introduction

Sandwich holography1-7 has been introduced byAbramson as a useful method within hologram inter-ferometry using cw lasers. Pulsed holography has beenwidely used for testing and inspection in factory envi-ronments because of the short time of exposure. Thepulse is short enough to create a hologram even of amoving object. However, sometimes fringes caused byrigid body motion of the object give difficulties in de-termining only deformation in a double pulsed holo-gram. There is also the problem of not knowing the signof displacement in an ordinary double exposed holo-gram.

Rigid body displacement effects may be canceled outin recording systems by using techniques presented byGates and Hall8 and Neumann and Penn,9 where how-ever no possibilities to manipulate the fringes existduring reconstruction.

Erf et al. made some efforts to solve those problemsusing dual reference beam pulsed holography. A fringecontrol procedure was possible, but the adjustmentswere not simple to effect. Only image plane hologramshave been used.

Pulsed sandwich holography, introduced in thispaper, has solved a lot of the problems discussed aboveand is no more difficult to use than ordinary doublepulsed holography. However, a special hologram holderfor exposure and a special holder for evaluation andbonding the two plates together are needed.

Method

In ordinary sandwich holography two holographicplates with the emulsion facing the object are exposed

The author is with the Royal Institute of Technology, Laser Re-search Group, Department of Production Engineering, S-100 44Stockholm 70, Sweden.

Received 13 November 1976.

simultaneously in a special holder. Then the object isdeformed. A new pair of plates are put in the holder,and an exposure of the deformed object is performed.After development the back plate from the first pair iscombined with the front plate from the second pair, andthey are glued together.

The fringe control technique is introduced by tiltingthe sandwich to compensate for rigid body motion or todetermine the sign of displacement (Fig. 1). Thistechnique has been used for some years, and it worksvery well.

Of course the described technique for cw laser ho-lography can also be used for pulsed holography if thepulses are separated by many seconds so that the platescan be changed between -the pulses. We made someearly experiments which prove that this methodworks.

The main purpose of the project described in thispaper was to develop a method for when the delay timebetween the two pulses is less than 1 msec, and theplates cannot be changed easily. The approach was toexpose a rotating pair of plates so that the two imagesfrom the two pulses could be separated at the sandwichplates and afterward combine the recording on the backplate from the first pulse with the recording on the frontplate from the second pulse. To separate the two im-ages from the two pulses a relatively high speed of theplates is needed.

The following calculation can be made to determinethe upper limit for exposing a rotating plate. Fringeseparation (d) on the plate is given by d = X/(2 sina),where 2 is the maximum angle between two intersec-tion beams at the plate. The time of exposure is t (theQ-switched laser pulse). If during the time of exposure,the holographic plate is allowed to move a quarter of thedistance between two fringes the speed (v) perpendic-ular to the fringes will be

v = /(8 t sina). (1)

June 1977 / Vol. 16, No. 6 / APPLIED OPTICS 1727

hol ogramsB1 F1

n n

object0.

first exposure

second exposure

p

d

B1 F2 02 01

hIM21 P l

i d Lreconstruction with fringes

'2 0201'2~~~~~~~~2 11

Eye 9\ )

fringefree reconstruction

Fig. 1. The top of the object 0 is tilted at angle Xl by the force P.Therefore, a speckle ray from one object point is moved the vertical

distance h from P1 to P2 . B and F are the emulsions of back and front

hologram plates, respectively. Glass plate thickness is d, and re-

fractive index is n. 02 and 'I'2 represent sandwich rotation around

a horizontal and vertical axis, respectively. Object translation is Xi.

Corresponding sandwich translation is X2 . The identical reference

and reconstruction beams are excluded in the figure.

If r is the radius of a circular plate and the speed at theedge is v, the highest rotation speed is

4 = 15 - A (rpm). (2)4 *, -r t * r -sina

Another aspect is the distance (D) between the twospots on the plate that is possible to obtain for a giventime delay (T) between the pulses.

D = v*T.

D = (r n r T)/30.

Fig. 2. Exploded view of the holder for the rotating plates.

Fig. 3. Rotating plate holder.

(3)

(4)

Example:X = 0.6943 Azm (wavelength of the ruby laser);t = 20 nsec (Q-switched pulse);

T = 0.5 msec (pulse separation);r = 45 mm (the largest radius from a rotating 9 X

12-cm plate);2a = 20°.

Equation (1) gives v = 25.0 m/sec;Equation (2) gives n = 5303 rpm;Equation (3) gives D = 12.5 mm.

At the spindle of an electric motor a disk equipped

with a plate holder was attached (Figs. 2 and 3). Theplate holder has three points (ball bearings) making

contact with the plate surface and three pins fixing itssides. Three clamping springs with the forces actingat the ball bearings through the plates were used to holdthe plates. The plates were also pressed to the threepins by two springs on the opposite sides. The positionof the plates at the disk is such that the centrifugal forcepresses the plates toward the three pins during rotation.The disk with the plates is well balanced. In front ofthe disk a metal sheet with a variable slit is placed closeto the rotating plates. This metal sheet also acts as aprotection device should the plates break during rota-tion. As a matter of fact, we have had no plates dam-aged even at rotation speeds of around 8000 rpm.

1728 APPLIED OPTICS / Vol. 16, No. 6 / June 1977

I '1 11 i

, ..

Fig. 4. Experimental arrangement. Seen from the left are rotatingcylinder, reference surface, and reference mirror.

Experimental ConditionsThe experimental setup consists of a rotating, white

painted cylinder ( = 110 mm and n = 0.75 rpm), a fixedreference surface also painted white and a mirror todirect the reference beam to the plates (Fig. 4). Thelight from a ruby laser (Holobeam 651) was spread bya negative lens to illuminate the objects. The rotatingplate holder was placed in front of the object (Fig. 5).The plates used for the experiments were without anyantihalo coating so that the back plate could be exposedthrough the front plate. No triggering is needed. Atany time the rotating plates can be exposed. For theresults shown in this paper the plates were exposed atn = 6800 rpm, and the time separation between the twopulses from the laser was 600 Asec.

The separation between the two spots at the plateswas 16 mm at a distance of 45 mm from the axis ofrotation. The quality of the holographic images at therotating speed of 6800 rpm was as good as if the plateshad been exposed at rest.

IIREFERENCE I

MIRROR I

REFERENCE 1SURFACE

ROTATINGCYLINDER \

Evaluating EquipmentThe evaluating equipment was designed to make it

possible to combine the image from the first pulse withthe image from the second pulse and to bond the platestogether. Therefore a holder was made similar to theone used for exposing the plates. The position of thepins, ball bearings, and clamping devices was the same.However, there was one holder for each plate so that theplates could be rotated in relation to each other aroundthe same axis as the axis of rotation during exposure(Figs. 6 and 7).

After positioning the developed plates in the holder,a coarse rotation of one plate was made to align the firstand second image. Then by observing the holographicimage and using a micropositioning device for fine ad-justment of rotation, fringes appeared. The adjust-ments were finished when the reference surface wasfringe free. After that the plates were glued together.The bonded plates were removed from the holder, andby hand the plates could be evaluated like ordinarysandwich holograms (Fig. 8).

The procedure to attach the plates in the evaluatingholder was to rotate them for interference between thetwo pulses and glue them. This took only about 5 minfor one pair.

When looking at the sandwich hologram, vertical,parallel fringes appeared on the cylinder because of itsrotation. Figure 9 shows a photograph of the pulsedsandwich hologram. These fringes could be taken awayby tilting the sandwich around a vertical axis in thesame direction as the rotation of the cylinder (Fig. 10).By introducing a known horizontal tilt of the sandwichthe inclined fringes at the cylinder also give informationon the direction of rotation (Fig. 11). The limited sizeof the hologram makes it sometimes better to project animage on a screen for evaluation.

Because the reconstruction is made by means of aHe-Ne laser an error is introduced in the fringe patterncaused by the difference in wavelength between theHe-Ne laser and the ruby laser. As seen in Fig. 9 thefringes at the cylinder are slightly bent. Therefore,when using this technique the evaluation should bemade using the same wavelength as during exposure.

NEGATIVELENS

RUBY LASER Fig. 5. Experimental setup.

SLIT

HOLOGRAPHICPLATES

June 1977 / Vol. 16, No. 6 / APPLIED OPTICS 1729

Fig. 6. Exploded view of the holder for evaluation and bonding theplates.

Fig. 9. Reconstructed sandwich hologram with fringe free referencesurface.

Fig. 7. Evaluation holder.

Fig. 10. Fringe free reconstruction of the rotating cylinder from thesame sandwich. hologram as in Fig. 9. The hologram is tilted in the

same direction as the object was rotated.

Fig. 8. Pulsed sandwich hologram.

Fig. 11. Reconstruction with horizontal fringes at the reference

surface. The inclined fringes at the cylinder also give informationon the direction of rotation.

1730 APPLIED OPTICS / Vol. 16, No. 6 / June 1977

This is not a big problem, because ruby lasers and a Nd. YAG laser exist followed by a lithium niobate fre-

quency-doubling crystal with a pulse repetition rate of50 Hz. At the evaluating stage it could also be possibleto use a dye laser operating at the same wavelength asthe pulsed laser.

DiscussionThe method presented seems to be a good way to

make pulsed hologram interferometry even more usefulin factory environments and for engineering pur-poses.

Advantages of the Method

It provides a simple way for fringe control in pulsedholography.

It makes possible determination of the sign of a vi-bration amplitude or a displacement.

It is applicable to workshop investigations.Evaluation can be made by holding the sandwich by

hand in a laser beam.An ordinary holographic setup can be used. The only

additional equipment is the rotating holder.It is not limited to only two pulses. Many pulses can

be separated and during evaluation combined at ran-dom.

Disadvantages of the Method

A special hologram holder and special bonding holderare needed.

The size of the hologram is limited.The angle between reference and object beam must

be limited in relation to the rotation speed of theplates.

Some time is needed to align and glue the plates.Exposure and evaluation should be made using the

same wavelength to avoid errors.

ConclusionEfforts are going on to improve the technique. For

example it is possible to use many slits in front of theplates instead of only one to increase the size of the il-luminated part of the holographic plate. The slitsshould be of the same width as the rotation distanceduring the time delay between the two pulses. Then

it should be possible to get good holograms even if thetime delay is very short. It is also possible to use ashutter, synchronized to the laser output, in front of theslit of the hologram holder to prevent fogging of theplates by the ambient light.

Another possible way to make a pulsed sandwichhologram is to use the rigid sandwich technique pre-sented by Hariharan and Hegedus.7

The rigid sandwich consists of one fast plate and aslow plate, glued together, with their emulsions facingoutward, to two sheet glass spacers. The sandwich isexposed with the slower emulsion facing the object.The back plate is covered by a strip of black paper in thegap between the plates during the first exposure. Thesecond exposure of much shorter duration is madewithout the black paper in the gap and without appre-ciably effecting the front plate.

We propose making pulsed sandwich holography byusing pulses with unequal output energy and a rigidsandwich with either a material in the gap between theplates that is darkened by the first (weak) pulse fromthe laser or a shutter that covers the back plate duringthe first (in this case strong) pulse from the laser.

The author thanks Nils Abramson for his valuablesupport and Bertil Colding, the director of the depart-ment. The Swedish Board for Technical Developmentsponsored this project and supplied the ruby laser.

References1. N. Abramson, in Proceedings Electro-Optics, International '74

Conference, Brighton, England (1974), 35.2. N. Abramson, J. Opt. Soc. Am. 64, 552 (1974).3. N. Abramson, Appl. Opt. 13, 2019 (1974).4. N. Abramson, Appl. Opt. 14, 981 (1975).5. I. Sollid, Opt. Eng. 14, 460 (1975).6. N. Abramson, Appl. Opt. 15, 200 (1976).7. P. Hariharan and Z. S. Hegedus, Appl. Opt. 15, 848 (1976).8. I. W. C. Gates and R. G. N. Hall, in The Engineering Uses of

Holography (Cambridge U. P., London, 1970), p. 115.9. D. B. Neumann and R. C. Penn, J. Opt. Soc. Am. 62, 1373

(1972).10. R. K. Erf, R. M. Gagosz, and I. P. Waters, in The Engineering

Uses of Holography (Cambridge U. P., London, 1976), p. 73.

From The Dry Rotof our AcademicBiology, byW M Wheeler,Science 57pp 61-71 (1923).

Many of us coddle our graduate students till the more impression-able of them develop the most sodden types of the father-complex.Some of us even wear out a layer of cortical neurones annually,correcting their spelling and syntax. One fussy old guru of myacquaintance has destroyed both of his hemispheres, his corpuscallosum and a large part of his basal ganglia hunting stray com-mas, semicolons, dashes, parentheses and other vermin in doctors'dissertations.

Reproduced from A RANDOM WALK IN SCIENCEcompiled by R. L. Weber published by Institute of Physics 1973

June 1977 Vol. 16, No. 6 APPLIED OPTICS 1731