Holography, Art and Design Conference

Royal Photographic Society

 

Royal College of Art - Saturday 23rd March 2002

 

The PTI Holocamera - Research and Development

 

Mike Anderson     

Introduction

The PTI Holocamera was designed primarily for the higher education market, but is also a useful system, which may be used by anyone with an interest in holography. 

 

Design Philosophy

I considered using laser diodes about 5 years ago and was encouraged to put more effort into research into this area during 1999 when I was contacted by Michael Hunter of the Precision Tool and Instrument Company who asked me to consider the possibility of designing an inexpensive holographic system for educational purposes following on from discussions with science education suppliers.  Michael Hunter is in the audience and will be pleased to respond to any questions of a business nature at the end of my talk.  PTI is well known for manufacturing a range of laboratory equipment including optical benches, spectrometers, travelling microscopes, cathetometers and the "Holography Laboratory", a flat-bed holographic system which I had designed for PTI in 1993.  

 

A feasibility study was carried out in respect of possible configurations, which involved a considerable amount of CAD work after the usual hand drawn sketches.  Three prototypes were constructed, the first two for reflection holography only, and the last version being capable of making reflection and transmission holograms.  The production Holocamera is essentially the same as the final prototype with the laser diode unit being extendible from the main structure to optimise light distribution over the film and object.

 

As may be observed, the Holocamera has a vertical reference beam which was first used by the speaker in earlier designs, viz. two versions of the Holomex Viewcam.  In this instance, however, the design is an open one, which enables students to visualise the beam paths from a wide range of viewing positions.  As well as being very didactic, this concept helps to keep costs low, which is further aided by the use of a laser diode in place of a laser which is of primary importance in the design philosophy of the Holocamera.

 

Laser Diodes

Early tests with available laser diodes, both in modular form and also using separate drivers, produced holograms with a range of quality varying from very good to disasters.  In some cases there was pronounced fringing due, as I subsequently discovered, to mode hopping, but there was sufficient indication that the idea was viable.  At the time, I knew was fairly ignorant of the science and technology of laser diodes and it is true to say that I am only a little wiser now.  After consultation with a number of experts, they were of the opinion that I was wasting my time using laser diodes for holography since their output was insufficiently stable or coherent for an exposure time of a few seconds.  I was informed that laser diodes were prone to mode hopping i.e. the lasing frequency can suddenly flip from one mode to another, sometimes in fairly rapid succession - which is not a common characteristic of lasers and certainly unacceptable for holographic purposes.  Being of a cussed nature, I persevered.

 

A major problem to solve, was choosing the best laser diode for the purpose, bearing in mind that there are a number of fundamentally different types, made by a number of manufacturers all with wide product ranges.  Also, they are available as separate items or as modules containing a driver and collimating optics.  Laser diodes have to be powered-up with very stable, purpose designed or commercial drivers from a stable DC power supply.  Laser diode modules are quite expensive, generally not very well heat sinked and for my purposes had to have their integral collimation systems removed which is rather wasteful.

 

A considerable period of time was involved in developing my own driver since I had considerable difficulty in finding an economically viable commercial one at the time.  It is important to mention that anyone wishing to become involved in laser diode research, should take note that laser diodes are very sensitive to voltage spikes, over-voltage, over-current and static electricity, resulting in a blow-up!  My circuitry eventually worked very well, but only after the demise of a great number of laser diodes.  For production purposes, however, it was decided to use a commercial driver plus some additional components.

 

            Laser Diode Parameters

            Wavelength - maximum of 635nm to maximise viewing brightness 

            Output - maximum 5mW for financial and safety reasons

            Beam divergence 8o x 30o - not much choice here

            Uniform beam with no blemishes

            Single frequency

            Coherence length - greater than about 100mm

            Good frequency vs temperature stability - large heat sink

            Good frequency vs current stability - stable driver and power supply

            Good frequency vs voltage stability - stable driver and power supply

 

The first four parameters are not difficult to satisfy, although it is not unknown to find a particular specimen with a non-uniform beam.  With regard to coherence, some laser diodes appear to be better than others -  budget types are more variable. 

 

Heat sinking is very important, since a very small change of temperature can result in mode hopping.  Following a discussion with David McGuinness of Photonic Products and looking at manufacturers data, there are typically about 1.5 mode hops/oC.  Even with good heat sinking and allowing adequate time for thermal equilibrium to be reached (5 minutes or so), there is always the possibility that a mode hop may occur during exposure.  The laser diode in the Holocamera is enclosed within a fairly large mass of aluminium alloy plates to dissipate heat. 

 

The Holocamera uses a stabilised 3V DC mains power supply (battery operation is also possible) plus a very stable driver unit to ensure good control of current and voltage.  It should be noted that the output from laser diodes is also monitored and controlled by an internal photodiode.

 

Laser Diode Optics

From my original research with a commercial laser diode module, I discovered that good quality holograms could be made with the collimator removed, which made it unecessary to use a beam expander/spatial filter.  This had the added benefit of keeping costs down.  The elliptical beam profile of a laser diode may be seen on the screen using my latest creation, a 20mW laser diode module with excellent heat sinking. 

 

In the Holocamera, the elliptical beam is split into two by intercepting it with a mirror, which provides the object beam.  The other half of the beam is reflected from a second mirror, which provides the reference beam.  A beam splitter is not required, although this does mean that the beam balance ratio cannot be changed without recourse to using ND filters.  In practice, however, this has not been found to be necessary especially since the equipment has been designed for demonstration purposes rather than as a research tool. 

 

By placing a cylindrical convex lens in front of the laser diode beam along its major axis, as shown on the screen, the beam may be converged to have an approximately circular profile.  The beam has a central brightness which is approximately 3X that of the raw beam, and in the Holocamera, it may be used to expose Denisyuk holograms as well as providing a reference beam of enhanced brightness for reconstruction.  A cylindrical lens is swung into the light path at the end of a rotating arm operated by a knob on the top of the laser diode unit.

 

Construction of the Holocamera

The Holocamera was designed to be as simple as possible so that students could quickly change the optical configuration to make Denisyuk or double beam reflection holograms as well as double beam transmission holograms.  Therefore the reference beam angle has been fixed at approximately 450. and there are only two positions for the object beam mirror, one for reflection holograms and the other for transmission holograms.   By the simple expedient of standing the Holocamera on end, the film plane may be positioned horizontally, which is ideal for making Denisyuk holograms of small objects such as keys, jewellery and watches etc.

 

As mentioned previously, the laser diode unit has a knob controls the beam output for single or double beams as well as bringing a shutter into the beam.  There is no timer in this particular version.

 

One interesting innovation is the magnetic film holder, which applies the correct degree of pressure to the film by means of four powerful magnets within the vertical posts which attracts the swinging plate which fitted with steel bars at the top and bottom.  Maximum film size is 4in x 5in.

 

Vibration damping is effected with 8 rubber buffers and background tone may be changed from dark to light by the addition of a white tile.

 

Holograms are normally viewed "in situ" since the processing chemistry has been designed for reconstruction at 635nm.  Reflection holograms may of course be illuminated with a mini-spotlight in the normal way.

 

Holographic Film and Processing

A major current problem in holography is that of film availability after the demise Agfa 8E75 film.  The only viable alternative at the time of my early work was to use Slavich PFG-01.  On test, this proved to have much lower sensitivity than Agfa 8E75 by a factor of about 4X.  Initially, holograms were of poor quality with a high level of noise when developed in a range of formulations, which had been used successfully with Agfa film.  After discussing the problem with Amanda Ranalli of HMD, she recommended that I make contact with Ian Holmes, who confirmed that CW-C2 developer worked very well.  Tests proved that this was the best developer so far, although quality was still not as high as using Agfa film.  Initial evaluation of results was made more difficult by the uncertainty of not knowing whether it was the laser diode or the developer which was at fault.

 

After many tests, an effective formula was eventually achieved and is a re-formulating of CW-C2.  It is used as a concentrated one-shot system using dispensers fitted to the Developer and Activator bottles (12ml Developer + 12ml Activator).  The bleach is Ferric EDTA, which is of course re-usable.  Processing is carried out using a plastic container with sealed lid.

 

 

Sales enquiries to:

Michael Hunter                                                             

 

The Precision Tool and Instrument Company                   

Brett Drive                                                                    

Bexhill on Sea                                                              

E. Sussex                                                                    

TN40 2JP                                                                     

Telephone: 01424 732 674                                             

e-mail:       michael@solgroup.demon.co.uk                                         

 

Technical information from:

Mike Anderson

MA-Techs

4 Borrowdale Avenue

Harrow

HA3 7PZ

Telephone: 020 8427 9685

e-mail:       michael.anderson@triaster.co.uk