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Ruby Pulse Holography 2006
Rwmopa8 ruby laser has been running as a single frequency TEM00 mode laser with 6mm beam size and 1.1 to 1.8 joule single pulse output of 12-20nsec. The laser was capable of much higher intensities from a power supply/flashlamp ability but was kept at this moderate level to reduce chance of amplifier damage by keeping the peak intensity below 6-10j/cm2. When needed, extra power can be applied. The amplifier rods due to negative lensing, the higher they are pumped with power levels, the increased beam divergence helps lower the beam power density automatically for higher output powers. But due to the amps having 3/8 inch rods, eventually the spatial diffraction ripple increases due to this aperture restricting size. Clean optics and enough aperture for the beam passage through the amplifiers help minimize diffraction ripple in the spatial beam but become unavoidable at higher levels due to rod size.
Adding dye/ethanol to q switch and monitoring with oscilloscope to make sure single pulse before a hologram session and the output energy level is verified.dyecell.jpg
More info on laser.ruby.htm
Projected Wall test of near full power laser output. wall.jpg
Changes to old holography setup. holoreflect3.gif
link to original setup info holosetup.htm
Setup with new 16 inch paraboloid mirror. At F7.5 and 119.84 inch focal length the overall path lengths were measured at 156 inch for the reference beam and 157" for the object beams. mirrors.jpg
New configuration of beam splitter uses first beam splitter for ref and obj and second BS for obj split. The beam splitter setup was designed to insure a good reference beam quality by using only a single surface reflection and adding versatility to the beam ratio ability by inserting various lens which changes the beam ratios to improve fringe contrast. The target ratio was set to a 3/2; reference/object intensities .
Picture of energy meter sampling of output beam. Secondary reflection from reference splitter is caught instead of wasted to allow energy output measurement. beamsamp.jpg
Picture of laser firing though beamsplitter setup using ccd camera with OD3 neutral density filter. testsplit.jpg
Diffusion screens during a test fire of the holographic setup.
Reference beam coverage of film plate during test fire. ref.jpg
Image of reference beam at film plate with lower power to better visualize coverage. ref2.jpg
Image of reference beam at parabolic mirror . refp.jpg
One technique used to shield subject from the reference beam is asking subject if they can see a red light through the film plate is the hene alignment beam source and the stool is raised until they are not able to see the source light. Based on distance from film plate and height the reference beam should not be able to be seen. The beam boards will block any chance of the beam over reaching the film plate and hitting the subject. In order to fill a 30cm x 40cm film plate the reference beam is set to 18 inch diameter. bref.jpg
Even though the laser operates in single frequency mode and the coherence length should be around 3-6 meters or better, the path length differences between obj and reference should be less than half for best result. Also based on the oscillator resonator length the ideal distance of 2L was calculated so that the maximum intensity of the interference fringe on the plate would be closer to the 2L multiple path length, if a second mode should develop. The worst case would be odd multiple of L path length. In this case the path lengths were brought closer to the 10L where L was 22.42 inches of effective resonator length. 10L was 152.58 inch beyond the laser since amplifiers and other beam mirrors inside rwmopa8 made for an additional 71.62 inch laser internal path length. A Tektronix 2465B analog 400mhz scope was used to analyze the pulse shape to see if mode beating occurred. It was found to only occasionally develop a second mode based on clean pulse modulation harmonics present on the pulse envelope. With two modes and L length of 22 inches, this would mean that the coherence length at most would drop to 22 inches. If the second mode was at the same intensity but instead was found be no stronger than 50% of the fundamental mode and generally around 5% of the single mode amplitude. 80 percent of the test shots showed no pulse envelope modulation indicating single mode laser shots.
The ruby rods were setup with c plane horizontal so the output is vertical E field polarization this means S polarized for beam steering mirrors inside the laser and for beam splitters and mirrors. This is how normal optical bench type setup can be done to minimize the polarization angle of the two beams arriving at the film plate since the beam arrive in a S polarized orientation. The main object mirrors and reference paraboloid mirror in this setup are tilted downward and the vertical polarized beam is now a P polarized beam due to the now vertical (downward steering of the beams) and the angles will effect the fringe maximum. Unfortunately this setup has both horizontal and vertical beam steering. So one could evaluate which polarization orientation would produce less possible angle between object beam and reference beam. The laser was built with vertical laser polarization and was kept as default but noted as an issue of concern. Calculations in this setup with a general idea of 25 degree P polarized incidence of the reference beam to the film plate and the film plate at a 5 degree tilt toward the subject, 25-5 = 20 degrees polarization angle and the fringe visibility due to polarization was calculated in excel as =cos(20*pi()/180) or about 94 percent.
Beam splitter 1 used a clear wedge window for reference with a calculated uncoated S polarized at 30 degrees of about 6 percent. Secondary surface 6 percent was blocked Care was taken to make sure any secondary reflections were blocked and additionally the laser and splitters are mounted overhead.
Additionally the setup allowed different lens to be used for object and reference beams to control beam ratio intensities. The reference beam was spread further to give more even illumination.
The object beam was fixed with a -50mm and 275mm telescope at 54mm separation with some adjustment to separation.
The following reference lens gave these results:
-25mm below 1 to 1 ratio as measured at film plate.
-30mm around 1 to 1 ratio
-38.6mm around 3 to 4 to 1 ratio.
Two Thorlabs DET210 detectors with ND filters and a digital storage oscilloscope were used to record the beam intensities and ratios in the setup. test.jpg
The -30mm reference lens gave a very smooth reference beam and maximum contrast for the fringes with ratios approaching 1 to 1. Agfa 8E75HD film was used with about 30uj/cm2 exposure level. A 1 to 1.3 joule single laser pulse gave moderately bright sharp images with additional film developing to OD2.0 using the SM-6/Pyrochrome beach method shifting the playback frequency from 694nm to around 600nm giving a nice bright orange look. For the best in image quality the laser must produce a single pulse. Multiple pulses with 100s of microsecs between them gave holograms of lower quality due to interferometer like effects.
The object beam and reference beam have their own expanding lenses, so the object beam lens focal length can be reduced to allow more subject area to be illuminated without getting too high of a beam ratio. The setup with tight object beams approaches 1 to 1 ratio the wider object beams move the ratio closer to 2 or 3 to 1 if a larger area is needed.
Measured and calculations showed the following:
Path length 157 inch to plate. Within 1 to 5 inches of max temporal intensity of 2L.
Reference beam size at 16 inch paraboloid mirror was 25 inches.
Diffusers at 127 inch with 16inch spot size.
Diffuser to subject was 13 inch and then 12 inches to the plate
Reference lens to paraboloid mirror was 94 inch, which is below the 119 inch focal length to yielding a slight diverging reference beam. Reference beam size at film plate was 18 inch.
1/73 of the incident object beam on the diffuser reflected to the film plate from the subject of fair skin and light clothing. Energy at the film plate was roughly at 10 to 25uj/cm2 reference based on power levels and 8 to 10uj/cm2 object lighting. The safety ANSI extended source MPE was calculated at 500nanojoule/cm2 MPE times the extended source correction factor in this setup (16 inch diffused spot size at 13 inches from subject) 500nanojoule/cm2 x 10098 = 5049 microjoules/cm2. At 1 joule level, object beam 1 had a density of 800 microjoule/cm2 incident on the diffuser and object beam 2 had a 160uj/cm2 level, either beam and in total was below MPE. Calculations were made based on the beam energy of 800mj object beam energy at a viewing dist of 13 inch from a 16 inch diffuser with the ANSI formula for MPE. These calculations for an extended source, which uses the apparent visual angle created, and Lambert's law adjusted with a Dp/2 term per ANSI when the viewing distance is within 10 source diameters. The resultant energy is 56 times lower than the allowable maximum energy that will produce a hazardous diffuse source. Now this is not true of the reference beam whose average power is 20uj/cm2 which for ruby pulses is 40 times over the allowed MPE and will visually appear as a small source. Therefore the subject's eyes must not be hit with the reference light and proper care taken to prevent it by adjustment of subject height so beam falls on chest. Use of transmission H1 master and H2 Reflection technique helps to reduce this problem of using a direct reflection holographic technique.
A comparison of film and reference angles etc is presented: http://www.holography.co.uk/hcon1hb1.pdf
According to this study using a single beam reflection hologram, Agfa 8E75HD diffraction efficiency should be around 6 % for a 21.5 degree reference beam and reversal bleaching. If instead the rehalogenated bleach and 31 degree reference angle is used the efficiency could be as high as 15%. Even though the diffraction efficiency is higher, about twice, the apparent brightness at 694nm would be much less than a 600-625nm playback from reversal beach shrinkage as much as 30 to 1.
Overall the quality of the hologram was good. The brightness was moderate for the reflection volume hologram. The reference illumination was smooth without hot spots. In the sun the image was sharp and the graininess of the development was low to moderate. Good quality 30cm x 40cm film and plates can be made with this setup.