Using Eq. (3) variation in diffraction efficiency () with wavelength
at Bragg’s angle for different values of depth of refractive
index modulations of holocon recording has been plotted. While
drawing the curves care has been taken to ensure that criteria for
thick phase transmission holocons are fulfilled for which Eq. (3)
holds good [36]. A holocon is said to be thick if its Q parameter
(Q = 2d/n2) is greater or equal to 10 [37].
4. Experimental
4.1. Recording of holocons
Holocons are recorded using two coherent waves derived
from the same laser source. Out of two coherent waves one
is spherical wave and the other is a plane wave. Schematic
of the recording geometry is shown in Fig. 1 and photographs
of the recorded holocons in presence of fluorescent tube are
shown in Fig. 2(a)–(c). For present work holocons have been
recorded on high resolution silver halide plate PFG-01 (film
thickness d = 7 m and average refractive index n = 1.61) using
a He–Ne Laser of power 2 mW. The exposed film was processed
using standard procedure [38,39]. In order to study the
feasibility of spectral characteristics and performance analysis
of chromatic characteristics of holocons, three typical holocons
were recorded with different depth of refractive index modulations
(n1 = 0.014, 0.018 and 0.024). For spectral responses they
were illuminated with available wavelength sources ( = 488 nm,
514.5 nm, 532 nm and 632.8 nm) at the optimum Bragg position
for maximum efficiency operation and for chromatic characteristics
they were played back by a white light coming from a LED
source.
5. Optical characterization
5.1. Spectral characteristics
Using Eq. (3) variation in diffraction efficiency () with wavelength
at Bragg’s angle for different values of depth of refractive
index modulation are plotted in Fig. 3.
Fig. 3 shows that low depth of refractive index modulation of
holocon exhibits maximum efficiency in lower wavelength (ultraviolet)
region whereas its high depth of refractive index modulation
exhibits maximum efficiency in higher wavelength region (near
infrared) at a particular film thickness and at the same time the
intermediate depth of refractive index modulation are quite suitable
for visible range of the spectrum.
5.1.1. Feasibility studies
Recorded holocons were illuminated with available four different
wavelength sources ( = 488 nm, 514.5 nm, 532 nm and
632.8 nm) and angle of illuminations were optimized for maximum
diffraction efficiency in each case to ensure illumination on
Bragg’s angle [13]. Fig. 4 shows experimental curves for variation in
diffraction efficiency versus wavelength on illumination atthe optimum
Bragg position. Simulation curves are also drawn on the same
plot with the same specifications as that of the typically recorded
holocons using Kogelnik formula for diffraction efficiency. Experimental
curves are very close to simulated curves. In order to realize
all other simulation curves shown in Fig. 3, one has to achieve
Fig.