We use volume holography
to create spectrally specific, selective filters for the identification
of substances such as toxic or explosive materials. The identification
method is spectroscopy (such as IR or Raman spectroscopy) where the
identity of molecules is found in the detailed absorption or emission
spectra. Volume holographic filters are able to improve the sensitivity
and speed of the measurement by detecting multiple absorption (or emission)
spectral lines of the given substance simultaneously. The operation
is based on the Bragg selectivity and multiplexing ability of volume
holograms. It’s well known that within the dynamic range of the
holographic recording medium, multiple holograms can be superimposed,
or multiplexed, in the same volume, which makes it possible to construct
a holographic filter whose wavelength selectivity curve (spectral response
curve) is matched precisely to the absorption spectrum of a given substance.
In order to achieve this, a special recording exposure schedule must
be carefully designed such that the strength and spectral bandwidth
of individual hologram are matched precisely to those of the corresponding
peak in the spectrum. With multiple peaks detected simultaneously, it’s
expected the detection sensitivity and speed will be increased greatly
compared with traditional methods, and the required data volume will
decrease by several orders of magnitude, which makes it very attractive
for remote sensing applications.
Figure
1. Experimental
setup for recording volume holographic filters in symmetric transmission
geometry using a 514nm Argon laser. M1, M2 and M3 are mirrors. Multiple
holograms are recorded in sequence by rotating the two mirrors M2 and
M3 mounted on rotation stages. The filter is used in the reflection
geometry with IR illumination.
The experimental setup for recording a holographic filter with prescribed
filtering characteristics (spectral response) is shown in Figure 1.
A 488nm Argon laser beam is split into two mutually coherent beams with
equal intensity by a Beam Splitter. A hologram (grating) is recorded
inside the recording medium by interfering these two beams. The superposition
of multiple holograms is realized by introducing multiple pairs of recording
beams in sequence. This is done by rotating the two mirrors M2 and M3
after each hologram recording. Two 4-f systems (not shown in figure
for simplicity) between the two mirrors and the medium are needed to
ensure the two recording beams always overlap at the same position inside
the medium. The desired spectral response is written into the hologram
by breaking it up into a sequence of peaks of varying amplitude and
width and then recording a separate grating for each of the peaks. The
spectral center of each grating is determined by the incident angles
of the two recording beams. The strength of each grating is controlled
by the exposure time during hologram formation. Finally, the spectral
bandwidth of each grating can be controlled by either the amplitude
of the index modulation (strong grating regime) or the effective hologram
thickness (weak grating regime). A filter constructed in the way described
so far has a spectral response with multiple peaks at specified positions
and with specified relative strengths and widths.
To prove the concept, we recorded seven gratings in a 2¥1¥0.5cm
LiNbO3:Fe crystal. The recording intensity is 5mW/cm2. The exposure
schedule is 75min -> 60min -> 50min -> 40min -> 30min ->
20min -> 10 min. The measured diffraction efficiency is shown in
Figure 2. As can be seen, the diffraction efficiency of the weakest
grating is still above 40%. The sidelobes on the short wavelength side
of each peak are believed to be due to the ‘dc’ refractive
index apodization.
Figure 2. Measured
diffraction efficiency of a volume holographic filter. The filter is
recording by multiplexing seven Bragg gratings in the same LiNbO3:Fe
crystal.
