Biomedical Engineering Reference
In-Depth Information
The invention in the late 1960s of digital detectors such as charge-coupled devices (CCDs) and active-
pixel sensors (also known as CMOS sensors) provided the ideal solution for digital recording of optical
holograms. These two-dimensional devices directly convert light intensity to electric charges that can
be quantized by a built-in analog-to-digital converter. More importantly, they can be manufactured at
relatively low cost and they easily provide video or better frame rate. Today, most of the work carried in
digital holography is based either on CCD or CMOS sensors.
Despite completely new technology that sees digital sensors replacing photographic plates, schemes
for recording of holograms in digital holography are very similar, if not identical, to those in classical
holography. In turn, recording of SHG holograms is very much alike recording of digital holograms in
bright-field digital holography, with the main differences being attributed not to the digital recording
medium, but to all the required apparatus to generate the second-harmonic object and reference waves.
9.3.1 Setting up a Holographic SHG Microscope
All digital holographic microscopes, no matter how they are physically implemented, comprise the
same functional groups of elements. In this section, we present implementations of transmitted light
(Figure 9.3a) and reflected light (Figure 9.3b) off-axis holographic SHG microscopes.
It should however be noted that to this day, we are unaware of any reflected light holographic SHG
microscope and only the transmitted light types have yet been reported. In this sense, the description of
(a)
(b)
M
M
M
M
BS
BS
BE
M
M
M
M
M
M
M
M
M
M
BE
L
L
FDC
FDC
DL
DL
L
L
CL
CL
S
BE
BE
TL
CCD
F
o
L
BS
MO
L
r
M
CC
BS
r
MO
CC
M
o
F
S
TL
CCD
FIgurE 9.3
Schematics of typical holographic microscopes in a Fresnel configuration. (a) Transmission holo-
graphic SHG microscope and (b) reflection holographic SHG microscope. BS, beam splitter; L, lens; M, mirror; BE,
beam expander; FDC, frequency doubler crystal; CL, condenser lens; MO,
∞
-corrected microscope objective; TL,
tube lens;
o
, object wave;
r
, reference wave; S, specimen; F, filter, and CCD digital sensor. Beam envelopes are rep-
resented by uniform gray shades (dark for fundamental and pale for the second harmonic), while image formation
is represented by dashed rays.
