Phase-Sensitive Optical Coherence Microscopy (OCM) - Biomedical Optical Phase Microscopy and Nanoscopy - page 264

Reference [20] presents a phase OCM setup in which phase noise is canceled by
measurement of motions relative to a nearby surface. An unequal-arm Michelson
interferometer compensates for the path delay between sample and reference reflections,
and a differential measurement scheme that uses a passive reference gap compensates for
phase noise in the Michelson interferometer.
To assess the noise performance of the interferometer, a glass coverslip was used such that
the light reflected from the bottom surface of the glass interfered with light from the top
surface and the interference pattern was recorded. A test measurement at 1 kHz bandwidth
gave phase noise of 0.069 mrad, corresponding to displacement of 8.5 pm. To measure the
fiber activity, a walking leg nerve with 1-mm diameter and 50-mm length from an
American lobster ( Homarus americanus ) was dissected and placed in an acrylic nerve
chamber. The chamber contained five wells filled with a saline solution. Between wells the
nerve was surrounded by an insulating layer of petroleum jelly to maximize interwell
resistance. A compound action potential was generated by a current pulse from a stimulus
isolator and was detected at the other end by an amplifier with a gain of 10 4 . In the central
well, the nerve rested upon a small glass platform such that it was not submerged in the
saline solution.
The optical signal showed a peak height of 5 nm and FWHM duration of 10 ms, with a
direction corresponding to an upward displacement. The noise of the displacement
measurement was B 0.25 nm for 1-kHz bandwidth. The displacements were observed in
approximately half of the nerve preparations and varied in amplitude from 0 to 8 nm for
5-mA, 1-ms stimulation.
Figure 13.7 shows the nerve displacement and electrical potential that were measured using
the system. Positive displacements correspond to an increase in the height of the nerve
surface.
Other studies on giant axon of the squid using phase OCM with and without dyes can be
found in Ref. [21] .
13.4.7 Measurements of Red Blood Cells
In Ref. [22] , we used a low-coherence spectral domain phase microscopy (SDPM) system
for accurate quantitative phase measurements in red blood cells (RBCs) for the prognosis
and monitoring of disease conditions that manifest in mechanical and structural changes
in RBCs. Using the system, a comparison was done on the cellular dynamics of healthy
RBCs and glutaraldehyde-treated RBCs that have lower amplitude of vibrations, and
the membrane vibrational fluctuations were measured in time to reflect their membrane
stiffness. Due to its common-path geometry, the OPD stability of SDPM is less than