Phase Modulation at 125 kHz in a Michelson Interferometer Using an Inexpensive Piezoelectric Stack Driven at Resonance

Fast phase modulation has been achieved in a Michelson interferometer by attaching a lightweight reference mirror to a piezoelectric stack and driving the stack at a resonance frequency of about 125 kHz. The electrical behavior of the piezo stack and the mechanical properties of the piezo-mirror arrangement are described. A displacement amplitude at resonance of about 350 nm was achieved using a standard function generator. Phase drift in the interferometer and piezo wobble were readily circumvented. This approach to phase modulation is less expensive by a factor of roughly 50 than one based on an electro-optic effect

Improved Phase Modulation for an En-face Scanning Three-dimensional Optical Coherence Microscope

We have previously described an inexpensive method for modulating the interferometer of an en-face scanning, focus-tracking, three-dimensional optical coherence microscope (OCM). In this OCM design, a reference mirror is mounted on a piezoelectric stack driven at a resonance frequency of about 100 kHz. We perform a partial discrete Fourier transform of the digitally sampled output fringe signal. In the original design, we obtained the amplitude of the backscattered light by summing the powers in the fundamental (1ω) and first harmonic (2ω) of the modulation frequency. We used the particular piezoamplitude that eliminates the effects of interferometer phase drift. However, as the reference mirror was stepped to scan at different sample depths, variations in the back-coupled reference power added noise to the fringe signal at the fundamental piezodriving frequency. We report here a technique to eliminate the effects of this piezowobble by using instead the sum of the 2ω and 3ω powers as a measure of the backscattered light intensity. Images acquired before and after this improvement are presented to illustrate the enhancement to image quality deep within the sample

Limits to Performance Improvement Provided by Balanced Interferometers and Balanced Detection in OCT/OCM Instruments

We compare the dynamic range of OCT/OCM instruments configured with unbalanced interferometers, e.g., Michelson interferometers, with that of instruments utilizing balanced interferometers and balanced photodetection. We define the dynamic range (DR) as the ratio of the maximum fringe amplitude achieved with a highly reflecting surface to the root-mean-square (rms) noise. Balanced systems achieve a dynamic range 2.5 times higher than that of a Michelson interferometer, enabling an image acquisition speed roughly 6 times faster. This maximum improvement occurs at light source powers of a few milliwatts. At light source powers higher than 30 mW, the advantage in acquisition speed of balanced systems is reduced to a factor of 4. For video-rate imaging, the increased cost and complexity of a balanced system may be outweighed by the factor of 4 to 6 enhancement in image acquisition speed. We include in our analysis the beat-noise resulting from incoherent fight backscattered from the sample, which reduces the advantage of balanced systems. We attempt to resolve confusion surrounding the origin and magnitude of beat-noise , first described by L. Mandel in 1962. Beat-noise is present in both balanced and unbalanced OCT/OCM instruments

An optical coherence microscope for 3-dimensional imaging in developmental biology

An optical coherence microscope (OCM) has been designed and constructed to acquire 3-dimensional images of highly scattering biological tissue. Volume-rendering software is used to enhance 3-D visualization of the data sets. Lateral resolution of the OCM is 5 mm (FWHM), and the depth resolution is 10 mm (FWHM) in tissue. The design trade-offs for a 3-D OCM are discussed, and the fundamental photon noise limitation is measured and compared with theory. A rotating 3-D image of a frog embryo is presented to illustrate the capabilities of the instrument

Role of Beat Noise in Limiting the Sensitivity of Optical Coherence Tomography

The sensitivity and dynamic range of optical coherence tomography (OCT) are calculated for instruments utilizing two common interferometer configurations and detection schemes. Previous researchers recognized that the performance of dual-balanced OCT instruments is severely limited by beat noise, which is generated by incoherent light backscattered from the sample. However, beat noise has been ignored in previous calculations of Michelson OCT performance. Our measurements of instrument noise confirm the presence of beat noise even in a simple Michelson interferometer configuration with a single photodetector. Including this noise, we calculate the dynamic range as a function of OCT light source power, and find that instruments employing balanced interferometers and balanced detectors can achieve a sensitivity up to six times greater than those based on a simple Michelson interferometer, thereby boosting image acquisition speed by the same factor for equal image quality. However, this advantage of balanced systems is degraded for source powers greater than a few milliwatts. We trace the concept of beat noise back to an earlier paper [J. Opt. Soc. Am. 52, 1335 (1962)]

Motion-Sensitive 3-D Optical Coherence Microscope Operating at 1300 nm for the Visualization of Early Frog Development

We present 3-dimensional volume-rendered in vivo images of developing embryos of the African clawed frog Xenopus laevis taken with our new en-face-scanning, focus-tracking OCM system at 1300 nm wavelength. Compared to our older instrument which operates at 850 nm, we measure a decrease in the attenuation coefficient by 33%, leading to a substantial improvement in depth penetration. Both instruments have motion-sensitivity capability. By evaluating the fast Fourier transform of the fringe signal, we can produce simultaneously images displaying the fringe amplitude of the backscattered light and images showing the random Brownian motion of the scatterers. We present time-lapse movies of frog gastrulation, an early event during vertebrate embryonic development in which cell movements result in the formation of three distinct layers that later give rise to the major organ systems. We show that the motion-sensitive images reveal features of the different tissue types that are not discernible in the fringe amplitude images. In particular, we observe strong diffusive motion in the vegetal (bottom) part of the frog embryo which we attribute to the Brownian motion of the yolk platelets in the endoderm