An apparatus and method are provided. In particular, at least one first electro-magnetic radiation may be provided to a sample and at least one second electro-magnetic radiation can be provided to a non-reflective reference. A frequency of the first and/or second radiations varies over time. An interference is detected between at least one third radiation associated with the first radiation and at least one fourth radiation associated with the second radiation. Alternatively, the first electro-magnetic radiation and/or second electro-magnetic radiation have a spectrum which changes over time. The spectrum may contain multiple frequencies at a particular time. In addition, it is possible to detect the interference signal between the third radiation and the fourth radiation in a first polarization state. Further, it may be preferable to detect a further interference signal between the third and fourth radiations in a second polarization state which is different from the first polarization state. The first and/or second electro-magnetic radiations may have a spectrum whose mean frequency changes substantially continuously over time at a tuning speed that is greater than 100 Tera Hertz per millisecond.

Method and apparatus for performing optical imaging using frequency-domain interferometry

A digital holographic technique is implemented in a microscope for
three-dimensional imaging reconstruction. The setup is a
Mach–Zehnder interferometer that uses an incoherent light source to
remove the coherent noise that is inherent in the laser sources. A
phase-stepping technique determines the optical phase in the image
plane of the microscope. Out-of-focus planes are refocused by
digital holographic computations, thus considerably enlarging the depth
of investigation without the need to change the optical focus
mechanically. The technique can be implemented in transmission for
various magnification ratios and can cover a wide range of
applications. Performances and limitations of the microscope are
theoretically evaluated. Experimental results for a test target are
given, and examples of two applications in particle localization and
investigation of biological sample are provided.

Improved three-dimensional imaging with a digital holography microscope with a source of partial spatial coherence.

We present an interferometer topology based on 3 x 3 fiber couplers that gives instantaneous access to the magnitude and phase of die interferometric signal. We demonstrate its performance in heterodyne and homodyne detection with a broadband light source.

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Instantaneous quadrature low coherence interferometry with 3 x 3 fiber optic couplers

Quantitative Phase Imaging

A method and system for determining a characteristic of a biological sample including directing light at the biological sample and receiving that light; directing the light at a reference reflecting device and receiving that light; adjusting an effective light path length to facilitate an interference of the light reflected from the biological sample corresponding to a first depth and the light reflected from the reference reflecting device; and detecting the broadband light resulting from the interference, to provide an interference signal. The method also includes: determining a first phase associated with the interference signal corresponding to the first depth; varying the effective light path length to define a second depth; determining a second phase associated with the interference signal corresponding to the second depth; and determining the characteristic of the biological sample from the phases.

Low coherence interferometry for detecting and characterizing plaques

Quadrature detection techniques have been applied to images obtained from a Mach-Zehnder interferometer with differently polarized beams to yield the real and the imaginary parts of the diffracted fields simultaneously. This approach eliminates the need for phase retrieval by providing complete information on the complex amplitude of the diffracted signal. We present results in which we demonstrate our ability to reconstruct two- and three-dimensional microscopic objects from their complex diffraction patterns.

Three-dimensional images generated by quadrature interferometry

An optical quadrature interferometer is presented. The optical quadrature interferometer uses a different state of polarization in each of two arms of the interferometer. A light beam is split into two beams by a beamsplitter, each beam directed to a respective arm of the interferometer. In one arm, the measurement arm, the light beam is directed through a linear polarizer and a quarter wave plate to produce circularly polarized light, and then to a target being measured. In the other arm, the to reference arm, the light beam is not subject to any change in polarization. After the light beams have traversed their respective arms, the light beams are combined by a recombining beamsplitter. As such, upon the beams of each arm being recombined, a polarizing element is used to separate the combined light beam into two separate fields which are in quadrature with each other. An image processing algorithm can then obtain the in-phase and quadrature components of the signal in order to construct an image of the target based on the magnitude and phase of the recombined light beam.

Optical quadrature interferometry utilizing polarization to obtain in-phase and quadrature information

An underground object detector can locate and identify the shape of underground objects using a pulsed laser (12) to generate acoustic waves in soil. A remotely located microphone detects an acoustic signal (24) between the soil surface and subsurface objects (10). The amplitude of this acoustic signal is correlated with the position of the laser beam source (14) and output on a visual display, resulting in an acoustic map of the ground (26). A raster scanning of the suspect ground in the vicinity reveals the shape of the underground object, allowing the operator to discriminate benign and natural objects.

Optical pulse induced acoustic mine detection

A novel optically based magnetic field sensing technique is developed that uses a thin magnetic film as the sensing element and the longitudinal and transverse magneto-optic Kerr effects in an optical read technique. The linear system response is tested for small magnetic fields using a simple free-space ellipsometry measurement system. Measurements taken at 8 mHz showed a magnetic field sensitivity of 4 x 10-3 Oe, with a system dynamic range of over 60 dB. The noise spectrum was dominated by 1/f noise. Calculations for a shot-noise-limited measurement system indicate that a magnetic field sensitivity of 10-6 Oe/Hz1/2 may be possible through enhancement of the sensing element parameters and 100-mW light sources.

Magnetic field measurements using magneto-optic Kerr effect sensors

A technique for measuring small magnetic fields using a thin amorphous magnetic film and the magneto‐optic Kerr effect has been developed. A thin film having in‐plane uniaxial anisotropy is aligned such that an external magnetic field rotates the film magnetization in a controllable manner. This magnetization rotation is ‘‘read’’ optically by a coherent homodyne measurement of the amount of polarization rotation caused by the magneto‐optic Kerr effect during reflectance of polarized light from the film surface, using one polarization as the local oscillator. The resulting signal is linear in magnetic field. Initial noise equivalent magnetic field sensitivity measurements of the technique yield 2 mOe/Hz1/2 at a frequency of 8 mHz. Calculations indicate that sensitivities of order 10−6 Oe/Hz1/2 are obtainable using this technique.

Scott C. Lindberg

Papers

Three-dimensional images generated by quadrature interferometry

Optical quadrature interferometry utilizing polarization to obtain in-phase and quadrature information

Optical pulse induced acoustic mine detection

Magnetic field measurements using magneto-optic Kerr effect sensors

Measurement of magnetic fields using the magneto‐optic Kerr effect