The diffraction tomography theorem is adapted to one-dimensional length measurement. The resulting spectral interferometry technique is described and the first length measurements using this technique on a model eye and on a human eye in vivo are presented.

Measurement of intraocular distances by backscattering spectral interferometry

Manuscript received April 14, 1983; revised June 24, 1983. N. B. Abraham is with the Department of Physics, BrynMawr College, Bryn Mawr, PA 19010. P. Mandel is with the Service de Chimie Physique 11, Universite Libre de Bruselles, Brussels, Belgium. L. W. Casperson is with the Department of Electrical Engineering and Applied Science: University of California, Los Angeles, CA 90024. Editor’s Note: A similar correction by Dr. F. Holigner and Prof. Dr. H. Weber of the Universitat Kaiserslautern was received on May 4, 1983. tions can be expressed as BR = y + p P (3a j

Principles of lasers

A topical review of numerical and experimental studies of supercontinuum generation in photonic crystal fiber is presented over the full range of experimentally reported parameters, from the femtosecond to the continuous-wave regime. Results from numerical simulations are used to discuss the temporal and spectral characteristics of the supercontinuum, and to interpret the physics of the underlying spectral broadening processes. Particular attention is given to the case of supercontinuum generation seeded by femtosecond pulses in the anomalous group velocity dispersion regime of photonic crystal fiber, where the processes of soliton fission, stimulated Raman scattering, and dispersive wave generation are reviewed in detail. The corresponding intensity and phase stability properties of the supercontinuum spectra generated under different conditions are also discussed.

Supercontinuum generation, photonic crystal fiber

Quantum sensing is highly attractive for accessing spectral regions in which the detection of photons is technically challenging: sample information is gained in the spectral region of interest and transferred via entanglement into another spectral range, for which highly sensitive detectors are available. This is especially beneficial for terahertz radiation, as the corresponding photon energy lies in the range of a few meV - an energy where no semiconductor detectors are available and coherent detection schemes or cryogenically cooled bolometers have to be employed. Here, we report on the first demonstration of quantum sensing in the terahertz frequency range in which the terahertz photons interact with a sample in free space and information about the sample thickness is obtained by the detection of visible photons. A nonlinear single-crystal interferometer setup with a periodically poled lithium niobate crystal (PPLN) and a 660 nm pump source is used, generating visible (signal) photons and associated (idler) photons in the terahertz frequency range. Separation from the pump photons and detection of the visible signal photons is achieved by using highly efficient and narrowband volume Bragg gratings and an uncooled scientific complementary metal-oxide-semiconductor (sCMOS) camera. The acquired frequency-angular spectra show quantum interference in the Stokes as well as the Anti-Stokes part of collinear forward generation caused by spontaneous parametric down-conversion (SPDC) and down-conversion as well as up-conversion of thermal photons. The information encoded in the quantum interference can be used to determine the thickness of coatings or functional layers that are mainly transparent in the terahertz spectral range. As a first demonstration, we show layer thickness measurements with terahertz photons based on induced coherence without induced emission.

Terahertz Quantum Sensing

An optical evaluation technique is described that is suitable for determining the positions and magnitudes of reflection sites within miniature optical assemblies. This method utilizes the coherence effects exhibited by a broadband optical source and is referred to as optical coherence-domain reflectometry. Background theory is given, and experimental results have demonstrated a resolution of 10 μm with an optical dynamic range of more than 100 dB.

Optical coherence-domain reflectometry: a new optical evaluation technique

Mid-infrared (mid-IR) light scatters much less than shorter wavelengths, allowing greatly enhanced penetration depths for optical imaging techniques such as optical coherence tomography (OCT). However, both detection and broadband sources in the mid-IR are technologically challenging. Interfering entangled photons in a nonlinear interferometer enables sensing with undetected photons, making mid-IR sources and detectors obsolete. Here we implement mid-IR frequency-domain OCT based on ultra-broadband entangled photon pairs spanning from 3.3 to 4.3 µm. We demonstrate 10 µm axial and 20 µm lateral resolution 2D and 3D imaging of strongly scattering ceramic and paint samples. By intrinsically being limited only by shot noise, we observe 106 times more sensitivity per integration time and power of the probe light. Together with the vastly reduced footprint and technical complexity, our technique can outperform conventional approaches with classical mid-IR light sources.

Frequency-domain optical coherence tomography with undetected mid-infrared photons

Optical technology in the mid-infrared wavelength range is currently a rapidly developing field initiated by the availability of novel high-power and spatially coherent sources. Non-destructive testing techniques based on these sources are very promising for industrial and medical applications. However, there are still many engineering problems due to the technical challenges and high prices of the optical elements suitable for the mid-infrared region. In this paper, we report the development and performances of the first mid-infrared Fourier-domain optical coherence tomography based on a supercontinuum source and low-cost pyroelectric detector. The system is designed to operate in the spectral region around 4 μm. Experimental results are demonstrated for detections of embedded microstructures in ceramic materials and subsurface oil paint layers.

Mid-infrared Fourier-domain optical coherence tomography with a pyroelectric linear array

Chemical mapping was demonstrated with a mid-infrared (MIR) microspectroscopy setup based on a supercontinuum source (SC) emitting in the spectral range from 1.55 to 4.5 µm and a MEMS-based Fabry-Perot filter spectrometer. Diffraction limited spatial resolution in reflection geometry was achieved. A multilayer film consisting of different polymers and mixtures thereof was measured and results were compared to those gained with a conventional FTIR microscope equipped with a thermal MIR source. Results show that compared to thermal sources, the application of the SC source results in higher signal-to-noise ratios together with better spatial resolution and faster scanning. Furthermore, diffraction limited imaging of red blood cells was demonstrated for the first time in the MIR spectral region in reflection mode. The distinctive characteristics of the MIR spectral region in conjunction with the high brightness, spatial coherence and broadband nature of supercontinuum radiation show the potential for improving infrared microscopy significantly.

Diffraction limited mid-infrared reflectance microspectroscopy with a supercontinuum laser.

Fourier transform infrared (FT-IR) spectrometers have been the dominant technology in the field of mid-infrared (mid-IR) spectroscopy for decades. Supercontinuum laser sources operating in the mid-IR spectral region now offer the potential to enrich the field of FT-IR spectroscopy due to their distinctive properties, such as high-brightness, broadband spectral coverage and enhanced stability. In our contribution, we introduce this advanced light source as a replacement for conventional thermal emitters. Furthermore, an approach to efficient coupling of pulsed mid-IR supercontinuum sources to FT-IR spectrometers is proposed and considered in detail. The experimental part is devoted to pulse-to-pulse energy fluctuations of the applied supercontinuum laser, performance of the system, as well as the noise and long-term stability. Comparative measurements performed with a conventional FT-IR instrument equipped with a thermal emitter illustrate that similar noise levels can be achieved with the supercontinuum-based system. The analytical performance of the supercontinuum-based FT-IR spectrometer was tested for a concentration series of aqueous formaldehyde solutions in a liquid flow cell (500 µm path length) and compared with the conventional FT-IR (130 µm path length). The results show a four-times-enhanced detection limit due to the extended path length enabled by the high brightness of the laser. In conclusion, FT-IR spectrometers equipped with novel broadband mid-IR supercontinuum lasers could outperform traditional systems providing superior performance, e.g., interaction path lengths formerly unattainable, while maintaining low noise levels known from highly stable thermal emitters.

Sensitivity-Enhanced Fourier Transform Mid-Infrared Spectroscopy Using a Supercontinuum Laser Source

Recent developments and commercial availability of low-noise and bright infrared (IR) supercontinuum sources initiated intensive applied research in the last few years. Covering a significant part of near- and mid-infrared spectral ranges, supercontinuum radiation opened up unique possibilities and alternatives for the well-established imaging technique of optical coherence tomography (OCT). In this contribution, we demonstrate the development, performance, and maturity of a cost-efficient dual-band Fourier-domain IR OCT system (2 µm and 4 µm central wavelengths). The proposed OCT setup is elegantly employing a single supercontinuum source and a pyroelectric linear array. We discuss adapted application-oriented approaches to signal acquisition and post-processing when thermal detectors are applied in interferometers. In the experimental part, the efficiency of the dual-band detection is evaluated. Practical results and direct comparisons of the OCT system operating within the employed sub-bands are exhibited and discussed. Furthermore, we introduce the 2 µm OCT sub-system as an affordable alternative for art diagnosis; therefore, high resolution and sensitive measurements of the painting mock-ups are presented. Finally, potentials of the dual-band detection are demonstrated for lithography-based manufactured industrial ceramics.

Ivan Zorin

Papers

Frequency-domain optical coherence tomography with undetected mid-infrared photons

Mid-infrared Fourier-domain optical coherence tomography with a pyroelectric linear array

Diffraction limited mid-infrared reflectance microspectroscopy with a supercontinuum laser.

Sensitivity-Enhanced Fourier Transform Mid-Infrared Spectroscopy Using a Supercontinuum Laser Source

Dual-band infrared optical coherence tomography using a single supercontinuum source