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

We present a polarization-sensitive optical coherence-domain reflectometer capable of characterizing the phase retardation between orthogonal linear polarization modes at each reflection point in a birefringent sample. The device is insensitive to the rotation of the sample in the plane perpendicular to ranging. Phase measurement accuracy is ±0.86°, but the reflectometer can distinguish local variations in birefringence as small as 0.05° with a distance resolution of 10.8 μm and a dynamic range of 90 dB. Birefringence-sensitive ranging in a wave plate, an electro-optic modulator, and a calf coronary artery is demonstrated.

Polarization-sensitive low-coherence reflectometer for birefringence characterization and ranging

Fiber optic interferometers to sense various physical parameters including temperature, strain, pressure, and refractive index have been widely investigated. They can be categorized into four types: Fabry-Perot, Mach-Zehnder, Michelson, and Sagnac. In this paper, each type of interferometric sensor is reviewed in terms of operating principles, fabrication methods, and application fields. Some specific examples of recently reported interferometeric sensor technologies are presented in detail to show their large potential in practical applications. Some of the simple to fabricate but exceedingly effective Fabry-Perot interferometers, implemented in both extrinsic and intrinsic structures, are discussed. Also, a wide variety of Mach-Zehnder and Michelson interferometric sensors based on photonic crystal fibers are introduced along with their remarkable sensing performances. Finally, the simultaneous multi-parameter sensing capability of a pair of long period fiber grating (LPG) is presented in two types of structures; one is the Mach-Zehnder interferometer formed in a double cladding fiber and the other is the highly sensitive Sagnac interferometer cascaded with an LPG pair.

/pdf/interferometric-fiber-optic-sensors-2akpm29ydq.pdf

Interferometric Fiber Optic Sensors

Optical fibers have been involved in the area of sensing applications for more than four decades. Moreover, interferometric optical fiber sensors have attracted broad interest for their prospective applications in sensing temperature, refractive index, strain measurement, pressure, acoustic wave, vibration, magnetic field, and voltage. During this time, numerous types of interferometers have been developed such as Fabry-Perot, Michelson, Mach-Zehnder, Sagnac Fiber, and Common-path interferometers. Fabry-Perot interferometer (FPI) fiber-optic sensors have been extensively investigated for their exceedingly effective, simple fabrication as well as low cost aspects. In this study, a wide variety of FPI sensors are reviewed in terms of fabrication methods, principle of operation and their sensing applications. The chronology of the development of FPI sensors and their implementation in various applications are discussed.

https://www.mdpi.com/1424-8220/14/4/7451/pdf-vor

Chronology of Fabry-Perot interferometer fiber-optic sensors and their applications: a review.

The future of imaging is the integration of function and anatomy Hongki Yoo et al have successfully done just that by combining two existing intravascular imaging techniques into a single catheter-based system Their dual-modality intra-arterial catheter uses a combination of optical frequency domain imaging and near-infrared fluorescence imaging to simultaneously provide molecular information in the context of the surrounding three-dimensional microanatomy of the artery wall

/pdf/intra-arterial-catheter-for-simultaneous-microstructural-and-2nb6uolbta.pdf

Intra-arterial catheter for simultaneous microstructural and molecular imaging in vivo

A miniature Fabry-Perot (FP) interferometric fiber-optic sensor suitable for high-temperature sensing is proposed and demonstrated. The sensor head consists of two FP cavities formed by fusion splicing a short hollow-core fiber and a piece of single-mode fiber at a photonic crystal fiber in series. The reflection spectra of an implemented sensor are measured at several temperatures and analyzed in the spatial frequency domain. The experiment shows that the thermal-optic effect of the cavity material is much more appreciable than its thermal expansion. The temperature measurements up to 1000 degrees C with a step of 50 degrees C confirm that it could be applicable as a high-temperature sensor.

Miniature fiber-optic high temperature sensor based on a hybrid structured Fabry-Perot interferometer

We report the development of an all-fiber multimodal system, based on a double-cladding fiber (DCF) and related devices, suitable for simultaneous measurements of optical coherence tomography (OCT) and fluorescence spectroscopy (FS). The DCF together with a DCF coupler and a single-body DCF lens has assisted in the realization of a multimodal but single-unit probe for the combined system. The DCF lens allowed simultaneous focusing of input beams for OCT and FS and also the effective collection of both signal beams from a sample. The DCF coupler could extract the OCT signal via the core channel and the FS signal through the cladding channel. The OCT image and the fluorescence spectra of a plant tissue were then simultaneously measured to validate the performance of the proposed multimodal system.

Combined system of optical coherence tomography and fluorescence spectroscopy based on double-cladding fiber.

We present a method for simultaneously measuring the thickness and the group refractive index of a specimen using self-referenced spectral-domain fiber-based interferometry. By removing the scanning part and using the fiber-based configuration, the system complexity and stability could be significantly improved. To minimize the system drift, we utilized the signals originated from the fiber ends of both arms. Implementing in a self-referenced configuration, we could improve the measurement accuracy down to a decimal place. Experimental measurements were made with a 1.555 mm thick fused silica plate. At 814 nm the thickness was measured as 1.5546 +/- 0.0002 mm, and at the same time, the group index was obtained as 1.4627 +/- 0.0002.

Self-referenced spectral interferometry for simultaneous measurements of thickness and refractive index

A full-field optical coherence tomography (FF-OCT) system utilizing a simple but novel image restoration method suitable for a high-speed system is demonstrated. An en-face
image is retrieved from only two phase-shifted interference fringe images through using the mathematical Hilbert transform. With a thermal light source, a high-resolution FF-OCT system having axial and transverse resolutions of 1 and
2.2 μm, respectively, was implemented. The feasibility of the proposed scheme is confirmed by presenting the obtained en-face images of biological samples such as a piece of garlic and a gold beetle.
The proposed method is robust to the error in the amount of the phase shift and does not leave residual fringes. The use of just two interference images and the strong immunity to phase errors provide great advantages in the imaging speed and the system design flexibility of a high-speed high-resolution FF-OCT system.

Image restoration method based on Hilbert transform for full-field optical coherence tomography

We present a novel wavelength-swept laser source for optical coherence tomography (OCT) which is based on the conventional laser diode technology of the vertical-cavity surface-emitting laser (VCSEL). In our self-heating sweep VCSEL (SS-VCSEL), a VCSEL device is simply driven by ramped pulses of currents in direct intensity modulation. The intrinsic property of VCSEL produces a frequency-swept output through the self-heating effect. By the injected current, the temperature of the active region is gradually increased in this effect. Consequently, it changes the wavelength of the laser output by itself. In this study, various characteristics of our SS-VCSEL were experimentally investigated for low-cost instrumentation of a swept source OCT system. A low-cost SS-VCSEL-based OCT system was demonstrated in this research that provided an axial resolution of 135 μm in air, sensitivity of -91 dB and a maximum imaging range longer than 10 cm when our source was operated at a sweep repetition rate of 5 kHz with an output power of 0.41 mW. Based on the experimental observations, we believe that our SS-VCSEL swept source can be an economic alternative in some of low-cost or long-range applications of OCT.

Eun Seo Choi

Papers

Miniature fiber-optic high temperature sensor based on a hybrid structured Fabry-Perot interferometer

Combined system of optical coherence tomography and fluorescence spectroscopy based on double-cladding fiber.

Self-referenced spectral interferometry for simultaneous measurements of thickness and refractive index

Image restoration method based on Hilbert transform for full-field optical coherence tomography

VCSEL-based swept source for low-cost optical coherence tomography