Multimode Fiber Output Research Articles

Multimode fiber ruler for detecting nanometric displacements

Light is a perfect tool for numerous metrology applications. To deliver light to hard-to-reach places, fiber probes are widely used. Hair-thin endoscopes based on multimode fibers offer exceptional performance in terms of information density and instrument footprint. Here, we integrate optical metrology into a flexible fiber probe and present a multimode fiber ruler for detecting nanometric displacements. A fast single-shot measurement demonstrates two-dimensional resolving power of 1.8 nm, which is 670 times smaller than the diffraction limit of the optical system and 24 times smaller than the demagnified image pixel size. The multimode fiber ruler does not require detailed field mapping; therefore, low-magnification optical systems can be used to increase the light intensity on a sensor. Moreover, the proposed approach does not rely on any special structures, such as optical grating or metasurfaces. A high-resolution two-dimensional fingerprint is naturally “printed” on the multimode fiber output facet. Our results enable fiber-based displacement measurements with nanometer precision, establishing a new benchmark for fiber-based optical alignment sensors and metrology.

Modelling of ATR-FTIR MEMS Spectrometer Under Partially-Coherent Multimode-Fiber Illumination

Miniaturization of attenuated total reflectance (ATR) spectrometers has been an emerging field applying such powerful surface sensing method for in-situ spectroscopic analysis. In this work, we present a model for field propagation through ATR element under the illumination of multimode fiber (MMF) in a micro-electro-mechanical-systems (MEMS) based ATR Fourier transform infrared (FTIR) spectroscopic system. The core spectrometer is based on micro-fabricated Michelson interferometer using deep etching technology, in which the light propagates in-plane with respect to the silicon substrate. An ATR multiple reflection crystal is illuminated with infrared (IR) thermal blackbody radiation source through an MMF. The output light of the crystal is fiber-coupled to the MEMS interferometer then to an IR broadband photodetector. The optical system is modelled using scaler Fourier optics, where the fiber output field is represented by a group of spatially shifted elementary sources, to predict the ATR absorbance response, taking into account the partial spatial coherence nature of the MMF output. The model output leads to the requirements on the ATR measurement conditions and numerical aperture (NA) of the system. The model is compared to practical results of MEMS spectrometer which is experimentally characterized over the mid-infrared (MIR) wavenumber range from 5000 cm−1 to 2100 cm−1, lower limited by the used fiber and photodetector cut-off. Spectra of liquid samples are obtained using two different crystals and total internal reflection (TIR) angles showing good agreement with theoretical prediction.

Homogenization with coherent light illuminated beam shaping diffusers for vision applications: spatial resolution limited by speckle pattern

BackgroundHomogenizing properties of beam shaping diffusers illuminated with coherent laser light was studied with special regard on their use in vision applications.ExperimentalTwo diffusers projecting circular pattern at different scattering angles (Thorlabs Engineered Diffuser™) were illuminated with different spatial intensity distribution (elliptical Gaussian, nearly flat-top and granular multimode fiber output, respectively) collimated beams.ResultsThe intensity distribution measured at high angular resolution with a CMOS sensor revealed the granular (speckle) pattern of the projected light having contrast of nearly 1. The smallest speckle size (highest speckle density) could be obtained at the largest aperture size using nearly flat-top and multimode fiber illumination.ConclusionAs an important measure of the homogeneity for vision applications the apparent local contrast was calculated as the function of the spatial averaging. Although, the homogeneity requirements depend on the actual application, the usability of such diffusers in combination with coherent light sources is in general strongly limited in megapixel resolution imaging applications, unless time-varying speckle averaging methods can be applied.

Space-time focusing in a highly multimode fiber via optical pulse shaping.

Fields propagating through a highly scattering material will be distorted in both space (intensity speckles) and time (spectral and temporal speckles), inhibiting tasks such as imaging and communication in both the optical and radio frequency regions. In optics, research thus far has demonstrated spatial focusing, image transmission, and short pulse delivery through bulk scattering materials and multimode fibers by taking advantage of spatial wavefront-shaping techniques. Here, we exploit spectral phase shaping for reference-free characterization of spectral and temporal speckle, and space-time focusing of broadband ultrafast pulses distorted by modal dispersion in a multimode fiber. We show that temporal speckle fields at different multimode fiber output locations are uncorrelated and demonstrate the ability to focus a short pulse at a specific output spatial location, while keeping the field at other output locations noise-like, offering opportunities to expand multimode fiber imaging and communication capacity.

Adaptive control of input field to achieve desired output intensity profile in multimode fiber with random mode coupling

We develop a method for synthesis of a desired intensity profile at the output of a multimode fiber (MMF) with random mode coupling by controlling the input field distribution using a spatial light modulator (SLM) whose complex reflectance is piecewise constant over a set of disjoint blocks. Depending on the application, the desired intensity profile may be known or unknown a priori. We pose the problem as optimization of an objective function quantifying, and derive a theoretical lower bound on the achievable objective function. We present an adaptive sequential coordinate ascent (SCA) algorithm for controlling the SLM, which does not require characterizing the full transfer characteristic of the MMF, and which converges to near the lower bound after one pass over the SLM blocks. This algorithm is faster than optimizations based on genetic algorithms or random assignment of SLM phases. We present simulated and experimental results applying the algorithm to forming spots of light at a MMF output, and describe how the algorithm can be applied to imaging.

Design of System for Rugged, Low-Noise Fiber-Optic Access to High-Temperature, High-Pressure Environments

This paper details a new design for fiber-optic access to harsh environments, with a particular focus on absorption spectroscopy in high-temperature, high-pressure combustion systems. A key component is a fiber-fused rod lens assembly which tolerates temperatures above 1100°C and enables simple optical access without interferences from etalons or ambient absorbers. The fibers, rod lenses, and supporting ports are designed to provide line-of-sight optical access without the need for adjustable optomechanics. Fiber modal noise was minimized by actively aligning a photodiode to the multimode fiber's output facet, resulting in a shot-noise-limited SNR of 53 000 for the system.

Higher-Order Modal Dispersion in Graded-Index Multimode Fiber

Previously, we proposed a field-coupling model for propagation in graded-index multimode fiber (MMF), analogous to the principal states model for polarization-mode dispersion (PMD) in single-mode fiber. That model was based on the concept of first-order principal modes, which have well-defined group delays that depend on the strength of the mode coupling. That first-order model predicts a linear relationship between the intensity waveforms at the MMF input and output. Here, we extend that model to account for higher order modal dispersion. The higher-order model predicts several effects analogous to higher-order PMD: pulse broadening, filling-in between peaks of the pulse response, depolarization and pattern blurring at the MMF output, and a nonlinear relationship between input and output intensity waveforms.

Enhanced multimode fiber link performance using a spatially resolved receiver

In this letter, we demonstrate a new spatially resolved photodetection and equalization technique that compensates for differential modal delay in multimode fiber (MMF) links. We experimentally demonstrate in excess of a two-fold bandwidth increase using a simple two-segment photodetector that retains all of the advantages associated with multimode fiber including alignment tolerance. Furthermore, we characterize the MMF output in the far-field demonstrating that the temporal response depends upon spatial coordinate and allowing numerical optimization of the technique.