A Fiber-Based Chromatic Dispersion Probe for Simultaneous Measurement of X-Axis and Z-Axis Displacements with Nanometric Resolutions

In this paper, a fiber-based chromatic dispersion probe for simultaneous measurement of X-axis and Z-axis displacements with nanometric resolutions by using the full width at half maxima (FWHM) of the detected spectral signal has been proposed and demonstrated. For X-axis, FWHM is employed for indicating the X-axis displacement based on the fact that the FWHM remains almost constant with the varying Z-axis displacement of the fiber detector and shows a linear relationship with the X-axis displacement within a specific Z-axis displacement range. For the Z-axis, the linear relationship between the centroid wavelength λ of the detected spectral signal and the Z-axis displacement is employed for indicating the Z-axis displacement based on the fact that the sensitivity (slope of the λ-Z curve) is also linear with X-axis displacement within a certain X-axis displacement range. Theoretical and experimental investigations have verified the feasibility of the proposed chromatic dispersion probe, which yields X- and Z-axis measurement ranges of 2.3 μm and 15 μm and X- and Z-axis measurement resolutions of better than 25 nm and 50 nm, respectively. Experiments were further performed to evaluate the basic performance of the prototype probe and the maximum measurement errors were less than 10 nm and 60 nm for X- and Z-axis displacements, respectively.

A Fiber-Based Chromatic Dispersion Probe for Simultaneous Measurement of Dual-Axis Absolute and Relative Displacement

This paper presents a fiber-based chromatic dispersion probe for the simultaneous measurement of dual-axis absolute and relative displacement with nanometric resolutions. The proposed chromatic dispersion probe is based on optical dispersion. In the probe, the employed light beam is split into two sub-beams, and then the two sub-beams are made to pass through two optical paths with different optical settings where two identical single-mode fiber detectors are located at different defocused positions of the respective dispersive lenses. In this way, two spectral signals can be obtained to indicate the absolute displacement of each of the dual-axes. A signal processing algorithm is proposed to generate a normalized output wavelength that indicates the relative displacement of the dual-axis. With the proposed chromatic dispersion probe, the absolute and relative displacement measurements of the dual-axis can be realized simultaneously. Theoretical and experimental investigations reveal that the developed chromatic dispersion probe realizes an absolute measurement range and a measurement resolution of approximately 180 μm and 50 nm, respectively, for each axis. Moreover, a relative displacement measurement range and a measurement resolution of about 240 μm and 100 nm, respectively, are achieved for the dual-axis.

Dispersion-Diversity Multicore Fiber Signal Processing

Beyond playing a primary role in high-capacity communication networks, multicore optical fibers can bring many advantages to optical and microwave signal processing, as not only space but also chromatic dispersion are introduced as new degrees of freedom. The key lies in developing radically new multicore fibers where the refractive index profile of each individual core is tailored properly to provide parallel dispersion-diversity signal processing with application in a variety of scenarios such as parallel channel equalization, analogue-to-digital conversion, optical computing, pulse generation and shaping, multiparameter fiber sensing, medical imaging, optical coherence tomography, broadband measurement instrumentation, and next-generation fiber-wireless communications. Here, we experimentally prove, for the first time to our knowledge, reconfigurable two-dimensional dispersion-managed signal processing performed by a novel dispersion-diversity heterogeneous multicore fiber. The fiber comprises seven different trench-assisted cores featuring a different refractive index profile in terms of both radial geometry and core dopant concentration. As a representative application case, we demonstrate reconfigurable microwave signal filtering with increased compactness as well as performance flexibility and versatility as compared to previous technologies.

Depth of focus extension in optical coherence tomography using ultrahigh chromatic dispersion of zinc selenide

We report a novel technique to overcome the depth-of-focus limitation in optical coherence tomography (OCT) using chromatic dispersion of zinc selenide lens. OCT is an established method of optical imaging, which found numerous biomedical applications. However, the depth scanning range of high-resolution OCT is limited by its depth of focus. Chromatic dispersion of zinc selenide lens allows to get high lateral resolution along extended depth of focus, because the different spectral components are focused at a different position along axes of light propagation. Test measurements with nanoparticle phantom show 2.8 times extension of the depth of focus compare to the system with a standard achromatic lens. The feasibility of biomedical applications was demonstrated by ex vivo imaging of the pig cornea and chicken fat tissue.

Na4B8O9F10: A Deep-Ultraviolet Transparent Nonlinear Optical Fluorooxoborate with Unexpected Short Phase-Matching Wavelength Induced by Optimized Chromatic Dispersion

Exploring significant ultraviolet/deep-ultraviolet nonlinear optical (NLO) materials is hindered by rigorous and contradictory requirements, especially, possessing a moderate optical birefringence to meet phase-matching (PM). Except for suitable birefringence, small chromatic dispersion is also crucial to blue-shift the PM wavelength. Here, the introduction of fluorinated tetrahedral boron-centred chromophore strategy was proposed to optimize the chromatic dispersion. Herein, [BF4]- unit with large HOMO-LUMO band gap was introduced to Na-B-O-F system and Na4B8O9F10 was designed and synthesized successfully for the first time. Na4B8O9F10 with optimized chromatic dispersion can achieve a short second harmonic generation PM wavelength of 240 nm with a relatively small birefringence (cal. 0.036@1064 nm). Notably, Na4B8O9F10 is the first acentric crystal with [BF4]- unit among reported metal-fluorooxoborate system, involving isolated [BF4]- and novel [B7O10F6]5- fundamental building blocks.

Group Refractive Index of Nanocrystalline Yttria-Stabilized Zirconia Transparent Cranial Implants

Transparent “Window to the Brain” (WttB) cranial implants made from a biocompatible ceramic, nanocrystalline Yttria-Stabilized Zirconia (nc-YSZ), were recently reported. These reports demonstrated chronic brain imaging across the implants in mice using optical coherence tomography (OCT) and laser speckle imaging. However, optical properties of these transparent cranial implants are neither completely characterized nor completely understood. In this study, we measure optical properties of the implant using a swept source OCT system with a spectral range of 136 nm centered at 1,300 nm to characterize the group refractive index of the nc-YSZ window, over a narrow range of temperatures at which the implant may be used during imaging or therapy (20–43°C). Group refractive index was found to be 2.1–2.2 for OCT imaging over this temperature range. Chromatic dispersion for this spectral range was observed to vary over the sample, sometimes flipping signs between normal and anomalous dispersion. These properties of nc-YSZ should be considered when designing optical systems and procedures that propagate light through the window, and when interpreting OCT brain images acquired across the window.

Quantum enhancement of accuracy and precision in optical interferometry

White-light interferometry is one of today's most precise tools for determining the properties of optical materials. Its achievable precision and accuracy are typically limited by systematic errors due to a high number of interdependent data-fitting parameters. Here, we introduce spectrally resolved quantum white-light interferometry as a novel tool for optical property measurements, notably, chromatic dispersion in optical fibres. By exploiting both spectral and photon-number correlations of energy-time entangled photon pairs, the number of fitting parameters is significantly reduced, which eliminates systematic errors and leads to an absolute determination of the material parameter. By comparing the quantum method to state-of-the-art approaches, we demonstrate the quantum advantage of 2.4 times better measurement precision, despite requiring 62 times fewer photons. The improved results are due to conceptual advantages enabled by quantum optics, which are likely to define new standards in experimental methods for characterising optical materials.

Characterization of Chromatic Dispersion and Refractive Index of Polymer Optical Fibers

The chromatic dispersion and the refractive index of poly(methyl methacrylate) polymer optical fibers (POFs) have been characterized in this work by using a tunable femtosecond laser and a Streak Camera. The characterization technique is based on the measurement of the time delays of light pulses propagating along POFs at different wavelengths. Polymer fibers of three different lengths made by two manufacturers have been employed for that purpose, and discrepancies lower than 3% have been obtained in all cases.

Wavelength-coded volume holographic imaging endoscope for multidepth imaging

A wavelength-coded volume holographic imaging (WC-VHI) endoscope system capable of simultaneous multifocal imaging is presented. The system images light from two depths separated by 100  μm in a tissue sample by using axial chromatic dispersion of a gradient index probe in combination with two light-emitting diode sources and a multiplexed volume hologram to separate the images. This system is different from previous VHI systems in that it uses planar multiplexed gratings and does not require curved holographic gratings. This results in improved lateral imaging resolution from 228.1 to 322.5  lp/mm. This letter describes the design and fabrication of the WC-VHI endoscope and experimental images of hard and soft resolution targets and biological tissue samples to illustrate the performance properties.

Accurate Measurements of the Zero-Dispersion Wavelength in Optical Fibers

We have developed a frequency-domain phase shift system for measuring the zero-dispersion wavelength and the dispersion slope of single-mode optical fibers. A differential phase shift method and nonlinear four-wave mixing technique were also investigated. The frequency-domain phase shift method is used to produce Standard Reference Materials that have their zero-dispersion wavelengths characterized with an expanded uncertainty (k = 2) of ± 0.060 nm.