In this paper, we propose a reflective long-period grating-based sensor with a Sagnac fiber loop mirror (SFLM) for simultaneous measurement of refractive index (RI) and temperature. By cascading the SFLM to the end of a long-period fiber grating (LPFG), the LPFG works as a reflection operation, which is convenient in some applications. Further, the SFLM and the LPFG have different sensitivities to RI and temperature. As a result, RI and temperature measurement can be simultaneously achieved by monitoring the wavelength shifts of the LPFG and the SFLM's dips in the reflection spectrum. Experimental results show that the temperature sensitivity can reach 1.533 nm/°C, and the RI sensitivity is from 16.864 nm/RIU (refractive index unit) to 113.142 nm/RIU when the RI range is from 1.333 to 1.430. The application for 40 km long-distance RI and temperature measurement shows that the sensor has potential application in long-distance sensing.

Reflective long-period fiber grating-based sensor with Sagnac fiber loop mirror for simultaneous measurement of refractive index and temperature

Radius of curvature measurement based on wavefront difference method by the point diffraction interferometer

A method for measuring the focal length of the lens by a Fizeau interferometer is proposed. Based on the Gaussian imaging equation and the longitudinal displacements of the object point and image point, a precise formula for focal length calculation is deduced. The longitudinal displacement of the object points is determined by the wavefront difference method with a subnanometer resolution. An experimental system for focal length measurements is set up to verify the principle. The sources of uncertainty in measurement are discussed. Both the positive and negative lens experimental results indicate that the measurement accuracy is less than 0.16% under normal experimental environment.

Focal length measurement based on the wavefront difference method by a Fizeau interferometer

The invention discloses a holographic measuring method of two-dimensional Jones matrix parameters and an implementation device. The method comprises the following steps: two mutually irrelevant lasers are subjected to diffraction light splitting; two zero level diffraction light rays subjected to the light splitting are converted into linearly polarized light rays with the vibration directions being orthogonal, and the linearly polarized light rays irradiate a polarization-sensitive sample and penetrate through the sample to become object light waves containing the polarization information of the sample; meanwhile, smoothing is performed on the primary diffraction light rays of two light sources; the two primary diffraction light rays of the two light sources are respectively reserved and converted into linearly polarized light rays of which the vibration directions form angles of 45 degrees and minus degrees with the polarization directions of the zero level diffraction light rays; the four linearly polarized light rays are taken as reference light to interfere with the object light waves, so that a four-channel Jones matrix hologram containing amplitudes, phases and polarization information of the object light waves is formed; data processing is performed on the four-channel Jones matrix hologram to obtain the complex amplitude distribution of all four matrix elements of the two-dimensional Jones matrix of the polarization-sensitive sample. Through the holographic measuring method of the two-dimensional Jones matrix parameter and the implementation device, high measuring efficiency is realized as the complex amplitude spatial distribution of all the four parameters of the Jones matrix of the to-be-measured sample can be measured only through one step.

Holographic measuring method of two-dimensional Jones matrix parameters and implementation device

Focal length measurement by fiber point diffraction longitudinal interferometry

A refractive index measurement method by measuring the longitudinal displacement of a point light source (PLS) after passing through a plane-parallel-plate sample is proposed. The displacement is derived by applying two point-diffraction longitudinal shearing interferometric measurements if the distance from the PLS to the exit pupil is predetermined. With two fibers to simulate the ideal PLS, an experimental system for solid and liquid sample tests is proposed to verify the principle. The experimental results indicate that its accuracy is in the order of 10(-4). Ways to improve the accuracy are discussed based on the detailed error analysis.

Refractive index measurement by fiber point diffraction longitudinal shearing interferometry

A non-immersive lens refractive index measurement method based on fiber point-diffraction longitudinal interferometry is presented. The lens imaging process is simplified to the single refraction of the back surface if the object point is located at the vertex of the front surface. The lens refractive index is derived through measuring its thickness, radius of curvature of the back surface, the distance between the object point and the image point. Experiments indicate its accuracy is better than 2.2 × 10⁻⁴. Since the front surface is excluded in the imaging process, even an aspherical lens could be accurately measured by this method.

Lens refractive index measurement based on fiber point-diffraction longitudinal interferometry

The invention discloses a refractive index measurement technology and a device based on dual-fiber point-diffraction phase-shift interference, and belongs to the technical field of optical measurement. The refractive index measurement technology comprises the following steps: firstly regulating a measurement fiber so that the measurement fiber is conjugated with a reference fiber relative to a film spectroscope, wherein no interference fringe is positioned on a CCD (Charge Coupled Device) camera at the time; acquiring and processing a phase-shift interference pattern to obtain the fitting coefficients of the wave disparity of a pupil plane; then inserting a measured sample processed into a parallel plate between the measurement fiber and the film spectroscope, so as to change the optical path difference of the measurement fiber and the reference fiber, wherein concentric ring-shaped interference fringes appear on the CCD camera at the time; acquiring and processing the phase-shift interference pattern to obtain the fitting coefficients of the wave disparity of the pupil plane again; computing the axial displacement of a fiber point light source after the parallel plate is introduced by measuring the distance from the reference fiber to the CCD camera; and further computing the refractive index of the measured sample on the basis of the thickness measurement of the parallel plate. The refractive index measurement device disclosed by the invention is simple in structure and can accurately, easily and conveniently measure the refractive index of a solid or liquid.

Lingfeng Chen

Papers

Focal length measurement by fiber point diffraction longitudinal interferometry

Refractive index measurement by fiber point diffraction longitudinal shearing interferometry

Lens refractive index measurement based on fiber point-diffraction longitudinal interferometry

Refractive index measurement technology and device based on dual-fiber point-diffraction phase-shift interference