A pulsed-laser range finding based on differential optical-path is proposed, and the mathematical models are developed and verified. Based on the method, some simulations are carried out and important conclusions are deduced. (1) Background power is suppressed effectively. (2) Compared with signal-to-noise ratio (SNR) of traditional method, SNR of the proposed method is more suitable than traditional method in long-range finding and large tilt angle of target. (3) No matter what the tilt angle of target is, it always has optimal sensitivity of zero cross as long as the differential distance is equal to the light speed multiplied by the received pulse length and there is an overlap between two echoes.

Differential optical-path approach to improve signal-to-noise ratio of pulsed-laser range finding.

Photoacoustic microscopy (PAM) is an imaging modality well suited to mapping vasculature and other strong absorbers in tissue. However, one of the primary drawbacks to PAM when used for high-resolution imaging is the relatively poor axial resolution due to the inverse dependence on the transducer bandwidth. While submicron lateral resolution PAM can be achieved by tightly focusing the excitation light, the axial resolution is fundamentally limited to 10s of microns for typical transducer frequencies. Here we present a multiphoton PAM technique called transient absorption ultrasonic microscopy (TAUM), which results in a completely optically resolved voxel with an experimentally measured axial resolution of 1.5 microns. This technique is demonstrated by imaging individual red blood cells in three dimensions in blood smear and ex vivo tissues. To the best of our knowledge, this is the first demonstration of fully resolved, volumetric photoacoustic imaging of erythrocytes. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Volumetric imaging of erythrocytes using label-free multiphoton photoacoustic microscopy

Accurate and reliable peak extraction of axial response signals plays a critical role in confocal microscopy. For axial response signal processing, nonlinear fitting algorithms, such as parabolic, Gaussian or sinc2 fitting may cause significant systematic peak extraction errors. Also, existing error compensation methods require a priori knowledge of the full-width-at-half-maximum of the axial response signal, which can be difficult to obtain in practice. In this paper, we propose a generalised error compensation method for peak extraction from axial response signals. This full-width-at-half-maximum-independent method is based on a corrected parabolic fitting algorithm. With the corrected parabolic fitting algorithm, the systematic error of a parabolic fitting is characterised using a differential equation, following which, the error is estimated and compensated by solving this equation with a first-order approximation. We demonstrate, by Monte Carlo simulations and experiments with various axial response signals with symmetrical and asymmetrical forms, that the corrected parabolic fitting algorithm has significant improvements over existing algorithms in terms of peak extraction accuracy and precision.

Corrected parabolic fitting for height extraction in confocal microscopy

This Letter proposes a laser differential confocal paraboloidal vertex radius measurement (DCPRM) method for the high-accuracy measurement of the paraboloidal vertex radius of curvature. DCPRM constructs an autocollimation vertex radius measurement light path for the paraboloid by placing a reflector in the incidence light path. This technique is based on the principle that a paraboloid can aim a parallel beam at its focus without aberration and uses differential confocal positioning technology to identify the paraboloid focus and vertex accurately. Measurement of the precise distance between these two positions is achieved to determine the paraboloid vertex radius. Preliminary experimental results indicate that DCPRM has a relative expanded uncertainty of less than 0.001%.

Laser differential confocal paraboloidal vertex radius measurement

A laser reflection-confocal focal-length measurement (LRCFM) is proposed for the high-accuracy measurement of lens focal length. LRCFM uses the peak points of confocal response curves to precisely identify the lens focus and vertex of the lens last surface. LRCFM then accurately measures the distance between the two positions to determine the lens focal length. LRCFM uses conic fitting, which significantly enhances measurement accuracy by inhibiting the influence of environmental disturbance and system noise on the measurement results. The experimental results indicate that LRCFM has a relative expanded uncertainty of less than 0.0015%. Compared with existing measurement methods, LRCFM has high accuracy and a concise structure. Thus, LRCFM is a feasible method for high-accuracy focal-length measurements.

Measuring the lens focal length by laser reflection-confocal technology

A new laser differential reflection-confocal focal-length measurement (DRCFM) method is proposed for the high-accuracy measurement of the lens focal length. DRCFM uses weak light reflected from the lens last surface to determine the vertex position of this surface. Differential confocal technology is then used to identify precisely the lens focus and vertex of the lens last surface, thereby enabling the precise measurement of the lens focal length. Compared with existing measurement methods, DRCFM has high accuracy and strong anti-interference capability. Theoretical analyses and experimental results indicate that the DRCFM relative measurement error is less than 10 ppm.

Laser differential reflection-confocal focal-length measurement

The invention belongs to the technical field of optical precision measurement and relates to a method for measuring the focal length of a reflection type differential confocal lens. The method comprises the following steps of: precisely positioning the positions of the surface vertex and the focal point of the lens by being matched with a planar reflecting mirror through a differential confocal measuring method, and further measuring the vertex focal length and the focal length of the lens. The invention firstly provides a method for realizing precise positioning of the focal length by virtue of a characteristic that a differential confocal response curve corresponds to the focal point and the surface vertex of the tested lens when passing the zero point, and expands the differential confocal measuring technology into the field of measurement on the focal length of the lens; and the method has the advantages of high measuring precision and strong ambient interference resistance, and can be used for high-precision detection of the local length of the lens.

Method for measuring focal length of reflection type differential confocal lens

The invention belongs to the technical field of optical precision measurement, and relates to a method for measuring central thickness of a reflecting type differential confocal lens. According to the method, a vertex on the front surface and a vertex on the rear surface of the lens are precisely positioned by matching a differential confocal measuring method with a plane reflecting mirror, so that the central thickness of the lens is measured according to a ray tracing formula. The vertex on the front surface and the vertex on the rear surface of the lens are highly precisely focused and collimated for the first time by combining light beams emitted by the measured lens with a differential confocal technology, the assembling and adjusting difficulty of the measured lens is greatly reduced, and the method has the advantages of high measuring precision and high capacity of resisting environmental disturbance, and can be used for highly precisely detecting the central thickness of the lens.

Hualing Wu

Papers

Laser differential reflection-confocal focal-length measurement

Method for measuring focal length of reflection type differential confocal lens

Method for measuring central thickness of reflecting type differential confocal lens

Focal length measuring method of auto-collimating differential confocal lens

Spherical aberration measurement method of differential confocal system