Measuring optical power transmission near the critical angle for sensing beam deflection

Two-Axial Measurement of the Angular Microdeflection of a Laser Beam Using One Single-Axis Sensor

The majority of current methods for measuring the angular deflection of a laser beam enable measurement only in one selected plane. However, there are tasks in which measurements of laser beam deflections in 3D are required. In this paper, we present a way of enabling two-axial measurements of the deflection of a beam based on a single-axis sensor. The key idea is to direct a laser beam, alternately, into one of two arms of a measurement system. In the first arm, the beam is transmitted directly to the angular sensor, while in the second, the beam is directed to the sensor via a special optical element that rotates the plane of the beam deflection; in other words, this element changes the deflection in the horizontal plane into a deflection in the vertical plane, and vice versa. To alternate the path of the beam, a variable phase retarder and a polarising beamsplitter are used. The proposed technique was experimentally verified, and the results confirm its effectiveness.

An efficient analysis of oblique reflection of airborne ultrasound beams from thin membranes for gas sensing

The use of the angular spectrum method (ASM) to simulate the reflection of airborne ultrasound beams from a thin membrane separating air from a mixture of air and another gas is examined. The main advantage of this method is its high computing speed and efficiency for practical design calculations, suitable for sensing applications. The implemented ASM code is validated against custom Rayleigh integral code in a pure propagation simulation. In addition, ultrasound beam reflection calculations using ASM with finite element numerical results and experimental measurements are compared, finding good agreement in both cases. Then, ASM is used to estimate the sensitivity of specular reflection signals to variations in the composition of the incidence medium as a function of the angle of incidence. Conditions for which a reflection signal using inexpensive commercial ultrasound emitter/receiver at 40 kHz, in a simple configuration, offer a high enough sensitivity suitable for monitoring air quality indoors are found.

Interference sensor for ultra-precision measurement of laser beam angular deflection

A new interference sensor for ultra-precise measurement of laser beam angular deflection is proposed. The angular tilt of the measuring device, in relation to the beam axis, can also be measured. The theoretical basis of the angle evaluation presented is based upon analyzing the fringe period, as well as the suggested optical setup and design of the device. Calculating algorithms for the evaluation of the beam angle change, the influence of the fringe phase on angle measurement, and the metrological feasibilities are also detailed herein. The main source of measurement errors is explored, and the influence of the sensor's positioning procedure in relation to the laser beam on the repeatability of the processing function is statistically analyzed. The results show that the sensor resolution reaches 25 nrad with an accuracy better than 1% in the measuring range 350 µrad. Finally, the potential applications of the interference sensor are described.

Air wedge with variable refractive index for precise laser beam steering in a small range

This paper provides the results of work on a new method for metrological laser beam steering for the purpose of angular stabilization. Frequency stabilized metrological lasers are used for interference measurements of length and angle. Angular deviation of the beam may cause cosine errors when measuring displacements. This paper presents a method for metrological laser beam deflection with an air wedge of variable refractive index. The change in the index is achieved by changing the air pressure in the wedge volume. The implemented system produced a range of angular displacements of the 0.1 mrad (10^-4  rad; 100 μrad) beam with a resolution of 10^-7  rad.

Interference method for ultra-precision measurement and compensation of laser beam angular deflection

A new interferometric method for ultra-precise measurement of laser-beam angular deflection is proposed. The angular tilt of a measuring device in relation to the beam axis also can be measured. The method is based on interference fringe period analysis in the selected plane of measurement. The theoretical basis and experimental verification of the method are presented. It is shown that by using the proposed technique, it is possible to measure the laser beam angular deflection or instability with ultrahigh resolution reaching single nanoradians. The proposed method allows the measurement and further compensation of laser beam deflections in a very compact design.

High-resolution optical angle sensors: approaching the diffraction limit to the sensitivity

We carry out a detailed analysis of angle-sensitive devices based on the critical-angle effect. We consider their use in measuring small angular deflections of a laser beam. We establish the diffraction limit to the sensitivity for optical-angle sensors based on reflection and transmission of a laser beam. We find that this limit is identical to that of the triangulation scheme when using a position-sensitive detector or the autocollimation scheme. We analyze the main proposals to date of optical-angle sensors based on the critical-angle effect, focusing on their maximum sensitivity and their polarization dependence in practical conditions. We propose and analyze theoretically a novel and simple angle-sensitive device for sensing optical-beam deflections with very low polarization dependence and a maximum sensitivity close to the diffraction limit when used with typical laser beams. We discuss the basic principles for designing this type of device, provide numerical results, and point out a convenient fabrication procedure.

Reflectivity of a gaussian beam near the critical angle with external optically absorbing media

One can use the angle-modulated reflectance of a gaussian beam near the critical angle to sense with high resolution the index of refraction of the external medium. We analyze in detail the reflectivity of a gaussian beam near the critical angle and its dependence on the optical absorption of the external medium. The given formulation is relatively simple and is useful in discerning the effects of the various parameters involved on the reflectivity and its differentials with respect to the angle of incidence. The results presented can be readily used for the quantitative design of novel sensors based on modulated reflectance near the critical angle. We provide a simple algebraic expression for the loss of sensitivity of modulated reflectance near the critical angle as the sample's absorption coefficient increases. We find that, in a typical case, the sensitivity has decreased to approximately half its value for transparent samples when the absorption coefficient has increased to 25 cm(-1). We conclude that modulated reflectance near the critical angle remains a competitive technique for monitoring the index of refraction of an external medium with an absorption coefficient of as much as 120 cm(-1). We compared experimentally obtained curves of the first differential of the reflectivity with respect to the angle of incidence with theory and found good agreement.

Sensitivity of optical sensors based on laser-excited surface-plasmon waves

We investigate the effect of the divergence of a Gaussian laser beam on the resonance curve and the sensitivity of optical sensors based on surface-plasmon resonance (SPR). For He-Ne laser beams it is found that, for beams with a waist radius of less than 300 microm, the SPR-curve characteristics differ appreciably from the case in which a plane wave is considered. Simple expressions for the sensitivity of (bio)chemical sensors are given. A simple Lorentzian model is used to estimate the maximum possible sensitivity when a multilayer system is used to enhance the resonance peak. It was found that the sensitivity can reach its highest value when the width of the SPR curve is equal to the laser-beam divergence. The results could be particularly important when a SPR curve is used to measure the absolute value of the refractive index of a sample or the dielectric constant and the thickness of a metal layer.