JPH02228566A - Measurement of speed and density of fluid using photothermal effect-disturbance removing type heterodyne interference method optical fiber probe - Google Patents

Abstract

PURPOSE:To achieve a higher sensitivity by using an optical heterodyne interferometer for the measurement of a phase change in a fluid generated by a photothermal effect. CONSTITUTION:A linearly polarized light with a frequency nu emitted from a light source He-Ne laser Ia.HN is divided into two beams with a beam splitter PBs to be incident on two acoustic optical elements (AOM1 and AOM2) respectively. Beams nu1 and nu2 are emitted from the AOM1 and AOM2 to be incident into a fiber. An interference beat between the lights nu1 and nu2 is detected on a detector PD1. Likewise, an interference beat is detected with a detector PD2. A light source Ia.C for excitation uses a continuous light beam matched with an absorption line of particles used being pulsated with chopper. In the case of measuring a density, a phase change caused by a photothermal effect is detected with a heterodyne interference probe set overlapped at a point involved to determine it from an initial value thereof. A speed is determined by a analyzing a change of a curve of the phase change thus obtained.

Description

[Detailed Description of the Invention] Until now, laser Doppler velocimeters (LDVs) have been widely used for non-contact measurement of fluids due to their excellent features such as high spatial resolution, quick response, and no need for calibration. ing. However, when the LDV contains or does not contain scattering particles in the fluid, it is necessary to disperse the scattering particles by a person, and the LDV is subject to various usage limitations. Recently, photothermal spectroscopy has attracted attention as a method for measuring gas velocity and concentration that does not require scattering particles.

- A laser beam is irradiated into a gas, and part or most of the energy is converted into thermal energy, causing an increase in the temperature of the gas. photo-thermal deflection, which measures the refractive index change (photothermal effect) caused by this from the deflection of another probe beam.
Onspectroscopy has been performed. However, this method measures two-phase gradients, and although it is a convenient method, it directly analyzes the phenomenon, and has problems such as being difficult to understand and having slightly low accuracy.

The method of this patent uses an optical heterodyne interferometer with blowing force in order to measure the phase change itself in the fluid caused by the photothermal effect and improve sensitivity. Optical heterodyne interferometry is attracting attention among optical interferometry because it establishes a linear relationship between the measured phase and the interferometer output and has high sensitivity.

However, while heterodyne interferometry has high sensitivity.

As is common with interferometers, a reference optical path is required;
Because it is different from the measurement optical path, it is easily affected by disturbances.This problem can be solved by using an optical fiber to construct the optical system! This becomes important when converting to i and making it into a probe. This is because the optical fiber optical path is easily affected by temperature changes and vibrations. Therefore, in this patent, in order to be less susceptible to disturbances, we developed a new optical fiber common optical path interferometer that uses two Twyman interferometers to eliminate the effects of disturbances in the optical fiber.

A schematic diagram of the measurement optical system of this patent is shown in FIG. First.

This section describes the heterodyne interferometer probe section. Light source He
The linearly polarized light of frequency ν emitted from the -N e laser la, )IN is split into two beams by a amplifier PBS, and each beam enters two acousto-optic elements (AOMI, AOM2). 110 MHz + from AOMI
438 kHz, 110 MH from AOM2
z frequency shifted ν1. A beam of ν is emitted. In order to reduce the influence of disturbance, each beam is made to enter the polarization maintaining optical fibers fl and f2 with their polarization directions aligned with the principal axes of refractive index.

In order to increase the interference output when outputting from fl and f2, the principal axes of the refractive index of each fiber were aligned. Detector P
On the light-receiving surface of D+, the light of frequency ν1 which has been emitted by rl and propagated as beam splitter BS+ ÷ mirror ml -+BS+ +PD1, and the light of frequency ν1 which has been emitted by f2 and propagated to beam splitter BS2 as measured fluid Ce, ÷BS+ +PD1
An interference beat between the light and the propagated light of frequency ν2 is detected. Similarly, an interference beat is detected by detector PD2.

Due to the interference beat between the two, AOMI, 2. fl, 2.
The phases of are included with the same sign.

The phase between BSI and In1, the phase between BS2 and 1II2, and the phase between BSI and BS2 to be measured are included with opposite signs. When a phase meter detects the phase difference between the two interference beats, the phase of the same sign portion of the former is canceled out.

The latter part to be measured is detected twice as much. Therefore, a pulsed light source is used that can eliminate the influence of disturbances on the optical fiber section. In the case of concentration measurement, the phase change caused by the photothermal effect is detected by the heterodyne interference probe placed over the point, and determined from the initial value. The speed is determined by analyzing the change in the phase change curve. In addition, the velocity is determined using a time flight method by setting the position of the detection probe downstream from the excitation point.

4. Brief description of the drawings In FIG. 1, a disturbance rejection type heterodyne interferometry optical fiber probe was used for detection.

Optical system for measuring fluid velocity and concentration using photothermal effects.

Claims (1)

[Claims] Making measurements of fluid velocity and concentration using heterodyne interferometry.