JPH0625774B2 - Measurement Method of Fluid Velocity and Concentration Using Photothermal Effect-Disturbance Rejection Type Heterodyne Interferometry Optical Fiber Probe- - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Laser non-contact Doppler velocimeters (LDVs) have been widely used for non-contact measurement of fluids because of their excellent features such as high spatial resolution, fast response, and no need for calibration. ing. However, when the LDV contains or does not contain the scattering particles in the fluid, it is necessary to mix and disperse them, and there are various application restrictions in use. Recently, photothermal spectroscopy has been attracting attention as a method of measuring gas velocity and concentration that does not require scattering particles. Irradiation of a laser beam into a gas causes some or most of its energy to be converted to thermal energy, which causes a temperature rise in the gas. The change in refractive index (photothermal effect) caused by this is measured from the deflection of another probe beam.
pectroscopy has been done. However, it is a simple method to measure the phase gradient, but it directly analyzes the phenomenon,
There were problems such as incomprehension and a little inaccuracy.

In the method of this patent, an optical heterodyne interferometer is used as a probe in order to measure the phase change itself in the fluid caused by the photothermal effect and to improve the sensitivity. The optical heterodyne interferometry is drawing attention because it has a linear relationship between the measurement phase and the interferometer output among the optical interferometry and has high sensitivity. However, while heterodyne interferometry is highly sensitive, it is common to interferometers.
Since it requires a reference light path and is different from the measurement light path, it is easily affected by disturbance. This problem becomes serious when an optical system is used to simplify and probe an optical system. This is because the optical fiber optical path is easily affected by temperature changes and vibrations. Therefore, in this patent, in order to reduce the influence of disturbance, a new optical fiber common optical path interferometer using two special Mach-Zehnder interferometers was newly developed to eliminate the influence of disturbance in the optical fiber.

A schematic diagram of the measurement optics of this patent is shown in FIG. First, the heterodyne interferometer probe section will be described. Light source He-Ne
The linearly polarized light of the frequency ν emitted from the laser la.HN is split into two beams by the beam splitter PBS and is incident on the two acousto-optic elements (AOM1, AOM2) respectively. 4 from AOM1
0MHz + 438kHz, from AOM2, ν ₁ , which is 40MHz frequency deviation,
A beam of ν ₂ is emitted. In order to reduce the influence of disturbance, the respective beams are incident on the polarization-maintaining optical fibers f1 and f2 with their polarization directions aligned with the principal axes of the refractive indices.
In order to increase the interference output even when outputting from f1 and f2,
The principal axes of refractive index of each fiber were aligned. On the light receiving surface of the detector PD1, the light of the frequency ν ₁ which is emitted from f1 and propagated through the beam splitter BS1 → mirror m1 → BS1 → PD1 and f2 is emitted from the beam splitter BS2 → measurement fluid ce. → BS1 → PD
An interference beat with light having a frequency of ν ₂ propagating with 1 is detected. Similarly, an interference beat is detected by the detector PD2. The interference beats of both contain the phases of AOM1,2, f1,2, with the same sign, and the phase between BS1 and m1, the phase between BS2 and m2, and the phase between BS1 and BS2 to be measured. The phase is included with the opposite sign. When the phase difference between the two interference beats is detected by the phase meter, the phase of the former same-sign portion is canceled, and the latter portion to be measured is doubled. Therefore, it is possible to eliminate the influence of disturbance on the optical fiber portion.

As the excitation light source la.C, a continuous light source matched to the absorption line of the fluid used is pulsed with a chipper, or a pulsed light source is used. In the case of the concentration measurement, the phase change caused by the photothermal effect is detected by the heterodyne interference probe placed at the point and is found from its initial value. The velocity is obtained by analyzing the change in the curve of the phase change. Regarding the velocity, the position of the detection probe is set at a position downstream from the excitation point, and the velocity is calculated by the time flight formula.

[Brief description of drawings]

Fig. 1 shows an optical system for measuring the velocity and concentration of a fluid by the photothermal effect, which uses a disturbance-removing heterodyne interferometry optical fiber probe for detection.

Claims (1)

[Claims] Claim: What is claimed is: 1. A differential type device for detecting the photothermal effect, which is placed on the focal point of an excitation laser or is installed in a wake of the laser to measure the velocity and concentration of a fluid. It is a heterodyne interferometer. The interferometer is
With a common optical path type configuration using two special Mach-Zehnder interferometers, in order to cancel the phase of the guide path to the interferometer of two frequency-shifted laser beams, the laser
Allowing the beam to pass through the fluid in opposite directions and causing the measurement beam and the reference beam to interfere with each other;
A device consisting of two symmetrically arranged beam splitters that serve to split the measurement beam and the reference beam, and two symmetrically arranged total reflection mirrors that reflect the reference beam.