JPH08152475A - Modulated-laser doppler speedometer - Google Patents

Abstract

PURPOSE: To obtain a modulated-laser Doppler speedometer which can be miniaturized and whose application range is expanded by intensity-modulating a laser beam at a high frequency so as to be cast on a moving object and heterodyne-detecting reflected waves which have been returned and a reference signal. CONSTITUTION: A laser beam is intensity-modulated at a reference signal frequency (p) which is transmitted by an oscillator, and it is transmitted (an injection current is modulated by a semiconductor laser) so as to be cast on a target (a moving object). In reflected waves which has been returned, two Doppler frequency transitions are generated by the movement of the target with reference to the intensity-modulated laser beam. One is a Doppler frequency transition Δω generated by the frequency ω of the laser itself (as a carrier), and the other is a Doppler frequency transition Δp generated by a modulation signal (at a frequency (p)). Out of them, p+Δp is detected by a detector, a heterodyne detection is performed by a mixer between it and the reference signal (p), the frequency transition Δp is detected, and the speed of the target is found.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is a small and simple laser Doppler velocimeter, and can be expected to have a wide range of fields of application in process control in plants and various industrial and environmental measurement applications.

[0002]

2. Description of the Related Art A conventional laser Doppler velocimeter irradiates a laser by dividing it into two beams, a reference beam and a transmitted beam, and returns the transition frequency of the reflected beam to a heterodyne of the reflected beam and the reference beam. It was obtained by detection. The conventional laser Doppler velocimeter uses the Doppler frequency transition due to the moving object of the laser itself, and therefore has a drawback that a very large Doppler frequency transition occurs and it can be applied only to a low-speed moving object. Further, the conventional laser Doppler velocimeter uses an infrared laser having a long wavelength in order to drop the frequency transition, which is very difficult to handle. Moreover, since the system is composed of an optical system, the equipment is very complicated and expensive.

[0003]

SUMMARY OF THE INVENTION The present invention is a laser Doppler velocimeter with few optical components,
Since most of the system is made up of electronic components, it can be expected to be considerably miniaturized. In addition, the Doppler transition frequency is considerably lower than that of the conventional one.
Measurement with a relatively low-speed sampler such as a D converter is also possible. Further, since visible / ultraviolet lasers can be applied, a wide range of unprecedented applications such as measurement of air flow using Mie scattering of aerosol can be expected.

[0004]

The conventional laser Doppler velocimeter detects the Doppler transition frequency from the coherent heterodyne detection by utilizing the Doppler shift due to the wave nature of the laser itself. In the present invention,
Irradiate a moving object by applying high-frequency intensity modulation to the laser,
The Doppler transition frequency of the moving object is detected by performing heterodyne detection on the returned modulated wave and the reference signal.

[0005]

DETAILED DESCRIPTION OF THE INVENTION The present invention is a new type of laser Doppler velocimeter designed to alleviate and overcome the drawbacks of the conventional laser Doppler velocimeter while inheriting the excellent features thereof.

If the velocity of the moving object is V and the velocity of the medium is c, the frequency transition Δf of the Doppler effect is (1)
It is shown like the formula. In the equation (1), f is the frequency of the electromagnetic wave to be irradiated.
Thus, the Doppler frequency transition Δf is determined by the frequency f of the emitted laser.

A conceptual diagram of a conventional laser Doppler velocimeter is shown in FIG. As can be seen from FIG. 1, the conventional laser Doppler velocimeter measures the velocity of a moving object using the Doppler transition frequency of the moving object of the laser itself.

FIG. 2 is a conceptual diagram of the modulated laser Doppler velocimeter of the present invention. In the present invention, unlike the conventional laser Doppler velocimeter, rather than utilizing the Doppler transition frequency due to the frequency of the laser itself, the laser is subjected to high-frequency intensity modulation of a specific frequency, and its reflected signal and reference signal are The speed of movement of the object is measured by heterodyne detection.

The present invention utilizes the Doppler effect of modulated waves. FIG. 3 shows the intensity-modulated wave that has undergone the Doppler effect. What is shown as “Original” in FIG. 3 is a transmitted wave that is not subjected to the Doppler effect,
What is expressed as pressed is the reflected wave that has undergone the Doppler shift. In FIG. 3, p and ω are frequencies of the modulation wave and the laser (carrier wave). The p and ω are subjected to the Doppler effect by the moving object, and are subjected to the Doppler shifts of Δp and Δω. The Doppler shifts of Δp and Δω are determined by p and ω, and their values are shown in equations (2) and (3). In the modulated laser Doppler velocimeter, the velocity of the moving object is obtained by utilizing the Doppler shift Δp of the intensity modulation signal shown by the equation (2). In addition, frequency transition due to the Doppler shift of the laser also occurs as in equation (3), but for the photodetector that senses light energy,
This degree of Doppler shift has no effect. Rather, as in the case of the heterodyne detection of the conventional laser Doppler velocimeter, the interference beat generated when the reflected light other than the target has a very strong returning light is a problem. However, the beat frequency Δω caused by the interference is
Since the frequency is considerably higher than Δp, it is considered possible to cut with a low-pass filter.

[0010]

As described above, according to the present invention, since the Doppler transition frequency of the intensity modulation signal is used, the detected Doppler transition frequency can be made low, and the conventional laser Doppler velocimeter can be used. It can be applied in the visible / ultraviolet range, which was difficult to apply in Japan. Also,
Since the system is an electronic system, miniaturization is easy.

[Procedure amendment]

[Submission date] March 24, 1995

[Document name to be amended] Statement

[Name of item to be corrected] Brief description of the drawing

[Correction method] Change

[Correction content]

[Brief description of drawings]

FIG. 1 is a plan view of a conventional laser Doppler velocimeter. A conventional laser Doppler velocimeter detects Δf caused by Doppler transition of a frequency f of laser light (carrier) by optical heterodyne detection, and determines the transition frequency Δf.
The speed of the target was measured from f.

[Explanation of symbols] f: Frequency of laser (carrier) Δf: Transition frequency caused by Doppler effect

FIG. 2 is a plan view of a modulated Doppler velocimeter according to the present invention. In the present invention, the laser light is intensity-modulated and transmitted by the reference signal frequency p transmitted by the oscillator (the injection current is controlled in the semiconductor laser). For the intensity-modulated laser light, two Doppler frequency transitions occur due to the movement of the target. One is the Doppler frequency transition Δω generated by the frequency ω of the laser (carrier) itself, and the other is the Doppler frequency Δp generated by the modulation signal (frequency p). Of these, p
+ Δp is detected by the Detector (ω + Δω
Is considered to be weak due to its high frequency, but it can be removed by a low-pass filter. ), And the Mixer performs heterodyne detection with the reference signal p to detect Δp. This Δ
Calculate the target speed from p

[Explanation of Codes] p: Frequency of reference signal ω: Frequency of laser (carrier) Δp: Doppler transition frequency of modulation signal Δω: Doppler transition frequency of laser (carrier)

FIG. 3 is a plan view showing a conceptual diagram of a Doppler transition frequency of a modulated wave. Upper C
“Ompressed” is the waveform of a modulated wave that has undergone frequency transition due to Doppler shift. Original
Compared with the waveform of the (original modulated wave), the carrier has a frequency of Δω and the modulated signal has a frequency of Δp. Each Doppler transition frequency depends on the frequencies ω and p of the carrier and modulation signals.

[Explanation of Codes] V: Velocity of target c: Light velocity p: Frequency of modulation signal ω: Frequency of laser (carrier) Δp: Doppler transition frequency of modulation signal Δω: Doppler transition frequency of laser (carrier)

Claims (1)

[Claims] 1. A laser light is intensity-modulated at a specific frequency to irradiate a moving object, and the Doppler transition frequency of the returning reflected signal is detected by heterodyne detection between the reference signal and the transition frequency. A remote speed measuring device that measures the speed of a moving object.