JPS60203821A - Optical fiber vibrator - Google Patents

Abstract

PURPOSE:To measure vibration only based upon a multimode optical fiber by making coherent light modulated at its amplitude incident upon the multimode optical fiber and detecting the projected light through a specific filtering means. CONSTITUTION:Light modulated in amplitude by a signal from a tracking generator 9 and emitted from a semiconductor light emitting diode 8 is made incident upon the multimode optical fiber 10 to be a vibration sensor, the projected light is received by a photodetector 12 through the mode filtering means 11 and the oscillation frequency is found out from the electric signal by a spectrum analyzer 1. The means 11 increases the variation of the amplitude of the photodetected signal at the vibration of the fiber 10 by increasing the its insersion loss to measure only the oscillation frequency synchronously with a modulation signal (a) of the generator 9. Thus, the constitution can be simplified.

Description

BACKGROUND AND OBJECTS OF THE INVENTION The present invention relates to an optical fiber vibrometer, and particularly to an optical fiber vibrometer suitable for measuring even minute vibrations with sieve sensitivity.

Conventional vibration meters that use electricity receive signals output from a sensor section via a metal cable. This is a configuration that detects whether However, as mentioned above, the sensor part uses a mechanical mechanism, so
The amplitude of the ball's movement varies depending on the frequency; for example, as the frequency increases, the ball becomes unable to respond, and the distance the ball moves, that is, the amplitude, decreases. Therefore, it is necessary to calibrate the relationship between frequency and amplitude before using K, which measures accurate amplitude. However, when the frequency of the object to be measured is unknown or when a plurality of frequencies contribute to a random frequency, accurate calibration cannot be performed. In addition, the sensor unit can only detect the vibration frequency at the installation location, so if you do not know which part of the object to be measured is vibrating, such as when investigating the vibration of a cable,
There is an inconvenience that a large number of sensor sections must be attached. Furthermore, since signals are received via metal cables, noise due to electromagnetic induction is likely to occur, making it difficult to perform highly accurate measurements. - By the way, on the other hand, there is a vibration meter that uses light and is configured with a measurement system as shown in FIG. This converts light from a light source 1 into linearly polarized light using a polarizing plate 2, enters the polarizing beam splitter 6, and splits the light into P-polarized light and S-polarized light.
One polarized light enters a polarization-maintaining optical fiber 4 as a vibration sensor, the other polarized light enters a reference polarization-maintaining optical fiber 5, and the output from these two polarization-maintaining optical fibers 4 and 5. The emitted light is interfered with, and the light is made to enter the light receiver 7 through the pinhole 6 to detect the intensity of the light and measure the amount of vibration.

However, since the conventional measurement system shown in FIG. 1 uses two optical fibers 4.5, it requires a large number of optical systems, which poses a problem in terms of cost.

The present invention has been made in view of the above, and its purpose is to be able to measure vibrations using only a multimode optical fiber as a vibration sensor, and to be able to measure vibrations (with very small vibrations) with high sensitivity. The purpose of the present invention is to provide a fiber optic vibrometer that can measure

[Summary of the invention]

The features of the present invention include a light source that outputs amplitude-modulated coherent light, a multimode optical fiber as a vibration sensor that receives the light from this light source, and a system that converts the light emitted from this optical fiber into an electrical value. a mode filtering mechanism that increases fluctuations in the amplitude of an optical signal received by the light receiver when the optical fiber provided between the light receiver and the output end of the optical fiber vibrates; The structure consists of a spectrum analyzer that manually calculates the frequency of the output of the light receiver.

〔Example〕

The embodiment of the present invention shown in FIG. 2 and FIGS. 6 and 4 will be described below.
This will be explained in detail using figures.

FIG. 2 is a configuration explanatory diagram showing an embodiment of the optical fiber vibrometer of the present invention. In FIG. 2, reference numeral 8 denotes a light source composed of a semiconductor light emitting diode that emits coherent light, and the light from the light source 8 is amplitude-modulated by an electrical signal from a tracking generator 9. Coherent amplitude-modulated light from a light source 8 is incident on a multimode optical fiber 10 serving as a vibration sensor, and an optical signal from the multimode optical fiber 10 is received by a light receiver 12 via a mode filtering mechanism 11. Convert to electrical signal. This electrical signal is manually input to the spectrum analyzer 13, where the frequency is measured.

The mode filtering mechanism 11 is provided to increase the sensitivity of measurement, and FIG. 6 is a diagram showing the effect of the mode filtering mechanism, and shows the case where the frequency and amplitude are constant. The horizontal axis is the insertion loss of the mode filter, and the vertical axis is the amplitude of the electrical signal at that time.Increasing the insertion loss of the mode filter increases the amount of change in optical power, which increases the amplitude of the electrical signal. You can see that it also gets bigger.

From this, the mode filtering mechanism 11. It can be understood that measurement sensitivity can be increased by providing .

Note that coherent light is transmitted through a multimode optical fiber 10.
When the light enters the optical fiber, the light interferes due to the difference in the propagation constant between the modes, and when looking at the cross section of the optical fiber 10, as shown in FIG. 4, the intensity of light called speckle occurs. Fourth
In the figure, 14 is a cladding, 15 is a core, and 16 is a speckle, and this speckle 15 is a part of the optical fiber 10.
When it vibrates, it moves within the core 15 of the optical fiber 10. Therefore, the light emitted from the multimode optical fiber 10 is transmitted through the mode filtering mechanism 11 before entering the light receiver 12. In this case, the mode filtering mechanism 11 imparts an axial shift to the optical fiber 10. When the axis misalignment is applied, the multimode optical fiber 10 vibrates, causing
As the speckle 16 moves, the coupling efficiency of the off-axis portion changes, and the light receiver 12 receives a light intensity signal having the same frequency as the vibration, in addition to the modulated optical signal. Therefore, if the drowsiness signal from the light receiver 12 is manually input to the spectrum analyzer 16 and synchronized with the modulation signal α, only the frequency can be measured. Here, an actual measurement example is shown in FIG. Figure 5 Ca
) is for the vibration meter of the present invention.

Figure (6) shows the case of a conventional vibration meter shown for comparison. As shown in FIG. 5, if vibration occurs, the optical power changes, and the electrical signal from the light receiver 12 also changes at the same frequency as the vibration frequency.

According to the embodiment of the present invention described above, a signal with the same amplitude can be obtained even if the measured vibration frequency is different, and even if the vibration frequency is unknown, the amplitude can be calibrated. Moreover,
Since only the multimode optical fiber 10 is used as the vibration sensor, the configuration is relatively simple. Even minute vibrations can be measured with high sensitivity by increasing the loss of the mode filtering mechanism 11.

Note that the modulation signal may be a direct current. However, in this case,
Use an oscilloscope instead of a spectrum analyzer.

〔Effect of the invention〕

As explained above, according to the present invention, vibrations can be measured using only a multimode optical fiber as a vibration sensor, and even minute vibrations can be measured with high sensitivity. This has the effect of simplifying the configuration of the device.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of the configuration of a conventional optical fiber vibrometer. Fig. 2 is a configuration explanatory diagram showing one embodiment of the optical fiber vibrometer of the present invention, Fig. 3 is a diagram for explaining the effect of the mode filtering mechanism shown in Fig. 1, and Fig. 4 is a diagram showing the effect of the mode filtering mechanism of Fig. 2. A diagram showing a speckle pattern on the end face of a multimode optical fiber,
FIG. 5 is a diagram showing an example of measurement using the optical fiber vibrometer of the present invention. 8; light source; 9; tracking generator. 10; Multimode optical fiber; 11; Mode filtering mechanism; 12; Photoreceiver; 16; Spectrum analyzer.

Claims (1)

[Claims] (1) A light source that outputs amplitude-modulated coherent light, a multimode optical fiber as a vibration cell into which the light from the light source enters, and converts the light emitted from the optical fiber into electricity. a mode filtering mechanism that increases fluctuations in the amplitude of an optical signal received by the light receiver when the optical fiber provided between the light receiver and the output end of the optical fiber vibrates; An optical fiber vibrometer characterized by comprising a spectrum analyzer and a spectrum analyzer that measures the frequency by manually inputting the output of a light receiver.