JP2961165B2 - Optical fiber sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial applications]

The present invention relates to an optical fiber sensor having a laser that demodulates a signal separately from a laser that supplies light to the sensor, and more particularly to an optical fiber sensor that is suitable when the phases of two lasers are matched by injection locking.

[Prior art and problems]

In a conventional optical fiber sensor, a two-beam interferometer is generally used to read out a phase change of light. It has only a laser that supplies light to the sensor as a light source, and the phase of light propagating through one of two optical fibers separated before entering the sensor portion changes to reflect an external physical phenomenon. On the other hand, the light of the light source propagates as it is in the other optical fiber without being affected by external physical phenomena and is multiplexed again.The signal phase-modulated by the interference is converted into intensity modulation and detected as a sensor output. You. Therefore, the two interfering lights are basically the same except for a phase change reflecting an external physical phenomenon, and signal demodulation is achieved with a high s / n ratio. However, two optical fibers are required as basic components of the sensor, and the influence of temperature, sound, and mechanical vibration other than the object to be measured is likely to be mixed in from the two optical fibers, and the length of the sensor is limited. That is, the conventional optical fiber sensor has only a laser that supplies light to the sensor as a light source, and a part thereof is used for signal demodulation by bypassing the sensor part.
In particular, phase synchronization is not required, but instead, two necessary fibers are required as basic components of the sensor, and there are problems associated with the configuration, price, s / n ratio, and the like. The present invention solves the above-mentioned problems, and is an interference type optical fiber sensor that requires only one optical fiber as a basic constituent element. It is an object of the present invention to provide an optical fiber sensor which is hardly affected by temperature, influence and mechanical vibration.

[Means for Solving the Problems]

In order to achieve the above object, the present invention provides an optical fiber sensor that utilizes the fact that the phase of light propagating in an optical fiber changes by reflecting an external physical phenomenon, and supplies light to the sensor fiber at one end of the sensor fiber. A sensor laser and an injection locking system having a slave laser at the other end of the sensor fiber, wherein the sensor fiber and the slave laser are connected by an optical fiber coupler, and each output port of the optical fiber coupler detects light intensity. And a signal processing device following the device, and obtains an electric signal proportional to an optical phase change reflecting a physical phenomenon.

[Action]

According to the present invention, the sensor can be configured simply by providing a light source, a single sensor fiber, and a slave laser at the end of the fiber, so that the configuration can be simplified. The signal attenuated by the propagation can be demodulated with less noise, and the effect of temperature, sound, and vibration other than the object to be measured is less likely to be mixed, and the sensor can be made longer easily. The effects of various physical phenomena on the fiber can be detected with high accuracy at the wavelength level of light. In this case, in the present invention, the signal light carrier is injection-locked with the synchronous laser (slave laser). As a result, the frequency of the synchronous laser (slave laser) matches the frequency of the signal light carrier and the phase of the synchronous laser (slave laser). Is also synchronized with the phase of the signal light carrier. For example, when light of a wavelength lambda of a light source is injected into a fiber laser oscillating by an oscillation wavelength lambda of a ring laser, the oscillation wavelength of the fiber laser changes from the oscillation lambda of the ring laser to the wavelength lambda of the light source. , Injection synchronization is established. what if,
When the light from the light source is subjected to sinusoidal phase modulation of the frequency ω due to the physical phenomenon to be measured, the optical angular frequency has a component of 2πf ± ω × integer in addition to the fundamental wave 2πf. here,
If ω is selected so that all ω × integers are out of the lock range of the injection locking system, the injection locking occurs for the fundamental wave, and the phase modulation component does not affect the injection locking.

【Example】

Hereinafter, embodiments of the present invention will be described with reference to the drawings. The most significant feature of the present invention is that an interference sensor is basically composed of only one optical fiber by using a laser for demodulating a signal separately from a laser for supplying light to the sensor. What is newly needed here is to match the phase of the laser that supplies light to the sensor with the phase of the laser that demodulates the signal. For this purpose, the second major point is that injection locking is used. It is a feature. First Embodiment FIG. 1 is a basic configuration diagram of a first embodiment, in which reference numeral 1 denotes a light source, 2 denotes a fiber, 3 denotes a sensor unit, 4 denotes a signal to be measured (physical phenomenon), 5 denotes a coupler, 6 is a synchronous laser, 7 is a bypass fiber, and 8 is a detector. The light from the light source 1 (master laser) undergoes a phase change by a signal under measurement 4 in a sensor section 3 formed in a part of the fiber 2. On the other hand, the signal light is
Is branched into a bypass fiber 7 and a synchronous laser (slave laser) by a coupler 5 disposed in the
Are combined. Therefore, the signal light carrier is injection-locked with a synchronous laser (slave laser). As a result, the frequency of the synchronous laser (slave laser) matches the frequency of the signal light carrier, and the phase of the synchronous laser (slave laser) is also synchronized with the phase of the signal light carrier. By the way, many studies have been made on the injection locking phenomenon in the microwave region, and the same phenomenon has been reported in the optical region (reference: CLRuthroff, “Injection-Locked Os”).
cillator FM Receiver Analysis ", Bell System Technic
al Journal, pp1653-1661, Oct., 1968). What is important here is that injection locking does not occur for the signal light but for the signal light carrier. As a result, the synchronous laser can reproduce the light from the light source 1 (master laser) before receiving the phase modulation due to the phenomenon to be measured. On the other hand, the light (signal light) that has undergone phase modulation due to the phenomenon to be measured is transmitted through the fiber 2
And the coupler 5 arranged via the bypass fiber 7 to the coupler 5 '. Accordingly, light before being subjected to phase modulation (reference light) and light after being subjected to phase modulation (signal light) are multiplexed by the coupler 5 ', and the phase modulation is converted into intensity modulation on the same principle as interference. . Here, since the phase relationship between the two lights combined by the coupler 5 'depends on the length of the bypass fiber 7, the length of the bypass fiber 7 needs to be stabilized. In this case, as the synchronous laser 6, a traveling wave type semiconductor laser amplifier or various fiber lasers (ring type or Fabry-Perot type) can be used. Second Embodiment FIG. 2 is a basic configuration diagram of the second embodiment. The components shown in FIG. 2 that are the same as those shown in FIG. 1 are given the same reference numerals to simplify the description. In the second embodiment, a ring laser is formed as a whole by arranging an optical amplifier 10 on a part of a fiber 9 serving as a fiber ring as a synchronous laser (slave laser). In this case, as the optical amplifier 10, an erbium-doped fiber optical amplifier or a traveling-wave semiconductor optical amplifier can be used. The difference between the second embodiment and the first embodiment is that, in the second embodiment, the optical path directly passing through the fiber coupler constituting the ring laser is given the same role as the bypass fiber 7 in FIG. Thus, the configuration can be simplified. In the second embodiment, the phase relationship between the light before being subjected to phase modulation (reference light) and the light after being subjected to phase modulation (signal light) depends on the dependence on the length of the bypass fiber 7 in FIG. The advantage is that there is no corresponding instability factor. Third Embodiment FIG. 3 is a diagram showing the basic configuration of the third embodiment. The components shown in FIG. 3 that are the same as those shown in FIG. In the figure, reference numeral 11 denotes an excitation light source, 12 denotes a wavelength selection mirror, 13 denotes an erbium-doped fiber, 14 denotes an optical wavelength filter, and 15 denotes an isolator. In the third embodiment, an erbium-doped fiber optical amplifier is used in the ring laser portion of the second embodiment. The erbium-doped fiber 13 is connected to the coupler 5 provided in the optical fiber 2 in a ring shape.
15 is inserted. Further, the erbium-doped fiber
An excitation light source 11 is connected to 13 via a wavelength selection mirror 12. When a certain amount or more of excitation light is input to the erbium-doped fiber 13, the fiber ring oscillates as a laser. The ring laser in this state is a synchronous laser (slave laser)
It is what becomes. As described above, in any of the first embodiment of FIG. 1, the second embodiment of FIG. 2, and the third embodiment of FIG. 3, the phase-modulated signal light is transmitted from the fiber 2 to the synchronous laser. The principle of signal demodulation when injected is basically the same. Hereinafter, the configuration of FIG. 3 will be described in more detail as an example. First, the oscillation wavelength lambda (2) (not shown) of the ring laser is set to a value as close as possible to the wavelength lambda (1) (not shown) of the light source 1. To this end, while adjusting the setting of the optical wavelength filter 14 disposed in the erbium-doped fiber ring, the temperature of the fiber ring is controlled by external temperature,
It is necessary to stabilize so as not to be affected by vibration. Thereby, of the laser oscillation modes determined by the fiber ring optical path length and the optical amplification gain distribution of the erbium-doped fiber, only the mode closest to the wavelength lambda (1) (wavelength lambda (2)) can be selectively oscillated. it can. With such a configuration, when light of the lambda (1) is injected into the fiber laser oscillating by the wavelength lambda (2), the oscillation wavelength of the fiber laser changes from the lambda (2) to the lambda (1). Then, injection synchronization is established. If the light from the light source 1 undergoes sinusoidal phase modulation of the frequency ω by the signal under measurement 4 in the sensor unit 3, the optical angular frequency becomes the fundamental wave 2
In addition to πf (wavelength lambda (1)), 2πf ± Nω (where N
Is an integer) component. Where all Nω
Can be selected so as to deviate from the lock range of the injection locking system used here, the injection locking occurs for the fundamental wave, and the phase modulation component does not affect the injection locking. As a result, the coupler 5 multiplies the signal light of the fundamental frequency ω that is phase-modulated by the signal under measurement and the synchronous light of the frequency ω that does not include the phase modulation component. What is important here is that the phase relationship between the multiplied light waves is ideally fixed by injection locking. Here, besides 2 × 2 couplers, 3
Multiplication (= interference) can be performed with either the × 3 coupler or the 4 × 4 coupler. However, it is a multiplication.
When the phase difference between the two light waves is maintained at 90 degrees, the demodulation can be performed with the highest sensitivity and with good linearity.
For the most efficient demodulation, a 3 × 3 coupler or 4
A x4 coupler is advantageous. The reason is described below. FIG. 4 explains this relationship. The same reference numerals are given to the same components as those in FIGS. 1 to 3 to simplify the description. In the figure, the signal light and the ring laser light enter the 3 × 3 coupler 5 ′. Assuming that both frequencies are f and the phase difference is θ, when B1 = B2 = B3, the output fluctuation components are I1: cos (2πf + θ) I2: cos (2πf + θ + 120 degrees) I3: cos (2πf + θ−120 degrees) . Therefore, except for the output port to the ring laser, 2
The light intensity from the two output ports does not disappear at the same time. While injection locking is established, the phase difference θ between the two changes in the range of −90 degrees to +90 degrees in proportion to the difference between the natural oscillation frequency of the ring laser and the frequency of the light source 1. Above that it is known that synchronization is lost. Now, if the phase of the phase-modulated injection light is newly set to θ with reference to the phase of the ring laser light (synchronous light) multiplexed by the coupler, Δ
θ: phase difference between injected light and synchronous light, ψcospt: can be expressed as θ = Δθ + ψcospt using phase modulation by a signal to be measured riding on the injected light. Therefore, if I2 and I3 are independently taken out by two light receivers and the signal processing unit electrically generates I2 + I3 and I2-I3, an output proportional to ψ can be finally obtained. If Δθ = −30 degrees, I1: cos (2πf−30 degrees) I2: cos (2πf + 90 degrees + ψcosptθ) I3: cos (2πf−150 degrees + ψcosptθ), and if the output I2 is detected, ψ is the most sensitive and Demodulation can be performed stably with good linearity. Since Δ = θ−30 degrees, the limit of the injection locking band (out-of-synchronization point)
There is room for 60 degrees. In the case of a 4 × 4 coupler, I1: cos (2πf + θ) I2: cos (2πf + θ + 90 degrees) I3: cos (2πf + θ + 180 degrees) I4: cos (2πf + θ + 270 degrees) 2πf + 90 ° + ψcosptθ) or I4: cos (2πf + 270 ° + ψcosptθ) is obtained, and demodulation under ideal conditions is also possible. At this time, since Δθ = 0 degrees, the operating point of demodulation is at the center of the synchronization width, and a more stable operation can be performed than in the case of the 3 × 3 coupler.

【The invention's effect】

As described above, according to the present invention, the following effects can be obtained. Unlike a general two-beam optical interferometer, a sensor can be configured only by providing a light source, one sensor fiber, and a slave laser at an end of the fiber, so that the configuration can be simplified. In addition, by utilizing the strong light intensity of the synchronous laser, it is possible to demodulate a signal attenuated while propagating over a long distance with less noise. In addition, since there is only one sensor fiber, the effects of temperature, sound, and mechanical vibration other than those to be measured are less likely to be mixed,
The length of the sensor can be easily increased. Therefore, over a long distance of, for example, 10 km or more,
It is possible to construct a sensor capable of detecting the effects of various physical phenomena on the fiber with high accuracy at the wavelength level of light.

[Brief description of the drawings]

FIG. 1 is a diagram showing a basic configuration of a first embodiment of the present invention, and FIG.
FIG. 3 is a diagram showing the basic configuration of the second embodiment, FIG. 3 is a diagram showing the basic configuration of the third embodiment, and FIG. 4 is a 3 × 3 coupler or 4
FIG. 4 is a diagram for explaining that a × 4 coupler is advantageous for efficient demodulation. 1 ... light source, 2 ... fiber, 3 ... sensor part, 4 ...
Signal to be measured, 5: Coupler, 6: Synchronous laser, 7:
... Bypass fiber, 8 ... Detector, 9 ... Fiber,
10 optical amplifier, 11 excitation light source, 12 wavelength selective mirror, 13 erbium-doped fiber, 14 optical wavelength filter, 15 isolator.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-2-176509 (JP, A) JP-A-2-135323 (JP, A) (58) Fields investigated (Int.Cl. ⁶ , DB name) G01J 9/00-9/04 G01N 21/00-21/74 G02B 6/00 JOIS

Claims (1)

(57) [Claims] An optical fiber sensor utilizing the fact that the phase of light propagating in an optical fiber changes by reflecting an external physical phenomenon. A master laser for supplying light to one end of the sensor fiber, The other end of the sensor fiber has an injection locking system having a slave laser. The sensor fiber and the slave laser are connected by an optical fiber coupler, and each output port of the optical fiber coupler has a light intensity detector and a signal following it. An optical fiber sensor having a processing device and obtaining an electric signal proportional to an optical phase change reflecting a physical phenomenon.