JPH0346053B2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to an optical measurement device that can accurately measure a physical quantity to be measured without depending on optical transmission line loss, coupling loss, fluctuation, etc. .

[Technical background of the invention and its problems]

Conventionally, various optical measuring devices have been known that optically measure physical quantities by utilizing a phenomenon in which physical quantities such as temperature, pressure, magnetic field, electric field, etc. cause changes in the intensity and phase of light waves. One of them is an optical measurement device that transmits a constant light output through an optical fiber, causes a loss in a sensor section according to the physical quantity to be measured, and measures the physical quantity based on the change in the amount of light. In such a device, as a sensor, for example, a polarization-maintaining fiber whose radiation loss of two principal axes changes depending on temperature, a magnetic sensor combining a magneto-optical element and a polarizer, etc. are used.

However, when using changes in optical loss, changes in loss in the optical transmission path, coupling loss in optical elements, etc. directly appear as measurement errors.

Therefore, for example, in a temperature sensor that utilizes the temperature dependence of the fundamental absorption characteristic of a semiconductor element, as shown in _FIG . In other words, a method has been proposed in which a reference light of wavelength λ ₂ , which is not affected by loss fluctuations due to temperature, is simultaneously transmitted, and loss fluctuations in the transmission line and optical coupling section are compensated for by calculating the difference in the amount of received light between the two. ing.

However, such a device has disadvantages in that its field of application is limited and its configuration is complicated because it requires a multiplexer, a demultiplexer, and the like.

[Purpose of the invention]

The present invention was made based on the above problem, and its purpose is to provide an optical measuring device in which the amount of loss varies depending on the physical quantity to be measured, without complicating the structure. It is an object of the present invention to provide an optical measurement device that enables compensation for loss fluctuations in transmission paths and coupling parts and can be applied to a wide range of fields.

[Summary of the invention]

The present invention is characterized by the following configuration.

That is, by guiding light waves from a light source that emits light waves while sweeping the wavelength to a polarization-maintaining fiber via an optical isolator and a non-polarizing beam splitter, the polarization state is changed to the output end of the polarization-maintaining fiber. generates periodically changing light waves, which are introduced into the sensor section. The sensor section has a reflective film at the end, and returns the light wave whose polarization state changes periodically to the incident end via the reflective film, and also returns the light wave whose polarization state is perpendicular to each other to the input end. The loss difference changes depending on the physical quantity to be measured. Then, the feedback wave from this sensor section is received by a photoelectric conversion element via the polarization maintaining fiber and the non-polarization beam splitter, and the amplitude value of the AC component of the output of this photoelectric conversion element is calculated based on the DC component. The method is characterized in that the measured value of the physical quantity to be measured is obtained by normalizing the physical quantity.

〔Effect of the invention〕

If the loss of the feedback wave generated in the sensor section differs depending on the polarization direction, when a light wave whose polarization state changes periodically is incident on this sensor section, the intensity of the feedback wave will vary depending on the polarization state of the incident light wave. Changes periodically based on periodic changes. Therefore, the output of the photoelectric conversion element also becomes a signal containing an alternating current component. The amplitude of this AC component, that is, the loss difference in the polarization direction of the feedback wave is:
Since it changes depending on the physical quantity to be measured, the physical quantity to be measured can be obtained from this loss difference.

Incidentally, the output of the photoelectric conversion element varies not only due to the loss in each polarization direction of the feedback wave, but also due to optical transmission line loss, coupling loss, photoelectric conversion loss, and the like. Although the latter losses lead to measurement errors, they affect the DC level and the amplitude of the AC component of the output of the photoelectric conversion element in equal proportions.

In the present invention, the amplitude value of the AC component is normalized by a value based on the DC component, and the measured value is obtained based on this value, so transmission line loss, etc. that can lead to measurement errors is compensated for, and the measurement accuracy is excellent. We can provide equipment that Furthermore, the present invention can be applied to all optical measurement devices that utilize the fact that the amount of loss changes depending on the physical quantity to be measured, so it has a wide range of applications, and it is not necessary to use multiple light waves with different wavelengths. Since there is no multiplexer or demultiplexer required,
It also has the advantage of simple structure.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing an example of a temperature sensor using a polarization maintaining fiber in the sensor section.

In the figure, the wavelength swept light source 11 is composed of, for example, a distributed feedback semiconductor laser (DFB laser) in a single mode and capable of changing the oscillation wavelength by several angstroms. For sweeping the oscillation wavelength, for example, a thermal wavelength sweeping method using a microheater submount, which was previously proposed by the present inventors, can be used. In other words, according to the thermal wavelength sweep method, the oscillation output can be kept constant by APC (automatic park control), and stable wavelength sweep is possible. The sweep waveform can be changed arbitrarily, but in this example,
Assume that the wavelength is swept in proportion to time. Therefore, a sawtooth wave-like wavelength sweep signal is provided from the sawtooth wave generator 12 to the wavelength sweep light source 11 . In addition, by the DC bias power supply 13,
The wavelength swept light source 11 is made to emit light at a constant rate.

The light from the wavelength swept light source 11 passes through an optical isolator 14 and further passes through a non-polarizing beam splitter 15, and then enters a polarization maintaining fiber 16. At this time, in order to evenly distribute the electric field of the light from the wavelength swept light source 11 to the main axis of the polarization maintaining fiber 16, the polarization direction of the wavelength swept light source 11 and the main axis direction of the polarization maintaining fiber 16 are When light is incident at an angle of 45°, the polarization state at the output end of the polarization-maintaining fiber 16 changes periodically from linearly polarized light to circularly polarized light and then to linearly polarized light orthogonal to the linearly polarized light according to the wavelength sweep. change. This light wave is incident on the sensor section 17.

The sensor section 17 is a polarization-maintaining fiber whose cutoff wavelength is at least 0.2 μm shorter than the wavelength of the light source, and the polarization-maintaining fiber of the sensor section 17 is aligned with the polarization-maintaining fiber 1 for optical transmission with respect to its main axis.
The main axes of the polarization maintaining fibers are arranged at 45° to each other so that the electric field of the light from the fibers 6 is evenly distributed. In the polarization maintaining fiber of the sensor section 17, the radiation loss of the propagating light is different between the first principal axis and the second principal axis, and the loss difference between the two becomes smaller as the temperature applied to the sensor section 17 increases. It has the following properties. That is, in general, polarization-maintaining fibers have a refractive index change in the principal axis direction due to residual stress in the core during cooling during manufacturing. Therefore, when the temperature of the fiber is raised, the stress difference in the two principal axis directions becomes smaller, so the refractive index difference in both principal axis directions also becomes smaller, and the difference in radiation loss also becomes smaller.

A light wave whose polarization state changes periodically as described above is introduced into the sensor section 17 . A reflective film 18 is formed at the output end of the sensor section 17 .
Since the polarization preserving fiber 16 and the sensor section 17 are arranged so that their principal axes form an angle of 45 degrees, linearly polarized light is alternately incident on the first principal axis and the second principal axis of the sensor section 17. . Since the radiation loss in both principal axis directions is different, the feedback wave reflected by the reflective film 18 and returned to the input end of the polarization maintaining fiber 16 for light transmission becomes an optical signal with periodic intensity changes.
The amplitude becomes smaller as the temperature applied to the sensor section 17 becomes higher.

The feedback wave is reflected by the non-polarizing beam splitter 15 whose splitting ratio changes within 1% even if the polarization state at the input end of the polarization-maintaining fiber 16 changes, and is reflected by the photoelectric converter 19. Photoelectrically converted. The output of the photoelectric converter 19 is amplified by an amplifier 20 and introduced into a signal processing circuit 21 .

FIG. 2 is a diagram showing signal waveforms input to the signal processing circuit 21. When the temperature applied to the sensor section 17 is low, the difference in loss in both polarization directions in the sensor section is large, so the amplitude value A of the AC component is large, as shown in FIG. As the temperature applied to the sensor section 17 becomes higher, the difference in loss in both polarization directions in the sensor section 17 becomes smaller, so the amplitude value A of the AC component becomes smaller. The signal processing circuit 21 A/D converts this signal, detects the maximum value and minimum value, and calculates the level B of the DC component of the signal from these values.
The amplitude value A is normalized by the calculation A/B.

In this way, according to this embodiment, not only can loss fluctuations in the transmission line and the coupling portion be compensated for, but also fluctuations in the photoelectric converter 19 can be compensated for, and extremely accurate temperature measurement is possible.

FIG. 3 is a diagram showing an example in which the present invention is applied to a magnetic sensor using a Faraday element in the sensor section.

The difference between this embodiment and the previous embodiment is the sensor section 31.

That is, the sensor section 31 is provided through a rod lens 32 at the output end of the polarization maintaining fiber 16 for optical transmission. 33 in the figure is a polarization separating prism, whose transmission axis is the polarization preserving fiber 1.
It is arranged at an angle of 45° to the main axis of 6. The light transmitted through the polarization separation prism 33 is guided to another polarization separation prism 35 via a Faraday element 34. The transmission axis of this polarization separation prism 35 is arranged so as to coincide with the transmission axis of the polarization separation prism 33. Therefore, if no magnetic field is applied to the Faraday element 34, the light wave that has passed through the Faraday element 34 also passes through the polarization separation prism 35 and is reflected by the reflective film 36 provided at the base end of this prism 35. , and is fed back again via the polarization separation prism 35 - the Faraday element 34 - the polarization separation prism 33 - the rod lens 32 - the polarization preserving fiber 16. Furthermore, when a magnetic field is applied to the Faraday element 34, the plane of polarization of the light wave propagating through the Faraday element 34 rotates, so that the propagating light is transferred to the polarization separation prism 3.
5, 33 suffers loss due to reflection. Therefore, the magnetic field can be measured based on the amount of loss of the feedback light.

On the other hand, the light wave reflected by the polarization separation prism 33 passes through a waveguide 37 for adjusting the optical path, is reflected by a reflective film 39 provided at the base end of this waveguide 37, and is again passed through the waveguide 37 to Polarization separation prism 33~
It is fed back via the rod lens 32 and the polarization maintaining fiber 16. The feedback light that returns through this path is not affected by the magnetic field at all.

The light wave incident on the polarization separation prism 33 from the rod lens 32 is a light wave whose polarization state changes periodically as the wavelength sweep of the wavelength swept light source 11 is performed, as in the above-described embodiment. exhibits a switch effect that alternately repeats transmission and reflection of light. Therefore, as the wavelength is swept, the optical path changes periodically, causing a change in loss, and the feedback light incident on the photoelectric conversion element 19 can have a periodic intensity change.

In this embodiment, the return light reflected from the reflective film 38 is the brightest and does not depend on the magnetic field, so it is used as a reference signal. In addition, the reflective film 3
As the applied magnetic field increases, the amount of the feedback light reflected from the magnetic field 6 decreases. Therefore, the signal processing circuit 2
No. 1 can compensate for losses caused by measurement errors by normalizing the amplitude value of the AC component of the input signal by the maximum value (reference signal).

As described above, the present invention can reduce transmission line loss and coupling loss by normalizing the amplitude value of the AC component of the output signal of a photoelectric conversion element by the maximum value, minimum value, or their average value, which is not influenced by physical quantities. It is possible to compensate for losses that lead to measurement errors such as

Note that the present invention is not limited to the embodiments described above. For example, as a sensor part, in addition to those mentioned above, electric field sensors such as LiNbO ₃ and LiTaO ₃ ,
It can be applied to an extremely wide range of applications, including voltage sensors and temperature sensors that utilize the basic absorption of semiconductors.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of a temperature sensor according to one embodiment of the present invention, FIG. 2 is a main waveform diagram of the sensor, and FIG. 3 is a diagram showing the configuration of a magnetic field sensor according to another embodiment of the present invention. The block diagram shown in FIG. 4 is a diagram for explaining a conventional optical measuring device. 11... Wavelength swept light source, 14... Optical isolator,
15...Non-polarizing beam splitter, 16...Polarization maintaining fiber, 17,31...Sensor section, 18,3
6, 38... Reflection film, 19... Photoelectric converter, 20... Amplifier, 32... Rod lens, 33, 35... Polarization separation prism, 34... Faraday element, 37...
waveguide.

Claims (1)

[Claims] 1. A light source that emits light waves while sweeping the wavelength, an optical isolator that allows the light waves from this light source to pass in only one direction, and branches the light waves that have passed through the optical isolator regardless of the polarization state. A non-polarizing beam splitter, and guiding the light waves introduced through the non-polarizing beam splitter to a measurement point, and periodically changing the polarization state of the light waves at the output end by wavelength sweeping the light source. A polarization-preserving fiber for optical transmission, a light wave from this polarization-maintaining fiber, which is guided back to the input end via a reflective film provided at the end, and the loss difference between the returned waves in mutually orthogonal polarization directions. a sensor section whose value changes according to a physical quantity to be measured; a photoelectric conversion element which receives a feedback wave from the sensor section via the polarization preserving fiber and the non-polarizing beam splitter and converts it into electricity; and the photoelectric conversion element. means for normalizing the amplitude value of the AC component of the output of the element by a value based on the DC component to obtain a measured value of the physical quantity to be measured,
2. An optical measuring device, wherein the polarization maintaining fiber for optical transmission is arranged such that the angle between its main axis and the polarization direction of the wavelength swept light source is 45 degrees. 2. The sensor section includes a polarization-maintaining fiber whose loss difference in two principal axes changes depending on temperature and whose cutoff frequency is at least 0.2 μm shorter than the wavelength of the light source, and a reflective fiber provided at the end of the polarization-maintaining fiber. The polarization-maintaining fiber of the sensor section is arranged such that its main axis direction forms an angle of 45° with respect to the main axis of the polarization-maintaining fiber for optical transmission. The optical measuring device according to item 1. 3. The sensor unit includes a polarization splitting prism that alternately distributes the light waves from the polarization maintaining fiber according to periodic changes in the polarization state, and a polarization splitting prism that distributes the light waves from the polarization maintaining fiber in one polarization direction. a magneto-optical element that rotates the polarization plane of a light wave by an applied magnetic field; a waveguide that propagates the light wave in the other polarization direction distributed by the polarization separation prism while maintaining the polarization direction; 2. The optical measuring device according to claim 1, comprising the magneto-optical element and a reflective film provided at each end of the waveguide.