JPH02238334A - Interference optical measuring apparatus wherein scale factor is stabilized - Google Patents

Abstract

PURPOSE:To prevent errors due to temperature change by controlling a current so that the light generating amount of a semiconductor light source which generates monochromatic light is constant, while obtaining the wavelength based on the current value, and correcting a scale factor. CONSTITUTION:Monochromatic light from a light source 1 is split into two light beams in an interference optical system 2. The light beams are inputted into a photodetector 3 by way of the different paths. Then the phase difference thetaof the interference light beams is obtained in a signal detecting circuit 4. The difference is divided by a scale factor, and the intended physical quantity is obtained. The scale factor is the function of a wavelength lambda. The wavelength lambdais changed with temperature. The value is corrected as follows. An amount P of generated light is found through a half mirror 13 and a photodetector 14. The current of the semiconductor 1 is controlled so that the generated light amount P becomes constant with a control part 8. The current value is obtained with the voltage drop in a resistor 9. Since the wavelength lambda is determined based on the current value, the scale factor is corrected with a correcting circuit 11.

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Technical field The present invention relates to a measuring device that uses optical interference to measure physical quantities such as rotational angular velocity, velocity of a moving object, intensity of underwater sound, temperature, and pressure. Find the temperature variation of the wavelength λ of light,
This relates to what is intended to obtain correct measurements.

When a monochromatic element is split into two parts, passed through different optical paths, and then recombined, the lights interfere.

The difference between the optical paths of the two lights appears as a phase difference on the light receiving element surface.

Once the physical conditions are determined, the phase difference θ between the two lights is determined.

When physical conditions change, the phase difference θ changes. The variation in the phase difference θ can be determined as an increase or decrease in the amount of light incident on the light receiving element. Therefore, the physical quantity can be determined from the phase difference θ.

Examples of such devices include fiber optic gyros, fiber optic hydrophones, and fiber optic laser velocimeters.

The optical path difference changes due to changes in physical quantities.

However, when the light enters the light receiving element, the amount of light changes due to the change in the phase difference θ. The phase difference θ is the optical path difference divided by the wavelength λ of the light. Therefore, when the wavelength λ changes due to temperature fluctuation, the proportionality constant between the optical path difference and the phase difference changes.

Generally, the relationship between the size of a physical quantity to be measured and the size of the output of a measuring device is called a scale factor.

In a measurement system using optical interference, the wavelength λ is included in the scale factor. Therefore, when the wavelength changes, the school factor changes.

Such a measurement system will be briefly explained.

(a) Optical fiber gyro Two monochromatic lights are separately introduced into both ends of an optical fiber coil in which a single mode optical fiber is wound around a core many times.

These lights propagate inside the coil both counterclockwise and clockwise. Although they each come out at opposite ends, they are combined and made incident on the light receiving element.

When the coil rotates at an angular velocity Ω, a phase difference θ occurs between the two lights.

It is. λ is the wavelength of monochromatic light, n is the core refractive index of the optical fiber, L is the total length of the optical fiber, a is the coil radius, and C is the speed of light in vacuum. For example, when θ is simply interfered with,
It enters the light receiving element output in the form of cos θ. Although there are other variations of the optical fiber gyro kζ, θ is the observable quantity. The ratio between this and the physical quantity Ω to be measured is the scale factor, which includes the wavelength λ of the light emitted by the light source.

(c) Optical fiber hydrophone This is also a single-mode optical fiber wound into a coil, allowing monochromatic light to enter from one end and emit light from the other end. Make two similar coils, and soak one coil in water to use it as a sensor coil.

The other coil is kept under constant conditions and serves as a reference coil. Monochromatic light is split into two parts by a beam splitter, and then input from one end of both the sensor coil and reference coil. The light emitted from the other end is combined and interfered with.

Although they are equivalent coils, the sensor coil causes a refractive index change & optical path length change Δl due to water pressure. Therefore, the two lights produce a phase difference θ, which is? π θ =■(nΔ7+7Δn) (2) Given by λ. Here, l is the total fiber length of the fiber coil, and n is the refractive index. Since Δl and Δn are proportional to the water pressure p, the strength of the underwater sound wave can be determined from the amplitude of the change in θ. Equation (2) also includes the wavelength λ of light in the scale factor.

In this way, a system that uses a sensor coil and a reference coil, divides monochromatic light into two, passes them through both coils, and causes them to interfere can also be used to measure pressure and temperature. pressure p,
From the variation k of temperature T, l. This is because n changes.

(d) Optical fiber laser speed meter. measures the velocity V of an object using the Doppler effect. Light emitted from a light source that produces monochromatic light,
It is passed through an optical fiber, and the light is emitted from the microlens system at the tip and hits the object. The light reflected from the object and the light reflected by the microlens system return in opposite directions through the optical fiber. This light is taken out to the side by a beam splitter and is made incident on a light receiving element where it interferes.

When an object is moving at a speed V in a direction forming an angle θ with respect to the optical axis of the lens, the frequency of the reflected light from the object shifts due to the Doppler effect. This frequency shift is equal to the frequency of intensity change (beat frequency) of the interference light. This can be written as. The denominator of this equation also includes λ.

g) Problems to be Solved by the Invention The problem is the stability of the wavelength of the light source in the measurement system using these interferences. It must emit monochromatic light and it must have a certain degree of coherency.

When a gas laser is used as a light source, wavelength stability is excellent. However, since it becomes a large device, it becomes difficult to use in practice.

When a semiconductor laser or a light emitting diode (superluminescent diode) is used as a light source, it can be made smaller and lighter. However, in semiconductor light sources, the wavelength λ fluctuates due to temperature changes.

A change in the wavelength λ means a change in the scale factor.

For example, a semiconductor laser with a center wavelength of 780 nm,
The wavelength change is Q, 3 nm/°C. If the temperature range used is, for example, 50° C., the wavelength will vary by as much as 15 nm in this range.

In other words, the scale factor fluctuates by about 2% within this range.

If the temperature fluctuation is large, the scale factor changes, so
The measurement accuracy of physical quantities deteriorates.

Conventionally, elements with little temperature change have been selected and used, or the temperature has been kept constant. This is because if the temperature is kept constant, λ will also be constant.

However, it is not easy to always maintain a semiconductor light emitting device at a constant temperature. In some cases, a semiconductor light-emitting element is attached to an element that can freely generate heat and absorb heat, such as a Beltier element, and the Peltier element is energized in a direction that cancels out temperature fluctuations. However, this requires a complicated circuit and increases costs.

Additionally, because there is a time delay, it may not be possible to keep up with sudden changes in the environment.

Even if the temperature is not kept constant, it should be possible to correct the scale factor if the oscillation wavelength λ is known.

However, if we want to measure the wavelength λ, we must use a spectrometer. A device for this purpose is large-scale and cannot be easily realized.

The wavelength λ is a difficult quantity to measure.

a) Purpose It is an object of the present invention to provide a measuring device using interference using a semiconductor light emitting element as a light source; it is an object of the present invention to provide a device capable of correcting changes in scale factor caused by wavelength fluctuations due to temperature changes. be.

Rather than suppressing temperature changes, it suppresses scale factor fluctuations.

(e) Composition Semiconductor lasers and superluminescent diodes are
It can emit light of a single wavelength. Sometimes it is necessary to have strong coherence, and sometimes it is not. It depends on the measurement method.

As the temperature rises, the bandgap of the semiconductor narrows,
The refractive index increases. Therefore, as the temperature rises, the oscillation wavelength becomes longer.

However, in the case of semiconductor lasers, there is a condition that the length of the resonator is an integral multiple of the wavelength, so as the wavelength becomes longer,
Portrait mode changes. For this reason, the wavelength does not become longer continuously, but "jumps" occur at a certain wavelength. Even in new modes, as the temperature rises, the wavelength becomes longer. Again, a jump occurs at a certain point.

Because it changes intermittently in this way, temperature is not a monovalent function of wavelength. Furthermore, when transitioning between different longitudinal modes, there may be hysteresis.

This is true, but within a certain temperature range, it can be said that the wavelength increases linearly with temperature.

This is simply called the λ-T characteristic.

In the case of light emitting diodes, as well as temperature, within a certain range,
The wavelength increases linearly.

Another factor is that semiconductor light emitting devices have temperature characteristics. It is the amount of light emitted. As the temperature increases, the amount of light emitted decreases.

In the case of a semiconductor laser, the threshold current becomes high, making it difficult to oscillate, and even if a current larger than the threshold current flows, the amount of light emitted becomes small.

This also applies to light emitting diodes.

In this case, although the concept of threshold current does not exist, the relationship between current and light emission amount decreases as the temperature increases. When the temperature is high, the number of minority carriers increases in each region of p%n, so
This is because recombination at the non-junction increases and effectively used injection current decreases.

Therefore, if the amount of light emission P is controlled to be constant, when the temperature T increases, the electric current must be increased. This is because the luminous efficiency decreases as the temperature rises.

Then, under the control condition of keeping P constant, it can be said that the temperature T has a linear relationship with the electric current within a certain range.

This is simply called the IT characteristic.

If you know the λ-T characteristic and the A-T characteristic under control conditions with P constant, you will know the relationship between λ and E.

The oscillation wavelength λ is a difficult quantity to measure, but the electric current is an easy quantity to measure. The scale factor has λ in the denominator. current I
From this, the wavelength λ is determined, and the scale factor is corrected using this. This is the principle of the invention.

The configuration of the present invention will be explained with reference to FIG.

This includes a light source 1, an interference optical system 2, a light receiving element 3, a signal detection circuit 4, a control section 8, a resistor 9, an amplifier circuit 10, and a correction circuit 1.
Consists of 1 etc.

The light source 1 is a semiconductor light emitting element that emits monochromatic light, such as a semiconductor laser or a superluminescent diode.

The interference optical system 2 is a part that splits monochromatic light into two parts using a mirror beam splitter or the like, passes them through different paths, and then combines them to cause interference. It represents the parts of the interference optical system, such as the optical fiber gyro, optical fiber hydrofon, and optical fiber laser velocimeter, which have already been explained. Target physical quantities include angular velocity, underwater sound waves, temperature, pressure, and velocity.

The light receiving element 3 converts the intensity of the interference light into an electrical signal. This intensity includes some form of the phase difference θ between the two interfered lights.

Monochromatic light emitted from a light source 1 enters an interference optical system 2.

As mentioned above, this light is split into two lights, which pass through different paths and then combine again to enter the light receiving element 3.

The output of the light receiving element 3 is processed by a signal detection circuit 4 and output as an electrical signal. This includes amplifier circuits, etc. What is obtained by this is the phase difference θ of the interference light.

The target physical quantity is obtained by dividing θ by the scale factor. Conventionally, the scale factor was assumed to be constant, so θ expresses the size of the target physical quantity. Therefore, the signal detection circuit 4'! So, the device was completed.

In the present invention, a correction circuit 11 for correcting the scale factor is provided after the signal detection circuit 4.

In order to correct, the wavelength λ must be known. Mechanisms for this purpose include a control section 8, a resistor 9, an amplifier circuit 10, and the like.

The control unit 8 causes a current I to flow through the semiconductor light source 1 to cause it to emit light. A resistor 9 is connected in series between the control section 8 and the light source 1. Electric wires 5, 6 connect these.

The control unit 8 controls the light emission amount P of the light source 1 to be constant. This is important. For this reason, the intensity of the light source must be monitored. For example, the amount of light emitted P is known from the half mirror 13 and the light receiving element 14.

If the temperature T rises, the electric current required to produce the same amount of light emission P will automatically increase. This appears as an increase in the voltage RI between the terminals of the resistor 9. This is amplified by the amplifier circuit 10.

RI is linear with temperature within a certain range. Since λ is linear with temperature, λ can be found from RI. Depending on the value of λ,
A correction circuit 11 corrects the scale factor.

(h) Production The light emission amount P of the light source 1 is controlled to be constant.

When the temperature is low, the injection current I becomes small, and when the temperature is high, the injection current I becomes large.

Let o be the injection current at a certain reference temperature To.

In a certain range, the injection current at temperature T is ■=α
(''To) + Io (4) This holds true on the condition that the amount of light emission P is kept constant. α is a positive constant. Since it differs depending on the element,
Measure and find it in advance.

On the other hand, if the oscillation wavelength of the semiconductor light source at the reference temperature To is λ0, then at the temperature T the following relationship holds: λ = β(T-To)+λo(5). β is a positive constant. β also differs from element to element. Moreover, this formula only holds true within a certain temperature range. This is the range in which mode jumps do not occur.

The temperature T can be eliminated from (4) and (5), resulting in the following.

The current I is a current that the control unit 8 supplies to the light source 1. This can be determined by the voltage RI between the terminals of the resistor 9. The calculation of equation (6) is immediately performed by the amplifier circuit 10.

The correction circuit 11 obtains the wavelength λ obtained by (6), and simply adds λ. to the phase difference θ obtained by the signal detection circuit 4. to obtain the measured value of the desired physical quantity.

(41, (5), and (6) are simple linear relationships.

However, this does not mean that the method of the present invention cannot be applied unless such a simple relationship holds. There may be a quadratic term instead of (6).

It may be even more complex. Since the relationship between the function and λ can be determined in advance, any functional relationship may be used. The amplifier circuit 10 is a little complicated, but even if it is difficult to do so with an analog circuit, it is sufficient to make it a digital circuit and store the relationship between λ and λ in a memory.

i) Effect In an interference optical device using a semiconductor light emitting element, the wavelength λ
When , the scale factor changes.

λ is a quantity that cannot be easily measured.

In the present invention, the wavelength λ is determined by the change kζ in the injection current, and the scale factor is corrected.

Although it has a simple structure, it can accurately measure physical quantities.

[Brief explanation of drawings]

FIG. 1 shows a schematic configuration 1 of the interference optical measurement device of the present invention...
...Light source 2 ...Interference optical system 3 ... Light receiving element 4 ... Signal detection circuit 5, 6 ... Electric wire 8 ...・Control unit 9... Resistor 10... Amplifier circuit 11... Correction circuit 13... Half mirror 14... Light receiving element inventor Hiroko Nishiura Sho Hayashi

Claims (4)

[Claims] (1) Light source 1 that generates monochromatic light and light source 1 that generates 2
An interference optical system 2 which is divided into two parts, passed through different paths, and then combined again to interfere with each other so that the interference light includes a phase difference θ corresponding to the physical quantity, and an interference optical system 2 that receives the interference light emitted from the interference optical system 2. A light-receiving element 3 converts the signal into an electrical signal, a signal detection circuit 4 that calculates the phase difference θ of interference light from the output of the light-receiving element, and a light-receiving element 3 that controls the light source 1 to keep the light emission amount P of the light source 1, which is a semiconductor light-emitting element, constant. A control unit 8 that supplies the current I, an amplifier circuit 10 that amplifies a value proportional to the current I to determine the oscillation wavelength λ of the light source, and a scale factor that determines the relationship between the phase difference θ and the physical quantity to be measured using the wavelength λ. An interferometric optical measurement device with a stabilized scale factor, characterized by comprising a correction circuit 11 for correction. (2) An interference optical measuring device with a stabilized scale factor as set forth in claim (1), wherein the interference optical system 2 measures the rotational angular velocity based on the Sagnac effect. (3) Claim (1) wherein the interference optical system 2 measures the speed of the moving object by using the Doppler frequency shift of the light irradiated onto the moving object or the light irradiated from the moving object onto the road surface. Interference optical measurement device with stabilized scale factor. (4) The interference optical system 2 detects underwater acoustics by detecting a phase change that occurs due to distortion due to sound pressure applied to an optical fiber.
1) An interference optical measurement device with a stabilized scale factor as described in item 1).