JPH059772B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to an optical modulator that can be used, for example, to detect rapidly changing phenomena.

``Prior art'' For example, it is often possible to change the phase of light passing through an optical waveguide by applying an electric field to an optical waveguide formed in an optical crystal such as lithium niobate and changing the electric field. Are known.

By utilizing this phenomenon, an optical modulator can be configured, and by measuring the amount of light emitted from the optical modulator, the voltage or current can be converted into a voltage, and the current can be converted into a voltage. Alternatively, various physical quantities can be converted into voltages and then measured.

Since the response speed of the optical modulator is extremely fast compared to other voltage detection means, it has the characteristic of being able to detect high-speed phenomena that cannot be measured with conventional methods.

FIG. 3 shows the structure of a conventional optical modulator. The optical modulator is constructed by forming optical waveguides 2 and 3 on an optical crystal substrate such as lithium niobate, and forming electric field application electrodes 4A and 4B along one of the optical waveguides 2. It can be constructed by inputting a laser beam or the like, combining the lights that have passed through the two optical waveguides 2 and 3, and making the combined light enter the light receiver 6.

A signal source 7 to be measured is connected to the electric field applying electrodes 4A and 4B, and a voltage to be measured is applied from the signal source 7 to apply an electric field to the optical waveguide 2, thereby changing the phase of light passing through the optical waveguide 2. is changed by an amount corresponding to the electric field.

This phase change causes a phase difference between the light that is combined through the other optical waveguide 3, and the two lights interfere according to this phase difference, and the amount of light that enters the light receiver 6 increases. The light modulation operation is performed by changing according to the phase difference. In other words, when two lights are combined with the same phase, the amount of light I _P entering the photoreceiver 6 becomes maximum as shown at point A _P in Figure 2, and the phase difference increases.
When the angle is 180°, the amount of light I _P is 0 as shown at point A _B in Figure 2.
becomes.

Therefore, the voltage of the signal source 7 to be measured can be measured based on the amount of light detected by the light receiver 6.

``Problems to be Solved by the Invention'' In this type of optical modulator, it is clear from curve A in FIG. 2 that the modulation sensitivity is highest at the position where the phase difference between the two lights is π/2.

In order to perform the optical modulation operation centered on the position of π/2, the phases of the lights passing through the optical waveguides 2 and 3 must have a phase difference of π/2.

Conventionally, as a means for providing this phase difference, as shown in FIG. For example, as shown in FIG. 5, a dielectric material 8 or the like is deposited along the wave path, and the refractive index of this portion is changed by depositing the dielectric material 8. A method has been considered in which the phase of the light passing through is changed to give a phase difference to the light passing through the two optical waveguides 2 and 3.

In the case of the former method, since the wavelength λ of the light is extremely short, the difference in optical path length that provides the phase difference π/2 is an extremely small value.

For example, the difference in optical path length for giving a phase difference of π/2 to light with a wavelength λ=0.8 μm is 0.8×1/4=0.2 μm. The processing technology to provide a difference of 0.2 μm in optical path length has not yet been established and is difficult to realize.

Also in the latter method, in order to shift the phase of light passing through either one of the two optical waveguides 2 and 3 by π/2, the length over which the dielectric material 8 is coated must be precisely defined. This method also requires difficult processing technology and is difficult to implement.

"Means for Solving the Problem" This invention provides two optical waveguides with an optical path length difference that can be processed, and a light source that provides light to these two optical waveguides that can change the wavelength. By changing the wavelength of the light emitted from the light source, the light passing through the two optical waveguides becomes π/
The structure is such that it can be adjusted to have a phase difference of 2.

Therefore, according to the present invention, the optical path length difference between the two optical waveguides can be selected to be an appropriate length that can be processed, and the two optical waveguides can be easily manufactured.

However, the phase of the light passing through the two optical waveguides is π/2.
In order to have a phase difference of , it is easy to adjust the wavelength of the light emitted from the light source.

Therefore, according to the present invention, it is possible to provide an optical modulator that is easy to manufacture and easy to use.

"Embodiment" FIG. 1 shows an embodiment of the present invention. In the figure, 1 is a crystal substrate made of, for example, lithium niobate crystal;
2 and 3 indicate optical waveguides formed on this crystal substrate 1.

In the present invention, an extension 3A is formed in either one of the two optical waveguides 2 and 3 to provide an optical path difference of a processable length. In other words, this example shows a case where the extension 3A is formed in the optical waveguide 3 on the side where the electrodes 4A and 4B for applying the voltage to be measured are not formed.

When the light emitted from the light source 5 passes through the optical waveguide 3, the optical path length difference Δ between the optical waveguides 2 and 3 due to the formation of the extension portion 3A is several tens of wavelengths with respect to the phase of the light that has passed through the optical waveguide 2. The length may be set such that a delay amount (phase difference) of approximately several hundred wavelengths occurs.

Here, in this invention, a light source that can change the wavelength of light is used as the light source 5. It is known that the wavelength of the light of a laser light source changes when the temperature of the light emitting part or the current applied to the light emitting part is changed, and laser light sources with variable wavelengths are also commercially available.

In this invention, the wavelength of the light given to the two optical waveguides 2 and 3 is changed using the light source 5 whose wavelength can be adjusted in this manner.

As described above, the two optical waveguides 2 and 3 are given a sufficiently large optical path length difference compared to the wavelength λ of the light. When the wavelength λ of the light source 5 is changed to λ(1+δ), the amount of change d(Δφ) in the phase difference Δφ between the optical waveguides 2 and 3 is d(Δφ)=360×(Δ・n/λ−Δ・
n/λ(1+δ))≒360Δ・n/λ・δ……(1). However, δ≪1, and n is the optical waveguide 2 and 3.
It is the refractive index within. As a concrete value, λ=
If we choose 0.83 μm, n=2.25, and Δ=75 μm and shift the wavelength to λ′=0.831 μm (δ=1.2×10 ⁻³ ), then d(Δφ)≈990°.

Adjustment so that the phase difference Δφ becomes π/2 is performed as follows.

Light having a suitable wavelength λ is applied from a light source 5 to two optical waveguides 2 and 3, and the combined light is applied to a light receiver 6. The wavelength λ of the light from the light source 5 is changed while the light receiver 6 receives the multiplexed light.

As the wavelength λ changes, the phase difference between the lights passing through the two optical waveguides 2 and 3 changes according to the first equation.

Therefore, the electrical output to the light receiver 6 changes according to the curve A shown in FIG. 2, and the peak point _AP and minimum point _AB of the electrical output are detected. That is, the peak point A _P indicates an in-phase state, and the minimum point A _B indicates a state where the phase difference is π.

Therefore, by setting the wavelength at which the electrical output of the photoreceiver 6 is halfway between the peak point A _P and the minimum point A _B , that is, 1/2 of the value of the electrical output I _P at the peak point A _P , This means that the phase difference Δφ between the lights passing through the two optical waveguides 2 and 3 is set to π/2.

By applying the voltage signal B to be measured (curve B in Figure 2) to the electrodes 4A and 4B at a position where the phase difference Δφ is π/2, the voltage signal B to be measured changes the phase of the light passing through the optical waveguide 2. let Due to the change in phase, the phase difference Δφ changes, and the amount of light incident on the light receiver 6 changes in accordance with the slope of curve A.
An electrical output signal shown as is obtained.

"Effects of the Invention" As described above, according to the present invention, the two optical waveguides 2 and 3 have an optical path that is sufficiently long compared to the wavelength λ of the light to be passed therethrough, for example, several tens to several hundred times the wavelength λ. In addition to providing a length difference, the two optical waveguides 2 and 3 are configured to have a configuration in which the wavelength of the light from the light source 5 that provides light is changed, and by adjusting the wavelength of the light emitted from the light source 5, the two optical waveguides can be separated. Since the configuration is such that the phase difference Δφ between the lights passing through wave paths 2 and 3 can be adjusted freely, the center of the optical modulation operation can be easily set at the position of the phase difference of π/2 where the modulation sensitivity is highest. I can do it.

Therefore, according to the invention, two optical waveguides 2 and 3
The difference in optical path length given to the wavelength λ of light is several tens to several hundreds
The optical waveguides 2 and 3 can be easily manufactured since the number can be selected to be about twice as large.

However, since the phase difference Δφ is not uniquely determined by the optical path difference between the two optical waveguides 2 and 3, the accuracy of the optical path difference does not require high precision. Therefore, manufacturing is easy in this respect as well.

Furthermore, since the operating point of optical modulation can always be operated at a position of π/2, it can always be operated at a position with high modulation sensitivity, and the voltage to be measured can be detected with high sensitivity.

Furthermore, since the modulation operation can be performed at a position with a phase difference of π/2, the modulated output signal matches the waveform of the signal to be measured, making it possible to obtain an optical modulator with good linearity.

[Brief explanation of the drawing]

FIG. 1 is a block diagram for explaining one embodiment of the present invention, FIG. 2 is a graph for explaining the operation of the present invention, FIG. 3 is a block diagram for explaining the conventional technique, and FIG. 5 and 5 are plan views for explaining the conventional technology. 1... Optical crystal substrate, 2, 3... Optical waveguide, 4
A, 4B...electrode, 5...light source, 6...light receiver,
7...Measurement signal source.

Claims (1)

[Claims] 1 A. Two optical waveguides are provided that split the light emitted from the light source into two, combine the two-branched light again, and input the light into the receiver, and either one of the optical waveguides An electrode is provided along the optical waveguide, and a modulation signal is applied to this electrode. The modulation signal changes the phase of the light passing through one of the optical waveguides, and the amount of combined light is determined by the change in phase. In the optical modulator configured to change the wavelength of the light, B: giving the two optical waveguides an optical path difference that is sufficiently longer than the wavelength of the light passing through them, and using a light source that can change the wavelength of the light source; An optical modulator characterized in that the operating point of the optical modulator can be set by adjusting the phase difference of the light passing through the two optical waveguides.