JPS5845006B2 - light modulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical modulator that temporally modulates the phase and amplitude of a light beam, and particularly to an optical modulator that uses the electro-optic effect of a dielectric crystal.

Optical modulators that modulate light beams at high speed according to electrical signals are essential elements for systems that transmit and process high-speed, large-capacity information, such as optical communications and optical information processing.

Electro-optic light modulators that use the Pockels effect, in which the refractive index of a crystal changes in proportion to the applied electric field, are more effective than light modulators that use other effects, such as mechanical vibration or acousto-optic effects. Because they have a wide bandwidth, they can modulate light at much faster speeds.

Therefore, it is becoming widely used for the above purpose.

The principle of conventional optical modulators using electro-optic effects is widely understood as described below.

For example, ferroelectric crystal LiT, which is currently very commonly used.
In the aO3 crystal, an electric field is applied in the Z-axis direction, which is the direction in which the maximum electro-optic effect is obtained, and a light beam is transmitted from a direction perpendicular to the Z-axis direction, for example, the X-axis direction.

Depending on the strength Fz of the applied electric field, the refractive index in the y-axis direction and the Z-axis direction for the transmitted light wave is n = n - n
3r13E2/2, n2=n e-n c3r33E
It becomes z/2.

here. . no is the refractive index for ordinary rays and extraordinary rays, and r13+r33 are electro-optic constants.

The polarization plane of the transmitted light beam is linearly polarized light inclined at 45° from the y-axis and the Z-axis.

The phase difference φ that occurs between the photoelectric red components in the y-axis and Z-axis directions at the exit surface of the crystal is φ-(2π/in)・l・(n2'-n
,) becomes.

Here, λ is the wavelength of light, and l is the length of the crystal in the light propagation direction.

If an electric field is applied so that the phase difference φ becomes π, the polarization plane of the emitted light becomes linearly polarized light orthogonal to that of the incident light.

By passing the emitted light through the analyzer, the intensity of the light passing through the analyzer changes depending on the strength of the applied electric field.
Amplitude modulation of the light is achieved.

The phase difference before and after the electric field is applied is π The applied voltage ■π is ■π = person (dll)/(no3r33 no3r13)
is given by

Here, d is the distance between the electrodes.

■π when dll=1 is the half-wavelength electric field distance product (E-
It is called CI input/Z and is one of the indicators representing the electro-optical performance of a modulation medium.

L IT a 03 crystal is an excellent electro-optic crystal,
Currently, it is widely used as a material for optical modulators based on the above principle.

However, conventional optical modulation has several drawbacks.

For example, the phase difference φ at the front end includes a period (2π/in) l (n e no ) which is independent of the strength of the applied electric field and is due to so-called natural birefringence.

Therefore, the modulation operation becomes unstable due to changes in ambient temperature.

In order to compensate for the temperature dependence of this natural birefringence term, two crystals of equal length are inserted, as is well known, into two modulation crystals of equal length and an inlet/half plate inserted between them. Attempts have been made to make the electric field application axes, for example, the C-axes orthogonal to each other in the above example.

These methods can compensate for the above effects if the two crystals are ideally arranged geometrically; If there is any imperfection, complete compensation cannot be made, which increases the complexity of processing and assembling the crystal during actual fabrication.

Furthermore, if there is a portion with the above-mentioned non-uniform natural birefringence inside the crystal used, the degree of extinction will deteriorate.

For this reason, c) it may be inconvenient for use in things like optical shacks that require a high n10ff load rating.

An object of the present invention is to solve the above-mentioned difficulties and provide a high-performance and stable electro-optic light modulator.

According to the invention, P bo HxNb 205 (],
, 5 ≤ A highly stable light modulation element can be obtained.

The above-mentioned difficulty with conventional optical modulators, namely the influence of natural birefringence, is that the combined vibration locus of two independent photoelectric components vibrating in the crystal axis direction is changed by an external electric field. The country of birth can be traced to the use of the word.

On the other hand, by adopting the configuration of the present invention, the above-mentioned influence can be avoided.

The principle of the present invention is as described below.

In other words, by applying an external electric field between the photoelectric different components that vibrate in the direction of the principal axis of the crystal, which propagate independently and orthogonally to each other in the absence of an electric field, we can create electro-optic constant tensor components that cause coupling. By using a crystal as a light modulation medium and applying an electric field by providing electrodes in appropriate directions, incident linearly polarized light having an oscillating electric field component only in the direction of the main axis of the crystal will change to a straight line perpendicular to it as it passes through the crystal. It is converted into polarized light.

Light intensity modulation is achieved by transmitting the emitted light through an analyzer.

This process does not utilize the phase difference between the two orthogonal photoelectric different components as described above, but utilizes a change in amplitude, so there is no effect of the above-mentioned natural birefringence.

In order for all of the energy of the above incident light to be converted into components perpendicular to the vibration direction, it is necessary for the two components to have the same propagation constant, that is, the refractive index, or to have a wave number vector equal to the difference in propagation constant. It is necessary that there be a constant periodicity (or periodicity of the applied electric field).

In many known electro-optic crystals, the propagation constants of two orthogonal photoelectric different components, that is, the refractive indexes in the principal axis direction of the refractive index ellipsoid are different.

Therefore, in order to construct an optical modulator based on the above principle, electrodes for applying a periodic electric field must be provided.

However, in a normal crystal, this difference in refractive index is large, so the periodic spacing of the electrodes must be made fine, and this makes it difficult to apply a uniform electric field to the crystal.

Technology for confining optical energy near the surface of a dielectric crystal substrate As has become clear in the field of so-called optical integrated circuit technology, the propagation constants of many light propagation modes propagating near the surface of a substrate are very close to each other. Therefore, if the substrate has an electro-optic effect, the above operation can be achieved with electrodes with relatively coarse periodic intervals, but as is well known, the light beam emitted by a light source such as a laser is Coupling the guided mode into the nearby propagating mode and extracting it back into the air as a beam involves considerable technical difficulty, and it is difficult to provide a simple optical modulator that is easy to adjust. Can not.

The feature of the present invention is a new ferroelectric crystal PbO-xNb205 (1,
5≦X≦2.1 (x is the molar ratio) as an electro-optic modulation medium to realize a light modulation element having the above-mentioned configuration.

Regarding the method of growing this crystal, please refer to the patent application No. 49-3017.
Please refer to the detailed explanation in No. 9.

Further, its growth and crystallographic studies are described in Journal of Crystal Growth, Volumes 24/25 (197
4), p. 445, and the same journal, Vol. 26 (1974), p. 319.

The composition range stated in this academic journal is 1.5≦X≦
At room temperature, the crystals in 3.1 have two groups with different crystal lattice constants, that is, a composition range of 1.5□x <
It is divided into the groups 2.1 and 2.1≦X≦3.1.

The crystals included in each group are so-called solid solutions that have similar lattice constants even if their compositions differ within the group.

Although any of the crystals belonging to the above two groups exhibits excellent electro-optic properties, the electro-optic light modulation of the configuration of the present invention requires a crystal of 1.5≦x<2.1 due to its optical properties. Only crystals belonging to the group are valid.

In one embodiment of the present invention, x=1.9, that is, Pb0.1.
9Nb20. The case where this is used will be explained with reference to drawings.

In FIG. 1, 1 is the crystal axis a. Pb0.1.9Nb20. shaped into a parallelepiped along the b and c axes and polished. It is a crystal.

An electric field is applied in the a-axis direction on the two opposing three faces of the crystal, and at regular intervals in the a-axis direction in the traveling direction of the light beam 2, so that the direction of the electric field changes in the light propagation direction. Then, vertically opposing electrodes I are provided, and the output of the modulation signal generator 6 is alternately connected to adjacent electrodes as shown in FIG.

The crystallographic symmetry of the P bo HxNb205 (1,5≦x<2.1) crystal is C2V, and its electro-optical constant tensor is.

When the applied electric field is zero, linearly polarized incident light 2 whose polarization plane vibrates in the C-axis direction exits the crystal while maintaining its polarization plane.

Since the analyzer 5 is arranged so as to transmit only the photoelectrically different component vibrating in the a-axis direction, the intensity of the modulated light 4 becomes zero at this time.

When an electric field is applied, as the incident light 2 propagates through the crystal,
The energy of the photoelectric component only in the C-axis direction is converted into a component that vibrates in the a-axis direction.

r51 in the electro-optic constant matrix mentioned above is involved in this.

The magnitude of r51 governs the magnitude of the coupling between the two optical electric fields.

r5. of Pb0.1.9Nb205 crystal. = 45
10−” m/v, and the maximum electro-optical constant of L t T a 03 crystal r33 = 30 × 10−12 m
Greater than /v.

Even if the constant is large as described above, the coupling between the photoelectroclosing components will not be 10% no matter how long the action length is.

This is because the velocities in the corresponding crystals, that is, the propagation constants are not the same.

In order to eliminate this phase mismatch, a lattice equal to the propagation constant mismatch is created.

A grating with a period J~ may be formed in the light propagation direction.

This grating may change the refractive index with respect to light.

That is, this is possible by dividing the electrodes to which the electric field is applied, as shown in FIG. 1, at intervals of the spaced holes, and by reversing the direction of electric field application for each adjacent electrode.

Another method may be to divide the crystals and arrange them in the direction of propagation of light.

However, in this case, there is a reflection loss of light at each crystal end face, so it is not a good idea.

An optical modulator having the configuration shown in FIG.
About 3 nm conventional electro-optic crystals such as LiN
b When configured with Oa, the electro-optic constant involved in such an effect is r51-r42" 28 X 10-
Since it is not very high at 12rn/V and the principal refractive index difference nene involved is large at 8X10-2, the lattice spacing J~ determined from the above equation is as short as 6 μm, and the electric field applied evenly within the crystal cross section is not distributed.

FIG. 2 shows the dispersion characteristics of the principal refractive index of PbO.1.9 Nb2O5 with respect to the optical wavelength.

All crystals belonging to PbO.x N b 205 (1,5≦x<2.1) have the same dispersion time as shown in FIG.

That is, it becomes na-no at a specific wavelength, indicating a uniaxial crystal, and at other wavelengths, it is a biaxial crystal.

At wavelengths near this specific wavelength, na and n.

This specific wavelength changes depending on the value of the composition ratio X of the crystal.

The principal refractive indices involved in the operation of the optical modulator in the embodiment of FIG. 1 are na and n.

It is. For a light wavelength of 633 nm, this difference is 8X1
0-'.

From this, the periodic hole of the electrode is 0.8 rnm. Therefore, even if the thickness of the crystal in the a-axis direction is, for example, 0.3 TLm, the applied electrolyte is uniformly distributed in the thickness direction and contributes to modulation.

The crystal length in the b-axis direction, that is, the light propagation direction, is ll = 20my.
If n, the applied voltage is as low as about 15V, which is sufficiently practical.

Of course, a specific wavelength, for example Pb0.1.9 N b 205
For a crystal with a composition of 600 nm, there is no need for the electrode to have a periodic structure.

In addition, for different wavelengths to be used, the above-mentioned periodic interval of the electrodes may be changed, or a crystal having a different composition ratio and having a different specific wavelength, which is a -axial crystal, may be used.

The reason for limiting the composition ratio electro-optic constant r
This is because 61 or r6□ does not exist.

This is also because when x<1.5, high quality crystals cannot be easily obtained using the crystal growth method exemplified above.

As described above, according to the present invention, a high-performance and stable optical modulator can be obtained.

[Brief explanation of drawings]

FIG. 1 is a principle block diagram of an embodiment of the present invention, in which 1 is a modulation medium, 2 is an incident light beam, 3 is an output light beam, 5 is an analyzer, 6 is a modulation signal generator, and 7 is for applying an electric field. It is an electrode. Figure 2 shows pbo・1.9 Nb used in this example.
FIG. 2 is a diagram showing the dispersion characteristics of refractive index with respect to optical wavelength of 05 crystal.

Claims (1)

[Claims] 1 PbO-XNb2O, (1,5
≦X≦2.1, X is the molar ratio) using a single crystal,
An optical modulator that transmits a light beam in the b-axis direction of the crystal, and has electrodes arranged in a southern pattern along the light beam transmission direction on two opposing surfaces perpendicular to the a-axis. .