JPH0324649B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical modulation method for modulating the polarization state of a light wave using the electro-optic effect of a crystal.

Optical information processing systems such as optical communication systems, optical memory devices, and laser recording devices require optical modulators that electrically modulate the amplitude of light wave beams generated from lasers and light emitting diodes at high speed. Also,
Optical communication systems require optical switches that can switch optical paths at high speed. Conventionally, optical modulators that use the acousto-optic effect, magneto-optic effect, or electro-optic effect have been used as optical modulators, but the optical modulator that uses the electro-optic effect of crystals is the fastest. Optical switches include methods that change the optical path by mechanically moving optical parts such as mirrors, methods that change the optical path by diffracting the light using the acousto-optic effect, and methods that use an optical modulator that uses the electro-optic effect to change the optical path. There is a method of converting the polarization state and switching the optical path using a polarization separator using a birefringent material. Among these, an optical switch using an optical modulator and a polarization separator using the electro-optic effect of a crystal is the fastest.

Optical modulators using the electro-optic effect of crystals include (1) an optical modulator that converts the polarization state of light by electrically controlling the amount of phase delay between the polarization components in the two main axis directions of the electro-optic crystal; (2) There are optical modulators that convert incident linearly polarized light into orthogonal polarized light components by applying a periodic electric field in the light transmission direction using the electro-optic effect that causes rotation of the refractive index ellipsoid. . In addition,
Both of the optical modulators (1) and (2) use a polarizer when modulating the amplitude of incident light. Among the above optical modulators, the modulator (1) is usually made using a crystal with a relatively large electro-optic constant, such as an ADP crystal, a KDP crystal, a Bi ₁₂ SiO ₂₀ crystal, or a lithium tantalate (LiTaO ₃ ) crystal. It will be done. However, ADP crystals
KDP crystals are difficult to handle due to their deliquescent nature. Furthermore, when ADP crystal, KDP crystal, or Bi ₁₂ SiO ₂₀ crystal is used, there is a drawback that high voltage is required. When using LiTaO ₃ crystal, the applied voltage may be low, but the drawback is that a high extinction ratio cannot be obtained because the polarization state changes when the ambient temperature changes even without applying a voltage. this
In the optical modulator (1) using LiTaO ₃ crystals, a method is known in which temperature compensation is achieved by using two crystals in series, but this requires crystal homogeneity and high manufacturing precision. Not practical.

On the other hand, the optical modulator in (2) can be constructed using LiTaO ₃ crystal, etc., so it operates at a relatively low voltage, and the polarization state is always constant regardless of temperature when no voltage is applied. Therefore, it has the advantage of being able to obtain a relatively high extinction ratio. Therefore, the optical modulator (2) is excellent not only as a simple optical modulator but also as an optical modulator for configuring an optical switch as described above. Furthermore, the optical modulator (2) has the advantage that the operating wavelength range can be set arbitrarily by selecting the structure of the electrodes installed. Therefore, for example, by increasing the length of the electrode in the light transmission direction and making the electrode period constant over the entire length, it is possible to obtain light modulation that also has a wavelength filter effect that modulates only a very narrow range of wavelengths. When the above-mentioned optical modulator is combined with a polarization splitter, an optical switch that switches only a specific wavelength can be obtained, which can be used in a wavelength division multiplexing communication system that handles light of a plurality of wavelengths. Furthermore, in order to further narrow the operating wavelength range, the electrodes may be made longer to increase the number of electrode periods. For this purpose, a compact optical modulator can be obtained by installing an optical waveguide on a lithium niobate (LiNbO ₃ ) crystal and placing an electrode with a minute period on top of the optical waveguide. As described above, the optical modulator (2) also has unique functions not found in other optical modulators. As mentioned above, when using an optical modulator or optical switch in an optical communication system or optical information processing system, in addition to being able to drive at high speed, there is also a requirement that a certain optical output state or a certain optical switching state can be maintained constant. may be done. For example, in an optical modulator used in a laser photographic recording device, it is necessary to keep the optical output constant for a long time when recording a white, black, or constant density screen for adjustment and setting of a reference level. Further, in an optical communication system, when an optical path switched by an optical switch is kept constant for a long time, the output of the optical modulator needs to be constant.
As mentioned above, in order to maintain the output of an optical modulator at a certain level, conventionally a DC voltage was applied to the optical modulator, but at this time a phenomenon called DC electric field drift occurs, and the internal electric field of the crystal gradually changes. Because it ends up
It was not possible to maintain a certain light output level for more than about 0.5 seconds. Therefore, in order to solve this problem, a method of simply reversing the polarity of the applied voltage periodically, that is, a method of maintaining a steady state by applying an AC rectangular voltage, can be considered, but when reversing the polarity, the point where the voltage is 0 A problem arises in that the amount of light instantly drops to 0 as the light passes through the light. As mentioned above, the optical modulator (2) has excellent properties, but in the past, its applications were greatly limited due to the above-mentioned drift state.

The present invention provides an optical modulation method using an optical modulator as described in (2) above, which can maintain a steady light output state for a long time without any change in the amount of light. A plurality of electrodes having a period equal to the light transmission direction are installed in series in the light transmission direction on a crystal that has an electro-optic effect that causes rotation of the main axis, such as LiTaO ₃ crystal or LiNbO ₃ crystal. This is characterized by applying alternating current voltages or rectangular voltages having different phases to each other.

Embodiments of the present invention will be described with reference to the drawings.

FIG. 1 shows an embodiment of the optical modulation system according to the present invention. FIG. 1a shows an example of an optical modulator used in the present invention, and FIGS. 1b and 1c show examples of applied voltages. In FIG. 1a, on the X-plate LiTaO ₃ crystal 1 formed to transmit light in the Y-axis direction, comb-shaped electrodes 2 and 3 having a period Λ in the y-direction and installed in series in the Y-direction, and The interdigitated electrodes 4 constitute an interdigital electrode. The comb-shaped electrodes 2 and 3 are connected to signal generators 5 and 6, respectively, and the comb-shaped electrodes 4 and 3 are connected to signal generators 5 and 6, respectively.
The electrode deposited on the lower surface of the LiTaO ₃ crystal 1 is grounded. In this example, the thickness of crystal 1 is 0.05
The period Λ is approximately 0.3 μm, and the period Λ is sufficiently larger than the thickness of the crystal 1. When a voltage is applied between the comb-shaped electrode 4 and the comb-shaped electrode 2 or 3 in the optical modulator shown in FIG. The X-direction polarization component of the incident light 7 is converted into a Z-direction polarization component by γ ₅₁ and becomes the output light 9. Here, the refractive index of LiTaO ₃ crystal for ordinary light and extraordinary light is n _p・
Letting n _e and the optical wavelength be λ, the period Λ is selected so as to satisfy equation (1).

2π/Λ (2π/λ) ( _ne −n _p ) (1) The conversion efficiency between the X and Z polarization components is determined by the magnitude of the applied voltage, and can be increased to 100% by adjusting the applied voltage. A conversion efficiency close to . When modulating the intensity of incident light with the above optical modulator, a polarizer is inserted on the output side or on both the input and output sides, and when configuring an optical switch, the optical path is separated depending on the polarization direction. Insert a polarization separator, for example a birefringent material, for this purpose. By the way, in the conventional optical modulator, the comb-shaped electrodes 2 and 3 are connected, and when keeping the optical output level constant, the comb-shaped electrodes 2 and 3 are connected.
3, since the constant voltage was applied, the emitted light changed due to the DC electric field drift phenomenon as described above, and it was not possible to keep it constant for a long time. However, in the embodiment of the present invention, , a rectangular voltage shown in FIG. 1b is applied to the comb-shaped electrode 2, and a rectangular voltage shown in FIG. It is something. Therefore, in this embodiment, voltage is applied alternately to the comb-shaped electrodes 2 and 3 as described above, and by making the lengths of the comb-shaped electrodes 2 and 3 in the Y direction equal, voltage is applied to only one of the comb-shaped electrodes 2 and 3. For example, when the first
In Figures b and c, the conversion efficiency at time t ₁ or t ₃ can be made equal. Also, at time t ₂ intermediate between them, the comb-shaped electrodes 2 and 3 each have half (V/
2) voltage is applied, but since the conversion efficiency of the optical modulator is determined by the product of the applied voltage and the length of the part to which the voltage is applied in the Y direction, the conversion efficiency at time t ₂ is equal to the conversion efficiency at time t ₁ , Equal when t ₃ . Therefore, in this example, the first
By applying voltages as shown in Figures b and c, a constant optical output can be obtained at all times.

The operating wavelength λ of the optical modulator shown in FIG. 1a is determined by equation (1), and the wavelength range becomes narrower as the number of electrode fingers increases. Therefore, when trying to obtain an optical modulator with a narrow operating wavelength width like a wavelength filter using crystals of the same length, birefringence (n _e − n _p )
Λ can be made small by using a crystal with a large value of . LiNbO ₃ crystal has birefringence 20 times or more than LiTaO ₃ crystal, so it is suitable for the above purpose.
FIG. 2 shows another embodiment of the optical modulator used in the present invention, in which an X-plate LiNbO ₃ crystal 11 is used for the above purpose. On the LiNbO ₃ crystal 11, comb-shaped electrodes 12 and 13 are placed as in FIG. However, the period Λ ₁ of the comb-shaped electrodes 12, 13, 14
is less than 1/20 of the period Λ of the optical modulator shown in FIG. 1a. In addition, as mentioned above, an electric field is generated only near the surface by electrodes with a minute period, so in order to effectively apply an electric field to the light wave, the incident light is applied to the surface of the LiNbO ₃ crystal 1 by Ti diffusion method etc. The light is confined in the formed optical waveguide 15 and propagates. The operation of the optical modulator of this embodiment is almost the same as that shown in FIG. 1a. In this embodiment, the comb-shaped electrodes 12 and 13
Rectangular voltages having phases different by π from each other as shown in FIG. 1b and c are applied to each of them, and the optical output is kept constant.

FIG. 3 shows yet another embodiment of an optical modulator that can be used in the present invention. In this example, a LiNbO ₃ crystal is used as in FIG. 2, but in this example, a Z-plate LiNbO ₃ crystal 21 is used, and a comb-shaped crystal is used to generate an electric field in a direction parallel to the upper surface (Y direction). Electrodes 22 and 23 and a comb-shaped electrode 24 are arranged to face each other. Further, an optical waveguide 25 is installed as in FIG. 2. The operating principle of this embodiment is exactly the same as that of FIGS. 1 and 2, and by applying the electrodes b and c of FIG. A steady light output state can be maintained.

As described above, according to the present invention, it is possible to obtain a light modulation method that can maintain a steady light output state for a long time without any change in the amount of light.

Note that the form of the optical modulator and the applied voltage waveform used in the present invention are not limited to the above-mentioned embodiments. For example, in the optical modulator shown in FIG. 1a, an electrode pattern similar to that on the upper surface may be formed on the lower surface. Further, the number of electrodes installed in the light transmission direction is not limited to two as in the above embodiment. Three electrodes may be arranged in succession and rectangular voltages having different phases of 2π/3 may be applied to each other. The applied voltage waveforms are sin ² ωt and
It may be a voltage proportional to cos ² ωt. Here, ω is frequency and t is time. The applied voltage waveform may be an alternating current voltage or a rectangular voltage having a period of 0.5 seconds or less without causing a direct current drift phenomenon. Note that as the material of the optical modulator used, barium titanate crystal, barium/sodium niobate crystal, strontium/barium niobate crystal, etc. can be used.

[Brief explanation of the drawing]

FIG. 1a is a perspective view showing an embodiment of the optical modulator used in the present invention, FIGS. 1b and c are diagrams showing an example of the applied voltage waveform according to the present invention, and FIGS. 2 and 3 are respectively in accordance with the present invention. FIG. 3 is a perspective view showing another embodiment of the optical modulator used in the present invention. In the figure, 1, 11, 21 are crystals, 2, 3,
4, 12, 13, 14, 22, 23, and 24 are comb-shaped electrodes, 5 and 6 are signal generators, and 15 and 25 are optical waveguides.

Claims (1)

[Claims] 1. On a crystal that has an electro-optical effect that causes rotation of the principal axis of the refractive index ellipsoid of the crystal, a plurality of electrodes having a period equal to the light transmission direction are installed in series in the light transmission direction, and each electrode is out of phase with each other
An optical modulation method characterized by applying periodic voltages of the same polarity with the same period differing by 180 degrees.