JPH0756199A - Polarization-independent waveguide type optical switch - Google Patents

Abstract

PURPOSE:To provide a polarization-independent type waveguide type optical switch which does not depend on a polarization state of input light among waveguide type optical switches utilizing an electro-optical effect. CONSTITUTION:This polarization-independent type waveguide type optical switch using the electro-optical effect is composed of an LiNbO3 substrate 1, optical waveguides 2 formed in the LiNbO3 substrate and electrodes 3, 4 formed on the LiNbO3 substrate 1. Half-wave plates 6 of a thin-film type are inserted into at least one point of the optical waveguides.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical switch for optical communication and optical information processing, and more specifically, it does not depend on the polarization state of input light in a waveguide type optical switch utilizing the electro-optical effect. The present invention relates to a polarization independent waveguide type optical switch.

[0002]

2. Description of the Related Art Optical switches are very important optical parts for optical communication or information processing, and are being actively researched and developed. As its type, a mechanical switch that mechanically moves a prism or an optical fiber, a switch using a thermo-optical effect used in a silica glass waveguide, or the like,
There is an electro-optical switch used in a waveguide having an electro-optical effect such as a Ti-diffused LiNbO ₃ waveguide. The mechanical switch and the thermo-optical switch among them have no polarization dependence, but have a problem that the response speed thereof is as slow as about 1 msec or more.

On the other hand, a switch using the electro-optical effect has a characteristic that the response speed is extremely fast. However, since the change in the refractive index of the electro-optic effect differs depending on the polarization direction of light, when an electrode is provided on the arm with a Mach-Zehnder interferometer type switch, its operation depends on the polarization state of the input light. There was a problem that it would end up.

In the waveguide type switch using the electro-optic effect, the polarization dependence is the greatest problem, and many researchers have been trying to improve it. As a result, polarization-independent switches have now been realized by the type of switch in which the directional coupler is loaded with electrodes (RCAlferness "Pol
arization-independent optical directional coupler
switch using weighted coupling "Appl.Phys.Lett.35
(10), 15,1979 or M.Kondo et al. "LOW-DRIVE-VOLTA
GE AND LOW-LOSS POLARIZATION-INDEPENDENT LiNbO ₃ OP
TICAL WAVEGUIDE SWITCHES "Electron.Lett.Vol.23, No.
21, 1987).

FIG. 13 shows the configuration of the switch described in the latter document. In FIG. 13, reference numeral 1 is LiN
bO ₃ plate, 2 a Ti diffused LiNbO ₃ optical waveguide, 3 a switch positive electrode side electrode, and 4 a switch negative electrode side electrode. Here, the switch shown in FIG. 13 has an electrode length of 1
To switch light with a wavelength of 1.3 μm at 9 mm, 1
It requires a voltage of 8V. However, this switch voltage imposes a heavy burden on the driving power source in order to switch lines at high speed.

[0006]

As described above, the conventional switch shown in FIG. 13 requires a switch voltage as high as 18 V in order to realize a polarization-independent electro-optical switch. This switch voltage imposes a heavy burden on the driving power supply when switching at high speed. In view of the above problems, it is an object of the present invention to provide a polarization independent waveguide type optical switch that can be driven at a low voltage using the electro-optic effect.

[0007]

The structure of a polarization-independent waveguide type optical switch according to the present invention which achieves the above-mentioned object is Li
And NbO ₃ substrate, and the optical waveguide fabricated in the LiNbO ₃ substrate, in polarization-independent waveguide-type optical switch using an electro-optical effect consisting of an electrode formed on the LiNbO ₃ substrate, A thin film type wave plate is inserted into at least one location of the optical waveguide.

[0008]

According to the structure of the present invention, the polarization filter is provided on the way, so that the polarization is corrected and the switch voltage becomes about half as compared with the conventional structure. The reason is as follows. As for the electro-optic effect in the Z-cut LiNbO ₃ crystal, TM mode has about three times as high efficiency as TE mode. Therefore, considering only TMmode as shown in FIG. 5, the switch voltage is about 6V. However
TEmode requires a high voltage for the switch and requires 18V. Therefore, the voltage required to switch both polarizations is 18V. On the other hand, a half-wave plate is inserted in the center of the electrode as a polarization filter and the first half is TM
(TE), and after performing mode conversion on the wave plate, TE
(TM) voltage VTE + TM required for the switch when propagating
Is given by the formula shown below.

[0009]

[Equation 1] Modulation efficiency ηTM = π / (L · VTM) Modulation efficiency ηTE = π / (L · VTE) VTE + TM = 2 · π / (L · (ηTE + ηTM)) = 2 / (1
/ VTE + 1 / VTM) where L: electrode length ηTM, ηTE: modulation efficiency of TM light and TE light VTM, VTE: voltage required to switch TM light and TE light with the configuration of the prior art VTM + TM: configuration of the present invention Switch voltage required at

When VTM = 6V and VTE = 18V are inserted in the above equation, VTM + TM = 9V. This value is in agreement with the experimental result. Therefore, with the configuration according to the present invention, the waveguide type polarization independent switch requires only about half the switch voltage as compared with the conventional configuration.

As described above, the waveguide-type polarization-independent optical switch of the present invention requires only about half the switch voltage of the conventional switch, and as a result, the load on the switch drive power supply can be reduced, and further, A faster switch can be realized.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the polarization independent waveguide type optical switch according to the present invention will be described below.

(Embodiment 1) FIG. 1 shows a waveguide type polarization independent optical switch using the electro-optic effect according to a first embodiment of the present invention. 2 is an enlarged sectional view taken along the line AA 'in FIG. 1, and FIG. 3 is an enlarged sectional view taken along the line BB' in FIG. Here, in FIGS. 1 to 3, reference numeral 1 is LiN.
bO ₃ substrate, reference numeral 2 is Ti-diffused LiNbO ₃ optical waveguide,
Reference numeral 3 is a positive electrode for switching, reference numeral 4 is a negative electrode for switching, reference numeral 5 is a thin film wave plate insertion groove, and reference numeral 6 is a thin film 1.
/ 2 wave plate, reference numeral 7 respectively shows wire bonding for electrode connection.

A method of manufacturing the switch will be briefly described below. A Ti film of 1000 liters was formed on the optically polished LiNbO ₃ substrate 1 by the lift-off method and the electron beam evaporation method. This was diffused in a humidified oxygen atmosphere at 1000 ° C. for 7 hours to prepare a Ti-diffused LiNbO ₃ optical waveguide 2. SiO ₂ is deposited as a buffer layer on this by CVD
Then, a gold electrode (positive electrode side electrode 3 for switch, negative electrode side electrode 4) was formed thereon. The electrode length is 20mm,
The electrode interval was 10 μm. After that, a thin film wave plate insertion groove 5 for inserting the wave plate is formed on the electrode 4 with a dicing saw.
The thin film ½ wavelength plate 6 manufactured by extending the polymer film at the center of the groove is processed so as to be perpendicular to the waveguide, and the groove is formed so that its optical axis is 45 ° with respect to the substrate surface. Inserted in 5. After the half-wave plate 6 was inserted, it was fixed with an ultraviolet curable resin. In addition, the groove was tilted about 5 ° from the vertical with respect to the substrate in order to suppress reflected return light. Moreover, after fixing the half-wave plate 6, the electrodes cut by the electrode connecting wire bonding 7 were connected.

FIG. 4 shows the switching characteristics when using 1.3 μm light. Here, the horizontal axis represents the voltage applied to the electrodes, and the vertical axis represents the cross port optical output. It can be seen that the switch characteristics are completely the same for the two polarized waves TE and TM. The switch voltage at this time was about 9V.

For comparison, the switch characteristics when the wave plate is removed are shown in FIG. As shown in the figure, the configuration without the wave plate is the same as that shown in the prior art, and the switch is in the bar state (the state in which the cross port optical output is lost) with two polarizations of TM and TE. This occurred when the voltage applied to the electrodes was 18V. From this result, it can be seen that the switch voltage is halved by inserting the wave plate in the center of the electrode.

(Second Embodiment) FIG. 6 shows a waveguide type polarization independent switch according to a second embodiment of the present invention. In contrast to the switch shown in the first embodiment that is of the directional coupler type, the switch of this embodiment has a 3 dB directional coupler.
The Mach-Zehnder interferometer type used. The basic principle that the switch voltage is small is the same as that of the first embodiment.

FIG. 7 shows the switch characteristics, and FIG. 8 shows the switch characteristics when the wave plate is removed for comparison. Figure 7
8 and FIG. 8 are compared, while FIG. 7 operates at 11 V, in FIG. 8 the switch plate has a high switch voltage of 22 V and the wave plate is inserted when performing the switch operation independent of the polarization state. In this case, it can be seen that the switch voltage is about half that in the case without the wave plate. In the configuration using the Mach-Zehnder interferometer, since the electrode length and the directional coupler can be designed separately, it is possible to lengthen the electrode and reduce the switch voltage.

(Embodiment 3) FIG. 9 shows a waveguide type polarization independent switch according to a third embodiment of the present invention. This is a structure for preventing polarization dependence in the switch characteristics in a high frequency region where microwave attenuation is a problem when performing a high speed switch using a traveling wave type electrode. That is, in the method in which the wave plate as the mode converter is inserted only in the center of the electrode, the light in the TM mode in the first half where the microwave is strong has good switching characteristics up to a relatively high frequency, whereas the microwave voltage is Light that becomes a TM mode with good modulation efficiency in the latter half after being considerably attenuated (at the stage of entering the switch
In TE mode light, the attenuation of microwaves directly causes the deterioration of the switch characteristics. That is, polarization dependence occurs in the switch characteristics in the high frequency region where microwave attenuation is a problem.

In the present embodiment, in order to eliminate this, the half-wave plate 6 as a mode converter is inserted at two places. At this time, the lengths of the three electrodes separated are L1, L
2 and L3. The respective electrodes are connected by wire bonding 7. When the switch speed is low, L1 + L3 = L2 is set in order to perform polarization independent switch operation. The attenuation of the microwave for driving the switch becomes a problem from about 5 GHz. If switching is performed at such a high frequency, it is necessary to consider attenuation of microwaves in the electrodes.

FIG. 10 shows the inter-electrode voltage when the microwave of 5 GHz is incident on the traveling wave electrode in the longitudinal direction of the electrode. Here, L1 and L are arranged so that the sum of the areas of the portions L1 and L3 indicated by the diagonal lines is equal to the area of the portion L2.
2 and L3 were set. FIG. 11 shows the switch frequency characteristic when the TM mode is incident and the switch frequency characteristic when the TE mode is incident at this time. For comparison, the switch frequency characteristic in the configuration shown in FIG. 1 is shown in FIG.

From the comparison between FIG. 11 and FIG. 12, the difference in the extinction ratio due to the polarization when the switch is driven at high speed is suppressed by using two wave plates as the polarization mode converter. I understand. Further, in order to reduce the difference due to the polarization of the switch characteristics in the high frequency region, two or more wave plates may be inserted between the electrodes. However, it is necessary to consider that the insertion of the wave plate also increases the microwave loss and the optical insertion loss at the electrode connection portion.

The embodiments of the waveguide type optical switch using the electro-optic effect in LiNbO ₃ have been described above.
In addition to this, it goes without saying that the configuration of the present invention is very effective for an electro-optical switch using an organic material and a switch using an acousto-optic effect.

Further, it has been pointed out that even in the waveguide type optical switch using the thermo-optic effect, a slight polarization dependency is caused by the stress caused by local heat. The configuration of the present invention is also effective as a method of eliminating this polarization dependence.

[0025]

As described above, according to the present invention, a polarization filter can be provided on the way to correct the polarization.
It has become possible to realize a waveguide-type polarization-independent switch that can switch at a lower voltage than before.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a waveguide type polarization independent optical switch according to a first embodiment of the present invention.

FIG. 2 is an enlarged cross-sectional view taken along the line AA ′ of FIG.

FIG. 3 is an enlarged sectional view taken along the line BB ′ in FIG.

FIG. 4 is a diagram showing a cross-port optical output characteristic when a voltage is applied to the electrodes of the waveguide type polarization independent switch shown in FIG. 1, in which a wavy line and a solid line represent TE mode and
The characteristics when the TM mode is incident are shown.

5 is a diagram showing the cross-port optical output characteristics when the thin-film wave plate is removed and a voltage is applied to the electrodes in the switch of FIG. 1, where the wavy line and the solid line respectively enter the TE mode and the TM mode. The characteristics are shown.

FIG. 6 is a schematic view of a waveguide type polarization independent optical switch according to a second embodiment of the present invention.

7 is a diagram showing cross-port optical output characteristics when voltage is applied to the electrodes of the waveguide type polarization independent switch shown in FIG. 6, in which wavy lines and solid lines represent TE mode and
The characteristics when the TM mode is incident are shown.

8 is a diagram showing the cross-port optical output characteristics when the thin-film wave plate is removed and a voltage is applied to the electrodes in the switch of FIG. 6, in which the wavy line and the solid line respectively enter the TE mode and the TM mode. The characteristics are shown.

FIG. 9 is a schematic view of a waveguide type polarization independent optical switch capable of high speed switching according to a third embodiment of the present invention.

FIG. 10 is a diagram showing a manner in which, when a 5 GHz switch driving microwave is incident on an electrode, the voltage is attenuated as the voltage propagates inside the electrode.

11 is a diagram showing deterioration of a switch extinction ratio when high-speed switching is performed by the switch shown in FIG.

12 is a diagram showing deterioration of a switch extinction ratio when high-speed switching is performed by the switch shown in FIG.

FIG. 13 is a schematic view of a conventional waveguide type polarization independent optical switch.

[Explanation of symbols]

1 LiNbO ₃ Substrate 2 Ti Diffused LiNbO ₃ Optical Waveguide 3 Positive Electrode for Switch 4 Negative Electrode for Switch 5 Thin Film Wave Plate Insertion Groove 6 Thin Film 1/2 Wave Plate 7 Wire Bonding for Electrode Connection

Front page continuation (72) Inventor Hiroshi Takahashi 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (1)

[Claims] 1. A LiNbO ₃ substrate and the LiNbO ₃
In a polarization-independent waveguide type optical switch using an electro-optical effect, which is composed of an optical waveguide produced in a substrate and an electrode formed on the LiNbO ₃ substrate, at least one location of the optical waveguide A polarization independent waveguide type optical switch having a thin film type wave plate inserted therein.