JPS6015251B2 - optical demultiplexer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a TM/TE mode duplexer used in optical transmission systems or optical integrated circuits.

A prism made of a birefringent crystal is conventionally known as an element for separating linearly polarized light whose polarization planes differ by 90 degrees.

For example, Wallaston prism,
There are R ratio hon (rotion) prisms, and when parallel light from these prisms is incident, the direction of passing through it is 2.
The light is divided into two parts, each of which becomes a linearly polarized light that vibrates perpendicularly to the other. However, since these prisms have a bulk structure and are not formed into a waveguide, it is difficult to apply them to optical transmission systems or optical integrated circuits. There is a device shown in FIG. 1 that has the effect of splitting light by changing the polarization direction of light waves propagating in a waveguide structure such as an optical fiber or an optical waveguide.

It has a structure in which a metal film 3 is loaded on the surface of an optical waveguide 2 formed near the surface of a substrate 1. The metal film produces a very large absorption of TM mode propagating light having a polarization plane 5-1 perpendicular to the substrate. On the other hand, the plane of polarization 5 parallel to the substrate
For the TE mode with -2, almost no absorption occurs. Therefore, of the light incident on the waveguide 2, only the TE mode component passes through with low propagation loss, and the TM mode component cannot pass through. That is, it acts as a TE mode passing filter. This structure does not allow a TM mode pass filter. FIG. 2 shows a filter that can pass only one of the TM and TE modes. It utilizes optical coupling between two waveguides, and planar waveguides 2 and 4 are created on the surface of the substrate 1.
In particular, the planar optical waveguide 4 has a tapered shape whose thickness changes in a direction perpendicular to the light propagation direction.

6 is an intermediate layer having a smaller refractive index than both waveguides.

Let us now assume that the propagation constants of the waveguides 2 and 4 are 3, 32, and the optical coupling coefficient is C. By appropriately selecting the thickness and refractive index of the waveguide, it is possible to obtain 8,=82 for the TM mode, and l3,-82AI》C for the TM mode. As a result, only the TM mode is coupled between both waveguides, and the optical power is transferred to the other waveguide. Therefore, the light 7 in the waveguide 2
When the TE mode is incident, the TE mode passes through the waveguide 2 as light 8 without being coupled. On the other hand, since the TM mode is coupled to the waveguide 4 and the waveguide 4 has a tapered structure, the coupled propagation light 9 of the TM mode bends the optical path and travels, so the TM mode and the TE mode' spatially separated. Even if the TM mode and the TE mode are exchanged, the same thing will hold true if the propagation constant is set in this way. In this case, there was a problem in that it was impossible to make the waveguide three-dimensional, and it was impossible to connect it to an optical fiber. Therefore, it is an object of the present invention to improve the above-mentioned drawbacks of the technology, and its features include a substrate made of a substance having an electro-optic effect, and a bonding region having a length of 1 in parallel and close to each other near the surface of the substrate. A pair of optical waveguides with the same configuration and a metal film electrode loaded on the surface of one optical waveguide, mlo<1 (m+2
) lo (m is a natural number, lo is 100% coupling length for the TE mode of the coupling region), and the length 1 is divided into (m+1) in the light propagation direction according to the value of m that satisfies the TE mode of the coupling region. an electrode provided on top of another optical waveguide or parallel to the optical waveguide at a position offset from the optical waveguide; The optical demultiplexer includes means for providing different potential differences. Examples will be described below with reference to the drawings. FIG. 3 shows an embodiment of the present invention, in which two straight waveguides 2 and 4 are formed on the surface of a substrate 1 made of a substance having an electro-optic effect.

Let the length of the coupling region 10 in which both waveguides are parallel and close to each other be 1. A metal film 11 is loaded on the surface of the waveguide 4 in the form of a strip. Further, a fiber layer 12 is loaded on the waveguide 2, and a strip-shaped electrode 13-1 with a length of 1/2 is loaded.
, 13-2 are provided. Here, the substance having an electro-optic effect is a ferroelectric crystal represented by LINO03 or a semiconductor crystal represented by GaAs.
It has the characteristic that the refractive index of the substance changes depending on the influence of an external electric field. Hereinafter, a description will be given using LiNb03 as an example. Li
When a single crystal of N is used as the substrate, the waveguides 2 and 4 can be created by thermally diffusing patterned Ti metal or the like. Guided light having a plane of polarization 5-1 perpendicular to the substrate surface is referred to as TM mode, and guided light having a plane of polarization 5-2 parallel to the substrate surface is referred to as TE mode. 100% for TE mode light
The length of the bond is lo, and the length of the bond is lo x 1 milk o
Assume that the relationship holds true. 1 for TM mode light
There is no limit to 00% bond length. Generally, when considering optical coupling between two waveguides, when the number of propagation carriers of each waveguide (here 2, 4) is a, 82, l8, -3
2 l》C, no optical coupling occurs between the two. C is a coupling coefficient, C=Tai/21. is given by Furthermore, in general, when a metal film is loaded on the surface of a waveguide, a large amount of absorption occurs for TM mode light, and the propagation constant becomes extremely small. For TE mode, the effect of metal film loading is small. Therefore, for TM mode I8, -82 l
>>Condition C is satisfied, and no optical coupling occurs between the waveguides.
On the other hand, for the TE mode, since the change in the propagation constant due to metal film loading is small, optical coupling occurs almost equally between 8 and 82. As mentioned above, the connection regarding TE mode is 1
. Ku1ku3. A relationship has been established. At this time, the coupling region is divided into two, and the phase differences with different signs are applied to the waveguides 2 and 4.
100% binding can be achieved by inducing between Regarding this, please refer to 5EE, Joumal
of Q Female nt Yamam ElectronicsVol1
2, p396 (1976). As explained in the paper by Kogelnik et al. For this purpose, two fly poles 13-1 and 13-2 are provided on the waveguide 2. In order to eliminate the influence that the electrode has on the guided light, such as changes in light absorption and propagation constant, a buffer layer 12 with a smaller refractive index and smaller than that of the waveguide is used.
is provided. AI203 or S is used as a buffer layer.
i02 can be used. The metal film loaded on the waveguide 4 is used as an electrode for common grounding, and the electrode 13
TE mode'
On the other hand, 100% coupling can be achieved. In the substrate 1, the polarization axis 14 is perpendicular to the substrate surface. When a voltage is applied from the outside, electric fields with different signs concentrate directly below the electrode. This situation is shown in FIG. FIG. 4 is a sectional view of FIG. 3, and the same parts as in FIG. 3 are given the same numbers. 15 is an applied electric field.

Therefore, the propagation constants of the waveguides 2 and 4 increase for one and decrease for the other. As a result, a phase difference occurs between the two waveguides, and the sign of the phase difference generated in the coupling region divided into two differs depending on the sign of the applied voltages y, , V2. Therefore, in FIG. 3, when the laser beam 7 is incident on the waveguide 2, the TM mode light is not coupled to the waveguide 4, but proceeds through the waveguide 2 and is extracted as an output beam 8.

On the other hand, the TE mode light is 100% coupled to the waveguide 4 and the output light 9
is extracted as The change in propagation constant that occurs when a voltage is applied to produce 100% coupling for TE mode light is sufficiently small compared to the change in propagation constant for TM mode light induced by metal film loading; The effect on light is small. Therefore, the non-coupling condition of the TM mode is not destroyed even by applying a voltage. Although the case where the degree of coupling with respect to the TE mode is lo<1<3o has been described here, the same applies to the case where mlo<1<1<1>lo. Here, m=1, 2, 3, . . .
Next, FIG. 5 shows another embodiment of the present invention, and the same parts as in FIGS. 3 and 4 are given the same numbers.

In this case, the divided electrode shows the case where m=1, but the same applies to other cases. Split electrodes 13-- and 13-
2 is provided not on the waveguide 2 but near the waveguide 4 loaded with the metal film 11. Therefore, the buffer layer 12 shown in the embodiment of FIG. 3 is not required. The applied electric field 15 is concentrated in the waveguide 4 and its propagation constant 82
By changing , a desired phase difference can be given between the waveguides. The light 7 incident on the waveguide 2 exits from the waveguide 2 without being coupled with the TM mode light 8. On the other hand, the TE mode light 9 is transferred to the waveguide 4 by applying a predetermined voltage.
0% binding. In FIG. 5, the divided electrode 13-1.1
3 and 2 are provided outside the waveguide 4;
The same applies if it is provided outside of the . FIG. 6 shows another embodiment of the invention, in which the polarization axis 14 of the substrate 1 is parallel to the substrate surface.

Generally, the electro-optic effect produces a refractive index change most efficiently when the applied discharge field is parallel to the polarization axis. Therefore, in this case, the divided electrodes 13-1 and 13-2 are provided so as to sandwich the waveguide 2 therebetween. Buffer layer 12 is not required. In the waveguide 2, the refractive index is changed by an applied electric field 15 parallel to the polarization axis, and a desired phase difference can be provided. When the light 7 enters the waveguide 2, the TM mode light 8 is emitted from the waveguide 2 without coupling. Also, TE mode light 9 is 10
0% coupling occurs and the light is emitted from the waveguide 4. Next, experimental results regarding the embodiment of the structure shown in FIG. 3 will be described.

Using a C plate LiNb03 as a substrate, approximately 500 Ti metals were sputtered and then patterned into two straight waveguides. The dimensions are a waveguide width of 8 squares, a coupling length of 1=16 ribs with a spacing of 51. By thermally diffusing this in the atmosphere at 1030° C. for 1q hours, an optical directional coupler is created. After polishing both end faces of the substrate, a strip of AI with a thickness of about 3000 Å is loaded onto one waveguide (marked as ■). Next, a peak 203 is deposited to a thickness of about 2000 Å as a buffer layer, and then a two-part electrode is formed using AI on the other waveguide (indicated by ■). Each electrode has a length of 8 bars and a width of 10 peaks. Before forming the metal film and electrodes, laser light was applied and the coupling characteristics were determined. As a result, it was found that for 1.15 mountain light, the TM mode was 10312 columns and the TE mode was 10-8 sails. Therefore, for the TE mode, the relationship 1<o holds true. When voltages of about 18 V and 24 V were applied to each electrode, 100% coupling occurred in the TE mode. TM
The mode passed through the input waveguide without coupling. The obtained crosstalk is approximately 2 in both TM and TE modes.
It was MB. As explained above, the present invention comprises '1'3
2) TM and TE modes can be demultiplexed at the same time, and {3} each demultiplexed mode is spatially separated. It can be used in fiber or optical integrated circuits.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a conventional TE mode filter, Fig. 2 is a block diagram of a conventional TM/TE mode duplexer, Fig. 3 is a block diagram of an embodiment of the present invention, and Fig. 4 is a block diagram of a conventional TE mode filter. The sectional view of FIG. 3, and FIGS. 5 and 6 are configuration diagrams of other embodiments of the present invention. 1...Substrate, 2...Waveguide, 3...
... Metal film, 4 ... Waveguide, 5 ... Polarization plane, 6 ... Intermediate layer, 7 ... Incident light, 8
......Uncombined light, 9...... Combined light, 10.
...Binding region, 11...Metal film, 12...
...Buffer layer, 13...Divided electrode, 14.
... Polarization axis, 15 ... Applied electric field. Figure 1 Figure 2 Figure 3 Figure 5 Figure 6 Figure 4

Claims (1)

[Claims] 1. A substrate made of a substance having an electro-optic effect, a pair of optical waveguides having the same configuration and having a parallel and closely spaced coupling region of 1 provided near the surface of the substrate, and loaded on the surface of one of the optical waveguides. Metal film electrode and ml_0<l<(m
+2) Divide the length l into (m+1) in the light propagation direction according to the value of m that satisfies l_0 (m is a natural number, l_0 is the 100% coupling length for the TE mode of the coupling region), and create a buffer for each divided region. an electrode provided parallel to the optical waveguide on top of another optical waveguide or shifted from the optical waveguide via a layer; and an electrode provided alternately in each divided region to at least one optical waveguide via the electrode and the metal film electrode. An optical demultiplexer comprising means for providing different potential differences.