JPS6219810A - Optical multiplexer - Google Patents

Abstract

PURPOSE:To provide an optical waveguide with a polarized wave separating function, to reduce loss and to improve safety by composing an optical multiplexer of an optical directional coupler formed of two waveguides on a substrate and a metallic film mounted on one waveguide. CONSTITUTION:The optical multiplexer consists of glass 21 as the substrate, optical waveguides 22 and 23, and Al metallic film 24, and input linear polarized light 25 which excites a TM mode to the waveguide 22 which do not has the film 24 and linear polarized light 26 which excites a TE mode to the waveguide which has the film 24. Therefore, when the film 24 is mounted to coupling length with 100% of the TE mode is obtained, the TE mode light in the waveguide is coupled with the waveguide by 100%, but the TM mode light in the waveguide 22 travels straight, so that the TE mode light and TM mode light are multiplexed together eventually. Thus, the optical waveguides are given the polarized wave separating function to realize the optical multiplexer which has small loss and high stability.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an optical multiplexer. More specifically, it relates to an optical element that combines two wavelengths of light using polarization.

Conventional Technology As the manufacturing technology of optical fibers has progressed, their transmission loss has decreased significantly, and as a result, various application fields for optical fibers have been developed, including optical communication technology that replaces traditional telecommunications technology. , some of which have already been put into practical use.

Looking at the remarkable advantages of optical fiber communication systems, the first thing to mention is that information can be transmitted over long distances with almost no relays. In addition, compared to conventional communication systems, the amount of information that can be transmitted per unit time is large (several orders of magnitude larger), and a wide variety of information including image information can be transmitted, so it will become the center of future communication technology. It is thought that this role plays a role.

Various optical fiber application devices such as optical fiber systems mainly consist of optical fibers as oscillators, receivers, and transmission lines, but in reality, they are modulated and mixed in the same way as various electrical circuit components in conventional electronic and electrical circuits. , optical elements are required to perform various types of coupling such as demodulation, amplification, branching, and multiplexing, as well as processing optical information.

An optical multiplexer is one such optical element. An example of a conventional optical multiplexer (reflective film type) is shown in FIG. As can be seen from FIG. 2, there is a polarization separating prism 11, and lenses 12, 13, 1 are arranged in a T-shape in three directions of the prism, respectively.
It is composed of optical fibers 15, 16, and 17 attached through the prism 11, and a polarization separation multilayer film 18 provided on the diagonal surface of the prism 11. The optical fibers 15, 16 arranged in orthogonal relation to each other are polarization maintaining optical fibers, and 17 is constituted by a single mode optical fiber. As shown in FIG. 2, the linearly polarized light vibrating parallel to the plane of the paper that propagates through the polarization-maintaining optical fiber 15 passes through the polarization separation multilayer film 18, is coupled to the optical fiber 17, and is then polarized. The linearly polarized light propagating through the wave-maintaining optical fiber 16 and vibrating perpendicular to the plane of the paper is reflected by the polarization separation multilayer film 18 and coupled to the optical fiber 17.
As a result, the two lights are combined by the optical fiber 17.

In this method, since the waves are combined through the lenses 12, 13, and 14, there is a drawback that in addition to the difficulty in aligning the axes, the axes may be misaligned due to external factors such as vibrations.

Problems to be Solved by the Invention As mentioned above, the development of optical fiber application technology has been remarkable as the transmission loss of optical fibers has decreased. There is an urgent need to develop and improve various optical elements useful for carrying out this process.

Conventional products such as optical multiplexers, which are one type of optical element, involve the use of lenses, and therefore have various drawbacks as described above.

Therefore, the development of an optical multiplexer with a new configuration that can solve these difficulties has been awaited, and it is an object of the present invention to provide such an optical multiplexer.

That is, an object of the present invention is to realize a low-loss, highly stable optical multiplexer by imparting a polarization separation function to an optical waveguide.

Means for Solving the Problems In view of the current state of conventional optical multiplexers, the inventors of the present invention have developed a system that does not use lenses, can eliminate troublesome alignment operations, and can be used even under the influence of external forces. As a result of various studies aimed at developing an optical multiplexer that does not cause axis misalignment, we found that when the optical absorption characteristics of the optical waveguide are changed by loading a metal film on the optical waveguide, the propagation constant of the TM mode changes significantly. In contrast, the present invention was completed based on the discovery that it is advantageous to utilize the property that the propagation constant of the TE mode does not show large changes.

That is, the optical multiplexer of the present invention is composed of an optical directional coupler consisting of a substrate and two waveguides formed on the substrate, and a metal film loaded on one of the waveguides. Features.

The optical multiplexer of the present invention has a configuration as shown, for example, in FIG. 1 attached. As is clear from FIG. 1, the optical multiplexer includes an optical directional coupler A consisting of a substrate 21 made of glass, LiNbO3, etc., and two optical waveguides 22 and 23 formed thereon; It is composed of a metal film 24 loaded on one optical waveguide 23.

Here, the waveguide can be formed by various conventionally known methods. For example, a material having a refractive index higher than that of the substrate is formed on a substrate by various epitaxy methods (vapor phase, liquid phase, molecular beam, organometallic CVD). Alternatively, the desired waveguide pattern can be obtained by depositing by vacuum evaporation, sputtering, etc., and patterning by photolithography. Of course, it is also possible to form a waveguide by solid phase diffusion using impurity ion diffusion, ion exchange, or the like.

As the metal film, AI can be cited as a typical example (however, other materials can also be used), and these can be obtained according to various methods such as vacuum evaporation, sputtering, and ion blating. However, vacuum evaporation is sufficient.

This metal film is loaded onto one waveguide of the optical directional coupler in a predetermined shape and size, and the patterning method can be carried out using photolithography, for example, in the same manner as in the waveguide formation described above. This can also be done by vapor depositing metal through a mask having a pattern of , or by using a lift-off method. The thickness of the metal film is 50
If the thickness is 0 Å or more, a sufficient effect can be expected.

However, this lower limit is not critical and is merely a limit in film formation technology. Therefore, if it is a homogeneous thinner film, there is no problem even if it is 500A or less. If the above conditions are satisfied, the requirement that only TE mode is 100% coupled to a waveguide loaded with a metal film can be advantageously achieved.

Operation The thus obtained optical multiplexer of the present invention allows TE mode light to be directly introduced into the waveguide loaded with a metal film through the optical fiber from its input end under conditions in which only the TE mode is 100% coupled. By similarly introducing TM mode light into the waveguide not loaded with a film from its input end through an optical fiber, TE mode light and TM mode light are transmitted from the output end of the waveguide not loaded with a metal film. The combined output light can be extracted.

As in the above structure, when a metal film is loaded on the optical waveguide, T
The M mode suffers a large loss and at the same time its propagation constant changes significantly. Now, in order to clarify this point, Pt"1.0 from the output end of the waveguide 22 of the optical directional coupler A.
When the input light is introduced, the output lights from the waveguides 22 and 23 (denoted as P1 and β2, respectively) can be expressed by the following equation.

Pl=1-β2 (1) Here, β1 and β2 are the propagation constants of each waveguide 22.23, C is the coupling coefficient, L is the 100% coupling length, and L is the coupling length. Therefore, the loss coefficients of waveguides 22 and 23 are β1 and β
2, the light intensity of Pl and β2 is πL/2L.

When plotted against , the relationship as shown in FIG. 3 is obtained. As understood from this result, (β1-β2)/C
It can be seen that when is large, no optical coupling occurs regardless of the magnitude of (αyawα2)/C.

Therefore, the bond length (L
), when a metal film is loaded on the waveguide 23, T
Since the E mode light is almost 100% coupled to the waveguide 22, and the TM mode light travels straight, the TE mode light and the TM mode light are combined as a result (see later embodiments).

Thus, according to the optical multiplexer of the present invention, TE mode light and TM mode light can be combined by directly introducing each light into the waveguide on the optical directional coupler from the optical fiber that propagates the light without using a lens. Since waves can be realized, there is no need for troublesome alignment, etc., and it is stable against the action of external forces.

The optical multiplexing path of the present invention can be used not only for multiplexing light but also as an optical filter depending on the embodiment.

Example The present invention will be explained in more detail with reference to Examples below.

FIG. 1 shows a preferred configuration example of the optical multiplexer of the present invention, which includes a glass 21 as a substrate, optical waveguides 22 and 23, and an AI
The linearly polarized light 25 that excites the TM mode is placed in a waveguide 22 that is composed of a metal film 24 and does not have an AI film, and the linearly polarized light 26 that excites the TE mode is sent to a waveguide 23 that is loaded with an AI film 24.
The multiplexed light 27 can be obtained.

In FIG. 4, n9=
When a waveguide with a refractive index of 2.206 is provided and an A1 film of nt=1.48 J9.0 is loaded on one of the waveguides (β).

- β6)/βb and α (loss coefficient) and the waveguide thickness (where β4 is the general propagation constant of the waveguide loaded with the A1 film, while β5 is the general constant of the waveguide loaded with the AI film). ). As is clear from the figure, TM
The loss of the mode is extremely large, but that of the TE mode is negligibly small.

Therefore, when the metal film 24 is loaded at a coupling length such that the TE mode is 100% in FIG.
In contrast, the TM mode light in the waveguide 22 travels straight, and as a result, the TE mode light and the TM mode light are combined.

In addition, FIG. 5 shows that the optical multiplexer A of the present invention (see FIG. 1) is
Attaching polarization-maintaining optical fibers 28.29 for introducing each polarized light 25 and 26 into the waveguide 22.23, respectively,
On the other hand, a state in which an output optical fiber 30 (single mode optical fiber) for propagating the multiplexed light 27 outputted from the right end of the waveguide 22 is attached is shown. All of them are characterized by using bulk type waveguides, and the use of lenses can be completely eliminated.

Effects of the Invention As explained in detail above, according to the optical multiplexer of the present invention, an optical directional coupler consisting of a substrate and two waveguides provided on the substrate is used, and a metal is placed on one of the waveguides. By using a membrane-loaded configuration, alignment with the optical fiber can be achieved without using a lens. Therefore, it is extremely easy to use, and since there are few connection points, it can be made compact with less axis deviation due to external disturbances such as vibrations.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a preferred configuration example of the optical multiplexer of the present invention, FIG. 2 is a diagram for explaining the configuration of a conventional optical multiplexer, and FIG. 3 is a diagram of an optical directional coupler. This is a graph plotting the relationship between the coupling length and the output light intensity, and Figure 4 shows the changes in the propagation constants and losses of the TM mode and TE mode when a metal film is loaded on one side of the waveguide, and the loss in the waveguide. 5 is a graph plotting the relationship with the thickness of the wave path, and FIG. 5 is a diagram showing a state in which an optical fiber is attached to the optical multiplexer of the present invention. (Main reference numbers) 11...Polarization separation prism, 12.13.14...Lens, 15.16.28.29...Polarization maintaining optical fiber, 17
．． 30...Output optical fiber, 18...Polarization separation multilayer film, 21...Substrate, 22.23...Waveguide, 24...Metal film, 25...TM mode excitation linearly polarized light, 26...TE mode Excitation linearly polarized light, 27... Output light

Claims (4)

[Claims] (1) Optical multiplexing comprising an optical directional coupler consisting of a substrate and two waveguides formed on the substrate, and a metal film loaded on one of the waveguides. vessel. (2) Under conditions where only TE mode is 100% coupled, TE mode light is introduced from the input end of the waveguide loaded with the metal film, while TM mode light is introduced from the waveguide end not loaded with metal film. The optical multiplexer according to claim 1, wherein the optical multiplexer according to claim 1 is characterized in that the output light, which is a combination of the TE mode light and the TM mode light, is extracted from the other end of the waveguide which is not loaded with a metal film. . (3) The optical multiplexer according to claim 1 or 2, wherein the metal film is an aluminum film formed by a vapor deposition method. (4) The optical multiplexing waveguide according to claim 3, wherein the substrate is glass or LiNbO_3.