JP2728421B2 - Optical waveguide - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Summary] Resonant reflection composed of a pair (one or more) of a first cladding layer having a lower refractive index than the core and a second cladding layer of the same medium as the core between the core and the substrate. This is a substantially single-mode optical waveguide that is provided with a clad and guides light of all polarization states having a wide wavelength range with low loss.

[Industrial applications]

The present invention relates to a planar dielectric optical waveguide formed on a substrate having a high refractive index such as a semiconductor, and more particularly to an optical waveguide useful for an optical integrated circuit.

[Conventional technology and problems to be solved by the present invention]

In recent years, optical communication and optoelectronic devices have rapidly and widely developed. Not only simple systems consisting only of conventional light emitting / receiving elements and transmission lines (optical fibers), but also modulators, isolators, optical switches, and branching devices A high-performance system to which various optical circuits such as a coupler, a demultiplexer, and a multiplexer are added has been required.

However, the optical circuit used in such a system is currently composed of a combination of individual elements such as microlenses and prisms, and the assembly process including high-precision optical axis alignment requires enormous labor and labor. It is considered difficult to meet the anticipated mass demand in the future.

In order to overcome such a problem, an optical integrated circuit technology that enables a large number of optical functional elements to be integrated and integrated mass-production is indispensable. In particular, the development of a semiconductor substrate optical integrated circuit that can be integrated with a light emitting / receiving element or an electronic circuit is an urgent issue.

However, when a conventional total reflection type optical waveguide using a refractive index difference between a core and a clad is manufactured on a semiconductor substrate as an optical waveguide which is a basic optical integrated circuit, light confinement in the core is weak, so that light is not transmitted. Part of the electromagnetic field seeps into the cladding and is radiated to the high refractive index semiconductor substrate, resulting in high loss. FIG. 4 shows an example of such a total reflection type optical waveguide.
11 is a substrate with a high refractive index such as a semiconductor, 12 is a cladding layer, 13 is a core layer, 14 is a refractive index distribution, 15 is a field distribution of guided light, and 16 black-painted portions are radiation loss light penetrating into the substrate. Represents.

To reduce such radiation loss light, the cladding layer
12 has to be thicker, which has manufacturing difficulties. In addition, it is necessary to control a small difference in refractive index between the core and the clad, and there is a problem that the optical waveguide itself has no light control function.

To solve these problems, a new resonant reflection type optical waveguide (ARROW) has been proposed.

FIG. 5 illustrates this, wherein 17 is a semiconductor substrate having a high refractive index n _S , 18 is a first cladding layer having a refractive index n ₁ and a thickness d ₁ , and 19 is a first cladding layer having a refractive index n _{2 and a} thickness d ₂ . The second cladding layer 20 has a refractive index n _{C and a} thickness d _C
, 21 denotes a refractive index distribution, and 22 denotes a field distribution of guided light. The following relationship exists between the refractive indices. However,
n ₀ is the refractive index of the medium above the core (usually air).

n _C > n ₀ , n ₁ > n _C , n ₁ > n ₂ , n _C ≧ n ₂ , n ₂ <n _S The resonant reflection type optical waveguide of FIG. 5 has a relatively low refractive index such as SiO _2. As a cladding layer between the dielectric core layer 20 and the semiconductor substrate 17 having a thickness, an interference reflection cladding having a thickness of about 2 μm and comprising a first cladding layer 18 having a high refractive index and a second cladding layer 19 having a low refractive index is used. Light is guided using the high reflectance (99.9% or more) characteristic of interference reflection. Since the higher-order mode has a significantly larger radiation loss than the fundamental mode, a single-mode waveguide can be substantially realized.

The resonant reflection type optical waveguide shown in Fig. 5 is manufactured because 1) low loss over a wide wavelength range, 2) thin cladding layer (less than 1/2 of conventional), and a wide allowable thickness for obtaining low loss. Easy, 3) Waveguide for optical integrated circuit, such as a single-mode waveguide is practically possible even with a thick film core suitable for high-efficiency coupling with optical fiber, 4) Integration with semiconductor elements and electronic circuits, etc. It has many features suitable for

Since the waveguide loss of the resonant reflection type optical waveguide has strong polarization direction dependence and wavelength dependence, it can be applied to a polarizer (polarization filter), an optical demultiplexer, and the like.

However, such a strong dependence on the polarization direction and the wavelength is not always a preferable characteristic when light is simply guided. If the waveguide has polarization dependence, only the polarization direction component is guided with low loss, and other orthogonal polarization direction components become radiation loss. Therefore, it is necessary to control the polarization direction of the incident light. is there. Further, if there is wavelength dependency, various restrictions occur such that light of a wide range of wavelengths cannot be multiplexed and guided.

The present invention has a feature that is not inferior to the above-described resonant reflection type optical waveguide, and has a small polarization dependence and wavelength dependence of the waveguide loss, and is applicable to a wide range of optical integrated circuits. It is an object of the present invention to provide a single-mode optical waveguide.

[Means for solving the problem]

The present invention has a structure in which a first clad layer and a second clad layer are provided between a core layer and a substrate, but the first clad layer in contact with the core layer has a lower refractive index than the core layer. The refractive index of the two cladding layers is higher than that of the first cladding layer. As a result, part of the light propagating through the core layer is
Light that is totally reflected by the cladding layer and partially penetrates the first cladding layer is interference-reflected by the second cladding layer.

FIG. 1 shows the principle of the present invention. In the figure, 1 is the refractive index
The semiconductor substrate of n _S, 2 the first cladding layer of refractive index n ₁ thickness d _1, 3
Is a second cladding layer having a refractive index n _{2 and a} thickness d ₂ , and 4 is a refractive index n _{C and a} thickness d _C
Represents a refractive index distribution, and 6 represents a field distribution of guided light. The following relationship exists between the refractive indices of the respective layers.

n _C > n ₀ , n ₁ <n _C , n ₁ <n ₂ , n _C ≧ n ₂ , n ₂ <n _S [Operation] The optical waveguide according to the present invention has a structure similar to that of the conventional resonant reflection type optical waveguide. With. A high refractive index material such as a semiconductor is used for the substrate, and a set of two layers of interference reflection cladding is interposed between the core layer and the substrate, and light is guided using the extremely high reflectance. However, in the case of the conventional resonant reflection type optical waveguide,
The first cladding layer is a medium having a higher refractive index than the core layer, for example,
Since Si (n ₁ = 3.5) or TiO ₂ (n ₁ = 2.3) is used, the reflection caused by the interference reflection cladding is a simple interference reflection caused by a high refractive index difference between the two cladding layers.

On the other hand, in the optical waveguide of the present invention, the refractive index of the first cladding layer is smaller than that of the core layer and the second cladding layer.
Therefore, the light is partially totally reflected at the boundary between the core layer and the first cladding layer, but the partially leaked light causes interference in the second cladding layer. Then, each layer is set to an optimum thickness, and the loss difference between the fundamental mode and the higher-order mode is made large, whereby a substantially single mode is obtained.

The optical waveguide of the present invention has low loss, strong optical confinement,
Highly efficient coupling with an optical fiber is possible. In addition, it has the same features as a resonant reflection type optical waveguide, such as integration with semiconductor elements, and the polarization dependence of the waveguide loss can be reduced to 1/10 or less of the resonance reflection type optical waveguide. Low loss characteristics are obtained for light in a wide wavelength range.

〔Example〕

Shows the calculation results waveguiding loss characteristics of each mode to the thickness d ₁ of the first clad layer normalized by the wavelength as an example in Figure 2. The horizontal axis of the figure is the thickness d ₁ ′ (= d ₁ / λ) of the first cladding layer normalized by wavelength, and the vertical axis is α (dB · λ / m) of the loss normalized by wavelength. .

Here, it is assumed that the core layer and the second cladding layer are made of C7059 glass (n ₂ = 1.54), which is often used as a waveguide material, and the first cladding layer is SiO ₂ (n ₁ = 1.46). N _S = 3.8
When 5, n _C = 1.54 and the normalized core layer d _C ′ (d _C / λ) is 6.3, at d ₁ ′ = 0.4, the loss of the fundamental mode for both TE and TM is 1 dB at a wavelength of 0.633 μm. A practical single mode of less than / cm and higher-order mode loss of more than 10 dB / cm is obtained.

Generally, the loss difference between the fundamental mode TE wave and the TM wave is as small as several times, whereas the loss difference between the fundamental mode TE wave and the higher-order mode TE wave or the higher-order mode TM wave is small. Since a loss difference between two or more digits can be obtained, a substantially single-mode waveguide can be realized.

Further, since the loss d ₁ ′ monotonically decreases, the minimum loss is not limited by the refractive index of the first cladding layer as in the case of the resonant reflection type optical waveguide, and any loss value can be set. is there. Therefore, for light of any wavelength,
A low-loss waveguide can be obtained depending on the thickness of the first cladding layer.

FIG. 3 shows the optical power distribution of the TE fundamental mode when d ₁ ′ = 0.4 under the same conditions as in FIG. It can be seen that the light confinement in the core is extremely strong, as in the case of the resonant reflection type optical waveguide.

 Next, a production example will be described.

For a wavelength of 0.633 μm, the core layer of C7059 glass was 4 μm in thickness, the first cladding layer of SiO ₂ was 0.3 μm in thickness, and the second cladding layer of C7059 glass was 2 μm in thickness. Each layer was continuously formed using high frequency sputtering.

Measurement was performed using He-Ne laser light (λ = 0.633 μm), and a single mode was confirmed from the near-field image of the guided light.
The measurement results of the waveguide loss at this time are shown in FIG.
Indicated by ●. 0.5dB / cm in TE mode, 0.7dB / cm in TM mode
The low loss characteristic of was obtained.

By combining the optical waveguide of the present invention with a conventional resonant reflection type optical waveguide, it becomes possible to configure monolithic optical integrated circuits on various semiconductor substrates.

〔The invention's effect〕

ADVANTAGE OF THE INVENTION According to the optical waveguide of this invention, it is low loss with respect to the light of any polarization state of a wide wavelength range, and the loss difference between a fundamental mode and a high-order mode is made large, and a substantially single mode Can guide light.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining the principle of the present invention, FIG. 2 is a diagram showing a waveguide loss characteristic of an optical waveguide according to one embodiment of the present invention, and FIG. 3 is a diagram showing an optical power distribution of a TE fundamental mode according to one embodiment of the present invention. FIG. 4 is an explanatory view of a conventional total reflection type optical waveguide, and FIG. 5 is an explanatory view of a conventional resonance reflection type optical waveguide. In FIG. 1, 1: substrate (n _S ) 2: first cladding layer (n ₁ , d ₁ ) 3: second cladding layer (n ₂ , d ₂ ) 4: core device (n _C , d _C ) 5 : Refractive index distribution 6: Field distribution of guided light

Claims (1)

(57) [Claims] A substrate (1), a core layer (4) made of a dielectric material for guiding light, a core layer (4) and a substrate (1)
And an interference reflection cladding comprising a first cladding layer (2) and a second cladding layer (3) provided in the middle of the core layer (4) and the second cladding layer (2). The refractive index of the substrate (1) is made lower than any of the refractive indices of the layer (3), and the refractive index of the substrate (1) is made second.
An optical waveguide characterized by having a refractive index higher than the refractive index.