JP3103417B2 - Waveguide type optical demultiplexing circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type optical demultiplexing circuit, and more particularly, to a demultiplexer for separating a plurality of lights having different wavelengths which have propagated in the same optical waveguide for each wavelength, and The present invention relates to an optical demultiplexing / photodetection integrated circuit on a semiconductor substrate in which the demultiplexer and the photodetector are integrated.

It should be noted that the present invention relates to TE (Transverse Electric: electric field parallel to the substrate) propagating in an optical waveguide, T
A waveguide type polarizer that radiates one of two polarized lights, M (Transverse Magnetic: magnetic field is parallel to the substrate), to the substrate and passes the other, and the intensity of guided light by current injection or voltage application It also relates to an optical modulator (on / off switch) for modulating a level.

[0003]

2. Description of the Related Art An optical communication system currently in practical use is a simple system in which a laser diode on a transmitting side and a photodetector on a receiving side are connected one-to-one by an optical fiber. However, in the future, with the introduction of optical LANs and subscriber systems, it is certain that high-speed and large-capacity optical fiber communication will be required. In recent years, in order to realize this, an ultra-high-speed AM modulation system, a coherent optical communication system, or a wavelength division multiplexing (WDM for short) which multiplexes and transmits light having different wavelengths on the same optical fiber in these systems. A new communication method such as a method has been proposed.
In the wavelength division multiplexing system, a system in which the wavelength interval is extremely narrow and the multiplicity is extremely high is frequency multiplexing (Frequency Dv.
This is called an ision multiplexing (FDM for short) or dense WDM (DWDM for short) method. These communication systems require a high-performance system to which various optical circuits such as an optical demultiplexer, an optical multiplexer, an optical switch, and a polarizer are added in addition to a laser diode and a photodetector. However, in order to realize these optical circuits with the current technology, it is necessary to combine individual elements manufactured individually, and there are difficulties in mass productivity and stability. Therefore, in the future, it is expected that these elements will be replaced by an optical integrated circuit in which these elements are organically integrated for the purpose of stable and high performance, low cost, mass production, and the like.

[0004] Each of the above elements has high performance if manufactured individually, but most of them are difficult to manufacture monolithically on an optical integrated circuit. In an optical integrated circuit, various elements are coupled by an optical waveguide. Therefore, these elements are optimally of an optical waveguide type or a combination with an optical waveguide.

In the above-described wavelength multiplexing method, a plurality of lights having different wavelengths are multiplexed and transmitted through one optical fiber.
An optical demultiplexer that separates light of a specific wavelength from a plurality of transmitted lights of different wavelengths is indispensable. recent years,
A WDM system with a relatively small number of wavelength multiplexes is about to be put into practical use, but in the future, DWDM or FDM with a very narrow wavelength interval to be multiplexed and a very large number of multiplexes will be used in the future.
It is expected to shift to the system.

As a waveguide type duplexer for the DWDM or FDM system, a Mach-Zehnder interference type (J. Li
ghtwave Technol., vol. 6, pp. 1003-1010 (1988)), grating type (IEEE J. Quantum Electron., QE-15, No. 7,
p.632 (1979); IEEE Trans. Circuits Syst., CAS-26, No.1
2, p.1049 (1979)), asymmetric directional coupler (Appl. Phys.
Lett., Vol. 33, No. 2, pp. 161-163 (1978); IEEE J. Quantum
Electron., Vol. QE-14, No. 11, pp. 843-847 (1978)) and the like have been actively studied.

As shown in FIG. 14, the Mach-Zehnder interference type duplexer is constructed by combining a plurality of asymmetric Mach-Zehnder interference systems having two optical paths having different lengths. Light of a plurality of wavelengths input to one input port is separated for each wavelength and output from each output port. In this type, many wavelengths can be separated at intervals of about several tens of wavelengths, but the maximum dimension exceeds 1 cm even with one Mach-Zehnder interferometer. Therefore, when the number of channels is large, the number of combinations increases, and the size of the entire circuit becomes large. The height is at least several cm. Therefore, it is difficult to use it as one element of an optical integrated circuit including various elements.

Next, the basic structure of the grating type is shown in FIG.
It is shown in FIG. The one shown in this figure is the most high-performance type of multi-wavelength type duplexer, in which a grating unit and a photodetector are grouped in a cascade array. Also in this type, it is possible to separate the individual wavelengths and detect the light at a wavelength interval of about several mm. However, the guided light is scattered in the grating portion, and the detection efficiency particularly at the array terminal is reduced. In addition, a grating having a two-dimensional spread is used for the wavelength selection action, and the waveguide must also be expanded to the width, so that there is a limit to miniaturization of the element.

The asymmetric directional coupler type duplexer is shown in FIG.
As shown in (a), the configuration is made up of a directional coupler composed of two waveguides having different core refractive indices and widths. FIG. 1 shows the wavelength dependence of the propagation constants β _a and β _b of each waveguide.
6 (b), and β _a = β at the selected wavelength λ ₁
becomes _b . The light of λ ₁ incident on the waveguide a is coupled from the waveguide a to the waveguide b by phase matching. The length of the coupling portion in the complete coupling length L ₁ with respect to the wavelength lambda _1, the light of wavelength lambda ₁ is outputted from the waveguide b. When the wavelength is separated from the λ _1, β _a
And β _b are different, and no coupling occurs due to phase mismatch. Therefore, the output intensity from the waveguide b with respect to the wavelength is as shown in FIG. Even in this type of duplexer, the selected wavelength width is about several Å, but as shown in FIG.
Side lobes occur on the left and right sides of the selected wavelength, which is a major problem. This weak coupling occurs even at a wavelength slightly shifted from the selected wavelength lambda _1, to the length of the coupling portion is constant, complete coupling length is to vary greatly with respect to the wavelength. Furthermore, generally, in these duplexers,
Since the demultiplexing characteristic is slightly shifted between the TE polarized light and the TM polarized light, the TE component of the two wavelengths whose wavelengths are slightly shifted and the TM
Components tend to cause crosstalk. Therefore, some measure is needed to remove the polarization dependence of the demultiplexing characteristic.

On the other hand, as for the waveguide type polarizer, as a type of TE passing / TM radiation (or absorption), a metal clad loaded type, an anisotropic optical crystal loaded type, an ARROW type polarizer and the like have been proposed. I have. Among these, the ARROW type is particularly excellent in characteristics, but when the thickness of the core is set to be substantially the same as that of the optical fiber, the function of the polarizer is reduced. In addition, TM passing
The TE radiation function can be realized by using an anisotropic optical crystal loaded type, but this element is not suitable for mass production and is expensive.

[0011]

SUMMARY OF THE INVENTION The present invention has been made in view of such a situation, and an object of the present invention is to solve the above-mentioned various problems in the conventional optical demultiplexer and to make the selected wavelength width extremely small. Optical splitter suitable for optical integrated circuits that is narrow, does not generate spectral characteristics, has high efficiency, can be miniaturized, is easy to manufacture, and can be integrated with photodetectors. Is to provide a waveguide type optical demultiplexer capable of performing the following.

[0012]

The waveguide type optical demultiplexer of the present invention, which achieves the above object, is basically a kind of multilayer clad optical waveguide. FIG. 1 is a diagram showing a sectional view (a) and a refractive index distribution (b) of the most basic structure of a duplexer according to the present invention. This is a multi-layer clad waveguide composed of 3, 4 and the core 1. The relationship between the refractive indexes of the respective layers is such that the semiconductor substrate 5 has a high refractive index, and the core layer 1 and the cladding layer 3 are relatively higher than the cladding layers 2 and 4. FIG.
Shows numerical examples of the thickness and the refractive index of each layer for setting the selected wavelength to 1.3 μm, but this is only one example. Each of the cladding layers 2, 3, and 4 will be referred to as a first cladding layer 2, a second cladding layer 3, and a third cladding layer 4 in order from the top. Propagating radiation loss for wavelengths of the waveguide (loss by radiation to the substrate 5) becomes the characteristic as shown in FIG. 2, although the loss is very large in the wavelength lambda _1, if slightly moves away wavelength from lambda ₁ , Losses drop sharply. Accordingly, the waveguide it is understood that selected center wavelength acts as a narrow-bandwidth demultiplexer lambda _1.

The principle of the waveguide according to the present invention having such characteristics will be described using the concept of the asymmetric directional coupler described above. Paying attention to the refractive index of each layer in FIG. 1, since the second cladding layer 3 has a higher refractive index than the adjacent upper and lower layers 2 and 4, if the upper core layer 1 is removed, the three-layer cladding becomes the second cladding layer. It can be considered as a waveguide in which the two clad layers 3 are a core and the first clad layer 2 and the third clad layer 4 are clad. That is, in the structure of the waveguide of the present invention including the core 1 and the three cladding layers 2 to 4, the two waveguides having the core layer 1 and the second cladding layer 3 as cores are approaching in parallel. It can be considered as a kind of asymmetric directional coupler. Therefore, the waveguide mode in which the second clad layer 3 is regarded as a core is referred to as a second clad mode. Is called a core mode. When this is compared with the directional coupler of FIG. 16 shown in the prior art, the core layer 1 corresponds to the waveguide a and the second cladding layer 3 corresponds to the waveguide b. Waveguides a and b are completely confined waveguides,
In the case of the device of the present invention, the core mode and the second cladding mode are leak modes in which a part of light leaks to the outside, that is, to the substrate 5. However, in the core mode, the degree of leakage is very small. In the second cladding mode, the degree of light emission to the substrate 5 is small because the second cladding layer 3 is close to the substrate 5 having a high refractive index. It is very high and can hardly guide light. Therefore, the difference between the waveguide type optical demultiplexer of the present invention and the conventional asymmetric directional coupler type demultiplexer lies in the presence of the high refractive index layer (substrate) 5 below this.

FIG. 3 shows an example of characteristics of the propagation constants β _c and β ₂ of the core mode and the second cladding mode with respect to wavelength.
When the refractive index and the thickness of the second cladding layer 3 are largely different from those of the core layer 1, the characteristics of β _c and β ₂ are largely different, but β _c = β ₂ at the center wavelength λ ₁ . Now, as shown in FIG. 1, when light having a wavelength of λ ₁ enters the waveguide of the present invention, the waveguide mode, that is, the core mode, has the same propagation constant as the second cladding mode. And the power is transferred from the core layer 1 to the second cladding layer 3. However, the power that has shifted to the second cladding mode is immediately radiated to the substrate 5. The light is not radiated to the substrate 5 after the power is completely transferred to the second cladding layer 3, but is radiated to the substrate 5 sequentially from the light transferred to the second cladding layer 3. Most of the light is emitted to the substrate 5 until the light is completely coupled to the second cladding layer 3. According to the numerical analysis, the complete coupling length at wavelength λ ₁ is 1
mm or less, if the length of the waveguide is that much, 99.9% or more of the light having the wavelength λ ₁ is emitted to the substrate 5. On the other hand, as shown in FIG. 2, in the wavelength wavelength deviates from lambda _1, since the beta _c and beta ₂ are different, even when incident on the waveguide of the present invention, coupled to the second cladding mode by the phase mismatch Go through without low losses. Therefore, the selected wavelength width of the waveguide type duplexer of the present invention is very narrow, and light of the selected wavelength is radiated to the substrate 5 with high efficiency, and light of other wavelengths propagates with low loss. Become. Since light having a wavelength slightly deviated from λ ₁ causes imperfect coupling to the second cladding mode, in a conventional asymmetric directional coupler, this is the same as in FIG.
In the device of the present invention, light incompletely coupled to the second cladding 3 is immediately radiated to the substrate (high-refractive-index layer) 5 to cause side lobes as shown in FIG. Since it does not return to 1, no side lobe occurs.

If the thickness and the refractive index of the core layer 1 and the second cladding layer 3 are greatly different from each other, the difference (傾 き β _{c) in FIG.}
/ ∂λ-∂β ₂ / ∂λ) increases, and the loss characteristics in FIG. 2 become sharper. Further, in the conventional asymmetric directional coupler type duplexer, side lobes are generated as shown in FIG. 16C, whereas in the device of the present invention, the second cladding layer 3 moves from the substrate 5 parallel to the side lobe. Since light is radiated to the side, no side lobe can be generated. Further, in the case of the duplexer of the present invention, the fundamental mode of the core mode can be coupled to a mode of an arbitrary order of the second cladding mode. The second cladding mode is a pseudo-guiding mode, and may include higher-order modes such as first-order and second-order modes in addition to the zero-order mode. In general, the higher-order mode has wavelength dependence of the propagation constant (に 対 し て β / ∂) with respect to the fundamental mode.
λ) is large and the radiation loss is large, so that coupling to this higher-order mode narrows the wavelength selection width of the duplexer and increases the radiation efficiency at the selected wavelength. Further, as will be described later, the polarization dependence of the duplexer of the present invention is very large,
The splitting wavelength for polarized light is very far from the splitting wavelength for TM polarized light. This means that, in other words, the film thickness for demultiplexing the TE polarized light of the same wavelength is significantly different from the film thickness for demultiplexing the TM polarized light. Therefore, as will be described in detail later, by arranging a plurality of wavelength demultiplexers for TE polarized light and the same wavelength demultiplexer for TM polarized light in a cascade, there is a feature that polarization diversity reception becomes possible.

The principle of the duplexer according to the present invention has been described above with the concept of a directional coupler. Strictly speaking, the core mode and the second clad mode described above are based on the case where these cores exist independently of each other. The radiation loss characteristic of the coupled waveguide cannot be calculated without using the theory of a general multilayer clad waveguide. For example, the radiation efficiency of the second cladding mode depends on the thickness d ₃ of the third cladding 4.
Is considered to be as large as possible when the thickness is reduced, and the radiation efficiency at the selected wavelength is increased. In practice, not only d ₃ but also the thickness d ₁ of the first clad 2 and the thickness d of the second clad 3 _{If 2} is not set to an appropriate thickness, the radiation efficiency will not increase. Therefore, in order to optimally design the duplexer of the present invention, it is considered that a layer (substrate) for emitting light is simply approached to one waveguide of the asymmetric directional coupler to emit the combined light. Cannot be designed.

In the duplexer of the present invention, only the selected wavelength is emitted to the substrate. If a photodetector is manufactured on that portion of the substrate (semiconductor), only light of that wavelength can be detected. it can. That is, it is significant that the duplexer of the present invention is integrated with a photodetector on a semiconductor substrate. In most systems using an optical demultiplexer, the separated light is immediately detected by a photodetector, and when the demultiplexed light is processed and then detected, the former circuit may be replaced. Many.

In the case of multiple wavelengths, as shown in the cross section in FIG. 4, the parameters of the three-layer cladding C, in particular, the thickness of the coupling layer 3 are sequentially changed so that the duplexers 11, 12, and 13 for each wavelength are changed. And integrating the photodetectors 7 at the corresponding positions of the substrate 5 and connecting the demultiplexing / photodetection integrated elements in an array so as to individually detect light of many wavelengths. Can be.

So far, the application of the multilayer clad waveguide according to the present invention to an optical demultiplexer has been described.
Applications to polarizers and optical switches are conceivable.

In the waveguide according to the present invention, it is the TE or TM polarized light that increases the radiation efficiency at the selected wavelength, and the other polarized light is transmitted with low loss. That is, although the wavelength dependence is large, the present invention can be applied to a polarizer of TE passing / TM radiation or a polarizer of TM passing / TE radiation having a high extinction ratio and low insertion loss. Alternatively, as described above, a polarization separation / TE (or TM) polarization detector can be configured by integrating with a photodetector on a substrate. Note that this integrated element also has a wavelength demultiplexing function, and has functions of polarization separation, light demultiplexing, and light detection.

Next, in the waveguide according to the present invention, the propagation loss largely changes with respect to the change in the refractive index of the second cladding layer, similarly to the fact that the propagation loss largely depends on the wavelength.
Therefore, for example, a material whose refractive index can be controlled electrically is used as the second cladding layer, and the refractive index of this layer is appropriately changed electrically, so that the intensity of light passing through this element is increased or decreased. Since it can be changed, it can be applied as a light intensity modulator (in particular, an optical switch). Even if the element length is short, a high extinction ratio can be obtained by only slightly changing the refractive index of the coupling layer, so that low drive voltage / current and high-speed modulation can be achieved.

That is, the waveguide type optical demultiplexing circuit of the present invention comprises at least a core layer, three or more cladding layers, and a substrate in order from the surface of the waveguide, and a specific intermediate layer other than the outermost cladding layer. The refractive index of the cladding layer is relatively higher than that of the other cladding layers to form the coupling layer, and the refractive index of the substrate is higher than that of the core layer and the coupling layer, so that the propagation mode of the core and the coupling layer By setting the propagation constant of the propagation mode to match only at a specific wavelength, light of a specific wavelength in light propagating in the core is separated and coupled to a coupling layer, and the separated light is The radiation is emitted from the coupling layer to the substrate.

In this case, it is desirable that the substrate is formed of a semiconductor and a photodetector for detecting light emitted on the substrate is integrated. Also, at least one of the core layer, the coupling layer, and the cladding layer other than the coupling layer is formed of a variable refractive index material, and by changing the refractive index of the layer, the intensity modulation or switching of the propagation light is performed. You can also.

Further, another waveguide type optical demultiplexing circuit of the present invention comprises at least a core layer, three or more cladding layers, and a substrate in order from the surface of the waveguide, and an intermediate layer other than the outermost cladding layer. The specific cladding layer has a relatively high refractive index compared to other cladding layers to form a coupling layer, and
By setting the refractive index of the substrate higher than that of the core layer and the coupling layer, and setting the propagation constant of the propagation mode of the core and the propagation mode of the coupling layer to coincide only with TE or TM polarized light of a specific wavelength, the core T of a specific wavelength in light propagating in
E or TM polarized light is separated and coupled to the coupling layer, and the separated light is emitted from the coupling layer to the substrate.

In this case as well, it is desirable to form the substrate from a semiconductor and integrate a photodetector for detecting light emitted on the substrate. Then, by connecting a plurality of the above-mentioned waveguide type optical demultiplexing circuits having different wavelengths having the same propagation constants in tandem, polarization separation and wavelength demultiplexing can be performed together.

[0026]

According to the first aspect of the present invention, an optical signal in which a plurality of lights having different wavelengths and narrow wavelength intervals are multiplexed can be separated for each wavelength. By doing so, the separated light of each wavelength can be radiated to the photodetector on the substrate and converted into an electric signal by the photodetector. By setting the thickness of a specific layer in the multilayer cladding layer to be tapered in the waveguide direction, the selected wavelength width can be widened.

Further, by applying an electric action or the like to the variable refractive index coupling layer in the multi-layer cladding layer, the waveguide loss is significantly fluctuated, and the intensity of light incident on the core layer is modulated or switched. Can be.

According to the second aspect of the present invention, for the incident light containing two polarized lights of TE and TM, the TM polarized light is radiated to the substrate with high efficiency, and the TE polarized light is transmitted with low loss. Alternatively, TE polarized light can be emitted and TM polarized light can be passed. Or separate for each polarization,
TE and TM polarizations can be individually detected. If the thickness of a specific layer in the multilayer clad is changed in a tapered shape in the waveguide direction, the wavelength dependence of the polarizer function can be reduced.

[0029]

Next, specific examples of the waveguide type optical demultiplexing circuit of the present invention will be described.

The general structure of the waveguide according to the present invention is, as shown in cross section in FIG. 5, a specific intermediate layer 3 other than the outermost layer 3 (not limited to one layer) in a multilayer clad C composed of three or more layers. No.) is a multilayer clad waveguide with a relatively high refractive index and a coupling layer. This is a selected wavelength when it is used as a demultiplexer, and a used wavelength when it is applied to a polarizer or an optical switch. In, the refractive index and thickness of each layer are set such that the propagation constants of the core mode of the core layer 1 and the mode of the second cladding layer 3 match. Integrated optical demultiplexing / detection device,
When applied to a polarization separation / photodetection integrated device, a photodetector is manufactured below this waveguide using the substrate 5 as a semiconductor. The core mode is limited to the fundamental mode, but the order of the second cladding mode coupled with the core mode is arbitrary. Since the selected wavelength width and the radiation efficiency to the substrate differ depending on the mode order to be coupled, an appropriate mode order is selected according to the purpose of use. In general, the higher the mode order, the narrower the wavelength selection range and the higher the radiation efficiency. Although the number of claddings is basically three, the number of cladding layers is not limited to three, for example, a non-reflection layer is provided at the boundary with the substrate 5 in order to increase the radiation efficiency.

FIG. 1 shows a basic structure of the waveguide of the present invention and numerical examples of the refractive index and the layer thickness for setting the selected wavelength to 1.3 μm. In this example, the second cladding layer 3 is a bonding layer. The refractive indexes of the first cladding layer 2 and the third cladding layer 4 are lower than the refractive indexes of the core 1 and the second cladding 3. The refractive index of the second cladding layer 3 is determined by the refractive index of the core n _c =
For 1.54, n ₂ = 2.6, which is large, and conversely, for the core thickness d _c = 14 μm, d ₂ = 0.665 μm, which is very thin. The thickness of each cladding has been optimized by numerical calculations. As shown in FIG. 3, the slopes of the characteristic curves of the propagation constants β _c and β ₂ with respect to the wavelengths of the core mode and the coupling mode (second cladding mode) are greatly different, and β _c = β _{2 at a} wavelength of 1.3 μm. Become. In this example, the core mode is coupled to the second mode of the second cladding mode. However, these are the cases of TE polarized light. FIG. 2 shows the propagation loss (radiation loss) with respect to the wavelength of the waveguide calculated based on the theory of the multilayer cladding. At a center wavelength of 1.3 μm, the loss exceeds 100 dB / cm, and the waveguide light is attenuated to -100 dB or less per cm, and this light is emitted to the substrate 5. When the wavelength is slightly away from 1.3 μm, the loss decreases sharply and the loss is 1d
The wavelength width of B / cm or more is as very narrow as about 40 °.
As the wavelength departs from 1.3 μm, the loss decreases, and no side lobe occurs in the wavelength range practically 0.8 μm to 1.7 μm for optical communication. In this structure example, the core thickness is as large as 14 μm, and even when directly connected to the optical fiber, the matching with the propagation mode of the optical fiber is good and the loss is small.
On the other hand, the thickness of each cladding layer is relatively thin, which is advantageous in manufacturing.

Next, when the core thickness is reduced, matching of the field distribution with the optical fiber is deteriorated, and a spot size matching device is required. However, the radiation efficiency at the selected wavelength is further increased without substantially changing the selected wavelength width. Can be higher. FIG.
FIG. 3 shows the loss characteristics when the core thickness is 8 μm in the structure of FIG. 300 dB / c loss at 1.3 μm center wavelength
m and the element length is about 1 mm, the optical power necessary for practical use can be guided to the substrate 5. On the other hand, as the element length is shorter, the insertion loss of light of a non-selected wavelength (passing wavelength) is reduced.

FIG. 4 shows a case where light of multiple wavelengths is demultiplexed.
As shown in FIG.
If the integrated elements of the device 7 are arranged in an array along the waveguide
good. As shown in the loss characteristics in FIG.
Thickness, especially the thickness d of the bonding layer _TwoJust adjust the center
The wavelength can be set arbitrarily. With this circuit,
Detecting multiple wavelengths of light in a waveguide individually
Can be.

If it is a problem that the selected wavelength width is narrow, if the coupling layer 3 is made of a material close to the refractive index of the core 1,
As shown in the loss characteristics in FIG. 8, the selected wavelength width becomes wider.
In this example, n ₂ = 1.6 and d ₂ = 3.78 μm, the refractive index and the thickness are closer to the core 1 than in the example of FIG. 1, and the propagation constants of the core mode and the coupling mode (second cladding mode) are different. This is because the slope of the wavelength characteristic (∂β / ∂λ) becomes closer. However, the steepness of the loss characteristic is lost as shown in FIG. Therefore, as can be seen from FIG. 7, when the thickness of a specific layer (not limited to one layer) in the cladding layer is changed, the selected wavelength shifts, so that the thickness of this layer is tapered in the light propagation direction. In this case, loss characteristics with a sharp rise and fall and a wide wavelength selection range can be obtained. For example, in the example of FIG. 1, as shown in the sectional view of FIG. 9, the d ₂ 0.663
If the wavelength is gradually changed from μm to 0.667 μm in the waveguide direction, the selected wavelength is 1.295 μm to 1.305 μm in a width of radiation loss of 1 dB / cm or more, and the selected wavelength width is widened to about 100 °.

Further, as shown in FIG. 2, at the selected wavelength, TE polarization has a very high loss while TM polarization remains at a very low loss. That is, this waveguide structure is capable of transmitting light having a wavelength of 1.3 μm through TM transmission and TE.
Acts as a polarizer of radiation. On the other hand, if the thickness of the cladding layer is appropriately set, as shown in FIG.
A TE-pass and TM-radiation polarizer can be constructed. In each case, a sufficient extinction ratio is obtained even though the core is as thick as 14 μm. When the magnitude of the wavelength dependence of the polarizer function becomes a problem, the thickness of the specific cladding layer may be changed in a tapered shape in the waveguide direction, as in FIG. Further, if integrated with the photodetector 7 on the substrate, as shown in the cross section in FIG.
It becomes 5. Since these elements have an optical demultiplexing function, these TE polarization demultiplexers 16 and TM polarization demultiplexers 17 are
If combined like this, optical demultiplexing for multiple wavelengths, TE, T
An M polarization individual detection circuit (polarization diversity type demultiplexing / photodetection integrated device) can be configured. As a result, since a specific wavelength can be demultiplexed and each polarization of TE and TM can be individually detected, this element is a very useful element in a wavelength multiplexing coherent optical communication system or the like.

Next, FIG. 13 shows a structure similar to that of the above-mentioned example having a core thickness of 8 μm (FIG. 6), in which the wavelength is fixed to 1.3 μm and the refractive index n ₂ of the coupling layer (second cladding layer) 3 is changed. It is a propagation loss characteristic when it is changed. For n ₂ = 2.6,
Although the loss is 300 dB / cm or more and hardly propagates light, the loss is sharply reduced to 0.1 dB / cm or less by changing n ₂ by only 0.2%. Therefore, if a material having a variable refractive index is used for the coupling layer 3 and the refractive index of the coupling layer 3 is made variable by voltage application or current injection, an optical intensity modulator, particularly an optical switch, can be constructed. Since a small change in the refractive index greatly changes the propagation loss, a low drive voltage and current are possible, and the off state (n ₂ = 2.
Since the propagation loss in 6) is very large, the element length can be shortened even if the extinction ratio is sufficiently large, and the stray capacitance can be reduced, so that high-speed modulation becomes possible.

The waveguide type optical demultiplexing circuit and the like of the present invention have been described based on the principle and some embodiments.
The present invention is not limited to these embodiments, and various modifications are possible.

[0038]

As is apparent from the above description, according to the waveguide type optical demultiplexing circuit of the present invention, the optical demultiplexing circuit required for the future FDM or DWDM type optical communication system in which the degree of wavelength multiplexing is very high. An optical demultiplexing / detection function can be realized on an optical integrated circuit.

When the waveguide type optical demultiplexing circuit of the present invention is applied to a polarizer, the conventional polarizer function obtained by an individual element such as a crystal prism or a polarization beam splitter is achieved on an optical integrated circuit. be able to. By being integrated with the photodetector on the substrate, not only can TE and TM polarizations be individually detected, but they can also have an optical demultiplexing function.

When the waveguide type optical demultiplexing circuit of the present invention is applied to an optical switch, a high extinction ratio, a low driving voltage / current,
Light intensity modulation of high-speed modulation becomes possible.

[Brief description of the drawings]

1A and 1B are a cross-sectional view and a refractive index distribution of a most basic structure of a waveguide type optical demultiplexing circuit according to the present invention.

FIG. 2 is a diagram illustrating a propagation radiation loss characteristic with respect to a wavelength of the waveguide of FIG. 1;

FIG. 3 is a diagram illustrating an example of characteristics of a propagation constant of a core mode and a second cladding mode of the waveguide of FIG. 1 with respect to wavelength;

FIG. 4 is a cross-sectional view of an example of a multi-wavelength demultiplexing / photodetection integrated circuit.

FIG. 5 is a sectional view showing a general structure of a waveguide according to the present invention.

FIG. 6 is a diagram illustrating propagation radiation loss characteristics when a core is thinned.

FIG. 7 is a diagram showing propagation radiation loss characteristics when the thickness of a coupling layer is changed.

FIG. 8 is a diagram showing propagation radiation loss characteristics when the refractive index and thickness of the coupling layer are close to the refractive index and thickness of the core.

FIG. 9 is a cross-sectional view of an example in which the thickness of the coupling layer is tapered in the waveguide direction.

FIG. 10 is a diagram illustrating propagation radiation loss characteristics when TM polarized light is targeted.

FIG. 11 is a cross-sectional view illustrating an example of an optical demultiplexing / TE / TM polarization individual detection circuit.

FIG. 12 is a cross-sectional view showing an example of a multi-wavelength optical demultiplexing / TE / TM polarization individual detection circuit.

FIG. 13 is a diagram illustrating an example of a propagation loss characteristic when the refractive index of the coupling layer is changed.

FIG. 14 is a diagram showing a configuration of an example of a conventional Mach-Zehnder interference type duplexer.

FIG. 15 is a diagram showing a configuration of an example of a conventional grating type duplexer.

FIG. 16 is a diagram showing a configuration and characteristics of an example of a conventional asymmetric directional coupler type duplexer.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Core 2 ... 1st cladding layer 3 ... 2nd cladding layer (coupling layer) 4 ... 3rd cladding layer 5 ... Substrate C ... Multi-layer cladding 7 ... Photodetector 11-13 ... Branching filter 14 ... TE polarization detector 15 TM polarization detector 16 TE polarization splitter 17 TM polarization splitter

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G02B 6/12-6/14 JICST file (JOIS)

Claims (6)

(57) [Claims] 1. A refractive index of at least a core layer, three or more clad layers, and a substrate in order from the surface of the waveguide, and the refractive index of a specific intermediate clad layer other than the outermost clad layer is compared with other clad layers. And relatively high to the bonding layer, and the refractive index of the substrate is higher than the core layer and the bonding layer,
By setting the propagation constants of the propagation mode of the core and the propagation mode of the coupling layer so as to match only at a specific wavelength, light of a specific wavelength in the light propagating through the core is separated and coupled to the coupling layer. A waveguide type optical demultiplexing circuit, wherein the separated light is emitted from the coupling layer to the substrate. 2. The waveguide type optical demultiplexing circuit according to claim 1, wherein the substrate is formed of a semiconductor, and a photodetector for detecting light emitted on the substrate is integrated. 3. The method according to claim 1, wherein at least one of the core layer, the coupling layer, and the cladding layer other than the coupling layer is formed of a variable refractive index material.
3. The waveguide type optical demultiplexing circuit according to claim 1, wherein intensity modulation or switching of propagation light is performed by changing a refractive index of the layer. 4. A refractive index of at least a core layer, three or more cladding layers, and a substrate in order from the surface of the waveguide, and a refractive index of an intermediate specific cladding layer other than the outermost cladding layer is compared with other cladding layers. And relatively high to the bonding layer, and the refractive index of the substrate is higher than the core layer and the bonding layer,
By setting the propagation constant of the propagation mode of the core and the propagation mode of the coupling layer to coincide only with the TE or TM polarized light of a specific wavelength, the TE or TM polarized light of a specific wavelength in the light propagating through the core is set. Wherein the separated light is coupled to the coupling layer, and the separated light is emitted from the coupling layer to the substrate. 5. The waveguide type optical demultiplexing circuit according to claim 4, wherein the substrate is formed of a semiconductor, and a photodetector for detecting light emitted on the substrate is integrated. 6. A plurality of waveguide-type optical demultiplexing circuits according to claim 4 or 5, wherein a plurality of waveguide type optical demultiplexing circuits are connected in tandem to perform both polarization separation and wavelength demultiplexing. A waveguide type optical demultiplexing circuit characterized by the above-mentioned.