JPS63221304A - Optical circuit - Google Patents

Abstract

PURPOSE:To obtain an optical circuit which functions as an optical demultiplexer by the formation of thin film only by providing a region for separating wavelength permitting the transmission of light having at least one wavelength selectively from light in a specified wavelength region. CONSTITUTION:A second clad layer 41 of a low refractive index is formed on a semiconductor substrate 40 comprising a material having a high refractive index, and a resonance reflecting clad is formed by forming a first clad layer 42 of a high refractive index on the clad layer 41. A core section 43 for propagating light beams is formed by covering a transparent dielectric layer having a lower refractive index than the refractive index of the layer 42 on the clad layer 42. The light to be transmitted is reflected totally at an interface between the core section 43 and an air layer. The light is reflected simultaneously by the interference in the resonance reflecting clad between the core section 43 and the semiconductor substrate 40, and propagated in the core section 43. The reflectance of the resonance reflecting clad to the light propagated through the core depends on the refractive index of the clad layers 42, 41, film thickness of the layers, refractive index of the core section 43, film thickness thereof, and the wavelength of light to be transmitted. By selecting above-described factors appropriately, the reflectance may be regulated to almost 1.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to an optical demultiplexer that separates a plurality of lights of different wavelengths propagating in the same optical waveguide, or This invention relates to an optical circuit that acts as an optical multiplexer for combining light.

(Prior art) The optical communication system currently in practical use is a unidirectional optical fiber that connects a light source that emits signal light and a photodetector that receives the signal light and converts it into an electrical signal. This is the simplest one-to-one format. However, this simple system is being further developed into network optical communication that connects one-to-many or multiple-to-multiple terminal devices. In this network optical communication, two-way communication between terminals is performed using only one optical fiber, and different types of signals (e.g., audio, image data, etc.) are transmitted using one optical fiber. transmits light of different wavelengths through one optical fiber,
A wavelength division multiplexing communication system is used in which two-way communication or one-way multiplexing communication is performed by separately placing signals on each wavelength. This wavelength multiplexing communication requires an optical multiplexer that combines lights of different wavelengths into one optical fiber, and conversely, an optical multiplexer that combines lights of different wavelengths into one optical fiber, and conversely, an optical multiplexer that separates lights of different wavelengths transmitted through one optical fiber into their respective wavelengths. A demultiplexer and an optical multiplexer/demultiplexer having both of these functions are used.

Conventionally, an optical circuit shown in FIG. 9 is known as an optical multiplexer/demultiplexer. The input terminal of the first optical fiber 1 that transmits light with a wavelength of 0.89 μm is connected to a focusing Rondo lens 2 and a prism 3.
It is then coupled to a glass block 5 via a dielectric multilayer filter 4 that transmits only light with a wavelength of 0.89μ and reflects light with other wavelengths.

Similarly, light is transmitted from the input terminal of the second optical fiber 6 through which light with a wavelength of 0.81 μm is transmitted to the glass block 5 via a focusing rod lens 7, a prism 8, and a filter 9 that transmits only light with a wavelength of 0.81 μm. Join.

Then, the intersection of the optical axis of the first optical fiber system and the optical axis of the second optical fiber system is made to coincide on the surface of the glass block 5, and the light having a wavelength of 0.89 μm is merged.

Furthermore, a third filter 11 that reflects only the steel light with a wavelength of 0.89 μm and the light with a wavelength of 0.81 μm is coupled to the glass block 5 via a prism 10, and the above-mentioned light is combined by this third filter 11. Light focusing rod lens 1
2 and is transmitted to the outside via the common terminal 13. Furthermore, light with a wavelength of 1.2 μm and light with a wavelength of 1.3 μm are input from the common terminal 13 and directed toward the third filter 11 . A mirror 14 that reflects both the light with a wavelength of 1.2 μm and the light with a wavelength of 1.3 μm is arranged behind the third filter 11, and the light with a wavelength of 1.2 μm and the light with a wavelength of 1.3 μm that have passed through the third filter 11 are The light is reflected toward the fourth filter 15. This fourth filter 15 has a characteristic of transmitting only light with a wavelength of 1.2 μI and reflecting light of other wavelengths.

Therefore, light with a wavelength of 1.2 μm passes through the fourth filter 15, passes through the converging Rondo lens 16, and is sent to the third optical fiber 17.
is output to.

On the other hand, the light having a wavelength of 1.3 μm is reflected by the fourth filter 15 and outputted to the fourth optical fiber 20 via the fifth filter 18 and the focusing rod lens 19.

Furthermore, an optical circuit shown in FIG. 9 is known as another optical multiplexer/demultiplexer. In this optical circuit, an optical fiber guide groove 31 and a filter guide groove 32 are formed on a silicon substrate 30, and an optical fiber 33 that acts as an input/output boat is installed in each fiber guide groove 31, and an optical fiber 33 that acts as an input/output boat is installed in each fiber guide groove 32. A dielectric multilayer filter 34 that transmits or reflects only light of a specific wavelength is inserted perpendicularly to the substrate to perform optical demultiplexing and optical multiplexing.

(Problems to be Solved by the Invention) The optical multiplexer/demultiplexer shown in FIG. 8 described above uses various optical elements such as microlenses, prisms, and filters.
The optical axes of these optical elements must be made to intersect or coincide with each other when mounted on the glass block, which requires high assembly precision and is not suitable for mass production. Furthermore, in the optical multiplexer/demultiplexer shown in Fig. 9, multilayer filters must be individually installed in minute grooves with a width of a few μm, which similarly requires troublesome assembly work and is poor in mass production. There were drawbacks.

Therefore, an object of the present invention is to eliminate the above-mentioned drawbacks, provide an optical circuit that can be manufactured without complicated assembly work, and can be integrally integrated including a photodetector etc. .

(Means for Solving the Problems) An optical circuit according to the present invention includes a core portion for propagating a light beam, and a first refractive index adjacent to the core portion and having a refractive index higher than that of the core portion.
It has a cladding layer and a second cladding layer adjacent to the first cladding layer and has a lower refractive index than the first cladding layer, and has strong reflection characteristics toward the core for light in one or more specific wavelength ranges. a resonant reflective cladding section having a resonant reflective cladding section; and a resonant reflective cladding section formed by partially changing the film thickness of at least one of the first cladding layer and the second cladding layer, and capable of transmitting only light in at least one wavelength range of the light in the specific wavelength range. It is characterized by having a wavelength separation region that transmits light.

(Function) FIG. 1 is a diagram showing an example of a resonant reflection type leading waveguide which is the basic configuration of the optical circuit according to the present invention. A second cladding layer 41 with a low refractive index is formed on a semiconductor substrate 40 made of a high refractive material, and a first cladding layer with a high refractive index is formed on the second cladding layer 41 to form a resonant reflective cladding. layer 4
2, a transparent dielectric layer having a refractive index lower than that of the first ground layer is deposited on the core portion 43 for propagating the light beam.
form.

The light to be transmitted is totally reflected at the interface between the core section 43 and the air layer, and is also reflected by the interference effect in the resonant reflective cladding between the core section 43 and the semiconductor substrate 4o, and propagates within the core section 43. The reflectance of the resonant reflective cladding for light propagating within the core is determined by the first and second cladding layers 42 and 4.
1 and its film thickness, the refractive index and film thickness of the core portion 43, and the wavelength of the light to be transmitted, and appropriately select the film thickness and refractive index of each cladding layer constituting the resonant reflective cladding. By doing so, it is possible to obtain a reflectance close to 1. The reflectivity of this resonant reflective cladding is close to 1, but slightly smaller than 1, so that an extremely small amount of light is transmitted through the resonant reflective cladding and radiated to the substrate. The light emitted to this substrate results in radiation loss, but if a resonant reflective cladding that satisfies the optimal film thickness condition is used, the reflectance becomes approximately 1 and the loss can be suppressed to several dB/am or less.

For a detailed explanation regarding this point, please refer to Japanese Patent Application No. 61-206698, which was previously proposed by the applicant.

On the other hand, the reflectance of this resonant reflective cladding changes depending on the film thickness of each cladding layer and the wavelength of propagated light, so by partially changing the film thickness of each cladding layer, it is possible to increase the reflectivity to transmit only a specific wavelength. A loss region (high transmittance region) can be partially formed. Therefore, if the film thickness of each layer of the resonant reflective cladding is partially changed to create a state of low reflectance (high transmittance) for a specific wavelength stone, only the light of wavelength λ1 will be transmitted toward the substrate in this region. is emitted. On the other hand, for light with a different wavelength λ2, this region does not have a low reflection condition (high loss condition), so this wavelength λ2
Most of the light is reflected by the resonant reflective cladding and propagates further into the waveguide. As described above, the optical circuit according to the present invention constitutes a resonant reflective cladding by laminating dielectric layers having a predetermined refractive index and thickness on a semiconductor substrate, and also controls the thickness of the cladding layer constituting this resonant reflective cladding. The function as an optical demultiplexer can be achieved by only changing the parts, thus eliminating the need for complex assembly work.Furthermore, the photodetector and related electronic circuit elements can be formed on the semiconductor substrate in advance. ,
By laminating a dielectric layer on this semiconductor substrate, it becomes possible to monolithically integrate an optical demultiplexer and a photodetector.

(Embodiment) FIG. 2 is a diagram showing the configuration of an example of an optical circuit according to the present invention. Two photodetectors 51a on the silicon substrate 50
, 51b and related electronic circuit elements (not shown) such as an amplifier are formed in advance. These photodetectors 51a.

51b and its associated electronic circuitry can be easily fabricated using known silicon planar technology.

Sin, which is a low refractive index material, is formed on the silicon substrate 50.

A second cladding layer 52 consisting of a layer (refractive index 1.45) is formed, and a first cladding layer 53 consisting of an rtoz layer (refractive index 2.3) which is a high refractive index material layer is formed on this second cladding layer 52. The second cladding layer 52 and the first cladding layer 53 constitute a resonant reflective cladding. Further, on the first cladding layer 53, 830. Core 54 consisting of layers
form. These first and second cladding layers 53 and 5
2 and the core 54 can be formed by sputtering, vapor phase growth, electron beam evaporation, or the like. As described above, the reflectance of the resonant reflective cladding for light propagating through the core 54 is determined by the thickness d of the first and second cladding layers 53 and 52, the refractive index nl+n2, and the thickness dc of the core 54.
Determined by and refractive index nC, maximum reflectance (minimum loss)
The thickness d of the first cladding layer and the thickness dt of the second cladding layer when dt is given are given by the following equation for the TE mode.

・(2N+1) ・・・
...(1) (M, N=0.1.2...) Here, λ represents the wavelength of the light to be propagated, and d ell
is the equivalent core film thickness considering the amount of electromagnetic field seeping out of the core. Since the electromagnetic field hardly seeps into the resonant reflective cladding under low-loss conditions (high reflectance), the refractive index of the cladding layer (air in this example) above the core is set to n.
When o is assumed, the equivalent film thickness dc1 of the core 54 can be approximately approximated by the following equation.

λ d cm + □ (3) 2π, 7.
．． 1 In this way, the propagation loss of the resonant reflective optical waveguide changes depending on the film thickness of the resonant reflective cladding consisting of two layers. next,
The radiation loss characteristics of the TE fundamental mode with respect to the film thickness d of the first cladding layer 53 were calculated and shown in FIG. In this calculation, the film thickness of the core 54 is 4μ steel and the film thickness of the second cladding layer 52 is 2μI. In the figure, the horizontal axis shows the film thickness d1 of the first cladding layer 53, and the vertical axis shows the loss ( The solid line shows the calculation result for light with a wavelength of 0.633μm, the broken line shows the calculation result for light with a wavelength of 0.85μm, and the dashed-dotted line shows the calculation result for light with a wavelength of 1.3μm. The calculation results are shown below. Considering light with a wavelength of 0.633 μm, dt' = 0.088 μm + 0.26
A high reflectance state (low loss Lit) occurs near the three film thickness conditions of 4 μm + 0.44 am, and a low radiation loss state with a loss of 1 dB/cm or less occurs near these film thicknesses.

Also, in the vicinity of d, = 0.176μ-, 0.352μm, there is a very large radiation loss state of several tens of dB/cm or more (
high transmittance state). On the other hand, d+−0,088 μm
, 0.264 μm, and 0.44 μm, the reflectance is almost high for light with wavelengths of 0.85 μm and 1.3 μm, so under these film thickness conditions, the three types of It becomes highly reflective for wavelength light, and can form a resonant reflective waveguide. Also d! -0,1
0.8 under the condition of 76μ, 0.352μ-
Since it is in a high reflectance state for wavelength light of 5 μ- and wavelength light of 1.3 μI, under this film thickness condition, the reflectance is 0.63
Only the 3μ-wavelength light is transmitted toward the substrate, and the other wavelengths are reflected by the interference reflective cladding and propagated within the core 54. Furthermore, when considering the wavelength light of 0.85μ steel, it becomes a high transmittance state near dl-0.23μl, while it has a high reflectance state for wavelength light of 0.633μm and 1.3μ-wavelength light. Become. Therefore ti+-0,
By setting it to 23 μl, only light with a wavelength of 0.85 μm is transmitted. Further, under the film thickness condition of d+=0.33 μl, the film exhibits high transmittance only for light with a wavelength of 1.3 μl, and high reflectance for light of other wavelengths. As a result, by setting dz=0.33 μm, only 1.3 μl of wavelength light is transmitted, and light of other wavelengths is reflected by the resonant reflective cladding and guided within the core. An optical demultiplexer that separates only light of a specific wavelength by partially changing the thickness of the first cladding layer and forming a wavelength separation region in the resonant reflective waveguide that transmits only light of a specific wavelength. It will function as The film thickness conditions of the first and second cladding layers that achieve this high transmittance state are given by the following equation.

......(4) (N, M=1.2.3...) The difference between equations (1), (2) and equations (4), (5) is that equation (1) ), (2) is an odd multiple of some least common divisor, formula (4)
, (5) is an even number multiple. Also, to create a high loss condition, both dl and d2 can be calculated using equations (4) and (5) at once.
There is no need to bring it close to the value of , it is sufficient to bring only one of them close. Since the first cladding layer is usually thinner and easier to control, the thickness of the first cladding layer is changed.

In this example, from the incident end of the core 54, the wavelength λ1 and the wavelength λ2 are
It is assumed that light of different wavelengths is input, and light of wavelengths of λ-fold and λ-multiplex are demultiplexed. The film thickness of the second cladding layer 53 is partially changed, and the first wavelength separation region 55a transmits only λ-fold wavelength light and reflects λ-fold wavelength light, and the λ-fold wavelength light is reflected and λ-fold wavelength light is reflected. A second wavelength separation region 55b that transmits only wavelength light is formed.

These first and second wavelength separation regions are formed at positions corresponding to the formation positions of photodetectors 51a and 51b.

These two wavelength separation regions 55. Based on the results shown in FIG. 3, the portions other than a and 55b are formed using the first and second cladding layers 53 so as to be in a high reflectance state (low loss state) for light with multiple wavelengths of λ and λ. and 52 film thickness conditions are set. Both the λ multiple wavelength light and the λ multiple wavelength light incident on the core 54 propagate through the core 54 while repeating reflection, and the λ multiple wavelength light is transmitted to the first and second cladding layers 53 in the first wavelength M SJf region 55a. and 52 respectively, and the substrate 5
The λ-multiplexed wavelength light passes through the first and second cladding layers 53 and 52, respectively, in the second wavelength separation region 55b, and enters the photodetector 51b.

Next, a specific example of the design will be explained.

A case will be considered in which light having a wavelength of 0.78 μm and light having a wavelength of 0.89 μm are separated. This time, the thickness of the first cladding layer is 0.13 μm, 0.24 μm steel, and 0.271 μm, respectively.
FIG. 4 shows the wavelength dependence of the TE fundamental mode loss when the fiber is closed at 1 m+. Here, the core thickness dc is 4am, the second
The thickness d2 of the cladding layer was 2 μm. In addition, the refractive index of each layer is 5in2 for the core and first cladding materials,
The material of the cladding layer is Ti1t, and the refractive index data of each material at each wavelength is published in the literature handbook.
I found it from Optical Constance of Solids (published by Academic Press, 1985) and used it. In the example of FIG. 4, d.

-0.13μ field part wavelength 0.78μ■ and 0.89μ
Both lights of the cabinet are guided, and light with a wavelength of 0.78 prn is emitted to the substrate at a portion of d = 0.24 μm, and d
A wavelength of 0.89 μl can be emitted to the substrate in the +=0.27 μm portion. At this time, the loss curve in Fig. 4 is steep on the short wavelength side and gentle on the long wavelength side, so for example, when cascading demultiplexers with wavelengths of 0.78 μm and 0.89 μm, the light with a wavelength of 0.78 μm is placed first. By splitting the wavelength into 0.89 μm, greater isolation can be achieved for light with a wavelength of 0.89 μm. As a design example, if there is a radiation loss of 2006B/cm in the demultiplexer for a wavelength of 0.78μm,
The isolation is 19d8 or more, and the coupling efficiency to the photodetector is about 90%.

Furthermore, in the loss characteristics shown in Fig. 3, the range of film thickness d+ in which high radiation loss (high transmittance) can be obtained is very narrow;
In order to obtain a loss of 06B/c11 or more, several people are required to control the film thickness with high precision. Therefore, in order to expand the allowable range of film thickness accuracy, anti-reflection coating layers 60a and 60b are provided between the second cladding layer 52 and the substrate 50, respectively, as shown in FIG. It becomes possible to expand the width.

FIG. 6 shows the characteristics when the anti-reflection coating layers 60a and 60b are provided. The horizontal axis shows the film thickness of the first cladding layer 53, and the vertical axis shows the radiation loss per unit length. The wavelength of the light used was 0.633 μm, the solid line shows the characteristics without the anti-reflection coating layer, the broken line shows the characteristics when the Tie layer is used as the anti-reflection coating layer, and the dashed line shows the characteristics without the anti-reflection coating layer. The characteristics are shown when C7059 glass with a refractive index of 1.52 is used as the reflective coating layer.

The materials and thicknesses of the first cladding layer, second cladding layer, and core are the same as those used in FIG. 3. As is clear from FIG. 6, anti-reflection coats 60a and 60b
By providing this between the substrate 50 and the second cladding layer 52, the allowable range of film thickness for obtaining high transmittance characteristics is expanded, and film thickness control in the manufacturing process becomes easier.

In the loss curves shown in Figures 3 and 4, the high reflectance characteristic part (low loss characteristic part) is flat over a wide range with respect to the film thickness of the second cladding layer or the propagation wavelength, resulting in high transmittance. A steeper section (high-loss characteristic section) is extremely advantageous in terms of design because it allows greater isolation when constructing a duplexer. In order to achieve this purpose, the resonant reflective cladding may be configured from one set of two layers, a second cladding layer with a low refractive index and a first cladding layer with a high refractive index, to two sets of four layers or a multilayer structure. .

FIG. 7 shows an example in which the optical circuit according to the present invention is applied to coherent optical communication. When the optical circuit according to the present invention having a demultiplexing light-receiving integrated circuit element is applied to coherent optical communication signals, the light from the local oscillator (light source) and the signal light must be mixed before entering the photodetector. For this purpose, the optical circuit shown in FIG. 7 is suitable. Three transmission lines 101, 102, and 103 are formed on the incident side of the substrate 100. Signal lights of wavelengths λ1' and λ2' whose wavelength ranges are separated from each other to some extent are made incident on the first transmission line 101 from an external transmission line such as an optical fiber. It is assumed that these lights of λ8' and λ2' are highly precisely wavelength-controlled, modulated, and have a slightly broadened spectrum. The wavelength λ close to the wavelength λ1' is frequency-controlled with high precision from the second transmission line 102.

incident light. First transmission line 101 and second transmission line 1
02 by the optical directional coupler 104, the light of λ1 and the light of λ1' + λ2' are mixed, and the light beams of λ,'+λ2'+λ are combined as the fourth and fifth beams as two light beams. Transmission line 1
05 and 106, respectively.

On the output sides of these fourth and fifth transmission lines 105 and 106, optical elements 107 and 108 are formed, respectively, which integrally form a wavelength separation region and a photodetector that transmit only light of λ and λ1', These optical elements 107 and 108 perform coherent detection of signal light of λ,'.

On the other hand, since the optical element 108 formed in the transmission path 106 reflects the signal light of λ2' toward the core by the resonant reflective cladding, the signal light of λ2' is further guided and travels. This transmission line 106 is connected to the third transmission line 1 by a directional optical coupler 109.
03, the light of λ2 and the signal light of λ2' are mixed into a light beam of λ2+λ2', which is transmitted to two photodetectors 110 and 111 via two transmission paths. Then, the two photodetectors 110 and 111 coherently detect the beat signal of the signal light of λ2. Generally, the signal lights λ1' and λ2' are λ.

The intensity is several tens of dB or less than the light of λ2, so 1
07, 108.110. The intermediate frequency beat signal coherently detected by the photodetectors 111 and 111 contains an extremely large DC component. Therefore, by using two photodetectors for each signal light, the DC component can be balanced and removed. If the optical circuit according to the present invention is applied to such optical communication, each transmission path and optical element can be integrally integrated on a silicon substrate.

The present invention is not limited to the embodiments described above, and various modifications are possible. For example, in the embodiment described above,
Although a silicon substrate was used as the substrate, a GaAs substrate or an I
An nP substrate or the like can also be used.

Furthermore, in the above-described embodiment, the light is reflected and guided by the air layer and the resonant reflective cladding, but it is of course also possible to form a resonant reflective cladding on both sides of the core and guide the light. It is.

(Effects of the Invention) As explained above, according to the present invention, a resonant reflective cladding is formed by sequentially depositing a low refractive index cladding layer and a high refractive index cladding layer on a substrate, and a low refractive index cladding layer is formed on the resonant reflective cladding king. It is possible to form a core part by depositing a refractive index transparent dielectric layer, and partially change the film thickness of one of the cladding layers to form a wavelength separation region that transmits only light of a specific wavelength toward the substrate. can.

As a result, it is possible to create an optical circuit that functions as an optical demultiplexer by simply forming a thin film, and no complicated assembly work is required except for the coupling part with the optical fiber.

Furthermore, if a semiconductor substrate such as silicon is used as the substrate,
The guide waveguide, optical demultiplexer, and photodetector can be monolithically integrated on the substrate.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the configuration of an example of a resonant reflective optical waveguide used in the present invention, FIG. 2 is a diagram showing the configuration of an example of an optical circuit according to the present invention, and FIG. 3 is a diagram showing the film thickness of the first cladding layer. FIG. 4 is a graph showing the relationship between propagation wavelength and radiation loss characteristics. FIG. 5 is a diagram showing the configuration of a modified example of the optical circuit according to the present invention. FIG. FIG. 7 is a graph showing the relationship between the film thickness of the first cladding layer and loss characteristics when a reflective coat is provided; FIG. 7 is a diagrammatic perspective view showing an example in which the optical circuit according to the present invention is applied to coherent optical communication; FIG. 9 and 9 are diagrams showing the configuration of a conventional optical demultiplexer and optical multiplexer. 40... Semiconductor substrate 4L 52... First cladding layer 42, 53... Second cladding layer 43, 54... Core part 50... Silicon substrate 51a, 51b... Photodetector 55a, 55b...Separation areas 60a, 60b...Non-reflective coating Figure 3
Figure 4 Figure 5 Figure 6

Claims (1)

[Claims] 1. A core portion through which a light beam propagates, a first cladding layer adjacent to the core portion and having a refractive index higher than the refractive index of the core portion, and a first cladding layer adjacent to the first cladding layer. a resonant reflective cladding portion having a second cladding layer having a refractive index lower than that of the first cladding layer or the second cladding layer and having a strong reflection characteristic toward the core for light in one or more specific wavelength ranges; An optical circuit characterized by having a wavelength separation region formed by partially changing the thickness of at least one of the cladding layers and transmitting only light in at least one wavelength range among the light in the specific wavelength range. . 2. The optical circuit according to claim 1, wherein a plurality of the first cladding layers and the second cladding layers are alternately stacked. 3. The optical circuit according to claim 1, characterized in that a plurality of wavelength separation regions are formed, each of which transmits a plurality of light beams having different wavelengths. 4. The optical circuit according to claim 1, wherein the second cladding layer, the first cladding layer, and the core portion are formed on a semiconductor substrate. 5. A silicon substrate is used as the semiconductor substrate, a photodetector is formed on the silicon substrate, and the wavelength separation region is formed at a position corresponding to the position of the photodetector. The optical circuit according to item 4. 6. The optical circuit according to claim 5, characterized in that an anti-reflection coating layer is formed between the wavelength separation region and the photodetector.