JP2856525B2 - Optical waveguide polarizer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to two types of polarized waves, TE (electric field is parallel to a substrate) and TM (magnetic field is parallel to a substrate), propagated in an optical waveguide. The TM mode relates to an optical waveguide polarizer that radiates to the substrate side and passes only the TE mode.

[Conventional technology]

In general, a semiconductor laser that is a light emitting element oscillates in a TE mode, and many other optical functional elements operate only with respect to polarization of either TE or TM. On the other hand, light that has propagated in a single-mode fiber is generally elliptically polarized light, and contains both TE and TM polarized light components. It is necessary to remove any polarized light or to separate the propagation paths of TE and TM polarized light. Therefore, polarizers and polarization mode splitters are important components of optical circuits. The polarizer is also used for forming an isolator.

By the way, it is expected that an optical circuit combining optical elements will become an optical integrated circuit that integrates elements organically, does not require optical axis adjustment, is stable, has high performance, is low cost, and is suitable for mass production in the future. However, at present, the polarizer which is one of the components has a very high performance as an individual component, but is not suitable for miniaturization, and it is difficult to monolithically integrate it into an optical integrated circuit.

In an optical integrated circuit, since various elements are coupled by an optical waveguide, a waveguide polarizer is indispensable as a polarizer for an optical integrated circuit. As a polarizer of this type, ninth
A structure in which a metal M is clad on the surface of an optical waveguide WG as shown in the figure as a clad directly or via a buffer layer is known as the simplest structure. As a reported example, the extinction ratio of a single mode glass slab waveguide prepared by K ⁺ ion exchange loaded with aluminum is 40-60 dB / c.
Although m can be obtained, the insertion loss in the TE mode becomes as large as 4 to 6 dB / cm. Further, as shown in FIG. 10, a polarizer loaded with an anisotropic optical crystal C such as calcite (CaCO ₂ ) as a clad has a refractive index of the anisotropic optical crystal C higher than that of the core for the TM mode. Since it is high and low for the TE mode, it results in transmission of the TE mode and leakage of the TM mode, and a high extinction ratio can be obtained with low insertion loss. However, it is required that the optical crystal be polished with high precision, the refractive index of the core is limited, and since it cannot be monolithically integrated, it is not suitable for mass production and is expensive.

Meanwhile, the resonant reflection type optical waveguide (hereinafter, referred to as ARROW) proposed by the present inventors is a single mode optical waveguide, and unlike the conventional optical waveguide, FIG. 8 shows the relationship between the cross section and the refractive index. As shown, between the high refractive index semiconductor substrate 3 and the low refractive index dielectric core 1, a first cladding layer 21 of a high refractive index layer and a second cladding layer of a low refractive index layer (the same as the core 1). The material has a structure in which the resonant reflection cladding 2 of a two-layer set of materials having a refractive index of 22 is narrow, has a strong polarization dependence in the basic structure, and has a property of guiding only the TE mode with low loss. is there. Looking at the loss characteristics with respect to the thickness of the first clad layer, the thickness of the first clad layer is reduced to around 0.1 μm as shown in FIG. 5 by the dashed line for the TE mode and the dashed line for the TM mode. If set, extinction ratio 50dB per cm at 1.3μm wavelength,
An insertion loss of 0.3 dB is obtained. In FIG. 5, λ is the wavelength used, n _c is the refractive index of the core, d _c is the thickness of the core, n ₁ is the refractive index of the first cladding layer, and d ₁ is the first cladding layer. Is the thickness of the layer, d ₂ is the thickness of the second cladding layer, and Δα is T
It represents the difference in loss per unit length between M mode and TE mode. Hereinafter, the same applies.

[Problems to be solved by the invention]

However, in order to obtain such characteristics in ARROW, it is necessary to reduce the thickness of the core to 4 μm. Therefore, when light is input from an optical fiber having a core diameter of usually about 8 μm, light from the optical fiber is The spot size must be reduced by conversion. Conversely, if the thickness of the core is made as thick as that of the optical fiber, the loss in the TM mode decreases (the loss in the TE mode also decreases).
It cannot be used as a polarizer.

The present invention has been made in view of such a situation, and its object is to provide a high extinction ratio, a low insertion loss, and easy manufacture with respect to the above-described conventional polarization element using ARROW. An object of the present invention is to provide an optical waveguide polarizer of a TE mode transmission type and a TM mode leakage type which is optimal for an optical integrated circuit.

[Means for solving the problem]

Hereinafter, the configuration of the optical waveguide polarizer of the present invention will be described with reference to the drawings.

FIG. 1 shows the basic structure of the optical waveguide polarizer 30 of the present invention.

Since the premise of the optical waveguide polarizer 30 is ARROW (resonant reflection optical waveguide) shown in FIG. 8, this optical waveguide will be described first. As described above, this single mode optical waveguide is different from a conventional optical waveguide in that a high refractive index semiconductor substrate (GaAs, InP, Si, etc.) 3 and a low refractive index dielectric core (SIO ₂ ) 1 are used. In between, a set of _two layers of a first cladding layer (TiO ₂ , poly-Si, etc.) 21 of a high refractive index layer and a second cladding layer (material having the same refractive index as the core 1) 22 of a low refractive index layer The resonance reflection cladding 2 has a narrow structure, and confine light to the core 1 by an extremely high reflectance due to the resonance reflection of the resonance reflection cladding 2. The reflectance of the resonant reflective cladding 2 depends on the thickness, refractive index, and wavelength of each layer of the vibrating reflective cladding 2. When these satisfy the optimum conditions, the reflectance due to the resonance reflection is extremely close to 1, so that a low loss of 1 dB / cm or less can be achieved. Conversely, under the condition where the reflectance is low, most of the light passes through the resonant reflection cladding 2 and is emitted to the substrate 3, resulting in a very high loss. FIG. 3 shows the radiation loss of the TE mode with respect to the thickness d ₁ of the first cladding layer 21 at the wavelength λ = 0.633 μm.
It largely depends on the thickness d ₁ and the refractive index n ₁ . Also, TM
The mode has polarization dependence in which the loss is larger (fifth mode
One-dot chain line and two-dot chain line in the figure). The loss takes the maximum value when the value of d ₁ is almost the same in both the TE mode and the TM mode. However, as shown in FIG. 3, as the refractive index of the first cladding layer 21 increases, the maximum point moves to the left. .

By the way, instead of forming the first cladding layer by a single layer 21 as shown in FIG. 8, as shown in FIG.
And the low-refractive-index layer L are alternately layered, the TM-mode equivalent refractive index of the first cladding layer 21 'is smaller than the TE-mode equivalent refractive index. . As a result, the first cladding layer 2 where the loss in the TM mode is maximized
The thickness d _{1 of} 1 ′ becomes relatively thick. Therefore, if the thickness of the first cladding layer is set to a thickness at which the loss of the TM mode is maximized, the TE mode maintains a low loss, and the TE mode transmission,
An optical waveguide polarizer with TM mode leakage can be configured. FIG. 4 shows that at a wavelength of 1.3 μm, the first cladding layer 21 is a single layer (n ₁ = 3.5),
FIG. 4 shows the loss characteristics of each mode of the polarizer of the present invention in which 1 ′ is composed of three layers (n = 3.5 / 1.46 / 3.5). In the case of three layers, the total thickness is determined by the thickness d _{1 of} the first cladding layer 24 ′.
And In the case of a single layer, the thickness d ₁ of the first cladding layer 21 at which the loss of TE and TM is maximized is equal, but the maximum point of the TM mode is relatively right even if the number of layers is relatively small as three layers. Deviate.
Therefore, in the vicinity of d ₁ = 0.3 μm, the waveguide loss is 30 dB in the TM mode.
A high-performance optical waveguide polarizer with a waveguide loss of 0.01 dB / cm or less in the TE mode or more and a TE mode of 0.01 dB / cm or less can be constructed.

As for the manufacture of the present device, the first optical waveguide type polarizer can be easily manufactured because the first cladding layer of ARROW which is easy to manufacture is constituted only by three layers.

Further, as shown in FIG. 2, the optical waveguide polarizer 30 according to the present invention comprises a conventional ARROW or ARROW-B (the first cladding layer has a lower refractive index than the core, and the second cladding layer is almost the same as the core). It can be easily integrated with 40 and 41 having the same refractive index, and the loss at the input and output ends of the optical waveguide polarizer 30 of the present invention can be made almost zero. Furthermore, the second
As shown in the figure, since all the TM mode light is radiated to the substrate 3, the photodetector 4 is integrally formed on the semiconductor substrate 3, and the TM mode light radiated to the substrate 3 is converted by the photodetector 4. By performing detection, a polarization separation / light detection integrated circuit for TM mode detection in which the polarizer 30 and the photodetector 4 are integrated can be configured. As a result, the TM and TE modes can be separately detected, so that the element configured as described above can be widely applied to a polarization diversity receiver for coherent communication and the like.

[Action]

According to the optical waveguide device of the present invention, since the first cladding layer is composed of a multilayer thin film in which the high refractive index layers and the low refractive index layers are alternately stacked, the equivalent refraction of the first cladding layer in the TE mode is achieved. Since the equivalent refractive index of the TM mode is smaller than the refractive index and the thickness of the first cladding layer at which the loss of the TM mode is maximized is relatively large, the thickness of the first cladding layer is set to TM. If the thickness is set so that the loss of the mode is maximized, the TE mode maintains a low loss, and the TE mode is transmitted and the TM mode is leaked.
Therefore, light containing two polarizations of TE and TM is separated,
The TM mode radiates to the substrate with very high efficiency, and allows only the TE mode to pass with low loss.

〔Example〕

Next, examples of the optical waveguide polarizer of the present invention will be described.

As described above, the basic configuration of this optical waveguide polarizer is such that the first cladding layer 21 of the resonant reflection type optical waveguide (ARROW) shown in FIG. It is composed of a multilayer thin film 21 'in which a refractive index layer (refractive index _nH ) H and a low refractive index layer (refractive index _nL ) L are alternately stacked. As the difference between n _L and n _H is larger, the equivalent refractive index difference between the TE mode and the TM mode is larger, and the difference between the maximum loss points of the TE mode and the TM mode is larger. TiO ₂ (for short wavelength band) and poly-Si (for long wavelength band) may be used for the high refractive index layer H, and SiO ₂ or the like may be used for the low refractive index layer L. FIG.
FIG. 7 shows radiation losses of fundamental modes of TE mode (broken line) and TM mode (solid line) in the polarizer of the present invention;
At a wavelength λ = 1.3 μm for optical communication, the core thickness is d _c = 8.2 μm, which is almost the same as the core diameter of the optical fiber, and the first clad multilayer film 21 ′ is made of Si (n _H = 3.5) and SiO ₂ (n _L = 1.46) 3 layers of a combination of _{(n H / n L / n} H, assumes what thickness d ₁₎ of the total. The greater the number of layers of the multilayer film 21 ', the greater the difference between the maximum loss points in each mode, but a sufficient difference occurs with only three layers. Therefore, by setting the thickness of the first cladding layer 21 'to around d ₁ = 0.3 μm, a high-performance polarizer having a loss in the TM mode of 30 dB / cm or more and a TE mode of 0.01 dB / cm or less is formed. it can. However, since the allowable range of the error of the film thickness is narrow, it is necessary to control the film thickness with high accuracy of about 5%. In addition, the loss of TE _2nd higher order mode TE ₂ mode is 1dB
/ cm or less, and is no longer a substantially single mode.

When the high-order mode becomes a problem, or when it is necessary to obtain a higher extinction ratio may be performed using a relatively thinned same structure core thickness d _c. However, when connecting to an optical fiber, the spot diameter needs to be reduced. With the same structure as above, the core thickness is d _c = 4 μm
And a single layer (AR
FIG. 5 shows the loss characteristics in the case of (basic structure of ROW). Even with a single layer, the loss difference between each mode is 50dB / cm
This is a conventional ARROW type polarizer obtained as described above. If the first cladding layer is formed into a multilayer according to the present invention, d ₁ = 0.2
High performance with loss difference of 100 dB / cm or more and TE mode insertion loss of 0.8 dB / cm or less in a wide range of 5 to 0.36 μm, wide tolerance of film thickness, and higher TE mode of 10 dB / cm or more. , Which is essentially a single mode. Further, by controlling the film thickness with high precision, a very high-performance polarizer of 300 dB / cm or more can be realized. If the core thickness is further reduced, an extinction ratio with a higher extinction ratio can be obtained while keeping the insertion loss small to some extent.

Next, if the optical waveguide polarizer having the structure of the present invention is used,
A high-performance polarizer can be constructed even for light in a short wavelength band.
FIG. 6 shows TiO ₂ (n _H = 2.3) and SiO ₂ (n _L = 1.46)
This is a loss characteristic at a wavelength λ = 0.85 μm in a structure in which layers (n _H / n _L / n _H ) are stacked to form a first cladding 21 ′. The larger the value of n _H, the better, but since there is absorption in the short wavelength band, high refractive index Si (n = 3.5) is not used. In the case of FIG. 6,
Although the first cladding layer 21 'is composed of only three layers, the maximum thickness of the TM mode is shifted to a sufficiently large one, and d ₁ = 0.3 to 0.32 μm.
At m, a loss difference between modes of 30 dB / cm or more is obtained. Furthermore, reducing the core thickness d _c in the same manner as described above in 3.0 [mu] m, as shown in FIG. 7, it is a single layer loss difference 30 dB / cm
However, in the case of a multilayer, d ₁ = 0.28 to 0.35 μm
A loss difference of 100 dB / cm or more can be realized in the vicinity, and the higher-order TE mode becomes 20 dB / cm or more, and a substantially single mode can be maintained.

The basic structure and some embodiments of the optical waveguide polarizer of the present invention have been described above, but the present invention is not limited to these, and various modifications are possible. For example, it is also possible to provide a similar set of two layers of cladding above and below a slab-shaped core, in which the upper and lower cladding layers having a high refractive index, which are in contact with the core, can be multilayered according to the present invention. In addition, the present invention can be applied to a snap index type optical fiber having a circular cross section to be configured for mode separation. Further, the same effect can be obtained by using a birefringent thin film instead of the first clad layer having a multilayer structure.

〔The invention's effect〕

According to the optical waveguide device of the present invention, since the first cladding layer is composed of a multilayer thin film in which the high refractive index layers and the low refractive index layers are alternately stacked, the equivalent refraction of the first cladding layer in the TE mode is achieved. Since the equivalent refractive index of the TM mode is smaller than the refractive index and the thickness of the first cladding layer at which the loss of the TM mode is maximized is relatively large, the thickness of the first cladding layer is set to TM. If the thickness is set so that the loss of the mode is maximized, the TE mode maintains a low loss, and the TE mode is transmitted and the TM mode is leaked.
Therefore, light containing two polarizations of TE and TM is separated,
The TM mode radiates to the substrate with very high efficiency, and allows only the TE mode to pass with low loss.

Therefore, the polarizer function conventionally obtained by individual elements such as an optical crystal prism and a polarizing beam splitter can be realized by an optical integrated circuit according to the present invention. This improves the performance of the integrated optical isolator and the like. Further, by integrating a photodetector manufactured on a semiconductor substrate and the polarizer of the present invention, the present invention can be applied to a polarization separation / photodetection integrated circuit for TM mode detection and the like.

[Brief description of the drawings]

FIG. 1 is a diagram showing a basic structure of an optical waveguide polarizer of the present invention, FIG. 2 is a diagram showing a configuration of an example of an optical integrated circuit incorporating an optical waveguide polarizer according to the present invention, and FIG. FIG. 4 is a diagram showing the loss characteristics of the TE mode with respect to the thickness of the first clad layer of the resonant reflection type optical waveguide on the premise of the present invention. FIG. 4 is a diagram showing one embodiment of the optical waveguide type polarizer of the present invention and the resonant reflection type. FIG. 5 is a diagram showing a loss characteristic with respect to the thickness of the first clad layer of the optical waveguide;
FIG. 4 is a diagram showing the loss characteristic with respect to the thickness of the first clad layer when the core thickness is reduced in the case of FIG. 4, and FIG. 6 is a diagram showing an embodiment of the optical waveguide polarizer of the present invention for a short wavelength band. FIG. 7 is a diagram showing the loss characteristics with respect to the thickness of the first clad layer of the resonant reflection type optical waveguide, and FIG. 7 is a diagram showing the loss characteristics with respect to the thickness of the first clad layer when the core thickness is reduced in the case of FIG. FIG. 8 is a diagram showing the relationship between the cross section of the resonant reflection type optical waveguide and the refractive index, and FIGS. 9 and 10 are diagrams showing the configuration of a conventional optical waveguide type polarizer. 1 ... core, 2 ... resonant reflection cladding, 3 ... substrate, 4
... Photodetector, 21, 21 'first cladding layer, 22 second cladding layer, 30 optical waveguide polarizer, 40, 41 resonance reflection optical waveguide, H high refraction Index layer, L: low refractive index layer

Claims (5)

(57) [Claims] A substrate, a second cladding layer having a relatively low refractive index,
In a resonant reflection type optical waveguide in which a first clad layer having a relatively high refractive index and a core layer having substantially the same refractive index as the second clad layer are laminated in order, the first clad layer is composed of a high refractive index layer and a low refractive index layer. An optical waveguide comprising a multi-layered film in which are alternately stacked, and radiating the TM polarized light out of the incident TE polarized light and the TM polarized light to the substrate and guiding the TE polarized light. Type polarizer. 2. The optical waveguide polarizer according to claim 1, wherein the substrate is made of a semiconductor. 3. The optical waveguide polarizer according to claim 1, wherein the first cladding layer comprises three layers of a high refractive index layer, a low refractive index layer, and a high refractive index layer. 4. The first cladding layer according to claim 1, wherein a birefringent layer is used instead of a multilayer film in which high refractive index layers and low refractive index layers are alternately stacked. An optical waveguide polarizer according to any one of the preceding claims. 5. The optical waveguide polarizer according to claim 1, wherein a photodetector is provided integrally with the substrate so as to receive the TM polarized light radiated on the substrate. .