JP3462014B2 - Asymmetric directional coupler wavelength filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [0001] The present invention relates to an asymmetric directional connection.
The present invention relates to a combiner type wavelength filter. Learn more
This invention is useful as an optical waveguide type wavelength filter.
Concerning a new asymmetric directional coupler type wavelength filter
It is. [0002] 2. Description of the Related Art Optical communication using optical fibers
Low transmission loss and large transmission bandwidth
As it always has excellent features, it is
As a result, its practical use is rapidly progressing. Currently this
In optical communications using optical fibers such as
Light source and light that receives this optical signal and converts it into an electrical signal
One-way, one-to-one connection between a detector and a single optical fiber
Communication, that is, one-way communication between one-to-one terminal devices
Has been put to practical use. However, such one-way one-to-one communication
In telecommunications, the amount of information that can be transmitted is very limited.
We do not use fiber efficiently. So, even bigger
One-to-many or many-to-many for capacity communication and high functionality
Network optical communication linking various terminal devices
It has been issued and is being put to practical use. This network Hikari
In communication, for example, only one optical fiber
For two-way communication between terminals, or for different types of
Multiple signals such as audio, video, data, etc.
In the case of transmission using only one optical fiber, the wavelength
Are transmitted through a single optical fiber.
By assigning each signal to the wavelength separately,
The wavelength multiplexing communication method that performs directional multiplexing communication is adopted.
You. Also, in one-way one-to-one communication, the transmission capacity is skipped.
This method of wavelength division multiplexing is used for
I'm trying to be. [0004] Generally, such wavelength division multiplexing communication includes a wavelength.
For combining different light beams into one optical fiber
Wavelength, or conversely, the difference in wavelength transmitted over one optical fiber.
Optical demultiplexing to separate different light into different wavelengths
Optical multiplexer / demultiplexer with both functions
A long filter is used. Conventionally, this wavelength filter
For example, multilayer filters and diffraction gratings
Individual elements such as chlorens and prisms are combined
Individual element assembly type, or formed perpendicular to the optical waveguide
A multi-layer filter is inserted into a micro groove with a width of several tens of μm.
Hybrid integrated type. However, individual
Element-assembled wavelength filters require fine optical axis alignment.
It is very difficult to manufacture
In addition, the hybrid integrated wavelength filter has
The process of inserting a multilayer filter into a micro-micron groove is extremely
There was a problem that it was difficult. Therefore, such a conventional wavelength filter has
In order to solve the problem, it was formed on a single substrate
Wavelength filters constructed using optical waveguides have been developed
I have. As an optical waveguide type wavelength filter, for example,
For example, an asymmetric directional coupler type wavelength filter as illustrated in FIG.
There is a filter. This asymmetric directional coupler type wavelength filter
Is the refractive index difference Δ between the core and the clad, the width w of the core and
A first optical waveguide (1) and a second light having different thicknesses t from each other;
It has a structure combined with the waveguide (2). This
With respect to the wavelength λ, as shown in FIG.
Dispersion characteristics, which are dependencies of the propagation constant β, are different from each other,
A certain center wavelength λ₀Only the first optical waveguide (1) and the second optical waveguide
Propagation constant β of each wave path (2)₁And β_TwoWho matches
It constitutes a directional coupler. [0006] In a directional coupler, in general, an eigenstate electric power is applied.
The magnetic field has two peaks in the same direction as the transverse electromagnetic field distribution.
Even mode (propagation constant β_e) And two in the opposite direction
Odd mode forming a peak (propagation constant β_o)
Therefore, the following equation [0007] (Equation 1) [0008] The coupling length of the even mode and the odd mode given by
L₀And the first optical waveguide (1) and the second optical waveguide (2)
Designed so that the length L of the directional coupling part (3) is equal.
And thus the center wavelength λ₀Only the optical signal of
The light is transferred to another optical waveguide different from the optical waveguide and emitted. Toes
For example, as shown in FIG.
λ₀, Λ₁, Λ_Two... λ_NOptical signal is the first optical waveguide
When the light is input from the input port (4) of (1), the directional connection
In the joint (3), the center wavelength λ₀Only the optical signal is demultiplexed
And is transferred to the second optical waveguide (2).
The light exits from the exit port (7) of (2) and has a center wavelength λ.₀
Wavelength λ other than₁, Λ_Two... λ_NOptical signal
The light is emitted from the emission port (6) of one optical waveguide (1).
In the multiplexing operation, for example, the second optical waveguide (2)
Wavelength λ from input port (5)₀Optical signal
Then, this wavelength λ₀Is transmitted to the directional coupler (3).
, The wavelength λ from the second optical waveguide (2).₁, Λ_Two...
λ_NMultiplexed into the first optical waveguide (1) through which the optical signal of
And exits from the exit port (6) of the first optical waveguide (1).
Fired. Such multiplexing / demultiplexing operation is performed by ADD / DR
A wave that is called an OP operation and performs the ADD / DROP operation
The long filter is called ADD / DROP type wavelength filter
ing. This ADD / DROP type wavelength filter is a lightwave
Waves that control the destination of optical signals by associating addresses with lengths
It is an essential element for long multiple access networks. Toko
In general, the polarization of light incident on an optical waveguide is
TE polarized light in which the vector vibrates in a plane parallel to the substrate and the substrate
And TM polarized light that oscillates in a plane perpendicular to. However,
In optical communication, an optical signal is being transmitted through an optical fiber that is a transmission path.
Photoelastic effect to receive external forces from various directions
Is elliptically polarized light, and this elliptically polarized light is
From the asymmetric directional coupler type wavelength filter.
Incident on an optical waveguide type wavelength filter such as
And TM polarized light. Therefore, an asymmetric directional coupler type wavelength filter
To function as a wavelength filter as described above.
Is two simultaneous for both TE and TM polarizations
Propagation constant of optical waveguide is center wavelength λ₀Matches only in
Must be designed and manufactured. But
Conventionally, an asymmetric directional coupler type wavelength filter has
Each of the two optical waveguides that compose it is TE polarized light and
Mode birefringence with different propagation constant for TM polarized light
For both TE polarized light and TM polarized light
The propagation constant of the two optical waveguides to a certain center wavelength λ₀smell
It is very difficult to design
Was. The difficulty of such a coincident design is, in particular, the core and
The difference in refractive index between the two
This is largely attributable to the mode birefringence of the optical waveguide. FIGS. 3A and 3B are each illustrated in FIG.
Of the first asymmetric directional coupler type wavelength filter
The core of each of the optical waveguide (1) and the second optical waveguide (2)
FIG. 2 illustrates a conceptual diagram of a clad. This figure 3
The first optical waveguide (1) illustrated in FIG.
(8) the refractive index is n_Two, The refractive index of the core (9) is n₁And
And the thickness of the core (9) is t₁And the refractive index of the core (9)
n₁Is the refractive index n of the cladding (8)_TwoGreater than
You. Further, the second optical waveguide (2) illustrated in FIG.
Means that the refractive index of the cladding (8) is n_Four, Core (9) refraction
Rate is n_ThreeAnd the thickness of the core (9) is t_TwoAnd the core
(9) refractive index n_ThreeIs the refractive index n of the cladding (8)_FourThan
Is also large. Furthermore, both optical waveguides have infinite width.
It has a flat plate structure. Also, a core in the first optical waveguide (1)
Refractive index difference Δ between (9) and cladding (8)₁And the second light guide
The bending between the core (9) and the cladding (8) in the waveguide (2)
Fold difference Δ_TwoAnd [0013] (Equation 2) Is defined by Here, the first optical waveguide
Refractive index n of cladding (8) in (1)_Two1.44
7, the refractive index n of the core (9)₁1.593, core (9)
Thickness t₁Is set to 0.728 μm. FIG. 4 shows the state at this time.
Equivalent refractive index n for the wavelength λ of the first optical waveguide (1)_eqTo
This is an example. This equivalent refractive index n_eqIs the propagation constant
β is the propagation constant k of a plane wave in vacuum₀Is given by [0015] (Equation 3) This is a value standardized as shown in FIG. Also this
In FIG. 4, the solid line represents the equivalent refractive index for TE polarized light,
The dotted line indicates the equivalent refractive index for TM polarized light. this
Between the core (9) and the clad (8) of the first optical waveguide (1)
Refractive index difference Δ₁Is about 21% of light such as general optical fiber
Since it is very large compared to a waveguide, its dispersion characteristics tend to decrease.
Is very large as illustrated in FIG. Thus, the dispersion characteristics of the first optical waveguide (1)
If the inclination of the second light guide is very large,
The slope of the dispersion characteristic of the wave path (2) should be small.
For example, the refractive index difference between the core (9) and the clad (8)
Δ_TwoIs about 0.3 to 2 which is the refractive index difference of a general optical waveguide.
%, A narrow band asymmetric directional coupler type wave
Long filters can be configured. However, as described above, the refraction between the core and the clad is
An optical waveguide with a small index difference has a refractive index difference between the core and cladding.
Requires high-precision thin film formation technology to precisely control
Is very difficult to form.
there were. Optical waveguide that does not require high-precision refractive index control
ARROW type optical waveguide has already been developed
You. This ARROW type optical waveguide is located between the core and the substrate.
In addition, the refractive index is much higher than the core
The first layer, which always has a small refractive index, and the same
Interference reflection of two layers consisting of a second layer having the same refractive index
It has a structure in which the film is sandwiched.
Therefore, light is strongly confined in the core. This AR
The ROW type optical waveguide is a leaky waveguide with radiation to the substrate.
Although it is a kind, the radiation loss to the substrate in the fundamental mode is actually
Radiation loss of high-order mode, small enough for practical use
Is large enough to realize quasi-single mode
And the slope of the dispersion characteristics is small.
Has features. Therefore, the slope of the dispersion characteristic is small.
Used as the second optical waveguide, the first optical waveguide having a large dispersion characteristic.
Asymmetric directional coupler type wave by combining with waveguide
Long filters can be configured. However, this ARROW type optical waveguide
For example, the slope of the dispersion characteristic illustrated in FIG.
Asymmetric directionality by combining with the first optical waveguide (1)
When a coupler type wavelength filter is configured, it is illustrated in FIG.
As described above, the mode birefringence of the ARROW type optical waveguide is small.
ARROW type optical waveguide as the second optical waveguide
Wavelength λ of TE and the first optical waveguide (1) for TE polarized light
₁And λ for TM polarization_TwoIs very different
The same center wavelength λ₀To make the propagation constants match
And it was very difficult. In FIG. 5, ARRO
The W-type optical waveguide has a refractive index n of the core._c1.53, the core
Thickness t_cIs set to 5.20 μm, and the first optical waveguide
Folding ratio n₁Is 1.593 and the refractive index n of the cladding is_TwoTo 1.4
47, core thickness t₁Is set to 0.740 μm. Accordingly, the present invention solves the above problems.
It was made to solve this problem
It can be easily manufactured without the need for
Both TE polarized light and TM polarized light of each optical waveguide constituting the filter
New propagation constants for polarized light at the same wavelength
To provide a new asymmetric directional coupler type wavelength filter
It is an object. [0021] SUMMARY OF THE INVENTION The present invention has been made to solve the above problems.
As a solution to the problem, the refractive index higher than the refractive index of the core layer
A first layer having a modulus and a thickness less than the thickness of the core layer.
Has a refractive index lower than the refractive index and is thinner than the core layer thickness
The intermediate layer sandwiched by the second layer is inserted into the core layer
And the dispersion characteristics of the optical waveguide
Optical waveguides having scattering characteristics.
The propagation constant for both TE and TM polarized light in the waveguide is
Asymmetry characterized by coincidence at the same wavelength
A directional coupler wavelength filter is provided. [0022] BRIEF DESCRIPTION OF THE DRAWINGS FIG.
Examples are provided to further explain the embodiments of the present invention.
explain. [0023] 6 (a) and 6 (b) each show a non-pair of the present invention.
Optical waveguide constituting a directional coupler type wavelength filter
And a second optical waveguide. In FIG. 6 (a)
The illustrated first optical waveguide has a refractive index n of the cladding (8)._Two
Is 1.447, the refractive index n of the core (9)₁Is 1.593
Yes, its thickness t₁Is 0.728 μm. FIG.
The second optical waveguide illustrated in FIG.
Folding ratio n_TwoIs 1.447, the refractive index n of the core (9)_FourIs 1.
53, refraction of the first layer (10) constituting the intermediate layer (12)
Rate n_ThreeIs 2.3, the refractive index n of the second layer (11)_TwoIs 1.4
47, thickness t of the intermediate layer (12)_IIs 0.78 μm, core
Thickness of the entire layer t_CIs 5.2 μm, refraction of the first cladding layer
Rate n_TwoIs 1.447 with thickness t_TwoIs 0.913 μm, the second
Refractive index n of cladding layer_FourIs 1.53 and the thickness t_ThreeIs 2.6
μm. The optical waveguide illustrated in FIG.
A) having a refractive index higher than that of (9), and
The first layer (10), which is thinner than the thickness of (9), forms the core (9).
And has a lower refractive index than the thickness of the core (9).
In the middle between two thin second layers (11)
An interlayer (12) is inserted into the core (9). This light
In the waveguide, the thickness t of the intermediate layer (12)_IAnd fix
Thickness t of the first layer (10)_HWhen only changing, etc.
The change in the valence refractive index is as illustrated in FIG. This figure
7, the thickness t of the first layer (10)_HIncrease
The equivalent refractive index for TE polarized light and TM polarized light
The difference from the equivalent refractive index for
The equivalent refractive index becomes very large. The equivalent bending of the first optical waveguide shown in FIG.
The refractive index is the equivalent refractive index for TE polarized light as illustrated in FIG.
The refractive index is larger than the equivalent refractive index for TM polarized light.
Therefore, the equivalent refractive index of the first optical waveguide shown in FIG.
6 (b), the equivalent refractive index of the second optical waveguide is TE polarized light and T
Match at the same wavelength for both M polarized light
Next, the second layer constituting the intermediate layer (12) in the second optical waveguide will be described.
Thickness t of one layer (10) _HCan be designed. FIG. 8 shows the first optical waveguide of FIG.
(B) of the present invention comprising the second optical waveguide.
For wavelength in symmetric directional coupler wavelength filters
This is an example of an equivalent refractive index, that is, dispersion characteristics. However
Then, an intermediate layer (12) is formed in the second optical waveguide.
Thickness t of the first layer (10)_HAnd thickness of the intermediate layer (12)
t_IAre 0.04638 μm and 0.72, respectively.
8 μm, and the thickness of each of the other layers is shown in FIG.
In (b), the thickness of each layer is the same. this
As is clear from FIG. 8, the first optical waveguide and the second optical waveguide
Equivalent refractive index for both TE polarized light and TM polarized light
The propagation constant is the wavelength λ₀Match only in
I understand. That is, it is inserted into the core (9) of the second optical waveguide.
Of the first layer (10) constituting the intermediate layer (12)
By adjusting the thickness, TE polarized light and TM polarized light can be
Asymmetric directional coupling that can multiplex and demultiplex at one wavelength
The combiner wavelength filter can be easily manufactured. Mochi
Of course, the present invention is not limited to the above examples,
It goes without saying that various aspects are possible for details.
Nor. [0028] As described in detail above, according to the present invention,
Easy to manufacture without the need for precise refractive index control
Can be emitted from the optical fiber that is the transmission path
Optical signals of different wavelengths that are elliptically polarized light can be accurately detected.
A new asymmetric directional connection capable of multiplexing and demultiplexing.
A combiner-type wavelength filter is provided.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a conceptual diagram showing an example of a non-directional coupler type wavelength filter. FIG. 2 is a diagram illustrating the dispersion characteristics of two optical waveguides in the non-directional coupler type wavelength filter illustrated in FIG. 1; FIGS. 3A and 3B are conceptual diagrams respectively illustrating the cores and claddings of a first optical waveguide and a second optical waveguide in the asymmetric directional coupler type wavelength filter illustrated in FIG. 1; . FIG. 4 is a diagram illustrating an example of an equivalent refractive index n _eq with respect to a wavelength λ of the first optical waveguide illustrated in FIG. FIG. 5 exemplifies an equivalent refractive index n _eq with respect to a wavelength λ of each optical waveguide in an asymmetric directional coupler type wavelength filter composed of an ARROW type optical waveguide and a first optical waveguide illustrated in FIG. FIG. FIGS. 6A and 6B are conceptual views respectively illustrating a first optical waveguide and a second optical waveguide constituting an asymmetric directional coupler type wavelength filter according to the present invention. FIG. 7 is a diagram illustrating a dispersion characteristic that is an equivalent refractive index n _eq with respect to a wavelength λ of the second optical waveguide illustrated in FIG. 6B; FIG. 8 shows the first optical waveguide illustrated in FIG.
FIG. 9 is a diagram exemplifying a dispersion characteristic that is an equivalent refractive index n _eq with respect to a wavelength λ of the asymmetric directional coupler type wavelength filter of the present invention constituted by the second optical waveguide illustrated in (b). DESCRIPTION OF SYMBOLS 1 First optical waveguide 2 Second optical waveguide 3 Directional coupling part 4 Input port 5 Input port 6 Output port 7 Output port 8 Cladding 9 Core 10 First layer 11 Second layer 12 Intermediate layer

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-6-186448 (JP, A) JP-A-63-63006 (JP, A) JP-A-61-231511 (JP, A) International Publication 96/06372 (WO, A1) EP 721117 (EP, A1) Heismann et. a l. , Applied Physics Letters, May 2, 1994, Vol. 64 No. 18, pp. 2335-2337 S.C. T. Chu et. al. , Electronics Letters, January 5, 1995, Vol. 31 No. 1, pp. 33-35 (58) Field surveyed (Int.Cl. ⁷ , DB name) G02B 6/12-6/14 G02F 1/29-1/313

Claims (1)

(57) Claims 1. A first layer having a refractive index higher than the refractive index of the core layer and having a thickness smaller than the thickness of the core layer has a refractive index lower than the refractive index of the core layer. Having a thickness smaller than the thickness of the core layer.
An intermediate layer sandwiched between the layers is constituted by an optical waveguide inserted into the core layer and an optical waveguide having a dispersion characteristic different from the dispersion characteristic of the optical waveguide, and TE polarization and TM polarization of the two optical waveguides. Characterized in that the propagation constants for both polarizations are the same at the same wavelength.