JPH11248948A - Asymmetrical directional coupler type wavelength filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the asymmetrical directional coupler type wavelength filter which has such stepped spectrum characteristics that the transmission wavelength range is flat and the side lobe is low by providing the core of one of two optical waveguides with the width or film thickness tapered along the propagation direction. SOLUTION: A channel optical waveguide and an ARROW type optical waveguide are coupled as an upper optical waveguide 1 and a lower optical waveguide 2 at right angles with respect to the substrate surface. The width of the ARROW type optical waveguide is tapered so that the width at the coupling start part of the directional coupling part is wider than that at the coupling end part. The transmission wavelength band ranges from the lower- limit wavelength at which the equivalent refractive index of the width of the coupling start part of the ARROW type optical waveguide is equal to the equivalent refractive index of the width of the channel optical waveguide to the upper- limit wavelength at which the equivalent refractive index of the width of the coupling end part of the ARROW type optical waveguide is equal to the propagation constant of the width of the channel optical waveguide.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an asymmetric directional coupler type wavelength filter. More specifically, the invention of this application is useful in a WDM system or the like, and is capable of uniformly causing power transition in all transmission wavelength regions, and has a step-like spectral characteristic having a flat transmission wavelength region and extremely low side lobes. The present invention relates to a new asymmetric directional coupler type wavelength filter having:

[0002]

2. Description of the Related Art Optical communication using an optical fiber has very excellent features such as low transmission loss and large capacity communication with a wide transmission band. Is rapidly progressing. At present, in optical communication using such an optical fiber, a single optical fiber connects a light source that emits an optical signal to a photodetector that receives the optical signal and converts the optical signal into an electric signal. One-to-one communication has been put to practical use.

[0003] However, in such one-way one-to-one communication, the amount of information that can be transmitted is extremely limited, and the broadband characteristics of optical fibers are not efficiently used. Therefore,
Network optical communication for connecting one-to-many or many-to-many terminal devices has been actively researched and developed for further large-capacity communication and high functionality, and is being put to practical use. In this network optical communication, for example, when two-way communication between terminals is performed using only one optical fiber, or when a plurality of different types of signals are transmitted using only one optical fiber, the wavelengths differ. 2. Description of the Related Art Wavelength division multiplexing (WDM), which performs two-way communication or one-way multiplex communication by transmitting light to one optical fiber and separately assigning each signal to each wavelength, is employed. Also, in one-way one-to-one communication, this WDM is being used to dramatically increase the transmission capacity.

Conventionally, in a subscriber WDM system, it has been desired to realize a filter that divides two channels having wavelengths of 1.3 μm and 1.55 μm. This filter needs to have a sufficient broadband characteristic to include the allowable range of the laser emission wavelength. As such a wavelength filter, for example, Adiabatic (adiabatic)
Spectral properties optimized using the combination, i.e.
The transmission efficiency in the transmission wavelength range is flat and uniform, and the transmission efficiency in the stop wavelength range is 0.
A wavelength filter having a step-like spectral characteristic in which the transmission efficiency sharply changes between two wavelength ranges has been proposed.

The spectral characteristics of such a wavelength filter can also be obtained in an asymmetric directional coupler type wavelength filter as illustrated in FIG. The asymmetric directional coupler type wavelength filter is composed of a first optical waveguide (3) and a second optical waveguide (4) in which the refractive index difference Δ between the core and the clad, the width w of the core, and the thickness t of the core are different from each other. According to this structure, as shown in FIG. 16, the dispersion characteristics, which are the dependence of the propagation constant β on the wavelength λ, are different from each other, and only at a certain center wavelength λ ₀ , the first optical waveguide ( propagation constant beta ₁ of 3) and the propagation constant beta ₂ of the second optical waveguide (4) is matched constitutes a directional coupler. Further, between the propagation constant β and the equivalent refractive index n _eq ,

[0006]

(Equation 1)

There is a relationship represented by Here, λ is the wavelength in vacuum. Therefore, the vertical axis _{shows the} equivalent refractive index n _eq
Taking the dispersion of the two waveguides as shown in FIG.
It is as shown in the equivalent refractive index n _eq of the equivalent refractive index n _eq of the first optical waveguide (3) ⁽¹⁾ and the second optical waveguide (4)
⁽²⁾ also coincides only at the center wavelength λ ₀ . In the directional coupler, the electromagnetic field in the eigenstate generally has an even mode (propagation constant β _e ) in which the lateral electromagnetic field distribution forms two peaks in the same direction, and an odd mode in which two peaks form in the opposite direction. Mode (propagation constant β _o )

[0008]

(Equation 2)

[0009] and coupling length L _o given by the first optical waveguide (3) and the second directional coupler of the optical waveguide (4) (5) of length L are equal (or an odd multiple) so as designed, thereby, only the optical signal having a center wavelength lambda _o is emitted possessed a different optical waveguide from the incident optical waveguide. However, in such a conventional asymmetric directional coupler wavelength filter, the incident wavelength is maximum coupling efficiency is smaller than 100% deviates from the center wavelength lambda _o, also shortened and coupling length L _o, between the optical waveguide However, there has been a problem that the power transfer in the first embodiment changes periodically to increase the side lobe.

Therefore, the invention of this application has been made in view of the above circumstances, and it is possible to cause the power transition to be uniform in the entire transmission wavelength range, to have a flat transmission wavelength range and to have a side lobe. It is an object of the present invention to provide a new asymmetric directional coupler wavelength filter having a very low step-like spectral characteristic.

[0011]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems by providing a width or a film in which a core of one of two optical waveguides is tapered along a propagation direction. An asymmetric directional coupler type wavelength filter characterized by having a thickness is provided.

Further, according to the invention of this application, in the above-mentioned wavelength filter, the two optical waveguides are vertically coupled to the substrate surface (claim 2), and one of the two optical waveguides is provided. Is an ARROW type optical waveguide or a channel optical waveguide, the other optical waveguide is a channel optical waveguide (Claim 3), and the ARROW type waveguide has a width or a film thickness that is tapered along the propagation direction. What is done (claim 4) is also provided as an embodiment.

[0013]

FIG. 1 illustrates an asymmetric directional coupler type wavelength filter according to the invention of the present application. The asymmetric directional coupler type wavelength filter of the present invention shown in FIG. 1 is constituted by, for example, a pair of optical waveguides which are vertically coupled on a substrate, and the upper optical waveguide (1) or the lower optical waveguide. Either (2) has a width that changes in a tapered shape along the propagation direction.

For example, an SLC-ARROW optical waveguide having a multilayer structure formed on a substrate as illustrated in FIG. 2 and a channel optical waveguide formed on an ARROW optical waveguide as illustrated in FIG. It is assumed that wavelength filters are vertically coupled as a lower optical waveguide (2) and an upper optical waveguide (1), respectively. SLC-
The width W ′ of the ARROW type optical waveguide is changed from the initial width W0 ′ to W.
When the value changes to 0 ′ + ΔW ′ or W0′−ΔW ′, the equivalent refractive index increases as the width increases and decreases as the width decreases, as illustrated in FIG. Further, in the channel optical waveguide, the width W is changed from the initial width W0 to W0 +.
When changing to ΔW or W0−ΔW, SLC-ARRO
Similar to the W-type optical waveguide, the equivalent refractive index changes as illustrated in FIG.

Here, for example, as illustrated in FIG. 6, the width of the SLC-ARROW type optical waveguide is such that the width W1 'of the coupling start end in the directional coupling portion is the width W1 of the coupling end portion.
Assuming a tapered shape wider than 2 ′, the transmission wavelength range is SLC-ARROW as illustrated in FIG.
The wavelength at which the equivalent refractive index of the width W1 'of the optical waveguide and the equivalent refractive index of the width W0 of the channel optical waveguide coincide with each other is a lower limit, SLC
-The wavelength range is an upper limit of the wavelength at which the equivalent refractive index of the width W2 'of the ARROW type optical waveguide and the propagation constant of the width W0 of the channel optical waveguide match.

Therefore, the filter characteristic of the asymmetric directional coupler type wavelength filter having such a structure is a step-like one having a flat transmission band and a very low side rope as illustrated in FIG. Conversely, as illustrated in FIG. 9, the channel optical waveguide has a width in which the width W1 of the coupling start end in the directional coupling portion is wider than the width W2 of the coupling end portion, and is tapered along the propagation direction. If you have
As illustrated in FIG. 10, the transmission wavelength range is a lower limit of the wavelength at which the propagation constant of the width W2 of the channel optical waveguide and the propagation constant of the width W0 ′ of the SLC-ARROW type optical waveguide match, and the width W1 of the channel optical waveguide. Propagation constant and SLC-ARROW
The wavelength range is the upper limit of the wavelength where the propagation constant of the width W0 'of the optical waveguide coincides with the upper limit, and thus has a step-like filter characteristic as illustrated in FIG.

That is, the asymmetric directional coupler type wavelength filter according to the present invention is configured such that one of the pair of optical waveguides has a width (or film thickness) that changes in a tapered shape along the propagation direction. , A step-like filter characteristic having an arbitrary transmission wavelength range can be easily obtained. Further, each wavelength in the transmission wavelength region obtained as described above is coupled at a position where the equivalent refractive index of each optical waveguide matches in the propagation direction in the coupler. Therefore, the power transition occurs uniformly over the entire transmission wavelength range.

As the lower optical waveguide (2), a channel optical waveguide may be used in addition to the above-mentioned ARROW type optical waveguide. Hereinafter, embodiments will be described with reference to the accompanying drawings, and embodiments of the present invention will be described in further detail.

[0019]

EXAMPLE A filter characteristic was obtained in the case where each optical waveguide parameter of the asymmetric directional coupler type wavelength filter of the present invention illustrated in FIG. 1 was designed to have the values illustrated in FIG. Here, the channel optical waveguide and the AR
ROW type optical waveguides are vertically coupled to the substrate surface as an upper optical waveguide (1) and a lower optical waveguide (2), respectively.

Since the dispersion characteristic of the ARROW type optical waveguide is relatively flat, the width of the ARROW type optical waveguide is largely changed to obtain a wavelength filter operable in a wide band, and the dispersion is substantially equal to that of the channel optical waveguide. It is necessary to change the propagation constant. Therefore, the width of the ARROW type optical waveguide is linearly tapered. The change in the tapered width of the ARROW type optical waveguide, that is, the change from the coupling start end width W1 ′ to the coupling end width W2 ′ of the directional coupling portion illustrated in FIG. 6 is from 10.0 μm to 5.0 μm. 9.
5 μm to 5.5 μm, 9.0 μm to 6.0 μm
In the case of a pattern, the full width at half maximum of the characteristic is 32.
7, 25.3 and 18.3.

ARRO having such a tapered width
In the W-type optical waveguide, both the upper core layer (21) and the lower core layer (23) have a thickness t = 3.5 μm, a refractive index n = 1.5365, and a stripe lateral confinement layer (= S).
LC layer) (22) has a film thickness of 0.6 μm and a refractive index n =
1.4512, the first cladding (24) has a thickness t = 0.8
μm and the refractive index n = 1.
5): film thickness t = 3.5 μm and refractive index n = 1.536
5. The silicon substrate (26) has a refractive index n = 3.4780.

On the other hand, the width of the channel optical waveguide is 1.30.
μm, and the core (11) has a thickness t =
1.3 μm and refractive index n = 1.6150, refractive index n of cladding (12) = 1.4512, SLC-ARRO
The cladding layer thickness between the upper core layer (21) of the W-shaped waveguide and the core (11) of the channel waveguide is 0.5 μm.

The coupling length L, that is, the length L of the directional coupling portion is fixed at 30 mm. FIG. 13 exemplifies the interaction between such optical waveguides, and qualitatively expresses the coupling strength K (z) and the detuning Δβ (λ, z) depending on the device length. As exemplified in FIG.
The detuning Δβ (λ, z), that is, the change in the waveguide width changes linearly. On the other hand, the coupling strength is constant over the device length, that is, the length L of the directional coupling portion, and decreases smoothly near both ends. As the wavelength changes, this curve shifts vertically and the tuning point (Δβ (λ, z) = 0) moves to a different position.

FIG. 14 exemplifies filter characteristics obtained in each tapered width change pattern. The filter characteristics were derived using strict modes of individual optical waveguides, and the power transfer characteristics were obtained using mode coupling theory. As is apparent from FIG. 14, the asymmetric directional coupler type wavelength filter of the present invention has a step-like shape in which the transmission efficiency is flat and uniform, and the transmission wavelength is substantially zero. It can be seen that the transmission wavelength range is determined by the tapered width value of the optical waveguide. That is, in the wavelength filter of the present invention, the wavelength bandwidth can be easily controlled by controlling the rate of change of the optical waveguide width.

Of course, the present invention is not limited to the above-described example, and it goes without saying that various embodiments are possible in detail.

[0026]

As described in detail above, according to the present invention, the power transition can be uniformly generated in the entire transmission wavelength range, and the transmission wavelength range is flat and has a step-like spectral characteristic with a very low side lobe. , A new asymmetric directional coupler type wavelength filter is provided.

[Brief description of the drawings]

FIG. 1 is a perspective view illustrating the configuration of an asymmetric directional coupler type wavelength filter according to the present invention.

FIG. 2 is a configuration diagram illustrating an SLC-ARROW type optical waveguide;

FIG. 3 is a diagram illustrating equivalent refractive index characteristics of an SLC-ARROW type optical waveguide.

FIG. 4 is a configuration diagram illustrating a channel optical waveguide;

FIG. 5 is a diagram illustrating equivalent refractive index characteristics of a channel optical waveguide.

FIG. 6 is a plan view showing an example of an asymmetric directional coupler wavelength filter according to the present invention.

FIG. 7 is a diagram illustrating an equivalent refractive index characteristic of the asymmetric directional coupler wavelength filter of FIG. 6;

FIG. 8 is a diagram illustrating filter characteristics of the asymmetric directional coupler type wavelength filter of FIG. 6;

FIG. 9 is a plan view showing another example of the asymmetric directional coupler type wavelength filter of the present invention.

FIG. 10 is a diagram illustrating an example of equivalent refractive index characteristics of the asymmetric directional coupler type wavelength filter of FIG. 9;

11 is a diagram illustrating filter characteristics of the asymmetric directional coupler type wavelength filter of FIG. 9;

FIG. 12 is a diagram showing an example of each optical waveguide parameter in the asymmetric directional coupler type wavelength filter of the present invention.

FIG. 13 is a diagram exemplifying an interaction between optical waveguides in the asymmetric directional coupler type wavelength filter of FIG.

FIG. 14 is a diagram illustrating filter characteristics of the asymmetric directional coupler wavelength filter of FIG. 12;

FIG. 15 is a conceptual diagram illustrating a conventional asymmetric directional coupler type wavelength filter.

FIG. 16 is a diagram illustrating a dispersion relation of propagation constants of two optical waveguides in the asymmetric directional coupler wavelength filter of FIG. 15;

17 is a diagram illustrating a dispersion relation of equivalent refractive indices of two optical waveguides in the asymmetric directional coupler wavelength filter of FIG.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 upper optical waveguide 11 core 12 clad 2 lower optical waveguide 21 upper core layer 22 stripe lateral confinement layer 23 lower core layer 24 first clad 25 second clad 26 silicon substrate 3 first optical waveguide 4 second optical waveguide 5 directional coupling Department

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yasuo Kokubu 1-3-3 Kosugaya, Sakae-ku, Yokohama-shi, Kanagawa Prefecture (72) Inventor Shinya Sato 3-6-16, Sakado, Takatsu-ku, Kawasaki-shi, Kanagawa d. Kusselheim 305 (72) Inventor Takeshi Maeda 1-1101 Mineokacho, Hodogaya-ku, Yokohama-shi, Kanagawa Prefecture 102-1 Shizuto Building 102 (72) Inventor Takashi Kato 2-46 Futamatagawa Midorigaokaso Asahi-ku, Yokohama-shi, Kanagawa Prefecture 201

Claims (4)

[Claims] An asymmetric directional coupler type wavelength filter characterized in that the core of one of the two optical waveguides has a width or a film thickness that is tapered along the propagation direction. . 2. The asymmetric directional coupler type wavelength filter according to claim 1, wherein the two optical waveguides are vertically coupled to the substrate surface. 3. The optical waveguide according to claim 1, wherein one of the two optical waveguides is an ARROW type optical waveguide or a channel optical waveguide, and the other optical waveguide is a channel optical waveguide.
Or 2 asymmetric directional coupler type wavelength filters. 4. The asymmetric directional coupler wavelength filter according to claim 3, wherein the ARROW type waveguide has a width or a film thickness that is tapered along the propagation direction.