JP2694011B2 - Waveguide type wavelength filter - Google Patents

Description

TECHNICAL FIELD The present invention relates to a waveguide type wavelength filter having a steep selection characteristic used in a wavelength division multiplexing optical transmission system or an optical integrated circuit.

(Prior Art) As a waveguide type tunable wavelength filter used in a wavelength division multiplexing optical transmission system, a diffraction grating or a diffraction grating including a quarter wavelength phase shift unit is conventionally used in a semiconductor-based waveguide structure. Some are formed and some are applied distributed feedback semiconductor laser (DFB).

The principle of operation is that when a diffraction grating is formed in the waveguide structure, the equivalent refractive index of the waveguide is Neff and the period of the diffraction grating is A.
Then, the fact that the light of wavelength λB given by the Bragg condition λB = 2Neff · A is selectively reflected is used.

In addition, when a diffraction grating including a quarter wavelength phase shift part is formed, the selected wavelength light is transmitted (reference: Applied Physics
(See Letter, 53, 83, 1988).

In order to make the selected wavelength variable, the equivalent refractive index Neff
Can be controlled in some way. In the former wavelength filter, a pn junction is formed and the plasma effect by injecting carriers into the waveguide is applied to change the refractive index of the waveguide. Alternatively, it is also possible to apply a reverse bias voltage and control the refractive index of the waveguide section by the electro-optic effect.

The principle of operation is the same as in the case of a wavelength filter using a DFB semiconductor laser.

(Problems to be Solved by the Invention) However, in these wavelength filters, the wavelength of the selected light differs depending on the polarization plane direction of the light. That is, the selected wavelength was different depending on the TE mode or TM mode of the guided light. Therefore, a polarizing plate for controlling the plane of polarization of incident light is required in advance, which causes a problem that the configuration is complicated.

There is also a development example using a lithium niobate single crystal as a wavelength filter that does not depend on polarization. This wavelength filter incorporates a comb-type electrode structure and a TE-TM mode converter based on the electro-optic effect. Therefore, it is necessary to constantly apply a relatively large bias voltage, and the operating voltage for changing the wavelength is as high as 100 V, which is more problematic than a wavelength filter using a semiconductor.

The present invention has been made in view of such circumstances,
It is an object of the present invention to provide a waveguide type wavelength filter that can vary the selected wavelength without depending on the direction of the plane of polarization.

(Means for Solving the Problem) In order to achieve the above object, in claim (1), the core region for confining light is a first stripe-shaped optical waveguide made of a semiconductor crystal of a superlattice structure, and the core region is the above-mentioned. A second stripe-shaped optical waveguide made of a semiconductor crystal in which a superlattice structure is mixed, and a cladding region on one side of the first stripe-shaped optical waveguide is a core region of the second stripe-shaped optical waveguide. And the clad region on one side of the second striped optical waveguide is integrally formed so as to be constituted by the core region of the first striped optical waveguide. Diffraction gratings each having a period given by a function of an equivalent refractive index are arranged in the vicinity of the first and second striped optical waveguides.

Further, in claim (2), a pn junction is provided in or near the striped optical demultiplexing region so that carrier injection or reverse bias voltage application to the optical demultiplexing region can be effectively performed.

(Operation) According to claim (1), the light incident on the striped light demultiplexing region is automatically generated by the polarization direction, and the TE mode light is the first striped light guide having a superlattice structure. The TM mode light is propagated to the core region of the waveguide by being divided into the core regions of the second stripe optical waveguide having a mixed crystal structure.
That is, TE mode light and TM mode light are spatially demultiplexed from the incident light without applying a bias voltage from the outside.

Further, for example, light having the same wavelength is selected in each of the striped optical waveguides by the diffraction grating having a unique period, which is arranged close to the first and second striped optical waveguides.

In this way, the light having the wavelengths selected by demultiplexing the first and second striped optical waveguides for each polarization plane is propagated through the first and second striped optical waveguides, and then the wavelength filter. Is output from.

According to claim (2), carriers are injected into the striped optical demultiplexing region via the pn junction or a reverse bias electric field is applied. As a result, the refractive index of the striped optical waveguide changes, and the wavelength of the selected light changes.

(Embodiment) FIG. 1 is a configuration diagram showing a first embodiment of a waveguide wavelength filter according to the present invention. In FIG. 1, reference numeral 1 denotes a substrate, which is made of, for example, a GaAs single crystal. 2 is the substrate 1
The buffer layer formed on the GaAs substrate 1 is made of Al _x Ga _1-x As (eg, x = 0.3) having a lower refractive index than the GaAs substrate 1.

Reference numeral 3 is a first striped optical waveguide (hereinafter referred to as a first optical waveguide), which is GaAs and Al _y Ga _1-y As (for example, y = 0.
It is composed of a superlattice layer with a repeating structure of thin layers of about 100Å in 3).

Reference numeral 4 denotes a second striped optical waveguide (hereinafter referred to as a second optical waveguide), which has an average Al composition z obtained by destroying a superlattice layer similar to that of the first optical waveguide 3 by a method described later.
It is composed of a mixed crystal of Al _z Ga _1-z As (for example, when the layer thickness is repeated about 100Å, z = 0.15).

These first and second optical waveguides 3 and 4 are formed on the upper surface of the buffer layer 2 and have a refractive index of the buffer layer 2
It has a higher refractive index.

Reference numeral 5 denotes a striped optical demultiplexing region, in which the cladding region on one side of the first optical waveguide 3 is constituted by the core region of the second optical waveguide 4 and the cladding on one side of the second optical waveguide 4 is formed. The first and second optical waveguides 3 and 4 are integrally formed so that the region is constituted by the core region of the first optical waveguide 3. The shape is formed into a ridge-type stripe structure having a width W (several μm) and a height H (several 1000Å to 1 μm) by a method described later.

Reference numeral 6 denotes a diffraction grating with a 1/4 wavelength phase shift portion, which is formed on the upper surface of the first optical waveguide 3 by a method described later.
The period Λ1 is given by Λ1 = λB / 2Neff (TE), where Neff (TE) is the equivalent refractive index of the propagation light of the first optical waveguide 3 and λB is the transmission wavelength.

Reference numeral 7 denotes a diffraction grating with a 1/4 wavelength phase shift portion, which is formed on the upper surface of the second optical waveguide 4 by a method described later.
The period Λ2 is given by Λ2 = λB / 2Neff (TM) where Neff (TM) is the equivalent refractive index for the propagation light of the second optical waveguide 4 and λB is the transmission wavelength.

Next, a method of manufacturing the waveguide type wavelength filter having the above structure will be described.

First, using a crystal growth method such as molecular beam epitaxy (MBE) or metal organic chemical vapor deposition (MOCVD) capable of controlling the film thickness at the atomic layer level, a buffer layer 2 and then a superlattice are formed on the substrate 1. The layer is epitaxially grown on the entire surface.

Next, Ga ion implantation or selective deposition of a SiO ₂ film is performed only on the region to be the second optical waveguide 4, and then high temperature annealing treatment is performed to produce mixed crystals. here,
Mixed crystal means that the superlattice structure becomes a mixed crystal having a uniform composition by diffusing impurities or by some kind of artificial operation applied here. This mixed crystal widens the energy gap as compared with the superlattice structure.

Then, by adopting an appropriate etching method such as reactive ion etching or wet etching,
A stripe structure having the above-described width W and height H of the structural parameters is formed.

Next, a resist pattern of a diffraction grating with a 1/4 wavelength phase shift section having a period of Λ1 and Λ2 is formed on the upper surface of each ridge-type optical waveguide by an electron beam exposure apparatus, and then a concave-convex diffraction grating is formed by reactive ion etching. And

The equivalent refractive indices Neff (TE) and Neff (TM) of the first and second optical waveguides (3, 4) are obtained by preliminary experiments. The order of forming the diffraction grating and the ridge structure may be the reverse of the above.

Next, the operation of the above configuration will be described step by step with reference to FIGS.

FIG. 2 is a diagram showing a lateral refractive index distribution in the striped light demultiplexing region 5 of FIG. In Fig. 2, the solid line indicates the refractive index of TE mode light whose polarization direction is parallel to the substrate 1 surface, and the broken line indicates the refractive index of TM mode light whose polarization direction is perpendicular to the substrate 1 surface. ing. Further, A and D indicate the air region, B indicates the region of the second optical waveguide 4, and C indicates the region of the first optical waveguide 3.

Here, when the refractive index of the first optical waveguide 3 is Ns and the refractive index of the second optical waveguide 4 is Na, it is known that the two have the following relationship.

The refractive index Na of the second optical waveguide 4 is substantially the same for the TE and TM mode lights for light longer than the wavelength corresponding to the band gap, that is, for light transparent to the semiconductor crystal.
That is, the relationship of Na (TE) = Na (TM) is satisfied.

However, the refractive index Ns of the first optical waveguide 3 has polarization dependency and satisfies the following relationship.

TE mode light: Ns (TE)> Na (TE) TM mode light: Ns (TM) <Na (TM) Also, its refractive index is about 2 × 10 ^-3 (for optical properties, held in 1987. International Conference on Laser Electro-Optics (CL
EO) paper presented in WQ-4))).

As can be seen from FIG. 2, in the lateral refractive index distribution of the optical demultiplexing region 5, the peak value of the refractive index for the TE mode light is in the region C of the first optical waveguide 3. On the other hand, the peak value of the refractive index for TM mode light is in the region B of the second optical waveguide 4.

In addition, generally, a core region made of a material having a high refractive index is
In an optical waveguide surrounded by a cladding region made of a material having a low refractive index, light has the property of propagating through a core region having a high refractive index.

Therefore, the light incident on the striped light demultiplexing region 5 is automatically demultiplexed according to the polarization direction, and the TE mode light travels along the first optical waveguide 3 having the superlattice structure portion as the core region. The TM mode light that has been propagated is propagated along the second optical waveguide 4 having the mixed crystal structure portion as the core region. That is, TE mode light and TM mode light are spatially demultiplexed from the incident light.

Further, FIG. 3 is a diagram showing a light output distribution after passing through the striped light demultiplexing region 5. In FIG.
The solid line 'shows the output distribution of TE mode light, and the broken line' shows the output distribution of TM mode light. Also, A and D
Indicates the air region, B indicates the region of the second optical waveguide 4, and C indicates the region of the first optical waveguide 3.

According to FIG. 3, the peak value of the TE mode light is near the region C of the first optical waveguide 3, and the peak value of the TM mode light is near the region B of the second optical waveguide 4. I understand.

Furthermore, on the upper surface of each of the first and second optical waveguides 3 and 4, diffraction with a 1/4 wavelength phase shifter having periods Λ1 and Λ2 determined by the equivalent refractive index for TE mode light and TM mode light is provided. Since the gratings 6 and 7 are formed, light of the same wavelength is selected in each of the first and second optical waveguides 3 and 4.

In this way, the light that has been demultiplexed into the first and second optical waveguides 3 and 4 for each polarization plane and the wavelength of which has been selected is
And after being propagated through the second optical waveguides 3 and 4, the wavelength filter outputs the wavelength.

As described above, according to the first embodiment, the TE and TM mode lights can be spatially separated without applying the bias voltage from the external bias power source.
The same wavelength can be selected for both mode lights. That is, a waveguide wavelength filter that does not depend on the direction of the plane of polarization is realized.

FIG. 4 is a second view of the waveguide type wavelength filter according to the present invention.
FIG. 3 is a configuration diagram showing an example of the embodiment. In the second embodiment, in addition to the structure of the first embodiment, an input section 8 and a striped optical waveguide formed of striped optical waveguides are provided at both ends of the striped optical demultiplexing region 5 in the light propagation direction. The output units 9 are connected to each other.

In such a configuration, the light input to the wavelength filter is propagated to the input unit 8 and then is incident on the striped light demultiplexing region 5. The light incident on the optical demultiplexing region 5 is demultiplexed according to its polarization plane, the TE mode light is propagated along the first optical waveguide 3 having the superlattice structure portion as the core region, and the TM mode light is mixed. The light is propagated along the second optical waveguide 4 having the crystal structure portion as the core region.

Next, the light transmitted through the light demultiplexing region 5 is output from the wavelength filter after being propagated through the output unit 9.

The striped optical waveguide forming the input / output units 8 and 9 may be an optical waveguide having a superlattice structure, as in the first optical waveguide 3, or similar to the second optical waveguide 4. Further, it may be a mixed crystal optical waveguide in which a superlattice structure is converted into a liquid crystal. The manufacturing method thereof is the same as the method described in the first embodiment.

Also in the second embodiment, as in the first embodiment, a waveguide type wavelength filter that does not depend on the direction of the polarization plane of incident light is realized.

Also, in order to electrically change the wavelength of the transmitted light,
A pn junction is provided in or near the striped light demultiplexing region 5 in which the diffraction grating is formed, and a plasma effect by injecting carriers into the striped light demultiplexing region 5 via the pn junction is obtained. The equivalent refractive index may be changed by using it or by using the electro-optic effect by applying a reverse bias voltage.

(Effects of the Invention) As described above, according to claim (1), the core region for confining light is the first stripe-shaped optical waveguide made of a semiconductor crystal having a superlattice structure, and the core region is for the superlattice structure. A second stripe-shaped optical waveguide made of a mixed crystal of semiconductor crystals, and a cladding region on one side of the first stripe-shaped optical waveguide is formed as a core region of the second stripe-shaped optical waveguide. And the clad region on one side of the second striped optical waveguide is integrally formed so as to be constituted by the core region of the first striped optical waveguide to form a striped optical demultiplexing region. , Further, the first
Since a diffraction grating having a period given by the function of the equivalent refractive index is arranged close to the second and second striped optical waveguides, the TE mode light and the TM mode light are converted from the incident light by the bias voltage from the external bias power supply. Since it is possible to spatially separate the light without applying the light, it is possible to select the same wavelength for both mode lights, and to provide a waveguide type wavelength filter that does not depend on the direction of the polarization plane. is there. .

According to claim (2), in addition to the effect of claim (1), the selection wavelength can be made variable.

Therefore, the waveguide type wavelength filter according to the present invention can be applied to a wavelength division multiplexing optical transmission system or an optical integrated circuit.

[Brief description of the drawings]

1 is a configuration diagram showing a first embodiment of a waveguide type wavelength filter according to the present invention, FIG. 2 is a diagram showing a lateral refractive index distribution in a striped optical demultiplexing region of FIG. 1, and FIG. FIG. 4 is a diagram showing the optical output distribution after passing through the striped optical demultiplexing region of FIG. 1, and FIG. 4 is a configuration diagram showing a second embodiment of the waveguide type wavelength filter according to the present invention. In the figure, 1 ... GaAs substrate, 2 ... buffer layer, 3 ... first
Striped optical waveguide, 4 ... second striped optical waveguide, 5 ... striped optical demultiplexing region, 6,7 ... 1/4 wavelength phase-shifting diffraction grating, 8 ... input section, 9 ...... Output section.

Claims (2)

(57) [Claims] 1. A first stripe-shaped optical waveguide in which a core region for confining light is made of a semiconductor crystal having a superlattice structure, and a second stripe-shaped optical waveguide in which a core region is made of a mixed crystal of the superlattice structure. A waveguide, wherein a cladding region on one side of the first striped optical waveguide is constituted by a core region of the second striped optical waveguide, and one side of the second striped optical waveguide. To form a striped optical demultiplexing region by integrally forming a clad region of the first striped optical waveguide into a core region of the first striped optical waveguide. A waveguide type wavelength filter characterized in that diffraction gratings each having a period given by a function of an equivalent refractive index are arranged close to each other. 2. The waveguide type wavelength filter according to claim 1, wherein a pn junction is provided at or near the striped light demultiplexing region.