JPH0365906A - Semiconductor optical wavelength filter - Google Patents

Abstract

PURPOSE:To obtain the steep wavelength selectivity and sufficient wavelength blocking characteristics of the directional coupler type optical electrode filter and to facilitate the production thereof by forming the filter in such a manner that at least one of the waveguides has a shutting off area in a zero order waveguide mode. CONSTITUTION:The W type waveguide 2 which is the 1st waveguide has the shutting off area in the zero order waveguide mode and, therefore, the equiv. refractive index of the zero order mode decreases considerably with an increase in the wavelength. The dispersion curve of the W type waveguide, therefore, intersects steeply with the dispersion curve of the step type waveguide which is the 2nd waveguide having a less change in the equiv. refractive index. The filter has the sufficient wavelength selectability accordingly. The control of the refractive index of the cores, clads, etc., of the respective waveguides is facilitated simply by successively laminating the respective layers 2, 3 of the waveguides with this SW type filter. The steep wavelength selection characteristics and the wide blocking area are obtd. in this way and the production is facilitated.

Description

Detailed Description of the Invention [Industrial Fields of Application] The present invention relates to a wavelength multiplexing discrimination type semiconductor light receiving element capable of discriminating and receiving a plurality of lights of different wavelengths, and a semiconductor optical amplifier that uses spontaneous emission light. This invention relates to a semiconductor optical functional device to be cut.

[Prior art and its issues] As a conventional transmission type optical wavelength filter using a waveguide,
Mainly, two types are known: (1) λ/4 shift Bragg waveguide type optical wavelength filter and (2) directional coupler type optical wavelength filter.

The λ/4 shift Bragg waveguide type optical wavelength filter has a periodic structure with a first-order coupling order at the interface between the substrate serving as the cladding and the waveguide layer serving as the core. It has a periodic structure whose phase is shifted by π. In this type of optical wavelength filter, the reflectance of the periodic structure is 0 for light at the Bragg wavelength where the propagation constant difference δβ between the two eigenmodes propagating in the waveguide layer is O, and the reflectance of the periodic structure is 0.
It operates as an optical filter that transmits light at the Bragg wavelength.

However, this type of optical wavelength filter has drawbacks such as the narrow stopband that exists before and after the transmission wavelength range, which not only causes light to pass through again outside the stopband, but also that the amount of attenuation in the stopband is not sufficient. .

On the other hand, a directional coupler type optical wavelength filter consists of a directional coupler with two waveguide layers having a step-type refractive index distribution, as shown in FIG. 9, for example. In FIG. 9, symbol I is a cladding, and symbols 2 and 3 are both waveguide layers.

This directional coupler has a structure in which the thickness, refractive index, length, etc. of each waveguide layer are set so that the respective dispersion curves of the waveguide layer 2 and the waveguide layer 3 intersect at a certain specific wavelength λC. The parameters are determined.

Therefore, at a specific wavelength λC, the propagation constant difference δβ of the eigenmodes propagating through each waveguide 2.3 becomes 0, and only at this specific wavelength λC, the difference in propagation constant between the waveguide FJ2 and the waveguide layer 3 is 100.
% binding occurs. The wavelength λ incident on the waveguide layer 2.

Of the light of ~λ, only light with a wavelength around λC is emitted from the waveguide layer 3, and operates as a transmission type optical wavelength filter.

In addition, an optical wavelength filter having a structure in which a periodic structure, a so-called grating, is provided in the cladding l between the waveguide layer 2 and the waveguide layer 3 has also been reported. in this case,
The propagation constants between the two waveguides 2.3 do not match at wavelength λC, and the propagation constant difference δβ of the eigenmode is not O, but by appropriately determining the period A of the grating, the propagation constant difference δβ can be reduced to O. Then, guided wave F! 100% coupling between J2 and the waveguide layer 3 can be achieved. Therefore, similar to the directional coupler type optical wavelength filter shown in FIG.
It operates as a transmission type optical wavelength filter.

However, the directional coupler type optical wavelength filter shown in Fig. 9 has the disadvantage that the dispersion curves of each waveguide layer intersect gently, the transmission wavelength range is wide, and the amount of attenuation in the stopband is not sufficient. Ta. In addition, in a directional coupling type optical wavelength filter equipped with a grating, the filter length is IIIffi,
It is reported that the full width at half maximum δλ of the transmission wavelength range is 6.5n- (R
,C,^1lerness et a! .

+! 1TEGRATED AND GtllDED-1
1AVE 0PTIC31989, VOL.

4. Explosion A6-1. pp. 215-218), but it has the drawback that it is necessary to carve the grating using an electron beam or the like during the crystal growth process, and the manufacturing process is complicated.

An object of the present invention is to overcome the insufficiency of the Bragg waveguide type optical wavelength filter, which has a narrow blocking wavelength range, and the broadband property of the wavelength transmission characteristic, which has a wide transmission wavelength range, of the directional coupler type optical wavelength filter. It is an object of the present invention to provide a transmission type semiconductor optical wavelength filter that solves the above problems, has steep wavelength selectivity and sufficient wavelength blocking characteristics, and is easy to manufacture.

[Means for Solving the Problems] The present invention provides that a first single mode semiconductor waveguide and a second single mode semiconductor waveguide are sequentially stacked in the crystal growth direction of a semiconductor crystal, Waveguide parameters are determined such that the effective refractive index of the single mode semiconductor waveguide and the second single mode semiconductor waveguide match at a specific wavelength, and the first single mode semiconductor waveguide is wave path and second
A directional coupler type optical wavelength filter that completely couples with a single mode semiconductor waveguide and does not completely couple at other wavelengths, wherein at least one of the single mode semiconductor waveguides is a zero-order waveguide. The solution was to have a cutoff region in the wave mode.

In other words, as a waveguide constituting a semiconductor directional coupler that does not have a grating, etc., a waveguide that has a cutoff region for the 0th-order waveguide mode, such as a waveguide that has a W-type layer refractive index distribution or an asymmetric step-type refractive index distribution, is used. By using this, the dispersion curves of both waveguides are made to intersect steeply.

[Example 1] Figures 1 and 2 both show the layer structure and refractive index distribution of each layer of an example of the semiconductor optical wavelength filter of the present invention.

The semiconductor optical wavelength filter shown in FIG. 1 is an example in which a W-shaped waveguide is used as a method of forming a semiconductor waveguide having a cutoff region in the 0th-order waveguide mode, and the one shown in FIG. This is an example using an asymmetric step type waveguide.

In FIGS. 1 and 2, the symbol l is the cladding, the symbol 2 is the core of the first single mode semiconductor waveguide (hereinafter simply referred to as the first waveguide), and the symbol 3 is the second single mode semiconductor waveguide. A core of a mode semiconductor waveguide (hereinafter simply referred to as a second waveguide),
Reference numeral 4 indicates an inner cladding of the first waveguide, and reference numeral 5 indicates an asymmetric side cladding of the first waveguide.

In this example, Ggx ASI-X rny P with a refractive index of n4 is placed on an InP wafer with a refractive index of 3.165.
+-V was laminated by the MO-CVD method to form a clad 1, and on this clad 1, the core 3 of the second waveguide, the clad 1, and the core 2 of the first waveguide were sequentially deposited by the MO-CVD method. . The core 3 of the second waveguide contains GaxAs+-xInyPry with a refractive index of n2, and the core 2 of the first waveguide contains GaxAs+-xInyPr-y with a refractive index of n3.
y P +−Y, and the inner cladding 4 of the first waveguide and the ! The asymmetric side cladding 5 of the waveguide has a refractive index nl
=3.165 InP was used, respectively.

3 to 6 all show the relationship between the dispersion characteristics and the power transfer rate of the directionally coupled □-type optical wavelength filter.

Figure 3 shows the dispersion characteristics of the semiconductor optical wavelength filter (hereinafter abbreviated as SW type filter) using the W-shaped circular wave path of the present invention shown in Figure 1, and Figure 4 shows the power of the SW type filter. Figure 5 shows the dispersion characteristics of the conventional directional coupling type optical wavelength filter shown in Figure 9, and Figure 6 shows the power transition of the conventional directional coupling type optical wavelength filter. The rates are shown for each.

In the SW type filter of the present invention, the first waveguide is W! ! !
Since the waveguide has a cutoff region for the 0th-order waveguide mode, the equipolarized refractive index of the 0th-order mode decreases significantly as the wavelength increases. Therefore, as shown in FIG. 3, the dispersion curve of the W-shaped ring waveguide sharply intersects the dispersion plane line of the step-type waveguide, which is the second waveguide, in which the change in equipolarized refractive index is small. Therefore, the power transfer rate becomes as shown in FIG. 4, and sufficient wavelength selectivity is obtained.

On the other hand, since the conventional directional coupler type optical wavelength filter uses a step waveguide with little change in equipolarized refractive index, the intersection of these dispersion curves is as shown in Fig. It is extremely gentle compared to that of the SW type filter of the present invention shown in the figure. Therefore, its power transfer rate is as shown in FIG. 6, and its wavelength selectivity is low.

Referring to FIGS. 3 to 6, the influence of the dispersion characteristics of the waveguide on the characteristics of the directional coupling type optical wavelength filter will be explained, and the operation of the SW type filter of this embodiment will be described below.

A wavelength λ is applied to the second waveguide of the directional coupler type optical wavelength filter.

Assume that light of ~λ is incident. At this time, the power transfer rate from the second waveguide to the first waveguide is proportional to sin'' (βcL), where L is the filter length, βc = (to' + Δ3
). The coupling coefficient between the first waveguide and the second waveguide satisfies L=π/2, and Δ=(β1-β2)/2, where βl and β2 are the coupling coefficients of the first waveguide and the second waveguide, respectively. This is the propagation constant of the zero-order waveguide mode. As can be seen from the above formula,
The power transfer rate is maximum when Δ=0, that is, β!
= β2, and as the difference δβ between β1 and β2 increases, the power transfer rate decreases. Therefore, βl−
If the condition β2 occurs only with light of wavelength λC, then
The wavelength of light incident on the second waveguide. The wavelength λC of the light of ~λ
Only nearby light is transmitted to the first waveguide, thus operating as an optical wavelength filter.

By the way, the change in propagation constant difference δβ with respect to wavelength is θβ/θλ
It is clear that the larger the wavelength λC, the more rapidly the power transfer rate of light other than the wavelength λC decreases, resulting in a sharp power transfer characteristic near the wavelength λC. Therefore, as shown in FIGS. 3 and 4, the SW type filter of the present invention has an extremely large change in the propagation constant difference δβ with respect to the wavelength compared to the conventional directional coupler type optical wavelength filter, and the change in the propagation constant difference δβ with respect to the wavelength is extremely large, and It can be seen that it has a steep power transition characteristic, that is, a steep wavelength selection characteristic.

Furthermore, since the SW type filter of the present invention is made of a semiconductor and can be manufactured by simply laminating each layer of the first waveguide and the second waveguide in sequence in the crystal growth direction of the semiconductor, the core, cladding, etc. of each waveguide can be It is easy to control the refractive index, the control range is wide, and the transmission wavelength λC and the transmission wavelength range δλ
Another advantage is that there is a large degree of freedom in design.

7 and 8 show the results of a numerical study of the wavelength dependence of the propagation constant difference δβ and the wavelength selection characteristics of the SW type filter of the present invention shown in FIG. 1, respectively. Note that the waveguide parameters in this numerical example are as shown in Table 1. Here, the semiconductor material GaxAG+-X t ny P
The +-y material dispersion was also taken into account.

In this numerical example, the full width at half maximum δλ of the wavelength transmission range was analyzed to be 4.7 nm under the condition that the filter length was 1 mm. As a result of manufacturing the same structure based on this design, the full width at half maximum was achieved as designed.

(Below, margin) Table! Note that although the explanation has been made using a GaAs1nP semiconductor, the operating principle is exactly the same even if other semiconductor materials are used. Further, in this embodiment, only the first waveguide has a cutoff region for the 0th-order waveguide mode, but the semiconductor optical wavelength filter of the present invention has a cutoff region for the 0th-order waveguide mode. Not only the waveguide but also the second waveguide can have a cutoff region in the zero-order waveguide mode, and the present invention is not limited to this embodiment.

[Effects of the Invention] As explained above, the semiconductor wavelength filter of the present invention has the following effects:
At least the waveguide in a directional coupler type optical filter! Since it uses a waveguide that has a cutoff region for the 0th-order waveguide mode, it has a steeper waveguide than a conventional Bragg waveguide type optical wavelength filter or a conventional directional coupler type optical wavelength filter. Has wavelength selection characteristics and a wide wavelength stopband,
It also has the advantage of being easy to manufacture.

Furthermore, it also has the potential to be developed by providing gain to one waveguide of the directional coupler to realize a filter function (semiconductor optical amplifier, etc.).

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing the layer structure and refractive index distribution of one embodiment of the semiconductor optical wavelength filter of the present invention, and FIG. 2 is the layer structure of another example of the semiconductor optical wavelength filter of the present invention. Fig. 3 is a graph showing the dispersion characteristics of the semiconductor optical wavelength filter shown in Fig. 1, and Fig. 4 is a diagram showing the semiconductor optical wavelength filter shown in Fig. 1. Figure 5 is a graph showing the dispersion characteristics of a conventional directional coupler type optical wavelength filter. Figure 6 is a graph showing the power transfer rate of a conventional directional coupler type optical wavelength filter. The graph shown in FIG. 7 is a graph showing the wavelength dependence of the propagation constant difference δβ of the semiconductor optical wavelength filter shown in FIG. 1 and the coupling coefficient of the first waveguide and the second waveguide.
Fig. 8 is a graph showing the wavelength selection characteristics of the semiconductor optical wavelength filter shown in Fig. 1, and Fig. 9 is a schematic diagram showing the layer structure and refractive index distribution of an example of a conventional semiconductor optical wavelength filter. It is. ■...Clad, 2nd...Core of waveguide, 3rd...
- Core of the second waveguide, 4... Inner cladding of the first waveguide, 5... Asymmetric side cladding of the first waveguide.

Claims (1)

[Claims] A first single mode semiconductor waveguide and a second single mode semiconductor waveguide are sequentially stacked in the crystal growth direction of the semiconductor crystal, and the first single mode semiconductor waveguide and the second single mode semiconductor waveguide The waveguide parameters are determined such that the effective refractive index of the semiconductor waveguide is the same at a specific wavelength, and the first single mode semiconductor waveguide and the second single mode semiconductor waveguide are formed only at the specific wavelength. A directional coupler type optical wavelength filter that completely couples and does not completely couple at other wavelengths, wherein at least one of the single mode semiconductor waveguides has a cutoff region in the zero-order waveguide mode. Features of semiconductor optical wavelength filter