JPS63212905A - Waveguide type optical filter - Google Patents

Abstract

PURPOSE:To make a wavelength region of transmitted light through a waveguide type optical filter flat and to make its selectively for wavelength steep by forming a first and a second periodical structure having each specified Bragg wavelength, length, and coupling factor on an optical waveguide. CONSTITUTION:A clad layer 8 comprising SiO2 and a core layer 9 comprising SiO2 are formed on a substrate 7, and an optical filter 10 is formed on the core layer 9. A first periodical structure 11 having lambda1 Bragg wavelength L1 length, and N coupling factor, a periodical structure having by pi lagged phase 12, and a second periodical structure 13 having lambda1 Bragg wavelength L2 length, and N coupling factor are formed on the core layer 9. Thus, light of lambda1 Bragg wavelength and light having lambda2 cut-off wavelength out of said Bragg wavelength are made incident to the optical waveguide. In this stage, the light having lambda1 wavelength passes through an optical filter 10 and the light having lambda2 wavelength is reflected by the effect of the first and second periodical structure 11, 13, so the optical filter functions as an optical branching filter. if the number M of the first periodical structure 11 is increased, the transmitting loss can be decreased. Thus, the wavelength region of transmitted light is widened and the selectivity for wavelength is made steep.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Minute Hand> The present invention relates to a waveguide type optical filter having steep wavelength selection characteristics and flat transmission wavelength characteristics.

<Conventional technology and its problems> A typical example of a conventional waveguide optical filter is the Bragg waveguide applied to distributed feedback semiconductor lasers! be. Figure 4 shows the structure of this Bragg waveguide, where 1 is an n-1nP substrate, 2 is a waveguide layer, 3 is an active layer, 4 is a buffer layer, and 5 is a P
- Summer nP layer 6 is a periodic structure with a first-order coupling order provided at the interface between the n-1nP substrate 1 and the waveguide layer 2. FIG. 5 is an enlarged model diagram of the periodic structure of the Bragg waveguide shown in FIG. 4, and the phase of the periodic structure is shifted by π in the middle of the periodic structure. FIG. 6 shows the power reflectance characteristics at the left end of this periodic structure. however,
Δβ#β−β1, β=π/(period of the periodic structure), β is the propagation constant of the guided light, and L is the length of the periodic structure.

Δβ indicates the deviation from the Bragg wavelength λ (=2π/β). Further, χ is a coupling coefficient, which is generated when two modes of interest among the eigenmodes propagating in the waveguide are perturbed in the periodic structure portion. Here, the two modes include modes that have the same eigenvalue but differ only in the propagation direction. In FIG. 6, the vertical axis shows the deviation of the guided light wavelength from the standardized Bragg wavelength, and the vertical axis shows the reflectance by the optical filter. As is clear from this figure, the optical filter has a reflectance of zero for light at the Bragg wavelength where Δβ=0, and operates as an optical filter that transmits the light at the Bragg wavelength. However, when used as an optical multiplexer/demultiplexer, the transmission wavelength is limited to a narrow band near the Bragg wavelength, and the attenuation in the stopband is not large enough.

<Objective of the Invention> The object of the present invention is to solve the narrow-band characteristic of the wavelength transmission characteristic in which the transmission wavelength of a conventional optical filter using Bragg reflection is only the Bragg wavelength, and to have a flat transmission wavelength range and a steep transmission wavelength. An object of the present invention is to provide a waveguide-type optical filter using Bragg reflection having excellent wavelength selectivity.

Means for Solving the Problems〉 The structure of the present invention to achieve the above object is a first optical waveguide having a Bragg wavelength λ8 and a length Ll formed on an optical waveguide and whose coupling order is Nth.
Before and after cascade-connecting M periodic structures with a phase difference of π/N, a second periodic structure with a Bragg wavelength λ1 and a length L2 with a coupling order of Nth is connected with a phase difference of π/N. so that it becomes 1
The main feature is that they are connected in cascade one by one.

Embodiment> FIG. 1 is a diagram for explaining a first embodiment of the present invention, in which FIG. 1(a) shows an optical demultiplexer using the present invention;
Figure (bl is a cross-sectional view of the optical filter 10 with a periodic structure. A first-order periodic structure was used as the periodic structure. In the figure, 7
is a substrate, 8 is a cladding layer, 9 is a core layer, 10 is an optical filter of the present invention, 11 is a first periodic structure having a length Ll,
12 is a phase shift part of π of the periodic structure, 13 is the length L2
This is the second periodic structure portion.

In this embodiment, a silicon wafer with a thickness of 0.5 mm is used as the substrate 7, and the cladding layer 8 is 5LO2 with a refractive index of 1.46 formed by thermal oxidation in wet 0. The core layer 9 is made of Corning 7059 with a refractive index of 1.53.
Glass was formed by sputtering. The periodic structures 11 and 13 of the optical filter 10 are formed by processing the upper part of the core layer into a waveform using an interference exposure method and a reactive ion beam etching method. The periodic structure 11.13 is tilted by 1 degree with respect to the optical axis in order to separate incident light and reflected light.

The operation of this embodiment is shown below.

Light having a Bragg wavelength λ, which is a wavelength passed through the optical filter 10,
Light having a cutoff wavelength λ shifted from the Bragg wavelength is input to the optical waveguide. When the waveguide reaches the optical filter 10 of the periodic structure 11, 13, the light of wavelength λ passes through the optical filter 10, and the light of wavelength λ2 is reflected. In this case, it operates as an optical demultiplexer, but it is clear that if the incident light is reversed, it operates as an optical multiplexer.

The results of a theoretical study of the relationship between the parameters of the periodic structure and the transmission characteristics of the optical filter 10 are shown in FIGS. 2 and 3. In both figures, the horizontal axis represents the deviation Δβ of the guided light propagation constant from the Bragg wavelength, normalized by the coupling coefficient χ, and the vertical axis represents the passage loss of the optical filter. FIG. 2 shows the characteristics with respect to the number M (M>l) of the first periodic structures 11, where M=1
The case corresponds to the case of the Bragg waveguide of the distributed feedback semiconductor laser shown in FIG. For M=1, the Bragg wavelength Δ
At β/χ = 0, the passing loss becomes zero and thereafter the passing loss increases, but when M is increased, a point occurs where the passing loss becomes zero even at other than Δβ/χ = 0 within the passband of Δβ/χ < 0.3. At the same time as the ripple appears, the passing loss in the cutoff region around Δβ/χ=1 increases.

In this way, in the invention, the first periodic structure 110 number M
By increasing , it is possible to widen the pass wavelength range, steepen the wavelength selection characteristics, and reduce crosstalk in the cutoff wavelength range.

FIG. 3 shows the length of the first periodic structure 11.

and the length 2 of the second periodic structure 13, /L.

It is a characteristic for L2/L, <0.5, Δβ/
No ripple occurs within the transmission range of χ<0.3, and L2/L1
>0.5, the transmission range becomes wider and the ripple height also becomes higher. However, since the change is smaller than when M is changed, the length ratio of the periodic structure L2/L
.

By adjusting , it is possible to finely change the wavelength passing characteristics of the optical filter.

<Effects of the Invention> As explained above, in the present invention, compared to conventional Bragg reflection filters, it is possible to widen the passing wavelength range and sharpen the wavelength selectivity, and the wavelength of the light source is not affected by factors such as temperature. This has the advantage that it can be applied to an optical multiplexer/demultiplexer whose characteristics do not change even when the characteristics change.

[Brief explanation of the drawing]

FIG. 1 (al(b)) is a perspective view and a sectional view of the first embodiment of the present invention, in which 7 is a substrate, 8 is a cladding layer,
9 is a core layer, 1o is an optical filter of the present invention, 11 is a first periodic structure section, 12 is a phase shift section of the periodic structure, and 13 is a second periodic structure section. FIG. 2 is a transmission characteristic diagram for the number of first periodic structures, and FIG. 3 is a transmission characteristic diagram for the ratio of the length of the first periodic structure to the length of the second periodic structure. FIG. 4 is a structural diagram of a conventional Bragg waveguide applied to a distributed feedback semiconductor laser. In the figure, 1 is an n-1nP substrate, 2 is a waveguide layer, 3 is an active layer, and 4 is a buffer layer. , 5 is a P-InP layer, and 6 is a periodic structure. Fifth
The figure is a model diagram of the periodic structure of the Bragg waveguide shown in FIG. 4, and FIG. 6 is a power reflectance characteristic diagram of the periodic structure.

Claims (1)

[Claims] M first periodic structures with a Bragg wavelength λ_1 and a length L_1 and a coupling order of Nth order (N≧1) formed on an optical waveguide are arranged so that the phase difference between adjacent periodic structures is π/N. (
M≧1) One second periodic structure with a Bragg wavelength λ_1 and a length L_2 and a coupling order of N order is connected in cascade, and one second periodic structure with a coupling order of Nth order and a Bragg wavelength λ_1 and a length L_2 is connected before and after the M first periodic structures connected in cascade. 1. A waveguide-type optical filter characterized in that the waveguide-type optical filters are cascade-connected so that the phase difference between adjacent periodic structures is π/N.