JPH0797193B2 - Variable wavelength filter - Google Patents

Description

TECHNICAL FIELD The present invention relates to a tunable wavelength filter used for wavelength-multiplexed optical transmission and the like.

(Prior Art) A tunable wavelength filter that selects an optical signal of an arbitrary wavelength from a wavelength multiplexed optical signal is one of the key devices that can be applied to a wide range of applications in optical transmission, optical switching, optical information processing, and the like. is there. In any of the applications, sufficient wavelength selectivity and wide tuning range of the selected wavelength are required as the characteristics of the variable wavelength filter. Further, since it is inevitable to form an optical integrated circuit as a structure, it is also necessary to use a transmission type wavelength selection filter that transmits only an arbitrary desired wavelength.

BACKGROUND ART Heretofore, some studies have been made on wavelength selective filters. Among them, filters using semiconductors are drawing attention from the aspect of integration. For a filter having a uniform diffraction grating on a semiconductor substrate, a structure having gain (Applied Physics Letters (Applie
d Physics Letters, Vol. 50, p. 66, 1987) and structure without gain (Electronics Letters)
s) Volume 23, p. 622, 1987).

(Problems to be Solved by the Invention) However, a filter having a uniform diffraction grating on a semiconductor substrate has the following drawbacks.

In the case of a filter having a gain, the gain and the transmission wavelength are changed by injecting the current, but the injection current is limited to the oscillation threshold or less. Therefore, the variable wavelength range was as small as several Å.

On the other hand, in the case of a filter without gain, the variable wavelength range is ~ 100.
It is thought that even Å is possible, but it is basically a reflection type filter, and when viewed as a transmission type filter, it becomes a band cut filter. Therefore, in order to extract only the optical signal of a specific wavelength, the input side of the filter must be branched and the reflected light from the filter must be detected. However, sufficient light is necessary for the loss due to the branch and the reflection at the branch point. Difficult to get output.

It is an object of the present invention to provide a band-transmissive variable wavelength filter that solves the above problems and that has a large variable wavelength and that transmits only an optical signal of an arbitrary wavelength.

(Means for Solving the Problems) Means provided by the present invention for solving the above-mentioned problems are as follows.
A band transmission type variable wavelength filter composed of a Mach-Zehnder interferometer, wherein a band cut variable wavelength filter is inserted into a first optical path of two optical paths after branching, and an optical path is included into a second optical path. It is characterized in that it includes an optical element whose length changes according to an external signal.

(Function) Among the filters in which a uniform diffraction grating is provided on the semiconductor substrate, a filter having no gain does not cause oscillation even if a current is injected. Therefore, the variable wavelength range is limited only by the heat generated by current injection, and a variable width of about 100Å can be obtained. Such a filter is a band cut filter, but has a large variable wavelength range. FIG. 3 shows a band-transmissive variable wavelength filter of the present invention configured to extract only an optical signal of a specific wavelength by using such a band-cut variable wavelength filter.

The beam splitter 11, the mirrors 12 and 14, and the beam combiner 15 are arranged as shown in FIG. This is a Mach-Zehnder interferometer. The beam splitter divides the optical path into two, and the band cut filter 30 is arranged on one optical path 41. The optical path length adjuster 16 is arranged on the other optical path 42.

When a wavelength-multiplexed optical signal is input to this system, the beam splitter 11 splits the optical path into two. The beam passing through the optical path 41 is attenuated by the band cut filter 30, for example, only the signal of λ ₂ and becomes a spectrum like point B. On the other hand, in the beam passing through the optical path 42, only the optical signal of a specific wavelength is not attenuated. The beam combiner 15 combines the beams passing through the optical paths 41 and 42. If the optical path length adjuster 16 adjusts the optical path length, only the signal of λ ₂ can be taken out by the interference of the _two beams that have passed through the optical paths 41 and 42, respectively.

With the above-described configuration, it is possible to obtain a filter having a large variable wavelength range of about 100 Å and capable of extracting only an optical signal having an arbitrary wavelength. Here, the following points should be noted. The transmittance T of the Mach-Zehnder interferometer is
Frequency f _{i of} optical signal, refractive index n of optical path, optical path difference Δl, speed of light c
Between There is a relationship. When the optical path difference Δl = 0, T = 1 is always satisfied from the equation (1), so that an optical signal of an arbitrary frequency f _i (wavelength λ _i ) can be sent. On the other hand, when Δl ≠ 0, in order to set T = 1, the frequency interval Δf of the wavelength multiplexed signal is calculated from the equation (1). Must be set to satisfy the relationship.

(Embodiment) FIG. 1 is a block diagram showing an embodiment of the present invention.

In FIG. 1, the beam splitter 11, the mirrors 12 and 14, and the beam combiner 15 can constitute a Mach-Zehnder interferometer.
As an input wavelength multiplexed signal, 50 waves are lined up at intervals of 2Å 1.5
An optical signal in the μm band is incident. The beam splitter 11 splits the incident beam into optical paths 41 and 42. The optical path 42 includes a beam splitter 11, a mirror 12, an optical path length adjuster 16 and a mirror 14.
It is an optical path that goes through to the beam combiner 15. The band-cut filter 30 has a uniform diffraction grating formed on a semiconductor substrate, crystals of a guide layer and a clad layer grown on the diffraction grating, and then an embedded structure for lateral mode control. In order to narrow the band width in the band cut filter 30, the height of the diffraction grating is made low, that is, the coupling constant k is kept low. The 3dB downband width was 1Å. When a current of 100 mA was injected, the transmission blocking wavelength moved 100Å toward the short wave side. The beams passing through the optical paths 41 and 42 are combined by the beam combiner 15. By adjusting the optical path length with the optical path length adjuster 16, only specific wavelengths can be transmitted. By adjusting the optical path length and then injecting a current into the band cut filter 30, only the optical signal of an arbitrary wavelength can be extracted from the input signal of 50 wavelengths.

FIG. 2 is a plan view showing another embodiment of the present invention. In this embodiment, each element is integrated on one common semiconductor substrate 20, and a diffraction grating region 23 is formed in a part of the semiconductor substrate 20. In the fabrication of this embodiment, after the guide layer and the clad layer are grown on the semiconductor substrate 20, the light guide 2
The growth layer is etched leaving 1, 24, the optical path length adjusting region 22 and the diffraction grating region 23. After that, embedded growth is performed. The electrodes are separately provided in the optical path length adjusting region 22 and the diffraction grating region 23 so that current can be independently injected. After the wavelength-multiplexed signal is incident on the light guide 21, the optical path length adjustment region
It is divided into a diffraction grating region 23 and a diffraction grating region 23, and after passing through the respective regions, they are superposed by a light guide 24. Optical path length adjustment area 22
The optical length can be changed by injecting a current into
Further, if a current is injected into the diffraction grating region 23, the transmission blocking wavelength changes, so that a transmission filter is formed according to the same principle as that of the embodiment of FIG.

In the embodiment shown in FIG. 1, the Mach-Zehnder interferometer is constructed by using the beam splitter, the mirror, and the beam combiner, but in the present invention, it is possible to use an optical fiber, a fiber coupler or the like instead. Absent. Further, the band cut filter is not limited to the semiconductor, and any variable wavelength band cut filter may be used in the present invention. The integrated filter having the structure shown in FIG.
Not only semiconductors but also materials other than semiconductors can be manufactured.

(Effects of the Invention) As described in detail above, according to the present invention, it is possible to provide a band transmission type tunable wavelength filter having a large wavelength tunable range of 100Å and approximately 50 channels.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an embodiment of the present invention, FIG. 2 is a plan view showing another embodiment of the present invention, and FIG. 3 is a wavelength-multiplexed optical signal propagating through a filter of the present invention. It is a conceptual diagram which shows a mode. 11 …… Beam splitter, 12,14 …… Mirror, 15 …… Beam combiner, 16 …… Optical path length adjuster, 20 …… Board, 21,24
...... Light guide, 22 ...... Optical path length adjustment area, 23 ...... Diffraction grating area, 30 ...... Band cut filter, 41, 42 ...... Optical path.

Claims (1)

[Claims] 1. A band transmission type variable wavelength filter comprising a Mach-Zehnder interferometer, wherein a band cut variable wavelength filter is inserted into a first optical path of two optical paths after branching, and a second optical path. A tunable wavelength filter including an optical element whose optical path length changes according to an external signal.