The present invention relates to a light-sensitive detector chiefly used for wavelength multiplexing of optical communication, and an optical demultiplexer using the light-sensitive detector.
Examples of a diffraction grating used as an optically demultiplexing element include: a planar or concave diffraction grating; a wave guide diffraction grating using an arrayed wave guide (AWG); a Bragg diffraction grating having a refractive index periodic structure provided in a fiber itself; and so on. Particularly, an optical demultiplexer using a reflection type planar or concave diffraction grating to constitute a space-optical system is used widely in an optical spectrum analyzer, a monochromater, and so on.
In the field of optical communication, an optically demultiplexing element is required for demultiplexing a wavelength-multiplexed optical signal propagated in an optical fiber in dense wavelength division multiplexing (DWDM) communication, so that a diffraction grating is used widely. There has been proposed an optical demultiplexer in which a wavelength-multiplexed optical signal propagated in an optical fiber is demultiplexed by a diffraction grating an light with each wavelength (channel) is made incident on a corresponding photodetector so that the quantity of light in each channel is monitored (for example, republished Patent WO99146629). In the optical demultiplexer used for monitoring the quantity of light with each or wavelengths demultiplexed as described above, photo detected is performed by a photodetector array.
FIG. 8 shows an example of the configuration of the optical demultiplexer. In FIG. 8, the optical demultiplexer is constituted by an optical fiber 4, a collimator lens 2, a planar diffraction grating 3, and a photodetector array 11. The photodetector array 11 includes a plurality of photodetectors 10 which are arranged for performing photoelectric conversion individually, and the photodetector array 11 is generally formed on a common semiconductor substrate and constituted by one semiconductor chip. From the point of view to maintain long-term reliability and facilitate handling properties, this chip is generally used in a state of being mounted on a package such as a CAN package or a DIP package (dual-in-line package).
Here, a total body including the photodetectors 10 or the array 11 thereof and the package 13 for mounting the photodetectors 10 or the array 11 thereon is called xe2x80x9clight-sensitive detector 1xe2x80x9d. The light-sensitive detector 1 has a light-transmissible window 12 in a surface opposite to the light-receiving surface of the photodetector array 11 so that the light-transmissible window 12 serves as means through which light entering the respective photodetectors 10 passes. Incidentally, the light-sensitive detector 1 needs to further have an electric wiring means for taking the photoelectrically converted electric signal to the outside. The electric wiring means includes lead frames and pins (terminals) generally embedded in the package, and bonding wires for connecting the respective photodetectors to the lead frames and pins. The electric wiring means is, however, omitted in FIG. 8.
Incident light 51 from the optical fiber 4 diverges in accordance with the numerical aperture of the optical fiber and is converted into collimated light 52 by the collimator lens 2. The collimated light 52 is made incident on the planar diffraction grating 3. The diffracted light 53 demultiplexed into channels in accordance with the wavelengths by the planar diffraction grating 3 passes through the collimator lens 2 again and is converted into converged light 54. As a result, light is condensed onto the light-receiving surfaces of the photodetectors 10, so that photoelectric conversion is performed.
In the optical demultiplexer configured as described above, the collimator lens 2 can play a role of collimating the light 51 diverged from the optical fiber 4 and a role of condensing the light 53 diffracted by the diffraction grating 3 onto the light-receiving surfaces of the photodetectors 10. Hence, the optical demultiplexer configured thus has a feature that the number of constituent parts is small. Moreover, in order to obtain an optical system which is high in diffraction efficiency and low in cost and has good stability against temperature, the optical fiber 4 and the light-sensitive detector 1 are mounted in the form of mounting near to so-called Littrow mounting in which the optical fiber 4 and the light-sensitive detector 1 are put as close to each other as possible so that the angle of light incident on the diffraction grating 3 is substantially equal to the angle or light made to exit from the diffraction grating 3.
The aforementioned background-art technique, however, has a problem in giving a serious influence on optical characteristic. That is, the light-sensitive detector 1 has the photodetector array 11 mounted in the inside of the package 13 having a finite size. Hence, even in the case where the optical fiber 4 and the photodetector array 11 are intended to be provided in the form of ideal Littrow mounting, the package 13 becomes a hindrance to the intention so that the distance between the mounted optical fiber 4 and the mounted light-sensitive detector 1 cannot be reduced any more than a predetermined value.
In such a case, a problem in optical characteristic occurs as follows. Light rays from the collimator lens 2 toward the diffraction grating 3 and light rays diffracted toward the light-sensitive detector 1 by the diffraction grating 3 are abaxial light rays viewed from the collimator lens 2. Hence, such light rays are influenced by abaxial aberration of the collimator lens 2, that is, coma aberration, astigmatism, and so on. As a result, the shape of a spot condensing into each of the photodetectors 10 may be enlarged or deformed so that efficiency of condensing the light onto the light-receiving surface of the photodetector 10 is lowered. In addition, in the photodetector array, there may arise a problem that crosstalk among the respective photodetectors (channels) is worsened.
As a measure to solve the above-mentioned problems, the package 13 may be removed so that the photodetector array 11 is treated as the semiconductor chip. However, if it is arranged so, a problem in reliability occurs so that a steady operation for a long term cannot be guaranteed. In addition, in a state in which the semiconductor chip is exposed to the outside, difficulty to handle the semiconductor chip increases when the optical demultiplexer is assembled.
On the other hand, measures to reduce the size of the package 13 as sufficiently as much may be conceived. From the point of view of reliability, however, a sealing structure cannot be avoided. For this reason, reduction of the size is limited.
The present invention is devised upon attention to the aforementioned problems in the background art. An object of the present invention is to provide a light-sensitive detector and an optical demultiplexer chiefly used in wavelength multiplexing of optical communication, in which the positional relation between the optical fiber and the photodetector array to be mounted in the light-sensitive detector can be determined in accordance with the design of an optical system without any limitation on the shape of the light-sensitive detector package to thereby achieve ideal optical characteristic.
A light-sensitive detector according to the present invention comprises at least one photodetector, a package on which the photodetector is mounted, and light-transmissible means for making detection light incident on a light-receiving surface of the photodetector, wherein the package is provided with means which is formed in a position adjacent to the photodetector and which allows transmission of light rays substantially parallel to the detection light therethrough.
The means for allowing transmission of light rays is a light-transmissible window which is constituted by an aperture formed in a part of the package made of an opaque member and which is sealed with a light-transmissible member. Alternatively, at least one part of the package may be made of a light-transmissible member. In addition, the package and the light-transmissible means for making the detection light incident on the light-receiving surface of the photodetector may be integrally molded as a light-transmissible member.
This light-sensitive detector may be used in connection not only with multiplexed light but also with non-multiplexed light.
An optical demultiplexer according to the present invention is constituted by an optical system in which incident light containing a plurality of wavelengths is made incident on a diffraction grating so that light demultiplexed by the diffraction grating is made to enter a light-sensitive detector having a plurality of photodetectors mounted therein. The light-sensitive detector used here includes photodetectors, a package on which the photodetectors are mounted, and light-transmissible means for making the demultiplexed light incident on light-receiving surfaces of the photodetectors, wherein the package is provided with means formed in a position adjacent to the photodetectors for allowing transmission of incident light entering the photodetectors so that light is made incident on the diffraction grating through the means for allowing transmission of the incident light.
The means for allowing transmission of the incident light is a light-transmissible window constituted by an aperture which is formed in a part of the package made of an opaque member and which is sealed with a light-transmissible member. Alternatively, at least one part of the package may be made of a light-transmissible member. When an end surface of an optical fiber is mounted near to the means for allowing transmission of the incident light, the incident light can be supplied easily.
The present disclosure relates to the subject matter contained in Japanese patent application No. 2000-394406 (filed on Dec. 26, 2000), which is expressly incorporated herein by reference in its entirety.