Optical demultiplexer and method of assembling same

Details Optical demultiplexer and method of assembling same Optical demultiplexer and method of assembling same

1999-11-10

2001-05-29

Pascal, Leslie (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C359S572000

Reexamination Certificate

active

06239891

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an optical demultiplexer primarily for use in wavelength division multiplex optical communications.
BACKGROUND ART
One conventional optical demultiplexer for use in wavelength division multiplex optical communications include an optical demultiplexer having optical fibers as input and output mediums. For example, an optical demultiplexer disclosed in Japanese laid-open patent publication No. 9-243855 has input and output optical fibers positioned at the focal point of a collimator lens. A light beam emitted from the input optical fiber is converted by the collimator lens into a parallel beam, which is applied to a Littrow reflective diffraction grating. The parallel beam applied to the Littrow reflective diffraction grating is demodulated by chromatic dispersion characteristics thereof into light beams that are focused by the collimator lens onto the end surface of the output optical fiber in respective channels. The above optical demultiplexer has performed the optical demultiplexing function as described above.
There are also known an optical demultiplexer having an optical fiber as an input medium and an optical waveguide as an output medium as disclosed in Japanese laid-open patent publication No. 8-75948, and an optical demultiplexer having an optical fiber as an input medium and a photodetector array as an output medium as disclosed in Japanese laid-open patent publication No. 7-30485.
It is important for the optical demultiplexers to achieve a desired level of accuracy for the alignment of various components. Particularly, the Littrow optical demultiplexer demands such alignment accuracy.
In the case of optical demultiplexers with reflective diffraction gratings, it is found difficult to monitor the state of a light beam applied to the diffraction grating. Therefore, efficient adjustments are hard to perform in the assembling process. When such an optical demultiplexer malfunctions, the cause of the malfunction cannot easily be identified, making it impossible to conduct quick troubleshooting.
An operation monitor such as an optical fiber amplifier for use in wavelength division multiplex optical communications is required to evaluate an average light quantity per channel. Conventional optical demultiplexers need to electrically add output power levels in respective channels and divide the sum of the output power levels by the number of channels from which the output power is obtained. Accordingly, the conventional optical demultiplexers are disadvantageous in that their electric circuit is complex and expensive.
It is therefore an object of the present invention to provide an optical demultiplexer which has a reflective diffraction grating and is capable of monitoring the state of a light beam applied to the diffraction grating, and a method of assembling such an optical demultiplexer.
DISCLOSURE OF THE INVENTION
The above object can be achieved by an optical demultiplexer according to the present invention which has a light transmitting area formed in a portion of a reflective diffraction grating.
Specifically, according to a first aspect of the present invention, an optical demultiplexer having light input means, a collimator lens, a substrate with a diffraction grating formed thereon, and light detecting means for detecting light demultiplexed by the diffraction grating, characterized in that said diffraction grating comprises a reflective diffraction grating having a reflecting surface, said light input means and said light detecting means face said reflective diffraction grating with said collimator lens interposed therebetween, said reflecting surface including a light transmitting area in at least a portion thereof.
According to an invention described in claim
2
, in an optical demultiplexer according to claim
1
, said light transmitting area is an area where said reflecting surface is not formed.
According to an invention described in claim
3
, in an optical demultiplexer according to claim
1
, said light transmitting area is an area of low reflectivity.
According to an invention described in claim
4
, in an optical demultiplexer according to claim
1
, said substrate is disposed such that an angle at which light applied to said light transmitting area passes through said diffraction grating and is applied to the reverse side of said substrate becomes smaller than an angle of total reflection of the light.
According to an invention described in claim
5
, an optical demultiplexer according to claim
1
further comprises a photodetector for measuring a quantity of light, behind the substrate of said diffraction grating in alignment with said light transmitting area. The photodetector should preferably be directly mounted on the reverse side of the substrate.
According to an invention described in claim
6
, in an optical demultiplexer according to claim
1
, said light input means comprises an optical fiber.
According to an invention described in claim
7
, in an optical demultiplexer according to claim
1
, said light detecting means comprises one selected from a group consisting of an optical fiber array comprising a plurality of optical fibers, a waveguide array comprising a plurality of waveguides, and a photodetector array comprising a plurality of photodetectors.
According to an invention described in claim
8
, an optical demultiplexer according to claim
1
further comprises a lens disposed behind the substrate of said diffraction grating, for focusing light having passed through said light transmitting area, and light detecting means disposed behind the substrate of said diffraction grating, for detecting the light which has passed through said light transmitting area and has been focused.
According to an invention described in claim
9
, in an optical demultiplexer according to claim
1
, said light detecting means comprises one selected from a group consisting of an optical fiber array comprising a plurality of optical fibers, a waveguide array comprising a plurality of waveguides, and a photodetector array comprising a plurality of photodetectors.
As described above, the present invention resides in that in the optical demultiplexer employing the reflective diffraction grating, the reflective diffraction grating has the light transmitting area for allowing applied light to be observed from the reverse side of the diffraction grating.
According to another aspect of the present invention, there is provided a method of assembling an optical demultiplexer having an input optical fiber, a collimator lens, a diffraction grating, and light detecting means for detecting light demultiplexed by the diffraction grating, characterized in that said diffraction grating comprises a reflective diffraction grating having a reflecting surface, said reflecting surface includes a light transmitting area in a portion thereof, and the optical axis of the diffraction grating is positionally adjusted while light having passed through the light transmitting area is being monitored.


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Kobayashi, K., et al., “Microoptic Grating Multiplexers and Optical Isolators for Fiber-Optic Communications”,IEEE—Journal of Quantum Electronics, QE-16(1), pp. 11-22, (Jan. 1980).
Lipson, J., et al., “A Six-Channel Wavelength Multiplexer and Demultiplexer For Single Mode Systems”,Journal of Lightwave Technology, LT-3(5), pp. 1159-1163, (Oct. 1985).
Masahiro

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