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.