1. Field of the Invention
The present invention relates to an optical device suitable for use as a wavelength-multiplex optical terminal for demultiplexing a light beam in a particular wavelength range from signal light beams in various wavelength range, i.e., light beams of communication service signal or video signal light beams, transmitted through an optical fiber.
2. Description of the Related Art
With the recent development of wavelength-multiplex communications using a fiber amplifier, it has become customary to monitor the amounts of optical signals at respective wavelengths, adjust the amounts of optical signals, and then have an amplifier amplify the optical signals in order to maintain desired amplifier characteristics.
Various processes are known for monitoring the amounts of optical signals. According to the known processes, since monitor devices are associated with respective optical fibers, they are alone required to have a considerable size.
There have been demands for a small-size monitor device which is packaged with high density. A monitor device monitors an optical signal by extracting a portion of the optical signal. It is desirable for a monitor device to be able to monitor an optical signal without significantly attenuating the optical signal.
Heretofore, solutions to the above problems and demands have been proposed in Japanese Laid-Open Patent Publication No. 2003-294990, Japanese Laid-Open Patent Publication No. 2003-294992, Japanese Laid-Open Patent Publication No. 2003-295000, WO03/096095, WO03/060584, and WO03/098293.
In recent years, attempts have been made to add broadcasting services as new value-added services to FTTH (Fiber To The Home) services. A broadcasting downstream signal wavelength is added to upstream and downstream signal wavelengths of IP (Internet Protocol) services through a single optical fiber in the new services. In other words, combined services for communication and broadcasting by the single-core, three-wavelength optical fiber have been proposed.
In order to realize such combined communication and broadcasting services, a customer premises facility is required to include a B-ONU (Broadband Optical Network Unit) that is need for conventional IP services and a V-ONU (Video-Optical Network Unit) for receiving broadcasting services. Stated otherwise, the subscriber needs to have a wavelength-multiplexing optical terminal for processing upstream and downstream signals having different wavelengths. In the future, there is a possibility to transmit a downstream video signal at another wavelength, and a wavelength-multiplexing optical terminal for three wavelengths will be required to handle all those signals.
The wavelength-multiplexing optical terminal mainly comprises a WDM (Wavelength Division Multiplexing) filter, a light-detecting unit (e.g., a photodiode) for receiving a downstream signal, and a light-emitting device (e.g., a laser diode) for transmitting an upstream signal.
For demultiplexing wavelengths with WDM filters, ordinary filters are required to have an attenuating level of about 25 dB in the cut-off range, but filters designed to meet special specifications are required to have an attenuating level of about 40 dB in the cut-off range. As shown in OPTCOM, March 2004, page 38, a bandpass filter (rejection filter) may be inserted between a WDM prism and a light-detecting unit.
Conventional wavelength-multiplexing optical terminals are mainly classified into a microoptics design using a lens and a PLC design using an optical waveguide, as shown in OPTORONICS, January 2004, page 173.
Since both designs employ a light transmitting component such as a lens or an optical waveguide for guiding optical signals to a WDM filter, it is necessary to meet a most important requirement for optical components, i.e., optical alignment between the optical fiber and the light transmitting component. Accordingly, the cost of the resultant assembly tends to be high and the connection is liable to be of low reliability.
For providing combined communication and broadcasting services inexpensively, there has been proposed a V-ONU-compatible optical device having an integrated combination of a WDM function based on the microslitting technology (see “Standards of optical fiber arrays and future trend”, OPTICAL ALLIANCE, November 2005) for directly slitting an optical fiber, and a light-detecting function.
The V-ONU-compatible optical device fabricated according to the microslitting technology comprises a wavelength-demultiplexing thin-film substrate inserted in an oblique silt directly defined in an optical fiber and a light-detecting component disposed above the optical fiber for detecting an optical signal beam demultiplexed by the wavelength-demultiplexing thin-film substrate. The V-ONU-compatible optical device thus constructed is inexpensive to manufacture, and yet incorporates an integrated combination of the wavelength-demultiplexing function and the light-detecting function.
A bandpass filter may be disposed on the optical path of the demultiplexed optical signal beam in order to increase the optical signal beam attenuation in the cut-off range.