1. Field of the Invention
The present invention relates to the structure of an optical communication apparatus and, more particularly, to a structure included in an optical communication apparatus for mounting an optical circuit.
2. Description of the Related Art
An optical circuit for the above application includes optical parts, optical fibers, splices and optical connectors. The optical circuit may additionally include photoelectric parts, an electric control circuit, Peltier elements for cooling, a Peltier driver, and a heat sink, as needed. These optical elements of the optical circuit are, in many cases, mounted on a single flat plate as far as possible. When all the optical elements as well as associated structural elements are mounted on a single flat plate, they are easy to assemble and adjust. Further, the optical fibers connecting the optical parts cannot be bent at acute angles, and must be laid with a radius of curvature greater than a preselected one. Generally, the minimum radius of curvature of the fibers must be 30 mm or above although it depends on the kind of the fibers. Therefore, arranging the optical circuit on a single flat plate additionally meets such a requirement relating to the optical fibers and facilitates the design of the optical circuit.
The optical circuit may be received in a box-like casing, as proposed in the past. The casing, or module, is sized, e.g., 10 cm.times.10 cm.times.1 cm or 20 cm.times.50 cm.times.3 cm. After all the optical parts and optical circuit have been arranged in the casing, the casing is closed by a protection cover. This kind of scheme protects the optical circuit from damage and promotes easy handling after assemblage.
In the above module structure, a surface for mounting the optical circuit is implemented as a single surface or as a plurality of surfaces. For a single mounting surface, use is made of a printed circuit board received in the casing or a surface corresponding to the protection cover. For a plurality of mounting surfaces, the inside of the casing is provided with a multilayer structure, e.g., a two-storied or three-storied structure. The multilayer structure reduces the widthwise and lengthwise dimensions of the module, but increases the thicknesswise dimension.
The optical parts, as distinguished from electrical parts, cannot be easily connected by a printed circuit board or the like, and must be connected by optical fibers in a network configuration. However, it is impossible to connect the optical parts by fibers whose length is the same as the distance between the optical parts, i.e., the fibers each needs an extra length. While the extra portions of the fibers must be rearranged and mounted, the prerequisite is that all the fibers including their extra portions be smoothly laid with a radius of curvature greater than the minimum radius of curvature. As a result, the total space for accommodating the extra portions of the fibers is equal to or even broader than the space for accommodating the optical parts.
Although the previously stated conventional mounting structures each takes account of the above characteristic of optical circuit mounting, they have the following problems left unsolved. When the entire optical circuit is mounted on a single surface, a mounting design matching with the above characteristic is achievable. However, the mounting area, as seen in a plan view, increases with an increase in the scale of the optical circuit. On the other hand, because the mounting space is limited by various conditions including the overall dimensions of an apparatus, the circuit scale is limited. Particularly, in parallel with the progress of optical communication technologies, an optical amplifier circuit and other circuits of the type using a wavelength multiplexing scheme or erbium doped fibers are increasing in scale. This type of optical fibers cannot be sufficiently coped with by the predominant arrangement in which the entire optical circuit is mounted on a single surface.
The module scheme with a multilayer structure is capable of coping with the increasing scale of the optical circuit. However, because each layer of the module is implemented by a sheet metal or similar structural member, the surface for mounting the optical circuit is divided into a first surface, second surface and so forth lying one above the other. The optical parts mounted on the consecutive layers or floors are interconnected by optical fibers. This connection is made permanent by splices involving a minimum of optical signal loss. Therefore, the connection between the consecutive layers is extremely difficult and restricted in the aspect of design and manual work. In addition, it is more difficult to disassemble, adjust or repair the completed module.
Japanese Patent Laid-Open Publication No. 4-359202 proposes a solution to the above problems. The solution consists in dividing a tray for receiving the extra portions of optical fibers into a plurality of trays. The adjacent trays are connected together by hinges at one side thereof. Such trays are stacked one upon the other in a shelf configuration, so that the top of a desired tray can be uncovered for easy access. This promotes easy assemblage and easy disassemblage, adjustment and repair after the assemblage.
However, the arrangement taught in the above document simply deals with the extra portions of optical fibers and does not pay attention to the connection between the consecutive layers or floors on which the optical circuit is mounted. Specifically, a number of optical fibers are permanently connected between the layers by splices. When the hinges are opened and closed in such a condition, the fibers must be protected from damage. Further, maintenance, e.g., removing a desired part from the structural member constituting the mounting surface without cutting the permanent connection is not practicable. Moreover, because the above hinge structure does not allow three or more layers to be opened at the same time, the assemblage must be manipulated layer by layer. For these reasons, the conventional hinge scheme is little better than the simple multilayer scheme as to design freedom and easy assemblage.