This invention relates to fiber optic devices and, more particularly, to devices for monitoring, switching, attenuating or distributing optical signals in fibers.
Optical transmission systems are being developed to handle large amounts of communications traffic. One type of optical transmission system uses optical fibers as the transmitting media. Optical fibers with sufficiently low loss for practical long distance optical communication systems have been developed only recently.
Any communication system requires both transmitting and receiving apparatus. It is desirable that the transmitting source have a constant power output that is independent of temperature changes and component aging. One source of optical power is a laser which changes output power in response to changes of both temperature and aging. Output power of the laser can be stabilized by monitoring the light actually coupled into the fiber and by using such light to develop a signal for feedback control of the laser. To effectively monitor such light in the fiber for controlling the laser, a tap should monitor the transmitted light without altering the characteristic of the transmitted power. All transmitted modes should be monitored equally because any substantial preference of one mode or modes over another mode or modes is unsuitable for accurate feedback control of the laser. Available fiber optic taps substantially prefer one optical mode or modes over another mode or modes.
Fiber optic systems need optical tap devices for economically monitoring and controlling the optical power. A wide variety of devices are being developed to fill these needs. Development of a device may result in a specific and perhaps optimum component for each particular application. Such a specific development can be costly. As an alternative a designer may desire to convert the optical signal to an electrical signal and process the latter electronically taking advantage of well developed and versatile electronic technology. This electronic approach introduces noise into the system as well as requiring a source of external power.
Problems created by developing specific components for each application and by converting optical signals to electronic signals for processing are avoided by the use of an optical fiber access port in a tap for monitoring and controlling optical power and for building many optical control devices without substantial further development effort regardless of changes introduced by new fiber materials and structures.
A practical optical communications system requires at least a few different optical devices for establishing desirable interconnection arrangements. Optical switches can be used so that part of the system can be rearranged conveniently either for testing or for providing a variety of communications needs. Additionally, a variable optical attenuator can be used for adjusting the magnitude of power being transmitted in any part of the system. Such attenuators are helpful in setting levels for testing and for establishing some branching arrangements. Another device which facilitates the design of optical communication systems is a multibranch distributor. Such a distributor divides incident light, transmitted along one fiber, into portions for further transmission along two or more other fibers.
As in the case of the optical fiber tap, the variable attenuator and the multiple branch distributor should perform their designated functions without substantial preference of some mode or modes over another mode or modes. Accurate transmission of information often requires faithful communication of the input signal characteristic throughout the system.
Available fiber optic devices are relatively complicated and expensive devices whereas it is desirable for reasons of cost and ease of operation to have simple, inexpensive fiber optic devices. Such simple, inexpensive devices enhance the practicality of optical communications systems arrangements.
These problems of complicated design, high cost and mode selectivity of the existing fiber optical devices are solved by a group of fiber optic devices, each including an access port.
It is an object to provide a family of simple fiber optic devices for coupling optical power between fibers.
It is a further object to provide fiber optic devices which are substantially insensitive to optical modes carried by a fiber.
It is another object to provide a basic unit for constructing different optical control devices.