Optical Communication System With a Fiber-Optic Amplifier

1. Field of the Invention

The present invention relates to a system for transmitting an optical signal over an optical waveguide containing a fiber-optic amplifier.

2. Description of the Prior Art

A system for transmitting a first optical signal over an optical waveguide containing a fiber-optic amplifier which includes an amplifying length of fiber, a pump source, and a wavelength-selective fiber-optic coupler for coupling the pump source to the amplifying length of fiber is known from "EOCC '89", Fifteenth European Conference on Optical Communication, Sept. 10-14, 1989, Gothenburg, Sweden, Proceedings, Vol. 1, Regular Papers, TuA 5-7, pages 86 to 89.

In the above-referenced article, two different system configurations are explained. In the first ("co-propagating configuration"), the light from the pump source is launched into the Er.sup.3+ -doped length of fiber via a coupler which is located in front of the amplifying length of fiber when viewed in the direction of transmission of the optical signal to be amplified. In the second system configuration ("counter-propagating configuration"), the coupler is located behind the length of fiber when viewed in the direction of transmission of the signal to be amplified. Sometimes it is desirable or necessary in transmission systems to transmit, in addition to the first optical signal, a second optical signal from the location of the fiber-optic amplifier in one direction or the other.

It is an object of the invention to provide a system suitable for the above purposes.

This object is attained by coupling an optical transmitter to an unused port of the fiber-optic coupler to provide a second optical signal to the unused port, whereby the second optical signal is provided to the optical waveguide.

FIG. 1 shows an embodiment for transmitting the second optical signal in a direction opposite to that of the first optical signal, and

FIG. 2 shows an embodiment for transmitting the second optical signal in the same direction as the first optical signal.

FIG. 1 shows a transmission system for transmitting an optical signal from a point A to a point B. At the end point A there is an optical transmitter (not shown), and at the end point (B) there is an optical receiver (not shown). The optical signal to be transmitted from A to B has a wavelength .lambda..sub.1 to 1540 nm. The transmission link is implemented with a single-mode optical waveguide 15 which has sufficiently good transmission properties at the wavelength .lambda..sub.1. The system of FIG. 1, like the prior art system referred to above, includes a fiber-optic amplifier 10 consisting of an Er.sup.3+ -doped length of fiber 11, a wavelength-selective fiber-optic coupler 12, and a pump source 13. The coupler 12 has four ports 1 to 4. Port 1 is connected to the optical waveguide 15 coming from the end point A, port 2 is connected to the Er.sup.3+ -doped length of fiber 11, and port 3 is connected via an optical waveguide to the pump source 13. The coupler 12 is a wavelength-selective coupler which has the property of coupling the optical signal of wavelength .lambda..sub.1, which is to be transmitted from A to B, from port 1 to port 2 with minimum loss, and of coupling the pump light generated by the pump source 13, which has a wavelength .lambda..sub.2 of 980 nm, from port 3 to port 2 with minimum loss. Thus, not only the optical signal to be amplified but also the pump light necessary for amplification is fed into the Er.sup.3+ -doped length of fiber 11.

According to the invention, the additional coupler port 4 is used to transmit a second optical signal from the location of the fiber-optic amplifier in the direction of the end point A of the transmission process. The electric signals to be transmitted in this direction, such as signals from service or backward channels, are converted by an optical transmitter 14 to an optical signal having a wavelength .lambda..sub.3 of 1300 nm, which is transmitted from the optical transmitter 14 through an optical waveguide to coupler port 4.

The coupler 12, which is optimized for the above-described coupling of the pump light, has the property of performing the additional function--without the need for special steps--to couple the optical signal from port 4 with low loss to port 1, whence it is transmitted over the optical waveguide 15 in the direction of the end point A of the system.

In the system according to the invention shown in FIG. 2, parts having the same functions as in FIG. 1 are designated by similar reference characters and, therefore, need no separate explanation. The coupler 12 is located behind the Er.sup.3+ -doped length of fiber 11 when viewed in the direction of transmission of the signal to be transmitted from A to B. As in FIG. 1, it couples the signal from port 1 to port 2 and the pump light from port 3 into the length of fiber 11, the latter view port 1 instead of port 2 as in FIG. 1. In this configuration, an optical transmitter 14 having a wavelength .lambda..sub.3 of 1300 nm is connected via an optical waveguide to coupler port 4, so that a second optical signal can be transmitted from the location of the fiber-optic amplifier 10 in the direction of the end point B of the transmission system. The second optical system of wavelength .lambda..sub.3 travels in the same direction as the first optical signal of wavelength .lambda..sub.1 ; in the system of FIG. 1, it travels in the opposite direction.

Instead of only one coupler in front of or behind the Er.sup.3+ -doped length of fiber 11, couplers may be provided on both sides as in the case of the above-mentioned prior art system, so that it is possible to transmit an additional signal from the location of the fiber-optic amplifier in one direction or the other (FIG. 1) or a first additional signal in one direction and a second additional signal in the other (FIG. 2).

It is also possible to supply the Er.sup.3+ -doped length of fiber 11 with pump light from both sides, i.e., from two pump sources each located on one side of the length of fiber.

The wavelengths specified in the embodiments described are only examples of wavelengths for which the available system components are suitable. It is possible, of course, to use other wavelengths provided that the system components are suitable for them.