Optical Transmission System

1. Field of the Invention

The present invention relates to optical transmission systems in general and in particular to an optical transmission system comprising a method and apparatus for cancelling noise, increasing the stimulated Brillouin scattering (SBS) threshold in a standard single mode fiber, and using an external modulator in combination with a distributed feedback (DFB) laser and an erbium doped fiber amplifier (EDFA) to transmit 1550 nanometer (nm) light over standard single mode fiber with a dispersion null at 1310 nm.

2. Description of the Related Art

Conventional prior known optical transmission systems comprising lasers, optical amplifiers, fiber optic cables, and the like, are found to have a number of deficiencies. For example, one of the deficiencies which has been found in prior known optical transmission systems is that the optical radiation from a light source comprising a distributed feedback (DFB) laser and an erbium doped fiber amplifier (EDFA) may be too noisy for some applications.

Another deficiency in prior known optical transmission systems, specifically those comprising a directly modulated DFB and an EDFA as described above which are coupled to a standard single mode fiber with a dispersion null at 1310 nm, is that the EDFA is operable only at wavelengths longer than 1310 nm, e.g. 1550 nm, and that at those wavelengths composite second order (CSO) distortions are generated in the fiber. The standard single mode fiber with a dispersion null at 1310 nm referred to herein corresponds to the SMF-28 fiber which is commercially available from Corning Incorporated, Corning, N.Y.

Heretofore, the methods and apparatus used to reduce noise and compensate for CSO distortions has been limited generally to electrical and optical compensation circuits such as disclosed in applicants' U.S. Pat. No. 5,115,440, applicants' U.S. patent application Ser. No. 07/787,830, both of which are assigned to the assignee of the present application, and a paper presented by Richard B. Childs and Vincent A. O. Byrne in the OPTICAL FIBER COMMUNICATION CONFERENCE, 1990 Technical Digest Series, Vol. 1, Conference Edition, January 1990.

Another deficiency in prior known optical transmission systems has to do with the effect of stimulated Brillouin scattering in a standard single mode fiber.

Stimulated Brillouin scattering (SBS) is a phenomenon which restricts the amount of optical power which can be transferred in a standard single mode fiber with a dispersion null at 1310 nm. That is to say, the SBS threshold, i.e. the maximum amount of optical power which can be transferred in a standard single mode fiber, is limited to a certain amount of power at a given wave length in a certain bandwidth for a given length of the fiber such that, as the length of the fiber or the wavelength increases, its ability to handle the power decreases. For example, the amount of optical power at 1310 nm that can be transferred in a standard single mode fiber with a dispersion null at 1310 nm, 50 kilometers (km) long is approximately 5 milliwatts (mW). At 1550 nm, the amount of power that can be input to the same fiber is approximately 4 mW.

Still another deficiency noted in prior known conventional optical transmission systems arises from the use of yttrium aluminum garnet (YAG) and yttrium lanthanum fluoride (YLF) lasers with standard nonlinear external modulators. Among the problems in these types of systems is the problem that nonlinear external modulators have a relatively low depth of modulation, e.g. 2%, thus providing a relatively low signal/carrier-to-noise ratio, and the problem that the YAG and YLF lasers, while providing relatively high power, e.g. 175 mW and 100 mW, respectively, are relatively expensive and have relatively short life times.

In view of the foregoing, a principal object of the present invention is an optical transmission system comprising a method and apparatus for cancelling noise in the output of an optical source comprising a directly modulated distributed feedback (DFB) laser coupled to an erbrium doped fiber amplifier (EDFA).

In an embodiment of the present invention for cancelling noise as described above, there is provided a means for sampling the output of the EDFA to obtain a sample of the noise therein, converting the sample to an electrical noise signal, inverting the electrical noise signal, combining the inverted electrical noise signal with a second modulation signal, i.e. data/information, and modulating the output of the EDFA with the combined signal to obtain a noise free modulated data signal. The noise free modulated data signal is then further amplified in another EDFA if desired.

Another object of the present invention is an optical transmission system comprising a method and apparatus for increasing the stimulated Brillouin scattering (SBS) threshold in a standard single mode fiber with a dispersion null at 1310 nm and stabilizing a 1550 nm DFB laser coupled thereto.

In an embodiment of the present invention for increasing the SBS threshold in a standard single mode fiber with a dispersion null at 1310 nm and stabilizing a 1550 nm DFB laser coupled thereto there is provided a means for directly modulating the laser with a constant amplitude signal having a predetermined frequency, e.g. 1 GHz, to thereby broaden the line width of the laser from approximately 40 MHz to approximately 2 GHz, i.e. .+-.1 GHz.

Another object of the present invention is an optical transmission system comprising a method and apparatus for using an external linearized modulator with a DFB laser which has the advantage of avoiding composite second order (CSO) distortions at 1550 nm in a standard single mode fiber with a dispersion null at 1310 nm while providing a carrier with a high signal/carrier-to-noise ratio.

The above and other objects, features and advantages of the present invention will become apparent from the following detailed description of the accompanying drawings, in which:

FIG. 1 is a block diagram of an embodiment of the present invention for cancelling noise in the output of a light source comprising a DFB laser and an erbium doped fiber amplifier (EDFA);

FIG. 2 is a block diagram of another embodiment of the present invention for increasing the line width of a DFB laser coupled to an EDFA and, thus, the stimulated Brillouin scattering (SBS) threshold of a standard single mode fiber with a dispersion null at 1310 nm coupled to the EDFA;

FIG. 3 is another embodiment of the present invention comprising a distributed feedback laser, an EDFA and a linearized external optical modulator for modulating the output of the EDFA; and

FIG. 4 is a drawing showing the relationship between the SBS threshold versus length of a fiber optic cable for various wavelengths of optical radiation.

Referring to FIG. 1, there is provided in accordance with the present invention an optical transmission system designated generally as 1. In the transmission system 1 there is provided a source of light 2, an optical beam splitter 3, a detector 4, an inverter 5, a combiner 6, an external modulator 7 and an erbium doped fiber amplifier (EDFA) 8 coupled to a fiber optic cable 9.

In the source 2 there is provided a distributed feedback (DFB) laser 15 and an erbium doped fiber amplifier (EDFA) 16.

In operation, the DFB 15 generates coherent radiation which is amplified in the EDFA 16. To cancel noise in the output of the EDFA 16 a portion of the light output of the EDFA 16 is directed by the beam splitter 3 to the detector 4 wherein it is converted to an electrical noise signal. The electrical noise signal is then inverted in the inverter 5, i.e. the phase of the noise signal is shifted 180.degree. relative to the noise in the output of the EDFA 16. The output of the inverter 5 is then coupled to the signal combiner 6. In the signal combiner 6 the output of the converter 5 is combined with data/input on the input line 17 of the combiner 6. The output of the combiner 6 is provided to the modulating signal input of the external modulator 7. In the external modulator 7 the light from the EDFA 16 is modulated by the output from the combiner 6. Because the output of the combiner 6 comprises a noise component which is 180.degree. out of phase with the noise component in the output of the EDFA, the noise component is cancelled in the output of the modulator 7 such that the EDFA 8 amplifies an essentially noise-free data signal for transmission over the fiber optic cable 9.

Referring to FIG. 2, there is provided in accordance with another embodiment of the present invention an optical transmission system designated generally as 20. In the transmission system 20 there is provided a distributed feedback laser (DFB) 21. Coupled to the output of the DFB 21 there is provided an EDFA 22. Coupled to the output of the EDFA 22 there is provided an external modulator 23. Coupled to the output of the external modulator 23 there is provided an EDFA 24. The output of the EDFA 24 is coupled to a fiber optic cable 25. The DFB 21 is a directly modulated laser receiving a modulating signal on a signal line 26 from a modulator 27.

In practice, the fiber optic cable 25 is limited in the amount of power it will accept at a given wavelength in a certain bandwidth for a given length of the fiber optic cable. This limit is the result of a phenomenon known as stimulated Brillouin scattering (SBS) the threshold of which, as indicated above, is a function of the wavelength of the light and the length of the fiber optic cable over which the light is being transmitted.

Referring to FIG. 4, there is shown a diagram of the SBS limit for a fiber optic cable versus the length of the cable for two different wavelengths of coherent radiation. As can be seen from FIG. 4, as the length of the cable and the wavelength of the light increases, the SBS limit decreases.

Referring again to the optical transmission System 20 of FIG. 2, it has been found that by modulating the DFB 21 with a modulating signal having a predetermined frequency, e.g. 1 GHz, with a constant amplitude, e.g. 4 milliamps, peak to peak, the line width of the radiation provided by the DFB 21 relative to a center frequency of 1550 nm is broadened from approximately 40 MHz to approximately 2 GHz, i.e. .+-.1 GHz. The broadening of the line width of the output of the DFB 21 is found to provide a corresponding increase in the SBS threshold in the fiber optic cable 25. It is also found to reduce noise in the output of the DFB 21 and therefore stabilize the laser. As described above with respect to the apparatus of FIG. 1, the external modulator 23 is provided to receive a modulating signal on an input modulating signal line 28 for modulating the light from the EDFA 22. The post modulation amplifier EDFA 24 is then used to further amplify the light before it is transmitted over the fiber optic cable 25.

Referring to FIG. 3, there is provided in another embodiment of the present invention an optical signal transmission system designated generally as 30. In the system 30 there is provided a DFB 31. Coupled to the output of the DFB 31 there is provided an EDFA 32. Coupled to the output of the EDFA 32 there is provided an external modulator 33. Coupled to the output of the external modulator 33 there is provided an EDFA 34. Coupled to the output of the EDFA 34 there is provided an optic fiber cable 35. The external modulator 33 includes a modulation signal input line 36.

In operation, the DFB 31 provides coherent radiation to the EDFA 32. The EDFA 32 amplifies the radiation which is then modulated by a modulating signal on the input line 36 in the external modulator 33. The modulated signal is then provided to the EDFA 34 which amplifies the signal and provides it on the fiber optic cable 35.

The optical transmission system 30 of FIG. 3 has several advantages. One of the advantages is that the system uses a distributed feedback (DFB) laser which in comparison to known yttrium aluminum garnet (YAG) and yttrium lanthanum fluoride (YLF) lasers, while providing less power, is considerably less expensive and vastly more reliable.

Another advantage of the embodiment of FIG. 3 is that the external modulator 33 comprises a linearized external modulator instead of a standard non-linear modulator as used in prior art systems. Linearized modulators of the type described herein are commercially available as Model No. SW115P from Crystal Technology located in Palo Alto, Calif. The linear modulator, while having a significant insertion loss compared to a standard modulator, e.g. 10 db versus 4 db, has approximately twice the depth of modulation, e.g. 4% versus 2%, of the standard modulators. The increased depth of modulation significantly increases the signal/carrier-to-noise ratio.

To compensate for the lower power output of the DFB lasers and the signal loss in the modulator 33, the EDFA 32 amplifies the output of the DFB 31 and the EDFA 34 amplifies the output of the modulator 33.

A further advantage of the external modulator 33 is that the external modulator 33 avoids inducing composite second order (CSO) distortions in the standard single mode fiber 35 with a dispersion null at 1310 nm. Since optical amplifiers such as EDFA 32 can operate only at 1550 nm and directly modulated DFBs would produce CSO at that wavelength in a standard single mode fiber, the use of the external modulator 33 has the further advantage of avoiding the CSO distortions in the fiber 35.

While preferred embodiments of the present invention are described above, it is contemplated that numerous modifications may be made thereto for particular applications without departing from the spirit and scope of the present invention. Accordingly, it is intended that the embodiments described be considered only as illustrative of the present invention and that the scope thereof should not be limited thereto but be determined by reference to the claims hereinafter provided.