Externally modulated laser optical transmission system with feed forward noise cancellation

Abstract

An optical transmitter for generating a modulated optical signal for transmission over a fiber optic link to a remote receiver including a laser; a modulator for externally modulating the optical output signal with a RF signal to produce an optical signal including a modulated information-containing component; and a phase modulator coupled either to the output of the modulator or directly to the output of the laser for canceling the noise signals generated in the laser.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of this invention will be better understood and more fully appreciated by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein:

FIG. 1 is a highly simplified block diagram of an externally modulated optical transmission system as known in the prior art;

FIG. 2 is a highly simplified block diagram of a first embodiment of the optical transmission system according to the present invention; and

FIG. 3 is a highly simplified block diagram of a second embodiment of the optical transmission system according to the present invention.

Claims

1. An optical transmitter for generating a modulated optical signal for transmission for transmission over a fiber optic link to a remote receiver comprising: a laser for producing a baseband optical signal including noise spread over the frequency spectrum;a modulator for modulating the optical signal with a RF signal to produce a modulated information-containing component; anda phase modulator coupled to the output of the modulator for canceling the phase noise associated with the baseband optical signal and producing an output signal.

2. A transmitter as defined in claim 1, wherein the laser is a semiconductor laser and the phase modulator cancels the phase noise component in the baseband optical signal.

3. A transmitter as defined in claim 1, wherein the RF signal is a digital signal.

4. A transmitter as defined in claim 1, where the modulator is a Mach-Zehnder modulator.

5. A transmitter as defined in claim 1, further comprising a frequency discrimination circuit having an input connected to the output of the laser and an output coupled to a photodiode so that the phase noise in the optical signal is converted to a modulating electrical signal applied to the phase modulator such that effective phase noise cancellation occurs.

6. A transmitter as defined in claim 1, wherein the wavelength of the light output of the laser is in the 1530 to 1570 nm range.

7. A transmitter as defined in claim 1, wherein the RF signal is a broadband analog signal input that has a bandwidth greater than one octave and includes a plurality of distinct information carrying channels.

8. A transmitter as defined in claim 1, wherein the phase modulator is selectively adjustable to compensate for the distortion produced by the dispersive fiber optic link depending upon the length of the link.

9. An optical transmitter for generating a modulated optical signal for transmission for transmission over a fiber optic link to a remote receiver comprising: a laser for producing a baseband optical signal including noise spread over the frequency spectrum;a phase modulator coupled to the output of the laser for canceling the phase noise associated with the baseband optical signal and producing an output signal; anda modulator for modulating the optical signal with a RF signal to produce a modulated information-containing optical signal.

10. A transmitter as defined in claim 9, wherein the laser is a semiconductor laser and the phase modulator cancels the phase noise component in the baseband optical signal.

11. A transmitter as defined in claim 9, wherein the RF signal is a digital signal.

12. A transmitter as defined in claim 9, where the modulator is a Mach-Zehnder modulator.

13. A transmitter as defined in claim 9, further comprising a frequency discrimination circuit having an input connected to the output of the laser and an output coupled to a photodiode so that the phase noise in the optical signal is converted to a modulating electrical signal applied to the phase modulator such that effective phase noise cancellation occurs.

14. A transmitter as defined in claim 9, wherein the wavelength of the light output of the laser is in the 1530 to 1570 nm range.

15. A transmitter as defined in claim 9, wherein the RF signal is a broadband analog signal input that has a bandwidth greater than one octave and includes a plurality of distinct information carrying channels.

16. A transmitter as defined in claim 9, wherein the phase modulator is selectively adjustable to compensate for the distortion produced by the dispersive fiber optic link depending upon the length of the link.