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Wavelength division multiplexing system    
United States Patent5953139   
Link to this pagehttp://www.wikipatents.com/5953139.html
Inventor(s)Nemecek; Joseph E. (Worcester, MA); Noonan; Michael J. (Shrewsbury, MA); Mahapatra; Amaresh (Acton, MA)
AbstractAn analog lightwave communication system comprises at least two optical transmitters for providing optical information signals at different optical wavelengths. A dense wavelength division multiplexer includes at least two inputs for receiving the optical information signals from the optical transmitters and multiplexes the optical information signals to a composite optical signal at an output. Each input of the dense wavelength division multiplexer comprises at least one optical resonant cavity comprising first and second reflecting materials spaced to permit resonance at a selected wavelength. A fiber optic transmission system coupled to the output of the dense wavelength division multiplexer receives the composite optical signal.
   














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Drawing from US Patent 5953139
Wavelength division multiplexing system - US Patent 5953139 Drawing
Wavelength division multiplexing system
Inventor     Nemecek; Joseph E. (Worcester, MA); Noonan; Michael J. (Shrewsbury, MA); Mahapatra; Amaresh (Acton, MA)
Owner/Assignee     CFX Communications Systems, LLC (Northboro, MA)
Patent assignment
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Publication Date     September 14, 1999
Application Number     08/670,722
PAIR File History     Application Data   Transaction History
Image File Wrapper   Patent Term   Fees
Litigation
Filing Date     June 21, 1996
US Classification     398/79 398/91 398/188
Int'l Classification     H04J 014/02
Examiner     Bacares; Rafael
Assistant Examiner    
Attorney/Law Firm     Hamilton, Brook, Smith & Reynolds, P.C.
Address
Parent Case     RELATED APPLICATIONS This application is a continuation-in-part of International Application No. PCT/US96/03199, filed on Mar. 6, 1996, the entire contents of which is incorporated herein by reference.
Priority Data    
USPTO Field of Search     359/124 359/127 359/129 359/133 359/131 359/580 359/181 359/183 385/24 385/17 385/18
Patent Tags     wavelength division multiplexing
   
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 Technical Review Submit all comments and votes
 Claims Submit all comments and votes
 


What is claimed is:

1. An analog lightwave communication system comprising:

at least two optical transmitters for providing analog optical information signals at different optical wavelengths;

a first wavelength division multiplexer having at least two inputs for receiving the optical information signals from the optical transmitters and for multiplexing the optical information signals to a composite optical signal at an output, each input of the first wavelength division multiplexer comprising at least one optical resonant cavity comprising first and second reflecting materials spaced to permit resonance at one of the different optical wavelengths;

an oscillator circuit providing a single tone modulation signal:

a phase modulator having an optical input coupled to the output of the first wavelength division multiplexer for receiving the composite optical signal and an electrical input coupled to the oscillator circuit for receiving the single tone modulation signal, the single tone modulation signal driving the phase modulator to modulate the composite optical signal such that stimulated Brillouin scattering threshold in the system is increased; and

a fiber optic transmission system coupled to the output of the first wavelength division multiplexer for receiving the composite optical signal.

2. The system of claim 1 wherein each optical transmitter comprises a DFB laser source which provides a continuous wave optical signal; an external modulator coupled to the laser source having an RF electrical input; and an RF driver coupled to the RF electrical input of the external modulator which provides an RF information signal for modulating the continuous wave optical signal to produce the optical information signal.

3. The system of claim 1 further comprising an optical receiver coupled to the output of the phase modulator for receiving a portion of the phase modulated composite optical signal and a noise threshold detector coupled to the optical receiver for detecting noise level in the composite optical signal to control the level of the single tone modulation signal.

4. The system of claim 2 wherein the RF information signal comprises at least 40 cable television channels.

5. The system of claim 1 further comprising an optical amplifier coupled between the output of the first wavelength division multiplexer and the fiber optic transmission system.

6. The system of claim 5 wherein the optical amplifier is an erbium-doped fiber amplifier.

7. The system of claim 1 wherein the fiber optic transmission system comprises a second wavelength division multiplexer for demultiplexing the composite optical signal to the optical information signals at different wavelengths and at least two optical receivers, each receiver for receiving one of the optical information signals at a respective wavelength.

8. The system of claim 1 wherein each optical transmitter comprises a DFB laser source that is directly modulated with an RF information signal.

9. The system of claim 8 wherein the RF information signal comprises at least 40 cable television channels.

10. The system of claim 1 wherein the first wavelength division multiplexer provides a wavelength separation of less than 2 nm.

11. The system of claim 10 wherein the wavelength separation is less than 1.6 nm.

12. An analog lightwave communication system comprising:

at least two optical transmitters for providing analog optical information signals at different optical wavelengths, each optical transmitter comprising:

a DFB laser source which provides a continuous wave optical signal;

an external modulator coupled to the laser source having an RF electrical input; and

an RF driver coupled to the RF electrical input of the external modulator which provides an RF information signal for modulating the continuous wave optical signal to produce the optical information signal;

a first wavelength division multiplexer having at least two inputs for receiving the optical information signals from the optical transmitters and for multiplexing the optical information signals to a composite optical signal at an output;

an oscillator circuit providing a single tone modulation signal; and

a phase modulator having an optical input coupled to the output of the first wavelength division multiplexer for receiving the composite optical signal and an electrical input coupled to the oscillator circuit for receiving the single tone modulation signal, the single tone modulation signal driving the phase modulator to modulate the composite optical signal such that stimulated Brillouin scattering threshold in the system is increased.

13. The system of claim 12 further comprising an optical receiver coupled to the output of the phase modulator for receiving a portion of the phase modulated composite optical signal and a noise threshold detector coupled to the optical receiver for detecting noise level in the composite optical signal to control the level of the single tone modulation signal.

14. The system of claim 12 further comprising a fiber optic transmission system coupled to the output of the first wavelength division multiplexer for receiving the composite optical signal.

15. The system of claim 14 wherein the fiber optic transmission system comprises a second wavelength division multiplexer for demultiplexing the composite optical signal to the optical information signals at different wavelengths and at least two optical receivers, each receiver for receiving one of the optical information signals at a respective wavelength.

16. The system of claim 14 further comprising an optical amplifier coupled between the output of the first wavelength division multiplexer and the fiber optic transmission system.

17. The system of claim 16 wherein the optical amplifier is an erbium-doped fiber amplifier.

18. The system of claim 12 wherein the RF information signal comprises at least 40 cable television channels.

19. An analog lightwave communication system comprising:

at least four optical transmitters for providing analog optical information signals at different wavelengths between 1530 and 1565 nm;

a first dense wavelength division multiplexer having at least four inputs for receiving the optical information signals from the optical transmitters and for multiplexing the optical information signals to a composite optical signal at an output;

an oscillator circuit providing a single tone modulation signal;

a phase modulator having an optical input coupled to the output of the first wavelength division multiplexer for receiving the composite optical signal and an electrical input coupled to the oscillator circuit for receiving the single tone modulation signal, the single tone modulation signal driving the phase modulator to modulate the composite optical signal such that stimulated Brillouin scattering threshold in the system is increased;

an optical amplifier for amplifying the composite optical signal;

a fiber link for transporting the amplified composite optical signal to a remote site; and

a second dense wavelength division multiplexer coupled to the fiber link at the remote site for demultiplexing the composite optical signal to the optical information signals.

20. The system of claim 19 wherein the optical amplifier is an erbium-doped fiber amplifier.
 Description Submit all comments and votes
 


BACKGROUND OF THE INVENTION

The invention relates generally to optical communications, and more particularly to the transmission of analog signals in cable television systems using wavelength division multiplexing.

Analog lightwave communication systems are typically used in today's CATV trunking and distribution networks. Trunking applications typically are limited to about 40 video channels per fiber due to CSO and CTB considerations. Thus, to carry 80 video channels and maintain transmission quality requires the use of two fibers.

A need exists for an analog lightwave communication system than can transport a greater number of video channels over a single fiber without degrading CSO and CTB performance.

SUMMARY OF THE INVENTION

The present invention provides an improved analog lightwave communication system for use in CATV trunking applications to provided increased channel capacity over longer distances without performance degradation.

In accordance with an aspect of the invention, a lightwave communication system comprises at least two optical transmitters for providing optical information signals at different optical wavelengths. A first wavelength division multiplexer includes at least two inputs for receiving the optical information signals from the optical transmitters and multiplexes the optical information signals to a composite optical signal at an output. Each input of the first wavelength division multiplexer comprises at least one optical resonant cavity comprising first and second reflecting materials spaced to permit resonance at a selected wavelength. A fiber optic transmission system coupled to the output of the first wavelength division multiplexer receives the composite optical signal.

A preferred embodiment of the system uses at least two, and preferably 3 or 4 sources, each operating at different wavelengths in the range of 1530 nm to 1565 nm. The sources can include a temperature control feedback circuit to maintain the output of each laser at a wavelength corresponding to one of the fixed wavelengths of the multiplexer.

According to another aspect of the present invention, each optical transmitter includes a DFB laser source which provides a continuous wave optical signal and an external modulator coupled to the laser source. An RF driver coupled to an RF electrical input of the external modulator provides an RF information signal for modulating the continuous wave optical signal to produce the optical information signal.

According to another aspect of the invention, an oscillator circuit providing a single tone modulation signal for driving a phase modulator having an optical input coupled to the output of the first wavelength division multiplexer. The single tone modulation signal drives the phase modulator to modulate the composite optical signal such that stimulated Brillouin scattering threshold in the system is increased.

According to another aspect of the invention, an optical monitoring receiver coupled to the output of the first wavelength division multiplexer receives a portion of the composite optical signal and controls the level of the single tone modulation signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views.

FIG. 1 is a schematic block diagram of an optical communication system in accordance with the present invention.

FIG. 2 is a schematic block diagram of an optical transmitter for use in the system of FIG. 1.

FIG. 3 is a schematic diagram of a dense wavelength division multiplexer for use in the system of FIG. 1.

FIG. 4 schematically illustrates a resonant cavity filter arrangement for the dense wavelength division multiplexer of FIG. 3.

FIG. 5 is a schematic diagram of an oscillator circuit for providing single tone phase modulation in the system of FIG. 1.

FIG. 6 is a schematic diagram of an automatic bias control circuit for use in the system of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a communication system 10 is shown which illustrates the principles of the present invention. The system 10 generally comprises optical transmitters 12, dense wavelength division multiplexers 14 and 22, optical amplifier 18 and optical receivers 24. The optical transmitters 12, described in more detail below, output optical information signals 13 each at a different wavelength (.lambda..sub.1, .lambda..sub.2 . . . .lambda..sub.N). The optical information signals 13 are coupled to the WDM 14 which multiplexes the signals to produce a composite optical signal 15 at its output. The composite optical signal 15 includes the optical information signals at the input wavelengths .lambda..sub.1 -.lambda..sub.N. The WDM 14, described in detail further below, is a dense WDM that includes narrowband interference filter technology to achieve channel spacing on the order of two nanometers or less.

The composite optical signal 15 is coupled to the optical amplifier 18 which provides optical gain. The optical amplifier 18 is preferably an erbium-doped fiber amplifier such as an Italtel AF18A device which provides 18 dBm output power. The amplified composite optical signal 19 is coupled to a remote dense WDM 22 over fiber transmission link 21. The fiber transmission link 21 in the preferred embodiment is a CATV long-haul fiber trunk. The remote WDM 22 demultiplexes the composite optical signal to its constituent optical information signals 23 at the respective wavelengths .lambda..sub.1 -.lambda..sub.N. The optical receivers 24 each receive a particular optical information signal 23.

Stimulated Brillouin scattering (SBS) is a phenomenon which limits the amount of optical power that can be effectively coupled into a fiber. SBS is particularly difficult in external modulation systems which use an optical source having a relatively narrow optical linewidth. To further exacerbate the problem, systems which employ optical amplifiers can produce gain which surpasses the SBS threshold, typically about 8 dBm in single mode fiber. It should be noted that SBS is a