# Patent

## Title
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 reducing or canceling the noise signals generated in the laser.

## Background
REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of U.S. patent application Ser. No. 11/366,936 filed Mar. 2, 2006, and U.S. patent application Ser. No. 11/701,742 filed Feb. 2, 2007.


This application is also related to U.S. patent application Ser. No. 10/946,056 filed Sep. 21, 2004, and assigned to the common assignee.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an optical transmission system for analog or digital signals, and in particular to a system using an externally modulated solid-state laser. Moreover, the invention relates to the cancellation of noise components in the system arising from a number of possible sources such as Brownian motion of charge carriers within the semiconductor laser (white noise) or noise arising from fluctuations in the bias current or the thermal environment of the laser (which varies inversely with frequency and therefore is typically referred to as “1/f” noise).
2. Description of the Related Art
Directly modulating the analog intensity of a light-emitting diode (LED) or semiconductor laser with an electrical signal is considered among the simplest methods known in the art for transmitting analog signals, such as voice and video signals, on optical fibers. Although such analog transmission techniques have the advantage of substantially smaller bandwidth requirements than digital transmission, such as digital pulse code modulation, or analog or pulse frequency modulation, the use of amplitude modulation typically places more stringent requirements on the noise and distortion characteristics of the transmitter.
For these reasons, direct modulation techniques have been used in connection with 1310 nm lasers where the application is to short transmission links that employ fiber optic links with zero dispersion. For applications in metro and long haul fiber transmission links, the low loss of the link requires that externally modulated 1550 nm lasers be used, typically over very long distances (100 km) and high frequencies (over 900 MHz). A limiting factor in such links can be the conversion of residual phase noise from the laser which is converted into amplitude noise via the dispersion present in the fiber link. The present invention is therefore addressed to the problem of providing a simple and low cost system for noise cancellation associated with the phase noise of a laser so that the analog optical output can be used in metro and long haul optical networks, especially for analog transmission of broadband RF signals.
Direct current modulation of lasers is also known for use in digital optical transmission systems such as dense wavelength division multiplexing (DWDM) systems. See, for example, Kartalopoulos, DWDM Networks, Devices, and Technology (IEEE Press, 2003), p. 154.
In addition to the low noise characteristics required for an analog Optical transmission system at 1550 nm, the system must be highly linear. Distortion inherent in certain analog transmitters prevents a linear electrical modulation signal from being converted linearly to an optical signal, and instead causes the signal to become distorted. These effects are particularly detrimental to multi-channel video transmission, which requires excellent linearity to prevent channels from interfering with each other. A highly linearized analog optical system has wide application in commercial analog systems, such as broadcast TV transmission, CATV, interactive TV, and video telephone transmission.
As an alternative to direct current modulation, external modulators of a continuous wave (CW) laser are known in the prior art for use in optical transmission systems. U.S. Pat. No. 5,699,179 describes an externally modulated, feedforward linearized analog optical transmitter for reducing the fiber-induced composite second order (CSO) distortion components.
Linearization of optical and other nonlinear transmitters has been studied for some time, but proposed solutions suffer from practical disadvantages. Most applications discussed above have bandwidths which are too large for many practical implementations. Feedforward techniques for linearization require complex system components such as optical power combiners and multiple optical sources. Quasi-optical feedforward techniques suffer from similar complexity problems and further require extremely well matched parts. However as discussed below, feedforward techniques for phase noise cancellation are practical techniques that can be implemented using many well developed technologies.
Prior to the present invention, there has not been an application of a phase modulator coupled to an externally modulated laser for the purpose of canceling phase noise components arising from the various noise sources in the semiconductor structure of the laser. It should be noted that semiconductor lasers exhibit noise in both their amplitude (often referred to as relative intensity noise) and their phase. These noise properties are essentially independent of the lasing wavelength, although the noise can manifest itself differently at different wavelengths in single mode fiber transmission. The primary internal mechanism leading to phase and amplitude noise is spontaneous emission within the active region of the laser. Since spontaneously emitted photons have no particular phase relationship to those photons produced via stimulated emission, both the amplitude and the phase of the resultant optical field are affected. The process of spontaneous emission is well understood and has been shown to be described by a Brownian motion process in which the noise spectrum is essentially constant (white noise) within the frequencies of operation. External to the laser, environmental effects such as micro-phonics, temperature fluctuations, and bias current noise can also produce phase noise in the optical field. These events typically lead to optical phase noise which exhibits a noise spectrum with a “1/f” dependence.
The present invention seeks to minimize the inherent phase noise from the semiconductor laser through feedforward cancellation regardless of the driving mechanism of the noise.
SUMMARY OF THE INVENTION
1. Objects of the Invention
It is an object of the present invention to provide an improved optical transmission system using an externally modulated laser.
It is another object of the present invention to compensate for noise in a laser used in an optical transmission system.
It is also another object of the present invention to provide an external phase modulator for use in an externally modulated 1550 nm analog optical transmission system to improve phase noise reduction.
It is still another object of the present invention to provide a highly linear optical transmission system suitable for long haul dispersive optical fiber media using an externally modulated laser together with a phase corrective circuit and a phase modulator coupled to the optical signal.
It is still another object of the present invention to provide a phase shifting circuit for reducing the residual phase noise from an externally modulated laser in an analog optical transmission system suitable for long haul dispersive optical fiber media.
It is also an object of the present invention to provide a phase noise compensation process in a broadband analog optical transmission system.
Some implementations or embodiments may achieve fewer than all of the foregoing objects.
2. Features of the Invention
Briefly, and in general terms, the present invention provides 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 laser with a RF signal to produce an optical signal including a modulated information-containing component; and a phase modulator coupled to the output of the modulator for canceling the noise signals generated in the laser.
In another aspect, the present invention provides an optical transmitter for generating a modulated optical signal for transmission over a dispersive fiber optic link to a remote receiver having a semiconductor laser for producing an optical signal with associated phase noise; a noise cancellation circuit coupled to the output of the laser including an optical phase modulator for reducing the phase noise generated in the laser; and an external modulator coupled to the output of the phase modulator and for receiving a broadband analog radio frequency signal input and modulating the optical signal.
In another aspect, the present invention provides an optical transmission system for use over dispersive fiber optic links, including an optical transmitter with an analog or digital RF signal input; a semiconductor laser; a modulation circuit for externally modulating the laser, and a circuit for canceling phase modulation components of the optical associated with the noise generated by the semiconductor laser.
In another aspect of the invention, there is provided a noise cancellation circuit for reducing phase noise in the transmission of analog signals that splits an output optical signal from the external modulator of a semiconductor laser into two paths, one to a phase modulator and the other to a frequency discriminator. The phase modulation cancellation signal is adjusted in amplitude and phase to match the frequency or phase dependence of the phase noise by the laser. The phase of the signals are synchronized by a delay or phase adjustment element in one of the paths. The primary and secondary signals are then recombined by a phase modulator to produce a single optical signal having only amplitude modulation. Thus, the phase modulator modulates the primarily signal from the semiconductor laser in such a way that the resultant phase noise is minimized thus making the analog signals suitable for transmission over dispersive fiber optic links.
Additional objects, advantages and novel features of the present invention will become apparent to those skilled in the art from this disclosure, including the following detailed description as well as by practice of the invention. While the invention is described below with reference to preferred embodiments, it should be understood that the invention is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional applications, modifications and embodiments in other fields, which are within the scope of the invention as disclosed and claimed herein and with respect to which the invention could be of significant utility.



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.



The novel features and characteristics of the invention are set forth in the appended claims. The invention itself, however, as well as other features and advantages thereof, will be best understood by reference to a detailed description of a specific embodiment, when read in conjunction with the accompanying drawings.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Details of the present invention will now be described, including exemplary aspects and embodiments thereof. Referring to the drawings and the following description, line reference numbers are used to identify like or functionally similar elements, and are intended to illustrate major features of exemplary embodiments in a highly simplified diagrammatic manner. Moreover, the drawings are not intended to depict every feature of actual embodiments nor the relative dimensions of the depicted elements, and are not drawing to scale.
FIG. 1 is a block diagram of a prior art optical transmitter as represented in U.S. Pat. No. 5,699,179 utilizing an external modulator. The transmitter, shown generally at 10, transmits an optical signal to a remote receiver 60 over an optical fiber path 30. The transmitter 10 includes a semiconductor laser 12, which produces a continuous wave (CW) output. Typical examples of such lasers are distributed feedback (DFB) laser/or Fabry-Perot lasers, that produce an output optical beam, typically at a wavelength of 1550 nm. The unmodulated optical signal from the laser is coupled to a modulator 16 by optical fiber 14. The modulator 16 may be a single modulator such as a Mach-Zehnder modulator, a cascaded MZ modulator or more than one modulator such as in a feed-forward linearizer. The modulator 16 also receives, via terminal 18 and line 20, a broadband RF signal such as an amplitude modulated vestigial sideband (AM-SDB) cable television (CATV) or video signal. Moreover, when a feed-forward linearizer is used, a depolarizing signal is provided to the modulator 16 via terminal 22 and line 24. The depolarizing signal is used to depolarize the optical input to an error-correcting modulator (not shown) in the modulator 16.
The modulated optical signal which carries the video data is coupled by a fiber link 26 to an amplifier 28. The amplifier 28 is typically an erbium doped fiber amplifier (EDFA). The amplified optical signal is coupled to a fiber optical transmission line 30 to the receiver 60. The optical fiber transmission line 30 may be a long-distance link extending over several kilometers. In this case, line amplifiers such as EDFA 28 may be provided at spaced intervals along in the line in order to boost the signal to desired levels. At the receiver 60, an amplifier (not shown) may also be provided to boost the incoming optical signal. The boosted signal is then applied to a photodetector and demodulated at the receiver 60 to produce an electrical signal, which represents the original video or data signal at line 50.
FIG. 3 is a block diagram of an optical transmitter according to a first embodiment of the present invention utilizing an external modulator. The transmitter, shown generally at 200, transmits an optical signal to a remote receiver over an optical fiber path 30. The transmitter 200 includes a semiconductor laser 101, which produces a continuous wave (CW) output. Typical examples of such lasers are distributed feedback (DFB) laser/or Fabry-Perot lasers, that produce an output optical beam, typically at a wavelength of 1550 nm.
The edge-emitting semiconductor laser used in the system of FIG. 3 is Preferably a distributed feedback laser (DFB), although a Fabry-Perot (FP) laser may be used as well. DFB lasers are the preferred approach since their optical output is primarily contained in a single lasing mode whereas the FP laser has its optical energy spread among many modes. In a preferred embodiment, the laser is an external cavity laser within the wavelength of the light output of the laser in the 1530 to 1570 nm range.
The unmodulated optical signal from the laser is split into two parts; the first part is coupled to a modulator 102 by optical fiber 103. The modulator 102 may be a single modulator such as a Mach-Zehnder modulator, a cascaded MZ modulator or more than one modulator such as in a feed-forward linearizer. The modulator 102 receives, via terminal 104 and line 105, a broadband RF signal such as an amplitude modulated vestigial sideband (AM-SDB) cable television (CATV) or video signal, or a digital signal. The analog signal may have a bandwidth greater than one octave and carry a plurality of channels. Moreover, when a feed-forward linearizer is used, a depolarizing signal is provided to the modulator similar to the arrangement of FIG. 1. The depolarizing signal is used to depolarize the optical input to an error-correcting modulator (not shown) in the modulator 102.
As noted above, the optical signal output of the laser is split into two parts: one part is applied to a modulator 102; the other part is coupled by optical fiber 106 to a frequency discriminator 107.
The output of the frequency discriminator 107 is applied to an attenuator 108 to appropriately adjust the amplitude of the signal to be commensurate with that of the phase modulated components introduced by the phase noise characteristics of the laser 101.
The output of the attenuator 108 is then connected to a phase shift circuit 109. The circuit 109 corrects for the time lag of the signal output applied to circuit elements 107, 108 compared to that signal directly applied to the modulator 102. In the video transmission band of interest (50 MHz-1000 MHz for traditional CATV systems), the phase noise of the semiconductor laser 101 is “white”; i.e., the spectral power density of the noise is independent of frequency. In this case, the phase correction path 106, 107, 108, 109 would need to have a constant (adjustable) gain with its delay precisely matched to that of the primary path 103, 102, and 110. One aspect that needs to be accounted for is the effect of the frequency discriminator 107 on the signal, specifically the result of the optical to electrical conversion process in the phase correction path. When the optical signal is detected by a photodiode, a phenomenon known as shot noise is observed. This noise results from the statistical process of absorbing a photon in the photodiode to generate an electron-hole pair. This noise is, for all practical purposes, unavoidable. Therefore, shot noise will impose a lower limit on the amount of phase noise cancellation achievable.
The output of the phase shift circuit 109 is then applied to the phase modulator 110, to thereby introduce phase corrections into the optical signal by phase modulation thereof to thereby correct or compensate for the noise generated by the laser.
The spectral noise density of the generated photocurrent from a photodiode is given as

<in2>=2eIp 

where e is the electron charge and Ip is the DC photocurrent. One skilled in the art will immediately appreciate the fact that the noise power has a linear dependence on the received optical power and therefore the signal to noise ratio of a shot-noise dominated process improves as the received power increases. This represents a fundamental design trade-off in the proposed invention. More power tapped into the phase correction path 106, 107, 108, 109 . . . will improve the ultimate phase noise cancellation at the expense of the transmitter's optical output power.

The output of the phase modulator 110 is coupled over a fiber 111 to an amplifier 112, which is then connected to the optical fiber or link 30. At the remote end, the optical fiber or link 30 is connected to the receiver which converts the received optical signal into an RF signal, similar to that as represented in FIG. 1.
FIG. 2 is a block diagram of an optical transmitter according to a second embodiment of the present invention utilizing an external modulator. The transmitter, shown generally at 100, transmits an optical signal to a remote receiver over an optical fiber path 30. The transmitter 100 includes a semiconductor laser 101, which produces a continuous wave (CW) output. Typical examples of such lasers are distributed feedback (DFB) laser/or Fabry-Perot lasers, that produce an output optical beam, typically at a wavelength of 1550 nm. The unmodulated optical signal from the laser is split into two parts; one is coupled to a phase modulator 110 by optical fiber 103. The phase modulator 110 introduces phase corrections into the optical signal to thereby correct or compensate for the noise generated by the laser. The CW output of the phase modulator 110 is coupled to a modulator 102, to produce a modulated information-containing optical signal.
The modulator 102 may be a single modulator such as a Mach-Zehnder modulator, a cascaded MZ modulator or more than one modulator such as in a feed-forward linearizer. The modulator 102 receives, via terminal 104 and line 105, a broadband RF signal such as an amplitude modulated vestigial sideband (AM-SDB) cable television (CATV) or video signal, or a digital signal. Moreover, when a feed-forward linearizer is used, a depolarizing signal is provided to the modulator similar to the arrangement of FIG. 1. The depolarizing signal is used to depolarize the optical input to an error-correcting modulator (not shown) in the modulator 102.
As noted above, the optical signal output of the laser is split into two parts: one part is applied to a phase modulator 110; the other part is coupled by optical fiber 106 to a frequency discrimination circuit 107.
The output of the frequency discriminator 107 is applied to an attenuator 108 to appropriately adjust the amplitude of the signal to be commensurate with that of the phase modulated components introduced by the phase noise characteristics of the laser 101.
The output of the attenuator 108 is then connected to a phase shift circuit 109. The circuit 109 corrects for the time lag of the signal output applied to circuit elements 107, 108 compared to that signal applied to the phase modulator 110. In the video transmission band of interest (50 MHz-1000 MHz for traditional CATV systems), the phase noise of the semiconductor laser 101 is “white”; i.e., the spectral power density of the noise is independent of frequency. In this case, the phase correction path 106, 107, 108, 109 would need to have a constant (adjustable) gain with its delay precisely matched to that of the primary path 103, 102, and 110. One aspect that needs to be accounted for is the frequency discriminator 107, specifically the optical to electrical conversion process in the phase correction path. When the optical signal is detected by a photodiode, a phenomenon known as shot noise is observed. This noise results from the statistical process of absorbing a photon in the photodiode to generate an electron-hole pair. This noise is, for all practical purposes, unavoidable. Therefore, shot noise will impose a lower limit on the amount of phase noise cancellation achievable.
Many variations and modifications will be apparent to those skilled in the art without departing from the spirit and scope of the invention. For example, although described and illustrated in the context of a video or multi-channel TV signal modulating a laser or light emitting diode, other nonlinear devices such as amplifiers may have inherent distortion largely cancelled by this technique. The fine adjustment of the relative phase of the signals in the primary and secondary paths is in the secondary path in the illustrated embodiment, but this could also be in the primary path with the coarse adjustment. The secondary path is preferred since such a delay in the primary path may have an inappropriate impedance for this path.
Various aspects of the techniques and apparatus of the present invention may be implements in digital circuitry, or in computer hardware, firmware, software, or in combinations of them. Circuits of the invention may be implemented in computer products tangibly embodied in a machine-readable storage device for execution by a programmable processor, or on software located at a network node or web site which may be downloaded to the computer product automatically or on demand. The foregoing techniques may be performed by, for example, a single central processor, a multiprocessor, one or more digital signal processors, gate arrays of logic gates, or hardwired logic circuits for executing a sequence of signals or program of instruction to perform functions of the invention by operating on input data and generating output. The methods may advantageously be implemented in one or more computer programs that are executable on a programmable system including at least one programmable processor coupled to receive data and instructions from, and to transmit data and instructions to, a data storage system, at least one in/out device, and at least one output device. Each computer program may be implemented in a high-level procedural or object-oriented programming language, or in assembly or machine language if desired; and in any case, the language may be a complied or an interpreted language. Suitable processors include, by way of example, both general and special purpose microprocessors. Generally, a processor will receive instructions and data from read-only memory and/or random access memory. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including by way of example, semiconductor devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and CD-ROM disks. Any of the foregoing may be supplemented by or incorporated in, specially designed application-specific integrated circuits (ASICS).
It will be understood that each of the elements described above, or two or more together, also may find a useful application in other types of constructions differing from the types described above.
While the invention has been illustrated and described as embodied in an optical transmission system, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.
Without further analysis, from the foregoing others can, by applying current knowledge, readily adapt the present invention for various applications. Such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

## Claims

1. An optical transmitter for generating a modulated optical signal for transmission over a fiber optic link to a remote receiver comprising:
a laser for producing a baseband optical signal including noise spread over a frequency spectrum;
a modulator for modulating the baseband optical signal with a RF signal to produce a modulated information-containing optical signal;
a phase modulator coupled to an output of the modulator for receiving the modulated information-containing optical signal, for modifying the modulated information-containing optical signal by reducing phase noise associated with the baseband optical signal, and for producing an output optical signal that is coupled to the fiber optic link; and
a phase correction circuit coupled between the laser and the phase modulator, the phase correction circuit including a frequency discriminator having an input coupled to receive a portion of the baseband optical signal and an output coupled to a photodiode to convert the phase noise associated with the baseband optical signal to a modulating electrical signal for controlling the phase modulator to provide feedforward cancellation of the phase noise.

2. The optical transmitter as defined in claim 1, wherein the laser is a semiconductor laser and the phase modulator cancels or reduces the phase noise associated with the baseband optical signal.

3. The optical transmitter as defined in claim 1, wherein the RF signal is a quadrature amplitude modulated signal.

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

5. The optical transmitter as defined in claim 1, wherein a wavelength of the baseband optical signal output from the laser ranges from 1530 nm to 1570 nm.

6. The optical 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.

7. The optical transmitter as defined in claim 1, wherein the phase modulator is selectively adjustable to compensate for distortion produced by dispersion in the fiber optic link depending upon a length of the fiber optic link.

8. The optical transmitter as defined in claim 1, wherein an amount of power tapped into the phase correction circuit from the laser is adjusted depending on an amount of phase correction needed compared to a desired transmitter optical output power.

9. The optical transmitter as defined in claim 1, wherein the phase correction circuit is responsive to the phase noise in the baseband optical signal so that the phase modulator is controllably adjustable to substantially cancel the phase noise associated with the baseband optical signal.

10. An optical transmitter for generating a modulated optical signal for transmission over a fiber optic link to a remote receiver, the optical transmitter comprising:
a laser for producing a baseband optical signal including noise spread over a frequency spectrum;
a phase modulator coupled to an output of the laser for producing a phase modulated output optical signal;
a phase corrective circuit coupled to the output of the laser to provide an electrical input signal to the phase modulator for modifying the baseband optical signal by controllably reducing phase noise associated with the baseband optical signal; and
a modulator for modulating the phase modulated output optical signal produced by the phase modulator with a RF signal to produce a modulated information-containing optical signal.

11. The optical transmitter as defined in claim 10, wherein the laser comprises a semiconductor laser and the phase modulator provides phase noise cancellation in the baseband optical signal.

12. The optical transmitter as defined in claim 10, wherein the RF signal is a digital signal.

13. The optical transmitter as defined in claim 10, where the modulator is a Mach-Zehnder modulator.

14. The optical transmitter as defined in claim 10, wherein the phase corrective circuit is coupled to the phase modulator and includes a frequency discriminator having an input connected to the output of the laser and a photodiode coupled to the frequency discriminator so that the phase noise in the baseband optical signal is converted to a modulating electrical signal for controlling the phase modulator.

15. The optical transmitter as defined in claim 10, wherein a wavelength of the baseband optical signal output from the laser ranges from 1530 nm to 1570 nm.

16. The optical transmitter as defined in claim 10, 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.

17. The optical transmitter as defined in claim 10, wherein the phase modulator is selectively adjustable to compensate for distortion produced by dispersion in the fiber optic link depending upon a length of the fiber optic link.

18. The optical transmitter as defined in claim 10, wherein the phase modulator is to adjust both an amplitude and a phase of the baseband optical signal.

19. The optical transmitter as defined in claim 10, wherein the phase corrective circuit is responsive to the phase noise in the baseband optical signal so that the phase modulator is controllably adjustable to substantially cancel the phase noise associated with the baseband optical signal.