# Patent

## Title
Optical transmission system with directly modulated laser and 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; an input coupled to the laser for directly amplitude modulating the laser with an RF signal to produce an optical signal including an amplitude modulated information-containing component; and a phase modulator coupled 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.
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 signals, and in particular to either a directly modulated or externally modulated solid-state laser. Moreover, the invention related to the cancellation of white noise components 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 method 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 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.
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 is a practical technique which can be implemented using many well developed technologies.
As noted above, external modulators are known for use in optical transmission systems in the prior art. 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.
Prior to the present invention, there has not been an application of a phase modulator coupled to a directly (current) 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. This 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 a directly modulated laser.
It is another object of the present invention to compensate for noise in a laser used in an analog optical transmission system.
It is also another object of the present invention to provide an external Mach Zehnder modulator for use in a 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 analog optical transmission system suitable for long haul dispersive optical fiber media using a directly modulated laser with a phase corrective circuit.
It is still another object of the present invention to provide a phase shifting circuit for reducing the residual phase noise from the 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 dispersive fiber optic link to a remote receiver having an input for receiving a broadband analog radio frequency signal input; a semiconductor laser for producing an optical signal with associated phase noise; and a noise cancellation circuit including an optical phase modulator for reducing the phase noise in the optical transmitter's output and thereby the distortion in the signal present at the receiver end of the fiber optic link due to phase modulation noise components.
In another aspect, the present invention provides an optical transmission system for use over dispersive fiber optic links including an optical transmitter with analog signal input; a semiconductor laser; a modulation circuit for directly modulating the laser, and phase shifting circuit for canceling phase modulation components of the optical associated with an external modulator for an optical signal noise generated by the semiconductor laser.
In another aspect, the present invention further provides a low-cost direct modulation technique, preferably including a circuit for controlling an optical phase modulator reducing the phase noise components produced by a 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 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 the optical phase modulator to produce a single optical signal having only amplitude modulation. Thus, the phase modulator modulates the primary 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(a) is a highly simplified block diagram of an externally modulated optical transmission system as known in the prior art;
FIG. 1(b) is a highly simplified block diagram of a directly modulated optical transmission system as known in the prior art;
FIG. 2 is a highly simplified block diagram 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(a) is a block diagram of a prior art optical transmitter as represented in the U.S. Pat. No. 5,699,179 utilizing an external modulator. The transmitter, shown generally at 10, transmits an optical signal to a 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, at a wavelength of 1,550 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 an electrical signal, which represents the original video or data signal at line 50.
FIG. 1(b) is a block diagram of a prior art optical transmitter utilizing direct current modulation of the laser. The broadband RF analog signal is applied directly to the laser 12. The modulated optical signal from the laser 12 is coupled by a fiber link 26 to an amplifier 28, such as an EDFA. The amplified optical signal is coupled to a fiber transmission line 30 to the receiver 60. At the receiver, the optical signal is converted to an electrical signal, representing the original video or data signal at line 50.
FIG. 2 is a highly simplified block diagram of the optical transmission system 100 according to the present invention. There is shown an analog RF signal input source 101, such as broadband signal including a plurality of channels, and a pre-distortion circuit 105. The RF signal applied to the laser 102 is appropriately pre-distorted by the use of a pre-distortion circuit 105, as is known in the prior art for modifying the RF signal applied to the laser to compensate for the nonlinear response of the laser affecting the signal at the remote receiver. The output of the pre-distortion circuit 105 is applied to the laser 102 to modulate it. The modulation of the laser 102 in the present invention may be an AM-VSB modulator, or a quadrature, amplitude modulator. The optical signal output 110 of the laser is split into two parts: one part is applied to a phase modulator 111; the other part is applied to a frequency discrimination circuit 115.
The edge-emitting semiconductor laser used in the system of FIG. 2 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. Moreover, the broadband analog signal input has a bandwidth greater than one octave and includes a plurality of distinct information carrying channels.
The output of the frequency discrimination 115 is applied to a signal conditioning circuit 103 which consists of a series connected sequence of circuits performing distinct operations on the output RF signal of the frequency discriminator. The RF signal is applied to an attenuator 116 to appropriately adjust the amplitude of the signal to be commensurate with that of the phase shifted components introduced by the phase noise characteristics of the laser 102.
The output of the attenuator is then connected to a phase shift circuit 117. The circuit 117 corrects for the time lag of the signal output applied to circuit elements 115, 116, 117 compared to that signal applied to the modulator 111. In the video transmission band of interest (50 MHz-1000 MHz for traditional CATV systems), the phase noise of the semiconductor laser is “white”, i.e., the spectral power density of the noise is independent of frequency. In this case the phase correction path would need to have a constant (adjustable) gain with its delay precisely matched to that of the primary path. One aspect that needs to be accounted for is the frequency discriminator, 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.
The output of the phase shift circuit 117 is then applied to the phase modulator 111, to thereby introduce phase corrections into the optical signal to thereby correct or compensate for the noise generated.
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 will improve the ultimate noise cancellation at the expense of the transmitter's optical output power.

The output of the modulator 111 is coupled over a fiber 112 to an amplifier 113, which is then connected to the optical fiber or link 114. At the remote end, the optical fiber or link 114 is connected to the receiver which converts the received optical signal into an RF signal.
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 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 implements 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 complied or 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;
an input coupled to the laser for directly modulating the laser with an RF signal to produce an optical signal including an amplitude modulated information-containing component and a phase modulated component; and
a phase modulator coupled to an output of the laser and operative to receive a phase modulation cancellation signal for reducing phase noise associated with the optical signal.

2. A transmitter as defined in claim 1, wherein the laser is a semiconductor laser and the phase modulator is an external component spaced apart from the semiconductor laser.

3. A transmitter as defined in claim 1, wherein the phase modulator increases the SBS threshold of the received optical signal at the remote receiver.

4. A transmitter as defined in claim 1, further comprising a phase correction circuit coupled to the laser and the phase modulator, the phase correction circuit operative to generate the phase modulation cancellation signal, the phase correction circuit including a frequency discriminator having an input coupled to receive a portion of the optical signal output from the laser and a photodiode coupled to the frequency discriminator so that a phase modulated component in the optical signal is converted to a biasing electrical signal for biasing the phase modulator.

5. A transmitter as defined in claim 1, wherein a wavelength of the optical signal output from the laser is in the 1530 to 1570 nm range.

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

7. A transmitter as defined in claim 1, further comprising a pre-distortion circuit for modifying the RF signal applied to the laser to compensate for a nonlinear response of the laser affecting the received optical signal received at the remote receiver.

8. A 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. A transmitter as defined in claim 1, wherein the modulation of the laser is quadrature amplitude modulation.

10. A transmitter as defined in claim 1, wherein the phase correction circuit comprises a frequency discriminator coupled to receive a portion of the optical signal and a signal conditioning circuit coupled to an output of the frequency discriminator.

11. 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 phase noise spread over a frequency spectrum;
an input coupled to the laser for directly modulating the laser with an RF signal to produce an optical signal including an amplitude modulated information-containing component and phase noise;
an electro-optical element coupled to an output of the laser to receive a first portion of the optical signal, the electro-optical element for reducing the phase noise associated with the optical signal; and
a biasing circuit coupled to the output of the laser to convert a second portion of the optical signal into an electrical signal and coupled to the electro-optical element to control the electro-optical element based on the electrical signal to thereby reduce the phase noise in a received optical signal received at the remote receiver.

12. A transmitter as defined in claim 11, wherein the laser is a semiconductor laser and wherein noise characteristics of the laser introduce the phase noise.

13. A transmitter as defined in claim 11, wherein the electro-optical element increases an SBS threshold of the received optical signal received at the remote receiver.

14. A transmitter as defined in claim 11, wherein the biasing circuit includes a frequency discriminator coupled to the output of the laser to receive the second portion of the optical signal, an attenuator coupled to an output of the frequency discriminator to adjust an amplitude of the output of the frequency discriminator, and a phase shift element to synchronize a time lag of the second portion of the optical signal relative to the first portion of the optical signal applied to the electro-optical element.

15. A transmitter as defined in claim 11, wherein a wavelength of the optical signal output from the laser is in the 1530 to 1570 nm range.

16. A transmitter as defined in claim 11, 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. A transmitter as defined in claim 11, further comprising a pre-distortion circuit for modifying the RF signal applied to the laser to compensate for a nonlinear response of the laser affecting the received optical signal received at the remote receiver.

18. A transmitter as defined in claim 11, wherein the biasing circuit controllably adjusts a reduction of phase noise associated with the optical signal.

19. A method for reducing noise in an optical communication system in which an optical signal is transmitted over a fiber optic link to a remote receiver, the method comprising:
directly modulating a laser with an information signal to produce an output optical signal including an amplitude modulated information-containing component;
coupling the output optical signal to a phase modulator externally coupled to the laser;
generating an electrical signal based on a determined phase noise associated with the optical signal; and
phase modulating the output optical signal with the phase modulator controlled by the electrical signal to reduce the determined phase noise associated with the output optical signal.

20. The method of claim 19, wherein the determined phase noise is reduced in a received optical signal received at the remote receiver.

21. The method of claim 19, further comprising:
splitting the output optical signal into a first portion routed along a primary path and a second portion routed along a secondary path,
wherein the first portion of the output optical signal is coupled to the phase modulator,
wherein the electrical signal is generated based on the second portion of the output optical signal.

22. The method of claim 19, wherein directly modulating the laser with the information signal comprises directly modulating the laser using quadrature modulation.

23. A transmitter as defined in claim 1, wherein the phase modulation cancellation signal is adjusted in amplitude and phase to match the phase associated with the phase noise of the optical signal.

24. 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;
an input coupled to the laser for directly amplitude modulating the laser with an RF signal to produce an optical signal including an amplitude modulated information-containing component and a phase modulated component;
a phase modulator coupled to an output of the laser for reducing phase noise associated with the optical signal; and
a biasing circuit coupled to the phase modulator, including 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 optical signal is converted to a biasing electrical signal applied to the phase modulator.