JP2008193709A - Directly modulated or externally modulated laser optical transmission system with feed forward noise cancellation - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical transmitter for generating a modulated optical signal for transmission over a fiber optic link to a remote receiver including a laser. <P>SOLUTION: The optical transmitter includes: a modulator for directly amplitude modulating the laser 102 with an analog RF signal to produce an optical signal including an amplitude modulated information-containing component; and a phase modulator 111 coupled to the output of the laser for canceling the noise signals generated in the laser. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an analog signal light transmission system, and more particularly to a solid-state laser that is either directly modulated or externally modulated. In addition, the present invention provides for the cancellation of white noise components (white noise) resulting from the Brownian motion of charge carriers in a semiconductor laser, eg, a semiconductor laser, or the noise (which is the frequency from the bias current or the thermal environment of the laser). For this reason, it relates to the cancellation of (typically referred to as “1 / f” noise).

  Direct modulation of the analog intensity of a light emitting diode (LED) or semiconductor laser with an electrical signal is one of the simplest methods known as prior art for transmitting analog signals such as audio and video signals on optical fibers. It is considered one. Such analog transmission techniques have the advantage of requiring significantly less bandwidth than that required by digital transmission such as digital pulse code modulation or analog or pulse frequency modulation, but the use of amplitude modulation is typical. Presents more stringent requirements regarding the noise and distortion characteristics of the transmitter.

For these reasons, direct modulation techniques have been used for 1310 nm lasers applied to short transmission links using optical fibers with zero dispersion. For metro and long haul fiber transmission links, use of an externally modulated 1550 nm laser, typically over very long distances (100 km), to reduce the link loss, high frequency ( 900 MHz) is required. In such links, conversion of residual phase noise from a laser converted to amplitude noise through dispersion appearing in the fiber link may be a limiting factor.
Therefore, the present invention provides a simple and low-cost system for noise cancellation related to laser phase noise that allows analog light output to be used for analog transmission of metro and long-haul optical networks, particularly broadband RF signals. It tackles the issues it provides.

  Laser direct current modulation is known for use in digital optical transmission systems such as Dense Wavelength Division Multiplexing (DWDM) systems. See, for example, page 154 of Kartalopoulos, “DWDM Networks, Devices, and Technologies” (IEEE Press, 2003).

  In a 1550 nm analog optical transmission system, in addition to requiring low noise characteristics, the system must be highly linear. The distortion inherent in some analog transmitters prevents the linear electrical modulation signal from being linearly converted to an optical signal and causes distortion in the signal instead. These effects are particularly detrimental to multi-channel video transmissions that require a high degree of linearity to prevent inter-channel interference. Highly linearized analog optical systems can be widely applied to commercial analog systems such as broadcast TV transmission, CATV, interactive TV, and video telephone transmission.

  Although linearizing optical transmitters and other nonlinear transmitters has been studied over a period of time, the proposed solution has encountered practical disadvantages. For most of the applications described above, the bandwidth is too large for many practical implementations. Feedforward technology for linearization requires complex system configurations such as an optical power combiner and multiple light sources. Quasi-optical feedforward techniques have encountered similar complex problems and require additional components that are very well matched. However, as will be described later, the feedforward technique for phase noise cancellation is a practical technique that can be implemented using a number of well improved techniques.

  As mentioned above, external modulators 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 hybrid second order (CSO) distortion components induced by a fiber.

  Prior to the present invention, there was no direct (current) modulated laser coupled to a phase modulator for the purpose of canceling phase noise components arising from various noise sources in the laser's semiconductor structure. It was. Semiconductor lasers exhibit noise in both amplitude (sometimes referred to as relative intensity noise) and phase. While the noise itself appears to be different at different wavelengths in single mode fiber transmission, the noise characteristics are essentially independent of the laser wavelength. The main internal mechanism that causes phase and amplitude noise is spontaneous emission within the active region of the laser. Since spontaneously emitted photons do not have a specific phase relationship with photons generated by stimulated emission, both the amplitude and phase of the resulting light field are affected. The process of spontaneous emission is well understood and is explained by the Brownian motion process whose noise spectrum is essentially constant (white noise) within the operating frequency. Outside the laser, environmental effects such as micro-phonics, temperature fluctuations, and bias current noise can also generate phase noise in the optical field. These events typically result in optical phase noise representing a noise spectrum with “1 / f” dependence. The present invention seeks to minimize intrinsic phase noise from a semiconductor laser through feedforward cancellation regardless of the noise drive mechanism.

  It is an object of the present invention to provide an improved optical transmission system that uses a directly modulated laser.

  Another object of the present invention is to compensate for noise in lasers used in analog optical transmission systems.

  Another object of the present invention is to provide an external Mach-Zehnder modulator for use in a 1550 nm analog optical transmission system to improve phase noise attenuation.

  Yet another object of the present invention is to provide a highly linear analog optical transmission system suitable for long distance transmission dispersive optical fiber media using a direct modulation laser with a phase correction circuit.

  Still another object of the present invention is to provide a phase shift circuit for reducing residual phase noise from a laser in an analog optical transmission system suitable for long distance transmission dispersion optical fiber media.

  It is a further object of the present invention to provide a phase noise compensation process in a broadband analog optical transmission system.

  Briefly and in general terms, the present invention provides a modulated optical signal for transmission over a distributed fiber optical link to a remote receiver having an input for receiving a broadband analog radio frequency signal input. An optical transmitter that generates, a semiconductor laser that generates an optical signal with phase noise, and a signal that reduces phase noise at the output of the optical transmitter and therefore appears on the receiver side of the fiber optic link due to phase modulation noise components And a noise cancellation circuit including an optical phase modulator for reducing distortion.

  In another aspect, the invention provides an optical transmission system for use on a distributed fiber optical link including an optical transmitter with an analog signal input, a laser semiconductor, a modulation circuit for directly modulating a laser, and a semiconductor A phase shift circuit for canceling a phase modulation component of light associated with an external modulator for optical signal noise generated by a laser is provided.

  In another aspect, the present invention further provides a low cost direct modulation technique, preferably including circuitry for controlling an optical phase modulator that reduces the phase noise component produced by the laser. .

  In another aspect of the present invention, the output optical signal from the semiconductor laser is divided into two paths, a path to the phase modulator and a path to the frequency discriminator, to reduce phase noise in analog signal transmission. A cancellation circuit is provided. The amplitude and phase of the phase modulation cancellation signal are adjusted according to the frequency or phase dependence of the phase noise caused by the laser. The phase of the signal is synchronized by a delay or phase adjustment element in one of the paths. The primary and secondary signals are then recombined by an optical phase modulator to produce a single optical signal having only amplitude modulation. That is, the phase modulator modulates the primary signal from the semiconductor laser so that the resulting phase noise is minimized and produces an appropriate analog signal for transmission over the dispersion fiber optical link.

  Further objects, advantages and novel features of the present invention will become apparent to those skilled in the art from the present disclosure, including the following detailed description, as well as 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 skilled in the art who have received the teachings herein will recognize other applications, modifications, and variations that are described and claimed herein, and which are within the scope of the invention, so as to make the invention sufficiently useful. It will be appreciated that embodiments in other fields are possible.

  Details of the invention including exemplary aspects and embodiments thereof are described herein. Referring to the drawings and the following description, like reference numerals are used to indicate like or functionally similar elements, and illustrate important features of example embodiments in a very concise manner. Is intended. Furthermore, the drawings do not represent all features of an actual embodiment, nor represent the relative dimensions of the represented elements, nor are they represented at a fixed ratio.

  FIG. 1A is a block diagram of a prior art optical transmitter described in US Pat. No. 5,699,179 using an external modulator. The transmitter, indicated generally by “10”, transmits an optical signal through the optical fiber path 30 to the receiver 60. The transmitter 10 includes a semiconductor laser 12 that generates a continuous wave (CW) output. A typical example of such a laser is a distributed feedback (DFB) laser / or Fabry-Perot laser that produces an output light beam with a wavelength of 1550 nm. The unmodulated optical signal from the laser is coupled to the modulator by optical fiber 14. Modulator 16 can be a single modulator such as a Mach-Zehnder modulator, a cascaded MZ modulator, or two or more modulators such as those in a feedback linearizer. Modulator 16 also receives broadband RF signals, such as amplitude modulated residual sideband (AM-SDB) cable television (CATV) signals or video signals, via terminal 18 and line 20. In addition, when a feedforward linearizer is used, a depolarized signal is provided to modulator 16 via terminal 22 and line 24. This depolarized signal is used to depolarize the optical input to an error correction modulator (not shown) within the modulator 16.

  The modulated optical signal carrying the video data is coupled to an amplifier 28 by a fiber link 26. Amplifier 28 is typically an erbium doped fiber amplifier (EDFA). The amplified optical signal is coupled to a fiber optic transmission line 30 for the receiver 60. The fiber optic transmission line 30 can be a long distance link extending over several kilometers. In this case, a line amplifier, such as EDFA 28, is provided at a spatial interval along the line to boost the signal to the desired level. In the receiver 60, an amplifier (not shown) is provided to boost the incoming optical signal. The boosted signal is then provided to a photodetector, demodulated into an electrical signal at receiver 60, and represents the original video or data signal at line 50.

  FIG. 1 (b) shows a block diagram of a prior art optical transmitter utilizing direct current modulation of a laser. The broadband RF analog signal is applied directly to the laser 12. The modulated optical signal from laser 12 is coupled by fiber link 26 to an amplifier 28 such as an EDFA. The amplified optical signal is coupled to the fiber transmission line 30 for the receiver 60. At the receiver, the optical signal is converted to an electrical signal representing the original video or data signal on line 50.

  FIG. 2 is a very simple block diagram of an optical transmission system 100 according to the present invention. An analog RF signal input source 101, such as a broadband signal including multiple channels, and a predistortion circuit 105 are shown. The RF signal applied to the laser 102 is predistorted to modulate the RF signal applied to the laser and compensate for the nonlinear response of the laser that affects the signal at the remote receiver, as is known in the prior art. Appropriate predistortion is provided by using circuit 105. The modulation of the laser 102 in the present invention may be an AM-VSB modulator, or a quadrature or amplitude modulator. The optical signal output 110 of the laser is divided into two parts. One is supplied to the phase modulator 111 and the other is supplied to the frequency discriminating circuit 115.

  The edge-emitting semiconductor laser used in the system of FIG. 2 is preferably a distributed feedback laser (DFB), but a Fabry-Perot (FP) laser can be used as well. While FP lasers have light energy that diffuses in many modes, the use of DFB lasers is a preferred technique because its light output is primarily contained in a single lasing mode. is there.

  In a preferred embodiment, the laser is an external cavity laser that is within a laser light output wavelength in the range of 1530 to 1570 nm. Furthermore, the broadband analog signal input has a bandwidth greater than one octave and has a plurality of different information carrying channels.

  The output of the frequency discriminator 115 is provided to a signal conditioning circuit 103 comprised of a series of series connected circuits that perform a separate operation on the frequency discriminator output RF signal. The RF signal is commensurate with the amplitude of the phase shift component provided by the phase noise characteristics of the laser 102, with the amplitude of the signal applied to the attenuator 116 appropriately adjusted.

  The output of the attenuator is then connected to the phase shift circuit 117. This circuit 117 corrects the time lag of the signal output given to the circuit elements 115, 116, 117 compared with the signal given to the modulator 111. In the video transmission band in question (50 MHz to 1000 MHz with respect to the conventional CATV system), the phase noise of the semiconductor laser is “white”, that is, the density of the noise spectrum intensity is independent of the frequency. In this case, the phase correction path is required to have a constant (adjustable) gain with a delay that exactly matches that of the primary path. One aspect that needs to be considered is a frequency discriminator, in particular an optical device for the electrical conversion process in the phase correction path. When an optical signal is detected by a photodiode, a phenomenon known as shot noise is seen. This noise arises from a statistical process that absorbs photons in the photodiode and produces electron hole pairs. This noise is unavoidable for all practical purposes. Therefore, shot noise imposes a lower limit on the amount of phase noise cancellation that can be achieved.

  The output of phase shift circuit 117 is then provided to phase modulator 111, thereby providing phase correction in the optical signal, resulting in correction or compensation of the generated noise.

The spectral noise density of the photocurrent generated from the photodiode is given by:
<I _n ^2> = 2eI _p

Here, e is the charge amount of electrons, and I _p is a DC photocurrent. Those skilled in the art will immediately appreciate that the noise power is linearly dependent on the received optical power, and therefore the signal-to-noise ratio of the shot noise dominant process improves with increasing received power. I will. This represents a fundamental design compromise in the present invention. The additional power tapped into the phase correction path will improve extreme noise cancellation at the expense of transmitter optical output power.

  The output of modulator 111 is coupled to amplifier 113 through fiber 112, which is then coupled to an optical fiber or link 114. At the remote end, an optical fiber or link 114 is connected to the receiver and 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 as a TV signal that modulates a laser or light emitting diode, other non-linear devices such as amplifiers have inherent distortions that can be significantly reduced by this technique. Fine adjustment of the relative phase of the signals in the primary and secondary paths is done in the secondary path in the illustrated embodiment, but this fine adjustment is placed in the primary path along with the coarse adjustment. You can also. A secondary path is preferred because such a delay in the primary path will have an impedance that is inappropriate for this path.

  Many aspects of the techniques and apparatus of the present invention can be implemented in digital circuitry, computer hardware, firmware, software, or combinations thereof. The circuit of the present invention is implemented in a computer product tangibly embodied in a machine readable storage device executed by a programmable processor and on software located on a network node or website downloadable to the computer product. Or on demand. The technique described above is a function of the present invention by, for example, executing a sequence of a single central processing unit, a microprocessor, one or more digital signal processors, instruction signals or instruction programs and manipulating input data to produce output Can be executed by a gate array of logic gates or a wiring logic circuit. The method includes at least one programmable processor coupled to received data and instructions from and transmitted data and instructions to the data storage system, at least one input / output device, and at least one output device. Can be advantageously implemented in one or more computer programs executable on a programmable system including: Each computer program can be implemented in a high level procedural or object oriented programming language, or it can be implemented in assembly or machine language if desired. In any case, the language is a compiled or interpreted language. Suitable processors include, by way of example, general purpose and special purpose microprocessors. Generally, a processor will receive instructions and data from a read-only memory and / or a random access memory. Storage devices suitable for tangibly embodied computer program instructions and data include all non-volatile memory, such as semiconductor devices such as EPROM and EEPROM, flash memory devices, built-in hard disks and removable disks. Examples include magnetic disks, magnetic optical disks, and CD-ROM disks. Any of the above can be supplemented by specially designed application specific integrated circuits (ASICs) or as embedded.

  It will be appreciated that each of the above-described elements, or two or more, may also find beneficial application in other forms of construction different from the above form.

  Although the invention has been illustrated and described as embodied in an optical transmission system, various modifications and changes in structure are possible without departing from the spirit of the invention in any way and will not be described in detail. It is not intended to be limited.

  Without further analysis, the above-mentioned matter may be considered by a third party without omitting features that are deemed to constitute the essential characteristics of the general or specific features of the invention correctly from the point of view of the prior art. It fully illustrates the gist of the present invention that can be easily adapted to many applications by applying current knowledge. Accordingly, such adaptations are intended and intended to be embraced within the meaning and scope of the equivalents of the claims that follow.

1 is a simplified block diagram of an externally modulated optical transmission system known in the prior art. FIG. 1 is a simplified block diagram of a directly modulated optical transmission system known in the prior art. 1 is a simplified block diagram of an optical transmission system according to the present invention.

Claims (7)

An optical transmitter that generates a modulated optical signal that is transmitted over a fiber optic link to a remote receiver,
A laser that generates a baseband optical signal containing noise that spreads over the frequency spectrum;
A modulator that directly modulates the laser with an analog RF signal to generate an optical signal including an amplitude modulation information holding component and a phase modulation component;
And a phase modulator coupled to the output of the laser for canceling the phase noise associated with the optical signal.   The transmitter according to claim 1, wherein the laser is a semiconductor laser, and the phase modulator cancels the noise component in the laser output signal.   The transmitter of claim 1, wherein the phase modulator increases an SBS threshold of a received optical signal at the remote receiver.   A frequency discrimination circuit having an input connected to the output of the laser and an output coupled to a photodiode, whereby the phase noise in the optical signal is converted into a modulated electrical signal provided to the phase modulator; The transmitter of claim 1, wherein effective phase noise cancellation is generated.   The transmitter according to claim 1, wherein the wavelength of the optical output of the laser is in the range of 1530 to 1570 nm.   The transmitter of claim 1, wherein the broadband analog signal input has a bandwidth greater than one octave and has a plurality of identifiable information carrying channels.   The transmission of claim 1, further comprising a predistortion circuit for modulating the RF signal applied to the laser to compensate for the nonlinear response of the laser acting on the signal at the remote receiver. vessel.

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