CN101277154B - For generation of the optical launcher of the optical modulation for being transferred to remote receiver via optical fiber link - Google Patents

Abstract

The invention provides a kind of for generation of optical modulation for the optical launcher being transferred to remote receiver via optical fiber link, it comprises: laser; Modulator, it carries out external modulation to produce the optical signalling comprised containing modulation intelligence component with RF signal to described optical output signal; And phase-modulator, its output being coupled to described modulator or be directly coupled to described laser output for reduction or eliminate the noise signal produced in described laser.

Description

Optical transmitter for generating a modulated optical signal for transmission to a remote receiver via a fiber optic link

technical field

The present invention relates to an optical transmission system for analog or digital signals, and in particular to a system using externally modulated solid-state lasers. Furthermore, the invention relates to the elimination of noise components in said system (white noise) generated by many possible sources within the semiconductor laser, such as Brownian motion of charge carriers, or noise generated by fluctuations in the bias current of the laser or in the thermal environment (which varies inversely with frequency, and is therefore often referred to as "1/f" noise).

Background technique

Directly modulating the analog intensity of a light emitting diode (LED) or semiconductor laser with an electrical signal is considered the simplest method known in the art for transmitting analog signals such as voice and video signals over optical fibers. While such analog transmission techniques have the advantage of having significantly smaller bandwidth requirements compared to digital transmissions such as digital pulse code modulation or analog or pulse frequency modulation, the use of amplitude modulation usually imposes stricter requirements on the noise and distortion characteristics of the transmitter requirements.

For these reasons, direct modulation techniques have been used in conjunction with 1310 nm lasers in the case of applications for short transmission links employing fiber optic links with zero scatter. For applications in metropolitan and long-haul fiber-optic transmission links, low link loss requires the use of externally modulated 1550nm lasers, typically over very long distances (100km) and high frequencies (over 900MHz). A limiting factor for such links may be the conversion of residual phase noise from the laser, which is converted to amplitude noise via scattering present in the fiber link. The present invention thus focuses on the problem of providing a simple and low-cost system for noise cancellation associated with the phase noise of lasers, so that analog optical outputs can be used in metropolitan and long-distance optical networks, especially for analogue broadband RF signals transmission.

Direct current modulation of lasers is also known for use in digital optical transmission systems, such as Dense Wavelength Division Multiplexing (DWDM) systems. See (eg) "DWDM Networks, Devices, and Technology" by Kartalopoulos (IEEE Press, 2003, p. 154).

In addition to the low noise characteristics required for a 1550nm analog optical transmission system, the system must also be highly linear. Distortion inherent in certain analog transmitters prevents a linear electrical modulation signal from being linearly converted to an optical signal, but instead distorts the signal. These effects are particularly detrimental for multi-channel video transmission, which requires excellent linearity to prevent channels from interfering with each other. Highly linearizable analog optics are widely used in commercial analog systems such as broadcast TV transmission, CATV, interactive TV and video telephony transmission.

As an alternative to direct current modulation, it is known in the prior art to use external modulators of continuous wave (CW) lasers in optical transmission systems. US Patent No. 5,699,179 describes an externally modulated feedforward linearized analog optical transmitter for reducing fiber-induced compound second order (CSO) distortion components.

The linearization of optical and other nonlinear emitters has been studied for some time, but the proposed solutions have drawbacks in practice. Most of the applications discussed above have bandwidths that are too large for many practical implementations. Feedforward techniques for linearization require complex system components such as optical power combiners and multiple light sources. Quasi-optical feed-forward techniques suffer from similar complexity problems and additionally require extremely well-matched parts. However, as discussed below, the forward technique for phase noise cancellation is a practical technique that can be implemented using many well-developed techniques.

Prior to the present invention, phase modulators coupled to externally modulated lasers for the purpose of canceling phase noise components produced by various noise sources in the semiconductor structure of the laser had not been applied. It should be noted that semiconductor lasers exhibit noise both in their amplitude (often referred to as relative intensity noise) and in their phase. These noise characteristics are largely independent of laser wavelength, but noise can appear in different ways at different wavelengths in single-mode fiber transmission. The main internal mechanism leading to phase and amplitude noise is spontaneous emission within the active region of the laser. Since spontaneously emitted photons have no specific phase relationship to those generated via stimulated emission, both the amplitude and phase of the resulting light field are affected. Spontaneous emission processes are well known and have been shown to be described by Brownian motion processes where the noise spectrum is substantially constant within the operating frequency (white noise). Outside the laser, environmental effects such as microphonics, temperature fluctuations, and bias current noise can also generate phase noise in the optical field. These events typically result in optical phase noise, which exhibits a noise spectrum with a "1/f" correlation.

The present invention seeks to minimize the inherent phase noise from semiconductor lasers by feed-forward cancellation regardless of the driving mechanism of the noise.

Contents of the invention

1. Purpose of the invention

It is an object of the present invention to provide an improved optical delivery system using externally modulated lasers.

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

Yet another object of the present invention is to provide an external phase modulator for external modulation of a 1550 nm analog optical transmission system for improved phase noise reduction.

It is a further object of the present invention to provide a highly linear optical transmission system suitable for long-distance dispersive fiber optic media and using an externally modulated laser together with a phase correction circuit and a phase modulator coupled to the optical signal.

It is yet another object of the present invention to provide a phase shifting circuit for reducing residual phase noise from an externally modulated laser in an analog optical transmission system suitable for long-distance dispersive fiber optic media.

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

2. Features of the invention

Briefly and broadly stated, the present invention provides an optical transmitter for generating a modulated optical signal for transmission to a remote receiver via a fiber optic link, comprising: a laser; a modulator for pairing the signal with an RF signal The laser is externally modulated to generate an optical signal comprising a component containing modulation information; and a phase modulator coupled to an output of the modulator for canceling a noise signal generated in the laser.

In another aspect, the present invention provides an optical transmitter for generating a modulated optical signal for transmission to a remote receiver via a dispersive optical link, having: a semiconductor laser for generating an optical signal with associated phase noise signal; a noise cancellation circuit coupled to the output of the laser and including an optical phase modulator for reducing phase noise generated in the laser; and an external modulator coupled to the output of the phase modulator and used It receives a broadband analog radio frequency signal input and modulates the optical signal.

In another aspect, the present invention provides an optical transmission system for use via a dispersive fiber link, comprising: an optical transmitter with an analog or digital RF signal input; a semiconductor laser; a modulation circuit for externally modulating the a laser; and circuitry for canceling an optical phase modulation component associated with noise generated by the semiconductor laser.

In another aspect of the present invention, there is provided a noise cancellation circuit for reducing phase noise in analog signal transmission, which splits the output optical signal from the external modulator of the semiconductor laser into two paths, one leading to the phase modulation device and the other goes to the frequency discriminator. The phase modulation cancellation signal is adjusted in amplitude and phase to match the frequency or phase dependence of the laser generated phase noise. The phases 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 with amplitude modulation only. Thus, the phase modulator modulates the main signal from the semiconductor laser in a way that minimizes the resulting phase noise, making the analog signal suitable for transmission via a dispersive fiber link.

Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art from this disclosure, including the following detailed description, and through practice of the invention. While the invention is described hereinafter with reference to preferred embodiments, it should be understood that the invention is not limited thereto. Those skilled in the art having access to the teachings herein will recognize additional applications, modifications, and embodiments in other fields that are within the scope of the inventions disclosed and claimed herein, and which Can have significant utility relative to it.

Description of drawings

These and other features and advantages of the present invention will be better understood and more fully understood by reference to the following detailed description, taken in conjunction with the accompanying drawings, in which:

Figure 1 is a highly simplified block diagram of an externally modulated optical delivery system known in the art;

Figure 2 is a highly simplified block diagram of a first embodiment of an optical delivery system according to the present invention;

Figure 3 is a highly simplified block diagram of a second embodiment of an optical delivery system according to the present invention.

The novel features and characteristics of the invention are set forth in the appended claims. However, the invention itself, together with other features and advantages, can best be understood by referring to the detailed description of specific embodiments taken in conjunction with the accompanying drawings.

Detailed ways

Details of the invention, including exemplary aspects and embodiments thereof, will now be described. Referring to the drawings and the following description, like reference numerals are used to identify identical or functionally similar elements, and it is intended to illustrate the main features of the exemplary embodiments in a highly simplified diagrammatic manner. Furthermore, the drawings are not intended to depict every feature of actual embodiments or the relative dimensions of the depicted elements, and the drawings are not drawn to scale.

Figure 1 is a block diagram of a prior art optical transmitter utilizing an external modulator as presented in US Patent No. 5,699,179. The transmitter, shown generally at 10 , transmits an optical signal to a remote receiver 60 via fiber optic path 30 . Transmitter 10 includes a semiconductor laser 12 that produces a continuous wave (CW) output. Typical examples of such lasers are Distributed Feedback (DFB) lasers and/or Fabry-Perot lasers, which typically produce an output beam at a wavelength of 1550 nm. The unmodulated optical signal from the laser is coupled to modulator 16 through optical fiber 14 . Modulator 16 may be a single modulator such as a Mach-Zehnder modulator, cascaded MZ modulators, or more than one modulator such as in a feed-forward linearizer. Modulator 16 also receives a broadband RF signal, such as an amplitude modulated vestigial sideband (AM-SDB) cable television (CATV) or video signal, via terminal 18 and line 20 . Additionally, a depolarized signal is provided to modulator 16 via terminal 22 and line 24 when a feedforward linearizer is used. The depolarized signal is used in modulator 16 to depolarize the optical input to an error correction modulator (not shown).

The modulated optical signal carrying the video data is coupled through fiber optic link 26 to amplifier 28 . Amplifier 28 is typically an erbium-doped fiber amplifier (EDFA). The amplified optical signal is coupled to a fiber optic transmission line 30 to a receiver 60 . The fiber optic transmission line 30 may be a long-distance link extending over several kilometers. In this case a line amplifier such as an EDFA 28 may be provided at spaced distances therein along the line in order to boost the signal to the required level. 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 receiver 60 to produce an electrical signal representative of the original video or data signal at line 50 .

3 is a block diagram of an optical transmitter utilizing an external modulator according to a first embodiment of the present invention. The transmitter, shown generally at 200 , transmits an optical signal via fiber optic path 30 to a remote receiver. Transmitter 200 includes a semiconductor laser 101 that produces a continuous wave (CW) output. Typical examples of such lasers are Distributed Feedback (DFB) lasers and/or Fabry-Perot lasers, which typically produce an output beam at a wavelength of 1550 nm.

The edge emitting semiconductor lasers used in the system of Fig. 3 are preferably distributed feedback lasers (DFB), but Fabry-Perot (FP) lasers could equally be used. DFB lasers are the preferred approach because their optical output is mainly contained in a single laser mode, whereas FP lasers have optical energy spread among many modes. In a preferred embodiment, said laser is an external cavity laser with a laser light output wavelength in the range of 1530 to 1570 nm.

The unmodulated optical signal from the laser is split into two parts; the first part is coupled to modulator 102 through optical fiber 103 . Modulator 102 may be a single modulator such as a Mach-Zehnder modulator, cascaded MZ modulators, or more than one modulator such as in a feed-forward linearizer. Modulator 102 receives via terminal 104 and line 105 a broadband RF signal or a digital signal such as an amplitude modulated vestigial sideband (AM-SDB) cable television (CATV) or video signal. Analog signals may have a bandwidth greater than one octave and carry multiple channels. Also, when using a feed-forward linearizer, a depolarized signal is provided to the modulator similar to the configuration of Figure 1 . The depolarized signal is used in modulator 102 to depolarize the optical input to an error correction modulator (not shown).

As mentioned above, the optical signal output of the laser is split into two parts: one part is applied to the modulator 102;

The output of the frequency discriminator 107 is applied to an attenuator 108 to properly adjust the amplitude of the signal to be commensurate with the amplitude of the phase modulation component 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 the skew of the signal output applied to the circuit elements 107 , 108 compared to said signal applied directly to the modulator 102 . In the concerned video transmission frequency band (50MHz-1000MHz for traditional CATV systems), the phase noise of the semiconductor laser 101 is "white", that is, the spectral power density of the noise is independent of frequency. In this case, the phase correction paths 106 , 107 , 108 , 109 will need to have constant (adjustable) gain with a delay that exactly matches that of the main paths 103 , 102 and 110 . One aspect that needs to be addressed 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 a photodiode detects an optical signal, a phenomenon known as shot noise is observed. This noise is generated by the statistical process of absorbing photons in the photodiode to create electron-hole pairs. This noise is unavoidable for all practical purposes. Therefore, shot noise will place a lower limit on the amount of phase noise cancellation that can be achieved.

The output of the phase shift circuit 109 is then applied to a phase modulator 110 to thereby introduce a phase correction into the optical signal through its phase modulation to thereby correct or compensate for laser generated noise.

The spectral noise density of the photocurrent generated from the photodiode is given as

<i _n ² >=2eI _p

where e is the electron charge and I _p is the DC photocurrent. Those skilled in the art will immediately appreciate the fact that noise power has a linear dependence on received optical power, and thus the signal-to-noise ratio of a process dominated by shot noise improves as received power increases. This represents a fundamental design tradeoff in the proposed invention. Tapping more power into the phase correction paths 106, 107, 108, 109... will improve the final phase noise cancellation at the expense of the optical output power of the transmitter.

The output of the phase modulator 110 is coupled via an optical fiber 111 to an amplifier 112 which is then connected to an optical fiber or link 30 . At the far end, an optical fiber or link 30 connects to a receiver that converts the received optical signal to an RF signal, similar to that presented in FIG. 1 .

2 is a block diagram of an optical transmitter utilizing an external modulator according to a second embodiment of the present invention. The transmitter, shown generally at 100 , transmits an optical signal via fiber optic path 30 to a remote receiver. Transmitter 100 includes a semiconductor laser 101 that produces a continuous wave (CW) output. Typical examples of such lasers are Distributed Feedback (DFB) lasers and/or Fabry-Perot lasers, which typically produce an output beam at a wavelength of 1550 nm. The unmodulated optical signal from the laser is split into two parts; one part is coupled to the phase modulator 110 through the optical fiber 103 . Phase modulator 110 introduces phase correction to the optical signal to thereby correct or compensate for noise produced by the laser. The CW output of phase modulator 110 is coupled to modulator 102 to produce an optical signal containing the modulated information.

Modulator 102 may be a single modulator such as a Mach-Zehnder modulator, cascaded MZ modulators, or more than one modulator such as a feed-forward linearizer. Modulator 102 receives a broadband RF signal such as an amplitude modulated vestigial sideband (AM-SDB) cable television (CATV) or video signal or a digital signal via terminal 104 and line 105 . Also, when using a feed-forward linearizer, similar to the configuration of Figure 1, a depolarized signal is provided to the modulator. The depolarized signal is used in modulator 102 to depolarize the optical input to an error correction modulator (not shown).

As mentioned above, the optical signal output of the laser is split into two parts: one part is applied to the phase modulator 110;

The output of the frequency discriminator 107 is applied to an attenuator 108 to properly adjust the amplitude of the signal to be commensurate with the amplitude of the phase modulation component 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 the skew of the signal output applied to the circuit elements 107 , 108 compared to said signal applied to the phase modulator 110 . In the concerned video transmission frequency band (50MHz-1000MHz for traditional CATV systems), the phase noise of the semiconductor laser 101 is "white", that is, the spectral power density of the noise is independent of frequency. In this case, the phase correction paths 106 , 107 , 108 , 109 will need to have constant (adjustable) gain with a delay that exactly matches that of the main paths 103 , 102 and 110 . One aspect that needs to be addressed is the frequency discriminator 107, specifically the optical-to-electrical conversion process in the phase correction path. When a photodiode detects an optical signal, a phenomenon known as shot noise is observed. This noise is generated by the statistical process of absorbing photons in the photodiode to create electron-hole pairs. This noise is practically unavoidable. Therefore, shot noise will impose a lower limit on the amount of phase noise cancellation that can be achieved.

Many changes and modifications will be readily 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, the inherent distortion of other non-linear devices such as amplifiers can be largely eliminated by this technique. 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 can also be in the primary path with coarse adjustment. The secondary path is preferred because such delays in the primary path may have undue impedance to this path.

Aspects of the techniques and devices of the present invention may be implemented in digital circuitry, or in computer hardware, firmware, software, or combinations thereof. The circuitry of the present invention may be implemented in a computer product (tangibly embodied in a machine-readable storage device for execution by a programmable processor) or in software located at a network node or website (which may be downloaded automatically or on demand to computer products). The foregoing techniques may be performed by, for example, a single central processing unit, multiple processors, one or more digital signal processors, gate arrays of logic gates, or hardwired logic circuits for executing a sequence of signals or instruction programs to be The data is manipulated and output is generated to perform the functions of the invention. The method may advantageously be implemented in one or more computer programs executable on a programmable system comprising at least one computer coupled to receive data and instructions from and transmit data to a data storage system. and a programmable processor of instructions, at least one input/output 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 compiled or translated language. Suitable processors include, by way of example, both general and special purpose microprocessors. Generally, a processor will receive instructions and data from a 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 means may be supplemented by or incorporated in a specially designed Application Specific Integrated Circuit (ASIC).

It will be appreciated that each of the elements described above, or two or more together, may also be effectively applied to other types of configurations than those described above.

Although the invention has been illustrated and described as being implemented 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 from the spirit of the invention in any way.

Without further analysis, the foregoing will present the gist of the invention in such a way that others, by applying current knowledge, can, without omitting those aspects which, from a standpoint of the prior art, fully constitute essential characteristics of a general or particular aspect of the invention It is easy to adapt it for various applications without any features, and thus such adaptation is properly and intended to be included within the equivalent meaning and range of the appended claims.

Claims (17)

1. An optical transmitter for generating a modulated optical signal for transmission to a remote receiver via an optical fiber link, comprising: a laser for generating a baseband optical signal including a noise spread over the spectrum; a modulator for modulating said baseband optical signal with an RF signal to produce an optical signal containing modulated information; a phase modulator coupled to an output of the modulator for receiving the modulated information-containing optical signal and for adjusting the modulated information-containing optical signal by canceling phase noise associated with the baseband optical signal an optical signal, and produces an output optical signal coupled to said 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 and an output, the input of the frequency discriminator being coupled to receive the A portion of a baseband optical signal, the output of the frequency discriminator coupled to a photodiode to convert phase noise associated with the baseband optical signal into a modulated electrical signal for controlling the phase modulator to provide the Feed-forward cancellation of phase noise. 2. The optical transmitter of claim 1, wherein the laser is a semiconductor laser and the phase modulator cancels phase noise associated with the baseband optical signal. 3. The optical transmitter of claim 1, wherein the RF signal is an amplitude modulated vestigial sideband cable television or video signal. 4. The optical transmitter of claim 1, wherein the modulator is a Mach-Zehnder modulator. 5. The optical transmitter of claim 1, wherein the baseband optical signal output from the laser has a wavelength ranging from 1530 to 1570 nm. 6. The optical transmitter of claim 1, wherein the RF signal is a broadband analog signal input having a bandwidth greater than one octave and comprising a plurality of distinct information-carrying channels. 7. The optical transmitter of claim 1, wherein the phase modulator is selectively adjustable depending on the length of the fiber optic link to compensate for distortion caused by chromatic dispersion in the fiber optic link. 8. The optical transmitter of claim 1, wherein tapping more power into the phase correction circuit improves final phase noise cancellation at the expense of optical output power of the transmitter. 9. An optical transmitter for generating a modulated optical signal for transmission to a remote receiver via a fiber optic link, the optical transmitter comprising: a laser for generating a baseband optical signal including a noise spread over the spectrum; a phase modulator coupled to the output of the laser for generating a phase modulated output optical signal; a phase correction circuit coupled to the output of the laser to provide an electrical input signal to the phase modulator for use by the phase modulator by canceling phase noise associated with the baseband optical signal to adjust the baseband optical signal; and a modulator for modulating the phase modulated output optical signal generated by the phase modulator with an RF signal to generate an optical signal containing modulation information. 10. The optical transmitter of claim 9, wherein the laser comprises a semiconductor laser and the phase modulator provides phase noise cancellation in the baseband optical signal. 11. The optical transmitter of claim 9, wherein the RF signal is a digital signal. 12. The optical transmitter of claim 9, wherein the modulator is a Mach-Zehnder modulator. 13. The optical transmitter of claim 9, wherein the phase correction circuit is coupled to the phase modulator and includes a frequency discriminator having an input connected to the output of the laser and coupled to the a photodiode of a frequency discriminator to convert the phase noise in the baseband optical signal into a modulated electrical signal for controlling the phase modulator. 14. The optical transmitter of claim 9, wherein the baseband optical signal output from the laser has a wavelength ranging from 1530 to 1570 nm. 15. The optical transmitter of claim 9, wherein the RF signal is a broadband analog signal input having a bandwidth greater than one octave and comprising a plurality of distinct information-carrying channels. 16. The optical transmitter of claim 9, wherein the phase modulator is selectively adjustable according to the length of the fiber optic link to compensate for distortion caused by chromatic dispersion in the fiber optic link. 17. The optical transmitter of claim 9, wherein the phase correction circuit is for adjusting the amplitude and phase of the baseband optical signal.