Abstract:
A method and system for dispersion control of electromagnetic signals in communication networks by narrowing the widths of electromagnetic pulses such as modulated laser signals. Generally, in the preferred embodiment, the present invention utilizes a feedback loop based on dither frequency modulation which dynamically adjusts the alignment of the laser center frequency with the filter passband. In this way, there is an acceptable tradeoff between optical power and pulse width, so a higher power laser can be used to generate a narrower optical pulse. The narrower pulses then travel farther in the fiber link before reaching their dispersion limit. It is believed that, by using this invention, existing link distance could be doubled, while re-using existing installed singlemode fiber. The systems employing the feedback loop may be information carrying or control systems.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention generally relates to methods and systems that generate and process electromagnetic signals in the radio frequency, microwave and optical frequency spectra. More specifically, the invention relates to such methods and systems that modulate these electromagnetic signals and prepare the modulated signals for transmission over significant distances. Even more specifically, the present invention relates to such method and systems that are particularly well suited for enabling such electromagnetic signals to carry data at a rate of forty gigabits and above for long distances. 
   2. Description of the Prior Art 
   The demand for bandwidth has driven the introduction of higher data rate protocols such as ten Gigabit Ethernet technology and Synchronous Optical Networks (SONET). Serial links at this data rate are limited by dispersion to a few km&#39;s on singlemode fiber; the next generation of forty Gbit/s and beyond data links will face even more severe distance limitations. Because this limit is caused by dispersion, not link budget, it cannot be overcome by a brute force approach of increasing the laser power. In any case, high power lasers on singlemode fiber are prone to additional problems such as nonlinear noise sources. Since many existing data centers have been planned with much longer link distances, there is expected to be a need for ten and forty gigabit repeaters and channel extenders; however these devices are expensive and must be located at inconvenient places along the link path. It is desirable to overcome the dispersion limit of high speed data links, and thereby achieve much longer working distances without repeaters on singlemode fiber. 
   In order to compensate for singlemode dispersion, a method is required for narrowing the widths of optical pulses being launched into the fiber, without resorting to a special type of expensive laser device. It is known that launching a Gaussian optical pulse through a Gaussian wavelength selective bandpass filter will reduce the pulse width. There is a tradeoff of pulse width vs. optical power. A higher power transmitter is required, but this can be easily achieved with current transceiver designs simply by increasing the laser bias current. However, it is not practical to implement this tradeoff unless a controlled method exists for matching the center wavelength of an arbitrarily chosen laser to the center of a filter passband. Otherwise, the optical loss between the laser and filter becomes too great and any advantages from reducing the pulse width are lost. One method and system for narrowing the widths of optical pulses is disclosed in copending patent application Ser. No. 09/865,256, for “Apparatus and Method for Wavelength-Locked Loop for Systems and Applications Employing Electromagnetic Signals,” filed May 22, 2001, the disclosure of which is hereby incorporated herein in its entirety by reference. The present invention is an improvement on the system described in this copending application. 
   SUMMARY OF THE INVENTION 
   An object of this invention is to extend the distance for high data rate protocols such as ten and forty Gigabit Ethernet Links and Synchronous Optical Networks (SONET) using dispersion compensation. 
   Another object of the present invention is to overcome the dispersion limit of high speed optical data links, and thereby achieve much longer working distances without repeaters on singlemode fiber. 
   A further object of this invention is to compensate for dispersion of a modulated laser signal on a singlemode fiber by narrowing the width of optical pulses being launched into the fiber. 
   These and other objectives are attained with a method and system for dispersion control of electromagnetic signals in communication networks by narrowing the widths of electromagnetic pulses such as modulated laser signals. Generally, in the preferred embodiment, the present invention utilizes a feedback loop based on dither frequency modulation which dynamically adjusts the alignment of the laser center frequency with the filter passband. In this way, there is an acceptable tradeoff between optical power and pulse width, so a higher power laser can be used to generate a narrower optical pulse. The narrower pulses then travel farther in the fiber link before reaching their dispersion limit. It is believed that, by using this invention, existing link distance could be doubled, while re-using existing installed singlemode fiber. 
   The systems employing the feedback loop may be information carrying or control systems employing electromagnetic waves including those waves at radio frequency, microwave frequency and optical frequency portions of an electromagnetic frequency spectrum. Thus, the electromagnetic signals may comprise radio frequency signals, microwave signals, and optical signals. 
   When employed in laser/optical networks, the system and method of the present invention may be used to tune laser diode devices, and/or compensate for any type of wavelength-selective element in the network, including wavelength selective filters, attenuators, and switches, in fiber Bragg gratings, ring resonators in optical amplifiers, external modulators such as acousto-optic tunable filters, or array waveguide gratings. This applies to many other optical components in the network as well (for example, optical amplifiers that may act as filters when operating in the nonlinear regime). Furthermore, the system and method of the invention may be used to implement less expensive devices for all of the above application areas. 
   Alternately, the system and method of the invention may be implemented to tune such devices for Wavelength Division Multiplexing (WDM) and optical network applications, in real-time, during manufacture, e.g., tuning all lasers to a specific wavelength. This would significantly increase lot yields of laser devices which otherwise may be discarded as not meeting wavelength specifications as a result of manufacture process variations, for example. 
   Further benefits and advantages of the invention will become apparent from a consideration of the following detailed description, given with reference to the accompanying drawings, which specify and show preferred embodiments of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  generally illustrates pulse narrowing 
       FIG. 2  shows a system embodying this invention 
       FIG. 3  depicts an alternate system embodying the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The technique of pulse narrowing by using optical filters is shown in  FIG. 1 . Although it is not intuitive, it can be shown that a Gaussian shaped optical pulse becomes narrowed when passing through a Gaussian bandpass filter, even if the filter bandwidth is larger than the original laser pulse bandwidth. Mathematically, this happens because both the laser pulse shape and the filter response are Gaussian shaped in both time and frequency domains; passing light through such a filter thus results in the convolution of two Gaussian pulses, which is itself a Gaussian pulse with narrower width. Details of such pulse narrowing are given in many standard optics texts such as P. Das, Optical Signal Process, Springer-Verlag (191) or H. Dutton, Optical Communication, Academic Press (1993). 
   In principle, a Gaussian bandpass filter can be used to narrow the width of output pulses from a direct modulated semiconductor laser diode; narrower pulses would be able to tolerate higher dispersion (pulse spreading) and thus overcome the dispersion distance limits on an optical link. However, due to inherent limitations in the mass production of optical filters and lasers, a range of optical center wavelengths is obtained. If there is a mismatch between the laser and filter center wavelength, then the significant excess optical loss (3–4 dB or more) negates any benefits obtained from the dispersion compensation. This invention presents a method for precisely aligning the center wavelengths of an arbitrary laser and filter combination, making dispersion compensation practical for high data rate links using direct modulated semiconductor lasers. 
     FIG. 2  shows a preferred system  10  embodying this invention. Generally, in ten Gbit and above systems, a distributed feedback laser diode is often used for direct current modulation application. An optical filter is employed having a Gaussian bandpass filter. More specifically, optical system  10  includes a light source such as laser diode  12  driven with both a bias voltage  14  from a voltage bias circuit  16 , and modulated data  18  from a data source (not shown). The laser diode generates an optical (laser light) signal  20  that is received by a bandpass filter  22  or other suitable frequency selective device including but not limited to: thin film optical interference filters, acousto-optic filters, electro-optic filters, diffraction gratings, prisms, fiber Bragg gratings, integrated optics interferometers, electroabsorption filters, and liquid crystals. The laser diode itself may comprise a standard Fabry Perot or any other type (e.g., Vertical Cavity Surface Emitting (VCSEL)), light emitting diodes, or may comprise a Distributed Feedback semiconductor laser diode (DFB) such as commonly used for wavelength multiplexing. 
   Preferably, the laser diode emits light in the range of 850 nm to 1550 nm wavelength range. As mentioned, the bandpass filter may comprise a thin film interference filter comprising multiple layers of alternating refractive indices on a transparent substrate, e.g., glass. 
   As further shown in  FIG. 2 , there is an added sinusoidal dither modulation circuit or oscillator  24  for generating a sinusoidal dither modulation signal  27  that modulates the laser bias voltage. The sinusoidal dither signal may be electronically produced, e.g., by varying the current for a laser, or mechanically, by varying the micro-electromechanical system&#39;s (MEMS) mirror to vary the wavelength. The dither modulation frequency is on the order of a few kilohertz (kHz) but may range to the Megahertz range. Preferably, the dither modulation frequency is much less than the data rate which is typically on the order of 1–10 GHz. Modulation of the laser diode bias current  14  in this manner causes a corresponding dither in the laser center wavelength. Modulated data is then imposed on the laser, and the optical output passes through the bandpass filter  22 . 
   Preferably, the filter  22  is designed to tap off a small amount of light  30 , for example, which is incident upon a photo detector receiver device, e.g., P-I-N diode  32 , and converted into an electrical feedback signal  34 . The amount of light that may be tapped off may range anywhere between one percent (1%) to five percent (5%) of the optical output signal, for example, however, skilled artisans will appreciate that other amounts of laser light above the noise level that retain the integrity of the output signal including the dither modulation characteristic, may be tapped off. The remaining laser light passes on through the filter  22  to the optical network (not shown). 
   As the PIN diode output  34  is a relatively weak electric signal, the resultant feedback signal is amplified by amplifier device  36  to boost the signal strength. The amplified electric feedback signal  40  is input to a multiplier device  42  where it is combined with the original dither modulation signal. The cross product signal  44  that results from the multiplication of the amplified PIN diode output (feedback signal) and the dither signal includes terms at the sum and difference of the dither frequencies. The result is thus input to a low pass filter device  46  where it is low pass filtered and then averaged by integrator circuit  50  to produce an error signal  52  which is positive or negative depending on whether the laser center wavelength is respectively less than or greater than the center point of the bandpass filter. 
   This error signal is then used to adjust the dither modulation of the laser diode, adjusting its center wavelength to more closely align with the center wavelength of the optical filter. In particular, the error signal may be applied to the laser bias voltage device  16 , where it may be added (e.g., by an adder device, not shown) in order to correct the laser bias current  14  in the appropriate direction. In this manner, the bias current (and laser wavelength) will increase or decrease until it exactly matches the center of the filter passband. Alternately, the error signal  52  may be first converted to a digital form, prior to being applied to the bias voltage device  16 . 
   When the two center wavelengths are in alignment, the feedback error-signal is frequency-doubled and there is no change to the laser center wavelength. In this manner, the laser and filter center wavelengths are kept in alignment with each other through a dynamic feedback loop. In one embodiment, the laser/filter combination is aligned when the optical link is initialized (this can be done under the control of software  54 , for example) and the loop keeps the laser and filter in alignment while data is being transferred. In another embodiment, the feedback loop is deactivated after the initial alignment is completed; this assumes the environment is stable enough to maintain the laser and fiber center wavelengths in alignment while the link is operating, and that realignment is not required until the link is re-initialized at some later time. An application with frequency re-initializations such as multi-gigabit datacom link might benefit from the second example. This approach is viable for long wavelength laser diodes in the 1300 nm to 1550 nm range, which are typically highly coherent and offer relatively narrow input spectra to begin with. 
   An alternate embodiment uses wavelength locked loops in a dual pulse compression system  60  as shown in  FIG. 3 . This implementation is only possible for long wavelength (1300 nm or 1550 nm) laser diodes as it requires both normal and anomalous dispersion to occur in the pulse compression system. It has been shown previously that the output of a laser diode can be compressed in time if the signal is passed through alternating regions of positive and negative group velocity dispersion, with an intermediate step of spectral filtering. Such an approach is shown in  FIG. 3 , where the dispersion is achieved through a combination of optical fiber  62  and a pair of diffraction gratings  64 ,  66  with a spatial filter  70  in the grating plane. Incident light passes through the fiber  62 , which induces a linear frequency chirp; the gratings then spread the optical spectrum out in space (the grating output plane is actually the Fourier transform of the incident light). 
   A filter  70  in this plane selectively adjusts the optical spectrum components, and a mirror  72  then reverses the process; the output light is taken from a beam splitter  74 , and is significantly compressed in time. This type of pulse compression has, per se, been described elsewhere; see for example C. DeCusatis and P. Das, “Spread spectrum techniques in optical communication using transform domain processing”, IEEE Journ. Selected Areas in comm. (special issue on microelectronics and photonics in communications), vol. 8, p. 1608–1616 (1990). One of the limitations in optical pulse compression schemes is the transform domain filtering, which can lead to high optical loss. In this embodiment, a wavelength locked loop is incorporated into the pulse compression scheme as shown. In this manner, the spectrum of the input optical pulse which has passed through the fiber and gratings is exactly matched to the filter bandpass by the wavelength locked loop. This overcomes the excessive loss in the pulse compression system, and results in narrower optical output pulses than can otherwise be achieved. 
   The present invention may be embodied in many different types of specific systems. As described above, the invention may be used with Ethernet technology. This invention can also be used with other high data rate protocols such as Synchronous Optical Networks (SONET). 
   While it is apparent that the invention herein disclosed is well calculated to fulfill the objects previously stated, it will be appreciated that numerous modifications and embodiments may be devised by those skilled in the art, and it is intended that the appended claims cover all such modifications and embodiments as fall within the true spirit and scope of the present invention.