Abstract:
An optical transmitter is disclosed including an optical signal source generating a frequency modulated signal encoding data. An optical spectrum reshaper is positioned to receive the frequency modulated signal and converts the frequency modulated signal into a reshaped signal having increased amplitude modulation relative to the frequency modulated signal. A third-order dispersive element is positioned to receive the reshaped signal and is adapted to impose third-order dispersion on the reshaped signal to generate a compensated signal having third-order dispersion effective to compensate for second-order dispersion caused by an optical fiber positioned between the optical transmitter and a receiver.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application Ser. No. 60/921,402 filed Apr. 2, 2007. 

   BACKGROUND OF THE INVENTION 
   1. The Field of the Invention 
   This application relates to optical transmitters and, more particularly to systems and methods for overcoming chromatic dispersion. 
   2. The Relevant Technology 
   The ability to transmit data over optical fibers over long distances is limited by various factors. For example, at intermediate stages between transmitter and receiver a signal may be amplified, which introduces noise into the signal. The signal may also be converted to an electrical signal and then retransmitted, which is subject to detection errors. 
   The primary limitation on long-haul transmission of optical signals is dispersion within the optical fiber itself Inasmuch as the optical fiber has a wavelength dependent index of refraction, different frequency components of a signal travel at different speeds. Transmitted pulses will therefore tend to broaden, causing the peak amplitude of 1 bits to be reduced and the amplitude of adjacent 0 bits to increase thereby making the transmitted symbols indistinguishable from one another. 
   U.S. patent application Ser. No. 11/084,633, filed Mar. 18, 2005, and entitled “Method and apparatus for transmitting a signal using simultaneous FM and AM modulation,” described a laser transmitter including a directly modulated laser that emits frequency modulated pulses through an optical spectrum reshaper that converts a portion of the frequency modulation to amplitude modulation. In this application, the frequency modulated signal includes frequency excursions from a base frequency to a peak frequency. The application discloses that for frequency excursions equal to between 0.25 and 0.75 times a bit rate, 1 bits separated by an odd number of 0 bits will destructively interfere as they broaden and begin to overlap, which makes an intervening 0 bit more readily distinguishable. Although this method is particularly useful for promoting proper detection of the 101 bit sequence, it does not provide any benefit for isolated 1 bits among multiple 0 bits. 
   In view of the foregoing, it would be an advancement in the art to provide a laser transmitter having a directly modulated laser and optical spectrum reshaper that reduces errors caused by chromatic dispersion for a plurality of bit patterns, particularly isolated 1 bits. 
   BRIEF SUMMARY OF THE INVENTION 
   In one aspect of the invention, an optical transmitter includes a digital signal source configured to output a data signal. An optical signal source, such as a DFB laser receives a data signal from the digital signal source and generates a frequency modulated signal encoding the data signal. An optical spectrum reshaper is positioned to receive the frequency modulated signal and converts the frequency modulated signal into a reshaped signal having increased amplitude modulation relative to the frequency modulated signal. A third-order dispersive element is positioned to receive the reshaped signal and is adapted to impose third-order dispersion on the reshaped signal to generate a compensated signal. An optical fiber has a first end positioned to receive the compensated signal and a second end coupled to a receiver. The third-order dispersive element imposes third-order dispersion on the reshaped signal effective to compensate for second-order dispersion caused by the optical fiber. 
   In another aspect of the invention, the third-order dispersive element is a filter having a Gaussian profile and wherein the reshaped signal has a frequency profile positioned relative to the transmission function of the Gaussian profile such that the reshaped signal experiences third-order dispersion. 
   These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     To further clarify the above and other advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
       FIG. 1A  illustrates an optical transmitter having dispersion compensating components formed of Gire-Tournois filters in accordance with an embodiment of the present invention; 
       FIG. 1B  illustrates an optical transmitter having dispersion compensating components formed of ring resonator filters in accordance with an embodiment of the present invention; 
       FIG. 2A  illustrates the effect of third-order dispersion on a Gaussian pulse; 
       FIG. 2B  illustrates the effect of second-order dispersion on a frequency reshaped pulse; 
       FIG. 3A  illustrates an eye diagram for a frequency reshaped pulse after transmission through 400 km of fiber; 
       FIG. 3B  illustrates an eye diagram for a frequency reshaped pulse having third-order dispersion compensation after transmission through 400 km of fiber. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIGS. 1A and 1B , an optical transmitter  10  includes a laser  12  coupled to a digital signal source  14 . The laser  12  may include a distributed feed back (DFB), distributed bragg reflector (DBR), or other type of laser. The laser  12  is preferably directly modulated such that the output of the laser encodes a data signal from the digital signal source  14 . In a preferred embodiment, the output of the laser  12  includes adiabatically chirped pulses having both frequency and amplitude modulation. The laser  12  is preferably biased above its lasing threshold such that transient chirp caused by modulation of the laser is reduced. 
   The output of the laser  12  is transmitted through an optical spectrum reshaper (OSR)  16 . The OSR  16  converts at least a portion of frequency modulation in the output of the laser  12  to amplitude modulation. The output of the OSR  16  may also remain frequency modulated. The OSR  16  may be embodied as one or more filters, including, but not limited to, a coupled multi-cavity (CMC) filter, a periodic multi-cavity etalon, a fiber Bragg grating, a ring resonator filter or any other optical element having a wavelength-dependent loss. The OSR  16  may also include a fiber, a Gire-Tournois filter, or some other element with chromatic dispersion. 
   The OSR  16  preferably has a frequency dependent transmission profile such that the frequency modulation bandwidth of the laser  12  lies on a sloped portion or “transmission edge” of the transmission profile. The laser  12  may be modulated to generate frequency excursions from a base frequency to a peak frequency in order to generate frequency modulated pulses. One or both of the base and peak frequency preferably lie on the transmission edge. 
   The transmission function of the OSR  16  and the base and peak frequency of the laser  12  may be chosen such that the duty cycles of the amplitude modulation and frequency modulation are not equal. In particular, the duty cycle of the frequency modulation at the output of the OSR  16  may be shorter than that of the amplitude modulation. For example, the duty cycle of the frequency modulation may be at least fifteen percent, preferably at least 25 percent, shorter than the duty cycle of the amplitude modulation. In this manner, the leading and trailing portions of a pulse will have a lower frequency, which tends to keep the optical energy at the center of the pulse. Distortions will therefore tend to propagate away from the center of the pulse and isolated 1 bits will be narrower after propagation. 
   The laser  12  and OSR  16  may include any of the lasers, OSRs, and modulation methods described in the following applications, which are hereby incorporated herein by reference: 
   (i) U.S. patent application Ser. No. 11/272,100, filed Nov. 8, 2005 by Daniel Mahgerefteh et al. for POWER SOURCE FOR A DISPERSION COMPENSATION FIBER OPTIC SYSTEM; 
   (ii) U.S. patent application Ser. No. 10/308,522, filed Dec. 3, 2002 by Daniel Mahgerefteh et al. for HIGH-SPEED TRANSMISSION SYSTEM COMPRISING A COUPLED MULTI-CAVITY OPTICAL DISCRIMINATOR; 
   (iii) U.S. patent application Ser. No. 11/441,944, filed May 26, 2006 by Daniel Mahgerefteh et al. for FLAT DISPERSION FREQUENCY DISCRIMINATOR (FDFD); 
   (iv) U.S. patent application Ser. No. 11/037,718, filed Jan. 18, 2005 by Yasuhiro Matsui et al. for CHIRP MANAGED DIRECTLY MODULATED LASER WITH BANDWIDTH LIMITING OPTICAL SPECTRUM RESHAPER; 
   (v) U.S. patent application Ser. No. 11/068,032, filed Feb. 28, 2005 by Daniel Mahgerefteh et al. for OPTICAL SYSTEM COMPRISING AN FM SOURCE AND A SPECTRAL RESHAPING ELEMENT; 
   (vi) U.S. patent application Ser. No. 11/084,633 filed Mar. 18, 2005 by Daniel Mahgerefteh et al. for METHOD AND APPARATUS FOR TRANSMITTING A SIGNAL USING SIMULTANEOUS FM AND AM MODULATION; and 
   (vii) U.S. patent application Ser. No. 11/084,630, filed Mar. 18, 2005 by Daniel Mahgerefteh et al. for FLAT-TOPPED CHIRP INDUCED BY OPTICAL FILTER EDGE. 
   An imaging lens  18  and isolator  20  may be positioned between the laser  12  and the OSR  16  to focus the laser output on the OSR and to prevent back reflection into the cavity of the laser  12 , respectively. 
   A wavesplitter  22  positioned between the laser  12  and the OSR  16  directs some of the output of the laser  12  to a photodiode  24 . A second wavesplitter  26  positioned on the opposite side of the OSR  16  from the wavesplitter  22  directs a fraction of the output of the OSR  16  to a second photodiode  28 . The outputs of the photodiodes  24 ,  28  are input to a controller that controls the temperature of a thermoelectric cooler (TEC)  30  to which the laser  12  is mounted. The temperature of the TEC is controlled to maintain the frequency of the laser in alignment with the transmission edge of the OSR  16  by ensuring that a ratio of the outputs of the photodiodes  24 ,  28  remains at a predetermined value. 
   Referring to  FIGS. 2A and 2B  while still referring to  FIGS. 1A and 1B , the output of the OSR  16  is particularly tolerant to second-order dispersion. Isolated 1 bits in the frequency reshaped output of the OSR  16  have a center frequency higher than the wings, the pulse therefore propagates through a dispersive fiber  36  without experiencing significant second-order dispersion upon detection by a receiver  38 . 
   As is apparent in  FIG. 2A , a pulse having a typical Gaussian profile will develop a spurious peak  32  at its leading edge following propagation through a fiber having only third-order dispersion. However, as is apparent in  FIG. 2B , a pulse output from the OSR  16  of the transmitter  10  will develop a similar spurious peak  34  after experiencing only second-order dispersion. In the illustrated example, the dispersion in the example  FIG. 1B  has a sign opposite that in the example of  FIG. 1A  such that the spurious peak  34  occurs on the trailing edge. 
   Accordingly, the output of the OSR  16  is passed through a third-order dispersive element  40  that imposes third-order dispersion on the output of the OSR  16  effective to reverse the spurious peaks caused by the second-order dispersion of the fiber. In some embodiments the third-order dispersive element  40  imposes third-order dispersion on the output of the OSR  16  effective to reverse third-order dispersion within the OSR  16 . In still other embodiment, the third-order dispersion of the element  40  imposes third-order dispersion sufficient to compensate for third-order dispersion of the OSR  16  and for spurious peaks caused by second-order dispersion within the fiber. 
   The required third-order dispersion may be accomplished by means of a filter having a Gaussian profile. In a preferred embodiment, the frequency band (e.g. a band containing 98% of the optical energy) of the output of the OSR  16  is preferably located on the Gaussian transmission profile of the filter such that output signals will experience third-order dispersion. For a Gaussian transmission profile, third-order dispersion occurs near the peak transmission frequency. 
   Referring to  FIG. 3A , without the use of the third-order dispersive element  40 , the signal detected at a receiver  38  following propagation through 400 km of fiber manifests a large amount of noise and an indiscernible data eye. In contrast, as is apparent in  FIG. 3B , where a third-order dispersive element  40  is used the data eye remains open after 400 km. 
   Referring again to  FIGS. 1A and 1B , in some embodiments, an all-pass filter  42  receives the output of the third-order dispersive element  40 . The all-pass filter  42  imposes second-order dispersion on optical signals transmitted therethrough. The all-pass filter  42  preferably imposes second-order dispersion having a sign opposite that of the optical fiber  36 . The all-pass filter is preferably designed to have a substantially frequency independent transmission function across the bandwidth of the output of the OSR  16 . 
   Referring specifically to  FIG. 1A , the OSR  16 , third-order dispersive element  40 , and all-pass filter  42  may be embodied as solid multi-cavity etalons, such as Gire-Tournois filters  44 ,  46 , and  48 , respectively. The spectral response of the filters  44 ,  46 ,  48  may be tuned by adjusting the angle of the filters  44 ,  46 ,  48  and then bonding them in place. Further tuning may be accomplished by adjusting the temperature of the filters  44 ,  46 ,  48 , either through localized heating or a thermoelectric cooler underlying all of the filters  44 ,  46 ,  48 . In the embodiment of  FIG. 1A , an optical isolator  50  may be positioned between the OSR  16  and third-order dispersive element  40  to suppress back reflection. The all-pass filter  42  includes two Gire-Tournois filters  48  in the illustrated embodiment. In the embodiment of  FIG. 1A  the optical transmitter  10  is coupled to the fiber  36  by means of coupling optics such as a fiber pigtail  52 . A lens  54  may be used to focus the output of the transmitter on the fiber pigtail  52 . 
   Referring specifically to  FIG. 1B , the OSR  16 , third-order dispersive element  40 , and all-pass filter  42  may also be embodied as ring resonator filters  56 ,  58 , and  60 , respectively, integrated in a planar lightwave circuit (PLC)  62 . In such embodiments, signals from the laser  12  may be transmitted into an input waveguide  64 . A tap splitter  66  diverts some of the light from the input waveguide  64  to photodiode  24 . The ring resonator filter  56  of the OSR  16  couples a portion of the light from the input waveguide  64  to an output waveguide  72 . In the illustrated embodiment, the ring resonator filter  56  functioning as the OSR  16  includes multiple ring resonators such as are described in U.S. patent application Ser. No. 11/702,436 filed Feb. 5, 2007. A tap splitter  74  directs a portion of light transmitted through the output waveguide  72  to the photodiode  28 . 
   The ring resonator filter  58  functioning as the third-order dispersive element  40  is positioned adjacent the output waveguide  72 . The ring resonator filter  60  serving as the all-pass filter is likewise positioned adjacent the output waveguide  72 . In the illustrated embodiment, the ring resonator filter  60  includes two sets of three resonator rings, each set having one resonator ring adjacent the output waveguide  72 . 
   Coupling optics, such as a fiber pigtail  52  couple the optical fiber  36  to the PLC  62  such that light from the output waveguide  72  is transmitted into the fiber  36 . 
   The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.