Abstract:
An optical signal is transmitted, optically amplified, optically filtered, and received. The optical filtering rejects optical noise such as Amplified Spontaneous Emission noise and is configured to pass a single side band optical signal, and multimode filtering may be applied. A change in the optical signal may be detected, and the characteristics of the optical filtering may be altered based on the detected change.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/354,721, entitled “SSB SOLITON AND DISPERSION MANAGED SOLITON TRANSMISSION” filed on Feb. 5, 2002, which is incorporated herein by reference in its entirety. This application claims the benefit of U.S. Provisional Application No. 60/356,072, entitled “A FIBER OPTIC AUTO WAVELENGTH TRACKING FILTER FOR OPTICAL REGENERATORS AND RECEIVERS” filed on Feb. 11, 2002, which is incorporated herein by reference in its entirety.  
         [0002]    This application is a continuation-in-part of U.S. patent application Ser. No. 10/052868, filed Jan. 16, 2002; U.S. patent application Ser. No. 10/053478, filed Jan. 16, 2002; U.S. patent application Ser. No. 10/050635, filed Jan. 16, 2002; U.S. patent application Ser. No. 10/050751, filed Jan. 16, 2002; U.S. patent application Ser. No. 10/050641, filed Jan. 16, 2002; and U.S. patent application Ser. No. 10/050749, filed Jan. 16, 2002, all of which are incorporated herein by reference in their entirety. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0003]    1. Field of the Invention  
           [0004]    This invention relates to filtering noise in optical signal transmission.  
           [0005]    2. Discussion of Related Art  
           [0006]    Transmission of data over long distances, especially transoceanic telephone transmission, is increasingly effected optically using optical fibers. This has significant advantages compared to electrical transmission; in particular losses are low and there is less signal distortion. In such telecommunication systems, in particular for undersea telecommunications, there is a need for longer range optical signal transmission without degrading signal quality while remaining within reasonable cost limits.  
           [0007]    Optical data signals traveling in optical fibers must be amplified at points along the way in order to compensate for the intrinsic attenuation losses of fiber. Such amplification in today&#39;s systems is accomplished by using Raman- or Erbium-Doped Fiber Amplifiers (EDFA).  
           [0008]    The process of signal amplification is imperfect. Amplifiers such as EDFAs boost the signals but also add noise known as Amplified Spontaneous Emission (ASE). The spectrum of this noise is broadband, typically 30 nm in wavelength domain, and constitutes a background white noise for the signal spectrum. As a typical fiber optic link usually needs an optical amplifier every 50 or 100 km, a 1000 km link will contain approximately 10 to 20 EDFAs. The accumulation of noise generated by EDFAs can be a problem, as most optical data transmission links are ultimately limited by ASE noise.  
           [0009]    One common way to reduce the ASE noise in optical links is to insert a narrow pass-band optical filter at (e.g., after) each of some or all of the amplifiers in the line. The filter is narrow enough to allow signal spectrum to pass while rejecting most of the ASE noise. FIG. 1 illustrates a typical case in which a Double Side Band (DSB) 10 Gb/s return-to-zero (RZ) data channel occupies approximately 25 GHz (0.2 nm) as signal spectrum, and a 100 GHz (0.8 nm FWHM (Full-Width Half-Maximum)) pass band filter is applied to reduce ASE noise. The data channel&#39;s spectrum usage is depicted as rectangle  110  and the filter&#39;s spectral effect is depicted as a dome shape (“dome”)  112  centered or nearly centered around the optical carrier (OC) frequency of the signal spectrum (the dome not necessarily having any particular characteristics other than a high point with sides extending outward and downward from the high point).  
         SUMMARY OF THE INVENTION  
         [0010]    The invention provides a system and methods for filtering noise in optical signal transmission. In certain embodiments, the invention provides a system and method for fiber optic auto wavelength tracking and filtering for optical regenerators and receivers.  
           [0011]    According to one or more aspects of the invention, an optical signal is transmitted, optically amplified, optically filtered, and received. The optical filtering rejects optical noise such as Amplified Spontaneous Emission noise and is configured to pass a single side band optical signal, and multimode filtering may be applied. A change in the optical signal may be detected, and the characteristics of the optical filtering may be altered based on the detected change. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    FIGS.  1 - 3  are spectral diagrams illustrating pass bands; and  
         [0013]    FIGS.  4 - 8  are block diagrams of optical signal filtering systems according to certain embodiments of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0014]    The present invention provides improved systems and methods for transmitting optical signals. Among other things, preferred embodiments of the invention include optical regenerators or receivers having noise filtering systems that, in certain embodiments, detect a change in the optical signals (the change possibly being gradual, as in the case of OC wander) and adapt in response to the change.  
         [0015]    As optical fiber spans are increasing in distance and optical carriers intend to send signals in excess of 4000 km, it is useful to provide narrower pass band filters for the reduction of ASE. As shown in FIG. 2, at 10 Gb/s, the pass band of the filter can be narrowed to 0.1 nm FWHM as depicted by dome  212  that is narrower than dome  112  of FIG. 1 (as noted above, each dome does not necessarily have any particular characteristics other than a high point with sides extending outward and downward from the high point). In fact, the pass band of the filter may be narrow enough to permit only one data side band of the data channel to pass or to allow Single Side Band (SSB) reception at the receiver to improve the signal to noise ratio of the system. As shown in FIG. 1, pass band dome  112  allows passage of all spectral components of the data channel, including OC “0”, left side band (LSB) “1”, and right side band (RSB) “2”, and respective data side band pairs “01”-“02”, “11”-“12”, and “21”-“22”. As shown in FIG. 2, the center of pass band dome  212  is offset to the left of OC “0” and dome  212  is narrower than dome  112  such that pass band dome  212  eliminates RSB “2” and its data side band pair “21”-“22”, and allows passage of an SSB signal having OC “0” and LSB “1” and respective data side band pairs “01”-“02” and “11”-“12”.  
         [0016]    The use of the narrower pass band reduces the amount by which the ASE overlaps the signal pass band. Furthermore, tight pass band filtering is beneficially applicable to many or all spectrally efficient schemes such as Carrier Suppressed RZ (CSRZ) or SSB. Removal of any redundant spectral components, through filtering, in data signals lowers ASE and simultaneously increases the modulation efficiency. In addition, narrow pass band filtering reduces the required pump power in EDFA and Raman amplification processes. This is shown in FIG. 2, in which a narrower pass band filter is used to remove redundant spectral components in the data signals and to further reduce ASE overlap.  
         [0017]    Optical transmitter center frequencies (or wavelengths) are not completely stable and often vary in the range of ±25 GHz (±0.2 nm), which can complicate or hinder the use of fixed range narrow band filters. A variance in the center frequency may cause the narrow band filter to be excessively or inadequately offset with respect to the transmitted signal and to thereby improperly cut off desirable signal such as leftmost band “11” or rightmost band “02” of FIG. 2.  
         [0018]    It is desirable to provide a stable, narrow-band filter that helps to avoid ASE noise accumulation.  
         [0019]    [0019]FIG. 8 illustrates an embodiment of the present invention in which an example optical transmission system  806  includes an optical transmitter  810  that transmits optical signals via regenerators including regenerators  812 ,  814  to optical receiver  816 . One or more of the regenerators and/or receiver  810  may include optical filter/optical amplifier combinations such as options A, B, C as illustrated and now described. In the case of option A, optical filter  818  receives and acts upon the optical signals before the optical signals reach optical amplifier  820 . In the case of option B, optical amplifier  822  receives and acts upon the optical signals before the optical signals reach optical filter  824 . In the case of option C, optical filter  826  receives and acts upon the optical signals before the optical signals reach optical amplifier  828 , which receives and acts upon the optical signals before the optical signals reach optical amplifier  830 . One or more of filters  818 ,  824 ,  828 ,  826 ,  830  may have characteristics, including ASE noise rejection characteristics, as described below.  
         [0020]    In a case in which a receiver such as receiver  810  includes one or more of the filter/amplifier combinations described above, the one or more combinations may be positioned to act upon the optical signal before the optical signal reaches a demodulator or a photodetector in the receiver.  
         [0021]    In order to maximize ASE noise rejection for a variety of data formats (e.g. SSB, CSRZ or DSB) of long haul optical fiber links, an optical pass band filter (which may serve as one or more of the filters of FIG. 8) is provided to address wavelength or frequency shift or wander of the transmitters in optical link. Moreover, in at least some embodiments, the filtering action can further be improved to remove ASE components within the signal pass band itself and thus further reject the ASE within the pass band.  
         [0022]    A filter such as a multimode or band-reject filter may be used that is highly tailored to passing a valid signal while rejecting noise. FIG. 3 depicts an example effect of an example reshaped optical pass band filter that can be used for removing more ASE noise power from the signal pass band. In particular, as shown in FIG. 3, since the filter&#39;s pass band shape has a “rabbit ears” shape  310 , the filter passes much less ASE between spectral components “12” and “01” than is passed by a rectangular pass band shape. The rabbit ears shape  310  includes dual domes  312 ,  314 , overlapping or joined at the bottom, with a notch  316  therebetween. The dual domes may be the result of the filter having multiple modes, and each of the dual domes may be sufficiently high and sufficiently spectrally wide to capture (or substantially capture) the OC and one side band. The notch may have a spectral width approximately equal to the frequency difference between the OC and the side band.  
         [0023]    As shown in FIG. 8, a filter (e.g., a having the pass band shape depicted in FIG. 3) may be placed inside an optical regenerator or in front of a receiver on a per lambda (i.e., per channel) basis. (In an optical communication system such as a WDM system, each fiber can have many different channels, each channel being at a different optical center frequency or wavelength, known as “lambda”.)  
         [0024]    [0024]FIG. 4 shows a block diagram of an optical transmission system  410  according to an embodiment of the present invention. System  410  may have some or all of the characteristics of optical signal handling techniques disclosed in one or more of the following patent applications, each of which is hereby incorporated herein by reference in its entirety: U.S. patent application Ser. No. 10/052868, filed Jan. 16, 2002; U.S. patent application Ser. No. 10/053478, filed Jan. 16, 2002; U.S. patent application Ser. No. 10/050635, filed Jan. 16, 2002; U.S. patent application Ser. No. 10/050751, filed Jan. 16, 2002; U.S. patent application Ser. No. 10/050641, filed Jan. 16, 2002; U.S. patent application Ser. No. 10/050749, filed Jan. 16, 2002; and U.S. patent application entitled “FORMING OPTICAL SIGNALS HAVING SOLITON PULSES WITH CERTAIN SPECTRAL BAND CHARACTERISTICS, which is being filed simultaneously herewith.  
         [0025]    System  410  includes a tunable infinite impulse response (IIR) filter  412 , such as a rotatable etalon, that can shift its center frequency, e.g., by rotation, upon command signals  413  from a Decision Circuit (DS)  414  that may include a microprocessor. Portions of the optical signals are tapped off (e.g., by couplers) from both optical input  416  and output  418  parts of filter  412  and are detected electronically by the DS through the use of Optical to Electrical converters (O/Es)  420 ,  422  (e.g., photodiodes). The pass band of the filter can be set to pass only an optical carrier and two data sidebands (a DSB signal), or carrier and one data sideband (an SSB signal), or no carrier and two data sidebands (a CSRZ signal). Components including filter  412 , the DS, the couplers, and converters  420 ,  422  may have some or all of the characteristics described in one or more of the patent applications incorporated by reference above.  
         [0026]    Preferably, the spectral pass band of transmission system  410  has a flat top rectangular shape, or a rabbit-ear shape as shown in FIG. 3. Multi-cavity etalon structures can be used to create such spectral pass band shapes. Pertinent principles are described in H. van de Stadt and J. M. Muller, “Multimirror Fabry-Perot interferometers,” J. Opt. Soc. Am. A, 2, pp. 1363 et seq., 1985.  
         [0027]    Referring to FIG. 4, when the center frequency of the incoming optical signal shifts (e.g., due to drift or purposeful alteration), the DS detects the shift by detecting a change in optical power received from tapping output  418 . DS causes filter  412  to tune its center frequency to the new center frequency. The DS also monitors the signal received from tapping input  416  to determine whether the change in output tap power was due to a change in input optical power to the device. If so, it is determined that the change in output optical power was not due to a shift in the center frequency of the incoming optical signal, and no resulting action is taken to tune the filter.  
         [0028]    [0028]FIG. 5 shows an embodiment  510  of system  410 . Embodiment  510  uses a bulk optics approach (i.e., a free space beam propagation technique) that may have some or all of the characteristics described in one or more of the patent applications incorporated by reference above. A high finesse Fabry perot etalon  512  is disposed between a first collimator  514  and a second collimator  516 .  
         [0029]    Etalon  512  is responsible for establishing the pass band, which allows only certain frequencies of light to pass, centered about a center frequency of the positioned etalon. The pass band width is substantially fixed due to results of the filter design such as the thickness of the etalon and the optical properties of the material used. However, the center frequency of the filter&#39;s pass band can shift and be adjusted, by rotating the etalon. By rotating the etalon, the effective thickness of the etalon through which light passes changes causing the center frequency of the pass band to shift and allowing different frequencies to pass through the etalon. In certain preferred embodiments, a multi-mirror etalon is used. Such an etalon may be used to create a more rectangular pass band shape.  
         [0030]    Collimator  516  receives the passed optical signals from the etalon and provides them to optical tap  518 . Optical tap  518  (e.g., a beam splitter) receives the “tuned” signal and provides an output signal, which may be transmitted onto an optical fiber  520 , and an identical feedback optical signal which is received by an optical-to-electrical converter  522 , such as a photo diode detector. Converter  522  then provides an electrical version of the signal to a decision circuit  524 . Circuit  524 , among other things, is responsible for tuning the filter by causing the etalon  512  to rotate. Circuit  524  may detect the energy or power of the feedback signal. In certain embodiments, the amount of energy or power is at maximum when the filter is tuned to capture as much of the optical signal (e.g., SSB signal) as will fit within the pass band of the filter.  
         [0031]    [0031]FIG. 6 illustrates another embodiment  610  of system  410 . Embodiment  610  uses an electronically tunable liquid crystal Fabry-Perot filter  612  that may have some or all of the characteristics described in one or more of the patent applications incorporated by reference above. Optical signals of arbitrary polarization on link  609  are received by optical tap  613  which provides input signals to first collimator  614  and a tap signal on optical link  615  to O/E  617  of decision circuit  619 . Collimator  614  transmits the input signals to first polarization beam splitter (PBS)  616  which divides the light into two paths  618 ,  620 . Light on path  618  passes through first half wave plate  622  so that light on paths  620  and  624  have states of polarization that are aligned to the optical axis of liquid crystal cell  612 . Since the liquid crystal Fabry-Perot filter  612  is a polarization sensitive element, aligning the light allows it to be tuned by the filter. The filter light is emitted as paths  626 ,  628  which are recombined into the output fiber using a second half wave plate  630 , second PBS  632  and second collimator  634 . Optical tap  636  receives the optical signal from collimator  634  and provides the output signal on link  638  and provides a feedback signal on optical link  640  to O/E  642  of decision circuit  619 . The O/Es  617 ,  640 , the decision circuit  619 , and other components may have some or all of the characteristics described in one or more of the patent applications incorporated by reference above. Electrical stimulus on control line  650  causes the filter  612  to change its filtration properties and thus allows the filter to track the wandering center frequency of the signals on link  609 . For example, the index of refraction of the filter  612  changes in response to electrical stimulus.  
         [0032]    [0032]FIG. 7 shows an embodiment  710  of system  410 . Embodiment  710  includes a tuning element  712  that includes a grating filter  714 . Grating filter  714  may have some or all of the characteristics described in one or more of the patent applications incorporated by reference above. Optical signals are received from link  716  by tap  713  which provides input signals to grating filter  714  and a tap signal on optical link  715  to O/E  717  of decision circuit  719 . Grating filter  714  operates to provide filtration on the input signals so that output signals having frequencies of interest pass through the grating on link  718 . The output signals are received by tap  720  which provides output signals on link  722  and provides feedback signals on link  724  to O/E  726 . The tap and feedback signals are received by O/Es  717 ,  726  which provide respective electrical versions thereof to decision circuit  719 . The decision circuit  719  may use control signal  730  to tune the grating filter  714  and/or to cause the center frequency of the pass band of grating filter  714  to shift.  
         [0033]    Filter Tuning  
         [0034]    Regarding detection of frequency shifts, if the center frequency of a channel changes, the channel&#39;s signal may drift partially or entirely out of the pass band of the filter. Where such drifting out occurs, the output signal of the filter becomes attenuated, which attenuation is manifested in the feedback signal and is detected by the O/E and decision block. In such a case, the decision block acts to tune the filter in response to the frequency shift so that the pass band of the filter more closely matches the new center frequency of the channel. The decision block may also monitor the input signal of the filter to help determine whether the attenuation, if any, in the output signal corresponds to attenuation in the input signal. If it is determined that the attenuation detected in the output signal output corresponds to attenuation in the input signal (rather than a drifting out of the pass band), the filter may not be tuned.  
         [0035]    Variations  
         [0036]    In connection with the above, the transmission technology may be modified in many ways. For example, one or more finite impulse response filters (FIRs), e.g., as described in one or more of the patent applications incorporated by reference above, may be used in addition to or in place of filters described above, e.g., to help prevent or reduce intersymbol interference (ISI). For example, a non-tracking and/or non-tunable filter may be used, e.g., where the optical signal is highly stable. For example, arrangements described above were illustrated with single filtering devices (e.g., filters) for the most part to avoid clutter. For example, the filters may be implemented as a cascaded arrangement of filters as well. Moreover, though not shown in the FIGS. to avoid clutter, gaining elements may be incorporated into the implementations, e.g., to compensate for any insertion loss from various components of the implementations. For example, the insertion loss of a device may be compensated by Erbium doped optical fiber amplifiers or the like, which may be placed before, after or within a filter block.  
         [0037]    The transmission technology may use, in whole or in part, one or more of the filtration techniques described in one or more of the patent applications incorporated by reference above, e.g., for noise reduction or for another purpose.  
         [0038]    It will be further appreciated that the scope of the present invention is not limited to the above-described embodiments, but rather is defined by the appended claims, and that these claims will encompass modifications of and improvements to what has been described.