Abstract:
A method and device for bandwidth efficient multi-channel optical single sideband modulation with suppressed carrier and a corresponding method and apparatus for demodulation are disclosed. The modulator includes a bandwidth efficient coder/modulator, a Microwave/millimeter-wave upconverter and an OSSB modulator, an optical notch filter and a chirped fiber grating. The demodulator uses at least one narrowband optical bandpass filters in combination with at least one baseband optical receiver to recover the transmitted baseband digital signals. The modulation method and device are suitable for short or long-distance optical fiber transmission systems.

Description:
This application claims priority from U.S. Provisional patent application No. Ser. No. 60/187,383 filed on Mar. 7, 2000 which is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a method and apparatus for modulation of broadband optical signals. More particularly, the present invention relates to an interleaved single sideband modulation technique. 
     2. Description of Related Art and General Background 
     Conventional optical fiber transmission systems, such as optical fiber community access television (“CATV”) transmission systems can carry multiple channels on a single optical fiber communication line. The channels are transmitted modulated on a wideband signal made up of a plurality of frequency division multiplexed carriers. A wideband optical detector or photo-receiver receives the wideband signal. Each individual channel can be recovered by a heterodyne tuner along with an appropriate microwave filter. An optical fiber transmission system using this type of modulation technique can transmit analog or digital signals and is known as a sub-carrier multiplexed (“SCM”) optical transmission system. FIG. 1 shows a schematic diagram of a typical SCM system which is described in detail in W. I. Way, Subcarrier Multiplexed Lightwave Systems for Subscriber Loop Applications, Journal of Lightwave Technology, 1988, pp. 1806-1818. 
     High spectral efficiency digital modems may be used to greatly increase the spectral efficiency of conventional SCM techniques. For example, an optical transmitter with a 1 GHz bandwidth can transmit 100 sub-carrier 6 MHz 64-QAM (quadrature amplitude modulation) channels. Since each channel can carry 30 Mb/s of data, 4.98 Gb/s of data may be transmitted, which gives a spectral efficiency of approximately 5 bits/sec/Hz. In comparison, the same transmitter can transmit only 1.4 Gb/s of on-off keying data for a spectral efficiency of only about 1.4 bits/sec/Hz. 
     There are two important problems to overcome when using a broadband optical transmitter to transport a large quantity of digital data using SCM technology. The first is that the receiver must be a very wideband photoreceiver, which tend to have high spectral noise density and require a complicated and expensive heterodyne receiver. The second is that SCM is an optical double-sideband modulation (ODSB) technique, as shown in FIG.  2 A. This means that half of the bandwidth is wasted, as each of the upper and lower sidebands are carrying the same information. One solution to this problem, as shown in Olshansky (U.S. Pat. No. 5,301,058), is to eliminate the lower side band to produce an optical single-sideband signal (OSSB) as shown in FIG.  2 B. One may then combine many OSSB modulators, using multiple carrier signals, to more efficiently use the available optical fiber transmission spectrum. This is illustrated in FIG.  2 C. This is known as OSSB-DWDM, or optical single side band, dense wavelength division multiplexing. Using double OSSB (D-OSSB), the upper and lower sidebands carry different signals, as shown in FIG.  2 D. Thus, the required number of carriers is only half of that required by the OSSB modulation shown in FIG.  2 C. 
     When amplifying the transmitted signal in a conventional multiplexing method, the carrier signal is likewise amplified. Amplification of the carrier signal represents a waste of amplifier gain, since gain is used to amplify a signal that carries no information. Moreover, as power density in the transmission fiber is increased, signal losses due to nonlinear effects are also increased. Elimination of the carrier signal can significantly decrease the total signal power, thereby reducing the total power density and nonlinear effects. 
     One method for suppressing the carrier signal is disclosed by Jopson (U.S. Pat. No. 5,745,273). Jopson makes use of a dual path modulator arranged in an optical loop. The light is divided by a coupler which provides a portion of the signal to an optical fiber traveling in each direction around the loop. The signal in one direction is modulated to create a carrier and sidebands while the other is solely the carrier. Upon recombining the two optical signals in a combiner, a signal is produced in which the two carrier signals cancel each other and leave only the modulated signal. One drawback of the Jopson arrangement is the requirement of extremely strict tolerances with respect to the lengths of the paths of the loop so that the two signals will arrive at the combiner having the carrier signals exactly out of phase. This requirement makes the Jopson device difficult to implement in practice. 
     Even these solutions are imperfect. Use of an OSSB supressed carrier (OSSB-SC) modulation method, when applied to multi-channel, long-distance optical fiber transmission systems, presents three additional problems. First, conventional narrowband optical filters have a slow roll-off which makes it likely that an optical filter used in the receiver will allow portions of adjacent channels to enter into the filtered window, producing noise in the signal, as illustrated in FIGS. 3A and 3B. Second, as illustrated in FIG. 3B, residual images are produced due to imperfections in the 90° phase shift of the high frequency electrical modulating signal or in the phase shift of the optical signal between the arms of the Mach-Zehnder modulator. Third, dispersion causes self- and external phase modulations which tend to produce distortions in signals transmitted over long distances at 1550 nm, due to beating among the several optical channels. This last problem may be reduced by the use of dispersion reduction techniques such as use of a chirped fiber grating or dispersion compensating fibers. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the needs identified above by providing an interleaved optical single sideband communications system including a modulator, constructed and arranged to accept an incoming optical carrier. The modulator includes a splitter which splits the incoming optical signal into a first optical carrier and a second optical carrier. The modulator also includes a first AC phase modulator to apply a first electrical signal carrying a plurality of first channels to modulate the first optical signal and a second AC phase modulator to apply a second electrical signal carrying a plurality of second channels to modulate the second optical signal, each first channel corresponding to one of the second channels, and each first channel being phase shifted 90° relative to each corresponding second channel. Additionally, the modulator includes a first DC phase modulator to modulate the first optical signal and a second DC phase modulator to modulate the second optical signal. The first and second DC phase modulators are constructed and arranged to modulate an optical carrier component of the first optical signal to be phase shifted 90° relative to a corresponding optical carrier component of the second optical signal. The modulator also includes a combiner which combines the modulated first and second optical signals to form a combined optical signal having an optical carrier component, such that alternate channels of the combined optical signal are substantially cancelled to produce an interleaved, optical single-sideband signal which reduces both adjacent channel interference and residual image problems as discussed above in relation to conventional techniques. An example of a spectrum for an I-OSSB transmission is illustrated in FIG.  3 C. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in an constitute a part of this specification illustrate an embodiment of the invention and together with the description, explains the objects, advantages, and principles of the invention. 
     FIG. 1 is a schematic diagram of a conventional subcarrier multiplexed lightwave system. 
     FIGS. 2A through 2D are spectral diagrams comparing spectral efficiency of various modulation techniques. 
     FIG. 3A is a spectral diagram showing optical double sideband transmission. 
     FIG. 3B is a spectral diagram showing optical single sideband transmission. 
     FIG. 3C is a spectral diagram showing interleaved optical single sideband transmission according to the present invention. 
     FIG. 4A is a schematic diagram of an optical frequency division multiplexed lightwave system according to the present invention. 
     FIG. 4B is a schematic diagram of an optical frequency division multiplexed lightwave system having an array of optical filters according to the present invention. 
     FIG. 4C is a schematic diagram of an optical frequency division multiplexed lightwave system including a broadband optical receiver according to the present invention. 
     FIG. 5A is a schematic diagram of a prior art dual electrode Mach-Zehnder modulator. 
     FIG. 5B is a spectral diagram showing input and output of the modulator shown in FIG.  5 A. 
     FIG. 5C is a schematic diagram of a dual-electrode Mach-Zehnder modulator as employed in the present invention. 
     FIG. 5D is a spectral diagram showing input and output of the modulator shown in FIG.  5 C. 
     FIG. 6A shows four channel dual-electrode Mach-Zehnder modulator as employed in the present invention. 
     FIGS. 6B-6E show spectral diagrams of input and output signals from the modulator shown in FIG.  6 A. 
     FIG. 7 is a schematic diagram showing an interleaved optical single sideband suppressed carrier optical transmitter according to the present invention. 
     FIG. 8 is a schematic diagram of a transmitter according to the present invention. 
     FIG. 9 is a schematic diagram of a multiple light source optical communication system according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description, for purposes of explanation and not limitation, specific details are set forth such as particular optical and electrical circuits, circuit components, techniques, etc. in order to provide a thorough understanding of the present invention. However, the invention may be practiced in other embodiments that depart from these specific details. In some instances, detailed descriptions of well-known devices and circuits may be omitted so as not to obscure the description of the present invention with unnecessary details. 
     Referring now to FIG. 1, a conventional subcarrier multiplexing transmitter and receiver pair are shown. A plurality of modulators  2 ,  4 ,  6 ,  8 , which may be analog, digital or any combination thereof, produce signals corresponding to a plurality of channels. Each channel is frequency division multiplexed by using local oscillators  10 ,  12 ,  14 ,  16  of different radio frequencies, known as subcarriers. The signal for each channel is processed by a band pass filter (not shown) to attenuate components of the signal which are outside of the channel (e.g. harmonics). The several channels are amplified by an amplifier  26  and combined, and the combined signal is amplified once more and used to drive a light emitting device which is conventionally a directly or externally modulated laser diode acting as part of an optical transmitter  36 . Preferably the light emitting device has a fast response time and can produce a narrow linewidth with good coherence. 
     The combined signal is transmitted through an optical fiber  38  to a broadband optical receiver  40 . The optical fiber  38  is preferably single mode fiber to reduce modal dispersion and other modal noise problems. It may alternately be a conventional single mode fiber having zero dispersion at 1310 nm or any other single mode fiber. For wavelength division multiplexing applications, or other broadband applications, the dispersion slope is also preferably small. The signal proceeds to a heterodyne tuner which typically includes a tunable local oscillator  46  which is used to selectively tune to one of the channels which may then be demodulated with an appropriate analog or digital demodulator. Preferably, a band pass filter (not shown) may be included in the receiver to better select the desired channel and exclude noise from neighboring channels. 
     FIG. 4A shows a multiple channel transmission system consistent with an aspect of the present invention. Baseband signals are modulated by a plurality of modulators  52 ,  54 ,  56 . The modulators may be, for example, a simple modulator such as an amplitude shifted keying (ASK) modulator, a frequency shifted keying (FSK) modulator, a differential phase shift keying (DPSK) modulator, a differential quadrature phase shift keying (DQPSK) modulator, or a duobinary modulator. 
     The modulated signals are each passed through an intermediate frequency band pass filter  58 ,  60 ,  62 , then modulated using a plurality of upconverters including local oscillators  64 ,  66 ,  68 . The channels are combined, amplified and passed through an amplifier  70  to an optical transmitter  72  which may be optical transmitter  36  described above with respect to FIG.  1 . Optionally, the transmitter  72  may include an erbium-doped fiber amplifier (EDFA, not shown) to increase the signal strength. The combined optical signal passes through a length of optical fiber  74 , which is preferably single mode optical fiber. It is optionally pre-amplified with an optical amplifier  76 , which is preferably an EDFA. A tunable channel optical filter  78  selects a particular channel which is then received by a baseband optical receiver  80 . The optical filter  78  also helps to reduce spontaneous emission noise produced by the EDFA pre-amplification process. The baseband optical receiver  80  produces an electrical signal which is demodulated by a demodulator  82 . 
     A second embodiment of a transmission system is shown in FIG.  4 B. The system shown in FIG. 4B is similar to the system of FIG.  4 A. However, the single tunable optical filter  78  is replaced by a plurality of optical filters  84 ,  86 ,  88  each of which is preferably a fixed filter, although each may also be tuneable filters adapted to pass only a single selected channel. Each channel signal proceeds to a baseband optical receiver  90 ,  92 ,  94  which in turn, passes the resulting electrical signal to a demodulator  82 . 
     Where tuneable filters are used, they preferably include a feedback circuit to ensure that the filter passband always locks on to the center of the desired channel, despite any wavelength drift of the laser diode. This provides an advantage over conventional DWDM systems in which all optical transmitters require a stringent wavelength locker. 
     Another alternate arrangement of the transmission system is shown in FIG.  4 C. In this embodiment, one of the channels carries a plurality of low bit-rate channels  96 ,  98 ,  100 . The plurality of low bit-rate subcarrier channels  96 ,  98 ,  100  are multiplexed onto a single band having a bandwidth which is preferably of a similar size to each of the high bit-rate channels, though this is not necessary. Other components of the device are similar to those shown in FIGS. 4A and 4B, with the exception of the receivers. For each channel which contains low bit-rate subcarrier channels, the baseband optical receiver  90  is replaced with a broadband optical receiver  102 . The broadband optical receiver  102  provides the sub-channel signals to demodulators (not shown) which are then used to extract each of the individual sub-channels. 
     The plurality of low bit-rate channels shown in FIG. 4C can preferably use spectrally efficient modems ( 96 ,  98 ,  100 ) such as M-ary quadrature amplitude modulated (QAM) modems, quadrature phase shifted keying (QPSK) modems, orthogonal frequency division multiplexing (OFDM) modems or M-ary vestigial sideband (VSB) modems. One skilled in the art will recognize that other spectrally efficient modems may be employed. 
     To better understand the present invention, it is useful to discuss OSSB and D-OSSB transmission. In an OSSB system carrying one channel, the channel is modulated onto the optical carrier signal with a modulator shown in detail in FIGS. 5A and 5B. A dual electrode Mach-Zehnder modulator, indicated generally at  104 , forms the basis of the system. An incoming light signal λ IN  is split into a first optical signal λ 1  and a second optical signal λ 2 . An RF alternating current electrode  106  modulates the two optical signals with the channel signal to be transmitted (i.e. f 1 ), however, f 1  is applied to the carrier such that the signal applied to the upper arm of the modulator is phase-shifted 90° with respect to the signal applied to the lower arm. Subsequently, a DC electrode  108  further modulates the carriers such that the two arms are also shifted 90° with respect to each other. That is, the carriers of the two arms are in quadrature with each other. The two signals are then combined to produce an output signal λ out  in which only the carrier and the lower side band are present. This process may be easily modified so that the lower side band is cancelled and the upper side band is transmitted. 
     Referring now to FIG. 5B, spectra of the signals at various stages are shown. Initially, λ IN  includes only the carrier. After both the AC and DC electrodes  106 ,  108  have applied an electric field to the carrier signal in the upper arm, λ 1  has an upper and a lower side band, the upper side band at 90° and the lower side band at −90°, along with the carrier at 0°. Likewise, after passing through both electric fields, the lower arm signal 2 has a carrier at −90°, an upper side band at −90° and a lower side band at −90°. When the two signals λ 1  and λ 2  are combined to form λ OUT  the two upper side bands cancel each other, leaving only the lower side band and the carrier. 
     FIGS. 5C and 5D illustrate D-OSSB transmission. Just as in OSSB, a dual-electrode Mach-Zehnder modulator  104  is used. An incoming light signal λ IN  is split into a first optical signal λ 1  and a second optical signal λ 2 . An RF alternating current electrode  106  is used to modulate the two optical signals with a first channel m 1 , to be transmitted, however, the signal is applied to the carrier in such a way that the m 1  component of the first and second optical signals are phase-shifted 90° with respect to each other. At the same time, the RF alternating current modulates the two optical signals with a second signal m 2 , with the m 2  component of the first and second optical signals phase-shifted 90° with respect to each other. Moreover, in each arm of the modulator, m 1  is phase-shifted 90° with respect to m 2 . Subsequently, a DC electrode  209  further modulates the carriers such that the two arms are also shifted 90° with respect to each other, that is the carriers of the two arms are in quadrature with each other. The two signals are then combined to produce an output signal λ OUT  in which contains the carrier, m 2  as the upper side band and ml as the lower side band. 
     As shown in FIG. 5D, λ IN  includes only the carrier. After both the AC and DC electrodes have applied an electric field to the carrier signal in the upper arm, λ 1  can be represented by the sum of the two spectra shown. A first spectrum of λ 1  has an upper and a lower side band each carrying m 1 , the upper side band at 90° and the lower side band at −90°, along with the carrier at 0°. A second, carrying m 2 , has an upper side band at 0° and a lower side band also at 0°. Likewise, after passing through both electric fields, the lower arm signal λ 2  can be represented by the sum of two spectra. A first λ 2  spectrum carrying m 1  has a carrier at −90°, an upper side band at −90° and a lower side band at −90°. A second, carrying m 2 , has a carrier at −90°, an upperside band at 0° and a lower side band at 180°. When the two signals λ 1  and λ 2  are combined to form λ OUT  the two upper side bands of m 1  cancel each other, leaving only the lower side band and the carrier. Similarly, the two lower m 2  sidebands cancel each other, leaving only the upper side band and the carrier. Thus, λ OUT  contains the carrier and the two side bands, the lower carrying m 1  and the upper carrying m 2 . The system can be easily modified to reverse the order such that the lower side band will carry m 2  and the upper will carry m 1 . 
     As discussed above, ODSB transmission has the drawback that an optical filter will have a spectrum  109  which tends to overlap multiple channels, introducing noise into the decoded signal, as shown in FIG.  3 A. Further, ODSB requires allocating one-half of the bandwidth to images of the primary information since each side band carries the same information. As shown in FIG. 3B, the OSSB technique shown in FIGS. 5A-B fail to completely solve these problems. Though the lower side band is available for additional channels as in D-OSSB, the problem with the filter overlap remains, and a second problem is introduced. Since it is difficult to produce perfect quadrature in the multiplexer, cancellation of the unwanted side band will often be incomplete, resulting in residual images  110 . These residual images  110  produce additional noise, which when added to the noise resulting from the filter&#39;s slow roll off, can seriously interfere with reception of the transmitted data. 
     As shown in FIG. 3C, by interleaving channels with empty channels, the problem of slow band bass filter roll off can be eliminated and the problem of residual images can be substantially reduced. Since no channel is directly adjacent to another, the filter can properly capture a single channel without also picking up portions of the neighboring ones. Since there are only two residual images  110  on each side band (in this example using four channels), the filter will pick up a smaller amount of noise from the images. Note how in FIG. 3C, only tails of each residual image are within the filter range  109 . In contrast, in FIG. 3B, nearly two entire residual images are within the filter range  109 . 
     A modulator consistent with the present invention for interleaving channels to produce I-OSSB modulation is illustrated in FIGS. 6A-E. An input optical signal λ IN , includes only the carrier as shown in FIG.  6 B. The AC electrode  106  of a Mach-Zehnder multiplexer  104  applies an electric field to the carrier signal in the upper arm, λ 1  containing the channels to be transmitted. After further application of a DC field by the DC electrode  108 , the output can be represented by the spectrum shown in FIG.  6 C. Four separate signals f 1 , f 2 , f 3 , and f 4  are multiplexed onto the carrier, each producing both an upper side band and a lower side band. Adjacent channels are 90° out of phase with each other. 
     Similarly, the lower arm has four separate signals f 1 , f 2 , f 3 , and f 4  multiplexed onto the carrier, as shown in FIG.  6 D. Each of the signals, f 1 , f 2 , f 3 , and f 4 , is applied to the lower arm in quadrature with the corresponding signal f 1 , f 2 , f 3 , and f 4  in the upper arm and each is 90° out of phase with its adjacent channel. Each arm is then placed in quadrature with the other by the DC electrode  108 . 
     When the two signals λ 1  and λ 2  are combined to form λ OUT  the f 1  and f 3  signals are cancelled in the upper side band, leaving only f 2  and f 4 . Likewise, in the lower side band, f 2  and f 4  signals are cancelled leaving only f 1  and f 3 . Thus, λ OUT  contains the carrier and the two side bands, the lower side band carrying f 1  and f 3  and the upper side band carrying f 2  and f 4 . The system can be easily modified to reverse the order such that the lower side band will carry f 2  and f 4  and the upper will carry f 1 , and f 3 . As can be appreciated from the spectrum shown in FIG. 6E, this result corresponds to the spectrum shown in FIG.  3 C and each channel has no directly adjacent channels, that is, every other channel has been cancelled. 
     The I-OSSB modulator of FIGS. 6A-E may be used in a transmission system as illustrated in FIG. 7. A continuous wave light source  112 , such as a laser diode, produces a light signal. The light si passes through a polarization controller  114  and a polarization maintaining optical fiber  115  which maintains a particular polarization of the light. The light signal is processed by an I-OSSB optical modulator  116  as described above, producing, in the example as shown, four multiplexed channels. A notch filter  118  is disposed downstream from the modulator  116 . The notch filter  118  is a bandreject filter which is selected to eliminate the carrier without interfering with the signals of the channels. Optionally, an EDFA amplifier  120  may follow the notch filter  118  to boost the signal strength. Preferably, the system includes a dispersion compensating device  122  which helps to reduce the signal loss due to dispersion and intermodulation (i.e. four wave mixing). This dispersion compensating device  122  may be, for example, a chirped fiber Bragg grating (CFBG), as shown in the FIG., in which the period of the grating varies linearly with position. As a result, the grating reflects different wavelengths at different points along its length which produces a wavelength dependent delay in the signal. In a wide band application, it may be necessary to employ multiple CFBGs in order to produce sufficient delay across a broad frequency range. Alternately, a dispersion compensating fiber may be used, however, dispersion compensating fibers generally have the drawback that attenuation is very high. After passing through the dispersion compensating component  122 , the signal may be amplified again by an amplifier  120 , then it is transmitted through the optical fiber  123 , which is preferably single mode fiber. 
     FIG. 8 shows additional detail of the electrical portion of a transmitter according to the present invention. A plurality of modulators (for purposes of illustration, four)  124 ,  126 ,  128 ,  130  produce a signal for each of a plurality of channels. Each channel signal is preferably filtered with a low pass filter  132 ,  134 ,  136 ,  138  prior to upconversion by a local oscillator  140 ,  142 ,  144 ,  146 . Next, the signals are preferably filtered again with a band pass filter  148 ,  150 ,  152 ,  154  prior to (optional) amplification by an amplifier  156 . A hybrid coupler  164  is used to split each channel into two signals at 90° to each other. Two of the 90° signals are passed to a first summer  166  and two to a second summer  168 . Likewise, two of the 0° are passed to each summer  166 ,  168 . By way of example, the 90° of channels  1  and  3  are passed along with the 0° of channels  2  and  4  to the first summer  166 , while the 90° of channels  2  and  4  are passed along with the 0° of channels  1  and  3  to the second summer  168 . The summed signals may then be used to modulate a light signal from light emitting device  168  at the carrier frequency in a dual-arm Mach-Zehnder modulator  170  as shown in FIGS. 6A-E. The summers may also be replaced by wideband microwave/millimeter wave directional couplers to increase the number of combined channels. 
     It may be useful in practice to provide a system according to the present invention which combines the I-OSSB modulator with dense wavelength division multiplexing to provide extremely high bandwidth transmission, as shown in FIG. 9. A plurality of light emitting devices  172 ,  174 ,  176 ,  178  supply carrier signals for a plurality of I-OSSB modulators  180 ,  182 ,  184 ,  186 , each transmitting multiple channels. The multiplexed signals are preferably passed through a dispersion compensating device  188  before or preferably after entering a multiplexer  190 , which may be of conventional design. The multiplexer  190  can also be replaced by a wideband optical coupler whenever applicable. The multiplexed signal is transmitted over a single mode fiber  192  and treated, as appropriate, with an amplifier  194  such as an EDFA. A demultiplexer  196 , which may be of conventional design, separates the carrier signals, which are then filtered by an optical filter  198  and received with a receiver  200  according to the present invention, such as is shown in FIG. 4B or  4 C. In place of a conventional demultiplexer  196 , the demultiplexer  196  may be custom designed to accommodate various wavelength windows. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment, but on the contrary it is intended to cover various modifications and equivalent arrangement included within the spirit and scope of the claims which follow.