Abstract:
An apparatus includes multiple signal paths for optically converting an optical signal to multiples of the optical signal at different respective carrier frequencies for reducing interference between wireless transmissions of the multiples of the optical signal. Preferably, the converting includes a first modulator for modulating the optical signal into a first optical carrier and an initial first-order sideband signal with a frequency spacing twice that of the first optical carrier and a first interleaver for separating the first optical carrier and the initial first-order sideband signal. The converting also includes a second phase modulator for modulating the first optical carrier into a second optical carrier and a second first-order sideband signal with a frequency spacing twice that of the second optical carrier.

Description:
[0001]     This application claims the benefit of U.S. Provisional Application No. 60/804,666, entitled “Reduction of Physical layer Interference in a DWDM Radio Over Fiber Network by using Multiple Time Remodulation”, filed on Jun. 14, 2006, the contents of which is incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates generally to optical communications, and, more particularly, to reduction of physical layer interference in dense wavelength division multiplexing DWDM radio-over-fiber network by using multiple time re-modulation.  
         [0003]     The application of radio-over-fiber (ROF) for broadband wireless access has attracted much attention recently because it provides the mobile broadband services, wireless local area networks LANs, and fixed wireless access services such as Local Multipoint Distribution Service LMDS that uses microwave signals to transmit and receive data. A few key issues such as all-optical up-conversion, down-conversion and network architecture have been solved. However, one more important issue merits consideration in the future radio-over-fiber ROF network. Referring to the diagram  100  shown in  FIG. 1 , areas A, B and C are neighbouring channel transmission regions,  107   ch1 ,  107   ch2  and  107   ch3 . These adjacent channel regions include optical to electrical O/E converters and radio frequency RF transmitters,  109   ch1 ,  109   ch2 ,  109   ch3 , that have overlapped wireless RF transmission areas. The wavelength division multiplexing WDM signals  101  are up-converted by using an external modulator  103  based on an optical carrier suppression (OCS) modulation scheme and the RF frequency (or only “RF”) is f. After up-converting to the frequency f the signals are separated or multiplexed by the arrayed waveguide grating AWG  105  as ch 1 , ch 2  and ch 3 . The RF frequency of the optical mm-wave for all channels ch 1 , ch 2  and ch 3  is identical and equal to 2f, which means that the customer units in area A, B and C use the same RF frequency. When the wireless signals are broadcast in these areas, the customer unit in the overlapped area would accept two or three different signals which have the same RF frequency. After down-conversion, these signals would interfere with each other when the customer unit receives them.  
         [0004]     If the RF carrier frequency in area A, B, and C can be set to different frequencies, the physical layer interference would be mitigated. For example, the RF carrier frequency in area A, B, and C can be set to 59 GHz, 59.5, and 60 GHz, respectively. In this way, only one RF frequency signal can be effectively down-converted at each customer unit in the overlapped region.  
         [0005]     Accordingly, there is a need to overcome the problem of physical layer interference caused in a radio-over-fiber network with multiple channels at the same carrier frequency.  
       SUMMARY OF THE INVENTION  
       [0006]     In accordance with the invention, an apparatus includes multiple signal paths for optically converting an optical signal to multiples of said optical signal at different respective carrier frequencies for reducing interference between wireless transmissions of said multiples of said optical signal. In a preferred embodiment, the converting includes a first modulator for modulating the optical signal into a first optical carrier and an initial first-order sideband signal with a frequency spacing twice that of the first optical carrier and a first interleaver for separating the first optical carrier and the initial first-order sideband signal. The converting also includes a second phase modulator for modulating the first optical carrier into a second optical carrier and a second first-order sideband signal with a frequency spacing twice that of the second optical carrier.  
         [0007]     In another aspect of the invention, a method includes optically converting an optical signal to multiples of said optical signal at different respective carrier frequencies for reducing interference between wireless transmissions of said multiples of said optical signal. Preferably, the converting includes modulating the optical signal into a first optical carrier and an initial first-order sideband signal with a frequency spacing twice that of the first optical carrier and separating the first optical carrier and the initial first-order sideband signal. The method further includes converting modulating the first optical carrier into a second optical carrier and a second first-order sideband signal with a frequency spacing twice that of the second optical carrier.  
         [0008]     In yet another aspect of the invention, a method includes converting an optical signal into a first optical carrier and an initial first-order sideband signal with a frequency spacing twice that of the first optical carrier, and separating the first optical carrier and the initial first-order sideband signal for subsequent converting of the first optical carrier into a second optical carrier and a second first-order sideband signal with a frequency spacing twice that of the second optical carrier. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0009]     These and other advantages of the invention will be apparent to those of ordinary skill in the art by reference to the following detailed description and the accompanying drawings.  
         [0010]      FIG. 1  is schematic of a dense wavelength division multiplexing DWDM radio-over-fiber network illustrating physical layer interference;  
         [0011]      FIG. 2  is a schematic showing the inventive time re-modulation with different frequencies to reduce the physical layer interference shown in  FIG. 1 ;  
         [0012]      FIG. 3  is a diagram of an experimental setup for two time modulation with a RF frequency of 20 and 19.5 GHz in accordance with the present invention, with inserted optical spectra at a resolution of 0.01 nm;  
         [0013]      FIG. 4  shows optical eye diagrams ( a ), ( b ), ( c ) and ( d ) (100 ps/div) after up conversion at respective points ( a ), ( b ), ( c ) and ( d ) in  FIG. 3 ; and  
         [0014]      FIG. 5  is a graph of bit-error-rate BER curves for the experimental setup of  FIG. 3 .  
     
    
     DETAILED DESCRIPTION  
       [0015]     The schematic  200  of  FIG. 2  shows an exemplary embodiment of an inventive all optical carrier re-modulation to different carrier frequencies for reducing the physical layer interference in overlapped transmission regions. A phase modulation PM 1 , PM 2 , PM 3  is used along with interleaving IL 1 , IL 2 , IL 3  to realize the DWDM signal up-conversion. After modulation of the incoming optical signal carrier  203   ch1 ,  203   ch2 ,  203   ch3  driven by a small RF signal with frequency f 1 , f 2 , f 3  the optical spectrum of each channel contains an optical carrier and the first order sideband signal  205   ch1 ,  205   ch2 ,  205   ch3  with a respective frequency spacing  2   f   1 ,  2   f   2 ,  2   f   3  as shown in  FIG. 2 . Then an interleaver IL 1 , IL 2  is used to separate out the remaining optical carrier  203   ch2 ,  203   ch3  and the first-order sideband signal  207   ch1 ,  207   ch2 ,  207   ch3 . At the final wireless transmission stage  211 , with optical-to-electrical conversions  211   f1 ,  211   f2 ,  211   f3 ,  211   fn1 ,  211   fn3  where the re-modulated signals are transmitted wirelessly, the carrier frequencies of the transmitted signals in overlapped regions shown are different and can be selectively filtered out by tuning in the desired channel.  
         [0016]     Referring again to  FIG. 2 , there are three distinct paths shown: a first path PM 1 , IL 1 , fiber link  215  and arrayed waveguide grating AWG 1 ; a second path PM 2 , IL 2 , fiber link  217 , arrayed waveguide grating AWG 2 ; and a third path PM 3 , IL 3 , fiber link  219 , arrayed waveguide grating AWG 3 .  
         [0017]     In the first path, after modulation by the phase modulator PM 1  driven by a small RF signal with frequency (f 1 ), the optical spectrum of the channel only contains an optical carrier and the first order sideband signal  205   ch1  with a frequency spacing of  2   f   1 . Then an interleaver IL 1  separates out the remaining optical carrier  203   ch2  from the first-order sideband signal  207   ch1 . The remaining two tones of the first order sideband signal  207   ch1  generate an optical millimeter wave (mm-wave). This optical millimeter wave is sent over a fiber link  215  to an array waveguide grating AWG 1  which multiplexes the optical signal as first channel ch 1  at a carrier frequency  2   f   1  to multiple optical-to-electrical converters  211   f1 ,  211   fn1  for wireless transmission. Since all ch 1  transmissions are on the same carrier frequency, the wireless transmission regions  211   f1 ,  211   fn1  transmitting on ch 1  should be apart enough so there is no overlap in their wireless transmission regions.  
         [0018]     The remaining optical carrier  203   ch2  from the first interleaver IL 1  is re-modulated by the second phase modulator PM 2  driven by a second RF frequency f 2 . After the second phase modulation PM 2  the optical spectrum contains an optical carrier and the first-order sideband signal  205   ch2  with a spacing of  2   f   2 . The second interleaver IL 2  separates out the optical carrier  203   Ch3  from the first-order sideband signal  207   ch2 . The first-order sideband signal or optical millimeter wave (mm-wave)  207   ch2  provided by the second interleaver IL 2  is sent over a fiber link  217  to an array waveguide grating AWG 2  which multiplexes the optical mm-wave  207   ch2  as channel ch 2  on a carrier frequency  2   f   2  to an optical-to-electrical converter for wireless transmission. Since the ch 2  transmission is on a different carrier frequency than the ch 1  transmission there is no interference between their respective transmission regions  211   f2  for ch 2  and regions  211   f1 ,  211   fn1  for ch 1 .  
         [0019]     The remaining optical carrier  203   ch3  from the second interleaver IL 2  is modulated by a third phase modulator PM 3  driven by a third RF frequency f 3  to produce an optical carrier and first order sideband signal  205   ch3 . The optical carrier is separated out by the third interleaver IL 3  to leave only the first order sideband signal  207   ch3 . After the third interleaver IL, the optical mm-wave, i.e., first order sideband signal  207   ch3  at frequency  2   f   3 , is sent over a fiber link  219  to an array waveguide grating AWG 3  which multiplexes the millimeter wave as channel ch 3  on a carrier frequency  2   f   3  to optical-to-electrical converters for wireless transmission in regions  211   f3 ,  211   fn3 . Since the ch 3  transmission is on a different carrier frequency than the chi and ch 2  transmissions there is no interference between their respective transmission regions  211   f2  for ch 2 , transmission regions  211   f1 ,  211   fn1  for ch 1  and transmission regions  211   f3 ,  211   fn3  for ch 3 .  
         [0020]     The exemplary embodiment of  FIG. 2  demonstrates that the successive phase modulation and interleaving IL can be used for multiple wavelength operation to realize DWDM signal multi-time re-modulation. When these signals are delivered to the optical-to-electrical converter, arrayed waveguide grating (AWG) can be used to route the optical mm-wave to different antennas, and make the each antenna at an overlapped region transmit at a different RF carrier frequency. The elements shown in the schematic  200  of  FIG. 2  can be physically located or grouped in a variety of configurations. The preferred physical location would be to have the phase modulator PM 1 , PM 2 , and PM 3  and interleaver IL 1 , IL 2 , and IL 3  located in a central office along with the signal source generator  201 . The fiber links  215 ,  217  and  219  can be from the central office to a remote station containing the arrayed waveguide grating AWG 1 , AWG 2 , and AWG 3 .  
         [0021]     An experiment setup  300  for generating optical mm-wave signals at different RF frequencies by using multiple time re-modulation in accordance with the invention is shown in  FIG. 3 .  FIG. 4  shows corresponding optical eye diagrams  400  (100 ps/div) after up-conversion at different points labeled in  FIG. 3 . Eye diagrams of ( a ), ( b ), ( c ) and ( d ) are obtained from points ( a ), ( b ), ( c ) and ( d ), respectively, noted in the experimental setup in  FIG. 3 .  
         [0022]     A distributed feedback laser DFB laser at 1549.3 nm was modulated by a LN Mach-Zehnder modulator (LN-MZM) driven by a 2.5 Gbit/s electrical signal with a PRBS length of 2 31 −1. Then this 2.5 Gbit/s base-band non-return-to-zero NRZ source was amplified EDFA (erbium-doped fiber amplifier)  31  and then modulated by a phase modulator  32  driven by a 20 GHz sinusoidal clock with peak-to-peak amplitude of 3V. The optical spectrum after the phase modulator PM  32  is shown in  FIG. 3  as inset (i). The half-wave voltage of this phase modulator is 8V. Since the driving voltage is much smaller than half-wave voltage of the phase modulator, the second order sideband is 25 dB lower than the first order sideband; therefore the second order sidebands have little effect on the transmission of the optical mm-wave in single mode fibers SMF.  
         [0023]     An optical interleaver IL with two output ports, shown as ( a ) and ( b ) in  FIG. 3 , and 25 GHz bandwidth was used to suppress the optical carriers and convert the modulated DWDM lightwaves to DWDM optical mm-waves. After the optical interleaver IL, the carrier suppression ratio is larger than 15 dB as shown in inset (iii) in  FIG. 3 , and the repetition frequency of the optical mm-wave is 40 GHz. The corresponding eye diagram is shown in  FIG. 4 ( b ). The total power of the optical mm-wave signals is larger than 1 dBm. The remaining optical carrier from the other port (a) of the interleaver is shown in  FIG. 3  as inset (ii). The eye diagram of the separated optical carrier is shown in  FIG. 4 ( a ). There only exists the basement signal, and the RF carrier is negligible.  
         [0024]     The remaining optical carrier was re-modulated by the second phase modulator PM  33  with a frequency of the RF signal to drive the phase modulator at 17.5 GHz. The optical spectrum after the second time modulation is shown in  FIG. 3  as inset (iv). The output from the second time modulation is passed through an optical circulator to a fiber Bragg grating (FBG), path ( c ) in  FIG. 3 , to separate the remaining optical carrier and the first sideband signals. The optical spectra after this separation are shown in  FIG. 4  as inset (v) and (vi). In this way, a 35 GHz optical mm-wave signal was generated and realized with the second time modulation. The eye diagram after the second time modulation is shown in  FIG. 4 ( d ), where it can be seen that the repetitive frequency of the RF signal is 35 GHz.  
         [0025]     Through switching the optical mm-waves, either 40 GHz or 35 GHz, were amplified 35 by an EDFA to obtain a power of 5 dBm and then they were transmitted over variable length single mode fiber SMF  34 . At the receiver end, the optical mm-wave signals were filtered by a tunable optical filter TOF 1  with a bandwidth of 1.2 nm, then they were pre-amplified by an EDFA  36  with a gain of 30 dB at small signal, and then filtered by a tunable optical filter TOF 2  with a bandwidth of 0.5 nm before optical-to-electrical O/E conversion via a PIN PD  37  with a 3 dB bandwidth of 60 GHz. The converted electrical signal was amplified by an electrical amplifier EA  38  with a bandwidth of 10 GHz centered at 40 GHz. An electrical LO signal at 40 GHz was generated by using a frequency multiplier from 10/8.75 to 40/35 GHz. The electrical LO signal and a mixer were used to down-convert the electrical mm-wave signal. The down-converted 2.5 Gbit/s signal was detected by a bit error rate BER tester  39 .  
         [0026]     The fiber length was changed and the BER performance of the optical mm-wave after the first modulation  32  and the second modulation  33  was measured. The measured BER curves  500  are shown in  FIG. 5 . The power penalties for the 40 GHz mm-wave after the first-time modulation and transmission over 10 and 20 km are 0 and 0.7 dB, respectively. While the power penalties for the 35 GHz millimeter wave after the second-time re-modulation and after transmission over 10 and 20 km are 0 and 0.5 dB, respectively. These results show that the optical mm-wave signals after the second-time re-modulation have very good transmission performance.  
         [0027]     The present invention has been shown and described in what are considered to be the most practical and preferred embodiments. For example, the exemplary embodiment employed three all optical time re-modulation paths to provide transmissions with three different carrier frequencies f 1 , f 2 , f 2 , however, that departures may be made there from and that obvious modifications will be implemented by those skilled in the art. It will be appreciated that those skilled in the art will be able to devise numerous arrangements and variations which, although not explicitly shown or described herein, embody the principles of the invention and are within their spirit and scope.