Abstract:
A system for simultaneously transmitting multifrequency signals including a laser light source having a shared gain section and an addressable gain section. A first group of signals modulating the addressable gain section of the laser produces a plurality of wavelength division multiplexed channels. The second group of signals modulates the shared gain section of the laser such that each of the multiplexed channels is modulated with the signals from the second group.

Description:
This application is a continuation of U.S. patent application Ser. No. 08/918,931, filed Aug. 25, 1997 now U.S. Pat. No. 6,147,784. 
    
    
     RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 60/024,634, filed Aug. 27, 1996. 
     FIELD OF INVENTION 
     The present invention generally relates to optical communication systems and more particularly to systems that utilize a laser light source for simultaneously transmitting a plurality of multifrequency signals. 
     BACKGROUND 
     Optical communication systems are used to transmit information signals over optical fibers or waveguides. In order to increase the capacity of these systems, wavelength division multiplexing (WDM) is used to transmit multiple signals at different wavelengths over a single waveguide. A waveguide grating router laser (WGRL) such as disclosed in U.S. Pat. No. 5,373,517 entitled “Rapidly Tunable Integrated Laser” issued to Dragone et al., can be used as a multiple wavelength source for high performance WDM local access networks. The WGRL can simultaneously transmit N equally spaced WDM channels, each modulated with an information signal. The WGRL includes N separately addressable gain sections and an integral 1×N waveguide grating router (WGR) preferably integrated on a semiconductor wafer. 
     When A WDM local access network includes feeder fiber(s) from a central office to a remote node consisting of a waveguide-grating router (WGR), which is in turn connected to remote terminals (or optical network units) via multiple distribution fibers, the local access network is referred to as a WGR-based WDM passive optical network. By matching the wavelengths of the WGRL transmitter, located at the central office, to the WGR at the remote node, distinct broadband signals modulating each wavelength can be transmitted to each of N remote terminals. 
     WGR-based WDM passive optical networks are thus well suited to transmit distinct broadband signals to each of a multiplicity of terminals or subscribers (thereby establishing broadband point-to-point connections). However these networks are not optimized for the simultaneous transmission of broadcast signals, such as video, to all subscribers. A passive splitter based network can be used in addition to a WGR-based WDM passive optical network to transmit these broadcast signals. However, a drawback associated with this approach is that it requires a separate network to transmit the broadcast signals thereby requiring increased cost and complexity. 
     Another approach is to take advantage of spectral slicing through the WGR at the remote node to provide broadcast video signals without altering the outside fiber plant. Spectral slicing refers to a technique known in the art, whereby a light source (such as an LED), having a broad spectral output, is employed in conjunction with a WDM demultiplexer (or WGR demultiplexer) to generate a multiplicity of spectral bands. Thus, in the case of a WGR-based passive optical network, a single broadband source is modulated with broadcast information. The WGR at the remote node slices this broad spectrum and directs an equal fraction of the modulated optical spectrum to each subscriber. A disadvantage associated with this approach is that it requires an additional light source at the central office which increases the optical bandwidth of the system as well as requiring coarse WDM couplers at both the central office and at each subscriber&#39;s optical network unit. 
     Accordingly, there is a need for an optical network which simultaneously delivers both broadband and broadcast signals without requiring an additional overlayed network or an additional light source. 
     SUMMARY OF THE INVENTION 
     The system in accordance with one embodiment in accordance with the present invention meets these needs and avoids the disadvantages and drawbacks of prior systems by providing a system for simultaneously transmitting multifrequency signals where a first group of signals modulate a light source to produce a plurality of information bearing wavelength division multiplexed channels. A second signal modulates the multiplexed channels. 
     In another embodiment in accordance with the invention, a system is provided for simultaneously transmitting multifrequency signals including a light source having a shared gain section and an addressable gain section. A first group of signals modulating the addressable gain section of the laser produces a plurality of wavelength division multiplexed channels. A second group of signals modulates the shared gain section of the laser such that each of the multiplexed channels is modulated with the second signal group. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a block diagram of a system in accordance with a first embodiment of the present invention. 
     FIG. 2 is a block diagram of a system in accordance with a second embodiment of the present invention, including a feed forward circuit. 
     FIG. 3 is an exemplary RF spectrum graph recorded at the output of the receiver portion of the system. 
     FIG. 4 is an exemplary bit-error-rate graph of transmitted WDM channels with and without modulation of the broadcast video signals. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a simplified block diagram of the system  10  in accordance with one embodiment of the present invention. The system  10  is adapted to combining broadband and baseband signals. The baseband signals modulate addressable gain sections of a waveguide-grating router laser (WGRL) and broadband signals modulate a shared gain section of the WGRL so as to combine the broadband signals with each modulated baseband channel. A waveguide grating router demultiplexes the received channels and a receiver circuit splits the baseband and broadband signals. In this manner, system  10  can simultaneously transmit N WDM channels, each channel modulated with a broadband signal. 
     Generally, FIG. 1 illustrates a waveguide grating router laser  20 , a data generator or like baseband signal source  30 , a broadband signal source  40 , a waveguide or light transmission medium  50 , a waveguide-grating router  90  and receiver circuits  60 . The WGRL  20  includes separately addressable gain sections  21   1 ,  21   2 ,  21   3  . . .  21   N , where N is equal to the number of baseband signals received from source  30 . Each addressable gain section  21   1 ,  21   2 ,  21   3  . . .  21   N  is coupled to waveguide grating router  22  which is a routing device operating as a multiplexer/ demultiplexer of optical frequencies. The output at  22 ′ of WGR  22  is coupled to a shared gain section  23 . 
     A baseband signal source  30 , such as a data generator, produces baseband signals filtered by low pass filters  31   1 ,  31   2 ,  31   3 , . . .  31   N  . The baseband signals received from source  30  and lowpass filtered at filters  31   1 . . .  32   N , are transmitted over lines  32   1 ,  32   2 ,  32   3  . . .  32   N  to modulate each of the respective addressable gain sections  21   1  . . .  21   N  of WGRL  20  thereby producing N WDM channels. By way of example, the inventors of the present invention utilized four channels of a 1.5 μm wavelength integrated eight wavelength device having eight separately addressable gain sections, an 8×1 WGR and a fiber-pigtailed output facet. The baseband signals received from source  30  were distinct 2 23 −1 pseudo random bit streams (PRBS) at 150 Mb/s. The laser&#39;s round-trip frequency was approximately 2.5 GHz. The four addressable gain sections of WGRL  20  used were biased at 30.6 mA, 29.3 mA, 27.4 mA, and 29.1 mA which produced four WDM channels with 200 GHz spacing. 
     Broadband signals transmitted from source  40  include, for example, digital video signals received from a broadcast system. Depending on the signal transmission from source  40 , these signals may be in digital form and may contain several video and audio channels occupying a predetermined bandwidth. For example, the broadband signals may be 16 quadrature phase shift keying (QPSK) subcarriers containing 79 MPEG video channels and 29 audio channels which occupy a 500 MHZ band which are downconverted to the 350-850 MHZ range in order to fit within the bandwidth of the transmitter used. 
     A broadband subcarrier circuit  80  is used, if needed, to adjust the amplitude of the received broadband signals. Depending on the broadband signal source  40  and the signals transmitted therefrom, circuit  80  can include: cascaded amplifiers  81 ,  82 , and  83  for maintaining, within acceptable levels, the signal to noise ratio of the broadband signals; a local oscillator  89  and mixer  84  for downconverting, if necessary, the broadband signals to fit within the bandwidth of WGRL  20 ; and an attenuator  85  and high pass filter  86 . The output of circuit  80  is transmitted over line  88  and modulates shared gain section  23  of WGRL  20 . For example, the shared gain section  23  may be biased at 70.4 mA for modulating the broadband signals. In this manner, each of the WDM channels modulated at the addressable gain sections  21   1  . . .  21   N  of WGRL  20  are simultaneously modulated with the broadband RF signals from shared gain section  23  of WGRL  20  using a single RF drive circuit. 
     The WDM channels modulated with the broadband signals are output from WGRL  20  to fiber isolator  70  and transmitted over waveguide medium  50  to a remote node which includes a waveguide-grating router  90  for optically demultiplexing the transmitted WDM channels. Temperature tuning of WGRL  20  may be required to align the channels of WGR  90  and WGRL  20 . Distribution fibers  91   1  . . .  91   N  coupled to WGR  90  route the WDM channels according to wavelengths to each of N receivers  60 . Each circuit  60  includes a photodetector receiver  100 , such as an avalanche photodiode receiver, which is coupled to one of the outputs  91   1  . . .  91   N  of the waveguide grating router  90 , to convert the optically received signal into an electrical signal. The output  105  of receiver circuit  100  is coupled to splitter  110  which splits the received electrical signals permitting separate filtering of the baseband point-to-point and the broadband broadcast portions of the electrical spectrum into its baseband and broadband signals. The baseband signals are transmitted to amplifier  120  which is coupled to splitter  110 . Low pass filter  125  is coupled, at its input, to amplifier  120  and at its output to data receiver  130  which receives the transmitted baseband signals. The broadband signals split at splitter  110  are amplified by amplifier  135  and transmitted to RF receiver  140 . Alternatively, a high pass filter  145 , shown in broken lines, connected in series with mixer  150  and local oscillator  146 , also shown in broken lines, can be disposed between amplifier  135  and RF receiver  140  depending upon the frequency bandwidth of the broadcast signals received. 
     FIG. 2 illustrates a feed forward circuit scheme as disclosed in C. R. Doerr, et al., “Elimination of Signal Distortion and Crosstalk from Carrier Density Changes in the Shared Semiconductor Amplifier of Multifrequency Signal Sources,” IEEE Phot. Technology Lett., Vol. 7, pp. 1131-1133, 1995. This scheme employed with the system of FIG. 1 is used to eliminate or reduce crosstalk due to carrier density fluctuations in the shared gain section  23  of WGRL  20 . Baseband signals received from source  30  are filtered by low pass filters  31   1 ,  31   2 ,  31   3  . . .  31   N  and transmitted over lines  32   1 ,  32   2 ,  32   3  . . .  32   N  to splitters  200   1 ,  200   2 ,  200   3 , . . .  200   N . The baseband signals are split with one portion directly modulating the addressable gain sections  21   1  . . .  21   N  of WGRL  20  and the other portion transmitted over lines  210   1 ,  210   2  . . .  210   N . The signals are amplified by amplifiers  220   1  . . .  220   N  and combined with similar portions of each of the remaining modulation signals in a feed forward scheme by splitters  230   1  . . .  230   N . For example, a portion of the broadband signals transmitted on lines  210   N  and  210   3  amplified by amplifiers  220   N  and  220   3  respectively and combined at splitter  230   N  and outputted onto line  231 ; the broadband signals transmitted on lines  210   1  and  210   2  are combined at splitter  230   1  and outputted onto line  232 . Similarly, the outputs on lines  231  and  232  from splitters  230   1  and  230   N  respectively, are combined at splitter  240  and transmitted over line  245  to attenuator  250 . The number of splitters and baseband signals to be combined is dependent on N which is the number of baseband signals received from source  30 . 
     Splitter  260  combines the output signal from attenuator  250  with the broadband signals transmitted over line  88  and outputs the combined signals which modulate shared gain section  23  of WGRL  20 . The WDM channels modulated with the broadband signals are output from WGRL  20  to a fiber isolator  70  and transmitted over a waveguide medium  50  to a WGR  90 , and then via distribution fibers  91   1  . . .  91   N  to N receiver circuits  60  as described with reference to FIG.  1 . 
     Turning now to FIG. 3, an exemplary graph of the RF spectrum recorded at output  105  of receiver circuit  100  using the system described in FIGS. 1 and 2 illustrates that 150 Mb/s baseband data occupies the frequency band from DC to approximately 200 MHZ and the video subcarriers occupy the band from 350 MHZ to 850 MHZ. This exemplary measurement was taken using four addressable gain sections of a 1.5 μm wavelength integrated eight wavelength WGRL  20  biased at 30.6 mA, 29.3 mA, 27.4 mA, and 29.1 mA respectively which produced four WDM channels with 200 GHz spacing. The baseband signals used from source  30  were 2 23 −1 pseudo random bit streams (PRBS) at 150 Mb/s. This graph shows that multiple information bearing WDM channels, each channel simultaneously modulated with broadband broadcast signals from source  40 , can be successfully transmitted using a multiwavelength waveguide-grating router laser and received by a receiver circuit  60  after being demodulated by WGR  90 . 
     FIG. 4 illustrates the bit-error rate (BER) performance of the four WDM channels referenced in FIGS. 2 and 3 both with and without the broadband signals. A BER curve is a standard measure of the performance of an optical communication system in which the probability of receiving an error is plotted as a function of received optical power. The BER data taken without the broadband signals are represented by open symbols, while data taken with the broadband signals are represented by closed symbols. Prior to measuring the exemplary BER performance of the system, the optical modulation depth of both the baseband and broadband signals were adjusted such that the received optical power in each WDM channel required for a 10 −9  BER was roughly equal to the received optical power at which a noticeable degradation of video picture quality was observed. The baseband optical modulation depth for all four WDM channels was set in the range from 55-58%. The broadband optical modulation depth was set such that the received optical power required for excellent picture quality was −32 dBm for all four WDM channels. This operation point was chosen to ensure that one service did not unduly suffer at the expense of the other. Baseband network performance was then evaluated by measuring the BER for each WDM channel, both with and without the broadband signals. The single-channel baseline curve shows the output of the photodetector receiver  100  and the −35.5 dBm receiver sensitivity at 10 −9  BER for a single 150 Mb/s channel with 100% modulation depth. The power penalty of 2-3 dB was measured for the four channels without the broadband signals, as expected for a decrease in modulation depth from 100% to 55-58% range. The addition of the broadband signals to the baseband signals resulted in an additional 1 dB of power penalty to each channel. The inset in FIG. 4 is an optical spectrum analyzer trace of the four WDM channels. 
     The present invention illustrates that a WDM passive optical network with a WGRL located, for example, at a central office and a WGR located, for example, at a remote node, can provide high speed virtual point-to-point connectivity. The present system illustrates that a WGRL can be used as a source to provide both virtual point-to-point baseband connections with a simultaneous broadband service.