Abstract:
A hybrid WDM-TDM optical link employing a hybrid modelocked multi-wavelength semiconductor which provides approximately 4 to approximately 20 wavelength channels that makes possible modulated multiplexed data which when demultiplexed by ultra fast optical demultiplexing provides rates suitable for conventional electronic photo receivers. The link uses single-stripe GaAs/AlGaAs semiconductor optical amplifiers which simultaneously generate from approximately four to more than approximately twenty tunable WDM channels. Diode laser can also include InP, InGaAlP, InGaAsP, InGaP, InGaAs. A four channel version transmits approximately 12 picosecond pulses at approximately 2.5 GHz for an aggregate pulse rate of 100 GHz. When generating approximately 20 wavelength channels, each transmitting approximately 12 picosecond pulses at a rate of approximately 600 MHz, there is provided optical data and transmission systems operating at rates in excess of 800 Gbits/s.

Description:
This is a Divisional of application Ser. No. 09/344,422 filed Jun. 25, 1999 now U.S. Pat. No. 6,314,115. 
    
    
     This invention relates to an ulra-high speed data and communication link, and in particular to a hybrid WDM-TDM transmitter capable of simultaneously generating up to twenty or more tunable WDM channels using a single-striped Ga As/AlGaAs type semiconductor optical amplifier, and was funded in part by the National Science Foundation ECS contract number 9522267, and is a Continuation-in Part of U.S. patent application Ser. No. 09/096,271 filed Jun. 11, 1998, now U.S. Pat. No. 6,256,328. This application claims the benefit of provisional applicaition No. 60/0855,566. 
     BACKGROUND AND PRIOR ART 
     Ultra high speed data links will become wide spread with the deployment of broadband switched digital networks and services, e.g., teleconferencing, video telephony, and computer services. The key hurdle in the commercial development of these networks is the availability of cost effective photonic technologies that will speed up the generation, transmission and processing of these vast amounts of data. Present state of the art optical communications and signal processing rely on a wavelength division multiplexed (WDM) or a time division multiplexed (TDM) hardware platform. 
     Thus, hardware based on multiwavelength optical signal sources capable of generating ultrashort and highly synchronized picosecond pulses are of great interest in novel photonic networks utilizing combined optical wavelength division multiplexed (WDM) and time division multiplexed (TDM) data formats. To date, multiwavelength generation has been demonstrated by either using spectral filtering of femtosecond optical pulses, or semiconductor laser based devices. See Morioka et al., “Multi WDM-Channel pulse generation from a Single Laser Source utilizing LD-pumped Supercontinuum in Optical Fibers”, PhotonTech. Lett., vol.6, no.3, 365˜368, 1994: and Nuss et al. “Dense WDM with Femtosecond Laser Pulse”, IEEE/LOS 1994 Annual Meeting, Boston, Mass., 1994 
     In these approaches, the main idea is to generate femtosecond optical pulses at a low channel pulse rate. These approaches generally suffer from limited wavelength channels, the need of high power laser source, system complexity, and low single channel rates. 
     Furthermore, there have been various patents of general interest in this area that also fail to overcome the problems described above. U.S. Pat. No. 4,435,809 to Tsang et al. describes a passively mode locked laser having a saturable absorber that only has a single wavelength operation mode, with multiple longitudinal modes. U.S. Pat. No. 4,446,557 to Figueroa describes a mode-locked semiconductor laser with tunable external cavity where a user adjusts the cavity length which modifies the longitudinal mode spacing to generate a single wavelength output. U.S. Pat. No. 5,115,444 to Kirkby et al. describes a multichannel cavity laser where each wavelength is generated from a common cavity with each wavelength experiencing a different optical path length. Simultaneous generation of each wavelength is not feasible since the gain competition in the final optical amplifier stage will complicate and prevent simultaneous multiwavelength generation. U.S. Pat. No. 5,228,050 to LaCourse et al. describes an integrated multiple-wavelength laser array, each wavelength having its own cavity that is length adjustable to allow lasing at different wavelengths, and requires an array of lasers for the multiple wavelength generation. U.S. Pat. No. 5,319,655 to Thornton describes a multiwavelength laterally-injecting type lasers which requires the sources to be precisely aligned to one another. U.S. Pat. No. 5,524,012 to Wang et al. describes a tunable, multiple frequency laser diode that uses a multistripe semiconductor laser array to generate several wavelengths, and requires a grazing incidence angle on the diffraction grating. Using the grazing incidence angle prevents simultaneous wavelength generation. U.S. Pat. No. 5,524,118 to Kim et al. describes a wavelength-varying multi-wavelength optical filter laser using a single pump light source, which requires using an erbium doped fiber amplifier. U.S. Pat. No. 5,561,676 to Goldberg describes a compound-cavity high power, tunable modelocked semiconductor laser, that generates a single wavelength output that does not allow for multiple wavelength generation. 
     Another problem with multichannel generation from femtosecond lasers is that the multiple channels are generated by spectrally filtering the laser output after the optical pulse is generated. This is inefficient because the filtering process eliminates, or throws away, energy that was used in making the optical pulse. 
     In approaches relying on the spectral filtering of super-continuum generation, some wavelength channels may experience excess spectral incoherence, rendering them useless for data transmission. 
     SUMMARY OF THE INVENTION 
     The first objective of the present invention is to provide an ultra-high speed data and communication link based on an actively modelocked laser with multiple wavelength generation. 
     The second object of this invention is to provide an ultra-high speed photonic circuit based on multiwavelength generation from a semiconductor diode laser source. 
     The third object of this invention is to provide an ultra high-speed data and communication link based on generating multiwavelengths simultaneously from a single stripe semiconductor diode laser source. 
     The fourth objective is to provide a method for switching and routing, multiple wavelengths simultaneously. 
     The subject invention hybrid WDM-TDM optical link includes a hybrid modelocked multiwavelength semiconductor laser that can simultaneously generate over 20 independent wavelength channels at rates greater than approximately 5 Gbits per second. Additionally, ultrafast optical demultiplexing that relies on an all optical clock recovery technique and nonlinear optical loop mirrors is used to demultiplex multiwavelength data down to rates suitable for electronic photoreceivers. The temporal duration of the optical pulses would mean that aggressive temporal interleaving can lead to optical data and transmission systems operating at rates in excess of approximately 800 Gbits per second, based solely on the semiconductor optical amplifier applications. 
     Further objects and advantages of this invention will be apparent from the following detailed description of a presently preferred embodiment which is illustrated schematically in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a schematic representation of the novel high speed hybrid WDM-TDM optical link. 
     FIG. 2 a  is a schematic setup of the multiwavelength modelocked semiconductor diode laser transmitter capable of up of the generation of up to 20 simultaneous wavelength channels (the output of which is connected to a delay line unit including a multiplexer). 
     FIG. 2 b  shows a slit mask that can be used with the embodiment of FIG. 2 a.    
     FIG. 3 a  show the spectral output of the multiwavelength modelocked diode laser of FIG. 2 a.    
     FIG. 3 b  show the temporal output of the multiwavelength modelocked diode laser of FIG. 2 a.    
     FIG. 4 is an oscilloscope trace of a recovered clock superimposed on the input data train. 
     FIG. 5 a  is an oscilloscope trace of the capability of rate conversion of the optical clock recovery oscillator of FIG.  1 . 
     FIG. 5 b  is an oscilloscope trace showing the performance of the all optical switch of FIG.  1 . 
     FIG. 5 c  is an oscilloscope trace showing the operation of the optical switch(wavelength domain). 
     FIG. 6 shows a multiwavelength optical switch based on a nonlinear optical loop mirror that uses a semiconductor optical amplifier as the nonlinear optical element. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Before explaining the disclosed embodiment of the present invention in detail it is to be understood that the invention is not limited in its application to the details of the particular arrangement shown since the invention is capable of other embodiments. Also, the terminology used herein is for the purpose of description and not of limitation. 
     FIG. 1 is a schematic of a setup  1  of a high speed hybrid WDM-TDM optical link, showing the multiwave transmitter(fully discussed and described with respect to FIGS. 2 a  and  2   b ) connected to an optical clock recovery oscillator and the all optical switch. The multiwavelength laser transmitter features the multiwavelength modelocked semiconductor diode laser disclosed in parent U.S. patent application Ser. No. 09/096,271 filed Jun. 11, 1998, which is incorporated by reference. 
     Referring to FIG. 1, modulator  12  can be an electro-optic modulator such as a lithium niobium intensity modulator(by United Technologies, Inc.), and the like. Alternatively, modulator  12  can be semiconductor based modulator such as a semiconductor electro-absorption modulator, and the like. Optical multiplexer  14  can be wavelength division multiplexing devices such as an arrayed waveguide grating router(by PIRI, Inc.), a bulk diffractive grating devices(by Instruments SA, Inc.), and the like. Data link  16  can be a single optical fiber either single mode or multimode fiber, multiple fiber bundles(both coherent and incoherent), parallel optical busses(such as fiber by Corning), data links such as free-space propagation, and the like. Optical amplifier  18  can be a semiconductor optical amplifier, an erbium doped fiber, and the like. Tap  26  can be an optical fiber splitter, an optical coupler, an optical beamsplitter, and the like. Referring to FIG. 1, demultiplexer  28  is where the multwavelength switching in the invention is occuring. Demultiplexer  28  can be an all optical switch such as a nonlinear optical interferometer, a lithium niobate electro-optic switch, and the like. 
     Referring to FIG. 1, the multiwave laser generator  10  is capable of generating up to 20 different wavelengths, each of which can be modulated by modulator  12 , which enables data to be transmitted at a data rate of 5 Gbits/s. The data then passes through an all optical multiplexer  14  to a 100 Gbits/s data link  16  to an optical amplifier  18 . Owing to the modelocked nature of the semiconductorlaser, the optical data is transmitted in the R-Z format. The R-Z data format is also referred to as a return-to-zero data format, and is an optical data format that uses a pulse of light to represent a logical state. This is in contrast to a continuous light intensity to represent a logical state or non-return-to-zero data format NR-Z. The R-Z data format is relevant to the subject invention because the laser output light is in the return to zero format. R-Z data also offers advantages in all optical signal processing because it contains an RF spectral component at the clock frequency(which is not found in the NR-Z data format). 
     At the receiver  20 , the data is demultiplexed by employing an all-optical clock recovery oscillator  22  in conjunction with an ultrafast nonlinear optical loop  24  in conjunction with an optical tap  26  and an all optical demultiplexer  28  which utilizes a semiconductor optical amplifier as the nonlinear element. The data then goes to a bandpass filter element  30  before reaching the receiver  20 . Owing to the temporal duration of the data bits(approximately 10 picoseconds or 10 to minus 11 th  seconds), aggressive temporal interleaving of the data should allow for single wavelength channel data rates of approximately 40 Gbits/s, implying a total aggressive data transmission of approximately 840 Gbit/s from a single semiconductor laser diode. To achieve approximately 840 Gbits/s occurs by combining both the 21 independent wavelengths or WDM channels with the time division multiplexing or interleaving optical pulses of 10 picosecond pulses to single wavelength rates of approximately 40 GHz would imply a combined data rate of approximately 21×40 Gb/s or 840 Gb/s. Thus, this hybrid approach uses or combines both wavelength division multiplexing and time division multiplexing. 
     FIG. 2 a  is a schematic of the multiwavelength modelocked semiconductor diode laser transmitter system  200 . SOA, semiconductor optical amplifier  230 , can be a GaAs(gallium arsenide), AlGaAs(aluminum gallium arsenide), and the like. G grating  250  can be approximately 1800 lines per mm. Spatial filter, SF  270  can be a spatial transmission mask with variable intensity transmission, variable phase delay(or optical path length) or a combination of both amplitude and phase variation. Lens, L  260  can be a 150 mm achromatic lens for collimating and focussing the light. M 2 .  210  and M 1 .  280  are end mirrors. Lenses  222 ,  226 , and  242  can be an 8 mm achromatic lens for collimating and focussing the light. Resistor  232  and capacitor  236  can be configured as a bias tee, such the ones manufactured by Mini Circuits. DC bias  233  can be approximately 172 mA. RF sinusoidal signal  235  can have a frequency of approximately 600 MHz. Component  258  can be a broadband reflective mirror. Multiplexer  290  can be a configuration of 3 cascaded modified Michelson interferometers, or any type of optical splitters such as but not limited to beam splitters, and the like. Multiplexer  290  can further be an optical delays such as but not limited to retroreflecting corner cubes with mechanical position stages, and the like. Multiplexer  290  can further be an optical recombiners such as but not limited to beamsplitters, recombiners, and the like. 
     FIG. 2 a  schematically shows the setup  200  of actively modelocked optical pulses generated from an angled-strip GaAs/AlGaAs SOA  230  by incorporating an intracavity spectral filter  270  to define the individual spectral components. In FIG. 2 a,  components  210  through  280  correspond to the multiwavelength laser  10  shown in FIG.  1 . Referring to FIG. 2 a,  the end mirror M 1   280  reflects the selected spectral components back to the gain device  230  to complete the multiple wavelength generation. A co-linear composite multiwavelength pulse train is coupled out from the zeroth order grating reflection  250  for diagnostics  257 . Modelocking occurs by injecting approximately 1 W of an RF sinusoidal signal  235  at a frequency of approximately 600 MHz with approximately 172 mA of DC bias current  233  into the diode chip  230 , while diode chip  230  is impedance-matched for optimum power delivery. 
     FIG. 2 b  shows the filter mask  270  that can be used with the embodiment  200  of FIG. 2 a.  The filter mask  270  can be constructed as a pure amplitude mask with more than 20 slits to accommodate 20 wavelength channels with wavelength spacing of approximately 0.52 nm. Referring to FIG. 2 a,  the black lines represent areas where light can pass through the mask. The horizontal direction is parallel to the direction of spectral dispersion determined by the grating; the vertical direction is perpendicular to the dispersion plane determined by the grating. The operation of the components in FIG. 2 a  will now be described. 
     Referring to FIG. 2 a,  light is generated in SOA  230  with dc bias current  232 . The light is collected and collimated by lenses  226  and  242 . Light is focussed on to mirror M 2 - 210  using lens  222 . This light is then reflected back into the gain medium by retracing the optical path. Light from  242  is directed to grating G- 250 . The diffracted light is directed to the spectral filter which is comprised of lens  260 , spatial filter  270 , and mirror M 1 - 280 . The specularly reflected light(the zeroth order diffracted light) is sent to the diagnostic  253 . The reflected light from mirror M 1 - 280  is directed back into the gain element  230  by retracing the optical path. This completes the optical cavity. The bias current  233 , passed through cap  232  is increased to 175 mA to bring the laser cavity close to lasing threshold. An additional RF current  235  is applied to modulate the gain at a rate that is inversely proportional to the round trip time. This additional current establishes modelocking. The twenty wavelengths are defined by the optical filter. As modelocking occurs, it is transient unsaturated gain which allows the simultaneous production of the multiple wavelength. When the wavelengths are equally spaced, nonlinear four wave mixing occurs in the SOA chip  30  which produces four wave mixing components that are injected into the lasing wavelengths. This effect contributes to establishing as phase relation between the independent wavelength channels. The zeroth reflection  253  is also used as the output  285 . The light is then passed through the multiplexer unit  290  to increase the pulse repetition frequency. G grating  50  can be approximately 1800 lines per mm. SF  70  (Spatial filter) is a transmission mask which can be fabricated photographically, or can be construed by fabricating thin slits in a thin, optically opaque medium. The mask can be a transmission mask, a phase mask, or a combination of amplitude and phase masks. Lens  60  can be a 150 mm achromatic lens, end mirrors M 2 ,  10  and M 1 .  80 . Lens  22  can be a 8 mm focal length lens for collimating or focusing the light from a laser diode. Lens  26  can be a 8 mm focal length lens for collimating or focusing the light from a laser diode. Lens  42  can be a 8 mm focal length lens for collimating or focusing the light from a laser diode. Resistor  32  and capacitor  36  are found in commercially available bias tees from Mini Circuits, Hewlett Packard, or Picosecond Plus Labs, (the resistor-capacitor combination is a “bias tee”). DC bias  33  can be approximately 175 mA, and RF sinusoidal signal  35  can have a frequency of approximately 2.5 GHz. 
     Diagnostics  257  can pass to external components such as a spectrometer  292  such as a Jarrel Ashe spectrometer or a Hewlett Packard Optical Spectrum Analyzer, and a diode array  294  such as a linear detector array such as a silicon detector array manufactured by Epitaxial Products, E.G.&amp;G, or Princeton, that are used to measure the optical spectrum. Diagnostics  257  can further include a high speed sampling oscilloscope  298  such as a Tektronix high speed sampling oscilloscope, and a high speed photodetector  296  such as a Picometrix pin photodiode with 20 picosecond response time, which is used to measure the optical pulse train. Diagnostics  257  can further include an external optical autocorrelator, such as a commercially available autocorrelator from Femtochrome, which measures individual optical pulse characteristics. 
     Referring to FIG. 2 a,  the output spectrum can be monitored using a spectrometer  292  in conjunction with a diode array  294 , while the temporal characteristics can be measured using a fast photodetector  296  with a sampling oscilloscope  298  and nonlinear optical autocorrelation techniques. Nonlinear optical autocorrelation techniques is a method that can provide information about the temporal intensity of a short optical pulse. This method uses a modified interferometer and nonlinear crystals such as Lithium Iodate(LiIO3)(manufactured by Femtochrone). 
     The output of the modelocked diode laser  200  is directed into a temporal interleaver  291  (delay unit  290 ) to achieve high pulse repetition rates between approximately 5 and approximately 15 GHz. As earlier discussed, an ultrafast optical demultplexer  28  is required in FIG.  1 . Temporal interleaver  291  can be comprised of beamsplitters and optical delays to generate a pulse train and subsequently combine them with an appropriate delay, to increase the overall repetition rate of the pulse train. For that purpose, an optical clock recovery oscillator  22 , based on a passively modelocked semiconductor is employed such as a well known optical recovery circuit shown in FIG. 1 of U.S. Pat. No. 5,548,433 to Smith, which is incorporated by reference. The well known optical clock recovery can be achieved by injecting light from a modulated data stream  26 A into the passively modelocked laser of the optical clock recovery oscillator  22 , with output clock pulses passing out at arrow  22 A. Arrows  18 A,  26 A,  22 A,  26 B, and  28 A show the direction through the nonlinear optical loop  24 . For this application, a small amount of optical power from the WDM-TDM laser transmitter  10  is used to injection lock the passively modelocked diode laser. Typical operating modes of the optical clock recovery oscillator are minimum injection powers of 3 microwatts for robust recover, a fractional locking bandwith of approximately 3×10−3, immunity to bit patterns of 2 50 −1, and &gt;8:1 data to clock rate conversion. To demonstrate the robustness of the clock recovery oscillator  22 , FIG. 4 shows and oscilloscope trace of clock recovery with repetitive data packets containing a single login “one” data bit followed by 49 clock cycles of logic zero. The generation of stable clock recovery suggests that the clock recovery process is immune to pseudo random bit patterns of greater than 2 50 −1. 
     The capability of rate conversion is shown in FIG. 5 a.  In this oscilloscope trace, the data transmitter is displayed operating at a rate equal to eight times the free running clock rate, along with the recovered, synchronized optical clock pulse train. This shows the ability of the clock recovery process to lock onto a signal with a data rate equal to a multiple of the fundamental clock rate, thus enabling the clock to be employed in demultiplexing applications. Upon achieving robust clock recovery, the subsequently generated optical modelocked optical pulses are amplified in a semiconductor optical amplifier and used as control pulses in a nonlinear amplifier asymetrically displaced from the center of the optical loop. Typical launch pulse energies into the optical switch are approximately 10 pJ, not including coupling losses into the SOA. Upon achieving robust clock recovery, the subsequently generated clock pulses are amplified in a semiconductor optical amplifier and are used as control pulses in an all optical switch shown and described in reference to FIG.  6 . The optical switch consists of a nonlinear optical loop mirror with a semiconductor optical amplifier asymmetrically displaced from the center of the loop. Complete data switching is achieved with control pulse energies as low as 1.6 picojoules, not including coupling losses in the semiconductor optical amplifier. 
     FIG. 5 b  is an oscilloscope trace showing the performance of the all optical switch. FIG. 5 b  shows the temporal output of the optical switch when an optical logic pulse of state “one” is present at the input of the switch. The two traces represent the output of the switch when the control; or switching pulse is either present or absent. The relative ratio of the peak detected signal represents the switching contrast, which in this case is greater than approximately 10 dB. 
     The multiwavelength optical switch depicted in FIG. 6 is based on a nonlinear optical loop mirror that uses a semiconductor optical amplifier  307  as the nonlinear optical element. The operation of the switch is schematically represented in FIG.  6 . It should be noted that the same functionality can be achieved by employing all fiber based optical components. Multiwavelength data packets, which are generated from the multi wavelength laser transmitter  10  are injected into the nonlinear loop(comprised of beamsplitter  302 , mirrors  303 ,  304 , polarization beam splitters  305 ,  309 , semiconductor optical amplifier  307 , lenses  306 ,  308 , optical delay(mirrors  310 ,  311 ,  312 ,  313 ) and mirror  314 . Control or grating pulses  332  are generated from the injection modelocked laser  10 . The output  305 A of the injection modelocked control laser  322  injected into an out of the nonlinear loop using polarizing beam splitters  305 ,  309 , respectively. The position of the SOA  307  is placed off-center x in the loop. The control pulse  322  induces an index change so that the data pulse is ejected out  315  of the loop at beamsplitter  302 . It should be noted that without the control pulse, the data pulses  322  are reflected back toward the multiwavelength laser  10  and are blocked or filtered out by isolator  301 . Aperature  316  is used to spatially filter the output signals for achieving high switching contrast. The mirrors, isolator, aperature and polarizing beamsplitters are commercially available off the shelf from Newport Corporation. The SOA  307  can be the same as SOA  230 . 
     FIG. 5 c  shows multiwavelength switching across a wide spectral bandwidth with high contrast. The solid trace shows the output of the switch in its “on” state, when a control pulse  322  is present, showing the presence of the multiples wavelength channels being switched at the output. The dashed trace shows the switch in its “off” state when a control pulse  322  is absent, showing a minimal amount of light leakage through the switch. The switching contrast ranging from approximately 13 to approximately 23 dB is measured in the spectral domain, illustrating the capability of switching, routing and processing multiwavelength channels in parallel, simultaneously. 
     The correlated temporal synchronization described above has applications in optical clock recovery  22  shown in FIG. 1, and synchronized WDM networks. The optical clock recovery can be a generation of a train of stable optical pulses  22 A at a well defined repetition rate by injecting an optical data stream  26 A into a passively modelocked laser which is part of optical clock recovery oscillator  22 . The synchronized WDM network can be a network of optical fiber links and switches which use plural wavelengths which are pulsed at a well defined repetition rate to send optical data. The WDM-TDM network is a network of optical fiber links and switches which use many wavelengths which are pulsed at a selected repetition rate and that the optical pulses can be temporally interleaved with each other to send optical data. 
     Furthermore, the ultrafast temporal modulation shows that intracavity dynamics plays a role in establishing phase coherence across the composite multiwavelength spectrum. 
     In addition, the foregoing invention shows that modelocked multiwavelength single-stripe diode laser can be useful WDM-TDM generators of multifrequency wavelengths networks such as the optical links described above. 
     Although the mutiwavelength laser generator and the passively modelocked diode laser for demultiplexing are described using semiconductor laser diodes such as GaAs and AlGaAs, and the like; other lasers can be used such as but not limited to optical fiber lasers such as erbium, praesodimium, and the like. Solid state lasers can also be used such as but not limited to YAG, fosterite, LiSAF, and Ti:Al 2 O 3 , and be used. Other semiconductor lasers can also be used such as but not limited to vertical cavity lasers, InP, InGaAs, InGaAsP, GaN, InGaAlN, and the like. Still other semiconductor lasers can be used such as InAlGaAs, and InAlGaP, and the like. Still other lasers can be used such as but not limited to diode pumped lasers, gas lasers, waveguide lasers, and the like. 
     In summary, it has been shown that there are now, as a result of employing a hybrid WDM-TDM modelocked multiwavelength semiconductor, optical links capable of generating over 20 independent wavelength channels at rates greater than 5 Gbit/s which lead to optical data and transmission lines systems operating at rates in excess of approximately 800 Gbit/s. 
     While the invention has been described, disclosed, illustrated and shown in various terms of certain embodiments or modifications which it has presumed in practice, the scope of the invention is not intended to be, nor should it be deemed to be, limited thereby and such other modifications or embodiments as may be suggested by the teachings herein are particularly reserved especially as they fall within the breadth and scope of the claims here appended.