Abstract:
A channel-obtaining device takes the desired channel and uses a resonant semiconductor device to mix the channel to a different frequency. The dropped channel is sufficiently removed from the other channels in frequency that it can be effectively separated.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of the U.S. Provisional Application Nos. 60/070,547, filed on Jan. 6, 1998, which is incorporated herein by reference. 
    
    
     STATEMENT AS TO FEDERALLY SPONSORED RESEARCH 
     This work was supported by a grant from ARPA (Contract # DAAL 01-94-K-03430) and National Science Foundation (Grant # ECS-9412862). The government may have certain rights in this invention. 
    
    
     BACKGROUND 
     The present disclosure describes a system for copying an optical channel in an optical communications system. More specifically, the present system describes a special kind of four-wave mixing element which enables dynamic copying of single or multiple channels. 
     Fiber optic cable has the capability of carrying an enormous amount of information. Demand for bandwidth in long distance communication lines is growing rapidly because of applications like the Internet. 
     To accommodate this bandwidth growth, commercial fiber optic systems now employ wavelength division multiplexing “WDM” in which information is carried on many optical wavelengths spanning a wide optical bandwidth. Each wavelength is called a channel and can carry substantial amounts of information. 
     It may be desirable at some times to copy the information in one channel or a subset of channels. 
     For example, in applications such as next-generation-Internet, a fiber optical cable will carry multiple wavelengths and use packet switching techniques. Reading header information on a packet, for example, could require a device that can copy the channel which contains the packet. Once copied to a specific wavelength in the optical spectrum, the information can be conveniently filtered using a fixed wavelength filter and then read (i.e., detected). Clearly, it is important for the copying process to be non destructive. 
     SUMMARY OF THE INVENTION 
     The present system teaches a technique of modifying certain channel information in a way that allows the modified channel information to be separated from the other information. 
     The preferred mode carries this out by using a non-linear medium to mix the desired channel with a pump wave that shifts the information in the desired channel to a different frequency that corresponds to a resonant frequency of a cavity or resonator. The preferred non linear medium is a semiconductor four wave optical mixer that also provides optical gain for the resonator. The system also includes an optical Bragg filter, to create resonant enhancement of only a specified frequency sifted wave. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects of the invention will now be described in detail with reference to the accompanying drawings, wherein: 
     FIG. 1 shows a block diagram of the overall system; and 
     FIG. 2 shows an optical spectrum of a preferred optical resonator system; 
     FIG. 3 shows relationship between pump power and response. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A block diagram of the overall embodiment is shown in FIG.  1 . Input fiber  100  represents the input to the system. The input includes several optical channels propagating in the same fiber. These channels are shown to include λ 1  . . . λ 4  for simplicity. However, the system may include hundreds of channels. These channels are coupled into an optical mixing device, preferably a four-wave mixing device  110  of a special type which is equipped for selective injection-locked enhancement of a resonant converted signal that is generated in the device. The resonance renders the four-wave injection device  110  wavelength selective, e.g., so that it enables modifying a specific channel. 
     The system generates a wavelength-converted replica of less than all of the channels. The preferred mode wavelength converts only a single channel, which we here define as λ 2 . 
     A preferred mode uses a fiber-Bragg-grating coupled laser diode. The grating can alternatively be a grating formed on a chip, e.g. a semiconductor substrate. The optical gain medium in this case functions as both the nonlinear mixing medium and the amplifying mechanism to enable laser action. The gain medium is a semiconductor in the preferred embodiment. A pump laser 112 produces a pumping wavelength λ p    114 . The pump tunes the four-wave mixer, to produce a four-wave mixing replica λ 2 *=λ res =2λ p −λ 2 . This value is selected to be resonant with a mode of the laser cavity formed by the Bragg grating and the back facet of the semiconductor chip—the resonance wavelength of the semiconductor optical amplifier 116 within the four-wave mixer. 
     An alternative uses an integrated tunable grating in which the resonance wavelength λ res  itself is tuned to pick up the desired converted channel, while the pump wavelength λ p  is fixed. 
     While the present system describes the special case of a single resonance at λ 2 , it should be noted that several resonances could be used simultaneously in order to remove a plurality of input channels. 
     A frequency selective tap  130  is used at the output of the converter  110  to isolate the converted signals from the original channels. FIG. 1 shows the converted signal wavelength λ 2 *=λ res  being fixed and tuned according to λ p . Hence, the frequency-selective tap  130  in this embodiment can be implemented using another fiber-Bragg-grating having a narrow reflection wavelength centered at λ res . 
     In the alternative embodiment in which the pump wavelength λ p  is fixed and tuned according to λ res , the operation could be realized with a properly-designed wavelength division multiplexer that routes all wavelengths shorter than λ p  to one output port and all wavelengths longer than λ p  to the other. 
     An optical recirculator  140  couples the optical waves as shown such that the input wave  100  is coupled to the SOA  116 , and the reflection from the SOA is coupled to the copy line  132  which is connected to the frequency selective tap  130 . 
     The result is that one out of the several wavelength-division-multiplexed channels has been dynamically selected. This one channel is converted to a new wavelength λ res . This new wavelength is far enough from the other channels to allow the one channel to be filtered. The new wavelength can be separated from the other wavelengths. The original data stream is left unaffected, i.e. the copying process in non destructive). Hence, the removed channel  145  is centered on λ res  as converted from λ 2 . The output channel  147  remains unaffected, and is shown including λ 1 . . . 4 . 
     This same system can also be used to drop channels by separating only the desired channels, down converting those desired channels, and using those new channels as the new signal. 
     Such a device would have many applications, including use as a dynamic network interface. In this case, channels from a conventional C-band WDM network are dynamically converted to long wave length L-band WDM network. By tuning the pump wavelength λ p  and the resonant wavelength λ res , the device can dynamically select the desired channels and also the converted wavelength. 
     The device  116  is preferably formed from a semiconductor optical amplifier  116  that includes a cavity operating as a semiconductor laser. One end of the laser cavity includes a high-reflection coating  118 . The other end  120  is anti-reflection coated. The other end is also coupled via a narrow band pigtailed fiber-Bragg-grating  119 . 
     The input signal  100  is assumed to be carrying digital information. Both this input signal  100  and the four-wave mixing pump wave  114  are injected into the laser-active region  117  within the semiconductor optical amplifier  116 . The pump wave  114  can be coupled using an optical circulator  139  which can be separate from or the same as the optical circulator  140 . 
     The pump wave is preferably at ω p  and ω q  respectively. These waves are selected to lie outside of the reflection band of the Bragg-grating  119  and hence are passed by the Bragg-grating into the semiconductor optical amplifier. For these waves, the semiconductor laser simply acts as a folded path semiconductor optical amplifier mixer. 
     The pump and signal frequencies are preferably also chosen so that the corresponding four-wave mixing signal frequency ω s =2ω p −ω q  lies within the Bragg reflection band. This frequency ω s  is also closely tuned to a resonance frequency of the laser cavity  117 . 
     Then, when the laser is biased near threshold, the FWM signal can injection seed the lasing mode and is correspondingly enhanced by the lasing action. While this is similar to injection locking, here the master laser beam is not directly injected. Rather, the master is generated inside the slave laser medium  117  by a non-linear interaction between the input waves. 
     This enhancement can be modeled by solving the coupled mode wave equations for four-wave mixing subject to boundary conditions. This shows that the conversion efficiency H, the ratio between the in-fiber converted signal output power to the input signal power coupled to the circulator is                η     η   |       R   BG     ⇀   0         =       1   -       R   BG          ω   s                  1   -     C            R   BG          (     ω   s     )              G        (     ω   s     )                 ΔΦ        (     ω   s     )                  2               (   1   )                                
     where R BG  is the power reflectivity of the Bragg mirror, C is the fraction of light that is coupled from the fiber into the SOA (and vice versa), and G and ΔΦ are the net gain and phase shift respectively experienced by the field amplitude in a round trip in the SOA. The reflectivity of the high-reflection coated facet is taken to be unity. Furthermore, η|R BG→0  denotes the FWM conversion efficiency of the same device under identical operating conditions but without the FBG, so that Eq. (1) is a measure of the performance enhancement associated with the optical feedback. In the case of standard laser operation, this same factor gives the ratio of the output power of the laser to the amount of spontaneous-emission noise power emitted into the lasing mode. At threshold, where the gain nearly equals the losses, and for ω s  close to a resonance frequency of the laser cavity, it becomes very large, resulting, in the present case, in a strong resonance enhancement of the FWM conversion efficiency (as well as signal-to-noise ratio). This mechanism is different from injection locking which has recently been considered for several all-optical switching applications. Here, the “master” laser beam is not directly injected, but rather is generated inside the “slave” laser cavity by the FWM interaction between the input waves. 
     FIG. 2 shows a typical optical spectrum measured at the output of the device at resonance. The wavelength downshift is 4 nm and a total optical power of about 4 dBm is input into the SOA. This data shows that conversion efficiency and signal-to-noise ratio are greatly enhanced. 
     In fact, taking into account the amplification experienced by the input signal in the folded-path SOA, the conversion efficiency is close to unity in this case (near threshold the fiber-to-fiber gain is approximately equal to 1/R BG ≈6 dB). The measured variation of the conversion efficiency with the FWM signal frequency is illustrated in FIG. 2, where several resonances, associated with several modes of the external cavity, are clearly seen. The spectral width of these resonances (the locking range) is found to be quite small, less than 2 Ghz for all the peaks in the figure. 
     The bandwidth of the injection-locking resonances, and hence the maximum bit rate, can be increased by modification of the laser cavity. In order to see how this can be achieved, it is convenient to recast the conversion efficiency enhancement of Eq. (1) in terms of directly measurable parameters. For ω s  close to any resonance frequency (as will be assumed from here on), Eq. (1) is well approximated by a Fabry-Perot transmission peak with FWHM δν L  (a measure of the locking range) and maximum value E max  (the peak enhancement in the FWM conversion efficiency) given by                        δ   vL     ≈                      Δ                 v     π        1     -            Γ                 A                 Δ                 NL              Γ                 A                 Δ                   NL   /   2               ,                 E     m                 a                 x       ≈                  1   -       R   BG          (     v   th     )             (     1   -          Γ                 A                 Δ                 NL         )     2                     (   2   )                                
     where A is the material differential gain, r is the confinement factor, L is the SOA length, ν th  is the resonance frequency under consideration at threshold, and Δν=c/(l BG (ν th )+2L) is the cold-cavity free spectral range near ν th  (l BG  denotes the effective optical path length from the antireflection coated facet of the SOA into the Bragg mirror and back to the same facet). Finally, ΔN (&lt;0 under injection-locking conditions) is the deviation of the steady-state carrier density from its value at threshold, given by                Δ                 N     =           (     I   -     I   th       )     /     (     qV   a     )       -       g   th            P   _     /     (     Γℏω                   S   m       )               1   /     T     s                 ρ         +     A          P   _     /     (     ℏω                   S   m       )                     (   3   )                                
     where I is the laser bias current, V a  is the volume of the active region, S m  is the modal cross-sectional area, T sp  is the spontaneous and nonradiative recombination lifetime, g th  is the modal gain coefficient at threshold (g th =γ+1/L log(1/(C sqrt (R BG )), γ being the SOA scattering loss coefficient, and {overscore (P)} is the average optical power in the SOA. 
     Under the present experimental conditions, the contribution to {overscore (P)} from the mixing signal power can be neglected, since it is considerably smaller than the pump contribution. This has the important consequence that the entire locking range predicted by the present steady-state analysis is stable against small carrier density fluctuations otherwise, in the context of standard injection-locking, if the laser gain is saturated by the injection-locked beam itself, a fraction of the steady-state locking range will actually become unstable for nonzero line width enhancement factor. The average power {overscore (P)} can be easily related to the in-fiber input power Pn using the FWM coupled-mode equations; the result is                P   _     =         P     i                 n         CR   BG                     2      Γ                 A                 Δ                 NL            C   2          R   BG           (       g   th     +     Γ                 A                 Δ                 N     -   γ     )        L                 (   4   )                                
     (here R B  is evaluated at the frequency of the lasing mode in the absence of externally injected waves). ΔN can then be expressed as a (implicit) function of P in  by eliminating {overscore (P)} between Eqs. (3) and (4). 
     A first observation that follows from Eq. (2) is that the locking range increases with increasing ΓA|ΔN|L; the FWM signal enhancement, however, correspondingly undergoes a (larger) decrease. To illustrate this point, we plot in FIG. 3 δ VL  and E max /E min  (the peak-to-valley ratio of the resonance, given by ((1+e ΓAΔNL ) versus the input power (|ΔN| increases with increasing P in  as confirmed by Eqs. (3) and (4)). The circles in the figure are experimental values (corresponding to the resonance peak centered at the lasing frequency); the continuous lines are theoretical fits based on the expressions above, with all the relevant parameters as listed in the caption. The quantity ΓA|ΔN|L should then be optimized to achieve the largest possible locking range for which the FWM conversion efficiency remains larger than a given threshold (e.g., large enough to support error-free wavelength conversion in a given link). As clearly indicated by FIG. 3, this can be simply accomplished by varying the input pump power (and any change in any other laser parameter involved in the expression for ΓA|ΔN|L will cause a change in this optimum input pump power, but will not otherwise affect the maximum bit rate at which the wavelength converter can operate). 
     Instead, the maximum bit rate can be increased by increasing the free-spectral range Δ ν  (in particular by minimizing the optical path length in the fiber l BG , which, in the present device, accounts for most of the measured 7 Ghz free spectral range), since the locking range increases linearly with it, while the conversion efficiency is not affected. In practice this means, for instance, writing the grating as close to the lensed fiber tip as possible, and reducing its length (the corresponding decrease in reflectivity, while increasing the laser threshold, is if anything advantageous for this application since it increases E max . In particular, based on the present experimental results, if Δ ν  could be increased by a factor of three, the device would become well suited to operation at 2.5 Gbit/sec, the prevalent bit rate per channel currently used in telecom systems. On the other hand, for operation at 10 Gbit/sec and faster, more dramatic design innovations are likely required, such as, for instance, using an on-chip DBR to provide the optical feedback. 
     Although only a few embodiments have been described in detail above, other embodiments are contemplated by the inventor and are intended to be encompassed within the following claims. In addition, other modifications are contemplated and are also intended to be covered.