Abstract:
Apparatus for wavelength conversion includes a first optical be splitter, a first leg and a second leg each splitting from the first optical beam splitter, a first optically passive nonlinear device disposed in the first leg, a second optically passive nonlinear device disposed in the second leg and a second optical beam splitter joining the two legs beyond the optically passive nonlinear devices. An unmodulated optical signal splits between the two legs. When no other beam is in the second leg, all optical signals exit the second beam splitter at a first port. When another (modulated) beam is imparted to the optically passive nonlinear device in the second leg, an imbalance is created at the second beam splitter such that some portion of the unmodulated optical signal exits at a second port of the second beam splitter in a modulated manner.

Description:
FIELD OF INVENTION 
     This invention relates to optical transmission systems and, more particularly, to devices which perform wavelength conversion of optical signals from a first wavelength to a second wavelength. 
     BACKGROUND OF INVENTION 
     Optical devices such as wavelength shifters and wavelength converters have been used to “change the wavelength” of an optical data signal in optical transmission systems. This change in the wavelength is more properly exemplified as a transfer of data or information from a carrier signal at a first wavelength to a different carrier signal at a second wavelength. Wavelength shifters or converters are integrated into lightwave transmission networks as such networks employ wavelength division multiplexing and wavelength routing of optical signals. The shifters or converters help to overcome the capacity limitations of such networks by rearranging (wavelength channel interchange) and reallocating the optical wavelength channels for efficient use of the limited optical bandwidth of the network. 
     Optical wavelength conversion has been demonstrated in the past by using a traveling wave semiconductor optical amplifier (SOA) which performs the conversion on intensity modulated optical signals by employing either four-wave mixing or the gain saturation effect. These devices suffer from common shortcomings. The signal extinction ratio (a ratio of the high level of a signal in the optical transmission system divided by the low level of said signal) on the converted signal and/or the conversion efficiency are neither very high nor attractive over a wide range of wavelengths without the expenditure of relatively high input signal power. 
     Semiconductor optical amplifiers have been used more recently in both Mach-Zehnder and Michelson interferometers to provide wavelength conversion. Amplifiers were arranged in both legs of each interferometer. These configurations exploit the phase shift caused by the refractive index variation associated with gain saturation in the optical amplifiers. As such, the interferometers transfer attendant phase modulation into an amplitude (intensity) modulated signal. This technique apparently operates at low power while improving the signal quality of the converted signal with respect to extinction ratio and chirp. However, both techniques employ SOAs which require the conversion of the optical signal into an electrical signal. Such conversion results in signal degradation or loss that is inherent in the physical devices that perform the conversion. The conversion devices also add cost, complexity and electrical delays in the systems in which they are integrated. 
     SUMMARY OF THE INVENTION 
     These and other deficiencies of the prior art are overcome according to the principles of the invention in an apparatus for wavelength conversion that includes a first optical beam splitter, a first leg and a second leg each splitting from the first optical beam splitter, a first optically passive nonlinear device disposed in the first leg, a second optically passive nonlinear device disposed in the second leg and a second optical beam splitter joining the first leg and second leg beyond the first and second optically passive nonlinear devices. The first and second optically passive nonlinear devices are slabs of nonlinear optical media and in one embodiment of the invention they are Kerr media. 
     The apparatus additionally has a first mirror in the first leg and a second mirror in the second leg. The first mirror is disposed between the first beam splitter and the first optically passive nonlinear device; the second mirror is disposed between the second optically passive nonlinear device and the second beam splitter. 
     The apparatus also has an optical source connected to the first beam splitter. The optical source generates an unmodulated optical signal. A condition is established in the apparatus wherein the unmodulated optical signal is split between the first leg and the second leg and where no other beam is in the second leg, all optical signals exit the second beam splitter at a first port. A second condition is established wherein the unmodulated optical signal is split between the first and second leg and where another beam is imparted to the optically passive nonlinear device in the second leg, an imbalance is created at the second beam splitter such that some portion of the unmodulated optical signal exits at a second port of the second beam splitter in a modulated manner. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1 depicts a schematic diagram of a wavelength shifting device in accordance with the subject invention; 
     FIG. 2 depicts a schematic diagram of an optical switch incorporating the wavelength shifting device in accordance with the subject invention; and 
     FIG. 3 depicts a schematic diagram of a second embodiment of the optical switch incorporating the wavelength shifting device in accordance with the subject invention. 
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The subject invention will be primarily described within the context of an optical wavelength shifting device which may be used in wavelength division multiplexed (WDM) and dense WDM (DWDM) optical communication systems. However, it will be appreciated by those skilled in the art that the invention may be advantageously employed in any optical communications system in which it is desirable to minimize components and costs in such systems when optical-electrical-optical conversion of signals in the system is not desirable. 
     FIG. 1 depicts a schematic diagram of an optical wavelength shifter in accordance with the present invention. The shifter  100  is a device adapted for receiving an unmodulated signal of a second carrier wavelength, a modulated signal of a first carrier wavelength, and outputting the information encoded on the first carrier signal onto the second carrier signal. 
     Wavelength shifter  100  includes a first beam splitter  104  adapted for connection to an unmodulated optical beam source  102 . Unmodulated optical beam source  102  provides the second carrier wavelength upon which the data will eventually be modulated upon. A first optical communication path or leg  106  of the shifter  100  extends from first beam splitter  104  and acts as a control or balance leg for the shifter  100 , which is described in greater detail below. The first leg  106  includes first nonlinear media  110  in optical communication with the first beam splitter  104  via a first optical fiber  124   1 . Although the interconnection of elements in the subject wavelength shifter  100  is described by optical fibers  124   n , one skilled In the art will appreciate alternate methods of interconnections including using free space between each element with appropriate input and output couplers. The first nonlinear material  110  is subsequently in optical communication with a second mirror  116  via a second optical fiber  124   2 . Finally, first leg  106  terminates with second mirror  116  in optical communication with a second beam splitter (acting as a coupler or combiner)  118  via a third optical fiber  124   3 . 
     A second optical communication path or leg  108  extends from first beam splitter  104  and acts as a modulated leg. Second leg  108  includes a first mirror  114  in optical communication with first beam splitter  104  via fourth optical fiber  124   4 . First mirror  114  is subsequently in optical communication with second nonlinear material  112  via a fifth optical fiber  124   5 . The nonlinear material of the second nonlinear material  112  is substantially similar to the first nonlinear material  110 . In one embodiment of the invention, nonlinear media  110  and  112  are Kerr media; however, one skilled in the art will realize that any type of optically nonlinear material displaying the desired characteristics may be substituted for Kerr media. For example, “engineered” materials, such as those created through CVD, MBE or other techniques, (possibly involving quantum dots), that have suitable optical characteristics, especially a nonlinearity exceeding those of existing known materials may be used in place of the Kerr media. Nonlinear materials such as organic liquids, organic glasses (such as Se-based chalcogenide glasses) are acceptable materials. Second leg  108  continues from second nonlinear material  112  via a sixth optical fiber  124   6  and is in optical communication with second beam splitter (coupler/combiner)  118 . One skilled in the art will realize that there are various configurations that can achieve the desired results. For example, the first and second legs  106  and  108  may be arranged substantially parallel to each other thus eliminating the need for first and second mirrors  114  and  116 . The invention can be physically realized by incorporating the shifter  100  on a substrate (in one example, an InGaP substrate). In one embodiment, the substrate is micro-machined, and miniature cubes of nonlinear material are inserted and cemented in place. That way a micro-optical circuit hybrid can be fabricated out of integrated waveguides and cubes or beamsplitters of other nonlinear materials. 
     Second beam splitter  118  is so configured and adapted to receive input information from first leg  106  and second leg  108  and is also provided with two output ports: first output port  120   1  and second output port  120   2 . Depending upon the state of the wavelength shifter, output signals will be available at by either first output port  120   1  or second output port  120   2  as described in greater detail below. Second nonlinear material  112  is so adapted and configured so as to receive an input modulated optical signal of a first carrier wavelength. That is, the data that is to be transposed from the first carrier wavelength to the second carrier wavelength is carried by input modulated beam B 1  for eventual superposition or modulation upon unmodulated optical beam B 2  from unmodulated optical source  102 . 
     The wavelength shifter  100  comprises, illustratively, a Mach-Zehnder interferometer. The interferometer has a nonlinear characteristic by virtue of the first and second nonlinear media  110  and  112 , respectively. When the interferometer is properly balanced, the unmodulated optical input beam B 2  from optical source  102  exits the wavelength shifter  100  at second output port  120   2  of second beam splitter  118 . That is, when the interferometer is balanced, no light exits through the first port  1   20 , of second beam splitter I 1   8 . For example, when the input modulated beam B 1  is off (denoting a “0”) in a string of ones and zeros of digital data, the interferometer is balanced and no light exits from first output port  120   1  (i.e., beams B 1  and B 2  constructively interfere with one another 100%). When the input beam B 1  is on (denoting a logical “1” of digital data), a phase shift is induced in second nonlinear media  112  by beam B 1  interacting therewith. The phase shift unbalances the interferometer, thus causing light from the unmodulated optical source  102  to exit the first output port  120   1  of second beam splitter  118  (i.e., the constructive interference is reduced to less than 100%). It is noted that the maximum amount of light passing from the first output port  1   20 , corresponds to approximately a half wavelength of the beam B 2 . As such, the modulated input optical beam B 1  controls the phase shift in the second leg  108  of the nonlinear interferometer (wavelength shifter  100 ) causing the unmodulated input beam B 2  traveling through the first leg  106  to be modulated. The newly modulated beam MB 2  then exits the interferometer as a copy of the modulation scheme seen in the originally modulated input optical beam B 1 . 
     The benefits realized by the subject invention are readily revealed to those skilled in the art and informed by the teachings of the present invention. Specifically, since there is no conversion of the optical signals to electrical signals such as is done in semiconductor optical amplifier shifter type devices, signal degradation or loss during the conversion process is eliminated. Additionally, the bandwidth of operation can be very large (on the order of several hundred nanometers) as long as the nonlinearities are “matched” in the two legs of the interferometer. Another advantage of the invention is that the output (newly modulated signal) is strong, even with a relatively weaker input signal (which may create a smaller induced optical phase shift, as long as the unmodulated optical source (source providing optical beam B 1  in the specific description) is relatively strong. 
     One specific application for the subject invention is to create an all optical switch having dense wavelength division signals (DWDM) as inputs and outputs. The switch allows for cross connection of signals from any wavelength of any input signal to any wavelength of any output signal. As such, the application and scaling of such a device is powerful (in its broad application) yet reasonable as it does not unduly add cost or complexity or additional electrical components to a system using such a high volume switch. For example, FIG. 2 shows one embodiment of such an all optical switch  200 . The wavelength shifter  100  is adapted to receive the modulated input optical beam B 1  as previously described. Additionally, the wavelength shifter  100  is also adapted to receive a plurality of unmodulated input beams B 2   1 , B 2   2  . . . B 2   n  from a plurality of unmodulated optical beam sources  102   1 ,  102   2  . . .  102   n . Each such unmodulated optical beam source  102   n  is connected to the wavelength shifter  100  via a corresponding optical source switch mirror  202   n . That is, depending upon which unmodulated beam is desired to become the output modulated beam, a corresponding switch  202   n  is activated to bring the beam B 2   n  into directional alignment with the wavelength shifter  100  via input mirror  204 . In one embodiment, the mirror switches  202   n  are moveable micromirrors oriented in an arrayed fashion as shown in FIG.  2 . Other commonly employed methods for selective wavelength introduction into an optical opponent are also possible to those skilled in the art. The wavelength shifter  100  operates in an substantially similar method to that described above once the desired unmodulated beam B 2   n  is selected thereby outputting a modulated beam B 2   n  corresponding in wavelength to unmodulated beam B 2   n  and having the modulation characteristics of input modulated beam B 1 . 
     One skilled in the art can also appreciate that such an optical switch  200  can have an advanced configuration to accept multiple input modulated beams B 1   n  to affect and even higher switching capability than the embodiment shown in FIG.  2 . Specifically, FIG. 3 shows an optical switch  200  having a plurality of input modulated beams B 1   n  controlled by a corresponding plurality of input beam controlled mirrors  206   n  with the remaining portion of the switch  200  remaining substantially similar to that described in FIG.  2 . In this manner, it is easily seen that any desired input modulated beam B 1   n  (on any one of a desired number of corresponding wavelengths) may be selected to be shifted to any one of a number of unmodulated beams B 2   n  (having a different wavelength). The resultant output modulated beam MB 2   n  contains the modulated characteristics of the input beam B 1   n  modulated on the wavelength of the previously unmodulated input beam B 2   n . In either of the embodiments seen and described in FIGS. 2 and 3, optical mirror switches  202   n  and  206   n  can be controlled by any means known to those skilled in the art and, in one example, may be computer controlled based on the desirability of switching to various wavelengths to increase usable bandwidth or resolution of a particular portion of a bandwidth (channel) to establish a highly effective and dynamic tool for optical signal communication systems. 
     Although various embodiments that incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.