Optical switch and gate apparatus and method

Some exemplary embodiments of one version of the present invention provide an optical switch including: a splitting device having first, second, third and fourth terminals; a nonlinear element; an attenuator; and an optical loop associated with the third and fourth terminals, the optical loop including the attenuator and the nonlinear element, the nonlinear element being displaced from a mid-point of the optical loop, wherein the splitting device is able to receive an input signal via one of the first and second terminals and an activating signal via one of the first and second terminals and to provide an output signal at one of the first and second terminals in response to the activating signal. In exemplary embodiments of alternative versions of the present inventions, the optical switch may include a threshold device. In some versions, the optical switch may be operated with a continuous wave or a pulse based activating signal.

FIELD OF THE INVENTION

The invention relates to optical communication devices and systems and, more particularly, to optical gates and switches.

BACKGROUND OF THE INVENTION

In the field of optical communication, there is a need for fast transmission of large volume information. The optical fibers used today to carry the information in optical communication networks may be efficient in carrying large volume of information at high rates. Unfortunately, the transmission rate of the optical communication networks is not determined by the transmission rate of the optical fibers; rather, the transmission rate is determined by the switching rate of the switching devices in the networks. The switching devices used today are electronic switches that operate at relatively slow rates and are not fast enough to switch the information at higher desired rates, which can be carried by the optical fibers. Accordingly, the electronic switches used today are the bottleneck of the existing optical communication networks. In addition, the use of electronic switches requires the integration of Optical-Electrical-Optical (O-E-O) converters that are expensive.

SUMMARY OF THE INVENTION

It is an object of some exemplary embodiments of the present invention to provide All-Optical gates and switches.

It is another object of some exemplary embodiments of the present invention to provide fast All-Optical gates and switches.

It is yet another object of some exemplary embodiments of the present invention to provide All-Optical gates and switches that are activated by electronic signals.

It is still another object of some exemplary embodiments of the present invention to provide All-Optical gates and switches that are activated by optical signals.

In one version, exemplary embodiments of the present invention provide an optical switch including:

a threshold device having first, second and third terminals, the first terminal including a nonlinear element and a coupling device for coupling an activating signal from a fourth terminal into the first terminal,

wherein the threshold device is arranged to receive an input signal at the first terminal and to produce an output signal at one of the second and third terminals in response to the activating signal.

In an alternative version, exemplary embodiments of the present invention provide an optical switch including:

an input and first and second outputs;

first and second signal guiding branches, each branch having first and second terminals, at least one of the first and second branches comprising a non-linear optical element and one of the first and second branches including a coupling device for coupling an activating signal from an activating terminal into one of the first and second branches;

first and second optical couplers, at least one of the first and second optical couplers comprising an asymmetric optical coupler, the first optical coupler configured to split an input signal from the input into a first signal portion propagating through the first branch and a second signal portion propagating through the second branch,

wherein the second optical coupler configured to combine the first and second signal portions from the second terminals of the first and second branches, respectively, thereby to produce an output signal at one of the first and second outputs in response to the activating signal.

In an alternative version, exemplary embodiments of the present invention provide an optical switch including:

a splitting device having first, second, third and fourth terminals;

a nonlinear element;

an attenuator; and

an optical loop associated with the third and fourth terminals, the optical loop including the attenuator and the nonlinear element, the nonlinear element being displaced from a mid-point of the optical loop,

wherein the splitting device is able to receive an input signal via one of the first and second terminals and an activating signal via one of the first and second terminals, and to provide an output signal at one of the first and second terminals in response to the activating signal.

In an alternative version, exemplary embodiments of the present invention provide an optical switch including:

a splitting device having first second third and fourth terminals;

a nonlinear element;

a coupling device; and

an optical loop associated with the third and fourth terminals, the optical loop including the nonlinear element, displaced from a mid-point of the optical loop, and the coupling device, which is able to produce loss in the optical loop and to couple a continuous wave activating signal from an activating terminal into the nonlinear element,

wherein said splitting device is arranged to receive an input signal via one of said first and second terminals and to provide an output signal at one of said first and second terminals in response to said continuous wave activating signal.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1aschematically illustrates a graph5000having coordinates of output intensity Io and output relative phase change Δφ versus input intensity Ii. Graph5000depicts ideal and practical transmission curves5002and5004, respectively, illustrating the relationship between output and input intensities, Io and Ii, respectively, of a nonlinear medium, e.g., a Non-Linear Element (NLE) such as, for example, an optical amplifier, an Erbium Doped Fiber Optic Amplifier (EDFA), a Solid state Optical Amplifier (SOA), a Linear Optical amplifier (LOA), an optical limiter, or any other suitable nonlinear device or material. Curve5006schematically illustrates the relationship between the output phase change Δφ and the input intensity Ii in optical devices such as, for example, the above-mentioned amplifiers, limiters, or nonlinear media.

As shown inFIG. 1a,curve5004has a linear region5008, a nonlinear knee region5010, and a quasi-flat saturation region5012. For relatively low level input signals Ii, in range5008, the corresponding output signals Io are substantially linearly proportional to the input signal Ii. For intermediate levels of input signals Ii, e.g., in range5010, the output signals Io are no longer linearly proportional to the input signals. For relatively high-level input signals Ii, e.g., in the range5012, the output signals Io are saturated, generally fixed, and independent of the intensity of the input signals Ii.

Curve5006shows a phase change Δφ, which may correspond to a change of the refractive index ΔN, at the output of the non-linear device. The phase change Δφ depends on the change of the refractive index ΔN, the wavelength λ, and the length of the amplifier/limiter L. The phase change may be given by:
Δφ=2·π/λ·ΔN·L(1)

Thus, for fixed values of wavelength λ and length L, the phase change Δφ may be linearly proportional to the change of the refractive index ΔN.

At the range of low-level input signals, the output phase change Δφ depends linearly on the input signals Ii as indicated by range5014, which corresponds to intensity range5008. At the range of medium level input signals, the change of Δφ is a sub-linear function of the input intensities Ii, as indicated by range5016which corresponds to intensity range5010. At the range of relatively high input signals, the output phase shift Δφ is saturated and is almost fixed and does not depends on the input intensities Ii, as indicated by range5018, which corresponds to intensity range5012.

FIG. 1bschematically re-illustrates transmission curve5004ofFIG. 1a,where with exemplary output signals Io versus input signals Ii are indicated, as well as curve5006ofFIG. 1a,where exemplary output phase changes Δφ versus inputs signals Ii are indicated.FIG. 1bfurther illustrates the relationship between exemplary input signal patterns,5020and5028, and their corresponding output signal patterns,5020A and5028A. In analyzingFIG. 1bandFIG. 1cfor two different types of input signals, namely, low-level input signals within the linear range of the NLE (e.g., ranges5008and5014ofFIG. 1a) and high-level input signals within the saturation range of the NLE (e.g., ranges5012and5018ofFIG. 1a), the following observations are made:

Input signal pattern5020is a low level input signal and the pulses of signal5020(i.e., pulses5022and5026and pulse5024), having intensities Ii1and Ii2, respectively, are within range5008(or5014) ofFIG. 1a.Thus pulses5022,5024and5026are transmitted linearly according to curve5004, resulting in output signal pattern5020A having intensities Io1and Io2, respectively. The pulses of signal5020A (i.e., pulses5022A,5024A and5026A) are also within the linear range5014(or5008) ofFIG. 1aand are, thus, transmitted linearly according to curve5006. As shown inFIG. 1b,the lower amplitude pulses5022A and5026A have a phase shift Δφ1and the higher amplitude pulse5024A has a phase shift of Δφ2. Since the pulses5022A,5024A and5026A are all with low amplitudes, the phase shifts Δφ1and Δφ2are both very small. The difference Δφ1-Δφ2is even smaller and may be ignored for the purpose of the present invention. Accordingly, for the purpose of the present invention, the pulses5022A,5024A and5026A of pattern5020A may be considered to have substantially the same phase shift Δφ.

Input signal pattern5028represents an intensity amplification of signal pattern5020. The pulses of signal5028(i.e., pulses5030and5034and pulse5032), have intensities Ii3and Ii4, respectively, and are within the high level, i.e., saturated, intensity range5012(or5018) ofFIG. 1a.Thus, pulses5030,5032and5034are transmitted according to curve5004with quasi-equal intensities Io3and Io4, and quasi-equal phase shifts Δφ3and Δφ4, resulting in output pulses5030A,5032A and5034A, respectively, of output signal pattern5028A.

FIG. 1cschematically illustrates a graph similar to that ofFIG. 1b,showing the same input and output patterns5020and5020A; however, instead of amplified pattern5028,FIG. 1cillustrates transmission of an input pattern5029, which is produced by a lower amplification of input pattern5020than that of pattern5028. Due to the lower amplification of pulse pattern5020, only the higher amplitude5033of pattern5029has an intensity Ii4in the saturated region5012(or5018) ofFIG. 1a.However, the intensity Ii3of the other amplitudes, namely, the intensity of amplitudes5031and5035, is within the linear region5008(or5014) ofFIG. 1a.Accordingly, the non-linear device applies a lower effective amplification factor to amplitude5033compared to the amplification factor applied to amplitudes5031and5035, and results is larger phase difference, Δφ4-Δφ3, between the output pulse5033A and output pulses5031A and5035A of output pattern5029A, respectively.

I. Optical Threshold Devices Using an Adaptation of a Non-Linear MZI

FIG. 2aschematically illustrates a threshold device5040according to exemplary embodiments of one aspect of the present invention. The device illustrated inFIG. 2amay include a continuous sequence of optical components connected by light guiding media such as, for example, optical fibers, planar waveguides, or planar circuits (PLC) that may be fabricated using integrated optic techniques and/or on-chip manufacturing. Alternatively, device5040may be constructed from discrete components, in which case the optical fibers may be replaced by open space and the directional couplers, discussed below, may be replaced by beam splitters. A low level input pulse5042may propagate through input terminal5044of an asymmetric directional coupler5046having an amplitude splitting ratio of 1:m , wherein m may be any positive number). Coupler5046may split pulse5042into two pulses,5042aand5042b,which may propagating in separate output branches,5048and5050, respectively. The normalized amplitudes of pulses5042aand5042bin branches5048and5050are thus m and 1, respectively, in relative units as defined herein. Pulse5042amay propagate through phase shifter5052and may enter a directional coupler5060via an input branch5056. Pulse5042bmay propagate through amplifier5054and may enter coupler5060via an input branch5058. Phase shifter5052may be adjusted to produce a phase shift Δφ to ensure that pulse5042adestructively interferes with pulse5042bat an output port5062of coupler5060. The amplitude gain G of amplifier5054may be adjusted to maintain an amplitude magnitude of pulse5042b,at input branch5058of coupler5060, that will cause pulses5042aand5042bto null each other by the destructive interference between them at output port5062of coupler5060.

The phase shift Δφ produced by phase shifter5052may ensure that pulses5042aand5042benter coupler5060with a phase difference of π/2 radians. This means that Δφ may compensate for the differences in optical paths caused by the differences between branches5048and5050, the terminals of coupler5046and5060, and the phase shift of amplifier5054, which may include a SOA, LOA, or EDFA, as are known in the art, such that the relative phase between pulses5042aand5042bat output port5062of coupler5060will be π radians. At the same time, input ports5058and5056of combiner5060contribute their amplitudes to output port5062in a ratio of 1:n, wherein n represents any positive number, respectively, to produce equal amplitude pulses with opposite phases. When the required conditions for Δφ and the amplitudes are maintained, the amplitude at port5062may be given by:
I5062=1×G−m×n=0  (2)

To assure that I5062will be zero, the amplification G of amplifier5054should be equal to mxn when n is the splitting/combining ratio of coupler5060. Accordingly, in embodiments of the invention, both couplers5046and5060may be asymmetric couplers, wherein m, n≠1 and mxn=G). Alternatively, one of couplers5060and5046may be an asymmetric coupler while the other coupler may be a symmetric coupler, wherein either n=1 and m≠1 or m=1 and n≠1 and mnx=G. For example, when coupler5060is a symmetric coupler (i.e., n=1), gain G may be equal to m.

To compensate for possible changes in the relative phases of pulses5042aand5042bin coupler5060due to influence by external parameters, for example, environmental temperature changes, the relative phase may be controlled by a closed loop5070that may control phase shifter5052to maintain the proper phase shift Δφ. A coupler5072may tap a fraction of the intensity from port5062into optical guide5064, which may transmit the tapped light to a controller5066, which may monitor the tapped light and produce a corresponding electronic control signal that may be sent via lead5074to electrode5068. The electronic control signal may be used as feedback for adjusting phase shifter5052. For the range of low-level input signal5042, the output signal at port5062should be substantially zero. A substantially zero-level output may be maintained by closed loop control5070by adjusting shifter5052using controller5066.

In embodiments of the invention, closed loop5070maintains the desired steady state phase relationship between the signals at ports5056and5058, respectively. The response time of closed-loop phase control5070may be considerably longer than the time duration of the signals propagating in device5040and thus, the dynamic influence of loop5070on the phases of these signals may be negligible. To maintain the above mentioned steady-state conditions by sampling short-duration optical signals, controller5066may monitor and average the tapped light, e.g., by integration over a predefined range, producing an electronic control signal corresponding to the average of the optical signals, as tapped, arriving at optical guide5064from coupler5072.

In the range of low-level input signals, the change of the phases produced by amplifier5054is small and there is no change in the amplifier gain G. This means that while gain G and phase shift Δφ of threshold device5040may be adjusted to produce a zero-level output signal for inputs at a certain low level amplitude, the amplifier actually maintains an output signal level of substantially zero in a range of low-level input intensities that includes the specific intensity for which device5040is adjusted to produce the zero-level signal. The range of low-level input intensities may be defined as the range of amplitudes below a certain amplitude level for which the threshold device may be designed to yield substantially zero-level output signals.

The magnitude of the amplitude for which the threshold device is designed to yield a zero-level output may be determined by the values of gain G and phase shift Δφ. For amplitudes significantly higher than the above discussed low-level inputs, as discussed below with reference toFIG. 2b,gain G may be reduced to a saturated value Gsatand the phase shift Δφ may be increased to a saturated value Δφsat, i.e., the requirement for Equation 2 above are not fulfilled. Instead, in the range of high-level input signal, device5040may transmit the signals at a non-zero output level, which may be given by:
I5062=1×G−m×n≠0  (3)

Thus, the gain G and the phase shift Δφ may control the “turn on” point of the threshold device. The “turn on” (e.g., threshold) point may be defined as a point on the axis of input amplitudes (intensities) at which the transmission function of the threshold device, i.e., the output signal as a function of the input signal, begins to increase sharply.

FIG. 2billustrates threshold device5040, as inFIG. 2a,but describes operation of device5040for both low and high level ranges of input signals that may be carried by input pulse pattern5029. The input pattern signal5029may be as illustrated inFIG. 1c,i.e., it may include lower level pulses5031and5035with magnitudes within the linear range of amplifier5054and a higher-level pulse5033with a magnitude in the saturation range of amplifier5054. Lower level pulses5031and5035of input pattern5029may have amplitudes substantially the same or similar to the amplitude of pulse5042inFIG. 2a.Accordingly, as explained above with reference to pulse5042ofFIG. 2a,there would be substantially no output signal at port5062of device5040in response to input pulses5031and5035. It will be appreciated that the above discussion relating to lower level input pulse5042is also applicable to lower level input pulses5031and5035inFIG. 2b.

In contrast to the low-level pulses, pulse5033may be split by coupler5046into two pulses,5033aand5033b,propagating along branches5048and5050, respectively. The amplitude of pulse5033amay be about in times higher than the amplitude of pulse5033b;however, the amplitude of pulse5033bis still in the saturation range of amplifier5054. As explained above, in the saturation range, the gain Gsatof amplifier5054may be much lower than gain G in the linear region. This means that, in the range of high-level input signals, the ratio between the amplitudes of pulses5033dand5033c,carried by input branches5058and5056of coupler5060, respectively, may be much smaller than the ratio between these pulses in the range of low-level input signals. Accordingly, in contrast to the ratio maintained between pulses5033dand5033cto substantially null the output signal at port5062for the low-level input signals, the ratio between pulses5033dand5033cfor the high-level input signals may be changed to a value which results in a significantly non-zero output signal at port5062. In addition, the phase shift produced by amplifier5054in the saturated region may be much higher than the phase shift produced by the amplifier in the linear region. It can be seen from Equation 1 that the phase difference between pulses5033cand5033dat inputs5056and5058of coupler5060, respectively, may be reversed, e.g., from the value of π/2 radians for low-level signals to a value of −π/2 radians for the high-level signals, by appropriate selection of the length L of amplifier5054. The phase difference between pulses5033cand5033dat inputs5056and5058of coupler5060may also be adjusted by adjusting the excitation level of amplifier5054, which may determine the saturation level of the amplifier. Changing the polarity of the relative phase shift between pulses5033cand5033d,from a positive value at low-level signals to a negative value at high-level signals, results in a change from destructive interference to constructive interference, respectively, between pulses5033cand5033d,at port5062. This means that for low-level input signals, the output signals at port5062may “cancel out” by destructive interference, while the high-level input signals may interfere constructively to produce non-zero output signals at port5062. Therefore, in this case, the phase difference between the pulses at the input terminals of coupler5060may be opposite the phase difference between the same terminals in the case of lower level input amplitudes (e.g., pulse5042ofFIG. 2aor pulses5031and5035ofFIG. 2b).

It should be note that, even if the phase difference between pulses5033cand5033dis not reversed, the output signal at output port5062, i.e., the expression I5062=(1×Gsat−mxn) may not be zero because Gsatmay not be equal to mxn. In addition, the phase difference between pulses5033cand5033dmay be reversed, e.g., pulse5033dmay be drawn “upside down” relative to pulse5033c,to indicate a reverse phase polarity, as schematically illustrated inFIG. 2b.Thus, for high-level input signals, the intensity at output port5062may be produced by constructive interference, rather than by destructive interference, when operating on low amplitude level signals. Accordingly, in the case of relatively high level input signals, an output signal5082at output port5060may be significantly different from zero and may be given by: I5062=1×Gsat+mxn≠0, where Gsatis the amplitude gain at the saturated region of amplifier5054.

In embodiments of the invention, output signal5082may be further amplified to any desired intensity to produce a stronger signal, represented by pulse5084.

FIG. 2cillustrates a threshold device5041, which is an exemplary variation of the threshold device5040illustrated inFIGS. 2aand2b.In this variation, the 1:m directional coupler5046ofFIGS. 2aand2bis replaced with a symmetric directional coupler5045and the 1:m ratio between the amplitudes at branches5050and5048, respectively, may be obtained by appropriately different attenuation of the two branches, e.g., using different attenuators5092and5094, respectively.

Device5040ofFIGS. 2aand2band device5041ofFIG. 2care described in accordance with two different operational design requirements. It should be appreciated, however, that appropriate adjustment of parameter settings in device5041may produce the threshold operation described above with reference to device5040, and vice versa, as well as other threshold operations not explicitly described herein.

In device5040ofFIGS. 2aand2b,the output signals for higher level input signals are controlled by the gain and phase changes produced by amplifier5054when it is operated in the saturated region. In device5041ofFIG. 2c,in contrast, the signals for the higher-level input signals may be controlled only by the change in the gain of amplifier5054when it is operated in its a deeply saturated range.

The input pulse pattern in the embodiment ofFIG. 2cmay be of a type such as pattern5028ofFIG. 1b,i.e., of the type in which both the lower level input pulses5030and5034and the higher level input pulse5032are in the saturated range of amplifier5054. To produce such an input, an amplifier5086may be used in conjunction with a variable attenuator5088to produce an amplifier with variable gain, whereby the input gain may be adjusted to convert pattern5028into the type of pattern5021, which includes low-level pulses5023and5027and high amplitude pulse5025. After amplification and attenuation (hereinafter: “net amplification”) of input pattern5028into pattern5021, if such amplification is needed, pattern5021may be split by coupler5045into pulses5025aand5025b,propagating in branches5048and5050, respectively. In embodiments of the invention, the relative attenuations of attenuators5092and5094may be set to produce an amplitude ratio of 1:m between the signals at branches5050and5048, respectively. The pulse pattern at branch5050may pass through amplifier5054when the lower level pulses have amplitudes within the saturation region of amplifier5054. Thus, the pulse pattern may arrive at input5058of coupler5060with a gain of G′ and with, e.g., the maximum possible phase shift that amplifier5054can produce. The pulse pattern at branch5048passes through phase shifter5052and may arrive at input5056of coupler5060with a phase shift as produced by phase shifter5052, which may be adjusted to produce appropriately destructive interference between interfering pulses from inputs5056and5058at output5062. In addition, the ratio of 1:m may be adjusted such that m may be equal to G′/n. Accordingly, the output signal for lower-level input signals of device5041may be given by: I5062=1×G′−mxn=0, where n is the splitting ratio of coupler5060. For example, if coupler5060is a symmetric coupler (n=1), then G′ may be equal to m.

With higher-level input signals, such as pulse5032of pattern5028, the operation of device5041may be generally similar to its operation with lower-level input signals, except for a different gain of amplifier5054. Since higher-level pulse5025bis significantly within the saturated region, the gain of amplifier5054for this signal, G″, may be different from gain G′. However, the phase shift produced by amplifier5054for pulse5025bmay be the same as the phase shift produced for the lower level pulses, and may be the maximum possible phase shift. Accordingly, high-level pulses5025dand5025cfrom inputs5058and5056, respectively, may interfere destructively at output port5062as in the case described above of low-level pulses. However, in the case of high-level pulses, in accordance with embodiments of the invention, pulse5025dmay be amplified by amplitude gain G″, which may be significantly lower than G′, whereby output signal5082may be significantly different from zero and may be given by:
I5062=1×G″−m×n=G″−G′≠0.

Since, for higher-level input signals, device5041does not rely on phase inversion to produce an output signal5083, in such a situation, the amplitude of the output signal may be smaller than the amplitude of output signal5082discussed above with reference toFIG. 2b.Accordingly, amplifier5090may be used to enhance pulse5083and, thereby, to produce a higher amplitude signal5085.

In analogy to the control of the “turn on” point discussed above with reference to device5040, the “turn on” point of device5041may also be adjusted by varying the values of the amplifier length L, the splitting ratios m and n and the saturated level of amplifier5054, and/or by adjusting gains G′ and G″. The saturation level of amplifier5054may be varied by changing the excitation level of the amplifier, e.g., by adjusting optical pumping power in the case of EDFA and LOA, or by adjusting current injection level in the case of SOA. Accordingly, by adjusting the above mentioned parameters, e.g., the values of m, n, G′, G″, and the excitation level, it is possible to determine the amplitude for which the following equations are fulfilled:
I5062=1×G′−m×n=0 andI5062=1×G″−m×n=G″−G′≠0  (4)
The amplitude deduced from the value of G′ in Equations 4 may be defined as the “turn on” point of device5041.

Reference is now made toFIGS. 2d,3a,and3b.FIG. 2dillustrates threshold device5043in accordance with further exemplary embodiments of the present invention.FIGS. 3aand3billustrate the amplitude and phase transmission functions of a NLE (e.g., SOA, LOA, or EDFA) of device5043for two, respective, excitations levels. The threshold device5043in accordance with the embodiment ofFIG. 2dmay have a structural design generally similar to the structural design of device5041ofFIG. 2c,with the following differences. In the component structure of the device, attenuator5092ofFIG. 2cis removed and attenuator5094ofFIG. 2cis replaced by an amplifier5098. Additionally, device5043may be designed to operate in accordance with two different modes as detailed below.

In the first mode of operation of device5043, couplers5045and5060may be symmetric couplers (e.g., m=1, n=1). Amplifiers5054and5098may be generally identical; however, the excitation level (e.g., optical pumping or current injection level) of amplifier5098may be lower than the excitation level of amplifier5054. Thus amplifier5098may have a lower saturation level. The transmission functions and the saturation levels of amplifiers5098and5054are depicted denoted by symbols5100and5102, respectively. Lower input pulses5400and5037and high-level pulse5039of input signal pattern5027may be amplified and attenuated by amplifier5086and attenuator5088, respectively, to produce a variable input gain, if necessary. Lower input pulses5400and5037, which may be split by splitter5045into branches5048and5050, may be amplified and their phase may be shifted by amplifiers5098and5054. Phase shifter5052may control the phase of pulses within the range of lower level amplitudes such that the pulses enter port5056in a phase that ensures a desired destructive interference at port5062. In this design, lower-level pulses substantially cancel each other out at output port5062, resulting in a zero-level output signal from coupler5060. Higher-level input pulse5039may also be split by splitter5045into pulses5039aand5039b,propagating along branches5048and5050, respectively. Pulse5039bmay be amplified by amplifier5054to produce pulse5039d.Pulse5039amay be amplified by amplifier5098, which may have a saturation level lower than the saturation level of amplifier5054and, thus, may already be saturated at the amplitude magnitude of pulse5039a.Accordingly, the amplitude of pulse5039cthat is produced by amplifier5098is smaller than the amplitude of pulse5039dproduced by amplifier5054. The difference between the amplitudes of pulses5039dand5039cis enough to produce a significantly non-zero output signal at port5062. In addition, the phase shift of pulse5039c,which may be in the saturated region of amplifier5098, may be greater than the phase shift of pulse5039d,which may be in the linear region of amplifier5054. In this scenario, the different shifts of the phases of pulses5039cand5039dfurther enhance output signal5087, for higher level input signal, because the interference at port5062may not be perfectly destructive. Amplifier5090may be used to enhance pulse5087and, thereby, to produce a higher amplitude signal5089.

FIGS. 3aand3billustrate transmission functions of output intensity, Io, and output phase shift, Δφ, versus input intensity, Ii, corresponding to amplifiers5054and5098, respectively. Solid line5200inFIG. 3a,which corresponds to amplifier5054, illustrates the output phase shift Δφ versus the input intensity Ii with saturated and linear regions,5202and5204, respectively. Broken line5206inFIG. 3aillustrates the output intensity Io versus the input intensity Ii of amplifier5054with saturated and linear regions,5208and5210, respectively. Similarly, solid line5212inFIG. 3b,which corresponds to amplifier5098, illustrates the output phase shift Δφ versus the input intensity Ii with saturated and linear regions,5214and5216, respectively. Broken line5218ofFIG. 3billustrates the output intensity Io versus the input intensity Ii of amplifier5098with saturated and linear regions,5220and5222, respectively.

It can be seen that amplifier5054with the higher excitation has a gain slope G1that is steeper than the gain slope G2of amplifier5098with the lower excitation. On the other hand, the slope of the phase shift, K1, in amplifier5054is less steep than the slope of the phase shift, K2, in amplifier5098. This means that even if amplifiers5054and5098are designed to be identical, the different excitation levels of the two amplifiers result in different gains and different phase shifts for the two amplifiers. Accordingly, device5043may operate in a mode that produces an output signal in response to higher-level input signals, when amplifier5098is saturated and amplifier5054is not saturated, resulting in the two amplifiers having different gains and phase shifts. When device5043receives at its input5044signals in the range of lower level amplitudes, the resultant signals at branches5056and5058may cancel each other out at output port5062. However, since amplifiers5054and5098have different gain slopes, G1and G2, respectively, and different phase shift slopes, K1and K2, respectively, the resultant signals at terminals5056and5058have different gains and phase shifts, as explained above, even in the range of lower level input signals. Accordingly, while in the lower range amplifiers5054and5098compensate for each other's results, their mutual compensation may not be accurate and the signals of branches5056and5058may not completely cancel each other out at port5062to produce zero-level (or close to zero-level) signals across the range of lower level input signals.

An improvement to the performance of device5043, in a second mode of operation, may be achieved by using asymmetric couplers5045and5060to produce substantially zero-level output signals across the range of lower-level inputs. In the second mode of operation of device5043ofFIG. 2d,asymmetric couplers may be used for couplers5045and5060instead of the symmetric couplers used in the first mode of operation of the design of device5043inFIG. 2cs above.

Coupler5045may receive input signals from terminal5044and may split them at a ratio of 1:m, where the larger split portion (m) is directed toward branch5050, which leads to amplifier5054with the less steep phase shift slope K1, and the smaller split portion (1) is directed toward branch5048, which leads to amplifier5098with the steeper phase shift slope K2. The ratio 1:m may be chosen to be similar to the ratio K1:K2. Thus, the product 1·K2=m·K1may be fulfilled, thereby assuring that substantially the same phase shift would be produced by both of amplifiers5054and5098across the range of lower level input signals, at least over the amplitude range in which amplifier5098is substantially linear.

Since, under the above conditions, amplifiers5054and5098produce substantially the same phase shift across the range of lower level input signals, phase shifter5052may be adjusted to maintain the relative phase shift between the pulses at branches5056and5058such that the pulses from the two branches may interfere destructively at output port5062. However, maintaining the same phase shift for both amplifiers5054and5098requires that the smaller split amplitudes (fraction 1 from coupler5045) be directed towards amplifier5098via branch5048with the lower amplitude gain G2. At the same time, the larger split amplitudes (fraction m from coupler5045) are directed toward amplifier5054via branch5050with the higher amplitude gain G1. This means that the amplitudes with the smaller fraction (1) at terminal5056may be amplified by the smaller gain G2, resulting in significantly smaller amplitudes than the amplitudes at terminal5058that are produced from the larger split fraction (m) amplified by the larger gain G1.

To ensure that the amplitudes from terminals5056and5058are recombined with substantially equal amplitudes at output port5062, combiner (directional coupler)5060may be asymmetric with a combining ratio of 1:n, where the larger n portion arrives at port5062via branch5056and the smaller 1 portion arrives to that port via branch5058. In the range of low level input signals, the amplitude at port5062should be substantially zero and may be given by:
I5062=1·G2·n−m·G119 1=0  (5)
which may be reduced to: G2·n=m·G1

For higher-level input signals, such as pulse5039, amplifier5098may be saturated, its gain is reduced, and its phase shift is no longer equal to the phase shift of amplifier5054. This results in a significantly non-zero output signal5087at output port5062because the interference in port5062in this scenario is not completely destructive and the condition that G2·n=m·G1, derived from Equation 5, is no longer fulfilled.

From the above discussion, it is clear that the second design (mode) of device5043, using asymmetric couplers5045and5060, may be advantageous over the design using symmetric couplers because asymmetric design is clearly capable of maintaining the output signal5087at port5062at an amplitude of substantially zero across the entire range of lower level input signals.

In devices5040,5041, and5043ofFIGS. 2a-2d,the “turn on” point in both the symmetric coupler design and the asymmetric coupler design, may be adjusted by adjusting the saturation level of amplifiers5098and5045, e.g., by optical pumping or current injection. The excitation levels of amplifiers5089and5045may be different. Additional adjustable parameters that may determine the “turn on” point include gain G and the length L of amplifiers5054and5098, the splitting ratios m and n of couplers5045and5060, and the attenuation level of attenuators5088,5094and5092, which attenuation level may be different for each attenuator.

The “turn on” point of devices5040,5041and5043may actually be a threshold level. For low-level input signals, e.g., in the range below the “turn on” threshold, the output signal may be strongly attenuated by destructive interference at the output ports of the devices. This may result in a transmission function between the input and the output of the devices including a generally monotonic range with a relatively shallow slope. For high-level input signals, e.g., in a range above the “turn on” threshold, the output signal at the output port of the devices may increase sharply, whereby the transmission function between the input and the output of these devices may include a range with a steep slope.

In some embodiments of the invention, the amplitude at branch5050may be attenuated by a factor of 1/n prior to entering branch5058. In such embodiments, a symmetric (i.e., 1:1) coupler may be used instead of asymmetric (1:n) coupler5060. Similarly, in some embodiments of the invention, asymmetric coupler5045(1:m) may be replaced by a symmetric coupler with additional attenuators, in analogy to the configuration of device5041inFIG. 2cwhere symmetric coupler5045is used in conjunction with attenuators5092and5094.

In analogy to device5040ofFIG. 2a,the devices5040,5041, and5043ofFIGS. 2b,2c,and2d,respectively, may include a continuous sequence of optical components connected by light guiding media such as, for example, optical fibers, planar waveguides, or planar circuits (PLC), which may be fabricated using integrated optic techniques and/or on-chip manufacturing. Alternatively, devices5040,5041, and5043may be constructed from discrete components, in which case the optical fibers may be replaced by open space or a non-solid medium, e.g., a gas medium, and the directional couplers may be replaced by any suitable alternative components, e.g., beam splitters. It should be understood that, in embodiments of the invention, some or all of the couplers, amplifiers and/or attenuators used may include variable and/or adjusted components.

II. Optical threshold Devices Using an Adaptation of a Non-Linear Optical Loop

Reference is made toFIG. 4a,which schematically illustrates an optical threshold device, denoted5300, in accordance with exemplary embodiments of another aspect of the present invention. Reference is also made toFIG. 4b,which schematically illustrates an attenuator5314that may be used in conjunction with exemplary embodiments of the device ofFIG. 4a.The design of device5300may be beneficial because it is generally insensitive to the phase of the light signals and thus does not require a phase shifter or phase control. Device5300includes a symmetric directional coupler5305having an input terminal5309and an output terminal5307. Input5309may be directly coupled, by coupler5305, into input terminal5304of symmetric directional coupler5302. Directional coupler5302has an additional output terminal5306. Additional two terminals5308and5310of coupler5302may be connected to each other via a loop5312in a configuration similar to a loop mirror, as described below. Loop5312may include an amplifier5316and attenuator5314. Amplifier5316may include any suitable type of amplifier, for example, a SOA, LOA, or EDFA. Attenuator5314, which may be connected between connection points5313and5315on loop5312, may include any suitable type of attenuator, for example, a Variable Optical Attenuator (VOA). It should be appreciated that the attenuators and/or VOA's used in conjunction with embodiments of the present invention may be implemented in the form of any type of device that causes attenuation of signals, including devices not conventionally used for attenuation purposes. For example, in some embodiments, an attenuation function may be implemented by an optical amplifier, e.g., a SOA, a LOA, or an EDFA, excited to levels at which the amplifier absorbs rather than amplifies input signals. In some exemplary embodiments, attenuator5314may include a fixed or variable coupler5314A, connected between connection points5313and5315, as illustrated schematically inFIG. 4b.The attenuation factor of attenuator5314may be adjustable and may depend on the fraction of energy that coupler5314A may transmit between points5313and5315as well as the fraction of energy that coupler5314may couple out via a set of terminals, denoted5317and5317A. When an input pulse, such as pulse5320, is received at input5309and transmitted, via coupler5305, to input5304of device5300, the input pulse may be split by symmetric coupler5302, e.g., at a splitting ratio of 1:1, into ports5308and5310, respectively. A split pulse5330transmitted by port5310may propagate counterclockwise (i.e., in the direction of arrow5324) and its phase may be shifted, by coupler5302, π/2 radians (i.e., crossbar transmission or crossover transmission). The split pulse5328transmitted by port5308may propagate clockwise (i.e., in the direction of arrow5326) and its phase may be not be shifted by coupler5302(i.e., bar transmission).

It should be noted that if loop5312does not include a NLE component, such as amplifier5316, the pulses5330and5328that propagate counterclockwise and clockwise, respectively, complete their travel around loop5312and return to ports5308and5310, respectively, with equal amplitudes and the same relative phases. The relative phase is maintained because both pulses5328and5330, which propagate in mutually opposite directions, travel exactly the same distance, i.e., the length of loop5312. The amplitudes of pulses5328and5330returning to ports5310and5308, respectively, are equal to each other because they travel through the exact same medium, which is symmetric and linear for both propagation directions. This means that pulse5330that returns to port5308is π/2 radian ahead with respect to pulse5328that returns to port5310. On their return paths, each of pulses5328and5330, upon arrival at ports5310and5308, respectively, may be re-split into ports5306and5304, e.g., at a 1:1 ratio for each split, wherein the crossover split produces a phase shift of π/2 radians and the bar split does not produce any phase shift. Accordingly, the crossbar split of pulse5330from port5308may destructively interfere with the bar split of pulse5328from port5310, thereby to produce substantially zero output at output port5306. At the same time, the crossbar split of pulse5328from port5310may constructively interfere with the bar split of pulse5330from port5308, thereby to produce a reflected signal that carries substantially the entire energy of pulse5320reflected back to input port5304. Part of the signal reflected back into port5304may be coupled, by coupler5305, into terminal5307to form there a reflected output signal. Normalizing the input energy of pulse5320to a value of 1, the energy at output port5306, when loop5312does not includes NLE5316, may be given by:

I5306=A·[12·12+j2·j2]2=0(6)
Where j indicates a phase shift of π/2 radians, and A is the intensity attenuation factor of attenuator5314. The energy reflected back to input port5304may be given by:

Part of energy I5304reflected back into port5304may be coupled, by coupler5305, into terminal5307to form there a reflected output signal.

FIG. 4cschematically illustrates circulator5305A having three terminals connected to connecting points5305B,5305C, and5305D. Connecting points5305B,5305C, and5305D ofFIG. 4care the same points5305B,5305C, and5305D ofFIG. 4aand thus circulator5305A may replace coupler5305inFIG. 4a.Replacing coupler5305by circulator5305A has the advantage of reducing signal intensity loss by transmitting most of the energy from point5305C to point5305B and from point5305B to point5305D. A circulator such as circulator5305A may be used to replace any coupler in the designs of the embodiments according to the present invention. For example, a circulator such as circulator5305A may replace coupler5305ofFIG. 6, coupler5305ofFIG. 7, coupler5568ofFIG. 8b,couplers5652and5656ofFIG. 9b,coupler5305ofFIG. 10a,and couplers5305,5718,5740and5736ofFIG. 10b.

FIG. 5schematically illustrates a graph showing the relative phase shift and intensity of the output signals of a NLE, for example, amplifier5316ofFIG. 4a,versus the input signals for two different amplitudes of pulses that propagate in opposite directions.FIG. 5is useful in analyzing the operation of device5300inFIG. 4awhere loop5312includes amplifier5316. In analogy to the graph inFIG. 1a,the graph ofFIG. 5shows the transmission function of the output intensity Io and the output phase shift Δφ of NLE amplifier5316versus the input intensity Ii. When lower level input pulse5320having a normalized field amplitude value of 1 is received, from input5309via coupler5305, by input5304of device5300inFIG. 4a,the field amplitude of split pulse5330, denoted5400inFIG. 5, propagating in the counterclockwise direction indicated by arrow5324inFIG. 4a,is 1/√{square root over (2)} at the entrance of amplifier5316. Further, in this scenario, the field amplitude of split pulse5328, denoted5402inFIG. 5, propagating in the clockwise direction indicated by arrow5326inFIG. 4a,is √{square root over (A/)}{square root over (2)} at the entrance to amplifier5316. Factor A represents the level of power intensity attenuation resulting from attenuator5314. Since both pulses, i.e., pulses5400and5402, may be within the linear range of amplifier5316, the two pulses may be amplified by amplifier5316by the same intensity gain factor Gliner. The two pulses are also attenuated by the same factor A at attenuator5314. Accordingly, both pulses return to ports5308and5310after undergoing substantially the same attenuation, A, and the same amplification, Glinear. Thus, the amplitudes of the two pulses, after amplification and attenuation, may be substantially equal to each other.

As described above, pulses5400and5402enter amplifier5316ofFIG. 4awith different field amplitudes, e.g., 1/√{square root over (2)} and √{square root over (A)}/√{square root over (2)}, respectively. Accordingly, amplifier5316may shift the phases of pulses5400and5402by different amounts. However, since pulses5400and5402are low amplitude pulses, their phases may be shifted only by small shifts, Δφ2and Δφ2; respectively, yielding an even smaller additional relative phase shift, d(Δφ2)=Δφ2−Δφ2; between the pulses. The influence of such additional relative phase shift is generally insignificant for the purposes of the invention. Accordingly, the additional relative phase shift produced by amplifier5316between pulses5400and5402is negligible and pulses5400and5402may return to ports5308and5310with amplitudes that are substantially equal to each other and with a relative phase shift substantially equal to their original relative phase shift, i.e., similar to the relative phase shift originally produced by coupler5302, e.g., a phase shift of about π/2 radians.

Because the amplitudes of the pulses returning to ports5308and5310are substantially equal to each other, and due to the small influence of amplifier5316on the relative phases of pulses5400and5402for low level input signals, the behavior of device5300in this case may be generally similar to that of an analogous device (not shown) without amplifier5316in loop5312. Accordingly, in the case of low level input signals, substantially all the energy of pulse5320, after amplification by gain Glinearand attenuation A, may be reflected back to input5304. Based on the above, the intensity I5306at output port5306and the intensity I5304reflected back to port5304may be given by the following equations:

I5306=Glinear·A·[12·12+j2·j2]2=0⁢⁢I5304=Glinear·A·[12·j2+j2·12]2=Glinear·A(8)
Part of energy I5304reflected back into port5304may be coupled, by coupler5305, into terminal5307to form there a reflected output signal, wherein Glinearrepresents the intensity amplification gain within the linear range.

For higher-level input pulses, for example, pulse5322inFIG. 4a,having field amplitude H, the counterclockwise split pulse5404may enter amplifier5316with a field amplitude H/√{square root over (2)}, which falls within the saturation range of amplifier5316. The clockwise split pulse5406may enter amplifier5316with a field amplitude √{square root over (A)}·H/√{square root over (2)}, which falls within the linear range of amplifier5316. Counterclockwise split pulse5404is amplified by amplifier5316by intensity gain factor Gsat, which is smaller than Glineardue to the reduced gain in the saturation region, and the phase of pulse5404is shifted by the same amplifier5316by Δφ1=Δφsat. Clockwise split pulse5406is amplified by amplifier5316by gain factor Glinear, in the linear region, and the phase of pulse5406is shifted by the same amplifier5316by Δφ1′. Although the ratio between low amplitude pulses5400and5402may be similar to the ratio between higher amplitude pulses5404and5406, namely, a ratio equal to one divided by the field amplitude attenuation factor √{square root over (A)}, the difference between the amplitudes of pulses5404and5406may be much larger than the difference between the amplitudes of pulses5400and5402. Accordingly, the relative phase shift between high level pulses5404and5406, denoted d(Δφ1)=(Δφsat−Δφ1′), may be much larger than the relative phase shift between low level pulses5400and5402, denoted d(Δφ2). This means that pulses5404and5406return to ports5308and5310with different field amplitudes √{square root over (Gsat)}·√{square root over (A)}·H/√{square root over (2)}, √{square root over (Glinear)}·√{square root over (A)}·H/√{square root over (2)}, respectively, and significant different phase shifts, Δφsatand Δφ1, respectively.

Thus, for such high level inputs, when choosing the proper length of amplifier5316, d(Δφ1) may be adjusted to be equal to π radians while still maintaining a negligible value, d(Δφ2), of the relative phase shift for low-level input amplitudes. When d(Δφ1) is equal to π radians, a relatively large fraction of the energy of the higher-level input pulse5322may be emitted out by device5300through its output5306and only a small fraction may be reflected back through input5304. In this case, the output intensity I5306and the intensity I5304reflected back into input5304may be given by:

I5306=H2·A·[Glinear2·12+Gsat2·12]2≠0⁢⁢I5304=H2·A·[Glinear2·j2-j2·Gsat2]2(9)
Part of energy I5304reflected back into port5304may be coupled, by coupler5305, into terminal5307to form there a reflected output signal.

In the above discussion, device5300is analyzed for the case where the reduced amplitude pulse5406is in the linear region of amplifier5316and the unreduced amplitude pulse5404is in the saturated region of that amplifier. It should be noted that there are at least two additional settings relevant to describing effective operation of device5300. In a first additional setting, pulses5406and5404have the same gain Glinear; however, the phase sifts produced for the two pulses by amplifier5316are different. In a second additional setting, amplifier5316shifts the phases of pulses5406and5404by the same amount Δφ1=Δφsat; however, the gains produced for the two pulses by amplifier5316are different.

It should be appreciated that the analysis of device5300for the two additional settings of device5300, in the case of low level input signals, may be generally the same as discussed above with reference to the case where no output signal is produced. Therefore, the two additional settings of device5300are not further analyzed herein in the context of low-level input signals.

Analyzing device5300in the range of high input signals, according to the first additional setting, it is noted that pulses5406and5404are both in the linear region of amplifier5316. In this case, when amplifier5316is sufficiently long, when the length of the amplifier is appropriately adjusted and when attenuation factor A is adjusted to produce the proper ratio between pulses5404and5406, the relative phase shift d(Δφ1) may be adjusted to be equal to π radians even when the amplitude of pulse5404is still in the linear range. Accordingly, pulses5404and5406are amplified by the same factor Glinear. Therefore, Gsatmay be replaced by Glinearin the above equations 9, taking into account phase inversion. In this first additional setting, for high-level input signals, the entire energy may be emitted from output port5306and substantially no energy may be reflected back through input5304. In such a case, coupler5305does not transmit any signal from terminal5304to terminal5307and thus no reflected output signal is produced at port5307.

According to the second additional setting, analyzed for the case of high level input signals, the amplitude of pulse5406may be sufficiently high to be included in the saturated range of amplifier5316and, thus, amplifier5316may not produce any relative phase shift d(Δφ1) between pulse5406and pulse5404, because both pulses are in the saturated region of amplifier5316. However, since pulse5404may be at a much deeper saturation level than pulse5406, pulse5404may have a gain, Gsat1, that is much lower than the gain, Gsat2, of pulse5406. In this case, the transmitted intensity I5306and the reflected intensity I5304may be given by:

Accordingly, device5300may operate as a threshold device that produces substantially no output signal for lower level input signals, while emitting a large fraction of the energy of higher level input signals through its output5306. It is clear that, for all the versions of device5300described above, the larger the ratio between pulses5404and5406, the larger the relative phase shift d(Δφ1) between the pulses and the larger the different between Glinearand Gsat, resulting in improved operation of device5306for the higher level input signals. It should be appreciated that, in device5300according to exemplary embodiments of the present invention, there may be virtually no limitation on the ratio between pulses5404and5406, and the ratio may be as desired, for example, equal to one over the attenuation factor of attenuator5314. Further, in view of the above analysis, it should be appreciated that although the use of a large attenuation factor, i.e., a small value for A, may improve the performance of device5300in the range of higher level input signals, such large attenuation does not degrade the performance of device5300in the range of lower level input signals.

Referring again toFIG. 4a,a virtual mid point5318divides loop5312into two halves, wherein each half has an equal length, S, representing the distance from port5310to mid point5318or from port5308to mid point5318. We may also refer to mid point5318of loop5312as center5318of loop5312. It should be clear that in the descriptions of the embodiments according to the present invention the terms mid point of the loop and center of the loop are the same geometrical point and the use of those terms is done alternatively. It is noted that the counterclockwise pulse5330and the clockwise pulse5328inherently meet and overlap each other at mid point5318. When streams of pulses that are separated from each other by time periods, T, enter loop5312of device5300, and split into clockwise and counterclockwise streams, a pulse in the counterclockwise stream, such as pulse5330, meets a pulse in the clockwise stream, such as pulse5328, every half time period, T/2. This means that after every distance X=T/2·C/n, wherein C is the speed of light in vacuum and n is the refractive index of the optical guides, there is a meeting (“collision”) point between pulses that propagate in loop5312in opposite directions. To avoid such collisions from occurring at the NLE, e.g., at amplifier5316, the location of the NLE should be off center by a distance δS that may be given by:
l·X<δS<m·X(11)
where X is the above given distance between two adjacent meeting (collision) points and l and m are consecutive integers. For the specific example of l=0 and m=1, Equation 11 may be reduced to: δS<X.

When a low amplitude pulse, such as pulse5406, enters amplifier5316first, the pulse does not deplete an inverse population of the amplifier and, thus, a higher amplitude pulse5404may enter the NLE immediately following the exit of pulse5406. In a situation when the order of the locations of amplifier5316and attenuator5314is reversed, the higher amplitude pulse may enter NLE5316first. In this reverse order case, the higher amplitude pulse may deplete the inverse population of amplifier5316and, thus, a recovery time A -may be needed for amplifier5316to build an inverse population before entry of a lower amplitude pulse. Therefore, in the latter case, or in a situation where the stream of input pulses includes only high amplitude pulses, T/2 may be longer than Δτ.

As discussed above, the efficiency of device5300may be improved by increasing the ratio between the higher and the lower levels included in the input signal. Further, the output signals produced by device5300that correspond to different levels of input pulses have a more distinctive amplitude ratio than the ratio between their respective input pulses. Accordingly, an improved threshold system in accordance with exemplary embodiments of the present invention may include a configuration of a more than one device5300, for example, at least two devices5300connected in series, wherein the output signals from one device5300may be fed directly into the input of a subsequent device5300. Such a configuration may be used to improve threshold capability by further accentuating the distinction between lower and higher amplitude pulses.

Referring toFIG. 6, a threshold device5301in accordance with further exemplary embodiments of the invention is shown. The design of device5301is a modified version of the design of device5300. In addition to the NLE-attenuator functionality, which may be performed by amplifier5316and attenuator5314, as described above with reference to device5300, device5301includes additional NLE-attenuator functionality, which may be embodied in the form of an amplifier5316aand an attenuator5314a.As discussed above with reference to optimizing the operation of device5300, the length of amplifier5316may be adjusted to produce a relative phase shift d(Δφ1) equal to π radians. However, since the required adjusted length for amplifier5316in device5300may not be commercially available and may be difficult to produce, the additional set of amplifier5316aand attenuator5314amay be added to enable such adjustment. In this case the required length of each amplifier (5316or5316a) of device5301may be about half of the required length required for the single amplifier5316in device5300. In some alternative embodiments, similar relative phase shifting may be achieved by adding only amplifier5316a,i.e., without using attenuator5314a;however, the addition of attenuator5314amay useful to enable a further increase of the amplitude ratio between the counterclockwise and the clockwise signals propagating in loop5312.

FIG. 7schematically illustrates a device5303, which is a variation of the design of device5300ofFIG. 4a.Device5303may enable expansion of the range of lower level input signal for which the very high performance and output signals very close to zero may be obtained. As shown inFIG. 7, device5303has generally the same structure as device5300, with the addition of an amplifier5316band an attenuator5314b.Except for amplifier5316band attenuator5314b,identical reference numerals are used inFIGS. 4aand7to indicate components with identical or similar structure and functionality. The parameters of attenuator5314band amplifier5316bmay be generally identical to those of attenuator5314and amplifier5316, respectively; however, amplifier5316bmay be excited to a higher excitation level than amplifier5316. Transmission functions of amplifiers5316band5316are roughly illustrated by symbols5502and5500, respectively, inFIG. 7.

For lower level input signals, such as pulse5320, amplifiers5316band5316both operate at their linear region in a similar way and, thus, loop5312may be quasi-symmetric and the entire energy of the input signal may be reflected back into input5304. Coupler5305may transmit part of the signal reflected back into terminal5304to terminal5307to produce there a reflected output signal. However, the range of the low level input signals for which the output signals are very close to zero is expanded in device5303relative to device5300. This range expansion is possible because the quasi-symmetric configuration of loop5312is maintained in device5303for a wider range of input amplitudes due to a phase shift compensation produced by amplifier5316bto compensate for the small phase shift that amplifier5316may produce, as described in detail above. Since amplifiers5316and5316bare excited to different levels of excitations, their gain and phase shifts may not be identical and, therefore, it is appreciated that the phase shift compensation of amplifier5316bapplied to the phase shift of amplifier5316may not be perfect. However, since the phase shifts produced by amplifiers5316and5316bin the range of low level input signals is generally small, the difference between these phase shifts (after the compensation) is smaller yet and has no significant influence on the operation of device5303over a wider range of lower level input signals.

For higher-level input signals, such as pulse5322, the additional amplifier5316bis still within the range of small phase shifts in the linear region and may operate quasi-symmetrically for both counterclockwise and clockwise pulses, such as pulses5330and5328. Thus the set of amplifier5316band attenuator5314bmaintains their quasi-symmetry even for the higher-level input signals. However, amplifier5316having a saturation level that is lower than the saturation level of amplifier5316bis driven into a saturation state by the counterclockwise pulses5330it receives, yet the amplifier is not driven into saturation by the clockwise pulses5328it receives. Accordingly, in this situation, the set of amplifier5316and attenuator5314“breaks” the symmetry of loop5312in a way similar to that explained above with reference to device5300ofFIG. 4a.At the same time, the set of amplifier5316band attenuator5314bhas little influence on the symmetry of loop5312. Accordingly, in this situation, for higher-level input signal, only amplifier5316and attenuator5314have a significant role in the production of output signals, whereby device5303operates in this range in a manner similar to the operation of device5300as discussed above with reference toFIG. 4a.

In accordance with embodiments of the invention, each of devices5301and5303may have a “turn on” point, which may function as a threshold level. For low-level input signals in the range, e.g., below the “turn on” threshold level, output signals are strongly attenuated by destructive interference at the output port of the devices and the transmission function between the input and the output of these devices includes a monotonic range with a shallow slope. For high-level input signals, e.g., in a range above the “turn on” threshold level, the output signal at the output port of the devices increases sharply and the transmission function between the input and the output of these devices may include a range having a steep monotonic slope.

Adjustable parameters that may be used to adjust the “turn on” threshold may include but are not limited to the gain G and the length L of amplifiers5316,5316aand5316b,and the attenuations of attenuators5314,5314aand5314b.The excitation levels, the gains, and the attenuations of the different amplifiers and attenuators may be different for each amplifier and/or attenuator.

III. All Optical gates and Switches Using Threshold Devices

Referring toFIGS. 2b,2c,2d,4a,6and7, illustrating devices5040,5041,5043,5300,5301and5303, respectively. Devices5040,5041, and5043ofFIGS. 2b,2c,and2d,respectively, have two output ports5061and5062. When respective input signals5029,5028, and5027are above and below the threshold levels of devices5040,5041, and5043ofFIGS. 2b,2c,and2d,these input signals are respectively transmitted or blocked, by devices,5040,5041, and5043on their way out to be emitted by port5062as output signals5082,5083, and5087. When devices5040,5041and5043are used as threshold devices in the way described above, only output port5062is used to transmit and block output signals5082,5083, and5087ofFIGS. 2b,2c,and2d,respectively.

However, output port5061may be used as well. Ports5061and5062are the output terminals of same combiner (coupler)5060and thus are complimentary ports. This means that in a situation in which at one of the ports,5061or5062, a constructive interference is produced, then a destructive interference is produced at the other port. Accordingly, when respective input signals5029,5028, and5027are above or below the threshold levels of devices5040,5041, and5043ofFIGS. 2b,2c,and2d,these input signals are respectively blocked or transmitted, by devices,5040,5041, and5043on their way out to be emitted by port5061.

It can be seen that outputs5061and5062operate as alternating ports. For input signals above the threshold level, port5061is a blocking port and port5062is a transmitting port. Similarly, for input signals below the threshold level, port5061is a transmitting port and port5062is a blocking port.

Similarly, while threshold devices5300,5301, and5303ofFIGS. 4a,6, and7, respectively, are described above with respect to only one output5306, they may operate in a way similar to devices5040,5041, and5043ofFIGS. 2b,2c,and2d,respectively, with two output ports operating alternatively. When input signal, such as, signal5322ofFIGS. 4a,6(not shown) and7is above the threshold level of devices5300,5301, and5303, loop5312transmits the input signal to output port5306and no signal is emitted by output port5307. In a situation where the input signal, such as, signal5320ofFIGS. 4a,6(not shown) and7is below the threshold level of devices5300,5301, and5303, loop5312reflects the input signal back into coupler5305that couples part of the reflected input signal into output port5307and no signal is emitted by output port5306.

It can be seen that outputs5307and5306operate as alternating ports. For input signals above the threshold level, port5307is a blocking port and port5306is a transmitting port. Similarly, for input signals below the threshold level, port5307is a transmitting port and port5306is a blocking port.

Accordingly, it can be seen that each threshold device5040,5041,5043,5300,5301, and5303ofFIGS. 2b,2c,2d,4a,6, and7, respectively, has two alternating outputs. Only one of these outputs produces an output signal when the input signal is above the threshold level and only the other output produces an output signal when the input signal is below the threshold level.

III. I. Electronically Activated All-Optical Gates and Switches

Referring toFIG. 8a,illustrating switch5500that may be used as a gate as well. Switch5500includes input5502connected to NLE5504that is connected to input5506of threshold device5508having output terminals5510and5512. NLE5504may be an optical amplifier, such as, SOA, LOA, or EDFA. NLE5504has an electronic terminal5514through which electrical current5524is injected into NLE5504. Threshold device5508is a block diagram presentation that may represent any optical threshold device and in particular the threshold devices according to the present invention. Input terminal5506and output terminals5510and5512of threshold device5508may represent input terminal5044and output terminals5062and5061of threshold devices5040,5041, and5043ofFIGS. 2b,2c,and2d,respectively. Similarly, Input terminal5506and output terminals5510and5512of threshold device5508may represent input terminal5309and output terminals5306and5307of threshold devices5300,5301, and5303ofFIGS. 4a,6, and7, respectively.

Input signal5516is received at input5502and passes through NLE5504to appear, as signal5518, at input5506of threshold device5508. When NLE5504is, for example a SOA, injection current5524at port5514of NLE5504may be adjusted to have current level C1; a current that produces an optical amplification, at NLE5504, which causes signal5518at input5506of device5508to be above the optical threshold level of device5508. In such a case signal5518appears, at output port5510of device5508, as signal5520and no output signal is produced at port5512. In another situation where injection current5524at port5514of NLE5504is adjusted to have a current level C2; a current that produces a different optical amplification (or even optical loss), at NLE5504, which causes signal5518at input5506of device5508to be below the optical threshold level of device5508, signal5518appears, at output port5512of device5508, as signal5522and no output signal is produced at port5510.

Accordingly, device5500operates as a “1 by 2” (1×2) all optical switch that may route the signal from input5502either to output5510or to output5512, depends on the electrical current5524injected via terminal5514of NLE5504. For electrical injection current5524with current level C1at terminal5514, the output signal appears, as signal5520, at port5510and no output signal is produced at port5512. For injection current5524with current level C2at terminal5514, the output signal appears, as signal5522, at port5512and no output signal is produced at port5510. Thus, optical switching between input5502and outputs5510and5512is produced by changing electrical current5524, at terminal5514, between the values C1and C2.

When only one output terminal5510or5512is of interest for monitoring either output signal5520or5522, respectively, device5500operates as a gate that may transmit or block the signal, at the only used output port, according to electrical current5524injected at terminal5514of NLE5504.

The total activation time of a switch electronically activated is the sum of: (a) The time needed for the controller to change the level of the current activating the switch. (b) The time needed for the optical switch to change its state. The discussion below relates only to the switching time of the switch itself (clause (b)) without taking into account the time needed to change the state of the controller (clause (a)). The activation time of switch (or gate)5500depends on the time needed to change injected current5524from C1to C2and vice versa. The time needed to change injection current5524from C1to C2depends of the lifetime of the injected electrons and is in the range of 200 ps. The time needed to change injection current5524from C2to C1is very short and depends of the time that takes to the stimulated emission in NLE5504to deplete the inverse population of the charge carriers in NLE5504.

Though the activation time of 200 ps for switch5500is fast, it limits the switching rate to be 5 Gbps=1/200 ps at the most. The activation time can be even shorter when using optical activation, as used in device5550ofFIG. 8b,instead of the electronic activation of device5500ofFIG. 8a.

FIG. 8billustrate a switch (or gate)5550that is activated optically. Switch5550receives signal5552at input5554that may includes optical isolator5556. Signal5552propagates from input5554via isolator5556, guide5558, and coupler5560into NLE5564. From NLE5564signal5552propagates via coupler5568to appear as signal5574at input5572of threshold device5576having outputs5578and5580. Threshold device5576is a block diagram illustration that represents threshold devices similar to the threshold devices that block5508ofFIG. 8arepresents. Isolator5556may be used to block back reflection of radiation reflected from device5576toward input5554. NLE5564may be an optical amplifier, such as, SOA, LOA, and EDFA.

Electrical terminal5566is used to inject current into NLE5564. When NLE5564is, for example a SOA, the injected current from terminal5566is adjusted to produce amplification, at NLE5564, that produces signal5574at input5572that is above the threshold level of device5576. In such a case, signal5574from input5572appears, as signal5582, at output port5578and no signal is produced at output port5580.

To switch the output that emits the signal from output port5578to output port5580, optical signal5586having intensity P is coupled into optical guide5570. Optical signal5586propagates from guide5570to coupler5568that couples signal5586into guide5572and from there into NLE5564. Signal5586may have a wavelength that is different from the wavelength of input signal5552. Accordingly, when using wavelength sensitive couplers, signal5586may be coupled completely, by coupler5568, from guide5570into guide5572prior to the entrance of signal5586into NLE and may be coupled out completely, by coupler5560, from guide5558to guide5562. At the same time, signal5552may pass from guide5558to guide5572, through couplers5560and5568, without loosing any energy due to wavelength sensitive coupling process in couplers5560and5568.

When signal5586having intensity P is coupled, by coupler5568from guide5570into guide5572and from there into NLE5564, it depletes the excitation level of NLE5564and may drive NLE5564into a saturation state. When the intensity P of signal5586is high enough to deplete and/or saturate NLE5564, the amplification of NLE5564for input signal5552reduces and signal5552appears, at input5572, as signal5574having amplitude that is below the threshold level of device5576. In such a case, signal5574appears at output5580as signal5584and no signal is produced by output port5578.

Accordingly, device5550operates as a “1 by 2” all optical switch that may route the signal from input5552either to output5580or to output5578, depends whether optical signal5586is or is not coupled into guide5570, respectively. When optical signal5586is coupled into terminal5570, the output signal appears, as signal5584, at port5580and no output signal is produced at port5578. In the absence of optical signal5586at terminal5570, the output signal appears, as signal5582, at port5578and no output signal is produced at port5580. Thus, optical switching between input5552and outputs5578and5580is produced by the presence or absence of optical signal5586at terminal5570.

When only one output terminal5578or5580is of interest for monitoring either output signal5582or5584, device5550operates as a gate that may transmit or block the signal, at the only used output port, according to the presence or absence of signal5586at terminal5570.

The activation time of switch (or gate)5500depends on the time needed to change the state of NLE5564. The time needed to change the state of NLE5564. into a depleted state (saturated by signal5586) is the time that takes to the stimulated emission in NLE5504to deplete the inverse population of the charge carriers in NLE5504. This process is very fast and happens almost instantly. The time that takes NLE5564to return to its normal state from its depleted state when signal5586is removed from port5570, is the recovery time of NLE5564, which is in the range of 10 ps. Activation time of 10 ps corresponds to a switching rate of 100 Gbps=1/10 ps.

It can be seen that the longest activation time of device5550that is activated optically is 10 ps while the longest activation time of device5500that is activated electronically is 200 ps.

Threshold devices5300,5301, and5303ofFIGS. 4a,6, and7are phase insensitive. Accordingly, it should be clear that when threshold devices5508and5576in systems5500and5550ofFIGS. 8aand8b,respectively, are of the type of threshold devices5300,5301, and5303ofFIGS. 4a,6, and7, then switches (or gates)5500and5550are phase insensitive as well.

III. III. Additional Electronically and Optically Activated All-Optical Gates and Switches

FIGS. 9aand9bare schematic illustrations of switches (or gates) in a configuration similar to the configurations of threshold devices5040,5401, and5043ofFIGS. 2b,2c,and2d,respectively. Accordingly the same referral numeral are used for the same structures illustrated byFIGS. 2b,2c,2d,9a,and9b.

FIG. 9ais a general schematic illustration5600that may represent all the configurations of devices5040,5041, and5403ofFIGS. 2b,2c,and2d,respectively. Components5602and5606illustrated by broken lines are optional and may or may not be included in device5600. When components5602and5606are not included in device5600, device5600represents device5040ofFIG. 2b.When components5602and5060represent optical attenuators, device5600represents device5041ofFIG. 2c.Similarly, when component5602represents an optical amplifier and component5060is not included, device5600represents device5043ofFIG. 2d.For the clarity of the drawings and to avoid crowdedness, closed loop phase control5070ofFIGS. 2b,2c,and2dis not illustrated byFIGS. 9aand9bbut, loop5070may be included inFIGS. 9aand9bfor dynamically controlling the phase of phase shifter5052.

In the normal state of switch5600input signal5616may be adjusted to be either above or below the threshold level of device5600to produce output signal5614or5612only at one output port5062or5061, respectively. NLE5054may be an optical amplifier, such as, SOA, LOA, and EDFA. Electronic current signal5610may be injected into NLE5054via terminal5608. The current level5610, the phase shift of shifter5052, and the intensity of input signal5616may be adjusted to cause switch5600to emit the output signal either from port5061or5062.

For example and without any limitations, the normal state of switch5600may be adjusted to produce an output signal5614at port5062where electrical current5610is injected into NLE5054through terminal5608is adjusted to have a current level C3. In this case, as explained above for devices5040,5041, and5403in the descriptions accompanied toFIGS. 2b,2c,and2d,respectively, the optical components of input signal5616propagating in branches5048and5050are combined constructively and destructively, by coupler5060, at ports5062and5061, respectively, to produce signal5614at port5062, and no signal is produced at port5061.

By changing injection current5610to a value C4, the phase shift that NLE5054produces may change by π radians and causes the optical components of input signal5616propagating in branches5048and5050to be combined constructively and destructively, by coupler5060, at ports5061and5062, respectively, to produce signal5612at port5061, and no signal is produced at port5062.

Thus switching the output signal between ports5062and5061is performed by changing the amount of injected current5610from level C3to C4and vice versa. For maintaining the situation in which output signal is emitted substantially only from one output port5061or5062of switch5600, the following conditions should be fulfilled:

2. NLE5054should maintain substantially the same gain and π radians phase difference, while injecting alternating currents5610of levels C3to C4, into NLE5054. For maintaining condition (2) above, the excitation levels of NLE5054, produced by injection currents with levels C3and C4, should be high enough to avoid the optical component of input signal5616, propagating in branch5050, to drive NLE into a saturation state.

When operating switch5600as a gate, only one output5061or5062is of interest for monitoring output signal5612or5614. To alternatively produce an “on” and “off” state at the selected output of gate5600, indicated by the presence or absence of signal at this port, injected current5610levels should be changed from C3to C4and vice versa.

It should be clear that if the “on” and “off” situations should be maintained only at one port5061or5602when switch5600operates as a gate, then coupler5060may be a symmetric (n=1) or an asymmetric (n≠1) coupler.

The time response of switch (gate)5600equals to the time response of NLE5054and may be equal to 200 ps. To increase the speed of switch5600, the state change of NLE5054may be changed optically as illustrated by switch5650ofFIG. 9b.

FIG. 9billustrates switch5650similar to switch5600ofFIG. 9awith the exception that the state of NLE5054is changed optically by coupling optical signal5658into NLE5054via input5654and coupler5652, instead of changing injected current5610injected to NLE5054through terminal5608as shown inFIG. 9a.Since device5650is similar to device5600ofFIG. 9a,the explanation provided in the description toFIG. 9astands also forFIG. 9band will not be repeated here.FIG. 9bis different fromFIG. 9ain the way that NLE5054is excited to produce a state change of switch5650. This process is described below.

In the normal state of device5650ofFIG. 9b,input signal5662appears only at one output5061or5062as signal5664or signal5666, respectively. For example and without any limitations, the emitting port in the normal state may be port5062that emits signal5666and no signal is produced at port5061. In the normal state of device5650, NLE5054is excited, by injection current supplied to NLE5054via terminal5608. The injected current is adjusted to excite NLE5054above the excitation level in which the optical component of input signal5662propagating in branch5050can drive NLE5054into a saturation state. To switch between the emitting ports5061and5062of switch5650, optical signal5658is coupled to port5654and from there via coupler5652to NLE5054. Signal5658may be coupled out, by coupler5656, into guide5660. When couplers5652and5656are wavelength sensitive, signal5658may be completely coupled into NLE5054and out from NLE5054without affecting the propagation of the optical component of input signal5662in branch5050having a wavelength that is different from the wavelength of signal5658. Alternatively, an optical isolator or an optical filter may be included in input terminal5044to avoid the propagation of signal5658toward the source of input signal5662. Alternatively, as described above, circulators may replace couplers5652and5656to avoid the propagation of signal5658toward the source of input signal5662.

Signal5658having intensity P1that may deplete, by stimulated emission, the inverse population of charge carriers produced by the excitation of the current injected to NLE5054via terminal5654. Accordingly, the injection of optical pulse5658into NLE5054is equivalent to a reduction of the injection level of the electrical current injected into NLE5054from terminal5608. In the presence of optical pulse5658at port5654, the phase shift that NLE5054produces may be changed by π radians and cause the optical components of input signal5662propagating in branches5048and5050to be combined constructively and destructively, by coupler5060, at ports5061and5062, respectively, to produce signal5664at port5061, and no signal is produced at port5062.

Thus switching the output signal between ports5062and5061is performed by the presence or absence of optical signal5658at port5654.

While in switch5600ofFIG. 9athe switching is performed by changing the injected current5610at terminal5608from level C3to C4and vice versa, the normal state of switch5650ofFIG. 9bmay use the same injection current level C3as a constant operating current, while the switching is performed by injecting optical signal5658into NLE5054via port5654that effectively changes the excitation level of NLE5054into a similar level that the injected current with level C4produces. It should be clear that the current injected via terminal5610of switch5650is maintained fixed during the switching and the effective excitation is only changed by the injection of optical signal5658to NLE5054via port5654.

The effect of the optical injection may be analyzed in an alternative way. When injected signal5658is present inside NLE5054at the time that the optical component of input signal5662is propagating in NLE5054, the intensities of both optical signals inside NLE5054are superimposed and the component of input signal5662appears in NLE5054as being with higher intensity. The effective higher intensity of the optical component of signal5662may experience, in NLE5054, a phase shift that is greater by π radians than the phase shift that this optical component would experience, in NLE5054, in the absence of signal5658.

In the normal state of switch5650, the excitation level of NLE5054is higher than the excitation of NLE5054when optical activating (control) signal5658(or electronic activating signal5610) depletes part of the inverse population of NLE5054. A reduction in the inverse population may reduce the gain of NLE5054and my cause to signal5664produced at port5061in the presence of activating signal5658or5610to be smaller than signal5666produced at port5062in the absence of activating signals5658and5610. To maintain equal intensity for signals5664and5666at ports5061and5062, respectively, intensity equalizer device5665may be used at port5062to prevent from signal5666to be stronger than signal5664. Equalizing device5665may be an attenuator or intensity limiter based on a saturated optical amplifier.

An equalizing device, such as equalizing device5665ofFIG. 9b,may be used as well in the output terminals of the switches illustrated byFIGS. 8a,8b,9a,10a,and10bto make sure that the intensities of the output signals of these switches at the normal state would be equal to the intensities of the output signals in the activated state.

As explained above switch5650may be operated as a gate when only one output port is used to monitor output signal5664or5666. The response time of switch (or gate)5650is equal to the recovery time of NLE5054and is in the range of 10 ps.

IV. Phase Insensitive, Electronically and Optically Activated All-Optical Gates and Switches

FIGS. 10aand10billustrate phase insensitive switches (or gates) activated electronically and optically, respectively. Devices5700and5701ofFIG. 10aand10b,respectively, are similar to device5300ofFIG. 4aand thus the same referral numeral are used for the same structures and signals illustrated byFIGS. 4a,10a,and10b.

Device5700ofFIG. 10aillustrated in rectangle5712that may represent a packaging box having external input terminal5702and external terminals5704and5706that may be external Input/Output (I/O) terminals. External terminals5702,5704, and5706are connected to input terminal5309, terminal,5306, and terminal5307, respectively. NLE5316may be an optical amplifier, such as, SOA, LOA, and EDFA and having electronic terminal5708through which excitation current5707is injected into NLE5316. Attenuator5314may be of the type illustrated byFIG. 4b,such as, optical amplifier of one of the types of SOA, LOA, and EDFA. Attenuator5314may be excited into very low excitation level by an excitation current injected into attenuator5314via electronic terminal5710. The low excitation level by which attenuator5314is operated, determines the amount of attenuation that attenuator5314produces.

The excitation currents injected into NLE5316and attenuator5314may be adjusted to be with levels of C5and C6, respectively, (C5>C6), determining the threshold level of device5700. Input signal5724at input port5702may be adjusted to have amplitude that is above the threshold level of device5700, such as, the amplitude of signal5322shown inFIG. 4a.In this case, as explained above for device5300ofFIG. 4a,the optical components of input signal5724propagating clockwise and counterclockwise in loop5312experience phase shifts that are different by )radians, producing an output signal5726received at external I/O port5704, via port5306, and no signal is produced at port5307or external I/O port5706. The optical components of input signal5724, propagating clockwise and counterclockwise in loop5312, enters NLE5316prior and post to their attenuation by attenuator5314and thus have different amplitudes similar to amplitudes5400and5404illustrated byFIG. 5. Accordingly these optical components experience phase shifts that are different by the amount similar to d(Δφ1-Δφ2) ofFIG. 5that is equal to π radians.

To switch the output signal from being emitted by port5704, as signal5726, to be emitted from port5706, as signal5728, while input signal5724at port5702is the same, the relative phase shift between the optical components of input signal5724propagating clockwise and counterclockwise in loop5312should be changed from π radians either to zero radians, denoted as zero relative phase, or to 2·π·m, where m is an integral number (1, 2, . . .).

A zero relative phase shifts between the optical components of input signal5724propagating clockwise and counterclockwise in loop5312may be achieved by reducing injection current5707injected to NLE5316via terminal5708to be at a level C7. In this case, the current level C7of signal5707at port5708is adjusted to create lower excitation level of NLE5316in which both of the optical components of input signal5724propagating clockwise and counterclockwise in loop5312are in the saturated region of NLE5316, resulting with the same phase shift, produced by NLE5316for both of the optical components in loop5312. Thus the optical components of input signal5724at loop5312are combined, by coupler5302, to produce output signal5728received at external I/O port5706from port5304via coupler5305and port5307. In this case no signal is produced at port5704.

A relative phase shifts of 2·π·m radians (m=1, 2, 3 . . .) between the optical components of input signal5724propagating clockwise and counterclockwise in loop5312may be achieved by adjusting injection current5707injected to NLE5316via terminal5708to be at a level C8. In this case, the current level C8of signal5707is adjusted to create excitation level of NLE5316in which the optical components of input signal5724propagating clockwise and counterclockwise in loop5312experience different phase shifts in NLE5616which are different by an amount greater than the amount which is produced when NLE5316is excited by injection current5707of level C5. When the current level C8of injection current5707is adjusted properly, the relative phase shift between the optical components in loop5312may be 2·π·m radians instead of being π radians as when excitation current5707at port5708is with current level of C5. Thus the optical components of input signal5724at loop5312are combined, by coupler5302, to produce output signal5728received at external I/O port5706from port5304via coupler5305and port5307. In this case no signal is produced at port5704.

It can be seen that by switching the amount of current5707injected to NLE5316via port5708from being with level C5to have level C7or C8, the output signal switches the port from which it is emitted from port5704, as signal5726(at current5707with current level C5), to be emitted by port5706, as signal5728(at currents5707with current level C7or C8).

The response time of switch5700equals to the lifetime of the charge carriers in NLE5316and is in the range of 200 ps. To shorten the response time of switch5700, the current5707injected to NLE5316may be decreased from current level C5to CXwhile the amount of current injected to attenuator5314may be increased from C6to CX. In this situation, NLE5316and attenuator5314have the same injection current CXand operate similarly. Thus, in this situation, loop5312is a symmetric loop and acts as a simple mirror loop that reflects the signal back into terminal5304and from there to device terminal5706, as signal5728, via coupler5305and guide5307. The difference between levels C5and CXand between levels CXand C6used to perform the switching by changing both of the injected current, may be smaller than the difference between levels C5and C7or between levels C5and C8used to perform the switching by changing only injected current5707injected to NLE5316, via terminal5708. Thus when changing both of the injected currents, current5707injected to NLE5316and the current injected to attenuator5314, the switching time of switch5700may be reduced to be shorter than 200 ps. To make the response time of switch5700even faster, an optical activation may be used as illustrated byFIG. 10b.

Switch5700may be operated as a gate by monitoring only one of the devices' outputs,5704or5706.

The use of loop5312makes device5700phase insensitive.

FIG. 10billustrates switch5701similar to switch5700ofFIG. 10awith the exception that the state of NLE5316is changed optically by coupling optical signal5734into NLE5316via input guide5716and coupler5718, instead of changing the injected current5707injected to NLE5316through terminal5708as shown inFIG. 10a.Since device5701is similar to device5700ofFIG. 10a,the explanation provided in the description toFIG. 10astands also forFIG. 10band will not be repeated here.FIG. 10bis different fromFIG. 10ain a way that NLE5316is excited to produce a state change of switch5701. This process is described below.

In the normal state of device5701, input signal5730propagates from input port5702and appears only at one output5704or5706as signal5732or signal5733, respectively. For example and without any limitations, the emitting port in the normal state may be port5704that emits signal5732and no signal is produced at port5706. In the normal state of device5701, NLE5316is excited, by injection current5707supplied to NLE5316via terminal5708. Injected current5707is adjusted to excite NLE5316above the excitation level in which the small optical component of input signal5730propagating in loop5312can drive NLE into a saturation state. To switch between the emitting ports5704and5706of switch5701, an optical signal5734is coupled to port5716and from there via coupler5718into NLE5316. Signal5734may be coupled out, by coupler5722, into guide5720. When couplers5718and5722are wavelength sensitive, signal5734may be completely coupled into NLE5316and out from NLE5316without affecting the propagation of the optical components of input signal5730in loop5312having a wavelength that is different from the wavelength of signal5734. Alternatively, an optical isolator or an optical wavelength sensitive filter may be included in input terminal5702to avoid the propagation of signal5734toward the source of input signal5730. Such wavelength sensitive filters may be included in ports5704and5706to separate between signals5732and5733and activation signal5734.

The use of wavelength filters at the input port and/or output terminals may be applied to all of the embodiments according to the present invention for separating the information signals from the activating signals, even if it was not specifically illustrated by the drawings.

Signal5734having intensity P2that may deplete, by stimulated emission, the inverse population of charge carriers produced by the excitation of current5707injected to NLE5316via terminal5708. Accordingly, the injection of optical pulse5734into NLE5316is equivalent to a reduction of the injection level of current5707injected into NLE5316from terminal5708. In the presence of optical pulse5734at port5716, the phase shift that NLE5316produces may be changed by π radians and causes the optical components of input signal5730propagating in loop5312to be combined constructively and destructively, by coupler5302, at ports5304and5306, respectively, to produce signal5733at port5706when no signal is produced at port5704.

Thus switching the output signal between ports5704and5706is performed by the presence or absence of optical signal5734at port5716.

While in switch5700ofFIG. 10athe switching is performed by changing injected current5707at terminal5708from level C5to C7and vice versa, the normal state of switch5701may be operated with injection current5707having a constant level of C5while the switching is performed by injecting optical signal5734to NLE5316, via port5716, that effectively change the excitation level of NLE5316into a similar level that injection current5707with current level C7produces. It should be clear that the current injected via terminal5708of switch5701is maintained fixed during the switching and the effective excitation is only changed by the injection of optical signal5734to NLE5316, via port5716.

The effect of the optical injection may be analyzed in an alternative way. When injected signal5734is present inside NLE5316at the time that the optical components of input signal5730is propagating in NLE5054, the intensities of both optical signals in NLE5316are superimposed and the component of input signal5730appear in NLE5316as being with higher intensity. The effective higher intensity of the optical component of signal5730may experience, in NLE5316, a phase shift that is greater by π radians than the phase shift that these optical components would experience, in NLE5316, in the absence of signal5734in NLE5316.

The state of NLE5316may change by injecting signal5734from ports other than port5716. Signal5734may be injected via port5742to be coupled into I/O terminal5304via coupler5740, guide5307and coupler5305. Similarly, signal5734may be injected via port5738to be coupled into I/O terminal5306via coupler5736. In both cases, signal5734is split, by coupler5302, into two components propagating clockwise and counterclockwise in loop5312. The optical components of signal5734in loop5312change the excitation state of NLE5316or are added to the optical components of input signal5730in loop5312to change the switching state of switch5701in a way similar to the explained above when optical signal5734is coupled to NLE5316via port5716. Alternatively, the injection of signal5734into guide5307or5306may be done by injecting signal5734directly into I/O port5733or5732, respectively.

As explained above, switch5701may be operated as a gate when only one output port is used to monitor the output signal. The response time of switch (or gate)5701is equal to the recovery time of NLE5316and is in the range of 10 ps.

It should be clear that all the electronic activating signals, such as, signals5524,5610, and5707ofFIGS. 8a,9a,and10a,my be in a form of electrical pulses or in a form of dc current or voltage. Similarly, all the optical activating signals, such as, signals5586,5658, and5734ofFIGS. 8b,9b,and10b, may be in a form of optical pulses or in a form of Continuous Wave (CW).

When signal5734is in a form of CW radiation, it may fill the whole medium of loop5312and its components propagate simultaneously through NLE5316and thus experience the same phase shift produced by NLE5316. Accordingly, no relative phase shift is produced between the components of CW signal5734in loop5312. In this case, signal5734is reflected back from loop5312via the same port from which it is coupled into loop5312. If switch5701is operated as a gate in which the output signal5733is monitored only at port5706and signal5734is injected from port5704, there is no mixing between output signal5733and activating signal5734. When device5701operates as a switch, both ports5704and5706are used to monitor output signals and its activating signal is coupled from one of ports5704or5706, the separation between activating signal5734and output signal5732or5733may be done by using optical isolators at port5704or5706(not shown) to block signal5734reflected back into one of these ports. Alternatively, signal5734may be with a wavelength that is different from the wavelength of signals5732and5733and the separation between signal5734and signals5732and5733may be done by wavelength sensitive filters that are included in ports5704and5706or at one of these ports (not shown).

The design ofFIG. 10bwhere coupler5718is included in optical loop5312containing attenuator5314may be modified by removing attenuator5314. In this case, coupler5718may be adjusted to produce attenuation similar to the attenuation of attenuator5314that has been removed from loop5312. Accordingly, coupler5718performs two functions, the first is to couple activating signal5734from port5716into loop5312and the second is to produce signal intensity loss by coupling part of the energy out of loop5312in a way similar to the illustrated byFIG. 4bwhere coupler5314A ofFIG. 4bperforms as attenuator5314inFIG. 4a.The attenuation that coupler5718produces in loop5312may be adjusted using variable coupler5718to select the proper coupling ratio needed to produce the desirable loss in loop5312. While the modified design ofFIG. 10bthat does not include attenuator5314may resemble the structure of the Terahertz Optical Asymmetric Demultiplexer (TOAD), in fact they are very different one from another. The coupling device according to the modified design ofFIG. 10bdiffers from the TOAD in the following aspects:

1. The TOAD is designed to couple the activating signal into the loop without loss. The TOAD cannot work as designed when loss is introduced in its optical loop by the coupling of the activating signal. The modified design ofFIG. 10bintroduces loss at the coupling point of the activating signal on purpose, a loss which may be further adjusted to a proper value.

2. The activating signal of the TOAD must be a pulse modulated signal that is synchronized with the information signals at its optical loop. The TOAD can not be operated by a CW activating beam. The activating signal of the modified design ofFIG. 10bmay be a CW beam.

Switches5700and5701ofFIGS. 10aand10b,respectively, are phase insensitive due to the use of optical loop and are unique in the sense that in spite of the use of optical loop, they may perform the switching even while the activating signals5707or5734are in the form of dc current (or voltage) and CW radiation, respectively.

In all the switches and gates according to the present invention, there is an output port which emits the output signal in a normal state of the switch. In the presence of an activating signal, the switch changes the output port from which the output signal is emitted. This change lasts as long as the activating signal is present. The presence time of the activating signal is equal to the time width of the pulse of the activating signal or to the time period in which a dc or CW activating signal is present.

It should be noted that the numerical data mentioned within the embodiments described above, for example, the specific NLE activation time or response time or recovery time, are all examples of a certain NLE technology. As NLE technology improves, the NLE devices' inherent timings and speeds may still limit the switching speed of the described switches and gates, but may produce faster switching responses than mentioned.

All the embodiments according to the present, may include a continuous sequence of optical components connected by light guiding media such as, for example, optical fibers, planar waveguides, or planar circuits (PLC), which media may be fabricated using integrated optic techniques and/or on-chip manufacturing. Alternatively, All the embodiments according to the present may be constructed from discrete components, in which case the optical guiding media may be replaced by open space, e.g., vacuum, or by a non-solid, e.g., gaseous media, and the directional couplers may be replaced with beam splitters. It should be understood that all amplifiers and attenuators may include variable and/or adjustable components. It should be clear that all amplifiers may made of amplifying media and devices and in particular are made of SOA's, LOA's and EDFA's. It should be appreciated that all attenuators are made of attenuating media and devices and in particular are made of couplers, directional couplers and absorbing amplifiers.

It should be clear that the embodiments according to the present invention may include in their output ports equalizing devices and specifically attenuators to equalize the intensities of the signals from the different output ports.

It should be understood that the switches according to the present invention may include wavelength filters in their inputs and/or outputs for separating the information signal from the activating signal.

It should be clear that while some of the coupling devices are illustrated as couplers they may be circulators as well.

It should be understood that while some of the activating signals are illustrated as either being in a form of pulses or CW beam they may all be each of them or both of them, i.e. in a form of pulses and/or CW beam.

It should be clear that the activating signals according to the present invention may be with the same or different wavelength as the wavelength of the information signal.