Spatiotemporal optical multiplexer and associated methods

Abstract

A spatiotemporal optical multiplexer includes a storage optical cavity having a plurality of storage-cavity modes that are transversely separated, a dump optical cavity having a plurality of dump-cavity modes that are transversely separated, and a phase modulator located in the dump cavity and overlapping all of the dump-cavity modes. Each dump-cavity mode is coupled to a respective storage-cavity mode. The storage-cavity modes have the same storage-cavity resonant frequency while the dump-cavity modes have dump-cavity resonant frequencies that are distinct. The phase modulator is driven to simultaneously tune the dump-cavity resonant frequencies. At any time, no more than one dump-cavity mode is resonant with its respective storage-cavity mode. When this resonance occurs, the energy of the respective storage-cavity mode is coupled out of the storage cavity and through the dump cavity as an output pulse.

Description

SPATIOTEMPORAL OPTICAL MULTIPLEXER AND ASSOCIATED METHODSRELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 63 / 567,200, filed on March 19, 2024, which is incorporated herein by reference in its entirety.STATEMENT REGARDING FEDERALLYSPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with government support under grant number W911NF-23-1-0053-P00001, awarded by the Army Research Office, and grant number FA9550-22- 1-0279, awarded by the Air Force Office of Scientific Research. The government has certain rights in the invention.BACKGROUND

[0003] Single-photon cameras are frequently used to image trapped atoms in quantum computers based on atom arrays, among other applications. Many prior-art single-photon cameras use image sensors based on electron-multiplying charge-coupled devices (EMCCDs). While these EMCCD-based cameras provide up to 106pixels with near-unity quantum efficiency, they typically operate with a maximum frame rate of no more than a few kilohertz.

[0004] Single-photon detectors are single-pixel or few-pixel arrays that offer temporal resolutions below 1 ns and near-unity quantum efficiency. Examples of prior-art single-photon detectors include single-photon counting modules (SPCMs), superconducting-nanowire singlephoton detectors (SNSPDs) and single-photon avalanche diodes (SPADs). These single-photon detectors offer temporal resolution below one nanosecond and near-unity quantum efficiency.SUMMARY

[0005] The present embodiments include spatiotemporal optical multiplexers and demultiplexers, and associated methods, that may used to re-multiplex the spatial information of an optical input signal (or signals) into the time domain of an output optical field. Advantageously, the present embodiments lose very little light, as compared to other devices and techniques known in the art. The embodiments herein use a buffer array, an array of controllable optical buffers that introduce various time delays. The buffer array may be used with a spatially large (i.e., multimode) single-pixel photodetector to create a high-speed camera.

[0006] In embodiments, a spatiotemporal optical multiplexer includes a storage optical cavity having a plurality of storage-cavity modes that are transversely separated, a dump optical cavity having a plurality of dump-cavity modes that are transversely separated, and an intracavity phase modulator in the dump optical cavity. The intracavity phase modulator spatially overlaps all of the plurality of dump-cavity modes. Each of the plurality of dumpcavity modes is spatially coupled to a respective one of the plurality of storage-cavity modes. The plurality of storage-cavity modes all have the same storage-cavity resonant frequency. The plurality of dump-cavity modes have a respective plurality of dump-cavity resonant frequencies that are distinct (i.e., no two of the dump-cavity resonant frequencies are the same).

[0007] The combination of the intracavity phase modulator and the dump optical cavity is one implementation of the buffer array mentioned above. The phase modulator is driven to simultaneously tune the dump-cavity resonant frequencies. At any time, no more than one dump-cavity mode is resonant with its respective storage-cavity mode. When this resonance occurs, the energy of the respective storage-cavity mode is coupled out of the storage cavity and through the dump cavity as an optical output pulse. This process repeats for all of the dumpcavity modes to create a temporal sequence of optical output pulses.

[0008] In other embodiments, a method for spatiotemporal optical multiplexing, includes coupling light into a storage optical cavity to excite one or more of a plurality of storage-cavity modes that are transversely separated. The method also includes driving an intracavity phase modulator to sequentially couple each of the plurality of storage-cavity modes to a respective one of a plurality of dump-cavity modes of a dump optical cavity. The intracavity phase modulator is located in the dump optical cavity and spatially overlaps all of the plurality of dump-cavity modes. The plurality of dump-cavity modes are transversely separated. The dump optical cavity, in response to said driving, transmits a temporal sequence of output optical pulses generated from the plurality of storage-cavity modes, respectively.

[0009] In some embodiments, the spatiotemporal optical multiplexer further includes an active 1 X N optical switch or other type of optical device for coupling the temporal sequence of output optical pulses into a single propagation mode (e.g., via a combining cavity or directional waveguide coupling). These embodiments may be used, for example, for timedivision multiplexed optical interconnects and networks.

[0010] Other applications of the present embodiments include, but are not limited to, quantum computers, quantum repeaters, squeezed-light-based microscopes and sensors with parallel readout, and parallelized fluorescence microscopy (e.g., fluorescence-lifetime imaging microscopy, or FLIM).BRIEF DESCRIPTION OF THE FIGURES

[0011] FIG. 1 is a schematic diagram of a spatiotemporal optical multiplexer that multiplexes N spatially separate optical input signals to generate a single time-multiplexed optical stream.

[0012] FIG. 2 is a schematic diagram of a spatiotemporal optical multiplexer, in embodiments.

[0013] FIG. 3 is a schematic diagram of a spatiotemporal optical multiplexer, in embodiments.

[0014] FIG. 4 is a schematic diagram of a spatiotemporal optical multiplexer, in embodiments.

[0015] FIG. 5 is a schematic diagram of a spatiotemporal optical multiplexer, in embodiments.

[0016] FIG. 6 is a schematic diagram of a spatiotemporal optical multiplexer, in embodiments.

[0017] FIG. 7 is a flow chart of a method for spatiotemporal optical multiplexing, in embodiments.DETAILED DESCRIPTION

[0018] FIG. 1 is a schematic diagram of a spatiotemporal optical multiplexer 100 that multiplexes N spatially separate optical input signals 102 to generate a single time-multiplexed optical stream 110. Each of the N optical input signals 102 varies temporally with a signal bandwidth 5sig, where N is an integer greater than 1. For clarity in FIG. 1 (and several other figures herein), only N = 6 optical input signals 102 are shown, of which only a first optical input signal 102(1), a second optical input signal 102(2), and a third optical input signal 102(3) are labeled. However, the number N of optical input signals 102 may be any integer greater than one. In particular, N may have a value as large as several thousand, or more.

[0019] The spatiotemporal optical multiplexer 100 includes an A / -way switch 104 that, at any given time, routes only one of the N optical input signals 102 therethrough to generate the time-multiplexed optical stream 110. Each of the N optical input signals 102 defines a respective channel of the multiplexer 100. Therefore, the multiplexer 100 has N channels that form a one-to-one correspondence with the N optical input signals 102 (and therefore a one-to- one correspondence with the N inputs of the IV- way switch 104). Each of the N optical input signals 102 may also referred to herein as an optical input mode.

[0020] FIG. 1 shows an optical detector 106 detecting the time-multiplexed optical stream 110. The detector 106 has a bandwidth Bdetthat is larger than the signal bandwidth Bsigsuch that Bdet> NBsig. In this case, it should be possible to time-multiplex the optical input signals 102 to generate the optical stream 110, and subsequently detect the optical stream 110 with the detector 106, without losing any information. However, the / V-way switch 104 wastes all but approximately 1 / N of each optical input signal 102(t), where i is an integer between 1 and N. The reason for this loss is that when the A -way switch 104 is not connected to a specific channel, the optical input signal from that channel is lost (i.e., absorbed or reflected). Thus, at any moment in time, the information of all but one of the N optical input signals 102 is lost.

[0021] FIG. 2 is a schematic diagram of a spatiotemporal optical multiplexer 200, in accordance with some of the present embodiments. Advantageously, and unlike the spatiotemporal optical multiplexer 100 of FIG. 1, the multiplexer 200 multiplexes the N optical input signals 102 to create a time-multiplexed optical stream 208 without losing any information. To achieve this, the multiplexer 200 includes a buffer array 202 with N buffers 204 that form a one-to-one correspondence with the N optical input signals 102. Therefore, the multiplexer 200 also has N channels. Each buffer 204(7) stores its corresponding optical input signal 102(f) for a different duration before releasing the light. Equivalently, each buffer 204(f) delays the optical input signal 102(f) by an Ithtime delay that is unique to the fthchannel. The buffer array 202 is controllable in that light is emitted from the buffer array 202 in response to an electrical signal (e.g., a modulation signal) or an optical control signal (e.g., a pump beam).

[0022] When all of the N optical input signals 102 are cotemporaneous (i.e., received by the spatiotemporal optical multiplexer 200 at the same time), the buffer array 202 temporally separates the N optical input signals 102 such that the buffer array 202 outputs, at any given time, no more than one of the N optical input signals 102. The signals outputted by the buffer array 202 are shown in FIG. 2 as N optical output pulses 206. While FIG. 2 shows six optical output pulses 206, only a first optical output pulse 206(1), a second optical output pulse 206(2), and a third optical output pulse 206(3) are labeled in FIG. 2 for clarity. Similarly, only a first buffer 204(1), a second buffer 204(2), and a third buffer 204(3) are labeled in FIG. 2. The N optical output pulses 206 form a “stream” in the sense that they are temporally separated and propagate along the same propagation mode (e.g., in free space or along an optical waveguide).

[0023] The spatiotemporal optical multiplexer 200 further includes the A / -way switch 104 of FIG. 1, which combines the N optical output pulses 206 to form the single time- multiplexed optical stream 208. The switch 104 is controlled synchronously with the N timedelays such that the switch 104 connects each optical output pulse 206(f) to the output stream 208 when the optical output pulse 206(i) arrives at the switch 104. By operating in this manner, none of the information of the N optical input signals 102 is lost. For example, in FIG. 2 the switch 104 is first controlled to optically connect an output of the switch 104 to the first buffer 204(1) before the first optical output pulse 206(1) arrives at the switch 104. The switch 104 maintains this first connection while the first optical output pulse 206(1) passes through the switch 104 to fill a first time slot TS1 of the optical stream 208. The switch 104 is then subsequently controlled to optically connect the output of the switch 104 to the second buffer 204(2) before the second optical output pulse 206(2) arrives at the switch 104. The switch 104 is maintained in this second position while the second optical output pulse 206(2) passes through the switch 104 to fill a second time slot TS2 of the optical stream 208. The switch 104 is then subsequently controlled to optically connect the output of the switch 104 to the third buffer 204(3) before the third optical output pulse 206(3) arrives at the switch 104. The switch 104 is maintained in this third position while the third optical output pulse 206(3) passes through the switch 104 to fill a third time slot TS3 of the optical stream 208. This process continues through all N channels of the multiplexer 200 to fill N sequential time slots of the optical stream 208. These N sequential time slots form a frame, similar to other time- multiplexed signals known in the art.

[0024] FIG. 3 is a schematic diagram of a spatiotemporal optical multiplexer 300, in accordance with some of the present embodiments. The multiplexer 300 is similar to the spatiotemporal optical multiplexer 200 of FIG. 2 except that an optical waveguide 302 replaces the / V-way switch 104. All of the optical output pulses 206 are coupled into the optical waveguide 302, along which they the optical output pulses 206 propagate to form the single time- multiplexed optical stream 208. The optical waveguide 302 may be sized such that all of the optical output pulses 206 excite a single transverse mode of the optical waveguide 302. In this case, the optical output pulses 206 may be detected with a single-mode optical detector that is mode-matched to the optical waveguide 302 (e.g., a fiber-optic-coupled photodetector).

[0025] FIG. 4 is a schematic diagram of a spatiotemporal optical multiplexer 400, in accordance with some of the present embodiments. The multiplexer 400 is similar to the spatiotemporal optical multiplexer 200 of FIG. 2 and the spatiotemporal optical multiplexer 300 of FIG. 3 except that a combining optical cavity 402 replaces the N-way switch 104 of FIG. 2 and the optical waveguide 302 of FIG. 3. The combining optical cavity 402 is positioned relative to the buffer array 202 such that all of the optical output pulses 206 are coupled to one longitudinal mode of the combining optical cavity 402. In the example of FIG. 4, the combiningoptical cavity 402 is a Fabry-Perot cavity formed from a first mirror 404 and a second mirror 406 that face each other. The time-multiplexed optical stream 208 is then emitted from the one longitudinal mode of the combining optical cavity 402.

[0026] FIG. 5 is a schematic diagram of a spatiotemporal optical multiplexer 500, in accordance with some of the present embodiments. The multiplexer 500 implements the buffer array 202 of FIGS. 2-4. The multiplexer 500 includes a storage optical cavity 504 that has a plurality of storage-cavity modes 516. Each of the storage-cavity modes 516 is a longitudinally resonant standing- wave mode having a longitudinal axis that extends parallel to an optical axis 502. In FIG. 5, the optical axis 502 and longitudinal axes of the storage-cavity modes 516 all extend parallel to the z axis of a right-handed Cartesian coordinate system 550. The storagecavity modes 516 are transversely separated in that they are spaced along a direction perpendicular to the optical axis 502. In the example of FIG. 5, the storage-cavity modes 516 are transversely separated along the y axis of the coordinate system 550. Alternatively or additionally, the storage-cavity modes 516 are transversely separated along the x axis of the coordinate system 550. For clarity in FIG. 5, only a first storage-cavity mode 516(1), a second storage-cavity mode 516(2), and a third storage-cavity mode 516(3) are labeled.

[0027] The spatiotemporal optical multiplexer 500 also includes a dump optical cavity 514 that has a plurality of dump-cavity modes 522. Each of the dump-cavity modes 522 is a longitudinally resonant standing-wave mode having a longitudinal axis that extends parallel to the optical axis 502. The dump-cavity modes 522 are also transversely separated along a direction perpendicular to the optical axis 502. The dump-cavity modes 522 form a one-to-one correspondence with the storage-cavity modes 516 in that each dump-cavity mode 522(i) is spatially coupled to a respective storage-cavity mode 5 16(t ).

[0028] In the example of FIG. 5, the storage optical cavity 504 includes a first mirror 506 and a second mirror 508 that face each other and are longitudinally separated (i.e., along z), much like a Fabry-Perot cavity. However, the storage optical cavity 504 further includes a lens array 518 located between the mirrors 506 and 508. The lens array 518 transversely separates the storage-cavity modes 516. The storage-cavity modes 516 all have the same longitudinal length (i.e., along z) between the mirrors 506 and 508 and therefore the same storage-cavity resonant frequency and the same storage-cavity linewidth Ts.

[0029] Also in the example of FIG. 5, the dump optical cavity 514 includes a third mirror 509 and a fourth mirror 512 that approximately face each other and are longitudinally separated. The dump optical cavity 514 also includes a lens array 524 that is located between the mirrors 509 and 512 and that transversely separates the dump-cavity modes 522. However,a normal vector of the fourth mirror 512 is slightly misaligned relative to the optical axis 502. Accordingly, the longitudinal lengths of the dump-cavity modes 522 are distinct (i.e., no two of the dump-cavity longitudinal lengths are the same), and therefore the dump-cavity modes 522 have respective dump-cavity resonant frequencies that are distinct (i.e., no two of the dumpcavity resonant frequencies are the same). It is assumed that the dump-cavity modes 522 have the same dump-cavity linewidth Td.

[0030] For clarity in FIG. 5, the tilt of the fourth mirror 512 relative to the optical axis 502 is exaggerated. In practice, the longitudinal lengths of the dump optical cavity 514 may vary by as little as the optical wavelength A, or less (e.g., A / 2). To ensure optimal dumping (e.g., to avoid reflection of incident light off of the storage optical cavity 504) the time-averaged transmission of the dump optical cavity 514 may be equal to the transmission of the second mirror 508 (i.e., the input mirror of the storage optical cavity 504). While FIG. 5 depicts this technique implemented in one dimension by tilting the fourth mirror 512, the fourth mirror 512 may be additionally tilted in two dimensions.

[0031] In FIG. 5, the second mirror 508 is a first reflective surface of a two-sided mirror 507. Similarly, the third mirror 509 is a second reflective surface, opposite the first reflective surface, of the two-sided mirror 507. However, the mirrors 508 and 509 may alternatively he individual (i.e., physically separate) optical elements, such as single-sided mirrors.

[0032] The spatiotemporal optical multiplexer 500 also includes an intracavity phase modulator 510 located in the dump optical cavity 514. The phase modulator 510 is sized and positioned to spatially overlap all of the dump-cavity modes 522. Electrically driving the phase modulator 510 changes the refractive index of the phase modulator 510, which is turn changes the resonant frequencies of the dump-cavity modes 522. While the phase modulator 510 is driven with a single RF tone, one dump-cavity mode 522(f), at most, becomes resonant. When this occurs, the dump-cavity mode 522(0 efficiently couples to the storage-cavity mode 516(0, thereby transferring the energy of the storage-cavity mode 516(0 through the dump optical cavity 514 to form the corresponding optical output pulse 206(0.

[0033] The dump optical cavity 514 may therefore be thought of as an electrically controllable transmission filter through which the energy of each storage-cavity mode 516(0 is transmitted when the corresponding dump-cavity mode 522(0 has a resonant frequency that is the same as that of the storage-cavity mode 516(0. Ignoring any loss in the dump optical cavity 514, the energy of the storage-cavity mode 516(0 may be advantageously transmitted through the dump optical cavity 514 with near-unity transmission. When the dump-cavity mode 522(f)is not resonant, the transmission filter provides essentially zero transmission for the energy of the storage-cavity mode 516(t). In this case, the storage-cavity mode 516(f) is maintained due to the reflectivity of the second mirror 508 (i.e., the storage-cavity mode 516(f ) does not “see” the dump-cavity mode 522(f) that lies outside of the storage optical cavity 504).

[0034] When the phase modulator 510 is driven with a single-tone modulation signal, the dump-cavity modes 522 become sequentially resonant with the dump optical cavity 514, with no more than one of the dump-cavity modes 522 being resonant with its corresponding storage-cavity mode at any time. Therefore, the dump-cavity modes 522 sequentially couple to the storage-cavity modes 516 to sequentially transfer the energies of the storage-cavity modes 516 through the dump optical cavity 514 and form the optical output pulses 206.

[0035] For each dump-cavity mode 522(f) to couple energy out of the corresponding storage-cavity mode 516(f), the resonant frequency of the dump-cavity mode 522(f) should match the resonant frequency of the storage-cavity mode 516(f). To achieve this, the dumpcavity linewidth Fdmay be made larger than the storage-cavity linewidth Fs(i.e., Fd> Ts), which is equivalent to the storage optical cavity 504 having a storage time greater than that of the dump optical cavity 514. This relationship helps ensure that the spectral transmission of the dump optical cavity 514 is broad enough to coincide with the resonant frequency of the storagecavity mode 516(f). For example, the storage-cavity linewidth Fsmay be 1 MHz while the dump-cavity linewidth Fdmay be 500 MHz. However, the storage-cavity linewidth Tsand dump-cavity linewidth Fdmay have other values without departing from the scope hereof.

[0036] To ensure that no more than one of the storage-cavity modes 516 is coupled to the dump optical cavity 514 at any given time, the storage optical cavity 504 and dump optical cavity 514 may both have finesses greater than or equal to N. Combined with the above relationship Fd> Ts, this implies that the dump-cavity free spectral range FSRdexceeds the storage-cavity free spectral range FSRs(i.e., FSRd> FSRs). For example, FSRdmay be 50 GHz while FSRsmay be 200 MHz. However, FSRdand FSRsmay have other values without departing from the scope hereof. In embodiments, the dump-cavity line width Tdis greater than the storage-cavity free-spectral range FSRs.

[0037] The relationship FSRd> FSRsimplies that the longitudinal length ldof the dump optical cavity 514 is less than the longitudinal length lsof the storage optical cavity 504 (i.e., ld< ls), where ldrepresents the average longitudinal length of the dump optical cavity 514. For example, the storage optical cavity 504 may have a longitudinal length lsof 75 cm while the dump optical cavity 514 may have a longitudinal length ldof 2.5 mm. However, thelongitudinal lengths ldand lsmay have other values without departing from the scope hereof. Note that the optical cavities 504 and 514 are not shown in FIG. 5 with the relationship ld< ls.

[0038] The modulation frequency of the modulation signal driving the intracavity phase modulator 510 is low enough such that there is sufficient time for each dump-cavity mode 522(f) to transfer the energy of the corresponding storage-cavity mode 516(i) through the dump optical cavity 514. On the other hand, the modulation frequency is high enough to ensure that all of the storage-cavity modes 516 are sequentially coupled to the dump optical cavity 514 before the energies of the storage-cavity modes 516 decay from the storage optical cavity 504. In embodiments, the modulation frequency is greater than the storage-cavity linewidth Tsand less than the dump-cavity linewidth Td. For example, where Fsis 1 MHz andis 500 MHz, the modulation frequency may be 20 MHz. However, the modulation frequency may have another value without departing from the scope hereof.

[0039] The intracavity phase modulator 510 may be an electro-optic modulator, an elasto-optic modulator, or another type of optical phase modulator known in the art. In some embodiments, the amplitude of the modulation signal equals the half-wave voltageof the phase modulator 510, which provides an optical phase shift of +n to the dump-cavity modes 522 (i.e., the modulation depth is TT). When the phase modulator 510 is driven with a singletone modulation signal, the phase modulator 510 may be electrically resonant at the frequency of the single-tone modulation signal to reduce the amplitude of the modulation signal.

[0040] In FIG. 5, the optical output pulses 206 propagate through free space. A lens 520 focuses all of the optical output pulses 206 onto a focal point, where they may be detected with a spatially multi-mode optical detector (e.g., see the optical detector 106 of FIG. 1). Examples of such optical detectors include, but are not limited to, single-photon counting modules, singlephoton avalanche detectors, superconducting nanowire single-photon detectors, avalanche photodiodes, photomultiplier tubes, silicon photomultipliers, and pin photodiodes. The lens 520 replaces the Al- ay switch 104 of FIG. 2, the optical waveguide 302 of FIG. 3, and the combining optical cavity 402 of FIG. 4. It should be understood that the spatiotemporal optical multiplexer 500 may alternatively be used with the Al-way switch 104, the optical waveguide 302, or the combining optical cavity 402.

[0041] The storage time of the dump optical cavity 514 may be larger than the inverse bandwidth of the optical detector. For photon-counting applications, the dark counts of the optical detector are distributed amongst the pixel of the optical detector so that each pixel has a reduced dark count rate compared with the optical detector itself. This occurs because thespatiotemporal optical multiplexer 500 effectively decreases the temporal width of the signal, which in turn increases its spectral width. This is effectively trading the temporal resolution of the optical detector (i.e., all counts in a time interval T are compressed into a shorter time interval for detection). This technique may be used in a single-pixel configuration to advantageously reduce the dark counts of a photon counter, assuming that its full temporal resolution is not needed for the application at hand.

[0042] In some embodiments, the intracavity phase modulator 510 is a nonlinear optical material having a third-order susceptibilityThis nonlinear material is optically pumped by a control beam at a different wavelength than the optical signals 102 (as opposed to being electrically controlled with an electrical modulation signal). When the intensity of the control beam is ramped, the refractive index of the nonlinear material changes, thereby sequentially bringing the dump-cavity modes 522 into resonance with the storage-cavity modes 516.

[0043] FIG. 6 is a schematic diagram of a spatiotemporal optical multiplexer 600, in accordance with some of the present embodiments. The multiplexer 600 is similar to the spatiotemporal optical multiplexer 500 of FIG. 5 except that the dump optical cavity 514 is formed with a stepped mirror 612 instead of a tilted mirror (e.g., the fourth mirror 512 of FIG. 5). With the stepped mirror 612, the dump optical cavity 514 has a longitudinal length that varies with each “step” along the direction transverse to the optical axis (i.e., along the y axis). For example, the first dump-cavity mode 522(1) has a first longitudinal lengththe second dump-cavity mode 522(2) has a second longitudinal length (2different from lr, and so on up to the IVthdump-cavity mode 522(W), which has an / Vthlongitudinal length lN. Due to these step- wise changes in the longitudinal length between the third mirror 509 and the N steps of the stepped mirror 612, the dump-cavity modes 522 have different resonant frequencies.

[0044] In another embodiment, the fourth mirror 512 of the spatiotemporal optical multiplexer 500 of FIG. 5 is a planar mirror that is not tilted with respect to the optical axis 502. The dump optical cavity 514 further includes an intracavity optical wedge through which each of the dump-cavity modes 522 passes with a different physical length. Due to these different lengths, the dump-cavity modes 522 will have different resonant frequencies.

[0045] FIG. 7 is a flow chart illustrating a method 700 for spatiotemporal optical multiplexing, in accordance with some of the present embodiments. In the step 710 of the method 700, light is coupled into a storage optical cavity to excite one or more of a plurality of storage-cavity modes that are transversely separated. In one example of the step 710, the input optical signals 102 are coupled into the storage optical cavity 504 of the spatiotemporal opticalmultiplexer 500 of FIG. 5 to excite one more of the storage-cavity modes 516. As shown in FIG. 5, the storage-cavity modes 516 are transversely separated along y (see the right-handed Cartesian coordinate system 550 in FIG. 5).

[0046] In the step 720 of the method 700, an intracavity phase modulator is driven to sequentially couple each of the plurality of storage-cavity modes to a respective one of the plurality of dump-cavity modes of a dump optical cavity. The intracavity phase modulator is located in the dump optical cavity and spatially overlaps all of the plurality of dump-cavity modes. The plurality of dump-cavity modes are transversely separated.

[0047] In one example of the step 720, the intracavity phase modulator 510 of FIG. 5 is driven to sequentially couple each storage-cavity modes 5 16(i) to its respective dump-cavity mode 522(i), where i is an integer index that runs from 1 to N. The dump-cavity modes 522 are formed from the dump optical cavity 516 and are transversely separated along y (see coordinate system 550 in FIG. 5). The phase modulator 510 is located in the dump optical cavity 514 and spatially overlaps all of the dump-cavity modes 522.

[0048] In response to the step 720, the dump optical cavity transmits a temporal sequence of output optical pulses generated from the plurality of storage-cavity modes, respectively. As an example of this response, the dump optical cavity 514 of FIG. 5 transmits the A output optical pulses 206. Each output optical pulse 206(i) is derived from an excitation of the corresponding storage-cavity mode 516(i). The A output optical pulses 206 form a temporal sequence in that no more than one of the A output optical pulses 206 is outputted at any time. As shown in FIG. 5, it is not necessary that the output optical pulses forming the temporal sequence occupy the same propagation mode.

[0049] In some embodiments, the method 700 further includes the step 730, in which the temporal sequence of output optical pulses is coupled into a single propagation mode, thereby forming a time-multiplexed optical stream. In one example of the step 730, the A output optical pulses 206 are coupled into the A input ports of the A-way switch 104 to create the time-multiplexed optical stream 208, as shown in FIG. 2. In another example of the step 730, the A output optical pulses 206 are coupled into the optical waveguide 302, as shown in FIG. 3. In another example of the step 730, the A output optical pulses 206 are coupled into one longitudinal mode of the combining optical cavity 402, as shown in FIG. 4. Other optical devices may be used to couple the temporal sequence of output optical pulses into a single propagation mode without departing from the scope hereof.

[0050] The embodiments described herein may he operated in reverse to demultiplex atime-multiplexed optical stream into spatially separated optical signals. Accordingly, any system or device embodiment described herein as a “multiplexer” may also function as a demultiplexer. Similarly, any method embodiment described here as “multiplexing” may be reversed to perform demultiplexing.Applications

[0051] In one application, any of the spatiotemporal optical multiplexers of the present embodiments is combined with a single-photon detector to create a high-speed (e.g., MHz frame rate) single-photon-sensitive camera. This camera may be used, for example, to read out, an atom-array quantum information processor in real time. Advantageously, this camera has a much lower dark noise than the electron-multiplying charge-coupled devices (EMCCDs) conventionally used to read out atom-array processors.

[0052] In another application, any of the spatiotemporal optical multiplexers of the present embodiments is used as a high-speed spatial light modulator (SLM) by operating the multiplexer as a demultiplexer (i.e., in the reverse direction as described above). This two- dimensional (2D) SLM could be used to address many atoms (or other objects) in parallel with several beams of light of the same frequency (acousto-optic deflectors do not have this capability) and with each of these beam being controlled independently and fast (conventional SLMs cannot attain these speeds) using a single high-bandwidth SLM on the input.

[0053] In another application, any of the spatiotemporal optical multiplexers of the present embodiments is used to reduce dark counts of any detector at the cost of reduced bandwidth. Conventional low-light-level detectors have timing resolutions as low as 300 ps, but many cold-atom and ultracold- atom applications do not require timing resolutions less than ~30 ns. A factor of 300 reduction in dark counts can be obtained by temporally compressing optical pulses using the spatiotemporal optical multiplexers disclosed herein.

[0054] The spatiotemporal optical multiplexers of the present embodiments may also be used as optical serializers and deserializers for optical networking (e.g., in the C-band), optical interconnects within a single optical information processing unit, parallel detection of quantum dots, and bio-imaging, among others.

[0055] The spatiotemporal optical multiplexers of the present embodiments may also be implemented using nanophotonics. Advantageously, these implementations may offer higher bandwidths than their free-space counterparts. For example, microlens cavity arrays (e.g., the 518 and 524 of FIG. 5) may be replaced with nanophotonic arrays. Alternatively or additionally, free-space modulators may be replaced with waveguide-based devices and components.Combinations of Features

[0056] Features described above as well as those claimed below may be combined in various ways without departing from the scope hereof. The following examples illustrate possible, non-limiting combinations of features and embodiments described above. It should be clear that other changes and modifications may be made to the present embodiments without departing from the spirit and scope of this invention:

[0057] (Al) A spatiotemporal optical multiplexer includes a storage optical cavity having a plurality of storage-cavity modes that are transversely separated, a dump optical cavity having a plurality of dump-cavity modes that are transversely separated, and an intracavity phase modulator in the dump optical cavity. The intracavity phase modulator spatially overlaps all of the plurality of dump-cavity modes. Each of the plurality of dump-cavity modes is spatially coupled to a respective one of the plurality of storage-cavity modes. The plurality of storage-cavity modes all have the same storage-cavity resonant frequency. The plurality of dump-cavity modes have a respective plurality of dump-cavity resonant frequencies that are distinct.

[0058] (A2) In the spatiotemporal optical multiplexer denoted (Al), the storage optical cavity includes a first mirror, a second mirror facing the first mirror, and a first lens array located between the first mirror and the second mirror. Furthermore, the dump optical cavity includes a third mirror, a fourth mirror facing the third mirror, and a second lens array located between the third mirror and the fourth mirror. Furthermore, the intracavity phase modulator is located between the third mirror and the fourth mirror.

[0059] (A3) In the spatiotemporal optical multiplexer denoted (A2), the fourth mirror includes a stepped mirror.

[0060] (A4) In the spatiotemporal optical multiplexer denoted (A2), the dump optical cavity includes an optically transmissive wedge located between the third mirror and the fourth mirror.

[0061] (A5) In the spatiotemporal optical multiplexer denoted (A4), the fourth mirror includes a planar mirror oriented perpendicularly to an optical axis of the dump optical cavity.

[0062] (A6) In the spatiotemporal optical multiplexer denoted (A2), the fourth mirror includes a planar mirror that is tilted relative to an optical axis of the dump optical cavity.

[0063] (A7) In any of the spatiotemporal optical multiplexers denoted (A2) to (A6), the second mirror is a first reflective surface of a two-sided mirror. Furthermore, the third mirror is a second reflective surface, opposite the first reflective surface, of the two-sided mirror.

[0064] (A8) In any of the spatiotemporal optical multiplexers denoted (A2) to (A6), all of the plurality of storage-cavity modes have the same storage-cavity longitudinal length, the plurality of dump-cavity modes has a respective plurality of dump-cavity longitudinal lengths that are distinct, and the storage-cavity longitudinal length is greater than all of the plurality of dump-cavity longitudinal lengths.

[0065] (A9) In any of the spatiotemporal optical multiplexers denoted (A2) to (A8), the first and second mirrors establish a first optical axis therebetween, the plurality of storage-cavity modes are transversely separated along a first transverse direction that is perpendicular to the first optical axis, the third and fourth mirrors establish a second optical axis therebetween, and the plurality of dump-cavity modes are transversely separated along a second transverse direction that is perpendicular to the second optical axis.

[0066] (A10) In any of the spatiotemporal optical multiplexers denoted (Al) to (A9), the intracavity phase modulator is an electro-optic modulator.

[0067] (Al l) In any of the spatiotemporal optical multiplexers denoted (Al) to (A10), the spatiotemporal optical multiplexer further includes a combining optical cavity coupled to the dump optical cavity such that all of the plurality of dump-cavity modes are coupled to one longitudinal mode of the combining optical cavity.

[0068] (A12) In any of the spatiotemporal optical multiplexers denoted (Al) to (Al l), the spatiotemporal optical multiplexer further includes an optical waveguide coupled to all of the plurality of dump-cavity modes.

[0069] (A13) In any of the spatiotemporal optical multiplexers denoted (Al) to (A12), each of the storage optical cavity and the dump optical cavity has a finesse that is greater than the number of the plurality of storage-cavity modes.

[0070] (A14) In any of the spatiotemporal optical multiplexers denoted (Al) to (A13), the spatiotemporal optical multiplexer further includes an optical detector configured to detect light emitted by the dump optical cavity.

[0071] (A15) In the spatiotemporal optical multiplexer denoted (A14), the dump optical cavity has a dump time that is larger than an inverse bandwidth of the optical detector.

[0072] (A16) In any of the spatiotemporal optical multiplexers denoted (Al) to (A14), the storage optical cavity has a storage-cavity linewidth. Furthermore, the dump optical cavity has a dump-cavity linewidth that is greater than the storage-cavity linewidth.

[0073] (Bl) A method for spatiotemporal optical multiplexing includes coupling light into a storage optical cavity to excite one or more of a plurality of storage-cavity modes that are transversely separated. The method also includes driving an intracavity phase modulator tosequentially couple each of the plurality of storage-cavity modes to a respective one of a plurality of dump-cavity modes of a dump optical cavity. The intracavity phase modulator is located in the dump optical cavity and spatially overlaps all of the plurality of dump-cavity modes. The plurality of dump-cavity modes are transversely separated. The dump optical cavity, in response to said driving, transmits a temporal sequence of output optical pulses generated from the plurality of storage-cavity modes, respectively.

[0074] (B2) In the method denoted (B l), the plurality of storage-cavity modes all have the same storage-cavity resonant frequency. Furthermore, the plurality of dump-cavity modes have a respective plurality of dump-cavity resonant frequencies that are distinct.

[0075] (B3) In either of the methods denoted (Bl) and (B2), said driving the phase modulator includes driving an electro-optic modulator.

[0076] (B4) In the method denoted (B3), said driving the electro-optic modulator includes driving the electro-optic modulator at a modulation frequency that is greater than a linewidth of the storage optical cavity.

[0077] (B5) In either of the methods denoted (B3) and (B4), said driving the electrooptic modulator includes resonantly driving the electro-optic modulator.

[0078] (B6) In any of the methods denoted (Bl) to (B5), the method further includes coupling the temporal sequence of output optical pulses into a longitudinal mode of a combining optical cavity.

[0079] (B7) In any of the methods denoted (Bl) to (B6), the method further includes coupling the temporal sequence of output optical pulses into an optical waveguide.

[0080] (B8) In any of the methods denoted (Bl) to (B7), the method further includes detecting the temporal sequence of output optical pulses with an optical detector.

[0081] (B9) In the method denoted (B8), the optical detector has an inverse bandwidth that is less than a dump time of the dump optical cavity.

[0082] (B10) In any of the methods denoted (Bl) to (B9), each of the storage optical cavity and the dump optical cavity has a finesse that is greater than the number of the plurality of storage-cavity modes.

[0083] (Bl l) In any of the methods denoted (Bl) to (B10), the storage optical cavity has a storage-cavity linewidth. Furthermore, the dump optical cavity has a dump-cavity linewidth that is greater than the storage-cavity linewidth.

[0084] Changes may be made in the above methods and systems without departing from the scope hereof. It should thus be noted that the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limitingsense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method and system, which, as a matter of language, might be said to fall therebetween.

Claims

CLAIMSWhat is claimed is:

1. A spatiotemporal optical multiplexer, comprising: a storage optical cavity having a plurality of storage-cavity modes that are transversely separated; a dump optical cavity having a plurality of dump-cavity modes that are transversely separated; and an intracavity phase modulator in the dump optical cavity, the intracavity phase modulator spatially overlapping all of the plurality of dump-cavity modes; wherein: each of the plurality of dump-cavity modes is spatially coupled to a respective one of the plurality of storage-cavity modes; the plurality of storage-cavity modes all have the same storage-cavity resonant frequency; and the plurality of dump-cavity modes have a respective plurality of dump-cavity resonant frequencies that are distinct.

2. The spatiotemporal optical multiplexer of claim 1, wherein: the storage optical cavity comprises a first mirror, a second mirror facing the first mirror, and a first lens array located between the first mirror and the second mirror; the dump optical cavity comprises a third mirror, a fourth mirror facing the third mirror, and a second lens array located between the third mirror and the fourth mirror; and the intracavity phase modulator is located between the third mirror and the fourth mirror.

3. The spatiotemporal optical multiplexer of claim 2, the fourth mirror comprising a stepped mirror.

4. The spatiotemporal optical multiplexer of claim 2, the dump optical cavity includingan optically transmissive wedge located between the third mirror and the fourth mirror.

5. The spatiotemporal optical multiplexer of claim 4, the fourth mirror comprising a planar mirror oriented perpendicularly to an optical axis of the dump optical cavity.

6. The spatiotemporal optical multiplexer of claim 2, the fourth mirror comprising a planar mirror that is tilted relative to an optical axis of the dump optical cavity.

7. The spatiotemporal optical multiplexer of claim 2, wherein: the second mirror is a first reflective surface of a two-sided mirror; and the third mirror is a second reflective surface, opposite the first reflective surface, of the two-sided mirror.

8. The spatiotemporal optical multiplexer of claim 2, wherein: all of the plurality of storage-cavity modes have the same storage-cavity longitudinal length; the plurality of dump-cavity modes has a respective plurality of dump-cavity longitudinal lengths that are distinct; and the storage-cavity longitudinal length is greater than all of the plurality of dump-cavity longitudinal lengths.

9. The spatiotemporal optical multiplexer of claim 2, wherein: the first and second mirrors establish a first optical axis therebetween; the plurality of storage-cavity modes are transversely separated along a first transverse direction that is perpendicular to the first optical axis; the third and fourth mirrors establish a second optical axis therebetween; and the plurality of dump-cavity modes are transversely separated along a second transverse direction that is perpendicular to the second optical axis.

10. The spatiotemporal optical multiplexer of claim 1, the intracavity phase modulator comprising an electro-optic modulator.

11. The spatiotemporal optical multiplexer of claim 1, further comprising a combiningoptical cavity coupled to the dump optical cavity such that all of the plurality of dumpcavity modes are coupled to one longitudinal mode of the combining optical cavity.

12. The spatiotemporal optical multiplexer of claim 1, further comprising an optical waveguide coupled to all of the plurality of dump-cavity modes.

13. The spatiotemporal optical multiplexer of claim 1, each of the storage optical cavity and the dump optical cavity having a finesse that is greater than the number of the plurality of storage-cavity modes.

14. The spatiotemporal optical multiplexer of claim 1, further comprising an optical detector configured to detect light emitted by the dump optical cavity.

15. The spatiotemporal optical multiplexer of claim 14, the dump optical cavity having a dump time that is larger than an inverse bandwidth of the optical detector.

16. The spatiotemporal optical multiplexer of claim 1, wherein: the storage optical cavity has a storage-cavity linewidth; and the dump optical cavity has a dump-cavity linewidth that is greater than the storagecavity linewidth.

17. A method for spatiotemporal optical multiplexing, comprising: coupling light into a storage optical cavity to excite one or more of a plurality of storage-cavity modes that are transversely separated; and driving an intracavity phase modulator to sequentially couple each of the plurality of storage-cavity modes to a respective one of a plurality of dump-cavity modes of a dump optical cavity, the intracavity phase modulator being located in the dump optical cavity and spatially overlapping all of the plurality of dumpcavity modes, the plurality of dump-cavity modes being transversely separated; wherein the dump optical cavity, in response to said driving, transmits a temporal sequence of output optical pulses generated from the plurality of storage-cavity modes, respectively.

18. The method of claim 17, wherein:the plurality of storage-cavity modes all have the same storage-cavity resonant frequency; and the plurality of dump-cavity modes have a respective plurality of dump-cavity resonant frequencies that are distinct.

19. The method of claim 17, where said driving the phase modulator comprises driving an electro-optic modulator.

20. The method of claim 19, wherein said driving the electro-optic modulator comprises driving the electro-optic modulator at a modulation frequency that is greater than a linewidth of the storage optical cavity.

21. The method of claim 19, wherein said driving the electro-optic modulator comprises resonantly driving the electro-optic modulator.

22. The method of claim 17, further comprising coupling the temporal sequence of output optical pulses into a longitudinal mode of a combining optical cavity.

23. The method of claim 17, further comprising coupling the temporal sequence of output optical pulses into an optical waveguide.

24. The method of claim 17, further comprising detecting the temporal sequence of output optical pulses with an optical detector.

25. The method of claim 24, the optical detector having an inverse bandwidth that is less than a dump time of the dump optical cavity.

26. The method of claim 17, each of the storage optical cavity and the dump optical cavity having a finesse that is greater than the number of the plurality of storage-cavity modes.

27. The method of claim 17, wherein: the storage optical cavity has a storage-cavity linewidth; and the dump optical cavity has a dump-cavity linewidth that is greater than the storagecavity linewidth.

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