In this Internet era, rapid transitioning of many aspects of medical, defense and commercial applications to digital platforms dictates their heavy reliance on optical communication systems. This trend places severe challenges on contemporary communication systems, expected to provide wide-bandwidth services and high degree of information security, with minimum energy dissipation. To meet the rising bandwidth demand, current bandwidth limited technology requires implementation of parallel processing; however, energy dissipation of such a system is globally and environmentally significant. A solution to this problem is envisioned in a form of an optical processor, a device recognized for its high speed, low loss and signal quality preservation. In this dissertation we investigate two optical processor roles: direct signal processing role (all-optical switching) and preprocessing role in preamplified few photon detection. \tIn the direct signal processing role, an optical switch controls the state of one wave by a low-energy control signal. It is realized in a highly nonlinear fiber, a medium acclaimed for its inherent femtosecond response time, a low loss and a high figure of merit. The switching functionality is implemented by employing the four wave mixing process in the depleted pump regime. To maximize the energy exchange, we revisit the saturated parametric mixer phase matching condition and propose a novel dispersion engineering method enhancing the performance. This study advances the understanding of the phase-matched interaction in the saturated parametric mixer and enables the first realization of a few-photon controlled silica-fiber-based device. \tIn the preprocessing role, the all-fiber photonic sampler is implemented prior to detection of a preamplified few photon signal with a low duty cycle; the sampler function is limited to amplified spontaneous emission noise alleviation. The proposed few photon detection method shows an order of magnitude improvement in the Bit Error Ratio and detection efficiency with respect to conventional preamplified receiver, and operates at speed beyond the reach of current single photon detector technology.
In this Internet era, rapid transitioning of many aspects of medical, defense and commercial applications to digital platforms dictates their heavy reliance on optical communication systems. This trend places severe challenges on contemporary communication systems, expected to provide wide-bandwidth services and high degree of information security, with minimum energy dissipation. To meet the rising bandwidth demand, current bandwidth limited technology requires implementation of parallel processing; however, energy dissipation of such a system is globally and environmentally significant. A solution to this problem is envisioned in a form of an optical processor, a device recognized for its high speed, low loss and signal quality preservation. In this dissertation we investigate two optical processor roles: direct signal processing role (all-optical switching) and preprocessing role in preamplified few photon detection. \tIn the direct signal processing role, an optical switch controls the state of one wave by a low-energy control signal. It is realized in a highly nonlinear fiber, a medium acclaimed for its inherent femtosecond response time, a low loss and a high figure of merit. The switching functionality is implemented by employing the four wave mixing process in the depleted pump regime. To maximize the energy exchange, we revisit the saturated parametric mixer phase matching condition and propose a novel dispersion engineering method enhancing the performance. This study advances the understanding of the phase-matched interaction in the saturated parametric mixer and enables the first realization of a few-photon controlled silica-fiber-based device. \tIn the preprocessing role, the all-fiber photonic sampler is implemented prior to detection of a preamplified few photon signal with a low duty cycle; the sampler function is limited to amplified spontaneous emission noise alleviation. The proposed few photon detection method shows an order of magnitude improvement in the Bit Error Ratio and detection efficiency with respect to conventional preamplified receiver, and operates at speed beyond the reach of current single photon detector technology.
The stagnating increase in data transmission capacity in optical communication systems combined with the ever growing demand of transmission bandwidth is leading to an impending capacity crunch, referring to the point in time after which the available bandwidth of the individual user starts to decrease. To postpone this point in time, existing technologies in terms of data transmission through optical fibers must be optimized and new degrees of freedom must be introduced to continue the exponential increase in available bandwidth; space-division multiplexing is believed to be the strongest candidate for another degree of freedom in transmission fibers. This thesis is two-fold: firstly, starting at Maxwell’s equations and basic principles of quantum mechanics, a semi-classical model of the noise properties of fiber optical parametric amplifiers and frequency converters is presented. The model accounts for multiple effects present in nonlinear fibers such as four-wave mixing, Raman scattering, distributed loss, and dispersion, and it is valid in the depleted pump regime. After validating the model against well-known results of quantum models, the model is used to predict the impacts of Raman noise, loss, and pump depletion on the noise properties of parametric frequency conversion and phase-insensitive and phase-sensitive parametric amplification. An important part of realizing space-division multiplexing is the ability of optical signal processing so the second part of this thesis addresses few-mode Raman fiber amplifiers and parametric amplifiers and frequency converters. A model of weak random linear mode coupling in the pump of a two-mode distributed Raman fiber amplifier is presented and it is shown that an amplification noise figure induced by mode coupling increases with the degree of mode coupling and that this tendency increases as the pump depletes. Also, a very low mode-dependent gain of 0.25 dB per 10 dB gain is experimentally demonstrated in a two-mode distributed Raman fiber amplifier by exciting the pump in a combination of two modes. A comprehensive model of four-wave mixing in two-mode fibers acvi counting for six simultaneous processes is derived, and the conversion efficiency from signal to idler in the four-wave mixing processes of phase conjugation and Bragg scattering in two two-mode fibers with different phase matching properties are experimentally investigated. A conversion efficiency of > −2.70 dB is demonstrated for Bragg scattering in the conversion of a signal in the LP01-mode to the idler in the LP11-mode; the signal-to-idler separation is ∼ 25 nm. Good qualitative agreement between experiments and theory is found for both processes in both fibers.
Parametric processes in materials presenting a second- or third-order nonlinearity have been widely used to demonstrate a wide range of all-optical signal processing functionalities, including amplification, wavelength conversion, regeneration, sampling, switching, modulation format conversion, optical phase conjugation, etc.. The recent evolution of optical fiber communication systems towards advanced modulation formats making use of the phase dimension, as well as polarization and, more recently, space-multiplexing, has created new requirements, as well as new opportunities, for parametric all-optical signal processing. In this presentation, we will review our recent results on the demonstration of all-optical parametric signal processing using different nonlinear platforms, including highly nonlinear optical fibers (HNLFs), silicon nanowires, and periodically-poled lithium niobate (PPLN) waveguides. In particular, we will show how phase-sensitive processes can be engineered to demonstrate phase-quadrature separation, which we have recently demonstrated in HNLFs and PPLN waveguides. Silicon nanowires are particularly attractive for signal processing thanks to their compact size, CMOS compatible fabrication process, degrees of freedom in dispersion engineering, and high nonlinear coefficient. However, the detrimental effect of free-carrier absorption induced by two-photon absorption has so far prevented them from being used for the demonstration of phase-sensitive processing. Thanks to the introduction of p-i-n junctions across silicon waveguides, we have recently been able to demonstrate phase-sensitive extinction ratios as high as 20 dB, allowing the phase regeneration of phase-modulated signals under continuous wave pumping operation. One of the well-known limitations of planar waveguide devices for all-optical signal processing is their inherent polarization sensitivity. We will show how the introduction of polarization-diversity circuits relying on efficient and wideband polarization splitters and rotators can overcome this limitation. Finally, we will also discuss the introduction of signal processing functionalities that are compatible with the novel dimension of space multiplexing. More specifically, we will show how mode-selective wavelength conversion based on four-wave mixing can be realized in a multimode silicon waveguide.
In the context of rapid technological advancements, progress in chip manufacturing technology is propelling electronic devices toward miniaturization and high efficiency at an unprecedented pace. The fundamental principles of quantum mechanics are beginning to play a crucial role in modern electronic devices, providing the foundation for new technological methods and research directions. Optical quantum signal processing, as an important branch of quantum technology, utilizes the quantum properties of photons for information processing and transmission, offering advantages such as high bandwidth, low noise, and strong resistance to interference. However, current optical quantum measurement and control instruments face significant challenges in terms of real-time performance and programming complexity, resulting in reduced data processing efficiency and significantly increased development costs. This study implements a real-time processing algorithm for optical quantum signals on a Field-Programmable Gate Array (FPGA). The algorithm possesses advantages of real-time performance, low power consumption, and high integration. It allows for rapid functionality switching and efficient optical quantum signal processing. The design specifically aims to enhance the signal-to-noise ratio, significantly improving measurement accuracy and reliability. Experimental results indicate that this method substantially enhances the measurement precision and signal-to-noise ratio of optical quantum signals. This research provides an efficient and reliable new method for optical quantum signal processing, laying a solid foundation for further studies in quantum communication, quantum computing, and quantum precision measurement.
Optical parametric amplifiers rely on second-order susceptibility (three-wave mixing) or third-order susceptibility (four-wave mixing) in a nonlinear process where the energy of incoming photons is not changed (elastic scattering). In the latter case, two pump photons are converted to a signal and to an idler photon. Under certain conditions, related to the phase evolution of the waves involved, this conversion can be very efficient, resulting in large amplification of an input signal. As the nonlinear process can be very fast, all-optical applications aside from pure amplification are also possible. If the amplifier is implemented in an optical input-phase-sensitive manner, it is possible to amplify a signal wave without excess noise, i.e., with a noise figure of 0 dB. In this paper, we will provide the fundamental concepts and theory of such amplifiers, with a focus on their implementation in highly nonlinear optical fibers relying on four-wave mixing. We will discuss the distinctions between phase-insensitive and phase-sensitive operation and include several experimental results to illustrate their capability. Different applications of parametric amplifiers are also discussed, including their use in optical communication links.
Q uantum entanglement refers to the nonclassical dependency of physically separated quantum systems. It is an essential resource that must be freely available for implementing many of the novel functions of quantum information processing, such as database searching, clock synchronization, teleportation, computing and cryptography.1 The efficient generation and transmission of quantum entanglement is therefore of prime importance. With the ubiquitous standard optical fiber serving as the transmission medium and the widespread availability of efficient active and passive fiber devices, technological synergy between the generation and propagation components of the overall quantum network can be achieved by deploying sources of entanglement that rely on the nonlinearity of the fiber itself. Such fiber-based sources of quantum entanglement would also have the advantage of modal purity over their crystal counterparts,2,3 which is very important for realizing complex networks that involve many quantum operations. In our quest to develop fiber-based devices for quantum information processing, we have recently demonstrated an allfiber source of quantum-correlated photon pairs.4 The operating physical mechanism in this source is nondegenerate fourphoton mixing, wherein two pump photons scatter through the Kerr nonlinearity of the fiber to create simultaneous signal and idler photons. Our experiment is conducted near the zero-dispersion wavelength of standard dispersion-shifted fiber, where such scattering is enhanced owing to phase matching of the photon wave functions. It must be pointed out here that in a conventional wavelength-divisionmultiplexed classical optical communication line, one strives to suppress the fourphoton or four-wave mixing process, which otherwise causes cross talk between the wavelength channels and sets limits on the total data capacity of the communication line. In our experiment, the four-photon mixing takes place in a nonlinear-fiber Sagnac interferometer (NFSI), which we previously used to generate quantum-correlated twin beams in the fiber.5 The pump is a mode-locked train of ~3-ps-long pulses. The pulsed operation serves two important purposes: the NFSI amplifier can be operated at low average powers and the production of the signal/ idler fluorescence photons is confined to well-defined temporal windows, allowing use of a gated detection scheme to increase the signal-to-noise ratio. To measure the nonclassical (i.e., quantum) correlations between the signal and the idler photons, one must effectively suppress the pump photons from reaching the detectors. Since a typical pump pulse contains ~108 photons and we are interested in detecting ~0.01 signal (idler) photons/pulse, a pump-to-signal (idler) rejection ratio in excess of 100 dB is required. We achieved this specification by sending the fluorescence photons through a free-space double-grating spectral filter that separates the signal and idler photons from each other and from the pump photons not rejected by the NFSI. To demonstrate the quantum nature of the four-photon scattering at the single photon level, we assembled a photonFigure 1.C oincidence rates as a function of the single-photon rates in two different cases: signalidler fluorescence produced by a pump pulse (diamonds) and signal-idler fluorescence produced by two consecutive pump pulses (triangles). T he line represents the calculated accidental counts. T he inset shows a plot of the detected idler photons as a function of the injected pump photons (hollow circles). A second-order polynomial is shown to fit the experimental data. T he contributions of the dark counts, linear scattering and quadratic scattering are plotted separately as well. counting apparatus for detecting the signal and idler photons in coincidence. This apparatus is based on commercial InGaAs avalanche photodiodes operating in the gated Geiger mode.4 In the inset in Fig. 1, we show the number of scattered photons, NS, versus the number of pump photons, NP, injected into the NFSI. We fit the experimental data with a second-order polynomial fit, including the number of dark counts during the gate interval, ND. The fit clearly shows that the quadratic scattering ( ∝NP) in the fiber can dominate over the residual linear scattering ( ∝NP) of the pump, owing to imperfect filtering. The main body of Fig. 1 shows the coincidence-counting results. The diamonds represent the rate of coincidence counts versus the rate of the signal (idler) photons generated during the same pump pulse. The triangles represent the measured coincidence rate versus the signalphoton count rate when the signal is delayed with respect to the idler by one pulse period. For two independent photon sources, each with a count rate RS <<1, the accidental coincidence rate RC is given by RC = RS 2, regardless of the photon statistics of the sources. This quadratic relation is plotted as the solid line in Fig. 1, which fits the delayed-coincidence data (triangles) very well. These measurements show that while the fluorescence photons produced by the adjacent pump pulses are independent, those coming from the same pump pulse show a strong correlation, which is a signature of their non-classical behavior. This work was supported by the Department of Defense Multidisciplinary University Research Initiative program administered by the Army Research Office under Grants DAAD19-00-1-0177 and DAAD19-00-1-0469, and by the U.S. Office of Naval Research under Grant N00014-91-J-1268.
This dissertation explores the role of optimized resources in facilitating efficient optical quantum information processing and sensing. Three key applications are considered: quantum interferometry, detector characterization, and multi-photon processing. For each application, we examine the underlying quantum resource states and quantum technology resources. Careful attention is paid to how these resources can be optimized to maximize the efficiency of the overarching application in terms of resources consumed, whether photonic, temporal, or infrastructural. Such optimization is an important step in fully realizing the potential of quantum science as it emerges from the laboratory and into the mainstream. For quantum interferometry, we investigate the traditional trade-offs between two-photon quantum interference and single-photon classical interference in terms of measurement resolution and resilience against optical loss and background. We show how we can overcome these trade-offs by replacing conventional single photons with highly non-degenerate energy-entangled photons, which are more optimized for achieving high information content on a per-photon basis. In combination with a specialized dual-wavelength interferometer, our high per-photon information efficiency allows us to achieve nanometer-scale resolution in a matter of seconds with only O(10^4) photon pairs. We then show how this entanglement-enhanced quantum sensing modality can allow for new studies in challenging regimes, such as characterizing lossy thin films or detecting distant vibrations. Then, for detector characterization, we consider the instrumental role of photon detectors in various optical quantum science protocols. As these protocols expand into spectral bands with growing technological applications, such as the mid-infrared, there is a developing need to characterize suitable detectors, with their efficiencies being one of the most important metrics. Typical methods for efficiency characterization, whether classical or quantum, can face the challenge of being infrastructure-intensive, with some requiring resources such as independent reference standards or large bulk-optics apparatuses. To help realize a resource-efficient characterization system for mid-infrared detectors, we have developed a compact and portable solution involving highly non-degenerate photon pairs produced by a fiber-coupled waveguide device. We evaluate the performance of early prototypes and identify next steps for improved device engineering. Lastly, for multi-photon processing, we focus on the single-photon source, given its key role in preparing the required photonic states. From a scalability perspective, efficient single-photon generation is essential for performing many large-scale multi-photon protocols within useful timescales. To meet this requirement, we explore how multiplexing techniques allow us to pair heralded single photons with photon routing, storage, and counting technologies to realize an efficient single-photon source. We then discuss our efforts to implement and integrate optimized versions of these technologies to address limitations faced by earlier implementations with regard to preparing pure single photons at a high rate. This discussion includes an in-depth analysis of our work related to realizing an efficient photon-number-resolving detector.