This thesis describes a series of experimental research primarily revolving around the applications of periodically poled lithium niobate (PPLN). Initially, a set of work is described regarding the fabrication and characterisation of PPLN ridge waveguides designed for frequency doubling into the blue/UV range. These waveguides are fabricated via diffusion of zinc into the crystal structure. A precision dicing process creates the sidewall definition for modal confinement and also optical facet preparation. Preliminary results with the most efficient waveguide fabricated produced a maximum conversion efficiency of 6.9%/W in a 14 mm-long device, generating 391 nm light. This was performed with a novel near-IR VECSEL; designed and built as part of a collaboration with the research undertaken in this thesis to investigate new routes for miniaturised UV sources. The development of a non-destructive route to characterise the physical dimensions of a ridge waveguide for nonlinear applications is also presented. This presented methodology uses white light interferometry and standard edge recognition algorithms to determine the waveguide width profile. In combination with numerical propagation constant data for these waveguides, a numerical phasematching spectra can be calculated. A comparison to the experimental phasematching spectra of the second-harmonic generation process enables the discrimination between issues in the fabrication processes. Using bulk PPLN, a final application is presented for communications using mid-IR radiation and all commercial components. This configuration involves the use of differencefrequency generation followed by sum-frequency generation to create the transmitter and receiver systems, respectively. This enables final detection of 809 nm light (firmly within the silicon absorption band). Bit error ratios as low as 10−7 from single-pass nonlinear conversion for both systems without the need for an additional optical amplifier in the receiver. Finally, the propagation of relative intensity noise within the system as the source lasers propagate through a two step nonlinear conversion is presented and discussed within.
This thesis describes a series of experimental research primarily revolving around the applications of periodically poled lithium niobate (PPLN). Initially, a set of work is described regarding the fabrication and characterisation of PPLN ridge waveguides designed for frequency doubling into the blue/UV range. These waveguides are fabricated via diffusion of zinc into the crystal structure. A precision dicing process creates the sidewall definition for modal confinement and also optical facet preparation. Preliminary results with the most efficient waveguide fabricated produced a maximum conversion efficiency of 6.9%/W in a 14 mm-long device, generating 391 nm light. This was performed with a novel near-IR VECSEL; designed and built as part of a collaboration with the research undertaken in this thesis to investigate new routes for miniaturised UV sources. The development of a non-destructive route to characterise the physical dimensions of a ridge waveguide for nonlinear applications is also presented. This presented methodology uses white light interferometry and standard edge recognition algorithms to determine the waveguide width profile. In combination with numerical propagation constant data for these waveguides, a numerical phasematching spectra can be calculated. A comparison to the experimental phasematching spectra of the second-harmonic generation process enables the discrimination between issues in the fabrication processes. Using bulk PPLN, a final application is presented for communications using mid-IR radiation and all commercial components. This configuration involves the use of differencefrequency generation followed by sum-frequency generation to create the transmitter and receiver systems, respectively. This enables final detection of 809 nm light (firmly within the silicon absorption band). Bit error ratios as low as 10−7 from single-pass nonlinear conversion for both systems without the need for an additional optical amplifier in the receiver. Finally, the propagation of relative intensity noise within the system as the source lasers propagate through a two step nonlinear conversion is presented and discussed within.
The quantum nature of light at the single photon level allows for unique applications that classical physics neither predicts nor describes. Most notably, appropriate conditions may cause the states of photons with indistinguishable properties to become entangled, enabling novel approaches to quantum computation, secure communications, and metrology. \n \nAny operational quantum information network transporting entangled-photon states must establish a high-fidelity link between its distant nodes despite the inherent fragility of entangled states. However, the lack of a universal operating wavelength for all optical devices makes this a substantial challenge. Telecommunication fibers, entangled photon pair sources, quantum optical memories, and quantum repeaters all function in disparate spectral windows dictated by the properties of the materials used to fabricate them. Quantum frequency conversion (QFC), the single-photon limit of nonlinear parametric sum and difference frequency generation in optics, offers a bridge between spectral regions with full preservation of quantum state character, including entanglement. \n \nPeriodically poled optical waveguides in ferroelectric crystals are versatile tools in nonlinear optics. Confining the nonlinear interaction to a waveguide greatly enhances its efficiency compared to bulk optics. Periodic poling, the process of periodically inverting the domains of a ferroelectric medium using a strong electric field, enables a variety of quasi-phase matching configurations to engineer a desired nonlinear interaction. \n \nThis work concentrates on the design, development, fabrication, and characterization of a titanium-diffused periodically poled lithium niobate (Ti:PPLN) waveguide device. This device is designed to execute single-step quantum frequency conversion from 369.5nm to 1550nm in order to facilitate a quantum state transfer between standard fiber telecommunication wavelengths and an atomic quantum memory system employing trapped ytterbium ions. \n \nThe creation of phase matching conditions for such an extreme distance in frequency demands precise control of high magnitude electric fields on the single-micron scale. As the first demonstration of its kind in Ti:PPLN, the development of these devices included novel improvements to existing fabrication methods, improving the state-of-the-art of precision, quality, and yield for poling of ferroelectric crystals.
Entanglement is the hallmark of quantum mechanics. Quantum entanglement -- putting two or more identical particles into a non-factorable state -- has been leveraged for applications ranging from quantum computation and encryption to high-precision metrology. Entanglement is a practical engineering resource and a tool for sidestepping certain limitations of classical measurement and communication. Engineered nonlinear optical waveguides are an enabling technology for generating entangled photon pairs and manipulating the state of single photons. This dissertation reports on: i) frequency conversion of single photons from the mid-infrared to 843nm as a tool for incorporating quantum memories in quantum networks, ii) the design, fabrication, and test of a prototype broadband source of polarization and frequency entangled photons; and iii) a roadmap for further investigations of this source, including applications in quantum interferometry and high-precision optical metrology. \n \nThe devices presented herein are quasi-phase-matched lithium niobate waveguides. Lithium niobate is a second-order nonlinear optical material and can mediate optical energy conversion to different wavelengths. This nonlinear effect is the basis of both quantum frequency conversion and entangled photon generation, and is enhanced by i) confining light in waveguides to increase conversion efficiency, and ii) quasi-phase matching, a technique for engineering the second-order nonlinear response by locally altering the direction of a material's polarization vector. Waveguides are formed by diffusing titanium into a lithium niobate wafer. Quasi-phase matching is achieved by electric field poling, with multiple stages of process development and optimization to fabricate the delicate structures necessary for broadband entangled photon generation. \n \nThe results presented herein update and optimize past fabrication techniques, demonstrate novel optical devices, and propose future avenues for device development. Quantum frequency conversion from 1848nm to 843nm is demonstrated for the first time, with >75% single-photon conversion efficiency. A new electric field poling methodology is presented, combining elements from multiple historical techniques with a new fast-feedback control system. This poling technique is used to fabricate the first chirped-and-apodized Type-II quasi-phase-matched structures in titanium-diffused lithium niobate waveguides, culminating in a measured phasematching spectrum that is predominantly Gaussian (R^2 = 0.80), nearly eight times broader than the unchirped spectrum, and agrees well with simulations.
We describe the design and application of domain-engineered, periodically poled lithium niobate (PPLN) for use to produce entangled photons and for other tools in quantum information and communications. By specially designing and controlling the PPLN poling pattern, multiple nonlinear optical processes can be simultaneously phasematched. This capability can be used to generate polarization-entangled photon pairs through type-II spontaneous parametric downconversion. The single PPLN crystal is designed to produce both the |HV〉 and |VH〉 states where the downconverted photons are distinguishable by wavelengths, which enables generation of post-selection-free, polarization-entangled twin photons. We describe the design and fabrication of the PPLN crystal, and initial experimental results for downconversion of a 775 nm pump to 1532 nm and 1567 nm orthogonally polarized photons. We also discuss other applications of engineered optical frequency conversion for quantum information including the use of dual-wavelength upconversion as a beamsplitter to route or analyze photons.
Periodically poled lithium niobate waveguides (PPLN/W) are considered to be one of the most useful toolboxes for enabling quantum communication experiments. Thanks to the high optical confinement over longer lengths than in bulk configurations (a few cm in our case), such structures provide highly efficient non-linear interactions, i.e., in parametric downconversion, or sum and difference frequency generation regimes. Within the framework of long-distance quantum communication at telecom wavelengths, PPLN/Ws have therefore been proved to be a key ingredient for building ultrabright sources of time-bin, as well as polarization entangled photons, and for photonic quantum interfaces providing coherent wavelength conversions from telecom to visible wavelength range, and conversely. During the presentation, we will discuss some recent experimental advances regarding polarization entanglement sources and quantum interfaces.
Summary form only given.There been increasing interest in realizing optical signal processing functions in optical communication systems based on nonlinear optical parametric processes. In the proposed ultrashort pulse optical code-division multiple access (CDMA) systems, in order to realize multiplexing and demultiplexing for multichannel operation, a nonlinear element is needed to distinguish between correctly decoded signals and incorrectly decoded multiaccess interference, which have comparable average optical power but carry different spectral information. It had been proposed to realize all-optical recognition of optical phase-coded ultrashort waveforms by using second-harmonic generation in long nonlinear crystals, which could have advantages in contrast ratio, operating power and detection efficiency over other proposed schemes. While high contrast ratios have been demonstrated based on such a scheme using bulk periodically poled lithium niobate (PPLN) crystals, the high efficiency has not yet been demonstrated. We use a simple combination of PPLN SHG waveguides and conventional photodetectors to realize all-optical recognition of spectrally phase coded optical pulses, which shows the potential of much higher conversion efficiency compared to previous bulk experiments, which is crucial for real-time operation with a high signal to noise ratio.
Broadband nonlinear optical devices play a critical role in both classical and quantum optics. Here, we design and fabricate a 6.82-mm-long step-chirped periodically poled lithium niobate (CPPLN) waveguide on lithium niobate on insulator, which enables quasi-phase matching over a broad bandwidth for second-harmonic generation (SHG) and spontaneous parametric down-conversion (SPDC). The SHG achieves an average efficiency of 91%/W/cm 2 over the first-harmonic wavelength range of 1510 nm–1620 nm, paving the way for realizing SPDC across a wide range of pump wavelengths. For SPDC, by tuning the pump wavelength to 775 nm, 780 nm, and 785 nm, we achieve broadband photon-pair generation with a maximum full bandwidth and brightness up to 99 THz (846 nm) and 21 MHz/mW/nm, respectively. Our findings provide an efficient and experiment-friendly approach for generating broadband photon pairs, which holds significant promise for advancing applications in quantum metrology.