Recent works have demonstrated how silicon nitride integrated photonics can be endowed with photoinduced second-order nonlinearities for efficient frequency conversion. Here we will showcase how highly-efficient second-harmonic generation in a microresonator can be combined with self-injection locking to a DFB laser to create a standalone dual-wavelength displaying high output power, conversion efficiency and hertz-level coherence in an integrated fashion. We will also cover how the photoinduced nonlinearity can trigger cascaded effects, expanding the operation range and functionality of the microresonator, and discuss how silicon nitride microrings can be further mode engineered to provide combined high efficiency and wide tunability of the nonlinear processes.
The centrosymmetric structure of stoichiometric silicon nitride inhibits the realization of second-order nonlinear processes in this low-loss, complementary-metal-oxide-semiconductor fabrication-compatible platform. Nevertheless, linear electro-optic modulation is an essential functionality desired for implementation in photonic integrated circuits. This study presents the successful achievement of electro-optical modulation in a silicon nitride microring resonator, employing thermally assisted electric-field poling. With an inscribed electric field of 100 V/μm within the silicon nitride waveguide, an effective second-order susceptibility of 0.45 pm/V is induced. Leveraging silicon nitride as the active material for electro-optic modulation, we determined the operational bandwidth of the device, constrained by the electrode design, to be 78 MHz. Furthermore, we demonstrate the capability of the device to modulate data at bitrates of up to 75 Mb/s. Our findings highlight the potential of linear electro-optical modulation in the silicon nitride integrated platform.
The rapid development of photonic applications calls for scalable, miniaturized power efficient integrated circuits. Thin film lithium niobate (TFLN) now emerges as a major photonic platform for integration of advanced functionalities such as based on nonlinear optics. We report experimentally efficient second harmonic generation in periodically poled lithium niobate waveguides and design rules for nonlinear frequency conversion, including a study on tolerances on the dimensions and poling parameters. Our work aims at establishing reliable and versatile nonlinear building blocks for scalable TFLN photonic integrated circuits.
On-chip coherent light generation has wide-ranging applications in metrology, spectroscopy, quantum optics, etc. In this study, we demonstrate the generation of coherent light from a silicon-nitride microring resonator using cascaded nonlinear processes. This involves a telecom pump laser and its efficiently generated second harmonic through the coherent photogalvanic effect. By leveraging second- and third-order nonlinear effects such as harmonic generation, (stimulated) four-wave mixing, and optical parametric oscillation, we achieve the generation of UV, visible, and near-infrared light. This study highlights the potential of silicon nitride integrated photonics in producing broad-spectrum light sources at wavelengths beyond the capabilities of conventional lasers.
KEYWORDS: Interferograms, Angular momentum, Cameras, Signal intensity, Receivers, Interferometry, Superposition, Signal detection, Holography, Signal processing
Orbital angular momentum (OAM) spectrum diagnosis is a fundamental building block for diverse OAM-based systems. Among others, the simple on-axis interferometric measurement can retrieve the amplitude and phase information of complex OAM spectra in a few shots. Yet, its single-shot retrieval remains elusive, due to the signal–signal beat interference inherent in the measurement. Here, we introduce the concept of Kramers–Kronig (KK) receiver in coherent communications to the OAM domain, enabling rigorous, single-shot OAM spectrum measurement. We explain in detail the working principle and the requirement of the KK method and then apply the technique to precisely measure various characteristic OAM states. In addition, we discuss the effects of the carrier-to-signal power ratio and the number of sampling points essential for rigorous retrieval and evaluate the performance on a large set of random OAM spectra and high-dimensional spaces. Single-shot KK interferometry shows enormous potential for characterizing complex OAM states in real time.
Integrated optics has shown itself very convenient for exploiting nonlinear processes as it results in high confinement factor, freedom of dispersion engineering and compactness. However, the choice of materials is crucial for the development of nonlinear systems. Ideally, one looks for a platform that offers high second and/or third order nonlinearities, low loss and ease of fabrication. Silicon nitride (Si3N4) is now proven to be a good platform for frequency conversion based on third order nonlinearity. Supercontinuum generation (SCG) was obtained in the near-IR and mid-IR regions by pumping waveguides with common fiber lasers. It resulted in broadband coherent combs extending in the mid-IR thanks to dispersive wave generation. Yet, Si3N4 does not exhibit any second order nonlinearity desirable for comb self-referencing via second-harmonic generation (SHG). On the other hand, lithium niobate (LiNbO3) is widely used in integrated photonics for second order nonlinear processes. In our work, we exploit a hybrid Si3N4-LiNbO3 photonic integrated platform that combines maturity and dispersion engineering capabilities of Si3N4 integrated photonics with second-order nonlinear properties of LiNbO3 bypassing challenging lithium niobate etching. We study numerically and experimentally the potential of SCG and SHG for frequency comb self-referencing on this platform when pumping with a fiber laser operating at 2 μm for mid-IR operation, a window useful for sensing as it contains many molecular signatures.
We cover recent work on photo-induced second order nonlinearities in silicon nitride microresonators. We show that, like in waveguides, all-optical poling occurs in resonators via the photogalvanic process, resulting in high efficiencies owing to resonant enhancement but without sacrificing versatility and tunability. We also show that, owing to the large efficiency, cascaded nonlinear effects occur in normal dispersion microresonators once second and third order nonlinearities are combined, expanding the scope and potential of silicon nitride nonlinear photonics.
Silicon nitride offers many advantages for integrated nonlinear photonics. Pushed by recent progress in fabrication, we now have access to very low loss waveguides while maintaining large flexibility in terms of dispersion engineering, both essential for the design of efficient nonlinear systems. As such many nonlinear optical demonstrations, mainly based on 3rd order effects in the telecom band, have been performed. In this talk I will cover our recent work on dispersion engineered systems based on the inherent 3rd order effects for efficient and controlled frequency conversion either through four-wave mixing or supercontinuum generation.
We demonstrate that the all-optical inscription of second-order nonlinearity through the coherent photogalvanic effect allows not only degenerate but also non-degenerate sum-frequency generation in silicon nitride waveguides. Several multiphoton absorption processes can occur simultaneously, such that their quantum interference allows for the inscription of distinct charge gratings for quasi-phase matching of several second-order nonlinear processes within the same waveguide. In this work, we investigate the effect theoretically and experimentally validate the generalized sum-frequency generation.
We demonstrate a polarization-maintaining passively mode-locked thulium-doped fibre laser that can operate at two different repetition rates (dual-comb) simultaneously. Based on the presented approach, we observe beat notes with a free spectral range of 1.97 kHz and aim to realize a free-running dual-comb source in the 2 μm band.
Stoichiometric silicon nitride (Si3N4) constitutes a mature platform for integrated photonics. Its pertinent properties, including wide transparency window from the visible to the mid-IR, low propagation loss, and high third-order nonlinearity, are exploited in many linear and nonlinear applications. However, due to the centrosymmetric nature of the Si3N4, the absence of the second-order susceptibility (χ(2)) impedes a realization of three-wave mixing processes as well as the linear electro-optic effect, relevant for many applications on an optical chip. Here, we implement the electric-field poling technique to induce the effective χ(2) inside a Si3N4 waveguide, thus enabling the linear electro-optic modulation. Using numerical simulations, we estimated the concentration and the diffusion coefficient of the charges responsible for the space-charge electric field formation. In addition, the DC third-order susceptibility of Si3N4 previously unknown in the literature is measured using a free-space Mach-Zehnder interferometer.
We demonstrate operation of a tunable mode-locked thulium-doped fiber laser, based on a wavelength-selective chirped fiber Bragg grating (CFBG). By applying strain to the CFBG, we shift its reflection band and can thereby tune the emission-wavelength of the fiber laser between 2022 nm and 2042 nm. We obtain a pulse train at 9.4 MHz repetition rate and a pulse duration between 9.0 and 12.8 ps. To the best of our knowledge, we report the first tunable mode-locked thulium-doped fiber laser using a tunable CFBG as wavelength-selective element.
All-optical poling leads to an effective second-order nonlinearity (χ(2)) in centrosymmetric materials without the need for sophisticated fabrication techniques or material processing, through the periodic self-organization of the charges. The absence of the inherent χ(2) in prevailing silicon-based platforms can be surmounted through all-optical poling. Using the induced effective χ(2) in silicon nitride (Si3N4) waveguides, nonlinear frequency up-conversion processes, such as second-harmonic generation, were previously demonstrated on Si3N4. Here, we report near- and non-degenerate difference-frequency generation in all-optically poled Si3N4 waveguides. We show the agreement between the theory and the measurements and optimize achievable QPM bandwidth range, reaching conversion efficiency of 1 %/W.
It is now well established that silicon nitride offers many advantages for integrated nonlinear photonics. Pushed by recent progress in fabrication, we now have access to very low loss waveguides while maintaining large flexibility in terms of dispersion engineering, both essential for the design of efficient nonlinear systems. As such many nonlinear optical demonstrations, mainly based on third order effects in the telecom band, have been performed. Pushing the applications over the entire accessible spectral range of silicon nitride, from the visible to the middle infrared, as well as offering completely new horizon of applications by inducing effective second-order effects, would provide new and essential element to nonlinear integrated photonic toolbox. In this talk I will review progress in reaching the middle infrared for sensing applications as well as leveraging all-optical poling to enhance the typically weak second-order nonlinearities of the platform.
We present the architecture and code design for a highly scalable, 2.5 Gbis per user optical code division multiple access
(OCDMA) system. The system is scalable to 100 potential and more than 10 simultaneous users, each with BER of less
than 10-9. The system architecture uses a fast frequency-hopping, time-spreading codes. Unlike frequency and phase
sensitive coherent OCDMA systems, this architecture utilizes standard on-off keyed optical pulses allocated in the time
and wavelength dimensions. This incoherent OCDMA approach is compatible with existing WDM optical networks and
utilizes off the shelf components. We discuss the novel optical subsystem designs for encoders and decoders that enable
the realization of a highly scalable incoherent OCDMA system with rapid reconfigurability. A detailed analysis of the
scalability of the two dimensional code is presented. Broadcast and select network deployment architectures for
OCDMA are discussed.
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