Lithium Niobate Waveguide Research Articles

Highly efficient second-harmonic generation in a double-layer thin-film lithium niobate waveguide

Highly efficient second-harmonic generation in a double-layer thin-film lithium niobate waveguide

Wavelength Stabilization of Entangled Biphotons Using Dynamic Temperature Compensation for Quantum Interference Applications

AbstractIn this paper, a dynamic temperature compensation method is presented to stabilize the wavelength of the entangled biphoton source, which is generated on spontaneous parametric down‐conversion from a magnesium oxide doped periodically poled lithium niobate waveguide. Utilizing the dispersive Fourier transformation technique, the photon wavelength variation is monitored in case of conventional static temperature control, revealing a long‐term wavelength drift up to 556.8 pm over a 14‐h measurement period. A Hong‐Ou‐Mandel (HOM) interferometer is constructed to assess the impact on quantum applications, showing a decrease in visibility from 95.5% to 69.4%. To address this issue, a digital proportional‐integral‐differential algorithm is implemented to dynamically compensate the working temperature variation of the waveguide, thereby instantly stabilizing the wavelength to a peak‐to‐peak fluctuation of +138.05 pm/‐127.61 pm with the standard deviation being 30.49 pm. The wavelength stability shows more than a hundredfold enhancement in terms of Allan deviation, reaching at an averaging time of 10000 s. With the dynamic control in operation, the HOM interference visibility turns to stable at 96.1% ± 0.6%. The method provides a simple and accessible solution for precisely controlling and stabilizing the wavelength of entangled biphotons, thus improving performance in various quantum information processing applications.

2D sheets of MoSe2 for waveguide based second harmonic generation

Two-dimensional transition metal dichalcogenides show a strong second-order nonlinear response, which can only be exploited if the interaction between light and 2D material is maximized as it happens in waveguiding structures. Such an approach also allows the addition of a second order nonlinear response to linear integrated optics components by just placing a sheet of transition metal dichalcogenides material on top. Here, we analyze the second harmonic generation in MoSe2 sheets combined with silicon nitride ridge and thin-film waveguides for both modal phase matching and quasi-phase-matching. The calculated second harmonic generation efficiency is comparable with that in lithium niobate waveguides and can further be enhanced by increasing the number of MoSe2 layers on top of the waveguides. This also holds true for flux correlated photons generated by spontaneous parametric down conversion as we show by quantitative analysis.

High-efficiency second-harmonic generation in reverse-polarization dual-layer lithium niobate waveguides.

Second-harmonic (SH) generation is an important way to generate short-wavelength light sources. Thin-film lithium niobate (TFLN) resonators and waveguides featuring tight light confinement are flexible to realize mode- or quasi-phase-matching (MPM or QPM) and thus efficient SH generation. Here, we report an efficient SH with an absolute conversion efficiency of 45% in a reverse-polarized double-layer TFLN waveguide. Such a high conversion efficiency without periodic poling was realized with the pump of 67 mW in the 1550 nm band, benefiting from the employment of the largest nonlinear coefficient d33 and the significant nonlinear interaction of the fundamental transverse electric (TE) mode and the first-order TE mode in the vertical direction. This work paves the way toward on-chip integrated nonlinear optics in terms of harmonic generation and quantum light sources across the telecom and visible bands.

Modal phase-matching in thin-film lithium niobate waveguides for efficient generation of entangled photon pairs

Thin-film lithium niobate (TFLN) waveguides have emerged as a pivotal platform for on-chip spontaneous parametric down-conversion (SPDC), serving as a crucible for the generation of entangled photon pairs. The periodic poling of TFLN, while capable of generating high-efficiency SPDC, demands intricate fabrication processes that can be onerous in terms of scalability and manufacturability. In this work, we introduce a novel approach to the generation of entangled photon pairs via SPDC within TFLN waveguides, harnessing the principles of modal phase-matching (MPM). To address the challenge of efficiently exciting pump light typically in a higher-order mode, we have engineered a mode converter that couples two asymmetrically dimensioned waveguides. This converter adeptly transforms the fundamental mode into a higher-order mode, demonstrating a conversion loss of 1.55 dB at 785 nm with a 3 dB bandwidth exceeding 30 nm. Subsequently, we have showcased the device’s capabilities by characterizing the pair generation rate (PGR), coincidences-to-accidentals ratio (CAR), and spectral profile of the entangled photon source. Our findings present a simplified and versatile method for the on-chip generation of entangled photon sources, which may pave the way for the application in the realms of quantum information processing and communication technologies.

Broadband polarization-independent infrared photon upconversion within nanophotonic lithium niobate waveguides by higher-order-mode engineering.

A broadband polarization-independent infrared photon upconversion scheme based on nanophotonic waveguides on the lithium niobate on insulator (LNOI) platform is proposed. With the introduction of higher-order-mode dispersion engineering, polarization-independent and broadband photon upconversion with a maximum bandwidth of nearly 46 nm is achieved in this device. The proposed broadband polarization-independent photon upconversion structure shows great potential applications in on-chip infrared photon detection and nonlinear photonics.

Topological parametric gain and two-color edge states in equidistant lithium niobate waveguide arrays.

We analyze parametric χ2 processes in equidistant finite-size arrays of thin-film lithium niobate waveguides, where the fundamental harmonic (FH) field supports topological edge states due to the specific interplay between inter- and intra-modal couplings of two families of guided modes, while the second-harmonic (SH) field only supports bulk modes. Regimes of topological parametric gain are identified, where the gain only occurs in the edge states of the FH field, regardless of the spatial distribution of the pump SH field. The topological gain of the FH component generally triggers localization of the SH field near an edge of the array in the optical parametric oscillation dynamics. In small-size arrays, parametric gain at both edges can be observed even when pumped at one side. This process can lead to an anomalous "tunneling" of the SH field to the opposite edge. We also analyze the existence and stability of two-color nonlinear edge states (solitons), in which both FH and SH fields are localized at an edge of the array. Depending on the phase-matching condition, such solitons either emerge from the linear FH edge state without a power threshold or exist above a certain power threshold dictated by the coupling strength in the SH field.

High sensitivity optical pressure sensor based on graphene/molybdenum disulfide composite film.

In this paper, a high sensitivity optical pressure sensor based on a graphene/molybdenum disulfide (MoS2) composite film is proposed. The sensor is composed of a polydimethylsiloxane (PDMS) pyramid structure, graphene/MoS2 composite film, and lithium niobate waveguide. The pressure deforms the PDMS pyramid structure, which leads to the change of the refractive index of the graphene/MoS2 composite film, and finally be detectable sensitively by the variation of the interference spectrum. Experiments have been carried out using our sensor prototype, and the sensitivity is up to 575.233 nm/kPa in the pressure range of 0 kPa-0.123 kPa, which is much higher than that of typical optical pressure sensors. This shows the advantages of high sensitivity optical pressure sensors based on the graphene/MoS2 composite film, which is expected to be applied in highly sensitive pressure detection environments.

Lithium niobate waveguide-integrated Bi2Te3/graphene heterojunction photodetector.

We demonstrate an on-chip photodetector by integrating a graphene and topological insulator Bi2Te3 heterostructure on a thin-film lithium niobate waveguide. Lithium niobate on insulator (LNOI) waveguides are fabricated by the photolithography-assisted chemical mechanical etching method. The bismuth telluride (Bi2Te3) and graphene heterostructure design provides enhanced photocurrent due to the effective photocarrier generation. The lithium niobate waveguide-integrated Bi2Te3/graphene heterojunction presents a high absorption coefficient of 2.1 dB/µm. The Bi2Te3/graphene heterojunction photodetector exhibits a responsivity of 2.54 mA/W without external bias at a 1.55 µm wavelength, which is enhancement of sevenfold as compared to the pure graphene-based photodetector. The photodetector has a 3 dB bandwidth of over 4.7 GHz. This work provides a potentially viable method for a self-powered, high responsivity, and fast response of the photodetector integrated with the LNOI photonic platform.

Purifying quantum-dot light in a coherent frequency interface

Abstract Quantum networks typically operate in the telecom wavelengths to take advantage of low-loss transmission in optical fibres. However, bright quantum dots (QDs) emitting highly indistinguishable quantum states of light, such as InGaAs QDs, often emit photons in the near infrared thus necessitating frequency conversion (FC) to the telecom band. Furthermore, the signal quality of quantum emissions is crucial for the effective performance of these networks. In this work we report a method for simultaneously implementing spectral purification and frequency shifting of single photons from QD sources to the c-band in a periodically poled lithium niobate waveguide. We consider difference frequency generation in the counter-propagating configuration to implement FC with the output emission bandwidth in units of GHz. Our approach establishes a clear path to integrating high-performance single-emitter sources in a hybrid quantum network.

Design and analysis of single-electrode integrated lithium niobate optical phased array for two-dimensional beam steering

The realization of high-speed, low-power optical phased array (OPA) on thin-film lithium niobate on insulator (LNOI) is considered an ideal solution for the next generation of solid-state beam steering. Most reported on-chip two-dimensional optical phased arrays suffer from issues such as large antenna spacing, high power consumption and complex wiring due to independent control of array elements. To address these challenges while fully utilizing the benefits of the LNOI platform, we propose a two-dimensional beam-scanning OPA based on lithium niobate (LN) waveguides. We design a multi-layer cascaded domain engineering structure inside the LN waveguide, combined with wavelength tuning, to enable two-dimensional beam scanning with single electrode controlling the OPA. Through simulation, we achieve a 42°×9.2° two-dimensional beam steering. Compared to existing on-chip integrated OPAs, this work offers significant advantages in increasing integration, simplifying control units and reducing power consumption.

Multiplexed quantum frequency conversion.

We study multiplexed quantum frequency conversion (m-QFC) on a single waveguide, where a single pump beam simultaneously converts multiple signal beams at distinct wavelengths. Using a three-peak periodically poled lithium niobate (PPLN) waveguide, we demonstrate the multiplexed upconversion in the telecom band with internal efficiencies up to 73.6%. Tested with a multichannel photon-pair source, the quantum correlation is preserved well after the conversion, with coincidence-to-accidental ratio reaching 767. These results highlight a viable device approach to multiplexed quantum key distribution, quantum sensing, and quantum computing.

High-efficiency mid-infrared CW-seeded optical parametric generation in PPLN waveguide

Mid-infrared optical sources have been extensively used in a variety of applications, including gas sensing and metrology, by exploiting the fingerprint fundamental vibrational absorption bands of molecules in this spectral region. Parametric frequency conversion techniques, such as optical parametric generation (OPG) and optical parametric oscillator (OPO), represent common methodologies for mid-infrared generation. Notably, continuous-wave (CW)-seeded OPG exhibits superior stability and performance compared to conventional OPG. Here, we present a simple method for mid-infrared source generation based on femtosecond OPG in periodically poled lithium niobate (PPLN) waveguides. In addition to its robustness and simplicity, mid-infrared CW-seeded OPG features a high quantum conversion efficiency of 46.5% and a low threshold. The phase of CW-seeded OPG was passively referenced to the CW laser, making it suitable for dual-comb applications. This system shows great promise in the miniaturization of mid-infrared combs and will be applied to non-laboratory applications, including open-path atmospheric gas sensing and portable environmental monitoring.

Measurement of transition frequencies and hyperfine constants of molecular iodine at 520.2 nm

We measured the transition frequencies of the hyperfine components in the four lines (P(34) 39-0, R(36) 39-0, P(33) 39-0, and R(35) 39-0) of the B-X transitions of molecular iodine at 520.2 nm. The 520.2 nm laser was generated by wavelength-converting the output of a 1560.6 nm external-cavity diode laser using a dual-pitch periodically poled lithium niobate (PPLN) waveguide. The frequencies were measured by counting the heterodyne beats between the laser stabilized at the frequencies of the hyperfine components and a frequency comb synchronized with a hydrogen maser. We determined the transition frequencies of the a1 components with relative uncertainties of 1×10−11; the uncertainty was limited by the impurity of the molecular iodine in the cell. From the measured hyperfine splitting frequencies, we calculated the hyperfine constants of these four transitions to obtain the rotational dependence of the excited-state hyperfine constants.

Chirped periodically poled lithium-niobate-on-insulator micro-waveguides for broadband second-harmonic generation.

Periodically poled lithium niobate (LN) waveguides based on quasi-phase matching schemes, benefiting from their high nonlinear coefficient (d33) and strong optical confinement, are widely employed for implementing efficient second-harmonic generation (SHG). Here, we report broadband SHG in z-cut chirped periodically poled lithium-niobate-on-insulator (CPPLNOI) ridge micro-waveguides. Nearly 90-nm-wide SHG at the telecom band is achieved, along with an averaged normalized efficiency of 7.5%/(W·cm2). We also demonstrate simultaneous generation of second as well as cascaded third and fourth harmonics under direct pumping of femtosecond pulses. This work would benefit applications for frequency conversion of a wideband coherent light source.

Tunable pattern recognition of optical QPSK data using optical correlation and direct detection.

Performing pattern recognition via correlation in the optical domain has potential advantages, including: (i) high-speed operation at the line rate and (ii) tunability and scalability by operating on the optical wave properties. Such pattern recognition might be performed on quadrature-phase-shift-keying (QPSK) data transmitted over an optical network, which generally requires using coherent detection to distinguish the phase levels of the correlator output. To enable simpler detection, we combine optical correlation with optical biasing to experimentally demonstrate tunable and scalable QPSK pattern recognition using direct detection. The pattern is applied by adjusting the relative phases of the local pumps. Delayed QPSK signals, a coherent bias tone, and local pumps undergo nonlinear wave-mixing in a periodically poled lithium niobate (PPLN) waveguide to perform optical correlation and biasing. The biased correlator output is captured using direct detection, where the highest power level corresponds only to the pattern. Multiple QPSK pattern recognitions are achieved error-free over 3072 symbols using power thresholding values of (i) 0.78 at a 5-Gbaud rate and 0.73 at a 10-Gbaud rate for 2-symbol pattern recognition and (ii) 0.81 at a 5-Gbaud rate and 0.79 at a 10-Gbaud rate for 3-symbol pattern recognition.

Low Dark Current, High Responsivity, and Self‐Powered MoTe2 Photodetector Integrated With a Thin Film Lithium Niobate Waveguide

AbstractThin film lithium niobate is one of the most crucial platforms in the next generation of integrated optoelectronics because of its ability to integrate tight optical confinement, low optical loss, and active optical functions. A current challenge in the field of thin film lithium niobate photonics is the development of high‐performance photodetectors to achieve multifunctional photonic integrated circuits. This study introduces a high‐performance MoTe2 photodetector integrated on a thin film lithium niobate waveguide. The photodetector achieves the lowest dark current and highest on/off ratio among waveguide‐integrated photodetectors on a thin film lithium niobate platform. Due to a slight asymmetry in the behavior of the two electrode contacts of the device, the photodetector also achieves self‐driven. At zero bias and a wavelength of 1310 nm, a dark current of ≈20 pA, and an on/off ratio exceeding 105 are achieved. At a bias voltage of 1 V, the measured dark current, responsivity, and on/off ratio are 1.13 nA, 309 mA/W, and 1.8 × 104, respectively. Notably, the device exhibits fast rise and fall times of 9.3 and 13.5 µs, respectively, accompanied by a high detectivity of 2.58 × 1011 W−1.

Ultraviolet astronomical spectrograph calibration with laser frequency combs from nanophotonic lithium niobate waveguides

Astronomical precision spectroscopy underpins searches for life beyond Earth, direct observation of the expanding Universe and constraining the potential variability of physical constants on cosmological scales. Laser frequency combs can provide the required accurate and precise calibration to the astronomical spectrographs. For cosmological studies, extending the calibration with such astrocombs to the ultraviolet spectral range is desirable, however, strong material dispersion and large spectral separation from the established infrared laser oscillators have made this challenging. Here, we demonstrate astronomical spectrograph calibration with an astrocomb in the ultraviolet spectral range below 400 nm. This is accomplished via chip-integrated highly nonlinear photonics in periodically-poled, nano-fabricated lithium niobate waveguides in conjunction with a robust infrared electro-optic comb generator, as well as a chip-integrated microresonator comb. These results demonstrate a viable route towards astronomical precision spectroscopy in the ultraviolet and could contribute to unlock the full potential of next-generation ground-based and future space-based instruments.

Tunable optical matrix convolution of 20-Gbit/s QPSK 2-D data with a kernel using optical wave mixing.

Compared to its electronic counterpart, optically performed matrix convolution can accommodate phase-encoded data at high rates while avoiding optical-to-electronic-to-optical (OEO) conversions. We experimentally demonstrate a reconfigurable matrix convolution of quadrature phase-shift keying (QPSK)-encoded input data. The two-dimensional (2-D) input data is serialized, and its time-shifted replicas are generated. This 2-D data is convolved with a 1-D kernel with coefficients, which are applied by adjusting the relative phase and amplitude of the kernel pumps. Time-shifted data replicas (TSDRs) and kernel pumps are coherently mixed using nonlinear wave mixing in a periodically poled lithium niobate (PPLN) waveguide. To show the tunability and reconfigurability of this approach, we vary the kernel coefficients, kernel sizes (e.g., 2 × 1 or 3 × 1), and input data rates (e.g., 6-20 Gbit/s). The convolution results are verified to be error-free under an applied: (a) 2 × 1 kernel, resulting in a 16-quadrature amplitude modulation (QAM) output with an error vector magnitude (EVM) of ∼5.1-8.5%; and (b) 3 × 1 kernel, resulting in a 64-QAM output with an EVM of ∼4.9-5.5%.

A Sixteen‐user Time‐bin Entangled Quantum Communication Network With Fully Connected Topology

AbstractQuantum key distribution (QKD) promises unconditionally information‐theoretic secure communication guaranteed by the laws of physics and has become one of the most crucial candidates in future security aspects. Developing a large‐scale network with a scalable and integrated scheme is of great significance for expanding the advantages of QKD protocols among multiple users. Here, a sixteen‐user fully connected quantum network by using a novel time‐bin entangled source implemented in the integrated multi‐channel periodically poled lithium niobate waveguides is proposed. Based on this entangled source, the quantum processor prepares 120 entangled photon pairs to allocate 15 links to each user by utilizing dense wavelength division multiplexing technology. To enable the users’ communication with each other simultaneously, a phase‐compensated Mach‐Zehnder Interferometer based on a Fourier‐transform setup to control the relative phase of the interferometer for all the involved wavelength channels is developed. The experimental results show that the network can support sixteen users for long‐distance communication with each other simultaneously. The novel scheme of time‐bin entangled sources paves an efficient way for implementing large‐scale quantum resources in a compact integrated platform, and the time‐bin entangled network promises a new potential for constructing large‐scale and extensible quantum networks with an integrated photonic architecture.