Reconfigurable AWGR based on LNOI with a tunable central wavelength and bandwidth used in elastic optical networking

Elastic optical networking introduces elasticity and adaptation into the optical domain, which highly depends on reconfigurable optical devices. In this paper, a tunable 4×4 arrayed waveguide grating router based on lithium niobate on insulator is designed. By using the electro-optic effect of lithium niobate, we design electrode regions with specific shapes in the array waveguide region to realize the tuning of the routing wavelength and bandwidth of the third output channel. The designed arrayed waveguide grating router (AWGR) has a dense channel spacing of 0.8 nm, and the minimum insertion loss is 2.3 dB. Experiments show that the tuning range of the central wavelength can reach 3.2 nm, and the 3 dB bandwidth can be expanded from 0.2 to 0.6 nm.

Highly efficient, modal phase-matched second harmonic generation in a double-layered thin film lithium niobate waveguide

In this contribution, we numerically investigate second harmonic generation in double-layered lithium niobate on the insulator platform by means of the modal phase matching. The modal dispersion of the ridge waveguides at the C waveband of optical fiber communication is calculated numerically and analyzed. Modal phase matching can be achieved by changing the geometric dimensions of the ridge waveguide. The phase-matching wavelength and conversion efficiencies versus the geometric dimensions in the modal phase-matching process are investigated. We also analyze the thermal-tuning ability of the present modal phase matching scheme. Our results show that highly efficient second harmonic generation can be realized by the modal phase matching in the double-layered thin film lithium niobate ridge waveguide.

Integrated O- and C-band silicon-lithium niobate Mach-Zehnder modulators with 100 GHz bandwidth, low voltage, and low loss

Broadband integrated thin-film lithium niobate (TFLN) electro-optic modulators (EOM) are desirable for optical communications and signal processing in both the O-band (1310 nm) and C-band (1550 nm). To address these needs, we design and demonstrate Mach-Zehnder (MZ) EOM devices in a hybrid platform based on TFLN bonded to foundry-fabricated silicon photonic waveguides. Using a single silicon lithography step and a single bonding step, we realize MZ EOM devices which cover both wavelength ranges on the same chip. The EOM devices achieve 100 GHz EO bandwidth (referenced to 1 GHz) and about 2-3 V.cm figure-of-merit (V _π L) with low on-chip optical loss in both the O-band and C-band.

Supervised learning of soliton X-junctions in lithium niobate films on insulator

In this Letter, the first implementation, to our knowledge, of X-junctions between photorefractive soliton waveguides in lithium niobate-on-insulator (LNOI) films is reported. The experiments were performed on 8 µm thick films of congruent undoped LiNbO₃. Compared with bulk crystals, the use of films reduces the soliton formation time, allows more control over the interaction between the injected soliton beams, and opens a route to integration with silicon optoelectronics functions. The created X-junction structures show effective supervised learning, directing the signals propagated inside the soliton waveguides into the output channels highlighted by the control assigned by the external supervisor. Thus, the obtained X-junctions have behaviors analogous to biological neurons.

All-polymer monolithic resonant integrated optical gyroscope

Resonant integrated optical gyroscopes (RIOGs) can integrate discrete optical components as a promising candidate for high-performance micro-optical gyroscopes. However, the current RIOG still consists of discrete elements due to the difficulty and complexity of heterogeneous integration of resonator and modulators. This paper presents on-chip integration of optical functional components including modulator, resonator, beam splitter, and coupler for the organic-polymer-based RIOG. Simple integrated optical processes such as spin coating, lithography, and etching can realize RIOG chips with low cost, size, weight, and power (CSWaP) features. Thereinto, the electro-optic modulator (EOM) fabricated by self-synthesized electro-optic (EO) polymer (side chain bonded polyurethane imide) exhibits less than 2 V half-wave voltage, which is half of the lithium niobate (LiNbO₃) modulator. With respect to the resonator, a quality factor of approximately million was achieved using low-loss fluorinated polymer. In addition, the angular velocity sensing of RIOG was also investigated. By demonstrating the monolithic integration of the resonator and modulators, such an all-polymer RIOG chip prototype builds the technical foundation for the precision fully integrated optical gyroscope.

AlGaAs Nonlinear Integrated Photonics

Practical applications implementing integrated photonic circuits can benefit from nonlinear optical functionalities such as wavelength conversion, all-optical signal processing, and frequency-comb generation, among others. Numerous nonlinear waveguide platforms have been explored for these roles; the group of materials capable of combining both passive and active functionalities monolithically on the same chip is III–V semiconductors. AlGaAs is the most studied III–V nonlinear waveguide platform to date; it exhibits both second- and third-order optical nonlinearity and can be used for a wide range of integrated nonlinear photonic devices. In this review, we conduct an extensive overview of various AlGaAs nonlinear waveguide platforms and geometries, their nonlinear optical performances, as well as the measured values and wavelength dependencies of their effective nonlinear coefficients. Furthermore, we highlight the state-of-the-art achievements in the field, among which are efficient tunable wavelength converters, on-chip frequency-comb generation, and ultra-broadband on-chip supercontinuum generation. Moreover, we overview the applications in development where AlGaAs nonlinear functional devices aspire to be the game-changers. Among such applications, there is all-optical signal processing in optical communication networks and integrated quantum photonic circuits.

Compact MZI modulators on thin film Z-cut lithium niobate

In this paper, we designed, implemented, and characterized compact Mach-Zehnder interferometer-based electro-optic modulators. The modulator utilizes spiral-shaped optical waveguides on Z-cut lithium niobate and the preeminent electro-optic effect which is applied using top and bottom electrodes. Optical waveguides are made of rib etched lithium niobate waveguides with bottom silicon oxide cladding, while SU8 polymer covers the top and sides of the rib waveguides. The proposed implementation resulted in low optical losses < 1.3 dB/cm. Moreover, we achieved compact modulators that fit 0.286 cm and 2 cm long optical waveguides in 110 µm × 110 µm and 300 µm × 300 µm areas, respectively. For single arm modulation, the modulators achieved a VπL of 7.4 V.cm and 6.4 V.cm and 3-dB bandwidths of 9.3 GHz and 2.05 GHz, respectively. Push-pull modulation is expected to cut these VπL in half. The proposed configuration avoids traveling wave modulation complexities and represents a key development towards miniature and highly integrated photonic circuits.

Quantifying the refractive index of ferroelectric domain walls in periodically poled LiNbO3 single crystals by polarization-sensitive optical coherence tomography

Domain walls (DWs) in ferroelectric (FE) and multiferroic materials possess an ever-growing potential as integrated functional elements, for instance in optoelectronic nanodevices. Mandatory, however, is the profound knowledge of the local-scale electronic and optical properties, especially at DWs that are still incompletely characterized to date. Here, we quantify the refractive index of individual FE DWs in periodically-poled LiNbO₃ (PPLN) single crystals. When applying polarization-sensitive optical coherence tomography (PS-OCT) at 1300 nm using circular light polarization, we are able to probe the relevant electro-optical properties close to and at the DWs, including also their ordinary and extraordinary contributions. When comparing to numerical calculations, we conclude that the DW signals recorded for ordinary and extraordinary polarization stem from an increased refractive index of at least Δn > 2·10^-3 that originates from a tiny region of < 30 nm in width. PS-OCT hence provides an extremely valuable tool to decipher and quantify subtle changes of refractive index profiles for both inorganic and biomedical nanomaterial systems.

Broadband and compact polarization beam splitter in LNOI hetero-anisotropic metamaterials

In this paper, theoretical modeling and numerical simulations of a high-performance polarization beam splitter (PBS) based on hetero-anisotropic metamaterials are proposed on the lithium-niobate-on-insulator (LNOI) platform. The hetero-anisotropic metamaterials constructed by sub-wavelength gratings (SWGs) can be regarded as effective anisotropy medium, which exhibits strong birefringence without breaking the geometrical symmetry, contributing to the formation of PBS. Rather than the principle of PBS based on beat-length difference of transverse electric (TE) polarization and transverse magnetic (TM) polarization, the device can realize polarization beam splitting in single beat length, and the footprint of the proposed PBS can be reduced to 8 µm × 160 µm (with S-bend). The simulation results show that the bandwidth is 185 nm (1450∼1634 nm) for TE polarization while the bandwidth is 85 nm (1490∼1575 nm) for TM polarization when the polarization extinction ratio is >20 dB. Furthermore, the insertion loss is less than 1 dB in the range of 1450 to 1650 nm, for both TE and TM polarization. Additionally, the proposed device proves strong robustness of the fabrication tolerance.

“Seeing Is Believing”—In-Depth Analysis by Co-Imaging of Periodically-Poled X-Cut Lithium Niobate Thin Films

Nonlinear and quantum optical devices based on periodically-poled thin film lithium niobate (PP-TFLN) have gained considerable interest lately, due to their significantly improved performance as compared to their bulk counterparts. Nevertheless, performance parameters such as conversion efficiency, minimum pump power, and spectral bandwidth strongly depend on the quality of the domain structure in these PP-TFLN samples, e.g., their homogeneity and duty cycle, as well as on the overlap and penetration depth of domains with the waveguide mode. Hence, in order to propose improved fabrication protocols, a profound quality control of domain structures is needed that allows quantifying and thoroughly analyzing these parameters. In this paper, we propose to combine a set of nanometer-to-micrometer-scale imaging techniques, i.e., piezoresponse force microscopy (PFM), second-harmonic generation (SHG), and Raman spectroscopy (RS), to access the relevant and crucial sample properties through cross-correlating these methods. Based on our findings, we designate SHG to be the best-suited standard imaging technique for this purpose, in particular when investigating the domain poling process in x-cut TFLNs. While PFM is excellently recommended for near-surface high-resolution imaging, RS provides thorough insights into stress and/or defect distributions, as associated with these domain structures. In this context, our work here indicates unexpectedly large signs for internal fields occurring in x-cut PP-TFLNs that are substantially larger as compared to previous observations in bulk LN.

Graphene-coated double D-type low loss optical fiber modulator

A graphene-coated double D-type low loss all-fiber modulator is proposed. The modulator is improved on the basis of standard fiber. Only the cladding is processed without grinding the original core structure. The upper and lower cladding are cut same distance. This can ensure that the mode field does not deviate in one direction, so that most of the mode field is still tied to the core, which greatly reduces the device loss. The existence of the double graphene layer can also ensure a very excellent modulation efficiency. The calculation results show that the mode loss of our proposed dual-D modulator under X polarization is 0.125 dB/mm, and the mode field mismatch loss is 0.25%. The mode loss in Y polarization is 0.033 dB/mm, and the mode field mismatch loss is 0.32%. When the modulation voltage is 5 V, the modulation depth is 78.4% under the condition of five-layer graphene, while the modulation speed can reach 15.38 GHz. Besides maintaining low modulation voltage and higher modulation efficiency, this structure makes full use of the advantages of good fiber coupling, and will be widely used in future fiber communications and all-fiber systems.

Effects of the Domain Wall Conductivity on the Domain Formation under AFM-Tip Voltages in Ion-Sliced LiNbO3 Films

The specified domain patterns were written by AFM-tip voltages in LiNbO3 films composing LNOI (LiNbO3-on-insulator). The domain wall conductivity (DWC) was estimated in the written patterns. This estimate was based on the effects of load resistors RL inserted between DWs and the ground, on the features of occurring domains. In this case, the domain formation is controlled by the ratio between RL and the DWs’ resistance RDW. Starting from the comparison of patterns appearing at different RL, the value of RDW in a specified pattern was estimated. The corresponding DWC is of σDW ≈ 10−3 (Ohm cm)−1 which exceeds the tabular bulk conductivity of LiNbO3 by no less than twelve orders of magnitude. A small DW inclination angle of (10−4)0 responsible for this DWC is not caused by any external action and characterizes the domain frontal growth under an AFM-tip voltage.

Thin-film lithium niobate electro-optic modulator on a D-shaped fiber

We propose a low-insertion-loss electro-optic modulator formed with LNOI bonded on a D-shaped SMF. The proposed modulator employs high-performance Mach-Zehnder interferometer (MZI) formed with ridge LNOI waveguides and driven by travelling-wave electrodes. The light from the fiber core is coupled into a thin strip LNOI waveguide and then launched into the MZI via a ridge LNOI waveguide with tapered slab height and vice versa. Such all-fiber configuration exempts the need of the butt-coupling with an SMF. The calculated results show that our proposed modulator is capable of achieving a low insertion loss of less than 1.5 dB, an EO modulation efficiency (Vπ·L) of 2.05 V·cm, and a 3-dB modulation bandwidth of larger than 80 GHz. Our all-fiber LNOI modulator is feasible in practice and opens a new door to realize high-speed fiber devices by the integration of an optical fiber and thin film LN.

Tri-layer gradient and polarization-selective vertical couplers for interlayer transition

We demonstrate and optimize a tri-layer vertical coupler for a silicon nitride (Si₃N₄) multilayer platform operating at a 2 µm band. The large spacing between the topmost and bottommost layers of a gradient structure enables ultra-low crossing loss and interlayer crosstalk without affecting the efficiency interlayer transition. We achieve a 0.31 dB transition loss, ultra-low multi-layer crosstalk of -59.3 dB at a crossing angle of 90° with an interlayer gap of 2300 nm at 1950nm. With width optimization of this structure, the fabrication tolerances toward lateral misalignment of two stages in this coupler have increased 61% and 56%, respectively. We also propose a vertical coupler, based on this design, with mode selectivity and achieve an extinction ratio of < 15 dB for wavelengths in the 1910-1990 range. Meanwhile, a multi-layer interlaced AWGs centered at 1950nm and based on vertical coupler has been demonstrated. The proposed vertical couplers exhibit potential for application in large-scale photonic-integrated circuits and broadly in photonic devices.