Efficient sum-frequency generation of a yellow laser in a thin-film lithium niobate waveguide.

Yellow lasers with high efficiency and tunability play an essential role in many applications. Here, we demonstrate the sum-frequency generation (SFG) of yellow light on a periodically poled thin-film lithium niobate (PP-TFLN) waveguide. Taking advantage of large χ(2) nonlinearity, a high normalized conversion efficiency of 10,097% (W·cm2) is obtained with pump wavelengths of 1317.7 and 1064 nm. An absolute conversion efficiency of 24.17% is recorded with on-chip pump powers of 10.4 dBm (O-band) and 13.5 dBm (1064 nm).

Virtual-State Model for Analyzing Electro-Optical Modulation in Ring Resonators.

Ring resonators play a crucial role in optical communication and quantum technology applications. However, these devices lack a simple and intuitive theoretical model to describe their electro-optical modulation. When the resonance frequency is rapidly modulated, the filtering and modulation within a ring resonator become physically intertwined, making it difficult to analyze the complex physical processes involved. We address this by proposing an analytical solution for electro-optic ring modulators based on the concept of a "virtual state." This approach equates a lightwave passing through a dynamic ring modulator to one excited to a virtual state by a cumulative phase and then returning to the real state after exiting the static ring. Our model simplifies the independent analysis of the intertwined physical processes, enhancing its versatility in analyzing various incident signals and modulation formats. Experimental results, including resonant and detuning modulation, align with the numerical simulation of our model. Notably, our findings indicate that the dynamic modulation of the ring resonator under detuning driving approximates phase modulation.

Ultra-broadband 1 × 2 3 dB power splitter using a thin-film lithium niobate from 1.2 to 2 µm wave band.

The 3 dB power splitters are fundamental building blocks for integrated photonic devices. As data capacity requirements continue to rise, there is a growing interest in integrated devices that can accommodate multiple spectral bands, including the conventional O-, C-, and L-bands, and the emerging 2 µm band. Here we propose and experimentally demonstrate a 3 dB power splitter based on adiabatic mode evolution using a thin-film lithium niobate, with ultra-broadband operation bandwidth from 1200 to 2100 nm. The fabricated power splitter exhibits low insertion losses of 0.2, 0.16, and 0.53 dB for wavelengths at 1310, 1550, and 2000 nm, respectively. The measured 1 dB bandwidth covers 1260-1360, 1480-1640, and 1930-2030 nm, which we believe that the proposed device is capable of operating in both O-, C-, L-, and 2 µm bands.

Thermo-optic tunable optical filters with GHz-bandwidth and flat-top passband on thin film lithium niobate platform.

Lithium niobate on insulator (LNOI) is a new photonic integrated platform that provides high optical confinement and retains the inherent excellent properties of lithium niobate (LN). Tunable filters are one of the indispensable devices for integrated optics. Here we design and fabricate a thermo-optic (TO) tunable optical filter using two cascaded racetrack microring resonators (MRRs) based on LNOI. The filter shows a narrow and flat top passband with intra band ripple less than 0.3 dB, 3 dB bandwidth of 4.8 GHz and out-of-band rejection of about 35 dB. The insertion loss of the filter is about -14 dB, including grating coupling loss about -6.5 dB and on-chip loss less than -1 dB. The heating power for center wavelength shift of the filter is about 89.4 mW per free spectral range (FSR). Relevant applications of such filters include optical information processing and microwave photonics.

All-optical wavelength conversion of a 92-Gb/s 16-QAM signal within the C-band in a single thin-film PPLN waveguide.

Tunable all-optical wavelength conversion (AOWC) within 151 nm bandwidth is demonstrated in a thin-film periodically poled lithium niobate (PPLN) waveguide, which utilizes the cascaded second-harmonic generation and difference-frequency generation (cSHG/DFG) process. Also, in the same waveguide, AOWC of a 92-Gb/s 16-ary quadrature amplitude modulated (16-QAM) signal within the C-band is successfully achieved. For Bit-error ratio (BER) measurements, we obtain a negligible optical signal-to-noise ratio (OSNR) penalty (<0.2 dB) for the converted idler wave at a BER of 1e-3.

Photonic sampling analog-to-digital conversion based on time and wavelength interleaved ultra-short optical pulse train generated by using monolithic integrated LNOI intensity and phase modulator.

High-speed analog-to-digital conversion (ADC) is experimentally demonstrated by employing a time and wavelength interleaved ultra-short optical pulse train to achieve photonic sampling and using wavelength division demultiplexing to realize speed matching between the fast optical front-end and the slow electronic back-end. The sampling optical pulse train is generated from a cavity-less ultra-short optical pulse source involving a packaged device that monolithically integrates an intensity modulator and a phase modulator into a chip based on lithium niobate on insulator (LNOI). In the experiment, the fiber-to-fiber insertion loss of the packaged modulation device is measured to be 6.9 dB. In addition, the half-wave voltages of the Mach-Zehnder modulator and the phase modulator in the LNOI-based modulation device are measured to be 3.6 V and 3.4 V at 5 GHz, respectively. These parameters and the device size are superior to those based on cascaded commercial devices. Through using the packaged modulation device, two ultra-short optical pulse trains centered at 1541.40 nm and 1555.64 nm are generated with time jitters of 19.2 fs and 18.9 fs in the integral offset frequency range of 1 kHz to 10 MHz, respectively, and are perfectly time interleaved into a single pulse train with a repetition rate of 10 GHz and a time jitter of 19.8 fs. Based on the time and wavelength interleaved ultra-short optical pulse train, direct digitization of microwave signals within the frequency range of 1 GHz to 40 GHz is demonstrated by using a two-channel wavelength demultiplexing photonic ADC architecture, where the effective number of bits are 5.85 bits and 3.75 bits for the input signal at 1.1 GHz and 36.3 GHz, respectively.

Enhanced bound states in the continuum in a DBR-assisted photonic crystal slab.

Bound states in the continuum (BICs) are perfectly confined resonances within the radiation continuum. The novel characteristics of single BICs have been studied in great detail in various wave systems, including electromagnetic waves, acoustic waves, water waves, and elastic waves in solids. In practice, the performance of BICs is limited by the finite size of the structure, while the combination of multiple BICs can further improve the localization of resonances. In this study, we experimentally demonstrate the combination of Fabry-Perot and symmetry-protected BICs at near infrared wavelengths by employing a compound photonic crystal system composed of a photonic crystal slab and a distributed Bragg reflector, resulting in an enhanced high quality factor.

Direct Imaging of Integrated Circuits in CPU with 60 nm Super-Resolution Optical Microscope.

Far-field super-resolution optical microscopies have achieved incredible success in life science for visualization of vital nanostructures organized in single cells. However, such resolution power has been much less extended to material science for inspection of human-made ultrafine nanostructures, simply because the current super-resolution optical microscopies modalities are rarely applicable to nonfluorescent samples or unlabeled systems. Here, we report an antiphase demodulation pump-probe (DPP) super-resolution microscope for direct optical inspection of integrated circuits (ICs) with a lateral resolution down to 60 nm. Because of the strong pump-probe (PP) signal from copper, we performed label-free super-resolution imaging of multilayered copper interconnects on a small central processing unit (CPU) chip. The label-free super-resolution DPP optical microscopy opens possibilities for easy, fast, and large-scale electronic inspection in the whole pipeline chain for designing and manufacturing ICs.

Efficient polarization splitter-rotator on thin-film lithium niobate.

Recently, thin-film lithium niobate coherent modulators have emerged as a promising candidate for the next generation coherent communication system. High performance polarization splitter-rotators (PSRs) are essential to further achieve dual polarization coherent modulators. Here we present a PSR on the lithium niobate on insulator (LNOI) platform with the measured insertion loss less than 1 dB, extinction ratio exceeding 26.6 dB and 19.6 dB for TE0 and TM0 modes, working bandwidth of 1520-1580 nm and total length of 440 µm. In addition, a relatively large fabrication tolerance for waveguide width is also proved. This demonstrated PSR can find its potential application in polarization-division multiplexing (PDM) optical transmitter based on LNOI.

High-efficient coupler for thin-film lithium niobate waveguide devices.

Lithium niobate (LN) devices have been widely used in optical communication and nonlinear optics due to its attractive optical properties. The emergence of the thin-film lithium niobate on insulator (LNOI) improves performances of LN-based devices greatly. However, a high-efficient fiber-chip optical coupler is still necessary for the LNOI-based devices for practical applications. In this paper, we demonstrate a highly efficient and polarization-independent edge coupler based on LNOI. The coupler, fabricated by a standard semiconductor process, shows a low fiber-chip coupling loss of 0.54 dB/0.59 dB per facet at 1550 nm for TE/TM light, respectively, when coupled with an ultra-high numerical aperture fiber (UHNAF) of which the mode field diameter is about 3.2 µm. The coupling loss is lower than 1dB/facet for both TE and TM light in the wavelength range of 1527 nm to 1630 nm. A relatively large tolerance for optical misalignment is also proved, due to the coupler's large mode spot size up to 3.2 µm. The coupler shows a promising stability in high optical power and temperature variation.

Strongly Enhanced Second Harmonic Generation in a Thin Film Lithium Niobate Heterostructure Cavity.

Boosting second-order optical nonlinear frequency conversion over subwavelength thickness has long been pursued through optical resonance in micro- and nanophotonics. However, the availability of thin film materials with high second-order nonlinearity is limited to III-V semiconductors, which have low transparency in the visible. Here, we experimentally demonstrated strongly enhanced second harmonic generation in one-dimensional heterostructure cavities on thin film lithium niobate. A guided-mode resonance resonator and distributed Bragg reflectors are combined for both efficient coupling and electromagnetic field localization. Over 1200 times second harmonic generation enhancement is experimentally realized compared with flat thin film lithium niobate through optimizing the trade-off between quality factor and mode volume, leading to a record high normalized conversion efficiency of 2.03×10^{-5} cm^{2}/GW under 1.92 MW/cm^{2} pump intensity. Our approach could inspire the miniaturization and integration of compact resonant nonlinear photonic devices on thin film lithium niobate.

Photonic crystal enabled manipulation of optical and electric field in germanium avalanche photodetectors.

We demonstrate the use of a photonic crystal (PhC) structure to improve the performance of a germanium avalanche photodetector (APD) by simultaneously manipulating the distribution of the optical and electric fields. The PhC is fabricated at the top center of the vertical germanium APD. For a 14 µm diameter device, the 1550 nm responsivity increases from 0.2 to 0.6 A W-1 at unity gain, owing to the resonance-enhanced absorption. Moreover, the structure separates the absorption and multiplication regions of the device, resulting in an increase of the avalanche gain and the gain-bandwidth product. Under -10 dBm input optical power, a 3 dB bandwidth of 34 GHz before avalanche and a clear 40 Gbps eye diagram under avalanche demonstrates good high-speed performance of the device.

Precisely ordered Ge quantum dots on a patterned Si microring for enhanced light-emission.

Semiconductor microcavities can greatly enhance the light-emission of embedded quantum dots (QDs). Here, a new route toward the microcavity-QD system by fabricating microcavities followed by growing ordered QDs on a patterned microresonator is proposed, which keeps QDs from being etched. Self-assembled Ge QDs prefer to form at the rims of Si microrings or microdisks. The Ge QDs on the pit- or groove-patterned microring resonator (MRR) show better size uniformity and position accuracy. These features are explained by the evolutions of surface morphology and surface chemical potential distribution. Sharp photoluminescence peaks in the telecommunication band with the quality factors in the range of 450-850 from groove-patterned MRR are observed at 295 K due to efficient overlap between Ge QDs and resonant modes. Our schemes shed light on the exactly site-controlled growth of QDs on micro- and nano-structures, which further facilitates the investigation of light-matter interactions.

Fundamental mode hybridization in a thin film lithium niobate ridge waveguide.

The high performance of thin film lithium niobate on insulator (LNOI) platform shows potential for electro-optical signal processing and nonlinear optics systems. To realize precise polarization management for sub-wavelength devices, we theoretically and experimentally investigate fundamental transverse electric (TE) and transverse magnetic (TM) mode hybridization in an x-cut LNOI ridge waveguide. Sudden jumps in the free-spectrum-range (FSR) of these modes in a fabricated microring resonator demonstrate the mode hybridization. The measured Q-factor of the lithium niobate (LN) microring is 1.78 million near the critical coupling condition. The hybridization wavelength was designed at 1562 nm and observed at 1537 nm. Potential applications include fundamental mode conversion, polarization rotation, polarization splitter, and polarization insensitive waveguides in optical receiver module.

Integrated four-channel directly modulated O-band optical transceiver for radio over fiber application.

We have fabricated a compact and integrated 4-channel analog optical transceiver for radio over fiber application. In the fabricated module, the transmitter optical sub-assembly is composed of four directly modulated DFB laser chips integrated with an optical multiplexer based on an arrayed waveguide grating (AWG) using silica-based planar lightwave circuit (PLC) technology. The receiver optical sub-assembly consists of a PIN photodiode array integrated with an AWG-PLC-type optical de-multiplexer. For all the lanes, the 3 dB bandwidth exceeds 19.1 GHz and the measured spurious-free dynamic range (SFDR) is up to 90.5 dB⋅Hz2/3 when the input RF frequency is from 2 GHz to 14 GHz. Meanwhile, the electrical inter-channel crosstalk of the transceiver is less than -20 dB when the carry frequency is below 18.5 GHz. This module shows a great transmission performance in radio over fiber system. Under simultaneous 4-channel different 600 Mb/s 5-band 64QAM-OFDM RF signal operation, the measured error vector magnitude (EVM) performance below 8% is achieved after 15.5 km single-mode fiber propagation for all lanes. This scheme has potential applications in guiding high-dense, cost-effective and high-linearity analog optical transceiver design to realize high-capacity radio over fiber transmission systems.

Electromagnetically induced transparency and absorption in a compact silicon ring-bus-ring-bus system.

We have theoretically and experimentally demonstrated electromagnetically induced transparency (EIT) and electromagnetically induced absorption (EIA) phenomena in a compact silicon ring-bus-ring-bus (RBRB) system. The two ring resonators in our RBRB system are both directly coupled and indirectly coupled through an asymmetric tricoupler. The coherent interference between a radiant mode and a subradiant mode in the two rings results in EIT and EIA effects at the through and drop ports, respectively. A theoretical model is established to analyze the proposed system based on temporal coupled mode theory. Finite-difference time-domain method is also employed to simulate the characteristics of this system. Consequently, RBRB structures were fabricated on a silicon-on-insulator platform and EIT and EIA transmissions have been observed simultaneously in the two outputs. The experimental results agree with our theoretical modeling and numerical simulations.

Two-mode contra-directional coupler based on superposed grating.

We propose and demonstrate a novel two-mode grating assisted contra-directional coupler (TGACC), which is capable of filtering two modes channels simultaneously by superposed grating with two superposed grating components. Finite-difference time-domain simulation is employed to study the structure. The influences of main structural parameters are analyzed, and apodization is employed to reduce the band sidelobes, crosstalk and back-reflections. We experimentally present a mode-channel switchable TGACC for 2.54nm-wide wavelength band centered at 1548.0nm by 50K thermal-optic tuning. With two channels combined into one device, the TGACC can help to enrich the functionality and reduce the footprint of mode-division multiplexing (MDM) systems.

Femtogram scale high frequency nano-optomechanical resonators in water.

A femtogram scale nanobeam optomechanical crystal (OMC) resonator operating in water is designed and demonstrated. After immersing the device in water, the mechanical Q-factor reduces to 6.6 from 2285 in air. The thermomechanical motion of the highly damped mechanical resonance in water can be resolved using the sensitive cavity optomechanical readout. The mechanical frequency is shifted to 5.251 GHz from 5.3 GHz in air due to the added motional inertia. From the thermomechanical noise spectrum of the mechanical resonance, a noise floor of 9.33am/Hz is achieved in water. Through 2D finite element method (FEM) simulations, the acoustic dissipation dominates the low mechanical Q-factor of the device during the interaction between the mechanical resonance and surrounding water. The mass sensitivity of the present device is estimated to be 1.33ag/Hz in water.

Silicon nitride O-band (de)multiplexers with low thermal sensitivity.

In this paper, four-channel cascaded Mach-Zehnder interferometer-based wavelength (de)multiplexers in the O-band are demonstrated experimentally by utilizing silicon nitride (SiN) optical waveguides. By reference to the commonly used 100 Gigabit Ethernet standards, two types of (de)multiplexer devices with different channel spacings are designed and fabricated. Both the devices exhibit low insertion loss and flat passbands. The lower thermo-optical coefficient provided by SiN brings benefits of reduction in thermal sensitivity. The fabricated (de)multiplexers show a temperature-dependent wavelength shift of about 18.5 pm/°C, which is reduced by 75% compared to the standard silicon-based devices.

Transmission of 2.86 Tb/s data stream in silicon subwavelength grating waveguides.

In this paper, we perform an investigation of terabit-scale data transmission in silicon subwavelength grating (SWG) waveguides for wavelength-division multiplexing (WDM) optical signals. Silicon SWG waveguide is capable of decreasing the light confinement in silicon core by engineering the geometry, leading to relatively lower optical nonlinearity compared to silicon wire waveguide. We demonstrate ultrahigh-bandwidth 2.86 Tb/s data transmission through the fabricated 2-mm-long silicon SWG waveguide over a wide range of launch powers. In the experiment, 75 WDM channels are utilized with each carrying 38.12 Gb/s orthogonal frequency-division multiplexing (OFDM) 16-ary quadrature amplitude modulation (16-QAM) signal. With the benefit of efficient reduction on optical nonlinearity, the optimum launch power is increased by 8 dB in SWG waveguide, indicating higher tolerance to the nonlinear impairments, compared to a silicon wire waveguide with identical length. With the optimum launch power, all 75 channels exhibit bit-error rate (BER) values less than 4e-5 after SWG waveguide transmission. We also evaluate the terabit-scale data transmission performance through four silicon SWG waveguides with different lengths (1 mm, 2 mm, 4 mm and 12 mm). The required optical signal-to-noise ratios (OSNRs) to achieve BER level of 1e-3 are around 15.27, 15.47, 16.66 and 20.38 dB, respectively.