Electro-optic Tuning Research Articles

Optimized domain-engineered lithium niobate for electro-optic spectral tuning in a multi-wavelength optical parametric oscillator

We report on an electro-optically (EO) spectrum tunable, multiline optical parametric oscillator (OPO) based on a nonperiodically poled lithium niobate (NPPLN). Besides being an efficient optical parametric down converter (OPDC) for multi-wavelength signal generation, the NPPLN can function as an EO modulator for fast spectral tuning based on a unique asymmetric-duty-cycle (ADC) equivalent domain structure. We have developed a genetic algorithm to construct and optimize such an ADC equivalent domain configuration in NPPLN to maximize its EO spectral tuning rate with the least compromise of its nonlinear gain for conducting the multiple down-conversions. When the novel EO NPPLN device with an equivalent ADC of 29%/71% works in an OPO intracavity pumped by a diode-pumped, 1064-nm Nd:YVO4 laser, we measured EO tuning rates of ∼0.5 and ∼0.53 nm/(kV/mm) for the down-converted dual signals at 1540 and 1550 nm, respectively. The tuning rates have been three orders of magnitude higher than a conventional one without an engineered domain asymmetry and 12% greater than another domain asymmetric LN device designed by a simulated annealing optimization method. The EO tuning technology developed in this study can be potentially implemented in micro/nano-photonic LN waveguide circuits.

Tunable narrow band-rejection and band-pass filter based on long-period waveguide grating on a thin-film lithium niobate rib waveguide

We propose and demonstrate experimentally an electro-optic (EO) and thermo-optic (TO) tunable wavelength filter with band-rejection and band-pass dual-function. Our proposed filter is based on a long-period waveguide grating (LPWG) formed on a lithium niobate on insulator (LNOI) rib waveguide with a channel-shaped polymer cladding waveguide. The LPWG formed on the surface of the LNOI core enables efficient mode coupling between the two fundamental modes of the LNOI waveguide and the polymer cladding waveguide and hence dual-function filtering. For the z-/x-polarized input light, our fabricated best filter shows an 18.1-dB/8.2-dB band-rejection contrast and a 15.7-dB/17.4-dB band-pass contrast at 1565.26/1594.24 nm wavelength, with a relatively narrow 3-dB bandwidth of 2.5/2.3 nm, respectively. In addition, the fabricated filter also shows an EO tuning efficiency of ∼32 pm/V and a TO tuning efficiency of 0.465 nm/°C, respectively. Our proposed filter could find applications in wavelength division multiplexing, temperature sensing, and other fields of optical communication and optical information processing.

Fast and low energy-consumption integrated Fourier-transform spectrometer based on thin-film lithium niobate

Abstract Integrated miniature spectrometers have impacts in industry, agriculture, and aerospace applications due to their unique advantages in portability and energy consumption. Although existing on-chip spectrometers have achieved breakthroughs in key performance metrics, such as, a high resolution and a large bandwidth, their scanning speed and energy consumption still hinder practical applications of such devices. Here, a stationary Fourier transform spectrometer is introduced based on a Mach–Zehnder interferometer structure on thin-film lithium niobate. Long and low-loss spiral waveguides with electro-optic tuning are adopted as the optical path scanning elements with a half-wave voltage of 0.14 V. A high resolution of 2.1 nm and a spectral recovery with a bandwidth of 100 nm is demonstrated under a high-speed and high-voltage scanning in the range of −100 V to +100 V at up to 100 KHz. A low energy consumption in the μJ scale per scan is also achieved.

Electro-optic tuning in composite silicon photonics based on ferroionic 2D materials

Tunable optical materials are indispensable elements in modern optoelectronics, especially in integrated photonics circuits where precise control over the effective refractive index is essential for diverse applications. Two-dimensional materials like transition metal dichalcogenides (TMDs) and graphene exhibit remarkable optical responses to external stimuli. However, achieving distinctive modulation across short-wave infrared (SWIR) regions while enabling precise phase control at low signal loss within a compact footprint remains an ongoing challenge. In this work, we unveil the robust electro-refractive response of multilayer ferroionic two-dimensional CuCrP2S6 (CCPS) in the near-infrared wavelength range. By integrating CCPS into silicon photonics (SiPh) microring resonators (MRR), we enhance light-matter interaction and measurement sensitivity to minute phase and absorption variations. Results show that electrically driven Cu ions can tune the effective refractive index on the order of 2.8 × 10−3 RIU (refractive index unit) while preserving extinction ratios and resonance linewidth. Notably, these devices exhibit low optical losses and excellent modulation efficiency of 0.25 V.cm with a consistent blue shift in the resonance wavelengths among all devices for either polarity of the applied voltage. These results outperform earlier findings on phase shifters based on TMDs. Furthermore, our study demonstrates distinct variations in electro-optic tuning sensitivity when comparing transverse electric (TE) and transverse magnetic (TM) modes, revealing a polarization-dependent response that paves the way for diverse applications in light manipulation. The combined optoelectronic and ionotronic capabilities of two-terminal CCPS devices present extensive opportunities across several domains. Their potential applications range from phased arrays and optical switching to their use in environmental sensing and metrology, optical imaging systems, and neuromorphic systems in light-sensitive artificial synapses.

Chalcophosphate metasurfaces with multipolar resonances and electro-optic tuning.

We computationally analyze the electro-optic response of metasurfaces consisting of interconnected nanoantennas with multipolar resonances using a chalcophosphate, Sn2P2S6, as the active material. Sn2P2S6 has large electro-optic coefficients and relatively low Curie temperature (<70 °C), allowing for strong changes in the refractive index of the material under moderate electric fields if the temperature can be finely controlled in proximity of the Curie point. Through numerical simulations, we show that metasurfaces designed with this nanostructured material demonstrate a significant shift of multipolar resonances upon biasing, despite moderate refractive-index values of the chalcophosphate and reduced mode localization due to this. The magnetic octupolar resonance of a dense array provides the strongest shift of spectral features upon changes in the refractive index, and we attribute it to the high mode localization of this higher-order multipole. We numerically demonstrate that narrow lattice resonances of collective nature do not provide an advantage in shifting the spectral features because of the nonlocal and delocalized nature of the modes, which are spread in the nanoantenna surrounding rather than confined inside nanoantennas. Both in-plane and out-of-plane biasing of the chalcophosphate crystals are similarly efficient with suitable electrode design, choice of electrode material, and crystal orientation within the nanoantennas. These designs exhibit optical properties similar to those of metasurfaces with isolated nanoantennas in the dense array.

Design of optical phased array with low-sidelobe beam steering in thin film lithium niobate

Optical beam-steering devices are crucial components of complex optical control systems, which are widely used in diverse information-processing applications. Optical phased arrays (OPAs) as the promising beam-steering devices, have proven to alleviate current performance limitations in beam steering devices for ultra-small solid-state light detection and rangings (LiDARs) and free-space communication systems. The high electro-optic (EO) coefficient (γ33 = 30.9 pm/V) of lithium niobate (LiNbO3, LN) can be a useful platform for phase tuning of the OPAs, which is conducive to enable phase shifting for non-mechanical beam steering with high-speed and low power consumption. The design, configurations and LN-OPA corresponding performances are discussed in this paper. The OPA consisting of different waveguides array is optimized with a side lobe suppression lower than - 10 dB at the wavelength of 1.55 μm. Meanwhile, the OPA with grating emitters or adding silica cavity further improves the deflection quality of the beam. Then optical beam steering with an angle of 50° in one dimension controlled by the LN-OPA has been demonstrated. The proposed OPA on the LN platform in this work might hold significances for advanced LiDAR schemes based on EO tuning.

Comparative analysis of lithium niobate and barium titanate material platforms for implementing electro-optically tunable general-purpose photonic processors

The field of general-purpose photonic processors (GPPPs) has been gaining momentum as a promising area for reconfigurable integrated photonic hardware. Among various tuning mechanisms, the electro-optic tuning mechanism can enable the implementation of high-speed GPPPs with faster reconfiguration, larger bandwidth, and reduced dependence on high-performance blocks. Lithium niobate (LN) and barium titanate (BTO) are promising electro-optic platforms that enable electro-optically tunable GPPPs. However, the inherent anisotropy associated with the two materials and polarization domain formation in BTO makes the study and analysis of these effects on device performance essential, as GPPPs involve tunable devices in three different orientations, which must perform similarly. This article explores and comparatively analyzes the potential of using two material platforms to enable the implementation of high-speed GPPPs. Various applications implemented on LN and BTO GPPPs are also discussed and compared. Based on our results, a method to select the optimal device orientation for implementing a high-speed GPPP with all devices performing similarly in the two platforms has been proposed. The results indicate that both platforms have their own pros and cons, and the choice of platform depends on the application.

Electro-optical tunable interleaver in hybrid silicon and lithium niobate thin films.

The interleaver was one of the key devices in dense wavelength division multiplexing (DWDM) applications. In this study, an interleaver with an asymmetrical Mach-Zehnder interferometer structure was designed, fabricated, and characterized in hybrid silicon and lithium niobate thin films (Si-LNOI). The interleaver based on Si-LNOI could be fabricated by mature processing technology of Si photonic, and it was capable of the electro-optical (E-O) tuning function by using the E-O effect of LN. In the range of 1530-1620 nm, the interleaver achieved a channel spacing of 55 GHz and an extinction ratio of 12-28 dB. Due to the large refractive index of Si, the Si loading strip waveguide based on Si-LNOI had a compact optical mode area, which allowed a small electrode gap to improve the E-O modulation efficiency of the interleaver. For an E-O interaction length of 1 mm, the E-O modulation efficiency was 26 pm/V. The interleaver will have potential applications in DWDM systems, optical switches, and filters.

Ultra-narrow linewidth laser system based on intra-cavity electro-optic crystal frequency stabilization of external-cavity diode laser

An external-cavity diode laser based on an intra-cavity electro-optic modulator tuning is developed in this paper. Electro-optic crystal lithium niobate (LiNbO3), with no hysteresis effect and high response speed, is used to replace the traditional tunable part piezoelectric transducer (PZT). The linewidth of this laser at free-running mode is about 133 kHz. In addition, in the ultra-narrow linewidth laser system, the error signal is fed back to the driving voltage of LiNbO3 to realize laser locking, which can avoid the problem of laser output power fluctuation caused by feeding back to the laser injection current. The self-developed crystal-tuned laser has been successfully used in an ultra-narrow linewidth laser system, and the experimental results show that the linewidth of its beat frequency signal is about 1.5 Hz.

Integrated optical pattern generation on thin-film lithium niobate with electro-optic modulators and phase-change material cells

Reconfigurable photonic integrated circuits enable high-bandwidth signal shaping with the prospect for scalability and compact footprint. Cointegration of electro-optical tunability with nonvolatile attenuation through functional materials allows for implementing photonic devices that operate on both phase and amplitude. Based on this approach, we propose an integrated photonic design for optical pattern generation deploying a continuous-wave laser and a single electrical function generator. We employ the nonvolatile and reconfigurable phase-change material Ge2Sb2Te5 (GST) as a tunable attenuator for an integrated photonic circuit on the lithium-niobate-on-insulator (LNOI) platform. The GST can be switched between its amorphous and crystalline phases, leading to an optical contrast of ≅18dB. Combining this with integrated electro-optical modulators with a 4 GHz bandwidth in LNOI enables the generation of short optical pulses, based on the principles of inverse discrete Fourier transform.

Electro-optically tunable optical delay line with a continuous tuning range of ∼220 fs in thin-film lithium niobate.

We demonstrate fabrication of a 30-cm-long thin-film lithium niobate (TFLN) optical delay line (ODL) incorporated with segmented microelectrodes of 24-cm total length using the femtosecond laser lithography technique. The transmission spectra of the unbalanced Mach-Zehnder interferometers (MZIs) reveal an ultra-low propagation loss of 0.025 dB/cm. The device exhibits a low half-wave voltage of 0.45 V, corresponding to a voltage-length product of 10.8 V·cm, which is equivalent to 5.4 V·cm in the push-pull configuration. We also demonstrate a high electro-optic (EO) tuning efficiency of 3.146 fs/V and a continuous tuning range of 220 fs in the fabricated ODL.

Versatile Tunning of Compact Microring Waveguide Resonator Based on Lithium Niobate Thin Films

With the advancement of modulation technology and the requirement for device miniaturization and integration, lithium niobate on insulator (LNOI) can be a versatile platform for this pursuit, as it can confine the transmitted light at the nanoscale, leading to a strong light–matter interaction, which can sensitively capture external variations, such as electric fields and temperature. This paper presents a compact microring modulator with versatile tuning based on X-cut LNOI. The LNOI modulator equipped with electrodes with a coverage angle of 120∘ achieved a maximum electro-optic (EO) tuning efficiency of 13 pm/V and a maximum extinction ratio of 11 dB. The asymmetry in the static or quasi-static electro-optic tuning of the microring modulator was also analyzed. Furthermore, we measured the thermal-optic effect of the device with a sensitivity of 26.33 pm/∘C, which can potentially monitor the environment temperature or compensate for devices’ functional behavior. The demonstrated efficient and versatile compact microring modulator will be an important platform for on-chip active or passive photonic components, microring-based sensor arrays and integrated optics.

Investigation of High-Q Lithium Niobate-Based Double Ring Resonator Used in RF Signal Modulation

In recent years, millimeter-wave communication has played a crucial role in satellite communication, 5G, and even 6G applications. The millimeter-wave electro-optic modulator is capable of receiving and processing millimeter-wave signals effectively. However, the large attenuation of millimeter waves in the air remains a primary limiting factor for their future applications. Therefore, finding a waveguide structure with a high quality factor (Q-factor) is critical for millimeter-wave electro-optic modulators. This manuscript presents the demonstration of a double ring modulator made of lithium niobate with the specific goal of modulating an RF signal at approximately 35 GHz. By optimizing the microring structure, the double ring resonator with high Q-factor is studied to obtain high sensitivity modulation of the RF signal. This manuscript employs the transfer matrix method to investigate the operational principles of the double ring structure and conducts simulations to explore the influence of structural parameters on its performance. Through a comparison with the traditional single ring structure, it is observed that the Q-factor of the double ring modulator can reach 7.05 × 108, which is two orders of magnitude greater than that of the single ring structure. Meanwhile, the electro-optical tunability of the double ring modulator is 6 pm/V with a bandwidth of 2.4 pm, which only needs 0.4 V driving voltage. The high Q double ring structure proposed in this study has potential applications not only in the field of communication but also as a promising candidate for a variety of chemical and biomedical sensing applications.

Controlling single rare earth ion emission in an electro-optical nanocavity

Rare earth emitters enable critical quantum resources including spin qubits, single photon sources, and quantum memories. Yet, probing of single ions remains challenging due to low emission rate of their intra-4f optical transitions. One feasible approach is through Purcell-enhanced emission in optical cavities. The ability to modulate cavity-ion coupling in real-time will further elevate the capacity of such systems. Here, we demonstrate direct control of single ion emission by embedding erbium dopants in an electro-optically active photonic crystal cavity patterned from thin-film lithium niobate. Purcell factor over 170 enables single ion detection, which is verified by second-order autocorrelation measurement. Dynamic control of emission rate is realized by leveraging electro-optic tuning of resonance frequency. Using this feature, storage, and retrieval of single ion excitation is further demonstrated, without perturbing the emission characteristics. These results promise new opportunities for controllable single-photon sources and efficient spin-photon interfaces.

Ultrafast tunable lasers using lithium niobate integrated photonics

Early works1 and recent advances in thin-film lithium niobate (LiNbO3) on insulator have enabled low-loss photonic integrated circuits2,3, modulators with improved half-wave voltage4,5, electro-optic frequency combs6 and on-chip electro-optic devices, with applications ranging from microwave photonics to microwave-to-optical quantum interfaces7. Although recent advances have demonstrated tunable integrated lasers based on LiNbO3 (refs. 8,9), the full potential of this platform to demonstrate frequency-agile, narrow-linewidth integrated lasers has not been achieved. Here we report such a laser with a fast tuning rate based on a hybrid silicon nitride (Si3N4)–LiNbO3 photonic platform and demonstrate its use for coherent laser ranging. Our platform is based on heterogeneous integration of ultralow-loss Si3N4 photonic integrated circuits with thin-film LiNbO3 through direct bonding at the wafer level, in contrast to previously demonstrated chiplet-level integration10, featuring low propagation loss of 8.5 decibels per metre, enabling narrow-linewidth lasing (intrinsic linewidth of 3 kilohertz) by self-injection locking to a laser diode. The hybrid mode of the resonator allows electro-optic laser frequency tuning at a speed of 12 × 1015 hertz per second with high linearity and low hysteresis while retaining the narrow linewidth. Using a hybrid integrated laser, we perform a proof-of-concept coherent optical ranging (FMCW LiDAR) experiment. Endowing Si3N4 photonic integrated circuits with LiNbO3 creates a platform that combines the individual advantages of thin-film LiNbO3 with those of Si3N4, which show precise lithographic control, mature manufacturing and ultralow loss11,12.

LAN Wavelength Division Multiplexer on Silicon-Lithium Niobate Hybrid Integration Platform

We demonstrate an 8-channel LAN wavelength division multiplexer (LWDM) on silicon-lithium niobate (Si-LN) hybrid integration platform. The device consists of three-stage cascaded MZIs. The phase-error corrections and precise wavelength alignment of 4.5 nm channel spacing are achieved by employing the electro-optic tuning effect on bonded LN waveguides on silicon. The proposed hybrid LWDM with controllable phase shifters has potential applications in optical links of ultra-large transmission capacity with integration of hybrid modulators of the same technology.

Spiral waveguide Bragg grating modulator on thin-film Z-cut lithium niobate

Next-generation photonic integrated circuits require compact electro-optic modulators (EOMs) that achieve high performance and efficient use of the on-chip area simultaneously. We demonstrate a compact EOM based on a spiral-shaped waveguide Bragg grating on thin-film lithium niobate. The modulator utilizes the grating’s optical filter characteristics along with electro-optic tuning of the central Bragg wavelength to achieve simple and efficient intensity modulation. The spiral Bragg gratings were realized on Z-cut lithium niobate and modulated using top and bottom electrodes. The integrated design wrapped a 2.2 mm long grating into a 120×120µm2 area. The modulator bandgap, with an extinction ratio of over 35 dB at 1550 nm, could be efficiently tuned with a sensitivity of 8.36 pm/V and a 3 dB operating bandwidth of 25 GHz.

Electro-optic tunable grating-assisted optical waveguide directional coupler in lithium niobate

In this paper, a high-performance electro-optical tunable grating-assisted directional coupler (GADC) is proposed and demonstrated experimentally. Our proposed GADC consists of a two-mode waveguide (TMW) and a single-mode waveguide (SMW) formed with lithium niobate (LN). By introducing a long-period waveguide grating into the side-wall of the TMW to compensate for the phase mismatch of the fundamental modes of the SMW and TMW, the fundamental modes of the two waveguides can be coupled efficiently each other at a specific wavelength. Furthermore, push–pull electrode structure is introduced to achieve the electro-optic (EO) tuning function featuring high speed, low driving voltage, and large tuning range. The best LN GADC we fabricated on x-cut LN substrate using annealing proton exchange process shows a high isolation of ~ 34 dB at 1532.9 nm wavelength, quite large EO tuning efficiency of 1.195 nm/V (1526.4–1549.1 nm) or 1.736 nm/V (1576.1–1602.2 nm), and a thermo-optic tuning efficiency of 0.128 nm/°C (22–60 °C). Our proposed LN GADC can find applications in the fields of high-speed tunable wavelength filtering, mode filtering, and EO modulation.

Phase-change material-assisted all-optical temporal differentiator.

This paper proposes a novel microring resonator (MRR)-based all-optical tuning temporal differentiator (DIFF). Specifically, the DIFF uses nonvolatile phase-change material Ge2Sb2Te5 (GST) to achieve low energy consumption and high-speed optical control of the state of the MRR, avoiding the traditional electro-optic (EO) and thermo-optic (TO) tuning designs. By changing the crystallinity of GST to changing the coupling regimes of the MRR, a broad range for the differentiation order α, i.e., 0.47-1.64 can be realized. The intensity response and phase response of the GST-assisted MRR, and normalized intensity in the output of the temporal DIFFs for Gaussian optical pulses have been obtained by simulation. Furthermore, input pulse width and detuning influence on the differentiation order and output deviation are discussed. Finally, our structure can effectively reduce the chip area and power consumption compared with the traditional EO and TO tuning designs.

Photo- and electro-controllable 2D diffraction gratings prepared using electrohydrodynamic instability in a nematic polymerizable mixture

2D diffraction gratings were prepared by polymerization-induced fixation of electrohydrodynamic convection patterns in a nematic mixture. Photo- and electro-optical tuning of their diffraction efficiency was demonstrated.