Integrated frequency-modulated optical parametric oscillator.

Optical frequency combs have revolutionized precision measurement, time-keeping and molecular spectroscopy1-7. A substantial effort has developed around 'microcombs': integrating comb-generating technologies into compact photonic platforms5,7-9. Current approaches for generating these microcombs involve either the electro-optic10 or Kerr mechanisms11. Despite rapid progress, maintaining high efficiency and wide bandwidth remains challenging. Here we introduce a previously unknown class of microcomb-an integrated device that combines electro-optics and parametric amplification to yield a frequency-modulated optical parametric oscillator (FM-OPO). In contrast to the other solutions, it does not form pulses but maintains operational simplicity and highly efficient pump power use with an output resembling a frequency-modulated laser12. We outline the working principles of our device and demonstrate it by fabricating the complete optical system in thin-film lithium niobate. We measure pump-to-comb internal conversion efficiency exceeding 93% (34% out-coupled) over a nearly flat-top spectral distribution spanning about 200 modes (over 1 THz). Compared with an electro-optic comb, the cavity dispersion rather than loss determines the FM-OPO bandwidth, enabling broadband combs with a smaller radio-frequency modulation power. The FM-OPO microcomb offers robust operational dynamics, high efficiency and broad bandwidth, promising compact precision tools for metrology, spectroscopy, telecommunications, sensing and computing.

Picojoule-level supercontinuum generation in thin-film lithium niobate on sapphire.

We demonstrate ultraviolet-to-mid-infrared supercontinuum generation (SCG) inside thin-film lithium niobate (TFLN) on sapphire nanowaveguides. This platform combines wavelength-scale confinement and quasi-phasematched nonlinear interactions with a broad transparency window extending from 350 to 4500 nm. Our approach relies on group-velocity-matched second-harmonic generation, which uses an interplay between saturation and a small phase-mismatch to generate a spectrally broadened fundamental and second harmonic using only a few picojoules of in-coupled fundamental pulse energies. As the on-chip pulse energy is increased to tens of picojoules, these nanowaveguides generate harmonics up to the fifth order by a cascade of sum-frequency mixing processes. For in-coupled pulse energies in excess of 25 picojoules, these harmonics merge together to form a supercontinuum spanning 360-2660 nm. We use the overlap between the first two harmonic spectra to detect f-2f beatnotes of the driving laser directly at the waveguide output, which verifies the coherence of the generated harmonics. These results establish TFLN-on-sapphire as a viable platform for generating ultra-broadband coherent light spanning from the ultraviolet to mid-infrared spectral regions.

Temperature Dependence of the Mechanical Dissipation of Gallium Bonds for Use in Gravitational Wave Detectors.

The mirror suspensions in gravitational wave detectors demand low mechanical loss jointing to ensure good enough detector performance and to enable the detection of gravitational waves. Hydroxide catalysis bonds have been used in the fused silica suspensions of the GEO600, Advanced LIGO, and Advanced Virgo detectors. Future detectors may use cryogenic cooling of the mirror suspensions and this leads to a potential change of mirror material and suspension design. Other bonding techniques that could replace or be used alongside hydroxide catalysis bonding are of interest. A design that incorporates repair scenarios is highly desirable. Indeed, the mirror suspensions in KAGRA, which is made from sapphire and operated at cryogenic temperatures, have used a combination of hydroxide catalysis bonding and gallium bonding. This Letter presents the first measurements of the mechanical loss of a gallium bond measured between 10 K and 295 K. It is shown that the loss, which decreases with temperature down to the level of (1.8±0.3)×10^{-4} at 10 K, is comparable to that of a hydroxide catalysis bond.

Low Mechanical Loss TiO_{2}:GeO_{2} Coatings for Reduced Thermal Noise in Gravitational Wave Interferometers.

The sensitivity of current and planned gravitational wave interferometric detectors is limited, in the most critical frequency region around 100 Hz, by a combination of quantum noise and thermal noise. The latter is dominated by Brownian noise: thermal motion originating from the elastic energy dissipation in the dielectric coatings used in the interferometer mirrors. The energy dissipation is a material property characterized by the mechanical loss angle. We have identified mixtures of titanium dioxide (TiO_{2}) and germanium dioxide (GeO_{2}) that show internal dissipations at a level of 1×10^{-4}, low enough to provide improvement of almost a factor of 2 on the level of Brownian noise with respect to the state-of-the-art materials. We show that by using a mixture of 44% TiO_{2} and 56% GeO_{2} in the high refractive index layers of the interferometer mirrors, it would be possible to achieve a thermal noise level in line with the design requirements. These results are a crucial step forward to produce the mirrors needed to meet the thermal noise requirements for the planned upgrades of the Advanced LIGO (Laser Interferometer Gravitational-Wave Observatory) and Virgo detectors.

Analysis of two-level systems and mechanical loss in amorphous ZrO2-doped Ta2O5 by non-cage-breaking and cage-breaking transitions.

The energy landscape of ZrO2-doped amorphous Ta2O5 is explored in this work. With models corresponding to experimental concentrations of 50% Zr and 50% Ta cations, we search for, gather, and analyze two-level systems (TLSs) from molecular dynamic simulations. The mechanical loss function is calculated for each TLS individually. The results show that TLS with low asymmetry and large elastic coupling constants contribute the most to mechanical loss. We identify these as "bad actors." The higher barriers relate to the mechanical loss at higher temperatures. The concept of the oxygen cage that describes the local structural environment surrounding a metal ion is introduced. The existence of a drastic change in local environment, or a cage-breaking process, enables us to understand the double peaks present in the asymmetry distribution and provides a pictorial interpretation to distinguish two types of TLS. Quantitatively, a cage-breaking event is related to at least one large distance change in an atom-atom pair, and non-cage-breaking transitions have only small rearrangements. The majority of TLSs are cage-breaking transitions, but non-cage-breaking TLS transitions show higher average mechanical loss in ZrO2-doped Ta2O5. By decomposing the contributions to mechanical loss, we find that the low temperature loss peak near 40 K mainly comes from non-cage-breaking TLS transitions and the second loss peak near 120 K originates from cage-breaking TLS transitions. This finding is important for understanding the interplay between the atomic structure of TLS and mechanical loss.

Performance scaling of a 10-GHz solid-state laser enabling self-referenced CEO frequency detection without amplification.

A simple and compact straight-cavity laser oscillator incorporating a cascaded quadratic nonlinear crystal and a semiconductor saturable absorber mirror (SESAM) can deliver stable femtosecond modelocking at high pulse repetition rates >10 GHz. In this paper, we experimentally investigate the influence of intracavity dispersion, pump brightness, and cavity design on modelocking with high repetition rates, and use the resulting insights to demonstrate a 10.4-GHz straight-cavity SESAM-modelocked Yb:CALGO laser delivering 108-fs pulses with 812 mW of average output power. This result represents a record-level performance for diode-pumped femtosecond oscillators with repetition rates above 10 GHz. Using the oscillator output without any optical amplification, we demonstrate coherent octave-spanning supercontinuum generation (SCG) in a silicon nitride waveguide. Subsequent f-to-2f interferometry with a periodically poled lithium niobate waveguide enables the detection of a strong carrier-envelope offset (CEO) beat note with a 33-dB signal-to-noise ratio.

Growth and characterization of Sc2O3 doped Ta2O5 thin films.

We present the optical and structural characterization of films of Ta2O5, Sc2O3, and Sc2O3 doped Ta2O5 with a cation ratio around 0.1 grown by reactive sputtering. The addition of Sc2O3 as a dopant induces the formation of tantalum suboxide due to the "oxygen getter" property of scandium. The presence of tantalum suboxide greatly affects the optical properties of the coating, resulting in higher absorption loss at λ=1064nm. The refractive index and optical band gap of the mixed film do not correspond to those of a mixture of Ta2O5 and Sc2O3, given the profound structural modifications induced by the dopant.

Pilot-tone-based self-homodyne detection using optical nonlinear wave mixing.

An all-optical pilot-tone-based self-homodyne detection scheme using nonlinear wave mixing is experimentally demonstrated. Two scenarios are investigated using (1) multiple wavelength-division-multiplexed channels with sufficient power of the pilot tones and (2) a single channel with a low-power pilot tone. The eye diagram and bit error rate of the system are studied by tuning various parameters such as pump power, relative phase, and pilot-to-signal ratio.

Pump-linewidth-tolerant wavelength multicasting using soliton Kerr frequency combs.

We experimentally demonstrate pump-linewidth-tolerant wavelength multicasting using microresonator-based soliton Kerr frequency combs. When Kerr comb lines serve as coherent pumps in a periodically poled lithium niobate waveguide, the linewidth of the multicast signal almost remains that of the original signal at different linewidths of Kerr combs, ranging from 100 kHz to 1 MHz. However, in conventional multicasting where free-running (FR) pumps are used, the linewidth of the converted signal significantly increases. Furthermore, the error vector magnitude (EVM) performance demonstrates eight-fold error-free multicasting of 10 Gbaud 16-quadrature amplitude modulation signals, even when the linewidths of the Kerr combs are as broad as 1 MHz (no Kalman filtering algorithm in the receiver). In contrast, the EVM performance of the signal copy is degraded with an FR laser as a dummy pump.

Simultaneous all-optical phase noise mitigation and automatically locked homodyne reception of an incoming QPSK data signal.

Simultaneous phase noise mitigation and automatic phase/frequency-locked homodyne reception is demonstrated for a 20-32 Gbaud QPSK signal. A phase quantization function is realized to squeeze the phase noise of the signal by optical wave mixing of the signal, its third-order harmonic, and their corresponding delayed variant conjugates, converting the noisy input into a noise-mitigated signal. In a simultaneous nonlinear process, the noise-mitigated signal is automatically phase- and frequency-locked with a "local" pump laser, avoiding the need for feedback or phase/frequency tracking for homodyne detection. Open eye-diagrams are obtained for in-phase and quadrature-phase components of the signal and ∼2 dB OSNR gain is achieved at BER 10-3.

Demonstration of optical multicasting using Kerr frequency comb lines.

We experimentally demonstrate optical multicasting using Kerr frequency combs generated from a Si3N4 microresonator. We obtain Kerr combs in two states with different noise properties by varying the pump wavelength in the resonator and investigate the effect of Kerr combs on multicasting. Seven-fold multicasting of 20 Gbaud quadrature phase-shift-keyed signals and four-fold multicasting of 16-quadrature amplitude modulation signals have been achieved when low-phase-noise combs are input into a periodically poled lithium niobate waveguide. In addition, we find that the wavelength conversion efficiency in the PPLN waveguide for chaotic combs with high noise is similar to that for low-noise combs, while the signal quality of the multicast copy is significantly degraded.

Experimental demonstration of tunable homodyne detection of WDM and dual-polarization PSK channels by automatically locking the channels to a local pump laser using nonlinear mixing.

This Letter proposes a method for tunable automatically locked homodyne detection of wavelength-division multiplexing (WDM) dual-polarization (DP) phase-shift keyed (PSK) channels using nonlinear mixing. Two stages of periodically poled lithium niobate (PPLN) waveguides and an LCoS filter enable automatic phase locking of the channels to a local laser.

Nonlinear diffusion model for annealed proton-exchanged waveguides in zirconium-doped lithium niobate.

Photorefractive-damage- (PRD) resistant zirconium-oxide-doped lithium niobate is investigated as a substrate for the realization of annealed proton-exchanged (APE) waveguides. Its advantages are a favorable distribution coefficient, PRD resistance comparable to magnesium-oxide-doped lithium niobate, and a proton-diffusion behavior resembling congruent lithium niobate. A 1D model for APE waveguides was developed based on a previous model for congruently melting lithium niobate. Evidence for a nonlinear index dependence on concentration was found.

Tunable delay control of entangled photons based on dispersion cancellation.

We propose and demonstrate a novel approach for controlling the temporal position of the biphoton correlation function using pump frequency tuning and dispersion cancellation; precise waveguide engineering enables biphoton generation at different pump frequencies while the idea of nonlocal dispersion cancellation is used to create the relative signal-idler delay and simultaneously prevents broadening of their correlation. Experimental results for delay shifts up to ±15 times the correlation width are shown along with discussions of the performance metrics of this approach.

Optical channel de-aggregation of quadrature-phase-shift-keying and eight-phase-shift-keying data using mapping onto constellation axes.

An eight-phase-shift-keying signal is experimentally de-aggregated onto two four-pulse amplitude modulation signals using nonlinear processes in optical elements. Quadrature-phase-shift-keying signals are similarly de-multiplexed into two binary phase shift keying signals by mapping the data points onto the constellation axes. De-multiplexing performance is evaluated as a function of the optical signal-to-noise ratio of the incoming signals. The effect of phase noise is also studied.

Tunable radio frequency photonics filter using a comb-based optical tapped delay line with an optical nonlinear multiplexer.

A radio frequency (RF) photonic filter is experimentally demonstrated using an optical tapped delay line (TDL) based on an optical frequency comb and a periodically poled lithium niobate (PPLN) waveguide as multiplexer. The approach is used to implement RF filters with variable bandwidth, shape, and center-frequency.

Phase noise mitigation of QPSK signal utilizing phase-locked multiplexing of signal harmonics and amplitude saturation.

We demonstrate an all-optical phase noise mitigation scheme based on the generation, delay, and coherent summation of higher order signal harmonics. The signal, its third-order harmonic, and their corresponding delayed variant conjugates create a staircase phase-transfer function that quantizes the phase of quadrature-phase-shift-keying (QPSK) signal to mitigate phase noise. The signal and the harmonics are automatically phase-locked multiplexed, avoiding the need for phase-based feedback loop and injection locking to maintain coherency. The residual phase noise converts to amplitude noise in the quantizer stage, which is suppressed by parametric amplification in the saturation regime. Phase noise reduction of ∼40% and OSNR-gain of ∼3 dB at BER 10(-3) are experimentally demonstrated for 20- and 30-Gbaud QPSK input signals.

Generation of biphoton correlation trains through spectral filtering.

We demonstrate the generation of two-photon correlation trains based on spectral filtering of broadband biphotons. Programmable amplitude filtering is employed to create biphoton frequency combs, which when coupled with optical dispersion allows us to experimentally verify the temporal Talbot effect for entangled photons. Additionally, an alternative spectral phase-filtering approach is shown to significantly improve the overall efficiency of the generation process when a comb-like spectrum is not required. Our technique is ideal for the creation of tunable and high-repetition-rate biphoton states.

Tunable optical correlator using an optical frequency comb and a nonlinear multiplexer.

We experimentally demonstrate a tunable optical correlator to search for multiple patterns among QPSK symbols. We utilize an optical frequency comb to generate the coherent signals and multiplex them coherently in a single PPLN waveguide. Multiple patterns with different lengths are successfully searched within QPSK symbols in a 40-Gb/s signal.

Experimental characterization of phase tuning using fine wavelength offsets in a tunable complex-coefficient optical tapped-delay-line.

We use fine-detuning of pump wavelengths to adjust the tap phases in a complex-coefficient optical tapped-delay-line that utilizes conversion/dispersion-based delays and nonlinear wave mixing. Full 2π phase tuning is demonstrated by detuning the frequency of laser pumps by <20 GHz, which shows close agreement with theory.