Electro-optically tunable single-frequency lasing from neodymium-doped lithium niobate microresonators.

Tunable light sources are a key enabling technology for many applications such as ranging, spectroscopy, optical coherence tomography, digital imaging and interferometry. For miniaturized laser devices, whispering gallery resonator lasers are a well-suited platform, offering low thresholds and small linewidths, however, many realizations suffer from the lack of reliable tuning. Rare-earth ion-doped lithium niobate offers a way to solve this issue. Here we present a single-frequency laser based on a neodymium-doped lithium niobate whispering gallery mode resonator that is tuned via the linear electro-optic effect. Using a special geometry, we suppress higher-order transverse modes and hence ensure single-mode operation. With an applied voltage of just 68 V, we achieve a tuning range of 3.5 GHz. The lasing frequency can also be modulated with a triangular control signal. The freely running system provides a frequency and power stability of better than Δ ν=20MHz and 6 %, respectively, for a 30-minute period. This concept is suitable for full integration with existing photonic platforms based on lithium niobate.

Pockels-effect-based adiabatic frequency conversion in ultrahigh-Q microresonators.

Adiabatic frequency conversion has some key advantages over nonlinear frequency conversion. No threshold and no phase-matching conditions need to be fulfilled. Moreover, it exhibits a conversion efficiency of 100 % down to the single-photon level. Adiabatic frequency conversion schemes in microresonators demonstrated so far suffer either from low quality factors of the employed resonators resulting in short photon lifetimes or small frequency shifts. Here, we present an adiabatic frequency conversion (AFC) scheme by employing the Pockels effect. We use a non-centrosymmetric ultrahigh-Q microresonator made out of lithium niobate. Frequency shifts of more than 5 GHz are achieved by applying just 20 V to a 70-µm-thick resonator. Furthermore, we demonstrate that with the same setup positive and negative frequency chirps can be generated. With this method, by controlling the voltage applied to the crystal, almost arbitrary frequency shifts can be realized. The general advances in on-chip fabrication of lithium-niobate-based devices make it feasible to transfer the current apparatus onto a chip suitable for mass production.

Frequency Comb Generation via Cascaded Second-Order Nonlinearities in Microresonators.

Optical frequency combs are revolutionizing modern time and frequency metrology. In the past years, their range of applications has increased substantially, driven by their miniaturization through microresonator-based solutions. The combs in such devices are typically generated using the third-order χ^{(3)} nonlinearity of the resonator material. An alternative approach is making use of second-order χ^{(2)} nonlinearities. While the idea of generating combs this way has been around for almost two decades, so far only few demonstrations are known, based either on bulky bow-tie cavities or on relatively low-Q waveguide resonators. Here, we present the first such comb that is based on a millimeter-sized microresonator made of lithium niobate, that allows for cascaded second-order nonlinearities. This proof-of-concept device comes already with pump powers as low as 2 mW, generating repetition-rate-locked combs around 1064 and 532 nm. From the nonlinear dynamics point of view, the observed combs correspond to Turing roll patterns.

Tunable single-frequency lasing in a microresonator.

Whispering-gallery-mode resonators made of laser-active materials can serve as efficient microphotonic coherent light sources. However, the majority of experimental realizations relies on expensive pump light sources like narrow-linewidth or pulsed laser systems, which is inappropriate for most applications. In order to overcome this, we present a whispering-gallery laser system without the need for an expensive pump light source and at the same time with unprecedented laser performance: A laser-active resonator made of Nd:YVO 4 is non-resonantly excited, employing a low-cost laser diode without any external frequency stabilization, emitting up to 100 mW optical power around 810 nm wavelength. Continuous-wave single-frequency lasing at 1064 nm wavelength is achieved with directed laser light emission in the mW-regime. The temporal power and frequency stability are within ±1.5 % and ±30 MHz, respectively. Modehop-free frequency fine-tuning is achieved exceeding 11 GHz tuning range by changing the temperature of the cavity. Faster tuning can be expected when applying geometric or electro-optic instead of thermal tuning.

Multiwavelength holography: height measurements despite axial motion of several wavelengths during exposure.

Interferometric measurements of rotating objects face an axial motion component if the optical axis of the measurement system is not pointing towards the axis of rotation. In a typical interferometer, axial motion of half a wavelength reduces the interference contrast to zero. Our setup compensates for this axial component by an adapted variation of the reference path length during exposure utilizing a piezoelectric actuator. We present off-center measurements on a cylinder, rotating with different angular velocities. The repeatability of these measurements is dominated by motion blur, which demonstrates that the compensation of the axial motion works accurately.

Scattering-loss reduction of ridge waveguides by sidewall polishing.

Ridge waveguides provide a large refractive index contrast and thus strong mode confinement, making them highly attractive for building compact photonic integrated circuits. However, ridge waveguides suffer from scattering losses. We demonstrate scattering-loss reduction of ridge waveguides made of lithium-niobate-on-insulator (LNOI) substrates by more than one order of magnitude. This is achieved by gently polishing of the ridge's sidewalls and simultaneous protection of the top surfaces by a metal layer. Whispering-gallery-resonator loss measurements reveal ultra-low losses down to 0.04 dB/cm of the processed waveguides. Our approach pushes ridge waveguides further towards their fundamental absorption-loss limit, enabling highly efficient integrated devices.

Quasi-phase-matched self-pumped optical parametric oscillation in a micro-resonator.

Lasing and self-pumped optical parametric oscillation (self-OPO) are achieved in a high-Q whispering-gallery-mode micro-resonator, made of neodymium-doped lithium niobate. A laser process providing 5 mW output power at 1.08 µm wavelength is sufficient to pump a self-OPO process within the same high-Q cavity. At 6 mW lasing output power, the sum of signal and idler output powers exceeds 1.2 mW. The wavelength of the generated light ranges from 1.5 to 3.8 µm. Phase matching is provided by a radial quasi-phase-matching structure, which is generated by a current-controlled calligraphic poling technique. To the best of our knowledge, this is the first demonstration of a quasi-phase-matched self-pumped nonlinear optical process in a micro-resonator, as well as the first self-OPO in a micro-resonator. The concept bears the potential for a highly integrated and wavelength-tunable coherent light source at low cost.

Control of mode anticrossings in whispering gallery microresonators.

Optical microresonators attract strong interest because of exciting effects and applications ranging from sensing of single atoms and molecules to quantum and nonlinear optics. For all this, control and tuning of the discrete resonances are vital. In resonators made of anisotropic materials that are beneficial for nonlinear-optical applications, anticrossings of ordinarily (o) and extraordinarily (e) polarized modes occur regularly. This effect is badly understood and harmful for mode control and tuning. We show that the anticrossings are inherent in the o- and e-modes because of the vectorial properties of Maxwell's equations. Within a novel pertubative approach employing a strong localization of the modes near the resonator rim, we have quantified the anticrossings. The values of avoidance gaps strongly exceeding the linewidths and selection rules for the interacting modes are predicted. The inferred values of the avoidance gaps are confirmed experimentally in resonators made of lithium niobate. Furthermore, based on theory, we have eliminated the anticrossings completely by spatially-controlled introduction of defects. This paves the way for unperturbed tuning of anisotropic microresonators.

Continuous-wave whispering-gallery optical parametric oscillator based on CdSiP2.

Continuous-wave (cw) optical parametric oscillators (OPOs) are ideally suited for applications, for example high-resolution spectroscopy, that need coherent sources combining narrow-linewidth emission with good wavelength tunability. Here, we demonstrate for the first time cw OPOs based on a millimeter-sized whispering gallery resonator (WGR) made of cadmium silicon phosphide (CdSiP2). By employing a compact laser diode at 1.57-µm wavelength for pumping, a cw OPO with wavelength tunability from 2.3 µm to 5.1 µm is realized based on such a resonator. The oscillation thresholds are in the milliwatt range. The maximum total power conversion efficiency reaches more than 15%. The intrinsic quality factor at 1.57 µm is determined to be 3.5 × 106. This work suggests that CdSiP2 is a very promising alternative for constructing mid-infrared parametric devices.

Frequency comb up- and down-conversion in synchronously driven χ(2) optical microresonators.

Optical frequency combs are key to optical precision measurements. While most frequency combs operate in the near-infrared (NIR) regime, many applications require combs at mid-infrared (MIR), visible (VIS), or even ultra-violet (UV) wavelengths. Frequency combs can be transferred to other wavelengths via nonlinear optical processes; however, this becomes exceedingly challenging for high-repetition-rate frequency combs. Here it is demonstrated that a synchronously driven high-Q microresonator with a second-order optical nonlinearity can efficiently convert high-repetition-rate NIR frequency combs to VIS, UV, and MIR wavelengths, providing new opportunities for microresonator and electro-optic combs in applications including molecular sensing, astronomy, and quantum optics.

Cascaded second-order optical nonlinearities in on-chip micro rings.

We demonstrate cascaded Stimulated Raman Scattering (SRS), Second-Harmonic Generation (SHG), and Sum-Frequency Generation (SFG) in integrated on-chip whispering-gallery resonators (WGRs). These lithium niobate-based WGRs are fabricated using highly-parallel semiconductor manufacturing techniques coupled with specialized polishing as a post-processing step and thus represent a novel means for batch fabrication of this family of non-linear devices. We achieved record high Q-factors for on-chip lithium niobate WGRs reaching up to 3 × 106. Furthermore, we present a flexible but stable coupling scheme, which gives us the opportunity to optimize the coupling regarding the non-linear optical processes we observe.

Upconversion-enabled array spectrometer for the mid-infrared, featuring kilohertz spectra acquisition rates.

Mid-infrared spectroscopy is an essential analytical method in science and industry. Unlike in the near-infrared range, grating spectrometers for the mid-infrared are rarely employed, mostly due to the limited availability and performance of suitable line array detectors. In this work, continuous-wave nonlinear-optical upconversion is used to enable mid-infrared spectroscopy. A broad spectral window between 3.7 and 4.7 µm is upconverted to 825 - 867 nm for detection on a silicon-camera-based near-infrared grating spectrometer with a high sensitivity down to sub-picowatt of input power. A theoretical model is presented that accurately describes the upconversion process and the total system behavior. Spectroscopic flame emission measurements demonstrate the applicability towards the analysis of highly dynamic processes.

Self-gated mid-infrared short pulse upconversion detection for gas sensing.

Pulsed nonlinear-optical upconversion is used for mid-infrared signal detection. A setup for both mid-infrared generation and upconversion based on a single pump laser enables sensitive light detection and is utilized for gas spectroscopy. With the demonstrated pulsed setup, quantum efficiencies above 80 % for the upconversion of a Gaussian beam signal and 25 % for the upconversion of backscattered radiation are achieved and in agreement with theoretical predictions. Combined with efficient background suppression due to spectral and temporal gating, this results in highly sensitive detection of the infrared signals. As a demonstration of application, the presented system is used for methane sensing in an open path geometry, highlighting the potential for stand-off leak detection with a concentration resolution better than 1.5 ppm·m.

Self-frequency doubling in a laser-active whispering-gallery resonator.

Lasing and self-frequency doubling are achieved in a millimeter-sized laser-active whispering-gallery resonator made of neodymium-doped lithium niobate. A low-cost 808-nm laser diode without external frequency stabilization is sufficient to pump the neodymium ions. Laser oscillation around 1.08 µm drives a frequency-doubling process within the same cavity providing green light. The electrical-optical efficiency of the system reaches up to 2×10-4. To the best of our knowledge, this is the first demonstration of combining lasing and χ(2) frequency conversion in a single high-Q whispering-gallery resonator. This approach is general and can be applied to other materials and other nonlinear optical processes.

Digital holography on moving objects: interference contrast as a function of velocity and aperture width.

Digital holographic measurements on planar moving objects are investigated. We discuss the dependence of the interference contrast on velocity and aperture width for both diffusely and specularly reflecting objects. Using spatial phase shifting, the experimental results for motion in parallel and perpendicular to the optical axis are in good agreement with the theoretical considerations. Measurements with object velocities of up to 100 mm/s are conducted using only less than 1 mW of continuous-wave laser light. These considerations are used to determine the optimal angle between the direction of motion and the illuminating beam, resulting in the lowest decrease in contrast with increasing velocity.

Pseudo-type-II tuning behavior and mode identification in whispering gallery optical parametric oscillators.

Wavelength tuning of conventional mirror-based optical parametric oscillators (OPOs) exhibits parabolically-shaped tuning curves (type-0 and type-I phase matching) or tuning branches that cross each other with a finite slope (type-II phase matching). We predict and experimentally prove that whispering gallery OPOs based on type-0 phase matching show both tuning behaviors, depending on whether the mode numbers of the generated waves coincide or differ. We investigate the wavelength tuning of optical parametric oscillation in a millimeter-sized radially-poled lithium niobate disk pumped at 1 µm wavelength generating signal and idler waves between 1.7 and 2.6 µm wavelength. Our experimental findings excellently coincide with the theoretical predictions. The investigated whispering gallery optical parametric oscillator combines the employment of the highest nonlinear-optical coefficient of the material with a controlled type-II-like wavelength tuning and with the possibility of self-phase locking.

Impact of the photorefractive and pyroelectric-electro-optic effect in lithium niobate on whispering-gallery modes.

Whispering-gallery resonators made of undoped and MgO-doped congruently grown lithium niobate are used to study electro-optic refractive index changes. Hereby, we focus on the volume photovoltaic and the pyroelectric effect, both providing an electric field driving the electro-optic effect. Our findings indicate that the light-induced photorefractive effect, combining the photovoltaic and electro-optic effect, is present only in the non-MgO-doped lithium niobate for exposure with light having wavelengths of up to 850 nm. This leads to strong resonance frequency shifts of the whispering-gallery modes. No photorefractive effect was observed in the MgO-doped material. One has to be aware that surface charges induced by the pyroelectric effect result in a similar phenomenon and are present in both materials.

Broadband infrared spectroscopy using optical parametric oscillation in a radially-poled whispering gallery resonator.

We demonstrate optical parametric oscillation in a millimeter-sized whispering gallery resonator suitable for broadband infrared spectroscopy. This nonlinear-optical process is quasi-phase-matched using a radial domain pattern with 30 µm period length, inscribed by calligraphic poling. The output wavelengths are selected in a controlled way over hundreds of nanometers. We achieve this by increasing the temperature of the resonator in steps such that the azimuthal mode number of the pump wave rises by one. As a proof-of-principle experiment, we measure a characteristic resonance of polystyrene in the spectral range of 2.25 - 2.45 µm.

Continuous-wave whispering-gallery optical parametric oscillator for high-resolution spectroscopy.

We achieve a continuous operation of a whispering gallery optical parametric oscillator by stabilizing the resonator temperature T on the mK level and simultaneously locking the pump frequency to a cavity resonance using the Pound-Drever-Hall technique. The millimeter-sized device converts several mW of a pump wave at 1040 nm wavelength to signal and idler waves around 2000 nm wavelength with more than 50% efficiency. Over 1 h, power and frequency of the signal wave vary by <±1% and by <±25 MHz, respectively. The latter can be tuned over 480 MHz without a mode hop by changing T over 120 mK. In order to prove the suitability for high-resolution spectroscopy, we scan the signal frequency across the resonance of a Fabry-Perot interferometer resolving nicely its 10 MHz linewidth.

Second-harmonic generation of light at 245 nm in a lithium tetraborate whispering gallery resonator.

A millimeter-sized, monolithic whispering gallery resonator made of a lithium tetraborate, Li2B4O7, crystal was employed for doubly resonant second-harmonic generation with a continuous-wave laser source at 490 nm. An intrinsic quality factor of 2×10(8) was observed at the pump wavelength. A conversion efficiency of 2.2% was attained with 5.9 mW of mode-matched pump power. In the lithium tetraborate resonator, it is feasible to achieve phase-matching of second-harmonic generation for pump wavelengths between 486 and 506 nm.