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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 $\Delta\nu =20\,$MHz and 6%, respectively, for a 30 minute period. This concept is suitable for full integration with existing photonic platforms based on lithium niobate., Comment: 18 pages, 5 figures

Drawing around 60 attendees and 20 presenters to a virtual lecture room, April’s CHI-2 Photonics in Microresonators and Beyond conference explored recent progress in the use of microresonators and integrated photonic devices exhibiting second-order nonlinearity for optical frequency conversion.

Optical frequency combs have become a key technology for optical precision measurements [1] and, moreover, bear potentials to boost the bandwidth of telecommunication networks [2] , to be applied for molecular spectroscopy [3] , and to be used in modern quantum information systems [4] . Owing to their feasibility for such a vast application range, frequency comb technology has seen major advances in terms of complexity, power requirements and size, leading to micro- to millimeter-sized frequency comb generators emitting continuous-wavepumped Kerr combs and, more lately, soliton frequency combs based on third-order ( χ (3) ) nonlinearities [2] .

Lithium niobate (LN) is a material with outstanding optical properties including large second-order nonlinear coefficients; strong linear electro-optic effects; and excellent transparency at visible and near infrared wavelengths. It is thus widely used for a range of optical frequency conversion techniques; recent examples comprise frequency combs [1] – [4] and tunable integrated optical parametric oscillators [5] . We recently demonstrated a novel frequency conversion method, Pockels-effect-based adiabatic frequency conversion (AFC), in a LN whispering gallery microresonator [6] . By applying a voltage pulse faster than the lifetime of the photons in the cavity, we change the resonator eigenfrequency and hence the frequency of the trapped light such that no threshold has to be overcome, no phase-matching conditions need to be fulfilled and no mode hopping can occur.

There have been impressive advances in the area of microresonator-based frequency combs (microcombs) during the last decade, making their way into applications in a wide variety of fields [1] . Although microcombs are commonly generated making use of third-order ( χ (3) ) nonlinear processes, several groups have recently started to investigate frequency comb generation based on the second-order ( χ (2) ) nonlinearity as it bears potential benefits like the linear electro-optic effect for either tuning or stabilization, or the possibility to convert light into more challenging spectral regions like the UV or mid-infrared [2] , [3] . For the latter, one approach is to start with optical parametric oscillation (OPO) and to operate it close to degeneracy. Then, a series of second-order nonlinear processes takes place, generating equidistant comb lines around both the pump frequency and the sub-harmonic frequency [2] , [4] . A promising χ (2) material to move into the mid-infrared region is cadmium silicon phosphide (CdSiP 2 ) which is transparent from 1 to 6.5 µm and has a high nonlinearity of 84.5 pm/V. Furthermore, OPO has already been shown for a CdSiP 2 mm-sized whispering gallery resonator pumped at telecom wavelength [5] . Based on these observations, we investigate OPO operation close to the point of degeneracy and this way, the possibility to generate frequency combs both around 1.5 µm and 3 µm. In this work, we have observed OPO with thresholds below 100 µW and conversion efficiencies of around 17 %. We could tune the OPO output wavelengths from 2.3 to 5 µm by coupling the pump light into different modes of the resonator and by controlling its temperature. One of the pump modes investigated enabled operation near degeneracy. Measured signal and idler wavelengths for this pump mode are plotted in figure 1a ) for different temperatures. However, OPO near degeneracy is prone to instabilities, as several parametric oscillations can be excited under similar conditions. Here, by tuning the temperature in the mK range and the pump detuning with respect to the resonance in the MHz range, the minimum obtained difference between signal and idler frequencies was 120 GHz (4 FSRs). Nonetheless, in this state we have already observed sidebands generated around the pump corresponding to SHG as shown in figure 1b ). Why operation closer to the point of degeneracy is hampered, is currently under investigation. A possibility to gain further insight and control on OPO operation might be to make use the linear electro-optic effect for fine tuning of the relative position of the resonances between the pump and the sub-harmonic frequencies.

Owing to the discrete frequency spectrum of whispering gallery resonators (WGRs), the resonance and phase-matching conditions for the interacting waves in the case of second-harmonic generation (SHG) cannot generally be fulfilled simultaneously. To account for this, we develop a model describing SHG in WGRs with non-zero frequency detunings at both the pump and second-harmonic frequencies. Our model predicts strong distortions of the line shape of pump and second-harmonic resonances for similar linewidths at both frequencies; for much larger linewidths at the second-harmonic frequency, this behavior is absent. Furthermore, it describes the SHG efficiency as a function of detuning. Experimentally, one can change the WGR eigenfrequencies, and thus the relative detuning between pump and second-harmonic waves by a number of means, for example electro-optically and thermally. Using a lithium niobate WGR, we show an excellent quantitative agreement for the SHG efficiency between our experimental results and the model. Also, we show the predicted distortions of the pump and second-harmonic resonances to be absent in the lithium niobate WGR but present in a cadmium silicon phosphide WGR, as expected from the linewidths of the resonances involved.

Whispering gallery resonators made out of lithium niobate allow for optical parametric oscillation and frequency comb generation employing the outstanding second-order nonlinear-optical properties of this material. An important knob to tune and control these processes is, e.g., the linear electro-optic effect, the Pockels effect via externally applied electric fields. Due to the shape of the resonators a precise prediction of the electric field strength that affects the optical mode is non-trivial. Here, we study the average strength of the electric field in z-direction in the region of the optical mode for different configurations and geometries of lithium niobate whispering gallery resonators with the help of the finite element method. We find that in some configurations almost 100% is present in the cavity compared to the ideal case of a cylindrical resonator. Even in the case of a few-mode resonator with a very thin rim we find a strength of 90%. Our results give useful design considerations for future arrangements that may benefit from the strong electro-optic effect in bulk whispering gallery resonators made out of lithium niobate.

Microresonator frequency combs have seen remarkable progress in the past decade. In this work, we investigate optical parametric oscillation in a mm-sized cadmium silicon phosphide (CdSiP2) whispering gallery resonator for X(2) frequency comb generation in the mid-infrared region. The comb formation starts with optical parametric oscillation (OPO) near degeneracy followed by a series of internally pumped second-order nonlinear processes. The first step of OPO has been achieved with thresholds below 100 μW, efficiencies of up to 17 % and wavelength tunability from 2.3 to 5 μm. The control of the wavelength tuning is hampered when the OPO is operated close to degeneracy. Nevertheless, we have observed sideband generation around the pump wavelength, indicating that frequency combs can be generated using this scheme. However, improved control of the wavelength tuning near degeneracy is needed to increase the number of sidebands, leading to broadband frequency combs in the mid-infrared.

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.

Whispering gallery resonators (WGRs) are ideally suited for the realization of miniaturized lasers. Due to their high quality factor and small mode volume, they allow for low-threshold and narrow-linewidth emission from (sub)millimeter-sized cavities made of laser-active materials. However, so far the majority of experimental realizations relies on expensive pump light sources like narrow-linewidth or pulsed laser systems, impeding most applications. We demonstrate two whispering-gallery-based single-frequency lasers pumped by compact spectrally multimode low-cost laser diodes. The spheroidally-shaped millemeter-sized WGRs are made of Pr:LiLuF4 and Nd:YVO4. They provide quality factors beyond 107 at the lasing wavelengths (640 nm and 1064 nm, respectively). The pump light is focused onto the rim of the WGR. We observe single frequency emission at milliwatt output powers. The temporal stability of the output power and of the output frequency are determined to be ±1:5 % and ±30 MHz within 30 min, respectively. By changing the temperature of the cavity, we achieve mode-hop-free tuning exceeding 11 GHz.

A rather unknown method to perform optical frequency tuning is the adiabatic frequency conversion. But this method has some appealing advantages compared to conventional frequency conversion schemes, i. e. nonlinear- optical based ones: The internal conversion efficiency can reach unity even on a single-photon level. No threshold and no phase-matching conditions need to be fulfilled. Previous realizations of adiabatic frequency conversion suffer from short photon lifetimes, limited tuning range and challenging experimental setups. Here, we employ the Pockels effect for adiabatic frequency conversion (AFC) in a non-centrosymmetric ultrahigh-Q microresonator made out of lithium niobate. With a 70-μm-thick resonator we observe frequency shifts of more than 5 GHz by applying a moderate voltage of 20V. In contrast to former schemes our setup is considerably simplified and provides a linear electric-to-optical link that enables us to generate also arbitrary waveforms of frequency shifts. Furthermore, our presented conversion scheme is well-suited for on-chip fabrication. Volume fabrication and application of larger electric fields for reasonable voltages become possible. By doing this, it is feasible to achieve tuning on the order of hundreds of GHz.

Whispering gallery resonators made out of crystalline materials exhibiting second-order nonlinearities enable frequency mixing such as optical parametric oscillation with high efficiency at low optical input powers and are ideally suited to realize versatile and compact optical frequency converters. Recent achievements stimulate this field further: Frequency conversion not only at a single wavelength or with few wavelengths is possible, entire frequency combs can be transferred into different spectral domains, e.g., allowing the realization of frequency combs in spectral regions that are suitable for multicomponent analytics like the mid-infrared region. Furthermore, these resonators are also supposed to be the ideal host for cascaded nonlinearities allowing the build-up of frequency combs based on second-order nonlinearities. All this comes with new and better schemes to tune whispering gallery resonators, providing advanced opportunities to modulate the laser wavelengths with nanosecond speed employing the Pockels effect. Juggling with light: We will summarize in the presentation these recent achievements, demonstrating that in the field of whispering gallery resonators still many discoveries are ahead of us.

Optical frequency combs are revolutionising modern time and frequency metrology. In the past years, their range of applications has increased substantially, driven by their miniaturisation through microresonator-based solutions. The combs in such devices are typically generated using the third-order $\chi^{(3)}$-nonlinearity of the resonator material. An alternative approach is making use of second-order $\chi^{(2)}$-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 millimetre-sized microresonator made of lithium niobate, that allows for cascaded second-order nonlinearities. This proof-of-concept device comes already with pump thresholds as small as 2 mW, generating repetition-rate-locked combs around 1064 nm and 532 nm. From the nonlinear dynamics point of view, the observed combs correspond to the Turing roll patterns., Comment: Article with supplementary information, 12 pages, 8 figures. V2 contains few missing umlauts

We investigate the threshold of CH(2) frequency comb generation in lithium niobate whispering gallery microresonators theoretically and experimentally. When generating a frequency comb via second-harmonic excitation, also commonly known as second-harmonic generation, the threshold for the onset of cascaded second-order processes leading to a comb is found to be ∼85 µW. The second-harmonic generation efficiency up to this value is in excellent agreement with a previously known theoretical framework. This framework is extended here, showing that the onset of cascaded CH(2) processes and the maximum of the second-harmonic generation efficiency coincide. The model introduced here allows us to determine the frequency comb generation threshold analytically. Furthermore, we observe that the frequency distance between the comb lines is a function of the pump power. It changes from four free spectral ranges at the oscillation threshold to one free spectral range at 590 µW.

Light states relevant to the χ ( 2 ) comb generation in optical microresonators possess typically dual backgrounds for coupled first-harmonic (FH) and second-harmonic (SH) envelopes. Stability and/or instability of these backgrounds is crucial for realization of stable χ ( 2 ) combs and also for efficient SH generation. We explore the properties of the dual backgrounds and their instability for the cases of FH and SH pumping of the resonator. In contrast to optical parameteric oscillation, the instability is controlled by a fourth-degree characteristic equation for the increment. Coefficients of this equation depend not only on wavenumbers of the perturbations, pump power, and dispersion parameters, but also on FH-SH group velocity difference (temporal walk-off). Our results include characterization of the regions and conditions of stability for the FH and SH pumping cases and different spectral ranges.

Microresonator frequency combs based on the $\chi^{(3)}$ nonlinearity are nowadays well understood and making their way into different applications. Recently, microresonator frequency combs based on the $\chi^{(2)}$ nonlinearity are receiving increasing attention, as they promise certain benefits, but still require further study. Here, we demonstrate the generation of a $\chi^{(2)}$ frequency comb, initiated via optical parametric oscillation (OPO) in a lithium niobate mm-sized microresonator. By pumping at 532 nm with 30 $\mu$W of power, we observe 1-THz-wide comb spectra around 1064 nm with degenerate and non-degenerate states. We also show that comb generation requires signal and idler waves to be degenerate in mode numbers and how the fulfillment of this condition can be identified from the temperature tuning curves. The results demonstrate the potential to directly generate frequency combs via OPO beyond 3 $\mu$m wavelengths in the mid-IR by puming in the near-IR region., Comment: 8 pages, 7 figures

Investigations of the frequency combs in χ(3) microresonators have passed a critical point when the soliton based regimes are well established and realized on different platforms. For χ(2) microresonators, where the first harmonic (FH) and second harmonic (SH) envelopes are coupled via the SH generation and optical parametric oscillation, the comb-soliton studies are just starting. Here we report on a vast accessible dual χ(2) soliton-comb family in high-Q microresonators with the SH and FH combs centered at the pump frequency ω p and its half ω p /2. Vicinity of the point of equal FH and SH group velocities λ c , available via proper radial poling, is found to be the most advantageous for the generation of spectrally broad dual FH-SH combs. Our predictions as applied to lithium niobate resonators include the dependence of comb and dissipative soliton parameters on the pump power, the deviation λ p − λ c , the modal quality factors and frequency detunings, and the necessary parameters of radial poling of the resonator. These predictions form a solid basis for the realization of χ(2) frequency combs.

Experimental and theoretical studies of nonlinear frequency combs in \chi^(3) optical microresonators attracted tremendous research interest during the last decade and resulted in prototypes of solitonbased steadily working devices. Realization of similar combs owing to \chi^(2) optical nonlinearity promises new breakthroughs and is a big scientific challenge. We analyze the main obstacles for realization of the \chi^(2) frequency combs in high-Q microresonators and propose two families of steadystate nonlinear solutions, including soliton and periodic solutions, for such combs. Despite generic periodicity of light fields inside microresonators, the nonlinear solutions can be topologically different and relevant to periodic and antiperiodic boundary conditions. The found particular solutions exist owing to a large difference in the group velocities between the first and second harmonics, typical of \chi^(2) microresonators, and to the presence of the pump. They have no zero-pump counterparts relevant to conservative solitons. Stability issue for the found comb solutions remains open and requires further numerical analysis., Comment: 10 pages, 7 figures

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

Whispering gallery-mode microresonators guide light by total internal reflection and exhibit exceptionally high quality factors and small mode volumes. If such a microresonator is fabricated from a nonlinear-optical material, one can benefit from these properties for efficient optical frequency conversion. One of the well-known second-order nonlinear-optical materials for these microresonators is periodically-poled lithium niobate (PPLN) because of its high second-order susceptibility and wide transmission window. Thus PPLN-based whispering gallery-mode microresonators are efficient platforms for higher-harmonic generation of ultraviolet light using a near-infrared pump source in the telecom range [1], for low-input-power optical parametric oscillation [2] and for frequency comb up- and down-conversion from the near-infrared region to the visible, ultraviolet and mid-infrared regions [3].

Optical frequency combs have become the key technology in optical precision measurements [1], bear a high potential for broadband, high-resolution molecular spectroscopy [2] and are promising to advance future optical clocks [3]. For low-power, compact and mobile applications, high-repetition-rate combs such as continuous-wave-pumped Kerr combs [4] and soliton frequency combs based on a third-order nonlinearity [5] have turned out to be especially useful. Thus far, most of these frequency combs operate in the near-infrared (NIR) spectral regime around 1550 nm for reasons such as chromatic material dispersion. Many applications ranging from astronomical spectroscopy, optical clocks and quantum physics all the way to molecular sensing, however, require the frequency combs to be centered in the ultraviolet (UV), visible (VIS) and mid-infrared (MIR) spectral regimes [3,6–8].

Microresonators are a versatile and efficient system platform for optical frequency conversion. Here, the non-linear response of material polarization is used for frequency synthesis. For example in WGRs made of non-centrosymmetric materials, tunable OPOs have been realized, and in WGRs made of centrosymmetric materials frequency combs were generated. Another way for frequency conversion is adiabatic tuning. Here, the optical length of the circumference of the resonator is changed during its ringdown time. This induces a frequency shift of the circulating light. So far different methods have been demonstrated. Free electrons generated by laser pulses, induce a change of the refractive index[1]. By this method the wavelength can be tuned by some nanometers. The free electrons however lower the quality factor of the resonator. Moreover the repetition rate is limited by the time it takes for the free electrons to recombine. Another method is to change the frequency adiabatically via the AC-Kerr effect[2]. Here, again second pump laser is needed. With this method the quality factor is not affected, but the achieved frequency shift is smaller compared to that for the free-electron method.

Optical frequency combs are a key technology for optical precision measurements. So far, most frequency combs operate in the near-infrared regime (NIR). Many applications, however, require combs in the ultraviolet (UV), visible (VIS) or mid-infrared (MIR) spectral ranges. This can be achieved by making use of nonlinear-optical processes. In this contribution, we demonstrate the efficient conversion of frequency combs with a repetition rate of 21 GHz to UV, VIS and MIR wavelengths in a synchronously driven high-Q microresonator with second-order optical nonlinearity. This opens up a new path for applications including, but not limited to, molecular sensing and quantum optics.

Optical parametric oscillators (OPOs) convert the frequency of laser light to almost arbitrary values. Nowadays, they serve as wavelength-agile light sources for spectroscopic applications as well as photon generators for quantum-optical experiments. Conventional OPOs are based on a nonlinear-optical crystal surrounded by a mirror cavity. Regarding the cavity, one often finds the following rule of thumb: the more waves are resonant, the lower is the oscillation threshold and the more difficult is the wavelength tuning. In whispering-gallery-resonator-based OPOs (WGR OPOs), light is guided by total internal reflection in a millimeter-sized spheroidally-shaped nonlinear-optical crystal. Thus, these devices are intrinsically triply resonant. Indeed, they provide microwatt-level oscillation thresholds, i.e., the lowest values of all OPO configurations. However, following the abovementioned rule, their applicability in fields beyond fundamental science might be questionable, because the most striking feature of an OPO, i.e., the wavelength tunability, is hampered. Nevertheless, several experimental studies revealed that the output wavelengths of WGR OPOs could be tuned in well-defined steps over hundreds of nanometers by temperature variation. Combined with strategies for mode-hop free tuning, it is possible, e.g., to tune the output wavelength in a controlled way until it meets MHz-wide resonances. This is sufficient for high-resolution spectroscopy. WGR OPOs are nowadays operated around various center frequencies, covering the visible-to-mid-infrared spectral range. These light sources - despite of their intrinsic triple resonance - might serve as compact and wavelength-agile devices for various applications.

We propose a vectorial perturbation theory for axially symmetric, generally nonspherical whispering gallery resonators made of isotropic and anisotropic optical materials. It is based on analysis of the leading terms in the coupled equations for independent light-field components, as derived from Maxwell's equations, and true boundary conditions. Strong localization of the whispering gallery modes (WGMs) near the resonator rim, controlled by the azimuth modal number m, is the main prerequisite for our analysis. The theory gives high-precision expressions for the WGM frequencies and modal functions, including the evanescent effects. One of important applications of the theory is analysis of anticrossings of the WGM resonances in anisotropic resonators detected in experiments. Simple relations for the frequency avoidance gaps during the anticrossings are derived and compared with experimental data obtained in lithium-niobate-based WGM resonators. We show also that the vectorial effects substantially restrict the field of applicability of the scalar WGM models.

We review the progress in the development of frequency converters based on three-wave mixing in whispering gallery resonators (WGRs). The theoretical description, given in a unified notation for all basic processes, reveals that the phase-matching condition known from conventional devices is replaced by several selection rules and, furthermore, the fact that conversion efficiencies of more than 25% can be reached in the overcoupled regime only. Experimentally, the conversion efficiencies exceed 50% already at milliwatt input powers. This is achieved, however, so far in bulk resonators only since today the on-chip devices have two orders of magnitude lower quality factors. Regarding the stability of the conversion process, one has to consider impurities left from the crystal growth and material specific effects like photoconductivity, photorefractivity, and pyroelectricity. The impressive experimental progress paves the way that micrometer-sized frequency converters based on WGRs will find the way out of the lab into real-world applications.

Frequency combs in χ ( 2 ) optical microresonators caused by cascaded second-order nonlinear processes currently attract great research interest. In contrast to χ ( 3 ) resonators, two light amplitudes relevant to the first and second harmonics (FH and SH), two dispersion coefficients, and a considerable difference of the FH and SH group velocities (the walk-off) must be taken into account to investigate localized coherent structures (solitons) propagating with a common velocity. Finding such comb solutions taking into account external pumping is a crucial step toward the χ ( 2 ) combs. We report on two new families of driven soliton-comb solutions for χ ( 2 ) microresonators. They are strongly localized, corresponding to spectrally broad combs, and possess well-defined FH and SH frequency detunings, propagation velocities, and, generally, nonzero spatial backgrounds.

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.

Optical frequency combs are key to optical precision measurements. While most frequency combs operate in the near-infrared regime, many applications require combs at mid-infrared, visible or even ultra-violet 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 near-infrared frequency combs to visible, ultra-violet and mid-infrared wavelengths providing new opportunities for microresonator and electro-optic combs in applications including molecular sensing, astronomy, and quantum optics.

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.

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.

We report lasing at 1064 nm in an LED-pumped high-Q whispering-gallery resonator (WGR) made from Nd:YVO4. A single LED is sufficient to achieve quasi-cw operation. Furthermore, self-pumped three-wave-mixing is demonstrated in high-Q WGRs made from laser-active lithium niobate. When pumping the resonator with a free-running laser diode without any external frequency stabilization, self-frequency doubling of the 1.08-μm laser line into the green spectral region is observed. With the application of quasi-phase matching techniques, the nonlinear optical processes can be selected at will, which enables compact and versatile self-pumped frequency synthesizers. As a proof-of-principle experiment, self-pumped optical parametric oscillation is demonstrated in a radially poled WGR made from neodymium-doped lithium niobate.

High-quality whispering gallery resonators (WGRs) made of AgGaSe2 and CdSiP2 bulk crystals are fabricated. With femtosecond laser matching and subsequent fine polishing, the intrinsic Q-factors of these resonators are approaching values of 106 to 107 . By adjusting the coupling condition, maximum coupling efficiencies of 60% and 30% for AgGaSe2 and CdSiP2 resonators could be obtained. In addition, thermal effects on the distortion of the mode spectra of these resonators are also investigated. The results in this work reveal great potentials of WGRs made of non-oxide crystals for mid infrared applications.

The tunability of cw optical parametric oscillators can be extended in the mid-infrared spectral range by using whispering gallery resonators (WGRs) made of non-oxide crystals. We demonstrate the fabrication of WGRs from silver gallium selenide, cadmium silicon phosphide and orientation-patterned gallium phosphide with quality factors above 106 . The resonators made of the first two provide optical parametric oscillation in the mid-infrared pumped by a compact laser diode at 1.57 μm wavelength. The wavelength tunability of the device based on silver gallium selenide provides a tuning range beyond 8 μm wavelength. These achievements are considered as the first steps of cw optical parametric oscillators into this important spectral region.

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.

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 × 10

Continuous-wave optical parametric oscillators (cw OPOs) combine narrow-linewidth emission with a large wavelength tuning range far beyond that of lasers. Conventionally, these devices are based on a nonlinear-optical crystal surrounded by a mirror cavity. In this configuration, cw OPOs are nowadays operated from the visible to the mid-infrared with watt-level output powers. They are ideally suited not only for high-resolution spectroscopy. Enabled by the pioneering work [1] of researchers from the Jet Propulsion Lab, a new OPO configuration was demonstrated some years ago [2] — whispering gallery optical parametric oscillators (see Fig. 1a). Here, the light is guided by total internal reflection in a spheroidally-shaped nonlinear-optical crystal. These monolithic devices do not require any reflective or anti-reflection coating, and they are easily miniaturized down to sub-millimeter diameters. Furthermore, due their low oscillation thresholds, they can be pumped by compact laser diodes. However, they are intrinsically triply-resonant, i.e. all three interacting waves are simultaneously circulating the cavity. It might be assumed that the wavelength tuning of WGR OPOs is much more difficult to control compared with that of conventional mirror-based singly-resonant systems. Hence, it was questionable if they are applicable in fields beyond fundamental science.

Perovskite potassium tantalate-niobate mixed crystals (KTa 1−x Nb x O 3 with 0 ≤ x ≤1, KTN) undergo a phase transition from a paraelectric cubic to a ferroelectric tetragonal structure with decreasing temperature [1]. By adjusting the Ta/Nb content one can tune the phase-transition temperature of KTN and thus also its main properties at a given temperature. For example, the phase transition occurs around room temperature for x = 0.4, accompanied by changes in the dielectric and electro-optic (EO) properties [2]. This extremely promising material is of great interest because of its large EO effect and excellent nonlinear optical performance [2, 3]. However, due to the fact that the crystals grown have compositions being different from those of the molten ingredients, high-quality and homogeneous single-crystalline KTN is difficult to produce, which is limiting the application of this material [4, 5].

Whispering-gallery resonators (WGRs) are most promising for nonlinear-optical frequency-conversion due to their intensity enhancement by small mode volumes and high Q-factors. This has been shown frequently by millimeter-sized diamond-blade cut and polished bulk WGRs. For reproducible batch fabrication, however, the integration of WGRs into lithium-niobate-on-insulator (LNOI) substrates became of great interest. Here we report on integrated WGRs made by batch processes like lithography and reactive-ion etching. Since the Q-factor of integrated WGRs is limited by scattering losses, we focused on developing a polishing process for the waveguide sidewalls that allowed us to enhance the unloaded Q-factors already to more than 106 with room for further improvements. Furthermore we employ a coupling scheme with two waveguide chips, one comprising a linear coupling waveguide and one with the integrated WGR. By adjusting the distance between the coupling waveguide and the WGR, we can reproducibly and stably tune the coupling-efficiency between 0 and 95 %.

A single source emitting single-frequency light with tunable color and without the need of changing toxic chemicals, as it is the case for dye lasers, is desired by many scientists. The Department of Microsystems Engineering IMTEK at the University of Freiburg together with the Fraunhofer Institute for Physical Measurement Techniques IPM now extend the tuning range of continuous-wave optical parametric oscillators into the visible spectrum by integrated, intra-cavity frequency doubling. Combination of these two nonlinear-optical effects enables the operator to deliberately choose any wavelength between 430 and 690 nm.

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.

Broadband infrared spectroscopy employing optical parametric oscillation in bow-tie cavities, including a periodically- poled lithium niobate (PPLN) crystal, is well known. We demonstrate, however, that such spectroscopy is also possible using 2-mm-size monolithic whispering gallery resonators (WGRs). This is achieved in a radially-poled WGR by controlling wavelength tuning despite triple resonance of pump, signal, and idler light. Simulated and measured tuning characteristics of the Type-0 OPOs, pumped at about 1 μm wavelength, coincide. Tuning branches, which are crossed or curved at degeneracy, are present over a spectral range of up to 0.9 µm. As a proof-of-principle experiment, we show that all spectroscopic features of ethanol can be resolved using the idler light between 2.2 and 2.55 μm.