Bidirectional Raman soliton-like combs with unidirectional pump in a spherical microresonator.

We experimentally demonstrate bidirectional Raman soliton-like combs in a whispering gallery mode microresonator with a unidirectional pump for the first time, to the best of our knowledge. We develop a relatively simple theoretical model and find an analytical solution for forward- and backward-propagating Raman sech2-shaped solitons in an anomalous dispersion region under unidirectional pumping in a normal dispersion region. Raman solitons exist, thanks to the balance between losses and Raman gain from a CW wave (which is equal in both directions) as well as between dispersion and Kerr nonlinearity.

Raman Lasing in a Tellurite Microsphere with Thermo-Optical on/off Switching by an Auxiliary Laser Diode.

The generation of coherent light based on inelastic stimulated Raman scattering in photonic microresonators has been attracting great interest in recent years. Tellurite glasses are promising materials for such microdevices since they have large Raman gain and large Raman frequency shift. We experimentally obtained Raman lasing at a wavelength of 1.8 µm with a frequency shift of 27.5 THz from a 1.54 µm narrow-line pump in a 60 µm tellurite glass microsphere with a Q-factor of 2.5 × 107. We demonstrated experimentally a robust, simple, and cheap way of thermo-optically controlled on/off switching of Raman lasing in a tellurite glass microsphere by an auxiliary laser diode. With a permanently operating narrow-line pump laser, on/off switching of the auxiliary 405 nm laser diode led to off/on switching of Raman generation. We also performed theoretical studies supporting the experimental results. The temperature distribution and thermal frequency shifts in eigenmodes in the microspheres heated by the thermalized power of an auxiliary diode and the partially thermalized power of a pump laser were numerically simulated. We analyzed the optical characteristics of Raman generation in microspheres of different diameters. The numerical results were in good agreement with the experimental ones.

Experimental demonstration of Kerr optical frequency comb generation in a tellurite microsphere.

We experimentally demonstrate optical frequency comb generation in a tellurite microsphere, for the first time to the best of our knowledge, for tellurite glass microresonators. The TeO2-WO3-La2O3-Bi2O3 (TWLB) glass microsphere has a maximum Q-factor of 3.7 × 107, which is the highest ever reported for tellurite microresonators. We obtain a frequency comb containing seven spectral lines in the normal dispersion range when pumping the microsphere with a diameter of 61 µm at a wavelength of 1.54 µm.

Comprehensive Numerical Analysis of Temperature Sensitivity of Spherical Microresonators Based on Silica and Soft Glasses.

In recent years, the use of optical methods for temperature measurements has been attracting increased attention. High-performance miniature sensors can be based on glass microspheres with whispering gallery modes (WGMs), as their resonant frequencies shift in response to the ambient parameter variations. In this work, we present a systematic comprehensive numerical analysis of temperature microsensors with a realistic design based on standard silica fibers, as well as commercially available special soft glass fibers (GeO2, tellurite, As2S3, and As2Se3). Possible experimental implementation and some practical recommendations are discussed in detail. We developed a realistic numerical model that takes into account the spectral and temperature dependence of basic glass characteristics in a wide parameter range. To the best of our knowledge, spherical temperature microsensors based on the majority of the considered glass fibers have been investigated for the first time. The highest sensitivity dλ/dT was obtained for the chalcogenide As2Se3 and As2S3 microspheres: for measurements at room temperature conditions at a wavelength of λ = 1.55 µm, it was as high as 57 pm/K and 36 pm/K, correspondingly, which is several times larger than for common silica glass (9.4 pm/K). Importantly, dλ/dT was almost independent of microresonator size, WGM polarization and structure; this is a practically crucial feature showing the robustness of the sensing devices of the proposed design.

Thermo-Optical Control of Raman Solitons in a Functionalized Silica Microsphere.

The investigation of optical microcavity solitons is in demand both for applications and basic science. Despite the tremendous progress in the study of microresonator solitons, there is still no complete understanding of all features of their nonlinear dynamics in various regimes. Controlling soliton properties is also of great interest. We proposed and investigated experimentally and theoretically a simple and easily reproducible way to generate Raman solitons with controllable spectral width in an anomalous dispersion region in a functionalized silica microsphere with whispering gallery modes (WGMs) driven in a normal dispersion regime. To functionalize the microsphere, coating (TiO2 + graphite powder) was applied at the pole. The coating is used for effective thermalization of the radiation of an auxiliary laser diode launched through the fiber stem holding the microsphere to control detuning of the pump frequency from exact resonance due to the thermo-optical shift of the WGM frequencies. We demonstrated that the thermo-optical control by changing the power of an auxiliary diode makes it possible to switch on/off the generation of Raman solitons and control their spectral width, as well as to switch Raman generation to multimode or single-mode. We also performed a detailed theoretical analysis based on the Raman-modified Lugiato-Lefever equation and explained peculiarities of intracavity nonlinear dynamics of Raman solitons. All experimental and numerically simulated results are in excellent agreement.

Thermo-Optical Sensitivity of Whispering Gallery Modes in As2S3 Chalcogenide Glass Microresonators.

Glass microresonators with whispering gallery modes (WGMs) have a lot of diversified applications, including applications for sensing based on thermo-optical effects. Chalcogenide glass microresonators have a noticeably higher temperature sensitivity compared to silica ones, but only a few works have been devoted to the study of their thermo-optical properties. We present experimental and theoretical studies of thermo-optical effects in microspheres made of an As2S3 chalcogenide glass fiber. We investigated the steady-state and transient temperature distributions caused by heating due to the partial thermalization of the pump power and found the corresponding wavelength shifts of the WGMs. The experimental measurements of the thermal response time, thermo-optical shifts of the WGMs, and heat power sensitivity in microspheres with diameters of 80-380 µm are in a good agreement with the theoretically predicted dependences. The calculated temperature sensitivity of 42 pm/K does not depend on diameter for microspheres made of commercially available chalcogenide fiber, which may play an important role in the development of temperature sensors.

Extreme Concentration and Nanoscale Interaction of Light.

Concentrating light strongly calls for appropriate polarization patterns of the focused light beam and for up to a full 4π solid angle geometry. Focusing on the extreme requires efficient coupling to nanostructures of one kind or another via cylindrical vector beams having such patterns, the details of which depend on the geometry and property of the respective nanostructure. Cylindrical vector beams can not only be used to study a nanostructure, but also vice versa. Closely related is the discussion of topics such as the ultimate diffraction limit, a resonant field enhancement near nanoscopic absorbers, as well as speculations about nonresonant field enhancement, which, if it exists, might be relevant to pair production in vacuum. These cases do require further rigorous simulations and more decisive experiments. While there is a wide diversity of scenarios, there are also conceptually very different models offering helpful intuitive pictures despite this diversity.

Cascade Brillouin Lasing in a Tellurite-Glass Microsphere Resonator with Whispering Gallery Modes.

Brillouin microlasers based on microresonators with whispering gallery modes (WGMs) are in high demand for different applications including sensing and biosensing. We fabricated a microsphere resonator with WGMs from a synthesized high-quality tellurite glass with record high Q-factors for tellurite microresonators (Q ≥ 2.5 × 107), a high Brillouin gain coefficient (compared to standard materials, e.g., silica glasses), and a Brillouin frequency shift of 9 ± 0.5 GHz. The high density of excited resonance modes and high loaded Q-factors allowed us to achieve experimentally cascade Stokes-Brillouin lasing up to the 4th order inclusive. The experimental results are supported by the results of the theoretical analysis. We also theoretically obtained the dependences of the output Brillouin powers on the pump power and found the pump-power thresholds for the first five Brillouin orders at different values of pump frequency detuning and Q-factors, and showed a significant effect of these parameters on the processes under consideration.

Thermo-optical control of L-band lasing in Er-doped tellurite glass microsphere with blue laser diode.

Miniature lasers based on rare-earth ion-doped tellurite microsphere resonators with whispering gallery modes (WGMs) are promising devices for basic research and applications. However, the excitation of WGMs using an external pump is not a simple task requiring passive or active control. We propose and demonstrate the implementation of thermo-optical control of the L-band laser generation in an Er-doped in-band pumped tellurite glass microsphere using a cheap low-power blue laser diode and a constant-wavelength telecom laser as a pump. The proposed scheme ensures simplification and cost reduction of microlasers.

Lenslet array-free efficient coherent combining of broadband pulses at the output of a multicore fiber with a square core grid.

An efficient optical scheme for coherent combining of radiation from the output of a multicore fiber (MCF) with a square array of cores in the out-of-phase supermode is proposed. The scheme uses only simple optical elements and is suitable for an arbitrary number of MCF cores. In a proof-of-concept experiment broadband pulses transmitted through a 25-core fiber were combined with 81% efficiency and good beam quality. In numerical modeling a close to unity efficiency is obtained for a large number of cores. The proposed scheme can be used in a reverse direction for efficient beam splitting and launching the out-of-phase supermode into the MCF.

Demonstration of a fiber optical communication system employing a silica microsphere-based OFC source.

The fabrication of microsphere resonators and the generation of optical frequency combs (OFC) have achieved a significant breakthrough in the past decade. Despite these advances, no studies have reported the experimental implementation and demonstration of silica microsphere OFCs for data transmission. In this work, to the best of our knowledge, we experimentally for the first time present a designed silica microsphere whispering-gallery-mode microresonator (WGMR) OFC as a C-band light source where 400 GHz spaced carriers provide data transmission of up to 10 Gbps NRZ-OOK modulated signals over the standard ITU-T G.652 telecom fiber span of 20 km in length. A proof-of-concept experiment is performed with two newly generated carriers (from 7-carrier OFC) having the highest peak power. The experimental realization is also strengthened by the modeling and simulations of the proposed system showing a strong match of the results. The demonstrated setup serves as a platform for the future experimental implementation of silica microsphere WGMR-OFC in more complex WDM transmission system realizations with advanced modulation schemes.

Tunable Raman lasing in an As2S3 chalcogenide glass microsphere.

We demonstrate experimentally Raman lasing in an As2S3 chalcogenide glass microsphere pumped by a C-band narrow line laser. Single-mode Raman lasing tunable from 1.610 µm to 1.663 µm is attained when tuning a pump laser wavelength in the 1.522-1.574 µm range. When the pump power significantly exceeds the threshold, multimode cascade Raman lasing is achieved with the maximum Raman order of four at a wavelength of 2.01 µm. We also report an up-converted wave generation at 1.38 µm which is interpreted as the result of four-wave mixing between the pump wave and the wave generated in the second Raman order. The numerical results based on the simulation of the Lugiato-Lefever equation agree with the experimental results.

Chalcogenide fibers for Kerr squeezing.

We propose and investigate theoretically the Kerr squeezing of light at a wavelength of 2 µm in chalcogenide fibers with large nonlinearity and-this is the advance-with much reduced attenuation. We present suitably realistic but straightforward designs of low-loss step-index single-mode fibers with the nonlinear Kerr coefficient 3 to 4 orders of magnitude higher than for standard telecommunication fibers, and we give estimations of optimal squeezing for continuous wave laser signal in the considered fibers based on As2S3 or As2Se3 glasses.

Single-shot laser pulse reconstruction based on self-phase modulated spectra measurements.

We report a method for ultrashort pulse reconstruction based only on the pulse spectrum and two self-phase modulated (SPM) spectra measured after pulse propagation through thin media with a Kerr nonlinearity. The advantage of this method is that it is a simple and very effective tool for characterization of complex signals. We have developed a new retrieval algorithm that was verified by reconstructing numerically generated fields, such as a complex electric field of double pulses and few-cycle pulses with noises, pedestals and dips down to zero spectral intensity, which is challenging for commonly used techniques. We have also demonstrated a single-shot implementation of the technique for the reconstruction of experimentally obtained pulses. This method can be used for high power laser systems operating in a single-shot mode in the optical, near- and mid-IR spectral ranges. The method is robust, low cost, stable to noise, does not require a priori information, and has no ambiguity related to time direction.