This PDF file contains the front matter associated with SPIE Proceedings Volume 10090, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

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 %.

Micro spherical resonators have attracted significant attention in recent years due to their interesting optical properties and the range of applications for which they can be used. Most of the publications dedicated to micro spherical Laser are devoted to lasing effects in different materials where the spectral properties of the emission depends on (i) the choice of dopant (e.g. Er³⁺, Yb³⁺, Tm³⁺) and (ii) the host matrix (e.g. silica, fluoride, phosphate or telluride glass) in which the dopant is embedded. Yet, the dynamics of theses Lasers are still to be studied. This paper shows experimental results on the amplitude fluctuations of a Whispering Gallery Mode Laser, also known as relative intensity noise (RIN). It gives information about the dynamics inside the cavity, such as photon lifetime, effective pumping rate and noise sources. We use as active medium Er³⁺ doped fluoride ZBLALiP glass and also industrial IOG-1 Yb³⁺- Er³⁺ co-doped phosphate glass. Theses glasses are well adapted to the development of micro spherical Laser operating in the infrared region, in particular with emission wavelengths falling respectively in the C-band and C+L band. We have observed that the RIN can provide insurance about the emission of the Laser. Moreover, we have shown that a single-mode emission comes with the presence of multiple harmonics of the relaxation frequency, which is the signature of a Laser with high noise levels. In this particular case, the second and higher orders of intensity fluctuations cannot be neglected any longer in the small-signal analysis.

Microresonators with high quality factors have recently attracted much attention due to their ability to dramatically enhance light intensity by confining light within a small mode volume for a long period of time. They provide a versatile platform for researching on fundamental physics and practical applications ranging from nonlinear and quantum optics to ultrasensitive sensing. Lithium niobate (LN) is a artificial crystalline material with large electro-optical coefficients and high second-order nonlinearity, therefore, it is a good candidate for active photonic devices. Here, we report on our recent progresses on the mass fabrication of monocrystalline LN microdisk resonators with Q factors higher than 1e6 and LN-silica hybrid microdisk resonators with Q factors of the order of 1e5. The active tunable characteristics of the resonance wavelengths of the fabricated LN microdisk resonators and its based transmission modulations were demonstrated based on the electro-optic and thermo-optic effects of LN crystal.

Mode coupling is a important issue in a resonant system. A concept of dissipative coupling is proposed in the frame of coupled mode theory. This is a indirect coupling based on coupling of both modes to a highly lossy mode. It is shown that the lossy mode provide a equivalent coupling between two coherent modes. As an example, the theory is applied to a micro-ring to break its chiral symmetry. By carefully designing dissipation and scattering coupling we break chial symmetry of light in the modes-coupling system. The resonant frequencies and modes both in theoretical and numerical results show good agreements. The reflection spectrum show also asymmetry feature. Moreover, the dissipation is usually considered to be harmful for applications and should be avoid in the designation of photonic systems. We believe our finding of symmetry breaking by dissipation coupling will provoke people to utilizing dissipations as a tool for manipulating photons.

A comprehensive modeling of the mode-locked laser is presented. The methodology, based for a large part on space-time analogy, applies to any cavity, but may be particularly important for short cavities with particular emphasis where very tight parameter control is essential. Unlike earlier models the beam deformation by nonlinear index in time and space is completely accounted for. It is shown that the mechanism responsible for starting mode-locking is not only Kerr lensing but also Kerr deflection. The problem of directionality in a ring laser is addressed. Will the operation be bidirectional, or unidirectional, and in the later case in which direction? It is suggested that this question can be addressed by considering the analogy between a ring laser and a quantum mechanical two level system. While it is generally taken for granted that multi-GHz combs can only be obtained by miniaturization of the laser, it is shown that a high frequency comb can be generated in a mode-locked laser by inserting a glass etalon.

Narrow-linewidth lasers are key elements in optical metrology and spectroscopy. From their spectral purity, the measurements accuracy and the overall quality of collected data critically depend. Crystalline micro-resonators have undergone an impressive development in the last decade, opening new ways to photonics from the mm to the μm scale. Their wide transparency range and high Q-factor make them suitable for integration in compact apparatuses for precision spectroscopy from the visible to the mid-IR. Here, we present our recent results on frequency stabilization and linewidth narrowing of quantum cascade lasers using crystalline Whispering Gallery Mode Resonators for mid-IR precision spectroscopy.

Optical frequency comb technology has been the cornerstone for scientific breakthroughs such as precision frequency metrology, re-definition of time, extreme light-matter interaction, and attosecond sciences. Recently emerged Kerr-active microresonators are promising alternatives to the current benchmark femtosecond laser platform. These chip-scale frequency combs, or Kerr combs, are unique in their compact footprints and offer the potential for monolithic electronic and feedback integration, thereby expanding the already remarkable applications of optical frequency combs.

In this talk, I will first report the generation and characterization of low-phase-noise Kerr frequency combs. Measurements of the Kerr comb ultrafast dynamics and phase noise will be presented and discussed. Then I will describe novel strategies to fully stabilize Kerr comb line frequencies towards chip-scale optical frequency synthesizers with a relative uncertainty better than 2.7×10^-16. I will show that the unique generation physics of Kerr frequency comb can provide an intrinsic self-referenced access to the Kerr comb line frequencies. The strategy improves the optical frequency stability by more than two orders of magnitude, while preserving the Kerr comb’s key advantage of low SWaP and potential for chip-scale electronic and photonic integration.

Substrate-transferred crystalline coatings are a groundbreaking new concept for the fabrication of ultralow-loss mirrors. The single-crystal lattice structure of these substrate-transferred GaAs/AlGaAs Bragg mirrors exhibits the lowest mechanical losses and hence unmatched Brownian noise performance, which nowadays limits the stability of precision optical interferometers. Another outstanding feature of these coatings is the wide spectral coverage of the GaAs/AlGaAs material platform. Limited by interband absorption at short wavelengths and the reststrahlen band at long wavelengths, crystalline coatings can be employed as low-loss multilayers from approximately 900 nm up to 5 μm and beyond. Excellent optical performance has been demonstrated in the near-infrared with excess optical losses (scatter + absorption) as low as 3 parts per million (ppm), enabling cavity finesse values up to 360,000 at 1.55 μm. Our first attempts at applying crystalline coatings in the mid-infrared has resulted in mirrors with excess optical losses of 159 and 242 ppm at 3.3 and 3.7 μm, respectively. Remarkably, these results are already on par with current state-of-the-art amorphous mirror coatings. Absorption measurements based on photothermal common-path interferometry (PCI) reveal that the optical losses are largely dominated by optical scatter. Via, PCI, we have confirmed absorption losses below 10 ppm at 3.7 μm, showing the enormous potential of GaAs/AlGaAs Bragg mirrors at mid-infrared wavelengths. An optimized fabrication process, which is currently under development, can efficiently suppress optical scatter due to accumulated growth defects on the surface. Ultimately, we foresee excess losses significantly less than 50 ppm in the mid-infrared spectral region.

Optical cavities are able to confine and store specific wavelengths of light, acting as optical amplifiers at those wavelengths. Because the amount of amplification is directly related to the cavity quality factor (Q) (or the cavity finesse), frequency comb research has focused on high-Q and ultra-high Q microcavities fabricated from a range of materials using a variety of methods. In all cases, the comb generation relies on a nonlinear process known as parametric frequency conversion which is based on a third order nonlinear interaction and which results in four wave mixing (FWM). Clearly, this approach requires significant optical power, which was the original motivation for using ultra-high-Q cavities. In fact, the majority of research to date has focused on pursuing increasingly high Q factors. However, another strategy is to improve the nonlinearity of the resonator through intelligently designing materials for this application. In the present work, a suite of nanomaterials (organic and inorganic) have been intelligently designed with the explicit purpose to enhance the nonlinearity of the resonator and reducing the threshold for frequency comb generation in the near-IR. The nanomaterials do not change the structure of the comb and only act to reduce the comb threshold. The silica microcavity is used as a testbed for initial demonstration and verification purposes. However, the fundamental strategy is translatable to other whispering gallery mode cavities.

Dissipative Kerr solitons (DKS) generated in optical microresonators have attracted significant attention over recent years in the areas of optical frequency metrology, spectroscopy and coherent communication. DKS allow for fully coherent, high repetition rate broadband optical frequency combs (soliton combs) and provide access to stable ultrashort pulses of tunable duration. The formation process and the dynamics of such dissipative solitons strongly depend on the interplay of high-order nonlinear and thermal properties of the microresonator, and in many aspects significantly deviate from the behavior of solitons in optical fibers. This talk will focus on the fundamental principles of DKS dynamics, and cover the range of various unique phenomena discovered in such systems.

Dissipative Kerr soliton mode locking in high-Q silica micro cavities is reviewed including resonator dispersion optimization. Phenomena relating to soliton propagation in the micro cavity are studied including dispersive wave generation and soliton trapping. Applications of the soliton comb are described.

Nonlinear wave mixing in optical microresonators offers a route to chip-level optical frequency combs with many promising applications. The properties of the combs generated depend crucially on the interaction between nonlinearity and dispersion. This paper will discuss our research on Kerr comb generation in silicon nitride chip-scale microresonators, with an emphasis on distinct features observed in the normal and anomalous dispersion regimes. The topics covered include comb initiation, comb coherence and mode-locking, power conversion efficiency, and second-harmonic involved comb generation.

We investigated the effect of Raman scattering in multimode whispering gallery mode (WGM) microcavities. First, we discuss the competition between the effects of four-wave mixing (FWM) and stimulated Raman scattering (SRS). Thanks to the different gain bandwidths of FWM and SRS, we can switch between the FWM and SRS dominant states by changing the pump power or by changing the coupling quality factor (Q). Next, we investigated the transverse mode interaction that occurs during SRS comb formation. We found that transverse mode coupling occurred when we pumped in a low-Q mode but a comb with a single-mode family was generated when we pumped in a high-Q mode. This finding will allow us to obtain or suppress a dual comb in a single WGM microcavity. Finally, we demonstrated broad bandwidth visible light generation by third harmonic generation (THG) following the generation of a broadband SRS comb. The generation and good understanding of the SRS comb will offer us various possibilities such as dual comb generation and broad bandwidth visible light generation.

Chip-scale frequency comb sources are key elements for a variety of applications, comprising massively parallel optical communications and high-precision optical metrology. In this talk, we give an overview on our recent progress in the area of integrated optical comb generators and of the associated applications. Our experiments cover modulator-based comb sources, injection locking of gain-switched laser diodes, quantum-dash mode-locked lasers, as well as Kerr comb sources based on cavity solitons. We evaluate and compare the performance of these devices as optical sources for massively parallel wavelength division multiplexing at multi-terabit/s data rates, and we report on comb-based approaches for high-precision distance metrology.

Kerr frequency combs in optical passive microresonators promise new breakthroughs in photonics. Such combs result from multiple hyper-parametric four-wave mixing processes when reaching a threshold of modulational instability. These combs however have chaotic nature. It was revealed in recent experiments, theoretical and numerical analysis that transition form these chaotic states to highly ordered states associated with dissipative Kerr solitons is possible. In this report we discuss theoretical approaches to analyze these soliton states and reveal methods of reliable transition to single soliton states. Latest experimental results with soliton combs are reported.

The propagation of whispering gallery modes along an optical fiber is fully controlled by nanoscale variation of the effective fiber radius. In the present work we demonstrate the possibility of the creation, tuning, translation and annihilation of arbitrary-shaped transient photonic elements, such as miniature slow light delay lines, dispersion compensators and dispersionless optical buffers, at the surface of an initially regular optical fiber. This is achieved by means of local heating of the fiber with low-power focused CO₂ laser radiation, which introduces nanoscale change to the effective radius of the fiber because of thermo-refractive coupling and thermal expansion. The CO₂ laser beam is swept along the fiber, with its position and intensity programmably controlled by an acousto-optical deflector, so that the shape and the settling speed for these structures are constrained only by thermal relaxation processes inside the irradiated fiber. Possible realization of a similar technique on a chip, with laser beam heating substituted by on-chip DC heaters, is analysed. The potential application of this method to the on-the-fly fine tuning of the shape of pre-created Surface Nanoscale Axial Photonics (SNAP) elements, particularly providing gates for switching on and off coupling of optical delay elements to a photonic circuit, is also discussed.

Due to their high quality factors, which result in large circulating optical intensities, microcavities are an attractive platform for creating frequency combs. Over the past decade, in an attempt to achieve both a high Q and a high third order susceptibility, many different material systems have been explored including silica, silicon, silicon nitride, and fluorides. However, these devices are ultimately limited by the material’s fundamental performance. In contrast, entirely new physical phenomena have been realized with nanomaterials. One strategy to leverage these emerging nanomaterials to enhance frequency comb generation is to create hybrid optical cavities in which novel nanomaterials are coated on or attached to the surface of a microresonator.

In the present work, we demonstrate a hybrid platform consisting of a gold nanoparticle coated whispering gallery mode silica microsphere. The hybrid device supports Q factors above 10 million at 1550nm, indicating that the nanoparticles are interacting with the optical field. Additionally, we demonstrate that the nanoparticles enhance the optical field in comparison to a plain silica optical cavity-based frequency comb, further reducing the comb threshold and increasing the comb span. The effect is studied over a range of gold nanoparticle concentrations. The mechanism and enhancement is further elucidated with finite element method modeling.

All-semiconductor photonic crystal surface-emitting lasers (PCSELs) operating in CW mode at room temperature and coherently coupled arrays of these lasers are reviewed. These PCSELs are grown via MOVPE on GaAs substrates and include QW active elements and GaAs/InGaP photonic crystal (PC) layer situated above this active zone.

Atoms of triangular shapes have been shown to increase optical power from the PCSEL but are also shown to result in a competition between lasing modes. Simulation shows that the energy splitting of lasing modes is smaller for triangular atoms, than for circles making high power single-mode devices difficult to achieve.

In this work we experimentally investigate the effect of lateral optical feedback introduced by a facet cleave along one or two perpendicular PCSEL edges. This cleavage plane is misaligned to the PC resulting in a periodic variation of facet phase along the side of the device.

Results confirm that a single cleave selects the lowest threshold 2D lasing mode, resulting in a ~20% reduction in threshold current and favours single-mode emission. The addition of a second cleave at right-angles to the first has no significant effect upon threshold current.

The virgin device is shown to have a symmetric far-field (1 degree) whilst a single cleave produces a 1 degree divergence perpendicular to cleave and 5 degree parallel to cleave. The second orthogonal cleave results in the far field becoming symmetric again but with a divergence angle of 1 degree indicating that single-mode lasing is supported over a wider area.

Whispering gallery mode (WGM) microresonators have been intensively studied in many areas such as sensing, lasing, and fundamental study. WGM microresonators are always coupled by a tapered fiber, and the coupling is controlled by a 3D nanotranslation stage. We always suffer from the instability of coupling condition, which means it is difficult to put microresonators in practical applications. Hence, we present an efficient way to package on-chip ultrahigh-Q microresonators. Stimulated Raman Scattering is achieved in this packaged microresonator, which means we have a portable, narrow linewidth laser and it can be used to expand the working wavelength of a laser. In addition, by coupling two whispering-gallery modes (WGM), which is simultaneously excited in the packaged microtoroid resonator, we can observe an electromagnetically induced transparency (EIT) effect for the first time in a portable WGM structure. This packaged microresonator can be used for real quantum communication applications. Furthermore, highly sensitive sensing can benefit from the high Q-factor and its stability.

Funnelling the light emitted from quantum emitters like atoms, molecules, or defect centers into the guided mode of a single mode optical fiber is highly important for scaling up quantum optics experiments, since it provides the possibility to interconnect experiments at different locations and ensures high mode overlap of photons from different sources. Here, we present a photonic nanocavity on a tapered optical fiber. The cavities are formed by two Bragg mirrors fabricated by an ion beam [1]. Characterization in terms of transmission, reflection, and polarization are performed and compared with numerical simulations [2]. The quality factors of the fabricated devices can reach values over 300 while the mode volume is smaller than the cubic wavelength. Simulations indicate that a Purcell enhancement of 19.1 with 82 % coupling efficiency can be reached using this cavities. A comparison of cavities fabricated using a gallium beam is compared with cavities made using a helium beam giving insights about implantation of gallium in the ion beam milling fabrication of resonators. Using the knowledge from experiment and simulation, new designs for nanofiber Bragg grating cavities are developed and tested.

[1] A W Schell et al. Sci. Rep. 5, 9619 (2015)

[2] H Takashima et al. Opt. Express 24, 15050-15058 (2016)

We report experimental studies on Kerr optical frequency generated with a a magnesium fluoride whispering gallery-mode disk-resonator. We particularly focus on microwave generation obtained after extracting the beat-note of the comb through fast photodetection. Primary comb patterns are excited through detuning the laser frequency with regards to the cold-cavity resonance, and they provide microwaves with different frequencies and high spectral purity.

We demonstrate a nanoscale device platform in GaAs that establishes a link between the radio frequency (RF) and optical domains through acoustic waves, mediated by the piezoelectric and photoelastic effects. First, interdigitated transducers (IDTs) convert 2.4 GHz RF photons into 2.4 GHz propagating surface acoustic waves. These acoustic waves are routed through phononic crystal waveguides and are coupled to a nanobeam optomechanical cavity that supports both a highly localized 2.4 GHz breathing mechanical mode and a high quality factor 1550 nm optical mode. In contrast to non-resonant excitation of photonic structures with IDTs, here the phononic waveguide preferentially excites a localized mechanical mode, which in turn strongly interacts with the optical mode through the photoelastic effect. Finally, the optical mode can be out-coupled or excited via an optical fiber taper waveguide. Using this platform, we demonstrate preparation of the breathing mode in a coherent state at any location in phase space, and optically read out an average coherent intracavity phonon number as small as one-twentieth of a phonon. In the time-domain, we show that RF pulses are mapped to optical pulses, forming a resonant acousto-optic modulator with a sub-Volt half-wave voltage. We also observe a novel acoustic wave interference effect in which RF-driven motion is completely cancelled by optically-driven motion, enabling the demonstration of interferometric opto-acoustic modulation in which acoustic wave propagation is gated by optical pulses. Efforts to improve upon the efficiency of the different transduction processes and integration with quantum dot gain media will be discussed.

Non-reciprocal devices, such as circulators and isolators, are indispensable components in classical and quantum information processing in an integrated photonic circuit. Aside from those applications, the non-reciprocal phase shift is of fundamental interest for exploring exotic topological photonics, such as the realization of chiral edge states and topological protection. However, incorporating low optical-loss magnetic materials into a photonic chip is technically challenging. In this study, the non-reciprocal transmission in an optomechanical resonator is experimentally demonstrated for the first time. The underlying mechanism of the non-reciprocity demonstrated in this study is actually universal and can be generalized to any traveling wave resonators with a mechanical oscillator, such as the integrated disk-type microresonator coupled with a nanobeam. Considering that higher cooperativity and cascading of the optical devices have been reported in a photonic integrated chip, non-reciprocity in such an microresonator has applications for integrated photonic isolators and circulators, which will play important roles in a hybrid quantum Internet.

Microscale resonators that simultaneously exhibit high-Q optical and mechanical resonances are routinely used to study the coupling between light and vibration. We have learned recently that Brillouin scattering (traveling-wave light-sound interactions) within these resonators can enable nonreciprocal optical transmission through a waveguide, which can be reconfigured optically and on demand. In this talk, we describe the basic theory and experimental demonstrations of Brillouin Optomechanics, and describe how it allows the breaking of time-reversal symmetry by means of traveling phonon modes. We experimentally demonstrate ultra-low loss optical isolation using a simple resonator system. Our results demonstrate that chip-scale optical isolation is much more accessible than previously thought.

We present the implementation results of the liquid-crystal on silicon phase-only spatial light modulator, which features high resolution, high reflectivity and improved damage threshold. The data on diffraction efficiency, flatness and temporal noises is presented as well.

Wireless power transmission technology based on high power diode and fiber lasers have a high potential for Earth and space applications. Narrow infrared laser beam can deliver up to 1 kW of electrical power to a photoelectric receiver with dimensions 10-20 cm at distance 1-10 km. To achieve high efficiency it is necessary to fit the beam to the dimension of receiver moving with angular velocity in range 0.5-3 /sec in all range of distance. Thus beam shaping system has to provide fast control of shape, dimensions and position of the beam with high accuracy in condition of atmosphere which distorts the beam. The description and results of the experimental testing of optical system corresponding to the declared characteristics is submitted.

The demand for a uniform intensity distribution in the focal region of the working beam is growing steadily, especially in the field of laser material processing. To generate such a top-hat beam profile, it was shown in the past, that the use of refractive beam shaping solutions provides very good results. In this work, existing beam shaping knowledge is combined with an intelligent modular approach to create a new beam shaping solution, that simplifies both, handling and integration into existing set-ups. Furthermore, the present system enables not just a flattop intensity distribution, but even donut shaped beam profile without adding any further components to the system. Additionally, this beam shaping system is built and successfully tested. Some results of the characterization are presented.

The development in ultra-intense lasers aims at achieving the highest laser intensity on the target.

To ensure highest intensity, one has to accurately control spatial phase to get the smallest focused spot. The spatial phase is controlled using adaptive optics systems with a wavefront sensor to measure spatial phase and a deformable mirror to correct it. This adaptive optics system is commonly placed at the output of the laser chain (just before or just after the compressor) and it now becomes a standard feature on high-power laser chains. The usual strategy of adaptive optics correction is to separate a small fraction of the main beam and to measure its wavefront using a wavefront sensor. However such strategy only ensures that the laser beam is free from aberrations at the location of the wavefront sensor. Aberrations induced by the optical elements located downstream of the wavefront sensor, for instance focusing optics, are not measured and therefore are not corrected by the adaptive optics loop. These aberrations contribute to final focal spot degradation. In order to get the highest intensity on the target, an aberration-free wavefront in the interaction chamber after the focusing optics is required.

We will present a simple, direct and automated method using a standard focal spot camera and phase retrieval algorithms in order to measure and correct wavefront directly on the focal spot itself. This method is simple as it does not require additional hardware and can be used with spectral bandwidth larger than 200 nm.

A novel micro optical element is introduced, allowing coupling of light from several emitters of a laser diode bar into an optical fiber at high brightness. The monolithic fiber coupler is designed with individual segments for each emitter of the laser diode bar, providing two refractive surfaces for each emitter. By means of the monolithic fiber coupler, very cost effective fiber coupled laser diode modules based on bars are feasible. Consequently, approaches based on laser diodes bars can also compete with single emitter solutions for pumping application. Further applications of laser modules with monolithic fiber couplers may also be for direct material processing or as components in projection and illumination systems.

Design methods are illustrated to produce beam shaping elements in which the refractive index is a continuous function of position. An index profile yielding a desired gradual transformation of the field can be computed in two ways. A ray theory approach yields a solution consistent with the eikonal equation, while diffraction effects can be incorporated into the index profile by using a split step representation of the medium and performing a series of phase retrieval calculations. The methods are demonstrated in an example of mode conversion and coherent laser beam combining, where a near-unity conversion efficiency can theoretically be achieved.

Traditional optical communication systems optimize multiplexing in polarization and wavelength both trans- mitted in fiber and free-space to attain high bandwidth data communication. Yet despite these technologies, we are expected to reach a bandwidth ceiling in the near future. Communications using orbital angular momentum (OAM) carrying modes offers infinite dimensional states, providing means to increase link capacity by multiplexing spatially overlapping modes in both the azimuthal and radial degrees of freedom. OAM modes are multiplexed and de-multiplexed by the use of spatial light modulators (SLM). Implementation of complex amplitude modulation is employed on laser beams phase and amplitude to generate Laguerre-Gaussian (LG) modes. Modal decomposition is employed to detect these modes due to their orthogonality as they propagate in space. We demonstrate data transfer by sending images as a proof-of concept in a lab-based scheme. We demonstrate the creation and detection of OAM modes in the mid-IR region as a precursor to a mid-IR free-space communication link.

The utilization of membrane deformable mirrors has raised its importance in laser materials processing since they enable the generation of highly spatial and temporal dynamic intensity distributions for a wide field of applications. To take full advantage of these devices for beam shaping, the huge amount of degrees of freedom has to be considered and optimized already within the early stage of the optical design. Since the functionality of commercial available ray-tracing software has been mainly specialized on geometric dependencies and their optimization within constraints, the complex system characteristics of deformable mirrors cannot be sufficiently taken into account yet. The main reasons are the electromechanical interdependencies of electrostatic membrane deformable mirrors, namely saturation and mechanical clamping, that result in non-linear deformation. This motivates the development of an integrative design methodology. The functionality of the ray-tracing program ZEMAX is extended with a model of an electrostatic membrane mirror. This model is based on experimentally determined influence functions. Furthermore, software routines are derived and integrated that allow for the compilation of optimization criteria for the most relevant analytically describable beam shaping problems. In this way, internal optimization routines can be applied for computing the appropriate membrane deflection of the deformable mirror as well as for the parametrization of static optical components. The experimental verification of simulated intensity distributions demonstrates that the beam shaping properties can be predicted with a high degree of reliability and precision.

We thoroughly investigate the Fabry-Pérot resonator, avoid approximations, and derive its generic Airy distribution, equaling the internal resonance enhancement, and all related Airy distributions, such as the commonly known transmission. We verify that the sum of the mode profiles of all longitudinal modes is the fundamental physical function characterizing the Fabry-Pérot resonator and generating the Airy distributions. We investigate the influence of frequency-dependent mirror reflectivities on the mode profiles and the resulting Airy distributions. The mode profiles then deviate from simple Lorentzian lines and exhibit peaks that are not located at resonant frequencies. Our simple, yet accurate analysis greatly facilitates the characterization of Fabry-Pérot resonators with strongly frequency-dependent mirror reflectivities.

Ultra high quality optical resonators have enabled accumulation of exceptionally high intensities of light from low input powers. This feature opens new horizons in low power observation of physical phenomena such as lasing, sensing and radiation pressure driven oscillations. Radiation pressure instability facilitates transfer of energy from photons to mechanical degree of freedom in optical resonators. In high quality toroidal micro cavities, radiation pressure is demonstrated in the form of "dynamic back action" and results mechanical oscillations with sub-Hz linewidth. Since the toroidal cavities are symmetrical in nature, the exerted radiation pressure can mainly excite radially symmetric modes such as the first cantilever mode and the radially breathing mode. Study of these modes reveals important information about interaction of light and mechanical mode as well as intrinsic properties of the resonator as a mechanical oscillator. However, there are some unexcited mechanical modes that in some cases have even higher mechanical quality factors compared to the usually excited ones. Most of the properties of these mechanical modes remain unknown because the radially symmetric force does not provide a component to excite them. In this research, we have developed a novel method to fabricate asymmetric toroidal resonators (minor and major diameters), which enables us to regeneratively excite unobserved asymmetric modes. One key feature is that the optical quality factor is relatively high despite the asymmetry. As a result, we are able to excite the asymmetric modes with sub-mW threshold powers. Complementary modeling is also performed, confirming the experimental findings.

Resonant optical sensors have enabled the label-free measurement of nanoparticles suspended in liquids, down to the resolution of individual viruses and large molecules, but are only able to quantify optical properties (refractive index, scattering, fluorescence). Additionally, these sensors are fundamentally limited by the random diffusion of particles to the sensing region, and thus only quantify a tiny fraction of the analyte. We have developed a microfluidic optomechanical resonator capable of sensing flowing nanoparticles using the action of phonons that are coupled to light. The phonon mode of the system casts a nearly perfect net for measuring density, viscoelasticity, and compressibility of the particles that flow through, without being limited by random diffusion. Information on the particle mechanical properties is encoded in the light scattered from the thermal fluctuations of the phonon mode, and measurements at a timescale of below 20 milliseconds have been demonstrated previously. In this work, we develop a new experimental method for improving the signal-to-noise ratio (SNR) and sensing speed achievable with this technique, by implementing electro-opto-mechanical transduction. We demonstrate real-time particle transit measurements as fast as 400 microseconds, a factor of 50x improvement in speed, without any post-processing. We discuss how this novel technique can be used for ultra-high throughput analysis of mechanical properties of biological particles in liquids, enabling a new form of flow cytometry.

Distributed-feedback (DFB) laser resonators are widely recognized for their advantage of generating laser emission with extremely narrow linewidth. Our investigation concerns ytterbium-doped amorphous Al₂O₃ channel waveguides with a corrugated homogeneous Bragg grating inscribed into its SiO₂ top cladding, in which a λ/4 phase-shift provides a resonance and allows for laser emission with a linewidth as narrow as a few kHz. Pump absorption imposes a thermal chirp of the grating period, which has implications for the spectral characteristics of the resonator. Thermal effects on the spectral response of a DFB passive resonator were investigated via simulations using Coupled Mode Theory by considering (i) a constant deviation of the grating period or (ii) a chirp with a linear profile. We report an increase of the resonance linewidth up to 15%. This result is due to two factors, namely changes of the grating reflectivity at the resonance frequency up to 2.4% and of the shift of resonance frequency up to 61 pm due to an accumulated phase shift imposed on the grating by the chirp profile. The linewidth decrease due to gain is on the order of 10⁶, which is a much larger value. Nevertheless, according to the Schawlow-Townes equation the linewidth increase of the passive resonator due to a thermal chirp quadratically increases the laser linewidth.

Ultrasensitive optical detection of nanoparticles is highly desirable for applications in early-stage diagnosis of human diseases, environmental monitoring and homeland security, but remains extremely difficult due to ultralow polarizabilities of small-sized, low-index particles. Optical whispering-gallery-mode (WGM) microcavities, with high Q factors up to 108, provide a promising platforms for label-free detection of nano-scaled objects, due to significantly enhanced light-matter interaction. The mechanisms of the conventional WGM sensors, based on the reactive (or dispersive) interaction, measure the mode shift induced by the environmental variations of refractive index, which may fail to detect low-index nanoparticles. In this work, we propose a different dissipative sensing scheme, reacting as linewidth change of WGMs, to detect single nanoparticle using a silica toroidal microcavity fabricated on a silicon substrate. In experiment, detection of single gold nanorods in aqueous environment is realized by monitoring simultaneously the linewidth change and shift of cavity mode. Besides a good consistent with the theoretical predictions, the experimental result shows that the dissipative sensing achieves a better signal-to-noise-ratio compared to the dispersive mechanism. Remarkably, by setting the probe wavelength on and off the surface plasmon resonance of the gold nanoparticles, the great potential of the dissipative sensing method to detect single lossy nanoparticles is demonstrated. This dissipative sensing method holds great potential in detecting lossy nanoparticles, and may become a promising lab-on-a-chip platform for detecting small-sized, low-index particles with ultralow polarizabilities.

We propose a principle to achieve a high-resolution temperature sensor through measuring the central frequency shift in the single-frequency Erbium-doped fiber ring laser induced by the thermal drift via the optical heterodyne spectroscopy method. We achieve a temperature sensor with a sensitivity about 9.7 pm/°C and verify the detection accuracy through an experiment. Due to the narrow linewidth of the output singlefrequency signal and the high accuracy of the optical heterodyne spectroscopy method in measuring the frequency shift in the single-frequency ring laser, the temperature sensor can be employed to resolve a temperature drift up to ~5.5×10^-6 °C theoretically when the single-frequency ring laser has a linewidth of 1 kHz and 10-kHz frequency shift is achieved from the heterodyne spectra.

Whispering Gallery Mode (WGM) silica microresonators are a particularly unique group of microcavities in the sense that they can confine light inside the device for an extended period of time while maintaining a high quality (Q) factor due to the total internal reflection. As a result, WGM resonators have high circulating optical power, which can cause nonlinear optical processes such as stimulated Raman scattering (SRS). It has been demonstrated that SRS has been observed in various WGM silica microresonators with the sub-mW Raman lasing threshold. However, in case of the Raman lasing efficiency, it is limited by the intrinsic property of silica itself, which is the Raman gain coefficient. Therefore, in the present work, we introduce a hybrid silica toroidal microcavity in order to enhance the Raman lasing efficiency. First, we synthesize a suite of silica sol-gels doped with a range of Zirconium (Zr) concentrations and integrate the material with silica toroidal microresonator. The intrinsic Raman gain of the Zr-doped silica is measured using Raman spectroscopy, and the values show a clear dependence on Zr dopant concentrations. The lasing performance is characterized using a 765 nm pump source, and the Raman emissions for the coated devices are detected at 790 nm and longer. The lasing emission and characteristic threshold curves are quantified using both an optical spectrum analyzer and an optical spectrograph. The lasing slope efficiency of exhibits a marked increase from 3.37% to 47.43% as the Zr concentration increases due to the Raman gain improvement. These values are particularly notable as they are the unidirectional, not bidirectional, lasing efficiencies.

We report on a study of performance of both active and passive optical gyroscopes based on high finesse crystalline whispering gallery mode (WGM) resonators. We show that the sensitivity of the devices is ultimately limited due to the nonlinearity of the resonator host material. A gyroscope characterized with 0.02 deg/hr^^1/2 angle random walk and 2 deg/hr bias drift is demonstrated.

Thermal properties of a photonic resonator, determined by both intrinsic properties of materials and the geometry and structure of the resonator, play important roles in various applications including radiation detection, biosensing, and microlaser. In this work, we propose and demonstrate a method to measure the thermal relaxation time and thermal conductance of an optical microresonator. The method utilizes the optothermal effect of two nearby optical modes in the transmission spectrum of the same resonator to extract the thermal properties of the resonator. We show that the thermal relaxation time, as well as thermal conductance, can be tailored by changing the geometric parameters of the resonator. Furthermore, we provide an analytical model that can be used to estimate the thermal relaxation time of a microtoroid resonator given its geometric parameters. The experimental results agree well with the analytical predictions. Our method can be exploited to characterize and optimize the thermal properties of other types of optical microresonators.

In recent years, the concept of parity-time (PT) symmetry has received considerable attention in the field of optics and photonics. In PT-symmetric arrangements, the interaction between gain/loss-contrast and coupling leads to the formation of exceptional points in parameter space. At these junctures, not only the eigenvalues but also the eigenvectors tend to merge, resulting in a sudden reduction of the dimensionality of the eigen-space. Consequently, in the vicinity of such points, the eigenfrequencies are strongly affected by external perturbationsas the system regains its original dimensionality. This unique behavior can be utilized to fundamentally enhance the sensitivity of micro-resonators. Here, we experimentally investigate this effect in integrated semiconductor PT-symmetric microring lasers that are biased at exceptional points. Using this arrangement, we demonstrate >10- fold enhancement in sensitivity. Our results also show that unlike standard microcavities, the parity-time symmetric system responds to the square-root of the perturbation. Our work provides a new avenue for enhancing the sensitivity of optical integrated sensors.

In this article, we apply the coupled-mode theory to vertically-coupled micro-disk resonators presenting an asymmetric distribution of refractive index and a multilayer separation region between the two waveguide cores, resulting in an effective propagation constant phase-mismatch in the coupling region. We introduce a criterion which, given the coupler overall permittivity distribution, clarifies how to best choose the individual decomposition index profiles among the various possible solutions. Following our recent experimental demonstration we subsequently exploit the derived decomposition to evaluate the theoretical transmission characteristics of an AlGaAs/AlOx-based structure as function of wavelength and as function of the position of the resonator relative to the access waveguide.We show that the resonant dips of the intensity transmission, spaced by the cavity FSR, are modulated by an envelop which governs the coupling regime of the resonator-waveguide system.

New spherical resonators with internal defects are introduced to show anomalous whispering gallery modes (WGMs). The defect induces a symmetry breaking spherical cavity and splits the WGMs. A couple of defects, a hollow sphere (bubble), and a hollow ring, have been studied. The hollow sphere was fabricated and the splitting of WGM was observed. In this paper, this "non-degenerated WGMs (non-DWGMs) resonance" in a microsphere with hollow defect structure is reviewed based on our research. The resonance of WGMs in a sphere is identified by three integer parameters: the angular mode number, l, azimuthal mode number m, and radial mode number, n. The placement of the defect such as a hollow ring or single bubble is shown to break symmetry and resolve the degeneracy concerning m. This induces a variety of resonant wavelengths of the spherical cavity. A couple of simulations using the eigenmode and transient analyses propose how the placed defects affect the WGM resonance in the spherical cavity. For the sphere with a single bubble defect, the experimentally observed resonances in Nd-doped tellurite glass microsphere with a single bubble are clarified to be due to the splitting of resonance modes, i.e., the existence of "non-DWGMs" in the sphere. The defect bubble plays a role of opening the optically wide gate to introduce excitation light for Nd³⁺ pumping using non-DWGMs in the sphere efficiently.

We demonstrate an automatic channel-switched intracavity- absorption acetylene sensor via Sagnac loop filter based on the mode-competition in a ring fiber laser. When the photonic crystal fiber gas cell is filled with 1% acetylene, the corresponding absorption intensity can be ~14.0 dB and ~7.2 dB at 1532.83 nm and 1534.01 nm, respectively. Compared with the single transmission pass method, the sensitivity can be improved up to more than 10 times. It spends 50 seconds in scanning the absorption spectra through applying gradient voltage to the tunable F-P filter.

We report the realization of precise spatial polarization control of light via a priori optimization of the polarization ratio and retardation modulation in a vectorial optical field generator. For the polarization ratio calibration, we generate 45° linearly polarized light, measure the intensities of the vertical and horizontal components of the output beam and calculate the ratio of them to obtain the modification coefficient. After several iterations, the corresponding coefficient converges to an optimized value based on the criterion that the measured intensities are equal to each other. As for the retardation calibration, circularly polarized light is generated and letting the modulated beam propagate through a circular polarization analyzer. The modification value is adjusted by dichotomy until the detected intensity of the output beam from the circular polarization analyzer approaches extinction. Several typical kinds of vectorial optical beams are generated with the obtained modification parameters and the measured Stokes parameters demonstrate that this method is practicable and beneficial for the performance improvement of the vectorial optical field generator.

In this research, the temperature characteristics of a 10 cm long iodine cell used in diode-pumped Nd:YAG laser system has been investigated. Here, the laser system were frequency stabilized by locking their frequency-doubled output at 563 THz. The absolute frequency measurement of the a₁₀ hyperfine components of molecular iodine R(56) 32-0 line with effect to various temperatures is reported. With the iodine cell temperature control, the output frequency is tuned in the range of standard frequency at 563.2602235 THz with about 10^-9 of stability.

We investigate the ability to focus the laser beam (λ=0.65μm) propagated through the scattering suspension of polystyrene microspheres in distilled water by means of bimorph deformable mirror. Shack-Hartmann sensor was used to measure the local slopes of the Poynting vector, while the CCD camera was used to measure the intensity of the focal spot in the farfield. Bimorph deformable mirror with 14 electrodes was applied in order to increase the intensity of the focal spot in the far-field. We investigated the efficiency of the laser beam focusing improvement by means of three techniques: LSQ fiterror minimization by Shack-Hartmann sensor, Hill-climbing optimization by Shack-Hartmann sensor and Hill-climbing optimization by far-field CCD camera.

The microchip lasers, being sources of coherent light, suffer from one serious drawback: low spatial quality of the beam, strongly reducing the brightness of emitted radiation. Attempts to improve the beam quality, such as pump-beam guiding, external feedback, either strongly reduce the emission power, or drastically increase the size and complexity of the lasers. Here we propose that specially designed photonic crystal in the cavity of a microchip laser, can significantly improve the beam quality. We experimentally show that a microchip laser, due to spatial filtering functionality of intracavity photonic crystal, improves the beam quality factor M² reducing it by factor of 2, and thus increase the brightness of radiation by a factor of 4. This comprises a new kind of laser, the "photonic crystal microchip laser", a very compact and efficient light source emitting high spatial high brightness radiation.

The high gain Diode Pumped Alkali Laser (DPAL) system will require an unstable resonator with high Fresnel number and high output coupling to achieve excellent beam quality. Coupling of the diode pump and laser radiation fields is dramatic in the DPAL system. Merging flow field analysis of the gain medium with wave optics resonator simulations requires new techniques. We develop a wave-optics simulation of confocal, positive-branch unstable resonators for the DPAL gain media to assess the limitations on far field beam quality. The design and analysis of the DPAL resonator and the influence of spatial variations in gain medium on far field beam quality are developed. The relative advantages of longitudinal and transverse flow geometries to beam quality are evaluated. A systematic study of the influence of gain medium aberrations, flow geometry, magnification, and resonator design on far field beam quality is reported.

A multi-stage linearly polarized (PM) (15 dB) pulsed fiber laser system at 1550 nm capable of operating at repetition rates between 3 and 20 kHz was investigated. A narrow linewidth seed source was linewidth broadened to approximately 20 GHz and pulses were created and shaped via an electro-optic modulator (EOM) in conjunction with a home built arbitrary waveform generator. As expected, a high repetition rate pulse train with a near diffraction limited beam quality (M2~1.12) was achieved. However, the ability to store energy was limited by the number of active ions within the erbium/ytterbium doped gain fiber within the various stages. As a result, the maximum energy per pulse achievable from the system was approximately 0.3 and 0.38 mJ for 300 ns and 1 μs pulses, respectively, at 3 kHz. Because the system was operated at high inversion, the erbium/ytterbium doped optical fiber preferred to lase at 1535 nm versus 1550 nm resulting in amplified spontaneous emission (ASE) both intra- and inter-pulse. For the lower power stages, the ASE was controllable via a EOM whose function was to block the energy between pulses as well as ASE filters whose purpose was to block spectral components outside of the 1550 nm passband. For the higher power stages, the pump diodes were pulsed to enable strategic placement of an inversion resulting in higher intrapulse energies as well as an improved spectrum of the signal. When optimized, this system will be used to seed higher power solid state amplifier stages.

In recent years wavefront measurements using a Shack-Hartmann Sensor became a fast and easy way to analyze the change of laser beam characteristics over a wide range of parameters. This method is well known for nearly Gaussian laser beams while the wavefront analysis of broadarea semiconductor lasers is still an open field of current research. Detailed analysis of the wavefront gives an additional path to get insight into the modal composition of semiconductor lasers, which has a dominant impact on the output parameters of the devices. For our investigations we utilize lasers based on the material system of GaAs emitting light in the near infrared. These types of laser emit typically more than one optical mode. The composition of these modal structures is highly affected by thermal and electric effects inside the active medium. By using a simulation software the intensity distribution at various diode currents can be associated with an assembly of Hermite Gaussian modes and thus gives insight into the basic modal structure. Additionally the change of modal composition can be recorded within the wavefront deflection. This delivers an extra track of information to the light emission. The aim of our research is to associate the wavefront with the modal structure gained by measuring the intensity distribution under changing working conditions. Furthermore we use a lens system to receive a magnified image of the beam and investigate the spatial evolution of the intensity and wavefront distribution of the laser emission along the propagating axis.

We present an experimental technique to generate partially coherent vortex beams with an arbitrary azimuthal index using only a spatial light modulator. Our approach is based on digitally simulating the intrinsic randomness of broadband light passing through a spiral phase plate. We illustrate the versatility of the technique by generating partially coherent beams with different coherence lengths and orbital angular momentum content, without any moving optical device. Consequently, we study its cross-correlation function in a wavefront folding interferometer. The comparison with theoretical predictions yields excellent agreement.

M-square measurements since the inception of the ISO 11146-1 measurement standard of 1996 has been one that has been difficult even for a seasoned veteran of such measurements. Variations of more than 10% are not uncommon for the same measurement tool on the same laser being measured. Much of the variation comes from alignment, the motion involved (time averaged based), complex attenuation techniques which often include variable neutral density filters and the type of sensors employed. Moreover, setup times for the instrument can take hours and the measurements themselves many minutes. Measurement of a laser or a laser systems' M-square should be as simple as measuring the power of the laser. In that one aligns the laser to the device; put the device in self calibration mode; make a measurement.

In 2012 the authors developed a passive optical design that provided real-time M-square measurement of a laser or laser system but nevertheless still required calibration of the key optics within the system: a Fabry-Perot etalon pair and their spacing in order to obtain an accurate M-square result. Using existing data from the sensor along with a simple ray tracing technique, the etalon spacing can be determined with high accuracy through the deconvolution of the data from the sensor; thereby eliminating a separate time consuming calibration. The key calibration information can now be obtained in a fraction of a second without any effort on the part of the user.

We report on experiments with conical refraction of laser beams possessing a high beam propagation parameter M². With beam propagation parameter values M²=3 and M²=5, unusual Lloyd’s distributions with correspondingly three and five dark rings were observed. In order to explain this phenomenon, we extend the dual-cone model of the conical refraction that describes it as a product of interference of two cones that converge and diverge behind the exit facet of the crystal. In the extended model, these converging/diverging cones are represented as the cone-shaped quasi-Gaussian beams possessing the M² parameter of an original beam. In this formalism, a beam-waist of these cone-shaped beams is proportional to the M² value and defines the area of their interference which is a width of the Lloyd’s ring. Therefore, the number of dark rings in the Lloyd distribution is defined by the M² value and can be much greater than unity. The results of the numerical simulations within the extended dual-cone model are in excellent agreement with the experiment.