This PDF file contains the front matter associated with SPIE Proceedings Volume 10518, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

Most centrosymmetric materials exhibit a neglectable electro-optic response; the Pockels coefficients are zero, while the DC-Kerr coefficients are typically small. For a standard material such as silica they are on the order of 10⁻²² m²/V^{2 . 1} Thus, refractive index tuning is achieved by thermal or mechanical means. Potassium tantalateniobate (KTa_1−xNb_xO₃ with 0 < x < 1, KTN) is a rare exception. It offers outstandingly high DC-Kerr coefficients in the 10⁻¹⁵ m²/V² range.² In this contribution, the first-ever KTN whispering gallery resonators (WGR) are demonstrated with quality factors Q > 10⁷ and electro-optic eigenfrequency tuning of more than 100 GHz at λ = 1040 nm for moderate field strengths of 250 V/mm. These results potentially pave the way for applications such as electro-optically tunable Kerr frequency combs.

Biology is a longstanding frontier for quantum metrology, where high optical intensities are frequently required to get sufficient signal, but lead to specimen damage [1]. In this regime quantum-noise limited sensitivity allows higher signal to noise or reduced optical intensities. I will give an overview of efforts in the Queensland Quantum Optics Laboratory to apply nanofiber based quantum-noise limited biosensor to biological measurements and our recent progress in nanofiber characterization. We demonstrate a quantum noise limited biosensor capable of detecting single unlabelled biomolecules with radii below 5 nm [2]. The nanoparticles are illuminated through a microscope objective and the scattered light is collected by a nanofiber. The collected light is detected with a heterodyne detector and obtains a quantum-noise limited performance down to 5 Hz and particle tracking with 1 kHz bandwidth. This detection performance opens for observation of biomolecular movements avoiding large biomarkers. We present a method for directly measuring the evanescent field of an optical nanofiber [3]. Unlike SEM-based methods, this method is non-destructive and yields a full profile of the nanofiber within minutes of fabrication. We obtain the radius with a resolution of 0.7 nm in 10 milliseconds. The method allows quality control and calibration of optical nanofibers for precision experiments for example as in the above mentioned biosensor. References [1] K. C. Neuman et al. Biophysics Journal 77, 2856-2863 (1999). [2] N. Mauranyapin, L. Madsen et al. Nature Photonics, in press July (2017) [3] L. Madsen, et al. Nano Letters, 16, 7333−7337 (2016)

As a result of their ability to amplify input light, ultra-high quality factor (Q) whispering gallery mode optical resonators have found numerous applications spanning from basic science through applied technology. Because the Q is critical to the device’s utility, an ever-present challenge revolves around maintaining the Q factor over long timescales in ambient environments. The counter-approach is to increase the nonlinear coefficient of relevance to compensate for Q degradation. In the present work, we strive to accomplish both, in parallel. For example, one of the primary routes for Q degradation in silica cavities is the formation of water monolayers. By changing the surface functional groups, we can inhibit this process, thus stabilizing the Q above 100 million in ambient environments. In parallel, using a machine learning strategy, we have intelligently designed, synthesized, and verified the next generation of small molecules to enable ultra-low threshold and high efficiency Raman lasing. The molecules are verified using the silica microcavity as a testbed cavity. However, the fundamental design strategy is translatable to other whispering gallery mode cavities.

In recent years, whispering gallery mode (WGM) devices have extended their functionality across a number of research fields from photonics device development to sensing applications. Here, we will discuss some such recent applications using ultrahigh Q-factor hollow resonators fabricated from pretapered glass capillary. We will discuss device fabrication and different applications that can be pursued such as bandpass filtering, nanoparticle detection, and trapping. Finally, we will introduce our latest results on visible frequency comb generation.

We investigate the application of integrated micro-combs in RF photonic systems and demonstrate a microwave photonic intensity differentiator based on a Kerr optical comb generated by a compact integrated micro-ring resonator. The on-chip Kerr optical comb is CMOS-compatible and contains a large number of comb lines, which can serve as a high-performance multi-wavelength source for the transversal filter, thus greatly reduce the cost, size, and complexity of the system. The operation principle is theoretically analyzed, and experimental demonstrations of fractional-, first-, second-, and thirdorder differentiation functions based on the principle are presented.

Temporal dissipative solitons in continuous-wave (CW) laser-driven Kerr-nonlinear microresonators have led to the generation of highly-coherent optical frequency combs and ultra-short optical pulses with repetition rates in excess of 10 GHz. Applications of such sources include optical telecommunication, microwave signal generation and optical spectroscopy. Here, a novel nonlinear optical Fabry-Perot microresonator is synchronously driven by picosecond laser pulses (instead of a CW laser) resulting in the formation of temporal dissipative solitons at 10 GHz repetition rate. As opposed to the conventional CW-driven case, single or multiple solitons form deterministically ‘on-top’ of the resonantly enhanced driving pulses, which significantly increases conversion efficiency. The solitons lock to the driving pulse, which enables stable operation and coherent actuation of the solitons’ repetition rate and carrier-envelope offset frequency. The Fabry-Perot microresonator with 10 GHz free-spectral range is based on a short length of standard optical fiber whose end-facets are coated with dielectric Bragg mirrors. Mounted inside a fiber-optical ferrule, the resonator can be interfaced directly with other fiber optical components. While being equivalent to whispering-gallery mode and ring-type resonators regarding nonlinear optical phenomena, the Fabry-Perot microresonator allows for straightforward design of group velocity dispersion, coupling ratio and nonlinearity via choice of fiber and dielectric mirrors. In summary, the presented results links the fields of CW driven microresonators, synchronously driven optical parametric oscillators as well as pulsebased non-resonant supercontinuum generation. Amongst others, they open new perspectives for microresonator-based frequency combs generation and for nonlinear photonics driven by temporally and spectrally structured light.

In this communication, present some of our latest results related to Kerr optical frequency comb generation. We investigate the conditions under which the energy conversion from the lightwave to the microwave spectral ranges is optimized. Our main finding is that the optimal regime features a pump-to-sidemode ratio smaller than 4 dB, corresponding to a conversion efficiency better than 40 %.

Gallium phosphide is an attractive material for non-linear optics because of its broad transparency window (E_b = 2.26 eV) and large Kerr coefficient (n_2 = 6*10^-18 m^2/W). Though well-established in the semiconductor industry as a substrate for visible LEDs, its use for chip-scale photonics remains limited due to fabrication challenges. Here we demonstrate unprecedentedly low loss (Q > 10^5) GaP-on-SiO2 waveguide resonators which have been dispersion-engineered to support Kerr frequency comb generation in the C-band. Parametric threshold is observed with as little as 10 mW injected power, followed by 0.1 THz frequency comb generation over a bandwidth exceeding 30 THz, in addition to strong 2nd and 3rd harmonic generation. Building on this advance, we discuss the prospects for low-noise, sub-mW-threshold microresonator frequency combs with center frequencies tunable from mid-IR to the near-IR. Applications of such devices range from precision molecular spectroscopy to ultrafast pulse generation to massively parallel coherent optical communication.

Switching waves (SWs) are walls that connect two homogeneous stable states of a multistable system. They are typically transient: a single SW will travel until the more stable state invades completely the less stable one. A single SW is only ever stationary at the so-called Maxwell point, a set of parameters where the two states have the same marginal stability. For parameters close to the Maxwell point, two approaching SWs can form a stable stationary structure provided that they have oscillatory tails through which they interlock. Such structures have recently been observed in the normal dispersion regime of microresonators and are known to correspond to dark pulse Kerr frequency combs. However, the small physical size of microresonators means that quantitative study of the transient dynamics that leads to their formation is difficult. In this contribution, we overcome this challenge by performing systematic experiments and simulations in a synchronously-driven macroscopic fiber ring resonator, and report on observations of transient SW dynamics. Our resonator is made of normal dispersion fiber, which allows for the coexistence of two homogeneous steady states and their connection through a SW. We have measured the SW velocities across a wide range of parameters, and close to the identified Maxwell point, observed interlocked SWs persisting for 100’s of resonator photon lifetimes. Our results provide significant experimental insights on the transient dynamics of SWs that have been theoretically predicted to underpin the formation of Kerr frequency combs in normally dispersive microresonators.

The combination of a high-Q optical microcavity and a saturable absorber is expected to enable mode locking between different transverse modes in a microcavity. This work studies saturable absorption in carbon nanotubes (CNTs) on microtoroids. The CNTs are selectively grown on silica microtoroids by chemical vapor deposition after the cobalt catalysis is selectively deposited on the surface. A Raman spectrum measurement showed that the grown CNTs are about 1.0 nm in diameter, and that the device is applicable for use in the 1550 nm band. The saturable absorption by CNTs is investigated with a counter-propagating pump-probe experiment.

Over the last two decades, integrated whispering-gallery-mode resonators have been increasingly used as the basic building blocks for selective filters, high-sensitivity sensors, and as nonlinear converters. In the latter two cases, optimum performance is achieved when the intra-cavity power or the resonance feature contrast are maximum. For devices with transversely singlemode resonator and access waveguides, the above-mentioned conditions are obtained when the system is critically coupled i.e. when the coupler power transfer rate corresponds to the single-pass intra-cavity loss. Designing coupled resonators for which critical-coupling is maintained over a large spectral range is therefore attractive to facilitate sensing or nonlinear frequency conversion.

In this paper, we theoretically show, using a generic model based on the universal description of the device spectral characteristics and a coupled-mode theory treatment of the coupling section, that access-waveguide-coupled resonators can exhibit a wideband critical-coupling bandwidth when their constitutive resonator and access waveguides are different i.e. when they are phase-mismatched. To illustrate this, we have calculated the spectral response of Si₃N₄/SiO₂ racetrack resonators and have found that, when the coupler beat-length becomes achromatic, the device critical-coupling bandwidth is expanded by more one order of magnitude compared to their phase-matched counterpart.

Microresonator-based frequency combs have attracted a great deal of attention in recent years. Traditional generation scheme could be slow due to the operation of tunable lasers and thermal effects. In some spectral ranges, it is also difficult to find a tunable laser with a certain tuning range. In this paper, we propose a fast and simple method for Kerr comb generation without laser detuning and local cooling. In this way, the generation time can be reduced to tens of nanoseconds, three orders of magnitude faster.

Whispering gallery mode microcavities stand out among other microdevices due to their long photon lifetime and strong light confinement characteristics. In particular, such microcavities made of polymers come up with additional advantages, e.g. ease of processing/shaping and their wide variety of properties. However, the fabrication of polymeric nano/microdevices with the desirable performance remains challenging. In this work, we report laser action in Rhodamine B doped whispering gallery mode microcavities fabricated by femtosecond laser induced two-photon polymerization. The host material is an acrylic based polymeric hollow microcylinder, which is on-chip integrable and exhibits good structural quality and smooth sidewall surfaces (Q-factor of 1x10^5 for undoped microcavities around 1550 nm). Rhodamine B is directly incorporated into the polymeric matrix, which does not affect significantly the optical properties of the dye. As a result of their high surface optical quality and the low absorption of the acrylate polymer around 600 nm, the dye microlasers exhibited lasing threshold at a pump energy as low as 12 nJ for free space pulsed excitation at 532 nm. Such performance was attained for microcavities fabricated in a single step of femtosecond laser writing, which, as far as we know, had not been demonstrated previously. In addition, a comparison analysis of the emission spectra collected from microstructures featuring different geometries was carried out, showing that random lasing effects do not play a role in the measured threshold. This work therefore opens promising avenues to fabricate low threshold dye microlasers, which are appealing for soft photonics applications.

The strong confinement obtained in whispering gallery mode (WGM) resonators is highly conducive to nonlinear effects, due to both resonant enhancement of the fields and a large modal overlap. This confinement can, however, also lead to difficulties; it is difficult to efficiently pump the nonlinear interaction whilst also extracting the signal light, especially when very different wavelengths are involved. The common coupling mechanisms to WGM resonators are all evanescent, and the coupling rates inherit the exponential decay of the evanescent field. The decay length of these fields is proportional to the wavelength, so longer wavelength modes will tend to couple more than their shorter counterparts. Experimentally this hinders efficiency and, by extension, observation of nonlinear processes. Through the use of a birefringent prism, and the different phase-matching conditions it imposes on coupling orthogonally-polarized modes, we can independently control the coupling rates of a pump mode and its second harmonic in an x-cut lithium niobate resonator. We thereby critically couple pump and signal in kind, increasing the process’s efficiency fifteen-fold. This selective coupling can easily be applied to birefringent resonators, where the birefringence is large enough, through use of a prism of the same material. This can also be used with isotropic media, if a suitable birefringent material can be found.

Optical square wave sources are particularly important for applications in high speed signal processing and optical communications. In most realizations, optical square waves are generated by electro-optic modulation, dispersion engineering of mode-locked lasers, polarization switching, or by exploiting optical bi-stability and/or optical delayed feedback in semiconductor diode lasers, as well as vertical-cavity surface-emitting lasers (VCSELs). All such configurations are bulky and cause significant timing jitters. Here we demonstrate the direct generation of optical square waves from a polarization-maintaining figure-eight nonlinear amplifying loop mirror (NALM) configuration that uses an embedded high index glass micro-cavity as the nonlinear element. Such a NALM mimics the behavior of a saturable absorber and has been used to reach passive mode-locking of pico- and even nano-second pulses. In our method, the NALM, including a high-Q micro-ring resonator, acts as an ultra-narrowband spectral filter and at the same time provides a large nonlinear phase-shift. Previously we have demonstrated that such a configuration enables sufficient nonlinear phase-shifts for low-power narrow-bandwidth (~100 MHz FWHM) passive mode-locked laser operation. Here we demonstrate the switching of stable optical square wave pulses from conventional mode-locked pulses by adjusting the cavity properties. In addition, the square wave signal characteristics, such as repetition rate and pulse duration, can be also modified in a similar fashion. The source typically produces nanosecond optical square wave pulses with a repetition rate of ~ 120 MHz at 1550nm. In order to verify the reach of our approach, we compare our experimental results with numerical simulations using a delay differential equation model tailored for a figure-eight laser.

Photonic synthesis of radio frequency waveforms revived the quest for unrivalled microwave purity by its seducing ability to convey the benefits of the optics to the microwave world. In this contribution, we will present a high-fidelity transfer of frequency stability between an optical reference and a microwave signal via a low-noise fiber-based frequency comb and cutting-edge photo-detection techniques. We will show the generation of the purest microwave signal with a fractional frequency stability below 6.5×10^-16 at 1 s and a timing noise floor below 41 zs Hz^-1/2 (phase noise below -173 dBc Hz^-1 for a 12 GHz carrier). This outclasses existing sources and promises a new era for state-of-the-art microwave generation. The characterization is achieved through a heterodyne cross-correlation scheme with lowermost detection noise. This unprecedented level of purity can impact domains such as radar systems, telecommunications and time-frequency metrology. The measurements methods developed here can benefit the characterization of a broad range of signals.

Interband and Quantum Cascade Lasers are key sources for MIR molecular sensing. Understanding their noise features and stabilizing their emission is of fundamental importance for applications like precision spectroscopy and metrology. High-Q crystalline Whispering Gallery Mode Resonators have proven to be powerful tools for characterization and stabilization of lasers from the UV to the MIR. Here, we report our recent results on Whispering Gallery Mode Resonators used for frequency characterization, stabilization and linewidth narrowing of Interband and Quantum Cascade Lasers. These results pave the way to new classes of compact MIR sources usable in Space missions, Metrology and Fundamental Physics.

To implement solid-state quantum information processing, precision control of quantum states in single quantum dots including charges, spins and wavefunctions are highly desirable. For a single quantum dot based photocurrent device, single hole spins were initialized with a high fidelity by controlling carrier tunneling rates with a resonant excitation, The tunneling rates are tuned by controlling the wavefunciton spread with an external magnetic field. In addition, many-body exciton states in a coupled system with a single self-assembled quantum dot and a wetting layer are observed by strong anomalous diamagnetic shifts. A tremendous positive diamagnetic coefficient is observed when an electron in the wetting layer combines with a hole in quantum dot, which is nearly one order of magnitude larger than that of the excitonic states confined in quantum dots. When the electrons recombine with holes within quantum dot in the coupled system, a peculiar negative diamagnetic effect is observed. To scale up the quantum dot-based quantum photonic network, photonic crystal cavities with high quality factors around 10000 are fabricated with single quantum dots located in antinode position of the cavity. Selective spin enhancement from single quantum dots in the cavity are observed for each branch in Zeeman splitting because of the Purcell effect, particularly can be precisely controlled with a vector magnetic field. Strong coupling for cavity QED between different excitonic states in a single quantum dot and the cavity will be presented, which has potential applications in realizing quantum nodes for quantum photonic network.

Optical whispering-gallery modes (WGMs) have been extensively investigated in solid micro-cavities of various geometries and materials demonstrating impressive quality (Q) factors. The peculiarity of WGMs supported by such solid structures is that they can be excited via evanescent-wave coupling while resonant light travels along closed paths at the boundary between the surface of the resonator and the surrounding environment. Here, we use micro-cavities made directly from small, vertically-suspended liquid droplets realizing excitation of their whispering-gallery modes by free-space laser beams and demonstrating laser-frequency locking on corresponding optical resonances with various liquids for sensing applications. From direct cavity photon lifetime measurements, we show intrinsic optical Q-factors > 10⁷ for highly-transparent liquid polymers in the visible, that may be limited by scattering due to thermal-induced surface distortions and residual optical absorption. On the other hand, the interaction between light and mechanical motion is also investigated in these droplets. Based on our recent experimental results, liquid microresonators exhibit interesting properties that potentially allow for optical stimulation of mechanical vibrations.

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.

Lasing whispering gallery mode resonators, such as dye doped microspheres and microcapillaries, have shown tremendous potential for refractive index sensing and biosensing applications owning to the narrower resonances achieved upon lasing. This has enabled higher resolution on the determination of the resonance wavelength shift induced by a variation of the surrounding refractive index and as a consequence to reach lower detection limit compared with their fluorescent counterparts. The sensing procedure in both cases relies on tracking the wavelength shift of individual modes, therefore requiring high resolution spectral analysis. This stringent requirement not only prevents any viable commercial prospects due to high equipment cost but more importantly imposes a technological limit, related to the equipment spectral resolution, on the achievable detection limit.

In this paper, we show for the first time that the lasing threshold and eventually the resonances intensity can be used for inferring changes of refractive index around a 15 μm dye doped polystyrene instead of the mode tracking procedure. The sensing mechanism relies on the spoiling of the resonator Q factor upon change of refractive index which eventually increases the lasing threshold. In addition to allow free space excitation and collection, alleviating the need for phase matched prism or fiber taper, this novel approach promises to reach lower detection limit by suppressing the need of high resolution spectral analysis of the whispering gallery mode spectra but instead relying on cost effective and highly sensitive intensity measurements.

High quality whispering gallery mode resonators can greatly enhance the optical field by trapping the light through total internal reflection, which makes these resonators a promising platform for many areas of research, including optical sensing, frequency combs, Raman lasing and cavity QED. Among these resonators, silica microtoroidal resonators are widely used because of their ability to be integrated and to achieve ultrahigh quality factors (above 100 million). However, quality factors of traditional silica toroids gradually decrease over time because there is an intrinsic layer of hydroxyl groups on the silica surface. This layer of hydroxyl groups attracts water molecules in the atmosphere and results in high optical losses. This property of silica degrades the behavior and limits the applications of the integrated silica toroids. In this work, we address this limitation by fabricating integrated microtoroids from silicon oxynitride. The surface of silicon oxynitride has a mixture of hydroxyl groups and fluorine groups. This mixture prevents the formation of a layer of water molecules that causes the optical losses. Our experiments demonstrate that the quality factors of the silicon oxynitride toroids exceed 100 million, and these values are maintained for over two weeks without controlling the storage conditions. As a comparison, quality factors of traditional silica toroids fabricated and stored under same conditions decayed by approximately an order of magnitude over the same duration.

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.

We propose a sensitivity-enhanced intracavity-absorption gas sensor based on the phenomenon of mode competition in the dual-wavelength ring fiber laser. The laser configuration possesses the sensing and reference wavelengths as 1530.372 nm and 1532.168 nm, respectively. When the hollow-core photonic crystal fiber (HC-PCF) is filled with 1000-ppmv acetylene, a sudden change on absorption intensity of more than 30 dB can be achieved by adjusting the optical loss in the laser cavities, resulting from the mode competition between the sensing and reference wavelengths. The minimum detectable acetylene concentration (MDAC) of 29.53 ppmv is obtained in experiment, one order of magnitude higher than former works.

Whispering Gallery Mode (WGM) biosensors have been widely exploited over the past decade, owing to their unprecedented detection limits and label free capability. WGM based sensing mechanisms, such as resonance frequency shift, linewidth broadening, and splitting of the two counter-propagating WGMs, have been extensively researched and applied for bio-chemical sensing. However, the mode-splitting of the originally degenerate WGMs from different equatorial planes on a fluorescent microsphere has not been fully investigated. In this work, we break the symmetry of the surrounding environment outside the microsphere by partially embedding the sphere into a high-refractive-index medium (i.e. glue), to lift the degeneracy of the modes from different WGM planes. The split-modes from multiple planes of the fluorescent microsphere are indiscriminately collected. It is found that the effective quality factor Q of the WGMs increases non-conventionally as the Refractive Index (RI) of the probing liquid increases up to the point where it is equal to that of the glue. This presents a new methodology for quantifying changes in the probing environment based on the Q spoiling of the resonances as determined by the RI difference between the environment and that of the reference glue. Furthermore, we find that this sensing platform opens the door to simple self-referenced sensing techniques based on the analysis of the spectral positions of subsets of the split modes.

In recent years, the development of a reliable, low cost, miniaturized optical gyroscope has been a subject of research and development in the scientific and industrial communities. Since the sensitivity of the optical gyroscopes is reduced with the sensor footprint, multi-ring configurations, such as the coupled resonator optical waveguide (CROW) and the triple ring resonator configuration (TRR), were suggested to increase the sensitivity of the small foot print optical gyroscopes. In this work, a dual coupler coupled cavities configuration is suggested to be used in optical gyroscopes to enhance their performance. The configuration consists of two resonators directly coupled to each other. The passive cavity is a dual coupler ring resonator and is inserted in the main cavity such that the circulating light is fed back to the main cavity by the drop port of the coupled cavity. This leads to a smaller full width half maximum (FWHM) and hence better sensitivity. The passive resonator is studied to find analytical expressions for the output electric field and the effect of rotation on this field. The shot noise-limit sensitivity is calculated and compared to the conventional passive gyroscope in two cases: for a constant source power and for a constant detected power. The effect of propagation loss is also studied. An order of magnitude potential enhancement in the sensitivity is shown for silica-on-silicon technology.

For high-resolution spectroscopy, a stable, narrow linewidth and high power output laser is desirable in order to pump different types of resonant optical parametric oscillators, which is the goal of the present work. Typical single frequency pump lasers are in the range of 10 watt output power whereas, depending on application and OPO type, higher power (>20 W) is desirable. Here we demonstrate a high-power single frequency laser based on off the shelf standard Nd:YAG pump modules. Two closely spaced, diode-side-pumped Nd:YAG rods were used in a mode-filling configuration to form a CW polarized ring resonator with TEM₀₀ beam quality and output power of 105 W. The output power achieved is, to our knowledge, the highest reported for continuous polarized, fundamental-mode ring lasers using standard side-pumped Nd:YAG modules. The resonator allowed for power tuning over a large dynamic range and achieved excellent beam quality, using a half wave plate between both rods for birefringence compensation. Single frequency operation was achieved using a TGG crystal and an etalon, with a preliminary output power of 40 W.

Frequency stabilized light sources with narrow linewidth are mandatory for atom interferometry based experiments. For compact experiment designs used on space platforms, tunable DFB diode lasers are often used. These lasers combine low energy consumption with small sizes, but lack long-term frequency stability. This paper presents an FPGA based laser frequency stabilization system for highly variable target frequencies using frequency modulated Rb-spectroscopy achieving latencies below 100 μs. The system consists of a DFB laser, a Rb-spectroscopy cell, a laser current controller and an FPGA board with an analog-digital conversion board. The digital part of the frequency stabilization system is a SoC mapped on an FPGA. The SoC consists of a processor, enabling user interaction via network connection, and the dedicated frequency stabilization module. This module consists of a demodulation stage, digital filters, a frequency estimator and a controller. To estimate the frequency, small ramps of the laser frequency are generated using a high-speed DAC connected to the laser current controller. The absorption spectroscopy output of this beam is sampled using a photodiode and a high-speed ADC. After signal conditioning with digital filters, the frequency estimator extracts the present mid-frequency of the laser applying pattern matching with a prerecorded reference spectrum. The frequency controller adjusts the mean laser current based on this estimation. The performance as well as the accuracy of the proposed laser stabilization system and its FPGA resource and power consumption are evaluated.

Based on the condition of eigenmode self-consistency, the physical model of the fast light enhanced in ring resonator has been established, and the sensitivity enhancement factor of the fast light enhanced laser gyroscope is defined with the concept of group delay. Combining with the Taylor series expansion, the expression of group delay is derived and the dependence of the group delay and output frequency difference on the Gaussian beam divergence angle in the total reflection prism ring resonator is revealed. Furthermore, the influence of prism size on group delay and output frequency difference is also analyzed.

Diode-pumped alkali metal vapor lasers (DPAL) offer significant promise for high average power. The DPAL system has high gain and will high output coupling and an unstable resonator to achieve excellent beam quality. We analyze the Rb-He system using average equations for the pump, laser and populations, including amplified spontaneous emission. We extend the formulation to include flow and temperature release and study its effects on the laser efficiency and 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. A systematic study of the influence of gain medium aberrations, flow geometry, and resonator design on far field beam quality is reported. The relative advantages of longitudinal and transverse flow geometries to beam quality are evaluated. Finally, coupling of the pump and laser radiation fields is dramatic in the DPAL system. The standard approaches to merging CFD analysis of the gain medium with wave optics resonator simulations will require new techniques.

Wavelengths in the yellow-orange range are of significant interest due to their application potential in the medical and biomedical areas, as well as for applications in laser displays and in remote sensing. These wavelengths can be obtained by frequency-doubling or sum-frequency generation of lasers with near-IR emission like VCSELS, fiber lasers, and OPOs. However, all these alternatives have several limitations that justify the development of alternative methods. As a possible solution for these limitations, a configuration of an intracavity converted Raman laser may be developed to obtain two wavelengths, 1163 nm, and 1147 nm, with high efficiency and good beam quality. This paper presents a configuration of a side-pumped intracavity converted Raman laser to achieve these objectives. A Nd:YLiF₄ crystal was used as fundamental wavelength gain crystal. The side-pumped configuration guarantees practicability and cost reduction while allowing good efficiency and fundamental mode laser beam. The intracavity conversion configuration allows high fundamental wavelength power at the Stokes crystal in order to facilitate the obtention of the Stokes wavelengths and enables optimization of its efficiency. As a result an output power at 1163 nm of 3.8 W in the multimode regime was obtained, corresponding to a pump to Stokes efficiency of 9.6%. The TEM₀₀ diode to Stokes efficiency was 7%. For the emission at 1147 nm, 1.5W of output power with a diode to Stokes efficiency of 3.7% was achieved. The side-pumped Nd:YLF/KGW intracavity Raman laser configuration is reported for the first time, to our knowledge.

A promising concept to achieve high-output power together with a narrow spectral line-width and a diffraction limited beam quality is the use of tapered diode lasers consisting of a distributed Bragg reflector (DBR) mirror as rear side wavelength selective mirror. The DBR mirror provides high reflectivity for the selected emission wavelength and besides contributes to spatial mode filtering. A ridge waveguide (RW) section supports the fundamental lateral mode suppressing higher order modes and the tapered gain region is used for amplification. The taper angle had to be designed with respect to the properties of the RW section. The implementation of separate electrical contacts allows a flexible output power together with fundamental mode operation. In this work, high brightness DBR tapered diode lasers with separate electrical contacts for the wavelength range between 630 nm and 1180 nm will be presented.

For red emitting devices, around 637 nm an output power up to about 1 W is achieved, limited by the properties of the semiconductor material. Devices emitting in the longer wavelength range around 1000 nm reach output powers up to 15 W. For all the above manufactured diode lasers about 80% of the emitted laser light is within the diffraction limited central lobe. Here, e.g. 10 W diffraction limited power was measured.

The devices are successfully implemented into experiments for non-linear frequency conversion, e.g. SHG and the pumping of an OPO, as an excitation source for absorption spectroscopy, as pump source for fs-lasers, and their emission is efficiently coupled into single mode fibers.

We investigated the ability to focus laser beam (λ = 0.65 nm), propagated through the scattering suspension of polystyrene microspheres in distilled water, by means of two bimorph mirrors. Shack-Hartmann sensor was used to measure the local slopes of the Poynting vector, and the CCD camera was used to measure the intensity of the focal spot in the far-field. Correction efficiency of the two bimorph deformable mirrors — with 14 and 31 control channels — were compared. Numerical and experimental investigation of the focusing improvement of the laser beam propagated through the scattering medium was performed.

The intensity distribution of a laser beam has a high impact on laser processing applications. In many applications, the emitted light of a laser beam source is transformed from a Gaussian intensity distribution into other intensity distributions with the intent to improve the process quality. Numerous approaches have been pursued for this purpose in the past. The vast majority of these optical systems is static. As a result the laser material processing process is limited to a specific intensity distribution. Different systems like membrane deformable mirrors can be used for shaping multiple intensity distributions. However, the control of such systems is complex and requires a deep understanding of the underlying operating principle of the specific mirror system. In this paper a new approach for active beam shapers made of catalog components, like spherical and cylindrical lenses, is introduced. Two optical systems for active beam shaping are designed which can change between two different intensity distributions by moving an individual spherical/ cylindrical lens along the beam path. One system forms a laser spot with Gaussian like intensity distribution and a TopHat shaped intensity distribution respectively. The second optical system is capable of forming a laser spot with Gaussian intensity distribution and a homogenous line shaped intensity distribution respectively. Also the mechanical housing for these optical systems is presented.

We experimentally demonstrate a class of non-diffracting beams with state of polarization (SoP) and intensity that can both be controlled along the propagation direction. The beams are composed of a superposition of equal frequency co-propagating Bessel beams (BBs) with different transverse and longitudinal wavenumbers. The BBs are weighted by suitable complex coefficients derived from closed-form analytic expressions. The desired polarization states (i.e., linear, radial, azimuthal and elliptical) are each independently encoded onto a set of BBs with the suitable polarizations. For experimental generation, the resulting field is decomposed into two orthogonal polarizations (horizontal and vertical). Via constructive (and destructive) interference of BBs, specific SoPs can be designed to switch on (and off) during propagation. This effectively alters the resultant SoP and intensity of the beam throughout propagation. We envision our proposed method to be of great interest in many applications, such as optical tweezers, atom guiding, material processing, microscopy, and optical communications.

Adaptive Optics (AO) is a key technology for ground-based astronomical telescopes, allowing to overcome the limits imposed by atmospheric turbulence and obtain high resolution images. This technique however, has not been developed for small size telescopes, because of its high cost and complexity. We realized an AO system based on a Multi-actuator Adaptive Lens and a Shack-Hartmann wavefront sensor (WFS), allowing for a great compactness and simplification of the optical design. The system was integrated on a 11” telescope and controlled by a consumer-grade laptop allowing to perform Closed-Loop AO correction up to 400 Hz.

The transformation of an intensity distribution from Gaussian to a flattop, doughnut, etc. still is a very interesting and important task. And the necessary result could be obtained with the use of adaptive optics that changes the phase of the beam and modifies the shape of the focal spot in the far-field zone. In this paper, we present the flattop and doughnut beam formation result with the use of a bimorph and stacked-actuator deformable mirrors as well as LC phase modulator. The experimental results are also given.

Possibility of creating a laser in the sky has been a hotly debated topic. Rather than a traditional laser with a cavity, what is generally understood is super-radiant emission, where a long and narrow gain volume produces coherent directional stimulated emission. Some filaments can provide the required geometry for an extended gain medium, necessary condition to create lasing in air. Ideal pulse parameters, including wavelength, duration energy and repetition rate will be discussed. Shadowgraphy is presented as a powerful method to analyze the shock wave created by the filament and its guiding-antiguiding properties. The best filaments to create a waveguide are not necessarily the same that will produce the optical excitation of air molecules leading to optical gain. The solution to ‘lasing in air” may reside in creating two color or “nested” filaments. It is shown that such combination can form a stable stationary waveguide. The gain produced by 800 nm femtosecond filaments is analyzed in different conditions of pulse duration, energy and pressure. At certain wavelengths, lasing in the sky will be bet achieved at the low pressures found in the upper atmosphere. Time resolved high resolution spectroscopy of nitrogen plasma emission excited with 800 nm filament reveals the contribution of rotational wave packets in the nitrogen ion emission. The ultrashort 800nm pulse launches a wavepacket in all the occupied states of neutral and ionized molecule. As the states in B and X of the ion evolve with different phase and respond with opposite polarity to the applied field, the emission alternates between “P” and “R” branch across the accessible rotational manifold. The rovibrational transition can be retrieved back to understand the process by which optical gain is achieved.

Multimode fibers provide a means of scaling the peak power of ultrafast fiber lasers by orders of magnitude. While large mode area (LMA) fibers have been widely utilized in fiber amplifiers, these fibers often sacrifice practical benefits of fiber, such as flexibility, and increase system cost and complexity. In addition, the mode-field area of effectively single-mode LMA fibers is smaller than what can be achieved in multimode fibers. Recent work has shown that nonlinear interactions in multimode graded-index fiber can cause a highly multimode field to self-organize into the fundamental mode in a condensation-like process that is robust even with fiber perturbations or disorder. Building on this, we developed a series of Yb:fiber, mode-locked lasers utilizing normal-dispersion, multimode graded-index fiber. In experiments, we observe that the transition from continuous wave lasing to mode-locking is characterized by a beam cleaning process, whereby the highly multimode (speckled) beam of the continuous wave field transforms into a low-order mode beam. Remarkably, experiments and numerical simulations show that the pulses can consist not just the fundamental mode, but can even comprise multiple transverse modes. Our theoretical analysis shows this to be a consequence of a surprising kind of mode-locking – spatiotemporal mode-locking - which relies on strong intermode interactions and spatial filtering. Our initial experiments yield MW-power pulses after external compression, rivaling the best results with flexible LMA fibers. Meanwhile, simulations show that nearly GW peak powers should be possible, making spatiotemporal mode-locking extremely attractive for high-power ultrafast laser development.

Ultrafast femtosecond laser systems have enabled many breakthroughs in the fields of science and technology. However due to the large spectral bandwidth necessary for creating short pulses, it is quite difficult to manipulate their transverse mode structure. Here we present successful femtosecond transverse mode conversion from the fundamental mode TEM₀₀ to modes TEM₀₁, TEM₁₀, and TEM₁₁ with the use of an achromatic phase mask based on diffractive optics. The femtosecond source had ~12.8 nm of bandwidth and pulse duration of <100 fs.

The homogenization of light is widely applied in various industrial sectors. The uniform high power processing of large areas requires a high degree of homogeneity. Sophisticated beam transformation techniques are used to optimize the illumination of standard optical diffusers such as microlens arrays and decrease the contrast of interference. Novel design techniques take advantage of a multimodal approach which is especially adapted to the characteristic properties of the laser light source. We show how anamorphic beam shaping is employed to transform the high power light source in order to meet the required level of homogeneity suited for the respective application.

For nearly Gaussian beams the characterization of changes in the beam parameters with variation of the operating conditions using a Shack-Hartmann sensor is a well-known technique. For broad-area semiconductor lasers the beam analysis based on wavefront detection is still a field of high innovation potential for quality control since the wavefront gives an additional insight to the modal composition of the laser which is strongly influenced by the processes inside the resonator. The diode lasers used for investigation are based on the material system of GaAs (λ=808nm-980nm). They show a multimodal behavior strongly affected by thermal and electrical effects inside the active medium resulting in a complex structure of the intensity and wavefront pattern. The investigations in this paper have been carried out using a so called Gaussian telescope which allows a magnification of the beam cross section as well as the investigation of the spatial evolution of the intensity and wavefront distribution in vicinity to the beam waist. Furthermore the pattern of the detected wavefront is associated with Legendre polynomials to obtain a quantitative expression of the pattern. Additionally simulation software is used to connect the modal composition of the intensity with a set of Hermite Gaussian modes. The aim of our work is to combine these two tracks of information to find a way to forecast whether the laser under test shows a stable or instable intensity distribution over the whole operation current range.

Given the lack of an accuracy standard for measuring a laser’s M-square value, what can a laser user utilize that will instil confidence that the M-squared measurement being made is as accurate as possible? While the ISO 11146-1 provides a method for making a measurement, there is no means to insure the M-square measurement is the most accurate possible. Most mainstream M-square measurement systems have higher then desirable variability in their measurements and consequently puts into question the accuracy of the result. Variabilities of 5 to 10% are not uncommon and therefore undesirable for many users. As there is no perfect laser for which an accuracy standard could perhaps be utilized, the repeatability of a measurement can therefore be the next best metric in providing the upper and lower limits in the measurement’s accuracy. The predominate source of variability should be limited to that of the inherent stability of the laser and not the variability caused by the measurement device itself. The mainstream M-square measurement systems have higher than desirable variability due the constantly changing attenuation and camera exposure times from motion of optics through the beam caustic and the resulting time averaging of the measurement. Having an instrument operating in real-time with fixed attenuation/exposure time and thus eliminating any time averaging of beam caustic data points, would therefore provide the best repeatability possible and limit any variation to that of the inherent stability of the laser source.

We will demonstrate a Runge Kutta based algorithm for solving the Mone-Ampère-Problem. The method is utilizing its parabolic type. We will show the exact numerical solving method down to each step. The derived solution is used in the context of laser beam shaping to reshape any given input distribution to arbitrary output distributions free of any constraints other than diffraction, which includes a true black in said distribution. The presented algorithm will give the required phase which will be used to verify the solution theoretically via diffraction integral and practically via Spatial Light Modulator (SLM).

Mathematical self-similar fractals manifest identical replicated patterns at every scale. Recently, fractals have found their way into a myriad of applications. In optics, it has been shown that manipulation of unstable resonator parameters such as cavity length, curvatures of mirrors, the design of aperture and its transverse position can reveal self-similar fractal patterns in the resonators eigenmodes. Here, we present a novel laser resonator that can generate self-similar fractal output modes. This resonator has a special plane termed self-conjugate, during each round trip inside the cavity, is imaged upon itself with either a magnification or demagnification depending on the direction of beam propagation inside the cavity. By imaging an aperture placed in the self-conjugate plane inside the cavity, we qualitatively show the fractal behaviour occurring at various scales which, are given by powers of the magnification at the self-conjugate plane. We computed the fractal dimension of the patterns we generated and obtained non-integer values, as is expected for fractals.

We propose a strategy to control the propagations of Airy beams in photonic lattices relying on optical Bloch oscillations (BOs). We predict the existence of optical BOs of Airy beams in photonic lattices with a refractive index ramp. It is found that Airy beams undergoing optical BOs show an alternatively switched concave and convex trajectory as well as a periodical revival of beam profiles. Moreover, the reconfigurability of the photonic lattices enables us to control optical BOs of Airy beams by varying the transverse index gradient or the orientation of the lattice, which offers opportunities to steer the Airy beam to a specific output channel.

We discuss the implications of the modeling and the design of diffractive and refractive freeform surfaces in nonparaxial regions of the fields to shape the profile of a laser beam in its far field, its focus or any other region. The fast physical optics approach employed enables the inclusion of freeform surfaces and diffractive beam shaping elements in the modeling. The design of beam shaping elements follows an inverse physical optics approach. We will discuss the pros and cons of refractive and diffractive solutions together with examples.

Liquid Crystal on Silicon (LCoS) Spatial Light Modulators (SLMs) are used as programmable adaptive optical elements^1,2,3 in many applications involving high power lasers. In some cases, LCoS SLMs may be exposed to laser radiation that can cause permanent, irreparable damage to the SLM. The damage arises from a number of parameters including laser wavelength, pulse duration, pulse repetition rate, beam diameter, spatial profile, temporal profile, and even angle of incidence. This paper is an introduction and practical guide to understanding laser damage mechanisms and expected damage threshold levels for LCoS SLMs.

Advances in 3D fabrication have afforded new freedoms in the design of custom optical elements. We explore a design space where the refractive index may vary freely with position in a material by considering applications in coherent beam shaping. First, we will describe experimental results where we have used femtosecond laser writing to fabricate custom GRIN waveguides in a planar geometry. We show a 2D device that converts a Gaussian beam to a flat top in one transverse direction, and is a single-mode waveguide in the other. Second, we describe a numerical method for designing 3D refractive index profiles. As an example, we will use the 3D method to design a device that transforms a circular beam with a Gaussian intensity profile into a square beam with a flat top.

The use of beams carrying orbital angular momentum (OAM) has become ubiquitous and topical in a variety of research fields. More recently, there has been a growing interest in exotic OAM carrying beams with spatially variant polarization, so called Poincare sphere beams, with the well known cylindrical vector beams (CVBs) a particular example; for example, they can be used to obtain tighter focus in applications ranging from optical trapping and tweezing, to laser material processing. Here we outline how to generate such beams in a deterministic manner directly from a solid state laser by employing intra-cavity geometric phase control. Further, we show how to detect and quantify such beams and introduce a new beam quality factor for vector beams. Finally, we consider the effects of amplification on the quality of such beams. We show that the amplification process can be used to maintain, degrade or improve the overall quality of vector beams.

We investigate in detail the resonant properties of a 2D dielectric cavity with an equilateral triangle shape, using a numerical integral equations approach and a semiclassical approach. The homogeneous Müller boundary equations are used to calculate the resonant modes of a dielectric triangle in a wide range of frequencies. It is shown that the modes of dielectric triangles localized on families of periodic orbits can be well described in terms of a semiclassical superscar model. Special attention is given to the families of resonances based on the inscribed triangle periodic orbit.

An Ultra Large Mode Area (ULMA) erbium doped optical fiber having a 52 micron core was investigated as the high power stage of a 1550 nm pulsed fiber laser system. The ULMA fiber was seeded with 1550 nm light from a 4 stage all fiber pulsed laser system with pulse energies of 11.35 and 6.25 μJ at pulse widths of 300 ns and repetition rates of 2 and 10 kHz, respectively. The ULMA fiber stage was counterpumped continuous wave in-band with a 1480 nm Raman laser. Maximum pulse energies (and amplifications) of 360 (15 dB) and 130 μJ (13.2 dB) at 2 and 10 kHz, respectively, were found when pumped with approximately 50 W of 1480 nm pump. More than 90% of the power at both repetition rates was found to reside in the pulse which indicated lower levels of amplified spontaneous emission. A lower conversion efficiency of approximately 53% -- which was significantly less than the theoretical maximum of 95% -- was estimated for the ULMA fiber amplifier. Possible causes are significant contamination on the order of 25% of the 1480 nm pump by lower Stokes orders, low coupling fractions of the 1480 nm pump into the core of the ULMA fiber, as well as pair induced quenching of Er due to clustering. Finally, creation of pump dumps at both the 1480 nm pump and 1550 nm signal ends by roughening the surface of the optical fiber enabled higher pump levels and pulse energies than would otherwise be the case.

In this paper, we present a stable dark soliton with its spectrum spanning about one octave generated in the silicon nitride microring resonator with normal dispersion. The dark soliton can be generated straightforwardly from noise. Then, we investigate the dependence of bandwidth and power level on pump power, the second-order dispersion, the ratio of coupling and intracavity loss coefficient and free spectral range.

We developed a longitudinally excited CO₂ laser with a tail-free short pulse and a high quality beam. The laser system had a longitudinal discharge tube, a long optical cavity and a simple driver circuit. The discharge tube was made of an alumina ceramic pipe with an inner diameter of 16 mm and a length of 45 cm, two metallic electrodes and two windows. Gas medium was a 20:1 mixture of CO₂/N₂ at a pressure of 2.6 kPa. The optical cavity with the length of 180 cm was formed by an output coupler with a reflectivity of 70% and a high-reflection mirror. The optical cavity had an aperture or did not. The driver circuit consisted of −600 V pulse power supply and a primary capacitance of 9.6 μF, a step-up transformer, a storage capacitance of 700 pF, a spark gap and the discharge tube. The CO₂ laser produced a tail-free short pulse with the width of 281 ns. The CO₂ laser beam was a ununifomly distributed circular beam with the M² factor of 22.8 in the laser system without an aperture, a top-hat shape of circular beam with the M² factor of 9.61 in the laser system with a φ11-mm aperture, 5.40 in the laser system with a φ9-mm aperture, or a Gaussian shape of circular beam with the M² factor of 3.01 in the laser system with a φ7-mm aperture.

In this paper we investigate several silica-based suspended core microstructured fibers optical (SC-MOFs) in regards to providing the high efficiency coupling for broadband dispersion measurement. We present free-space optic and butt coupling setup capable of coupling signal from standard single mode fibers (SMF) into SC-MOF with core diameter of less than 4 μm and coupling efficiency over 50%. We then investigate SC-MOF’s effective mode area, nonlinearity coefficient and chromatic dispersion curve, using both modeling of the fibers and measurements. Lastly, we have investigated the effects of the aspheric lens on broadband coupling for the chromatic dispersion measurement.

This work presents, for the first time, research of new cavity topologies for mode-locked Er-doped fibre lasers. The proposed drop- and B-shaped cavities using dual-fibre optical collimator allow a relatively simple ring cavity with continuously adjustable length. Demonstrated is high-quality nonlinear-polarisation-evolution mode locking in an Erdoped fibre laser based on the new cavity configurations with pulses of several hundred femtosecond duration. Possibility smooth variation of pulse repetition rate allоws to stabilise this rate and apply femtosecond fibre lasers with proposed new cavity topologies in metrological and other fields where high stability of pulse repetition rate is required.

Formation and correction of the given laser beam intensity and phase is an important practical and scientific problem. Semipassive bimorph flexible mirror is one of the most widely used devices for this purpose. But the key disadvantage of these kind of mirrors is their low spatial resolution of the corrected phase. Mainly this problem occurs when one deals with the small aperture wavefront correctors. In this work we present two approaches to overcome this problem – one to use a multilayer bimorph (multimorph) mirrors and another to put higher density of control electrodes and use a special technique (ultrasonic welding) to make the wire connection to these electrodes. Here we also present a numerical model to simulate bimorph correctors, based on a variation approach of the finite elements method.