Proceedings Volume 9343

Laser Resonators, Microresonators, and Beam Control XVII

Proceedings Volume 9343

Laser Resonators, Microresonators, and Beam Control XVII

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Volume Details

Date Published: 16 April 2015
Contents: 15 Sessions, 47 Papers, 0 Presentations
Conference: SPIE LASE 2015
Volume Number: 9343

Table of Contents


Table of Contents

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  • Front Matter: Volume 9343
  • Microresonator Optomechanics I
  • Microresonator Optomechanics II
  • Microresonator Combs, THz, and RF Photonics II: Joint Session with Conferences 9343 and 9347
  • Microresonator Sensors
  • Beam Shaping I
  • Beam Shaping II: Joint Session with Conferences 9343 and 9356
  • Beam Shaping III
  • Microresonator Fundamentals, Novel Topologies, and Devices I
  • Microresonator Fundamentals, Novel Topologies, and Devices II
  • Coupled Resonators and Phased Arrays
  • Diagnostics
  • Laser Mode Control I
  • Laser Mode Control II
  • Poster Session
Front Matter: Volume 9343
Front Matter: Volume 9343
This PDF file contains the front matter associated with SPIE Proceedings Volume 9343, including the Title Page, Copyright information, Table of Contents, Invited Panel Discussion, and Conference Committee listing.
Microresonator Optomechanics I
Asymmetric response function of the transduction spectrum for a microsphere pendulum
Jonathan M. Ward, Ramgopal Madugani, Yong Yang, et al.
The optical response of a silica microsphere pendulum evanescently coupled to a tapered optical fiber is studied. The pendulum oscillation modulates the microsphere’s whispering gallery mode (WGM) resonance frequencies (dispersive shift) and the external coupling rate (dissipative effect). These effects combine to give an observable modulation in the transmitted optical power so that the tapered fiber also acts as the mechanical motion transducer. This unique mechanism leads to an asymmetric response function of the transduction spectrum, i.e. the amplitude of the transmitted noise depends on laser detuning. This phenomenon can be explained using coupled mode theory with a Fourier transformation. The transduction of the mechanical motion and its relation to the external coupling gap was experimentally investigated and showed good agreement with the theory.
Microresonator Optomechanics II
Dynamical characteristics of AlGaInAs/InP microdisk lasers subject to optical injection
Yong-Zhen Huang, Ling-Xiu Zou, Bo-Wen Liu, et al.
The dynamic characteristics are investigated for an 8-μm-radius directional-emission microdisk laser subject to optical injection. Single mode operation with lasing mode wavelength of 1540.1 nm and the side mode suppression ratio of 35 dB is realized for the microdisk laser at the biasing current of 10 mA and the temperature of 288 K. Under the optical injection from a tunable laser, optical injection locking and the enhancement of the 3dB bandwidth of small signal modulation response from 3.4 to 13.7 GHz are observed for the microdisk laser biased at 7 mA with an injected optical power of 0.5 mW. By varying the wavelength detuning between the injection light and the lasing mode, we demonstrate the nonlinear states of four-wave mixing, period-one and period-two oscillations from the lasing spectra. Multiple peaks are observed for the period-one and period-two oscillation states.
Connecting microwave and optical frequencies with a vibrational degree of freedom
R. W. Andrews, R. W. Peterson, T. P. Purdy, et al.
We describe the construction of a device that converts electromagnetic signals from microwave (7 GHz) to optical (282 THz) frequencies, and vice-versa. The frequency converter relies on a flexible silicon nitride membrane that is coupled via radiation pressure to both a microwave circuit and a Fabry-Perot cavity. The frequency converter achieves conversion efficiencies of ∼10%, and is potentially capable of frequency conversion of quantum signals.
Microresonator Combs, THz, and RF Photonics II: Joint Session with Conferences 9343 and 9347
Terabit/s communications using chip-scale frequency comb sources
Christian Koos, Tobias J. Kippenberg, Liam P. Barry, et al.
High-speed optical interconnects rely on advanced wavelength-division multiplexing (WDM) schemes. However, while photonic-electronic interfaces can be efficiently realized on silicon-on-insulator chips, dense integration of the necessary light sources still represents a major challenge. Chip-scale frequency comb sources present an attractive alternative for providing a multitude of optical carriers for WDM transmission. In this paper, we give an overview of our recent progress towards terabit communications with chip-scale frequency comb sources. In a first set of experiments, we demonstrate frequency comb generation based on silicon-organic hybrid (SOH) electro-optic modulators, enabling line rates up to 1.152 Tbit/s. In a second set of experiments, we use injection locking of a gain-switched laser diode to enerate frequency combs. This approach leads to line rates of more than 2 Tbit/s. A third set of experiments is finally dedicated to using Kerr nonlinearities in integrated nonlinear microcavities for frequency comb generation. We demonstrate coherent communication using Kerr frequency comb sources, thereby achieving line rates up to 1.44 Tbit/s. Our experiments show that frequency comb generation in chip-scale devices represents a viable approach to terabit communications.
Towards efficient octave-spanning comb with micro-structured crystalline resonator
Optical frequency combs, typically produced by mode locked lasers, have revolutionized many applications in science and technology. Frequency combs were recently generated by micro resonators through nonlinear Kerr processes. However, the comb span from micro resonators was found to be limited by resonator dispersion and mode spectrum. While dispersion engineering has been reported in on-chip devices, monolithic crystalline resonators offer an advantage of high optical quality factor. Moreover, most resonators used for comb generation support many mode families, leading to unavoidable crossings in resonator spectrum. Such crossings strongly influence comb dynamics and may prevent stable coherent mode-locking and soliton states. We report a new crystalline resonator approach supporting dispersion control and single mode spectrum while maintaining high quality factor. Dispersion engineering by waveguide micro-structuring is used to flatten the dispersion in our MgF2 resonator. Both absolute magnitude of dispersion and its slopes can be altered over a wavelength span exceeding an octave. Dispersion flattening leads to generation of an octave-spanning frequency comb with repetition rate of 46 GHz and coupled pump power below 100 mW. We also demonstrate that the micro- structuring dispersion engineering approach can be used to achieve flattened and anomalous dispersion in a CaF2 resonator near 1550 nm wavelength. In addition, we describe observation of discrete steps between the modulation instability states of the primary comb and on the three-stage comb unfolding dynamics. The micro-structured resonators may enable efficient low repetition rate coherent octave spanning frequency combs without external broadening, ideal for applications in optical frequency synthesis, metrology, spectroscopy, and communications.
Advances in Kerr optical frequency comb generation
Guoping Lin, Khaldoun Saleh, Souleymane Diallo, et al.
We report and discuss some of the latest advances in Kerr optical frequency comb generation. We principally focus on widely tunable primary combs, and on the role played by the eigenmode family of the modes of interest. Our work shows that there is a giant dispersion shift between the various radial families of modes, and that we can use them to generate primary combs with significantly different intermodal spacings.
Microresonator Sensors
Hydrogen gas sensing using palladium-coated microdisk microresonators
Mustafa Eryurek, Yasin Karadag, Nevin Taşaltın, et al.
An SU-8 polymer microdisk resonator coated with a palladium (Pd) layer and coupled to a single-mode optical waveguide is used to as a hydrogen (H2) gas sensor. In the presence of H2, a red shift is observed in the spectral positions of the microdisk whispering gallery modes (WGMs) due to the expansion in the Pd lattice. H2 concentrations below the flammable limit (4%) down to 0.3% could be detected in nitrogen atmosphere at room temperature. For H2 concentrations between 0.3 − 1%, WGM spectral positions shifted linearly with H2 concentration at a rate of 32 pm/%H2. Average response time of the devices was measured to be 50 s for 1% H2. The proposed device concept can also be used to detect different chemical gases by using appropriate sensing layers.
Non-linear fluorescence excitation of Rhodamine 6G and TRITC labeled IgG in whispering gallery mode microresonators
Carme Pastells, M. Pilar Marco, David Merino-Arranz, et al.
We report the non linear fluorescence real-time detection of labeled IgG covalently bonded to the surface of a microspherical whispering gallery mode resonator (WGMR). The immunoreagents have been immobilized onto the surface of the WGMR sensor after being activated with an epoxy silane and an orienting layer. The developed immunosensor presents great potential as a robust sensing device for fast and early detection of immunoreactions. We also tested the potential of microbubbles as nonlinear enhancement platform. The dyes used in these studies are tetramethyl rhodamine isothiocyanate and Rhodamine 6G. All measurements were performed in a modified confocal microscope.
Optically active silica and polymeric materials for microcavity lasers and sensors
A. M Armani, N. Deka, S. Mehrabani, et al.
Silica and silica-doped high quality factor (Q) optical resonators have demonstrated ultra-low threshold lasers based on numerous mechanisms (eg rare earth dopants, Raman). To date, the key focus has been on maintaining a high Q, as that determines the lasing threshold and linewidth. However, equally important criteria are lasing efficiency and wavelength. These parameters are governed by the material, not the cavity Q. Therefore, to fully address this challenge, it is necessary to develop new materials. We have synthesized a suite of silica and polymeric materials with nanoparticle and rare-earth dopants to enable the development of microcavity lasers with emission from the near-IR to the UV. Additionally, the efficiencies and thresholds of many of these devices surpass the previous work. Specifically, the silica sol-gel lasers are co- and tri-doped with metal nanoparticles (eg Ti, Al) and rare-earth materials (eg Yb, Nb, Tm) and are fabricated using conventional micro/nanofabrication methods. The intercalation of the metal in the silica matrix reduces the clustering of the rare-earth ions and reduces the phonon energy of the glass, improving efficiency and overall device performance. Additionally, the silica Raman gain coefficient is enhanced due to the inclusion of the metal nanoparticles, which results in a lower threshold and a higher efficiency silica Raman laser. Finally, we have synthesized several polymer films doped with metal (eg Au, Ag) nanoparticles and deposited them on the surface of our microcavity devices. By pumping on the plasmonic resonant wavelength of the particle, we are able to achieve plasmonic-enhanced upconversion lasing.
High frequency ultrasound detection with ultra-high-Q silica microspheres
Maria V. Chistiakova, Andrea M. Armani
Due to the nondestructive and noninvasive nature of ultrasound imaging, the technique has a variety of applications in many fields, most notably in healthcare and electronics. Ultrasound detection based on optical microcavities has emerged as one accurate and sensitive method. While previous research using polymer microring cavities showed detection based on device deformation, the approach presented here relied on the photoelastic effect. In this effect, the ultrasound wave induces a strain in the medium leading to a refractive index change. This effect was shown experimentally and in a COMSOL simulation with the use of ultra high quality factor silica microspheres. With an increase in quality factor and input power from previous research, the device response is increased and the noise equivalent pressure is decreased. The simulations presented use the finite element method and integrate acoustic and optics components of the system. The predictive accuracy of the simulation is also presented.
Beam Shaping I
Generation of shape-invariant flat-top laser beams
A great number of laser applications need in place of the usual Gaussian beam a flat-top intensity profile in the focal plane of a focusing lens. In general the transformation of the laser beam from the Gaussian to the flat-top shape is made by a diffractive beam shaping technique. It is worthwhile to note that this transformation occurs in the vicinity of the focal plane. If a flat top laser beam keeping its shape during propagation is needed then this can be obtained by a weighted incoherent mixing of LG00 and LG01 eigenmodes. Here, we consider the generation of these two transverse modes by a solid-state laser axially pumped by a laser diode. The idea is to design the laser cavity so as to make identical the losses of LG00 and LG01 modes. To reach this objective we have used two techniques. The first one called as diffractive lies to insert an adequate amplitude mask inside the cavity. The second one called as interferometric consisted to couple the laser to an external cavity. It is important to note that LG00 and LG01 modes are not spatially in concurrence, i.e. the peak of the LG00 appears in the dip of the LG01 mode. As a result, the energy extraction from the amplifying medium is improved increasing thus the laser slope efficiency. Theory and experimental verifications have been done for the diffractive and interferometric techniques allowing the generation of a flat-top laser beam keeping its shape from the near-field to the far-field.
Optical trapping with superfocused high-M2 laser diode beam
G. S. Sokolovskii, V. V. Dudelev, Vasileia Melissinaki, et al.
Many applications of high-power laser diodes demand tight focusing. This is often not possible due to the multimode nature of semiconductor laser radiation possessing beam propagation parameter M2 values in double-digits. We propose a method of ‘interference’ superfocusing of high-M2 diode laser beams with a technique developed for the generation of Bessel beams based on the employment of an axicon fabricated on the tip of a 100 μm diameter optical fiber with high-precision direct laser writing. Using axicons with apex angle 1400 and rounded tip area as small as ~10 μm diameter, we demonstrate 2-4 μm diameter focused laser ‘needle’ beams with approximately 20 μm propagation length generated from multimode diode laser with beam propagation parameter M2=18 and emission wavelength of 960 nm. This is a few-fold reduction compared to the minimal focal spot size of ~11 μm that could be achieved if focused by an ‘ideal’ lens of unity numerical aperture. The same technique using a 1600 axicon allowed us to demonstrate few-μm-wide laser ‘needle’ beams with nearly 100 μm propagation length with which to demonstrate optical trapping of 5-6 μm rat blood red cells in a water-heparin solution. Our results indicate the good potential of superfocused diode laser beams for applications relating to optical trapping and manipulation of microscopic objects including living biological objects with aspirations towards subsequent novel lab-on-chip configurations.
Formation of the desired light intensity distribution on the target with bimorph deformable mirror
It is desired to use some special intensity distribution on the target for various industrial applications. The adjusting of the intensity profile can be implemented by means of adaptive optics. In this paper we present laser beam control in the focal plane of lens with bimorph deformable mirrors. Hill-climbing method and stochastic algorithm are compared. Advantages and disadvantages of two methods are discussed.
Transverse intensity transformation by laser amplifiers
Lasers beams with a specific intensity profile such as super–Gaussian, Airy or Dougnut-like are desirable in many applications such as laser materials processing, medicine and communications. We propose a new technique for laser beam shaping by amplifying a beam in an end-pumped bulk amplifier that is pumped with a beam that has a modified intensity profile. Advantages of this method are that it is relatively easy to implement, has the ability to reshape multimode beams and is naturally suited to high power/energy beams. Both three and four level gain materials can be used as amplifier media. However, a big advantage of using three level materials is their ability to attenuate of the seed beam, which enhances the contrast of the shaping.

We first developed a numerical method to obtain the required pump intensity for an arbitrary beam transformation. This method was subsequently experimentally verified using a three level system. The output of a 2.07 μm seed laser was amplified in a Ho:YLF bulk amplifier which was being pumped by a 1.89 μm Tm:YLF laser which had roughly a TEM10 Hermit Gaussian intensity profile. The seed beam was amplified from 0.3 W to 0.55 W at the full pump power of 35 W. More importantly, the beam profile in one transverse direction was significantly shaped from Gaussian to roughly flat-top, as the model predicted. The concept has therefore been shown to be viable and can be used to optimise the beam profile for a wide range of applications.
Beam Shaping II: Joint Session with Conferences 9343 and 9356
Optical alignment influenced aberrations in laser beam delivery systems and their correction
Michael Scaggs, Gil Haas
Industrial high power laser systems are often evaluated based upon spatial profile of the beam before they are brought to focus for processing materials. It is therefore often assumed that if the raw beam profile is good that the focus is equally as good. The possibility of having good optics and poor alignment or bad optics and good alignment and therefore not achieve a good focal spot is quite high due to the fact that a raw beam spatial profile does not manifest third order aberrations. In such instances the focal spot will contain aberrations when there are slightly misaligned, poor quality, high power optics in the system such as a beam expander or eye piece and objective of a 3-axis galvo. Likewise, if the beam itself is not on axis, the third order aberrations of astigmatism and coma are likely to appear but again not be seen in the unfocused beams spatial profile. The third order aberrations of astigmatism, coma and spherical aberration can significantly alter both the size and spatial profile at the focus resulting in out of spec performance. The impact of beam and zoom expanders and their alignment in beam delivery systems is investigated by measuring both the far field unfocused and the far field focus beams using an all passive beam waist analyzer system.
Beam uniformity of flat top lasers
Chao Chang, Larry Cramer, Don Danielson, et al.
Many beams that output from standard commercial lasers are multi-mode, with each mode having a different shape and width. They show an overall non-homogeneous energy distribution across the spot size. There may be satellite structures, halos and other deviations from beam uniformity. However, many scientific, industrial and medical applications require flat top spatial energy distribution, high uniformity in the plateau region, and complete absence of hot spots. Reliable standard methods for the evaluation of beam quality are of great importance. Standard methods are required for correct characterization of the laser for its intended application and for tight quality control in laser manufacturing. The International Organization for Standardization (ISO) has published standard procedures and definitions for this purpose. These procedures have not been widely adopted by commercial laser manufacturers. This is due to the fact that they are unreliable because an unrepresentative single-pixel value can seriously distort the result. We hereby propose a metric of beam uniformity, a way of beam profile visualization, procedures to automatically detect hot spots and beam structures, and application examples in our high energy laser production.
Beam Shaping III
Minimal-effort planning of active alignment processes for beam-shaping optics
Sebastian Haag, Matthias Schranner, Tobias Müller, et al.
In science and industry, the alignment of beam-shaping optics is usually a manual procedure. Many industrial applications utilizing beam-shaping optical systems require more scalable production solutions and therefore effort has been invested in research regarding the automation of optics assembly. In previous works, the authors and other researchers have proven the feasibility of automated alignment of beam-shaping optics such as collimation lenses or homogenization optics. Nevertheless, the planning efforts as well as additional knowledge from the fields of automation and control required for such alignment processes are immense. This paper presents a novel approach of planning active alignment processes of beam-shaping optics with the focus of minimizing the planning efforts for active alignment. The approach utilizes optical simulation and the genetic programming paradigm from computer science for automatically extracting features from a simulated data basis with a high correlation coefficient regarding the individual degrees of freedom of alignment. The strategy is capable of finding active alignment strategies that can be executed by an automated assembly system. The paper presents a tool making the algorithm available to end-users and it discusses the results of planning the active alignment of the well-known assembly of a fast-axis collimator. The paper concludes with an outlook on the transferability to other use cases such as application specific intensity distributions which will benefit from reduced planning efforts.
FPGA-accelerated adaptive optics wavefront control part II
S. Mauch, A. Barth, J. Reger, et al.
We present progressive work that is based on our recently developed rapid control prototyping system (RCP), designed for the implementation of high-performance adaptive optical control algorithms using a continuous de-formable mirror (DM). The RCP system, presented in 2014, is resorting to a Xilinx Kintex-7 Field Programmable Gate Array (FPGA), placed on a self-developed PCIe card, and installed on a high-performance computer that runs a hard real-time Linux operating system. For this purpose, algorithms for the efficient evaluation of data from a Shack-Hartmann wavefront sensor (SHWFS) on an FPGA have been developed. The corresponding analog input and output cards are designed for exploiting the maximum possible performance while not being constrained to a specific DM and control algorithm due to the RCP approach.

In this second part of our contribution, we focus on recent results that we achieved with this novel experimental setup. By presenting results which are far superior to the former ones, we further justify the deployment of the RCP system and its required time and resources. We conducted various experiments for revealing the effective performance, i.e. the maximum manageable complexity in the controller design that may be achieved in real-time without performance losses. A detailed analysis of the hidden latencies is carried out, showing that these latencies have been drastically reduced. In addition, a series of concepts relating the evaluation of the wavefront as well as designing and synthesizing a wavefront are thoroughly investigated with the goal to overcome some of the prevalent limitations. Furthermore, principal results regarding the closed-loop performance of the low-speed dynamics of the integrated heater in a DM concept are illustrated in detail; to be combined with the piezo-electric high-speed actuators in the next step
New concepts of electro-optical light deflection: EO-slab and phased EO-array
Volker Wirth, Alexei Krasnaberski, Mikhail M. Ivanenko
Deflection and modulation of a laser beam for Q-switching or material processing can be realized in many ways. Today, one task is still the speed of these components. Especially for spatial pulse separation in ultrashort pulse laser applications the deflection must be faster (MHz). Promising solutions are deflectors based on the electro-optic effect.
Microresonator Fundamentals, Novel Topologies, and Devices I
Femtosecond laser direct writing of high-Q microresonators in glass and crystals
Jintian Lin, Yingxin Xu, Jiangxin Song, et al.
We report on fabrication of microresonators of high quality (high-Q) factors in both glass and crystalline materials by femtosecond laser 3D micromachining. Based on this novel approach, we obtained high-Q microresonators of non-in-plane geometries in glass materials such as fused silica and Nd: glass and demonstrated lasing at a pump power as low as 69 microwatts. We also fabricated on-chip microresonators of sub-100 μm diameters in crystalline materials including calcium fluoride and lithium niobate, and demonstrated efficient second harmonic generation using the high-Q lithium niobate microresonator. Furthermore, femtosecond laser 3D micromachining allows direct integration of the microresonators with other functional microcomponents, such as a microfluidic mixer and a microheater, leading to compact microdevices with enhanced functionalities. Our technique opens new avenues for fabricating high-Q microresonators with either 2D or 3D geometries on various types of dielectric materials.
Hybrid silicon unidirectional-emission microspiral disk lasers for optical interconnect applications
Andrew W. Poon, Yu Zhang, Lei Zhang
In this paper, we report our recent progress on hybrid silicon uniport-waveguide-emission microspiral disk lasers for optical interconnects using bisbenzocyclobutene (BCB) die-to-die bonding. We numerically and experimentally investigate different silicon microspiral disk cavities directly coupled with an output-waveguide, with III-V multiple-quantum-well (MQW) gain mediums of different circular microdisk geometries vertically coupled on the top via a sub- 100nm BCB layer, and ring-shaped injection electrodes along the rim of the hybrid integrated structure in order to enhance the spatial overlap between the injection current distribution in the III-V gain microdisk and the whispering-gallery-like mode field in the hybrid silicon microcavity. We design and experimentally investigate an integrated thermal shunt connecting a 90o-arc of the III-V gain microdisk to the silicon substrate underneath in order to lower the thermal resistance of the lasers. We observe transverse-electric (TE)-polarized mode dominated multimode lasing emission out-coupled from the silicon waveguide of the hybrid silicon lasers under a pulsed injection at room temperature, with a threshold current of 30 mA and a side-mode-suppression-ratio (SMSR) of 25 dB at 50 mA.
PDMS quasi-droplet microbubble resonator
Jonathan M. Ward, Yong Yang, Síle Nic Chormaic
A thin-walled microbubble whispering gallery resonator was fabricated and filled with a polymer (Polydimethylsiloxane : PDMS) to form a polymer quasi-droplet optical microcavity. The thermal shifting of the whispering gallery modes (WGMs) was studied pre and post curing of the polymer. In both cases, large thermal shifts were observed. However, the sign of the shift changed as the polymer hardened. The final state of the cavity showed a large red shift of the modes. Complex mode mixing was also observed which results in EIT (electromagnetically induced transparency) and Fano-like resonances. The time response of the polymer-filled bubble was investigated demonstrating regenerative self-oscillation for a fixed laser detuning with a low threshold power.
Microresonator Fundamentals, Novel Topologies, and Devices II
Optimal coupling to high-Q whispering gallery modes with a sub-wavelength metallic grating coupler
Y. Zhou, Bobo Gu, X. Yu, et al.
Gold grating patterned on the end facet of an optical fiber is able to excite whispering gallery mode (WGM) in a silica microsphere. With a direct pathway of the metal reflection, the coupled WGM is able to superimpose and create an asymmetric Fano resonance. Since multiple resonances are present – the WGM, grating reflection, and a weak Fabry-Perot resonance along the diameter of the sphere – it is difficult to evaluate the power efficiency directly from the measured spectrum. Using temporal coupled-mode theory, a general model is constructed for the end-fire coupling from a grating to a WGM resonator.
Strong localization of whispering gallery modes in an optical fiber via asymmetric perturbation of the translation symmetry
Recently introduced Surface Nanoscale Axial Photonics (SNAP) is based on whispering gallery modes circulating around the optical fiber surface and undergoing slow axial propagation. In this paper we develop the theory of propagation of whispering gallery modes in a SNAP microresonator, which is formed by nanoscale asymmetric perturbation of the fiber translation symmetry and called here a nanobump microresonator. The considered modes are localized near a closed stable geodesic situated at the fiber surface. A simple condition for the stability of this geodesic corresponding to the appearance of a high Q-factor nanobump microresonator is found. The results obtained are important for engineering of SNAP devices and structures.
Cladding modes fiber coupling to silica micro-resonators based on long period gratings
Daniele Farnesi, F. Chiavaioli, F. Cosi, et al.
A novel method based on long period fiber gratings (LPGs) for coupling light to high-Q silica whispering gallery mode (WGM) resonators is presented. An LPG couples the fundamental mode of a fiber to higher order LP cladding modes at selected frequencies. At an adiabatically tapered section of the fiber following the LPG we demonstrated effective coupling of these cladding modes to WGMs both in silica microspheres and microbubbles. The taper is about one order of magnitude thicker than standard tapers used for the same purpose. Therefore this new method offers improved robustness for practical applications.
Coupled Resonators and Phased Arrays
Uncovering the physical origin of self-phasing in coupled fiber lasers
We studied coherent beam combining in a specific laser cavity architecture in which two Ytterbium-doped fiber amplifiers are passively coupled using a homemade binary phase Dammann grating. Our experimental results show that coherent beam combining is robust against phase perturbation in such a laser cavity architecture when the operating point is sufficiently above the lasing threshold. We observed redistribution of energy within the supermode of this laser cavity in response to an externally applied path length error. The energy redistribution is accompanied by an internal differential phase shift between the coherently coupled gain arms. Self-phasing mitigates or even completely neutralizes the externally applied optical path length error. We identify the physical origin of the observed self-phasing with the resonant (gain related) nonlinearity in the gain elements under our experimental conditions.
Phase dynamics of high radiance fiber laser arrays with active phase control
Erik Bochove, Brendan Neschke, Niketh Nair, et al.
The existing model of the LOCSET technique for the active phase synchronization of fiber laser arrays (T. Shay, Opt. Express, 2006) is extended to include relevant physical properties of the system, such as inherent optical path differences (OPD), line-width and group velocity dispersion (GVD), and we also include phase “jitter” of the master oscillator’s output in the model, which in experiments is implemented to induce spectral broadening for suppression of nonlinear frequency conversion. Linearization of the phase error signal, which incorrectly predicts convergence to a synchronous equilibrium state, is not performed. Instead, the closed-loop control dynamics are shown to be described by differential equations of Kuramoto type when phase corrector response dynamics are negligible. Linear stability analysis indicates that there is always one and no more than one dynamically stable state. The latter is shown to be normally synchronous, except when strong “jitter” is applied. A Liapounov function is found as subject to the validity of certain symmetry conditions.
Resonator modes and mode dynamics for an external cavity-coupled laser array
Niketh Nair, Erik J. Bochove, Alejandro B. Aceves, et al.
Employing a Fox-Li approach, we derived the cold-cavity mode structure and a coupled mode theory for a phased array of N single-transverse-mode active waveguides with feedback from an external cavity. We applied the analysis to a system with arbitrary laser lengths, external cavity design and coupling strengths to the external cavity. The entire system was treated as a single resonator. The effect of the external cavity was modeled by a set of boundary conditions expressed by an N-by-N frequency-dependent matrix relation between incident and reflected fields at the interface with the external cavity. The coupled mode theory can be adapted to various types of gain media and internal and external cavity designs.
Time-resolved deformation measurement of Yb:YAG thin disk using wavefront sensor
Michal Chyla, Shiva S. Nagisetty, Patricie Severová, et al.
Even though thin-disk medium mounted on a diamond substrate is generally used for high average power operation, we found that the pulsed pumping of the Yb:YAG thin-disk mounded on a copper-tungsten heatsink could improve both optical-to-optical O-O efficiency and beam quality. We are expecting that the increase of O-O efficiency is caused by the suppression of ASE. However, the mechanism of beam quality improvement is not clear. We developed a precise measurement system of thin-disk deformations based on a Hartmann-Shack wavefront sensor. Investigating thin-disk dynamics under pulsed pumping can help to greatly improve the mode matching and allow obtaining higher output energy.
Single shot M2 measurement for near infrared high energy laser pulses
Siva Sankar Nagisetty, Taisuke Miura, Martin Smrz, et al.
Since the principle of M2 measurement is to scan the beam discretely along with propagation direction, measurement time of several minutes is required which is not suitable for pulsed lasers. Several single-shot techniques have been proposed to measure M2 by using diffraction gratings and wavefront sensor, but were shown to be more complex and yield inaccurate results for multimode beams. Another approach to measure the M2 uses Rayleigh scattering from gas or liquid-filled cell. The scattered image by laser light in the cell, however, contains lots of speckle patterns which degrade the accuracy of M2 measurement. We developed a single shot M2 measurement based on a photosensitive glass. The measurement system consists of the photosensitive glass plate and the imaging camera with macro lens. When the pulsed laser beam focused into the cross-sectional direction of photosensitive glass plate, the visible fluorescence of the glass plate indicates the focusing property of laser beam. Then the visualized beam propagation in the glass is imaged precisely to measure the beam diameters around beam waist. Since the coherent laser beam is converted to the incoherent fluorescence, the beam propagation image is free from speckle patterns. The M2 can be calculated from the image within less than a second. This simple technique allows the possibility of the real time monitoring of the beam quality. We obtained M2=1.10 from a fiber coupled diode laser that is close to the actual value of M2=1.18 using the standard scanning method.
Characterization of vorticity in fluids by a spatially shaped laser beam
Anton Ryabtsev, Shahram Pouya, Manoochehr Koochesfahani, et al.
Measuring the speed and direction of vortices is of great importance in fluid dynamics. We report on the use of a CW laser beam with a superposition of Laguerre-Gaussian (LG) modes generated by a phase mask imprinted on a two-dimensional spatial light modulator. The shaped beam is then guided and scattered of a sample which is rotated; the rotational frequency is extracted from spectral analysis of the scattered light. This method allows for virtually real-time determination of vorticity characterization in a fluid.
Laser Mode Control I
Analysis of thermal depolarization compensation using full vectorial beam propagation method in laser amplifiers
Rainer Hartmann, Christoph Pflaum, Thomas Graupeter
We developed a complex physical model for simulating laser amplifiers to numerically analyze birefringence effects. This model includes pump configuration, thermal lensing effects, birefringence, and beam propagation in the laser amplifier. Temperature, deformation, and stress inside the laser crystal were calculated using a three-dimensional finite element analysis (FEA). The pump configuration is simulated using a three-dimensional ray tracing or an approximation based on super-Gaussian functions.

Our simulations show the depolarization of a linearly polarized electromagnetic wave in a cylindrical laser crystal. These simulations were performed using a three-dimensional full vectorial beam propagation method (VBPM). Stress induced birefringence can be compensated well for moderate pumping powers. High power amplification requires sensitive alignment. Our simulation technique calculates the influence of the photo-elastic effect inside the laser crystal accurately. Detailed knowledge about beam waist and depolarization is needed to develop compensation techniques for high power output beams with low depolarization losses.
Laser Mode Control II
Red-emitting external cavity diode laser with high slope efficiency and narrow bandwidth
Jiyeon Park, Hong Joo Song, Hong Man Na, et al.
External cavity diode laser with broad-area laser diode is operated up to the output power of 160 mW at the injection current of 850 mA and the bandwidth of 80 pm at a wavelength of 648 nm in external cavity. High slope efficiency of output power and narrow bandwidth using broad-area laser (BAL) diode, the width of active layer in the slow axis is too broad to select a specific wavelength. In this paper, more efficient wavelength selection method is investigated by confirming the tendency of grating grooves and designing to set up the wavelength dispersion direction along the fast axis of a solitary laser diode (LD) geometrically. Thus, the tunable external cavity diode laser module by using BAL diode is designed with an overall size of 49 mm x 52 mm x 48.5 mm. From injection current in the range of 650-900 mA, ECDL showed excellent wavelength locking behavior without a non-shift of the peak wavelength. Here, the tuning range is 4 nm with maintaining the narrow bandwidth of 80 pm and up to the output power of 100 mW. A side-mode-suppression of 36.5dB is also achieved at the output power of 160 mW and the injection current of 850 mA.
Wavefront control in high average-power multi-slab laser system
Jan Pilar, Stefano Bonora, Martin Divoky, et al.
A high average power cryogenically-cooled diode-pumped solid-state laser system for Hilase centre in Czech Republic is being developed by Central Laser Facility at Rutherford Appleton Laboratory, England in collaboration with Hilase team. The system will deliver pulses with energy of 100 J at 10 Hz repetition rate and will find applications in research and industry. The laser medium and other elements of the system are subject to heavy thermal loading which causes serious optical aberrations and degrade the output beam quality. To meet the stringent laser requirements of this kWclass laser, it is necessary to implement adaptive optics system, which will correct for these aberrations. During our research the sources of aberrations have been identified and analyzed. Based on this analysis, a suitable adaptive optics system was proposed. After finalizing numerical models, simulations and optimizations, the adaptive optics system was developed, characterized and installed in a cryogenically-cooled multi-slab laser system running up to 6 J and 10 Hz. The adaptive optics system consists of 6x6 actuator bimorph deformable mirror and wavefront sensor based on quadriwave lateral shearing interferometry operated in closed loop. The functionality of the system was demonstrated at full power.
Ultra stable carbon fiber high power CO2 laser with high quality laser beam and AOM implementation
High security printing as well as ultra high precision engraving need laser resonators with very stable laser beams (600 - 800W) especially in combination with AOMs. Based upon a unique carbon fiber structure - stable within the sub-micrometer range - a new resonator has been developed, accompanied by most recent thermo-mechanical FEM calculations. The resulting beam is evaluated on an automated optical bench allowing to optimize the complete beam path with collimators and AOM. Synchronous on-line evaluation with PyroCams and thus knowledge about how to minimize distortions within the nonlinear elements is presented in this paper.
Poster Session
Selective higher order fiber mode excitation using a monolithic setup of a phase plate at a fiber facet
Johannes Wilde, Christian Schulze, Robert Brüning, et al.
Controlling the modal content coupled into an optical fiber can be desirable in many situations, e.g. for adjusting the sensitivity of the guided field distribution to external perturbations1. For this purpose we used a monolithic setup of a phase plate at a fiber input facet to excite selectively higher order modes, which theoretically can provide a mode purity of more than 99%. We investigated the capabilities of this approach by complete modal decomposition of the fiber output signals, considering the achievable mode purity with respect to several possible imperfections of the setup. The experiments are compared with detailed numerical simulations and show a high agreement. Additionally a comparison with a well known setup with free space phase plates2–4 was undertaken. This showed the monolithic setup to be energetically twice as efficient.
Investigation of the impact of fiber Bragg grating bandwidth on the efficiency of a Raman resonator
Leanne J. Henry, Michael Klopfer, Ravinder K. Jain
Significant spectral power leakage was found to occur around the high reflectivity fiber Bragg gratings (FBGs) defining a 1121 nm Raman resonator cavity comprised of PM 10/125 germanosilicate fiber. This cavity was part of a Raman system pumped with broad linewidth 1069 nm and seeded with narrow linewidth 1178 nm. The 1069 nm upon entering the resonator cavity was Raman converted to 1121 nm which then amplified the 1178 nm as it passed through the cavity. Spectral leakage of 1121 nm light from the resonator cavity resulted in sub-optimal amplification of 1178 nm which forced usage of longer resonator cavities having a decreased threshold for Stimulated Brillouin Scattering. Upon study of 1121 nm linewidth broadening as a function of resonator length for cavities employing 3 nm FBGs, differences in the percentage of 1121 nm power spectrally leaking past the output FBG as a function of the 1121 nm intracavity power propagating in the forward direction are not experimentally discernible for resonator cavities longer than 40 m. But, for cavity’s shorter than 40 m, the percentage of 1121 nm power spectrally leaking past the output FBG decreased significantly for similar 1121 nm intracavity power levels. For all cavity lengths, a nearly linear relationship exists between percent 1121 nm power leakage and intracavity power levels. Also, cavities employing broader bandwidth FBGs experience less 1121 nm power leakage for similar 1121 nm intracavity power levels. Finally, modeling predictions of Raman system performance are greatly improved upon usage of experimentally derived effective FBG reflectivities.
Improving the intensity of a focused laser beam
Sofiane Haddadi, Michael Fromager, Djelloul Louhibi, et al.
Let us consider the family of symmetrical Laguerre-Gaus modes of zero azimuthal order which will be denoted as LGp0 . The latter is made up of central lobe surrounded by p concentric rings of light. The fundamental mode LG00 is a Gaussian beam of width W. The focusing of a LGp0 beam of power P by a converging lens of focal length f produces a focal spot keeping the LGp0 -shape and having a central intensity I0= 2PW2/(λf)2 whatever the value of the radial order p. Many applications of lasers (laser marking, laser ablation, …) seek nowadays for a focal laser spot with the highest as possible intensity. For a given power P, increasing intensity I0 can be achieved by increasing W and reducing the focal length f. However, this way of doing is in fact limited because the ratio W/f cannot increase indefinitely at the risk of introducing a huge truncation upon the edge of the lens. In fact, it is possible to produce a single-lobed focal spot with a central intensity of about p times the intensity I0. This result has been obtained by reshaping (rectification) a LGp0 beam thanks to a proper Binary Diffractive Optical Element (BDOE). In addition, forcing a laser cavity to oscillate upon a LGp0 can improve the power extract due to a mode volume increasing with the mode order p. This could allow envisaging an economy of scale in term of laser pumping power for producing a given intensity I0. In addition, we have demonstrated that a rectified LGp0 beam better stand the lens spherical aberration than the usual Gaussian beam.
Laser beam shape converter using spatially variable waveplate made by nanogratings inscription in fused silica
Titas Gertus, Andrejus Michailovas, K. Michailovas, et al.
In this work we present a beam shaping technique based on a spatially variable phase retardation plate inscribed inside bulk of fused silica glass by femtosecond laser pulses. Formation of self-assembled periodic nanostructures was exploited to fabricate the converter. During the fabrication process we control induced nanogratings orientation and retardance. Combination of a spatially variable waveplate and a polarizer acts as a spatially variable transmission filter. With a converter fabricated to transform an initially Gaussian beam to a flat-top beam we preserve more than 50% of initial laser power. Theoretically, the efficiency of the proposed converter could be up to 70%. The proposed converter with no absorbing elements possesses resistance to optical damage similar to that of fused silica. Additionally, the already-fabricated converter allows for on-the-fly adjustment of the beam shape from flat-top to a shape with a dip in the middle. The shaped beam was tested in a high power picosecond pulse amplifier.
Role of geometry in optothermal response of toroidal ultra-high-Q cavities
Ultra-high quality factor (UHQ) resonant cavities are able to store light for long periods of time, resulting in high circulating intensities. As a result, numerous nonlinear optical phenomena appear, such as radiation pressure oscillations and lasing. However, deleterious behaviors also occur, such as optothermal broadening of the resonant linewidth. The degree of distortion is directly related to the circulating power in the cavity, the material absorption, and the thermo-optic coefficient of the cavity material. Specifically, a portion of the circulating power is absorbed by the material and converted to heat. This thermal energy is able to induce a refractive index change in the cavity which is experimentally observed as a resonant wavelength change. This behavior has been observed in numerous cavities, but one interesting case is the toroidal cavity, as it has a particularly complex geometry providing multiple thermal transport pathways. To accurately capture this complex behavior, we have developed a COMSOL Multiphysics model which combines the thermal and optical components. The model uses the non-uniform optical mode profile as the heat source. As such, changes in device geometry and wavelength are inherently captured. To verify the modeling, we characterize the optothermal threshold for a series of toroidal cavities across a range of wavelengths and device geometries. Additionally, the thermal time constant of the structure is explored. Of note, the membrane thickness is shown to play a critical role in the optothermal behaviors.
Whispering-gallery mode lasers for biosensing: a rationale for reducing the lasing threshold
Alexandre François, Nicolas Riesen, Hong Ji, et al.
Whispering-gallery modes have been studied extensively for biosensing applications. Whilst the vast majority of work undertaken has focused on high Q factor resonators, with the main improvement being a reduction of the resonator size to improve sensitivity, we have chosen a different pathway by starting with resonators that exhibit extremely high refractive index sensitivity but low Q factor. A way forward to overcome this limitation is to introduce a gain medium and operate the resonator above its lasing threshold. This has been shown to result on average in a 5 fold increase in the Q factor. With the lasing threshold itself being dependent on the Q factor, amongst other parameters, the Q factor enhancement can be exploited to either reduce the lasing threshold or alternatively enable smaller resonators to be operated above their lasing threshold. As a demonstration we present a 10 μm diameter polystyrene microsphere lasing in aqueous solution for refractive index sensing applications, which to the best of our knowledge is the smallest polystyrene microsphere laser ever demonstrated in these conditions.
High-speed transient sensing using dielectric micro-resonators
Amir R. Ali, Volkan Otugen, Tindaro Ioppolo
In this paper, we demonstrate the use of whispering gallery mode (WGM) resonators for high-speed transient sensing. In the typical WGM sensor, the micro-resonator modes are interrogated by coupling light from a tunable laser through a single mode optical fiber. The laser is tuned over a narrow range by thermo-optic effect, and mode shifts in the transmission spectrum through the fiber are observed. For high-speed applications, thermal inertia of the optical system impedes the proper tuning of the laser, limiting the WGM sensor applications to slow varying phenomena. In order to use the sensors for high-speed transient applications, we tune the DFB laser using a harmonic rather than a ramp waveform and calibrate the laser response at various input frequencies and amplitudes using a Fabry-Perot interferometer. WGM shifts are tracked using a fast cross-correlation algorithm on the transmission spectra. We demonstrate dynamic force measurements up to 10 kHz using this approach. We also present a simple lumped-heat capacity thermal model to predict the laser response.
Short-and-long-term highly stable oscillation and amplification of linearly polarized passively mode-locked solitonic fiber laser resonators
M. Brotons-Gisbert, G. E. Villanueva, J. Abreu-Afonso, et al.
In this work, we present a short- and long-term operation and environmentally-stable, all-polarization-mantained, Fabry-Pérot resonator, passively mode-locked fiber laser operating in an all-anomalous-dispersion solitonic regime. Our results confirm that the highly stable operation of the laser oscillator is maintained after amplifying the laser output with a conventional EDFA. Such stability has been studied by a variety of measurements in the temporal and spectral -both optical and electrical- domains before and after amplification. Pulse durations of 540 fs, period-relative time jitters of ~0.015‰, and long-term uninterrumped operation with <1.8% variation of the individually photodetected pulse peak powers are obtained for the laser oscillator. After amplification, dispersion-induced pulse durations of ~244 fs, period-relative time jitters of ~0.019‰ and an average output power of 20mW are obtained, while maintaining the 100 dB signal-to-noise ratio (SNR) measured at 500 Hz offset from the fundamental harmonic frequency of the photodetected signal. We have also carried out a theoretical validation of the emission properties of our laser oscillator based on solutions of the Nonlinear Schröedinger Equation that take into account wavelength and z-position dependence of the active medium gain.
Predicting the whispering gallery mode spectra of microresonators
Jonathan M. M. Hall, Shahraam Afshar V., Matthew R. Henderson, et al.
The whispering gallery modes (WGMs) of optical resonators have prompted intensive research efforts due to their usefulness in the field of biological sensing, and their employment in nonlinear optics. While much information is available in the literature on numerical modeling of WGMs in microspheres, it remains a challenging task to be able to predict the emitted spectra of spherical microresonators. Here, we establish a customizable Finite-Difference Time-Domain (FDTD)-based approach to investigate the WGM spectrum of microspheres. The simulations are carried out in the vicinity of a dipole source rather than a typical plane-wave beam excitation, thus providing an effective analogue of the fluorescent dye or nanoparticle coatings used in experiment. The analysis of a single dipole source at different positions on the surface or inside a microsphere, serves to assess the relative efficiency of nearby radiating TE and TM modes, characterizing the profile of the spectrum. By varying the number, positions and alignments of the dipole sources, different excitation scenarios can be compared to analytic models, and to experimental results. The energy flux is collected via a nearby disk-shaped region. The resultant spectral profile shows a dependence on the configuration of the dipole sources. The power outcoupling can then be optimized for specific modes and wavelength regions. The development of such a computational tool can aid the preparation of optical sensors prior to fabrication, by preselecting desired the optical properties of the resonator.
Manufacture of refractive and diffractive beam-shaping elements in higher quantities using glass molding technology
Michael Wolz, Ullrich Blöcher, Gerhard Dross, et al.
Laser beam shaping elements can be used e.g. for material processing. The results of these processes can be improved when the usually Gaussian profile of the laser is transformed into a top hat profile, which can be circular or rectangular in shape. Another frequently used type of beam-forming devices are beam splitters for parallel processing using only one laser. These types of beam formers can be implemented as diffractive or refractive elements. So far these optics are produced either directly by means of lithography e.g. in glass or in plastic using a hot embossing process or nanoimprint technology. Elements produced in this way have either the disadvantage of high costs or they are limited in temperature range, laser power or wavelength. A newly developed molding process for glass allows the manufacture of larger numbers of optics with reduced cost.

The production of molds for refractive top hat beam shaping devices requires very high precision of the applied grinding process. Form deviations below 100 nm are necessary to obtain a homogeneous illumination. Measurements of the surface topography of gauss to top hat beam shaping elements using white light interferometry are presented as well as results of optical measurements of the beam profile using a camera.

Continuous diffractive beam shaping elements for beam splitting applications are designed to generate several sub-beams each carrying the same energy. In order to achieve this, form deviations of less than 50 nm are required. Measurements of the surface of a 1 x 5 beam splitter are compared with ideal beam splitter profiles. The resulting beam intensity distribution of a molded element is presented.
Wavelength-tunable laser based on electro-optic effect
Pengfei Wu, Suning Tang
Currently available wavelength-tunable lasers have technical difficulty in combining high-speed, continuous and wide wavelength tunability with high output power. We demonstrated a high-speed wavelength-tunable laser based on a fast electro-optic effect. We observed that the wavelength-swept speed exceeds 107 nm/s at center wavelength of 1550 nm with continuous wavelength tunability. Moreover, the maximum output power is over 100 mW and the wavelength tuning range is near 100 nm with a full width at half maximum of less than 0.5 nm.