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A novel optical concept is introduced with standard components for highly efficient coherent beam combining a system of (N x N) beams. In a proof-of-principle experiment a well-defined setup with microlens arrays (MLAs) is used to create a beam matrix of 5 x 5 beams. For the combination step the same setup is employed, and the created 25 beams are combined. A combination efficiency above 90% is achieved. Furthermore, the concept allows for dynamic beam combination, i.e., the resulting number of beams and corresponding positions can be controlled by the absolute phases of the array of input beams. A proof-of-principle experiment shows excellent agreement with the model. © COPYRIGHT SPIE

We present theoretical framework to describe Brillouin lasing in microcavities in the case of a significant mismatch between the Brillouin shift and the cavity intermode spacing. We show that despite an increase of the lasing threshold a significant increase of the Brillouin power in comparison with the resonance case is achievable. A necessary condition for this effect is the optimal value of the pump frequency detuning from the cavity mode frequency. An increase of the Brillouin threshold is accompanied by narrowing of the spectrum range where the Brillouin signal could be generated in non-resonant case. Besides, with the optimal pump frequency detuning the Brillouin signal noise level is reduced. Analytical results are in quantitative agreement with the results of numerical simulations.

A conical refraction (CR) laser based on a separate gain medium (Nd:YVO4) and an intracavity CR element (KGW) was demonstrated. The decoupling of the gain and CR media enabled the laser to produce a well-behaved CR laser beam with excellent quality, while reducing the complexity of the pumping scheme. The proposed laser setup has the potential for power scaling using the efficient diode pumping approach and the properties of the generated CR beam are independent from the laser gain medium.

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.

Achieving ever narrower linewidths in diode lasers has become of paramount importance for coherent communications. Coupling a laser chip to a fiber however introduces external optical feedback (EOF) to the cavity which is notoriously detrimental to the stability of the device, in all but very specific and difficult conditions. A better understanding of the interplay between EOF and the laser dynamics is thus crucial for designing narrow- linewidth diode lasers to be used in field-testing scenarios. The standard formalism of EOF in diode lasers relies on the assumption that feedback is present on one side of the laser cavity. However, given the currently available integration technologies, this assumption is no longer justified. In this work a revision of EOF theory is explored based on the updated assumption that feedback can be introduced from both sides of the laser cavity. This is done by obtaining the dynamic equations of the proposed system, including an expression for the power spectral density and linewidth. Additionally, the stability of the system is discussed. Results show that the proposed revised theory can yield stable laser performance while simultaneously reducing laser linewidth.

We have demonstrated the highest conical refraction (CR) laser output power to date by placing a CR crystal inside of a diode-pumped Nd:YVO laser cavity. The CR crystal did not have a significant influence on laser output power as well as efficiency. The CR laser produced the maximum output power of 3.68 W with the slope efficiency of 42 % and opticalto- optical efficiency of 34 %. Therefore, this approach could be an attractive pathway for further power scaling of the CR lasers.

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

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

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

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 highprecision direct laser writing. Using axicons with apex angle 1400 and rounded tip area as small as similar to 10 μm diameter, we demonstrate 2-4 μm diameter focused laser ` needle' beams with approximately 20 mu m 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 5-6μm 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. Publisher PDF

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.

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. Published version

Optical interconnect and optical packet switching systems could take advantage of small footprint, low power lasers and optical logic elements. Microdisk lasers, with a diameter below 10μm and fabricated in InP membranes with a high index contrast, offer this possibility at the telecom wavelengths. The lasers are fabricated using heterogeneous integration of InP membranes on silicon-on-insulator (SOI) passive waveguide circuits, which allows to combine the active elements with compact, high-index contrast passive elements. The lasing mode in such microdisk lasers is a whispering gallery mode, which can be either in the clockwise (CW) or counter clockwise direction (CCW) or in both. The coupling to the SOI wire waveguides is through evanescent coupling. Predefined, unidirectional operation can be achieved by terminating the SOI wires at one end with Bragg gratings. For all-optical flip-flops, the laser operation must be switchable between CW and CCW, using short optical pulses. Unidirectional operation in either direction is only possible if the coupling between CW and CCW direction is very small, requiring small sidewall surface roughness, and if the gain suppression is sufficiently large, requiring large internal power levels. All-optical flip-flops based on microdisk lasers with diameter of 7.5μm have been demonstrated. They operate with a CW power consumption of a few mW and switch in 60ps with switching energies as low as 1.8fJ. Operation as all-optical gate has also been demonstrated. The surface roughness is limited through optimized etching of the disks and the large internal power is obtained through good heat sink.

Organic electro-optic materials offer exceptional processability (both from solution and the gas phase) that permit fabrication of flexible and conformal device structures and the integration of organic materials with a wide range of disparate materials. In addition, organic electro-optical materials have fundamental response times that are in the terahertz region, and useable electro-optic coefficients that are approaching 300 pm/V (at telecommunication wavelengths). In addition to fabrication by traditional lithographic methods, multiple devices on a single wafer have been fabricated by soft and nano-imprint lithography. In this presentation, we review the fabrication and performance evaluation of a number of all-organic and organic-silicon photonic ring microresonator devices. Both electrical and thermal tuning of devices, including both single and multiple ring micro-resonators, are demonstrated.

We propose and analyze a new type of a resonator in an annular geometry which is based on a single defect surrounded by radial Bragg reflectors on both sides. We show that the conditions for efficient mode confinement are different from those of the conventional Bragg waveguiding in a rectangular geometry. A simple and intuitive approach to the design of optimal radial Bragg reflectors is proposed and employed, yielding chirped gratings. Small bending radii and strong control over the resonator dispersion are possible by the Bragg confinement. A design compromise between large Free Spectral Range (FSR) requirements and fabrication tolerances is suggested., Comment: 19 pages, 12 figures

Planar photonic crystals are constructed by combining two-dimensional periodic structures with high refractive index contrast slabs. By suppressing the loss in these structures due to imperfect confinement in the third dimension, one can fully take advantage of their relatively simple fabrication, and achieve the functionality of three-dimensional photonic crystals. One of the greatest challenges in photonic crystal research is a construction of optical nanocavities with small mode volumes and large quality factors, for efficient localization of light. Beside standard applications of these structures (such as lasers or filters), they can potentially be used for cavity QED experiments, or as building blocks for quantum networks. This paper will address our theoretical and experimental results on optical nanocavities based on planar photonic crystals, with mode volumes as small as one half of cubic wavelength of light in material, and with Q factors even larger than 1 x 10^4.

The quantum noise limits of a semiconductor laser with dispersive loss are examined theoretically and experimentally. Using optical feedback from Cs vapor as a dispersive loss element we demonstrate almost 6 orders of magnitude reduction in the linewidth, to a 44 Hz level, and 1.9 dB amplitude noise squeezing below the standard quantum noise limit.

The Whispering Gallery Modes (WGMs) of fused silica microspheres hold the promise for simultaneous acheivement of high Q ( > 10^9) and small mode volumes (≾ 10^(-14) m^3) necessary for strong coupling in cavity QED. Here we present results for high Q measurements into the MR along with some progress towards experimental implementation in cavity QED.

Thermal nonlinearity can produce oscillatory instability in optical microspheres. We experimentally demonstrate this instability and analyze the conditions needed to observe this regime. The observed behavior is in good agreement with the results of numerical simulation. In pure fused silica with low optical absorption the thermal oscillations are suppressed owing to an interaction of thermal and Kerr nonlinearities. We also describe experimentally observed slow and irreversible thermo-optical processes in microspheres.