Single Optical Beam Research Articles

Tight focusing of the vector optical field with polarization varying along complex curves of the Poincaré sphere.

Different from the scalar optical field with spatially uniform polarization, the vector optical field exhibits inhomogeneous distribution of polarization on the cross section. Manipulating the variation of polarization in a single optical beam is important to acquire a flexible and controllable focused optical field. Previous studies mainly focused on the vector optical field with its polarization varying along a circular trajectory of the Poincaré sphere. Here, we demonstrate the tight focusing behaviors of the vector optical field with the polarization varying along complex curves of the Poincaré sphere, which is generated by the joint modulation of azimuthal phase and amplitude distributions of orthogonally polarized components. The longitudinal polarization component with a multipolar pattern in rotational symmetry can be achieved with similar distribution of the total focused field. The transverse and longitudinal spin angular momentum distributions in the focal space are discussed. Approximately pure transverse spin angular momentum can be constructed and manipulated in the focal space, which provides the possibility to manipulate the 3D spin flux for the applications of nano and spin photonics.

Generation of Orbital Angular Momentum Light by Patterning Azopolymer Thin Films

Orbital angular momentum (OAM) encoding is a promising technique to boost data transmission capacity in optical communications. Most recently, azobenzene films have gained attention as a versatile tool for creating and altering OAM-carrying beams. Unique features of azobenzene films make it possible to control molecular alignment through light-induced isomerization about the azo bond. This feature enables the fabrication of diffractive optical devices such as spiral phase plates and holograms by accurately imprinting a phase profile on the incident light. By forming azobenzene sheets into diffractive optical elements, such as spiral phase plates, one can selectively create OAM-carrying beams. Due to the helical wavefront and phase variation shown by these beams, multiple distinct channels can be encoded within a single optical beam. This can significantly increase the data transmission capacity of optical communication systems with this OAM multiplexing technique. Additionally, holographic optical components made from azobenzene films can be used to build and reconstruct intricate wavefronts. It is possible to create OAM-based holograms by imprinting holographic designs on azobenzene films, which makes it simpler to control and shape optical beams for specific communication requirements. In addition, azobenzene-based materials can then be suitable for integration into optical communication devices because of their reconfigurability, compactness, and infrastructure compatibility, which are the main future perspectives for achieving OAM-based technologies for the next generation, among other factors. In this paper, we see the possible use of azobenzene films in the generation and modification of OAM beams for optical communications through light-induced isomerization. In addition, the potential role of azobenzene films in the development of novel OAM-based devices that paves the way for the realization of high-capacity, OAM-enabled optical communication networks are discussed.

Coexistence of Quantum and 1.6 Tbit/s Classical Data Over Fibre-Wireless-Fibre Terminals

In this paper a fibre free-space fibre link that allows a quantum key distribution (QKD) signal and high-speed data to be transmitted in a single optical beam is presented. This setup has the potential to provide ultra-high data rate secure wireless connectivity within indoor spaces. Both quantum and classical channels were transmitted using wavelengths in the C band, over a range of 2.5 m in free space and in the presence of a 200 lx background illumination. Raman noise caused by the light propagating in the optical fibres which provide connections to the free-space segment, together with geometric loss of the free-space link are the major impairments. Results yield secret key rates of 2.343 kbit/s when the QKD signal is sent without high-speed data (and associated Raman noise), and rates of 1.305 - 1.996 kbit/s when high-speed data is also transmitted (depending on spectral separation of QKD and classical information). Further, it is shown that the secret key rate can be optimised by increasing loss to attenuate the Raman noise by misalignment of the link. Improvement of the secret key rate from 1.305 kbit/s to 1.887 kbit/s was shown using this method.

Optoelectronic Readout of STT-RAM Based on Plasmon Drag Effect

An opto-electronic readout of the memory state in spin transfer torque random access memory (STT-RAM) cells is proposed. A single optical beam illuminating an array of STT-RAM cells creates electrical potential along each STT-RAM cell due to optical rectification that can be further enhanced through plasmon drag effect (PLDE). Then, the photo-induced voltage is assumed to be measured by conventional electronics. Here, a theoretical modeling is performed, where the results provide an estimate of the photo-induced voltage in each memory cell and its variation with respect to the changes in state of the memory cell (parallel vs anti-parallel). Our study shows that the plasmonic enhancement facilitates ${\sim }$ 20 times enhancement in the voltage change due to state change of the each memory cell, compared to the case of out-of-resonance excitation. This enhancement potentially improves the memory readout rate if a proper supporting electronic circuit is available.

Continuous rotational motion in birefringent particles using two near-orthogonally polarized optical tweezers beams at different wavelengths with low ellipticity

Continuous yaw (in-plane) rotation of an optically trapped birefringent particle is conventionally achieved by illuminating with light of high ellipticity. This ellipticity has to be high enough to make the tilt of the washboard potential bypass the confining effect of the corrugation. Here by using a novel technique, we generate continuous rotations of a 3 µm sized spherical birefringent particle with ellipticity lower than that permissible with a single optical tweezers beam. We use two lasers, near-orthogonally polarized, with similar intensities at wavelengths of 1064 and 975 nm to obtain continuous rotations, just by making one of the beams elliptically polarized, but well lower than that permissible by the washboard potential. The maximum rotation frequency we achieve with the present configuration is 5 Hz which can be increased by simultaneously increasing the two laser intensities.

Review of key vertical-cavity laser and modulator advances enabled by advanced MBE technology

In this special issue honoring Professor Arthur Gossard, I am delighted to be able to review a small segment of the work he has enabled while at UCSB on the subject of the title, but further limited to devices grown all-epitaxially. When he arrived in 1987 from Bell Labs, he had already been consulting on the installation of our new Gen-II MBE that we intended to use for vertical-cavity Fabry–Pérot modulators, devices somewhat similar to those he had grown at Bell Labs. However, within a couple of years, we obtained leading results on reflection modulators, moving the on/off contrast from prior values of less than 5:1 to more than 50:1 with insertion losses of less than 2 dB, required voltages in the 2–4 V range, and changes in reflection per volt to ∼20%/V. These had multiple-quantum-well (MQW) active regions to phase shift and partially absorb the resonant lightwaves within a cavity formed between two distributed-Bragg-reflector (DBR) mirrors all formed in the AlGaAs/GaAs system. Also in this same period, novel vertical-cavity surface-emitting laser (VCSEL) structures analogous to the modulators were developed. They had strained InGaAs/GaAs MQW actives and AlGaAs/GaAs DBRs and operated near 980 nm. The initial new idea was to place active quantum wells only at the maxima of the cavity E-field standing wave, which provides nearly a doubling of the modal gain they contribute. These designs quickly led to leading results in threshold current (<1 kA/cm2—1990 and Ith < 1 mA with Po > 1 mW—1991), power out (up to 113 mW cw—1993), and temperature stability with gain offset (constant output over 50 °C—1993). Additional notable results in the 1990s included a selective oxidation of AlGaAs to form lens-like intra-cavity apertures for dramatic reductions in optical cavity loss; the first strained layer InGaAlAs/GaAs 850 nm VCSELs; and an 8-wavelength division multiplexing VCSEL array integrated within a 60 μm diameter for direct emission into a multimode fiber. In the 2000s, results included all-epitaxially grown 1310 nm and 1550 nm VCSELs that employed AlGaAsSb DBRs and AlGaInAs actives with tunnel junctions to enable two n-type contacts on InP for low thermal and electrical resistance; multi-terminal VCSELs for polarization modulation to double the information output on a single optical beam; and a novel high-speed, high-efficiency design that incorporated sophisticated bandgap engineering in the DBRs and carbon doping for low optical loss and electrical resistance, midlevel Al-content mirror layers near the cavity for deep oxidation to reduce capacitance, and a redesigned lens-like aperture for reduced mode volume. This latter design gave record modulation bandwidth and efficiency results then, and it is still being used around the world for the leading results today. In the most recent decade, InGaAsSb/AlGaAsSb/GaSb materials for VCSELs and photonic ICs have been studied for emission in the 2–4 μm wavelength range.

Pitch-rotational manipulation of single cells and particles using single-beam thermo-optical tweezers.

3D pitch rotation of microparticles and cells assumes importance in a wide variety of applications in biology, physics, chemistry and medicine. Applications such as cell imaging and injection benefit from pitch-rotational manipulation. Generation of such motion in single beam optical tweezers has remained elusive due to the complexities of generating high enough ellipticity perpendicular to the direction of propagation. Further, trapping a perfectly spherical object at two locations and subsequent pitch rotation hasn't yet been demonstrated to be possible. Here, we use hexagonal-shaped upconverting particles and single cells trapped close to a gold-coated glass cover slip in a sample chamber to generate complete 360 degree and continuous pitch motion even with a single optical tweezer beam. The tweezers beam passing through the gold surface is partially absorbed and generates a hot-spot to produce circulatory convective flows in the vicinity which rotates the objects. The rotation rate can be controlled by the intensity of the laser light. Thus such a simple configuration can turn the particle in the pitch sense. The circulatory flows in this technique have a diameter of about 5 μm which is smaller than those reported using acousto-fluidic techniques.

Simultaneous trapping of low-index and high-index microparticles using a single optical fiber Bessel beam

We propose and experimentally demonstrate a fiber-based Bessel optical trap for the simultaneous trapping of both high-refractive-index particle and a low-refractive-index particle. We obtain the Bessel beam by incident 980 nm beam into the designed integrate fiber structure, which is fabricated by coaxial splice a section of step-index multimode fiber to the single mode fiber. A high refractive index glass microsphere, which is stuck on the tip of multimode fiber, is utilized to converge the Bessel beam to produce a bright focal spot and a dark cage along the axial direction. The high-refractive-index particle and the low-refractive-index particle are trapped in the bright focal spot and the dark cage, respectively. Our simulation verifies the feasibility that particles with high refractive index and low refractive index can be trapped and manipulated at the same time. The proposed optical trap which can simultaneously trap the particles with different refractive index makes it easier to manipulate cells or molecules with different properties and explore the multi-molecule interaction, which can facilitate the research related to biology and chemistry.

Continuous-variable entangled states of light carrying orbital angular momentum

The orbital angular momentum of light, unlike spin, is an infinite-dimensional discrete variable and may hence offer enhanced performances for encoding, transmitting, and processing quantum information. Hitherto, this degree of freedom of light has been studied mainly in the context of quantum states with definite number of photons. On the other hand, field-quadrature continuous-variable quantum states of light allow implementing many important quantum protocols not accessible with photon-number states. Here, we realize a scheme based on a $q$-plate device for endowing a bipartite continuous-variable Gaussian entangled state with nonzero orbital angular momentum. We then apply a reconfigurable homodyne detector working directly with such nonzero orbital angular momentum modes in order to retrieve experimentally their entire quantum-state covariance matrix, thus providing a full characterization of their quantum fluctuation properties. Our work is a step towards generating multipartite continuous-variable entanglement in a single optical beam.

Multi-target CW interferometric acoustic measurements on a single optical beam.

We present a free-space, continuous-wave laser interferometric system capable of multi-target dynamic phase measurement at acoustic frequencies up to a Nyquist bandwidth of 10.2 kHz. The system uses Digitally-enhanced Heterodyne Interferometry to range gate acoustic signals simultaneously from multiple in-line reflections while isolating coherent cross-talk between them. We demonstrate sub-nanometer displacement sensitivity across the audio band for each individual reflection surface and 1.2 m resolution between successive surfaces. Signals outside the 1.2 m range-gate of the system were suppressed by greater than 30 dB in amplitude, enabling high fidelity independent acoustic measurements.

Vortex avalanche in the perturbed singular beams.

We have theoretically and experimentally considered transforming a single optical vortex beam into the vortex avalanche caused by weak local perturbations of the holographic grating responsible for the beam shaping. The vortex avalanche is accompanied by a sharp change of the orbital angular momentum (OAM) so that its dependence on the holographic grating perturbations forms the OAM spectrum. We revealed that the restored vortex beam has anomalous regions of the OAM spectrum in a form of sharp dips and bursts (resonances) that occur when the integer perturbation parameter coincides with a topological charge of one of the vortex modes. We found also that the intensity of the perturbed beam is nonuniformly distributed among the vortex modes with positive and negative topological charges. They form two groups of satellites with clearly marked intensity maxima. As the grating perturbation increases, the initial beam intensity is almost completely pumped over into the vortex avalanche with nearly the same energy redistribution among the satellites.

Optical Dipole Trap for Laser-Cooled Lithium-7 Atoms

In this work, we discuss a far-off resonance optical dipole trap for cold lithium-7 atoms. A single optical trapping beam was produced by a continuous-wave fiber laser. Using our experimental data, we obtained important parameters of the trap, such as size of the cold atomic cloud, and evaluate the dipole potential and the rate of the trap losses. Information on temperature is acquired from observation of parametric resonances. We investigate the parametric resonances obtained with strong modulation of the trap potential and record superharmonics. We plan to prepare ultra-cold gas of highly-excited lithium atoms and study interactions between Rydberg atoms.

A common-path polarization-based image-inversion interferometer.

We present a collinear, common-path image-inversion interferometer using the polarization channels of a single optical beam. Each of the channels is an imaging system of unit magnification, one positive and the other negative (inverted). Image formation is realized by means of a set of anisotropic lenses, each offering refractive power in one polarization and none in the other. The operation of the interferometer as a spatial-parity analyzer is demonstrated experimentally by separating even- and odd-order orbital angular momentum modes of an optical beam. The common-path configuration overcomes the stability issues present in conventional two-path interferometers.

Implement Algorithm to Reduce Effect of Snowy Weather in North of Iraq on Fso Signal

Free space optic (FSO) is now a well-established access technology that commonly start using in wireless data transmission because of its large advantage like cost effective and Line-Of-Sight (LOS) permission which lead this technology to be more secure and get own to everyone. While radio frequency (RF) travels using optical fiber through weather atmospheric, large phenomena like (Rain-fog/haze-snow and temperature) works to block laser beam from reaching its receiver. Optical code division multiple access (OCDMA) is a hot topic due to its large advantage such as permitting large number of user to transmit large amount of data using single optical laser beam , but multiple users share single laser beam lead to increase multiple access interference (MAI) which primary deteriorates the performance of OCDMA. Sub-Carrier Multiplexing (SCM) is used in the system to multiplex multiple data stream on single laser beam. Spectral Amplitude Coding (SAC) technique used to enhance FSO signal by allocate own spectrum to each user, this technique generate code called Multi-Diagonal (MD) code that allow zero interaction between user data. This paper focused on the effect of snowy weather in north of Iraq on the FSO signal by using (SCM-SAC-OCDMA) based on multi-diagonal code and determined the distance between the transverse with approach bit error rate (BER) in the receiver side. Based on the system result data can travel through [0.4 - 4] Km as transmission distance under different snow rate with BER between [].

3-dimensional dark traps for low refractive index bio-cells using a single optical fiber Bessel beam.

We proposed and experimentally demonstrated 3-dimensional dark traps for low refractive index bio-cells using a single optical fiber Bessel beam. The Bessel beam was produced by concatenating single-mode fiber and a step index multimode fiber, which was then focused by a high refractive index glass microsphere integrated on the fiber end facet. The focused Bessel beam provided two dark fields along the axial direction, where stable trapping of low refractive index bio-cells was realized in a high refractive index liquid bath. The all-fiber and seamlessly integrated structure of the proposed scheme can find ample potential as a micro-optical probe in in situ characterization and manipulation of multiple bio-cells with refractive indices lower than that of the liquid bath.

Nearly Degenerate Four-Wave Mixing in Single-Mode Fabry–Pérot Laser Diode Subject to Single Beam Optical Injection

In this paper, nearly degenerate four-wave mixing created by pump–probe structure with small frequency spacing and enough high power is demonstrated and discussed by introducing single optical beam into single-mode Fabry–Perot laser diode, in which the injected beam is called the probe wave, whose frequency detuning is controlled within ±50 GHz, and lasing resonance mode such as dominant mode or arbitrarily residual side mode is selected as the pump wave. In the case of dominant mode like the pump wave, conjugate signals around the pump–probe wave are easily observed for positive and negative probe detuning by judiciously tuning injected probe power. While external injection mode is moved towards the side mode selected as the pump wave, newly generated signals only occurred with small positive probe detuning. As the probe wave with positive frequency detuning is close to the pump wave, higher order conjugate signals are gradually grown. Under the condition of side mode as the pump wave, the strongly nonlinear mixing of the pump–probe wave also gives rise to new frequency signals on both sides of the dominant mode with enough high peak level. Against the change of correspondingly initial power and frequency detuning of the probe wave, outcome spectrum, output power, and conversion efficiency of conjugate signals are analyzed in detail, while dominant or one side mode is, respectively, selected as the pump wave.

Development of a compact, fiber-coupled, six degree-of-freedom measurement system for precision linear stage metrology.

A compact, fiber-coupled, six degree-of-freedom measurement system which enables fast, accurate calibration, and error mapping of precision linear stages is presented. The novel design has the advantages of simplicity, compactness, and relatively low cost. This proposed sensor can simultaneously measure displacement, two straightness errors, and changes in pitch, yaw, and roll using a single optical beam traveling between the measurement system and a small target. The optical configuration of the system and the working principle for all degrees-of-freedom are presented along with the influence and compensation of crosstalk motions in roll and straightness measurements. Several comparison experiments are conducted to investigate the feasibility and performance of the proposed system in each degree-of-freedom independently. Comparison experiments to a commercial interferometer demonstrate error standard deviations of 0.33 μm in straightness, 0.14 μrad in pitch, 0.44 μradin yaw, and 45.8 μrad in roll.

Regular oscillations and random motion of glass microspheres levitated by a single optical beam in air.

We experimentally report on optical binding of many glass particles in air that levitate in a single optical beam. A diversity of particle sizes and shapes interact at long range in a single Gaussian beam. Our system dynamics span from oscillatory to random and dimensionality ranges from 1 to 3D. The low loss for the center of mass motion of the beads could allow this system to serve as a standard many body testbed, similar to what is done today with atoms, but at the mesoscopic scale.

Efficient production of an 87Rb F = 2, mF = 2 Bose-Einstein condensate in a hybrid trap

We have realized Bose-Einstein condensation (BEC) of 87Rb in the F=2, m_F=2 hyperfine substate in a hybrid trap, consisting of a quadrupole magnetic field and a single optical dipole beam. The symmetry axis of the quadrupole magnetic trap coincides with the optical beam axis, which gives stronger axial confinement than previous hybrid traps. After loading 2x10^6 atoms at 14 muK from a quadrupole magnetic trap into the hybrid trap, we perform efficient forced evaporation and reach the onset of BEC at a temperature of 0.5 muK and with 4x10^5 atoms. We also obtain thermal clouds of 1x10^6 atoms below 1 muK in a pure single beam optical dipole trap, by ramping down the magnetic field gradient after evaporative cooling in the hybrid trap.

Self-organization in cold atomic gases: a synchronization perspective.

We study non-equilibrium spatial self-organization in cold atomic gases, where long-range spatial order spontaneously emerges from fluctuations in the plane transverse to the propagation axis of a single optical beam. The self-organization process can be interpreted as a synchronization transition in a fully connected network of fictitious oscillators, and described in terms of the Kuramoto model.