Passive Optical Element Research Articles

Passive anti-sunlight glare traffic signs with non-axisymmetric freeform lens design.

This paper proposes a passive optical brightening element design, a non-axisymmetric freeform lens (NAFL), arranged and assembled on a traditional traffic sign. NAFL is the first optical design which can effectively solve the traffic problem that direct sunlight affects the driver's inability to look directly at the traffic sign. The NAFL can converge the sunlight behind the traffic sign and diverge forward to 150 meters away. In this way, the NAFL array combinations on the traffic sign can directly rely on sunlight as image information pixels. According to the simulation, the optical efficiency of the NAFL can be as high as 81.5%. Besides, the angular tolerance is also analyzed to evaluate the working hours of the NAFL. Finally, we made the prototype and proved that such passive brightening components could effectively improve the traffic sign's visibility in harsh sunlight.

Linear bosonic quantum channels defined by superpositions

A minimal energy quantum superposition of two maximally distinguishable, isoenergetic single mode Gaussian states is used to construct the system-environment representation of a class of linear bosonic quantum channels acting on a single bosonic mode. The quantum channels are further defined by unitary dynamics of the system and environment corresponding to either a passive linear optical element U_{BS} or two-mode squeezing U_{TM}. The notion of nonclassicality distance is used to show that the initial environment superposition state becomes maximally nonclassical as the constraint energy is increased. When the system is initially prepared in a coherent state, application of the quantum channel defined by U_{BS} results in a nonclassical state for all values of the environment energy constraint. We also discuss the following properties of the quantum channels: 1) the maximal noise that a coherent system can tolerate, beyond which the linear bosonic attenuator channel defined by U_{BS} cannot impart nonclassical correlations to the system, 2) the noise added to a coherent system by the phase-preserving linear amplification channel defined by U_{TM}, and 3) a generic lower bound for the trace norm contraction coefficient on the closed, convex hull of energy-constrained Gaussian states.

Ultra-wide frequency response measurement of an optical system with a DC photo-detector.

Precise knowledge of an optical device's frequency response is crucial for it to be useful in most applications. Traditional methods for determining the frequency response of an optical system (e.g. optical cavity or waveguide modulator) usually rely on calibrated broadband photo-detectors or complicated RF mixdown operations. As the bandwidths of these devices continue to increase, there is a growing need for a characterization method that does not have bandwidth limitations, or require a previously calibrated device. We demonstrate a new calibration technique on an optical system (consisting of an optical cavity and a high-speed waveguide modulator) that is free from limitations imposed by detector bandwidth, and does not require a calibrated photo-detector or modulator. We use a low-frequency (DC) photo-detector to monitor the cavity's optical response as a function of modulation frequency, which is also used to determine the modulator's frequency response. Knowledge of the frequency-dependent modulation depth allows us to more precisely determine the cavity's characteristics (free spectral range and linewidth). The precision and repeatability of our technique is demonstrated by measuring the different resonant frequencies of orthogonal polarization cavity modes caused by the presence of a non-linear crystal. Once the modulator has been characterized using this simple method, the frequency response of any passive optical element can be determined to a fine resolution (e.g. kilohertz) over several gigahertz.

Four-channel optically pumped atomic magnetometer for magnetoencephalography.

We have developed a four-channel optically pumped atomic magnetometer for magnetoencephalography (MEG) that incorporates a passive diffractive optical element (DOE). The DOE allows us to achieve a long, 18-mm gradiometer baseline in a compact footprint on the head. Using gradiometry, the sensitivities of the channels are < 5 fT/Hz1/2, and the 3-dB bandwidths are approximately 90 Hz, which are both sufficient to perform MEG. Additionally, the channels are highly uniform, which offers the possibility of employing standard MEG post-processing techniques. This module will serve as a building block of an array for magnetic source localization.

Novel roles for the lens in preserving overall ocular health

Outside the traditional roles of the lens as an important refractive element and a UV filter, it was David Beebe’s group that first demonstrated that the lens acts an oxygen sink that protects the tissues of the anterior segment of the eye from oxygen or oxygen metabolites. In this review, we follow on from this work, and present new evidence from our laboratory to demonstrate that the lens serves as a reservoir for the release of the antioxidant glutathione (GSH) into the aqueous humor to provide a source of GSH and/or its precursor amino acids to nearby tissues that interface with the aqueous humor, or to remove toxic metabolites from the eye via the aqueous outflow pathway. In addition to GSH release, our laboratory and others have shown that ATP is released from the lens under hyposmotic conditions to activate purinergic signalling pathways in an autocrine manner to alter lens function. In this review, we raise the idea that ATP and/or its subsequent degradation product adenosine may exert a paracrine function and influence purinergic signalling systems in other tissues to alter aqueous humor outflow. These new secondary roles indicate that the lens is not just a passive optical element, but a highly dynamic and active tissue that interacts with its neighbouring tissues, through modifying the environments in which these tissues function. We believe that the lens actively contributes to the ocular environment and as a consequence, removal of the lens would alter the functionality of neighbouring tissues. We speculate that a long term effect of lens removal may be to inadvertently increase the exposure of anterior tissues of the eye to oxidative stress due to elevated oxygen levels and a reduction in the availability of GSH and purinergic signalling molecules in the aqueous humor. Since cataract surgery is now being performed on younger patients due to our increasing diabetic population, over time, we predict these changes may increase the susceptibility of these tissues to oxidative stress and the incidence of subsequent ocular pathologies. If our view of the lens is correct, the actual loss of the biological lens may have longer term consequences for overall ocular health than currently appreciated.

Refractive and diffractive neutron optics with reduced chromatic aberration

Thermal neutron beams are an indispensable tool in physics research. The spatial and the temporal resolution attainable in experiments are dependent on the flux and collimation of the neutron beam which remain relatively poor, even for modern neutron sources. These difficulties may be mitigated by the use of optics for focusing and imaging. Refractive and diffractive optical elements, e.g. compound refractive lenses and Fresnel zone plates, are attractive due to their low cost, and simple alignment. These optical elements, however, suffer from chromatic aberration, which limit their effectiveness to highly monochromatic beams. This paper presents two novel concepts for focusing and imaging non-monochromatic thermal neutron beams with well-known optical elements: (1) a fast mechanical transfocator based on a compound refractive lens, which actively varies the number of individual lenses in the beam path to focus and image a time-of-flight beam, and (2) a passive optical element consisting of a compound refractive lens, and a Fresnel zone plate, which may focus and image both continuous and pulsed neutron beams.

Steerable pencil beams for multi-Gbps indoor optical wireless communication.

We report a novel optical wireless communication (OWC) system solution that supports multi-Gbps (Gigabit-per-second) capacity for indoors. Narrow beams, termed as pencil beams, are directed to wireless users using a tunable laser and a passive diffractive optical element. This enables a wide coverage of ultra-high-capacity communication links to serve multiple network users simultaneously. Experimental results demonstrating data rates of up to 10 Gbps, with on-off keying modulation format, over a distance of more than 2.5 m, are reported. Error-free links beam-steered over a total wavelength range of 130 nm, with steering angle of 17.16°, have been achieved. This system is proposed for short-range OWC and is promising for seamless integration in in-building optical networks.

Photovoltaic–thermal solar energy collectors based on optical tubes

In this work, we examine the use of oil filled tubular optical concentrators coupled with a model organic bulk heterojunction photovoltaic: poly-3-hexathiophene-[6,6]-phenyl-C61-butyric-acid methyl-ester (P3HT:PCBM) to create a photovoltaic–photothermal hybrid solar collector. The organic photovoltaic cells were fabricated onto one half of a tubular light pipe and then silicone oil was flowed inside the pipe. This allows solar energy in the visible wavelengths to be effectively converted into electricity by photocell while simultaneously; the silicone oil captures the infrared radiation (IR) part of the spectrum as heat energy. The VIS–IR power conversion efficiency for this model organic system, under normally incident AM1.5G illumination was found to be: PCE∼28%, which is combined by the photovoltaic efficiency (PCE∼2%) and the photothermal efficiency (PCE∼26%). We further show that the oil filled tube, acts as a passive optical element that concentrates the light onto the photovoltaic and thereby increases its overall efficiency but also the range of incident angles in which the light is efficiently captured.

Time magnification of electrical signals using chirped optical pulses

The authors report a method for stretching electrical signals in time. A high chirp rate is imposed on the electrical signal by mixing it with a dispersed ultra-short optical pulse in an electro-optic intensity modulator. This is followed by a passive optical dispersion element to produce a time-magnified copy of the input electrical signal.

Simple diffuser for production of laser speckle

A passive optical element that is a simple, inexpensive, and effective way for producing laser speckle is described. The optical element, a random phase plate, was developed in our laboratory. The statistics of the speckle pattern it produces were measured and are discussed in this paper. The phase plate was used to produce a speckled beam using a 5-mW cw He-Ne laser with a Gaussian beam profile and using a pulsed Nd:YAG laser with a nonuniform intensity profile for which the mean intensity was 10(9) W/cm(2).

Passive optical element with selective angular reflection

This work is related to the development of passive selective transmission materials that will contribute to regularize the solar thermal gain. We propose an original solution to the problem of seasonal control of energetic input into buildings through windows. A passive optical element with selective angular reflection is used to solve this problem. This optical element allows sunlight to enter windows during the fall and winter, whereas, owing to the different astronomical path of the sun, it stops and rejects direct sunlight by means of the optical effect called total internal reflection (TIR) during the central spring-summer period. The purpose of this paper is to describe the optical element in some detail, to develop the principal design equations, and give the results of the optimization of optical and geometrical parameters.

Laser Wavelength Stabilization with a Passive Interferometer

A control system has been devised for stabilizing the output wavelength of a laser by reference to an external passive optical element. This element, consisting of two spherical mirrors, forms an off-axis resonator that, when broadly illuminated by a laser beam, functions as a wavelength-sensitive discriminator. The stabilization control loop is closed by using a signal from this discriminator to tune the laser by moving one of its mirrors with a piezoelectric transducer. The error signal is proportional to changes in wavelength of the incident laser beam and is derived from the discriminator without deliberate frequency or amplitude modulation of the laser The optical arrangement does not return any light in the direction of the source and thus avoids wavelength pulling due to spurious reflections. Two independent helium-neon lasers operating at 6328 A were stabilized against a common reference interferometer using this system. Their relative stability was studied by heterodyning the two outputs and analyzing the beat spectrum. Each feedback loop had a gain of 60 dB at dc falling off to 46 dB at 200 c/s and to 20 dB at 2 kc/sec. The resulting wavelength stability was about Deltalambda/lambda = 2 x 10(-10). The residual instability was mainly owing to room noise occurring at frequencies above the response of the feedback loops.