Improving the Optical Quality Factor of the WGM Resonator

New iterative annealing and polishing increases the resonator’s finesse over the fundamental limit.

Resonators usually are characterized with two partially dependent values: finesse (F) and quality factor (Q). The finesse of an empty Fabry-Perot (FP) resonator is defined solely by the quality of its mirrors and is calculated as

F = πR1/2/(1 – R).

The maximum up-to-date value of reflectivity R ≈ 1 – 1.6 × 10–6 is achieved with dielectric mirrors. An FP resonator made with the mirrors has finesse F = 1.9 × 106. Further practical increase of the finesse of FP resonators is problematic because of the absorption and the scattering of light in the mirror material through fundamental limit on the reflection losses given by the internal material losses and by thermodynamic density fluctuations on the order of parts in 109. The quality factor of a resonator depends on both its finesse and its geometrical size. A one-dimensional FP resonator has Q = 2 F L/λ, where L is the distance between the mirrors and λ is the wavelength. It is easy to see that the quality factor of the resonator is unlimited because L is unlimited. F and Q are equally important.

Posted in: ptb catchall, Tech Briefs, Briefs, Photonics, Mirrors, Performance upgrades

Ultra-Stable Beacon Source for Laboratory Testing of Optical Tracking

A prototype laser beacon assembly provides reference for testing tracking and pointing systems.

The ultra-stable beacon source (USBS) provides a laser-beam output with a very low angular jitter and can be used as an absolute angular reference to simulate a beacon in the laboratory. The laser is mounted on the top of a very short (≈1 m) inverted pendulum (IP) with its optical axis parallel to the carbon fiber pendulum leg. The 85-cm, carbon fiber rods making up the leg are very lightweight and rigid, and are supported by a flex-joint at the bottom (see figure). The gimbal-mounted laser is a weight-adjustable load of about 1.5 kg with its center of rotation co-located with the center of percussion of the inverted pendulum. This reduces the coupling of transverse motion at the base of the pendulum to angular motion of the laser at the top.

Posted in: ptb catchall, Tech Briefs, Briefs, Photonics, Lasers, Optics, Lasers, Optics, Composite materials, Mountings, Test equipment and instrumentation

Delaying Trains of Short Light Pulses in WGM Resonators

Delays would not be limited by resonator ring-down times.

Suitably configured whispering-gallery-mode (WGM) optical resonators have been proposed as delay lines for trains of short light pulses. Until now, it has been common practice to implement an optical delay line as a coiled long optical fiber, which is bulky and tends to be noisy. An alternative has been to implement an optical delay line as a coupled-resonator optical wave-guide (a chain of coupled optical resonators), which is compact but limits the width of the pulse spectrum to the width of an optical resonance and thereby places a lower limit on the duration of a pulse. In contrast, a delay line according to the proposal could be implemented as a single WGM resonator, and the pulses delayed by the resonator could be so short that their spectral widths could greatly exceed the spectral width of any single resonance.

Posted in: ptb catchall, Tech Briefs, Briefs, Photonics, Architecture, Fiber optics, Architecture, Fiber optics, Performance upgrades, Acoustics, Acoustics

Creating Patterned Multispectral Filters

In recent years the explosion in demand for multispectral imaging has coupled with the industry’s insatiable need for weight reduction, there-by greatly increasing the demand for more sophisticated approaches to producing optical filters that are used in these systems. One method to meet the challenge of reducing the weight of a multispectral system is to eliminate beam-splitting optics and multiple detectors by patterning a filter array on a single substrate, or directly on the CCD itself.

Posted in: Features, ptb catchall, Articles, Photonics, Imaging, Imaging and visualization, Optics, Imaging, Imaging and visualization, Optics, Coatings Colorants and Finishes, Coatings, colorants, and finishes, Materials properties

Implementing Thermal Bumps in New Product Designs

Thermal issues are dominating today’s electronic product design landscape as never before. It is easy to see this in Intel’s move to a multi-core architecture as a methodology to manage their thermal problems. Of course, less than optimal solutions lead to less than optimal results. Thermoelectric devices (TECs) have been used in the optoelectronics industry for thermal management, but have not found wide-spread acceptance in electronic product design. Thermal management solutions implemented with these active devices, however, offer a broad potential for implementation including the following:

Posted in: Applications, ptb catchall, Application Briefs, Photonics, Electronic equipment, Electronic equipment, Thermal management, Thermal management, Materials properties

NbxTi1–xN Superconducting-Nanowire Single-Photon Detectors

Potential applications include optical communications and quantum cryptography.

Superconducting-nanowire single-photon detectors (SNSPDs) in which NbxTi1–xN (where xerve as the superconducting materials have shown promise as superior alternatives to previously developed SNSPDs in which NbN films serve as the superconducting materials. SNSPDs have potential utility in optical communications and quantum cryptography.

Posted in: ptb catchall, Tech Briefs, Briefs, Photonics, Cryptography, Wireless communication systems, Cryptography, Wireless communication systems, Conductivity, Nanomaterials

Miniature Incandescent Lamps as Fiber-Optic Light Sources

These lamps can be used without coupling optics.

Miniature incandescent lamps of a special type have been invented to satisfy a need for compact, rapid-response, rugged, broadband, power-efficient, fiber-optic-coupled light sources for diverse purposes that could include calibrating spectrometers, interrogating optical sensors, spot illumination, and spot heating. A lamp of this type (see figure) includes a re-entrant planar spiral filament mounted within a ceramic package heretofore normally used to house an integrated-circuit chip. The package is closed with a window heretofore normally used in ultraviolet illumination to erase volatile electronic memories. The size and shape of the filament and the proximity of the filament to the window are such that light emitted by the filament can be coupled efficiently to an optical fiber without intervening optics.

Posted in: ptb catchall, Tech Briefs, Briefs, Photonics, Fiber optics, Optics, Fiber optics, Optics, Fabrication

Prism Window for Optical Alignment

Prism windows could be generally useful in manufacture of optical instruments.

A prism window has been devised for use, with an autocollimator, in aligning optical components that are (1) required to be oriented parallel to each other and/or at a specified angle of incidence with respect to a common optical path and (2) mounted at different positions along the common optical path. The prism window can also be used to align a single optical component at a specified angle of incidence. Prism windows could be generally useful for orienting optical components in manufacture of optical instruments.

Posted in: ptb catchall, Tech Briefs, Briefs, Photonics, Optics, Optics

Range-Gated Metrology With Compact Optical Head

A compact, single-fiber optical head requires minimal internal alignment.

This work represents a radical simplification in the design of the optical head needed for high-precision laser ranging applications. The optical head is now a single fiber-optic collimator with dimensions of order of 1×1×2 cm, which can be easily integrated into the system being measured with minimal footprint. Previous heads were significantly larger, with multiple optical elements requiring careful alignment. The new design has only one optical fiber per head, rather than four, making it much easier to multiplex between tens or hundreds of heads. It is capable of subnanometer precision, consistent with the demanding requirements of new missions.

Posted in: ptb catchall, Tech Briefs, Briefs, Photonics, Downsizing, Fiber optics, Lasers, Fiber optics, Lasers, Parts

Performance of 1mm² Silicon Photomultipliers

A silicon photomultiplier (SPM) is a new type of semiconductor detector that has the potential to replace the photomultiplier tube (PMT) detector in many applications. In common with a PMT detector, the output of an SPM is an easily detectable current pulse for each detected photon and can be used in both photon counting mode and as an analogue (photocurrent) detector. However, the SPM also has a distinct advantage over PMT detectors. The photon-induced current pulse from a PMT varies greatly from photon to photon, due to the statistics of the PMT multiplication process (excess noise). In contrast, the current pulse from an SPM is identical from photon to photon. This gives the SPM a distinct advantage in photon counting applications as it allows the associated electronics to be greatly simplified. Identical pulses also mean that the SPM can resolve the number of photons in weak optical pulses, so-called photon number resolution. This is critical in a number of applications including linear-optics quantum computing.

Posted in: Features, ptb catchall, Articles, Photonics, Imaging, Imaging and visualization, Optics, Imaging, Imaging and visualization, Optics, Semiconductors

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