How does a Half wave plate work?

A half-wave plate (or any arbitrary-wavelength plate) works on the principle of Birefringence, which is that the crystal in question has a different refractive index for a different polarization of light.

Most materials that you deal with in basic physics and engineering classes are isotropic, meaning that the refractive index is the same no matter which direction the light travels and no matter what its polarization is.

Most wave plates are made with uniaxial crystals — the refractive index for one polarization direction is different from for other polarization directions. They can also be made with biaxial crystals, but those are usually more expensive. If memory serves me correctly, the most common material for wave plates in inexpensive optics is Calcite since it has decent uniaxial birefringence (that is, the axis with a different refractive index is more than just a tiny bit different from the other axes).

The wave plate consists of a crystal that is cut along its optic axes, so that the optic axis (the axis with a different refractive index) is in the plane of the crystal, so that there will be no separate refraction.

One of the meanings of refractive index is relative phase velocity of light — speed of light is equal to vacuum speed of light divided by refractive index — v = c/n.

Any light entering the wave plate at normal incidence will have two polarization components — the component along the optic axis, and the component orthogonal to the optic axis. The component along the axis with higher refractive index will travel slower than the component with a lower refractive index. The axis with lower refractive index is called the fast axis, and the one with higher refractive index is called the slow axis.

If the input light is polarized in a direction that is not exactly along one of the two crystal axes will have one of the polarization components be slower than the other, which changes the relative phase between them through the crystal.

It starts with linear polarization, then the phase retardation changes it to elliptical, then (if it is linearly polarized with equal fast and slow axis components) to circular, then back to elliptical, and finally to linear but rotated to its supplementary angle. This is described with nothing but the maximum components of the two orthogonal polarization directions and the phase difference between the fast and slow polarizations.

The phase difference is accumulated over length.

The phase difference is equal to the distance times the difference between the fast and slow wavenumbers:

phase difference phi = d*(k_slow – k_fast).
k = n * 2*pi/wavelength
(or k_fast uses the lower n, and k_fast uses the higher n).

At a phase difference of 0 it is linear, and at a phase difference of pi/2 it is linear but at a complementary angle to the 0 phase difference. If the magnitudes of the two orthogonal polarizations are the same, then a phase difference of pi/4 then it will be circular polarization.

Half-wave plates will rotate arbitrarily rotate polarizations according to how you rotate the plate — that is, they add a phase difference of pi/2. Quarter wave plates change 45 degree linearly polarized light into circularly polarized light.

This article comes from quora edit released

Selecting the Optimum Pockels Cell for Q-Switching

For Q-Switching applications, the device must be able to withstand extremely high intra-cavity optical power densities. These are usually many times higher than those indicated by the peak output power which is measured outside the cavity because the circulating optical field is enhanced by the cavity Q. In many cases, only those devices based on KD*P will withstand such high intensities, although lithium niobate has been shown to be effective in moderately high power applications around 1μm wavelength.

Next is the choice of longitudinal or transverse field device. This is often dictated by the required aperture of the Pockels cell. Large apertures are more easily obtained in longitudinal devices as the half wave voltage is effectively independent of crystal dimensions. For a transverse field device, the half wave voltage is determined by amongst other things, the ratio of length to aperture (higher being more favorable). The apparent benefits of lower switching voltage are however often outweighed by other factors. In particular, most of the transverse field devices require multiple crystal designs to counter effects of birefringence and sometimes walk-off as well which occur in devices where the beam does not propagate along the optic axis. For most applications, longitudinal cells offer the simplest solution and are thus often preferred.

Miniature 8mm Pockels cell for Q-switchingAnother consideration is safety. Will the Pockels cell be in an exposed position where the user may have access to the device? If so, then great care must be exercised in the choice of interconnect used on the cell. For exposed positions we recommend the use of the EM5XX range where high voltage BNC type connectors are employed. For those units which are embedded within an enclosed section of the laser system, or where other engineering safety controls are in place, a more open style of interconnect may be used such as the simple or stud pin terminals used on the EM508M and EM510M devices. This also allows a more compact packaging and can additionally present lower capacitance to the Pockels cell driver.

This article comes from eoc-inc edit released

MWIR Camera Lens – Infrared Optics with Low Distortion

MWIR camera lens are a family of midwave infrared lenses specifically designed to operate in the 3-5 ?m wavelenght region with InSb FPA Focal Plane Arrays. The lenses offer a bayonet standard interface or, alternatively, they can be equipped with a custom mount interface at no additional cost.

In the design of the lenses, great importance was attached to a good image quality and a large aperture (small F-number).

These lenses, mounted on a MWIR camera lens, are the perfect choice for a variety of applications, including imaging through fog, high-speed thermal imaging, thermography, R&D (MWIR range), non-destructive testing.

This article comes from stemmer-imaging edit released

Photon amplification, emission observed in plastic scintillation materials

A research team has observed, in polystyrene-based scintillation materials, photon amplification and emission that cannot be explained with the established scintillation mechanism. Photon yield from the polystyrene-based scintillation materials was found to increase in accordance with a power-law of concentration of fluorescent molecules doped in polystyrene.

The results of this research can have a variety of applications such as environmental background radiation / radiation measurements and elementary particle / atomic nucleus experiments, and it requires new interpretations of the scintillation mechanism in plastic scintillation materials, which are expected to have further applications in the future. It is also expected to help to develop high performance radiation detectors.

The results of this research were published in Applied Physics Letters, the weekly journal of the American Institute of Physics, on December 28, 2012.

Plastic scintillation materials made by doping fluorescent molecules to plastics have been used in a wide variety of applications in order to obtain high sensitivity in detecting radiation. The scintillation mechanism of these materials was established in the middle of 20th century and it is explained with the so-called “ladder”, in which ultra violet light emitted from plastics by radiation is converted stepwise into visible light by fluorescent molecules. This means that fluorescent molecules whose absorption wavelength overlaps with the emission wavelengths of plastics are required in making plastic scintillation materials.

In order to develop high performance plastic scintillation materials, the team focused on polystyrene, which emits ultra violet light when exposed to ionizing radiation. In a departure from the established prerequisites, para-terphenyl was used as fluorescent molecule despite the fact that its absorption wavelength has a small overlap with the emission wavelength of polystyrene.

In the established scintillation mechanism, light emitted from plastics should be absorbed by fluorescent molecules and the light should be attenuated every time when it is re-emitted. In addition, when the relation between emission wavelength of polystyrene and absorption wavelength para-terphenyl is taken into account, photon yield emitted from polystyrene-based scintillation materials should be smaller than photon yield emitted from polystyrene having no fluorescent molecules added.

However, the photon yield emitted from the polystyrene-based scintillation materials is found to be more than that from polystyrene having no fluorescent molecules added. In other words, the birth of new luminescence was shown. Furthermore, it is clearly shown that photon yield from the scintillation material increases in accordance with a power-law of fluorescent molecule concentration (which is changed to the maximum of 5-orders). These phenomena cannot be explained with the present scintillation mechanism of plastic scintillation materials and requires new interpretations.

The product of this research will lead the way to the improved performance of radiation detectors which use a plastic scintillation material is used for such applications as natural environmental radiation / radiation measurements and elementary particle / atomic nucleus experiments.

This article comes from phys edit released

Lithium Tantalate Wafers

Lithium Tantalate Wafers (LT/LiTaO3) has been recognized as an ideal substrate material for the RF surface acoustic wave (SAW) filter, a key component within wireless communication devices as mobile phones. Its stability and size make it the preferred option among other piezoelectric materials.

Lithium Tantalate Wafers exhibits unique electro-optical, pyroelectric and piezoelectric properties combined with good mechanical and chemical stability and , wide transparency range and high optical damage threshold. This makes LiTaO3 well-suited for numerous applications including electro-optical modulators, pyroelectric detectors, optical waveguide and SAW substrates, piezoelectric transducers etc.

Lithium tantalate Wafers (LiTaO3), has properties which make it useful for SAW (surface acoustic wave) devices and, to a different specification, for optical use. Wafers can be produced with varying properties, such as “black” wafers free of pyroelectric discharge, 4″ (100mm), orientation 36°, 39°, 42 °, 46°, 48°, X-CUT detailed specifications, or with higher physical strength to withstand processing during manufacture, resulting in higher yields.

This article comes from roditi edit released

Round Thin Film Laser Polarizers

Thin Film Laser Polarizers separate the s- and p-polarization components. They are designed for use in high energy lasers. Due to high damage threshold, reaching 10 J/cm2 @ 1064 nm 8 ns, Thin Film Laser Polarizers are used as an alternative to Glan Laser Polarizing Prisms or Cube Polarizing Beamsplitters.

Typical applications are intracavity Q-switch hold-off polarizer or extracavity attenuator for Nd:YAG lasers.

Thin Film Laser Polarizers can be used at an > 40° angle of incidence, but polarization is most efficient and appears in a broad wavelength range at 56° AOI (Brewster angle). Typical polarization ratio Tp/Ts is 200:1.

Standard size is up to Ø50 mm (2”), while max. available dimensions are 100×200 mm. For optimal transmission a Thin Film Polarizer should be mounted in an appropriate holder for angular adjustment.

This article comes from eksmaoptics edit released

Infrared Optics Lenses with Large Field of View

Opto Engineering LWIR is a family of longwave infrared Optical lenses specifically designed to operate in the 8-14 µm wavelenght region with uncooled detectors (a-si, vOx). The infrared Optical lenses can be equipped with any custom mount interface at no additional cost.

In the design of the infrared Optical lenses, great importance was attached to a good image quality and a large aperture (small F-number).

These infrared Optical lenses, mounted on an uncooled LWIR camera are the perfect choice for a variety of applications spanning from industrial to military, including temperature measurement for process quality control and monitoring, predictive maintenance, imaging through smoke and fog, medical imaging.

This article comes from stemmer-imaging edit released

A Small-Animal PET System Based on LYSO Crystal Arrays

A positron emission tomography system for small animals has been designed for research purposes, and developed at Instituto de Fisica, UNAM, Mexico. Its detection modules are based on pixelated scintillator LYSO crystal arrays coupled to Hamamatsu H8500 position-sensitive photomultiplier tubes.

The front-end electronics are based on nuclear instrumentation modules (NIM) and in-house built readout circuits. Peak signal digitization is performed with a commercial analogue-to-digital acquisition (DAQ) board. The system has been characterized for spatial, timing and energy resolution, system dead time, absolute sensitivity, scatter fraction and noise equivalent count rate (NEC).

The results indicate that the detection modules are able to identify individual crystals (out of 400) with up to 8-to-1 peak-to-valley ratios with individual crystal energy resolution ranging from 7 to 15% at 511 keV. The timing resolution is 1.9 ns and the system dead time was found to be 16.8 s and 42.1 s for 0.5 ml and 10 ml volume sources, respectively. The measured absolute system sensitivity is 0.11% and the scatter fraction from a glass capillary inside a 2.5 cm diameter mouse phantom is 21.5%. A true NEC maximum value was not achieved with the system due to saturation of the PS-PMT output signals for activities above 0.27 mCi. Results from a Na18 F PET bone scan of a 30 g mouse are shown.

This article comes from ieeexplore edit released

Optical Grade Sapphire Windows

Sapphire Windows are manufactured from single crystal sapphire, making them ideal for demanding applications (such as laser systems) because of their extreme surface hardness, high thermal conductivity, high dielectric constant and resistance to common chemical acids and alkalis. Sapphire is the second hardest crystal next to diamonds and, because of their structural strength, sapphire windows can be made much thinner than other common dielectric windows with improved transmittance. Chemically, sapphire is single crystal aluminum oxide (Al2O3) and is useful in a transmission range from 0.2 – 5.5μm.

  • Features Extreme Surface Hardness Chemical Resistance
  • Transmits Wavelengths Ranging From UV to Mid-Infrared
  • Thinner and Stronger than Standard Glass Windows

This article comes from edmundoptics edit released

Radiation Detection with Plastic Scintillators

Background

It has been established opinion since the 1950s that organic crystals and liquid scintillators can work for detecting neutrons, but that plastics are not suitable for neutron detection.

For years, plastic materials have been used in large, low-cost detectors for portals and high-energy physics facilities, and while they could detect neutrons and gamma rays, they have been incapable of distinguishing one from the other, which is key to identifying nuclear substances such as uranium and plutonium from benign radioactive sources.

Organic crystals serve as one of the best neutron detectors, but the crystals can be difficult to grow and obtain in large volumes. Liquid scintillators present some hazards that hinder their use.

Gas detectors that rely on helium-3, a byproduct of tritium’s radioactive decay, have run into problems because the United States now produces markedly less tritium.

However, plastics have more flexibility in their composition and structure than crystals, as well as having none of the hazards associated with liquid scintillators.

Description

Scientists at Lawrence Livermore National Laboratory have developed a plastic that can detect neutrons, something previously thought impossible.

Livermore scientists demonstrated a plastic scintillator that can discriminate between neutrons and gamma rays with a polyvinyltoluene (PVT) polymer matrix loaded with a scintillating dye, 2,5-diphenyloxazole (PPO). They have found that plastic scintillators have a roughly 20 percent finer resolution for neutron-gamma ray discrimination than liquid scintillators.

Advantages

New plastic that detects neutrons could be far cheaper and more flexible than traditional detectors.

Given the material’s low cost, huge plastic sheets could be formed easily into dramatically larger surface areas than other neutron detectors currently used and could aid in the protection of ports, stadiums and other large facilities.

The plastic scintillators may turn out to be best for detecting neutrons once the factors of usage in the field, cost, and performance are taken into consideration

Potential Applications

Plastic scintillators could assist in detecting nuclear substances such as plutonium and uranium that might be used in improvised nuclear devices. As well as help in detecting neutrons in major scientific projects.

This article comes from ipo edit released