Light can be understood as a transverse electromagnetic wave composed of oscillating electric and magnetic fields. The direction of the electric field in a light wave is described by the polarization of the light. Many light sources, including sunlight, halogen lamps, and LED spotlights, are considered unpolarized because the direction of their electric fields fluctuates randomly in time. Laser sources, on the other hand, are usually linearly polarized.

Laser polarizationapp

This property is exploited in devices called birefringent tuners, which narrow the transmitted optical bandwidth and reflect other wavelengths of light from the propagating intracavity laser beam. The tuner is placed at Brewster's angle and then rotated about an axis perpendicular to the surface interface to induce retardation at the narrowed transmission wavelength. The wavelength of interest exhibits only a p-polarized component, resulting in minimal or no reflection losses. In contrast, other wavelengths are delayed differently, become s-polarized, and experience reflection loss. The tilt angle of the tuner affects the shift of the wavelength to minimize power loss and is adjusted on a stand equipped with micrometer screws for precise modifications.

Linearly polarized light

Most modern microscopes have eyepieces with the number of field of view at 20 or 22. The diameters of the field of view are listed:

A single-plate tuner with an incident angle of Brewster's angle is able to produce wavelength-dependent polarization changes through the transmission loss of the s-polarized component of the laser beam. The thicker the tuner, the smaller the free spectral range that defines the spectral interval between two transmission values and provides better spectral resolution. A propagating laser beam experiences slightly different propagation within the filter than its ordinary and extraordinary rays, where the former passes unchanged while the latter is refracted at a certain angle. This results in birefringence and uneven overlapping contributions to the output beam. Rotating the tuner about an axis perpendicular to the surface changes the transmission curve and inevitably changes the direction of the anomalous component of the light.

S-polarization vs p-polarization

Incoherent polarization combining involves sending beams of different polarizations through a polarization beam combiner (PBC) to obtain an unpolarized beam that displays the combined optical power and almost double the brightness of the incident beam. This technique involves a polarized input beam and unpolarized emission and is therefore not suitable for repeatable power scaling. For example, a vertically polarized beam and a horizontally polarized beam can be sent through a thin film polarizer, causing one of the beams to be reflected and the other to be transmitted, with both beams propagating in the same direction.

Linearpolarization

The concept of nonlinear polarization comes into play in the field of frequency-doubled fiber lasers. In some lasers that use Nd: YAG and other nonlinear crystals as the lasing medium, frequency doubling produces a wave with an optical frequency twice that of the pump light.

For example: Diameter of  the field of view (mm) = 20 / 40 = 0.50, where 20 is the field number of eyepiece, and 40 = objective mag.

Incoherent beam combining can be applied to end-pumped solid-state lasers, where the pump light is injected along the direction of the beam rather than transversely. Laser crystals made from materials such as Nd: YAG that can absorb pump radiation in both polarization directions can take advantage of this technology. For example, a diode laser stack consisting of multiple diode strips arranged in a 2D array of emitters to produce multi-kilowatt output power suffers from a lower overall brightness than that of a single diode strip. Combinations of incoherently polarized beams can be applied to the aroma layer to increase brightness, which in turn pumps high-power solid-state bulk lasers.

Unpolarizedlaser

Birefringence describes an optical property that occurs in certain transparent media in which the refractive index depends on the polarization of light, causing the light to become birefringent in the material. This phenomenon can be caused by many factors, including the crystal structure of the medium, inherent or induced stress fields in the material, and the application of additional electromagnetic fields.

Diameter of the field of view (mm) = F / M, where F is the number of field of view (FOV) of the eyepiece, and M is the magnification (mag.) of the objective.

Understanding the polarization of laser light is important for many applications because polarization affects reflection, focusing of the laser beam, and other optical behaviors that affect the final application of the laser. Although most laser sources are linearly polarized, other types of polarization can also be produced, such as circular, elliptical, and radial polarization.

Laserdiodepolarization

Birefringent tuners can also be composed of multiple plates. The extra number of plates allows the laser to operate with minimal target loss. This is because, by increasing the thickness of the plate, the free spectral range decreases and the transmission minimum over the wavelength range moves closer. To further increase reflection losses at unwanted wavelengths, polarizers can be inserted between the plates. This configuration with a certain limiting plate thickness becomes a so-called Lyot depolarizer, which depolarizes incident polychromatic polarized light.

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Also known as polarization coupling, this technique stacks multiple linearly polarized laser beams and can be divided into two subcategories: coherent and incoherent polarization combinations.

In the case of coherent polarization combination, the orthogonal polarization states of two coherent beams are superimposed, resulting in purely linear laser emission. Because of this output, the technique can be repeated multiple times to increase the total power emitted by the laser, which is also known as power scaling. This technology is used in power amplifier laser systems.

Most people use put a transparent plastic ruler under microscope to perform this task. Actually, the microscope itself provides pretty accurate measurements ranging from 0.2 - 10 mm. Based on optical physics, the diameter of the field of view can be reliably derived by a simple formula:

The field area (A) is calculated by . If the cell is used as unit (instead of mm), the total number of cells in the field of view can be derived from the number of cells on the diameter line with the formula. This is an excellent approximation for most of our histological samples.  For example, if 16 positive cells and 28 total cells are on the line across diameter, respectively. Then Positive cells (Pi x 82 =201) divided by total cells number (Pi x 142 = 616) in entire field would be % of positive cells (201/ 616= 32.6%) (Figure).