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Gentec Electro-Optics offers a great range of power detectors based on silicon or germanium photodiodes for powers up to 750 mW.

If you use the polarizer in the way it was designed, it doesn't really matter, because your eyes are unable to detect polarization anyways. However, it changes if you flip it around. Then the light from a linearly-polarized source such as an LCD display gets first shifted in a color-dependent way, and then, based on this shift, filtered out by the linear polarizer. As a result, the flipped photographic "circular polarizer" applied to linearly polarized light will filter out some colors more and some colors less.

In some cases, additional properties of photodiodes have to be observed, such as linearity of response over a wide dynamic range, the spatial uniformity of response, or the shape of the dynamic response (e.g. optimized for time domain or frequency domain), or the noise performance.

Photodiodecircuit

The quantum efficiency of a photodiode is the fraction of the incident (or absorbed) photons which contribute to the photocurrent. For photodiodes without an avalanche effect, it is directly related to the responsivity <$S$>: the photocurrent is

There is a number of ways you can find out whether you are dealing with a polarizer of a given kind, this is because 1) light reflected off surfaces is partially linearly polarized, 2) polarizers of a certain kind polarize your light and you can use that to compare with your polarizer, and 3) LCD screens emit linearly polarized light. As a result, you can do the following to tell which polarizer you have

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AMS Techno­logies carries an exceptionally broad range of photodiodes (PDs), based on various materials, cooled or uncooled, featuring single devices as well as photodiode arrays or assemblies:

The electronics used in a photodiode-based photodetector can strongly influence the performance in terms of speed, linearity, and noise. As mentioned above, current amplifiers (transimpedance amplifiers) are often a good choice.

In the photovoltaic mode (see the line for a 1-kΩ load resistor), the response is nonlinear. In the photoconductive mode, shown here for a simple circuit with a reverse bias applied through a load resistor, a very linear response is achieved. The same holds for a constant reverse bias (not shown).

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Menlo Systems offers a series of photodetectors for lowest light level signals. From avalanche to PIN photodiodes, you can find the detector that is best for your specific application.

So-called waveguide photodiodes contain an optical waveguide which confines light along its path through the absorbing region. That region can then again be very thin, and nevertheless one can obtain efficient absorption in a short length. By minimizing the length of the active region, one can also minimize the electrical capacitance and reach a very high bandwidth.

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Photodiodes are frequently used photodetectors, which have largely replaced the formerly used vacuum phototubes. They are semiconductor devices which contain a p–n junction, and often an intrinsic (undoped) layer between n and p layers. Devices with an intrinsic layer are called p–i–n or PIN photodiodes. Light absorbed in the depletion region or the intrinsic region generates electron–hole pairs, most of which contribute to a photocurrent. The photocurrent can be quite precisely proportional to the absorbed (or incident) light intensity over a wide range of optical powers.

For a high responsivity of a photodiode, one should have a material with a strong absorption for the optical wavelength of interest. When using a thicker layer for obtaining efficient absorption, one may lose a lot of the generated carriers and therefore still not substantially improve the responsivity.

Apparently, this piece of equipment gets rid of complications a simple linear polarizer causes with (partially) reflective parts inside the camera.

The combination of high bandwidth (tens of gigahertz) and high photocurrents (tens of milliamperes) is achieved in velocity-matched photodetectors, containing several small-area photodetectors, which are weakly coupled to an optical waveguide and deliver their photocurrents into a common RF waveguide structure.

Figure 1 schematically shows the typical design of the photodiode on p–i–n type. Here, one has an intrinsic region between an n-doped and a p-doped region, where most of the electric carriers are generated. Through the electrical contacts (anode and cathode), the generated photocurrent can be obtained. The anode may have a ring shape, so that the light can be injected through the hole. A large active area can be obtained with a correspondingly large ring, but that tends to increase the capacitance, thus reducing the detection bandwidth, and increases the dark current; also, the efficiency may drop if carriers are generated too far from the electrodes.

Sandwich detectors may be used for remote temperature measurements, for example, where you uses the ratio of signals from two photodiodes: the higher the temperature, the higher is the relative amount of radiation at shorter wavelengths.

On one hand, You can distinguish between the two as the linear polarizer cancels light reflected in a surface at the Brewster angle of incidence, i.e, you cannot see the reflexion. The transmission axis should be in the vertical direction. On the other hand, the circular polarizer differentiates circular polarized light that rotates counterclockwise or clockwise direction. Circular polarizers are often used as antireflection coatings in displays screens, as the light impinging in the display is circularly polarized rotating in one direction when previously transmitted by the circular polarizer, but after reflection then rotates in the opposite direction, and the circular polarizer cancels transmission to the observer.

Photodiodecharacteristics

A photodiode is sometimes integrated into the package of a laser diode. It may detect some light getting through the highly reflecting back facet, the power of which is proportional to the output power. The signal obtained can be used, e.g., to stabilize the output power, or to detect device degradation.

Current amplifiers, which are also available as OEM devices, can also have very good noise properties. The relevant figure is the noise-equivalent input current, which can be well below 1 pA/Hz1/2.

However, I have just learned that in photography, the word "circular polarizer" is actually used for a linear polarizer with a quarter wave-plate. In other words, "circular polarizer" in photographic equipment means an optical element which ideally does the following:

Commercially available laboratory current amplifiers help to make power measurements very flexible by providing many different sensitivity settings, and thus a huge dynamic range with low-noise performance, and also possibly a built-in display, adjustable bias voltage and signal offset, adjustable filters, etc.

Using our advertising package, you can display your logo, further below your product description, and these will been seen by many photonics professionals.

The speed (bandwidth) of a photodiode is typically limited either by electrical parameters (capacitance and external resistor) or by internal effects such as carrier transit time in the depletion region. (In some cases, relatively slow diffusion of carriers generated outside the depletion region limits the bandwidth.) The highest bandwidths of tens of gigahertz are usually achieved with small active areas (diameters well below 1 mm) and small absorption volumes. Such small active areas are still practical particularly for fiber-coupled devices, but they limit the photocurrents achievable to the order of 1 mA or less, corresponding to optical powers of ≈ 2 mW or less. Higher photocurrents are actually desirable for suppression of shot noise and thermal noise. (Higher photocurrents increase shot noise in absolute terms, but decrease it relatively to the signal.) Larger active areas (with diameters up to the order of 1 cm) allow for handling of larger beams and for much higher photocurrents, but at the expense of lower speed.

Nevertheless, the second step, the quarter-wave plate, works exactly as described above only for one fixed wavelength of light $\lambda_0$ (this could be e.g. the wave-length of orange light). For other wavelengths (other colors of light), it introduces a slightly different phase shift.

Photodiodeworking principle

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Ultrafast photodetectors from ALPHALAS for measurement of optical waveforms with rise times starting from 10 ps and total spectral coverage from 170 to 2600 nm (VUV to IR) have bandwidths from DC up to 30 GHz. Configurations include free-space, fiber receptacle or SM-fiber-pigtailed options and have compact metal housings for noise immunity. The UV-extended versions of the Si photodiodes are the only commercial products that cover the spectral range from 170 to 1100 nm with a rise time < 50 ps. For maximum flexibility, most models are not internally terminated. A 50 Ohm external termination supports the highest speed operation, while a high impedance load generates large amplitude signals. Applications include pulse form and duration measurement, mode beating monitoring and heterodyne measurements. Balanced photodiodes complement the large selection of more than 70 unique models.

PINphotodiode

In some cases, the electrode structure is made such that it forms an electrical waveguide, where the electric wave can propagate in parallel with the optical wave in the optical waveguide. Such traveling-wave photodiodes can reach a bandwidth well above 100 GHz.

The noise performance of photodiodes can be very good. For high photocurrents, it can be limited by shot noise, although thermal noise in the electronics is often stronger than that. For the detection of very low light levels (e.g. for photon counting), the dark current can also play a role.

Photodiodediagram

Some photodiodes are available in the form of photodiode arrays of one-dimensional or two-dimensional kind. Two-dimensional detector arrays, e.g. for use as image sensors, may be realized with photodiodes or with other kinds of photodetectors.

For substantially increased responsivity, one may either use avalanche photodiodes (see below) or phototransistors; these are based on quite different operation principles.

Some semiconductor materials are intrinsically better suited than others for fast photodiodes. For example, indium gallium arsenide (InGaAs) is particularly suitable because that direct band gap material (in contrast to silicon, for example) exhibits a rather short absorption length, allowing the realization of very thin absorbing layers, in which the photocarriers can be quickly collected. For fast avalanche photodiodes, it is also important to have a low ratio of the impact ionization coefficients for holes and electrons.

I have a polarizer which has CIRCULAR PL written on it. This suggests it is a circular polarizer even though I am supposed to have a linear one.

For a particularly high detection bandwidth in the gigahertz region, advanced photodiode designs are used. For example, some devices contain an optical resonator around the thin absorbing section. In that way, one can achieve efficient absorption and thus a high quantum efficiency despite a rather small thickness of the intrinsic region, as is chosen for reducing the drift time.

Further, we have many interesting case studies on the same page, with topics mostly in fiber optics. Concrete examples cases, investigated quantatively, often give you much more insight!

A circular polarizer is often a combined $\lambda$/4- retardader, and a linear polarizer having its transmision axis at 45 deg with the optical axis of the retardader. A linear polarizer cannot transmit light having the electric field vibrating perpendicular to its transmision axis.

You can learn more about polarization reading Hecht's textbook "Optics, 5th ed" (Pearson education, 2016, or previous editions), or a more especiliazed texts such as that from Goldstein "Polarized ligh" (Marcel Dekker, 2003), as well as the more recent from Jose Jorge Gil and Ravizgor OssiKovski "Polarized light and the Mueller matrix approach" (CRC press, 2016)

In order to avoid that trade-off, one often uses a current amplifier (also called transimpedance amplifier). Such an amplifier, which is typically realized with an operational amplifier (op-amp), keeps the voltage at the diode nearly constant (e.g. near zero, or at some possibly adjustable reverse bias), so that the photodiode's capacitance loses much of its relevance. The residual voltage variations at the photodiode are inversely proportional to the gain of the used operational amplifier. Still, it is good to minimize the input capacitance when requiring a high detection bandwidth; for example, it is better to directly connect a photodiode to the current amplifier, instead of using a long cable connection.

More specific terms: avalanche photodiodes, Geiger mode photodiodes, lateral effect photodiodes, quadrant photodiodes, p–i–n photodiodes, silicon photodiodes, germanium photodiodes, InGaAs and GaAs photodiodes

Photodiodesymbol

The same principle may also be applied with photodiodes made of the same material because at longer wavelengths (closer to the bandgap) the top photodiode will not absorb all light. One again gets a wavelength-dependent ratio of signals from two photodiodes.

In other words, if you ordered a photographic polarizer, it will be most likely either be a linear polarizer, or a linear polarizer with a quarter-wave plate. Both of them should dim and brighten an LCD screen when rotated and used in the designed direction, but you can tell between the two when you flip it. Then the simple linear polarizer will work exactly the same, but the circular one will work different; it should show less dimming/brightening when rotated and possibly some color variations to the light when flipped.

with the quantum efficiency <$\eta$>, the electron charge <$e$> and the photon energy <$h\nu$>. The quantum efficiency of a photodiode can be very high – in some cases more than 95% – but varies significantly with wavelength. Apart from a high internal efficiency, a high quantum efficiency requires the suppression of reflections e.g. with an anti-reflection coating.

EDIT: The meaning of the word "circular polarizer" as I use it in the first part of the answer above is an optical element which filters out the light of a given circular polarization and leaves only the complementary one. (In practice this could be for instance achieved by a combination of a quarter-wave plate, a linear polarizer, and a 3/4-wave plate.)

Photodiodes are available not only as single-segment detectors. There are dual and quadrant photodiodes, which can be used for precision sensing, and also one-dimensional and two-dimensional photodiode arrays. For more details, see the article on position-sensitive detectors.

Photodiodesensor

CSRayzer offers different kinds of photodiodes used in high speed, ultra-low light detection, and laser range finding, LIDAR and free space communications.

I.e., if linearly polarized light of various wavelengths enters the quarter-wave plate, it will emerge from it with various degrees of elliptical polarization (essentially a mix of linear+circular polarizations) from it, dependent on its wavelength.

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Photodiodeapplication

There are so-called sandwich detectors or two-color photodiodes consisting of two (or more) photodiodes in a sequence. The top photodiode is made from the materials with the largest band gap energy and absorbs short-wavelength light while transmitting much of the light with longer wavelengths, which cannot be absorbed. That transmitted light then hits a further photodiode. The ratio of powers detected by the photodiodes depends on the wavelength.

In the simple circuit according to Figure 3, the magnitude of the bias voltage drops with increasing photocurrent due to the voltage drop at the load resistor. While that has little influence on the linearity, it leads to a charging or discharging of the photodiode's capacitance whenever the incident light intensity changes, so that the detection bandwidth is reduced; it may become RC-limited. That introduces a trade-off between detection bandwidth and responsivity: a high bandwidth requires a small load resistor, which leads to a low responsivity and also a higher noise-equivalent power, which is often limited by thermal noise (Johnson noise) of the load resistor.

Even when used in photoconductive mode, photodiodes are usually not understood to be photoconductive detectors, which have a significantly different operation principle.

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A higher responsivity (although sometimes at the cost of lower quantum efficiency) can be achieved with avalanche photodiodes. These are operated with a relatively high reverse bias voltage so that secondary electrons can be generated (as in photomultipliers). The avalanche process increases the responsivity, so that noise influences of subsequent electronic amplifiers are minimized, whereas quantum noise becomes more important and multiplication noise is also introduced.

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