FAQ – Photonics

The most Frequently Asked Questions (FAQ) we get about Photonics. Let us know if there is any question not found below which you would like to have the answer to.

The detector is taking too long to respond/time constant is too slow

The first step when this issue comes up is to check the source intensity. If the intensity of the source is too high (beyond 1mW/cm2), the detector material will heat up to a point that changes the detector characteristics. This change forces the detector to take longer than it originally took to take a measurement since essentially the detector is re-sensitized which changes the photoconductive properties of the detector. The level of impact on the detector performance depends on what the ratio is of the heated/changed area to the entire detector surface area.

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Are the detectors linear?

Typically, when we talk about linearity we are referring to responsivity linearity. When the film runs out of free carriers that would be forced into conduction by incoming photons, the detector is pushed into the non-linear region. Incoming photons do not increase the detector response anymore. Please refer to technical note titled ‘Linearity’ for a graph.

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What is the maximum temperature that the detector surface can reach and still maintain it photoconductive properties?

The maximum rated element temperature for PbS detectors is +65°C and for PbSe it is +55ºC. This refers to the actual temperature limits that the detector surface can reach, not temperature that radiates onto the detector. Based on our data PbS detectors are much more sensitive to temperature than PbSe detectors. There is a time vs. temperature relationship when it comes to the element degradation. For example, if the detector reaches +80°C for a few seconds, the heat will not necessarily change the detector characteristics. On the other hand, if it sits at +65ºC for an extended period of time the detector may degrade. As a general rule, we use the above stated temperatures as limits.

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Can detectors be used for pulsed laser profiles or positioning?

Yes, as long as the pulse width of the laser is long enough, the wavelength is in the correct range, and the power density is within limits. Please note the detector time constant of the model you are using/considering. The detector time constants are in the microsecond range. If the pulse width is significantly faster than this range, the detector will not have enough time to respond to the laser pulse.

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I am currently using an InGaAs array and would like to look at switching to a PbS array. What is the quantum efficiency of a PbS array?

Typically photoconductive detectors (PbS) are not characterized by quantum efficiency because, theoretically, they are completely efficient (100% QE), unlike photovoltaic detectors (InGaAs). A better way to compare PbS and InGaAs arrays is with detectivity (D*), which is really normalized signal to noise ratio. Generally, PbS detectors have slightly better detectivity than extended InGaAs detectors. However, if you are looking at shorter wavelengths (<1.7µm), standard InGaAs tends to have a higher D* value than PbS.

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How do the specifications of PbS/PbSe detectors compare with standard and extended InGaAs?

(Note: These specifications are approximate figures that provide data for a general comparison).

TemperatureParameterInGaAs (std.)InGaAs (ext.)PbSPbSe
300°Kλ range (µm)0.7 – 1.71.2 – 2.61.0 – 3.01.0 – 5.5
300°Kλ pk. (µm)1.52.32.23.8
300°Kτ (µS)0.10.12003.0
300°KD*λpk (cm√Hz/w)5 x 10125 x 10101 x 10113 x 109
300°KModePVPVPCPC

 

TemperatureParameterInGaAs (std.)InGaAs (ext.)PbSPbSe
243°Kλ range (µm)0.7 – 1.71.2 – 2.60.8 – 3.51.0 – 5.5
243°Kλ pk. (µm)1.52.32.54.0
243°Kτ (µS)0.0010.1100010
243°KD*λpk (cm√Hz/w)3 x 10132 x 10112 x 10111.5 x 1010
243°KModePVPVPCPC

(PV = photovoltaic, PC = photoconductive)

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What is detectivity?

Detectivity is defined as the signal to noise ratio of the detector normalized by detector area. As the detector temperature decreases, the detectivity increases. Please refer to Section 4 in the detector technical notes for a graph of detectivity vs. temperature for both PbS and PbSe detectors.

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What is the spectral response of your detector?

Spectral response defines the sensitivity of a detector to radiation at a various wavelengths. PbS detectors are sensitive in the 1-3µm region and PbSe detectors are sensitive in the 1-5.5µm region. Cooling the detector shifts the spectral response of PbS and PbSe detectors to longer wavelengths. Please refer to Section 3 of our detector technical notes for a graph of detectivity vs. wavelength at different temperatures.

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How does the detector operating temperature impact the detector characteristics?

As the detector operating temperatures decreases, the detectivity, responsivity, and spectral response improves.

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Can the detectors be used without a chopper if used to measure the radiation of a pulsed laser beam?

The detectors do not require a chopper. A chopper is often used to modulate the source, allowing band limited amplification of the signal and thereby increasing the signal/noise ratio. If the source is already modulated (a pulsed laser beam) then there is no need to use a chopper. Care must be taken to insure that the laser power on the detector film isn’t high enough to damage the film. (Furthermore, for this application to work, the pulse must be within the response time of the detector.).

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What is the typical frequency response of PbS/PbSe detectors?

The detector has a 1/f noise characteristic, so the lower frequencies have higher noise and consequently a lower D* value. The signal level stays relatively constant as the chopping frequency is increased, up to the detector response limit. Therefore, if the system is not band limiting the noise measurement (wide band amplification) the Signal/Noise (D*) will be constant below the response limit. If the noise reading is band limited around the chopping frequency, D* will increase up to the point that the detector response starts to roll off because of its time constant. For PbSe detectors, 1 kHz is chosen as a standard chopping frequency because it is away from the 1/f noise but lower than the time constant response limit. You can find a graph of the typical frequency response of the detectors in section 8 of our technical notes.

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What are some recommendations for the bias circuit?

For our testing, we use a voltage divider bias circuit with a load resistor set at either 1 Megohm or set at a resistance equal to the dark resistance of the detector. The opamp can be nearly anything with a high input impedance. We recommend an FET input op amp which typically have an input resistance of approximately 10^12 ohms and work well. Please see our detector technical notes section for a diagram of a simple test bias circuit.

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What should the load resistor value be set at?

The load resistor value should be selected to match the detector dark resistance.

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