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.

Source (with permission to reproduce): http://optodiode.com/resources.html

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.

Source (with permission to reproduce): http://optodiode.com/resources.html

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.

Source (with permission to reproduce): http://optodiode.com/resources.html

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.

Source (with permission to reproduce): http://optodiode.com/resources.html

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.

Source (with permission to reproduce): http://optodiode.com/resources.html

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).

Temperature Parameter InGaAs (std.) InGaAs (ext.) PbS PbSe
300°K λ range (µm) 0.7 – 1.7 1.2 – 2.6 1.0 – 3.0 1.0 – 5.5
300°K λ pk. (µm) 1.5 2.3 2.2 3.8
300°K τ (µS) 0.1 0.1 200 3.0
300°K D*λpk (cm√Hz/w) 5 x 1012 5 x 1010 1 x 1011 3 x 109
300°K Mode PV PV PC PC

 

Temperature Parameter InGaAs (std.) InGaAs (ext.) PbS PbSe
243°K λ range (µm) 0.7 – 1.7 1.2 – 2.6 0.8 – 3.5 1.0 – 5.5
243°K λ pk. (µm) 1.5 2.3 2.5 4.0
243°K τ (µS) 0.001 0.1 1000 10
243°K D*λpk (cm√Hz/w) 3 x 1013 2 x 1011 2 x 1011 1.5 x 1010
243°K Mode PV PV PC PC

(PV = photovoltaic, PC = photoconductive)

Source (with permission to reproduce): http://optodiode.com/resources.html

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.

Source (with permission to reproduce): http://optodiode.com/resources.html

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.

Source (with permission to reproduce): http://optodiode.com/resources.html

How does the detector operating temperature impact the detector characteristics?

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

Source (with permission to reproduce): http://optodiode.com/resources.html

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.).

Source (with permission to reproduce): http://optodiode.com/resources.html

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.

Source (with permission to reproduce): http://optodiode.com/resources.html

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.

Source (with permission to reproduce): http://optodiode.com/resources.html

What should the load resistor value be set at?

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

Source (with permission to reproduce): http://optodiode.com/resources.html

What are PC detectors?
Photoconductive Devices (PC) are detectors based on the photoconductive effect. Infrared radiation generates charge carriers in the semiconductor active region decreasing its resistance. The resistance change is sensed as a voltage change by applying a constant current bias. The optimum bias current is specified in the Final Test Report and depends on the detector size, operating temperature and spectral characteristics. The PC detectors we offer comes with different spectral response.

Source (with permission to reproduce): https://vigo.com.pl/en/support/faq/

What is an immersion lens and how do they impact measurement of IR radiation?

Optical immersion is achieved by using high refractive index microlenses in order to improve performance of the devices but may limit acceptance angle. Optical immersion is a monolithic integration of a detector element with hyperhemispherical microlens (basic configuration) that makes optical linear size of detector 11 times larger compared to its physical size. This results an improvement of D* by one order of magnitude and electric capacitance by a factor of two orders of magnitude less compared to a conventional detector of the same optical area. We are glad to help you further to determine if you’re not sure if you should use an immersion lens in your system or not.

Source (with permission to reproduce): https://vigo.com.pl/en/support/faq/

How do thermoelectric coolers (TEC) work?

Operation of thermoelectric coolers is based on Peltier effect. Two-, three- and four-staged thermoelectric cooler are available. TEC is biased with DC current supply. The parameters of TEC depend on temperature of the hot side of cooler. It is typically specified for 300K.

Source (with permission to reproduce): https://vigo.com.pl/en/support/faq/

What are standard TEC parameters
Parameter
/Cooling
-2TE -3TE -4TE
Tdet [K] ~230 ~210 ~195
Vmax [V] 1.3 3.6 8.3
Imax [A] 1.2 0.45 0.5
Qmax [W] 0.36 0.27 0.28
ΔTmax [K] 92 114 125

Source (with permission to reproduce): https://vigo.com.pl/en/support/faq/

What types of windows can I choose from for my detector?

Our supplier VIGO offers thermoelectrically cooled detectors which are typically provided with:

  • 3° wedged Al2O3 windows (wAl2O3)
  • 3° wedged ZnSe AR coated windows (wZnSeAR)

3° wedge prevents “fringing” – interference from stray back reflections and there is also windows optimized for different spectral bands. Windows can be anti-reflection (AR) coated on two surfaces. We can mount windows provided by the user as well as filters.

VIGO has summarized their detector windows in the figure below.Source (with permission to reproduce): https://vigo.com.pl/en/support/faq/

What temperatures can III-V materials withstand?

There is a possibility to cool it down to liquid helium temperature. As for the opposite side, we can heat it up to 200°C what makes it convenient for some of the applications.

Source (with permission to reproduce): https://vigo.com.pl/en/support/faq/

What is the lifetime of VIGO’s detectors?
Lifetime strongly depends on temperature of storage and temperature of operation. At normal temperatures lifetime exceed 5 years but many customers are using detectors for more than 10 years in laboratory applications. Usually at field applications, mechanical wear-out of optoelectronic system is main reason for end-of-life replacements.
Source (with permission to reproduce): https://vigo.com.pl/en/support/faq/

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