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Photodetectors - Frequently Asked Questions

1. Why am I not seeing a signal on my oscilloscope or spectrum analyzer?

a. The incident light on the photodetector may be insufficient to generate a signal. For our non-amplified photodetectors we recommend a minimum of 1 mW of incident power and for our amplified photodetectors we recommend a minimum incident power of 50 µW.

b. The photodetector may not be suitable for the wavelength being observed. Check the spectral response curve for the photodetector to determine if the wavelength range is suited for your application.

c. The photodiode may be damaged. Visually inspect the photodiode and look for any signs of burning around the photodiode.

d. If you are working with a battery-biased photodetector, it may be possible that the batteries need replacing. Take the top plate off the photodetector and use a volt meter to measure the voltage. If it is less than the specified bias voltage, it may be necessary to replace the batteries. EOT's battery biased photodetectors use 3 V Duracell DL2430 batteries.

e. Make sure that you are using a DC-coupled 50 Ω termination.

f. The signal may be off the visible portion of the oscilloscope. Adjust your oscilloscope to see if this is the case.

g. The laser may not be aligned with the active area. The active area may not actually be behind the center of the window, so sweep the area in the vicinity of the center.

h. See question #7.

2. Why does the output voltage on my oscilloscope not increase when I increase the incident power on the photodetector?

The photodetector may be saturated. Try using the photodetector with a lower incident power. As a general rule of thumb, the saturation of the photodetectors occurs at output voltages that are about 1/3 that of the bias voltage.

3. Why does it appear that my pulses are being broadened or the fall time of the photodetector is not meeting its specification?

a. If you have a battery-biased photodetector, it may be a sign that the batteries are depleting. Remove the top plate on the photodetector and measure the voltage of the batteries, compare it with the bias voltage specification for the photodetector.

b. This problem can be seen with long pulse applications (>1 µs), particularly if the rep rate of the laser is high (>1 kHz). Basically, the decoupling capacitor in the photodetector does not have enough time to recharge between pulses. EOT recommends that you decrease the incident power.

c. If you are not using a 50 Ω termination the fall time will increase. If you are using AC coupling, the photodiode will not be biased.

4. Why am I seeing ringing from the photodetector?

a. This may be caused by an impedance mismatch. All EOT photodetectors are designed to be terminated into 50 Ω. Check your oscilloscope to determine if the 50 Ω termination is being used.

b. Ringing can also be caused by a coaxial cable that is too long. EOT recommends that coaxial lengths be <4 feet.

5. Can I use my EOT photodetector with an LED?

Generally speaking, no. The output of an LED is not collimated enough, nor does it place enough photons incident on the photodetector to generate a signal. This typically is true of both the non-amplified and amplified photodetectors. Larger area diodes may work.

6. Can I measure power with my EOT photodetector?

This is possible with our photodetectors having larger active areas. The ET-2040 and ET-2070 can be used for this application. Please visit our Photodetector Technical Notes on this website to reference our Power Conversion Calculator.

7. Why am I measuring a DC output from my photodetector when no optical input is present?

The photodiode is probably shorted, allowing all or some of the bias voltage to appear at the output of the photodetector. A shorted diode results from too much optical power.

8. Can I order custom photodetectors from EOT?

Yes. EOT can typically design custom photodetectors for your application. Please contact EOT for pricing and information on custom photodetectors.

9. Most of your photodetectors are free space. What if I want a fiber-coupled photodetector?

Most of our photodetectors can be purchased with an FC receptacle upon request. Some models are also available with permanently attached fiber. Please contact EOT for pricing and information.

10. What happens if I use a termination other than 50 Ω?

A higher termination will give you a proportionately higher voltage output (voltage output = photodetector current x termination resistance). But, the frequency response of the photodetector will decrease by an approximately proportionate amount. This is because the RC time constant of the termination resistance and photodiode junction capacitance determines the response time of the photodetector. This could also cause ringing.