Operation
Thorlabs’ DET series is ideal for measuring pulsed and CW light sources. The DET02AFC includes a reversed-
biased PIN photodiode, and a bias battery packaged in a rugged housing. The SMA output signal is the direct
photocurrent out of the photodiode anode and is a function of the incident light power (P) and wavelength (
λ
). The
spectral responsivity [
ℜ
(
λ
)] can be obtained from Figure 1 to estimate the amount of photocurrent to expect. Most
users will wish to convert this photocurrent to a voltage (V
OUT
) for viewing on an oscilloscope or DVM. This is
accomplished by factoring in an external load resistance (R
LOAD
). The output voltage is derived as:
V
OUT
= P *
ℜ
(
λ
) * R
LOAD
It should be noted that the load resistor will react with the photodetector junction capacitance (C
J
) to limit the
bandwidth. For best frequency response, a 50
Ω
terminator should be used. The bandwidth (f
BW
) and the rise-time
response (t
R
) can be approximated using the diode capacitance (C
J
) and the load resistance (R
LOAD
) as shown
below:
f
BW
= 1 / (2 *
π
* R
LOAD
* C
J
)
t
R
= 0.35 / f
BW
For maximum bandwidth, we recommend using a 50
Ω
coax cable with a 50
Ω
terminating resistor at the opposite
end of the coax. This will also minimize ringing by matching the coax with its characteristic impedance. If bandwidth
is not important, you may increase the amount of voltage for a given input light by increasing R
LOAD
.
Circuit Schematic
Figure 2
Setup
•
Unpack the optical head and install the battery as described in “Battery Replacement.”
•
(Optional)
Install a Thorlabs TR series ½
″
diameter post into one of the #8-32 (M4) tapped holes, located on
the bottom and side of the sensor, and mount into a PH series post holder.
•
Attach a 50
Ω
coax cable to the output of the DET. Select and install a terminating resistor to the remaining
end of the cable and connect to a voltage measurement device. See the “Operation” section to determine
resistor values. Thorlabs sells a 50
Ω
terminator (T4119) for best frequency performance, a variable
terminator (VT1) for output voltage flexibility, and SMA-to-SMA (CA29xx series) and SMA-to-BNC (CA28xx
series) coaxial cables. Note that the input impedance of your measurement device will act as a terminating
resistor. A load resistor is not necessary when using current measurement devices.
•
Install a fiber source to the DET02AFC photodetector FC/PC connector. Thorlabs recommends cleaning the
end of the fiber each time it is connected with the detector.
15829-S01 Rev D 3/18/2008
Page 2 of 5
Содержание DET02AFC
Страница 5: ...15829 S01 Rev D 3 18 2008 Page 5 of 5...
