Amplified InGaAs Detector
Chapter 4: Operation
Rev M, September 7, 2021
Page 5
4.3. Bandwidth Filter Adjustment
The PDA10DT also includes an adjustable low-pass filter with settings from
500 Hz to 1 MHz in 8 steps. This filter allows the user to optimize the PDA10DT to
operate at the lowest amount of high-frequency optical and electrical noise. The
filter is adjusted by rotating the filter control knob, located on the side of the unit.
To adjust the filter, follow the steps below:
1. Determine the maximum bandwidth required.
2. Set the filter bandwidth switch setting just above the desired bandwidth.
4.4. Thermoelectric Cooler
The thermoelectric cooler built into the detector is factory set to cool the detector
to -10 °C with a thermistor providing feedback to maintain a constant temperature.
The housing is used as a heat sink and includes a fan to increase the cooling
capacity. It is important to note that the cooling fan will keep the heat sink at room
temperature. Without it, the heat sink will warm up, causing a higher temperature
drop from the heat sink to the detector element, resulting in larger TEC currents.
Without the fan, the TEC current will operate at its limit (~820 mA) and the detector
element will no longer be temperature stabilized. Offsets will increase and
fluctuate, and output noise will increase. For best results do not block, limit airflow
to, or stop the cooling fan. This operation is automatic and requires no input or
adjustment by the user.
4.5. Light-to-Voltage Conversion
The Spectral Responsivity,
(
), can be obtained Spectral Response Curve on
page 15 to estimate the amount of output voltage to expect. The light-to-Voltage
conversion can be estimated by factoring the wavelength-dependent responsivity
of the InGaAs detector with the gain as shown below:
V
out
(V) = Gain
𝑉
𝐴
∗ ℜ(λ)
𝐴
𝑊
∗ Input Power (W)
For terminators with low resistance, <5 k
Ω or 1% error, an additional factor needs
to be included in the above formula. As described above, the output includes a 50
Ω series resistor (R
S
). The output load creates a voltage divider with the 50
Ω
series resistor as follows:
Scale Factor =
𝑅
𝐿𝑂𝐴𝐷
(𝑅
𝐿𝑂𝐴𝐷
+ 𝑅
𝑆
)
Where R
LOAD
is the terminating resistor and R
S
= 50
Ω. For a standard 50 Ω
terminator, the gain will be scaled by 0.5 as shown below:
Scale Factor =
50 Ω
(50 Ω + 50 Ω)
= 0.5
𝑉
𝑜𝑢𝑡
(𝑉) = Gain
𝑉
𝐴
∗ ℜ(λ)
𝐴
𝑊
∗ Input Power (W) ∗ 𝑆𝑐𝑎𝑙𝑒 𝐹𝑎𝑐𝑡𝑜𝑟
