Conserving Signal Strength - New Focus 1621 User Manual

162x nanosec revB.fm Page 8 Friday, August 17, 2001 5:51 PM

Conserving Signal Strength

If you have a low-power signal, 50 Ω of transimped-
ance gain can lead to very small output voltages. If you
need to conserve signal strength, it is important to
focus your optical beam onto the photodiode. Adjust-
ing the photodiode position for optimal signal is best
performed as a two-step procedure:
1. Set the load resistance to 10 k Ω or Open .
2. Hook the detector output directly into an oscil-
3. Adjust the photodiode position (in both trans-
4. Select the 50- Ω resistor, or supply your own
To compute the approximate output voltage for a
given input optical power use the relationship
where P
photodetector's responsivity in A/W (see page 6 for
typical responsivities), and G is the load resistance (V/
A). For example, with 10 mW of optical power, a
responsivity of 0.5 A/W, and with the load set to 50 Ω ,
the photodetector will produce an output voltage of
approximately (10 mW) · (0.5 A/W) · (50 V/A) = 0.25 V.
8 • Operation
loscope or other voltage-measurement device
with a large input impedance.
verse and focus directions) for maximum DC
voltage.
For optical signal levels above about 1 mW (at 10
k Ω ), the voltage will saturate. Remember, the out-
put voltage can only rise up to the battery volt-
age—9 V when the battery is fresh. If you use the
1-M Ω input impedance of an oscilloscope, satura-
tion is even easier to reach.
small load resistance suitable to your gain and
frequency-response requirements. Make fine
adjustments to the photodiode position until
the maximum signal is achieved.
V = P
out
is the input optical power in watts, R is the
in
· R · G ,
in
NEW FOCUS, Inc.