Page 188 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Stability and Tempco Issues

Stability and Tempco Issues 181

R L1
(a)
+15V
Signal
-
+A Z-inputs Output:
PD1 AD534 10V (Z -Z )
2 1
R L2 (X -X ) + Y1
Y-inputs 1 2
Ref.
-
+A X-inputs
PD2
-15V
10k
10k
(b) Ref. - Output:
Signal + Signal - Ref.
10k
10k
Figure 8.16 Analog division of signal and reference is straightforward,
but limited to about 1 percent precision, even at low frequencies. If
signal and references are equal in magnitude, subtraction can offer
higher precision and greater bandwidth.

-40

-50
-60 One detector
Voltage (dBm) -70 Balanced detectors

-80

-90
-100
Dark
-110

-120
0 250 500 750 1000 1250
Frequency (Hz)
Figure 8.17 Noise spectra of a 1.55 mm diode laser source illuminating just one detector, or
two detectors arranged for equal photocurrents. The shot noise level at this photocurrent is
-98 dBm.

Figure 8.17 shows intensity noise from a 3mW 1.55mm distributed feedback
laser, detected on an InGaAs photodiode and 1MW transimpedance. With 5V
output (5mA photocurrent) the shot noise limit should be 1.27mV/ Hz , or
-98dBm in the 4.6Hz bandwidth used. The traces show the measured low-
frequency noise level with the photodiode masked (lowest curve), and with the

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