Page 87 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Interlude: Alternative Circuits and Detection Techniques

80 Chapter Four

(a)
+ V (t)
o
I (t)
-
p

(b) R V (t) (c) LED V (t)
o
o
PD
I (t) I (t)
p
p
(d) (e)
PD LED V (t) DAC ADC V (t)
o
o
1k +
I (t) I (t)
- Microprocessor
p
p
I = V /220
220 LED o
Figure 4.1 Alternative realizations of a resistor for use in transimpedance amplifiers.

called a resistor. Figure 4.1d attempts to correct the nonlinearity of V o (t)/I p (t)
by driving the LED with an operational current source, illuminating the pho-
todiode as before. Figure 4.1e shows another “resistor” embodiment for digital
fans.
The idea of synthesizing a resistor using a source/photodetector pair has been
reinvented several times since about 1971 and can be applied very usefully to
high-performance photodetection systems. Figure 4.2 shows a transimpedance
amplifier with optical feedback. The transimpedance resistor has been replaced
with an optical feedback path. This approach can significantly improve per-
formance where receiver bandwidth and noise performance are limited by the
parasitic capacitance of the feedback resistor. The capacitance and thermal
noise of the feedback resistor have been removed completely. Although the feed-
back photodiode adds to the capacitance at the amplifier input, its effect can be
small. Typically, a very low capacitance chip photodiode whose capacitance will
be swamped by the signal photodiode and input FET gate capacitance will be
used. There is still the dark current to contend with, and both photodiodes will
show the full shot noise of the detected photocurrent. Dark current will be min-
imized through the use of a very small, low-leakage silicon device for the feed-
back, even if the signal photodiode must use ternary semiconductors for
operation beyond 1mm wavelength. In addition, the chip photodiode can often
be physically much smaller than a high-value resistor and can benefit from
reduced microphonic sensitivity and electrical leakage.
The next issue is dynamic range, or the ratio between the highest and the
lowest detectable powers. This is important in many systems with large varia-
tions in transmitter-to-receiver loss such as optical fiber networks that have

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