Bookham New Focus 2051, Руководство пользователя

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14 • Frequency Response and Noise NEW FOCUS, Inc.
Another way to characterize the noise is with the
photocurrent noise (
I
n
), which is related to NEP by
I
n
= R · NEP ,
where R is the photodetector’s responsivity (in A/W).
The photocurrent noise is independent of wavelength
because it gives the noise of the photoreceiver with the
photodetector’s responsivity factored out.
To characterize the noise of the photoreceiver, the
output electrical noise spectrum is measured with a
spectrum analyzer. This voltage noise spectrum is
converted to an equivalent optical photocurrent noise
by dividing the voltage noise by the transimpedance
gain (V/A). The photocurrent noise,
I
n
(ƒ), has units of
pA/ and is plotted in Figure 3 and Figure 4 using
the expression
20·log[In(ƒ)/1 A]
.
Calculating NEP
The noise equivalent power (NEP) can be calculated by
dividing the photocurrent noise by
R
, the detector’s
responsivity (see page 6).
From DC to 150 kHz the average photocurrent noise
for the Model 2051 on the high gain setting is about
0.34 pA/
, corresponding to an average NEP at
900 nm of 0.68 pW/ . The integrated noise
equivalent power from DC to 150 kHz is then obtained
by multiplying the average NEP by
, the square
root of the bandwidth.
The expression
BW = 2
π
ƒ
3-dB
/4
for a one-pole low-
pass filter is useful for calculating the equivalent noise
bandwidth. Using the high-pass filter set 1 decade
below the low-pass cutoff reduces noise-equivalent
bandwidth by approximately 10 %. For the Model 2051
with a 3-dB bandwidth of 150 kHz, the equivalent
noise bandwidth is 235 kHz. This gives an optical noise
equivalent power of about 330 pW, so the minimum
detectable optical signal at 900 nm (with a signal-to-
noise ratio of 1) for the Model 2051 on the highest gain
Hz
Hz
Hz
BW
205x 10MHz Adj Rcvr revA.fm Page 14 Monday, January 10, 2005 9:29 AM
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