Biomedical Engineering Reference
In-Depth Information
where is the quantum efficiency of the detector, q
e
is the electron charge, !
0
is the
source center angular frequency,
„
is the Planck constant divided by 2,andP is
the integral of the source power spectral density. I
N
corresponds to the overall noise
of the optical detection system and has three significant sources: shot noise (I
SH
),
optical intensity noise or relative intensity noise (I
RIN
), and thermal noise (I
TH
).
Therefore, the total noise current is the sum of squares of each of the noise
contributions and can be expressed as
I
N
D
I
SH
C
I
RIN
C
I
TH
:
(5.29)
However, in the OCT system, the optical detection scheme employed is hetero-
dyne. Therefore, it can be arranged to achieve shot noise-limited optical detection,
whereby the reference arm optical field is much higher than that in the sample
arm. This is often the case in OCT when imaging highly scattering and absorbing
biological specimens. In a shot noise-limited case, the relative intensity noise and
the thermal noise become negligible. So the total noise can be expressed as
D
p
2q
e
I
DC
B;
I
N
D
I
SH
(5.30)
where I
SH
is the root mean square (RMS) shot noise, B is the detection bandwidth,
and I
DC
is the DC photocurrent and is assumed to be entirely due to the reference
arm's optical power. In case of shot noise limit, the overall noise variance can be
expressed as
r
q
e
h
v
0
2
PB
:
I
N
D
(5.31)
Therefore, substituting Eqs. (
5.28
)and(
5.31
) into Eq.
5.27
yields
P
2h
v
0
B
:
SNR
D
(5.32)
Generally, the SNR is quoted as a figure of merit in logarithmic unit and is related
to the power SNR by
SNR
D
10 log
10
:
P
2h
v
0
B
(5.33)
For a single detector, the measurement bandwidth B
D
1= is a function of the
acquisition time detector. Therefore, this expression implies that to increase either
image acquisition speed or image resolution, which increases the measurement
bandwidth, higher source power is required to maintain the SNR performance of
the system.
Search WWH ::
Custom Search