Biomedical Engineering Reference
In-Depth Information
Time bandwidth relation of femtosecond laser
250
x
xxx
1300nm
ooo
1060nm
830nm
200
o
150
x
100
o
x
x
o
50
x
o
x
o
x
x
o
x
o
o
o
0
10
20
30
40
50
Time (fs)
60
70
80
90
100
Fig. 5.8
Pulse width vs. bandwidth of femtosecond lasers
require additional pump laser sources, water cooling, and an experienced operator
to align and maintain them. The ultrashort pulses from these lasers can be coupled
into new tapered [
35
], microstructured [
36
], or ultrahigh-NA fiber [
37
] to achieve
even broader bandwidths and hence higher axial resolution imaging. The bandwidth
and pulse duration of a mode-locked laser can be calculated by
.nm/
D
1:47
10
3
0
.nm/
.fs/
:
(5.34)
Figure
5.8
plots the relation between time and bandwidth. For achieving high
axial resolution, the femtosecond laser should emit less than 20 fs pulses at 800 nm.
Currently, the Ti:sapphire laser, with pulses down to 7 fs, has been commercialized.
With a compact design and low cost, Ti:sapphire lasers at 800 nm with axial
resolution up to 1:2 m in the tissue have been utilized for corneal and retinal
imaging [
38
]. However, at 1 and 1:3 m, the available lasers can emit pulses only
above 50 fs. Especially for the 1:3-m laser, bulkiness and maintenance issues
continue to be problematic, particularly outside of a research laboratory.
5.4.5.5
Supercontinuum Light Source
Supercontinuum (SC) lasers are a new type of light source based on the nonlinear
optical phenomenon. Supercontinuum light is generated by invoking high optical
nonlinearity in a material. Typically, mode-locked pulsed laser sources are used in
the near infrared (1,064 nm) in the nanosecond, picosecond, or femtosecond range,
ensuring high peak powers to drive the nonlinear effect in a material, which “breaks”
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