Building Hyperspectral Light Sources
Thilo Krätschmer, Joachim W. Walewski, and Scott T. Sanders
Instead of wavelength sweeps, here we use a Fourier approach (CW c-FTS): all colors
always on, each modulated at a unique frequency. In principle, drawbacks 1-9 are eliminated.
Recent hyperspectral sources developed:
Short, spectrally broad pulse dispersed by long fiber: ‘chirp’
wavelength [nm]
30
transmitted power [W]
18-km fiber
60
cell contents:
100% N
10,000
averages
25
2
98% N
2
2% C H
20
2 2
15
10
5
0
0
20
40
60
80
100
time [ns]
40
FWHM = 600 ps
20
0
0
1
2
3
4
5
time [ns]
drawbacks: 4, 6, 7, 9
absorbance
LED output power [mW]
1550 1540 1530 1520 1510 1500
1.2
0.8
10,000
averages
0.4
0.0
1505 1510 1515 1520 1525 1530 1535 1540
wavelength [nm]
These sources share the
following advantages:
1.
2.
Fourier Transform of Time Traces…
Setup, Ring Cavity with Grating Compressor
Inexpensive, for lasers (<$25k)
Wavelength = a consistent f(time)
But all suffer from one or more of
the following drawbacks:
1.
2.
3.
4.
5.
6.
7.
8.
9.
Solution: Control intracavity programmable
spectral filter to broaden spectral coverage:
Moving parts
One scan direction superior to other
Unreliable at rapid sweep speeds
Wavelength sweeps generally too fast
Undesirable polarization effects
Unsuitable outside of telcom bands
Sweep rates constrained
Spectral resolution unstable
Spectral resolution = f(wavelength)
Intracavity-dispersion mode-locked laser: ‘dispersion locker’ Yamashita et. al., CLEO 2006
Setup, Ring Cavity with Optical
Programmable Profiler, Closed Loop Control
Picture of Setup
… Agrees with Spectrometer Data
Max. hold optical spectra
SOA
Challenge: Semiconductor gain media like our
linear optical amplifier (LOA) tend to cover
only ~ 8 nm
Comparison of Spectra:
Output temporal waveform (5 kHz)
~250 MHz
DCF
Synthesizer
20mV/div
Results
Amp
PC
drawbacks: 2, 3, 7, 9
90%
10%
50s/div
Isolator
Output
Fourier domain mode locked ‘scan amplifier’ Huber et. al., CLEO 2006
Time Trace I and Io of CW c-FTS
•
•
•
•
CW c-FTS spectrum recorded in 1 ms.
Required data acquisition bandwidth: 15 MHz
With improved laser stability, expect total
acquisition times limited only by Fourier
principles
With broader spectral coverage ~ 50 nm, the
source is suitable to many sensing and
imaging applications
drawbacks: 1, 2, 5, 6, 7, 8
University of Wisconsin Engine Research Center
Next steps:
•
•
Try version based on fiber Raman amplification
rather than LOA gain medium. Raman ring should
offer improved comb stability.
Perform more simulations to forecast performance
limits of CW c-FTS.