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Building Hyperspectral Light Sources. 20mV/div. SOA. 50 m s/div. ~250 MHz. DCF. Synthesizer. Amp. PC. 90%. 10%. Isolator. Output. Thilo Krätschmer, Joachim W. Walewski, and Scott T. Sanders.

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building hyperspectral light sources
Building Hyperspectral Light Sources




~250 MHz









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’

These sources share the following advantages:

Setup, Ring Cavity with Grating Compressor

Fourier Transform of Time Traces…

Challenge: Semiconductor gain media like our linear optical amplifier (LOA) tend to cover only ~ 8 nm

Solution: Control intracavity programmable spectral filter to broaden spectral coverage:

18-km fiber



Inexpensive, for lasers (<$25k)

Wavelength = a consistent f(time)

But all suffer from one or more of

the following drawbacks:

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)

Setup, Ring Cavity with Optical

Programmable Profiler, Closed Loop Control



drawbacks: 4, 6, 7, 9

Intracavity-dispersion mode-locked laser: ‘dispersion locker’

Yamashita et. al., CLEO 2006

Picture of Setup

… Agrees with Spectrometer Data

Max. hold optical spectra

Comparison of Spectra:

Output temporal waveform (5 kHz)


drawbacks: 2, 3, 7, 9

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

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.

drawbacks: 1, 2, 5, 6, 7, 8