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Simple Multiwavelength Time-Division Multiplexed Light Source for Sensing Applications

Simple Multiwavelength Time-Division Multiplexed Light Source for Sensing Applications. Thilo Kraetschmer and Scott Sanders Engine Research Center Department of Mechanical Engineering University of Wisconsin 14 th Gordon Research Conference August 12, 2007. Outline. Motivation

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Simple Multiwavelength Time-Division Multiplexed Light Source for Sensing Applications

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  1. Simple Multiwavelength Time-Division Multiplexed Light Source for Sensing Applications Thilo Kraetschmer and Scott Sanders Engine Research Center Department of Mechanical Engineering University of Wisconsin 14th Gordon Research Conference August 12, 2007

  2. Outline • Motivation • How this laser works • Experimental results • Comparison to multiplexed diode lasers

  3. Time Division Multiplexing (TDM) Light Source Sample Detector Sample Light Source Detector

  4. 1 -6 10 1 2 3 ... ... 17 18 19 1 2 3 ... ... 17 18 19 0.8 -7 10 0.6 -8 10 Signal [V] Spectral Power [a.u.] 0.4 -9 10 0.2 -10 10 0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 -11 10 m Time [ s] 1330 1340 1350 1360 1370 1380 Wavelength [nm] Desired optical output • A sequence of pulses, each at a unique wavelength

  5. r r e e w w o o BOA* p p *Booster optical amplifier, a form of semiconductor optical amplifier: n time Understanding the laser design • 4-step upgrade from a CW fiber laser to a 2-color TDM source

  6. FBG FBG BOA r r e e w w o o p p n time Step 1: Replace mirrors with Bragg gratings • Customization of laser wavelength, linewidth

  7. L R L R L R 1 period r r e e w w FBG BOA FBG o o p p n time Step 2: Pulsed operation • Pulsed operation

  8. 1 period 1 period r r e e w w 1 period o o p p FBG FBG BOA n time Step 3: Add second grating pair • Pulsed operation at an additional wavelength with a modified pulse pattern L R L R L R L R L R L R

  9. 1 period BOA r r e e w w o o p p FBG FBG n time Step 4: Use the same gratings on both ends • Still a linear cavity laser, enforced within ring arrangement by the pulse pattern

  10. Animation of 3-color TDM source for animation: right click on the figure, select play

  11. x-t diagram familiar to gasdynamicists • Color-map of density in a shock tube experiment: He-air-CO2, M = 2.5 time [s] distance [m]

  12. TDM source x-t diagram • ASE only, no FBGs

  13. TDM source x-t diagram • Main reflections only, 3 FBGs

  14. TDM source x-t diagram • All signals, 3 FBGs

  15. Schematic of 19-color realization • fiber roundtrip length ~ 3 km • repetition rate ~ 66 kHz

  16. 1 1 2 3 ... ... 17 18 19 0.8 0.6 Signal [V] 0.4 0.2 0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 m Time [ s] Experimental Results • Time trace of TDM output • 19 pulses of ~ 200 ns duration • Optical power is ~ 10 mW during each pulse

  17. -6 10 1 2 3 ... ... 17 18 19 -7 10 -8 10 Spectral Power [a.u.] -9 10 -10 10 -11 10 1330 1335 1340 1345 1350 1355 1360 1365 1370 1375 1380 Wavelength [nm] Experimental Results • Spectrum of TDM output • the gain of each wavelength was adjusted to obtain a flat output spectrum • the active linewidth is ~ 5 times narrower than the passive linewidth

  18. 1 0.9 0.8 0.7 0.6 0.5 Spectral Power [a.u.] 0.4 0.3 0.2 0.1 0 1330 1335 1340 1345 1350 1355 1360 1365 1370 1375 1380 Wavelength [nm] Experimental Results • Spectrum of TDM output • the BOA injection current pulse pattern was customized to form a ramped output spectrum

  19. Experimental Results • High-speed detection strategy

  20. 1 0.8 0.6 Signal [V] 0.4 0.2 0 0 1 2 3 4 5 6 7 8 9 10 m Time [ s] Experimental Results • Liquid phase Methanol, I and Io

  21. 1 Methanol Measurement 0.9 Methanol Reference Isopropanol Measurement 0.8 Isopropanol Reference 0.7 0.6 0.5 Absorbance [a.u.] 0.4 0.3 0.2 0.1 0 1330 1335 1340 1345 1350 1355 1360 1365 1370 1375 1380 Wavelength [nm] Experimental Results • Measured spectra of Methanol and Isopropanol • single shot measurement • 66 kHz rep. rate • standard deviation of 100 consecutive shots: ~0.0013

  22. Laser features • no moving parts • individual tunability of each wavelength (typ: 1 nm) • narrow spectral linewidth of each channel (< 1 GHz) • small longterm spectral drift of each channel (< 1 GHz) • fiber coupled output, typical in 10 – 100 mW range To build this laser you need only: • gain medium (preferably with a broad gain bandwidth and fast switching times) • custom waveform generator applying modulation (preferably to the gain medium) • matched compressor / stretcher (preferably as part of a long laser cavity)

  23. Comparison to Multiplexed Diode Lasers • Advantages of TDM source over multiplexed diode lasers • straightforward to reach high wavelength count N: 100s to 1000s • single gain medium (for N wavelengths that lie within the gain bandwidth of a single gain medium) • modulation decoupled from wavelength-selective element • no external couplers / multiplexers needed • simple and stable wavelength control • broad tunability • more options for custom-wavelength lasers • opportunities for high-power lasers • Advantages of multiplexed diode lasers over TDM source • long fiber not required • some diode lasers are very inexpensive • direct scanning by current modulation Questions?

  24. The original 19 wavelengths were chosen to align with H2O peaks – now we choose the N wavelengths differently

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