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 14 th Gordon Research Conference August 12, 2007

2. MotivationDesignResultsConclusions 14 th GRC 2007 2 Outline  Motivation  How this laser works  Experimental results  Comparison to multiplexed diode lasers

3. MotivationDesignResultsConclusions 14 th GRC 2007 3 Sample Light Source Detector Light Source Detector Sample Time Division Multiplexing (TDM)

4. MotivationDesignResultsConclusions 14 th GRC 2007 4 Desired optical output 00.511.522.533.544.5 0 0.2 0.4 0.6 0.8 1 Time [  s] Signal [V] 1 2 3...... 17 18 19  A sequence of pulses, each at a unique wavelength

5. MotivationDesignResultsConclusions 14 th GRC 2007 5 Understanding the laser design BOA* time p o w e r p o w e r  4-step upgrade from a CW fiber laser to a 2-color TDM source *Booster optical amplifier, a form of semiconductor optical amplifier:

6. MotivationDesignResultsConclusions 14 th GRC 2007 6 Step 1: Replace mirrors with Bragg gratings time p o w e r p o w e r  Customization of laser wavelength, linewidth BOAFBG

7. MotivationDesignResultsConclusions 14 th GRC 2007 7 Step 2: Pulsed operation time p o w e r p o w e r BOAFBG 1 period L R FBG  Pulsed operation

8. MotivationDesignResultsConclusions 14 th GRC 2007 8 Step 3: Add second grating pair time p o w e r p o w e r BOAFBG 1 period  Pulsed operation at an additional wavelength with a modified pulse pattern L R

9. MotivationDesignResultsConclusions 14 th GRC 2007 9 Step 4: Use the same gratings on both ends time p o w e r p o w e r BOA FBG 1 period  Still a linear cavity laser, enforced within ring arrangement by the pulse pattern

10. MotivationDesignResultsConclusions 14 th GRC 2007 10 Animation of 3-color TDM source for animation: right click on the figure, select play

11. MotivationDesignResultsConclusions 14 th GRC 2007 11 distance [m] time [s] x-t diagram familiar to gasdynamicists  Color-map of density in a shock tube experiment: He-air-CO 2, M = 2.5

12. MotivationDesignResultsConclusions 14 th GRC 2007 12 TDM source x-t diagram  ASE only, no FBGs

13. MotivationDesignResultsConclusions 14 th GRC 2007 13  Main reflections only, 3 FBGs TDM source x-t diagram

14. MotivationDesignResultsConclusions 14 th GRC 2007 14  All signals, 3 FBGs TDM source x-t diagram

15. MotivationDesignResultsConclusions 14 th GRC 2007 15 Schematic of 19-color realization  fiber roundtrip length ~ 3 km  repetition rate ~ 66 kHz

16. MotivationDesignResultsConclusions 14 th GRC 2007 16 00.511.522.533.544.5 0 0.2 0.4 0.6 0.8 1 Time [  s] Signal [V] 1 2 3...... 17 18 19 Experimental Results  Time trace of TDM output  19 pulses of ~ 200 ns duration  Optical power is ~ 10 mW during each pulse

17. MotivationDesignResultsConclusions 14 th GRC 2007 17 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. MotivationDesignResultsConclusions 14 th GRC 2007 18 13301335134013451350135513601365137013751380 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Wavelength [nm] Spectral Power [a.u.] Experimental Results  Spectrum of TDM output  the BOA injection current pulse pattern was customized to form a ramped output spectrum

19. MotivationDesignResultsConclusions 14 th GRC 2007 19 Experimental Results  High-speed detection strategy

20. MotivationDesignResultsConclusions 14 th GRC 2007 20 012345678910 0 0.2 0.4 0.6 0.8 1 Time [  s] Signal [V] Experimental Results  Liquid phase Methanol, I and I o

21. MotivationDesignResultsConclusions 14 th GRC 2007 21 Experimental Results  Measured spectra of Methanol and Isopropanol  single shot measurement  66 kHz rep. rate  standard deviation of 100 consecutive shots: ~0.0013

22. MotivationDesignResultsConclusions 14 th GRC 2007 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. MotivationDesignResultsConclusions 14 th GRC 2007 23 Comparison to Multiplexed Diode Lasers  Advantages of TDM source over multiplexed diode lasers 1.straightforward to reach high wavelength count N: 100s to 1000s 2.single gain medium (for N wavelengths that lie within the gain bandwidth of a single gain medium) 3.modulation decoupled from wavelength-selective element 4.no external couplers / multiplexers needed 5.simple and stable wavelength control 6.broad tunability 7.more options for custom-wavelength lasers 8.opportunities for high-power lasers  Advantages of multiplexed diode lasers over TDM source 1.long fiber not required 2.some diode lasers are very inexpensive 3.direct scanning by current modulation Questions?

24. MotivationDesignResultsConclusions 14 th GRC 2007 24

25. MotivationDesignResultsConclusions 14 th GRC 2007 25 The original 19 wavelengths were chosen to align with H2O peaks – now we choose the N wavelengths differently