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

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

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

MotivationDesignResultsConclusions 14 th GRC Desired optical output Time [  s] Signal [V]  A sequence of pulses, each at a unique wavelength

MotivationDesignResultsConclusions 14 th GRC 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:

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

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

MotivationDesignResultsConclusions 14 th GRC 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

MotivationDesignResultsConclusions 14 th GRC 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

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

MotivationDesignResultsConclusions 14 th GRC 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

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

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

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

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

MotivationDesignResultsConclusions 14 th GRC Time [  s] Signal [V] Experimental Results  Time trace of TDM output  19 pulses of ~ 200 ns duration  Optical power is ~ 10 mW during each pulse

MotivationDesignResultsConclusions 14 th GRC 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

MotivationDesignResultsConclusions 14 th GRC 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

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

MotivationDesignResultsConclusions 14 th GRC Time [  s] Signal [V] Experimental Results  Liquid phase Methanol, I and I o

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

MotivationDesignResultsConclusions 14 th GRC 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)

MotivationDesignResultsConclusions 14 th GRC 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?

MotivationDesignResultsConclusions 14 th GRC

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