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

ConclusionsResultsDesignMotivation 214th GRC 2007

Outline

Motivation

How this laser works

Experimental results

Comparison to multiplexed diode lasers

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SampleLight Source

Detector

Light Source

Detector

Sample

Time Division Multiplexing (TDM)

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Desired optical output

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Wavelength [nm]

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.u.]

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Time [ s]

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A sequence of pulses, each at a unique wavelength

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Understanding the laser design

BOA*

time

pow

er

pow

er

4-step upgrade from a CW fiber laser to a 2-color TDM source

*Booster optical amplifier, a form of semiconductor optical amplifier:

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Step 1: Replace mirrors with Bragg gratings

time

pow

er

pow

er

Customization of laser wavelength, linewidth

BOAFBG FBG

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Step 2: Pulsed operation

time

pow

er

pow

er

BOAFBG

1 period

L R L R L R

FBG

Pulsed operation

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Step 3: Add second grating pair

time

pow

er

pow

er

BOAFBGFBG

1 period 1 period

1 period

Pulsed operation at an additional wavelength with a modified pulse pattern

L R L R L R L R L R L R

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Step 4: Use the same gratings on both ends

time

pow

er

pow

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BOA

FBGFBG

1 period

Still a linear cavity laser, enforced within ring arrangement by the pulse pattern

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Animation of 3-color TDM sourcefo

r an

imat

ion:

rig

ht c

lick

on t

he f

igur

e, s

elec

t pl

ay

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distance [m]

time

[s]

x-t diagram familiar to gasdynamicists

Color-map of density in a shock tube experiment: He-air-CO2, M = 2.5

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TDM source x-t diagram

ASE only, no FBGs

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Main reflections only, 3 FBGs

TDM source x-t diagram

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All signals, 3 FBGs

TDM source x-t diagram

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Schematic of 19-color realization

fiber roundtrip length ~ 3 km

repetition rate ~ 66 kHz

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Experimental Results

Time trace of TDM output 19 pulses of ~ 200 ns duration

Optical power is ~ 10 mW during each pulse

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Wavelength [nm]

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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

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Experimental Results

Spectrum of TDM output the BOA injection current pulse pattern was customized to form a ramped output spectrum

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Experimental Results

High-speed detection strategy

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Experimental Results

Liquid phase Methanol, I and Io

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.]

Methanol Measurement

Methanol ReferenceIsopropanol Measurement

Isopropanol Reference

Experimental Results

Measured spectra of Methanol and Isopropanol

single shot measurement

66 kHz rep. rate

standard deviation of 100

consecutive shots: ~0.0013

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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)

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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?

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ConclusionsResultsDesignMotivation 25

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

N wavelengths differently