1. Wavelength Tuning of a 2 µm Tm-Ho fiber laser based CO2 Sensor
Marjorie Dallmann, Renjie Zhou, and Prof. Lynford Goddard
Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign
Goals
Theory
Design and build a stretch tuning fiber Bragg grating (FBG) circuit
Tune the output wavelength of a 2 µm Tm-Ho fiber laser by stretching the FBG
Use wavelength modulation absorption spectroscopy (WMAS) to measure CO2 concentration for monitoring spoilage of stored grain
FBG – a reflector inside an optical fiber that selectively reflects one wavelength and transmits all others
The highest absorption peak for CO2 with low-cost fiber is around µm, the 2 µm fiber laser must be tuned to ~1 nm around this peak for WMAS
By stretching the FBG, the fiber laser output wavelength can be changed to allow for WMAS and CO2 concentration measurement
FBG Stretching Set-Up
12V power source
Fiber Bragg Grating
90o Rotation
Stepper Motor
Both motors are fixed on a stationary base
Arduino Uno
Stepper Motor Driver
Experimental Results
1.5 µm FBG tuning
Arduino Control Code
Main Body Rotation function
void rotateDeg(float deg, float vel){
int dir = (deg > 0)? HIGH:LOW;
digitalWrite(dirpin, dir);
int steps = abs(deg)*(1/0.225);
float usDelay = (1/vel) * 70;
for(int i=0; i < steps; i++){
digitalWrite(steppin, HIGH);
delayMicroseconds(usDelay);
digitalWrite(steppin, LOW);
delayMicroseconds(usDelay);} }
int dirpin = 2;
int steppin = 3;
void rotateDeg(float deg, float vel);
void setup() {
pinMode(dirpin, OUTPUT);
pinMode(steppin, OUTPUT);
rotateDeg(90, 1); }
void loop() {}
Original transmission spectrum
Transmission spectrum after tuning
Future Work
Perform wavelength modulation with 2 µm FBG and test setup
Begin to test the device for CO2 concentration measurement
References
Arduino Function Code:
Stepper Motor, Arduino, and Easy Driver Picture:
R. Zhou et al, CO2 sensing with a 2005 nm thulium holmium co-doped fiber laser, Optical Sensors, OSA Technical Digest, paper STh2B.4, 2012.