October-2011

Narrow Laser Linewidth Measurement Using

Delayed Self Heterodyne Method

A project report submitted

By:RVBEENAN.V

National Photonics Fellow-2011

Under the guidance ofDr. Deepa Venkitesh

Indian Institute of Technology MadrasChennai 34

Key vord : Linewidth laser, heterodyne detection, Lorentzian linewidth, power spectrum

CONTENTS

Introduction 2

Linewidth Measurement Techniques

2.1 Heterodyne Detection

2.2 Frequency Discriminator

2.3 Delayed Self-Homodyne Technique

2~4Delayed Self-heterodyne Detection

2.4.1 Linewidth interpretation

2.4.2 Effect of fiber delay in DSHI

2.4.3 Advantage disadvantages ofDSHI Technique

2.5 Comparison of Techniques

6

7

8

11

13

14

15

15

16

Laser Linewidth Measurements Using Self-heterodyneDetection Technique

3.1 Theory of Laser Linewidtb

3. l.I Schawlow- Townes linewidth

3.1.2 Phasor derivation of laser linewidth

17

17

17

18

3.1.3 The Laser Field Spectrum 23

3.2 Delayed Self-Heterodyning of Laser Fields 26

3.2.1 Power Spectrum of Delayed self-heterodyne Interferometer 27

.4 Acousto-optic modulator versus Phase modulator 30

Experimental Implementation of DSm

.1. Experimental-setup

ystem Configuration and Key Components

4.2.1 The laser

4.2.2 The coupler

4.2.3 Frequency Shifter

4.2.4 Polarization control

32

32

33

33

33

34

3

A) The Quality Factor 49

B) Total number of transitions per second into one mode 51

4.2.5 The Low Noise Amplifier (LNA) 36

4.2.6 Electrical Spectrum Analyzer 37

4.3 System Design and Optimization 37

4.3.1 Estimation of parameter: COm 37

4.3.2 Estimation of parameter: Td 38

4.3.3 System intensity control 39

4.4 Experimental observation and Result Interpretation 39

4.4.1 Frequency domain - spectrum 39

4.4.2 Vortex II (narrow linewidth laser). 40

4.4.2 a) Linewidth Versus Different delay length. 41

4.4.2 b) Linewidth Versus Different Power. 42

4.4.3 Tunable Laser Source (81689A). 44

4.4.3 a) Linewidth Versus Wavelength oflaser. 45

4.4.3 b) Linewidth Versus power oflaser 45

4.4.4 Resolution of system 46

Conclusions and Future Work 48

5.1 Conclusions 48

5.2 Future Work 48

rences 55

LIST OF TALBLES AND FIGURES

ill linewidth relation 41

vidth versus different power 41

r power Vs 3 dB laser linewidth for 25krn delay . 43

ill linewidth relation 44

'idth versus Wavelength 45

'idth versus power of laser 45

lay versus Resolution 47

~~13ti of the setup for optical heterodyne detection 7

l..(lm-olution of narrow linewidth laser and signal spectrum 8

~,...,,~~~ frequency discriminator for linewidth measurement 9

layed self-homodyne measurement set-up for laser

Linewidth measurement. (a) Mach-Zehnder interferometer.

(b) Michelson interferometer. (c) low-finesse Fabry-Perot filter 12

la. ed self-homodyne mixing of the laser field itself 13

·~j1en13tic etup for optical delayed self-heterodyne detection 13

elf-heterodyne mixing of the laser field 14

-. -Power spectrum for various values of r/rc] 5] 15

.l- Phasor Model: Phase change due to one photon 19

~--- Phasor model- Phase change due to N spontaneous emissions 20

." - Delayed self-heterodyne mixing of the laser field 27

Laser linewidth measurement setups with a phase modulator 31

. - Laser linewidth measurement setups-Self-heterodyne setup

with an acousto-optic modulator (AOM). 32

- - The coupler 34

- ~- Orientation of wave fronts 35

- - Re ult of a measurement performed with the heterodyne technique with a

delay less than coherence length of laser 39

- : - D III experimental set up 40

. L er power Vs 3 dB, 10dB and 20 dB laser linewidth 42

er power Vs 3 dB laser linewidth for 25km delay 43

- - Phase noise spectra of Vortex II 44

- - Phase noise spectra ofTLS (81689A). 46

D pendence of resolution on fiber delay 47

- A rectangular cavi ty of dimensions 2a x 2b x d 51