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Measurement of the frequencylinewidth of single-mode lasers

by means of a quadrature signalinterferometer

Th. Kinder, Th. Mller-Wirts: TEM Messtechnik GmbH, HannoverK.-D. Salewski, J.-U. Gnther: University of Greifswald

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What is (spectral) laser linewidth?

Laser - Meter

Spectral density

laser linewidth

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Why is there a laser linewidth?

[ ]

W HWB HWB

HWB HWB

t jt j

Lc

t

E E

E f f f

j

E dt e E dt et E f

11

24

2

1

)(4)()()(

)(2

)()(

222

20

2

20

20

22

20*2

0 0 0

0)(20

2 0

==

=

+=

+

==+

===

+

bzw.

reiteHalbwertsbderBestimmung

ProfilLorentz

Classical model: The oscillations of an atomic dipole decay due to the emission of radiation.As a result, the emitted wave is of finite length.

E02

2

0

f ( ) 2

Fourier Transform: After a Fourier transform, we find a spectral density function withfinite width, centered about the eigen frequency of the dipole 0.

decay time:

ct clt ee E t E W t jt ====

1)( 020 length of the wave:

Determination of the spectral width

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Linewidth measured with FPILaser

optical frequency

photo current I

photo detector

Fabry Perot interferometer

I

Advantage:

direct signal detection

Disadvantage:

measurement only at distinct center frequencies

no measurement while tuning the laser

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Linewidth measured by beatOften cited statemant of Dirac:Each photon ... interferes only with itself. Interference between different photonsnever occurs.

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Linewidth measured by beat

Advantage:

fast signal detection

Disadvantage:

2 lasers required

no measurement while tuning the laser

suppression of disturbations that affect bothlasers synchronously

Often cited statemant of Dirac:Each photon ... interferes only with itself. Interference between different photonsnever occurs.

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Interferometric linewidth measurement

Interferometer Signal:

opt. frequency, D geometric path difference,

n refractive index, c0 speed of light in vacuum

I = I o (1 + cos ) with 00

4

+=

c Dn

Albert A. Michelson(1852-1931)Quelle: http://www.sil.si.edu

Lichtquelle

Detektor D

Principle of operation:

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Interferometric linewidth measurement

opt. frequency, D geometric path difference,n refractive index, c0 speed of light in vacuum

I = I o (1 + cos ) with 00

4

+=

c Dn

Albert A. Michelson(1852-1931)source: http://www.sil.si.edu

constant and known: nD = c0 / 4

= c0 / 4 nD

Incremental laser interferometer

D , constant and known:

Michelson-Morley-Experiment

c0 = 4 nD /

D constant and known:

Measurement of Laser frequency changeSource: http://utf.mff.cuni.cz/Relativity

=0 !

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Optical phase quadratureGeneration of a pair of

Sy = S y0 sin and

Sx = S x0 cos

l Laser Wavelength

Quadrature Signals

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Interpretation of quadrature signals1)

2)

3)

1) regular laser frequency scan

2) a mode hop occurs

3) a multi-mode state occurs

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Interferometer set-up (1)

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iScanTEM Messtechnik GmbHCoSyThomas KinderThomas Mller-WirtsKlaus-Dieter SalewskiFaserinterferometerLinienbreiteADI

absolute distance interferometryDBR diode current driverlaser diode driverfiber interferometerfibre interferometerDFB diode current driverlaser frequency stabilisation

Interferometer set-up (2)a

FPI

TC TemperatureStabilisation

NormalisationPhoto Detectors

Quadrature SignalPhoto Detectors

WedgedBeam Splitter

laser beam toexperiment

Fabry-Perot-Interferometer

BS

PB A PB B

b

IbIa

Advantage:

fast signal detection

measurement with 1 laser

measurement possiblewhile tuning the laser

Disadvantage:

calculation of phase fromx/y coordinates

Patents:US 6,178,002DE 197 43 493 A 1

principle fairly unknown(until now)

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Linewidth measurement in the

presence of noise1. Interferometer path difference2. Detector upper cut-off frequency

Observed noise in x-y-planeFourier transform of the phase noise

Noise spectrum does not exceed the detector frequency range.

MHz f m L G 3005,0 == d.h. MHz f El 250=

laser onlaser off

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Linewidth measurement in the

presence of noise

== 774,122 HBW El R Las

MHz HBW MHz HBW Las El 29,419,1 ==

1. Recording of phase values for 0,2 ms

2. Calculation of the standard deviationfor both cases (laser on and laseroff)

3. Calculation of the laser linewidth :

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ResultsLinewidth of a DBRlaser diode

Frequency noise and

linewidth during lasertuning