74th Annual Scientific Conference of the University of Latvia FOTONIKA-LV Friday, 4th March 2016

Inga Brice, Aigars Atvars, Janis Alnis

Laser spectrum narrowing below 1kHz line width with broadband Fabry-Pérot resonators

Fabry-Pérot resonator - two parallel mirrors.Condition for a resonance is

Reflected light from the resonator mirror:

where L is mirror separation distance, λ – wavelength of a light and n – integer number.

Free spectral range:

where r is the reflectance of mirrors.

5 10 15 200,00,20,40,60,81,0

|F(

)|

, arb. units

Finesse: If r = 0.95 then F = 1500.

Reflectance from a FP resonator

Optical resonators

Pound-Drever Hall technique for laser stabilization to a resonance peak=

Modulate the incoming light phase:

Expansion is in Bessel functions:

4 5 6 7 8 9 10

1 .0

0 .5

0 .5

1 .0Reflected light from the resonator:

Goal was to design low-cost resonator for laser stabilisation in broad wavelength range

In-stock mirrors for TiSa lasers Layertec Nr 103521Reflectivity 99.85%, 650…1100 nm.

Zerodur spacer from Schott Glass.

HS1 - thermal shield 1HS2- thermal shield 2GT – Zerodur glass spacer PS - plastic screwsHH – holes with heat spectra isolation TS1- temperature sensor 1L- legPT1- Peltier element 1PT2-Peltier element 2 W1 and W2 – windows with antireflectance coatingM- mirrorTS- temperature sensor 2Vacuum pump 1 for low vacuumVacuum pump 2 for high vacuum.

Schematic of resonator assembly

Double-stage Peltier is thermally linking outer shield bottom plate with the vacuum chamber.

Fabry-Perot resonator with thermal shields and vacuum chamber

Light coupling and transmission modes

The first task was to obtain TEM00 mode state for the resonator by mirror alignment.

Test laser used at 972 nm. TEM00 mode has a slight admixture of higher order mode visible on the left side of the picture. Means that spacer length is not chosen very well and TEM00 owerlaps with some higher order mode.

Principal schema of experimental setup

F-Pol – fiber polarizatorL/2 - λ/2 plate M- mirrorBS – beam splitterL/4 - λ/4 plateFP – Fabry–Perot resonatorPD1, PD2 – PhotodiodesCam – video cameraL-lensAmp- amplifierMixer – signal mixer,PS – phase shifterLP-F – lowpass filterPDH Signal – Pound-Drever-Hall signalRF-Generator – radio frequency modulation generatorATT – attenuatorR1, R2 – resistors

2 4 6 8 10 12

-3

-2

-1

0

frequency, arb. units

sign

al in

tens

ity, a

rb. u

nits

-4

0

4

8

12

PH

D s

igna

l, ar

bitra

l uni

ts

measured and theoretical signals

Measurements on oscilloscope

Modulation frequency used is 7 MHz applied to laser diode current. FP transmission full width at half maximum line width is 0.7MHz when scanning laser across the resonance. Values for theoretical simulations r= 0.98.

Optical beat note spectrum with another stable laser

-10 -5 0 5 10-80

-70

-60

-50

-40

-30

-20

-10

Bea

t not

e si

gnal

, dB

m

Beat note frequency - const (MHz)

Laser is locked to FP. Beat note 20 MHz span. Resolution 20 kHz. Modulation sidebands -20dB (100 times) less than carier.

Beatnote line width of the central peak

-10 -8 -6 -4 -2 0 2 4 6 8 100

10

20

30

40

50

Bea

t not

e si

gnal

, rel

. uni

ts

Beat note frequency - const (kHz)

no desk vibration damping

-5 -4 -3 -2 -1 0 1 2 3 4 50

10

20

30

40

50

60

with desk vibration damping

Bea

t not

e si

gnal

, rel

. uni

tsBeat note frequency - const (kHz)

Signal FWHM is 1 kHz.

Optimal temperature selection

22 24 26 28 30 32 34

-2

0

2

4

6

8

10

12

Bea

t fre

quen

cy -

cons

t., M

Hz

T, oC

polinomial fit

Tc=27,6 (1) oC

Zerodur material has zero expansion somewhere around room temperature.

Long-term stability

0,0 0,5 1,0 1,5 2,0 2,5 3,0-5

-4

-3

-2

-1

0zerodur FP drift 2 MHz/ day or 23 Hz/s

Bea

t fre

quen

cy -

cons

t., M

Hz

No. of Day - const.

Beat frequency signal during long-term measurements.A long-term drift of about 2 MHz/day or 23 Hz/s is seen.Resonator length increases with time due to crystialization of Zerodur.Drift rate decreases within a couple of years as water layer evaporates from mirrors

Stability at different time scales

1E-3 0,01 0,1 1 10 100100

1000

10000

linear drift 70 Hz/sremoved

Alla

n va

rianc

e, H

z

Averaging time, s

bump

For our diode laser λ0 = 780 nm, the optical frequency is about 3.8 1014 Hz. The relative stability of the frequency of our system seen from Allan deviation is 10-12.Thermal noise limit is 15 digits, and we plan to construct resonator with better mirrors to reach this. Gravitational wave detectors reach 21 digits due to long arms.

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