Pound-Drever-Hall locking A conceptual introduction to laser - PowerPoint PPT Presentation

Pound-Drever-Hall locking A conceptual introduction to laser frequency stabilization Jesse Zhang Internal Colloquium, 22 nd November 2019 | | Internal colloquium 1

Interests in laser stabilization f L Laboratory for Positron and Positronium Physics | | Internal colloquium 22.11.19 2

Relating laser frequency to cavity length Cavity Laser PD PZT Error signal Servo Electronic feedback Laboratory for Positron and Positronium Physics | | Internal colloquium 22.11.19 3

Transmission and Reflection through cavity !"# = %/2( Eric D. Black. An introduction to Pound-Drever-Hall laser frequency stabilization . Am. J. Phys., 69, 2000. Laboratory for Positron and Positronium Physics | | Internal colloquium 21.11.19 4

Using the reflection signal ▪ Hold reflection at zero ▪ Problem: symmetric signal Reflected intensity [a.u.] ▪ decrease or increase frequency? ▪ Derivative is antisymmetric ? ▪ Vary frequency and see the response on the reflected beam Compare variation of reflected intensity and frequency variation Frequency [a.u.] and feed this measurement back to laser/cavity. Laboratory for Positron and Positronium Physics | | Internal colloquium 22.11.19 5

The PDH error signal ▪ Modulate laser frequency driven by oscillator ▪ Pick of reflected light with photodetector ▪ Mix PD signal with oscillation signal ▪ -> PDH error signal Laboratory for Positron and Positronium Physics | | Internal colloquium 21.11.19 6

The PDH setup EOM Eric D. Black. An introduction to Pound-Drever-Hall laser frequency stabilization . Am. J. Phys., 69, 2000. Laboratory for Positron and Positronium Physics | | Internal colloquium 21.11.19 7

Real world problems Theory Reality Laboratory for Positron and Positronium Physics | | Internal colloquium 21.11.19 8

99 problems but error signal ain’t one Laboratory for Positron and Positronium Physics | | Internal colloquium 21.11.19 9

Our PDH setup PZT EOM Servo electronics PD for reflected light Laboratory for Positron and Positronium Physics | | Internal colloquium 21.11.19 10

Electro-optic modulator (EOM) ▪ Pockels effect : ▪ Refractive index changes proportional to electric field strength ▪ -> need high voltage ▪ Modulation of 5 MHz ▪ Can pass through cavity for further use downstream Laboratory for Positron and Positronium Physics | | Internal colloquium 22.11.19 11

Thank you for your attention! | | Internal colloquium 12

Backup: Constructive interference Cavity Laser Frequency f 2L = n & Cavity length L ⟺ ( = ) * +, Laboratory for Positron and Positronium Physics | | Internal colloquium 21.11.19 13

Back up: Modulating the beam: side bands § Modulate phase rather then frequency § ! "# = ! % & '()*+ , -"# .*) ≈ ! % 1 % 2 & 3)* + 1 5 2 & ' )+. * − 1 5 2 & ' )3. * Reflected intensity [a.u.] § Creates two FM sidebands § Reflected light yields to beats at modulation frequency Ω § On resonance beats are 180° out of phase 8 8 - Ω 8 + Ω Frequency [a.u.] Laboratory for Positron and Positronium Physics | | Internal colloquium 21.11.19 14