Measuring small absorptions exploiting photo-thermal self-phase modulation

TL;DR

This paper introduces a novel optical measurement technique that detects small absorption coefficients by analyzing photo-thermal self-phase modulation effects in optical cavities, enabling highly sensitive absorption measurements.

Contribution

The paper presents a new method for measuring tiny optical absorption coefficients using cavity deformation caused by photo-thermal effects, with demonstrated application to lithium niobate doped with MgO.

Findings

Measured absorption coefficient of lithium niobate: (5.9 +/- 0.9) *10^-4 /cm

Method can detect absorption coefficients as low as 10^-8 /cm with high finesse cavities

The technique is applicable to materials with very low absorption levels.

Abstract

We present a method for the measurement of small optical absorption coefficients. The method exploits the deformation of cavity Airy peaks that occur if the cavity contains an absorbing material with a non-zero thermo-refractive coefficient dn/dT or a non-zero expansion coefficient ath . Light absorption leads to a local temperature change and to an intensity-dependent phase shift, i.e. to a photo-thermal self-phase modulation. The absorption coefficient is derived from a comparison of time-resolved measurements with a numerical time-domain simulation applying a Markov-chain Monte-Carlo (MCMC) algorithm. We apply our method to the absorption coefficient of lithium niobate (LN) doped with 7mol% magnesium oxide (MgO) and derive a value of alphaLN = (5.9 +/- 0.9) *10^-4/cm . Our method should also apply to materials with much lower absorption coefficients. Based on our modelling we estimate that with cavity finesse values of the order 10^4, absorption coefficients of as low as 10^-8 /cm can be measured.