Shot noise limited heterodyne detection of CARS signals

M Jurna,E Büttner,H L Offerhaus,I Rimke,C Otto,J P Korterik

doi:10.1117/12.761641

Abstract

We demonstrate heterodyne detection of CARS signals using a cascaded phase-preserving chain to generate the CARS input wavelengths and a coherent local oscillator. The heterodyne amplification by the local oscillator reveals a window for shot noise limited detection before the signal-to-noise is limited by amplitude fluctuations. We demonstrate an improvement in sensitivity by more than 3 orders of magnitude for detection using a photodiode. This will enable CARS microscopy to reveal concentrations below the current mMolar range.

Highlights

By probing the internal vibrational structure of molecules, the coherent anti-Stokes Raman scattering (CARS) process enables label-free identification and chemically selective microscopy
In this paper we demonstrate that CARS spectroscopy is possible at the shot noise limit using a compact setup that exploits the phase coherence of the CARS process for heterodyne detection [7]
For an Avalanche Photo Diode (APD) the benefit from using heterodyne detection is lower since the noise levels are lower and the quantum efficiency is higher

Summary

Introduction

By probing the internal vibrational structure of molecules, the CARS process enables label-free identification and chemically selective microscopy. In this paper we demonstrate that CARS spectroscopy is possible at the shot noise limit using a compact setup that exploits the phase coherence of the CARS process for heterodyne detection [7]. The idler is used for the generation of the CARS whereas the signal is used as the local oscillator. These wavelengths bear no direct phase relation to each other, we demonstrate that the CARS output wavelength is phase-locked to the signal wavelength creating an appropriate scheme for heterodyne detection. The 532nm synchronously pumps an Optical Parametric Oscillator (OPO) [9, 10] generating pulses of ∼ 6.5ps at the signal and idler wavelength. Received 24 May 2007; revised 9 Jul 2007; accepted 28 Aug 2007; published 1 Nov 2007 12 November 2007 / Vol 15, No 23 / OPTICS EXPRESS 15208

Heterodyne detection

Results and Discussion

Conclusions