Spectrally resolved cavity ring down measurement of high reflectivity mirrors using a supercontinuum laser source

Abstract

We investigate a cavity ring down setup that offers the possibility to measure the spectrally resolved reflectivities of high reflectivity mirrors. The setup consists of a resonator (ring down cavity) and an intensified CCD camera system combined with a spectrograph for spectral resolution. A commercial supercontinuum laser (350-1750 nm) is applied as a compact excitation source. It is based on a photonic crystal fiber that is pumped by a q-switched microchip laser (1.6 ns pulse duration, 25 kHz repetition rate). This combination allows simultaneously recording the transmittance of the cavity over a wide wavelength range determined by the excitation source and the spectral sensitivity of the detector. The photon lifetimes inside the cavity (ring down times) are measured with high spectral resolution by means of an intensified camera system. Subsequently shifting the "gate" of the image intensifier from short to long delay times after the excitation pulse allows calculation of the reflectivity spectrum of the mirrors. Comparison of these results with measurements using a conventional setup (laser diode 675 nm and photomultiplier tube) clearly shows the high potential of the method due to the multichannel excitation and the detection scheme.

PCF-Based Cavity Enhanced Spectroscopic Sensors for Simultaneous Multicomponent Trace Gas Analysis

“…This light is initially strong, but decays at much faster rate due to lower values of the reflection coefficient. Similar problems seem common for other SC-CRDS spectrographs [49]. …”

“…Although this idea is essentially not new and, as mentioned above, has been already demonstrated by earlier investigations of Stelmaszczyk et al [45,48], the new aspect of the work [49,50] is the SC light source itself, because the authors for the first time use SC radiation generated in a PCF. Nonetheless, it has never been proved whether or not such PCF-based supercontinuum sources are also eligible in ring-down setups that are designed to detect molecular absorption.…”

“…More recently, the Supercontinuum Cavity Ring-Down Spectrography (SC-CRD-Spectrography) has been proposed as a broadband diagnostic tool for highly reflective mirror coatings [49,50]. Although this idea is essentially not new and, as mentioned above, has been already demonstrated by earlier investigations of Stelmaszczyk et al [45,48], the new aspect of the work [49,50] is the SC light source itself, because the authors for the first time use SC radiation generated in a PCF.…”

Spectrally resolved measurement of small optical losses by cavity enhanced spectroscopy techniques

“…With every round trip the trapped light intensity decreases because of the optical losses in the cavity [3]. The decrease is described by a single exponential decay.…”

“…A second setup was realized with the goal of significantly reducing the recording time for a complete loss spectrum. In this setup a super continuum fiber laser source (fiberware ILUM 100) was used for simultaneous excitation with a broad spectrum of wavelengths so that it was possible to detect the spectral losses with only one measurement [3]. This laser source emitted ns pulses with a spectrum in the 390 -2500 nm range.…”