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Gain-assisted broadband ring cavity enhanced spectroscopy
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Paper Abstract

Incoherent broadband cavity enhanced spectroscopy can significantly increase the effective path length of light-matter interaction to detect weak absorption lines over broad spectral range, for instance to detect gases in confined environments. Broadband cavity enhancement can be based on the decay time or the intensity drop technique. Decay time measurement is based on using tunable laser source that is expensive and suffers from long scan time. Intensity dependent measurement is usually reported based on broadband source using Fabry-Perot cavity, enabling short measurement time but suffers from the alignment tolerance of the cavity and the cavity insertion loss. In this work we overcome these challenges by using an alignment-free ring cavity made of an optical fiber loop and a directional coupler, while having a gain medium pumped below the lasing threshold to improve the finesse and reduce the insertion loss. Acetylene (C2H2) gas absorption is measured around 1535 nm wavelength using a semiconductor optical amplifier (SOA) gain medium. The system is analyzed for different ring resonator forward coupling coefficient and loses, including the 3-cm long gas cell insertion loss and fiber connector losses used in the experimental verification. The experimental results are obtained for a coupler ratio of 90/10 and a fiber length of 4 m. The broadband source is the amplified spontaneous emission of another SOA and the output is measured using a 70pm-resolution optical spectrum analyzer. The absorption depth and the effective interaction length are improved about an order of magnitude compared to the direct absorption of the gas cell. The presented technique provides an engineering method to improve the finesse and, consequently the effective length, while relaxing the technological constraints on the high reflectivity mirrors and free-space cavity alignment.

Paper Details

Date Published: 20 February 2017
PDF: 7 pages
Proc. SPIE 10110, Photonic Instrumentation Engineering IV, 101100X (20 February 2017); doi: 10.1117/12.2251099
Show Author Affiliations
Mahmoud A. Selim, Ain Shams Univ. (Egypt)
George A. Adib, Ain Shams Univ. (Egypt)
Si-Ware Systems (Egypt)
Yasser M. Sabry, Ain Shams Univ. (Egypt)
Si-Ware Systems (Egypt)
Diaa Khalil, Ain Shams Univ. (Egypt)
Si-Ware Systems (Egypt)

Published in SPIE Proceedings Vol. 10110:
Photonic Instrumentation Engineering IV
Yakov G. Soskind; Craig Olson, Editor(s)

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