Abstract
Fiber laser sources offer interesting possibilities for gas sensors since
they can operate over an extended wavelength range, encompassing the near-IR
absorption lines of a number of important gases but a major problem is that
overtone absorption lines of gases in the near-IR are relatively weak. In
order to enhance sensitivity, we present here a simple method of
intra-cavity absorption spectroscopy (ICAS) which makes use of the amplified
spontaneous emission (ASE) already present within a fiber laser cavity. The
ASE also provides a convenient broadband source for the simultaneous
interrogation of several gases within the gain-bandwidth of the fiber laser.
The key principle is based on adjusting the cavity attenuation to select an
appropriate inversion level where the fiber gain curve is flat. Under this
condition, the ASE undergoes multiple circulations within the fiber laser
cavity, enhancing the effective path-length of a gas cell placed within the
laser cavity. A theoretical model of system operation is given and we have
experimentally demonstrated the principle of operation with acetylene and
carbon dioxide using a simple erbium fiber laser system containing a 6 cm
path-length, fiber coupled, intra-cavity, micro-optic gas cell. We have
experimentally simultaneously observed 16 absorption lines for 1% acetylene
gas in the 1530 nm region and detected the very weak carbon dioxide lines in
this same wavelength region. A path length enhancement of ${\sim}500$ in the linear regime has been demonstrated transforming the 6 cm
micro-optic cell into an effective path length of ${\sim}30$ m. We also demonstrate how the enhancement factor may be
calibrated by use of a simple fiber-optic interferometer. Apart from the
OSA, all components are inexpensive and the system is very simple to
construct and operate.
© 2011 IEEE
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