Intra-cavity spectroscopy using amplified spontaneous emission in erbium fibre lasers

摘要

Fibre 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 laser absorption spectroscopy (ICLAS) which makes use of the amplified spontaneous emission (ASE) already present within a fibre laser cavity. The ASE also provides a convenient broad-band source for the interrogation of several gases within the gain-bandwidth of the fibre laser. The key principle is based on adjusting the cavity attenuation to select an appropriate inversion level and hence flatten the erbium-fibre gain curve. Under this condition, the ASE undergoes multiple circulations within the fibre laser cavity, enhancing the effective path-length of a gas cell placed within the laser cavity. We have experimentally demonstrated the principle of operation with acetylene gas, using a simple erbium fibre laser system containing a 6cm path-length, fibre coupled, intra-cavity, micro-optic gas cell. For 1% acetylene gas, we have experimentally observed 16 absorption lines in the 1530nm region and a path length enhancement of ～60 has been demonstrated, transforming the 6cm micro-optic cell into an effective path length of ～3.5m. Apart from the OSA, all components are inexpensive and the system is very simple to construct and operate.