A novel composite cavity fibre laser (CCFL) design is being investigated for its use in sensing applications. A CCFL can
be fabricated simply by writing three spectrally matched Bragg gratings directly into a continuous length of doped fibre.
When strained evenly, so that the strain experienced by both cavities are the same, the strain response of the CCFL is
expected to be similar to that of a FBG or single cavity fibre laser sensor. However, if the internal cavities are strained
unevenly, simulations derived from theoretical analysis suggest that the wavelength-encoded and intensity-encoded
sensitivities can become significantly different from that of a typical single cavity fibre laser. A 3cm/9cm CCFL was
tested using three different straining formats, and results agree well with theoretical expectations.
The composite cavity fibre laser (CCFL) is demonstrated for its capability in maintaining single longitudinal mode
operation, whilst having longer cavity length than typical distributed Bragg reflector or distributed feedback fibre lasers,
and hence also higher output power. These two attributes should enable the CCFL to be useful in sensing applications. A
long cavity length CCFL can be fabricated simply by writing three spectrally matched Bragg gratings directly into a
continuous length of doped fibre. This is analogous to the use of feedback cavities in semiconductor laser designs to
maintain single longitudinal mode. Results from in-house fabrications show that long cavity length CCFLs can be
fabricated to have single longitudinal mode and narrow linewidth characteristics similar to that of a distributed feedback
fibre laser, and also significantly higher output power.
An in-line fibre ring cavity is fabricated by writing two blazed gratings next to each other to form a Fabry Perot cavity. A
visibility of fringes as good as 0.032 in the reflection spectrum and 0.76 for transmission is obtained for the interference
between the forward propagating guided mode and the reverse propagating ghost mode of the blazed grating. The ability
to measure the external refractive index and the variability of this response with cavity length is demonstrated.
This paper reports the development of a very compact and very-high sensitivity optical fibre hydrophone system using a distributed-feedback fibre laser with a cavity length of 10cm. A theoretical system design making use of a Mach-Zehnder interferometer, homodyne demodulation scheme and digital signal processing is described. At the time of writing, the system is only partially completed; therefore the content of this paper will focus on the distributed-feedback fibre laser sensor head. Results for noise spectrum below 100kHz are presented, as well as discussions on some key issues with designing such hydrophone systems. Although not the intention for the system, initial results also indicate the suitability of the DFBFL for intensity modulated sensing.