A compact low-beat-frequency dual-polarization distributed Bragg reflector (DBR) fiber laser for high-frequency
ultrasound detection has been demonstrated. The laser was fabricated in high germanium concentration, small-core
erbium-doped fiber with very small birefringence. Induced birefringence to the fiber during the UV inscription process is
small (~10<sup>-7</sup>) because of the small fiber core (4.2-μm) and consequently the laser beats at a low frequency of ~20 MHz,
making frequency down-conversion unnecessary. The beat frequency can be adjusted by controlling the side-exposure
time of the UV light irradiating the gain cavity, providing a simple approach to multiplex a large number of DBR fiber
lasers of different frequencies in series using frequency division multiplexing (FDM) technique.
We report the fabrication of extremely short linear-cavity distributed Bragg reflector fiber lasers (DBR-FLs). It has a
total length of 7 mm, with the nominal cavity of only 0.4 mm. The FL has a linewidth and polarization beat frequency of
220 Hz and 18.9 MHz, respectively. The relaxation oscillation frequency and its relative peak are 120 kHz and
-75dB/Hz, respectively. The FL exhibited low-noise characteristics, with an intensity noise of -107 dB/Hz at 1 MHz. We
constructed a FL sensor array that has great potentials for large-scale, high sensitivity sensing applications.
We present recent work and progress on Spectrally Coded Multiplexing (SCM). SCM as a generic multiplexing
technique adds new dimension of freedom and provides additional flexibility and channel capability needed for fibre and
fibre grating based sensor systems. We show a few examples of our newly developed SCM techniques based on
specially designed fibre gratings.
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.
A multiplexing technique using amplitude-modulated chirped fibre Bragg gratings (AMCFBGs) is presented. This
technique realises the multiplexing of spectrally overlapped AMCFBGs with identical centre Bragg wavelength and
bandwidth. Since it is fully compatible with the wavelength division multiplexing scheme, the number of gratings that
can be multiplexed can be increased by several times. The discrete wavelet transform is used to demodulate such
multiplexed signal. A wavelet denoising technique is applied to the multiplexed signal in conjunction with the
demodulation. Strain measurements are performed to experimentally demonstrate the feasibility of this multiplexing
technique. The absolute error and crosstalk are measured. An application to simultaneous two-parameter sensing is also