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.
We propose a simple method for achieving a desirable chirp in the grating based on refractive index direct current
component (RIDCC) modulation. The method uses dual-scan technique with an UV beam to obtain an increase of
refractive index direct current component in the grating along the grating length. The effect of RIDCC modulation on the
group delay characteristic of fiber grating is analyzed and the relationship between the RIDCC modulation and group
delay of grating is deduced. Also, the numerical simulation results are given by using coupled mode theory and they
agree very well with theoretical values. By controlling the RIDCC modulation function and modulation coefficient, we
can realize different group delay profile using the same phase mask. These gratings can be used to compensate
dispersion and dispersion slope of transmission fiber in high-speed optical communication systems.
The chemical sensing principle based on Fiber Bragg Grating (FBG) is presented. By deriving the relation between
effective core index and surrounding index of etched FBG, the relation between Bragg wavelength and concentration of
chemical solutions is analyzed, which is demonstrated in the experiment by measuring propylene glycol solutions. The
factors influencing the sensitivity of chemical sensing is investigated. The results show that, the sensitivity of the Bragg
wavelength of FBGs to the surrounding index can be enhanced with decreasing the diameter of FBGs by etching the
fiber with HF solution. The sensitivity can be appreciably enhanced by etching the fiber diameters as small as 5.0 μm. A
maximum sensitivity of 911.8 nm/riu is achieved. Since a shift of 0.01 nm can be detected, this leads to a detectable
change of 1.1×10<sup>-5</sup> in the surrounding index.
On the basis of non-linear coupled-mode theory, the dispersion relation of a dimensional periodic structure is analyzed
and slow Bragg soliton solutions are given. The approximation approach for non-linear coupled-mode theory is studied
by assuming adiabatic evolution of the soliton passed through fiber Bragg grating which is apodized by using a
hyperbolic tangent apodization function in the middle of the FBG. And in the presence of apodization, the velocity
equation of Bragg soliton is found. In this paper, we have simulated the motion tracks and velocities of Bragg solitons,
analyzed effects of different parameters on motion track and velocity of Bragg soliton, and obtained different time-displacement
curves and time-velocity curves. It is shown that the final steady velocity of soliton can be found by
choosing specifical parameters, thereby, slow Bragg solitons whose propagate velocity through the grating is far lower
than the speed of light in glass can obtained. We have presented detailed simulative results that agree with theoretical
A facilitated fiber Bragg grating detection system for detecting temperature is presented. The scheme uses a processing fiber Bragg grating that is stuck on a cantilever beam and is identical to one used as a sensor to demodulate the temperature signal. A step motor that is controlled by PLC (Programmable Logic Controller) stretches the cantilever beam periodically, and the system of two gratings produces a carrier at this frequency with its phase modulated by the temperature signal applied to the sensing grating. The temperature signal, which is demodulated by this system, is transmitted to PC then processed and displayed in PC. This system’s configuration is simple, sensitivity is high, and cost is low.