New approaches for multiplexing self-mixing interference (SMI) in fiber ring laser (FRLs) are presented. The expression
of output powers with optical feedback in FRL with parallel dual-channel and multi-channel is analyzed. The gain
competition, the intensity alternation among different channels and the attainable performance are discussed. The
experimental results show good agreement with the theory. Therefore, two-channel and multi-channel sensing or
measurement will be realized simultaneously.
According to the theory of self-mixing interference (SMI) in fiber ring lasers (FRLs), Fourier Transform method is
adopted for vibration measurement. The harmonic components increase with the increase of the amplitude of vibration.
According to the theory of self-mixing speckle effect (SMSE) in FRLs, the similarity to Gaussian form of probability
density distribution increases as the velocity increases. A method based on the fractal boxes of a section of speckle
waveform is adopted for demodulation. The relationship between the numbers of boxes and velocities of the object is
linear. Dual-channel SMI is presented demonstrating the potential for multiplexing in FRLs. The dual wavelengths of the
FRL are selected by two fiber Bragg gratings (FBGs). The output power in one channel is modulated by that in the other
channel, which is resulted in the mode competition between the two channels. The experimental results show a good
agreement with the simulation results, and indicate that SMI with parallel dual-channel is an efficient approach for
multiple displacement measurement.
In this paper, we present a demodulation of Fabry-Perot pressure sensor method based on radial basis function
network(RBF). RBF network is a kind of three layers frontal feedback neural network with single connotative layer. It is
proved that RBF is able to approach random continuous function with random precision. The cavity length variation is
simulated from 473 to 483 µm with the step of 0.5 µm and the simulation result shows that the relative error of this new
method is less than 0.02% and the maximum absolute error is less than 0.1 µm. The MEMS Fabry-Perot pressure sensor
is also demodulated by the experiment. In the experiment, we change the pressure from 0 to 2 MPa with the step of 0.1
MPa. The experimental result shows that its linearity of the cavity length versus pressure achieves 0.98858 and the
standard deviation between measured pressures and real pressures is less than 0.05 Mpa. By the experiment we can see
that, this RBF network method can obtain upper precision and can reach the practice demand. This new method adapts to
the practice demand with its higher resolution and less calculation time.
In this paper, the FFT (Fast Fourier Transform) demodulation method conbined with CZT(Chirp
Z-Translation) is adopted here for Fabry-Perot pressure sensors and its principle and error are analyzed
theoretically. CZT is a kind of DFT (Discrete Fourier Transform) in some special condition and it is
usually used to thicken the frequency spectrum. CZT can do nothing to reduce the error from FFT and
itself may induce error but the error here from FFT and CZT will influence the demodulation resolution
a little. In the simulation, cavity lengthes from 397 to 403 μm with the space of 0.5 μm are simulated
and the result shows that the relative error of this new method is less than 0.01% and the maximum
absolute error is less than 0.05 μm. In the experiment of the demodulation of the MEMS Fabry-Perot
pressure sensor, whose metrical range is from 0 to 3 Mpa, its resolution reaches 0.01 MPa, its linearity
between the length of the cavity and pressure achieves 0.99316 and the standard deviation between
measured pressures and real pressures is less than 0.005 Mpa. By the experiment we can see that this
new method adapts to the practice demand with its higher resolution and less calculational quantity.