In this paper, total harmonic distortion (THD) of phase generated carrier demodulation algorithm by digital arctangent
approach (PGC-DAT algorithm) is analyzed. THD due to the deviations of modulation depth and carrier phase delay is
investigated both theoretically and experimentally. The analytical expression of THD as the function of carrier
modulation depth, signal amplitude, initial phase and deviations of modulation depth and carrier phase delay has been
derived. According to our analysis, the maximum THD for different initial phases exists, which reaches the peak value
when the signal amplitude is close to 1.2 rad. The best modulation depth to minimize the maximum THD is 2 rad.
Finally, an experimental system is set up with the mean THD below -60 dB. The analytical expression of THD derived in
this paper agrees well with the experimental results.
The noise floor of the phase demodulation scheme using a 3×3 coupler was analyzed, where the signal was demodulated
by linear combination of the three outputs and digital inverse arctangent approach. According to our study, the noise
floor has connection with the noise intensity, noise correlation and initial phase. The approximate expressions to
calculate the noise floor were deduced and validated by simulations with input Gaussian white noise. Experiment results
show that the predicted noise floor distribution as a function of initial phase agreed well with the experiment results. The
noise floor of the experiment system was below 0.70 μrad/√Hz@1kHz.
An improved demodulation method by solving a quadratic equation was proposed for the phase generated carrier (PGC)
scheme with frequency modulation to suppress the impact of laser intensity modulation (LIM). The influence of LIM on
total harmonic distortion (THD) was analyzed for PGC algorithm considering laser intensity modulation coefficient
(LIMC), signal amplitude, initial phase and modulation depth. According to our analysis, the maximum THD of PGC
algorithm is more than -24dB when LIMC is 0.1. While the maximum THD is less than -65dB by using the improved
method when LIMC is less than 0.5, which is limited by the low-pass filter. For an experiment system with 0.285 LIMC,
the THD was -9dB for PGC algorithm. By using the improved method, the THD approached -50dB.
Phase Generated Carrier (PGC) with directly Frequency Modulation (FM) is one of the most important
demodulation methods for optical fiber interferometric sensor system. Previous research has confirmed that system
performance using Orthogonal Demodulation Type PGC (ODT-PGC) method is determined by many parameters,
such as signal phase delay, FM depth, laser intensity accompanying modulation. This article proposes a new PGC
demodulation method based on Fixed Phase Delay (FPD-PGC) by 3x2 directional coupler, using second-harmonic
components of two interferometric signals to demodulate. The demodulation principle of the new method is described
in detail and its performances have been studied. Theoretical analysis and experimental results show that the new
method combines main advantages of directional coupler method and ODT-PGC method, and eliminates, to a great
extent, the impacts of FM depth, signal phase delay, intensity modulation. Signal-to-total-Harmonic Ratio (SHR) of
new method increases more than 30dB compare with ODT-PGC method under the condition of intensity modulation
coefficient is 0.4. Besides that, Signal to Noise Ratio (SNR) also improves significantly.
An improved demodulation method for the phase-generated carrier (PGC) system by eliminating laser-intensity modulation (LIM) is proposed. The influence of LIM is analyzed theoretically and experimentally. Two parameters are used to describe the effect of LIM: the LIM coefficient (LIMC) and the LIM phase delay (LIMPD). Good stability of the LIMC and LIMPD is confirmed by experimentation with an actual system. The demodulation signal using the traditional method has a much greater higher harmonic component than the improved method due to LIM. The increase of the signal-to-total-harmonic ratio (SHR) using the improved method is >23 dB, and there is a corresponding improvement of 19 dB to the signal to noise and distortion (SINAD) and signal-to-noise ratio (SNR). A new prototype system using the improved PGC method for marine seismic sensing capable of demodulating multiple channels in parallel, simultaneously, is demonstrated. The SHR is stable at 56 dB when the LIMC is <0.5. Similar results are obtained for the SINAD and SNR. The demodulated signal's upper limit is ~100 rad at 100 Hz and 12 rad at 1 kHz, giving a dynamic range reaching 130 dB at 100 Hz. The system's SINAD is stable within 1 dB, whereas the SHR is stable within 2 dB in field application.
A new demodulation method is proposed to eliminate the light intensity disturbance(LID) due to the Phase Generate Carrier(
PGC) modulation method for the optical marine seismometer which is based on the fiber interferometer. A look up table by
the use of a nonlinear function has been demonstrated. The experimental results show that the performance of the system has
been improved with the harmonic suppression ratio(HSR) of the demodulation results increasing 30dB and the dynamic range
of the system increasing 26dB, comparing with the Differential Cross-Multiplying(DCM) algorithm for the PGC method. The
new method approaches a dynamic range of 130dB@100Hz with the HSR more than 50dB.
In this paper the parameters of a CW double-clad fiber laser are theoretically analyzed, which is of important references to designing the kindred double-clad fiber lasers. This paper also offers a personalizing design scheme of double-clad fiber lasers, with the influence of some important parameters on the output power clarified.