Narrow linewidth lasers have important applications in optical communication and sensing. To accurately measure the linewidth, we built a laser linewidth measurement system based on 3×3 coupler. It uses the demodulation algorithm of Differential Cross Multiplication to get the phase noise, and then obtains the linewidth value according to the β-separation line method. At the same time, we introduced wavelet denoising in the signal processing in order to reduce the influence of low frequency noise. Two wavelet bases, db4 and sym11, which were more effective for this system were identified through simulation, and experiments were conducted using a 2 μm band laser. The results prove that the frequency noise power spectral density and linewidth values are reduced after using wavelet denoising.
A free space optical communication system at 2 μm band was preliminary demonstrated. The fiber laser source is a homemade continuous wave thulium-doped fiber laser with single-wavelength output. The center wavelength of the fiber laser is 2048.15 nm, and the fluctuation of the wavelength and optical power is less than 0.02 nm and 0.893 dB, respectively. By modulating and demodulating the radio frequency signal, the text and real-time video transmission was realized. The transmission distance in free space is ~1 m, and the transmission rate is less than 66 kbit/s. With further improvement, the system may be well suited in free space optical communication system.
We demonstrate a polarization maintaining fiber (PMF) sensing tape capable of measuring transverse-force (TF) and temperature simultaneously, based on the distributed polarization crosstalk analysis (DPXA) technique. Using a self developed automatic birefringence axis alignment equipment, a PMF is fixed on a transparent PET strip with the highest sensitivity to TF and high polarization crosstalk consistency along the axial direction. 6 preset crosstalk points with an adjacent spatial distance of 2 m along the PMF are produced through fusion splicing, and the temperature spatial resolution of 2 m is determined. The distributed TF and temperature sensing can be achieved by independently monitoring the intensities of TF-induced polarization crosstalk peaks and the distance-resolved delay spacing of adjacent preset crosstalk peaks, respectively. The maximum temperature measurement error is measured to be 6.84 ℃, which can be seen as the temperature resolution of sensing. The experimental results indicate that our sensing tape can be practical after several improvements.
For the first time, we experimentally study the transversal-stress (T-stress) induced polarization crosstalk behaviors in polarization maintaining fibers (PMFs) including the linearity, sensitivity, response time and recovery time, using a distributed polarization crosstalk analysis (DPXA) system. Using two Panda PMFs with or without polyacrylate buffer coating and one Bow-tie PMF with golden polyimide coating as experimental samples, we find that: I) the polarization crosstalk can be highly linear with the T-stress for PMFs no matter with or without coating; II) the polyacrylate coating can reduce the crosstalk sensitivity of naked PMFs by more than hundreds of times, while replacing the polyacrylate coating with polyimide coating can increase the sensitivity by tens or even hundreds of times; III) the polyacrylate coating can induce a significant recovery time of crosstalk when a T-stress is removed after a long loading time compared with that in naked PMFs or golden PMF with polyimide coating, however the crosstalk response speed is too fast to be measured by the DPXA system. Additionally, we also find that the current polyimide coating technique still needs to be improved further to reduce the crosstalk base level. This work will be very useful for PMF-based distributed sensing applications and sensing PMF manufacturing.
Optical fiber devices and applications in the 2-μm band have been investigated extensively due to its unique advantages, such as eye safety. A fiber sensor based on tilted fiber Bragg grating with a grating plane angle of 2 deg is fabricated and experimentally tested. To the best of the authors’ knowledge, this is the first tilted fiber Bragg grating sensor to realize simultaneous measurements of temperature, axial strain, and a certain range of surrounding refractive index (SRI) at the 2-μm band. This paper uses the wavelength detection method and selects three independent resonant wavelengths as the investigated parameters. Results show that perturbations of temperature, axial strain, and SRI can shift the wavelengths of the core mode resonance and cladding mode resonance to some degree. The temperature sensitivities of the core mode and cladding mode are nearly the same, but their axial-strain sensitivities are different. Furthermore, the core mode is insensitive to the change in SRI. The sensitivities of SRI, temperature, and strain can thus be obtained by experimentally determining the wavelength shifts of the three independent resonance peaks. A 3 × 3 matrix containing the relationship coefficients between the disturbances of temperature, axial strain, and SRI and wavelength shifts is constructed. By reversely solving the matrix equation, variations in temperature, strain, and SRI can be obtained using the experimental determination of wavelength drifts.
Polarization crosstalk is a phenomenon that the powers of two orthogonal polarization modes propagating in a polarization maintaining (PM) fiber couple into each other. Because there is certain mathematical relationship between the polarization crosstalk signals and external perturbations such as stress and temperature variations, stress and temperature sensing in PM fiber can be simultaneously achieved by measuring the strengths and locations of polarization crosstalk signals. In this paper, we report what we believe the first distributed temperature sensing demonstration using polarization crosstalk analysis in PM fibers. Firstly, by measuring the spacing changes between two crosstalk peaks at different fiber length locations, we obtained the temperature sensing coefficient (TSC) of approximately −0.73 μm/(°C•m), which means that the spacing between two crosstalk peaks induced at two locations changes by 0.73 μm when the temperature changes by 1 °C over a fiber length of 1 meter. Secondly, in order to bring different temperature values at different axial locations along a PM fiber to verify the distributed temperature sensing, four heating-strips are used to heat different fiber sections of the PM fiber under test, and the temperatures measured by the proposed fiber sensing method according to the obtained TSC are almost consistent with those of heating-strips measured by a thermoelectric thermometer. As a new type of distributed fiber temperature sensing technique, we believe that our method will find broad applications in the near future.
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