Time-expanded phase-sensitive (TE-φ )OTDR is a distributed optical fiber sensing (DOFS) technique that takes advantage of the dual-frequency comb technology to offer distributed, dynamic, and high-spatial resolution measurements. The performance delivered by this recent approach is unmatched by any other DOFS, combining the high resolution of OFDR with the potential for long range and fast sampling of φ OTDR. In this contribution, we present an optimized TE-φ OTDR scheme with important improvements with respect to the traditional one. In particular, the new architecture uses electrooptical phase modulation instead of intensity modulation, increasing the energy-efficiency. Additionally, it employs an optical hybrid to double the spectral efficiency of the system, which in practical terms results in doubling the spatial resolution for the same interrogating comb bandwidth. The proposed architecture has been experimentally validated through a scheme providing 5 mm of spatial resolution, 80 m of range and 70 Hz sampling rate with a simple, compact and low-cost setup using field-programmable gate arrays (FPGA) and relatively low bandwidth photodetection (2 MHz).
Frequency combs, analogue to digital sampling, and free space optical communications are emerging applications that can benefit from advances in elegant, compact, and high-performance miniature mode-locked lasers. By virtue of their ‘chip-scale’, and multiplicity of adjacent waveguides, compact planar waveguide devices are suited to efficiently operate in the ~0.5 GHz to 10 GHz pulse regimes, and to have key advantages when employed as dual-frequency combs. The erbium ytterbium cerium co-doped ZBLAN chips we have developed have achieved transform limited mode-locked output near 1.55 μm, pulse lengths down to 180 fs, and operation up to 5 GHz. This paper will also cover our more recent work on demonstrating a dual-frequency comb based on simultaneously mode-locking adjacent waveguide lasers in a common-cavity setup and demonstrate its spectroscopic potential by rapidly (~ms) acquiring high resolution and broad (<2 THz) molecular spectra of molecules.
Passively mode-locked sub 200 fs pulses are generated from Er-Yb co-doped ZBLAN waveguide laser using a semiconductor saturable absorber mirror repetition rates of up to 533 MHz. At 156 MHz and 1556 nm central wavelength, the chip laser operates with a broad 25 nm bandwidth. The waveguides were written in the Er-Yb co-doped ZBLAN glass by using ultrafast laser inscription.
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