We propose a precision measurement method of laser linewidth based on short-delay self-homodyne, using the second peak-valley difference (SPVD) feature of the coherent power spectrum to fit laser linewidth. The SPVD model of the self-homodyne coherent envelope spectrum was established. One-to-one correspondence among the values of SPVD, the delay length, and the laser linewidth was determined theoretically and through simulations, while the reliability and stability of the method was verified experimentally. By comparing the detected results, it is found that the fitted laser linewidth obtained by the self-homodyne system is closer to its true value than that obtained by the self-heterodyne system. Hence, the simpler structure of the short-delay self-homodyne coherent envelope laser linewidth measurement method proposed is expected to substitute the previous laser linewidth measurement method, including complex short-delay self-heterodyne coherent envelope laser linewidth measurement method and traditional self-homodyne/heterodyne laser linewidth measurement method, to achieve more precise laser linewidth value.

We propose a novel design of photonic crystal fiber to achieve the single-polarization single-mode transmission. The topology of this design only employs, with a view to simplicity and stability, uniformly sized circular air holes, and they are all regularly latticed in a pure-silica background. Guiding properties are studied by use of the full-vector finite element method, combined with perfectly matched layers as absorbing boundary conditions. It has been found that, increasing the hole-to-pitch ratio can be performed to broaden the SPSM band, however subject to a qualification that the radio should be inadequate to open a bandgap. In addition, influences of the ring number and the inner cladding coverage on transmission characteristics have been thoroughly investigated. An optimized sample is deliberately designed to offer an SPSM band with width of 120nm, centered at =1.55μm. Moreover, the proposed design demonstrates low confinement loss, high nonlinear coefficient, as well as excellent bending resistance.

Ultrasensitive refractive index (RI) sensing based on novel Vernier-effect demodulation through a single fiber Fabry–Perot interferometer (FFPI) is proposed and demonstrated. A core-offset fiber pair is used as an FFPI for RI sensing. An artificial reference spectrum (ARS) obtained by modulating and extracting a sensing spectrum twice is demonstrated. The Vernier effect is realized by superimposing the sensing spectrum with the ARS. By optimizing the cavity length of the FFPI and the free spectrum range (FSR) of the ARS, an ultrahigh sensitivity of 133948.33 nm/refractive index unit (RIU) with a magnification factor of 118.38 in the RI range of 1.3340~1.3354 is obtained. The magnification factor based on the ARS can be conveniently adjusted. Furthermore, the proposed scheme is suitable for various interferometric sensing applications.

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Based on enhanced Vernier effect, a compact fiber sensor with ultrahigh sensitivity is proposed for simultaneous transverse load (TL) and temperature measurements. A single mode fiber (SMF) is spliced with a segment of hollow-core fiber (HCF) coated with polydimethylsiloxane (PDMS), some PDMS is injected into the HCF, forming a Vernier sensor with an air cavity adjacent to a PDMS cavity. It is shown that TL and temperature changes give rise to opposite and remarkable different variations in lengths of the two cavities, thereby enhancing Vernier effect and in favor of simultaneous measurements of TL and temperature. Moreover, the limited sensitivity magnification due to the length mismatch between the two cavities is compensated for by reconstructing the Vernier envelope with a broadened free spectrum range (FSR) from output signal. As a result, the highest TL sensitivity reported so far of −2637.47 nm/N and a good condition number of 69.056 for the sensitivity coefficient matrix have been achieved.

In this paper, we demonstrate an active metamaterial manifesting electromagnetically induced transparency (EIT) effect in the microwave regime. The metamaterial unit cell consists of a double-cross structure, between which a varactor diode is integrated. The capacitance of the diode is controlled by a reversed electrical bias voltage supplied through two connected strip lines. The diode behaves as a radiative resonant mode and the strip lines as a non-radiative resonant mode. The two modes destructively interference with each other through conductive coupling, which leads to a transmission peak in EIT effect. Through electrical control of the diode capacitance, the transmission peak frequency is shifted from 7.4 GHz to 8.7 GHz, and the peak-to-dip ratio is tuned from 1.02 to 1.66, demonstrating a significant tunability.