•Based on compressed sensing with non-subsampled contourlet transform (NSCT) and the total variation model, an effective method is proposed to evaluate the optical properties of real photonic crystal fibers (PCF).•The cross section images of real PCFs are rebuilt effectively by using only 36% image data.•Optical properties of commercial PCFs are evaluated efficiently without requirement of expensive apparatus and long samples.A real photonic crystal fibers (PCFs) evaluation approach based on compressed sensing with non-subsampled contourlet transform (NSCT) and the total variation model is proposed for modeling optical properties of the real PCFs. The classical images of a commercial large mode area PCF and polarization-maintaining PCF are used to verify the effectiveness of the proposed method. Experimental results demonstrate that the cross section images of real PCFs are rebuilt effectively by using only 36% image data for evaluating the optical properties with the same accuracy as by 100% data. To the best of our knowledge, this is the instance of applying the compressed sensing with the NSCT and total variation to reconstruct the cross section images of PCFs for quickly evaluating the optical properties of real PCFs without the requirement of long fiber samples and expensive measurement apparatuses.

•A highly-nonlinear polarization-maintaining photonic quasi-crystal fiber (PQF) based on As2Se3 glass is proposed.•The nonlinearity coefficient reaches up to 2079 W−1km−1 while the birefringence reaches up to the order of 10−2.•The spectral width for mid-infrared x- and y-polarized SC covers 1 µm–10.2 µm and 1 µm–12.5 µm, respectively.•The polarization state can be well maintained when the incident angle of the input pulse changes within ±2°.We propose an As2Se3-based photonic quasi-crystal fiber (PQF) with high nonlinearity and birefringence. By optimizing the structure parameters, a nonlinear coefficient up to 2079 W−1km−1 can be achieved at the wavelength of 2 μm; the birefringence reaches up to the order of 10−2 due to the introduction of large circular air holes in the cladding. Using an optical pulse with a peak power of 6 kW, a pulse width of 150 fs, and a central wavelength of 2.94 μm as the pump pulse, a mid-infrared polarized supercontinuum is obtained by using a 15 mm long PQF. The spectral width for x- and y-polarizations covers 1 μm–10.2 μm and 1 μm–12.5 μm, respectively. The polarization state can be well maintained when the incident angle of the input pulse changes within ±2°. The proposed PQF, with high nonlinear coefficient and birefringence, has potential applications in mid-infrared polarization-maintaining supercontinuum generation.

•Based on the total variation (TV) model in Haar wavelet domain, an effective method is proposed to evaluate the optical properties of real photonic crystal fibers (PCFs).•The noise suppression ratio is improved and the cross-section images of PCFs are rebuilt with higher accuracy in comparison with traditional denoising methods.•Optical properties of commercial PCFs and homemade side-leakage PCF are evaluated efficiently without requirement of expensive apparatus and long samples.•Propose total variation model in wavelet domain to denoise cross sections of PCF.•Rebuild cross sections of PCF with high accuracy. Improve evaluating accuracy of optical properties of PCF.An evaluation method based on the total variation model (TV) in wavelet domain is proposed for modeling optical properties of real photonic crystal fibers (PCFs). The TV model in wavelet domain is set up to suppress the noise of the original image effectively and rebuild the cross section images of real PCFs with high accuracy. The optical properties of three PCFs are evaluated, including two kinds of PCFs that supplied from the Crystal Fiber A/S and a homemade side-leakage PCF, by using the combination of the proposed model and finite element method. Numerical results demonstrate that the proposed method can obtain high noise suppression ratio and effectively reduce the noise of cross section images of PCFs, which leads to an accurate evaluation of optical properties of real PCFs. To the best of our knowledge, it is the first time to denoise the cross section images of PCFs with the TV model in the wavelet domain.

•Sm3+ and Eu3+ doped SLP can emit orange-red and red light, respectively.•Energy transfer from Sm3+ to Eu3+ in SLP occurs.•Emission color and intensity of SLP:Sm3+, Eu3+ can be tuned by tuning relative content of Sm3+/Eu3+.A series of Sm3+, Eu3+, and Sm3+–Eu3+ doped Sr3La(PO4)3 (SLP) are synthesized by using a high temperature solid-state method. SLP:Eu3+ emits red light, and the strongest peak locates at 613 nm under the 392 nm radiation excitation, which is attributed to the 5D0 → 7F2 transition of Eu3+. SLP:Sm3+ produces red-orange light, and the peak locates at 598 nm under the 400 nm radiation excitation, which is assigned to the 4G5/2 → 6H7/2 transition of Sm3+. The energy transfer from Sm3+ to Eu3+ in SLP has been validated and the critical distance (Rc) of Sm3+ to Eu3+ in SLP is calculated to be 1.50 nm. With increasing the Eu3+ doped concentration, the energy transfer efficiency (Sm3+ → Eu3+) gradually increases to 59.6%. Moreover, the emission color and emission intensity of SLP:Sm3+, Eu3+ can be tuned by properly tuning the relative doped composition of Sm3+–Eu3+.

A wavelength-tunable fiber laser based on a twin-core fiber (TCF) comb filter is proposed and demonstrated. The TCF comb filter is fabricated by splicing a 0.85 m long TCF between two standard single mode fibers (SMFs) and with exhibits a good linear strain characteristic with a sensitivity of 1.23 pm/με. The wavelength of the laser can be linearly tuned from 1558.04 nm to 1553.62 nm by applying an axial strain to the TCF comb filter. The optical signal-to-noise ratio (OSNR) of the fiber laser reaches 45 dB. The 3 dB bandwidth is 0.02 nm. The fluctuation of the laser peak in the output power and the wavelength is less than 0.5 dB and within 0.05 nm, respectively. The fiber laser has the advantages of having a simple structure and stable operation under room temperature.Highlights► A wavelength-tunable fiber laser based on a twin-core fiber comb filter is proposed and set up. ► TCF comb filter is fabricated by splicing a 0.85 m long TCF between two standard single mode fibers. ► TCF comb filter exhibits a good linear strain characteristic, and its sensitivity is 1.23 pm/με. ► Laser wavelength can be linearly tuned from 1553.62 to 1558.04 nm by changing the axial strain. ► The optical signal-to-noise ratio of the fiber laser reaches 45 dB and its 3 dB bandwidth is 0.02 nm.

A novel refractive index (RI) sensor based on an asymmetrical fiber Mach–Zehnder interferometer (AFMZI) was realized by concatenating single-mode abrupt taper and core-off section. Compared with the normal two-taper MZI, the proposed sensor enlarges the evanescent field in the surrounding medium through the core-offset section. As a result, the RI sensitivity to the surrounding medium can be improved. Experimental results show that the RI sensitivity of the proposed sensor is 28.2 nm/RIU (refractive index unit) and 59.2 nm/RIU for interferometer length of 30 mm and 50 mm, respectively. It is much higher than the RI sensitivity of the normal two-taper MZI which is 14.7 nm/RIU and 19.1 nm/RIU for the corresponding interferometer length. Such kinds of low cost and highly sensitive RI sensor would find application in chemical and biological sensing fields.Highlights► An interferometer was proposed for refractive index sensing. ► Enlarged evanescent filed by core-offset section gives an enhanced sensitivity. ► The high order cladding mode is efficiently suppressed by the core-offset section. ► The highest sensitivity is 59.2 nm/RIU.

The mode coupling in the tilted long period fiber grating (TLPG) is analyzed by using the full vector complex coupled mode theory (FV-CCMT). Compared with the non-tilted LPG, new resonance bands corresponding to the LP1m cladding modes are observed in the transmission spectrum of the TLPG. The coupling to LP1m cladding modes is the combination of the coupling to degenerate vector modes TE0m, TM0m and HE2m with different propagation constants. The maximum coupling between LP01 and LP1m modes occurs at a tilt angle of ∼87°. Numerical results show that the state of polarization (SOP) of the input light would induce the shift of the new resonance wavelength. Also, the RI sensitivity of the new resonance wavelength is much higher than that of the normal resonance wavelength corresponding to the LP0m cladding modes. Such TLPG would find application in chemical and biological sensing fields.Highlights► Tilted long period fiber grating is investigated using the full vector complex coupled mode theory. ► New resonance bands corresponding to LP1m are found in tilted long period fiber grating. ► Coupling to LP1m is the combination of the coupling to degenerate modes TE0m, TM0m and HE2m. ► State of polarization of the input light would induce the shift of the new resonance wavelength. ► RI sensitivity of new resonance wavelength is higher than that of normal resonance wavelength.

A switchable multi-wavelength fiber ring laser is proposed and experimentally demonstrated with a novel side-leakage photonic crystal fiber (SLPCF) based filter incorporated into the ring cavity at room temperature. Stable multi-wavelength laser operations can be achieved due to the spatial mode beating, polarization hole burning and spectral hole burning effects. By adjusting the polarization controller appropriately, the laser can be switched among the single-, dual- and triple-wavelength lasing oscillations whose signal-to-noise ratio is up to 50 dB. In addition, the lasing wavelength can be also tuned and switched by applying the strain to the filter.Highlights► Switchable multi-wavelength fiber ring laser was proposed and demonstrated. ► Side-leakage photonic crystal fiber was used to construct the filter. ► The laser can be switched among the stable single-, dual- and triple-wavelength lasing operations. ► Signal-to-noise ratio of the lasing operation is up to 50 dB. ► Lasing wavelength can be also tuned and switched by applying strain to the filter.

An in-fiber modal interferometer is demonstrated by splicing a section of homemade dual-concentric-core photonic crystal fiber (DCCPCF) and two segments of single-mode fibers (SMFs) with collapsing the air holes in the splice regions. By analyzing the transmission spectra and the spatial frequency spectra of the interferometers, it's observed that the main interfering modes are the high-order ring modes, which are different from the previously reported interferometers using the single-, few- or multi-mode fibers. The influences of the lengths of DCCPCF and the first collapsed region on the interferometers were investigated experimentally. The strain, temperature and refractive index characteristics of the interferometer were analyzed.Research Highlights► Dual-concentric-core photonic crystal fiber was used to construct an interferometer. ► The interfering modes were identified by analyzing the spectra of interferometers. ► The polarization dependent loss of the interferometer was measured. ► We analyzed effects of the fiber and first collapsed region length on interferometer. ► The sensing characteristics of the interferometer were analyzed.

Our recent research on designing microstructured fiber with novel dispersion properties is reported in this paper. Two kinds of photonic crystal fibers (PCFs) are introduced first. One is the highly nonlinear PCF with broadband nearly zero flatten dispersion. With introducing the germanium-doped (Ge-doped) core into highly nonlinear PCF and optimizing the diameters of the first two inner rings of air holes, a new structure of highly nonlinear PCF was designed with the nonlinear coefficient up to 47W−1·km−1 at the wavelength 1.55 μm and nearly zero flattened dispersion of ±0.5 ps/(km·m) in telecommunication window (1460–1625 nm). Another is the highly negative PCF with a ring of fluorin-doped (F-doped) rods to form its outer ring core while pure silica rods to form its inner core. The peak dispersion −1064 ps/(km·nm) in 8 nm full width at half maximum (FWHM) wavelength range and −365 ps/(km·m) in 20 nm (FWHM) wavelength range can be reached by adjusting the structure parameters. Then, our recent research on the fabrication of PCFs is reported. Effects of draw parameters such as drawing temperature, feed speed, and furnace temperature on the geometry of the final photonic crystal fiber are investigated.

•Fiber loop mirror sensor using a side-leakage photonic crystal fiber is proposed.•The sensor is sensitive to torsion, strain and temperature.•Simultaneous measurement of torsion, strain and temperature is achieved.A fiber optic sensor for simultaneous measurement of multi-parameter is proposed. The fiber sensor is realized by a fiber loop mirror based on a piece of homemade side-leakage photonic crystal fiber. Affected by the group birefringence of the side-leakage photonic crystal fiber, which induced by the elliptical germanium-doped core and the linear side-leakage defect, the fiber loop mirror has different wavelength responses to torsion, strain and temperature at different interference fringe valleys. By monitoring the wavelength shifts of three selected valleys in the transmission spectrum, simultaneous measurement of torsion, strain and temperature is achieved. The maximal sensitivities to torsion, strain and temperature are 151.14 pm/°, 1 pm/με and 71 pm/°C, respectively.