High-sensitivity mode interferometer enabled precisely tunable and switchable thulium-doped fiber laser.

A tunable multi-wavelength narrow-linewidth thulium-doped fiber laser employing two-stage cascaded Sagnac loop mirrors is proposed and experimentally demonstrated. The designed fiber laser is composed of a pump source, wavelength division multiplex, circulator, thulium-doped fiber, polarization controllers (PCs), couplers and polarization-maintaining fibers (PMFs). Two cascaded Sagnac loops are used as the cavity reflector and filter, and the proposed filter is fabricated using two sections of PMFs with 2-m and 1-m lengths, respectively. In the experiment, the laser threshold is 110 mW, and laser can emit single, double, triple, quadruple and quintuple wavelengths in the spectral range of 1873–1901 nm through the simultaneous adjustment of the two PCs. The power fluctuations and 3-dB linewidth are less than 2.1 dB and 0.2 nm, respectively, over 10 min at room temperature, and the side-mode suppression ratio is greater than 20 dB. The proposed laser will be useful in various fields, such as spectral analysis, fiber sensing and optical communication.

Tunable and switchable multiwavelength fiber laser incorporating tapered few-mode fiber Bragg gratings

We propose and demonstrate a tunable self-excited multiwavelength Brillouin-erbium fiber laser based on a novel in-line Mach–Zehnder interferometer sensor (NMZIS) with variable temperature. The configuration of the NMZIS is a section of uncoated single-mode fiber sandwiched between an up-taper and multimode fiber. With the temperature changing from 30 °C to 90 °C, the transmission spectrum of the NMZIS shifts towards longer wavelengths. By embedding the NMZIS to act as a fiber filter in the ring cavity, the wavelength of the output multiwavelength comb is tuned from 1552.35 to 1564.48 nm. The corresponding generation of 21 Brillouin–Stokes lines with 25 dB optical signal-to-noise ratio is achieved.

Based on a piece of highly-nonlinear near-zero-dispersion-flattened photonic crystal fiber (PCF), a broadly tunable multiwavelength erbium-doped fiber laser is proposed by using a bi-directionally pumping scheme. This kind of PCF induces the modulation-instability-assisted four-wave mixing to generate new wavelengths. The proposed laser with excellent stability is tunable and switchable by adjusting the fiber Bragg gratings and the variable optical attenuators. The outstanding merits of the proposed multiwavelength laser are the flexible tuning and the ultrabroad spectral range over 150 nm. Especially, the proposed laser source can work at the wavelength of less than 1460 nm, overcoming the limit of gain bandwidth of erbium-doped fiber.

We experimentally demonstrate a switchable multiwavelength thulium-doped fiber laser with cylindrical vector beam (CVB) emission, which is the first report, to our best knowledge, on the generation of CVBs in a 2 µm wavelength band. The Sagnac loop filter, made up of a 50:50 coupler, a section of polarization-maintaining fiber and a polarization controller, serves as a periodical filter and accounts for the excitation of multiwavelength channels. The mode-selective coupler consisting of both tapered single-mode and four-mode fibers has been designed and fabricated as a mode converter from LP01 to LP11 mode with a purity of >80%. By adjusting the polarization controllers, laser outputs with 1–3 switchable wavelengths have been realized in the spectrum ranging from 1988–1998 nm. The laser could find miscellaneous applications in wavelength/mode-division multiplexing system, etc.

In this paper, a stable highly tunable and switchable multi-wavelength fiber laser is presented. It is based on the filtering characteristics of the Sagnac loop mirror and the polarization-dependent devices, the polarization hole burning effect is induced to generate multi-wavelength output with high optical signal-to-noise ratio and narrow 3 dB linewidth, optical signal-to-noise ratio is about 50 dB and linewidth is less than 0.02 nm. The tunable bandwidth is about 30 nm, which covers almost the entire C-band. By the adjusting polarization controller, the number and wavelength spacing of laser emissions can be controlled and switched. The single-wavelength exhibits good power and wavelength stability, the peak power fluctuation is less than 0.04 dB and the wavelength drift is less than 0.04 nm, and the wavelength spacing of the dual-wavelength can be tuned from 0.67 to 27.4 nm. In the proposed structure, multi-wavelength up to 6 laser emissions can be obtained span from 1528 to 1562 nm.

A novel optical fiber humidity sensor based on a Mach–Zehnder Interferometer (MZI) is proposed and experimentally demonstrated. The MZI is fabricated by three sections of single mode fiber (SMF) and two sections of multimode fiber (MMF), where the two MMFs are spliced among the three SMFs. To obtain an excellent sensor, two sections of MMF are etched with hydrofluoric acid, then sodium alginate is coated on the corroded areas and immersed in calcium chloride solution to form calcium alginate (CaAlg) hydrogel by fully crosslinking sodium alginate with calcium ions. While humidity is between 15%~80%, the sensitivity of the sensor can reach 0.48346 dB/%RH. In addition, the proposed sensor shows good performances in the repeatability, stability, response and recovery time, which can be potentially used in the medical storage field.

A reflective tactile sensing scheme based on fiber specklegram analysis is proposed in this study. The specially designed sensor probe with a reflector inside was 3D printed from elastic material. A section of single mode fiber (SMF) and a section of multimode fiber (MMF) were embedded under the reflector. The light emitted from the SMF is reflected by the reflector and the reflected light illuminates the MMF. At the output facet of the MMF, specklegram is detected by a camera. The reflector consists of two soft mirrors that are perpendicular to each other originally. The contact force on the sensor probe causes the variation of the angle between the two mirrors. The transmission path of the light from SMF to MMF changes accordingly, which results in a changed specklegram. In this way, the zero-mean normalized cross-correlation coefficients of the fiber specklegrams decrease linearly with the increase of contact force. The sensitivity and measurement range of contact force are examined with MMFs of different diameters under two operating wavelengths. The negligible effect of fiber bending on the force sensing was also experimentally confirmed. In addition, the convolutional neural network is used to classify the specklegrams of three contact forces applied at nine positions of the sensor probe, and the average accuracy of classification is 100%, which shows its capability to identify the contact position on the probe. On the whole, the proposed tactile sensor is confirmed with a high sensitivity of force sensing in a proper measurement range and a capability to localize the contact position. The reflective structure also facilitates the integration with robot arm and other intelligent structures.

An all-fiber Mach–Zehnder interferometer curvature sensor based on a helical four-core fiber (HFCF) is proposed. The sensor is constructed by splicing two sections of single-mode fiber with a short section of HFCF. A simulation model of the HFCF is established using the beam propagation method, and the consistency between experimental and simulation results is verified. Experiments show that the proposed sensor exhibits a curvature sensitivity of 4.1545 nm m −1 within the curvature range of 0.8–188 m −1 °C –1 within the temperature range of 25 °C–90 °C. Furthermore, the sensor has a simple structure and low manufacturing cost. The use of helical core fiber offers a novel and feasible solution for designing fiber curvature sensors.

Multi-wavelength fiber lasers are under intense research due to their potential in many application fields like wavelength-division-multiplexing communication, fiber-based sensing, ranging, microwave photonics and terahertz generation, etc. In this manuscript, we will present our detailed investigation on tunable high power multi-wavelength double-clad fiber laser pumped by high-power multimode laser diode. The fiber laser is constructed in a ring-cavity. By adjusting the polarization-controller inside the cavity, the laser can operate at single-wavelength, dual-wavelength and triple-wavelength regime. The maximal output power is 5.5 watt and the conversion efficiency is 68.8%.

Hybrid configuration of Sagnac and modal interferometer with high extinction ratio

Tunable multiwavelength Brillouin-Raman fiber laser in a linear cavity with spectrum reshaped by Rayleigh scattering

A switchable multi-wavelength thulium-doped fiber laser (TDFL) using a four-mode fiber based Sagnac loop filter was proposed and demonstrated. The Sagnac loop, incorporating a 4.8 m long four-mode step-index fiber, acting as a comb filter was analyzed theoretically and experimentally. By adjusting two polarization controllers, eight stable single-wavelength operations and five stable dual-wavelength operations were obtained with easy switching between the different operation modes. The wavelength range covered during the single-wavelength switchable operation with a signal-to-noise ratio of up to 55 dB was ∼28.51 nm. In dual-wavelength switchable operation, the maximum and minimum wavelength spacings were ∼21.78 nm and ∼1.50 nm, respectively. Due to the abundant wavelength resources, the proposed TDFL has great potential in wavelength division multiplexing free-space optical communications, optical fiber sensing, and photonic generation of microwave signals.

A tunable and switchable multi-wavelength random fiber laser (MW-RFL) is proposed and experimentally demonstrated in this work, which consists of a random distributed feedback mechanism, an improved Lyot filter, and a nonlinear optical loop mirror (NOLM). The theoretical derivation and simulation analysis of the improved Lyot filter are carried out by using the transfer matrix theory. The results show that by adjusting the polarization controller (PC) in the filter, not only the channel interval can be switched between 0.9 and 0.45 nm, but also the extinction ratio (ER) of the transmission spectrum can be adjusted and the wave peak position can be moved. The feedback mechanism of the proposed laser is provided by the randomly distributed backward Rayleigh scattering generated in the single-mode fiber. By changing the pump laser power, the number of output laser channels will be linearly increased. When the channel interval is kept constant, the wavelength can be tuned within a certain range by carefully adjusting the PC3 in the NOLM. When the channel interval of the spectrum is 0.9 nm, the wavelength tuning ranges of the three and four laser channels are 3.6 and 3.5 nm, respectively. When the channel interval of the spectrum is 0.45 nm, the wavelength tuning ranges of the three and four laser channels are 2.8 and 2.77 nm, respectively. Then the variations of the output spectrum when using 25 km single-mode fiber (SMF) are studied. It is observed that the linewidth and side mode suppression ratio (SMSR) of the laser channel are significantly increased. Finally, the stability of the random laser is discussed, and the maximum central wavelength drift and peak power fluctuation measured at room temperature are 0.05 nm and 0.34 dB, respectively. The average peak power and SMSR are maintained at −4.47 dBm and 34.6 dB, respectively, which confirms that the laser has good stability.

A tunable and switchable multi-wavelength Erbium–Brillouin random fiber laser (MW-EBRFL) with a half-open cavity structure and incorporating a segment of highly nonlinear fiber (HNLF) is proposed and experimentally demonstrated. Due to HNLF being with high nonlinear coefficient, high Brillouin gain coefficient and high Rayleigh scattering (RS) coefficient, a reduced length of HNLF can provide a strong random distributed feedback (RDFB) and high stimulated Brillouin scattering (SBS) gain. In addition, an optical fiber amplify loop (OFAL) including a high-power erbium-doped fiber amplifier (HP-EDFA) and an optical attenuator (ATT) is constructed to provide a strong Brillouin pump (BP) and controllable optical feedback. Using the presented MW-EBRFL, up to 11 orders of stable Stokes laser lines with a wavelength spacing of 0.074 nm and the minimum optical signal-to-noise ratio (OSNR) of 41.22 dB are obtained. System parameters including BP power and HP-EDFA pump power influence on output of MW-EBRFL are also investigated. By adjusting BP wavelength, multi-wavelength output with 6–11 channel numbers can be tuned in a wide wavelength range spanning 36 nm. The proposed MW-EBRFL also can realize switchable multi-wavelength generation of 1 ∼ 11 order Stokes with an increasing wavelength number in turn, only by simply adjusting HP-EDFA pump power. Furthermore, when a polarization-maintaining fiber loop mirror filter (PMF-LMF) is cascaded with output port of the cavity, it can be very convenient to achieve wavelength switching between single- and double-Brillouin frequency spacing (BFS), only by tuning polarization controller in PMF-LMF.