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We proposed and demonstrated a novel few-mode concentric-ring core fiber (FM-CRCF) for axial micro-strain measurement with fiber ring laser based on few-mode-singlemode-few-mode fiber structure. The core area of CRCF consists of four concentric rings which refractive indices are 1.448, 1.441, 1.450, 1.441, respectively. LP 01 and LP 11 are two dominated propagating mode groups contributing in the CRCF. In this few-mode-singlemode-few-mode structure, two sections of CRCF act as the mode generator and coupler, respectively. The basis of sensing is the center single mode fiber. Moreover, this structure can be used as an optical band-pass filter. By using fiber ring cavity laser, the axial micro-strain sensing system has high intensity (∼20 dB), high optical signal to noise ratio (∼45 dB) and narrow 3 dB bandwidth (∼0.1 nm). In the axial micro-strain range from 0 to 1467 μ ε , the lasing peak wavelength shifts from 1561.05 nm to 1559.9 nm with the experimentally sensitivity of ∼ 0.81 pm / μ ε . [ABSTRACT FROM AUTHOR]

A challenge for optical fiber biosensor is to achieve ultrahigh sensitivity with narrow full width at half maximum (FWHM) of the spectrum. To address this challenge, an ultrahigh-sensitivity microfiber interferometer fiber ring laser (FRL) biosensor is proposed and investigated for Listeria monocytogenes (L. monocytogenes) detection. The fiber biosensor is composed of a singlemode- tapered no core-singlemode (STNS) fiber configuration, which is functionalized with the anti- L. monocytogenes antibodies. An Erbium Doped Fiber Amplifier is applied to the sensor to excite laser and thus reduce the FWHM of the spectrum, which significantly improved the limit of detection (LoD). The proposed STNS FRL biosensor has excellent reproducibility, specificity and sensitivity for L. monocytogenes. The developed STNS FRL biosensor can directly detect L. monocytogenes cells with LoD as low as 1.0 cell/mL, indicating the capability for detecting single cell of L. monocytogenes. Real lettuce and milk samples have been tested and test result in lettuce and milk samples has deviations within ± 30 % from that of Phosphate-buffered saline (PBS) for L. monocytogenes concentrations vary from 101 to 103 cells/mL(g). The developed STNS FRL biosensor has ultrahigh sensitivity, good stability, reproducibility, and specificity, which has potential applications in diseases/medical diagnostics. • Developed new STNS fiber ring laser configuration capable detecting 1.0 cell/mL Listeria monocytogenes solution. • Provide very narrow FWHM of 0.15 nm due to the introduction of fiber ring laser configuration. • Demonstrated detecting 10 cells/ml Listeria monocytogenes in complex food matrices of real milk and lettuce samples. • The sensor structure has potential for medical diagnostics and food safety inspection. [ABSTRACT FROM AUTHOR]

A novel refractive index (RI) sensor has been demonstrated based on a simple dual-wavelength fiber ring laser by incorporating two fiber Bragg grating (FBG) filters. The grating filters are made of two FBGs with different cladding thicknesses. By inserting the filters into the fiber ring cavity, a stable dual-wavelength fiber laser can realize at room temperature. The cladding-etched FBG (thinned) acted as the RI sensing elements, while the normal one as temperature reference. As the RI increases, the laser wavelength corresponding to the thinned FBG moves toward longer wavelength, while that corresponding to the normal FBG is unchanged. Then, RI changes can be determined by measuring the separation of the two lasing wavelengths. Moreover, compared with the conventional thinned FBG-based sensors, the fiber laser sensor has a narrow bandwidth, which is beneficial to high precision sensing. The RI measurement resolution of proposed sensor is about $1.44\times 10^{{ {-5}}}$ RIU. [ABSTRACT FROM PUBLISHER]

A continuous-wave tunable thulium-doped all-fiber ring laser near 2 μm under in-band pumping at 1.6 μm has been demonstrated in the filterless configuration, specifically to act as a compact and simple laser source for gaseous carbon dioxide detection. The laser parameters suitable for tunable operation have been identified through numerical modelling and subsequent experimentation. A laser power of 8 mW with a tunable range of 36 nm (in between 1968 to 1932 nm) is demonstrated by intracavity loss variation. This laser configuration can be superior to the FBG-based configurations due to the possibility of a wider tuning range as compared to the latter. [ABSTRACT FROM AUTHOR]

A review for optical fiber sensors based on fiber ring laser (FRL) demodulation technology is presented. The review focuses on the principles, main structures, and the sensing performances of different kinds of optical fiber sensors based on FRLs. First of all, the theory background of the sensors has been discussed. Secondly, four different types of sensors are described and compared, which includes Mach–Zehnder interferometer (MZI) typed sensors, Fabry–Perot interferometer (FPI) typed sensors, Sagnac typed sensors, and fiber Bragg grating (FBG) typed sensors. Typical studies and main properties of each type of sensors are presented. Thirdly, a comparison of different types of sensors are made. Finally, the existing problems and future research directions are pointed out and analyzed.

• A switchable and tunable multi-wavelength EDFL based on single STS filter is proposed and experimentally demonstrated. • The STS filter has a comb spectrum with obvious envelope and uniform fluctuation. • By adjusting the PC, the fiber laser can work in adjacent or non-adjacent multi-wavelength state. • The proposed multi-wavelength EDFL has the advantages of simple structure, low cost, easy adjustment and good stability. A novel switchable and tunable multi-wavelength erbium-doped fiber ring laser (EDFL) is proposed and experimentally demonstrated by using a single SMF-TMF-SMF (STS) structure filter. The STS filter, based on mode interference, is fabricated by sandwiching a section of two-mode fiber (TMF) between single mode fibers (SMF) through core-offset splicing, which has a comb spectrum with obvious envelope and uniform fluctuation. The transmission characteristics of the STS filter can be changed by adjusting the polarization controller (PC) wrapped on the TMF. Based on the cooperation of the STS filter and polarization dependent loss (PDL), the fiber laser can be individually switchable and widely tunable. By adjusting the PC, the outputs can be switched from single-wavelength to adjacent eight-wavelength and can be tuned in the range of 1528 ∼ 1561 nm. Furthermore, non-adjacent dual-, three- and four-wavelength with different intervals can also be achieved by appropriately adjusting the PC. With the assistance of four wave mixing (FWM) effect, the fiber laser has a stable output. The tuning and switching of the proposed multi-wavelength EDFL is simple and flexible. [ABSTRACT FROM AUTHOR]

We reported an erbium-doped fiber ring laser (FRL) displacement sensor based on a single-mode fiber (SMF) loop tied into the laser cavity. The proposed fiber loop structure acts as the sensing head as well as the filter of the FRL. A good linear relationship is obtained both in principle and in experiments between the displacement of one end of the loop and the central wavelength of the proposed FRL. The average sensitivity of this sensor reaches 227.5 pm/mm in the displacement range of 0–30 mm. High optical signal-to-noise ratio ( $\sim 52$ dB) and narrow 3-dB bandwidth (<0.7 nm) are also achieved due to the FRL structure. Moreover, the macrofiber loop, as the key sensor head of the proposed sensing system, is tied with simple SMF without removing the coating, which has obvious advantages of firm structure with a wide range of displacement measurement and easy fabrication with low-cost SMF. [ABSTRACT FROM PUBLISHER]

We present a new method for multigas detection based on multiband fiber ring laser. Wavelength division multiplexing is used to realize multiband laser operation. Laser intracavity absorption spectroscopy combined with wavelength modulation technique is used to improve the measurement sensitivity. The second harmonic signal is extracted by the software-based lock-in algorithm for gas concentration retrieval. The relationship between the harmonic amplitudes and gas concentrations is investigated under both weak absorption and strong absorption to improve the accuracy of concentration retrieval. When the system is applied to detect the mixed gases of acetylene, carbon monoxide and carbon dioxide in C + L band, the minimum detectable concentrations are 0.6 ppm, 17.4 ppm and 19.2 ppm respectively. [ABSTRACT FROM AUTHOR]

In this letter, we investigate a novel refractive index sensor based on a simple fiber ring laser incorporating a bent fiber filter. The bent fiber structure acted as a fiber filter and a sensing element. The bent fiber filter is made of a section of semicircular bare standard single-mode fiber with a selected bending radius. The gain medium (erbium-doped fiber) is pumped by a 980 nm wavelength laser. Laser emission around 1557.3 nm with a low pumping threshold is obtained when a bent fiber filter with 4.5 mm bending radius is used. The 3 dB bandwidth of the laser is less than 0.3 nm, which is beneficial to high precision sensing. The medium refractive index varied from 1.3259 to 1.3730, and the relationship between the lasing wavelength and the medium refractive index exhibited linear behavior, where good linearity of about 60 nm/RIU was obtained. Finally, we fabricated sensor prototypes with different bending radii. When a 4 mm bending radius was selected, the maximum sensitivity is 124 nm/RIU. The fabrication of the proposed refractive index sensor is simple and cost-effective, which makes it a good candidate for the potential sensing applications. [ABSTRACT FROM PUBLISHER]

• A temperature measurement method with error suppression is performed by a dual-wavelength FRL with an intracavity Sagnac interferometer. • A dual-wavelength filter is formed by cascading two DWDMs. • The intensity difference of two peaks is measured by difference method. • Compared with single-wavelength configuration, the measurement error of dual-wavelength is reduced from 0.443% to 0.066%. The sensors based on intracavity intensity modulation of fiber ring laser (FRL) have shown excellent performance in sensing resolution and detection limit, but suffer from large measurement errors. The measurement errors mainly result from the power fluctuation of pump source and environmental interference. Therefore, the methods of error suppression are of great significance for FRL sensors. In this paper, a temperature measurement method with error suppression is performed by a dual-wavelength FRL with an intracavity Sagnac interferometer. The sensing principle of single- and dual-wavelength FRL sensors have been analyzed. By the sensing manner of intensity difference of two excited wavelengths, the measurement error has been decreased from 0.443% to 0.066% compared with the single-wavelength configuration. The temperature sensitivity and measurement stability have been further discussed. This method can be extended to similar FRL sensors to suppress measurement errors. [ABSTRACT FROM AUTHOR]

In this letter, a narrow line-width ytterbium-doped fiber ring laser with a fiber Bragg grating inside the cavity is proposed and experimentally demonstrated. In order to realize single longitudinal mode (SLM) output, an un-pumped ytterbium-doped fiber is used as an SLM selector. First, the effect of the coupler output ratio on the output power and the threshold power is discussed in this proposed laser, and the coupler output ratio of 30% is adopted at last. Furthermore, the spectrum and frequency spectra are analyzed to obtain the line width and frequency stability. Finally, the output power with fluctuation in half an hour is measured to evaluate the power stability. The results show that the output power of 7 mW, signal-to-noise ratio greater than 65 dB at 1069 nm, line width less than 500 Hz, and power stability below 1.5% can be achieved for the laser. [ABSTRACT FROM PUBLISHER]

A fiber ring laser with a carbon nanotube mode-locker that allows bidirectional oscillation is experimentally demonstrated in the L-band. Through leveraging a double-peak intracavity gain spectrum, two pulse trains in two nonoverlapping wavelength bands are shown to counter-propagate in the simple ring cavity. Ultrashort soliton pulses of \(\sim 800\) fs or less can be stably generated in both directions. It is also demonstrated that bidirectional operation within one of the wavelength bands can also be achieved with much smaller center wavelength separations by tuning the gain spectrum. The repetition rates of the two pulse trains have a slight difference due to the intracavity dispersion. Thus, the directional- and wavelength-multiplexed output from such a cavity-diversity-enabled ultrafast laser could potentially be leveraged in those applications requiring a dual-comb source. [ABSTRACT FROM PUBLISHER]

Abstract: A stable single longitudinal mode (SLM) fiber ring laser that incorporates polarization maintaining erbium doped fiber (PM EDF) acting as gain medium and saturable absorber is proposed and demonstrated. Both theoretical deviation and experimental result prove that optical intensity is time invariant when the laser operates in single longitudinal mode, the SLM operation can be approximately verified by optical intensity analysis, which is practical to analyze longitudinal mode in the application of engineering. The linewidth of the fiber ring laser is measured by the delayed self-heterodyne interferometery, and an acousto-optic frequency shifter is employed to shift the lasing frequency and eliminate zero frequency interfere. The SLM operation is verified by the scanning Fabry–Perot interferometer, whereas the lasing frequency drift slowly during a period of 2h, PM EDF is proved to be effective on suppressing mode hopping and selecting longitudinal mode. [Copyright &y& Elsevier]

Mode instability acts as a common feature in single-frequency fiber ring lasers. The mechanism of coherence collapse by mode instability is theoretically analyzed and demonstrated with an unbalanced fiber Michelson interferometer utilizing phase modulation, which is illuminated by a single-frequency erbium-doped fiber ring laser. Multiform mode instability phenomena accompanied with coherence collapse are observed and discussed in detail by tracing the dynamics of the interference fringe visibility. The results show that mode instability would introduce extra phase noises like a false alarm to interferometric fiber optic sensing systems. [ABSTRACT FROM AUTHOR]

We have demonstrated a switchable dual wavelength fiber ring laser with a high degree of polarization output by using an intracavity 3-stage all fiber Lyot filter. The filter is formed by concatenating four 45° tilted fiber gratings separated by polarization maintaining fibers with a length ratio of 1:2:4 (20, 40, and 80 cm), giving a compact integrated configuration with reduced bandwidth. Switchable dual wavelength or single wavelength output at 1533.5 and 1563.3 nm has been achieved. The output lasing is considerably stable owing to the in-phase mode-selecting function of the multistage Lyot filter, and has a very high degree of polarization higher than 99.9%. [ABSTRACT FROM PUBLISHER]

Highlights: [•] The monitoring technique is based on erbium-doped fiber ring laser. [•] The wavelength of chaotic laser can be continuously tuned to match with the channel of the WDM-PON. [•] The range of chaotic lasing wavelength can cover the C-band and L-band working region. [•] The spatial resolution is up to an order of centimeter which is independent with measuring range. [ABSTRACT FROM AUTHOR]

ABSTRACT A new tunable 1060-nm fiber ring laser based on a hybrid amplifier is proposed and experimentally demonstrated. The hybrid amplifier includes an ytterbium-doped fiber amplifier with a 70-cm Yb-doped fiber and a semiconductor optical amplifier. The proposed laser can provide low relative intensity noise and a narrow linewidth. According to experimental results, the laser output power can exceed 3 dBm and the wavelength tuning range reaches 45 nm from 1035 to 1080 nm under an optical side-mode suppression ratio greater than 55 dB. Moreover, stable and flat laser output with negligible power fluctuations can be maintained for over 1 h. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:2892-2896, 2013 [ABSTRACT FROM AUTHOR]

This paper proposed and experimentally demonstrated an all-fiber tunable and programmable bandpass filter using a linearly chirped fiber Bragg grating (CFBG). The center wavelength and spacing of the transmission peaks could be independently tuned via computer. The tunable range is about 18 nm. With this filter we demonstrated a tunable fiber ring laser which has an output power of about − 7 dBm, full-width at halfmaximum linewidth of ∼0.017 nm which is limited by the resolution of the optical spectrum analyzer (OSA). Furthermore, a spacing tunable dual-wavelength fiber laser was achieved with the same setup. This all-fiber laser features advantages like simple structure, low cost, flexible and digital tuning capability. [ABSTRACT FROM AUTHOR]

Abstract: Self-mixing interference in dual-wavelength fiber ring laser (FRL) with serial connection of fiber Bragg gratings (FBGs) is presented. Wavelength division multiplexing and active sensing is achieved by extracting single wavelength and adding feedback to the system. The expression of the optical output power of dual-wavelength fiber ring laser is analyzed when optical feedback is introduced. The gain competition and the intensities alternation among the applied channels are discussed. We apply the developed system for displacement measurements of two moving objects and investigate the influence on output signals under different feedback conditions. The experimental results show that this system has improved efficiency to expand the channels and maintains many advantages of self-mixing interference, validating the feasibility for implementation in a dual-channel displacement sensor at the same time. [Copyright &y& Elsevier]