Fiber Loop Mirror Research Articles

Dissipative soliton resonance in a normal dispersion all-polarization-maintaining thulium-doped fiber laser

We report a first demonstration of dissipative soliton resonance (DSR) in a normal dispersion thulium-doped fiber laser (TDFL) constructed entirely of polarization-maintaining fibers. A nonlinear fiber loop mirror with a combination of normal and anomalous dispersion fibers inside the loop ensures a self-starting mode-locking operation in the DSR regime. The oscillator generated linearly polarized optical pulses with a fundamental repetition frequency of 5.735 MHz and a nearly constant peak power clamped at ∼ 4.35 W. The pulse duration extends from 217 ps to 6 ns, when the pump power increases from 417 mW to 1.86 W. The maximum average output power of the DSR pulses was 151 mW, corresponding to the single pulse energy of 26 nJ.

Switchable red, blue-green emission using polarized optical fibre loop mirror in upconversionPr3+/Yb3+-doped ZBLAN fibre laser system

This manuscript experimentally demonstrates a switchable upconversion visible fibre laser system. The laser system is based on a linear fibre laser cavity with Praseodymium–Ytterbium codoped fluorozirconate (ZBLAN) optical fibre as a gain medium. The fibre laser cavity integrates a fibre loop mirror with a polarization controller to switch between multiple visible spectra. The optical fibre laser source provides dual blue and green emission with a centred wavelength of 490 nm and 520 nm, as well as red emission with a bandwidth of 115 nm, from orange to deep red with peaks centred at 605 nm, 635 nm, 693 nm, and 715 nm. The mentioned emissions can be switched and combined to generate almost pure white emission, white emission with high deep-red contribution, and vivid red. In addition, it demonstrated visible emission separation in an upconversion laser system via polarization, generating a wideband switched red-blue-green light source.

Polarization Influence on Er3+-Doped Multi-Wavelength Brillouin Fiber Laser Based on Fiber Loop Mirror

Polarization influences on the performance of multi-wavelength Brillouin Er3+-doped fiber laser are investigated by adjusting the polarization controller (PC) in the fiber loop mirror (FLM), where the linear laser cavity is composed of a fiber-tailed mirror and an FLM, and the stimulated Brillouin scattering (SBS) and the Er3+-doped fiber amplification (EDFA) simultaneously serve as the cavity gain. We realized 1–7 Brillouin laser lines by increasing the 980 nm pump power. For the first-order Brillouin laser, the signal–noise ratio (SNR) and optical intensity present a sinusoidal envelope; the conversion efficiency changes significantly from 0.56465 dBm/mw to 0.44975 dBm/mw by adjusting the ring’s angle in the PC; the first-order SBS thresholds are 20.4 mw, 36.1 mw and 28.5 mw at different angles θ2 = 36°, 276° and 300°, respectively; flatness between the two Brillouin lasers change obviously from 2.863 dB to 41.801 dB with different ring angles; the second-order Brillouin laser is suppressed and disappears finally at Δθ2 = −64° to −84° and 106°~136° angle variation. For the fifth-order Brillouin laser, the highest-order Brillouin laser line is seriously suppressed until it disappears at some angle variations similarly. The powers and wavelength stabilities for one-, three- and seven-wavelength Brillouin fiber lasers were measured in 1 h, and the obtained Er3+-doped multi-wavelength Brillouin fiber laser (MWBFL) worked stably during that time, but the stabilities become worse with higher SBS orders.

Burst-mode pulse generation in passively mode-locked all-fiber green/orange lasers at 543 nm and 602 nm

We report on the experimental realization of, to the best of our knowledge, the first green and orange passively mode-locked all-fiber lasers. Stable mode-locking in the burst-pulse status is achieved at the wavelengths of 543.3 nm and 602.5 nm. The figure-9 cavity comprises the fiber end-facet mirror, gain fiber (Ho3+:ZBLAN fiber or Pr3+/Yb3+:ZBLAN fiber), and fiber loop mirror (FLM). The FLM with long 460 HP fiber is not only used as an output mirror, but also acts as a nonlinear optical loop mirror for initiating visible-wavelength mode-locking. The green/orange mode-locked fiber lasers with the fundamental repetition rates of 3.779/5.662 MHz produce long bursts containing ultrashort pulses with the 0.85/0.76 GHz intra-burst repetition rates, respectively. The ultrashort intra-burst pulses stem from the dissipative four-wave-mixing effect in the highly nonlinear FLM as well as the intracavity Fabry–Perot filtering. Long bursts of 22.2/11.6 ns with ultrashort pulses of 87/62 ps are obtained in our experiment. The visible-wavelength passively mode-locked lasers in an all-fiber configuration and burst-mode would represent an important step towards miniaturized ultrafast fiber lasers and may contribute to the applications in ablation-cooling micromachining, biomedicine imaging, and scientific research.

Plastic Optical Fiber Spectral Filter Based on In-Line Holes

We propose a spectral filter based on a plastic optical fiber with micro-holes as a low-cost, robust, and highly reproducible spectral filter. The spectral filter is explored for two configurations: a fiber extended in a straight line and a fiber optic loop mirror scheme configuration. The transmission traces indicate a spectral blue shift in peak transmission, at 587 nm, 567 nm, 556 nm, and 536 nm for zero, one, two, and three holes in the fiber, respectively. The filter exhibits a bandpass period of approximately 120 nm. Additionally, we conduct a comparison of the transmission with holes separated by distances of 1 cm and 500 μm. The results demonstrate that the distance between holes does not alter the spectral transmission of the filter. In the case of the fiber loop mirror configuration, we observe that the bandpass can be adjusted, suggesting the presence of multimode interference. Exploring variations in the refractive index within the holes by filling them with glucose solutions at various concentrations, we determine that the filtering band and spectral shape remain unaltered, ensuring the stable and robust operation of our spectral filter.

Tunable narrow linewidth fiber laser based on parity-time symmetric double rings and variable optical attenuator

A tunable narrow linewidth fiber ring laser based on parity-time (PT) symmetric double rings and variable optical attenuator (VOA) is demonstrated. The laser is composed of a fiber loop mirror (FLM), which creates two coupled rings: the reflective ring and the transmissive ring. The loss of the transmissive ring can be controlled by tuning the VOA, which balances with the gain of the reflective ring. When the PT symmetry is broken, a single-longitudinal-mode (SLM) laser is realized. The laser can be tuned by an optical tunable filter (OTF), and the tuning range is 38 nm, spanning from 1525 nm to 1563 nm. In the whole tuning process, the linewidth of the laser is at the sub-kHz level, and the optical signal to noise ratios (OSNR) is over 53 dB. The fluctuation of wavelength and power are less than 0.02 nm and 0.52 dB in an hour, respectively.

Wavelength continuously tunable and wavelength spacing variable comb filter based on a High-Birefringence fiber loop mirror

We demonstrated an all-fiber comb filter that can continuously tune the wavelength for four switchable wavelength spacings. The comb filter consists of a fiber loop mirror (FLM) that is inserted with three polarization maintaining fibers (PMFs) and three polarization controllers (PCs). The transmission characteristics of the comb filter was theoretically analyzed. The output characteristics of the comb filters shows a dependence of the orientation angles of the PCs. With the angles being optimally set, the comb filter with four different wavelength spacings can be achieved. The wavelength continuously tunable and wavelength spacing variable mechanisms are elaborated. The three PCs are employed to enable the tunable and switchable functions. The two PCs between the PMFs serve as a polarization rotator to align the birefringent axes of the PMFs for varying the wavelength spacing while the PC adjacent to the optical coupler can be used for wavelength tuning function. When the two quarter-waveplates (QWPs) of the PC are optimally set at π/4 and - π/4, the half-waveplate (HWP) acts as a wavelength tuning element, and when the QWPs are set at zero degrees, the HWP serves as an amplitude tuning element. The center wavelength of the comb filter could be linearly tuned with the rotation angle of the HWP, and a π/2 change in the rotation angle leads to a wavelength change in one free-spectral-range (FSR). A π/4 change in the rotation angle could bring the normalized peak transmission from its maximum at 1 to minimum at 0. The proposed tunable technique has been verified by our experimental results. We experimentally demonstrated the comb filters can switch among FSRs of 0.75 nm, 1.01 nm, 1.51 nm, and 3.1 nm with tunable functions.

Optimization of Dual-Wavelength linear cavity Erbium-Doped fiber laser based on pairs of fiber Bragg gratings

There are few reports in the field of linear cavity dual-wavelength fiber lasers about the laser output characteristics when 3-dB fiber loop mirror (FLM) and Fiber Bragg grating (FBG) are used as full reflective mirrors, respectively. Additionally, there is scarce research on further tuning of the FBG parameters (reflectivity and center wavelength interval) to study dual-wavelength fiber lasers. In this paper, we compare the performances of dual-wavelength lasing outputs between the 3-dB FLM-FBG cavity and the FBG-only cavity in a linear-cavity erbium-doped fiber laser (EDFL) based on the polarization hole burning effect. The results show that the optical signal-to-noise ratios (OSNRs) of the dual-wavelength lasing outputs with the FBG-only cavity are approximately 4 ∼ 5 dB higher than those with the 3-dB FLM-FBG cavity. Furthermore, the lasing output performances based on the FBG-only cavity are compared when the reflectivities of the output-mirror FBGs are the same or different. It is observed that the dual-wavelength laser with the same output reflectivity leads to higher OSNR, slope efficiency, and stability. Moreover, by varying the center wavelength interval of pairs of FBGs in the FBG-only cavity to values of 4 nm, 8 nm, and 12 nm respectively. It is shown that wider intervals result in higher OSNRs but lower laser slope efficiency. Finally, we demonstrate a dual- wavelength linear cavity EDFL operating at room temperature with two lasing wavelengths of 1550 nm and 1562 nm. The laser results in OSNRs around 50.62 dB and 50.93 dB; center wavelength variations of less than 0.030 nm and 0.035 nm; output power fluctuations of less than 0.061 mW and 0.059 mW, respectively.

Polarization-Maintaining Fiber Loop Mirror Based Ultrasound Sensing Technique With Enhanced Signal-to-Noise Ratio

Polarization-Maintaining Fiber Loop Mirror Based Ultrasound Sensing Technique With Enhanced Signal-to-Noise Ratio

Simultaneous displacement, temperature and strain sensing system based on fiber macro-bend loss and edge filter

We proposed a multi-parameter fiber sensing system. The macro-bending loss of cascaded single-mode-no-core-single-mode (SNS) fiber was used for displacement sensing, and the combination of the SNS and fiber loop mirror (FLM) was constructed to improve the dynamic range of measurement. We designed the single-mode-dispersion-compensation-single-mode (SDS) fiber edge filter for fiber bragg grating (FBG) demodulation. The steep linear transmission region of the edge filter was used to improve the sensitivity of FBG for measurement of temperature and strain. The two measurement methods were applied the multi-parameter fiber sensing system to realize simultaneous measurement of displacement, temperature and strain. The system can overcome the crosstalk between displacement and temperature.

Vernier Effect in a Compact Strain Sensor Based on Fiber Loop Mirrors Using a 3 × 3 Coupler

In this paper, a new configuration based on two fiber loop mirrors in series using a 3×3 coupler is demonstrated. This configuration is compact and can explore the Vernier effect. The results obtained were a sensitivity of 74.9±0.2 pm/με for the Vernier envelope and 6.41±0.01 pm/με for the Vernier carrier. These results were compared to the sensitivity of the setup using only one fiber loop mirror interferometer, obtaining 14.41±0.01 pm/με. This means a magnification factor of 5.2 for the Vernier envelope. Theoretical treatment of the system was carried out, using Jones matrix calculus, and the subsequent simulation results are also presented, having obtained good agreement with the experimental data.

A Temperature-Insensitive Multipoint Displacement Sensing System Based on Fiber Macro-Bending Loss

We proposed a novel, temperature-insensitive, low-cost multipoint displacement sensing system. The system includes three parts: fiber displacement sensor, fiber loop mirror (FLM) and optical time domain reflectometer (OTDR). The fiber displacement sensor is designed by cascading single-mode-no-core-single-mode (SNS) fiber, the SNS is attached to FLM that is constructed by a 3 dB coupler to improve measurement dynamic range. The variation of displacement will significantly change the value of SNS macro-bending loss, which cause the variation of FLM reflected energy to achieve displacement measurement. Each FLM constitutes a displacement sensing unit, only 5% pulsed light energy is input for each sensing unit to increase the number of reusable sensors. The results show that each sensor can maintain a similar sensing effect, the measurement dynamic range can reach 15.613 dB. The peak energy of each FLM is almost unaffected by changes in external temperature. The proposed system is an ideal solution to engineering problems of displacement measurement, which can be widely used in the field of structural health monitoring.

Paracetamol concentration-sensing scheme based on a linear cavity fiber laser configuration

A paracetamol concentration-sensing scheme based on a linear cavity fiber laser configuration is demonstrated experimentally. The laser cavity has a fiber sensor at one end, that allows refractive index measurements. The refractometer consists of a cleaved fiber tip combined with an FBG functioning as a reflecting mirror. The combination of a fiber loop mirror at the other end allows to reflect all the light from the FBG and refractometer, forming a linear cavity. By measuring the intensity variation of the Fresnel reflection at the fiber-to-liquid interface, the measured concentration is linear and have a concentration sensitivity of -8.74±0.34×10-5µW/(g/kg), over a range of 52.61 to 219.25 g/kg, and with a resolution of 2.77 g/kg.The results obtained present high stability and prove the potential of the fiber laser system to performed real-time measurements of concentration, in a non-invasive way.

Measurement of the birefringence variation induced by dihydrogen diffusion into a polarization-maintaining fiber.

We report continuous measurements of the transmission spectrum of a fiber loop mirror interferometer composed of a Panda-type polarization-maintaining (PM) optical fiber during the diffusion of dihydrogen (H2) gas into the fiber. Birefringence variation is measured through the wavelength shift of the interferometer spectrum when the PM fiber is inserted into a gas chamber with H2 concentration from 1.5 to 3.5 vol.% at 75 bar and 70°C. The measurements correlated with simulation results of H2 diffusion into the fiber lead to a birefringence variation of -4.25 × 10-8 per mol m-3 of H2 concentration in the fiber, with a birefringence variation as low as -9.9×10-8 induced by 0.031 μmol m-1 of H2 dissolved in the single-mode silica fiber (for 1.5 vol.%). These results highlight a modification of the strain distribution in the PM fiber, induced by H2 diffusion, leading to a variation of the birefringence that could deteriorate the performances of fiber devices or improve H2 gas sensors.

An aluminum coated all-fiber mirror and a Mach–Zehnder interferometer for developing a switchable and tunable multi-wavelength laser

In this paper, a tunable and switchable multi-wavelength erbium doped linear fiber laser, based on an optical fiber loop mirror containing a Mach–Zehnder interferometer (MZI) and an aluminum coated optical fiber cleaved end is presented. The MZI, obtained by core-offset splicing a piece of photonic crystal fiber (PCF) between two segments of a single mode fiber (SMF), acts as a wavelength selective filter (WSF). The full reflecting mirror implemented is obtained by thermal evaporation of aluminum on a cleaved fiber end, technique which has not been deeply explored for compact fiber laser configurations. In this way, both optical loop mirror and the aluminum coated fiber work together to make a resonant cavity. By rotating the wave plates of a polarization controller, the polarization state of the cavity modes diversifies, and the laser setup can emit one, two, three or four laser wavelengths, which can be switched from 1526 to 1562 nm. In addition, this laser arrangement can be tuned within a 5 nm interval and has a side-mode suppression ratio of 42 dB for single-wavelength operation. Finally, the proposed laser configuration achieved a high stability for more than an hour at room temperature.

Fiber Loop Mirror Based on Optical Fiber Circulator for Sensing Applications

In this paper, a different Fiber Loop Mirror (FLM) configuration with two circulators is presented. This configuration is demonstrated and characterized for sensing applications. This new design concept was used for strain and torsion discrimination. For strain measurement, the interference fringe displacement has a sensitivity of (0.576 ± 0.009) pm‧με-1. When the FFT (Fast Fourier Transformer) is calculated and the frequency shift and signal amplitude are monitored, the sensitivities are (-2.1 ± 0.3) × 10-4 nm-1 με-1 and (4.9 ± 0.3) × 10-7 με-1, respectively. For the characterization in torsion, an FFT peaks variation of (-2.177 ± 0.002) × 10-12 nm-1/° and an amplitude variation of (1.02 ± 0.06) × 10-3/° are achieved. This configuration allows the use of a wide range of fiber lengths and with different refractive indices for controlling the free spectral range (FSR) and achieving refractive index differences, i.e., birefringence, higher than 10-2, which is essential for the development of high sensitivity physical parameter sensors, such as operating on the Vernier effect. Furthermore, this FLM configuration allows the system to be balanced, which is not possible with traditional FLMs.

Numerical design of a polarization-independent nonlinear optical loop mirror

We present the numerical design of an input polarization-independent nonlinear optical fiber-loop mirror with elliptical birefringence. We found real physical parameters of circular and linear induced-birefringences to eliminate the input polarization dependence of the nonlinear polarization rotation effect caused by self-phase and cross-phase modulation. To show the input polarization independency, we present the results for a nonlinear optical fiber-loop mirror which includes a 65/35 coupler, and a 70-m fiber loop with 0.75 turns per meter twist ratio, and 5 cm bend radii. The nonlinear optical fiber-loop mirror presents less than 40 dB transmission polarization dependency. The numerical model was made by the transfer matrix method and solved by the split-step Fourier method. The results of this numerical design have multiple potential applications such as mode-locking pulsed lasers, and optical communications, among others.

A Mechanophotonic Approach toward an Organic, Flexible Crystal Optical Interferometer

AbstractOrganic crystals are poised to be a potential alternative to conventional silicon and its derivatives due to their utilization as microscale photonic components like active and passive waveguides, ring‐resonators, modulators, directional couplers, add‐on filters, interferometers (IMs), and reconfigurable circuits. Among these, IMs are important for flexible organic photonic integrated circuits. Here, the design and synthesis of extremely flexible (E)‐1‐(((5‐bromopyridin‐2‐yl)imino)methyl)naphthalen‐2‐ol (BPyIN) crystals are demonstrated. The interactions stabilizing the crystal packing and crystal‐substrate adhesion dictate the elasticity and pseudo‐plasticity, respectively. The extremely flexible BPyIN crystals disclose bending geometry‐dependent optical modes in the fluorescence spectra. This understanding is extended for fabricating a fiber loop mirror (FLM)‐like geometry using a BPyIN single crystal. Mechanophotonics approach facilitates the integration of FLM‐like crystal with a doubly bent waveguide to construct a first‐of‐its‐kind IM. The light propagating path‐length‐dependent optical interference within the IM is established. These proof‐of‐principle experiments illustrate the usefulness of mechanophotonics to fabricate miniature photonic devices beyond the traditional fabrication approach.

Graded-index multimode fiber in nonlinear absorbing loop mirror as saturable absorber for noise-like pulse generation in 2 μm region

This investigation explores the potential use of graded-index multimode fiber (GIMF) in a nonlinear absorbing loop mirror (NAbLM) as an artificial saturable absorber for ultrashort pulse generation. The saturable absorption mechanism of GIMF-NAbLM was based on a combination of nonlinear phase shift and multimodal interference that was satisfied by the asymmetric positioning of the GIMF in the single-mode fiber loop mirror. The proposed saturable absorber was integrated in a mode-locked thulium-doped fiber laser with figure-of-eight cavity. A noise-like pulse of 164 fs spike pulse duration at 1950 nm central wavelength was attained in the net anomalous dispersion with a 10 m long GIMF. The GIMF-NAbLM structure demonstrates immense prospects as an effective saturable absorption device for generating stable ultrashort pulse in the 2 µm region due to its simplicity.

Passively Mode-locked Erbium-doped Fiber Laser Based on Nonlinear Fiber Loop Mirror

Passively Mode-locked Erbium-doped Fiber Laser Based on Nonlinear Fiber Loop Mirror