Double-clad Research Articles

Development of a coaxial DCF-GRIN fiberoptic Raman probe for enhancing in vivo epithelial tissue Raman measurements.

We report on the development of a novel, to the best of our knowledge, coaxial double-clad-fiber (DCF) and graded-index (GRIN) fiberoptic Raman probe for enhancing epithelial tissue Raman measurements in vivo. The ultra-thin (140 µm outer diameter) DCF-GRIN fiberoptic Raman probe is designed and fabricated with an efficient coaxial optical configuration, whereby a GRIN fiber is spliced onto the DCF to enhance both the excitation/collection efficiency and depth-resolved selectivity. We demonstrate that the DCF-GRIN Raman probe can be used to acquire high-quality in vivo Raman spectra from various oral tissues (e.g., buccal mucosa, labial mucosa, gingiva, mouth floor, palate, and tongue) covering both the fingerprint (800-1800 cm-1) and high-wavenumber (2800-3600 cm-1) regions within sub-seconds. The subtle biochemical differences between different epithelial tissues in the oral cavity can also be detected with high sensitivity, suggesting the potential of the DCF-GRIN fiberoptic Raman probe for in vivo diagnosis and characterization in epithelial tissue.

Investigation of bandwidth in multimode W-type microstructured plastic optical fibers

By employing the time-dependent power flow equation (TD PFE), we examine the bandwidth in multimode W-type (double clad) microstructured plastic optical fibers (mPOFs) with a PMMA (polymethyl methacrylate) solid core for parametrically varying depth and width of the intermediate layer (IL) (inner cladding). The investigated W-type mPOF's bandwidth was calculated for various configurations of the air-holes in the inner cladding and varied launch excitations. We obtained that results for smaller inner cladding air-holes at longer fiber lengths exhibit greater bandwidth. On the other hand, for shorter fibers, the launch beam which only excites guided modes, has no effect on the bandwidth due to the air-hole size. The W-type mPOF with a narrower inner cladding has a greater bandwidth. The bandwidth is also larger for a narrow launch that only excites guided modes as opposed to a wider launch that excites both guided and leaky modes. Therefore, the bandwidth increases as the width of the IL is reduced, or by decreasing the diameter of air-holes in the IL, or by exciting only guided modes. W-type mPOFs can be more easily tailored for a specific use in optical fiber sensors and communications because to their programmable properties.

Consecutive 1015-1105-nm wavelength tunable "figure-of-9" mode-locked Yb:fiber oscillator.

A widely wavelength tunable mode-locked Yb-doped fiber oscillator based on nonlinear amplifier loop mirror (NALM) is reported, in which only a piece of short (∼0.5 m) single-mode polarization-maintaining (PM) Yb-doped fiber is employed, instead of the frequently used long (a few meters) double cladding (DC) fiber in previous papers. Experimentally, the center wavelength can be consecutively tuned from 1015 to 1105 nm by tilting the silver mirror, corresponding to a tuning range of 90 nm. To the best of our knowledge, this is the broadest consecutive tuning range in Yb:fiber mode-locked fiber oscillator. In addition, the mechanism of wavelength tuning is tentatively analyzed and attributed to the combined action of the spatial dispersion induced by a tilting silver mirror and the limited aperture in the system. Specific to the wavelength of 1045 nm, the output pulses with 13-nm spectral bandwidth can be compressed to 154 fs.

Dark Rectangular Pulses From a Dumbbell-Shaped Mode-Locked Double-Clad Er:Yb Laser

The formation of dark rectangular pulses from a dumbbell-shaped mode-locked double-clad erbium-ytterbium fiber laser is experimentally demonstrated. The laser operates under a large net-anomalous cavity dispersion regime, with a passive mode locking mechanism that relies on a nonlinear amplifying loop mirror (NALM) along with a nonlinear optical loop mirror (NOLM). This design facilitates the control of nonlinear effects within the cavity and enables the generation of dark pulses as the dominant mode locking regime. The variations of dark-rectangular pulse emission and its spectral evolution are experimentally investigated as a function of the pump power, revealing a dependence analogous to bright dissipative soliton resonance (DSR) pulses, where the dark pulse duration increases with increasing pump power, a behavior that is opposite to previously reported dark rectangular pulses. At the maximum pump power of 9.5 W, the mode-locked fiber laser generates dark rectangular-shaped pulses with duration of 571 ns, average output power of 600 mW, fundamental repetition rate of 320 kHz.

Dual Band Active Nanostructured Core Fiber for Two-Color Fiber Laser Operation

We present the experimental verification of a new type of an active nanostructured fiber devoted to simultaneous laser emission at two wavelengths: 1040 nm and 1534 nm in the fundamental mode. The fiber core is formed with two types of nanorods made of a phosphate glass doped with either Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> or co-doped with Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> /Er <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> ions, respectively. A distribution of the nanorods in the core allows to control relative gain and modal properties of the fiber at both wavelengths and to maintain the single mode performance with a high quality beam generated in a single fiber core. The double-cladding fiber structure allows efficient excitation with a single 975 nm diode pump laser. As a proof-of-concept we demonstrated an active fiber with 15 µm core diameter composed of 6649 nanorods with an individual diameter of 160 nm. The fiber was used in a laser setup. The laser operation was verified for simultaneous two-color laser generation at output power levels of 0.78 W and 0.42 W with slope efficiency of 23.0% and 9.8% at 1040 nm and 1534 nm, respectively.

Comparative study on the continuous-wave double-cladding Yb-doped fiber amplifiers operating around 978 nm and 982 nm

The comparative study on the continuous-wave double-cladding Yb-doped fiber (DCYF) amplifiers operating around 978 nm and 982 nm is carried out in this paper. By means of numerical and experimental studies, it is revealed that compared with 978-nm amplifiers, the amplified spontaneous emission (ASE) around 1030 nm can be better suppressed in 982-nm amplifiers because of the lower requirement of population inversion for the amplification of 982-nm signal light, and thus, the larger optimum active fiber length and slope efficiency can be obtained in 982-nm amplifiers. This should be the first experimental demonstration that the slope efficiency of 982-nm amplifiers can be larger than that of 978-nm amplifiers, to the best of our knowledge. The effect of signal wavelength on the slope efficiency of amplifiers is also numerically studied. It is found that the optimum signal wavelength varies from the 982 nm to 980 nm with the increment of pump light. It is also found that the variation of optimum fiber length with the pump power in the case of signal wavelength longer than 986 nm is different from that with the signal wavelength shorter than 984 nm. The pertinent explanation is also given.

Double-Clad Bismuth-Doped Fiber with a Rectangular Inner Cladding for Laser Application

In this paper, we report the latest research results on the fabrication of double-clad Bi-doped germanosilicate core fibers with a rectangular inner cladding design for improved laser performance in the near-IR spectral region. Detailed comparative analysis of the absorption characteristics of the Bi-doped fibers with a circular- and rectangular-shaped inner cladding was performed. A series of cladding-pumped, Bi-doped fiber lasers emitting near 1.46 µm was developed using the semiconductor’s multi-mode fiber-coupled laser diodes at λ = 808 nm. The peculiarities of the laser parameters of the fabricated active fibers with the double-clad design were thoroughly studied by analyzing the dependencies of the slope efficiency of the lasers, namely the pump power, active fiber length and core-to-inner-cladding area ratio. The obtained results show that the rectangular design provided enhanced cladding absorption and improvements in laser performance. In particular, we achieved maximal slope efficiencies of 5.5% and 4.3% for the absorbed pump power introduced into the inner cladding with cross-section areas of 80 × 80 µm2 and 125 × 125 µm2, respectively. Multi-wavelength lasing operation in a free-running cavity due to a few modes’ propagation regimes was found using the Bi-doped fiber with an 80 × 80 µm2 inner cladding.

Generation of 15 nJ pulse energy by a sub-150 fs thulium-doped fiber Mamyshev oscillator.

Mamyshev oscillators have pushed the frontiers in output parameters of ytterbium- and erbium-based ultrafast fiber oscillators in the spectral region around 1 µm and 1.5 µm within the last few years tremendously. In order to expand the superior performance toward the 2 µm spectral region, we present in this Letter an experimental investigation of the generation of high-energy pulses by a thulium-doped fiber Mamyshev oscillator. Generating highly energetic pulses is enabled by a tailored redshifted gain spectrum in a highly doped double-clad fiber. The oscillator emits pulses with an energy of up to 15 nJ, which can be compressed to 140 fs.

Multiparameter Sensor Based on DCF Composite Tapered Structure Cascaded LPFGs for the Simultaneous Measurement of Refractive Index, Temperature, and Axial Strain

In this article, a fiber optic sensor based on double-cladding fiber (DCF) composite tapered structure cascaded long-period fiber gratings (LPFGs) for measuring refractive index (RI), temperature, and axial strain is designed. The two ends of a Section of DCF are, respectively, fused with single-mode fiber (SMF) and LPFG1 to form two up-tapers, and then the DCF is formed into a down-taper. Another end of LPFG1 is cascaded with LPFG2, and the center wavelengths of the two LPFGs are different. LPFG1, LPFG2, and tapered DCF generate three resonance peaks, namely, dip1, dip2, and dip3, respectively, whose sensitivity in response to different parameters is different. When external parameters change, the dips will have spectral shifts. The experimental results indicate that, when the RI is changed within 1.3355–1.3715, the sensitivity of dip1, dip2, and dip3 is –52.12, – 132.58, and 37.61 nm/RIU, respectively. When the temperature varies from 30 °C to 80 °C, the sensitivity of dip1, dip2, and dip3 is 74.05, 98.02, and 26.33 pm/°C in turn, respectively. When the axial sensitivity of dip1, dip2, and dip3 is – 1.95, – 2.58, and –3.24 pm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> , respectively. The resolutions for RI, temperature, and axial strain are 0.0005 RIU, 1.3375 °C, and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$14.0886 \mu \varepsilon $ </tex-math></inline-formula> , respectively. The three parameters can be monitored simultaneously using the coefficient sensitivity matrix. The designed sensor has high repeatability and good linearity, and it is practical in the field of multiparameter detection.

High-power tunable red laser based on double-cladding Pr3+-doped fiber

A high power visible laser is produced by combining double-cladding fluoride fiber, a specially fiber structured suitable for high power visible laser generation. Here, a double-cladding Pr3+-doped red fiber laser with an output power of 3.65 W and slope efficiency of 22 % had been developed in this research, making it the highest output power ever obtained in fiber lasers. In addition, the first tunable double-cladding fiber laser was also created, with a tuning range of near 9 nm from 632.60 nm to 641.48 nm. The free-running laser also achieved more than 500 mW at the center wavelength of 636.38 nm. With this tunable laser, new ground is broken in the field of double-cladding fiber tuning.

HIGH BRIGHTNESS 1908nm TM-DOPED FIBER LASER WITH POWER SCALING TO &gt;75W

In this paper, we have studied monolithic diode-pumped Tm-doped fiber laser to be used as a pump source for Ho-YAG systems. The cavity is designed to achieve high optical-to-optical efficiency and robustness against amplified spontaneous emission (ASE) induced parasitic lasing via optimizing the doped fiber length and cavity parameters. Experimentally, we have demonstrated 1907.7 nm fiber laser with an output power of 79 W from 10/130 μm Tm-doped double clad fiber, enabling high brightness and radiance density as well. The laser cavity has a slope efficiency of ~55%, ASE suppression of &gt; 40 dB and a near-diffraction-limited beam quality of M2~1.07.

Coaxial Mach-Zehnder Digital Strain Sensor Made from a Tapered Depressed Cladding Fiber.

An in-line digital optical sensor was proposed. It was built from a tapered depressed-cladding single-mode fiber and modeled as a coaxial Mach–Zehnder interferometer. The principle of operation of the optical digital sensor is based on the computation of the number of optical power transfer turning points (PTTP) from the transmission data of the component. Biconic tapers with high values of PTTP, high spectral resolution, high extinction ratio, and low insertion loss were modeled, fabricated, and characterized. As a proof of concept, an in-line digital strain sensor was fabricated and characterized. It presents a free spectral range of 1.3 nm, and produced 96 PTTP, at λ0 = 1.55 μm, under stretch of ΔL = 707 µm, therefore producing a digital resolution of 7.4 µm/PTTP. The sensor also produced a quasi-symmetric response to stretch and compression.

Highly stable Q-switched and mode-locked pulse generation from an all-PM figure-9 fiber laser.

A highly stable figure-9 Yb-doped fiber laser with all polarization-maintaining (PM) double-cladding fiber is demonstrated. Through leveraging the saturable absorption effect of a nonlinear amplifying loop mirror, both the Q-switched and mode-locked operation are realized by adjusting the pump power. With increasing the pump power from the threshold to the maxima, the repetition rate of the Q-switched pulses is linearly increased from 14.9 kHz to 138.0 kHz with the pulse duration accordingly reduced from 3.9 µs to 970 ns. The corresponding maximum average power and pulse energy are respectively 2.34 W and 17 µJ, which are more than ten times larger than the common material-based Q-switched all-fiber lasers. In addition, in the process of increasing and decreasing the pump power, an optical bistability that manifested as a significant power jumping effect is observed, while its effect on the pulse repetition rate and duration is trivial. Whereas for the single pulse mode-locked operation, a maximum output power of 56.3 mW with a fundamental repetition rate of 12.5 MHz is realized, corresponding to a pulse energy of 4.5 nJ. To the best of our knowledge, it is much higher than the most of previous works concerning figure-9 all-PM-fiber lasers of which the emitted pulse energy is generally less than 1 nJ. After being compressed by a pair of diffraction grating, a minimum pulse width of 378 fs and a maximum peak power of 9.76 kW are respectively obtained. In addition, through characterizing the spectral and temporal properties of the laser source, the excellent stability of both the Q-switched and mode-locked operations is verified.

Experimental investigation on a hundred-Watt monolithic fiber laser operating near 980 nm with 20/125-μm double-cladding Yb-doped fiber

• A hundred-Watt 980-nm MOPA fiber laser is demonstrated with the 20/125-μm DCYF. • In-band ASE limits the power up-scaling of the MOPA fiber laser. • Increasing seed power is beneficial to suppress the in-band ASE. • Split-step seed loading method is presented, with which 109-W output power is achieved. • The fiber laser provides a cost-effective solution for high-power core-pumping source. In this paper, a hundred-watt monolithic fiber laser operating near 980 nm is demonstrated by using a double-cladding Yb-doped fiber (DCYF) with 20-μm core and 125-μm inner-cladding diameter. The fiber laser is fabricated with the master oscillator power amplifier (MOPA) configuration to scale up the output power. In our experiment, it is found that the power scalability of the fiber laser is limited by the in-band amplified spontaneous emission (ASE) which can be effectively suppressed by increasing the seed power. Then, with the split-step seed loading method, 109-W output power around 980 nm has been achieved with in-band and 1030-nm ASE suppressed to 17.1 and 40.6 dB, respectively. The beam quality has also been measured, and the M 2 factor is about 1.9 at the maximum output power. Our work provides a cost-effective solution to achieve hundred-Watt fiber lasers operating near 980 nm.

Synthesis of Carbon Nanodots from Sugarcane Syrup, and Their Incorporation into a Hydrogel-Based Composite to Fabricate Innovative Fluorescent Microstructured Polymer Optical Fibers

Carbon nanodots (CNDs) are interesting materials due to their intrinsic fluorescence, electron-transfer properties, and low toxicity. Here, we report a sustainable, cheap, and scalable methodology to obtain CNDs from sugarcane syrup using a domestic microwave oven. The CNDs were characterized by infrared spectroscopy, dynamic light scattering, atomic force microscopy, absorption, and emission spectroscopies. The CNDs have 3 nm in diameter with low polydispersity and are fluorescent. A fluorescent hydrogel–CNDs composite was obtained using gelatin polypeptide as the polymeric matrix. The new hydrogel–CNDs composite was incorporated in the cavities of a double-clad optical fiber using an innovative approach that resulted in a microstructured polymer optical fiber with intrinsic fluorescence. This work shows a promising alternative for the fabrication of fluorescent materials since the CNDs synthesis is sustainable and environmentally friendly. These CNDs might substitute the rare-earth and other heavy metals of high cost and toxicity, which are usually incorporated in double-clad fibers for applications on lasers, amplifiers, and spectroscopy.

Stable orbital angular momentum mode generator based on helical long-period fiber grating

We developed a helical long-period fiber grating (HLPG), which was formed by twisting step-index double-clad fiber (SDF) under the irradiation of a CO2 laser, and can generate stable orbital angular momentum (OAM) mode. The mode coupling and transmission characteristics of the SDF-HLPG were investigated both theoretically and experimentally. The results prove that this grating can couple the light wave into the inner cladding mode of the SDF, and directly generate OAM mode. Subsequently, the tolerance of the SDF-HLPG to environmental disturbance was also evaluated. The OAM mode has low sensitivities to mechanical torsion and bending, and has almost no responses to strain and surrounding refractive index. This indicates that the SDF provides a channel for the OAM mode to avoid high external interference. Therefore, this kind of SDF-HLPG has the potential for communication applications.

High power passively mode-locked laser with Sb2Te3 deposited tapered fiber in Er/Yb doped fiber laser

Antimony telluride (Sb2Te3), one of the materials from the topological insulator (TI) family, was studied as a saturable absorber (SA) for producing high-power ultrafast lasers in the C-band region. In order to achieve a mode-locked operation, a tapered fiber with a tapered waist of 6 μm was coated with Sb2Te3 to provide the interaction of the evanescent field with the material. Non-linear optical properties of the constructed SA include a 4.05 % modulation depth and a saturation intensity of 0.35 MW/cm2. Mode-locked pulses were generated when the SA was placed inside the cavity. The mode-locked pulses were solitonic, having a center wavelength of 1561.5 nm and a pulse duration of 1.8 ps at a pump power of 461 mW. The fundamental frequency measured was 18 MHz within pump power ranging from 461 mW to 657 mW. The maximum average output power, pulse energy, and peak power obtained from the generated mode-locked pulses were 269 mW, 14.9 nJ, and 8.3 kW, respectively. The measured signal to noise (SNR) of 55 dB indicates the excellent stability of the mode-locked pulses. This is the first demonstration of mode-locked pulses utilizing Sb2Te3 coated tapered fiber as an SA in a double-clad erbium-ytterbium doped fiber (DC-EYDF) laser cavity that we are aware of.

Temperature characteristic of a gain fiber core in distributed side-coupled cladding-pumped fiber amplifiers with different pump schemes

In this paper, we study the temperature characteristic of a gain fiber core in distributed side-coupled cladding-pumped (DSCCP) fiber amplifiers by optical frequency domain reflectometry (OFDR). To the best of our knowledge, this is the first temperature distribution measurement of a fiber core in DSCCP fiber lasers. The temperature distributions are also firstly measured among forward, backward, and bidirectional pump schemes. In addition, the bidirectional pump scheme has lower temperature distribution than unidirectional pump schemes. The experimental fiber core temperature is in according with the numerical simulation, which was once introduced for the temperature study of DSCCP fiber amplifiers. By experimental measurement and theoretical modeling, it is found that the maximum core temperature is several tens of centimeters away from the injected end of the DSCCP fiber, such that it will distribute the heat load along the longitudinal direction and reduce the maximum thermal load with better thermal management advantages than ordinary double cladding fiber (DCF).

Analysis of transmission spectra and cladding mode evolution of etched double-cladding fiber long-period grating

We demonstrate the inscription of long-period gratings (LPGs) in a fluorine-doped double-cladding fiber (DCF) with a low refractive index trench. The grating has a single resonance dip in the wide wavelength range from 1200 nm to 2400 nm. The transmission spectra and cladding mode evolution of the DCF-LPG during the cladding etching process are investigated theoretically and experimentally. The mode order of the cladding modes decreases with one order of every 20μm diameter etching, while the resonance wavelength of the DCF-LPG barely shifts when the fiber diameter decreases gradually. Only LP11 mode can be excited in the etched DCF-LPG when the fiber diameter is less than 40μm. The characteristics of the DCF-LPGs are investigated experimentally. The torsion sensitivity was measured to be −0.154 nm/(rad/m) and −0.271 nm/(rad/m) at 1.55μm and 2.0μm wavebands, respectively, which are one order of magnitude higher than that of the conventional LPG. The proposed DCF-LPG has potential application as a tunable band-rejection filter in broad wavebands.

More than 6 kW near single-mode fiber amplifier based on a bidirectional tandem pumping scheme.

We demonstrate the first all-fiber monolithic bidirectional tandem pumping amplifier, to the best of our knowledge, based on a 30/250 µm conventional ytterbium-doped double-clad fiber. By optimizing the bidirectional pumping power distribution, an output power of 6.22 kW is obtained with near single-mode beam quality (M2=1.53), and no transverse mode instability is observed. This work could provide an excellent reference for high-power, higher-brightness fiber lasers.