All-solid Photonic Bandgap Fiber Research Articles

Large-mode-area multi-resonant all-solid photonic bandgap fiber with low bending loss and robust single-mode operation

In this paper, we have experimentally demonstrated a novel large-mode-area multi-resonant all-solid photonic bandgap fiber (AS-PBGF) which is specifically optimized for conventional applications of fiber lasers around 1 μm, with a hexagon core that is measured to be 46 μm in the corner-to-corner direction and a broad bandwidth from 970 nm to 1180 nm in the 3rd photonic bandgap (PBG). Bending loss lower than 0.1 dB/m of a 5-m-length fiber is obtained in the case of bending radius R greater than 25 cm. Robust single-mode (SM) operation of the fiber is verified within the whole PBG. In addition, we also report a phenomenon about the abnormal variation of M2 within the low loss region of the 3rd PBG. The evolution of beam quality within the 3rd PBG appears as a U-shaped curve rather than a constant. Finally, a preliminary conclusion about the relationship between the beam quality and SM characteristics in AS-PBGF which is quite different from that in conventional SIFs is verified.

High-power single-frequency single-mode all-solid photonic bandgap fiber laser with kHz linewidth.

There have been several demonstrations of single-frequency single-mode ytterbium-doped fiber lasers operating at a few hundred watts of power. A narrow spectral linewidth of these lasers is critical for many applications but has never been properly measured before at high powers. In this work, we report the first spectral linewidth measurement at kHz resolution of high-power single-frequency fiber lasers using a heterodyne technique and can confirm that these lasers can indeed operate at a few kHz spectral linewidth. Furthermore, we have improved the power from single-frequency single-mode all-solid photonic bandgap fiber lasers to 500 W using an improved photonic bandgap fiber.

Spectrally U-shaped profile of beam propagation factor in all-solid photonic bandgap fiber

We found the beam quality factor M2 of the fundamental mode as a function of wavelength is U-shaped in the working photonic bandgap (PBG) of an all-solid PBG fiber (AS-PBGF) for the first time, to the best of our knowledge, and our simulation results also match well with the phenomenon. The normal band that is near the high-frequency edge of the third PBG integrates the lowest M2 and single-mode operation simultaneously, while the other two edge regions suffer from anomalous variation of M2 versus wavelength. The general applicability of this finding can be further extended to other PBGs and also other representative structures in the AS-PBGF field.

KW-level monolithic single-mode narrow-linewidth all-solid photonic bandgap fiber amplifier.

Further power scaling of narrow-linewidth fiber lasers is critical for beam combining. Using all-solid photonic bandgap fibers with large effective mode areas and strong higher-order-mode suppression is an interesting approach. Previously, we demonstrated ∼400W single-frequency single-mode power at 1064 nm from a 50/400 photonic bandgap fiber amplifier, limited only by transverse mode instability (TMI). In this work, we demonstrate a TMI-limited single-mode power of 1.37 kW from a monolithic fiber amplifier with a 25/400 photonic bandgap fiber, the highest output power from a photonic bandgap fiber demonstrated to date, to the best of our knowledge. The spectral linewidth is broadened to ∼8GHz to suppress stimulated Brillouin scattering.

Leakage channels enabled multi-resonant all-solid photonic bandgap fiber for effective single-mode propagation.

All-solid photonic bandgap fiber (AS-PBGF) has been fully demonstrated to be a promising candidate of large-mode-area fiber for its mode-dependent selectivity and spectral filtering mechanism. In the present work, the concepts of multiple-resonant coupling and leakage channels are taken into consideration simultaneously for mode area scaling of AS-PBGF. The single-mode performance and bending resistance of a modified structure, called leakage channels enabled multi-resonant AS-PBGF (LC-PBGF), are evaluated numerically. Robust single-mode transmission is guaranteed by a specially designed microstructure cladding with only four layers of germanium-doped rods. Multi-resonant cores in the inner layers and leakage channels in the outermost layer, resulting from missing rods in the microstructure cladding, are employed to generate modal dissipation of high-order modes under bent configuration. The missing germanium-doped rods in each layer are properly designed to eliminate the dependence on bending direction, leading to differential bending loss between fundamental mode and high-order-modes with high loss ratio. In addition, some typical derivative structures based on the LC-PBGF concept have also been proved to have great potential for effective single-mode operation.

Improvement of 2-μm Thulium-Doped Fiber Lasers via ASE Suppression Using All-Solid Low-Pass Photonic Bandgap Fibers

All-solid photonic bandgap fibers (PBGFs) were designed and fabricated to work as low-pass filters in 2-μm region. They were used for manipulating thulium doped fiber laser (TDFL) operating in the 2-μm region via amplified spontaneous emission suppression. A sharp lasing peak at 1915 nm was achieved using a single-stage TDFL with full width at half maximum and optical signal-to-noise ratio of 0.3 nm and 39.5 dB, respectively. This lasing is achieved using the fabricated PBGF low-pass filter with cutoff wavelength at 1945 μm and loss of more than 25 dB. In this respect, the resultant TDFL performed better than the one that did not employ the low-pass filter. In this paper, we present a few representative cases involving the all-solid PBGF. We have shown that by increasing the diameter of the fabricated PBGF from 142 to 189 μm, their cutoffs related to their bandgaps shifted from 1727 nm toward longer wavelengths, 2079 nm. As a result, lasers with different wavelengths were generated within 2-μm region.

Highly efficient cladding-pumped single-mode three-level Yb all-solid photonic bandgap fiber lasers.

Efficient cladding-pumped three-level Yb fiber lasers are difficult to achieve due to the competing four-level system and necessary high inversions. We demonstrate an efficiency of ∼62.7% versus a coupled pump, a record for cladding-pumped fiber lasers with a single-pass pump. 84W at ∼978 nm with ∼1.12M2 was achieved, a record power for flexible fibers. Amplified spontaneous emission was suppressed by >40 dB. The efficiency is quantum-limited ∼94% versus an absorbed pump. This is made possible by the use of a photo-darkening-free Yb phosphosilicate core and recent progress in single-mode large-core all-solid photonic bandgap fiber designs, which provide the necessary large core-to-cladding ratio and suppression of the four-level system.

Dual-Resonance-Coupling and Sensing Characteristics in an All-Solid Photonic Bandgap Fiber Interferometer Based on LP01 Cladding Supermodes

A dual-resonance-coupling phenomenon and the sensing characteristics of an in-fiber modal interferometer based on cascaded long period gratings inscribed in an all-solid photonic bandgap fiber are demonstrated. Theoretical and experimental investigations reveal that the dual-resonance dips result from the coupling between fundamental core mode and two different kinds of LP01 cladding supermodes. Due to the deep grooves and asymmetric index distribution along the long period grating caused by side illumination, this dual-resonance interferometer is sensitive to polarization and twist, and the dual-resonance dips according to different LP01 cladding supermodes exhibit different responses to twist. Furthermore, the transmission loss and the resonance wavelength of each interference dip show discrimination in sensitivities of temperature and twist, making it a good candidate for multiple physics parameters measurements.

Mode amplification fiber laser based on photonic band gap fiber

We theoretically propose the structure of all-solid photonic band gap fiber to support LP01 mode at 980 nm and four modes of LP01, LP11, LP02 and LP21 at famplification of characteristics of the photonic band gab and apply photonic fiber as gain medium to fiber lasers, finally realizing separate amplification of LP11 and LP21 high-order modes with single-mode stable pump light of LP01 mode at 980 nm. Besides, we realize LP11 mode and LP21 mode laser output of dual-wavelength.

Suppressing 1.06-[formula omitted]m spontaneous emission of neodymium ions using a novel tellurite all-solid photonic bandgap fiber

In order to take advantage of the 1.3-μm emission of neodymium (Nd3+) ions for many practical applications, we propose a tellurite all-solid photonic bandgap fiber (ASPBGF) to filter out the competing emission at 1.06 μm which is most prominent in the emission spectrum of Nd3+ ion. A novel Nd3+-doped tellurite ASPBGF is fabricated by using our developed tellurite glasses which have high compatibility of thermal properties and their refractive index difference is 0.096 at 1320 nm. The fiber is designed with 4 layers of high-index rods to have low confinement loss. The measured transmission spectrum of a 2.2-cm-long section of the fabricated fiber exhibits high transmission bands near 0.75 and 1.33 μm (about −20 dB and −19 dB) and a low transmission band in the vicinity of 1.06 μm which is about −27 dB. By using our fabricated Nd3+-doped tellurite ASPBGF, it is demonstrated for the first time that the 1.06-μm emission peak due to the 4F3∕2→4I11∕2 transition of Nd3+ ions is greatly suppressed about 12 times as compared to that obtained by using a bulk samples with the same doping concentration.

Few-period helically twisted all-solid photonic bandgap fibers.

We present a type of few-period helically twisted all-solid photonic bandgap fiber (AS-PBGFs). The helical structure leads to orbital resonance of a cladding rod light, which couples with the core mode. A two-period twist structure exhibits an extremely strong resonant dip of up to 30dB. A series of samples with twist periods of 3.31-7.92mm (yielding twist rates of 1.90-0.79 rad⋅mm-1) in association with different resonance orders are fabricated and demonstrated. The inherent physical mechanism underlying the resonance is analyzed. Moreover, the responses of the resonance to mechanical torsion, strain, and temperature are investigated. The twisted AS-PBGFs feature high reproducibility, stability, and robustness, and have great potential in tunable in-fiber filters and sensors.

Efficient 240W single-mode 1018nm laser from an Ytterbium-doped 50/400µm all-solid photonic bandgap fiber.

Lowering the quantum defect by tandem pumping with fiber lasers at 1018nm was critical for achieving the record 10kW single-mode ytterbium fiber laser. Here we report the demonstration of an efficient directly-diode-pumped single-mode ytterbium fiber laser with 240W at 1018nm. The key for the combination of high efficiency, high power and single-mode at 1018nm is an ytterbium-doped 50μm/400μm all-solid photonic bandgap fiber, which has a practical all-solid design and a pump cladding much larger than those used in previous demonstrations of single-mode 1018nm ytterbium fiber lasers, enabling higher pump powers. Efficient high-power single-mode 1018nm fiber laser is critical for further power scaling of fiber lasers and the all-solid photonic bandgap fiber can potentially be a significant enabling technology.

Large-mode-area neodymium-doped all-solid double-cladding silicate photonic bandgap fiber with an index step of ～0.5%

A large-mode-area neodymium-doped silicate photonic bandgap fiber was theoretically designed and experimentally demonstrated. The relative index step between the high-index rods and the background glass was ∼0.5%, which is the lowest cladding index difference reported on rare-earth-doped all-solid photonic bandgap fibers to our knowledge. An output power of 3.6 W with a slope efficiency of 31% was obtained for a 100-cm-long fiber.

Coupling characteristics of twin-core few-mode all-solid photonic bandgap fiber and their application in mode separation

We theoretically investigate the coupling characteristics of a few-mode twin-core all-solid photonic bandgap fiber. We show that extrema in the coupling length, reversal of the ordering of supermodes, and complete decoupling can occur in high-order modes. Using significant differences in the coupling characteristics between different modes, we can separate degenerate mode pairs such as LP11a and LP11b, and LP21a and LP21b, as well as modes with the same azimuthal quantum number, e.g., LP01 and LP02. By setting a suitable photonic-crystal period, we can realize an optical fiber coupler with mode separation properties in the C-band.

Effect of submicron deformations on the transmission of all-solid photonic bandgap fibre

Waveguide losses in all-solid photonic bandgap fibre are studied numerically using both vectorial mode solver and vectorial beam propagation methods. Confinement loss, including material losses, is comprehensively evaluated for defect modes of hexagonal lattice photonic bandgap fibre. The excitation of the index-guiding modes at the bandgap edges leads to a narrowing of the transmission bands. Submicron deformations of the transverse structure of the fibre lead to a significant reduction in the width of the transmission bands, yet the minimum value of the losses within these bandgaps remains practically unchanged. Longitudinal variation of the fibre profile increases the effective losses by up to tens of dB/m.

Investigation on single taper-based all-solid photonic bandgap fiber modal interferometers.

We demonstrate a single taper-based all-solid photonic bandgap (AS-PBG) fiber modal interferometer that consists of a central tapered fiber region connected to the untapered via two abrupt transitions. Modal interference is given by superimposing the bandgap-guided fundamental core mode with a lower effective index and a specific index-guided cladding supermode with a higher effective index. A series of interferometers with taper diameter of 50μm ~60μm and device length of ~3mm are fabricated and studied in contrast to the conventional counterparts. The temperature coefficient of the interferometer is closely determined by the fraction of the cladding supermode energy localized within the index-raised regions of the fiber. The refractive index (RI) responsivities associated to fiber taper sizes are investigated. The measured maximal RI sensitivity is ~3512.36nm/RIU at the taper diameter of 50μm around RI = 1.423. This research gives a deep understanding to the modal-interferometric AS-PBG structure, which we believe to be valuable for the future application of the related device.

Large-Mode-Area All-Solid Photonic Bandgap Fibers for the Mitigation of Optical Nonlinearities

There is still significant need for power scaling of fiber lasers. Large-mode-area fibers are a key for the mitigation of optical nonlinearities. In recent years, mode instability has shown itself to be an additional significant limiting factor for single-mode power scaling in the regime of a few hundred watts to kilowatts. It is better appreciated now that further power scaling requires significant high-order-mode suppression in addition to a large effective mode area in a fiber. In recent years, we have shown that all-solid photonic bandgap fibers are a superior approach due to their unsurpassed higher-order-mode suppression in large-mode-area designs, making them well suited for applications at high average powers. We will review of some of the recent progress, challenges, and prospects of all-solid photonic bandgap fibers in this invited paper.

Extreme large mode area in single-mode pixelated Bragg fiber.

This paper reports the design and the fabrication of an all-solid photonic bandgap fiber with core diameter larger than 100 µm, a record effective mode area of about 3700 µm2 at 1035 nm and robust single-mode behavior on propagation length as short as 90 cm. These properties are obtained by using a pixelated Bragg fiber geometry together with an heterostructuration of the cladding and the appropriated generalized half wave stack condition applied to the first three higher order modes. We detail the numerical study that permitted to select the most efficient cladding geometry and present the experimental results that validate our approach.

Three-level Nd3+ luminescence enhancement in all-solid silicate glass photonic bandgap fiber

We report a neodymium-doped all-solid silicate glass photonic bandgap fiber which enhances three-level laser transition F-4(3/2)-I-4(9/2) (890-940 nm) by suppressing the four-level laser transition F-4(3/2)-I-4(11/2) (1000-1100 nm) using the bandgap effect. The laser output was not obtained due to the low absorbed pump power, but our result provides a new method for a new wavelength band fiber laser.

Supermode Bragg grating combined Mach-Zehnder interferometer for temperature-strain discrimination.

We report on a compact sensor by integrating a Mach-Zehnder interference and a cladding Bragg grating in a same section of all-solid photonic bandgap fiber. Theoretical investigation reveals that the Bragg grating resonance stems from the coupling of counter-propagating cladding LP01-like supermodes and the Mach-Zehnder interference works between a LP01-like supermode and LP01 core mode. Compared with the interference fringe, such supermode grating dip responses to axial strain in a more sensitive and opposite-direction manner. Whereas, the interference fringe shows a higher temperature sensitivity than the supermode grating dip. By means of these different responses, this device finds a useful application in the discrimination of temperature and axial strain.