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Here, we propose an all-fiber actively mode-locked Bismuth (Bi)-doped fiber laser based on the use of an acousto-optic modulator. The mode-locked Bi-doped fiber laser produces 13ns pulses with a repetition rate of 1.683MHz at 1340nm. Higher harmonic mode-locking is achieved simply by changing the operating frequency of the acousto-optic modulator. The output power of the laser is 7mW and this is further amplified to 101mW using an external Bi-doped fiber amplifier. A peak power of 4.6W with a pulse energy of 60nJ is achieved after the master oscillator power amplifier. The stability of the laser is studied using an RF spectrum analyzer and an SNR of more than 60dB at the fundamental frequency of 1.683MHz was recorded. Furthermore, wavelength tuning of the Bi-doped fiber laser is explored and demonstrated from 1300 to 1370nm.

We report a double-pass L-band erbium-doped fiber amplifier providing ≥20dB gain from 1565-1625nm, with 46dB maximum gain at 1600nm. At 1625nm, the NF, OSNR, gain coefficient, and temperature-dependent-gain-coefficient were 7.2dB, 25dB, 0.045dB/mW, and -0.037dB/oC, respectively.

A tunable dual-wavelength Brillouin-bismuth fiber laser with a triple-Brillouin frequency spacing of ∼37.8 GHz was demonstrated by employing modular structures. The modular designs can be divided into two main cavities, whereby cavity 1 consisted of a 2-km single-mode fiber (SMF) and a loop mirror that was able to generate a single-spaced Brillouin shift, while cavity 2 comprised of 10-km dispersion compensating fiber (DCF) that was able to produce a double-spaced Brillouin shift. Based on this design, a stable dual-wavelength lasing was established with a low peak power fluctuation of 0.13 dB and 0.6 dB for the Brillouin pump (BP) and third Brillouin Stokes line (BS3), respectively. By varying the center wavelength of the Brillouin pump, the dual-wavelength laser can be tuned across a 40 nm span, ranging from 1300 nm to 1340 nm. The frequency spacing of the Brillouin-bismuth fiber laser can also be easily switched between ∼12.6 GHz, ∼25.2 GHz, and ∼37.8 GHz by utilizing these modular structures. The proposed dual-wavelength laser has a promising potential for application as a low phase noise source for microwave generation.

Alternate-mark-inversion (AMI) has been used in the past to improve the transmission performance of binary intensity modulation formats. In this work, we improved the tolerance of O-band 4-level pulse amplitude modulation (PAM4) transmission to channel impairments through the adoption of an AMI scheme. Improvements in receiver sensitivity were shown in both semiconductor optical amplifier (SOA) and bismuth-doped fibre amplifier (BDFA) equipped links. We confirmed the capability of 4-level AMI to extend the reach in the O-band, which was achieved through the mitigation of nonlinear and dispersive effects. We experimentally demonstrated a 4.28-dB Q-factor improvement for transmission over 60 km of SMF-28e. To offer an insight to the mechanisms leading to the improved tolerance to transmission impairments, we presented numerical simulations that illustrate the underlying method of action of AMI4.

We implemented a bismuth-doped fiber amplifier (BDFA) based optical recirculating loop to investigate the performance of amplified O-band transmission over appreciable distances. Both single-wavelength and wavelength-division multiplexed (WDM) transmission were studied, with a variety of direct-detection modulation formats. We report on (a) transmission over lengths of up to 550 km in a single-channel 50-Gb/s system operating at wavelengths ranging from 1325 nm to 1350 nm, and (b) rate-reach products up to 57.6 Tb/s-km (after accounting for the forward error correction redundancy) in a 3-channel system.

In this article, we experimentally demonstrate an all-fiber optical amplifier using Bi-doped phosphosilicate fiber (BPSF) operating in both the O-band and the E-band, providing >20 dB gain from 1345 to 1460 nm for an input signal power of −23 dBm. A maximum gain of 31 dB with a noise figure (NF) of 4.8 dB was achieved at 1420 nm for −23 dBm input signal, whereas for an input signal power of −10 dBm it was 26 dB with a NF of 5.3 dB. The in-band OSNR of the proposed Bi-doped fiber amplifier (BDFA) was >22 dB from 1345 to 1460 nm for a −23 dBm input signal. The gain coefficient was found to be 0.04 dB/mW for a −23 dBm input signal. Also, the temperature dependent gain and NF performance of the BDFA within −40 to +60 °C were characterized. The temperature-dependent-gain (TDG) coefficient for a −23 dBm input signal was found to be −0.015 dB/°C at a signal wavelength of 1420 nm where the BDFA gain is maximum, while for an input signal power of −10 dBm it was −0.01 dB/°C at 1420 nm. The TDG coefficient of the proposed BDFA from 1350–1460 nm was in the range from −0.079 to −0.008 dB/°C. The BDFA gain and NF performance were also measured and compared using BPSFs with different unsaturable loss (UL) characteristics.

Objective Laser technology in urology is currently used for both stone lithotripsy and prostate enucleation. Thulium fiber laser (TFL) is a novel laser, with initial studies showing potential benefits over other lasers both in terms of its effectiveness and safety profile. Material and methods In the first part of this review, a descriptive analysis of the theoretical concepts behind TFL was performed. This part focused on the physics and laser parameters as applied to the clinical practice. These were interpreted in the context of other lasers, namely, the Holmium:YAG laser to highlight the theoretical advantages as well as potential pitfalls offered by the TFL. In the second part of the review, a narrative synthesis of in-vitro studies regarding TFL and its modifications is performed assessing stone-related parameters, namely, ablation rate, operative time retropulsion, and safety. Results TFL achieved high ablation rates in most studies and performed better than Holmium:YAG laser across a range of different settings and ablation modes when the two lasers were compared. Moreover, its ability to use low pulse energy ensures minimal stone retropulsion with the retropulsion threshold estimated to be 2-4 times higher than that of Holmium:YAG laser. From a safety viewpoint, TFL poses no additional risks than other lasers, although it does potentially lead to slightly higher temperatures in the surrounding tissues during lithotripsy. Conclusion The unique properties of TFL have made it an attractive alternative to conventional laser techniques currently used in urology. Clinical studies are required before its application can become more widespread.

In this paper, we present a comparative study on temperature-dependent spectroscopic characteristics and L-band amplifier performance for aluminum-rich erbium-doped fiber (EDF) and in-house fabricated phosphorus co-doped EDF. Different pumping configurations were studied to conclude that the pump wavelength of 980nm with unequal forward/backward pump powers exhibited better temperature stability. Phosphorus EDF provided 19.41.4dB gain and 4.60.2dB noise figure (NF) from 1575-1615nm at room temperature (RT), for a multi-channel input signal of -25dBm in each channel, whereas the aluminum-rich EDF provided 20.35.1dB gain and 5.30.8dB NF. Using a single-channel input signal of -25dBm at 1625nm, phosphorus EDF maintained >10dB gain with a 9.6dB and 12dB gain increment than aluminum-rich EDF at RT and -60oC, respectively. The temperature-dependent gain (TDG) coefficient from 1575-1615nm was in the range -0.006 to -0.044 dB/oC for phosphorus EDF and 0.011 to -0.023 dB/oC for aluminum-rich EDF, over the temperature range -60 to +80oC. We propose a hybrid L-band amplifier concatenating aluminum-rich EDF with phosphorus EDF, to suppress the temperature dependence of phosphorus EDF and improve the gain bandwidth restriction of aluminum-rich EDF. The hybrid EDF exhibited multi-channel 20.93.9dB gain and 3.70.6dB NF from 1575-1615nm at RT. The TDG coefficient of the hybrid EDF remained almost constant from 1585-1615nm, contributing to a temperature-insensitive gain flatness.

Here we present a fully fiberized Bi-doped ring laser incorporating a fiber Bragg grating (FBG) operating at a wavelength of 1340nm, which provides 172mW of continuous-wave output power with a slope efficiency of 35%. In addition, we demonstrate a wavelength-tunable version of the laser, which can be tuned from 1320nm to 1370nm by employing a home-made mechanically tunable FBG. Within the whole 50nm tuning band the laser maintains the output power of more than 100mW and the slope efficiency of more than 25%. A maximum output power of ~165mW is achieved over the wavelength range of 1330-1340nm. The linewidth of tunable laser is 0.04nm.

We propose a novel approach of exploiting the LP02 mode in ultra-high NA confined-doped M-type fibers to ensure effective single-mode operation, thereby breaking the trade-off between mode-area scaling and ultra-high NA. This novel approach opens the door to highly rare-earth doped fibers suitable for power scaling applications. Therefore enabling, simultaneously scaling of power and slope efficiency of fiber lasers.

In this article, we experimentally demonstrate dual O+C-band direct-detection wavelength division multiplexed (WDM) transmission over 50 km of standard single-mode fibre, wherein 4 × 40-Gb/s Nyquist on-off keying (OOK) modulation is used in the O-band whilst 4 × 40-Gb/s single-sideband (SSB) subcarrier modulation (SCM) with Kramers-Kronig (KK) detection is used in the C-band. A bismuth-doped fibre amplifier (BDFA) is used as the pre-amplifier in the O-band to extend the reach of the direct-detection system. It is demonstrated that the O-band exhibits superior receiver sensitivity relative to the C-band and its transmission performance is limited by the chromatic dispersion (CD), especially when longer reaches and wavelengths in the longer edge of the band are explored. In contrast, due to the adoption of the SSB format, the C-band shows enhanced tolerance to CD effects. In this case, some receiver sensitivity is sacrificed to accommodate the adoption of the KK detection which is used to eliminate the signal-to-signal beating interference of the SSB-SCM format. The inferior receiver sensitivity in the C-band is caused by (1) the higher CSPR requirement of the KK detection and (2) the restriction of small-signal modulation for the dual-drive Mach-Zehnder modulator to generate optical SSB signals. The results indicate the feasibility to further extend the reach of O-band transmission systems, which enables the possibility to realise dual O+C-band WDM transmission in intermediate- to long-haul optical networks.

We propose and demonstrate a bidirectional Vanilla recurrent neural network (Vanilla-RNN) based equalization scheme for O-band coarse wavelength division multiplexed (CWDM) transmission. Based on a 4×50-Gb/s intensity modulation and direct detection (IM/DD) system, we demonstrate the significantly better bit error rate (BER) performance of the Vanilla-RNN scheme over the conventional decision feedback equalizer (DFE) for both Nyquist on-off keying (OOK) and Nyquist 4-ary pulse amplitude modulation (PAM4) formats. It is shown that the Vanilla-RNN equalizer is capable of compensating for both linear and nonlinear impairments induced by the transceiver and the single-mode fiber (SMF). As a result, up to 100-km and 75-km SMF transmission can be achieved for OOK and PAM4 transmission, respectively. Furthermore, through the comparison with other equalization schemes, including the linear equalizer, 3rd-order Volterra equalizer, and Volterra+DFE, it is demonstrated that the Vanilla-RNN equalizer achieves the best BER performance. In the meantime, it also exhibits lower implementation complexity when compared to Volterra-based schemes. Our results show that the Vanilla-RNN scheme is a viable solution for realizing simple and effective equalization. This work serves as an exploration and offers useful insights for future implementations of reach-extended O-band CWDM IM/DD systems.

The recent emergence of bismuth-doped fiber amplifiers (BDFAs) enables the reach extension of high-speed O-band communication links. In this work, our recent efforts on amplified O-band direct-detection (DD) transmission using BDFAs are reviewed. We first present the performance of a four-channel O-band coarse wavelength-division multiplexed (CWDM) transmission over 50-km length of single-mode fiber (SMF), showing that the chromatic dispersion (CD) which has long been neglected in O-band transmission, will degrade the transmission performance of dispersive O-band channels. In this context, we quantify the relative benefits of different DD formats at 50 Gb/s over up to 70-km length of SMF in a BDFA pre-amplified O-band system. The considered formats include Nyquist on-off keying (OOK), Nyquist 4-ary pulse amplitude modulation (PAM4), and Kramers–Kronig (KK) detection assisted subcarrier modulation using quadrature phase-shift keying (QPSK), and they are evaluated over both the low CD 1320-nm wavelength and the more dispersive 1360-nm wavelength. It is demonstrated that OOK exhibits the best receiver sensitivity over all distances up to 70 km at the 1320-nm channel, whereas KK-QPSK achieves the optimal transmission performance at the 1360-nm wavelength when the reach is extended beyond 50 km. We consider that our results provide useful insights into possible future implementations of longer-reach O-band WDM systems.

We demonstrate the first single-sideband 50-Gb/sA coarse WDM direct-detection transmission in the O-band. It is shown that the Kramers-Kronig-detection assisted single-sideband transmission exhibits significant OSNR sensitivity improvements over double-sideband transmission, enabling up to 100-km reach.

Bismuth (Bi)-doped fibers provide great potential for developing optical amplifiers in the low-loss transmission window, 1250-1650nm, of silica fibers. In this paper, we review recent progress of Bi-doped fiber amplifiers (BDFAs) covering 1345-1460nm spectral region. We also present the BDFA performance at different ambient temperatures.

We report on what is to the best of our knowledge the longest 50-Gb/s/λ O-band wavelength-division multiplexed (WDM) transmission. A pair of in-house built bismuth-doped fiber amplifiers (BDFAs) and the use of Kramers-Kronig detection-assisted single-sideband transmission are adopted to overcome the fiber loss and chromatic dispersion, respectively, in a reach-extended O-band coarse WDM (CWDM) system with a channel spacing of ∼10 nm. Through experiments on an amplified 4×50-Gb/s/λ direct-detection system based on booster and pre-amp BDFAs, we show the superior performance of single-sideband transmission in terms of both optical signal-to-noise ratio sensitivity and uniformity in performance amongst CWDM channels relative to double-sideband transmission after both 75-km and 100-km lengths of single-mode fiber. As a result, up to 100-km reach with comparable performance at all 50-Gb/s channels was achieved without the need for in-line optical amplification.

The recent emergence of efficient O-band amplification technologies has enabled the consideration of O-band transmission beyond short reach. Despite the O-band being a low chromatic dispersion (CD) window, the impact of CD will become increasingly significant when extending the reach of direct-detection (DD) systems. In this work, we first numerically investigate the 3-dB bandwidth of single-mode fibers (SMF) and the CD-restricted transmission reach in intensity-modulation DD systems, confirming the significant difference between low- and high-dispersion O-band wavelengths. We then carry out experimental transmission studies over SMF for distances of up to 70 km at two different wavelengths, the low-dispersion 1320 nm and the more dispersive 1360 nm, enabled by the use of an O-band bismuth-doped fiber amplifier as a preamplifier at the receiver. We compare three 50-Gb/s optical DD formats, namely, Nyquist on-off keying (OOK), Nyquist 4-ary pulse amplitude modulation (PAM4) and Kramers–Kronig detection-assisted single-sideband quadrature phase shift keying (KK-QPSK) half-cycle subcarrier modulation. Our results show that at both wavelengths, OOK and QPSK exhibit better bit error rate performance than PAM4. When transmitting over 70-km of SMF at the less dispersive wavelength of 1320 nm, 50-Gb/s OOK modulation offers more than 1.5-dB optical power sensitivity improvement at the photodiode (PD) compared to 50-Gb/s QPSK. Conversely, at 1360 nm, the required optical power to the PD can be reduced by more than 3 dB by using QPSK instead of OOK.

The recent emergence of bismuth-doped fiber amplifiers (BDFAs) offers the potential to transmit high-speed WDM signals over long distances in the O-band spectral region, thereby greatly enhancing the scope of systems utilizing these wavelengths. In this paper, we present a comprehensive experimental study on several basic characteristics of an O-band BDFA based on a phosphosilicate optical fiber, including the frequency-dependent noise figure, gain tilt (static and dynamic), transient response, and polarization dependent gain. We discuss our findings and their implications on the use of BDFA technology in high bit-rate multichannel systems.

In this work, a Figure-9 (F9) bismuth-doped fiber laser (BiDFL) operating in the dissipative soliton resonance (DSR) regime is presented. The 1338 nm laser used a BiDF as the active gain medium, while a nonlinear amplifying loop mirror (NALM) in an F9 configuration was employed to obtain high energy mode-locked pulses. The wave breaking-free rectangular pulse widened significantly in the time domain with the increase of the pump power while maintaining an almost constant peak power of 0.6 W. At the maximum pump power, the mode-locked laser delivered a rectangular-shaped pulse with a duration of 48 ns, repetition rate of 362 kHz and a radio-frequency signal-to-noise ratio of more than 60 dB. The maximum output power was recorded at around 11 mW with a corresponding pulse energy of 30 nJ. This is, to the best of the author’s knowledge, the highest mode-locked pulse energy obtained at 1.3 μm as well as the demonstration of an NALM BiDFL in a F9 configuration.

We report a double-pass bismuth-doped fiber amplifier operating in the E-band providing 38dB gain with 6dB NF for -23dBm input signal at 1430nm. The amplifier 3-dB bandwidth and temperature-dependent-gain coefficient are 40nm and -0.006dB/℃, respectively.

This work proposes a new route to overcome the limits of the thermal poling technique for the creation of second order nonlinearity in conventional silica optical fibers. We prove that it is possible to enhance the nonlinear behavior of periodically poled fibers merging the effects of poling with the nonlinear intrinsic properties of some materials, such as MoS2, which are deposited inside the cladding holes of a twin-hole silica fiber. The optical waves involved in a second harmonic generation process partially overlap inside the thin film of the nonlinear material and exploit its higher third order susceptibility to produce an enhanced SHG., SCOPUS: ar.j, info:eu-repo/semantics/published

High power fibre lasers have established their strong presence in the manufacturing sector and for scientific applications. The key to the power scaling of fibre lasers can largely be attributed to the progress in large-mode-area (LMA) fibre designs by offering a higher threshold to the non-linear effects and development of high performance rare-earth doped fibres. In LMA fibres, a guiding discrimination between the fundamental mode and higher-order modes can ensure an effective single-mode operation. In this talk, Professor Sahu will review the progress in rare-earth doped fibres suitable for efficient high power laser operation in the 1µm and 2µm wavelength regions.

Cladding-pumped Tm-doped fiber lasers operating below 1950 nm have difficulty matching the high-efficiency, power-scalable output that can be achieved at longer wavelengths. This challenge arises due to the strong three-level behavior at short wavelengths and strong competition from higher-gain long wavelength emission. In this Letter, we demonstrate a nested-ring fiber design in which a highly doped Tm ring is embedded within a larger undoped core. The fiber is specifically tailored for highly efficient and high power short-wavelength operation ( < 1950 n m ). The nested-ring Tm fiber laser has generated 62 W of single-mode 1907 nm output with up to 65% (70%) slope efficiency with respect to launched (absorbed) pump power.

The sensitivity of an interferometric fiber optic gyroscope (IFOG) scales with the length of the sensing optical path. Thus, IFOG development history has seen much work devoted to shrinking ever-increasing lengths of optical fiber into a fixed volume. Indeed, the success of the IFOG as a guidance and navigation technology is founded, to a large extent, on the many advancements in fiber-optics which were required to compact numerous state-of-the-art components – including a multi-kilometer length of optical fiber – to within the size of a teacup.An exciting technology which promises to continue this trend is multicore optical fiber, in which multiple, independent optical waveguides (cores) are placed within the same glass cladding which would ordinarily contain only one core. The dense arrangement of cores in such fibers can be exploited in an IFOG by connecting them in series, and thereby increasing the instrument sensitivity proportionally. As originally proposed by Bergh [1], these features present an opportunity to increase sensitivity while reducing the sensor footprint and simplifying the optical fiber coil - a key driver of cost and complexity in IFOGs.Here we detail performance characteristics of an all-fiber multicore IFOG employing a bend-insensitive, single-mode, 7-core fiber in the sensing coil. Like the recent, first-ever demonstration by Mitani et al. [2], [3], we employ an open-loop testbed architecture with a depolarized sensing loop, in which fiber cores are connected in series via a pair of multicore fan-in/fan-out devices. Here however, the fan-in/fan-out components are tapered fiber devices, packaged in conventional fiber-optic component sleeves, and with the core interconnections made via standard fusion splices [4]. Measurements of noise and long-term stability of the instrument show that its performance is commensurate with the 7X enhanced sensitivity afforded by the optical path length increase. For this 7-core, 154 m long, 10 cm diameter fiber coil, we show long-term gyro bias stability under 0.02 deg/hr and angle random walk of $2.4\, \text{mdeg} /\sqrt {\text{hr}}$. This compares favorably with both noise models and performance measurements in IFOGs of similar scale factor, thus confirming the sensitivity improvement conferred by use of 7-core fiber.The all-fiber configuration of the sensing loop makes this instrument highly practicable as a drop-in replacement for current IFOGs, with no change to existing front-end designs. Moreover, as multicore fiber technology continues to push the frontiers of optical fiber transmission capacity, future designs may benefit from even greater core multiplicity – an exciting prospect for the next generation of compact, low-cost, high-accuracy IFOGs.

We show that optical fibers doped with as little as 7 mole% P 2 O 5 have 27% lower temperature sensitivity of accumulated phase (relevant in interferometry) as compared to standard optical fibers. We believe full temperature insensitivity can be reached with higher levels of doping.

Multiwavelength fiber lasers operating in the 1.3 μm wavelength region are of great interest as they complement existing systems in the conventional optical communication band of 1.55 μm. In this work, a multiwavelength Brillouin fiber laser with single- and double-Brillouin frequency spacings operating in 1.3 μm is proposed and demonstrated using a dispersion compensating fiber (DCF) as the Brillouin gain medium. A bismuth-doped fiber serves as the laser gain medium. At the maximum pump power of 1021 mW and with the Brillouin pump set at 11 dBm, up to 7 Brillouin Stokes lines with a wavelength spacing of 0.074 nm are obtained in the single-spaced configuration. In the case of the double-spaced configuration, 14 Stokes lines are generated with a wavelength spacing of 0.15 nm between the even Stokes lines. By tuning the central wavelength of the Brillouin pump, both laser configurations are wavelength-tunable with a range of 93 nm and 19 nm for the single-spaced and double-spaced Brillouin lasers respectively. This is, to the best of the author's knowledge, the first demonstration of a multiwavelength/Brillouin laser using a bismuth-doped fiber as the laser gain medium.

We present the fabrication and laser performance of thulium-holmium co-doped silica fibers when cladding pumped at ∼790 nm. By using the hybrid gas phase-solution doping process in conjunction with the MCVD preform fabrication technique, the doping concentration and the Tm:Ho ratio were varied to study the energy transfer efficiency from Tm3+ to Ho3+. Our study indicates that for a thulium concentration that has resulted in an efficient two-for-one cross- relaxation process with 790 nm pumping, and while maintaining a Tm:Ho concentration ratio in the range ∼ 10 to 20, the energy transfer efficiency has reached above 75%. In a free-running laser cavity, the pump power limited laser output of 38W with a slope efficiency of 56% at 2.1 microns is demonstrated.

In this article, we report high-speed multi-band direct-detection (DD) transmission over a hollow-core nested antiresonant nodeless fiber (NANF). Thanks to the ultrawide bandwidth of the NANF, we demonstrate dual-band transmission across the O- and C-bands over a ∼1-km length of a hollow-core fiber for the first time. Eight wavelength-division multiplexed (WDM) channels were transmitted using 100-Gb/s/λ Nyquist 4-ary pulse-amplitude modulation (PAM4) signals, which to the best of our knowledge, is the highest aggregate capacity ever transmitted in a DD hollow-core fiber-based transmission system. Optical pre-amplification was adopted for signal reception in both bands, achieved using an in-house built bismuth-doped optical fibre amplifier (BDFA) and a commercial erbium-doped fiber amplifier (EDFA) in the O- and C-band, respectively. We further demonstrate beyond 100-Gb/s/λ adaptively-loaded discrete multitone (DMT) transmission over the S+C+L-bands using the same NANF, without the use of optical amplification. Our experiments show that apart from fiber loss, the use of the NANF did not introduce any additional transmission penalties. The demonstrated results validate the ultrawide bandwidth and excellent modal purity of the fabricated NANF, which allow beyond 100-Gb/s/λ penalty-free transmission over multiple bands, highlighting the potential of this fiber technology for high-speed short- to intermediate-reach applications.

Double-clad silica fibres used in high power lasers typically comprise a core doped with a laser active ion, a silica inner-cladding pump guide and a low refractive index outer polymer coating for protection and low loss pump guidance. For efficient pump absorption in the active-ion doped core, the inner-cladding must be shaped in order to scramble the pump radiation to maintain overlap with the core. This shaping is traditionally undertaken via diamond, or ultrasonic, milling of the fibre preform into an octagon or hexagon, leaving a rough surface which is subsequently fire polished before drawing into a fiber. We report on our developments of an alternative approach for shaping the inner-cladding using a 10.6µm pulsed CO2 laser to machine the fibre preform. This process is shown to allow fabrication of N sided polygon shaped fiber claddings as well as novel cladding structures, which include concave and convex surfaces, as well as core to cladding area ratio adjustment. Processing speed is significantly increased whilst maintaining improved surface qualities that remove the requirement of further fire polishing prior to fibre drawing. We will discuss recent developments of novel cladding geometries such as Reuleaux polygon cladding shapes that allow use of automated x-y profilometers on the draw tower whilst maintaining a shaped cladding for pump scrambling.

A route to high power continuous-wave generation around 1.7 µm has been explored using an Er-only doped LMA fiber laser for high brightness core pumping of a thulium-doped fiber laser (TDFL). The maximum output power of 47 W with a slope efficiency of 80% was achieved at 1726 nm which, to the author’s best knowledge, is the highest recorded power in this wavelength region from a TDFL. The motivation for an Er-only pump source is scalability; these results show that this configuration has significant potential for further power scaling towards hundred-watt class systems at 1.7 µm and beyond.

Output power levels from 1907 nm Tm fiber laser sources are restricted by low lasing efficiency and challenging thermal management. Here we develop a novel fiber core structure where the Tm dopant material is confined to a ring within the larger undoped glass core. This evolution of dopant profiling has tripled the fiber device lengths and produced 131 W of 1907 nm emission with 64.5% absorbed slope efficiency. This, to the best of our knowledge, is the most efficient 1907 nm Tm fiber laser that has been demonstrated with the capability to perform beyond 100 W output power.

The increasing demand for novel fiber design has brought major challenges with the current fabrication methods. Additive manufacturing is an innovative fabrication process that has been attracting attention to the optical fiber community. 3D printed porous bodies from commercial silica powder were produced based on selective laser sintering (SLS). Complex structures such as antiresonant fiber (ARF), photonic crystal fiber (PCF) and multicore fiber (MCF) preforms were produced by this method. Additionally, a multi-material fiber with a silica cladding and a GeO2-SiO2 core was fabricated from a 3D printed preform. The optical and physical properties of the fabricated 3D printed structures were reported.

Thulium-doped fibers pumped with 79xnm high power diodes enable a cross-relaxation process to achieve two excited ions for one pump photon, reaching a quantum efficiency up to 200% in the two-micron band. For high power operation at longer wavelengths

Thermal poling, a technique to create permanently effective second-order susceptibility in silica optical fibers, has recently been improved by the discovery of an "induction poling" technique1 and the adoption of liquid electrodes2, allowing for poling fibers of any length and geometry. Nevertheless, the nonlinearity created via thermal poling is always limited by the ooelig;'(3)of the optical fiber material and by the maximum electric field that can be frozen inside the glass. For these reasons research is ongoing to determine routes for further improving the nonlinear effects due to the thermal poling process. In this work, we propose to enhance the effects of the thermal poling by exploiting the intrinsic nonlinear properties of some 2D materials3, which are deposited inside the cladding holes of a twin-hole silica fiber. The materials we focused on are 2D Transition Metal Chalcogenide (2D TMDC) MoS2 and WS2 and the technique adopted to realize the deposition inside the cladding channels of a twin-hole step index silica fiber consists of a thermal decomposition process4 of the precursor ammonium tetrathiomolybdate (NH4)2MoS4 in 6% H2/Ar flow. The technique has allowed us to uniformly coat the two cladding channels for a length of 25 cm with a film nominally consisting in a bi-layer of the 2D materials. A Raman based analysis has been used to test the morphology of the coating. The fiber deposited with 2D materials was later thermally poled and periodically erased via exposure to UV light to reach the QPM condition at a wavelength of 1550 nm. The effective oœ'(2) of the fiber was measured via SHG for both the deposited and the pristine fiber, showing an enhancement of the nonlinearity in favor of the deposited one. The phenomenon can be explained by the exploitation of a higher oœ'(3) seen by the pump wave due to the presence of the 2D layer deposited inside the cladding holes and opens the possibility of exploiting the higher intrinsic material oœ'(2), in case of a periodic patterning/synthesis of the TMDC., SCOPUS: cp.p, info:eu-repo/semantics/published

We present a tunable CW Bi-doped fiber laser from 1305-1375nm by utilizing a fiber-pigtailed– tunable-filter in a ring cavity. A maximum output power of 150mW at 1330±5nm with a slope efficiency of 35% is demonstrated.

We report a bismuth-doped fiber amplifier providing >20dB gain from 1345nm- 1460nm with 31dB maximum gain and 4.8dB NF at 1420nm for a -23dBm signal. The gain coefficient and temperature-dependent-gain coefficient are 0.042dB/mW and -0.015dB/°C, respectively.

We present a detailed characterisation of the emergent bismuth-doped fibre amplifier, characterising performance in terms of electrical noise figure, polarisation dependent gain, and cross gain modulation. The results support its suitability for high capacity links.

We experimentally compare 50-Gb/s single- and double-sideband formats in an O-band BDFA-amplified transmission system over different distances. The results show that single-sideband transmission is beneficial for distances beyond ~50km at more dispersive Oband wavelengths. © 2020 The Author(s)

38 μI pulse energy with peak power of 1.3 kW and 0.3 nm spectral width is obtained at a short-wavelength of 1851 nm by actively Q-switching a thulium fiber laser cladding-pumped at 792 nm.

We present a Bi-doped fiber laser (BDFL) at 1340nm with >170mW output power and 35% slope efficiency. A wavelength-tunable version of the BDFL facilitated by an FBG is demonstrated from 1315-1340nm with >120mW output power.

We numerically investigated LP02 mode in M-type fibres for mode-area scaling. A comparison with step-index fibres ensures up to 56% increment in mode-area with effective single mode operation for ultra-high NAs (~0.30) M-type fibres.

In this work, a multi-wavelength Bismuth-doped fiber laser (MWLBDFL) operating in the 1.3 µm wavelength region is presented and experimentally demonstrated. The MWLBDFL leveraged on a 60 m Bismuth-doped fiber as an active gain medium, a compact two-mode fiber filter (TMFF) as a comb filter and a 2 km single-mode fiber section to induce four-wave mixing in the system. By optimising the polarization controller, the MWLBDFL generated 18 stable lasing output channels within 10 dB from the highest lasing line and a reasonably flat spectrum over a range of 11 nm. The stability of the MWLBDFL tested for a 150-minute duration showed a peak power fluctuation of less than 1 dB with a negligible wavelength drift. The extinction ratio of the TMFF was approximately 8.6 dB while the highest and lowest optical-signal-to-noise ratios (OSNRs) of the MWLBDFL were ~ 55 dB and ~ 50 dB, respectively. To the best of the author’s knowledge, the proposed MWLBDFL demonstrates the highest number of output channels compared to other MWLs operating in the same wavelength region.

A tunable passively Q-switched fiber laser operating at 1.3 μm was demonstrated using bismuth-doped fiber (BDF) as the gain medium and using niobium carbide (Nb2C) as the saturable absorber (SA). The Nb2C was prepared using the solution casting method, and then fabricated into a film form for ease of integration into the laser cavity. Stable Q-switched pulses were observed as the pump power was increased from 820 to 1037 mW, generating pulses with increasing repetition rates from 10.1 kHz to 13.8 kHz and decreasing pulse widths from 17.54 to 7.58 μs. The all-fiber laser had a center wavelength at 1314 nm with a broad 3-dB bandwidth of 8.35 nm. The maximum output power and pulse energy of the Q-switched laser were 0.74 mW and 53.7 nJ, respectively. The laser was stable when tested for its long-term stability, where the peak frequency remained consistent at 13.8 kHz and the SNRs were maintained to be more than 60 dB throughout the entire test period. To the best of our knowledge, this is the first demonstration for a passively Q-switched fiber laser operating at 1.3 μm wavelength region using BDF as the gain medium.

High power, short-wavelength operation of a thulium-doped silica fiber laser at 1726 nm has been demonstrated in a core-pumped monolithic (all-fiber) resonator configuration, in-band pumped by a high power erbium-only fiber laser operating at 1580 nm. The thulium fiber laser yielded 46.7 W in a single-spatial-mode output beam for 65 W absorbed pump power. The corresponding slope efficiency with respect to absorbed pump power of 79.8% compares favorably with the theoretical maximum (Stokes) efficiency of 91.5%. The prospects for further scaling of single-mode power in this wavelength regime to >100 W are considered, as well as the potential applications for high power lasers operating in this difficult-to-reach wavelength band.

The emission band of holmium-doped silica fibers extends beyond 2200 nm, which means these lasers have the potential of covering considerable parts of the atmospheric transmission window between ∼2100 nm and 2250 nm. However, efficient operation toward 2200 nm is challenging due to absorption in fused silica at the laser wavelength. Here we present a holmium-doped fiber laser specifically targeting long-wavelength operation. The laser is implemented as a high-feedback wavelength selective ring cavity and is tunable from 2025 nm to 2200 nm. A maximum slope efficiency of 58% is obtained at 2050 nm and a slope of 27% is obtained at 2200 nm. A power of 5.5 W from a single aperture (8.9 W total) is demonstrated at 2200 nm. Our results represent extended coverage of the 2 μm spectral band with multiwatt-level silica fiber lasers. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Thermal poling of optical fibres is a well-known technique to create second order nonlinearity inside silica optical fibres, otherwise characterized by negligible nonlinear properties in the electric dipole approximation. Some recent work, realized by F. De Lucia et al., has introduced a new technique, designated as "Induction poling" [1] and with the adoption of liquid materials as embedded electrodes (both metallic and non-metallic) [2], allows thermal poling of optical fibres with any length and geometry. Despite these advances, thermal poling still represents a technological challenge that needs to be continuously optimized and simplified. In this work we focus our attention on the optimization of the electrical configuration of thermal poling of single mode optical fibres. We consider the single-anode (S-A) configuration, where a single electrode is embedded inside one of the two cladding channels of the optical fibre and connected to the desired electrical potential, and the double-anode (D-A) configuration, introduced for the first time by W. Margulis et al. in 2009 [3] and later commonly adopted by the scientific community. Fig. 1(a) shows the dependence (numerically calculated with COMSOL Multiphysics) of the X2 eff on the poling duration for both electrode configurations and at two different positions. The key result of these simulations is that the final value (for extended poling times) of the X2 eff in S-A configuration is approximately double with respect to the one obtained in the D-A approach. Furthermore, the value at the centre of the fibre is almost zero in D-A configuration. We hypothesize that this behaviour arises from the mutually competitive evolution of the space-charge formation due to the presence of two anodes. In contrast, the S-A configuration does not suffer from this limitation. Experimentally for the first time the X2 eff was measured in a process of second harmonic generation (SHG) at 1550 nm in a fibre periodically poled in S-A configuration. The nonlinearity has been periodically erased via exposure to a UV light generated by a frequency doubled Argon-ion laser (CW, 244 nm). Fig. 1(c) shows the spectrum of the SHG light.

With ytterbium doped fiber (YDF) lasers and amplifiers have reached continuous wave output power of multi-kW with direct diode pumping [1] and 10kW with tandem pumping [2] in a good beam quality, today fiber lasers are becoming the laser choice for many industrial applications and processes, defense, and scientific research. One of the challenges of high average power fiber laser is to maintain a long-term stability of the output power. In particular, photodarkening (PD) is seen as a power loss in YDF gain medium that can significantly influence the operation lifetime of the device. Moreover, it was observed that the PD is related to the transverse mode instability (TMI) that can limit the output power of the laser [3].