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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.

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.

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.

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.

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.

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.

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 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.

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].

Strong C-H bond absorption [1], and moderate water absorption in the spectral region around ∼1.7 μm make this wavelength band of interest for a range of applications, including polymer processing and laser surgery [2]. Despite being a growing area of interest, development of high-power laser sources in this region has been limited. Daniel et al [3], showed that high-power, continuous-wave generation between 1.66–1.75 μm could be achieved by inband pumping thulium-doped fibre (TDF) at 1565 nm using an erbium/ytterbium co-doped (Er/Yb) fibre laser, displaying 12.6 W at 1726 nm. Yb is added as a co-dopant to Er-doped fibres (EDF) to increase pump absorption efficiency. However, parasitic lasing in the Yb emission band at 1 μm can limit efficiency and the onset of self-pulsing can cause catastrophic damage, limiting single-mode output power and operating lifetimes. Here we report the use of a cladding-pumped Er-only fibre laser (EDFL) to core pump a thulium-doped fibre laser (TDFL), efficiently generating >25 W of single-mode output at 1726 nm.

Exploring the new wavelength bands is of great importance for many different applications such as medicine, astronomy, material processing, defence and optical fiber communication. Ytterbium (Yb), Erbium (Er), and Thulium (Tm) or Holmium (Ho)-doped fibers covers the wavelength bands around 1, 1.5 and 2μm, respectively. However, the band between Yb and Er is mostly not covered by the rare earth (RE)-doped silica fibers. Recent years, there is a growing interest to use Bi-doped fibers with different host materials such as aluminosilicate, phosphosilicate and germanosilicate to explore the wavelength band from 1150–1500nm. Also, Bi-doped germanosilicate fibers with a high concentration of germanium (up to 50mol%) were used to cover the band from 1600–1800nm. Many optoelectronic devices including amplifiers, lasers, amplified spontaneous emission (ASE) sources and pulsed lasers were reported using Bi-doped fibers [1–3]. However, less work is being done to develop tunable Bi-doped fiber lasers. Using a Bi-doped germanosilicate fiber a tunable laser operating from 1360 to 1510nm was reported. The cavity was formed by a loop reflector and an external plane diffraction grating [4].

High power Ho-doped fibre laser (HDFL) operating >2.1 μm spectral region that exhibits excellent atmospheric transmission, can find significant advantages for applications in remote sensing, defence and as pump source for mid-IR generation. HDFLs can be cladding pumped at ∼1.95μm by high brightness thulium-doped fibre lasers (TDFLs) for power scaling beyond 2.1-μm [1]. In a double clad HDF, a fluorosilicate glass cladding is needed rather than the low-index polymer commonly used in double clad fibres. This is to avoid absorption of the pump wavelength at 1.95μm by the polymer coating materials that brings additional challenges in fibre fabrication. In addition, the use of TDFL as a pump source for HDFLs will limit the overall optical to optical conversion efficiency. An alternative approach is to use Tm3+ as a sensitizer for HDFs which can be directly diode pumped at 790nm. So far, the cladding pumped Tm:Ho co-doped silica fibre laser system had been demonstrated with slope efficiencies up to 42% [2].

Optical filters have numerous applications in various optical systems due to their wavelength based discriminative transmission losses. These applications include detection of an emitted weak signal while avoiding strong excitation signal in any spectroscopy system, laser beam diagnostics, and tailoring the spectrum of a broadband light source etc in different sectors such as astrophysics, chemical analysis, imaging systems, communication, aerospace, and optics labs etc. A fiber based filter is highly desirable as it can be easily spliced to other fibers leading to an integrated optical systemthanks to the low splice losses. Several fiber based filters have been demonstrated based on birefringence [1], tapering [2], dispersion [3], and absorption [4–5] etc. The most important characteristic of a filter is a steep absorption edge with a low insertion loss, therefore it must offer a very high absorption in attenuated band compared to transmission band. The rare-earth absorption based fiber filters can offer very high absorptions with steep edge. Moreover, they are superior in quality than others as they can afford to handle much higher power thanks to the long length of the fiber. A first demonstration of fiber filter based on holmium doped silica fiber was demonstrated by Farries et al. [4]. In this paper, we propose for the first time a bismuth (Bi) doped silica fiber as a long pass filter with a cut-off wavelength of ∼1050 nm. Bi doped silica fibers are emerging as an attractive gain medium for lasers and amplifiers in wavelength range of 1100 nm to 1500 nm [5–6]. However, their use for filtering application unlike holmium remains unexplored. Bi doping has been investigated along with different co-dopant such as AhO 3 and P 2 O 5 in silica and germania hosts [5–6]. The crucial role of doping concentration of Bi, codopants (such as AhO 3 or P 2 O 5 ), and the host matrix (SiO 2 or GeO 2 ) in the absorption spectrum of the fiber has been shown in [5–6]. The steepness of the curve and order of discriminative losses in forbidden and transmission band decides the suitability of fiber for filtering application. Recently, we fabricated a series of bismuth and alumina doped silica fibers using optimized MCVD process in conjunction with solution doping technique [7]. A doping level of up to 5mol % of alumina was achieved with a core diameter of 8 μm with a 100 μm cladding diameter and the relative index difference between core and cladding is 0.008 [7]. Figure 1(a) shows the measured absorption spectra of two Bi-Al doped fibers (different Bi doping). Fiber 366 (red curve in Fig. 1(a)) has a broad absorption till ∼775 nm with an additional peak at ∼980 nm. Fiber 367 (black curve in Fig. 1(b)) offers a very sharp edge between ∼640 nm and ∼735 nm. Further, fiber366 was tested at high power level. A high power supercontinuum source (output power >1W, bandwidth 600 nm to 2400 nm) was used for further characterization. Figure 1 (b) shows the measured spectrum of the output of supercontinuum light. A second spectrum shown in figure 1(b) was obtained by coupling the output of the SC source to a 54 m long Bi-Al doped fiber. The spectrum clearly shows the complete suppression of light before 1050 nm. This clearly shows the potential of Bi doped fiber for filtering applications.

There are a number of spectral features around 1700 nm which are very attractive to the laser community. Strong C-H bond absorption and moderate water absorption lend this region to many applications such as polymer processing and laser surgery. Despite being a growing area of interest, development of high-power, laser sources in this region is quite challenging. Here we present preliminary results of in-band pumping a thulium-doped fiber laser (TDFL) with erbium-only doped fiber laser (EDFL) to generate 1700 nm Erbium-only was chosen to avoid the power scaling limitations of co-doping with ytterbium. Parasitic lasing on the ytterbium band at 1030 nm results in efficiency roll-off and self-pulsing. Because of this, single-mode Er/Yb laser systems are generally limited to moderate powers of ~20W and reliability remains an issue. An in-house double-clad large mode-area fiber, with a fundamental mode-field diameter of 20 um, was cladding pumped at 975 nm. Maximum power at 90 W launched was 31 W and the slope efficiency was 44.2% (35.4%) with respect to absorbed (launched) pump power. Suitability for high-brightness core pumping of a TDFL was confirmed with an M2 measurement of 1.1 ± 0.1. For 1700 nm generation, the 1580 nm pump light was free-space coupled into an in-house TDF (0.2 wt.% dopant concentration). For a maximum launched power of 23 W, 15 W of 1726 nm output was generated with a slope efficiency of 67.1% (64.7%) with respect to absorbed (launched). To the author’s best knowledge, this is the highest recorded power in this wavelength region from a TDFL.

We report a hybrid process by combining both vapor-phase and solution-doping techniques of rare-earth doped preform fabrication in conjunction with the MCVD technique, in order to fabricate highly efficient Tm-doped laser fibers. The proposed fabrication route takes advantage of co-doping silica with high alumina content through the vapor-phase doping process, which is otherwise difficult to achieve using conventional solution doping technique. In addition, by employing the solution doping method, high-purity thulium halide precursors that have low vapor pressures up to several hundred degree Celsius. These high-purity thulium halide precursors can be used to dope the fiber core region with a high thulium concentration that is optimized for an efficient two-for-one cross-relaxation process for 79xnm diode pumped thulium-doped fiber laser. Fibers fabricated using the hybrid approach show more homogeneous and flat-top dopant profiles, compared with the conventional approach, where both aluminum and thulium are incorporated in the core through solution doping. This will ensure that more doped region will take part in the cross-relaxation process. Superior laser performance with a slope efficiency of >70% in the two-micron band has been demonstrated when diode pumped at ~790nm.

We report a double-pass bismuth-doped fiber amplifier operating in the O-band with a 31dB gain and a 7dB NF for −10dBm input signal. The amplifier power conversion efficiency and gain-coefficient are 11% and 0.06dB/mW, respectively.

A cladding-pumped Tm fibre is designed and fabricated with a ring-structured fibre core geometry, optimised for power-scalable, single-mode 1907 nm operation with 67.0% slope efficiency and demonstrated in an all-fibre laser oscillator configuration.

Recent advances of development of rare-earth (RE) doped optical fibers have become increasingly important due to their applications in various optoelectronic devices such as high power optical fiber amplifiers, fiber lasers, etc. We present a review of recent progress on development of RE-doped glass host based optical fiber materials with special emphasis on large mode area fibers for efficient laser and amplifier. The challenging task is to manage high power inside the active core otherwise output power instability would affect the beam quality. Best solution is to use suitable dopants in addition to active elements during the process of fabrication. Modified chemical vapour deposition (MCVD) process combined with solution doping technique is deployed to develop RE-doped nano-engineered glass based optical fibers through suitable thermal annealing of optical preforms. The developed doped fibers are characterized by DTA, TEM, XRD, XAS, EPMA and EXAFS to assess the structural parameters. This new class of optical fiber materials will open up new possibilities for extended functionality and greater optoelectronic integration.

A planarized optical fibre designed for two-dimensional bend monitoring and external refractive index sensing is presented. The approach uses two single-mode waveguides each containing a set of spectrally multiplexed Bragg gratings. To achieve sensitivity to bending and external refractive index, the cladding material of the fibre is partially removed through physical machining. This acts to offset the neutral axis, increasing bend sensitivity, and exposes the evanescent field of the guided mode, permitting external refractive index monitoring. Through collective monitoring of the Bragg grating array, real time, multiparameter sensing is attainable, allowing new capability for intelligent monitoring.

A continuous-wave erbium-doped ytterbium-free fiber laser generates a record-breaking pump-power-limited output power of 656 W at ~1601 nm when cladding-pumped by 0.98 μm diode lasers. The slope efficiency was 35.6% with respect to launched pump power, and the beam quality factor (M2) was ~10.5. This M2 value excludes a fraction ~25% of the power which emerged from the cladding, which we attribute in part to mode coupling between the 146 μm core and 700 μm inner cladding. Whereas these parameters are adequate for in-band tandem pumping of Tm-doped fiber lasers, we predict that an output power of over 1 kW is possible by pumping with state-of-the-art 0.98 μm diode lasers, even with a smaller core that allows for improved beam quality.

Thulium-doped aluminosilicate fiber fabricated by MCVD process combining both solution doping and gas phase. Laser characterization report a slope efficiency of ∼72% with respect to launched pump power at 2069nm emission wavelength.

We report an efficient thulium-doped fiber offering a laser efficiency >70% with respect to the absorbed pump power over a wide wavelength band of 1890 to 2080nm when cladding pumped at 793nm.

We demonstrate an efficient ytterbium-doped alumino-phospho-silicate low-NA fiber fabricated by modified solution-doping process that shows continuous-wave laser efficiency of 81%. In ps-MOPA, the fiber provides 35kW peak power with repetition-rate 1.46MHz, and average power 9.1W.

Bismuth (Bi)-doped fiber (BDF) lasers and amplifiers have been demonstrated in different glass hosts (i.e., aluminosilicate, phosphosilicate and germanosilicate) covering the 1150–1800nm wavelength region [1-3]. Pulsed Bi-doped fiber lasers (BDFLs) are of great interest owing to their potential applications in medicine, material processing and optical fiber communications. The first Bi-doped pulsed fiber laser was demonstrated in 2007 with 50ps pulses at 1161nm in a Bi-doped silicate fiber using SESAM as a saturable absorber (SA) [4]. A number of studies have since reported on BDFLs [3-6], however, the pulse dynamics in BDFs have not been fully understood due to the unsaturable loss and excited state absorption present in these fibers [7]. In this paper, we present an all fiber self-mode-locked BDFL operating at 1340nm with a minimum pulse width of 1.5ns, the first demonstration of its kind.

Gd-doped silica and phosphosilicate fibers were pulled from preforms fabricated using the rod-in-tube technique and the solution doping technique, respectively. Ultraviolet (UV)-B luminescence from trivalent Gd at around 312 nm given by transition from first excited state to ground state were observed under deep UV excitations.

Optical fiber lasers operating in the near infrared (IR) and visible spectral regions have relied on the spectroscopic properties of rare earth ions such as Yb3+, Er3+, Tm3+, Nd3+ and Sm3+. Here, we investigate Gd3+ doping in phosphosilicate and pure silica fibers using solution doping and sol-gel techniques respectively for potential applications in the ultraviolet (UV). Photoluminescence spectra for optical fiber bundles and fiber preforms were recorded and compared. Emissions at 312 nm (phosphosilicate) and 314 nm (pure silica) were observed when pumping to the Gd3+ 6DJ, 6IJ, and 6PJ = 5/2, 3/2 energy levels. Oxygen deficient center was observed in solution doping sample with a wide absorption band centered at around 248 nm not affecting pumping to 6IJ states.

Multi-trench fiber (MTF) is a novel large mode area fiber design for high power fiber laser applications. This fiber design allows very high suppression of the higher order modes by offering high losses and delocalizing them out of the core. MTFs allow the core refractive index to be higher than the surrounding cladding as compared to other structures such as photonic crystal fibers, photonic bandgap fibers, and Bragg fibers. This feature of MTFs dramatically reduces the complexity associated with doped fiber fabrication. The MTF design is an all-solid structure with cylindrical symmetry, which provides easy cleaving and splicing with other fibers. In this paper, we present the first experimental demonstration of the MTF. S $^{2}$ measurements indicate single mode operation with very high suppression of the higher order modes.

We will review the influence of fiber fabrication and selection of pump wavelengths on gain, noise figure and laser efficiency of Bismuth (Bi)-doped fiber amplifiers and lasers operating in the wavelength region of 1150-1400nm.

Bismuth (Bi) doped fibers have shown promising potential for lasers and amplifiers in the 1150-1500 nm and 1600-1800nm wavelength region. Bi-doped aluminosilicate, phosphosilicate and germanosilicate fibers provide luminescence around 1150 nm, 1300 nm and 1450 nm, respectively. Recent results have demonstrated the possibility to extend the Bi luminescence window beyond 1600 nm using Bi-doped high (≥ 50 mol %) germanosilicate fibers. These spectral regions can serve a wide range of applications in medicine, astronomy, defense and to extend the optical fiber communication. However, Bi-doped fiber lasers and amplifiers are still far from their optimum performance owing to the unclear nature of the near-infrared emitting Bi active centers. In this paper, we review the current state of the art of Bi-doped silica fiber lasers (CW and pulsed) and amplifiers in different wavelength bands. Also, we present our work on the development of Bi-doped aluminosilicate and phosphosilicate fiber lasers and amplifiers in the 1180 nm and 1330 nm bands. These lasers and amplifiers find applications in generating visible light sources and to access the second telecommunication window. The fibers used here were fabricated by modified chemical vapor deposition-solution doping technique and characterized for their unsaturable loss. Moreover, we present the influence of pump wavelengths on the gain, noise figure and laser efficiency of these Bi-doped fiber amplifiers and lasers. We also discuss Bi-doped fibers for pulsed laser application and demonstrate a mode-locked Bi-doped fiber laser operating at 1340 nm.

A cladding-pumped multi-element fiber (MEF) comprising 4-Er/Yb-doped signal fibers and single multimode pump fiber has been used to demonstrate a wide band amplifier. The MEF amplifier has been characterized for different pump powers and signal fiber lengths. A maximum gain of 37dB and corresponding noise figure of 7.3dB has been obtained with an input signal of -23dBm in the C-band. The signal fibers have also been cascaded to provide >17dB gain over 80nm bandwidth covering wavelength range of 1536-1615nm.

We demonstrate a 60µm core diameter single-trench Yb free Er-La-Al doped fiber having 0.038 ultra-low-NA, using conventional MCVD process in conjunction with solution doping process. Numerical simulations ensure an effective single mode, the effective area varies from 1,820µm2 to 1,960µm2 for different thicknesses of trenches and resonant rings. This fiber has been fabricated with conventional fabrication process, which can dramatically reduce the fabrication cost, hence suitable for mass production. Moreover, all solid structure ensures easy cleaving and splicing. Experimental measurements demonstrate a robust effective single mode operation. Furthermore, this fiber in 4%-4% laser cavity shows a record efficiency of 46% with respect to absorbed power.

We present a suite of silica-based thulium-doped fiber amplifiers (TDFAs) operating across the 1650–2050nm waveband. High gain and low NF are achieved by combining several TDFA designs optimized for short, central and long wavelength operation.

A high-power Er,Yb double-clad ribbon fiber laser pumped by a 9-diode-bar pump module is reported. The laser yielded 102 W of continuous-wave output at 1566 nm for a launched pump power of 244 W, corresponding to a slope efficiency of ~ 44% with respect to launched pump power. Tunable operation was achieved using a simple external feedback cavity with a diffraction grating and the operating wavelength could be tuned from 1533 nm to 1567 nm. Temperature distribution in the ribbon fiber geometry and prospects of power scaling will be discussed.

This review summarizes the very recent work on development of Yb2O3 doped nano-crystalline YAG in a silica based preforms made by modified chemical vapour deposition (MCVD)-solution doping technique and under suitable thermal annealing of the preforms for next generation of optical fiber devices. There is a great need to engineer the composition as well as doping levels of different elements within the core glass during the preform making stages to generate phase-separated Yb2O3 doped yttria-alumino rich silica nano particles, with or without crystalline in nature, in the fibre. The discussion concentrates on making of Yb2O3 doped yttria-alumino rich YAG nano-crystals containing silica glass based perform, drawing of fibers containing phase-separated dielectric nano-particles along with material characterizations, study of spectroscopic properties, photo-darkening phenomena and lasing characteristics. Also, approaches that may be applied for the formation of yttria alumina rich phase-separated crystalline nano-particles and dielectric nano-particles within the preform and fiber core region respectively are discussed in this review.