Fiber Gain Medium Research Articles

Advancing Q-switched pulse generation in the 1.9 µm wavelength region using novel thulium-doped fiber gain medium and MXene Nb2CTx saturable absorber

This paper presents the first demonstration of a Q-switched Thulium-doped fiber laser (TDFL) utilizing our newly developed Thulium-doped fiber (TDF) and a MXene niobium carbide - polyethylene oxide (Nb2CTx-PEO) thin film as a gain medium and saturable absorber (SA), respectively. The TDF core glass contains 6.15 wt% Al2O3, 2.2 wt% Y2O3, 0.47 wt% Tm2O3, 2.12 wt% GeO2, 1.57 wt% HfO2 and 0.05 wt% Bi2O3 with the absorption of 30 dB/m at 1550 nm. The outcome was the realization of a stable pulsed laser operating at a central wavelength of 1888.3 nm. The Q-switched pulses exhibited a maximum repetition rate of 46.7 kHz and a minimum pulse width of 1.995 µs at a pump power of 731.41 mW. The Q-switched operation maintained a signal-to-noise ratio (SNR) of 45 dB, confirming its stability. This breakthrough highlights the laser system's potential for diverse applications, including free-space optical communication, spectroscopy, and medical fields.

Graphene-polyvinyl alcohol polymer based saturable absorption at 2000 nm region

A <span>graphene-polyvinyl alcohol (PVA) composite saturable absorption is demonstrated at 2000 nm region. Graphene suspension is produced using low-cost electrochemical exfoliation process. The suspension is mixed with PVA host polymer in 1:1 ratio and left evaporated at room temperature which finally produced graphene-PVA thin film. Thulium doped fiber (TDF) gain medium has been shown to produce a stable Q-switched pulse with a highest repetition rate of 54 kHz, a short pulse duration of 2.89 µs, a maximum peak power of 16 mW, and an estimated maximum pulse energy of 49 nJ. Apparently, at 2000 nm region, superior performances of graphene-PVA composite have been recorded which was largely contributed by meticulous composite preparation and homogenous mixture with PVA host</span>.

Light Management for Enhancing Optical Gain in a Solar‐Pumped Fiber Laser Employing a Solid‐State Luminescent Solar Concentrator

Optical gain of 8.3 km−1 is obtained in a neodymium‐doped silica fiber under simulated sunlight (intensity = 2.1 kW m−2). The fiber gain medium (length = 50 m) is coiled and attached to a solid‐state luminescent solar concentrator (LSC) disk (diameter = 180 mm). The disk/fiber unit is placed in a cavity comprising highly reflective (HRM) and dichroic (DM) mirrors to increase the photon confinement by a factor of 2.2 compared with a liquid LSC (also employing HRM/DM). The enhancement is mainly due to the air gap between HRM/DM and LSC, which affords total internal reflection (TIR) in the solid‐state LSC and an ideal boundary condition for the multilayer mirrors. The numerical calculations indicate that an additional 2.2‐times enhancement is achievable with a side wall exhibiting 90% diffuse reflection. This has been experimentally confirmed (1.9‐times optical‐gain enhancement is realized by adding a polytetrafluoroethylene side‐wall reflector). Overall, the solid‐state LSC‐based solar‐pumped laser (SPL) comprising reflective side walls exhibits a gain factor of 4.2, which is superior to its liquid‐state predecessor. Further improvements employing a solid‐state LSC design and fiber optimization will facilitate the market penetration of SPLs.

Comparison of two low-noise CEO frequency stabilization methods for an all-PM Yb:fiber NALM oscillator

We present a comparison of two low-noise carrier-envelope offset (CEO) frequency stabilization methods studied using an ytterbium (Yb) fiber laser oscillator based on a nonlinear amplifying loop mirror. We first investigate the phase locking performance achieved with cross-gain modulation (XGM) via injection of an auxiliary low-power continuous-wave (CW) laser into the fiber gain medium. Amplification of the injected CW laser light cross-modulates the gain of the oscillator, resulting in an intra-cavity power modulation, thus providing control of the CEO frequency. The XGM method is then compared with the conventional pump-current modulation scheme. Both stabilization methods provide similar locking performances with sub-200-mrad of integrated residual carrier-envelope-phase (CEP) noise (10 Hz to 1 MHz), suitable for high-resolution comb spectroscopy applications.

Mid-infrared luminescence of dysprosium-doped gallium-antimony-sulfur-iodine chalcohalide glasses and fibers

Dy3+ doped Ga-Sb-S-I chalcohalide glasses were prepared by melt-quenching method. The thermal stability and mid-infrared (MIR) luminescence of the glass samples were investigated systematically. Dy3+ dopants in Ga-Sb-S-I glasses exhibited intense 2.92-μm emission and 4.41-μm emission. High quantum efficiency and large stimulated emission cross section were observed for these two emission bands. Finally, Dy3+ doped Ga-Sb-S-I glass fibers were fabricated, using a core composition of 0.05 wt.% Dy3+: Ga8Sb32S55I5 and a cladding composition of Ga8Sb31S56I5. MIR attenuation and luminescence of the fibers were characterized. The fiber shows the minimum loss of 1.9 dB/m at 6 µm. Strong 3.0-µm and 4.4-µm emission bands were seen. It indicates that Dy3+ doped Ga-Sb-S-I glass fiber is promising for mid-infrared fiber gain medium usage.

Passively Q-switched S+/S band fiber laser with copper telluride saturable absorber

A Q-switched S-band fiber laser based on a thulium-fluoride fiber gain medium and copper telluride (Cu2Te) based saturable absorber (SA) is demonstrated. The Cu2Te based SA is fabricated as a thin film and at the maximum pump power, the generated Q-switched output has a minimum pulse width of 3.2 µs and highest pulse energy of 56.7 nJ. The pulse train varies from 20.9 kHz to 30.2 kHz at the highest pump power. The wavelength of the generated output can be tuned over a range of 32 nm from 1470 nm to 1502 nm. The demonstrated laser has a high potential for industrial applications focusing on the S-band region.

1.8 µm passively Q-switched thulium-doped fiber laser

This work demonstrates a simple and reliable passively Q-switched thulium-doped fiber laser capable of generating an output with a peak wavelength at 1.8 µm. Lasing is realized from the shorter wavelength region of a thulium-doped silica fiber gain medium by suppressing lasing at longer wavelengths in combination with a tunable bandpass filter. The laser is modulated by a single-walled carbon nanotube film saturable absorber. The generated Q-switched laser is capable of achieving a high repetition rate of 67.9 kHz and a narrow pulse width of 1.2 µs. As most work on the 1.6–1.8 µm waveband focuses on continuous-wave and mode-locked lasers, to the best of the author’s knowledge, this is the first demonstration of a passively Q-switched thulium-doped fiber laser operating at 1.8 µm with an all-silica fiber setup. The demonstrated laser has potential for various applications including biomedical surgery and polymer-based welding applications.

All-fiber passively Q-switched erbium fiber laser implementing erbium-ytterbium-thulium co-doped saturable absorber fiber

We demonstrate a passive Q-switched fiber laser using an erbium-ytterbium-thulium (Er-Yb-Tm) doped fiber. The length of the Er-Yb-Tm co-doped fiber is varied from 0.4 m to 1.4 m to study the effect of this passive gain medium for pulse laser performance. The proposed laser operates in the pulse operation regime within a certain range of pump powers, which is dependent on the length of the Er-Yb-Tm co-doped fiber. A longer fiber employed in the laser cavity results in a higher laser pump power threshold, but a wider pump power range of pulse laser operation. The experimental outcome shows that a 1.0 m Er-Yb-Tm co-doped fiber offers the best performance in this work with a Q-switched pump power threshold of 45.1 mW, the widest pulse energy output range of 0.0439 μJ from 0.0167 μJ to 0.0606 μJ, with the supplementary advantage of 120.7 mW Q-switched laser operating range. This fiber gain medium analysis contributes to the generation of short pulse laser source without employing any chemical materials and complex laser setup.

A narrow linewidth tunable single longitudinal mode Ga-EDF fiber laser

A tunable ring cavity single longitudinal mode (SLM) fiber laser incorporating Gallium-Erbium co-doped fiber (Ga-EDF) gain medium and several mode filtration techniques is demonstrated. With Ga-EDF, high emission power was accorded in short fiber length, allowing shorter overall cavity length and wider free spectral range. Tunable bandpass filter, sub-ring structure, and cascaded dissimilar fiber taper were utilized to filter multi-longitudinal modes. Each of the filter mechanism was tested individually within the laser cavity to assess its performance. Once the performance of each filter was obtained, all of them were deployed into the laser system. Ultimately, the 1561.47 nm SLM laser achieved a narrow linewidth laser, optical signal-to-noise ratio, and power fluctuation of 1.19 kHz, 61.52 dB and 0.16 dB, respectively. This work validates the feasibility of Ga-EDF to attain a stable SLM output in simple laser configuration.

The generation of Q-switched erbium-doped fiber laser using black phosphorus saturable absorber with 8% modulation depth

We report a generation of the Q-switched laser operating in 1.55-micron region by using black phosphorus (BP) as a saturable absorber (SA). A 980-nm laser diode was pumped into Erbium-doped fiber (EDF) gain medium in ring cavity configuration. The BP-based SA was prepared by mechanically exfoliating the BP crystal using scotch tape. The obtained BP-tape SA has a modulation depth of 8 %. To realize a Q-switching operation, a small piece of the tape is then integrated into between two fiber ferrule tips. A stable Q-switching operation started at 40 mW. The maximum repetition rate obtainable at 28.57 kHz, with pulse width of 5.35 μs. This finding shows the BP is one of the promising material to work as an SA for pulsed laser generation.

All-fiber dual-wavelength Q-switched and mode-locked EDFL by SMF-THDF-SMF structure as a saturable absorber

We demonstrate all-fiber dual-wavelength Q-switched and mode-locked erbium-doped fiber laser (EDFL) by utilizing the thulium-holmium-doped fiber (THDF) as a fiber saturable absorber (SA) and also a Mach-Zehnder interferometer (MZI) element. The 19cm long THDF has a core diameter of 11.5µm, refractive index difference of 0.005, and cutoff wavelength of 1810nm. Stable dual-wavelength Q-switching operation was generated at 1555.14nm and 1557.64nm with free spectral range (FSR) of 2.5nm. The repetition rate of 14.45–78.49kHz was obtained between 12 and 100mW pump power. At maximum pump power, the maximum output power and pulse energy were 2.58mW and 32.87 nJ, respectively. By adding 195cm long SMF in the same cavity, the stable dual-wavelength mode-locking operation was started at 166mW and continue stable to 201mW pump power. This mode-locking operation produced stable dual-wavelength pulses at 1530.34nm and 1532.84nm with a repetition rate of 1MHz with a pulse duration of 128ns and signal-to-noise ratio (SNR) of 62dB. It shares the same value of FSR in Q-switching operation. The highest output power of 1.57 nJ corresponds to the maximum output power of 1.57mW was obtained. Our results validate the linear absorption characteristic at C-band region and multimode fiber effect of THDF can be utilized as SA to generate stable all-fiber dual-wavelength pulsed lasers. Remarkably, these findings expand a fiber gain medium application in short pulse generation.

Femtosecond-Pulse Fiber Based Amplification Techniques and Their Applications

Three kinds of pulse amplification techniques including chirped-pulse amplification (CPA), parabolic pulse amplification, and divided-pulse amplification (DPA) based on rare-earth doped fiber gain medium are reviewed. Large core area fiber and cladding-pumping technique compose the foundation of high average power, high pulse energy fiber CPA technology. Femtosecond pulses have been demonstrated at average powers of up to 830 W, pulse energies of up to 2.2 mJ and compressed pulse duration of 23 fs from different fiber CPA systems. The state-of-the-art performance of fiber CPA system to date is the simultaneous generation of 530 W, 1.3 mJ compressed femtosecond pulses at 400 kHz repetition rate. The balance between fiber gain, dispersion and nonlinearity is required for the self-similar propagation and thus the formation of pulses with parabolic intensity profile. Sub-33 fs pulses were demonstrated from a gain-cascaded parabolic amplification system. A newly emerging DPA technique is also reviewed. In addition, the applications of pulsed fiber amplifiers in the areas of supercontinuum generation and material micromachining system are also introduced.

Direct observation of Kramers–Kronig self-phasing in coherently combined fiber lasers

A highly stable coherent beam-combining system has been designed to measure self-phasing in fiber lasers due to nonlinear effects. Whereas self-phasing in previous coherent combination experiments has been principally attributed to wavelength shifting, these wavelength effects have been efficiently suppressed in our experiment by using a dual-core fiber with closely balanced optical path lengths. The self-phasing from nonlinear effects could then be measured independently and directly by common-path interferometry with a probe laser. The Kramers-Kronig effect in the fiber gain media was observed to induce a phase shift that effectively canceled the applied path length errors, resulting in efficient lasing under all phase conditions. This process was demonstrated to result in robust lasing over a large range of pump conditions.

Gain smoothening filter in two-segment fiber-optical parametric amplifier

This paper demonstrates a method to remove the gain ripple profile specifically for a two-segment fiber optical parametric amplifier arising from the dispersion differences between the fiber gain medium and the standard single mode fiber attached to the optical components. This is achieved by using a simple design that incorporates a gain smoothening filter at the mid-stage of the amplifier. This simple yet practical method is useful when isolators are adopted for stimulated Brillouin scattering suppression but it comes with the expense of parametric gain.

High power fiber lasers: current status and future perspectives [Invited

The rise in output power from rare-earth-doped fiber sources over the past decade, via the use of cladding-pumped fiber architectures, has been dramatic, leading to a range of fiber-based devices with outstanding performance in terms of output power, beam quality, overall efficiency, and flexibility with regard to operating wavelength and radiation format. This success in the high-power arena is largely due to the fiber’s geometry, which provides considerable resilience to the effects of heat generation in the core, and facilitates efficient conversion from relatively low-brightness diode pump radiation to high-brightness laser output. In this paper we review the current state of the art in terms of continuous-wave and pulsed performance of ytterbium-doped fiber lasers, the current fiber gain medium of choice, and by far the most developed in terms of high-power performance. We then review the current status and challenges of extending the technology to other rare-earth dopants and associated wavelengths of operation. Throughout we identify the key factors currently limiting fiber laser performance in different operating regimes—in particular thermal management, optical nonlinearity, and damage. Finally, we speculate as to the likely developments in pump laser technology, fiber design and fabrication, architectural approaches, and functionality that lie ahead in the coming decade and the implications they have on fiber laser performance and industrial/scientific adoption.

Tunable dual wavelength fiber laser incorporating AWG and optical channel selector by controlling the cavity loss

In this paper, we propose and demonstrated a dual wavelength fiber laser (DWFL) based on the use of an erbium doped fiber (EDF) gain medium as well as an 1 × 24 Arrayed Waveguide Grating (AWG) together with two optical channel selectors (OCS) to provide channel spacing tunability. The output power of the two wavelengths is equalized by controlling the cavity loss in the DWFL using two Programmable Optical Attenuators (POAs). The widest spacing obtained from the DWFL is 18.13 nm while the narrowest spacing is 0.8 nm. The DWFL has good stability with only minor power fluctuations of less than 1.5 dB and a Side Mode Suppression Ratio (SMSR) of approximately 69.1 dB with peak fluctuations of less than 2.3 dB.

Multiwavelength fiber ring laser based on a semiconductor and fiber gain medium

We report a novel multi-wavelength laser source based on hybrid gain medium, with a semiconductor optical amplifier and erbium doped fiber amplifier, and a double-ring structure. More than 60 lines with more than 50 dB signal-to-noise ratio (SNR) have been achieved in our setup. The wavelength interval between the wavelengths is approximately 0.32 nm. The 3 dB spectral width of each lasing line is approximately 0.019 nm. The laser can be tuned from approximately 1526 nm to approximately 1562 nm by adjusting the loss in the cavity.

High-power rod-type photonic crystal fiber laser

We report on a novel ytterbium-doped fiber design that combines the advantages of rod and fiber gain media. The fiber design has outer dimensions of a rod laser, meaning a diameter in the range of a few millimeters and a length of just a few tens of centimeters, and includes two important waveguide structures, one for pump radiation and one for laser radiation. We obtained 120-W output power in single-mode beam quality from a 48-cm-long fiber cane that corresponds to an extracted power of 250 W/m. The fiber has significantly reduced nonlinearity, which therefore allows for scalability in the performance of a high-peak-power fiber laser and amplifier system.

Demonstration of a rapidly strain tuned Er^3+-doped fiber laser for sensitive gas detection

We describe a novel combination of a diode-pumped, wavelength-modulated Er(3+)-doped fiber laser light source with a sensitive noise cancellation circuit for detection of acetylene and ammonia. The laser tuning element, a fiber Bragg grating, was mounted in such a way that it could be strained controllably and rapidly, allowing noise cancellation techniques to be applied to the wavelength-modulated output of the fiber laser. The experimental setup is relatively simple and can be extended to other fiber laser wavelengths for which semiconductor lasers are not readily available by selection of a different fiber Bragg grating and gain medium.

Transient dynamics in a self-starting passively mode-locked fiber-based soliton laser

We describe measurements of spectrally resolved turn-on and turn-off dynamics in a self-starting passively mode-locked fiber soliton laser. An intracavity quantum well optical amplifier whose bias is modulated changes the cavity losses, thereby switching the laser between continuous wave and mode-locked states. We show that during the transients, the laser undergoes a period of self Q switching lasting a few hundreds of microseconds (as determined by the relaxation oscillation frequency in the fiber gain medium) before it reaches steady state.