nJ Pulse Research Articles

25 nJ, 634 ps and 1 MHz all-fiber passively mode-locked fiber laser based on a GaAs saturable absorber

We demonstrate an all-fiber passively mode-locked Tm-Ho co-doped fiber laser based on a GaAs saturable absorber (SA) fiber laser at 1875.2 nm. With a 10% output coupler, the fiber laser outputs 1.13 MHz, 634 ps and 25 nJ pulses at 1875.2 nm with a 3 dB-bandwidth of 18.4 nm. The Q-switched mode-locking (QML), the mode-locking at fundamental repetition rate (FRR) and the mode-locking at second harmonic of repetition rate (SHRR) are also observed with the increase of the pump power. The main reason of the SHRR is soliton pulse energy quantization effect.

172 fs, 24.3 kW peak power pulse generation from a Ho-doped fiber laser system.

We demonstrate a high-peak-power femtosecond fiber laser system based on single-mode holmium (Ho)-doped fibers. 833fs, 27.7MHz pulses at 2083.4nm generated in a passively mode-locked Ho fiber laser are amplified and compressed to near transform-limited 172fs, 7.2nJ pulses with 24.3kW peak power. We achieve this performance level by using the soliton effect and high-order soliton compression. To the best of our knowledge, this is the first demonstration of sub-200fs pulses, with peak power exceeding 10kW from a Ho-doped single-mode fiber laser system without using bulk optics compressors.

Q-switched thulium/holmium fiber laser with gallium selenide

A passively Q-switched thulium/holmium fiber laser with a gallium selenide (GaSe) saturable absorber (SA) is proposed and demonstrated for the first time. The GaSe based SA is prepared by mechanical exfoliation and inserted into the proposed laser cavity to generate Q-switched pulses. Stable Q-switching operation is achieved at 1986.0 nm, with output pulse repetition rates ranging from 22.9 kHz to 32.3 kHz over a pump power range of 112.0 mW to 235.0 mW. The generated Q-switched pulses have a maximum pulse energy of 120.3 nJ and minimum pulse width of 6.9 μs. The proposed thulium/holmium fiber laser with GaSe SA will be able to cater to multiple applications requiring pulsed laser outputs in the 2.0 μm region.

Inkjet-printing of graphene saturable absorbers for ~2 μm bulk and waveguide lasers

A technique for inkjet-printing of graphene saturable absorbers (SAs) for ~2-μm bulk and waveguide lasers is presented. Based on distillation-assisted solvent exchange to fabricate high-concentration graphene inks, this technique is capable of producing few-layer graphene films of arbitrary shape. Absorption saturation of graphene printed on glass is demonstrated at ~1.56 µm for picosecond and femtosecond pulses indicating a large fraction of the saturable losses. Inkjet-printed transmission-type graphene SAs are applied in passively Q-switched nanosecond thulium (Tm) microchip and planar waveguide lasers. The Tm microchip laser generates 136 ns / 1.2 µJ pulses at 1917 nm with a repetition rate of 0.37 MHz with a Q-switching conversion efficiency reaching 65%. The planar waveguide laser generates 98 ns / 21 nJ pulses at 1834 nm at a repetition rate in the MHz-range. The inkjet-printing technique is promising for production of patterned SAs for waveguide lasers.

Fiber-Based High-Energy Femtosecond Pulses Tunable From 920 to 1030 nm for Two-Photon Microscopy

We demonstrate a fiber-based femtosecond laser source, which can deliver 37 MHz, ~100 fs laser pulses tunable from 920 nm to 1030 nm with up to 10 nJ pulse energy. Pumped by an ultrafast Yb-doped fiber laser, a short piece of tapered photonic crystal fiber with lower dispersion was used to generate self-phase-modulation filtered for the tunable pulses. We believe that this new type of high-energy ultrafast fiber laser source can be an ideal solution for the two-photon fluorescence microscopy applications.

Femtosecond-laser-written Microstructured Waveguides in BK7 Glass

There is a deficiency of low-loss microstructured waveguides that can be fabricated with a single laser-pass to minimize stress build-up, which can enable enhanced functionality and higher compactness for integrated optical devices. We demonstrate, for the first time, a series of multi-ring claddings each with a pair of cores in BK7 glass. Each waveguide was fabricated using only a single laser-pass at 1 MHz pulse repetition rate, 5 mm/s translation speed, 250 fs pulse width, over a set of pulse energies. We obtained the lowest-reported propagation loss of 0.062 dB/cm, measured at 1155 nm wavelength from the waveguide written with 340 nJ pulse energy. The maximum observed numerical aperture is 0.020, measured at 1155 nm wavelength from the waveguide written with 620 nJ pulse energy. Such waveguides could be incorporated in integrated Raman laser platforms for biomedical applications.

Noise-like pulses generated from a passively mode-locked fiber laser with a WS2 saturable absorber on microfiber

We report on a noise-like pulse (NLP) generation in a mode-locked erbium-doped all-fiber laser using a WS2-deposited microfiber saturable absorber (SA). The prepared WS2 SA can serve as an excellent mode-locked and highly nonlinear device in the all-fiber laser cavity. At proper pump power and polarization states, typical NLPs can be generated. Adjusting the polarization also yields special NLPs with second-order harmonic mode-locking. For typical NLPs, 37 mW maximum output power and 8.8 nJ pulse energy are achieved at 334 mW pump power. The microfiber-based WS2 component could be a potential device for exploring nonlinear dynamics, such as NLPs.

Core-pumped mode-locked ytterbium-doped fiber laser operating around 980 nm

In this letter, we first demonstrate a core-pumped passively mode-locked all-normal-dispersion ytterbium-doped fiber oscillator based on nonlinear polarization evolution operating around 980 nm. The dissipative soliton fiber laser pulse can be compressed down to 250 fs with 1 nJ pulse energy, and the slope efficiency of the oscillator can be as high as 19%. To improve the dissipative soliton laser output spectrum smoothness, we replace the birefringent plate based intracavity filter with a diffraction-grating based filter. The output pulse duration can then be further compressed down to 180 fs with improved spectral-smoothness. These schemes have potential applications in seeding cryogenic Yb:YLF amplifiers and underwater exploration of marine resources.

Efficient Optical Gain from Near‐Infrared Polymer Lasers Based on Poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)

AbstractState‐of‐the‐art near‐infrared lasers based on poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)] (PCDTBT) and poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole) (F8BT) blends are reported. Polymer light‐emitting diodes (PLEDs) based on PCDTBT/F8BT blends with 30 wt% PCDTBT content exhibit a maximum radiance of 64.3 W sr−1 m−2 and external quantum efficiency of 2.11% with Commission Internationale de L'Eclairage (CIE) coordinates (x = 0.69, y = 0.30). Using an optimal blend ratio of 15 wt% PCDTBT in F8BT, a maximum gain value of 28.2 cm−1 at 710 nm is achieved, a remarkable value given that PCDTBT is a low emissive polymer extensively employed as donor polymer in organic photovoltaics. The lowest amplified spontaneous emission (ASE) and laser thresholds exhibited by the blends are 590 nJ pulse−1 (21 µJ cm−2) and 63.1 nJ pulse−1 (201 µJ cm−2). Transient absorption spectroscopy confirms efficient Förster resonant energy transfer from F8BT to PCDTBT which, together with the large miscibility of PCDTBT in F8BT, enables PCDTBT emission enhancement and optical gain. Furthermore, a ternary blend system composed of F8BT, PCDTBT, and poly(3‐hexylthiophene) is demonstrated, in which the ASE wavelength can be tuned in a 60 nm range from 650 to 710 nm at a very low threshold level via control of the blend content ratio.

Pre-chirp managed, core-pumped nonlinear PM fiber amplifier delivering sub-100-fs and high energy (10 nJ) pulses with low noise.

We demonstrate a pre-chirp managed amplification (PCMA) system that is based on two stages of core-pumped, polarization maintaining (PM) fiber amplifiers. It produces output pulses with <65 fs duration and >10 nJ pulse energy from single-mode fibers. Tailoring of the spectra in the amplification chain enables pulse compression to near-perfect transform limited pulses (Strehl-ratio >0.9) and low intensity noise levels (0.008%) despite B-integrals >40 rad in the PCMA amplifier. Design strategies are presented. We expect this PCMA system to become an easy to implement add-on to a variety of existing sources while maintaining the advantages of the robustness of the PM standard fiber format.

High-power Yb-based all-fiber laser delivering 300 fs pulses for high-speed ablation-cooled material removal

We report on a 72W Yb all-fiber ultrafast laser system with 1.6GHz intra-burst and 200kHz burst repetition rate developed to demonstrate ablation-cooled material removal at high speeds. Up to 24W is applied on Cu and Si samples with pulses of ∼300 fs, and record-high ablation efficiencies are obtained, compared to published results to date, despite using only ∼100 nJ pulses. Ablation speeds approaching 1 mm3/s are reported with 24W of average power, limited by available laser power and beam scanning speed. More significantly, these results experimentally confirm the theoretically expected linear scaling of the ablation-cooled regime to higher average powers without sacrificing efficiency, which implies that further scaling is possible with further increases in laser power and scanning speeds.

High energy passively Q-switched Er-doped fiber laser based on Mo_05W_05S_2 saturable absorber

A passively Q-switched Er-doped fiber laser based on a novel transition metal sulfide (TMDs) Mo0.5W0.5S2 absorber is reported for the first time. There are two different types of Mo0.5W0.5S2-based saturable absorbers (SAs) that achieve Q-switching operation, one of which is depositing the Mo0.5W0.5S2 on the surface of the tapered fiber, and the maximum single pulse energy of 172.91 nJ and minimum pulse width of 1.42 µs is obtained. While the other is transferring Mo0.5W0.5S2-PVA(polyvinyl alcohol: PVA) SA film onto a standard FC/APC fiber end face, the high single pulse energy of 246.5 nJ and the minimum pulse width is 1.92 μs is obtained, and the modulation depth of SAs detected by the twin-detector method is 15%. The experiment results show that Mo0.5W0.5S2 can be considered as a promising candidate for pulse fiber laser applications and other photoelectric devices.

Dynamic fiber delivery of 3 W 160 fs pulses with photonic crystal hollow core fiber patchcord.

We report output characteristics of a connectorized hollow core photonics crystal fiber when it is subjected to coiling down to a 50 mm radius, bending, and torsion. We achieved coupling efficiency up to 73% with an output average power of 2 W and 24 nJ pulse energy. With optimized coupling, depolarization was as low as 7%. Coiling and bending of the photonic crystal patchcord introduces little distortion; torsion, however, changes the polarization drastically. To our knowledge, this is the first report on dynamic fiber delivery of fs pulses.

175 fs-long pulses from a high-power single-mode Er-doped fiber laser at 1550 nm

Development of Er-doped ultrafast lasers have lagged behind the corresponding developments in Yb- and Tm-doped lasers, in particular, fiber lasers. Various applications benefit from operation at a central wavelength of 1.5 μm and its second harmonic, including emerging applications such as 3D processing of silicon and 3D printing based on two-photon polymerization. We report a simple, robust fiber master oscillator power amplifier operating at 1.55 μm, implementing chirp pulse amplification using single-mode fibers for diffraction-limited beam quality. The laser generates 80 nJ pulses at a repetition rate of 43 MHz, corresponding to an average power of 3.5 W, which can be compressed down to 175 fs. The generation of short pulses was achieved using a design which is guided by numerical simulations of pulse propagation and amplification and manages to overturn gain narrowing with self-phase modulation, without invoking excessive Raman scattering processes. The seed source for the two-stage amplifier is a dispersion-managed passively mode-locked oscillator, which generates a ∼40 nm-wide spectrum and 1.7-ps linearly chirped pulses.

Er-fiber laser enabled, energy scalable femtosecond source tunable from 1.3 to 1.7 µm.

We demonstrate that energetic femtosecond pulses tunable from 1.3 to 1.7 µm can be achieved using self-phase modulation enabled spectral broadening followed by spectral lobe filtering. Based on a home-built 5-W Er-fiber laser system operating at 31-MHz repetition rate, we obtain femtosecond pulses that can be continuously tuned from 1.3 to 1.7 µm with >4.5 nJ pulse energy. We further optimize the spectral broadening process using a fiber with larger mode area and scale up the pulse energy to >10 nJ; the resulting pulse duration is as short as ~50 fs. Such a widely tunable, energetic femtosecond source is well suited for driving a laser scanning microscope to perform deep tissue multiphoton microscopy.

Multiphoton imaging with blue-diode-pumped SESAM-modelocked Ti:sapphire oscillator generating 5 nJ 82 fs pulses.

We report a directly blue diode pumped Ti:Sapphire oscillator that generates 5 nJ pulses. This is five times higher pulse energy than previously reported for a directly diode pumped Ti:sapphire laser. With 460 mW of average power at 92 MHz and 82 fs pulses, its peak power reaches 61 kW, also several times higher the value than previously published. Direct diode pumping significantly reduces the complexity and therefore the footprint and the cost of the laser, while SESAM modelocking ensures reliable selfstarting and robust operation. Such a laser is ideally suited for biomedical imaging and nanostructuring applications. As a demonstration of sufficient peak power for microscopy applications, we perform different modalities of nonlinear microscopy of biological samples.

A High Performance Deep Blue Organic Laser Gain Material

High performance organic lasers with both good optical lasing property and desirable carrier mobility still remains a big challenge for realizing electrically pumped organic lasing. In this work, we have developed a carbazole‐end‐capped ladder‐type oligo‐(p‐phenylene) based hybrid oligomer as solution‐processable deep blue laser gain media to reach a good balance between optical property and carrier transporting ability, by introducing moderate steric hindrance into the pi‐conjugated planar oligo‐(p‐phenylene) backbone. This material demonstrates excellent laser performance and moderate hole mobility of 1 × 10–3 cm2 V−1 s−1. The 1‐D distributed feedback (DFB) laser exhibits very low lasing threshold of 0.6 nJ pulse−1 and high slope efficiency of 5.9 ± 0.3% (50% fill factor). More importantly, the prominent ASE performance maintained at very thin film samples. The amplified spontaneous emission (ASE) threshold remains very low (16 nJ pulse−1) in the 45 nm‐thickness film. These results might indicate possibilities for electrically pumped gain materials.

1.56 µm sub-microjoule femtosecond pulse delivery through low-loss microstructured revolver hollow-core fiber

We report for the first time, to the best of our knowledge, on ~1 MW peak power femtosecond pulse delivery through ≈10 m-long air-filled microstructured revolver hollow-core fiber (RHCF) in the telecom spectral band near 1.56 µm wavelength. We have developed a high-power all-fiber master oscillator power amplifier source based on the novel large-mode area erbium-doped double-clad fiber with 980 nm multi-mode diode pumping that emits up to 530 nJ pulses shorter than 400 fs with 1.42 W maximum average power. These pulses have been further launched into low-loss (<30 dB km−1) RHCF with eight non-touched cylindrical capillaries-based cladding and 61 µm core size with more than 80% efficiency. Owing to low dispersion and nonlinearity of the RHCF developed, the output pulse characteristics (spectral and temporal) are close to the input ones for low and moderate pulse energies. However, we have observed significant nonlinear spectral filtering together with pulse shortening (down to 353 fs) at the maximum output average power of 0.94 W. We believe that the system developed may be highly promising for high-precision material processing and other high-energy and high-power laser applications.

Polarization multiplexing in large-mode-area waveguides and its application to signal enhancement in multiphoton microscopy

Soliton self-frequency shift (SSFS) is an efficient technique of generating broadband tunable femtosecond optical pulses. By using large-mode-area (LMA) waveguides such as photonic-crystal (PC) rods or LMA fibers, SSFS is capable of generating solitons with tens of or even &amp;gt;100 nJ pulse energy, enabling deep-tissue multiphoton microscopy (MPM) with the unprecedented imaging depth. MPM signals are proportional to the repetition rate of the laser. Here, we demonstrate an efficient technique of enhancing MPM signals in LMA waveguides, through polarization multiplexing, in both a PC rod with no polarization-maintaining (PM) structure and a PM LMA fiber. The collinear output soliton pulses with orthogonal linear polarizations show similar pulse energy, pulse width, and spectrum. We also demonstrate the application of this polarization multiplexing technique to MPM signal enhancement in biological tissues. Compared with single-polarization soliton excitation, excitation with polarization-multiplexed solitons can efficiently boost MPM signals in different modalities of MPM.

Advances in Single-Crystal Fibers and Thin Rods Grown by Laser Heated Pedestal Growth

Single-crystal fibers are an intermediate between laser crystals and doped glass fibers. They have the advantages of both guiding laser light and matching the efficiencies found in bulk crystals, which is making them ideal candidates for high-power laser and fiber laser applications. This work focuses on the growth of a flexible fiber with a core of dopant (Er, Nd, Yb, etc.) and a polycrystalline clad of yttrium aluminum garnet (YAG) that will exhibit good wave guiding properties. Direct growth or a combination of growth and cladding experiments are described. Scattering loss measurements at visible wavelengths, along with dopant profile characterization with damage threshold results, are also presented. For single-pass amplification, a single-pass linear gain of 7.4 was obtained for 29 nJ pulses of 5 ns duration at 1 MHz repetition rate. We also obtained a laser efficiency of over 58% in a diode-pumped configuration. These results confirm the potential for single-crystal fibers to overcome the limitations of the glass fibers commonly used in fiber lasers, making them prime candidates for high-power compact fiber lasers and amplifiers.