Mode-locked Laser Operation Research Articles

Single-channel 5.12 Tbit/s (1.28 Tbaud) DQPSK transmission over 300 km using non-coherent Nyquist pulses.

We present a single-channel 5.12 Tbit/s polarization-multiplexed DQPSK transmission over 300 km at 1.28 Tbaud using a non-coherent Nyquist pulse. An ultrafast OTDM demultiplexer for 1.28 Tbaud Nyquist pulses was newly developed with a mode-locked fiber laser operating in the L band as a control pulse source. Thanks to the high PMD tolerance of Nyquist pulses, a 300 km transmission was successfully demonstrated for the first time at such a high symbol rate.

Study of a harmonic mode lock stability under external continuous-wave injection

In this paper, we investigate experimentally the effect of an injected continuous external optical laser in a stable passive harmonic mode-locked fiber laser operating in the anomalous dispersion regime. Under specific conditions, the continuous-wave significantly increases the stability of the harmonic mode-locked regime. This occurs for a discrete set of wavelengths and below a critical injected power.

Molybdenum Disulphide Tape Saturable Absorber for Mode-Locked Double-Clad Ytterbium-Doped All-Fiber Laser Generation**Supported by the PPP Grant Scheme of University of Malaya under Grant No PG098-2014B.

We demonstrate the generation of passive mode-locked double-clad ytterbium-doped fiber laser operating in a 1-micron region. We prepare the saturable absorber from commercial crystal of molybdenum disulphide (MoS2). Without chemical procedure, the MoS2 is mechanically exfoliated by using a clear scotch tape. A few layers of MoS2 flakes are obtained on the tape. Then, a piece of 1 × 1 mm tape containing MoS2 thin flakes is inserted between two fiber ferrules and is integrated in the ring cavity. Stable mode-locking operation is attained at 1090 nm with a repetition rate of 13.2 MHz. Our mode-locked laser has a maximum output power of 20 mW with 1.48 nJ pulse energy. These results validate that the MoS2 has a broad operating wavelength which covers the 1-micron region, and it is also able to work in a high-power cavity.

Crystal growth, spectroscopic characterization, and sub-100 femtosecond mode-locked operation of a Yb:LiLuF_4 laser

We present a comprehensive growth, spectroscopic, and laser performance investigation of a Yb:LiLuF4 crystal, including CW laser experiments, and, for the first time to the best of our knowledge, mode-locked laser operation. Pumping with two 400 mW, polarization-combined, single-mode fiber-coupled laser diodes, we demonstrated efficient CW emission with 140 mW maximum output power and up to 59% slope efficiency. Using a semiconductor saturable absorber mirror for inducing the mode-locked regime and a pair of fused silica prisms for intracavity group delay dispersion compensation, we obtained soliton mode-locking pulses with 90 fs duration for the emission with polarization parallel to the crystal c-axis and 113 fs for the orthogonal polarization, with average power exceeding 30 mW in both cases at about a 90 MHz repetition rate.

Statistical characterization of the internal structure of noiselike pulses using a nonlinear optical loop mirror

In this work we study statistically the internal structure of noiselike pulses generated by a passively mode-locked fiber laser. For this purpose, we use a technique that allows estimating the distribution of the amplitudes of the sub-pulses in the bunch. The technique takes advantage of the fast response of the optical Kerr effect in a fiber nonlinear optical loop mirror (NOLM). It requires the measurement of the energy transfer characteristic of the pulses through the NOLM, and the numerical resolution of a system of nonlinear algebraic equations. The results yield a strongly asymmetric distribution, with a high-amplitude tail that is compatible with the existence of extreme-intensity sub-pulses in the bunch. Following the recent discovery of pulse-energy rogue waves and spectral rogue waves in the noiselike pulse regime, we propose a new way to look for extreme events in this particular mode of operation of mode-locked fiber lasers, and confirm that rogue wave generation is a key ingredient in the complex dynamics of these unconventional pulses.

Broadband wavelength tunable mode-locked thulium-doped fiber laser operating in the 2 μm region by using a graphene saturable absorber on microfiber

A broadband wavelength tunable mode-locked Tm3+-doped fiber laser operating in the 2 μm region based on a graphene saturable absorber is experimentally investigated. A section of graphene film is transferred on a microfiber, which allows light-graphene interaction via evanescent field. The microfiber based graphene not only acts as an excellent saturable absorber for mode-locking, but also induces a polarizing effect to form an artificial birefringent filter for wavelength selection. By tuning the polarization states in the laser cavity, the laser exhibits tunable wavelength mode-locked pulses over a wide range from 1880 to 1940 nm. Such a system provides a compact, user friendly and low cost wavelength tunable ultrashort pulse source in the 2 μm region.

Generation of high energy square-wave pulses in all anomalous dispersion Er:Yb passive mode locked fiber ring laser.

We have experimentally demonstrated square pulses emission from a co-doped Er:Yb double-clad fiber laser operating in anomalous dispersion DSR regime using the nonlinear polarization evolution technique. Stable mode-locked pulses have a repetition rate of 373 kHz with 2.27 µJ energy per pulse under a pumping power of 30 W in cavity. With the increase of pump power, both the duration and the energy of the output square pulses broaden. The experimental results demonstrate that the passively mode-locked fiber laser operating in the anomalous regime can also realize a high-energy pulse, which is different from the conventional low-energy soliton pulse.

Generation of Q-Switched Mode-Locked Erbium-Doped Fiber Laser Operating in Dark Regime**Supported by the Fund from University of Malaya under Grant No RU007/2015, LRGS(2015)/NGOD/UM/KPT, and MOSTI under Grant No SF014-2014.

We demonstrate a stable Q-switched mode-locked erbium-doped fiber laser (EDFL) operating in dark regime based on the nonlinear polarization rotation technique. The EDFL produces a pulse train where the Q-switching envelope is formed by multiple dark pulses. The repetition rate of the Q-switched envelope can be increased from 0.96 kHz to 3.26 kHz, whereas the pulse width reduces from 211 μs to 86 μs. The highest pulse of 479 nJ is obtained at the pump power of 55 mW. It is also observed that the dark pulses inside the Q-switching envelope consist of two parts: square and trailing dark pulses. The shortest pulse width of the dark square pulse is obtained at 40.5 μs when the pump power is fixed at 145 mW. The repetition rate of trailing dark pulses can be increased from 27.62 kHz to 50 kHz as the pump power increases from 55 mW to 145 mW.

Real-Time Intensity Domain Characterization of Fibre Lasers Using Spatio-Temporal Dynamics

Fibre lasers are light sources that are synonymous with stability. They can give rise to highly coherent continuous-wave radiation, or a stable train of mode locked pulses with well-defined characteristics. However, they can also exhibit an exceedingly diverse range of nonlinear operational regimes spanning a multi-dimensional parameter space. The complex nature of the dynamics poses significant challenges in the theoretical and experimental studies of such systems. Here, we demonstrate how the real-time experimental methodology of spatio-temporal dynamics can be used to unambiguously identify and discern between such highly complex lasing regimes. This two-dimensional representation of laser intensity allows the identification and tracking of individual features embedded in the radiation as they make round-trip circulations inside the cavity. The salient features of this methodology are highlighted by its application to the case of Raman fibre lasers and a partially mode locked ring fibre laser operating in the normal dispersion regime.

Er3+-doped nano-engineered yttria stabilized zirconia–alumino silicate fiber for efficient CW and mode-locked laser operation

We report on the development of efficient continuous wave (CW) and mode-locked ring fiber lasers utilizing an $\text{Er}^{3+}$ - doped nanoengineered yttria-stabilized zirconia alumino (YSZA)-silicate fiber as the gain element. With the proper design of the material composition, the YSZA fiber host presents superior features to eliminate the $\text{Er}^{3+}$ cluster effects and enhance the radiative emission over commercial silica fibers, thus significantly increasing the fiber laser efficiencies. Through cavity analysis and optimization, we successfully demonstrate a 975-nm pumped single-mode $\text{Er}^{3+}$ - doped CW YSZA-silicate fiber laser with a slope efficiency of 43%, which is corresponding to the quantum efficiency of 69%. Because of the special dispersion property of the $\text{Er}^{3+}$ - doped YSZA-silicate fiber, we are able to build a stable soliton mode-locked 1565-nm fiber laser with 3.2-nJ pulse energy, 644-fs pulsewidth, and 4.96-kW peak power.

Femtosecond mode-locked Nd(3+)-doped Ba(Zr,Mg,Ta)O(3) ceramic laser.

We have demonstrated continuous wave (CW) laser operation and the first, to the best of our knowledge, sub-200 fs mode-locked laser operation of Nd(3+)-doped Ba(Zr,Mg,Ta)O(3) ceramic. Its disordered crystalline nature exhibits a broad gain bandwidth of 30 nm with a high-emission cross section. It also has higher thermal and mechanical properties than Nd:glass. In CW operation, a maximum output power of 1.5 W under 6.2 W of absorbed pump power was obtained. In mode-locked operation, a pulse duration of 196 fs with an average power of 60 mW was successfully achieved. The laser spectrum straddled both fluorescence peaks of A-site and B-site Nd(3+) ions.

Graphene Saturable Absorber Based on Slightly Tapered Fiber With Inner Air-Cavity

We experimentally demonstrate a passively mode-locked fiber laser operation by using a saturable absorber based on graphene deposited on slightly tapered optical fiber with inner air-cavity. Such a saturable absorber enables a strong light–graphene interaction owing to the evanescent field of the excited cladding mode, and as a result, stable passively mode-locked laser pulses with pulsewidth of 674 fs have been generated, and in addition, a harmonic mode-locking operation can also be achieved.

Mechanically exfoliated black phosphorus as a new saturable absorber for both Q-switching and Mode-locking laser operation.

Black phosphorus (BP), an emerging narrow direct band-gap two-dimensional (2D) layered material that can fill the gap between the semi-metallic graphene and the wide-bandgap transition metal dichalcogenides (TMDs), had been experimentally found to exhibit the saturation of optical absorption if under strong light illumination. By taking advantage of this saturable absorption property, we could fabricate a new type of optical saturable absorber (SA) based on mechanically exfoliated BPs, and further demonstrate the applications for ultra-fast laser photonics. Based on the balanced synchronous twin-detector measurement method, we have characterized the saturable absorption property of the fabricated BP-SAs at the telecommunication band. By incorporating the BP-based SAs device into the all-fiber Erbium-doped fiber laser cavities, we are able to obtain either the passive Q-switching (with maximum pulse energy of 94.3 nJ) or the passive mode-locking operation (with pulse duration down to 946 fs). Our results show that BP could also be developed as an effective SA for pulsed fiber or solid-state lasers.

Mode-locked laser operation of Indium-modified Yb:KY(WO₄)₂ single crystal.

We report the first pulsed laser operation of an Indium-modified Yb:KY(WO4)2 crystal. Indium incorporation enlarges the broadening of the Yb3+ optical bands, reduces crystal lattice parameters and increases n(p) refractive index. A KY0.8In0.07Yb0.13(WO4)2 crystal pumped at 981 nm with a Ti-sapphire laser in a SESAM modulated resonator produces at 300 K self-starting and stable mode-locking. The shortest laser pulses achieved were centred at λ = 1041.1 nm, have a duration of 96 fs with average power of 134 mW and repetition rate of 103.5 MHz (1.3 nJ/pulse).

Long cavity and low repetition rate passively mode-locked fiber laser with high-energy right angle trapezoid shaped soliton in anomalous dispersion regime

A long cavity passively mode locked fiber laser in the anomalous dispersion regime is reported. Nonlinear polarization rotation technique is employed to achieve the mode locking in our experiments. The output pulse from the fiber laser has the Gaussian profile spectrum and right angle trapezoid shape. Stable mode locking is achieved without using any dispersion-compensation components. The single pulse with an energy of 652 nJ and a repetition rate of 836 kHz at the pump power of approximately 500 mW is obtained and the duration of the output pulse increases linearly with the pump power. Different from the conventional low-energy soliton pulse, experimental results demonstrate that the passively mode locked fiber laser operating in the anomalous regime can also realize high energy pulse.

High-Energy Square Pulses in a Mode-Locked Yb-Doped Fiber Laser Operating in DSR Region

We report the experimental observation of high-energy square pulses in all-normal-dispersion passively mode-locked Yb-doped fiber laser under dissipative soliton resonance (DSR) condition using a nonlinear polarization rotation technique. The generated square pulses are mode locked with fundamental cavity repetition rate of 1.86 MHz. The maximum output average power and the corresponding pulse energy are 62.1 mW and 34 nJ, respectively. The output square pulses are wave-breaking-free and can be tuned in pulsewidth from hundreds of picoseconds to several nanoseconds by changing the pump power. The multistability of DSR in all-normal-dispersion cavity has been first demonstrated in experiment, which is in agreement with theoretical prediction. The observed experimental results will enhance the understanding of fundamental physics of DSR phenomenon.

Frequency-agile dual-frequency lidar for integrated coherent radar-lidar architectures.

We propose a novel architecture for implementing a dual-frequency lidar (DFL) exploiting differential Doppler shift measurement. The two frequency tones, needed for target velocity measurements, are selected from the spectrum of a mode-locked laser operating in the C-band. The tones' separation is easily controlled by using a programmable wavelength selective switch, thus allowing for a dynamic trade-off among robustness to atmospheric turbulence and sensitivity. Speed measurements for different tone separations equal to 10, 40, 80, and 160GHz are demonstrated, proving the system's capability of working in different configurations. Thanks to the acquisition system based on an analog-to-digital converter and digital-signal processing, real-time velocity measurements are demonstrated. The MLL-based proposed architecture enables the integration of the DFL with a photonic-based radar that exploits the same laser for generating and receiving radio-frequency signal with high performance, thus allowing for simultaneous or complementary target observations by exploiting the advantages of both radar and lidar.

Soliton dynamic patterns of a passively mode-locked fiber laser operating in a 2 μm region

We experimentally report on the soliton dynamics in a passively mode-locked thulium-doped fiber laser based on the nonlinear amplifying loop mirror with all-abnormal dispersion. In fundamental mode-locked regime, stable soliton mode-locked pulses, with central wavelength of 1965.67 nm and repetition rate of 1.72 MHz, have been observed. In addition, by changing the pump power and appropriately rotating the intra-cavity polarization controllers, several distinctive multi-soliton operations and interactions have been observed, such as different types of soliton rain, the release of solitons, the conversion between soliton rain and soliton release, and the emission of quasi-rectangular pulses.

Passively mode-locked fiber lasers at 1039 and 1560 nm based on a common gold nanorod saturable absorber

We demonstrated passively mode-locked ytterbium and erbium doped fiber lasers operating at 1039 and 1560 nm by using a common gold nanorods (GNRs) saturable absorber (SA). The GNRs with designed aspect ratios were mixed with sodium carboxymethylcelluose to form the GNRs SA film. The film had broadband longitudinal SPR (surface plasmon resonance) absorption from 800 to 1800 nm. By inserting the same film into a ytterbium or erbium doped fiber laser cavity pumped by a 980 nm laser diode, stable passively mode-locked laser operation at 1039 or 1560 nm was achieved for a threshold pump power of ~100 or ~70 mW, respectively. The pulse width, the output power, and the repetition rate of the 1039 nm mode-locked laser were 460 ps, 1.47 mW, and 43.5 MHz for a pump power of ~110 mW, respectively. The corresponding output parameters of the 1560 nm mode-locked laser were 2.91 ps, 2 mW, and 35.6 MHz for a pump power of ~74 mW, respectively. Our results showed that one GNRs SA could be used for constructing broadband mode-locked lasers.

Picosecond pulse generation from a Yb:CaF2–YF3 mode-locked laser

We experimentally investigated a picosecond (ps) diode-pumped passively mode-locked (ML) laser operation of a 5 at.% Yb, 5 at.% Y:CaF2 crystal obtained by the vertical Bridgman method. Without extra negative dispersion elements, the ML laser emitted stable 1.3 ps pulses at a repetition rate of 60 MHz with 514 mW average output power under the absorbed pump power of 6.43 W. The continuous wave (CW) operation was also reported with the average output power of 1.48 W, corresponding to the optical slope efficiency of 31.3%.