Quantum-dot Saturable Absorber Research Articles

Spatiotemporal mode-locking of an all-fiber laser based on InP quantum dot saturable absorber

Passive mode-locking based on saturable absorbers (SAs) is an effective way to generate ultrafast pulses, while the spatiotemporal mode-locking (STML) based on SAs are rarely studied. We construct an all-fiber laser with InP quantum dots (QDs) SA, and realize multi-mode Q-switching (MMQS) and STML. In addition, by adjusting the polarization controller (PC) in the laser cavity, we obtain two different single-pulse STML states. The narrowest pulse widths of the two states are 57 ps and 32 ps, respectively, and the pulse width can be tuned continuously with increasing the pump power. By decoupling the effects of the PC and the SA in our experiment, we find that the polarization plays a key role in the selection of transverse modes. The effect of independent polarization on spatiotemporal mode-locked pulses has not been investigated before. To further analyze and understand the experimental results, we numerically solve the nonlinear Schrodinger equation, and compare the numerical simulation with the experimental results. In addition to passively adjusting cavity loss, the InP QD SA can balance the large walk off between different transverse modes, due to the spatial and spectral filtering effects. Our work has important implications for the design and application of all-fiber STML lasers.

Ti3C2Tx MXene Quantum Dots with Surface-Terminated Groups (-F, -OH, =O, -Cl) for Ultrafast Photonics.

Transition metal carbides and nitrides (MXenes) have attracted significant attention in photoelectric applications due to their highly tunable electronic and optical properties influenced by a flexible compositional or surface functional group regulation. Ti3C2Tx MXenes (-F, -OH, =O terminated) used in previous ultrafast photonic studies are usually synthesized via a generic hydrofluoric acid (HF) etching strategy, which may cause numerous defects and thus impedes the optoelectronic properties of Ti3C2Tx. In this contribution, inspired by a much higher conductivity and carrier mobility of Ti3C2Tx (-F, -OH, =O, -Cl terminated) prepared from a minimally intensive layer delamination method (MILD) etching strategy, we further optimized it with a liquid-phase exfoliation (LPE) method to synthesize pure Ti3C2Tx quantum dots (QDs) for ultrafast photonic. Compared to the other QDs saturable absorber (SA) devices performed at 1550 nm, our SA device exhibited a relatively low saturation intensity (1.983 GW/cm−2) and high modulation depth (11.6%), allowing for a more easily mode-locked pulse generation. A distinguished ultrashort pulse duration of 466 fs centered at the wavelength of 1566.57 nm with a fundamental frequency of 22.78 MHz was obtained in the communication band. Considering the SA based on such a Ti3C2Tx QDs tapered fiber is the first exploration of Er3+-doped fiber laser (EDFL), this work will open up a new avenue for applications in ultrafast photonics.

Wavelength-tunable noise-like fiber laser based on PbS quantum dot saturable absorber film.

A wavelength-tunable noise-like pulse (NLP) erbium-doped fiber laser incorporating PbS quantum dot (QD) polystyrene (PS) composite film as a saturable absorber (SA) is experimentally demonstrated. The wavelength tuning is implemented via a Lyot filter consisting of a segment of polarization-maintaining fiber (PMF) and a 45° tilted fiber grating. By adjusting the polarization state of the ring cavity, the laser can deliver NLP with a continuous wavelength-tunable range from 1550.21 to 1560.64 nm. During continuous wavelength tuning, the output power varies between a range of 30.88-48.8 mW. Worthwhile noting is that the output power of 48.8 mW is the reported highest output power for wavelength-tunable NLP operation in an erbium-doped fiber laser using composite film as a saturable absorber.

PbS Quantum Dots Saturable Absorber for Dual-Wavelength Solitons Generation.

PbS quantum dots (QDs), a representative zero-dimensional material, have attracted great interest due to their unique optical, electronic, and chemical characteristics. Compared to one- and two-dimensional materials, PbS QDs possess strong absorption and an adjustable bandgap, which are particularly fascinating in near-infrared applications. Here, fiber-based PbS QDs as a saturable absorber (SA) are studied for dual-wavelength ultrafast pulses generation for the first time to our knowledge. By introducing PbS QDs SA into an erbium-doped fiber laser, the laser can simultaneously generate dual-wavelength conventional solitons with central wavelengths of 1532 and 1559 nm and 3 dB bandwidths of 2.8 and 2.5 nm, respectively. The results show that PbS QDs as broadband SAs have potential application prospects for the generation of ultrafast lasers.

Bismuth quantum dots as an optical saturable absorber for a 1.3 μm Q-switched solid-state laser.

Two-dimensional bismuth has attracted extensive attention for its unique properties, such as a moderate bandgap, high carrier mobility, and high stability. In this work, a uniform bismuth quantum dots saturable absorber (BiQDs-SA) was successfully fabricated by the liquid-phase exfoliation method. It was successfully employed in a passively Q-switched bulk laser operating at the telecommunication wavelength of 1.3μm. The as-fabricated BiQDs-SA-based 1.3μm Q-switched laser not only exhibits low timing jitter but also shows long-term stability for several days. The maximum average output power of 125mW was obtained with the shortest pulse width of 510ns and repetition rate of 135kHz, corresponding to single-pulse energy and pulse peak power of 0.9μJ and 1.8W, respectively. The results indicate that BiQDs-SA is a potential optical modulator for passively Q-switched solid-state lasers.

Femtosecond Alexandrite laser passively mode-locked by an InP/InGaP quantum-dot saturable absorber

An Alexandrite laser passively mode-locked using an InP/InGaP quantum-dot semiconductor saturable absorber mirror (QD-SESAM) was demonstrated. The laser was pumped at 532nm and generated pulses as short as 380fs at 775nm with an average output power of 295mW. To the best of our knowledge, this is the first report on a passively mode-locked femtosecond Alexandrite laser using a SESAM in general and a QD-SESAM in particular.

Passively Q-switched green laser operation using CdTe/CdS quantum dots (Invited Paper)

The direct generation of passively Q-switched lasers at a green wavelength has rarely been investigated in the past. In this Letter, we demonstrate a passively Q-switched praseodymium-doped yttrium lithium fluoride green laser at 522 nm using CdTe/CdS quantum dots as a saturable absorber. A maximum average output power of 33.6 mW is achieved with the shortest pulse width of 840 ns. The corresponding pulse energy and peak power reached 0.18 μJ and 0.21 W, respectively. To the best of our knowledge, this is the first demonstration in regard to a quantum dots saturable absorber operating in the green spectral region.

Diode-pumped ultrafast Yb:KGW laser with 56 fs pulses and multi-100 kW peak power based on SESAM and Kerr-lens mode locking

A high-power sub-60 fs mode-locked diode-pumped Yb:KGW laser based on hybrid action of an InGaAs quantum-dot saturable absorber mirror and Kerr-lens mode locking was demonstrated. The laser delivered 56 fs pulses with 1.95 W of average power corresponding to 450 kW of peak power. The width of the generated laser spectrum was 20.5 nm, which was near the gain bandwidth limit of the Yb:KGW crystal. To the best of our knowledge, these are the shortest pulses generated from the monoclinic double tungstate crystals (and Yb:KGW laser crystal in particular) and the most powerful in the sub-60 fs regime. At the same time, they are also the shortest pulses produced to date with the help of a quantum-dot-based saturable absorber. High-power operation with a pulse duration of 90 fs and 2.85 W of average output power was also demonstrated.

Quantum-dot saturable absorber and Kerr-lens mode-locked Yb:KGW laser with >450 kW of peak power.

The hybrid action of quantum-dot saturable absorber and Kerr-lens mode locking in a diode-pumped Yb:KGW laser was demonstrated. Using a quantum-dot saturable absorber with a 0.7% (0.5%) modulation depth, the mode-locked laser delivered 90 fs (93 fs) pulses with 3.2 W (2.9 W) of average power at the repetition rate of 77 MHz, corresponding to 462 kW (406 kW) of peak power and 41 nJ (38 nJ) of pulse energy. To the best of our knowledge, this represents the highest average and peak powers generated to date from quantum-dot saturable absorber-based mode-locked lasers.

Saturable absorber mirrors for ytterbium mode-locked femtosecond lasers

We present a femtosecond, diode-pumped Yb:KYW laser configured with a range of saturable absorber mirrors, both commercially available and state-of-the-art structures. The laser performance is characterized for different output coupler (OC) transmission – for the typically used OC transmission between 0.5-10% the laser power increases with the increasing transmission but the spectrum gets narrower at the same time, resulting in longer pulses and the laser pulse peak power having a maximum. Full Text: PDF References C. Hönninger, R. Paschotta, M. Graf, F. Morier-Genoud, G. Zhang, M. Moser, S. Biswal, J. Nees, A. Braun, G.A. Mourou, I. Johannsen, A. Giesen, W. Seeber, U. Keller, "Ultrafast ytterbium-doped bulk lasers and laser amplifiers", Appl. Phys. B 69, 3 (1999). CrossRef A. Lagatsky, C. Brown, W. Sibbett, "Highly efficient and low threshold diode-pumped Kerr-lens mode-locked Yb:KYW laser", Opt. Expr. 12, 3928 (2004). CrossRef H. Liu, J. Nees, G. Mourou, "Diode-pumped Kerr-lens mode-locked Yb:KY(WO4)2 laser", Opt. Lett. 26, 1723 (2005). CrossRef P. Wasylczyk, C. Radzewicz, "Design and alignment criteria for a simple, robust, diode-pumped femtosecond Yb:KYW oscillator", Laser Phys. 19, 129 (2009). CrossRef S. Meyer, J. Squier, S. Diddams, "Diode-pumped Yb:KYW femtosecond laser frequency comb with stabilized carrier-envelope offset frequency", Eur. Phys. J. D 48, 19 (2008). CrossRef D. Kopf, G. Zhang, R. Fluck, M. Moser, U. Keller, "All-in-one dispersion-compensating saturable absorber mirror for compact femtosecond laser sources", Opt. Lett. 21, 486 (1996). CrossRef R. Paschotta, G. Spuhler, D. Sutter, N. Matuschek, U. Keller, M. Moser, R. Hovel, V. Scheuer, G. Angelow, T. Tschudi, "Double-chirped semiconductor mirror for dispersion compensation in femtosecond lasers", Appl. Phys. Lett. 75, 2166 (1999). CrossRef A. Jasik, P. Wasylczyk, M. Dems, P. Wnuk, A. Wójcik-Jedlińska, K. Reginski, Ł. Zinkiewicz, K. Hejduk, "A passively mode-locked, self-starting femtosecond Yb:KYW laser with a single highly dispersive semiconductor double-chirped mirror for dispersion compensation", Laser Phys. Lett. 10, 085302 (2013). CrossRef A. Schmidt, S. Rivier, W. Cho, J. Yim, S. Choi, S. Lee, F. Rotermund, D. Rytz, G. Steinmeyer, V. Petrov, U. Griebner, "Sub-100 fs single-walled carbon nanotube saturable absorber mode-locked Yb-laser operation near 1 µm", Opt. Expr. 17, 20109 (2009). CrossRef E. Rafailov, S. White, A. Lagatsky, A. Miller, W. Sibbett, D. Livshits, A. Zhukov, V. Ustinov, "Fast quantum-dot saturable absorber for passive mode-locking of solid-State lasers ", IEEE Photon. Technol. Lett. 16, 2439 (2004). CrossRef A. Jasik, J. Muszalski, J. Talalaev, M. Kosmala, K. Pierściński, M. Bugajski, "Low-temperature grown near surface semiconductor saturable absorber mirror: Design, growth conditions, characterization, and mode-locked operation", J. Appl. Phys. 109, 053101 (2009). CrossRef G. Sobon, J. Sotor, I. Pasternak, K. Grodecki, P. Paletko, W. Strupinski, Z. Jankiewicz and K. Abramski, "Er-Doped Fiber Laser Mode-Locked by CVD-Graphene Saturable Absorber", J. Light. Technol. 30, 2770 (2012). CrossRef

Transient behaviour of quantum-dot saturable absorber mirrors at varying excitation fluence

We present results from studying the carrier dynamics in self-assembled InAs/GaAs quantum-dot saturable absorbers intended for mode-locking of solid-state lasers. Four samples are examined, featuring controlled variations in the resonance condition of the electric field inside the absorber, the number of quantum-dot (QD) layers and the thickness of the GaAs barriers between these QD layers. Pump-probe experiments are conducted at a wide range of excitation fluences and reveal a fast relaxation component of the initial bleaching at low excitation fluences, while a slowly relaxing induced transparency becomes dominant at higher excitation fluences. Time-resolved photoluminescence measurements reveal a large and slowly relaxing induced transparency due to a capture of excess carriers from the barrier bands into the QDs and a slow radiative recombination there. The resonance condition as well as the thickness of the barriers between the QD layers can be used to control the relaxation behaviour. The fastest response is obtained in a structure with an increased number of QD layers at each individual anti-node of the electric field, which is attributed to the appearance of efficient non-radiative recombination channels and capture centres. These centres are probably related to dislocations and other defects appearing in thick QD stacks.

Gigahertz repetition rate mode-locked Yb:KYW laser using self-assembled quantum dot saturable absorber

Fundamental mode-locking is achieved in a 1.036 GHz cavity using a semiconductor quantum dot saturable absorber mirror with a fast relaxation time component of down to 550 fs. The dispersive cavity delivers 1.7 ps wide pulses with spectra supporting sub-picosecond pulse durations and an M² of 1.3. An average output power of up to 339 mW at wavelengths around 1,032 nm is achieved and the saturable absorber’s damage threshold is identified as a limitation for further power scaling.

Growth of PbSe Quantum Dots Within High‐Index Lead‐Phosphate Glass for Infrared Saturable Absorbers

PbSe quantum dots (QDs) were grown in high‐refractive‐index low‐melting‐temperature lead‐phosphate glass. The lowest energy exciton transition of the QDs was tuned over a wide range within the infrared spectral region (0.93–2.75 μm) by a controlled heat treatment. The measured QD radius ranged between 2 and 5.3 nm, with a time (t) dependence of t0.29 for long dwelling times during the heat treatment, indicating that the QD growth mechanism tends to follow Lifshitz‐Slyozov‐Wagner theory. The QD saturable absorber behavior at 1.2 μm had a measured saturation fluence of ~2.1 μJ/cm2.

High-power integrated ultrafast semiconductor disk laser: multi-Watt 10 GHz pulse generation

Presented is an optically pumped modelocked integrated external-cavity surface emitting laser (MIXSEL) with a pulse repetition rate of 10 GHz, generating picosecond pulses at 2.4 W average output power at a centre wavelength of 963 nm. The MIXSEL structure integrates both the absorber and the gain layers within the same wafer. The saturable absorber is a single layer of self-assembled InAs quantum dots (QD) and the gain is obtained with seven InGaAs quantum wells. It is shown that the picosecond pulse duration is limited by the slow recovery time of the integrated QD saturable absorber.

P-i-n junction quantum dot saturable absorber mirror: electrical control of ultrafast dynamics

We report on nonlinear optical properties of a p-i-n junction quantum dot saturable absorber based on InGaAs/GaAs. Absorption recovery dynamics and nonlinear reflectivity are investigated for different reverse bias and pump power conditions. A decrease in absorption recovery time of nearly two orders of magnitude is demonstrated by applying a voltage between 0 and -20 V. The saturable absorber modulation depth and saturation fluence are found to be independent from the applied reverse bias.

A negative feedback controlled hybrid mode locked Nd : YAG laser using a semiconductor quantum dot saturable absorber

The parameters of a solid-state PbS quantum dot saturable absorber, optical elements, resonator and feedback circuit have been adjusted for stable and highly efficient hybrid mode-locking Nd : YAG laser operation. A highly stable microsecond pulse train (40–50 μs) containing about 5000 ps pulses and 3 mJ total train energy was generated with a repetition rate of 10 Hz. This all solid-state hybrid mode-locked Nd : YAG laser was used for synchronous pumping of a femtosecond Ti : sapphire laser.

All-Optical Switch Using Quantum-Dot Saturable Absorbers in a DBR Microcavity

We propose and analyze an all-optical switch based on quantum-dots (QDs) embedded in a distributive Bragg reflector (DBR) cavity. The QDs behave as saturable absorbers that modify the cavity reflectivity. The device exploits the guided in-plane modes of the DBR to fully separate the control pulse from the input signal. We analyze the performance of the switch under realistic experimental conditions. We calculate the signal loss and switching intensity of the device, specializing to the case of InAs QDs embedded in GaAs/AlGaAs DBR structure, and show that the switching power can be as low as 19 μW. Noise injection due to spontaneous emission is also analyzed and shown to be small. We also investigate the possibility of improving the switching speed via nonradiative recombination and show that switching speeds of up to 112 GHz with low energy consumption of 3.91 fJ per bit are possible.

High-power MIXSEL: an integrated ultrafast semiconductor laser with 64 W average power

High-power ultrafast lasers are important for numerous industrial and scientific applications. Current multi-watt systems, however, are based on relatively complex laser concepts, for example using additional intracavity elements for pulse formation. Moving towards a higher level of integration would reduce complexity, packaging, and manufacturing cost, which are important requirements for mass production. Semiconductor lasers are well established for such applications, and optically-pumped vertical external cavity surface emitting lasers (VECSELs) are most promising for higher power applications, generating the highest power in fundamental transverse mode (>20 W) to date. Ultrashort pulses have been demonstrated using passive modelocking with a semiconductor saturable absorber mirror (SESAM), achieving for example 2.1-W average power, sub-100-fs pulse duration, and 50-GHz pulse repetition rate. Previously the integration of both the gain and absorber elements into a single wafer was demonstrated with the MIXSEL (modelocked integrated external-cavity surface emitting laser) but with limited average output power (<200 mW). We have demonstrated the power scaling concept of the MIXSEL using optimized quantum dot saturable absorbers in an antiresonant structure design combined with an improved thermal management by wafer removal and mounting of the 8-µm thick MIXSEL structure directly onto a CVD-diamond heat spreader. The simple straight cavity with only two components has generated 28-ps pulses at 2.5-GHz repetition rate and an average output power of 6.4 W, which is higher than for any other modelocked semiconductor laser.

Optical bistability in a two-section InAs quantum-dot laser

Room temperature, continuous-wave bistable operation is achieved in two-section 1.24 μm InAs quantum-dot (QD) lasers with integrated intracavity QD saturable absorbers (SA). It is found that the hysteresis width is narrowed with increasing reverse bias voltage, and broadened with increasing length of saturable absorber. This can be explained by the competition between QD absorption and electroabsorption in the SA section. In addition, a larger hysteresis width is realized than other reports so far, which can be attributed to a greater number of stacked layers of active region in our case. The experimental results can be explained by a modified threshold current model.

Temperature dependence of electroabsorption dynamics in an InAs quantum-dot saturable absorber at 1.3 μm and its impact on mode-locked quantum-dot lasers

We report temperature-dependent absorption recovery times in an InAs p-i-n ridge waveguide quantum-dot modulator under low reverse bias, investigated via subpicosecond pump-probe measurements. The measured decrease in absorption recovery time with increasing temperature (293–319 K) is in excellent agreement with a thermionic emission model. A similar trend in pulse duration with increasing temperature is also observed from a two-section mode-locked quantum-dot laser fabricated from a similar epitaxial structure. These measurements confirm the key role of the absorber recovery time in the reduction in the pulses generated by two-section mode-locked quantum-dot lasers, both at room and elevated temperatures.