Suppression of Q-switched mode locking and break-up into multiple pulses by inverse saturable absorption

Suppression of Q-switching during mode locking is extremely important for the successful development of continuous-wave mode-locked solid-state and fiber lasers. In this paper we show that the use of inverse saturable absorption, such as two-photon absorption and free carrier absorption, can suppress Q-switching instabilities and expand the continuous-wave operating regime in solid-state and fiber lasers. Conversely, an excess of inverse saturable absorption may lead to pulse-energy limiting and induce a break-up into multiple pulses. To analyze the advantages and limitations of adding this nonlinearity, we derive and discuss the mode-locking stability criteria in the presence of inverse saturable absorption. Useful asymptotic formulas for the absorber design employing inverse saturable absorption are also derived.     

Efficient graphene Q switching and mode locking of 1.34 μm neodymium lasers

We demonstrate that few-layered graphene sheets used as a saturable absorber can provide efficient Q-switching and mode-locking modulation in 1.34 μm Nd:GdVO(4) bulk lasers. The minimum Q-switched pulses were 450 ns for 260 mW average power, 43 kHz repetition rate, and 2.5 μJ pulse energy. For the mode-locked laser, an average power of 1.29 W was achieved with 11 ps pulse duration and 13 nJ pulse energy. To our knowledge, this average power is the highest yet obtained from a graphene mode-locked laser, and the corresponding optical-optical efficiency of 23% is the best result among 1.3 μm neodymium mode-locked lasers. The quality factor M(2) of the Q-switched beam was 1.4 and 1.6 in the horizontal and longitudinal planes, respectively, and the M(2) of the mode-locked beam reached 1.1 and 1.0. These results clearly indicate the advantages of few-layered graphene as a saturable absorber.     

Mode-locked CW dye lasers

Mode-locking of CW dye lasers are described. Three different methods: active mode-locking by acoustic loss modulation, pumping with mode-locked pulses and passive mode-locking by a saturable absorber, all resulted in stable continuous trains of tunable mode-locked pulses of various duration. Best results were obtained using passive mode-locking by a saturable absorber. The pulse lengths for this case were measured by auto-correlation technique to be as short as 1.5 ps. Factors influencing the mode-locking and possible improvements are discussed.     

半导体可饱和吸收镜作为被动调Q吸收体的发展状况

简介了近年来发展起来的若干种新型固体激光器被动调Q用吸收体：掺Cr4+系列，Cr，Nd∶YAG自调Q激光晶体，人眼安全激光器被动调Q用吸收体，GaAs吸收体，半导体可饱和吸收镜。着重介绍了固体激光器和光纤激光器调Q用半导体可饱和吸收镜的原理、研制方法及应用状况。     

Emerging 2D Semiconducting Materials: Tin Diselenide for Ultrafast Photonics

SnSe₂, a novel 2D semiconducting material with good photoelectric properties and tunable bandgaps structure, has been widely studied in high-performance photodetectors and superconductivity applications, etc. Herein, another excellent property, nonlinear optical absorption, is applied to ultrafast photonics. The measured nonlinear optical absorption curve of SnSe₂ with modulation depth of 5.52% is obtained. Q-switching and mode-locking in Er-doped fiber lasers based on SnSe₂ is reported. A Q-switched fiber laser can be obtained with a 0.5-m-long gain fiber and a 50:50 optical coupler. With 0.75-m-long active fiber and a 40:60 optical coupler, mode locking can be obtained. Furthermore, a third-order bound-state soliton pulse with a pulse duration of 882 fs can be achieved. The experimental results show that SnSe₂ is an excellent optical saturable absorber for the applications of ultrafast photonics.     

Passive Q-switching and mode-locking for the generation of nanosecond to femtosecond pulses

The passive and hybrid Q-switching and mode-locking of solid-state lasers, dye lasers, semiconductor lasers and gas lasers is reviewed. The dynamics of saturable absorbers and reverse saturable absorbers is illustrated. The nanosecond pulse generation by passive and hybrid Q-switching of low-gain active media is described. The picosecond and femtosecond pulse generation by passive and hybrid mode-locking in low-gain and high-gain active media is analysed. The performance data of passively and hybridly mode-locked cw femtosecond dye lasers are collected. The pulse shortening of ultra-fast pulses with saturable absorbers in intra-cavity and extra-cavity configurations is discussed.     

快速与慢速饱和吸收体被动锁模掺铒光纤激光器的理论分析

利用半导体饱和吸收波导作为慢速饱和吸收体与非线性快速饱和吸收体相结合，建立了共同被动锁模掺铒光纤激光器的理论模型-分析了半导体饱和吸收波导、光纤自相位调制和自振幅调制对锁模脉冲宽度和脉冲啁啾的影响- 关键词：     

New regime of inverse saturable absorption for self-stabilizing passively mode-locked lasers

The reflectivity of a semiconductor saturable absorber mirror (SESAM) is generally expected to increase with increasing pulse energy. However, for higher pulse energies the reflectivity can decrease again; we call this a roll-over of the nonlinear reflectivity curve caused by inverse saturable absorption. We show for several SESAMs that the measured roll-over is consistent with two-photon absorption only for short (femtosecond) pulses, while a stronger (yet unidentified) kind of nonlinear absorption is dominant for longer (picosecond) pulses. These inverse saturable absorption effects have important technological consequences, e.g. for the Q-switching dynamics of passively mode-locked lasers. A simple equation using only measurable SESAM parameters and including inverse saturable absorption is derived for the Q-switched mode-locking threshold. We present various data and discuss the sometimes detrimental effects of this roll-over for femtosecond high repetition rate lasers, as well as the potentially very useful consequences for passively mode-locked multi-GHz lasers. We also discuss strategies to enhance or reduce this induced absorption by using different SESAM designs or semiconductor materials. PACS 42.60.Fc; 42.70.Nq; 78.20.Ci     

Stability regime study of a nonlinear mirror mode-locked laser

A diode array pumped nonlinear mirror mode-locked (ML) laser is developed and the regime of the stable operation for such a system is found to occur for a little detuning of intra-cavity second harmonic phase-matching. The existing theory of mode-locking using a fast saturable absorber (FSA) is used to derive corresponding equivalent parameters for a fast nonlinear mirror saturable absorber. The condition for stable single pulse ML operation is evaluated in terms of small signal gain of the laser medium and small signal nonlinear loss. The analysis explains the experimental observations and differentiates the regime of operation in ML, Q-switching and cw mode-locking.     

Graphene oxide-COOH as a new saturable absorber for both Q-switching and mode-locking fiber lasers

Graphene oxide carboxylic acid(COOH), a novel two-dimensional(2D) layered material with its unique optical and electronic properties, is discovered to exhibit the saturation of optical absorption under laser illumination.Applying the liquid-phase exfoliation method, we prepare graphene oxide-COOH dispersions with deionized water and fabricate graphene oxide-COOH polyvinyl alcohol polymer composite film. We further obtain stable Q-switching pulse and mode-locked laser operation with a 22.7 MHz repetition rate and a 1.5 ps pulse duration by incorporating the graphene oxide-COOH-based saturable absorbers into the all-fiber erbium-doped fiber laser cavity. The experimental results show that the proposed graphene oxide-COOH material can act as an effective absorber for pulsed fiber lasers, which demonstrate potential applications in the area of ultrafast optics.     

Single-walled carbon nanotubes on side polished fiber as a universal saturable absorber for various laser output states

We demonstrated four different laser operational states in the same passive fiber ring laser cavity by controlling solely the polarization state in a saturable absorber. The demonstrated laser operational states were continuous wave, mode-locking, Q-switching, and Q-switched mode-locking. The saturable absorber was fabricated by spin-coating a single-walled carbon nanotubes (SWCNT)/polymer composite on a side polished fiber providing enhanced nonlinear interaction with the SWCNT. The wide tuning range of the saturable absorber allows the simple means to generate different forms of laser output from a single laser cavity.     

Contrastive study of two SESAMs for passive mode-locking in Nd:YVO4 laser with low pump power

Two semiconductor saturable absorber mirrors (SESAMs), of which one is coated with 50% reflection film on the top and the other is not, were contrastively studied in passively mode-locked solid-state lasers which were pumped by low output power laser diode (LD). Experiments have shown that reducing the modulation depth of SESAM by coating partial reflection film, whose reflectivity is higher than that between SESAM and air interface, is an effective method to get continuous wave (CW) mode-locking instead of Q-switched power LD, in which no water-cooling system was used, could obtain CW mode-locking by the 50% reflector coated SESAM with average output power of ～ 20 mW.     

基于金属/介质混合反射镜的高调制深度半导体可饱和吸收镜锁模的飞秒光纤激光器

介绍了一种新型半导体可饱和吸收镜(SESAM)的原理与特性,并用它来实现掺铒光纤激光器的锁模飞秒脉冲的生成.这种新型宽带SESAM调制深度可以达到20%,用在1.5μm波段的环形腔掺铒光纤激光器里,实现自启动锁模.锁模光谱以1559 nm为中心,半峰全宽为9 nm,锁模激光脉冲串重复频率为25.6 MHz,输出功率14 mW,自相关仪测得脉宽为170 fs.     

147 fs碳纳米管倏逝场锁模全光纤掺铒光纤激光器

利用化学腐蚀法在光纤包层表面成功制备了调制深度为3.9%的单壁碳纳米管饱和吸收体.组建了环形腔结构的全光纤掺铒光纤激光器,以制备的单壁碳纳米管薄膜为锁模元件,利用倏逝场锁模实现了锁模输出.锁模脉冲的中心波长为1556 nm,3 dB光谱带宽为24 nm,脉冲宽度为147 fs,重复频率为150 MHz.在520mW抽运功率下,平均输出功率为21 mW,相应的单脉冲能量为0.14 nJ.     

High-quality, large-area monolayer graphene for efficient bulk laser mode-locking near 1.25 μm

High-quality monolayer graphene as large as 1.2×1.2?cm² was synthesized by chemical vapor deposition and used as a transmitting saturable absorber for efficient passive mode-locking of a femtosecond bulk solid-state laser. The monolayer graphene mode-locked Cr:forsterite laser was tunable around 1.25?μm and delivered sub-100?fs pulses with output powers up to 230?mW. The nonlinear optical characteristics of the monolayer graphene saturable absorber and the mode-locked operation were then compared with the case of the bilayer graphene saturable absorber.     

Er-doped all-fiber laser mode-locked by graphitic carbon nitride nanosheets

Few-layer graphitic carbon nitride(g-C_3N_4) nanosheets were fabricated and utilized as a saturable absorber for mode-locking in an Er-doped fiber laser with net normal dispersion. The g-C_3N_4/polyvinyl alcohol(PVA) hybrid-film-based saturable absorber has a modulation depth of 4.01% and a saturation intensity of 7.5 MW/cm~2. By integrating g-C_3N_4-PVA mode-locker into the laser cavity, a mode-locked operation could be obtained. The achieved mode-locking pulse centered at 1530.3 nm has a pulse width of 530 ps. Its repetition rate is 40.8 MHz, and the corresponding signal-to-noise ratio is about 55 dB.     

Side-pumped continuous-wave passively mode-locked Nd:YAG laser by using non-antiresonant SESAM

A new type semiconductor saturable absorber mirror (SESAM), which is not fulfilled antiresonance condition, was used in a side-pumped Nd:YAG laser. Experiment results show that stabilized continuous-wave mode-locking can successfully be obtained in picosecond regime by a nonantiresonant SESAM. Because the bandwidth of picosecond regime mode-locked pulse is not broad; therefore, the antiresonant factor of SESAMs, which are used in picosecond mode-locked lasers, can be omitted when the SESAMs are designed and fabricated. Without the limit of antiresonance condition, the difficulty of design and growth will decrease significantly and there will be more freedom in considering the modulation depth.     

Saturable absorber mode-locking and laser pulse compression

The theoretical predictions of two alternative models of saturable absorber mode-locking in dye lasers are compared. The predicted mode-locking and pulse compression regimes are considered for both flashlamp-pumped and cw dye lasers, and some questions raised thereby are tested by numerical simulation of pulse propagation.     

Experimental Study of a Pulsed Ytterbium-Doped Fibre Laser with Fast and Slow Saturable Absorbers in a Linear Cavity

We present a linear-cavity stretched-pulse fibre laser and by semiconductor saturable-absorber mirrors. A with mode locking by a nonlinear polarization rotation Q-switched mode-locking cw train and a mode-locking pulse train are obtained in the experiment. We investigate the effects of the equivalent fast saturable absorber and the slow saturable absorbers in experiment. It is found that neither the nonlinear polarization evolution effect nor a semiconductor saturable absorber mirror is enough to produce the stable cw mode-locking pulses in this experiment. A nonlinear polarization evolution effect controls the cavity loss to literally carve the pulses; semiconductor saturable absorber mirrors provide the self-restarting and maintain the stability of the modelocking operation.     

MoS_2 saturable absorber prepared by chemical vapor deposition method for nonlinear control in Q-switching fiber laser

Due to the remarkable carrier mobility and nonlinear characteristic,MoS_2 is considered to be a powerful competitor as an effective optical modulated material in fiber lasers.In this paper,the Moare prepared by the chemical vapor deposition method to guarantee the high quality of the crystal lattice and uniform thickness.The transfer of the films to microfiber and the operation of gold plated films ensure there is no heat-resistant damage and anti-oxidation.The modulation depth of the prepared integrated microfiber-Moabsorber is 11.07%.When the microfiber-Mo S_2 saturable absorber is used as a light modulator in the Q-switching fiber laser,the stable pulse train with a pulse duration of 888 ns at 1530.9 nm is obtained.The ultimate output power and pulse energy of output pulses are 18.8 mW and 88 nJ,respectively.The signal-to-noise ratio up to 60 dB indicates the good stability of the laser.This work demonstrates that the Moabsorber prepared by the chemical vapor deposition method can serve as an effective nonlinear control device for the Q-switching fiber laser.