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.     

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     

Optical characteristics of semiconductor saturable absorber mirrors investigated by the reflection Z-scan technique

Recently, the semiconductor saturable absorber mirror (SESAM) has become a key component of passive mode-locked solid-state lasers. Here we present a simple method based on the reflection Z-scan technique to measure the key optical parameters of SESAM such as saturation fluence and modulation depth. The experimental results demonstrate that our method is able to perform with a high accuracy of 10⁻⁴ and a dynamic range of over four orders of magnitude.     

On the possibility of mode-locking a dye laser by a fast saturable absorber

The problem of mode-locking a fully relaxing saturable (dye) laser medium by an instantaneously saturating (fast) absorber is theoretically considered when group velocity dispersion and Kerr-type nonlinearity are present. Off-resonant chirped solutions are found whose minimum duration is determined by the relative losses (saturable and nonsaturable) and by the saturation depth of the absorber. The addition of nonlinear elements can only broaden the stability range of the solution with respect to the group velocity dispersion.     

Femtosecond Er doped fiber laser using high modulation depth SESAM based on metal/dielectric hybrid mirror

We proposed and fabricated a metal/dielectric mirror based high modulation depth semiconductor saturable absorber mirror (SESAM), for ease of the growth restrictions. Using such a SESAM, we obtained self-starting mode-locking in a ring cavity fiber laser, which produced 102 fs pulses at a repetition rate of 36.2 MHz.     

In situ characterization of semiconductor saturable absorber mirrors in an operating Yb:KGW mode-locked laser

We report in situ characterization of a semiconductor saturable absorber mirror (SESAM) in an operating Yb:KGW mode-locked laser. The technique may be described as a pump-probe experiment in which the intracavity beam acts as a pump beam while the output of the same laser is used as a test beam for the SESAM reflectivity. At zero delay, the probe pulse overlaps in time with the subsequent intracavity pulse. The method is an alternative to standard pump-probe measurements in situations where the intracavity parameters such as energy fluence onto the SESAM, pulse length, and center wavelength cannot be achieved simultaneously with available lasers.     

含负克尔效应半导体可饱和吸收镜的五镜腔飞秒钛宝石激光器的理论分析

基于半导体可饱和吸收镜(SESAM)的负克尔效应，对含SESAM的五镜腔进行了理论计算和分析.结果表明，由于SESAM的负克尔效应，与没有SESAM的相比，锁模激光器的稳区在上稳区的内边缘有明显扩展，这使得激光器锁模启动的工作点更加靠近直流运转时的稳区边缘，从而获得更大的非线性因子.这说明SESAM的可饱和吸收机理具有锁模自启动效应外，其负克尔效应也有助于锁模的自启动. 关键词： 半导体可饱和吸收镜 负克尔效应 五镜腔     

Subpicosecond pulse generation from a 1.56 μm mode-locked VECSEL

Near-transform-limited subpicosecond pulses at 1.56 μm were generated from an optically pumped InP-based vertical-external-cavity surface-emitting laser (VECSEL) passively mode-locked at 2 GHz repetition rate with a fast InGaAsNSb/GaAs semiconductor saturable absorber mirror (SESAM). The SESAM microcavity resonance was adjusted via a selective etching of phase layers specifically designed to control the magnitude of both the modulation depth and the intracavity group delay dispersion of the SESAM. Using the same VECSEL chip, we observed that the mode-locked pulse duration could be reduced from several picoseconds to less than 1 ps with a detuned resonant SESAM.     

Sub-100 femtosecond pulses from a SESAM modelocked thin disk laser

We present the first passively modelocked thin disk laser (TDL) with sub-100-fs pulse duration using the broadband sesquioxide gain material Yb:LuScO₃ and an optimized SEmiconductor Saturable Absorber Mirror (SESAM). In this proof-of-principle experiment, we obtained 5.1 W of average power at a repetition rate of 77.5 MHz and a pulse duration of 96 fs. We carefully explored and optimized the different parameters on the soliton pulse formation process for the generation of short pulses. In particular, SESAMs combining fast recovery time, high modulation depth and low nonsaturable losses proved crucial to achieve this result even though they are expected to only play a minor role in soliton modelocking. To our knowledge, these are the shortest pulses ever obtained with a modelocked TDL, reaching for the first time the sub-100-fs milestone. This result opens the door to sub-100-fs oscillators with substantially higher power levels in the near future.     

Noise conversion from pump to the passively mode-locked fiber lasers at 1.5 μm

We characterize the noise conversion from the pump relative intensity noise (RIN) to the RIN and phase noise of passively mode-locked lasers at 1.5 μm. Two mode locking mechanisms, nonlinear polarization rotation (NPR) and semiconductor saturable absorber mirror (SESAM), are compared for noise conversion for the first time. It is found that the RIN and the phase noise of both types of lasers are dominated by the noise converted from the pump RIN and thus, can be predicted with the measured pump RIN and noise conversion ratios. The SESAM laser is found to show an excess noise conversion from the laser RIN to the laser phase noise due to the slow saturable absorber effect.     

Passively mode-locked erbium-doped fiber laser via a D-shape-fiber-based MoS_2 saturable absorber with a very low nonsaturable loss

We report on the generation of conventional and dissipative solitons in erbium-doped fiber lasers by the evanescent field interaction between the propagating light and a multilayer molybdenum disulfide(MoS_2) thin film. The MoS_2 film is fabricated by depositing the MoS_2 water–ethanol mixture on a D-shape-fiber(DF) repetitively. The measured nonsaturable loss, saturable optical intensity, and the modulation depth of this device are 13.3%, 110 MW/cm~2, and 3.4% respectively.Owing to the very low nonsaturable loss, the laser threshold of conventional soliton is as low as 4.8 mW. The further increase of net cavity dispersion to normal regime, stable dissipation soliton pulse trains with a spectral bandwidth of 11.7 nm and pulse duration of 116 ps are successfully generated. Our experiment demonstrates that the MoS_2-DF device can indeed be used as a high performance saturable absorber for further applications in ultrafast photonics.     

Ultrafast Nonlinear Optical Properties of a Graphene Saturable Mirror in the 2 μm Wavelength Region

Mid‐infrared ultrafast lasers have emerged as a promising platform for both science and industry because of their inherent high raw power and eye‐safe spectrum. 2D nanostructures such as graphene have emerged as promising photonic materials for laser mode‐locking to generate ultrashort pulses. However, there are still many unanswered questions about graphene's key advantages to be practical devices, especially over the matured semiconductor saturable absorber mirror (SESAM). In this work, we conducted systematic comparisons on the nonlinear optical properties of graphene and that of a commercial SESAM at 2 μm wavelength. Our results showed that graphene has significant advantages over the commercial SESAM, exhibiting ∼28% less absorptive cross‐section ratio of excited‐state to ground‐state and ∼50 times faster relaxation time. This implies that graphene can be exploited as a better mode‐locker than the current commercial SESAM for high power, high repetition rate and ultrafast mid‐infrared laser sources.     

Intracavity-frequency-doubled Q-switched and mode-locked Nd:Lu0.15Y0.85VO4/KTP green laser with AO modulator and central SESAM

By using mixed crystal Nd:Lu_0.15Y_0.85VO₄ as laser medium, KTP as frequency-doubling crystal, a diode-pumped intracavity-frequency-doubled Q-switched and mode-locked (QML) Nd:Lu_0.15Y_0.85VO₄/KTP green laser with acousto-optic (AO) modulator and central semiconductor saturable absorption mirror (C-SESAM) is realized by using a V-type cavity. The QML laser characteristics such as the pulse width, single-pulse energy, have been measured for different modulation frequencies of the AO modulator. In comparison with the singly passively QML green laser with central SESAM, the doubly QML green laser can generate more stable and shorter pulses with higher peak power. Based on the coupled rate equations for a diode-pumped doubly QML green laser with AO and C-SESAM, in which the saturated absorption mechanism of C-SESAM is considered, the recurrence relation of the relative amplitude of the mode-locking pulses is given and the related numerical simulations are in good agreement with the experimental results.     

半导体可饱和吸收镜锁模的钒酸盐混晶Nd:Gd0.1Y0.9VO4激光器

采用Nd∶Gd_0.1Y_0.9VO₄晶体作为增益介质和Z形腔结构,分析比较了腔内加入自行设计的镀和不镀高反膜的半导体可饱和吸收镜（SESAM）对激光锁模的影响.在腔内加入镀膜SESAM后,激光锁模阈值由1.69W下降为1.45W,并且锁模更稳定.在2W抽运功率下,在1064nm中心波长处获得了双端250mW的连续锁模输出,光光转换效率为12.5%,重复频率为142.25MHz. 关键词： 0.1Y_0.9VO₄激光器')" href="#">Nd∶Gd_0.1Y_0.9VO₄激光器 半导体可饱和吸收镜 连续锁模     

Self-stabilized energy-scalable dissipative soliton fiber laser

We report on the self-stabilized dissipative-soliton laser with an energy scalability by using a semi-conductor saturable absorber mirror (SESAM) as the pulse stabilizer for the first time. By tailoring the fluence on the SESAM relying on the two-photon absorption effect so that its reflectivity has a negative slope with respect to the fluence, the intra-cavity pulse dynamics is self-stabilized to produce single pulses only, resulting in a 48-nJ pulse energy and an unprecedented pump-to-output power conversion efficiency of 63%. Therefore, it resolves a long standing problem of the formation of multiple pulses in high-energy mode-locked lasers and thus opens a way to build high repetition-rate oscillators with ≥ microjoules pulse energies without any external amplifiers.     

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.     

Experimental study on doubly QML Nd:Lu<Subscript>0.15</Subscript>Y<Subscript>0.85</Subscript>VO<Subscript>4</Subscript> laser with AO modulator and central SESAM

By using double-mixed crystal Nd:Lu_0.15Y_0.85VO₄ as laser medium, a diode-pumped doubly Q-switched and mode-locked (QML) Nd:Lu_0.15Y_0.85VO₄ laser with acousto-optic (AO) modulator and central semiconductor saturable absorption mirror (SESAM) is realized for the first time. The Q-switched envelope modulation depth is nearly 100%.The average output power and the pulse width of the Q-switched envelope etc. for different AO modulator repetition rates have been measured. The experimental result show that Nd:Lu_0.15Y_0.85VO₄ crystal is an excellent laser medium for doubly QML lasers.     

基于铋可饱和吸收体的超快激光产生

采用磁控溅射沉积法在微纳光纤表面上镀一层纳米级厚度的铋薄膜,制备了一种微纳光纤-铋膜结构的可饱和吸收体.在1.5μm处的非线性光调制深度为14%.将其应用到掺铒光纤激光器中,在1.5μm波段获得稳定的超快脉冲激光产生,脉宽为357 fs,输出功率为45.4 mW,单脉冲能量为2.39 nJ,信噪比为84 dB.实验结果表明,利用磁控溅射法可制备出大调制深度的可饱和吸收体,为获得高能量超短脉冲激光输出提供新方案.     

Carboxyl graphene oxide solution saturable absorber for femtosecond mode-locked erbium-doped fiber laser

The carboxyl-functionalized graphene oxide(GO-COOH) is a kind of unique two-dimensional(2 D) material and possesses excellent nonlinear saturable absorption property and high water-solubility. In this paper, we prepare saturable absorber(SA) device by depositing GO-COOH nanosheets aqueous solution on a D-shaped fiber. The modulation depth(MD) and saturable intensity of the SA are measured to be 9.6% and 19 MW/cm~2, respectively. By inserting the SA into the erbium-doped fiber(EDF) laser, a passively mode-locked EDF laser has been achieved with the spectrum center wavelength of 1562.76 nm. The pulse duration, repetition rate, and the signal-to-noise ratio(SNR) are 500 fs, 14.79 MHz, and 80 dB,respectively. The maximum average output power is measured to be 3.85 mW. These results indicate that the GO-COOH nanosheets SA can be used as a promising mode locker for the generation of ultrashort pulses.     

Yb-doped passively mode-locked fiber laser with Bi_2Te_3-deposited

In this study we present an all-normal-dispersion Yb-doped fiber laser passively mode-locked with topological insulator(Bi2Te3) saturable absorber. The saturable absorber device is fabricated by depositing Bi2Te3 on a tapered fiber through using pulsed laser deposition(PLD) technology, which can give rise to less non-saturable losses than most of the solution processing methods. Owing to the long interaction length, Bi2Te3 is not exposed to high optical power, which allows the saturable absorber device to work in a high power regime. The modulation depth of this kind of saturable absorber is measured to be 10%. By combining the saturable absorber device with Yb-doped fiber laser, a mode-locked pulse operating at a repetition rate of 19.8 MHz is achieved. The 3-d B spectral width and pulse duration are measured to be 1.245 nm and317 ps, respectively.