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Optical parametric chirped-pulse amplification (OPCPA) is prone to undesired spatiotemporal couplings. This Letter studies a family of OPCPA couplings resulting from the nonlinear phase shift induced by frequency-dependent phase mismatch. These OPCPA couplings manifest as pulse-front deformation, transversely varying pulse duration, and spectrally varying wavefront curvature, which are directly linked with the phase–mismatch dispersion terms. The numerical study in this Letter also reveals that the focused signal intensity severely degrades with increasing signal bandwidth and pump depletion.

Ultrafast lasers with both high peak-power and high average-power will open new avenues for many applications. While conventional technologies of Ti:sapphire laser amplification and optical parametric amplification can achieve several tens of watts of average-power, scaling to a higher average-power is challenging due to thermal limitations. Here, we demonstrate that the quasi-parametric chirped-pulse amplification (QPCPA) can break this average-power barrier. QPCPA is proven robust against the thermal dephasing by obstructing the back-conversion effect. Numerical simulations show that QPCPA based on a Sm:YCOB crystal can support peak powers of 3 TW at 5 kHz and 13.5 PW at 1 Hz, with average powers exceeding 150 W in both cases. We also discuss the prospects of QPCPA with the recently proposed configuration of temperature-insensitive phase matching, which is promising to simultaneously achieve higher peak-power and higher average-power.

An ultrashort mid-infrared (IR) source beyond 5 µm is crucial for a plethora of existing and emerging applications in spectroscopy, medical diagnostics, and high-field physics. Nonlinear generation of such sources from well-developed near-IR lasers, however, remains a challenge due to the limitation of mid-IR crystals. Based on oxide L a 3 G a 5.5 N b 0.5 O 14 (LGN) crystals, here we report the generation of femtosecond pulses tunable from 3 to 7 µm by intrapulse difference-frequency generation of 7.5 fs, 800 nm pulses. The efficiency and bandwidth dependences on pump polarization and crystal length are studied for both Type-I and Type-II phase-matching configurations. Maximum pulse energy of ∼ 10 n J is generated at 5.2 µm with a conversion efficiency of ∼ 0.14 % . Because of the few-cycle pump pulse duration, the generated mid-IR pulses are as short as about three cycles. These results, to the best of our knowledge, represent the first experimental demonstration of LGN in generating mid-IR ultrashort pulses.

Optical vortex is a promising candidate for capacity scaling in next-generation optical communications. The generation of multi-vortex beams is of great importance for vortex-based optical communications. Traditional approaches for generating multi-vortex beams are passive, unscalable and cumbersome. Here, we propose and demonstrate a multi-vortex laser, an active approach for creating multi-vortex beams directly at the source. By printing a specially-designed concentric-rings pattern on the cavity mirror, multi-vortex beams are generated directly from the laser. Spatially, the generated multi-vortex beams are decomposable and coaxial. Temporally, the multi-vortex beams can be simultaneously self-mode-locked, and each vortex component carries pulses with GHz-level repetition rate. Utilizing these distinct spatial-temporal characteristics, we demonstrate that the multi-vortex laser can be spatially and temporally encoded for data transmission, showing the potential of the developed multi-vortex laser in optical communications. The demonstrations may open up new perspectives for diverse applications enabled by the multi-vortex laser.

Optical parametric chirped-pulse amplification (OPCPA) provides an efficient route to push ultrafast pulses toward ultrahigh peak powers. However, in high average power regime, the unavoidable thermal effects in nonlinear crystals will intrinsically destroy the phase-matching (PM) condition, which fundamentally limit average power scaling. Here, we present a theoretical design of high average power OPCPA based on simultaneous temperature and wavelength insensitive PM. By regulating the temperature set for PM as well as the wavelength of interacting waves in lithium triborate crystal, the noncollinear angles for temperature insensitive PM and wavelength insensitive PM will coincide and thus ensures simultaneous temperature and wavelength insensitive PM condition. We investigate the performances of the proposed PM scheme in comparison with traditional wavelength insensitive noncollinear PM. Because of the larger temperature bandwidth, higher average power can be anticipated in OPCPA with the proposed PM scheme. Besides, the large spectral bandwidth also allows the amplification of sub-10 fs pulses in the visible spectral range. The proposed PM scheme will provide a promising approach in generating ultrashort pulses with high peak and average powers.

In strong‐field physics experiments with high‐intensity lasers, single‐shot characterization of the temporal contrast between the laser pulse peak and its temporal pedestal is important; this allows fast optimization of the pulse contrast and meaningful comparison with theory for each pulse shot. To date, high contrast ratios of 1010 have been demonstrated in single‐shot measurements for petawatt (PW) lasers. However, ultrahigh contrast ratios of ≈1013, as required for the planned 200 PW lasers, pose challenges to high‐intensity laser technologies and have thus far remained open for investigation. This article reports a pilot demonstration of ultrahigh‐contrast measurements by adapting a single‐shot cross‐correlator (SSCC). An evaluation method for the SSCC detection limit is introduced. The strategy mimics the test beam with known spatial contrast, whose cross‐correlation is equivalent to that of a test pulse with ultrahigh temporal contrast. The ultimate contrast measurement limit of 1013 is achieved, which corresponds to the highest pulse intensity by optical damage and the lowest temporal pedestal by single‐photon detection. The photon noise of the detector is observed and becomes dominant as the temporal pedestal of the optical pulse decreases. The demonstrated detection ability is applied to a high‐contrast laser system, suggesting the accessibility of ultrahigh‐contrast measurements.

Both femtosecond and dual-wavelength picosecond operations of Nd,La:SrF2 disordered crystal laser have been demonstrated for the first time. In femtosecond operation, the laser generated mode-locked pulses with pulse duration of 156 fs, which is an average power of 90 mW, and a repetition rate of 108 MHz at the central wavelength of 1057 nm. In dual-wavelength picosecond operation, the laser generated synchronously mode-locked pulses with an average power of 500 mW and pulse duration of 9.9 ps. The mode-locked spectrum clearly shows two peaks at 1050.8 and 1059.0 nm, respectively. A periodic beat frequency of 2.2 THz is observed from the intensity autocorrelation trace, indicating the synchronous dual-wavelength mode-locking. Our experimental results show that Nd,La:SrF2 disordered crystal is an excellent laser material for femtosecond and dual-wavelength lasers near 1 μm wavelength.

Back conversion is an intrinsic phenomenon in nonlinear frequency down-conversion processes. However, the physical reason for its occurrence is not well understood. Here, we theoretically reveal that back conversion is the result of a π-phase jump associated with the depletion of one interacting wave. By suppressing the idler phase jump through a deliberate crystal absorption, the back conversion can be inhabited, thus enhancing the conversion efficiency from the pump to the signal. The results presented in this Letter will further the understanding of nonlinear parametric processes and pave the way toward the design of highly efficient down-conversion systems.

Radially polarized lasers, in contrast to the conventional Gaussian laser mode, possess unique features such as sharp focusing and strong longitudinal fields. Thus far, radially polarized femtosecond pulses have been produced only by low-power devices such as mode-locked resonators and segmented half-wave plates. It is imperative to solve the bottleneck problem in generating higher powers and shorter durations. This paper reports on a polarization-insensitive, high-gain optical parametric amplifier for radially polarized femtosecond pulses, which works at type-II phase-matching and approximately degenerate wavelength. We experimentally demonstrate > 1000 -fold amplification of radially polarized ∼ 400 f s pulses at 1610 nm, via chirped ∼ 280 f s pumping at 800 nm, with the axially symmetric intensity profile and radial polarization state both being well-maintained within a 20 nm spectral range. With currently available high-energy picosecond pumping, the demonstrated amplification scheme will be promising to create radially polarized femtosecond pulses with ultrahigh powers and may facilitate future applications such as strong-field physics.

Resonant cascaded nonlinearity (RCN) induced by optical parametric amplification (OPA) in a chirped quasi-phase-matching chip can be applied to control the group velocity of ultrafast lasers. However, the group delay produced in a single-stage OPA is limited to the pulse duration, and its sign cannot be altered. In this study, we propose a tandem RCN configuration with multiple OPA stages that can produce large-magnitude and sign-controllable group delays. The group delay produced in the multi-stage configuration is shown to be a linear superposition of each single-stage group delay. By virtue of the byproduct idler in the OPA process, the signal-group delay can be altered from positive to negative (and vice versa) with the same chip structure and pump condition. In the numerical simulation with two OPA stages, both a positive and negative group delay of six-fold pulse duration were achieved for 100-fs pulses at 1550 nm. A much larger group delay can be achieved by increasing the number of OPA stages.

The coherent noise inherent to optical parametric chirped-pulse amplification (OPCPA) poses a fundamental limitation on the temporal quality of current ultrashort ultraintense lasers as well as their applications in strong-field physics research. Here we present a peer comparison of coherent noise between OPCPA and one of its alternatives — quasi-parametric chirped-pulse amplification (QPCPA). In the initial amplification stage termed as small-signal regime, the spectrum of chirped signal pulse in either OPCPA or QPCPA is exposed to the imperfections in pump pulse and optics, resulting in coherent noise pedestals in the time domain. While in the final amplification stage featured by amplifier saturation, the nonlinear growths of coherent noise become sharply different. We demonstrate that the non-parametric saturation behavior of QPCPA can greatly smooth the spectral distortions of signal, and thus bring down the coherent noise. Consequently, QPCPA scheme allows an improvement of pulse contrast by 2–3 orders of magnitude. The results presented in this paper provide a new perspective on high-contrast amplification of ultrashort pulses.

Mid-infrared femtosecond vortex beams generated by optical parametric oscillators (OPO) are reported for the first time, to the best of our knowledge. Order-tunable femtosecond Hermite–Gauss beams from the first to sixth order are produced from a synchronously pumped OPO and then converted into the corresponding first through sixth-order femtosecond vortex beams by a cylindrical lens mode converter. By slightly tuning the cavity length, the wavelength of the vortex beam can be continuously tunable in the range from 2323 to 2382 nm, and the pulse duration can be changeable from ∼ 400 f s to ∼ 1.1 p s . The work provides a flexible and reliable way to generate mid-infrared femtosecond vortex beams, and is of special significance for expanding the wavelength range of femtosecond vortex beams and their application fields.

We demonstrated a femtosecond mode-locked Er:ZrF4-BaF2-LaF3-AlF3-NaF (Er:ZBLAN) fiber laser at 2.8 μm based on the nonlinear polarization rotation technique. The laser generated an average output power of 317 mW with a repetition rate of 107 MHz and pulse duration as short as 131 fs. To the best of our knowledge, this is the shortest pulse generated directly from a mid-infrared mode-locked Er:ZBLAN fiber laser to date. Numerical simulation and experimental results confirm that reducing the gain fiber length is an effective way to shorten the mode-locked pulse duration in the Er:ZBLAN fiber laser. The work takes an important step towards sub-100-fs mid-infrared pulse generation from mode-locked Er:ZBLAN fiber lasers.

Frequency-domain optical parametric amplification (FOPA) is a new scheme that enables extremely broadband amplification of ultraintense pulses. The spatiotemporal coupling property of signal pulses can make the coherent noise of FOPA sharply different from that of conventional OPCPA. This paper presents a first theoretical study on the coherent noise produced in a FOPA system. We reveal that the coherent noise acquires the spatiotemporal coupling, and thus distinguishes the compressed signal pulse not only in time but also in space, which allows the suppression of coherent noise via optical manipulations in the spatial domain. The quantitative impacts of spatiotemporal coherent noise originated from the imperfections in either pump laser or crystal surfaces, are numerically studied. The result provides a new perspective on improving the coherent contrast of ultraintense lasers.

Pulse contrast is a crucial parameter of high peak-power lasers since the prepulse noise may disturb laser–plasma interactions. Contrast measurement is thus a prerequisite to tackle the contrast challenge in high peak-power lasers. This paper presents the progress review of single-shot cross-correlator (SSCC) for real-time contrast characterization. We begin with the key technologies that enable an SSCC to simultaneously possess high dynamic range ($10^{10}$), large temporal window (50–70 ps) and high fidelity. We also summarize the instrumentation of SSCC prototypes and their applications on five sets of petawatt laser facilities in China. Finally, we discuss how to extend contrast measurements from time domain to spatiotemporal domain. Real-time and high-dynamic-range contrast measurements, provided by SSCC, can not only characterize various complex noises in high peak-power lasers but also guide the system optimization.

There is a growing interest in developing ultrafast laser sources around 2-μm wavelength range. Here, we experimentally demonstrate a diode-pumped passively mode-locked Tm:YAG ceramic laser using graphene as a mode locker. By transferring graphene synthesized by chemical vapor deposition onto Au-coated mirror as saturable absorber, continuous wave mode locking is successfully obtained in the Tm:YAG ceramic laser. The generated mode-locked pulses in 2-μm spectral region have a pulse width as short as 2.8 ps, with a maximum average output power of 158 mW and repetition rate of 98.7 MHz. The mode-locked mid-infrared pulses are very stable having a multitude of potential applications such as ultrafast molecule spectroscopy, mid-IR nonlinear frequency conversion, etc.

The pulse contrast of state-of-the-art petawatt lasers is limited by coherent noise. This paper reports on a new family of noise, termed nonlinear beat noise, which is generated by the nonlinear mixing of two kinds of coherent noise in optical parametric chirped-pulse amplification (OPCPA). We theoretically study the various nonlinear beat noises and reveal their intensity evolutions in an OPCPA amplifier. The results suggest that nonlinear beat noise will be destructive to the future hundred-petawatt lasers.

Quasi-parametric chirped pulse amplification (QPCPA) is a new scheme that enables the amplification of chirped signal pulses without back conversion by depleting the idler pulses. In this paper, we present a numerical study on the bandwidth, efficiency, and robustness of QPCPA. Self-locked phase among the interacting waves is found to be the underlying mechanism for the suppression of back conversion, which allows signal efficiency approaching to the quantum limit even under the phase-mismatch condition, and thus greatly increases the phase-mismatch tolerance of QPCPA. We demonstrate that QPCPA can break through the trade-off between the efficiency and bandwidth encountered in conventional optical parametric amplification, hence supporting highly efficient amplification of few-cycle pulses.

We experimentally demonstrated a diode-pumped sub-50 fs Yb,Na:CNGG disordered crystal laser. Pumped by a 980 nm distributed Bragg-reflector tapered diode laser and passively mode-locked with a semiconductor saturable absorber mirror (SESAM), soliton pulses as short as 62 fs and 45 fs were obtained without and with external compression, respectively. The ultrashort pulses had a repetition rate of ~104 MHz at the central wavelength of 1061 nm. To the best of our knowledge, this is the first demonstration of sub-50 fs pulses from the Yb3+-doped CNGG type disordered crystal lasers.

Optical parametric phase (OPP), inherently induced in optical parametric chirped-pulse amplification (OPCPA), will significantly degrade pulse compression in the few-cycle pulse regime. While OPP is a well-known nonlinear phenomenon, its nonlinearity origin has remained unclear in physics. Here, we present a systematic theoretical investigation on OPP and demonstrate that OPP originates from the cascaded quadratic nonlinearity in a broadband OPCPA with intrinsic phase-mismatch. Because the intrinsic phase-mismatch is frequency dependent, OPP manifests itself as a nonlinear spectral phase. We show that OPP increases with the parametric gain, approaches to half the intrinsic phase-mismatch in the high-gain limit, and thus directly links with the crystal dispersion. We also find that the effect of OPP in the spatial domain is typically negligible, which makes the OPP compensation necessary only in the spectral domain. The results presented in this paper are of importance to guide the design of few-cycle intense OPCPA systems.

Separating lights into different paths according to the polarization states while keeping their respective path’s polarizations with high purification is keen for polarization multiplex in optical communications. Metallic nanowire gratings with multi-slits in a period are proposed to achieve polarized beam splitters (PBSs) in reflection and diffraction. The setting of multi-slits largely reduces the reflection of photons with a transverse magnetific field via the plasmonic waveguiding effect, which leads to highly polarized output lights with extinction ratio larger than 20 dB in each channel. The proposed reflection/diffraction PBSs enrich the approaches to control the polarization states with the advantages of wide incident angles and flexible beam splitting angles.

We studied the optical scattering noise, measurement artifacts, and their suppression in single-shot measurements of prepulse contrast. The measurement noise due to air scattering and the two kinds of measurement artifacts resulting from multiple reflections of the generated correlation signal and correlation between the pulse under test and the internally reflected sampling pulse were experimentally explored. The scattering noise and measurement artifacts significantly degrade the measurement fidelity. We demonstrate the clean measurement of prepulse contrast in a single-shot cross-correlator by attenuating the main peak of the correlation beam with a dot mirror and designing the correlating process based on a periodically-poled lithium niobate crystal embedded in an unpoled wafer. The standard time-scanning measurements of pulse contrast are also performed to confirm the experimental results of our single-shot measurement.

Optical parametric chirped-pulse amplification (OPCPA) has been demonstrated to be a promising approach for pushing femtosecond pulses towards ultra-high peak powers. However, the future success of OPCPA strongly relies on the ability to manipulate its phase-matching (PM) configuration. When a high average power pump laser is involved, the thermal effects in nonlinear crystals induce phase-mismatch distortions that pose an inherent limitation on the conversion efficiency. Here, we demonstrate that the noncollinear configuration previously adopted for wavelength-insensitive PM can be employed for temperature-insensitive PM when the noncollinear angle is properly reset. Simultaneous temperature- and wavelength-insensitive PM is realized for the first time by imposing such a temperature-insensitive noncollinear configuration with an angularly dispersed seed signal. Based on the lithium triborate crystal, the proposed noncollinear achromatic PM has a thermal acceptance 6 times larger than that of the conventional wavelength-insensitive noncollinear PM and has a sufficient spectral acceptance to support pulse durations of ~20 fs at 800 nm. These achievements open new possibilities for generating ultra-high peak power lasers with high average power.

A two-stage 2.2 μm OPCPA system is built with amplified chirped-pulse energy of 5 J. After compression, 1.8 J, 82 fs mid-IR pulses will be potentially obtained, providing an important driving source for strong-field physics. Article not available.

We investigate pulse cleaning behaviors in short-pulse-pumped optical parametric amplifiers (OPA). We theoretically study the contrast enhancement of amplified signal pulse and generated idler pulse, and reveal their dependence on the parametric gain in both the regimes of small signal and saturated amplifications. The signal contrast enhancement is nearly equal to the parametric gain, while the idler contrast is approximately equal to the product of the contrasts of the pump and signal pulses in a low gain OPA and increases with the gain. The effects of group-velocity mismatch and group-velocity dispersion on the contrast enhancement are also investigated. The results presented in this paper are of value for pulse cleaning.

We propose and experimentally demonstrate a novel second-harmonic-generation (SHG) scheme capable of supporting temperature-insensitive phase-matching (PM) at various laser wavelengths. This versatile temperature-insensitive PM is designed by using two cascaded crystals with opposite signs of temperature derivation of phase-mismatch, in which the temperature-induced phase-mismatch in the first crystal is well compensated in the second crystal. Two application examples are studied at the typical wavelengths of ~ 1.0 and 1.5 μm, respectively. The proof-of-principle experiment, using two crystals of LiB3O5 (LBO) and KH2PO4 (KDP), demonstrates that the temperature-acceptance of PM can be 2 – 3 times larger than that of using a traditional single crystal. The proposed two-crystal design may provide a promising route to high-average-power SHG at various laser wavelengths.

In this paper, precise synchronization of the femtosecond and picosecond laser regenerative amplifiers with different wavelengths and independent seed oscillators was achieved using the electronic phase-locked loop and global clock techniques. The root-mean-square relative timing jitter of the two regenerative amplifiers was measured as 0.66 ps using a modified statistical method based on the error propagation relation between the independent variables and the conventional optical cross-correlation technique. The results suggest that this method is more accurate and requires simple optical setups for low-pulse repetition rate lasers.

Slow and fast light are ubiquitous in optical amplifiers. In this Letter, we show for the first time, to the best of our knowledge, that optical parametric amplification (OPA) in chirped quasi-phase-matching structures can act as a platform for group-velocity control in the femtosecond regime. The resonant cascaded nonlinear phase underlies the group-velocity control, which manifests an unusual effect that both slow and fast light can be achieved under the normal condition of signal amplification. As numerically demonstrated in the OPA based on the lithium niobate crystal, the signal and idler pulse can be significantly delayed in time comparable to the signal duration and can also keep high fidelity for durations down to 100 fs until the crystal dispersion becomes effective. The broad bandwidth, large group delay, and direct compatibility with integrated optics will make the proposed platform attractive to both fundamental research and applied science.

In this Letter, we study short-wavelength conical refraction (CR) via sum-frequency generation (SFG) in the femtosecond regime, a previously unaddressed topic. Based on biaxial crystal of KGd(WO4)2 whose dispersion of optical-axis orientation is negligible in near-IR, conventional femtosecond lasers at 800 and 1054 nm are transformed into CR beams, respectively. Femtosecond CR beams at 454 nm are generated via SFG with the near-IR CR beams. While the generated sum-frequency ring is typically incomplete, a full-ring distribution can be achieved by adopting Type-II SFG with a large phase mismatch. We find that the femtosecond sum-frequency ring under various phase-matching conditions evolves as typical CR beams.

Efficient frequency-tripling of phase-modulated broadband Nd:glass laser pulses is of interest to inertial confinement fusion. We report and theoretically study an efficient frequency-tripling scheme for 351 nm broadband pulses generation by use of broadband and narrowband Nd:glass lasers. Based on the conventional two-crystal doubling and tripling baseline configuration, employing an additional narrowband laser can increase the bandwidth acceptance of Nd:glass up to 320 GHz, which is 3.5 times larger than that of using the conventional configuration. The proposed scheme may also dramatically reduce the conversion of amplitude modulation from the frequency modulation. The efficiency sensitivities on both the fundamental intensity and the doubler orientation are discussed for practical applications.

We have experimentally demonstrated a diode-pumped passively mode-locked Nd:CLNGG-CNGG hybrid laser for the first time. By employing Nd:CLNGG and Nd:CNGG disordered crystals together as laser gain media, the mode-locked laser could generate much shorter pulses. With a semiconductor saturable absorber mirror (SESAM) as a mode locker, the laser emitted stable mode-locked pulses with a pulse width of 534 fs, a repetition rate of 88 MHz, and an average output power of 60 mW. To our best knowledge, this is the shortest pulse generated from SESAM mode-locked Nd-doped crystal lasers.

We have experimentally demonstrated a diode-pumped passively mode-locked Nd:CaNb2O6 laser for the first time to our best knowledge. With a semiconductor saturable absorber mirror (SESAM) as a passive mode locker, the laser generated stable mode-locked pulses with pulse duration of 17.3 ps and repetition rate of 88.4 MHz. With a singe-emitter laser diode pumping, the maximum average output power of the mode-locked laser was 0.843 W, with a slope efficiency of 23%. The experimental results show the Nd:CaNb2O6 crystal is a promising laser gain medium for picosecond pulse generation.

With the proposal of dual-wavelength pumping (DWP) scheme, DWP Er:ZBLAN fiber lasers at 3.5 μm have become a fascinating area of research. However, limited by the absence of suitable saturable absorber, passively Q-switched and mode-locked fiber lasers have not been realized in this spectral region. Based on the layer-dependent bandgap and excellent photoelectric characteristics of black phosphorus (BP), BP is a promising candidate for saturable absorber near 3.5 μm. Here, we fabricated a 3.5-μm saturable absorber mirror (SAM) by transferring BP flakes onto a Au-coated mirror. With the as-prepared BP SAM, we realized Q-switching and mode-locking operations in the DWP Er:ZBLAN fiber lasers at 3.5 μm. To the best of our knowledge, it is the first time to achieve passively Q-switched and mode-locked pulses in 3.5 μm spectral region. The research results will not only promote the development of 3.5-μm pulsed fiber lasers but also open the photonics application of two-dimensional materials in this spectral region.

Second-harmonic generation (SHG) provides a promising route for generating vortex beams of both short wavelength and large topological charge. Here we theoretically investigate the efficiency optimization and beam characteristics of focused vortex-beam SHG. Owing to the increasing beam divergence, vortex beams have distinct features in SHG optimization compared with a Gaussian beam. We show that, under the noncritical phase-matching condition, the Boyd and Kleinman prediction of the optimal focusing parameter for Gaussian-beam SHG remains valid for vortex-beam SHG. However, under the critical phase-matching condition, which is sensitive to the beam divergence, the Boyd and Kleinman prediction is no longer valid. In contrast, the optimal focusing parameter for maximizing the SHG efficiency strongly depends on the vortex order. We also investigate the effects of focusing and phase-matching conditions on the second-harmonic beam characteristics.

Quasi-parametric chirped-pulse amplification (QPCPA) can improve the signal amplification efficiency and stability by inhibiting the back-conversion, in which the idler absorption plays a critical role. This Letter theoretically studies the impacts of idler absorption on the QPCPA performance in both the small-signal and saturation regimes. We demonstrate that there exists an optimal idler absorption that enables the achievement of maximum pump depletion within a minimum crystal length. To overcome the reduction in small-signal gain induced by idler absorption, the configuration of gradient idler absorption is proposed and demonstrated as a superior alternative to constant idler absorption. The results provide guidelines to the design of state-of-the-art QPCPA lasers.

High-power ultrafast fiber lasers operating at the 2 μm wavelength are extremely desirable for material processing, laser surgery, and nonlinear optics. Here we fabricated large-core (LC) double-cladding Tm-doped silica fiber via the sol-gel method. The sol-gel-fabricated Tm-doped silica (SGTS) fiber had a large core diameter of 30 μm with a high refractive index homogeneity (Δn=2×10−4). With the newly developed LC SGTS fiber as the gain fiber, high-power mode-locking was realized. By using a semiconductor saturable absorber mirror (SESAM) as a mode locker, the LC SGTS fiber oscillator generated mode-locked pulses with an average output power as high as 1.0 W and a pulse duration of 23.9 ps at the wavelength of 1955.0 nm. Our research results show that the self-developed LC Tm-doped silica fiber via the sol-gel method is a promising gain fiber for generating high-power ultrafast lasers in the 2 μm spectral region.

Combining the configurations of collinear and small-angle noncollinear phase-matching, a mid-infrared (MIR) femtosecond optical parametric amplifier (OPA) with extended tunability is proposed and demonstrated. A tuning range from 2.0 µm to 4.7 µm is demonstrated by a two-stage LiNbO3-based femtosecond OPA, which well exceeds the typical tuning range reported so far. MIR femtosecond pulses with energy up to 30 µJ are obtained.

Beam combining of fiber array with hexagonal ring distribution has been studied in detail. The theoretical analysis and numerical calculation results are given to illustrate the propagation properties of the resulting beam through free space. A comparison between the coherent and the incoherent case shows that high peak irradiance and good beam quality for coherent combining can be obtained in the far field. The effects of phase errors and beam quality M 2 factor are also studied. Results indicate that the element numbers should be increased to achieve high power and the space between adjacent elements should be reduced to maintain a good beam quality. This is basically the same for both coherent and incoherent beam combining.

We report on a compact high-power passively mode-locked Nd:LuVO4 laser by use of semiconductor saturable absorber mirror (SESAM). The laser generated clean CW mode-locked pulses with an average output power of 8.6 W, a repetition rate of 144 MHz, and a pulse duration of 13.2 ps. By employing a three-mirror cavity scheme, the laser setup has only a footprint size of 70×30 cm2, which can be packaged to a portable laser system for convenient applications.

Passively cw mode-locked laser operation of Yb-doped NaY(WO4)2 (Yb:NaYW) crystal was demonstrated with an average power of 400 mW. This is the highest power achieved from the mode-locked Yb:NaYW laser to our knowledge. The laser was end-pumped by a high power fiber pigtailed laser diode bar and the mode-locking was enabled by a semiconductor saturable absorber mirror (SESAM). The laser pulse duration was 1.1 ps, centered at the wavelength of 1019.7 nm with a bandwidth of 1.6 nm, and nearly transform-limited.

Cascaded nonlinearity based soliton pulse compression in the process of femtosecond difference frequency generation is studied theoretically. A set of simplified coupled wave equations under the conditions of large phase mismatch and matched group velocities is obtained, which reveals the physical mechanism of soliton compression in this process. Numerical simulations demonstrate that in the presence of group velocity dispersion and equivalent cross phase modulation, both the pump and the signal pulses can be compressed with a high compression ratio.

In this paper, we studied efficient second-harmonic generation (SHG) of femtosecond pulses in both phase- and group-velocity-matched structures. Obtained results show that phase matching becomes more critical under conditions required for high levels of conversion efficiency. And the imperfect phase mismatch caused by mismatched group-velocity dispersion (GVD) will limit conversion efficiency as well as bandwidth of generated second-harmonic (SH) pulses. The spectral characteristics of the generated SH pulses and its conversion efficiency in the strong pump regime are investigated in detail. The acceptance bandwidth of nonlinear crystal in the high-efficiency SHG is redefined in the paper, and the definition is much closer to the practical application of design.

We report a novel frequency tripler for efficient conversion of broadband high-power laser pulses at 1 µm. The deuteration level can be used as a degree of freedom to alter the phase-matching wavelength of a partially deuterated KDP crystal. It is shown that a segmented partially deuterated KDP crystal with discrete values of deuteration is capable of enhancing the acceptance bandwidth of frequency tripling. A five-segment design is presented, which is applicable to the efficient frequency tripling of uncompressed chirped pulses with a bandwidth of ~4 nm, and hence suitable for a Nd:glass petawatt laser system. The tolerances on the design parameters are also evaluated.

We show numerically that when a soliton circulating in the laser cavity experiences dynamical bifurcations, extra sets of sidebands appear in the soliton spectrum. The new sidebands have clearly different characteristics to those of the conventional soliton sidebands, indicating that there exists a new mechanism of sideband generation in the lasers.

We have experimentally studied the passive Q-switching performance of a Nd:YAG ceramic laser with GaAs saturable absorbers. It was found that, depending on the laser intensity in the wafer, the Q-switched pulses obtained could have very different pulse width and pulse energy. The shortest pulse obtained has 4.7 ns pulse width, 6.25 µJ pulse energy and 1.33 kW peak power. Furthermore we show that, by adding a slit in the cavity, the Q-switched pulse stability could be significantly improved.

In this paper, the multi-stage compression of picosecond pulses by cascaded quadratic nonlinearity is studied theoretically, and the dependence of pulse compression on phase-mismatch, laser intensity, and crystal characteristics has been discussed in detail. We demonstrate that the multi-stage pulse compression is much more efficient than the single-stage with a same total crystal length. Pulses as short as ∼150 fs can be generated by compressing 30-ps initial pulses in a two-stage configuration under the realistic crystal and laser conditions, and shorter pulses of ∼30 fs may be obtained by three-stage compression. Pulse compression performances with BiBO and BBO crystals are compared and discussed finally.

A mid-infrared saturable absorber mirror is successfully fabricated by transferring the mechanically exfoliated black phosphorus onto the gold-coated mirror. With the as-prepared black phosphorus saturable absorber mirror, a continuous-wave passively mode-locked Er:ZBLAN fiber laser is demonstrated at the wavelength of 2.8 μm, which delivers a maximum average output power of 613 mW, a repetition rate of 24 MHz, and a pulse duration of 42 ps. To the best of our knowledge, this is the first time a black phosphorus mode-locked laser at 2.8 μm wavelength has been demonstrated. Our results demonstrate the feasibility of black phosphorus flake as a new two-dimensional material for application in mid-infrared ultrafast photonics.

Stable 30 fs pulses centered at 1068 nm (less than 10 optical cycles) are demonstrated in a diode pumped Yb:CaYAlO4 laser by using high-quality chemical vapor deposited monolayer graphene as the saturable absorber. The mode locked 8.43 optical-cycle pulses have a spectral bandwidth of ~ 50 nm and a pulse repetition frequency of ~ 113.5 MHz. To our knowledge, this is the shortest pulse ever reported for graphene mode-locked lasers and mode-locked Yb-doped bulk lasers. Our experimental results demonstrate that graphene mode locking is a very promising practical technique to generate few-cycle optical pulses directly from a laser oscillator., 15 pages,6 figures

Black phosphorus, a newly emerged two-dimensional material, has attracted wide attention as novel photonic material. Here, multi-layer black phosphorus is successfully fabricated by liquid phase exfoliation method. By employing black phosphorus as saturable absorber, we demonstrate a passively Q-switched Er-doped ZBLAN fiber laser at the wavelength of 2.8 {\mu}m. The modulation depth and saturation fluence of the black phosphorus saturable absorber are measured to be 15% and 9 {\mu}J/cm2, respectively. The Q-switched fiber laser delivers a maximum average power of 485 mW with corresponding pulse energy of 7.7 {\mu}J and pulse width of 1.18 {\mu}s at repetition rate of 63 kHz. To the best of our knowledge, this is the first time to demonstrate that black phosphorus can realize Q-switching of 2.8-{\mu}m fiber laser. Our research results show that black phosphorus is a promising saturable absorber for mid-infrared pulsed lasers.

Passively mode-locked lasers provide a convenient way to generate ultrafast laser pulses with high peak power. In this report, we gave introduction of progress of passively mode-locked mid-infrared solid-state lasers at 2 μm region in our laboratory.