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We theoretically and experimentally demonstrate a novel mode-locked ytterbium-doped fiber laser with a saturable absorber based on nonlinear Kerr beam cleanup effect. The saturable absorber was formed by a 2-m graded-index multimode fiber and a single-mode fiber segment served as a diaphragm. With an all-normal-dispersion-fiber configuration, the laser generated dissipative soliton pulses with pulse duration of 26.38 ps and pulse energy more than 0.25 nJ; output pulses could be compressed externally to 615.7 fs. Moreover, the self-starting mode-locking operation of this laser exhibited a high stability with a measured signal-to-noise ratio of 73.4 dB in the RF spectrum., 9 pages, 5 figures

In advanced biomedicine and microfluidics, there is a strong desire to sort and manipulate various cells and bacteria based on miniaturized microfluidic chips. Here, by integrating fiber tweezers into a T-type microfluidic channel, we report an optofluidic chip to selectively trap Escherichia coli in human blood solution based on different sizes and shapes. Furthermore, we simulate the trapping and pushing regions of other cells and bacteria, including rod-shaped bacteria, sphere-shaped bacteria, and cancer cells based on finite-difference analysis. With the advantages of controllability, low optical power, and compact construction, the strategy may be possibly applied in the fields of optical separation, cell transportation, and water quality analysis.

A continuous-wave singly resonant optical parametric oscillator employing a volume Bragg grating as one of the end mirrors is demonstrated. Due to the high intracavity power, the minute absorption of the grating leads to the thermal induced surface deformation and reflection peak shift. In this letter, the former effect is confirmed to be a cause of beam quality degradation of the resonant signal, while the latter, together with the thermal effects of the nonlinear crystal, leads to unconventional behaviors of power and stability. To the best of our knowledge, the joint effects of the grating and the nonlinear crystal are studied for the first time in details by changing the thermal effects of grating in varied strengths.

We experimentally investigate characteristics of saturable absorbers (SAs) based on nonlinear Kerr beam cleanup effect (NL-KBC). The SAs are formed by a long graded-index multimode fiber (GRIN MMF) with a short single-mode fiber served as a diaphragm. We studied the evolution of output spectrum and beam profiles from the GRIN MMF in order to investigate the mechanism of these SAs. We further performed saturable absorption measurements to evaluate their modulation depths and saturation intensities. We experimentally observed and first theoretically analyzed the “relaxation oscillation” behavior of the optical transmittance with increasing input intensity. We also studied their nonlinear polarization dynamics and observed the repolarized effect in NL-KBC regime. Our results confirm the optical properties of the SAs based on NL-KBC, and these SAs can find applications in Q-switched and mode-locked lasers.

We present an efficient method for polarized 963 nm emission based on second-harmonic generation from a high-power, polarized thulium-doped fiber laser using periodically poled MgO:LiNbO3 as the nonlinear material. The maximum 963-nm output power of 710 mW has been achieved with a pulse duration of 34 ns at the repetition rate of 100 kHz and a Gaussian spatial beam profile. The frequency doubling efficiency is up to 56%, and the total optic-to-optic conversion efficiency is more than 12.9%. In addition, third- and fourth-harmonic generations have been observed.

We present a compact efficient intracavity MgO:PPLN optical parametric oscillator (OPO) with a volume Bragg grating output coupler pumped by a Q-switched Nd:YVO4 laser operating in 2.1- $\mu $ m region. Narrowband 2.1- $\mu $ m output power of 3.7 W is achieved with a pulse duration $\mu $ m is obtained at 17.5-W LD pump power. The 2.1- $\mu $ m beam quality factors $M^{2}$ of 1.85 and 2.37 in horizontal and vertical directions are obtained. To the best of our knowledge, the efficiency is the highest ever achieved from an intracavity OPO in 2- $\mu $ m region.

We report a narrow-bandwidth near-degenerate MgO:PPLN optical parametric oscillator (OPO) based on a volume Bragg grating (VBG) output coupler (OC) intracavity pumped by a linearly polarized Q-switched Nd:YAG laser. Maximum 2.1 μm output power of 7.1 W with beam quality factor M2 of 2.0 and 2.3 in horizontal and vertical directions is achieved. 23.5 W of 1 μm radiation with fundamental mode is obtained simultaneously.

We develop a two-stage end-pumped Nd:YVO4 amplifier seeded by a passively Q-switched microchip laser. An average output power of 13.5 W with repetition rate up to 7 kHz and pulse duration of ∼1.24 ns is obtained, corresponding to a pump extraction efficiency of 16.1% (19.5% for the second stage) and peak power of ∼1.5 MW. The beam quality factors at maximum output power are measured to be M x 2 = 1.56 and M y 2 = 1.48 . We introduce an analytical model to estimate gain and beam quality after amplification. This model focuses on the influence of ratio of seed spot radius to pump spot radius when designing an amplifier. Moreover, our experiments reveal that the re-imaging system in the double-pass configuration can be used to enhance the beam quality.

Based on the high-quality polycrystalline ZnGeP 2 (ZGP) synthesized by the optimized two-temperature-zone method, ZnGeP 2 single crystals with dimensions of 16 mm diameter and 70 mm length were successfully grown using the modified vertical Bridgman method. The etch pit density (EPD) of as-growth crystals can be reduced to a level of 510 cm −2 ; the full width at half maximum (FWHM) of the X-ray rocking curve of the crystals is 20″; the transmittance of the annealed and polished wafers (1.7 mm thick) can reach about 57% in the wavelength range 3–8 µm and the optical absorption coefficient is about 0.1 cm −1 at 2.1 µm; using KTP optical parametric oscillator (OPO) pumping source at 2.1 µm, the output power of ZGP OPO is 2.8 W at 3–5 µm. These results suggest that this modified vertical Bridgman method can grow high structural and optical quality ZGP single crystals for OPOs.

We demonstrate an effective and simplified theoretical model of the in-band-pumped gain-switched thulium-doped fiber laser in which the effective pump power and effective population density are defined as the approximation of spatial variables. The numerical simulation results based on the model are in good agreement with the experimental data. The parameters of the pump source and the cavity configuration are considered in our numerical model for the optimization of the laser system. The pump condition in which the pulse distortion can be avoided is presented.

A high-energy, azimuthally polarized (AP) and actively Q-switched Nd:YAG laser is demonstrated. The thermal bipolar lensing effect in the Nd:YAG laser rod is used as a polarization discriminator, and a c-cut YVO4 crystal is inserted into the laser cavity to increase the mode-selecting ability of the cavity for AP mode. The laser generated AP pulses with maximum pulse energy as high as 4.2 mJ. To the best of our knowledge, this is the highest pulse energy obtained from an actively Q-switched AP laser. The pulse energy remained higher than 1 mJ over a wide range of repetition rates from 5 kHz to 25 kHz.

The effects of cavity-dispersion noncoaxiality (CDN) on the generation of ultrabroadband femtosecond pulses in KLM Ti:sapphire laser were investigated theoretically and experimentally. It was predicted that when the laser sub-cavity works near the coaxial operation point, the limitation of CDN on the bandwidth broadening is minimum, which is favorable for ultrabroadband pulse generation. On the basis of this prediction, femtosecond pulses with bandwidth of 650 to 1000 nm were directly generated from a home built KLM Ti:sapphire laser. To our knowledge, they are the broadest bandwidth pulses produced from KLM Ti:sapphire laser with similar oscillator configuration and gain crystal length of 3 mm.

A high average-power 2 μm laser with good beam quality based on an intracavity potassium titanium oxide phosphate (KTP) optical parametric oscillator (OPO) is demonstrated. A concave lens is used in the 1064 nm Nd:YAG pumped laser cavity to compensate for the thermal lensing of the laser rod. The cavity length of the KTP OPO is enlarged to improve the 2 μm beam quality. The maximum average output of the 2 μm laser is up to 18 W at 7 kHz with M 2 less than 6 and pulse width of 70 ns. The FWHM of the signal and idle lights are both less than 3 nm.

An elliptically polarized absorption quantum-beat spectroscopy is developed and used to observe electron-spin coherence relaxation in an intrinsic bulk GaAs in Voigt geometry. The temperature dependence of electron-spin coherence lifetime is first obtained.

We present an all-fiberized polarized mode-locked thulium-doped fibre laser system operating at the 2 μm region. Passively mode-locking by a semiconductor saturable absorber mirror, the laser generated 20.2 ps pulses at a repetition rate of 10.22 MHz. The maximum average output power of 56 mW with pulse energy of 5.48 nJ was achieved, and the output beam was more than 99.6% linearly polarized. To the best of our knowledge, it is the first mode-locked polarized thulium-doped fibre laser with all-fiberized configuration.

We demonstrate a high-efficiency near-degenerate periodically poled MgO:LiNbO(3) (PPMgLN) optical parametric oscillator (OPO) using a volume Bragg grating (VBG) output coupler (OC) pumped by a multilongitudinal Q-switched Nd:YVO(4) laser at 20 kHz repetition rate. A total parametric power of 4.3 W with a conversion efficiency of 60% is achieved in a double-pass pump configuration. The output power improvement over the case of a single-pass pump is nearly 60%. Both the signal and the idler bandwidths are less than 40 GHz and are confined within 170 GHz bandwidth at 2128.8 nm. Such efficiency is, to our knowledge, the highest ever achieved from a degenerate OPO using a VBG OC.

An intracavity quasi-phase-matched optical parametric oscillator (OPO) has been developed for the purpose of generating radiation with high average power and high repetition rate in the 2 μm regime. The device is a degenerate OPO based on a 3 mm thick MgO-doped periodically poled LiNbO(3) (PPMgLN) crystal, which is pumped in turn within the cavity by a diode side-pumped, Q-switched 1 μm Nd:YAG laser operating at 10 kHz. Up to 20 W broadband 2 μm radiation can be generated with a compact configuration under the crystal temperature of 115 °C. The beam profile is close to circularly symmetric with M(2) ~ 10.

The laser oscillation and polarization behavior of a side-pumped Nd:YAG laser are studied theoretically and experimentally by a thermal model for a working cavity. We use this model along with the Magni method, which gives a new stability diagram, to show important characteristics of the resonator. High-power radially and azimuthally polarized laser beams are obtained with a Nd:YAG module in a plano-plano cavity. Special regions and thermal hysteresis loops are observed in the experiments, which are concordant with the theoretical predictions.

We demonstrate a modified zero-additional-phase spectral phase interferometry for direct electric-field reconstruction setup, where two quasi-monochromatic auxiliary pulses employed for sum-frequency generation are derived from a 4f zero-dispersion compressor. With adequate flexibility and convenience to change the spectral shear and the frequencies of the auxiliary pulses, this device can cover a wide variety of ultrashort pulses with diverse central wavelengths, bandwidths, and durations. We show its applicability by measuring different pulses from three femtosecond sources.

The cavity dispersion noncoaxiality (CDN) effects on broadband few-cycle pulse generation of a Kerr-lens mode-locked Ti:sapphire laser is investigated theoretically and experimentally. It is found that the influence of CDN is comparable with that of self-focusing and self-phase-modulation in the frequency-dependent mode size (FDMS) effects. Spectra extending from 680 nm to 1020 nm with pulse duration shorter than three optical cycles are favorably generated under the minimum CDN in the vicinity of the coaxial point of the sub-cavity.

Circularly polarized absorption quantum-beat spectroscopy is used to study excess-energy dependence of electron-spin coherence dynamics in intrinsic GaAs. Absorption quantum beats are observed. The quantum-beat oscillatory frequency is used as a high precision measure of electron g factor. Double linear energy dependence of g factor is obtained and disagrees with the prediction of k∙p theory. A theoretical calculation reveals the double linear energy dependence reflects the energy dependence of g factor of electrons measured through light-hole- and heavy-hole-electron transitions, respectively. The phase of the quantum beats provides the key information to distinguish the two transitions experimentally.

Temperature dependence of electron-spin coherence dynamics is investigated for an intrinsic bulk GaAs in the Voigt geometry using the elliptically polarized absorption quantum-beat spectroscopy. Temperature dependences of spin coherence and recombination lifetimes as well as g factor of electrons are reported over a temperature range from 8.1to260K. The temperature dependence of spin coherence lifetime (T2*) agrees well with a reported theoretical calculation and can be fitted well by a relationship T2*∼T−1∕2, which provides an evidence to support electron-spin decoherence dominated by the Bir-Aronov-Pikus mechanism. The temperature dependence of g factor also agrees well with reported results.

The elliptically polarized absorption spectroscopy is further developed to observe electron spin coherence dynamics in an intrinsic bulk GaAs in Voigt geometry. Absorption saturation quantum beats are observed and explained as spin-precession-dependent state filling. An analytical model is developed quantitatively to describe the quantum beats and used to fit experimental data to retrieve spin coherence lifetime. Comparing to the reported data reveals that the spin coherence dynamics of the real and imaginary of a complex refractive index is much different.