Figure-eight actively-passively mode-locked erbium-doped fiber laser

The advantages of using nonlinear optical loop mirror (NOLM) to compress pulse with slight amplitude fluctuation and reflected energy loss are analyzed in theory. Experimentally the NOLM is placed in an actively mode-locked erbium-doped fiber ring laser to form a figure-eight actively and passively mode-locked fiber laser. 12 ps mode-locked pulses centered at 1.543 μm were obtained with the modulation frequency of 2.498748700 GHz. 3.715 mW output power is achieved with 50 mW pump power.     

Tunable femtosecond near-infrared source based on a Yb:LYSO-laser-pumped optical parametric oscillator

We demonstrate a widely tunable near-infrared source from 767 nm to 874 nm generated by the intracavity second harmonic generation(SHG) in an optical parametric oscillator pumped by a Yb:LYSO solid-state laser. The home-made Yb:LYSO oscillator centered at 1035 nm delivers an average power of 2 W and a pulse duration as short as 351 fs. Two Mg O doped periodically poled lithium niobates(Mg O:PPLN) with grating periods of 28.5–31.5 μm in steps of 0.5 μm and19.5–21.3 μm in steps of 0.2 μm are used for the OPO and intracavity SHG, respectively. The maximum average output power of 180 m W at 798 nm was obtained and the output pulses have pulse duration of 313 fs at 792 nm if a sech2-pulse shape was assumed. In addition, tunable signal femtosecond pulses from 1428 nm to 1763 nm are also realized with the maximum average power of 355 m W at 1628 nm.     

Noise-like rectangular pulses in a mode-locked double-clad Er:Yb laser with a record pulse energy

Generation of noise-like rectangular pulse was investigated systematically in an Er–Yb co-doped fiber laser based on an intra-cavity coupler with different coupling ratios. When the coupling ratio was 5/95, stable mode-locked pulses could be obtained with the pulse packet duration tunable from 4.86 ns to 80 ns. The repetition frequency was 1.186 MHz with the output spectrum centered at 1.6 μm. The average output power and single pulse energy reached a record 1.43 W and1.21 μJ, respectively. Pulse characteristics under different coupling ratios(5/95, 10/90, 20/80, 30/70, 40/60) were also presented and discussed.     

Fabrication of 16W all-normal-dispersion mode-locked Yb-doped rod-type fiber laser with large-mode area

A mode-locked ytterbium-doped rod-type fiber laser with 85 μm core diameter is developed based on the nonlinear polarization evolution in an all-normal-dispersion ring cavity, in which a uniaxial birefringent plate is used as the spectral filter. Average power up to 16 W is obtained at the repetition rate of 58 MHz, and the pulse duration is compressed to 182 fs with a grating-pair compressor. The output laser pulses show very good beam quality and power stability.     

Flash-lamp-pumped picosecond Nd:YAG regenerative amplifier

A flasli-lamp-pumped Nd:YAG regenerative amplifier has been developed at 1.06 μm, seeded with 10-ps pulses from a diode-end-pumped and mode-locked Nd:YAG oscillator with homemade semiconductor saturable absorber mirror (SESAM). The pulse energy was amplified to 2 mJ by the regenerative amplifier at 10-Hz repetition rate. In two-stages amplifier the regenerative amplified pulse energy was amplified to 100 mJ, and 35-mJ double frequency at 532 nm was obtained by extra-cavity double frequency with a KTP crystal.     

High repetition rate picosecond LiNbO_3 THz parametric amplifier and parametric gain

A high repetition rate, picosecond terahertz(THz) parametric amplifier with a LiNbO_3(LN) crystal has been demonstrated in this work. At a 10 k Hz repetition rate, a peak power of 200 W and an average power of 12 μW have been obtained over a wide range of around 2 THz; at a 100 k Hz repetition rate, a maximum peak power of 18 W and an average power of 10.8 μW have been obtained. The parametric gain of the LN crystal was also investigated, and a modified Schwarz–Maier model was introduced to interpret the experimental results.     

Absolute cryogenic radiometer for high accuracy optical radiant power measurement in a wide spectral range

An absolute cryogenic radiometer(ACR) with a detachable optical window was designed and built for high accuracy optical radiant power measurement and photodetector spectral responsivity calibration. The ACR receiver is an electroplated pure copper cavity with a 50-μm-thick wall and inner surface coated with a specular black polymer material mixed with highly dispersible carbon nanotubes. The absorptivity of the cavity receivers was evaluated to be ≥0.9999 in the 250 nm–16 μm wavelength range and ≥0.99995 in 500 nm–16 μm. The cavity receiver works at the temperature of ～5.2 K with nanowatt-level noise-equivalent power. The relative standard uncertainty is 0.041% for the measurement of ～100 μW optical radiant power(250 nm–16 μm) and 0.015% for～1 m W(500 nm–16 μm).     

Mid-infrared Er:ZBLAN fiber laser reaching 3.68 μm wavelength

We report a continuous-wave Er:ZBLAN fiber laser with the operation wavelength reaching 3.68 μm.The midinfrared Er:ZBLAN fiber laser is pumped with the dual-wavelength sources consisting of a commercial laser diode at 970 nm and a homemade Tm-doped fiber laser at 1973 nm.By increasing the launched pump power at1973 nm,the laser wavelength can be switched from 3.52 to 3.68 μm.The maximum output power of 0.85 W is obtained with a slope efficiency of 25.14% with respect to the 1973 nm pump power.In the experiment,the laser emission at 3.68 μm is obtained with a significant power of 0.62 W,which is the longest emission wavelength in free-running Er:ZBLAN fiber lasers.     

550 MHz carbon nanotube mode-locked femtosecond Cr:YAG laser

We demonstrate a femtosecond Cr:YAG laser mode-locked by a carbon nanotube saturable absorber mirror(CNT-SAM) at a repetition rate of 550 MHz. By employing the CNT-SAM, which exhibits a modulation depth of 0.51% and a saturation fluence of 28 μJ∕cm~2 at 1.5 μm, we achieved a compact bulk Cr:YAG laser with selfstarting mode-locked operation near 1.5 μm, delivering an average output power of up to 147 m W and a pulse duration of 110 fs. To our knowledge, this system provides the highest repetition rate among reported CNT-SAM mode-locked Cr:YAG lasers and the shortest pulse duration among saturable absorber mode-locked Cr:YAG lasers with repetition rates above 500 MHz.     

All-fiber,wavelength-tunable ultrafast praseodymium fiber laser

The interest in tunable ultrafast fiber lasers operating in the 1.3 μm region has seen a significant increase due to rising demands for bandwidth as well as the zero-dispersion characteristic of silica fibers in this wavelength region. In this work, a tunable mode-locked praseodymium-doped fluoride fiber(PDFF) laser using single-walled carbon nanotubes as a saturable absorber is demonstrated. The mode-locked pulses are generated at a central wavelength of 1302 nm with a pulse repetition rate of 5.92 MHz and pulse width of 1.13 ps. The tunability of the mode-locked PDFF laser covers a tuning range of 11 nm.     

High-performance InAlGaN/GaN enhancement-mode MOS-HEMTs grown by pulsed metal organic chemical vapor deposition

Pulsed metal organic chemical vapor deposition was employed to grow nearly polarization matched InAlGaN/GaN heterostructures. A relatively low sheet carrier density of 1.8×10~(12)cm~(-2), together with a high electron mobility of1229.5 cm~2/V·s, was obtained for the prepared heterostructures. The surface morphology of the heterostructures was also significantly improved, i.e., with a root mean square roughness of 0.29 nm in a 2 μm×2 μm scan area. In addition to the improved properties, the enhancement-mode metal–oxide–semiconductor high electron mobility transistors(MOSHEMTs) processed on the heterostructures not only exhibited a high threshold voltage(VTH) of 3.1 V, but also demonstrated a significantly enhanced drain output current density of 669 m A/mm. These values probably represent the largest values obtained from the InAlGaN based enhancement-mode devices to the best of our knowledge. This study strongly indicates that the InAlGaN/GaN heterostructures grown by pulsed metal organic chemical vapor deposition could be promising for the applications of novel nitride-based electronic devices.     

Diode-Pumped Self-Starting Mode-Locked Nd：YVO4 Laser with Semiconductor Saturable Absorber Output Coupler

By using a semiconductor saturable-absorber output coupler as a mode-locking device, we experimentally realized the operation of a diode-pumped passively mode-locked Nd:YVO4 laser. Stable laser pulses with duration of 2.3ps were generated at the output power of about 1W. With increasing the pump power to 9 W, the maximum mode-locked power of 1.7 W was obtained, which corresponds to a slope conversion efficiency of 44 % and opticalto-optical conversion efficiency of 19%.     

Effect of persistent high intraocular pressure on microstructure and hydraulic permeability of trabecular meshwork

As the aqueous humor leaves the eye, it first passes through the trabecular meshwork(TM). Increased flow resistance in this region causes elevation of intraocular pressure(IOP), which leads to the occurrence of glaucoma. To quantitatively evaluate the effect of high IOP on the configuration and hydraulic permeability of the TM, second harmonic generation(SHG) microscopy was used to image the microstructures of the TM and adjacent tissues in control(normal) and high IOP conditions. Enucleated rabbit eyes were perfused at a pressure of 60 mm Hg to achieve the high IOP. Through the anterior chamber of the eye, in situ images were obtained from different depths beneath the surface of the TM. Porosity and specific surface area of the TM in control and high IOP conditions were then calculated to estimate the effect of the high pressure on the permeability of tissue in different depths. We further photographed the histological sections of the TM and compared the in situ images. The following results were obtained in the control condition, where the region of depth was less than55 μm with crossed branching beams and large pores in the superficial TM. The deeper meshwork is a silk-like tissue with abundant fluorescence separating the small size of pores. The total thickness of pathway tissues composed of TM and juxtacanalicular(JCT) is more than 100 μm. After putting a high pressure on the inner wall of the eye, the TM region progressively collapses and decreases to be less than 40 μm. Fibers of the TM became dense, and the porosity at 34 μm in the high IOP condition is comparable to that at 105 μm in the control condition. As a consequent result, the permeability of the superficial TM decreases rapidly from 120 μm2to 49.6 μm2and that of deeper TM decreases from 1.66 μm2to0.57 μm2. Heterogeneity reflected by descent in permeability reduces from 12.4 μm of the control condition to 3.74 μm of the high IOP condition. The persistently high IOP makes the TM region collapse from its normal state, in which the collagen fibers of the TM are arranged in regular to maintain the physiological permeability of the outflow pathway. In the scope of pathologically high IOP, the microstructure of the TM is sensitive to pressure and hydraulic permeability can be significantly affected by IOP.     

Investigation on mode matching including thermal effects in LD end-pumped passively mode-locked Nd：YVO_4 laser

We design a laser diode （LD） end-pumped passively mode-locked solid-state laser with a semiconductor saturable absorber mirror. Mode-matching efficiency in the active medium including the thermal effect is analyzed, and the optimum mode-matching coefficient is obtained by optimizing the pump mode. In the experiment, a total power of 4.2 W stable pulse sequences without multipulse at 1064 nm is obtained with a pulse repetition rate of 86 MHz and a pulse width of 13 ps, corresponding to a high optical to optical efficiency of 44%.     

A Stable Porous Silicon Dielectric Reflector with a Photonic Band Gap Centred at 10μm

By pulsed anodic etching at low temperature, we prepared a porous silicon reflector with a photonic band gap centred in the long-wavelength infrared spectral region （centred at about 12 μm）. After proper oxidation process, the stable reflector structure, which can reflect electromagnetic wave from 8 μm to 12 μm （centred at 10 μm） within wide incidence angles （about 50°）, is obtained. The wavelength shift of absorption peak of Si-H and Si-O shows the influence of oxidation process and indicates the stability of oxidized porous silicon dielectric reflector, which offers possible applications for the room temperature infrared sensor.     

Passively Mode-Locked Femtosecond Disordered Crystal Laser with Nd:CGA as Gain Medium

We present a laser-diode-pumped passively mode-locked femtosecond disordered crystal laser by using Nd:CaGdAIO_4(Nd:CGA) as the gain medium.With a pair of SF6 prisms to control the dispersion compensation,laser pulses as siort as 850 fs at 1079 nm are obtained with a repetition rate of 124.6MHz.The measured threshold pump power is 1.45 W.A maximum average output power of 122 mW is obtained under the pump power of 5.9 W.These results show that Nd:CGA could be a promising laser medium for generating femtosecond ultrashort pulse at about 1 μm.     

Experimental investigation on Q-switching and Q-switched mode-locking operation in gold nanorods-based erbium-doped fiber laser

We report on the generation of Q-switched and Q-switched mode-locked(QML) pulses in an erbium-doped fiber ring laser by using a polyvinyl alcohol(PVA)-based gold nanorod(GNR) saturable absorber(SA). The PVAbased GNR SA has a modulation depth of ～4.8% and a non-saturable loss of ～26.9% at 1.5 μm. A Q-switched pulse train with a repetition rate varying from 18.70 to 39.85 k Hz and a QML pulse train with an envelope repetition rate tuning from 20.31 to 31.50 k Hz are obtained. Moreover, the lasing wavelengths of the Q-switched pulses can be flexibly tuned by introducing a narrow bandwidth, tunable filter into the laser cavity. The results demonstrate that the GNRs exhibit good optical performance and can find a wide range of applications in the field of laser technology.     

Dark pulse emission in nonlinear polarization rotation-based multiwavelength mode-locked erbium-doped fiber laser

We experimentally show dark pulse generation in all-normal dispersion multiwavelength erbium-doped fiber laser（EDFL） with a long cavity of figure-of-eight configuration. The EDFL generates a stable multiwavelength laser with 0.47 nm spacing at 24 m W threshold pump power, while the number of lines obtained increases with the pump power. A dark pulse emission is observed as the pump power is increased above 137 m W, with fundamental repetition rate of 29 k Hz and pulse width of 2.7 μs. It is observed that the dark pulse train can be shifted to second-, third-, and fourth-order harmonic dark pulses by carefully adjusting the polarization controller. For the fundamental dark pulse, the maximum pulse energy of 32.4 n J is obtained at pump power of 146.0 mW.     

High-Energy Rectangular Pulse Dissipative Soliton Generation in a Long-Cavity Sigma-Shaped Configuration Mode-Locked Fiber Laser

The high-energy rectangular pulse dissipative soliton （DS） is achieved in a long-cavity sigma-shaped configuration erbium-doped fiber laser. A long fiber is inserted in the linear arm of the sigma-shaped configuration to extend the resonator length twice, which effectively improves the pulse energy. The nonlinear polarization rotation technique is employed to realize the mode-locked state. Stable rectangular pulses with pulse energy of 421.22 nJ are obtained at the pump power of 620mW. To the best of our knowledge, this high pulse energy is a new record of the rectangular pulse mode-locked DS fiber lasers.     

Low noise, continuous-wave single-frequency 1.5-μm laser generated by a singly resonant optical parametric oscillator

We report a low noise continuous-wave (CW) single-frequency 1.5-μm laser source obtained by a singly resonant optical parametric oscillator (SRO) based on periodically poled lithium niobate (PPLN). The SRO was pumped by a CW single-frequency Nd:YVO₄ laser at 1.06 μm. The 1.02 W of CW single-frequency signal laser at 1.5 μm was obtained at pump power of 6 W. At the output power of around 0.75 W, the power stability was better than ±1.5% and no mode-hopping was observed in 30 min and frequency stability was better than 8.5 MHz in 1 min. The signal wavelength could be tuned from 1.57 to 1.59 μm by varying the PPLN temperature. The 1.5-μm laser exhibits low noise characteristics, the intensity noise of the laser reaches the shot noise limit (SNL) at an analysis frequency of 4 MHz and the phase noise is less than 1 dB above the SNL at analysis frequencies above 10 MHz.