Pump Pulse Energy Research Articles

Hybrid pumped gain-switched thulium fiber laser at a high repetition rate.

A gain-switched all-fiber thulium laser at 2μm with high repetition rate has been demonstrated under a hybrid pumping scheme combined of a pulsed pump at 1.56μm and CW pump at 793nm. The in-band pulsed pump at 1.56μm triggers the gain-switching pulses while the CW pump at 793nm facilitates the energy storage. Therefore, the seed cavity delivers high energy pulses allowing the elimination of multistage amplification. Such hybrid pump configuration is effective for generating gain-switched pulses of high average power with better slope efficiency and pulse width of a few hundreds of a nanosecond. The optimized cavity under such hybrid pump configuration provides output power of 5.92W from the gain-switched oscillator with slope efficiency of 60% and pulse width of 300ns at a repetition rate of 344kHz. Maximum pulse energy of 17.2μJ and peak power of 53.9W has been achieved at this repetition rate. Stable gain-switched pulses at reduced pump pulse energy have been achieved by the use of a CW pump at 793nm. This novel pump configuration facilitates gain switch at higher repetition rates where enough pump pulse energy may not be available. The gain-switched laser also operates at 520kHz and 1.3MHz repetition rate by changing the 1.56μm pulsed pump and cavity length.

Tunable amplified spontaneous emission from dye doped hollow polymer optical fiber

We report temperature induced tuning of amplified spontaneous emission from dye doped hollow polymer optical fiber. The dye doped Poly methyl methacrylate hollow fibers with different radii are fabricated by heat drawing technique from hollow preforms. Amplified spontaneous emission obtained by transverse optical pumping with second harmonic of Nd:YAG laser. Pump pulse energy and fiber diameter dependence on amplified spontaneous emission from the hollow fibers are also analyzed. The tuning of amplified spontaneous emission is carried out by changing the surrounding temperature from 30 °C to 72 °C. With increase in temperature, amplified spontaneous emission from the hollow polymer fiber shows a linear red shift with a temperature sensitivity of 0.09 nm/ °C for a wavelength range of 3.74 nm. Temperature induced tuning of amplified spontaneous emission is found to be reversible.

Partially coherent noise-like pulse generation in amplified spontaneous Raman emission.

Amplified spontaneous Raman emission under picosecond pulse pumping is studied in detail both experimentally and numerically. It is found that the Raman output pulses are noise-like with partial coherence, which has important implications for various applications. Numerical simulation reproduces the finding well, along with the temporal and spectral dependence of the Raman pulse on the pump pulse energy. The numerical simulations of the temporal, spectral, and energy evolution of the Raman and pump pulse along the fiber gives insight on how to design such sources.

Terahertz parametric oscillator with the surface-emitted configuration in RbTiOPO4 crystal

Terahertz parametric oscillator based on RbTiOPO4 (RTP) crystal utilizing the surface-emitted configuration under high pumping pulse energy and low pulse repetition rate is investigated. The obtained THz wave was discontinuously tuned from 3.01 to 3.09 THz, from 3.53 to 4.17 THz, from 4.54 to 4.66 THz, and from 5.35 to 5.89 THz, respectively. The maximum output THz wave pulse energy was 1.77 μJ at 3.85 THz. It was obtained when the pumping pulse energy was 120 mJ and the pumping beam diameter was 3.0 mm. The polarization of THz wave was along the z axis of the RTP crystal. The THz beam on its output surface was approximately elliptical with a major axis of 6.5 mm and a minor axis of 3.5 mm. The divergence angles were 2.45° in the horizontal direction, and 3.88° in the vertical direction.

Carrier-envelope phase stability of a polarization-encoded chirped pulse Ti:Sapphire amplifier

The scheme of polarization-encoded chirped pulse amplification (PE-CPA) reduces the gain narrowing effect in Ti:Sapphire (Ti:Sa) amplifiers by utilizing both crystal axes of Ti:Sa. Hence, the carrier-envelope phase (CEP) of a PE-CPA pulse originates from two othogonal polarization directions. The CEP stability of PE-CPA pulses was investigated for various amplification conditions by varying the pump pulse energy. The CEP stability is directly compared to the conventional CPA scheme under the same laser parameters. A quasi-common-path interferometer was realized inside a four-pass amplifier stage, ensuring exceptional geometrical path length stability and high spectral phase sensitivity. The spectral phase noise of PE-CPA pulses showed a minor increase of 4 mrad compared to the conventional scheme, while at unsaturated amplification of a net gain of 13, it revealed a CEP stability better than 63 mrad.

Experimental investigation on pulse-contrast degradation caused by surface reflection in optical parametric chirped-pulse amplification

In this Letter, we experimentally explore the pulse-contrast degradation caused by surface reflection in optical parameter chirped-pulse amplification. Different pump-to-signal conversion efficiencies and post-pulses with different intensities are obtained by changing the seed-pulse or pump-pulse energy and inserting etalons with different reflection coefficients, respectively. The contrast measurements show that the generated first pre-pulse intensity is proportional to the product of the surface reflection intensity ratio and the square of the pump-to-signal conversion efficiency.

Eye-Safe KGd(WO4)2 Raman Lasers: Comparative Study of Pumping of Nd:KGd(WO4)2 Lasers With 4F3/2 → 4I11/2 and 4F3/2 → 4I13/2 Working Transitions**

This is a comparative study of eye-safe extracavity KGd(WO4)2 (KGW) Raman lasers that convert multimode emission from pulsed Nd:KGW lasers operating on the 4F3/2 → 4I11/2 and 4F3/2 → 4I13/2 transitions into the third Stokes component at λ = 1500 nm and the first Stokes component at λ = 1507 or 1538 nm. For equal sizes of the Raman and laser sections of a Raman laser emitting the third Stokes component, the pulse energy is higher (~14.2 mJ) and the divergence of the Stokes beam is lower (~9.4 mrad) for a lower energy of the Nd:KGW laser (~6.7 J). For pump pulse energies of 29–34 mJ, all the Raman lasers have essentially the same optical lasing efficiency of up to 36%. In terms of the electrical energy delivered to the flashlamp, however, the overall efficiency of the Raman laser for the third Stokes component is a factor of ~1.7 higher. The duration of the Raman pulses is comparable to that of the pump pulses and is ~20 ns for the first Stokes components. The pulse in the third Stokes component is shorter by a factor of 2.5–3. Given the different pump levels and the differences in the thermallyinduced optical distortions of the Nd:KGW laser components, the energy density distribution for beams of the first Stokes components is elliptical, while that of the third Stokes components is almost circular. The possibility of simultaneous lasing at wavelengths of 1507 and 1538 nm with parallel and mutually orthogonal polarizations of the Stokes waves is demonstrated.

Efficient Preparation of D2 Molecules in v=2 by Stimulated Raman Pumping

We report the preparation of D2 molecules in v=2 level in molecular beam condition. A single longitudinal mode laser system was used for excitation of D2 from (v=0, j=0) to (v=2, j=0) with the scheme of stimulated Raman pumping. An excitation efficiency of 25.2% has been achieved, which was determined by the scheme of resonance-enhanced multiphoton ionization. Dependence of relative excitation efficiency on laser energy has been measured. We found that the increasing rate of excitation efficiency became slower as pulse energy of Stokes laser increase, while the excitation efficiency still increases approximately linearly with pump pulse energies up to 60 mJ. The spectral line shapes of Raman transition was also measured at different laser energies and considerable dynamical Stark effect was observed. A single peak was found on the three dimension surface of relative excitation efficiency, indicating the process occurred in the present study is a process of stimulated Raman pumping instead of stimulated adiabatic Raman passage.

Ag nanowire-assisted low threshold WGM lasing from polymer optical fiber.

Whispering Gallery Mode (WGM) emission has been observed from Ag nanowire doped polymer optical fiber laser. Low threshold lasing and high photostability of the active medium has been noticed with a given concentration of Ag nanowires in the microcavity of the fiber. Quantum yield and lifetime measurements of the dye (active medium) with and without nanowires confirm that presence of nanowires enhance the rate of radiative decay of the fluorophore, thereby providing low pump pulse energy for the excitation of lasing modes in the cavity, as compared with a bare dye-doped polymer fiber laser.

Observation of molecular rotation during femtosecond laser filamentation in air by a pump-probe longitudinal diffraction method

SynopsisUtilizing a pump-probe longitudinal diffraction method, we have experimentally observed dynamics of air molecular rotation during femtosecond filamentation. By adjusting only the pump pulse energy, a recurrence of phase shift variation of the probe throughout a range of delay of at least 100 ps was achieved.

Near diffraction-limited performance of an OPA pumped acetylene-filled hollow-core fiber laser in the mid-IR.

We investigate the mid-IR laser beam characteristics from an acetylene-filled hollow-core optical fiber gas laser (HOFGLAS) system. The laser exhibits near-diffraction limited beam quality in the 3 μm region with M2 = 1.15 ± 0.02 measured at high pulse energy, and the highest mid-IR pulse energy from a HOFGLAS system of 1.4 μJ is reported. Furthermore, the effects of output saturation with pump pulse energy are reduced through the use of longer fibers with low loss. Finally, the slope efficiency is shown to be nearly independent of gas pressure over a wide range, which is encouraging for further output power increase.

Evolution of energy deposition during glass cutting with pulsed femtosecond laser radiation

We report on investigations of the energy deposition in the volume of thin glass during an ablation cutting process with pulsed femtosecond laser radiation by time-resolved pump-probe shadowgraphy. For a single laser pulse, the temporal evolution of the transient electronic excitation of the glass volume is imaged up to 10 ps after initial excitation. For an increasing number of laser pulses, the spatial excitation of the glass volume significantly changes compared to single pulse irradiation. Sharp spikes are observed, which reduce the transmission of the illuminating probe pulse. This indicates local maxima of the absorption and, therefore, energy deposition of the pump pulse energy in the glass volume. Furthermore, for an increasing number of pulses, different shapes of the surface ablation crater are observed. To study the correlation between the shape of the surface ablation crater and the energy deposition in the glass volume, simulations of the spatial intensity distribution of the pump pulse are executed by means of linear beam propagation method. We show that the transient excitation spikes observed by pump-probe shadowgraphy can be explained by refraction and diffraction of the laser radiation at the surface ablation crater. Our results provide an experimental validation for the physical reason of an ablation stop for an ablation cutting process. Moreover, the simulations allow for the prediction of damage inside the glass volume.

Mid-infrared Supercontinuum Generation to ∼4.7 μm in a ZBLAN Fiber Pumped by an Optical Parametric Generator

We present mid-infrared (mid-IR) supercontinuum (SC) generation from a step-index fluorozirconate fiber pumped at 2.25 μm. When 70 ps pulses with a peak power of 128 kW were launched into a ZBLAN fiber with a zero dispersion wavelength at 1.603 μm, SC was extended to 4695 nm, with spectral flatness below 10 dB, in the 2.00–4.56 μm range. To the best of our knowledge, this is the first time such a broad and flat mid-IR SC spectrum from a ZBLAN fiber pumped with ps pulses has been achieved. The SC evolution as a function of pump wavelength and pump pulse energy was also experimentally demonstrated.

Experimental investigation of a pulsed Rb–Ar excimer-pumped alkali laser

We present experimental results of an exciplex-pumped alkali laser (XPAL) at 780 nm based on the 52P3/2 → 52S1/2 transition of the Rb atom in mixtures of Rb vapor and Ar. A laboratory-built Ti:sapphire laser with a pulse repetition rate of 3 kHz and a pulse width of 100 ns is used as the pump source. The maximum laser pulse energy of 0.26 µJ at 780 nm is obtained under an absorbed pump pulse energy of 42 µJ at 755 nm in mixtures of Rb vapor and Ar at a temperature of 423 K, corresponding to an optical conversion efficiency of 0.62%. Further experiments show that the output laser at 780 nm can always be detected for pump wavelengths ranging from 754 to 759 nm, indicating that Rb–Ar mixtures can be effectively pumped by commercial laser diodes (LDs) with a bandwidth of 5 nm.

1.57 MW peak power pulses generated by a diode-pumped Q-switched Nd:LuAG ceramic laser

The pulse characteristics of a laser diode dual-end-pumped electro-optic Q-switched Nd:LuAG ceramic laser at various repetition rates are presented. The largest output pulse energy of 11 mJ is realized at the repetition rate of 100 Hz with pump energy of 84.3 mJ, and the slope efficiency in respect to pump pulse energy is 18.6%. The single pulse peak power reaches up to 1.57 MW. Using Nd:LuAG ceramic as the amplification medium seeded by an Nd:YAG laser of 5.2 mJ, a 10.3 mJ amplified pulse is obtained with pump pulse energy of 42.8 mJ, corresponding to an extraction efficiency of 11.9%.

Monolithic all-fiber repetition-rate tunable gain-switched single-frequency Yb-doped fiber laser.

We report a monolithic gain-switched single-frequency Yb-doped fiber laser with widely tunable repetition rate. The single-frequency laser operation is realized by using an Yb-doped distributed Bragg reflection (DBR) fiber cavity, which is pumped by a commercial-available laser diode (LD) at 974 nm. The LD is electronically modulated by the driving current and the diode output contains both continuous wave (CW) and pulsed components. The CW component is set just below the threshold of the single-frequency fiber laser for reducing the requirement of the pump pulse energy. Above the threshold, the gain-switched oscillation is trigged by the pulsed component of the diode. Single-frequency pulsed laser output is achieved at 1.063 μm with a pulse duration of ~150 ns and a linewidth of 14 MHz. The repetition rate of the laser output can be tuned between 10 kHz and 400 kHz by tuning the electronic trigger signal. This kind of lasers shows potential for the applications in the area of coherent LIDAR etc.

Peculiarities of the inverse Faraday effect induced in iron garnet films by femtosecond laser pulses

The inverse Faraday effect in iron garnet films subjected to femtosecond laser pulses is experimentally investigated. It is found that the magnitude of the observed effect depends nonlinearly on the energy of the optical pump pulses, which is in contradiction with the notion that the inverse Faraday effect is linear with respect to the pump energy. Thus, for pump pulses with a central wavelength of 650 nm and an energy density of 1 mJ/cm2, the deviation from a linear dependence is as large as 50%. Analysis of the experimental data demonstrates that the observed behavior is explained by the fact that the optically induced normal component of the magnetization is determined, apart from the field resulting from the inverse Faraday effect, by a decrease in the magnitude of the precessing magnetization under the influence of the femtosecond electromagnetic field.

Multi-Cycle Terahertz Emission from β-Barium Borate

We report the observation of strong narrowband terahertz emission at 10.6 THz from the common nonlinear second harmonic generation crystal β-barium borate crystal excited by 40 fs laser pulses at a 800 nm center wavelength. The emitted THz field amplitude is linear with incident pump energy with no saturation observed up to 2 mJ of pump pulse energy, and exhibits a crystal azimuthal angular dependence consistent with an optical rectification process. The narrowband emission can be explained by the large dispersion caused by optical phonons in the vicinity of the emission energy.

Passive Q-switching of a Tm:YLF laser with a Co2+ doped silver halide saturable absorber

We report a successful passive Q-switching of a Tm:YLF laser operating at λ = 1.9 μm, using a Co2+:AgCl0.5Br0.5 saturable absorber. Approximately 200-ns long, 150 μJ pulses were obtained. Increase in pump energy resulted in repetitive pulsing, with a repetition rate approximately proportional to the pump pulse energy. Room-temperature optical transmission saturation curves measured in ∼1-mm thick Co2+:AgCl0.5Br0.5 plates yielded a ground state absorption cross section σgs=(7.8±0.5)×10−18cm2, and an excited state absorption cross section σes=(3.3±0.3)×10−18cm2, at λ = 1.9 μm. The lifetime of the A2(4F) second excited-state of the octahedral O symmetry was τ∗=(0.6±0.06)ns.

Ultrafast Saturable Absorption of Core/Shell Colloidal Quantum Dots

Ultrafast saturable absorption (SA) materials that are capable of blocking the optical absorption under strong excitation have extensive applications in photonic devices. This work presents core/shell colloidal quantum dots (CQDs) which have the quantized energy levels, excellent band gap tunability, and possess significant SA performance. When the band gap is close to the pump pulse energy, the CQDs show significant resonant SA response. At the same excitation conditions, the core/shell CQDs dispersions show better SA response than graphene dispersions, and comparable to the recently reported molybdenum disulfide. The carrier dynamics of the SA of the CQDs is analyzed systematically. The research has also found that the two-photon absorption of the CQDs show nearly cubic power law of the band gap, while the SA performance keeps almost the same in the nonresonant regime. Further, superior passive Q-switched laser behavior is observed using the CQDs as a saturable absorber. The results directly reveal the physical processes of this basic problem and broaden the applications of CQDs in photonic devices.