Pulse Build-up Time Research Articles

Injection-seeded high-power Yb:YAG thin-disk laser stabilized by the Pound-Drever-Hall method.

We demonstrate an injection-seeded thin-disk Yb:YAG laser at 1030 nm, stabilized by the Pound-Drever-Hall (PDH) method. We modified the PDH scheme to obtain an error signal free from Trojan locking points, which allowed robust re-locking of the laser and reliable long-term operation. The single-frequency pulses have 50 mJ energy (limited to avoid laser-induced damage) with a beam quality of M2 < 1.1 and an adjustable length of 55-110 ns. Heterodyne measurements confirmed a spectral linewidth of 3.7 MHz. The short pulse build-up time (850 ns) makes this laser suitable for laser spectroscopy of muonic hydrogen, pursued by the CREMA collaboration.

Some experimental studies on the UV spark pin-array pre-ionization in TEA CO2 laser

The output characteristics of a UV pin array pre-ionized transversely excited atmospheric CO2 laser have been experimentally studied in detail. The effects of the applied voltage and capacitance of the main capacitors on the laser performance was investigated. By increasing the voltage at a constant capacitance, not only the laser output energy increases, but also the pulse build-up time, width and tail length decrease. Also, the stability conditions of the discharge were investigated and the cross section of the laser output beam at different input energies was compared.

Radiation robustness of laser ceramics and single crystal for microchip laser remote analysis

Laser pulse energy and pulse build-up time were measured during gamma irradiation at a dose rate of 150 Gy h−1 to evaluate the radiation-induced effects and robustness on a laser remote inspection system using ceramics and single crystal microchip laser. Results showed a time-dependent decrease in pulse energy and an increase in pulse build-up time during irradiation. Both effects were larger for ceramics than for single crystals due to the amount of generated optical loss. The behaviors of pulse energy and build-up time with the increase in optical loss in the gain medium were simulated using rate equation calculations. The radiation effect on build-up time was larger than that on pulse energy for both specimens. Therefore, build-up time measurement is highly sensitive and effective for the evaluation of the radiation effect generated in laser medium, especially for radiation robust specimens or at low radiation dose rate.

Pump pulse characteristics of quasi-continuous-wave diode-side-pumped Nd:YAG laser

The influence of pumping laser pulse on the property of quasi-continuous-wave (QCW) diode-side-pumped Nd:YAG laser is investigated theoretically and experimentally. Under remaining a fixed duty cycle, the average output power increases, and the corresponding thermal focal length shorten with the increase of the pump pulse duration, which attributes to the decrease of the ratio of pulse buildup time to the pulse duration. At a pump power of 146 W, the laser output power changes from 65.1 W to 81.2 W when the pulse duration is adjusted from 150 μs to 1000 μs, confirming a significant enhancement of 24.7%. A laser rate equation model incorporating the amplified spontaneous emission is also utilized and numerically solved, and the simulated results agree well with the experimental data.

Coherent Anti-Stokes Raman Scattering Spectroscopy Using a Double-Wavelength-Emission Electronically Tuned Ti:Sapphire Laser.

Coherent anti-Stokes Raman scattering (CARS) spectroscopy is a powerful tool for Raman imaging technology. In contrast, conventional spontaneous Raman spectroscopy is often used for biological analysis with multivariate analysis. This study develops a new type of CARS instrument with a double-wavelength-emission, background-free, electronically tuned Ti:sapphire laser (DW-ETL). DW-ETL generates two laser pulses with different wavelengths simultaneously within its single resonator. The pulse wavelength and buildup time are regulated by acousto-optical tunable filter in the resonator. The present DW-ETL CARS system is free from any mechanical movement to measure a CARS spectrum by controlling each laser pulse of the emission throughout the fingerprint region. Consequently, it is theoretically able to provide stable CARS spectra to apply multivariate analysis in biological applications. The present study demonstrates that the DW-ETL CARS system provides spectra of biomedical samples in the full finger-print region, and the stability and controllability of the system are evaluated.

Criterion for optimizing high-power acousto-optically Q-switched self-Raman yellow lasers with repetition rates up to 500 kHz

The criterion for optimizing the high-power acousto-optically ${Q}$Q-switched self-Raman yellow laser is originally explored for the repetition rate within 100-500 kHz. The minimum allowed value for the gate-open time is experimentally verified to be determined by the pulse buildup time. By using the minimum allowed gate-open time, the highest conversion efficiency can be achieved to raise the output power by approximately 20% in comparison with the conventional results. At a repetition rate of 200 kHz, the maximum output power at 588 nm can be up to 8.8 W at an incident pump power of 26 W. Furthermore, a practical formula is developed to accurately calculate the threshold pump power as a function of the gate-open time for a given repetition rate.

Single-axial-mode Nd:YAG laser with precisely controllable laser pulse output time

In recent years, high-energy single-axial-mode Q-switched lasers have been widely studied and applied because of their wide applications such as in nonlinear optics, laser spectroscopy and light detection and ranging (LIDAR). Many applications require a Q-switched pulse that has not only single axial mode but also can be synchronized with an external system. But two most commonly used methods (the build-up time reducing technique and ramp fire technique) are difficult to achieve single-axial mode operation. In this work, we apply the ramp-hold-fire technique to an injection-seeded Nd:YAG laser. The slave oscillator is a self-filtering unstable resonator (SFUR). The SFUR oscillator can achieve a smooth spatial profile TEM&lt;sub&gt;00&lt;/sub&gt; transverse mode. An RTP electro-optical crystal is adopted for intracavity phase modulator to modify the effective optical path length of the slave oscillator cavity. The seed-injection locking is realized by the ramp-hold-fire technique. The laser driver generates a pumping pulse. After a suitable time delay the driver is fired, a linear ramp voltage is applied to the RTP crystal. A photodiode detector monitors the interference signal. As soon as the interference peak is detected, the controlling electronics produces a stop signal. The ramp voltage on the RTP crystal is stopped and held at a fixed value. Then the Q-switch is fired at a set time, and finally single axial mode laser is demonstrated. Combining the advantages of intracavity phase modulation and Q-switch exact synchronization of the ramp hold fire technique, we obtain a narrow linewidth single-axial-mode laser pulse with precisely controllable output time. The laser is capable of generating 1064 nm pulse energy large than 50 mJ. The pulse build-up time is reduced by 31 ns to 48 ns. The pulse firing time is precisely controlled with jitter less than 1%. Then the frequency spectrum of the 1064 nm laser is measured with a commercial Fizeau wavemeter HighFinesse WS7. The multi-beam interference patterns of the pulse are shown to be smooth in the wavemeter. The wavelength is measured to be 1064.40416 nm and the linewidth is less than 0.5 pm which is limited by the instrument resolution. Meanwhile, the frequency stability is measured to be less than 0.1 pm (V-V) over 1700 pulses with a working frequency of 0.1 Hz.

Short pulse generation in active Q-switched Yb-doped all fiber laser and its amplification

Generation of stable sub-cavity round-trip time pulses have been achieved in all-fiber Ytterbium-doped Q-switched fiber laser by controlling the modulation window ON-time of acousto-optic modulator (AOM). Short pulses (anomalous pulses) of duration in the range of 50–55 ns have been achieved in experimental configurations with the cavity round-trip time of ∼130 ns, when modulation window “ON-time” of AOM was equal to or shorter than the pulse build-up time of the normal Q-switched pulses. In the case of normal AO Q-switching, pulses of duration more than 1.3 µs with an average power of 30 mW and pulse energy of 0.38 µJ at 80 kHz of repetition rate were obtained at 2 W of pump power. At the same value of the pump power, the anomalous pulses of ∼55 ns with an average power of 12 mW were generated, which are ∼24 times shorter than pulses obtained with conventional Q-switching process (normal pulses). The peak power of the anomalous pulses are higher than the normal pulses by the factor of eight at the same value of the pump power. The anomalous pulses from the AO Q-switched oscillator with 55 ns pulse duration have been amplified in single mode Ytterbium doped all fiber amplifier in two stages. Amplified average output power of ∼19 W was achieved with the output pulse energy and peak power of 240 µJ and 4.4 kW respectively in all fiber configuration.

Experimental and theoretical study on characteristics of pulse excitation in T-burners

Pulse excitation is the key to measure the pressure-coupling response function of composite propellant. It is also a key trigger factor for nonlinear combustion instability. This paper aims at understanding characteristics of pulse excitation in T-burners. Pulse excitation is provided by black powder (BP). D2 law is used to calculate BP burning properties. Firstly, the experimental pressure history of a pulse excitation is analyzed. Pressure pulse and mean pressure increment are introduced to describe pulse excitation. Secondly, the modified zero-dimension model and one-dimension model of pressure pulse are established based on energy conservation and modification. The results of models indicate that the modified zero-dimensional model can accurately predict the pressure pulse. The modified zero-dimension model demonstrates that the pressure pulse is determined by pulse build-up time threshold, volume coefficient, effective weight fraction of BP, weight of BP et. al. When burning time of BP is larger than the threshold, volume coefficient is equal to 2, and effective weight fraction of BP is less than 1. The pressure pulse is approximately linear correlation with weight and effective weight fraction of BP. Otherwise, volume coefficient is larger than 2, and effective weight fraction of BP is equal to 1. The pressure pulse is approximately linear correlation with volume coefficient and BP weight. Thirdly, a zero-dimensional prediction model of mean pressure is established based on conservations of energy and mass. The prediction models of pressure pulse and mean pressure are validated by T-burner experiments. Finally, effects of BP burning properties on pressure pulse and mean pressure increment are studied. The results show that both pressure pulse and mean pressure increment increase with increasing BP weight, linearly. The pressure pulse is more sensitivity to the variations of burning time of BP. As burning time of BP decreases, the mean pressure increment gradually increases to the maximum, and the pressure pulse can become a very large value.

Spectral dynamics of a diffusive random laser under two photon pumping

The spectral dynamics of a diffusive random laser under two photon excitation is investigated by using an experimental set-up which allows for fine mode discrimination and time resolution. The diffusive scattering system consists of a di-ureasil hybrid matrix powder doped with Rhodamine B. The anti-Stokes random laser emission obtained by pumping at 1064 nm shows a larger output pulse duration, build-up time, and threshold than the one produced under one photon pumping at 532 nm using the same experimental conditions. The time resolved analysis of the single shot emission pulse demonstrates the inherent stochastic nature of its lasing modes as well as the existence of modal relaxation oscillations. The outcome of our experiments also suggests that mode saturation appears with the increase of pump power.

Study of Offset Frequency Locking and Active Frequency Stabilization for Two RF-Excited Waveguide CO&lt;sub&gt;2&lt;/sub&gt; Laser With Common Ground Electrode

In this paper, we designed the offset frequency locking and active frequency stabilization system for the continuous-wave (CW) and electro-optical Q-switched RF-excited waveguide CO 2 lasers to provide an ideal light source for coherent radar. The CW laser is used as a local laser, and the pulsed laser is used as a main laser, which can produce Q-switched and cavity-dumped laser synchronously. A closed-loop laser frequency control system was used to stabilize the Q-switched laser at the gain curve center frequency and to lock the two lasers at a selected heterodyne frequency. The system includes two parts: 1) pulsed laser frequency stabilization and 2) offset frequency locking. The frequency stabilization part can keep the Q-switched laser oscillating at the peak of the gain curve by measuring the pulse build-up time continuously and controlling the cavity length. The offset frequency locking circuits keep the heterodyne frequency around the selected frequency by measuring the frequency of the beat signals and controlling the CW laser cavity length. In the experiment, the long-term frequency shift of the pulse laser was measured to be less than ±16 MHz and the offset frequency fluctuation was less than ±2 MHz when the frequency control system was in a closed-loop operation.

Hybrid-pumped, gain-switched 1120 nm ytterbium-doped fiber laser

A monolithic, hybrid-pumped, gain-switched, single-mode, 1120 nm Yb-doped fiber laser is reported. Pumped by a 1064 nm pulse fiber source and a continuous-wave 976 nm laser diode, a record output pulse energy of 91.7 μJ at repetition rate of 10 kHz is obtained. The full width at half-maximum of the pulses is 4.1 μs, and the slope efficiency corresponding to the 1064 nm laser is more than 46%. The pulse build-up time and a pulsewidth decrease with an increase in the pump pulse energy are observed. In the gain-switched pulses, mode-lock-resembling pulses are detected. The subpulse's duration is about 415 ns, including the cavity round-trip time, and the width varied from 80 to 320 ns, which is longer than the traditional mode-locked laser.

Study of frequency stabilization for electro-optical Q-switched radio-frequency-excited waveguide CO2 laser using build-up time method

An electro-optical Q-switched RF-excited Z-fold CO(2) waveguide laser was designed, which can output a Q-switched laser and a cavity-dumped laser synchronously. The build-up time method is presented to stabilize the laser frequency. A closed-loop control system was designed to keep the laser oscillating at the peak of the gain curve by measuring the pulse build-up time continuously and controlling the cavity length. In the experiment, the variations for the pulse build-up time and cavity-dumped laser output power with time were recorded in a period of time. The frequency fluctuation is less than ±16 MHz.

Comparative study of diode-pumping self-injection and injection-locking Tm:YAG lasers

A comparative study of the laser characteristics of self-injection and injection-locking Tm:YAG lasers is given in this paper. At a pump energy of 145 mJ and Q-switched repetition rate of 100 Hz, an output energy of 2.39 mJ was obtained for an injection-locking Tm:YAG laser, with a pulse width of 403.2 ns and a pulse building-up time of 2.12 μs. Under the same conditions, the output energy, pulse width and pulse build-up time for a self-injection Tm:YAG laser were 2.21 mJ, 407.0 ns and 3.95 μs, respectively. The threshold of the Q-switched injection-locking Tm:YAG laser was much lower than that of the self-injection laser, and the pulse width was narrower and the pulse build-up time shorter. Additionally, the output spectrum was much purer for the injection-locking laser.

Analysis of Output Pulse Characteristics in $Q$-Switched Yb-Doped Fiber Laser

Characteristics of the output pulses of a high-peak-power <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> -switched Yb-doped fiber laser are presented. A <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> -switched fiber laser with 1.3 kW peak power and with typical pulse duration of 240 ns at 20 kHz modulation frequency was realized, variations of pulse duration, average power, peak power, and pulse energy with variation in the modulation frequency were studied. The effect of the modulation duty cycle (DC) on the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> -switched pulse characteristics was analyzed both theoretically and experimentally. It is shown that, for a particular pump power and repetition rate, the DC of modulation needs to be optimized to achieve high-peak-power single-pulse output. If the DC of modulation is long, satellite pulses of low peak power are also observed along with the main peak, when the DC is made short enough, the satellite pulses disappear and single peak pulses appear in the output. Theoretical analysis and simulation to predict the pulse build-up time, output pulse shape, and effect of modulation window time on the output pulse characteristic were carried out. Simulation results show good agreement with experimental results. This paper reported here will be useful in the development of high-peak-power <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> -switched fiber lasers.

Influences of ASE on the performances of Q-switched ytterbium-doped fiber lasers

The output characteristics of actively Q-switched ytterbium-doped fiber lasers (YDFL) are investigated in this paper. Our experimental results show that, the combined effect between the short switching time and the gain transient property of doped fiber causes the initial amplified spontaneous emission (ASE) power fluctuation, forming the multi-peak structure in the output pulse for either ring or linear cavity Q-switched YDFL. The pulse buildup time decreases with the rising of the pump. Moreover, since the broad-band ASE generated in the YDF is very high, it may saturate the doped fiber once the switch has been opened, making it difficult to achieve Q-switched laser oscillation for both fiber lasers. By using ASE filter to suppress the initial ASE, the gain supplied by the YDF can be greatly enhanced, which can not only decrease the threshold, but also greatly decrease the duration, and enhance the peak power of the Q-switched laser pulse as well. By using such an ASE filter, the Q-switched laser pulse with the peak power of 40.7 W and the duration of 30 ns has been achieved for the linear cavity YDFL pumped with 160 mW.

Nd:YAG 레이저의 제 2조화파로 종여기하는 펄스형 Ti:sapphire 레이저 발진기와 이를 이용한 단일경로 형태의 Ti:sapphire 증폭기의 출력특성

레이저 분광용 고출력 파장가변 레이저로 이용할 Nd:YAG 레이저의 제 2조화파로 종여기한 평행평면형 공진기 구조의 펄스형 Ti:sapphire 레이저 발진기와 이를 이용한 단일경로 형태의 Ti:sapphire 레이저 증폭기의 출력특성을 조사하였다. 발진기의 경우에는 여기광의 에너지, 공진기 길이, 출력거울의 반사율을 변화시키면서 출력스펙트럼, 펄스 발생시간, 펄스폭, 출력에너지를 측정하였다. 그리고 증폭기의 경우, 발진기의 레이저 광과 증폭기 여기광이 증폭매질에 들어오는 시간차이, 그리고 발진기의 여기에너지와 증폭기의 여기에너지를 각각 변화시키면서 증폭기의 출력에너지를 측정하고 이를 분석하였다. 이 결과 발진기와 증폭기의 여기에너지가 18 mJ/pulse일 때 두 여기광의 시간차가 35 ns까지는 지속적으로 증폭기의 에너지가 증가했으며, 발진기의 여기광에 대한 증폭기의 기울기 효율은 23.5 %이고, 증폭기의 여기광에 대한 증폭기의 기울기 효율은 11.6 %였다. The various output characteristics of a pulsed Ti:sapphire laser oscillator with a plane-parallel resonator, pumped longitudinally by the second harmonic wave of a Nd:YAG laser, and the output of a Ti:sapphire laser amplifier operated along the single path of the oscillator beam were investigated and analyzed. In the case of the oscillator, we measured the spectrum, the pulse buildup time, the temporal duration time of the pulse, and the output energy according to the variation of the pumping energy, resonator length, and the reflectance of the output coupler. And, in the case of the amplifier, we investigated and analyzed the output energy of the amplifier as a function of the time difference between the two pump beams of the oscillator and the amplifier, the pumping energy of the oscillator, and the pumping energy of the amplifier When pump energies of both the oscillator and the amplifier were 18 mJ/pulse, we could find that the output energy of the amplifier increased linearly and gradually up to the time difference of 35 ns. Finally, we determined that the slope efficiencies of the oscillator and the amplifier were 23.5 % and 11.6 %, respectively.

Concentration dependence of fluorescence and lasing efficiency in Cr 2+:ZnSe lasers

We investigated the effect of Cr 2+ concentration on the fluorescence lifetime, fluorescence efficiency, and lasing performance of Cr 2+:ZnSe lasers. In the experiments, polycrystalline Cr 2+:ZnSe samples with concentrations of 0.34 × 10 18–66 × 10 18 ion/cm 3 were prepared by diffusion doping. Spectroscopic measurements performed with Cr 4+:YAG and Tm 3+:fiber lasers show that the fluorescence efficiency decreases monotonically with increasing active ion concentration. Lifetime measurements and lasing characterization were also performed with a pulsed 1570-nm KTP optical parametric oscillator. In gain-switched operation, the highest output energy was obtained with the Cr 2+:ZnSe sample having a Cr 2+ ion concentration of 14 × 10 18 cm −3. The power performance degrades with increasing concentration due to larger passive losses at the lasing wavelength. Furthermore, the pulse build-up time and the output pulsewidth decrease with pump energy and ion concentration. Finally, with a single set of optics, tunable output could be obtained in the 2520–3050 nm wavelength range.

Characterization of laser-diode end-pumped intracavity frequency doubled, passively Q-switched and mode-locked Nd:YVO 4 laser

Simultaneous Q-switching and mode-locking in a laser-diode end-pumped intracavity frequency doubled Nd:YVO 4/KTP green laser using Cr 4+:YAG saturable absorber is experimentally demonstrated. The influence of the initial transmission ( T 0) of the Cr 4+:YAG crystal on the Q-switched mode-locked green pulses as well as on the average green power is characterized by using Cr 4+:YAG crystal with various T 0. The effect of T 0 on the pulse build-up time in intracavity second harmonic configuration is theoretically investigated. It was found that the depth of modulation for the mode-locked pulses is greatly improved at the second harmonic wavelength as compared to that for the fundamental wavelength. The average pulse duration of the individual mode-locked pulse for the second harmonic beam measured to be less than 500 ps with a repetition rate of 400 MHz .

Fast wavelength tuning of a self-seeded Fabry-Perot laser diode with a Fabry-Perot semiconductor filter

In this letter, we describe a new and simple method for achieving fast wide-range wavelength tuning in a self-seeded Fabry-Perot laser diode (FPLD). The method relies on the provision of optical feedback to a self-seeded laser diode from a Fabry-Perot semiconductor filter (FPSF). Wavelength selection is realized by electrically tuning the comb-like spectral response of the FPSE Using an ordinary FPLD biased below its threshold as an FPSF, we have demonstrated stepwise wavelength tuning over a range of 9.0 nm with a sidemode suppression ratio higher than 15 dB throughout the range, and a pulse buildup time as short as a few microseconds.