LiSAF Laser Research Articles

Dual-wavelength synchronously mode-locked Cr:LiSAF laser with a tunable beating frequency and a central wavelength.

We demonstrate a versatile dual-wavelength synchronous mode-locking of a diode-pumped Cr:LiSAF laser for the first time, to our knowledge. A two-color mode-locked operation is achieved by using intracavity birefringent filters (BRFs) or etalons as frequency-selective elements. Using filters with different thicknesses and hence different free spectral ranges (FSRs), wavelength separation in two-color mode-locking could be selected between 1 and 9 nm, with corresponding beating frequencies in the 0.4-3.5 THz range. Moreover, the central wavelength of the two-color output could be tuned smoothly between 840 and 875 nm, only limited by the bandwidth of the semiconductor saturable absorber mirror (SESAM) used for mode-locking. The method, which enables easy adjustment of the central wavelength and beating frequency of a dual-wavelength operation, is suitable for use in other laser gain media as well.

Investigation of pulse-to-pulse timing jitter in an ultrafast Cr:LiSAF laser for synchronization of radiation pulses

Timing jitter is closely related to the phase noise of ultrafast laser pulse train under different mode-locking regimes. Investigations have been made towards timing jitter for pulse-to-pulse Cr:LiSAF femtosecond laser, for usage as a seed laser in synchrotron light source. The laser produced about 180 fs of pulses with an 83.27 MHz pulse repetition rate, which corresponds to a 1.80 m cavity length. Mode-locking was passively achieved using semiconductor saturable absorber mirror (SESAM) optimized for low threshold and stability operation. Active timing-jitter stabilization to an external high precision electronic oscillator was attained with a cavity length control loop using a piezoelectric actuator. Phase noise detection has been evaluated using a timing stabilizer system (CLX-1100) and a phase detector (AD8302). According to the results, the AD8302 phase detector has superior timing jitter sensitivity than the CLX-1100 timing stabilizer system. Ultra low-noise oscillator was introduced to improve the performance of the CLX-1100 timing stabilizer system, which has been evaluated from the phase noise of spectral density and reflected in the root mean square timing jitter. The low timing jitter of the laser pulses was sufficient to accomplish the synchrotron of the radiation source.

Broadly tunable (402–535 nm) intracavity frequency-doubled Cr:LiSAF laser

Broadly tunable (402–535 nm) intracavity frequency-doubled Cr:LiSAF laser

LED-pumped Cr:LiSAF laser system operating at 100 Hz based on a multipass amplifier.

The LED-pumping technology is used for the first time, to the best of our knowledge, to develop a complete master oscillator power amplifier (MOPA) system including a multipass amplifier. A pumping head using an original slab architecture is developed integrating a Cr:LiSAF slab pumped by 2112 blue LEDs via a Ce:YAG luminescent concentrator. The slab configuration enables the reaching of a large number of passes-up to 22-together with access to efficient cooling, allowing for a repetition rate scale up. For 22 passes, the amplifier delivers pulses with energy up to 2.4 mJ at 10-Hz repetition rate with a gain of 4.36 at 825 nm. A complete study of the MOPA is described, concluding in nearly constant performances versus the repetition rate, up to 100 Hz.

Cavity-dumped nanosecond Cr:LiSAF laser in the 985\u20131030\xa0nm region for versatile seeding of Yb-based amplifiers

We report nanosecond (ns) cavity-dumped operation of a low-cost diode-pumped Cr:LiSAF laser around 1000 nm. The system is pumped with one 1-W single-emitter multimode diode at 665 nm. A Pockell cell (PC) and thin-film-polarizer (TFP) combination placed inside the cavity chops up an adjustable portion of the intracavity power and creates a variable time-dependent output coupler. Via adjusting the length and magnitude of the electrical signal going into the PC, output pulses with pulsewidths in the 2.5–500 ns range and with peak power levels above 10 W are generated at repetition rates up to 100 kHz. The central wavelength of the pulses could be smoothly tuned in the 985–1030 nm region, and is only limited by the anti-reflection coating bandwidth of the current PC and TFP. This versatile nanosecond source with 100 nJ level energies could serve as an attractive low-cost seed source for Yb-based amplifiers, including the cryogenic Yb:YLF systems.

455-nm tunable narrow linewidth Q-switched frequency-doubled Cr:LiSAF laser

We present a narrow linewidth frequency-doubled Cr:LiSAF laser with a 450- to 460-nm tunability and maximum repetition frequency (RF) of 63 kHz. Under a pump power of ∼900 mW, the fundamental wavelength could be tuned at the range of 883 to 1020 nm, with a maximum output power of 180 mW at 910 nm. The pulsed operation was achieved by using an acousto-optical modulator. An LBO crystal was adopted for intra-cavity frequency doubling and a maximum output power of 44.8 mW was obtained at 455 nm, indicating a slope efficiency of 11.2%. The spectral linewidth was <0.1 nm in the whole tuning range.

Electro-optic Q-switched Cr:LiSAF laser

We demonstrate electro-optic Q-switched solid state laser with Cr:LiSAF active medium. A single 50 ns pulse with 14 mJ of output energy is demonstrated. Simultaneous generation of several peaks with a step of 1.4 nm within the spectrum envelope with a full width at half maximum of 10.3 nm is demonstrated. For an electro-optic Q-switched mode a Pockels cell is used. Demonstrated laser can be used in differential absorption lidar systems.

Mode-locked Cr:LiSAF laser far off the gain peak: tunable sub-200-fs pulses near 1 µm.

We report, to the best of our knowledge, the first mode-locking results of a Cr:LiSAF laser near the 1 µm region. The system is pumped only by a single 1.1 W high-brightness tapered diode laser at 675 nm. A semiconductor saturable absorber mirror (SESAM) with a modulation depth of 1.5% and non-saturable losses below 0.5% was used for mode-locking. Once mode-locked, the Cr:LiSAF laser produced almost-transform-limited sub-200-fs pulses with up to 12.5 mW of average power at a repetition rate of 150 MHz. Using an intracavity birefringent filter, the central wavelength of the pulses could be smoothly tuned in the 1000-1020 nm range. Via careful dispersion optimization, pulse widths could be reduced down to the 110-fs level. The performance in this initial study was limited by the design parameters of the SESAM used, especially its passive losses and could be improved with an optimized SESAM design.

Tunable Q-switched mode-locked Cr:LiSAF laser

We have achieved stable passively Q-switched mode-locked (QML) operation in a Cr: LiSAF laser. In the experiments, a 10 mm long, %1.5 Cr-doped LiSAF crystal was pumped by two 150 mW single-mode diodes at 660 nm. An AlGaAs based saturable Bragg reflector (SBR) with a central wavelength of 850 nm and a modulation depth of 0.8% was used for initiating and sustaining QML operation. The QML laser was self-starting and robust against environmental fluctuations. In QML operation, the system produced sub-5-ps long mode-locked pulses at 150–200 MHz, inside 2–2.5 μs long Q-switched envelopes with 30–70 kHz repetition rate. The modulation depth of mode-locking stayed around 50%. We have also observed variation of mode-locked pulse parameters within the Q-switched envelope and also from Q-switched pulse to pulse. At an absorbed pump power level of around 280 mW, the QML pulses had an average power of 50 mW, whereas the estimated peak power of the mode-locked pulses were around 0.5 kW. Simple rotation of an intracavity birefringent tuning plate (BRF) enabled tuning of the central wavelength of the pulses between 835 and 865 nm. To our knowledge, this study is the first work focusing on QML operation in Cr: LiSAF lasers. The findings in this initial study is an initial attempt to understand the dynamics of QML operation in Cr:LiSAF. Further work will focus on reducing the mode-locked pulsewidths to sub-1-ps level and also on increasing the modulation depth of mode-locking for further peak power scaling.

Detailed investigation of absorption, emission and gain in Yb:YLF in the 78–300 K range

We present spectroscopic measurements focusing on a detailed investigation of temperature dependence of absorption, emission and gain in the uniaxial Yb:YLF laser gain medium. Measurements are carried out in the 78&#x2013;300 K range, but we especially targeted our attention to the 78&#x2013;150 K interval, which is the desired working range of liquid nitrogen cooled cryogenic Yb:YLF lasers/amplifiers. A tunable (770&#x2013;1110 nm) Cr:LiSAF laser with around 100 mW continuous-wave output power and sub-0.2 nm bandwidth is used as an excitation source. The average power of the Cr:LiSAF laser is low enough to prevent heating of the sample, and its spectral flux (W/nm) is high enough to enable large signal-to-noise ratio measurements. Measured absorption data is used to cross-check the validity of the emission measurements, while the measured temperature dependent small-signal gain profile provided a second independent confirmation. The acquired absorption cross section curves match the previous literature quite well, whereas the measured strength of c-axis emission is stronger than some of the earlier reports. Direct measurements of small signal gain confirmed the emission cross section data, where single pass gain values above 50 have been measured for the 995 nm transition of E//c axis at 78 K. We further provide simple analytic formulas for the measured temperature dependence of absorption and emission cross section. We hope the presented results to be useful for the development of next generation of cryogenic Yb:YLF laser and amplifier systems.

21 fs Cr:LiSAF laser mode locked with a single-walled carbon nanotube saturable absorber

We report the shortest femtosecond pulses directly generated from a solid-state laser that is mode locked by using a single-walled carbon nanotube saturable absorber (SWCNT-SA). In the experiments, we used a 660nm diode-pumped, low-threshold extended-cavity Cr:LiSAF laser operating around 850nm with a repetition rate of 47.9MHz. The SWCNT-SA mode-locked Cr:LiSAF laser produced 21fs pulses with a time-bandwidth product of 0.56 by using only 210mW of pump power. Pump-probe spectroscopy measurements showed that the SWCNT-SA exhibited saturable absorption with slow and fast decay times of 2.7ps and 0.4ps. The single-pass modulation depth and saturation fluence of the SWCNT-SA were further determined as 0.3% and 45 μJ/cm2 at the pump wavelength of 850nm.

Tunable UV source based on an LED-pumped cavity-dumped Cr:LiSAF laser.

We developed a light-emitting diode (LED)-pumped Cr:LiSAF laser operating in Q-switched and cavity-dumped regimes. The laser produces 1.1 mJ pulses with a pulse duration of 8.5 ns at a repetition rate of 10 Hz on a broad spectrum centered at 840 nm with a full width at half maximum of 23 nm. After frequency tripling in two cascaded LBO crystals, we obtained 7 ns pulses with an energy of 13 µJ at 280 nm and with a spectral width of 0.5 nm, limited by the spectral acceptance of the phase matching process. By rotating both LBO crystals, UV emission is tuned from 276 nm to 284 nm taking advantage of the broad infrared spectrum of the Cr:LiSAF laser.

LED-pumped passively Q-switched Cr:LiSAF laser.

We report, to the best of our knowledge, the first light-emitting diode (LED)-pumped Cr:LiSAF laser, in both quasi-continuous-wave (QCW) and passively Q-switched operation. This Letter is based on the recent development of LED-pumped luminescent concentrators (LCs). Combining the capacity of high-density integration of blue LEDs with the excellent properties of Ce:YAG LCs, this new pump source can deliver high irradiance (7.3 kW/cm2) in the visible to pump Cr:LiSAF. The Cr:LiSAF laser demonstrates an energy of 8.4mJ at 850nm in QCW (250μs pulses at 10Hz). A small signal gain per roundtrip of 1.44 at 850nm and a wavelength tunability between 810 and 960nm have been performed. A passively Q-switched oscillator is also presented using a Cr:YAG saturable absorber. A peak power of 3.1kW is obtained with a pulse energy of 130μJ and duration of 41.6ns.

High-efficiency tunable dual-wavelength Cr:LiSAF laser with external grating feedback

A high-efficiency tunable all-solid-state dual-wavelength Cr:LiSAF laser is demonstrated. A V-folded main cavity combined with an external grating feedback was used to improve the efficiency and tunability. With one wavelength fixed at 862 nm, the other wavelength could be tuned from 840 nm to 882 nm. The output power in dual-wavelength operation mode reaches 195 mW with a pump power of 735 mW, indicating an optical–optical efficiency of 26.5%.

100-nm tunable femtosecond Cr:LiSAF laser mode locked with a broadband saturable Bragg reflector.

We report broad tunability of a femtosecond (fs) diode-pumped Cr:LiSAF laser mode-locked with a broadband saturable Bragg reflector (SBR). The SBR had seven pairs of AlxOy(n∼1.5) and Al0.17Ga0.83As(n∼3.5) layers in its Bragg stack, enabling a 250nm reflectivity bandwidth around 850nm. A 6-nm-thick strained In0.15Ga0.85As quantum well placed between Al0.17Ga0.83As cladding layers was used as a broadband saturable absorber in the 800-920nm wavelength range. The laser was pumped by 6 single mode diodes-four at 640 nn and two at 660nm. Mode locking was self-starting and fs pulses could be continuously tuned from 800nm to 905nm by an intracavity birefringent filter with an out-of-plane optic axis. The pulse widths varied from 70fs to 255fs as the laser was tuned. The laser had an 85.5MHz repetition rate and the output power varied from 80mW to 180mW with tuning.

Generation of sub-20-fs pulses from a graphene mode-locked laser.

We demonstrate, what is to our knowledge, the shortest pulses directly generated to date from a solid-state laser, mode locked with a graphene saturable absorber (GSA). In the experiments, a low-threshold diode-pumped Cr3+:LiSAF laser was used near 850 nm. At a pump power of 275 mW provided by two pump diodes, the Cr3+:LiSAF laser produced nearly transform-limited, 19-fs pulses with an average output power of 8.5 mW. The repetition rate was around 107 MHz, corresponding to a pulse energy and peak power of 79 pJ and 4.2 kW, respectively. Once mode locking was initiated with the GSA, stable, uninterrupted femtosecond pulse generation could be obtained. In addition, the femtosecond output of the laser could be tuned from 836 nm to 897 nm with pulse durations in the range of 80-190 fs. We further performed detailed mode locking initiation tests across the full cavity stability range of the laser to verify that pulse generation was indeed started by the GSA and not by Kerr lens mode locking.

Continuous-wave, quasi-continuous-wave, gain-switched, and femtosecond burst-mode operation of multi-mode diode-pumped Cr:LiSAF lasers

In this study, we have investigated the power scaling potential of diode-pumped Cr:LiSAF lasers upon pulsed pumping at varying pump pulse widths (5 μs to 10 s) and duty cycles (2%–100%). Compared to continuous-wave (cw) pumping, pulsed pumping allows the laser crystal to cool down while the pump is in the off state, and enables better control of thermal effects and efficient laser operation in the pump active period. Depending on the parameters of the pulsed pump source, the Cr:LiSAF laser operates in cw, quasi-cw, or gain-switched regimes. In cw laser experiments, using a 20 mm long crystal with a relatively low Cr doping of 0.8%, output powers as high as 2.4 W could be obtained at an absorbed pump power of 5.5 W. In quasi-cw operation, laser output powers as high as 3.1 W have been achieved, which represents a 30% improvement compared to the cw case. In gain-switched operation, pulses in the 1.35–5 μs range with peak powers up to 30 W have been observed at kHz repletion rates. Moreover, pulsed pumping has also been investigated in a cavity that is mode locked with a saturable Bragg reflector to investigate the advantage of femtosecond burst-mode operation compared to cw mode locking. In femtosecond burst-mode operation, 250 fs long pulses around 815 nm at a 106 MHz repletion rate with a burst average power of 1.1 W have been acquired. The corresponding pulse energy and peak power of the femtosecond pulses were 10.3 nJ and 36.6 kW, respectively. This represents a twofold improvement in pulse energies compared to cw mode locking. The power levels obtained in this study were limited by the available pump power, and future studies in this direction have the potential to scale the output power of the Cr:LiSAF lasers above the 10 W level.

Controlling the Temporal Pulse Shape of the Passively Q-Switched Laser by Nonlinear Feedback Control

Q-switched lasers are high intensity laser pulses with extremely short durations of a few nanoseconds. The application of such laser pulses is ubiquitous [1,2]. In many dermatological procedures, intense energy pulses of Q-switched lasers are used to evaporate pigmented cells with high precision while the ultra-short pulse duration prevents damaging the surrounding healthy tissues. These lasers are also used in many industrial applications that require precision cutting, ranging from laser ablation and micromachining to the high-speed drilling of holes. However, the necessary precision of desired pulse characteristics has become increasingly demanding. Ever shrinking dimensions of micromachining call for reliable pulses with predictable properties. Currently, the difficulty of producing identical pulses of prescribed intensity, shape, and duration limits the use of Q-switched lasers. While the authors have previously worked on actively Q-switched lasers [3], this work focuses on passive Q-switching, where saturable absorber materials function as the internal switch in the laser cavity. In this paper theoretical tools from the field of nonlinear dynamical control systems [4] are applied to design a feedback controller circuit, which can produce a Q-switched pulse with customized characteristics by varying the pump rate, which can be influenced by the pump power. The pump power is considered as the input to the system and the photon number is the output of the system. The population inversion, the saturable absorber concentration and the photon number are considered as the states of the system. First it is demonstrated that by applying nonlinear state feedback, the Q-switched laser system can be transformed into a pair of two cascaded integrators. Therefore, the desired output can be customized by applying its second derivative as the input. Cr:YSO Q-switched Cr:LiSAF laser is used as an example to demonstrate our method. Both theoretical analysis of the method and the numerical simulation results are presented.

Numerical Investigations of Laser Diode End-pumped Solid-state Cr\(^{3+}\):LiSAF Lasers Passively Q-switched with Cr\(^{4+}\):YSO Crystal

We have numerically investigated, for the first time, the optical performance and characteristics of diode-end-pumped passively Cr\(^{4+}\):YSO Q-switched solid-state Cr\(^{3 + }\):LiSAF lasers using a rate equation system extended to multi-wavelength. Homogeneously broadened emission spectra of the laser crystal and broad absorption spectra of Cr\(^{4+}\):YSO saturable absorber have been respected in the investigation. A tunable passively Q-switched laser operation is also developed with a birefringent filter plate used as an intracavity selective element. As a result, improved understanding of passively Q-switched Cr\(^{4+}\):YSO solid-state Cr\(^{3+}\):LiSAF lasers has been obtained in broadband and tunable laser operations. Characteristics of the tunable passively Q-switched Cr\(^{3 + }\):LiSAF laser were demonstrated to be clearly dependent on wavelength. The spectro-temporal evolution of a single pulse emitted from the passively Q-switched Cr\(^{3 + }\):LiSAF laser has been reported.

Design and Development of the Folded 4-Mirror Resonators for Diode End-pumped Solid-State Cr:LiSAF Laser

We present the design and development of folded four-mirror resonators for diode end-pumped solid-state Cr:LiSAF lasers. The astigmatic effects due to the Brewster-cut laser crystal and two curved mirrors used at oblique incidence were taken into consideration. The obtained results showed the stability regions of resonator, the beam parameters in the resonator and within the laser crystal as well as other sensitive parameters and their variation ranges. Furthermore, the designed folded four-mirror resonators for diode end-pumped Cr:LiSAF laser were experimentally evaluated. The CW Cr:LiSAF laser characteristics in threshold, efficiency and beam quality as well as comparative studies are presented.