Enhanced resolution optoacoustic microscopy using a picosecond high repetition rate Q-switched microchip laser.

Conventional optoacoustic microscopy (OAM) instruments have at their core a nanosecond pulse duration laser. If lasers with a shorter pulse duration are used, broader, higher frequency ultrasound waves are expected to be generated and as a result, the axial resolution of the instrument is improved. Here, we exploit the advantage offered by a picosecond duration pulse laser to enhance the axial resolution of an OAM instrument. In comparison to an instrument equipped with a 2-ns pulse duration laser, an improvement in the axial resolution of 50% is experimentally demonstrated by using excitation pulses of only 85 ps. To illustrate the capability of the instrument to generate high-quality optoacoustic images, en-face, in-vivo images of the brain of Xenopus laevis tadpole are presented with a lateral resolution of 3.8 µ m throughout the entire axial imaging range.

Edge-enhanced optical parametric generation in periodically poled LiNbO3.

We demonstrate enhanced optical parametric gains occurring at the edge of periodically poled LiNbO3 (PPLN) regions. Experiments performed in MgO-doped PPLN samples, pumped at 532 nm with parametric signal outputs around 800 nm and 1550 nm, exhibit good agreement with numerical simulations of the nonlinear wave dynamics in the system, based on the assumption of an average refractive index increase Δn = 5.3×10-5 in the PPLN region. Excitation in proximity to the PPLN edge with a pump power of 8.1 mW results in a 3.6-fold output power increase with respect to parametric generation inside the PPLN area.

1.34 µm VECSEL mode-locked with a GaSb-based SESAM.

Mode locking of a 1.34 µm vertical external cavity surface emitting laser is demonstrated using a GaSb-based semiconductor saturable absorber mirror (SESAM). The SESAM includes six AlGaSb quantum wells (QWs) with an absorption edge at ∼1.37 µm. The proposed approach has two key benefits: the QWs can be grown lattice matched, and only a small number of Bragg reflector layers is required to provide high reflectivity. Pump-probe measurements also reveal that the AlGaSb/GaSb structure exhibits an intrinsically fast absorption recovery on a picosecond timescale. The mode-locked laser pulse train had a fundamental repetition rate of 1.03 GHz, a pulse duration of ∼5 ps, and a peak power of ∼1.67 W. The demonstration paves the way for exploiting GaSb-based SESAMs for mode locking in the 1.3-2 µm wavelength range, which is not sufficiently addressed by GaAs and InP material systems.

Ho:KY(WO4)2 thin-disk laser passively Q-switched by a GaSb-based SESAM.

A Holmium thin-disk laser based on a 3 at.% Ho:KY(WO4)2 / KY(WO4)2 epitaxy and single-bounce pumping by a 1960 nm Tm-fiber laser is passively Q-switched with a GaSb-based quantum-well semiconductor saturable absorber mirror. It generates an average output power of 551 mW at 2056 nm with a slope efficiency of 44% (with respect to the absorbed pump power). The best pulse characteristics (energy and duration) are 4.1 µJ / 201 ns at a repetition rate of 135 kHz and the conversion efficiency with respect to the continuous-wave regime is as high as 93%.

Sub-10 optical-cycle passively mode-locked Tm:(Lu2/3Sc1/3)2O3 ceramic laser at 2 µm.

A Tm-doped mixed sesquioxide ceramic laser is mode-locked near 2 µm using InGaAsSb quantum-well semiconductor saturable absorber and chirped mirrors for dispersion compensation. Maximum average output power of 175 mW is achieved for a pulse duration of 230 fs at a repetition rate of 78.9 MHz with a 3% output coupler. Applying a 0.2% output coupler pulses as short as 63 fs are generated at 2.057 µm.

87 fs mode-locked Tm,Ho:CaYAlO4 laser at ∼2043 nm.

We report on, to the best of our knowledge, the first sub-100 fs mode-locked Ho3+-laser in the 2 µm spectral range employing a disordered co-doped Tm,Ho:CaYAlO4 (Tm,Ho:CALYO) crystal as a gain medium. Pulses as short as 87 fs are produced with an average output power of 27 mW at 80.45 MHz repetition rate. An output power of 96 mW is reached for a pulse duration of 98 fs.

Sub-50 ps pulses at 620 nm obtained from frequency doubled 1240 nm diamond Raman laser.

We report a monolithic 1240 nm diamond Raman laser producing pulses with duration of 42-62 ps at 100 kHz repetition rate, and maximum average power of 246 mW. The Raman laser is formed by a 0.5-mm thick planar diamond, coated on both sides and pumped by ~100 ps pulses from a Q-switched 1064 nm laser. The maximum conversion efficiency from 1064 nm to 1240 nm was about 25%. The 1240 nm signal was frequency-doubled in single-pass configuration through a 10-mm long LBO crystal, enabling generation of pulses with a duration of 29-46 ps at 620 nm. The maximum average power at 620 nm was 128 mW, and the maximum conversion efficiency from 1240 nm to 620 nm was 50%. The Raman laser provides an efficient and flexible way to extend short pulse operation to wavelengths in spectral domains difficult to reach, such as 620 nm and in addition provides a simple pulse shortening mechanisms.

Diode-pumped Tm:KY(WO4)2 laser passively modelocked with a GaSb-SESAM.

We present the first diode-pumped modelocked thulium (Tm3+) laser based on a double-tungstate crystalline gain material. The solid-state laser consists of a Tm:KY(WO4)2 crystal as gain medium and a GaInSb/GaSb quantum well saturable absorber for self-starting passive mode locking. The laser is pumped by a multi-mode fiber-coupled laser diode at a wavelength of 793 nm. An average output power of 202 mW is achieved at a center wavelength of 2032 nm. Pulses with duration of 3 ps are generated at a repetition rate of 139.6 MHz. We also report on the first noise evaluation of a modelocked solid-state laser operating in the 2-µm wavelength range. We measured a timing jitter of sub-100 fs and a relative intensity noise of only 0.04% (frequency range from 500 Hz to 1 MHz).

Thulium doped LuAG ceramics for passively mode locked lasers.

Passive mode-locking of a thulium doped Lu3Al5O12 ceramic laser is demonstrated at 2022 nm. By applying different near surface GaSb-based saturable absorber mirrors, stable self-starting mode-locked operation with pulse durations between 2 and 4 picoseconds was achieved at a repetition rate of 92 MHz. The SESAM mode-locked Tm:LuAG ceramic laser exhibits an excellent stability with a fundamental beat note extinction ratio of 80 dB above the noise level. Furthermore, spectroscopic properties of Tm:LuAG ceramics at room temperature are presented.

Frequency-doubled passively Q-switched microchip laser producing 225 ps pulses at 671 nm.

We report a 671 nm laser source emitting 225 ps pulses with an average power of 55 mW and a repetition rate of 444 kHz. The system consists of a 1342 nm SESAM Q-switched Nd:YVO4 microchip master oscillator and a dual-stage Nd:YVO4 power amplifier. The 1342 nm signal was frequency-doubled to 671 nm using a periodically poled lithium niobate crystal. This laser source provides a practical alternative for applications requiring high energy picosecond pulses, such as time-gated Raman spectroscopy.

Broadly tunable mode-locked Ho:YAG ceramic laser around 2.1 µm.

A passively mode-locked Ho:YAG ceramic laser around 2.1 µm is demonstrated using GaSb-based near-surface SESAM as saturable absorber. Stable and self-starting mode-locked operation is realized in the entire tuning range from 2059 to 2121 nm. The oscillator operated at 82 MHz with a maximum output power of 230 mW at 2121 nm. The shortest pulses with duration of 2.1 ps were achieved at 2064 nm. We also present spectroscopic properties of Ho:YAG ceramics at room temperature.

GaSb-based SESAM mode-locked Tm:YAG ceramic laser at 2 µm.

Tunable and mode-locked laser operation near 2 µm based on different Tm-doped YAG ceramics, 4 at.% and 10 at.%, is demonstrated. Several designs of GaSb-based surface-quantum-well SESAMs are characterized and studied as saturable absorbers for mode-locking. Best mode-locking performance was achieved using an antireflection-coated near-surface quantum-well SESAM, resulting in a pulse duration of ~3 ps and ~150 mW average output power at 89 MHz. All mode-locked Tm:YAG ceramic lasers operated at 2012 nm, with over 133 nm demonstrated tuning for continuous-wave operation.

Mode-locked Tm,Ho:KLu(WO(4))(2) laser at 2060 nm using InGaSb-based SESAMs.

Passive mode-locking of a Tm,Ho:KLu(WO(4))(2) laser operating at 2060 nm using different designs of InGaAsSb quantum-well based semiconductor saturable absorber mirrors (SESAMs) is demonstrated. The self-starting mode-locked laser delivers pulse durations between 4 and 8 ps at a repetition rate of 93 MHz with maximum average output power of 155 mW. Mode-locking performance of a Tm,Ho:KLu(WO(4))(2) laser is compared for usage of a SESAM to a single-walled carbon nanotube saturable absorber.

SESAM mode-locked red praseodymium laser.

We present the first semiconductor saturable absorber mirror (SESAM) mode-locked praseodymium solid-state laser. The laser is based on a Pr(3+):LiYF(4) crystal as gain medium and a GaInP-quantum well-based SESAM. Self-starting continuous-wave mode-locked laser operation with an average output power of 16 mW is achieved at a center wavelength of 639.5 nm. The laser operates at a repetition rate of ∼85.55 MHz and emits pulses with a duration of ∼18 ps.

Ultrahigh precision nonlinear reflectivity measurement system for saturable absorber mirrors with self-referenced fluence characterization.

Measurement of nonlinear optical reflectivity of saturable absorber devices is discussed. A setup is described that enables absolute accuracy of reflectivity measurements better than 0.3%. A repeatability within 0.02% is shown for saturable absorbers with few-percent modulation depth. The setup incorporates an in situ knife-edge characterization of beam diameters, making absolute reflectivity estimations and determination of saturation fluences significantly more reliable. Additionally, several measures are discussed to substantially improve the reliability of the reflectivity measurements. At its core, the scheme exploits the limits of state-of-the-art digital lock-in technology but also greatly benefits from a fiber-based master-oscillator power-amplifier source, the use of an integrating sphere, and simultaneous comparison with a linear reflectivity standard.

Mode-locked VECSEL emitting 5 ps pulses at 675 nm.

A picosecond GaInP/AlGaInP/GaAs vertical external-cavity surface-emitting laser (VECSEL) at 675 nm is reported. The laser is mode-locked with a GaInP/AlGaInP/GaAs saturable absorber mirror and emitted ~5.1 ps pulses at a 973 MHz repetition rate and an average power of 45 mW. To our knowledge, this is the first demonstration of a passively mode-locked VECSEL emitting fundamental laser radiation at the visible part of the spectrum.

11 W single gain-chip dilute nitride disk laser emitting around 1180 nm.

We report power scaling experiments of a GaInNAs/GaAs-based semiconductor disk laser operating at ~1180 nm. Using a single gain chip cooled to mount temperature of ~10 °C we obtained 11 W of output power. For efficient thermal management we used a water-cooled microchannel mount and an intracavity diamond heat spreader. Laser performance was studied using different spot sizes of the pump beam on the gain chip and different output couplers. Intracavity frequency-doubling experiments led to generation of ~6.2 W of laser radiation at ~590 nm, a wavelength relevant for the development of sodium laser guide stars.

Picosecond passively mode-locked GaSb-based semiconductor disk laser operating at 2 µm.

We report on a passively mode-locked optically pumped GaSb-based semiconductor disk laser producing stable picosecond optical pulses at a 1.95 µm wavelength. The gain mirror was comprised of a 15 quantum well InGaSb/GaSb structure. A fast semiconductor saturable absorber mirror with three InGaSb/GaSb quantum wells was used to attain self-starting mode-locked operation at a fundamental repetition rate of 881.2 MHz. The laser produced pulses with 30 pJ energy and a duration of 1.1 ps within a factor of 2 of the Fourier limit.

Carbon nanotube films for ultrafast broadband technology.

Mode-locked sub-picosecond operation of Yb-, Er- and Tm:Hodoped fiber lasers operating at 1.05 microm, 1.56 microm and 1.99 microm, respectively, is demonstrated using the same sample carbon nanotube-based saturable absorber mirror. A mesh of single-walled carbon nanotubes was deposited on an Ag-mirror using a one-step dry-transfer contact press method to combine broadband saturable absorption and high reflectance properties. The novel fabrication method of the polymer-free absorber and device parameters determined using nonlinear reflectivity measurement are described in detail. To our knowledge the observed operation bandwidth of approximately 1 microm is the broadest reported to date for a single carbon nanotube-based saturable absorber.

Harmonically mode-locked VECSELs for multi-GHz pulse train generation.

We report on optically-pumped vertical-external-cavity surface-emitting lasers passively mode-locked with a semiconductor saturable-absorber mirror. The potential of harmonic mode-locking in producing pulse trains at multigigahertz repetition rates has been explored. The results present first systematic study of multiple pulse formation in passively mode-locked VECSELs.