Single-frequency microchip Nd lasers

Subnanosecond visible two-stage optical parametric generator and amplifier based on MgO:PPLN and LBO crystals at strong pump depletion

Theoretical and experimental investigation of two-stage optical parametric generator based on magnesium oxide doped periodically poled lithium niobate (MgO:PPLN) crystal and optical parametric amplifier based on lithium triborate (LBO) crystal is presented. The first stage crystal was pumped by the subnanosecond fundamental harmonic at 1064 nm wavelength. In the theoretical description, the input signal and idler photons are described by the quantum model and their further amplification is tracked by simulating the nonlinear coupling equations. Such description allows the analysis of pulsed beam evolution during the propagation in the nonlinear crystal under strong pump depletion regime. The second stage crystal was seeded by the output signal wave of the first stage and pumped by the third harmonic of the laser radiation. Experimentally, tuning in the visible wavelength ranges and high pulse power (up to 20 mW at 475 nm wavelength) were achieved.

Chip-scale high-peak-power semiconductor/solid-state vertically integrated laser

Compact lasers capable of producing kilowatt class peak power are highly desirable for applications in various fields, including laser remote sensing, laser micromachining, and biomedical photonics. In this paper, we propose a high-peak-power chip-scale semiconductor/solid-state vertically integrated laser in which two cavities are optically coupled at the solid-state laser gain medium. The first cavity is for the intra-pumping of ytterbium-doped yttrium aluminum garnet (Yb:YAG) with an electrically driven indium gallium arsenide (InGaAs) quantum well, and the second cavity consists of Yb:YAG and chromium-doped yttrium aluminum garnet (Cr:YAG) for passive Q-switching. The proposed laser produces pulses as short as 450 ps, and an estimated peak power of 57.0 kW with a laser chip dimension of 1 mm³. To the best of our knowledge, this is the first monolithic integration of semiconductor and solid-state laser gain mediums to realize a compact high-peak-power laser.

Here the authors demonstrate chip-scale high-peak-power lasers by vertical integration of semiconductor and solid state laser gain mediums to reach the same maturity level as existing semiconductor lasers, which are suitable for miniaturization and cost-effective mass production.

Ultra-compact diode-pumped single-frequency Ti:sapphire laser

In this Letter, we report on the development of an ultra-compact single-frequency Ti:sapphire laser under direct diode pumping. Single-longitudinal-mode operation is realized from a miniature plane-parallel resonator using a volume Bragg grating as an output coupler. InGaN laser diodes operating at around 470 nm and 490 nm with a combined power of 6.7 W are used as an optical pump. A maximum output power of 700 mW is generated during single-frequency operation at 813.4 nm. A laser linewidth of 2.4 MHz is measured during free-running operation, which is reduced to about 180 kHz when the laser is locked to an external reference cavity.

Super-collimation by axisymmetric diffractive metamirror

We propose and demonstrate experimentally super-collimation of light beams by an axisymmetric diffractive metamirror-an axisymmetric concentric dielectric ring structure positioned in front of a mirror at a distance of several micrometers. By super-collimation, we mean the formation of a well-collimated beam characterized by a substantial enhancement of its axial component in the far-field domain. In the reported experiments, the axial intensity of the field was enhanced by around six times. Such axisymmetric super-collimators could be especially useful for improving the emission spatial quality of micro-lasers, when integrated as one (or both) resonator mirrors.

Single-Mode Semiconductor Nanowire Lasers With Coupled Cavities

Semiconductor nanowires are one of the most fascinating topics over the past few decades. As miniaturized coherent light sources, semiconductor nanowires have been attracting tremendous attention in recent years for scientific and technological interest as potential ultra-compact, low cost, high efficiency, and low power consumption. Among different types of lasers, one-dimensional nanowires are of great interest as a promising material for next-generation nanophotonics and nanoelectronics applications due to their unique optical and electrical properties. Semiconductor nanowire lasers with single-mode output are vital in a variety of practical applications ranging from signal processing, spectroscopy, displays, optical sensing, on-chip communications, and biological studies. This article reviews the basic technology and research progress of single-mode semiconductor nanowire lasers. Afterward, the key methods and development of the different types of coupling to achieved single-mode laser output are elaborated. Finally, the challenges faced by each scheme are summarized.

Elimination of spatial hole burning in solid-state lasers using nanostructured thin films

Multimode laser emission generally takes place in homogeneously broadened gain media placed inside a standing-wave resonator due to spatial hole burning. Solutions proposed to eliminate this phenomenon have so far involved the use of intracavity elements, such as an etalon, wave plates or saturable absorbers. We propose a monolithic solution, wherein birefringent layers of TiO₂ are deposited on both laser mirrors. This solution enables one to control the contrast of the interference pattern of the standing wave inside the resonator, and thus the strength of the spatial hole burning, by rotating one mirror around the optical axis. A monochromatic laser emission is demonstrated in a quasi-continuous-wave laser-diode-pumped Yb³⁺:YAG laser experiment.

Single-mode oscillations of diode-pumped mid-infrared Er:Y2O3 ceramic microchip lasers at 2.7 μm

Two compact mid-infrared microchip lasers at 2717 and 2740 nm have been demonstrated using a Er:Y₂O₃ ceramic as laser gain medium with thickness of 800 μm, for the first time to our knowledge. Under a 976-nm diode laser pumping, the 2717 nm microchip laser with linewidth of about 0.16 nm is achieved with a maximum output power of 234.8 mW and slope efficiency of about 10.9%. The laser beam quality expressed by M² factor is measured to be about 1.23 and 1.45 in x and y directions. A single wavelength at 2740 nm with linewidth of about 0.15 nm is also achieved with maximum output power of 102 mW and slope efficiency of about 4.9%. Beam quality of the 2740 nm laser is found to be about 1.15 and 1.26 in x and y directions. Using a mechanical chopper to modulate the pump laser for thermal mitigation, the maximum output powers can be further improved to 312 mW for 2717 nm laser and 145 mW for 2740 m laser at higher pump powers. Such a mid-infrared microchip laser source with very compact size could be have great potential in various eye-safety-related applications.

Single-longitudinal-mode 1521 nm passively q-switched Er:Yb:YAl3(BO3)4 pulse microchip laser

A single-longitudinal-mode 1521 nm pulse microchip laser Q-switched by a Co²⁺:MgAl₂O₄ saturable absorber was demonstrated in an Er:Yb:YAl₃(BO₃)₄ crystal. The influence of the waist radius of pump beam at 976 nm on the laser performance was investigated. At an incident pump power of 6.54 W and pump beam waist radius of 60 μm, a 1521.4 nm single-longitudinal-mode pulse laser with average output power of 434 mW, energy of 16.5 μJ, repetition frequency of 26.3 kHz and width of 2.9 ns was obtained. The result shows that caused by the mode selection of the saturable absorber and large cavity losses, a single-longitudinal-mode 1.55 μm pulse microchip laser can also be realized in the Er:Yb:YAl₃(BO₃)₄ crystal.

A single-frequency intracavity Raman laser

A continuous-wave (CW) single-longitudinal-mode (SLM) intracavity Raman laser is demonstrated for the first time, by virtue of the spatial hole-burning free nature of stimulated Raman scattering (SRS) gain. By using a single etalon in the Nd:GdVO₄ fundamental laser cavity, the spectral linewidth of the multimode fundamental field is suppressed below the Raman linewidth of Raman crystal BaWO₄; hence power in all the longitudinal modes of fundamental field can be extracted by one single Stokes mode. Therefore, the hole-burning free SRS gain exhibits a spectral cleanup effect whereby a stable SLM Stokes field is derived from the multimode fundamental field within a simple standing-wave cavity arrangement. The low-threshold SLM Raman laser delivered 3.42 W SLM Stokes and 1.53 W SLM one-way yellow harmonic at the guide-star wavelength of 589.16 nm. The results here provide a new approach to SLM laser operation with good simplicity and power dynamic range. Further engineering for power scaling and better stability is also discussed.

940 mW 1564 nm multi-longitudinal-mode and 440 mW 1537 nm single-longitudinal-mode continuous-wave Er:Yb:Lu2Si2O7 microchip lasers

An Er:Yb:Lu₂Si₂O₇ microchip laser was constructed by placing a 1.2 mm thick, Y-cut Er:Yb:Lu₂Si₂O₇ microchip between two 1.2 mm thick sapphire crystals, in which input and output mirrors were directly deposited onto one face of each crystal. End-pumped by a continuous-wave 975.4 nm diode laser, a 1564 nm multi-longitudinal-mode laser with a maximum output power of 940 mW and slope efficiency of 20% was realized at an absorbed pump power of 5.5 W when the transmission of output mirror was 2.2%. When the transmission of the output mirror was increased to 6%, a 1537 nm single-longitudinal-mode laser with a maximum output power of 440 mW and slope efficiency of 12% was realized at an absorbed pump power of 4.3 W. The results indicate that the Er:Yb:Lu₂Si₂O₇ crystal is a promising microchip gain medium to realize a single-longitudinal-mode laser.

Randomly polarised beam produced by magnetooptically Q-switched laser

Diode-pumped solid-state micro lasers are compact (centimetre-scale), highly stable, and efficient. Previously, we reported Q-switched lasers incorporating rare-earth substituted iron garnet (RIG) film. Here, the first demonstration of the magnetooptical (MO) Q-switch in an Nd:YAG laser cavity is performed. We fabricate a quasi-continuous-wave (QCW) diode-pumped Nd:YAG laser cavity, which is shortened to 10 mm in length and which contains an RIG film and a pair of small coils. This cavity yields a 1,064.58-nm-wavelength pulse with 25-ns duration and 1.1-kW peak power at a 1-kHz repetition ratio. Further, the polarisation state is random, due to the isotropic crystal structure of Nd:YAG and the fact that the MO Q-switch incorporating the RIG film does not require the presence of polarisers in the cavity. This is also the first report of an MO Q-switch producing random polarisation.

Enhanced 3 μm luminescence properties based on effective energy transfer Yb3+ : 2F5/2→Dy3+ : 6H5/2 in fluoaluminate glass modified by TeO2

Enhanced 3 μm luminescence of Dy³⁺ based on the effective process of Yb³⁺:F_5/22→Dy³⁺:H_5/26 with a higher energy transfer coefficient of 7.36×10^-39  cm⁶/s in fluoaluminate glass modified by TeO₂ was obtained. The energy transfer efficiency from Yb³⁺ to Dy³⁺ in Dy³⁺/Yb³⁺ codoped glass was as high as 80%, indicating the effective energy transfer of Yb³⁺. The higher temperature of the glass transition (T_g) and larger characteristic temperatures (ΔT,K_gl) revealed better thermal properties of the prepared glasses compared with the traditional fluoaluminate glasses, which is of great benefit to fiber drawing. The lower hydroxyl content (15.7 ppm) indicated better fluorescence properties of the glass. It was noted that the longer lifetime of 572 μs and higher emission cross section of 5.22×10^-21  cm² along with the bandwidth of 245 nm around 3 μm proved potential applications in mid-IR laser materials of the present glass.

Magneto-optical Q-switching using magnetic garnet film with micromagnetic domains

High-power giant pulses can be used applied in various applications with Q-switched micro-lasers. This method can shorten the pulse duration; however, active control is currently impossible in micro-lasers. To achieve precise pulse control while maintaining compactness and simplicity, we exploit the magneto-optical effect in magnetic garnet films with micromagnetic domains that can be actively controlled by a pulsed magnetic field. Our Q-switching technique enhances the output power by a factor of 4 × 10³. Moreover, the device itself is smaller than other Q-switching devices. This novel type of active Q-switch can be combined with a micro-laser to obtain megawatt-order pulses.

Exploring transverse pattern formation in a dual-polarization self-mode-locked monolithic Yb: KGW laser and generating a 25-GHz sub-picosecond vortex beam via gain competition

Formation of transverse modes in a dual-polarization self-mode-locked monolithic Yb: KGW laser under high-power pumping is thoroughly explored. It is experimentally observed that the polarization-resolved transverse patterns are considerably affected by the pump location in the transverse plane of the gain medium. In contrast, the longitudinal self-mode-locking is nearly undisturbed by the pump position, even under the high-power pumping. Under central pumping, a vortex beam of the Laguerre-Gaussian LGp,l mode with p = 1 and l = 1 can be efficiently generated through the process of the gain competition with a sub-picosecond pulse train at 25.3 GHz and the output power can be up to 1.45 W at a pump power of 10.0 W. Under off-center pumping, the symmetry breaking causes the transverse patterns to be dominated by the high-order Hermite-Gaussian modes. Numerical analyses are further performed to manifest the symmetry breaking induced by the off-center pumping.

Plasmon-Assisted Nd(3+)-Based Solid-State Nanolaser

Solid-state lasers constitute essential tools in a variety of scientific and technological areas, being available in many different designs. However, although nanolasing has been successfully achieved for dyes and semiconductor gain media associated with plasmonic structures, the operation of solid-state lasers beyond the diffraction limit has not been reported yet. Here, we demonstrate room temperature laser action with subwavelength confinement in a Nd(3+)-based solid-state laser by means of the localized surface plasmon resonances supported by chains of metallic nanoparticles. We show a 50% reduction of the pump power at threshold and a remarkable 15-fold improvement of the slope efficiency with respect to the bulk laser operation. The results can be extended to the large diversity of solid-state lasers with the subsequent impact on their applications.

High-gain mid-infrared optical-parametric generation pumped by microchip laser

High-gain mid-infrared optical-parametric generation was demonstrated by simple single-pass configuration using PPMgLN devices pumped by giant-pulse microchip laser. Effective mid-infrared wavelength conversion with 1 mJ output energy from 2.4 mJ pumping using conventional PPMgLN could be realized. Broadband optical-parametric generation from 1.7 to 2.6 µm could be also measured using chirped PPMgLN.

Self-mixing thin-slice solid-state laser Doppler velocimetry with much less than one feedback photon per Doppler cycle

The drastic shortening of a photon lifetime as compared with normal TEM00 operations has been shown to be associated with the formation of annular mode operations in a thin-slice Nd:GdVO4 laser with tilted laser diode end pumping. The 15 dB enhancement of the signal-to-noise ratio, owing to the shortened photon lifetime, has been demonstrated in the self-mixing laser Doppler velocimetry experiment in comparison with the TEM00 operations, where the minimum intensity feedback rate from a target to the laser for successful measurements was estimated to be -123  dB, which corresponds to 0.007 photon per Doppler cycle.

Single-longitudinal-mode Er:GGG microchip laser operating at 2.7 μm

We reported on a diode-end-pumped single-longitudinal-mode microchip laser using a 600-μm-thick Er:GGG crystal at ∼2.7  μm, generating a maximum output power of 50.8 mW and the maximum pulsed energy of 0.306 mJ, with repetition rates of pumping light of 300, 200, and 100 Hz, respectively. The maximum slope efficiency of the laser was 20.1%. The laser was operated in a single-longitudinal mode centered at about 2704 nm with a FWHM of 0.42 nm. The laser had a fundamental beam profile and the beam quality parameter M(2) was measured as 1.46. These results indicate that the Er:GGG microchip laser is a potential compact mid-infrared laser source.

RTP Q-switched single-longitudinal-mode Nd:YAG laser with a twisted-mode cavity

This paper describes a high pulse repetition frequency Nd:YAG twisted-mode laser that uses an RTP crystal as the electro-optic Q-switch. Stable single-longitudinal-mode laser beams at 1, 5, and 10 kHz were obtained with a linewidth less than 0.1 GHz. Under an incident pump power of 7.5 W and a PRF of 10 kHz, the maximum output power of the single-longitudinal-mode laser was 1.19 W. The corresponding conversion efficiency, single pulse energy and pulse peak power were 15.8%, 119 μJ, and 2.5 kW, respectively.

Tm:KLu(WO(4))(2) microchip laser Q-switched by a graphene-based saturable absorber

We report on the first Tm-doped double tungstate microchip laser Q-switched with graphene using a Tm:KLu(WO₄)₂ crystal cut along the N_g dielectric axis. This laser generates a maximum average output power of 310 mW with a slope efficiency of 13%. At a repetition rate of 190 kHz the shortest pulses with 285 ns duration and 1.6 µJ energy are achieved.

In-band-pumped Ho:KLu(WO4)2 microchip laser with 84% slope efficiency

We report on a continuous-wave Ho:KLu(WO4)2 (KLuW) microchip laser with a record slope efficiency of 84%, the highest value among the holmium inband-pumped lasers, delivering 201 mW output power at 2105 nm. The Ho laser operating at room temperature on the (5)I8→(5)I7 transition is in-band-pumped by a diode-pumped Tm:KLuW microchip laser at 1946 nm. Ho:KLuW laser operation at 2061 and 2079 nm is also demonstrated with a maximum slope efficiency of 79%. The microchip laser generates an almost diffraction-limited output beam with a Gaussian profile and a M2<1.1. The laser performance of the Ng-cut Ho:KLuW crystal is very similar for pump light polarizations ‖Nm and Np. The positive thermal lens plays a key role in the laser mode stabilization and proper mode-matching. The latter, together with the low quantum defect under in-band-pumping (∼0.08), is responsible for the extraordinary high slope efficiency.

Diode-pumped microchip Tm:KLu(WO₄)₂ laser with more than 3 W of output power

A diode-pumped microchip laser containing a quasi-monolithic plano-plano cavity is realized on the basis of a Tm:KLu(WO₄)₂ crystal. The maximum CW output power is 3.2 W (at an absorbed pump power of 6.8 W) and the slope efficiency as high as 50.4%. The laser is operating at 1946 nm in the TEM₀₀ mode with a M²<1.05. Microchip operation with Tm:KLu(WO₄)₂ is, in principle, due to a special crystal cut along the N(g) optical indicatrix axis. This crystal cut possesses positive near-spherical thermal lens that provides the required mode stabilization in the plano-plano cavity. Sensitivity factors of the thermal lens, "generalized" thermo-optic coefficients and constants describing the photoelastic effect are determined for the monolithic Tm:KLu(WO₄)₂ crystal.

Long-haul self-mixing interference and remote sensing of a distant moving target with a thin-slice solid-state laser

An effective long-haul self-mixing interference effect has been observed in a thin-slice LiNdP₄O₁₂ (LNP) laser due to Doppler-shifted optical feedback from a distant target. The narrow spectral linewidth of the LNP laser, which was evaluated to be 16 kHz by heterodyne measurements, led to successful self-mixing laser Doppler velocimetry and vibrometry of targets placed 2.5 km away from the laser through single-mode optical fiber access.

Laser optical feedback imaging controlled by an electronic feedback loop

In autodyne interferometry, the beating between the reference beam and the signal beam takes place inside the laser cavity and therefore the laser fulfills simultaneously the roles of emitter and detector of photons. In these conditions, the laser relaxation oscillations play a leading role, both in the laser quantum noise, which determines the signal-to-noise ratio (SNR), and also in the laser dynamics, which determines the response time of the interferometer. In the present study, we have experimentally analyzed the SNR and the response time of a laser optical feedback imaging (LOFI) interferometer based on a Nd(3+) microchip laser, with a relaxation frequency in the megahertz range. More precisely, we have compared the image quality obtained when the laser dynamics is free and when it is controlled by a stabilizing electronic feedback loop using a differentiator. From this study, we can conclude that when the laser time response is shorter (i.e., the LOFI gain is lower), the image quality can be better (i.e., the LOFI SNR can be higher) and that the use of an adapted electronic feedback loop allows high-speed LOFI with a shot-noise limited sensitivity. Despite the critical stability of the electronic feedback loop, the obtained experimental results are in good agreement with the theoretical predictions.

Cleaved-coupled nanowire lasers

The miniaturization of optoelectronic devices is essential for the continued success of photonic technologies. Nanowires have been identified as potential building blocks that mimic conventional photonic components such as interconnects, waveguides, and optical cavities at the nanoscale. Semiconductor nanowires with high optical gain offer promising solutions for lasers with small footprints and low power consumption. Although much effort has been directed toward controlling their size, shape, and composition, most nanowire lasers currently suffer from emitting at multiple frequencies simultaneously, arising from the longitudinal modes native to simple Fabry-Pérot cavities. Cleaved-coupled cavities, two Fabry-Pérot cavities that are axially coupled through an air gap, are a promising architecture to produce single-frequency emission. The miniaturization of this concept, however, imposes a restriction on the dimensions of the intercavity gaps because severe optical losses are incurred when the cross-sectional dimensions of cavities become comparable to the lasing wavelength. Here we theoretically investigate and experimentally demonstrate spectral manipulation of lasing modes by creating cleaved-coupled cavities in gallium nitride (GaN) nanowires. Lasing operation at a single UV wavelength at room temperature was achieved using nanoscale gaps to create the smallest cleaved-coupled cavities to date. Besides the reduced number of lasing modes, the cleaved-coupled nanowires also operate with a lower threshold gain than that of the individual component nanowires. Good agreement was found between the measured lasing spectra and the predicted spectral modes obtained by simulating optical coupling properties. This agreement between theory and experiment presents design principles to rationally control the lasing modes in cleaved-coupled nanowire lasers.

Optimization of an autodyne laser interferometer for high-speed confocal imaging

In autodyne interferometry, the beating between the reference beam and the signal beam takes place inside the laser cavity and therefore the laser fulfills simultaneously the roles of the emitter and the detector of photons. In these conditions, the laser relaxation oscillations play a leading role, both in the laser quantum noise that determines the signal-to-noise ratio (SNR) and also in the laser dynamics that determine the response time of the interferometer. In the present study, we have theoretically analyzed the SNR and the response time of a laser optical feedback imaging (LOFI) setup based on an autodyne interferometer. More precisely, we have compared the image quality of two lasers having the same output power and the same relaxation frequency, but having two different values of the LOFI gain induced by two different values of the laser response time. From this study, we have finally determined the best laser dynamical parameters and the best experimental conditions for high-speed imaging at the shot-noise limit. Finally, we conclude that a laser diode with a very short response time (in the nanosecond range) seems to be an interesting candidate compared to solid-state microchip laser with a response time of several tens of microseconds. Analytical predictions are confirmed by numerical simulations.

Hybrid Q-switched broadband laser source with low timing jitter

We present a novel broadband laser source based on a dual cavity in which a subnanosecond passively Q-switched microchip laser is coupled with a long cavity including an acousto-optic modulator (AOM) and a microstructured optical fiber working as a non linear medium. This active-passive Q-switched laser source emits pulses as short as those emitted by the free running microchip laser (~600 ps). The time pulse emission is governed by the AOM allowing tunable repetition rate from 0 to more than 4 kHz with a temporal jitter reduced to less than 50 ns, i.e. a 600-fold reduction compared to that of the free running microchip. Furthermore, thanks to spectral broadening in the microstructured fiber, this source emits a supercontinuum from 700 nm to 1700 nm.

Experimental comparison of autodyne and heterodyne laser interferometry using an Nd:YVO₄ microchip laser

Using an Nd:YVO₄ microchip laser with a relaxation frequency in the megahertz range, we have experimentally compared a heterodyne interferometer based on a Michelson configuration with an autodyne interferometer based on the laser optical feedback imaging (LOFI) method regarding their signal-to-noise ratios. In the heterodyne configuration, the beating between the reference beam and the signal beam is realized outside the laser cavity, while in the autodyne configuration, the wave beating takes place inside the laser cavity, and the relaxation oscillations of the laser intensity then play an important part. For a given laser output power, object under investigation, and detection noise level, we have determined the amplification gain of the LOFI interferometer compared to the heterodyne interferometer. LOFI interferometry is demonstrated to show higher performance than heterodyne interferometry for a wide range of laser powers and detection levels of noise. The experimental results are in good agreement with the theoretical predictions.

Airy beams from a microchip laser

It is theoretically shown that an end-pumped microchip laser formed by a thin laser crystal with plane-plane but slightly tilted facets can emit, under appropriate pumping conditions and near a crystal edge, a truncated self-accelerating Airy output beam.

Comparative study of autodyne and heterodyne laser interferometry for imaging

For given laser output power, object under investigation, and photodiode noise level, we have theoretically compared the signal-to-noise ratios of a heterodyne scanning imager based on a Michelson interferometer and of an autodyne setup based on the laser optical feedback imaging (LOFI) technique. In both cases, the image is obtained point by point. In the heterodyne configuration, the beating between the reference beam and the signal beam is realized outside the laser cavity (i.e., directly on the detector), while in the autodyne configuration, the wave beating takes place inside the laser cavity and therefore is indirectly detected. In the autodyne configuration, where the laser relaxation oscillations play a leading role, we have compared one-dimensional scans obtained by numerical simulations with different lasers' dynamical parameters. Finally, we have determined the best laser for LOFI applications and the experimental conditions for which the LOFI detection setup (autodyne interferometer) is competitive compared to a heterodyne interferometer.

A linearly-polarized Nd:YVO4/KTP microchip green laser

We described the principle and the fabrication of a Nd:YVO(4)/KTP microchip for the linearly-polarized green laser and verified its availability by manufacturing and characterizing the green laser using the microchip. Under the driving condition having the modulation frequency of 60 Hz and the duty ratio of 25%, the laser showed the stable linear polarization, the maximum average power of 37 mW, yielding the high electrical-to-optical efficiency of 10.9%.

Stable single-frequency output at 2.01 microm from a diode-pumped monolithic double diffusion-bonded Tm:YAG nonplanar ring oscillator at room temperature

We demonstrate a monolithic double diffusion-bonded monolithic Tm:YAG nonplanar ring laser pumped by a fiber-coupled laser diode. Up to 867 mW single-frequency output at 2.01 microm was obtained from the Tm:YAG system at room temperature, with a slope efficiency and an optical-optical efficiency of 31.6% and 19.2%. The power stability of the single frequency laser was 0.32% within 30 min.

Efficient high-peak-power AlGaInAs eye-safe wavelength disk laser with optical in-well pumping

We have demonstrated an efficient high-peak-power AlGaInAs eye-safe wavelength disk laser at 1555 nm. The quantum defect and the thermal load are significantly reduced by pumping the quantum well directly. The overall conversion efficiency is enhanced over three times compared with the barrier pumping method. With a pump peak power of 3.7 kW, an output peak power of 0.52 kW is generated at a pulse repetition rate of 20 kHz.

Compact efficient multi-GHz Kerr-lens mode-locked diode-pumped Nd:YVO4 laser

We demonstrate the compact efficient multi-GHz Kerr-lens mode locking in a diode-pumped Nd:YVO(4) laser with a simple linear cavity without the need of any additional components. Experimental results reveal that the laser system can be characterized in stable single-pulse and multiple-pulse mode-locked operations. With a pump power of 2.5 W, the compact laser cavity produces average output powers greater than 0.8 W with a pulse width less than 10 ps in the range of 2-6 GHz.

Single-longitudinal mode Nd:YVO4 microchip laser with orthogonal-polarization bidirectional traveling-waves mode

We present a single longitudinal mode, diode pumped Nd:YVO(4) microchip laser where a pair of quarter-wave plates (QWPs) sandwich Nd:YVO(4) and the principle axes of QWPs are oriented at 45 degrees to the c-axis of Nd:YVO(4). Three pieces of crystals were optically bonded together as a microchip without adhesive. Owing to large birefringence of Nd:YVO(4), two standing waves with orthogonal polarizations compensate their hole-burning effects with each other, which diminish total spatial hole-burning effects in Nd:YVO(4). The maximum pump power of greater than 25 times the threshold for single longitudinal mode operation has been theoretically shown and experimentally demonstrated. The power of output, slope efficiencies and temperature range of single longitudinal mode operation are greater than 730 mw (at 1.25 W pump), 60% and 30 degrees C, respectively.

Static gain saturation in quantum dot semiconductor optical amplifiers

Measurements of saturated amplified spontaneous emission-spectra of quantum dot semiconductor optical amplifiers demonstrate efficient replenishment of the quantum-dot ground state population from excited states. This saturation behavior is perfectly modeled by a rate equation model. We examined experimentally the dependence of saturation on the drive current and the saturating optical pump power as well as on the pump wavelength. A coherent noise spectral hole is observed with which we assess dynamical properties and propose optimization of the SOA operating parameters for high speed applications.

Achieving cellular resolution for in vivo retinal images of transgenic GFAP-GFP mice via image processing

In vivo retinal images of transgenic mice, expressing GFP under the control of the GFAP (glial fibrillary acidic protein) promoter, have very poor signal-to-noise ratio (SNR) and cellular resolution such that the analysis of GFAP-GFP expressing retinal cells from these images can be a very challenging task. We report an image averaging method based on a pixel rank matching criterion which significantly enhances both these image attributes. We also show that it compares favorably against direct image averaging and a commercial averaging routine available from the Heidelberg Retinal Angiograph 2 software.

Diode-pumped multi-frequency Q-switched laser with intracavity cascade Raman emission

A diode-pumped actively Q-switched mixed Nd:Y(0.3)Gd(0.7)VO(4) laser with an intracavity KTP crystal is developed to produce cascade SRS emission up to the fourth order. With an incident pump power of 14 W and a repetition rate of 50 kHz, the average output powers at the first, second, third and fourth Stokes modes are approximately 0.05 W, 0.61 W, 0.25 W, and 0.11 W, respectively. The maximum peak power is greater than 2 kW.

Continuous-wave laser characteristics of a Nd3+:LaB3O6 cleavage microchip and the influence of thermal effects

With pumping by a Ti:sapphire laser, the cw laser characteristic of a 5.6 at. %, 0.5 mm thick Nd3+:LaB3O6 cleavage microchip in an end-pumped plano-plano resonator is researched. A slope efficiency of up to 49% and a maximum output power of 530 mW are obtained at an absorbed pump power of 1.16 W and an optimal output coupler transmission of 3.2%. Choppers with different duty cycles are used to change the thermal effects in the microchip, and the influences of thermal effects on the laser characteristics of the Nd3+:LaB3O6 cleavage microchip are investigated.

Crystal Structure, Judd−Ofelt Analysis, and Spectroscopic Assessment of a TmAl3(BO3)4 Crystal as a New Potential Diode-Pumped Laser near 1.9 μm

TmAl3(BO3)4 crystallizes in the trigonal system R32 (No. 155) with a = b = 9.2741(13) A, c = 7.218(3) A, alpha = beta = 90 degrees , gamma = 120 degrees , V = 537.7(2) A(3), D(c) = 4.494 g cm(-3), and Z = 3. The absorption spectrum of this crystal was recorded at room temperature. The Judd-Ofelt (J-O) theory was applied to the absorption intensities of TmAl(3)(BO3)4 to obtain the three J-O parameters: Omega(2) = 2.40 x 10(-20) cm(2), Omega(4) = 0.48 x 10(-20) cm(2), and Omega(6) = 1.09 x 10(-20) cm(2). The radiative probabilities, radiative lifetimes, and branching ratios of TmAl3(BO3)4 were calculated. The absorption and emission cross sections, together with the potential laser gain near 1.9 microm, were investigated. The potential laser gain curves indicate that the tunability range is about 200 nm.

Rare-earth-doped silica microchip laser fabricated by sintering nanoporous glass

We report a new method for fabricating rare-earth-doped silica glasses for laser materials obtained by sintering nanoporous silica glasses impregnated with rare-earth-doped ions. The fabricated materials have no residual pores and show good optical and mechanical properties. Good performance from a Nd3+ -doped silica microchip laser operating at 1.064 microm is successfully demonstrated, suggesting that the fabricated silica glasses have potential for use as active materials for high-power solid-state lasers.

Frequency-shifted optical feedback in a pumping laser diode dynamically amplified by a microchip laser

Compared with conventional optical heterodyne detection, laser optical feedback imaging (LOFI) allows for a several orders of magnitude higher intensity modulation contrast. The maximum contrast amplification is typically 10(3) for a diode laser in the gigahertz range and 10(6) for a microchip laser in the megahertz range. To take advantage of the wavelength tunability of a laser diode and of the lower resonant detection frequency of a microchip laser, we used LOFI modulation induced by the frequency-shifted optical feedback in a laser diode as a modulated pumping power for a microchip laser for resonant dynamic amplification. In this way, we were able to transfer the optical feedback sensitivity of the laser diode to the megahertz range. Application to telemetry is also reported.

Resonant excess quantum noise in lasers with mixed guiding

We show experimentally that the combination of soft-edged gain and index guiding can lead to resonant excess quantum noise. Resonances with excess noise factors close to 100 are observed in end-pumped Nd3+:YVO4 lasers for cavity lengths in which two modes experience similar gain. An associated increase in the relaxation oscillation damping rate demonstrates that the fluctuation enhancement is indeed caused by excess quantum noise and not by dynamic instabilities.

Erbium fiber laser-pumped continuous-wave microchip Cr(2+):ZnS and Cr(2+):ZnSe lasers

Efficient continuous-wave (cw) lasing of Cr(2+):ZnS and Cr(2+):ZnSe crystals in external hemispherical cavities and in a microchip configuration under Er-fiber-laser pumping at room temperature are reported. The key result is what is believed to be the first successful demonstration of cw Cr(2+):ZnS and Cr(2+):ZnSe microchip lasers with maximum output powers of 63 and 100 mW at 2320 and 2520 nm, with slope efficiencies of 53% and 20%, respectively.

Laser relaxation-oscillation frequency imaging

We describe a new imaging technique based on modification of laser relaxation frequency induced by coherent optical feedback from an external target. A direct comparison (both theoretical and experimental) is made with laser feedback interferometry techniques, in which there is a modification of the laser's steady state. We show that, for a laser with a cavity damping rate gamma(c) higher than the population damping rate, gamma(1) , the modification of the laser relaxation frequency can be several orders of magnitude more sensitive than the perturbation of the laser's output power. Application of this technique to imaging is reported.

Continuous-wave operation and Q-switched mode locking of Cr(4+):YAG microchip lasers

Cw-operation, gain-switched, and passively Q -switched mode locking of Cr(4+): YAG microchip lasers with output powers of several hundred milliwatts is demonstrated experimentally in the eye-safe region near 1.5microm . Requirements for cw mode locking of such lasers are investigated by numerical simulations.

Self-mode-locked pulsed monomode laser

We predict the existence of a new pulsed-laser operation regime, when the phases and polarizations of the two coupled cold-cavity eigenstates of a monomode solid-state laser are taken into account in the derivation of the Maxwell-Bloch equations. This monomode pulsed regime is experimentally observed, without any normal mode locking or Q switching occurring inside the cavity. We obtain close agreement between experiments and theory, even in the simple case of a Nd:YAG microchip laser, for which sech(2) pulses at nearly megahertz repetition rates are readily observed.