Outcoupling Mirror Research Articles

Self-mode-locked AlGaInP-VECSEL

We report the mode-locked operation of an AlGaInP-based semiconductor disk laser without a saturable absorber. The active region containing 20 GaInP quantum wells is used in a linear cavity with a curved outcoupling mirror. The gain chip is optically pumped by a 532 nm laser, and mode-locking is achieved by carefully adjusting the pump spot size. For a pump power of 6.8 W, an average output power of up to 30 mW is reached at a laser wavelength of 666 nm. The pulsed emission is characterized using a fast oscilloscope and a spectrum analyzer, demonstrating stable single-pulse operation at a repetition rate of 3.5 GHz. Intensity autocorrelation measurements reveal a FWHM pulse duration of 22 ps with an additional coherence peak on top, indicating noise-like pulses. The frequency spectrum, as well as the Gaussian beam profile and the measured beam propagation factor below 1.1, shows no influence of higher order transverse modes contributing to the mode-locked operation.

The transfer function of generic linear quantum stochastic systems has a pure cascade realization

This paper establishes that generic linear quantum stochastic systems have a pure cascade realization of their transfer function, generalizing an earlier result established only for the special class of completely passive linear quantum stochastic systems. In particular, a cascade realization therefore exists for generic active linear quantum stochastic systems that require an external source of quanta to operate. The results facilitate a simplified realization of generic linear quantum stochastic systems for applications such as coherent feedback control and optical filtering. The key tools that are developed are algorithms for symplectic QR and Schur decompositions. It is shown that generic real square matrices of even dimension can be transformed into a lower 2×2 block triangular form by a symplectic similarity transformation. The linear algebraic results herein may be of independent interest for applications beyond the problem of transfer function realization for quantum systems. Numerical examples are included to illustrate the main results. In particular, one example describes an equivalent realization of the transfer function of a nondegenerate parametric amplifier as the cascade interconnection of two degenerate parametric amplifiers with an additional outcoupling mirror.

Continuous wave vertical cavity surface emitting lasers at 2.5 μm with InP-based type-II quantum wells

A concept for electrically pumped vertical cavity surface emitting lasers (VCSEL) for emission wavelength beyond 2 μm is presented. This concept integrates type-II quantum wells into InP-based VCSELs with a buried tunnel junction as current aperture. The W-shaped quantum wells are based on the type-II band alignment between GaInAs and GaAsSb. The structure includes an epitaxial GaInAs/InP and an amorphous AlF3/ZnS distributed Bragg reflector as bottom and top (outcoupling) mirror, respectively. Continuous-wave operation up to 10 °C at a wavelength of 2.49 μm and a peak output power of 400 μW at −18 °C has been achieved. Single-mode emission with a side-mode suppression ratio of 30 dB for mesa diameters up to 14 μm is presented. The long emission wavelength and current tunability over a wavelength range of more than 5 nm combined with its single-mode operation makes this device ideally suited for spectroscopy applications.

Variable energy, high peak power, passive Q-switching diode end-pumped Yb:LuAG laser

A new method to control the pulse energy in a passively Q-switched laser was proposed and experimentally verified for a diode-end-pumped Yb:LuAG laser. By changing the pumping area parameters it was possible to demonstrate generation of a wide range of output energies with a single laser configuration consisting of a gain medium, passive Q-switch and out-coupling mirror. The range of available energies 0.15–0.51 mJ with maximum peak power of 113 kW in simple Q-switching regime by means of a Cr:YAG saturable absorber and a Yb:LuAG gain medium pumped by a 20 W laser diode emitting at 968 nm was demonstrated.

Micro-cavity 2-μm GaSb-based semiconductor disk laser using high-reflectivity SiC heatspreader

An optically pumped GaSb-based semiconductor disk laser (SDL) emitting at 2.05 μm has been realized with a very short (380 μm long) laser cavity by high-reflectivity coating the intra-cavity SiC heatspreader, which then serves as the outcoupling mirror. Room-temperature output powers in excess of 750 mW have been demonstrated in multimode operation and still 100 mW in TEM00 emission, which is a more than 100× increase in output power compared to previous reports on GaSb-based micro-cavity (μC) SDLs. Mode-hop-free tunable single-frequency emission with linewidths <7 MHz has been achieved which makes this type of miniaturized SDLs attractive for sensing applications requiring small-size 2-μm laser sources.

On the measurement of the thermal impedance in vertical-external-cavity surface-emitting lasers

A detailed and systematic analysis of the loss mechanisms in vertical-external-cavity surface-emitting lasers is presented with the goal to correctly determine the amount of pump power that is converted to heat. With this input, the accuracy of a recently proposed method for measuring the thermal impedance based on roll-over characteristics is shown to be very high for devices with and without dielectric coating. Potential errors arising from non-heating losses can be determined by performing experiments with different out-coupling mirrors.

Influence of the pump threshold on the single-frequency output power of singly resonant optical parametric oscillators

We demonstrate that for a given pump source, there is an optimum pump threshold to achieve the maximum single-frequency output power in singly resonant optical parametric oscillators. Therefore, cavity losses and parametric amplification have to be adjusted. In particular, continuous-wave output powers of 1.5 W were achieved with a 2.5 cm lithium niobate crystal in comparison with 0.5 W by a 5 cm long crystal within the same cavity design. This counter-intuitive result of weaker amplification leading to larger powers can be explained using a model from L.B. Kreuzer (Proc. Joint Conf. Lasers and Opt.-Elect., p. 52, 1969). Kreuzer also states that single-mode operation is possible only up to pump powers which are 4.6 times the threshold value. Additionally, implementing an outcoupling mirror to increase losses, single-frequency waves with powers of 3 W at 3.2 µm and 7 W at 1.5 µm could be generated simultaneously.

Scaling behavior of bipolar cascade VCSELs

We present theoretical and experimental results on the scaling properties of bipolar cascade vertical-cavity surface-emitting lasers with respect to active region stacking and out-coupling mirror reflectivity. The measurement results at around 980-nm wavelength confirm the theoretically expected behavior of an almost linear increase in slope efficiency and a reduction in threshold current density with the number of stages. A three-stage laser shows a differential quantum efficiency of 128% in continuous-wave operation at room temperature. For the threshold current density, we obtain a decrease from 3.75 kA/cm/sup 2/ for the conventional VCSEL to about 1.2 kA/cm/sup 2/ for a three-stage device. Such multi-diode cascade VCSELs are highly interesting for optical links, analog systems, and high-power applications.

Loss rates of laser cavities

The loss rate of a laser cavity can differ from the loss rate of the same cavity mode in the absence of gain. It is shown that this difference arises from longitudinal inhomogeneities in the gain, the loss or the energy flow density. A similar ambiguity arises in the definition of the partial loss rate through the out-coupling mirror. This affects the calculation of the equivalent Schawlow–Townes linewidth of an unstable laser cavity from experimentally accessible quantities, and thereby the determination of the excess noise factor of the set-up.

InP-based 1.55 µm resonant cavity light-emitting diode with two epitaxial mirrors

We report on the fabrication and characterisation of a resonant cavity light-emitting diode (RCLED) operating at 1.55 µm. As compared to conventional LEDs such devices merit from a higher emission intensity with a more narrow linewidth and are therefore promising as light sources in fibre optical communication systems. The present design is based on an InGaAsP multi-quantum well structure and utilises two InGaAsP/InP epitaxial distributed Bragg reflectors (DBRs), one 50 periods high reflectivity bottom mirror and a top out-coupling mirror with lower reflectivity. This gives the advantage of top-emission combined with the possibility of achieving very narrow linewidth. The process is fully monolithic and full wafer scale compatible.

Spectral properties of a Tm:Ho:YAG laser in active mirror configuration

:Ho3+:YAG laser is operated in active mirror configuration. This laser is characterised by a rather thin crystal (3 mm thickness in our experiment) and a very short resonator (about 3.2 mm). Therefore this laser is well suited for operation in a single longitudinal mode. Single-frequency operation is demonstrated with and without an intracavity etalon. It is further shown that single-frequency emission is reached with a stable cw emission. With a slight detuning of the outcoupling mirror, spiking can be achieved, resulting in an emission on different lines. The assignment of the observed laser lines to the known transitions between Stark sublevels of the 5I7 and the 5I8 level is discussed and compared with predictions in the literature.

Beam divergence studies on hard edge unstable resonators for a long pulse XeCl excimer laser

The focusability of a long pulse XeCl excimer laser has been improved using confocal positive branch unstable resonators where the outcoupling is done through the convex mirror. A nearly diffraction limited output beam is obtained from hard edge unstable resonators. An improvement of the far field energy distribution is achieved with partial reflecting hard edge outcoupling mirrors. The phase delay between the central part of the beam and the beam edge caused by these partial reflecting mirrors can be reduced by using phase unifying mirrors. Using unstable resonators a brightness of 1.4 × 10 15 W/cm 2 sr has been obtained from a long pulse XeCl excimer laser

The effect of transverse modes in a wave-guide resonator on the resonance condition of a Compton FEL

In a free electron laser where the resonator has a wave-guide structure the phase velocity of the light has a significant impact on the wavelength of the output radiation. This effect is well known in Raman-type FEL's. It is however less known that this effect can also play an important role in Compton-type FEL's. This effect has been experimentally verified in our 6 MeV Compton-type FEL, having a wave-guide resonator with a hole in the outcoupling mirror. Successive transverse modes, which have distinct phase velocities, show up at different wavelengths in the output spectrum. The difference appeared to be as large as 5% of the resonance wavelength. In normal lasers this difference is not more than about 0.1%.

Optical cavities for IR-FELs at the FELI

Abstract The g -parameters of two optical cavities whose length is 6.718 m are designed to be stable. The FEL beam is outcoupled from the upstream mirror with an aperture by hole output coupling. The cavity mirrors are spherical and gold-coated on copper substrate. The outcoupling mirror has a hole of 0.5 mm diameter in the center. Our alignment system concept is that the optical axis of two mirrors is adjusted to coincide with the electron beam position. The electron beam position in the beamduct of the undulator is detected using three OTR plates and images of OTR at the plates are observed by CCD cameras.

Generation of tunable narrow-band midinfrared radiation in a type I potassium niobate optical parametric oscillator

We have demonstrated a type I phase-matched KNbO3 optical parametric oscillator (OPO) pumped by the fundamental of a Q-switched Nd:YAG laser. The wavelengths of the OPO’s signal and idler radiation are tuned continuously in the spectral regions of 1.45–2.01 and 2.27–4.0 μm, respectively. With a 7.8-mm-long crystal the energy density at threshold is 65 MW cm−2 for 9-ns-long laser pulses and a transmission of the outcoupling cavity mirror of 10%. At pump pulse energies of 82 mJ (two times above threshold) the OPO generates pulses of up to 6 mJ at the signal wave and 5.4 mJ at the idler wave. This output corresponds to a total conversion efficiency of 14%. The measured spectral width of the signal wave increases from 2.5 nm (at 1.56 μm) to more than 30 nm (at 1.97 μm). Injection seeding with the idler radiation of a 355-nm-pumped narrow-band β-barium borate OPO reduced the spectral width of the KNbO3 OPO output to a single longitudinal mode.

Temperature dependence of efficiency and thermal lensing of diode-laser-pumped Nd:YAG lasers

Efficiencies, lasing thresholds and thermal lensing of a Nd:YAG rod side-pumped with two 10 W diode-laser-arrays are measured for various rod temperatures between 80 K and 380 K. The threshold is reduced by a factor of 2.5–4 on cooling from room temperature to about 100 K, while the slope efficiency is increased by between 0% and 50% depending on the reflectivity of the outcoupling mirror. Extrapolation to a 240 W pumped laser gives an increase in optical efficiency of 25% to 30%. The thermal lensing measured with a beam deflection and an interferometric method is reduced by a factor of 10.

Passive stabilization of a synchronously pumped mode-locked dye laser with the use of a modified outcoupling mirror

Recently a new passive stabilization technique for synchronously pumped dye lasers was reported that allows one to generate nearly transform-limited pulses. The idea of this technique is to split off an extremely small fraction of the laser pulse and to superimpose it back on itself slightly advanced in time. In these experiments the seeding pulse was provided by an external cavity, which led to an undesirable frequency hopping. Here we present a simple modification of an outcoupling mirror that integrates the generation of the seeding pulse into the main laser cavity. In this way frequency-stable, nearly transform-limited pulses are generated.

Observation of optical chaos in a phase-conjugate resonator

The outcoupled power of a phase-conjugate resonator that uses four-wave mixing in BaTiO(3) is observed to undergo transitions from steady to periodic and from periodic to chaotic behavior as a function of time. Period doubling, period-three cycles, and period-five cycles are observed. Control parameters for the transitions are the diameter of an intracavity pinhole aperture, the length of the resonator, and the radius of curvature of the outcoupling mirror.

Mirror problems of a cw argon-ion laser at high output power

We report upon some experiments done on a continuous-wave argon-ion laser which offers 300 W output power in the visible and 34 W in the ultraviolet region of the spectrum, limited by the degradation of the resonator mirrors. To avoid destruction of the outcoupling mirror, a folded three-mirror instead of a two-mirror cavity was tried, resulting in nearly the same output power.

Gain induced stability of active plane-parallel resonators

It has been observed for a plane-parallel resonator with a saturated medium that under certain conditions confined beams mainly concentrated near the axis are present. The experiments have been done with a sealed-off plane-parallel CO2 laser of one meter length and with an internal diameter of 20 mm. Without gain variations such a system is unstable because of thermal defocussing by the heated gas. But due to sufficiently high gain variations the beam converges nevertheless. Since the gain variation increases with decreasing reflectivity confined beams near the optical axis are only observed at sufficiently low reflectivity. The experiments were done with a totally reflecting mirror and an outcoupling mirror having a reflectivity of, respectively, 90%, 80%, 50%, and 36%. For the high reflectivities the outcoupled beam is mainly concentrated near the edge with irregular density distributions. For the low reflectivity the beam is more or less radially symmetric, concentrated near the axis having decreasing intensity with the distance from the axis. The behaviour can be understood by an analysis in which the constant and quadratic term of the complex propagation constant near the optical axis due to gain variations, dispersion and heat effects are taken into account.