Nonabsorbing Mirrors Research Articles

Multiphysics analysis of chip and resonator designs for increased damage threshold of external cavity high-power laser diodes.

We present a detailed analysis of multiphysics simulation results to evaluate the threshold for catastrophic optical damage (COD) of high-power laser diodes under misaligned external optical feedback. Three different chip designs are investigated: the non-injecting mirror concept, the non-absorbing mirror concept and the introduction of an additional energy barrier within the waveguide near the front facet. Furthermore, a modification of the external resonator that promises a lower sensitivity towards misalignments is considered. The dependence of the COD threshold on the additional design parameters (bandgap change, modification length, focal length) and the impact of the different approaches on electro-optical efficiency as well as beam quality are analyzed. Compared to the initial design, the different chip design concepts promise an increase of the achievable output power by 8%, 27% and 27% respectively, whereas the modified resonator fully prevents feedback-induced failure.

Facet Cooling in High-Power InGaAs/AlGaAs Lasers

Several factors limit the reliable output power of a semiconductor laser under CW operation, such as carrier leakage, thermal effects, and catastrophic optical mirror damage (COMD). Ever higher operating powers may be possible if the COMD can be avoided. Despite exotic facet engineering and progress in non-absorbing mirrors, the temperature rise at the facets puts a strain on the long-term reliability of these diodes. Although thermoelectrically isolating the heat source away from the facets with non-injected windows helps lower the facet temperature, data suggests the farther the heat source is from the facets, the lower the temperature. In this letter, we show that longer non-injected sections lead to cooler windows and biasing this section to transparency eliminates the optical loss. We report on the facet temperature reduction that reaches below the bulk temperature in high power InGaAs/AlGaAs lasers under QCW operation with electrically isolated and biased windows. Acting as transparent optical interconnects, biased sections connect the active cavity to the facets. This approach can be applied to a wide range of semiconductor lasers to improve device reliability as well as enabling the monolithic integration of lasers in photonic integrated circuits.

High-power broad-area buried-mesa lasers

Broad area lasers emitting near 940 nm are fabricated using a process based on two-step epitaxy. The n-side of the layer structure and the active layer are grown during the first epitaxial step, the p-side during the second. Between the first and the second step a shallow etching is used to remove the active layer from the two sides and at the two facets. This simple approach allows the creation of buried mesa lasers with non-absorbing mirrors, resulting in a reduced lateral current leakage, lower threshold current and higher efficiency, plus an increased robustness with respect to catastrophic optical damage.

InGaAs/GaAs quantum well intermixing using proton irradiation for non-absorbing mirror

We report a proton irradiation-induced intermixing on InGaAs/GaAs quantum well (QW) heterostructures with thick upper cladding layers. Proton irradiation was performed with different dose, followed with annealing. Proton irradiated and annealed sample shows the blue-shift of the photoluminescence (PL) spectra and the blue-shift energy was increased up to about 30 meV with increasing dose, but was insensitive to the annealing temperature. Finally, we calculated that the relative absorption coefficient at the laser wavelength for the irradiated sample with dose of 1 × 1016/cm2 was 0.045, suggesting such proton irradiation-induced intermixing is a promising approach for fabricating non-absorbing mirror.

Theoretical analysis of the inter-diffusion coefficients in the AlGaInP material quantum well intermixing

Quantum well intermixing (QWI) has been widely used in modifying the bandgap of semiconductor materials, post-growth; it has been investigated in fabricating non-absorbing mirror regions of laser cavities to improve output power. In this work, the QWI mechanism is briefly introduced. A concentration distribution function in multiple quantum wells is mathematically obtained for odd and even wells respectively. In addition, a 650nm AlGaInP/GaInP quantum well wafer is fabricated by metal organic vapor phase epitaxy, and a series of quantum well intermixing experiments is accomplished by Zn impurity diffusions. Based on experimental data, a concentration distribution function is simulated and an inter-diffusion coefficient between Al and Ga is calculated. Finally, the effects of QWI on the inter-diffusion coefficient are discussed.

Output Power Limitations and Improvements in Passively Mode Locked GaAs/AlGaAs Quantum Well Lasers

We report a novel approach for increasing the output power in passively mode locked semiconductor lasers. Our approach uses epitaxial structures with an optical trap in the bottom cladding that enlarges the vertical mode size to scale the pulse saturation energy. With this approach we demonstrate a very high peak power of 9.8 W per facet, at a repetition rate of 6.8 GHz and with pulse duration of 0.71 ps. In particular, we compare two GaAs/AlGaAs epilayer designs, a double quantum well design operating at 830 nm and a single quantum well design operating at 795 nm, with vertical mode sizes of 0.5 and 0.75 μm, respectively. We show that a larger mode size not only shifts the mode locking regime of operation towards higher powers, but also produces other improvements in respect of two main failure mechanisms that limit the output power: the catastrophic optical mirror damage and the catastrophic optical saturable absorber damage. For the 830 nm material structure, we also investigate the effect of non-absorbing mirrors on output power and mode locked operation of colliding pulse mode locked lasers.

Analytical model for low finesse, external cavity, fiber Fabry–Perot interferometers including multiple reflections and angular misalignment

We present an analytical model for single mode, multiply reflected, external cavity, optical fiber Fabry-Perot interferometers in the low finesse regime using simple geometry and the Gaussian beam approximation. The multiple reflection model predicts attenuation of the peak-to-peak interference as the fiber to mirror distance approaches zero, as well as fringe asymmetry in the presence of nonabsorbing mirrors. A series of experiments are conducted in which a series of fiber Fabry-Perot cavities are constructed using uncoated, single mode glass fibers, and mirrors of varying reflectivity. The cavity length is swept, and the predictions of the model are found to be in good agreement with the experimental interferograms.

Non-Absorbing Mirrors for Quantum Well Colliding Pulse Mode-Locked Lasers

We present results from GaAs/AlGaAs quantum-well colliding-pulse mode-locked lasers emitting pulses at 830 nm with a repetition rate of 20 GHz. In particular we investigate the effects of nonabsorbing mirrors on the output power characteristics of these devices in mode-locking operation. Under best mode-locking conditions, the lasers with nonabsorbing mirrors show ~10% shorter pulses (1.84 ps) and narrower radio-frequency peaks but operate with less average power.

Phononic and photonic band gap structures: modelling and applications

Photonic crystals (PhCs) are artificial materials with a permittivity which is a periodic function of the position, with a period comparable to the wavelength of light. The most interesting characteristic of such materials is the presence of photonic band gaps (PBGs). PhCs have very interesting properties of light confinement and localization together with the strong reduction of the device size, orders of magnitude less than the conventional photonic devices, allowing a potential very high scale of integration. These structures possess unique characteristics enabling to operate as optical waveguides, high Q resonators, selective filters, lens or superprism. The ability to mould and guide light leads naturally to novel applications in several fields.Band gap formation in periodic structures also pertains to elastic wave propagation. Composite materials with elastic coefficients which are periodic functions of the position are named phononic crystals. They have properties similar to those of photonic crystals and corresponding applications too. By properly choosing the parameters one may obtain phononic crystals (PhnCs) with specific frequency gaps. An elastic wave, whose frequency lies within an absolute gap of a phononic crystal, will be completely reflected by it. This property allows realizing non-absorbing mirrors of elastic waves and vibration-free cavities which might be useful in high-precision mechanical systems operating in a given frequency range. Moreover, one can use elastic waves to study phenomena such as those associated with disorder, in more or less the same manner as with electromagnetic waves.The authors present in this paper an introductory survey of the basic concepts of these new technologies with particular emphasis on their main applications, together with a description of some modelling approaches.

Thermal runaway and optical efficiency in InAs/GaAs quantum dot lasers

In contrast to quantum well lasers, we show thermal runaway effects are prominent for quantum dot (QD) lasers emitting at 1300 nm. In addition to the surface states at the cleaved facet and the effect of strain relaxation, which can couple confined states with surface states, the exposed relaxed QDs at the cleaved facet can themselves contribute to extra nonradiative surface recombination, aggravating the facet heating problem. Notable improvement in optical efficiency after the formation of nonabsorbing mirrors by a laser annealing technique highlights the significance of the thermal runaway problem for 1300 nm QD lasers, and demonstrates an effective postfabrication solution.

Strained-Layer Quantum-Well Lasers

This tutorial article explains the two important reasons for the introduction of strain into the active region of a quantum-well laser. First, it reduces the density of states at the top of the valence band, which allows population inversion to be obtained at a lower carrier density. Second, it distorts the 3-D symmetry of the crystal lattice and matches it more closely to the 1-D symmetry of the laser beam. Together these effects greatly enhance almost all characteristics of semiconductor lasers and make possible a wide range of applications. Combinations of compressive and tensile strain can also be used, for example, to produce nonabsorbing mirrors and polarization-insensitive semiconductor optical amplifiers.

Solution to causality paradox upon total reflection

A dispute about the existence of an additional time (named as the Goos-Hanchen time) associated with the Goos-Hanchen shift in total reflection has recently arisen. At the same time, an inconsistency between the optical ray model and the electromagnetic theory also appears in the optical planar waveguide. By analyzing light propagation in an optical planar waveguide with both the zigzag-ray model and the electromagnetic theory, this paper shows that the Goos-Hanchen time really exists, and the total time delay upon total reflection upon an ideal nonabsorbing plasma mirror is the sum of the group-delay time and the Goos-Hanchen time. The causality paradox of total reflection of a TM wave upon an ideal nonabsorbing plasma mirror is also solved taking into consideration the negative Goos-Hanchen shift. Finally, the expression of the group velocity of the guided mode in optical planar waveguide was obtained, which clearly shows that the time delay upon total reflection is the sum of the group-delay time and the Goos-Hanchen time at given any time.

High power (>10 W from 100 µm aperture) high reliability 808 nm InAlGaAs broad area laser diodes

Single stripe InAlGaAs laser diodes with non-absorbing mirrors (NAMs) operating at 808 nm have been fabricated and tested. The inclusion of the NAM more than doubles the catastrophic optical damage power and greatly improves the reliability of the laser diode. A maximum CW power of over 10 W has been achieved from an 808 nm broad area laser with a 100 μm stripe width.

Solution to causality paradox upon total reflection in optical planar waveguide

A dispute about the existence of an additional time associated with the Goos-Hänchen shift has recently arisen. By analyzing light propagation in an optical planar waveguide with both the zigzag-ray model and the electromagnetic theory, we show in this paper that the Goos-Hänchen time really exists, and the total time delay upon total reflection is the sum of the group delay time and the Goos-Hänchen time. The causality paradox of total reflection of a TM wave upon an ideal nonabsorbing plasma mirror is also solved with the consideration of a negative Goos-Hänchen shift.

Time delay associated with total reflection of a plane wave upon plasma mirror

By employing both the ray model and the electromagnetic theory in a slab optical waveguide, we show that the Goos-Hänchen time, which has been recently argued in the literature, really exists and the time associated with total reflection of a plane wave upon nonabsorbing plasma mirror is exactly the sum of the group delay time and the Goos-Hänchen time. Based on this concept, it is also indicated that the causality is preserved not only for the frustrated Gires-Tournois interferometer case but also for the case of total reflection of a plane TM wave on a nonabsorbing plasma mirror.

Improved catastrophic optical damage level from laser with nonabsorbing mirrors

The authors demonstrate an improved catastrophic optical damage (COD) level from a ridge laser with nonabsorbing mirrors (NAMs) fabricated by quantum well intermixing. Under destructive testing conditions, the COD level of the NAM laser was improved by a factor of 2.6 compared to the standard laser, attributed to reduced absorption induced facet degradation. Verification of the degradation mechanism was confirmed by inspection and removal of damaged facets.

980-nm master oscillator power amplifiers with nonabsorbing mirrors

High-power master oscillator power amplifiers with nonabsorbing mirrors have been fabricated. These devices exhibit 1.0 and 1.5 W of single-frequency diffraction-limited continuous-wave optical powers at relatively low-amplifier currents of 1.75 and 2.6 A, respectively. These devices operate with a single longitudinal mode with a 40-dB sidemode suppression ratio over the entire amplifier current range without any longitudinal mode hops as a result of the very low optical feedback from the PA facet. Coupling of over 500 mW into a single-mode fiber is also achieved.

A new structure of 780 nm AlGaAs/GaAs high power laser diode with non-absorbing mirrors

A novel high-power AlGaAs laser structure, which is named MBG (melt-back-grown)-NAM (non-absorbing mirror), has been proposed. The NAM regions are formed on both sides of the resonant facets to prevent the catastrophic optical damage due to the optical absorption, which causes local temperature rise of the facets. Instead of conventional chemical wet etching, the melt-back process technique during the liquid phase epitaxy was used to form window regions to prevent the interface degradations. It is observed that the maximum output power of the laser diodes with the new structure was three times higher than that of the conventional-structure laser. Therefore, the new method of forming the non-absorbing mirror region can be considered quite effective to reduce the interface defects between the NAM and internal active region.

980 nm high power, high slope efficiency distributedfeedback lasers with nonabsorbing mirrors

High-power InGaAs-GaAs distributed feedback lasers with nonabsorbing mirrors have been fabricated. These devices exhibit over 215 mW of single frequency CW optical power at a bias current of 300 mA and an operating temperature of 25°C. Low threshold currents of 15 mA and high slope efficiencies of 0.9 W/A near threshold are also obtained.

High-power InGaAs-GaAs-AlGaAs distributed feedbacklasers with nonabsorbing mirrors

High power InGaAs-GaAs distributed feedback (DFB) lasers with nonabsorbing mirrors have been fabricated with over 200 mW of single frequency CW optical power at 25°C (110 mW at 95°C). Unlike their distributed Bragg reflector counterparts, the DFB devices exhibit relatively linear optical power against current curves. Low threshold currents of ~10 mA were also obtained.