Mark Lichtner

Measurement and Simulation of Distributed-Feedback Tapered Master-Oscillator Power Amplifiers

The spectral and spatial behavior of monolithically integrated distributed-feedback tapered master-oscillator power amplifiers emitting around 973 nm is experimentally and theoretically investigated. We demonstrate a good agreement between experiments and theory and analyze peculiarities of the observed dynamical regimes.

Stripe-array diode-laser in an off-axis external cavity: theory and experiment.

Stripe-array diode lasers naturally operate in an anti-phase supermode. This produces a sharp double lobe far field at angles +/-alpha depending on the period of the array. In this paper a 40 emitter gain guided stripe-array laterally coupled by off-axis filtered feedback is investigated experimentally and numerically. We predict theoretically and confirm experimentally that at doubled feedback angle 2alpha a stable higher order supermode exists with twice the number of emitters per array period. The theoretical model is based on time domain traveling wave equations for optical fields coupled to the carrier density equation taking into account diffusion of carriers. Feedback from the external reflector is modeled using Fresnel integration.

Theoretical Investigation of Striped and Non-Striped Broad Area Lasers With Off-Axis Feedback

We report a method to improve the beam quality of broad area lasers by using a V-shaped external cavity formed by two off-axis feedback mirrors that allow to select a single transverse mode with transversally modulated intensity distribution. In the case when one of the two feedback mirrors is absent a spontaneous formation of self-induced transverse population grating leading to a reduction of the lasing threshold is observed. Most favorable conditions for stabilization of a single transverse supermode and creation of a high power and high brightness plane wave traveling in the extended cavity are obtained for equal reflectivities of the two external reflectors.

Novel Approach to Finite-Aperture Tapered Unstable Resonator Lasers

We present a novel concept of a finite-aperture tapered unstable resonator laser based on two distributed Bragg reflectors at the rear and front facets with different lateral widths. An analysis of the passive resonator, as well as a spatiotemporal simulation of the active device, is presented. A variation of the ratio between the widths of both Bragg reflectors changes the losses of the lateral modes differently, leading to certain geometries with good modal discrimination and beam quality. The principle of operation is demonstrated by fabrication and experimental characterization of a first-generation of device.

Dynamic simulation of high brightness semiconductor lasers

High-power tapered semiconductor lasers are characterized by a huge amount of structural and geometrical design parameters, and they are subject to time-space instabilities like pulsations, self-focusing, filamentation and thermal lensing which yield restrictions to output power, beam quality and wavelength stability. Numerical simulations are an important tool to find optimal design parameters, to understand the complicated dynamical behavior and to predict new laser designs. We present a fast dynamic high performance parallel simulation tool suitable for model calibration and parameter scanning of the long time dynamics in reasonable time. The model is based on traveling wave equations and simulation results are found to be in satisfactory agreement with experimental data.

High-brightness emission from stripe-array broad area diode lasers operated in off-axis external cavities

Phase-coupled stripe-array diode lasers show a strong double-lobed far-field because adjacent stripes tend to operate in the anti-phase supermode. One way to achieve a stable phase relationship and global coupling of the emitters of such a stripe-array is off-axis feedback. In this work several off-axis external cavity designs are discussed. A 400 μm wide emitter stripe array consisting of 40 stripes with a pitch of 10 μm was investigated. By operating this device in a Littrow type off-axis external cavity, more than 2 W of output power of near diffraction-limited, single longitudinal mode emission with a brightness as high as 88 MW/cm2-str could be achieved. The technique of off-axis feedback was also adapted to realize spectral beam combining of 25 emitters of a laser bar. The experimental data are compared with numerical simulations using a new theoretical model including feedback.

A novel approach to finite-aperture tapered unstable resonator lasers

A novel concept of a Finite Aperture Tapered Unstable Resonator Laser (FATURL), based on two distributed Bragg reflectors (DBRs) at the rear and front facets with different lateral widths, is presented. The passive resonator analysis using Fresnel-Kirchhoff diffraction theory shows that with the grating widths the losses of the different lateral resonator modes change differently. This leads to resonator geometries, characterized by nearly integral Fresnel-numbers with higher modal discrimination of the fundamental lateral mode. In spatial-temporal simulations of the active device the influence of carrier-photon interaction on that effect is studied showing that the inhomogeneous index change by spatial hole-burning in the carrier density may become dominant over the geometric modal discrimination. Both, the influence of the grating widths on the suppression of higher order modes as well as the influence of spatial hole-burning is found in the experimental characterization of a first generation of fabricated devices.

Wann geht der Laser an

Ein Laser ist eine Lichtquelle, bei der durch Zufuhr von elektrischer Energie Licht besonderer Gute oder Reinheit (einfarbig, koharent, polarisiert) erzeugt werden kann. Das dynamische Verhalten des Lasers hangt von der Energiezufuhr ab. Sie bestimmt, ob der Laser zum Beispiel dauerhaft Licht konstanter Intensitat liefert, ob er dauerhaft flackert, ob er nach kurzem Aufflackern ausgeht, oder ob er noch anderes Verhalten zeigt.

Improving the stability of distributed-feedback tapered master-oscillator power-amplifiers

We consider theoretically the lasing properties of distributed-feedback master-oscillator power-amplifiers which are compact semiconductor laser devices capable of emitting a high brilliance beam at an optical power of several Watts. Based on a traveling wave equation model we calculate emitted optical power and spectral maps for increasing current injections of the power amplifier. We show that a proper choice of the Bragg grating allows to optimize the laser operation, so that the laser emits a high intensity continuous wave beam and shows no previously observed mode jumps or dynamic instabilities when injection currents are tuned.

Calculation of improved features of distributed-feedback tapered master-oscillator power-amplifiers

During recent years, compact lasers emitting single-frequency, diffraction limited continuous-wave CW beams at an optical power of several Watts have received considerable attention regarding several applications, such as frequency conversion, free-space communications, and pumping of fiber lasers and fiber amplifiers. A device which is capable to maintain a good beam quality and wavelength stability in the Watt range is the monolithically integrated master-oscillator power-amplifier (MOPA), where either a distributed Bragg reflector (DBR) [1] or a distributed feedback (DFB) [2] laser and a flared (or tapered) gain-region amplifier are combined on a single chip. Figure 1 shows the structure of the DFB MOPA device that has been considered. It consists of an index guided DFB ridge-waveguide (RW) laser and a gain-guided tapered amplifier. Both regions are separately contacted and can be driven independently. The PA has a total flare angle of 6°. In order to understand the origin of different instabilities and transitions between different states we analyze and simulate a mathematical model based on travelling wave equations for the complex slowly varying amplitudes of the counterpropagating optical fields coupled to a diffusion equation for the excess carrier density and an equation for gain dispersion [2, 3]. Figure 2 (left) shows calculated optical spectra with increasing PA injection currents. The diagrams show a basic red shift of the lasing wavelength with increasing injected current as well as periodic changes of the lasing modes. To understand the properties of the laser, we consider the laser dynamics in terms of bifurcation diagrams. We perform a bifurcation analysis by using the power amplifier current as bifurcation parameter. We find Hopf bifurcation points and a scenario compatible with the quasiperiodic route to chaos is obtained as the injection current is increased.