Sigurgeir Kristjansson

Grating coupled surface emitters: integrated lasers, amplifiers, and beam shaping outcouplers

Grating coupled surface emitting lasers (GCSELs) are in- plane lasers monolithically integrated with grating outcouplers for beam shaping and image generation. Highly directional and efficient outcouplers can be formed using various grating and waveguide geometries. Beam shaping features are incorporated using a computer generated waveguide hologram that allows the wavefront of the emitted light to be tailored for the required beam shape. Requirements on the integrated in-plane laser include a wide and spatially coherent guided wave with a minimum of wavefront distortion and a stable emission wavelength. Promising lasers for this purpose are master-oscillator power-amplifier configurations as well as various kinds of unstable resonator designs. Here we present results from modeling and experiments on the key elements of GCSELs as well as fully integrated GCSELs of linear and circular geometries.

Circular grating coupled DBR laser with integrated focusing outcoupler

We report on the fabrication and operation of a circular grating coupled, AlGaAs-InGaAs surface-emitting distributed Bragg reflector (DBR) SQW laser with an integrated focusing outcoupler. Electron beam lithography and reactive ion beam etching were used to fabricate the laser with a first-order resonant grating for optical feedback and a separate detuned, chirped second-order grating for outcoupling and focusing the beam.

Surface-emitting tapered unstable resonator laser with integrated focusing grating coupler

We report on the fabrication and operation characteristics of a grating-based, surface-emitting, tapered unstable resonator laser with a trumpet shaped taper and a focusing outcoupler. The laser has a low threshold current density of 225 A/cm/sup 2/. It operates with a spatially coherent field and the near-field intensity indicates stable, filamentation free behavior up to at least 3/spl times/ the threshold current under continuous operation. At low drive currents the laser produces a nearly diffraction limited focused spot. With increasing current, the focused spot size increases and multiple side-lobes develop at high currents. We attribute this to a change in the phase of the emitted wavefront associated with thermal effects in the gain region.

Circular grating-coupled surface-emitter with high-quality focused output beam

We have investigated the effect of resonator size and geometry on the performance of focusing circular grating-coupled surface-emitting lasers. Both the effects of reducing the resonator diameter and introducing an unpumped indentation in the center of the resonator have been studied. We have found that inclusion of such an unpumped central indentation can greatly improve the spatial-mode behavior, resulting in a very uniform near-field intensity, single wavelength emission, and a focused spot size only 1.5 times the diffraction limit for a laser oscillating in the fundamental circular mode. Reducing the resonator diameter of lasers without an unpumped central indentation does not offer similar improvement.

Infrared-sensitive InGaAs-on-Si p-i-n photodetectors exhibiting high-power linearity

We report on the device fabrication and measured performance of p-i-n photodiodes made from wafer-bonded InGaAs-on-Si material. Dark currents below 38 pA and 3-dB bandwidths above 11 GHz were measured for a -4-V bias and for an active area diameter of 20 /spl mu/m. The thermal conductivity of silicon enables a 200-MHz 1-dB compression current of 76.5 mA, measured on nonoptimized test devices. These devices dissipate upwards of 612 mW of electrical power. High-yield wafer-scale fabrication of InGaAs-on-Si p-i-n photodetectors is demonstrated.

Optical field analysis of a circular grating-coupled surface-emitting laser with integrated focusing outcoupler

The lasing characteristics of a circular grating-coupled surface-emitting laser (GCSEL) with an integrated focusing outcoupler are investigated. Measured near and far fields suggest lasing in two near orthogonal and mutually incoherent linear elements, rather than ideal circular modes of the planar circular resonator, resulting in a degradation of the focused spot quality. Modeling of the evolution of the emitted optical field by Fresnel diffraction calculations, using a new orthogonal projection scheme to account for polarization, supports this assumption. Using the theory, we have also estimated the degradation of the focused spot quality induced by random errors in the period of the focusing outcoupler caused by the limited resolution of the electron-beam lithography system used to fabricate the device. We found this effect to be small compared to the effect of the nonideal mode characteristics.

Observation of stable cylindrical modes in electrically pumped circular grating-coupled surface-emitting lasers

We present the results of a comprehensive experimental study of the modal behavior of electrically pumped circular grating-coupled surface-emitting lasers with resonant second-order gratings. Our results show stable cylindrical mode operation with azimuthal orders m = 1 and m = 52. We have found that, when a passive, unpumped 0.7-microm-deep indentation is included at the center of the circular resonator contact, the modal behavior becomes more stable with respect to increasing current. This result manifests itself in the lasing of two orthogonal m = 1 modes along with one higher-order mode from threshold to 2.2 times threshold or more. The spatial coherence of the cylindrical modes over the area of the resonator leads to better beam quality. Lasers without passive central indentations exhibit mode hopping between quasi-linear modes and higher-order cylindrical modes as the current is increased.

Effects of feedback from collimating, focusing, and spot-array generating outcoupler gratings in surface-emitting semiconductor lasers.

We experimentally demonstrate that a grating outcoupler used for complex beam shaping (spot-array generation) can produce unintentional optical feedback that severely disturbs the integrated in-plane laser that illuminates the coupler. Simulations show that these outcouplers, in contrast to conventional collimating or focusing outcouplers, tend to produce high levels of feedback in spite of the detuning used to suppress feedback. Further, this feedback is focused to high intensity in the laser gain medium. This focused light acts as a seed for the nonlinear self-focusing that causes wave-front distortion and filamentation, which degrades the beam quality.

Grating-based surface-emitting tapered unstable resonator laser simulations and experiments

We have fabricated and characterized tapered unstable resonator lasers with two slightly different resonator geometries, one with a linear taper and one with a nonlinear taper, to investigate the effect of resonator geometry on the performance characteristics. Special attention has been paid to the modal behavior and its dependence on drive current. We investigate the modal behavior by monitoring the focused spot produced by an integrated focusing grating coupler. To aid in the design of the lasers and the analysis of the measurement results, we implemented numerical simulations based on the beam-propagation method and obtained good agreement between measurements and simulations. We find that the threshold current, the efficiency and the optical field stability, including filamentation threshold, are significantly improved by adjusting the shape of the current injection region to the diverging field profile in the tapered section. Improvements in threshold current and efficiency are attributed to a more efficient use of the injected carriers, while improvement in optical field stability is attributed to reduced carrier induced phase front distortion. We conclude that a relatively small modification of the resonator geometry can lead to large improvements in laser performance.

Surface-emitting tapered unstable resonator laser with integrated focusing grating coupler comparing linear and trumpet shaped taper

Here we compare the performance of InGaAs SQW tapered unstable resonator lasers (TURL) with linear-shaped-taper with lasers that have a trumpet-shaped-taper that follows the shape of the optical mode as it expands while propagating in the laser resonator. The laser resonator is defined by two first-order feedback gratings and is completely separated from the grating outcoupler.