Mode Vertical-cavity Surface-emitting Laser Research Articles

Microscopic modeling and simulation of transverse-mode dynamics of vertical-cavity surface-emitting lasers

The dynamics of transverse modes of vertical-cavity surface-emitting lasers were simulated by use of a model that incorporates microscopically computed gain and refractive index with many-body effects. The model equations were solved by finite-difference methods in two-dimensional space and time domains without any a priori assumptions of symmetry of solutions or types and number of modes. Simulation was carried out for devices with and without index guiding at various pumping levels. We show that index-guided vertical-cavity surface-emitting lasers involve more transverse modes than purely gain-guided devices at the same pumping level. Both time-resolved and time-averaged near-field patterns over several time scales are investigated. Complicated spatial and temporal dynamic behaviors occur at higher pumping levels that include azimuthal rotating waves and intensity oscillations owing to dynamic competition between modes of the same order and those of different orders.

Spectral identification of transverse lasing modes of multimode index-guided vertical-cavity surface-emitting lasers

The identification of lasing modes in vertical-cavity surface-emitting lasers (VCSELs) is usually based on the polarization-resolved near-field pattern identification, a method difficult to apply in the presence of multiple transverse modes. We study lasing modes of a multitransverse mode index-guided VCSEL under dc pumping conditions by considering in detail the modes’ spectral positions. The experimentally observed spectra are compared with theoretical calculations, based on a recently developed method for vectorial VCSEL eigenmodes [D. Burak and R. Binder, Appl. Phys. Lett. 72, 891 (1998)]. This allows for the identification of transverse lasing modes as a function of the pumping current. Two main effects that influence the spectral characteristics of the VCSEL are considered in our approach: temperature-induced linear shift of lasing wavelengths with increasing pumping current and switching of the lasing mode between different eigenmodes of the VCSEL cavity. The obtained temperature-dependent shift of the lasing frequencies agrees reasonably well with the value known from previous experimental estimations.

Polarization-controlled single-mode VCSEL

Relative intensity noise (RIN) in a vertical-cavity surface-emitting laser (VCSEL) was greatly reduced through the use of polarization control to eliminate competition between two orthogonal polarization states by ensuring there was only one polarization state. Polarization was stable with optical feedback of up to 10%. Polarization control was achieved by inducing a small loss anisotropy in fundamental transversal mode VCSEL's. Anisotropic post structures, such as a rectangular post, an oblique post, or a zigzag-sidewall post, were found to be effective in creating loss anisotropy without serious degradation of other VCSEL characteristics such as light-output power or beam profile.

Selection and modulation of high-order transverse modes in vertical-cavity surface-emitting lasers

A theoretical proposal to double the pulse repetition frequency by alternating current modulation of two orthogonal high-order transverse modes of a vertical-cavity surface-emitting laser (VCSEL) is presented. Static and dynamic characteristics of weakly index-guided VCSELs in a multitransverse mode regime are analyzed. An efficient model that takes into account all the transverse modes supported by the waveguide is developed. Mode partition noise in a current modulated VCSEL is studied, taking into account the azimuthal degree of freedom. Different transverse modes can be excited with a probability that is numerically calculated. For high injection currents, modes that are not favored in the steady state can be excited with higher probability. The excitation probability is similar for azimuthally orthogonal modes. This symmetry can be broken by selecting a particular high-order transverse mode by using azimuthal-dependent current profiling. This selection can be achieved over current ranges as wide as ten times the threshold current. Current modulation of this transverse mode is then analyzed. Alternate current modulation of two orthogonal high-order transverse modes is also studied. Alternating modulation of these modes can double the pulse repetition frequency obtained by modulating just one high-order transverse mode, without increasing injected current density levels. This current-induced spatial switching leads to high-frequency beam steering in the laser azimuthal direction.

Strong polarization selectivity in 780-nm vertical-cavity surface-emitting lasers grown on misoriented substrates

We report that the 780 nm quantum well vertical-cavity surface-emitting lasers (VCSELs) grown on a 2° off misoriented (001) substrate toward (111)A exhibit a high polarization suppression ratio over a few hundred. The main polarization is always along the [−110] direction for all the lasers over the entire operating currents. To understand the physical origin of this polarization selectivity, the gain/loss difference between two competing polarization modes in VCSELs is investigated by measuring the subthreshold spectral linewidth. The obtained modal gain/loss difference is about 3.0 cm−1, which is sufficiently large for polarization stabilization and amounts to 4% of the threshold modal gain. Comparison with the subthreshold measurement and previous theoretical work shows significant discrepancy, which implies the possibility of other polarization selection mechanisms inducing such large gain/loss differences in 780 nm quantum wells grown on a misoriented substrate. In addition, it is found that the 780 nm VCSEL made of a bulk active medium grown on a misoriented substrate also shows a high polarization selectivity as quantum well lasers.

Suppression of higher-order transverse modes in vertical-cavity lasers by impurity-induced disordering

Etched pillar, bottom emitting vertical cavity surface emitting lasers (VCSELs) employing a zinc diffused and disordered spatial mode filter are fabricated and tested. The filters successfully suppress lasing of higher order transverse modes in VCSELs with diameters greater than 11 /spl mu/m, while obtaining output powers of up to 1.5 mW with threshold currents as low as 2.7 mA. Emission is entirely single mode, with mode suppression ratio greater than 25 dB up to the highest output power levels.

Continuous wave single transverse mode vertical-cavity surface-emitting lasers fabricated by helium implantation and zinc diffusion

A single transverse mode vertical-cavity surface emitting laser with moderate power has been fabricated using helium implantation and zinc diffusion. Devices with a 4 μm diameter active region exhibited a fundamental transverse mode (TEM00) operation with a maximum continuous wave output power of 1.5 mW and a quantum efficiency as high as 70%.