Highly Coherent, High Power 1 µm GaAs VeCSELs

, by Arnaud Garnache, Mikhael Myara

VeCSEL Design Basics

Usually, our VeCSEL devices are formed by the gain mirror (1/2-VCSEL), a mm-cm air gap, and a commercial flat mirror with a reflectivity of 99% . The chip is soldered on a Peltier element to stabilize the chip temperature with a precision of 10-3 K. A Piezoelectric Transducer (PZT) is used to continuously tune the cavity length, thus the laser frequency. The external mirror is held by an ultra-stable mirror mount. The 1/2-VCSEL structure is optically–pumped in cw, by means of single-mode pump diode, multimode pump diode or commercial fibre–coupled high power multimode laser diode. The pump beam is focused on a circular spot size some tens or hundreds microns with two commercial achromat lenses at a typical brewster incidence angle. The components are glued on a home made breadboard and inserted in a metallic box for thermal and acoustic noise isolation. The following shows a typical VeCSEL set-up as made at IES, with a photography of a prototype typically developed for advanced studies of the laser coherence.

These lasers can easily select a single transverse mode (|k>) thanks to the high finesse / low losses optical cavity design and the higher orders modes filtering by the pump be localisation. This permits to reach very high beam quality with M2 propagation factors down to 1.1, limited by the measurement. Moreover, this single-transverse mode regime permits to reach a single-frequency operation (single longitudinal mode selection: |z>) because these photonic components behave in a first approximation like ideal homogeneous gain lasers, with very reduced non-linear effects, due to the laser geometry (homogeneous gain, thin active medium and "empty" cavity). The single-frequency regime is thus obtained without requiring any intracavity filter, as it can be reached by the gain-curvature induced modes competition. The polarization state is also strongly linear (|E>), owing to slight gain dichroism and high finesse cavity. This leads to the conclusion that these lasers easily select a unique state of the light :

This ultra pure light state exhibits exciting features such as very high SMSR values, ultra-low intensity noise and narrow linewidth (<50 kHz) free running.

High Power, High Coherence VeCSEL Design

For many reasons, high power operation usually disturbs the single transverse mode single frequency operation of most lasers. It is obvious that High Power VeCSELs require thermal management at the level of the 1/2-VCSEL and high quality semiconductor layers over large surfaces. A high pump power usually induces thermal lens effects which changes the stability conditions of the cavity modes. In order to stabilize only the fundamental transverse mode in high power vecsels, we in fact take advantage of this pump induced thermal lens, without here the need of a concave mirror.

With such a set-up, multiwatt operation has been obtained while high coherence were observed, even concerning the propagation factor in spite of the use of the thermal lens, the intensity noise and the linewidth.

VeCSELs properties

Here follows the optical output power as a function of the pump power for a thermal-lens stabilized VeCSEL cavity. The apparent high threshold and non-linear slope efficiency is due to bad pump to laser overlapping without thermal lens,
leading to a > 100% actual efficiency as the laser waist reduces : the cavity stability was calculated for high pump power. Note that QW absorption is
negligible and quickly bleached here, as laser waist is not much bigger than pump waist above threshold ( 20× transparency density at peak intensity) and laser intensity rapidly stronger than the absorption saturation intensity respectively.

This result was obtained with a high coherence beam, with TEM00 gaussian beam (M2 < 1.2) and high SMSR value (> 45 dB, limited by the spectrum analyzer apparatus function). Typical optical spectrum and beam shapes are shown below.

The high finesse cavity permits to reach ultra high SMSR values (> 60 dB) at the quantum limit, while exhibiting a highly coherent low divergence beam.