Monolithic Microlens VCSELs With High Beam Quality

In this paper, we report on high beam quality vertical-cavity surface-emitting lasers (VCSELs) featuring a monolithically integrated aspherical microlens. This lens is fabricated by a one-step diffusion-limited wet etch process. VCSELs with microlenses with different curvature radius are designed, fabricated, and characterized, as well as reference VCSEL devices without the microlens. Significant improvement of beam divergence and single-mode performance are demonstrated in microlens VCSELs. The divergence angles of a conventional VCSEL are
${\text{14.5}}$
°,
$16.0$
°, and
$17.0$
° (full width at D4
$\sigma$
) at 2 mA, 3 mA, and 4 mA, respectively, with a
$M^2$
value measured to be 6.9 at 3 mA. Under the same current injection, the divergence angles of a comparable VCSEL with a micro-lens (curvature radius of 31.8 
$\mu$
m) are reduced to
$4.8$
°,
$5.5$
°, and
$5.7$
°, respectively. The
$M^2$
value of the lensed VCSEL is as low as 1.9 at 3 mA.