A new method to stabilize the polarization of vertical-cavity surface-emitting laser(VCSEL) was proposed and characteristics of the laser were investigated.
Polarization-stabilizing structure consists of fine grating pattern etched at the top layer of distributed Bragg reflector(DBR). On the sidewalls of the grating grooves, aluminium was thermally deposited to enhance polarization selectivity. The tested periods of the grating grooves were 0.9 ㎛, 1.2 ㎛, and 1.5 ㎛, and the widths were half of the periods. After the deposition, lasers with window sizes of 10 ㎛, 15 ㎛, 20 ㎛, 30 ㎛ were fabricated in usual procedure.
After the fabrication, L-I curve and transverse mode image of VCSEL were investigated with a polarizer.
Polarization-stabilized VCSEL's whose grating periods are 1.2 ㎛ and 1.5 ㎛ lased in TM mode only. The polarization ratio was well over 25:1, and threshold currents were almost the same with those of VCSEL's without the grating. VCSEL with 0.9-㎛-grating did not lase in stabilized polarization, which means that exact calculation of diffraction theory should be conducted for each groove width and depth to fully understand the function of the metal-clad grating.
On the contrary, polarization of VCSEL without the grating was not stabilized. Polarization of higher transverse mode was orthogonal to that of fundamental mode, and therefore, two polarization mode lased simultaneously in high current region. Moreover, near field image taken by CCD camera revealed that higher modes tend to lase in spatially exclusive manner.
