High Power, Low RIN, 1.55μm DFB Laser for Analog Applications

In this paper we present a directly modulated laser (DML) designed for high dynamic range analog link [1]. These devices are of great interest for local oscillator distribution or receiver signal remoting. Use of direct modulation is simpler and less expensive than external modulation. To get high power and high efficiency we focused in a first time on the reduction of internal losses in the cavity by decreasing the optical confinement in p-doped indium phosphide (InP). The main constraint was to maintain a sufficient overlap between the optical field and the quantum wells to have a low relative intensity noise (RIN) and a large modulation bandwidth. A compromise has been done on the optical confinement to get in the same time good static performances (power and efficiency) and dynamic performances (RIN and bandwidth). In a second step we tried to reduce the beam divergence and, above all, the ellipticity of the mode. Wafers were processed in dual channel shallow ridge DFB. 1mm long cavities were cleaved and facets were anti-reflective (AR)/ highly reflective (HR) coated. Lasers are mounted p-up on AlN submounts integrating coplanar lines for both DC and RF characterization. Maximum power was 135 mW at 600 mA bias current. The efficiency defined as (Power/(Current-Threshold current)) was up to 0.3 W/A at a bias current as high as 500 mA. RIN measurement has showed a RIN level below -155 dB/Hz on the 40 MHz to 20 GHz range at 450 mA bias current. The modulation bandwidth is up to 6.5 GHz. The side mode suppression ratio (SMSR) exceeds 55dBm. Due to the use of shallow ridge structure linearity of P-I and injection current is very good but the divergence of the beam FHWM of 14°x31°. Future improvements will focus on the reduction of the divergence and the ellipticity of the beam, necessary for a better coupling into an optical fibre