Optical Millimeter Wave Generation And Data Transmission Using A Dual-polarization Emission External Cavity Diode Laser

Abstract

Tunable millimeter wave generation by photomixing one TE and one TM longitudinal mode of a dual-polarization external cavity diode laser is presented. Data transmission at 140 Mbit/s onto a 60 GHz beat carrier is demonstrated by externally modulating one of the modes. Introduction Optical mixing of two laser modes is a straight-forward technique for the generation of millimeter waves. Potential applications of this method include optoelectronic metrology such as the characterization of high-speed electronic and photonic devices [l] and signal transmission at millimeter wave carrier frequencies via fibers [2,3]. Here, we report on the generation of millimeter waves and their use for data transmission by means of beating the two modes of a dual-polarization emitting external cavity diode laser (ECDL). This technique offers the advantage of narrow free-running mode linewidths, a wide beat frequency tuning range potential up to several THz, and an easy separation of the two modes for external modulation and control. Generation of tunable millimeter waves The experimental setup of the dual-polarization ECDL is shown in Fig. 1 [4]. The laser diode is of a 1.55pm InGaAsPflnP ridge-waveguide structure designed for having a polarization-insensitive modal gain. In order to achieve optimum coupling between the active device and the two external resonators, a two-layer AR-coating was applied on one facet. Its output emission is collimated, subsequently split in a polarization beam splitter (PBS 1) and diffracted back by two 1200 l/mm gratings forming a mode and polarization-selective twoarm external cavity. In order to be able to adjust the feedback levels of the two modes corresponding to the two arms, variable attenuators were inserted. The laser output of the uncoated facet is coupled to a lens-ended polarization maintaining fiber (PMF) and subsequently split in PBS2 for power monitoring and recombined for the detection by a monochromator (M), a Fabry-Perot spectrum analyzer (FP-SA) and a fast photodetector (PD) When the two spectral modes are mixed in the fast photodetector a distinct line at the beat frequency between the two modes is observed in the RF-spectrum. The beat note detection with a preceding polarization filter.