POLARIZATION STATE OF THE EMISSION OF EXTERNAL GRATING DIODE-LASERS

The laser emission of transverse electric (TE) and transverse magnetic (TM) polarization, in alternating successive spectral ranges, that ca n occur when external grating diode lasers are wavelength tuned, is th eoretically and experimentally investigated. The modulated threshold g ain curves of the external cavity for the two polarizations can be mad e to intersect with each other if a grating is used in such a way as t o practically eliminate the difference between the laser diode losses and confinement factors for TE and TM polarization. The result is a se quence of wavelength ranges where the threshold gain is alternately lo wer for the TE resp. TM polarization. Within each range, a wavelength tunable single mode laser emission is obtained. The phenomenon is desc ribed, taking into account the characteristics of the key components o f the external cavity, i.e., laser diode structure, antireflective (AR ) coating, and grating efficiency. Design tools are proposed for the c haracteristics of the components necessary for the development of such an external cavity. The theoretical and design concepts are experimen tally confirmed in a number of external cavities, using diode lasers e mitting around 1.5 mum, with different types of antireflective coating s, as well as different grating configurations.