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.