Proceedings Article | 8 February 2007
KEYWORDS: Semiconductor lasers, Diffraction, Near field, Continuous wave operation, High power lasers, Mirrors, Output couplers, Laser damage threshold, Laser optics
Tapered lasers offer both high-power, together with good beam quality. They contain a ridge waveguide, which acts as a
modal filter, and a tapered section of increasing width, which provides high power. Our lasers are based on Al-free
active region and the material structure, which was grown by Metallorganic Chemical Vapor Deposition, has very low
internal losses of 0.5 cm-1, a very low transparency current density of 86 A/cm2, a high internal quantum efficiency of
86%, and a high characteristic temperature T0 of 171 K. Based on these good results, we have realised fully index-guided
single emitters (IG1) with a narrow output width of a few tens of microns, a narrow taper angle of less than 1°,
which deliver a maximum power of 1 W CW, together with a good beam quality parameter M2&sgr;&sgr; =3 at &lgr;=915nm.
In order to obtain higher power, we have realized an array of N=6 fully index-guided tapered diode lasers. They deliver
a maximum output power of 4W CW. The emitters of the free-running array are not optically coupled to each other, as a
consequence, the array has a highly beam quality parameter M2 which is at least equal to N times the single emitter one.
In order to improve beam quality of diode arrays, several approaches have been investigated to combine them
coherently such as evanescent coupling [1], intracavity spatial filtering [2, 3, 4], or a combining technique using a
binary phase grating [5] and also the Talbot effect. For the Talbot effect, both monolithic [6] as well as external Talbot
cavity [7] configurations have been demonstrated. The Talbot effect refers to a diffraction phenomenon and consists of
a reproduction of the field of an illuminated periodic object at certain distances away from the object plane. These
distances are multiples of the Talbot distance ZT=2d2/&lgr;, where d is the spatial periodicity of the object and &lgr; the
wavelength. It was studied for many kinds of lasers such as CO2 lasers [8] or semiconductor lasers [9]. Particular
interest was placed on semiconductor lasers because of their small size and high efficiency.
Here, we demonstrate for the first time the coherent operation of an array of tapered diode lasers placed in an external
Talbot cavity. The in phase supermode is selected by tilting the reflecting mirror. The divergence of the central peak is
0.4° FWHM.
