M. Lecomte

Optimization of an inductively coupled plasma etching process of GaInP∕GaAs based material for photonic band gap applications

In this article, we investigate the dry etching of GaInP∕GaAs based material system using an inductively coupled plasma (ICP) etching system. In a view to develop a suitable ICP process for the etching of aluminum-free material, ridge waveguides have been fabricated and the effects of the ICP parameters have been assessed. The coil power and the platen power have been varied at constant pressure and temperature for a chlorine-based process. The surface quality, sidewall profile, and selectivity have been reported. We also demonstrate the optimization of the chlorine-based process for deep etching and its subsequent implementation in photonic band gap device fabrication for 1.55μm optical applications. The optimized process has been shown to provide a high aspect ratio and a good selectivity for 250nm diam holes with a depth of 3μm in the GaInP∕GaAs material system. The influence of the ICP parameters on this material system have been analyzed mainly by scanning electron microscopy with particular attentio...

High-power and high-brightness laser diode structures using Al-free active region

High bit-rate, WDM, networks are reliant on Er or Er/Yb doped fiber amplifiers. Reliable, high power laser diodes at 980nm and 1480nm are key devices for pumping these amplifiers. We have developed several 980 nm laser diode structures at 980 nm, using an Aluminum free active region and standard AR/HR coatings on the facets. Our laser show low optical losses, low threshold current density and a high external differential efficiency. We demonstrate a mini-bar of small angle index guided tapered laser diodes (emissive width of 3 mm) with an optical output power of 20W at 33A under CW operation (25°C). The far field of the slow axis has a Gaussian single lobed shape, with a FWHM of 3.5° at maximum power, which is two times less than obtained with multimode broad area lasers. With such a device, we expect to couple 10W into a 100μm diameter fiber. We also demonstrate a large aperature gain-guided tapered laser with an output power of 2.4W and a calculated M21/c2 = 3, the M21/c2 factor being calculated with the method based on measurements of the fields profiles widths at 1/c2.

High-power and high-brightness laser diode structures at 980 nm using Al-free materials

High bit rate, WDM, networks use intensively Er or Er/Yb doped fibre amplifiers. Reliable, high power laser diodes at 980nm and 1480nm are key devices for pumping these amplifiers. We have developed different structures of laser diodes at 980nm, using Aluminium free materials. Our laser structure shows low optical losses together with a low threshold current density and a high external differential efficiency. We demonstrate a mini-bar of broad area laser diodes (emissive width of 2.6mm) with an optical output power of 19W at 25A under CW operation. We have also developed a mini-bar of small angle index guided tapered laser diodes (W=2.6mm). We demonstrate 17W at 27.6A under CW operation at 20 degree(s)C. Slow axis far field has a Gaussian single mode shape, with a FWHM of 3.3 degree(s) (at 15A, 11W), which is two times less than obtained on multimode broad area lasers. With such a device, we expect to couple 10W into a 100micrometers diameter fiber. We also demonstrate, on a large aperture gain-guided tapered laser, an output power of 1.3W with an M2 of 3.3

High-Power Narrow Linewidth Distributed Feedback Lasers With an Aluminium-Free Active Region Emitting at 852 nm

In this letter, we demonstrate the design, fabrication, and characterization of single-mode distributed feedback (DFB) lasers emitting at 852 nm for atomic clocks and spatial applications. The epitaxial structure comprises an aluminium-free active region with the DFB fabrication technology based on an epitaxial regrowth concept. Initial results carried out on uncoated broad-area devices showed low internal losses (<3 cm-1), a high internal efficiency (95%), and for antireflection/high-reflectivity coated broad-area lasers, an output optical power of 5.5 W was measured at 8.5 A. Ridge waveguide structures were then fabricated with a ridge width of 4 mum showing typical single spatial mode emission with the M2 factor <1.5. Based on these preliminary results, DFB ridge waveguides were then processed and characterized. Single-mode emission was achieved at 852.12 nm corresponding to the D2 cesium transition, with an optical output power of 40 mW at 140 mA. Linewidth measurement was also carried out on these devices with a linewidth of 0.9 MHz measured at 70 mA.

Clarinet laser: Semiconductor laser design for high-brightness applications

High-power and high-brightness continuous-wave (cw) operation has been achieved with an optimized design of fully index-guided tapered laser emitting at 975 nm. The device achieves simultaneously negligible astigmatism and stable low divergence in the lateral axis at high-power operation. By using a quasi-three-dimensional simulation model, the different mechanisms modifying the slow axis beam divergence at high power have been carefully balanced in the clarinet design, easing the use of collective optics in laser bars. The devices consist of a relatively long ridge-waveguide filtering section coupled to a relatively short tapered section with an aperture angle of 2°. InGaAs∕InGaAsP lasers were fabricated with this design, demonstrating an output power of 1 W cw, a maximum wall-plug efficiency of 50%, negligible astigmatism, a slow-axis far-field divergence (measured at 1∕e2) of 5° at 1 W and beam quality parameter M2<3.

High-brightness tapered lasers with an Al-free active region at 1060 nm

High-brightness diode lasers at 1060 nm are useful in display applications (to provide green light by frequency doubling) and in free-space optical communications. On Al-free active region laser structures, we have obtained low optical losses of 0.9 cm-1, a high internal quantum efficiency of 98% and a low transparency current density of 64 A/cm2. On uncoated broad-area lasers (2 mm x 100 μm) at 20°C CW, we have obtained a high maximum wall-plug efficiency of 66%, and an optical power higher than 3W per facet. Based on these good results, we have realized 3.7 mm long gain-guided tapered lasers, delivering a high power of 3W at 10°C CW, together with a low M2 of 3 at 1/e2 and a high maximum wall-plug efficiency of 57%. We have also realized separate electrode lasers, in which the ridge and tapered sections are biased separately. In this configuration, the current through the ridge section is only a few tens mA while the current on the tapered section is several Amps. This allows to control a large output power with only a small change of the ridge current. By moving the ridge current from 0 to 50 mA, keeping a constant 4A current through the tapered section, we have obtained a large change of the output power from 0.09 W to 2.6 W, which corresponds to a high modulation efficiency of 50 W/A under static operation. In dynamic regime, the separate electrode laser can be operated at 700 Mbps, showing a high modulation efficiency of 19 W/A, optical modulation amplitude of 1.6 W and extinction ratio of 19dB [1]. These modulation efficiencies are, to our knowledge, record values.

High-power Al-free active region (λ = 852nm) DFB laser diodes for atomic clocks and interferometry applications

We have developed single frequency and single spatial mode laser structures with high optical power, using an aluminium free active region, which are to our knowledge the first demonstration for Cs pumping at 852 nm.

Very narrow linewidth of high power DFB laser diode for Cs pumping

We obtain (Fig. 1) a low threshold current at 30°C around 50mA and a high slope efficiency of 1W/A. At this measurement temperature, we have also obtained the D2 line of Cs (852.12nm) for a current of 170mA and so, a high optical intensity of 110mW (Fig. 2). The side mode suppression ratio (SMSR) for these operating conditions is higher than 50dB. We also measured by the self-heterodyne method the linewidth of our DFB laser at 30°C. We obtained at the D2 line of Cs (Fig.3) a very narrow linewidth of 200kHz for a white noise approximation (lorentzian fit) and 446kHz for a low frequency noise approximation (Gaussian fit). Furthermore, we can see (Fig.4) the usual decrease of the linewidth with the optical power, without any further increase at high power. From this linewidth evolution with power the Henrys factor is evaluated to be around 2.5.

In-phase coherent coupling of tapered lasers in an external Talbot cavity

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.

High-power, high-reliability, and narrow linewidth, Al-free DFB laser diode for Cs pumping (852nm)

The development of techniques such as atom optical pumping, for atomics clocks or precise gyroscopes, requires laser diodes with high power and excellent spectral (narrow linewidth) and spatial qualities together with high reliability. We have realized a six months ageing test on Al-free DFB lasers emitting at 852nm for Cs pumping. Ten DFB lasers were aged at 40°C and 20mW. The extrapolated lifetimes at 40°C, based on 20mW operating current, of our DFB lasers are higher than 500000 hours which confirms the excellent potential of this Al-free technology for long life spatial mission. Furthermore, the evolution of the operating current (initially around 70mA), after six months, is less than 5% (corresponding to 3mA). We obtain a very good stability of optical spectra: an average variation of the Side Mode Suppression Ratio (SMSR) of less than 2dB and a variation of the wavelength of less than 0.12 nm. We also measured the linewidth of our DFB lasers with the delayed self-heterodyne method after the six months ageing: we obtain a very narrow linewidth at 25°C (measurement temperature) around 215kHz (lorentzian fit, white noise) or 330kHz (gaussian fit, 1/f noise).