Diode lasers beam profile

Fig. 3 (A) Frequency and intensity profile of a diode laser beam, from left to right unmodulated, modulated with no absorption, and modulated with absorption by the upper sideband. (B) Schematic diagram of an instrument for FMS. The frequency modulated diode laser output is converted to an amplitude modulation after passing through a gas sample, which absorbs at a particular wavelength. The amplitude modulation is proportional to gas concentration and can be phase sensitively detected and related to oxygen concentration. (From Ref " p. 41. Copyright 2002 Advanstar Communications Inc.)...

The light from the source, in this case a laser diode, is transferred to the fibre input cross section by a transfer lens system. The first lens is the laser collimator, with a focal length, fl, which is normally a few mm. If the collimated beam is focused into a fibre by a lens of a longer focal length, 12, all aberrations in the laser beam profile are magnified by a factor M = 12 / fl. This requires a fibre of a eorrespondingly large diameter. However, the NA of the beam coupled into the fibre, and eonsequently the pulse dispersion in the fibre, is reduced by the same ratio. 

The diffusion-layer imaging technique which was developed by McCreery is another method for studying intermediates in the diffusion layer [71-75]. A laser beam is directed in a parallel direction through the diffusion layer of the electrode and the light is then magnified and focused on a diode-array detector. With this method, spatial resolution of the diffusion layer of 1.25 pm is achieved, and concentration profiles in the diffusion layer are mapped. A detailed description of mass transport processes as well as the kinetics and spectra of intermediates can be obtained. Diffusion coefficients and extinction coefficients for, for example, the benzophenone radical anion were measured with this technique [74, 75]. 

Diode lasers have an extremely small cavity. Most lasers in the power range below 200 mW (CW) are single-mode lasers, i.e. the height and width are so small (a few pm) that only one transversal mode is excited. This implies that the radiation ean, in prineiple, be focused into a diffraction-limited spot. However, beeause the eavity is only a few pm long, the light is emitted over a wide angle. The general beam profile of a laser diode is shown in Fig. 7.1. 

A laser beam focused into the diffusion layer in combination with a diode array detector may even be used to directly image the concentration profile inside the diffusion layer, as described by Posdorfer et al. [76,77]. 

L.S. Mayboudi, M. Chen, G. Zak, A.M. Birk, P.J. Bates, Characterization of Beam Profile for High-power Diode Lasers with Application to Laser Welding of Polymers, AimiC, ITIA (2006)... 

Figure 3 Laser beam 1-D profiles around focus plane for Rofm-Sinar DLxl6 diode laser...

Bates, P.J., Characterization of beam profile for high-power diode lasers with apphcation to laser welding of polymers, SPE, ANTEC, 2274-2278 (2006). 

Laser powers (P) used in this research were in the range of 2 to 160 W depending on the scanning speed, part thickness and material type. Two scanning speeds (V) of 3 and 6 m/min were used. The focal point of the beam was positioned to be at the interface between the natural and black PA mXD6 parts. Information on the beam profile of this diode laser can be found in reference 13. In the absence of any scattering, the focused beam has nominal dimensions of 0.6 x 1.4 mm. The 1.4 mm dimension was oriented perpendicular to the direction of beam travel. 

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