Diode laser, coupling, into fibre

PDT was performed by a KTP pumped dye laser (Laser Sonic) or a diode laser emitting at 652 nm (Diomed IW). Light was delivered by bare fibres coupled into a modified balloon system (Lajat/Patrice), by a spherical distributor or by a 20 mm long cylinder for interstitial treatment (Medlight Switzerland). The power density varied from 2500 mW s (KTP-dye) to 870 mW s (diode laser). For interstitial... 

Fig. 7.22 Trading NA against fibre diameter when coupling a diode laser into a fibre...

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. 

If a lens of short foeal length is used, e.g. a seeond laser diode collimator, magnification of the aberrations is avoided. Now the laser ean be coupled into a thin fibre. However, the NA is large, and so is the pulse dispersion. An example is shown in Fig. 7.23. Pulses from a 650 nm, 45 ps diode laser were sent through a 1 mm fibre of 2 m length. The pulse shape shown left is for an NA of 0.3, the right pulse shape is for an NA of < 0.1. 

H. Sakaguchi, N. Seki, S. Yamamoto, Power coupling from laser diodes into single-mode fibres with quadrangular pyramid-shaped hemiellipsoidal ends. Electron. Lett. 17(12), 425 26 (1981)... 

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