EFECTIVIDAD DE LA PRÁCTICA RECONSTRUIDA

The conversion of pump power into useful Nd-doped laser power is plotted in figure 8.6. The slope efficiency was measuied to be 48% by assuming a pump launch efficiency of 65%. The coupling efficiency of 65% was assumed by comparison to those obtained using undoped fibres of similar dimensions. The deviation from the ideal quantum efficiency of 74% is mainly

due to the difference in pump and signal mode-field distributions in the active fibre, and also the radiative transitions at 920 and 1320nm serve to reduce this value. There is also some evidence of excited-state absorption processes which lead to emission at shorter wavelengths and this results in a weak ‘whitish’ sidelight.

The measured fluorescence spectrum obtained from the Nd^+:fibre is shown in figure 8.7(a). As expected, the spectrum is much broader than that obtained with Nd^+xrystal/glass lasers^*^!. The broad gain spectrum suggests that a large tuning range should be possible and to

study this tuning range the mirror was replaced by a diffraction grating (see figure 8.2(b). The

is consistent with the eailier Rigrod analysis on inclusion of the -70% reflectivity of the grating. Finally the tuning range was recorded by rotating the grating about its vertical axis (see figure 8.7(b)).

100-

200 400 600 800 1000 1200 1400 0

Launched power (mW) Figure 8.6 Output power from a Nd-doped fibre laser pumped at 810nm

1025 g, 150 1040 1060 1---' 1 1 1080 1100 1120 Wavelengtli (nm) Figure 8.7. (a) Fluorescence spectrum and (b) tuning range of the neodymium-doped fibre laser.

The pump power was 1.2W (780mW launched) @ 810nm.

The tuning range is characterised by a rapid increase to high power output at short wavelengths, a broad plateau region, then a gradual decrease in power to longer wavelengths. In

identified as originating from the effects of the inherent birefringence of the active fibre^^^. When the grating was blocked significant channelling was observed on the amplified spontaneous emission (ASE) spectrum. On manipulation of the fibre position the internal polarisation state could be altered, and this caused variations in the details of the channelling. Specific manipulation resulted in an unmodulated ASE spectrum, and under these conditions a flat laser tuning curve was obtained. Hence for optimal performance some foim of fibre polarisation c o n t r o l l e ri ^ s should be included in this type of laser.

An integrated fibre-grating device^® was also incorporated as the Nd^+ifibre laser internal mirror (see section 2.8.2 in chapter 2). The fibre grating had a reflectivity of >90% and a bandwidth of less than Inm centred on lOBOnm. This reflector restored the output power decrease which occurred for the bulk grating, and figure 8.8 summarises the laser output characteristics for all of the Ndrfibre laser cavity configurations considered.

400 n

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300 600 900 1200

0 1500

Incident power (mW)

Figure 8,8. Power transfer characteristics Nd^+:fibre lasers where the internal reflector was (a)

diffraction grating (T]g=44%), (b) a 90% reflective mirror (T|g=48%), and (c) an mtegrated fibre grating (T|g=51%).

Further experiments were undertaken using a cavity with a highly reflecting internal mirror and a 50m length of doped fibre to maximise pump absorption. The excess unpumped portion of fibre was essentially transparent to the generated laser signal apart from a small additional loss due to intrinsic fibre absorption. This configuration did not significantly alter the

laser performance although the increased pumping level of -'2.25W (1.5W launched) enabled an output power of >800mW to be obtained. An interesting feature of this laser was that >100mW output power could be obtained over a pump wavelength tuning range of some 130nm (720- 850nm). This range included five obvious peaks with another existing below 720nm. The longest wavelength used (850nm) was due to the optics-related tuning range of the TiiSapphire pump laser, and would be expected to increase significantly from examination of the fibre absorption

spectrum.