It is particularly interesting to consider the spectral emission of the free-running 451 nm Fabry-Perot diode laser since devices operating at this wavelength have only recently become available, and no report of their performance had previously been published in the literature. Figure 2.6 shows the spectral output from the 451 nm Fabry-Perot diode laser operating without extended-cavity feedback at different diode injection currents ranging from below threshold (45 mA) up to operating current (58 mA). The width of the individual laser modes in the plot is determined by the spectrometer resolution and does not represent the actual line-width of the laser. The spectrum shown in plot a) corresponds to an injection current of 45 mA, which was just below the threshold current at a diode operating temperature of 40° C. Below threshold, the device acts as a light emitting diode (LED) and a large number of longitudinal modes are observed in the spectral profile, with a mode spacing of around 58 pm. This is in fair agreement with the mode spacing of 45 pm (Table 2.1) that was calculated for an assumed diode chip length of 0.70 mm (Nakamura et al. 1997). At the threshold current of 46 mA (plot b) two dominant modes are observed, with a number of weaker side modes present. As the injection current is further increased (plots c-f) the positions of the strongest modes shift to higher wavelengths. Single mode lasing is observed only for certain specific combinations of current and temperature as shown, for example, in plot c). The observed multi-mode behaviour is one of the reasons why it is desirable to use the Fabry-Perot laser in an extended cavity configuration for high resolution spectroscopy
2. Extended-cavity diode laser development applications. Another advantage is that the range of coarse wavelength tuning, for a specific diode, can be extended by a few nm beyond what is possible by tuning temperature and current only. The mode spacing observed above threshold current in the present laser matches the regular Fabry-Perot spacing observed below threshold. This is in contrast to previous observations for some violet diode lasers operating in the 400-420 nm region, where mode spacings several times greater than those estimated from the Fabry-Perot cavity length were observed (Nakamura et al. 1997; Jiang and Lin 1999).
Figure 2.6 Spectral output from a Fabry-Perot diode laser, emitting at around 451 nm, as a function of diode injection current: a) corresponds to a current just below the laser threshold; b) is just above threshold current; plots c) to f) correspond to a range of injection currents from a little above threshold up to operating current. The relative intensity scale of each spectrum is indicated to the right.
A comparison of the spectral emission of the 410 nm diode laser, before mounting in the extended cavity, with that of the 451 nm diode laser is also worthwhile. In Figure 2.7, the spectral output of the free-running Fabry-Perot diode lasers is shown as a function of temperature and of injection current.
The parameters (β) required for the evaluation of current tuning rates were extracted from the current-tuning plots.
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2. Extended-cavity diode laser development
Figure 2.7 Spectral emission of the Fabry-Perot diode lasers as a function of a) temperature for the 451 nm laser, b) temperature for the 410 nm laser, c) current for the 451 nm laser, d) current for the 410 nm laser.
2. Extended-cavity diode laser development In the grey-scale plots shown in Figure 2.7, regions of darker shading correspond to higher intensities. Figure 2.7a) shows the spectral emission of the free-running 451 nm Fabry-Perot diode laser for diode temperatures ranging from 25 to 60 °C (the injection current was held constant during this experiment). From this plot it is apparent that, at most temperatures, between five and fifteen Fabry-Perot diode modes are active, whereas single mode emission is only achieved within a very narrow range of operating conditions.
The individual modes appear to tune continuously with temperature over intervals of 10-20 °C. From the slope of the individual modes, a wavelength tuning rate of 16 pm/°C can be estimated. A shift of the gain curve to higher wavelengths with increasing temperatures is also apparent (around 40 pm/°C) since the overall mode pattern is shifting to the right.
In Figure 2.7b) the corresponding plot for the 410 nm Fabry-Perot laser is shown. There are several distinct differences compared to the behaviour of the 451 nm diode. For most temperatures there are only a few modes active, often separated by a multiple of the expected mode spacing. For a particular temperature there are normally between two and four groups of active modes, each consisting of one or two neighbouring modes. These groups are separated by gaps spanning two or three inactive modes. Individual modes tune to higher wavelengths with increasing temperature at a rate of around 15 pm/°C. Most modes tune for about 3 °C after first appearing, they then disappear and become inactive for about 3 °C before reappearing again and tuning for another 3 °C. The gaps in the centre of each mode tuning, thus lead to the spacing between the groups of modes observed in Figure 2.7b). From the overall slope of the mode pattern seen in this figure, the rate at which the gain curve changes with temperature was estimated to be around 60 pm/°C
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2. Extended-cavity diode laser development which is thus higher than the tuning rate of the individual modes, as it was in the case of the 451 nm laser.
The effect of current tuning on the spectral output of the Fabry-Perot diode lasers is shown in Figure 2.7c) and d). With increasing injection current a larger number of modes become active and the positions of the individual modes also shift to higher wavelengths. As has been addressed above, the reason for this is that at higher injection currents, the local temperature of the diode junction is slightly elevated. The parameter for the rate of wavelength tuning with injection current can therefore be evaluated from the plots: β=4.3 pm/mA for the 451 nm laser; β=4.0 pm/mA for the 410 nm laser.
The spectral output of the free-running 410 nm diode laser at just below threshold current is similar to the one shown for the 451 nm diode laser in Figure 2.6a). In this case, the free-spectral-range is 38 pm, which is in good agreement with the value of 34 pm (Table 2.1), which was calculated for an assumed diode cavity length of 0.70 mm (Nakamura et al. 1997).
2.3.2 Wavelength scanning of blue extended cavity diode lasers