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
2. Extended-cavity diode laser development the scan. The Fabry-Perot etalon transmission traces have been normalised by the simultaneously recorded laser output power, which is also shown.
Figure 2.8 a) Transmitted fringe patterns through two different Fabry-Perot interferometers (FSR=3.1 GHz and 7.5 GHz) during a wavelength scan of the 451 nm ECDL. For the lower FSR etalon, 37 interference fringes are observed, corresponding to 110 GHz mode-hop free tuning. The laser output power is also shown. b) Mirror spacing of scanning Fabry-Perot interferometer (FSR=7.5 GHz) used to validate single-mode emission during ECDL scanning.
c)-f) Scanning Fabry-Perot traces recorded at four different times during a single laser wavelength scan, the corresponding times are indicated in fig b).
During the scan shown in Figure 2.8a) the 7.5 GHz confocal etalon was used with a fixed mirror separation. A separate experiment was performed to check that the laser emission remained single-mode throughout the entire ECDL scan. The laser wavelength tuning was performed as before, at a rate of 4 Hz, but now the mirror spacing of the high-finesse confocal etalon was
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2. Extended-cavity diode laser development modulated at a rate of 100 Hz, as shown in Figure 2.8b). The amplitude of the mirror displacement was adjusted to ensure that the same laser mode appeared at least twice during each scan. Four of the resulting Fabry-Perot etalon scans are shown in Figure 2.8c)-f), and in each of Figures 2.8c)-f), two narrow peaks of similar intensity are visible; the spacing of these two peaks is the same in each case. This therefore confirms that the laser was emitting on a single longitudinal mode at each of the positions of the laser wavelength scan.
Although the results of only four of the twenty-five confocal etalon scans are shown as a representative sample, the others all show the same features and thus confirm the absence of side-modes from the ECDL emission over the entire 110 GHz wavelength scan.
It is worthwhile to compare this mode-hop free tuning range to those previously achieved for blue/violet diode lasers. Extended-cavity designs employing anti-reflection coated GaN diodes, pivot-point grating tuning, and injection-current tuning have achieved a continuous tuning range of around 50 GHz (Hildebrandt et al. 2003). Similar diodes without anti-reflection coating, used in combination with optimised pivot-point grating mounting and injection-current modulation have achieved continuous tuning ranges of 10-25 GHz (Leinen et al. 2000). Finally, uncoated diodes in combination with un-optimised pivot point grating mounting have typically achieved continuous tuning ranges below 10 GHz (Conroy et al. 2000; Gustafsson et al. 2000).
The output power of the laser is also shown in Fig. 2.8a), it decreases steadily during the 110 GHz long scan shown here, almost reaching zero at one end of the scan. Such a reduction in laser output-power during scanning is an inherent feature of the current tuning process and restricts the scanning range in all ECDL devices that rely on this principle. At one end of the
injection-2. Extended-cavity diode laser development current scan, the maximum recommended current is reached, whereas at the other end of the scan, the current falls below the threshold value required for laser action. In the present case the injection current was modulated around a mean value of 51.9 mA, resulting in an output power from the ECDL of around 0.9 mW at the centre of the scan. It is evident that the fractional decrease in laser power would be less significant for shorter wavelength scans.
For the 451 nm ECDL the extended-cavity and diode parameters (defined above, and shown in Figure 2.5) are summarised in Table 2.2.
l1 45.0 mm l2 22.5 mm lC 24.0 mm
α 41 nm/V
β 4.25 pm/mA
Table 2.2. Important dimensions and tuning parameters for the 451 nm ECDL.
This leads to the following predicted ratios between the modulation voltages for optimised tuning (Eqns. 2.18 and 2.19): ∆VA/∆VB=2.43, and ∆Ι /∆VB =-0.23. It was found that a wide range of ratios between ∆VA and ∆VB resulted in broad mode-hop free tuning ranges. The actual ratios used for the 110 GHz mode-hop free scan shown in Fig. 2.8 were: ∆VA/∆VB=1.56 and
∆I/∆VB = -0.27 mA/V. Mode-hop free tuning ranges exceeding 80 GHz were observed for ∆VA/∆VB ranging from 1.0 to 2.5. This indicates that tuning of the grating angle is of less importance in the present experiment; precise matching of the extended-cavity length tuning with the diode cavity length tuning is more critical. This appears to suggest that the overlap between the
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2. Extended-cavity diode laser development active diode mode and the active extended cavity mode is of greater importance than the exact matching of the peak of the grating feedback profile to the cavity modes. A possible explanation for this may be that the front-facet reflectivity of the present diode laser is appreciable, and the spectral profile of the grating feedback is relatively wide compared to the extended-cavity mode spacing. It should be noted that the grating feedback profile is still sufficiently narrow though, to favour only one of the diode Fabry-Perot modes. It should be noted that more care would have to be taken to synchronise the grating tuning with the extended cavity length and current tuning when using either anti-reflection coated diodes or gratings providing a narrower feedback profile. This therefore represents a disadvantage of using anti-reflection coated diodes.
In contrast to the grating angle tuning, the exact matching of the diode current and extended-cavity length tuning was found to be critical to achieve even modest mode-hop free tuning ranges with the present ECDL. For the ratio
∆VA/∆VB=1.56, used for the scan in Fig. 2.8a), Eqn. 2.19 predicts
∆I/∆VB = -0.23, which is close to the experimentally observed ratio of -0.27.
This close agreement was observed for a wide range of values of ∆VA/∆VB. The scan shown in Fig. 2.8 was recorded at a laser scan rate of 4 Hz to allow a sufficient number of confocal Fabry-Perot etalon scans to be recorded during one wavelength sweep. The same tuning ranges, however, were achieved at laser scanning rates of up to 60 Hz, above which the maximum tuning range gradually decreases with increasing scanning frequency due to piezo-response non-linearities, to mechanical resonances, and to limited diode heat transfer.
2. Extended-cavity diode laser development