Experimental set-up

Two extended-cavity diode lasers (ECDL) systems were constructed during this research. The diode laser used in the first ECDL system was a multi- mode Fabry-Perot GaN device (Nichia) operating at around 451 nm, with a maximum output power of approximately 5 mW. The laser used in the second ECDL was also a Fabry-Perot GaN device (Nichia). At room temperature it operated at around 410 nm, with a maximum output power of around 30 mW. The reflectivities of the unmodified front facets of these commercial devices were not disclosed by the supplier. For devices similar to the 410 nm laser, a reflectivity of around 4% has been estimated (Hildebrandt 2004).

A schematic illustration of the design of the extended cavity is shown in Figure 2.4. The diode laser was housed in a thermo-electrically temperature- controlled mount (Thorlabs). An anti-reflection coated aspheric lens (focal length=4.5 mm, numerical aperture=0.55) was used to collimate the output

2. Extended-cavity diode laser development

beam. A mount with a shallow screw-thread (Thorlabs, S1TM09) was used to finely adjust the distance of the collimating lens from the diode laser. A holographic grating (1800 lines-per-mm; Edmund Scientific) was mounted in a piezo-electric kinematic mount (Thorlabs, KC1-PZ), which was attached directly to the laser diode mount by three steel rods. The mount incorporated three piezo-electric actuators, thus allowing the grating angle and extended cavity length to be tuned independently. The grating was mounted at the angle calculated from Eqn. 2.12 and listed in Table 2.1. The zeroth order served as the laser output beam and was reflected by a small mirror mounted on the same plate as the grating, as shown in Figure 2.4. This ensured that the direction of the output beam remained constant even for large changes in the grating angle associated with coarse wavelength tuning (Hawthorn et al. 2001). The same thing could be achieved through the use of a transmission grating (Merimaa et al. 2000), but the transmission grating required is not a standard commercially-available component and has to be manufactured specially, thus complicating the set-up.

2. Extended-cavity diode laser development

Figure 2.4 The extended cavity diode laser configuration

Fine adjustment to the grating alignment was performed to optimise feedback to the diode, following a method described in detail in the literature (MacAdam et al. 1992). This was done using a piece of white card with a small aperture. The card was held inside the extended cavity so that the diode laser beam passed through the aperture. The beam retro-reflected from the grating was then incident on the card, but the grating could be aligned (coarse adjustment with thumb-screws) so that this beam passed back through the aperture. This rough alignment was then improved upon by reducing the diode injection current to just above threshold and then very carefully adjusting the grating angle so that the intensity of the beam coupled out of the extended cavity was maximised. The better alignment of the cavity caused a reduction in the laser threshold current, so the injection current was again reduced to just above the new threshold value, and the fine optimisation of the grating angle was done again. This process was repeated several times until

2. Extended-cavity diode laser development

no further reduction in the threshold current could be obtained. Naturally, the vertical angle adjustment was more critical, since there is a small range of horizontal grating angles at which the laser can operate.

The temperature and injection-current of the diode laser were controlled using low-noise drivers (Tektronix). A three-channel high-voltage supply (Thorlabs) was used to control the lengths of the three piezo-electric actuators in the kinematic mount. Wavelength scanning was achieved by tuning the piezo-actuators with two triangular waveforms, which were in phase and whose amplitudes could be accurately adjusted. One of these waveforms was used to modulate the piezo marked ‘A’ in Fig. 2.4, and the other was applied to both of the two piezos marked ‘B’, thus allowing simultaneous rotation and translation of the grating. No anti-reflection coating was applied to the front facet of the diode. Instead the effective length of the internal Fabry-Perot diode cavity was tuned synchronously by ramping the diode injection current with a third triangular waveform. The triangular waveform modulation signals were generated by use of a circuit containing three variable resistors, which divided the output signal from a single function generator. The variable resistors permitted an empirical optimisation of the ratios between the amplitudes of the tuning signals.

A glass plate was used to direct a part of the output beam towards a solid quartz Fabry-Perot etalon with a low finesse and a free spectral range of 3.1 GHz. The transmitted intensity pattern allowed the relative frequency change of the ECDL output during scans to be measured. In addition to this, the main part of the laser beam was directed into a scanning confocal Fabry-Perot etalon (FSR=7.5 GHz, F=30), which was used to monitor the spectral output of the ECDL to determine whether single-mode operation was maintained

2. Extended-cavity diode laser development

during the scans. The power of the ECDL output beam was also continuously measured, by directing a reflection from a second glass plate onto a photo- diode.

A measurement of the laser line-width was made by tuning the wavelength so that the transmitted intensity through the confocal etalon was about 50% of the maximum, corresponding to the location of the steepest gradient on an interference fringe. As a result, any change in laser frequency would result in a change in the intensity of transmitted light, which was monitored with a photo-diode. The gradient of the slope, for a particular value of transmitted intensity, could be determined from a single mode scan, which was performed immediately prior to the line-width measurement.

In a separate experiment, a characterisation of the spectral output of the free- running Fabry-Perot diode lasers was performed using a 1.26-metre-long spectrometer with a 2400 lines-per-mm grating operating in first order, fitted with a back illuminated un-intensified CCD camera (Wright Instruments, 384 x 576 pixels) for signal detection. In each case, the responses of the diode to tuning of the injection current and of temperature were investigated.