Laser performance

Violet ECDL No tunable sources could be found within budget, so the ECDL was

constructed in-house. Laser diodes are sold for use in ECDLs. They usually include an anti-reflective coating to increase the MFHTR, although the effectiveness of this has been disputed.193 _{A relatively inexpensive and powerful violet diode (120 mW,}

Roithner LaserTechnik) was mounted in a temperature controlled laser diode mount (laser diode starter set, Thorlabs). A grating (1200 lines/mm, 400 nm blaze) was mounted on an aluminium holder opposite a small beam steering mirror, placed in a 3-axis piezo controlled kinematic mount (Thorlabs). Figure 2.2 shows the ECDL in operation. A short focal length lens (f= 1.45 mm) was chosen in order to collimate the beam into a less elliptical profile.194 The smaller lens was also curved instead of flat on the rear side, which would prevent another cavity forming between the diode and the lens. The lens was AR-coated for the violet region, but the shape was designed for 780 nm so the lens was slightly astigmatic at 408 nm. However, the lens was near impossible to focus by hand so was replaced with a larger one (f= 3.1 mm) for easier operation. The grating was aligned by placing a pinhole in front of the lens and adjusting the vertical axis until the tuning effect of horizontal movement was maximised (range of≈2 nm), using a spectrograph (SR163, Andor) to see the changes in wavelength.

The modes of the ECDL were too closely spaced to be resolved by a normal grating spectrometer. To see the mode structure the beam was passed through a slit (100µm) and lens (f= 100 mm) onto a grating (3600 lines/mm), from where the diffracted beam was projected onto fluorescent fabric at a distance of 9.3 m, to create an

FIGURE2.2: Photograph of the ECDL in operation. Laser diode is on right, grating and beam steering mirror in centre and tuned wavelength output on left.

FIGURE 2.3: Highly dispersed modes of the ECDL projected onto a fluorescent screen. Contrast and brightness are adjusted to see modes more clearly. Scale in cm.

extremely high resolution spectrometer. The ECDL mode structure could be clearly seen (Figure 2.3), but attempts to derive quantitative results from this setup were unsuccessful.

A Bruker 70 Fourier Transform Spectrometer was used to obtain high resolution spectra ( 0.7 cm−1, 0.01 nm), with an acquisition time of 30 s and Happ-Genzel apodisation to flatten the baseline. The temperature of the diode was set to 22 °C for all spectra shown below.

The emission of the ECDL at high output power (30 mW) is shown in Figure 2.4. It consists of about 16 narrow modes and not of the single mode required for efficient frequency conversion and spectroscopy. It was not known if all of these modes were

0 0.2 0.4 0.6 0.8 1 408.8 409 409.2 409.4 409.6 409.8 410 normalised intensity wavelength (nm)

FIGURE2.4: ECDL emission spectrum at 120 mA injection current and 30 mW output power, showing 16 dominant modes spread over 1 nm.

38 40 42 44 46 48 407.6 407.8 408 408.2 408.4 408.6 408.8 409 409.2 409.4 current (mA) wavelength (nm) 0 0.2 0.4 0.6 0.8 1 407.6 407.8 408 408.2 408.4 408.6 408.8 409 409.2 409.4 normalised intensity wavelength (nm)

FIGURE2.5: (a) Free running diode emission with no feedback grating or lens at different currents. (b) Diode emission at 41 mA, showing close to SLM operation.

active simultaneously, or if they were emitted in rapid succession following tiny fluctuations in the power supply.

To find the source of the modes, spectra were taken of the bare diode with no grating or collimating lens. The light was collected by a parabolic mirror about 30 cm from the diode and the metal bench was covered with black fabric to eliminate back reflections as even small reflective surfaces could cause feedback into the diode. Figure 2.5 shows how the emission spectrum of the free-running diode changed with current. At very low currents the diode has the broad emission of an LED, but when the lasing threshold is reached (41 mA) one mode dominates at 408.25 nm. The current range where one mode dominates is only 3 mA. Increasing the current further red-shifts the output and also stimulates more modes. At the maximum injection current of 120 mA, beyond the range of this plot, the spectrum was similar to that of the ECDL in Figure 2.4.

38 40 42 44 46 48 407.6 407.8 408 408.2 408.4 408.6 408.8 409 409.2 409.4 current (mA) wavelength (nm) 0 0.2 0.4 0.6 0.8 1 407.6 407.8 408 408.2 408.4 408.6 408.8 409 409.2 409.4 normalised intensity wavelength (nm)

FIGURE2.6: (a) Diode emission with a collimating lens at a range of currents. (b) Diode emission at 44 mA, showing close to SLM operation.

Positioning a collimating lens (f=3.1 mm, plano-aspheric) in front of the diode caused some feedback, and this changed the emission to that in Figure 2.6. The lasing threshold is higher at 44 mA, and the current range over which one mode dominates is even smaller (1 mA). Lancaster et al.195 observed that the feedback from their lens encouraged single longitudinal mode (SLM) operation; this was not the case for our diode.

The ideal current for operation of the diode is at the point where it approaches SLM, as this gives the greatest wavelength selectivity. Spectra of the diode with grating-selected feedback (i.e. the ECDL in operation) could not be collected at such low currents in this setup because the output was too weak ( 8µW). While it would have been possible to obtain spectra by rearranging the apparatus to collect more light, and possibly replacing the beamsplitter of the spectrometer with one designed for blue light, the spectrum at such low output is of little interest as it would be too low for the Hg monitor. No further effort was made to obtain SLM output of the ECDL.

Red DPSS In place of a red diode (and a second ECDL, which would be too expen-

sive) a 300 mW Diode-Pumped Solid-State (DPSS) laser (Roithner LaserTechnik) was tested. The red DPSS was intended to be a powerful fixed wavelength. The maximum power output was 350 mW at 671 nm. Unfortunately the power supply was highly unstable. Even after warming up for 4 hours it still flickered on and off. Figure 2.7 shows the output at full power and at slightly reduced power. The shift of wavelength is probably due to heating effects. It was also extremely sensitive to feedback, with significant changes in both longitudinal and transverse modes with tiny changes in reflections from apparatus or the optical bench. This laser was therefore unsuitable for use in atomic spectroscopy as it was too unstable.

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 670.8 670.9 671 671.1 671.2 671.3 671.4

approx. intensity (arb. u

nits)

wavelength (nm)

full power reduced power

FIGURE2.7: Emission spectrum of the red DPSS laser