for pumping an Optical Parametric Oscillator
As described in Chapter 4, an OPO device that is pumped by a violet diode laser would be desirable, not only for accessing a previously unobtainable region of the spectrum with these types of devices, but also to allow investigations into the spectral properties of the OPO. The blue diode laser that is used here is a Nichia Corporation NLHV3000E device with the manufacturers’ specifications detailing a nominal C-W output of 30mW at 405nm.
10 20 30 40 50 60 0 5 10 15 20 25 30 Ou tp ut Po w er (m W )
Input Current (mA)
Injection Current (mA) Output P
ow
er (mW)
Figure 5.1: Output Power versus input current for a free running Nichia violet diode laser
The sharp change in the slope indicates the threshold injection current and marks the beginning of laser operation.
injection current. The injection current was increased whilst the output power was recorded using a Newport power meter. The threshold current for the laser is 37mA, and the maximum output power is 27mW. This output power was typical for blue diodes. Red and IR diode lasers typically have output powers greater than 100mW, but with generally higher threshold currents.
Figure 5.2 shows the wavelength tuning range for the Nichia violet diode. For a variation in injection current the change in the wavelength of the laser output was recorded. The operating current of the diode laser was fixed at 45mA. The range of wavelengths obtainable is over 1.4nm with the largest range of mode-hop-free tuning for the temperature variation between 17 and 20 degrees, where the wavelength is 406.5nm.
After characterisation in free running mode shown in figures 5.2 and 5.1, the laser diode was incorporated within a home built Littrow external cavity configuration
10 15 20 25 30 35 406.0 406.2 406.4 406.6 406.8 407.0 407.2 407.4 407.6 W av ele ng th (n m) Temperature (0C) W av elength(nm)
Injection Current (mA)
Figure 5.2: Wavelength versus temperature for a free running Nichia violet diode laser
Short continuous segments of similar wavelength values show the tuning of a single longi- tudinal mode as a result of varying the optical path length.
using a holographic UV grating (1800 lines/mm) and collimated over a large distance (∼4m). The diode was held within a standard mount and the grating contained within a machined holder which was mounted using a commercial 1 inch adjustable mirror mount which allowed the grating orientation to be altered in both the vertical and horizontal direction. The laser diode was temperature stabilised using a Peltier thermoelectric cooler (TEC) which was sandwiched using heatsink paste between the laser mount and an aluminium baseplate. At first the thermistor that provided the feedback to stabilise the temperature was placed at the top of the aluminium mount, however this was not sufficiently close to the laser diode for the feedback to the temperature controller to operate properly. To alter this the thermistor probe was then placed within the mirror mount, closer to the laser diode.
One of the benefits of introducing optical feedback within a laser diode device is a reduction in the threshold operating current. In order to obtain the lowest threshold current for the ECDL configuration, the first order diffracted beam from the grating
must be aligned with the diode output. This is achieved practically by placing the grating within a kinematic mount allowing orientation of the grating in the horizontal and vertical directions. In the alignment procedure, the laser is operated just below threshold and the second, diffracted, less intense beam is observed to return to the centre of the collimating lens. If this beam is observed separately to the main diode laser beam then not all of the diffracted beam is being coupled back to the diode, showing that the ECDL is not in perfect alignment. By careful adjustment of the grating, the two beams can be coincident upon each other, ensuring that the diffracted light passes through the centre of the collimation lens and couples back into the diode. Through careful alignment the threshold of the laser was reduced from 35mA in free running mode to 27.8mA in the external cavity mode, demonstrating the reduction in threshold as a result of using an external cavity.
26 28 30 32 34 36 38 40 42 44 46 407.0 407.2 407.4 407.6 407.8 408.0 W av ele ng th (n m)
Injection Current (mA)
Injection Current (mA)
W
av
elength
(nm)
Figure 5.3: Wavelength-Current Characteristic of a Nichia violet extended cavity diode laser
The wavelength maintains a step function, but over a far smaller wavelength range
Figure 5.2 shows the wide tuning range and the ‘step’ characteristics of the wave- length of a free running diode laser. In contrast, Figure 5.3 shows the wavelength tuning range of the diode when operated in an external cavity configuration. The diode current was varied from 28mA to 45mA with the temperature stabilised at 20
degrees. The step function remains but the wavelength range has reduced consider- ably to 0.1nm which demonstrates that the laser oscillates preferentially on a single longitudinal mode.
To observe the spectral output from the diode laser both in free running and extended cavity configurations, the output from the laser diode was observed on an optical spectrum analyser (OSA). The laser light was aligned and coupled into the OSA via an optical fibre probe. The spectra recorded are shown in Figure 5.4.
a) b) Wavelength (nm) Wavelength (nm) P ow er ( W) m P ow er ( W) m 0000 0720 1440 2160 2880 3600 405.65 406.65 407.65 0000406.19 407.19 408.19 0600 1200 1600 2400 3000
Figure 5.4: Comparison of spectral output from the free running and exter- nal cavity diode laser
The multimode output from the free running laser (a)and the single mode output from the ECDL (b) are clearly shown.
The spectral outputs shown in Figure 5.4 show the difference in the spectral output from the diode laser when operated free running and extended cavity. For the free running diode there are 4 modes shown and the laser supports oscillation at these four modes, over a wavelength range of 0.9nm. In contrast the ECDL oscillates on a single mode over a range of 0.2nm. The feedback introduced by the extended cavity encourages the single mode oscillation.
Phase continuous tuning of the diode laser was also attempted. Without the use of the electronic circuit, the mode hop free tuning range was 6GHz. An electronic circuit was used in a method similar to that described in [1]. The circuit is used to vary the length of the solitary and extended cavity within the same ratio. The mode hop free tuning range achieved was 16.5GHz shown in Figure 5.5.
1.5GHz 0 11
Frequency (GHz)
T
ransmission I
ntensity (a.u)
Figure 5.5: 16.5GHz Mode-Hop-Free Tuning of Nichia Violet ECDL
Transmitted fringe pattern of the violet ECDL through a low finesse Fabry-Perot interferom- eter (FSR=1.5GHz) with a fixed length. There are 11 equally spaced fringes corresponding to 16.5GHz of mode-hop-free tuning. (Smooth tuning trace obtained by D. Gawron)