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In the last section results were presented which demonstrate that single-pass frequency-doubling of the OPO produces efficient conversion into the visible. This system also has the benefit that it is easy to switch between the signal and visible pulses very quickly. However, if higher visible power is required then it is advantageous to use intracavity doubling. This process is more complex than the extracavity case and utilises the lai'ge intracavity powers available within the OPO cavity to achieve substantially higher conversion efficiencies into the visible and therefore higher output powers.

An intracavity-frequency-doubled picosecond OPO based solely on LBO is described in the following section. Efficient single-pass, frequency-doubling of the signal into the red can be achieved by using temperature-tuned NCPM in LBO, with two different crystal geometries being required to frequency-double the entire signal branch. For both arrangements each LBO crystal was situated at a second intracavity focus.

6.6.1 Experimental configuration

The pump source for the frequency-doubled picosecond LBO OPO was a

commercial self-mode-locked Ti:sapphire laser (Spectra-Physics, Tsunami) which was configured for picosecond operation. See Chapter 4 for a detailed description of this laser. The intracavity frequency-doubling was performed in two 16 mm-long LBO crystals located at the second intracavity focus of a standing-wave five-mirror resonator similar to that of Reid et al [8] and is shown in Figure 6.15. The singly-resonant OPO cavity comprised a plane mirror, two r = -200 mm curved mirrors foiming the primary focus in the OPO crystal and two r = -100 mm mirrors that formed the secondary focus in the doubling crystal. The type I doubling crystal was cut in an identical geometry to the 30 mm-long LBO used as the OPO crystal. Both these crystals were cut for non-critical propagation along the x-axis (<j)=0°, 0=90°). The type II doubling crystal was cut for non-critical propagation along the z-axis ((j)=0°, 0=0°). All crystals had AR-

coated end faces centred at 1.4 pm. The frequency-doubling was performed using a combination of type I and type II phase-matching with temperature- tuning. Because of the limitations imposed by the design of the oven, in order to provide continuous coverage in the visible it was necessary to use two crystals with type I and type II phase-matching so as to maintain the SHG process above room temperature. A more suitable oven design based on Peltier cooling (discussed later) will allow frequency-doubling across the available tuning range

Chapter 6 Visible Picosecond Optical Parametric Oscillators

Picosecond Self -Mode-locked Ti:sapphire laser

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2 Lens r = -200 mm r = -200 mm f = 10 cm ^s \ \ "■ “ P*"' r = -100 mm r = -100mm Figure 6.15

Schematic o f the frequency-doubled picosecond Ti:sapphire-pumped LBO OPO, with the cavity parameters displayed.

with a single crystal cut for type I phase-matching. To access the demonstrated visible range, two sets of OPO mirrors were used with highly reflecting (R>99.7%) dielectric coatings centred at 1180 and 1400 nm. Both mirror sets also had high transmission (T>95%) centred at 800 nm. The parametric oscillator was initially aligned without the doubling crystal present and, after external orientation, the LBO crystal was inserted into the OPO cavity.

6.6.2 Experimental results

6.6.2.1 Tuning characteristics

In Figure 6.16 the experimental tuning range of frequency-doubled LBO OPO is shown as a function of phase-matching temperature. By using a combination of pump and temperature-tuning, the second harmonic output tuned from 584 to 771 nm, for Tiisapphire pump wavelengths covering 770-800 nm and OPO

crystal temperatures from 110 to 230 °C. The corresponding SHG phase- matching temperatures which were in the range 20 to 120 °C are shown in Figure 6.17. This figure also indicates the extent to which the visible tuning range is split up between the two mirror sets used and between type I and type II frequency-doubling. The solid curves represent the predicted tuning range derived from the Sellmeier equations of Reference 2 and the temperature- dependence of the refractive indices given in Reference 3. It can be observed

Chapter 6 Visible Picosecond Optical Parametric Oscillators 800 800 750 775 780

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770 600 550 220 100 120 140 160 180 200 Temperature (°C) Figure 6.16

Visible range of the internally frequency-doubled LBO OPO; closed circles: 1.4 }lm mirror set, open circles: 1.18 pm mirror set, fo r Ti:sapphire pump wavelengths from 770-800nm.

that there is very little extension to the tuning range accessed by the extracavity

case. This implies that the main restriction to the tuning was the OPO mirror réflectivités and not other losses in the cavity. The tuning could also be extended if a complementary Ti: sapphire laser mirror set was available in the laboratory. The visible range of the OPO can potentially be extended to wavelengths as short as 500 nm by using a mirror set for the Ti:sapphire laser with high reflectivity from 700-850 nm.

Chapter 6 Visible Picosecond Optical Parametric Oscillators

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