Establishment of 355 nm pump source

Theoretically it is well known, that sum frequency mixing in a nonlinear medium can be obtained by meeting the conditions of photon energy conservation and momentum conservation (please see Appendix I). By the energy conservation relations

and in particular for the case of frequency mixing of the 1.064 jam infrared wave and its second harmonic wave we know that the optimum ratio of the green light intensities to the infrared light intensities in the mixing crystal should be 2il. The momentum conservation condition depends upon what type of geometry is to be employed in the

CHAPTER 4 Pump sources

mixing crystals, which further determines the polarisation directions of these two radiations. For efficiently generating the third harmonic wave, other conditions also need to be met as we have discussed in the previous chapter, and there can be summarised in terms of a figure of merit.

A type II frequency mixing geometry has recently been identified in LBO for the generation of 355 nm radiation from incident 1.064 jam and 532 nm radiation[14]. The advantage of the type II geometry compared to the type I geometry is a reduced walkoff angle, and as is apparent from the calculations, a minimum value for the type II geometry of 0.55^ is obtained under the phase matching conditions (please see Appendix HI). The other advantage of this geometry is the comparatively large angular acceptance bandwidth which is calculated to be 2.5 mrad in the yz phase matching plane, and is, of course infinite, to first order, in the orthogonal plane. The effective nonlinear coefficient has been calculated to be 0.8 pm/V. It is slightly lower than the type I geometry, but, considering all the above parameters we find that the figure of merit of the type II geometry is considerably larger than the type I geometry.

2(0 3(0

Fig. 4-4 Type II phase matching geometry for sum frequency mixing of 1.064 jim and 532 nm radiation in LBO.

The type II geometry SFM LBO crystal used had dimensions of 5 x 5 x 1 5 mm^,

and was also supplied by FC Castech Inc., Fuzhou, China. This crystal was cut at 0 = 42.2°, (j) = 90°. The polarisation states of 1.064 qm (œ), 532 nm (2(o), and 355 nm

(3CO) waves are shown in figure (4-4) along with the principal axes of the crystal. The Sellmeier equation coefficients used in this calculation was from Chen et al[15].

For the experimental investigation we have used two different tripling geometries as shown in figure (4-5). These two geometries are believed to be the best selections. The

first geometry uses a high efficiency KTP doubling crystal, but the problem is that the

second harmonic wave and the residual fundamental wave polarisation directions are

not ideal for the following type II frequency mixing crystal. This mismatch reduces the conversion efficiency. The second geometry uses a type I NCPM LBO as a doubling medium. In this alternative frequency tripling geometry the polarisation states of both the generated second harmonic radiation and remaining fundamental radiation are

ideally matched to the subsequent sum frequency mixing process in the CPM type II LBO crystal. However, because the doubling efficiency is lower in the LBO crystal at this modest pump level compared to the situation in a KTP, the third harmonic conversion efficiency was not improved. These conclusions are, of course, dependent on the power available at the fundamental frequency, and, as this increases, the LBO

may eventually be superior to KTP as a doubler. However, a KTP(II)/LBO(II)

frequency tripling geometry has been used to efficiently generate the 355 nm ultraviolet

and has been used as an OPO pump source throughout this project.

2co

8p = taii\lX 2)

Type n SHG Type II SFM

2co