Grades CVD diamond

Figure 3. Raman spectrum of an optical grade CVD diamond specimen measured with a Renishaw instrument with an excitation wavelength of 633 nm.

Figure 4. Transmission spectrum for an optical grade CVD diamond window 1.0 mm thick in the UV-visible-near IR spectral range compared to that of a high quality natural Type Ila window 0.5 mm thick.

Figure 10. Young modulus of an Optical Grade CVD diamond specimen in the 20°C to 800°C temperature range [37].

The current standard material for optical elements in CO2 lasers is ZnSe [27] because of its very low intrinsic absorption at 10.6 pm [14,15,34]. Table 3 compares the critical parameters of CVD diamond and ZnSe showing that CVD diamond, for the reasons discussed above, has the potential to handle much greater beam powers. This has been long recognized [51] and the thermal effects in diamond laser windows have been theoretically modelled in previous work [27,51-53]. Some of the earlier results [51-53] were derived before reliable data were available on the properties of polycrystalline diamond and were therefore very speculative. The following is an up-date of the predicted thermal effects in optical grade CVD diamond and ZnSe windows where some of the earlier calculations are revisited. 

The absolute bulk absorption of optical grade CVD diamond windows has been found to be in the range 0.1-0.03 cm [27]. A value of 0.05 cm has been assumed for this calculation. 

The analysis has been done for a continuous wave, 5 kW incident beam that is assumed to have a Gaussian profile with a 14 mm diameter at 14% (1/e ) of maximum intensity. For this level of power the temperature increases are relatively low and the temperature dependence of parameters such as dn/dT [54] have been ignored, but the temperature dependence of the thermal conductivity for optical grade CVD diamond (Fig. 11), although not very pronounced, has been taken into account. 

Figure 11 shows the temperature dependence of the thermal conductivity for different grades of CVD diamond optical grade, and three grades of thermal... 

Values of the dielectric loss of CVD diamond have been measured over the past 3 years as a suitable material grade for dielectric window applications was being developed [5]. For open resonant cavity measurements, samples are usually required to be of at least 30 mm in diameter and of thickness in excess of 0.87 mm depending on the measurement frequency and the accuracy required. For recent CVD diamond, values of tan 5 below 10 have been achieved. A specific example is a window 100 mm in diameter and 1.6 mm thick which exhibited a tan 8 value of 0.6 ( 0.2) 10 . This is the lowest value so far reported for CVD diamond and would enable the material to be used as output windows in Gyrotron tubes of powers in excess of 2 MW as discussed in 2.4. 

Property CVD Diamond (high optical quality) ZnS (FLIR grade) ZnS (Multi-spectral) Germanium... 

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