Hollow Glass Waveguides
Laser Power Delivery
Hollow waveguides are ideal for high power laser delivery because of the inherently high damage threshold of an air-core structure. For most medical and some low-power industrial applications, it is sufficient to be able to deliver up to 100 W of laser power. For these purposes, the HGWs can be used without cooling although it is often helpful if an inert gas is used to purge the bore of the guide. In Figure 1, we show the results of low-power, CO2 laser power delivery through 700 and 530 µm bore HGWs.
Figure 1 - Low CO2 laser power delivery for two bore size HGWs with no cooling.
This is the typical laser power used in medical lasers and for low power
cutting and marking
It is also possible to delivery higher CO2 laser powers through the guides if active cooling is incorporated. In Figure 2, we show the results of high-power CO2 laser transmission through a 700-µm-bore HGW that has a water jacket surrounding the guide. The maximum laser power delivered through the guide was just over 1000 W.
Figure 2 - High CO2 laser power delivery for a 700 µm bore HGW with a water
cooling jacket. Note that the maximum power is just over 1000 W
The 2.94-µm, pulsed Er:YAG laser is becoming an important medical laser because the depth of ablation is very shallow and, therefore, this laser has great potential in surgical applications involving precise cutting and ablation. In Figure 3 we show the average 3 µm laser power delivered by the 1000 µm-bore HGW. These data were obtained using a multimode Er:YAG laser made by Continuum. The maximum average output power of about 8 W represents a substantial average power for this wavelength. This power is sufficient for most surgical and dental applications.
Figure 3 - Power delivery though a HGW using an Er:YAG laser
The output beam profile of the HGW is important for many applications. In principle the HGWs are nearly single mode because the higher order modes are attenuated by the factor (Uo)2 (see Eqn. 1). In practice, however, mode distortion can occur even with a TEM00 input beam. The spatial profile can worsen on bending due to increased coupling into higher order modes. The amount of coupling into higher order modes is a function of the diameter of the waveguide, the roughness of the surface, and the refractive indices of the material. The spatial profile of a 530-µm-bore HGW is shown in Figure 4 A and B. From the data we see that it is possible to generate a single-mode HE11 output when the guide is straight or bent (Figure 4A), but, at other times when the guide is bent, low-order modes can be generated resulting in the modal pattern in Figure 4B. An important point is tha t the smaller the bore size the better the modal purity even on bending. A 250-µm-bore, straight or bent guide, for example, retains a nearly perfect single mode output. The near single-mode output from the glass waveguides is very important when small spot sizes are needed for precise cutting or marking.
Figure 4 - (a) High quality output beam profile from a bent 530 µm bore HGW
(b) A similar HGW but with the output beam profile distorted somewhat
by pushing against the side of the guide