Changes

== Gas Purification System ==

== Gas Purification System ==

[[Image:laser_cavity.png|center|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]

[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]

Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.

Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.

As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).  

As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).  

[[Image:GP2000b.png|center|thumb|250px| Figure: Average laser output as a function of total shots fired.]]  

[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]  

An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.