Q-switched CO2 lasers deliver power

Stephen Lee

Recent work takes advantage of the high peak power potential and high beam quality available from this new class of gaseous lasers

Figure 1. The first commercial sealed-off, Q-switched CO2 laser.Click here to enlarge imageHighly efficient, high-power CO2 lasers operating in the mid-infrared spectrum can deliver large amounts of power for materials processing. These lasers' wall-plug efficiencies are rated on the order of 10-15. CO2 lasers emit at wavelengths between 9 and 11 µm to allow a wide range of materials processing. Some materials absorb better at 9.6 and 9.25 µm, significantly speeding processing time while decreasing thermal and collateral damage.1 Decreasing thermal and collateral damage, important factors in materials processing, can also be accomplished by increasing the amount of peak power delivered to the target by laser Q-switching.Now, for the first time, a robust, production-grade Q-switched CO2 laser is available for commercial materials processing applications.

Figure 2. High peak power pulse from Q-switched CO2 laser.

Click here to enlarge imageQ-switchingQ-switching, a technique to create short pulses of laser light at very high peak power, is accomplished by first creating a very high-loss lasing cavity with a low "Q-factor." Q-factor refers to the amount of loss in a lasing cavity. A laser cavity with a high Q-factor has a low loss and vice-versa. A laser cavity with enough loss allows the gain medium to be inverted to a very large population level of photons while preventing stimulated emission. Consequently, the gain cannot saturate and therefore grows to an extremely large small-signal level. Now, if the Q-factor of the cavity is switched from low to high, laser oscillation occurs. This creates a small-signal gain much greater than the threshold gain required for laser oscillation. The cavity irradiance then grows intensely, resulting in a very large stimulated emission in a short pulse. Switching the Q-factor of the cavity, or Q-switching, results in high-power short pulses of light.

The Q-factor is typically switched by one of two common methods: A rotating cavity mirror or electro-optical (EO) modulation. Rotating mirror techniques are slow and require moving parts and thus are less attractive solutions in a solid-state, high-tech world.

EO modulators are very fast response solid-state shutters that prevent feedback and create loss in the cavity through a change in the state of polarization of the light circulating in the cavity, effectively preventing gain saturation, enabling small-signal gain build-up.

Figure 3. GEM Q-400 power vs. rep rate.Click here to enlarge imageQ-switched EO modulated resonatorBoth short-pulse and cavity-dumped CO2 laser technology has been around since the middle of the 1970s, limited to various military and government applications. A major hurdle associated with developing a robust, production-grade Q-switch for a CO2 laser is finding materials that can operate at CO2 wavelengths without being damaged or destroyed by the high circulating power in the laser cavity. Coherent-DEOS (Santa Clara, CA) has developed the first commercially available Q-switched CO2 laser that solves this problem. The GEM-Series Q-switched CO2 laser is based upon an EO modulated resonator design. The resonator consists of a radio frequency (RF) excited, folded 5-pass Al2O3 waveguide gain cell. The Q-switch is a low-loss CdTe EO modulator with optically contacted ZnSe windows. The entire unit has a high damage threshold, allowing it to survive the extremely high circulating fluence of the cavity. The wavelength-selective ZnSe optics ensure lasing at 9.25 microns. The output coupler extracts the resulting Q-switched pulse train, which exhibits both high beam quality and excellent pointing stability. The system is illustrated in Figure 1. Figure 2 illustrates the high voltage pulse and the associated Q-switched pulse.

Coherent's GEM Series Q-switched lasers have demonstrated 150 ns full-width half-maximum (FWHM) pulses at 100 kHz pulse repetition frequency (PRF) repetition rates with pulse energies of 0.5 mJ. This results in high peak powers in excess of 3 kW. Figure 3 illustrates typical power-repetition rate performance.

ApplicationsIn general, CO2 laser drilling accounts for approximately 80 percent of the laser microvia market.2 Therefore this is the first significant commercial application for Q-switched CO2 lasers. Other commercial applications include skiving polyimide from flex circuitry and marking glass.

Figure 4. No charring on these polyimide samples.Click here to enlarge image

The laser was used to skive polyimide from copper on a flex circuit board. A galvanometer was used to raster scan a focused laser beam, with a calculated spot size of 50 micron, located 60 mm below the scanner. A number of tests were conducted using a variety of scan speeds, pulse duration times, PRFs and scan line spacing to determine the optimum operating parameters, which are listed in the table.

No charring was observed and the polyimide was successfully removed from large areas, especially in places where copper leads and circuit patterns were covered. Moreover, no additional cleaning of the polyimide was required from the flex circuitry. Non-tenacious debris created was removed using conventional cleaning techniques, such as permanganate de-smearing.

Large amounts of energy delivered in short pulses also benefits glass marking applications where thermal shock on glass is a concern. CO2 lasers cut, drill and mark materials by vaporizing the material through a process called photothermal ablation, which causes vibrational bond-breakage of the material. The efficiency of the photothermal ablation process is highly dependent upon the absorption of the material as well as the amount of power and dwell time of the power delivered to the material.

Click here to enlarge imageAll materials, because of their inherent physical and chemical properties, absorb light differently at different wavelengths. Less-absorptive materials, requiring longer dwell times at a given power, can cause substantial thermal damage, smaller thermal "wrinkles" or even melting in the surrounding material. This latter problem also leaves a lot of debris. Shorter dwell times, combined with high peak powers at optimum wavelengths, eliminate these problems. Q-switched CO2 pulses allow glass marking through efficient vaporization of the material without inducing collateral thermal damage.SummaryRecent materials processing applications demand shorter processing times to increase processing speeds while reducing the amount of thermal and collateral damage to areas surrounding processing points. Newer CO2 laser technology achieves this capability by shifting to a shorter processing wavelength, which exploits the absorption that many materials exhibit in the 9-micron region, and by Q-switching the laser to provide substantially higher peak powers.

Material processing of polyimide, silicon and glass at nine microns, coupled with the ability to deliver large amounts of energy in short pulses through Q-switching, will enable faster and more efficient flex circuit machining, micro-machining of plastics, microvia formation in printed circuit boards, silicone tube cutting and glass marking.

These capabilities are now available for the first time in a production-grade, robust Q-switched CO2 laser designed specifically for commercial materials processing applications.ReferencesStephen Lee, "CO2 processing at 9 microns," Industrial Laser Solutions, March 2002.

Sri Venkat, "The meteoric growth of laser microvias," Printed Circuit Europe, 3rd Quarter 2001.

Stephen Lee is CO2 product manager at Coherent Inc., Santa Clara, CA. Contact him via e-mail at [email protected] or by telephone at (408) 764-4633.