Q-Switches

Acousto-Optic Q-switches are used to generate short duration, high-peak power pulses from continuous wave (CW) Nd:YAG lasers. A Q-switched laser can perform many applications which would be impossible for a CW laser system. For instance, in resistor trimming, a thin layer of resistive materials must be vaporized without damaging the underlying substrate. The short, intense pulses from a Q-switched laser can easily achieve this, whereas with a continuous beam, the power required to vaporize the resistive film would also cause heat damage to the substrate. 

Every optically transmissive material has an index of refraction which determines the angle at which a light beam will bend when it travels into and out of it. Sound waves can exert enough pressure to slightly compress the material and temporarily change its index of refraction. Thus, when a light beam travels through material at the same time as a sound wave, it will be deflected from the path it would take if no sound waves were present. Q-Switches exploit this acousto-optic phenomenon to switch a laser beam on and off. RF power (sound waves in the radio frequencies-megahertz) is pumped into the Q-switch crystal, deflecting the beam away from one of the cavity mirrors, changing the “Q” or quality factor of the laser cavity enough to extinguish the lasing action. Rapidly switching the RF sound waves on and off switches the laser beam on and off. In this way, the laser can be made to emit short duration (nanosecond range) laser pulses at high repetition rates (Kilohertz range). The analogy of a pinched water hose is useful for better understanding this process: when a water hose is pinched, high back pressure develops (increasing gain). Releasing the pinch results in a high velocity surge of water (high power pulse). The longer the pinch is held, the higher the back pressure and velocity of the water surge will be (slower rep rates = higher gain and higher power pulses). 

The pulses have high peak power because, while the system is not lasing, the rod is being pumped to a higher level of gain by the arclamp. When the RF power is removed from the Q-switch crystal, the beam is no longer deflected, (the cavity “Q” is restored) and a high peak pulse is emitted. The peak power is affected (and can be controlled) by the repetition rate. Slower pulse rates yield higher power pulses because the rod has a longer time between pulses to build gain. The high peak-power and short duration of these pulses are what make them useful for material processing. The high peak-power is sufficient to vaporize small quantities of material from a surface for resistor trimming, marking, etc. The short pulse duration minimizes the amount of heat input to the workpiece, reducing thermal damage to the bulk material or substrate.

Acousto-Optic Material, Antireflective Coatings, Acoustic Transducers, Bonding Process

A good acousto-optic material will have a high index of refraction and photoelastic constant to achieve good deflection of the beam. High optical transmission and low absorption of sound waves are also critical. Rapid response time is crucial for high speed switching. Many materials such as fused silica, lead molybdate and dense flint glass can be used. Directed Light’s Q-Switches are manufactured with crystalline quartz, which, while more expensive, best satisfies all of these requirements. The quartz crystal is polished to an extremely precise finish – 10-5 Scratch-Dig and l/8 flatness – and coated with an antireflective coating. This limits reflection loss to less than 0.2% per surface. The excellent quality of the quartz crystal limits overall insertion loss to less than 10%. The antireflective coatings have a damage threshold of 100Megawatts/cm2.

When the acoustic signal is switched on, sound waves traveling through the acousto-optic crystal create a train of compressions that act as a diffraction grating. This deflects the laser beam away from one of the mirrors, extinguishing lasing and allowing gain to build in the laser rod.

The acoustic transducer is also crucial to the proper performance of the Q-switch. The transducer is a piezoelectric device: it expands and contracts when voltage is applied to it. Thus, when RF power is applied to the transducer, it expands and contracts at RF frequencies (many megahertz) and this, in turn, sends sounds waves into the quartz crystal. Directed Light’s Q-switches are made with high quality lithium niobate transducers. For connection to the RF power source, a film of gold is vacuum evaporated onto the thin plate of lithium niobate. The gold makes a durable, non-corroding, non-tarnishing connector. 

One of the most critical aspects of Q-switch design and manufacture is the bond between the transducer and the acousto-optic crystal. In order to reduce costs, some Q-switch manufacturers use organic glues or epoxies to attach the transducer to the crystal. In our Q-switches, the transducer is joined to the crystal by a metal bonding process. Thin metallic films are evaporation deposited on the quartz crystal and lithium niobate transducer. The two pieces are then “cold-welded” together under vacuum and high pressure. This type of joining process has two significant advantages over epoxies: the metal bond joint is more efficient at transmitting sound waves, meaning more RF power goes into the crystal for better laser beam holdoff, and less waste heat is generated, and the metal bond is much more resistant to thermal cycling, making our Q-switches the most durable available. 

Circular polarizing mirrors are a special type of bending mirror which convert a linearly polarized beam into a circularly polarized beam. Simply speaking, every laser beam has an associated electric field. The orientation of this electric field with respect to the direction of beam propagation can greatly affect how the focused beam cuts or welds a given material. In most CO2 laser systems, the electric field of the output beam is oriented in a plane at 45o from the horizontal or vertical. This is known as ‘S’ or ‘P’ linear polarization. A linearly polarized beam will interact differently with the work material depending on the direction of travel. For instance, it may cut nicely in the X direction but yield excessive burring or a nonperpendicular kerf in the Y direction. For consistent cutting or welding in all directions, a circularly polarized beam is required. Quarter-wave and eighth-wave circular polarizing mirrors were developed for this purpose. 

For CO2 lasers, several substrate materials are available. Silicon is used on low to medium power systems when cost is a big consideration. Copper, with its superior thermal conductivity is used for very high-powered systems. Molybdenum is used in dirty environments where durability and repeated cleanings are required. 

Historically, coatings of enhanced and protected gold or silver were the industry standards. In recent years, however, enhanced multilayer dielectric coatings have been developed offering higher reflectivities and greater durability. These coatings are becoming increasingly popular.