Understanding solid-state laser

A laser, which is an acronym for Light Amplification by Stimulated Emission of Radiation, is a device that converts electrical or optical energy into light. Electrical or optical energy is used to excite atoms or molecules, which then emit monochromatic (single wavelength) light.

Laser consists of a cavity, with plane or spherical mirrors at the ends, that is filled with lasable material. This material can be excited to a semi-stable state by light or an electric discharge. The material can be a crystal, glass, liquid, dye, or gas as long as it can be excited in this way. A solid state laser is one that uses a crystal, whose atoms are rigidly bonded, unlike a gas. The crystal produces laser light after light is pumped into it by either a lamp or another laser.

The simplest cavity has two mirrors, one that totally reflects and one that reflects between 50 and 99%. As the light bounces between these mirrors, the intensity increases. Since the laser light travels in the same direction as an intense beam, the laser produces very bright light. Laser beams can also be projected over great distances, and can be focused on a very small spot.

The type of mirror determines the type of beam. A very bright, highly monochromatic and coherent beam is produced when one mirror transmits only 1-2% of the light. If plane mirrors are used, the beam is highly collimated (made parallel). The beam comes out near one end of the cavity when concave mirrors are used. The type of beam in the first case makes lasers very useful in medicine since these properties allow the doctor to target the desired area more accurately, avoiding damage to surrounding tissue.

One way to excite the atoms to a higher energy level is to illuminate the laser material with light of a higher frequency than the laser light. Otherwise known as optical pumping, these solid state lasers use a rod of solid crystalline material with its ends polished flat and parallel and coated with mirrors to reflect the laser light. Ions are suspended in the crystalline matrix and emit electrons when excited.

The sides of the rod are left clear to admit the light from the pumping lamp, which may be a pulsed gas discharge producing flashing light. The first solid-state laser used a rod of pink ruby and an artificial crystal of sapphire. Two common solid state lasers used today are Nd:YAG (neodymium:yttrium aluminium garnet) and Nd:glass. Both use krypton or xenon flash lamps for optical pumping. Brilliant flashes of light up to thousands of watts can be obtained and operating lifetimes are near 10,000 hours.

Since laser light can be focused to a precise spot of great intensity, enough heat can be generated by a small pulsed laser to vaporize different materials. Thus, lasers are used in various material removal processes, including machining. For instance, ruby lasers are used to drill holes in diamonds for wire drawing dies and in sapphires for watch bearings.

Raw materials

Optical, mechanical, and electronic components made of various materials (crystals, metals, semiconductors, etc.) are usually supplied by other manufacturers. Outsourcing varies from laser manufacturer to manufacturer. A solid state laser consists of two major components, or "boxes”. One component contains the optics (lasing crystal and mirrors), and the other contains the electronics (power supply, internal controls). Sometimes these two components are integrated into one box.

Manufacturing process

1 Usually, all or most of the components are manufactured elsewhere. For instance, crystal growers provide the lasing material. To grow an Nd:YAG crystal, a high-purity oxide powder compound of the desired elements is placed in a crucible and melted in a radio frequency furnace at high temperatures. A seed crystal is then brought into contact with the liquid surface. When the seed crystal is slowly lifted, rotated, and cooled slightly, a single crystal of the desired composition emerges at the rate of about 0.02 in (0.5 mm) per hour.

Typical Nd:YAG crystals range from 2.4-3.1 in (60-80 mm) in diameter by 6.9-8.9 in (175-225 mm) in length. Rods, wafers and slabs in various geometries are extracted from the grown crystal, then fabricated, polished, and coated to customer specifications. Finished products range from rods as small as 0.02 in (0.5 mm) in diameter by I in (25 mm) long to slab geometries as large as 0.3 x 1.5 in (8 x 37 mm) in cross section by 9.2 in (235 mm) long. The most common Nd:YAG rod geometry is a right circular cylinder.