Solid State Laser

Background

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. A 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 aluminum 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.

History

The concept behind lasers was first proposed by Albert Einstein, who showed that light consists of mass-less particles called photons. Each photon has an energy that corresponds to the frequency of the waves. The higher the frequency, the greater the energy carried by the waves. Einstein and another scientist named S. N. Bose then developed the theory for the phenomenon where photons tend to travel together. This is the principle behind the laser.

Laser action was first demonstrated in the microwave region in 1954 by Nobel Prize winner Charles Townes and co-workers. They projected a beam of ammonia molecules through a system of focusing electrodes. When microwave power of appropriate frequency was passed through the cavity, amplification occurred and the term microwave amplification by stimulated emission of radiation (M.A.S.E.R.) was born. The term laser was first coined in 1957 by physicist Gordon Gould.

A year later, Townes worked with Arthur Schawlow and the two proposed the laser, receiving a patent in 1960. That same year, Theodore Maiman, a physicist at Hughes Research Laboratories, invented the first practical laser. This laser was a solid state type, using a pink ruby crystal surrounded by a flash tube enclosed within a polished aluminum cylindrical cavity cooled by forced air. The ruby cylinder was polished on both ends to be parallel to within a third of a wavelength of light. Each end was coated with evaporated silver. This laser operated in pulsed mode. Two years later, a continuous ruby laser was made by replacing the flash lamp with an arc lamp.

After Maiman's laser was successfully demonstrated, other researchers tried a variety of other substrates and rare earths, including erbium, neodymium, and even uranium. Yttrium aluminum garnet, glass, and calcium fluoride substrates were tested. The development of powerful laser diodes (a device that forms a coherent light output using electrodes or semiconductors) in the 1980s led to all-solid-state lasers in the continuous-wave regime that were more efficient, compact and reliable. Diode technology improved during the 1990s, eventually increasing output powers of solid state lasers to the multikilowattt level.

Nd:YAG and ruby lasers are now used in many industrial, scientific and medical applications, along with other solid state lasers that use different type of crystals. Nd:YAG lasers are also being used for monitoring pollution, welding and other uses. This type of crystal is the most widely used—more than two-thirds of crystals grown are this type. Other crystals being grown include Nd:YVO4 (yttrium orthovanadate), Nd:glass, and Er:YAG.

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.

Design

The design of the laser cavity is determined by the application. Typically, the research and development group develops the design. This design determines the operating characteristics, including power, wavelength, and other beam properties. The designers also incorporate safety features as required by the Food and Drug Administration(FDA).

The 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)

  

  A solid state laser consists of a cavity with plane or spherical mirrors at each end that is filled with a crystal, whose atoms are rigidly bonded. After light is pumped into it by either a lamp or another laser, the crystal produces light that bounces between the mirrors, increasing intensity and producing a very bright light.
  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.

Assembly

• 2 Once the laser is designed and the components received, the optics are integrated with the mechanical components. A technician follows a blueprint, placing the optical components in the desired positions, using metal holders or mounting devices. This procedure is performed in a clean room environment to avoid contamination of the optical components.

Alignment

• 3 Next, the lasing cavity is aligned so it operates at the desired specifications. This is performed on a test table by another technician, using another laser to help with the alignment.

Final testing

• 4 Before shipping the laser to the customer, it goes through a step called end testing, which basically checks the laser for proper operation, including output power, beam quality and other characteristics. The laser is operated for a number of hours to make sure it passes inspection.

Quality Control

Most laser manufacturers follow international quality standards that provide feedback loops throughout the manufacturing process. The laser also goes through several major testing procedures as previously described.

All laser devices distributed in the United States must be certified as complying with the federal laser product performance standard and reported to the Center for Devices and Radiological Health (CDRH) Office of Compliance prior to distribution to end users. This performance standard specifies the safety features and labeling that all lasers must have in order to provide adequate safety to users. Each laser must be certified that it complies with the standard before being introduced to the market. Certification means that each unit has passed a quality assurance test that complies with the performance standard. Those that certify lasers assume responsibility for reporting and notification of any problems with the laser.

Byproducts/Waste

Since suppliers of the various components usually follow total quality management procedures, the laser manufacturer does not test the components for defects and there is little waste. If defective components are found, they are sometimes sent back to the manufacturer.

The Future

Solid state lasers are being designed that have higher power, are faster, have shorter wavelengths, and better beam quality, which will expand their applications. For instance, lasing materials are being developed that will be able to squeeze many billions of pulses into one second, resulting in femtosecond lasers delivering dozens of pulses in each nanosecond. Solid state lasers that can provide power on the terawatt or petawatt level are also being tested for producing nuclear reactions, with the potential of being used in nuclear medicine applications such as CAT scanning. Nd:YAG lasers are expanding into the electronics industry for drilling, soldering and trimming applications. Lasing crystals continue to be made to last longer.

The world laser system market is expected to increase from $4.7 billion in 2000 to $8 billion in 2005, with the solid state laser market reaching over $1.1 billion, compared to $4.6 billion for diode lasers. Solid state lasers are replacing dye, ion and HeNe type lasers in certain markets. Other analysts predict flashlamp-pumped solid state lasers will grow to $660 million and diode-pumped solid state lasers to $312 million by 2003. The latter type of laser will become more popular for such industrial applications as general-purpose marking and materials processing, as costs come down and higher powers become available. These lasers are also being designed with minimal maintenance.