Industrial application of laboratory-cultivated gemstones - Ruby laser rods

Industrial application of laboratory-cultivated gemstones - Ruby laser rods

Shortly after the idea of the laser was proposed, rubies were first used to make the world's first laser. The working substance is artificial ruby: The matrix of the ruby crystal is Al2O3, and approximately 0.05wt% Cr2O3 is doped within the crystal. Cr3+ replaces the position of Al3+ in the crystal, and optically it belongs to a negative uniaxial crystal. The common sizes of ruby rods range from 0.5cm to 2cm in diameter and 4cm to 16cm in length. It may look like a very light pink glass rod or a very deep reddish-brown color, depending on the Cr doping concentration of the rod. The excitation source is a xenon lamp. Under the irradiation of an Xe (xenon) lamp, the electrons originally in the ground state E1 in the ruby crystal absorb the photons emitted by the Xe lamp and are excited to the E3 energy level.

The average lifetime of particles at the E3 energy level is very short (about 10-9 seconds). Most electrons reach the energy level E2 through radiation-free transitions. The lifetime of electrons at the E2 energy level is very long, up to 3×10-3 seconds. Therefore, a large number of particles accumulate at the E2 energy level, forming a particle number inversion between E2 and E1. At this time, the crystal has an amplification effect on photons with a frequency ν that satisfies hν=E2 - E1, that is, it has a gain for the light of this frequency. When the gain G is large enough to meet the threshold condition, there is a laser output of 694.3nm at some mirror ends.

Although the efficiency of the ruby laser is not high, only 0.1%, and it generates dark red 694.3nm light, due to its extremely simple and representative structure, which is consistent with the structure of the most widely used YAG laser at present, and the energy level (3-level system) is simpler, it is easier to analyze and understand. The ruby rod, as thick as a pen lead and as long as a finger, can easily Pierce through the iron sheet and reflect back from the lunar surface to be detected. These lasers were widely used in laser cutting machines and drilling machines before the invention of the much more efficient YAG laser rods (1%-3%), and many military non-lethal weapons also adopted smaller ruby rods.

The single crystal of ruby has high strength, great hardness, good wear resistance, excellent thermal conductivity, a small coefficient of expansion, outstanding thermal stability, high-temperature resistance, corrosion resistance and high dielectric properties. It has a high pass rate within a wide spectral range (250-5500nm). It is widely applied in the production of high-power laser Windows, infrared Windows and multispectral Windows of various shapes and specifications, as well as in high-tech fields such as missile hoods, light-transmitting rod mirrors, optical lenses and medical surgical blades.

The Birth of the Ruby Laser

The ruby laser was born in 1960 at the Hughes Research Laboratories in the United States, where physicist Theodore Maiman was conducting research on the amplification of microwave radiation through stimulated emission. At that time, scientists already had a certain understanding of atomic energy levels and the principles of stimulated emission. However, applying these theories to the optical frequency range to achieve a light source with high intensity and coherence remained a significant challenge.

Maiman turned his attention to ruby crystals. Ruby is a form of aluminum oxide (Al₂O₃) doped with chromium ions (Cr³⁺). Its unique crystal structure and energy level properties made it an ideal candidate for generating laser light. Maiman coated both ends of a ruby rod with silver to serve as mirrors in a resonant optical cavity. He then used a high-intensity flashlamp to "pump" the ruby crystal.

When the flashlamp emitted intense light, the chromium ions in the ruby absorbed the energy and transitioned from the ground state to an excited state. As more chromium ions reached the excited state, they began undergoing stimulated emission, releasing red photons with a wavelength of 694.3 nanometers. These photons bounced back and forth within the optical cavity, stimulating additional chromium ions to emit identical photons. This chain reaction produced a powerful, highly directional, and coherent beam of laser light.

In this way, the world's first ruby laser was created. The bright red beam it emitted pierced the darkness like a dawn of new technological promise, ushering in the age of the laser.

The working principle of laser

From quantum mechanics, it is known that the energy of an atom based on electrons outside the nucleus is discontinuous and is divided into individual energy levels. When atoms receive specific energy, they will transition from the ground state to the corresponding excited state. If a photon (a beam of light) with appropriate energy is irradiated onto an atom in an excited state, the atom will descend to its lower energy level and emit a second photon exactly the same as the former.
 

In the laser, rare earth particles (gain substances) are placed between two mirrors. The photons produced by stimulated radiation propagate between the mirrors and act as stimulated radiation sources to generate new photons. In this way, laser can be continuously produced. And based on this, the photons produced by the laser are all the same, having the same energy, direction and phase. This is how lasers work.

Profound Impact of the Ruby Laser

The birth of the ruby laser sent shockwaves throughout the scientific and technological communities. It provided unprecedented tools for research in fields such as physics, chemistry, and biology. The laser's ability to generate high-energy, tightly focused beams revolutionized industrial processes like cutting, welding, and drilling, dramatically improving efficiency and precision.

In the medical field, laser surgery has become a common practice, particularly in ophthalmology and cosmetic procedures, offering patients reduced trauma and faster recovery. In communications, the high frequency and bandwidth of laser light laid the foundation for fiber-optic technology, enabling faster and farther transmission of information—bringing the concept of a "global village" closer to reality.

In the military sphere, the development of laser weapons has become a key focus for many nations. Their high precision, speed, and resistance to electromagnetic interference suggest they could play a pivotal role in future warfare.

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Synthetic Ruby Rods Cr:Al₂O₃ Available in 2mm/4mm Dia10mm/20mm Lengths