The first laser to be successfully used was the Ruby laser, it was invented by Maimanin 1960. Ruby laser is a Solid state laser, in which a three-stage scheme is used for population inversion, which makes laser action possible.
History of Ruby laser
Lasers are based on the principle of stimulated emission of radiation.This principle predicted and give by Albert Einsteinin 1917 Which lays the foundation for laser by theoretical understanding. Although before discovery of laser, scientist demonstrated this principle on amplification of microwaves in 1950s. This called MASER (Microwave Amplification by Stimulated Emission of Radiation), which amplified microwave radiation instead of visible light.
The idea of light amplification by stimulated emission was given by various scientists but in 1960 American physicist Theodore Maiman built successfully demonstrated the first working laser using a synthetic ruby crystal. Ruby laser is the first ever working laser.
Construction of Ruby Laser
Ruby laser mainly has three parts.
- Working material
- Resonant cavity
- Optical pumping system
Working material
Ruby rod is made from a crystal of aluminum oxide (Al2O3), in which 0.05% chromium oxide (Cr2O3) is added. Due to this, some aluminum ions are replaced by chromium ions, due to which laser action is possible, and due to these chromium ions the color of the ruby crystal becomes pink.
Resonance cavity
The ruby crystal is converted into a rod of 10 cm long and 0.8 cm diameter with both ends flat and parallel to each other. One end is made fully reflective while the other is made partially reflective. This rod is placed in a cylindrical casing and liquid nitrogen and water are used in the casing to keep the temperature constant.
Optical pumping system
The optical pumping system is a coil shaped tube filled with xenon gas, which is wrapped around a glass casing and is connected to a suitable power supply.
Working of ruby laser
Here is a step-by-step working process of the Ruby laser
Pumping Process (Absorption of Energy)
Chromium ions are in normal ground state, when light from the xenon flash lamp falls on the ruby rod, the incident radiation is absorbed by the chromium ions and rises to the excited state. In a ground state chromium ion can absorb a photon of wavelength 5500A∘ and go to an excited state.
Non-Radiative Transition
The life time of excited state is very short 10-8second. So the Cr3+ions at the excited state quickly lose energy (non-radiatively) and drop to the metastable state.
Stimulated Emission
We have Cr3+ions at metastable state which have a lifetime of 10-3seconds. When a photon of wavelength 694.3 nm interacts with these ions in the metastable state, it stimulates the ion to drop back to the ground state. As the ion transitions, it emits a coherent photon of the same wavelength (694.3 nm), phase, and direction as the stimulating photon.
Optical Resonance
The emitted photons by stimulated emission travel along the axis of the ruby rod. There are fully reflective and partially reflective mirrors at the ends of the rod which reflect the photons back and forth, stimulating further emission and amplifying the light.
Laser Output
The partially reflective mirror allows some of the amplified light to exit the ruby rod as a pulsed laser beam. Here we get laser output which is pulsed and has a wavelength of 694.3 nm (red light)
Ruby laser energy level diagram
Chromium ion goes to excited state states and stays for 10−8 second and then makes a non-radiative transition to metastable states which have a very long lifetime of 10−3seconds. The number of atoms in these states keeps on increasing and at the same time, the number of atoms in the ground state keeps on decreasing due to photo pumping. Thus population inversion is achieved between metastable state and ground state. At room temperature, if the radiation is kept constant, then during the process of pumping, the population at the A2 level is about 15 more than that at the A1 level. Some excited atoms in excited states return to the ground state but with a low probability. Light amplification starts in the resonant cavity when the population inversion occurs. When excited atoms at metastable state transition to the ground state, there are two weak lines at 6943A∘ (A2 → E1) and 6928A∘ (A1 → E1) each having width ≈6A∘. But under lasing conditions, the line 6943A∘ dominates over 6928A∘. This shows that the pumping transitions are spectrally broad while the emission transitions are narrow. The wavelengths of the two spectral lines 6943A∘ and 6928A∘ are temperature-dependent.
Advantages of ruby lasers
The advantages are based on their unique properties derived from the material used and their simple design. Some of the advantages are-
High Energy Pulses
The high-energy pulse output of ruby laser is highly effective for material drilling, welding, and cutting hard materials like metals and ceramics.
Narrow Wavelength Emission
Output of a specific wavelength of 694.3 nm is used in holography, where coherent and monochromatic light is necessary. Also used in dermatology, particularly in treating certain skin conditions like pigmentation, where specific absorption by melanin is required.
High Coherence and Directionality
Ruby lasers exhibit excellent coherence and directionality of the emitted light used for optical measurements, such as interferometry and holographic imaging, requiring high coherence over large distances.
Simple and Robust Design
Ruby lasers have a relatively simple and robust construction, which makes them easy to maintain and operate. It has low maintenance requirements since there are few complex components.
Disadvantages of ruby lasers?
There are some disadvantages that make this laser less suitable for certain applications compared to other modern lasers.
Low Efficiency
Ruby lasers have low optical-to-optical conversion efficiency. Typically less than 1%. A significant portion of the input energy is lost as heat. Due to low efficiency, ruby lasers require high-intensity pumping, resulting in high energy consumption and limited continuous operation.
Thermal Problems
Because of large amount of input energy is lost as heat, ruby lasers tend to generate excessive heat during operation. We have to add efficient cooling systems to prevent damage to the laser components.
Low Gain Medium Efficiency
Ruby crystals have relatively low gain compared to other modern laser materials. This means that achieving population inversion requires much higher pumping energy, leading to inefficiency in low-energy applications.
High Maintenance Requirements
Since ruby lasers rely on optical pumping, the flashlamps degrade over time so we need frequent replacement which increases maintenance costs.
Wavelength Limitation
Ruby lasers emit at a fixed wavelength of 694.3 nm (deep red) but modern lasers and tunable dye lasers offer a wider range of wavelengths.
Limited Repetition Rate and Lifespan
Ruby lasers operate in a pulsed mode, the high energy required to excite the ruby crystal. Although ruby crystals are physically durable, their optical properties can degrade over time due to repeated high-energy pumping.
Uses of Ruby Laser
Ruby laser is one of the earliest types of laser developed. Still used in various specialized applications due to its unique characteristics.
Holography
Because of coherence and monochromaticity at a wavelength of 694.3 nm, Ruby lasers are widely used in holography. The high coherence length allows the laser to produce detailed and accurate holograms over large distances.
Dermatology and Medical Treatments
Ruby lasers are effective in tattoo removal, where the specific wavelength is absorbed well by darker pigments, breaking down ink particles under the skin. The pulsed operation of ruby lasers delivers high energy in short bursts, minimizing damage to surrounding tissue while targeting specific skin pigments.
Micromachining and Material Processing
Excellent focusability makes them suitable for micromachining in drilling microscopic holes in hard materials like metals and ceramics. Cutting and scribing thin layers of materials with high precision. Also used in fabricating intricate components on semiconductor wafers.
Rangefinding and LIDAR
Ruby laser pulses can travel long distances and reflect off distant objects, allowing accurate measurement of distance. Military rangefinders for targeting and surveillance. LIDAR systems for mapping and surveying terrain.
Photography and Imaging
For the photography and imaging of extremely fast-moving objects, we use ruby lasers to provide intense, short-duration flashes of light. This technique is used in specialized high-speed photography.
Metrology
For measuring extremely small displacements, surface irregularities, and variations in the refractive index we use ruby laser because these applications demand highly stable, monochromatic light sources
Scientific Research
Ruby lasers play a significant role in the study of plasma physics, utilizing high-intensity laser pulses to create and examine plasma. Also used in laser cooling and trapping, where specific wavelengths are used to manipulate atoms and molecules.
Question and Answers
Q: What is a ruby laser?
A: A ruby laser is a solid-state laser that uses a synthetic ruby crystal as its gain medium. It was the first lasers to be invented.
Q: What is the lasing medium in a ruby laser?
A: The lasing medium is a synthetic ruby crystal, which is aluminum oxide (Al2O3) doped with chromium ions (Cr3+).
Q: What type of pumping is used in a ruby laser?
A: Optical pumping, typically using a xenon flash lamp.
Q: What is the role of the chromium ions in a ruby laser?
A: The chromium ions are the active centers where the laser action occurs. They absorb the light from the flash lamp and undergo transitions that lead to light amplification.
Q: How is population inversion achieved in a ruby laser?
A: By using a strong flash of light from a xenon flash lamp to excite the chromium ions to a higher energy level.
Q: How does a ruby laser differ from a helium-neon (He-Ne) laser?
A: A ruby laser uses a solid crystal as the gain medium and is typically pulsed, while a He-Ne laser uses a gas mixture and is typically continuous wave.