A Comprehensive Guide on Laser Principles and Applications

I. Principles of Lasers

Basic structure of a laser:

1. Working substance

The laser-active medium refers to the material system used to achieve population inversion and amplify the stimulated radiation of light. It is sometimes also referred to as the laser gain medium. These can be solid (crystals, glass), gas (atomic gas, ionic gas, molecular gas), semiconductors, and liquid media.

The main requirement for the laser-active medium is to achieve as much population inversion as possible between specific energy levels of its working particles, and to maintain this inversion as effectively as possible throughout the laser emission process. For this purpose, the active medium should have an appropriate energy level structure and transition characteristics.

2. Pumping source

The excitation (pumping) system refers to the mechanism or device that provides an energy source to achieve and maintain particle inversion in the laser working material. Depending on the working material and operating conditions of the laser, different excitation methods and devices can be adopted. There are four common types:

①. Optical excitation (optical pumping). This involves using light emitted from an external light source to irradiate the working material to achieve particle inversion. The entire excitation device is typically composed of a gas discharge light source (such as a xenon lamp or krypton lamp) and a condenser.

②. Gas discharge excitation. This involves using the gas discharge process occurring within the gas working material to achieve particle inversion. The entire excitation device is usually composed of discharge electrodes and a discharge power supply.

③. Chemical excitation. This involves using the chemical reaction process occurring within the working material to achieve particle inversion. This usually requires appropriate chemical reactants and corresponding triggering measures.

④. Nuclear energy excitation. This involves using the fission fragments, high-energy particles, or radiation produced by a small-scale nuclear fission reaction to excite the working material and achieve particle inversion.

3. Resonant cavity – To increase the propagation distance of light waves in the gain medium

An optical resonator is typically constructed by combining two mirrors with specific geometric shapes and optical reflection characteristics in a particular manner. Its functions are:

①. To provide optical feedback capability, allowing stimulated emission photons to travel back and forth multiple times within the cavity to form coherent sustained oscillations.

②. To limit the direction and frequency of the oscillating light beams within the cavity, ensuring that the output laser has certain directionality and monochromaticity.

The first function of the resonator is determined by the geometric shape (the radius of curvature of the reflective surface) and the relative combination method of the two mirrors that usually make up the cavity. The second function is determined by the given resonator type’s different selective loss characteristics for light with different travel directions and frequencies within the cavity.

II. Classification of Lasers

By laser working medium:

• Solid-state lasers(Fiber lasers)
• Gas lasers
• Semiconductor lasers
• Dye lasers
• Free electron lasers

Laser operation modes:

• Continuous
• Pulse: Single pulse; Repetitive frequency; Quasi-continuous

By chemical composition:

• Atomic lasers
• Molecular lasers
• Ion lasers
• Free electron lasers
• Excimer lasers

Laser modulation methods:

• Free running
• Q-switching
• Mode locking

III. Typical Lasers

1. Solid-state Lasers

They are divided into two categories: crystal and glass, made by doping activating ions in the base material.

Currently, more than 200 different base-doped systems have realized laser oscillation as the working material, but the three types that are widely used and have good performance are as follows:

(1) Neodymium glass laser

Rare earth element neodymium is doped in glass as the working material, λ = 1.053 μm. Because large volume and good uniformity neodymium glass can be obtained, it can be made into large devices, achieving high energy and power lasers. A laser with an output power of 1014W has been produced.

(2) Ruby laser

• Working material: Ruby crystal
• Output wavelength: λ=694.3nm
• Output linewidth:Δλ=0.01~0.1nm
• Operating mode: continuous, pulsed
• Divergence angle: θ ≈ 10^-3rad, generally multi-mode output; Pump power >threshold 10~20%→single mode

(3) Neodymium-doped Yttrium Aluminum Garnet (Nd:YAG)

• Working material: YAG crystal doped with the rare earth element Neodymium
• Output wavelength: λ=1064nm, 914nm, 1319nm
• Working mode: Continuous, high repetition rate pulse

Due to the ability to dope with a high concentration of neodymium, the working material can provide a higher laser power per unit volume, and the laser can be made smaller. If a semiconductor laser is used as the pump source, the device volume can be even smaller.

(4) Continuous Wave Tunable Titanium Sapphire Laser

3900S CW Tunable Ti:sapphire Laser

The high-performance,tunable,solid state IR laser

Output wavelength ranges from 675 to 1100nm

Pumped by an Ar laser or LD pumping a 532nm laser

TEM₀₀ output power can reach up to 3.5Wcw

Applications:

• Spectroscopy
• Fiber laser research
• Telecommunications research
• Semiconductor studies

2. Gas Lasers

• Working material: Various mixed gases, good optical uniformity.
• Gas lasers are superior in monochromaticity and beam stability compared to solid-state, semiconductor, and liquid lasers.
• The spectrum lines have reached thousands of types (160nm~4mm).
• Working mode: Continuous operation (most cases)

Most gas lasers have a weakness of not having high instantaneous power.

Reason: Typically, the gas pressure is low, resulting in fewer particles per unit volume.

(1) Helium-Neon Laser

Working Material: Mixture of Helium and Neon gases

The laser is emitted by neon atoms, while helium improves the conditions of gas discharge, thus enhancing the output power of the laser.

Output Wavelength: The commonly used is 632.8nm

Depending on the chosen operating conditions, the laser can output near-infrared, red, yellow, and green light.

(λ=3.39μm; λ=1.15μm)

(2) CO₂ Laser

Working Material: A mixture of CO₂, He, N₂, and Xe gases

The laser is emitted by CO₂ molecules, while the other gases assist in improving the working conditions of the laser, enhancing the output power, stability, and lifespan of the laser.

Output Wavelength: λ=10.6μm

The CO2 laser is the highest power output gas laser, with a continuous output of 50kW; and a pulse output of 1012W.

(3) Argon Ion Laser

Argon/Krypton Ion Laser, Stabilite2017 Argon/Krypton Ion Laser

Output Wavelength:

• λ =488nm；
• λ =514.5 nm ；

Highest output power in the visible light area

Output power ranges from a few watts to a few hundred watts.

3. Helium-Cadmium Laser

Using cadmium metal vapor as the emitting material, it mainly has two continuous spectral lines, namely, ultraviolet radiation with a wavelength of 325nm and blue light of 441.6nm. The typical output power is respectively 1~25mW and 1~100mW. Its main application areas include letterpress printing, blood cell counting, integrated circuit chip inspection, and laser-induced fluorescence experiments, etc.

(1). Copper Vapor Laser

Typically excited through electron collision, its two main working spectral lines are the green light at 510.5nm wavelength and the yellow light at 578.2nm. With a typical pulse width of 10 to 50nS, the repetition frequency can reach up to 100KHz. At the current level, the energy of a single pulse is around 1mJ. This implies that the average power can reach up to 100W, while the peak power can soar to as high as 100KW.

(2). Nitrogen Molecular Laser

The pulse discharge excitation outputs violet external light, with a peak power reaching tens of megawatts, pulse width less than 10nS, repetition frequency from tens of Hz to thousands of Hz. It is primarily used as a pump source for dye lasers, and can also be used for spectral analysis, detection, medicine, and photochemistry. Common wavelengths: 337.1nm, 357.7nm.

3. Semiconductor Laser

Made of semiconductor materials of different components.

Laser with the active area and the constraint area.

Characteristics: Smallest in size, lightest in weight, long service life, effective usage time exceeds 100,000 hours.

Output wavelength range: Ultraviolet, visible, infrared

Output power: mW, W, kW.

Schematic Diagram of DFB Semiconductor Laser

Schematic Diagram of DBR Semiconductor Laser

Vertical Cavity Surface Emitting Laser (VCSEL)

Quantum cascade lasers, QCLs

A new type of unipolar semiconductor device based on the principle of electron transition between sub-bands in semiconductor quantum wells and phonon-assisted resonant tunneling.

Fiber Coupled (Pigtail Package)

Semiconductor Laser Devices

ProLite Type Fiber Coupled Single Emission Laser

IV. Applications of Lasers

1. Industrial Applications

• Precision measurement (distance, displacement)
• Laser processing (cutting, welding, drilling, engraving)
• Spectral analysis

2. Medical Applications

• Ophthalmology
• General Surgery
• Dentistry
• Dermatology

3. Military Applications

• Laser Rangefinding
• Laser Reconnaissance
• Atmospheric Laser Communication
• Laser Guidance
• Laser Weapons

4. Daily Applications

• Laser Printers
• Computer Optical Drives
• Barcode Scanners
• Laser Anti-counterfeiting
• Laser Neon Lights

5. Applications in the Communication Field

• Space Laser Communication
• Optical Fiber Communication

V. Mechanisms of Damage from Laser Weapons

1. Ablation Effect – Local High Temperature

2. Shockwave Effect

3. Radiation Effect – Strong Electromagnetic Field

VI. Advantages of Laser Weapons

1. No need for ballistic calculation

2. No recoil

3. Easy operation, agile, and versatile usage

4. No radioactive pollution, high cost-effective ratio

Almost all lasers used in optical communication are semiconductor lasers, with only a small number of CATV systems using 1310 nanometer or 1550 nanometer LD pumped solid-state lasers.

Lasers used in communication are mainly of two types: pump light sources used in optical fiber amplifiers and signal light sources used in transmitters.

Lasers used in Free Space Optics (FSO) communication are available in two types: 850nm and 1550nm.

VII. Laser Ranging

Make use of the monochromaticity, strong coherence, and directivity of lasers to achieve high-precision measurements and inspections, such as measuring length, distance, speed, and angles.

VIII. Laser Welding

IX. Laser Rapid Prototyping

X. Laser Engraving

XI. Laser Nuclear Fusion

XII. Laser Medical Treatment

XIII. Laser Communication

As the frequency of light waves is several orders of magnitude higher than that of radio waves, a very thin optical fiber can carry an amount of information equivalent to what a cable of this thickness in the picture can carry.

XIV. Laser Weapons

XV. Laser Display