Basics of Lasers and Coherent Light: From Helium-Neon to Excimer
The term “LASER” stands for “Light Amplification by Stimulated Emission of Radiation,” a groundbreaking technology that amplifies light through stimulated emission. Because lasers combine three key properties—monochromaticity, directionality, and coherence—they behave differently from conventional light sources. The combination of these characteristics has made lasers indispensable across a wide range of fields, from medicine to industry.
This article provides a comprehensive overview of lasers, covering the basic structure of laser light, the characteristics of different types of lasers, and their applications. By exploring specific types such as helium-neon lasers, argon lasers, and excimer lasers, even beginners can gain a clear understanding of how each functions. Additionally, the article introduces compact, high-efficiency technologies like semiconductor lasers, highlighting their roles in a wide variety of applications.
Examining laser light in detail allows one to appreciate the importance of coherence, where the light waves’ phases are aligned, and the energy concentration enabled by stimulated emission. Acquiring this foundational knowledge makes it easier to understand both the potential applications of laser-based technologies and future advancements. Through this article, readers can systematically explore the world of lasers and coherent light, gaining insight into their connections with cutting-edge technological fields.
1.Characteristics of Laser Light: Monochromaticity, Directionality, and Coherence
First, let’s summarize the key characteristics of laser light to understand why it stands apart from other types of light.
Monochromaticity refers to the property of laser light in which it oscillates around a nearly single wavelength. Unlike ordinary light sources, which emit a broad spectrum of wavelengths, laser light has an extremely narrow spectral width, producing highly pure light. In industrial and research settings, this monochromaticity allows precise control over transmission and absorption properties, enabling experiments and processing with high accuracy. It is particularly useful for observing subtle differences and performing precision machining, enhancing practical utility.
High directionality means that laser light travels as a very narrow beam over long distances. Unlike a flashlight, which spreads out as it travels, laser light has minimal spatial divergence, concentrating energy on a specific point. This makes it ideal for applications requiring high power, such as material processing, or for illuminating and analyzing microscopic areas. It also provides advantages in medical devices and communication technologies.
Coherence is another major characteristic of laser light. It refers to the condition in which the phases of light waves are aligned, enabling phenomena such as interference and holography. By leveraging coherent interactions, ultrahigh-precision measurements and specialized imaging techniques become possible, making laser light widely applicable across cutting-edge research and industrial applications.
2.Differences Between Laser Light and Ordinary Light
By clarifying the differences between laser light and ordinary light sources, we can better understand the advantages and unique characteristics of lasers.
Conventional light sources such as incandescent bulbs or fluorescent lamps emit a wide range of wavelengths in an unstructured manner, either as point or line sources. In contrast, lasers achieve monochromaticity, high directionality, and coherence simultaneously through their excitation mechanisms. These properties enable high-precision processing, measurement, and communication that ordinary light sources cannot provide, making lasers a crucial technological foundation in our daily lives.
In terms of energy efficiency and safety, lasers also allow high energy to be used only where necessary, thanks to specialized control technologies. Ordinary light sources tend to illuminate a broad area at once, whereas lasers can easily focus the beam narrowly or select specific wavelengths according to the application. This capability makes lasers indispensable in fields requiring pinpoint illumination, such as microfabrication or medical treatments.
In recent years, the miniaturization and power increase of lasers have advanced rapidly, greatly improving portability and usability. Lasers are now used in a wide range of industrial equipment and consumer devices, and the development of semiconductor and diode lasers has accelerated this trend. The proliferation of laser technology is not only a difference in light sources but also a driving force for creating new applications across industries.
What Is Coherence, Where the Phases of Light Are Aligned?
Coherence refers to the property of light waves (electromagnetic waves) in which their phase relationships are aligned. In ordinary light sources, individual molecules or atoms emit light independently, so the phases are random, making interference difficult to achieve. In contrast, lasers align the phases of light through stimulated emission, enabling phenomena such as interference and holography. This greatly enhances the precision of measurement and imaging technologies. These characteristics make lasers indispensable for high-precision metrology and communication applications.
3.Mechanism of Laser Oscillation
This section explains how lasers amplify light and produce phase-aligned beams.
Laser oscillation begins by supplying energy from an external source to excite the atoms or molecules in the lasing medium, promoting light emission. As the light repeatedly reflects within the resonator, waves with aligned wavelengths and phases are amplified. This process is the origin of a laser’s characteristic monochromaticity and high directionality.
Excitation can be achieved through electrical energy, optical pumping, or chemical reactions. Lasing media vary widely—including solids, gases, and semiconductors—and are selected based on the intended application and wavelength. The combination of these diverse oscillation methods and media has enabled the development of versatile laser technologies.
Through repeated stimulated emission, the population inversion is maintained, allowing continuous generation of high-energy laser light. Understanding this principle provides a foundation for efficiently learning the characteristics and applications of different types of lasers.
Excitation, Stimulated Emission, and Population Inversion Processes
First, through excitation, energy is injected from an external source, causing electrons to rise to higher energy levels. Next, stimulated emission occurs when external photons interact with these excited electrons, prompting the release of light with the same phase and wavelength. In this way, light of identical wavelength and phase is amplified. As it repeatedly reflects within the resonator while maintaining population inversion, a chain reaction generates high-power laser light.
4. Classification of Laser Media: Solid-State, Gas, and Semiconductor
The type of laser medium greatly affects the output wavelength and characteristics. Let’s review some typical media and their features.
Laser media can be classified into solid-state, gas, semiconductor, and even liquid types. A well-known example of a solid-state laser is the Nd:YAG laser, which is widely used in applications such as metal processing and medical laser surgery. Selecting a medium that suits the intended application while considering cost and maintenance is key.
Common gas lasers include helium-neon (HeNe) lasers, argon lasers, and carbon dioxide (CO₂) lasers. These lasers produce well-defined wavelengths with high beam quality, making them ideal for precise measurements and scientific experiments. The red light of HeNe lasers and the blue-green light of argon lasers are particularly visible, often serving as measurement standards.
Semiconductor lasers, or diode lasers, generate light using a semiconductor junction and are compact and highly efficient. They are widely used in communication devices, barcode scanners, and consumer optical drives, making them closely integrated into daily life. Recent technological advancements have improved both output power and beam quality, and semiconductor lasers are expected to see expanding use in industrial applications as well.
5. Principle and Applications of the Helium-Neon (He-Ne) Laser
The helium-neon (He-Ne) laser, a representative of gas lasers, is widely used due to its stable output and high beam quality.
The He-Ne laser operates by exciting a mixture of helium and neon gases. The most common type emits red light at 632.8 nm and is widely used in applications requiring precise beam position and phase, such as measurements, optical experiments, and holography. Its beam stability and long lifespan make it indispensable for both research and industrial applications.
In the medical field, low-power He-Ne lasers are sometimes used in rehabilitation and wound treatment. Their stable output and moderate stimulatory effects make them suitable for such purposes. Additionally, because they are relatively easy to handle, He-Ne lasers remain an important testbed for beginners learning the principles of laser light.
Although He-Ne lasers are not high-power, their excellent beam quality and high coherence make them useful for interference experiments and position detection in robotics. They are convenient for small-scale laboratory setups, though some maintenance considerations, such as gas replenishment and cooling, must be taken into account.
6. Characteristics and Applications of Argon Lasers
Argon lasers, which operate in the visible light range, are utilized across scientific, industrial, and medical fields, taking advantage of their striking color properties.
An argon laser is a gas laser that emits light by exciting argon gas through electrical discharge, primarily producing blue and green visible light. Within the visible spectrum, it is highly valued for applications such as spectroscopy and display technologies. Its excellent beam quality makes it particularly effective in research and measurement fields that demand high precision.
In the medical field, argon lasers have been used in ophthalmic surgeries and dermatological treatments. For example, in retinal photocoagulation, specific wavelength bands of argon light can be mechanically controlled to affect biological tissue accurately. Compared to red light, the transmission and absorption characteristics differ, allowing the wavelength to be selected according to the surgical method or treatment purpose.
In industrial applications, argon lasers are employed in lithography and fine processing. They can operate in continuous-wave mode with a stable beam, and with proper temperature management, long-duration operation is achievable. Leveraging these strengths, argon lasers serve as essential enabling technology from research and development to mass production processes.
7. How Excimer Lasers Work and Their Applications
Excimer lasers, which emit high-energy ultraviolet light, are indispensable in cutting-edge technologies such as semiconductor manufacturing and corneal refractive surgery.
Excimer lasers operate through a unique mechanism in which excited dimer molecules (excimers) release ultraviolet light as they return to their ground state. Typical examples include ArF (193 nm) and KrF (248 nm) lasers, which can emit powerful energy at extremely short wavelengths—a major advantage of these lasers.
In semiconductor manufacturing, excimer lasers are widely used in lithography, where a high-precision, short-wavelength light source is required to pattern extremely fine circuits. Patterning with excimer lasers is essential for enhancing the performance of highly integrated semiconductor chips.
In the medical field, excimer lasers are used in procedures such as LASIK corneal refractive surgery, precisely removing corneal tissue to improve vision. The short wavelength allows fine control over tissue interaction, enhancing both the safety and accuracy of the procedure. Excimer lasers, capable of operating at the nanometer scale, truly represent a hallmark of advanced technology.
8. Structure and Advantages of Semiconductor Lasers
Semiconductor lasers, characterized by their compact size and low power consumption, are widely used in devices ranging from communication equipment to barcode readers.
These lasers operate by amplifying and resonating light generated when electrons move through a semiconductor crystal. They function in extremely small devices, with both optical and electrical inputs serving directly as driving energy, enabling high energy efficiency. As a result, semiconductor lasers are incorporated into a wide variety of products, including optical disc drives for CDs and DVDs, laser printers, and communication devices.
The emission wavelength can also be selected relatively easily, allowing designs in the infrared or visible spectrum as needed. Furthermore, as mass production and cost reduction of diode devices have progressed, their adoption in mobile devices has expanded. Infrared lasers used in smartphone face recognition and sensor technologies, for example, are often based on semiconductor laser technology, and demand is expected to continue growing.
On the other hand, achieving high output while maintaining beam quality has historically been a challenge for semiconductor lasers. Recent innovations, such as laser diode arrays, have brought significant improvements. These advances now allow full-scale applications in industrial cutting and welding, enabling larger-scale and more efficient laser processing.
Latest Trends and Technological Innovations in Diode Lasers
In diode lasers, technologies that increase output power by arranging multiple laser elements in an array are attracting significant attention. At the same time, efforts are underway to enhance beam directionality and coherence by increasing the integration density. Moreover, improvements in cooling techniques and advances in semiconductor materials have made stable laser oscillation possible even under low- or high-temperature conditions, making them promising candidates for use as sensors in the aerospace industry and other extreme environments.
9. Main Applications of Lasers: Medical, Industrial, Communication, and Research
By understanding the comprehensive application areas of lasers—such as medical, industrial, communication, and research—we can appreciate the versatility and potential of laser technology.
In the medical field, lasers are widely used for surgeries, dermatological treatments, and dental procedures. By selectively irradiating specific wavelengths, it is possible to treat targeted areas without causing unnecessary damage to surrounding tissues. The precision of laser light is particularly valuable in advanced medical applications, such as LASIK eye surgery and cancer treatments.
In industry, lasers are employed for cutting, welding, drilling, and marking processes. Because laser processing is non-contact, there is minimal wear on parts, improving the maintenance efficiency of production lines. Lasers also offer strong adaptability to a variety of materials, including metals, plastics, and ceramics, and can perform three-dimensional processing, making them highly versatile tools in manufacturing.
In the communications sector, lasers—especially semiconductor lasers—play a central role in high-speed data transmission through optical fibers. Looking ahead to 5G, 6G, and beyond, lasers will remain a critical enabling technology for networks requiring higher bandwidths and lower latency.
In research and development, lasers are essential in applications such as spectral analysis and optical control in quantum computing, supporting a wide range of cutting-edge scientific fields and driving technological progress.
Summary
This section provides a comprehensive overview of laser characteristics, operating principles, and application areas, while also highlighting future technological prospects.
The unique properties of lasers—monochromaticity, directionality, and coherence—have already made them indispensable across a wide range of fields. Various types of lasers, including helium-neon lasers, argon lasers, excimer lasers, and semiconductor lasers, leverage their specific wavelength ranges and output characteristics to deliver significant value in medical, industrial, and communication applications.
Looking ahead, laser technology is expected to achieve even greater diversity in applications through improvements in light output, beam quality, and precision control. Advances in the manipulation of coherent light may accelerate breakthroughs in quantum technologies and cutting-edge measurement techniques. As a key technology supporting our digital society and driving future innovations, the impact of lasers is set to grow even further.
By gaining a comprehensive understanding of lasers, we can uncover numerous insights that contribute to next-generation industrial competitiveness, enhanced medical capabilities, and research breakthroughs. Acquiring a solid foundation in the use of coherent light enables flexible application toward solving real-world challenges and creating new technologies.