Category: Fiber Laser Technology

The methods for Fiber Bragg Grating inscription

Fiber Bragg Gratings: Photosensitivity and Inscription Methods

The development of fiber photosensitivity has opened up new opportunities. The photosensitivity permits fabricating fiber Bragg gratings (FBG), which are now prevalent in many applications such as multiplexers, demultiplexers, and optical add/drop filters, where individual wavelength selection and separation are required.

Principles and Methods of FBG Inscription

Fiber Bragg gratings are generated by “inscribing” or “writing” systematic changes of refractive index into the core of a special type of optical fiber using an intense ultraviolet source such as a UV laser. The type of FBG inscription that is most suitable depends on the type of grating to be manufactured. Generally, a germanium-doped silica fiber is used to inscribe fiber Bragg gratings. The germanium-doped fiber is photosensitive, which means that the refractive index of the core changes with exposure to UV light. The amount of the change depends on the intensity and duration of the exposure as well as the photosensitivity of the fiber.

The methods of FBG inscription consist of two types: holographic (interference) and nonholographic. The first type uses the amplitude or spatial splitting of the beam into two beams, which interfere in the fiber bundle. Nonholographic methods based on the periodic illumination of the fiber with the use of a pulsed source through an amplitude mask or point-by-point method.

Interferometric and Phase Mask Inscription Techniques

Interferometer inscription schemes use two-beam interference. The UV laser is split into two beams, which interfere with each other, creating a periodic intensity distribution along the interference pattern. The refractive index of the photosensitive fiber changes according to the intensity of light to which it is exposed. This method allows for quick and easy changes to the Bragg wavelength, which is directly related to the interference period and a function of the incident angle of the laser light.
In the scheme with a Lloyd interferometer (spatial beam splitting), the interference pattern is produced by using a mirror: one-half of the beam is reflected, while the other half is reflected at an angle. This interferometer comprises a dielectric mirror, which directs half of the UV beam to a fiber that is perpendicular to the mirror.
With a Talbot interferometer, by simultaneous rotation of additional mirrors located on rotation stages, wherein the fiber should be placed on a linear stage. This interferometer consists of two plane parallel mirrors and a diffractive element (a phase mask) with its surface aligned perpendicular to the mirrors. The illumination is arranged to recombine the first–order beams (or other orders such as the plus first and zero orders) to form the UV interference pattern.

Fiber Bragg gratings inscription using the phase mask technique is based on the diffraction of UV light by a mask placed close to the fiber. The phase mask technique is significantly simpler in comparison with other methods. It has a simple setup and uses low low-coherence UV laser beam.

The common method in FBG phase-mask inscription uses dedicated 193nm or 248nm excimer laser models with increased coherence. When illuminated with the laser beam, the phase mask generates a regular interference pattern. The enhanced spatial coherence length of the excimer laser provides good contrast behind the mask, which is typically placed at a distance of 100µm to 200µm from the fiber core.
The phase mask methods have quickly prevailed over other techniques due to the simplicity and flexibility of the application and the great reproducibility of the inscribed gratings.

Photonic Crystal Fiber technology (PCF)

Photonic Crystal Fibers: Structure, Principles, and Applications

Fundamental Properties and Light Guidance Mechanisms

Photonic crystal fibers and fiber lasers are two of the most rapidly developing spheres of optics and photonics over recent years. Photonic crystal fibers are a new class of optical fibers. They have a unique artificial-like crystal microstructure. They can guide light not only through a common total internal reflection mechanism but also with the use of the photonic bandgap effect.

Design Flexibility and Dispersion Engineering

The construction of PCFs is flexible. There are several parameters to manipulate: the period of the grating, the air hole shape, the refractive index of the glass, and the type of grating. Freedom of design permits one to obtain single-mode fibers, which are single-mode in all optical range, and a cut-off wavelength does not exist. By manipulating the structure, it is possible to design the required dispersion properties of the fiber. PCFs can be fabricated having zero, low, or anomalous dispersion at visible wavelengths. The dispersion can also be depressed over a large range. Regardless of the type of glass and structure, the usual method of photonic crystal fiber fabrication is multi-rate thinning.

Materials and Waveguiding Advantages

In a photonic crystal fiber, light is absorbed in the core, providing a better waveguide to photons than standard optical fiber. The polymers used instead of glass in PCF provide the advantage of a more flexible fiber, which provides easier and less expensive installation. Different photonic crystals according to various photonic gratings are produced, depending on the required properties of the propagated light.

Current Applications of Photonic Crystal Fibers

PCF is finding applications in fiber-optic communications, fiber lasers, nonlinear devices, high-power transmission, highly sensitive sensors, and other areas.
Significant features also make photonic crystal fibers very attractive for several specific areas in case as telecom components, quantum optics, the guidance of cold atoms, and Bragg fiber.
Photonic crystal fibers mix characteristics of photonic crystals and classical fibers. Research on photonic crystal fibers is still unexplored. Apart from already developed applications as fiber dispersion compensation, supercontinuum generation, and particle guidance is expected a series of new applications in telecommunication, sensing, beam delivery, surgery, spectroscopy, and fiber lasers is expected in the next few years.

Fiber laser light source global market development

Applications of Fiber Lasers in Industry and Medicine

Fiber lasers are used for material processing applications such as cutting and welding in the automotive, manufacturing, heavy industry, and consumer electronics sectors. They are also being used for medical applications. Apart from the mentioned, a fiber laser source is used for marking and engraving components for better traceability and for removing product falsification and inventory management.

Ensuring Laser Quality and System Integration

Nowadays, companies have become more careful about the quality of the laser delivered to the process. The quality of the laser is usually measured on the laser source. Most laser sources will be integrated into a system being used for a specific process, set of processes, or applications. That system can consist of optics and mechanical components used to shape, resize, and deliver the laser energy in many different ways, depending on how the laser is applied. In sum, the laser light from that source can interact with several different components, each affecting its quality.

Market Trends and Technological Innovations

Researchers of market forecast that the global fiber laser market to increase at a CAGR of 15.81% during the period 2016-2020. A new trend in the fiber laser market is the development of a ribbon-core fiber laser light source. Researchers have developed ribbon-core fiber lasers to increase the fiber laser output radically while keeping the beam quality. Ribbon-core fiber lasers are produced by a line of doped silica rods surrounded by undoped rods to increase the beam quality. The refractive index of the central ribbon core is high, which helps focus the light. These fiber lasers’ source can produce high levels of power without being thermally damaged.

Advantages and Market Drivers

A key market driver is the increased use of fiber lasers in materials processing. The flexibility of using a wide range of materials and the advantages of using lasers over other conventional processes have made them the first choice for many materials processing applications, such as metal welding, plastic welding, and brazing, mostly in automotive engineering, cladding or repair welding, and cutting of metals for the machine tooling, automobile, and medical sectors. Fiber laser sources are solid-state and offer high reliability and convenience. Materials processing involves chemical or mechanical steps for product manufacturing across industries such as automotive, general manufacturing, heavy industry, aerospace, and electronics.

Nonlinear Raman Scattering spectroscopic techniques

Linear vs. Nonlinear Spectroscopy

At the present day, many types of spectroscopic techniques are available to researchers. Standard spectroscopy, such as absorption and emission spectroscopy, is linear and uses one incident light beam. In this case, the observed light interactions have a tendency to be weak, and the spectra produced can be indefinite.
Nonlinear spectroscopy, such as Coherent Raman Scattering and Stimulated Raman Scattering, uses two or more light beams. Many nonlinear spectroscopic techniques can vary the light’s optical parameters, such as its amplitude, frequency, polarization, and phase.

Advantages of Nonlinear Spectroscopy

There are several advantages of using nonlinear techniques. Nonlinear spectroscopy uses multiple sources of light, meaning more information is obtained from samples tested using a nonlinear technique.
Nonlinear spectroscopy can be used to study interfacial and surface processes. It can be used to study interactions in areas of the electromagnetic spectrum not accessible to linear spectroscopic methods. Nonlinear spectroscopy is also useful for studying dynamic processes and can be used to understand nanoparticles and their unique optical properties.

Principles and Wavelength Considerations in Raman Spectroscopy

Raman spectroscopy provides information about molecular vibrations that can be used for sample identification and quantitation. The technique involves shining a monochromatic light source (i.e. laser) on a sample and detecting the scattered light.
Laser wavelengths ranging from ultraviolet through visible to near infrared can be used for Raman spectroscopy. The choice of laser wavelength has an important impact on experimental capabilities.

Factors Affecting Raman Spectroscopy Performance

Sensitivity. For example, an infra-red laser results in a decrease in scattering intensity by a factor of 15 or more when compared with blue/green visible lasers.
Spatial resolution. Thus, the achievable spatial resolution is partially dependent on the choice of the laser.
Optimization of the result based on the sample behavior. For example, Blue or green lasers can be good for inorganic materials and resonance Raman experiments, and surface-enhanced Raman scattering (SERS). Red or near infra-red (660-830 nm) is good for fluorescence suppression. Ultraviolet lasers for resonance Raman on biomolecules and fluorescence suppression.

Raman spectroscopy is used in many varied fields – in fact, any application where non-destructive, microscopic, chemical analysis and imaging are required. Whether the goal is qualitative or quantitative data, Raman analysis can provide key information easily and quickly. It can be used to rapidly characterize the chemical composition and structure of a sample, whether solid, liquid, gas, gel, slurry, or powder.

Quasi-continuous-wave (quasi-CW) technique

Quasi-Continuous-Wave Laser Operation

Quasi-continuous-wave (quasi-CW) operation of a laser means that its pump source is switched on only for certain time intervals, which are short enough to reduce thermal effects significantly. However, still long enough that the laser process is close to its steady state, for example, the laser is optically in the state of continuous-wave operation. The duty cycle may be, for example, a few percent, thus significantly reducing heating and all related thermal effects, such as thermal lensing and damage due to overheating. Therefore, quasi-CW operation allows the operation with higher output peak powers at the expense of a lower average power.

Quasi-CW Laser Implementations and Design Considerations

A quasi-continuous-wave operation is most often used with diode bars and diode stacks. Such devices are sometimes even designed specifically for quasi-CW operation: their cooling arrangement is designed for a smaller heat load, and the emitters can be more closely packed in order to obtain a higher brightness and beam quality. Compared with an ordinary continuous-wave operation, additional lifetime issues can result from the quasi-CW operation, related to higher optical peak intensities or to frequent temperature changes. Some doped-insulator solid-state lasers are also operated in quasi-cow operation. Such lasers are sometimes called heat capacity lasers.

Excimer Lasers and Their Practical Limitations

Vacuum-ultraviolet (VUV) coherent light has been proposed for different applications, as optical data storage, metrology, biomedical applications, fundamental spectroscopic research, and laser lithography. Conventional coherent VUV laser sources at this wavelength are excimer lasers. These lasers generate a high output power of more than 100 W; however, their structures are huge and complex. Moreover, high manufacturing and maintenance costs are required. The low repetition rate (usually several kHz) of such excimer laser systems also restricts the applicability of many shot statistics during data acquisition in spectroscopic measurements.

High-Repetition-Rate VUV Generation Techniques

The development of coherent radiation with high-repetition-rate (quasi-continuous wave) or continuous wave (CW) in VUV has to be based on the frequency conversion or exploitation of new laser materials. At present, the latter is a more challenging topic, and the former is the unique way to generate VUV laser light with a high-repetition-rate or continuous wave. Generally, techniques for optical frequency conversion for generating short-wavelength light are the second harmonic generation (SHG) and sum frequency generation (SFG) processes.

Developing and Producing Lasers: Fundamentals, Types and Applications

There are literally more than 10,000 types of lasers developed to date. Most of them are developed only in a laboratory, but some have found very broad applications.

Fiber Lasers and Pumping Technologies

Fiber lasers are an interesting class of solid-state lasers. Active media are the core of rare-earth (Er, Yb, etc.) doped fiber. Pump light can be in the core or the cladding. Fiber lasers can be very compact and rugged. They are becoming very popular with the advent of suitable diode pumps. Since the fiber core is very small, the threshold pump power is a few orders of magnitude less as compared to the bulk case. Since core-pumping requires high spatial quality lasers, diode bars and arrays cannot be used. This limits the pump power. To resolve this problem, high-power fiber lasers use pumping from the cladding of the fiber. Cladding pumped Nd and Yb doped fiber can yield ~10 W output power.

Single-Frequency Lasers for Precision Applications

Single-frequency sources are also attractive because they can be used for driving resonant enhancement cavities, e.g., for nonlinear frequency conversion, and for coherent beam combining. Typical applications of single-frequency lasers occur in the areas of optical metrology and interferometry, optical data storage, high-resolution spectroscopy (e.g., LIDAR), and optical fiber communications. In some cases, such as spectroscopy, the narrow spectral width of the output is directly important. In other cases, such as optical data storage, a low-intensity noise is required, thus the absence of any mode beating noise.

Ultrafast Lasers in Advanced Materials Processing

The unique characteristics of ultrafast lasers, such as picosecond and femtosecond lasers, have opened up new avenues in materials processing that employ ultrashort pulse widths and extremely high peak intensities. Thus, ultrafast lasers are currently used widely for both fundamental research and practical applications. Surface processing includes micromachining, micro- and nanostructuring, and nano-ablation, while volume processing includes two-photon polymerization and three-dimensional (3D) processing within transparent materials.
Tunable wavelength laser arrays find wide applications in fiber-optic networks, broadband sensors, biotechnology, and medical diagnostics due to their wide tuning range and stable lasing operation.

CO₂ Lasers for High-Power Industrial Use

CO2 laser is one of the most powerful. It is used very commonly for «hardcore» materials processing, like cutting and welding. Lasing action results from transitions between vibrational levels of CO2. These lasers are typically RF discharge-pumped. They can operate pulsed or CW. They are mostly used in industry and medicine, where high powers are needed.
Gas lasers, laser diodes, and solid-state lasers can be manufactured to emit ultraviolet rays, and lasers are available that cover the entire UV range. The strongest ultraviolet lines are at 337.1 nm and 357.6 nm, wavelength. Ultraviolet lasers have applications in industry (laser engraving), medicine (dermatology and keratectomy), chemistry (MALDI), free air secure communications, computing (optical storage), and the manufacture of integrated circuits.

Tunable Solid-State and Dye Lasers

Ti:Sapphire is the most widely used tunable and mode-locked solid-state laser. It has a bandwidth of about 400 nm, centered at 800 nm. Emissions result from 3d transitions. No diodes are available at this wavelength. This is the main drawback of Ti:Sapphire. Dye lasers use solutions of organic dyes as active media. Solvents can be alcohol, glycerol, or water. Due to the large wavelength range, dye lasers are used in many scientific spectroscopic applications. They are also used in medicine for retinopathy and curing dermatological diseases.

Deep ultraviolet (DUV) fiber lasers sources and applications

Deep-Ultraviolet Solid-State Lasers: Applications and Impact

Over the past decade, the generation of DUV radiation by solid-state lasers, including fiber lasers, has been extensively studied. Deep-UV light sources have a wide range of applications. Because shorter-wavelength light has higher photon energy, the photon energy of deep ultraviolet light sources is sufficient to kill bacteria and viruses and decompose harmful, stable substances, such as dioxin and polychlorinated biphenyls (PCBs), which have caused serious environmental problems worldwide. Therefore, deep-UV light sources are used in water purification, sterilization, and environmental protection equipment. In addition, since the focal point of light decreases with decreasing wavelength, deep ultraviolet light sources have potential for use in high-density optical data recording and nanofabrication technology. Furthermore, they are also expected to be used in medical procedures and analytical instruments.

Limitations of Conventional DUV Light Sources

Most DUV fiber lasers used are gas light sources, such as mercury lamps or excimer lasers. They contain toxic substances, which cause serious environmental problems, and are large in size and low in efficiency and reliability. Moreover, the emission wavelengths are fixed at 254 nm for mercury lamps and 193 nm for ArF excimer lasers.

Emerging Deep-UV Laser Technologies for Industry and Science

New laser sources operating in the deep-ultraviolet (DUV) range (the wavelength region below 300 nm) can help to streamline industrial and scientific applications. For example, state-of-the-art semiconductor lithography and inspection are currently performed using somewhat expensive pulsed excimer lasers at 193 nm. Actinic inspection of exposed wafers—that is, inspection at the exposure wavelength can benefit from continuous-wave light sources.

Scientific and Quantum Applications of Tunable DUV Lasers

On the scientific side, applications include angle-resolved photoemission spectroscopy (ARPES), where researchers need high photon energies for the measurement of large portions of the Brillouin zone of new materials. In addition, new applications are emerging in the field of Raman spectroscopy, such as protein structural analysis and Raman spectroscopy beyond the solar background.
In quantum technology, tunable DUV lasers are used for high-resolution spectroscopy and laser cooling. For instance, atomic clocks can be improved considerably with direct access to optical transitions in aluminum or mercury ions, and one can possibly realize nuclear optical clocks with thorium in the near future.

Narrow Spectral line width of high power lasers

High-Power Narrow-Linewidth Laser Sources

High-power laser diodes have been developed for applications including solid-state laser pumping, fiber laser pumping, and material processing. The spectral bandwidth of high-power diodes typically spans the range of 3-5 nm. A narrow emission spectrum in the range of 0.1-0.5 nm and a smaller wavelength tolerance can be extremely beneficial for special applications such as the spin-exchange optical pumping (SEOP), which is of great importance in the field of nuclear physics, atomic physics, laser cooling, and neutron scattering.

Spectral Linewidth Narrowing Techniques for High-Power Diode Lasers

A variety of external cavity techniques has been developed to narrow the spectral line width of high-power diode lasers. Two types of gratings are used to form the external cavity. The diffraction grating external cavity provides a large tunable range of the center wavelength of ~10 nm. Meanwhile, the volume holographic grating (VHG) external cavity has a smaller footprint with a limited tunable center wavelength range of <0.5 nm. The resonant wavelength of a VHG can be precisely tuned with temperature control. Using a thick VHG (14-18 mm), one can narrow the spectral line width down to a few GHz (7-10 GHz) with output power exceeding tens of watts.

Beam Quality and Fiber Coupling Limitations

Due to the low beam quality of high-power broad-area laser diodes, it is difficult to effectively couple the laser power into a multi-mode fiber without special beam shaping treatment. High-power, narrow spectral line-width diode lasers are useful for optical pumping of alkali metal vapors. High-power, spectrally narrowed lasers with good stability are the key to the success of SEOP. However, these laser diode arrays are not the best choice for high-efficiency optical pumping applications.

Single-Frequency Fiber Lasers as an Alternative Solution

Achieving the narrow spectral linewidth that is required for a long coherence length makes for complicated and expensive lasers, although complexity and size can be somewhat reduced by going with a semiconductor laser design. Now, Optromix single-frequency high-power fiber Laser Irybus-SF-1030-X series offers a wide-range thermal wavelength tuning and optional active wavelength control. Irybus-SF-1030-X is a single-frequency high-power low noise 1030 nm – 1100 nm Yb-doped fiber laser. Its key advantage is an ultra-narrow line width (<100 kHz) based on a longitudinal single mode. Irybus-SF-1030-X comes with a piezoelectric tuning, internal and external wavelength modulations at kHz bandwidth for locking purposes.

The Importance of Laser Beam Profiling Technology

Importance of Beam Profile Technology in Laser Applications

There are many applications of lasers in which the beam profile technology is of critical importance. When the beam profile is important, it is typically necessary to measure it to ensure that a suitable profile is present. For some lasers and applications, this may be necessary only during the design and fabrication phase of the laser. In other cases, it is necessary to continuously monitor the laser profile during laser operation. For example, scientific applications of lasers often push the laser to its operational limits, and continuous or periodic measurement of the beam profile is necessary to ensure that the laser is still operating as expected. Some industrial laser applications require periodic beam profile monitoring to eliminate scrap produced when the laser degrades. In other applications, such as some medical uses of lasers, the practitioner cannot tune the laser, and the manufacturers measure the beam profile in the design phase to ensure that the laser provides reliable performance at all times.

Fundamental Properties of Laser Beams

The temporal nature of a laser beam enables it to vary from a continuous wave to an extremely short pulse, providing very high power densities. The coherence of a laser enables it to travel in a narrow beam with a small and well-defined divergence or spread. This allows a user to define exactly the area illuminated by the laser beam technology. Because of coherence, a laser beam can also be focused to a very small and intense spot in a highly concentrated area. This concentration makes the laser beam useful for many applications in physics, chemistry, the medical industry, and industrial applications. Laser beams technology’s unique irradiance profile gives it very significant characteristics. The beam profile is the pattern of irradiance that is distributed across the beam or its cross-sectional irradiance profile.
Examples of two different types of ideal laser beams technology for different purposes are a Gaussian and a flat top beam. A Gaussian beam allows the highest concentration of focused light, whereas a flat top beam allows for a very uniform distribution of the energy across a given area.

Laser types for micro welding: selecting the right laser

Laser Types Used in Micro Welding Applications

Four types of lasers can be used for micro welding: pulsed neodymium-doped yttrium aluminum garnet (Nd: YAG), continuous wave fiber, quasi-continuous wave (QCW) fiber, and nanosecond fiber. Each type offers unique features that work best for specific applications. In some cases, several options may work: that’s when the cost of ownership and serviceability can tip the scales.

Pulsed Nd:YAG Lasers for Precision Micro Welding

With the Nd: YAG laser, the active gain medium is neodymium, which is doped into a host crystal of yttrium aluminum garnet. This solid rod of material is typically 0.1 to 0.2 inches in diameter and about 45 in long. Micro welding Nd: YAG lasers are optically pumped using flash lamps; they emit light with a wavelength of 1,064 nm, but can be frequency doubled (532 nm) to appear green. With excellent pulse control, the Nd: YAG laser also offers high peak powers in small laser sizes, which enables welding with a large optical spot size. The pulsed Nd: YAG laser is suitable for spot welding applications with less than 0.02-in. penetration and seam welding of heat-sensitive packages.

Continuous Wave Fiber Lasers for High-Speed Seam Welding

A fiber laser is generated within a flexible doped glass fiber that is typically 10 to 30 feet long and 10 to 50 microns in diameter. Ytterbium is used as the doping element because it provides good conversion efficiency and a near 1-micron output wavelength, which matches well with existing laser delivery components.
The efficient lasing process allows the fiber laser to be small, air-cooled, and offer high wall plug efficiencies. The fiber laser’s unique characteristics are its focusability and its beam qualities that can be fine-tuned for each welding application. The two ends of the beam quality spectrum are single-mode and multimode. Single mode is defined by a beam quality of M2 less than 1,2, while multimode generally is above M2 of 2.
For high-speed seam welding applications, the fiber laser is operated in CW mode. In other words, the laser output remains on until it is turned off. For spot welding either a single weld or seam, the laser output can be pulsed or modulated, which means the laser is turned on and off rapidly. CW fiber lasers are suitable for general seam welding up to 0.06 in. deep for a 500-W laser, high-speed seam welding of the same and dissimilar materials, and producing spot welds less than 100 microns in diameter.

Quasi-Continuous Wave (QCW) Fiber Lasers for Versatile Micro Welding

Quasi-continuous wave fiber lasers’ peak power and pulse width characteristics are similar to those of the Nd: YAG laser, through the parameter range is not quite as broad. Similar to CW fiber lasers, the QCW lasers offer single-mode to multimode options with spot sizes from 0.001 to 0.04 in. These lasers also shine in small spot sizes and small penetration applications, although they do offer fairly comprehensive coverage of many micro-welding applications.

Nanosecond Fiber Lasers for Welding Dissimilar Materials

A nanosecond fiber laser, typically used for laser marking applications, can be repurposed for certain welding applications. It provides multi-kilowatt peak power, but with a pulse width of 60 to 250 nanoseconds that can be delivered between 20 and 500 kilohertz. This high peak power enables welding of almost any metal, including steel, copper, and aluminum. The very short pulse widths enable very fine control for welding small parts, as well as the ability to weld dissimilar materials.