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Dec 18, 2023

Lasers: Understanding the Basics

The actual Q-switch device is an acousto-optical modulator or an electro-optical

The actual Q-switch device is an acousto-optical modulator or an electro-optical modulator (EOM). Both use crystals where an applied electric field produces some perturbation of the optical properties of the crystal. In the case of acousto-optical modulators, the applied electric field is a radio-frequency voltage that produces a high-frequency sound wave in the crystal. This sound wave diffracts the photons from the laser and prevents laser amplification. EOMs instead use an applied high voltage that modifies the crystal refractive index and alters the polarization of the incoming light; an appropriate combination of polarization-sensitive optics can be placed in the cavity to prevent light of altered polarization from circulating.Other types of lasers, such as excimer lasers, do not require a Q-switch to produce nanosecond pulses but rather rely on a transient pump pulse: Excimer laser pulses are produced by exciting the noble gas/halogen mixture with a powerful and short electric discharge. Ti:sapphire lasers can also produce nanosecond pulses if they are pumped with a nanosecond pulse of green light produced by a frequency-doubled, Q-switched YAG laser. This method is called gain switching because the cavity gain rather than the cavity loss is directly changed.Apart from a huge number of industrial applications, Q-switched lasers have important applications in scientific research. One is pumping of Ti:sapphire ultrafast amplifiers (described in the following section) by using the frequency-doubled (green) output of a Q-switched Nd:YAG or Nd:YLF at 1-10 kHz. Another one is using the YAG or YLF laser to produce energies per pulse in the joule range at 1-100 Hz. These lasers are often used with nonlinear optical generators that can produce tunable wavelengths in the UV, visible and IR region, enabling time- and wavelength-resolved studies. Nowadays most YAG or YLF lasers operating at >100 Hz are diode-pumped, while high-energy 10-Hz systems require pumping with a flashlamp because diodes are not suitable for producing high-energy output pulses.For some scientific applications, it may be desirable to have a narrow-linewidth Q-switched laser. In some cases, this can be accomplished using a combination of optical gratings and etalons; in other cases, the laser can be "seeded" with a low-power CW or Q-switched narrow-linewidth laser that is easier to control than the higher-power stage. This approach, called "injection seeding," uses a MOPA (master oscillator, power amplifier), conceptually splitting the linewidth selection and the high-power generation into two stages that are optimally designed for the two purposes.Ultrafast lasersUltrafast lasers are generally defined as lasers that produce pulses in the range of 5 fs to 100 ps (1 femtosecond = 10−15 seconds). If a laser is able to oscillate in many longitudinal modes, such short pulses can be produced with the so-called mode-locking technique. With this technique, the modes are locked in phase (mode-locking regime) and their coherent interference causes the intracavity optical field to collapse into a single pulse traveling back and forth in the laser cavity. Every time the pulse reaches the output mirror, part of it is coupled out and available.

Physics shows that the more modes that interfere, the shorter the pulse duration (Figure 7). Since larger lasing bandwidths support a larger number of oscillating modes, the pulse duration is inversely proportional to the bandwidth of the laser gain material. In the absence of dispersion, these pulses are time-bandwidth limited, i.e., have the shortest possible length for a given bandwidth.

Ultrafast pulses are highly useful in research; thanks to the short pulse duration and high peak power, the advent of femtosecond lasers in the 1990s enabled groundbreaking research leading to Nobel prizes for femtochemistry (pump-probe spectroscopy) and optical comb generation. Femtosecond lasers have also enabled multiphoton excitation (MPE) techniques that deliver three-dimensional imaging of live tissue. MPE is now widely used in several areas of biological research, most notably neuroscience.

Many important applications require the ultrafast pulses to be amplified using one of several methods such as regenerative amplification or a master oscillator power amplifier (MOPA) approach. Pulse amplification usually requires a reduction in repetition rate, so a pulse-picker selects the oscillator pulses to be amplified in one or more amplifier stages. In the case of femtosecond lasers, the high peak power of the amplified pulses can damage the laser optics. For this reason, the amplification is usually preceded by stretching the pulse (chirping) to 50 to 200 ps. The amplified pulse is then re-compressed to the fs domain. This is commonly referred to as chirped pulse amplification, or CPA.

In scientific research, amplified ultrafast pulses are used for a wide array of applications. These include photochemistry, pump-probe spectroscopy, terahertz (THz) generation and creating accelerated electrons and other small charged particles. The pulses can also drive nonlinear generation of extreme-UV light with pulse widths of tens of attoseconds.

In industrial applications, amplified ultrafast pulses are increasingly used in materials processing applications that require ablation or materials modification without any residual thermal effect and/or on a submicron spatial scale. Examples include thin-film patterning in the production of flat panel displays. Ultrafast lasers are also increasingly used to cut the toughened glass for touchscreens, using a process called filamentation cutting that cannot be performed with other lasers. This method produces unmatched edge quality and can create curved shapes and cutouts.

Ultrafast laser materials

Until recently, scientific ultrafast lasers have mainly relied on titanium:sapphire (Ti:sapphire) because of its large bandwidth and broad tuning range; turnkey commercial Ti:sapphire lasers can deliver pulses as short as 6 fs. Ti:sapphire lasers are typically pumped using a green-wavelength CW pump laser. Typical repetition rates of Ti:sapphire oscillators are 50 to 100 MHz, and peak powers as high as several hundred kilowatts.

The most common CPA systems based on Ti:sapphire operate at 1 to 10 kHz with the amplifier stages energized by nanosecond green lasers. Ti:sapphire CPA systems are unique in their ability to produce pulse energies of several millijoules with pulse widths as short as 20 fs. Custom CPA systems based on Ti:sapphire can produce even petawatt peak powers.

Industrial ultrafast lasers typically need high repetition rates and high power in order to sustain economically viable throughput in the application. Until recently, most of these have been MOPA systems based on Nd-doped bulk materials (e.g., YAG or glass) or fiber, or a combination of the two. These lasers and amplifiers are well-proven to provide the requisite combination of power and industrial reliability. However, the smaller the gain bandwidth of Nd means that they are limited to the ps regime. Their high peak power and high repetition rates find applicability in precision micromachining applications, particularly for thin films and/or for tough materials like chemically strengthened glass, using the filamentation method just mentioned.

Figure 7. When a very large number of laser modes that all have a "zero" in the same position interfere, the resultant superposition is an extremely narrow pulse.

In the past 10 years, femtosecond lasers and amplifiers using ytterbium (Yb) have become available to meet evolving market needs in both the scientific and industrial sectors. An example is the Monaco series of one-box amplifiers from Coherent.

Yb-doped materials combine to some extent the advantages of Ti:sapphire scientific lasers and Nd-based industrial lasers. For scientific research, the gain bandwidth of Yb means oscillator pulses can be as short as 50 fs, which is more than adequate for many applications, particularly in MPE microscopy. Unlike Ti:sapphire, Yb can be directly diode-pumped and used in a fiber format, enabling more scalable performance than bulk gain materials that are often limited by cooling and thermal lensing issues. This means that Yb-fiber MOPA-type amplifiers can deliver flexible repetition rates as high as tens of MHz. When used to pump optical parametric devices, the resulting output is fully tunable from UV to mid-IR wavelengths, providing advantages for applications such as spectroscopy of advanced materials, or functional biological imaging. It should be noted that for scientific applications needing extremely short (>6 fs) pulse widths and/or high pulse energies, Ti:sapphire currently remains the preferred gain material, and both media will co-exist for the foreseeable future.

For industrial applications, the main attraction of Yb-fiber amplifiers is the combination of high peak power and high average power in the femtosecond regime, unlike Nd systems with picosecond pulse widths. Femtosecond laser pulses have two advantages over picosecond pulses for materials processing. First, the material interaction involves many simultaneous photons and becomes reasonably wavelength insensitive, unlike with nanosecond linear absorption. Second, the short pulses and nonlinear interaction means that fs pulses can deliver even better edge quality and precision than ps pulses. As a result, Yb-fiber amplifiers are rapidly finding applications in micromachining of mixed layered substrates (e.g., polyimide on glass) as found in electronics and displays.

Frequency doubling and harmonic generationEven with the broad choice of commercially available lasers, it is not always possible to find one that exactly matches the wavelength required by a specific application. Ti:sapphire lasers are broadly tunable, but in most cases, they are too complex for industrial applications and unable to reach the all-important UV region of the spectrum. OPSLs are simple and can be designed at many wavelengths in the 920- to 1160-nm region but are not ideal for pulsed operation. To achieve the desired wavelength in just about any regime of operation — CW, pulsed, or ultrafast — the processes of harmonic frequency conversion and parametric generation provide wavelength flexibility when used in conjunction with the lasers described so far. All these processes are related and are called nonlinear phenomena since they depend nonlinearly on the laser peak power. That is, they are proportional to the square, third, or higher power of the laser output power.In simple terms, when an intense and/or tightly focused laser beam passes through a suitable crystal, its oscillating electric field interacts with the electrons of the crystal in several ways. One of these mechanisms distorts the electron cloud in the crystal, thereby polarizing the atoms at a frequency that is the same as that of the laser beam, but also at a frequency that is its double (nonlinear polarization). This frequency corresponds to a wavelength that is half that of the incoming laser. The nonlinear polarization is much smaller than the linear term, but it depends on the square of the laser power, therefore increasing more strongly in the presence of an intense laser pulse. It generates an optical field at double the frequency of the original laser beam, with the result that part of the incoming laser power will be converted to half the original wavelength (second-harmonic generation (SHG) or frequency doubling) (Figure 9). Since energy has to be conserved, any gain in the SHG beam is traded for a decrease in power of the original beam. In some cases, it is possible to achieve an almost total conversion of the original ("fundamental") beam into its second harmonic. Common crystals for SHG are BBO, LBO, and KDP. The most common example of SHG is the conversion of a Nd-based laser IR output at 1064 nm into a green output at 532 nm (green), constituting the most popular visible wavelength, used ubiquitously to pump Ti:sapphire lasers.

Nano-, pico-, and femtosecond OPOs are complex devices that are implemented in conjunction with pulsed and ultrafast pump lasers. CW OPOs are equally, if not more, complex. OPAs are easier to design and build but require a more energetic pump to produce the white light and one-pass amplification in the crystal. For this reason, they are pumped by CPA pico- or femtosecond amplifiers producing at least several microjoules. The addition to an OPA/OPO of one or more stages of harmonic generations and mixing yields a range of wavelengths that can cover 200 nm to 20 µm.Common laser typesFor many years, the most common CW laser was the helium neon laser, or HeNe. These low-power lasers (a few milliwatts) use an electric discharge to create a low-pressure plasma in a glass tube; nearly all emit in the red at 633 nm. In recent years, the majority of HeNe applications have switched to visible laser diodes. Typical applications include barcode readers, alignment tasks in the construction and lumber industries, and a host of sighting and pointing applications ranging from medical surgery to high-energy physics.In fact, the laser diode has become by far the most common laser type, with truly massive use throughout telecommunications and data storage (e.g., DVDs, CDs). In a laser diode, current flow creates charge carriers (electrons and holes) in a p-n junction. These combine and emit light through stimulated emission. Laser diodes are available as single emitters with powers up to tens of watts, and as monolithic linear bars with numerous individual emitters. These bars can be assembled into 2D arrays with total output powers in the kilowatt range. They are used in both CW and pulsed operation for so-called direct diode applications. But even more importantly, laser diodes now underpin many other types of lasers, where they are used as optical pumps that perform the initial electrical-to-optical power conversion.For example, higher-power visible and UV CW applications were originally supported by argon-ion and krypton-ion lasers. Based on a plasma discharge tube operating at high current, these gas-phase lasers are large and inefficient, generating a large amount of heat that must be actively dissipated. The tube also has a finite lifetime and thus represents a costly consumable. In most former applications, the ion laser emitting at blue or green wavelengths was displaced by DPSS lasers. Here, the gain medium is a neodymium-doped crystal (usually Nd:YAG or Nd:YVO4) pumped by one or more laser diodes. The near-IR fundamental at 1064 nm is then converted to green 532-nm output with the use of an intracavity doubling crystal. The DPSS laser, in turn, has been challenged by several newer technologies, with the OPSL the most successful of these. Here the gain medium is a large-area semiconductor laser that is pumped by one or more laser diodes. The OPSL offers numerous advantages, most notably wavelength and power scalability. Specifically, these lasers can be designed to operate at virtually any visible wavelength, at last freeing applications from the restrictions of limited legacy-wavelength choices (i.e., 488 and 514 nm from argon-ion lasers and 532 nm from frequency-doubled YAG lasers). Indeed, OPSLs represent a paradigm shift in lasers because they can be designed for the needs of the application instead of vice versa.OPSL is now a leading technology in low-power bioinstrumentation applications, most notably at 488 nm; the power scalability and inherent low noise of OPSL technology is now seeing multiwatt green and yellow OPSLs moving strongly into other applications, including scientific research, forensics, ophthalmology, and light shows.While YAG and other neodymium crystal hosts lend themselves to operation in CW, Q-switched and mode-locked operations, laser diodes, OPSL, and ion lasers do not support Q-switched operation and are virtually not used in mode-locked regime.At longer wavelengths, carbon dioxide (CO2) lasers, which use plasma discharge technology, emit in the mid-infrared around 10 µm. Most are CW or pseudo-CW, with commercial output powers from a few watts to several kilowatts. A similar technology is the carbon monoxide (CO) laser, which was originally developed in the 1960s, but only made truly practical for industrial use in 2015. CO lasers emit in the 5 to 6 µm spectral range. This shorter wavelength, mid-infrared output offers two important advantages for some applications as compared to CO2 lasers. The first is that many metals, films, polymers, PCB dielectrics, ceramics, and composites exhibit significantly different absorption at the shorter wavelength, which can sometimes be exploited to advantage. The second is that they can be focused to smaller spot sizes due to diffraction, which scales linearly with wavelength. Together, these characteristics enable the CO laser to deliver superior results in some glass processing, film cutting, and ceramic scribing applications.Another important technology is the fiber laser, which can be operated in CW, Q-switched, and mode-locked formats and typically emits at about 1 μm (when the fiber is ytterbium-doped). In a fiber laser, the resonator is formed by a large mode area, double-clad optical fiber (with the outer cladding containing the dopant) and fiber Bragg gratings for resonator mirrors. This is pumped from each end by a series of diode lasers, whose outputs are fiber-coupled into the gain fiber.

The fiber laser offers several important advantages. The first is that the output is naturally fiber-delivered, which makes it easy to couple into many laser machine tools and to integrate the laser with robotic delivery systems. Next, fiber laser beam quality is sufficient to couple it into small fibers, allowing the beam to be focused to small spots in order to obtain the high power densities required for metal welding, cutting, and other industrial processes. Fiber laser architecture also lends itself to power scaling. A single set of pumps and gain fiber can typically produce output powers of up to multiple kilowatts, but it is also possible to use fiber combiners to enable power scaling, achieving output powers exceeding 10 kW. Finally, fiber lasers have high wallplug efficiency (the conversion of input electrical energy into laser light) compared to CO2 and solid-state lasers, and can also have low maintenance requirements. This lowers cost of ownership.

Nd:YAG, CO2, fiber, and direct diode lasers are the workhorses of industrial laser applications. Direct diode lasers, in particular, offer the lowest cost per watt of any industrial laser type, as well as the lowest operating costs, due to their high electrical efficiency. Direct-diode lasers predominantly service low-brightness applications, such as heat treating, cladding, and some welding applications. On the downside, high-power laser diodes or arrays cannot deliver anything close to the diffraction-limited beam provided by other laser types.The advent of slab-discharge technology has allowed the size:power ratio of CO2 lasers to be greatly scaled down, increasing their utility in subkilowatt applications. Low-cost waveguide designs also support a healthy market for CO2 lasers with powers in the tens of watts, primarily in marking and engraving applications.

Over the past decade, high-power fiber lasers (>1 kW) have come to dominate metal cutting applications in the 4- to 6-mm thickness range because they typically offer excellent results, together with lower maintenance costs than CO2 lasers of similar power. Furthermore, near-infrared fiber lasers are advantageous when cutting certain metals, such as copper, aluminum, and brass, which are difficult to cut with CO2 because of their high reflectivity in the far-infrared.

CO2 lasers continue to be used for even thicker materials, but this is mostly because the processes have been optimized for this laser and manufacturers are slow to change a production process that works well. However, this is likely to change over time. CO2 lasers at 1 kW and below are still utilized in some thinner metal (2 to 4 mm) cutting applications. And CO2 lasers remain the first choice when both metals and nonmetals must be processed. This is because their longer wavelength is well absorbed by a wide range of nonmetallic materials, including wood, paper, leather, cloth, plastics, and many other organics, while the near-infrared fiber laser output is not.

Nd:YAG can deliver the high peak power for materials processing applications such as metal welding. In these heavy industrial applications, raw power is more important than beam quality, and for many years, these lasers were lamp-pumped. But the ever-increasing power and lifetime characteristics of laser diodes are causing these lasers to switch to diode pumping; i.e., DPSS lasers.Conversely, lower-power Q-switched DPSS lasers are often based on Nd:YVO4. These are usually optimized for high beam quality for micromachining and microstructuring applications with high repetition rates (up to 250 kHz) to support high throughput processes. They are available with powers up to tens of watts with a choice of near-IR (1064 nm), green (532 nm), or UV (355 nm) output. The UV is popular for producing small features in "delicate" materials because it can be focused to a small spot and minimizes peripheral thermal damage. Deep-UV (266 nm) versions are starting to be used in some applications, but their relatively high cost and the need for specialty beam delivery optics causes many potential applications to rely instead on 355-nm lasers optimized for short pulse duration, which can produce similar results in many materials.Excimers represent another important pulsed laser technology. They can produce several discrete wavelengths throughout the UV; depending on the gas combination, emission ranges from 157 to 348 nm. The deep-UV line at 193 nm is the most widely used source for lithography processes in the semiconductor industry. The 308-nm wavelength is used for annealing silicon in high-performance displays. The same wavelength is also key to generating a unique long-wear surface on the cylinder liners of high-performance diesel engines. And finally, excimers have a unique ability to produce high pulse energies — up to one joule per pulse. This enables direct writing of low-cost electronic circuits for applications such as medical disposables.Ultrafast lasers for scientific applications are dominated by Ti:sapphire, as already described. Ultrafast lasers are also a fast-growing technology for micromachining and other high-precision materials processing applications. While there is some diversity in the form and construction of commercially available industrial ultrafast lasers, they all utilize a certain basic configuration. Specifically, a passively mode-locked oscillator is used to generate output at the pulse widths of about 10 ps or shorter that are necessary to drive photoablation. However, most mode-locked oscillators produce relatively low energy pulses (in the nanojoule range) at repetition rates in the tens of megahertz. Best results for micromachining are achieved when the pulse-to-pulse overlap is in the range of 50% to 70%. In other words, the beam deflection mechanism moves the beam about one-third of the beam diameter before the arrival of the next ultrafast pulse. Consequently, a repetition rate in the range of tens of megahertz is too high to be used with existing scanning technology, so a pulse picker selects a fraction of these pulses. The energy of these pulses is then boosted in an amplifier to produce the final output. Most commercial picosecond products are based on one of the following architectures:• A fiber laser oscillator followed by a fiber- or rod-type amplifier• A fiber laser oscillator followed by a free-space amplifier• A diode-pumped solid-state oscillator followed by a free-space amplifierThe all-fiber (oscillator and amplifier) approach has the advantage of being relatively low cost and holds the potential of being robust. The big negative is that nonlinearities, scattering, and other effects in the fiber amplifier limit the maximum per-pulse energy that can be attained to about 10 µJ (at a 10-ps pulse width). This level of pulse energy can cater to some applications, but a large number of applications are served with pulse energies in the 100-µJ range. One can use specialty fibers to increase the mode inside the fiber and thereby allow for larger pulse energies, but such fibers lead to limited bend radii and hence bring their own packaging limitations. To achieve the higher pulse energies required for most applications, a fiber oscillator can be mated with a free-space amplifier. Because of the relatively low energy output from the fiber seed, a regenerative amplifier is often used. In a regenerative amplifier, a pulse undergoes a large number of passes and can therefore be amplified substantially. Regenerative amplifiers also have the advantage of being compact and delivering good beam performance. The third approach is to use a diode-pumped solid-state oscillator (usually with Nd:YVO4 as the gain medium), which can produce higher pulse energies than a fiber seed. This is followed by a free-space amplifier, typically in either a regenerative or multipass configuration. In fact, more than one amplifier stage can boost power to levels as high as 100 W.And finally, many other types of niche and exotic lasers exist that are beyond the coverage of this overview article. Examples include Raman lasers used in telecommunications, quantum cascade lasers used in some gas-sensing applications, and chemical lasers, which tend to be limited to military programs.