What Is an Ultrashort Pulse Laser?

The generation of ultra-short pulsed light is important because pulsed light can be generated by controlling the coherent light waves of the laser, and its time width is beyond the scope of electronics control. Broadly speaking, ultra-short pulsed light refers to pulsed light of less than 1 ns.

Chinese name: Ultrashort pulse laser

Foreign name: ultrashortpulsed laser

1 History and current status of ultrashort pulse laser technology

In laser, the generation of ultra-short pulse light is important because the pulse light can be generated by controlling the coherent light waves of the laser, and its time width is beyond the scope of electronics control. Broadly speaking, ultra-short pulsed light refers to pulsed light of less than 1 ns. In the mid-1960s, scientists conducted experimental research on mode-locked ultrashort pulsed light produced by ruby lasers and Nd-doped lasers that were pulsed by a flash lamp. Since then, the technology for generating short-pulse light has stepped from mode-locked subpicosecond pulses to femtosecond pulses. In recent years, ultra-short pulse light technology has gained popularity. Since the 1990s, various tunable ultra-short pulse mode-locked solid-state lasers have reached practicality. A tunable laser is a photon-terminated laser (Photon terminated laser) in which the energy level of the laser is in a vibration excitation state, which widens the oscillation frequency band. A typical Ti: sapphire laser is stable in operation and achieves ultra-short (minimum about 5 fs) pulsed light with an average output power of 1 W. If a Yb ion-doped laser crystal is used, a sub-picosecond pulse output with a higher average output power can be obtained. The semiconductor laser has the characteristics of fast relaxation and high-speed modulation of the pump (current), so even without mode-locking, ultra-short pulses in the picosecond region (10-10 to 10-12 s) can be generated by using the gain transition phenomenon. Light. ^[1]

The recently developed small picosecond and femtosecond pulsed lasers have made great progress in ultrashort pulsed light sources. Considering the requirements for ultra-short pulse light sources from the perspective of light utilization, whether to use the characteristics of the time domain (ultra high speed) effectively or to use the high peak intensity of short-time concentrated light energy are two major research directions. In practical applications, these two directions are closely related. From the above point of view, maximizing the performance of the light source, achieving shorter pulsed light generation and higher peak intensity are the driving forces behind the development of this technology. In addition, improving the performance of new light sources, popularizing new functions or phenomena found and making them practical applications; improving the reliability, stability, life and cost of light sources is also the key to technology development. In addition to increasing pulse width and pulse energy, improving beam quality is also an extremely important research topic. This has a great impact on the development of the technical field, such as maximizing coherence from time and space. ^[1]

People have accumulated a lot of experience in the research of the development of ultra-short pulse laser technology, such as effectively generating high-intensity pulses and obtaining high-energy pulses as much as possible in the phase of generating pulses; various attempts have been made to directly generate high-intensity ultra-short pulses, etc. And obtained research results, and contributed to the development of this field. However, in the process of generating and using high-intensity pulses, problems such as the coherence of optical pulses or the repetitiveness and reliability of waveforms and wavelengths are not ideal. Therefore, the high-repetition pulse output of the selected mode-locked laser oscillator and high magnification have become the mainstream. Although the energy of each pulse is small, the pulse generation source can easily obtain a pulse with good coherence by using a continuous oscillation mode-locked laser. ^[1]

2 Characteristics and application of ultra-short pulse laser 2 light source

Table 1 gives the characteristics and main application fields of ultra-short pulse lasers commonly used in recent years.Ultra-short pulse lasers have broad application prospects in medical and optical recording.Many applications are currently in the practical test stage, including Applications in physical science research.

Another feature of this technology is the wide range of pulses used.For example, in information communication applications, the ultra-high repetition frequency of a single pulse (pJ level) with low energy is above 100 GHz; in applications such as measurement, from nJ to mJ The energy range of the first stage works with high repetition frequency; in high-intensity quantum scientific research applications, a single peak pulse can reach the high peak intensity of the petawatt (PW) level. In terms of wavelength, through the conversion of the output wavelength of the ultra-short pulse laser, it can process from a soft X-ray region of a few nanometers to a THz pulse equivalent to a sub-millimeter wave. Considering the current status of ultra-short pulse lasers from the application perspective, they can be roughly divided into the following three categories. ^[1]

(1) Lasers for physical science research. This is the earliest established field of application of ultra-short pulse laser devices. Because this application imposes various requirements on pulse characteristics, such as wavelength, pulse width, and pulse energy, a variety of lasers can be used, including dye lasers and excimer lasers. In the case of focusing on performance and regardless of cost, solid-state lasers are mostly used. The performance of solid-state lasers is flexible (the tunable range of parameters such as pulse energy or repetition frequency is relatively wide), such as lasers used for nuclear fusion ignition or large-scale laser systems developed and used in various research equipment fall into this category. ^[1]

(2) It is expected to be used as a laser for industrial equipment. Mainly considered for measurement and processing. The short pulse laser can obtain the ideal processing results, but the reliability or maintainability and cost of the equipment must be considered. In recent years, with the improvement of the reliability of mode-locked solid-state lasers and the emergence of high-power fiber lasers, people have high hopes for the development of this field.

(3) Semiconductor lasers and fiber lasers as components of optical information communication systems. As far as this industrial application is concerned, it has the greatest social benefits, but it is also susceptible to social conditions such as market conditions and information and communication policies. People still remember the industry depression brought about by the bursting of the IT bubble. In addition to the performance of the device, issues such as reliability, cost, and environmental protection must also be considered, and technical requirements are strict. In the long run, the field of communications is one of the highest expectations. ^[1]

3 Ultra-short pulse laser 3 ultra-short pulse solid-state laser

Features of ultra-short pulse laser mode-locked solid-state lasers

Almost all ultra-short pulses produced by solid-state laser media are mode-locked. Compared with other media, solid-state lasers have the following characteristics: (1) Very short pulses of 5 fs (when using Ti3 +: gemstone lasers) can be obtained, which is the shortest pulse width directly obtained from all types of laser oscillators; ( 2) High average power and high energy density are available. Features (1) reflect that Ti3 +: gem wideband tunable laser has replaced almost all mode-locked dye lasers in 10 years. The main reason is that solid laser media is more durable and easier to operate than organic dye laser media (before Ti3 +: gem laser, used as a light pulse light source in the picosecond to 30 fs band). Feature (2) The light source used is a lamp-pumped Nd3 +: YAG mode-locked laser with a long history. In recent years, with the development of semiconductor laser pumping technology and the advent of small high-power Yb3 +: lasers, both the pulse width and power performance have been further improved, and its application range has also been expanded. Like solid-state lasers, the mode-locking technology of fiber lasers and semiconductor lasers has also been developed. Picosecond and sub-picosecond time-domain pulsed light sources have also incorporated these small lasers into communication systems or measurement equipment, and are being promoted and applied. For large mode-locked solid-state lasers, short pulses or high average powers that are difficult to achieve with other lasers, and short pulses with high peak power that effectively use (requires external amplification) energy storage functions have become the focus of technological development. ^[1]

Ultrashort Pulse Laser Mode Locking

Almost all solid-state laser oscillators are lamp or other light pumped. The flash can be used to pump pulses, and the transient oscillation can be performed for a shorter period of time equivalent to the life of the upper level or with a Q switch, and the arc pump can be continuously pumped to achieve stable continuous oscillation. Although the mode-locking method used makes the energy of each pulse larger, in recent years, a continuous-oscillation mode-locked laser with good coherence, repeatability and stability has been used. When high energy pulses are required, external amplification measures are taken for the output power of the oscillator. Although the lamp pump cost is low, there are problems such as life, aging, and noise caused by discharge. Therefore, high-power semiconductor laser pumps are used (in addition to direct pumps, there are also solid-state lasers pumped by semiconductor lasers. Indirect method of pump source) has become the mainstream method. ^[1]

When the pulse width is long (300 ~ 30 ps), the active mode-locking method is often used and the loss in the resonant cavity is modulated by external signals. In a typical solid-state laser resonator, the repetition frequency of the output pulse, which is equivalent to the round-trip time of the light pulses in the resonator, is 100 MHz and does not require every modulation. Raman-Nath diffraction acousto-optic modulators are usually used, and phase modulators can also be mode-locked to increase the repetition frequency.

When a shorter pulse width is required, passive mode or active mode locking can be used, and pulse-pumped solid-state laser modulation is often used to utilize the passive loss of saturable absorption (the property of reducing strong light absorption) dyes. In addition, the initial continuous-oscillation mode-locked solid-state lasers were unable to control the Q-switching during saturable absorption, so it was difficult to obtain stable mode-locked pulse sequences. In recent years, with the development of short-pulse technology and equivalent effect technology (obtained using non-resonant nonlinear refractive index saturable absorption), passive mode-locking methods have been practically used. Saturable absorption material is a semiconductor film with fast absorption recovery time, which can be used alone or in combination with other methods. The following describes mode-locked solid-state lasers. Solid-state laser media are widely used and can be broadly divided into: (1) lasers that require short pulse widths; (2) lasers that value efficiency and power. ^[1]

Various lasers featuring short pulses

Ti3 +: Gem Laser

There are two main factors limiting the laser pulse width: the gain band of the laser medium and the group velocity dispersion in the resonator. In recent years, a newly developed technology can compensate the material dispersion of solid laser crystals in a resonant cavity. The shortest pulse width that can be achieved depends on the gain band. Ti3 +: Al2O3 (Ti: sapphire) laser has the widest gain band and can oscillate in the 660 ~ 1100 nm band. Ti: sapphire is a medium that is expected to produce the shortest pulses. This medium can perform mode-locked oscillation in almost the entire range of the gain band, and a pulse width of about 5 fs is directly generated by the oscillator. Ti3 +: The gemstone laser uses the main technology of the mode-locked solid-state laser. This is a laser for the most detailed experimental research on mode-locking methods. To design the spatial mode of pump light and oscillating light, the minimum beam diameter in the crystal must be less than 100! M. In almost all cases, pulses are generated using the mode-locking mode of the nonlinear refraction effect in laser crystals. Therefore, the spatial mode of the multiple planar mirrors and concave mirrors that constitute the cavity must be considered during design. ^[1]

The most commonly used Kerr mirror mode-locking is based on laser crystal materials, and uses the non-linear refractive index under non-resonant conditions, so the response speed is extremely fast, and it is an ideal "fast absorbing material". However, in many cases, a continuous oscillation (CW) state and a mode-locked (ML) state exist at the same time, and the oscillation starts at the CW state, but the ML state cannot be self-starting. Therefore, in order to ensure the ML state, auxiliary methods are generally used.

SESAM is considered one of the most effective methods at present. The pulse width obtained by mode locking depends on the group delay dispersion in the cavity. In solid-state lasers, the material dispersion of the gain medium crystal is an order of magnitude larger than the jet sheet (thickness of about 0.2 mm) of the dye laser, so dispersion compensation technology must be used. The specific method is to use a Brewster prism pair or a dispersion compensation mirror with small loss to compensate the dispersion in the oscillator. The prism pair compensates the group delay dispersion by using the wavelength dependence of the refraction angle; the dispersion compensation mirror compensates the group delay dispersion when reflecting through a mirror. This dielectric multilayer film mirror is designed for ultra-short pulse lasers. The shortest pulse record is 5 fs, which can be obtained by optimizing the dispersion characteristics of the crystal, prism pair, and reflector to reduce the dispersion in the cavity and eliminate the pulse distortion factors as much as possible. Instead of using the ratio of the oscillation wavelength (800 nm) and the pump light wavelength (515 nm) to show the quality of the crystal, it is better to use the sensitivity value (FOM value). The FOM value of the crystal used in the mode-locked oscillator must be around 150. High-density crystals will lower the FOM value, so only the absorption coefficient for pump light was used in the early days! Crystals of about 1 cm (length 10 ~ 20mm). In recent years, the absorption coefficient of the crystal! = 6 cm has reached a very good quality, and the length may be shortened to 2 ~ 3 mm. Thin crystals are easy to compensate for the dispersion in the cavity and are conducive to the generation of short pulses. However, in high-density crystals, it is difficult to eliminate the heat generated by the pump and it is easy to generate thermal strain, so attention must be paid. 3.3.2. Cr3 +: LiSAF, Cr3 +: LiCAF and other solid-state lasers In recent years, miniaturization and high-power development of solid-state lasers using high-power semiconductor laser pumps have been developed. For so-called tunable solid-state lasers with energy levels accompanied by phonon radiation at the lower laser level, direct pumping with semiconductor lasers is conducive to miniaturization. The Ti3 +: gemstone laser requires a high pumping optical density, so it cannot be directly pumped with a green semiconductor laser. It is necessary to oscillate a Nd3 +: laser to convert it into a 2x wave pump source. In contrast, Cr3 +: LiSrAlF6 (Cr: LiSAF) lasers have an oscillatable wavelength range of 780 ~ 1000 nm (when pulsed, the continuous oscillation range is slightly narrower), which is narrower than that of Ti3 +: gemstone lasers, but requires oscillation The pump power density is much lower than that of Ti3 +: gemstone laser, and a semiconductor laser pump with a wavelength of about 680 nm can be used to achieve continuous oscillation and mode locking. Crystals such as Cr3 +: LiCaAlF6 (Cr: LiCAF) and Cr3 +: LiSrGaF6 (Cr: LiSGAF) can obtain the same mode-locked laser oscillation, but their characteristics are different. ^[1]

Many research reports have reported that using the advantages of this medium, the pulse width of a small, low-priced femtosecond laser oscillator directly pumped with a semiconductor laser has reached a record of less than 10 fs.

Long wavelength laser

For mode-locked solid-state lasers that use Ti3 +: gemstones or Cr3 +: LiSAF and oscillate at long wavelengths in the femtosecond region, when short pulses are particularly needed, Cr4 +: forsterite (Mg2SiO4) lasers (oscillation wavelength) 1170 ~ 1370 nm) or Cr4 +: YAG (Y3Al5O12) laser (oscillation wavelength: 1340 ~ 1560 nm). The Cr4 +: Mg2SiO4 mode-locked laser uses the same technology as the above types of lasers. The gain increases with the cooling of the crystal. Therefore, it is desirable to cool it within the allowable condensation range, and use it at a temperature of -15 ° C ~ 5 ° C. When the crystal temperature is 5 ° C, the output power of a mode-locked pulse pumped with a 6 W femtosecond region is about 100 mW. The transmittance of the output mirror is about 2%, and the loss in the resonator must be minimized. The thermal conductivity of the crystal is also lower than that of titanium sapphire, so the doping concentration cannot be too high, and the generally suitable absorption coefficient is 0.34 cm-1. However, when short pulses are generated, high-density crystals are often used, such as an absorption coefficient of 2.4 cm-1 to shorten the crystal to 5 mm. In order to reduce the residual absorption of the oscillation wavelength, the grown crystal is heat-treated. At present, high-density crystals are difficult to achieve high FOM values and stable quality. Typical FOM values (absorption ratios at 1064 nm and 1250 nm) are about 30. ^[1]

The semiconductor laser pumped Nd: YVO4 laser is used as the pump source to achieve the output of the Nd3 + -doped solid-state laser with a wavelength of about 1.06 "m. Studies on the direct continuous oscillation of semiconductor lasers have also been reported. When the mode-locking method requires more In the case of high output, it is desirable to increase the output power and brightness of a 1.06 "m wavelength semiconductor laser. Related scientists believe that the use of semiconductor fiber lasers (the same output wavelength of about 1.06 "m) as the pump source is conducive to miniaturization. The oscillation wavelength of ultra-short pulse light of this laser is beneficial to cell penetration, so it is expected to be used in biological It has been applied in medicine. Cr4 +: YAG mode-locked laser still obtains the oscillation wavelength from the color center laser. This laser has the advantages of stable operation, long life, and can work at room temperature without liquid nitrogen cooling, so it is favored by users. The shortest pulse obtained was 20 fs. ^[1]