Ⅰ What is a laser?
The full name of the laser is “Light amplification by stimulated emission of radiation”. A laser is a device that amplifies light through stimulated radiation. A complete laser mostly consists of three components: a pump source, a resonator, and a gain medium (with exceptions). Generally, according to the classification of gain media, lasers can be divided into solid-state lasers, fiber lasers, semiconductor lasers, gas lasers and so on.
Ⅱ how is the laser generated?
The most important condition for laser generation is the particle number inversion (colony number inversion). The corresponding pump source is required to pump the low energy level ions of the gain medium to the high energy level. And then ions fall to emit light wave twice as much as the pump light, which is called the stimulated radiation process. In addition, two conditions need to be met: the laser gain must be greater than the loss and can play a role in controlling the direction and frequency selection of the beam. It is not difficult to understand that the gain need to be greater than the loss, because of that can the laser come out of the cavity. There are many sources of loss, such as resonant cavity cavity loss, dielectric absorption loss, etc. The source of gain is a gain medium, which requires an energy level structure that can meet the stimulated radiation. Controlling the reverse propagation of the beam and the frequency selection mainly rely on the resonator, and the effective method is to adjust the cavity length of the resonator.
Fig. 1 Schematic diagram of stimulated radiation
Ⅲ what is a passively Q-switched laser
Passively Q-switched laser, as the name suggests, is a laser that applies passively Q-switched technology. This type of laser adds a saturation absorber (passively Q switch, non-modulator) to the typical laser. In general, this saturated absorber refers to a passively Q-switched crystal.
Passively Q-switched lasers emit high-energy short-pulse laser beams in the nanosecond range with repetition rates up to the order of kilohertz to megahertz. They are often used in laser material processing (e.g. cutting, drilling, laser marking), pumping nonlinear frequency conversion devices, ranging and remote sensing.
Fig. 2 Schematic diagram of a passively Q-switched laser
Ⅳ What is a passively Q-switched crystal?
Passively Q-switched crystal (saturated absorber) is used as laser cavity quality modulator in passively Q-switched laser rather than electronically controlled modulator. It is characterized by a lower unit saturation energy, which can be further reduced when used with convergent beams. When the passively Q-switched crystal is saturated, the loss decreases.
Common passively Q-switched crystals are, for example, Cr4+: YAG crystals (for 1μm lasers), V3+: YAG crystals (for 1.3 μm lasers), Co2+: Spinel crystals (for 1 .5μm laser) and so on.
Ⅴ What is the principle of passively Q-switched technology?
Passively Q-switched technology uses the characteristics of saturated absorber material (passively Q-switched crystal). At first, the passively Q-switched crystal was in an unsaturated state. The pump source excite gain medium to amplify the light. But due to the large loss, the gain was only slightly greater than the loss and was not enough to produce a laser. However, the gain still allows a large number of inverted particles to accumulate in the gain medium. Passively Q-switched crystal saturate under the action of light and then the loss decreases, but the gain remains high, which is why the “giant pulse” occurs. At the same time, a large amount of energy accumulated in the gain medium is consumed, and the gain begins to decline. At the gain and loss level, the pulse power reaches its highest point. Subsequently, the loss of the passively Q-switched crystal returns to a high loss state before the gain, and the gain medium begins to accumulate energy again. This process causes a delay in the next pulse. This cycle of this whole process produces periodic short pulse laser beams.
Fig. 3 Laser emission curve of passively Q-switched laser
Ⅵ What is the difference between Q-switched lasers and mode-locked lasers?
- The principles are different
The Q-switched laser regulates the loss, which means a control adjustment in the time domain. Mode-locked laser, also known as phase-locked laser, modulates the phase of light oscillating in the resonator. So that optical interference with a fixed mode is amplified to form a wideband, large energy, fixed frequency interval spectrum in the frequency domain, and then produce high-energy narrow pulses in the time domain.
- The pulse widths are different
The output pulse process of Q-switched lasers involves the accumulation and release of particle numbers. And generally, Q-switched laser outputs short pulse lasers in the order of nanoseconds. The mode-locked laser modulates the phase, which is closer to an instantaneous process than Q-switched one, so it generally outputs ultrashort pulse lasers in the order of picoseconds and femtoseconds.
- The applications are different
Q-switched lasers are typically used in applications which need high laser intensity and nanosecond pulses, such as metal cutting and pulsed holographic optics. With nonlinear laser devices, they can be used for optical storage and micromachining. In addition, Q-switched lasers are ideal for measurement applications such as laser ranging. In the field of scientific research, they are also commonly used for the study of chemical kinetics. Q-switched lasers are also used in the field of medical and cosmetic for tattoos removing, dark spots removing and other skin repair treatments.
Mode-locked lasers are typically used in ultra-short pulse applications with high laser intensity. Compared to Q-switched lasers, they act in more sophisticated fields. For example, there are nuclear fusion, generation of terahertz waves, optical storage, nanofabrication, two-photon microscopy, corneal surgery, photon sampling, etc.
- The processes are different
The process of Q-switched laser is relatively simple, especially the passively Q-switched laser, which mainly depends on the characteristics of saturated absorbing materials. Mode-locked lasers are relatively complex. And the faster the pulses are needed, the more complex the process required
- The costs are different
Q-switched lasers generally have low cost and are suitable for general industrial production and civil fields. Mode-locked lasers can obtain shorter pulses, but their cost is higher, suitable for some professional fields, industrial and medical fields that require shorter pulses.
Table 1 The difference between Q-switched laser and mode-locked laser
| Q-switched laser | Mode-locked laser | |
| Principle | Modulation loss | Modulation phase |
| Pulse width | Nanosecond | Picoseconds, femtoseconds, etc. |
| Application | High-energy nanosecond pulse applications | High-energy picosecond and femtosecond pulse applications |
| Process | Simpler | More complex |
| Cost | Cheaper | More expensive |
Ⅶ What is the difference between passively Q-switched laser and actively Q-switched laser?
- The materials are different
Passively Q-switched laser uses saturatible materials, such as passively Q-switched crystals. A passively Q-switched laser depends on the characteristics of the material itself and the recovery time of the material. And the effects are generally not adjustable. Actively Q-switched laser uses actively Q switches, the core of which is electro-optical or acousto-optical crystals, etc. It has high controllability but it needs extra driver control, which is more stable than passive Q-modulated laser.
- The energy is different
Passively Q-switched laser can produce large-energy pulses. But because of its material limitations, once the passively Q-switched crystals are saturated, they output pulses, making it difficult to fully reach the theoretical highs. The loss of the actively Q-switched laser is basically adjusted externally, which can be closer to the theoretical highest point of gain.
- The sizes are different
The structure of passively Q-switched laser is extremely simple, so it can be achieved in a very small size. And the gain medium in passively Q-switched laser can usually be bonded with passively Q-switched crystals for microchip lasers, and the total length of the cavity can reach the order of millimeters. Active Q-modulated lasers can generally achieve centimeters because they also need to carry drivers, and it is difficult to obtain smaller sizes.
- The costs are different
Passively Q-switched laser is cheaper, and only one more saturated absorbing material than ordinary lasers can achieve huge pulse output. Actively Q-switched laser is generally more expensive than passively Q-switched lasers.
Table 2 The difference between passively Q-switched laser and actively Q-switched laser
| Passively Q-switched laser | Actively Q-switched laser | |
| Material | Saturatible material (e.g. passively Q-switched crystals) | Actively Q-switch (e.g. electro-optical, acousto-optical crystals) |
| Energy | Big | Bigger |
| Size | Small | Larger |
| Cost | Cheap | More expensive |
Ⅷ What type of laser is suitable for passively Q-switched technology?
- General solid-state laser
Most doped insulator solid-state lasers are well suited for passively Q-switched technology. Because solid gain media generally have a long upscale life and high saturation energy. Thus, the general solid-state lasers are capable of storing large amounts of energy. Then they can obtain a high level of giant pulses.
- Microchip laser
Microchip lasers can actually be attributed to solid-state lasers, but deserve special instructions because of their high adaptability to passively Q-switched technology. Because the materials and processes required for passively Q-switched technology are the simplest. Only the bonding of passively Q-switched crystals and laser crystals can achieve the effect of small size as well as high-quality emitting giant pulses.
- Fiber laser
Compared with solid-state lasers, fiber lasers are generally compatible with passive Q-tuning technology, and their mode area and energy are not as large as those of solid-state lasers.
- Gas laser
Most gas lasers are not suitable for passively Q-switched technology, even Q-switched technology. Because they have insufficient energy storage in their gain medium. CO2 lasers are exceptions.
- Semiconductor laser
Semiconductor lasers are generally not suitable for Q-switched technology. They can apply Q-switched technology to obtain short pulses, but can only produce a fairly low pulse energy.
Table 3 The adaptability of various types of lasers to passively Q-switched technology
| General solid-state laser | ★★★★ |
| Microchip laser | ★★★★★ |
| Fiber laser | ★★ |
| Gas laser | ★ |
| Semiconductor laser | ★ |
Ⅸ About passively Q-switched laser, what indicators need to pay attention to?
- Pulse width
Pulse width is one of the most important indicators of pulsed lasers, describing the time of action of a single pulse of the laser. It roughly determines the performance of the pulsed laser.
- Repetition frequency
Repetition frequency describes the number of laser pulses per unit time. It determines the operating rate and efficiency of the system.
- Wavelength range
Wavelength range is one of the indispensable indicators of optics. It determines the operating range of the laser.
- Energy
Energy generally refers to the single pulse energy of a laser. In some places, it may label as average power. The average power is equal to the single pulse energy multiplied by the repetition frequency. Besides, the laser damage threshold of the optical path system is also closely related to the laser energy.
Ⅹ What are the precautions for passively Q-switched lasers in the process of use?
- Repetition frequency adjustment is not recommended
Theoretically, the pulse repetition frequency increases with the pump power for continuously pumped passively Q-switched lasers. However, the improper operation can lead to problems, such as energy and pulse width instability. Therefore, adjusting the repetition frequency during use is generally not recommended. In addition, the design of the laser has considered the heat dissipation at the rated frequency. If the repetition rate of the laser is adjusted, the thermal management may fail, and the laser may be damaged in serious cases. There will be many uncertain factors after adjustment, and we cannot guarantee that the parameters of the laser can be consistent with those on the color page of the product. Therefore, adjusting the frequency in the use process is generally not recommended.
- Safety should be taken care of during use
Even if the passively Q-switched laser with relatively small energy to other lasers still has a high energy to eyes, it must pay attention to safety when using it. Firstly, the most direct way is to avoid direct lasers into the eye. Secondly, for optical components in the optical path, their damage threshold range needs to be strictly calculated to avoid damage to the system. Thirdly, the overall cleanliness of the laser should be kept to avoid burning of dust particles.
References
[1] Laser. https://en.wikipedia.org/wiki/Laser#Laser_physics
[2] Q Switching. https://www.rp-photonics.com/q_switching.html
[3] Understanding Passively Q-Switched Solid-State Lasers. https://www.photonics.com/Articles/Understanding_Passively_Q-Switched_Solid-State/a56862
[4] Passive Q-switch crystals. https://4lasers.com/en/components/crystals/passive-q-switch-crystals
[5] Q-switching. https://en.wikipedia.org/wiki/Q-switching
[6] Mode locking. https://en.wikipedia.org/wiki/Mode_locking
[7] Detailed explanation of the technical principle and cost competition analysis of pulsed laser in lidar.http://news.eeworld.com.cn/mp/ICVIS/a73153.jspx
[8] The advantages and disadvantages of passive VS active Q-switching. https://www.rpmclasers.com/passive-vs-active-q-switching
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