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Trivalent rare earth ions Tm3+ and Ho3+ show extraordinary performance results for the high power continuous wave and pulsed laser operation in 2 µ wavelength range. Thulium fiber laser is, in general, better for CW operations, when holmium laser is preferred for pulsed and q-switched lasers operations because of its high gain. However, holmium can be excited around 1.9 µ for an efficient operation at 2.1 µ, or it needs to exploit some energy transfer from thulium or ytterbium.
Before recently, holmium laser products were created as co-doped systems, because there weren’t any laser diodes that could provide wavelength ranges for pumping Ho3+ ions. Mostly, thulium co-doping was used, because its ions cross relaxation process properties. Currently, the most potential option to reach the highest output powers is in-band pumping of Ho:YAG crystals. It is possible in 1.9 µ wavelength range.
There are a lot of different applications that need short laser pulses with high pulse energies or high CW powers at the 2.1 µ wavelength, which makes holmium lasers in high demand on the market. Mainly, holmium advanced laser systems are created using thulium crystals or fiber lasers for pumping. Moreover, 2.1 µ wavelength is eye safe, because the emission doesn’t reach the retina, which makes it not dangerous for eyes. This wavelength range makes holmium laser in demand for commercial use, especially for LIDAR systems, which operate similarly to radars. 2 µ wavelength allows one to absorb certain atmospheric gasses (e.g. H2O, CO2, N2O) to detect and to analyse them. One of the key advantages of this laser for LIDAR technology is the capability to detect specific lighter atmospheric gasses and molecules. It has a greater potential than thulium fiber laser in chemical and petroleum industries due to safety and quality control as well as in medicine and environmental research.
oreover, holmium laser is very promising in terms of medical applications. High water absorption allows performing extra precise surgeries; 2.1 µ emission coagulation effects minimize bleeding. Ho:YAG penetrates into a soft tissue at the depth of 300 µ. However, thulium fiber laser has its own advantages in performing surgical procedures. Holmium laser systems have a lot of potential and, definitely, require further developments and research. They can be applied in a lot of different industries for various purposes, including spectroscopy, sensing and surgery.

Q-switching technique is usually utilized in solid-state laser technology to generate nanosecond high energy pulses. It creates short pulses through regulating cavity losses. Q factor (quality factor) is a definition of an oscillation damping strength measurement.
There are two types of q-switching: passive and active.
Active q-switching technique uses an electrically controlled modulator (acousto-optic or electro-optic). It is applied to control optical losses which are high initially, but in the process of switching they are lowered abruptly. Pump phase and the gain-medium upper-state lifetime should be roughly the same to avoid losing energy in spontaneous emissions. Energy loss through spontaneous emissions becomes significant when laser gain is high. However, it is not the biggest concern when it comes to bulk lasers. A bigger issue is parasitic lasing because of unwanted reflections or q-switch. To avert the lasing a modulator should have a high pump phase when performing power losses.
It is possible to design smaller mode area lasers when the energy per pulse and energy stored are lowered due to high pulse repetition rates (10 kHz, 100 kHz or more). However, high repetition rates create another issue – collecting enough laser gain even when stored energy is low, because when the gain is low pulses become longer. Even in case of high average powers it can still be problematic, because it may need bigger beam areas. That is why it is better to select a crystal providing a higher laser gain, for example, Nd:YVO4.
Passive q-switching technique uses a saturable absorber instead of an electrical modulator. There is a high optical loss when it is in the unsaturated state. To start the lasing a laser gain has to reduce that loss. When the emission increases, it saturates losses, and the laser power grows fast, which leads to the gain saturation.
It may give an impression that it is poor because of the absorption; however this is not the case. Just a little portion of energy is required for an absorber transparency, when the laser gain medium saturation energy is higher than the absorber energy. One of the most common crystals used in this case is Nd:YAG.
The pumping in a passive q-switched laser continues up to a moment when a pulse build up begins. This process begins when there is enough energy stored in the gain medium. The main difference between passive and active q-switching technique is that in case of a passive one the pumping power change does not affect the energy pulse, it will only effect the timing. In active q-switching technique both will be affected, the energy pulse and timing.

Ultra-short pulsed femto- and picosecond lasers are in high demand today. Thulium-doped lasers, definitely, stand out because of their capability to generate emissions in a wide range. Thulium Fiber Laser has unique qualities which make it perfect for a lot of different applications, namely, medicine, spectroscopy, laser ranging and micromachining of transparent materials, in particular, semiconductors and laser for LIDAR. Presently, they are the most effective sources of a single-mode emission with the wavelength in 2 µ range.
Thulium Fiber Laser water absorption properties are outstanding. The main constituent of any biological tissues is water, which is why strong water absorption quality of thulium laser allows significant heating of small areas. In other words, cutting biological tissues becomes extra precise. All of the above make Thulium Fiber Laser irreplaceable when it comes to performing surgical procedures.
Thulium lasers compared to traditional optimal cost-efficient laser systems that operate at shorter wavelengths have undeniable advantages when using for free space applications. It gives them a high commercial value, especially as laser for LIDAR and gas sensing systems, optical communication applications and pumping lasers for the mid infra-red spectrum.
Thulium doped lasers are released either in fiber or crystal host materials. Depending on the host material the wavelength range spreads from 1840 nm to 2100 nm. Thulium fiber can either be a q-switched laser or continuous wave laser, and both develop significantly a high average power. On the other hand, thulium doped crystals have a broad emission spectrum, which allows a large wavelength tuning range. Thulium doped crystals are of really good quality with a few imperfections and defects. YAG and YFL crystals reveal the best quality nowadays; however, they are not the best choices because of their thermal conductivity and emission cross section. There is definitely a room for improvement, and further research has to be done to enhance their performance.
Despite the fact that there are so many different applications in a lot of industries only few laser technology providers can deliver this type of laser at the moment. Currently, Optromix is developing its own thulium laser and will release it shortly for commercial use (e.g. laser for LIDAR).

Nd:YLF laser active elements are made of yttrium lithium fluoride (YLiF4) crystal. YLF stands for Yttrium Lithium Fluoride. Usually neodymium-doped YLF crystal is used for YLF laser equipment; however, it can also be doped with rare earth elements, such as ytterbium (Yb), erbium (Er), thulium (Tm), holmium (Ho) or praseodymium (Pr).
Yttrium ions in YLF crystal may be substituted with laser-active rare earth ions, because of its similar size, without distorting the structure of crystal lattice. In neodymium-doped YLF crystal its (Nd3+) concentration is usually up to 1% of its total weight.
YLF crystal has a natural strong birefringence, which removes thermal polarization losses. Besides the emission wavelength and the gain of the Nd:YLF crystal waves are polarization-dependent, there is a stronger wave of 1047 nm and a weaker one of 1047 nm. It makes Nd:YLF crystal better for a less powerful laser equipment that require extra precision.
1053 nm wavelength matches the maximum for loop gain of phosphate laser glass, that contains neodymium ions, that is why Nd:YLF lasers are often used as a master oscillator and preamplifier for the subsequent stages of the neodymium phosphate glass amplifier.
Diode-pumping and lamp-pumping is possible for Nd:YLF laser. It has lower thermal conductivity in comparison with Nd:YAG laser, but its thermal distortions are weaker, which leads to a better beam quality and worse fracture resistance, hence it limits the output power in the laser equipment.

YAG stands for yttrium aluminum garnet, which is a synthetic crystal. Nd:YAG is an artificial cubic garnet crystal. YAG crystals were created shortly after ruby laser discovery. They have high gain and other unique properties which make them universal for a lot of different applications. YAG crystals increase the stability of the source, its efficiency and lifespan, reduce size and power consumption.
There are certain limitations connected with the high gain and the safety of the operating fluence. Mode-lock property creates pulses of very different widths – nanosecond to picoseconds, which enables it to make wide-ranging peak power for various applications.
YAG laser is a solid state diode pumped laser, its beam is in mid infra-red range and its wavelength is 1064 nm. Nd:YAG is an outstanding solution for scientific laser systems because it can reach extremely high powers in a pulsed mode, which is used in the oscillators to produce series of very short pulses to perform research with femtosecond time resolution.
Nd:YAG laser may be used with a frequency doubler, in this case its wavelength is 532 nm and its output power is lower.
In terms of applications, yag laser systems are very versatile. It has been widely used in the military, for example, for rangefinding or for target designation.
On the other hand, as mentioned before its pulse width and high power, it is extremely useful for scientific purposes, or as a pumping source for other lasers.
Medicine is another field where Nd:YAG is frequently used, namely dermatology and ophthalmology. Commercial purposes include applications such as ablation, spectroscopy, marking, nondestructive testing and others. Moreover, it is an optimal cost-efficient laser.
Key feature of the YAG lasers is its resilience to different environmental conditions; hence it suits perfectly well for remote sensing, bathymetry, gated imaging illumination, atmospheric and ocean studies, etc.
Other lasers or nonphotonic techniques may be used for most of the applications mentioned above, for example, diode or CO2 lasers. Nd:YAG allows to perform a broad range of applications, because of its one-of-a-kind properties, and also, not many lasers have an ability to function efficiently with diode pumping and to switch between pulsed or CW mode.

LIDAR (light detection and ranging) is a laser technology used for optical remote sensing which allows one to analyze scattered light properties in order to obtain certain information about a distant object.
These advanced laser systems are often used, for example, to collect precise information about Earth surface and its characteristics. The sensor sends out a pulse of light to travel to an object, it reflects off the object and travels back. When the light clashes into an object, the sensor detects the reflected pulse. Then it measures the time necessary for the reflected pulse to return. The light pulse travels with the speed of light which is known and constant; hence, the time is easily converted into distance or as it is called – the range. The information on the position and angle of the laser equipment allows calculating exact coordinates of the object reflected.
LIDAR technology can be applied in a lot of different areas, from geographical mapping to robotics due to its high configuration capabilities and wavelengths.
There are different types of LIDARs: rangefinder, DIAL and Doppler.
Rangefinders measure a distance between a sensor and a solid object.
DIAL (differential absorption) measures chemical concentrations in the atmosphere (ozone, water vapor, pollution). It emits pulses with two different wavelengths which are set in a specific way, so that a molecule can absorb one of them, but the other can’t. This way the molecule concentration is deduced.
Doppler technique measures an object velocity. When a light pulse travels to a moving object, its wavelength changes a little, and it is called Doppler shift. When the object is moving away from the sensor, the reflected wavelength will be longer, and when the object is moving towards the sensor, the reflected wavelength will be shorter.