Holmium Advanced Laser Systems – a Better Choice for Q-switched Operations than Thulium Fiber Laser

Holmium Advanced Laser Systems

Performance of Rare-Earth Ions

Trivalent rare earth ions Tm3+ and Ho3+ show extraordinary 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.

Evolution of Holmium Laser Products

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.

Industrial and LIDAR Applications

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.

Medical Applications

Moreover, 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.

Utilizing Nd:YAG Q-switched Lasers in Solid-State Laser Technology

NdYAG Q-switched Lasers

Introduction to Q-Switching

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.

Types of Q-Switching

There are two types of q-switching: passive and active.

Active Q-Switching

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

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.