What are Neodymium-doped Lasers?

A neodymium-doped laser is a type of laser that utilizes crystal/glass doped with neodymium (1%) as the laser gain medium to increase the gain of the laser beam when pumped by an external laser light (optical pumping). Neodymium is a rare earth ion. The neodymium-doped laser uses a diode-pumped solid-state laser (DPSSL) technology or solid-state dye laser (SSDL) technology. The neodymium-doped lasers are ideal for scientific, LIDAR & sensing, industrial (material processing), medical, semiconductors & microelectronics applications. 

In DPSS Lasers, the gain medium is pumped by a laser diode, diode bars, laser diode array, or stacks to increase the gain of the input laser beam. In SSDL, the gain medium is pumped by an argon-ion laser, diode lasers, frequency-doubled solid-state laser, excimer laser, nitrogen laser, copper vapor laser, or flash pumps to increase the gain of the input laser beam. Mostly, the neodymium-doped laser uses DPSS Laser technology.

The internal structure of a DPSS laser 

The lasing material or gain medium (i.e., Nd: YAG/ Nd: YLF/ Nd: YVO4/Nd: glass) usually used are in the shape of a rod or thin disk shape. This neodymium-doped crystal/class is placed between two optically coated mirrors to form the optical cavity.

Table 1: Commonly used materials in neodymium-doped laser and their output wavelength range

Advantages of neodymium-doped lasers

The neodymium-doped crystal/glass materials have a special feature that it will absorb maximum optical power at a particular wavelength. For example, the wavelength range for Nd: YAG is 800 to 802 nm. Hence, a simple GaAs/AlGaAs laser diode or diode array with a wavelength range of about 800 nm can resonate with the Nd: YAG absorption wavelength. Therefore more optical efficiency (50%) is possible when compared to flashlamp-pumped neodymium-doped laser. This is because a flashlamp emits light with a broad wavelength and hence most of the light cannot absorb by the neodymium-doped laser.

Absorption spectrum of Nd: YAG/glass lasers

The neodymium-doped lasers have good wall-plug efficiency/electrical to optical efficiency (10%), long lifetime (20,000 hours), high beam quality, and compactness as compared to the flashlamp-pumped laser, which has low electrical to optical efficiency (1%) and less lifetime (200 to 600 hours). When compared to the ytterbium-doped laser, the power efficiency of the neodymium-doped laser is high.  The higher wall-plug efficiency of the neodymium-doped laser ensures lower power supply demand and cooling demand.

How does a neodymium-doped laser work? 

Neodymium-doped laser (Energy level diagram)

Nd: YAG energy states

The neodymium-doped laser is a four-level laser system, which means that four energy levels (E1, E2, E3, E4, or E5) are involved in the laser action. The light from the external laser source is focused onto the neodymium-doped crystal/glass (i.e., Nd: AG/ Nd: YLF/ Nd: YVO4/Nd: glass) through the cylindrical lens (collimator). This cylindrical lens transforms the strongly diverging light beam into more aligned in a specific direction, i.e., into collimated light or parallel rays.

Nd: YAG energy states (emit light output with wavelength 1064 nm)

Based on the incoming wavelength of light, the Nd3+ ions in the ground state (E1) absorb the optical energy and jump into a higher energy state (E4 or E5). From these higher energy states (E4 or E5), the ions rapidly transit to the metastable state (E3), which is the energy that has a long lifetime (10^-3 seconds) as compared to the ordinary excited energy state (E4 or E5). The energy state (E4 or E5) lifetime is 10^-8 seconds. There is no radiation during this rapid transition (E4 or E5 to E3); hence, it is called non-radiative decay. In this way, the number of Nd3+ ions increases in the metastable state that results in population inversion occurs.

Now, after some time (10^-3 seconds), the ions in the metastable state move to the energy state E2 by emitting photons. This is known as spontaneous emission. These emitted photons reflect back and forth between the reflective surfaces and strikes more ions in the metastable state leads to more photons release (stimulated emission). Again, these newly emitted p