What are Nd:Glass Lasers?

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- GoPhotonics

Dec 29, 2022

Nd:Glass laser is a four-level solid-state laser. The output wavelength of Nd:glass lasers is 1.062 µm (for silicate glasses) and 1.054 µm (for phosphate glasses) which lies in the near-infrared region of the spectrum and emits energy in the order of several kilojoules. The Nd:glass laser is similar to Nd:YAG lasers in their structure and lasing action. Neodymium (Nd3+) ions are the dopant in the laser medium for Nd:glass lasers. These ions are doped with glass host materials like silicate and phosphate ions. The advantage of glass rods is that they can be grown to larger sizes than YAG crystals economically. The optical quality and doping uniformity are excellent for glass host materials, but the optical distortion is higher. Nd:glass lasers are optically pumped using krypton or xenon flash lamps, laser diodes, etc. The ultrashort pulses with narrow linewidth can be achieved in these lasers using the mode-locking phenomena.

Working of Nd:Glass laser

Figure 1: Construction of an Nd:Glass Laser

An Nd:Glass laser uses neodymium-doped silicate or phosphate glasses as the active medium. The active medium is kept in a resonator cavity containing two mirrors at both ends. One of the mirrors is highly reflecting and the other one is partially reflecting and it is used to couple the light outside the laser. A flash lamp or any other optical source is used to pump the active medium. A power supply is provided for the flash lamps to work. When pumping is given, population inversion takes place inside the active medium and lasing takes place. The output wavelength obtained depends on the glass material used. For silicate glasses, the emission wavelength is 1.062 µm and for phosphate glasses, the emission wavelength is 1.054 µm. Figure 1 shows the schematic of an Nd:glass Laser.

Energy level diagram of Nd:Glass laser

Figure 2: Energy level diagram of Nd:Glass Laser

Upon optical pumping, the molecules or ions absorb the photons and are excited to the upper absorbing levels of E3 from the ground state. The lifetime at this level is very short and hence nonradiative decay occurs from energy level E3 to energy level E2 which is a metastable state. The energy level E2 has a very long lifetime and therefore, ions get accumulated at this level.  Population inversion is achieved between the E2 and E1 which leads to stimulated emission and thus gives a laser transition at 1.062 µm or 1.054 µm. Then spontaneous emission takes place from the energy level E1 to the ground state E0.

Parameters of Nd:Glass Laser

Laser Wavelength1.054 - 1.062 µm
Laser transition Probability2.9 to 3.4 x 103/s
Upper Laser Level Lifetime290 - 340 µs
Stimulated Emission Cross Section2.9 to 4.3 x 10-24 m2
Spontaneous Emission Linewidth and Gain Bandwidth, FWHM7.5 x 1011/s
Inversion Density8 x 1023/m3
Small Signal Gain Coefficient3/m
Laser Gain Medium Length0.1 m
Single Pass Gain1.3
Doping Density4.6 x 1026/m3
Index of Refraction of Gain Medium1.50 - 1.57
Operating Temperature300 K
Thermal Conductivity of Laser Rod~1 W/m-K
Thermal Expansion Coefficient of Laser Rod8.5 to 14 x 10-6/K
Pumping MethodOptical (Flash lamp or Laser)
Pumping Bands300 - 900 nm
Output PowerUp to 10 kJ/pulse in large amplifiers
ModeSingle-mode or multi-mode

Advantages of Nd: Glass Laser

  • Larger linewidth
  • Higher lasing threshold
  • Suitable for high-energy pulsed operation
  • Flexible in physical parameters

Disadvantages of Nd:Glass Laser

  • Lower thermal conductivity
  • Larger size

Applications of Nd:Glass laser

Nd:Glass lasers are used in applications that require low pulse repetition rates and high pulse energies (up to 100 Joules per pulse). The pulsed operation of Nd:glass lasers are utilized in hole piercing and deep keyhole welding applications, etc. The 1060 nm Nd-doped phosphate glass laser is useful for laser drilling and laser welding, as well as other important material processing applications. In laser welding, thermal impact and deformation are very small and since Nd:glass has good optical uniformity, the optical processing is easier.

These lasers are used in laser-induced nuclear fusion. In laser fusion devices, usually, Nd:glass lasers are frequency tripled to 351 nm. The short pulses with high energy in Nd:glass laser can heat matter to thermonuclear temperatures thereby energy is generated in small controlled explosions. The Lawrence Livermore National Laboratory's (LLNL) NOVA 10-beam Nd:glass laser was able to deliver 150 kJ on target with pulse widths of about 1 ns. 

They are also used for applications like range finders, optical communication, etc.