A Volume Bragg Grating (VBG) laser diode is a type of laser diode that uses Bragg grating to achieve high spectral purity and wavelength stabilization. Volume Bragg Grating is a transparent medium that exhibits a periodic variation in its refractive index and provides narrow linewidth for laser diodes. It is used for pumping some solid-state lasers in which the laser crystal exhibits efficient pump absorption only in a narrow wavelength range. Volume bragg gratings are commonly used to lock the wavelength of high-power diode lasers and solid-state lasers, such as the Nd:YAG laser, to a specific wavelength.

The wavelength of a laser diode can be stabilized by using it together with VBGs. When a VBG is added to a laser diode, it creates additional optical feedback resulting in laser light emission within a narrow optical bandwidth that effectively sets the output wavelength of the VBG for all practical purposes. If no feedback was given, there might be a shift in wavelength due to temperature changes and the increasing age of the laser diode. By using this, the total optical bandwidth can be reduced to some nanometers which are useful for solid-state laser pumping. 

Structure of VBG Laser Diode

Figure 1: Structure of a VBG Laser Diode

The structure of a VBG laser diode typically consists of a laser diode, volume bragg grating, and collimating lens. Figure 1 depicts the structure of a VBG laser diode.

The laser diode contains a laser gain medium typically, a semiconductor material such as GaAs or InP formed between two mirrors. A pump source, especially a high-power laser diode emitting light at a shorter wavelength than the desired output wavelength, is used to excite the laser gain medium. The VBG is placed at one end of the laser cavity and it acts as an output coupler. The end faces of VBG are optically polished with an anti-reflection coating to prevent parasitic reflections, which is the unintended and undesirable oscillation that occurs in a laser resulting in noise or instability. There is also a collimating lens to collimate the laser beam. 

Working of VBG Laser Diode

Figure 2: Working of a VBG Laser Diode

When an electrical current is applied to the laser diode and the pump source, it excites the laser gain medium and emits a wide spectrum of wavelengths λ₁, λ₂,…λ_N. The laser light travels through the laser cavity and is focused using a collimating lens to fall on the VBG which is positioned at one end of the cavity. This allows the grating to act as an output coupler, reflecting only the desired wavelength of laser light back into the cavity while allowing all other wavelengths to pass through. This selective reflection is due to the periodic refractive index modulation of the VBG that acts as a wavelength-selective reflector. Therefore, VBG self-seeds the laser with a narrow light beam and the laser diode is forced to lase at this particular wavelength chosen by the bragg grating. Hence, the laser emits only one wavelength λ₁. This laser light exits the laser cavity through the output window, which is transparent to the desired output wavelength. The working of the VBG laser diode is shown in figure 2. 

Advantages of VBG Laser Diode

• Improved spectral stability
• Cost-effective and easy to implement
• Less cooling requirements
• Improved emission characteristics 
• Increased pumping efficiency
• Ability to operate at high powers 
• Not necessary that the source be coupled into the fiber
• Consistent pumping efficiency over wide temperature ranges and drive currents

Disadvantages of VGB Laser Diode

• Complex
• Extra cavity optical components add mechanical delicacy

Applications of VBG Laser Diode

VBG laser diodes have a wide range of potential applications across many different fields. They are used for materials processing and manufacturing, where high-power and efficient lasers are needed to cut, weld, and anneal a variety of materials. VBG laser diodes could also be used in medical applications, such as laser surgery or photodynamic therapy, where their spectral stability and high power output could improve the effectiveness and safety of these treatments.

These laser diodes can also be used in telecommunications, where their spectral stability and high efficiency could improve the performance of optical communication systems. They could also find use in entertainment, such as laser light shows, where their ability to produce high-power and stable beams could create stunning visual effects.