What exactly is Hydroxyl group? A lower hydroxyl content results in better transmittance in the infrared spectrum, lower transmission loss, and higher product quality.

The main component of glass is silicon dioxide (SiO₂). During the glass melting process, high-temperature flames are used. Since flame is a product of hydrogen and oxygen combustion, it introduces a significant amount of water vapor. This water vapor reacts chemically with the molten silicon dioxide at high temperatures, forming "silanol groups" (-SiOH), which remain trapped within the solid glass.

In simple terms, the hydroxyl groups in glass are essentially "burned" into it during the production process.

Hydroxyl groups have a significant negative impact on the properties of glass, especially its optical performance:

• Infrared Light Absorption: Hydroxyl groups exhibit very strong absorption peaks at specific infrared wavelengths (notably around 2.7 µm, 2.2 µm, and 1.38 µm). This severely attenuates transmitted infrared light signals.

• Reduced Transmission Efficiency: For optical fibers transmitting light signals or lasers transmitting laser energy, the absorption of light by hydroxyl groups converts it into heat, leading to energy loss and signal attenuation.

• Impaired Laser Performance: In high-power lasers (such as fiber lasers), the thermal effects generated by hydroxyl groups absorbing energy can result in decreased beam quality, reduced efficiency, and even damage to the components.

Which fields specifically require low-hydroxyl glass?

• Optical Fiber Communication: Signals travel long distances through fibers, where any minor loss is critical.

• High-Power Lasers: Such as fiber lasers used in industrial processing and medical applications.

• Optical Sensors: Which require high-precision optical systems.

Therefore, the hydroxyl content is one of the core indicators for measuring the quality of optical-grade and laser-grade glass tubes/fiber preforms. A lower hydroxyl content results in better transmittance in the infrared spectrum, lower transmission loss, and higher product quality.