What is the Comparison of Resin, Glass, and Silicon Carbide Waveguide Materials?

Earlier, we published an article titled "Comparison of Performance Characteristics of Resin, Glass, and Silicon Carbide Waveguides"(Click to View). Today, we will introduce the three materials in more detail.

Resin Material Waveguide

Optical Properties

1. **Refractive Index**: The refractive index of resin typically ranges from 1.49 to 1.60, which is lower compared to glass and silicon carbide. A lower refractive index means that the critical angle for total internal reflection in the waveguide system is larger, resulting in lower optical signal transmission efficiency than other materials with higher refractive indices. (n = sin i / sin r, where n is the refractive index, sin i is the sine of the incident angle, and sin r is the sine of the refracted angle.)

2. **Transmittance**: Resin materials exhibit good transmittance in the visible light range, but their optical uniformity is affected by molecular arrangement. Lower light scattering properties help maintain transparency, but prolonged use may lead to a decrease in transmittance due to aging.

3. **Birefringence**: Resins typically exhibit low birefringence because of their relatively random molecular structure and low internal stress, resulting in minimal phase differences during light propagation. However, under certain circumstances, external stress may induce uneven optical properties.

Mechanical Properties

1. **Hardness**: The hardness of resin is generally low (with a Vickers hardness of about 15-20 HV), making it more susceptible to mechanical damage compared to glass and silicon carbide, especially when the surface is subjected to friction. This lower hardness limits its lifespan in precision optical systems.

2. **Toughness and Elastic Modulus**: Resin exhibits relatively high toughness and an elastic modulus typically in the range of 2-5 GPa, which provides some resistance to cracking under impact. However, a decrease in the elastic modulus may weaken the structural stability of optical devices, particularly under high temperatures or prolonged use. (Elastic modulus: E = σ / ε, where the larger the elastic modulus, the better the material's elasticity, indicating a quick return to its original shape after stress.)

Chemical Properties

1. **Chemical Corrosion Resistance**: Resin exhibits good chemical stability and can resist the corrosion of many acids, bases, and organic solvents. However, it is highly sensitive to ultraviolet (UV) light, and prolonged exposure to UV radiation may lead to yellowing or embrittlement.

2. **Moisture Absorption**: Resin materials typically have a certain degree of hygroscopicity, which can lead to a decline in optical performance after water absorption, such as changes in refractive index and reduced transparency. Additionally, moisture absorption can weaken the mechanical properties of the material and increase the coefficient of thermal expansion.

Glass Material Waveguide

Optical Properties

1. **Refractive Index**: The refractive index of optical-grade glass typically ranges from 1.5 to 1.9, depending on its composition. Common optical glass, such as BK7, has a refractive index of 1.5168, allowing glass to provide superior optical transmission efficiency in total internal reflection waveguide designs and reducing light loss. (n = sin i / sin r, where n is the refractive index, sin i is the sine of the incident angle, and sin r is the sine of the refracted angle.)

2. **Dispersion Coefficient**: The Abbe number (V-value) of glass usually ranges from 50 to 60, indicating a low dispersion coefficient, which means that the change in refractive index across different wavelengths is minimal. Therefore, in AR waveguides, glass can effectively reduce dispersion phenomena, ensuring image clarity and color consistency. (σ = δλ * D * L, where δλ is the root mean square spectral width of the light source, D(λ) is the dispersion coefficient, and L is the length. The dispersion coefficient for single-mode optical fibers is generally around 20 ps/km·nm, meaning that longer fiber lengths result in greater total dispersion.)

3. **Transmittance and Absorption Coefficient**: High-quality optical glass has an extremely high transmittance, reaching over 95%. Its absorption coefficient is low, particularly in the visible light spectrum, with minimal absorption of light energy, thus ensuring efficient transmission of optical signals.

Mechanical Properties

1. **Hardness and Toughness**: Glass has a high hardness (typically 500-700 HV), providing excellent scratch resistance and durability. Although glass is relatively brittle, modern processing techniques such as chemical strengthening and physical tempering can significantly enhance its impact resistance and toughness.

2. **Elastic Modulus and Poisson's Ratio**: The elastic modulus of typical optical glass ranges from 70 to 85 GPa, with a Poisson's ratio of about 0.2-0.3. This combination allows glass to maintain good shape retention under mechanical loads, particularly in waveguide systems, ensuring the stability of the optical path. (The ratio of strain in the vertical direction (εl) to strain in the load direction (ε) is known as the Poisson's ratio. Denoted as v, it is defined as v = -ε1/ε. Within the elastic deformation phase, v is a constant. Theoretically, for isotropic materials, only two of the three elastic constants (E, G, v) are independent.) (Elastic modulus: E = σ / ε. The larger the elastic modulus, the better the material's elasticity, indicating a rapid return to its original shape after stress.)

Chemical Properties

1. **Chemical Stability**: Glass materials exhibit excellent stability in most chemical environments, especially in acids, bases, and organic solvents. Certain specialized types of glass (such as quartz glass) demonstrate exceptional stability even in high-temperature and corrosive conditions.

2. **Moisture Resistance**: Glass is virtually non-hygroscopic, ensuring that its optical performance remains unaffected in humid environments. Water vapor has no significant impact on the refractive index, transparency, or other optical parameters of glass.

Silicon Carbide Waveguide Material

Optical Properties

1. **Refractive Index**: Silicon carbide has a very high refractive index, approximately 2.65, which significantly enhances its ability to bend light paths in optical designs, making it suitable for systems requiring precise beam control. However, the high refractive index also results in a higher reflection rate for incident light, which may introduce more optical loss at interfaces.

2. **Transmittance**: Although silicon carbide has relatively low transmittance in the visible light range, it shows significant advantages in the infrared (IR) and ultraviolet (UV) bands. This property makes silicon carbide promising for the transmission of specific wavelengths in waveguide systems.

3. **Optical Bandgap**: Silicon carbide has a wide bandgap (approximately 3.0 eV), indicating strong stability under high-energy photon exposure, preventing photodegradation phenomena seen in glass and resin under high-frequency illumination. (αhν = B(hν - Eg)^m, where α is the molar absorption coefficient, h is Planck's constant, ν is the frequency of the incident photon, B is a proportionality constant, Eg is the optical bandgap of the semiconductor material, and m is related to the material and type of transition.)

Mechanical Properties

1. **Hardness**: Silicon carbide is one of the hardest known materials, with a hardness close to 2500 HV. Its extremely high hardness ensures minimal deformation under mechanical impact and friction, maintaining long-term optical precision and stability.

2. **Fracture Toughness**: Despite its hardness, silicon carbide has relatively low fracture toughness (typically around 3.0 MPa·m^0.5), making it susceptible to brittle fracture under strong impact or stress concentration. (Fracture toughness can be measured through fracture testing, with the formula: Fracture Toughness = Fracture Strength / Fracture Length.)

3. **Elastic Modulus**: Silicon carbide has an extremely high elastic modulus (approximately 410 GPa), which means it exhibits minimal elastic deformation under stress, ensuring structural stability and optical precision under high loads. (Elastic modulus: E = σ / ε. The larger the elastic modulus, the better the material's elasticity, indicating a rapid return to its original shape after stress.)

Chemical Properties

1. **High-Temperature Resistance**: Silicon carbide retains its structure and optical performance at extremely high temperatures, with a melting point above 2700°C. This allows it to maintain excellent optical properties in high-temperature environments, whereas glass and resin may undergo thermal deformation or degradation under similar conditions.

2. **Corrosion Resistance**: Silicon carbide exhibits excellent chemical corrosion resistance, able to withstand the effects of most acids, bases, and high-temperature oxidation, further enhancing its durability in harsh environments.

Summary

Resin waveguides are easy to process and have high toughness, but they have weak optical and mechanical properties, particularly low refractive index and insufficient hardness. Glass waveguides offer good overall optical performance and durability, making them suitable for precision optical systems, with average mechanical properties and strong chemical stability. 

Silicon carbide waveguides excel in mechanical performance and thermal stability, boasting extremely high hardness suitable for harsh environments, but they have poor optical transmittance in the visible light range and are difficult to process.