Optical notch filters are precision components designed to block a very narrow wavelength band while transmitting surrounding wavelengths. They are widely used in laser systems, Raman spectroscopy, fluorescence imaging, LIDAR, and optical sensing. While thin-film coatings usually get most of the attention, the substrate material plays a critical role in overall filter performance, durability, and application suitability.
Why Substrate Choice Matters
The substrate is not just a mechanical support. It directly affects:
- Optical transmission range
- Surface quality and coating adhesion
- Thermal stability
- Laser damage threshold
- Environmental durability
- Mechanical strength
Choosing the wrong substrate can limit performance—even if the coating design is perfect.
1. Optical Glass (BK7 / Borosilicate Crown Glass)
BK7 is one of the most commonly used substrates for visible and near-infrared notch filters.
Advantages:
- Excellent transmission from ~350–2000 nm
- High surface quality achievable
- Good homogeneity and low inclusions
- Cost-effective for large volumes
- Compatible with most coating processes
Typical applications: Visible laser rejection filters, fluorescence microscopy, machine vision systems.
Limitation: Not suitable for deep UV or mid-IR applications.
2. Fused Silica (Quartz)
Fused silica is the go-to material when performance demands go beyond standard optical glass.
Advantages:
- Wide transmission range: ~180 nm to 2.5 µm
- Very low thermal expansion
- Excellent thermal shock resistance
- High laser damage threshold
- Exceptional chemical stability
Typical applications: UV notch filters, high-power laser systems, spectroscopy.
Limitation: Higher cost than BK7.
3. Sapphire (Al₂O₃)
Sapphire is used when mechanical and environmental robustness are critical.
Advantages:
- Extremely high hardness (scratch resistant)
- Excellent thermal conductivity
- Withstands harsh environments
- Transmission from ~200 nm to 5 µm
Typical applications: Industrial sensing, harsh-environment optics, high-temperature laser systems.
Limitation: Birefringence can affect polarization-sensitive systems.
4. Silicon
Silicon substrates are common for infrared notch filters.
Advantages:
- Excellent transmission in 1.2–7 µm range
- Good thermal conductivity
- Mechanically robust
Typical applications: IR spectroscopy, thermal imaging systems, gas sensing.
Limitation: Opaque in the visible spectrum.
5. Germanium (Ge)
Germanium is a premium substrate for long-wave infrared (LWIR) filters.
Advantages:
- High transmission in 2–14 µm
- Excellent for thermal imaging
- High refractive index (useful in compact optical designs)
Typical applications: Thermal cameras, military optics, long-wave IR systems.
Limitation: Heavier and more temperature-sensitive than silicon.
In optical notch filters, coatings define spectral performance, but substrates define reliability and application compatibility.
- For visible systems → BK7
- For UV or high-power lasers → Fused Silica
- For rugged environments → Sapphire
- For infrared → Silicon or Germanium
Understanding the substrate ensures that your filter performs not only in the lab, but also in the field.