What is fused silica?
Fused silica: high purity amorphous silica for refractories, iron & steel and glass production.

What's the difference of Chinese Fused silica JGS1,JGS2,JGS3?
JGS1: High transmission for 185-2000nm
JGS2: High transmission for 200-2500nm
JGS3: High transmission for 260-3500nm

Highly Recommended Fused Silica Windows from Yutai Optics:
Optical Fused Silica Windows
Fused Silica Windows with Hole
Fused Silica Windows with Step
Coring Series Fused Silica Windows
Thin Fused Silica Glass Sheet