The most common pitfall when sourcing UV-grade fused silica substrates is making the decision based on a single 200–800 nm average transmission curve. Suppliers often present curves that look "beautiful": average visible-light transmission above 90% across the board. Yet once these substrates are installed as excimer laser windows, deep-ultraviolet (DUV) lithography illumination elements, or UV sensing windows, after a few hundred hours of operation they show declining transmission, localized heating, and even visible fluorescence. The problem usually lies neither in polishing accuracy nor in the vague claim of "high purity," but in the internal defect chemistry of the substrate—the combined intrinsic behavior of germanium impurities, hydroxyl groups, and UV absorption. The differences among JGS1, JGS2, and JGS3 grades are fundamentally differences in defect chemistry, not simple tiers of quality.

1. Is There an Absorption Shoulder at 243 nm?

Germanium in fused silica exists as a substitutional impurity. Germanium-related oxygen-deficient centers (GeODCs), structurally mainly ≡Ge–Ge≡ or ≡Ge–Si≡, are the source of the photosensitivity used in fiber Bragg grating writing, and simultaneously the main precursors of radiation-induced attenuation. Under UV excitation they convert into paramagnetic Ge–E' color centers (≡Ge·), with the unpaired electron localized in an sp³ orbital pointing toward an oxygen vacancy. In measured transmission spectra collected from 200–800 nm at a 0.5 nm step, JGS2 exhibits a pronounced strong absorption peak at 243 nm, while JGS1 and JGS3 show only a negligible signal in the same band.
The implication for substrate procurement is direct: whenever the operating wavelength falls below 250 nm, the bulk absorption of JGS2 becomes both a heat source and a loss source. During selection, buyers should request measured transmission curves and watch specifically the shoulder near 243 nm, rather than relying only on the average visible-light value. Between batches of the same nominal grade, the height of this "shoulder" is often the true source of transmission dispersion.

Figure 1. Comparison of UV-visible absorption spectra of JGS1 / JGS2 / JGS3 fused silica. JGS2 shows a strong absorption peak at 243 nm.
 

2. The 3670 cm⁻¹ Hydroxyl Band: Deciding Whether a Substrate Is "Radiation-Resistant" or "Prone to Conversion"

The second criterion comes from FTIR. Scanned over 500–4000 cm⁻¹, the absorption intensity at 3670 cm⁻¹ gives a clear ordering: JGS1 > JGS2 > JGS3. This band corresponds to IR-active hydroxyl species (≡Si–OH, ≡Al–OH, ≡Ge–OH, and interstitial molecular water). The network also hosts IR-inactive species, such as interstitial molecular hydrogen (H₂) and bonded hydrides (≡Si–H, ≡Ge–H); these produce no IR absorption but participate in the same defect chemistry. Taken together, these hydrogen-bearing configurations constitute the substrate's "hydrogen reservoir."
Under UV irradiation, the hydrogen reservoir is activated: through indirect excitation or local photolysis, these species release mobile hydrogen that either thermally hops along the inherent μ-oxygen bridge network of silica or slowly diffuses through interstitial channels as recombined H₂ molecules. The three grades therefore follow three different fates. JGS1 has the highest hydroxyl content, but its extreme purity lacks trapping sites such as Ge substitutional centers, so defect conversion cannot establish itself. JGS3 is severely depleted of structural hydrogen and is excluded from the hydrogen-shuttling mechanism. Only JGS2 possesses both Ge trapping centers and a sufficient hydrogen reservoir.
This conclusion is counterintuitive to end users: high hydroxyl content does not mean stability, and low hydroxyl does not mean radiation resistance. When selecting a substrate, first define the operating wavelength and loss mechanism, then work backward to determine which "defect combination" is required. Hydrogen-loading modification (to boost radiation resistance) and low-hydrogen processing (to suppress IR absorption) are two opposite routes at the substrate level. If the datasheet does not specify hydrogen speciation, buyers who look only at "OH content" can easily choose the wrong direction.

Figure 2. FTIR spectra of the three fused silica grades. The hydroxyl absorption intensity at 3670 cm⁻¹ follows the order JGS1 > JGS2 > JGS3.
 

3. A 0.5-Hour UV Exposure as a Substrate Grading Test

The first two criteria can be judged without running an experiment; the third requires measurement, but it is inexpensive. Place a 7 × 7 × 100 mm polished rectangular sample into an X-band EPR cavity (a 240–400 nm mercury lamp is introduced from below through a high-purity fused silica fiber) and irradiate continuously for 0.5 hour. Before irradiation, none of the three substrates shows a paramagnetic signal. After 0.5 hour, JGS1 and JGS3 still show no obvious response, while JGS2 exhibits a central Ge–E' peak (g∥ = 2.0018, g⊥ = 2.0008, line width only 0.7 G) together with two weak peaks—the classic 119 G hyperfine doublet, assigned to the Ge–E'H center elastically coupled to a trapped hydrogen atom (hyperfine coupling constants A ≈ [122.2, 117.2, 121.9] G).
This dataset can be directly converted into an incoming-inspection procedure: a short UV exposure plus an EPR baseline spectrum distinguishes the "UV responsiveness" of the three grades within half an hour. It is far cheaper than long-term aging tests and is well suited for batch-consistency sampling. For substrates already installed in running systems, the same baseline can indicate whether a part is in its "fresh state" or has already been "radiation-activated."
 

Figure 3. EPR spectra of the three fused silica grades before UV irradiation and after 0.5 h of UV exposure. Only JGS2 develops the central Ge–E' peak and the 119 G Ge–E'H doublet.
 

Grade	Preferred Spectral Range	Key Criteria	Short-term UV Response	Typical Substrate Applications
JGS1	Deep-UV transmission priority	Highest hydroxyl, but lacks Ge trapping centers; no absorption shoulder at 243 nm	No significant EPR response after 0.5 h	DUV windows, optical components insensitive to color centers
JGS2	UV optics, sensing, and laser transmission	Contains Ge substitutional sites; strong absorption at 243 nm; moderate hydroxyl	Rapidly forms Ge–E'; dark-state conversion to Ge–E'H; reversible	UV window plates, optical base materials for sensing paths (radiation history must be managed)
JGS3	IR and broadband priority	Minimal structural hydrogen; weak UV absorption	Lacks hydrogen reservoir; shuttle mechanism cannot be sustained	IR windows, broadband optical base materials

Table 1. Summary comparison of JGS1 / JGS2 / JGS3 UV-grade fused silica grades.
 

4. The Four-Point Substrate Selection Checklist

• Examine the measured 200–800 nm transmission curve, focusing on the absorption shoulder near 243 nm.
• Check the 3670 cm⁻¹ hydroxyl intensity and the declared hydrogen speciation (whether the part is H₂-loaded).
• Review trace impurity content such as Ge and the batch-to-batch consistency.
• Obtain a short-term UV EPR response baseline.

Only when all four checks are in place can a "grade designation" be converted into a "usable substrate part." Merely matching the grade name and the average transmission value means putting a material that "looks transparent but develops coloration after a few hundred hours of exposure" into the optical path.
 
 
 

Supply and Customization

JXT supplies UV-grade fused silica substrates and windows in JGS1 / JGS2 / JGS3 grades. Specifications cover conventional standard dimensions and large-format optical blanks, with support for laser cutting, custom geometries, and precision polishing. We can match the grade to your operating wavelength and provide test data on UV-visible transmission, hydroxyl content, and material grade. Our products target DUV lithography illumination, excimer laser windows, UV spectroscopy, and sensing optical paths, supporting both R&D prototyping and volume delivery.