art photonics Technical Note 007
Both polycrystalline silver halide fibers (PIR) and hollow waveguides (HWG) from art photonics transmit up to 85–90 % of CO₂ laser radiation (10.6 µm) through a 1 m high-power cable. Choose a PIR fiber when the cable must bend, move or run long distances: a full 360° loop costs only about 1.5 % of transmission. Choose a hollow waveguide when you need a low-divergence output beam (about 5.7° full angle) and the cable can stay fairly straight.
art photonics GmbH manufactures both technologies in-house in Berlin for high-power light delivery in the mid-infrared. They reach similar transmission when properly optimized, but they are built for different operating conditions. This article explains how each one works, compares their key parameters for CO₂ laser beam delivery, and gives a simple selection rule. Connector types (SMA, FC/PC, FC/APC), bending radii and other details are listed in the product datasheets linked at the end.
What are PIR fibers and hollow waveguides?
PIR fiber: solid-core polycrystalline silver halide fiber
A PIR fiber is a solid core/clad fiber made of silver chloride–silver bromide solid solutions (AgClxBr1−x). In art photonics PIR fiber, the core is AgCl0.25Br0.75 and the cladding is AgCl0.5Br0.5. Light is guided by total internal reflection.
PIR fibers have low bending losses and a high numerical aperture (NA). Because silver halides have a high refractive index, the bare fiber end faces would reflect part of the light (Fresnel reflection). art photonics suppresses these losses with an anti-reflective microstructure on the end faces, called SMART treatment, made during cable assembly.
Hollow waveguide (HWG): air-core silica capillary
A hollow waveguide is an air-core silica glass capillary with a reflective silver/silver iodide (Ag/AgI) coating on its inner wall. The capillary is protected by an acrylate layer and a polymer buffer. Light travels through the air core and is guided by reflection from the coated wall.
Because the core is air, an HWG has no end-face Fresnel reflection, and its output beam has low divergence. However, its transmission and beam quality depend on the coupling optics and on how much the waveguide is bent.
PIR vs HWG: key parameters at 10.6 µm (CW)
| Parameter | PIR fiber | Hollow waveguide (HWG) |
|---|
| Core / inner diameter available | 240–900 µm core (multimode) | 500–1000 µm bore (multimode) |
| Attenuation at 10.6 µm | 0.2–0.4 dB/m | 0.3–0.7 dB/m |
| Bending loss, one full 360° loop of 40 cm diameter 1 | ≈ 0.05 dB | ≈ 0.5–1 dB |
| Absolute transmission loss when bent (one 360° loop, 40 cm diameter) 1 | ≈ 1.5 % | ≈ 14 % |
| How transmission is improved | Anti-reflective microstructures on the fiber end faces (SMART treatment, made during cable assembly) | Coupling optics with the optimum f-number (NA) |
| Transmission of CO₂ laser radiation through a 1 m high-power cable 1 | Up to 85–90 % (900 µm core) | Up to 85–90 % (1000 µm bore) |
| Output NA / full divergence angle | Effective NA = 0.30 / 35° 2 | Effective NA ≈ 0.05 (follows the input NA) / 5.7° |
| Maximum cable length | 15 m | 3 m (manufacturing limit) |
| Recommended maximum CW power 3 | 35 W | 30 W |
| Additional features | - Improved transmittance at 8–17 µm
- Excellent long-term stability with minimal aging
- Non-hygroscopic and non-toxic
| - Hollow core
- No Fresnel reflection at the end faces
- Double polymer coating for high flexibility
|
| Recommended for | - Stable power delivery while the cable is bent
- Vacuum feedthroughs
- Systems with high NA or high beam divergence
| - Low-divergence laser beam delivery
- Gas spectroscopy with a long optical path through the waveguide
|
1 Measured at 10.6 µm with a launch NA of ≈ 0.05, for a 900 µm core PIR fiber with SMART treatment or a 1000 µm bore hollow waveguide.
2 Depends on cable length. With a low input NA (about 0.03–0.05), the PIR output NA can be smaller, about 0.10–0.20.
3 For a CO₂ laser (10.6 µm) with a launch NA of ≈ 0.05, for a 900 µm core PIR fiber with SMART treatment or a 1000 µm bore hollow waveguide. The recommended maximum CW power depends on the core or bore diameter; see the datasheets for each size.
What the differences mean in practice
Bending: PIR keeps transmission stable, HWG loses more
Bending is the biggest practical difference. Wound into one full loop of 40 cm diameter, a PIR cable loses about 1.5 % of its transmission, while a hollow waveguide loses about 14 %, roughly nine times more. PIR is therefore the better choice for cables in moving systems, and wherever the cable has to be routed around obstacles.
Output beam divergence: HWG delivers a narrower beam
A hollow waveguide preserves the low NA of the input beam: with a launch NA of about 0.05, the output full divergence is about 5.7°. A PIR fiber outputs an effective NA of 0.30, or about 35° full divergence (0.10–0.20 with a low input NA). When a tightly collimated or easily refocused beam matters, a hollow waveguide is the better choice.
Coupling tolerance and end-face losses
PIR fibers tolerate misalignment, high input NA and high beam divergence. Their high refractive index would cause Fresnel reflection at the end faces, which the SMART anti-reflective treatment suppresses. Hollow waveguides have no end-face reflection at all, but reach their best transmission and beam quality only with coupling optics of the optimum f-number.
Length, spectral range and environment
PIR cables are available up to 15 m long, hollow waveguides up to 3 m. PIR fibers also transmit well across 8–17 µm, are non-hygroscopic and non-toxic, age very little, and can pass through vacuum feedthroughs. The hollow core of an HWG can instead carry gas or air flow, or serve as a long optical path for gas spectroscopy.
Which technology should I choose?
Rule of thumb: if the cable must bend, move or be long, choose PIR; if the output beam must have low divergence and the cable can stay straight, choose HWG.
Choose a PIR fiber when:
- stable transmission under bending is important;
- the cable must be bent or routed through a moving system where bending losses are undesirable;
- longer cable lengths are required (up to 15 m);
- high input NA or high beam divergence must be accommodated;
- a vacuum feedthrough or broadband IR transmission (8–17 µm) is required.
Choose a hollow waveguide (HWG) when:
- low output divergence is the primary requirement;
- bending losses can be tolerated;
- the cable can remain relatively straight (up to 3 m);
- gas or air flow needs to be delivered through the cable;
- a long optical path through the hollow waveguide is needed, for example for gas spectroscopy.
Download Technical Note 007 (PDF)
Related products and datasheets
Not sure which option fits your laser system? Contact the art photonics team at sales@artphotonics.com for advice on fiber type, core or bore size, connectors and coupling optics.
