In photonics since 1999. Benelux technical support, answer within one working day.
Metallic or dielectric: choosing a laser mirror
A metallic mirror reflects broadly at around 96 to 98 per cent and tolerates being cleaned. A dielectric stack reflects above 99,5 per cent but only over a designed band and angle range. For a single laser line, use dielectric. For white light or several wavelengths at once, use metal.
THE SHORT ANSWER
A metallic mirror reflects broadly at around 96 to 98 per cent and tolerates being cleaned. A dielectric stack reflects above 99,5 per cent but only over a designed band and angle range. For a single laser line, use dielectric. For white light or several wavelengths at once, use metal.
| Dielectric reflectance | Above 99,5 % |
|---|---|
| Dielectric band | Designed, narrow |
| Metallic reflectance | 96-98 %, broadband |
| Typical substrate | Fused silica |
| Angle of incidence | Specified, commonly 0-45 degrees |
| Damage threshold | Higher for dielectric, per specification |
The choice is between bandwidth and efficiency, and you cannot have both.
Dielectric stacks
A dielectric high reflector is dozens of quarter-wave layers of alternating index. Reflections from every interface add in phase at the design wavelength, which is how reflectance above 99,5 per cent is reached without absorbing anything. Because the mechanism is interference, it works only where the phase relationship holds: inside the designed band, at the designed angle. Tilt it and the band moves.
Metallic coatings
Aluminium, silver or gold reflect by conduction, not interference, so the response is broad and flat and indifferent to angle. The cost is absorption. Two to four per cent of your beam becomes heat in the coating, which for a low-power alignment path is irrelevant and for a kilowatt beam is a destroyed mirror.
The specification that matters
Give the wavelength, the angle of incidence, the polarisation and the power density, including pulse length if the source is pulsed. A mirror quoted at 99,5 per cent for 633 nm at normal incidence tells you nothing useful about its behaviour at 45 degrees in your beam. That is the specification most often left out, and the most common reason an expensive mirror underperforms.
When to use it
- One known laser line and a need to keep every photon: dielectric
- Broadband, white-light or multi-wavelength paths: metallic
- High power density where absorption would distort the substrate: dielectric
- Alignment and pickoff duty where a few per cent loss does not matter: metallic
What to watch out for
- A dielectric coating designed for normal incidence shifts its band when you tilt it; specify the angle you will actually use
- Coating damage thresholds are quoted for a pulse length and wavelength: yours must match the quote
- Metallic coatings scratch easily, and a protected coating still is not a hard surface
- S and P polarisation do not reflect identically off a dielectric stack at 45 degrees
Still deciding?
Tell us the wavelength, the aperture and what you are measuring. We answer with a part number, a price and a lead time, usually the same working day.