Optical mirrors are among the most commonly used components in optical laboratories. Their quality, performance, and long-term reliability directly impact experimental accuracy and system stability. Selecting the right mirror requires careful consideration of reflectivity, laser damage threshold, coating durability, wavelength range, and environmental stability.
To meet diverse laboratory and industrial requirements, optical mirrors are generally classified into metallic mirrors, dielectric mirrors, and metal–dielectric mirrors.
Metallic Mirrors
Broadband metallic-coated mirrors are widely used as general-purpose mirrors due to their extremely broad spectral coverage, typically spanning 200 nm to 12 µm. They are:
- Largely insensitive to polarization
- Weakly dependent on angle of incidence
- Capable of providing a constant phase shift, making them suitable for ultrashort-pulse and femtosecond laser applications
However, metallic coatings are relatively soft and require careful handling and cleaning.
Silver-Coated Optical Mirrors
- Highest reflectivity in the visible (VIS) and near-infrared (NIR) regions
- Naturally soft and chemically reactive without protection
- Protective dielectric layers are mandatory to ensure stability
Using magnetron sputtering, dense protective coatings can be applied to silver mirrors, significantly improving environmental resistance. With proper protection, silver mirrors can achieve operational lifetimes exceeding 10 years under normal atmospheric conditions.
Gold-Coated Optical Mirrors
- Reflectivity comparable to silver in the NIR
- Chemically stable, but mechanically soft
- Protective coatings are required to allow safe cleaning
In many broadband applications, protected silver mirrors are preferred over gold due to:
- Wider usable wavelength range
- Slightly higher reflectivity
- More cost-effective pricing
Aluminum-Coated Optical Mirrors
- Relatively high and uniform reflectance across the VIS and NIR
- Best-performing metal in the UV region
- Native oxide layer absorbs in the deep UV
Because aluminum is soft, a protective overcoat is strongly recommended to improve durability and handling robustness.
Dielectric Mirrors
Dielectric mirrors provide extremely high reflectivity—often exceeding 99.9%—over a limited but well-defined spectral bandwidth (typically a few hundred nanometers). Compared to metallic mirrors, dielectric coatings offer:
- Higher laser damage thresholds
- Greater mechanical durability
- Easier cleaning and longer service life
They are commonly used in:
- General laboratory optics
- High-power Nd:YAG laser systems (1064 nm, 532 nm)
- UV and deep-UV applications
Quarter-Wave Stack Design
The most common dielectric mirror design is the quarter-wave stack, consisting of alternating high- and low-refractive-index layers with optical thickness:
n · t = λ / 4
This structure produces constructive interference of reflected waves at each interface, maximizing reflectivity at the design wavelength.
- High refractive index contrast → broader reflection band
- Low refractive index contrast → narrower reflection band
By adjusting the number of layer pairs and introducing non-quarter-wave layers, dielectric mirrors can be engineered for custom reflectivity values ranging from partial reflectors to near-total reflectors.
Metal–Dielectric Mirrors
Metal–dielectric mirrors combine metallic reflective layers with dielectric coatings to enhance performance and stability. In practice, almost all protected metallic mirrors are metal–dielectric systems.
Key Characteristics
- Dielectric layers improve chemical and mechanical durability
- Multilayer dielectric stacks can increase reflectivity beyond that of bare metal
- Offer extremely broadband reflectance with improved environmental resistance
Unlike single dielectric overcoats—which may reduce reflectivity—carefully designed multilayer dielectric stacks can enhance reflectance across targeted spectral regions.
Metal–dielectric reflectors are commonly used in:
- Astronomical telescopes
- High-performance broadband optical systems
- Applications requiring both wide spectral coverage and long-term stability
The graphs above show the reflectance spectra of a protected silver mirror and a metal dielectric silver mirror, both optimized for high reflectivity in the visible spectral range for applications in astronomical telescopes.
Choosing the Right Optical Mirror
When selecting an optical mirror, consider:
- Required wavelength range
- Desired reflectivity and bandwidth
- Laser power and pulse duration
- Environmental exposure and cleaning frequency
Each mirror type offers distinct advantages, and the optimal choice depends on balancing optical performance with durability and cost.