What is an Optical Mirror?
Mirrors utilize reflection to redirect, focus, and collect light. Optical mirrors consist of metallic or dielectric films deposited directly on a substrate such as glass, differing from common mirrors, which are coated on the back surface of the glass. Optical Mirrors are designed that they reflect light for a wide range of applications like illumination, imaging, etc. Optical mirrors with a metallic coating have high reflectivity over the widest spectral region, whereas mirrors with a broadband dielectric coating have a narrower spectral range of operation; the average reflectivity throughout the specified region is greater than 99%. Hot, cold, backside polished, ultrafast, D-shaped, elliptical, parabolic, concave, crystalline, and laser line dielectric-coated optical mirrors are available for more specialized applications. Dielectric materials can store charges, commonly used for energy storage in capacitors and to construct radio frequency transmission lines. The reflective surface of an optical mirror may be subject to environmental conditions, which means that durability and damage resistance must also be considered when choosing a mirror as well as how well it reflects light at the wavelength of interest. Therefore, the physical concept of reflection becomes important and is attributed to the mirror's optical component.
Reflection
Generally, when light reaches a planar interface between two media as shown in the above figure, a portion of it is reflected into the original incident medium and a portion is transmitted and refracted into the second medium. Absorption of the light in either medium is also possible, but non-absorbing media will be assumed here. The direction of the reflected light is governed by two laws. First, the incident ray reflected ray, and the normal to the interface must lie in the same plane. In this plane of incidence, the angle of incidence (θi) is always equal to the angle of reflection (θrfl). Reflection can occur from smooth surfaces such as those found on mirrors or from rough, uneven surfaces called diffuse reflection or scattering. Although both obey the same laws of reflection, specular reflection leads to rays that reflect as a group at the same angle, whereas diffuse reflection occurs at different angles off randomly oriented surfaces. This enables specular reflection to perform the useful operations of redirecting light.
The fraction of the incident light that is either reflected or transmitted at the interface is described by the Fresnel equations and depends on the angle of incidence as well as the index of refraction of the incident (n1) and refracting (n2) media. The fraction of the incident power reflected from the interface is called the reflectance or reflectivity (R), while the fraction refracted in the second medium is the transmittance or transmissivity (T). By assuming that both media are non-absorptive, the sum of R and T must be unity, thus allowing knowledge of one to provide information about the other. The Fresnel equations are greatly simplified for light at normal incidence, i.e., θi = 0, a situation of significant practical interest. At normal incidence, angular and polarization dependencies are removed from the formula for R (recall that T is complementary), leaving only the dependence on the indices of refraction:
Reflectivity (R) is the fraction of the incident power reflected from the interface.
The index of refraction is a complex value with both real, associated with refraction, and imaginary, related to the transition absorption cross-section components. Furthermore, there is a wavelength dependence associated with the index of refraction. Consequently, R is highly dependent on the materials making up either side of the interface, as discussed in the mirror characteristics section below.
Mirrors made up of planar surfaces, such as that shown in the figure above, are important components for directing light through the proper path in an optical system. Such mirrors can be combined to form optical components known as retroreflectors or corner cubes. These components consist of three mirror surfaces all perpendicular to one another. Such a geometry enables 180 degrees of reflection of the light, regardless of incidence angle, and therefore requires very little alignment compared to a single flat mirror. In addition to stationary mirrors, rapid redirection can be achieved by utilizing rotating planar mirror systems such as those found in scanners or on a smaller scale with micromirrors, which are used for switching in telecommunications and displays. Curved mirror surfaces also called concave reflectors can be exploited for the collecting, focusing, and imaging of light as illustrated in the figure below. These mirrors possess an advantage over lenses in that they perform satisfactorily across a broad-wavelength range without requiring refocusing. The reason for this is that reflection occurs at the surface of these optics, rather than passing through the optic as is the case with a lens, and so the dispersion of the index of refraction does not come into play. Simple spherical reflectors can be used to collect radiation from a source at the focal point located at half of the radius of curvature of the mirror and reflect it as a collimated beam parallel to the axis. Since spherical mirrors possess spherical aberration, a parabolic curved surface can be used instead to either collimate light from a focal point or focus light from a collimated beam as shown in the figure below. Ellipsoidal surfaces can focus light from one focal point to another as shown in the figure below.
The figure above shows concave reflectors with different surface shapes allowing for light collection and focusing. A paraboloidal reflector reflects light from the focus into a collimated beam (left). An off-axis paraboloidal reflector refocuses a collimated beam off the mechanical axis (middle). Ellipsoidal reflectors reflect light from one focus to a second focus, usually external (right).
Mirror Characteristics
Selecting the proper mirror for an application requires consideration of several factors, including reflectivity, laser damage resistance, coating durability, thermal expansion of the substrate, wavefront distortion, scattered light, and cost. These mirror characteristics depend on the optical coating, the substrate, and the surface quality. The optical coating is the most critical component of a mirror as it dictates its reflectivity and durability. Optical mirror coatings are typically made up of either metallic or dielectric materials. A common situation for mirror applications is when light is incident from the air (n1 = 1) onto the optical coating material and so the reflectivity given by the above equation is dictated solely by the material’s index of refraction (n2). By their conductivity, metals have a complex index of refraction with a large imaginary part over a very wide wavelength range. This gives rise to a large reflectivity that is relatively insensitive to wavelength, which gives metallic mirrors their shiny appearance. Metallic coatings are usually made of silver, gold, or aluminum and the resulting mirrors can be used over a very broad spectral range as shown in the figure below. Metallic coatings are relatively soft, making them susceptible to damage, and special care must be taken when cleaning. Mirrors with dielectric coatings are more durable, easier to clean, and more resistant to laser damage. However, as a consequence of their dispersive and predominantly real indices of refraction, dielectric mirrors have a narrower spectral reflectivity and are typically used in the VIS and NIR spectral region. When compared with metallic mirrors, a dielectric mirror can offer higher reflectivity over certain spectral ranges and can offer a tailored spectral response as shown in the figure below.
The above Reflection spectra of silver metallic mirrors show broadband reflectivity (left) and dielectric laser-line mirrors show two narrow reflection bands (right).
Most substrates upon which the coatings are deposited are dielectric materials and these substrates control the thermal expansion and transmission properties of mirrors. Certain materials have lower thermal expansion coefficients, e.g., PYREX® borosilicate glass or fused silica, than others, e.g. N-BK7 optical glass, but the cost of the material and ease of polishing must also be considered. If light transmitted through the substrate is not required, the backside of the substrate is typically ground to prevent inadvertent transmissions. However, for transmissive mirrors, a substrate material with a homogenous index of refraction is important, e.g. fused silica.
Before depositing the optical coating, the substrate’s surface must be ground and polished to the proper shape either planar or curved. The surface quality and flatness determine the fidelity of the mirror performance with the targeted application dictating the requirements for these parameters. Surface flatness is often specified in wavelengths, e.g. λ/10, over the entire usable area of the mirror. When preservation of the wavefront is critical, a λ/10 to λ/20 mirror should be selected, while less demanding applications can tolerate a λ/2 to λ/5 mirror with the associated reduction in cost. Surface quality is usually dictated by the severity of random localized defects on the surface. These are often quantified in terms of a “scratch and dig” specification, e.g. 20-10, with a lower value indicating improved quality and therefore lower scattering. For high-precision surfaces, such as those found within the cavity of a laser, a scratch-dig specification of 10-5 may be required since it would yield very little scattered light. Surface polishing tolerances in terms of irregularity, surface roughness, and cosmetic imperfections are verified using state-of-the-art metrology equipment. These same parameters and procedures are used to assess the quality and flatness of other optical components such as lenses or windows.
Optical mirrors are used to redirect light in various applications including spectroscopy, material processing, medical, beam guiding, and laser cavity, or alignment applications in UV, VIS, and IR spectral regions. Wavelength Opto-Electronic offers broadband, narrowband, metal, medical laser, ultrafast, scanning, phase retarder, cavity, and CO₂ mirrors of flat, wedged, or curved surfaces designed for 0° or 45° of the angle of incidence with robust dielectric, reflective and metal coatings. Therefore based on the above properties, mirrors are of the following types:
Flat Mirrors
Flat mirrors are made of a single pane of glass with no curve. This makes them less expensive to produce and install, but also provides less reflection and clarity. For this reason, they are most often used in commercial or industrial settings where uniformity is key. Flat mirrors are used for different applications ranging from interferometry, beam steering or folding, or as optical components within imaging systems.
As seen in the above figure for the ray starting from point A and travelling in a horizontal direction towards point E, the angle of incidence is 0, and hence it retraces its path. Similarly, the ray starting from A and travelling towards point C follows the law of reflection, according to which when the light rays fall on the smooth surface, the angle of reflection is equal to the angle of incidence, also the incident ray, the reflected ray, and the normal to the surface all lie in the same plane. When these rays are produced backwards, they appear to meet at a point E. Now this image is known as a virtual image. In a real image, the rays of light actually meet after reflection, while in a virtual image, it appears to meet but do not actually meet. A real image can be obtained on screen but not a virtual image.
Copper Mirrors
Copper mirrors reflect more light than other materials, making them ideal for use in lighting and other applications where brightness is important. Additionally, copper mirrors are less likely to corrode or tarnish over time, making them a more durable option for long-term use. If one is looking for some superior mechanical stability, copper mirrors are best.
IR Mirrors
IR Mirrors are Optical Mirrors designed for the reflectance of the IR spectrum (600 - 4000 cm-1). IR Mirrors are used in a variety of applications like beam steering. Infrared mirrors, also known as IR Mirrors are known to benefit in an array of applications such as beam steering.
Laser Line Mirrors
Laser line mirrors are designed with reflectivity R > 99.5% for Nd YAG laser applications. The laser line mirrors are available with us at fundamental line 1064nm with their harmonics 532nm, 355nm, and 266nm for a 45° angle of incidence. These mirrors have a higher damage threshold, and high optical quality, and are suitable for beam-guiding applications.
Laser line mirrors are fabricated with specialized coatings that offer high damage thresholds, making them well-suited for use with a range of high-powered CW or pulsed laser sources. Laser line mirrors are designed to withstand the high-intensity beams typically produced by Nd: YAG, Ar-Ion, Kr-Ion, and CO2 lasers.
Broadband Dielectric Mirrors
Broadband dielectric mirrors have excellent reflection over multiple spectral ranges. Broadband Metallic Mirror has a thin layer of a metal coating to provide an abrasion-resistant surface while maintaining performance. These are best suited for applications that required broadband reflection. Due to the oxidation property of Silver, metallic mirrors are much more suitable in a low-humid environment. Wavelength Opto-Electronic offers Metal Mirrors in two protective coatings (Aluminium and Silver) with high surface quality and flatness.
Broadband Mirrors
Broadband Mirror is available in three different spectral ranges Visible, NIR and IR regions. Its primary purpose is to redirect light to make optical systems compact in size. These are utilized in beam guiding with broadband reflection demanding applications in Visible, NIR, and IR ranges. Wavelength Opto-Electronic offers Broadband Mirrors with a high degree of surface flatness, low scattering, and high reflectivity.
Cavity Mirrors
The Rear Mirror, known for very high reflectivity (>99.7%), is the crucial optical component in the laser resonator. The Output Coupler is a partially reflective mirror to extract a portion of the laser beam from the laser resonator. They often require a slight wedge to prevent interference from multiple reflections inside the component. Rear Mirror and the output coupler are collectively known as Cavity Mirror.
CO2 Mirrors
Reflective laser optics mirrors must have low reflection losses, high optical quality, and good resistance against extreme optical intensity.
Off-Axis Parabolic Mirrors
Parabolic Mirrors are used to direct and focus all the incoming collimated light into a single focal point due to the surface shape parabola. It’s applicable when there is a need to either collimate or focus the beam and when the beam divergence is small. There are useful for telescopes, retroreflectors, fiber collimators, spectroscopy, lighting, and solar applications.
Off-axis parabolic mirrors are available with one of four coatings: UV-enhanced aluminum (250 - 450 nm), protected aluminum (450 nm - 20 µm), protected silver (450 nm - 20 µm), and protected or unprotected gold (800 nm - 20 µm). They are designed to focus or collimate broadband light.
Phase Retarders
90˚ phase retarders are used to convert linear polarization to circular polarization. Polarization (also polarization) is a property applied to transverse waves that specify the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of the wave. With 90˚ phase retarders used in cutting, the material can be removed uniformly regardless of cutting directions. Reflective Phase Retarders are used as beam-bending mirrors external to the laser cavity to establish and maintain circular polarization. This requirement is particularly vital for laser material processing applications where cut or scribed edge quality, and weld penetration, are critical to the consistency and precision of the final part. Zero-degree phase retarders maintain control over the circularly polarized beam.
Scanning Mirrors
Scanning mirror is lightweight rectangular mirrors used for high-speed two-axis laser scanning systems. The dimensions for each mirror are calculated accordingly with the laser beam size. The mirror is designed with a high reflectivity of 99.5% or above. The scanning mirror is generally mounted onto a galvanometer for scanning purposes. For two-axis scan mirrors, commonly the Y mirror has a bigger size compared to the X mirror. This is because the X mirror is used to scan the Y mirror rather than the object directly.
Ultrafast Mirrors
Ultrafast Mirror has high laser-induced damage thresholds, greater reflectance, and low Group Delay Dispersion effects on the ultrashort pulses. The change in the group delay to the frequency at center frequency ω0 is known as group delay dispersion (abbreviated as GDD) and is generally given in fs2. As a pulse travels through a material or coating and is reflected, the components are delayed depending on their frequency. These are utilized in the beam steering applications with Ti: Sapphire, Yb: YAG, Yb: KGW, and Er: Fiber laser fundamental wavelengths as well as their harmonics. These mirrors are used in various fields including spectroscopy, material processing, and the medical industry. Wavelength Opto-Electronic offers Ultrafast Mirrors in UV, Visible, and NIR regions.
Ultrafast Mirrors for Pulsed Lasers
Ultrafast mirrors are used with femtosecond and picosecond pulsed lasers. Low Group Delay Dispersion (GDD) mirrors are optimized for low dispersion and >99% reflectance when used with Ti: Sapphire, Ytterbium (Yb), Neodymium (Nd), Thulium (Tm), or Holmium (Ho) lasers. The Dual-Band Dielectric Mirror has a specialty coating that maximizes the reflectance of the mirror at both 400 nm and 800 nm. For picosecond Yb lasers, high-power mirrors featuring dielectric coatings with high laser damage threshold values are used. The chirped mirrors are designed specifically to correct for phase distortions that occur when ultrashort pulses travel through an optical system, correcting dispersion. Mirrors with ultrafast-enhanced silver coating are offered in addition to our protected silver mirrors and unprotected gold Plano mirrors.
Plano Metallic Mirrors
High-quality, metal-coated optical mirrors are available for use with light throughout the UV, VIS, and IR spectral regions. Their relatively wide bandwidth and high reflectivity make mirrors with metallic coatings ideal for applications like spectroscopy. We also offer silver-coated mirrors specifically designed for ultrafast applications in the fundamental wavelength range of femtosecond Ti: Sapphire lasers and gold-coated mirrors for CO2 experiments.
Dichroic Beamsplitters
Dichroic beamsplitters offer a splitting ratio that is dependent on the wavelength of the incident light. They are useful for combining/splitting laser beams of different colors.
Hot and Cold Mirrors
Hot and cold mirrors are ideal for use in situations where heat could severely damage an experimental setup. Hot and Cold UV fused silica mirrors offer increased transmission and reflectance, a lower coefficient of thermal expansion, and are wedged to reduce ghosting, a faint secondary reflection is also created from light striking the front of the glass. The coefficient of thermal expansion (CTE) refers to the rate at which a material expands with an increase in temperature.
Concave Mirrors
Spherical concave mirrors with metallic or dielectric coatings that together span the 250 nm - 20 µm spectral range, with focal lengths from 9.5 mm to 1000 mm. Here, mirrors with dielectric coatings offer >99% average reflectance in the 350 - 400 nm, 400 - 750 nm, 750 - 1100 nm, or 1280 - 1600 nm spectral ranges. Backside polished mirror options are also available for the dielectric coating ranges of 400 - 750 nm and 750 - 1100 nm. Metallic coatings are offered in UV-enhanced aluminum (250 - 450 nm), protected aluminum (450 nm - 2 µm), protected silver (450 nm - 2 µm), protected gold (800 nm - 20 µm), and mid-infrared enhanced gold for Herriott cell mirrors (2 - 20 µm).
Right-Angle Prism Mirrors
Hypotenuse-coated right-angle prism mirrors, which are also known as turning mirrors. Each mirror's hypotenuse is coated with either a broadband dielectric, metallic, or laser line coating. A variety of sizes are available, as well as prisms that come pre-mounted in a 16 mm cage, Ø 1/2" lens tube, or 30 mm cage Ø1" lens tube compatible housing. We also offer leg-coated right-angle prism mirrors with dielectric coatings for use in optical delay lines. knife-edge right-angle prism mirrors have metallic coatings and clear apertures that extend across the 90° angle between the coated surfaces. Retroreflecting hollow roof prism mirrors with metallic coatings redirect an incoming beam 180° back toward the source. Where a retroreflector is a device or surface that reflects radiation to its source with minimum scattering.
Retroreflectors
Retroreflectors reflect an image or beam back 180° toward its original direction. Prisms achieve this either through total internal reflections (TIR) or specular reflections, depending on whether the reflective faces are coated. TIR is the optical phenomenon in which waves arriving at the interface from one medium to another are not refracted into the second medium, but completely reflected into the first medium. "Hollow" retroreflector mirrors employ first-surface specular reflections to eliminate dispersion, chromatic aberrations, and material absorption inherent in prisms.
There are a variety of free-space optics, including retroreflector prisms and mirrors (a.k.a. corner cubes), a lateral transfer retroreflectors, hollow roof mirrors, right-angle (a.k.a. porro) prisms, and Dove prisms. Fiber optic retroreflector patch cables reflect light input through the connector backward through the fiber and can be used to create a fiber interferometer or to build a low-power fiber laser.
Mirror Blanks
Mirror blanks are designed to be used as front-surface mirrors when coated. It offers a very low coefficient of thermal expansion, ideal for applications that are sensitive to thermally induced beam drift. They have a surface quality of 10-5 scratch-dig and a flatness of <λ/10. Scratch-Dig is a qualitative technique for classifying a level of polish for small optics.
