Dichroic Mirrors
Author: the photonics expert Dr. Rüdiger Paschotta
Definition: mirrors with significantly different reflection or transmission properties at two different wavelengths
Alternative terms: dual-wavelength mirrors, dual-band mirrors, dichroic reflectors
More general term: dielectric mirrors
DOI: 10.61835/1un Cite the article: BibTex plain textHTML Link to this page LinkedIn
A dichroic mirror (or dual-band mirror, dual-wavelength mirror, dichroic reflector) is a mirror with significantly different reflection or transmission properties at two different wavelengths – actually meaning two wavelength regions of some often not so large width. The specifications often refer to frequently used laser lines, so that dichroic mirrors are often belonging to the category laser line optics.
There are also trichroic mirrors, having defined optical properties at three different wavelengths.
Some dichroic reflectors are used for broadband applications, e.g. for reflecting only ultraviolet light to some application but not so much infrared light which could lead to unwanted heating of the irradiated objects. Similar broadband devices are called hot mirrors or cold mirrors, depending on whether they reflect or suppress heat radiation.
The dichroic property relates to one of two possible meanings of the term dichroism.
Dichroic mirrors are applied for different purposes. Some examples:
- In a diode-pumped laser, a dichroic short-pass mirror in the resonator, placed next to the laser crystal, may be used for injecting pump light, while the circulating laser light is reflected to nearly 100%.
- In a laser with intracavity frequency doubling, a dichroic end mirror may couple out the harmonic light while fully reflecting the pump wave.
- In the case of external frequency doubling, a dichroic mirror may be used as harmonic separator (see Figure 1), i.e., as a kind of wavelength-dependent beam splitter.
- In laser microscopy (fluorescence microscopy), a dichroic mirror can be used for separating the fluorescence light (containing the image information) from the pump light.
- A similar situation occurs in various methods of spectroscopy, e.g. Raman spectroscopy.
Most dichroic mirrors are dielectric mirrors, but there are also crystalline mirrors where the multilayer structure consists of semiconductor materials. In both cases, the operation principle is that of a multilayer interference coating.
Designing Dichroic Mirrors
For designing mirror coatings, a flexible simulation and design software is indispensable. It must not only be able to calculate all relevant optical properties for a given design, but assist you in finding a suitable design for achieving given target properties. The RP Coating software is an ideal tool for such work, as it is particularly flexible. For example, you can start with a known type of mirror design, described with few parameters, and optimize those very efficiently – if necessary, followed by a local optimization of all layer thickness values.
Short-pass and Long-pass Mirrors
In electronics, the terms low-pass and high-pass filter are common, where “low” and “high” refers to the frequency. In optics, where it is more common to refer to wavelengths, one uses the terms short-pass and long-pass mirror. Here, a short-pass mirror (or shortpass mirror) is one which has a high transmittance at short wavelengths and high reflectance at longer wavelengths; it could also be called a high-pass filter (referring to optical frequencies).
It can be challenging to make mirrors such that the wavelength with high transmittance and the wavelength with high reflectance are close together, as e.g. in Figure 2. They need more sophisticated designs and often also a higher precision of coating fabrication.
Fabrication of Dielectric Mirrors
Most dichroic mirrors are fabricated as dielectric mirrors, e.g. with electron beam deposition, ion beam sputtering (IBS) or ion-assisted deposition (IAD). Semiconductor-based dichroic mirrors are fabricated with epitaxial techniques such as MOCVD or MBE.
Depending on the case, the design of the required layer structure may be possible based on analytical considerations, possibly followed by a numerical optimization, or entirely on numerical optimization, e.g. with a Monte-Carlo method. In many cases, the design involves a compromise between the obtained optical properties, the required number of layers, and the required growth precision.
For any dielectric mirror, the reflection spectrum (reflectance vs. wavelength) depends on the angle of incidence and (for non-normal incidence) also on the polarization of the input light. Only to a limited extent, mirror designs can be made such that the desired dichroic properties are achieved over some range of input angles.
As a dichroic mirror has to be transparent for at least one wavelength of interest, the quality (e.g. transmission losses) of the substrate material and possible reflections from the back side need to be considered. An anti-reflection coating on the backside can help to reduce such a reflection, and a slight wedge form of the substrate can often eliminate the effects of residual reflection.
Alternative Approach: Using Polarization
In situations where the two relevant wavelengths of two light beams are rather close, it may be difficult to achieve e.g. high transmissivity for one and high reflectivity for the other. It may then be easier to work based on polarization, if non-normal beam incidence can be used.
More to Learn
Encyclopedia articles:
Suppliers
The RP Photonics Buyer's Guide contains 76 suppliers for dichroic mirrors. Among them:
Knight Optical
Providing unparalleled transmission, reflection and absorption, our dichroic filters (offered as custom-made optics or from our wide range of stock) feature clear transitions between transmitted and reflected wavelengths.
Available in longpass, shortpass (which can both serve as hot and cold mirrors), bandpass, bandblocking and colour correction filters, as well as filter sets, our collection of dichroic filters offers minimal absorbance characteristics.
Ecoptik
Ecoptik offers dielectric laser mirrors including dichroic versions for various applications, covering wavelength ranges from UV to IR. There is a wide choice of round, elliptical or custom shapes as well as flat, spherical and cylindrical surfaces.
LASEROPTIK
LASEROPTIK offers different types of multiline coatings for applications like nonlinear frequency conversion (harmonic separators), OPOs and others.
EKSMA OPTICS
EKSMA Optics offers a wide choice of dichroic mirrors and IBS coated high transmission dichroic mirrors for different laser wavelengths.
Avantier
A dichroic mirror, also down as a dual-band mirror, dual-wavelength mirror, or dichroic reflector, is a specialized mirror which has different optical properties at two different wavelengths.
Vortex Optical Coatings
Our dichroic beamsplitters tend to operate at 45° angle of incidence in a system and are used to split off 2 bands/wavelengths at right angles. We design and manufacture custom dichroics as well as offering some common solutions in our web shop.
OPTOMAN
OPTOMAN's IBS-coated wavelength separators feature spectral drift-free performance, which is why very sharp edge configurations are feasible. IBS coated dichroic mirrors feature spectral drift-free performance that allows very sharp edge configurations. OPTOMAN can make mirrors with transmittance over 99% and reflectance over 99.9%. Our Dichroic mirrors are optimized for high power laser applications providing the best possible extinction ratio between transmitted and reflected laser beams and feature minimum pulse dispersion.
Standard dichroic mirrors can be found in OPTOSHOP.
Perkins Precision Developments
Perkins Precision Developments (PPD) manufactures high energy laser mirrors, including dichroic and trichroic versions, for both prototype and volume OEM requirements. We apply custom Ion Beam Sputtered (IBS) thin film coatings for visible to near-infrared wavelengths on customer supplied substrates. PPD utilizes IBS coating technology because it is ideal for complex spectral designs, high power lasers and applications where it is critical to minimize losses from absorption and scatter.
UltraFast Innovations
UltraFast Innovations (UFI®) provides a wide range of laser mirrors, including dichroic mirrors e.g., for input and output coupling in ultrafast laser systems. In addition, these dichroic mirrors can be used for building a so-called wave synthesizer. Based on that technology, we have developed the first commercial light field synthesizer.
Artifex Engineering
Artifex Engineering offers custom dichroic mirrors tailored to your requirements. Dielectric coatings are particularly developed for the respective application and enable precise wavelength ranges, so that individual wavelengths or selected ranges can be specified. Our mirrors can be manufactured in almost any shape: round, rectangular, elliptical, or any other shape including cut-outs and drill holes. Holes may also be drilled at an angle to the surface normal. Visit our product page for more information. We look forward to your inquiry.
Shanghai Optics
Shanghai Optics manufactures a wide range of optical filters or mirrors for engineering, scientific, and photographic applications. Our hard coated optical filters are dichroic filters/mirrors. They are composed of a single substrate with a dense hard coating. These hard sputtered filters are expensive to produce, but they have excellent performance and are ideal for applications where precision is of paramount importance. We use cutting-edge IBS technology to give our dichroic filters uniform dielectric coatings, and no adhesives whatever are involved in manufacture. This ensures a quality product with a very long filter life. Since they have only one substrate covered with a thin film, Shanghai Optics customized dichroic filters are much thinner than traditional coated filters and offer much higher transmission.
Bibliography
[1] | T. Amotchkina et al.,“Broadband beamsplitter for high intensity laser applications in the infra-red spectral range”, Opt. Express 24 (15), 16752 (2016); https://doi.org/10.1364/OE.24.016752 |
[2] | Design of a dichroic mirror with the RP Coating software |
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2020-07-10
What is the difference between dichroic and dielectric?
The author's answer:
The term dichroic denotes the function – treating different wavelengths differently – while dielectric states what materials are used for a mirror.