Fabry-Perot etalons
A Fabry-Perot etalon is an interferometer of the simplest
form, but it is that essential simplicity that makes it so useful in a wide
variety of optical applications.
Basically there are two types of etalon – planar and confocal. In both cases two matched reflecting surfaces face each other. In the planar etalon the two reflectors must be very flat and parallel. In the confocal etalon the reflectors are concave and of spherical form: the separation is such that the focal points coincide. When a light beam is incident, both types act as narrowband filters with a series of transmission maximums that are uniformly spaced when expressed in frequency units. Both types can be manufactured in fixed or scanning forms. In scanning etalons one reflector is moved rapidly backwards and forwards relative to the other, fixed reflector. This has the effect of scanning a range of frequencies in the incident beam. Fixed and scanning etalons of both types are available commercially but, in terms of units sold, fixed planar etalons constitute by far the most important variety. SLS Optics manufactures only this type of etalon.
The principal mathematical expressions that describe the characteristics of fixed planar etalons are shown at the foot of this section. The main features of interest to potential users are the free spectral range (FSR) and the finesse (F). The free spectral range is the spacing between transmission maximums, and this is a function only of the optical path length between the reflectors. As the path length is increased the FSR becomes smaller, and vice-versa. The finesse is a measure of the sharpness of the transmission peaks, and is defined as the ratio of FSR to transmission peak width at half-height (often referred to as “full width at half maximum” or FWHM). For perfectly flat and parallel reflectors, and an incident beam that is perfectly collimated and at normal incidence, etalon finesse is a function of mirror reflectivity - the higher the mirror reflectivity, the higher the finesse (defined as reflectivity finesse or Fr). If the mirrors are loss-less the etalon transmission maximums reach 100%. In practice, though, the reflectors are not perfectly flat and parallel, they are not loss-less, and the incident beam is not perfectly collimated and at normal incidence. All of these factors degrade the finesse, so that what is observed in practice is a smaller, effective finesse (Fe). At the same time the peak transmission of the etalon is reduced from 100%. Nevertheless, the peak transmission of a practical, fixed planar etalon, and its potential resolving power, make it a very powerful device compared with other filters. These characteristics open up a wide range of applications in optical systems in manufacturing industry and in scientific research.
SLS Optics has always recognized the need to employ the most sophisticated
manufacturing techniques and testing procedures in the production of etalons.
Not all applications require high finesse and therefore minimal defects.
But the best production processes and testing protocols will always enhance
customer confidence, and we are finding increasingly that customers wish
to push etalon performance to higher levels. This is well illustrated by
the fact that most of our production of etalons is now for use in the deep
ultraviolet region of the spectrum, where flatness and parallelism defects
become most critical and absorption in coatings, if not addressed, can have
catastrophic consequences for performance. For SLS Optics this means an
unending quest for ever-better quality in materials and manufacturing techniques.
This has spin-offs in terms of performance assurance and reliability in
less demanding etalon applications. SLS Optics probably produces more high-quality
planar etalons than any other manufacturer in the world, and certainly produces
a wider variety.
SLS Optics manufactures fixed planar etalons in two basic forms – air-spaced or solid. Both types can be designed for use at customer-defined wavelengths within the range 190nm to about 2 microns.
In an air-spaced etalon two reflector-coated plates face each other across an air gap formed by optically contacting spacer blocks to the plate peripheries. The back surface of each plate is usually wedged and anti-reflection coated, to avoid the possibility of interference from these surfaces. The plates need to be very flat, and the spacer blocks provide an extremely high level of parallelism. Unlike some manufacturers SLS Optics does not sub-contract any of the production processes. We consider it vital to maintain full control of manufacturing at all stages. An example of the importance of this is the fact that coating the plates changes their surface flatness. This can be allowed for at the plate figuring stage, but only if the coating process is carefully standardised. The coating materials are dielectrics, which are selected for low absorption. The plate material is invariably fused silica, which has the necessary stability for figuring to the required surface flatness and has good transparency in the wavelength region that we address. The spacer blocks are cut from discs of low-thermal-expansion materials (Schott Zerodur® or Corning ULE®) polished and figured to high levels of surface flatness and parallelism. Normally three spacer blocks are used, uniformly spaced around the plate periphery. Where the air-gap is very large we use six spacers, to provide maximum rigidity and therefore stability. (We can also provide air-spaced etalons with hermetically sealed gaps, if customers require etalons free from environmentally induced instability). The completed etalon is supplied resiliently mounted in a cell designed to afford the maximum protection and freedom from stress, which can adversely affect performance.
Structurally, a solid etalon is a simpler device. A fused silica substrate with very flat and parallel faces has carefully matched reflector coatings deposited on the faces. Like the air-spaced etalon, it is supplied resiliently mounted in a cell that provides protection and a certain amount of thermal isolation.
For high-FSR solid etalons, where the mirror spacer is too thin to be self-supporting, it is possible to make the entire etalon using dielectric coating techniques. On a suitable fused silica substrate, the first reflector coating is deposited, followed by the silica spacer and the second reflector. Such deposited etalons are very robust.
All of these fixed planar etalons can be supplied in various standard sizes, or alternatively we can provide custom designs to suit specific customer requirements. Naturally, customers will see a price benefit if they choose a standard size. Potential customers should also note that SLS Optics carries substantial stocks of standard air-spaced etalon plates and spacers: this means that finished etalons can be delivered quickly if the customer’s FSR requirement is modestly flexible. If a customer requires a very precise FSR we can provide this: it involves modelling the effects of certain coating parameters that are insignificant in normal circumstances. But please note that such modelling takes on a much more important role for high-FSR (small mirror spacing etalons). SLS Optics has extensive software facilities for calculating the theoretical performance of planar etalons in customer-specified conditions of use. If you have an appropriate requirement please let us know.
We also custom-manufacture devices, such as multiple beam splitters, that are not strictly etalons but have to be manufactured to etalon standards. If you have such a requirement please contact us.
Etalon definitions
| Free Spectral Range ( FSR ) | = | 1 | in wavenumbers |
| 2nd | |||
| = | c | in frequency | |
| 2nd | |||
| = | λ2 | in wavelength | |
| 2nd | |||
| Reflectivity Finesse (F r ) | = | π√R | |
| 1-R | |||
| Minimum Resolvable Bandwidth | = | FSR | |
| or Bandpass | F |
where:
- R = mirror reflectivity (fraction of unity)
- n = refractive index of cavity
- d = distance between mirror surfaces (cavity gap)
- c = speed of light
- λ = wavelength
