depolarizers (original) (raw)

Definition: optical device which convert polarized light to unpolarized light

Alternative term: pseudo-depolarizers

Category: general optics

• optical elements
  • polarization optics
    * polarizers
    * waveplates
    * Berek compensators
    * Brewster plates
    * depolarizers
    * Cornu depolarizers
    * wedge depolarizers
    * Lyot depolarizers
    * polarization scramblers
    * Faraday rotators
    * optical isolators
    * Faraday circulators
    * Faraday isolators
    * Faraday mirrors
    * Babinet–Soleil compensators
    * fiber polarization controllers
    * (more topics)

Related: polarization of lightunpolarized lightpolarizersdepolarization loss

Opposite term: polarizers

Page views in 12 months: 103

DOI: 10.61835/55f Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

Content quality and neutrality are maintained according to our editorial policy.

📦 For purchasing depolarizers, use the RP Photonics Buyer's Guide — an expert-curated directory for finding all relevant suppliers, which also offers advanced purchasing assistance.

Contents

What are Depolarizers?

Depolarizers are optical devices which can depolarize light, i.e., convert polarized light into unpolarized light. This can be useful for removing undesired polarization effects that affect the performance of optical systems.

Frequently, such devices do not generate truly unpolarized light, but only a kind of pseudo-unpolarized light — which, however, may be good enough for the intended purpose. In practice, it is often sufficient that light behaves as if it were truly unpolarized, even if it does not have the correct statistical properties. It is common to call such devices depolarizers, although strictly speaking they should be called pseudo-depolarizers.

There are also so-called polarization scramblers, which apply time-dependent polarization changes, and come closer to true depolarization.

Applications of Depolarizers

Usually, depolarizers are used to remove or mitigate detrimental effects of light polarization. Some examples:

• Depolarizers are used with imaging spectrometers for making the response polarization-independent.
• For optical amplifiers with polarization-dependent gain, a depolarizer before the amplifier can avoid the problem that the gain will depend on the (possibly variable) polarization of the optical input. Alternatively, one may depolarize the pump light (e.g. of Raman amplifiers) to avoid polarization-dependent gain.

Types of Depolarizers

Cornu Depolarizers

A Cornu depolarizer (a design invented by Marie Alfred Cornu) consists of two wedges of a birefringent crystal — typically quartz — which are optically contacted together, with their optic axes oriented at 90° relative to each other. The fast axes of each wedge are set at 45° with respect to the edges of the device. The endfaces of the double-wedge assembly are parallel and approximately perpendicular to the incident light beam, ensuring that the transmitted beam does not experience significant angular deviation or lateral displacement.

The device functions as a spatially varying birefringent element: Owing to the wedge geometry, the thickness of each part traversed by light varies along one direction. This means that the optical retardance changes continuously as a function of position across the beam. Each wedge acts similarly to a waveplate with locally defined thickness.

As a result, linearly (or otherwise) polarized input light is transformed such that the polarization state at the output is still deterministic, but strongly position-dependent — oscillating rapidly across the beam profile. The oscillation period is determined by the material properties, wedge angle and wavelength. Only at certain positions for a given wavelength, the polarization state is linear; generally, it is elliptical. Even for monochromatic light, spatial integration across the beam effectively averages over all polarization states, resulting in output light that is unpolarized e.g. in the sense that transmission through a linear polarizer is independent of its orientation. For polychromatic light with a sufficiently large optical bandwidth, the local polarization can be “smeared out” in addition.

Note that similar double-wedge devices are available where some construction details can differ from the original Cornu design, e.g. concerning the relative orientations of the optical axes of the two wedges.

A detailed analysis of such depolarizers is quite complicated, considering e.g. that refraction occurs at the interface of the two wedges, where the refractive index changes somewhat due to the birefringence. Therefore, four different waves with slightly different propagation directions contribute to the output.

This operation principle results in a kind of pseudo-depolarization, not involving randomness but a systematic variation of polarization state. As the modulation is in the spatial (rather than temporal) domain, the obtained light is also called spatially unpolarized light.

Note that spatial depolarization is associated with effects on the wavefronts: Plane wavefronts for the ($x$) and ($y$) components of the electric field (for propagation in ($z$) direction) would imply a uniform polarization state across the beam profile, which one does not have here. Therefore, spatial coherence and beam quality are also affected to some extent, and imaging artifacts may occur.

Depolarizers of similar kinds, based on the explained operation principle but often with some deviations in detail, are commercially available.

Wedge Depolarizers

There are wedge depolarizers which use essentially the same operation principle as Cornu depolarizers. Some of those use a second plate which is not birefringent. For example, they combine crystalline quartz with fused silica. Such a device does not depolarize if the input polarization is oriented in the direction of the fast or slow axis of the first plate. Therefore, one indicates the preferred input polarization direction (45° against the axis) on the housing.

There are also wedge depolarizers with two birefringent plates.

As the wedge angle is typically quite small, the period of polarization changes is accordingly longer.

Dual Babinet Compensator Depolarizers

A dual Babinet compensator depolarizer consists of two devices which are similar to Babinet–Soleil compensators, which differ in orientation by 45° and are usually made of quartz. In contrast to ordinary Babinet compensators, each device contains prisms with 90° angle between their axis orientations, resulting in spatially variable polarization changes as for the Cornu polarizer. So the dual Babinet compensator depolarizer can also be seen as a combination of two Cornu polarizers. It generates polarization changes in two dimensions rather than in one dimension only, which can be considered a more thorough pseudo-depolarization.

Lyot Depolarizers

Lyot depolarizers (invented by Bernard Lyot) use two or more waveplates — in that respect similar to Cornu depolarizers, but here not with a variable plate thickness, i.e., not with wedge shapes. Typically, the plate thickness is doubled from plate to plate, and the fast axis orientation is successively rotated by 45°.

With Lyot depolarizers, one fully relies on the finite optical bandwidth of the input light, exploiting the wavelength-dependent polarization changes. The design parameters — in particular, the plate thickness values — must be suitable for the spectral bandwidth of the light to be depolarized: The larger the bandwidth, the thinner the plates can be. Lyot depolarizers are mainly used with broadband light, e.g. from light-emitting diodes or superluminescent sources, less with lasers.

The same operation principle can be realized with fiber optics, using polarization-maintaining fibers as birefringent elements, which can simply be spliced together.

Like Cornu depolarizers, Lyot depolarizers do not randomize polarization in a stochastic sense. However, they are suitable for some applications. In contrast to Cornu depolarizers, the polarization remains uniform over the beam cross-section.

Photonic Metasurfaces

As an emerging technology, photonic metasurfaces can be made which provide spatially variable polarization changes [6], resulting in spatial depolarization, e.g. similar to that of Cornu-type depolarizers. One may also achieve quasi-random spatial polarization changes.

This technology may be particularly interesting for applications where miniaturized devices are needed.

Depolarization Based on Scattering

A more or less complete loss of polarization may occur when light is scattered on a suitable surface, possibly even multiple times. For example, integrating spheres enable polarization-independent measurements in that way. However, that operation principle of depolarization has the clear limitation that the spatial coherence is also completely lost.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is an optical depolarizer?

A depolarizer is an optical device that converts polarized light into unpolarized (or pseudo-unpolarized) light. It is used to remove or mitigate detrimental effects of light polarization in optical systems.

Do depolarizers create truly unpolarized light?

Often, they create pseudo-unpolarized light, which behaves as if it were unpolarized and is sufficient for many applications, even if it lacks the correct statistical properties of truly unpolarized light.

How does a Cornu depolarizer work?

A Cornu depolarizer uses two wedges of a birefringent material to create a spatially varying retardance. This causes the polarization state of transmitted light to vary rapidly and systematically across the beam profile.

What is the working principle of a Lyot depolarizer?

A Lyot depolarizer uses a sequence of waveplates with different thicknesses and axis orientations. It relies on the finite optical bandwidth of the light, causing wavelength-dependent polarization changes that average out to an unpolarized state.

What is the main difference between Cornu and Lyot depolarizers?

Cornu depolarizers produce a spatially varying polarization state and can work with monochromatic light. Lyot depolarizers rely on spectral averaging, require broadband light, and produce a spatially uniform output polarization.

Can depolarizers affect the beam quality?

Yes, depolarizers that create a spatially varying polarization state, such as the Cornu type, can affect the wavefronts. This may reduce the spatial coherence and beam quality of the light.

Suppliers

Bibliography

(Suggest additional literature!)

Questions and Comments from Users

Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.

Please do not enter personal data here. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.

By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.