group velocity delay compensation plates (original) (raw)

Acronym: GVD compensation plates

Definition: birefringent plates which are used for compensating the group delay between two ultrashort pulses

Alternative terms: group delay compensation plates, time delay compensator plates, GVD compensation plates

Categories: article belongs to category photonic devices photonic devices, article belongs to category light pulses light pulses

Related: group velocity mismatchbirefringencewaveplates

DOI: 10.61835/w4g Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

Function of Group Velocity Delay Compensation Plates

In some ultrafast optical applications, it is necessary to temporally overlap two ultrashort light pulses of different wavelengths that have developed a relative temporal offset. This offset typically arises from group velocity mismatch occurring as the pulses propagate through a dispersive or birefringent medium. A common scenario is the generation of frequency-doubled pulses using a mode-locked laser and a nonlinear crystal: The fundamental and frequency-doubled pulses usually experience different group velocities within the crystal, resulting in their arrival at slightly different times.

To restore temporal overlap — a prerequisite for efficient nonlinear interactions or high-precision time-resolved measurements –, such a delay can be compensated by passing both pulses through a birefringent plate. In this arrangement, each pulse must be polarized along one of the crystal axes. (The required relative polarization is automatically obtained for frequency doubling with type-I phase matching in the nonlinear crystal, but not for type-II phase matching.) Due to the birefringence, the group velocities for the two polarization directions differ significantly, producing a relative group delay, commonly in the range of hundreds of femtoseconds. The polarization-induced group velocity difference is typically much larger than the group velocity difference resulting from ordinary chromatic dispersion, which would typically tend to further increase (rather than decrease) the temporal offset.

Materials often used for group velocity delay compensation plates include calcite (CaCO3) and beta-BBO (β-barium borate), owing to their strong birefringence and high optical quality.

Terminology

The term group velocity delay compensation plates may be considered as not ideal, since what is compensated (modified) is not a group velocity difference but rather just a relative group delay. However, the modified term group delay compensation plates is not commonly used.

The abbreviated term GVD compensation plates is sometimes used. Note that in other contexts, GVD is typically understood to be group velocity dispersion, not group velocity delay.

Another possible term is time delay compensation plates or just time delay compensators.

Technical Details

Anti-reflection Coatings

To minimize parasitic reflections and related transmission losses, group velocity delay compensation plates should be equipped with anti-reflection coatings working at both relevant wavelengths — for instance, 800 nm for the fundamental pulse and 400 nm for the second harmonic. High-performance dual-wavelength AR coatings are commercially available, though achieving low reflectivity across a broad spectral range (as required for particularly short femtosecond pulses) can be technologically challenging.

It is worth noting that while both waveplates and delay compensation plates are constructed from birefringent materials, conventional waveplates generally lack dual-wavelength AR coatings and are not optimized in thickness to produce a precise group delay.

Tuning of Time Delay

For normal incidence and fixed wavelengths, a compensation plate introduces a fixed relative delay, determined by the material's group velocity mismatch and the plate thickness. Fine adjustment of the delay is possible by slightly tilting the plate with respect to the beam direction, which changes the effective path length and hence the accumulated group delay.

The tuning range of this technique can be limited by different factors:

When designing or selecting a delay compensation plate for an experiment, these constraints must be balanced against the required delay range and the characteristics of the ultrashort pulses being used.

Alternative Techniques

There are other techniques for compensating GVM effects. For example, one can use suitably designed chirped mirrors or chirped fiber Bragg gratings for obtaining different group delays for pulses at different wavelengths. When the pulses are spatially separated, one can apply a tunable optical delay line for one of them.

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 a group velocity delay compensation plate?

It is an optical component made from a birefringent material, designed to correct a relative temporal offset between two ultrashort light pulses, which typically have different wavelengths.

How does this compensation plate work?

It utilizes birefringence, where the group velocity depends on the light's polarization. By aligning each pulse's polarization with one of the crystal's axes, a differential group delay is introduced that can cancel out a pre-existing delay between the pulses.

What is a typical application for these plates?

A common use is after frequency doubling in a nonlinear crystal to restore the temporal overlap between the fundamental and the generated second-harmonic pulses, which is essential for further nonlinear processes or time-resolved measurements.

How can the delay be adjusted?

Fine adjustment of the relative group delay is possible by slightly tilting the plate with respect to the beam. This changes the effective path length within the material and thus the accumulated delay.

What materials are used for delay compensation plates?

Materials with strong birefringence and high optical quality, such as calcite (CaCO3) and beta-BBO (β-barium borate), are frequently used.

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