Features

• Dual Band Coating for 400 nm and 800 nm
• Low Group Velocity Dispersion (GVD)
• 400 nm Stack on Top Maximizes 400 nm Reflectance
• Ø1" Rounds
• Material: N-BK7

Specifications

Our UltraFast Mirror with low Group Velocity Dispersion (GVD) was designed and manufactured for an OEM customer that required a mirror with high reflectivity at 400 nm and 800 nm simultaneously. The mirror was used with a femtosecond pulsed laser in a meteorologic device that measures the thickness of an opaque film.

Custom Coating Capabilities
These mirrors highlight Thorlabs' custom coating abiliities. For OEM customers, Thorlabs is able to provide custom coatings from the UV through the near IR. Please feel free to contact us to discuss your specific needs if our coating capabilities would benefit your application.

GVD Data Download

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Phase vs. Group Velocity:

The commonly used formula v = c / n where c is the speed of light and n is the index of refraction of a material refers to the phase velocity of a wave (i.e. the velocity at which the phase of any one frequency component of the wave propagates). This speed must always be ≤c. However, there is another velocity, group velocity (v_g), which deals with the speed at which the modulation envelope advances. This velocity is often associated with the velocity at which energy or information is conveyed and is given mathematically by

Using the fact that ω = 2πf and ω = kv, it immediately follows that

For a nondispersive media in which v is independent of k, the second term in the equation above vanishes. In contrast, for a nondispersive medium in vacuum, it follows that

It should be noted that the group velocity may be greater than, equal to, or less than the speed of light.

Group Velocity Dispersion:

Qualitatively, the term group velocity dispersion (GVD) is used to refer to the spreading in time of short pulses; since group velocity depends on the index of refraction of a material as shown in the equation above (which in turn depends on the frequency of the light), different frequency components of the pulse will travel at different velocities. Therefore GVD is a measure of the frequency dependence of the group velocity in a medium. This temporal spreading of the pulse is of particular concern in the optical communications industry; a high dispersion will cause the group of pulses comprising a bit stream to merge together, making the information unintelligible. Consequently, GVD greatly limits the length of fiber through which a signal can be sent without regeneration.

Quantitatively, GVD is the derivative of the inverse group velocity with respect to either the angular frequency or the wavelength, which gives

or

respectively.