# Chromatic Dispersion

Author: the photonics expert Dr. Rüdiger Paschotta

Definition: the frequency dependence of the phase velocity in a transparent medium

More specific terms: normal and anomalous dispersion, material dispersion, waveguide dispersion, group delay dispersion, third-order dispersion, principal dispersion

The chromatic dispersion of an optical material is the phenomenon that the phase velocity and group velocity of light propagating in a transparent medium depend on the optical frequency. That dependency results mostly from the interaction of light with electrons of the medium, and is related to absorption in some spectral regions; see the article on Kramers–Kronig relations. A quantitative measure is the group velocity dispersion (as dispersion per unit length) or the group delay dispersion (for a complete optical element).

The attribute “chromatic” is used to distinguish that type of dispersion from other types, which are relevant particularly for optical fibers: intermodal dispersion and polarization mode dispersion.

Chromatic dispersion can also occur from geometrical effects, for example; see below the section on chromatic dispersion of optical components.

## Passive Fiber Optics | single-mode, multimode fibers, polarization, chromatic dispersion, nonlinearities

Part 10: Chromatic Dispersion of Fibers | Group Velocity

We discuss the chromatic dispersion of fibers: their origin, the contribution of waveguide dispersion, and effects of chromatic dispersion, e.g. in telecom applications.

## Mathematical Description of Chromatic Dispersion

### Dispersion Figures from Ernst Abbe

An early measure for the magnitude of chromatic dispersion was the *Abbe number* <$V_\textrm{D}$>, introduced by Ernst Abbe:

The denominator is also called the principal dispersion. The Abbe number depends on the refractive indices at only three different wavelengths:

- 486.1 nm (blue Fraunhofer F line from hydrogen)
- 589.2 nm (orange Fraunhofer D line from sodium)
- 656.3 nm (red Fraunhofer C line from hydrogen)

Large values of the Abbe number indicate *low* chromatic dispersion and vice versa. Such values can be used for the design of achromatic optical elements, but can of course give only a quite rough indication.

### Modern Parameters for Dispersion of a Medium

The modern way of quantifying chromatic dispersion is based on a Taylor expansion of the wavenumber <$k$> (change in spectral phase per unit length) as a function of the angular frequency <$\omega$>. The expansion is made around some center frequency <$\omega_0$>, e.g. the mean angular frequency of some light pulses:

$$k(\omega ) = {k_0} + \frac{{\partial k}}{{\partial \omega }}\left( {\omega - {\omega _0}} \right) + \frac{1}{2}\frac{{{\partial ^2}k}}{{\partial {\omega ^2}}}{\left( {\omega - {\omega _0}} \right)^2} + \frac{1}{6}\frac{{{\partial ^3}k}}{{\partial {\omega ^3}}}{\left( {\omega - {\omega _0}} \right)^3} + ...$$where the terms corresponding to the different orders have the following meaning:

- The zero-order term describes a common phase shift.
- The first-order term contains the inverse group velocity (i.e., the group delay per unit length) and describes an overall time delay without an effect on the pulse shape:

- The second-order (quadratic) term contains the second-order dispersion or group delay dispersion (GDD) per unit length:

- The third-order (cubic) term contains the third-order dispersion (TOD) per unit length:

Second-order dispersion is often specified in units of s^{2}/m. It is the derivative of the inverse group velocity with respect to angular frequency:

As an example, the group delay dispersion of silica is +36 fs^{2}/mm at 800 nm, or −22 fs^{2}/mm at 1500 nm. Zero group delay dispersion is reached close to 1270 nm.

Dispersion of third and higher order is called *higher-order dispersion*. When dealing with very broad optical spectra, one sometimes has to consider dispersion up to the fourth or even fifth and sixth order. Ultimately, the concept of Taylor expansion loses its value in this regime, where many dispersion orders have to be considered. It is therefore often more convenient e.g. in numerical modeling to work directly with a table of frequency-dependent phase changes.

The dispersion of various orders for a medium can most conveniently be calculated if the refractive index is specified with a kind of Sellmeier formula. Tabulated index data are less suitable, since the numerical differentiation is sensitive to noise.

For light propagating in waveguides such as optical fibers, one considers the phase constant <$\beta$> instead of the wavenumber <$k$>. The second-order dispersion, for example, is then given as the second derivative of <$\beta$> with respect to angular frequency. This is often called <$\beta ''$> or <$\beta_2$>.

### Dispersion of an Optical Element

The considerations in the previous section were based on the frequency-dependent wavenumber in a medium, which is the phase change per unit length. The resulting dispersion figures are then also related to dispersion effects per unit length. Frequently, however, one requires the complete dispersion effect of an optical element, such as a block of optical glass with a certain propagation length for light. In such a case, one performs the Taylor expansion as shown above for the total phase delay of the component (rather than for the wavenumber, i.e., the phase delay per unit length). Considering second-order dispersion, one then obtains the total group delay dispersion (in units of seconds squared) rather than the group delay dispersion per unit length, which is called group velocity dispersion.

The chromatic dispersion of an optical component may simply result from the dispersion of its parts (for example, by accumulation over a certain propagation length. In some cases, however, large amounts of dispersion can arise from interference effects and can then not be understood as accumulated effects. For example, a Gires–Tournois interferometer generates dispersion by interference effects in an optical resonator, and not primarily via material dispersion. The same happens in other types of interferometers. The amount of dispersion arising from such effects can be huge when large differences in propagation lengths are involved.

Chromatic dispersion can also result from geometric effects. An important example is that of a prism pair as is often used for dispersion compensation in mode-locked lasers. Here, chromatic dispersion arises from wavelength-dependent path lengths, caused by wavelength-dependent refraction at prism surfaces. Similar effects occur in laser resonators containing Brewster-angled optical components, and in pairs of diffraction gratings as used for dispersive pulse compression.

## Normal and Anomalous Dispersion

One distinguishes *normal dispersion* (for <$k'' > 0$>) and *anomalous dispersion* (for <$k'' < 0$>). Normal dispersion, where the group velocity decreases with increasing optical frequency, occurs for most transparent media in the visible spectral region. Anomalous dispersion sometimes occurs at longer wavelengths, e.g. in silica (the basis of most optical fibers) for wavelengths longer than the zero-dispersion wavelength of ≈ 1.3 μm.

Great care is recommended when the sign of dispersion is specified. The ultrafast optics community identifies that sign with the sign of <$k''$>. The opposite sign is usually used in optical fiber communications, where the dispersion is often specified with the *dispersion parameter*

in units of picoseconds per nanometer and kilometer (ps/(nm km)). The different signs result from using a frequency derivative in one case and a wavelength derivative in the other. Note also that the conversion factor depends on the wavelength.

Between wavelength regions with normal and anomalous dispersion, there is a zero dispersion wavelength. The region around this wavelength can be special in some respects, not only concerning weak dispersive pulse broadening.

## Case Study: Dispersion Engineering for Telecom Fibers

We explore different ways of optimizing refractive index profile for specific chromatic dispersion properties of telecom fibers, resulting in dispersion-shifted or dispersion-flattened fibers. This also involves automatic optimizations.

## Effects of Chromatic Dispersion

### Wavelength-dependent Refraction and Diffraction

A frequently encountered phenomenon is that dispersion causes wavelength-dependent refraction, which is responsible, e.g., for the occurrence of rainbows. Similarly, wavelength-dependent diffraction at a diffraction grating allows the spatial separation of different frequency components of light. That is exploited in grating spectrometers, for example.

### Dispersive Pulse Broadening and Chirping

Chromatic dispersion has an important impact on the propagation of light pulses because a pulse always has a finite spectral width (bandwidth), so that dispersion can cause frequency-dependent phase changes. Consequently, its frequency components propagate with different group velocities. Normal dispersion, for example, leads to a lower group velocity of higher-frequency components, and thus to a positive chirp (instantaneous frequency rising over time), whereas anomalous dispersion creates negative chirps.

Note that the spectral shape (i.e., the frequency dependence of the power spectral density of the optical field) is never changed by chromatic dispersion, since only phase changes are caused. However, if chromatic dispersion occurs together with optical nonlinearities, e.g. in an optical fiber, it can indirectly affect the spectral shape, since the phase changes modify the temporal shape and thus the effect of nonlinearities.

Polychromatic light, e.g. corresponding to ultrashort light pulses, may be described with complex amplitudes <$A$> in the time or frequency domain. If it propagates through a dispersive medium, the resulting change of complex amplitudes can be described with the following equations. The simpler description is found in the frequency domain, where second- and third-order dispersion generate phase changes proportional to the second or third power of frequency offset, respectively:

$$\frac{\partial }{{\partial z}}A(z,\omega ) = i\left( {\frac{{{\beta _2}}}{2}\Delta {\omega ^2} + \frac{{{\beta _3}}}{6}\Delta {\omega ^3} + ...} \right)A(z,\omega )$$In the time domain, those factors are replaced with time derivatives:

$$\frac{\partial }{{\partial z}}A(z,t) = \left( { - i\frac{{{\beta _2}}}{2}\frac{{{\partial ^2}}}{{\partial {t^2}}} + \frac{{{\beta _3}}}{6}\frac{{{\partial ^3}}}{{\partial {t^3}}} + ...} \right)A(z,t)$$The frequency dependence of the group velocity also has an effect on the pulse duration. If the pulse is initially unchirped, dispersion in a medium will generally increase its duration (*dispersive pulse broadening*). For an originally unchirped Gaussian pulse with the duration <$\tau_0$>, which experiences only second-order dispersion (= group delay dispersion) <$D_2$>, the pulse duration is increased according to

The approximation holds for the case of strong broadening, i.e., for <$D_2 \gg \tau_0^2$>. In that regime, shorter input pulses lead to longer output pulses. This is an effect of the increased pulse bandwidth.

As a numerical example, an initially unchirped 30-fs pulse at 800 nm becomes stretched to 45 fs after 10 mm of propagation in silica (with +360 fs^{2}). After 10 cm, the pulse duration would increase to 334 fs.

Dispersion of the opposite sign can be used later to recompress the pulse (*dispersive pulse compression*). This is important, e.g., in the context of chirped-pulse amplification. Depending on the sign and amount of required dispersion, and also on other factors such as the optical peak power, different devices are used as dispersive compressors. Examples are prism pairs, pairs of diffraction gratings, chirped mirrors or other dispersive mirrors, chirped Bragg gratings, and dispersive glass fibers.

Note that shorter pulses are increasingly sensitive to dispersion. Substantial broadening occurs when the square of the pulse duration is smaller than the group delay dispersion.

Higher-order dispersion causes more complicated changes of the pulse shape. A challenge in dispersion compensation is often to compensate not only the lowest (second) order of dispersion, but also higher orders for approaching the transform limit.

### Soliton Effects

Although dispersion alone leads to pulse broadening, dispersion combined with the Kerr nonlinearity can give rise to the formation of solitons and in this way assist the generation of extremely short pulses of light with a soliton mode-locked laser. However, this usually requires the careful management of dispersion over a wide wavelength range, i.e., taking care also of higher-order terms.

### Dispersion in Nonlinear Optics

In nonlinear optics, particularly concerning nonlinear frequency conversion, chromatic dispersion has manifold influences, which can be grouped into three categories:

- It determines the options for phase matching of parametric nonlinear processes.
- It limits the effective interaction length for short pulses via the group velocity mismatch (→
*temporal walk-off*, limited phase-matching bandwidth). - It can also limit the nonlinear interaction length via dispersive pulse broadening: eventually, the pulses become so long that their peak power is too low for efficient nonlinear interactions. However, the group velocity mismatch normally introduces the more stringent limitation.

## Waveguide Dispersion

The discussion above is based on the assumption of plane waves. In practice, significant deviations from this situation can occur, in particular in the context of waveguides. Here, the quantity of interest is usually not the magnitude of the wave vector (<$k$> vector) (which anyway is no longer well defined), but rather the value <$\beta$> (the imaginary part of the propagation constant), which specifies the phase change per unit length in the propagation direction. As <$\beta$> is influenced by the waveguide (particularly for mode diameters of only a few wavelengths or even less), the dispersion is also affected (→ *waveguide dispersion*). This is important e.g. in optical fibers, and particularly in photonic crystal fibers with very small effective mode areas. In some cases, waveguide dispersion makes the overall dispersion anomalous even in the visible wavelength region, where the material dispersion of silica alone is clearly in the normal dispersion regime. For telecom applications, fiber designs are often made for tailored dispersion properties, resulting in, e.g., dispersion-shifted fibers.

## Measurement of Chromatic Dispersion

There are several techniques for measuring chromatic dispersion:

- The pulse delay technique [2] (for fibers) is based on measuring the difference in propagation time (group delay) for pulses with different center wavelengths. This is typically done using hundreds of meters (or even some kilometers) of a fiber. The dispersion is obtained by differentiation of these data.
- The phase shift technique or “difference method” [4] (also for fibers): a light beam with a sinusoidally modulated intensity is sent through a fiber, and the phases of the oscillations of input and output power are compared. The group delay can be calculated from that phase, and the dispersion can be measured by performing the measurement at different wavelengths.
- Dispersion in the resonator of a wavelength-tunable passively mode-locked laser can be measured by monitoring changes in the pulse repetition frequency when the laser wavelength is changed, as this reveals the wavelength-dependent group delay.
- Different types of interferometry [5] (e.g. white-light interferometry [6] or spectral phase interferometry [10]) can be used to measure the phase delay caused by a dispersive component. The dispersion properties can be obtained from this phase by numerical differentiation. The method is normally used for dispersion measurements on dispersive laser mirrors and sometimes for fibers.

## More to Learn

- Tutorial on Passive Fiber Optics | single-mode, multimode fibers, polarization, chromatic dispersion, nonlinearities, Part 10: Chromatic Dispersion of Fibers | Group Velocity

Encyclopedia articles:

## Suppliers

The **RP Photonics Buyer's Guide** contains four suppliers for chromatic dispersion measurement devices. Among them:

### Thorlabs

Thorlabs manufactures a robust benchtop white light interferometer for characterizing reflective and transmissive dispersive properties of optics and coatings designed for ultrafast applications. The Chromatis™ dispersion measurement system covers 500 – 1650 nm, providing a means for measuring optics used for common femtosecond systems, including Ti:Sapphire systems as well as 1 µm and 1550 nm oscillators. The Chromatis compliments our ultrafast family of lasers, amplifiers, and specialized optics including nonlinear crystals, chirped mirrors, low GDD mirrors/beamsplitters, and dispersion compensating fiber.

### Bibliography

[1] | M. DiDomenico, “Material dispersion in optical fiber waveguides”, Appl. Opt. 11 (3), 652 (1972); https://doi.org/10.1364/AO.11.000652 |

[2] | L. G. Cohen and C. Lin, “Pulse delay measurements in the zero material dispersion wavelength region for optical fibers”, Appl. Opt. 16 (12), 3136 (1977); https://doi.org/10.1364/AO.16.003136 |

[3] | D. N. Payne and A. H. Hartog, “Determination of the wavelength of zero material dispersion in optical fibers by pulse-delay measurement”, Electron. Lett. 13 (21), 627 (1977); https://doi.org/10.1049/el:19770449 |

[4] | A. Sugimura and K. Daikoku, “Wavelength dispersion of optical fibers directly measured by ‘difference method’ in the 0.8–1.6 μm range”, Rev. Sci. Instrum. 50 (3), 434 (1979); https://doi.org/10.1063/1.1135825 |

[5] | M. Tateda et al., “Interferometric method for chromatic dispersion measurement in a single-mode optical fiber”, IEEE J. Quantum Electron. 17 (3), 404 (1981); https://doi.org/10.1109/JQE.1981.1071115 |

[6] | H.-T. Shang, “Chromatic dispersion measurement by white-light interferometry on metre-length single-mode optical fibres”, Electron. Lett. 17 (17), 603 (1981); https://doi.org/10.1049/el:19810424 |

[7] | B. Costa et al., “Phase-shift technique for the measurement of chromatic dispersion in single-mode optical fibres using LED's”, Electron. Lett. 19 (25), 1074 (1982); https://doi.org/10.1049/el:19830729 |

[8] | L. Cohen, “Comparison of single-mode fiber dispersion measurement techniques”, J. Lightwave Technol. 3 (5), 958 (1985); https://doi.org/10.1109/JLT.1985.1074327 |

[9] | L. Thevenaz et al., “All-fiber interferometer for chromatic dispersion measurements”, IEEE J. Lightwave Technol. 6 (1), 1 (1988); https://doi.org/10.1109/50.3953 |

[10] | C. Dorrer, “Chromatic dispersion characterization by direct instantaneous frequency measurement”, Opt. Lett. 29 (2), 204 (2004); https://doi.org/10.1364/OL.29.000204 |

[11] | I. A. Walmsley et al., “The role of dispersion in ultrafast optics”, Rev. Sci. Instrum. 72 (1), 1 (2001); https://doi.org/10.1063/1.1330575 |

[12] | B. P.-P. Kuo and S. Radic, “Highly nonlinear fiber with dispersive characteristic invariant to fabrication fluctuations”, Opt. Express 20 (7), 7716 (2012); https://doi.org/10.1364/OE.20.007716 |

## Questions and Comments from Users

2022-03-24

How can I calculate the FWHM duration of a Gaussian pulse at the output of multiple fibers concatenated in cascade, depending of the FWHM at the input?

The author's answer:

Assuming that only chromatic dispersion does the pulse broadening, and that the input pulse is bandwidth-limited, you can just calculate the total dispersion and use the formula in the article.

2022-07-13

If we assume the bandwidth of the laser pulse to be 9 THz, the (w − w_{0}) in each higher-order terms becomes bigger and bigger. How we can ignore the higher orders term in the Taylor series in general?

The author's answer:

It depends on the used units how large numerical values these terms have. If you use petahertz instead of hertz as frequency units, everything will look fine to you!

It is just that the higher-order derivatives turn out to become rapidly smaller, so that the Taylor expansion still makes sense.

2023-03-20

In this article you say “If the pulse is initially unchirped, dispersion in a medium will always increase its duration”. This seems to be in conflict with “This is equivalent to the pulse having a flat spectral phase, but does not always precisely lead to the shortest possible pulse duration in terms of full width at half maximum.” from the article on bandwidth-limited pulses. Am I mistaken here or is this a kind of technicality with a different definition of pulse duration?

The author's answer:

You are right, the first statement is actually not strictly true; therefore, I now replaced “always” with “generally”.

2023-06-02

Here you have provided GDD as D_{2} with the unit (time squared), but I have seen the unit of D_{2} (time per length) in the chirped fiber Bragg grating data sheet I am using. Now, I am a little bit confused with the parameter D_{2}.

The second question is that the grating has the ability to introduce higher-order dispersion as well. I have calculated the stretched pulse width as well, now how to know which dispersion is applied to the initial pulse? And is the equation you have provided valid for higher-order dispersion as well?

The author's answer:

From the given units, you can often find out what exactly is meant. However, “time per length” (units of s/m) would not fit to group delay dispersion – I would have expected something like s / (m nm) (seconds per meter of fiber and nanometer wavelength).

From what you reported I can't see any information on the grating's higher-order dispersion. So that question cannot be answered.

2023-09-07

I am still a little bit confused with the relationship between beta and D. According to literatures, beta is defined based on angular frequency while D based on lambda.

Also, we know the relationship between beta_{2} and D_{2} and according to that we can calculate higher orders of beta and D, but what is the relationship between beta_{1} and D_{1}?

The author's answer:

Different authors may use the same symbols differently. In my case, <$\beta$> is a phase constant (phase change per unit length).

Dispersion is usually considered as starting with <$D_2$>, which results from the second-order derivative of <$\beta$>. So we don't have <$D_1$>. The first-order derivative is related to the group velocity.

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2021-01-27

If the pulse duration becomes larger because of chromatic dispersion, will the spectral bandwidth become narrower?

The author's answer:

No, the spectral bandwidth is not modified by chromatic dispersion.