dispersion compensation (original) (raw)
Definition: the control of the overall chromatic dispersion of a system by adding optical elements with a suitable amount of dispersion
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lightwave communications, 
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Related: chromatic dispersiondispersion compensation modulesmode lockingsolitonsoptical fiber communicationsprism pairsGires–Tournois interferometersfiber Bragg gratingschirped mirrorsdispersive mirrorselectronic dispersion compensationpulse compression
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Contents
What is Dispersion Compensation?
Dispersion compensation essentially means canceling the chromatic dispersion of some optical element(s). However, the term is often used in a more general sense of dispersion management, meaning the control (but not necessarily the complete compensation) of the overall chromatic dispersion of some system. The goal can be, e.g., to avoid excessive temporal broadening of ultrashort pulses and/or the distortion of signals in optical fiber communications. Dispersion compensation is applied mainly in mode-locked lasers and in telecommunication systems, but also sometimes in optical fibers transporting light e.g. to or from some fiber-optic sensor.
Dispersion Compensation in Fiber-optic Communication Links
Dispersion compensation is an important issue for fiber-optic links, i.e., in the context of optical fiber communications. Here, strong dispersive broadening of modulated signals can occur in cases with high data rates. Without dispersion compensation, each symbol would be broadened so much that it would strongly overlap with a number of neighboring symbols. Even for moderate broadening, significant inter-symbol interference can strongly distort the detected signal. Therefore, it is essential to compensate the dispersion before detecting the signal.
For high data rates such as 40 Gbit/s or 160 Gbit/s, pulse broadening becomes much stronger than for 10 Gbit/s, for example. This is essentially due to two factors:
- The shorter symbol time means that less temporal spreading can already be relevant.
- The spectral bandwidth of the signal becomes larger, thus causing stronger temporal broadening.
It is then generally not sufficient to compensate the second-order dispersion only; one also needs to deal with higher-order dispersion. Problems can arise, for example, when dispersion-shifted fibers with a substantial dispersion slope are used, and only dispersion of second order is compensated. Figure 1 shows this effect for a single 2-ps pulse at 1550 nm after 10 km and 50 km of such a fiber. Mainly uncompensated third-order dispersion is responsible for the resulting distortions.
Figure 1: Distortion of a triple pulse after 10 km (solid curve) and 50 km (dashed curve) of a dispersion-shifted fiber, when only second-order dispersion is compensated. The simulation has been done with the software RP ProPulse.
The resulting pulse distortion may appear small. However, a simulation with triple 2-ps pulses at a spacing as applied for 160 Gbit/s demonstrates that even in that case significant signal distortions can arise due to inter-symbol interference.
Figure 2: Pulse distortion after 10 km (solid curve) and 50 km (dashed curve) of a dispersion-shifted fiber, when only second-order dispersion is compensated.
The dispersion of a transmission fiber can be compensated with sections of fibers with different designs or with other optical elements. Dispersion compensation modules (DCMs) can contain, e.g., long pieces of dispersion-compensating fibers or chirped fiber Bragg gratings. The latter have the advantage of compactness and comparatively low insertion losses.
To some limited extent, the effects of dispersion in a system for optical data transmission can also be mitigated in a cost-effective way by electronic dispersion compensation.
Dispersion Compensation in Mode-locked Lasers
In a mode-locked laser for femtosecond pulse generation, the chromatic dispersion introduced by the gain medium and other optical components in the laser resonator is often not desirable because it tends to broaden and chirp the generated pulses. While the naturally occurring chromatic dispersion is often in the normal dispersion regime (at least for lasers operating at short wavelengths), the desired dispersion may either be close to zero or even anomalous (for the formation of quasi-soliton pulses in the resonator). Such dispersion values can be achieved by introducing optical components with anomalous dispersion. In the case of bulk lasers, such components are usually either special dielectric dispersive mirrors (e.g. in the form of monolithic Gires–Tournois interferometers or chirped mirrors), or prism pairs.
For mode-locked fiber lasers, the dispersion can be compensated with special dispersive fibers (e.g. photonic crystal fibers or multimode fibers where a higher-order mode is used), with chirped fiber Bragg gratings, or sometimes with bulk components such as pairs of diffraction gratings.
For pulses with durations below roughly 30 fs, it is necessary to control not only the second-order dispersion but also the dispersion of higher orders. It may then be important e.g. to select the appropriate material for a prism pair, or an optimized geometry of a prism compressor. There are also special devices called GRISMs [7], with a grating structure on a prism, which make possible an optimized ratio of the strength of second- and third-order dispersion.
Dispersive Pulse Compression
Dispersive optical elements can also be used to dispersively (linearly) compress ultrashort pulses outside laser resonators. The article on pulse compression gives more details.
Frequently Asked Questions
What is dispersion compensation?
Dispersion compensation is the process of canceling or otherwise managing the chromatic dispersion of an optical element or system. Its goal is typically to prevent excessive temporal broadening of ultrashort pulses or the distortion of signals in optical communications.
Why is dispersion compensation crucial for fiber-optic communications?
In fiber-optic links, dispersion causes signal pulses to spread out in time. At high data rates, this broadening can cause pulses to overlap (inter-symbol interference), severely distorting the signal. Dispersion compensation is essential to reverse this effect before the signal is detected.
How can the dispersion of a telecom fiber be compensated?
The dispersion of a transmission fiber is typically compensated with a dispersion compensation module (DCM). Such modules may contain a long coil of a special dispersion-compensating fiber or a compact device like a chirped fiber Bragg grating.
What is the role of dispersion compensation in mode-locked lasers?
In mode-locked lasers generating femtosecond pulses, components in the resonator introduce dispersion that tends to broaden the pulses. Compensating elements are used to manage the total resonator dispersion, often making it near-zero or anomalous, which is crucial for stable ultrashort pulse formation.
What components are used for dispersion compensation in bulk lasers?
In bulk mode-locked lasers, anomalous dispersion is often introduced using special dielectric dispersive mirrors (such as chirped mirrors) or prism pairs.
When does higher-order dispersion become important?
Higher-order dispersion must be considered for systems with very broad bandwidths. This includes telecom systems with very high data rates (e.g., 160 Gbit/s) and lasers producing pulses with durations below roughly 30 fs, as it can cause significant pulse distortion.
Suppliers
Sponsored content: The RP Photonics Buyer's Guide contains 21 suppliers for dispersion compensation. Among them:
âš™ hardware
O/E Land's OEDCG-100 is a dispersion compensating module based on fiber Bragg gratings. It features a compact size, small insertion loss and customized dispersion slope. Our unique fiber grating apodization technology enables us to fabricate dispersion compensation grating filters with high isolation, low side lobes and low ripples.
âš™ hardware
The APE femtoControl is a compact motorized dispersion compensation unit for optimization of the duration of femtosecond laser pulses in the spectral range of Ti:sapphire lasers.
femtoControl compensates for material dispersion by applying the inverse amount of dispersion to the pulse. This is generated by a pair of prisms on motorized translation stages allowing continuous adjustment of the pulse length.
For example, femtoControl can help to achieve more crisp and clearer microscope images especially for multi-photon microscopy.
âš™ hardware
Thorlabs manufactures a suite of options for dispersion management, including a pre-compensation module, dispersion compensating fiber, chirped mirrors, and low GDD optics. For ultrafast applications where dispersion must be well known and managed, Thorlabs’ portfolio includes a robust benchtop white light interferometer for characterizing reflective and transmissive dispersive properties of optics and coatings. By utilizing two different detectors, the Chromatis™ dispersion measurement system covers a broad spectral range from 500 — 1650 nm, enabling characterization of optics used for common femtosecond systems, including Ti:sapphire systems as well as 1 µm and 1550 nm oscillators. The Chromatis complements our ultrafast family of lasers, amplifiers, and specialized optics including nonlinear crystals, chirped mirrors, low GDD mirrors/beamsplitters, and dispersion compensating fiber.
Bibliography
| [1] | E. B. Treacy, “Optical pulse compression with diffraction gratings”, IEEE J. Quantum Electron. 5 (9), 454 (1969); doi:10.1109/JQE.1969.1076303 |
|---|---|
| [2] | O. E. MartĂnez et al., “Negative group-velocity dispersion using refraction”, J. Opt. Soc. Am. A 1 (10), 1003 (1984); doi:10.1364/JOSAA.1.001003 |
| [3] | R. L. Fork et al., “Negative dispersion using pairs of prisms”, Opt. Lett. 9 (5), 150 (1984); doi:10.1364/OL.9.000150 |
| [4] | J. Kuhl and J. Heppner, “Compression of femtosecond optical pulses with dielectric multilayer interferometers”, IEEE J. Quantum Electron. 22 (1), 182 (1986); doi:10.1109/JQE.1986.1072855 |
| [5] | R. L. Fork et al., “Compression of optical pulses to six femtoseconds by using cubic phase compensation”, Opt. Lett. 12 (7), 483 (1987); doi:10.1364/OL.12.000483 |
| [6] | D. Garthe et al., “Adjustable dispersion equaliser for 10 and 20 Gbit/s over distances up to 160 km”, Electron. Lett. 30 (25), 2159 (1994); doi:10.1049/el:19941453 |
| [7] | S. Kane and J. Squier, “Grism-pair stretcher–compressor system for simultaneous second- and third-order dispersion compensation in chirped-pulse amplification”, J. Opt. Soc. Am. B 14 (3), 661 (1997); doi:10.1364/JOSAB.14.000661 |
| [8] | G. Tempea et al., “Dispersion control over 150 THz with chirped dielectric mirrors”, JSTQE 4 (2), 193 (1998); doi:10.1109/2944.686723 |
| [9] | N. Matuschek et al., “Theory of double-chirped mirrors”, JSTQE 4 (2), 197 (1998); doi:10.1109/2944.686724 |
| [10] | B. Golubovic et al., “Double Gires–Tournois interferometer negative-dispersion mirrors for use in tunable mode-locked lasers”, Opt. Lett. 25 (4), 275 (2000); doi:10.1364/OL.25.000275 |
| [11] | H. Lim et al., “Femtosecond ytterbium fiber laser with photonic crystal fiber for dispersion control”, Opt. Express 10 (25), 1497 (2002); doi:10.1364/OE.10.001497 |
| [12] | A. Isomäki and O. G. Okhotnikov, “All-fiber ytterbium soliton mode-locked laser with dispersion control by solid-core photonic bandgap fiber”, Opt. Express 14 (10), 4368 (2006); doi:10.1364/OE.14.004368 |
| [13] | N. M. Litchinitser et al., “Fiber-based tunable dispersion compensation”, J. Opt. Fiber Commun. Rep. 4, 41 (2007); doi:10.1007/978-0-387-48948-3_11 |
| [14] | R. Jones et al., “Silicon photonic tunable optical dispersion compensator”, Opt. Express 15 (24), 15836 (2007); doi:10.1364/OE.15.015836 |
| [15] | M. E. Durst et al., “Tunable dispersion compensation by a rotating cylindrical lens”, Opt. Lett. 34 (8), 1195 (2009); doi:10.1364/OL.34.001195 |
(Suggest additional literature!)
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