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Electro-optic Sampling

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Definition: an optical sampling technique based on the electro-optic effect

German: elektrooptisches Abtasten

Category: methods

How to cite the article; suggest additional literature

Electro-optic sampling (or electrooptic sampling) is an optoelectronic technique of optical sampling, which exploits the linear electro-optic effect (also called Pockels effect). When ultrashort optical pulses are used in the electro-optic probe, there is only a short time interval in which the electric field at the probe can influence the light. This effect – usually a change in polarization, which is turned into a change in optical power at a polarizer – can then be measured without requiring a very fast photodetector. A full waveform of a periodic signal can then be obtained by slowly varying the arrival time of the probe pulse, i.e., by sequential sampling of a repetitive signal. For each setting of the relative time delay, the signal obtained can be averaged over many pulses, so that noise is averaged out and a very high sensitivity is achieved. The principle is the same as for an electronic sampling oscilloscope, where however the electronic sampling gate is replaced with the electro-optic probe.

Balanced detection with two photodetectors and lock-in detection with a modulated trigger beam is also often used. Such techniques make it possible to achieve a sub-millivolt resolution (limited by shot noise) within a measurement time of 1 s, even though the half wave voltage of the electro-optic probe is typically in the kilovolt region.

The optical pulses required can be generated with a mode-locked laser (often a bulk laser, e.g. based on Nd:YAG), or with a gain-switched laser diode. The latter solution is much cheaper and fairly versatile owing to the compactness and the variable pulse repetition rate.

An alternative technique with similar capabilities is photoconductive sampling.

Types of Electro-optic Probes

In internal electro-optic sampling (or direct electro-optic sampling), the substrate e.g. of an integrated circuit is used as the electro-optic medium. Of course, the substrate has to be an electro-optic material in that case. Suitable substrates are gallium arsenide (GaAs) and indium phosphide (InP). Internal sampling is minimally invasive.

In external electro-optic sampling (or indirect electro-optic sampling), one uses an external electro-optic probe (proximity electrodeless modulator). This probe may be a plate of electro-optic material (used in transmission or reflection) or a small (micromachined) crystal, which may be mounted on a fiber (often with a GRIN lens in between). Often used crystal materials are lithium tantalate (LiTaO3), bismuth silicate (BSO), zinc telluride (ZnTe), and gallium arsenide (GaAs). GaAs is favorable owing to its high electro-optic coefficient, its capability to be micromachined (with mechanical or chemical methods), and its relatively small dielectric constant (reducing back action on the sample). Invasiveness of electro-optic probes is often a considerable concern, as the use of an electro-optic probe with high dielectric constant can cause time delays and reflections in the device under test.

Types of Electro-optic Sampling Systems

Pump–probe systems use two copies of optical pulse trains with a variable time delay. Mechanically changing the time delay allows sequential sampling of different portions of a waveform. The timing jitter of the mode-locked laser is not important, as only the relative timing between pump and probe pulses matters, and these pulses are derived from the same laser pulses.

Other systems use a single pulse train which is synchronized to an electronic (often microwave) oscillator. Here, the sequential sampling is achieved by introducing a small frequency offset between some harmonic of the pulse repetition rate and the electronic oscillator which drives the device under test.

Factors Determining the Measurement Bandwidth

The following factors are relevant for the achieved measurement bandwidth:

Attractions of Electro-optic Sampling

The main attractions of electro-optic sampling are:

Applications

Electro-optic sampling is applied in various areas:

Bibliography

[1]J. A. Valdmanis, G. Mourou and C. W. Gabel, “Picosecond electrooptic sampling system”, Appl. Phys. Lett. 41 (3), 211 (1982)
[2]J. A. Valdmanis, G. Mourou, and C. Gabel, “Subpicosecond electrical sampling”, IEEE J. Quantum Electron. 19 (4), 664 (1983)
[3]J. A. Valdmanis and G. Mourou, “Subpicosecond electrooptic sampling: principles and applications”, IEEE J. Quantum Electron. 22 (1), 69 (1986)
[4]K. J. Weingarten et al., “Picosecond optical sampling of GaAs integrated circuits”, IEEE J. Quantum Electron. 24 (2), 198 (1988)
[5]Q. Wu, and X. C. Zhang, “Free-space electrooptic sampling of terahertz beams”, Appl. Phys. Lett. 67 (24), 3523 (1995)
[6]P. Uhd Jepsen et al., “Detection of THz pulses by phase retardation in lithium tantalate”, Phys. Rev. E 53 (4), 3052 (1996)
[7]P. Gaal et al., “Measuring optical frequencies in the 0–40 THz range with non-synchronized electro-optic sampling”, Nature Photon. 1, 577 (2007)

(Suggest additional literature!)

See also: optical sampling, Pockels effect, photoconductive sampling
and other articles in the category methods

In the RP Photonics Buyer's Guide, 1 supplier for equipment for electro-optic sampling is listed.

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