Optical Sampling

Some typical applications of optical sampling are:

• testing integrated electronic circuits (ICs) or of other fast electronics (e.g. microwave circuits) by measuring waveforms at various points on the chips [4, 5]
• measuring the temporal response (bandwidth) of fast photodetectors
• analog-to-digital conversion (A/D conversion) with sampling rates of tens of gigahertz [11], where fast photoconductive switches allow interleaved operation of multiple electronic A/D converters; applications in wideband radar, optical fiber communications, and ultrafast oscilloscopes
• characterizing trains of ultrashort pulses at extremely high pulse repetition rates, as occur e.g. in time-multiplexed optical fiber communications systems
• measuring waveforms of terahertz pulses [6]
• terahertz spectroscopy, e.g. measuring terahertz transmission spectra [7] by Fourier transformation of time-domain data (comparing spectra with and without an absorbing sample between terahertz emitter and detector)
• generating microwave or millimeter wave signals, e.g. bursts with arbitrary waveforms, with photoconductive switches

The main attractions of optical sampling are:

• the extremely high measurement bandwidth, which can easily exceed 1 THz (1000 GHz)
• the capability of doing measurements on arbitrary points of electronic circuits with minimal back action on the device under test (particularly with electro-optic sampling)
• the capability for full time domain characterization

When data are recorded in the time domain for different delay times, the delay time is often varied with an optical delay line, realized e.g. with a corner cube prism on a motorized translation stage. This approach, however, called synchronous sampling, limits the speed with which a range of delays can be realized, and consequently the speed with which whole transmission spectra or the alike can be recorded.

Much faster data acquisition is possible with asynchronous sampling, using two different mode-locked lasers with slightly different pulse repetition rates. This automatically provides a temporally varying delay between the two pulses. The difference in pulse repetition rates determines how many times per second the measurement interval – given by the inverse average repetition rate – is scanned. This effective scan rate, however, must be low enough to allow for a sufficient temporal resolution, which is then limited by the detection bandwidth (not by the pulse duration). A high pulse repetition rate helps, provided that it is low enough to allow for a sufficiently large temporal range.

As an example, asynchronous sampling with two 1-GHz mode-locked lasers is sufficient for recording terahertz transmission spectra with a 1-GHz frequency resolution, corresponding to a temporal range of 1 ns. A 10-kHz difference of repetition frequency means that 10 000 spectra per second can be recorded. This allows one either to record data for many samples per second (e.g. to obtain position-dependent transmission spectra or even two-dimensional transmission images), or to average many spectra of a single sample within a few seconds, lowering the detection noise to a very low level. Using fast detection electronics, it is possible to record thousands of transmission spectra, ranging from virtually zero to several terahertz, within 1 s.

The asynchronous sampling method can be applied in essentially the same way to other pump–probe measurements, e.g. to measure the recovery dynamics of semiconductor saturable absorber mirrors or similar structures.

An additional advantage of asynchronous sampling is that mechanical noise of the delay time and position-dependent mode sizes are avoided. On the other hand, all the advantages obviously come at the cost of requiring two lasers instead of one.

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