High-Speed Detectors and Receivers - Bookham - #1

Our time-domain optimized high-speed detectors (Models 1444, 1454, and 1024 on pages 128-131) are commonly used for measuring the pulse shape of short-pulsed lasers or for generat­ing an optical trigger signal from short optical

pulses. Some important considerations must be taken into account when these types of measure­ments are made.

One important consideration in such measure­ments is the optical saturation level of the photo-receiver under pulsed-laser excitation. Saturation will

begin when the output signal reaches a certain level, and for all signal types (including pulses)

this level is given roughly by the cw input satura­tion power (P_cw) multiplied by the gain, G. For

pulses much shorter than the response time of the photoreceiver the output pulse will have a

width equal to the FWHM of the photoreceiver's impulse response. For pulses of period T then, the average power at saturation will be P_cw scaled by the duty cycle of the output signal, FWHM/T For example, a 1-mW, 10-MHz laser used with a 10-GHz photoreceiver (35-ps FWHM) with P_cw = 1 mW would need to be attenuated by a factor of 35x10^-12/100x10^-9 or 35 dB.

A second consideration of pulsed-laser measure­ments is offsets. Offsets might result from the

oscilloscope or a DC-coupled photoreceiver and

can lead to erroneous conclusions about low

frequency or slow signal components. For this

reason, it is important to subtract offsets from the impulse measurement, which can be accom­plished by subtracting the average background

signal level taken over some window prior to pulse arrival from the entire measured impulse.

To maintain the fidelity of your measurements, every component

in your system needs to have a bandwidth greater than the 3-dB bandwidth of your signal, or, equivalently, an impulse response

faster than the fastest part of your signal. (For time-domain

measurements, a good rule of thumb is to have a frequency

3-dB bandwidth greater than 0.44/t, where t is the full width at half maximum (FWHM) of the temporal pulse). For example, even a very fast (50-GHz) oscilloscope combined with a 6-ps photodetector will not produce a 6-ps trace. This is because the pulse width you see depends on the convolution of many band-widths, including those of the signal, the photodiode, and the

oscilloscope.

To estimate the FWHM of a 5-ps pulse with a 6-ps photo-

detector and 50-GHz oscilloscope, you can sum the squares of the individual pulse responses. (This is very accurate for Gaussian

pulses.) To do this, you will need to estimate the FWHM of the oscilloscope. Since FWHM=0.44/f_3-dB, where f_3-dB is the frequency 3-dB bandwidth, we can estimate the FWHM for a 50-GHz oscilloscope as approximately 9 ps.* The measured

signal will then have a FWHM of

5ps) +(6 ps) +(94 ps) = 12.2 ps.

Other important factors to keep in mind are the bandwidths of

your cables and connectors, and the pulse-to-pulse jitter of your

laser. Because the sampled oscilloscope trace is made up of data

taken from many different pulses, timing jitter can broaden the

measured signal.

To measure signals greater than 50 GHz, you can use methods

that are based on optical pump-probe configurations such as electro-optic sampling** By using electro-optic sampling and

exciting the photodetector with a 100-fs FWHM pulse, we mea­sured a response of 5-ps FWHM from a 60-GHz photodetector.

By using a 50-GHz oscilloscope to measure the photodetector's output with the same excitation pulse, the pulse width was 12 ps, as expected.

*K. Rush, S. Draving, and J. Kerley, "Characterizing High-Speed Oscilloscopes," IEEE Spectrum, (1990), pp. 38-39.

** For a discussion on electro-optic sampling, see K.J. Weingarten, M.J.W.

Rodwell, and D.M. Bloom, "Picosecond Optical Sampling of GaAs Integrated Circuits" IEEE Journal of Quantum Electronics, QE-24 (1988), pp. 198-220.

Ultrafast Laser

Probe

Pump

o

__^ SHG Crystal

I I -Photodetector

5 ps/div

N

Variable Delay

Photodiode

sales@newfocus.com fax: [40B1 9 19-6083