Calibration of lightwave detectors to 50 GHz - testing and calibrating HP 83440 Series instruments - Technical

Hewlett-Packard Journal, Feb, 1993 by David J. McQuate, Kok Wai Chang, Christopher J. Madden

Because they operate at much higher frequencies than previous products, new methods had to be found to test and calibrate the HP 83440 Series lightwave detectors. Three systems were developed. Their results agree closely.

The HP 83440 Series lightwave detectors described in the article on page 83 are a family of optical-to-electrical converters that includes models with bandwidths above 20 GHz. To test and calibrate these detectors, a higher-frequency test system was required. In this article we describe three systems that can be used to characterize high-speed lightwave converters. The first is a time-domain system that measures a photoreceiver's response to a short optical pulse. In the second system, two lasers are heterodyned to generate a test signal for a photoreceiver. The third system establishes an optical modulator as a calibrated source, which then is used to measure a photoreceivers bandwidth. We present results that show good agreement among the systems on measurements of a photoreceiver's frequency response.

Optical Impulse Test System

Picosecond pulses are generated by a system consisting of a mode-locked Nd:YAG laser and a fiber-grating pulse compressor (Fig. 1). The laser produces 80-ps FWHM (full width at half maximum amplitude) pulses at an 80-MHz rate, a wavelength of 1060 nm, and an average power of 20 watts. The pulse compressor uses self-phase modulation and positive group velocity dispersion in single-mode fiber to broaden the pulse spectrum and linearly frequency modulate (chirp) the pulse as it propagates through the fiber. 1

The diffraction grating pair introduces a time delay proportional to wavelength. When the chirped and spectrally broadened pulse is passed through the gratings, the pulse is compressed to about 2 ps FWHM.

We cannot measure the shape of such a short pulse directly, but we can measure its autocorrelation function, from which we can calculate the power spectral density of the pulse and make estimates of its width. The autocorrelator splits the input beam into two paths, which pass through rotating rectangular glass blocks arranged to introduce a periodically swept differential delay. The two beams then meet in a LiIO3 crystal which generates the second harmonic (at 530 nm), with an amplitude proportional to the product of the intensities of the two beams. This shorterwavelength light is detected by a photomultiplier tube. The output signal traces out the optical pulse's autocorrelation function as the glass blocks rotate.

An HP 54501A digitizing oscilloscope records the output of the autocorrelator. The Fourier transform of this time record is the power spectral density of the optical impulse. Our measurements show a drop in power of only three decibels at 60 GHz.

Measurement of Photoreceiver Impulse Response

Once we have characterized the optical impulse, we can measure a photoreceivers response to it. The optical impulse is focused on the photoreceiver. An HP 54120B sampling oscilloscope with an HP 54124A 50-GHz test set records the receiver's response. The measured trace is the convolution of the optical impulse with the impulse responses of the oscilloscope and the photoreceiver (Fig. 2).

The Fourier transform of the measured trace is the power spectral density of the impulse, but filtered by both the oscilloscope and the photoreceiver. The photoreceiver's frequency response can be obtained by dividing the measured spectrum by the optical pulse's power spectral density and by the oscilloscope's frequency response. The latter can be obtained by CW measurements using a microwave synthesizer and a microwave power meter, or by using two similar sampling oscilloscopes to measure each other's sampler impulse responses.2

Optical Heterodyne Test System

A second system for characterization of lightwave receivers to 50 GHz is shown in Fig. 3. An optical heterodyne source was built using a pair of diode-pumped Nd:YAG ring cavity lasers whose nominal wavelength of 1320 nm could be varied by changing the laser crystal temperature. The output beam from each laser passes through an optical isolator, a shutter, and a half-wave plate. The two beams are then combined in a fiber-optic 3-dB coupler/splitter. The isolators protect the lasers from feedback resulting from optical reflections. The shutters allow measurement of each individual laser's average power. The half-wave plates allow rotation of each laser's optical polarization. Good polarization alignment is obtained using an HP 8509A polarization analyzer and is maintained using a 3-dB coupler made from polarization preserving fiber. The output power (P1 and P2) is typically 1 mW from each laser at each of the coupler's outputs. Previous dual-YAG heterodyne systems measured photoreceiver frequency response to 22 GHz3 and 33 GHz.4

Let the difference between the laser frequencies be fd. If the laser outputs are linearly polarized and aligned, the optical power modulation envelope seen by a photoreceiver is:

[PROGRAM LISTING OMITTED]

If P1 - P2 the resulting signal is 100% modulated at the difference frequency.