A broadband, general-purpose instrumentation lightwave converter

Converting lightwave signals with wavelengths of 1200 to 1600 nanometers to electrical signals, this device serves as an optical front end for spectrum analyzers, network analyzers, bit error rate testers, and oscilloscopes.

APPLICATIONS OF HIGH-SPEED fiber optic transmission systems continue to grow. Digital lightwave telecommunication systems that can operate at rates of many gigabits per second are being developed. Currently, researchers are exploring fiber optic cable television (CATV) transmission to provide the gigahertz bandwidth required for all the channels and services, including high-definition television (HDTV), contemplated for future systems. Meanwhile, subcarrier modulated optical links are being investigated as alternatives to microwave links.

Special lightwave test instrumentation is required to test these high-speed systems. Often the systems and their components, such as laser diode transmitters, need to be characterized in both frequency and time domains. The HP 11982A amplified lightwave converter combines a highspeed pin photodetector with a low-noise preamplifier to provide a general-purpose instrumentation front end for lightwave frequency-domain and time-domain measurements on optical signals over the 1200-nm-to-1600-nm wavelength range. It can be used with spectrum analyzers, oscilloscopes, bit error rate testers, and network analyzers. Combining the dc-coupled HP 11982A with an HP 54120 Series oscilloscope, shown in Fig. 1, allows measurements of optical waveform characteristics such as pulse width, rise and fall times, extinction ratio, and eye diagrams. When this lightwave converter is operated with a spectrum analyzer, optical modulation characteristics such as signal strength and distortion, modulation bandwidth, and intensity noise can be measured. The HP 83810A portable lightwave signal analyzer, shown in Fig. 2, is formed when the HP 11982A is combined with the HP 8593A microwave spectrum analyzer and a downloadable lightwave personality stored on a credit-card-size ROM. The downloadable program includes frequency response corrections that allow calibrated measurements over the frequency range of 9 kHz to 22 GHz.

Converter Design

The primary design goals of the HP 11982A lightwave converter were that it be dc-coupled and that it have sufficient conversion gain, broad bandwidth, and good flatness. Only by simultaneously achieving all of these design objectives could the converter be considered truly general-purpose. Dc-coupled response is required to display the absolute levels of a time-domain waveform and to be able to determine the extinction ratio. In addition, the average optical power can be determined from the dc level.

Ensuring sufficient conversion gain, bandwidth, and flatness for the converter involved making trade-offs. The highest-bandwidth photoreceiver would just be a broadband pin photodiode, which would convert the photons of light to an electrical current without any amplification. However, a photodiode-only converter often does not have sufficient conversion gain to see low-level signals, particularly when used with spectrum analyzers that have relatively large noise figures. Too much gain, however, would degrade flatness and introduce additional gain slope, reducing the benefits of the amplification at frequencies up to 22 GHz.

The basic design of the HP 11982A was leveraged from the HP 70810A lightwave receiver in the HP 71400A lightwave signal analyzer.1 That design consisted of a pin photodiode followed by four microwave monolithic distributed amplifiers cascaded to provide a nominal 32 dB of amplification. However, the HP 70810A was not dc-coupled and its uncorrected frequency response was not acceptable for time-domain measurements. To provide the dc capability in the HP 11982A, a low-frequency dc-coupled amplifier is placed in parallel with a two-stage microwave amplifier in a split-band receiver configuration, as shown in Fig. 3. In the split-band approach, the low-frequency amplifier amplifies the detected signals from the photodiode below the crossover frequency, while the microwave amplifier amplifies signals above the crossover frequency. The outputs of the two amplifiers are resistively summed. The crossover frequency at approximately 2 MHz is formed by the parallel combination of output resistors and coupling capacitor according to the following formula:

Crossover Frequency [f.sub.c] = 1/2 Pi. (C.sub.2(r.sub.o [R.sub.5 ^^R.sub.Load)))

where [R.sub.o] is the output impedance of the microwave amplifier and [R.sub.Load] is the input impedance of the electrical measurement instrument.

The ideal operation of the two amplifiers in the neighborhood of the crossover is shown in Fig. 4. Well below the crossover, the high-frequency path has 90 degrees of phase lead and essentially zero amplitude, while the low-frequency path has zero degrees of phase and unity amplitude. At the crossover frequency, both paths have vectors that are complex conjugates and sum to produce a vector of unity with zero degrees of phase. Well above the crossover frequency the low-frequency path has 90 degrees of phase lag and essentially zero amplitude, while the high-frequency path has zero degrees of phase and unity amplitude. The low-frequency path also includes a zero about an octave above the pole at the crossover frequency. This zero is cancelled by adjusting the pole produced by C,.

A well-behaved crossover frequency response characteristic is important for good time-domain performance for signals that have spectral content in the neighborhood of the crossover frequency. Fig. 5 shows a measurement of the crossover frequency reponse characteristic of the HP 11982A, indicating that there is less than 1 dB amplitude variation through the crossover band.

To provide a reasonable compromise between sensitivity and frequency response, the microwave amplifier is designed to provide about 18 dB of gain. As shown in Fig. 6, this requires two stages of amplification with only a single interstage network. The interstage coupling is provided by a 1000-pF [TaO.sub.5], thin-film integrated capacitor in parallel with a [0.047-[micro-mF] ceramic capacitor. The integrated capacitor has good microwave performance and the large ceramic capacitor extends the low-frequency cut-off to 34 kHz, almost two decades below the crossover frequency. To reduce parasitics to ground, the interstage network is constructed with a short suspended-substrate transmission line segment. The capacitor that determines the crossover frequency is also a 1000-PF integrated capacitor.

The amplification in the low-frequency path is provided by a Comlinear CLC401 operational amplifier. This integrated circuit uses current feedback to provide high-speed operational amplifier performance. Because of the resistive divider network formed at the output by the 50-ohm input impedance of the receiver, the gain in this amplifier circuit has to be five times greater than the high-frequency path.

A FET follower is placed at the input to eliminate input off set voltage contributions from the input bias current of the CLC401. In addition, an LT1012, an amplifier with very low dc offset voltage, monitors the output and input voltages. This amplifier determines the output offset voltage, which is less than 1 mV. The thermistor shown is used to vary the gain of the low-frequency path to track temperature induced gain variations in the microwave amplifiers.

For maximum converter sensitivity, the photodetector is not back-terminated in 50 ohms. To mimimize the effects of mismatch loss at the higher frequencies, the detector is placed as close as possible to the microwave amplifier. The combination of the photodetector and the split-band amplifier provides conversion gain of typically 300 volts/watt. The overall frequency response roll-off of the HP 11982A is about 3 dB electrical at 11 GHz and 6 dB electrical, or 3 dB optical, at 15 GHz. The roll-off characteristic is gradual, as shown in the HP 11982A calibration chart (Fig 7). Each HP 11982A comes with its individually generated calibration chart, making it extremely useful for a number of frequency-domain lightwave measurements. The calibration is referenced to a Hewlett-Packard heterodyne laser system which produces a constant-amplitude optical modulation frequency.1 In addition, the gradual roll-off of the lightwave converter is beneficial in displaying time-domain waveforms without excess overshoot and ringing. Assuming that the roll-off is Gaussian, the full-width-at-half-maximum (FWHM) impulse response of the HP 11982A can be calculated using the following relationship:'

FWHM = 0.44/(Optical 3-dB Bandwidth).

This would predict an FWHM of approximately 30 picoseconds for a 15-GHz optical bandwidth.

Optical-Mechanical Design

The optical-mechanical design of the lightwave converter shown in Fig. 8 consists of two parts: the optoblock and the optical microcircuit. The function of the optoblock is to collimate the light at the input connector and refocus it onto the photodetector. The input to the optoblock uses the Hewlett- Packard fiber optic connector adapter system. This adapter design allows mating to any of five different connector systems: HMS-10/HP, FC/PC, ST, biconic, and DIN. It also allows easy access to the ferrule for cleaning. The connector is designed to provide a physical, low-return-loss contact to the input fiber. Internally, it contains a short piece of fiber that is cleaved at a small angle to prevent reflections at the glass-to-air surface from propagating back out of the connector. Exiting the rear of the input connector, the light passes into air. The diverging beam is first collimated into an expanded parallel beam. Next, it is reflected off a mirror positioned at a 45-degree angle, which directs the light to the output lens. The light is then focused onto the detector. As shown in Fig. 8, the mirror is partially transmissive, which allows the light to be aligned to the detector by viewing the reflected light from the illuminated detector with a microscope eyepiece.

The optical microcircuit containing the pin photodiode and the amplifier is mated to the optoblock. The pin detector and microwave amplifier are placed in one half of the package. This half is sealed with a rubber 0-ring gasket at the microcircuit-optoblock interface. A spiral-wound gasket is also placed at this interface to reduce the likelihood of any radiated electromagnetic interference (EMI) pickup. The low-frequency amplifier is constructed on a printed circuit board in the other half of the microcircuit package and is connected to the microwave amplifier with noncapacitive dc feeds.

Frequency-Domain Measurements

The HP 11982A can be used with an electrical spectrum analyzer to make a number of useful measurements on laser transmitters, such as modulation bandwidth, harmonic distortion, intensity noise spectrum, and linewidth, among many others." These measurements can be made most conveniently when the HP 11982A is configured as the HP 83810A lightwave signal analyzer or with an HP 8593A microwave spectrum analyzer when the lightwave corrections are downloaded into the analyzer.

The modulation response and harmonic distortion of a laser can be measured easily using the HP 83810A. Using a microwave generator to provide an electrical modulation stimulus for the laser under test, the optically modulated fundamental and harmonic components can be observed, as shown in Fig. 9. With the lightwave personality the modulated power levels can be measured electrically referenced to the input of the spectrum analyzer, or optically referenced to the o tical in ut of the lightwave converter.

This 1300-nm distributed feedback (DFB) semiconductor laser was producing second-harmonic distortion of approximately - 3 7 dB electrical at an optical modulation level of - 10 dBm. By varing the electrical modulation frequency to the laser, both its modulation bandwidth and its harmonic distortion as a function of frequency can be determined.

Low harmonic distortion is required for the transmission of video on fiber optic systems. However, this is only one of many important measurements that need to be made on CATV systems. In addition, the HP 83810A lightwave signal analyzer has the ability to make composite-triple-beat and carrier-to-noise measurements on fiber optic systems when the CATV personality is downloaded along with the lightwave personality.

Often very low-noise performance from the laser is necessary to meet the required signal-to-noise ratio for these transmission systems. A measurement of a laser intensity noise spectrum is shown in Fig. 10, revealing the characteristic noise peak at the relaxation oscillation frequency,5 which occurs at approximately 5 GHz for this laser. An important quantity used as a figure of merit for lasers is the relative intensity noise (RIN). It is the ratio of the mean square intensity fluctuation spectral density of the optical signal, [P.sub.n], to the square of the average optical power, [P.sub.avg.] This ratio is equivalent to the ratio of the electrical noise power referenced to a 1-Hz bandwidth measured on the spectrum analyzer to the average electrical power. Because the HP 11982A is dc-coupled, this latter quantity can be determined by measuring its output voltage into a 5011 load with a voltmeter, then equating it to (V.sub.dc) 2/50. Using this technique, the dc voltage was measured to be 200 mV and the electrical noise to be - 145.2 dBm at the noise peak. Thus, the RIN for this laser can be determined to be - 144.2 dB/Hz at 1 GHz.

Many future systems, especially those requiring coherent optical transmission techniques, will require lasers that operate in a single longitudinal mode and exhibit narrow linewidths. The linewidths of these single-frequency lasers, whose wavelength range is from 1250 nm to 1600 nm, can be measured with the lightwave signal analyzer when a fiber optic interferometer, such as the HP 11980A,6 iS connected in front of it as shown in Fig. 11. The interferometer functions as a frequency discriminator, converting optical phase or frequency deviations into intensity variations, which can be measured by the HP 83810A. A measurement of the linewidth of a single-frequency distributed feedback (DFB) laser is shown in Fig. 12. A linewidth of 40 MHz is indicated by the marker placed 3 dB down from the peak. This same basic technique can be used to measure frequency chirp (dynamic linewidth broadening under modulation) of lasers."

Time-Domain Measurements

A number of very useful time-domain measurements can be made with the HP 11982A lightwave converter when it is combined with a high-speed oscilloscope such as an HP 54120 Series digitizing oscilloscope. The lightwave converter's broad bandwidth with gradual roll-off minimizes the amount of instrumentation-introduced aberrations, such as overshoot and ringing, in the displayed waveform.

This amplified converter is especially useful in observing eye diagrams produced by pseudorandom binary sequence (PRBS) intensity modulation. Since these waveforms are produced by the time superposition of the pseudorandom bit stream triggered at the bit rate, persistence mode must be used. Thus, low-level signals cannot be averaged and pulled out of the noise. Fig. 13 shows an eye diagram comparison of a laser, intensity modulated with a 1.7 Gbit/s PRBS, whose output has been detected with the HP 11982A and a photodiode without amplification.

The broad bandwidth of the HP 11982A is helpful in the measurement of high-speed laser pulse characteristics. Typically, the pulse width of a laser is given by its FWHM. Shown in Fig. 14 is the measured FWHM of a mode-locked semiconductor laser producing pulses at a 1-GHz repetition rate. The measured FWHM was 39 picoseconds. From this, it is necessary to subtract the contributions of the lightwave converter and the HP 54120 Series digitizing oscilloscope using the following relationship:

FWHM.sup.2 laser = [FWHM.sup.2.sub.measured] [FWHM.sup.2.sub.HP11982A] - [FWHM.sup.2.sub.HP54120A]

= (39 ps)[sup.2 - (30 ps)[sup.2] - (16 ps)[sup.2]

Therefore,

[FWHM.sub.laser] = 19 PS - This calculation assumes a Gaussian pulse shape and agrees with the result determined from an autocorrelation measurement of the laser pulse width.

Acknowledgments

The efforts of a number of people at the HP Signal Analysis Division contributed to the successful introduction of the HP 11982A. Several of the key people and their responsibilities are as follows. Roberto Collins was the microwave engineer who had the design responsibility for the lightwave converter. Dean Carter performed the product design and implemented the mechanical changes to the optical microcircuit design. jim Young and Clyde Underwood coded the downloadable lightwave personality for the spectrum analyzer. Ron Koo provided production engineering support, and Ron Flatt assembled the prototype microcircuits. A special word of thanks goes to the "Redeye" team in microelectronics manufacturing and the rest of the new product introduction team.

References

1. C. Miller, "High-Speed Lightwave Signal Analysis," HewlettPackard Journal, Vol. 41, no. 1, February 1990, pp. 80-91.

2. Fiber Optics Handbook, Hewlett-Packard publication no. 59529654.

3. W. Radermacher, "A High-Precision Optical Connector for Optical Test and Instrumentation," Hewlett-Packard Journal, Vol. 38, no. 2, February 1987, pp. 8-11.

4. Application Note 371, Lightwave Measurements with the HP 71400 Lightwove Signal Analyzer, Hewlett-Packard Publication No. 5954-9137.

5. C. Miller, D. Baney, and L. Stokes, "Enhanced Intensity Noise and Spectral Measurements on Lasers Using Lightwave Signal Analysis," Hewlett-Packard RF, Microwave, and Lightwave Measurement Symposium, 1990.

6. D. Baney and W. Sorin, "Linewidth and Power Spectral Measurements of Single-Frequency Lasers," Hewlett-Packard journal, Vol. 41, no. 1, February 1990, pp. 92-96.

COPYRIGHT 1991 Hewlett Packard Company
COPYRIGHT 2004 Gale Group