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  • Passive Component Testing: From All-Loss to All-Parameter

Photonics Spectra
Dec 2002
As optical networks implement higher bit rates and move to denser channel spacings, new challenges for component testing emerge.

E.U. Wagemann and Gunnar Stolze

Fiber optics technology has taken a major leap forward with dense wavelength division multiplexing (DWDM). Today, as networks move to speeds as high as 40 Gb/s, with channel spacings of 25 GHz and below, time-domain properties of the signals become increasingly important. As a consequence, modern optical components must be specified for spectral and polarization-dependent loss, group delay and differential group delay.

Because narrow-channel loss characteristics produce steep dispersion characteristics, wavelength-selective devices challenge today’s applied methods of determining dispersion properties. Therefore, the precise characterization of these components requires a test method with high accuracy, high resolution and a large dynamic range for both loss and dispersion.

Agilent Technologies has developed a method that combines a tunable laser source with a low-noise output for loss measurements and swept homodyne interferometry for the measurement of dispersion properties. Using homodyne interferometry, group and differential group delay can be determined by measuring the phase delay through the device under test.

The road to 40-Gb/s networks

The need to provide higher bandwidth over a single fiber can be satisfied by different means. First, the deployment of additional wavelength bands can increase spectral bandwidth. Second, more and more channels can be squeezed onto a single band, decreasing channel spacing. Third, each channel can be modulated at a higher speed — 10 and 40 Gb/s are key here.

The deployment of 40-Gb/s networks presents a challenge to systems providers, network equipment manufacturers and component manufacturers — and thus to test and measurement systems.

At higher data speeds and narrower channel spacing, the “run time” properties of the signal as well as loss become important. As a rule of thumb, one-tenth of a bit period is an acceptable delay (or dispersion) for a system. For example, a bit period for 40 Gb/s is 25 ps, so 2.5 ps is an acceptable system delay. But a system has many components, so the delay contribution of each component must be smaller. New techniques, such as forward error correction, dispersion compensation, specialty fiber and digital postprocessing, will be essential in achieving 40-Gb/s networks because they allow additional margins for error-free data transmission.

Nevertheless, passive components are at the front line of the network and, as such, are crucial to performance. Therefore, the development of new methods for component testing is imperative.

Most passive components, such as fiber gratings, arrayed waveguide gratings and add/drop modules, either route or drop light. Excluding dispersion properties, compromises in the loss performance of these components are unacceptable. In advanced networks, components can lead to system penalties or even failure by not meeting the specifications of insertion and polarization-dependent loss, and of group and differential group delay. The ability to test all these parameters will be essential in passive optical devices, particularly at 10 and 40 Gb/s.

Testing for loss

Measurements of loss and dispersion traditionally have been treated as separate tasks and have been addressed by separate solutions. A fundamental shift in test instrumentation is needed to realize all-parameter analysis, wherein both loss and dispersion parameters are measured simultaneously with the highest possible accuracy. For advanced networks, test and measurement must precisely characterize all relevant device parameters, rather than a limited subset or single parameter.

Optical components are always tested for loss. For wavelength-dependent components, such as optical filters and multiplexers, spectral loss measurements determine the higher-order parameters that describe the quality of the optical filter, including its shape, bandwidth, crosstalk and passband ripple.

Passive components route, redirect or block light channels. Because their main purpose is wavelength routing, they must be characterized precisely for their spectral loss properties. Historically, DWDM components have been tested with a broadband source and an optical spectrum analyzer. Today, this commonly is addressed using a tunable laser and a power meter. To gain higher dynamic range and resolution, tunable laser sources have become the focus of considerable research.

Polarization-dependent loss

Tunable lasers with greatly reduced source spontaneous emission are commercially available. An improvement of more than 30 dB in the ratio of signal to total source spontaneous emission (compared with that of a typical tunable laser) enables the use of a simple, broadband power meter rather than a wavelength-selective receiver. A laser with low source spontaneous emission offers an improved measurement range compared with a standard design when a power meter is used as the receiver.

Because polarization is not fixed in optical networks, polarization-dependent loss has become more and more important. It represents the influence of polarization on the loss properties of optical components. Higher-order parameters of optical filters such as bandwidth, passband ripple or crosstalk also are polarization-dependent.

A typical measurement setup uses a source (a tunable laser source, if this loss is to be measured over wavelength), a polarization controller, the device under test and a power meter. The state of polarization is changed, and a series of measurements are performed to evaluate the polarization dependence of the device.

There are two approaches to these measurements: deterministic and nondeterministic. Deterministic techniques derive the device’s loss from its Mueller or Jones matrices, which are obtained by measuring the transmission properties of the device over a set of defined input polarization states. In contrast, nondeterministic techniques measure the minimum and maximum transmission through the device over a large number of input polarization states.

Using the “polarization scanning” technique, the device is exposed to all states of polarization, and a power meter measures the transmission. The maximum and minimum transmission through the device can be measured directly, and the polarization-dependent loss is the ratio of minimum and maximum transmission.

This technique is suitable for measurements at a few wavelengths because the scanning is performed at each wavelength individually. Exposing the instrument to all states of polarization is impossible because only a finite number can be approached. In practice, a large number of polarization states are generated at a scan rate that is suitable for the power meter’s averaging time. Increasing the measurement time, which allows the transmission through the device to be measured at more polarization states, reduces the uncertainty of the measurement.

The Mueller method, a deterministic measurement method, ascertains the loss by exposing the device to four well-known states of polarization. This is advantageous if the loss must be measured over wavelength with high resolution. The four polarization states typically are linear horizontal, linear vertical, linear +45° and right-hand circular. The Mueller matrix coefficients yield the average insertion loss, the minimum and maximum transmission, and, therefore, the polarization-dependent loss.

This method can be incorporated with transmission measurements over wavelength, where, at each of the four polarization states, the transmission over wavelength is recorded. Consequently, the Mueller method obtains accurate results in a very short time for a large number of wavelength points.

Although they are based on different approaches, both polarization scanning and the Mueller method should yield the same results. Both methods demonstrate that they can measure low polarization-dependent loss values, and the results over wavelength are in agreement. Measurement time becomes the dominant criterion for choosing the appropriate method.

The Mueller method has an advantage over polarization scanning because of its deterministic nature, which allows polarization-resolved measurements to be performed. In integrated optical components, such as arrayed waveguide gratings, two fundamental modes of propagation exist. These modes correspond to two orthogonal states of polarization. Lightwave signals with other states of polarization can be resolved into these fundamental modes, and the results of the Mueller method can be analyzed to yield the spectral loss at these modes.

Testing for dispersion

In high-speed optical networks, the “timing” properties of signals or parts of signals become important, making a precise characterization of dispersion properties of optical components necessary. Dispersion may be managed or compensated using special fibers or components, but a precise design for lowest dispersion and a precise characterization of all components is required.

Three dispersive phenomena are known to degrade network performance by broadening the digital pulses. In each phenomenon, the degradation is caused by a difference in arrival time of various components of the signal: spatial modes, colors or polarization modes.

Modal dispersion affects only multimode systems. Most networks use single-mode fibers, so this dispersion effect is of little concern.

Dispersion is observed when the index of refraction varies with wavelength, causing changes in the group velocity. A variation of group delay with wavelength causes delays in different frequency components of the signal by different amounts, stretching a light pulse as it travels along a fiber.

Polarization mode dispersion becomes a performance limitation in high-speed systems when special fibers or devices have compensated for chromatic dispersion. Pulse spreading is caused by the difference in propagation velocity between orthogonal polarization states.

It is important to note that the structure of a passive component itself can induce dispersion. In complex structures, such as multilayer films or arrayed waveguide gratings, in which multiple paths are possible, the addition of the various paths depends on the phase of the interference so that the combined “effective path length” can be wavelength-dependent.

Today, modulation phase shift is the standard method for measuring chromatic dispersion. A CW light source is intensity-modulated by a high-frequency sine wave. The modulated optical signal travels through the device under test and is demodulated by the receiver. The phase of the detected modulation is measured relative to the original electrical signal. Relative group delay is calculated from the variation of this phase with wavelength.

This method was first used to characterize fiber properties and can be used to measure various components. It is most accurate at the highest modulation frequencies — typically, frequencies in the gigahertz range are applied. The offset between the main signal and the side modes is determined by the modulation frequency: If the device’s dispersion properties vary in a scale comparable to the modulation signal bandwidth, the resolution of modulation phase shift is limited. This can happen for so-called group-delay ripple and at the edges of a passband filter.

Swept homodyne interferometry

Narrow-channel devices challenge today’s methods of determining dispersion properties because narrow-channel loss characteristics go hand in hand with steep dispersion traces. For advanced components, test requirements are therefore best described as “high accuracy, highly dynamic and high resolution” — for both loss and dispersion. To address this properly, our setup combines a tunable laser source with low-noise output for loss measurement and swept homodyne interferometry for the measurement of dispersion properties.

Figure 1.
A typical optical setup for measuring dispersion properties using swept homodyne interferometry requires no moving parts.

In swept homodyne interferometry, a laser source is wavelength-tuned, while the arm lengths of the interferometer remain fixed (Figure 1). One arm includes the device under test, and the second is used as reference. The optical signals are combined, and a diode detects a fringe pattern. No moving parts are needed for the optical setup, which can be extended easily to measure both the transmission and reflection properties of devices.

Interferometric pattern

This results in an interferometric pattern in the detector plane. The phase information of the device is extracted by mathematical means and translated into group delay.

Figure 2.
To demonstrate the ability to measure spectral insertion loss, a thin-film filter was used as the test device.

Swept homodyne interferometry acquires dispersion information with high-spectral resolution as the phase information of the device under test is obtained from a single wavelength. We have used the group delay characteristics of a hydrogen-cyanide gas cell peak to check the capabilities of this method. Gas cell peaks are based on molecular absorption lines and are used for wavelength calibration because of their narrow spectral width.

The technique’s high sensitivity is attributable to the fact that it compares optical, not electrical, phases. Because all lightwave signals travel as a group, and not at a single frequency, group delay is the parameter of interest. The group delay unit of measurement is time, usually measured in picoseconds. It can be calculated from the phase delay using the formula

and is a measure of how much a light pulse is stretched when passing a component. Differential group delay is a measure of the polarization dependence of group delay — so there is a similar correlation as between loss and polarization-dependent loss.

Figure 3.
The polarization-dependent loss of a 100-GHz thin-film filter (insertion loss for reference) was determined using the Mueller method.

In our setup, a polarization controller and a polarization-diversifying receiver record two traces of group delay, leading to a Jones matrix as a function of wavelength. From this, maximum and minimum group delay can be determined. This procedure ensures that the recorded group delay trace is always free of polarization effects that can result, for example, from a change in the input polarization state between two measurements.

Figure 4.
The group delay of a thin-film filter vs. wavelength has a typical double-dent structure.

To demonstrate the capability of the technique, a thin-film filter was selected as a device for the all-parameter test. These devices use a stack of layers with different optical path lengths for wavelength-selective transmission and reflection properties. The tunable laser/power meter approach can achieve a dynamic range of 70 dB (Figure 2). In the filter’s passband (approximately 400-pm-wide), the polarization-dependent loss was determined to be flat, as designed (Figure 3).

Figure 5.
The differential group delay is designed to be close to zero in a filter’s passband.

The group delay of a DWDM filter should be flat over wavelength and should have only small ripples because dispersion effects resulting from ripples cannot be compensated in long-haul networks (Figure 4). The differential group delay of a DWDM component is designed to be close to zero. This would imply no polarization dependence of group delay. Good components display values well below 1 ps in the passband of the filter (Figure 5).

Meet the authors

E.U. Wagemann is product manager at Agilent Technologies’ Optical Communications Measurement Div. in Böblingen, Germany, where he is responsible for advanced measurement solutions.

Gunnar Stolze is an applications expert at Agilent’s Optical Communication Measurement Div., specializing in optical component testing.

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