Aaron Van Pelt, Kathryn Li Dessau, Steve Cason, New Focus, and Kenneth Bystrom and Simon Cao, Avanex Corp.
To accommodate explosive growth in the telecommunications network market, the technology of wavelength-division-multiplexed (WDM) fiber-optic systems is continually advancing, with the telecommunications community pushing for narrower channel spacings and/or an expanded wavelength range. Current state-of-the-art dense WDM (DWDM) systems are using narrow 50-GHz (0.4 nm) channel spacing, with companies already discussing even narrower spacing.
The result of this increase in the number of channels is a proportional increase in the complexity and duration of time required to fully characterize network components, such as multiplexers and gain-flattening filters, many of which must be characterized not only at the channel of interest, but also over the entire wavelength band.
By design, devices in WDM systems have wavelength-dependent optical properties. Many WDM components must, therefore, be optimized during manufacturing to set the center (or channel) wavelength accurately or to minimize insertion loss across the wavelength range. Historically, methods such as those using a broadband source and an optical spectrum analyzer (OSA) have been used to monitor component performance during manufacture. Although this method is very fast, the channel spacing and high channel count of DWDM systems are straining the limits of these setups. To achieve higher resolution, a tunable laser and a detector can be used, but such a measurement requires taking a large number of data points. Stepping the laser across the full range of interest for a multiplexer is, therefore, a lengthy process, limiting the method to final device testing only.
A new approach based on a swept-wavelength laser can significantly decrease the measurement time-delivering the resolution of a laser with the speed of an OSA. In this method, the wavelength is swept continuously at a constant rate while the output is recorded, providing a very-high-resolution spectral picture-fast enough to observe changes as a component is being adjusted. While this has obvious applications in the final testing and quality control of DWDM components, it also creates a whole new opportunity to improve the manufacture of DWDM components by allowing in situ testing during the manufacturing process itself. Indeed, the capability of real-time, high-resolution measurements in the assembly environment can significantly improve the production of demultiplexers, multiplexers, and other DWDM devices. Wavelength characterization of these components requires source and detection test equipment to have characteristics far better than the components themselves-the swept-wavelength laser is, therefore, ideal for these measurements.
The swept-wavelength technique, although new to the optics regime, has been used in the RF and microwave community for many years. Network analyzers use this combination of a swept source and power meter to characterize insertion loss and return loss in the frequency domain. In the optical regime, a tunable laser continuously sweeps over the wavelength range while the output is recorded. Thus, the main requirement is a tunable modehop-free laser that has a very linear and repeatable scan.The Model 6428 Telecom-Test laser from New Focus satisfies this requirement and is specifically designed for swept-wavelength measurements. It delivers extremely linear, repeatable modehop-free tuning over the entire wavelength range between 1,520 and 1,570 nm and incorporates a patent-pending motor design to ensure highly linear, repeatable scans. This ensures that motion measured on the oscilloscope is due to the spectral change of the device under test. The typical power flatness is 0.04 dB over 5 nm, making the laser ideal for intensity versus wavelength measurements. Finally, the high output power allows the simultaneous measurement of multiple output channels in a device. Other components needed to complete a typical swept-wavelength system include a high-speed detector as the receiver and a data acquisition card for data capture and storage.
Many DWDM components are optimized during manufacture to set the center (or channel) wavelength accurately or to minimize insertion loss across the wavelength range. Avanex Corp. uses the swept-wavelength method to characterize its PowerMux product (see Figure). The swept-wavelength technique allows the company to make in-process alignment adjustments while seeing the results in a real-time display of the device output. The PowerMux high-performance wavelength processor is an interleaver product, separating odd and even channels into separate outputs. It can be used to multiplex or demultiplex DWDM channels based upon any of the new channel spacings-100 GHz, 50 GHz, 25 GHz, or even smaller, and can be used for any number of channels and any bit rate, with up to 80% utilization efficiency.
With the swept-wavelength system a measurement that would have taken a traditional step-and-measure laser approximately two hours can be performed in just one second with data update rates of up to 3 Hz (see Figure 2). The high linearity of the scans enables accurate testing over the entire wavelength range, with a typical absolute wavelength accuracy of 20 pm, a typical wavelength repeatability of 30 pm, and typical relative wavelength accuracy of 30 pm. Because this technique allows the entire spectral output of the device to be seen at once, it enables operators to make adjustments in real time and gives the manufacturer real-time process control.
An alternate technique consists of a broadband source, such as an edge-emitting, light-emitting diode (EELED) or the amplified spontaneous emission (ASE) of an erbium fiber amplifier, and an OSA. As stated earlier, this technique is fast (a typical scan can take 1 sec) and has a very wide wavelength range (>50 nm). However, its relatively low wavelength resolution (typically 0.1 nm due to limitations of the OSA) makes it unsuited to characterize some critical DWDM components. Consider, for example, a demultiplexer in a 50-GHz system with 80 channels. If the required isolation be tween channels is over 30 dB-that is, the device loss at 0.4 nm from line center is at least 30 dB-the measurement requires a resolution of 0.01 nm (10 pm).
Nonetheless, the OSA method does offer superb dynamic range because the OSA acts as a non-wavelength-selective light source. This method has, therefore, been used successfully in the sub-component assembly level where a relative measurement is sufficient and where the speed of the instrument allows technicians to optimize the alignment of subcomponents. Despite its limited resolution, this method is acceptable for devices that have little spectral information, such as isolators or taps. Devices that have more-intricate wavelength dependencies require more-complex methods.
In contrast, the step-and-measure method provides much higher resolution by using a tunable optical source, usually an external-cavity diode laser and a power meter or detector. This method shares the same high-resolution, high-power capability of the swept-wavelength method. It is considerably slower, however, because in this case, the laser is stepped incrementally over the de sired wavelength range and the optical throughput is measured at each step. The laser stops at each measurement point for a given amount of time with this step-and-measure method.
Measuring a broadband device covering the wavelength range of 1,520-1,570 nm with 1-nm resolution, for instance, will require about 50 data points. If the average step time is 400 msec, this type of scan will take 20 sec. A narrow-band device like a drop filter will typically require a 10-pm resolution over 3 nm yielding 300 measurement points-a measurement taking a few minutes. Finally, an 80-channel demultiplexer for a 50-GHz system requires characterizing the actual shape as well as the isolation over neighboring channels. Covering the 35-nm wavelength span with 0.01-nm resolution means the measurement must cover 3,500 points, which will take about 20 min-and this is just for a single output channel of the device at a single polarization. For polarization-dependent effects, the four Stokes parameters must be measured, taking at a minimum four times longer to measure. In comparison, the swept-wavelength technique with a New Focus Model 6428 laser scanning at 100 nm/sec takes less than a second to cover the 35-nm wavelength range.
In comparison to the OSA-based method, both laser-based systems have poorer dynamic range, with typical values of about 20 to 30 dB. Because of the external cavity of the laser, the spontaneous emission is suppressed by at least 40 dB. However, because the power meter is not wavelength selective, it will integrate all the received power over its usable wavelength range. The ASE of the laser, although heavily suppressed by the external cavity, will limit the dynamic range because the power meter will integrate all the power, including the ASE. The dynamic range of most tunable lasers without additional optical filters extends from 20 to 25 dB. An easy improvement is to use a tracking optical filter to eliminate the laser's ASE. For instance, by using a synchronized OSA as a filtered detector, the dynamic range can be improved to up to 80 dB.
Given the ever-narrower channel spacings in telecommunications systems, the efficiency of wavelength characterization measurements is becoming a major issue. Although the broadband source and OSA method is fast, it does not provide the resolution required to characterize complicated devices. Techniques based on step-and-measure lasers can provide both the required dynamic range and resolution but are very slow. The new swept-wavelength technique is considerably faster and has the resolution of a laser. This method is uniquely suited to both final testing of DWDM devices and to assembling DWDM components, especially as testing of these devices becomes more complicated. Techniques to speed up both the in-process manufacturing and the final testing of components such as multiplexers and gain-flattening filters clearly have enormous benefits.
Aaron Van Pelt is a technical-support engineer, Kathryn Li Dessau is a product manager, and Steve Cason is a technical sales engineer at New Focus (Santa Clara, CA), www.newfocus.com. Kenneth Bystrom is senior engineering manager and Simon Cao is senior vice president, product development, at Avanex Corp. (Fremont, CA), www.avanex.com.
This article appeared in the May 2000 issue of Laser Focus World, Lightwave's sister publication.