Performance Monitoring Advances Optical Layer Networking

ElectroPhotonics' Keith Beckley explains how different optical performance monitor designs operate in DWDM networks.

By: Keith Beckley, ElectroPhotonics Corp.

A critical function in managing complex, high channel-count dense wavelength division multiplexing (DWDM) networks is monitoring the optical signals. An embedded optical performance monitor (OPM) can accomplish this task by providing direct feedback on the health of the optical signal.

A performance monitor reports three critical parameters back to the network management layer, including wavelength and channel count, power, and optical signal-to-noise ratio (OSNR; see Figure 1). A typical design uses a 1% or 5% tap to feed DWDM signals to the monitor. The instrument produces either a table of carrier parameters or an array of wavelength versus power values, which is an output format similar to that of an optical spectrum analyzer (OSA).

Depending on the design of the network, the OPM can be placed in either all or a select number of locations (see Figure 2). Placing an OPM after an add/drop or cross-connect element allows the instrument to monitor the wavelength and power performance of these critical nodes.

Performance monitors are especially suitable for network commissioning. Inserting an OPM at both ends of a link enables the proper pre- and post-emphasis of the amplifier stages. Using the feedback from the monitors, the network can be brought up with all the channels at equal power and OSNR. Rapid fault sectionalization in case of a fiber failure is another obvious monitoring application. Finally, placing an OPM before the final demultiplexer allows for the measurement of the received OSNR, which is related to the signal's bit error rate. This is also an ideal place to monitor the long-term variability of the optical signals.

Performance specifications
The specification of an embedded performance monitor depends on the design of the system that it monitors. Some system vendors focus on accurate OSNR and are less interested in tight wavelength measurement. For other systems, peak wavelength is the critical factor, and OSNR measurement can be less accurate. These various requirements aside, an OPM for a typical 100 GHz DWDM system must meet a minimum performance for all measurement values.

An ideal OPM should measure wavelength to an accuracy approaching that of a wavelength meter, measure power to an accuracy approaching that of a power meter, and measure OSNR to an accuracy of an OSA. Typical specifications available today include wavelength accuracy of ±50 pm, peak power accuracy of ±0.5 dB, and an OSNR accuracy to within 1 dB of an OSA. Carrier wavelengths need to be measured from 1525nm to 1565nm and peak carrier power can range from +5dBm to –55dBm. For installed systems, the launched SNR at the transmitter is ~35 dB and the received SNR is typically >15 dB therefore this is the range for which an OPM must be accurate.

Another parameter required to measure OSNR is noise bandwidth. The noise bandwidth is defined as the distance, in wavelength, from the channel peak to the point at which the noise floor is to be measured. The noise bandwidth is typically half the channel spacing so, for example, in a 100 GHz spaced system the noise bandwidth would be 0.4 nm.

In addition to measurement accuracy, performance monitors must meet rigorous environmental and reliability specifications. Although designing accurate, reliable, and low-cost OPM devices is a challenge, several vendors have released products that operate between 0 and 60°C, feature internal self-calibration, and fit into a typical equipment rack.

Competing technologies—bulk gratings and blazed gratings
OPM technologies competing for market share fall into three broad categories. The first approach, based on the design of a traditional OSA, incorporates a bulk grating and a variable slit. This design is the best option for OSNR as the slit ensures excellent contrast. As with a traditional OSA, the problems lie in wavelength and power accuracy. The requirements of the long optical path length make it difficult to shrink the design to a small form factor. In addition, the design requires mechanical moving parts, which raise concerns about the long-term reliability.

The second approach is also based on diffraction. Unlike the first scenario, the diffractive component in this case is a linearly-chirped, blazed fiber Bragg grating. Much like an ordinary fiber Bragg grating, the device is written into the core of the fiber such that the grating is at an angle to the core. As the grating is chirped, different wavelengths are coupled out of the fiber at different physical locations along the fiber. This fiber device is then coupled to a linear detector array such that each successive pixel of the array corresponds to a unique wavelength.

The blazed grating technique produces a simple, low-loss, mechanically stable design, but there are drawbacks. Commercially available linear detector array technology has a length of 512 pixels. For the 35 nm erbium doped fiber amplifier (EDFA) band, each pixel corresponds to ~70pm of bandwidth. This broad bandwidth requires considerable digital signal processing to achieve accurate wavelength readings. The blazed grating also suffers polarization sensitivity, requires athermal, packaging and has difficulty measuring signals with large dynamic ranges.

Fabry-Perot filters
The third and perhaps most promising technique is based on tunable Fabry-Perot (FP) filters. FP filters are manufactured as either all-fiber or fiber-coupled bulk optic devices. In either case, the FP filter is mounted on a piezoelectric transducer and tuned with a ramp voltage.

Piezo transducers, while affording high reliability, suffer from hysteresis. This is typically overcome by using a feedback design in conjunction with a known wavelength source. A wavelength reference is generally required to ensure long-term calibration, and is an essential component of the design. These references are typically either a fixed etalon filter or an athermally packaged fiber grating.

The drawback of the FP design is the relatively wide skirts of the filter response, which allow adjacent channels to ‘bleed' through the filter, thereby contributing to the bandpass power. This results in erroneous power and OSNR measurements. These errors can be overcome by careful calibration of the filter's spectral properties, and by ensuring the filter shape remains the same over the entire tuning range.

There is little question of the need for embedded optical monitors for both fault detection and long-term network monitoring as DWDM networks continue to increase in complexity. The recent near-simultaneous announcement by several vendors of products designed to meet these needs signals the start of the market for these innovative devices.

About the author…
Keith Beckley is a project manager with ElectroPhotonics Corp., 7941 Jane St. Unit 200, Concord, ON L4K 4L6. Tel: (905) 669-4660, ext. 224, Fax: 905-669-3722; e-mail: kbeckley@electrophotonics.com.

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