Compact power monitors enable channel-monitoring applications

June 1, 2003

In WDM systems, monitoring of power in each wavelength channel is often needed in three areas: dynamic gain equalization (DGE), channel protection/loss of signal (LOS) detection, and power tuning at the transmitter and receiver. Dynamic gain equalization is applicable mostly to DWDM networks having multiple nodes with optical amplifiers. At each node, the power in each wavelength channel or band must be balanced before the signal gets optically amplified. Power in each channel must be measured, and the information sent to an array of variable optical attenuators (VOAs) in real time for power balancing.

Channel protection/LOS, applicable to DWDM and CWDM systems, is a concern in the event of a network failure such as laser burnout, fiber break, or system congestion, when operators must dynamically allocate the signal stream to another fiber or wavelength channel through an optical switch. Fast multichannel power monitoring provides information about the failure to the system software in real time.

Power tuning at the transmitter and receiver is also applicable to DWDM and CWDM, as well as to time-domain-multiplexing (TDM) systems. In systems with a wide range of optical power per channel, a VOA must control the optical power from the laser transmitter and power into the receiver. Information from an inline power monitor is necessary for the VOA to act appropriately.

System designers prefer four- and eight-channel granularity for metro and access DWDM and CWDM systems. Because the channels are managed (added or dropped) in multiples of four or eight, the power monitor modules are best fitted for system integration if they handle four or eight channels. The power monitors can come in two varieties: fiber-terminated power monitor (PM) or inline power monitor (IPM). In a PM, the fiber is attached to the photodiode directly, whereas in an IPM, the optical signal comes out through a second port, with a small amount of tapped signal going onto the photodiode. Such a detector is known as an integrated photodetector (IPD), and is almost the same size as the terminated photodiode (see Fig. 1). Filter chips inserted into the optical path of a photodiode or IPD create a DWDM/CWDM filter.

Compact multichannel power monitors would have a small form factor (approximately 1 × 1 in. or less) and low profile (less than 0.6 in.) for easy integration into larger systems. In either a PM or an IPM approach, the power in each wavelength is converted into a photocurrent using a photodiode. The photocurrent is further digitized with an analog-to-digital converter, corrected for temperature, dark current, and gain effects, and the data is made available through an output port. The math, control, and electronic housekeeping are performed with a microprocessor.

Whether the demultiplexing device stays inside or outside the compact power meter depends on the end user's choice, technology limitations, and WDM channel spacing. If the channel spacing is narrow, such as 50 or 25 GHz, then one would normally use arrayed waveguide gratings, which will be external to the monitor. If the channel spacing is wider, for instance 100 GHz and above (as is the case for most metro and access CWDM/DWDM networks), one normally uses thin-film filters (TFF) for demultiplexing. In that case, the demultiplexer can be integrated inside the compact power monitor.

Different architectures and applications for optical-channel monitoring (OCM) use compact power monitors. In the simplest application, a small part of the optical signal in the fiber is tapped with a fused-fiber coupler. The tapped signal is further demultiplexed and terminated into a PM (see Fig. 2). The same functionality may be achieved by splitting the tapped signal with a fused fiber or waveguide 1 × N splitter, and then terminating the split signals with a PM equipped with channel filters. If the 1 × N splitter hurts the power budget, one may use optical filters in an IPD-like configuration, and cascade them with fusion splices to achieve the same functionality.

In the previous architectures, the power information being retrieved from the channels is implicitly planned for use elsewhere in the network. Stand-alone OCMs, fault-detection/rerouting systems and power-controlled multichannel transmitters and receivers fall into this application category.

In applications such as dynamic gain equalizers, the power monitor stays within the framework of mux/demux and VOA. The incoming DWDM signal is demuxed and after passing through an array of VOAs is muxed again. The outgoing DWDM signal typically goes into an optical amplifier, and therefore the VOA array needs to balance the power in each channel for optimum amplification. In many cases, the VOA array has an optical tap incorporated (typical in waveguide VOAs), and the tapped signal is fed into the PM for power measurement. Information from this PM controls the VOAs in real time. If the VOA array does not have a tap port (as with most microelectromechanical systems VOAs), then the signal out of the VOA array may be passed through an IPM to achieve the same functionality.

In variable attenuator and multiplexer applications, the incoming DWDM signal is demuxed and dropped into receivers or transceivers. New information is then fed into the fiber from transmitters. These signals are passed through an array of VOAs, and then muxed together before they go on into the fiber or optical amplifier. An IPM in the optical path can control the VOAs.

Traditionally, power meters for channel monitoring are built by system designers on the system board using discrete optical taps, photodiodes, or integrated photodetectors, with the electronics designed around it. While such an approach gives the system designer maximum control of the device, it also hampers calibration, fiber and component management, and portability of the monitor. A solution should be modular, pluggable, and able to render calibrated data. Such a device should also be small, portable, inexpensive, and have popular serial I/O protocols.

Such PM and IPMs have been created in four-channel and eight-channel versions. In the eight-channel version, eight IPDs convert a part of the signal into a photocurrent (see Fig. 3). The photocurrents are converted into a voltage with logarithmic amplifiers. The use of logarithmic amplifier makes possible a wide dynamic detection range of approximately 55 dB. Linear amplifiers are limited to approximately 20 dB of dynamic range. The amplified signals are then fed into a microcontroller with integrated analog-to-digital converters and input/output ports. The device should be factory calibrated for -5°C to +70°C, photodiode dark currents, electronics parts variations, and all input optical powers. Power measurement accuracy can exceed 0.1 dB and measurement speed can exceed 1 ms/channel.

Saroj Sahu is director of fiberoptic subsystems at Santec USA, 433 Hackensack Ave, Hackensack, NJ 07601. He can be reached at [email protected].

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