Light relief for enterprise networks
12 June 2008
In enterprise networks, short haul multi-mode fibres (MMF) suffer from multi-path modal dispersion, where different modes of light arrive at the receiver at different times.
Increasing capacity requirements and the demand for high performance have required most of the market to upgrade from legacy MMF running at 1Gbit/sec to serial 10Gbit/sec 220m. Replacing the legacy OM1 fibre is too expensive and the installed base of legacy MMFs is severely bandwidth-limited due to modal dispersion, making it difficult to achieve 220m at 10Gbit/sec.
LX4 was initially deployed using 4x3.125Gbit/sec. Today, long-range multimode (LRM) at 10Gbit/sec is used for installed legacy fibres, requiring less than half the LX4 solution and significantly lower power consumption. LRM uses EDC (electrical dispersion compensation) to compensate for optical dispersion in the electrical domain. A linear TIA (transimpedence amplifier) guarantees that the distorted optical signal is amplified linearly instead of becoming saturated prior to reaching the EDC.
In LRM applications, for a reliable opticallink performance with feed-forward equalizer/decision feedback equaliser (FFE/DFE), EDC is the most common architecture. Adaptive analogue technology and DSPs are two methodologies that are commonly used to enable FFE/DFE. Equally important is the performance of the TIA. A linear TIA has the normal limiting TIA performance, like small sensitivity and overload, but also includes linear functions, like THD (total harmonic distortion), WDP (waveform distortion penalty) and relative noise.
TIA parameters
Earlier, TIAs were limited both in linearity and range. The linear range of a TIA depends on its transimpedence (Zt). The upper 3dB bandwidth measurement, for example, falls within the linear range. A measurement of - 20dBm, on the latest TIAs, with the Zt of 10k, is considered small signal bandwidth. For linear TIAs, using an AGC (automatic gain control) circuit to make the linear range from sensitivity range to overload range is preferred. AGC is an adaptive system where the average output signal level is fed back to adjust the gain to an appropriate level for a range of input signal levels. Linear TIAs should remain linear from small signals (sensitivity level) to large signals (overload level).
The bandwidth differs using different input power even when measured with the same device. Both the AGC circuit time constant vs. network analyser sweep time set and the AGC circuit frequency dependency can cause these significant peaks.
For some TIAs, the AGC time constant can be adjusted using external capacitance. With the capacitance range of 270pF to 1nF, the AGC time constant can be measured from 1msec to 3msec. The network analyser sweep time can be adjusted to be as slow as possible, allowing the AGC time to be neglected. No difference in the bandwidth measurement is evident as the sweep time changes. With frequency dependency, the test results are better when the TIA die uses an electrical signal of an optical signal. For an optical signal, the optical sine wave performance must be checked and adjusted using laser bias current and modulation current to guarantee the laser works in the linear range. For the electrical signal, the sine wave signal can be added directly to the TIA input using bias T. In the overload range, the output amplitude remains constant when the input signal is 1GHz and 2GHz sine wave. The output amplitude increases as the input signal changes to 4G and 5G at the overload current.
The image measures the linear TIA bandwidth with AGC circuit on using 1310nm at 100M sine wave frequency to turn on the AGC circuit and the 1550nm optical signal to measure the linear TIA bandwidth using the sweep process.
To verify that the set-up provides for the bandwidth measurement with AGC turned on, it is important to verify the 1310nm signal at 100M changes the TIA gain. With AGC circuit on, the bandwidth is measured with the -20dBm 1550nm optical signal from the HP8703.
First the 1550nm laser source is blocked, the TIA input is changed to get different TIA output, and the TIA AGC performance is verified. To measure the linear TIA THD, a linear 1310nm laser source is used modulated with a 1GHz sine wave. The laser bias and modulation can be adjusted to obtain appropriate signals. An EDFA may be needed to provide enough peak-to-peak overload input optical power, and the spectrum analyser must be monitored to guarantee that only a 1GHz sine wave signal is present without additional harmonics of the fundamental frequency. The procedure is the same if 2GHz and 3GHz sine wave signals are needed to modulate the laser.
After selecting the proper bias current and modulation current, the New Focus PD is replaced with the ROSA under test. From the spectrum analyser, the fundamental signal (1GHz), 2nd order harmonic (2GHz) and 3rd order harmonic (3GHz) signal power as P1, P2 and P3 are read. THD is calculated using: THD=sqrt(P2+P3)/sqrt(P1).
Relative noise
IEEE 802.3aq defined three stressed optical signals as precursor pulse, post cursor pulse, and split symmetric pulse. These parameters and related PIE-D values (a metric used to assess EDC capability to equalise the link) test the link performance using the linear TIA.
WDP is a deterministic dispersion penalty due to a particular transmitter with reference emulated multi-mode fibres and receivers. WDPi is the dispersion penalty of the TP3 test signal, and WDPo is the dispersion penalty measured at the ROSA output. The distortion contributed by the ROSA is determined by: dWDP= WDPo-WDPi
Circadiant Hydra test system ensures that difficult stressors are consistent in the semiconductor food chain. According to IEEE802.3aq the simulated fibre stressor are set to 4.1dB, 3.9dB and 4.2dB for pre-cursor, symmetric and post-cursor, respectively. It is better to measure the optical source WDPi for different stressors separately for improved accuracy.
Where RNi is the relative noise of the test signal characterised using O/E curve, the relative noise contribution of the module is calculated using the following: dRN=sqrt(RNo^2-RNi^2).
Noise performance and bandwidth are affected by PD capacitance and bond wire inductance on TIA input node. The same TIA assembled by different firms using different photo detectors can differ in performance. Selecting two different ROSAs, ROSA1 and ROSA2 results in different bandwidths.
The different ROSA BWs affect the ROSA’s dWDP and dRN data. Each parameter is unique for the linear TIA 1348TA, not for any TIA+EDC combined performance.
The most important test in LRM applications is the optical comprehensive stressed test. Optical comprehensive stressed receiver sensitivity and overload can be done using a Circadiant Hydra where an electrical signal is created using a specified pattern and impaired by Gaussian low pass filter or Gaussian white noise source or ISI (pre-cursor, post-cursor and split symmetric-cursor).
The signal is converted to an optical signal using a linear electrical/optical converter and the optical attenuator is connected before testing the linear ROSA and EDC for stressed receiver sensitivity and overload.
Recently, FFE/DFE (feed-forward equaliser/decision-feedback equaliser) based EDC techniques for ISI (Inter Symbol Interference) mitigation in optical networks have been commercialised. Today, SiGe process-based EDC is in production stage while power consumption and price motivated IC companies design CMOS process-based EDC.
Theoretically, EDC, by using adaptive analogue methods, can tolerate more ROSA noise and higher bandwidth ROSA. EDC using the DSP method prefers ROSA low noise and needs low bandwidth.
Newer linear TIAs deliver the same or better performance as CMOS-based EDCs. Linear TIA +EDC can be worked not only in LRM multi-mode fibre applications, but also in single-mode fibre applications like metro and long haul.
LIAN ZHAO is senior application engineer, ARIEL NACHUM is validation test engineer, and LOI NGUYEN is vice president of technology, Inphi Corporation
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