Keysight Technologies Wavelength and polarization dependence of 100G-LR4 components. Application Note

Transcription

1 Keysight Technologies Wavelength and polarization dependence of 100G-LR4 components Application Note

2 02 Keysight Wavelength-dependence measurements for 100G-LR4 components - Application Note O-band WDM The use of wavelength-domain multiplexing (WDM) for increasing the capacity of fiber links is applied extensively in the C-band and L-band, where erbium doped fiber amplifiers enable long-haul transmission. Over short distances, like within buildings, it has typically been more economical to use lower cost transceivers in the 850 nm band with multimode fiber and to increase capacity by adding parallel fibers. But now there is a growing need to provide economical high-rate links that achieve longer reach than with multimode fiber, especially for large data centers and also for some installations of fiber fronthaul or backhaul in wireless networks. This has resulted in increasing use of WDM within the O-band wavelength range of nm, corresponding to the shortest wavelengths for single-mode performance from standard single mode fiber (SSMF) and to its lowest chromatic dispersion range. WDM use in this wavelength range began with just two signal wavelengths at very large separation, such as using 1310 nm and 1550 nm for respectively uplink and downlink in passive optical networks (PON) for fiber to the home service. The wavelength-selective components here can be band-splitter filters, with a broad intermediate range of overlapping transmission that is not critical if no signal is present at these wavelengths. Another convention, coarse wavelength domain multiplexing (CWDM), was developed to support many wavelength channels over unamplified links, by spreading the channels over the full usable range of SSMF with wide passbands so that the wavelengths of the transmitters do not need to be determined and stabilized to a narrow tolerance. CWDM uses a grid based on 20 nm spacing, using channels centered between 1271 and 1611 nm. Not every link uses the full wavelength range. With the need for 100 Gb/s links in data communication, the IEEE Ethernet Working Group has included implementations for reach up to 10 km (100GBASE-LR4) or, with tighter tolerances, 30 km (100GBASE-ER4) by using four wavelength channels in single mode fiber. The wavelengths are assigned to a frequency-based grid, like the dense WDM (DWDM) grid used for telecommunications, but with a wider spacing of 800 GHz, which is roughly 4.5 nm. This is wider and more tolerant than the 50 GHz or 100 GHz grid usually used for DWDM, but narrower than CWDM, and is also called LAN-WDM. Spacing the wavelengths closer reduces skew between the wavelength channels that results from chromatic dispersion. With this spacing, it is also conceivable to use more than 4 wavelengths in the future. The channels, or lanes, are centered at: THz ( nm), THz ( nm), THz ( nm), THz ( nm). Spectral measurements for passive components The spectral response of components used in WDM links is a key factor in determining link performance at the physical level. The insertion loss (IL) of passive components influence the signal power budget. The wavelength selectivity of filters used for multiplexing and especially demultiplexing, characterized from traces of IL vs. wavelength with parameters like ripple or flatness in the passband and isolation of wavelength outside the passband, is important for signal stability and avoiding crosstalk. Reflections, parametrized as return loss (RL), can also degrade link performance and should be controlled. Low dependence of these response parameters on the polarization of the optical signal is also needed to avoid fluctuations in power, because the polarization state can change randomly along fiber links. So passive WDM components are typically tested and verified by measuing IL, PDL and often RL across the applicable wavelength range. Using a tunable laser source at the common side of an LR4 multiplexer, for example, allows all four lane ports to be measured simultaneously with synchronized power meters. A block diagram for such measurements is shown in Fig. 1, implemented using the N7700A-100 application software package. Details for the instrumentation are given further below.

3 03 Keysight Wavelength-dependence measurements for 100G-LR4 components - Application Note Figure 1. Block diagram for swept-wavelength IL & PDL measurements As shown in figure 2, the measurement results show the insertion loss spectrum for each output port, averaged over all states of polarization. That would be the IL of unpolarized input signal. Spectra of the polarization dependent loss are also determined. This can also be shown as two IL spectra for each port corresponding to the IL for the input polarization states for maximum and minimum transmission. For planar devices like wafer chips, this usually corresponds to polarization parallel or perpendicular to the chip surface (TE or TM). The N7700A software also provides for calculation of key analysis parameters for the passbands, like wavelength offset, bandwidth, isolation, ripple and maximum in-channel IL and PDL. Figure 2. Measurement result for a 4-port multiplexer, including data analysis

4 04 Keysight Wavelength-dependence measurements for 100G-LR4 components - Application Note Spectral measurements for components with integrated detectors Another class of components requiring similar measurements is increasingly important. The optical detectors used in receivers are also characterized with respect to relevant wavelength and polarization dependence, but the response usually doesn t have strong variation. However when the detectors are integrated with filters or other passive components, this assembly needs to be characterized in a similar way as for the individual components. An important example is the LR4 receiver optical subassembly (ROSA), which can include the demultiplexer optics, photodiodes for detecting each signal lane, and often some electronics for transimpedance amplification for the RF signal carried on the detected photocurrent. Such a structure is shown schematically in Fig. 3. Figure 3. Schematic diagram of an LR4-ROSA device The electrical contacts on the ROSA that are used for providing bias voltage to the photodiode detectors can also be used to access the photocurrent while an input optical signal is varied in wavelength and polarization to measure responsivity response parameters. Such a solution is shown in Fig. 4.

5 05 Keysight Wavelength-dependence measurements for 100G-LR4 components - Application Note Figure 4. Block diagram for measurements of wavelength and polarization dependent responsivity This measurement uses source/measure units to apply bias voltage and measure the photocurrent from the integrated detectors of the DUT. The results are then interpreted as responsivity in units of ma current per mw optical input power. So the absolute input optical power is measured with an optical power meter and then applied to the DUT. Again both the polarization-averaged response as well as the minimum and maximum responsivity vs. polarization are determined by the software. An example is shown in Fig. 5. Figure 5. Sample measurement of an LR4-ROSA device

6 06 Keysight Wavelength-dependence measurements for 100G-LR4 components - Application Note Test solution configuration The core of these measurement setups is the tunable laser source that provides wavelength sweeps at a constant speed. The Keysight 81606A or 81608A, with different accuracy tolerances, configured with Option 113 for the O-band, support these measurements with a combination of important features including: sweep rates up to 200 nm/s with excellent power flatness and specified wavelength accuracy and repeatability using the built-in real-time wavelength monitor, very low background spontaneous emission at wavelengths away from the laser line to measure filters with high dynamic range, and sufficient signal power level for use in setups that split the power for measuring return loss or multiple devices and for testing responsivity and crosstalk of detecting components with relevant input power. These laser modules are used with the 8164B mainframes that then have four slots for additional modular instruments like a switch to extend the wavelength range by using two or more lasers, the 81636B power sensor, or a return loss module that can be included in the path between N7786B and the DUT. The N7786B is a fast-switching polarization controller that can synthesize a chosen sequence of polarization states using the built-in polarimeter and then repeatedly run through this sequence while logging the output state of polarization (SOP) and power level and providing synchronization triggers for the detectors of the setup. For measuring polarization dependence vs. wavelength, a sequence of 6 SOP is repeated for each measurement point while the laser makes a continuous sweep across the chosen wavelength range. The resulting data can then be interpolated to the chosen wavelength grid and analyzed in a generalized implementation of the Mueller Matrix method to determine the maximum and minimum response in dependence on polarization. Higher level analysis, such as resolving the response spectra for the principal axes of the device (TE vs. TM) or calculating the shift of a filter s center wavelength due to polarization, is also available from the matrix data. For measuring passive optical devices that output optical signals, the N7744A 4-port or N4475A 8-port optical power meter instruments are used as detectors for the solution. These combine wide dynamic range with high bandwidth to allow fast sampling in synchronization with the polarization states. Multiple units can be used in the same setup to simultaneously measure devices with multiple output ports, like splitters and switches or for measuring multiple components at the same time. The unique clip-on quad-adapters speed up optical connections, even to unconnectorized bare fibers, and connections can be made to one set of adapters while another component is still being measured, to speed up throughput. For measuring detectors or integrated devices including optical detectors, the setup can use instruments to sample the output photocurrent from the device. One such instrument is a special customization of the N7745A with 4 photocurrent inputs and 4 optical inputs that has been set up with the special option E02. This takes advantage of the same features mentioned above and provides a compact solution for measuring signals from 4 photodiodes. But this special model has to date only been implemented to provide negative bias voltage and does not isolate the detectors from electrical ground. Many devices require more flexibility in the electrical connections, which can be provided by using the B2900 series source/measure units. These setups are controlled using the N7700A IL/PDL software engine (option 100). More details on the software and the configuration are available in the N7700A brochure at www. keysight.com/find/n7700. Other alternatives, like a simpler measurement without polarization dependence can be made at high repetition rates that can support alignment and calibration procedures, taking advantage of the fast bi-directional sweeping functionality of the 81606A and the continuous logging capability of the N774xA-series power meters, using the N7700A FSIL software engine (option 102).

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