Fabry-Perot tunable filters improve optical channel analyzer performance
Dense wavelength-division multiplexed systems with narrow channel spacings require new approaches in performance monitoring.
Calvin Miller Micron Optics
Siemens Information & Communication Networks
The parameters and accuracies necessary to monitor the performance of optical channels in a dense wavelength-division multiplexing (DWDM) system include wavelength to 䔸 pm, power to ۪.5 dB, and optical signal-to-noise ratio (OSNR, or simply S/N) to ۫ dB over a 10- to 30-dB range. Additionally, technicians must measure these parameters on as many as 128 channels spaced as close as 50 GHz with peak power as low as -30 dBm over a temperature range of typically 0 to +70C in a rack space no more than 2 inches wide. Shock and vibration requirements common to telecommunications equipment and a design lifetime of 10 years round out the incredibly difficult-to-achieve list of characteristics.
There are several technologies capable of measuring some of these parameters to the required accuracy necessary to monitor DWDM optical channels. But currently, the fiber Fabry-Perot tunable filter (FFP-TF), with finesse (ratio of tuning range to bandwidth) equal to 2000 and scan calibrated with an accurate multiwavelength reference, stands alone in its ability to meet all the above optical channel analyzer (OCA) requirements for DWDM at 50-GHz spacing.
A number of tools have been used to measure DWDM channel performance. For example, a conventional diffraction grating optical spectrum analyzer (OSA) has been used to perform power measurements. These OSAs have such a wide dynamic range that the difficult-to-quantify S/N parameter is usually referenced to an OSA with 0.1-nm resolution. But wavelength resolution and accuracy, as well as ruggedness and size, are usually inadequate with an OSA. Diffraction gratings in combination with closely spaced diode detector arrays have improved ruggedness and reduced size compared to a conventional OSA; however, performance at 50- or 100-GHz spacing is poor due to inadequate resolution.
Other equipment choices lack completeness. For example, a Michelson interferometer with a HeNe reference performs all measurements to the required accuracy except at 50-GHz spacing or with a large number of channels, where power and S/N measurements fail. Lensed Fabry-Perot tunable filters lack resolution, accuracy, and along with tunable interference filter technologies, have inadequate temperature stability. Long-period fiber Bragg gratings with long diode arrays do not qualify due to an overall lack of accuracy, especially for S/N. A fundamental problem with most of these technologies (except the OSA) is out-of-band optical rejection, which must be greater than 60 dB (30 dB for S/N, 20 dB for 100 channels of interfering signals, and 10 dB for operating S/N margin).
In principle, multipass Fabry-Perot interferometers and diffraction gratings could have the required resolution, accuracy, and out-of-band rejection. But assembly tolerances are much higher, and therefore thermal stability is worse than the already-inadequate single-pass devices. A comparison between the theoretical spectral response of a single-pass and a double-pass Fabry-Perot filter with the same bandwidth (resolution) indicates the double-pass Fabry-Perot has twice the contrast factor (out-of-band rejection); but these values have not been achieved in practice. Additionally, the finesse requirement for the double-pass filter is reduced by ÷ন -1. Therefore, two passes at a finesse of 1287 are required to achieve the same resolution as one pass at a finesse of 2000.
Admittedly, the foregoing technology assessment is oversimplified, focusing on areas of difficulty for each technology, and represents only today`s level of performance. Considering the variety of technologies, the critical requirement for channel performance monitoring of DWDM systems, and the vast effort being expended, any overview can only be a snapshot in time.
Figure 1 shows the lenseless all-fiber design of the FFP-TF. The short stub of fiber inside the etalon serves as a singlemode internal lens to eliminate extraneous cavity modes and reduce alignment sensitivities. Just as a butt-joint splice performs better than any lensed splice, the FFP-TF outperforms lensed Fabry-Perot filters with an ever-increasing margin at high finesse. In addition, passive temperature compensation and a temperature operating range from -20 to +80C have been standard with the FFP-TF for several years.
The high degree to which theoretical calculations of spectral response around the passband match measurements is shown in Figure 2(a). Figure 2(b) shows the same corresponding match all the way to the bottom of the stop band (out-of-band rejection). These characteristics allow accurate deconvolution of filter responses and its use with 100 or more 50-GHz spaced interfering channels, respectively.
FFP-TFs have been used by the thousands as wavelength-locked demultiplexers in WDM equipment since late 1995. Detailed queries concerning operating hours and field failures (none were reported) were made during the first two years of commercial use and reliability was calculated at 500 FITs (1 FIT is one failure in 109 hours or 114,077 years).
Based on FFP-TFs shipped and reported field failures over the last two years (again, none were reported) reliability is better than 100 FITs. This field reliability record is due to 100% temperature testing of every FFP-TF for field use over typically -20 to +80C or other customer-specified temperature range.
FFP-TF OCA technology
Variable voltage and thermal fluctuations (drifts), as well as the nonlinearity of the mechanical actuator (PZT) and fiber Fabry-Perot optics, necessitate constant calibration of the FFP-TF when used as an optical channel analyzer. This consistency can be achieved with a fixed fiber Fabry-Perot, called the multiwavelength reference, designed to have 25 to 75 wavelength peaks within the tuning range of the scanning FFP-TF. The spacing uniformity of these peaks enables wavelength calibration to better than 5 pm.
The construction of the reference consists of a short length of optical fiber (1 mm gives approximately 100-GHz spacing) with dielectric mirrors deposited directly on the fiber ends and pigtails bonded with accurate alignment. The thermal properties of this reference are extremely linear, allowing either conventional temperature stabilization or temperature measurement and correction. A fiber Bragg grating is used to uniquely identify one peak of the reference. Only the wavelength at each peak of the reference is used, but the accurate power and S/N value of the reference nulls could also be used in the future. The key to FFP-TF OCA technology lies in the stability and accuracy of the reference.
Figure 3 shows a block diagram of the FFP-TF OCA, which contains an optical switch to alternately scan the DWDM spectrum and the known wavelengths of the multiwavelength reference. Details of the wavelength correction algorithms and the thermal performance of this technology can be found in Laser Focus World, March 1998, page 119, another PennWell publication.
FFP-TF OCA performance
Wavelength and power measurement accuracies meet DWDM needs directly. But S/N measurements must be normalized to correspond to an OSA with 0.1-nm resolution. Additionally, the entire OCA spectrum can be deconvolved to partially remove the effects of the FFP-TF spectral response by as much as 9 dB.
The relatively wide skirts of the FFP-TF spectral response shown in Figures 2(a) and (b) collect additional noise power amounting to p/2 times an OSA with 0.1-nm resolution. However, the narrow FFP-TF bandwidth (0.04 nm in this case) reduces noise power by a factor of 2.5. These effects combine to require a 2.02-dB reduction in S/N measurements for a FFP-TF with 5-GHz bandwidth. Due to interfering signal power being present when measuring the noise power for large values of S/N with closely spaced DWDM channels, a given FFP-TF will reach a maximum S/N value denoted S/Nfilter.
This effect can be partially removed by subtracting the fractional value for S/Nfilter as follows:
10-0.1 S/N actual = 10-0.1 S/N measured -10-0.1 S/N filter
Figures 4(a) and (b) compare S/N measurements for an FFP-TF OCA with and without deconvolution, an FFP-TF OCA with deconvolution and correction for S/Nfilter, and a commercial Michelson interferometer. This data was taken using three unmodulated tunable lasers and an ASE noise source to simulate a DWDM system.
The peaked response of the FFP-TF shows spectral detail far greater than an equivalent squared response typical of a diffraction grating OSA. In Figure 5, the 10-GHz modulation sidebands of a 10-Gbit/sec channel are clearly resolved with a 4.4-GHz bandwidth FFP-TF. An ultimate need for DWDM network monitoring is to measure optical parameters required for determining bit-error-rate performance. Increased spectral resolution can only aid in this endeavor.
Compared to other OCA technologies, some mature and well-developed, the new FFP-TF OCA technology utilizing finesse 2000 and a multiwavelength reference has the best performance as an OCA for DWDM systems. Additionally, field worthiness and high reliability have been demonstrated; the fundamental filter element and the simplicity and stability of the reference may allow operation over several years without recalibration.
The existence of the lensed, bulk-optics, super cavity with finesse >40,000 and laboratory FFP-TF devices that reach finesses approaching 10,000 with existing manufacturing processes indicate devices with much higher finesse should appear in the future. Such high-finesse FFP-TF technology may be able to monitor not only wavelength, power, and S/N but also modulation format and other optical parameters necessary to determine bit-error-rate performance. In addition, laboratory FFP-TFs have operated reliably at scan rates exceeding 5 kHz, allowing measurement intervals below 0.2 msec. These capabilities could prove very valuable to the DWDM all-optical network of the future. u
Calvin Miller is president at Micron Optics Inc. (Atlanta, GA) and Lawrence Pelz is a senior optical engineer at Siemens Information & Communication Networks Inc. (Boca Raton, FL)