An all-optical dynamic gain tilt compensator (DGTC) is proposed and experimentally demonstrated. A single wide-band thin film filter and a pair of photodetector allow the DGTC to distinguish band add/drop position. Power fluctuations from EDFA gain tilt were reduced with fast electronic variable optical attenuators.
© 2006 Optical Society of America
The presence of saturated erbium doped fiber amplifiers (EDFA) in optical networks which experience abrupt change of data traffic optical add/drop will introduce power transients in the surviving channels  and is a potential issue for future reconfigurable networks . Dynamic transient control is therefore essential if reconfigurable optical add/drop multiplexers (ROADM) are to be used in optical mesh networks. Previous work on transient compensation, including all-optical control assisted by linearized electrical circuit  and link control scheme , have demonstrated that transients may be suppressed with minimal interruption in network traffic In a metro network, band add/drop will however also introduce a wavelength dependence in the gain transient because of the predominantly homogeneous broadened gain characteristic of EDFA . Dynamic gain tilt must therefore also be catered for to avoid the accumulation of excess optical power in optical channels in reconfigurable optical mesh networks. In this paper, we propose and perform a proof-of-concept demonstration of a per-link based dynamic gain tilt compensator (DGTC) employing a feed forward controlled electronic variable optical attenuator (EVOA). The EVOA controller uses a pair of thin film filter for low cost gain tilt monitoring and provides a stable reference for the dynamic gain tilt compensation . We demonstrate the operation of the system in a three band DWDM system, in which the wavelengths are added or dropped in one of three bands within the EDFA gain bandwidth.
2. Dynamic gain tilt compensation
We measure the dynamic gain tilt after dropping channels at different wavelengths in an EDFA link. In Fig. 1, for example, dropping of Band A introduces 0.40 dB power increase of channels in Band B while channels in Band C undergo 0.34 dB power increases. Due to the wavelength dependent gain characteristics of EDFAs, dropping of longer wavelengths (Band C) causes higher power fluctuation than that of shorter wavelengths resulting in dynamic gain tilt of the whole system. The resultant dynamic gain tilt can propagate and accumulate throughout the reconfigurable network, and is difficult to compensate at the receiver since the received traffic may have been routed all-optically between an arbitrary number of nodes and transmitted through a variable number of EDFAs.
The proposed dynamic gain tilt compensator (DGTC) needs to determine the position of band add/drop and generate the appropriate optical attenuation to compensate the change in gain in each of the surviving bands to maintain a near constant gain level immediately after each add or drop. The DGTC employs a pair of photodiodes with different spectral responses to measure the gain tilt. A small fraction of the power in the optical fiber transmission line is tapped, split and detected by the two photodiodes. One of the two photodiodes has the spectral filter placed in front of it as shown in Fig. 2. By measuring the relative output from each photodiode and using the known spectral response of the optical filter, it is possible to determine the wavelength band of the add/drop and generate the appropriate control signal for the predefined gain tilt compensation for the given EDFA link. The electronic control circuit operates with 2.2µs delay from the detectors to EVOA, and a fiber delay was inserted to cater for this delay.
The thin film filter used for the gain tilt monitor was designed by Essential Macleod  and was fabricated by an ion beam assisted e-beam deposition system. The filter consists of alternating layers of SiO2/Ta2O5. Using the measurement of average detected power from the filtered and unfiltered photodiodes, the control circuit shown in Fig. 3 generated the required outputs needed to drive the electronic variable optical attenuator. Deviation of power levels detected from the photodiode pair gave an indication of the wavelength position of the optical band that was added or dropped. The logarithmic amplifier produced a signal proportional to the log of the ratio of the photodiode signals. This was fed to a window comparator and analog switch which outputted the pre-calibrated current levels for the precise power adjustments needed for the gain tilt compensation.
3. Experimental results
We demonstrated the operation of the DGTC in a three-band system containing a single unclamped EDFA without additional gain control. Each band (namely Band A, B and C) consists of 8 DWDM channels (100 GHz spacing) with equalized optical power after the EDFA link as shown in Fig. 4(a). A near logarithmic filter response (near -0.3 dB/nm) was used with one of the photodiodes. The spectral response of all three bands after this logarithmic filter is shown in Fig. 4(b). The free space coupling loss of filter and photodiode was 3 dB.
Table 1 summarizes the reference level of the control circuit with fixed power ratio. With add/drop of different bands inducing average power fluctuation, the power ratio differs from the reference level and drives the EVOA to various attenuations. The EVOA may be biased at a non-zero attenuation so that adding of bands (or decrease of power in surviving channels) may be compensated by reducing the drive current.
We experimentally tested the gain tilt compensation by modulating Band A using an optical modulator at a frequency of 1 kHz. 24 channels carrying -35dBm optical power each were amplified by a saturated unclamped EDFA with 23dB gain for each channel. A flattened spectrum after EDFA was obtained after gain flattening technique as appear in Fig. 4 (left). The removal of Band A causes average power fluctuations in Band B (17%) and Band C (12%) after the constant-pump-current EDFA. With the DGTC, power fluctuations were reduced as shown in Fig. 5.
Finally we compare the 10 Gbit/s 231-1 PRBS bit error rate (BER) of a surviving channel in Band B with and without compensation. Figure 6 shows the BER after 3m and 40km optical fiber before and after the transient compensator is added. 3 dB power penalty improvement was observed after 3 meters fiber transmission. If the uncompensated signal is allowed to travel through 40km fiber (without dispersion compensation), the signal is degraded by the add/drop as shown in the inset of Fig. 6. The eye diagram of the uncompensated signal shows two-level amplitude induced by add/drop of Band A channels, and this is removed by the DGTC.
In the experiment an 80:20 fiber coupler was used to tap the signal from the transmission line and arrayed waveguide gratings were used for the wavelength multiplexer and demultiplexer. The total loss of the gain tilt compensator was about 7 dB comprising 1.5–2 dB insertion loss for the EVOAs, 1 dB loss from the 20% tap for the gain tilt monitor and about 5 dB insertion loss from the arrayed waveguide grating based wavelength demultiplexers and multiplexers. The insertion loss may be greatly reduced if lower insertion loss multiplexers/demultiplexers were used. A lower loss implementation based on commercially available thin film filter filters for the mulitplexers and demultiplexers  is proposed in Fig. 7. The thin film filter based fiber-pigtailed multiplexers and demultiplexers have transmission and reflection losses of 0.6 dB and 0.4 dB, respectively. With EVOA of less than 2 dB insertion loss, it should be possible to reduce the insertion loss of the proposed compensation scheme to less than 4 dB.
4. Discussion and conclusion
The gain tilt detection via the use of a pair of photodiodes and a thin film filter was found to operate reliably in the three-band system. The stability and sensitivity of dynamic gain tilt compensation may be improved by using a steeper slope filter. The technique provides optically transparent compensation and is suitable for re-configurable optical network at different bitrates. The tilt monitoring and compensation scheme has potential for low cost implementation by depositing the required thin film filter onto the photodiode die for minimizing the package size and potential integration with an electronic variable optical attenuator.
In summary, we proposed and demonstrated a dynamic gain tilt compensator (DGTC) using thin film filter monitoring technique for different band power fluctuation. The all-optical gain tilt compensation is potentially low cost and bitrate transparent and may make it attractive for future reconfigurable and banded DWDM networks.
This work was fully funded by RGC Earmarked grant CUHK4198/03E.
References and links
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8. See for example the mico-optic thin film filter (TFF) bandpass products at http://www.fibercom.com.tw