Recent advances in PLC functional devices
NTT Photonics Laboratories, NTT Corporation, Tokai, Naka, Ibaraki 319-1193,
Phone: +81-29-287-7492, Fax: +81-29-287-7882, E-mail: firstname.lastname@example.org
To achieve much larger capacity in point-to-point wavelength division multiplexing (WDM) transmissions, we must increase their bit rate per wavelength as well as their channel number. Optical networks are continuing to adopt rings, cross-connect, and photonic routing systems. These advanced optical systems require highly functional optical devices to exceed the electrical speed limit. I describe recent advances in planar lightwave circuit (PLC)  based functional devices, such as dispersion equalizers, large-scale arrayed-waveguide gratings (AWGs), and optical signal processing devices for communications .
Figure 1. Configuation of PLC dispersion slope equalizer for 8 x 40 Gb/s WDM transmission.
Figure 2. Configuration of PLC PMD compensator.
Figure 3. Configuration of 10 GHz-spaced 1010-channel AWG filter.
Figure 4. Configuration of D/A converter.
The precise compensation of the dispersion slope of dispersion-shifted fiber (DSF) or non-zero DSF in high bit rate WDM systems is becoming increasingly important. PLC dispersion equalizer, which we have developed, has unique characteristics including compactness, dispersion variability, and the ability to provide flexible and accurate dispersion compensation . Figure 1 shows the configuration of our PLC slope equalizer, which consists of an array of cascaded Mach-Zehnder interferometer equalizers with various compensation values and multiplexing AWG. We realized 8 x 40 Gb/s and 16 x 20 Gb/s equalizers, whose operational electrical bias power was greatly reduced to less than 1 W in total by employing local heating and quenching with high electrical power to thermo-optic heater , . Polarization mode dispersion (PMD) is also becoming a problem in long-haul high-speed transmission systems. We reported on the integrated-optic PMD compensator shown in Fig. 2. A deteriorated optical signal is input into an endless polarization controller and converted into TE and TM modes. The two modes are then introduced into a polarization dependent delay line for compensation and combined again. Detailed results will be reported at ECOC 01 .
Large-scale and high-density WDM filters are important in dense WDM systems. Figure 3 shows a 10 GHz-spaced 1010-channel WDM filter that covers both the C and L fiber bands, where ten 1 x 101 AWGs (AWG#k with k=1, 2, ... ,10) with a 10 GHz spacing are connected to a primary 1 x 10 flat-top AWG with a 1 THz channel spacing. The phase errors of the secondary 10 GHz AWGs were compensated for by the photoinduced refractive index change .
Optical signal processing devices for communications
Optical label recognition will be important for the coming packet-switched networks. Figure 4 shows a primitive circuit for recognizing a 10 Gb/s and 4-bit optical pulse pattern. The pulse train is split into four replicas that are delayed and weighted by mDt and 2m-3, respectively (m=0-3. Dt: pulse interval). One time slot of the multiplexed output pulses is extracted by gating. Thus the label is recognized by this digital-to-analogue (D/A) conversion in the optical region .
I described recent progress on PLC functional devices. I expect these devices to play important roles in the coming advanced optical networks.
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