T. Chiba, H. Arai, K. Ohira, H. Nonen, H. Okano and H. Uetsuka
Optoelectronic System Laboratory, Hitachi Cable, Ltd., Hitaka-cho 5-1-1, Hitachi-shi,
Ibaraki-ken 319-14, Japan
Tel: +81-294-25-3833, Fax: +81-294-43-7487 E-mail: chiba@lab.hitachi-cable.co.jp
A density of channels in WDM systems has been higher and higher to accommodate the exploded traffic demand. Recently, a 1010 channel AWG of which the channel spacing is 10GHz has been reported[1]. On the other hand, there is a flexible system configuration to upconvert or downconvert the WDM signal by using a interleave filter, which is also called wavelength splitter / slicer / interleaver, as shown in Fig. 1. In the illustration, the 1st stage is for a band separation, the 2nd stage for up/down conversion, and, at the 3rd stage, each WDM signal is separated by AWGs. In order to make this system operate, a wavelength splitter at the 2nd stage is required to have the Box-like characteristics such as the periodic response, flat-passband, low insertion loss and low crosstalk. In addition, low chromatic dispersion (CD) is indispensable for DWDM systems.
In this paper, we present wavelength splitters with a tandem configuration of Fourier transform-based MZIs. The circuit using PLC technique is fabricated with high index contrast waveguides of ?1.5%.
|
Figure 1. DWDM filter system. |
 |
Figure 2. Wavelength splitter with configuration of dispersion compensation. |
Figure 2 shows our PLC-type wavelength splitter circuit configuration. The filter consists of three identical 3-stage Fourier filter circuits. Taking the phase conjugate relationship into consideration, the port #9 of Circuit 2 is connected to Circuit 1, and the port of #10 of Circuit 2 is connected to the port #11 of Circuit 3. In this configuration, the port combinations of #3 <=> #2 and #3 <=> #6 allow CD-free optical performance over the entire wavelength range of interest[2,3]. The phase conjugate relationship is maintained in the case of the port combinations of #4 <=> #1 and #4 <=> #5.
The configuration is useful to not only CD compensation but also smart integration of two independent circuits. In general, if two same circuits are integrated on a substrate, each circuit is arranged side by side. In the case of the wavelength splitter, each circuit can share part of its components, such as delay lines and couplers between two circuits, which are separated only by port selections. So the unique characteristic can apply very well to simultaneous MUX/DEMUX operation[4].
We fabricated a 50GHz-to-100GHz wavelength splitter based on the above design by using a high index contrast silica-on-silica PLC process. The \delta is 1.5% and the minimum curve radius is 2 mm. Consequently, the chip size was able to be shrunken 37x25 mm^2, which is about 30% that with \delta 0.8%. We used a thermally expanded core (TEC) technique [5] to reduce the mode field miss match loss between a standard single mode fiber (SMF) and the\delta 1.5% waveguide.
 |
Figure 3. Measured spectral response of fabricated 3-stage tandem type (the paths #3 to #2 and #3 to #6). |
 |
Figure 4. Measured CD distribution and spectra (the paths #3 to #2 and #3 to #6). |
Figure 3 shows the measured spectral response of the wavelength splitter shown in Fig.2.
The fabricated chip demonstrates a periodic box-like response with low crosstalk and a, flat pass-band. The insertion loss is less than 2 dB. Figure 4 shows the measured CD distribution on the pass band of the 3-stage tandem-type circuit for the path between #3 and #2 and for the path between #3 and #6. The CD is almost compensated.
We have demonstrated a chromatic dispersion free Fourier transform-based wavelength splitter. The device meets performance requirement necessary for actual DWDM systems. This wavelength splitter is not only applicable to MUX/DEMUX, but also valid for such applications as optical add-drop filter. 5.