Super compact optical add-drop multiplexer for FTTH applications based on low-loss polymer waveguide materials N. Keil, H.H. Yao, C. Zawadzki, F. Beyer, O. Radmer, M. Bauer and C. Dreyer Four- and eight-channel arrayed-waveguide grating (AWG) and fixed optical add-drop multiplexer (OADM) devices with channel spacing of 1200 and 600 GHz have been fabricated using super-high refractive index contrast (Dn ¼ 0.020) triazine containing polymers. Accord- ingly, the size of the four-channel AWG was only 10  3 mm and the insertion loss was 3 dB. Introduction: Fibre-to-the-home (FTTH) is the ultimate broadband environment and has been developed at an accelerated rate in many countries recently. A key issue for FTTH systems is the availability of inexpensive optical devices, such as splitter, multiplexer=demultiplexer, and optical add-drop multiplexer (OADM). Optical polymer waveguide devices are attractive for implementing such components because they offer the potential of fairly simple and low-cost fabrication based on low-temperature processes and low-cost packaging based on passive alignment [1–3]. The optical loss of optical polymers has been reduced to less than 0.1 dB=cm recently [4].A critical cost factor is the producible number of chips per wafer being solely determined by the refractive index contrast (Dn) of the waveguide materials. Typical polymer waveguide devices have Dn below 0.011, leading to a rather large chip size. Higher Dn values not only result in smaller devices and higher integration density but would also counter- act the propagation losses, to yield a low-loss compact device. In this Letter, we report results on compact polymer waveguide devices for FTTH applications using the polymer material triaziacrylate featuring a loss of 0.3 dB=cm at 1550 nm wavelength and a refractive index contrast of Dn ¼ 0.020. Four and eight channel AWGs and OADMs with a channel spacing of 1200 and 600 GHz, respectively, have been realised. The device size of the four-channel AWG is only 10  3 mm, allowing the accommodation of more than 150 pieces on a 4-inch wafer and 400 pieces on a 6-inch wafer. The insertion loss amounts to 3 dB because of the small size, and the crosstalk was <25 dB. Design and layout: Using the super-high refractive-index contrast of Dn ¼ 0.020 for the polymer waveguide, a core size of 4.2  4.5 mm was chosen. The bending radius was 2.2 mm. The coupling loss between the standard singlemode fibre and the polymer waveguide of this index contrast is more than 1.5 dB=facet. To reduce this value, a laterally tapered waveguide is incorporated at each facet [5], enabling the coupling loss to be decreased to 0.25 dB if the endface width of the taper is around 1 mm. drop add input output l 1 , l l l 2 ,..., ,..., i N l 1 , l l l 2 ,..., ,..., i N l 1 l 2 l i l i l N Fig. 1 Schematic layout of fixed N-channel OADM The integrated compact fixed N-channel OADM consists of two cascaded AWGs as schematically depicted in Fig. 1. AWG1 is used as 1  N demultiplexer and AWG2 is used as N  1 multiplexer. The wavelengths of the N channels are l 1 , l 2 , ... , l N . The input signals are demultiplexed by AWG1 and the selected input optical signals at l i are dropped at the ith output port of AWG1 (drop port). The add signals at the wavelength l i are fed into the ith input port of the AWG2 (add port). The demultiplexed input signals together with the add signals at wavelength l i , except the drop signals, are fed to the corresponding input ports of AWG2. After multiplexing, these signals will be directed to the output port of AWG2 to form the common output signals of the OADM. In Fig. 1, only one channel is dropped and added. Obviously, any number of channels can be dropped and added as long as this channel number is less than or equal to the OADM channel number N. Fig. 2 depicts the mask layout for a four-channel fixed OADM with a channel spacing of 1200 GHz. For simplicity, only the first channel (l 1 ) is dropped and added. The eight-channel OADM with a channel spacing of 600 GHz has a similar mask layout, except that the AWG, which represents the key building block of the OADM architecture, has eight ports. Fig. 2 Mask layout of polymer four-channel OADM Experimental results: Novel triazine containing polymers with a refractive index contrast of Dn ¼ 0.020 and a propagation loss of a ¼ 0.3 dB=cm for the 1.55 mm wavelength region were used in this study. This waveguide material was deposited on polycyanurate polymer substrate by spin-coating to form all-polymer AWGs and all-polymer OADMs. Conventional photolithography and reactive ion etching were employed for waveguide fabrication. The spectral transmission characteristics of the fabricated devices were measured using a tunable laser and optical power meters. The spectrum of the four-channel AWG (channel spacing of 1200 GHz) is depicted in Fig. 3a. The measured insertion loss was as low as 3 dB for the central channels and the crosstalk was around 25 dB. The respective eight- channel AWG (channel spacing of 600 GHz) is shown in Fig. 3b. Again, the insertion loss was determined to be the same 3 dB for the central channels and the crosstalk around 28 dB. For this device, the chip size was 13  4.5 mm. P opt , dBm l, m m 1520 1530 1540 1550 1560 b 0 -10 -20 -30 -40 P opt , dBm a 0 -10 -20 -30 Fig. 3 Measured transfer characteristics a Four-channel, 1200 GHz AWG b Eight-channel, 600 GHz AWG In Fig. 4a the measured transfer characteristics for the four-channel OADM with a channel spacing of 1200 GHz is shown. The drop wavelength channel at l 1 passes AWG1 only, whereas the other channels at wavelengths l 2 , l 3 and l 4 pass AWG1 and AWG2 and appear at the output port. Therefore, these through channels exhibit narrower response than the drop channel. The measured insertion loss was 3.8 dB and the crosstalk < 25 dB. The chip size of the OADM measures only 18  2.5 mm. Fig. 4b shows the measured transfer characteristics of the eight-channel OADM. Here, the insertion loss and crosstalk were 4.5 dB for the central channels and <25 dB, respectively. Chip size is 22  4 mm. Fig. 5 displays these fabricated all-polymer devices, and Table 1 lists key parameters of the fabricated all-polymer AWGs and OADMs with a Dn ¼ 0.020. ELECTRONICS LETTERS 17th February 2005 Vol. 41 No. 4