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Ultrawide-Passband Tandem MZI-Synchronized AWG and. Group Delay Ripple Balancing Out Technique. M. Oguma, T. Kitoh, A. Mori, and H. Takahashi.
ECOC 2010, 19-23 September, 2010, Torino, Italy

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Ultrawide-Passband Tandem MZI-Synchronized AWG and Group Delay Ripple Balancing Out Technique M. Oguma, T. Kitoh, A. Mori, and H. Takahashi NTT Photonics Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi, Kanagawa, Japan, [email protected]

Abstract We propose and demonstrate a novel wide passband MZI-synchronized AWG and a simple way to eliminate its group delay ripple. We obtained a 0.5-dB passband of 69 GHz and a 0.5 ps total GDR for a dispersion balanced out pair.

Introduction The constantly growing demand for larger transmission capacity has driven progress on optical equipment and devices. Spectral efficiency must be improved for both the transmission format and optical multi/demultiplexers. Silica-based arrayed waveguide grating (AWG) filters are now widely used in commercial transmission equipment, such as reconfigurable optical add/drop multiplexers (ROADM), because of their large channel count, high reliability and mass producibility. However, the passband width of the conventional type is insufficient for higher bit rate systems operating at, for example, 40 or 100 Gbps, especially in ROADM systems where the signal passes through many AWGs. Some ways of enlarging the passband width have already been reported. Of these, the most effective method is to use an additional MZIbased interferometer that controls the precise position at which light is into the AWG according to the signal wavelength. To date, configurations employing a single Mach1,2 Zehnder interferometer (MZI) , a three delay 3 arm interferometer and three MZIs4 as additional interferometers have been proposed and demonstrated. The last of these achieved a wide passband comparable to that of a typical wavelength selective switches (WSS)5. However, when we select a wider passband configuration, the additional interferometer becomes more complex, and its fabrication error adjustment also becomes more complicated if we are to obtain a satisfactory phase property in the passband3. For this reason, only a single MZI-synchronized AWG has yet been commercialised. We have therefore proposed a tandem MZI synchronized AWG with a simple additional interferometer6. We achieved a large 0.5-dB passband width of 59 GHz and a group delay ripple (GDR) of 2 ps, which is sufficient for a 40Gbps DQPSK ROADM system. To realize 100Gbps or higher bit rates in the future, we need to

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expand the bandwidth further without any increase in GDR. In this paper, we report a new tandem MZIsynchronized AWG that is modified by using a 1st order mode assistance method7 to enlarge its passband. We also propose a simple way of balancing out its group delay ripple by using a pair of tandem MZI-synchronized AWGs as a multiplexer and a demultiplexer in the ROADM node. Moreover, we describe a compact integrated layout that is suitable for ROADM applications, which includes a dispersion reversed AWG pair in one chip. Design for wide passband width Figure 1 shows the schematic layout of our improved tandem MZI-synchronized AWG. As shown in the magnified view, this circuit employs a 1st mode converter and a multimode

Fig. 1: Schematic layout of tandem MZIsynchronized AWG employing 1st Mode converter

(a) (b) Fig. 2: Explanatory drawing showing the difference between (a) the previous work and (b) this work as regards circuit structures and their effects

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waveguide instead of a directional coupler for connection between the tandem MZI and the slab waveguide of the AWG. Figure 2 explains the effect of the connection difference. When we position the two waveguides of directional coupler too apart in order to obtain a wide passband, the combined spot shape has two peaks at around of channel centre frequency as shown in Fig. 2(a). Because this type of distortion from a Gaussian beam easily induces loss ripple in the passband, the design flexibility of this type of synchronized AWG is very limited. On the other hand, if we employ a structure consisting of a mode converter and a multimode waveguide, and even if we also increase the multimode waveguide width, the combined spot shape around the centre frequency has only one peak, and its spot size becomes slightly wider as shown in Fig 2(b). Because excess loss due to this type of distortion is smaller than that of the former one, we can design this type of circuit more freely and thus expand its passband. Experimental transmission results To confirm the above expectation, we designed and fabricated a 40-channel tandem MZI-synchronized AWG with a 100-GHz channel spacing by using a conventional GeO2doped silica-based waveguide with a relative index difference of 1.5% on silicon. The coupling ratios of the two directional couplers in the tandem MZI were designed to be 14%. The free spectral ranges (FSR) of the two delay parts in the tandem MZI were set at 50 and 100 GHz, respectively.

The transmission spectra of all 40 channels are shown in Fig. 4. Due to good synchronization with the tandem MZI and AWG, all the shapes were very similar. The worst and average values for all the channels are listed in table 1. We can see the good uniformity of all the properties.

Fig. 4: Transmission spectra of all 40 channels of fabricated tandem MZI synchronized AWG employing mode converter and multimode waveguide Table 1: Worst and average properties for all 40 channels Insertion Loss (dB) Loss Ripple (dB) Extinction Ratio (dB) Passband Width (GHz)

Fig. 3: Detailed spectrum of centre port

Figure 3 shows the measured transmission spectra of the fabricated tandem MZI synchronized AWG for channel 20 (centre channel). As expected it was wide and had a flat top. The fabricated circuit achieved a wide 0.5-dB passband width of 69 GHz with a small loss ripple of 0.13 dB within +/-30 GHz of the channel centre frequency. These values are 1.17 times greater and one-third smaller than 6 that of previous work .

demultiplexing

Definition

Worst

Average

ch centre

2.8

2.4

+/-20G +/-30G +/-20G +/-30G 0.5 dB 1 dB 20 dB

0.20 0.37 20.3 18.0 67.7 74.4 87.3

0.16 0.12 22.8 18.9 69.4 75.5 88.4

Dispersion balanced out design In addition to the amplitude properties, the phase response is an important factor in term of realizing good transmission quality, especially in a large-scale network that contains a lot of ROADM equipment. In general, an arbitrary cascaded MZI filter has a nonlinear chromatic dispersion response, which is measured as GDR or phase ripple. Thus there is also the risk that a tandem MZI-synchronized AWG will exhibit its undesirable characteristic. The GDR of the tandem MZI-synchronized AWG with improved amplitude properties was 6 ps, which is too large for use in an actual transmission system. Therefore we incorporated a dispersion

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cancelling method8 in this MZI-synchronized AWG. The method uses a special feature of the lattice form filter which consists of two waveguides. It is known that we can reverse the phase response while maintaining the amplitude properties, simply by simultaneously exchanging the input and output ports of this type of filter. Because a tandem MZI is this type of filter, the nonlinear group delay curve of the synchronized AWG can be reversed by exchanging ports as shown in Fig. 5. Taking care that the port connection between the tandem MZI and the mode converter are

(a)

(b)

Fig. 5: Port exchange to reverse group delay curve. White arrow indicates input port and black and gray arrows indicate output ports that should be connected to the converter input for 0th and 1st modes, respectively.

Fig. 6: Compact circuit layout of tandem MZIsynchronized AWGs which are dispersion balanced out pairs

Fig. 7: Group delay responses of circuit A, circuit B and the sum of the dispersion balanced out pairs, and transmittance of circuits A and B

exchanged, we can locate a dispersion balanced out pair of tandem MZI-synchronized AWGs in small chip that is 25 mm wide and 40 mm high. Total group delay response results Figure 7 shows the individual and total group delay response of the fabricated dispersion reversed pair. It also shows the transmission of one circuit to indicate the passband region. Although individual curves have large ripples, they cancelled out their respective nonlinear group delay responses so completely that their total GDR within the 3-dB passband became about 0.5 ps. Conclusions We proposed and successfully demonstrated an improved tandem MZI-synchronized AWG and reported the cancellation of its group delay characteristics. The fabricated circuit, which employs a 1st mode converter and a multimode waveguide at the connection between the tandem MZI and the slab region of the AWG, exhibited a wide 0.5-dB passband width of 69 GHz against a channel spacing of 100 GHz and a small loss ripple within +/-30 GHz of 0.13 dB at the centre channel. The measured transmission spectra of all 40 channels indicated such good synchronization between the tandem MZI and AWG that we obtained uniform demultiplexing properties over 40 channels. This simple way to balance out the group delay ripple also works so well that the total GDR within the 3-dB passband was about 0.5 ps for a multiplexing and demultiplexing pair. Moreover this dispersion balanced out pair was laid out on small 25 x 40 mm chip. We believe that this wide passband and dispersion balanced out pair of tandem MZIsynchronized AWGs can be applied to an ROADM system with 100-Gbps or higher bit rate transmission systems. References 1 C. Dragone, J. Lightwave Technol., 16, 1895 (1998) 2 C. R. Doerr, et al. Photon. Technol. Lett., 15, 920 (2003) 3 C. R. Doerr, et al., Photon. Technol. Lett., 18, 2308 (2006) 4 K. Maru, et al., Opt. Express, 17, 22260 (2009) 5 P. Wall, et al., OFC2008, OWC1 6 S. Kamei et al., ECOC2009, PD1.6 7 M. Kohtoku et al., Electron. Lett., 38, 792 (2002) 8 M. Oguma et al., J. Lightwave Technol., 22, 895 (2004)