Both high-density multiplexing and high-speed transmission are required for large capacity networks supporting broadband service penetration. Silica waveguide optical circuit technology can provide key optical devices for such systems because it has high controllability as regards optical amplitude and sufficient phase for realizing sophisticated optical interferometric circuits. This paper reviews recent progress on this technology, and reports experimental results for an ultra wideband AWG, an optical OFDM filter, a demodulator for a coherent receiver and a highly functional modulator.
© 2011 OSA
Transmission with high spectral efficiency is an indispensable technology for large capacity optical network systems because of the limited bandwidth of the transmission window in silica fibers and optical amplifiers . Ordinary dense wavelength division multiplexing (DWDM) systems provide 80 channels with a 50 GHz spacing and a bit rate of 10 Gb/s. In this case, the efficiency is 0.2 bit/s/Hz. We have two approaches for increasing the efficiency: one is to increase the bit rate per channel with multilevel modulation, and the other is to densely multiplex the channels with narrower spacings. For the former, we can introduce modulation formats such as dual polarization quadrature phase shift keying (DP-QPSK) and quadrature amplitude modulation (QAM) by using advanced modulators and digital coherent detection technologies. For the latter, we require ultra wideband DWDM filters and optical orthogonal frequency division multiplexing (OFDM) filters.
Since the modulation format is multi-level in such transmission systems, the distances between constellation points are shorter than those with binary on-off keying modulation. This means the bit error rate of the signal is easily increased by impairments such as ASE noise from EDFAs, band narrowing, crosstalk and phase distortion in optical DWDM demultiplexers, and phase error and polarization crosstalk in the demodulators in coherent receivers. Of course, the signal itself must be ideal by being generated with a modulator with high linearity. Namely, optical components should be closer to ideal when used in high-speed, high-capacity transmission systems employing multilevel modulation and coherent detection.
Silica glass waveguide-based planar lightwave circuit (PLC) technology  enables us to realize the high performance optical devices needed for the above systems because such waveguides have low propagation and fiber connection losses, mass producibility and a highly controllable propagation constant. These characteristics have been proven in a range of commercial products including splitters for fiber-to-the-home (FTTH), arrayed-waveguide-grating (AWG) wavelength multi/demultiplexers for DWDM, and integrated optical switch arrays for reconfigurable optical add/drop multiplexing (ROADM) systems. These facts show that silica waveguides have excellent propagation characteristics and light amplitude and phase in the waveguides can be precisely controlled, thus enabling us to fabricate interferometric optical circuit devices for use in the high capacity transmission systems described above.
This paper reviews recent progress on silica waveguide devices, focusing on ultra wideband AWG demultiplexers and optical Fourier transform-based OFDM filters, demodulators in coherent receivers, and highly functional modulators, recently developed for the large capacity transmission systems of the near future. The experimental results will show that silica PLC devices can be used in such systems.
2. Fabrication and design of silica optical waveguides
Our lightwave circuits consist of a germanium-doped silica (SiO2 glass) waveguide core embedded in a silica cladding layer on a silicon substrate as shown in Fig. 1 , fabricated with flame hydrolysis deposition (FHD), photolithography and reactive ion etching. Because the waveguide is kept away from the atmosphere, its propagation constant has good long-term stability, which is needed for phase sensitive interferometric devices.
Table 1 lists the characteristics of silica waveguides according to their core-cladding refractive index difference (Δ). As the difference increases, the minimum bending radius decreases, and we can easily design high-density optical circuits with high functionality. The coupling loss with a standard single mode fiber will increase as Δ increases. We can reduce the coupling loss by using a spot size converter . In the PLC devices reported later in this paper, we used 1.5% Δ waveguides.
We can use two optical phase adjustment methods for a silica waveguide circuit. One is based on the thermo optic effect . A thin film heater is mounted on the waveguide, and we can temporarily change the refractive index by heating. A π phase shift requires 100 mw of electrical power. The other method is UV-irradiation based on a germanium-oxygen bond defect . If we irradiate the waveguide with an argon fluoride excimer laser at a wavelength of 193 nm, the refractive index changes permanently. An irradiation time of 30 seconds is needed for a π phase shift in 3.5 mm long irradiation area. There is little excess loss or polarization dependence with either technique. The thermo-optic method is useful for dynamically tuning the optical characteristics for system operation, and the UV-irradiation method is useful for optical length trimming to obtain perfect interferometric characteristics in passive circuits.
Waveguide shape can be optimized with a powerful design method based on wavefront matching (WFM) [5,6]. As shown in Fig. 2(a) , the design area is composed of small pixels. The waveguide shape can be designed by deciding the refractive index of each pixel. Our goal is, for example, to create an index distribution that allows light to propagate from the input on the left to the offset output on the right. The blue lines indicate the wavefront calculated for the initial index distribution with the beam propagation method (BPM). The red lines show the wavefront for time-inversely propagated light from the output. Taking account of the two wavefronts, we adopted a high index (core index) at the pixel in between to reduce the light speed, and a low index (cladding index) to increase the light speed. The desired index pattern is automatically generated by repeating this calculation several times over the entire design area. With the WFM method, the waveguide shape quickly changes and forms the optimized pattern because the index at each pixel is selected deterministically. Figure 2(b) shows examples of the WFM-based optimized waveguide shape. An irregular rather than a smooth waveguide shape purposely induces mode conversion and forms the electrical field with twin peaks needed to reduce the scattering at a Y-branch. As a result, the branching loss is reduced. For similar reasons, the scattering loss and undesired crosstalk to the other waveguide is reduced at an X-crossing. Loss reduction generally means a reduction of the conversion to undesired modes such as the higher order modes and leaky modes. Namely, WFM leads to a reduction in the phase error caused by interference from the undesired modes. So the WFM helps us to control both optical amplitude and phase and is useful for obtaining ideal characteristics in interferometric optical circuits.
3. Rectangular passband AWG demultiplexer
As the modulation rate increases and the signal bandwidth becomes wider, the passband width of the AWG demultiplexer must become wider and the passband spectral shape rectangular. A useful approach is to add a Mach-Zehnder interferometer (MZI) in front of the AWG input port , and this technique has been used in commercial 10 Gb/s, 50 GHz-spacing DWDM systems. For a system operating at 100 Gb/s or higher, we have proposed a tandem MZI-synchronized AWG as shown in Fig. 3(a) , which has a more rectangular spectrum . The input waveguide connected to the first slab waveguide is multimode, and a combination of 0th and 1st modes forms a single-peak electrical field, which is perturbed depending on the wavelength due to the tandem MZI. As a result the focusing spot in the output slab waveguide remains at the center of the output waveguide entrance, thus attaining a flat transmission spectrum as shown in Fig. 3(b). The 0.5 dB bandwidth is as wide as 70 GHz (70% of the channel spacing) for all forty 100 GHz-spaced channels.
Another important factor is the group delay characteristics, which affect the high bit rate signals, and we have proposed a group delay cancellation technique. As shown in Fig. 3(a), two AWGs with slightly different waveguide layouts are placed in a single chip. In circuits A and B, the waveguides for 1st mode excitation are on the left and right side, respectively, of the multimode input waveguide. This difference inverts the group delay spectrum shown in Fig. 3(b). Red rectangles, blue crosses and green dots indicate the group delays for circuits A, B, and A plus B, respectively. As shown by the green dots, group delay is greatly reduced to less than 0.5 ps. The transmission curves for A and B are the same. If we use circuit A for the multiplexer and B for the demultiplexer, the total group delay ripple in the transmission line is cancelled out.
4. Optical OFDM filter
Orthogonal frequency division multiplexing (OFDM) in both the electrical and optical domains is actively investigated as a promising candidate for realizing denser multiplexing than DWDM. To demultiplex OFDM signals in the optical domain, an optical Fourier transform (FT) filter is required instead of the AWG used in DWDM. We have proposed two types of FT filters: discrete FT (DFT)  and fast FT (FFT) . The DFT filter can deal with an arbitrary channel count while the FFT filter is limited to a power-of-two channel count but its insertion loss is low. Figure 4(a) is a schematic of the waveguide circuit of the FFT type (N = 4). The input light is split into four delay lines and exchanged, coupled, exchanged and coupled in the same ways as the butterfly computation shown in the transfer function, where S(t) is an OFDM signal, S'(kΔt) is delayed S(t), dn is a demultiplexed signal, and Δt is a unit delay.
The chip is 17 x 70 mm2 in size and designed for four-channel OFDM with a 10 GHz spacing. Figure 4(b) is the measured transmission spectra for four output ports, showing that all the transmission dips for a certain port coincide with the transmission peak wavelengths of the other ports. This is a characteristic of an OFDM filter. We used a symmetric MZI with a thermo-optic phase shifter as the coupler to tune the coupling ratio to exactly 50%. This enabled us to realize a transmission dip of less than −25 dB. The insertion loss was about 2 dB. We input the 10 Gb/s x 4 OFDM signal and examined four output signals with a 10 Gb/s BER tester. A time gating device (25 ps optical gate switch) was placed in front of the tester to measure the BER in the time window when all the delayed signals overlapped. The power penalty was 0.3 to 2 dB, and no BER floor was observed.
Similar FFT-based OFDM filter is proposed by D. Hillerkuss . They use cascaded MZI structure which has the same spectral response as that of our result. Feature of our configuration is that the delay lines are arrayed in parallel and occupies smaller area.
5. Coherent receiver frontend
As shown in Fig. 5(a) , a coherent receiver is composed of many optical and electrical devices, and we need to integrate them to reduce cost and size. We proposed that the shortest way to achieve this was to integrate all the passive optical parts. The PLC chip (including a polarization splitter, a polarization rotator, a power splitter and two 90° optical hybrids) is called a dual polarization optical hybrid (DPOH), and it is shown schematically in Fig. 5(b). The polarization splitter is based on an MZI and the birefringence of one arm-waveguide is different from that of the other. This imbalance results in a polarization splitting function. As shown in Fig. 6(a) , the polarization extinction ratio is higher than 27 dB over a wide 100 nm range thanks to the special wideband coupler used in the MZI . A polyimide film waveplate is inserted across the waveguide and works as a polarization rotator. The 90° hybrid is composed of four 2 x 2 multi-mode interference (MMI) couplers connected to each other.
In our design, we balance out the wavelength dependence on the phase (the λ/4 path length in the hybrid is no longer 90° when the wavelength is changed) with the wavelength dependence in the 2 x 2 MMI coupler. So this 90° hybrid works with a phase accuracy of 90 ± 0.7 degrees over a 100 nm range . The excess optical loss of this chip is only 2 dB regardless of the complicated waveguide layout thanks to the low propagation loss of the silica waveguide and the WFM-based waveguide design. A version with low temperature dependence has recently been realized with a symmetric waveguide circuit design .
We fabricated a receiver frontend that included a DPOH chip, a micro lens array, and eight photo diodes (PD) and transimpedance amplifiers (TIA) . The PDs and TIAs were hermetically packaged in a ceramic case, which was attached to the DPOH chip via a micro lens array. As shown in Fig. 6(b), the mechanical size and optical and electrical specifications of the module are based on the OIF implementation agreement on coherent receivers. We input a 100 Gb/s DP-QPSK signal into the module and successfully retrieved the data by digitally processing the output from the module. The latest version with a wide dynamic range and high common-mode rejection is described in detail in reference .
Integrated coherent receiver frontend could be fabricated with other technologies such as InP-PIC  and silicon photonics  if a practical, low loss polarization splitter and rotator are also integrated in the optical chip. Although it needs external optical coupling to OE converters, our configuration is more practical because polarization elements are integrated.
6. Highly functional modulator
To generate multilevel signals such as QPSK and QAM optically (in many cases they are also polarization-multiplexed) requires complicated modulators. To cope with this, we have been developing a silica-lithium niobate (LN) hybrid technique, making use of both the high-speed EO effect of LN and the complicated waveguide circuit of a silica PLC . Straight LN waveguide array chips used as phase shifters are directly bonded to PLC chips with a connection loss about 0.2 dB/pt, which is realized with an integrated spot size converter in the silica PLC to match the mode field diameter of the silica waveguides with that of the LN waveguides.
One simple application is to DP-QPSK. As shown in Fig. 7(a) , two IQ modulators and a polarization beam combiner are integrated. This is now being developed for commercial 100G transmission systems. By using the configuration shown in Fig. 7(b), we can generate an optical 64-QAM signal directly from six independent binary electrical signals. We combine three different QPSK signals and can generate a 64-QAM signal thanks to the precise amplitude obtained with the WFM-based design asymmetric Y-branches. Figure 7(c) shows a multi-level modulator in which four push-pull modulators are connected in parallel and serially. By changing the tunable coupler setting, we can generate QPSK, 8-PSK, 8-QAM and 16-QAM signals (selectable). In every case, each drive signal is binary. Figure 7(d) shows a flexible-carrier modulator in which tunable MZI frequency filters and variable couplers are integrated . By changing the filter and coupler settings, we can generate 1-carrier 16-QAM, 2-carrier QPSK or 4-carrier BPSK as needed. This is useful for changing the modulation format according to the transmission distance.
Recent progress on a rectangular passband AWG, an optical OFDM filter, a coherent receiver frontend and highly functional modulators was reviewed. All the devices make use of the features of silica waveguides, namely the precise controllability of the optical amplitude and phase required for making complicated interferometric optical circuits. Since the waveguide design method, chip fabrication process and peripheral technologies such as fiber connection and packaging are already established, these devices can easily be prepared for implementation in large capacity transmission systems in the near future.
The author thanks his colleagues, T. Kitoh, M. Oguma, K. Takiguchi, T. Mizuno, M. Itoh, A. Mori, Y. Nasu, S. Mino, T. Saida, T. Goh, and H. Yamazaki for their efforts and discussions in relation to this work, and S. Suzuki and Y. Inoue for their advice and encouragement.
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