We demonstrated a high performance monolithically integrated multi-channel receiver fabricated on the SOI platform. This receiver is composed of a 1 x 8 Si-based ring-resonators filter and an array of high speed waveguided Ge-on-Si photodetectors. The optical channel spacing is about 1.5 nm. The responsivity of Ge-on-Si photodetector is about 1.0 A/W at the wavelength range of 1554 nm to 1564 nm. Each channel is capable of operating at a data rate of 20 Gbps, resulting in an aggregate data rate of 160 Gbps. At a BER of 1 × 10−11, the receiver showed an optical input sensitivity of between −20 dBm and −21 dBm for each channel at 10 Gbps data rate.
© 2010 OSA
The optoelectronics devices based on the Silicon-on-Insulator (SOI) platform have many advantages, such as low cost, high reliability, ultra-small size, and mature process technology which is compatible with complementary metal oxide semiconductor (CMOS). It is easy to realize the high volume manufacturability of the Si-based optoelectronics devices with low cost for the huge market demand. In the past decade, silicon photonic has attracted many research groups in the world and many optical silicon-based components have been demonstrated, including compact and low loss Si-based passive waveguide devices [1–4], high speed silicon modulator [5–8], silicon Raman lasers [9–11], thermo-optic devices [12,13] and SiGe Photodetector [14–20].
Wavelength division multiplexing (WDM) technology has been rapidly developed and widely deployed in optical communication system [21,22]. The receiver based on WDM technology enables multiple channel data transmission in a single fiber-optic link and can dramatically increase the aggregate data rate. In our previous work, we have reported the WDM silicon receiver based on arrayed waveguide grating (AWG) [19,20]. Our AWG-based receiver can transmit and receive 32 wavelength signals. AWG is a good choice for the filter with many output channels, such as more than 16. However, for the filter with less than 8 output channels, ring-resonator should be a better choice because of its smaller size.
In this work, a monolithically integrated multi-channel silicon photonic receiver chip was designed and fabricated. The receiver is composed of a 1 x 8 Si-based ring-resonators de-multiplexer (DEMUX) and an array of high speed waveguided Ge-on-Si photodetectors on the SOI wafer with 220 nm top Si layer and 2 μm buried oxide (BOX). The diameter of each ring-resonator is about 16 µm, so the entire size of the receiver is small. The ring-resonators DEMUX with 1.5 nm channel spacing is designed and the drop channel of each ring-resonator is connected to a waveguided Ge-on-Si photodetectors (WGPD).
2. Design and fabrication
The schematic of the multi-channel receiver is shown in Fig. 1 . It is composed of a DEMUX with 1 × 8 ring-resonators and an array of Ge-on-Si vertical p-i-n waveguided photodetectors. Each output of DEMUX is connected to a WGPD. A group of optical signals with 8 different wavelengths (λ1 ~λ8) are launched from the input waveguide of ring-resonators DEMUX. And then, the 8 optical signals are dropped into each output channels of 8 ring-resonators, respectively. Then, each divided optical signal is coupled into a WGPD and is transformed into electrical signal by this WGPD. If other wavelength signals are launched from the input waveguide, these signals will go through the through waveguide and be detected by the WGPD at the end of through waveguide. In order to maintain the single mode propagation in the waveguide and reduce the reflection in each ring resonator, ridge waveguides are designed in the coupling range, shown in Fig. 1. A nano-taper was used as the mode-size converter at the input. At the same time, the ridge waveguides are gradually transited into channel waveguides at the input/output of the ring-resonators DEMUX, in order to reduce the coupling loss between the nano-taper and the optical lensed fiber at the input while ensure mode confinement at the output. The channel waveguides are 500 nm wide and 220 nm tall while the ridge waveguides were similarly wide, with a slab thickness of 110 nm. In order to design around 1.5 nm channel spacing, the diameter of the first ring is 16.090 µm. The radius of each subsequent ring is larger 10.4 nm than the previous one, such that the diameter of the last ring is 16.235 µm. The free spectral range (FSR) is 12.4 nm, larger than the necessary spectral range of 12.0 nm. Compared to our previous work , we increase the Ge thickness up to 800nm to enhance the responsivity of the WGPD. Due to the high refractive index of Ge, 20 μm-long Ge on the 220nm-high Si waveguide can almost completely absorb the optical signal at 1550 nm. A Ge active area with longer length will cause higher capacitance and higher dark current. In order to balance the receiver’s speed, sensitivity and responsivity, the designed Ge active area is 5 μm wide and 25 μm long in each of our photodetectors.
The multi-channels receiver was fabricated on an 8-inch silicon-on-insulator (SOI) wafer with 220 nm top Si layer and 2 μm BOX. The Microscope image of the entire processed receiver is shown in Fig. 2 . In order to fabricate this multi-channels receiver, the PECVD SiO2 of 60 nm was first deposited as the hard mask (HM). Then, the ridge arrayed waveguides were formed by partially etching 110 nm of Si, shown in Fig. 2 (Inset 4). Figure 2 (Inset 3) is the SEM image of ring-resonator and Inset4 is the TEM image of the ring-resonator. The remaining 110 nm of Si was etched after a second lithography step to form the channel waveguides. To form the Ge photodetectors, separate masks were used to implant boron into the photodetector regions to form the p anode regions and the p + Ohmic contacts. The implants were activated via rapid thermal anneal of 1050 °C for 5 seconds prior to the selective epitaxial growth of Ge in an ultrahigh vacuum chemical vapor deposition (UHVCVD) epitaxy reactor. After depositing a thin layer of oxide, windows were opened in the Ge active regions by a combination of dry and wet etching to expose the underlying Si. After growing a thin SiGe buffer layer at 350 °C, Ge was selectively grown to a thickness of 800 nm at 550 °C. The n + ohmic contact was formed by implanting phosphorus into Ge, followed by an annealing at 500 °C for 5 min. For both of p + and n + Ohmic contacts, double implantations with different energies and doses were used to reduce the contact resistance. Then, a screen SiO2 of 100 nm and a SiN etch stop layer of 50 nm were deposited subsequently. After this, an inter-level dielectric (ILD) SiO2 layer of 1 µm was deposited and etched to form the contact hole. Finally, a TaN/Al metal stack was deposited and etched to form top and bottom contacts after contact holes opening. Figure 2 (Inset 2) is the SEM image of the WGPD and Inset1 is the TEM image of the WGPD.
3. Measured results and analysis
The stand-alone 1 × 8 ring-resonators DEMUX was evaluated first. The stand-alone reference ring-resonators DEMUX with identical parameters of the actual DEMUX in the multi-channels receiver was fabricated on the same chip, and was used for optical characterization. After dicing process, a polarization-maintaining (PM) lensed fiber with 2.5 µm spot diameter was used to couple the light into the nano-taper of the Si waveguide. The normalized power spectrum of transverse-electric (TE) for each of the 8 ring-resonators drop waveguides and the through waveguide is shown in Fig. 3 . The power spectra were normalized by the transmission power spectrum of a short waveguide test structure, so as to decouple the coupling losses. The on-chip transmission loss of the ring-resonators DEMUX is 0.3 dB and the non-uniformity of transmitted power between the 8 drop channels was less than 0.5 dB. Good crosstalk performances were obtained. The crosstalk between the dropped signals and the through signal is about 18 dB; and the adjacent crosstalk of the dropped signals is more than 20 dB. The measured FSR is 12.3 nm and the measured channel spacing is in the range of 1.5 ± 0.3 nm. The 1-dB bandwidth for each channel is about 12.5% of the average channel spacing.
The WGPD of the first channel of the multi-receiver is chosen to analyze the speed. For the frequency roll-off measurement, the RF signal generator is swept from 100 MHz to 18 GHz. The applied bias is −1 V and the wavelength is 1554.2 nm. The measured result is given in Fig. 4 , which shows that the frequency of the WGPD is about 15 GHz at the 3 dB roll-off point. We also measured the eye diagram of this WGPD. Figure 5 shows the eye diagram at a bit rate of 15 Gbps (left) and 20 Gbps (right). We used the Subminiature-A (SMA) RF cable with the 18 GHz transmission limitation for characterization, so the actual result of 20 Gbps should be better. The open eye diagram suggests that each channel of the receiver is capable of transmitting data at 20 Gbps, so the receiver can be capable of aggregately transmitting data rate of 160 Gbps.
The photocurrent spectrum of each channel of the receiver is collected by scanning the input optical wavelength at the bias of −1 V. The photocurrent spectra of 8 channels of the receiver are plotted in Fig. 6 . The optical power entering the input waveguide of the receiver is about −5 dBm, with the input coupling loss decoupled. The dark current of the Ge PD is less than 1 μA and the adjacent crosstalk is more than 20 dB, which is the same to the result of reference DEMUX tested by the external photodetector. The channel spacing of the receiver is also closed to the DEMUX performance. The photocurrent non-uniformity of the WDM receiver is about 1.0 dB; and it is also near to the optical non-uniformity of the reference DEMUX. Based on the photocurrent spectra, the WGPD responsivity for all 8 channels of the receiver is plotted in Fig. 7 . The responsivity of the receiver is about 1.0 A/W at the wavelength of 1554 ~1564 nm. At a bias of −1V, the responsivity decreases slightly with increasing wavelength. The uniform responsivities of the receiver also make the non-uniformity of the receiver closed to that of the DEMUX.
Later, the receiver chip is packaged with the transimpedance amplifier (TIA) of 10 Gbps on an electrical evaluation board to enable sensitivity measurement. The Al pad of the 1st channel is difficult for wire-bonding after scratched by the RF probe in the former test, so we used the second channel to analyze the sensitivity. The bias from the TIA into the photodetector is also −1 V. The bit error rate (BER) of channel 2 is measured for decreasing input optical power using a 231-1 pseudorandom bit sequence, shown in Fig. 8 . At the same time, according to the photocurrent spectra of the receiver, the optical input sensitivity of the receiver is extracted to be between −20 to −21 dBm for all 8 channels in the wavelength of 1554 ~1564 nm at a bit error rate (BER) of 1 × 10−11.
In conclusion, we have presented the design, fabrication and characterization of a high performance multi-channels silicon photonics receiver on the SOI platform. The receiver is composed of a 1 × 8 ring-resonators DEMUX and a Ge-on-Si waveguided photodetector array. The crosstalk between adjacent channels of the receiver is more than 20 dB. The responsivities of the receiver are about 1.0 A/W at the wavelength of 1554 ~1564 nm. With each channel being capable of operating at a data rate of at least 20 Gbps, the aggregate data rate of the receiver is at least 160 Gbps. At a BER of 1 × 10−11, the receiver showed an optical input sensitivity between −20 dBm and −21 dBm for all 8 channels for 10 Gbps data rate.
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