UWB monocycle pulse generation based on colourless silicon photonic integrated circuit
Fig. 2 and 56 mW electrical power was needed to tune the device over ∼ 8.9 nm without degrading the Q factor or the extinction ratio [7].
Introduction: Ultra-wideband impulse radio (UWB-IR) has been considered to be a promising technique for short-reach wireless communication because it can potentially allow new services to coexist with the existing narrowband wireless services with minimal interference, inherent immunity to multipath fading, low power spectral density (PSD), wide bandwidth (BW) and potentially a simple transceiver architecture [1]. The range of wireless UWB signals can be kept to less than tens of metres by maintaining a low radiation power density. UWB-over-fibre serves as an effective solution to distribute UWB-IR signals. For such optical distribution, it can be more cost effective to implement the generation and distribution of UWB-IR in the optical domain and only perform the conversion to the RF wireless domain at the remote antenna units, thus avoiding the need for multiple electrical-to-optical-to-electrical conversions in the generation and distribution of UWB-IR signals. A UWB monocycle pulse can be generated using discrete optical components by phase modulation to intensity modulation (PM–IM) conversion [2] and photonic microwave delayline filters with negative coefficients [3]. Silicon photonics [4] offers the potential for large volume low-cost production of chips to produce UWB-IR. Recently, UWB generation was reported based on two-photon absorption in silicon waveguides [5], but two light sources were needed. The monocycle pulse generation based on PM–IM conversion using a passive silicon microring resonator (MRR) that was reported recently [6] only operates at a predefined wavelength, whereas multichannel UWB systems using a wavelength division multiplexed (WDM) network will require the UWB generation to be wavelength transparent. In this Letter, we describe a colourless monolithically integrated UWB-IR generation from a silicon chip and the top-view micrograph is shown in Fig. 1. Similar to [6], the monocycle pulse generation relies on PM–IM conversion; however, here we employ a wavelengthtunable MRR and also integrate the optical-to-electrical conversion on the same chip. The tunability of the MRR over one free spectral range ensures that the device can operate at any wavelength and it is thus compatible with WDM distribution of UWB-IR signals. The total occupied chip area is less than 0.74 × 1.23 mm2. We also characterise the on–off keying (OOK) modulation of the generated UWB monocycle pulses with a fixed pattern.
P1
P2 P3
Fig. 1 Top-view micrograph of fabricated device Inset left: Ge photodetector; right: MRR with TiN heater
Experimental results and discussion: To measure the micoring through a port transmission response, we launched a TE polarised light from a narrow linewidth tunable laser with 0.01 nm resolution into port 3 (P3) and measured the optical power at port 1 (P1), as shown in Fig. 1. The Q factor measuring from 0 to − 3 dB was found to be ∼ 3500 and the extinction ratio was ∼ 22 dB. To characterise the thermal tuning of the ring resonance, the applied voltage and the generated current were measured by a source meter. The MRR resonance shift was plotted against the applied electrical power, as shown in the inset of
0 –5 wavelength tuning, nm
Ultra-wideband monocycle pulse generation is experimentally demonstrated on a silicon photonic circuit with a monolithically integrated wavelength-tunable phase-intensity modulation converter and a waveguide germanium photodetector. A fractional bandwidth of 166% is achieved and modulation with a 2.5 Gbit/s test data pattern was demonstrated.
normalised transmission, dB
Ke Xu, Zhenzhou Cheng, Chi Yan Wong and Hon Ki Tsang
–10 –15 –20
10 8 6 4 2 0 0 20 40 60 electrical power, mW
–25 1543
1544
1545 l, nm
1546
1547
Fig. 2 Normalised transmission of MRR through port measured by TE polarised light Inset: Wavelength tuning against applied electrical power
The experimental setup is depicted in Fig. 3. The laser output is fed into a phase modulator driven by a pre-programmed pulse sequence with 100 ps full-width half-maximum (FWHM) and 625 MHz repetition rate. The phase-modulated signal is amplified before being coupled onto the silicon chip at port 2 through a grating coupler ( ∼ 5 dB coupling loss). The coupling loss may be reduced by using an apodised grating structure. The thermal tuning is realised by passing an electrical current through the resistive metal heater via a pair of DC probes. The phase-modulated signal is converted to a monocycle pulse train when the centre of the spectrum slope is tuned to near the carrier wavelength. The UWB signal is generated by linear detection of the optical power in the integrated photodiode. The electrical output is coupled out from the chip through a high-speed RF probe. UWB signal EDFA
PC TL
PC
PM
thermal tuning Ge PD
PPG 10 GHz
Fig. 3 Experimental setup of UWB generation using MRR and integrated Ge photodetector TL: tunable laser; PC: polarisation controller; PM: phase modulator; PPG: pulse pattern generator
To generate a positive monocycle pulse, the carrier wavelength is located near the centre of the right slope. The Ge photodiode has −3 V bias and the output is connected directly to an RF spectrum analyser and a digital communication analyser. The measured spectrum is shown in Fig. 4a. The centre frequency (CF) is ∼ 3.8 GHz and the 10 dB BW is ∼ 6.3 GHz measured from 0.6 to 6.9 GHz. The corresponding fractional BW is ∼ 166%. Although the monocycle pulse may not match the Federal Communications Commission (FCC) mask perfectly, the generated UWB signal does satisfy the FCC requirements that the fractional BW should be larger than 20% or a 10 dB BW of at least 500 MHz should lie in the frequency range from 3.1 to 10.6 GHz. It is known that a triplet pulse can fit the FCC mask better and such triplet pulses may be produced by modifying the waveguide layout of our device to superimpose two asymmetric monocycle pulses with proper delay [8]. In addition, as the 3 dB BW of the MRR is ∼0.44 nm, it is compatible for an UWB signal which has a much larger BW. The signal-to-noise ratio (SNR) at the frequency range from 8 to 11 GHz is reduced by the limited BW of the integrated photodetector. The waveform of the pulse train shown in the inset of Fig. 4a demonstrates that a clean and open monocycle pulse is produced. The FWHM of the pulse is ∼ 100 ps and the repetition rate is ∼625 MHz. The negative polarity monocycle pulses are also generated as shown in Fig. 4b. The CF and the 10 dB BW are 3.8 and 6.3 GHz which are compliant with the FCC requirement as well.
ELECTRONICS LETTERS 26th September 2013 Vol. 49 No. 20 pp. 1291–1293
RF power, dBm
–20
positive
–30 –40
Conclusion: We have integrated a wavelength-tunable silicon ring resonator with a Ge photodetector for monocycle pulse generation and detection. We demonstrated the device operation for modulated UWB-IR generation from an optical signal carrying a data pattern. The integrated UWB generator has a small footprint and can be implemented in a colourless optical network unit for the UWB over the WDM-PON system.
negative
CF-3.8 GHz BW=6.3 GHz
CF-3.8 GHz BW=6.3 GHz
200 ps/div
200 ps/div
–50 –60 –70 –80 0
2
4 6 8 10 frequency, GHz
12
0
2
a
4 6 8 10 frequency, GHz
12
b
Fig. 4 Measured RF spectrum of positive monocycle pulse a Positive monocycle pulse b Negative monocycle pulse Insets: Corresponding waveforms with repetition rate of 625 MHz
In the measurement of the modulation performance of the tunable UWB generator, we employed a data pattern of ‘110100110110’, where ‘1’ represents ‘1000’ and ‘0’ represents ‘0000’. Hence, the pulse width is 100 ps, whereas the data rate is 2.5 Gbit/s. The measured back-to-back and 25 km transmission waveforms of the modulated monocycle pulse for both positive and negative polarities are shown in Fig. 5. The defined pattern can be clearly observed with good extinction ratio after monocycle pulse generation and detection. The phase modulator can also be integrated with the proposed device to further reduce the footprint and power consumption. Other than OOK modulation, the MRR used for PM-IM conversion can be embedded with a PN diode for bi-PM. 1
1
0
1
0 0
1
1 0
1
1
0 500 ps/div positive b2b
positive 25 km
negative b2b
negative 25 km
Fig. 5 Measured back-to-back and 25 km transmission waveform for both polarity monocycle pulses with OOK modulation Bit pattern is self-defined as ‘110100110110’
Acknowledgments: The authors thank the Institute of Microelectronics (IME), Singapore for device fabrication and C.W. Chow for helpful discussions. This work was supported by the Research Grants Council of Hong Kong under grant CUHK 416710. © The Institution of Engineering and Technology 2013 25 June 2013 doi: 10.1049/el.2013.2111 One or more of the Figures in this Letter are available in colour online. Ke Xu, Zhenzhou Cheng, Chi Yan Wong and Hon Ki Tsang (Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, Hong Kong, People’s Republic of China) E-mail:
[email protected] References 1 Yao, J., Zeng, F., and Wang, Q.: ‘Photonic generation of ultrawideband signals’, J. Lightwave Technol., 2007, 25, pp. 3219–3235 2 Dong, J., Zhang, X., Xu, J., and Huang, D.: ‘Ultrawideband monocycle generation using cross-phase modulation in a semiconductor optical amplifier’, Opt. Lett., 2007, 32, pp. 1223–1225 3 Bolea, M., Mora, J., Ortega, B., and Capmany, J.: ‘Optical UWB pulse generator using an N tap microwave photonic filter and phase inversion adaptable to different pulse modulation formats’, Opt. Express, 2009, 17, pp. 5023–5032 4 Chen, X., Li, C., and Tsang, H.K.: ‘Device engineering for silicon photonics’, NPG Asia Mater., 2011, 3, pp. 34–40 5 Yue, Y., Huang, H., Zhang, L., Wang, J., Yang, J.-Y., Yilmaz, O.F., Levy, J.S., Lipson, M., and Willner, A.E.: ‘UWB monocycle pulse generation using two-photon absorption in a silicon waveguide’, Opt. Lett., 2012, 37, pp. 551–553 6 Liu, F., Wang, T., Zhang, Z., Qiu, M., and Su, Y.: ‘On-chip generation of ultra-wideband monocycle pulses’, Electron. Lett., 2009, 45, pp. 1247–1249 7 Xu, K., Lei, G.K.P., Lo, S.M.G., Cheng, Z., Shu, C., and Tsang, H.K.: ‘Bit-rate-variable DPSK demodulation using silicon microring resonator with electro-optic wavelength tuning’, IEEE Photonics Technol. Lett., 2012, 24, pp. 1221–1223 8 Zhou, E., Xu, X., Lui, K.-S., and Wong, K.K.Y.: ‘A power-efficient ultra-wideband pulse generator based on multiple PM-IM conversions’, IEEE Photonics Technol. Lett., 2010, 22, pp. 1063–1065
ELECTRONICS LETTERS 26th September 2013 Vol. 49 No. 20 pp. 1291–1293
