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Optical short pulse processors enable several systems including optical code division multiple access networks, optical secure communications, and optical pulse shapers. For instance, compact and low cost integrated solutions for optical short pulse en/de-coders with large code length and capable of processing large optical bandwidth may enable realization of Tbps communication networks. Here, we report an integrated 128-bit short pulse processor capable of signal processing on large amount of optical bandwidth, demonstrated ≈ 25 nm, using sub-ps optical short pulse signal. The chip fabricated in a commercial foundry SOI CMOS process and includes more than 500 distinct optical components and over 150,000 distinct electrical components.
© 2016 Optical Society of America
1. Introduction
Optical short pulse shaping and processing play a key role in wide range of applications including optical code division multiple access networks [1,2], optical secure communication [3–5], optical pulse shapers [6,7], and time stretch imaging for cancer detection [8].
Figure 1 shows the schematic of a generic spectral processor for short optical pulses occupying large optical bandwidth. A first block separates frequency contents of the optical pulse into multiple wavelengths. The amplitude and phase of each wavelength component is then adjusted independently. A final block recombines these amplitude-and-phase adjusted wavelength components in to a single optical field. The shape of output waveform depends on the amplitude and phase settings. The wavelength separating and combining blocks may be realized as a prism, diffraction grating, array of wavelength-selective filters, etc. Key metric in such short pulse processors are pulse width, or equivalently total optical bandwidth that is processed, and the number of wavelengths that can be independently processed.
For example, optical code-division multiple-access (O-CDMA) is a promising optical access technology due to several attractive features including flexibility, reconfigurability, ease of network control, and potential for enhanced physical layer security [9]. The physical layer security relies on coexistence of multiple users and a large code space. Spectral phase encoded time-spreading O-CDMA is a coherent O-CDMA technique which applies spectral phase changes based on a unique code to each user’s data modulated optical pulse, causing the encoded waveform to spread in the time domain [9,10]. A receiver with full knowledge of the spectral code reconstructs the original short optical pulse and distinguishes the short pulse from other users’ data pulses. In such a system, doubling the code length, improves the bit-error-rate (BER) by almost two orders of magnitude for fixed number of users [10]. The optical pulse width plays key role in determining the bit rate of the system. In the literature, an O-CDMA system with 320 Gbps throughput (32 users × 10 Gbps) using on-off keying modulation, 64 bit code length, 450 fs laser pulse, and enhanced non-linear detectors is experimentally demonstrated [12] for error-free operation (BER < 10−11). The throughput can be improved further by incorporating hybrid modulation schemes in O-CDMA networks [11].
There have been two primary approaches to implement ultra-short pulse optical spectral processors. In a first approach, free-space diffraction grating followed by spatial light modulators (SLM) are used [9,12,13] either without control electronic (fixed masks) or without amplitude control capability. In a second approach, monolithic array waveguide gratings (AWG) or echelle gratings, serving as wavelength separator and combiner, along with on-chip phase and occasionally amplitude adjusters are used [7, 14–16]. The limitations of past realizations include high loss associated with AWG and echelle grating approaches for large number of channels and small channel spacing [7,14–16,19–22], the inability to adjust both amplitude and phase of each wavelength, and a large system size (especially in SLM realizations) [9,12,13].
Our approach combines the best features of the free-space and monolithic approaches while enabling scalability in the number of supported wavelengths and independent control of phase and amplitude for each wavelength. Specifically, free-space diffraction gratings are used as wavelength separation and combining blocks while a monolithic chip adjusts the phases and amplitudes of each wavelength independently. The monolithic chip uses a commercial foundry 180 nm SOI CMOS technology that is widely used in the realization of radio frequency switches for the front-end of cellular networks [18]. No pre- or post-processing steps are needed. Ideally, the entire shortpulse spectral processor should be monolithically integrated in a chip. This would require monolithic wavelength separation and combining blocks that support a large number of wavelengths. Limited quality-factor of on-chip filters, and sensitivity of AWG to process mismatches are some of the challenges that need to be overcome for such monolithic realization. Table 1 summarizes the performance of selected published state of the art integrated wavelength separation devices.
Table 1:. Performance Summary of Selected Published State of the Art Integrated Wavelength Separation Devices.
2. Monolithic optical short-pulse encoder/decoder
The Optical Short-pulse En/De-coder (OSPED) includes 128 channels where each channel consists of a pair of grating couplers to couple light in and out of the chip and optical amplitude and phase adjusters in between (Fig. 2). Optical short pulses that originate from a laser, are diffracted using an off-chip grating, and subsequently coupled to the chip for spectral processing. The processed light is coupled out and detected after all the channels are combined outside of the chip using a lens and diffraction grating. Control electronics for all the tunable components are integrated on the same chip at a distant location from the optical core, and the control electrical signals are routed to the active elements using the available metal lines in the CMOS process. Independent control of phase shift and amplitude of optical field at each element enables on-chip spectral processing and waveform engineering.
Fig. 2 (a) Schematic of the 128-bit monolithic optical short pulse en/de-coder with independent amplitude and phase control at each bit (wavelength-channel). Short pulse is diffracted off-chip and multiple wavelengths centered at 128 different wavelengths (colors) are coupled in to grating couplers. Each color is processed on chip using independent wavelength-dependent amplitude and phase control units, then coupled out of the chip using similar grating couplers; (b) Chip microphotograph of the fabricated 128-bit monolithic optical short pulse en/de-coder in a commercial 180 nm Silicon-On-Insulator (SOI) CMOS process featuring over 500 distinct optical components and over 150,000 distinct electrical components.
Performances of different types of photonics devices in this technology platform have been reported before [17,23,24]. We have optimized the devices for processing of very large optical bandwidth. For instance, input and output grating couplers are distanced far enough so that a reasonable channel isolation without causing too much optical power coupling loss is achieved. Each variable attenuator is designed in such a way that its passive response (without applying electrical stimuli) peaks at the nearest possible wavelength compared with the target channel center wavelength to save active power consumption. The electrical current used to tune active elements is controlled by a programmable electrical 7-bit Digital-to-Analog Converter (DAC) that is implemented on the same chip using MOSFETS with 180 nm and 250 nm channel lengths.
3. System elements and calibration
Each channel consists of a pair of grating couplers with tunable thermo-optical attenuator followed by a tunable thermo-optical phase shifter in between. Grating couplers are 4 μm wide and 30 μm long to ease coupling from diffracted light. The passive response of each grating coupler shows a 1 dB bandwidth of 26 nm [Fig. 3(d)]. The center-to-center distance between two adjacent grating couplers is 10 μm to give better than 20 dB channel isolation. To ease the requirements for free space optics to couple diffracted light in and out of the chip, 128 channels are laid out in 16 clusters of 8-channels in a staggered configuration occupying a total length of 1.575 mm. The edge to edge distance of adjacent clusters is 20 μm. The difference between design of each cluster is in the passive layout of tunable attenuator to shift the peak wavelength as close as possible to the target wavelength while satisfying lithography limitations of the process. The tunable phase shifter is realized as a 180 μm-long meandered silicon waveguide with four polysilicon heaters connected in parallel electrically [Fig. 3]. A 2π optical phase shift is achieved by consuming ≈ 31 mW power in the heater with 0.81 kΩ electrical resistance. The tunable attenuator is made of an MZI with compact 1 × 2 Multi-Mode Interferometric (MMI) power splitter/combiner in a total footprint of ≈ 29 μm× 7 μm. The longer arm is heated up by p-doped polysilicon heater with a measured electrical resistance of ≈ 1.1 kΩ. The variable attenuator provides a measured 16 dB amplitude tunability while consuming ≈ 16 mW DC power at a peak designed wavelength [Fig. 4]. In the tunable attenuator of each cluster, the length of longer arm in the MZI is slightly different (La has been adjusted) to set the peak wavelength at a desired value, so that collectively all clusters cover ≈ 30 nm optical bandwidth while saving active power consumption. The measured passive responses of attenuators of different clusters are shown in Fig. 4(b). The peak wavelength of each cluster from both measurement and simulations are presented in Fig. 4(d).
Fig. 3 (a) Thermo-optical variable phase shifter realized as a meandered waveguide with poly-silicon heaters in between to increase the power efficiency and decrease the voltage swing that is needed to cover 2π optical phase shift. (b) Typical measured phase and amplitude responses of a variable phase shifter as a function of heater power consumption at peak wavelength. (c) Grating coupler to couple light in or out of the chip with 4 μm width and 30 μm length for high isolation between adjacent channels. (d) Typical passive response of grating coupler with 1 dB bandwidth of 26 nm.
Fig. 4 (a) Optical variable attenuator is constructed as an interferometer with polysilicon heater in between. The relative phase change between the two paths, due to thermo-optical effect that is more prominent in the longer branch, results in amplitude tunability of around 16 dB. The arm length is slightly different (La has been adjusted) for each cluster to set the desired peak wavelength over the desired ≈ 30 nm wavelength range. (b) Typical passive response of attenuators of each cluster demonstrating peaks at different wavelengths. (c) Typical measured phase and amplitude responses of an optical variable attenuator as a function of heater power consumption at peak wavelength. (d) Simulated and measured peak wavelengths of variable attenuators of all clusters.
Calibration of phase and amplitude active response of each channel is critical for system level demonstration. The experimental setup shown in Fig. 5(a) is used for this calibration. Optical short pulses using a commercial mode locked laser with approximate pulse width of ≈ 320 fs and repetition rate of 100 MHz are shined to an off chip diffraction grating using fiber and collimating lens. As a result, approximately 25 nm optical bandwidth is diffracted and coupled to 128 channels on the chip. For each channel, the amplitude response is measured by using a lensed fiber at the top of the corresponding output. The response of peak wavelength (channel center wavelength) is used for amplitude characterization. Normalized spectrums of all individual channels are shown in Fig. 5(b). For phase calibration, the interference signal at the mid-wavelength of two adjacent channels that has the strongest interference is used. A phase shifter in one channel is tuned and calibrated with respect to the adjacent channel phase. For each channel, the phase change from mid to center wavelength is almost negligible since they are only around 80 pm away. An example of measurement result of such a characterization is shown in Figs. 5(b)–5(c).
Fig. 5 (a) Schematic of the measurement setup to characterize amplitude and phase responses of each channel; (b) Normalized measured spectrum of all channels plotted together after amplitude calibration. (c) Measured spectrum of channels 63 and 64 picked up by a lensed fiber at the top of grating couplers for two cases when phase shifter in channel 64 is tuned to give in or out of phase interference. The information at the mid-wavelength, here 1550.07 nm, is used to calibrate the phase shifter. (d) Example of normalized interference pattern at mid-wavelength of (c) as a function of active heater power of phase shifter in channel 64.
4. Electrical routing
The OSPED system has a total of 256 independent heaters, 128 for variable phase shifters and 128 for variable attenuators, that must be controlled independently. One terminal of all the heaters are connected electrically to a common terminal using the three bottommost available metal layers (bottommost copper and other two aluminum), and the other terminals are routed independently to the designated driver circuitry using the topmost available metal layer (aluminum). This topmost metal layer is 4 μm thick and 2 μm wide, and requires a minimum line-to-line spacing of 2.8 μm due to the lithography limitations. All the routing is done judiciously to minimize the effect of metal lines on the optical devices while satisfying the foundry metal density requirements. The digitally controlled integrated electronic consists of a programmable 7-bit Digital to Analog Converter (DAC) followed by the current drivers that drive the polysilicon heaters via the aforementioned metal interconnects similar to what was reported in [17]. The chip is programmed serially through a computer using clock, reset, and load signals.
5. Experimental results
5.1. Operation of OSPED measured with fast electro-optical detection
The performances of individual optical components, namely, the tunable optical phase shifter, the tunable optical attenuator, and the grating couplers were individually characterized in stand-alone structures concurrently fabricated in the same process. The measured optical losses for components are around 5.5 dB for grating coupler, 0.3 dB per 1×2 MMI, 2 dB per phase shifter, 0.8 dB per attenuator, and 2 dB for silicon waveguide (1.27 dB/mm). The total optical insertion loss of the chip is 15.8 dB. Phase and amplitude response of the channels were calibrated using the approach that is explained in the previous section. System level measurements are conducted using a setup that is shown in Fig. 6(a). The output light of channels after getting processed is combined using another off-chip diffraction grating and coupled to the output fiber. The total loss of free space setup together with on-chip loss is 20.3 dB. A low-dispersion optical amplifier (EDFA) is used at the output to boost the power level while preserving pulse shape. The amplified output is split to detect using either fast photodiode and fast electrical oscilloscope both with 50 GHz analog bandwidth (when number of on channels are small enough), or using optical short pulse auto-correlator. Fig. 6(b) shows photograph of chip mounted on PCB with required supply voltages and programming signals. Several thermal vias were placed under the die that is attached to the PCB using a thermally-conductive epoxy to improve the thermal conductivity.
Fig. 6 (a) Schematic of the measurement setup to measure processed optical pulse width using both very fast electro-optical detection and optical short pulse intensity auto-correlator; (b) Photograph of wirebonded chip that is mounted on PCB with required supply and programming signals as well as vias underneath chip for better thermal conductivity. (c) Measured optical pulse width when one, two, three, 10 and 128 channels are on and are in phase using very fast photodiode. (d) Detected optical pulse width using fast detector vs number of on channels. After three channels, pulse width is limited to electrical bandwidth of the detection system.
We measured electrically detected optical pulses when only 1, 2, 3, 4, 10 and 128 channels are on and they are all in phase. As shown in Figs. 6 (c)–6(d), after three channels, the detected pulse width is capped at around 23 ps in good agreement with simulation.
5.2. Operation of OSPED measured with optical short pulse auto-correlator
Because of bandwidth limitation of electronic detection, the pulse width of output processed signal can not be measured for optical bandwidth higher than ≈ 0.4 nm. Instead, the well known auto-correlation techniques can be used to measure the pulse width of such ultra short optical signal [25]. A commercial auto-correlator equipment, whose operation principle is based on recording the second-order correlation function with a Michelson interferometer, was used in our experiments. The experiment was performed for three different code lengths (number of active channels) of 64, 88, and 128 for two cases. In the first case, Gaussian amplitude profile and uniform phase profile are used across all the used channels. In the second case, the amplitude profiles remain unchanged while the phases are scrambled using Walsh orthogonal code [26] where the phase of active channels with associated bit “1” are changed by π compared with other active channels with associated bit “0”. The encoded data was normalized to the peak of constructed signal for each code length. As a result, intensity auto-correlation shows that both extracted short pulse width and intensity ratio of pulse peak to peak of encoded signal are correlated with code length as theoretically expected [10], i.e., pulse width scales down and intensity ratio of pulse peak-to-peak of encoded signal scales up linearly with code length. This relationship is intuitively explained in Fig. 7(d). The initial laser pulse width is extracted to be ≈ 320 fs assuming Gaussian pulse. Example of measured intensity auto-correlation of encoded pulse with 128 active channels is also shown in Fig. 7(c). As expected, the pulse is spread in time domain from ≈ 0.38 ps to ≈ 50 ps with a ratio of ≈ 128.
Fig. 7 (a) Experimental result of intensity auto-correlation of the output processed light for three code lengths, 64, 88 and 128, in two cases: one is successfully constructed short pulse, and second is encoded pulse with Walsh orthogonal code. The data is normalized to the peak of constructed signal and encoded signal for each code length is relatively normalized to corresponding constructed signal. (b) Extracted constructed pulse width and intensity ratio of pulse peak to peak of encoded signal versus number of used channels; (c) Example of measured intensity auto-correlation of encoded pulse with 128 channels; (d) Intuitive explanation of the effect of code length on intensity and intensity auto-correlation for Gaussian pulse.
6. Conclusion
The presented work is the first integration of a large scale optical short pulse en/de-coder, including the complete integrated control electronics, with independent amplitude and phase control for every channel capable of arbitrary waveform generation in a commercial CMOS SOI process. Table 2 shows the performance of this work in comparison with selected previously published fully- or semi- monolithic approaches.
Table 2:. Performance Summary in Comparison with other Semi- or Fully-Monolithic Spectral Light Processors.
Acknowledgments
The authors would like to thank Nankyung Suh Cockerham and Wes Hansford from MOSIS for supporting the chip fabrication. The authors also thank A. Samiei for designing the band-gap reference current and electrostatic discharge protection circuitries, and SungWon Chung, M. Yamagata, A. Imani, K. Datta, R. Chen, S. Subramanian, and F. Rezaiefar for feedback and comments about various aspects of this project.
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