Javier S. Fandiño

A monolithic integrated photonic microwave filter

Meeting the ever increasing demand for transmission capacity in wireless networks will require evolving towards higher regions in the radiofrequency spectrum, reducing cell sizes as well as resorting to more compact, agile and power efficient equipment at the base stations, capable of smoothly interfacing the radio and fiber segments. Photonic chips with fully functional microwave photonic systems are promising candidates to achieve these targets. Over the last years, many integrated microwave photonic chips have been reported in different technologies. However, and to the best of our knowledge, none of them have fully integrated all the required active and passive components. Here, we report the first ever demonstration of a microwave photonics tunable filter completely integrated in an Indium Phosphide chip and packaged. The chip implements a reconfigurable RFphotonic filter, it includes all the required elements, such as lasers, modulators and photodetectors, and its response can be tuned by means of control electric currents. This demonstration is a fundamental step towards the feasibility of compact and fully programmable integrated microwave photonic processors. Emerging information technology scenarios, such as 5G mobile communications and Internet of Things (IoT), will require a flexible, scalable and futureproof solution capable for seamlessly interfacing the wireless and fiber segments of communication networks [1,2,3]. Microwave photonics (MWP) [4,5],the interdisciplin ary approach that combines radiofrequency and photonic systems, is the best positioned technology to achieve this target. A very relevant example is 5G wireless communications, which targets an extremely ambitious range of requirements including [6,7], a 1000-fold increase in capacity, connectivity for over 1 billion users, strict latency control, as well as network flexibility via agile software programming. These objectives call for a paradigm shift in the access network to incorporate smaller cells, exploit the millimeter wave regions of the radiofrequency spectrum and implement massive multiple-input multipleoutput at the base stations (BTSs) [7]. The successful integration of the wireless and fiber segments thus relies on the possibility of implementing agile and reconfigurable MWP subsystems, featuring broadband operation, as well as low space, weight and power consumption metrics. The solution consists in resorting to integrated microwave photonics (IMWP) [8,9] chips allocated either in the BTS and/or the central office in combination with radio over fiber transmission in the fiber segment connecting them [10,11]. The two fundamental issues to be solved in IMWP are related respectively to technology and architecture. First, there is the need to identify the best material platform where to implement MWP chips. Second, whether it would be better to follow an application specific photonic integrated circuit (ASPIC) approach, where a specific architecture is employed to implement a specific functionality, or to resort to a generic programmable architecture. IMWP ASPICs with certain complexity have been reported to date mainly in four material platforms: indium phosphide (InP) [1214], Silicon-on-Insulator (SOI) [15-21], silicon nitride (Si3N4) [22-26] and chalcogenide glass [27,28]. Several functionalities have been demonstrated with a variable degree of photonic (20-60%) integration, as shown in Table 1. A different approach is based on generic processors [29,30], where a common architecture implements different functionalities by suitable programming. A recent paper [31] reported the design of a programmable optical core inspired by the concept of electronic field programmable gate arrays. This approach is based on an optical core composed implemented by a 2D waveguide mesh where the connections between waveguides 1 ar X iv :1 61 2. 06 99 9v 1 [ ph ys ic s. op tic s] 2 1 D ec 2 01 6 are controlled by means of tunable Mach-Zehnder interferometers (MZIs). Researchers fabricated a simplified version of the processor composed of two mesh cells, using a commercial Si3N4 waveguide technology known as TriPleX [32]. The reported processor featured a free spectral range of 14 GHz and is fully programmable. A band-pass filter with a tunable centre frequency that spans two octaves (1.6-6 GHz) and a reconfigurable band shape (including a notch filter and a flat-top resonance) was demonstrated. A reconfigurable processor implementing signal integration, differentiation and Hilbert transformation has also been recently reported in InP technology [33]. To the best of our knowledge however, none of the above contributions has reported to date the integration of all the required active (sources, modulators and detectors) and passive (splitters, optical filters and waveguides) photonic components in a single chip, either monolithically or following a hybrid approach. Here we report, , the design, fabrication, packaging and experimental demonstration of the first monolithic IMPW filter that integrates all these elements in the same substrate. The chip implements a reconfigurable RF-photonic filter that employs a tunable distributed Bragg reflector laser (DBR); a singlesideband optical modulator; a tunable optical filter based on a ring assisted Mach Zehnder interferometer (RAMZI)[34]; and an on-chip optical detector. This demonstration constitutes a fundamental step forward in the implementation of a fully integrated MWP filter, and opens the path for compact and programmable MWP signal processors, where the RAMZI filter will be replaced by a 2D reconfigurable mesh and multiple functionalities will be implemented by suitable programming of the mesh interconnections. Table 1: Overview of reported IMWP chips. AWG: Arbitrary waveform generation. BFM: Beamforming. TBPS: Tunable broadband phase shift. TTTD: Tunable true-time delay. IFM: Instantaneous frequency measurement. ADC: Photonic analog-to-digital conversion.

Photonics-based microwave frequency measurement using a double-sideband suppressed-carrier modulation and an InP integrated ring-assisted Mach Zehnder interferometer filter

A photonic system capable of estimating the unknown frequency of a CW microwave tone is presented. The core of the system is a complementary optical filter monolithically integrated in InP, consisting of a ring-assisted Mach-Zehnder interferometer with a second-order elliptic response. By simultaneously measuring the different optical powers produced by a double-sideband suppressed-carrier modulation at the outputs of the photonic integrated circuit, an amplitude comparison function that depends on the input tone frequency is obtained. Using this technique, a frequency measurement range of 10 GHz (5-15 GHz) with a root mean square value of frequency error lower than 200 MHz is experimentally demonstrated. Moreover, simulations showing the impact of a residual optical carrier on system performance are also provided.

Integrated InP frequency discriminator for Phase-modulated microwave photonic links

We report the design, fabrication and characterization of an integrated frequency discriminator on InP technology for microwave photonic phase modulated links. The optical chip is, to the best of our knowledge, the first reported in an active platform and the first to include the optical detectors. The discriminator, designed as a linear filter in intensity, features preliminary SFDR values the range between 67 and 79 dB.Hz(2/3) for signal frequencies in the range of 5-9 GHz limited, in principle, by the high value of the optical losses arising from the use of several free space coupling devices in our experimental setup. As discussed, these losses can be readily reduced by the use of integrated spot-size converters improving the SFDR by 17.3 dB (84-96 dB.Hz(2/3)). Further increase up to a range of (104-116 dB.Hz(2/3)) is possible by reducing the system noise eliminating the EDFA employed in the setup and using a commercially available laser source providing higher output power and lower relative intensity noise. Other paths for improvement requiring a filter redesign to be linear in the optical field are also discussed.

Arbitrary Coupling Ratio Multimode Interference Couplers in Silicon-on-Insulator

In this paper, we present the design, manufacturing, characterization, and analysis of the coupling ratio spectral response for multimode interference couplers in silicon-on-insulator (SOI) technology. The couplers were designed using a Si rib waveguide with SiO2 cladding, on a regular 220 nm film and 2μm buried oxide SOI wafer. A set of eight different designs, three canonical and five using a widened/narrowed coupler body, have been subject of study, with coupling ratios 50:50, 85:15, and 72:28 for the former, and 95:05, 85:15, 75:25, 65:35, and 55:45 for the latter. Two wafers of devices were fabricated, using two different etch depths for the rib waveguides. A set of six dies, three per wafer, whose line metrology matched the design, were retained for characterization. The coupling ratios obtained in the experimental results match, with little deviations, the design targets for a wavelength range between 1525 and 1575 nm, as inferred from spectral measurements and statistical analyses. Excess loss for all the devices is conservatively estimated to be approximately 0.6 dB in average. All the design parameters, body width and length, input/output positions and widths, and tapers dimensions are disclosed for reference.

Figures of merit for self-beating filtered microwave photonic systems.

We present a model to compute the figures of merit of self-beating Microwave Photonic systems, a novel class of systems that work on a self-homodyne fashion by sharing the same laser source for information bearing and local oscillator tasks. General and simplified expressions are given and, as an example, we have considered their application to the design of a tunable RF MWP BS/UE front end for band selection, based on a Chebyshev Type-II optical filter. The applicability and usefulness of the model are also discussed.

Two-port multimode interference reflectors based on aluminium mirrors in a thick SOI platform

Multimode interference reflectors (MIRs) were recently introduced as a new type of photonic integrated devices for on-chip, broadband light reflection. In the original proposal, different MIRs were demonstrated based on total internal reflection mirrors made of two deep-etched facets. Although simpler to fabricate, this approach imposes certain limits on the shape of the field pattern at the reflecting facets, which in turn restricts the types of MIRs that can be implemented. In this work, we propose and experimentally demonstrate the use of aluminium-based mirrors for the design of 2-port MIRs with variable reflectivity. These mirrors do not impose any restrictions on the incident field, and thus give more flexibility at the design stage. Devices with different reflectivities in the range between 0 and 0.5 were fabricated in a 3 um thick SOI platform, and characterization of multiple dies was performed to extract statistical data about their performance. Our measurements show that, on average, losses both in the aluminium mirror and in the access waveguides reduce the reflectivities to about 79% of their target value. Moreover, standard deviations lower than ±5% are obtained over a 20 nm wavelength range (1540-1560 nm). We also provide a theoretical model of the aluminium mirror based on the effective index method and Fresnel equations in multilayer thin films, which shows good agreement with FDTD simulations.

Analysis of System Imperfections in a Photonics-Assisted Instantaneous Frequency Measurement Receiver Based on a Dual-Sideband Suppressed-Carrier Modulation

Instantaneous frequency measurement receivers are a well-established technology that is used for the ultrafast characterization of pulsed microwave signals over a broad bandwidth. Recently, numerous photonic approaches to instantaneous frequency measurement (IFM) have been proposed and experimentally demonstrated, with the ultimate aim of leveraging the benefits of optical technology to improve the performance of already existent electronic solutions. Despite the numerous results, not so much attention has been paid so far to understand the subtle implications that system imperfections can have on realistic photonics-based IFM receivers. Here, we focus our attention on one of the most promising among these IFM techniques, which is based on optical power monitoring of a dual-sideband suppressed-carrier modulation after a Mach-Zehnder interferometer (MZI) filter. We develop a time-domain model for the rigorous analysis of all major optical and electrical effects, including amplitude imbalance and phase errors in the modulator and the MZI, as well as on-pulse RF phase/frequency modulation. Simulations are then used to illustrate the substantial effect that a nonperfectly suppressed optical carrier can have on system performance. More importantly, it is shown that in a nonideal situation, the system amplitude comparison function critically depends on input RF power, thus greatly limiting the dynamic range of the photonics-based receiver. Some approaches to solve these issues are also discussed.

Manufacturing Tolerance Analysis of an MMI-Based 90

A numerical study of the impact that manufacturing tolerances have on the performance of an InP 4 × 4 MMI working as a 90° optical hybrid is presented, including simultaneous variations of width, thickness, and refractive index over the C and L bands. Simulation results for different figures of merit, such as optical common-mode rejection ratios (CMRRs) and phase errors, are provided for both nominal and worst case scenarios. Additionally, system simulations are performed to compute imbalance-induced power penalty. Our results indicate that the combined effect of realistic foundry tolerances on device performance is significant. In particular, a fourfold reduction is predicted between nominal (≃40 nm) and worst cases (≃10 nm) when optical CMRRs and phase errors are compared against Optical Internetworking Forum specifications. By contrast, a much greater bandwidth is expected at the system level (≥ 40 nm) if a power penalty of less than 1 dB (@BER = 10-3) is to be allowed. In fact, worst case power penalties lower than 0.25 dB are predicted over the full C band, which further proves the great potential of integrated 4 × 4 MMIs as wide-bandwidth devices for mass production of coherent receivers using state-of-the-art integration technologies.

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We develop, analyze and apply a linearization technique based on dual parallel Mach-Zehnder modulator to self-beating microwave photonics systems. The approach enables broadband low-distortion transmission and reception at expense of a moderate electrical power penalty yielding a small optical power penalty (<1 dB).

Optical Hybrid for InP Integrated Coherent Receivers

In this paper we report on the design, simulation and practical aspects of a microwave photonic beat filter monolithically integrated on an Indium Phosphide platform. The chip contains all the components required for operation: tunable laser source, modulator, tunable filter and photo-detector. Onchip system simulations of the actual chip, using technology performance parameters, are provided. Expected filter response variations due to fabrication errors are also reported. Results and discussions on the use of double and single sideband modulation are given. Finally, details on the design of low temperature co-fired ceramics are provided.