Imad Hasan

Limitation Factor Analysis for Silicon-on-Insulator Waveguide Mach–Zehnder Interference-Based Electro-Optic Switch

The Mach-Zehnder interference (MZI) structure has played a significant role in research and development of the optical modulator/switch and silicon-on-insulator (SOI) waveguides have been increasingly developed to implement highly integrated photonic devices. In this paper, for the SOI-waveguide MZI-type electro-optic (EO) switch with free-carrier dispersion (FCD) effect, the extra optical absorption (EOA) loss caused by the FCD effect is analyzed and modeled. An intrinsic limitation factor existing in this device is found to be the tension between the EOA loss and the interaction length, resulting in a negative impact upon the device performance. The numerical calculations show that the millimeter-order interaction length has the lowest optical on-chip (OC) loss of about 0.8 and 1.8 dB at the OFF- and ON-state, respectively, and even a lowest OC imbalance of 1.0 dB between the two switching states. The influence of the coupling ratio of 3 dB waveguide directional coupler used in the MZI structure upon the switch performance is also studied, and a push-pull modulation scheme is proposed as an efficient solution to leveraging this intrinsic limitation caused performance decay with a combination of injection and depletion processes for the FCD effect. As a result, the optical OC loss is reduced to 1.0 dB, its imbalance is compressed to 0.2 dB, and the crosstalk at the OFF-state is also better than -21 dB. The relationship between the switching speed and the interaction length is also analyzed. As a vital condition for the FCD-based EO modulation of the switch, the dependence of free-carrier concentration modulation on the drive voltage and electrode gap is simulated via MEDICI software.

Free space and waveguide Talbot effect: phase relations and planar light circuit applications

Optical fields that are periodic in the transverse plane self-image periodically as they propagate along the optical axis: a phenomenon known as the Talbot effect. A transfer matrix may be defined that relates the amplitude and phase of point sources placed on a particular grid at the input to their respective multiple images at an image plane. The free-space Talbot effect may be mapped to the waveguide Talbot effect. Applying this mapping to the transfer matrix enables the prediction of the phase and amplitude relations between the ports of a Multimode Interference (MMI) coupler– a planar waveguide device. The transfer matrix approach has not previously been applied to the free-space case and its mapping to the waveguide case provides greater clarity and physical insight into the phase relationships than previous treatments. The paper first introduces the underlying physics of the Talbot effect in free space with emphasis on the positions along the optical axis at which images occur; their multiplicity; and their relative phase relations determined by the Gauss Quadratic Sum of number theory. The analysis is then adapted to predict the phase relationships between the ports of an MMI. These phase relationships are critical to planar light circuit (PLC) applications such as 90° optical hybrids for coherent optical receiver front-ends, external optical I-Q modulators for coherent optical transmitters; and optical phased array switches. These applications are illustrated by results obtained from devices that have been fabricated and tested by the PTLab in Si micro-photonic integration platforms.

Wireless enabled multi gas sensor system based on photonic crystals

In this paper we introduce a multi gas sensor system based on refractive index changes in a 2D slab photonic crystal. The sensor is formed by a L3 resonant cavity sandwiched between two W1.06 waveguides in the photonic crystal. The sensor configuration is similar to an Add-Drop filter structure. The transmission spectrums of the sensor with different ambient refractive indices ranging from n = 1.0 to n = 1.1 are calculated. The simulation results show that a change in ambient RI of Δn = 0.0008 is apparent with a corresponding change in output wavelength of the sensor of 2.4 nm. The properties of the sensor are simulated using the 3D finite-difference time-domain (FDTD) method. The Q-factor of the sensor is also optimized, with highest values reaching over 30,000. The sensor system is hybrid integrated with a wireless RF chip which processes the sensor data and transmits them in effect turning the entire system into a wireless sensor mote.

Gas sensing using slow light in photonic crystal waveguides

We introduce a novel gas sensor based on photonic crystal (PhC) waveguides where the gas sensing is based on the interaction between the slow light mode and the gas. Specifically, when the refractive index of the photonic crystal waveguide changes (due to a change in gas), the slow light regime of the photonic crystal waveguide is affected and shifts in wavelength. We have performed experiments with Helium and Argon gases to confirm the operation of the sensor, with Air being used as reference gas. Results show that the slow light regime typically shifts by 0.6 nm for Helium and 0.05nm for Argon.

Slow Light in Photonic Crystal Cavity Filled with Nematic Liquid Crystal

An innovative technique to tune the slow light propagated through photonic crystal cavity filled with E7 type nematic crystal has been simulated and presented. Observed propagating modes in the previously fabricated photonic crystal indicate that both slow and fast modes propagate in the waveguide. Design efforts were made to adjust the propagating modes as well as their group velocities. Numerical studies show that by inserting nematic liquid crystal, designer can achieve additional degree of freedom to tune the device by using external perturbation such as applying heat or electric field. Comparative studies have also been done to see the performance of the devices fabricated in two deferent material platforms (silicon and InP) with an objective to develop economic and efficient functional material systems for building robust integrated photonic devices that have the ability to slow, store, and process light pulses.

An intrinsic limitation to silicon-on-insulator waveguide Mach-Zehnder interference-based electro-optic devices

Mach-Zehnder interference (MZI) construction is broadly exploited to implement optical switches and modulators in the field of integrated optical/photonic technology. Silicon-on-insulator (SOI) waveguides have been increasingly developed to implement highly integrated photonic devices and systems. In this work, for the SOI-waveguide MZI-type electro-optic (EO) modulated devices with free-carrier dispersion (FCD) effect, the FCD-induced extra optical absorption (EOA) loss and its negative impact upon the device performance are studied. An intrinsic limitation to this type of device is found to be the tension between the EOA loss and the interaction length for a half-wave modulation. The numerical calculation and professional software simulation show the EOA loss of <1.0dB determines an interaction length of 4-mm. The performance decay processes of both EO switch and modulator due to the modulation-induced EOA loss are modeled. The numerical calculation shows the optical on-chip (OC) loss is 1.0dB and the isolation between two outputs is 21dB for the EO switch, while for the EO modulator the OC loss is 1.0dB and 2.5dB at the off- and on-state, respectively, and the extinction ratio only approaches 20dB. The negative influence of this intrinsic limitation to the bandwidth of EO modulator is also analyzed.

90 ◦ SOI optical hybrid for Radio-over-fibre links

Radio-over- fibre (RoF) technology is receiving large attention due to its ability to provide simple antenna front ends, increased capacity and increased wireless access coverage. Coherently detected RoF systems would enable the information to be carried in both the amplitude and phase or in different states of the polarisation of the optical field. Additionally, the selectivity of coherent receiver is very well suited for access networks. We present a 90° optical hybrid built on a silicon-on-insulator planar light-wave circuit, which can be used as the optical front end of the digital coherent receiver in a digitised RoF link and will lead to reduced receiver footprint and cost. The optical hybrid circuit includes 2 × 2 and 4 × 4 multimode interference (MMI) splitters, in a polarisation diversity configuration. The simulation results at vacuum wavelength 1,550 nm show polarisation independence and phase errors between the ports of less than 0.03°. The properties of the prototyped 4 × 4 MMI were measured over a wide range of wavelengths. The 2 × 2 and 4 × 4 MMI showed nearly equal splitting ratios. Measurements of the relative phase relationship between the ports for Transverse Electric mode polarisation are shown to match the simulation results.

Performance comparison between silicon-on-insulator curved waveguides and corner turning mirrors

Silicon-on-insulator (SOI) photonic integrated circuits have recently become a research topic of great interest due to their compact confinements and compatibility with the modern micro-electronics. As the dominant issues of the integration density of planar lightwave circuits on a single SOI chip, low-loss SOI curved waveguides and corner turning mirror (CTM) structures are attracting attention. This work aims at the performance comparison between the SOI curved waveguides and CTM structures. For this goal we have designed both SOI curved waveguides and SOI CTM structures and then fabricated them. The performance of these two devices, such as the propagation loss and polarization dependent loss, is measured and compared.

An accurate measurement method for optical radiation loss of silicon-on-insulator curved waveguides

Silicon-on-insulator (SOI) photonic integrated circuits have recently become a research topic of great interest due to their compact confinements and compatibility with the modern micro-electronics. Low-loss SOI curved waveguides are attracting attention as they offer solutions to the dominant issue of integration density of planar lightwave circuits on a single SOI chip. Propagation loss and radiation loss rate of SOI curved waveguides are accurately measured by using Fabry-Perot interferometric method with the help of an optical vector analyzer (Luna system). The presented technique provides a universal solution for optical loss measurement of photonic integrated chips.

Performance improvement to silicon-on-insulator waveguide directional-coupler based devices

For the SOI-waveguide directional coupler (WDC), optical access loss (OAL) and polarization dependence (PD) are two critical performance specifications which seriously affect the adoptability and deployment of a device, including optical on-chip loss (OCL), polarization dependent loss (PDL) and extinction ratio of a 3dB-coupler based device. In this work, using a commercial software tool - FIMMPROP, the performance of an SOI-WDC is simulated. Simulations find that the curved waveguides for the turning sections of a 3dB WDC not only enlarge the footprint size, but also seriously deteriorate the device performance. For instance, the two curved waveguide sections of a WDC induce an unpredictably large change in the 3dB-coupling length, increase an OAL of 0.4-0.9dB, and seriously deteriorate the PD, and these performance changes radically depend on rib size. After a corner-turning mirror (CTM) structure is introduced to a 3dB SOI-WDC, the experiments show both the footprint length and 3dB-coupling length are unchanged, the OAL of the 3dB coupler is only 0.5dB which is close to the simulation value. Therefore, for a 3dB-coupler based Mach-Zehnder interference (MZI) structure, the OCL will be controlled to be <1.0dB in device design and will not depend on rib size.