Ramesh K. Gupta

Bio: Ramesh K. Gupta is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topics: Silicon photonics & Power dividers and directional couplers. The author has an hindex of 4, co-authored 8 publications receiving 71 citations.

Wavelength-Independent Directional Couplers for Integrated Silicon Photonics

Abstract: It has been shown that the wavelength-dependent performance of a directional coupler (DC) in silicon-on-insulator (SOI) platform can be greatly engineered by suitable design optimizations. Semianalytical coupled mode theory is used to optimize a nearly wavelength-independent design of a DC in an SOI substrate with a device layer thickness of 220 nm, operating in TE-polarization ( $\lambda \sim$ 1550 nm). The transmission characteristics of fabricated DCs are found to be indeed wavelength independent over a bandwidth of ${\text{100 nm}}$ (1525 nm $\leq \lambda \leq$ 1625 nm), consistent with the theoretical predictions. The average excess loss of such directional couplers is evaluated as $\sim\text{0.8}$ dB and there are scopes for its further reduction. These DCs are then used further to demonstrate integrated optical building blocks like power splitters (2 $\times$ 2, 1 $\times$ 4), Mach–Zehnder interferometers (2 $\times$ 2), and all-pass microring resonators. Their performances are also found to be uniform within the wavelength range mentioned and, thus, making them suitable for integrated silicon photonics for broadband applications.

Dispersion Enhanced Critically Coupled Ring Resonator for Wide Range Refractive Index Sensing

Abstract: It is shown that the dispersion in transmission characteristics of a ring resonator designed with silicon-on-insulator waveguides in all-pass configuration can be enhanced significantly by increasing interaction length of the directional coupler. This in turn helps to single-out highly extinct resonance(s) at and around the critically coupled wavelength. Such a device is found to be useful for a wide range of refractive index sensing for the cladding materials/analytes ( ${\text{1.0}} ). As a proof of concept, the sensor devices were fabricated and characterization results are shown to be consistent with theoretical prediction. The fabricated devices have been also used successfully to determine unknown refractive index of a given analyte (Newport F-IMF-150) with an error limit of $\delta n \sim {\text{1.67}} \times {\text{10}}^{-2}$ RIU. Analyzing experimental results, it is shown that the limit of detection can be further reduced ( $\ll {\text{10}^{{-3}}}$ RIU), if the perimeter of the ring is increased without compromising the round-trip waveguide loss. The superiority of such a sensor device lies in its simpler design rule, easier operation, wider range, and nearly accurate detection mechanism.

Ultra-Broadband Add-Drop Filter/Switch Circuit Using Subwavelength Grating Waveguides

Abstract: An ultrabroadband add-drop filter/switch circuit is designed and demonstrated by integrating a pair of subwavelength grating waveguides in a $2\times 2$ Mach–Zehnder interferometer configuration using silicon photonics technology. The subwavelength grating is designed such that its stopband and passband are distinguished by a band-edge wavelength $\lambda _{\text{edge}} \sim$ 1565 nm, separating C and L bands. The stopband ( $\lambda ) is filtered at the drop port of the device, whereas the passband ( $\lambda >\lambda _{\text{edge}}$ ) is extracted either in cross port or in bar port. The device is designed to operate only in TE polarization. Experimental results exhibit a nearly flat-top band exceeding 40 nm for both stopband and passband. The stopband extinction at cross- and bar ports are measured to be $>$ 35 dB with a band-edge roll-off exceeding 70 dB/nm. Wavelength independent directional coupler design and integrated optical microheaters at different locations of the Mach–Zehnder arms for thermo-optic phase detuning are the key for stopband filtering at the drop port and switching of passband between cross- and bar ports with flat top response. Though the insertion loss of fabricated subwavelength grating waveguides are negligibly small, the observed passband insertion loss is $\sim$ 2 dB, which is mainly due to the combined excess loss of two directional couplers. Experimental results also reveal that the passband switching between cross- and bar ports of the device has been possible with an extinction of $>$ 15 dB by an electrical power consumption of $P_\pi \sim$ 54 mW. A switching time of 5 $\mu$ s is estimated by analyzing the transient response of the device. The passband edge could also be detuned thermo-optically at a rate of 22 pm/mW.

Performance analysis of metal-microheater integrated silicon waveguide phase-shifters

Abstract: A detailed theoretical and experimental study of metal-microheater integrated silicon waveguide phase-shifters has been carried out. It has been shown that the effective thermal conductance gw and the effective heat capacitance hw evaluated per unit length of the waveguide are two useful parameters contributing to the overall performance of a thermo-optic phase-shifter. Calculated values of temperature sensitivity, SH = 1/gw and thermal response time, τth = hw/gw of the phase-shifter are found to be consistent with the experimental results. Thus, a new parameter ℱH = SH/τth = 1/hw has been introduced to capture the overall figure of merit of a thermo-optic phase-shifter. A folded waveguide phase-shifter design integrated in one of the arms of a balanced MZI switch is shown to be superior to that of a straight waveguide phase-shifter of the same waveguide cross-sectional geometry. The MZI switches were designed to operate in TE-polarization over a broad wavelength range (λ ∼ 1550 nm).

Integrated silicon photonics directional couplers for WDM applications

Abstract: Supermode dispersion characteristics of directional couplers in 250-nm silicon-on-insulator substrate are investigated for silicon photonics WDM applications (λ ∼1550 nm). Experimental results of such a directional coupler based 2×2 wavelength interleavers and 1×4 WDM (δλ ∼ 10 nm) are shown to be consistent with semi-analytical theoretical prediction.

Optical Refractive Index Sensors with Plasmonic and Photonic Structures: Promising and Inconvenient Truth

Abstract: DOI: 10.1002/adom.201801433 Scientific American selects plasmonic sensing as the top 10 emerging technologies of 2018.[15] Almost every single new plasmonic or photonic structure would be explored to test its sensing ability.[16–29] These works tend to report the sensing performance of their own structure. Some declare that their sensitivity breaks the world record. However, there is still a missing literature on what the world record really is, the gap between the experiments and the theoretical limit, as well as the differences between metal-based plasmonic sensors and dielectric-based photonic sensors. To push plasmonic and photonic sensors into industrial applications, an optical sensing technology map is absolutely necessary. This review aims to cover a wide range of most representative plasmonic and photonic sensors, and place them into a single map. The sensor performances of different structures will be distinctly illustrated. Future researchers could plot the sensing ability of their new sensors into this technology map and gauge their performances in this field. In this review, we focus on optical refractive index (RI) sensors with no fluorescent labeling required. We will utilize two parameters to characterize and compare the performance of optical RI sensors: sensitivity to RI change (denoted by symbol SRI) and figure of merit (in short, FoM). For simplicity, we restrict our discussions to bulk RI change, where the change in RI occurs within the whole sample. There is another case where the RI variation occurs only within a very small volume close to the sensor surface. This surface RI sensitivity is proportional to the bulk RI sensitivity, the ratio of the thickness of the layer within which the surface RI variation occurs, and the penetration depth of the optical mode.[6] The bulk RI sensitivity defines the ratio of the change in sensor output (e.g., resonance angle, intensity, or resonant wavelength) to the bulk RI variations. Here, we limit our discussions to the spectral interrogations and the bulk RI sensitivity SRI is given by[3,5–7,30]

Ultra-Compact Broadband 2 × 2 3 dB Power Splitter Using a Subwavelength-Grating-Assisted Asymmetric Directional Coupler

Abstract: An ultra-compact broadband 2 × 2 3 dB power splitter is proposed and demonstrated by utilizing a subwavelength- grating (SWG)-assisted asymmetric directional coupler. In the present design, a subwavelength grating is inserted into the straight coupling region of an asymmetric directional coupler, so that the refractive index and the dispersion properties of this structure are engineered for achieving an ultra-broad bandwidth and an ultra-compact footprint. For the present power splitter with a straight coupling region as short as 5.25 μm, the bandwidth is as large as 300 nm (1400∼1700 nm) for achieving an imbalance of <0.4 dB and an excess loss of <0.33 dB for the fundamental transverse electrical (TE) mode. The fabricated splitter works well with an imbalance less than 0.5 dB and with an excess loss less than 1 dB in a broad wavelength-band from 1450 to 1650 nm.

Compact high-performance adiabatic 3-dB coupler enabled by subwavelength grating slot in the silicon-on-insulator platform

Abstract: We demonstrate a compact high-performance adiabatic 3-dB coupler for the silicon-on-insulator platform. The refractive index of the gap region between two coupling waveguides is effectively increased using subwavelength grating, which leads to high-performance operation and a compact design footprint, with a mode-evolution length of only 25 µm and an entire device length of 65 µm. The designed adiabatic 3-dB coupler has been fabricated using electron beam lithography and the feature size used in our design is CMOS compatible. The fabricated device is characterized in the wavelength range from 1500 nm to 1600 nm, with a measured power splitting ratio better than 3 ± 0.27 dB and an average insertion loss of 0.20 dB.

Wavelength-Independent Directional Couplers for Integrated Silicon Photonics

Abstract: It has been shown that the wavelength-dependent performance of a directional coupler (DC) in silicon-on-insulator (SOI) platform can be greatly engineered by suitable design optimizations. Semianalytical coupled mode theory is used to optimize a nearly wavelength-independent design of a DC in an SOI substrate with a device layer thickness of 220 nm, operating in TE-polarization ( $\lambda \sim$ 1550 nm). The transmission characteristics of fabricated DCs are found to be indeed wavelength independent over a bandwidth of ${\text{100 nm}}$ (1525 nm $\leq \lambda \leq$ 1625 nm), consistent with the theoretical predictions. The average excess loss of such directional couplers is evaluated as $\sim\text{0.8}$ dB and there are scopes for its further reduction. These DCs are then used further to demonstrate integrated optical building blocks like power splitters (2 $\times$ 2, 1 $\times$ 4), Mach–Zehnder interferometers (2 $\times$ 2), and all-pass microring resonators. Their performances are also found to be uniform within the wavelength range mentioned and, thus, making them suitable for integrated silicon photonics for broadband applications.

Subwavelength silicon photonics for on-chip mode-manipulation

Abstract: On-chip mode-manipulation is one of the most important physical fundamentals for many photonic integrated devices and circuits. In the past years, great progresses have been achieved on subwavelength silicon photonics for on-chip mode-manipulation by introducing special subwavelength photonic waveguides. Among them, there are two popular waveguide structures available. One is silicon hybrid plasmonic waveguides (HPWGs) and the other one is silicon subwavelength-structured waveguides (SSWGs). In this paper, we focus on subwavelength silicon photonic devices and the applications with the manipulation of the effective indices, the modal field profiles, the mode dispersion, as well as the birefringence. First, a review is given about subwavelength silicon photonics for the fundamental-mode manipulation, including high-performance polarization-handling devices, efficient mode converters for chip-fiber edge-coupling, and ultra-broadband power splitters. Second, a review is given about subwavelength silicon photonics for the higher-order-mode manipulation, including multimode converters, multimode waveguide bends, and multimode waveguide crossing. Finally, some emerging applications of subwavelength silicon photonics for on-chip mode-manipulation are discussed.