Integrated mode-evolution-based polarization rotators

Integrated visible-light polarization rotators and splitters for atomic quantum systems

In this work, we design and experimentally demonstrate the first, to the best of our knowledge, integrated polarization splitters and rotators at blue wavelengths. We develop compact and efficient designs for both a polarization splitter and rotator at a 422-nm wavelength, an important laser-cooling transition for 88Sr+ ions. These devices are fabricated in a 200-mm wafer-scale process and experimentally demonstrated, resulting in a measured polarization-splitter transverse-electric thru-port coupling of 98.0% and transverse-magnetic tap-port coupling of 77.6% for a compact 16-µm-long device and a polarization-rotator conversion efficiency of 92.2% for a separate compact 111-µm-long device. This work paves the way for more sophisticated integrated control of trapped-ion and neutral-atom quantum systems.

MEMS-tunable polarization management in photonic integrated circuits

Optical fibers are generally polarization-insensitive while photonic integrated circuits (PICs) often exhibit a large polarization dependence due to the high-aspect-ratio and high-index-contrast of integrated waveguides. As PICs become more mature there is an increasing need for tunable polarization management on-chip. Although micro-electro-mechanical systems (MEMS) are increasingly finding application in PICs for optical switching and phase shifting, they have so far not found wide application for polarization management. In this work we propose two optical MEMS architectures for polarization management enabling tunable polarization splitting and rotation - key functions so far lacking in PICs. The first structure consists of a directional coupler with a MEMS-tunable gap enabling a continuously-variable polarization splitting ratio. A second architecture consists of a symmetry-breaking MEMS perturber suspended over an air-cladded waveguide enabling tunable polarization rotation. For both architectures we simulate a polarization extinction exceeding 25 dB, and the operating bandwidth can be as large as 100 nm. We conclude with a discussion of actuation schemes and examine fabrication considerations for implementation in PIC foundries.

Compact and efficient polarization rotator using laterally asymmetric rib waveguides on a lithium-niobate-on-insulator platform

Photonic devices based on a lithium-niobate-on-insulator (LNOI) are current research hotspots; however, owing to the high refractive index contrast of the LNOI platform and inherent birefringence of lithium niobate itself, such photonic devices are generally polarization sensitive, affecting their further wide application. This paper proposes a simple, compact, and efficient polarization rotator (PR) based on a laterally asymmetric rib waveguide by depositing a layer of semi-infinite silicon nitride dielectric material on one side of the rib waveguide. The results show that a PR with a polarization rotation region length of 15.77 µm is achieved, and the polarization extinction ratio (PER), insertion loss (IL), and polarization conversion efficiency (PCE) are 38.57/68.95 and 0.2/0.22 dB, and 99.99%/almost 100%, respectively, for the fundamental transverse electric mode (T E 0) and transverse magnetic mode (T M 0) at a 1.55 µm wavelength. The operation bandwidth is around 120 nm for the T E 0 mode and T M 0 mode when the PER, IL, and PCE are greater than 20 dB, less than 0.32 dB, and more than 99%, respectively. Fabrication tolerances to the key structural parameters are investigated in detail. In addition, the evolution fields of the T E 0 mode and T M 0 mode along the propagation direction through the proposed device are presented.

Ultra-Short Polarization Rotator Based on Flat-Shaped Photonic Crystal Fiber Filled with Liquid Crystal

In this study we demonstrate a high-performance polarization rotator (PR) based on flat-shaped photonic crystal fiber. The flat surfaces of the fiber are plated on gold films as electrodes, and the core of the structure is filled with liquid crystal. The polarization rotation characteristics of the flat-shaped fiber can be effectively adjusted by applying external voltage. The optical properties are analyzed using the finite element method (FEM). The results show that the magnitude of the modulation voltage is closely related to the thickness of the flat fiber. When the fiber thickness is 20 μm, only 100 V is required to achieve the highest PR performance. In the wavelength of the 1.55 μm band (~200 nm bandwidth), the conversion length of the PR is only 3.99 μm, the conversion efficiency is close to 100%, and the minimum crosstalk value is -26.2 dB. The presented PR, with its excellent performance, might enable promising applications in the communication system and the photonic integrated circuits.

Design of Waveguide Polarization Convertor Based on Asymmetric 1D Photonic Crystals

Photonic crystals possess metastructures with a unique dispersion relation. An integrated optical circuit plays a crucial role in quantum computing, for which miniaturized optical components can be designed according to the characteristics of photonic crystals. Because the stable light transmission mode for a square waveguide is transverse electric or transverse magnetic polarization, we designed a half-waveplate element with a photonic crystal that can rotate the polarization direction of the light incident on a waveguide by 90°. Using the dispersion relation of photonic crystals, the polarization rotation length and the optical axis’s angle of deviation from the electric field in the eigenmode can be effectively calculated. Polarization rotators designed on the basis of photonic crystal structures can effectively reduce the insertion loss of components and exhibit favorable polarization rotation performance.

On-chip beam rotators, adiabatic mode converters, and waveplates through low-loss waveguides with variable cross-sections

Photonics integrated circuitry would benefit considerably from the ability to arbitrarily control waveguide cross-sections with high precision and low loss, in order to provide more degrees of freedom in manipulating propagating light. Here, we report a new method for femtosecond laser writing of optical-fiber-compatible glass waveguides, namely spherical phase-induced multicore waveguide (SPIM-WG), which addresses this challenging task with three-dimensional on-chip light control. Fabricating in the heating regime with high scanning speed, precise deformation of cross-sections is still achievable along the waveguide, with shapes and sizes finely controllable of high resolution in both horizontal and vertical transversal directions. We observed that these waveguides have high refractive index contrast of 0.017, low propagation loss of 0.14 dB/cm, and very low coupling loss of 0.19 dB coupled from a single-mode fiber. SPIM-WG devices were easily fabricated that were able to perform on-chip beam rotation through varying angles, or manipulate the polarization state of propagating light for target wavelengths. We also demonstrated SPIM-WG mode converters that provide arbitrary adiabatic mode conversion with high efficiency between symmetric and asymmetric nonuniform modes; examples include circular, elliptical modes, and asymmetric modes from ppKTP (periodically poled potassium titanyl phosphate) waveguides which are generally applied in frequency conversion and quantum light sources. Created inside optical glass, these waveguides and devices have the capability to operate across ultra-broad bands from visible to infrared wavelengths. The compatibility with optical fiber also paves the way toward packaged photonic integrated circuitry, which usually needs input and output fiber connections.

Compact broadband ( O, E, S, C, L & U bands) silicon TE-pass polarizer based on ridge waveguide adiabatic S-bends

A compact, ultra-broadband and high-performance silicon TE-pass polarizer is proposed and demonstrated experimentally. It is based on partially-etched (ridge) waveguide adiabatic S-bends, input/output tapers and side gratings on a silicon-on-insulator (SOI) platform. A compact footprint and weak back reflections are obtained due to the bent waveguide and the tapers, respectively. An extremely high extinction ratio is achieved by scattering the undesired light in the slab section using the side gratings. The 3D FDTD simulations show a TE loss less than 0.3 dB and an extinction ratio greater than 30 dB over a 500 nm wavelength range (1200 nm to 1700 nm). Measured results show a high TM loss (> 35 dB) and a low TE insertion loss (< 1.5 dB), over a 200 nm wavelength range (1450 nm to 1650 nm). The measured TE loss is < 0.6 dB at a communication wavelength of 1550 nm. The footprint of the optimized design is 65 µm × 20 µm.

Mid-infrared polarization rotator based on a Si3N4-CaF2 hybrid plasmonic waveguide with asymmetric metal claddings

In this paper, a Si₃N₄-CaF₂ hybrid plasmonic waveguide (HPW) with an asymmetric metal cladding is designed for the mid-infrared polarization rotator (PR). The mode characteristics and polarization rotation performances of the Si₃N₄-CaF₂ HPW-based PR are simulated by using the finite element method. Operating at the wavelength of 3.5 µm, the polarization conversion efficiency between two polarization modes (PM 1 and PM 2) is larger than 99% at a Si₃N₄-CaF₂ HPW length of 104 µm. The Si₃N₄-CaF₂ HPW-based PR maintains good polarization rotation performances within fabrication tolerances from -10 to 10 nm. The polarization rotator based on the Si₃N₄-CaF₂ HPW paves the way to achieve integrated waveplates, driving many important optical functions from free space onto a chip.

Active control of polarization state of the light in InP waveguide

We demonstrate an InP deep-ridge type multi-quantum-wells (MQWs) waveguide-based active polarization control scheme by using two positive-intrinsic-negative diode phase shifters. The polarization state of horizontal/vertical linearly polarized input light is rotated along the ± 45°-linear polarization axis on the Poincaré sphere by 45°-eigenmode-rotated first phase shifter 1 and subsequently rotated along the vertical/horizontal axis by second normal phase shifter 2. The rotation of the eigenmode axes is obtained by using the surface plasmonic effect. The effective index of the waveguide is changed via quantum-confined Stark effect in the MQW core. The length of each phase shifter is 240 µm. The applied reverse bias voltages are -2.7 V and -1.95 V for the phase shifter 1 and 2, respectively.

Ultra-wideband Ge-rich silicon germanium mid-infrared polarization rotator with mode hybridization flattening

In this work we investigate the implementation of ultra-wideband polarization rotator in the mid-infrared spectral region. A new design method of the rotation section is proposed, yielding a polarization rotator with an extinction ratio of at least 15 dB in a wavelength range of 2 µm. For a spectral range wider than 3.8 µm, an extinction ratio of at least 10 dB is achieved for this design. The device is 1660 µm long and the associated insertion loss is below 1.2 dB on the full operational wavelength range. The influence of geometrical parameters with respect to the design method to obtain such a broadband behavior is discussed. Finally, to increase the tolerance to fabrication errors, a tapered rotator design is proposed. Such a device can support up to ± 100 nm fabrication errors and still guarantees remarkable broadband behavior. To the best of our knowledge, this is the first time an integrated polarization rotator is designed to operate for the wavelength range of 4 to 9 µm with a bandwidth exceeding 2 µm.

Transmissive silicon photonic dichroic filters with spectrally selective waveguides

Many optical systems require broadband filters with sharp roll-offs for efficiently splitting or combining light across wide spectra. While free space dichroic filters can provide broadband selectivity, on-chip integration of these high-performance filters is crucial for the scalability of photonic applications in multi-octave interferometry, spectroscopy, and wideband wavelength-division multiplexing. Here we present the theory, design, and experimental characterization of integrated, transmissive, 1 × 2 port dichroic filters using spectrally selective waveguides. Mode evolution through adiabatic transitions in the demonstrated filters allows for single cutoff and flat-top responses with low insertion losses and octave-wide simulated bandwidths. Filters with cutoffs around 1550 and 2100 nm are fabricated on a silicon-on-insulator platform with standard complementary metal-oxide-semiconductor processes. A filter roll-off of 2.82 dB nm-1 is achieved while maintaining ultra-broadband operation. This new class of nanophotonic dichroic filters can lead to new paradigms in on-chip communications, sensing, imaging, optical synthesis, and display applications.

Ridge waveguide assisted highly efficient transverse-electric-pass polarizer based on a hybrid plasmonic waveguide

An ultra-compact and low-loss transverse-electric (TE)-pass polarizer is proposed by utilizing a hybrid plasmonic ridge waveguide structure on a silicon-on-insulator platform. Since the transmission of a plasmonic polarizer is greatly influenced by the interaction between the metal and the optical field, there is a large insertion loss (IL) in the polarizer based on a hybrid plasmonic waveguide. Here, the electric field distribution and the periodic coupling between the fundamental transverse magnetic mode and plasmonic mode are taken into consideration to decrease the IL and shorten the length of the device. For a 4.2 μm-long polarizer, numerical simulations with the finite-differential-time-domain method show that it has a large extinction ratio (ER) of ∼29.5  dB with a low IL of 0.18 dB at the central wavelength 1550 nm. In addition, a more than 23 dB ER is achieved across a wide bandwidth of 100 nm.

Ultrashort and broadband silicon polarization splitter-rotator using fast quasiadiabatic dynamics

We propose an ultrashort and broadband silicon mode-conversion polarization splitter-rotator (PSR) consisting of a taper and a Y-junction both designed by the fast quasiadiabatic dynamics (FAQUAD). The FAQUAD is used to homogeneously distribute adiabaticity over the length of the PSR, providing shortcut to adiabaticity at a shorter device length. The total length of the silicon PSR is 39.2 μm. For a wavelength range from 1.5 μm to 1.6 μm, the PSR exhibits a good performance with > 88% transmission and > 11.4 dB extinction ratio (ER). Simulations also show that the designed devices have good fabrication tolerance.

Integrated InP polarization rotator using the plasmonic effect

we report on an integrated InP based polarization rotator scheme using the plasmonic effect. It operates as a half-wave retarder in ridge waveguide structure. The rotation angle of the eigenmode axes of the half-wave retarder waveguide is determined by the position off a bottom corner of a metal layer placed above the waveguide core in the upper cladding region. The simple rotator structure enables an easy and tolerant fabrication process. The length of the fabricated device is less than 50 μm, and a polarization extinction ratio (PER) of 20 dB has been achieved.

Broadband integrated polarization rotator using three-layer metallic grating structures

In this work, we demonstrate broadband integrated polarization rotator (IPR) with a series of three-layer rotating metallic grating structures. This transmissive optical IPR can conveniently rotate the polarization of linearly polarized light to any desired directions at different spatial locations with high conversion efficiency, which is nearly constant for different rotation angles. The linear polarization rotation originates from multi-wave interference in the three-layer grating structure. We anticipate that this type of IPR will find wide applications in analytical chemistry, biology, communication technology, imaging, etc.

Graphene-assisted ultra-compact polarization splitter and rotator with an extended bandwidth

The high refraction-index contrast between silicon and the surrounding cladding makes silicon-on-insulator devices highly polarization-dependent. However, it is greatly desirable for many applications to address the issue of polarization dependence in silicon photonics. Here, a novel ultra-compact polarization splitter and rotator (PSR), constructed with an asymmetrical directional coupler consisting of a rib silicon waveguide and a graphene-embedded rib silicon waveguide (GERSW), on a silicon-on-insulator platform is proposed and investigated. By taking advantage of the large modulation of the effective refractive index of the TE mode for the GERSW by tuning the chemical potential of graphene, the phase matching condition can be well satisfied over a wide spectral band. The presented result demonstrates that for a 7-layer-graphene-embedded PSR with a coupling length of 11.1 μm, a high TM-to-TE conversion efficiency (>−0.5 dB) can be achieved over a broad bandwidth from 1516 to 1602 nm.

Compact polarization rotator for silicon-based cross-slot waveguides using subwavelength gratings

A compact and broadband polarization rotator (PR) for silicon-based cross-slot waveguides using subwavelength gratings (SWGs) is proposed and analyzed. To significantly break the symmetry of the waveguide structure, the diagonal regular Si wires of the cross-slot waveguides are replaced with the full etching SWGs. Moreover, the special properties of the SWGs-whose effective index is adjustable-can effectively enhance the modal birefringence between the two lowest-order hybrid modes, resulting in a more compact device. By utilizing interference effect of the hybrid modes, both transverse electric to transverse magnetic (TE-to-TM) and TM-to-TE conversion can be efficiently realized. Numerical results show that a PR of 12.6 μm in length at a wavelength of 1.55 μm is achieved, where the polarization conversion efficiency (PCE) and insertion loss (IL) are, respectively, 97.2% and 0.71 dB, and the reflection loss is below -20.5  dB for both cases. Moreover, a wide bandwidth of ∼260  nm for both polarizations is obtained for keeping the PCE over 90% and IL below 1 dB. In addition, fabrication tolerances to the structural parameters are analyzed in detail, and field evolution along the propagation distance is also presented.

Silicon photonic transceiver circuit for high-speed polarization-based discrete variable quantum key distribution

We demonstrate a silicon photonic transceiver circuit for high-speed discrete variable quantum key distribution that employs a common structure for transmit and receive functions. The device is intended for use in polarization-based quantum cryptographic protocols, such as BB84. Our characterization indicates that the circuit can generate the four BB84 states (TE/TM/45°/135° linear polarizations) with >30 dB polarization extinction ratios and gigabit per second modulation speed, and is capable of decoding any polarization bases differing by 90° with high extinction ratios.

The Origin and Limit of Asymmetric Transmission in Chiral Resonators

We observe that the asymmetric transmission (AT) through photonic systems with a resonant chiral response is strongly related to the far-field properties of eigenmodes of the system. This understanding can be used to predict the AT for any resonant system from its complex eigenmodes. We find that the resonant chiral phenomenon of AT is related to, and is bounded by, the nonresonant scattering properties of the system. Using the principle of reciprocity, we determine a fundamental limit to the maximum AT possible for a single mode in any chiral resonator. We propose and follow a design route for a highly chiral dielectric photonic crystal structure that reaches this fundamental limit for AT.

Mode-evolution-based coupler for high saturation power Ge-on-Si photodetectors

We propose a mode-evolution-based coupler for high saturation power germanium-on-silicon photodetectors. This coupler uniformly illuminates the intrinsic germanium region of the detector, decreasing saturation effects, such as carrier screening, observed at high input powers. We demonstrate 70% more photocurrent generation (9.1-15.5 mA) and more than 40 times higher opto-electrical bandwidth (0.7-31 GHz) than conventional butt-coupled detectors under high-power illumination. The high-power and high-speed performance of the device, combined with the compactness of the coupling method, will enable new applications for integrated silicon photonics systems.

Polarization rotator based on augmented low-index-guiding waveguide on silicon nitride/silicon-on-insulator platform

Using a newly proposed augmented low-index-guiding scheme with silicon nitride/silicon dual-core waveguide, we have designed, fabricated, and characterized a transverse electric (TE) to transverse magnetic (TM) and TM-to-TE compact polarization rotator. The polarization rotation is realized in an asymmetric directional coupler. The measured peak conversion efficiencies for the TE-to-TM and TM-to-TE rotations are approximately 97%. The measured polarization extinction ratio for the TE-to-TM rotation is greater than 20 dB over 50-nm bandwidth, while for the TM-to-TE rotation it is greater than 15 dB over the C-band.

Mode-evolution-based polarization rotation and coupling between silicon and hybrid plasmonic waveguides

Hybrid plasmonic (HP) modes allow strong optical field confinement and simultaneously low propagation loss, offering a potentially compact and efficient platform for on-chip photonic applications. However, their implementation is hampered by the low coupling efficiency between dielectric guided modes and HP modes, caused by mode mismatch and polarization difference. In this work, we present a mode-evolution-based polarization rotation and coupling structure that adiabatically rotates the TE mode in a silicon waveguide and couples it to the HP mode in a strip silicon-dielectric-metal waveguide. Simulation shows that high coupling factors of 92%, 78%, 75%, and 73% are achievable using Ag, Au, Al, and Cu as the metal cap, respectively, at a conversion length of about 5 μm. For an extremely broad wavelength range of 1300–1800 nm, the coupling factor is >64% with a Ag metal cap, and the total back-reflection power, including all the mode reflections and backscattering, is below −40 dB, due to the adiabatic mode transition. Our device does not require high-resolution lithography and is tolerant to fabrication variations and imperfections. These attributes together make our device suitable for optical transport systems spanning all telecommunication bands.

300 nm bandwidth adiabatic SOI polarization splitter-rotators exploiting continuous symmetry breaking

Adiabatic polarization splitter-rotators are investigated exploiting continuous symmetry breaking thereby achieving significant device size and losses reduction in a single mask fabrication process for both SOI channel and ridge waveguides. A crosstalk lower than -25 dB is expected over 300nm bandwidth, making the device suitable for full grid CWDM and diplexer/triplexer FTTH applications at 1310, 1490 and 1550nm.

Polarization rotation Bragg diffraction using Si wire waveguide grating and polarization rotator

We report polarization independent Bragg grating wavelength filter with high diffraction efficiency. A rib waveguide polarization rotator and antisymmetric grating structure for fundamental to first order diffraction are used to generate the polarization rotation Bragg diffraction. The diffraction efficiencies and peak wavelengths become the same for two orthogonal input polarizations. Strong diffraction is attained easily. The concept was verified by simulation and experiment. Polarization independent band-pass filter consisting of polarization beam splitter and polarization rotation Bragg diffraction was experimentally demonstrated.

Polarization rotation and coupling between silicon waveguide and hybrid plasmonic waveguide

We present a polarization rotation and coupling scheme that rotates a TE(0) mode in a silicon waveguide and simultaneously couples the rotated mode to a hybrid plasmonic (HP(0)) waveguide mode. Such a polarization rotation can be realized with a partially etched asymmetric hybrid plasmonic waveguide consisting of a silicon strip waveguide, a thin oxide spacer, and a metal cap made from copper, gold, silver or aluminum. Two implementations, one with and one without the tapering of the metal cap are presented, and different taper shapes (linear and exponential) are also analyzed. The devices have large 3 dB conversion bandwidths (over 200 nm at near infrared) and short length (< 5 μm), and achieve a maximum coupling factor of ∼ 78% with a linearly tapered silver metal cap.

Reflectionless compact plasmonic waveguide mode converter by using a mode-selective cavity

A compact transmissive plasmonic waveguide mode converter which aims for the elimination of reflection and transmission of unconverted mode is proposed. The proposed scheme exploits a cavity formed by mode selective mirrors, which only allows two output modes: the transmission of the target mode and the reflection of the input mode. By appropriately tuning cavity lengths, the reflection of the input mode can also be suppressed to near zero by destructive interference, thereby all the residual outgoing modes are suppressed. The proposed device might be useful in the design of integrated photonic system since it relaxes the problem of unwanted reflection.

Low-loss partial rib polarization rotator consisting only of silicon core and silica cladding

We present a low-loss and small-footprint polarization rotator based on mode evolution. The polarization rotator is composed of an asymmetric-rib waveguide and a tapered waveguide, both of which consist only of a silicon core and a silica cladding. The rotator is fabricated under the same design rules as other device blocks, such as rib-waveguide phase shifters for photonic integration. The polarization rotator is fabricated using CMOS-based processes and provides polarization rotations with an on-chip insertion loss lower than 0.5 dB from transverse-electric (TE) to transverse-magnetic (TM) polarization and a loss lower than 1.0 dB from the TM to TE polarization in a 200 nm wavelength range extending over C and L bands.

Ultra-compact and broadband Si photonics polarization rotator by self-alignment process

An ultra-simple polarization rotator is demonstrated on SOI platform with self-aligned process to enhance performance repeatability and manufactural yield. The polarization rotation is essentially achieved by the symmetry breaking of a channel waveguide with a single-sided slab. The two-step lithography enabling this structure is fully compatible with the mainstream process flow of Si photonic integration. A polarization conversion efficiency of 93% is obtained at 1560nm in less than 10μm light propagation length. The merit of flat-band operation (> 100nm) by using asymmetric waveguide for polarization rotation is inherited.

Short-length and robust polarization rotators in periodically poled lithium niobate via shortcuts to adiabaticity

Conventional narrowband spectrum polarization devices are short but not robust, based on quasi-phase matching (QPM) technique, in periodically poled lithium niobate (PPLN) crystal. In this paper, we propose short-length and robust polarization rotators by using shortcuts to adiabaticity. Beyond the QPM condition, the electric field and period of PPLN crystal are designed in terms of invariant dynamics, and further optimized with respect to input wavelength/refractive index variations. In addition, the stability of conversion efficiency on the electric field and period of PPLN crystal is also discussed. As a consequence, the optimal shortcuts are fast as well as robust, which provide broadband spectrum polarization devices with short length.

Mode-coupling polarization rotator based on plasmonic waveguide

A novel polarization rotator (PR) based on mode coupling in plasmonic waveguides is demonstrated by simulation. A silicon waveguide with asymmetric claddings of silicon oxide and metal is applied to induce a hybridization of the polarization modes. Operating at the telecommunication wavelength of 1.55 μm, polarization conversion efficiency of 99.7% can be achieved in a device at a length of 9.7 μm with an insertion loss of 2.2 dB. This PR can be easily fabricated by oblique deposition of the claddings after etching the silicon waveguide without precise alignment for two-step lithography as required in a previous design.

Design of a SiO₂ top-cladding and compact polarization splitter-rotator based on a rib directional coupler

A compact polarization splitter-rotator based on a silicon-on-insulator rib asymmetrical directional coupler with SiO2 top-cladding is proposed. Unlike previously reported PSRs which specifically required the top-cladding material to be different from the bottom cladding in order to break the symmetry of the waveguide cross-section, our proposed PSR has no such limitation on the top-cladding due to the horizontal asymmetry of the rib waveguide. In addition, the device is highly compact and has a total length as short as 24 μm. Numerical simulation shows that a high conversion efficiency of ~97% is obtained at the wavelength of 1550 nm. With the width variation of ± 15 nm and the gap variation of ± 50 nm, the PSR still has high ER of 12 dB at the cross-port, showing large fabrication tolerance. This device can be cascaded to improve the performance at the through port and an example of a two-stage PSR is presented. The mode conversion between the strip waveguide and the rib waveguide is also discussed.

Four-port integrated polarizing beam splitter

In this Letter, we report on the first integrated four-port polarizing beam splitter. The device operates on the principle of mode evolution and was implemented in a silicon-on-insulator silicon photonics platform and fabricated on a 300 mm CMOS line using 193 nm optical immersion lithography. The adiabatic transition forming of the structure enabled over a 150 nm bandwidth from λ~1350 to λ~1500  nm, achieving a cross-talk level below -10  dB over the entire band.

Mode control and mode conversion in nonlinear aluminum nitride waveguides

While single-mode waveguides are commonly used in integrated photonic circuits, emerging applications in nonlinear and quantum optics rely fundamentally on interactions between modes of different order. Here we propose several methods to evaluate the modal composition of both externally and device-internally excited guided waves and discuss a technique for efficient excitation of arbitrary modes. The applicability of these methods is verified in photonic circuits based on aluminum nitride. We control modal excitation through suitably engineered grating couplers and are able to perform a detailed study of waveguide-internal second harmonic generation. Efficient and broadband power conversion between orthogonal polarizations is realized within an asymmetric directional coupler to demonstrate selective excitation of arbitrary higher-order modes. Our approach holds promise for applications in nonlinear optics and frequency up/down-mixing in a chipscale framework.

Integrated polarization rotator/converter by stimulated Raman adiabatic passage

We proposed a polarization rotator inspired by stimulated Raman adiabatic passage model from quantum optics, which is composed of a signal waveguide and an ancillary waveguide. The two orthogonal modes in signal waveguide and the oblique mode in ancillary waveguide form a Λ-type three-level system. By controlling the width of signal waveguide and the gap between two waveguides, adiabatic conversion between two orthogonal modes can be realized in the signal waveguide. With such adiabatic passage, polarization conversion is completed within 150 μm length, with the efficiencies over 99% for both conversions between horizontal polarization and vertical polarization. In addition, such a polarization rotator is quite robust against fabrication error, allowing a wide range of tolerances for the rotator geometric parameters. Our work is not only significative to photonic simulations of coherent quantum phenomena with engineered photonic waveguides, but also enlightens the practical applications of these phenomena in optical device designs.

Ultra-short silicon-on-insulator (SOI) polarization rotator between a slot and a strip waveguide based on a nonlinear raised cosine flat-tip taper

A design of ultra-short integrated polarization rotator (PR) with nonlinear and flat-tip tapers is demonstrated theoretically. Based on mode-evolution theory and three-dimensional (3D) finite-difference time-domain (FDTD) simulation, raised cosine profiled tapers are introduced and optimized in the transition region, which improve the polarized modes coupling and decrease the conversion length to 6 μm for the wavelength of 1.55 μm. Numerical simulations obtain extinction ratio of 32 dB and loss of 0.14 dB with practical material parameters. The method and result presented here can be extremely valuable for applications in polarization diversity circuits.

Ultrashort slot polarization rotator with double paralleled nonlinear geometry slot crossings

The polarization rotation of an optical slot structure from the vertical to the horizontal direction, or vice versa, is demonstrated theoretically within an ultrashort conversion length. Based on mode-evolution theory and three-dimensional finite-difference time-domain simulation, two parallel nonlinearly tapered crossings are introduced and optimized in the transition region, which efficiently decreases the conversion length from 11 to 5 μm for the wavelength of 1.55 μm in a silicon-on-isolator system. More importantly, the extinction ratio of 22 dB and loss of 0.12 dB are numerically obtained using practical material parameters. The method and result presented here may be extremely valuable for the applications of integrated slot structures.

Wideband polarization splitter and rotator with large fabrication tolerance and simple fabrication process

We propose and demonstrate a polarization splitter and rotator (PSR) built on a silicon-on-insulator platform. The PSR is constructed with a tapered waveguide followed by a 2×2 multimode interferometer and can be simply fabricated in a single lithography and etching step. A low insertion loss (<2.5 dB with minimum insertion loss of 0.6 dB) and a low polarization crosstalk (<-12 dB) over a wide operation bandwidth (~100 nm) with a large fabrication tolerance (>50 nm) are experimentally demonstrated.

Experimental demonstration of self-aligned InP/InGaAsP polarization converter for polarization multiplexed photonic integrated circuits

Highly efficient, low-loss, and compact InP/InGaAsP polarization converter based on a half-ridge waveguide structure is fabricated and demonstrated experimentally. The device is fabricated by a simple self-aligned process and integrated with a ridge InP waveguide. Using a 150-μm-long device, we obtain the mode conversion of more than 96% and the on-chip loss of less than 1.0 dB over the broad wavelength range from 1510 to 1575 nm. The experimental results are explained quantitatively using the full-vector eigenmode calculation, which also reveals large fabrication tolerance of the demonstrated device.

Polarization splitter using horizontal slot waveguide

In this paper a compact and efficient polarization splitter using horizontal slotted waveguides is presented. The splitter is designed by finite-difference-time-domain simulation and realized experimentally. The splitter is built by using a direction coupler consisting of two horizontal slotted waveguides and achieves a high extinction ratio of 14.1 and 16.8 dB for cross and through ports. The optimal coupling length is found to be 15 µm. The device exhibits a good response of extinction ratio across C + L broadband. The splitter obtained is readily used for a polarization diversity circuit, particularly for platforms with horizontal slot waveguides.

Adiabatic microring modulators

In this work, we demonstrate and experimentally characterize a new class of high-performance silicon photonic modulators-the adiabatic microring modulator. The adiabatic microring modulator utilizes a vertical PN junction and interior electrical contacts, leveraging all the advantages of previously-demonstrated microdisk modulators. However, this device also incorporates an adiabatic transition from the wide, multimode contact region, to a narrow, single-mode coupling region, eliminating unwanted spatial modes common to microdisks. As a result, the adiabatic microring modulator demonstrated in this work is the smallest microring modulator demonstrated to date, with a diameter of only 4 μm, yielding a 6.92-THz uncorrupted free spectral range. Here, we perform an experimental comparative analysis between silicon adiabatic microring modulators, silicon microdisk modulators, and a commercial lithium-niobate Mach-Zehnder modulator. We show that the silicon adiabatic microring modulator using partial doping is capable of operating at 12.5-Gb/s data rates and beyond. This device combines the best of all modulator designs, leveraging the depletion-based method to maximize the speed, utilizing the vertical-junction configuration to minimize the power consumption, employing a unique adiabatic design to eliminate higher-order modes, and using partial doping to reduce resistance, further enhancing the speed of the device.

Design of a compact mode and polarization converter in three-dimensional photonic crystals

A mode and polarization converter is proposed and optimized for 3D photonic integrated circuits based on photonic crystals (PhCs). The device converts the index-guided TE mode of a W1 solid-core (SC) waveguide to the band-gap-guided TM mode of a W1 hollow-core (HC) waveguide in 3D PhCs, and vice versa. The conversion is achieved based on contra-directional mode coupling. For a 25 μm-long device, simulations show that the power conversion efficiency is over 98% across a wavelength range of 16 nm centered at 1550 nm, whereas the reflection remains below -20 dB. The polarization extinction ratio of the conversion is in principle infinitely high because both W1 waveguides operate in the single-mode regimes in this wavelength range.

Mode-evolution-based polarization rotator-splitter design via simple fabrication process

A mode-evolution-based polarization rotator-splitter built on InP substrate is proposed by combining a mode converter and an adiabatic asymmetric Y-coupler. The mode converter, consisting of a bi-level taper and a width taper, effectively converts the fundamental TM mode into the second order TE mode without changing the polarization of the fundamental TE mode. The following adiabatic asymmetric Y-coupler splits the fundamental and the second order TE modes and also converts the second order TE mode into the fundamental TE mode. A shallow etched structure is proposed for the width taper to enhance the polarization conversion efficiency. The device has a total length of 1350 µm, a polarization extinction ratio over 25 dB and an insertion loss below 0.5 dB both for TE and TM modes, over the wavelength range from 1528 to 1612 nm covering all C + L band. Because the device is designed based on mode evolution principle, it has a large fabrication tolerance. The insertion loss remains below 1 dB and the polarization extinction ratio remains over 17 dB with respect to a width variation of +/- 0.12 µm at the wavelength of 1570 nm, or +/- 0.08 µm over the entire C + L band.

Silicon polarization independent microring resonator-based optical tunable filter circuit with fiber assembly

We report the design, fabrication, photonic packaging and the characterization of a silicon polarization independent optical tunable filter circuit with fiber assembly. We demonstrate the polarization transparent filter characteristics with an insertion loss of ~13.4 dB, an extinction ratio of ~20 dB, and a 3 dB bandwidth of 0.2 nm. The tuning range is of ~11.72 nm, along with the tuning speed of less than 400 μs. The tuning efficiency is ~0.23 nm/mW. The use of polarization diversity scheme and the silicon photonic packaging bridges the gap between the silicon photonic circuits and the real applications.

A tunable polarization diversity silicon photonics filter

A tunable polarization diversity silicon waveguide based optical filter was demonstrated. With the polarization diversity scheme, less than 0.5dB polarization dependent loss of the silicon optical filter was achieved in the wavelength range from 1525nm to 1600nm. The insertion loss of the whole polarization diversity circuits is 6.3dB. The extinction ratio of the optical filter is more than 27dB.

Silicon-on-insulator polarization splitting and rotating device for polarization diversity circuits

A compact and efficient polarization splitting and rotating device built on the silicon-on-insulator platform is introduced, which can be readily used for the interface section of a polarization diversity circuit. The device is compact, with a total length of a few tens of microns. It is also simple, consisting of only two parallel silicon-on-insulator wire waveguides with different widths, and thus requiring no additional and nonstandard fabrication steps. A total insertion loss of -0.6 dB and an extinction ratio of 12 dB have been obtained experimentally in the whole C-band.

Efficient and compact TE-TM polarization converter built on silicon-on-insulator platform with a simple fabrication process

An efficient TE-TM polarization converter built on a silicon-on-insulator nanophotonic platform is demonstrated. The strong cross-polarization coupling effect in air-cladded photonic-wire waveguides is employed to realize the conversion. A peak TE-TM coupling efficiency of 87% (-0.6 dB insertion loss) is measured experimentally. A polarization conversion efficiency of >92% with an overall insertion loss of <-1.6 dB is obtained in a wavelength range of 40 nm. The proposed device is compact, with a total length of 44 μm and can be fabricated with one lithography and etching step.

Compact polarization rotator on silicon for polarization-diversified circuits

We demonstrate a polarization rotator based on adiabatic mode evolution on silicon for polarization-diversified circuits. The rotator has a device length of 420 μm, a polarization-conversion efficiency of more than 90%, and an insertion loss less than 1 dB for a wavelength range of 80 nm. Combining the rotator with a compact, broadband polarization beam splitter based on cascaded directional couplers enhances the polarization conversion extinction ratio to over 30 dB with less than 1.5 dB total insertion loss over a 60 nm spectral range.

Silicon waveguide based TE mode converter

A silicon waveguide based TE mode converter was designed for the mode conversion between a horizontal waveguide and vertical waveguide in the two-layer structure waveguide based polarization diversity circuit. The TE mode converter's performance was studied. The polarization mode converter with minimum length of 5 μm was demonstrated to provide the TE mode conversion while maintaining the polarization status. The insertion loss at the transition region was less than 2 dB.

Adiabatic microring resonators

A class of whispering-gallery-mode resonators, herein referred to as adiabatic microring resonators, is proposed and numerically demonstrated. Adiabatic microrings enable electrical and mechanical contact to be made to the resonator without inducing radiation, while supporting only a single radial mode and therein achieving an uncorrupted free spectral range (FSR). Rigorous finite-difference time-domain simulations indicate that adiabatic microrings with outer diameters as small as 4 μm can achieve resonator quality factors (Qs) as high as Q = 88,000 and an FSR of 8.2 THz, despite large internal contacts.

Control of optical mode properties in cross-slot waveguides

A cross-slot waveguide geometry provides high confinement of the mode field of both fundamental quasi-TE and quasi-TM modes in geometrically perpendicular slots. A unique possibility to tailor optical mode characteristics such as effective index and confinement factor quite independently for the two polarizations with geometric and material parameters is shown. Nonbirefringent cross-slot geometries are presented. Fabrication related tolerances of the cross-slot geometry for low birefringence operation are studied. Means to externally tune the birefringence by a thermo-optic effect is also analyzed. Fabrication of a cross-slot waveguide test structure is demonstrated.