Apodized fully-etched surface grating coupler using subwavelength structure for standard silicon-on-insulator waveguide

We design and demonstrate the fully-etched apodized grating couplers based on the silicon-on-insulator (SOI) platform using subwavelength structure for both transverse electric (TE) and transverse magnetic (TM) modes operation. The subwavelength grating (SWG) is used to engineer the refractive index using second-order effective medium theory (EMT). The whole designing procedure is given in details, especially a feasible and programmable method is developed to precisely manipulate the coupling strength of each grating cycle. A perfect Gaussian output beam can be synthesized for the TE mode operation, achieving a field overlap up to 98.3% with the Gaussian fiber mode. The simulated peak coupling losses are ?4.63 and ?2.99 dB for the TE mode and the TM mode, respectively, which are comparable with conventional shallowly etched grating couplers, realizing a fabrication simplification without performance penalty. The measured peak coupling loss is ?7.6 dB for the TE mode coupler with a 1 dB bandwidth of 45 nm, and ?6.1 dB for the TM mode coupler with a 1 dB bandwidth of 34 nm.     

Volume grating preferential-order focusing waveguide coupler

A volume grating focusing waveguide coupler has been designed, fabricated, and tested. The volume grating is designed to couple a 633-nm wave guided in a single-mode polyimide waveguide preferentially into the air cover. The beam is outcoupled normally to the surface of the waveguide and focused to a line 25 mm above the waveguide. The slanted-fringe volume grating is holographically recorded by the interference of two coherent 364-nm ultraviolet waves focused with a cylindrical lens to produce the chirped grating. The 1-mm-long volume grating coupler exhibits a preferential-coupling ratio of 0.98, a coupling efficiency of 95%, and an almost diffraction-limited focal line with a full width at half-maximum of 10.49mum.     

Stratified waveguide grating coupler for normal fiber incidence

We propose a new stratified waveguide grating coupler (SWGC) to couple light from a fiber at normal incidence into a planar waveguide. SWGCs are designed to operate in the strong coupling regime without intermediate optics between the fiber and the waveguide. Two-dimensional finite-difference time-domain simulation in conjunction with microgenetic algorithm optimization shows that approximately 72% coupling efficiency is possible for fiber (core size of 8.3 microm and delta=0.36%) to slab waveguide (1.2-microm core and delta=3.1%) coupling. We show that the phase-matching and Bragg conditions are simultaneously satisfied through the fundamental leaky mode.     

UV-written long-period waveguide grating coupler for broadband add/drop multiplexing

We demonstrate a UV-written polymer long-period waveguide grating (LPWG) coupler, which offers a bandwidth of ∼20 nm, a maximum coupling efficiency of ∼80% and ∼60% for the TE and TM polarizations, respectively, and a wavelength-tuning range over the (S + C + L)-band (∼140 nm) with a temperature control of ∼25 °C. The LPWG coupler has the potential to be developed into a practical broadband add/drop multiplexer for coarse wavelength-division-multiplexing applications.     

Continuously apodized fiber-to-chip surface grating coupler with refractive index engineered subwavelength structure

We demonstrate a fully etched, continuously apodized fiber-to-chip surface grating coupler for the first time (to our knowledge). The device is fabricated in a single-etch step and operates with TM-polarized light, achieving a coupling efficiency of 3.7 dB and a 3 dB bandwidth of 60 nm. A subwavelength microstructure is employed to generate an effective medium engineered to vary the strength of the grating and thereby maximize coupling efficiency, while mitigating backreflections at the same time. Minimum feature size is 100 nm for compatibility with deep-UV 193 nm lithography.     

An ultra‐compact multimode interference coupler with a subwavelength grating slot

Multimode interference couplers (MMIs) are fundamental building blocks in photonic integrated circuits. Here it is experimentally demonstrated, for the first time, a two‐fold length reduction in an MMI coupler without any penalty on device performance. The design is based on a slotted 2 × 2 MMI fabricated on a commercial silicon‐on‐insulator (SOI) substrate. The slot is implemented with a subwavelength grating (SWG) comprising holes fully etched down to the oxide cladding, thereby allowing single etch step fabrication. The device has been designed using an in‐house tool based on the Fourier Eigenmode Expansion Method. It has a footprint of only 3.5 μm x 23 μm and it exhibits a measured extinction ratio better than 15 dB within the full C‐band (1530 nm‒1570 nm). SWG engineered slots thus offer excellent perspectives for the practical realization of MMIs couplers with substantially reduced footprint yet with outstanding performance.     

High‐efficiency single etch step apodized surface grating coupler using subwavelength structure

Grating couplers are key elements enabling the coupling of light between planar waveguide circuits and optical fibers. In this work, it is demonstrated using simulations and experiments that a high coupling efficiency can be achieved for an arbitrary buried oxide thickness by judicious adjustment of the grating radiation angle. The coupler strength is engineered by subwavelength structure, allowing straightforward apodization and single etch step fabrication. The design has been implemented using Fourier‐eigenmode expansion and finite difference time domain methods. The measured coupling loss of a continuously apodized grating is −2.16 dB with a 3 dB bandwidth of 64 nm, therefore opening promising prospects for low‐cost and high‐volume fabrication using 193 nm deep‐ultraviolet lithography. It is also shown by simulations that a coupling loss as low as −0.42 dB is predicted for a modified coupler structure with bottom mirror.     

Machining of acrylic-based Y-branch plastic optical fiber coupler with suspended waveguide taper

An acrylic-based Y-branch plastic optical fiber (POF) with a middle suspended waveguide taper has been developed. The suspended high index contrast waveguide taper structure has been designed in such a way that it is surrounded by air-cladding. Non-sequential ray tracing has been performed on the device giving an insertion loss of 4.68 dB and coupling ratio of 50: 50. The middle waveguide taper is constructed on the acrylic block itself without using any additional optical waveguiding medium injected into the engraved taper region. Fabrication of the devices is done by producing the device structures on an acrylic block using high speed computer numerical control (CNC) machining tool. Input and output POF fibers are inserted into this device structure in such a way that they are passively aligned to the middle waveguide taper structure. The device shows an insertion loss of 5.9 dB, excess loss of 2.9 dB and a splitting ratio of 50: 50.     

High efficiency and broad bandwidth grating coupler between nanophotonic waveguide and fibre

A high efficiency and broad bandwidth grating coupler between a silicon-on-insulator (SOI) nanophotonic waveguide and fibre is designed and fabricated. Coupling efficiencies of 46\% and 25\% at a wavelength of 1.55~μ m are achieved by simulation and experiment, respectively. An optical 3~dB bandwidth of 45~nm from 1530~nm to 1575~nm is also obtained in experiment. Numerical calculation shows that a tolerance to fabrication error of 10~nm in etch depth is achievable. The measurement results indicate that the alignment error of ±2~μ m results in less than 1~dB additional coupling loss.     

High-efficiency optical coupling to planar photodiode using metal reflector loaded waveguide grating coupler

For the realization of optoelectronic integrated circuits, it is required to incident light perpendicularly to a planar Si photodiode. We propose a high-efficient vertical optical coupler using an amorphous Si optical waveguide grating coupler with top reflector, which is transparent at 850 nm wavelength range. The optical waveguide (width of 300 nm $\times $ height of 100 nm) coupler is analyzed by using finite element method. The coupling efficiency of 80 % is calculated at the grating period of 380 nm, the duty ratio of 0.75 and the depth of 35–65 nm with top metal reflector.     

Waveguide grating coupler with a tailored spectral response based on a computer-generated waveguide hologram

We propose a novel method that, for the first time to our knowledge, makes it possible to tailor the spectral response of an input grating coupler. This method is based on the idea of computer-generated waveguide holography. Grating couplers with different spectral responses have been designed and fabricated, and experimental results are presented that demonstrate the effectiveness of the proposed method.     

Temperature-independent silicon subwavelength grating waveguides

We demonstrate, by experiment and numerical calculations, temperature-independent subwavelength grating waveguides with a periodic composite core composed of alternating regions of silicon and SU-8 polymer. The polymer has a negative thermo-optic (TO) material coefficient that cancels the large positive TO effect of the silicon. Measurements and Bloch mode calculations were carried out over a range of silicon-polymer duty ratios. The lowest measured TO coefficient at a wavelength of 1550 nm is 1.8×10(-6) K(-1); 2 orders of magnitude smaller than a conventional silicon photonic wire waveguide. Calculations predict the possibility of complete cancellation of the silicon waveguide temperature dependence.     

Low-loss subwavelength metal C-aperture waveguide

We present a design of a linear optical waveguide that utilizes a C-shaped metallic nano-aperture that efficiently transports light while maintaining a spot size of lambda/10. The performance of a C-aperture waveguide is superior to both a regular ridge waveguide and other surface plasmon based metal nano-optical waveguides. The energy transport mechanisms are explained by the coupling of an aperture surface resonance and the thickness resonances inside the guide channel. Finite-difference time-domain simulations of gold C-aperture waveguides are performed for a 1.5 microm wavelength incident plane wave. The 1/e decay length in power transmission is predicted to be approximately 2.5 microm. The total power throughput is 1.66 for the 2.55 microm long guide, with an intensity 6 times that of the incident wave at a distance 120 nm from the exit plane, having a spot size of 150 nm.     

Multi-mode holographic waveguide coupler

A double-exposure thick-holographic-grating waveguide coupler is described in which two waveguide modes can be excited with a single incident beam. Single-grating decouplers are also designed to selectively remove one of the coupled waves.     

Gradient-index antireflective subwavelength structures for planar waveguide facets

We demonstrate the use of subwavelength gratings etched into the facets of silicon-on-insulator ridge waveguides as a means of reducing facet reflectivity by the gradient-index effect. Reflectivities as low as 2.0% and 2.4% for the fundamental TE and TM modes, respectively, are demonstrated experimentally for light of 1.55 microm wavelength, in agreement with both effective medium theory and finite-difference time domain calculations. Simulations show that facet reflectivites can be further reduced to less than 1% by increasing the grating modulation depth.     

Compact efficient broadband grating coupler for silicon-on-insulator waveguides

We have designed a high-efficiency broadband grating coupler for coupling between silicon-on-insulator (SOI) waveguides and optical fibers. The grating is only 13 microm long and 12 microm wide, and the size of the grooves is optimized numerically. For TE polarization the coupling loss to single-mode fiber is below 1 dB over a 35-nm wavelength range when using SOI with a two-pair bottom reflector. The tolerances to fabrication errors are also calculated.     

Polarization-independent grating coupler for micrometric silicon rib waveguides

Grating couplers are a promising approach to implement efficient fiber-chip coupling. However, their strong polarization dependence makes dual-polarization operation challenging. In this Letter we propose, for the first time, a polarization-independent grating coupler for thick rib silicon-on-insulator (SOI) waveguides. Coupling efficiency is optimized by designing the grating pitch and duty cycle, without varying the bottom oxide thickness, which significantly simplifies practical implementation. Directivity of the grating coupler is enhanced by a high reflectivity layer under the bottom oxide after the selective removal of the Si substrate. Dual-polarization coupling efficiency of -2.8 dB is shown.     

Ultra-low-loss inverted taper coupler for silicon-on-insulator ridge waveguide

An ultra-low-loss coupler for interfacing a silicon-on-insulator ridge waveguide and a single-mode fiber in both polarizations is presented. The inverted taper coupler, embedded in a polymer waveguide, is optimized for both the transverse-magnetic and transverse-electric modes through tapering the width of the silicon-on-insulator waveguide from 450 nm down to less than 15 nm applying a thermal oxidation process. Two inverted taper couplers are integrated with a 3-mm long silicon-on-insulator ridge waveguide in the fabricated sample. The measured coupling losses of the inverted taper coupler for transverse-magnetic and transverse-electric modes are ∼ 0.36 dB and ∼ 0.66 dB per connection, respectively.     

Single beam grating coupler with a large grating periodicity

Diffraction of a guided mode by a thin grating is analyzed by a graphical method in the general case where the coupled modes do not propagate in a symmetry plane of the grating. Predictions of the method are confirmed by experiments and single beam coupler configurations are proposed that do not require small grating periodicities.