Spot-size Converter Research Articles

2D spot size converter using a multilevel asymmetric coupler MAC structure for Si photonics planar technology

In this work, we present an architecture for the design of an integrated optical spot size converter using planar technology. The proposed architecture is based on the use of multilevel planar structures without using vertical tapers. The function of the vertical taper is replaced by a horizontal taper and a multilevel asymmetric coupler structure. The proposed technique, used to couple a single-mode fiber with a core diameter of 8.2 µm to a silica-over-silicon waveguide with a 2 µ m × 4 µ m cross section, has a coupling efficiency greater than 98% without the need for a vertical coupler. The coupling transformation of the spot of the standard 500 n m × 220 n m Si photonics waveguide to the fundamental mode of a 1 µ m × 1 µ m waveguide is also demonstrated with a coupling efficiency greater than 92%. This technique offers very high flexibility in the coupling between different waveguides with different cross sections.

Manufacture of Three-Dimensional Optofluidic Spot-Size Converters in Fused Silica Using Hybrid Laser Microfabrication

We propose a hybrid laser microfabrication approach for the manufacture of three-dimensional (3D) optofluidic spot-size converters in fused silica glass by a combination of femtosecond (fs) laser microfabrication and carbon dioxide laser irradiation. Spatially shaped fs laser-assisted chemical etching was first performed to form 3D hollow microchannels in glass, which were composed of embedded straight channels, tapered channels, and vertical channels connected to the glass surface. Then, carbon dioxide laser-induced thermal reflow was carried out for the internal polishing of the whole microchannels and sealing parts of the vertical channels. Finally, 3D optofluidic spot-size converters (SSC) were formed by filling a liquid-core waveguide solution into laser-polished microchannels. With a fabricated SSC structure, the mode spot size of the optofluidic waveguide was expanded from ~8 μm to ~23 μm with a conversion efficiency of ~84.1%. Further measurement of the waveguide-to-waveguide coupling devices in the glass showed that the total insertion loss of two symmetric SSC structures through two ~50 μm-diameter coupling ports was ~6.73 dB at 1310 nm, which was only about half that of non-SSC structures with diameters of ~9 μm at the same coupling distance. The proposed approach holds great potential for developing novel 3D fluid-based photonic devices for mode conversion, optical manipulation, and lab-on-a-chip sensing.

1.3 μm InGaAlAs/InP DFB Laser Integrated With SSC Having Reverse Mesa Ridge Waveguide

We report the fabrication of a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.3~\mu \text{m}$ </tex-math></inline-formula> InGaAlAs/InP multi-quantum well (MQW) distributed feedback (DFB) laser integrated with a laterally tapered spot size converter (SSC) having a reverse mesa ridge waveguide. Because the top waveguide width of the reverse mesa waveguide is notably larger than the bottom width, a narrow SSC tip width, which is key for high quality SSC, can be fabricated by using conventional photolithography easily, helping to lower the device cost. At 25 °C, the emission wavelength is around 1314 nm and side mode suppression ratio larger than 45 dB can be obtained when the current is larger than 100 mA. The lateral and vertical divergence angles measured from the SSC facet of the laser are 11.8 and 13.2, respectively, which helps to ease the outcoupling of the light power.

53 GBd PAM-4 fully-integrated silicon photonics transmitter with a hybrid flip-chip bonded laser

We present a fully-integrated single-lane 53 GBd PAM-4 silicon photonics (SiPh) transmitter (Tx) with a flip-chip bonded laser diode (LD). The LD is butt-coupled to a Si edge coupler including a SiO2 suspended spot-size converter. The coupled power exceeds 10 dBm with a 1 dB allowable misalignment of 2.3 µm. The RF and eye performances of the Tx are evaluated. Extinction ratio >5 dB is obtained at 3.5 Vppd voltage swing. Aided by silicon capacitors, the Tx decouples parasitic inductances leading to remarkable improvements in the eye openings and transmitter dispersion eye closure quaternary by 1.16 dB. By implementing the fully-integrated Tx with driver packaging, we successfully demonstrate 106 Gb/s real-time operation satisfying KP4-FEC threshold at -5 dBm receiver sensitivity.

Cross-section mismatch: metamaterials to the rescue

Coupling light into and/or out of a photonic integrated circuit is often accomplished by establishing a vertical link between a single-mode optical fiber and a resonant waveguide grating, which is then followed by a tapered and a single-mode waveguides. The tapered waveguide operates as a spot-size converter, laterally expanding or contracting the light beam between the single-mode waveguide and the resonant waveguide grating. In this work, we propose using subwavelength structures to achieve tapering functionalities. To this end, we designed a metamaterial structure that enables the modulation of the refractive index necessary to either expand or focus a beam of light. Furthermore, we simulated the metamaterial structure through adequate numerical methods and the expanding, and focusing performances were analyzed in terms of efficiency and mode profile matching. We achieved over 43 % and 48 % for the integral overlap with the transverse magnetic fundamental mode for the focusing and expanding configurations, respectively, out of 49 % and 51 % of power transferred.

Edge coupling for hybrid mono-crystalline silicon and lithium niobate thin films

The hybridization of mono-crystalline silicon and lithium niobate thin films (Si-LNOI) combines the excellent electrical properties and mature micro-nano processing technology of Si, as well as the remarkable optical properties of LN. The Si-LNOI platform will drive new and promising integrated photonics devices. High-efficiency fiber-waveguide optical coupling is necessary to realize the full potential of devices and practical applications. In this study, a spot-size converter (SSC) was designed and demonstrated for efficient edge coupling between a Si-LNOI waveguide and lens fiber. The SSC was fabricated by standard semiconductor process, which consisted of an inverted-tapered Si and a silicon-rich nitride (SRN) waveguide overlying the inverted-tapered Si. At a wavelength of 1550 nm, the TE and TM light achieved coupling losses of 1.9 and 2.1 dB/facet, respectively. The coupling efficiency was stable in the wavelength range of 1500–1600 nm. The tolerance of alignment was also evaluated.

The O-band 20-channel 800 GHz Arrayed Waveguide Grating based on silica platform for 1 Tb/s or higher-speed communication system

A 20-channel 800 GHz spacing silica based arrayed waveguide grating (AWG) is designed and fabricated. We extend the wavelength allocation in IEEE 802.3bs from 8 channels to 20 channels in the O-band, which improves the transmission capacity of the chip to meet the increasing demand of network traffic. The insertion loss is less than 1.59 dB by adopting a spot-size converter (SSC). The polarization-dependent loss (PDL) is less than 0.3 dB at the center wavelength of each channel. With a flat-top of spectrum, 1 dB bandwidth is more than 2.8 nm, and the adjacent crosstalk is less than −20 dB. The transmission of 28.9 Gb/s NRZ and 57.8 Gb/s PAM4 signals in a single channel are successfully realized.

Thin-film lithium niobate dual-polarization IQ modulator on a silicon substrate for single-carrier 1.6 Tb/s transmission

We successfully demonstrate a monolithic integrated dual-polarization in-phase/quadrature (IQ) modulator based on thin-film lithium niobate platform with a silicon substrate, which consists of IQ modulators, spot-size converters, and a polarization rotator combiner. When coupled with polarization maintaining fibers, the measured insertion loss of the modulator is 12 dB. In addition, we experimentally achieve a single-carrier 1.6 Tb/s net bitrate transmission.

Fabrication of vertical-taper structures for silicon photonic devices by using local-thickness-thinning process

This paper describes a simple fabrication process of vertical-taper structures which can locally tune the thickness of silicon photonic devices. For low-loss spot-size conversion, taper angles less than 10° are required. To fabricate the gradual-slope shape of the vertical tapers, we have developed a step-and-exposure lithography process, which is realized by repeated light exposure to photoresist and movement of the wafer stage by using commercial steppers. The process is conducted at a lower temperature (∼120 °C) than the conventional process and is compatible with the complementary metal-oxide-semiconductor process. Also, we have made a model of the lithography to predict the angle of the taper. Theoretical angles are consistent with the experimental results. We demonstrate the conversion of a 400 nm thick silicon waveguide to 220 nm, whose length was 2.4 μm and insertion loss was measured to be less than 0.3 dB. The process enables us to choose the optimal thickness for each silicon-photonic device.

Spot-Size Converter Based on Long-Period Grating

We propose a spot-size converter (SSC) by exploiting the high-efficiency mode conversion capability that a long-period grating (LPG) possesses. Our proof-of-concept LPG-based SSC, fabricated with optical polymer material on silicon nitride (Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) platform, has a length of ∼700 μm, can improve the coupling efficiency between the LP <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">^x</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> mode of an ultra-high numerical aperture fiber and the E <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">^x</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> mode of the Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> waveguide from ∼24% to ∼33% at 1548 nm wavelength, corresponding to 1.4-dB reduction in insertion loss. Therein, the LPG achieves a maximum mode conversion efficiency of 90% between the two E <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">^x</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> modes of the polymer cladding and the Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> core. Our proposed SSC provides a new path to achieve low-loss fiber-to-chip coupling.

Direct f-3f self-referencing using an integrated silicon-nitride waveguide.

We have achieved the simultaneous generation of a 2.6-octave-wide supercontinuum (SC) spectrum over 400-2500 nm and third-harmonic light solely by a dispersion-controlled silicon-nitride waveguide (SiNW). To increase the visible intensity of the SC light component, we fabricated low-loss 5-mm-long deuterated SiNWs with spot-size converters by low-temperature deposition. We succeeded in measuring the carrier-envelope-offset (CEO) signal with a 34-dB signal-to-noise ratio because this short deuterated SiNW provides a large temporal overlap between the f and 3f components. In addition, we have demonstrated this method of CEO locking at telecommunications wavelengths with f-3f self-referencing generated solely by the SiNW without the use of highly nonlinear fiber and an additional nonlinear crystal. Compared with the method of CEO locking with a highly nonlinear fiber and a standard f-2f self-referencing interferometer, this method is not only simple and compact but also stable.

Demonstration of high output power DBR laser integrated with SOA for the FMCW LiDAR system.

We demonstrated a high output power distributed-Bragg-reflector (DBR) laser integrated with semiconductor optical amplifier (SOA) for the frequency-modulated continuous-wave (FMCW) light detection and ranging (LiDAR) system. In order to acquire higher output power, different from the conventional SG-DBR laser, the front mirror in this work is a section of uniform grating to get higher transmissivity. Therefore, the output power of the laser reaches 96 mW when the gain current and SOA current are 200 mA and 400 mA, respectively. Besides, we fabricated a spot size converter (SSC) at the laser output port to enhance the fiber coupling efficiency, which reached 64% coupled into the lensed fiber whose beam waist diameter is 2.5 μm. A tuning range of 2.8 nm with free spectral range (FSR) of 0.29 nm and narrow Lorentzian linewidth of 313 kHz is achieved. To realize distance and velocity measurement, we use the iterative learning pre-distortion method to linearize the frequency sweep, which is an important part of the FMCW LiDAR technology.

Terahertz out-of-plane coupler based on compact spot-size converter

Low-loss dielectric terahertz (THz) chips are efficient platforms for diverse THz applications. One of the key elements in the chip is the coupler. Most of the available THz couplers are in-plane and couple the THz wave from the metal waveguide to the dielectric waveguide. However, out-of-plane couplers are more suitable for wafer-scale testing and tolerant of alignment variation. In this work, we propose an out-of-plane THz coupler for coupling the antenna to the dielectric waveguide. The device is constructed using a grating and a compact spot-size converter. As the conventional optical spot-size converters that apply directly to THz chips are too large, we have designed a compact spot-size converter based on a tapered waveguide with a lens. The total device is 2.9 cm long and can couple a 7 mm diameter THz beam to a 500 µm wide waveguide. The device can scan the THz beam, radiate the input rectangular waveguide mode to free space, and drive the rotation angle of the fan beam through the scanning frequency. We fabricated the device using a single lithography step on a silicon wafer. The out-of-plane coupling efficiency was found to be ∼5 dB at 194 GHz. The fan-beam steering range was found to be around 40° in the frequency range of 170–220 GHz. The proposed out-of-plane coupling technique may provide an effective way for THz wafer-scale testing with a higher degree of freedom for on-chip integration. Also, the proposed technique being non-mechanical, beam steering using it, may therefore find applications in THz radar, communication, and sensing.

Ultra-compact spot size converter based on digital metamaterials

Spot size converters (SSCs) are essential optical components that connect waveguides with different modal spot sizes for adiabatic light coupling. However, previous SSCs usually suffer from large sizes or limited coupling efficiencies, which are not conducive to develop large-scale and high-density photonic integrated circuits (PICs). Herein, we theoretically study an ultra-compact SSC with a footprint of only 1×2 μm2. The design is based on topologically optimized digital metamaterials, which is implemented by using a genetic algorithm. The device shows a transmission of −0.11 dB for TE0-mode spot size conversion with a 1-dB bandwidth of 600 nm. Moreover, the device supports the spot size conversion for TE0-mode and TM0-mode simultaneously with a transmission of more than −2 dB. With the high transmission, wide spectral bandwidth, and ultra-compact footprint, the device is expected to open a new avenue to boost the development of PICs.

Reconfigurable spot size converter for the silicon photonics integrated circuit.

Spot size converter (SSC) plays a role of paramount importance in the silicon photonics integrated circuit. In this article, we report the design of a reconfigurable spot size converter used in the hybrid integration of a DFB laser diode with a silicon photonic waveguide. Our SSC consists of subwavelength gratings and thermal phase shifters. Four subwavelength grating tips are used to improve horizontal misalignment tolerance. Meanwhile, the phase mismatch between two input waveguides is compensated by phase shifters to minimize insertion losses. Our simulated result has yielded a minimum insertion loss of 0.63 dB and an improvement of the horizontal misalignment from ±0.65 µm to ±1.69 µm for 1 dB excess insertion loss at the wavelength of 1310 nm. The phase shifters are designed to compensate any phase error in both the fabrication and bonding processes, which provides a completely new edge-coupling strategy for the silicon photonics integrated circuit.

1.3 µm InGaAlAs/InP laser integrated with laterally tapered SSC in a reverse mesa shape.

We report 1.3 µm InGaAlAs/InP lasers integrated with laterally tapered spot size converter (SSC) in reverse mesa shape. Because the top width is significantly larger than the bottom width for the reverse mesa ridge, high quality SSCs having narrow tip width can be fabricated through conventional photolithography with a high reproductivity. The Threshold current of the fabricated 1000 µm long SSC integrated device is 25 mA and 44 mW single facet optical power can be obtained at 300 mA current. The lateral and vertical divergence angles are as low as 8 ° and 11°, respectively.

Buried 3D spot-size converters for silicon photonics

In this article, an efficient spot-size converter (SSC) for low-loss optical mode transition between large and small waveguides based upon a buried three-dimensional (3D) taper is demonstrated. The SCC can pave the way for scalable, low-loss coupling between on-chip waveguides of different sizes and with external components such as optical fibers and III-V active components, and it can be a key element in solving the challenges surrounding the economic high volume packaging and assembly of photonic integrated circuits. Through the use of a bespoke fabrication process, continual tapering of the waveguide dimensions both in width and height is achieved, offering minimal perturbance of the optical mode throughout the structure. The SSC exploits the space of the buried oxide (BOX) on a standard silicon-on-insulator wafer, leaving a planar top wafer surface, meaning that, crucially, further processing of the wafer is not inhibited in any way. Fabricated proof-of-concept devices demonstrate coupling between standard single-mode 220 nm thick silicon waveguides and large-core waveguides with dimensions about 3 µm wide and 1.5 µm height with BOX thickness of 2 µm. Coupling losses as low as 0.56 dB are achieved, limited mostly by the material loss of the polysilicon used. Substantial improvements can be yielded by simply changing the infill material and through optimization of the fabrication process and design. The demonstrated SSC approach can further be applied to other photonic platforms such as silicon nitride on insulator and so on.

Ultra-Compact Low Loss Polymer Wavelength (De)Multiplexer With Spot-Size Convertor Using Topology Optimization

Polymer waveguides are considered to be good candidates for optical interconnects application due to their advantages such as flexibility, good compatibility, and versatility for realizing 3D and micro structures. However, constrained by the limited degrees of freedom provided by template-based design approaches, there lacks a satisfying solution which achieves small footprint and low loss simultaneously. Inverse design has been proposed to improve the performance. However, most inverse design methods target at silicon-based optical devices optimization which has a relatively high refractive index difference. In this paper, we demonstrate an ultra-compact low loss polymer wavelength (de)multiplexer with spot-size conversion structure to interposing Si and optical fiber using gradient-based topology optimization and downhill simplex method. The 2-channel wavelength (de)multiplexer displays an excess propagation loss of 0.84 dB and 1.06 dB at 1310 nm and 1550 nm, respectively, with a total footprint of 245 × 35 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The 4-channel wavelength (de)multiplexer for coarse wavelength division multiplexing applications with 20 nm spacing (1270, 1290, 1310, and 1330 nm) with an excess waveguide propagation loss of 1.72 dB within 271 × 50 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> has also been demonstrated and all the output ports of the polymer wavelength demultiplexing structure achieved a conversion efficiency over 90%.

Narrow-linewidth thermally tuned multi-channel interference laser integrated with a SOA and spot size converter.

A narrow-linewidth thermally tuned multi-channel interference (MCI) laser integrated with a semiconductor optical amplifier (SOA) and spot size converter (SSC) is demonstrated in this paper. A MCI laser integrated with SOA through chirped grating is successfully realized for the first time, which achieves a tuning range of more than 42.5 nm with side-mode suppression ratios (SMSRs) higher than 48 dB and Lorentzian linewidth below 100 kHz. InGaAlAs multiple quantum wells (MQWs) and thermal tuning are used to reduce linewidth. The integration of SSC greatly improves the coupling efficiency between the laser and a lensed fiber. The MCI laser integrated with SSC achieves more than 16 dBm output power coupled into a lensed fiber. Air layers are fabricated in the phase sections to increase the heating efficiency. The total thermal tuning power is below 20 mW across the whole tuning range.

Hybrid Integrated Silicon Nitride–Polymer Optical Phased Array For Efficient Light Detection and Ranging

Silicon nitride (SiN) optical phased arrays (OPAs) have emerged as a potential alternative to their silicon counterparts, owing to their low nonlinearity, easy scalability, and low propagation loss. However, the development of SiN OPAs for light detection and ranging (LiDAR) applications has attracted less attention due to the low thermo-optic (TO) effect of SiN. Moreover, SiN OPAs are considered to be undesirable for transmitters in LiDAR systems because of the limited longitudinal tuning efficiency of SiN grating antenna. We present a hybrid integrated SiN-polymer OPA for realizing an efficient LiDAR. Owing to the large TO effect of polymer, the proposed OPA with polymeric phase modulators resolves the power dissipation drawback of the SiN devices. The polymeric modulators are integrated with a SiN power splitter and grating antenna, and the coupling loss is alleviated via a spot-size converter. Additionally, a steering angle magnifying lens is used to enhance the tuning efficiency of the antenna, expanding the longitudinal beam steering range. The prepared OPA transmitter equipped with the lens exhibits decent two-dimensional beam steering, featuring stable emission characteristics over a 12° × 30° field of view. With the transmitter, efficient time-of-flight based LiDAR is demonstrated to provide a detection range of 10 m under a peak optical power of 55 dBm. The proposed SiN-polymer OPA is highly anticipated to circumvent the challenges of conventional SiN counterparts and potentially spearhead the development of a prominent OPA platform for advanced LiDAR schemes.