Integrated Spot-size Converters Research Articles

Polymer Microlens on Pillar Grown From Single-Mode Fiber Core for Silicon Photonics

A novel coupling device composed of a combination of microlens and pillar having an approximately $8~\mu \text{m}$ diameter and $20~\mu \text{m}$ height and grown from single-mode fiber (SMF) core end is proposed. This device was successfully fabricated by using a self-written waveguide and dipping method with UV-curable resin. Theoretical and experimental studies were done. A high coupling efficiency of more than −0.97dB was obtained for coupling to an ultrahigh numerical aperture (0.41) SMF with a spot diameter of $3.7~\mu \text{m}$ at a $1.55~\mu \text{m}$ wavelength in place of a silicon chip with an integrated spot-size converter. In contrast, it is only −4.0dB by a conventional SMF.

Vertically integrated spot-size converter in AlGaAs-GaAs.

We report on the demonstration of a spot size converter (SSC) for monolithic photonic integration at a wavelength of 850nm on a GaAs substrate. We designed and fabricated a dual-waveguide AlGaAs chip. The design consists of a lower waveguide layer for efficient end-fire coupling to a single-mode fiber, an upper waveguide layer for high refractive index contrast waveguides, and a vertical SSC to connect the two waveguide layers. We measured a SSC conversion efficiency of 91% (or -0.4 dB) between the upper and lower waveguide layers for the TE mode at a wavelength of 850nm.

An Overgrowth-Free Design for InGaAlAs Spot-Size-Converted Ridge Waveguide Lasers

A great deal of work has been done over the past several years toward the development of lasers with integrated spot-size converters, for better coupling directly to flat cleaved fiber or better alignment tolerance in lensed systems. Many of the techniques, such as butt-coupling or lateral-taper-vertical-shift, require etch-and-regrowth over the active region and as such are not applicable directly to Al-containing lasers. In this letter, we demonstrate a simple method to achieve narrow (15deg times 15deg) far fields in Al-containing devices with a moderate degradation of dc, dynamic and thermal characteristics. At room temperature, uncoated 300-mum-long devices have thresholds of about 18 mA and slope efficiencies of about 0.24 W/A, with 27% power coupled directly into flat cleaved fiber compared to ~10% for a conventional (>30deg far field) device. Comparison between calculated and measured far fields versus ridge width and number of quantum wells gives design curve information for optimizing far field performance.

Rearrangeable nonblocking SOI waveguide thermooptic 4/spl times/4 switch matrix with low insertion loss and fast response

A rearrangeable nonblocking 4/spl times/4 thermooptic silicon-on-insulator waveguide switch matrix at 1.55-/spl mu/m integrated spot size converters is designed and fabricated for the first time. The insertion losses and polarization-dependent losses of the four channels are less than 10 and 0.8 dB, respectively. The extinction ratios are larger than 20 dB. The response times are 4.6 /spl mu/s for rising edge and 1.9 /spl mu/s for falling edge.

Selective-area MOCVD growth for distributed feedback lasers integrated with vertically tapered self-aligned waveguide

The growth pressure and mask width dependent thickness enhancement factors of selective-area MOCVD growth were investigated in this article. A high enhancement of 5.8 was obtained at 130 mbar with the mask width of 70 μm. Mismatched InGaAsP (−0.5%) at the maskless region which could ensure the material at butt-joint region to be matched to InP was successively grown by controlling the composition and mismatch modulation in the selective-area growth. The upper optical confinement layer and the butt-coupled tapered thickness waveguide were regrown simultaneously in separated confined heterostructure 1.55 μm distributed feedback laser, which not only offered the separated optimization of the active region and the integrated spotsize converter, but also reduced the difficulty of the butt-joint selective regrowth. A narrow beam of 9° and 12° in the vertical and horizontal directions, a low threshold current of 6.5 mA was fabricated by using this technique.

A new spot-size converter concept using fiber-matched antiresonant reflecting optical waveguides

We report on a new concept for InGaAsP-InP 1.55-/spl mu/m lasers with integrated spot-size converters (SSCs) based on antiresonant reflecting optical waveguides (ARROW). The mode expanders consist of a laterally tapered active region on top of a fiber-matched passive slab waveguide. The large slab mode is laterally confined by an antiresonant configuration of a couple of lateral waveguides defined in the same fabrication process as the active ridge. This feature makes the presented spot-size transformer as simple to fabricate as a standard waveguide, only requiring a planar growth step and a single conventional etch process. The fabricated tapers exhibit a low transformation loss and reduce the coupling loss to standard single-mode fibers from 8 to 4 dB. We also analyze by simulation two variants of the concept proposed in this work, including a taper structure for a buried waveguide, which are expected to show better performance. Simulation results show fiber-coupling efficiencies as low as 2.4 and 1.1 dB for both variants.

Oxide-confined laser diodes with an integrated spot-size converter

We report a new technique for the monolithic integration of a GaAs-based InGaAs-GaAs strained quantum-well laser and a spot-size converter (SSC) to improve the fiber coupling characteristics. The selective wet oxidation of AlGaAs is used to simplify the fabrication scheme of the component to a single planar epitaxial growth step and one conventional noncritical etch. This approach also allows us to avoid the photolithography of narrow features. An excellent reproducibility of the fabrication scheme was found. The integrated SSC exhibits very low transformation losses and a low beam divergence of 7.5/spl deg//spl times/13.5/spl deg/. The coupling efficiency to a 980-nm single-mode fiber is improved from -6.34 dB for a reference laser to -1.49 dB for the tapered device. The -1-dB alignment tolerance is /spl plusmn/1.5 /spl mu/m in the transverse direction and /spl plusmn/1.6 /spl mu/m in the lateral direction, respectively.

Multiwavelength DFB laser array with integrated spot size converters

We describe the design, fabrication, and performance of a five-element quarterwave-shifted distributed feedback laser array with monolithically integrated spot size converters intended for use as a multiple-wavelength source in dense wavelength-division telecommunications systems. Facet power in excess of 10 mW with less than 150 mA bias and longitudinal side mode suppression greater than 40 dB were routinely achieved. Narrow far-field full-width at half-maximum angles of 6.9/spl deg//spl times/16.3/spl deg/ provided 3.5-dB coupling loss into single-mode fiber with 1.0-dB misalignment tolerances of /spl plusmn/2.0 /spl mu/m. With /spl plusmn/10/spl deg/C thermal tuning, 22 1555-nm channels spaced by 50 GHz were accessed with this device. Thorough field evaluation indicates that such a device is consistent with manufacturing requirements.

Flexible, reliable and simple fabrication of integrated spot size converters with shifting mask technique

The fabrication of integrated spot size converters employing standard contact photolithography in a novel way and dry etching technique is presented. During the exposure the mask is shifted parallel to the substrates surface. Exploiting the graduation properties the resist is not completely dissolved, so that a vertical structure remains with a profile that can easily be predetermined. The resist pattern is subsequently transferred into the semiconductor by means of ion beam etching. The technique has proven to be flexible, reliable and simple. The optical measurements of the fabricated spot size converters show that the loss of fibre-chip coupling is considerably reduced and the alignment tolerances are improved.

2 Gbit/s small form factor fiber-optic transceiver for single mode optical fiber

Small form factor (SFF) optical transceivers are expected to be commonly used in the near future for high-end (high bit-rate, single mode) applications as well as for low-cost applications. SFF optical transceivers require new packaging techniques for the optical portion, because the distance between input and output optical axes is much smaller than those of conventional transceivers, e.g., the "1by9" type. Two types of optical packaging are introduced, both of which are suitable for transceivers which have an MT-based receptacle for single mode applications. One of the packaging types, a fiber-bulk structure, takes full advantage in cost and reliability of standard, hermetically sealed TO-canned optical devices. A conventional TO-can packaged laser diode (LD) and photodiode (PD) are held with short optical fibers by low-cost, injection-molded polymer retainers. The short fibers are connected to an MT ferrule inside the transceiver. An alternative packaging type, a surface mount structure, is intended for drastic cost reduction in large volume production by using advanced highly integrated assembly. An LD chip with an integrated spot-size converter and a waveguide-structured PD chip are mounted on a Si-substrate. Optical coupling between the optical devices and fibers on V-groove is accomplished by passive alignment, which reduces assembling time. A prototype transceiver with an MPO receptacle has been developed using the former packaging structure. Evaluated results show performance which complies with the 2.125 Gbit/s Fiber Channel standard. SFF transceivers with these new packaging technologies are expected to increase the optical port density and also to reduce the cost of high-end systems with bit rate of 2 Gbit/s or higher per port.

First demonstration of 980 nm oxide confined laser with integrated spot size converter

A new technique for the monolithic integration of a laser with a spot size converter is presented. The design, fabrication and performance of the InGaAs/GaAs strained QW tapered oxide confined laser are discussed. The spot size converter effectively reduces the horizontal and vertical FWHM and exhibits very low radiation losses.

Grey-tone lithography and dry etching technique for the fabrication of integrated spot size converters

The fabrication of integrated spot size converters employing grey-tone lithography and dry etching technique is presented. The paper describes the generation of the semitransparent photomask by electron-beam lithography and the following photolithographical formation of wedge-shaped regions in photoresist. The vertical profile is subsequently transferred into the semiconductor by means of reactive ion beam etching and laterally patterned using reactive ion etching leading to a tapered waveguide structure. Optical measurements show that the coupling of a butt fibre with a taper considerably reduces the coupling loss and increases the tolerable fibre mismatch, compared to the coupling of a lensed fibre to a chip waveguide. Moreover the spot size converter exhibits polarization insensitivity of the coupling.

Ridge laser with spot-size converter in a single epitaxial step for high coupling efficiency to single-mode fibers

We report on a 1.55-μm InGaAsP MQW laser diode with an integrated spot-size converter fabricated in a single epitaxial step using conventional photolithography. The laser structure uses a conventional ridge guide for the active layers and a second larger ridge for the passive waveguide. Low-beam divergence of typically 9/spl deg/×9/spl deg/ results in about 3-dB coupling losses, with a cleaved optical fiber.

Temperature- and polarization-insensitive responsivity of a 1.3-μm optical transceiver diode with an integrated spot-size converter

We have demonstrated an optical transceiver diode integrated with a spot-size converter. We clarify that the design of a detection layer is essential for the temperature- and polarization-insensitive characteristics of the responsivity. Because the detection layer was introduced, 0.65 dB of responsivity deviation was obtained in the wide temperature range of -10/spl deg/C-85/spl deg/C with a high laser performance. A spot-size converter enables the coupling loss to be 1.8 dB for a flat-end optical fiber. The frequency response was also obtained up to 1 GHz in both laser and detector modes.

An evolutionary optimization procedure applied to the synthesis of integrated spot-size converters

An evolutionary algorithm is applied to the synthesis of an integrated spot-size converter. Evolutionary algorithms turned out to be well suited for the solution of very complex problems having strongly non-linear cost functions defined over the solution space. They mostly work faster than other optimization techniques like random search or the Monte-Carlo method because of their parallel search mechanism, also referred to as implicit parallelism. The intrinsic behaviour of the optimization is demonstrated using an example of a spot-size converter that is implemented as a non-periodic segmented waveguide structure. Only a small number of structures have to be evaluated to achieve a coupling loss below 1.3dB that is considered to be very good. A supervising method is proposed, introducing an evolution quality figure. This figure is used to visualize and to qualify the evolution of the algorithm. Based on this figure a termination condition is suggested.

Monolithic integration of spot-size converters with 1.3-μm lasers and 1.55-μm polarization insensitive semiconductor optical amplifiers

To reduce packaging costs, it is necessary to use passive alignment between the laser diodes and optical fiber. Such an alignment requires low-coupling loss and large positional alignment tolerances. This is achievable with integrated spot-size converters, which permit to match the near field of a laser to that of a flat-end single-mode fiber (SMF). In this paper, we first review briefly the different technological approaches to realize spot-size converters. Then, we focus on the double-core structure developed both for 1.3-/spl mu/m Fabry-Perot lasers and 1.55-/spl mu/m semiconductor optical amplifiers (SOAs). The spot-size expansion is simulated using a two-dimensional (2-D) beam propagation method analysis. Short spot-size converters (100 /spl mu/m) integrated with 1.3-/spl mu/m lasers and 1.55-/spl mu/m SOAs exhibit beam divergences as low as 12/spl deg//spl times/12/spl deg/ and 12/spl deg//spl times/15/spl deg/, respectively. The performances of devices with integrated spot-size converters are reported and discussed. A 2-in wafer process is used thanks to the versatility of the double-core structure and its compatibility with buried ridge stripe technology.