Mikel Agustin

Effective 100 Gb/s IM/DD 850-nm Multi- and Single-Mode VCSEL Transmission Through OM4 MMF

To cope with the ever increasing data traffic demands in modern data centers, new approaches and technologies must be explored. Short range optical data links play a key role in this scenario, enabling very high speed data rate links. Recently, great research efforts are being made to improve the performance of vertical-cavity surface-emitting lasers (VCSELs) based transmission links, which constitute a cost-effective solution desirable for massive deployments. In this paper, we experimentally demonstrate intensity-modulation direct-detection transmissions with a data rate of 107.5 Gb/s over 10 m of OM4 multimode fiber (MMF) using a multimode VCSEL at 850 nm, and up to 100 m of OM4 MMF using a single-mode VCSEL at 850 nm. Measured bit error rates were below 7% overhead forward error correction limit of 3.8e−03, thus, achieving an effective bit rate of 100.5 Gb/s. These successful transmissions were achieved by means of the multiband approach of carrierless amplitude phase modulation.

Single-Mode 850-nm VCSELs for 54-Gb/s ON–OFF Keying Transmission Over 1-km Multi-Mode Fiber

By combing Zn-diffusion and oxide-relief apertures with strong detuning (>20 nm) in our demonstrated short-cavity (λ/2) 850-nm vertical-cavity surface-emitting lasers (VCSELs), wide electrical-to-optical bandwidth (29-24 GHz), low-differential resistance (~100 Q), and (quasi) single-mode (SM) with reasonable output power (~1.4 mW) performances can be simultaneously achieved. Error-free ON-OFF keying transmission at 54-Gb/s data rate through 1-km OM4 multi-mode fiber can be achieved by using highly SM device with forward error correction and decision feedback equalization techniques. As compared with the reference device with a larger oxide-relief aperture and a multi-mode performance, the SM device exhibits lower bit-error rate (1 × 10-5 versus 1 × 10-2) at 54 Gb/s. This result indicates that modal dispersion plays more important role in transmission than that of output power does. We benchmark these results to an industrial 50-Gb/s SM VCSEL. It shows a higher bit-error-rate value ~3.5×10-3 versus ~1.4×10-4 under the same received optical power.

Continuous wave and modulation performance of 1550nm band wafer-fused VCSELs with MBE-grown InP-based active region and GaAs-based DBRs

We report for the first time on wafer-fused InGaAs-InP/AlGaAs-GaAs 1550 nm vertical-cavity surface-emitting lasers (VCSELs) incorporating a InAlGaAs/InP MQW active region with re-grown tunnel junction sandwiched between top and bottom undoped AlGaAs/GaAs distributed Bragg reflectors (DBRs) all grown by molecular beam epitaxy. InP-based active region includes seven compressively strained quantum wells (2.8 nm) optimized to provide high differential gain. Devices with this active region demonstrate lasing threshold current < 2.5 mA and output optical power > 2 mW in the temperature range of 10-70°C. The wall-plug efficiency (WPE) value-reaches 20 %. Lasing spectra show single mode CW operation with a longitudinal side mode suppression ratio (SMSR) up to 45 dB at > 2 mW output power. Small signal modulation response measurements show a 3-dB modulation bandwidth of ~ 9 GHz at pump current of 10 mA and a D-factor value of 3 GHz/(mA)1/2. Open-eye diagram at 30 Gb/s of standard NRZ is demonstrated. Achieved CW and modulation performance is quite sufficient for fiber to the home (FTTH) applications where very large volumes of low-cost lasers are required.

Application of nanophotonics to the next generation of surface-emitting lasers

Abstract Novel trends and concepts in the design and fabrication of vertical cavity surface-emitting lasers (VCSELs) and their integration in optical networks and implementation in integrated photonics applications are discussed. To serve these goals and match the growing bandwidth demands, significant changes are to be implemented in the device design. New lateral leakage-mediated single-mode VCSELs, including both devices confined by oxide layers and those confined by alloy-intermixed regions, are likely to be good candidates for light sources for the data networks of the future. An overview of the records in VCSEL transmission distances and transmission speeds is discussed in this context.

Relative intensity noise of single- and multi-mode 850 nm vertical-cavity surface-emitting lasers

In this paper we present the results of relative intensity noise (RIN) measurements for single- and multi mode 850 nm vertical cavity surface emitting lasers (VCSEL). The method applied for the RIN measurements is based on an electrical spectrum measurement of a biased and unmodulated laser. The conducted measurements show that the RIN values of around 150 dB/Hz can be expected from MM and SM VCSELs.

Vertical-Cavity Surface-Emitting 1.55-μm Lasers Fabricated by Fusion

The results of studies on fabrication of vertical-cavity surface-emitting 1.55-μm lasers by fusing AlGaAs/GaAs distributed-Bragg-reflector wafers and an active region based on thin In0.74Ga0.26 As quantum wells grown by molecular-beam epitaxy are presented. Lasers with a current aperture diameter of 8 μm exhibit continuous lasing with a threshold current below 1.5 mA, an output optical power of 6 mW, and an efficiency of approximately 22%. Single-mode lasing with a side-mode suppression ratio of 40–45 dB is observed in the entire operating current range. The effective modulation frequency of these lasers is as high as 9 GHz and is limited by the low parasitic cutoff frequency and self-heating.

Oxide-confined leaky vertical-cavity surface-emitting lasers for single-mode operation

Vertical-cavity surface-emitting lasers (VCSELs) that operate at 850nm and are based on oxide-confined apertures are widely used in optical interconnects in data centers, supercomputers, wireless backbone networks, and consumer applications.1 As the processor productivity in these applications increases, it is necessary to continuously improve performance and scale transmission speeds accordingly. In recent years, developers have produced a generation of devices capable of transmitting 40Gb/s at moderate current densities,2, 3 and they have recently demonstrated 54Gb/s non-return-to-zero transmission through 2.2km of multimode fiber.4 Now, 108Gb/s per wavelength transmission can be realized over 100–300m of multimode fiber through the use of advanced modulation formats: discrete multitone,5 multiCAP,6 and PAM4.7 All of these achievements are made possible through the use of VCSELs operating in a single transverse and longitudinal mode (SM VCSELs). When manufacturing SM VCSELs, developers typically make the oxide aperture in a VCSEL very small (around 2–3 m in diameter). This approach, however, may result in very low optical power, high resistance, and low manufacturing yield. To extend single-mode behavior toward more conventional aperture sizes (5–7 m), several alternative approaches have been proposed, including surface patterning, etching, overgrowth, and ion implantation in combination with photonic crystals.8, 9 These approaches require additional processing steps that must be precisely aligned (oxide aperture and surface pattern). The resulting complexity can reduce the yield and increase the cost of manufacturing. Figure 1. Radial distribution of the simulated electric field of oxideconfined leaky vertical-cavity surface-emitting laser (VCSEL) optical modes. (a) Fundamental optical mode. (b) First excited mode. An active region (magenta line) placed within the cavity is confined by aluminum gallium arsenide distributed Bragg reflectors. The structure contains oxide apertures (white lines). A semiconductor-air interface is shown as a dotted line in the figure. arb. u.: Arbitrary units.

Thermally stable surface-emitting tilted wave laser

Novel lasing modes in a vertical-cavity surface-emitting laser (VCSEL)-type structure based on an antiwaveguding cavity are studied. Such a VCSEL cavity has an effective refractive index in the cavity region lower than the average index of the distributed Bragg reflectors (DBRs). Such device in a stripe geometry does not support in–plane waveguiding mode, and all modes with a high Q-factor are exclusively VCSEL-like modes with similar near field profile in the vertical direction. A GaAlAs–based VCSEL structure studied contains a resonant cavity with multiple GaInAs quantum wells as an active region. The VCSEL structure is processed as an edge-emitting laser with cleaved facets and top contact representing a non–alloyed metal grid. Rectangular-shaped ~400x400 µm pieces are cleaved with perpendicular facets. The contact grid region has a total width of ~70 μm. 7 μm–wide metal stripes serve as non–alloyed metal contact and form periodic rectangular openings having a size of 10x40 μm. Surface emission through the windows on top of the chip is measured at temperatures from 90 to 380 K. Three different types of modes are observed. The longest wavelength mode (mode A) is a VCSEL–like mode at ~854 nm emitting normal to the surface with a full width at half maximum (FWHM) of the far field ~10°. Accordingly the lasing wavelength demonstrates a thermal shift of the wavelength of 0.06 nm/K. Mode B is at shorter wavelengths of ~840 nm at room temperature, emitting light at two symmetric lobes at tilt angles ~40° with respect to the normal to the surface in the directions parallel to the stripe. The emission wavelength of this mode shifts at a rate 0.22 nm/K according to the GaAs bandgap shift. The angle of mode B with respect to the normal reduces as the wavelength approaches the vertical cavity etalon wavelength and this mode finally merges with the VCSEL mode. Mode B hops between different lateral modes of the VCSEL forming a dense spectrum due to significant longitudinal cavity length, and the thermal shift of its wavelength is governed by the shift of the gain spectrum. The most interesting observation is Mode C, which shifts at a rate 0.06 nm/K and has a spectral width of ~1 nm. Mode C matches the wavelength of the critical angle for total internal reflection for light impinging from semiconductor chip on semiconductor/air interface and propagates essentially as an in–plane mode. According to modeling data we conclude that the lasing mode represents a coupled state between the TM–polarized surface–trapped optical mode and the VCSEL cavity mode. The resulting mode has an extended near field zone and low propagation losses. The intensity of the mode drastically enhances once is appears at resonance with Mode B. A clear threshold is revealed in the L–I curves of all modes and there is a strong competition of the lasing mechanisms once the gain maximum is scanned over the related wavelength range by temperature change.

Temperature stable oxide-confined 850-nm VCSELs operating at bit rates up to 25 Gbit/s at 150ºC

New applications in industrial, automotive and datacom applications require vertical-cavity surface-emitting lasers (VCSELs) operating at very high ambient temperatures at ultrahigh speed. We discuss issues related to high temperature performance of the VCSELs including temperature response and spectral properties. The influence of the gain-to-cavity wavelength detuning on temperature performance and spectral width of the VCSELs is discussed. Performance of the oxide-confined 850 nm VCSELs with increased temperature stability capable of operating at bit rates up to 25 Gbit/s at heat sink temperature of 150°C and 35Gbit/s at 130°C. Furthermore, opposite to previous studies of VCSELs with large gain-to-cavity detuning, which demonstrated strongly increased spectral width and a strong redistribution of the mode intensities upon current increase. VCSELs demonstrated in this work show good reproducibility of a narrow spectrum in a wide range of currents and temperatures. Such performance strongly improves the transmission distance over multi-mode fiber and can reduce mode partition noise during high speed operation.

100G Flexible IM-DD 850 nm VCSEL Transceiver with Fractional Bit Rate Using Eight-Dimensional PAM

We demonstrate a novel optical transceiver scheme with a net flexible bit rate up to 100Gbit/s with 5 Gbit/s granularity, using an eight-dimensional modulation format family, and investigate its performance on capacity, reach, and power tolerance.