Kai-Lun Chi

Single-Mode, High-Speed, and High-Power Vertical-Cavity Surface-Emitting Lasers at 850 nm for Short to Medium Reach (2 km) Optical Interconnects

The vertical-cavity surface-emitting lasers (VCSELs) with high single-mode (narrow linewidth) output power are essential to minimize chromatic dispersion and to further improve the bit-rate distance product in a multimode fiber, which has a significant propagation loss (~3.5 dB/km) at 850 nm wavelength. Here, we demonstrate the detailed design considerations and fabrication of a single-mode, high-power, and high-speed VCSELs at the 850 nm wavelength with oxide-relief and Zn-diffusion apertures for the application of short (0.3 km) to medium reach (2 km) optical interconnects. By optimizing the relative geometric sizes between two such apertures in our demonstrated 850-nm VCSELs, we can not only attain high single-mode output power (~6.5 mW), but also with a reasonable threshold current (<; 2.0 mA). Furthermore, the spatial hole burning effect induced low-frequency roll off can also be minimized in our optimized structure to obtain a maximum data rate up to 26 Gbit/s. The record-high bit rate-distance products for OM4 MMF transmission under ON-OFF keying (14 Gbit/s × 2.0 km) modulation formats have been successfully demonstrated by the use of our VCSEL.

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.

Dynamic Analysis of High-Efficiency InP-Based Photodiode for 40 Gbit/s Optical Interconnect Across a Wide Optical Window (0.85 to 1.55 μm)

The detailed dynamic analysis of novel high-speed InP-based photodiodes (PDs) has been performed. Such device can sustain an invariable high external efficiency (~74%; no antireflection coating) across a wide optical operation window (0.85 to 1.55 μm). Furthermore, compared with the traditional GaAs-based high-speed PD for optical interconnect applications, our proposed device structure can offer an enlarged device active diameter and eliminate the degradation in responsivity performance when the desired speed performance is increased. This is because the strong photoabsorption process and the elimination of slow hole drift in the In0.53Ga0.47As based collector layer at 0.85-μm wavelength operation. By measuring the dynamic performance of PDs with different active diameters across such wide optical window, we can accurately extract the electron drift-velocity under different wavelengths excitations in the In0.53Ga0.47As collector layer. This result indicates that at short-wavelength (0.85 μm) operation, the photogenerated electron in the In0.53Ga0.47As collector suffers from significant intervalley scattering effect due to its high excess energy. By using such device with diameter of optical window as large as 40 μm, 40 Gbit/s error-free transmissions have been successfully demonstrated through 5-km single-mode (SMF-28) and 0.1-km multimode (OM4) fibers at long- and short-wavelengths operations with reasonable sensitivity, respectively.

Single-Mode Vertical-Cavity Surface-Emitting Laser Array With High Power and Narrow Far-Field Divergence Angle

We demonstrate novel structures of an 850-nm vertical-cavity surface-emitting laser (VCSEL) array for high output power, single-lobe far-field pattern, and narrow divergence angle. By using the Zn-diffusion process with proper sizes of oxide-current-confined and Zn-diffusion apertures, each unit of VCSEL in the demonstrated array is highly single-mode (side-mode suppression ratio 30 dB) with a narrow far-field divergence angle (9°- 10°) and high maximum single-mode output power ( ~ 6.3 mW). Due to the high uniformity of single-mode performance of each VCSEL unit, the 6 × 6 array exhibits an excellent lasing phenomenon, which includes single-lobe far-field pattern, weak in-phase coupling, narrowing of divergence angle (from 9° to 4°), and output power as high as around 104 mW. Furthermore, by measuring the bias-dependent output optical spectra in different positions of our array, the high similarity of these spectra indicates the excellent uniformity of our fabrication process for single-mode VCSEL.

Strong Wavelength Detuning of 850 nm Vertical-Cavity Surface-Emitting Lasers for High-Speed (>40 Gbit/s) and Low-Energy Consumption Operation

The strong (>20 nm) wavelength detuning technique has been demonstrated to enhance the modulation speed and high-temperature characteristics (at 85 °C), as well as lower the required driving current density performance of oxide-relief 850-nm vertical-cavity surface-emitting lasers (VCSELs) for >40 Gbit/s operation. By increasing the wavelength detuning from 15 to 20 nm, a significant improvement in the electrical-to-optical (E-O) bandwidth (20 to 27 GHz) of the VCSEL can be observed. This detuning design (~20 nm) is incorporated along with a Zn-diffusion structure into our oxide-relief VCSEL with a miniaturized oxide-relief aperture (~3 μm). Highly single-mode, high-speed (26 GHz) operation, and moderate differential resistance (100 Ω) values can be simultaneously achieved. In addition, it is found that devices with a further larger detuning wavelength (>20 nm) and enlarged oxide-relief apertures (~8 μm) can sustain the same maximum E-O bandwidth (26 GHz) as that of a miniaturized (~3 μm) VCSEL, resulting in the lower driving current density (8 versus 18.8 kA/cm2) required for high-speed performance. Excellent transmission performance, which includes an extremely low energy-to-data rate ratio (EDR: 228 fJ/bit; over 100 m OM4 fiber) and record-low driving-current density (8 kA/cm2; 3.5 mA) has been successfully achieved for 41 Gbit/s error-free transmission for these VCSELs.

High-speed and duo-mode 850 nm VCSELs for 47 Gbps optical interconnect over 1 km OM4 fiber

Novel duo-mode 850nm VCSELs with short (λ/2) cavity, oxide-relief and Zn-diffusion apertures are demonstrated to balance the trade-off of speed and reaching bottleneck of VCSELs. Extremely-high data rate (47-Gbps) over 1005m OM4-fiber transmission is reported.

Single-Mode 850 nm VCSELs Array with High-Power, Single-Lobe Pattern, and Narrow Divergence Angle

A high-performance single-mode 850 nm VCSEL array is demonstrated. By using Zn-diffusion apertures with the proper array spacing, a circular-symmetric pattern with CW high-power (140 mW) and narrow divergence angle (~5°) have been simultaneously achieved.

Carrier Dynamics in High-Efficiency Blue GaN Light-Emitting Diodes Under Different Bias Currents and Temperatures

Distinct temperature-dependent dynamic behaviors of GaN-based blue light-emitting diodes (LEDs) are observed by use of the very-fast electrical-optical pump-probe technique. Our static and impulse response measurement results indicate that the behaviors of internal carrier dynamics under different ambient temperatures can be classified into three regimes covering a wide range of bias current densities (20-2000 A/cm2). The first regime is when the bias current density ranges from low to moderate (20-100 A/cm2). The measured external quantum efficiency (EQE) degrades dramatically from 57 to 44%, and the measured waveform and extracted time constants of measured impulse responses are invariable from room temperature (RT) to 200 °C, which indicates that the carrier leakage is not an issue for the observed droop phenomenon. When the bias current density further increases to near 1 kA/cm2, the droop phenomenon are mitigated (44 to 24%). However, a significant shortening of the measured impulse response happens under 200 °C operation due to the device-heating effect. This phenomenon is diminished when the bias current densities are further increased to over 1 kA/cm2, due to the screening of the piezoelectric field. The extracted time constants can also be used to explain the droop phenomenon in GaN LED under high bias currents.

50 Gb/s NRZ and 4-PAM data transmission over OM5 fiber in the SWDM wavelength range

The development of advanced OM5 wideband multimode fiber (WBMMF) allowing high modal bandwidth in the spectral range 840-950 nm motivates research in vertical-cavity-surface-emitting-lasers (VCSELs) at wavelengths beyond the previously accepted for short reach communications. Thus, short wavelength division multiplexing (SWDM) solutions can be implemented as a strategy to satisfy the increasing demand of data rate in datacenter environments. As an alternative solution to 850 nm parallel links, four wavelengths with 30 nm separation between 850 nm and 940 nm can be multiplexed on a single OM5-MMF, so the number of fibers deployed is reduced by a factor of four. In this paper high speed transmission is studied for VCSELs in the 850 nm – 950 nm range. The devices had a modulating bandwidth of ~26-28 GHz. 50 Gb/s non-return-to-zero (NRZ) operation is demonstrated at each wavelength without preemphasis and equalization, with bit-error-rate (BER) below 7% forward error correction (FEC) threshold. Furthermore, the use of single-mode VCSELs (SM-VCSELs) as a way to mitigate the effects of chromatic dispersions in order to extend the maximum transmission distance over OM5 is explored. Analysis of loss as a function of wavelength in OM5 fiber is also performed. Significant decrease is observed, from 2.2 dB/km to less than 1.7 dB/km at 910 nm wavelength of the VCSEL.

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.