High-speed Vertical-cavity Surface-emitting Lasers Research Articles

An Analytical Design Method for High-Speed VCSEL Driver With Optimized Energy Efficiency

A universal analytical design approach for laser diode drivers (LDDs) is presented and verified. All design parameters are derived analytically, taking the vertical-cavity surface-emitting laser (VCSEL) characteristics into account. Two optimization strategies are proposed. First, the power consumption is minimized for the highest achievable optical modulation amplitude (OMA) and extinction ratio (ER). Second, the highest achievable data rate (DR) is considered to maximize the energy efficiency. As a result, a simple differential amplifier can be implemented as an LDD to achieve energy efficiencies and DRs comparable or even higher than the one of more complex designs utilizing preemphasis, feedforward equalization, and pulse amplitude modulation. Therefore, drivers designed with this holistic analytical approach accommodate well the demands for high DRs, high energy efficiency, high compactness, high reliability, and low latency in future optical data links. A compact LDD is designed with an active area of only $0.15 \times 0.12$ mm2 in a 130-nm SiGe BiCMOS technology. A DR of 40 Gbit/s with bit error rates $ and an energy efficiency of 2.245 pJ/bit are achieved for an OMA of 1.1 mW and an ER of 10 dB using a 20-GHz 850-nm common-cathode VCSEL. The achieved values deviate less than 10% from the predicted ones, which clearly illustrates the effectiveness of the presented analytical approach. Further improvements are achieved by optimizing the modulation levels and the performance to the link requirements. At 45 and 30 Gbit/s, 1.8 and 1.17 pJ/bit are achieved, respectively.

Advanced VCSEL Technology: Self-Heating and Intrinsic Modulation Response

Experimental data and modeling results are presented for a new type of vertical-cavity surface-emitting laser (VCSEL) that solves numerous problems with oxide VCSELs. In addition, the new oxide-free VCSEL can be scaled to small size. Modeling shows that this small size can dramatically increase the speed of the laser through control of self-heating. Modeling compared with both the oxide and the new oxide-free VCSEL predict intrinsic modulation speed for room temperature approaching 80 GHz, indicating the potential for > 100-Gb/s data speed from directly modulated VCSELs. The modeling is one of if not the first to include self-heating and spectral detuning between the gain and the cavity resonance that results from self-heating. The modeling results accurately accounts for the saturation in modulation speed in very high-speed oxide VCSELs operating at high temperature and accounts for the temperature of differential gain and photon density in limiting speed. Saturation of the photon density is found to limit the ultimate intrinsic speed. Differential gain decreases with increasing temperature but with sufficiently weak temperature dependence that it is not found to limit the VCSEL speed.

1060 nm VCSELs for long-reach optical interconnects

Abstract Reach extension of high capacity optical interconnects based on vertical-cavity surface-emitting lasers (VCSELs) and multimode fibers (MMFs), as needed for large-scale data centers, would benefit from high-speed GaAs-based VCSELs at 1060 nm. At this wavelength, the chromatic dispersion and attenuation of the optical fiber are much reduced in comparison with 850 nm. We present single and multimode 1060 nm VCSELs based on designs derived partly from our high-speed 850 nm VCSEL designs. The single-mode VCSEL, with a modulation bandwidth exceeding 22 GHz, supports back-to-back data rates up to 50 Gbps at 25 °C and 40 Gbps at 85 °C under binary NRZ (OOK) modulation. Using mode-selective launch, we demonstrate error-free 25 Gbps transmission over 1000 m of 1060 nm optimized MMF. Higher data rates and/or longer distances will be possible with equalization, forward-error-correction, and/or multilevel modulation.

采用BCB平整技术的高速850 nm垂直面发射激光器

Vertical-cavity surface-emitting lasers(VCSELs) are widely used in short-reached optical interconnects and data communication links because of their low energy consumption and high modulation speed. Capacitance, as the parasitic parameters, affects the modulation bandwidth. In this paper, parasitic capacitance of VCSELs is reduced by using a low-k benzocyclobutene(BCB) planarization technique. The detail BCB planarization technique has been discussed with optimal process parameters, which is useful for high-speed VCSEL fabrication. The small signal modulation bandwidth of the low-k BCB planarization VCSEL with 7 μm oxide aperture has been achieved to 15.2 GHz.

Comparison Between Analog Radio-Over-Fiber and Sigma Delta Modulated Radio-Over-Fiber

With the continuously increasing demand of cost effective, broadband wireless access, radio-over-fiber (RoF) starts to gain more and more momentum. Various techniques already exist, using analog (ARoF) or digitized (DRoF) radio signals over fiber; each with their own advantages and disadvantages. By transmitting a sigma delta modulated signal over fiber (SDoF), a similar immunity to impairments as DRoF can be obtained while maintaining the low complexity of ARoF. This letter describes a detailed experimental comparison between ARoF and SDoF that quantifies the improvement in linearity and error vector magnitude (EVM) of SDoF over ARoF. The experiments were carried out using a 16-QAM constellation with a baudrate from 20 to 125 MBd modulated on a central carrier frequency of 1 GHz. The sigma delta modulator runs at 8 or 13.5 Gbps. A high-speed vertical-cavity surface-emitting laser (VCSEL) operating at 850 nm is used to transmit the signal over 200-m multimode fiber. The receiver amplifies the electrical signals and subsequently filters to recover the original RF signal. Compared with ARoF, improvements exceeding 40 dB were measured on the third order intermodulation products when SDoF was employed, the EVM improves between 2.4 and 7.1 dB.

850 nm Vertical-Cavity Surface-Emitting Laser Arrays With Enhanced High-Speed Transmission Performance Over a Standard Multimode Fiber

The functionality of novel parallel and series high-speed vertical-cavity surface-emitting laser (VCSEL) arrays, which can greatly relax the tradeoff between output power and modulation speed, is demonstrated. Both types of array structure allow improvement in the output power with no degradation in their maximum modulation speed as compared to a single reference unit. The observed invariant electrical–optical bandwidth shown for these array structures is mainly due to the effective reduction of the differential resistance and parasitic capacitance, arising from the Zn-diffusion and oxide-relief apertures fabricated in the VCSEL unit. This in turn minimizes the degradation in the RC -limited bandwidth with the VCSEL arrays. Furthermore, the dense packing of single VCSELs, with Zn-diffusion apertures for optical mode control, minimizes the coupling loss between the output from the array into a standard multimode fiber (MMF). Compared with the single VCSEL unit, the parallel VCSEL array shows a significant enhancement in transmission performance over a standard OM4 MMF, which includes a larger eye margin, a higher signal-to-noise ratio, as well as a higher error-free data rate (48 versus 44 Gbit/sec). The device modeling technique is used to perform device analysis. From the results we can conclude that the improvement with the parallel array is because the value of the internal impedance is closer to the 50-Ω signal source, which minimizes the microwave reflection induced timing jitter in the eye-patterns.

Heat dissipation effect on modulation bandwidth of high-speed 850-nm VCSELs

Vertical-cavity surface-emitting lasers (VCSELs) have been widely used in short-reach optical communication links and interconnects because of their high modulation speed and low energy consumption. In this paper, the effect of heat dissipation on the modulation bandwidth of high-speed 850-nm VCSELs has been investigated. With increasing bottom distributed Bragg reflector (DBR) diameter from 34 μm to 46 μm, optical output power increased as well as the −3 dB modulation bandwidth. The maximum small-signal modulation bandwidth of 16.2 GHz was achieved with an aperture of 5 μm. Thermal analysis showed that the heat dissipation of the bottom DBR diameter, which determined the differential gain of the quantum wells, has a strong effect on the modulation bandwidth.

High Speed VCSEL Transmission at 1310 nm and 1550 nm Transmission Wavelengths

Vertical cavity surface emitting lasers (VCSELs) operating at 1310 nm and 1550 nm are very promising sources for access and interconnections in telecommunication systems due to their technologically attractive properties such as low threshold currents and narrow spectral linewidth due to single mode operation. VCSELs are also being rapidly commercialized for single-mode fibre metropolitan area and wide area network applications. All these advantages leads to cost-effective wavelength-tunable lasers, which are essential for the future intelligent, all-optical networks. Direct modulation (DM) of VCSEL with separate optical and current apertures enables high modulation bandwidth operating at single mode at low current density. However, dispersion and attenuation is a major hurdle to VCSELs transmission at bit rate of 10 Gb/s and above. In this study, a 1310 and 1550 nm VCSELs were directly modulated with 10 Gb/s NRZ PRBS 2⁷-1 and transmitted over 25 km ITU. T G.652 and ITU. T G.655 fibres and optimized for metro-access distances. A low dispersion penalty was realized when a 1550 nm VCSEL was used on a G.655 fibre and when a 1310 nm source was transmitted over a G.652 fibre. The transmission system has been analyzed on the basis of different parameters, which are (Bit error rate) BER, Quality (Q) factor and Output power.

Comparison between high- and zero-contrast gratings as VCSEL mirrors

This study presents a comparison between high-contrast gratings (HCGs) and zero-contrast gratings (ZCGs) for high-speed vertical-cavity surface-emitting lasers (VCSELs). Both types of gratings exhibit high reflectivities beyond 99.5% due to the destructive interference at the output plane, but the HCG has a broader high reflectivity band. The HCG has a lower reflection delay time and smaller energy penetration length than the ZCG. The HCG has poorer mode selectivity for the VCSEL than the ZCG. The fabrication of the ZCG is less complex but with tight fabrication tolerances.

Noise in VCSEL-Based Links: Direct Measurement of VCSEL Transverse Mode Correlations and Implications for MPN and RIN

The correlation properties of vertical-cavity surface-emitting laser (VCSEL) transverse modes determine the noise impairments of VCSEL-based high-speed MMF links. Relative intensity noise and mode partition noise originate from the distinct and aggregate fluctuations of the lasing transverse modes of VCSELs. These noise mechanisms are often the limitations in VCSEL-based optical links particularly 25 Gbaud and higher OOK and PAM-4 links and are an integral part of any link budget including those of the IEEE 802.3 and fiber channel T11.2 standards efforts. Here, we report the direct measurement of the correlation properties of high-speed VCSELs. We show that the corresponding computed mode partition noise (MPN) and relative intensity noise (RIN) powers are inconsistent with the conventional models used by standards groups yet are consistent with link measurements.

Fabrication and Characterization of Integrable GaAs-Based High-Contrast Grating Reflector and Fabry–Pérot Filter Array With GaInP Sacrificial Layer

Integrable GaAs-based high-contrast gratings (HCGs) are fabricated and characterized, targeting applications in high-speed vertical-cavity surface-emitting lasers (VCSELs). A Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.51</sub> In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.49</sub> P sacrificial layer beneath the GaAs layer is employed to create a low index surrounding HCG strips by selective etching. Experimental results show that the finite-size HCG has a reflectivity of 93% from 1270 to 1330 nm for the transverse magnetic polarization, which is consistent with the calculated results. An HCG-based Fabry-Perot filter array formed by the different HCGs, air gap, and GaAs substrate is demonstrated. The measured resonance wavelengths of the filter arrays are consistent with the theoretical results, which implies that the resonance wavelength of such filters can be tuned by parameters of the HCG itself.

Very High Bit-Rate Distance Product Using High-Power Single-Mode 850-nm VCSEL With Discrete Multitone Modulation Formats Through OM4 Multimode Fiber

In order to investigate the tradeoff between optical spectral width and modulation speed of 850-nm Zn-diffusion vertical-cavity surface-emitting laser (VCSEL) and its influence on the performance of discrete multitone (DMT) modulation, two kinds of high-speed VCSEL structures with different cavity lengths (λ/2 and 3λ/2) are studied. By shortening the cavity length to λ/2, allocating the oxide layer in the standing-wave peak, and performing a Zn-diffusion aperture in our VCSEL structure, stable dual mode in the output optical spectra across the full range of bias currents with good high-speed performance (∼16-GHz bandwidth) can be achieved. Compared with its multimode reference, it shows far less roll-off with regard to the maximum data rate versus transmission distance over OM4 multimode fibers under forward error correction (FEC) threshold (BER $ ). On the other hand, for the 3λ/2 VCSEL structure, by using the same Zn-diffusion conditions as those of dual-mode counterpart, highly single-mode operation (side-mode suppression ratio> 35 dB) with high available power can be achieved over the full range of bias currents. Although such device shows a smaller 3-dB electrical-to-optical bandwidth (12 versus 16 GHz) than that of the dual-mode one, it exhibits a superior transmission performance by use of DMT modulation format. A record high bit-rate distance product (107.6 Gb/s·km) at nearly 50-Gb/s transmission under FEC threshold (BER $ ) through 2.2-km OM4 fibers has been successfully demonstrated by the use of single-mode VCSEL with optimized structures. In addition, error-free (BER $ ) transmission at 20.3 Gb/s with bit-rate distance product of 44.66 Gb/s·km has also been demonstrated.

A 50 Gb/s NRZ Modulated 850 nm VCSEL Transmitter Operating Error Free to 90 °C

We report on the properties of an 850 nm vertical cavity surface-emitting laser (VCSEL) transmitter running at 50 Gb/s with NRZ modulation from 30 °C to 90 °C. This is the highest transmitter operating temperature at 50 Gb/s for a VCSEL link of any wavelength. This achievement is made possible by a combination of a high speed VCSEL design and driver and receiver circuits that incorporate equalization. Without equalization, the highest temperature attained with bit error ratio <;1E-12 is 57 °C.

Noise performance of high-speed radio over fiber links employing vertical-cavity surface-emitting lasers

This study investigates the intensity noise in high-speed vertical-cavity surface-emitting lasers (VCSELs) and its contribution to the noise performance of radio over fiber (RoF) links. We evaluate the sinusoidal modulation of VCSELs in terms of the second-order harmonic distortion (2HD) and third-order intermodulation distortion (IMD3) in additions to the relative intensity noise (RIN). The spurious-free dynamic range of the proposed VCSEL is estimated. The noise performance of the RoF link is assessed by the noise figure. The modulation characteristics of the VCSEL and the gain and noise factor (NF) of the fiber link are compared under conventional and high-speed modulations of VCSELs. Also, we present comparison of the NF between short (300 m) and relatively long (2 km) fibers.

Study of high-speed semiconductor VCSELs based on AlInGaAs heterostructures with large gain-cavity detuning

The effect of the spectral detuning between the spectral gain maximum of an active region based on AlInGaAs nanoheterostructures and vertical microcavity resonance (gain-cavity detuning) on the steady-state- and dynamic characteristics of high-speed vertical-cavity surface-emitting lasers (VCSELs) for the 850-nm spectral range with two selectively oxidized current apertures is studied. For multimode devices with gain-cavity detuning exceeding 20 nm and a comparatively large current aperture (>6 μm), anomalous lasing onset via higher order modes is observed, with subsequent switching to lower order modes at high currents. Simultaneous lasing via both types of transverse modes, observed at intermediate pumping levels, leads to a nonclassical dependence of the maximum effective-modulation frequency on the pump current. An increase in the working temperature, as well as a decrease in the size of the current aperture of a VCSEL, leads to classical multimode lasing behavior. The observed phenomena are to be taken into account when optimizing the design of high-speed VCSELs.

Single-mode temperature and polarisation-stable high-speed 850nm vertical cavity surface emitting lasers

A new intracavity-contacted design to realize temperature and polarization-stable high-speed single-mode 850 nm vertical cavity surface emitting lasers (VCSELs) grown by molecular-beam epitaxy is proposed. Temperature dependences of static and dynamic characteristics of the 4.5 pm oxide aperture InGaAlAs VCSEL were investigated in detail. Due to optimal gain-cavity detuning and enhanced carrier localization in the active region the threshold current remains below 0.75 mA for the temperature range within 20-90°C, while the output power exceeds 1 mW up to 90°C. Single-mode operation with side-mode suppression ratio higher than 30 dB and orthogonal polarization suppression ratio more than 18 dB was obtained in the whole current and temperature operation range. Device demonstrates serial resistance less than 250 Ohm, which is rather low for any type of single-mode short- wavelength VCSELs. VCSEL demonstrates temperature robust high-speed operation with modulation bandwidth higher than 13 GHz in the entire temperature range of 20-90°C. Despite high resonance frequency the high-speed performance of developed VCSELs was limited by the cut-off frequency of the parasitic low pass filter created by device resistances and capacitances. The proposed design is promising for single-mode high-speed VCSEL applications in a wide spectral range.

Equivalent Circuit Model of High Speed VCSEL Implemented in Circuit Simulators

Optical interfaces have been recently standardized as the main physical layer interfaces for most short length optical communication systems, such as IEEE802.3ae, OIF-VSR, and the Fiber Channel. As interface speed increases, the requirements for forecasting the optical characteristics of direct modulated laser diodes (LDs) also increase because those standards define the specifications for physical layers with optical domains. In this paper, a vertical-cavity surface-emitting laser (VCSEL) equivalent electronic circuit model is described with which designers can simulate the I-L-V, S-parameter, and transient characteristics of LDs on a circuit simulator by improving convergence. We show that the proposed VCSEL model can model an 850-nm bandwidth VCSEL with 10-Gbps operation.

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.

III–V Oxidation: Discoveries and Applications in Vertical-Cavity Surface-Emitting Lasers

Since the discovery of III-V oxidation by Dallesasse and Holonyak in 1989, significant progress has been made, both technically and commercially, on the use of oxides in compound semiconductor devices. The process-induced modification of refractive index and conductivity allows control of the two carriers of information in optoelectronic systems, the photon and the electron, enabling wide-ranging device applications. Of great technical and commercial importance has been the use of oxidation for the fabrication of high-speed vertical-cavity surface-emitting lasers (VCSELs), first implemented by Deppe's group at The University of Texas at Austin (Austin, TX, USA). Here, the low refractive index III-V oxide's interaction with the optical modes inside the VCSEL creates an optimal overlap of gain and field, enabling lasers with ultralow threshold currents and desirable optical beam properties. The discovery of III-V oxidation, key technical milestones in the fabrication of photonic and electronic devices that use oxidation, and the application to VCSELs are reviewed.

High‐speed 850 nm VCSELs operating error free up to 57 Gbit/s

Error-free transmission is demonstrated at bit rates up to 57 Gbit/s back-to-back, up to 55 Gbit/s over 50 m fibre and up to 43 Gbit/s over 100 m fibre using an oxide-confined 850 nm high-speed vertical cavity surface-emitting laser with a photon lifetime optimised for high-speed data transmission.