InP-based Vertical-cavity Surface-emitting Lasers Research Articles

CW Operation of a Tunable 1550-nm VCSEL Integrating Liquid-Crystal Microcells

An InP-based Vertical-Cavity Surface-Emitting Laser (VCSEL) with a liquid crystal (LC) microcell monolithically integrated on its surface for spectral tuning is investigated. Unlike tunable VCSELs integrating a movable membrane, here the physical length of the cavity remains unchanged and only the voltage applied on the LC ensures a refractive index modification for a particular polarization emitted by the VCSEL. This tunable VCSEL operates in CW at room temperature and exhibits more than 23 nm wavelength tuning around $1.55~\mu \text{m}$ at a maximum applied voltage of 20 V. The measured laser threshold around 6.5 mW is still comparable to VCSEL without LC microcell, a clear indication that the optical losses related the LC are very low. On the other hand, for this first optically pumped device, the lasing characteristics suggest that the LC birefringence is lower than expected. To assess this hypothesis, thermo-optical simulations have been conducted.

Mirror design for long-wavelength vertical-cavity surface-emitting lasers

A kind of bottom mirror, fabricated using a nano-scale epitaxy method, is proposed for long-wavelength InP-based vertical-cavity surface-emitting lasers (VCSELs). The design of the bottom mirror combines the conventional dielectric distributed Bragg reflector structure with a fabrication procedure using nano-scale epitaxial lateral overgrowth (ELO). Hence, this ELO mirror structure can provide both a high reflectivity, to form an optical resonance cavity, and a virtual InP substrate for the growth of high quality active regions in the VCSEL. The dependence of the reflectivity on the epitaxial lateral overgrowth mirror structure parameters is analyzed in detail using numerical methods. The results demonstrate that a stopband at more than 99% reflectivity is achieved in the wavelength range from 1.49 to 2.09 µm, and the threshold optical gain can be as low as 202, 257, and 311 cm−1 for the internal loss coefficients of 10, 15 and 20 cm−1.

Effect of Cavity Length, Strain, and Mesa Capacitance on 1.5-μm VCSELs Performance

A systematic experimental analysis of the stationary and dynamic performance of 1.5-μm InP-based vertical-cavity surface-emitting lasers with two dielectric distributed Bragg reflectors is presented. The electrical, thermal, and optical characteristics are studied as a function of the cavity length, strain in the active region and mesa capacitance.

Single-Mode High-Speed 1.5-μm VCSELs

Single-mode 1.5-μm InP-based vertical-cavity surface-emitting lasers (VCSELs) with a 1.5-λ-long semiconductor cavity and two dielectric distributed Bragg reflectors (DBRs) are presented. The electrical, thermal, and optical characteristics are studied as a function of tunnel junction diameter and for different temperatures ranging from –10 up to 65 °C. Small-signal modulation bandwidths in excess of 21 GHz at room temperature are demonstrated for a DC power consumption below 10 mW. In this paper, the superior dynamic characteristics of these VCSELs are shown by demonstrating operation at data rates up to 50 Gb/s with bit-error-ratio slightly above 10 –12 in back-to-back configuration by nonreturn-to-zero modulation and without any equalization. Neither forward error correction nor digital signal processing was required.

Continuous wave vertical cavity surface emitting lasers at 2.5 μm with InP-based type-II quantum wells

A concept for electrically pumped vertical cavity surface emitting lasers (VCSEL) for emission wavelength beyond 2 μm is presented. This concept integrates type-II quantum wells into InP-based VCSELs with a buried tunnel junction as current aperture. The W-shaped quantum wells are based on the type-II band alignment between GaInAs and GaAsSb. The structure includes an epitaxial GaInAs/InP and an amorphous AlF3/ZnS distributed Bragg reflector as bottom and top (outcoupling) mirror, respectively. Continuous-wave operation up to 10 °C at a wavelength of 2.49 μm and a peak output power of 400 μW at −18 °C has been achieved. Single-mode emission with a side-mode suppression ratio of 30 dB for mesa diameters up to 14 μm is presented. The long emission wavelength and current tunability over a wavelength range of more than 5 nm combined with its single-mode operation makes this device ideally suited for spectroscopy applications.

Two-step growth of metamorphic GaAs/AlGaAs mirror on an InP substrate by MOCVD

This paper describes a high-reflectivity metamorphic undoped GaAs/Al0.98Ga0.02As distributed Bragg reflector grown on an InP substrate by metalorganic chemical vapor deposition (MOCVD). Optimal two-step growth conditions at low and high growth temperatures can provide a smooth surface morphology, leading to a high reflectivity (>99.5%) with little optical scattering loss. We also show that a large mismatched interface does not create dislocations in the active layer at a slow cooling rate after the growth sequence. These results indicate that metamorphic GaAs/Al0.98Ga0.02As directly grown on an InP substrate by MOCVD is promising for application to InP-based vertical cavity surface-emitting laser structures.

Long-Wavelength VCSEL Using High-Contrast Grating

Recent advances in high-contrast grating (HCG) vertical-cavity surface-emitting lasers (VCSEL) emitting at 1550 nm is reported in this paper. The novel near-wavelength HCG has an ultrathin structure and broadband reflectivity. It enables a monolithic, simple fabrication process for realizing InP-based VCSELs emitting at ∼1550 nm. We report 2.4-mW single-mode output under continuous-wave operation at 15 °C. We show that, despite broadened by the Brownian motion, the HCG-VCSEL has a total linewidth of 60 MHz or a coherent length of 5 m in air, and an intrinsic linewidth <20 MHz. Transmission of directly modulated 10 Gbps over 100-km dispersion-compensated single-mode fiber is demonstrated. Tunable HCG-VCSEL is demonstrated with the HCG integrated with a micro-electro-mechanical structure. Continuous wavelength tuning as wide as 26.3 nm is achieved. The tunable VCSEL was used as a source for external modulation for 40-Gbps differential-phase-shift-keyed signal and transmitted over 100-km dispersion-compensated link with negligible power penalty.

Sputtered hydrogenated amorphous silicon thin films for distributed Bragg reflectors and long wavelength vertical cavity surface emitting lasers applications

In this work, we report a study of hydrogenated amorphous silicon (a-SiH) films deposited by radio frequency magnetron sputtering for application in Vertical Cavity Surface Emitting Lasers (VCSEL) elaboration. The influence of the hydrogen dilution in the plasma during the deposition on the optical and surface properties is investigated. After selection of the deposition parameters, a-SiH films have been combined with amorphous silicon nitride (a-SiN x) films to provide high reflectivity Bragg reflectors. Distributed Bragg reflector (DBR) based on these quarter wavelength thick dielectric layers have been realized and characterized by optical measurements and compared with theoretical calculations based on the transfer matrix method. A maximum reflectivity of 99.2% at 1.6 μm and a large spectral bandwidth of 700 nm have been reached with only four and a half periods of a-SiH/a-SiN x deposited on a glass substrate. Residual absorption at 1.55 μm has been measured to be as low as 60 cm −1 with a-SiH layers, compared with 400 cm −1 loss with amorphous silicon without hydrogenation step. Finally, DBR comprising six a-SiH/a-SiN x periods have been included in an InP-based VCSEL. Laser emission is demonstrated at room temperature in continuous wave operation with a photopumping experiment.

1.55-$\mu$m VCSEL With Enhanced Modulation Bandwidth and Temperature Range

InP-based vertical-cavity surface-emitting lasers (VCSELs) at 1.55-mum emission wavelength with improved modulation bandwidth and temperature behavior are demonstrated. Utilizing an improved active region, thermal design, and reduced chip parasitics, a superior modulation-bandwidth >10 GHz is achieved up to 85degC. The new VCSEL device is compared in detail with our reference design, analyzing all bandwidth-limiting elements. With their improved temperature range at invariant output power and minimal threshold change with temperature these VCSELs are especially qualified for uncooled operation in passive optical networks. Potential bit rates of 12.5 or even 17 Gb/s are expected with this kind of devices for cost-effective 100-G Ethernet solutions at metro-range.

Wavelength modulation spectroscopy with a widely tunable InP-based 23 μm vertical-cavity surface-emitting laser

We report on the successful application of recently developed 2.3 microm InP-based vertical-cavity surface-emitting lasers with a buried tunnel junction in a wavelength modulation spectroscopy measurement for carbon monoxide (CO) detection. The electrically pumped devices operate at room temperature under cw operation with stable single-mode emission and should allow for parts in 10(6) (ppm) level resolution measurement of CO with a standard optical setup.

Threshold characteristics of bottom-emitting long wavelength VCSELs with photonic-crystal within the top mirror

We present self-consistent modeling of InP-based 1300-nm AlInGaAs photonic-crystal vertical-cavity surface-emitting diode laser. We investigate the influence of photonic-crystal parameters on threshold characteristics. We show that a shallow photonic crystal can significantly deteriorate the VCSEL threshold current. The significant reduction of threshold current can be assured by the photonic-crystal etched through the whole top DBR mirror.

Experimental characterization of the frequency modulation behavior of vertical cavity surface emitting lasers

The frequency response of the current-to-wavelength tuning rate (FM response) was measured for two different vertical cavity surface emitting lasers (VCSELs) up to frequencies of 1MHz: GaAs-based VCSEL (763nm) and InP-based VCSEL (1853nm). Both of them show the same qualitative FM response behavior, which can be described by a square root law and therefore cannot be modeled by a first order low pass. The square root law behavior is a significant advantage for laser spectroscopy applications with VCSELs because the decrease of the current-to-wavelength tuning coefficient is less severe as in the case of the first order low pass.

Optical Design of InAlGaAs Low-Loss Tunnel-Junction Apertures for Long-Wavelength Vertical-Cavity Lasers

We report on the optical design of thin selectively etched InAlGaAs tunnel-junction apertures for the realization of optically efficient long-wavelength vertical-cavity surface-emitting lasers (VCSELs). These apertures were designed to introduce minimal optical loss to the structure, facilitate single-mode operation, and yield optical mode diameters that better match the injected current density profile. We then demonstrate InP-based VCSELs emitting at 1304 nm utilizing these low-loss InAlGaAs apertures, resulting in optically efficient low-loss devices with differential quantum efficiencies of up to 60%.

MEMS-tunable 1.55-/spl mu/m VCSEL with extended tuning range incorporating a buried tunnel junction

We present an electrically pumped and micromechanically tunable InP-based vertical-cavity surface-emitting laser operating in the 1.55-/spl mu/m wavelength range. The current confinement is achieved by a buried tunnel junction. The GaAs-based movable top mirror membrane is fabricated separately, assembled on top of the device, and can be actuated electrothermally. A single mode output power of about 1.7 mW and a tuning range of 28 nm was obtained. By the use of an antireflection coating at the semiconductor-air-interface, we were able to extend the tuning range up to 60 nm as expected from one-dimensional simulations.

Simplified nonplanar wafer bonding for heterogeneous device integration

We demonstrate a simplified nonplanar wafer bonding technique for heterogeneous device integration. The improved technique can be used to laterally integrate dissimilar semiconductor device structures on a lattice-mismatched substrate. Using the technique, two different InP-based vertical-cavity surface-emitting laser active regions have been integrated onto GaAs without compromising the quality of the photoluminescence. Experimental and numerical simulation results are presented.

Design of Synthesized DBRs for Long-Wavelength InP-BasedVertical-Cavity Surface-Emitting Lasers

We report applications of a metallic film and a phase matching layer(PML) to increase the reflectivity of the cavity mirror in along-wavelength InP-based vertical-cavity surface-emitting laser(VCSEL). The synthesis of the InGaAsP/InP distributed-Bragg reflector(DBR) with an Au film and the InP PML leads to the decrease of periodsof the DBR multilayer stacks from 33 to 20 while keeping thereflectivity of the structure over 99%. The reflectivity over thewhole forbidden band is significantly increased and become flattercompared to the bare DBR. The use of smaller DBR periods in a longwavelength VCSEL makes the epitaxial growth well controllable, decreaseof the heat resistance, and decrease of the in-series electricalresistance of the devices. This can improve the reliability of theVCSEL growth and possibly cut down the cost of VCSEL devices.

Complete polarization mode control of long-wavelength tunable vertical-cavity surface-emitting lasers over 65-nm tuning, up to 14-mW output power

We experimentally demonstrate a stable polarization mode operation in long-wavelength tunable vertical-cavity surface-emitting lasers over a 65-nm tuning range and the entire output power range (< 14.6 mW) at room temperature. The polarization mode control was achieved by utilizing anisotropic gain properties of quantum wells due to the difference in bond lengths between the constituent atoms at the interfaces combined with uni-axial external strain induced by a stressor. The experiments were conducted to verify this newly proposed polarization control scheme based on the spin flip model (SFM) developed to incorporate the detailed gain properties, cavity standing wave effect, self-heating effect, and strain effect. The experimental results on the tuning characteristics of polarization switching behavior and output powers were reproduced in highly agreeable manner by simulations. The relative importance of the external strain, interfacial strain at quantum wells, and the wavelength dependence of gain anisotropy are also discussed. It is also shown that the fast spin relaxation times for InP-based vertical-cavity surface-emitting lasers (VCSELs) was responsible for the inhibition of elliptic polarization states often observed for GaAs-based VCSELs. The effectiveness of the polarization control scheme was highlighted by the observed high polarization suppression ratio of 34 dB maintained for the entire wavelength and pump power ranges during the reliability testing over 2000 h. The influence of the elliptic polarization state for the optical pump laser was detected which could be explained as a memory effect of the spin-polarized electrons, supporting the validity of the SFM.

InP-based wavelength tunable vertical cavity surface emitting laser structures

We report on the design, fabrication and characteristics of both hybrid and monolithic micro-electro-mechanically wavelength tunable 1.55 μm InP-based Vertical-Cavity Surface-Emitting Laser (VCSEL) structures. Photo-pumped tunable VCSELs are successfully realized using both configurations, and a design for electrically pumped tunable VCSEL is presented. To cite this article: I. Sagnes et al., C. R. Physique 4 (2003).

Demonstration of methane spectroscopy using a vertical-cavity surface-emitting laserat 1.68 µm with up to 5 MHzrepetition rate

An electrically pumped InP-based vertical-cavity surface-emitting laser (VCSEL) emittingat 1.685 µm (5935cm−1) was used to probe rovibrational transitions ofmethane in the 2ν3 band at pressures ranging from 10mbar to 1.3 bar. For the first time a long-wavelength VCSEL at 1.68µm was used for thedemonstration of high-speed gas sensing at repetition rates as highas 5 MHz. The observed tuning rate at 1 MHz was 5.2cm−1 µs−1. The maximum continuous currenttuning range was more than 16 cm−1 (4.5 nm), whichis very useful for measurements in high-pressure environments. At 1MHz the tuning range was as large as 2.6 cm−1, at 5MHz it was still approximately 0.36 cm−1. Themeasured tuning rates of the VCSEL were −3.05cm−1 mA−1 (+0.86 nmmA−1) and −0.40 cm−1K−1 (+0.11 nm K−1). Surveyspectra measured with the VCSEL were compared to calculations usingthe HITRAN2000 spectroscopic database and found to be in goodagreement. A VCSEL was directly contacted on a section of its waferto demonstrate ‘on-chip’ testing ability as opposed toedge-emitting lasers that have to be singularized and individuallymounted prior to characterization for spectroscopic applications. Itis expected that VCSELs will considerably expand the range ofoperation for diode lasers in rapidin situ species concentration measurements because of theirability of fast and far mode-hop free single longitudinal andtransverse mode tuning properties.

Low-threshold index-guided 1.5 μm long-wavelength vertical-cavity surface-emitting laser with high efficiency

A significantly improved InP-based vertical-cavity surface-emitting laser with record device performance is demonstrated. Utilizing a twofold epitaxial growth process, self-adjusted lateral current confinement and index guiding are accomplished by means of a buried InGa(Al)As tunnel junction. Front and back mirrors are realized by 35 epitaxial InGaAlAs/InAlAs layer pairs and a 1.5 MgF2/a-Si layer pair, respectively. At room temperature and under continuous wave condition, lasers with small aperture diameters of only 13 μm exhibit record output powers of 1.6 mW with quantum efficiencies around 25%. For these devices, threshold current and voltage are as low as 4 mA and 1.2 V, respectively, because of low series resistances around 70 Ω.