Current spreading modification to enhance single-fundamental-mode VCSEL operation at higher temperatures

Continuous-wave (CW) performance of modern oxide-confined (OC) vertical-cavity surface-emitting diode lasers (VCSELs) at room and elevated temperatures is investigated with the aid of the comprehensive fully self-consistent optical-electrical-thermal-gain model. A standard OC GaInNAs/GaAs double-quantum-well VCSEL emitting the 1.3-μm radiation is used as a typical modern VCSEL structure. The oxide aperture is placed at the anti-node position of an optical standing wave within a VCSEL cavity. The desired single-fundamental-mode (SFM) operation has been found to be expected only in VCSELs equipped with relatively small active regions of diameters equal or smaller than 10 μm. Therefore a proton implantation used as an radial additional confinement of the current spreading from the upper annular contact towards the centrally located active region is proposed and its impact on the VCSEL performance is investigated. The above structure modification has been found to enable a radical improvement in the VCSEL performance. In particular, in this case, the SFM VCSEL operation is possible even in VCSELs with quite large active regions and for much wider ambient-temperature range than in the standard OC VCSELs.     

Picosecond optical switching of a 1.5 μm active birefringent optical fiber loop filter using 1.3 μm control optical pulses and a 1.3 μm semiconductor optical amplifier

Less than 100ps, polarization-independent switching operation of an active birefringent optical fiber loop filter using 1.3 μm control optical pulses as well as a 1.3 μm semiconductor optical amplifier (SOA) has been demonstrated. In the proposed SOA-based active birefringent filter operating at 1.55 μm wavelength, 1.3 μm SOA is employed to control the polarization-mode dispersion in the loop part. By injecting 1.3 μm ps gain-switched optical control pulses into the SOA, 1.5 μm input signals can be switched from the transmission port to the reflection port with less than 100 ps rise time.     

Optimization of electro-optical characteristics of GaAs-based oxide confinement VCSEL

An experimental study has been presented of the oxide-confined vertical-cavity surface-emitting lasers (VCSEL) operating in the 850 nm region of the electromagnetic spectrum. In this regard, various relevant VCSEL samples with numerous oxide aperture sizes have been fabricated and characterized. Thorough investigations of the electrical as well as optical characteristics of the fabricated samples have been performed which includes the overall device performance as a function of the oxidize aperture sizes. It is reported that the VCSEL with oxide aperture size <10 μm require low threshold currents (<1 mA). Further, the differential quantum efficiencies up to 28% were measured for a number of these devices. It is found that devices employing oxide aperture of 10 to 15 μm shows promising electro-optical characteristics for 850 nm oxide VCSEL optimization.     

Verification of evanescent coupling from subwavelength grating pairs

Near-field evanescent wave coupling of various subwavelength grating pairs, using a 1.55 μm infrared semiconductor laser has been demonstrated for use as an optical MEMS sensor. Subwavelength grating pairs were fabricated on both glass and silicon substrates. When coupled together the effective grating period is not subwavelength and can exhibit several diffraction orders. The 1.55 μm infrared source was incident on the coupled pairs and the first-order output intensity was recorded and compared with the output intensity from simulated results. This demonstrated evanescent wave coupling concept can be applied to MEMS systems with nanometer gap separations (e.g., pressure sensors, biosensors, and accelerometers) to allow for subnanometer displacement detection.     

Sensitivity of a VCSEL threshold performance to inaccuracies in its manufacturing

The paper describes an impact of various possible inaccuracies in manufacturing of verticalcavity surface-emitting diode lasers (VCSELs), like thicknesses and compositions of their layers different from assumed ones, on VCSEL room-temperature (RT) continuous-wave (CW) threshold performance. To this end, the fully self-consistent comprehensive optical-electrical-thermal-recombination VCSEL model has been applied. While the analysis has been carried out for the 1.3-μm oxide-confined intra-cavity contacted GaInNAs/GaAs VCSEL, its conclusions are believed to be more general and concern most of modern VCSEL designs. As expected, the VCSEL active region has been found to require the most scrupulous care in its fabrication, any uncontrolled variation in compositions and/or thicknesses of its layers is followed by unaccepted RT CW lasing threshold increase. Also spacer thicknesses should be manufactured with care to ensure a proper overlapping of the optical standing wave and both the gain and lossy areas within the cavity. On the contrary, less than 5% thickness changes in distributed-Bragg-reflectors are followed by nearly insignificant changes in VCSEL RT CW threshold. However, exceeding the above limit causes a rapid increase in lasing thresholds. As expected, in all the above cases, VCSELs equipped with larger active regions have been confirmed to require more careful technology. The above results should enable easier organization of VCSEL manufacturing.     

Optical metamaterials at near and mid-IR range fabricated by nanoimprint lithography

Two types of optical metamaterials operating at near-IR and mid-IR frequencies, respectively, have been designed, fabricated by nanoimprint lithography (NIL), and characterized by laser spectroscopic ellipsometry. The structure for the near-IR range was a metal/dielectric/metal stack “fishnet” structure that demonstrated negative permittivity and permeability in the same frequency region and hence exhibited a negative refractive index at a wavelength near 1.7 μm. In the mid-IR range, the metamaterial was an ordered array of fourfold symmetric L-shaped resonators (LSRs) that showed both a dipole plasmon resonance resulting in negative permittivity and a magnetic resonance with negative permeability near wavelengths of 3.7 μm and 5.25 μm, respectively. The optical properties of both metamaterials are in agreement with theoretical predictions. This work demonstrates the feasibility of designing various optical negative-index metamaterials and fabricating them using the nanoimprint lithography as a low-cost, high-throughput fabrication approach. PACS 42.25.Bs; 81.16.Nd; 42.70.-a; 81.07.-b     

Design and simulation of apodized wavelength filters using Gaussian-distributed sidewall grating

An accurate design for an apodized integrated optical wavelength filter using Gaussian-distributed sidewall Bragg grating is proposed, 2-dimensionally simulated and analyzed using the finite-difference time-domain method. It is verified that for various grating periods, the central wavelengths of the reflection bands are all fixed at designed 1.55 μm with the side lobes well suppressed.     

Spectral and modulation properties of a largely tunable MEMS-VCSEL in view of gas phase spectroscopy applications

An extensive characterization of the spectral properties of a largely tunable laser in the 1.56-μm spectral range is reported. This device combines a vertical-cavity surface-emitting laser (VCSEL) with a micro-machined (MEMS) Bragg mirror in a very compact arrangement. The large tunability obtained by an electro-thermal actuation of the MEMS mirror makes this device very attractive for high-resolution spectroscopy. Relevant laser parameters for the implementation of wavelength modulation spectroscopy techniques in gas sensing, such as tuning and modulation properties, are presented. A preliminary gas spectroscopy experiment performed with this laser is also shown.     

Design of an InGaAs/InP 1.55 μm electrically pumped VCSEL

The design of an electrically pumped InGaAs quantum well based vertical cavity surface emitting laser (VCSEL) on InP substrate is presented. Such optically pumped VCSELs have already been demonstrated. To design electrically pumped VCSEL, three simulations steps are needed: optical simulation gives access to the standing-wave electric field distribution, to design the active region and the Bragg mirrors. Thermal simulation is helpful to design metallic contacts while the energy band diagram is obtained by electrical simulation to design the buried tunnel junction useful for carrier injection. All these simulations are compared to experiment.     

1.55-μm InGaAs/InGaAlAs MQW vertical-cavity surface-emitting lasers with InGaAlAs/InP distributed Bragg reflectors

High-performance InGaAs/InGaAlAs multiple-quantum-well vertical-cavity surface-emitting lasers (VCSELs) with InGaAlAs/InP distributed Bragg reflectors are proposed for operation at the wavelength of 1.55 μm. The lasers have good heat diffusion characteristic, large index contrast in DBRs, and weak temperature sensitivity. They could be fabricated either by metal-organic chemical vapor deposition (MOCVD) or by molecular beam epitaxy (MBE) growth. The laser light-current characteristics indicate that a suitable reflectivity of the DBR on the light output side in a laser makes its output power increase greatly and its lasing threshold current reduce significantly, and that a small VCSEL could output the power around its maximum for the output mirror at the reflectivity varying in a broader range than a large VCSEL does.     

Fabrication and characterization of 1.55 μm single transverse mode large diameter electrically pumped VECSEL

We report on the design, fabrication, and characterization of InP-based 1.55 μm wavelength large diameter (50 μm) electrically pumped vertical external cavity surface emitting lasers (EP-VECSELs). The hybrid device consists of a half vertical cavity surface emitting laser (1/2-VCSEL) structure assembled with a concave dielectric external mirror. The 1/2-VCSEL is monolithically grown on InP substrate and includes a semiconductor Bragg mirror and a tunnel junction for electrical injection. Buried (BTJ) and ion implanted (ITJ) tunnel junction electrical confinement schemes are compared in terms of their thermal and electrical characteristics. Lower thermal resistance values are measured for BJT, but reduced current crowding effects and uniform current injection are evidenced for ITJ. Using the ITJ technique, we demonstrate Room-Temperature (RT) continuous-wave (CW) single transverse mode laser operation from 50-μm diameter EP-VECSEL devices. We show that the experimental laser optical output versus injected current (L–I) curves are well-reproduced by a simple analytical thermal model, consistent with the thermal resistance measurements performed on the 1/2-VCSEL structure. Our results indicate that thermal heating is the main mechanism limiting the maximum CW output power of 50-μm diameter VECSELs, rather than current injection inhomogeneity.     

Analysis of optical crosstalk within optoelectronic integrated circuits including lasers and photodetectors

Optical crosstalk from a 1.3 μm laser to a 1.55 μm photodiode on a single InP substrate, and its suppression within 1.3 μm/1.5 μm Y-junction transceiver OEICs, has been analyzed experimentally. The results indicate that the optical crosstalk suppression is limited by the accumulated light in the OEIC substrate coming mainly from the spontaneous emission of the integrated laser and from stray light at the laser–waveguide butt joint interface. For OEICs, integrating lasers and photodetectors, the achievable optical intra-chip crosstalk at present will be in the range of 30–40 dB at the required small die dimensions. Received: 16 May 2001 / / Published online: 23 October 2001     

Optical and microwave analysis of mushroom-type waveguides for traveling wave electroabsorption modulators based on asymmetric intra-step-barrier coupled double strained quantum wells by full-vectorial method

The finite difference method is exploited for a full-vectorial analysis of mushroom-type waveguides for traveling wave electroabsorption modulators (TWEAM) based on asymmetric intra-step-barrier coupled double strained quantum wells (AICD-SQW). In this analysis, the discontinuities of the normal components of the electric field across abrupt dielectric interfaces which are known as the limitations of scalar and semivectorial approximation methods are considered. The optical field distributions in mushroom-type TWEAM based on AICD-SQW and conventional ridge-type TWEAM of the same active region for 1.55 μm operation are presented. The important parameters in the high-frequency TWEAM design such as optical effective index which defines optical velocity and transverse mode confinement factor are calculated. Then, the transmission line microwave properties (microwave index, microwave loss, and characteristic impedance) of TWEAMs are obtained. The modulation response of mushroom-type TWEAM is calculated using circuit model by considering interaction between microwave and optical fields in waveguide and compared with conventional ridge-type TWEAM. It is found that increasing the width of p-cladding layer with the same i-layer to reduce the resistance in p-i-n mushroom-type waveguide of TWEAM based on AICD-SQW can improve the microwave propagation loss and thus the high-speed electro-optical response.     

Heterogeneous integration of InGaAsP microdisk laser on a silicon platform using optofluidic assembly

Heterogeneous integration of InGaAsP microdisk lasers on a silicon platform is demonstrated experimentally using an optofluidic assembly technique. The 200-nm-thick, 5- and 10-μm-diameter microdisk lasers are fabricated on InP and then released from the substrates. They are reassembled on a silicon platform using lateral-field optoelectronic tweezers (LOET). The assembled laser with 5-μm diameter exhibits a threshold pump power of 340 μW at room temperature under pulse condition. The heterogeneously-integrated InGaAsP-on-Si microdisk laser could provide the much needed optical source for CMOS-based silicon photonics. The small footprint and low power consumption make them attractive for optical interconnect applications. The optofluidic assembly technique enables efficient use of the III–V epitaxial materials in silicon photonic integrated circuits.     

Luminescent properties of semiconductor composite systems composed of erbium triphthalocyanine molecules and a silicon slot structure in the near-infrared region

Photoluminescence spectra of organic semiconductors based on mono-, bis-, and triphthalocyanine containing erbium as a complexing agent have been obtained in the range of 1–1.8 μm. Comparison of the spectral characteristics has shown that erbium triphthalocyanine has the highest photoluminescence quantum yield at a wavelength of 1.5 μm. To enhance this effect, composite materials based on erbium triphthalocyanine and a silicon slot structure have been synthesized, in which an additional increase in the photoluminescence signal near 1.14 μm has been observed. At the same time, no photoluminescence signal has been observed near the wavelength of 1.5 μm. This can be explained by taking into account the interaction of the erbium triphthalocyanine molecules with the adsorption centers of the silicon matrix.     

Growth kinetics and polysilicon formation by aluminium-induced crystallization on glass-ceramic substrates

In this work, we present extended structural properties of poly-Si thin films fabricated by aluminium-induced crystallization (AIC) of amorphous silicon (a-Si) on high-temperature glass-ceramic substrates. The silicon nucleation kinetics on glass-ceramic substrates was investigated by optical microscopy. The crystalline quality of the films was studied by micro-Raman spectroscopy as a function of exchange annealing conditions. By means of electron backscattering diffraction (EBSD), we have analyzed the effect of thermal annealing on silicon grain size and its distribution, intra- and inter-grains defects, and on the grains preferential crystallographic orientation. The optimal thermal annealing condition, allowing 100% crystallized polysilicon large grains with an average grain size of 26 μm and 〈100〉 oriented, acquired a thermal budget of 475°C and 8 h.     

Synchronously pumped femtosecond optical parametric oscillator based on AgGaSe2 tunable from 2 μm to 8 μm

The operation of a continuous-wave mode-locked silver gallium selenide (AgGaSe₂) optical parametric oscillator (OPO) is reported. The OPO was synchronously excited by 120-fs-long pulses of 1.55-μm radiation at a repetition rate of 82 MHz. The 1.55-μm radiation is generated by a noncritically phasematched cesium-titanyl-arsenate (CTA)-OPO pumped by a mode-locked Ti:sapphire laser. The AgGaSe₂-OPO generates signal and idler radiation in the range from 1.93 μm to 2.49 μm and from 4.1 μm to 7.9 μm, respectively. Up to 67 mW of signal wave output power has been obtained. The experimentally determined pulse duration and chirp parameters are in reasonable agreement with results from a numerical model taking into account group velocity mismatch, group velocity dispersion, self phase modulation, and chirp enhancement. Received: 6 August 1999 / Revised version: 4 October 1999 / Published online: 3 November 1999     

Carrier Dynamics and Saturation Effect in (113)B InAs/InP Quantum Dot Lasers

Quantum dot (QD) lasers exhibit many interesting and useful properties such as low threshold current, temperature insensitivity or chirpless behavior. In order to reach the standards of long-haul optical transmissions, 1.55 μm InAs QD lasers on InP substrate have been developed. Based on time resolved photoluminescence (PL) measurements, carrier dynamics behavior is at first investigated. Electroluminescence (EL) results are then shown at room temperature exhibiting a laser emission centered at 1.61 μm associated to a threshold current density as low as 820 A/cm² for a six InAs QD stacked layers. Finally, a rate equation model based on the reservoir theory is used to model both time-resolved photoluminescence (TRPL) and electroluminescence results. It is shown that carrier dynamic calculations are in a good agreement with measurements since the saturation effect occurring at high injected power is clearly predicted. P. Miska: Previously at Laboratoire d’Etude des Nanostructures à Semiconducteurs.     

Mid-infrared ZGP OPO pumped by near-degenerate narrowband type-I PPKTP parametric oscillator

The total pulse energy of the signal and idler in a near-degenerate type-I periodically poled KTiOPO₄ (PPKTP) optical parametrical oscillator (OPO) was spectrally confined within a 2 nm spectral bandwidth at 2.13 μm. This was achieved by using a volume Bragg grating as the output coupler. Both the signal and the idler from the PPKTP OPO were then simultaneously used to pump a mid-infrared ZnGeP₂ (ZGP) OPO. The 2 nm bandwidth was narrower than the ZGP crystal acceptance bandwidth and, thus, made efficient conversion in the second OPO possible. A total slope efficiency of 10% from 1.06 μm to the 3.5–5 μm region was demonstrated, generating 250 μJ in the mid-IR with only 3.6 mJ of 1.06 μm pump energy. This corresponds to a Nd:YAG pump to mid-IR conversion efficiency of 7%. PACS 42.65.Yj; 42.72.Ai; 42.40.Eq     

Designing strategy to enhance mode selectivity of higher-output oxide-confined vertical-cavity surface-emitting lasers

Performance of modern oxide-confined (OC) vertical-cavity surface-emitting diode lasers (VCSEL s) is more sensitive to the construction details than in the case of other VCSEL s. In particular, a stable single-fundamental-mode operation is difficult to be achieved in these VCSEL s especially in higher-output large-size continuous-wave (cw) operating devices at higher temperatures. In the present paper, an operation of OC VCSEL s has been investigated with the aid of the comprehensive fully self-consistent model using the (GaIn)(NAs)/GaAs quantum-well VCSEL with two oxide apertures as a typical example. A new approach is proposed to enhance cw RT single-fundamental-mode low-threshold operation in higher-output OC VCSEL s. One of their oxide apertures should be shifted to the node position of the resonator standing wave where it is working as the electrical aperture only. Then diameters of both apertures may be changed independently giving an additional degree of freedom for VCSEL designing, which enables their optimisation. While the larger-diameter optical aperture placed in the anti-node position creates an efficient radial waveguiding effect, the smaller-diameter electrical aperture enhances a more uniform current injection into the VCSEL active region. Due to combining influence of both the apertures, the single-fundamental-mode operation is predicted in a large device with the 10-m-diameter active region even for 80 K active-region temperature increase over RT of the ambient. An impact of intentional detuning at room-temperature (RT) of VCSEL active-region gain spectrum towards shorter wavelengths with respect to the resonator mode improves mode selectivity is also analysed. PACS  42.55.Px; 02.60.Cb; 85.60.Bt