Characteristics Of Vertical-cavity Surface-emitting Lasers Research Articles

Modeling and simulation of VCSELs noise and its influence on noise performance of RF fiber links

Abstract In this paper, we present numerical simulations of the dynamics and noise of directly modulated vertical-cavity surface-emitting lasers (VCSELs) as primary light sources in RF links. Contribution of the VCSEL noise to the noise performance of the RF link is evaluated in terms of the noise figure. The VCSEL characteristics are investigated under a variety of conditions, including CW operation and sinusoidal modulation. Both single and two transverse-mode oscillations of VCSELs are studied. The obtained results show that the frequency spectra of the relative intensity noise exhibit peaks at the mode-competition frequency, the modulation frequency and the higher harmonics. The low-frequency level of relative intensity noise is minimum when the modulated VCSEL signal is continuous and uniform, but is pronounced when the signal is pulsing and non-uniform. The noise level of the two-mode VCSELs is almost 20 dB/Hz higher than that of the single-mode VCSEL. The noise factor of the fiber link decreases with the increase of both the modulation frequency and the depth. The contribution of the VCSEL noise to the noise factor of the fiber link is much greater than that of the photodiode.

Synchronization properties and effects of parameter mismatches in unidirectionally coupled chaotic vertical-cavity surface-emitting lasers

We numerically investigate the effects of parameter mismatches on chaos synchronization in vertical-cavity surfaceemitting lasers (VCSELs). We assume injection-locked chaos synchronization in a unidirectionally coupled and openloop optical feedback system. The accuracy of chaos synchronization is greatly affected by the mismatches of the device parameters and operation conditions between the two lasers. In particular, the oscillation frequency of the laser is one of the important parameters in a system of injection-locked chaos synchronization. However, the variations of the device characteristics of VCSELs are very large compared with those of other types of semiconductor lasers. We study the effects of parameter mismatches related to the oscillation frequency of VCSELs on chaos synchronization. We proved that mismatches in terms of the birefringence and the injection current play crucial roles for the quality of chaos synchronization.

Optical Feedback in Vertical-Cavity Surface-Emitting Lasers

Vertical-cavity surface-emitting lasers (VCSELs) are as sensitive to optical feedback (OF) as edge-emitting lasers as their much higher mirror reflectivity is compensated by the much smaller intracavity round-trip time resulting in a similar OF rate. VCSELs may emit light in multiple transverse modes and may exhibit polarization bistability which adds new features to their behavior when subject to OF. We present a survey on experimental and modeling results for three regimes of OF in VCSELs: long, short, and extremely short external cavities (ECs). We show that for long and short ECs, the Lang-Kobayashi model is in good agreement with experiments on OF-induced polarization switching, mode hopping, and antiphase oscillatory dynamics at the delay time. These last dynamics modify polarization residence-time distribution making it oscillatory. We demonstrate numerically and experimentally coherence resonance in such bistable, time-delayed optical system. We present results on low-frequency fluctuation and pulse-package dynamics in VCSELs and discuss the role of light polarization. We discuss polarization rotating OF for generating high-frequency pulses as a result of EC mode beating. Finally, we demonstrate how OF from extremely short EC can strongly modify VCSEL characteristics and can be used to effectively control their polarization properties by subwavelength changes of the EC length.

Influence of optical losses on the dynamic characteristics of linear arrays of near-infrared vertical-cavity surface-emitting lasers

The effect of the level of internal and external optical losses on the dynamic characteristics of vertical-cavity surface-emitting lasers (VCSELs) in the spectral region of 850 nm is studied. It is shown that an increase in optical losses leads to a decrease in the speed of laser response and to the predominance of thermal effects, while a decrease in losses for the output of radiation brings about an increase in the response speed of the laser and the dominance of damping of the effective modulation frequency. Linear matrix emitters with the 1 × 4 format based on fast-response VCSEL with individual element addressing are produced and studied. Individual laser emitters with a current-aperture diameter of 5–7 μm provide lasing in the continuous-wave mode at room temperature in the region of 850 nm with threshold currents no higher than 0.5 mA, a differential efficiency no lower than 0.6 W/A, a modulation frequency as high as 20 GHz, and a MCEF factor of ∼10 GHz/mA1/2.

Operation performance characteristics of vertical-cavity surface-emitting lasers (VCSELs) under high thermal neutron irradiated fields

To operate and read out even the innermost detectors under any particular conditions, electronics and optical components must be developed accordingly. For semiconductor lasers, on which we will concentrate here, it has been found that an inner temperature increase has a direct impact on the light power emitted by the device. It was found that the effects of radiation on the behavior of semiconductor lasers are convolved with those of temperature. An optimized coupling to the cooling of the laser device reduces the thermal effects in the material. Therefore, a test stand to qualify the effect of heat in the device and the adoption of the heat sink is realized. In this paper, we create a model describing the degradation of the light power and voltage characteristic of a semiconductor-laser undergoing irradiation where the high temperature effects are taken into account. This VCSEL-device model can be used to predict the behavior and operation-performance characteristics (rise time, 3 dB bandwidth, light power, resonance frequency, and transmission bit rate) of a laser being irradiated with different neutron doses. We check the robustness of the model against the high fluence (in excess of 1015 neutrons/cm2). We take into account the study of different semiconductor- and polymeric material-based VCSEL devices such as aluminum gallium arsenide (AlGaAs), indium gallium arsenide phosphors (InGaAsP), and polymeric polymethylmethacrylate (PMMA) under the same operating conditions.

Temperature characteristics of VCSEL with liquid crystal overlay

The 850-nm vertical-cavity surface-emitting laser (VCSEL) with liquid crystal overlay is presented utilizing the birefringence of nematic liquid crystal. Threshold current, peak of optical power and I-P characteristics at different temperatures were studied. At 293 K, the current between the first polarization hop and the second one has an increase of 2.2 mA after overlaying the liquid crystal. At 313 K and 3 mA, the optical power difference between the two orthogonally polarized lights increases from 133.6 to 248.8 μW. The experimental results show that the nematic liquid crystal enlarges the stable range and the discrete of optical power of orthogonally polarized state. Our research could provide insight into the design and fabrication of VCSEL with stable polarization at high temperatures.

Photo-chemical study and optical properties of microtips self- written on vertical laser diodes using NIR photo-polymerization

Near infra-red (NIR) self-guided photo-polymerization is investigated in the context of micro-optics photo-fabrication on VCSELs (Vertical-Cavity Surface Emitting Lasers). We present the optimized process we have developed to allow for a collective fabrication on III-V devices wafers under real-time optical monitoring. The influence of photo-chemical parameters on final micro-elements dimensions is studied for two types of single mode 760 nm VCSELs. The difference of the resulting tip shape between the two lasers is due to the strong differences of their emissions, as they are nicely reproduced by the computed near-field profiles. The tip shapes are also compared to those produced by the light emitted by an optical fiber and differences with VCSEL tips are discussed. Also the VCSEL characteristics with fabricated tips are discussed and found in good agreement with optical modeling.

Circularly Polarized Optical Feedback Effects on the Polarization of VCSEL Emission

This letter investigates experimentally the effects of circularly polarized optical feedback (CPOF) on the polarization characteristics of vertical-cavity surface-emitting lasers (VCSELs). It is shown that CPOF imparts up to 30% circular polarization to the VCSEL emission. The degree of circular polarization acquired by the VCSEL emission is determined by the feedback power ratio and the VCSEL bias current. The effects of COPF have been compared to the influence of other species of linearly polarized optical feedback.

원형 수직 캐비티 표면 광방출 레이저의 모드특성

Abstract The resonance properties of circular vertical-cavity surface-emitting lasers (VCSELs) are studied by using a newly developed equivalent network approach. Optical parameters, such as the stop-band or the reflectivity of periodic Bragg mirrors and the resonance wavelength, are explored for the design of these structures. To evaluate the differential quantum efficiency and the threshold current density, a transverse resonance condition of circular modal transmission-line theory is also utilized. This approach dramatically reduces the computational time as well as gives an explicit insight to explore the optical characteristics of circular VCSELs. Key Words : Circular Modal Transmission Line Theory, Circular VCSELs, Bragg mirrors * 정회원, 한성대학교 정보통신공학과접수일자 2012.1.8, 수정완료 2012.2.8.게재확정일자 2012.2.10Received: 8 January 2012 / Revised: 8 February 2012 /Accepted: 10 February 2012 * Corresponding Author: kwangho@hansung.ac.krDept. of Information and Communication Engineering, Hansung University, Seoul, Korea

Decreasing parasitic capacitance in vertical-cavity surface-emitting laser with selectively oxidized aperture

Results of a comparative study of the structural parameters and the static and dynamic characteristics of vertical-cavity surface-emitting lasers (VCSELs) with microresonators based on Al0.15Ga0.85As and Al0.8Ga0.2As are presented. It is established that the vertical oxidation of layers in the Al0.8Ga0.2As microresonator during formation of the current aperture leads to a significant increase in the oxide thickness. This leads to a considerable decrease in parasitic capacitance of the device and a 1.7- to 2-fold growth in the cut-off frequency of a low-frequency filter formed by parasitic elements of the equivalent electric scheme of the device.

Study on influence of oxide aperture shape on modal characteristics of VCSELs

The effects of oxide aperture shape whose size is small, on the mode characteristics of external cavity oxide-confined photonic crystal vertical cavity surface emitting laser (VCSEL) is studied extensively based on the finite-difference time-domain algorithm. A new three-dimensional model of optical field in photonic crystal VCSEL is built and the influence of the variation in aperture shape on the characteristics of far-field and spectrum is pointed out. It is found that the modal characteristics of PhC-VCSEL are influenced by the shape of oxide aperture, the influence on spectrum feature is more apparent. It can be explained from the distribution of modal field by the fact that the symmetrical characteristic of prismatic oxide aperture is not identical to that of high-order mode. But the influence is less apparent while the depth of photonic crystal hole is deeper. The reason for this matter is illustrated. The results can be used to guide the fabrication of external cavity oxide-confined photonic crystal VCSELs.

Lasing Characteristics of VCSELs Subject to Circularly Polarized Optical Injection

This paper reports experimental investigations of the effects of circularly polarized optical injection on the lasing characteristics of vertical-cavity surface-emitting lasers (VCSELs). The operating characteristics of the target VCSELs are changed using available optical injection control parameters including the strength and polarization state of the optical injection as well as the frequency detuning between the injected light and the target VCSEL. Using circularly polarized optical injection with frequency detuning around -10 GHz, the linear output polarization of the stand-alone VCSEL can be converted into circularly polarized or elliptically polarized emission when the VCSEL drive current is close to or below the threshold current of stand-alone VCSEL. In addition, highly nonlinear light-current characteristics are observed in the optically injected VCSEL in the vicinity of threshold.

Submicron surface relief fabrication technology for epitaxial GaAs structures with thin AlGaAs stop layers

Basic technology for the fabrication of submicron surface reliefs in epitaxial GaAs structures with thin AlGaAs stop layers has been developed using electron-beam lithography. High sensitivity of the characteristics of vertical-cavity surface-emitting lasers (VCSELs) with respect to the level of optical losses allows this technology to be used for controlling the mode composition and stabilizing the polarization of VCSEL radiation.

Vertical-cavity surface-emitting laser with liquid crystal overlay

We perform a theoretical study of the spectral and polarization threshold characteristics of Vertical-Cavity Surface-Emitting Lasers with Liquid Crystal overlay (LC-VCSEL) in three different configurations of the LC cell. Our model predicts the possibility of selecting between two orthogonal directions of linear polarization (LP) of the fundamental mode (x or y LP) by choosing appropriate LC length. It further predicts very strong polarization discrimination with LP mode threshold gain difference as large as several times the threshold gain of the lasing mode. We also numerically demonstrate an active control of light polarization by electro-optically tuning the LC director and show that either polarization switching between x and y LP modes or continuous change of the LP direction would be possible. Finally, we numerically demonstrate that LC-VCSEL would be capable of efficient wavelength tuning.

Investigation polarization characteristics of vertical-cavity surface-emitting lasers

We have improved transmission line laser model to study the polarization characteristics by incorporating into spin-flip model (SFM). Theoretical studies on the influence of the Bragg reflectivity on the polarization switching of vertical-cavity surface-emitting laser (VCSEL) have been discussed in this paper. We have demonstrated that reflectivity of Bragg reflectors can be optimized to provide suitable hysteresis loops width which indicates the tolerance of the input power.

Threshold control in VCSELs by proton implanted depth

The proton implantation is one of key procedures to confine the current diffusion in vertical cavity surface emitting lasers (VCSELs), in which the proton implanted depth and profile are main parameters. Threshold characteristics of VCSELs with various proton implanted depths are studied after optical, electrical and thermal fields have been simulated self-consistently in three dimensions. It is found that for VCSELs with confinement radius of 2 μm, increasing proton implanted depth can reduce the injected current threshold power and enhance the laser temperature in active region. Numerical results also indicate that there are optimal values for current aperture in proton implanted VCSELs. The minimum injected current threshold can be achieved in VCSELs with proton implantation near the active region and confinement radius of 1.5 μm, while the VCSELs with proton implantation in the middle of p-type distributed Bragg reflectors (DBRs) and confinement radius of 2.5 μm can realize the minimum temperature.

High-order circuit-level thermal model of vertical-cavity surface-emitting lasers

A high-order equivalent circuit model of vertical-cavity surface-emitting lasers (VCSEL) is introduced by virtue of the rate equations. This model accounts for both thermal and spatial dependences of VCSEL characteristics. A four-term Bessel-series expansion for the VCSEL carrier number is utilised to achieve good accuracy. A zero-order Laguerre–Gaussian beam is utilised for describing the photo distribution to simplify the computation. A general diode sub-circuit, instead of a complicated polynomial expression, is adopted to model the current–voltage relationship. This model is fast, accurate and scalable. Moreover, it can be readily used in SPICE-like circuit simulators. Numerical examples are included to validate this model and show that this model exhibits much improved accuracy than the previously published models.

40 Gbit/s modulation of 1550 nm VCSEL

The modulation characteristics of vertical-cavity surface-emitting lasers with a buried tunnel junction emitting in the important wavelength regime around 1.55 m are presented. For the first time it was possible to demonstrate bit rates up to 40 Gbit/s also in this waveband. The 3 dB bandwidth measured under continuous-wave driving conditions was higher than 19 GHz. The present devices were found to be somehow sensitive to the bit-pattern length, especially at high temperatures or data rates resulting in some error-flooring. Error-free performance could be demonstrated up to 35 Gbit/s at room temperature and up to 25 Gbit/s at 55°C.

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.

Light-current characteristics of vertical-cavity surface-emitting lasers with external optical feedback

The influence of external optical feedback (OFB) on the light-current characteristics of the vertical-cavity surface-emitting lasers (VCSELs) was investigated theoretically and experimentally. By calculating the OFB sensitivity parameter, the OFB sensibility of the VCSELs was compared with the edge emitting lasers. Based on the compound cavity theory, the light-current characteristic parameters of the VCSELs with external OFB, such as the threshold current and the slope efficiency, were calculated. The experimental results indicated that the threshold current of the VCSELs with different DBR reflectivities decreased to different degrees, accompanied with a decrease of slope efficiency when under 10% feedback ratio of the external OFB, which is in good agreement with the theoretical calculation.