Characteristics of ARROW VCSELs under external optical feedback

The generation mechanism of self-sustained pulsation in vertical cavity surface emitting lasers is analyzed theoretically. The influence of self-focusing, diffraction loss as well as optical feedback is taken into consideration. It is shown that the condition of self-sustained pulsation can be modified significantly by optical feedback. In addition, it is possible to obtain self-sustained pulsation in vertical cavity surface emitting lasers by using external mirror.

We present a numerical study of the effects of optical feedback on the static and dynamic characteristics of vertical-cavity surface-emitting lasers (VCSELs) under both single-mode and two-mode operations. Our model includes transverse effects such as carrier diffusion and spatial hole burning (SHB) and is therefore capable of including gain-saturation-induced coupling among transverse modes. For short external cavities (/spl sim/30 /spl mu/m), optical feedback can completely suppress higher-order transverse modes. For relatively long external cavities (/spl sim/1 cm), VCSELs follow a period-doubling route to chaos. Numerical results show that multimode VCSELs exhibit self-pulsations at higher feedback levels compared with the single-mode case.

This study introduces an accurate non-linear distortion analysis for vertical cavity surface emitting lasers (VCSELs) based on Volterra series approach in directly modulated scheme. Closed-form expressions of the optical response of VCSELs in terms of modulation response, harmonic distortions, and second- and third-order intermodulation distortions are derived. The impact of external optical feedback on the VCSEL's response and intermodulation distortion is investigated. Moreover, the suggested non-linear model avoids the numerical solution of laser rate equations and provides in detail a non-linear distortion analysis of VCSELs.

The round-trip operator technique is widely used for dispersionless optical resonators beginning from pioneering studies of Fox and Li. The resonator modes are determined as eigenfunctions of the round-trip operator and may be calculated by means of numerical linear algebra. Corresponding complex eigenvalues determine the wavelength shifts relative to reference value and threshold gains. Dispersion elements, for example, Bragg mirrors in a vertical cavity surface emitting laser (VCSEL) cause a dependence of the propagation operator on the wavelength and threshold gain. We can determine the round-trip operator in this case also, but the unknown values of the wavelength and threshold gain enter into the operator in a complicated manner. Trial-and-error method for determination of the wavelength shifts and the threshold gains is possible but it is rather time consuming method. The proposed approximate numerical method for calculation of resonator modes is based on the solution of linear eigenvalue problem for the round-trip operator with reference wavelength and zero attenuation. The wavelength shifts and threshold gains can be calculated by simple formulae using the eigenvalues obtained and the computed effective length of the resonator. Calculations for a cylindrical antiresonant-reflecting optical waveguide (ARROW) VCSEL are performed for verification of the model.

Random source is important for the security of key distribution. In this paper, a novel secure key distribution scheme based on unidirectional injection of vertical cavity surface emitting laser (VCSEL) system is proposed. In the proposed scheme, a chaotic signal without time delay signature is generated by a VCSEL subject to unidirectional optical injection, which is regarded as a master laser. The chaotic signal generated by the master VCSEL is further injected into two slave VCSELs to obtain synchronized bandwidth-enhanced chaotic signals. After that, by sampling, quantizing and XOR operation on the two synchronized chaotic signals, two key streams can be obtained. Based on the well-known spin-flip model, the time delay signature of chaotic signals generated by master VCSEL and the synchronization performance between the master VCSELs and two slave VCSELs are numerically investigated in detail. It is shown that by the unidirectional injection, the chaotic outputs can be achieved in the master VCSEL in a wide range of frequency detuning and coupling strength. More importantly, no time delay signature can be observed in the auto correlation function of the chaotic intensity time series generated by the master VCSEL. Besides, we find that high quality synchronization is achieved between the bandwidth-enhanced chaotic signals generated by two slave VCSELs under the common driving of master VCSEL. The synchronization coefficient between two slave VCSELs increases up to 0.99, and the synchronization coefficient between master VCSEL and salve VCSEL is only 0.74. Note that such a high quality synchronization between two slave VCSELs while relatively low quality synchronization between the master and slave VCSEL is conducible to ensuring the security of key distribution. In addition, the effects of tunable parameters on key bit error rate are considered, and two quantization methods are employed for comparison. Numerical simulation results show that the key bit error rate between two legitimate users is as low as 1%, and the key bit error rate between legitimate user and eavesdropper is higher than 10% in the single-threshold case; the bit error rate can even be as low as 10-6 in the double-threshold case. The influence of parameter mismatch on key bit error rate is also discussed, and it is suggested that two salve VCSELs should be finely matched to ensure low bit error rate. Finally, NIST randomness test is performed for the generated key streams. Hence, the proposed scheme enhances the security of key distribution, which is valuable for further developing the chaos communication systems.

Remarkable progress made in performance characteristics and reliability of high-speed (> 10 GHz) 850-nm multi-mode (MM) oxide-confined vertical cavity surface emitting lasers (VCSELs) during the last decade has led them to find applications in space satellite systems. The main advantage of deploying high-speed VCSELs in space satellites over directly modulated 850-nm edge emitting lasers is the absence of COMD (catastrophic optical mirror damage). In recent years, leading VCSEL manufacturers introduced higher speed (~ 20 GHz or 25 Gbps) VCSELs with encouraging characteristics. However, little has been reported on reliability and failure modes of these state-of-the-art VCSELs although it is crucial to understand failure modes and degradation mechanisms in these VCSELs through physics of failure investigation and subsequently develop VCSELs that exceed lifetime requirements for space satellite systems. For the present study, we performed short-term and long-term accelerated life-tests on 25 Gbps oxide-confined MM VCSELs to study reliability of these devices. Our goal is to extract credible reliability model parameters (thermal activation energy and current exponent factor) from these life-tests to determine suitability of these lasers for future space satellite systems. We also performed failure mode analysis on VCSELs at different stages of degradation using various techniques. We employed nondestructive techniques including optical beam induced current (OBIC) and electron beam induced current (EBIC) techniques as well as destructive techniques including focused ion beam (FIB) and high-resolution TEM techniques. Our detailed reliability and failure mode analysis results are reported along with our understanding on the physical origin of degradation in high-speed VCSELs with strained InGaAs quantum wells.

The high frequency modulation effects on the relative intensity noise (RIN) characteristics of 405 nm InAlGaN laser diodes have been investigated taking into account the strain and thermal effects on the laser diode itself as well as external optical feedback effects. The simulation results indicate that the critical external optical feedback for –125 dB/Hz RIN is around 0.1%, a typical order found in practical DVD operating environments.By optimizing the injection current levels and its frequency, one can reduce the RIN with 0.1∼1% external feedback effect by more than 15 dB/Hz. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Vertical cavity surface emitting laser (VCSEL) is an important laser source for their evident plentiful applications in optical communication. The present investigation reports a comparison of the modeling and optimization of long wavelengths 1310 nm and 1550 nm high speed short cavity VCSEL for continuous wave operation at various temperature (283-3230K) for various diameters. The continuous wave lasing is demonstrated for the device diameter from 2 to 5 μm with threshold current of 1.07-1.33 mA with threshold power consumption of 1.86-2.57 mW for 1310 nm and threshold current of 0.94-1.24 mA and threshold power consumption 1.67-2.1 mW for 1550 nm VCSEL. The results demonstrate that the threshold current, peak emitted power and power consumption increases with the increase in device diameter. The results confirm that VCSELs with 2 μm diameter is most suitable to achieve energy-efficient operation. Although rollover current increases with the diameter, but, due to the advantage of lower threshold current and power consumption, VCSEL having smaller diameter is best suited. The power conversion efficiency for proposed long wavelength VCSELs is approximately 50% which is extremely useful for low power applications. The proposed VCSELs are suitable for very short reach (<2 m) optical interconnects such as chip-to-chip and board-to-board communication in high performance computers.

We numerically investigate how polarization properties of vertical cavity surface emitting lasers (VCSELs) are affected by optical feedback from an extremely short external cavity. In order to do it we use a two modes rate equation model that accounts for infinite round trips in the external cavity. With it we perform maps of bistability finding out a modulation of the polarization switching currents when increasing the external cavity length with a period equal to half the wavelength of operation of the device. When the external mirror reflectivity is high enough there is a region within each period of modulation where the VCSEL polarization is stable at any injected current within the operation range. Moreover, using polarized feedback the map of bistability is channelled and the bistable region is almost suppressed. A preliminar study of the effects of temperature variations on the map of bistability is also carried out and presents polarized feedback as a more robust technic to obtain polarization stabilization. An experimentally obtained map of bistability supports the model showing very good agreement with it.

Tunable vertical cavity devices including an air-gap integrated in the cavity have been designed, fabricated, and investigated. The ultrawide wavelength tuning is realized by micromechanical actuation of Bragg mirror membranes. Based on optical and mechanical model calculations, the air-gap filters and vertical cavity surface emitting lasers (VCSELs) are designed for investigating mainly the optical tuning efficiency. In our research, we focus on two different mirror material systems, dielectric Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> /SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and InP/air-gap Bragg mirrors and on two tuning concepts, respectively. For the dielectric mirrors, continuous tuning is achieved by thermal actuation of the Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> /SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> mirror membranes, and for InP/air-gap mirrors, electrostatic actuation of the InP membranes is used. To verify the optical and mechanical simulations, InP/air-gap filters are characterized by measuring reflectance spectra and the tuning behavior. The measured results agree with the simulations used to optimize the micromechanical and optical characteristics of air-gap filters and VCSELs for optical communication applications.

We introduced the vertical cavity surface emitting laser (VCSEL) as the laser diode in tour external cavity system. Because VCSELs are now commercially available, and the External cavity diode laser (ECDL) systems using them are expected to improve their frequency stability, we have replaced a Fabry-Perot type laser diode with a VCSEL, and examined its oscillation-frequency stability. Therefore we were able to expect that the VCSELs with our double optical feedback system have good oscillation frequency stability. The obtained VCSEL's oscillation-frequency stability, i.e., the square root of Allan variance &sigma; was 4&times;10^-10, at an averaging time of &tau;=1 sec.

Multimode VCSEL(Vertical Cavity Surface Emitting Laser) coupling into MMF (Multimode Optical Fiber) and its influence on MMF link has been numerically studied. Considering the transverse misalignment between VCSEL and MMF, coupling efficiency between VCSEL higher-order modes and modes of MMF is calculated and the Mode Power Distribution in the MMF is obtained. Simulation of MMF link with multimode VCSEL indicates that the higher-order VCSEL modes lead to decreasing of the eye opening of received signal. The transverse misalignment of VCSEL between MMF greatly deteriorates the eye opening. With 300m OM3-MMF link , the EOP (eye-opening-penalty) is greater than 14dB when the misalignment between VCSEL and MMF is lager than 5&mu;m.

Improved carrier confinement and stimulated recombination rate in GaN-based vertical-cavity surface-emitting lasers with buried p-AlGaN inversion layer

This paper is going to review the state-of-the-art of the high-speed 850/940-nm vertical cavity surface emitting laser (VCSEL), discussing the structural design, mode control and the related data transmission performance. InGaAs/AlGaAs multiple quantum well (MQW) was used to increase the differential gain and photon density in VCSEL. The multiple oxide layers and oxide-confined aperture were well designed in VCSEL to decrease the parasitic capacitance and generate single mode (SM) VCSEL. The maximal modulation bandwidth of 30 GHz was achieved with well-designed VCSEL structure. At the end of the paper, other applications of the near-infrared VCSELs are discussed.

High-performance 808 nm double and triple-junction vertical cavity surface emitting lasers (VCSELs) are designed and investigated. The light-current-voltage (L-I-V) characteristics of 808-nm single, double, and triple-junction VCSELs are compared. In the paper, the double-junction VCSELs with two oxide apertures exhibit a very high slope efficiency (SE) of 2.32 W/A at 25 °C, and the peak SE of 2.51 W/A is achieved at 10 °C. The power conversion efficiency (PCE) of 53.14% is achieved for double-junction individual VCSEL emitter. The peak SE of the triple-junction VCSELs reach 3.40 W/A with the PCE of 53.02% for individual VCSEL emitter. Under narrow pulse test conditions (400 Hz, 60 ns pulse width), the peak output powers of the designed double-junction and triple-junction VCSEL arrays reach 216.11 W and 325.77 W under the injection current of 80 A, respectively. Such high-performance multi-junction 808-nm VCSELs are very important for pumping solid state lasers and medical aesthetics.