Vertical Cavity Surface Research Articles

Photonic convolutional reservoir computing based on VCSEL with multiple optical injections

In this paper, a photonic convolutional time delay reservoir computing (C-TD-RC) system based on vertical cavity surface emitting laser (VCSEL) with multiple optical injections has been proposed. Here, the convolutional neural networks (CNNs) and the time delay reservoir computing (TD-RC) are combined in the system. Using TD-RC instead of the fully connected layer in the traditional CNNs can solve the problem of gradient vanishing and gradient explosion in CNNs. Two classification tasks including handwritten digits recognition and spoken digits classification are solved by the C-TD-RC system. In addition, in order to achieve better classification performance of RC, a new post-processing method combining ridge regression and winner take all strategy has been adopted. Numerical results demonstrate that the minimum Word Error Rate (WER) can reach 0.0135 for handwritten digits recognition and 0.0213 for spoken digits classification tasks by the C-TD-RC system. Furthermore, the effects of the virtual nodes, the injection paths and the number of convolution kernels applied in the first and second convolution layers on the C-TD-RC system are considered. The proposed C-TD-RC scheme is of great significance to the performance enhancement of RC in the future.

Modeling array of MVCSELs With controlled emission direction based on microstructures

This paper presents a novel approach to achieve beam angle control in vertical cavity surface emitting lasers (VCSELs) using microstructures. The proposed microstructured VCSEL (MVCSEL) models are designed and optimized to simulate the effect of various microstructure parameters on beam diffraction. And the MVCSELs with different microstructure period lengths are modeled and their emission angles are characterized. By varying the microstructure period length from 18μm to 1μm, the emission angle of the MVCSELs is found to vary from ±3° to ±70°, resulting in a large scanning range of 134°. Additionally, the vertical divergence angle of these MVCSELs is controlled to be in the range of 0.67° to 0.23°. Integration of these MVCSELs in an array provides a promising option for miniaturization and integration of LiDAR and can be directly used as a transmitter chip for LiDAR applications. These results suggest that the proposed MVCSELs array chip could have significant potential in the development of high-performance and low-cost LiDAR systems.

Improvement of Optical Confinement for Terahertz Vertical-Cavity Surface-Emitting Laser with Square-Lattice Photonic Crystal Structure

A new method proposed to enhance the optical confinement of the terahertz band in a vertical cavity surface emitting laser involves introducing a square-lattice photonic crystal structure. This structure’s filling factor was optimized by computing the energy band structure and optical band values of the photonic crystal. The optimal optical band value is 0.436–0.528 a/λ. At a specific carrier concentration, the real part of dielectric constant of GaAs/AlGaAs materials will gradually increase with the increase of Al elements. By adjusting the length of the resonant cavity, a vertical cavity surface emitting laser with two wavelengths can be created without utilizing current injection. Additionally, the photonic crystal structure’s control effect on the transverse mode of the vertical cavity surface emitting laser and the release effect of the PN junction light confinement were analyzed. Numerical calculations indicated that incorporating a cubic photonic crystal structure in the vertical cavity surface emitting laser resulted in a 2× increase in the difference frequency intensity and a 6.33× increase in the optical field intensity.

Electrical polarization control of VCSELs based on an orthogonal switching liquid crystal layer

Stable polarization has been regarded as a key element for improving the beam quality of vertical cavity surface emitting lasers (VCSELs). However, gain isotropy in the active layer of VCSELs ineluctably leads to the uncertainty of the polarization direction. In this paper, a liquid crystal (LC)-VCSEL laser with polarization control characteristics is designed and fabricated based on the optical rotation effect and electrically controlled birefringence characteristics of orthogonal LC. The experimental results show that, electrically controlled LC could flexibly realize the scheme of polarization stable output and the switching ratio of polarization output could reach 164:1. Based on the electrically controlled birefringence characteristics of LC, the linear polarization angle of VCSEL is further regulated by applying a deflection voltage, and the polarization angle tuning range is between 0° and 90°.

High Slope Efficiency Double and Triple Junction 808 nm Vertical Cavity Surface Emitting Lasers

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.

Ultrahigh Capacity MMWoF Link Using 2-λ VCSEL Front-Haul Transmitter Encoded With Broadband Generalized FDM Data Stream

By employing a dual-wavelength (2-λ) vertical cavity surface emitting laser (VCSEL) directly encoded by novel quadrature amplitude modulation generalized frequency division multiplexing (QAM-GFDM) modulation schemes with optimized data capacity, a 28-GHz millimeter-wave-over-fiber (MMWoF) link is created for fusing the long-reach single-mode-fiber wired network and the beyond fifth generation (B5G) mobile wireless network without using any localized synthesizer and mixer at the remote receiver end. Detuning the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> subcarrier to 2 optimizes the electrical-to-electrical 5-GHz 64-QAM-GFDM link with an error vector magnitude (EVM) of 3.04%, a signal-to-noise ratio (SNR) of 30.3 dB and a bit error ratio (BER) of 2.1×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-13</sup> . By using this condition to perform the long-reach 50-km optical transmission, the 5-GHz 64-QAM GFDM carried by the 2-λ VCSEL exhibits an SNR of 23.8 dB, an EVM of 6.47% and a BER of 2.2×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> . By fixing the optimized <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> subcarriers, the maximal bandwidth and raw data rate of the 64-QAM-GFDM transmission can be achieved to 6.75 GHz and 40 Gbit/s, respectively. For the wireless transmission at the remote end, the photo-mixed MMW carrier at 28-GHz exhibits a peak power of -11.4 dBm and a carrier-to-noise ratio (CNR) of 45 dB. After performing the self-heterodyne down-conversion via power envelope detection, the receiving data amplitude is improved from 26.2 to 132 mV with adding a 3-dB attenuator for impedance matching. Under the 2-m wireless transmission via horn antenna pair, the maximal bandwidth and data rate of the received 4-QAM GFDM data respectively achieve 2 GHz and 4 Gbit/s.

Design and Fabrication of Single-Mode Multi-Junction 905 nm VCSEL With Integrated Anti-Phase Mode Filter

In this paper, a record-high single-mode output power from a multi-junction vertical-cavity surface emitting laser (VCSEL) emitting at 905nm is achieved. Single spectral mode emission of the VCSEL is realized by using an anti-phase mode filter, placed at the top surface of the device, that acts as a relief layer to suppress the higher order transverse modes. Placement of such anti-phase layer at the surface areas in which the higher order modes are expected to emit, leads to reduction in the effective mirror reflectivity at those areas, increased optical losses, and consequently, increased threshold gain for the higher order modes compared to the fundamental mode. The room-temperature light-current-voltage characteristics of a 6-junction 905nm VCSEL with an oxide aperture size of ∼8μm and mode filter size of 5μm shows a single-mode optical power of 12.55mW and maximum power conversion efficiency of 56.48%. Moreover, the emission spectrum of this device shows a stable side-mode suppression ration of >30dB in the single-mode operation regime. The VCSEL remains reliably single-mode at higher temperatures, generating a maximum optical output power of 9.48mW at 60°C. Such remarkable single-mode performance can be mainly attributed to the multi-junction epitaxial structure of the device, in which the output optical power is expected to proportionally increase according to the number of bipolar cascade active regions.

Time-resolved absorption spectroscopy in electron beam melting with blue vertical-cavity surface-emitting lasers.

This paper addresses the challenge of understanding the dynamics of the interaction between partially evaporated metal and the liquid metal melt pool in electron beam melting (EBM), an additive manufacturing technology. Few contactless, time-resolved sensing strategies have been applied in this environment. We used tunable diode-laser absorption spectroscopy (TDLAS) to measure vanadium vapor in the EBM of a Ti-6Al-4V alloy at 20 kHz. Our study includes, to our knowledge, the first-time use of a blue GaN vertical cavity surface emitting laser (VCSEL) for spectroscopy. Our results reveal a plume that is roughly symmetrical with a uniform temperature. Moreover, we believe this work presents the first application of TDLAS for time-resolved thermometry of a minor alloying element in EBM.

Vectorial far-field method for subwavelength grating based VCSELs

We proposed in detail a procedure for the vectorial far-field analysis of high-contrast subwavelength gratings (HCGs) for vertical cavity surface emitting lasers (VCSELs). The vectorial far-field analysis method (VFM) is based on the vectorial angular spectrum method. The transmission and reflection coefficients of the HCG are used to modulate the polarized far-field components, and the modulated far-field components are vectorially composited to obtain the final transmitted and reflected far-field distribution. The reflectance and transmittance of the HCG to a two-dimensional finite-size beam can be solved by the integral of the output beam. The near field of the transmitted and reflected beam can be solved by an inverse Fourier transform of the far-field distribution. The VFM can be effectively applied in the transmit and reflect far-field beam shaping design and polarization analysis of HCG-VCSELs. The near field reconstructed by VFM can be applied to the calculation of coupling efficiency and lens design. The results of VFM are verified by a three-dimensional finite-difference time domain based on the licensed RSoft FullWAVE tool. VFM only needs to calculate a single-period structure, and the response of HCG to various near-field distributions can be obtained by one calculation, which provides an efficient and accurate method for the design of polarized HCG-VCSELs. The VFM reveals the underlying physics of the shaping effect of subwavelength grating on polarized beams and can be easily extended to the far-field analysis of arbitrary periodical meta-surfaces.

Wide‐Wavelength Tunable Mid‐Infrared Lasing Based on Black Arsenic Phosphorus

AbstractMid‐infrared (MIR) semiconductor laser sources are essential for applications in gas detection, infrared imaging, and high‐speed free space optical communications. At present, the dynamic wavelength tunability of MIR lasers based on lead salt, antimonide, and quantum cascade lasers is limited and their compact size as well as heterogeneous integration are still challenging. Two‐dimensional (2D) materials, used as optical gain materials, have the flexible tunability and compatible van der Waals integrations—providing many new possibilities for constructing MIR laser sources of large tunability and high integration. Here, wide‐wavelength tunable MIR vertical cavity surface emitting lasers (VCSELs) integrated on silicon substrates are realized based on 2D black arsenic phosphorus (b‐AsxP1−x). The emission wavelength of the optically pumped laser device is demonstrated to be tuned from 3.42 to 4.65 µm at room temperature, which could be controlled by adjusting the alloy composition and thickness of the gain media b‐AsxP1−x. The tunable MIR VCSEL device would pave the way for further exploration of 2D materials‐based IR lasers working as on‐chip light sources.

VCSEL-based optical transceiver with a 90°-bent graded-index core polymer waveguide coupler for 25.78-Gb/s transmission

In this paper, we propose a low-loss assembly of optical elements [vertical cavity surface emitting laser (VCSEL)/photodiode (PD)] and a 90°-bent graded-index (GI) core polymer optical waveguide for compact high-speed optical transceivers. The air gap between the 90°-bent GI-core waveguide and VCSEL/PD chips are filled with a high-index resin, and we demonstrate a reduction not only of the Fresnel reflection loss but also of the coupling loss due to the lower divergence angle of VCSEL and fiber output beams. In this paper, the core size and NA of the 90°-bent GI-core in the polymer waveguide are redesigned to match the specific launch conditions realized with the gap filled with resins, resulting in theoretical estimates of insertion losses as low as 0.82 dB and 1.38 dB for the waveguides on the transmitter and receiver sides, respectively. Such low insertion losses are obtained not by step-index core but GI core waveguides. After redesigning the structure, 90°-bent GI-core polymer waveguides with a bending radius of 1 mm are experimentally fabricated using the Mosquito method, and we experimentally demonstrate that the insertion loss of the waveguide can be reduced by about 4 dB when the 100-µm air gap is filled with an optical adhesive (na = 1.51). In addition, an error-free 25.78-Gb/s data transmission is successfully demonstrated over a 100-m MMF link using the fabricated VCSEL based transceivers with 90°-bent GI core polymer optical waveguides.

Secondary ion mass spectrometry quantification: Do you remember when a factor of 2 was good enough?

Well, do you remember when a factor of 2 was good enough? Probably, not unless you have been around secondary ion mass spectrometry (SIMS) since the early 1960s, like one of us has been. This paper will give many references back to the early days of quantification when a local thermal equilibrium model was used to obtain the first results that were accurate to within a factor of 2, but only 60% of the time. It only worked for bulk silicate matrixes. People were not even using SIMS on semiconductors in those days. Several early references showing profiles of ion implants in Si will be referenced, but it was not until 1980 that the first paper was published that explicitly showed how to use ion implants as SIMS standards. People were using ion implants as standards before 1980, but only in limited cases, and with no formal published equations specifying how to use them. But, the samples for which ion implants were used as standards were for dilute concentrations in a single matrix, which had a uniform matrix composition with depth. The rest of the paper will show how we at Eurofins EAG Laboratories tackled the problem of quantification of both major and minor elements in nonuniform, multi-element matrices with continuously graded composition changes using point-by-point CORrected-SIMS. These include SiGe heterostructure bipolar transistors, AlGaAs vertical cavity surface emitting lasers, and B plasma-implanted poly-silicon gates.

Dynamics and feedback sensitivity of vertical-cavity surface-emitting lasers with arbitrary optical feedback intensity

Vertical cavity surface emitting lasers (VCSELs) are the key laser sources in the high-performance computers (HPCs) and data centers (DCs). The explosive growth of data traffic in the HPCs and DCs requires increased bandwidths of VCSELs. In this paper, we deduce the Time-Delay (T-D) model considering the multiple round trips of light in VCSELs with an optical feedback cavity to enhance the modulation bandwidth. The T-D model is suitable for the VCSEL with an arbitrary optical feedback intensity. The influences of the round trip time m of light in the optical feedback cavity on the intensity-modulation responses with strong and weak optical feedback are studied. The feedback coefficients considering the multiple round trips of light in the optical feedback cavity and the dispersions of the distributed Bragg reflector are deduced, and are suitable for the arbitrary optical feedback intensity. The feedback coefficients are influenced by the value of m and the optical feedback intensity, and decrease as the value of m under arbitrary optical feedback intensity increases. The feedback coefficients under strong optical feedback are larger than that under weak optical feedback with the same value of m. The theories are useful for the high-speed VCSELs with optical feedback.

High output power, single mode, and TEM00 operation of a multiple gain chip VECSEL using a twisted-mode configuration.

A vertical external cavity surface emitting laser (VECSEL) has been developed for a sodium guide star application. Stable single frequency operation with 21 W of output power near 1178 nm with multiple gain elements while lasing in the TEM00 mode has been achieved. Higher output power results in multimode lasing. For the sodium guide star application, the 1178 nm can be frequency doubled to 589 nm. The power scaling approach used involves using multiple gain mirrors in a folded standing wave cavity. This is the first demonstration of a high power single frequency VECSEL using a twisted-mode configuration and multiple gain mirrors located at the cavity folds.

Simulative study of high speed light modulated measured sources for high optical network transmission efficiency

Abstract This study reported the performance evaluation and simulation of passive optical networks for high transmission rates up to 10 Gb/s for possible distance up to 50 km. Laser rate equations, directly modulated laser measured and vertical cavity surface emitting laser (VCSEL) are employed for the measurement of the network performance. Different bit sequences are suggested to show the network performance parameters with various proposed optical sources. Optical and electrical powers are measured for 50 km network reach. In addition to max. Q factor is also measured for various optical sources and bits sequence variations. Laser rate equations are employed in this network to upgrade the Q factor criteria for maximum network reach of 100 km in the downstream direction.

Impact of thermal oxidation uniformity on 150 mm GaAs- and Ge-substrate VCSELs

Vertical cavity surface emitting laser (VCSEL) devices and arrays are increasingly important in meeting the demands of today’s wireless communication and sensing systems. Understanding the origin of non-uniform wet thermal oxidation across large-area VCSEL wafers is a crucial issue to ensure highly reliable, volume-manufactured oxide-confined VCSEL devices. As VCSEL wafer diameters approach 200 mm, germanium (Ge) is emerging as an alternative substrate solution. To this end, we investigate the uniformity of 940 nm-emitting VCSEL performance across 150 mm diameter GaAs- and Ge-substrates, comparing the oxidation method in each case. Nominally identical epitaxial structures are used to evaluate the strain induced wafer bow for each substrate type with Ge exhibiting a reduction of over 100 μm in the peak-to-valley distortion when compared with GaAs. This wafer bow is found to be the principal cause of centre-to-edge oxidation non-uniformity when utilising a conduction-heated chuck furnace, in comparison to a convection-heated tube furnace. Using on-wafer testing of threshold current, differential resistance, and emission wavelength, device performance is demonstrated for the first time across a 150 mm Ge wafer, and is shown to be comparable to performance on GaAs substrates, when the effects of oxidation uniformity are removed. These results provide evidence that there is a realistic path to manufacturing high yield VCSELs, over wafer diameters approaching those used in Si-photonics, via Ge substrates.

High minority carrier mobility and electronic diffusion length in annealed GaInAsSb/GaSb active layer in Vertical Cavity Surface Emitting Laser (VCSEL)

Owing to its important role as an active layer for Vertical Cavity Surface Emitting laser (VCSEL). We studied the transport properties in annealed GaInAsSb /GaSb by means of PTD technique. We remark that annealing time improve significantly the diffusion length and minority carrier mobility. For an annealed time duration of 60 min the diffusion length reach 25 µm (3.9%) and the minority carrier mobility reach 3673 cm2/V. (3.4%).We have observed that surface recombination velocity (SRV) decreased in respect of annealing time and was found to be 1450 m / s (± 3.6). Unfortunately, interface recombination velocity (IRV) is highly increased from 1196 m / s (± 5) to 6000 m/s (5%) for GaInAsSb as grown to that annealed during 60 min.

Experimental and theoretical investigations on the modulation capabilities of a sample of vertical cavity surface emitting laser diodes for atomic vapour applications

This paper discusses various frequency modulation and intensity modulation capabilities within a sample of direct laser current modulated identical vertical cavity surface emitting laser diodes. The presented analysis is based on measurements of the spectral amplitudes as a function of the applied modulation power at a constant modulation frequency of 3.517 GHz. Their evaluation by Bessel function fits produces the three modulation parameters: frequency modulation index, intensity modulation index (via the alpha parameter) and the side band asymmetry of the first order side bands. The variation of the laser diode’s modulation capability is discussed. It is found that the individual laser diodes show a significant variations in their modulation capabilities. Thus an experimental preselection of the laser diode is required to find laser diodes which are suitable as a light source for atomic vapour applications, with special emphasis on a coherent population trapping-based scalar magnetometers.

Stabilisation of spatially periodic states by non-Hermitian potentials

We uncover new families of stable periodic solutions by the introduction of non-Hermitian potentials in the universal complex Ginzburg–Landau equation. We perform a comprehensive analysis on the dynamics and stability of the system by determining and following these new solutions for a one-dimensional system, and demonstrate that the results hold for higher spatial dimensions and for the corresponding complex Ginzburg–Landau fractional order equation. We prove the robustness of the stabilisation within a broad range in parameter space. The universality of the CGLE allows extending these results to different actual systems described by other specific models. In particular, we provide results on the stabilisation for Vertical Cavity Surface Emitting Lasers.

Leader-laggard synchronization of polarization chaos in mutually coupled free-running VCSELs.

We systematically study the leader-laggard synchronization of polarization chaos in mutually coupled free-running vertical cavity surface emitting semiconductor lasers in two cases of parallel and orthogonal injection. Specifically, we quantitatively investigate the effect of critical external parameter mismatch such as the coupling intensity and frequency detuning on the leader-laggard relationship utilizing the cross-correlation function. When the difference between two main cross-correlation peak values exceeds 0.1, the leader-laggard relationship can be viewed to be stable. Our results demonstrate that compared with the coupling strength, the frequency detuning is the dominant factor in determining the stability of the leader-laggard relationship. The exchange of the leader-laggard role occurs within a frequency detuning region from -5 GHz to 5 GHz for both parallel and orthogonal injection. Once the leader-laggard relationship is stable, the difference between the two cross-correlation values can reach 0.242 for negative frequency detuning, but the corresponding value is only 0.146 under positive frequency detuning.