Mode Vertical-cavity Surface-emitting Laser Research Articles

Improved optical performance in circular-grating distributed feedback nanoplasmonic lasers

Optical modes control has been driven from vertical cavity surface emitting lasers (VCSELs) applications with multi-mode operation such as optical fiber communication or optical sensing. Optical gain materials coupling with optical resonators provides an effective way to tuning the optical mode of VCSELs. In this paper, a kind of novel circular-grating distributed feedback (CGDFB) nanoplasmonic laser is designed by using a special semiconductor nanorods array VCSEL coupled optically with a second-order distributed feedback (DFB) circular grating. In the device, the mode field is cooperatively controlled by the confinement of surface plasmon around the nanorod cavity and the spatial tuning with the DFB circular grating. By optimizing the design on the device comprehensively, the CGDFB nanoplasmonic laser exhibits the optimum lasing performance with a single-longitudinal mode peak at 511.2 nm with a side-mode suppression ratio of 45 dB. The 3 dB line-width is compressed to 1.98 p.m. at a driven current of four times of the threshold current of 124 mA. A six-fold rotational symmetric far-field mode corresponding to the third-order Laguerre Gaussian azimuth mode is produced by the DFB circular grating. This work paves a fundamental way for the mode engineering design of novel nanolaser in the technical applications of optical communication, sensing and integrated photonics.

Low Threshold Current and Polarization-Stabilized 795 nm Vertical-Cavity Surface-Emitting Lasers

Low threshold current and polarization-stabilized 795 nm vertical-cavity surface-emitting lasers (VCSELs) are fabricated by integrating a surface grating of high polarization selectivity and high reflectivity. The rigorous coupled-wave analysis method is used to design the surface grating. For the devices with a grating period of 500 nm, a grating depth of ~150 nm, and a diameter of the surface grating region of 5 μm, a threshold current of 0.4 mA and an orthogonal polarization suppression ratio (OPSR) of 19.56 dB are obtained. The emission wavelength of 795 nm of a single transverse mode VCSEL is achieved at a temperature of 85 °C under an injection current of 0.9 mA. In addition, experiments demonstrate that the threshold and output power also depended on the size of the grating region.

Enhanced Performance of Reservoir Computing Using Multiple Self-Injection and Mutual Injection VCSELs

We propose and numerically demonstrate two reservoir computing (RC) schemes using multiple vertical-cavity surface-emitting lasers (VCSELs) with different external injections, i.e., self-injection and mutual injection. The virtual nodes are obtained from the orthogonal polarization modes of multiple VCSELs. We evaluate the performance of our proposed RC systems by using the chaotic time-series prediction and memory capacity (MC). Similar to edge-emitting lasers (EELs), the multiple VCSEL-based RC scheme exhibits better computing performance compared with a single VCSEL using several delay times. More interestingly, numerical results show that our proposed scheme using dual-mode is superior to that using single-mode under the scenario of the same number of virtual nodes, and the scheme using mutual injection VCSELs outperforms that using self-injection VCSELs, in which the coupling mechanism between both polarization modes may play an important role. Further, the effects of different polarized feedback/coupling configurations on both reservoir schemes are discussed in terms of the prediction performance and MC. Additionally, we demonstrate for the first time that the polarization mode competition significantly affects the parallel VCSEL-based RC performance through analyzing its dependence on the feedback phase.

Design and fabrication of 940 nm vertical cavity surface emitting laser single-emitter device

As a key part of vertical cavity surface emitting laser (VCSEL), active region will seriously affect the threshold and efficiency of the device. To obtain the appropriate laser wavelength and material gain, the design of In<sub>0.18</sub>Ga<sub>0.82</sub>As strain compensated quantum well is optimized. The relationship between the lasing wavelength of multiple quantum wells (MQWs) and the thickness is calculated. Considering the influence between the active region temperature and the lasing wavelength, the thickness of the quantum well is chosen as 6 nm, and the quantum barrier thickness is chosen as 8 nm, corresponding to the lasing wavelength of 929 nm. The material gain characteristics of the MQWs at different temperatures are simulated by Rsoft. The material gain exceeds 3300/cm at 300 K, and the temperature drift coefficient of the peak wavelength is 0.3 nm/K. In this work, Al<sub>0.09</sub>Ga<sub>0.91</sub>As and A<sub>l0.89</sub>Ga<sub>0.11</sub>As are chosen as the high- and the low-refractive index material of distributed Bragg reflector (DBR), and 20 nm graded layer is inserted between two types of materials. The influence of the graded layer thickness of DBR on the valence band barrier and reflection spectrum are calculated and analyzed. The increase of graded layer thickness can lead the band barrier peak and the reflection spectrum bandwidth to decrease. The reflection spectrum and phase spectrum of P-DBR and N-DBR are calculated by the transmission matrix mode (TMM): the reflectance of DBR is over 99% and the phase shift is zero at 940 nm. The optical field distribution of the whole VCSEL structure is simulated, in which the standing wave peak overlaps with the active region, and the maximum gain can be obtained. Using the finite element method (FEM), the effect of oxidation confined layer on the injection current is simulated. The current in the active region is effectively limited to the position corresponding to the oxidation confined hole, and its current density is stronger and more uniform. The optical field distributions in different modes of photonic crystal-vertical cavity surface emitting laser (PC-VCSEL) are simulated, and different modes have different resonant wavelengths. The values of quality factor Q in different modes of VCSEL and PC-VCSEL are calculated, Q of the fundamental mode is higher than that of higher transverse mode. It is demonstrated that the photonic crystal air hole structure can realize the output of basic transverse mode by increasing the loss of high order transverse mode. The VCSEL and PC-VCSEL with oxidation hole size of 22 μm are successfully fabricated, in which the photonic crystal period is 5 μm, the air pore diameter is 2.5 μm, and the etching depth is 2 μm. Under continuous current test, the maximum slope efficiency of VCSEL is 0.66 mW/mA, the output power is 9.3 mW at 22 mA, and the lasing wavelength is 948.64 nm at 20 mA injection current. Multiple wavelengths and large spectrum width are observed in the spectrum of VSCEL, which is an obvious multi-transverse mode. The maximum fundamental transverse mode output of PC-VCSEL reaches 2.55 mW, the side mode suppression ratio (SMSR) is more than 25 dB, and the spectrum width is less than 0.2 nm, indicating that the photonic crystal air hole has a strong control effect on the transverse mode, and the laser wavelength is 946.4 nm at 17 mA.

Complex nonlinear dynamics of polarization and transverse modes in a broad-area VCSEL

Lasers can exhibit nonlinear and chaotic dynamics driven by the interaction of multiple lasing modes, and investigating the different scenarios of mode competition and bifurcations of their dynamics is of great interest on a fundamental level as well as in view of applications. We study the dynamics of a broad-area vertical-cavity surface-emitting laser (VCSEL) in solitary continuous-wave operation with a comprehensive investigation of its polarization state, lasing spectra, near-field distributions, and temporal dynamics. Fluctuations at the frequency of birefringence splitting and other frequency components develop in a series of bifurcations. The bifurcations coincide with changes of the transverse lasing modes and/or the polarization state, demonstrating the importance of both the spatial and polarization degrees of freedom for mode competition. As a consequence, the inherent nonlinear dynamics of broad-area VCSELs is significantly more complex than the dynamics of VCSELs with a single spatial mode.

Ultra-narrow linewidth vertical-cavity surface-emitting laser based on external-cavity weak distributed feedback.

We demonstrate an ultra-narrow linewidth vertical-cavity surface-emitting laser (VCSEL) based on external-cavity weak distributed feedback from Rayleigh backscattering (RBS). A single longitudinal mode VCSEL with the linewidth as narrow as 435 Hz and a contrast of 55 dB are experimentally achieved by RBS fiber with a feedback level of RBS signal of -27.6 dB. By adjusting the thermal resistance of the VCSEL from 4.5 kΩ to 7.0 kΩ, the laser wavelength can be tuned from 1543.324 nm to 1542.06 nm with a linear tuning slope of -0.506 nm/kΩ. In the tuning process, the linewidth fluctuates in the range of 553-419 Hz.

Angled Flip-Chip Integration of VCSELs on Silicon Photonic Integrated Circuits

An investigation of angled flip-chip integration of a singlemode 850 nm vertical-cavity surface-emitting laser (VCSEL) on a silicon nitride photonic integrated circuit (PIC) is presented. Using numerical FDTD simulations, we consider the conditions under which the VCSEL can be integrated at an angle over a grating coupler with high coupling efficiency and low optical feedback. With both coupling efficiency and feedback decreasing with increasing angle, there is a trade-off. With co-directional coupling, first-order diffraction loss sets in at a critical angle, which further reduces the coupling efficiency. No such critical angle exists for contra-directional coupling. We also experimentally demonstrate angled flip-chip integration of GaAs-based 850 nm single transverse and polarization mode VCSELs over grating couplers on a silicon-nitride PIC. At the output grating coupler, light is either collected by an optical fiber or converted to a photocurrent using a flip-chip integrated GaAs-based photodetector. The latter forms an on-PIC optical link. We measured an insertion loss of 21.9, 17.6 and 20.1 dB with a singlemode fiber, multimode fiber and photodetector over the output grating coupler, respectively.

High-power single-mode 894\xa0nm VCSELs operating at high temperature (>\u20092\xa0mW @ 365\xa0K)

In this study, we realize the high-power output of a single-mode 894 nm vertical-cavity surface-emitting laser (VCSEL) at high temperature. The effects of the dimensional parameters of oxide aperture and surface relief on the transverse mode and threshold gain of VCSEL were analyzed. Through collaborative optimization of the oxide aperture and relief, the VCSEL with 8 µm oxide aperture diameter and 5 µm surface relief inner diameter can operate at high temperature of 365 K with single-mode output power of 2.02 mW and side-mode suppression of 29.2 dB.

795-nm high-temperature and high-power operating vertical-cavity surface-emitting laser and application in atomic gyroscope

Single-transverse mode vertical-cavity surface-emitting lasers (VCSELs) are preferable optical sources for small low-power atomic sensors, including chip-scale atomic clocks, magnetometers, and gyroscopes.When VCSEL is used as the pump source of nuclear magnetic resonance gyroscope, it is required to have high single-mode output power. Oxide aperture diameter must be sufficiently small (< 4 µm) in a conventional oxide-confined VCSEL to support the fundamental mode alone. However, high series resistance (typically > 200 Ω for GaAs-based VCSEL) from the small aperture limits its output power and reliability due to excessive current-induced self-heating and high current density. It is a very attractive idea to achieve high power operation of an intrinsic single mode VCSEL based on a large oxide aperture by means of epitaxial structure design without introducing additional process steps. Transverse optical confinement in oxide-confined VCSELs crucially depends on the thickness of oxide layer and its position relative to standing wave. Modifying the structure reduces the overlap between the oxide layer and the standing wave as well as the difference in effective refractive index between core and cladding of the VCSEL, thereby reducing the number of transverse modes andincreasing the mode extension beyond oxide aperture. A 795-nm VCSEL is designed and fabricated based on this concept. A cavity structure of VCSEL with gain-cavity detuning of ~10.8 nm at room temperature is adopted in this paper. The effective refractive index and the standing wave distribution of the VCSEL are calculated, and the position of the oxide layer in the epitaxial structure of the VCSEL is optimized according to the standing wave distribution. Finally, the structure with low effective refractive index difference is obtained. The proposed device achieves high single-mode operation of 4.1 mW at 80 ℃, SMSR of 41.68 dB, and OPSR of 27.4 dB. The VCSEL is applied to a nuclear magnetic resonance gyroscope (NMRG) system as pump source due to its excellent device performance, and satisfactory test results are obtained. This paper presents a new method of designing single-mode high power VCSEL and its feasibility is also demonstrated through experimental results.

Targeted defect analysis in VCSEL oxide windows using 3D slice and view

We report on high resolution analysis of vertical cavity surface emitting lasers (VCSELs) to detect and assess defects in sub-surface layers. We employ a focussed ion beam scanning electron microscope (FIB-SEM) to sputter and image successive cross sections (slice and view technique) in order to produce a 3D reconstruction of the oxide aperture region. High resolution images and measurements of the multilayers and oxide apertures of VCSEL devices were obtained. The process took ∼2.5 h and produced over 270 slice SEM images for a device volume of approximately 13.2 × 16.0 × 13.8 μm3, with a voxel size of 50 nm. On-wafer, single mode VCSEL devices with high and low output powers were analysed to compare their oxide apertures and distributed Bragg reflector (DBR) layer structures. It was found that the low output power VCSEL had DBR layer defects and a 41.8% reduction of effective oxide aperture area, explaining the lower power obtained. The results provide evidence that oxide aperture area and structural defects are major factors that affect the optical output power of VCSEL devices. Outcomes in this work show FIB-SEM slice and view is a valuable method for 3D reconstruction of VCSEL devices, which enables top view, cross-sectional view and angled view of the whole device region as well as designated structures such as oxide aperture or structural defects in various layers. This work demonstrates a promising technique with high resolution (50 nm) 3D imaging for analysis of complex semiconductor devices.

Narrow linewidth VCSEL based on resonant optical feedback from an on-chip microring add-drop filter

We demonstrate a narrow linewidth vertical-cavity surface-emitting laser (VCSEL) by injecting resonant optical feedback into the lasing cavity. A single longitudinal mode VCSEL with a Lorentzian linewidth of 32.6 kHz and a purified optical spectrum is experimentally achieved by an on-chip microring add-drop filter with a quality factor of 1.36 million, where the feedback level is ${-}{47.77}\;{\rm{dB}}$. The frequency noise spectrum of the VCSEL demonstrates that the thermo-optic effect in the microring resonator can also stabilize the lasing frequency. A VCSEL with narrow linewidth and stable frequency provides a high-performance light source for a single VCSEL or VCSEL array-based application.

Comprehensive design and simulation of a composite reflector for mode control and thermal management of a high-power VCSEL

We propose a composite top reflector composed of a distributed Bragg reflector (DBR) and a subwavelength high-contrast grating (HCG) for a high-power 808-nm vertical-cavity surface-emitting laser (VCSEL). The DBR and HCG in the reflector are connected by an indium tin oxide (ITO) surrounding layer, which makes it possible for the reflector to improve current injection uniformity and reduce heat generation while providing high reflectivity. The angle-dependent reflectivity of the composite reflector is optimized to suppress the high-order transverse modes of VCSEL while ensuring sufficient fundamental mode feedback. The number of top DBR periods and the thickness of the ITO surrounding layer are optimized to reduce the loss and provide high out-coupling efficiency. The double resonator coupled by top DBR is designed to provide optimal resonant wavelength stability, longitudinal optical confinement factor, and thermoelectric characteristics. Optical simulation results demonstrate that the well-designed configuration can provide a highest fundamental mode reflectivity of 99.7%, an out-coupling efficiency of 65%, a wavelength stability rate of 0.011 with the thickness of the ITO layer, and a confinement factor of 0.05. The transverse modes with order greater than 2 are effectively suppressed. The result of the thermoelectric model shows that the composite reflector-based VCSEL has low operating temperature and uniform current injection; thermal resistance of 0.87 K/mW is realized. In this context, devices with high emission efficiency and beam quality can be expected.

Tailored Transverse Mode Vertical-Cavity Surface-Emitting Laser: Hollow Laser Beam Emission through Circle Aperture

Vertical-cavity surface-emitting lasers that can be controlled to emit hollow laser beam in certain LP21 mode are proposed and fabricated. A circular inverted surface relief with precisely designed diameter and thickness is introduced through theoretical simulation. The fabricated device shows a stable single LP21 mode operation with excellent robustness of mode and power to temperature. Preliminary analysis attribute the excellent performance of the device to the inverted surface relief, leading to hollow near-field spot distribution.

Experimental Investigation on Wideband Optical Frequency Comb Generation Based on a Gain-Switched 1550 nm Multi-Transverse Mode Vertical-Cavity Surface-Emitting Laser Subject to Dual Optical Injection

A scheme for generating wideband optical frequency comb (OFC) is proposed and experimentally investigated based on a master-slave system. In such a system, a tunable optical resource and a distributed feedback semiconductor laser are utilized as master lasers (MLs), and their outputs provide dual optical injection (DOI) into a slave laser (SL) which is a 1550 nm multi-transverse mode vertical-cavity surface-emitting laser (1550 nm-VCSEL) operating at gain-switched state. For the 1550 nm-VCSEL biased at a relatively high current, two low-order transverse modes LP 01 and LP 11 can simultaneously lase and possess comparable powers. After loaded a large signal modulation, the 1550 nm-VCSEL can be driven into gain-switched state, and two separated sub-combs originating from two transverse modes can be generated. Through further introducing DOI to the gain-switched 1550 nm-VCSEL, the two sub-combs can link up to form an overall OFC under suitable operation parameters. The experimental results show that, through selecting optimized operating parameters, the bandwidth of the integral OFC within 10 dB amplitude variation can achieve 150.0 GHz. Additionally, the effects of the wavelengths of the two injection lights on the bandwidth of the overall OFC are also analyzed.

VCSEL-Based Optical Frequency Combs Expansion Induced by Polarized Optical Injection

The present theoretical work studies optical frequency combs (OFCs) associated with both linear polarizations of a gain switching vertical-cavity surface-emitting laser (VCSEL) subject to optical injection of arbitrary polarization. The overall OFC, formed by the two orthogonally polarized sub-combs, can have an expanded optical span in agreement with experiments reported by Prior et al. We show that the maximum optical span is obtained for a certain angle between the linearly polarized optical injection and the direction of emission of the main mode of the free-running VCSEL. We find that this angle decreases as the injection strength increases. We also characterize the maximum expansion of the optical span of the overall comb. For VCSELs with large birefringence splitting the maximum expansion is the optical span of the free-running OFC, while for small birefringence splitting VCSELs the maximum expansion is the value of the birefringence splitting.

Numerical investigations on multi-channel wideband chaotic signal generation by a multi-transverse mode vertical-cavity surface-emitting laser subject to chaotic optical injection.

A multi-channel wideband chaotic signal generation scheme is proposed and numerically investigated based on a slave multi-transverse mode vertical-cavity surface-emitting laser (SL) subject to chaotic optical injection from a master multi-transverse mode vertical-cavity surface-emitting laser (ML) with optical feedback. Taking two low-order transverse modes, LP01 and LP11, as an example for numerical calculations, the simulated results show that under suitable optical feedback both the LP01 and LP11 modes (two-channel) of a ML can be driven into the chaotic states where their bandwidths are relatively narrow at a level about 8 GHz. Further injecting the two chaotic signals into a SL, for the case of the globally chaotic optical injection, the SL can output two-channel chaotic signals with wide bandwidths above 20 GHz under appropriate operation parameters. Moreover, the case of SL with mode-selective chaotic optical injection is also analyzed.

Polarization Multiplexing Reservoir Computing Based on a VCSEL With Polarized Optical Feedback

We propose to realize the parallel tasks processing by reservoir computing (RC) using two polarization-resolved modes in a single vertical-cavity surface-emitting laser (VCSEL). Two feedback cases, parallelly-polarized optical feedback (PPOF) and orthogonally-polarized optical feedback, are considered in the VCSEL. A time series prediction task and a waveform recognition task are employed to examine the parallel tasks processing performance in two polarization-resolved modes of the VCSEL-based RC system. Through numerical simulations, the dependences of parallel tasks processing performance of the VCSEL-based RC system on the feedback strength, injection strength are analyzed carefully and compared for the two feedback cases. Besides, the effects of injection current, frequency detuning, spontaneous emission noise, and the number of virtual nodes are also examined. It is found that, for both feedback cases, good performance of parallel tasks processing in two polarization modes of the VCSEL-based RC system could be obtained in broad parameter regions, and in these regions a better performance is achieved for the PPOF case. This proposed polarization multiplexing RC based on VCSEL permits parallel tasks processing in two polarization modes in a single VCSEL, and hence is interesting and valuable for low power consumption neuromorphic photonics systems.

Real-time 850 nm multimode VCSEL to 1 550 nm single mode VCSEL data routing for optical interconnects

Short-reach optical interconnects among massive serves in data centers have attracted extensive research recently. Increasing capacity, cost and power efficiency as well as wavelength switching between data center network nodes are still key challenges for current optical interconnects. In this work, we experimentally demonstrate the real-time inter-mode optical wavelength switching technique, for high-speed wavelength flexible data center interconnects. A 10 Gbit/s 1 550 nm single mode vertical cavity surface emitting laser (VCSEL) is optically injected and used to control a 10 Gbit/s multimode VCSEL carrier at 850 nm. Results show that a clearly open eye diagram is achieved at back-to-back analysis, implying a successful wavelength switch and error-free operation at 10 Gbit/s. A fully optical wavelength conversion of a multimode VCSEL operation at 850 nm using a single mode VCSEL subject to external optical injection at 1 550 nm is reported. This work opens new perspectives towards the development of a cost effective high-speed real-time inter-band wavelength switching technique between servers and network devices operating at different transmission windows at network nodes, for current and future optical interconnects.

VCSEL-based broadband wavelength converter for ultra-wideband flexible spectrum data routing in optical interconnects

Abstract High-speed real-time wavelength conversion at key data centre network nodes is an emerging fundamental functionality requirement for transparent content resolution, wavelength assignment and routing towards better utilization of network resources under dynamic traffic patterns. In this paper, we experimentally demonstrate the first vertical cavity surface emitting laser (VCSEL)-based broadband wavelength converter for ultra-wide wavelength routing in Datacom. An 850 nm multimode VCSEL is directly modulated with 8.5 Gbps data and successfully transmitted error-free over 100 m OM3 multimode fibre (MMF) with a receiver sensitivity of −16.04 dBm, and a total transmission penalty of 1.14 dB. The received data is then used to run two cascaded single mode VCSELs at 1550 nm within the network integration node, therefore achieving the first reported real-time inter-band wavelength converter to C-band. By exploiting VCSEL wavelength tuneability with changing bias current, we show that the converted wavelengths can be routed over 3.2 nm spectra range for integration into ITU-T 50 GHz flexible grid and beyond. The newly converted wavelengths are successfully transmitted over 24.7 km of G 655 single mode fibre, achieving a minimum receiver sensitivity of −14.28 dBm, with a maximum transmission penalty of 2.72 dB. This work offers a viable enabling development technology for broadband wavelength converters for real-time wavelength routing in optical interconnects to address content resolution and wavelength assignment problem for current and future Datacom.

Stable VCSEL mode discrimination realized by a ring-shaped dielectric relief

We demonstrated a method of transverse mode selection of vertical-cavity surface-emitting lasers (VCSELs). A ring-shaped anti-phase SiO2 layer was introduced to filter the modes whose intensity distribution overlapped with the extra layer. The effective index method considering the size of the oxide aperture was applied to decide the inner/outer radius of the ring. Conventional reference device and two-round of confirmatory studies of ring-shaped SiO2 VCSEL were fabricated and characterized. In comparison with the spectra of the conventional VCSEL, the VCSELs with ring-shaped SiO2 layer emitted two stable transverse modes over the entire current range. This method may offer a new idea to solve the device request for the few-mode optical communications in the future.