Photonic Crystal Vertical-cavity Surface-emitting Lasers Research Articles

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.

Static and Dynamic Properties of Coherently-Coupled Photonic-Crystal Vertical-Cavity Surface-Emitting Laser Arrays

We report progress on phased 850 nm vertical-cavity surface-emitting laser arrays that can be designed to operate reliably through control of individual injection current to each laser element. In this manner, nearly all fabricated arrays can be biased to be mutually phase locked. This paper attempts to visualize and characterize the locking region between two neighboring lasers by measuring far-field visibility to create a two-dimensional visibility map. Experimental data show that the locking region can be shifted or changed by ambient temperature, asymmetric array design, and fabrication imperfections. Additionally, the relative intensity noise and harmonic distortion are measured to characterize the dynamic behavior of coherently coupled laser arrays in the locking region. Two-dimensional maps of total relative intensity noise and total harmonic distortion are created to visualize laser array dynamics in the locking region. The data show that the laser array must be carefully designed, biased, and modulated within the locking region to achieve an output signal with low noise, low distortion, and improved modulation performance.

Self-Heating Analysis of 1570 nm InGaAsP Buried Tunnel Junction Photonic Crystal VCSEL

In this paper, the lattice temperature in an InP-based 1570 nm InGaAsP buried tunnel junction photonic crystal vertical-cavity surface-emitting laser (BTJ-PhC VCSEL) was varied between 280 K until 370 K and its effects on the characteristics of the device was investigated. The temperature profiles of the BTJ-PhC VCSEL are obtained iteratively by considering their temperature-dependent material properties and the spatial distribution of all the significant heat sources. The thermal resistance used to model the electrical contacts causes about 8 K temperature increment above the ambient temperature (300 k) at a bias of 3 V and a 10.865 % increase in the threshold current is observed with temperature increment. This paper provides key results of the device characteristics upon lattice temperature, including the light power versus electrical voltage, the threshold current versus temperature, the wall-plug efficiency and the differential quantum efficiency versus temperature. Furthermore, various elements of heat sources within the active region were analyzed upon the increment of lattice temperature.

Single fundamental mode photonic crystal VCSEL with high power and low threshold current optimized by modal loss analysis**Project supported by the National Basic Research Program of China (Grant Nos. 2010CB934104, 2009CB320300, and 2011CBA00608) and the National Natural Foundation of China (Grant Nos. 61604007, 61378058, 61376049, 61575008, and 61574011).

The characteristics of the photonic crystal vertical cavity surface emitting lasers (PhC-VCSELs) were investigated by using the full vector finite-difference time-domain (FDTD) method through the transverse mode loss analysis. PhC-VCSELs with different photonic crystal structures were analyzed theoretically and experimentally. Through combining the dual mode confinement of oxide aperture and seven-point-defect photonic crystal structure, the PhC-VCSELs with low threshold current of 0.9 mA and maximum output power of 3.1 mW operating in single fundamental mode were demonstrated. Mode loss analysis method was proven as a reliable and useful way to analyze and optimize the PhC-VCSELs.

37-GHz Modulation via Resonance Tuning in Single-Mode Coherent Vertical-Cavity Laser Arrays

We show a significant improvement of modulation bandwidth from $2\times 1$ photonic crystal vertical-cavity surface-emitting laser arrays. Control of injection bias conditions to array elements enables resonance tuning of each element with variation of the phase relation and coherence of the array, resulting in the ability to tailor the modulation response. A bandwidth of 37 GHz is obtained under highly single-mode coherent operation with narrow spectral width and increased output power while the laser array is biased at low current density. Lasers with such performance characteristics may greatly enhance high-rate data transfer in computer server, data center, and supercomputer applications with potentially long device lifetime.

Large mode-volume, large beta, photonic crystal laser resonator

We propose an optical resonator formed from the coupling of 13, L2 defects in a triangular-lattice photonic crystal slab. Using a tight-binding formalism, we optimized the coupled-defect cavity design to obtain a resonator with predicted single-mode operation, a mode volume five times that of an L2-cavity mode and a beta factor of 0.39. The results are confirmed using finite-difference time domain simulations. This resonator is very promising for use as a single mode photonic crystal vertical-cavity surface-emitting laser with high saturation output power compared to a laser consisting of one of the single-defect cavities.

OPTICAL GAIN AND ENERGY BAND STRUCTURE OF PHOTONIC CRYSTAL VCSELs WITH HIGH-INDEX-CONTRAST SUBWAVELENGTH GRATINGS

Photonic crystal vertical-cavity surface-emitting laser structure with incorporation of high-index-contrast subwavelength gratings is fabricated and the three quantum well active region of the structure is investigated with 8 band k.p model. The dispersion of energy sub-bands and wave functions in the conduction bands and valence bands indicate that effective overlapping and interactions between electrons and holes can be realized in the quantum wells and that the maximum spontaneous emission rate and optical gain occurs at the expected wavelength, which is confirmed by the measured photoluminescence spectrum.

Threshold Characteristics Enhancement of a Single Mode 1.55 µm InGaAsP Photonic Crystal VCSEL for Optical Communication Systems

In the present work, we investigate threshold characteristics of a single mode 1.55 μm InGaAsP vertical cavity surface emitting laser (VCSEL) with two different optical confinement structures. The device employs InGaAsP active region, which is sandwiched between GaAs/AlGaAs and GaAs/AlAs distributed Bragg reflectors (DBRs). The optical confinement introduced by the oxide aperture or a single defect photonic crystal design with holes etched throughout the whole structure, is compared with previous work. Photonic crystal VCSEL shows 30.86% and 57.02% lower threshold current than that of the similar oxide confined VCSEL and previous results, respectively. This paper provides key results of the threshold characteristics, including the threshold current and the threshold power. Results suggest that, the 1.55 μm InGaAsP photonic crystal VCSEL seems to be the most optimal one for light sources in high performance optical communication systems.

Honeycomb photonic crystal vertical-cavity surface-emitting lasers: coupled cavities enhancing the single-mode range

A new design is proposed to increase the single-mode current range of low threshold current, photonic crystal vertical-cavity surface-emitting lasers. Our method is based on the optical coupling between the usual central defect and six others of nearly identical sizes. The resulting honeycomb configuration aims to spread higher supermodes in transverse directions, and to reduce their overlap with a central current-injected region. Simultaneous confinement of the fundamental mode within the central cavity and the shift of higher modes toward outer trapping defects can lead to stable single-mode operation. Simulations confirm that either in-phase or out-of-phase coupled supermodes could be promoted by slightly modifying the areas of outer cavities, while thermal lensing always stabilizes the in-phase coupled supermodes. The single-mode current regime should be extended noticeably compared to the conventional single-defect lasers.

Error-Free Transmission Over 1-km OM4 Multimode Fiber at 25 Gb/s Using a Single Mode Photonic Crystal Vertical-Cavity Surface-Emitting Laser

With the separation of optical and current apertures, photonic crystal vertical-cavity surface-emitting lasers can reach a 3-dB small-signal modulation bandwidth of > 18 GHz while lasing in the fundamental mode. Because of reduced chromatic dispersion, such devices enable error-free transmission over 1-km OM4 multimode fiber at a data rate of 25 Gb/s and operating at a current density of 5.4 kA/cm2. This can potentially lead to a laser source that is useful for rack-to-rack transmissions in large data centers and potentially long device lifetime.

Planar Photonic Crystal Vertical-Cavity Surface-Emitting Lasers

Planar index-guided proton-implanted photonic crystal vertical-cavity surface-emitting lasers are fabricated and characterized. Index guiding from the photonic crystal improves the performance of the lasers by creating a stable light output versus current response, reducing the threshold current, and enhancing the differential quantum efficiency. Examination of the etch depth dependence of laser efficiency reveals various mechanisms that affect the laser performance and modal properties such as optical loss, Joule heating, and spectral gain-resonance alignment. Photonic crystal designs can be chosen which result in lasing operation only in the fundamental transverse mode from threshold to maximum power, even for the condition of blue-shifted gain spectrum relative to the cavity resonance. Suitable photonic crystal designs are shown to be manufacturable due to the planar device topology, the use of optical lithography in all processing steps, and compatibility with virtually any vertical-cavity surface-emitting laser epitaxial designs.

III-V-on-Si Photonic Crystal Vertical-Cavity Surface-Emitting Laser Arrays for Wavelength Division Multiplexing

Double photonic crystal mirror vertical-cavity surface-emitting laser (PCM-VCSEL) arrays for the heterogeneous integration on a complementary metal-oxide semiconductor are presented. The trimming of one-dimensional photonic crystal mirrors spectral and modal features allows a fine tailoring of the lightwave which results in compact VCSEL arrays exhibiting a fully controllable dense wavelength distribution. Optically pumped double PCM-VCSELs emit 1.55-μm single-mode beams within the C-band with sub-milliwatt thresholds and spectral purity, making these devices highly suitable for telecommunication-oriented applications. The intrinsic capability exhibited by PCM-VCSEL arrays for a selective addressing of dense wavelength division multiplexing (DWDM) channels suggests a strong perspective potential for several applications such as high-capacity ultra-wideband optical interconnects, passive optical networks, free-space data transmission and sensing.

Demonstration of enhanced side-mode suppression in metal-filled photonic crystal vertical cavity lasers

Previously reported simulations have suggested that depositing thin layers of metal over the surface of a single-mode, etched air hole photonic crystal (PhC) vertical-cavity surface-emitting laser (VCSEL) could potentially improve the laser's side-mode suppression ratio by introducing additional losses to the higher-order modes. This work demonstrates the concept by presenting the results of a 30 nm thin film of Cr deposited on the surface of an implant-confined PhC VCSEL. Both experimental measurements and simulation results are in agreement showing that the single-mode operation is improved at the same injection current ratio relative to threshold.

Photonic crystal VCSELs with narrow spectral width and high bandwidth operating at low current density

A photonic crystal vertical-cavity surface-emitting laser with a spectral width of 0.16 nm and a 3dB small signal modulation bandwidth of 16.2 GHz is demonstrated. Separation of optical and current apertures enables a narrow spectral width and an extremely low operating current density of ∼ 1 kA/cm2 simultaneously. It can potentially be used as a high-reliability laser source that enables error-free transmission over long multimode fibre links at high data rate.

Vertical cavity surface emitting laser transverse mode and polarization control by elliptical hole photonic crystal

The polarization of traditional photonic crystal (PC) vertical cavity surface emitting laser (VCSEL) is uncontrollable, resulting in the bit error increasing easily. Elliptical hole photonic crystal can control the transverse mode and polarization of VCSEL efficiently. We analyze the far field divergence angle, and birefringence of elliptical hole PC VCSEL. When the ratio of minor axis to major axis b/a = 0.7, the PC VCSEL can obtain single mode and polarization. According to the simulation results, we fabricate the device successfully. The output power is 1.7 mW, the far field divergence angle is less than 10°, and the side mode suppression ratio is over 30 dB. The output power in the Y direction is 20 times that in the X direction.

Low Threshold Current Single-Fundamental-Mode Photonic Crystal VCSELs

An optimization of oxide aperture diameter to match the light emission aperture (the central defect region) in the photonic crystal vertical-cavity surface-emitting laser (PhC-VCSEL) is presented in order to obtain low threshold current and high single-fundamental-mode output of the device. A 3-D finite-difference time-domain method has been applied to simulate the mode field distribution and quality factors (Q factors) of fundamental and high order transverse modes which correspond to the mode loss of the PhC-VCSEL cavity. Single-fundamental-mode operation of PhC-VCSEL with 1.7-mW output power and low threshold current of 0.7-mA is demonstrated after oxide aperture optimization.

Hydrogen Detection Using a Functionalized Photonic Crystal Vertical Cavity Laser

A thin layer of palladium (Pd) was deposited on the surface of an etched air hole photonic crystal (PhC) vertical cavity surface emitting laser (VCSEL) to form a hydrogen gas (H2) sensor. H2 reacts with Pd, forming PdHx, which induces a shift in the complex refractive index. With Pd on its surface, the reflectivity of the top distributed Bragg reflector depends on the H2 concentration. We present device operation theory and experimental results showing a 60% output power increase and a 52 pm redshift of the lasing wavelength at 4% H2. Since the PhC VCSEL laser is single mode and the wavelength shift is comparable to the laser linewidth, the sensor can provide an accurate quantification of the H2 concentration over the range of 0-4% H2.

Lowering the threshold current of photonic crystal vertical-cavity surface-emitting lasers

Lowering the threshold current of photonic crystal vertical-cavity surface-emitting lasers

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.

Asymmetric Design and Simulation of Ring-Defect Photonic Crystal Vertical-Cavity Surface-Emitting Lasers

In this letter, the optical modes of a ring-defect photonic crystal vertical-cavity surface-emitting laser are calculated by the fully numerical solution of the scalar Helmholtz equation. We explain the experimentally observed higher order mode emission on the basis of optical loss discrimination. Special photonic crystal patterns that favor even higher order optical modes are designed and simulated. These modes are beyond the scope of previous simplified analysis.