Te Li

High-Power Large-Aperture Bottom-Emitting 980-nm VCSELs With Integrated GaAs Microlens

Microlens-integrated bottom-emitting 980-nm vertical-cavity surface-emitting lasers (VCSELs) with an emitting window aperture of 400 mum have been fabricated. A novel material structure with nine InGaAs-GaAsP quantum wells and slightly decreased reflectivity of n-type distributed Bragg reflectors (n-DBRs) are employed to increase the output power. A convex microlens is fabricated by a one-step diffusion-limited wet-etching technique on the GaAs substrate. The diameter of the active layer is about 200 mum after lateral oxidation, and the nominal diameter of the microlens is 400 mum. The maximum output power is 200 mW at continuous-wave operation at room temperature. The far-field divergence angles thetas|| and thetasperp of the single device at a current of 4A are 8.7deg and 8.4deg, respectively. The optical beam performance between the microlens-integrated VCSEL and ordinary VCSEL is compared.

Design and characterization of a nonuniform linear vertical-cavity surface-emitting laser array with a Gaussian far-field distribution

A 980 nm bottom-emitting vertical-cavity surface-emitting laser array with a nonuniform linear arrangement is reported to realize emission with a Gaussian far-field distribution. This array is composed of five symmetrically arranged elements of 200 microm, 150 microm, and 100 microm diameter, with center spacing of 300 microm and 250 microm, respectively. An output power of 880 mW with a high power density of 1 kW/cm2 is obtained. The divergence angle is below 20 degrees in the range of operating current from 0 A to 6 A. The theoretical simulation of the near-field and the far-field distribution is in good agreement with the experimental result. The comparison between this nonuniform linear array, the single device, and the conventional two-dimensional array is carried out to demonstrate the good performance of the linear array.

Vertical-external-cavity surface-emitting lasers operating at different wavelength: design, numerical simulation, and characteristics

We describe design, numerical simulation and characteristics of high-power optical pumped VECSELs at different wavelength (980nm, and 1300nm). The device design realizes the integrating diode-pumped lasers with vertical-cavity surface-emitting laser structure, drawing on the advantages of both. With periodical gain element structure, optical pumped VECSEL is scalable to watt level output. The characteristics such as threshold condition and output power are calculated theoretically. An optimum number of quantum wells and external mirror reflectivity are obtained from the calculation results, and the thermal characteristic is also considered. Finally the calculation results also predict high output power in this kind of device structure.

High Power VCSEL Device with Periodic Gain Active Region

High power vertical cavity surface emitting lasers with large aperture have been fabricated through improving passivation, lateral oxidation and heat dissipation techniques. Different from conventional three quantum well structure, a periodic gain active region with nine quantum wells was incorporated into the VCSEL structure, with which high efficiency and high power operation were expected. The nine quantum wells were divided into three groups with each of them located at the antinodes of the cavity to enhance the coupling between the optical field and the gain region. Large aperture and bottom-emitting configuration was used to improve the beam quality and the heat dissipation. A maximum output power of 1.4W was demonstrated at CW operation for a 400μm-diameter device. The lasing wavelength shifted to 995.5nm with a FWHM of 2nm at a current of 4.8A due to the internal heating and the absence of active water cooling. A ring-shape farfield pattern was induced by the non-homogeneous lateral current distribution in large diameter device. The light intensity at the center of the ring increased with increasing current. A symmetric round light spot at the center and single transverse mode operation with a divergence angle of 16° were observed with current beyond 4.8A.

Central hole effect on Whispering-Gallery-Mode of Triangular Lattice Photonic Crystal Microcavity

Whispering-Gallery-Mode (WGM) photonic crystal microcavity is a kind of photonic crystal application and can potentially be used for miniaturized photonic devices, such as thresholdless lasers. In this paper we study the WGM of photonic crystal microcavities focusing on the so called H2 cavities which are formed by removing seven air holes. The WGM in these large-size cavities has some advantages compared with single defect WGM in the view of real device applications. We further add a central air hole in the cavity region to analyze the effect on WGM in the microcavity by finite difference time domain (FDTD) and plane wave expansion (PWE). It is found that the tolerance of WGM is large enough for the fabrication of electrical injection structure.

Design and optimization of DBR in 980 nm bottom-emitting VCSEL

According to the theory of DBR, with the P-type DBR as an example, the electrical characteristics and optical reflection of the DBR are analyzed by studying the energy band structure with various graded region widths and doping densities. The width and doping density of graded region are decided through a comparative study. The P-type DBR of 980 nm VCSELs is designed with Al0.9Ga0.1As and Al0.1Ga0.9As selected as the high and low refractive index material for the DBR. The 980 nm bottom VCSELs, which consists of 30 pairs P-type DBR and 28 pairs N-type DBR, are then fabricated. In P-type DBR, the width of graded region is 0.02 μm and the uniformity doping concentration is 2.5×1018cm−3. Its reflectivity is 99.9%. In N-type DBR, the width of graded region is also 0.02 μm and the uniformity doping concentration is 2×1018cm−3. Its reflectivity is 99.3%. The I–V curve shows that the series resistance of the device is about 0.05 Ω. According to the theory of DBR, with the P-type DBR as an example, the electrical characteristics and optical reflection of the DBR are analyzed by studying the energy band structure with various graded region widths and doping densities. The width and doping density of graded region are decided through a comparative study. The P-type DBR of 980 nm VCSELs is designed, with Al0.9Ga0.1As and Al0.1Ga0.9As selected as the high and low refractive index material for the DBR. The 980 nm bottom VCSELs, which consist of 30 pairs P-type DBR and 28 pairs N-type DBR, are then fabricated. In P-type DBR, the width of graded region is 0.02 μm and the uniformity doping concentration is 2.5×1018cm−3. Its reflectivity is 99.9%. In N-type DBR, the width of graded region is also 0.02 μm and the uniformity doping concentration is 2×1018cm−3. Its reflectivity is 99.3%. The I–V curve shows that the series resistance of the device is about 0.05 Ω.

A novel bottom-emitting VCSEL's one-dimension array

A novel 980nm bottom-emitting VCSELs array with high power density and good beam property of Gaussian far-field distribution is reported. This array is composed of 5 symmetrically-arranged elements of 200&μm,150μm and 100μm-diameterμwith the center spacings of 300μm and 250μm respectively. The maximum power is 880mW at a current of 4A, corresponding to 1KW/cm2 average optical power density. The differential resistance is Ω with a threshold of 0.56A. The novel array is compared with a 300μm-aperture-size single device and a 4*4 2-D array with 50μm element aperture size and 250μm centre spacing. The three devices have the same lasing area. The conclusion is that the novel array is better in the property of output power, threshold current, lasing spectra, far-field distribution etc.

Large aperture low threshold current 980nm VCSELs fabricated with pulsed anodic oxidation

Pulsed anodic oxidation technique, a new way of forming current blocking layers, was successfully used in ridge-waveguide QW laser fabrication. We apply this method in 980nm VCSELs fabrication to form a high-quality native oxide current blocking layer, which simplify the device process. A significant reduction of threshold current and a distinguished device performance are achieved. The 500μm-diameter device has a current threshold as low as 0.48W. The maximum CW operation output power at room temperature is 1.48W. The lateral divergence angle θparalleland vertical divergence angle θperpendicular are as low as 15.3° and 13.8° without side-lobes at a current of 6A.

High-power VCSELs single devices and 2-D arrays

The high power bottom-emitting vertical-cavity surface-emitting lasers (VCSELs) and laser arrays emitting at 980 nm are reported. Extensive investigations on size scaling behavior of thermal properties of single devices show limits of attainable output characteristics. The maximum continuous wave (CW) output power at room temperature of single devices with aperture size up to 500 μm is as high as 1.95 W. The key characteristics such as maximum output power, wavelength and thermal resistance are discussed. The bottom-emitting arrays of 16 elements and 200 μm aperture size of individual elements show output power of CW 1.35 W at room temperature. The far-field angle is below 17° for all driving current, which is very favorable for focusing or collimating optics.

Theoretical analysis of 980nm high power vertical external-cavity surface-emitting semiconductor laser (VECSEL)

By using bottom-emitting structure, we will develop laser diode (LD) pumped 980 nm VECSEL with active region of InGaAs/GaAsP/AlGaAs system. Because the thickness of barrier layer and absorption layer exceed that of quantum well, single well approximation model (KP method) can be used to calculate the band structure of VECSEL. The Schrodinger equation of finite deep potential well can be adopted to calculate the energy level structures of electron, heavy and light holes. According to the transition selection rule, we theoretically obtained the emitting wavelength of VECSEL and calculated quasi-Femi energy of valence band and conduction band based on the analysis of energy level structure of electron and holes. By analyzing the gain of strained quantum wells, we calculated the gain of VECSEL using transition matrix elements of electron, heavy and light holes. We give out the threshold gain, output power and other characteristic parameters. We will study the configuration of VECSEL and pumping scheme. We designed external cavity mirror, active region and bottom-emitting structure. A LD-pumped vertical external cavity surface-emitting laser whose output power is greater than 1.0 W can be predicted.