Viktor Khalfin

High-power high-efficiency 2D VCSEL arrays

We review recent results on high-power, high-efficiency two-dimensional vertical-cavity surface-emitting laser (VCSEL) arrays emitting around 980nm. Selectively oxidized, bottom-emitting single VCSEL emitters with 51% power conversion efficiency were developed as the basic building block of these arrays. More than 230W of continuous-wave (CW) power is demonstrated from a ~5mm x 5mm array chip. In quasi-CW mode, smaller array chips exhibit 100W output power, corresponding to more than 3.5kW/cm2 of power density. High-brightness arrays have also been developed for pumping fiber lasers, delivering a fiber output power of 40W. We show that many of the advantages of low-power single VCSEL devices such as reliability, wavelength stability, low-divergence circular beam, and low-cost manufacturing are preserved for these high-power arrays. VCSELs thus offer an attractive alternative to the dominant edge-emitter technology for many applications requiring compact high-power laser sources.

Efficient vertical-cavity surface-emitting lasers for infrared illumination applications

Infrared illumination is used in the commercial and defense markets for surveillance and security, for high-speed imaging, and for military covert operations. Vertical-cavity surface-emitting lasers (VCSELs) are an attractive candidate for IR illumination applications as they offer advantageous properties such as efficiency, intrinsically low diverging circular beam, low-cost manufacturing, narrow emission spectrum, and high reliability. VCSELs can also operate at high temperatures, thereby meeting the harsh environmental requirements of many illuminators. The efficiency and brightness of these VCSELs also reduce the requirements of the power supply compared to, for example, an LED approach. We present results on VCSEL arrays for illumination applications, as well as results on VCSEL-based illumination experiments. These VCSELs are used in illuminators emitting from a few Watts up to several hundred Watts. The emission of these VCSEL-based illuminators is speckle-free with no interference patterns. Infra-red illumination at up to 1,600ft (500m) from the source has been demonstrated using VCSEL-based illumination, without any optics.

High power 1550 nm distributed feedback lasers with 440 mW CW output power for telecommunication applications

Summary form only given. High power 1550 nm InGaAsP/InP single-frequency distributed-feedback lasers with record output powers are reported. Ex-facet power levels of 440 mW and single-mode fiber power of 340 mW have been achieved for CW operation at 20/spl deg/C.

High-power vertical-cavity surface-emitting lasers for solid-state laser pumping

Vertical-cavity surface-emitting lasers (VCSELs) have emerged as a promising candidate for pumping of solid-state lasers, as they can be configured into high-power two-dimensional arrays and modules of arrays. VCSELs emit in a circular, uniform beam which can greatly reduce the complexity and cost of coupling optics. Their narrow and stable emission spectrum is well suited to the narrow absorption spectrum generally observed for solid-state gain media. The superior reliability of VCSELs greatly enhances the robustness of solid-state laser systems and enables high-temperature operation. In this work, we discuss recent developments on kW-class VCSEL pumps for solid-state lasers. Results on VCSEL modules designed for end-pumping and for side-pumping are presented. More than 4kW in CW operation is demonstrated from a multi-array VCSEL module. We also present results on solid-state lasers using VCSEL modules as pumps. In an end-pumping configuration, more than 250W peak power at 1064nm is demonstrated, and in a sidepumping Q-switched configuration, more than 21mJ at 946nm is demonstrated for an Nd:YAG solid-state laser.

High-power vertical-cavity surface-emitting arrays

We present record output power levels (a few hundred Watts) in continuous-wave (CW) and quasi-CW (QCW) from 2D vertical-cavity surface-emitting laser (VCSEL) arrays, corresponding to power densities exceeding 1kW/cm2 in CW and 3.5kW/cm2 in QCW. These VCSEL arrays emit around 975nm with narrow spectral width (<1nm) and excellent wavelength stability (<0.07nm/K). Peak power conversion efficiency of properly designed arrays exceeds 50%. Additional features of these arrays include emission in a circular, low-diverging beam, and reliable high-temperature operation. These arrays can also be operated reliably in short pulses (<200nsec) at many times their roll-over CW current, making them useful for high-energy applications. VCSEL arrays with 2.2kW peak output power operating under 100nsec pulse-width have been demonstrated.

High-brightness pump sources using 2D VCSEL arrays

Many applications require laser pump sources with high output power (tens to hundreds of Watts) in the smallest spot, with the smallest divergence. Such high-brightness pump sources typically use edge-emitting semiconductor lasers. However, it is also possible to use high-power two-dimensional vertical-cavity surfaceemitting laser (VCSEL) arrays for this purpose. Using a single 976nm 2D VCSEL array chip in an external cavity configuration, combined with a matching micro-lens array, we have demonstrated more than 30W output power from a 50μm/0.22NA fiber, corresponding to a brightness of 10MW/cm2.sr. This represents a substantial reduction in module complexity compared to edge-emitter based modules with similar brightness. These novel high-brightness pump sources exhibit some well-known intrinsic VCSEL performance features such as wavelength stability and narrow spectrum. Power and brightness can be scaled up using polarization and spectral combining.

Modeling of the type-II InGaAs/GaAsSb quantum well designs for mid-infrared laser diodes by k•p method

Different type-II InGaAs/GaAsSb quantum well design structures on InP substrate for mid-infrared emission has been modeled by six band k•p method. The dispersion relations, optical matrix element, optical gain and spontaneous emission rate are calculated. The effects of the parameters of quantum wells (thickness, composition) and properties of cladding layers were investigated. For injected carrier concentration of 5×1012 cm-2, peak gain values around 2.6-2.7 μm wavelengths of the order of 1000 cm-1 can be achieved, which shows that type-II InGaAs/GaAsSb quantum wells are suitable for infrared laser operation beyond 2μm at room temperature.

High-power red VCSEL arrays

High-power red laser sources are used in many applications such as cosmetics, cancer photodynamic therapy, and DNA sequencing in the medical field, laser-based RGB projection display, and bar-code scanning to name a few. Verticalcavity surface-emitting lasers (VCSELs) can be used as high-power laser sources, as efficient single devices can be configured into high-power two-dimensional arrays and scaled into modules of arrays. VCSELs emit in a circular, uniform beam which can greatly reduce the complexity and cost of optics. Other advantages include a narrow and stable emission spectrum, low speckle of the far-field emission, and good reliability. However, developing efficient red VCSEL sources presents some challenges because of the reduced quantum-well carrier confinement and the increased Aluminum content (to avoid absorption) which increases thermal impedance, and also decreases the DBR index contrast resulting in increased penetration length and cavity losses. We have recently developed VCSEL devices lasing in the visible 6xx nm wavelength band, and reaching 30% power conversion efficiency. We fabricated high-power 2D arrays by removing the GaAs substrate entirely and soldered the chips on high thermal conductivity submounts. Such arrays have demonstrated several Watts of output power at room temperature, in continuous-wave (CW) operation. Several tens of Watts are obtained in QCW operation. Results and challenges of these high-power visible VCSEL arrays will be discussed.

High-power vertical-cavity surface-emitting lasers for diode pumped solid-state lasers

Vertical-cavity surface-emitting lasers can be processed in large two-dimensional arrays of single devices to scale up the power for solid-state laser pumping. These arrays emit in a circular, uniform beam, with a narrow and stable emission spectrum that is well suited to the absorption spectra of solid-state gain media. kW-class 808 nm QCW VCSEL pump modules were developed to pump compact Nd:YAG lasers. An end-pumped Nd:YAG laser was constructed that produced 7.1 W average IR power, as well as a dual side-pumped passively Q-switched frequency-quadrupled Nd:YAG laser that generated 0.8 mJ UV pulses at a 240 Hz repetition rate.

High Power 1300 nm Fabry-Perot and DFB Ridge Waveguide Lasers

In this paper we summarize the results on the development of high power 1300 nm ridge waveguide Fabry-Perot and distributed-feedback (DFB) lasers. Improved performance of MOCVD grown InGaAsP/InP laser structures and optimization of the ridge waveguide design allowed us to achieve more than 800 mW output power from 1300 nm single mode Fabry-Perot lasers. Despite the fact that the beam aspect ratio for ridge lasers (30 degree(s) x 12 degree(s)) is higher than that for buried devices, our modeling and experiments demonstrated that the fiber coupling efficiency of about 75-80% could be routinely achieved using a lensed fiber or a simple lens pair. Fiber power of higher than 600 mW was displayed. Utilizing similar epitaxial structures and device geometry, the 1300 nm DFB lasers with output power of 500 mW have been fabricated. Analysis of the laser spectral characteristics shows that the high power DFB lasers can be separated into several groups. The single frequency spectral behavior was exhibited by about 20% of all studied DFB lasers. For these lasers, side-mode suppression increases from 45 dB at low current up to 60 dB at maximum current. About 30% of DFB lasers, at all driving currents, demonstrate multi-frequency spectra consisting of 4-8 longitudinal modes with mode spacing larger than that for Fabry-Perot lasers of the same cavity length. Both single frequency and multi frequency DFB lasers exhibit weak wavelength-temperature dependence and very low relative intensity noise (RIN) values. Fabry-Perot and both types of DFB lasers can be used as pump sources for Raman amplifiers operating in the 1300 nm wavelength range where the use of EDFA is not feasible. In addition, the single-mode 1300 nm DFB lasers operating in the 500 mW power range are very attractive for new generation of the cable television transmission and local communication systems.