Slab-coupled optical waveguide lasers (SCOWLs) and amplifiers (SCOWAs) are inherently low-confinement structures
with large nearly-circular modes that are easily coupled to optical fibers or collimated for free-space applications.
Recently SCOWL powers have increased to 3 W by increasing the cavity length to 1 cm and improving the heat
removal. SCOWAs are coherently combined using active phase control to achieve a very high-brightness source. Our
coherent beam combining system consists of single-pass amplifiers with angled-facet SCOWAs that suppress feedback.
Single-pass, 5-mm long, SCOWAs have now been demonstrated with 1.5 W CW output with only 50 mW seed power.
Arrays of 47 SCOWAs have demonstrated a raw power of 57 W with 50 mW of seed power per element. A coherent
beam combining demonstration is currently being assembled.
In semiconductor lasers, key parameters such as threshold current, efficiency, wavelength, and lifetime are closely related to temperature. These dependencies are especially important for high-power lasers, in which device heating is the main cause of decreased performance and failure. Heat sources such as non-radiative recombination in the active region typically cause the temperature to be highly peaked within the device, potentially leading to large refractive index variation with bias. Here we apply high-resolution charge-coupled device (CCD) thermoreflectance to generate two dimensional (2D) maps of the facet temperatures of a high power laser with 500 nm spatial resolution. The device under test is a slab-coupled optical waveguide laser (SCOWL) which has a large single mode and high power output. These characteristics favor direct butt-coupling the light generated from the laser diode into a single mode optical fiber. From the high spatial resolution temperature map, we can calculate the non-radiative recombination power and the optical mode size by thermal circuit and finite-element model (FEM) respectively. Due to the thermal lensing effect at high bias, the size of the optical mode will decrease and hence the coupling efficiency between the laser diode and the single mode fiber increases. At I=10Ith, we found that the optical mode size has 20% decrease and the coupling efficiency has 10% increase when comparing to I=2Ith. This suggests SCOWL is very suitable fr optical communication system.
High power fiber lasers have strong potential for use in both commercial and military applications. Improved wall plug efficiency over Nd:YAG and CO2 lasers combined with up to a 10-fold improvement in beam quality, make fiber lasers extremely attractive for industrial applications such as welding and cutting. In military applications, fiber lasers offer a simplified logistic train, a deep magazine limited only by electric power, and a compact footprint, allowing theater defense and self-protection of combat platforms with speed of light engagement and flexible response. Commercial viability of these systems, however, is limited by the availability of compact, cost effective, and reliable diode laser pump sources in the multi-kilowatt regime. The relatively low brightness of diode laser sources has complicated the task of building high power pumps at a reasonable cost. In response to this need, Nuvonyx, Inc. in conjunction with the University of Illinois at Urbana-Champaign, has been developing a new technology for producing high power, single lateral mode devices which do not suffer form the instabilities mentioned above. The waveguide consists of a narrow section, approximately 2 μm wide, which flares to approximately 12 μm wide at the output facet. The flaring of the waveguide increases the gain volume and reduces the optical power density at the facet allowing for higher output power capability. The index guide is defined using an epitaxial process which allows the confinement of the mode to be reduced as the width of the guide expands. Thus, the mode is confined in a single mode waveguide throughout the cavity maintaining stability of the mode to the emitting facet. In November 2002, Nuvonyx, Inc. was awarded a contract with the Air Force Research Lab, Kirtland AFB, Albuquerque, NM, to transition these devices to production quality for use in high-power fiber laser pumps. Partnered with Alfalight, Inc. and the University of Illinois, we have begun initial device fabrication and testing of these devices with the goal of achieving production quality, long lifetime, 50W bars exhibiting stable single lateral mode operation. The goal of this program is to ultimately deliver multi-kilowatt fiber laser pumps and direct diode laser systems for both military and industrial applications. Currently, we are in the process of developing the necessary device growth, processing, and packaging technologies. Several devices have been made and tested yielding promising results. In this paper, we present some of these results along with an examination of the system implications and capabilities of these devices.
Novel waveguide structures are presented that facilitate high power, single lateral mode output in narrow stripe semiconductor lasers. Flared tapered waveguide lasers, fabricated by a metal-organic chemical vapor deposition (MOCVD) selective area epitaxy (SAE), are shown to attain output powers of 650mW with stable single lateral mode beam properties. Novel integrated mode filters, which induce mode selective lateral radiation loss via curvature, or frustration of the index guide, are shown to increase the threshold for the 1st order mode to prevent it from attaining threshold. The addition these unique mode filters, which do not increase the fabrication complexity, extends the range of single lateral mode operation in narrow stripe devices.
The dual-wavelength operation with tunable mode separations for InGaAs-GaAs ridge waveguide DBR lasers are studied. The DBR lasers consist of a common gain section and two separate DBR section. The two DBRs utilize uniform gratings with the same Bragg period, resulting in a single nominal lasing wavelength. Dual-wavelength operation is achieved by simultaneously lasing the front grating mode and the back grating mode. By employing a spacing section between the two DBR gratings and applying a tuning current on it, the interaction between two mode is minimized and the control over the wavelength turning is greatly improved. Three device dimensions were investigated. The relationships between two-mode operation parameters and biasing/tuning conditions for each type are analyzed. The promising applications of this multi-wavelength optical source, such as in optical generation of millimeter-wave, are discussed.