Publications
Volume 3, Number 3
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Laser Development at Lincoln Laboratory Since the early 1960s, Lincoln Laboratory has been a major contributor to the science and technology of lasers. Key accomplishments include semiconductor lasers for fiber communications and spectroscopy, wavelength-tunable ionic solid state lasers, and techniques for achieving spectral purity and frequency stability in semiconductor lasers as well as gas lasers. Coupled to this work on laser technology have been numerous fundamental studies in quantum electronics and materials on the one hand and applications in communications and radar on the other. Two-Dimensional Surface-Emitting Arrays of GaAs/AlGaAs Diode Lasers Hybrid and monolithic two-dimensional surface-emitting arrays of GaAs/AlGaAs diode lasers have been designed and fabricated. The hybrid devices consist of linear arrays of edge-emitting graded-index: separate-confinement-heterostructure singIe-quantum-well (GRIN-SCH-SQW) lasers mounted on a Si substrate containing integral 45° deflecting mirrors and microchanneIs for the flow of cooling fluid. With this design, CW output powers greater than or equal to 120 W/cm2 have been achieved. For quasi-CW operation (150 μs pulses), peak output powers greater than 400 W/cm appear to be achievable. Two types of monolithic arrays—the first of edge-emitting lasers with external-cavity deflecting mirrors adjacent to the laser facets and the second with intracavity 45° deflecting mirrors—have also been fabricated and tested. Microlens Integration with Diode Lasers and Coherent Phase Locking of Laser Arrays Novel submillimeter-size lenses have been fabricated in compound semiconductor substrates by mesa etching and heat treatment. These precision large-numerical-aperture microlenses are well suited for diode lasers and laser arrays in miniaturized systems. A laser array has been efficiently coupled to an external cavity by a microlens array to achieve coherent phase locking. As part of a semiconductor substrate, these microlenses have the potential for monolithic integration with diode lasers and detectors to create reliable integrated optoelectronic systems. III-V Diode Lasers for New Emission Wavelengths Two types of III-V diode lasers have been developed for new emission wavelengths. We have obtained emission at 0.9 to 1.0 μm from quantum-well lasers with a strained InGaAs active layer and AlGaAs confining layers. Organometallic vapor phase epitaxy (OMVPE) was used to grow the layers on GaAs substrates. These InGaAs/AlGaAs lasers have achieved threshold current densities as low as 65 A/cm2, differential quantum efficiencies as high as 90%, and, for devices 300 μm wide and 1000 μm long, continuous output powers up to 3.2 W and power efficiencies as high as 47%. We have obtained emission at 2.27 μm from lattice-matched double-heterostructure lasers with a GaInAsSb active layer and AlGaAsSb confining layers gtown by molecular-beam epitaxy (MBE) on GaSb substrates. These GaInAsSb/AlGaAsSb lasers have exhibited threshold current densities as low as 1.5 kA/cm2, differential quantum efficiencies as high as 50%, and pulsed output powers as high as 1.8 W. These efficiencies and power values are the highest ever reported for room-temperature operation of semiconductor lasers with emission wavelengths >2 μm. Emission from 1.8 to 4.4 μm can potentially be achieved by changing the GaInAsSb composition. Diode-Pumped Solid State Lasers The use of diode lasers instead of flash lamps as optical pump sources for solid state lasers offers significant advantages such as higher efficiency and longer lifetime. We have demonstrated three novel lasers based on this technology. The first is a zigzag slab laser pumped by hybrid planar microchannel-cooled diode arrays that allow high-repetition-rate operation in a pulsed mode. The second is an end-pumped laser that uses multiple diode lasers for power scalability while maintaining high efficiency and good beam quality. The third is a Yb:YAG laser, pumped by strained-layer InGaAs diode lasers, that offers advantages over AlGaAs-pumped Nd:YAG lasers. These advances should lead to lower-cost higher-power solid state lasers. Microchip Lasers Lincoln Laboratory has developed tunable, single-frequency microchip lasers fabricated from Nd-doped solid state crystals. Diode-laser-pumped Nd:YAG microchip lasers have linewidths of less than 7 kHz at center frequencies of 1.064 and 1.319 μm, and have operated in a single-frequency, single-polarization, fundamental transverse mode at output powers in excess of 50 mW. These lasers have been piezoelectrically tuned over a range of plus/minus 3OO MHz, with a flat-band tuning response of 0.6 MHz/V drive frequencies up to 300 kHz. Nd:YAG microchip lasers have also been Q-switched to produce output pulses as short as 6 ns; much shorter pulses are possible. Titanium Sapphire Lasers In 1982, researchers at Lincoln Laboratory operated a tunable laser based on Ti:Al203 for the first time. A wide variety of developments in Ti:Al203 laser technology then followed the advances in crystal growth that occurred during the mid-1980s. Since that time researchers have demonstrated high efficiency, wide tunability, frequency-stable continuous-wave operation, and generation of very short pulses (<10-13 s) with Ti:Al203 Iasers. Ti:Al203 lasers are now commercially available and are a valuable research tool found in many laboratories. This article reviews some of the developments in Ti:Al203 lasers and focuses on contributions made at Lincoln Laboratory. Fast Electro-Optic Wavelength Selection and Frequency Modulation in Solid State Lasers Electro-optic devices permit rapid wavelength selection and high-bandwidth frequency modulation in single-frequency solid state lasers. Laser dynamics limits both the speed of wavelength selection and the linearity of frequency modulation. The maximum speed of laser mode selection is determined by the buildup of oscillation from spontaneous emission. For frequency modulation, the minimum fractional deviation from chirp linearity is the ratio of the cavity round-trip time to the duration of the chirp. Nd:YAG and Ti:Al203 lasers built at Lincoln Laboratory have attained these theoretical limits. Ultrastable CO2 Lasers This article begins with a brief review of gas-laser research and the events that led to the development of ultrastable CO2 lasers at Lincoln Laboratory. Extremely high spectral purity and short-term stabilities of Δf/f < 1.5 X 10-13 have been routinely achieved with these ultrastable CO2 lasers. At MIT/Lincoln Laboratory, we have also invented a novel frequency stabilization method that enabled the long-term frequency locking of CO2 (and many other) molecular lasers to the center frequency of any regular- or hot-band CO2-isotope-laser transition. Consequently, line-center-stabilized CO2-isotope lasers have become the best secondary frequency standards known to date in the infrared domain. Some of the output characteristics, selected applications, and design features of the lasers are also described. The article concludes with a discussion of some of the advantages and limitations of the present laser design. |
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