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.
Recent progress towards the realization of high-power, non- cryogenic (quasi-)cw mid-IR lasers based on the `W' configuration of the active region is reported. Type-II diodes with AlGaAsSb broadened-waveguide separate confinement regions are the first III-V interband lasers to achieve room-temperature pulsed operation at a wavelength longer than 3 micrometers . For cw operation, Tmax was 195 K and Pout equals 140 mW was measured at 77 K. Optically- pumped W lasers recently attained the highest cw operating temperatures (290 K) of any semiconductor laser emitting in the 3 - 6 micrometers range. For a (lambda) equals 3.2 micrometers device at 77 K, the maximum cw output power was 0.54 W per uncoated facet. In order to maximize the absorption of the pump in the active region, an optical pumping injection cavity structure was used to create an etalon cavity for the 2.1 micrometers pump beam. The pulsed incident pump intensity at threshold was only 8 kW/cm2 at 300 K for this edge- emitting mid-IR laser. The differential power conversion efficiency was 9% at 77 K and 4% at 275 K, which indicates promising prospects for achieving high cw output powers at TE-cooler temperatures following further optimization.
In the work we continue our studies of broadened waveguide separate confinement InGaAsSb/AlGaAsSb quantum well diode lasers grown by MBE on n-GaSb substrates. To avoid the structure degradation associated with the miscibility gap in the 2.3 - 2.7 micrometer wavelength range, we used highly strained, 'quasi-ternary' InxGa1-xSb1-yAsy compounds with 0.25 < X < 0.38 and y approximately equals 0.02 as the material for QWs. From spontaneous emission measurements we have identified that the Auger process determines the rate of recombination in quantum well active region over the entire temperature range studied (15 - 110 degrees Celsius) for 2.6 micrometer lasers and at temperatures higher than 65 degrees Celsius for 2.3 micrometer lasers. Under these conditions, strong temperature dependence of Auger coefficient leads to the rapid increase of threshold current density with temperature (T0 approximately 40 degrees Celsius). In the range of 15 - 65 degrees Celsius for 2.3 micrometer devices we believe monomolecular non-radiative mechanism dominates and T0 is about 110 degrees Celsius. In addition, single-mode CW room temperature ridge-waveguide lasers with wavelength of 2.3 - 2.55 micrometer have been fabricated for the first time. The lasers display threshold currents around 50 mA with CW output powers of several milliwatts. Switching of the peak lasing position has been observed for both CW and pulsed operation and is related to second sub-band transitions. These results show that excess carrier energy distribution and their concentration are current dependent above threshold.
We review our recent progress in the design and operation of 2-micrometer InGaAsSb/AlGaAsSb quantum-well diode lasers. The devices have InGaAsSb quantum-well active regions and AlGaAsSb cladding layers, and all were grown lattice-matched to GaSb substrates using molecular-beam epitaxy. The broadened- waveguide (BW) design produces internal losses as low as 2 cm-1, which leads to external quantum efficiencies as high as 53%. Single-quantum-well lasers with 200-micrometer apertures and 2-mm-long cavities exhibit output powers of 1.9 W CW and 4 W quasi-CW. The lowest threshold current densities are 115 A/cm2. Small arrays of similar multi-quantum-well- diodes emit 10.6 W CW. The broadened-waveguide design should improve the performance of all mid-infrared diode lasers.
KEYWORDS: Modulation, Absorption, Semiconductor lasers, Temperature metrology, Sensors, Diodes, Gas lasers, Frequency modulation, Spectroscopy, Fermium
It is well known that DFB lasers tune by a factor of at least 10 times more with temperature than with current. The problem, however, is that the electrical modulation of the laser is easier and much faster than temperature modulation. This paper describes a novel technique to temperature modulate a DFB laser. A 1393 nm DFB laser chip is mounted directly on a single element thermo-electric cooler (TEC) which allows temperature modulation of the chip by passing a current in both the forward and reverse direction through the TEC. A +/- 40 mA modulation through the TEC at a rate of up to 30 Hz provides a frequency sweep of 46 GHz of the laser output frequency. The time constant of the setup is 10 ms.
Ramon Martinelli, Raymond Menna, Pamela York, Dmitri Garbuzov, Hao Lee, Joseph Abeles, Nancy Morris, John Connolly, S. Yegna Narayan, Jacobus Vermaak, Gregory Olsen, David Cooper, Clinton Carlisle, Haris Riris, Anthony Cook
We have fabricated single-frequency diode lasers from a number of III-V semiconducting compounds. These diode lasers were specifically designed for laser absorption spectroscopy. Their emission wavelengths span the internal of 0.76 to 2.7 micrometers . Water vapor, CO, CO2, NH3, CH4 HF, and O2 have been detected using them. After a brief review of their physical structure and principles of operation, we present representative output characteristics of these lasers, along with a discussion of several important applications.
We have demonstrated distributed-feedback, 1.39 micrometer wavelength InGaAsP/InP, multiple-quantum-well, folded-cavity surface-emitting, laser diodes with a low threshold current of 25 mA. These devices have greater than 45 dB side-mode suppression. Their wavelength tunes continuously with current over an interval of 1.5 angstrom at a rate of 0.11 angstrom/mA (-1.7 GHz/mA). These devices are useful for two-dimensional spectroscopic gas-sensing applications. Employing these compact lasers, we have detected atmospheric water-vapor at 1.3925 micrometer.
Calculation of the optical field distribution in 2 micrometer AlGaAsSb/InGaAsSb/GaSb multiple quantum well (MQW) lasers shows that incorporation of about 100 nm waveguide layers between quantum wells (QW) and cladding layers increases the optical confinement factor and reduces losses caused by mode penetration into doped cladding layers. Structures of this type have been grown by MBE. Both photoluminescence studies and measurements of laser diode parameters demonstrate that excess carriers confined in the waveguide are effectively collected and recombine in the QWs despite the small valence band offset at the interface of the QW and the waveguide, which is expected to be less than kT at 300 K.
Many simple molecules, such as H2O, CO2, CO, N2O, CH4, and HCN, have strong absorption bands at wavelengths between 2 and 3.5 micrometers . We are developing InGaAsSb/AlGaAsSb multi-quantum-well diode lasers operating from 2 to 3.5 micrometers as sources for trace-gas monitors. These devices are grown by molecular beam epitaxy, and they generally comprise four or five InGaAsSb quantum wells separated by AlGaAsSb barriers. The cladding layers are high-Al-content AlGaAsSb layers. Our longest-wavelength, room- temperature (20 degree(s)C) lasers operate at 2.78 micrometers in the pulsed mode, delivering 95 mW peak power. The highest temperature for pulsed-mode operation is 60 degree(s)C, at which the wavelength is 2.9 micrometers . Between 78 and 200 K they operate cw, and at 200 K the output is 3 mW at 2.66 micrometers in a dominant single mode. We discuss the properties of these lasers along with some initial applications to water-vapor detection.
KEYWORDS: Digital signal processing, Filtering (signal processing), Signal detection, Electronic filtering, Signal to noise ratio, Optical filters, Semiconductor lasers, Spectroscopy, Frequency modulation, Interference (communication)
In recent years there has been renewed interest in using diode laser based sensors for environmental monitoring, industrial process control, and medical diagnostics applications. Diode lasers have the advantages of small size, non-intrusiveness, speed, ease of use, and high detection sensitivity. Several spectroscopic detection techniques can be employed with diode lasers, and digital signal processing algorithms can be used to enhance the detection sensitivity of a system. In our laboratory we used the following digital signal processing techniques to enhance the sensitivity and accuracy of near- and mid-infrared diode laser sensors: digital bandpass, Wiener, Kalman, and matched filtering, and a general least-squares fit. These digital signal processing algorithms have enhanced the signal-to-noise ratio of our sensors by an order of magnitude.
We describe room-temperature 2.78-micrometers AlGaAsSb/InGaAsSb multi-quantum well lasers. Pulsed laser operation was observed with threshold currents of 1.1 A, a maximum power output of 95 mW, and a maximum differential quantum efficiency of 9%. Lasers operated pulsed up to 60 degree(s)C with a characteristic temperature of 58 K over the range of 0 - 40 degree(s)C. Continuous wave operation was observed up to 200 K, with a peak emission wavelength of 2.665 micrometers . To date, 2.78 micrometers is the longest emission wavelength for a room-temperature III-V laser.
Near-infrared diode-laser-based systems using laser-absorption molecular spectroscopy can sensitively monitor atmospheric gases, pollutants, and toxic gases. They can also monitor trace gases on the human breath for medical diagnostics. The detection levels are equal to or less than parts per million. Sarnoff/SRI has made and tested room-temperature InGaAsP/InP DFB lasers operating at 1.39, 1.6, and 1.65 micrometers . All of these devices had output powers of 10 mW or more. The current-tuning rates varied from -580 to -1240 MHz/mA. The temperature tuning rate was about 0.1 nm/K for all devices. Continuous tuning ranges were 7 nm for the 1.39 micrometers lasers and 5 nm for the 1.6 and 1.65 micrometers lasers. We observed H2O at 1.39 micrometers , CO and CO2 at 1.6 micrometers , and CH4 at 1.65 micrometers . We monitored the ratio of 13CO2 to 12CO2 on human breath samples as the initial step towards clinical trials for medical diagnostics.
Fabry-Perot and distributed-feedback emission from strained InGaAs/InP quantum well lasers has been examined over a temperature range of 100 K to 315 K. The active layer contains two 12 nm wide In0.75Ga0.25As quantum wells. Fabry-Perot lasers, operating at 39 degree(s)C, showed cw emission at 2.0 micrometers . A threshold current of 40 mA and an external differential quantum efficiency of 10% were measured from a laser with 6 mW of cw light output. A linear wavelength tuning rate of 0.72 nm/K was measured from 100 K to 300 K. The characteristic temperature, To, of the threshold current, exhibits an abrupt decrease at 250 K, from To equals 136 K to To equals 56 K. A similar decrease in the characteristic temperature of the external differential quantum efficiency is observed. The decrease in To values at 250 K indicates the onset of an additional loss mechanism. Distributed feedback lasers were fabricated from the same wafer. They showed single mode output from 190 to 300 K with a side-mode-suppression ratio of about 20 dB. The wavelength was 1.95 micrometers at 0 degree(s)C and tuned at a rate of 0.13 nm/K. The current-tuning rate was -340 MHz/mA.
The design and operating characteristics of strained-layer InGaAsP/InGaAs active-grating surface-emitting amplified diode lasers are presented. For the first time, we report cw operation of an active-grating amplifier at a single wavelength of 1.7 micrometers with a cw power output in excess of 100 mW. In addition, we discuss, theoretically, the possibility of laterally scaling these devices using antiguided laser-array structures.
We have observed laser action in InO7GaO3AsO72SbO28 IInPO.7SbO.3 double heterojunction, diode lasers at ? = 3.06 .tm. The maximum operating temperature was 35 K. The threshold current densities
were in the range of 200 - 330 A /cm These devices were grown by organometallic vapor-phase epitaxy.
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