Engineering topics
Publications and source records attributed to Hemmati, H..
Recent successful demonstrations of laser communications with spacecraft and Earth-orbiting satellites have demonstrated the feasibility of some of the key aspects of this technology.
We have performed a study on telecommunication systems for a hypothetical mission to Mars. The objective of the study was to evaluate and compare the benefits that microwave (X-band and Ka-band) and optical communications technologies afford to future missions.
In this Par-II report of the Advanced Communications Benefits study, two critical metrics for comparing the benefits of utilizing X-band, Ka-band and Optical frequencies for supporting generic classes of Martian exploration missions have been evaluated.
A rationale is provided for selecting wavelengths particularly suited for free space laser communications systems. Comparisons of wavelengths are based on propagation (especially through the atmosphere), optical background noise, and the needed technologies.
Photoresponse is a sensitive phenomenon that reflects the.
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Three 10-W fiber-coupled diode lasers were used to pump a single Nd:YAG laser crystal.
A modulated fiber-coupled diode laser assemby has been developed. This module will serve as the laser transmitter assembly for the Optical Communication Demonstrator (OCD) system. The laser is capable of greater than 20mW of output at 100 Mbps Q-PPM modulation.
A single Nd:YAG laser crystal was pumped by three 10-W fiber-coupled diode lasers. At 50 kHz pulse repetition frequency, average output powers exceeding 11 W of continuous-wase 1064 nm, and 3.5 W of 532-nm were achieved.
A diode-pumped laser with over 11 W of continuous-wave 1064 nm, and 3.5 W of frequency-doubled average power at 50 kHz pulse repetition frequency, has been developed.
Laser communication and optical remote sensing systems often require a highly sensitive detection scheme to measure extremely weak optical signals.
NASA's space missions of the future will be dominated by more moderate sized mini- and micro- spacecraft. Such missions will place stringent requirements on the mass and power consumption required for the various spacecraft subsystems.
Tunable continuous-wave and pulsed laser output was obtained from a Tm-sensitized Ho:YLiF4 crystal at subambient temperatures when longitudinally pumped with a diode laser array. A conversion efficiency of 42 percent and slope efficiency of approximately 60 percent relative to the absorbed pumped power have been achieved at a crystal temperature of 275 K. The emission spectrum was etalon tunable over a range of 16/cm centered at 2067 nm with fine tuning capability of the transition frequency with crystal temperature at measured rate of -0.03/cm/K. Output energies of 0.22 mJ per pulse and 22 ns pulse duration were recorded at Q-switch frequencies that correspond to an effective upper laser level lifetime of 6 ms, and a pulse energy extraction efficiency of 64 percent.
The operation of a diode-laser pumped thulium, holmium yttrium-lithium-fluoride laser (Tm,Ho:YLF) in Q-switched mode is reported. Output energies of 200 microjoules in pulses of 22 ns duration are recorded at Q-switch frequencies commensurate with an effective upper laser level lifetime of 6 ms. This lifetime is appreciably longer than that observed in other hosts permitting stored energy extraction of 64 percent, close to the projected maximum performance from these materials.
Simple optical coupling scheme locks array of gain-guided diode lasers into oscillation in single mode and with single-lobed output beam. Selective feedback from thin etalon self-injection-locks array into desired mode. One application of new scheme for pumping of neodymium: yttrium aluminum garnet lasers with diode-laser arrays.