Hamid Hemmati

Diode-pumped self-frequency-doubled neodymium yttrium aluminum borate (NYAB) laser

Over 50 mW of the fundamental-mode 531-nm laser output was obtained with approximately 4% optical-to-optical conversion efficiency from a self-frequency-doubling NYAB crystal when pumped with two 1-W diode lasers. The prospect of higher conversion efficiency is discussed. >

2.07-μm cw diode-laser-pumped Tm,Ho:YLiF 4 room-temperature laser

Continuous-wave action is obtained at 2.07 μm from a 2-mm-long Tm-sensitized Ho:YLiF4 crystal at room temperature when longitudinally pumped by a pair of diode-laser arrays. Laser output power at 300 K is 26 mW, with a 30% slope efficiency and a lasing threshold of 108 mW. A maximum output power of 187 mW is obtained from a 4-mm-long crystal at 77 K, with a 67% slope efficiency. A preliminary demonstration of cavity Q switching produced 165 μJ of pulse energy at a repetition rate of 100 Hz.

Deep-Space Optical Communications: Future Perspectives and Applications

The concept of deep-space optical communications was formulated shortly after the invention of lasers. The promise of laser communications, high data rate delivery with significantly reduced aperture size for the flight terminal, led to the pursuit of several successful experiments from Earth orbit and provided the incentive for further demonstrations to extend the range to deep space. This paper is aimed at presenting an overview of the current status of optical communications with an emphasis on deep space. Future perspectives and applications of optical communications related to near-Earth and interplanetary communications are also addressed.

3.5-W Q-switched 532-nm Nd:YAG laser pumped with fiber-coupled diode lasers.

A diode-pumped laser with greater than 11 W of cw 1064-nm output power and 3.5 W of frequency-doubled average power at a 50-kHz pulse repetition frequency has been developed. A single Nd:YAG rod was pumped with the combined output of three fiber-coupled diode-laser arrays. Each pump laser was capable of 10 W of cw output power. The fiber output of each pump laser was first collimated and then focused with a single lens onto one end of a Nd:YAG rod. The resonator mirrors for the L-shaped cavity were selected such that thermal lensing in the laser crystal was mostly compensated. The 532-nm output beam quality factor (M(2)) was less than 1.5.

Prospects for Improvement of Interplanetary Laser Communication Data Rates by 30 dB

NASAs Mars laser communications demonstration (MLCD) project (since discontinued) was designed to achieve data rates greater than 1 Mb/s from the farthest Mars range and greater than 30 Mb/s from the nearest Mars range. We present here pathways whereby ongoing optical technology advances should allow 30 dB of increased data rates relative to MLCD by 2020. In other words, technical advances should soon allow the return of deep-space data at rates of 1 Gb/s from the maximum Mars range, 100 Mb/s from Jupiter distances, and 10 Mb/s from Uranus, all many orders of magnitude greater than present radio-frequency capabilities. The potential for realizing the stated capability with less than a three times increase in dc power required from the spacecraft and no increase in payload mass is discussed.

Optical Satellite Communications

Satellite-based communication systems of today are increasingly capacity limited. Based on radio frequency or microwave (generically RF) technologies, current state-of-the-art satellite communications (satcom) are often constrained by hardware and spectrum allocation limitations. Consequently, mobile payload sensors and instruments (on satellite or aircraft) are often implemented with restricted capacity to better match that of the host platform. Such limitations are expected to worsen as future interplanetary, deep-space, and manned missions use more sophisticated data-intensive sensors and as the demand for information—and a bigger return on the space-exploration investment—continues to increase.

High repetition-rate Q-switched and intracavity doubled diode-pumped Nd:YAG laser

A Nd:YAG laser was end pumped with 2.2 W of continuous-wave (CW) diode laser output. Efficient operation of the laser at high repetition rates was emphasized. This laser provides 890 mW of TEM/sub 00/ CW output at 1064 nm, and 340 mW of 532 nm average power at a Q-switched repetition rate of 25 kHz. Experimental data are compared with analysis. >

Earth-image tracking in the IR for deep space optical communications

Sub-microradian level laser beam pointing to an Earth-based receiver is required for deep space optical communications. This requires a beacon emanated from Earth towards the spacecraft. The beacon could be a laser or reflected sunlight from Earth. Earth image tracking in the visible is hampered by significant albedo variations and/or crescent Earth image yielding large central errors. Here, we report results of Earth-image tracking in the infrared (8 to 13 micron) region of the spectrum with the aim of substantially alleviating the two challenges mentioned earlier.

Qualification and reliability testing of a commercial high-power fiber-coupled semiconductor laser for space applications

A compact microchip laser pumped by a single fiber-coupled semiconductor diode laser is developed for a space-borne scanning laser radar instrument. A commercial off-the-shelf component is used for the pump laser and undergoes a rigorous qualification approach to meet the requirements for the space-borne application. The qualification and testing process for the commercial pump laser is derived based on a nonstandard piece part screening plan and is presented along with the test results. These tests include mechanical, vibration, thermal cycling, and radiation tests as well as a full destructive parts analysis. Accelerated lifetests are also performed on the packaged device to demonstrate the ability to meet an operational lifetime of 5000 h. The environmental testing approach would be applicable to space qualification of a variety of commercial photonic systems, particularly in cost-constrained missions.

Active optical compensation of low-quality optical system aberrations

We describe a method for the correction of slowly varying wavefront aberrations of low-quality telescope mirrors by using a DM in an active optical compensation system. Our goal is to reduce the surface wavefront error of low-cost multimeter-diameter mirrors from approximately 10 waves peak-to-valley (P-V), at a 1 microm wavelength, to approximately 1 wave or less. In a proof-of-concept demonstration using a 0.3m telescope at a wavelength of 633 nm, the rms wavefront error improved to 0.05 waves (0.26 waves P-V) from the original value of 1.4 waves rms (6.5 waves P-V). The Strehl ratio improved to 89% from the original value of 0.08%. The types of aberrations corrected include astigmatism, coma, defocus, trefoil, and higher-order aberrations.