Photonic lanterns provide an efficient way of coupling light from a single large-core fiber to multiple small-core fibers. This capability is of interest for space to ground communication applications. In these applications, the optical ground receivers require high-efficiency coupling from an atmospherically distorted focus spot to multiple fiber coupled single pixel super-conducting nanowire detectors. This paper will explore the use of photonic lanterns in a real-time ground receiver that is scalable and constructed with commercial parts. The number of small-core fibers (i.e. an array of single or few-mode cores) that make a photonic lantern determines the number of spatial modes that they couple at the larger multimode fiber core end. For instance, lanterns made with n number of single-mode fibers can couple n number of spatial modes. Although the laser transmitted from a spacecraft originates as a Gaussian shape, the atmosphere distorts the beam profile by scrambling the phase and scattering energy into higher-order spatial modes. Therefore, if a ground receiver is sized for a target data rate with n number of detectors, the corresponding lantern made with single-mode fibers will couple n number of spatial modes. Most of the energy of the transmitted beam scattered into spatial modes higher than n will be lost. This paper shows this loss may be reduced by making lanterns with few-mode fibers instead of single-mode fibers, increasing the number of spatial modes that can be coupled and therefore increasing the coupling efficiency to single pixel, single photon detectors. The free space to fiber coupling efficiency of these two types of photonic lanterns are compared over a range of the free-space coupling numerical apertures and mode field diameters. Results indicate the few mode fiber lantern has higher coupling efficiency for telescopes with longer focal lengths under higher turbulent conditions. Also presented is analysis of the jitter added to the system by the lanterns, showing the few-mode fiber photonic lantern adds more jitter than the single-mode fiber lantern, but less than a multimode fiber.
Fiber amplifiers have been used in many laser communication applications due to their compactness and high efficiency. However, fiber amplifiers for free space laser communications have been limited to low peak power communication formats due to nonlinear effects such as Self Phase Modulation (SPM). At high peak powers, SPM can broaden the spectrum of free space laser communication signals to an unacceptable degree, moving much of the transmitted power outside the receiver’s designed bandwidth. The Laser Communication Relay Demonstration (LCRD) needed a fiber amplifier that could amplify a 3.5 GHz wide signal to peak powers of nearly a kilo-Watt with little to no spectral broadening due to SPM. We tested several different commercial amplifiers and found an Er-doped, Very Large Mode Area (VLMA) fiber amplifier, pumped by a 1480 nm Raman fiber laser, met the needs for both high peak power and minimal spectral broadening. The high peak power performance of the VLMA amplifier is enabled by the large effective area of ~ 1100 μm<sup>2</sup> . We will present a detailed analysis of the effects of SPM on the amplified signal spectrum for both pulse position modulation (PPM) and differential phase shift keyed (DPSK) communication formats at peak powers up to 1 kW. Additionally, Bit Error Ratio (BER) performance data taken with the LCRD modem showed the signal did not suffer a measureable penalty from amplification with the VLMA amplifier. The ability to reach the required high peak power with this fiber amplifier makes it possible to consider its use for deep space laser communication, e.g. with high order M-ary PPM formats where such high peak powers are needed.
The Lunar Lasercom Ground Terminal (LLGT) is the primary ground terminal for NASA’s Lunar Laser
Communication Demonstration (LLCD), which demonstrated for the first time high-rate duplex laser
communication between Earth and satellite in orbit around the Moon. The LLGT employed a novel
architecture featuring an array of telescopes and employed several novel technologies including a custom PM
multimode fiber and high-performance cryogenic photon-counting detector arrays. An overview of the LLGT
is presented along with selected results from the recently concluded LLCD.
Improvements to a ground-based 40W 1.55 micron uplink transmitter for the Lunar Laser Communications
Demonstration (LLCD) are described. The transmitter utilizes four 10 W spatial-diversity channels to broadcast 19.4 -
38.9 Mbit/s rates using a variable-duty cycle 4-ary pulse position modulation. At the lowest rate, with a 32-to-1 duty
cycle, this leads to 320 W peak power per transmitter channel. This paper discusses a simplification of the transmitter
that uses super-large-area single mode fiber and polarization control to mitigate high peak power nonlinear impairments.
A ground-based 40W 1.55μm uplink transmitter for lunar laser communications is described. The transmitter, which
generates wavelength multiplexed communication and beacon signals, is implemented using four 10W spatial-diversity
channels to reduce far-field atmospheric-turbulence-induced fading and facilitate high-power signal generation via
parallel-spatial-combining of commercially-available EDFAs. Each transmitter channel can generate a 1 kHz modulated
beacon for spatial acquisition, and a multi-rate 4-PPM communication signal at a 311 MHz slot rate with 16:1 and 32:1
duty cycles to support 38.9 Mbit/s and 19.4 Mbit/s channel rates, respectively. Details on the transmitter design,
including the mitigation of optical nonlinear effects are discussed.
Laser Guide Star Adaptive Optics (LGS AO) has been offered to Keck II visiting astronomers since November 2004. From the few nights of shared-risk science offered at that time, the LGS AO operation effort has grown to supporting over fifty nights of LGS AO per semester. In this paper we describe the new technology required to support LGS AO, give an overview of the operational model, report observing efficiency and discuss the support load required to operate LGS AO. We conclude the paper by sharing lessons learned and the challenges yet to be faced.
The image quality obtained using laser guide star adaptive optics (LGS AO) is degraded by the fact that the
wavefront aberrations experienced by light from the LGS and from the science object differ. In this paper we
derive an analytic expression for the variance of the difference between the two wavefronts as a function of angular
distance between the LGS and the science object. This error is a combination of focal anisoplanatism and angular
anisoplanatism. We show that the wavefront error introduced by observing a science object displaced from the
guide star is smaller for LGS AO systems than for natural guide star AO systems.
The Laser Guide Star Adaptive Optics (LGS AO) at the W.M. Keck Observatory is the first system of its kind being used to conduct routine science on a ten-meter telescope. In 2005, more than fifty nights of LGSAO science and engineering were carried out using the NIRC2 and OSIRIS science instruments. In this paper, we report on the typical performance and operations of its LGS AO-specific sub-systems (laser, tip-tilt sensor, low-bandwidth wavefront sensor) as well as the overall scientific performance and observing efficiency. We conclude the paper by describing our main performance limitations and present possible developments to overcome them.
The purpose of this paper is to report on new adaptive optics (AO) developments at the W. M. Keck Observatory since the 2004 SPIE meeting.<sup>1</sup> These developments include commissioning of the Keck II laser guide star (LGS) facility, development of new wavefront controllers and sensors, design of the Keck I LGS facility and studies in support of a next generation Keck AO system.
The purpose of this paper is to report on new adaptive optics (AO) developments at the W. M. Keck Observatory since the 2002 SPIE meeting. These developments include continued improvements to the natural guide star (NGS) facilities, first light for our laser guide star (LGS) system and the commencement of several new Keck AO initiatives.
In this paper we describe the operational strategy and performance of the Keck Observatory laser guidestar adaptive optics system, and showcase some early science verification images and results. Being the first laser guidestar system on an 8-10 m class telescope, the Keck laser guidestar adaptive optics system serves as a testbed for observing techniques and control algorithms. We highlight the techniques used for controlling the telescope focus and wavefront sensor reference centroids, and a wavefront reconstructor optimized for use with an elongated guidestar. We also present the current error budget and performance of the system on tip-tilt stars to magnitude R=17. The capability of the system to perform astronomical observations is finally demonstrated through multi-wavelength imaging of the Egg proto-planetary nebula (CRL 2688).
The W. M. Keck Observatory Adaptive Optics (AO) team recently celebrated a milestone first AO-corrected image with the new Laser Guide Star (LGS) system. This paper details focus and pointing changes implemented for the LGS AO system. The combination of variable sodium altitude, elevation-dependent distance to the LGS, off-axis projection, and equipment flexure require both focus and pointing adjustments to keep the laser spot located and its size minimized on the wavefront sensor. We will describe the current approach to LGS focus and pointing-compensation adjustments, and provide some insight into issues seen thus far during engineering activities at the W. M. Keck Observatory.
Full-field imaging with a developmental projection optic box (POB 1) was successfully demonstrated in the alpha tool Engineering Test Stand (ETS) last year. Since then, numerous improvements, including laser power for the laser-produced plasma (LPP) source, stages, sensors, and control system have been made. The LPP has been upgraded from the 40 W LPP cluster jet source used for initial demonstration of full-field imaging to a high-power (1500 W) LPP source with a liquid Xe spray jet. Scanned lithography at various laser drive powers of >500 W has been demonstrated with virtually identical lithographic performance.
The Engineering Test Stand (ETS) is an 'alpha-class' Extreme Ultraviolet (EUV) lithography tool designed to demonstrate full-field EUV imaging and provide data required to accelerate production-tool development. The illumination system of the ETS is based on a laser-produced plasma (LPP) source using a recirculating Xe target medium. A Nd:YAG laser focused onto a Xe-gas or liquid target creates a plasma producing 13.4 nm radiation, at the center of the Si/Mo multilayer mirror passband. A condenser system, comprised of multilayer-coated and grazing incidence mirrors, collects the EUV radiation and directs it onto a reflecting reticle. A 1500 W LPP source has been integrated with the ETS and used for lithography. Two Xe spray sources have been evaluated, a cluster jet and a liquid spray jet. The cluster jet Xe source output rapidly degraded from heating of the hardware by the plasma causing the Xe clusters to be too small for efficient conversion. The TRW-designed liquid spray jet operates stably for hours and with tripled conversion efficiency into the condenser optics, producing EUV in the ETS.