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Current technologies available for deep-space optical data transmission and networking are discussed in this paper, as well as ongoing experiments, future perspectives, and applications.

Phobos Laser Ranging (PLR) is a concept for a space mission designed to advance tests of relativistic gravity in the solar system. PLR's primary objective is to measure the curvature of space around the Sun, represented by the Eddington parameter γ , with an accuracy of two parts in 10 7 , thereby improving today's best result by two orders of magnitude. Other mission goals include measurements of the time-rate-of-change of the gravitational constant, G and of the gravitational inverse square law at 1.5-AU distances-with up to two orders-of-magnitude improvement for each. The science parameters will be estimated using laser ranging measurements of the distance between an Earth station and an active laser transponder on Phobos capable of reaching mm-level range resolution. A transponder on Phobos sending 0.25-mJ, 10-ps pulses at 1 kHz, and receiving asynchronous 1-kHz pulses from earth via a 12-cm aperture will permit links that even at maximum range will exceed a photon per second. A total measurement precision of 50 ps demands a few hundred photons to average to 1-mm (3.3 ps) range precision. Existing satellite laser ranging (SLR) facilities-with appropriate augmentation-may be able to participate in PLR. Since Phobos' orbital period is about 8 h, each observatory is guaranteed visibility of the Phobos instrument every Earth

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The Doppler-free, two-photon 5d 10 6s 2 Si/ 2 -5d 9 6s 22 D 5 / 2 transition in singly ionized Hg, attractive as an optical-frequency standard, has been observed for the first time on a small number of 198 Hg + ions confined in a radio-frequency trap. The radiative lifetime of the 2 D 5 / 2 state and the absolute wave number of the two-photon transition were measured to be 0.090(15) s and 17 757.152(3) cm" 1 , respectively. Optical amplitude-modulation sidebands, induced by the secular (thermal) motion of the harmonically bound ions, were observed also for the first time.PACS numbers: 32.70.Fw, 32.30.JcMicrowave or optical transitions of laser-cooled ions that are confined in electromagnetic traps offer the basis for frequency standards of high stability and accuracy. 1 " 5 The advantages of such devices are numerous: Very long interrogation times and, therefore, high transition-line (?'s can be achieved; fractional frequency perturbations that are due to the trapping fields can be held below 10~1 5 ; collisions with background gas and cell walls can be largely avoided; Doppler shifts are directly reduced by trapping and cooling; and finally, nearly unit detection efficiency of transitions to metastable states is possible so that the signal-to-noise ratio need be limited only by the statistical fluctuations in the number of ions that make the transition. 5 Details of ion traps and laser cooling have been published elsewhere. 1 " 3 A particularly attractive candidate for an optical-frequency standard is the Sd l %s 2 Sy 2 -5d 9 6s 22 D 5 / 2 VLg* transition driven either by two photons with a wavelength near 563 nm 4 or by one photon at half this wavelength. 5 The lifetime of the 2 D 5 / 2 state, which decays by the emission of electric quadrupole radiation at 281.5 nm, is calculated to be of order 0.1 s. 6 This gives a potential optical-line Q of about 7x 10 14 . In this Letter we describe the first results of our investigation of the two-photon transition in 198 Hg + (which is free of hyperfine structure) stored in a miniature rf trap.Our trap is similar to the small radio-frequency traps used in the ion-cooling experiments that were conducted at Heidelberg University on Ba + , 7 and at the University of Washington on Mg + and Ba + . 8 A cross section of the trap electrodes is shown in Fig. 1. It is interesting to note that, although the inner surfaces of our trap electrodes were machined with simple conical cuts, the trap dimensions were chosen to make the fourth-and sixth-order anharmonic contributions to the potential vanish. 9 The rf drive frequency was 21 MHz with a voltage amplitude V 0 ^ 1 kV. The partial pressure of the background gas, with the exception of deliberately added mercury and helium, was ^10~7 Pa (133 Pa = 1 Torr). After loading of 50-200 mercury ions the mercury vapor was frozen out in a liquid-nitrogen cold trap and the vacuum vessel was backfilled with He to the order of 10" 3 -10" 2 Pa. This was sufficient to cool the trapped Hg + collisionally to near room temperature as verified by the ...