The polarization characteristics of an artificial laser source in space were measured through space-to-ground atmospheric transmission paths. An existing Japanese laser communication satellite and optical ground station were used to measure Stokes parameters and the degree of polarization of the laser beam transmitted from the satellite. As a result, the polarization was preserved within an rms error of 1.6°, and the degree of polarization was 99.4±4.4% through the space-to-ground atmosphere. These results contribute to the link estimation for quantum key distribution via space and provide the potential for enhancements in quantum cryptography worldwide in the future.
© 2009 OSA
Recently, there have been many dangers and threats to society, such as frequent large-scale disasters and accidents, worldwide infectious diseases, recurrent terrorism, and the deterioration of homeland security. Besides being intellectual and industrial inventions and innovations, science and technology are also a means to create social values and can help provide answers for these crises that threaten the world’s security and safety . Because of sophisticated eavesdropping technology, the information and communications technology sector needs to be able to prevent information leakage and illegal access; therefore, quantum cryptography technology has become more important to information security.
Novel methods have been studied that utilize the principles governing quantum cryptography to ensure unconditional security [2,3]; these include quantum teleportation between distant parties , which theoretically cannot be broken according to the laws of physics. Technologies for quantum teleportation and quantum channel coding that defeats the classical Shannon limit have been verified . Fiber-based quantum cryptography systems for commercial use are already being sold by some venture companies: Cerberis, Vectis, and Clavis from Id Quantique in Switzerland; MagiQ QPN Security Gateway 7505 from MagiQ Technologies in the USA; and SQBox from SmartQuantum in France [6–8]. In Switzerland, an Internet vote using quantum cryptography was conducted in October 2007 for the mayoral election in Geneva . These details show the maturity of the technology level for quantum key distribution (QKD).
QKD using optical fibres is limited to a distance of approximately 100 km due to transmission losses, nonlinearity, and background noise. Using free-space QKD would enable the long-distance transmission of photons; therefore, satellite quantum cryptography is an ideal application of QKD . It is important to investigate the feasibility of such satellite QKD for future space applications. A low earth orbit (LEO) satellite usually orbits the Earth at about 7 km/s, which causes the Doppler shift. The polarization is the best means for satellite QKD to be used under the Doppler shift comparing with the time-bin method. However, the depolarization from space to ground has never been measured precisely so far.
In this paper, a highly polarized artificial laser source with a degree of polarization (DOP) of more than 99% onboard a satellite is used for measuring the polarization and the obtained polarization characteristics through space to ground are presented.
2. Effect of atmospheric turbulence on the polarization13] and the detected depolarized light amounting to less than 10−8 rad over 600 m path was reported .
Polarimetry, which is a field in astronomy, has been conducted by measuring some polarized standard stars on the ground [15,16]. The Hubble space telescope in space measured the polarized star by using the Near Infrared Camera and Multi-Object Spectrometer (NICMOS)  and the mean DOP of such polarized standard stars is less than 9.947% . The comparison between the ground based and space based measurements was made , however, there has been no polarization measurement using a laser source from space so far.
The small ice crystals at the upper atmosphere induce the depolarization on the scattered light, which were done mainly not for the transmitted light but the scattered and reflected lights [20,21]. The radiative transfer approach to measure the polarization through the atmosphere uses the scattered light as well [22–24]. The influence of the small ice crystals at the upper atmosphere might cause some effects on the polarization for the transmitted light but has not been measured yet through space-ground atmospheric paths directly. Therefore, it needs to be confirmed whether the polarization characteristics and DOP can be maintained after the beam passes through the space-to-ground atmospheric paths.
3. Configuration of the ground-to-satellite laser communications experiments
The National Institute of Information and Communications Technology (NICT, formerly CRL) measured the polarization characteristics using an artificial laser source in space. A LEO satellite, the Optical Inter-Orbit Communications Engineering Test Satellite (OICETS) Kirari, was used for this purpose . The laser communications experiments between the optical ground station developed by NICT—located in Koganei of downtown Tokyo—and OICETS were conducted in cooperation with the Japan Aerospace Exploration Agency (JAXA) in March, May and September of 2006; these were called the Kirari Optical Communication Demonstration Experiments with the NICT optical ground station (KODEN).
After the first trials, NICT noticed that it would be important to measure the polarization characteristics through the atmosphere for satellite QKD. Then, NICT initiated the KODEN experiments again and conducted the revival experiments from October 2008 to February 2009 for the confirmation of the polarization characteristics through space-to-ground atmospheric paths. The OICETS satellite was controlled by JAXA from the Kirari operation center in Tsukuba, as shown in Fig. 1 . The optical antenna onboard OICETS is a 26-cm diameter center-feed Cassegrain mirror-type telescope. The laser beam from the satellite is transmitted with a wavelength of 847 nm. The optical ground station has a 1.5-m telescope located in Koganei, Tokyo and operated by NICT.
4. Experimental results and discussions
4.1 System description
Figure 2 shows the laser light from the laser terminal onboard OICETS (bright spot at the center) as captured by a CCD video camera beside the 1.5-m telescope. The scattered light of the beacon laser beam with a wavelength of 808 nm transmitted from the ground station can be seen below the laser light. The optics for the beacon were mounted beside the 1.5-m telescope tube as shown in Fig. 3 . The communication beam with a wavelength of 815 nm was transmitted through the coude optical path and out the 1.5-m telescope. The backscattered lights from the uplink beams did not influence the polarization measurements because the aperture of the polarimeter was different from that of the uplink beams.
The beam divergence of the downlink laser beam with the wavelength of 847 nm from the satellite was only about 6 μrad, so the footprint of the optical beam was only 6 m on the ground at the link distance of 1000 km. A polarimeter with a 1.5-cm aperture diameter was equipped beside the 1.5-m telescope in the optical ground station. Data—such as the received optical power, Stokes parameters, and DOP—were directly measured through the beam expander as shown in Fig. 3 and recorded at a rate of 10 Hz. The specifications of the polarimeter are an input power range of −60 dBm to + 10 dBm, normalized Stokes accuracy of less than 0.005, and DOP accuracy of less than ± 0.5% at the wavelengths between 700 to 1000 nm.
4.2 Polarization measurements on the ground before the launch
The polarization characteristics of the laser beam onboard the OICETS satellite was measured during the thermal vacuum test before launch . The polarization of the emitted laser beam was right-handed circular polarization (RHCP|Optical) by the classical optics viewpoint, and the depolarization was within 0.49%. The definition of the polarization here is defined as Stokes parameters of (S0, S1, S2, S3) = (1, 0, 0, 1), which is RHCP|Optical by the classical optical viewpoint . Circular polarization is usually used for laser communications terminals because the received weak laser beam can be isolated from the transmitted powerful laser beam with orthogonal polarization by using a quarter wave plate in front of the internal optics. If we follow the definition by the Institute of Electrical and Electronics Engineers (IEEE), the classical optics definition of circular polarization is just the opposite of the IEEE definition . The left-handed circular polarization for RF signals (LHCP|RF) is defined as the counterclockwise direction of the electro-magnetic field at the fixed observation plane from the back-side view of the propagation direction. This definition is common for satellite communications; however, LHCP|RF is regarded as RHCP|Optical and vice versa according to convention.
4.3 Polarization measurements through space-to-ground laser links
Polarization measurements from the spacecraft were conducted from October 2008 to February 2009. Figure 4 shows the received optical power at the optical ground station and DOP measured in the night from 16:16:08 to 16:21:58 in the Universal Time on December 23, 2008. The minimum distance between the ground station and the satellite was 959.8 km at the maximum elevation angle of 35.3°. The duration of the experiment was 350 sec above 15° in the elevation angle of the satellite. The scintillation indices ranged from 0.05 to 0.4 according to the elevation angles and there was no cloud. Data with low optical power due to intermittent tracking losses were removed in order to estimate the polarization data correctly. DOP with an rms error was measured to be 99.4 ± 4.4%. The error of DOP could be attributed to the instrumental error, the backscattered light from the uplink beams, and the polarization effect in the atmosphere. As shown in Subsection 4.1, there was no influence of the backscattered light from the uplink beams. The polarization effect in the atmosphere due to ice crystals might be negligible because no significant difference as a function of the elevation angles could be observed. Therefore, the instrumental error is considered to be dominant in this measurement.
Figure 5 shows the Stokes parameters of (S1, S2, S3) measured during the experiment. The Stokes parameters with rms errors were measured as (S1, S2, S3) = (−0.054±0.109, −0.005±0.104, 0.987±0.009), which indicates circular polarization. Figure 6 shows the polarization characteristics on the Poincaré sphere. The rms angular error on the Poincaré sphere was measured to be within 3.2° in this experiment. One revolution of 360° on the Poincaré sphere experiences 180° in the rotation angle of the polarization; therefore, the rms angular error for the linear polarization becomes half of 3.2°. This value includes both the nature of the atmospheric slant path and the instrument error; however, if we calculate tan(3.2°/2) = 0.028, the cross leak component of the orthogonal polarization will be 2.8% from the main component, which can be considered as a quantum bit error rate (QBER) for QKD. In the past polarization measurements of a laser beam after propagation over a horizontal 144 km path, the QBER was measured to be 4.8 ± 1% which was caused by the various imperfection of their experimental setup . In this paper, however, it was measured from space so the beam went through many different atmospheric layers in contrast to Ref . According to QKD theory, the maximal tolerated error has an upper bound of 11% . Therefore, the error budget can be considered to be within this maximal tolerated error for the satellite-to-ground QKD systems. Thus, it is useful to estimate the link budget for satellite-to-ground QKD scenarios by using the results presented here.
The polarization characteristics of an artificial laser source in space were measured through space-to-ground atmospheric transmission paths. A LEO satellite and an optical ground station were used to measure Stokes parameters and the degree of polarization of the laser beam transmitted from the satellite. The polarization was preserved within an rms error of 1.6°, and the degree of polarization was 99.4±4.4% through the space-to-ground atmosphere. These results contribute to the link estimation for QKD via space and provide the upper bound based on the measurements and the potential for enhancements in quantum cryptography worldwide in the future.
The authors express their appreciation to the team members of the Japan Aerospace Exploration Agency, Space Engineering Development Co. Ltd. and NEC Toshiba Space Systems, Ltd., for their support in conducting the experiments.
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