I-A.

International and Commercial Interest

Optical space telecom has been developed since SILEX and Galileo projects in the early 1990s [1]. The first Optical Inter-Satellite Link (OISL) between SILEX and Artemis was demonstrated in 2001 [2]. Evolving internet market has re-launched in 2014 the interest in optical satellite telecom, following the plan of Google and Facebook to invest in the development of large constellations using optical telecom. There are tremendous potential commercial applications in particular to connect a considerable percent of the Earth population to the Internet. Examples of the current international projects are:

I-B.

MPB Experience with Optical amplifiers

MPB has started its projects in optical telecom for space within a contract for ESA reviewing the feasibility of fiber laser for OISL in the mid-1990s. There were many technical obstacles to solve. The following contracts with ESA in the early 2000s were to build EDF amplifiers at lower power levels for intra-satellite applications and as a source of lasers for fiber sensor Interrogation. The technology has greatly improved with optical fiber amplifiers at 20 W output in the 1550 nm range being built for terrestrial application. Recently MPB built the amplifier stages of the four very high power laser guide star (4x22W, 589 nm) within an ESO contract, and successfully tested them in April 2016 [3].

Table-1 summarizes the heritage and roadmap of EDFAs for space at MPB, with the amplifier output power and the TRL.

Table 1:

Summary Space Qualification Tests of MPB’s EDFAs

II-A.

Description and general parameters

Five separate tests were performed under several conditions. Two tests used the Sc-46 and Three the Co-60 (Table2 and Table3). Some EDFs and EYDFs were pumped, in forward or backward configuration, to obtain similar output as their final use as amplifiers. The power output level in these active configurations was between 18 dBm (medium power) and 33 dBm (High power EYDFs). Other fibers were kept in passive form not pumped during irradiation to simulate the redundant amplifier. One fiber the EDF1-MPB used in MPB commercial EDF products was a witness for all the five tests and all configurations.

Table 2:

Comparison of the test parameters between the five tests performed

Table 3:

Comparison of the test parameters between ESTEC1 and ESTEC2

Table-2 compares the parameters of the five tests and the number of fibers irradiated in each of them, and Table-3 compares the radioactive characteristics of the Sc-46 and Co-60

The objectives were to test:

Fig. 1 and Fig. 2 presents illustrate the irradiated fibers set up at Ecole Polytechnique and at ESTEC

Figure 1:

set up at Ecole Polytechnique Montreal using Sc-46-a) schematic and b) picture

Fig.2:

Pictures of the fibers under irradiation

a) ESTEC-1 on the panel (light On)

b) ESTEC-1, illuminating pumped fibers with the light off,

c) ESTEC-2, illuminating pumped fibers with the light off

d) ESTEC-2 on the panel (light On).

We can see in b) the low index shining polymer makes the interface between the Single mode passive fiber and the double cladding EYDF.

The source is formed of two small pellet separated by 3 cm distance to permit a uniform distribution over 4.5 cm height within ± 2%. The fibers are enrolled on a thin plexi-glass spool. A perch and two rods permitted to hold and monitor the spool

III-A.

Polytech-1 and Polytech-2 Measurements

The tests with the Sc46 permit the use of only small number of fibers. Four fibers were used, one MPB commercial fiber in passive mode (EDF1-MPB, medium doping Er) and three radhard fibers in active mode (AMP1, AMP2 from Ixfiber and Elite-TM from Draka). The characteristics of the radhard fibers are given in Table4, and the measurement results are shown in Fig.3, Fig.4, Fig.5 and Fig. 6.

Table 4:

Radhard Fibers Characteristics

Fig. 3:

Polytech-1 Total optical power evolution of the four fibers during the radiation from 0 to 102 krad, followed by an annealing of one week at 70°C, then 48 hours pumping (300 mW at 976 nm) and additional 56 Krad up to 158 krad.

Fig. 4:

Polytech-1 Gain spectra before (dashed lines) and after 158 krad the shape is kept

Fig. 5:

Polytech-2 Gain Losses fromthe totalpower measurements

All of the four fibers have similar losses at 100 Krad. During each measurement (25 and 60, 102 and 158 Krad), we waited about 90 minutes, with pumping to stabilize the gain. During this time the gain was increasing very probably with the effect of photo-bleaching. We note that the Gain spectra shape stayed practically the same for each fiber. The loss of the commercial fiber was about the same of that by the rad-hard.

In the test Polytech2 we used the AMP3 from Ixfiber in active forward pumping configuration, and the commercial fiber EDF1-MPB in three different configurations, passive, active forward and active backward configurations.

The AMP3 showed the best performance, the EDF1 backward pumping was less affected by the radiation. The EDF1 in passive configuration had about 1.5 dB more losses than those in active. In Polytech-2 the Sc-46 became to be relatively weak, the dose rate started at 52 rad/h and at the end of the test was about 19 rad/h with 129 days of irradiation. The major disadvantage of the backward pumping is the relative high Noise Figure as shown in Fig.6. The Noise Figure of the AMP3 is slightly higher than the EDF1, in similar configuration. The Noise Figure slightly increases with the radiation 0.2 to 0.5 dB

Fig. 6:

Polytech-2 Noise Figure before (dashed lines) and after 125 krad

III-B.

ESTEC-1 and ESTEC-2 Measurements

In these tests many more fibers could be tested at the same time, and the results are presented.

Fig.7 compares the gain loss of the EDF1-MPB in ESTEC-1and ESTEC-2 considering all the configurations. Fig.8 shows the spectra of an EDF-PM pumped at 650 mW during the irradiation then after the irradiation. The Gain loss is high about 4-5 dB, however the gain could be increased with additional pumping during 5 days. The EDF-PM in passive configurations lost more than 8 dB during the irradiation. The EYDF are backward pumped, they show much less gain loss< 2.5 dB in all cases, about 0.5 dB for the radhard ones (see Table5)

Fig. 7:

Comparison of EDF1-MPB Gain Loss between ESTEC-1 and ESTEC-2

Fig. 8:

Evolution of EDF-PM from OFS, Irradiated, pumped during irradiation at 650 mW

Table 5:

Summary of the EYDF radiation tests

Note that the gain loss is probably slightly smaller in ESTEC-2 test with the dose 110 rad/h compared with ESTEC-1 at 363 rad/h

III-C.

Proba2-Fiber Sensor Demonstrator Measurements

The EDF are being tested in a LEO environment. They are part of the Interrogation module which monitors the Fiber Bragg Grating sensors of the “Fiber Sensor Demonstrator” (FSD) payload on Proba2 satellite. They are in orbit since November 2009.

Fig. 9:

Total Radiation received in the FSD-during the 7 years flight.

The radiation received inside the Interrogation module is about 1 krad/year. The fluctuations are due to the temperature variations between the measurements as shown in Fig.10

Fig. 10:

Evolution of different components as function of the radiation from the Fig.9

There is less than 2 % reduction in the laser diode pump measurements –they are sensitive to temperature that is fluctuating between 18°C and 32°C. We compare the maximum intensity of three FBG sensors on three different fiber lines, one on the pressurized fuel tank, one on the thruster and one on the propulsion tank. The fiber sensors are at the end of the optical circuit lines after the laser diode pumps, the EDF amplifier, the Fabry Perrot Tuneable Filter and circulator. The FBG sensors show less than 3% decrease in its intensity.