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For implementing an optical free-space transmission system at data rates up to 10 Gbit/s, commercially available devices developed for terrestrial fiber optic applications may be employed to advantage. We designed, set up and tested a breadboard consisting of an NRZ/RZ transmitter with a booster amplifier of 1 Watt optical output power and an optically preamplified direct detection receiver. The main objective was to determine the receiver sensitivity and assess critical components. With the breadboard parameters optimized, the required average number of photons per bit at the receiver input was 55 for a bit error probability of BEP = 10-6 in case of NRZ coding. We further demonstrated a sensitivity improvement of about 1 dB by applying RZ coding. The measurements yielded excellent agreement with simulations performed with a tool especially designed for this purpose
Since the early seventies efforts towards optical space communications have been undertaken worldwide. Links from ground to space (and vice versa), deep space communications and inter-satellite links were considered .Our aim was to investigate the possibility of implementing an optical inter-satellite link using devices developed for terrestrial applications at data rates up to 10 Gbit/s.
There are two major contenders today for highly sensitive space borne optical receivers in the multi-Gigabit/s range: optically pre-amplified direct detection receivers and coherent receivers. Although the theoretical limit of the receiver sensitivity is lower for coherent detectors than for direct detection reception, the experimentally achieved values are better for the latter technology .Both technologies perform equally well with respect to sensitivity towards background radiation. However, in our opinion, considering system complexity and availability of critical system components, optically pre-amplified direct detection reception is the clear winner, especially at data rates beyond 1 Gbit/s.
Excellent optical amplifiers are available in the form of Erbium doped fiber amplifiers (EDFA). They can serve both as booster amplifier in the transmitter and as preamplifier in the receiver. When employing EDFAs, the system wavelength is set to 1,550 nm. Compared to links operating at λ = 850 nm or λ = 1,064 nm, the larger antenna beam widths can be cited as a disadvantage. Still, because of the excellent state-of-the-art of the available components and their robustness, we chose an optically pre-amplified direct detection system at 1,550 nm for our investigations. With a transmit power of 1 W and the high receiver sensitivity we achieved within this study, one may close a link with some 70 dB transmission loss if the data rate does not exceed 10 Gbit/s. For a low-earth orbit scenario with a link distance of some 5,000 km this would imply telescope diameters of some 6 cm, while for a geostationary inter-satellite link with a distance of 80,000 km the required telescope diameter amounts to 25 cm.
Review of optical amplifier technology
Although the Erbium Doped Fiber Amplifier is the amplifier of primary interest, because of its superior performance and higher maturity, all optical amplifier technologies have been considered.
Optical Fiber Amplifiers :
Doped Fiber Amplifiers (DFAs) use a doped optical fiber as a gain medium. The optical signal to be amplified and an optical pump are multiplexed into the doped fiber, and amplification is achieved by stimulated emission of photons from the dopant ions. The pump light excites electrons into a higher energy level, from where they decay via stimulated emission of a photon back to a lower energy level. The energy levels form a three or four level system, and include a non-radiative transition either from the highest energy level and/or back to the bottom energy level. The amplification window is the range of optical wavelengths for which the amplifier provides usable gain. It is imposed by the dopant ions, the glass structure of the fiber and the pump wavelength. The fiber is doped with rare-earth ions such as Erbium (Er3+), Neodymium (Nd3+), Ytterbium (Yb3+),Praseodymium (Pr3+), or Thulium (Tm3+). The common rare earth used to dope the fiber is Erbium (Er) that has a radiative transition around 1.55 μm. The amplifier gain bandwidth is given by the energy separation of the upper and lower sub-levels of the rare earth ions. Although the electronic transitions of a single ion are very well defined, electron level broadening occurs when the ions are incorporated into the fiber glass, and thus the range of wavelengths that can be amplified is also broadened. This leads to a gain spectrum that is not uniform against wavelength. . The principal source of noise in DFAs is Amplified Spontaneous Emission (ASE), which has approximately the same spectrum as the gain. Electrons in the upper energy level can decay by spontaneous emission, which occurs randomly, depending upon the glass structure and inversion level. Photons are emitted spontaneously in all directions, but a portion of them is captured and guided along the fiber. Those photons may interact with dopant ions, and thus be amplified by stimulated emission. ASE is emitted in both the forward and reverse directions; co-propagating ASE noise has a direct impact on system performance. Counter-propagating ASE can lead to performance degradation, by depleting the inversion level and there by reducing the gain.
The Erbium-Doped Fiber Amplifier (EDFA) is the most commonly used amplifier in telecommunication networks. It has become an essential building block in WDM applications. The Erbium is excited into population inversion by pumping with laser diodes at either 980 nm or 1480 nm, or both. Indeed, a combination of 980nm and 1480nm pumping is also utilized in EDFA's. EDFA's can be pumped in forward direction (i.e. with pump wave co-propagating with the signal wave), or in backward direction. The direction of the pump wave does not influence the small-signal gain, but the power efficiency of the saturated amplifier as well as the noise characteristics. Bidirectional pumping enables not only to provide high output power, but also to achieve a low noise figure at the same time. High gain optical amplifiers also need to be protected from any parasitic reflections, because these could lead to laser oscillation or fiber damage. EDFA's are typically equipped with optical isolators at the output and possibly at the input. The resistance to radiation of such doped fibers might be critical, and therefore it had to be assessed.
Cladding-pumped, Erbium-doped fiber amplifiers (CP-EDFA) :
For applications requiring high optical powers, novel amplifier designs had to be developed, as this can not be using a conventional EDFA. Three factors limit the power efficiency and the output level of a single-mode fiber EDFA:
- The coupling between the pump laser and the doped core of the FA
- The rare-earth material concentration, and
- The maximum power density in the fiber core
Clad-pumping technology solves the first and third limitation. The second can be dealt with by selecting the base material (glass) appropriately and introducing Ytterbium (Yb) together with Erbium. Clad-pumping is based on special fibers with two concentric cores (or clads). The pump is launched into a large multimode internal cladding (about 50 to 100 μm) surrounding the single mode, Erbium-Ytterbium co-doped core. As the pump propagates along the fibre, Yb absorbs it in the core. The amount of power absorbed by Yb is then transferred to Er ions leading to an efficient coupling between the pump beam and Er ions, even if their overlapping factor is rather poor. . Double-clad fiber has enabled efficient capture of the output of high-power multimode diodes. With the commercialization of high-power, broad-area lasers with stripe width as large as the multimode core diameter; clad-pumping has become the technique of predilection for high-power amplifiers. Such a technique allows for pumping powers in excess of a few Watts, and optical amplifiers with output powers greater than +30 dBm .There are several technology and design options that can be used to improve the CP-EDFA as use of codopants (Yb3+), multimode pumping and pump to fiber coupling techniques.
Erbium-Doped Tellurite Fiber Amplifiers (EDTFA):
Tellurite oxide (TeO2)-based glasses are of interest based on their properties when used for the fabrication of Er3+ optical amplifiers. EDTFAs present a large bandwidth gain covering C and L bands, with large gain per unit length, which allows the fabrication of under-meter amplifiers. The power efficiency is lower than standard silica amplifiers, and the NF is higher than 4 dB, typically in the 5-6 dB range. A serious alternative is the Al-silica fibers which allow higher Er3+ concentrations reducing the gain threshold, but high aluminum concentrations can be a problem under radiation.
space radiation effect on edfa for inter-satellite optical communication
Erbium-doped fiber amplifiers (EDFAs) are being widely deployed in terrestrial applications as repeaters for long-line or highly distributed telecommunications systems But EDFA had never been used in space, because EDFA contains a long erbium-doped fiber (EDF), and fiber is sensitive to space radiation Inter-satellite optical communication as an important study field has developed very fast in this decade .With the growth of message capacity ,the internal modulate technology cannot meet the need in a few years from now. Hence, it is certain that the external modulation technique will take the place of the internal modulation technique in the next step. So whether EDFA, which is the key component of the external modulation technique, can be suitable for inter-satellite optical communication becomes a very important issue. In these two decades, several teams have studied the radiation effect on EDFA.
The main purpose is to confirm the biggest radiation dose that EDFA in inter-satellite optical communication can withstand. So the actual cause of the radiation effect is unimportant. Considering all performance parameters of actual communication systems, there are three critical parameters. The radiation effect on output power and noise figure (NF) must be confirmed. And the central wavelength of signal light is the third important parameter of both WDM and normal communication technologies.
The breadboard we set up consisted of an optical transmitter with a transmit power of 1 Watt, of devices simulating free-space loss, and of an optically pre-amplified receiver. The nominal wavelength was λ = 1,550 nm, the data rate was R = 10 Gbit/s and the modulation format was either non-return-to-zero coding (NRZ) or return-to-zero coding (RZ). As far as possible, commercially available devices were used.
Figure 1: Block diagram of breadboard setup. (DFB…distributed feedback laser, EAM…electro-absorption modulator, EDFA…Erbium doped fiber amplifier).
Measurement and simulation results showed that one of the most critical parameters for system performance is the transmit pulse extinction ratio ζ (cf. Fig. 4). Pronounced sensitivity degradation is to be expected when ζ becomes less than 10 dB. To avoid this, the modulation voltage, the modulator bias, and the laser drive current have to be adjusted and controlled precisely. With the DFB+EAM module the maximum extinction ratio obtained was 11.8 dB. An additional modulator, driven with the data signal, may improves the extinction ratio and thus lead to higher system sensitivity.
Both measurement and simulation showed that laser emission wavelength deviations from the optimum value amounting to 20% of the bandwidth Bo of the optical filter will result in a sensitivity penalty of 1 dB. For the breadboard, a temperature stabilized laser diode mount was used to control the laser wavelength, thus providing sufficient wavelength stability. The optimum optical filter bandwidth for the bread boarded system was in the range of 15 to 20 GHz. On a wavelength scale, this corresponds to 120 to 160 pm.
For the link budget assumed for the breadboard, the influence of the booster EDFA´s ASE is negligible, while for other configurations, the booster noise might have to be considered
b. Free Space Simulation:
Link attenuation due to space loss was simulated in the laboratory. Coarse attenuation of about 40 dB was obtained by picking up only a small fraction of the beam diverging from the EDFA´s fiber pigtail with a single-mode fiber input face placed at a distance of a few mm. For fine adjustment we employed a commercial variable attenuator equipped with single-mode fiber interfaces. The overall attenuation, including the power splitter at the receiver input, amounts to some 70 dB.
In a well-designed setup the preamplifier EDFA causes the dominating noise in the receiver. Therefore its noise figure should approach the theoretical limit of 3 dB as close as possible.
The optical band pass was a critical component in the receiver setup. Its temperature dependent center wavelength and its bandwidth strongly influence the receiver sensitivity. Considering the results presented in Fig. 2 as well as temperature drift of the filter center wavelength and variations of the optical carrier frequency due to temperature changes and possibly Doppler shift, it is recommended to use filters with higher than optimum bandwidths. Another, technically-demanding, approach to overcoming these problems is to actively control the center wavelength of the filter.
The insertion loss of the circulator/grating combination plays a minor role because it attenuates the optical signal in the same way as the optical noise, and because the electrical noise is, in general, negligible.
For the bread boarded system, the data clock signal was taken from the pattern generator and fed to the error detector. In a real system, a clock recovery of its own will have to be implemented, as it is by now standard in terrestrial fiber systems.
The sampling instant and the decision threshold for the error detector were optimized manually for every measurement. The sampling instant, which is not that critical, could be fixed relative to the recovered clock signal. Using an automatic power control at the input of the photo-diode module could mitigate the problem of threshold optimization, since the decision threshold could then be set once and its optimum value would not change significantly for different receiver input power levels
Measurements with nrz-coded input signal
The receiver sensitivity S (defined here as the receiver input power required to achieve a bit error probability of BEP = 10-6) was determined as a function of the optical filter bandwidth by employing four different optical band pass filters. For each filter the laser emission wavelength was optimized separately to obtain maximum transmission and thus highest sensitivity (cf. Fig. 2). Figure 3 presents the four optimum sensitivity values together with the interval of measurement uncertainty (±0.18 dB, as guaranteed by the manufacturer of the optical power meter used). The receiver sensitivity is also expressed by the required number of photons per bit, nS, at the receiver input. The filter with a bandwidth of Bo = 16 GHz appears to be the best choice, the filter with Bo = 31 GHz leads to similar performance. The broadest filter is suboptimum because of the high amount of noise caused by amplified spontaneous emission (ASE), detected by the photo-diode module. On the other hand, the consequence of a too small filter bandwidth is pulse shape distortion and signal energy rejection, resulting in a clear sensitivity penalty. Figure 3 shows that increasing the filter bandwidth from its optimum value results in less sensitivity degradation than decreasing it by the same amount; if in doubt; broader filters have to be preferred.
Figure 2: Receiver sensitivity S vs. wavelength λ for four optical bandpass filters differing in bandwidth and center wavelength in case of NRZ coding with an extinction ratio of ζ = 10.8 dB. (Bo…optical bandwidth).
Figure 3: NRZ coding: Measurement and simulation results for the receiver sensitivity expressed either in dBm (S) or in photons/bit (nS) vs. optical filter bandwidth Bo. The intervals indicated by the vertical lines represent the measurement uncertainty of ±0.18 dB.
Taking into account the chosen scale of the ordinate, the simulation results included in Fig. 3 agree well with the measurement. The deviation is some 0.5 dB for broad filters, 0.1 dB for the optimum bandwidth and about 1 dB for the narrowest filter. The fact that the simulation results are worse than the measurement results may be explained by the noise model applied for the simulation: The assumed Gaussian distribution of the ASE results in a sensitivity error of some +0.5 dB compared to the correct theoretical model.
Sensitivity deviations for filters with small bandwidths may be caused by suboptimum laser wavelength setting and insufficient filter modeling. Our simulation did not take into account a possible asymmetry of the filter's transfer function. Such asymmetry leads to distortion with effects equal to that of inter-symbol interference, resulting in higher optimum filter bandwidth. Other reasons for the discrepancy may be imperfect modeling of the electrical filter characteristics: The diode module has been modeled as Bessel filter of 5th order after measuring the magnitude of the transmission function. No information on the phase was available. Another reason for the difference between measurements and simulation could be attributed to imperfect modeling of the transmit signal's pulse shape.
To determine the influence of the extinction ratio of the transmit pulse, the modulation voltage and the bias of the modulation voltage were varied and the bit error probability vs. receiver input power was measured. Figure 4 shows the sensitivity as a function of the extinction ratio ζ. The maximum extinction ratio we could achieve was ζ = 11.8 dB. This led to a receiver sensitivity of S = -41.5 dBm corresponding to nS = 55 photons/bit.
Measurements with rz-coded input signal
It was found analytically that there is an advantage in introducing return-to-zero coding (RZ coding) in systems with direct detection receivers even if the receiver bandwidth is kept equal to that optimal for NRZ signals. This was also confirmed by numerical simulations.
To generate an RZ-coded transmit signal some modifications in the transmitter were necessary (see Fig. 5). The EAM now serves as data modulator (cf. inserts (a) and (b)), while a Mach-Zehnder modulator (MZM) fed with a sinusoidal voltage of frequency equal to the data rate acts as NRZ-RZ converter (insert (c)). The bias of the MZM was chosen such that the resulting pulse form is a sine-square shaped RZ pulse with a duty cycle of about 0.5, as shown in insert (d) of Fig. 5. The duty cycle is defined as the ratio of pulse duration (full width half maximum) and bit duration Tbit. A polarization control was implemented to properly set the input polarization of the MZM.
For RZ-coded signals, the same measurements and simulations as for NRZ were performed. Figure 6 presents the measurement and calculation results for the receiver sensitivity as a function of the optical filter bandwidth for RZ coding (the results for NRZ coding are also displayed for comparison). In contrast to NRZ coding, the experimentally determined optimum filter bandwidth is now Bo = 31 GHz. The corresponding sensitivity is some 1 dB better than that for NRZ coding in case of equal extinction ratio of ζ = 10.8 dB but optimum filter bandwidth.
Figure 5: Block diagram of the breadboard transmitter for RZ modulation. The inserts (a) and (c) show the modulation voltages for the electro-absorption modulator (EAM) and the Mach-Zehnder modulator, respectively. (b) gives the optical power after the EAM, while (d) shows the RZ coded transmit signal. (Tbit…bit duration).
For RZ coding the difference between the measurements and calculation is less than 0.5 dB for large optical filter bandwidths. For narrow filters the differences are larger which may be explained by imperfect pulse and filter modeling. In both the simulation and measurement, RZ leads to better system performance.
Figure 6: NRZ and RZ coding: Calculated receiver sensitivity expressed as required input power S and as photons per bit, nS, vs. optical filter bandwidth Bo. The circles and diamonds represent the measurement results obtained with the available optical filters for NRZ and RZ coding, respectively.
eff ects leadind to the difference to the quantum limit
For NRZ coding the best sensitivity achieved in experiment and simulation is 3.1 dB worse than the quantum limit which amounts to nS = 27 photons/bit for BEP = 10-6 . We investigated the individual contributions of the effects leading to this difference by performing calculations with modified simulation parameters.
Setting the electrical noise and the noise from the booster EDFA zero, assuming ideal noise properties of the preamplifier EDFA (3 dB) and the transmit pulse (rectangular with infinite extinction ratio) leads to 0.5 dB sensitivity penalty relative to the quantum limit. This is attributed to the fact that the optical Bragg filter and the electrical Bessel filter are not matched to the signal pulse shape.
For a realistic noise figure of the preamplifier EDFA (e.g. 3.3 dB), a receiver setup without polarization filter, realistic electrical noise density (N0 = 1.8·10-8 V/√Hz) and when including the influence of booster ASE, a 1 dB sensitivity penalty has to be expected.
Taking into account, finally, the measured extinction ratio and pulse shape in the simulation, leads to a sensitivity penalty of 3.1 dB. This indicates again that the transmit signal shape is the most important parameter influencing system performance.
1. Erbium-doped amplifier (EDFAs) is the latest state of the art solution for amplifying signals in light wave transmission system.
2. They are used as booster amplifier on the transmitter side to get as much power as possible into the link, as inline amplifiers to overcome the loss of the fiber and as preamplifiers at the receiver end to boost signals to the necessary receiver levels.
3. EDFAa can use in signal wavelength transmission systems, in wave length division multiplexed (WDM) systems.
1 Design concepts for EDFAs in terrestrial and submarine WDM systems
2. Transmission fiber design and dispersion-management techniques for terabit/s systems
3 Amplified submarine-cable systems, including a brief history of submarine-cable communications and the investigation of terabit/s system technologies
4 Advanced concepts in the physics of noise in amplified light, noise figure definitions, entropy, and ultimate capacity limits.
A breadboard for an optical free-space communication system employing only off-the-shelf devices designed for terrestrial fiber applications was set up and tested. Core parts of the setup were a booster EDFA in the transmitter and an EDFA serving as optical preamplifier for the direct detection receiver. The booster amplifier provided an output power of 1 W, while the preamplifier, which appeared to be the dominating noise source of the system, had a noise figure of 3.3 dB, close to the theoretical minimum of 3 dB. Using a simulation tool we determined the optimum optical bandwidth of the receiver. With the parameters thus found, maximum receiver sensitivity was achieved.
Two modulation formats were implemented: NRZ and RZ. RZ coding lead to a performance gain of some 1 dB. It is less sensitive to suboptimum optical bandwidths at the receiver.
The best sensitivity we obtained was only some 3 dB above the quantum limit, proving the attractivity of pre-amplified receivers with direct detection for optical free-space communication.