Method and system for optical signal amplification

Abstract

To provide improved techniques for transmitting data via optical signals.SOLUTION: An optical signal amplification method is implemented in a system comprising: a transmission line (10) for transmitting optical signals between a first data processing device (11) and a second data processing device (12); and an optical amplifier (14) disposed at the transmission line, the optical amplifier comprising an active fiber section (15) and a pumping device (16). The method comprises: determining a target operating gain of an optical amplifier; determining a target maximum gain of the optical amplifier; determining an active fiber section length such that the optical signals are amplified with at most the target maximum gain; determining a core cross-sectional area size of the active fiber section; and determining an operating pumping power of the pumping device below the maximum pumping power such that the optical signals are amplified with the target operating gain.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a method for amplifying (and transmitting) optical signals, particularly for quantum key distribution. Additionally, a system for amplifying (and transmitting) optical signals, particularly for quantum key distribution, is disclosed. [Background technology]

[0002] Optical signals in optical fiber lines are gradually attenuated. Optical transmission systems that transmit optical signals over long distances require the use of optical amplifiers. The possibility of imperceptible signal diversion makes data transmission over optical signals unsafe.

[0003] Standard amplifiers are typically configured to maximize the efficiency of converting pump power to signal power by implementing a high pump absorption coefficient. In this situation, the amplifier has high gain sensitivity to pump increases. This leads to the possibility that an intruder could undetectably apply additional pumping and then divert excess signal.

[0004] Furthermore, standard amplifiers include components such as optical isolators and couplers that divert signals from the main channel to ensure stable and controlled operation. The use of such components can lead to significant signal loss, which may be unacceptable for secure data transmission. Security requirements become even more important for quantum signal transmission and quantum key distribution.

[0005] U.S. Patent Application Publication No. 2015 / 043056 discloses a communication system and method for generating light for quantum key distribution, which includes directing optical pulses to a semiconductor optical amplifier at a temperature above 0° C. The optical pulses include a wavelength and pulse region within the emission spectrum of the semiconductor optical amplifier selected to induce Rabi oscillations in the semiconductor optical amplifier and emit light at a frequency of at least 1 THz.

[0006] U.S. Patent No. 1,141,7998 relates to a gain fiber assembly for optical fiber amplification systems such as fiber amplifiers and fiber lasers. The gain fiber assembly uses an active fiber with a single glass cladding with an outer diameter of less than 80 μm. A passive double-clad input fiber has its outer cladding stripped and tapered to match the outer diameter of the bare fiber.

[0007] The glass-fluid or glass-vacuum interface along the taper provides pump guidance into and along the cladding of the bare fiber, with NA>1 for vacuum or gas and NA>0.8 for liquids, potentially allowing for significantly shorter fiber lengths to reach maximum signal power and higher pump conversion efficiencies.

[0008] In European Patent Application Publication No. 2535989, a concentration of at least 0.1 weight percent Nd is added to the central core region. 3+ Large-area active double-clad optical waveguides are described that are doped with Nd and can be used to amplify light at wavelengths between 1050 nm and 1120 nm. At doping concentrations sufficient to provide a net optical absorption of at least 3 dB / m for pump light at a particular wavelength, Nd 3+ is Yb 3+ It operates at a much lower inversion level than Nd 3+ The doped waveguide is subject to mitigation of pump bleaching or photodarkening. The pump light is guided by the second cladding and the optical signal to be amplified is guided by the first cladding, which has a very large mode area of at least 500 square micrometers.

[0009] Chinese Patent Publication No. 115189212 discloses a coherent array fiber laser structure. It includes multiple pump light sources, a fiber coupler, a high-inverse grating, an active fiber, a low-inverse grating, a passive dielectric layer, a transmission-promoting film, a mode-selective layer, and a reflective layer, which are arranged in sequence along an optical path. The high-inverse grating, the active fiber, and the low-inverse grating form a first resonant cavity. The first resonant cavity, the passive dielectric layer, the transmission-promoting film, the mode-selective layer, and the reflective layer form a second resonant cavity. The active fiber absorbs the pump light and generates laser light that enters the passive medium layer. Through the passive medium layer and the mode-selective layer, mode selection and external cavity feedback of the output laser are achieved to achieve efficient injection locking and uniform-mode output. This improves the injection feedback and mode-locking capabilities and realizes a uniform-mode coherent array fiber laser output.

[0010] U.S. Patent Application Publication No. 2003 / 197921 discloses a phosphate glass fiber that can be co-doped with high concentrations of ytterbium and erbium and exhibits high optical gain per unit length. By avoiding alkali metal oxides in the glass composition and adjusting the concentration of network modifiers such as BaO, a phosphate glass composition is provided that exhibits a near-zero temperature coefficient of refractive index. Therefore, high absorption of pump radiation and heat hardening caused by the pump laser can be avoided. Summary of the Invention

[0011] It is an object of the present disclosure to provide improved techniques for transmitting data via optical signals, particularly with respect to optical amplification of the optical signals.

[0012] To solve this problem, a method and a system for amplifying an optical signal are provided according to the independent claims. Further embodiments are disclosed in the dependent claims.

[0013] According to one aspect, there is provided a method for amplifying an optical signal. The method can be implemented in a system including a transmission line for conveying an optical signal (optical pulse) between a first data processing device and a second data processing device, and an optical amplifier disposed in the transmission line, the optical amplifier comprising an active fiber section and a pumping device. The method includes determining a target operating gain of the optical amplifier, determining a target maximum gain of the optical amplifier, determining a length of the active fiber section such that the optical signal is amplified with at most the target maximum gain, determining a core cross-sectional area size of the active fiber section based on a maximum allowable pulse shape distortion and a target maximum energy per pulse such that high-energy pulses having a target maximum energy per pulse are distorted to at most the maximum allowable pulse shape distortion, and determining an operating pump power of the pumping device that is lower than the maximum pump power such that the optical signal is amplified with the target operating gain.

[0014] According to another aspect, there is provided a system for amplifying an optical signal, the system comprising: a transmission line for conveying an optical signal between a first data processing device and a second data processing device; and an optical amplifier disposed in the transmission line, the optical amplifier comprising an active fiber section and a pumping device, the system configured in accordance with the optical signal amplification method.

[0015] As a result, it can be ensured that the amount of leaked / diverted optical signals / pulses is limited when, for example, an eavesdropper increases the pump power beyond the operating pump power. Thus, the potential information acquisition by an eavesdropper can be limited. At the same time, low-energy and high-energy pulses can be amplified equally, and pulse shape distortions can be prevented or mitigated, resulting in fewer errors and improved control when monitoring the transmission line. Therefore, the security of quantum key distribution can be improved.

[0016] Furthermore, the method may also relate to quantum key distribution. In particular, the method may further comprise determining a shared key between the first and second data processing devices by quantum key distribution comprising amplifying the optical signal via an optical amplifier by operating a pumping device at an operating pumping power.

[0017] The target maximum gain may be determined from the target operating gain and a gain increase value determined from the optical signal leakage, preferably the minimum detectable optical signal leakage. The method may include determining the (minimum detectable) optical signal leakage. The gain increase value may be an (additional) gain increase or a gain increase factor. The gain increase value may correspond to the gain increase when operating the pumping device at the operational pump power compared to when operating the pumping device at the maximum pump power. The gain increase value may also be verified accordingly.

[0018] target maximum gain G max and the target operating gain G op The ratio between G max / G op is the minimum optical signal leakage r that is preferably detectable from optical signal leakage. E min The target maximum gain G max and the target operating gain G op The ratio between G max / G op is 1+10 -7 and 1+10 -1 Between and, preferably 1+10 -6 and 1+10 -2 Between 1 and 10, preferably 1+10 -5 and 1+10 -3 It can be between.

[0019] target maximum gain G max and the target operating gain G op The difference between is the minimum detectable optical signal leakage r E min (Target operating gain G op The minimum detectable optical signal leakage r E mincan depend on the length of the transmission line. When the distance between adjacent optical amplifiers (amplifier spacing) is 50 km and the transmission line length is 1000 km, the target maximum gain G max and the target operating gain G op The difference between -6 If the transmission line length is 40,000 km, it can be equal to 10 -5 Therefore, the pumping device may be periodically operated at close to the maximum gain possible for the pumping device, which may limit the ability of an eavesdropper to increase the pumping power and partially divert the optical signal.

[0020] The target operating gain can be between 3 and 100, preferably between 5 and 20, more preferably between 9 and 11. The maximum operating gain is 3+10 -6 and 100+10 -3 Between 5 and 10, preferably 5+10 -6 and 20+10 -3 Between 9 and 10, preferably 9+10 -6 and 11+10 -3 It can be between.

[0021] The operating pump power may depend on the determined core cross-sectional area size of the active fiber section. For example, to achieve a similar population inversion density level, an increase in the core cross-sectional area size may correspond to an increase in the pump power. Determining the operating pump power may include comparing the optical pulse before and after amplification by the optical amplifier and / or adjusting the operating pump power based on the comparison.

[0022] The target maximum gain may be determined as the sum of the target operating gain and the gain increase, and / or as the product of the target operating gain and the gain increase factor.

[0023] For example, the target maximum gain G max is the target operating gain G op and the gain increase value (gain increase ΔG or gain increase coefficient δG), G max =G op +ΔG or G max =G opδG. Note that both equations may coincide if the gain is rescaled from linear to logarithmic units or vice versa. The target maximum gain may be determined, for example, via a gain increase exponent applied to the target operating gain.

[0024] The target maximum gain may correspond to the gain of the optical amplifier when operating the pumping device at a maximum pumping power. The maximum pumping power may correspond to at least one of the maximum pumping power achievable by the pumping device, the pumping power (above a predetermined threshold) at which components of the optical amplifier degrade, and the pumping power (above a further predetermined threshold) at which nonlinear optical effects appear in the transmission line. The target maximum gain may also correspond to a total population inversion density in the active fiber section (which may be calculated and / or measured). The method may include determining a maximum pumping power corresponding to the maximum target gain. An eavesdropper may wish to couple another pumping device to the active fiber section, which may be easier to detect than an eavesdropper simply gaining access to an existing pumping device, increasing its pumping power, and partially diverting optical pulses.

[0025] The target operating gain may be determined from the signal attenuation / signal decay of the optical pulses along the transmission line / optical fiber and / or the position of the optical amplifier in the transmission line.

[0026] The target maximum signal power can be determined from at least one of the maximum signal intensity of the optical pulse, the maximum number of photons (per pulse) of the optical pulse, the (minimum) pulse duration of the optical pulse, and the (maximum) signal frequency (or (minimum) signal wavelength) of the optical pulse. s may be, for example, between 0.1 ns and 1 μs. s may be, for example, between 300 nm and 2500 nm. The target maximum signal power may be a maximum average signal power. The maximum average signal power may be determined as the time-averaged signal power (e.g., over one or more optical pulses).

[0027] The target maximum gain can be achieved, for example, by adjusting the length of the active fiber section (active fiber section length). The active fiber section length can be determined so that an optical pulse having any target signal power is amplified at most by the target maximum gain for additional pumping power (where the achieved population inversion density level is close to 1). For example, the gain can be repeatedly determined and the active fiber section length adjusted until the target maximum gain is achieved.

[0028] In particular, determining the active fiber section length may include at least one of providing an initial length of the active fiber section, operating the pumping device at a maximum pumping power, repeatedly determining a gain value and adjusting the active fiber section length until the gain value is equal to the target maximum gain, and setting the active fiber section length as the active fiber section length whose gain value is equal to the target maximum gain. The initial length may be longer than the initial estimate of the active fiber section length by, for example, a factor between 1% and 20%, preferably between 5% and 15%, and more preferably between 9% and 11%. The maximum pumping power may be the maximum pumping power achievable by the pumping device. Thus, the maximum pumping power may depend on the pumping device.

[0029] Adjusting the active fiber section length may include shortening and / or lengthening the active fiber section. For example, the gain value may be determined from the input signal strength (and / or the number of photons per pulse and / or the signal power) to the optical amplifier and the output signal strength (and / or the number of photons per pulse and / or the signal power) from the optical amplifier.

[0030] The optical signal leakage can be determined through the transmission line. The optical signal leakage can be associated with the transmission line.

[0031] The optical signal leakage may be, for example, a maximum allowable signal leakage determined by a quantum key distribution protocol. In particular, the optical signal leakage may correspond to the maximum (allowable) number of diverted photons (per pulse and / or per unit time), i.e., the maximum number of photons that can be diverted / leaked, for example, by an eavesdropper (while unaudited optical pulses can be amplified to not fall below a target operating gain). The maximum signal leakage may also correspond to at least one of the maximum signal strength, the maximum energy (per pulse) that can be diverted, the maximum signal power that can be diverted, the ratio between the diverted signal strength and the non-diverted signal strength, and the ratio between the number of diverted photons and the number of non-diverted photons. For example, an eavesdropper may operate the optical amplifier at a gain up to a target maximum gain above a target operating gain, for example, by operating the pumping device at a pump power above the operating pump power up to the maximum pump power. However, even when operating the optical amplifier at the target maximum gain, the optical signal leakage, in particular the maximum number of diverted photons, may be limited.

[0032] Optical signal leakage can be determined by monitoring the accuracy and / or resolution of the method. In particular, optical signal leakage can correspond to a minimum fraction of the number of photons (per pulse and / or per unit time) and / or signal power and / or signal intensity, which can be detected by monitoring techniques and / or equipment.

[0033] Determining the core cross-sectional area size (core profile size, size of the cross-sectional area of the fiber core) of the active fiber section may include determining at least one of a core diameter, a core radius, and a core circumference of (the fiber core of) the active fiber section.

[0034] The core cross-sectional area (size), in particular the core diameter, may be constant along the active fiber section, or the core diameter may be the average core diameter or the maximum core diameter of the active fiber section.

[0035] The fiber core of the active fiber section may generally have a circular cross section, although non-circular cross sections (e.g., elliptical) may also be provided.

[0036] The core cross-sectional area size may be determined by a pulse shape metric (indicative of a pulse shape distortion of the high-energy pulse). In particular, determining the core cross-sectional area of the active fiber section may include at least one of transmitting a high-energy pulse having a target maximum energy per pulse through the active fiber section, determining a pulse shape metric indicative of a pulse shape distortion of the high-energy pulse, comparing the pulse shape metric value to a maximum allowable pulse shape distortion, and adjusting the core cross-sectional area size based on the comparison result.

[0037] Determining the core cross-sectional area size may include, inter alia, repeatedly performing the above steps until the pulse shape metric is less than the maximum allowable pulse shape distortion, and preferably greater than the maximum allowable pulse shape distortion minus a threshold value.

[0038] The target maximum energy per pulse may be, for example, 1 μJ to 10 μJ, preferably 3 μJ to 5 μJ. The duration of the high-energy pulse may be, for example, 0.1 ns to 20 μs, preferably 3 ns to 20 μs. The peak power of the high-energy pulse may be, for example, 10 mW to 300 mW, preferably 100 mW to 200 mW.

[0039] The high-energy pulse may be, for example, a test pulse for determining optical signal loss. An optical pulse may be defined as including an energy per pulse that is less than or equal to a target maximum energy per pulse.

[0040] Determining the pulse shape metric is the first form factor s of the optical pulse before amplification. in (t) and the second form factor s of the amplified optical pulse (amplified optical pulse)out (t) and determining a first form factor s in (t) is the pulse shape S of the optical pulse before amplification in The second form factor s can be determined by normalizing (t) (e.g., so that the time integral from the beginning of the pulse to the end of the pulse is 1). out (t) is the pulse shape S of the amplified optical pulse out (t) (e.g., so that the time integral from the beginning of the pulse to the end of the pulse is 1). For example, the first form factor s in (t) and / or second form factor s out (t) can be determined as follows:

[0041]

number

[0042] where t1 and t2 indicate the start and end times of the pulse, respectively. Each light pulse may include one or more pulses.

[0043] Pulse shape S of the optical pulse before amplification in (t) and / or the pulse shape S of the amplified optical pulse out (t) can be determined, for example, using a photodetector or an oscilloscope.

[0044] The pulse shape metric is the first form factor s in (t) and second form factor s out As the root mean square value of (t), it can be determined in particular as follows:

[0045]

number

[0046] Here, t1 and t2 indicate the start time and end time of the pulse, respectively.

[0047] Alternatively, the pulse shape metric may be determined as the difference between the ratio of the first form factor at the pulse start and the pulse end and the ratio of the second form factor at the pulse start and the pulse end, specifically as follows:

[0048]

number

[0049] Adjusting the core cross-sectional area size may include increasing the core cross-sectional area size (in particular, increasing any of the core diameter, core radius, and core circumference) based on determining that the pulse shape metric is greater than the maximum allowable pulse shape distortion. If the pulse shape metric is greater than the maximum allowable pulse shape distortion, it may be determined that insufficient energy is stored in the active fiber section to amplify the pulse.

[0050] By adjusting the cross section of the core, pulse shape distortions may be prevented or mitigated, and thus, for example, errors when monitoring a transmission line may be reduced and control of the transmission line may be improved.

[0051] The method may include, for example, repeatedly monitoring pulse shape distortion at regular time intervals, e.g., interval lengths of 1 μs to 10 s. Monitoring pulse shape distortion may include, for example, determining a pulse shape metric or determining a bit error rate (BER) of the pulse before and after amplification. Further, the method may include determining whether the pulse shape distortion is appropriate based on a difference in the bit error rates before and after amplification being greater than a predetermined threshold.

[0052] The allowed pulse shape distortion may be, for example, 0 to 0.1, preferably 0 to 0.01, or 0.9 to 1, preferably 0.99 to 1.

[0053] Additionally, the method may include determining (and / or monitoring) optical signal loss along the transmission path (e.g., optical pulse intensity loss and / or photon number loss), preferably as a function of position along the transmission path. In other words, the optical signal loss may be determined as a function of position along the transmission path. In particular, for each position along the transmission path, a corresponding optical signal loss may be determined. Thus, the optical signal loss may correspond to a signal loss profile.

[0054] The optical signal loss may be determined by optical time domain reflectometry. The optical signal loss may be determined during and / or before and / or after determining the shared key. The optical signal loss may be determined repeatedly. For example, the optical signal loss may be determined repeatedly every time interval of 10 ns to 50 s, preferably 10 ns to 50 s, particularly 100 ns to 100 ms, 500 ns to 500 ms, 100 ms to 1000 ms, 0.5 s to 5 s, and 5 s to 10 s.

[0055] The method may include determining an intrusion event based on a loss of optical signal and / or terminating determination of the shared key based on a loss of optical signal. The method may include revoking the shared key based on a loss of optical signal.

[0056] The optical signal (pulses) can include test pulses for determining optical signal loss and key-distribution pulses for determining the shared key, where the test pulses include a higher energy per pulse than the key-distribution pulses. In particular, each test pulse may include a higher energy than each key-distribution pulse. Alternatively, the test pulses and key-distribution pulses may include the same range of energy per pulse.

[0057] The test pulses and the key-distribution pulses can be separated in time (time multiplexing) or in different frequency channels (wavelength division multiplexing, WDM).

[0058] The maximum energy per test pulse can be, for example, from 1 μJ to 10 μJ, preferably from 3 μJ to 5 μJ. The duration of the test pulse can be, for example, from 0.1 ns to 20 μs, preferably from 3 ns to 20 μs. The peak power of the test pulse can be, for example, from 10 mW to 300 mW, preferably from 100 mW to 200 mW.

[0059] The maximum number of photons per key delivery pulse may be, for example, 2000 to 10000 photons, preferably 2500 to 3500 photons.

[0060] The maximum energy per key-delivery pulse can be, for example, from 2.56e-16 J to 1.28e-15 J, preferably from 3.20e-16 J to 4.5e-16 J. The duration of the key-delivery pulse can be, for example, from 0.01 ns to 100 ns, preferably from 0.1 ns to 10 ns. The peak power of the key-delivery pulse can be from 2.5 nW to 12.8 μW, preferably from 60 nW to 500 nW.

[0061] Furthermore, the method may include providing the transmission line with a physically unduplicable structure, and thus preferably with an unreproducible profile. For example, the transmission line may include a fiber core section (active and / or non-active) doped with, for example, at least one of Al, P, N, and Ge, preferably with a dopant concentration of 0.5 mol-% to 5.0 mol-%, more preferably 1.0 mol-% to 3.0 mol-%. Because the dopant concentration may vary depending on the position in the transmission line, it is preferable to provide a unique dopant pattern. Such a pattern may allow for authentication of the transmission line and more accurate monitoring of signal leakage, which may result in improved security.

[0062] Additionally, the method may include providing the active fiber section with at least one of an erbium-doped fiber section, a thulium-doped fiber section, a neodymium-doped fiber section, and an ytterbium-doped fiber section.

[0063] Furthermore, the method may include providing a transmission line that is free of optical isolators and tap couplers. In principle, the transmission line does not include optical components with losses above a maximum acceptable threshold.

[0064] Furthermore, the method may further comprise providing the optical amplifier as a bidirectional optical amplifier, so that optical pulses can be transmitted from the first data processing device to the second data processing device and from the second data processing device to the first data processing device via the same optical fiber.

[0065] Furthermore, the method may include arranging a plurality of optical amplifiers in the transmission path, each optical amplifier comprising a (further) pumping device and a (further) active fiber section having an active fiber section length. Preferably, determining the shared key may include amplifying the optical signal through the plurality of optical amplifiers by operating each of the further pumping devices at an operating pumping power. Furthermore, the method may include arranging the plurality of optical amplifiers such that an optical amplifier distance between two adjacent optical amplifiers is between 30 km and 200 km, preferably between 30 km and 60 km.

[0066] The pump wavelength of the pump radiation emitted from the pumping device can be tuned less than 10 nm, preferably less than 1 nm, and more preferably less than 0.1 nm away from the peak wavelength of the absorption spectrum of the active fiber section, thereby maximizing active medium inversion and minimizing the achievable difference between the target maximum gain and the target operating gain.

[0067] Additionally or alternatively, the pump wavelength of the pump radiation emitted from the pumping device may be stabilized, preferably by controlling the temperature and / or power supply of the pumping device. The temperature may be controlled to vary by less than 0.1°C, preferably less than 0.01°C. The power supply may be controlled to vary by less than 1%, preferably less than 0.1%, more preferably less than 0.01%. Thus, a stable operating gain factor may be achieved.

[0068] The active fiber section may be part of a transmission line. The active fiber section may be connected to an optical fiber (section) of the transmission line, for example via a splice connection. The pumping device may be coupled to the transmission line, in particular to the active fiber section, via a feed (optical) fiber and / or a coupling element. In particular, pump radiation emitted from the pumping device may pass through the feed fiber and / or be (subsequently) fed into the transmission line via the coupling element. The coupling element may be configured to direct radiation having different wavelengths and / or radiation from different optical fibers into a single optical fiber. For example, the coupling element may be a wavelength division multiplexing (WDM) system / element. The feed fiber may be a passive fiber.

[0069] The pumping device may include a diode and / or a laser. The pumping device may be configured to emit radiation at a pumping wavelength of the active fiber section. The system may or may not comprise first and second data processing devices and / or further data processing devices. The method may be performed in a data processing device, for example the first, second and / or further data processing device. Determining the target maximum gain and / or determining the active fiber section length and / or determining the core cross-sectional size of the active fiber section and / or determining the operating pump power may or may not be performed (at least in part) in a data processing device, such as the first, second and / or further data processing device. The first, second and / or further data processing device may be connected to the optical amplifier, in particular the pumping device. The optical pulses may be emitted from the first and / or second data processing device.

[0070] Determining the shared key may comprise at least one of: determining a bit sequence in the first data processing device using a random number generator; encoding the bit sequence into optical pulses (in particular key delivery pulses); transmitting the optical pulses via a transmission path to the second data processing device; amplifying the optical pulses by an optical amplifier having an operational pump power; receiving and measuring the optical pulses at the second data processing device; discarding non-deterministic signal bits from the (received) bit sequences at the first and second data processing devices; disclosing the bit sequence and / or a portion of the received bit sequence via the classical channel by the first and / or second data processing device; performing error correction on the bit sequence and the received bit sequence; and determining an amplified key sequence corresponding to the shared key from the error-corrected bit sequence using privacy amplification.

[0071] The above-described embodiments relating to the optical amplification method can also be applied to an optical amplification system. [Brief explanation of the drawings]

[0072] In the following, an embodiment will be described by way of example with reference to the drawings. [Figure 1] Diagram showing the arrangement of an optical amplification system and an eavesdropping device [Figure 2] Energy diagram showing optical amplification in an erbium-doped fiber section [Figure 3] Diagram showing the quantum key distribution method [Figure 4] Diagram showing the (sub)steps for determining a shared key through quantum key distribution [Figure 5] FIG. 1 shows an exemplary reflectogram obtained from optical time domain reflectometry corresponding to a signal loss profile. DETAILED DESCRIPTION OF THE INVENTION

[0073] (a) Quantum key distribution system In Figure 1, a layout diagram is shown comprising a quantum key distribution system and an additional eavesdropping device 13 ("Eve"), which comprises a transmission line 10 comprising an optical fiber for transmitting optical pulses, in particular classical and / or quantum signals, between a first data processing device 11 ("Alice") and a second data processing device 12 ("Bob").

[0074] The first data processing device 11 may comprise a first processor 11a and a first memory 11b, and the second data processing device 12 may comprise a second processor 12a and a second memory 12b. The first and second data processing devices 11, 12 may or may not be part of the system. The first data processing device 11 and the second data processing device 12 are connected to a transmission path 10. The transmission path 10 may comprise a quantum channel configured to transmit quantum signals. Additionally, a classical channel configured to transmit classical signals may be provided. The classical channel may be provided within the transmission path or separately.

[0075] The system may comprise a number of further data processing devices, in particular a third data processing device comprising a third processor and a third memory (not shown). The third data processing device may be connected to a transmission line 10. The third data processing device may exchange classical and / or quantum signals with the first data processing device 11 and / or the second data processing device 12 via further communication channels. If the system is configured to perform a step, such step may for example be performed in at least one of the first data processing device 11, the second data processing device 12 and the third data processing device.

[0076] The eavesdropping device 13, comprising an eavesdropping processor 13a and an eavesdropping memory 13b, represents a device external to the system that may have access to the transmission path 10. The eavesdropping device 13 may be positioned on the transmission path 10 such that the light pulses transmitted over the transmission path 10 are at least partially received and / or retransmitted by the eavesdropping device 13. The eavesdropping device 13 may also have access to further communication channels.

[0077] The first memory 11b, the second memory 12b, the third memory, and the eavesdropping memory 13b may each comprise a quantum memory for storing quantum signals and a classical memory for storing classical signals. The quantum memory may be implemented using optical delay lines, controlled reversible inhomogeneous broadening (CRIB), Duane-Lukin-Sirak-Zoller (DLCZ) schemes, revival of silenced echoes (ROSE), and / or hybrid photon echo rephasing (HYPER).

[0078] The first data processing device 11, the second data processing device 12, the third data processing device and the eavesdropping processing device 13 may each comprise means for transmitting and / or receiving quantum states via light pulses.

[0079] (b) Optical amplification The optical pulses are amplified / repeated by optical amplification by stimulated emission of radiation to compensate for signal attenuation. For this purpose, an optical amplifier 14 comprising an active fiber section 15 and a pumping device 16 is provided / disposed / installed in the transmission line 10. Furthermore, a plurality of optical amplifiers 14 may be provided in the transmission line 10. The optical amplifiers 14 may be arranged equidistantly along the transmission line 10, for example, with an amplifier spacing between 30 km and 200 km, preferably between 30 km and 100 km, more preferably between 30 km and 60 km. By increasing the amplifier spacing, crosstalk between the individual optical amplifiers 14 may be reduced or prevented. The distance between any of the optical amplifiers 14 and either of the first and second data processing devices 11, 12 may be at least 30 km.

[0080] Optical amplification is performed in the active fiber (section) 15 of the optical amplifier 14. For example, the active fiber section 15 is pumped at a pump wavelength of 980 nm via a pumping device 16, a supply fiber 17 and a coupling element 18, which contains erbium (Er) to allow amplification at a signal wavelength of about 1530 nm. 3+ )-doped fiber. Additionally or alternatively, thulium-doped fiber (pumped at 1450 nm to 1550 nm), neodymium-doped fiber (pumped at 800 nm to 930 nm), or ytterbium-doped fiber (pumped at 915 nm to 980 nm) may be used. The use of erbium-doped fiber is advantageous in that the amplified signal wavelength is well matched to the transmission window of standard silica fiber.

[0081] For example, pumping device 16 may be a diode or a laser, and is configured to emit radiation having a particular pump wavelength. Coupling element 18 is a beam splitter-like device for directing radiation of different wavelengths into a single optical fiber, and is connected to active fiber section 15 and pumping device 16. For example, coupling element 18 may be a wavelength division multiplexing (WDM) element. All fiber connections (e.g., between active and passive fiber sections of the transmission line) may be connected to minimize unwanted local fiber connection losses.

[0082] Unlike standard optical amplifier setups, this system does not include an optical isolator or coupler. Isolators are configured to pass light only in one direction and are typically used to minimize the risk of multiple reflections within the active fiber section 15, which could potentially cause the optical amplifier 14 to become a laser. A tap coupler typically directs approximately 1% of the optical pulses to a photodetector to monitor the optical amplifier 14 (in particular, the input and output power, operating mode, and gain coefficient). This portion could be intercepted by an eavesdropper. The connections between different sections of the transmission line can be optimized to mitigate back reflections and avoid lasing in the active fiber section 15.

[0083] 2 shows an energy diagram illustrating the amplification of light in an erbium-doped fiber section 15. Pump radiation 20, having a pump wavelength of, for example, 980 nm, emitted from a pumping device 16 penetrates the erbium-doped fiber section (pump absorption R 13 ) and the erbium ion is absorbed at the first (ground) level 21( 4 I 15 / 2 ) to the relaxation time τ 32 ≒20μs (short-lived) third level 23( 4 I 11 / 2 ) from the third level 23 with a longer relaxation time τ 21 ≒10ms (metastable) second level 22(4 I 13 / 2 )

[0084] A light pulse passing through an erbium-doped fiber section emits a light pulse (stimulated signal emission rate W 21 ), causing a stimulated transition from the second level 22 to the first level 21. The magnitude of the resulting signal amplification depends on the concentration of erbium ions, the length of the active fiber section 15 and the intensity of the pumping radiation.

[0085] The population density dynamics of the second level 22 can be explained by the following equation:

[0086]

number

[0087] where n1 denotes the relative distribution density of the first level 21, and W 12 denotes the absorption rate of the induced signal. (A 21 ) and from the third level 23 to the second level 22 (A 32 ) The rate of spontaneous transitions depends on the properties of the active medium. Ratio A 32 =1 / τ 32 is the ratio A 21 =1 / τ 21 , the population density of the third level may be neglected for certain considerations. The population density of the second level 22 is then determined mainly by the ratio of the pumping rate to the signal rate (which is determined by the pump power and the signal power when other parameters are fixed). Increasing the signal power increases the stimulated emission rate W 21 is pumped to higher levels R 13 The ion ejection rate to the inversion level can become larger than the ion ejection rate to the inversion level, and the inversion level begins to decrease.

[0088] Pump absorption and / or emissivity R 13、 R 31 depends on the pump power and the signal absorption or emissivity W12 , W 21 depends on the signal power. In particular, it is given by

[0089]

number

[0090] Here, the signal power is P s , the absorption cross section is σ a (ν s ), σ a (ν p ), and the emission cross section is σ e (ν s ), σ e (ν p ), and the pumping frequency is ν p , the signal frequency is ν s , the mode spot size is ω s , ω p and the envelope of the pumping / signal mode is Ψ p , Ψ s Let's say.

[0091] The gains can be approximately:

[0092]

number

[0093] where Γ is the overlap factor between the signal mode and the active medium, σ a (ν s ) is the signal frequency ν s is the absorption cross section at ρ, the concentration of the active ion, η s =σ e (ν s ) / σ a (ν s ), n 2,i is the population density of the second levels 22 (fraction of ions in the second levels 22) before the light pulse, and L is the length of the active fiber section. This approximation is valid for a uniform distribution of the population density of the second levels 22 over the entire volume of the active fiber section 15.

[0094] Taking into account the radial dependence of the distribution density of the second levels 22 and the overlap of the signal mode with the active medium (as well as spontaneous emission), the change in signal power when passing through the active fiber section 15 can be expressed as follows:

[0095]

number

[0096] where N2(r)=ρ(r)2, N1=ρ-N2, P0=hν is the power of the fictitious noise photons at the amplifier input in the band δν,

number

[0097] The pump absorption coefficient is:

[0098]

number

[0099] where σ a (ν p ) is the pumping frequency ν p For a given cross section, fiber section length L and concentration, the population density of second levels 22 can determine the value of gain G and pump absorption.

[0100] (c) Limitation of Increased Profits An eavesdropper could carry out an attack through the optical amplifier by gaining access to pumping device 16 and increasing the pumping power, thereby increasing the gain and therefore the signal power. The eavesdropper could then divert the resulting excess optical pulses, possibly without being noticed.

[0101] Standard amplifiers are usually configured for maximum efficiency in converting pump power to signal power. However, various factors limit this efficiency, particularly the value of the pump absorption coefficient in the active fiber section 15. Primarily, the concentration of active ions, the length of the active medium, and the value of the overlap coefficient between the pump mode and the active medium determine the value of the pump absorption coefficient. Therefore, the active fiber parameters can be determined for maximum pump power absorption.

[0102] If the active fiber section 15 has a certain length and concentration of erbium ions such that the resulting absorption coefficient is 15 dB, all ions in the active fiber section 15 can absorb this portion of the pump power (if all active ions are in the ground state). For a "transparent" active fiber section 15, the number of erbium ions in the ground state / first level 21 is approximately 50%, which corresponds to half the absorption coefficient. If 90% of the erbium ions absorb the pump radiation 20 and reach the third level 23 (and then relax to the second level 22), only 10% of the erbium ions can absorb the pump radiation 20. Therefore, a 150 dB absorption coefficient of a non-inverted fiber can be used to obtain the same efficiency as a 90% inverted medium. This can be achieved at the expense of excess active fiber section length or excess concentration of (erbium) ions in the active fiber section 15, where "excess" corresponds to additional absorption compared to the non-inverted case.

[0103] As long as there are enough ions present to effectively absorb the pump power, the optical amplifier 14 maintains a gain that is sensitive to changes in pump power. In such a situation, a pump power increase of, for example, 10% results in a gain increase of the same order of magnitude. In contrast, the optical amplifier 14 can be configured to be insensitive to increases in pump power. This can be achieved by a low pump absorption coefficient, i.e., providing pump radiation 20 with a large amount of pump power to the active fiber section while still only a small fraction is absorbed. In this case, the population inversion density approaches 100% (e.g., 99%). Therefore, excessive pump power results in a high level of population inversion density at the expense of energy efficiency. Even with an arbitrary increase in pump power level, the population inversion density does not increase by more than, for example, 1%. Therefore, the maximum gain increase also does not exceed 1%. As a result, an optical amplifier 14 that is insensitive to increases in pump power can be provided.

[0104] Nevertheless, the optical amplifier 14 can resist eavesdropping attacks by increasing the pump power only for relatively low intensity optical pulses, because high intensity or high energy pulses can reduce the population inversion density and return the optical amplifier 14 to a state where it is sensitive to increases in pump power.

[0105] (d) High-energy pulse The light pulses may have durations of 3 ns to 20 μs and pulse peak powers of up to 200 mW, and therefore relatively high energies of up to 4 μJ (e.g., for pulses for optical time-domain reflectometry). These light pulses are generated over a period of time equal to the lifetime of the erbium ions in the second level 22 / relaxation time τ of the erbium ions in the active fiber section 15 to the first level 21. 21The duration of these light pulses is shorter than 10 ms (approximately 10 ms). The energy of these light pulses is high enough to substantially affect the population density of the erbium population levels during the pulse's transit through the active fiber section 15. Because the light pulses are too short, they act differently than a continuous or quasi-continuous signal on the population density of the second levels 22.

[0106] A high-energy pulse passing through the active medium can remove a significant portion of the population inversion density to some extent when the active medium becomes transparent (i.e., when the gain is equal to 1). Thus, if a high-energy pulse arrives and there is insufficient energy stored in the active medium, the active medium cannot amplify the high-energy pulse with the same gain as a low-energy pulse.

[0107] High-energy pulses can be further distorted when passing through an active medium. In particular, if the leading edge of a high-energy pulse is amplified by the active medium, the local population density of second levels 22 can decrease, reducing the amplification of the middle part of the high-energy pulse. Furthermore, the trailing edge of a high-energy pulse cannot be amplified by simply passing through the (then transparent) active medium. The resulting distortion can cause errors during processing of the optical pulse, especially for transmission line monitoring and optical time-domain reflectometry.

[0108] Pulse distortion is

number

[0109] The energy stored in the inverted active medium can be approximated as:

[0110]

number

[0111] where ν denotes the frequency of the amplified signal, ρ denotes the concentration of active ions (total amount of active ions per unit volume), a denotes the radius of the active fiber section 15, L denotes the active fiber section length, and n 2,f denotes the distribution density of the second level 22. This corresponds to being in the transparent state (when the gain is equal to 1 and when essentially equal amounts of active atoms are in the first state 21 and the second level 22).

[0112] The distribution density of the second level 22 before the pulse passes can be calculated as follows.

[0113]

number

[0114] where τ is the lifetime of erbium in the second level, R 13 is the pump absorption rate, R 31 is the rate of stimulated emission from the third level 23 to the first level 21, A 32 is the non-radiative transition rate from the third level 23 to the second level 22, W 12 is the forced signal absorption rate, W 21 is the stimulated signal radiation rate (see E. Desurvire, Erbium-Doped Fiber Amplifiers, Principles and Applications (2002)).

[0115] The distribution density of the second levels 22 corresponding to the transparent state of the active medium can be approximated by the following equation:

[0116]

number

[0117] The optical amplifier may be configured using parameters according to (Table 1).

[0118] [Table 1]

[0119] where r=0 and Ψ p,s When (r)=1, the corresponding transition rate value is W 21 =446.6, R 31 =31337.8, W 12 361.7 and R 13 = 154495.3 [1 / c]. The corresponding value of the energy stored in the active medium is E = 4.8 μJ. For an optical pulse with a maximum peak power of 200 mW and a length of 20 μs, the energy is 4 μJ. Such an optical pulse therefore strongly influences the inversion. For long transmission lines 10, the peak pulse duration can reach several hundred μs with a similar peak current and even higher pulse energy.

[0120] The energy stored in the active fiber section 15 can generally be increased by increasing the active fiber section length. However, according to the proposed method, the active fiber section length is already determined for the target maximum gain.

[0121] The energy stored in the active fiber section 15 can also be increased by increasing the fiber core cross-sectional area size of the active fiber section 15, particularly its diameter. Increasing the fiber core cross-sectional area while keeping the pump power level constant will proportionally decrease the pump power density. It is the pump power density that determines the rate at which erbium ions transition to higher levels. To reach the inversion level before increasing the fiber core cross-sectional area, the pump power must be increased accordingly. In this case, more energy is stored in the active medium, automatically providing an opportunity to amplify large input pulse energies.

[0122] (e) Optical amplification for quantum key distribution In Figure 3, a graph of a quantum key distribution method based on the above considerations is shown.

[0123] In an initial step 30, a transmission line 10 is provided for transmitting optical pulses between a first data processing device 11 and a second data processing device, and an optical amplifier 14 having an active fiber section 15 and a pumping device 16 is arranged on the transmission line 10.

[0124] In a first step 31, a target operating gain for the optical amplifier 14 is determined. The target operating gain may correspond to signal attenuation along the transmission line 10. For example, if the number of photons in an optical pulse drops by a factor of 10 after 50 km of transmission (transmission factor T=0.1), then the target operating gain of the optical amplifier at 50 km is G op =10.

[0125] In a second step 32, a target maximum gain for the optical amplifier 14 is determined based on the optical signal leakage, eg, the minimum detectable optical signal leakage.

[0126] For example, if the maximum number of photons that can be diverted / leaked by an eavesdropper is determined, a gain increment corresponding to the target operating gain can be determined and then added to the target operating gain to obtain the target maximum gain. In particular, from the maximum number of photons that can be diverted, a corresponding maximum divertable signal power (e.g., P s =Nhν s / t s This maximum transferable signal power may correspond to the power that can be obtained by increasing the gain of optical amplifier 14 by a gain increment above the target operating gain.

[0127] Furthermore, the active fiber section length is adjusted so that increasing the pump power does not allow amplification beyond the target maximum gain. The dependence / correspondence between gain and pump power can be determined, for example, by varying the pump power and measuring the amplification of the corresponding optical pulses.

[0128] In a third step 33, a core cross-sectional area size of the active fiber section 15 is determined (e.g., via a core diameter of the active fiber section 15) based on the maximum allowable pulse shape distortion and the target maximum energy per pulse. To this end, a high-energy optical pulse having a target maximum energy per pulse is transmitted through the active fiber section, a pulse shape metric indicative of the pulse shape distortion of the high-energy optical pulse is determined, the pulse shape metric is compared with the maximum allowable pulse shape distortion, and the core cross-sectional area size is adjusted based on the comparison result.

[0129] In a fourth step 34, an operating pump power of the pumping device 16 is determined such that the optical pulses are amplified at a target operating gain. The determined target operating gain, target maximum gain, and corresponding pump powers limit the percentage of optical pulses that can be diverted, no matter how much an eavesdropper with access to the pumping device 16 increases the pump power above the operating pump power.

[0130] The shared key can then be determined between the first data processing device 11 and the second data processing device 12 by quantum key distribution along the transmission path 10, in which optical pulses (emitted from the first or second data processing device 11, 12) are amplified via the optical amplifier 14 by operating the pumping device 16 at an operating pumping power.

[0131] (f) Shared key determination In figure 4 a graphical representation of further steps for determining a shared key by quantum key distribution is shown, corresponding to step 35. The first data processing device 11 and the second data processing device 12 are connected via an authenticated (public) classical channel, the optical fiber of the transmission line 10 serving as the quantum channel.

[0132] In an initial step 40, an initial internal loss profile of the transmission line 10 (particularly its optical fiber segment) is determined, which essentially represents the natural signal loss in the transmission line 10. This preliminary step should ensure that there are no eavesdroppers accessing the transmission line 10. The initial loss profile may be shared between the first and second data processing devices 11, 12 via an authenticated classical communication channel.

[0133] In a first step 41, a physical loss control of the transmission path 10 is performed (e.g. by the first data processing device 11 and the second data processing device 12). In particular, an internal loss profile is determined and shared between the first data processing device 11 and the second data processing device 12, preferably via a classical channel.

[0134] By comparing this updated internal loss profile with the initial internal loss profile, the proportion of signals that could have been captured by an eavesdropper, r E For example, if the natural signal loss in the section of the transmission line 10 that does not include the optical amplifier 14 is denoted by r0, and an eavesdropper determines the intercepted portion of the signal, r E If we intercept the intercepted part r E is the relation (1-r t )=(1-r E )(1-r0) to get the total loss r t can be derived from

[0135] Intercepted part r E The protocol terminates when the effective key rate becomes too large and the legitimate user loses its information advantage over the eavesdropper. The termination of the protocol depends on the length of the transmission line 10 and the distance between the two optical amplifiers 14. In particular, the protocol can be terminated if the effective key rate falls below the target key rate value.

[0136] In a second step 42, a bit sequence of length l is determined in the first data processing device 11 using a random number generator.

[0137] In a third step 43, the bit sequence is encoded into an optical signal comprising a series of l key-distribution pulses and transmitted via transmission line 10 to second data processing device 12. Signal bits 0 and 1 correspond to coherent states |γ0> and |γ1>, respectively. The specific manner in which the 0 and 1 signal bits are encoded into the parameters of coherent states |γ0> and |γ1> may vary. For example, the signal bits may be encoded into coherent optical pulses with different intensities / photon numbers and the same phase, or may be encoded into coherent optical pulses with the same intensity but different phases.

[0138] In a fourth step 44 , the optical pulses are amplified by one or more optical amplifiers 14 located along the transmission line 10 .

[0139] In a fifth step 45, the optical signal is received and measured by the second data processing device 12. The corresponding received bit sequence is determined in the second data processing device 12.

[0140] In a sixth step 46, non-deterministic signal bits corresponding to quantum measurements determined to be non-deterministic in the second data processing device 12 are discarded from the bit sequence in the first data processing device 11 and from the received bit sequence in the second data processing device 12. For this purpose, the bit positions of the non-deterministic signal bits are communicated from the second data processing device 12 to the first data processing device 11 via a classical channel.

[0141] In a seventh step 47, by disclosing the bit sequence and / or a part of the received bit sequence via a classical channel, an error rate can be determined for the bit sequence and / or the received bit sequence by the first data processing device 11 and the second data processing device 12, respectively, and an error correction can be performed. The error correction can be performed, for example, using a Low Density Parity Check (LDPC) code, so that an error-corrected bit sequence is determined in the first and second data processing devices 11, 12.

[0142] In an eighth step 48, an amplified key sequence is determined from the error-corrected bit sequence using privacy amplification. The amplified key sequence is shorter than the error-corrected bit sequence, so that a potential eavesdropper has no or negligibly small information about the amplified key sequence. The amplified key sequence represents the shared key sequence between the first data processing device 11 and the second data processing device as a result of the quantum key distribution.

[0143] The first step 41 to the eighth step 48 are repeated (arrow 49) concatenating the amplified key sequences to the (entire) shared key until the total length of the shared key is as required for the current application.

[0144] During all steps 41 to 49, the transmission path 10 may be continuously controlled (arrow 40a). Thus, a signal loss profile is determined and shared between the first data processing device 11 and the second data processing device 12, preferably via a classical channel. If the integrity of the transmission path 10 is compromised to such an extent that there is a significant risk that an eavesdropper will be able to decipher the scattering loss, the protocol may be terminated.

[0145] (g) Transmission line control 5 shows an exemplary reflectogram obtained from an optical time-domain reflectometry measurement corresponding to a signal loss profile. The reflectogram shows the logarithm of the backscattered signal (particularly a high-intensity optical test pulse) power as a function of the distance between the reflectometer and the corresponding discontinuity. The reflectometer can be located within or near the first data processing device 11 and / or the second data processing device 12.

[0146] Natural signal loss along the transmission path 10 is due to homogeneous scattering, resulting in an exponential decay of power corresponding to a linear region 50. Reflectogram features 51-54 include deviations from the exponential decay of the reflectogram curve, and in particular sharp peaks and / or drops in the reflectogram curve, making it possible to classify signal loss at corresponding locations along the transmission path 10. This is particularly useful in the initial step 40 for determining a shared key, where it is important to identify and mitigate local losses for comparison with losses determined during key exchange.

[0147] Reflectogram features 51-54 generally correspond to imperfections in the transmission line 10 and may represent, for example, poor quality connections, bends, or different connectors. Scattering losses from such regions are localized with respect to the transmission line 10. Peaks in reflectogram features 51-54 may be due to excess scattering resulting from the test pulse undergoing Fresnel reflections in the case of physical connectors. The noisy region 55 on the right side of the reflectogram represents the end of the backscattered signal.

[0148] Additionally or alternatively, the transmission path control may include determining and analyzing the strength of test pulses transmitted by the first data processing device 11 and received by the second data processing device 12 in order to classify signal losses at respective positions on the transmission path 10.

[0149] The features disclosed in the specification, drawings and / or claims may, alone or in various combinations thereof, be material for realizing the various embodiments.

Claims

1. An optical signal amplification method, the optical signal amplification method being realizable in a system having the following configuration: a transmission path (10) for transmitting an optical signal between a first data processing device (11) and a second data processing device (12); an optical amplifier (14) disposed on the transmission line (10), the optical amplifier (14) comprising an active fiber section (15) and a pumping device (16); The optical signal amplification method includes: determining a target operating gain for the optical amplifier (14); determining a target maximum gain for the optical amplifier (14); determining an active fiber section length such that the optical signal is amplified at or below the target maximum gain; determining a core cross-sectional area size of the active fiber section (15) based on a maximum allowable pulse shape distortion and a target maximum energy per pulse such that a high-energy pulse having the target maximum energy per pulse is distorted up to the maximum allowable pulse shape distortion; determining an operating pump power of the pumping device (16) that is lower than a maximum pump power such that the optical signal is amplified at the target operating gain. Optical signal amplification method.

2. moreover, determining a shared key between the first data processing device (10) and the second data processing device (11) by quantum key distribution, which includes operating the pumping device (16) at the operating pump power to amplify the optical signal through the optical amplifier (14); 2. The method for amplifying an optical signal according to claim 1.

3. The target maximum gain is determined from the target operating gain and a gain increase value determined from the optical signal leakage or the minimum detectable optical signal leakage.

3. The optical signal amplification method according to claim 1 or 2.

4. The optical signal leakage is determined via the transmission path (10).

4. The method for amplifying an optical signal according to claim 3.

5. Determining the core cross-sectional area of the active fiber section (15) comprises: determining at least one of a core diameter, a core radius, and a core circumference of the active fiber section (15); 3. The optical signal amplification method according to claim 1 or 2.

6. Determining the core cross-sectional area size of the active fiber section (15) transmitting the high-energy pulses through the active fiber section (15) at the target maximum energy per pulse; determining a pulse shape metric indicative of a distortion in a pulse shape of the high-energy pulse; comparing the value of the pulse shape metric to the maximum allowed pulse shape distortion; and adjusting the size of the core cross-sectional area based on the results of the comparison.

6. The method for amplifying an optical signal according to claim 5.

7. determining the pulse shape metric includes determining a first form actor of the optical pulse before amplification and a second form actor of the optical pulse after amplification; 7. The method for amplifying an optical signal according to claim 6.

8. further comprising determining an optical signal loss along the transmission path (10) or as a function of position along the transmission path (10).

3. The method for amplifying an optical signal according to claim 2.

9. the optical signal includes test pulses for determining optical signal loss along the transmission path (10) or optical signal loss as a function of position along the transmission path (10), and key distribution pulses for determining the shared key, the test pulses including a higher energy per pulse than the key distribution pulses; 9. The method for amplifying an optical signal according to claim 8.

10. The method further includes providing the transmission line (10) with a physically unduplicable structure, the transmission line (10) including a fiber core section doped with at least one of Al, P, N, and Ge.

3. The optical signal amplification method according to claim 1 or 2.

11. further comprising providing the active fiber section (15) with at least one of an erbium-doped fiber section, a thulium-doped fiber section, a neodymium-doped fiber section, and an ytterbium-doped fiber section.

3. The optical signal amplification method according to claim 1 or 2.

12. Further, the method includes providing the transmission line (10) without an optical isolator and a tap coupler.

3. The optical signal amplification method according to claim 1 or 2.

13. further comprising providing the optical amplifier (14) as a bidirectional optical amplifier.

3. The optical signal amplification method according to claim 1 or 2.

14. further comprising disposing a plurality of optical amplifiers (14) in the transmission line (12), each optical amplifier (14) comprising a further pumping device (16) and a further active fiber section (15) having the active fiber section length; determining the shared key includes amplifying the optical signal through the plurality of optical amplifiers (14) by operating each of the additional pumping devices (16) at the operating pumping power; 3. The method for amplifying an optical signal according to claim 2.

15. the pumping wavelength of the pumping radiation emitted from the pumping device (16) is adjusted to be less than 10 nm away from the peak wavelength of the absorption spectrum of the active fiber section (15); 3. The optical signal amplification method according to claim 1 or 2.

16. the pump wavelength of the pump radiation emitted from the pump device (16) is stabilized by controlling the temperature and / or power supply of the pump device (16); 16. The method of amplifying an optical signal according to claim 15.

17. 1. An optical signal amplification system, comprising: a transmission path (10) for transmitting an optical signal between a first data processing device (11) and a second data processing device (12); The optical signal amplification system further comprises an optical amplifier (14) disposed on the transmission line (10), the optical amplifier (14) comprising an active fiber section (15) and a pumping device (16); 3. An optical signal amplification system configured in accordance with the optical signal amplification method of claim 1.

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