4. Conclusion

the BNF in synchronization channel can be implemented to RFCS method realization structure. In this case, it contains one narrowband photodetector with a 10 GHz band and filters tuned to frequencies at 0, 100 and 200 MHz from the carrier with a bandwidth of 10 MHz. The procedure for such analysis relates to in-line monitoring procedures, and the arrangement of frequency components at a frequency of 100 MHz from the frequency of the RFCS allows us to speak about identical for all three constituent CD parameters and the corresponding identical change in their

Using the algorithm for approximating BNF spectrum by the Gaussian curve at each measurement makes it possible to increase the accuracy of determining its

Without going into the details of the physical nature of the phenomena, the level of the main noise of the photodetectors is higher than the level of background noise of scattering in the fiber and elements of the installation and determines the possibility of detecting the received signal (Figure 5). The gain in improving the OSNR in comparison with single-frequency measurements on the RFCS is determined by

amplitudes as it increases or decreases.

Quantum Cryptography in Advanced Networks

central wavelength by an order.

the following expression [15–20, 27]:

G ¼

BW ðPD

2 4

(10 MHz) at frequencies of 0, 100–200 MHz.

data, the gain in OSNR can be 10–12 dB.

Figure 5.

6

0

S

3 5

, ð BW<sup>3</sup>

0

S þ

where S ¼ S f ð Þdf is the spectral noise density of the receiver, BWPD is the bandwidth of the PD equal to 10 GHz, and BW<sup>3</sup> is the frequency band of filters

in different frequency regions S( f ) for different types of photodetector (with bandwidths 10 GHz) and filters (with bandwidths 10 MHz), respectively. In filter regions near 100 and 200 MHz, there are only thermal and shot noises. Current noises are characterized only for narrow DC filter. Taking into account the given

Thus, when using in-line CD monitoring with a preliminary amplitude modulation of the RFCS, the gain relative to the OSNR compared to the monitoring

Illustration for OSNR gain catch for three-frequency probing method: Noise—spectral characteristics of noise for photodetector realizations bandwidth BWPD. Three-frequency probing method has complex gain parameters

and characterizes only by one disadvantage of large current noise filtering on DC component.

The gain is determined by the different filter bandwidth, nature, and noise level

ð fCL 100þBW<sup>3</sup> 2

S þ

ð 2fCL <sup>100</sup> <sup>þ</sup>BW<sup>3</sup> 2

S

3 7 7

5, (2)

2fCL <sup>100</sup> �BW<sup>3</sup> 2

fCL 100�BW<sup>3</sup> 2

We demonstrated a microwave photonic method for CD monitoring of a highspeed synchronization channel based on analyzing the reflectance spectrum of the built-in BNF with the preliminary amplitude modulation of the optical carrier and the RFCS method. The results of the simulation show that the proposed method of in-line monitoring makes it possible to determine the magnitude and direction of the filter shift by poly-harmonic probing. Thus, taking into account its position relative to the carrier, we will reduce the error in classical CD measuring which is usually equal to 10%/°C (1.5–3 dB). When using in-line CD monitoring with the preliminary amplitude modulation of the RFCS, the gain relative to OSNR compared to the monitoring at carrier frequency can be 10–13 dB. Thus, CD analysis by the RFSC method can be performed starting from the classical OSNR of 3–5 dB.

The application of such type monitoring methods for QKD-CMC system will allow us to suggest an even wider use in the transition to homodyne and heterodyne systems. It should be noted that their development is still carried out at a theoretical level and does not take into account the fact that with an increase in the length of telecommunication lines, the probability of increasing the influence of СD on the phase value of the coded components will increase significantly.
