**4.1. Basic characteristics**

14 Optical Communication

effect. [17,18].

**3.4. Decoder circuit** 

**Figure 10.** The configuration of the coding of the Y-00 transmitter

and key synchronization for encryption of Y-00 [17,18].

Figure11 shows the configuration of the decoder circuit in the Y-00 receiver. The Seed Key information and irregular mapping information are also provided in the receiver as common information. The basic circuit configuration of the decoder is the same as the encryption circuit of the transmitter. However, the receiver does not perform the decoding of KDSR. Therefore, from the decoder outputs a decoding signal (threshold value selection information) of Y-00. The threshold value controller distinguishes the signal on a bit by bit to generate a threshold value level with the best level and timing. Optimum adjustment is made for the threshold level and timing by the Automatic Gain Control (AGC) amplifier and the threshold value controller. Furthermore, the decoder establishes bit synchronization

synchronization is added. OSK processing is added to this signal to generate a main signal to be the original signal as described in section 3.1. On the other hand, multi-value level selection signal is generated as follows. Running Key is generated from the Seed Key as a first. Then a base selection signal to be the original signal is generated using the randomly mapped base configuration information. This selection signal generates a multi-level selection signal level after processing by KDSR. The multi-value level selection signal that is the same as the main signal is weighted by each driver circuit and added to determine the multi-value level and generate an encoding signal for encryption of Y-00. The operating principle of this final-stage processing is the same as that of the Digital to Analog (D/A) converter. By driving the optical external modulator using the Y-00 signal generated, the Y-00 encryption signal becomes an optical signal with valid quantum noise

> To verified the adaptability to existing optical communication networks as Y-00 encryption equipment. Prototype equipment was produced and evaluated based on the content in section 3. The prototype targeted standard specifications of OC-48 (Optical Carrier: SONET standard) optical communication as IEEE standard considering practical use. Table1 shows evaluation results [17]. The transmission rate of original plaintext data is 2.48832 Gbps conforming to OC-48 and the average optical output power is 0 dBm. This is achieved transmission distance of 50 km without relay.

> A receiving sensitivity of -15.3 dBm was obtained at a bit error rate (BER) of 1E-12 with a transmission rate of 2.48832 Gbps and an average optical output power of 0 dBm.


**Table 1.** Major characteristics

### **4.2. Transmission experiment**

Low-delay real-time transmission of encrypted uncompressed full-specification High-Definition-Television (HDTV) moving picture data was performed using the prototype Y-00 transmission equipment. Figure12 shows the BER in the back-to-back transmission. The minimum receiving sensitivity is -15.3 dBm when BER=1E-12. The average input power of the Y-00 transmission equipment (receiver) is approx -10 dBm and the margin of 5.3 dB. It enabling 40 km transmission when considering the optical fiber loss (approx. 0.25 dB/km). Figure13 shows the transmission system in the experiment. The transmitter converts the OC-48 optical signal from 1.5 Gbps moving picture data of signal source by the High Definition Serial Digital Interface to Synchronous Digital Hierarchy (HD-SDI/SDH) converter. And then encrypts the signal by the Y-00 cipher transmission equipment (transmitter). The encrypted optical signal is transmitted through a 40 km single mode fiber (SMF). The receiver decodes encryption signal. And then restores the original moving picture data by the HD-SDI/SDH converter. The latency of the transmission system shown in Figure13 is approx 500 μs. It is achieving secure real-time high-definition moving picture transmission that hardly shows visible delay in monitor images before and after transmission. This result has verified that the Y-00 cipher transmission equipment is applicable to medical sector and financial system networks which require real-time response [27,28].

**Figure 12.** Receiver sensitivity of the regular receiver

**Figure 13.** Real-time HDTV transmission experiment

#### **4.3. Field test using a commercial line**

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networks which require real-time response [27,28].

**Figure 12.** Receiver sensitivity of the regular receiver

**Figure 13.** Real-time HDTV transmission experiment

Y-00 transmission equipment (receiver) is approx -10 dBm and the margin of 5.3 dB. It enabling 40 km transmission when considering the optical fiber loss (approx. 0.25 dB/km). Figure13 shows the transmission system in the experiment. The transmitter converts the OC-48 optical signal from 1.5 Gbps moving picture data of signal source by the High Definition Serial Digital Interface to Synchronous Digital Hierarchy (HD-SDI/SDH) converter. And then encrypts the signal by the Y-00 cipher transmission equipment (transmitter). The encrypted optical signal is transmitted through a 40 km single mode fiber (SMF). The receiver decodes encryption signal. And then restores the original moving picture data by the HD-SDI/SDH converter. The latency of the transmission system shown in Figure13 is approx 500 μs. It is achieving secure real-time high-definition moving picture transmission that hardly shows visible delay in monitor images before and after transmission. This result has verified that the Y-00 cipher transmission equipment is applicable to medical sector and financial system

We made a transmission experiment using an existing commercial line optical fiber to obtain further prospect of practical use. Figure14 shows the system of the transmission experiment that was actually made. The distance of each transmission span is 48 km and the average span attenuation is 14.5 dB. We made a transmission experiment of total distance 192 km with relay at three location using optical fiber amplifiers (EDFA). Figure15 shows the result of receiving sensitivity measured at the reception end. Figure16 shows waveforms of encrypted and decoded signals. We verified a receiving sensitivity of -18.4 dBm and -19.4 dBm respectively at a BER of 1E-12 in 192 km bidirectional transmission. Also we verified adaptability to optical amplifier repeater transmission. In addition, we have confirmed that the encryption of Y-00 can be applied in Fiber Channel (FC) and Gigabit Ether (GbE). The measured latency value of the transmission system was 1.29 ms in total including the delay of fiber length. Furthermore, we made WDM transmission experiment multiplexing optical output signals from two opposed Y-00 units and verified error-free transmission at each wavelength [17,18,21].

**Figure 14.** 192 km relay Y-00 encrypted transmission through commercial fibers

**Figure 15.** Received optical power sensitivity (192 km)

**Figure 16.** Y-00 transmission wave pattern

### **4.4. Application to 10 Gbps transmission**

This section describes a trial toward large-volume transmission that is the trend of optical communication. Y-00 encryption transmission equipment for 10 Gbps transmission based on optical intensity modulation has been developed in japan [30,31]. The design concept of this equipment is the same as the above-mentioned 2.4 Gbps transmission equipment except that dedicated high-speed devices have been developed to realize the equipment. This section describes the result of 360 km transmission experiment using optical fibers (for experiment laid in Tamagawa University) installed in the field. Figure17 shows the configuration of the transmission system. The 360 km transmission path contains nine EDFA for relay at intervals of 40 km using standard single mode fibers (SMF). Dispersion values are adjusted by the dispersion compensating fiber (DCF) and the tunable dispersion compensator (TDC) to set the residual dispersion to +1170 ps/nm. The optical interface conforms to OC-192. The optical output power is -1.7 dBm in the back-to-back transmission and the full-amplitude extinction ratio is 2.5 dB. Furthermore, the encryption contains various types of randomization for enhance safety. The transmission path is also provided with an optical preamplifier and an optical bandpass filter in the receiver to ensure the S/N for normal receivers.

**Figure 17.** 360 km Y-00 transmission system

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**Figure 15.** Received optical power sensitivity (192 km)

**Figure 16.** Y-00 transmission wave pattern

**4.4. Application to 10 Gbps transmission** 

This section describes a trial toward large-volume transmission that is the trend of optical communication. Y-00 encryption transmission equipment for 10 Gbps transmission based on optical intensity modulation has been developed in japan [30,31]. The design concept of this equipment is the same as the above-mentioned 2.4 Gbps transmission equipment except that dedicated high-speed devices have been developed to realize the equipment. This section describes the result of 360 km transmission experiment using optical fibers (for experiment laid in Tamagawa University) installed in the field. Figure17 shows the configuration of the transmission system. The 360 km transmission path contains nine EDFA for relay at intervals of 40 km using standard single mode fibers (SMF). Dispersion values are adjusted by the dispersion compensating fiber (DCF) and the tunable dispersion compensator (TDC) to set the residual dispersion to +1170 ps/nm. The optical interface conforms to OC-192. The Figure18 shows transmission waveforms (eye diagram). They are encrypted waveforms with no eye-opening at each transmission distance. Figure 19 shows characteristic of normal receiver (back-to-back, 40 km, 60 km and 80 km of non repeater transmittion and optical amplifier repeater transmission of 360 km) and the BER of eavesdropper. The minimum receiving sensitivity is -12.2 dBm (BER=1E-12) as shown in Figure 19. And the BER of 360 km transmission is 5.0 × 1E-7. Furthermore, we obtained results that satisfy receiving sensitivity -4 dBm at a BER of 5.0 × 1E-5 which is the target specification considering code error correction under all conditions. We evaluated adjacent signal detection of multi-value signal in the back-to-back transmission to evaluate tapping capability. And obtained a satisfactory result of eavesdropper's BER larger than 0.4. This evaluation has proved that the Y-00 transmission equipment is sufficiently applicable to 10 Gbps transmission. Thus we could obtain prospects for high-speed transmission [29,30,31].

## **5. Conclusion (future prospects and possibilities)**

Based on the Yuen 2000 protocol (Y-00) theory as the research result of H.P.Yuen and O.Hirota, we have developed the Y-00 encryption transmission equipment using quantum noise effects and have verified the practicality of the equipment. We verified the safety and adaptability to existing systems based on trial production results of the equipment and obtained prospects for practical use. The results show the high completeness of the equipment. Hitachi Information & Communication Engineering has been engaged in the development of prototype equipment and is further improving the reliability of the Y-00 encryption transmission equipment for the productization (Figure20). Trial production results show that the Y-00 system can achieve long-haul, large-capacity, high-speed real-

**Figure 18.** 10 Gbps Y-00 transmission wave form

**Figure 19.** Bit error rate (10 Gbps)

time transmission with low latency. Thus application of the Y-00 system to various fields can be expected. The Y-00 system is also applicable to uncompressed high-definition image transmission in particular, which extends the range of use. Since conventional optical communication technologies that are being developed at present can be used technically, development trends (such as large capacity, downsizing, and power-saving) can be maintained in common. Furthermore, existing optical communication infrastructures are available and allowing co-existence and combined use with current systems and reducing initial costs. Thus we can expect the use of the Y-00 system in wide applications.

**Figure 20.** Y-00 encryption transmission equipment for the productization
