1. Field of the Invention
The present invention relates to an optical communication system and an optical communication method, for transmitting confidential information by an optical signal.
2. Description of the Related Art
Conventional optical communication merely receives an emitted light to transmit information, and accordingly, there has not been any device for keeping the confidentiality of transmitted information against a third party. In one-on-one optical communication between two points, usually there is not caused a failure even if information transmitted by a light is not kept confidential. However, in an optical network having an optical add/drop multiplexing (to be referred to as OADM hereunder) function of dropping or adding a specific wavelength of a wavelength division multiplexed (to be referred to as WDM hereunder) signal light or an optical cross-connecting (to be referred to as OXC hereunder) function of performing the routing of optical signals on their own without converting the optical signals into electric signals, since many users share a fiber network in the network, it is possible for a third party to fetch the optical signal to receive it. Further, it is also possible for this third party to amplify the original optical signal after fetching, so as to cover up the interception from an interested party. Optical signals communicated in the existing optical communication system are considered to be in a vulnerable state to network providers or hackers of malicious intent.
In the existing optical communication systems, the encryption processing by means of software is performed on an electric signal which is used for the optical modulation in accordance with transmitted information. However, such encryption by means of software may be defeated by means of software with a high possibility. Even if a cipher cannot be broken in real time, sometimes, the cipher can be broken if the encrypted signal is recorded to be processed for a long stretch of time utilizing a large-scale computer.
Contrary to such encryption of electric signal before the optical modulation, if it becomes possible to encrypt an optical signal itself, it is impossible to break a cipher unless a hardware corresponding to this cipher is not provided on the reception side. Namely, even if the encrypted optical signal is converted into an electric signal to be recorded by a light receiving element, since information at the optical level (for example, phase information or the like) is lost at the reception time, it is hard to break the cipher even if the large-scale computer is utilized.
As a proposal of technology for realizing the keeping of information confidential at the optical level, there is known a spread optical spectrum communication system or the like. This spread optical spectrum communication system is obtained by basically applying a spread spectrum communication system in radio transmission to an optical domain. It is well known that the spread spectrum communication system in radio transmission has the excellent confidentiality, as apparent from the background that it was initially developed for the military purpose.
However, the spread optical spectrum communication system as described above has a configuration significantly different from that of the existing optical communication system, and therefore, has a drawback in that it is difficult to perform the application thereof only by adding a simple hardware to the existing system or only by reconstructing the existing system. To be specific, for example, in the existing optical communication system in which a WDM signal light containing optical signals of 40 waves having different wavelengths is transmitted, after the system construction, it is difficult to apply the spread optical spectrum communication to the optical signals of some waves (for example, optical signals of 4 waves) among the optical signals of 40 waves, to add the physical confidentiality them.
The development of an optical communication system which transmits optical signals of high bit rate, for example 40 Gbit/s or the like, over a long distance, is now progressed. As one problem in such an optical communication system, there has been known the degradation of transmission characteristic due to an occurrence of polarization-mode dispersion (to be referred to as PMD hereunder). This PMD is a group delay time difference caused between two cross polarization mode components of an optical signal propagated through a birefringent medium. An optical fiber generally utilized as an optical transmission path is ideally a transmission medium with axial symmetry, but actually causes the birefringence due to the fluctuation from the roundness caused by the defectiveness in manufacture, the coating processing, the application of stress with the cable forming or the like. Therefore, in the case where the optical signals of high speed such as 40 Gbit/s are transmitted over a long distance using the birefringent medium such as the optical fiber, it becomes important to compensate for the transmission characteristic degradation due to the PMD. As a technology relating to the PMD compensation, there has been proposed in Japanese Unexamined Patent Publication No. 11-196046 the technology for dynamically compensating for the PMD by a PMD compensator disposed on the reception side of an optical fiber transmission path while monitoring an occurrence state of the PMD in a transmitted light.
Such a known PMD compensator as described above requires time for detecting the PMD which irregularly fluctuates due to a change in environment or the like, and therefore, is able to automatically compensate for the PMD which is fluctuated at a relatively slow speed or the constant PMD. This means, conversely, that it is difficult to perform the reception processing of optical signals in which the PMD irregularly fluctuating at a high speed occurred, even if the known PMD compensator is used. Therefore, as one means for keeping the information confidential at the optical level, the positive utilization of the PMD which has been regarded as a factor of the transmission characteristic degradation is considered to be possible.
The present invention has been accomplished in view of the above problem and has an object to provide an optical communication system and an optical communication method, for keeping information confidential at the optical level to ensure the high safety.