Abstract:
Systems and methods for transmitting quantum and classical signals over an optical network are disclosed, wherein the quantum signal wavelength either falls within the classical signal wavelength band, or is very close to one of the classical signal wavelengths. The system includes a deep-notch optical filter with a blocking bandwidth that includes the quantum signal wavelength but not any of the classical signal wavelengths. The deep-notch optical filtering is applied to the classical signals prior to their being multiplexed with the quantum signals to prevent noise generated by the classical signals from adversely affecting transmission of quantum signals in the transmission optical fiber. Narrow-band filtering is also applied to the quantum signals prior to their detection in order to substantially exclude spurious non-quantum-signal wavelengths that arise from non-linear effects in the optical fiber. The present invention allows for the quantum and classical signals to have wavelengths within just a few nanometers of one another, which has benefits for both classical and quantum signal transmission on a common transmission optical fiber.

Description:
CLAIM OF PRIORITY AND CROSS-REFERENCE TO RELATED APPLICATION 
     This is a divisional of U.S. patent application Ser. No. 11/352,046 filed on Feb. 10, 2006, and which issued on Jul. 24, 2007, as U.S. Pat. No. 7,248,695 B1, the content of which is relied upon and incorporated herein by reference in its entirety, and to which the benefit of priority under 35 U.S.C. § 120 is hereby claimed. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to optical networks and quantum cryptography, and in particular relates to transmitting quantum signals over an optical network populated by classical signals. 
     BACKGROUND ART 
     QKD involves establishing a key between a sender (“Alice”) and a receiver (“Bob”) by using either single-photons or weak (e.g., 0.1 photon on average) optical signals (pulses) called “qubits” or “quantum signals” transmitted over a “quantum channel.” Unlike classical cryptography whose security depends on computational impracticality, the security of quantum cryptography is based on the quantum mechanical principle that any measurement of a quantum system in an unknown state will modify its state. As a consequence, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the exchanged qubits will introduce errors that reveal her presence. 
     The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article “Quantum Cryptography: Public key distribution and coin tossing,” Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175-179 (IEEE, New York, 1984). Specific QKD systems are described in U.S. Pat. No. 5,307,410 to Bennett, and in the article by C. H. Bennett entitled “Quantum Cryptography Using Any Two Non-Orthogonal States”, Phys. Rev. Lett. 68 3121 (1992). The general process for performing QKD is described in the book by Bouwmeester et al., “The Physics of Quantum Information,” Springer-Verlag 2001, in Section 2.3, pages 27-33. 
     In a typical QKD system, Alice and Bob are optically coupled by an optical fiber that carries only the quantum signals. This facilitates detecting the quantum signals because there are no externally introduced sources of noise from other optical signals. However, incorporating QKD into existing optical networks requires that the quantum signals share the same optical fiber as “classical” (i.e., non-quantum) optical signals used in standard optical telecommunications. This complicates the QKD process because detecting the quantum signals is hampered by the presence of the classical signals as well as the relatively large amounts of noise they generate in the quantum channel. 
     One approach to transmitting quantum signals over an optical fiber populated by high-speed traffic in the form of classical signals is to transmit the quantum signals on a substantially different wavelength band than the classical signals. Such an approach is discussed in the article by Chapuran et al., entitled “Compatibility of quantum key distribution with optical networking,” Proc. SPIE Vol. 5815 (2005), which article is incorporated by reference herein. The present state of the prior art teaches that the quantum and classical signals should be transmitted in wavelength bands separated by at least 150 nm. However, it is preferable to transmit both signals at much closer wavelengths. For example, in some cases it is advantageous to transmit both types of signals within the third telecom window (i.e., wavelengths around 1500 nm). The quantum signals benefit from the extended reach gained by operating near the optical fiber&#39;s maximum transparency wavelength (1550 nm) in this band. The classical signals also benefit from low attenuation as well as the wider variety of quality optical components available for these wavelengths, such as optical amplifiers. 
     SUMMARY OF THE INVENTION 
     A first aspect of the invention is a method of transmitting, over an optical network, one or more classical signals having corresponding classical wavelengths and quantum signals having a quantum signal wavelength. The method includes receiving the quantum and classical signals traveling over a first common optical path of the optical network and placing the quantum signals and the classical signals onto separate classical and quantum optical paths. The method also includes, in the classical optical path, optically filtering the classical signals to form filtered classical signals, said optical filtering excluding the quantum signal wavelength but not any of the one or more classical signal wavelengths, wherein the quantum signal wavelength is within 100 nm of one of the classical signal wavelengths. The method also includes, in the quantum optical path, narrow-band filtering the quantum signals so as to substantially exclude non-quantum signal wavelengths, thereby forming filtered quantum signals, and providing the filtered quantum signals to a quantum key distribution (QKD) station so as to form relayed quantum signals. The method further includes combining the filtered classical signals and the relayed quantum signals onto a second common optical path of the optical network. 
     Another aspect of the invention is a system for transmitting, over a long-haul optical fiber network, classical signals having respective classical wavelengths representing a classical channel and quantum signals having a quantum signal wavelength representing a quantum channel. The system includes a demultiplexer adapted to separate the quantum channel and the classical channel. The system also includes, in the separated classical channel, an optical isolator, an optical amplifier optically coupled to the optical isolator, and a first narrow-bandpass optical filter optically coupled to the optical amplifier. The first optical filter has a blocking bandwidth centered on the quantum signal wavelength but not extending to any of the classical signal wavelengths, so as to form filtered classical signals from the inputted classical signals. The system also includes in the separated quantum channel, a second narrow band optical filter optically coupled to a quantum key distribution (QKD) station. The second optical filter substantially removes non-quantum signal wavelengths. The QKD system is adapted to receive the filtered first quantum signals from the second optical filter and form therefrom relayed quantum signals. The system also includes a multiplexer adapted to recombine the classical and quantum channels onto a transmission optical fiber that carries both the filtered classical signals and the relayed quantum signals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a schematic diagram of an example embodiment of an optical network according to the present invention that allows for the transmission of both quantum and classical signals at close wavelengths; 
         FIG. 2  is a plot of attenuation (dB) vs. wavelength (nm) for a first example of the deep-notch blocking optical filter as used in the optical network of  FIG. 1 ; 
         FIG. 3  is a plot of attenuation (dB) vs. wavelength (nm) for a second example of the optical filter ( 30 ) as used in the optical network of  FIG. 1 , as formed from a cascade of two 200 GHz and three 100 GHz filters with the same center wavelength; 
         FIG. 4  is a plot of attenuation (dB) vs. frequency (THz) showing the individual curves for two different types of optical filters used to form the optical filter ( 76 ); 
         FIG. 5  is a plot of the attenuation (dB) vs. frequency (THz) for the optical filter ( 76 ) formed by combining the two filters shown in the plot of  FIG. 4 ; and 
         FIG. 6  is a schematic diagram of a section of an optical network, illustrating an example embodiment of how the present invention is used to support the co-propagation of quantum and classical signals on a long-haul fiber network. 
     
    
    
     The various elements depicted in the drawing are merely representational and are not necessarily drawn to scale. Certain sections thereof may be exaggerated, while others may be minimized. The drawing is intended to illustrate an example embodiment of the invention that can be understood and appropriately carried out by those of ordinary skill in the art. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention includes systems and methods that allow for the transmission of both quantum (optical) signals QS at quantum signal wavelength and high-speed classical (optical) signals CS at classical signal wavelength over an optical fiber network, wherein the quantum and classical signal wavelengths are relatively close to one another. In a preferred embodiment, the optical coupling between elements as referred to below is accomplished using sections of optical fiber. 
       FIG. 1  is a schematic diagram of an optical network  10  according to the present invention. Network  10  includes a transmitter side T and a receiver side R optically coupled by a transmission optical fiber  12 . On receiver side R, network  10  includes a multi-wavelength transmitter  20  optically coupled to a dense wavelength division multiplexer (DWDM)  22 , which in turn is optically coupled to an erbium-doped fiber amplifier (EDFA)  26 . EDFA  26  is optically coupled to a “deep notch” optical filter  30  having a sharp, narrow blocking bandwidth preferably centered about the quantum signal wavelength. Optical filter  30  is in turn optically coupled to the express port (often called the “reflect port”)  34  of an optical add/drop multiplexer (OADM)  36 , which is optically coupled to transmission optical fiber  12 . 
     In an example embodiment, optical filter  30  has a 3 dB blocking bandwidth in the range from about 0.2 nm to about 100 nm, and preferably in the range from 0.2 nm to 10 nm, with the maximum attenuation at the blocking wavelength in excess of 100 dB.  FIG. 2  is a plot of attenuation (dB) vs. wavelength (nm) for a first example embodiment of optical filter  30 . The attenuation plot of  FIG. 2  has a 3 dB blocking bandwidth of about 0.9 nm with a 140 dB maximum attenuation at the blocking wavelength.  FIG. 3  is a plot of attenuation (dB) vs. wavelength (nm) for a second example embodiment of optical filter  30 . The attenuation plot of  FIG. 2  has a 3 dB blocking bandwidth of about 1.4 nm with a 120 dB maximum attenuation at the blocking wavelength. 
     In an example embodiment, optical filter  30  is formed from combining a number of different blocking filters. In an example embodiment, five optical-drop filters from AC Photonics, Inc., Santa Clara, Calif., were used to create two different optical filters  30  having the attenuation characteristics of  FIGS. 2 and 3 , respectively. The example embodiment of  FIG. 2  was created from five cascaded 100 GHz filters of the same center wavelength. The example embodiment of  FIG. 3  was formed from a cascade of two 200 GHz and three 100 GHz filters with the same center wavelength. Note that similar filters can be created in the same manner to place the blocking bandwidth at or near the optical fiber dispersion minimum wavelength of 1310 nm. 
     With reference again to  FIG. 1 , transmitter T also includes a QKD transmitter ALICE optically coupled to transmission optical fiber  12  via an add port  35  of OADM  36 . In an example embodiment, OADM  36  has multi-channel add/drop capability to accommodate sending synchronization signals SC between QKD transmitter ALICE and a QKD receiver BOB (introduced below). The synchronization signals SC are typically carried on a synchronization channel having a wavelength different from the quantum signal (channel) wavelength. Alternatively, the synchronization signals are added through DWDM  22 . 
     With continuing reference to  FIG. 1 , the receiver side R of network  10  includes an OADM  50 , similar to OADM  36 , optically coupled to transmission optical fiber  12 . The express (reflect) port  52  of OADM  50  is optically coupled to an isolator  54 , which is optically coupled to an EDFA pre-amplifier  56 , which in turn is optically coupled to a DWDM  60 . A multi-channel receiver  70  is optically coupled to DWDM  60 . Other example configurations contain only EDFA  56  or EDFA  26 , but not both. In the case where EDFA  56  is not present, there is no need for isolator  54 . 
     Receiver side R also includes an optical filter  76  optically coupled to the drop (pass) port  53  of OADM  50 , and a QKD receiver BOB optically coupled to the optical filter. In an example embodiment, optical filter  76  has a 2 GHz bandpass centered on the quantum signal wavelength. This bandpass is about 50× more narrow than a standard telecom filter. As OADM  50  is similar to OADM  36 , in an example embodiment OADM  50  also has multi-channel add/drop capability to accommodate sending synchronization signals SC between ALICE and BOB. Alternatively these synchronization signals can be retrieved via DWDM  60 . 
     Method of Operation 
     Optical network  10  operates as follows. On the transmitter side T, multi-wavelength transmitter  20  generates classical signals CS 1 , CS 2 , . . . CSN of different wavelengths. In an example embodiment, two classical signals CS 1  and CS 2  are generated having respective wavelengths of 1552.52 nm (the “C 31 ” wavelength) and 1546.12 nm (the “C 39 ” wavelength). 
     The classical signals CS 1 , CS 2 , . . . CSN are multiplexed by DWDM  22  after which they are denoted in  FIG. 1  as a single classical signal CS for simplicity. These signals are amplified by EDFA  26 , which also produces large amounts of amplified spontaneous emission (ASE) noise in the quantum signal band. The combination of crosstalk through DWDM  22  and amplifier ASE can pollute the quantum channel with optical noise having power many orders of magnitude higher than the power of the quantum signals. Since the noise is In-band with the quantum channel, no level of filtering at the receiver-side can separate the quantum signals from the background noise. Therefore, the background noise within the quantum band must be reduced to levels roughly 30 dB below the power level of the quantum signals themselves before the quantum and classical signals can be combined. 
     The needed reduction in background noise in the quantum channel is accomplished with blocking optical filter  30 . Narrow-band filtering with optical filter  30  about a narrow wavelength range surrounding the quantum signal wavelength provides a sufficiently “dark” quantum channel at a wavelength either within the classical signal channel wavelength band, or close to thereto, i.e., within about 100 nm or so, and more preferably very close thereto, i.e., within 10 nm or so. The amplified and filtered classical signals, denoted as CS′, are then coupled into transmission optical fiber  12  via OADM  36 . 
     Also at the transmitter side T, quantum signals QS (shown as a dashed-line optical pulse) are generated by ALICE. The present invention allows the operating wavelength of the quantum signals QS to be substantially closer to the classical signal wavelengths than prior art approaches. Both the quantum and classical signals can, for example, share the C-band within the third telecom window. In an example embodiment, the quantum signal wavelength falls within the bandwidth of the classical signals, i.e., is between at least two classical signal wavelengths. In an example embodiment, classical signals CS include wavelengths of ˜1552.52 nm and ˜1546.12 nm, while the quantum signal wavelength is ˜1549.32 nm. This represents a spectral separation of the classical signals from the quantum signals of only ±3.2 nm. In the instant example, this allows for both the classical signals and the quantum signals to be transmitted at wavelengths within just a few nanometers of each other and at or near the optical fiber maximum transparency wavelength of 1550 nm. 
     Quantum signals QS are coupled into transmission optical fiber  12  through OADM  36 . In an example embodiment, OADM  36  can also provide some additional pre-filtering to reduce any leakage from the DWDM (classical) channels and/or ASE from EDFA  26  at the quantum signal wavelength. 
     On the receiver side R, OADM  50  serves to separate the quantum signals QS and classical signals CS′ arriving from the transmitter side T. Classical signals CS′ are directed through isolator  54  to EDFA pre-amplifier  56  for amplification and are then separated by DWDM  60  and detected by multi-channel receiver  70 . Isolator  54  cuts down on backward-propagating ASE from EDFA preamplifier  56 , which when combined with Raleigh and Raman backscattering, can cause false clicks in Bob&#39;s single-photon detectors (not shown). 
     Meanwhile, quantum signals QS are directed from WDM  50  to narrow-band optical filter  76  that serves to reduce noise caused by spontaneous Stokes and/or anti-Stokes Raman scattering from the DWDM classical channels that could result in false counts at BOB. Since the optical bandwidth of the quantum signals is quite narrow, for example less than 1 GHz, using a standard 100 GHz telecom filter for optical filter  76  is rather ineffective. An example embodiment of optical filter  76  consists of or otherwise includes a combination of a 2 GHz Fabry-Perot filter with a 100 GHz free spectral range such as those manufactured by Micron Optics, Inc., Atlanta, Ga., together with two 100 GHz thin-film filters such as those produced by AC Photonics, Inc., Santa Clara, Calif. The attenuation of each of these individual components is shown in  FIG. 4 , with curve A representing the Fabry-Perot filter and curve B the thin-film filter. The attenuation of the combined assembly that forms optical filter  76  is shown in  FIG. 5 . The addition of the Fabry-Perot filter in this example reduces the number of false clicks caused by nonlinear scattering of classical signals into the quantum signal band by roughly a factor of 50. The filtered quantum signals QS, denoted as QS′, are then received and detected by BOB. 
     Long-Haul Network Example 
       FIG. 6  is a schematic diagram of a section  200  of an optical network, illustrating an example embodiment of how the present invention may be used to support the co-propagation of quantum and classical signals on a long-haul fiber network containing multiple optical amplifiers. The approach may use either optical-electrical-optical (OEO) regeneration of the QKD signals in a cascaded quantum key distribution system, or quantum repeaters. 
     Network section  200  includes OADM  50  optically coupled to a transmission fiber  12  connected at its opposite end to an upstream node (not shown). Likewise, network section  200  includes OADM  36  optically coupled to a transmission fiber  13  connected at is opposite end to a downstream node. OADMs  50  and  36  are used to separate and re-combine the classical and quantum channels. The classical channel includes isolator  54 , optical amplifier  26  optically coupled to the isolator, and optical filter  30  optical coupled to the optical amplifier. The quantum channel includes optical filter  76  optically coupled to a QKD station  78 . In an example embodiment, the optical network includes a number of sections  200 , with QKD stations  78  cascaded along the network. 
     In the operation of network section  200 , quantum signals QS and classical signals CS arriving from the previous (upstream) node (not shown) through transmission fiber  12  are separated by OADM  50 . Classical signals CS pass through isolator  54  and are amplified by EDFA  26  and are then filtered with optical filter  30  to form filtered classical signals CS′. Meanwhile, quantum signals QS pass through narrow-band optical filter  76  to form filtered quantum signals QS′, which are relayed or otherwise processed by QKD station  78  to form relayed quantum signals QS″. The relayed quantum signals QS″ and the filtered classical signals CS′ are then recombined with OADM  36  and sent out over transmission fiber  13  to the next (downstream) node or QKD station (not shown). This process is repeated for the different network sections  200  making up the optical network. 
     While the present invention has been described in connection with preferred embodiments, it will be understood that it is not so limited. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined in the appended claims.