multiple spectral-phase coders on a single chip
NOD presents novel compact and integrated on a single chip spectral-phase coders/decoders that can be used in optical CDMA systems as well as in similar applications where a precise spectral-phase control is required. These devices are based on digital planar holographic technology and are able to demonstrate virtually arbitrary complex spectral-phase profiles.
The use of advanced data formats in optical communications continues to become more important because of constantly increasing data rate requirements. In addition, the increasing employment of multiple access networks, such as passive optical networks (PONs), takes advantage of complex multiplexing techniques, such as optical CDMA, for achieving higher throughput, superior scalability, and increased data privacy.
Development of optical CDMA began in the mid-1980s as a more flexible and reconfigurable alternative to WDM. The idea of using orthogonal codes instead of different carrier wavelengths for multiplexing was taken from wireless communications, where CDMA systems demonstrated superior performance, spectral efficiency and low probability of intercept. One of several known approaches to optical CDMA uses the so-called spectral-phase codes.
Optical spectral-phase codes are created by applying specific, typically 0 or π, phase shifts to different parts of optical spectrum within the bandwidth of a coherent optical source used at the transmitter. Signals encoded with different codes can be mixed together, realizing code-division multiplexing. At the receiver side, the mix is passed through a decoder that decodes a particular code, while all other signals in the mix become suppressed due to orthogonality of codes.
Current integrated technologies for spectral-phase en/decoding realize one code per chip as in the case of ring-resonator-based coders and fiber Bragg grating-based coders. Future development of spectral-phase encoded optical CDMA systems requires more flexible and robust spectral-phase coders that support simultaneous en/decoding of several codes.
NOD presents novel compact and integrated on a single chip spectral-phase coders/decoders that can be used in optical CDMA systems as well as in similar applications where a precise spectral-phase control is required. Those devices are based on digital planar holographic technology and are able to demonstrate virtually arbitrary complex spectral-phase profiles. Advantages of NOD encoder/decoder compared with other known spectral-phase coding technologies such as fiber Bragg grating-based, are in an ability to obtain multiple codes with a single hologram anda simple one step lithography manufacturing and. An exemplary device is shown in the next figure.
It has one input and four outputs corresponding to the four orthogonal spectral-phase codes. This device realizes a full set of Walsh-Hadamard-4 spectral-phase codes. Its amplitude characteristics are shown in the next figure. As one can see there are four clearly distinguished wavelengths in each code, which have specific phase relations defining a particular code.
The codes’ phase relations are presented in the next figure. Four lines show the simulation results for the full set of codes, showing a good agreement with ideal Walsh-Hadamard codes.
According to the figure, the four realized codes are (0, 0, 0, 0), (0, π, 0, π), (0, π, π, 0), and (0, 0, π, π). Parameters of an exemplary device with 40 GHz channel spacing and its performance characteristics are summarized in the table below.
The main advantage of our holographic coder is in realization of the complete code set on a single chip, which significantly reduces the number of required passive optical components for full en/decoding of several channels. Moreover, the technology is extremely flexible and allows to produce complex spectral-phase patterns.
