Encoder/decoder for optical CDMA

A decoder for decoding an optical wavelength-multiplexed signal compliant with optical CDMA, the optical wavelength-multiplexed signal having been encoded with a code pattern, comprising:

• L optical fibers (where L is an integer equal to or greater than two) each having a plurality of gratings;
• and a circulator for coupling the plurality of optical fibers, wherein,
• the code pattern contains n time bins, including time bin 1, time bin 2, . . . , time bin m, . . . , and time bin n (where n is an integer equal to or greater than two);
• the number of wavelengths assigned to a time bin m (where 1≦m≦n;
• m is an integer) is defined as Q(m), which is a function of m;
• among the Q(m) wavelengths assigned to the time bin m, a wavelength identified by an index q (where 1≦q≦Q(m);
• q is an integer) is defined as λ(m, q), which is a function of m and q;
• and each of the L optical fibers is represented by an index s (where 1≦s≦L;
• s is an integer), wherein,
• in each optical fiber s (where 1≦s≦L),
• gratings respectively corresponding to the time bins m (where 1≦m≦n) are formed, and
• at a distant position from a position of a grating which reflects an optical signal of a wavelength λ(m, q1) corresponding to at least one time bin m included among the n time bins, another grating which belongs to the same time bin m and which reflects an optical signal of a wavelength λ(m, q2) (where 1≦q1≦Q(m);
• 1≦q2≦Q(m);
• q1≠q2;
• q1 and q2 are integers) different from the wavelength λ(m, q1) is formed;
• and
• in each optical fiber s, an optical path difference from the grating which reflects the optical signal of the wavelength λ(m, q1) to the other grating which reflects the optical signal of the wavelength λ(m, q2) is defined as d(m, q2, s), wherein a sum of the optical path differences d(m, q2, s) for all of the L optical fibers equals zero.