Decoupling of transverse spatial modes in microstructure optical fibers

A properly designed MOF can simultaneously exhibit large anomalous dispersion at visible and near infrared wavelengths and support numerous transverse spatial modes that are essentially decoupled from one another, even in the presence of significant perturbations. In a MOF that includes an inner cladding region comprising at least one thin layer of air holes surrounding a core region, the key is to achieve a relatively large wave vector mismatch between the lowest order modes by appropriate design of the size of the core region and of the effective refractive index difference between the core region and the inner cladding region. In accordance with one aspect of our invention, MOFs are designed to exhibit simultaneously relatively large anomalous dispersion and essentially decoupled transverse spatial modes by making the diameter of the core region less than about 6 mu m and the difference in effective refractive index between the core and cladding regions greater than about 0.1 (10%). Preferably, the cladding region contains no more than 2 layers of air holes, and the distance between the nearest edges of adjacent air holes is less than about 1 mu m. MOFs with these features enable several embodiments of our invention. One embodiment is a nonlinear optical system comprising an optical pump source, an optical signal source, a utilization device and an optical fiber transmission path that optically couples the sources to the device. The transmission path includes at least a section of MOF in which at least two transverse modes are decoupled from one another over the length of the MOF section, the output of the pump source propagating in one of the transverse modes and the output of the signal source propagating in another of the transverse modes. Another embodiment is an optical transmission system comprising an optical transmitter, a utilization device and an optical fiber transmission path that optically couples the transmitter and the utilization device. The transmission path includes at least a section of MOF in which at least two of the transverse modes are decoupled from one another over the length of the MOF section, and further includes a first modulator for impressing information on a first optical signal to be propagated along the MOF in one of the transverse modes and a second modulator for impressing information on a second optical signal to be propagated along the MOF in another of the transverse modes.