Wavelength-insensitive couplers in form of all-fibre Mach-Zehnder interferometer

For original article see K. Morishita and T. Tahara, ibid., vol.27, no.13, p.1200-1202 (1991). The original authors reported the fabrication of wavelength insensitive fused fibre couplers using a Mach-Zehnder configuration. The present authors report a similar device but with improved performance: 0.1 dB loss for a splitting ratio fluctuation of 0.4 dB over 300 nm; the corresponding coupling ratio is 50+or-2.5%.