A single-mode photosensitive waveguide Mach-Zehnder interferometer fabricated in Ge-doped planar silica is reported. 2 mm long identical-wavelength reflection-gratings using external UV beams have been written into each arm to demonstrate a four-port bandpass device for the first time using this technology. The imbalance in the arms after writing of the gratings is compensated by laser trimming of one photosensitive arm of the interferometer. 96.8% reflection at the output port is measured and, 58.8% of the total power available at the input at 1.5558 mum in the bandpass of 1 nm is transmitted at the second input port after laser trimming. The total fiber-to-fiber insertion loss of the device is measured to be 1.35 dB.