Consistent with the present disclosure a compact, integrated tunable filter is provided that can adjust the power levels of optical signals output from an optical amplifier, for example, so that the amplifier has a uniform spectral gain. The tunable optical filter includes a planar lightwave circuit (PLC) having cascaded Mach-Zehnder interferometers, each of which having corresponding differential optical delays. At least one of the differential optical delays is different than the rest. Alternatively, the differential optical delays are different from one another. Each of the Mach-Zehnder interferometers is connected to one another by a tunable optical coupler. Such a filter has an improved frequency response in that the number of shapes that the transmission spectrum may have is increased. Accordingly, the optical filter may be more finely tuned to more effectively flatten, for example, the output of the optical amplifier. In addition, since the tunable optical filter is realized with a planar lightwave circuit (PLC) component, manufacturing costs can be reduced, and a more compact device can be obtained.