Long-period grating device and tunable gain flattening filter having same

A tunable gain flattening filter includes a long-period grating device. The long-period grating device includes: an optical fiber that includes a core having a refractive index and a core guided mode with a first effective index, and a cladding surrounding the core and having a cladding mode with a second effective index that is less than the first effective index; a thermoelectric module, the optical fiber being mounted on the thermoelectric module; a thermoelectric cooler configured to precisely control temperature of the optical fiber; and a thermistor configured as a sensor to provide feedback for the thermoelectric module. A plurality of perturbations in refractive index are defined on the core spaced apart by a periodic distance so as to form a long-period grating with a center wavelength. Diameter of the optical fiber is tapered or etched by HF solution to about 6 to 10 [mu]m.