bandpass filter (BPF)
A bandpass(BP) is a frequency-dependent quadripole that only allows frequencies in a certain frequency range to pass. Since in optical transmission systems the wavelength follows the frequency, the statement applies equally to frequencies and wavelengths.
A bandpass filter is a series connection of a highpass and a lowpass. Such a filter allows only those signals to pass that are in the frequency range or wavelength range for which the filter is designed. The rest of the frequency spectrum cannot pass through the quadripole.
For example, if such a bandpass filter has a center frequency of 946.50 MHz and a bandwidth of 3 MHz, it will pass all frequencies between 945 MHz and 948 MHz with a maximum attenuation of 3 dB.
In audio engineering, there is a bandpass filter standardized by the Audio Engineering Society( AES) called AES17-1998 that is used to limit the measurement bandwidth. This filter has a frequency response between 10 Hz and 20 kHz, +/- 0.1 dB and an attenuation of at least 60 dB at 24 kHz.