In order to increase the amount of data transmitted along a fiber optic transmission line, multiple wavelength transmission systems are used.  Dense Wavelength Division Multiplexed (DWDM) optical systems utilize optical filters to combine or separate the optical signals in the network according to their wavelengths. 
Optical filters use the principle of interference.  Alternating layers of an optical coating are built up upon a substrate, selectively reinforcing certain wavelengths of light and interfering with other wavelengths.  By controlling the thickness and number of the layers, the frequency (or wavelength) of the passband of the filter can be tuned and made as wide or as narrow as desired.  Unwanted wavelengths are reflected or absorbed, depending on the material used. 
DWDM optical systems emerged in force in the mid 1990s out of an industry demand for communication systems with increased transmission capability.  Unlike Coarse Wavelength Division Multiplexed (CWDM) technology in which data was transmitted over two to eight channels with channel spacing of 20 nm, DWDM systems evolved to transmit over dozens of closely spaced channels with signals packed at intervals less than 200 GHz. 
“Flatness” is a term used to describe a filter profile, in which all wavelengths within the passband of the filter are passed in equal amounts.  To minimize distortions in the information being transmitted over a DWDM system, the filter profiles must be very flat.  This is particularly true if the system is carrying analog modulated optical signals, as these types of modulation formats are particularly susceptible to the deleterious effects of nonlinear distortions. 
To minimize these effects, the operators of DWDM optical networks strive to obtain flat narrowband filters with channel spacings of 100 GHz or smaller.  Optical filters are usually employed as part of the multiplexing (MUX) or de-multiplexing (DEMUX) hardware of the optical system.  Because of the current limitations of the filter manufacturing processes, DWDM networks with channel spacing below 200 GHz were generally not available for analog applications: excessive amounts of nonlinear distortions are created when signals pass through the narrowband optical channel filters with excessively large pass-band slopes.  If realized, flat narrowband filters with channel spacings of 100 GHz or smaller will open up DWDM networks utilizing wavelengths in the 1550 nm range, the 1450 nm range, and the low 1310 nm range to analog optical communication applications. 
FIG.  1.  illustrates a simplified optical DWDM optical network 100 that includes: a plurality of data inputs 102, 104, 106, and 108; transmitters 110, 112, 114, and 116; an optical multiplexer 118; an optical de-multiplexer 120; fiber optic transmission lines 130, 132, 134, 138, 140, 142, 144, and 146; optical signal receivers 122, 124, 126, and 128; and data signal outputs 148, 150, 152, and 154. 
In operation, transmitters 110, 112, 114, and 116 receive data from inputs 102, 104, 106, and 108 and transmit signals with respective characteristic wavelengths λ1-λ4 along respective fiber optic transmission lines 130, 132, 134, and 136 to multiplexer 118.  Multiplexer 118 combines the optical signals and forwards the combined multiwavelength signal to fiber optic transmission line 138.  The transmitted signal arrives at de-multiplexer 120 where it is split into a plurality of signals λ1-λ4 along individual fiber optic transmission lines 140, 142, 144, and 146.  De-multiplexer 120 acts as a gate for incoming signals, re-directing them to specific fiber optic transmission lines according to their individual wavelengths.  Receivers 122, 124, 126, and 128 collect signals λ1-λ4 from fiber optic transmission lines 140, 142, 144, and 146, respectively, and transmit the signals as data output 148, 150, 152, and 154. 
To prevent the generation of unwanted second and third order distortions (CSO and CTB) in DWDM systems transmitting analog modulated signals requires that the optical filters within the network have both a narrow pass-band and also have a level insertion loss across this entire pass-band, i.e., are filters that are both flat and narrowband.  Transmission of analog DWDM signals enable multiple separate transmission channels, whereas flat pass-bands minimize unwanted CSO and CTB distortions that are generated by the signal passing through an optical filter. 
Conventionally, DWDM optical communication systems require optical filters with pass-bands ranging from 200 GHz to as small as 25 GHz.  Due to technical and physical limitations during the manufacturing process, the production of such filters with very shallow pass-band slopes is quite difficult.  In the 1550 nm range (C or L bands) the excessive slopes that occur in the filter pass-bands are compensated for by using laser transmitters that have very small chirp values (below 90 MHz/mA).  The low chirp 1550 lasers are more expensive than lasers with mediocre chirp values but they nevertheless permit the use of optical channel filters with narrow pass-bands and larger slope magnitudes.  Furthermore, in the 1310 nm range, DWDM is not easily realized because of the unavailability of low chirp lasers and also the inability to manufacture narrowband optical channel filters with flat pass-bands to compensate for the large laser chirp values.  Although the lasers at 1310 nm are typically less expensive than those at 1550 nm the chirp values at 1310 nm are usually much higher as well (greater than 120 MHz/mA). 
FIG.  2, illustrates a pass-band 202 from a flat band-pass optical filter.  The band-pass optical filter prevents transmission of all wavelengths from 0 to less than λa, prevents transmission of all wavelengths greater than λb, and transmits all wavelengths between λa and λb.  Since all wavelengths between λa and λb within pass-band 202 are transmitted with equal strength, the corresponding optical filter would be considered a ‘flat’ pass-band filter.  In other words, ideally there is no slope, tilt, or ripple in the pass-band.  Conventionally, due to manufacturing technology limitations decreasing the spectral width of the pass-band of an optical band-pass filter in order to enable multiple separate transmission frequencies, requires a sacrifice of the filter's “flatness.”  When an optical filter has excessive slope, tilt or ripple, in its pass-band, CSO and CTB will be generated and corrupt the quality of the information being carried by the signal. 
FIG.  3 illustrates a typical narrow pass-band 302 response of a narrow band filter.  The band-pass optical filter prevents transmission of all wavelengths from 0 to less than λx, prevents transmission of all wavelengths greater than λz, and transmits all wavelengths between λx and λz.  However, the filter having pass-band 302 does not transmit all wavelengths between λx and λz (including λy) equally.  Consequently, the effects of CSO and CTB distort the signal.  The result is the overall degradation of the signal and loss of information. 
It is desirable to have a flat narrowband optical filter.  Such devices would allow DWDM with analog modulated signal transmissions.