Thin-film filter (TFF)

A thin-film filter (TFF), in fiber-optic usage, is a narrow bandpass interference filter deposited on a glass substrate and mounted between fiber collimators. It transmits one channel of a wavelength-division multiplexing system and reflects all others, so a single filter is a channel add/drop element and a chain of filters is a multiplexer or demultiplexer. For CWDM channels on the 20 nm grid the flat passband is typically about 13 nm wide; for 100 GHz DWDM the channels are 0.80 nm apart and the usable passband is a fraction of that. TFF modules dominate CWDM and short DWDM channel plans of up to about 8–16 channels; CWDM vs DWDM describes the two grids.

Structure

The filter is a stack of alternating high- and low-index layers, usually tantalum pentoxide and silica, each a quarter-wave thick at the design wavelength: at 1550 nm about 185 nm of Ta₂O₅ ($n \approx 2.1$) and 267 nm of SiO₂ ($n \approx 1.45$). A quarter-wave stack alone is a mirror with a broad stop band, as described under thin-film interference. Placing a half-wave (or multiple half-wave) spacer layer between two such mirrors makes a thin-film Fabry-Perot cavity, a solid etalon a few micrometers thick, whose single transmission peak inside the stop band is the passband.

One cavity gives a peaked, Lorentzian-like response that is too narrow at the top and too slow at the skirts for a channel filter. Coupling two, three or more cavities through intermediate mirror layers flattens the top and steepens the edges, so that a laser drifting within its channel sees nearly constant loss while the adjacent channels are strongly rejected. Three-cavity designs are the usual compromise for 100 GHz DWDM, and the layer count for such a filter typically runs to well over a hundred, and small thickness errors in any layer distort the passband. This is why the filters are grown by ion-beam sputtering with optical monitoring of each layer.

Angle tuning

A thin-film filter shifts to shorter wavelengths when tilted. The angle-of-incidence entry gives the relation

$$\lambda(\theta) = \lambda_0 \sqrt{1 - \left(\frac{\sin\theta}{n_\text{eff}}\right)^{2}} ,$$

where $n_\text{eff}$ is the effective index of the layer stack. For a 1550 nm filter with $n_\text{eff} = 2.0$ (a high-index spacer), a 1° tilt shifts the center by 0.06 nm and a 5° tilt by 1.47 nm; a shift of one full 100 GHz channel, 0.80 nm, takes a tilt of 3.7°. Manufacturers use this to trim each filter onto the channel grid during assembly, and the filter is in any case mounted at a small angle so that the reflected beam returns to a second fiber. The same sensitivity means the passband shape degrades if the incident beam is not well collimated, and it separates the s and p passbands at larger angles, which appears as polarization-dependent loss.

Packaging and loss

The basic package is a three-port device: a dual-fiber collimator carries the common and reflect ports on one side of the filter, and a single-fiber collimator collects the transmitted channel on the other. A multiplexer is built by cascading such stages, or by bouncing a beam in zigzag fashion along a row of filters bonded to one block. Each stage adds insertion loss of a few tenths of a dB up to about 1 dB, and the last channel in the chain collects the reflection losses of every stage before it: at 0.5 dB per stage, the eighth channel of a cascade loses 4 dB and the sixteenth 8 dB. An arrayed waveguide grating has a loss that is roughly uniform and nearly independent of channel count, which is why AWGs take over for dense systems with many channels.

Temperature stability

Ion-beam-sputtered films are dense and absorb almost no water, so their center wavelength is stable with humidity and time, and a suitable choice of substrate expansion coefficient compensates the film's thermo-optic shift. The resulting drift is of order 1 pm/K, or about 70 pm across a 70 K operating range, a small fraction of a 0.8 nm channel, so TFF modules need no temperature control. Silica AWGs and bare fiber Bragg gratings drift about ten times faster and rely on athermal packaging.

Common questions

How does a thin-film filter compare with an AWG and an FBG?

The TFF has a flat passband, low loss for a few channels and passive temperature stability; its loss grows with the number of cascaded stages. The AWG handles 40 or more channels in one element at a fixed loss of a few dB. An FBG reflects one narrow channel and needs a circulator to separate it, making it a precise single-channel drop element.

Why does the filter's spectrum depend on how it is mounted?

Any tilt from the design angle shifts the passband, and a diverging or converging beam samples a range of angles, which broadens and lowers the passband. The filter specifications therefore assume collimated light at a stated angle of incidence.

Is a thin-film filter the same as a dichroic mirror?

Both are multilayer interference coatings. A dichroic is an edge filter that splits a broad band into two, usually at 45°; a TFF channel filter is a multi-cavity bandpass used near normal incidence. The optical filter entry covers the general families.

References: H. A. Macleod, Thin-Film Optical Filters, 4th ed. (CRC Press, 2010); R. Ramaswami, K. N. Sivarajan and G. H. Sasaki, Optical Networks: A Practical Perspective, 3rd ed. (Morgan Kaufmann, 2010).