Birefringents waveplates are optical systems that can be used for several purposes. One purpose can be rotation of incident wave polarisation plane. For this the lambda/2 plate are used. Change of linear polarisation into circular one can be achieved by lambda/4 plate.

Another usage of birefringent waveplates can be as a delay plate that produces two half power pulses, separated by a defined interval, from one input pulse. In a waveplate the puls is devided into collinear ordinary and extraordinary pulses that propagate by different group velocities. That results in a time separation beetween them. The time separation interval

dt = (ne - no) . d / c

where
ne ... extraordinary refractive index of birefringent material
no ... ordinary refractive index
d .... plate thickness
c .... velocity of light

phase retardation oscilation:

True zero order waveplate

True zero order waveplate is very thin birefringent plate of order k = 0. That leads into slow phase vs. wavelength oscilation (see figure aside).

For instead lambda/2 waveplate has a retardation of phase of pi/2. The phase retardation of extraordinary and ordinary wave can be computed according to following equation

2p/l . d . (ne-no) = (2k-1) . p

where l is wavelength, d is plate thickness and k in an order (0, 1, 2, etc.) Having the order of 0 leads to slow change of retardation vs. wavelength.

The true zero order waveplate is commonly placed on some isotropic substrate (BK7) that enables of polishing of such thin elements.

Zero order waveplate

Zero order waveplate is actually compounded from two higher order waveplate with crossed fast axes (subtractive order). That leads to subtraction of phases so that only plates appears as true zero order plate.

Low order waveplate

Low order waveplate is a birefringent plate of low order k (eg. 7 or 8). That leads to one-piece optical system (contrary to zero order or true zero order waveplate) but results in faster change of retardation (see figure aside).

Achromatic waveplate

Achromatic waveplate is a birefringent optical system compound of two waveplates where each is made of material with different curve of birefringence vs. wavelength (ne(l) - no(l)). That enables optical designer to achieve a very low retardation with respect to wavelength (see figure aside).

Superachromatic waveplate

Superachromatic design of waveplate employs a Panchatnam order of plates. It is actualy a composition of three achromatic waveplates (6 elements in total). It result is superslow and superlow change of phase retardation (see figure aside).