
Introduction
what is zeroorder in terms of diffractive optics?
Zeroorder is the part of the energy from incident beam going through the diffractive optical element (DOE) without being “diffracted”, meaning part of the incident beam “obeying” only geometrical ray optics (reflection & refraction) equations. Although in some cases the zeroorder is part of the designed image (array of spots), there is still an issue with its energy variation compared to the design (nominal). Trying to bypass this issue can result in complex optical setups or abandoning the project all together due to higher costs or insufficient performances
The problem
In the real world, nothing is perfect, and zero order will be present (to some extent). Zeroorder “shooting” out of the uniformity spec of the image can cause serious problems in many any applications.
This mainly happens due to three types of effects:
 Designrelated effects:
 The validity of scalar theory for features (diffractive microstructures) smaller than 5λ
 Fabricationrelated effects:
 Fabrication errors (etch depth errors and lateral misalignment)
 The steepness of side walls
 Surface roughness
 Operation & integrationrelated effects
 The spectral width of the source
 The difference between the wavelength of the light source vs. optimal design of the DOE
 The collimation of source
 The incidence of the input beam (shadowing effects)
 The polarization state of the source (only relevant with item (1).a)
Solutions
We present here several alternatives to suppress the zeroorder, each one with its advantages and disadvantages:
 Using Noncollimated incident beam
 By inserting a noncollimated/diverging input beam to the DOE, the zeroorder will continue diverging like the source, thus spreading over a large region, “smoothing out” the hotspot. This technique works only for farfield applications, so the addition of a focusing lens is prohibited (or zeroorder will be focused as well). In addition, a new DOE design with internal diffractive lens might be needed.
 Zeroorder reduction by special diffractive design
 As most DOEs are designed using IterativeFourierTransformAlgorithm (IFTA) it is possible to give the zeroorder optimization parameter larger weight in the overall merit function, over uniformity, efficiency, etc.
 Three phase level diffractive structures instead of two
 The introduction of a third phase level to the original binary grating profile can significantly reduce the design sensitivity to profile depth error (this translates into lower sensitivity to spectral width resulting zeroorder).
 Lateral shifting of the desired output image relatively to zeroorder – Adding a diffractive prism
 By including diffractive micro prisms in the design, the possible energy of the zero order (i.e. 12%) continues on the optical axis, while the desired image is deflected in a certain angle according to the diffractive micro prism array. This solution will show lower sensitivity to misalignment compared to other alternatives.
 Focal shifting of the desired output image relatively to zeroorder – Adding a diffractive (Fresnel) lens
 By including a Fresnel type of lens the possible energy of zero order (i.e. 12%) spreads out and is evenly distributed over the entire region and appears as a small background noise. Usually, we choose the design so that the desired image will be before the original focal plane, thus, the zero order will be at focus at the focal plane (It is also possible to move the zero order to other plane and leave the image plane at the original EFL of the system).
 Mechanical mask to block the zeroorder at an intermediate image plane – UDOB module
 This method inserts a real engineered aperture in a virtual intermediate focal plane that physically blocks (by reflection) the zeroorder. This is often used to also block higher diffractive orders and stray light.
 Double sided DOE in Holo/Or’s homogenizer type products (HH / RH / XH)
 Holo/Or developed a new class of diffusers/homogenizers with enhanced performance referred to as the high homogeneity series (Homogeneity can be defined as the average intensity over an area unit). Its advantages are higher homogeneity, and lower zero order. This DOE design includes two diffractive surfaces. The first decreases the coherence of the incident beam and the second surface shapes the beam.
Conclusions
In Holo/Or we can offer all of the above different solutions for customers that require a very small zeroorder or precise values in a specific range.
These solutions are also good for customers that want to use the same product for wavelengths over a spectral range of about +/10% (or even more, depending on the setup and the type of solution).
The table below summarizes the different alternatives to handle the zeroorder effect:
Solution 
Advantage 
Disadvantage 
Using a Noncollimated incident beam 
Might not require alterations to the DOE 
Addition of a diffractive lens to diffractive design adds its own manufacturing tolerances, otherwise, output shape & transfer region increases, and this can cause overlap of spots if used with multispot 
Zeroorder reduction by special diffractive design 
Simplest integration 
Limited affectivity (relevant for small angle and small number of spots for MultiSpot) 
Three level diffractive structures instead of two 
Cheap relative to multilevel solutions (8 & 16) 
Useful only for multispectral applications 
Lateral shifting of desired output image relatively to zeroorder – Adding diffractive prism 
Lower sensitivity to centration compared to solutions with diffractive Fresnel lens 
Output image is laterally shifted compared to the optical axis.
More expensive than binary (2level) DOE due to the addition of a multilayer micro prisms. Limited to relatively small angles applications 
Focal shifting of desired output image relatively to zeroorder – Adding diffractive (Fresnel) lens 
On axis solution. Can achieve higher efficiency compared to other solutions 
Centration is more important.
More expensive than binary (2level) DOE due to the addition of a multilayer Fresnel lens 
Mechanical mask to block the zeroorder at an intermediate image plane – UDOB module 
Blocks more than 99% of the zeroorder 
A full solution in a multielements module instead of a single DOE 
Double sided DOE 
Simple to design. Significantly reduces zeroorder 
More expansive due to second diffractive surface. Limited to homogenizer type products 

