Long Working Distance Optical Systems

One particularly interesting application that employs long working distance (LWD) beam delivery is ceramics machining. The beam is split into two equal square portions, which are then passed through round apertures at a high BUF. The beamlets are demagnified by 15x, producing on-target spots of approximately 125µm, which are directed onto opposite sides of the part. Careful alignment of the two beamlets results in simultaneous double-sided drilling, minimizing vibration and shock, decreasing hole taper, and doubling drilling speed. An open frame X,Y table permits part movement without obstructing either beamlet. Direct write motion control using CAD/CAM software produces quick-turnaround, high-accuracy prototype parts.

The following table shows the features of several long-working-distance objective lenses.

	Plano-convex Singlet	Correct Doublet	Four Element Corrected	Multi Element Telecentric
Resolution	>10µm	~5µm	~2µm	~2µm
Field size (mm)	~10mm	~10µm	~5mm	Up to 25mm
Complexity	Very low	Low	Moderate	High
Cost	Very low	Moderate	Moderate	High
Losses	2 to 5%	5 to 10%	5 to 10%	>20%
Notes	Very expensive; barrel distortion	Dual wavelength operation, moderate distortion	Low distortion	Very large field of view, good depth of field

Advantages of LWD Systems

Small numerical apertures with large field sizes
Large depth of field
Resolution to 2µm
Low optics losses

Disadvantages of LWD Systems

Small demagnification factors
Long optical path lengths
On-target viewing is problematic
Large support structure required

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