Polygon scanning mirror: a rotating multi-facet mirror for fast one-direction laser scanning

01

Facet count against scan angle, line rate and duty cycle

Duma derives the total scan angle of a polygon mirror head as 4π/n for n facets, which gives 144° for a pentagonal mirror, and notes that motors reach 60 krpm with 120 krpm under development, n runs from 5 on commercial mirrors to 24, up to 128 for swept sources, so scan frequencies above 100 kHz are reachable. [1]

In the TU Wien set-up an 8-facet, 144 mm polygon gives an optical scan angle of ±21.8° and runs at a maximum of 1,000 lines/s; about 50 % of each rotation, where the beam does not straddle two facets, is usable for scanning. [2]

Editorial calculation: by Duma's 4π/n, 8 facets give a 90° total angle, of which the TU Wien system uses 43.6°, roughly consistent with its 50 % usable rotation; 1,000 lines/s from 8 facets is 125 rev/s, or 7,500 rpm. [1] [2]

Scanner Optics lists a polygon scan head of up to 50,000 rpm and up to 260 m/s, or 5,300 lines/s, combining a 1D polygon with a 1D galvanometer for area scanning and a start-of-facet detector. Editorial calculation: 50,000 rpm is 833 rev/s, so 5,300 lines/s would mean about 6.4 lines per revolution; the two maxima probably refer to different configurations, so ask for facet count and rpm together. [4]

02

Pyramidal error, facet-to-facet error and how wobble is corrected

Pyramidal error is the tilt of a facet relative to the rotation axis and differs from facet to facet; TU Wien cites 10 to 300 µrad, and facet-to-facet angle deviations of 25 to 150 µrad from the nominal 360°/N in industrial polygons. Each produces scan-line wobble or shifted start positions. [2]

Editorial calculation: 2 arc seconds is 2 × 4.85 = 9.7 µrad, at the low end of the range TU Wien cites. [3] [2]

TU Wien corrects wobble actively with a fast steering mirror driven by iterative learning control: line wobble falls by a factor of 38 to a scanning accuracy of ≤1.3 µm, with a sensitivity of about 1 µm of wobble per µrad of residual pyramidal error in its set-up. [2] Coherent diamond-machines polygons up to 300 mm in silicon, germanium, ZnSe, ZnS, aluminium, copper, nickel plating and plastics. [3]

03

Polygon mirror order data

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Polygon: [facets; inscribed diameter; facet height]
Geometry: [prismatic or pyramidal angle]
Tolerances: [pyramidal; facet-to-facet; flatness]
Material and coating: [substrate; wavelength]
Speed: [rpm; lines/s]
Motor and sync: [bearing; start-of-facet sensor]

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