Contribution to the development of optical scanners with rotating or oscillating elements.

Duma, Virgil-Florin ; Nicolov, Mirela ; Szantho, Lucian 等

1. INTRODUCTION

There are a myriad applications of optical scanners, from remote (e.g. airborne or satellite for surveillance, industrial) to input (e.g. barcode reading, optical inspection, confocal microscopy, optical coherence tomography (OCT), robot vision) and output ones (printing, marking and engraving, rapid prototyping). There are several classes of scanners: with rotating or oscillating mirrors, resonant, holographic, acusto-optic and electro-optic (Bass, 1995).

A decade of our investigations addressed scanners with rotating (plane or polygonal) or oscillating (galvanometer-based) mirrors (Beiser, 1995). This paper will present the scope of our researches, with a brief overview of the results obtained, both for 1D and 2D scanning systems for several applications.

2. POLYGON SCANNERS FOR DIMENSIONAL, INDUSTRIAL MEASUREMENTS

The scheme of the dimensional (online, industrial) measurements using rotating mirror scanners (Richter, 1992) is presented in figure 1. As the mirror (2) rotates, the laser ray (1) is transformed into a rotating one, that scans the first lens (3) and, thus, the probe space. The emergent laser beam has to remain parallel to the optical axis (O.A.) of both lenses (3). If this condition is fulfilled, than the time interval [DELTA]t for which the photodetector (4) receives no light signal will be a measure of the dimension "d" of the object in the scanning direction:

d = h([t.sub.0] + [DELTA]t)-h([t.sub.0]) = h([[theta].sub.0] + [DELTA][theta]) - h([[theta].sub.0]), [theta] = [omega]t (1)

where the scanning function (Fig. 1) h(t) represents the current position of the ray in the scanned space (Duma, 2006):

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

h([theta]) = R[square root of 2] - e - R/cos[theta]+ e x tan[theta] - L/tan2[theta] (1)

The graph of the scanning function is presented in figure 2. The two pairs of angles are highlighted: [[theta].sub.1] and [[theta].sub.2] = the angles for which the the laser ray reaches the margins of the first lens; [[theta].sub.min] and [[theta].sub.min] = the angles produced when the ray is reflected by the margins of a facet of the polygon. A detailed analysis has been performed on this system (Duma, 2005), regarding these angles, the scanning velocity v=dh/dt, and the duty cycle (the time efficiency of the system):

[eta] = [[theta].sub.2] - [[theta].sub.1]/[[theta].sub.max] - [[theta].sub.min] = n [[theta].sub.2] - [[theta].sub.1]/2[pi]. (2)

The designing calculus that was our scope was thus obtained (Duma, 2007), more detailed than in literature (Beiser, 1991), with the possibility of simplifying the first lens of the system. A problem that was also addressed is obtaining a linear scan that is a constant problem of scanners (Li, 1995). For the measuring system in figure 1, obtaining a one-parameter functioning characteristic d([DELTA][theta]) has also been solved (Duma, 2005), in several ways, both for the polygon-based and for the monogon scanner, as the two-parameters function d([[theta].sub.0], [DELTA][theta]) in Eq. (1) does not provide precision in the measuring process.

3. GALVANOMETER-BASED SCANNERS

The resonant (Fig. 3) and the galvanometer scanner (GS), was studied (Duma, 2008), with an enhanced duty cycle n with regard to the state-of-the-art (Gadhok, 1999) and a linear scan on its active portions. This solution was considered with regard to the scan (oscillating) frequency that should be enhanced without a severe decrease of [eta].

The dynamic equation of the mobile element is:

[??] + 2[xi][[omega].sub.0][??] + [[omega].sup.2.sub.0][theta] = [M.sub.a](t)/J, (3)

[FIGURE 3 OMITTED]

where [[omega].sub.0] = [square root of K / J]; [xi] = c / 2 [square root of Jk] are the natural pulsation and the unitless damping factor, respectevely. The command function is

i(t) = [M.sub.a](t) / BNS (4)

(B is the magnetic induction, N is the number of spires, and S is the area of the surface of the mobile coil), where the active torque results from Eq. (5):

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)

Therefore the i(t) function is obtained by imposing a convenient scanning function x(t), with linear active portions and fast stop-and-return parts (Fig. 4). This is actually the ideal way a scan should be performed for most applications.

The GS is somehow competing nowadays for high end applications, i.e. biomedical ones: optical coherence tomography (OCT) and/or confocal microscopy, with polygonal mirrors setups. 1D solutions that require a fast scan have revitalized the use of polygons, e.g. for swept source laser sources, with on-axis or off-axis (Oh, 2005) polygon scanners.

[FIGURE 4 OMITTED]

3. 2D SCANNING SYSTEMS

In figure 5 the scanning module of a confocal microscope is presented, for which the scanning functions were developed (Duma, 2007). The proper programming of the two 1D GSs used allow for the full scan of the plane surface of the probe, in contrast with the non-linear, e.g. cushion-like shaped of 2D scanned previously obtained (Li, 2008).

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Another solution is the one of two polygon scanners (Sc 1 and 2) 2D assembly, presented in figure 6 that will be studied based on the analysis previously developed.

4. CONCLUSIONS

Our future work addresses improvements of the devices presented, as the problem of increasing the duty cycle of galvoscanners for high scan frequencies is still a problem and simpler lenses for systems with polygon scanners must be obtained. Other applications of the polygon scanners, both for the industrial domain, e.g. for marking, engraving and robotics, and for high end applications, i.e. in biomedical imaging, will be approached.

5. ACKNOWLEDGEMENT

The research is supported by the Romanian Education and Research Ministry, within the PN II, Ideas Grant, NURC (National University Research Council) code 1896/2008.

6. REFERENCES

Bass, M. (1995). Handbook of optics, Mc.Graw-Hill, N. Y.

Beiser, L. (1991). Design equations for a polygon laser scanner, Proc. of SPIE 1454, 60-66, ISSN 0277-786X

Beiser, L. (1995). Fundamental architecture of optical scanning systems, Appl. Opt. 34, 7307-7317

Duma, V.F. (2005). On-line measurements with optical scanners: metrological aspects, Proc. SPIE 5856, pp. 606-617, ISSN 0277-786X

Duma, V.F. (2006). Precise dimensional measurements using optical scanners, Proc. IEEE/LEOS-ODIMAP V, pp. 253-258, ISBN 84-690-0938-9

Duma, V.F. (2009). Mathematical functions of a 2-D scanner with oscillating elements, Modeling, Simulation and Control of Nonlinear Engineering Dynamical Systems, Springer, pp. 243-253, ISBN 978-1-4020-8777-6

Duma, V.F., Podoleanu, A. Gh. (2008). Theoretical approach on a galvanometric scanner with an enhanced duty cycle, Proc. SPIE, ISSN 0277-786X

Gadhok, J. S. (1999). Achieving high-duty cycle sawtooth scanning with galvanometric scanners, Proc. SPIE 3787, 173-180, ISSN 0277-786X

Li, Y. (1995). Laser beam scanning by rotary mirrors. II. Conic-section scan patterns, Appl. Opt. 34, 6417-6430

Li, Y. (2008). Beam deflection and scanning by two-mirror and two-axis systems of different architectures: a unified approach, Appl. Opt. 47, 5976-5985

Oh, W. Y. et al. (2005). 115 kHz tuning repetition rate ultrahigh-speed wavelength-swept semiconductor laser, Opt. Lett. 30, 3159-3161

Richter, B. (1992). Laser Scan Devices for Industrial Application, WIRE 42, Meisenbach GmbH, Bamberg