igh-power diode lasers are a viable alternative asight sources for the visualization of industrial pro-esses. Diode lasers are very compact, and their op-ration can be modulated at high precision andepetition rate to produce practically any desiredemporal pattern of light pulses. In those respectshese lasers are superior to other light sources usedn the production of pulsed illumination, such elec-ronic flash lamps,1–3 Nd:YAG lasers,4,5 ruby lasers,6nd copper vapor lasers.5 In addition, the practi-ally monochromatic light of diode lasers producesmages of much higher quality than the broadbandight of flash lamps. For many visualization appli-ations the radiant flux of a single diode-laser emitters insufficient. Therefore a high-power diode-laseright source is manufactured by stacking togethereveral elementary diode-laser arrays �DLAs�, eachonsisting of a number of laser emitters. Conse-uently, the light of such a source is incoherent.his is, in fact, an advantage compared with coherent
asers, because no speckle or interference pattern,hich deteriorates the image quality, is generated.In the visualization of small and�or fast moving
The authors are with the Optics Laboratory, Institute of Physics,ampere University of Technology, P.O. Box 692, FIN-33101 Tam-ere, Finland. T. Alahautala’s e-mail address is [email protected].
bjects, it is important to launch light of sufficientlyigh irradiance to the scattering object. This re-uires a light source of high brightness. Unfortu-ately, a DLA has lower brightness than a single
aser emitter, and the brightness of a stack of DLAss still lower. With forward-scattered light �extinc-ion measurement, shadowgraphs,7 and schlieren im-ging7,8� moderate irradiance can be sufficient,hereas backscattered light requires irradiance9,10
hat is 2 or 3 orders of magnitude higher. Backscat-ered illumination is required, for example, for ex-raction of morphological information from the object.
technical aspect related to backscattered illumina-ion is that a single optical access can be used.
The brightness of stacked diode laser light sourcesan be improved with micro-optics, but it is expen-ive. Therefore there is an interest in finding wayso improve brightness without micro-optics. Oneay to do this is to reduce the spacing between the
aser emitters in a stack. In cw laser operation theossibilities are limited for thermal reasons. How-ver, in visualization the main interest is in pulsedperation, where cooling requirements are less strin-ent and, consequently, individual emitters can bepaced more densely.In the following sections we present a compact
tacking architecture of DLAs that improves bright-ess by a factor of 40, compared with traditional lasertacks without additional optics. Compact stackingf 670-nm state-of-the-art DLAs and commercial08-nm DLAs is described. A laser head was man-factured in which the light from several compact
aser stacks could be fiber coupled or the light could
bs
2
Td
wltD
wti
sn
ANsFot
tt0hcpwstansttdhTa
sip0Bws
pclDmfinlTc
Flp
e transformed to a highly uniform beam by use of aingle optical element.
. Compact Stacking of Semiconductor Light Sources
he brightness of a single diode-laser emitter, Be, isefined as
Be ��e
Ae�e, (1)
here �e is the radiant flux of the emitter, Ae is theight-emitting area and �e is the solid angle sub-ended by the radiation. The brightness of a stack ofLAs, Bs, is
Bs �N�e
As�s, (2)
here N is the number of emitters in the stack, As ishe total area of the emitting face of the stack, and �es the solid angle subtended by the radiation from the
ig. 1. �a� Stacking of surface-emitting LEDs with integratedens, �b� noncompact stacking of low-fill-factor DLAs, and �c� com-act stacking of high-fill-factor DLAs.
Table 1. Calculated Brightnes
B,W
mm2 sr Single Emitter
LEDa 4.6 � 10�2
Diode laser, noncompactb 8.4 � 104
Diode laser, compactc 8.4 � 104
a20 mW, Ref. 11, Lambertian radiation pattern, Ae 0.3 � 0.3b1 W per 100 m, � 0.3, � 0.24 sr, Ae 0.0005 � 0.1 mmc1 W per 100 m, � 0.75, � 0.24 sr, A 0.0005 � 0.1 mm
e
tack. Because �s � �e we get for relative bright-ess
Bs
Be�
NAe
As. (3)
simple calculation based on Fig. 1�b� shows thatAe �� As, i.e., brightness is deteriorated from a
ingle-emitter value and Bs�Be �� 1. Comparingigs. 1�b� and 1�c� it is easy to see that the brightnessf the stack architecture in Fig. 1�c� is greater thanhe one shown in Fig. 1�b�.
The fill factor ��� of DLA is defined as the ratio ofhe emitter and pitch lengths. With this definitionhe fill factor of commercial DLAs is typically � .2 to 0.5. To increase the brightness of DLA even-igher � values can be considered in quasi-ontinuous wave and pulsed operation. In thisaper we will refer a DLA as “high-fill-factor DLA”hen � � 0.5. The brightness of a laser stack can be
ubstantially improved by placing DLAs directly onop of each other without heat sinks in between. Andditional advantage is achieved by processing thin-er DLAs than normally. The concept of compacttacking, presented here Fig. 1�c��, incorporates allhese features: high-fill-factor DLAs with reducedhickness and stacking without heat sinks. The tra-itional architecture, where DLAs are stacked witheat sinks, is termed a “noncompact stack” Fig. 1�b��.he calculated brightnesses of different light sourcesnd stacking architectures are compiled in Table 1.As indicated in Fig. 1 and Table 1, the compact
tacking �� 0.75, dH 0.1 mm� gives a pronouncedmprovement in brightness by a factor of 41 as com-ared with the noncompact stacking �� 0.3, dL .14 mm, D 1.5 mm�. The relative brightnesss�Be of compact stack is Bs�Be 3.8 � 10�3,hereas the corresponding parameter of noncompact
tack is 9.1 � 10�5.Different stacking schemes also have different
roperties in fiber coupling of light �Fig. 2�. It isustomary to couple the emission of each individualaser emitter to a single fiber with a low-fill-factorLA Fig. 2�b��. In this case the result dependsuch on the fiber dimensions and the accuracy of
ber alignment. Approximately the same bright-ess value �Table 1� can be reached with coupling the
ight from a compact stack to a fiber bundle Fig. 2�c��.he accuracy of the fiber bundle alignment is notritical in this case.
emiconductor Light Sources
ingle DLA Stack Fiber Coupled
N�A 3.3 � 10�2 1.2 � 10�2
2.1 � 104 7.6 � 100 2.1 � 102
6.3 � 104 3.1 � 102 2.0 � 102
, with integrated lens in stacking. �0.14 � 1.5� � 0.1 mm2. �0.1 � 0.0� � 0.1 mm2.
Frwith dimensions in millimeters is shown on the right.
4
The temperature of each high-fill-factor DLA in aompact stack rises by approximately 50 °C during-ms operation. This time scale is long enough forroducing microsecond pulses in the megahertzange for short periods, i.e., in burst operation. Theverall thermal resistance of a compact stack deter-ines the maximum permissible duty cycle, which in
ractice may range from 1% to 10%. Thus, for ex-mple, 1- s pulses can be generated repetitively at0–100-kHz rate and 100-ns pulses at 0.1–1-MHzate. The above considerations suggest that thehermal properties of compact stacks meet the re-uirements for high-speed light sources both in burstnd repetitive operation.
. Diode Laser Head with Compact Stacking
light source based on compact stacking was de-igned for the purposes of generating uniform lightnd fiber-coupled illumination. The generation ofniform light from number of compact stacks is basedn a single optical element, a truncated paraboloidith a Lambertian scatterer �PLS�. The principle ofLS is described in more detail in Ref. 12. In thisarticular application it was important to manufac-ure several narrow compact stacks that could belaced in the periphery of �14-mm PLS. Solutionor this purpose was based on compact stacking ofarrow 670-nm or 808-nm DLAs consisting of fourmitters �Fig. 3, Table 2�.The measured data �Table 2� yields Bs 490 Wm�2 sr�1 for 670-nm compact stacks, which is 70
imes the calculated value for a noncompact stacknd almost twice the calculated value for a compacttack �Table 1�. A compact stack of 808-nm DLAsields a brightness of over 600 W mm�2 sr�1. Theasers were driven at higher power level than whatas specified in cw operation. Therefore brightness
ig. 2. �a� Fiber coupling of LEDs with integrated lens removed,b� fiber coupling of a low-fill-factor DLA, and �c� fiber coupling ofcompact stack.
alues higher than the values shown in Table 1 forompact stacking were achieved in practice.
A diode-laser head including multiple compacttacks in an annular arrangement was designed anduilt �Fig. 4�. The head can accommodate a maxi-um of 24 compact stacks with six four-emitter com-
act DLAs in each stack. Compact laser stacks ofwo different wavelengths can be operated simulta-eously and independently.The high-fill-factor DLAs at 670 nm were manu-
actured by Optoelectronics Research Centre at Tam-ere University of Technology and the 808-nm DLAsy Coherent Tutcore Ltd. The maximum power of30 W was produced by use of six compact stacks of70-nm lasers, and 800 W was produced by use of 12ompact stacks of 808-nm lasers.
The laser head was equipped with PLS, as shown inig. 4�b�. Light propagates to the Lambertian reflec-or, which acts as a secondary light source. The back-eflected rays are collimated by the paraboloid, andhey form spatially uniform light at a certain distance,s shown in Fig. 5. It is estimated that 69% of theotal radiant flux can be conducted to an area with a
ig. 3. Compact stack made by placing fifteen high-fill-factor-atio DLAs on top of each other. An inverted image of the stack
4-mm diameter. The maximum measured nonuni-ormity of irradiance within this area was �1.5%.ew improvements have been suggested to increase
he optical efficiency up to 85%.12 The laser head cane water cooled in order to ensure reliable operation inot measurement environments.The diode-laser light source has been used for the
etection of small fast-moving objects in over 20ndustrial processes so far. Depending on the case,he object was illuminated by uniform light or fiber-oupled light. Application examples, where back-cattered light was used, include, e.g., theisualization of cavitation in water faucet, thetudy of chip formation in turning,13 and the visu-lization of high-temperature objects in different
ig. 4. �a� Laser head open with six 670-nm compact stacks in-talled on it. The laser head can accommodate altogether 576aser emitters. The diameter of the head is 50 mm, and the lengths 165 mm. �b� Uniform output beam with a diameter of 14 mm.he optical components used for generation of uniform light arerawn into the figure. The dimensions are in millimeters.
ig. 5. Measured radial intensities at three distances z, in milli-eters, from the laser head.
aser materials processing applications. The oper-tion of the semiconductor camera and the laser areomputer controlled, which enables the continuousperation and collection of quantitative image in-ormation at a megahertz rate.
. Conclusions
ompact stacking architecture was developed in ordero produce high brightness light from multiple DLAsithout micro-optics. Compact stacks of 670 and08-nm lasers were manufactured and accommodatedn the laser head. The light source was successfullypplied to quantitative visualization of different indus-rial processes by use of backscattered light.
The 670-nm lasers gave brighter images than the08-nm ones, although the power of the 808-nm la-ers was almost twice the power of the 670-nm lasers.his is explained by the camera quantum efficiency,hich is approximately three times higher at 670 nm
han at 808 nm. In practice, a visible laser beamreatly facilitates optical alignment and improves op-rational safety. To the authors’ knowledge theresent light source, completed in 1999, is the firstodulated high-power diode-laser system operating
t a visible wavelength at 670 nm.Compact stacking architecture was not previously
ntroduced, probably because of the fact that theractical applications are related to visualization anded DLAs have recently been developed and manu-actured on a pilot scale. The advance of semicon-uctor camera technology, particularly the recentapid progress in high-speed complementary metal-xide semiconductor camera technology, may createew applications for modulated high-brightness vis-
ble light sources.
Pekka Savolainen manufactured the visibletate-of-the-art DLAs and participated in their de-ign. Antti Lepisto manufactured the laser head.rri Priimagi, Outi Torhamo, and Cedric Sire par-
icipated in laboratory and field measurements.he work was jointly funded by the National Tech-ology Agency of Finland and several industrialnterprises.
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