spinning cone water film power meter for high-power co2 lasers
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Optics & Laser Technology 39 (2007) 196–201
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Spinning cone water film power meter for high-power CO2 lasers
R.K. Soni�, V.K. Mandloie, M.B. Pote, A.K. Nath
Department of Atomic Energy, Industrial CO2 Laser Section, Centre for Advanced Technology, Indore-452013, India
Received 15 September 2004; received in revised form 25 February 2005; accepted 1 March 2005
Available online 31 May 2005
Abstract
A laser power meter based on water as an absorbing medium has been developed to measure multi-kilowatt CO2 laser power with
high sensitivity and accuracy. Water absorbs CO2 laser radiation readily within a very thin layer. Though water has large thermal
capacity, due to short absorption length, it could vapourize at high laser powers. In order to circumvent this problem, this power
meter has a centre cone and a rapidly spinning water film as the calorimetric medium. The unique feature in this development is the
centre cone, which diverges the beam and reduces the power density thus reduces the possibility of vapourization. This minimizes the
error in measurements. Due to the rapidly moving fluid film, it exhibits fast response at low as well as high power levels.
r 2005 Elsevier Ltd. All rights reserved.
Keywords: Electro-magnetic radiation; Absorption coefficient; Response time
1. Introduction
The increasing number of high-power industrialprocessing lasers created a need for a primary laserpower meter, which is capable to measure CO2 laserpower in 10–20 kw range and is reliable in long-termoperation. Several optical power meters are availablenow for measuring cw laser power up to few kilowatts.However, power measurement at higher power levels(multi-kilowatt) is still a challenge. In the literature[1–3], several approaches have been described toconstruct laser power meters, however, most havelimitations to measure high powers. At higher powerlevels, continuous monitoring is a bit difficult, as theradiation tends to damage the absorber coating materialin some power meters. This paper reports the design of aspinning cone water film power meter to overcome allsuch deficiency. It can measure power of cw CO2 laserbeam from a few watts to tens of kilowatts.
e front matter r 2005 Elsevier Ltd. All rights reserved.
tlastec.2005.03.003
ing author. Tel: +91731 2488384;
88380.
ess: [email protected] (R.K. Soni).
2. Background
A flowing fluid as the absorbing medium is desirablefor intense laser radiation at high power levels. Thispower meter is designed considering this concept. Thisapproach simplifies the design, heat transfer andproblem of calibration and/or recalibration. Water itselfor in combination with appropriately selected dissolveddyes or collides has been found to be a convenient andinexpensive absorbing fluid. Moreover, the water hashigh specific heat, almost constant density with tem-perature, and chemically stable and good absorber ofCO2 laser radiation. These properties have historicallymost often been the basic ingredient in calorimeters forelectromagnetic radiation. Liquid calorimeters have alsobeen previously employed for measurement of laserradiation [1–3]. However, high-power applications havebeen infrequent and generally limited to the visiblespectrum because of containment vessel and/or windowproblems. The utilization of a free falling surface waterfilm for the absorption of CO2 laser radiation has beenreported in the literature [4]. Experience has shown thatthis particular approach is limited to relatively lowpower levels of a few hundred watts, principally becauseintense laser radiation quickly disrupt and disperse free
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Water Inlet , Temp. Sensor T1& Flow Sensor
Water Outlet and Temp. Sensor T2
ASSEMBLY
Center Cone
Water CollectingJacket
Water layer formedon the surface of the cone
Laser Beam
Rotating Cone/Disc
Fig. 2. Schematic of power meter.
R.K. Soni et al. / Optics & Laser Technology 39 (2007) 196–201 197
falling liquid film dynamics. In addition, instant localvapourization and energy conduction to the substratecan compromise measurement accuracy. Seguin et al.have reported a spinning water film power meter formeasuring high laser power [5]. However, it has a fewlimitations. In this power meter the water is comingfrom the apex of the rotating cone. Water has to travelcertain distance so that the centrifugal force is strongenough to spread and form a uniform water film on therotating cone. Due to this the area around the apex maynot have uniform water film, suitable for measuringlaser beam of stable resonators (solid laser beam). Thewater film surface area on which laser beam falls, is alsolimited by the size of laser beam (A ¼ p� R� S, whereA is the cone surface area not including the base, R the
base radius, S the slant height ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiR2 þH2
p� �, H is the
height of cone) i.e. for small beam of higher power, wecan increase the water film surface area to reduce thepower density, as it is possible in our power meter byusing centre cone.
Moreover, the water flow rate cannot be increasedbeyond a certain limit as the water jet velocity doesnotallow the formation of uniform water film. We havedeveloped a modified spinning cone water film powermeter incorporating a highly reflecting centre cone toovercome the above deficiencies.
3. Basic principle
This paper reports on a new approach to theutilization of an absorbing water film for measurementsof output power of high-power cw infrared lasers. Themain feature of the power meter is the rapidly spinningwater film, which works as the calorimetric medium.Fig. 1 is the photograph of the power meter. Its
Fig. 1. Spinner cone water film power meter.
construction is schematically shown in Fig. 2. Anelectric motor of 0.35HP has been used to rotate theshaft, central cone and conical disc (Fig. 1). Waterenters from the inlet port and flows through the hollowshaft, centre cone and then finally spreads over conicaldisc due to centrifugal force and back stream velocitypressure imparted to water (Fig. 2). Laser beam falls onthe centre cone & reflected and diverges to the rotatingcone where water film is formed (shown in Fig. 2 as darkblack colour above rotating cone). Laser gets absorbedin the water film and the temperature of the waterincreases. Water is collected from the outlet port andtemperature is measured with the help of another sensorplaced at outlet port (Fig. 2). The centre cone is designedin such a way to take care of high intensity beam anddiverge it so that the threshold value of laser intensityfor localized boiling is avoided. This reduces thevaporization of water and hence minimizes the inac-curacies in measurements.
In its rapid transit over the cone surface to an outercollecting annulus, the liquid film experiences enormousaxial and radial acceleration. Consequently, it becomesexceptionally stable physically. The net effect of thisprocess is the generation of a continuous free surfacethin liquid film, which is unaffected even by strongexternal forces; including intense laser radiations. Dueto its high transport velocity, the film can easily absorbtens of kilowatts of cw or pulsed radiation without anydifficulty. The measurement of incident radiation isaccomplished through simultaneous determination of
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z
Iin Iout
Fig. 3. Laser absorption.
R.K. Soni et al. / Optics & Laser Technology 39 (2007) 196–201198
the fluid flow rate and the temperature differencedeveloped in the absorbing liquid. These measurementsof flow and temperature are performed, repeatedlyupdated, and correlated by electronic circuit, such thatthe real time power level can be continuously displayed.
Absorption of laser radiation in water film can begiven by (Fig. 3)
Iout ¼ I in e�az,
where Iout is the radiation intensity coming out fromwater film, I in the radiation intensity incident on to thewater film, a the absorption co-efficient, and z is thewater film thickness, For water and CO2 laser combina-tion, i.e., l is 10.6 mm, a is 800 cm�1 [6].
For the attenuation length za ¼ 1=a ¼ ð0:0125mmÞ,Iout=I in ¼ 0:3678.
It is clear from the above calculation that �63% oflaser radiation is absorbed in the just 12.5 mm thicknessof water film (0.0125mm), further for 99.99% absorp-tion require 9.21 times absorption thickness za (i.e.�0.115mm), also reported in literature [7]. The waterfilm thickness measured practically was found within1–2mm. The implication of the above data is thatnegligible radiation is directly reaching to the metalsurface beneath the water film.
4. Response time
All the units of this power meter and containedvolume of water is made so small to achieve the fastresponse time. The response time of the spinning conewater film power meter can be given by tr ¼ V=Q [5],where V is the volume of water contained in the rotatingcone/disc in the cone-shaped water film and Q is thecooling water mass flow rate. In our case V ¼ 67:35 cm3
and Q ¼ 10 lpm (167.67 cm3/s). The response timecomes out to be 0.4 s.
However, the actual response time will be determinedby the response time of the temperature sensors andwater flow meter, which is of the order of ten seconds.
5. Mechanical design
Power meter head consist of mainly conical disc,centre cone, water collecting jacket and a motor.Conical disc and centre cone has been made ofaluminum. Aluminum has a very good reflectivity forCO2 lasers in the range of 95–98%. The centre cone andconical disc has the diameter of 70 and 222mm and theangles are 901 and 1201, respectively. The areacorresponding to these angles is 308 cm2. These arerotating parts and continuously exposed to water andopen environment. Since the aluminum is a lightweightmaterial and has good corrosion resistance therefore thisparticular material is preferred for these components.Other components like bearing housing and shaft ismade of stainless steel. Two numbers of bearings havebeen used and oil seals are used to protect the bearingsfrom water. The centre shaft is hollow and coupled withthe motor through a flexible coupling. This coupling cantake the linear as well as small angular misalignment.
6. Electronics
For the faster and continuous real time monitoring oflaser power meter, suitable sensors measure the waterflow rate and the rise in water temperature, and the finallaser power is displayed electronically. It gives contin-uous digitized read out of laser power. The blockdiagram is shown schematically in Fig. 4.
Semiconductor sensors AD590 have been used for thetemperature measurement at water inlet and outlet portof power meter. The sensor AD590 operates over arange of �55–+135 1C with a resolution of 0.1 1C. Theaccuracy of the temperature sensors is 70.5 1C. Thenonlinearity of the sensor is o0.3 1C for the full range.This sensor generates 1mA signal for each degree Kelvinof absolute water temperature. This signal is convertedinto voltage by passing the current through a resistance.The sensors T1 and T2 determine the temperature ofinlet and outlet water, respectively. These sensors givemilli-volt signal corresponding to temperature and aninstrumentation amplifier is used to get the differentialoutput. Wheel flow meter (EUREKA make) has beenused for the water flow measurement in this powermeter. The flow range is 1.66–16.66 lpm. It gives outputsignal of 4–20mA with an accuracy of 71% for the fullrange. This current signal is converted to voltage signalby connecting a resistance. The flow meter gives aminimum output of 4mA, which gives an offset voltagesignal. This offset signal is nullified by using adifferential amplifier. The clock pulse is required foranalog to digital converter (ADC) and display driver ICfor this a 100 kHz clock pulse is generated by anoscillator IC. The Burr Brown’s special function IC 4302is utilized for multiplying the temperature signal and
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5V DC
5V DC
24V DC
+15V DC
-15V DC
0
0
00
0
U1
AD620 6
3
2
74
OUT
+
-
V+
V-
R1
U3
OP-07
3
2
74
6
+
-
V+
V-
OUT
V1DC V
T1
T2
tempraturesensors T1&T2
flow meter
4mA to 20mA
for
1.66 to 16.66 lpm
Differential ampto nullify offset voltagefrom flow meter
Anolog
multiplier
4302
delta T
Flow o/p
ADC and
7 segment
Display
Driver
7 segment
Display
ICL7135
BB
O/P
InstrumentationAmp 0
53
2.
+
-
Fig. 4. Block diagram of power meter display circuit.
R.K. Soni et al. / Optics & Laser Technology 39 (2007) 196–201 199
flow meter signal. This multiplied output is utilized togive digital read out. The signal from multiplier is givento IC 7135, which is a high precision monolithic 4-1/2digit A/D converter. Dual slope conversion reliability iscombined with 71 in 20,000 counts accurately and a2.0000V full-scale capability. It features high impedancedifferential input. The multiplexed BCD output anddigit drivers provide easy interface to external displaydrivers. IC 7447 seven-segment display driver is used togive digitized display of laser power.
7. Experimental results
It is clear from the Fig. 2 that incident laser radiationis falling on the centre cone and directs earlier to conicaldisc which are rotating at 3000RPM. A thin water filmis formed on the conical disc as stated above andincident laser radiation is absorbed completely on thewater film. The design parameters of this spinningcalorimeter are such that its thermal mass (centre cone,conical disc and annulus water collecting jacket andfluid film) is made as low as possible. This feature alongwith a very rapid fluid transport gives the device aninherent fast response, over large measurement range.Maximum sensitivity can be achieved simply by adjust-ment of the absorbing liquid flow rate. When operating
with a highly absorbing fluid, such as water and CO2
laser (10.6 mm radiation) near full absorption of the laserradiation occurs within a penetration depth of a fractionof a millimetre. Thus one needs only to vary flow rate toaccommodate the full range of measurement.
Tests have shown that the radiation penetration depthinto the transporting film is small with the appropriateselection of absorbing fluid, and at a high fluid flowrates, the metallic cone surface becomes effectivelyisolated from the incident radiation. Consequentlynegligible energy is transmitted or absorbed by thenonfluid part of the system.
Experiments were conducted on this power meter withan indigenously developed high-power CW CO2 laserover a wide range of power levels (1–15 kW) to ensure itsperformance. Laser power was measured with differentwater flow rates and rotational speed of 3000 rpm andcompared with a Macken Instruments make hand heldpower meter and Ophir make power meter. Ordinarytap water was used as the absorbing medium. Resultsare plotted in Figs. 5 and 6. Close examination of thedata reveals that the response of power meter is linear inthe measured laser power range at differing water flowrates.
Our experimental calculation shows that approxi-mately 0.7 lpm water flow rate is needed for per kilowattof measurement and a minimum of 3 lpm flow rate is
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0
1
2
3
4
5
6
0.95 1.55 2.5 3.6 4.3
Power Measured by Hand Held Power Meter (kW)
Pow
er M
easu
red
by D
evel
oped
Pow
er M
eter
(kW
)
3 lpm
4.5 lpm
6 lpm
Fig. 5. Performance curve of developed power meter with varying flow
rate.
0
2
4
6
8
10
12
14
16
10 15 20 25 30 35 37
Discharge Current (amp)
Las
er P
ower
(kW
)
New Power Meter
Ophir Make Power Meter
Hand Held Power Meter
Fig. 6. Comparison of developed power meter with other power
meters.
R.K. Soni et al. / Optics & Laser Technology 39 (2007) 196–201200
required in this device to form a uniform film thickness.With temperature sensor having range �55–135 1C andresolution of 0.1 1C, theoretically the resolution inpower measurement is �20W.
8. Instrument accuracy
Though this power meter is a primary instrument andshould not require calibration; still the instrument can
have error. The reasons of these errors are
(i)
inaccuracy in the measurement of temperature andwater flow sensors,(ii)
heat loss to the surroundings by conduction/convection through metal part,(iii)
evaporation of the water at high power level, (iv) radiation loss to the atmosphere due to scattering oflaser beam incident on the centre cone,
(v) water (absorbing fluid) loss at high speed ofrotation,
(vi) error in measurement due to change in rCv of waterwith temperature.
It has been described earlier that laser beam falls onthe centre cone and diverges to an area of 308 cm2. Wehave measured power up to 15 kW and power density atthis power level is 0.049 kW/cm2. It has been experi-mentally found that beyond 1 kW/cm2 power intensitylevel, the localized boiling of water takes place thereforein our case the possibility of vaporization is almostnegligible. It has also described earlier that 0.7 lpm ofwater is needed for the measurement of each kilowatt ofpower; therefore �10 lpm of flow rate is required tomeasure the 15 kW power. If we calculate the rise intemperature of water for the 15 kW of power measure-ment, then it comes out to be 20 1C. rCv of waterchanges 1% from 20 to 50 1C, therefore there can be themaximum of 1% error in measurement of power. Centrecone has a very sharp point of diameter 1mm and thebeam diameter is 65mm. A part of laser radiation whichwill be reflected back is 5–10W (max). It will give us theerror of 0.1% (max). Since the metal temperature is notvery high heat loss through radiation is nil in our case.
Considering all errors, we can say that the accuracy ofthis power meter is 73%.
9. Conclusions
In conclusion, a simple and reliable spinning conewater film power meter has been developed. The featuresof this device are fast response and high measurementaccuracy. The device has large power measurementrange and can be employed with wide variety of lasersby selection of an appropriate absorbing fluid.
Acknowledgements
The authors gratefully acknowledge all the Scientificand Technical staff of Industrial CO2 Laser Section,CAT (Indore) for their continuous support and sugges-tions.
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