First edition — August 2006
Bradfield Road, Lindfield, NSW 2070
PO Box 264, Lindfield, NSW 2070
Telephone: (61 2) 8467 3600
Facsimile: (61 2) 8467 3610
Web page: http://www.measurement.gov.au

© Commonwealth of Australia 2006

CONTENTS
1 Introduction ............................................................................................................ 1
2 The Integrating Sphere ........................................................................................... 1
3 Diode Laser System ............................................................................................... 4
4 Monitor Photodiode and Preamplifier ................................................................... 9
5 Data Acquisition System ...................................................................................... 10
6 Calibration of the Responsivity of the External Photodiode Detector ................. 11
7 Preliminary Experimental Results using this Laser-sphere System .................... 11
8 Conclusions .......................................................................................................... 14
9 Acknowledgements .............................................................................................. 15
10 References ............................................................................................................ 15
iii

iv

1 INTRODUCTION
Traditionally, radiometric sources have been based on lamps of various types or on blackbodies. This has certainly be so for diffuse sources and sources that have used diffusing devices such as integrating spheres or diffusing plates. However, more recently, there has been increasing interest in using lasers as components in diffuse sources. While there are many types of lasers with different optical characteristics
(in the time, frequency, spatial and spatial frequency domains), it is continuous-wave single-frequency visible and near infrared lasers that would be of greatest interest in radiometry. In particular, the use of a CW single-frequency laser together with an integrating sphere should allow generation of optical beams that are highly temporally-coherent but have almost no spatial coherence. Such sources would have many applications including, for example, the simulation of blackbodies or moltenmetal sources at a fixed wavelength, and in the calibration of photometric equipment at particular wavelengths. In the future, radiometric source standards based on other laser types and characteristics together with traditional passive devices, such as integrating spheres, will be developed. A major metrological application of such laser-sphere systems is likely to be in the development of thermodynamic temperature scales [1–4].
In this report, a 650 nm monochromatic diffuse source is described. This source serves both as a demonstration of the principles involved as well as acting as a source in two specific applications in the work of the temperature and radiometry groups, that is, in (a) absolute radiometry and in (b) radiation thermometry and thermodynamic temperature scales.
2 THE INTEGRATING SPHERE
When a flux,

illuminates a diffusing surface of reflectance, r, over an illuminated area, A, the radiance, R, seen within a total projected solid angle,

, is:
R =

r / A

(1)
Inside the sphere, the radiation undergoes multiple reflections so that the total flux incident over the entire sphere can be obtained by a power series expansion in r(1 – f), where f is the port fraction, which leads to the result that:
R = [

/ A

] { r / [ 1 – r (1 – f)]} which implies that the sphere multiplier is:
(2)
M = r / [ 1 – r (1 – f)] (3)
Therefore, from a knowledge of the port fraction and the effective sphere reflectance, the sphere efficiency and output radiance can be calculated. If r = 0.98 and f = 0.015, the multiplier is M = 28.2. This suggests that an average input laser power ~2 mW is sufficient for most of the radiation thermometry applications envisaged including as a
650 nm source simulating a blackbody near ~1000
0
C. For absolute radiometry applications, the power and radiance levels are non-critical.
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The sphere used was a cast brass item comprising two flanged halves and three flanged ports. The outside diameter is ~220 mm and the inside diameter of the coated sphere is 170 mm. These ports are used for the laser input, monitor photodiode and main optical output, and have diameters of 5.0, 5.0 and 38.0 mm, respectively
(Figure 1). Therefore, the port fraction is f = 0.0129 and the sphere multiplier is
M = 30.
This sphere was internally coated with an ~8 mm thick layer of fine PTFE powder
(Spectralon) applied as a compacted series of layers bound with silicone in the base layers but with no binders used in the surface layers. A suitable particle size distribution for the PTFE powder was established and the PTFE was sieved prior to its application to the sphere. This approach was found to produce a coating that was macroscopically smooth but microscopically rough. The smoothness of the surface leads to a uniform radiation spatial distribution. On the other hand, the roughness of the surface leads to an orientationally-independent enhanced scattering which not only smooths the radiation spatial profile but, more importantly, reduces interference, speckle, laser spatial mode structure and other coherence effects. Therefore, the combination of the microstructural and optical properties of the sphere coating together with the high-divergence but single-mode characteristics of the diode laser means that it has been possible to direct-couple the laser to the sphere thereby maximising coupling efficiency while producing an extremely uniform radiation spatial profile with negligible laser speckle. The exploitation of the micro-roughness of the sphere coating and the spatial mode of the laser is a novel aspect of this system, and will be the subject of future theoretical work. The profile of the optical beam from the sphere is Lambertian.
One method to suppress speckle and laser spatial mode structure is to fibre-couple the light to the sphere using a multimode fibre vibrated in a ultrasonic bath to modescramble the beam. The approach used in the present work achieves a source with spatial uniformity comparable to or better than the fibre-coupled approach but also with improved power stability and coupling efficiency.
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Figure 1. Configuration of the integrating sphere.
2

Absolute radiometry using a laser-sphere (Figure 2a) is based on an optical system comprising the source, an aperture to define the source radiance, an aperture to define the irradiance at the detector and a detector of calibrated response. A variant of this system (Figure 2b) can be used as an optical system for radiation thermometry.
Fundamentally, the two systems differ only in the detector configuration used — in absolute radiometry the detector is an apertured photodiode whereas in radiation thermometry the detector comprises a simple imaging system, a filter and a photodiode.
For radiation thermometry, the choice of operating wavelength is somewhat arbitrary.
However, for measurement of temperatures ~1000
0
C, wavelengths ~650 nm are typically used. In general, the approximate waveband used is normally chosen to be in the vicinity of the peak of the Planck distribution for the temperature of interest.
Figure 2. Optical layout for (a) absolute radiometry, and (b) radiation thermometry.
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3 DIODE LASER SYSTEM
A photograph of the laser-sphere source is shown in Figure 3 and a schematic of the system is shown in Figure 4. Basically it comprises:
 the sphere,
 diode laser,
 diode laser driver,
 laser temperature sensor,
 heater and thermoelectric cooler for laser temperature control,
 a temperature controller,
 a photodiode and photodiode preamplifier to monitor sphere power,
 four linear power supply units to power the above,
 status indication for laser power, laser current, heater, set temperature, actual temperature and temperature lock faults, and
 analogue and relay outputs for monitoring, fault detection and alarm conditions.
The laser is a Toshiba TOLD-9442M 5 mW index-guided single-mode laser diode.
This diode laser produces 5.0 mW at 37.9 mA and has an operating wavelength of
650.5 nm at 26.3
0
C. It has an internal monitor photodiode which generates 0.154 mA at 37.9 mA of laser current. Beam divergence is

 = 28.0
0
and

 = 8.8
0
. This diode laser can be operated in a single longitudinal mode with a single line emission and linewidth of ~5 MHz provided the laser drive current and temperature are sufficiently stable.
Figure 3. Photograph of the laser-sphere source.
The diode laser is at the top and the photodiode is on the right hand side.
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Figure 4. Schematic of the overall laser-sphere system.
The diode laser is mounted in a hole in a U-section aluminium bracket and clamped in place using a securing screw and a curved spring-steel clamp with M3 screws and tapped M3 clamping nuts. Heatshrunk tubing around the device pins insulate the device from the heatsink and electrical connections to the laser are made using a 3-pin transistor-type socket. The diode laser module is connected to the controller by a
4-core shielded cable of minimal length terminated by a 3-pin Canon plug. A 1 M
 resistor across the diode laser anode and cathode terminals prevents electrostatic damage if the laser is disconnected from the controller.
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The laser driver used is a Thorlabs EK1011 module which is intended for CW lasers and operates in an automatic power control (APC) mode using feedback from the laser’s on-chip monitor photodiode. This module provides a laser drive current of
0 to 250 mA, a compliance voltage of >2 V, power stabilisation to within <0.01% and current resolution of 1

A. The driver is powered by a regulated + 9 V linear supply designed for good transient response but minimal turn-on and turn-off spikes so as to protect the laser driver and hence the laser itself.
In order to achieve stable output power and lasing wavelength, it is crucial to temperature control the diode laser. Typically, these lasers have wavelength dependences of ~0.3 nm/
0
C so a wavelength stability of ±1 pm requires temperature control to ±0.003
0
C.
The requirements for laser temperature control involve a relatively wide temperature range of ~10 to 35
0
C for applications requiring tunability, and temperatures near ambient conditions (~20 to 25
0
C) for applications requiring a given wavelength where that wavelength corresponds to an operating temperature near ambient. Temperature control near ambient temperatures can often be difficult because of the low duty cycles or powers required for either heating or cooling. Therefore, the approach taken here was to use a fixed amount of resistive heating and to arrange for the temperature controller to adjust the amount of cooling to maintain the temperature. That is, the heater and the cooler are acting against each other. This method achieves excellent temperature control near ambient temperatures while allowing for a wide temperature range which is further extended to lower temperatures by simply disabling the heater power.
Laser temperature control is achieved using a switch-mode controller (Thorlabs
TCM1000T) with fine adjustment of the proportional and integral gain. It was found that the temperature control approach used here did not require differential gain control and so a full PID controller is not required. This controller drives a Peltiertype thermoelectric cooler (Melcor 4.5 V/ 1 A) with up to 3 W (3 V at 1A) in either a cooling or heating mode but only cooling is used in this system. Heating is achieved using a 22

/ 20 W resistor in a TO220 package bolted to the laser heatsink on the cold side of the Peltier device. The heater is turned on or off using a front panel mounted switch (Sw 2) and the status of the heater is indicated by a yellow led on the front panel. Both the Peltier and the resistor were clamped in place using thermallyconductive BeO paste.
Improved temperature control can be obtained, albeit at the expense of temperature range, by adding ballast resistors of 12

and 3.3

in the heating and cooling circuits, respectively. The origin of this enhanced performance is the reduction in the temperature variations that occur as the Peltier is switched on and off, and that these variations can be further suppressed by both the thermal mass of the laser heatsink and further adjustment of the PI gains.
Although the nominal temperature stability quoted for the TCM1000T is <0.1
0
C, it was found that the combination of the above approach and careful setting of the PI gains enabled long-term (8 hours) stabilities of 0.002
0
C to be obtained. The stabilisation time is ~20 minutes. Laser temperature is sensed with an encapsulated 10 k

NTC thermistor bolted immediately adjacent to the laser on the laser heatsink
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using BeO thermally-conductive paste. Although there is a small temperature difference between the laser chip and the thermistor due to the thermal impedance of the heatsink and the packaging of the devices, this temperature difference is constant and hence does not contribute to laser temperature drift. All the temperature control signal and power connections are made to the temperature controller using a 7-core (3
A rating) flexible cable and a 7-pin multipole plug and socket combination at the controller rear panel.
The laser wavelength can be easily temperature-tuned by varying the temperature setpoint on the temperature controller board. This tunes the laser over a range of
~646.0 to 653.5 nm (10 to 35
0
C) in a linear fashion with a slope of ~0.3 nmK
–1
. In most applications, the laser is not tuned but rather set at a fixed suitable wavelength and stabilised at this wavelength.
The laser controller has four separate linear power supplies (Figure 5). The temperature control supplies are isolated from the laser and amplifier supplies by using two separate transformers and hence two separate grounds (0 V). This eliminates the possibility of cross-talk, noise and transients on the laser driver and preamplifier due to switching transients associated with the temperature controller.
Mains power to the controller is supplied via a fused IEC connector with an integral
EMI filter, and an illuminated mains switch. All metal parts (e.g. connector bodies and internal metal mountings) are wired to mains earth. PSU 1 supplies
~300 mA at + 9 V to the diode laser driver.
The 24 V output of transformer T1 is half-wave rectified by B2 and filtered by C5 to obtain ~16 V dc. An LM317T adjustable 3-terminal regulator provides + 9 V determined by R2 and R3. Low output noise is guaranteed by C8 and C9. Diode D1 provides a discharge path for these capacitors on turn-off. The supply is turned on or off by switch Sw 1 and the green front panel mounted led indicates when power is on.
This supply has sharp clean turn-on and turn-off response with negligible overshoot thereby providing additional transient suppression for the diode laser driver and hence the diode laser itself. The slow turn-on function for the diode laser is provided by the driver. PSU 2 supplies ±12 V to the monitor photodiode preamplifier. This supply is a conventional design based on full-wave rectification of the output of T1 (B1), filtered
(C1 and C2) and regulated (7812T and 7912T). The temperature control supplies
(PSU 3 and PSU 4) drive the heater and the temperature controller, respectively. They are both based on the LM323T (3 A) 3-terminal regulators driven by the full-wave rectified (B3) and filtered (C10) output from a separate transformer (T2). LM323T regulators were chosen as they contain additional protection circuitry.
The measured performance parameters of the diode laser system are:
 wavelength
650.333 ± 0.005 nm
 linewidth
 power
<5 MHz
0 to 5 mW
 noise (1 Hz to 10 MHz) <0.003%
 power instability/drift (12 hours) <0.007%
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Figure 5. Circuit diagram of the laser controller’s power supply units
(PSU 1 to PSU 4 in Figure 4).
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4 MONITOR PHOTODIODE AND PREAMPLIFIER
Although the diode laser is power stabilised, small alignment changes and changes in sphere reflectance could lead to a small change in the output radiance. This variation can be corrected for by using an auxiliary photodiode mounted directly on the sphere to monitor the power transmitted by a small port in the sphere. Therefore, the power sampled by this photodiode will be directly proportional to the power leaving the main sphere output port. A windowed UDT UV-020 photodiode was used. The photodiode is mounted in a teflon-insulated carrier on a flange on the side of the sphere. The port size is 5 mm diameter and the photodiode’s active diameter is 10 mm ensuring that all light transmitted by the port, including diffracted light, is detected.
Interference effects normally associated with the use of windowed photodiodes with laser sources are not significant here because the light emerging from the monitor photodiode sphere port is diffuse. It is not necessary to calibrate the responsivity of this photodiode since it merely performs a monitoring and scaling function. However, the stability of the responsivity is important as it is necessary for the photodiode to consistently monitor power over time. Recent studies of long-term silicon detector drift using cryogenic radiometry [5, 6] have shown that silicon detectors drift by
<0.003%/year at wavelengths near 650 nm.
The photodiode output current is ~1 to 2

A for typical laser power levels used here.
Therefore, it is necessary to amplify this signal using a transimpedance preamplifier to obtain a voltage signal at a useful level. This is typically done using a single-ended current-to-voltage convertor. However, this circuit topology has a number of limitations and so a preamplifier based on a new design approach was used (Figure 6).
A fully differential arrangement is used with balanced inputs which ensures that the photodiode is optimally and actively biased at almost precisely zero volts (within
<10

V). This virtually eliminates the residual nonlinearity and biasing effects present in single-ended designs and enables the photodiode to be used in either polarity equivalently. An ultra-precision op-amp (Analog Devices OP177) is used. It has extremely low input offset voltage (<10

V) and input offset voltage drift (<100 nV/
0
C), low bias and offset currents (<1 nA), very low noise (120 nV rms, 3 pA rms), high gain (>10
7
) and a closed-loop bandwidth of 600 kHz.
Figure 6. Circuit diagram of the sphere monitor photodiode and preamplifier.
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The output voltage (V out
) is proportional to the photodiode current (I d
) and the gain setting resistors (R):
V out
= 2 R I d
.
(4)
For R = 475 k, the gain is 0.95 V/

A. By choosing matched 0.1% <25 ppm/
0
C metal film resistors, an overall gain accuracy of better than 0.1% for (20

10)
0
C was obtained without gain calibration. If the preamplifier is solely used in a monitoring function, gain accuracy is not critical but gain stability is. The performance parameters of the amplifier are:
 input current 0 to

10

A
 gain (0.950 0

0.000 7) V/

A
 output noise <200 nV
5 DATA ACQUISITION SYSTEM
Many of the applications of this laser-sphere system will be in areas where the the accuracy required is ~0.1% so 12-bit resolution (0.024% full scale) is suitable.
Therefore, the simple interface shown in Figure 7 was constructed and housed in a small enclosure with a DB25 cable to connect to the printer port of a computer.
A MAX192 12-bit serial output ADC was used in a conventional configuration. The
IC accepts six analogue inputs via six simple voltage dividers which set the full-scale analogue input voltage ranges to either 6 V or 20 V. The printer port reads the ADC serial output directly. Printer port pins 14 and 17 are programmed to give two digital outputs D1 and D2 via 1 k isolating resistors. For applications requiring higher accuracy ~0.02%, the laser-sphere system would be used in laboratory environments where there is already existing data acquisition system with >20-bit resolution.
Figure 7. Schematic of 6-channel 12-bit serial ADC adaptor.
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6 CALIBRATION OF THE RESPONSIVITY OF THE EXTERNAL
PHOTODIODE DETECTOR
In absolute radiometric measurements (Figure 2a), the detector is formed by a photodiode/aperture combination. Therefore, the absolute responsivity of the photodiode must be determined with an accuracy comparable to or better than the desired system accuracy. The photodiode (windowless Hamamatsu S6337) was calibrated by cryogenic radiometry using an Oxford ‘Radiox’ cryogenic radiometer and a krypton-ion laser operating at 647.089 nm. The measured responsivity is:
R
647.089 nm
= (0.35722 ± 0.00007) AW
–1
The final absolute responsivity at 650.333 nm was obtained by applying a correction based on the ideal linear wavelength dependence of the responsivity and by making the reasonable assumption that the quantum deficiency is approximately constant over the range 647 to 650 nm, so that:
R
650.333 nm
= (0.35984 ± 0.00009) AW
–1
If the laser is tuned to another wavelength near 650 nm, the photodiode absolute responsivity at this new wavelength is easily calculated by performing a new correction.
7 PRELIMINARY EXPERIMENTAL RESULTS USING THIS LASER-SPHERE
SYSTEM
The output power stability of the laser-sphere source was measured using the amplified output signal from the monitor photodiode. Over a period of ~1.4 hours the radiance was found to have a random drift of ~0.006% (Figure 8).
1202.8
1202.7
1202.6
1202.5
1202.4
1202.3
1202.2
0 1000 2000 time (s)
3000 4000 5000
Figure 8. Long-term drift behaviour of the output power from the laser-sphere.
The standard deviation of the points is

~0.006%.
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Spatial non-uniformity of the emitted radiation was measured using an aperturedetector combination (~ 10 mm aperture and a Hamamatsu S6337 photodiode (LAH2) or a 4-element transmission trap (TT05)) positioned ~1 m from the sphere port
(source aperture diameter of 38 mm) and translated in the horizontal plane. The normalised measured intensities agree very closely with the expected cos
4  dependence (Figure 9). The deviations from ideality are <±0.03% (Figure 10). These deviations are primarily due to sphere non-uniformity. However, this non-uniformity is easily corrected for leading to a final uncertainty in the spatial distribution of
<±0.01%.
1.000
measured
theoretical
0.995
0.990
0.985
0.980
0.975
-60 -40 -20 0 position (mm)
20 40 60
Figure 9. Comparison of the measured intensity as a function of lateral position with the theoretically-expected cos
4 
dependence.
0.04
0.03
0.02
0.01
0.00
-0.01
-0.02
-0.03
-0.04
-60 -40 -20 0 position (mm)
20 40 60
Figure 10. Relative deviations of the measured signal from the theoretical dependence.
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Stray light is commonly a major problem in absolute radiometry. This may originate from either within the experimental system (e.g. diffraction or interreflections from surfaces in the system involving light from the laser-sphere source) or externally
(e.g. stray room light or light from other sources in the lab). A detailed investigation of stray light in these radiometric systems is under way. However, for the immediate purposes, reduction of stray light was required to determine the performance limits of the laser-sphere system and its applications. This was achieved using a stray light elimination tube (SLET), shown in Figure 11. A SLET is basically a passive tubular device constructed from two (or more) blackened conical absorbers aligned back-toback.
Figure 11. The stray light elimination tube (SLET).
Areas of the source and detector apertures were measured with a position uncertainty
~±100 nm using an x–y scanning technique (UMIS) with a cylindrical probe or using a coordinate measuring machine (Leitz).
For absolute radiometry, the uncertainties obtained using this laser-sphere system are summarised in Table 1. The uncertainty is currently limited by the calibration of the absolute responsivity of the photodetector from cryogenic radiometry.
Table 1. Uncertainties for absolute radiometry
Aperture area <0.003%
Power instability <0.005%
Radiance uniformity (corrected) <0.01%
Distance
Photodiode calibration
Total
<0.01%
<0.02%
<0.025%
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An application of this laser-sphere system is as a source to replace a gold fixed point blackbody as a temperature standard near 1000
0
C and as a reference source for thermodynamic temperature measurement in this range. At present, the National
Measurement Institute uses the HTSP standard pyrometer to realise the ITS-90 temperature scale. This pyrometer uses two 650 nm (10 nm FWHM) interference filters, a Hamamatsu S1337 photodiode, a 0.8 mm aperture and a 50 mm focal length lens. The out-of-band rejection is >10
6
and the temperature uncertainties at 1064
0
C are ~40 mK for thermal sources emitting incoherent radiation. The spectral transmission of the filters used in the HTSP (Figure 12) was obtained using a 1 m monochromator with a bandwidth of 0.1 nm and a calibration accuracy of ±20 pm.
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
644 646 648 laser sphere
650 652 lambda /nm
654 656 658
Figure 12. The transmission spectrum of the HTSP tandem filter arrangement.
The slope at 650.333 nm is ~10%/nm giving an uncertainty of ~0.05%.
The laser-sphere is a partially coherent source. For such sources, it was found that the accuracy of the HTSP standard pyrometer was limited by interreflections between the planar optical surfaces within the pyrometer (e.g. filter, window and silicon surfaces).
The best achievable accuracy with this arrangement was ~0.1%. An improved pyrometer design should allow uncertainties to be reduced to ~0.025%.
8 CONCLUSIONS
A radiance source based on an integrating sphere illuminated by a single-mode diode laser operating at 650 nm is demonstrated. The source has power or radiance drift of
<±0.007%. The spatial non-uniformity is <±0.03% but can be corrected to within
<±0.01%. A simple absolute radiometric system based on this source together with two apertures and a calibrated photodiode was shown to have total system standard uncertainties of <±0.025%. The method was applied to radiation thermometry by replacing a fixed point blackbody with the laser-sphere source. It was shown that the use of such sources with pyrometers requires careful design of the pyrometer optical system to minimise interreflections between the internal optical elements.
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9 ACKNOWLEDGEMENTS
I am indebted to Mark Darlow for his patient work coating the integrating sphere with spectralon. I also thank Errol Atkinson and Mark Ballico for assisting with the measurements to characterise the laser-sphere system performance in the temperature laboratory. The SLET, which was used during the testing phase of this project, was made by Chris Freund.
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