Download NMI TR 10 A 650 nm Diode Laser-based Integrating Sphere System

NMI TR 10
A 650 nm Diode Laser-based Integrating
Sphere System for Absolute Radiometry
Dr Philip B. Lukins
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)]}
(2)
which implies that the sphere multiplier is:
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 ~10000C. 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.
Figure 1. Configuration of the integrating sphere.
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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 ~10000C, 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 26.30C. It has an internal monitor photodiode which generates 0.154 mA
at 37.9 mA of laser current. Beam divergence is  = 28.00 and   = 8.80. 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/0C so a wavelength stability of ±1 pm requires temperature
control to ±0.0030C.
The requirements for laser temperature control involve a relatively wide temperature
range of ~10 to 350C for applications requiring tunability, and temperatures near
ambient conditions (~20 to 250C) 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 <0.10C, it
was found that the combination of the above approach and careful setting of the PI
gains enabled long-term (8 hours) stabilities of 0.0020C 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
NMI TR 10
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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 350C) 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
<5 MHz
 power
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/0C), low bias and offset currents (<1 nA), very low noise (120 nV rms, 3 pA rms),
high gain (>107) 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 (Vout) is proportional to the photodiode current (Id) and the gain
setting resistors (R):
Vout = 2 R Id.
(4)
For R = 475 k, the gain is 0.95 V/A. By choosing matched 0.1% <25 ppm/0C metal
film resistors, an overall gain accuracy of better than 0.1% for (20  10)0C 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:
R647.089 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:
R650.333 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
signal (mV)
1202.6
1202.5
1202.4
1202.3
1202.2
0
1000
2000
3000
4000
5000
time (s)
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 cos4
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%.
measured
theoretical
1.000
normalised signal
0.995
0.990
0.985
0.980
0.975
-60
-40
-20
0
20
40
60
position (mm)
Figure 9. Comparison of the measured intensity as a function of lateral position
with the theoretically-expected cos4 dependence.
0.04
0.03
deviation (%)
0.02
0.01
0.00
-0.01
-0.02
-0.03
-0.04
-60
-40
-20
0
20
40
60
position (mm)
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
NMI TR 10
Aperture area
<0.003%
Power instability
<0.005%
Radiance uniformity (corrected)
<0.01%
Distance
<0.01%
Photodiode calibration
<0.02%
Total
<0.025%
13
An application of this laser-sphere system is as a source to replace a gold fixed point
blackbody as a temperature standard near 10000C 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 >106 and the temperature uncertainties at 10640C
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
laser
sphere
0.5
0.4
0.3
0.2
0.1
0.0
644
646
648
650
652
654
656
658
lambda /nm
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.
10
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