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AbstractA new spectrally tuneable source has been developed for calibrating radiometricdetectors in radiance, irradiance or power mode. It i s termed the Spectrally TuneableAbsolute Irradiance and Radiance Source (STAIRS) and consists of a supercontinuumlaser, wavelength tuneable bandpass filter, power stabilisation feedback control andoutput coupling optics (Figure 1) [1]. I t is relatively portable, and provides a tuneablemonochromatic alternative to lamps, blackbodies or lasers.

Introduction This paper presents a reasonably portable, monochromatic tuneable source that canbe configured as a laser beam (as a source of optical power), or as an irradiance orradiance source. In this way it can replace tuneable lasers or mon ochromator-basedsources in many radiometry applications. The source is based on a supercontinuumlaser (wavelength range 400 nm – 2500 nm) commercially available from Fianium.Such sources, combined with a monochromator, are increasingly being used forradiometric measurements e.g. [2]. In STAIRS, the supercontinuum laser is coupledinto a filter, which provides spectral tuneability in a convenient fibre-coupledmanner. The laser power is also actively controlled using a feedback photodiode,and a software-emulated PID controller fed into the internal optical amplifier of thesupercontinuum laser.

Stabilising the sourceWithout active stabilisation, the output power of the supercontinuum laserdrifts by ~1 % per hour due to the alignment- and temperature-sensitive opticalinstrumentation and complex physics of the sup ercontinuum laser generation. Withactive stabilisation, there is no apparent drif t, and the signal fluctuations are dueto white noise (Figure 2). For averaging times greater than 100 s, the uncertaintyassociated with the laser power, due to this noise, is <0.01 %.

Wavelength tuneable filter The large wavelength range of the supercontinuum laser means that if awavelength band of only a few nm is required, over 99 % of the radiation mustbe removed by a filter of some sor t. The filter must therefore have a very goodrejection of ‘out-of-band’ light.

Several different types of filter were considered during the development of STAIRS,including acousto-optic, grating monochromators, prisms and volume Bragggratings. The final choice was to use the Laser Line Tuneable Filter (LLTF) [3] basedon Volume Bragg Grating technology. The advantage of the LLTF is a low out-of-bandtransmission; its main disadvantage is its high cost. At present we have only obtainedone for the 400 nm – 1000 nm spectral region (Figure 3).

 The LLTF has a bandwidth of 1 nm – 2 nm, with the shape and full width halfmaximum of the bandpass function changing with wavelength.

Output coupling optics The supercontinuum laser is fully fibre coupled and can provide a laser beam fromthe fibre. STAIRS has already been used to calibrate the irradiance responsivity ofocean colour radiometers, to test the stray light responsivity of an array spectrometerand as a source for reflectance measurements. For these measurements the STAIRSradiation was used either directly from the fibre, or coupled into a small integratingsphere (Figure 3). We are currently also developing a flat-panel source with ourcommercial partner, Polymer Optics. This will create a tuneable monochromaticsource, in a box 30 mm deep and ~200 mm by ~220 mm, with an illuminated surface170 mm by 170 mm (Figure 4). The system has an inbuilt feedback photodiode andis anticipated to be uniform to 1 % across the illuminated surface. The source isintended for use within, for example, vacuum chambers.

ConclusionsNPL has developed an optical source based on a supercontinuum laser and a VolumeBragg Grating. The source can be used in power, irradiance or radiance mode and isactively stabilised and can be tuned, at present from 400 nm to 1000 nm. It operatesat power levels of 0.5 mW – 5 mW. Extension to longer wavelengths is planned.

Acknowledgements This work was funded by the National Measurement Office of the UK Department of Business,Innovation and Skills and by the European Metrology Research Programme (EMRP). The flat

panel output optics are being developed by Polymer Optics Ltd.

References1. Levick, A.P., C.L Greenwell et al. “Spectral Radiance Source Based on Supercontinuum Laser

and Wavelength Tunable Bandpass Filter: The Spectrally Tunable Absolute Irradiance andRadiance Source.” Applied Optics 53, no. 16 (201 4): 3508-3519.

2. J. T. Woodward, A. W. Smith, C. A. Jenkins, C. S. Lin, S. W. Brown and K. R. Lykke,Supercontinuum sources for metrology, Metrologia, 46, S277-S282, 2009.

3. S. Blais-Ouellette, E. H. Wishnow, P. L. Shopbell, W. van Breugel, K. Taylor and R. Smith, DoubleBragg grating tunable filter, Spie-Int Soc Optical Engineering, 2004.

The spectrally tuneable absoluteirradiance and radiance source (STAIRS)

Andrew Levick, Claire Greenwell, Jane Ireland, Emma Woolliams, Teresa Goodman and Nigel Fox

National Physical Laboratory, Teddington, United Kingdom

Corresponding e-mail address: claire.greenwell@npl.co.uk 

Figure 1. A typical setup for the STAIRS system.

Figure 4: Polymer Optics’ flat panel source, (a) without and (b) with laser light input

(a) (b)

Figure 2. Measurement with a photodiode that is independent of the feedback system, of the

stability of the STAIRS laser, (a) when not stabilised, (b) when actively stabilised. (c) and (d) are the

corresponding Allan Deviation plots. The gradient in (c) indicates drift, while that in (d) indicates

white noise.

Figure 3. The supercontinuum laser coupled into a small integrating sphere via fibre and LLTF, which

was tuned to a selection of wavelengths.

450 nm 500 nm 550 nm 600 nm 650 nm

control laptop shielding

LLTF

laser output

(coupled to fibre)

super continuum laser