# Presentation: 026990

## Uncertainty analysis of radiometric calibrations using correlated photons

**Uncertainty analysis of radiometric calibrations using correlated photons**
** **

**Jessica Cheung and Chris Chunnilall**
**Optical Radiation Measurement Group**

**31 March 2003                ****© **** 2003 British Crown Copyright**

## Overview

**Optical Radiation Measurement Group, NPL**
**Radiometric calibrations**
**Correlated photon technique**
**Detector calibration**
**Source radiance calibration**
**Current accuracies**
**Key uncertainty factors**
**Ultimate goal - Reducing these uncertainties**
**Current and Future plans**

## Optical Radiation Metrology at NPL

**Responsibility for calibration requirements concerning:**
- Optical detectors and sources (200 nm – 14 m)
- Materials – reflectance, transmittance, fluorescence etc, appearance (200 nm – 56 m)
- Primary standard – cryogenic radiometer
- Transfer standards

## Radiometric calibrations

- Current techniques – cryogenic radiometer
- Traceable to SI units of electricity
- Realised in few mW regime
-  Correlated photon techniques
- Direct realisation of scales (no calibration chain)
- Realised in photon-counting regime
- Responsivity & radiance scales realised independently
-  Ultimate goal
- New calibration facilities – absolute measurements in photon counting regime
- Reduce primary uncertainties to same/better level as current techniques – new primary scales

## Spontaneous parametric downconversion

- Simultaneous and Random process
- Conversion efficiency ~ 10-7

- 	***p** = ***1 + **2
**	****k**p = **k**1 + **k**2
- 	t1	 t2

## Detector calibration

*N*: number of photon pairs
*N*C: number of coincidences
*	***A = *N*C / *N*B
- A is DUT channel,B is trigger channel

**Key sources of uncertainty**
-  Measurement of NC – wide temporal window – jitter, others?
-  Accidental coincidences AC
-  Losses in channel A:  tA
-  Measurement of NB
-  False triggers due to shot noise, background: DB
-   Limiting optical elements can be placed in the trigger path B
-  Alignment – optimisation, stability, reproducibility
**A = (*N*C – AC) / [tA (*N*B – DB)]

## Source radiance 

**Key sources of uncertainty**
-  Loss in path from source to down-conversion region
-  Overlap of pump and injected source
-  Alignment – optimisation, stability, reproducibility

## Current accuracies:

|  | Cryogenic Radiometer 
(%) | Correlated Photons 
(%) |
| --- | --- | --- |
| Responsivity | 0.01  0.001 | 0.5 |
| Radiance | 0.3   0.01 | 1 |

- Required improvement:
-  <1 order of magnitude for direct calibration in photon-counting regime
- 1 to 2 orders of magnitude for realisation of primary scales

## Schematic of NPL set up for detector calibration

## Summary of key uncertainty factors

- Characterisation of optical path loss
- Measurement of coincidence counts ()
- Alignment issues:
- 		How well can we align system ?
- 		How stable is this alignment ?
- 		How reproducibly can this be carried out ?

## Characterisation of Optical path loss

**Want to drive to 0.01% or better:**
**Losses between crystal and detector**

**Current techniques**
- Classical (grating spectrometer based) spectrometry (0.03%)
- Laser-based techniques (0.01%)
**Developing improved techniques**
- Laser-based Gonioreflectometer (aim 0.01%)
- Mode locked laser technique (0.03%

## Laser based techniques: Coherence effects

- Coherence of laser radiation leads to errors in spectrophotometry of the order of several percent,
- e.g. in specular transmittance due to interference in reflected beams:

- Solution:
- Wedged sample
- Or
- Mode locked laser

## Mode locked laser techniques

- Mode-locked laser has pulses of width approx. 0.3 mm.
- No interference for sample thicker than 0.15 mm ??
- transmittance of glass plate at 800 nm with cw and with modelocked laser

## Limited by quality of crystal

- Map scan of 3mm thick BBO crystal (with AR coatings – 632nm)

## Measurement of coincidence counts

- Pre-existing photons hit DUT before twin
- DUT does not see twin but sees a subsequent ‘twin’ or background photon
- Electronic features ?
- Background -
- 	half wave plate
- Detection of false twins - 	careful choice of 	coincidence window

## Alignment issues

**Improvement to experimental facility:**
- Computer-control of detector positioning to optimise and maintain alignment
- Map spatial uniformity of detectors
- Intensity stabilisation of pump power during alignment process
- Try to improve/quantify effects of beam pointing instability
- Minimise effects due to temperature variation

## Current and future plans at NPL

- Optical losses – high accuracy measurements of transmittance losses
- Alignment and reproducibility issues addressed via the use of automated stages and control software
- Validate technique via
- In-house comparisons (cryogenic radiometer)
- Inter-comparisons with other NMIs (e.g. NIST, IEN)
- Develop calibration facility in photon counting regime
- Meet future requirements of technology for detector calibration
- Primary scales

## Acknowledgements

- Chris Chunnilall (NPL)
- Nigel Fox (NPL)
- NIST
- IEN
- UK National Measurement Directorate
- Principal contact: jessica.cheung@npl.co.uk