New Method for Spectral Irradiance and Radiance

Responsivity Calibration using Pulsed Tunable Lasers

The 11th International Conference on New

Developments and Applications in Optical Radiometry

September 19-23, 2011, Maui, Hawaii, USA

National Institute of Standards and Technology

Gaithersburg, Maryland

USA

Yuqin Zong

Steven Brown, George Eppeldauer, Keith Lykke, and Yoshi Ohno

Continuous wave (CW) tunable lasers

Shortcomings:

bulky

expensive

interference fringes

hard to operate and maintain

high power,

narrow bandwidth,

Being used for calibrations of primary detectors and

remote sensing instruments

Pulsed tunable lasers

Narrow pulse width, extremely low duty cycle (eg, 10-6)

Pulse to pulse variation, and hard to be stabilized.

Transimpedence amplifiers don’t work well.

Fully automated

Large tunable range

Finite bandwidth, no or less interference fringes

portable,

affordable.

Have not been used as calibration source yet!

Key questions to be answered

Will detectors be saturated?

Is a pulse laser equivalent a CW laser for

detector calibrations?

How to overcome fluctuation of a pulsed laser and

get repeatable results?

Can pulse lasers be used for calibration of

detectors with small uncertainties?

1000 Hz OPO

laser

Standard

Trap Detector

Test

Detector

Monitor

Detector

Ultrasound

bath

Computer

Schematic of the new measurement method

shutter

integrating

sphere

baffle

Electrometers (2)

Shutter controller

M

standard

M

teststandard /)()( QQRRtest

Optical

fiber

The OPO laser

210 nm to 2400 nm

tunable range,

1000 Hz repetitive

rate

5 ns pulse width

5 – 8 cm-1 bandwidth

Pulse waveform

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

-20 -10 0 10 20

Re

l. la

ser

po

we

r

Time, t (ns)

After 50 mmsphere

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

-20 -10 0 10 20

Re

l. la

ser

po

we

r

Time, t (ns)

After 5 mMM fiber

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

-20 -10 0 10 20

Re

l. la

ser

po

we

r

Time, t (ns)

OPO laser

Laser spectrum

0

0.2

0.4

0.6

0.8

1

1.2

348 349 350 351 352

Re

lati

ve S

PD

Wavelength, λ (nm)

350 nm

0

0.2

0.4

0.6

0.8

1

1.2

598 599 600 601 602

Re

lati

ve S

PD

Wavelength, λ (nm)

600 nm

0

0.2

0.4

0.6

0.8

1

1.2

1098 1099 1100 1101 1102

Re

lati

ve S

PD

Wavelength, λ (nm)

1100 nm

The electrometer

Charge measurement

function from 2 nC to 2 μC

High performance multichannel

switching card

< 3 fA bias current

< 20 μV burden voltage

No accurate timing and switching

Detector

+ -

DMM

C

V

Pulse train

i(t)

VCdttiQT

0 )(

With a switched integrator

transimpedance amplifier

Measurement timing

start

Electrometer’s synchronized

charge measurement

end

Laser shutter

open close

Measurement repeatability

Two S2281 Si photodiodes (PD)

standard deviation = 7 ppm!

0.999950

0.999970

0.999990

1.000010

1.000030

1.000050

0 5 10 15 20 25 30

Re

lati

ve c

har

ge r

atio

Measurement No., i

0.999950

0.999970

0.999990

1.000010

1.000030

1.000050

0 5 10 15 20 25 30

Re

lati

ve c

har

ge r

atio

Measurement No., i

One 3 Si PD trap and one

S2281 Si PD

standard deviation = 12 ppm!

1000 Hz OPO

laser

Test Detector

(S2281 PD)

Test

Detector

Ref. detect

(S2281 PD)

Optical

fiber

Computer

Linearity measurement

shutter

integrating

sphere

baffle

Electrometers (2)

Shutter controller

Ref.test /)( QQPr i

OD 2

attenuator

Result of detector linearity test

The relative responsivity is obtained by normalizing the charge

ratio r(Pi) of the test detector to reference detector .

0.9988

0.9990

0.9992

0.9994

0.9996

0.9998

1.0000

1.0002

1.00E-09 1.00E-08 1.00E-07 1.00E-06 1.00E-05

Re

lati

ve r

esp

on

sivi

ty

Averaged photocurrent, I (A)

OPO laser at 450 nm.

For a SS2281 PD,

Saturation starts at

Peak=100 mA,

Averaged=10-6 µA.

Points with regard to the linearity

1) Nonlinearity depends on the detector and the

laser wavelength.

2) The instantaneous photocurrent without causing

nonlinearity is several orders of magnitude

higher than the threshold nonlinear DC

photocurrent (0.1 – 1 mA typically).

3) The level of allowed averaged photocurrent is

several orders of magnitude lower than the

threshold nonlinear DC photocurrent.

Validation results using CW lasers

Difference in measured responsivity is only ≈ 0.02 %, well

within the instruments’ uncertainty (0.05 %).

0.000

0.010

0.020

0.030

0.040

0.050

400 450 500 550 600 650

R(λ

) D

iffe

ren

ce (

%)

Wavelength, λ (nm)

OPO/CW

gated CW/ CW

Uncertainties

Relative standard unc. (%)

Uncertainty component Type A Type B

Reference trap detector 0.028

Laser wavelength (0.01 nm) 0.005

Sphere source irradiance

uniformity 0.005

Detector reference plane 0.010

Detector linearity 0.005

Transfer to test detector 0.005

Electrometer (relative only) 0.01

Combined uncertainty (%) 0.033

Expanded uncertainty (k=2)

(%) 0.066

Conclusions

A new method using pulsed laser sources has been

developed for calibration of detectors and instruments.

The method has been validated and found to be

equivalent to CW laser method.

The averaged photocurrent should be kept several

orders of magnitude lower than the threshold

nonlinear DC photocurrent to avoid nonlinearity.

Pulsed laser sources have advantage over CW

lasers in reducing interference fringes.

Conclusions

This method can be used in other applications such

as measurement of material property of

transmittance and reflectance.

Compared to a monochromotor-based system,

calibration uncertainties are significantly lower

(eg, one order of magnitude).

THANK YOU