NAT'L INST. OF STAND & TECH

AlllOb M0M317

surement Services:

MIST

eEFERENCE pubucations

Calibration Service for spectral nist

Responsivity of Laser and Optical-Fiber pubnc^^^^^

Power Meters at Wavelengths 250-53

Between 0.4 fjim and 1 .8 ixm

John H. Lehman

QQ U.S. Department of Commerce

Technology Administration

National Institute of Standards and Technology

.U57

0.250-53

I 999 '
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NIST Special Publication 250-53

' NIST MEASUREMENT SERVICES:

b Calibration Service for Spectral

Responsivity of Laser and Optical-Fiber

Power Meters at Wavelengths

Between 0.4 (xm and 1.8 imm

John H. Lehman

Optoelectronics Division

Electronics and Electrical Engineering Laboratory

National Institute of Standards and Technology

325 Broadway

Boulder, CO 80303-3328

December 1999

U.S. Department of Commerce

William M. Daley, Secretary

Technology Administration

Dr. Cheryl L. Shavers, Under Secretary of Commerce for Technology

National Institute of Standards and Technology

Raymond G. Kammer, Director
National Institute of Standards and Technology Special Publication 250-53

Natl. Inst. Stand. Technol. Spec. Publ. 250-53, 41 pages (Dec. 1999)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1999

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
Contents Page

1. Introduction 1

2. 42164C Calibration Service Summary 2

3. Design Philosophy and Theory 4

4. Measurement System 8

4.1 Hardware 9

4.1.1 Light Source 9

4.1.2 Monochromator 9

4.1.2.1 Gratings 9

4.1.2.2 Glass Filters (Bandpass Filters) 10

4.1.3 Optical System 10

4.1.4 Enclosure 11

4.1.5 Data-Acquisition System 12

4.2 Software 12

4.3 Reference Detector (Transfer Standard) 12

4.3.1 Pyroelectric Wedge Trap Detector (PWTD) Design Attributes 12

4.3.2 Performance 13

4.3.3 Calibration and TraceabiUty 14

5. Operating Procedures 14

5.1 Wavelength Verification 14

5.2 Startup 14

5.3 Monochromator Alignment 15

5.4 Detector Alignment and Measurement Protocol 15

5.5 Data Acquisition 15

6. Uncertainty Assessment 17

6.1 Reference Detector 18

6.1.1 Calibration with Primary Standard (Type B) 18

6.1.2 Temperature Stability (Type B) 19

6.1.3 Spatial Uniformity (Type A, n = 16) 19

6.1.4 Linearity (Type B) 19

6.1.5 Noise (Type A, n = 40) 20

6.1.6 Repeatability (Type A, n = 3) 20

6.2 Source Uncertainties 20

6.2.1 Monochromator Errors 21

6.2.1.1 Wavelength Offset (Type A, n = 3; Type B) 21

6.2.1 .2 Stray Light and Order Overlap (Type B) 21

6.2.1.3 Bandwidth Effect (Type B) 23

6.2.2 Two-Position Mirror Reflectance (Type B) 23

6.2.3 Polarization (Type B) 24

6.2.4 Beam Size (Type B) 24

6.3. Optical-Fiber Connector Correction (Type A, n = 3) 25

7. Quality Control (Check Standard Operation) 26

8. Conclusion 28

9. References 28

Appendix A. Parts List 30

A.l Mechanical Fixmres 30

Appendix B. Example Calibration Report 33

iii

Calibration Service for Spectral Responsivity of Laser and Optical-Fiber Power

Meters at Wavelengths between 0.4 //m and 1 .8 /jm

John H. Lehman

National Institute of Standards and Technology

Optoelectronics Division

325 Broadway

Boulder, Colorado 80303

This document describes a calibration service for absolute spectral-responsivity

measxarements of laser power meters, optical-fiber power meters, and detectors used with

lasers and optical-fiber connectors at wavelengths between 400 nm and 1 800 nm. In addition

to a summary of the calibration procedure, a theoretical basis is given for the uncertaint}'

assessment, as well as an overview of the measurement system and operating procedures. A

sample calibration report is included in this document that is similar to that which is provided to the customer. The calibration report contains absolute responsivity in terms of

amps per watt at each wavelength increment and a summary of the uncertainty assessment

for the meter.

Keywords:

absolute spectral responsivity; laser power; optical-fiber power; optical detector calibration;

optical fiber connector; photodetectors; pyroelectric detectors

1. Introduction

This calibration service provides absolute spectral-responsivity measurements of

laser power meters, optical-fiber power meters, and detectors used with lasers and opticalfiber connectors at wavelengths between and beyond those available from other NIST laser calibration services. The measurements are made at 10 nm increments over wavelengths

from 400 nm to 1 800 nm using a monochromator-based system, and are normaH2ed to higher accuracy measurements made with laser sources in this range (see NIST calibration

services 42130C - 42131C and 42110C - 421 IIC) [1,2]. In each calibration, particular care is taken to evaluate the effects of source coherence, interference, and reflections on the qualit}^

of the calibration; these factors generally dominate estimates of uncertainty.

The appropriate calibration procedure and the calibration uncertainty must be

interpreted case-by-case, depending on the intended use and nature of the test meter [3,4]. This determination begins prior to the measurement, with a dialogue with the customer to understand the meter's intended use and physical configuration. For each meter (or detector

module) sent to NIST for calibration, the customer is provided a written calibration report.

The calibration report contains absolute responsivity in terms of amps per watt at each

wavelength increment, a brief description of the measurement system and procedure, and a summary of the uncertainty assessment. Information unique to the individual calibration will

1
also be included.

For silicon photodiode-based meters, the wavelength range for the service is normally 450 nm to 1 000 nm, and for meters that incorporate photodiodes made from

germanium or indium gaUivim arsenide, the range is 800 nm to 1750 nm. If necessary, this coverage may be shortened or slighdy extended. We have an ongoing effort to include into

the measurement procedure comparisons with a greater number of laser sources that are not

listed as part of 42130C and 42131C (currendy 670 nm, 780 nm, 850 nm, 980 nm, 1310 nm,

and 1550 nm). Such comparisons provide information that may reduce the measurement

uncertainty. The customer is invited to inquire about the use of other laser wavelengths for

which caHbration may be provided as a special test.

2. 42164C Calibtation Service Summary

When we calibrate an optical-fiber power meter for absolute spectral responsivity,

we acquire one data set from a monochromator-based measurement system and two

additional independent data sets from a laser-based measurement system, with and without

fiber connectors installed [5]. Our first data set is a measure of the meter's absolute spectral

responsivity over the entire wavelength range of the meter. These data are usually depicted

as a smooth curve consisting of absolute responsivity values at 10 nm increments. The

second set represents the meter's absolute responsivity at several discrete laser wavelengths,

for example 850 nm, 1310 nm, and 1550 nm. The third set is similar to the second, except

that the laser light is coupled to the meter and the transfer-standard detector using a fiber

optic cable and connector. Differences in the three data sets reveal possible offsets that may

result from the use of a particular fiber connector.

The detector input conditions for this procedure include at least two of the following

scenarios as shown in Fig. 1 . In scenario A of Fig. 1 , coUimated, coherent Hght is directed at the detector surface. In scenario B, Hght from either a coherent or incoherent source is focused at or near the detector surface. In scenario C, light is transmitted through a fiber

attached to the detector with a fiber connector. Typically, Hght from a single-mode fiber has

a divergence angle that is less than 14° (±7° from normal incidence). In most cases, the fiber

connector functions as an enclosure. Light from the fiber is reflected between the detector

and the fiber connector, and the reflectance of the fiber connector varies as a function of

wavelength. In the context of our caHbration service, an ideal laser power meter would have

the same absolute spectral responsivity for each of these three scenarios [6]. (Note: Thermal

detectors with a black absorbing surface, such as our pyroelectric-based reference detector,

often meet these conditions, but generaUy require the use of a chopper and are therefore not

suitable for fiber systems [7].)

2
detector case and window

\ detector surface \ I

. aperture

cap

incoming rays incoming and

reflected rays

Figure 1. Optical input conditions: a. collimated; b. converging; c. diverging from

fiber connector.

The correct results and the preferred calibration procedure depend on the intended

use of the customer's meter. Figure 2 shows an example of an absolute responsivity curve obtained using the monochromator-based measurement system. Also shown as soHd circles are the connectorized, laser-based measurement system results for the same detector. If the

laser or optical-fiber power meter is intended to be used with fiber connectors attached, we

shift the entire curve obtained using the monochromator-based system (without changing its shape), so that it agrees as closely as possible with the results obtained using the laser-based

measurement system. The amount of shift and the resulting uncertainty are calculated based on the difference between the laser-based measurement results and the monochromatorbased measurement results. In this example, the customer would be provided with absoluteresponsivity values corresponding to 10 nm wavelength increments, from 750 nm to 1800

nm (the data from which the curve is drawn). One uncertainty value would be given for the

entire curve.

3
700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800

Wavelength (nm)

Figure 2. Absolute spectral responsivity results. The line is obtained from the

monochromator-based measurement system (each datum spaced at 10 nm

increments). The solid circles are results from the fiber-connector, laser-based

measurement system.

3. Design Philosophy and Theory

A conventional technique for developing the measurement equation for an optical

radiation measurement is given in detail by Kostkowski and Nicodemus [8] . In this section

we briefly describe the measurement configuration (details are given in section 4) and

present a measurement equation along with assumptions that allow us to define a closed

solution. Using this foundation, we describe the measurement uncertainties in section 6, in a way that will hopefully be beneficial to the calibration customer.

The definition of spectral responsivity for an optical detector is based on the

observation that the electrical output (photocurrent) of a detector is proportional to the

optical input (flux, radiant flux, or radiance). The mathematical relationship for the electrical

output X(X), of an optical detector is the following:

X(X) = 0(X)-S{X), (1)

where 0(X) is the flux and S(^) is the detector responsivity. This measurement equation assumes that the detector responsivity S{X) is spatially uniform, and that the detector is

4
X

Figure 3. Ideal source.

Figure 4. Broadband lamp output.

uniformly irradiated by an ideal optical source. The ideal source has flux 0(X), is unpolarized, and has infinitely narrow bandwidth at wavelength A, (this is depicted in Fig. 3). The monochromator-based system does not satisfy the criterion for Fig. 3, so we

must modify the measurement equation so that it is appropriate for our monochromatorbased system. Using functions based on properties of the light source, monochromator, and

other optical elements, we progress toward a much more complicated expression for

approximately the same spectrum shape introduced in Fig. 3. We begin with a broadband

source having a spectrum that is approximately the shape of an ideal blackbody, shown in

Fig- 4. The monochromator throughput, shown in Fig. 5, varies as a function of wavelength

depending on the grating efficiency. We include an efficiency term, EO(A,) in the

measurement equation to account for this variability. The details are not shown in the figure,

but the radiance and polarization of Ught from the monochromator vary as a function of

wavelength. We later account for these affects by including the detector's linearity and

polarization sensitivity in our uncertainty assessment.

5
X

Figure 5. Lamp output, monochromatof efficiency, 0(A,) EO(A,).

<E)(X)E(X)

z(X-X')

/

^

/

Figure 6. Lamp output, monochromator efficiency, and slit scattering; 0(k)B.^(k),

Rotation of the grating relative to the monochromator slits determines the wavelength

that will pass a defined band, AX, through the exit slit. In addition, two other phenomena may

be seen at the exit sHt of the monochromator: (1) Hght that is predictable and unwanted, for

example, multiple orders of a given wavelength (coherent scattering); and (2) background Hght

such as incoherent scattering from the grating and other optical elements. We partially account

for this added complexity with a slit scattering function, z(k — X'). The superposition of the slit scattering on the lamp and grating efficiency is shown in Fig. 6. (note: The term slit scattering

is often used for this function, but it is not really descriptive and a bit confusing. The term refers to scattering from the grating, both coherent and incoherent, that appears at the exit slit of the

monochromator (not scattering by the slits)). Most of the coherent scattered light, depicted in Fig. 6, is eliminated using a longpass

filter as shown in Fig. 7. The filter transmittance function, along with the transmittance

function of other optical elements in the system such as mirrors outside the

monochromator, appears in the measurement equation as T(^).

6
0(X)ElX)T(X)

z(X-X')

Figure 7. Lamp output, monochromator efficiency, filter and mirror transmittance,

and sUt scattering; (l>(X)E%X)T(K), z(k-X' ). We use the terms introduced above and include another term Xsi in the

measurement equation which accounts for all scattered light, so that the measurement

equation is now:

X{X) = 0(A) • S(A) T(Z) E{X) + X^, {X) . (2)

Xxi represents an average of the out-of-band light in the path of the lamp output (the

preferred source light). Accordingly, we expand Xsi in the equation above to:

X^iiA)= f ^(A,')-S(A')-T(A')-E{A')-dA\ (3)

Note that the notation X' is switched with X to be consistent with eq (1). Also, beginning

with eq 2, O, E^, and T are constant over AX.

Now the measurement equation is:

X{X) = (^(X)-S{X)-T(X)-EiA)+ f ^(A,')-S(A')-T(X')-E(X')-dX', (4) JXoAA

where

E(A) = E\A) \ z(X-A')-dX\and E(A,') = E\X') { ziX-A")-dA'. (5)

JAX JAoAA

In real monochromators, there is some residual spectral component to z(X — X') outside the

bandpass due to scattered light, zero order leakage, and grating ghosts, but this component is difficult to fully evaluate and correct. (We estimate the uncertainty for the monochromator

efficiency and scattered light in section 6.2.1.2.) For more information, the efficiency and slit scattering functions are discussed by Kostkowski and Nicodemus [8]. Because our measurement configuration uses a single monochromator witii longpass filters and has

relatively high scattered Ught compared to double monochromator systems, we carry this residual through the final measurement equation.

To proceed, we revert to a form similar to our original equation, but, with an

7
understanding of what is included, we write a simplified measurement equation that is not

only suitable for monochromator-based spectral responsivity but also for the substitution (or

comparison) metJiod. In the following discussion, subscript t designates the test detector,

subscript s designates the standard reference detector, and the additional si is carried

throughout to designate the scattered-light residual as the integral in eq (5). In addition, the

A,-function notation is dropped for simplicity.

For detector electrical output X, responsivity S, and optical input M (at the detector),

we can write

X,=M,.5,+Jr,, and M,=^^^-i^. (6)

In our measurement configuration, the reference detector is the basis for M and because of

the short measurement duration at each wavelength of interest and the stabihty of the lamp

source, we assume (with some uncertainty given in section 6.1.2) that Mt = Ms. Therefore

In practice, offsets in the detectors and instrumentation reqviire the measurement of signals

without optical inputs to determine baseline levels, in effect, non-optical background. This is included where necessary and may be stated as

S, = ^

' \/ -S^. (8)

It is also common to include the gain G, of any electronic amplifiers used with the test and

reference detectors. For our measurement configuration, this leaves a final measurement

equation of

iX,-X„-Xf-')

(X,-X,,-Xf-') G,

S,=^ Vt^—^-5,. (9)

4. Measurement System

The spectral responsivity of the customer's meter is measured using the system

shown schematically in Fig. 8. The essential components include a tungsten filament lamp

source and a single-grating monochromator (Czerny-Turner configuration) with ordersorting colored-glass filters. The light path is extended using mirrors on both the input and

output of the monochromator, to accommodate a variety of shapes and sizes of meters. A

translation stage on the input side of the monochromator allows a chopper to be moved in

and out of the beam path, to accommodate the reference detector. A flat mirror on a rotary

stage on the output side of the monochromator allows light to alternate between the

reference detector and the test detector.

Normally, the beam is focused (F number = f/4) to a radius of approximately 2 mm

8
lamp

NIST reference

detector

Figure 8. Grating monochromator-based spectral responsivity measurement system.

at the position of, and normal to, the plane of the test meter. The bandpass of the

monochromator is less than 6 nm. A pyroelectric detector is used as the reference detector for all measurements. The

reference detector is actively thermally stabilized and is electrically connected to dedicated

current amplifier, lock-in amplifier, and optical chopper.

Optics and instrumentation used with the monochromator output are enclosed in a

light-tight box. The entire measurement system rests on a commercial, vibration-damping

tabletop. All instruments, including the customer's meter and the monochromator, are controlled using GPIB/IEEE hardware connections.

4.1 Hardware

4.1.1 Light Source

The broad-spectrum Ught source is a tungsten- filament, quartz-envelope light bulb.

The dimensions of the filament are specified by the manufacturer as 4.1 mm x 9.5 mm. The

bulb has a rated filament temperature of 3400 K, at 6.25 A and 24 V. The light source is controlled using a dedicated electrical power supply operating in a constant-current mode.

The drift of the lamp power output has been measured to vary less than 0.1 % over a 30

minute period.

4.1.2 Monochromator

4.1.2.1 Gratings

The monochromator grating is mounted in a Czemy-Tumer configuration. One of

9
two gratings is available, depending on the wavelength range of the desired calibration. The

ruling density of each grating is 600 lines/mm. One grating is blazed at 500 nm and the

other at 1000 nm. The useful range of the monochromator using both gratings is 400 nm to

1 800 nm. The grating is moved using a stepper motor coupled to an absolute encoder. The

stepper motor is coupled to a lead screw that drives a sine-bar mechanism. The grating is attached to a kinematic mount, and, because each grating has the same line density, the

grating position algorithm is calibrated using a single offset value. The stepper motor is capable of up to 50,000 steps per revolution. The encoder will resolve 16384 steps per

revolution. With the use of a grating ruled with 600 grooves per millimeter, the mechanical

relationship of the grating placement and the wavelength selection is 10 nm per revolution.

Therefore, the mechanism is capable of 1638.4 steps per nm. This implies a positioning

resolution of 0.0006 nm. However, the grating resolution is the practical limit. According to the grating equation, the resolution is theoretically 0.06 nm [9,10]. Realistically, the

measurement resolution is decreased by the need for increased throughput. To increase the

throughput, the monochromator input and output aperture sUts are widened to about 2 mm

and limit the resolution to about 6 nm for measiirement test conditions.

4.1.2.2 Glass Filters (Bandpass Filters)

Colored glass filters are used to reduce transinission of the unwanted orders reflected

by the gratings and to reject stray light. The gratings are chosen to maximize throughput

and also to minimize the magnitude of overlapping orders over the useful wavelength range.

Four filters are used [1 1]. The wavelength range of each filter is as follows:

450 nm - 670 nm: OptoSigma 079-3340

670 nm - 1075 nm: Schott LL 0650

1075 nm - 1575 nm: Schott LL 1000

1575 nm - 1800 nm: Schott RL 1500

The filters are mounted on a motorized disk that rotates each filter into the path of the input

Hght, for each wavelength range.

4.1.3 Optical System

Several mirrors are used outside of the monochromator to collect light from the

lamp and focus light onto each detector. The mirror surfaces are silver with a dielectric

overcoat and are located at nearly symmetrical positions at the monochromator input and

output. Two spherical mirrors with an f-number of f/2 are used to collect Hght and extend

the working distance from the source and detectors using a 1:1 image distance. The mirror

configuration is shown in Fig. 9.

10
side view

flat mirror

Figure 9. Geometrical ray tracing of input optics outside the monochromator in three

orthogonal views (output optics are identical).

When light is reflected from the first spherical mirror and is directed off-axis,

aberration is introduced. To achieve the theoretical resolution of the grating, this aberration

must be minimi2ed. The second spherical mirror directs the light in an orthogonal plane to

partially correct the aberration.

There are two more mirrors inside the monochromator. These mirrors are f/4.7 and

are approximately 150 mm x 150 mm square. One mirror collimates light from the input slit and directs it to the grating, while the other focuses Hght on the exit sHt of the

monochromator. These two mirrors are front-surface aluminum mirrors with a magnesuim

fluoride overcoat.

4.1.4 Enclosure

The monochromator grating, monochromator mirrors, bandpass filters, test and

reference detectors, and output optics and fixtures of the monochromator are enclosed in a

light-tight box. The only normally detectable Ught (more than a picowatt) transmitted to the

test and reference detector enters the measurement system through the sHt on the lamp side

of the monochromator. The lamp, data acquisition system, and chopper are the only

significant components that are not enclosed.

11
4.1.5 Data-Acquisition System

The electronic hardware is controlled using a personal computer based on an Intel

80486/ 120MHz processor and a GPIB card. Communications protocol is based on the

lEEE/GPIB interface standard.

4.2 Software

The software is written to control the electronic instrumentation and coUect data over the GPIB. The interface is written to resemble a virtual instrument with push buttons,

slider keys, and numerical and graphical displays. By use of the software one can set the

positioning stage locations, adjust the lamp current and voltage, control the detector

electronics, and set up data collection parameters. During a data-collection episode, the

wavelength scan range, time delays, file destinations, and number of data points to average can be input into the program.

4.3 Reference Detector (Transfer Standard)

4.3.1 Pyroelectric Wedge Trap Detector (PWTD) Design Attributes

The reference detector is a pyroelectric trap built and evaluated at NIST. The trap

design is based on a lithium tantalate (LiTaOs), pyroelectric disc coated with gold black,

positioned opposite a gold mirror in a wedge configuration. The low-reflectance gold black

coating, along with the wedge-trap structure, ensure that the pyroelectric disc absorbs 99 %

of the light entering the aperture [7]. A schematic figure of the reference detector design is shown in Fig. 10. The mviltiple-reflection wedge-shaped trap structure has a 5 mm diameter

input aperture and an f/4 field of view.

Figure 10. Cross-section view of the pyroelectric wedge-trap detector. Light rays

entering the trap cavity, within the f/4 field of view have at least four chances to be

absorbed by detector element (for specularly reflected rays).

gold mirror gold black coating

Nl electrode — LTa03 ——- Ni electrode

V

12
4.3.2 Performance

Pyroelectric detectors that are designed for radiometric applications typically

demonstrate spatial uniformity variations of ± 2 %, noise equivalent power (NEP) ranging

from 10"^ W/Hz^''^ (no window) to less than 10"^ W/Hz^/^ (small area, sealed container with

a window), and a wavelength sensitivity ranging from the visible to beyond 10 \im. Despite

years of development and commercial success, these numbers do not represent the

theoretical performance limits for pyroelectric detectors. Although we have not yet reached

the theoretical limits of performance, we have achieved a design compromise that optimizes

those detector properties that are important for high accuracy spectral responsivity and

absolute power measurements. The transfer standard for this measurement system has a responsivity that varies as a function of wavelength by less than 1% over a range from 0.4

fxm to 1.8 i^m and has a measured NEP of 5x10-8 W/Hzi/2.

Knowledge of the spatial uniformity of a detector is particularly important for

measurements that require varying beam-input conditions (such as diverging versus

collimated input beams or repeated detector alignment). We measured the spatial uniformity over the PWTD aperture area using a 0.3 mm diameter, circularly polarized, 674 nm laser beam as a probe. This probe beam was scanned across the aperture area at 0.1 mm intervals

while detector output data were collected. The data, plotted in Fig. 1 1, show that the

maximum variation in detector response is less than 1% over an area roughly 5 mm in

diameter.

Figure 11. Spatial uniformity of PWTD.

13
NIST Electrical Standards

Digital Voltmeter and Standard Resistor ^

C-series Calorimeter

633 nm, 670 nm, 850 nm, 1310 nm, 1550 nm

Calibration Reference

Pyroelectric Wedge-Trap Detector

Figure 12. Pyroelectric wedge-trap detector traceability.

4.3.3 Calibration and Traceability

In principle, the pyroelectric wedge trap (PWTD) responsivity is constant (uniform) over the entire spectrum of interest for this calibration service. We verify this assumption by

comparing the relative responsivity of the PWTD to several transfer-standard detectors that are traceable to NIST. In addition, the relative spectral responsivity of the reference detector

has been measured using a large-area pyroelectric detector, which incorporates a gold-plated

integrating hemisphere [12]. The primary traceability of the PWTD is absolute responsivity

measured using laser input sources at 633 nm, 670 nm, 850 nm, 1310 nm, and 1550 nm

compared to the NIST C-series calorimeter (see Fig. 12 below), which is traceable to NIST

electrical standards [2].

5 Operating Procedures

5.1 Wavelength Verification

The monochromator uses two different gratings. Though the kinematic mounts

bviilt into each grating are quite reliable, it is recommended that the calibration be checked

after each grating change. Several heHum neon laser Unes are used to check the wavelength

calibration at approximately 633 nm, 1165 nm, 1176 nm, and 1523 nm. The

monochromator is scanned over a region near these wavelengths, with a sHt width of 0.5

mm, to determine whether the monochromator shows an offset from expected peaks. If necessary, an offset is added to the control software that turns the grating stepper motor.

Two gratings cover the wavelength range of the calibration service, to minimi2e the

dependence of grating efficiency with increasing wavelength.

5.2 Startup

Before any measurement is performed, the system is allowed to warm up at full

14
power. In this case, full power means the lamp source is at 1 50 W, aU electronics are turned

on, the positioning stages are initialized, and the reference-detector thermoelectric cooler is on and is allowed to achieve the desired temperature setpoint. The control program is initialized and all instruments in the measurement system may be tested to insure the GPIB

is operating properly.

5.3 Monochfomator Alignment

After the system has warmed up for about an hour, slight adjustments may be made

to the first mirror, which collects Hght from the lamp source. Adjusting this mirror will

optimize the amount of power incident at the entrance sUt of the monochromator and,

ultimately, the amount of power incident on both detectors. For wavelength-calibration

purposes, the first mirror may also be rotated several degrees, to collect light from a laser or gas-lamp source.

5.4 Detector Alignment and Measurement Protocol

The calibration-measurement process begins by adjusting the monochromator so

that it transmits a visible filament image. This image is centered onto the reference detector.

The two-position mirror is then moved to the complementary position, and the test meter is aligned as necessary. The test detector must capture all the incident Hght to insure that an

absolute measurement is accurate. The detector must also be aligned so that the incident

Hght is perpendicular to the detector. Both detectors are enclosed in the Hght-tight box.

Next, the GPIB communication is estabHshed with the test meter, and appropriate settings are made, including any offsets. Using the scan parameters menu, starting wavelength, ending

wavelength, wavelength increment, test meterpausefor data, and the number of data points to average

is selected, ^egin wavelength scan may now be selected and the data wiU be acquired as foUows:

1 . Adjust monochromator grating and filters to starting wavelength.

2. Send chopper to home position for reference detector.

3. Send two-position mirror to home position for reference detector.

4. Pause and then acquire and store data from the reference detector (ns = 40).

5. Send chopper out of the Hght path.

6. Send two-position mirror to test position for test detector.

7. Pause and then acquire and store data from test detector (nt = 5).

8. Repeat steps 2 through 7, two more times.

9. Adjust monochromator by the wavelength increment

10. Repeat steps 2 through 8 until the ending wavelength.

5.5 Data Acquisition

The sequence and duration of the data acquisition represents a compromise that is an attempt to maximize data-averaging in a minimum amount of time. The random

uncertainty associated with the pyroelectric reference detector increases with decreasing

monochromator throughput. The random uncertainty will decrease as more data are acquired. However, to reduce the random uncertainty by an order of magmtude requires

several minutes of additional data at each wavelength increment (using a reference-detector

time constant of one second and acquiring forty samples per data point). Typically, acquiring

15
a complete set of spectral responsivity data for a semiconductor-based power meter will take

several hours.

For each wavelength interval, the data acquisition sequence is as follows. Light from

the monochromator is first directed at the reference detector. The acqviisition program

pauses approximately 40 s, until the reference detector achieves a nearly steady- state value.

Then, 40 data samples are taken, spaced 1 s apart, and the average value is recorded. Next,

light is directed at the test detector for a duration and sample time prescribed by the

manufacturer or customer; this data sample set is averaged and recorded. This process is repeated two more times for each wavelength. Ideally, the pause duration and sample-set

size for each detector and for each wavelength are based on achieving a normal distribution over some interval, centered over the sample-set average. Realistically, the distribution is an

approximation of the normal distribution, because practical time constraints limit the

number of data points that can be taken.

The format for the data acquisition is shown in table 1. Following this, in eq (10), is the formulation for calculating the responsivity of the test detector at a given wavelength.

The customer is provided a table with a calibration report (see Appendix B), indicating the

responsivity St(X,) for each X over the wavelength range.

Table 1. Data for calculation of responsivity for each wavelength increment.

Wavelength

(sampling repeated 3 times at each

wavelength)

NIST

reference data

NIST

reference

background*

Test data

Test

background*

S(l)

(X.1 = starting wavelength)

X(l)„, X(l)s2, b(l)s,, b(l)s2, X(l)t,, X(l)t2, b(l)u, b(l)t2, X(l)s,3,. . .X(l)s40 b(l),3,. . .b(l)s40 X(l)t3, . • .X(l)t5 b(l)t3, • • .b(l)t5

S(2)

(^1 + increment)

X(2)s,, X(2)s2, b(2)s,, b(2)s2, X(2)„, X(2),2, b(2)ti, b(2),2, X(2)s3,. . .X(2)s4o b(2)s3, . . .b(2)s24o X(2),3, .

. .X(2)t5 b(2)t3, • • .b(2)t5

S(3)

(kz + increment)

X(3)s,, X(3)s2, b(3)s,, b(3)s2, X(3),i, X(3)t2, b(3)t,, b(3)t2, X(3)s3,. . .X(3)s4o b(3)s3, .

. .b(3)s34o X(3)t3, .. .Xt5 b(3)t3, .. .b(3)t5

SO)

(^i-i + increment) . .

.

s(f)

(ending wavelength)

X(f)s,, X(f)s2, b(f)s,, b(f)s2, X(f)u, X(0t2, b(f)t,, b(f)^, b(f):3, X(f),3, ...X(f)s40 b(f)s3, ...b(f)s4l) X(f)t3, ...X(f)t5 ...b(f)t5

* combined b = Xsi + X"***^' as appropriate

16
50-), =1/3

( ^ 5 ^

/=i /=i

40 40

(=1 /=i 40 40 )

't^„ ILK

i=\ /=]

reposition

5 \ repeal

40 40 E-*'.. E*.

;=1 ;=1 40 40 ;

't^„ E*.

/=1 ;=1 +

reposition

5 \ repeal

40 40 E-^. E*»

(=1 V 40

;=140 )

(10)

400 600 800 1000 Wavelength (nm)

Figure 13. Spectral responsivity measurement results for a silicon photodiode-based

optical detector.

Three graphs are shown in Fig. 13, which represent the averaged data for a sample

measurement for the reference detector and for the test detector, as well as the calculated

absolute responsivity of the test detector.

6. Uncertainty Assessment

The uncertainty estimates for the NIST spectral responsivity measurements are assessed using guidelines from NIST Technical Note 1297, "Guidelines for Evaluating and

17
Expressing the Uncertainty of NIST Measurement Results" by Barry N. Taylor and Chris E.

Kuyatt, 1994 Edition [13]. To establish the uncertainty limits, the error sources are separated

into (1) Type B errors, for which magnitudes are determined by subjective judgement or

other nonstatistical method, and (2) Type A errors, for which magnitudes are obtained

statistically from a series of measurements.

All the Type B error components are assumed to be independent and to have

rectangular or uniform distributions (that is, each has an equal probability of being within

the region, ±6i, and zero probability of being outside that region. If the distribution is rectangular, the standard deviation, as, for each Type B error component is equal to 5i/3'/' and the total "standard deviation" is (Sas^) where the summation is performed over all Type B error components.

The Type A errors are assumed to be independent, and the standard deviation, Sr, for each component is

Sr

V 2 (2x)^

Lx -N -1

(II)

where the x values represent the individual measurements and Nr is the number of x values

used for a particular Type A error component. The standard deviation of the mean is Sr/N/-, and the total standard deviation of the mean is [E(Sr^/Nr)] where the summation

is carried out for all the Type A error components.

The expanded uncertainty is determined by combining the Type A and Type B

"standard deviations" in quadrature and multiplying this result by an expansion factor of 2. The expanded uncertainty, U, is then

U = ijnal + llf- . (12)

When the monochromator-based measurement results are shifted to match the laser- based measurement results, there is an associated uncertainty added to the overall estimate.

The value given with the calibration results, as a connector offset, is based on the residuals

remaining after the monochromator-based data is shifted. The residuals are added in

quadrature and the result is listed as a Type A uncertainty.

As illustrated in Table 3, the measurement uncertainty components are divided into

two main categories and sometimes a third category is included depending on the customer

request for calibration. The first category is devoted to the pyroelectric transfer standard

calibration. The other category is devoted to the monochromator, light source, and other

measurement equipment. In addition, there is uncertainty that is a consequence of properties

inherent to the customer-provided meter.

6.1 Reference Detector

6.1.1 Calibration with Primary Standard (Type B)

Using measurement results obtained from calibrations with the NIST C-series

calorimeter, the uncertainty of the absolute responsivity is 0.47 %. This measurement is available as a NIST Calibration Service and the uncertainty is well documented [2].

18
6.1.2 Temperature Stability (Type B)

Pyroelectric detectors have a strong temperature dependence (for lithium tantalate

this is nearly 0.25 %/K) and therefore the reference detector, without active thermal

stabilization, is the component in the measurement configuration most vulnerable to thermal

drift. Therefore, the temperature of the reference detector is stabilized using a thermoelectric

cooler in thermal contact with the insulated detector. The duration of the measurement for

each wavelength increment is less than ten minutes. To estimate the uncertainty due to thermal drift, we have measured the temperature dependence of the reference detector for

thirty minutes and calculated the responsivity drift to be 0.1 %. For a ten minute responsivity

measurement at a given wavelength increment, we assign an uncertainty of 0.03 %.

6.1.3 Spatial Uniformity (Type A, n = 16)

The beam position for the duration of a calibration is highly repeatable. However,

the PWTD was calibrated against the primary standard, using coUimated laser sources with

varying beam sizes, and was aligned manually several times for each wavelength. Therefore,

we must account for variation of responsivity as a function of beam position when using the

PWTD as a reference detector. This uncertainty contribution is an important consideration,

given that one of the first assumptions made in deriving the measurement equation was that

both the test and reference detectors have uniform spatial responsivity.

We measured the spatial uniformity (as stated in section 4.3) over the PWTD

aperture area by use of a 0.3 mm diameter, circularly polarized, 674 nm laser beam as a probe. This probe beam was scanned across the aperture area at 0.1 mm intervals, while

detector output data were collected. A total of 6400 data points were collected over an

8 mm X 8 mm area. The measurement results indicate that the maximum variation in

detector response over an area approximately 5 mm in diameter, is less than 1 %. A subset

of these data, 46 data points spaced 0.1 mm apart and 4.5 mm across the center of the

detector, was analyzed and the standard deviation was 0.3 %; this value is used in our

uncertainty estimate.

6.1.4 Linearity (Type B)

The radiant power throughput of the measurement system varies with wavelength. It

is possible, by means of external feedback control, to adjust the throughput to a constant

radiance. However, because of the concern for thermal stability and the need to minimize

the measurement duration, we choose not to actively maintain constant radiance. Therefore,

we must know the reference-detector linearity.

The radiant power output from the monochromator varies by nearly a factor of four

as a function of wavelength, over the wavelength range, for both the 600 nm and 1000 nm

blaze gratings. The typical worst-case range is approximately 25 |aW to 100 |j,W. The linearity was evaluated at NIST, using a commercially available optical power meter [14]. The

estimated value of this contribution was approximately 0.15 %.

19
6.1.5 Noise (Type A, n = 40)

During normal calibration at each wavelength, 40 samples are acqiiiced from the

reference detector and averaged. This is based on the standard deviation of the data XQsi,

X0s2, X0s3, • •X0S4O described in Data Acquisition, section 5.5. Environmental factors, such

as ambient acoustic noise (the reference detector, in addition to being pyroelectric is also

piezoelectric), current amplifier noise, chopper instability, and other factors, contribute to white noise that appears as randomly varying output signal from the reference. Other noise,

particxilarly 1/f noise, or drift is accounted for in the Temperature Stability component 6.1.2.

6.1.6 Repeatability (Type A, n = 3)

As stated in section 5.5, several samples from the test and standard reference

detector are averaged at each wavelength increment. The result is three ratios, each having

the form

(13)

A=7

/=1 7=1

40 40

/=1 (=1

This is repeated three times, once for each time the two-position mirror directs light to the

test detector. These three ratios will vary from calibration episode to episode, but a typical

worst-case value, based on the standard deviation of three 40-sample sets, over the 400 nm

to 1800 nm wavelength range, is approximately 0.1 %.

6.2 Source Uncertainties

The source, the description of which was given in section 3, is optical radiation incident on

the detector. Qualities of the source are affected by components such as the lamp, gratings,

and mirror. The following facts should be considered in regard to source uncertainties.

• The wavelength is known, and the bandwidth is constant over the wavelength range of

the measurement, with some tolerance, based on the physical construction of the

monochromator.

• White noise and drift exist due to temperature change, current input variation, humidity

change, and other factors. • Electronic noise and acoustic noise affect the output of the reference detector,

independent of wavelength.

• The lamp-filament image size is known and underfills the reference and test detectors. • Gratings and mirrors polarize the lamp output.

• The test- and reference-detector electrical signal varies nearly Hnearly with optical

input. J

20
6.2.1 Monochromatof Errors

6.2.1.1 Wavelength Offset (Type A, n = 3; Type B)

Worst-case uncertainty, based on wavelength calibration, can be measured by

selecting a wavelength region in which detector responsivity is changing rapidly. For

example, in the range from 1550 nm to 1560 nm, the absolute responsivity of a Ge

photodiode varies by nearly 1 %/nm. A conservative estimate of the wavelength offset and

mechanism backlash of the monochromator grating position is approximately 0.1 nm (using

a HeNe laser at 1523 nm). Multiplying the detector responsivity rate of change by the

wavelength offset yields a wavelength uncertainty of approximately 0.1 %.

6.2.1.2 Stray Light and Order Overlap (Type B)

The monochromator filters allow a certain amount of undesirable stray Hght and

other-order transmission. When using a reference detector that is sensitive to a broad range

of wavelengths, such as a pyroelectric detector, to measvire the spectral responsivity of

semiconductor-based photodetectors (and other thermal detectors), there is an uncertainty

contribution that varies with wavelength and filter selection.

The amount of Hght transmitted by a glass filter is only proportional to the uncertainty

contribution. For example, the short-wavelength cutoff filter may transmit as much as 0.1%

stray Hght at 400 nm. But, the efficiency of other elements of the measurement system (for

example, aluminum-mirror reflectance) may reduce this contribution further. Based on

estimates of the grating efficiency and transmitivity, the contribution of uncertainty is approximately 0.5 % [9,10]. The data shown in Fig. 14 indicate this is a conservative estimate

for most of the wavelength range for the measurement service.

The uncertainty at the shorter wavelength range of the monochromator is of special

interest because of properties inherent to the Czemy-Tumer monochromator configuration.

When using the Czemy-Tumer configuration, there is a measurable "out of band," re- entrant spectrum caused by reflection from the sHt jaws and filter. The result of the

reflection is unwanted stray Hght that is, over a broad spectrum, ranging above the filter cutoff

(that is, longer wavelengths). This effect manifests itself as a low-power-level lamp spectrum

propagated along the path of the desiced monochromator output. The re-entrant spectra are

significantiy reduced to the level of the baseHne-scattered Hght, by placement of a baffle at the focal point of the re-entrant spectra, within the monochromator. Two spectra are shown

in Fig. 15; one curve (the lower curve) shows the residual background from 450 nm to 1000 nm, the other shows the output of a 674 nm wavelength laser diode. These two curves

indicate that there is a smaU but measurable amount of background Hght, but there is no

re-entrant spectrum at shorter wavelengths.

The filters are not caHbrated in the strictest sense, rather they are evaluated to ensure

that aging has not changed the cut-off wavelength or that the transmission below (i.e., shorter

wavelengths) the cut-off has not increased. This evaluation is done by scanning a wavelength region of interest, using the reference detector, both with the filter in place and

with the filter not in place. The ratio of these two scans wiU indicate the filter transmittance

versus wavelength. At shorter wavelengths near 450 nm, this measurement becomes

increasingly difficult. One reason for the difficulty is that the monochromator throughput is low and the signal for the condition of no-filter-in-place, as well as the condition of filter-in- place, is very near the noise floor of the reference detector. Therefore, the information from

21
the ratio of filter versus no-filter is Hmited by the low signal-to-noise ratio of the reference

detector. Filters are selected to niiniini2e stray Ught and the magnitude of Hght, reflected by

the grating, that is any integer multiple of the wavelength of interest.

It is nearly impossible to quantify, experimentally or analytically, this uncertainty

component. Therefore we rely on a conservative estimate (Type B, 0.29 %) that is based on

our measured values from filter transmittance, as well as direct measurement of out-of-band

transmission using laser sources at 633 nm, 674 nm, and 1 523 nm.

.|Q-5 I • \ • 1 • ^ ^ • 1 1 • 1 • 1 200 400 600 800 1000 1200 1400 1600 1800

Wavelength (nm)

Figure 14. Measured, apparent (relative) glass-filter efficiency.

22
10-^

10-='4

? 10"^

4

10"

10-'

10

10-10

4

3

10-11 t- 4

10"

674SiAp

nilSiNF

/ \ / \

400 500 600 700 800

Wavelength (nm)

900 1000

Figute 15. Monochromator throughput shown with 674 nm diode laset input and

background.

6.2.1.3 Bandwidth Effect (Type B)

At any given monochromator-wavelength selection, the transmitted wavelength is actually a range AX of wavelengths that is approximately 6 nm wide (that is, the bandwidth is not infinitely narrow under any circumstances). Therefore, when the monochromator is set

at wavelength A, there are photons, ranging in wavelength from /I to (A ± A/l/2), incident on

the test detector. Photodiode sensitivity varies as a function of wavelength, and the amount

of current generated by the photodiode is proportional to a weighted average of the photon

energy within a band (the weighted average is a function of the slope of the detector

responsivity over the selected band). Our transfer standard is a pyroelectric detector, which

is a thermal detector and, therefore, does not exhibit the same behavior. However, a photodiode, even when calibrated against a thermal detector, may exhibit a change in

responsivity over ±3 nm (6 nm) that may vary as much as 0.5 % [15].

6.2.2 Two-Position Mirror Reflectance (Type B)

Using reflectance data for a coated aluminum mirror at 900 nm, where the

reflectance change is greatest, we have calculated an uncertainty contribution of 0.12 %. This

uncertainty is based on the mirror reflectance being a function of the monochromator

23
output polari2ation. Both the reference detector and the test detector are situated relative to the final mirror (see hardware section), at an angle of approximately 22°. The estimate for

the extent of misalignment (variation from 22°) is based on visual inspection of the beam

position as a function of stage rotation, which has arcsecond resolution. The arcsecond

resolution is well beyond what is perceivable. The uncertainty is based on the fact that,

changing the rotary stage by 1° results in a beam realignment that is perceptible. In other

words, 1° is within the resolution perceptible (a relative judgement) by the hviman operator.

6.2.3 Polarization (Type B)

The gratings used in the monochromator will sUghdy polarize the light from the

broadband source. We have not measured the efficiency of the grating for two orthogonal,

plane-polarization orientations; rather, we rely on the manufacturer's data [16]. The

polarization uncertainty will depend on the polarization sensitivity of the detector under test, so we choose a worst-case scenario. Based on manufacturer's data for two orthogonal,

plane-polarization orientations (normal and parallel to the grating surface) and on the worstcase polarization sensitivity of either gold-black-coated Hthium tantalate, silicon, germanivim,

or indium gallium arsenide, the uncertainty contribution is less than approximately 0.01 %.

6.2.4 Beam Size (Type B)

Beam size depends on the imaging optics of the monochromator system and the

filament dimensions. Using the mirror optics as described in earlier sections, the lamp

filament is focused (1:1) on each detector. In addition, constant-intensity reduction of the

beam size is accomplished by reducing the horizontal- and vertical-monochromator sHt widths.

To verify the beam size, the beam image was measured at the focal plane (where the

test or reference detector plane is located) using a scanning-sHt, beam profilometer (average

of 70 points for each slit-resolution increment). Data for the beam's spatial dimensions and

relative intensity were stored in a file, and then the data were analyzed. Integrating the beam

power over the scanning-slit, beam profilometer area indicates that 99.4 % of the beam is within a vertical dimension of 4 mm and 2 mm horizontal dimension. Therefore, less than

0.6 % of the beam is outside a rectangular beam image of 4 mm x 2 mm. If the test and

reference detectors have identical detector areas, aligrmient depends on visual perception

and the presence of the filament image on both the reference and test detector. Assuming

that one can align the filament image on the detector active area, the question remains: what

fraction of total input power does the perceived image represent? In other words, if properly

aligned to intersect the center of the detector area, does the image fit within the detector

area? The uncertainty is a function of the total optical power falling outside the detector

area. The reference detector aperture size is 5 mm in diameter. The size of test detectors

measured by use of the system varies from customer to customer. For test detectors having an aperture size other than 5 mm, the uncertainty due to beam size may have to be evaluated on a case-by-case basis. For this document, a 0.3 % type B uncertainty is used as a conservative estimate; 0.6 % would be assigned if the test detector has an active area much

larger (or smaller) than the reference.

24
6.3 Optical-Fiber Connector Correction (Type A, n = 3)

At each wavelength increment of the spectral scan, the output of the test detector is compared to the output of the NIST reference detector. This process is repeated a number

of times for each detector and the results are averaged (this is discussed in section 5.5). As

stated in the introduction, the test detector is also calibrated at several laser wavelengths

using the NIST optical-fiber power measurement system (a single-mode optical fiber is connected to the test detector in these measvirements). Example results from the

monochromator-based system and the laser-based system, shown in table 2, are used

together to obtain the results presented in table I of the Calibration Report. The residual

offset values shown in column (4), are added in quadrature to obtain a single number and

this number is considered a type A uncertainty, with n=3.

Table 2. Spectral responsivity measurement-offset residuals at three wavelengths

(example data).

(1)

Wavelength

(nm)

(2)

Monochromatorbased absolute

responsivity, shifted, interpolated

(A/W)

(3)

Fiber connector- based absolute

responsivity

(A/W)

(4)

Residual offset [(2)-(3)]/(3)

(%)

851.9 0.2199 0.2184 1.10 1310.0 0.7446 0.7352 1.28 1550.0 0.8294 0.8380 -1.03

quadrature sum, type-A uncertainty

(1.102 + 1.282 + (-1.03)2)1/2 1.98

25
Table 3. NIST measurement uncertainties.

Source Type B Type A

as (%) Sr (%) Nr

Pyroelectric working standard

Calibration with Primary Standard 0.47

Temperatvire stability 0.02

Spatial uniformity 0.30 45

Linearity 0.09

Repeatability 0.10 3

Noise 0.05 40

Spectral response measurements

Wavelength calibration 0.06 0.10 3

Stray light 0.29

Bandwidth (6 nm) 0.23

Two-position mirror alignment 0.07

Polari2ation 0.01

Beam size 0.17

Relative expanded uncertainty (k— 2) U =1.24%

Additional uncertainty due to optical-fiber

connector correction (t3rpical)

1.98

Relative expanded uncertainty (k— 2) U = 2.59 %

7. Quality Control (Check-Standard Operation)

In addition to the reference-detector calibration and monochromator calibration, a check-standard operation is performed for each detector calibration or batch of calibrations.

The operation follows the normal calibration procedure described in this document, over the

wavelength range from 800 nm to 900 nm, at 10 nm increments. The check-standard

operation is a quality control measure that verifies that the system performance results are comparable to previous results. Check-standard results are maintained in a separate record, one record for each customer.

The check standard is a six-element silicon photodiode trap detector, calibrated using

the NIST laser optimized cryogenic radiometer (LOCR). The silicon-trap detector is based on photodiodes having nearly unit quantum efficiency (QE) over the wavelength range 800

nm to 900 nm. Therefore, in addition to historical records, the absolute spectral responsivity

of the check standard can be quickly verified based on the LOCR calibration results, as well

as mathematical calculation of the expected absolute responsivity. Though the wavelength

range for the check-standard measurement is Umited, it does overlap the wavelength range

for each monochromator grating and represents a subset of coverage for any calibration

request but requires less time than a complete, normal calibration routine.

We have tested several six-photodiode traps. One trap was built using 10 mm square,

300 )J,m thick silicon photodiodes and has been evaluated repeatedly. The absolute spectral

current responsivity indicates that the optical-to-electrical energy conversion is quite high

26
and approaches the theoretically expected value for TJ — \, where the absolute responsivity,

i/P (current output/ optical input power) , is given by — = 77 —= TJ AW nm

P hv 1239.5 (14)

for electron charge e, Planck's constant h, light frequency v , and wavelength X. As an example, absolute spectral responsivity data for the 10 mm square

embodiment are shown in Fig. 16 along with calculated values of A/l 239.5 (for light

travelling in air). The results indicate that the measured responsivity value, measured at 10 nm increments, from 800 nm to 900 nm, is nearly 99 % of the theoretically expected

value based on equation 14 (unit QE). The measurement results have an uncertainty of

±1.5 % (coverage factor k = 2), based on evaluation of the spectral responsivity

measurement system using a NIST-calibrated reference detector.

>

'</) co

a.

in

a>

o

u> £>m

0.70 h

0.65

0.60

0.55

Small tunnel trap tt3 (hammatsu SI 337 - 1 1) measured spectral responsivity

unit quantum efficiency

680 700 720 740 760 780 800 820 840 860 880 900 920

wavelength (nm)

LUo

0.4

0.3

8

£ 0.2

680 700 720 740 760 780 800 820 840 860 880 900 920

Wavelength (nm)

Figure 16. Absolute spectral responsivity of the check standard.

27
8. Conclusion

Absolute spectral responsivity calibration of laser and optical-fiber power meters over the wavelength range from 400 nm to 1800 nm presents many technical challenges. A

single, continuously variable laser source is currently not available for this measurement, so we must rely on a broadband source and a monochromator, as well as several diode-laser sources for the power-meter input. The measurement uncertainty varies from customer to customer, depending on the power-meter detector configuration and the type of the fiber

connector provided by the customer. We compare the laser and optical-fiber power meter

response to a weU characterized reference detector that is spatially and spectrally uniform

and insensitive to varying input-beam coherence and geometry. The calibration service

provides a written calibration record to the customer that includes a description of the

measurement configuration, the absolute responsivity as a function of wavelength of the

customer meter, and a documented estimate of the measurement uncertainty.

9. References

[1] Lehman, J. H.; Pyroelectric trap detector for spectral responsivity measurements. Engineering and

Ijxboratoty Notes, Optics and Photonics News, Vol. 8, No. 11, 1997, archived in ^^5/)/. Opt. Vol. 36, No. 34: pp. 9117-9118, 1997.

[2] West, E.D.; Case, W.E.; Rasmussen, A.L.; Schmidt, L.B.; A reference calorimeter for laser energy

measurements. Journal of Research ofthe National Bureau ofStandards — A. Physics and Chemistry 76A, No.

1: pp. 13-26; January-Febmary 1972.

[3] Vayshenker, I.; Li, X.; Keenan, D.A.; Scott, T.R.; Errors due to connectors in optical fiber power

measurements. Symposium on Optical Fiber Measurements, Natl. Inst. Stand. TechnoL Spec. Publ 905: pp. 49-

52; 1996.

[4] GaUawa, R.L.; Li, X.; Calibration of optical fiber power meters: the effect of connectors. AppL Opt.

Vol. 26, No. 7: pp. 1170-1174; 1987.

[5] International Electrotechnical Comission (TEC); Calibration offibre opticpower meters — CEI/IEC

1315. Bureau Central de la Commission Electrotechnique Internationale, Geneva; 1995. (The

standard includes formal definitions and background treatment of uncertainty contributions, including

temperature dependence, nonlinearity, aging, beam geometry, connector effects, wavelength,

bandwidth, and other considerations.)

[6] Lehman, J.; Sauvageau, J.; Vayshenker, I.; Cromer, C; Foley, K.; Silicon wedge-trap detector for optical fibre power measurements. Meas. Sci. Technol 9: pp. 1694-1698; 1998

[7] Harris, L.; The optical properties of metal blacks and carbon blacks. Monograph Series No. 1: Massachusetts Institute of Technology, Cambridge, Mass; p. 47, 1967.

[8] Kostkowski, H.J.; Nicodemus, F.E.; An Introduction to the Measurement Equation, Chapter 5, Self-Study Manual on Optical Radiation Measurements: Part I—Concepts, Chapters 4 and 5. Nat. Bur Stand (U.S.), Tech. Note 910-2; 1978.

[9] Strong, J.; Concepts of Classical Optics, Appendix?: W.H. Freeman & Company, San Francisco; 1958.

[10] James, J. F.; Sternberg, R.S.; The Design of Optical Spectrometers: Chapman and HaU, Ltd., London; p. 70; 1969.

28
[11] We use txade names to specify the experimental procedure adequately and do not imply or intend

endorsement by the National Institute of Standards and Technology. Similar products by other

manufacmrers may work equally well.

[12] Day, G.W.; Hamilton, C.A.; Pyatt, K.W.; Spectral reference detector for the visible to 12-|J.m

region; convenient, spectrally flat. Appl. Opt. Vol. 15, No.7: pp. 1865-1868; 1976.

[13] Taylor, B.N.; Kuyatt, C.E.; Guidelines for Evaluating and Expressing the Uncertainty of NIST

Measurement Results. Natl. Inst. Stand. Tecbnol. Tech. Note 1297: p. 8 1994.

[14] Vayshenker, I.; Yang, S.; Li, X.; Scott, T.R.; NonUnearity of optical power meters. Natl. Inst. Stand. Technol Spec. Publ 905: pp. 101-104; 1996.

[15] Dereniak, E.L.; Crowe, D.G.; Optical radiation detectors: John Wiley and Sons, Inc., New York; pp. 105, 1984.

[16] McPherson Instrument, 7A Stewart Rd., Chelmsford, MA 01824.

29
Appendix A. Parts List

We use trade names to specify the experimental procedure adequately and do not imply

endorsement by the National Institute of Standards and Technology. Similar products by

other manufacturers may work equally well.

Monochromator, McPherson Model 207 with stepper motor and optical encoder

Grating, McPherson 140 mm x 140 mm, 600 Hnes/mm, 500 nm blaze

Grating, McPherson 140 mm x 140 mm, 600 Hnes/mm, 1000 nm blaze

Filter, 450 nm - 670 nm: OptoSigma 079-3340

Filter, 670 nm - 1075 nm: Schott LL 0650

Filter, 1075 nm - 1575 nm: Schott LL 1000

Filter, 1575 nm - 1800 nm: Schott RL 1500

Linear Positioning Stage, Newport PM500

Rotary Positioning Stage, Newport PM500

Optical Bench, Melles Griot

Lamp, Gilway Tungsten

Lamp Power Supply, HP

Lamp Hardware Mount, NIST

Lockin Amplifier, Stanford Research SR850

Current Amplifier, Stanford Research SR570

GPIB card, CEC • Software, CEC Testpoint

Pyroelectric Detector, NIST

Temperature Controller Electronics, ILX Lightwave

Temperature Controller Hardware Mount, NIST

Flat Mirror Mount, NIST

Flat Mirror, Front Surface Silver, New Focus

Gimbal Mirror Mount, NIST

Spherical Mirror, f/2 Front Surface Silver, Rocky Mountain Instrument

Light Tight box, NIST

Magnetic Component Base, Thorlabs

A.1 Mechanical Fixtures

Light Source Mount: The electrical connection of the bulb is attached to a ceramic

bracket. The ceramic bracket is rigidly attached to a steel post that is attached to a magnetic

base. The bulb is not enclosed or actively cooled.

Flat-Mirror Mounts: The flat mirrors are 50.8 mm in diameter and are epoxied to an aluminum L-shaped bracket mounted on a steel post. The steel post is mounted on a magnetic base and rests on the optical bench. The mirror is mounted such that it wiU rotate about the long axis of the steel post and the front surface of the mirror.

Spherical-Mirror Mounts: The f/2 spherical mirrors are mounted on gimbal mounts

that rotate about the center axes of a 50.8 mm diameter spherical mirror. The mirror is held

within an aluminum ring that is attached to a steel post. The steel post is mounted on a magnetic base and rests on the optical bench.

The f/4.7 spherical mirrors are mounted inside the monochromator. The mirror

mounts are made of steel and are L-shaped. The mirrors rest on kinematic mounts that are aligned by the monochromator manufacturer. These mirrors are normally not re -adjusted.

30
Rotary Position Stage: The rotary stage supports one flat mirror on the output side

of the monochromator. This stage supports the final mirror, which directs light alternately on each detector in the measurement setup. The manufacturer specification for the accuracy

of the rotary stage is 0.1 (iradian. Alignment of this mirror is critical to the accuracy of the

overall measurement, because Hght from the grating monochromator is polarized and the

mirror orientation is the only variable that changes between detectors for a given

wavelength. The accuracy of this alignment is discussed in the error analysis section.

The combination of the rotary stage and mirror is used for several reasons. The

primary reason is to accommodate the myriad commercial designs for optical-fiber power

meters. The size of the meter and associated instrumentation requires a variety of working

distances in proximity to the monochromator and reference detector. The two-position

mirror arrangement allows room for cumbersome alignments. During operation, the time of

stage travel from one measurement position to the other (from test position to reference

position) must be minimized to shorten the duration of the calibration over the entire

wavelength range of interest. The rotary stage, because of a small amount of necessary travel,

is ideal for this application.

Grating Fixture: The grating fixture is a proprietary design patented by the

manufacturer. The grating rests in a steel frame with three points of contact consisting of a ground-steel ball and two ground-steel cylinders that are embedded in the grating blank.

31

Appendix B. Example Calibration Report

Nisr

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

ELECTRONICS & ELECTRICAL ENGINEERING LABORATORY

Boulder, Colorado 80303

Report of Special Test

Spectral Responsivity

For

Optical-fiber Power Meter

Ge Detector

Wavelengths: 750 nm - 1800 nm

Submitted by:

Customer Name

Customer Address

Calibration Summary

The spectral responsivity of the germanium (Ge) detector was measured using the system shown schematically in Figure 1. The measurement system is designed to accommodate a variety of commercial instruments that cover the

entire spectrum of optical communication wavelengths (400 nm to 1 800 nm). The calibration procedure provides

absolute spectral responsivity relative to a known NIST-calibrated transfer standard. The NIST transfer standard is a pyroelectric wedge trap detector that is calibrated using the NIST C-series calorimeter.'

A tunable monochromatic light source composed of a filament lamp, a grating monochromator, and a set of

bandpass filters was used to calibrate the test meter. The output beam from the monochromator (transmitted

J.H. Lehman, "F^Toelectric trap detector for spectral responsivit> measurements," Engineering and Laboratory Notes. Optics and Photonics News, VoL 8, No. 1 L archived in Appl. Opt. Vol. 36, No. 34, pp. 91 17-91 18, 1997. E.D. West, et al., "A reference calorimeter for laser energy measurements," J. Res. Nat. Bur. Stand. - A (U.S.A.), 76A, No.l, pp. 13-26, 1972.

Page: 1 of 7 Test No.: Date of Report: Reference: P.O. No./Check No./Date

33
FIBER OPTIC POWER METER

Model , Serial No.

with Ge Detector, Serial No.

through air) was directed alternately onto the test meter and the NIST transfer standard with a two-position mirror.

The beam was focused {& = f/4) to a diameter of approximately 2 mm at the position of, and normal to, the plane

of the test meter. The bandpass of the monochromator was less than 6 nm.

The output of the test meter was compared to the output of the NIST detector at the wavelengths listed in Table I. The test detector was also calibrated at three laser wavelengths (85 1 .9 nm, 1 3 1 0.0 nm, 1 550.0 nm) using the NIST

fiber optic power measurement system (see Test no. 813577-2) in which an optical fiber was connected to the test detector.

Because the test detector is intended to be used with fiber connectors installed, the data obtained from the

monochromator-based system were shifted to match the data from the fiber optic power measurement system. This

shift was done using a least-squares minimization process. The resulting shift of 5.90% (multiplication factor of

1.0590) is presumed to be a consequence of the use of the fiber optic connectors in the fiber optic power

measurement system. By customer request, the shift was made and the calibration results are shown in Table I and

figure 2. The connector offset, which varies with wavelength, contributes significantly to the measurement

uncertainty estimate shown at the end of Table II.

lamp ^

bandpass filter

3 chopper

two-position mirror

test meter

NIST reference

detector

Figure 1. Grating monochromator-based spectral responsivity measurement system.

Page: 2 of 7 Test No.: Date of Report: Reference: P.O. No./Check No./Date

34
FIBER OPTIC POWER METER

Model , Serial No.

with Ge Detector, Serial No.

a>o

</> <

1.0

0.9

0.8

0.7

0.6

^ 0.5

0.4

0.3

0.2

0.1

0.0 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800

Wavelength (nm)

Figure 2. Detector responsivity plotted as a function of wavelength.

Page: 3 of 7 Test No.: Date of Report: Reference: P.O. No./Check No./Date

35
FIBER OPTIC POWER METER

Model , Serial No.

with Ge Detector, Serial No.

Table I. Absolute responsivity (±2.59%).

nm A/W nm A/W nm A/W

750.0 0.1233 1100.0 0.4919 1450.0 0.8525

760.0 0.1312 1 1 10.0 0.5049 1460.0 0.8637

770.0 0.1392 1120.0 0.5186 1470.0 0.8754

780.0 A 1 /I 0.1473 1 1 O A A

1 130.0 0.5320 1480.0 0.8884

790.0 A 1 C C A 0.1559 1 1 /I A A

1 140.0 0.5453 1490.0 0.9003

800.0 A ^ ^ A c 0.1645 1 1 Cf\ A1 150.0 A C C O 1 0.5581 1500.0 0.9099

O 1 A A 810.0 A 1 TO A 0.1734 1 1 y^A A

1 160.0 A C T A A 0.5700 1510.0 0.9197

820.0 A 1 0*^ A 0.1824 1 1 TA A

1 170.0 A C O 1 0.5812 1520.0 0.9232

O O A A 830.0 A 1 A 1 ^ 0.1916 1 1 OA A

1 180.0 A C A'^ A 0.5920 1530.0 0.9148

O /I A A 840.0 A ^ A 1 1 0.2011 1 AA A

1 190.0 0.6031 1540.0 0.8836

850.0 0.2106 1200.0 0.6134 1550.0 0.8242

860.0 A A 0.2203 1210.0 0.6233 1560.0 0.7456

870.0 A O O AT 0.2303 1220.0 0.6336 1570.0 0.6733

O O A A 880.0

r\ ^ A A-^ 0.2402 1230.0 0.6438 1580.0 0.6168 OAA A 890.0 A C A 0.2506 1 /I A A 1240.0 0.6537 1590.0 0.5751 AAA A 900.0 A 1 A 0.2610 1250.0 0.6637 1600.0 0.5413

910.0 0.2715 1260 0 0.6737 1610.0 0.5125

920.0 0.2821 1270.0 0.6833 1620.0 0.4877

930.0 A A O A 0.2930 1280.0 0.6930 1630.0 0.4627

940.0 A A1 O 0.3038 1290.0 0.7029 1640.0 0.4387

ACA A 950.0 A '> 1 /I A 0.3149 1300.0 0.7122 1650.0 0.4157

960.0 0.3262 1310.0 0.7212 1660.0 0.3933 ATA A 970.0 A O T7 1 0.3371 1320.0 0.7304 1670.0 0.3709

A OA A 980.0 A /I O O 0.3483 1330.0 A "T ATf 0.7397 1680.0 0.3486

990.0 0.3595 1340.0 0.7490 1690.0 0.3259

1000.0 0.3709 1350.0 0.7582 1700.0 0.3028

1010.0 0.3825 1360.0 0.7672 1710.0 0.2793

1020.0 0.3954 1370.0 0.7773 1720.0 0.2549

1030.0 0.4070 1380.0 0.7867 1730.0 0.2297

1040.0 0.4187 1390.0 0.7963 1740.0 0.2045

1050.0 0.4306 1400.0 0.8057 1750.0 0.1784

1060.0 0.4425 1410.0 0.8152 1760.0 0.1530

1070.0 0.4545 1420.0 0.8247 1770.0 0.1296

1080.0 0.4668 1430.0 0.8311 1780.0 0.1089

1090.0 0.4790 1440.0 0.8418 . 1790.0 0.0913

1800.0 0.0760

Page: 4 of 7 Test No.: Date of Report: Reference: P.O. No./Check No./Date

36
FIBER OPTIC POWER METER

Model , Serial No.

with Ge Detector, Serial No.

Uncertainty Assessment

The uncertainty estimates for the NIST spectral responsivity measurements are assessed using NIST Technical

Note 1297, "Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results" by Barry N.

Taylor and Chris E. Kuyatt, 1994 Edition. To establish the uncertainty limits, the error sources are separated into

(1) Type A errors whose magnitudes are obtained statistically from a series of measurements and (2) Type B errors whose magnitudes are determined by subjective judgements or other nonstatistical methods.

All the Type B error components are assumed to be independent and have rectangular or uniform distributions (that

is, each has an equal probability of being within the region, ± 6i, and zero probability of being outside that region).

If the distribution is rectangular, the standard deviation, as, for each Type B error component is equal to 5,/3

^' and

the total "standard deviation" is (Das^) ' where the summation is performed over all Type B error components.

The Type A errors are assumed to be independent and normally distributed and, consequently, the standard

deviation, Sr, for each component is

N -1

(1)

where the x values represent the individual measurements and Nr is the number of x values used for a particular

Type A error component. The standard deviation of the mean is Sr/N/', and the total standard deviation of the mean

is [I(Sr^/Nr)] where the summation is carried out for all the Type A error components.

The expanded uncertainty is determined by combining the Type A and Type B "standard deviations" in quadrature

and multiplying this result by an expansion factor of 2. The expanded uncertainty, U, is then

U - 2W^l + Y.^ . (2)

The values used to calculate the NIST uncertainties are listed in Table II.

The number of decimal places used in reporting the values in Table I and Table II was determined by expressing

the total NIST uncertainty to two significant digits. An additional significant digit is given for values of absolute

responsivity in Table II to avoid rounding errors if the data need to be interpolated or further manipulated by the customer.

Page: 5 of 7 Test No.: Date of Report: Reference: P.O. No./Check No./Date

37
FIBER OPTIC POWER METER

Model , Serial No.

with Ge Detector, Serial No.

Because the monochromator-based measurement results in Table I were shifted to match the laser-based

measurement results, there is an associated uncertainty added to the overall estimate. The value given in Table II as a connector offset is based on the residuals remaining after the monochromator-based data were shifted. The

residuals were added in quadrature and the result is listed as a Type A uncertainty. It is important to note that the

value given is only an estimate and has not been thoroughly evaluated.

Table II. NIST Measurement Uncertainties

Source Type B Type A

Sr

Pyroelectric Working Standard

Calibration with Primary Standard 0.47 %

Temperature stability 0.02 %

Spatial xiniformity 0.30 % 45

Linearity 0.09 %

Repeatability 0.10% 3 Noise 0.05% 40

Spectral Response Measurements

Wavelength calibration 0.06 % 0.10 % 3

Stray light 0.29 %

Bandwidth (6 nm) 0.23 %

Two-position mirror alignment 0.07 %

Polarization 0.01 %

Beam size 0.17%

Relative Expanded Uncertainty (k=2) 1.24%

Additional uncertainty due to optical-fiber connector correction (typical)

1.98 % 3

Relative Expanded Uncertaint}' (k— 2) 2.59 %

Page: 6 of 7 Test No.: Date of Report: Reference: P.O. No./Check No./Date

38
FIBER OPTIC POWER METER

Model , Serial No.

with Ge Detector, Serial No.

For the Director,

National Institute of Standards

and Technology

Report Reviewed By:

Gordon W. Day, Division Chief

Optoelectronics Division

Christopher L. Cromer, Project Leader

Sources and Detectors Group

Optoelectronics Division

Report Prepared and Meter Calibrated By: Report Reviewed By:

John H. Lehman, Mechanical Engineer

Sources and Detectors Group

Optoelectronics Division

Thomas R. Scott, Group Leader

Sources and Detectors Group

Optoelectronics Division

Page: 7 of 7

Test No.: Date of Report: Reference: P.O. No./Check No./Date

39

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