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L^°*-|«ve| germanium dete U5« NO. 85-3041 VI 986 C.1 NBS-PUB- EL GERMANIUM DETECTOR

TRANSFER STANDARD AT 1 .064pim

REFERENCE

NBS

PUBLICATIONS

Alvin L. Rasmussen

Douglas L. Franzen

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303

January 1986

-QC

100

• U56

85-3041

1986

NBS

EESEAP.CH INF05MATON

CEIiTES

NBSIR 85-3041

LOW-LEVEL GERMANIUM DETECTOR

TRANSFER STANDARD AT 1.064fxm

Alvin L. Rasmussen

Douglas L. Franzen

Electromagnetic Technology Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

January 1986

U.S. DEPARTMENT OF COMMERCE. Malcolm Baldrige. Secretary

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Director

Contents

Page

1. Introduction and Background 1

2. Spatial Scanning of the Germanium PIN Photodiodes 2

3. Construction of the Detection Systems 2

4. Modulator System Calibration 5

5. LED System Calibration 8

6. Results 9

7. References 10

i i i

Low-Level Germanium Detector Transfer Standard at 1.064 ym

Alvin L. Rasmussen

Douglas L. Franzen

Electromagnetic Technology Division

National Bureau of Standards

Boulder, Colorado 80303

Two germanium PIN photodiodes have been calibrated in the 1 to

250 fJ/cm2 range with 15 percent uncertainty for 1.064 ym laser

pulses of 10 to 100 ns duration. To do these calibrations, we used

(1) an acousto-opti cal ly modulated cw Nd:YAG laser beam and a silicon PIN photodiode transfer standard to provide low-level laser

pulses of known energy and (2) a pulsed 1.06 ym LED beam. A 1 cm 2 collecting lens and a ground-glass diffuser were placed in front of

each detector to improve sensitivity and spatial uniformity,

respectively. In the future, these detectors may also be useful as transfer standards at wavelengths out to 1.7 ym.

Keywords: beamsplitter attenuator; low-level laser measurements;

modulated cw measurement system; 1.064 ym laser pulse measurements;

germanium PIN photodiode transfer standard; pulse energy; pulsed

1.06 ym LED measurement system; spatial scanning of germanium

photodiode detector

1. Introduction and Background

Because of NdrYAG laser applications in low-level guidance receivers and

range finders, the National Bureau of Standards (NBS) developed APD and PIN

silicon photodiode transfer standards [1] to measure low-level peak power and

energy, respectively, for 10 to 100 ns duration, 1.064 ym laser pulses at 10

to 15 percent uncertainty. Without a collecting lens, the silicon PIN photodiode transfer standards can measure pulses of about 10 -14 J/cm2 to

10 -11 J/cm2 . Because of continuing interest in low-level measurements and the

desire to extend them to longer wavelengths, NBS has developed germanium PIN

photodiode transfer standards. At 1.06 ym, the latter are about 10 times more sensitive than the silicon PIN photodiode transfer standards.

1
In this article, we will describe spatial scanning of the germanium

photodiodes, construction of the detection systems, modulation calibrations,

and LED calibrations.

2. Spatial Scanning of the Germanium PIN Photodiodes

Using a cw 1.064 ym laser beam, we determined the spatial uniformity of

the germanium PIN photodiodes with and without a 600 grit, ground-glass diffuser in front of the detector window (figs. 1 and 2). The room in which the

measurements took place was almost totally dark. The beam was greatly attenuated, chopped, and focused to a diameter less than 1 mm. The 1/4 cm2 area

(6 mm diameter) germanium photodiode detectors were scanned in the order of

5 min. Detector output was fed into a lock-in amplifier, and the amplifier

output was read on an oscilloscope. Scans showed large variations in the spatial uniformity of the diodes when no diffuser was used (figs. 1 and 2). The

flat window in front of the diode may have contributed significantly to this

nonuniform response. A diffuser in front of the window greatly improves the

spatial uniformity of the diode (figs. 1 and 2).

3. Construction of the Detection Systems

The germanium detection systems were constructed as described below. The

detectors were commercial units with the germanium PIN photodiode detector at one end of the Dewar flask and the fill hole for liquid N

2 and electrical con- nections at the other end. The specifications are given in table 1.

Table 1. Manufacturer's specifications for germanium PIN photodiode detectors.

Operating temperature

Operating bias

Detector area

NEP

Responsi vity

(we integrated the output for

pulse energy measurements)

Time constant

Window

77 K -300 V 0.25 cm2 2 . 10-1^ and 1 . 10'is w Hz‘i/2

100 V/yW and 5000 V/yW

15 ys and 1 to 2 ms sapphi re Operating position vertical or horizontal

(we used them in a horizontal position)

2
(a)

(b)

Figure 1. (a) Spatial uniformity of germanium detector Ge 6-1 at a wavelength

of 1.064 ym; (b) same, with ground-glass diffuser.

3
(a)

(b)

Figure 2. (a) Spatial uniformity of germanium detector Ge 6-2 at a wavelength

of 1.064 ym; (b) same, with ground-glass diffuser.

4
The detectors were each sealed in a portable light-tight enclosure. The

entrance includes a 1.00 cm2 aperture and 5:1 beam reduction collimator. The

latter had a narrowband 1.064 ym anti reflection coated, positive lens followed

by a negative lens. Once inside the box the beam travels about 5 cm and then

passes through a unit consisting of a wedge, an aperture, and a diffuser

before entering the sapphire window of the detector. The wedge is SF-6 glass

with a 1 deg wedge angle. It was to provide a reflected beam for the purpose

of pulse shape measurements. The aperture had a 0.4 cm diameter and was parallel to the rear face of the wedge. The diffuser, a quartz glass with a 600 grit ground rear face, was oriented approximately 0 deg to the original

beam and was approximately 1/3 cm from the detector window.

4. Modulator System Calibration

The modulator system is a cw Nd:YAG laser beam acousto-opti cal ly modulated with a silicon PIN photodiode transfer standard in one of the beams of a beamsplitter attenuator to provide low-level laser pulses of known energy

(fig. 3). The beamsplitter attenuator, BA-1, attenuates the beam according to

the number of beam reflections between the front and back surfaces of a 2 deg

quartz wedge.

To calibrate the germanium detection systems, a silicon PIN photodiode

transfer standard in a higher order beam (attenuation about 900) monitors the

pulse energy to the germanium detector in a still higher order beam (attenuation about 27,000). Black painted tubes were placed in front of all detectors

to reduce the field of view; the room was kept fairly dark. A differential

amplifier oscilloscope plug-in was used to read out the germanium detectors.

The faster detector, Ge 6-1, was calibrated without problems. The slower

detector, Ge 6-2, however, was easily saturated, yielded unsteady output, and

could not follow rapid random changes in the laser output. Therefore, another

scheme, described in the following section, was devised to calibrate detector

Ge 6-2.

5
Figure 3. Low-level modulated cw measurement system.

6
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Figure

4.

Pulsed

LED

system

for

calibrating

low-level

detectors.
5. LED System Calibration

A pulsed LED system for calibrating low-level detectors (fig. 4) was used

to calibrate the Ge 6-2 detector against the Ge 6-1 detector and to measure

its linearity. One detector at a time was placed in the very stable LED beam.

The LED system generated light pulses at approximately 1.06 pm. The central

wavelength and line width of pulses [2] are somewhat different from the Nd:YAG

laser. Since the calibration of the silicon PIN photodiode varies rapidly

with wavelength, the silicon PIN photodiode could not be used in the LED

system to calibrate the germanium detectors. The latter, however, are much

less sensitive to changes in wavelength in this range and can be used in the

LED system to calibrate similar detectors.

A 12.5 cm diameter planoconvex lens placed about one focal length from

the LED collimated the beam. A 10 cm diameter, nominally 1.06 pm, interference filter and a detector followed the lens. The detector output changed

negligibly when moved 50 cm along the optical axis of the beam. The beam

cross section was uniform over an 8 to 10 cm diameter.

No spectrophotometer was available to evaluate the wavelength response of

the 1.06 pm filter. A rough evaluation of the filter was made as described

below. The filter was inserted in a low-level beam of the modulated cw system. The filter passed about 70 percent of the 1.064 pm beam. We used the

Ge 6-1 detector to measure any difference in calibration of a silicon PIN

photodiode between the LED and modulated cw systems. From this difference in

calibration and the manufacturer 1 s responsi vity-versus-wavelength curve, we estimated the effective wavelength of the LED output to be 1.08 pm.

Applying precisely decreasing steps of current to the LED, we measured

the output of the PIN 4-1 silicon transfer standard and the germanium (Ge 6-1

and Ge 6-2) PIN photodiode detectors. For the three detectors, measurement

ratios of each step output to the maximum output were within 1 or 2 percent of

each other. As each detector in the order of PIN 4-1, Ge 6-1 and Ge 6-2

descended into the noise, measurements were no longer possible. Since PIN 4-1

and Ge 6-1 were linear detectors, Ge 6-2 was also judged to be linear. We

used Ge 6-1 to calibrate Ge 6-2. When Ge 6-1 could no longer be used as a transfer standard, we computed the pulse energy into Ge 6-2 from its own calibration .

8
6. Results

Two germanium PIN photodiodes were calibrated from 1 to 250 fJ/cm2 with

15 percent uncertainty for 10 to 100 ns duration, 1.06 ym laser pulses

(tables 2 and 3). They are portable transfer standards, traceable to the

national standard [3] of laser power and energy, and about 10 times more sensitive than the silicon transfer standards at 1.064 ym. The latter systems

have 1 cm2 area. The spectral responsivity of the germanium detectors makes

them potentially useful to 1.7 ym.

Table 2. Ge 6-1 and Ge 6-2 germanium PIN photodiode system calibration.*

Detector Cali brati on J/V cm2 t

Number of

cal ibrations (n)

Error in

measurement average at

95% Ci (%)

Total

estimated

error at

95% Ci (%)

Energy

range

fJ/cm2 *

Readout

range

Vt

Estimated

noi se fJ/cm 2

Ge 6-1

(817S) at

H setting

5.1 . 10“i3 105 0.4 15.1 2.5-250 0.005-0.5 ~±0.2

Ge 6-2

(817L) at

H setting

5.0 . 10’ 13 28 0.3 15.4 1-60 0.002-0.1 ~±0 . 08

*Pulses were

and for Ge

less than

6-2, A = 0

200 ns at 1.06

.998 cm2 . ym. Collector areas were for Ge 6-1, A

x = 0.992 cm 2

tReadout TX 7904 oscilloscope [4], +CH, 7A22 plug-in, HF -3 dB point 1 MHz, LF -3 dB point

10 kHz. Measurements made from December 1984 through March 1985.

9
Table 3. Error budget Ge 6-1 and Ge 6-2.

Source of error Uncertainty (%)*

PIN 4-3, silicon photodiode transfer standard 8.0

(traceable to national standards)

Beamsplitter attenuator (beam ratio = 29.7) 0.7

PIN 4-3 oscilloscope readout 3.0

Ge 6-1 oscilloscope readout 3.0

Precision

Ge 6-1 0.4

Ge 6-2 0.3

Total error budget**

Ge 6-1 15.1

Ge 6-2 15.4

* At the 95 percent confidence interval.

** Over half of the error budget comes from the readout used. If a more accurate readout were obtained, the uncertainty could be significantly

improved.

In calibrating Ge 6-2, ratios of output readings of Ge 6-1 and Ge 6-2 were

used. Readout error was reduced to the measurement precision.

7. References

[1] Sanders, A. A.; Rasmussen, A. L. A system for measuring energy and peak

power of low-level 1.064 ym laser pulses. Nat. Bur. Stand. (U.S.) Tech.

Note 1058; 1982.

[2] Young, M. The use of LEDs to simulate weak YAG-laser beams. Nat. Bur.

Stand. (U.S.) Tech. Note 1031; 1981.

[3] Franzen, D. L.; Schmidt, L. B. Absolute reference calorimeter for measuring high power laser pulses. Appl . Opt. 15: 3115; 1976 December.

[4] Certain trade names are used in this report in order to specify the

experimental conditions used in obtaining the reported data. Mention of

these products does not imply recommendation or endorsement by the

National Bureau of Standards, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

10
NBS-1 14A (REV. 2-8CI U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 85-3041

2. Performing Organ. Report No. 3. Publication Date

January 1986

4. TITLE AND SUBTITLE

Low-Level Germanium Detector Transfer Standard at 1064 m

5. AUTHOR(S)

Alvin L. Rasmussen and Douglas L. Franzen

6. PERFORMING ORGANIZATION (If joint or other than NB S. see instructions;

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8 . Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

Department of Defense

Calibration Coordination Group

Aerospace Guidance and Metrology Center

Newark Air Force Station, Ohio

10. SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a si gn i fi cant bibliography or literature survey, mention it here) 2 Two germanium PIN photodiodes have been calibrated in the 1 to 250 fj/cm range

with 15 percent uncertainty for 1.064 pirn laser pulses of 10 to 100 ns duration.

To do these calibrations, we used (1) an acousto-optical ly modulated cw Nd:YAG

laser beam and a silicon PIN photodiode transfer standard to provide low-level

laser pulses of known energy and (2) a pulsed 106 pirn LED beam. A 1 cnr collecting

lens and a ground-glass diffuser were placed in front of each detector to improve

sensitivity and spatial uniformity, respectively. In the future, these detectors

may also be useful as transfer standards at wavelengths out to 1.7 ym.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semico lor. , beamsplitter attenuator; low-level laser measurements; modulated cw measurement system;

1.064 ym laser pulse measurements; germanium PIN photodiode transfer standard; pulse

energy; pulsed 1.06 ym LED measurement system; spatial scanning of germanium photodiode

13. AVAILABILITY

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Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402. LS Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

16

15. Pn

U S GOVERNMENT PRINTING OfnCE i <f