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13

S NBS TECHNICAL NOTE 987

U.S. DEPARTMENT OF COMMERCE/ National Bureau of Standards

ervo Cori

Electro-Optic Modulator

p for cw Laser Power

Stabilization and Control
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress March 3, 1901. The

Bureau's overall goal is to strengthen and advance the Nation's science and technology and

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and the Institute for Computer Sciences and Technology.

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The National Bureau of Standards was reorganized, effective April 9, 1978.
ERRATA TO ACCOMPANY NATIONAL BUREAU OF STANDARDS TECHNICAL NOTE 987 , A

SERVO CCKTROLLED ELECTRO-OPTIC ^DDULATOR FOR CW LASER POWER STABILIZATION

AND CONTROL, BY JOEL B. FOWLER, MICHAEL A. LIND AND EDWARD F. ZALEWSKI

CONSTRUCTION NOTES on Fig. 6 should read as follows:

1) BNC attached to lOQ heater. Heater epoxied to back of

detector close to center

2) Thermister 5K @ 25C heat sunk to back of detector

3) Cavity dimensioned for EG§G UV444B silicon detector

4) Use quartz oval < 0.5 ram thickness; wedged for best

results

r BnreaB of Sta'iiac:.^^ 19 197-9 A Servo Controlled Electro-Optic

Modulator for cw Laser Power

Stabilization and Control

Joel B. Fowler, Michael A, Lind, and

Edward F. Zalewski

Center for Radiation Research

National Measurement Laboratory

National Bureau of Standards

Washington, D.C. 20234

*i^'S^-" •'' Co,

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued April 1979
National Bureau of Standards Technical Note 987

Nat. Bur. Stand. (U.S.), Tech. Note 987, 16 pages (Apr. 1979) CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1979

For sale by the Superintendent of Documents, U.S. Government Printing OflBce, Washington, D.C. 20402

Stock No. 003-003-02049-4 Price $1.10

(Add 25 percent additional for other than U.S. mailing).

I
TABLE OF CONTENTS

Page

INTRODUCTION 1

MODULATOR 1

ELECTRONICS PACKAGE 2'

DETECTOR AND BEAM SPLITTER ASSEMBLY 8

OPERATION OF THE SYSTEM 8

REFERENCES 11

APPENDIX 12

LIST OF FIGURES

1. Schematic diagram of laser stabilizer circuitry:

Version 1 3

2. Schematic diagram of laser stabilizer circuitry:

Version 2 "..... 4

3. Printed circuit board component layout 5

4. Photographically reproducible printed circuit board

layout: Side One 6

5. Photographically reproducible printed circuit board

layout: Side Two • 7

6. Mechanical drawings of beam splitter and detector housing . 9

m

A SERVO CONTROLLED ELECTRO-OPTIC

MODULATOR FOR cw LASER POWER STABILIZATION AND CONTROL

Jodl B. FoLOlo,^, h\-i(LkoiQ,t A. li,nd* and EdiA)a^d F. Zal^M^ki.

RadA^om(2,tA,^c Phy6^ci V-lvA,6A^on

C2.nt2,n. Ion. Rad-iatZon RdidCiftdh

Hat-Lonat Mdamn-^mdnt Labon.aton,y

Tlvo i-lmptd ddi>i.Qvii> {^on. a. toM c.06t la^d/i

ittab-iti^zcit-lon 6yi>t<im aKn pn.2.6znt2.d. Thu i>y6-

t<im6 d<ii>(iKi.bQ,d an.2. capable o {^ itabi-tA^z-lng the.

poMzn. outpai 0^ a cm ta^^A. t-ind to buttdH. tkan

.05% i^om VC to 100 kHz In tkd 350 to 1150 nm

6pzctn.al n.angz.

Key Words: La^^n. poMzn. modulator.; tcihin. potvdh.

stabtttzzn.) photoddtdcton. chan.ac.t2.h.tzatton',

xadtomttn^tc tn6tn.uim2,ntatton.

INTRODUCTION

With the electro-optical crystals now commercially available it is

possible to build a simple low cost system to extra-cavity stabilize the

output of many cw lasers to better than .05% for stabilization bandwidths of DC to 100 kHz. [1,2,3]^ The systems currently employed at NBS

to stabilize the laser beam power used for high accuracy characterization of detectors are described. Each system contains three main components: a low voltage electro-optic light modulator, a thermally

stabilized silicon monitor detector and beam splitter assembly, and an electronics package to bias and servo-control the modulator crystal.

MODULATOR

Any one of a variety of electro-optic modulators can be used with

the system. The choice of a device depends strongly on its intended

application. The modulator used in the NBS system for stabilizing the

laser power is a Lasermetrics [4] 3033 FW. This transverse field modulator was chosen primarily for its wide optical bandwidth, high maximum

cw input power damage threshold, and relatively low, half-wave retardation voltage. The Appendix lists some of the more important specifications for the device.

*Present Address: Battelle, N.W., P.O. Box 999, Richland, WA 99352.

^Figures in brackets indicate the literature references at the end of

this paper.

1
The crystals as supplied from the factory (see the Appendix) are surrounded by an index matching fluid and mounted in a 50.8 mm diameter

cylinder that is 131 mm long excluding the polarizer. All windows in

the unit are made from Dynasil grade fused silica and have a wideband

MgFa coating to minimize interreflections. A Glan-laser polarizer is

mounted at the exit port of the housing (with the provision to add an input polarizer when using unpolarized lasers).

ELECTRONICS PACKAGE

Two versions of this stabilizer system are currently in use at

NBS. The circuit diagrams are shown in Figs. 1 and 2. Version 1 (Fig. 1) has been designed to include amplitude modulation capability

and complete loop stability. Version 2 (Fig. 2) is designed for

improved laser amplitude stabilization by sacrificing loop stability

(oscillation may occur under certain operating conditions) and modulation versatility. The printed circuit board component layout is illustrated in Fig. 3 and the double-sided board layout suitable for photographic reproduction is given in Figs. 4 and 5. The same board is used

for both systems with minor modifications for Version 2.

The electronics package has three main sections: a temperature

controller to maintain a constant substrate temperature of the silicon

monitor detector, a high gain servo-amplifier compensated to drive a particular detector/modulator system, and a high voltage amplifier to

supply the necessary retardation voltages to the modulator.

In arranging the printed circuit board and the three power supplies

within the electronics package, it is important to keep the ac power

input well separated from the feedback control circuitry. To further

minimize 60 Hz interference, the leads from the front panel BNC connections to the printed circuit board, especially the detector and thermistor inputs, should be as short as possible.

Since the temperature coefficient of the silicon monitor detector

becomes non-negligible in the uv and ir spectral regions, it is necessary to control the temperature of the detector substrate to better

than 0.1 °C if high dc stability is required.

The temperature of the silicon substrate is sensed with a thermistor (FENWAL GB41J1 ) [4] bridge circuit. A reference voltage in one leg

of the bridge is adjusted by R31 . The difference voltage is amplified

by Ul. The follower Ql drives a resistance heater (MINCO HK610-0189.48)[4]

attached to the back of the silicon detector. The temperature of the

detector substrate is set a few degrees above the ambient temperature

of the laboratory.

The silicon detector used in the NBS system (EG&G UV444B) [4] was selected for its high degree of uniformity and stability of response in

the uv spectral region [5] and also for its large active area. The

current output from this detector operated in a photovoltaic mode drives

a current to voltage converter U3. The output of U3 must be kept below

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1.4 volts (9.0 volts for Version 1) to insure stability of the servo

loop and help prevent clipping resulting in saturation. U2 detects any

voltage over this value and lights an LED which signifies an OVERLOAD

condition. A buffer amplifier for external monitoring of the preamp

signal is provided by U4.

The output of the current amplifier is summed with an adjustable

bias voltage via U6 and the external MODULATION INPUT connector. The

sum of these three signals is amplified in U6 and U7 and fed into the

high voltage, wide-bandwidth amplifier U8 (DATEL SYSTEMS AM302B or AM303B) [4]. This signal is then used to drive the modulator.

The bandwidth of the Version 1 system is dependent primarily on the

preamp gain. Depending on the type of modulator used, the system bandwidth varies from about DC to 10 kHz at the 10"^ A/V setting and from

DC to approximately 100 kHz at the 10"* A/V setting. The bandwidth of

Version 2 is higher but is not totally protected from instability (due

to the higher loop gain) necessitating the use of an oscilloscope to

monitor the high gain stages. Version 2 is the most versatile and

could actually have a variable gain in the 2nd stage (U7) higher than

the standard 100 in some applications. The printed circuit board has

been laid out to allow easy access to the critical components that control the servo gain characteristics. The component values shown in the

schematic diagram for the Version 1 design have been chosen to provide

a maximum flat transfer characteristic from the MODULATION INPUT to the

detected laser power when using the modulator crystal described above.

If another modulator crystal is used it may be necessary to modify these

values somewhat to achieve a flat response curve. The transfer characteristic for the Version 2 circuit is not necessarily flat since it was designed primarily for laser stabilization [6]. Although the higher

system gain results in a tighter servo action the possibility of

instability will exist.

DETECTOR AND BEAM SPLITTER ASSEMBLY

The beam splitter and detector housing is shown in Fig. 6. The

beam splitter itself is a quartz oval, wedged slightly to minimize the

effects of interference bands. The oval should be as thin as possible

for the least beam deflection at the exit port of the housing. The

recess at the entrance port of the assembly is to accommodate an iris

diaphragm or a precision aperture. The silicon detector is mounted in

the recessed portion of the removable cover plate. The heater and

thermistor are cemented to the back of the detector for the best

temperature control

.

OPERATION OF THE SYSTEM

The first step in the operation of the system is to place the modulator in the laser beam, preferably near the laser itself, and then

align it. During the alignment procedure the power level of the incident laser beam should not exceed 100 milliwatts in order to avoid

damage to the crystal. The modulator should be rigidly mounted after

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the beam is centered in both the front and rear apertures of the device.

As with most transverse field modulators, the crystal should be oriented

at 45° with respect to the incident polarization vector to obtain the

maximum contrast ratio. The integrally mounted Glan-laser polarizer is

then locked into position at ±45° with respect to the edge of the crystal (in some stabilization applications other angles may be more

suitable).

The beam splitter assembly should be placed into the beam as far

down the optical path as is feasible. The best stability is obtained

from the system when there are no optical elements in the beam after the

beam splitter housing. The reflected beam from the quartz plate should

be perpendicular to the polarization of the incident beam; e.g. for a vertically polarized beam the reflected beam should be in the horizontal

plane.

The HEATER OUT, TEMPERATURE SENSE and DETECTOR are connected to the

beam splitter assembly via BNC cables. The length of these cables should

be kept to a minimum to minimize noise pickup. The modulator is con- nected to the HIGH and LOW OUTPUTS of the electronics package by two more BNC cables. In order to insure that the capacitive load of the

modulator does not destroy the high voltage driver amplifier in the

electronics package, it is recommended that a 1 M^ resistor be placed

across the modulator leads as close as possible to the modulator itself.

Turn on the TEMPERATURE CONTROLLER and allow a few minutes for the

temperature of the silicon detector to stabilize. At the same time warm

up the preamplifier and servo electronics by turning the PREAMP switch

on. The HIGH VOLTAGE should be activated only when control of the beam

power is desired. The detector may be monitored at the MONITOR output

whenever the PREAMP switch is on.

Adjust the laser to the desired operating range not exceeding the

maximum of 15 watts specified for the modulator. Turn the PREAMP GAIN

control to the most sensitive position possible without the OVERLOAD

indicator lighting. It may be necessary to insert some kind of neutral

density filter between the detector and the beam splitter at high power

levels. In the visible spectral region several layers of white paper

make a satisfactory attenuator.

After the HIGH VOLTAGE is turned on, adjust the BIAS OUTPUT so that the meter remains somewhere within its operating range. If a suitable operating point cannot be found, then switch the HIGH and LOW

outputs leading to the modulator. The BIAS ADJUST controls the dc bias

on the modulator which in turn regulates the power transmitted through

the polarizer.

It is possible to inject a waveform into the MODULATION INPUT and

thereby modulate the laser beam. It may be necessary to adjust the

amplitude of the input signal and the BIAS ADJUST to avoid clipping of

the waveform.

10
In applications requiring stabilizing laser beams outside of the

spectral bandpass of the modulator such as frequency doubling experiments, it is necessary to place the modulator in the beam of the pump

laseV and the monitor detector in the doubled beam. The best stability

from a dye laser output is also achieved by modulating the pump laser

beam and monitoring the output beam.

REFERENCES

[1] R. L. Barger, M. S. Sorem, and J. L. Hall, Appl . Phys. Lett. 22,

573 (1973).

[2] R. L. Barger, J. B. West and T. C. English, Appl. Phys. Lett. 27_,

31 (1975).

[3] J. Geist, M. A. Lind, A. R. Schaefer and E, F. Zalewski, NBS

Technical Note 954, 1977 (U.S. Government Printing Office,

Washington, D. C. 20402, SD Cat. No. 013.46:954).

[4] The use of trade names in this paper in no way implies endorsement

or approval by NBS and is included only to define the procedure.

[5] M. A. Lind and E. F. Zalewski, Appl. Opt. 15, 1377 (1976).

[6] J. B. Fowler, Proc. of the Electro-Optics and Laser Conference

(Indust. and Sci. Conf. Mgmt. , Chicago, 1977) p. 690.

11
APPENDIX

Specifications of Modulator Used

Crystal material AD*P

Number of crystals 4

Square aperture 2.5 mm

Maximum transmittance 95%

Useful optical bandwidth 340-1200 nm

Useful electrical bandwi dth DC-lOO MHz

Maximum cw laser input 1 aser 15 W

Half-wave retardation vo Itage

488 nm 135 V

@ 515 nm 142 V

@ 633 nm 175 V

Typical capacitance 125 pf

12
RBJ-IUA \nt.V. \i-,il

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN 987JUjQovt. A^wsslon tto^ 3. 8a«igii«if$ A$<jt$s{o« «*»

. ,^t^.. >^. .^ .Xx •: >. .t^^^i 4. TITLE AND SUBTITLE

A Servo Controlled Electro-Optic Modulator for cw

Laser Power Stabilization and Control

5. Publication Date April 1979

7. AUTHOR(S)

Joel B, Fowler, Michael A. Lind, and Edward F. Zalewski

e. renorming urgan. riepoi i i'<io.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, state. ZIP) 13. Type of Report & Period Covered

15. SUPPLEMENTARY NOTES

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

16. ABSTRACT (A 200~woTd or teas factual BUTntnary of moat aighificant information. If document includes a significant bibliography or

literature aurvey. mention it here.) Two simple designs for a low cost laser stabilization system

are presented. The systems described are capable of stabilizing the

power output of a cw laser line to better than ,05% from DC to 100 kHz

in the 350 to 1150 nm spectral range.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the firat letter of the first key word unless a proper name;

separated by semicolons)

Laser power modulator; laser power stabilizer; photodetector characterization;

radiometric instrimientation.

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Penalty for Private Use, $300

POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE COM-215

SPECIAL FOURTH-CLASS RATE

BOOK

II