UDR-TM-86-02

Operation Manual

Contract NAS8-36646

LASER-PHOTODETECTOR TIMING STATION INSTRUCTION

AND MAINTENANCE MANUAL

By:

Edward A. Strader

University of Dayton Research Institute

Experimental and Applied Mechanics Division

Impact Physics Laboratory

Dayton, Ohio 45469

fNASA-CH-178835) LASER-PHOIODEIECIOB TIHING N86-24720

STATION INSTRUCTION AND flAIHTINANCE HANOA L

{Dayton Univ., Ohio,) 13 p HC A02/M1 A01

CSCL 14B Onclas

G3/14

April 1986

Prepared For:

George C. Marshall Space Flight Center

Marshall Space Flight Center, Alabama 35812

The University of Day ton
TABLE OF CONTENTS

SECTION PAGE

1 INTRODUCTION 1

2 SYSTEM DESCRIPTION 3

3 SETUP AND MAINTENANCE 5

MECHANICAL SETUP 5

ELECTRONIC AND OPTICAL SETUP 6

MAINTENANCE 9

4 HELPFUL HINTS 10
SECTION 1

INTRODUCTION

A laser-photodetector station is a very useful/ non-contact

device for detecting the arrival of a projectile at a specific

point along a ballistic range. In general/ such a station consists of a low-power visible (Class I) laser and a photodetector.

The laser is positioned so that its output beam is directed

across the trajectory of the projectile to be detected.

Normally/ the laser beam strikes the PIN diode photodetector and

produces a DC output signal that is proportional to laser beam

intensity. When a projectile interrupts the laser beam, the

photodetector output voltage diminishes rapidly to provide an

electrical signal which indicates passage of a projectile. If

the photodetector circuit is equipped with a comparator circuit/

it also produces a pulse which can be used to trigger various

other devices such as flash tubes/ counters/ or pulsed x-ray

systems.

The analog (or direct) photodetector output signal has

proven to be very useful when it has been recorded by an

electronic device having a large temporal range and high resolution/ e.g./ a digital oscilloscope or digital transient recorder.

In ballistics experiments/ particularly those using powder or two

stage/ light-gas guns/ the projectile is frequently preceded or

accompanied by a variety of debris. A solid projectile produces

a well defined signature as it passes through the laser beam.

When the resolution of the transient recorder is adequate/ a

fairly good approximation to the velocity of the projectile can

be obtained from a single laser-photodetector station by dividing

the length of the projectile by the time which the laser beam is

blocked by the projectile. However/ when a significant amount of

gases precede and/or accompany the projectile/ it may be difficult to distinguish the signature of the projectile from that

of the extraneous matter.
A laser-photodetector station can be employed on either an

open or evacuated range. They can also be used with very small

projectiles and at hypervelocities/ situations in which use of

contact detectors (such as foil switches) are prohibited because

of possible damage to the projectile.
SECTION 2

SYSTEM DESCRIPTION

Each laser-photodetector system consists of the following

components: (a) a Class I HeNe laser light source/ (b) a PIN

diode detector and associated electronic circuitry mounted in a

protective enclosure, (c) a mounting structure for positioning

and supporting the laser and photodetector assembly, and

(d) cables necessary to attach the detector assembly to its power

supply. The Spectra Physics Model 155 HeNe laser used with each

system, produces 0.5 mW of 632.8 nm radiation in a beam which is

1 mm in diameter as it emerges from the laser cavity and has a

divergence of 1 mrad.

WARNING

THE USER OF THIS DEVICE SHOULD CONSULT THE INSTRUCTION

MANUAL PROVIDED WITH THE LASER AND BE THOROUGHLY FAMILIAR

WITH THE SECTION ON LASER SAFETY.

WARNING

The laser and photodetector are mounted on an aluminum beam

which is attached to the ballistic range beam with clamps. Four

rubber shock mounts between the clamps and the beam effectively

isolate the laser and photodetector assembly from mechanical and

electrical shocks which might otherwise be transmitted to the

station. The laser is attached to the beam using two aluminum

angle brackets. Adjustment of the position of these brackets

allows the orientation of the laser to be changed. The

photodetector is housed in an aluminum box which is mounted on

the aluminum beam opposite the laser. A mounting post/ horizontal translation stage/ small base, and vertical translating post

holder permit the photodetector to be accurately centered on the

laser beam.

A Motorola MRD500 PIN diode is employed as the

photodetector. The photodetector assembly contains several
transistor amplifier stages/ a voltage comparator/ and an indicator LED. The photodetector assembly has two BNC output

connectors for analog and pulse signals and two operatoradjustable multi-turn potentiometers for setting the DC and

trigger levels. (These functions are described in detail in the

next section.) An indicating LED turns "on" when the light level

from the HeNe laser falls below a preset value.

A 12 VDC power supply is housed in a separate cabinet. The

input to the power supply is 115 VAC/ 60 Hz. A long cable for

connecting the photodetector assembly to the power supply is

provided.
SECTION 3

SETUP AND MAINTENANCE

MECHANICAL SETUP

The first portion of a laser-photodetector station to be

assembled is the aluminum cross beam that is clamped to the

ballistic range beam. Since range beam dimensions vary/ each

cross beam must be tailored to meet the needs of the particular

installation.

The laser is attached to the cross beam using two aluminum

angles. One of the angles has two slots and is attached vertically to the side of the cross beam using two 1/4-20 socket head

machine screws. The slots in this angle allow the vertical

position of the laser to be adjusted. The second angle mounts

horizontally at the top of the slotted angle using two 1/4-20

socket head machine screws. One of the mounting holes in this

second angle is elongated to allow the laser to be tilted as

necessary. Washers are used on all four of these screws to

insure a tight mechanical coupling. The laser is secured to the

horizontal angle by a 1/4-20 machine screw which passes through

the angle into the bottom of the laser case.

WARNING

THIS SCREW SHOULD NOT PROTRUDE MORE THAN 3/8 INCH INTO

THE BODY OF THE LASER. SERIOUS DAMAGE TO THE LASER

WILL RESULT IF TOO LONG A SCREW IS USED TO ATTACH THE

LASER TO THE ANGLE BRACKET.

WARNING

The orientation of the laser beam with respect to the

trajectory of the projectile can be adjusted by varying the

position of the two aluminum angles. The intersection of the

projectile trajectory and the laser beam can be verified by

placing a piece of white paper in the path of the laser and

bore-sighting the gun. As the paper is moved towards or away
from the laser/ the "spot" on the paper will move across the

field of view of the boresight. The only alignment condition

which the laser beam must satisfy is that is pass through the

trajectory of the projectile.

The photodetector assembly is supported by a 0.5-inchdiameter post which fits into a vertical translating post holder.

The post holder is attached to a slotted base and a horizontal

translation stage. The horizontal translation stage must be

oriented with its1 axis of motion parallel to the projectile

motion. Assembly of all photodetector support components is made

using 1/4-20 socket head cap screws.

IMPORTANT

IN THE FOLLOWING ALIGNMENT PROCEDURES BE SURE THAT THE

LONG AXIS OF THE PHOTODETECTOR ASSEMBLY REMAINS PARALLEL

TO THE LASER BEAM

IMPORTANT

Adjust the photodetector assembly vertically with the coarse

and fine adjustments of the vertical post holder until the aperture at the front of the assembly is in the same horizontal plane

as the laser beam. (A small piece of white paper held in front

of the assembly can be used to locate the beam.) Position the

photodetector assembly horizontally using the horizontal translation stage until the laser beam falls entirely in the aperture of

the photodetector assembly. It may be necessary to reposition

the base relative to the top of the translation stage to achieve

the desired laser/photodetector orientation.

ELECTRONIC AND OPTICAL SETUP

Attach the power lead from the photodetector assembly to the

power supply, plug in the power supply to any convenient 115 VAC

outlet, and turn the power supply on. The photodetector circuitry should be operational at this time. However, it is

advisable to wait about five minutes before making tHe following
adjustments to allow the circuitry to stabilize. The red indicator LED on the photodetector assembly may or may not be

illuminated at this time.

Attach a Volt-Ohm Meter (VOM) with a full scale voltage of

at least 15 VDC to the BNC connector marked "LIGHT" and located

at the rear of the photodetector housing. Use the horizontal and

vertical adjustments of the mount to maximize the voltage at this

connector. The maximum voltage should be between 5 and 12 VDC

with a new laser and the red indicator LED should be off. BE

SURE TO KEEP THE PHOTODETECTOR ASSEMBLY PARALLEL TO THE LASER

BEAM.

The multi-turn potentiometers have been preset at UDRI and

should require only a slight adjustment. However/ as the components age/ the following adjustments may be necessary. In

addition/ it is often useful to monitor the output signal

produced by the photodetector from time to time to ensure that

the maximum signal level is being maintained.

Step 1. Block the laser beam by inserting an opaque object

in front of the photodetector assembly.

Step 2. Attach a VOM with 15 to 20 VDC range to the BNC

connector marked "LIGHT".

Step 3. Adjust the potentiometer marked "BAL" until the

voltage/ with the beam blocked/ is approximately

0.2 VDC. The potentiometer is a 15-turn pot and

an adjustment of several turns may be required/

particularly if the components have aged significantly or there has been a considerable change

in ambient temperature.

Step 4. Remove the "blocker" from the path of the laser

beam and observe the increase in the monitor

signal level. The mechanical alignment of the

photodetector assembly should be adjusted until

the "LIGHT" signal level is maximized. This

maximum signal can be as high as 11 VDC but is
usually lower. A signal level of 3.5 VDC must be

considered to be the lowest acceptable value. If

a signal level of at least 3.5 VDC cannot be

achieved/ there is a serious problem with the

system (see MAINTENANCE section).

Step 5. Momentarily block the laser beam again/ noting the

maximum and the minimum "LIGHT" signal levels.

Step 6. The second potentiometer (marked "LEVEL") can now

be adjusted. This potentiometer is connected

between +12 VDC and ground and is used to adjust

the point at which a decrease in the photodetector

signal will cause a trigger signal to be generated. Since this potentiometer has 15 turns, the

triggering level will change by approximately one

volt per turn (12V -f, 15 turns). Normally, this

potentiometer is adjusted so that an 80 percent

reduction in the magnitude of the light signal

will cause a trigger signal to be generated. If

the minimum "LIGHT" signal is 0.2 VDC and the

maximum "LIGHT" voltage is 8 VDC then an 80 percent reduction in "LIGHT" signal level will

correspond to a setting that produces about

1.6 volts (or approximately 1.6 turns from the end

of the potentiometer). This completes the

electronic setup.

The indicator LED is very useful for functional testing of

the laser-photodetector station before each firing of a gun

system. Normally, this LED is dark. Passing an opaque object,

e.g., pencil, finger, etc., through the laser beam should always

cause the indicator LED to glow. If the LED does not come on

when the beam is blocked, the LEVEL potentiometer should be

readjusted.
MAINTENANCE

The laser-photodetector station requires minimal maintenance

except for regular monitoring of the DC level produced by the

photodetector. Low output signal level can be caused by degradation of range windows or a reduction in laser beam intensity.

Degradation of range windows should be detected during routine

cleaning. A reduction in laser beam intensity can occur

gradually or suddenly. In general/ a 0.5 mW laser cannot be

expected to last longer than about 2 years. If the range alignment of the photodetector assembly cannot increase the diode

signal to an acceptable level/ then the laser must be replaced or

repaired.
SECTION 4

HELPFUL HINTS

There are several sources of noise which may make the interpretation of the photodetector signal difficult: (1) intense

radiation produced by extremely bright propellant gases or plasmas/ (2) momentary changes in the index of refraction of the

medium in the path of the laser beam/ and (3) electrical noise

produced by poorly-shielded equipment being operated in conjunction with the gun or photodetectors.

In some cases/ the optical "notch" filter which is supplied

with the photodetector assembly does not provide adequate protection against the intense light which a gun firing can produce.

However/ a judicious selection of laser photodetector station

location(s) can minimize this difficulty.

The simplest means of reducing the optical signal from

extraneous light sources is to locate the photodetector assembly

further away from the range centerline by mounting it on some

support other than the one provided. The incoherent radiation

from light-emitting gases inside the range will fall off as the

square of the reciprocal of the distance between the

photodetector. This procedure has drawbacks/ however, since the

laser and the photodetector may be attached to separate mounts

and independent mechanical vibrations of the laser and

photodetector may result in unwanted and false signals.

A more effective method of reducing the amount of stray

light is to reduce the field of view of the photodetector.

Instead of allowing light from all angles to reach the

photodetector through a large window on the range/ an extension

tube or similar device with an opening some distance from the

photodetector aperture is positioned so that the angular field of

view of the photodetector is severly restricted. In this way/

only light from gases directly in front of the photodetector can

be "seen" by the photodetector. The diameter of the He-Ne Laser

10
beam is considerably less than 5 mm under nearly all circumstances so a relatively long/ small-bore tube can be used. A

further refinement of this technique can be made if a second/

larger bore tube with a blackened interior surrounds the laser

beam as it enters the range tankage. If the dimensions of this

second tube are chosen properly/ only the interior of the second

tube and the laser beam will be visible to the detector. This

arrangement has been effective/ for example/ when used just

inches away from the muzzle of a high-performance/ 20 mm powder

gun. In some applications it is possible to utilize a more

intense laser to overcome background light or obscuring gases.

Interference due to electrical noise/ however/ can only be

reduced by moving the photodetector assembly away from the gun

range. Because of the very low divergence of the laser beam/ the

photodetector assembly can actually be located a considerable

distance away from the range if an appropriate lens is used to

collect the laser light and direct it into the photodetector.

The photodetector assembly could also be housed in a Faraday cage

since the laser light can pass through a wire mesh with little

loss of intensity.

11