HP 5501A Service manual

OPERATING AND SERVICE MANUAL
5501A
LASER TRANSDUCER (LASER HEAD)
SERIAL PREFIX: 1736A
This manual appliesdirectly to Hewlett-Packard Model5501A
Laser Transducers having serial prefix 1736A.
SERIAL PREFIXES NOTLISTED
For serial prefixes above 1736A, a Manual Change sheet is
included with this manual. For lower serial prefixes, refer to
Section
VI of this manual.
Copyright HEWLETT-PACKARD COMPANY 1975
5301 STEVENS CREEK BLVD., SANTA CLARA, CALIF. 95050
Printed: MAY 1979

SAFETY
This product has been designedandtestedaccordingto InternationalSafetyRequirements.To ensuresafe
operation and to keep the product safe, the information, cautions, and warnings in this manualmust be
heeded. Refer to Section
7 for general safety considerations applicable to this product.
CERTIFICATION
Hewlett-PackardCompany certifies that this product met its publishedspecificationsat the time ofship-
ment from the factory. Hewlett-Packardfurther certifies that its calibration measurements are traceable
tothe UnitedStates NationalBureauofStandards, totheextentallowedbythe Bureau's calibration facility,
and to the calibration facilities of other InternationalStandards Organization members.
WARRANTY
This Hewlett-Packard product is warranted against defects in materialsand workmanship for a period of
90 days from date of shipment, except that in the case of certain components listed in Section 1 of this
manual, this warranty shall be for the specifiedperiod. Duringthe warranty period, HP will, at its option,
either repair or replace products which prove to
be defective.
Warranty serviceof this product will beperformedat Buyer's facility at nochargewithin HP servicetravel
areas. Outside HP service travel areas, warranty service will be performed at Buyer's facility only upon
HP's prior agreement and Buyer shall pay HP's round trip travel expenses. Inall other cases, products
must be returned to a service facility designated by HP.
For products returned to HP for warrantyservice, Buyer shall prepayshipping charges toHP and HP shall
pay shipping charges to returnthe productto Buyer. However, Buyershallpay all shippingcharges, duties,
and taxes for products returned to HP from another country.
LIMITATION OFWARRANTY
The foregoing warranty shall not apply to defects resulting from improper or inadequate maintenance
by Buyer, Buyer-suppliedsoftwareor interfacing, unauthorized modificationor misuse, operationoutside
of the environmental specifications for the product, or improper site preparation or maintenance.
NO OTHER WARRANTY
IS EXPRESSED OR IMPLIED. HP SPECIFICALLY DISCLAIMS THE IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
EXCLUSIVE REMEDIES
THE REMEDIES PROVIDED HEREIN ARE BUYER'S SOLE AND EXCLUSIVE REMEDIES. HP SHALL NOT
BE LIABLE FOR ANY DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES,
WHETHER BASED ONCONTRACT, TORT, OR ANY OTHER LEGAL THEORY.
ASSISTANCE
Product maintenance agreements and other customer assistance agreements are available for Hewlett-
Packardproducts.
For any assistance, contactyour nearest Hewlett-PackardSales andService Office. Addresses areprovided
at the back of this manual.

SAFETY PRECAUTIONS
This is a Safety Class I system. This system has been designed and tested according to IEC
Publication
348, "Safety Requirements for Electronic Measuring Apparatus".
LASER RADIATION
DO NOT STARE INTO BEAM
MAXIMUM OUTPUT:
1 mw
PULSE SPEC: continuous wave
LASER MEDIUM: helium neon
CLASS 11 LASER PRODUCT
BEAM CAN BE SHUT
OFF BY CLOSING
BLACK APERTURE
ALIGNMENT
APERTURE FOR
REDUCED BEAM
-1' CAUTION" - Laser radiation
when open and interlock failed
or defeated. DO NOT STARE
INTO BEAM.
This laser product conformsto Federal Bureau of
Regulations 21 CRF 1040.10.

Although this system has been designed in accordance with international safety standards,
this manual contains information, cautions, and warnings which must
be followed toensure
safe operation and toretainthe system insafe condition. Removalof thecoverand
rub!sequent
service or adjustmentsshould be performedonly by qualified service personnel.
L 1
WARNING
HIGH VOLTAGES ARE GENERATED WITHIN THE LASER HOUSING.
THE COVER OF THE MODEL
5500C LASER IS PROVIDED WITH AN
INTERLOCK TO PREVENT ACCIDENTAL ACCESS TO VOLTAGES.
FOR SAFETY, THERE ARE NO HIGH VOLTAGES ON THE INTER-
CONNECTINGCABLE.
t %
CAUTION
Any adjustment, maintenance, and repair of an opened instrument
under voltage should
be avoided as muchas possibleand, when in-
evitable, should
be carriedoutonly by a skilled personwho is aware
of the hazard involved.
Capacitors inside the instruments may still
be charged even if the
instruments have been disconnected from their source of supply.
Whenever
it is likely that the protection has been impaired, the
system must
be made inoperative and be secured against any un-
intendedoperation.
Useof controls or adjustments, or performanceof procedures other
thanthose specifiedherein may resultinhazardousradiationexposure.

LASER TUBE WARRANTY CLAIM
Shipping Instructions
Carefully wrap the tube inIM-inchthick cotton batting or other soft padding material.
Wrap the above in heavy kraft paper.
Pack wrapped tube inarigidcontainer which is at least 4 incheslargerthanthe tube ineach dimension.
Surround the tube with at least 4 inches of packed excelsior or similar shock absorbing material; be
sure the packing
is tight all around the tube.
FROM: DATE
COMPANY
For additional information, contact:
COMPANY
1. HP INSTRUMENT a. MODEL
b. SERIAL No.
2. TUBE SERIAL No.
3. Is defective tube original? YES NO
4. Date purchased (if available)
5.
Describe nature and/or symptoms of trouble
6.
Describe operating environment (i.e., temperature, humidity, etc.)
7. Remarks

Model 5501A
Table of Contents
TABLE OF CONTENTS
Section
I
Title Page
..................................................
GENERAL INFORMATION 1-1
. 1.1 Introduction ...................................................... 1-1
. 1.3 EquipmentDescription ............................................. 1-1
. 1.7 Identification ...................................................... 1-1
. 1.11 EquipmentSupplied ............................................... 1-1
. 1.13 Available Accessories ............................................... 1-2
. 1.15 Specifications ...................................................... 1-3
INSTALLATIONAND OPERATION
........................................... 2-1
2.1
. Introduction ...................................................... 2-1
2.3 . Unpacking and Inspection .......................................... 2-1
2.5 . Packagingfor Reshipment .......................................... 2-1
2.7
. Storage ........................................................... 2-1
2.9
. Installation ........................................................ 2-1
2.11
. Mounting ...................................................... 2-2
2.13
. Power and Signal Connections ................................... 2-2
2.15 . Cable Fabrication ............................................... 2-3
.........................................................
2.17 . Operation 2-3
2-1 9
. Controls, Connectors, and Indicators .............................. 2-3
2.21
. Power Application .............................................. 2-3
2.25
. Beam Alignment ................................................ 2-7
THEORY OF OPERATION
................................................... 3-1
3.1
. Introduction ...................................................... 3-1
...............................................
3.3 . General Description 3-1
3.15
. General Functional Description ..................................... 3-2
3.17
. Power Distribution ............................................... 3-2
.........................................
3.21 . Laser Current Regulation 3-3
...........................................
3.25 . Automatic PZT Tuning 3-3
...............................
3.34 . Control and Diagnostic Monitoring 3-4
MAINTENANCE
............................................................ 4-1
4.1
. Introduction ...................................................... 4-1
4.3
. Performancecheck ................................................ 4-1
4.6
. Instrument Access ................................................. 4-2
...................................................
.
4.15 Troubleshooting 4-4
4.17 . Adjustment Procedures ............................................. 4-8
4.19
. Lock Reference Board Adjustments ................................ 4-8
4.26
. Laser Tube Current Adjustment ................................... 4-10
REPLACEABLE PARTS
....................................................... 5-1
5.1
. Introduction ...................................................... 5-1
5-4 . Ordering Information .............................................. 5-1
MANUAL CHANGES AND OPTIONS
......................................... 6-1
6.1
. Introduction ...................................................... 6-1
6.3
. Manual Changes ................................................... 6-1
6.5
. New Instruments ................................................ 6-1
6.7
. Older Instruments ............................................... 6-1
6.9
. Options ........................................................... 6-3
........................................................ CIRCUIT DIAGRAMS 7-1
7.1
. Introduction ......................................................... 7-1

Model 5501A
Tables and Figures
TABLES
Table
Figure
Title Page
Available Accessories ......................................................... 1.2
................................................................
Specifications 1.3
Laser Head System Signal Chart
............................................... 2.5
DC UnbalancedFailures
...................................................... 3.2
lnput Voltage ................................................................ 4.1
Voltages at Diagnostic Connector
.............................................. 4.1
Indicator Conditions During Retune
........................................... 4.2
ReferenceSignal
............................................................. 4.2
ModuleAdjustment Requirements
............................................ 4.8
Replaceable Parts
............................................................ 5.3
......................................................
Manufacturers Code List 5.9
BackdatingChanges
.......................................................... 6-1
Power Supply Options
........................................................ 6.2
....................................................
Laser Head Module Listing 7.7
A1 Connector Board Signal List ................................................ 7.8
A2 High Voltage Assembly Signal List
.......................................... 7.12
........................................... A5 Lock Reference Board Signal List 7.14
A6 PZT Power Supply Assembly Signal List
...................................... 7.16
A7 Control BoardAssembly Signal List
......................................... 7.18
FIGURES
Title Page
Model 5501A Laser Transducer ................................................ 1.0
Mounting Details and Physical Characteristics
.................................. 2.2
Typical Transducer System Interconnections
.................................... 2.3
Interconnecting Plugs and Cable Details
....................................... 2.4
Controls, Connectors, and Indicators
.......................................... 2.7
......................................................
RetuneTiming Diagram 3.5
Troubleshooting Flow Chart
................................................... 4.5
Laser Current Loop FailureAnalysis
............................................ 4.6
....................................... Automatic Retune Loop FailureAnalysis 4.7
................................ A5 Lock Reference Board Adjustment Locations 4.9
................................................. Laser Beam Blocking Method 4.9
Laser Current Adjustment Locations
........................................... 4.11
Schematic Diagram Notes
..................................................... 7.2
IC Diagrams ................................................................. 7-3
Laser Head Assembly Locations
................................................ 7.6
.................................................... FunctionalCircuit Diagram 7.7
A1 Connector BoardComponent Locator ...................................... 7.10
A1 Connector BoardSchematic Diagram ....................................... 7.11
A2 H.V. Power Supply Assembly Schematic Diagram
............................ 7.13
A5 Lock Reference Board Component Locator
.................................. 7.15
A5 Lock Reference BoardSchematic Diagram
.................................. 7.15
.............................
A6 PZT Power Supply Assembly Schematic Diagram 7.17
A7 Control Board Component Locator
......................................... 7.19
A7 Control Board Schematic Diagram
.......................................... 7.19

Model 5501A
General l nformation
Figure
7-7. Model 5507A Laser Transducer
1-0

Model5501A
General Information
SECTION I
GENERAL INFORMATION
1-1. INTRODUCTION
1-2. This section of the manual providesadescription of theHP 5501A Laser Transducer, hereaftercalled
the laser head. Informationregardingidentificationof the manual, the laser head, andcircuitboardswithin
the unit, as well as alistof availableaccessories(Table1-I), generalapplicationsinformation, and complete
specifications (Table1-2),
is also included.
1-3. EQUIPMENT DESCRIPTION
1-4. The laser head (Figure1-1) is a high stability laser source that generates acoherent light beam toa
receiverthroughthe system optics. The laser headalsogeneratesan electricalreferencesignalREF which
is
compared with the received electrical signal MEAS (from a 10780A Receiver) to provide displacement
information.
1-5. As an additional feature, the laser headprovidesvisualand electricaldiagnostic outputsthat indicate
its operational status. Remotely activated tuning can be performed via the diagnostic circuits.
1-6. The laser headconsistsof three printed-circuit boards, twosealed modules, alaser tube, andinternal
opticsmountedinaNEMA type 12enclosure. These componentsmaintainoptimum laser beamoperation,
as well as provide continuous laser reference and status information.
1-7. IDENTIFICATION
1-8. This manual is identified on the title pagebyequipment description and nomenclature,manualpart
number, and publicationdata. Refertoinformation presented inthefollowing paragraphsandensurethat
this manual applies to equipment being serviced.
1-9. Hewlett-Packard instruments have a two-section, nine-digit plus one-letter serial number usually
attachedtotheinstrumentrearpanel.
the four-digit prefix (firstfourdigitsformtheleft)identifiesagroupof
instruments. The letter indicates the assembly plant location. The five-digit serial number
is different for
each instrument.Iftheserial prefixof your instrumentdiffersfromthat listedonthetitlepageofthismanual,
there are differences between this manual and your instrument.
1-10. The printed-circuit boards within the instrument are identified by a two-section, 10-digit part
number (e.g., 05501-60001) and a four-digit series number (e.g., "SERIES 1404"). The series number
identifies the electrical characteristicsof the complete printed-circuit assembly.
A replacementcircuit-
board assembly may have a different series number than the assembly originally supplied with the
instrument. Therefore, when troubleshooting acircuit-board assembly, ensure that the series number on
the schematic diagram matches the series number on the board assembly. If the series number on the
assembly
is lowerthanthenumberontheschematicdiagraminSectionVII, refertosectionVI ofthismanual
for change information. If the series number onthe assembly
is higher than the numberontheschematic
diagram, the change information
is provided inamanualchangesheet which is availablefromthe nearest
Hewlett-Packard Sales and Service Office.
1-11. EQUIPMENT SUPPLIED
1-12. Three plugsthat mate with the laser head rear-paneljacks are includedwith the laser head. System
interconnection of the laser head
is accomplished by fabricating suitable cables that terminatewith these
plugs. Refer to Section
II of this manual for cable fabricating instructions.

Model 5501A
General l nformation
1-13. AVAILABLE ACCESSORIES
1-14. Table 1-1 lists electrical, mechanical, and optical accessories that are availablefor usewith thelaser
head. Also included are part numbers for cables that are suitable for interconnecting a laser transducer
system.
Table
7-7. Available Accessories
Description
Coupler
Laser Transducer Interface
Laser Transducer Counter
Laser Transducer Counter with Resolution Extension
Extender Board
HP-IB Interface
Binary Interface
Compensation
l nterface
Manual Compensator
Counter
Receiver
Pulse Converter
Service Kit without Laser Assembly
Laser Assembly only
Numeric Display
33% Beam Splitter
50% Beam Splitter
Linear Interferometer
Retroreflector
Retroreflector
Single Beam Interferometer
Plane Mirror Interferometer
Beam Bender
Cable for RF signals,
4 conductors
(bulk, order required length)
Cable for power,
4 conductors
(bulk, order required length)
Cable for diagnostics, 21 conductors
(bulk, order required length)
Adjustable Mount for 10700A, 10701A,
10705A, and 10707A
Adjustable Mountfor 10702A and 10706A
HP Model
or Part Number
10740A
10741A
10742A
10742A
Opt. H01
10743A
10745A
10746A
10755A
10756A
10760A
10780A
10781A
10782A
10782A
Opt. 001
10783A
10700A
10701A
10702A
10703A
10704A
10705A
10706A
10707A
C05-59995A
C07-59995A
(308-59995A
10710A
10711A

Model 5501A
General Information
1-15. SPECIFICATIONS
1-16. All technical specifications for the laser head are listed inTable 1-2.
Table
1-2. Specifications
LASER:
Helium-Neon Type with Zeeman frequency split beam, zero warm-up time, automati-
cally tuned.
BEAM POWER OUTPUT:
1 milliwatt maximum.
BEAM DIAMETER:
7 mm (0.28 inches)
ELECTRICAL SPECIFICATIONS
UNIT POWER REQUIREMENTS:
$15 volts k 0.25 Vdc 0.6 amp
-15 volts
f 0.25 Vdc 0.5 amp
NOTE
Internal Laser Head 2 amp fuses permit power distribution to
other Laser Transducer units.
SAFETY FEATURES:
1. Front shutter to block Laser Beam.
2. Safety switch that disables high voltage when cover
is removed.
OUTPUTS:
Reference signal: Differential squarewave at approximately 1.8 MHz. Levels compatible
with all Laser Transducer System output accessories.
Diagnostics: Set of TTL level signals.
PHYSICAL SPECIFICATIONS
I
ENCLOSURE:
b!EMA Type 12.
WEIGHT:
3.9 kg (8.5 pounds)

Model 5501A
Installation and Operation
SECTION II
INSTALLATION AND OPERATION
2-1. INTRODUCTION
2-2. This section of the manual contains unpacking, inspection, storage, and shipping information, in
addition to detailed installation and operation instructions.
2-3. UNPACKINGAND INSPECTION
2-4.
Iftheshippingcarton is damaged, inspecttheunitforvisibledamage (scratches,dents, etc.). Iftheunit
is damaged, notify thecarrierand thenearestHewlett-PackardSalesandServiceOfficeimmediately(offices
are listed at the back of this manual). Keep the shipping carton and packing material for the carrier's
inspection.The HPSalesandServiceofficewillarrangeforrepairorreplacementofthe unitwithoutwaiting
for a claim against the carrier to besettled.
2-5. PACKAGING FOR RESHIPMENT
2-6. If it becomes necessaryto reshipthe unit, use goodcommercialpackaging.The same containersand
materialsusedfor factorypackagingcan beobtainedthroughtheHewlett-PackardSalesandServiceoffices
listedat theback of thismanual.Contract packagingcompaniesinmanycitiescanalsoprovidedependable
custompackagingonshort notice. Adhere tothefollowinggeneral instructionswhen repackingtheunit.
a. IfshippingtoaHewlett-PackardServiceOfficeor Center, attachatag indicatingthetypeof service
required, return address, model number and full serial number.
b. Wrap the unit inheavy paper or plastic.
c. Useastrongshippingcontainer.Adouble-wall cartonmadeof350poundtest material
is adequate.
d. Use enough shock-absorbing material (three to four inch layer) around all sides of the unit to
provide a firm cushion and prevent movement inside the container. Protect the front and rear
panels with cardboard.
e. Seal the shipping container securely.
f. Mark the shipping container FRAGILE to assure careful handling.
2-7. STORAGE
2-8. Ifthe instrument is tobestoredforanextendedperiodoftime, itshouldbeenclosedinaclean, sealed
container.
2-9. INSTALLATION
2-10. The followingparagraphsdescribemounting procedures, required power and signalinterconnec-
tions, and fabrication of interconnecting cables.

Model 5501A
Installationand Operation
I MOUNTING TA3 \ I /
MOUNTING TAB
MOUNTING TAB
-14 SCREW (6 MM) MAX
I
I
3.120
BEAM 179.25 MM)
I
A
5.020
-
(127.51 MM)
I
A
REAR
250
-
MM) 16-35 MM)
12.79 ---
(324.87 MM)
FRONT
SIDE VIEW
8-32 MACHINE SCREWS
BOTTOM
Figure 2-1. MountingDetails andphysicalCharacteristics
2-11. Mounting
2-12. Mount the laser head by means of the three slotted mounting tabs that extendoutward from the
bottomedges of the unit(referto Figure2-1for exact locationsanddimensions).Themountingtabs maybe
secured to the work surface in any fashion that will not transmit stresses to the main enclosure and,
consequently, tothe laser tubeor printed-circuit assemblies. If necessary, thetabs can beremovedandthe
unitcan bemountedon
a flat surfacewith 8-32 machinescrews; usethesame tappedholesthat are usedto
secure the tabs.
Do not clamp the unitat any point other than the mountingtabs.
To do so can damage the unit.
2-13. Power and Signal Connections
2-14. The laser head rear panel containsthree multi-pin connectorsthat are used for power and signal
connection ofthe laser headtothe remainingcomponentsof alaser transducersystem. (RefertoFigure2-2
for an interconnection diagramof atypicalsystem.)Table2-1listsallpowerandsignallinesthatareavailable
at the three rear-panel connectors. The laser transducer system manual gives additional system wiring
information.

Model 5501A
l nstallationand Operation
INTERFACE CARDS
Figure 2-2. Typical Transducer System interconnections
I
I
2-15. Cable Fabrication
t15V
COM
-15v
2-16. Powerand signal interconnecting cables of suitablelength must befabricatedusingthe three plugs
included with the laser head. Figure2-3 illustratesthe physicaldetailsof thethree suppliedplugsand gives
part numbers for boththe plugsandthe recommendedcables. RefertoTable2-1for rear-panelconnector
pin numbers of all available power and signal lines.
WWER SUPPLY
2-17. OPERATION
2-18. The following paragraphsdescribethe laser headcontrols, connectors, and indicators, anddescribe
the steps necessaryto apply power tothe laser headandtoverify that the laser beam
is alignedthroughthe
optical componentsof the system.
REF, r15V
-
2-19. Controls, Connectors, and Indicators
2-20. Figure2-4 identifiesanddescribeseach of the operatingcontrols, connectors, and indicators. Refer
to Table 2-1 for information regardingthe specific power and signal linesthat are availableat each of the
connectors.
-
2-21. Power Application
-
'I
L
2-22. The installed laser head unit is activated when +I5 Vdc and -15 Vdc are applied to the unit. After
connecting the POWER plug, measure the voltage at pinsA and
B of thediagnosticsconnector and ensure
that the power source is adjusted to conform to the following requirements.
Measured Voltage
+15V t 0.25 Vdc
-15V
t 0.25 Vdc
-
MEAS X
I1
+15v
-
ll
MEAS Y
+15v
52
-
t
HP 10780A
HP 5501A
I
I
J1
-
- I I u -
HP 10780A
RECEIVER
Y
AXIS
LASER
TRANSDUCER
HP 10780A
RECEIVER
Z
AXIS
RECEIVER
X
AXIS
CALCULATOR
OR
OTHER
CONTROLLER

Model 5501A
Installation and Operation
PlNS END
QUUWG
CABLE-SIDE VlEW
CABLE
END
STRAIN RELIEF
PART NO. 1251-3749
PlNS
END CABLE END
DIAGNOSTICS
HP Part No. 1251-2797
SIDE VlEW
(BOTH PLUGS)
A
- PINS END VIEWS -
B
C
C
HPSpec No.
(308-59995A
or equivalent
REFERENCE SIGNAL
Sixteen #22 wires
I
POWER
I
PLUG CABLE DESCRIPTION
I
Figure 2-3. InterconnectingPlugs andCable Details
REFERENCE SIGNAL
HP Part No. 1251-3447
POWER
HP Part No. 1251-3450
HP Spec No.
C05-59995A
or eauivalent
HPSpec No.
C07-59995A
or equivalent
Dual coax; two #26 twisted pair within inner shield, two #22
power wires. Everythingenclosed
by a second insulated shield.
Three #20 wires

Model 5501A
Installationand Operation
Table
2-7. Laser HeadSystem Signal Chart
Source
Ext. Pwr. Sup.
Ext. Pwr. Sup.
Ext. Pwr. Sup.
Accessory
Equipment
Function
Accessory +15V Operating Power
Accessory +15V Return
System Reference Measurement
signal, equal to the difference in
frequency between the Laser f, and
f,
components.
External +15V power input
External -15V power input
+5V power test point
Power Ground
+15V sample for diagnostics
-15V sample for diagnostics
+5V sample for diagnostics
System Common return
Active low signal; external input to
initiate PZT Tune/Check cycle
Active high diagnosticsignal indicating
failure of the PZT Tuning/Check cycle
Active high diagnosticsignal indicating
PZT Tuning/Check cycle
is in process
Active high diagnosticsignal indicating
Laser Tube current
is not within
minimum and maximum limits
Active high diagnosticsignal indicating
any or all of the following conditions:
1. Laser Tube current out of
specifications
2. PZT voltage out of specifications
3. Retune/Check cycle in process
Laser current sample for diagnostics
PZT voltage sample for diagnostics
Active low diagnosticsignal
indicating Laser
is properly tuned
--
Destination
Accessory
Equipment
Accessory
Equipment
Accessory
Equipment
Accessory
Equipment
(testuse only)
Accessory
Equipment
Accessory
Equipment
Accessory
Equipment
Accessory
Equipment
Accessory
Equipment
Accessory
Equipment
Accessory
Equipment
Accessory
Equipment
Accessory
Equipment
Accessory
Equipment
Accessory
Equipment
Signal Name
Fused +15V
Fused +15V
RET
!El
REF
+I 5V
-15V
+5V
GND
+I5 TEST
-15 TEST
+5 TEST
SYS COM
RETUNE
cMD
RETUNE
FAILURE
RETUNE
STATUS
LASER
CURRENT
ERROR
ERROR
L
I MON
TEST
PZT MON
TEST
REF
OK
STATUS
Input
12-A
12-8
J2-D
J3-E
Output
Jl-A
)I-B
J1-C
Jl-D
12-C
J3-A
J3-B
J3-C
13-D
13-F
J 3-1
13-K
J3-L
J 3-M
13-N
J3-P

Model 5501A
1 nstallationand Operation
b
1
2
3
4
10
11
1. +I5 UNBAL Lights when +I5 Vdc power is high by approximately one volt or more; or when
(LED Lamp) the -15 Vdc power input
is low (i.e., less negative) by one volt or more.
2. -15 UNBAL Lights when -15 Vdc power
is high (i.e., more negative) by approximately one
(LED Lamp) one volts or more; or when the +I5 Vdc power input
is low (i.e., less positive)by
one volt or more.
3. +I5 FUSE Lightswhen +I5 Vdc fuse blows.
(LED Lamp)
4. -15 FUSE Lights when -15 Vdc fuse blows.
(LED Lamp)
5. POWER ON Lights when external power has been applied to Laser Head.
(LED Lamp)
6. LASER CURRENT Lights to indicate Laser Tube current
is not within appropriate operating limits.
(LED Lamp)
7. RETUNE FAILURE Lights to indicate the Laser Tube is not properly tuned or that retune cycle did not
(LED Lamp) retune the laser.
8. RETUNE Lights to indicate that retuning is required or that retune cycle is in progress.
(LED Lamp) Light should go out approximately
9 seconds after the RETUNE pushbutton is
depressed. If light remainson, a tuning fault is indicated.
9. RETUNE Manually activates Retunesequence.
(Pushbutton)
10. REFERENCE SIGNAL Allows the reference signals (REF and REF)and +15V power to be interconnected
(Connector,
JI) to other system components.
11. POWER Providesconnection point for external power supply input.
(Connector, 12)
12. DIAGNOSTIC Allows monitoring of diagnostic signals and application of RETUNE command
(Connector,
J3) by external equipment.
Figure
2-4. Controls, Connectors, and Indicators
2-6

Model 5501A
Installationand Operation
2-23. Ifadelay exists betweenapplication of the+15 Vdc and-15 Vdc power, it
is preferabletoapply +15
Vdc after the -15 Vdc. This providesautomatic retuneof the laser headuponpower turn-on. If+15 Vdc is
appliedfirst, the retunecycle is initiatedbyeither manuallydepressingtheRETUNEswitch(atthe rearof the
laser head) or applying a logic low RETUNE CMD signal to pin
E of the DIAGNOSTIC connector.
2-24. After power
is applied, the POWER ONand RETUNE lampsat the rearof the laser headwill remain
on. After
9 seconds the RETUNE lamp goes out and only the POWER ON lamp remains lit.
2-25. Beam Alignment
2-26. Rotatetheturretat thefront of thelaser headso that the beam exitsthroughthe largeaperture.DO
NOT STARE DIRECTLY INTO THE BEAM APERTURE. Presence of the red laser beam can be verified by
placingapieceof paperinthe beampathandobservingthebeamreflectiononthepaper.Thecomponents
of the laser transducer system must now be aligned so that the laser beamstrikesthephotodetector of the
receiver (HP Model 10780A). Refer to the documentation supplied with the receiver and to the Laser
Transducer System Manual for further alignment information.

Model 5501A
Theory of Operation
SECTION Ill
THEORY OF OPERATION
3-1. INTRODUCTION
3-2. This sectionprovidesthe theory of operation for the laser head. The first part presentsgeneral laser
theory as it applies to the laser head. Subsequent paragraphs described detailed laser head operation;
functional analysis of the laser head
is included.
3-3. GENERAL DESCRIPTION
3-4. The laser head transmits acoherent light beam(all light waves are inphase)that is used by the laser
transducer system to generatedisplacementmeasurementsignals. Inaddition tothisbeam, the laser head
generatesan electricalreference(REF)signal, and acceptsand producesinterfaceanddiagnosticsignalsfor
accessoryequipment. The laser headaccepts +15 Vdc and-15 Vdcoperating powerfromanexternalsource
and distributesfused +I5 Vdc and -15 Vdc to other units of the transducer system.
3-5. The laser headbasicallyconsistsof alaser tube assembly, regulatorcircuitsthat ensureoptimum laser
operation, and diagnostic circuits.
3-6. The laser tube contains a Helium-Neon gas that
is excited when high voltage is supplied. A laser
current control circuit maintains the appropriate laser tube current by monitoringcathodecurrent and
adjustingthe high voltage accordingly.
3-7. The laser tubeconsistsof the anode, cathode, mirrors, aspring, anda
piezoelectrictransducer.These
elements are enclosed inthe Helium-Neon environment. As aresult of the exictation, light energy inthe
form of photons are spontaneously emitted by the excited Neon atoms. These photons, traveling
approximatelyat the speed of light, are reflected by the mirrors and collidewith Neon atoms that are ina
metastablestate. This collision resultsinthe stimulatedemissionof severalphotonsbytheNeonatoms.This
event occurs repeatedly and
is responsiblefor the laser phenomenon; LightAmplification by Stimulated
Emissionof Radiation. Furtherphotoncollisionscause increasedcoherent emission. Thesechainreactions,
ultimatelycreatean in-phase, or coherent lightenergy levelwhich
is sufficienttogenerateabeamthrough
the laser tube aperture.
3-8. The laser frequency
is determined by the transition between energy levelsof the Neonatoms. The
distancebetweenmirrorsestablishesacavitylengthwhichisadjustedtosupportlongitudinaloscillationsat
a
wavelength of 6328 Angstroms (5x 1014 Hz). This wavelength lies in the red region of the visible light
spectrum.
3-9. A small amount of resonant cavity length tuning is provided by the piezoelectric transducer (PZT)
which is infront of the rear mirror. A springbehind the mirror forces itagainst the PZT. The PZT has the
property of expanding to a thickness which
is proportional to the amount of positivedc voltage applied
through
a stem connection at the rear of the tube. The expandingPZT pushesthe mirror tothe rearof the
tube, thereby creating a longer resonant cavity. The longer cavity sustains oscillations at a slightly lower
frequency. Therefore, the laser tube respondstoa more positivePZT voltageby tuning to aslightlylower
frequency. Conversely, the tube responds to a less positive PZT voltage by tuning to a higher laser
frequency. This PZT control potential ranges from +270V to +1800V.
3-10. A magnetthat surroundsthe laser tubecausesZeemansplittingof itsfrequencysymmetrically about
f,, the normal laser centerfrequency.This resultsintwocircularly polarizedfrequencycomponentsexisting
inthesame beam.Onecomponent
is left-handcircularlypolarized(LHCP)and is approximately 1 MHzfrom
the center operating frequency of the tube (f,). The other beam frequency component
is right-hand
circularly polarized (RHCP)and
is approximately 1 MHz from f,, inthe other direction.
3-11. The laser beam, containing the two circularly polarized frequency components (f, and f,), passes
through aA/4 plate
(A =wavelength). Thiscauses thef, andf,components tobecomelinearlypolarizedand
mutuallyperpendicular, ororthogonal. Thesefrequencycomponentsthenpassthrougha
A/2 platewhich is
factory-adjusted to compensatefor the imperfect orthogonal positioningof the f, and f, signals.

Model 5501A
Theory of Operation
3-12. The laser beam, containing the vertically polarized f, (thelower of the two frequencies) and the
horizontally polarized f, components, passes through a collimating telescope. This device consists of a
compound lens which spreads the extremely narrow laser beam into a parallel 0.28 inch
(7 mm) output
beam.
3-13. A devicecalledabeamsplitter divertsasmall portionof the output beamand routesthis sampletoa
polarizing beam splitter. This splitter partiallyseparatesand applies the f, and f, signals to the PZTcontrol
circuit.The PZTcontrolcircuit comparesthesignallevelof thef, andf,samples. Ifthelevelsofthesesamples
are not equal, an appropriate (dc)
PZT control voltage is generated to tune the laser tube and cause
equalizationof the f, and f,components. Inadditionto
providingthisautomatictuningcontrol,thecontrol
circuitsextractthe difference frequency betweenf, andf, andgenerateanelectricalreferencemeasurment
signal for use by the transducer accessory equipment.
3-14. Additional circuitswithinthe laser head monitor PZT voltage, laser current, andperformanceof the
automatictuning circuits, as well
as external operating power inputs. These circuits drive fault lightsand
provide diagnosticsignalsto accessory equipment. A retunecommandsignal,from accessoryequipmentor
manually activated at the laser head, interruptstheautomaticPZT tuning and forces the laser totune tothe
centerof its mechanicalrangefor
3 seconds. Control is then returnedtothe automaticcontrolcircuits.This
retune capability is provided to bringthe laser operating frequency within the range of automaticcontrol
when, due to extreme environmental changes, the laser attempts to tune beyond the automatictuning
range.
3-15. GENERAL FUNCTIONAL DESCRIPTION
3-16. The following text provides a detailed functional analysis of laser head operation. All descriptions
relate to Figure
7-4 unless otherwise specified. Operation of the laser head can be grouped into the
following functions:
Power Distribution
Laser Current Regulation
Automatic (PZT)Tuning
Control and Diagnostic Monitoring
3-17. Power Distribution
3-18. When +15 Vdc and -15 Vdc are applied to the laser head, the POWER ON light-emitting-diode
indicator (on the A1 Connector Board)
is forward biased and illuminates. The dc input potentials also
produce avirtual groundlevelat thejunction of two15Kohmresistors.This balancedconditionresultsinan
open circuit at both outputs of the comparator switch. An unbalancedcondition exists when either the
positive or negative dc input deviates from 15 volts by approximately
1 volt or more.This offset causes a
correspondingcomparatorswitchoutput to go toground, resultinginalit UNBAL indicator.Table 3-1lists
the unbalanced dc voltage conditions and the resulting unbalancedindications.
Table
3-7. DC UnbalancedFailures
3-19. Ifeither the+I5 Vdcor -15 Vdc two-amp fuseopens, current
is diverted through the associatedLED
indicator, turning that fuse indicator on.
DC Input Voltage
+I5 Vdc
-15 Vdc
Unbalanced Condition
(See Note)
High (more positive)
Low (less positive)
High (more negative)
Low (less negative)
Unbalanced Indicators
+15V UNBAL -15V UNBAL
ON OFF
OFF ON
OFF ON
ON OFF