THORLABS Vytran GPX4000LZ User manual

GPX4000LZ
Glass Processor
User Guide

GPX4000LZ Glass Processor
Table of Contents
Chapter 1 Warning Symbol Definitions ............................................................................................. 1
Chapter 2 Safety................................................................................................................................... 2
Chapter 3 Description ......................................................................................................................... 4
3.1. Introduction................................................................................................................... 4
3.2. Features ........................................................................................................................ 4
3.2.1 GPX4000LZ Standard Features ...........................................................................................4
3.2.2 GPX4000LZ Optional Features.............................................................................................4
3.3. GPX4000LZ Accessories.............................................................................................. 5
3.4. System Overview.......................................................................................................... 5
3.4.1 External Workstation Connections ........................................................................................6
3.4.2 Fiber Holding Blocks .............................................................................................................7
3.4.3 End Cap Holders...................................................................................................................8
3.4.4 Splice Head ...........................................................................................................................9
3.4.5 Mirror Tower ........................................................................................................................12
3.4.6 Imaging System ..................................................................................................................13
Chapter 4 Initial Installation, Setup, Introduction and Training ................................................... 14
Chapter 5 Operation .......................................................................................................................... 15
5.1. System Power ............................................................................................................. 15
5.2. Control Panel .............................................................................................................. 15
5.3. Back Panel .................................................................................................................. 16
5.4. Software Interface....................................................................................................... 16
5.5. Graphical User Interface ............................................................................................ 16
5.5.1 Main Menu...........................................................................................................................18
5.5.2 Command Bar .....................................................................................................................26
5.5.3 Camera Bar.........................................................................................................................27
5.5.4 Process Bar.........................................................................................................................28
5.5.5 Macro Bar............................................................................................................................28
5.5.6 Quick Open File Bar............................................................................................................29
5.5.7 Main Toolbar .......................................................................................................................29
5.5.8 Splice Process List..............................................................................................................31
5.5.9 Camera Image ....................................................................................................................31
5.5.10 Movement Control Bar ........................................................................................................32
5.5.11 Status Bar............................................................................................................................34
5.6. Initialization and Shutdown........................................................................................ 35
5.7. Storage and Transportation....................................................................................... 36
5.8. System Configuration................................................................................................. 38
5.8.1 Furnace Configuration.........................................................................................................38
5.8.2 Objective Lens configuration...............................................................................................38
5.9. Operating the GPX4000LZ.......................................................................................... 38
5.9.1 CO
2
Splice Head Configuration...........................................................................................38
5.9.2 CO
2
End-Cap Head Configuration ......................................................................................39
5.9.3 Filament Head Configuration ..............................................................................................39
5.10. Steps of a Basic Splice with One Button Process.................................................... 40

GPX4000LZ Glass Processor
5.10.1 Basic One Button Splice .....................................................................................................40
5.10.2 Modifying the Splice Routine...............................................................................................41
5.11. Measurement Tools .................................................................................................... 42
5.11.1 Measurement Guide............................................................................................................42
5.11.2 End View Quality Tool.........................................................................................................43
5.11.3 Click-Measure Tool .............................................................................................................43
5.12. Hot View Imaging........................................................................................................ 45
5.13. Laser Controls ............................................................................................................ 46
5.13.1 Laser On and Laser Warm-up ............................................................................................46
5.14. Additional Process Notes .......................................................................................... 46
5.14.1 Manual Alignment ...............................................................................................................46
5.14.2 Edge Alignment...................................................................................................................46
5.14.3 Active XY Alignment............................................................................................................46
5.14.4 Core Alignment ...................................................................................................................46
5.14.5 End View Alignment (or PM alignment) ..............................................................................46
5.14.6 Active Rotation Alignment ...................................................................................................47
5.14.7 Multiple-Stage and Multiple File Splicing ............................................................................48
5.14.8 Drawing Tapers...................................................................................................................49
5.14.9 Taper Properties .................................................................................................................49
5.14.10 Tension Monitor..............................................................................................................52
5.14.11 Thermal Core Expansion................................................................................................53
5.14.12 Fiber Lensing..................................................................................................................53
5.14.13 Mode Adaptors ...............................................................................................................53
5.14.14 Taper Tension Process ..................................................................................................54
5.14.15 Pre-Tension Process ......................................................................................................54
5.14.16 Tension Hold Process ....................................................................................................54
Chapter 6 Maintenance ..................................................................................................................... 55
6.1. General Care ............................................................................................................... 55
6.2. Keep the System Clean .............................................................................................. 55
6.3. Fiber Handler Maintenance ........................................................................................ 55
6.3.1 Changing Transfer Inserts:..................................................................................................55
6.3.2 Cleaning the Fiber Holding Block Insert..............................................................................56
6.3.3 Adjusting the Transfer Insert Block .....................................................................................56
6.4. Furnace Assembly Replacement Procedure ............................................................ 56
6.4.1 Changing a Filament Furnace.............................................................................................56
6.4.2 Changing a Laser Furnace..................................................................................................58
6.5. Changing the Lens Objective..................................................................................... 59
6.6. Cleaning the Mirrors................................................................................................... 60
6.6.1 Procedure............................................................................................................................60
Chapter 7 Appendix ........................................................................................................................... 61
7.1. Top Inserts .................................................................................................................. 61
7.2. Bottom Inserts ............................................................................................................ 62
7.3. Fiber Holder Insert Size Selection Guide .................................................................. 63
Chapter 8 Specifications ................................................................................................................... 64
Chapter 9 Regulatory ........................................................................................................................ 67
Chapter 10 Thorlabs Worldwide Contacts ........................................................................................ 68

GPX4000LZ Glass Processor Chapter 1: Warning Symbol Definitions
Rev I, September 15, 2021 Page 1
Chapter 1 Warning Symbol Definitions
Below is a list of warning symbols you may encounter in this manual or on your device.
Symbol Description
Direct Current
Alternating Current
Both Direct and Alternating Current
Earth Ground Terminal
Protective Conductor Terminal
Frame or Chassis Terminal
Equipotentiality
On (Supply)
Off (Supply)
In Position of a Bi-Stable Push Control
Out Position of a Bi-Stable Push Control
Caution: Risk of Electric Shock
Caution: Hot Surface
Caution: Risk of Danger
Warning: Laser Radiation
Caution: Spinning Blades May Cause Harm

GPX4000LZ Glass Processor Chapter 2: Safety
Page 2 TTN102049-D02
Chapter 2 Safety
WARNING
The filament in the glass processor furnace generates intense light when it is in operation. It is
strongly advised that the user does NOT look directly at it during splicing or tapering operations.
CAUTION: HOT SURFACE
The furnace gets extremely hot when the filament is turned on. Do not touch the furnace during or
immediately after the filament has been on for a few seconds, such as after a splice or tapering
operation.
WARNING
The fiber handlers and the splice head can exert a fair amount of force as they move from side to side.
Do not put your fingers between them, or between the fiber handlers and the slot end walls, while the
machine is in operation .
DANGER
The camera tower on the glass processors can generate a fair amount of force as it moves forward
and backward, and up and down. Do not put fingers between it and the furnace or mirror tower while
the machine is in operation.
CAUTION
Be sure your electrical power and gas supplies meet the criteria detailed in the Chapter 8.
DANGER
System contains a CO
2
laser. Do not override the safety mechanism if not trained how to do so
safely. In the unusual event that panels or the top plate need to be removed, remove the key from the
front of the unit and follow the lockout-tagout procedure to ensure that the laser cannot turn on.
WARNING
Turn off the main power using the rocker switch on the side before conducting any maintenance
tasks.
!
!
!
!
!
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GPX4000LZ Glass Processor Chapter 2: Safety
Rev I, September 15, 2021 Page 3
WARNING
Do not stand or climb upon the unit, or on shelves attached to the unit.
WARNING
After moving the unit into position, lock two of the wheels by pressing down on the black levers.
Before attempting to move the unit, unlock the wheels by pulling up on the black levers.
!
!
!
!

GPX4000LZ Glass Processor Chapter 3: Description
Page 4 TTN102049-D02
Chapter 3 Description
3.1. Introduction
Thank you for purchasing a Vytran
®
GPX4000LZ Glass Processor. This manual provides the basic instructions
necessary for operating the glass processor.
The Vytran GPX4000LZ Glass Processor is a versatile glass processing platform for the fabrication of splices,
tapers, terminations, couplers, and combiners. This systems incorporates a CO
2
laser and an optical furnace head
assembly that provides a stable high temperature heat source for precise control of glass processing conditions
while also maintaining the capability of using a filament heat source. An embedded real-time control system and
powerful machine-level macro programming language allow the user to develop unique event-driven routines for
fast and flexible process development. All high level system communication is through a user-friendly, PC-based
graphical interface that provides for easy operation and convenient data storage. This level of control combined
with the consistency of CO
2
laser delivery (or filament fusion technology) make the glass processor a multipurpose
platform for the production of fiber lasers, components, and many other specialty fiber applications.
The Vytran GPX4000LZ Glass Processor is designed to perform high-quality fusion splicing and tapering of
standard and specialty fibers. The system consists of a heating source (CO
2
laser or filament), precision stages
with multi-axis control, microscopic high-resolution CCD imaging system and a personal computer. The fusion area
can heat optical fiber to temperatures up to 3000 °C, which makes it possible to fuse and process various fiber
types and sizes. The multi-axis stages precisely control and aid the alignment and positioning of the fibers for high
quality splicing, end-capping and tapering. The imaging system displays magnified fiber images with sub-micron
resolution. The fiber can be viewed both from fiber side and fiber end. All Vytran glass processors have the ability
to rotate fibers; making it possible to align PM fibers, eccentric core fibers or non-circular fibers.
The Vytran GPX4000LZ Processor is the ultimate combination of a CO
2
Laser system and a filment fusion glass
processing system mounted and interfaced together within a cart, including all the required system components.
3.2. Features
3.2.1 GPX4000LZ Standard Features
•Automated Splice Heads with CO
2
or Filament Heat Source
•Fiber Splicing and Tapering
•Dissimilar Fiber Size Splicing and End Capping
•True XY Alignment with Rotary Capability
•Camera Tower with Illuminator
•Tension Monitoring and Control System
•Integrated into Wheeled Cart with One Side Shelf
3.2.2 GPX4000LZ Optional Features
•Liquid Cooling System (Item # GPXLZWCS)
•External Illuminators that Plug into the Top Plate
•Additional Side Shelf
•Component Arm with Shelf

GPX4000LZ Glass Processor Chapter 3: Description
Rev I, September 15, 2021 Page 5
3.3. GPX4000LZ Accessories
Component Description
Cart Assembly Housing for Electronics, PC, and CO
2
system
CO
2
Optical Furnace Heads Laser Splice Head and Laser End-Cap Head
Workstation Glass Processor Workstation with Fiber Holding Blocks and Furnace
PC Computer, including Monitor, Keyboard and Mouse
Vacuum Pump Vacuum Pump for Vacuum V-Grooves
(Alternative Model Supplied for 230 V Markets)
Power Supply Built into Electronics Enclosure
Gas Tank Regulator Regulator for Argon Tank (European DIN 477 Number 6 Also Included )
An Adaptor May be Required in other Areas or Countries
Gas Line 1/8" PTFE Line for Argon Gas
RS-232 Communication Cable 9-pin D-sub Cable for RS-232 Communication
Camera Cable 6 pin IEEE-1394 Fire Wire Cable
Tool Kit Hex Wrenches, Optical Targets and Inserts Replacement
Fiber Holding Block Inserts Top and Bottom Inserts for Fiber Holding Blocks
Filament Assemblies
(For Filament Fusion Only) Set of Filament Assemblies
CO
2
Laser Dump CO
2
Laser Dump for Laser Warmup
3.4. System Overview
The Vytran GPX4000LZ has three basic sub-components, integrated into one system: 1. a splicing and glass
processing workstation, 2. a computer (PC) for interface and control and 3. a CO
2
laser with optics to guide and
deliver the CO
2
beam to the furnace. The GPX4000LZ workstation is illustrated in Figure 1.
Figure 1 GPX4000LZ Workstation

GPX4000LZ Glass Processor Chapter 3: Description
Page 6 TTN102049-D02
3.4.1 External Workstation Connections
All the system interconnections are invisible to the user and contained within the cart and its electronics enclosure.
The user will only have to connect the power cable coming out of the cart and the network cable (if desired) as
shown in Figure 2. The Argon line will need to be connected to the Argon tank when using the filament heating
method.
Power Cable Network Cable
Figure 2 GPX4000LZ Back Panel Connections
Power
The glass processing systems are powered from a dual voltage supply enclosed in the cart. The GPX4000LZ can
accommodate line cords of all types (for different regions of the world). The appropriate power cord must be selected
at time of purchase. The system connects directly to any 110 – 220 V wall outlet.
Communications
The GPX4000LZ has an Ethernet connection and two analog outputs.
•Ethernet: A 15’ cable allows the system to be connected to the network.
•Analog Output: 2 BNC connectors permit the operator to use active feedback for alignment.
Gas (Argon)
For filament splicing, there is a 1/8" quick-connect bulkhead for argon gas.
Cooling Liquid
A liquid cooling system will be installed by the Service Engineer if that option is selected.

GPX4000LZ Glass Processor Chapter 3: Description
Rev I, September 15, 2021 Page 7
3.4.2 Fiber Holding Blocks
Mechanical handlers or Fibers Holding Blocks (FHBs) are used for holding, positioning and moving the fibers during
the various processes performed on the glass processor workstation. The GPX4000LZ FHB can align fibers prior
to splicing. True XY alignment can be performed using either the outer edge of the material being processed, or the
core of a fiber. The GPX4000LZ FHB can also perform a rotary alignment. Rotary alignment is required for aligning
the stress members of PM fibers, for aligning the cores of eccentric core fiber, and for aligning surface features of
non-circular fiber or multicore fibers.
GPX4000LZ Fiber Holding Blocks
The GPX4000LZ FHBs are capable of rotating each fiber up to 200°. They can also move up and down and front
and back for fiber centering.
Figure 3 Rotary Fiber Holding Block
Fiber Holding Block (FHB) Inserts
Multiple sizes and style bottom V-grooves and top U inserts are available to accept all of the standard coating fiber
diameters and materials currently being offered. For available insert sizes, please see the selection chart in Chapter
7. In addition to the standard insert sizes, custom inserts can be designed and fabricated. Contact Thorlabs for
more information.
Figure 4 Bottom (Left) and Top (Right) Fiber Holding Block Inserts

GPX4000LZ Glass Processor Chapter 3: Description
Page 8 TTN102049-D02
Fiber Holding Block (FHB) Transfer Inserts
The FHB transfer insert used on the glass processors make splicing faster and easier because it facilitates
repeatable fiber positioning when used in conjunction with a LDC400 Large-Diameter Fiber Cleaver. For example,
if the operator uses transfer inserts to hold fibers during cleaving, the operator can then transfer each of those
cleaved fibers to a glass processor in the same insert that held them during cleaving and ensure that the ends of
the fibers will be exactly at the same location every time. This is due to the fact that the transfer inserts are designed
to repeatedly mechanically seat (or mate) on both the cleaver and the glass processor, and a magnetic lid on each
transfer insert prevents axial movement of the fiber during transport from one piece of equipment to the other.
Figure 5 Transfer Inserts for Fiber Handling
3.4.3 End Cap Holders
There are several different devices available for holding end caps in the GPX4000LZ. For end caps with outer
diameters of 1.0 mm, 1.5 mm or 2.0 mm, Thorlabs offers steel tubes that can hold the end caps in place using
vacuum suction. For end caps with outer diameters of 4.0 mm, 5.0 mm or 8.0 mm, Vytran offers holders with flexure
clamps that can grab end caps as the clamp screw is tightened. Finally, for end caps with outer diameters between
7.6 mm and 9.5 mm, the ECH8L holder uses a magnetic lid to capture the end cap. An adjustable setscrew in the
lid makes it possible to clamp on end caps with varying outer diameters within the specified range.
Item # Type Accepted Length
Accepted Diameter
Min.
Typ.
Max
ECH1V
a
Vacuum 2.5 mm – 5.0 mm
0.95 mm 1.0 mm 1.05 mm
ECH15V
a
1.45 mm 1.5 mm 1.55 mm
ECH2V
a
1.95 mm 2.0 mm 2.05 mm
ECH4C
Flexure Clamp 2.5 mm - 10 mm
3.8 mm 4.0 mm 4.08 mm
ECH5C
4.8 mm 5.0 mm 5.08 mm
ECH8C
7.8 mm 8.0 mm 8.08 mm
ECH8L
Magnetic Lid 2.5 mm - 10 mm 7.6 mm 8.0 mm 9.50 mm
NOTE
The ECH1V, ECH15V and ECH2V End-Cap Holders use the same vacuum pump when plugged into the
back panel socket as the bottom fiber holding block inserts. Arrange your work flow so that no empty
holders are connected to the vacuum pump during alignment.
a
This holder requires both a VHA20 top insert and a VHE20 bottom insert to secure the holder within the fiber
holding block.

GPX4000LZ Glass Processor Chapter 3: Description
Rev I, September 15, 2021 Page 9
Figure 6 ECH8C Flexure Clamp Holding An 8 mm End Cap
3.4.4 Splice Head
The Splice Head consists of two parts: the Furnace Tower and the Mirror Tower. The splice head moves along the
Z-axis to allow for viewing and heating fibers.
Figure 7 Splice Head (Optical End-Cap Head Shown)
Furnace Tower
The Furnace Tower can be fitted with the Optical Splice Head, the Optical End-Cap Head or the Filament assembly.
After the splice head moves to the correct position, the Furnace Tower raises in the Y-axis to a predefined height
to match the fiber line. The Furnace assemblies mechanically mate to the top of the furnace tower and are held in
place with screws. When using the filament furnace assembly, additional clamps are used to secure the filament
and limit the influx of room air into the splicing area.
To change a furnace assembly, please see Section 6.4.1 - Changing a Filament .
Mirror tower
Furnace tower

GPX4000LZ Glass Processor Chapter 3: Description
Page 10 TTN102049-D02
Optical Splice Head
The Optical Splice Head is the assembly used for splicing or tapering with the CO
2
Laser system. It consists of a
horseshoe-shaped mirror to direct the incoming CO
2
donut-shaped beam to a cone-shaped mirror which focuses
the beam on the fibers to be spliced.
Optical End-Cap Head
The Optical End-cap Head is the assembly used for producing end caps with the CO
2
Laser system. It consists of
a horseshoe mirror to direct the incoming CO
2
donut shaped beam to the cone mirror which focuses the beam onto
the face of the large fiber/rod (end-cap) to be spliced.
Figure 8 Optical Heads
Figure 9 Laser Light Impinging on Large Fiber Prior to Splicing
Optical Splice Head Optical End-Cap Head

GPX4000LZ Glass Processor Chapter 3: Description
Rev I, September 15, 2021 Page 11
Filaments
The filaments are omega-shaped resistive heaters. Thorlabs offers standard filaments in a variety of sizes as well
as different materials optimized for different applications.
Graphite filaments are capable of achieving the high temperature required for splicing large-diameter silica fibers.
Graphite uses less argon and outgases very little compared to the other filament materials (Tungsten or Iridium).
Iridium filaments are best for low melting point or "soft glass" fibers and fiber couplers applications.
Figure 10 Splice Head Outfitted with Filament Assembly
Proper filament selection and splice parameters are essential for optimum splice performance. For filament
selection guidelines, see the table on the next page. In addition to the standard sizes, custom filaments can be
designed and fabricated. Contact Vytran for more information.
Figure 11 Filament Heater Element
Filament Assembly
Thorlabs supplies standard filaments pre-assembled in a filament body as shown in Figure 12. This is called a
filament assembly. Different filament bodies are used with different filaments, and are distinguished by a version
number engraved on the front of the housing.
Figure 12 V and T Filament Body Versions
Filament

GPX4000LZ Glass Processor Chapter 3: Description
Page 12 TTN102049-D02
The following table lists the standard graphite filament types as a function of the fiber size and application.
Item # Filament Material Application Cladding Diameter (Min/Max)
FTAV2
Graphite
Splice
80 µm / 250 µm
FTAV4 125 µm / 600 µm
FTAV5 250 µm / 1000 µm
FTAV6 400 µm / 1300 µm
FTAT3 Tapering 250 µm / 1500 µm
FTAT4 400 µm / 1800 µm
FRAV1
Iridium Splice
≤200 µm
FRAV3 ≤400 µm
FRAV5 250 µm / 1050 µm
3.4.5 Mirror Tower
The mirror tower houses three mirrors and an LED. The first set of two mirrors makes it possible to view the sides
of the fiber. They provide the "Front View" and "Back View" when using the 5X and 10X objective. A second larger
Side mirror allows viewing of the "Side View" when using the 2X objective. The other set of mirrors formed as a
wedge are used to view the ends of the fibers. The LEDs provide backlighting when viewing the fiber directly in
"Center View" for 5X and 10X and, "Top View" and "Side View" with 2X objectives. The Mirror Tower moves in the
X-axis independently, providing the ability to view both the side view and end view of the fibers. The Mirror Tower
also moves with the furnace assembly in the Z-axis to allow for the 2X position viewing.
Figure 13 Mirror Tower
Cent
er
-
View LED
End
-
View Mirrors
Front
-
View Mirror
Back
-
View Mirror
Side
-
View Mirror for 2X
Top
-
View LED
for 2X
Side
-
View LED
for 2X

GPX4000LZ Glass Processor Chapter 3: Description
Rev I, September 15, 2021 Page 13
3.4.6 Imaging System
The GPZ4000LZ Glass Processor is equipped with an automated camera assembly used for imaging the fibers.
This tower is capable of moving forwards and backwards as well as up and down. It is equipped with a ring
illuminator that provides backlighting for the front and back views of the fiber when using the 5X and 10X objectives
and is not used when using the 2X objective. The camera objective is swapped out to change the magnification
and/or field of view. The ring illuminator mounts on the 5X or 10X objective and connects to the underside of the
camera tower to provide current to the built-in LEDs.
Figure 14 Imaging Camera Assembly
Camera Objective
Ring Illuminator
Camera Tower

GPX4000LZ Glass Processor Chapter 4: Initial Installation, Setup, Introduction and Training
Page 14 TTN102049-D02
Chapter 4 Initial Installation, Setup, Introduction and Training
The initial GPX4000LZ system installation and setup will be performed at the customer site by a Thorlabs Service
Engineer.
Upon successful uncrating and installation, the Service Engineer will commission the system based on the
customer’s specific applications request, as specified on the purchase order.
An introduction and training session for the customer in the proper usage of the GPX4000LZ will also be provided.

GPX4000LZ Glass Processor Chapter 5: Operation
Rev I, September 15, 2021 Page 15
Chapter 5 Operation
5.1. System Power
The GPX4000LZ is turned on using the rocker switch on the left side of the unit. This switch powers up the
electronics enclosure, two exhaust fans, and the vacuum pump. Power is delivered to other system components
as well, but they will not turn on until other switches on the control panel are moved to the “ON” position.
5.2. Control Panel
The GPX4000LZ features a control panel (shown in Figure 15) with the following features:
•Four warning lights (red LEDs) that indicate conditions in which the laser will not emit radiation:
oA light labelled “COVER OPEN” that tells the user when the cover is open.
oA light labelled “LASER FAULT” that tells the user if the laser is not ready yet.
oA light labelled “TEMPERATURE WARNING” that tells the user if the laser is too hot to operate.
oA light labelled “VOLTAGE WARNING” that tells the user if the laser cannot lase due to improper
input voltage.
•A rocker switch labelled “DEVICE POWER” that provides power to the GPX4000LZ.
•A keyswitch which enables the laser. When the laser is enabled, a white LED above the key will glow.
•A red lamp labelled “LASER ON” that flashes when the laser is actually emitting radiation.
•An emergency stop button that kills power to the laser and the GPX4000LZ when depressed.
•A temperature controller that controls the laser cooling fans
Figure 15 Control Panel

GPX4000LZ Glass Processor Chapter 5: Operation
Page 16 TTN102049-D02
5.3. Back Panel
The back panel has the following ports:
•An opening with a power cable for accessories such as the LDC401, which the user may wish to place on
a shelf attached to the cart.
•A bulkhead connector for Argon gas (required for filament splicing applications).
•A port that provides vacuum for a vacuum-suction end-cap holder (Item # ECH2V).
•Two analog inputs that can be used for active fiber alignment.
•An opening for three cables which go to the PC monitor (power, data, USB).
Figure 16 Ports in the back panel
5.4. Software Interface
The GPX4000LZ is controlled with Vytran’s FFS3 software package which is used across Vytran’s entire range of
fusion splicers. FFS3 provides a fully integrated graphical user interface (GUI) for control and analysis together with
an SQL database for data collection and performance tracking.
5.5. Graphical User Interface
The GUI is the main window when working with the glass processor. It has multiple menus and toolbars that are
explained later in this document. The buttons are described in this chapter from top to bottom, as indicated in Figure
17.