Note: Descriptions are shown in the official language in which they were submitted.
1232028
Background of the Invention
In recent years laser instruments have gained wide
acceptance in the field of ophthalmological surgery, due primarily
to the ability of laser systems to accomplish surgical tasks
within the eye while causing little or no unwanted residual surge-
eel trauma.
Surgical laser systems are used for such diverse pun-
poses as photo coagulation of tissue to cutting and removal of
tissue within the eye. on the former case, the procedure requires
a relatively long laser illumination at a relatively low power
intensity. In the latter case, it is necessary to use pulses of
light energy of extremely short duration and high peak energy.
Such pulses are capable of destroying the tissue at and around the
focal point without causing undesirable thermal effects. It is
generally true that systems capable of long-term laser illumine-
lion are not capable of generating the short, high intensity
pulses required for some procedures.
A prior art system which exemplifies the state of the
art of the latter case systems is disclosed in US. Patent No.
4,309,998. This system is a Q-switched, mode-locked laser employ-
in a YAW crystal to produce pulses of sufficient brevity and
intensity to permit surgical cutting by optical puncture. Such a
system, however, produces a series of closely spaced pulses in
relatively uncontrolled fashion. It is difficult to select pro-
wisely the desired output energy or number of pulses produced.
Furthermore, this laser cannot be operated to produce the thermal
or
~23~
effects which are necessary in some ophthalmological surgical
procedures.
Also, the prior art system mentioned above is typical of
the prior art in that it requires a cooling system to remove the
heat generated in the relatively inefficient laser resonator. A
cooling system is a mechanical system which requires Montanans,
and which is subject to eventual failure. The cooling system also
increases substantially the size of the laser system, making it
more bulky and difficult to package in a usable and convenient
form.
~23%~%8
Summary of the Present Invention
The present invention generally comprises a laser system
adapted for ophthalmological use which is compact in ~onfigura-
lion, simple to use and easy to maintain, and is capable of accom-
polishing surgical cutting by optical puncture as well as thermal
effects such as photo coagulation and the like. The laser resow-
atop of the present invention uses a novel optical pumping cavity
to reduce the length of the resonator and thereby reduce the
output pulse width. The novel cavity configuration also alluvia-
ales the need for a cooling system, thereby greatly simplifying
the mechanical design of the system.
The optical pumping chamber includes a closed spherical
cavity in which a Nd:YAG crystal and an excitation flash lamp are
mounted in non-parallel fashion in the cavity. The small
diameter, highly doped laser crystal is configured as an unstable
resonator by a concave rear mirror and a convex front output
mirror. A Q-switch is interposed between the crystal and one of
the mirrors to permit selectively Q-switched and non-Q-switched
operating modes. A continuous output gas laser is directed
through the rear mirror and the laser crystal to provide a pilot
beam for aiming and focusing. The laser outputs are directed to
a focusing system through a slit lamp assembly, and both are
directed into the eye of a patient to a common focus. The laser
power supply includes a plurality of capacitors arrayed in banks
and controlled by a microprocessor to deliver pulses of preselect
ted voltage to the flash lamp singly or sequentially to produce
~232~28
single pulses, bursts of pulses, or repetitive pulses on command.
A control system also operates the Q-switch in synchronism with
the flash lamp, when Q-switching is desired. The system may be run
in a thermal mode in which the laser crystal is stimulated by
closely spaced repeated lamp flashes to cause the stimulated
electron population inversion to be maintained. The resulting
long train of pulses within a predetermined time-power envelope
closely approximates a continuous output and produces the thermal
physiological effects which are desirable for some laser surgical
procedures. In the Q-switched mode, the combination of MicroPro
censor control, a short optical pumping cavity, an unstable resow-
atop configuration, and a small diameter, short length, high gain
crystal produce very narrow pulses of high intensity, high beam
quality, and precise duration, spacing, and energy.
123~,028
Brief Description of the Drawing
Figure 1 is a front elevation of the laser system for
ophthalmological surgery of the present invention.
Figure 2 is a side elevation of the laser system depict
ted in Figure 1.
Figure 3 is a bottom view of the laser assembly of the
present invention, taken along line 3-3 of Figure 2.
Figure 4 is a partially cutaway perspective view of the
optical pumping chamber of the laser of the present invention.
Figure 5 is a layout view of the control panel of the
laser system of the present invention.
Figure 6 is a functional block diagram of the control
system of the laser system of the present invention.
Figure 7 is a functional block diagram of the control
and supply circuits for the flash lamp firing system of the present
invention.
Figure 8 is a cross-sectional elevation of the optical
output assembly of the laser system of the present invention.
~232~328
Description of the Preferred Embodiment
The present invention generally comprises a laser system
for ophthalmological surgical use which is compact in configure-
lion, simple and easy to use and maintain, and capable of provide
in both tissue cutting and thermal effect outputs. The laser
system also includes a microprocessor control system which pro-
vises precise selection of output pulse energy, pulse width, and
number of pulses. The laser can be operated in the Q-switch mode
to generate single or multiple pulse trains, or it can be operated
in a free-running "thermal" mode to mimic a continuous output and
produce the thermal physiological effects of long term laser
illumination.
With reference to Figure 3, the laser configuration of
the present invention includes a base plate 11 from which the
operating components are suspended. The base plate, which is
approximately 6 inches by 10 inches may be formed of Invar or
similar material which exhibits a very low coefficient of thermal
expansion. Secured to the base plate 11 is a novel optical pump-
in assembly 12, which is shown in greater detail in figure 4.
The assembly includes a generally cubic body formed of a pair of
rectangular solid members 13 and 14. Each of the members 13 and
14 includes a hemispherical cavity 16 and 17 formed in confronting
faces thereof and disposed to be in exact registration when the
members are joined together. In the preferred embodiment, the
cubic body is approximately 1.5~ inches in side dimension, and the
spherical cavity 18 formed by the two hemispherical cavities is
approximately 1 inch in diameter. The members 13 and 14 are
~L232~)~8
formed of so id aluminum or material of similar thermal and struck
tubal properties, and are joined by screws received in tapped
holes.
Each of the members 13 and 14 includes a bore 19 and 21.
respectively, extending through the cavities 16 and 17. In the
preferred embodiment the bores 19 and 21 are disposed in orthogo-
net fashion, although this arrangement is not critical for opera-
lion. The bores are disposed closely adjacent to the confronting
faces of the members 13 and I Secured in the bore 19 is a
flash lamp 22, with the output portion of the flash lamp disposed
within the cavity 18 and the electrodes 23 and 24 protruding from
the assembly 12. Secured in the bore 21 is a laser rod 26. In
the preferred embodiment the laser rod is a Nd:YAG crystal which
is highly doped with neodymium to provide a high amplification
factor. The small diameter of the rod 26 enhances the efficiency
of the system and the beam quality of the output.
The interior surface of the cavity 18 is treated with a
highly efficient, diffusely reflective coating, the reflectivity
being greater than 99~. A reflective material such as barium
sulfate powder mixed with a binder may be applied directly to the
interior surface of the cavity I For greater durability, the
cavities 16 and 17 may be lined with glass hemispheres, and high
purity barium sulfate powder without any binder may be secured
between the outer surface of the hemispheres and the interior
surfaces of the cavities.
The assembly 12 is secured to the base plate 11 with the
bore 21 and laser rod 26 aligned precisely with the optical axis.
'l~3~2~
The optical pumping assembly is disposed within a laser resonator
defined by mirror assemblies 32 and 33. Mirror assembly 32 in-
eludes a 50 cm radius spherical radius of curvature mirror 34 with
a concave surface 35 which is coated with multi layer dielectric
materials to be greater than 99~ reflective at the YAW laser
wavelength of 1.064 micrometers. Mirror assembly 33 includes a
convex surface 36 of 33cm spherical radius of curvature. This
mirror is formed on a meniscus substrate 37 with anti reflective
coatings 38 (at the YAW laser wavelength) applied to both surfaces
of the substrate. At the center of the convex surface a coating
which is highly reflective at the YAW laser wavelength is depose
tied over a 2.2 mm diameter spot to form a reflective spot 39.
It may be appreciated that the focus of the beam from
the mirror 34 is proximate to the spot 39, so that a portion of
the laser output is reflected back into the laser rod. The annum
far surround 38 so anti reflective material serves as an output
coupler for the laser beam which passes there through and on toward
the beam utilization apparatus. In the unstable laser resonator
defined by the mirror assemblies, all of the potential laser
output is realized in the fundamental mode, so that all of the
available laser energy is focused into the smallest possible
spot. Furthermore, the spot output coupler delivers a high pro-
portion of the beam, reducing multiple reflections axially
through the laser rod and permitting the generation of extremely
short pulses.-
The laser assembly also includes a Q-switch 41 inter-
posed along the optical axis between the laser rod and one of the
mirrors 34 and 39. In the preferred embodiment the Q-switch is
l pi
12,3~8
disposed between the laser rod and the output mirror assembly 33,
so that the laser rod is more fully illuminated by the wider
portion of the laser beam reflecting between the mirrors. The Q-
switch may comprise a transverse field electrooptic modulator
using a lithium niobate crystal and a single, multi layer Delco-
trig thin film polarizer 42, as is known in the prior art In Q-
switched operation the crystal is biased at a positive voltage to
its quarter-wave retardation level to block laser action. Switch-
in action is provided by a negative-going step pulse which is
generated upon command by the control circuitry to shift the beam
polarization and permit laser action. However, it is also quite
possible to achieve non Q-switched laser operation without remove
- in the Q-switch or the thin film polarizer.
Also secured to the base plate 11 is a continuous output,
visible light, low power laser 46, preferably a helium-neon (Hone)
gas laser. The output beam from the laser 46 is directed along
the optical axis to a 180 reflector assembly 47. The assembly 47
includes a pair of mirrors 48 and 49 disposed at an angle of 45
to the incident beam from the laser 46~ A diverging lens 40 and a
converging lens 50 disposed at the entrance and exit, respective-
lye of the reflector assembly 47 form a collimator which equalizes
the diameter of the Hone beam to the diameter of the YAW beam.
The Hone beam exits from the lens 50 and is directed through the
mirror 35, and through the laser rod, which are both substantially
transparent to the Hone wavelength, 633nm. The output mirror 33
is also transparent to the Hone beam, so that the two laser beams
exit from the mirror assembly 33 in colinear alignment.
From the mirror assembly 33 the beams are directed
~;~3~(~28
through a beam spreading lens 60 to a mirror assembly 51 where a
mirror 52 is used to reflect the beams 90 upwardly. The beam
spreading effect permits a highly converging focus at the surgical
site within the eye, so that only the tissue desired to be cut or
treated is affected by the laser pulses The spread beam also
reduces the criticality of the subsequent mirror and lens surfaces
and materials by reducing the energy density of the beam. Secured
to the assembly 51 is a photo sensor assembly 53, which receives
approximately 1% of the beam energy transmitted through the mirror
52. The output of the photo sensor is connected to the control
system to provide-a closed loop feedback system for recalibrating
the energy output of the laser with respect to the light energy
input, as will be explained in the following description
- With reference to Figure 1 and 2, the invention also
comprises a complete system for utilizing the laser describe above
to perform ophthalmological surgery. The apparatus includes a
cabinet 56 which is adapted to house the electronic power supplies
and controls of the system. The cabinet includes a top surface 57
on which a control panel 58 is supported. A cantilever table 59
extends outwardly from one side of the cabinet. The table 59 is
supported by the cabinet in vertically translatable fashion to
adjust to the height of the patient to be treated. The open end
configuration of the table 59 permits access to and use of the
instrument by individuals confined to a wheelchair. The feature
is significant when it is considered that many patients requiring
laser ophthalmological surgery are aged and frequently physically
disabled. The base plate of the laser assembly is secured to the
table 59 in inverted fashion beneath the top thereof. Supported
1~3~:~2~
on the table 59 is a binocular examining microscope 61, a standard
slit lamp assembly 62, and a frame 63 adapted to brace and
restrain the head of the patient to be treated It may be apple-
elated that the entire laser surgical system, requiring no anvil-
lazy equipment for operation and no cooling system. is fixedly
secured to the slit lamp biomicroscope, and is translated Verdi-
gaily therewith. Thus misalignment problems are reduced, and
moving mirrors, a source of failure in prior art systems, are
eliminated entirely. Furthermore, the entire system occupies the
space of a small desk.
With reference to Figure 8, as the beams travel from the
mirror 52 they pass through a hole in the top 57 and pass into a
beam delivery assembly 66. The assembly 66 includes a pair of
mirrors 67 and 68 aligned in generally parallel fashion to de-
liver the laser beams to a lens doublet 69 which focuses the
beams. They are then reflected by reflecting mirror 71 to a focus
inside the eye of the patient. The slit lamp projector is
directed toward a mirror 72 which reflects that light source into
the eye through the mirror 71. The surgeon's microscope 61 is
directed through the mirror 71 and about the sides of mirror 72 to
view the convergence of the beams within the eye, and the position
and size of the focal spot. To enable the Hone alignment beam to
be conveniently focused at the same point as the YAW pulses, the
doublet is designed and fabricated to have the same focal length
at 1064nm and 633nm. In the preferred embodiment, a selection of
such achromatized lenses is made available to enable the focal
spot diameter to be varied according to the requirements of the
ophthalmological procedure.
~23~(~28
Due to the fact that the laser assembly is secured
directly to the slit lamp assembly, there is little opportunity
for alignment problems to occur in the present system. This close
proximity also reduces the number of mirrors used, especially
compared to prior art articulated arm delivery systems, thereby
further increasing overall reliability.
A salient feature of the present invention is the
sophisticated control system which permits precise selection of
the pulse energy, pulse width, and number of pulses delivered by
the laser to the surgical site. The control system is depicted
schematically in Figure 6, wherein the large functional circuit
blocks are subdivided into functional units where appropriate.
Also, the many electrical power supplies or the circuits are not
shown, for clarity and brevity.
With reference to Figure 6, a significant feature of the
control system is the provision of a microprocessor controller 76,
complete with the necessary ROM, RAM, and programming to carry out
the functions described in the following. The microprocessor
controller also includes an input/output (I/O) section 77, a relay
operating section 78, and an analog/digital (ADO) converter sea-
lion 79. The control system also includes circuitry 81, comprise
in all of the electrical devices mounted on the laser assembly
except the Hone laser. This circuitry includes the flash lamp 22,
the photo diode 53 which senses the energy of the YAW output beam,
and the Q-switch 41. In addition, a thermistor 82 is secured to
the laser body 12 to measure thy temperature therein. The
circuitry 81 also includes the solenoids to operate a laser beam
attenuator 83 and a shutter I both being standard items in the
14
~2321)28
prior art and neither being shown for the sake of clarity. The
relay driver section 78 of the microprocessor controller is con-
netted to both the attenuator 83 and the shutter 84. The relay
section I is also connected to the Hone power supply 85, which in
turn energizes the Hone laser 46.
The electrical system further includes a pulse forming
network 87 which generates high voltage pulses of preselected
voltage, spacing, and number. The high voltage pulses are fed
through an inductor 88 and through normally open relay contacts 89
to the flash lamp 22~ The relay 91 which operates the contacts 89
is connected to the relay driver section of the microprocessor
controller. The pulse forming network receives the high voltage
required for the pulses from a charging power supply 96 through
the charge circuit 104.
A flash lamp control circuit 97 is also provided to
operate the flash lamp 22. The circuit 97 includes a timer section
98 which delays and controls the firing of the negative going
pulse which operates the Q-switch driver and power supply 101. In
Q-switched operation the Q-switch is opened approximately 70
microseconds after the flash lamp is fired by the high voltage
pulse, to permit laser action to peak before the pulse is
delivered. A trigger section 99, connected to the pulse forming
network 87, is also provided to actuate the pulse forming network
upon command from the microprocessor controller 76.
The flash lamp control circuit 97 further includes a
section 102 (simmer- L/S) which starts the flash lamp and maintains
an ionized state in the flash lamp thereafter by providing a
"simmer" current of approximately Moe from a controlled current
~23~28
source. The lamp start procedure is accomplished by the section
102 delivering a sharp pulse to the pulse transformer 103. The
pulse transformer generates a pulse of several kilovolts, suffice
tent to create a discharge through the flash lamp and begin opera-
lion. (The Q-switch and the shutter remain closed.) The simmer
current is then sufficient to maintain the lamp in readiness to be
flashed by the 200-400 volt pulses from the pulse forming network
87. It may be appreciated that the relay contacts 89 are main-
twined open during the lamp start procedure, so that the high
voltage starting surge will not damage the pulse forming network.
The circuit 97 also includes a signal conditioning section 106
which is connected to the thermistor 82 and the photo diode sensor
53. The section 106 conditions the signals from the thermistor
and the photo diode, and delivers them to the analog/digital con-
venter 79 of the microprocessor controller 76. The microprocessor
controller integrates the photo diode signal to derive the beam
energy of the YAW laser directly after it is fired. The therms-
ion signal is monitored reiteratively to assure that the tempera-
lure of the laser system is not exceeding the operating parameters
stored in memory. Furthermore, the controller is programmed with
a formula which determines the voltage from the charging power
supply which must be applied to the flash lamp 22 to generate a
laser pulse of desired energy. If the photo diode senses that the
generated pulse differs significantly in beam energy from the
desired setting, the microprocessor controller is programmed to
alter the formula to agree more closely with measured output.
Thus a closed feedback loop is constantly recalibrating the laser
system output to be as precise and exact as possible.
16
Swahili
The control system also includes the control panel 58
connected to the microprocessor controller which permits the sun-
goon to select the operating mode, pulse energy and spacing, and
the like. The panel 58 includes indicator lights 111, LED or LCD
displays 112, and numerical and functional setting switches 113.
In addition, the control panel circuitry includes a foot switch
operating circuit 114 connected to a foot switch 116 which permits
control of the firing of the laser system by pedal rather than
manual control of the surgeon.
With reference to Figure 7, the pulse forming network
trigger circuit, generally indicated at reference numeral 99,
includes an address decoder 121 which receives from the MicroPro
censor controller the numerical addresses of one or more of a
plurality of pulse forming networks, and their respective trigger
circuits 122. In the preferred embodiment there are 56 pulse
forming networks (PUN), each having their own trigger circuit 122.
Forty of the Puns are connected in five arrays of eight each,
primarily to ease interfacing with a binary digital
microprocessor. Twelve of the Puns are individually operable, and
four are spares which can be substituted by the microprocessor
controller for any PUN that may fail. Each trigger circuit is
connected to its respective PUN in similarly arrayed fashion.
Each PUN trigger 122 includes an opto-isolator 123 which
is connected to the base of transistor 124. A capacitor 126 is
connected between the collector of transistor 12~ and limiting
resistor 127, which in turn is connected to the emitter. The
capacitor is connected to a low voltage charging line 128. when a
network 122 is selected by the microprocessor to provide a pulse
3;2
to the flash lamp, all eight of the Puns in that array are charged
to a voltage determined by the microprocessor controller through
the charging line 128. The address of the selected network is
sent to the decoder 121r which grounds the signal line 129 of the
appropriate PUN trigger. The LED of the opto-isolator is caused
to actuate, thereby switching on the transistor 124. when tray-
sister 124 is caused to conduct the charge on the capacitor is
applied to the gate of the SIR of the respective PUN. Each PUN
within the functional block 87 includes a large capacitor charged
by the power supply go and connected through an SIR to the flash-
lamp. When the SIR is actuated, the resulting discharge produces
a flash lamp pulse of approximately 50 microsecond duration, the
intensity of the pulse being related in a predetermined, empire
icily derived manner to the voltage of the discharge. This known
relationship permits the microprocessor controller to select the
appropriate charging voltage to cause the requisite flash intent
sty to produce the desired laser pulse energy.
It may be appreciated that each flash lamp pulse will
produce one laser output pulse. The microprocessor controller may
fire the Puns in single fashion, or in serial, spaced apart
fashion to produce a pulse train of predetermined pulse energy,
spacing, and pulse width. For each output pulse in the Q-switched
mode, the Q-switch is opened approximately 70 microseconds after
the lamp discharge commences. The laser system of the preferred
embodiment is capable of delivering pulses of 5-10 nsec duration.
These pulses can be delivered singly, or in bursts of 1-10 pulses
in a 10 millisecond interval, or may be generated in repetitive
fashion at a 3 Ho rate.
18
~Z3Z~8
A significant feature of the operation of the present
invention is that it is capable of operating in a free-running
mode in which the output produces the thermal physiological
effects which are required for photo coagulation and the like. In
this "thermal" mode, the laser system can deliver a series of 10~-
500 my pulses of 50 microsecond duration in a period of 1-10
milliseconds. This operation is accomplished by charging the
required Puns to the necessary voltage, opening the Q-switch and
the shutter, and firing the Puns at 50-200 microsecond intervals.
The first lamp discharge causes a significant population of the
neodymium electrons to jump to the laser emission level, and a
laser pulse is generated. However, the electron population invert
soon in the emission band persists briefly, for approximately 400
microseconds. The rapid flash lamp firing of the thermal mode
takes advantage of this population inversion persistence by cause
in restimulation of the laser before the energy put into stab-
fishing the electron population inversion is lost. As a result,
the present invention operates very efficiently in the free-
running, "thermal" mode, and this mode is achieved without any
cooling system. The overall envelope of the thermal mode pulses,
considering their energy versus time characteristics, produce
physiological effects identical to long term thermal pulses
delivered by gas lasers and the like. Thus the present invention
is capable of a flexibility in operating modes which has heretofore
been unobtainable.
With reference to Figure 5, the control panel 58
includes an LED readout 131 which displays the desired pulse
energy setting of the YAW laser. Companion setting buttons 13~
~2320~
are provided to permit the surgeon to increment the setting
upwardly or downwardly. An LED readout 133 displays both the
pulse width setting and the number of pulses desired. Setting
buttons are provided to select either display and to increment the
settings. LED readout 136 displays the number of pulses delivered
by the laser, and is recitable by button 137. A plurality of
buttons 138 are provided to select the laser output in the cutting
mode in which tissue is severed by optical puncture. These but-
tons may select either a single pulse, a continuous pulse train at
a 3 Ho rate, or a burst of pulses. Selector button 139 permits
the surgeon to-select the thermal operating mode in which the
laser provides a pulse train which produces the effect of a
single, long term thermal pulse. Switches 141 and 142 enable the
YAW laser and operated the shutter, respectively. Warning lights
143 and 144 indicate a problem with the laser emission and with
the overall system, respectively. Switch 14D is a key operated
on-off switch.
