Diode Lasers for Medical Applications - White Paper - Coherent
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Diode Lasers for Medical Applications White Paper
White Paper
Diode Lasers for Medical Applications
Lasers are widely used throughout the field of medicine, from diagnostic imaging and clinical testing, to surgical treat-
ments and the latest aesthetic procedures. For therapeutic medical procedures in particular, diode lasers have now
become the dominant laser type in use. This is because these workhorse devices provide lower cost of ownership, a wider
choice of output wavelengths and power levels, compact rugged packaging, and superior (semiconductor) reliability when
compared to other laser types. In this article, we briefly examine the main features and advantages of typical diode laser
types, and then survey some of the leading medical applications that currently benefit from Coherent diode lasers.
Laser Diode Advantages
The diode laser is a monolithic semiconductor device This wide choice of output wavelengths enables tailoring
that directly converts electrical energy into laser light. of the laser system to best match the needs of each
It also provides unique levels of power and wavelength specific application, e.g., to maximize blood coagula-
scalability that combine to support a very wide range of tion, to tighten collagen, to maximize tissue ablation, to
medical applications. Selected wavelengths are enabled maximize penetration depth in soft tissue or to limit it to
by different semiconductor compositions; the output surface treatment, to burst target cells, and so on. For
wavelength can be set to be in the blue, green, red, or every application, there is one or more optimum wave-
near and mid infrared ranges, with near-infrared de- length which delivers the best selectivity, i.e., where the
vices generally offering the highest power levels. Other laser produces a maximum effect while minimizing any
than a small gap between green and red, the majority unwanted collateral effects.
of medically interesting wavelengths can be produced.
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White Paper
Diode Lasers for Medical Applications
A Coherent White Paper
a) b)
Figure 1: a) 8- and 12-bar stacks, suitable for industrial water cooling in long pulse (5-400 msec) operation for hair removal, b) Backside of such a stack, showing, in addition to the mounting
and alignment holes, the water-inlet and -outlet, suitable for O-ring sealing.
Figure 1: a) 8- and 12-bar stacks, suitable for industrial water cooling in long pulse (5-400 msec) operation for hair
removal, b) backside of such a stack, showing, in addition to the mounting and alignment holes, the water-inlet and -
Theoutlet, suitable
versatility for O-ring
of diode laserssealing.
is further enhanced by their power scaling: from single emitters at the watt level to bars
emitting tens to hundreds of watts, all the way to stacks of bars delivering multiple kilowatts. Plus, diode lasers are more
electrically efficient than any other laser type; in many cases >55% of the electrical input can be converted into useful laser
output. However, they also generate heat, which could lead to a reduction in efficiency and output power, and hence to re-
The versatility of diode lasers is further enhanced by their power scaling: from single emitters at
duced lifetimes. For this reason, cooling is a key consideration in any diode laser and diode laser system. De-ionized water
hasthe watt level
previously beento bars
the emitting
industry tens
standard to hundreds
since this providesofthe
watts, all the way
low electrical to stacks
conductivity of barsThis
necessary. delivering
unfortunately
adds to the cost
multiple of operation.
kilowatts. Plus,Indiode
addition, de-ionized
lasers water electrically
are more has to be controlled
efficientcarefully,
than any in order to prevent
other corrosion
laser type; in
and leakage in the cooling micro-channels; the control of water conditioning further adds to the running cost. But micro-
many casesstill
channel-coolers >55%offerofthe
the electrical
best thermal input canand
efficiency behence
converted
supportinto useful
highest CW laser
outputoutput.
powers.However, they
also generate heat, which could lead to a reduction in efficiency and output power, and hence to
“
Coherent has successfully addressed this cooling vs.
reduced lifetimes. For this reason, cooling is a key consideration in any diode laser and diode
cost challenge by using an architecture that separa-
Coherent
laser system. haswater
De-ionized successfully
has previously been the industry standard
tes the cooling watersince
from this providesThis
the electricity. theallows
efficient cooling using only clean, industrial-grade
lowaddressed the cooling
electrical conductivity necessary. vs. cost
This unfortunately adds to the cost of operation. In addition,
water, lowering the cost of ownership. And avoiding
de-ionized water has to be controlled carefully, in order to prevent
DI water corrosion
minimizes andfor
the need leakage
careful in the of
control
challenge by using an architec-
cooling micro-channels; the control of water conditioning thefurther
water condition.
adds to This
thesimplifies
runninginfrastructure
cost. But
requirements, extends expected device lifetime, and
ture that separates the cooling
micro-channel-coolers still offer the best thermal efficiency and the
lowers hence
costsupport highest
of ownership CW output
further.
powers.
water from the electricity.
”
Coherent has successfully addressed this cooling vs. cost challenge by using an architecture that
separates the cooling water from the electricity. This allows efficient cooling using only clean,
industrial-grade water, lowering the cost of ownership. And avoiding DI water minimizes the need 2
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for careful control of the water condition. This simplifies infrastructure requirements, extends
White Paper
expected device lifetime, and lowers the cost of ownership further.
Another important advantage of diode lasers is that their output wavelengths (from 405 nm up
Diode Lasers for Medical Applications
to ~2.3 µm) are deliverable through low-cost glass fiber optics. Except for applications requiring
large area illumination, most Coherent diode laser products packaged for the medical market
include some type of fiber coupling; they are supplied in a pre-aligned package with a standard
fiber output
Another connector,
important ordiode
advantage of withlasers
a pigtailed (attached)
is that their fiber. This
output wavelengths eliminates
(from alignment
405 nm up to challenges
~2.3 μm) are deliverable
through
for thelow-cost
system glassbuilder
fiber optics.
and Except for applications
allows convenient requiring large
fiber area illumination,
delivery most Coherent and
for laparoscopic diode laser
robotic
products packaged for the medical market include some type of fiber coupling; they are supplied in a pre-aligned package
procedures,
with a standard as well
fiber as connector,
output enablingorstraightforward connection
with a pigtailed (attached) toeliminates
fiber. This some type of handpiece
alignment with
challenges for the an
system builder and allows convenient fiber delivery for laparoscopic and robotic procedures, as well as enabling straight-
imaging (e.g., telescope) capability. Conversely, some applications such as hair removal actually
forward connection to some type of handpiece with an imaging (e.g., telescope) capability. Conversely, some applications
need
such aslarge area illumination
hair removal andarea
actually need large ourillumination
modulesandwithout fiberwithout
our modules coupling
fibermatch
couplingthis purpose.
match this purpose.
Table: A summary of the wavelengths of diode lasers that Coherent provides to support diverse medical therapeutic
applications.
“
Hair Removal
Another important advantage of diode lasers is that
their output wavelengths (from 405 nm up to ~2.3 μm)
Hair removal is one of the most popular non-surgical aesthetic laser procedures worldwide. Like
are deliverable through low-cost glass fiber optics.
other aesthetic laser procedures, selectivity is a paramount concern – obtaining maximum
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”
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White Paper
Diode Lasers for Medical Applications
A Coherent White Paper
Hair Removal
cy (killing the hair follicle) without damage or scarring of surrounding tissue, and with
mum discomfort to is
Hair removal the
onepatient.
of the mostActively cooling the
popular non-surgical skin’s
aesthetic lasersurface during
procedures theLike
worldwide. procedure
other aesthetic laser
procedures, selectivity is a paramount concern – obtaining maximum efficacy (killing the hair follicle) without damage or
ces the pain of the
scarring patient. Selectivity
of surrounding is further
tissue, and with minimum enhanced
discomfort to by optimizing
the patient. Activelypulse
coolingduration, so during the
the skin’s surface
he follicle procedure
cells arereduces the pain of the patient. Selectivity is further enhanced by optimizing pulse duration, so that the follicle
killed without causing thermal damage to the dermis – see figure 2. This
cells are killed without causing thermal damage to the dermis – see figure 2. This requires a pulse width somewhere
res a pulse width
between somewhere
5 and between
100 milliseconds, which is 5 and
very 100
simple milliseconds,
to achieve which
with the fast is very simple
on/off capabilities to
of diode lasers.
ve with the fast on/off capabilities of diode lasers.
Figure 2: Laser hair removal schematically explained.
2: Laser hair removal schematically explained.
The specific wavelength requirements also serve to make diode lasers a great match for hair removal. The color in hair
specific wavelength requirements also serve to make diode lasers a great match for hair
comes from the presence of melanin, which absorbs visible and near-IR light, from approximately 320 nm to 1200 nm.
val. The color in hair
Conversely, comes
blood from
absorbs mostthe presence
strongly of melanin,
at wavelengths which
below 600 absorbs
nm, and the watervisible and near-in soft tissue
that is omnipresent
starts to absorb strongly at wavelengths longer than 1100 nm. Thus, there is a wide potential wavelength window for
ht, from approximately 320 nm
this application between to and
600 nm 1200
1100nm. Conversely,
nm. System blood
builders have absorbs
converged most
on four strongly
diode at
laser wavelengths for this
application:
lengths below 600 755nm,nm,
and808 the
nm, 940 nm, and
water 1060
that is nm. Their instruments
omnipresent typically
in soft combine
tissue up toto
starts three of these wavelengths
absorb
since experience has shown that this type of multi-wavelength device yields the best results in the broadest range of
gly at wavelengths longer thana wide
patients, accommodating 1100variety
nm.ofThus, there
skin tones andisnatural
a wide hairpotential
colors. wavelength window
is application between 600 nm and 1100 nm. System builders have converged on four diode
wavelengths for this application: 755 nm, 808 nm, 940 nm, and 1060 nm. Their instruments
ally combine up to three of these wavelengths since experience has shown that this type of
-wavelength device yields the best results in the broadest range of patients, accommodating
e variety of skin tones and natural hair colors.
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Diode Lasers for Medical Applications
A Coherent White Paper
These different wavelengths are generated by diffe-
rent diode laser materials and until recently, this has
created difficulties for system builders because the L-I Curves for Four Wavelengths
slope (output versus current) was different for each
of the wavelengths. Setting and maintaining a power 100
balance was therefore often a challenge. Coherent
has developed a unique semiconductor solution
to this problem, so that all four wavelengths have 80
nearly identical slopes and provide the same power
per bar at the operating point / operating current –
60
Power (W)
see figure 3. These are all packaged in an identical
stack format, preferred by system builders in this
application.
40
755nm
808nm
20 940nm
1060nm
0
0 50 100
current (A)
Figure 3: Coherent diode lasers at the four hair removal wavelengths all exhibit
Figure 3: Coherent diode lasers at the four hair removal wavelengths all exhibit the same slope, simplifying power
the same slope, simplifying power balance.
balance.
These different wavelengths are generated by different diode laser materials and until recently,
Other Aesthetic Procedures this has created difficulties for system builders because the slope (output versus current) was
different for each of the wavelengths. Setting and maintaining a power balance was therefore
Lipolysis (laser body fat removal) is a growing application for the transdermal
often a challenge. application
Coherent has developed of light
a unique using 1060solution
semiconductor nm viatodirect
this problem, so
and fiber-coupled diode lasers. The 1060 nm light that
optimizes selectivity by
all four wavelengths maximizing
have theslopes
nearly identical ratio and
of absorption in fatpower
provide the same tissueper bar at the
versus absorption in water. At the appropriate intensity, thispoint
operating wavelength
/ operatingelevates
current –the
see temperature
figure 3. These in
areadipose fat tissue
all packaged in an identical stack
just below the dermis, resulting in cell degeneration. Overpreferred
format, a period byof a fewbuilders
system weeks,inthe
this body then naturally removes the
application.
degenerated adipose tissue, giving the skin time to adjust and often avoiding the loose baggy skin appearance typically
resulting from liposuction or some weight loss surgeries.
Other Aesthetic Procedures
Discomfort and dermal damage are avoided by using surface
Lipolysis (lasercooling
body fatof the skin,
removal) is a as in hair
growing removal.
application forAthe
novel tool serves
transdermal application of light
both to implement the cooling and maximize process usingefficiency
1060 nm via fordirect
fat removal. Specifically,
and fiber-coupled diode the applicator
lasers. The 1060 nmprovides an
light optimizes selectivity
illuminated area of approximately 10x15 cm², water bycooling
maximizingto cool theofilluminated
the ratio absorption inskin, as well
fat tissue as absorption
versus a gold coated reflector
in water. At the appropriate
to recycle some of the 1060 nm light which inevitably will be
intensity, reflected
this wavelength from the skin.
elevates Coherent in
the temperature supports this
adipose fat application
tissue just below the dermis,
with a fiber-coupled diode laser module supplied with a detachable (800 microns diameter) optical fiber, simplifying sys-
tem assembly, cleaning and service. Alternatively, 1210 nm can also be used for fat removal, as the fat absorption of this
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Skin tightening is another popular aesthetic procedure. Often performed as a standalone treatment or as an adjunct after
lipolysis to tighten the skin following adipose removal, it works through the heating and consequent photocoagulation
(tightening) of collagen. Conversely, wrinkle removal and skin resurfacing depend on the laser light being absorbed by the
dermis cells and shorter wavelengths enable efficient melanin absorption. These skin resurfacing and wrinkle removal pro-
cedures are often performed using 810 and 915 nm, although some system manufacturers prefer 1550 nm where water
absorption (rather than melanin absorption) is the dominant mechanism.
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Diode Lasers for Medical Applications
Yet, another variant of skin resurfacing uses a 1550 nm diode laser together with an infrared CO2 laser at 10.6 μm, in a
procedure called “fractional resurfacing” - see figure 4. In this two-pronged approach, the CO2 laser is focused to a small
spot and pulsed as the spot is moved across the skin, resulting in myriad small holes in the skin. The purpose of the
1550 nm diode laser is to cause photocoagulation of the collagen. The combination of softening and tightening is also
used in a less publicized “vaginal rejuvenation” procedure.
A Coherent White
MostPaper
varicose vein removal is still a non-laser application,
but even these procedures are beginning to change. Smaller
veins that feed a larger varicose vein are often treated trans-
dermally using 940, 980 or 1470 nm diode lasers. Larger
veins are then treated using conventional surgical means or
sometimes by using a 940 nm diode laser endovenously.
Other aesthetic applications include acne treatment,
primarily using blue-green lasers at either 450 nm or 520
nm, or 1470 nm which is highly absorbed on the skin’s
surface, and teeth whitening using 810 nm or 980 nm.
Figure 4: In Fractional Resurfacing, a matrix of laser spots removes damaged
tissue, but leaves the intervening area intact to promote recovery. A diode laser at 1550 nm
then tightens the underlying collagen.
gure 4: In Fractional Resurfacing, a matrix of laser spots removes damaged tissue, but leaves the intervening area
act to promote recovery. A diode laser at 1550 nm then tightens the underlying collagen.
Surgical
ost varicose vein removal Application Example
is still a non-laser application,–but
Treating Benign
even these Prostate
procedures are Hyperplasia
eginning to change. Smaller veins that feed a larger varicose vein are often treated
Like940,
ansdermally using many
980other age-related
or 1470 nm diodeconditions, Benign
lasers. Larger veinsProstate
are thenHyperplasia (BPH) is a problem affecting a growing segment of
treated using
the male
onventional surgical meanspopulation as by
or sometimes lifeusing
expectancy
a 940 nmcontinues
diode lasertoendovenously.
increase. In a procedure called transurethral resection of the prostate
(TURP), laser light is fiber-delivered through the urethra and the laser surgery site is actively cooled with flowing saline.
Typically, two diode laser wavelengths are used: 980 nm and 1470 nm. Both lasers are used to remove soft tissue primar-
ther aesthetic applications include acne treatment, primarily using blue-green lasers at either
ily by heating the water within cells and causing them to burst. As noted earlier, strong laser absorption in water occurs
50 nm or 520 nm, or 1470 nm which is highly absorbed on the skin’s surface, and teeth whitening
at wavelengths longer than 1200 nm. So, 1470 nm is absorbed very efficiently by soft tissue, including prostate tissue.
sing 810 nm or 980 nm.
In contrast, 980 nm is absorbed less and consequently penetrates deeper. The 980 nm at a higher power level removes
tissue quickly. This is followed by more precise removal of tissue using the 1470 nm wavelength.
urgical Application Example – Treating Benign Prostate Hyperplasia
ke many other age-related conditions, Benign Prostate Hyperplasia (BPH) is a problem affecting
Photodynamic Therapy
growing segment of the male population as life expectancy continues to increase. In a
ocedure called transurethral resection of the prostate (TURP), laser light is fiber-delivered
In photodynamic therapy (PDT), a laser is used in conjunction with a chemical or pharmaceutical photosensitizer agent
rough the urethra and the laser surgery site is actively cooled with flowing saline. Typically, two
to cause selective cell degeneration. The photosensitizer is given to the patient systemically, locally or topically with the
ode laser wavelengths are used: 980 nm and 1470 nm. Both lasers are used to remove soft
goal of causing preferential accumulation in the target cell type. When the photosensitizer is illuminated by laser light it
ssue primarily by heating the
undergoes water within cellsorand
a photo-physical causing them transformation
photochemical to burst. As noted earlier,
that creates some type of toxic species – usually a type
rong laser absorption in water occurs at wavelengths longer than 1200 nm. So, 1470
of reactive oxygen – that kills the host cell. Further selectivity is enabled nm is by locally directing laser light. When originally
bsorbed very efficiently by softPDT
introduced, tissue,
wasincluding
seen asprostate tissue.
a potential In contrast,
silver 980 treating
bullet for nm is absorbed
a broad range of cancers. Instead, PDT is used today to
treat only a few selective cancer types and has actually found more success in treating non-malignant ailments.
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Diode Lasers for Medical Applications
Red wavelengths (635 nm, 652 nm, 670 nm and 752 nm) are common in PDT. First, this matches the light absorption
spectrum of common photosensitizers, many of which are porphyrin derivatives. Just as important, these red wavelengths
penetrate deeper through soft tissue than blue or green wavelengths.
PDT is also sometimes used to treat the wet form of age related macular degeneration (AMD), although improved pharma-
ceutical options are reducing the use of PDT for this purpose. Here, the photosensitizer Visudyne is injected in the patient
and accumulates in blood vessels in the retina. The laser then acts to seal the leaking vessels that are obscuring the retina.
Coherent diode lasers with longer wavelengths beyond the normal retina response are typically used in the treatment,
specifically 689 nm. Power levels as low as 0.5 Watt are generally sufficient.
PDT is also used for wound healing, although there is some debate about the true efficacy of it for this purpose. Both
violet (405 nm) and near-IR (980 nm) Coherent diode lasers are used in these systems.
Summary
The unique characteristics of diode lasers, including small size, high electrical efficiency and semiconductor reliability and
longevity, have led to their widespread adoption in diverse therapeutic medical applications. Just as important, the unique
ability to scale both the power and wavelength of diode lasers, together with the option of either free space or fiber
delivery, enables Coherent to offer products that are optimally matched to the needs of specific medical procedures. The
end results are better outcomes, higher patient satisfaction, shorter treatment time, as well as shorter hospital stays, and
reduced costs.
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