CW, Pulsed, Ultrapulse: Three Modes, One Tube

Halfway down the CF-01 spec table sits a row that reads Laser Operating Mode: CW, UP, CPG. Three abbreviations. No explanation. Yet that row decides more about what the machine can and can't do than the power figure above it. And here's the odd part: all three modes come out of the same sealed tube, emitting the same 10600 nm beam. Nothing about the gas physics changes. What changes is timing — how long the energy is allowed to sit on tissue before it stops.

One Tube, Three Timing Schemes

A CO2 laser is, at heart, a water-heating instrument. At 10600 nm soft tissue absorbs the beam within a very shallow layer at the surface, so whatever energy arrives gets dumped into thin, water-rich tissue. What happens next depends on rate. Deliver the energy slowly and that layer warms, cooks, and conducts heat sideways into its neighbours. Deliver it fast enough and the layer flashes to vapour before conduction gets a vote.

That is the whole difference between the modes. The electronics gate one output in three patterns: an uninterrupted stream (continuous wave), trains of pulses, or brief high-energy bursts (ultrapulse) — with the fractional scanner, the CPG on the spec line, placing those bursts in a grid. Our engineering archive adds a detail worth memorising: on this class of platform the tube runs at full output, and the console sets energy by widening or narrowing the pulse. You're not turning power up and down. You're deciding how long each bite lasts.

One formula covers the rest. Fluence equals power times time, divided by spot area. Same tube, same spot — the only lever left is time.

Continuous Wave: the Heat Has Nowhere to Go

CW is the simplest scheme. Press the footswitch and the beam stays on. Tissue at the centre vaporizes, but heat keeps flowing outward for as long as the beam dwells, and tissue conducts it in every direction. The coagulated margin around the ablation crater grows. Dwell longer still and you get carbonisation — char — which absorbs the beam even more strongly and drives temperatures higher again.

For some jobs that's a feature, not a flaw. An uninterrupted beam cuts smoothly and seals small vessels as it goes, which is why CW remains the working mode for incisional and debulking tasks where a dry field matters more than a narrow thermal margin.

For resurfacing, it's the failure mode. Early continuous-output resurfacing put broad, poorly controlled thermal injury into the dermis, and the delayed scarring and pigment problems that followed gave CO2 skin work a reputation it took years to shed. The lesson was never that CO2 is dangerous. The lesson was that dwell time was too long.

Chopped pulses are not really pulses

One trap sits between CW and true pulsed operation. Any continuous source can be switched on and off rapidly — our archive calls this the chopped-pulse scheme, and it's blunt about the result: peak power never rises above the CW level, so each brief on-period carries too little fluence to vaporize cleanly. Tissue heats instead of ablating. You keep the thermal downsides and gain almost nothing. When a budget spec sheet says pulsed, this is the first thing to question.

Thermal Relaxation Time, Minus the Math

To see why ultrapulse works you need exactly one concept, and no equations. Every heated object cools at a rate set by its size. Anderson and Parrish formalised this in their 1983 selective photothermolysis paper in Science: the thermal relaxation time is roughly how long a heated target takes to lose half its excess heat, and it shrinks steeply — with the square of target size — as targets get smaller. Small structures shed heat in a fraction of the time bulky ones need.

The operating rule follows directly. Put your energy in faster than the target can shed it, and the damage stays inside the target. Take longer, and heat leaks into whatever sits next door. Because the layer a CO2 beam heats is so shallow, it cools quickly, and the delivery window is correspondingly short. Miss the window and the energy that should have gone into vaporisation goes into conduction instead.

Ultrapulse: Finish Before the Heat Spreads

Ultrapulse is the engineering answer: raise peak power well above the CW level and compress delivery into a burst short enough to beat the clock. Our engineering notes define an ultrapulse CO2 pulse as one shorter than 2 ms. The CF-01 operates well inside that boundary, at 0.067 to 0.67 ms, with 2 to 200 mJ available per pulse. Each burst vaporizes its column of tissue and is finished before meaningful conduction begins. What's left is a clean crater with a thin coagulation rim — enough rim to seal capillaries and trigger remodelling, not enough to cook the surroundings.

Fractional mode is ultrapulse plus geometry. The scanner distributes bursts into a grid of micro-columns with untouched skin between them — the wound-healing concept Manstein and colleagues published in Lasers in Surgery and Medicine in 2004. Intact tissue between columns supplies the repair, which is what makes ablative resurfacing workable as an outpatient procedure rather than a hospital one.

The pulse interval row matters too

Look at the CF-01 line reading Pulse Interval: 1–100 ms. That's cooling time between consecutive bursts. Fire pulses too close together and heat accumulates shot by shot — bulk heating through the back door, even when every individual pulse is perfect. Randomised scanning serves the same end: adjacent grid points aren't hit back to back, so each site cools while distant ones are being treated.

Matching Mode to Clinical Task

Where each mode earns its keep

• CW: incisional work, debulking of larger benign growths, coagulation-heavy tasks where bleeding control is the priority.
• Ultrapulse, focal: precise ablation of small benign epidermal lesions. Our archive recommends short-pulse superficial ablation for benign epidermal growths and superficial pigmented lesions such as seborrhoeic keratoses — narrow margins, minimal char, quick healing.
• Fractional (CPG): gridded resurfacing — atrophic acne scarring, surgical and traumatic scars, textural photodamage. The acne scar and scar treatment pages cover the indication side in detail.

Two cautions belong in any honest version of this list. Any pigmented lesion that looks atypical gets a biopsy or a dermatology referral before a laser touches it — ablation destroys the histology you'd otherwise be able to read. And this article is an engineering explainer, not medical advice; treatment decisions belong with a trained clinician who has the patient in front of them.

Reading the Mode Line Before You Buy

Mode nomenclature isn't standardised across vendors. One brochure's ultrapulse is another's superpulse, and a third may quietly mean gated CW. Compare numbers, never names. Four questions get you there:

1. Is pulsed operation true high-peak-power pulsing, or gated CW? Ask for peak power versus rated CW power. If they're the same figure, it's a chopped beam.
2. What's the shortest pulse width, and how much energy is available at it? A short pulse with no energy behind it can't ablate. The two numbers only mean something together.
3. Is the pulse interval adjustable, and does the scanner randomise? Both are your defences against cumulative heating in fractional work.
4. Does every listed mode actually fire at handover? Run all three on a test target during acceptance, not on your first patient.

For the full platform parameters behind the examples here — spot sizes, scan geometries, tube type, delivery arm — see the CF-01 CO2 fractional laser page. And if you're still weighing whether 10600 nm is the right wavelength for your caseload at all, the aesthetic laser wavelength map shows where it sits among the alternatives.