Pulse duration is the length of time a laser emits light during a single pulse, measured at the point where the power reaches half its peak value on both sides of the pulse. This measurement, called full width at half maximum (FWHM), is the standard way the laser industry defines how long a pulse lasts. Pulse durations range from milliseconds in basic industrial lasers down to attoseconds in cutting-edge research systems, and the choice of pulse duration fundamentally shapes what a laser can do.
How Pulse Duration Is Measured
A laser pulse isn’t a perfect on-off switch. The power ramps up, hits a peak, and ramps back down. To standardize the measurement, engineers draw a line at half the peak power and measure the time between where the pulse crosses that line on the way up and again on the way down. That time window is the pulse duration.
The units involved are often unfamiliar outside of physics. Nanoseconds (billionths of a second) and femtoseconds (millionths of a billionth of a second) are the most common. Here’s a quick scale:
- Millisecond (ms): one thousandth of a second (10⁻³ s)
- Microsecond (μs): one millionth of a second (10⁻⁶ s)
- Nanosecond (ns): one billionth of a second (10⁻⁹ s)
- Picosecond (ps): one trillionth of a second (10⁻¹² s)
- Femtosecond (fs): one quadrillionth of a second (10⁻¹⁵ s)
- Attosecond (as): one quintillionth of a second (10⁻¹⁸ s)
Pulses from a few tens of nanoseconds up to hundreds of microseconds are generally classified as “long” pulses. Those in the picosecond-to-femtosecond range qualify as “ultrashort.” Everything in between falls under “short.”
Why Shorter Pulses Hit Harder
The key relationship is simple: peak power equals pulse energy divided by pulse duration. If you pack the same amount of energy into a shorter burst, the instantaneous power skyrockets. A laser with modest total energy can deliver enormous peak power if the pulse lasts only femtoseconds. This is why ultrashort pulse lasers can vaporize material at a precise point without heating the surrounding area, and why they’re essential for applications that demand extreme precision.
How Different Pulse Durations Are Created
Two main techniques generate short and ultrashort pulses. Q-switching stores energy inside the laser cavity and releases it in a single burst, typically producing pulses in the nanosecond range. Specialized versions, like diode-pumped microchip lasers, can push Q-switched pulses down to around 270 picoseconds.
Mode-locking takes a different approach. It forces many light frequencies inside the laser cavity to oscillate in sync, and their interference creates a train of extremely short pulses. Active mode-locking produces pulses in the 10 to 100 picosecond range. Passive mode-locking with fast-responding materials can go much shorter. In titanium-sapphire lasers, this technique has produced pulses as brief as about 6.5 femtoseconds, which is close to the theoretical minimum for that type of laser.
Pulse Duration in Medicine
In medical lasers, pulse duration is chosen based on a concept called thermal relaxation time: the time it takes for a target structure (a blood vessel, a pigment particle, a layer of tissue) to cool by releasing absorbed heat to its surroundings. When the laser pulse is shorter than this relaxation time, energy stays confined to the target and doesn’t spread to nearby tissue. When the pulse is longer, heat diffuses outward and can damage surrounding structures. Research has identified 1.0 microsecond as a critical threshold below which thermal effects on surrounding tissue become negligible.
Eye Surgery
Femtosecond lasers are now standard for creating corneal flaps in LASIK and similar procedures. The optimal pulse duration for eye microsurgery sits in the range of 150 to 500 femtoseconds. Going below 100 femtoseconds actually causes problems: nonlinear optical effects like self-focusing can distort the beam and reduce cut quality.
Tattoo Removal
Tattoo removal illustrates the practical difference between nanosecond and picosecond pulses clearly. Traditional nanosecond lasers operate with pulse durations around 50 nanoseconds. Newer picosecond systems deliver pulses of 375 to 450 picoseconds. The ink particles embedded in skin are small enough that their thermal relaxation time is shorter than a nanosecond pulse, meaning nanosecond lasers aren’t actually confining energy efficiently to the ink. Picosecond pulses match the relaxation time more closely, shattering particles more effectively with less collateral heating.
Pulse Duration in Manufacturing
When cutting, drilling, or engraving materials at a microscopic scale, the enemy is the heat-affected zone: the area around the cut where heat changes the material’s properties, warps it, or leaves rough edges. Ultrashort pulse lasers (femtosecond range) minimize this zone dramatically. The pulse deposits its energy and the material ablates before heat has time to conduct into the surrounding area. This makes femtosecond lasers the tool of choice for micromachining electronics, medical devices, and precision components where tolerances are measured in micrometers.
Longer pulse lasers, in the microsecond to millisecond range, are still widely used for welding, cutting thick metals, and other industrial tasks where some heat spread is acceptable or even desirable for melting and fusing material.
Pulse Duration in Fiber Optic Communication
In data transmission through fiber optic cables, pulse duration determines how tightly you can pack information. As light pulses travel through fiber, they spread out over distance. A fiber that spreads pulses by 200 picoseconds per kilometer limits signaling to about 2.5 billion symbols per second over 1 km, but only 250 million symbols per second over 10 km. Shorter, sharper pulses can be packed closer together, but they’re also more susceptible to spreading. This tradeoff between pulse duration, fiber length, and data rate is one of the core engineering challenges in telecommunications.
Laser Safety and Pulse Duration
Safety limits for laser exposure depend directly on pulse duration. The American National Standards Institute sets maximum permissible exposure (MPE) levels that change with how long each pulse lasts. For visible and near-infrared lasers, pulses between 1 nanosecond and 18 microseconds follow one formula, while sub-nanosecond pulses follow different, more complex rules. Shorter pulses concentrate energy into smaller time windows, so even low-energy pulses can exceed safe thresholds. For pulsed lasers firing repeatedly, the safety calculation must account for three separate rules covering single-pulse exposure, average power, and the cumulative thermal effect of multiple pulses, with the most restrictive rule winning.
The Shortest Pulse Ever Created
The current record for the shortest laser pulse stands at 19.2 attoseconds, a soft X-ray flash produced by researchers pushing the boundaries of ultrafast science. At that timescale, the pulse acts as the fastest camera in existence, capable of capturing the movement of electrons around atoms in real time. Electrons govern chemical reactions, electrical conductivity, biological energy transfer, and quantum technology, but they move too fast for any other measurement tool. These attosecond pulses are opening direct observation of processes in photovoltaics, catalysis, and quantum devices that were previously invisible.