Continuous-wave (CW) vs pulsed lasers

A continuous-wave (CW) laser emits an output power that is constant in time, apart from noise; a pulsed laser emits its energy in pulses separated by intervals with little or no output. The distinction is about the time structure of the output, and the same gain medium can often run either way. A pulsed laser is described by its average power $P_\text{avg}$, repetition rate $f_\text{rep}$, pulse duration $\tau$, pulse energy $E$ and peak power $P_\text{peak}$. Typical durations range from about 10 fs for mode-locked Ti:sapphire to tens of nanoseconds for Q-switched lasers, and repetition rates from single shots to several gigahertz.

Ways of making pulses

Q-switching holds the cavity loss high while the pump builds up a large inversion, then switches the loss low so the stored energy leaves in one pulse of a few to a few hundred nanoseconds, at rates from hertz to hundreds of kilohertz. Mode locking fixes the phases of many longitudinal modes so that they add up to a train of picosecond or femtosecond pulses at the cavity round-trip rate, typically tens of megahertz to gigahertz. Gain switching pulses the pump of a laser diode so fast that only the first relaxation spike escapes, giving pulses of 10–100 ps. A CW laser can also be pulsed externally by a modulator or a chopper; this gives pulses only as short as the switch and never raises the peak above the CW power.

Between the two regimes is quasi-CW (QCW) operation, in which diode bars and fiber lasers are driven with long pulses, typically 100 µs to a few milliseconds at a duty cycle of a few percent. Each pulse reaches steady state, so the laser behaves as a CW source during the pulse, while the average heat load stays far below that of continuous operation.

Average power, energy and peak power

The three powers are linked by

$$E = \frac{P_\text{avg}}{f_\text{rep}}, \qquad P_\text{peak} = k\,\frac{E}{\tau},$$

where $k$ depends on the pulse shape: 0.881 for sech², 0.939 for Gaussian and 1 for a rectangular pulse. The duty cycle $D = \tau f_\text{rep}$ gives $P_\text{peak} \approx P_\text{avg}/D$.

A mode-locked oscillator at 1 W average power and 80 MHz has

$$E = \frac{1\ \text{W}}{80\ \text{MHz}} = 12.5\ \text{nJ},$$

and with 100 fs sech² pulses the peak power is about 110 kW, at a duty cycle of 8 × 10⁻⁶. A Q-switched laser giving 1 mJ pulses of 10 ns at 1 kHz also averages 1 W. Its peak power, taking a Gaussian shape, is about 94 kW, close to the oscillator's, but each pulse carries 80 000 times more energy. A CW laser at 1 W delivers 1 W at every instant. The comparison shows why the average power alone says little about what a laser can do to a material or a nonlinear crystal.

Where each is used

CW lasers serve where a steady, narrow-band output matters: telecom transmitters and the carriers of coherent links (whose data modulation is imposed on a CW source), high-resolution spectroscopy, interferometry and metrology, pumping of other lasers and amplifiers, and much of materials processing such as cutting and welding with multi-kilowatt fiber lasers. The narrowest linewidths come from CW operation, since a pulse of duration $\tau$ has a spectral width of at least $0.315/\tau$ to $0.441/\tau$, depending on its shape.

Pulsed lasers serve where peak power, energy per pulse or timing matters. Nonlinear optics, including harmonic generation and multiphoton microscopy, scales with peak intensity. Short pulses deposit energy before heat diffuses, which gives clean machining with picosecond and femtosecond lasers. Lidar and range finding use the time of flight of nanosecond pulses. Pulsed operation is also a measurement technique: short current pulses at low duty cycle characterize a laser diode without self-heating, as described in Pulsed vs CW LIV measurement.

Pitfalls

A thermal power meter reads the average power of a pulsed laser, and it is easy to forget that the peak may be 10⁵ times higher; optics and detectors rated for CW power can be damaged by pulses of the same average. Conversely, a fast photodiode or oscilloscope with too little bandwidth shows a pulse that is longer and lower than it is. Amplified spontaneous emission or leakage between pulses adds to the average power without adding to the pulse energy, which inflates the computed peak.

Common questions

What is the difference between a CW and a pulsed laser?

A CW laser emits steady power; a pulsed laser emits short bursts of energy. For the same average power, a pulsed laser reaches a peak power that is higher by roughly the inverse of its duty cycle.

What does quasi-CW mean?

The laser is driven with long pulses, usually 100 µs or more at a few percent duty cycle, so it reaches steady state during each pulse while running cooler on average than in true CW operation.

References: A. E. Siegman, Lasers (University Science Books, 1986); O. Svelto, Principles of Lasers, 5th ed. (Springer, 2010); B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics, 3rd ed. (Wiley, 2019).