Pulsed Lasers — Abridged Guide

Quick-reference guide to pulsed lasers — Q-switching, pulse parameters, thermal effects, and technology selection. For full derivations and worked examples, see the Comprehensive Guide.

Comprehensive Pulsed Lasers Guide →

1.Introduction to Pulsed Lasers

Pulsed lasers concentrate energy into brief bursts, achieving peak powers thousands to millions of times higher than average power. The peak-to-average power ratio equals the inverse of the duty cycle: P_peak/P_avg = 1/D.

When comparing pulsed laser specs, first calculate the duty cycle. A 10 ns pulse at 10 kHz gives D = 10⁻⁴ — meaning peak power is 10,000× the average power. This single number tells you more than any marketing spec.

This guide covers nanosecond-and-longer pulses produced by gain switching, Q-switching, and cavity dumping. For sub-nanosecond (picosecond and femtosecond) pulses produced by mode-locking, see the Ultrafast Lasers guide.

2.Pulsed Laser Classification

Duty Cycle

D = \tau_p \cdot f_{\text{rep}}

Three methods generate ns–μs pulses: gain switching (simplest, least controlled), Q-switching (dominant method, highest energy), and cavity dumping (shortest pulses without mode-locking, lowest energy).

For most nanosecond applications, Q-switching is the default. Choose active Q-switching (AO or EO) when you need precise timing and repeatability. Choose passive Q-switching when compactness and simplicity matter more than timing control.

Method	Duration	Energy	Key Trade-off
Gain switching	μs – ns	nJ – mJ	Simple but pulse coupled to pump
Active Q-switch (AO)	5–200 ns	μJ – 100 mJ	Robust; switching time limits min. pulse
Active Q-switch (EO)	1–50 ns	μJ – J	Fastest; complex HV electronics
Passive Q-switch	0.5–50 ns	μJ – mJ	Compact; higher jitter
Cavity dumping	0.1–5 ns	nJ – 100 μJ	Shortest; low energy

3.Temporal Pulse Characteristics

Time-Bandwidth Product

\Delta\nu \cdot \tau_p \geq K

K = 0.4413 (Gaussian), 0.3148 (sech²).

Pulse duration is defined as FWHM per ISO 11554. A Gaussian pulse carries ~6% more energy than a square pulse at the same FWHM and peak power — check whether datasheets assume Gaussian or square shape.

Most multimode Q-switched lasers operate far above the transform limit (TBP >> K). This only matters if you need narrow linewidth for frequency conversion or spectroscopy — otherwise, ignore TBP.

Real Q-switched pulses are asymmetric: the leading edge (buildup) is steeper than the trailing edge (gain depletion decay). Rise time matters for plasma formation and time-resolved measurements.

4.Energy and Power Relations

Peak Power (Gaussian)

P_{\text{peak}} \approx \frac{0.94 \cdot E_p}{\tau_p}

Peak Fluence (Gaussian beam)

F_0 = \frac{2 E_p}{\pi w_0^2}

The simplified P_peak ≈ E_p/τ_p overestimates Gaussian peak power by ~6%. For damage threshold calculations, use the Gaussian correction factor (0.94) and always use peak fluence (factor of 2 over average fluence).

Two lasers with identical average power but different rep rates are very different sources. At fixed P_avg, doubling the rep rate halves the pulse energy and peak power. Always check pulse energy at the actual operating rep rate, not the maximum energy at minimum rep rate.

Relationship	Formula
Average power	P_avg = E_p × f_rep
Peak power (Gaussian)	P_peak ≈ 0.94 E_p / τ_p
Duty cycle	D = τ_p × f_rep
Peak fluence (Gaussian)	F₀ = 2E_p / (πw₀²)
Peak-to-average ratio	P_peak / P_avg = 1/D

5.Q-Switched Laser Dynamics

Cavity Photon Lifetime

t_c = \frac{2L}{c \cdot \delta}

Pulse Duration (Siegman)

\tau_p \approx \frac{3.5 \cdot t_c}{g_i - 1}

Q-switched pulse duration scales with cavity photon lifetime. Shorter cavities and higher output coupling produce shorter pulses but may reduce extraction efficiency. The buildup time — delay from Q-switch opening to pulse peak — is typically 1–3 cavity lifetimes.

To shorten Q-switched pulses: reduce cavity length, increase output coupling, or increase the initial gain-to-threshold ratio. Each option has trade-offs with energy and beam quality.

6.Repetition Rate and Thermal Effects

Thermal Lens (rod)

f_{\text{th}} = \frac{\pi K A}{P_{\text{heat}} \cdot (dn/dT)}

The upper-state lifetime sets the maximum useful rep rate. For Nd:YAG (τ_f = 230 μs), pulse energy drops above ~5 kHz. Nd:YVO₄ (τ_f ≈ 100 μs) maintains energy to ~10 kHz. Above these thresholds, pulse energy falls as ~1/f_rep at constant pump power.

When a datasheet lists max rep rate and max pulse energy separately, they usually cannot be achieved simultaneously. Always request the energy-vs-rep-rate curve.

Metric	Good	Typical	Meaning
Energy stability (RMS)	<1%	1–3%	σ/μ over stated window
Energy stability (P-P)	<3%	3–10%	Max excursion
Timing jitter (active Q)	<1 ns	1–5 ns	Trigger-to-pulse variation
Timing jitter (passive Q)	1–10% of period	—	Inherent to bleaching dynamics

7.Resonator Architectures

Architecture determines the performance ceiling. Free-space resonators deliver the highest pulse energy and peak power (kW–GW) but require alignment maintenance. Fiber MOPAs deliver the best beam quality and stability with moderate pulse energy (nJ–mJ) but are peak-power-limited by fiber nonlinearities. Microchip cavities are the most compact and lowest-cost option at the lowest energy.

If your application needs >1 mJ pulse energy at nanosecond duration, you need a free-space resonator or a hybrid fiber-seed/free-space-amplifier system. If it needs <100 μJ with high rep rate and zero maintenance, fiber MOPA is almost always the right choice.

Parameter	Free-Space	Fiber MOPA	Microchip
Pulse energy	mJ – J	nJ – mJ	nJ – μJ
Peak power	kW – GW	W – kW	kW
Beam quality	1.0–3.0	1.0–1.3	~1.0
Maintenance	Moderate–High	Low	Very low
Size	Bench	Rack/bench	Thumb-sized

8.Pulsed Laser Technologies

DPSS Nd:YAG/YVO₄ (1064 nm + harmonics) dominates scientific and moderate-energy applications. Fiber MOPAs (Yb at 1.0 μm, Er at 1.5 μm) dominate high-rep-rate industrial applications. Excimer lasers (193–351 nm) serve UV-specific applications where solid-state harmonics are insufficient.

For wavelength flexibility beyond the fundamental and its harmonics, consider an OPO (optical parametric oscillator) pumped by a ns DPSS source — these provide continuously tunable output across the visible and near-IR.

9.Pulse Measurement and Diagnostics

Bandwidth Requirement

\text{BW} \geq \frac{0.35}{\tau_{\text{rise}}}

The measurement system (detector + oscilloscope) must have sufficient bandwidth to resolve the pulse. For a 10 ns pulse with 3 ns rise time, the system needs >117 MHz bandwidth. Insufficient bandwidth makes pulses appear longer and lower than they actually are.

Cross-check your temporal measurement by integrating the waveform numerically and comparing to an energy meter reading. If they disagree by more than ~10%, your measurement bandwidth is likely insufficient.

Detector	Wavelength Range	Speed	Use For
Si photodiode	200–1100 nm	<1 ns rise	Visible/NIR pulse shape
InGaAs photodiode	900–1700 nm	<1 ns rise	SWIR pulse shape
Pyroelectric	Broadband	Single-shot	Per-pulse energy
Thermopile	Broadband	Slow (avg)	Average power → derived energy

10.Application-Driven Laser Selection

Start with the application, not the laser. Four parameters narrow the field: wavelength (material absorption), pulse energy/fluence (process threshold), repetition rate (throughput), and pulse duration (thermal vs. ablative regime).

The most common datasheet trap: maximum pulse energy and maximum rep rate listed separately, implying both can be achieved simultaneously. They usually cannot. Demand the energy-vs-rep-rate curve.

Application	Wavelength	Energy	Rep Rate	Best Technology
Laser marking (metals)	1064, 532 nm	0.1–1 mJ	20–200 kHz	Fiber MOPA or DPSS YVO₄
LIBS	1064, 532 nm	10–200 mJ	1–20 Hz	DPSS Nd:YAG
Eye-safe LIDAR	1550 nm	10–500 μJ	1–100 kHz	Er fiber MOPA
LASIK	193 nm	1–5 mJ	100–500 Hz	ArF excimer
Polymer micromachining	248, 355 nm	1–100 mJ	1–200 Hz	KrF excimer or DPSS 355 nm
Pump for OPO	1064, 532 nm	10–100 mJ	10–100 Hz	DPSS Nd:YAG

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The Comprehensive Guide includes 6 worked examples, 6 SVG diagrams, 3 data tables, and 12 references.