Laser

How lasers work

Common laser types

JavaScript — laser concepts

// Coherence length (rough)
function coherenceLength(linewidth_Hz, c = 3e8) {
  return c / linewidth_Hz;
}

// Single-mode HeNe: ~1 MHz linewidth → ~300 m coherence
console.log(`HeNe coherence: ${coherenceLength(1e6).toFixed(0)} m`);

// Cavity stability: ABCD matrix etc — complex; just g-parameter
function cavityStability(g1, g2) {
  // 0 ≤ g1·g2 ≤ 1 for stable cavity
  return g1 * g2 >= 0 && g1 * g2 <= 1;
}

// Photon lifetime in cavity
function photonLifetime(L_meters, R_mirror1, R_mirror2, n = 1) {
  // τ = -L·n / (c·ln(R1·R2))
  // For R1·R2 close to 1
  const c = 3e8;
  return -L_meters * n / (c * Math.log(R_mirror1 * R_mirror2));
}

// 1m cavity, R1 = 0.99, R2 = 0.99
console.log(`Cavity τ: ${(photonLifetime(1, 0.99, 0.99) * 1e9).toFixed(2)} ns`);

// Laser intensity (CW)
function laserIntensity(power_W, beam_diameter_mm) {
  const area = Math.PI * Math.pow(beam_diameter_mm * 1e-3 / 2, 2);
  return power_W / area;  // W/m²
}

// 1 mW pointer, 2 mm beam
console.log(`1 mW pointer: ${laserIntensity(0.001, 2).toExponential(2)} W/m²`);

// 1 W laser, 1 mm beam (industrial cutting)
console.log(`1 W cutting laser: ${laserIntensity(1, 1).toExponential(2)} W/m²`);

// Pulse energy (Q-switched)
function qSwitchPulse(repetitionRate_Hz, avgPower_W) {
  return avgPower_W / repetitionRate_Hz;  // J per pulse
}

// 100 W average, 1 kHz rep rate, 5 ns pulses → peak power = J/(5e-9)
const pulse_J = qSwitchPulse(1000, 100);
console.log(`Pulse energy: ${pulse_J} J = ${pulse_J / 5e-9} W peak`);

// LASIK: very precise UV ablation
function lasikSettings() {
  return {
    laser: 'ArF excimer',
    wavelength: '193 nm',
    pulse_energy: '~1 mJ',
    pulse_rate: '~500 Hz',
    spot_size: '~1 mm',
    application: 'Reshape cornea, ~50 µm tissue removal'
  };
}
console.log(lasikSettings());

// Free electron laser parameters (rough)
function felParameters() {
  return {
    accelerator: 'Linac, ~10+ GeV electrons',
    undulator: 'Periodic magnetic structure',
    wavelength_tunable: '~10 nm to ~1 nm (X-ray)',
    pulse_duration: 'Femtosecond',
    peak_power: '~10 GW'
  };
}

Where lasers matter

• Communications. Fiber optics (entire internet); laser diodes in transmitters/receivers.
• Medicine. Surgery, ophthalmology (LASIK), cosmetic, diagnostic imaging.
• Manufacturing. Cutting, welding, drilling, marking. Industrial fiber lasers replacing many traditional methods.
• Research. Atomic physics, ultrafast processes, fusion, gravitational waves, atomic clocks.
• Imaging. Confocal microscopy, OCT, lidar, holography.
• Consumer. Optical drives, barcode scanners, laser printers, laser pointers, light shows.
• Defense. Range-finding, target designation, directed energy weapons (early stages).

Common mistakes

• Treating any concentrated light as laser. Lasers specifically use stimulated emission for coherence. Sunlight can be focused but isn't coherent or monochromatic.
• Believing lasers are dangerous to look at universally. Class 1 lasers (most consumer) are safe in normal use. Higher classes (3R, 3B, 4) require eye protection. Always check class.
• Confusing laser with LED. LED — spontaneous emission, broad spectrum, not coherent. Laser — stimulated emission, narrow spectrum, coherent.
• Treating "monochromatic" as exact. Real lasers have finite linewidth. Highly stable lasers can have linewidth < Hz; typical lab lasers ~kHz to MHz.
• Forgetting nonlinear effects. At high intensity, materials respond non-linearly — second harmonic generation, self-focusing, multiphoton absorption. Important for ultrafast / high-power lasers.
• Confusing coherence with monochromaticity. Related but distinct. Coherence = phase relationship; monochromaticity = single frequency. Laser is both, but they can be partially separated (broadband coherent sources exist).