Q-switching and mode-locking are powerful techniques for generating intense laser pulses. Q-switching builds up energy before releasing it in a short burst, while mode-locking synchronizes laser modes to create ultra-short pulses.

These methods enable applications like laser cutting, range finding, and studying ultrafast phenomena. Q-switching offers high energy pulses, while mode-locking produces incredibly short pulses, each with unique advantages for different uses.

Q-Switching Techniques

Q-switching for laser pulses

• Modulates Q-factor (quality factor) of laser cavity represents ratio of energy stored to energy lost per oscillation cycle
• Builds up large population inversion in gain medium before lasing occurs
• Suddenly increasing Q-factor (switching) releases stored energy in short, intense pulse
• Enables pulsed laser ablation for material processing, laser range finding, LIDAR (Light Detection and Ranging)
• Allows nonlinear optics experiments requiring high peak power

Active vs passive Q-switching

• Active Q-switching methods:
  • Electro-optic Q-switching uses electro-optic modulator (Pockels cell) to control cavity losses changes polarization state of light in response to applied electric field
  • Acousto-optic Q-switching employs acousto-optic modulator (AOM) to control cavity losses diffracts light using sound waves, effectively acting as fast shutter
• Passive Q-switching methods:
  • Saturable absorber Q-switching uses material with intensity-dependent absorption (saturable absorber) inside cavity initially attenuates light, preventing lasing
  • As intensity builds up, absorber becomes saturated (transparent), allowing pulse to develop
  • Simpler and more compact than active methods
  • Offers less control over pulse timing and repetition rate compared to active methods

Mode-Locking Techniques

Principle of mode-locking

• Establishes fixed phase relationship (locking) between longitudinal modes of laser cavity
• Constructive interference of locked modes generates train of short, intense pulses
• Laser cavity supports multiple longitudinal modes with slightly different frequencies
• If modes oscillate independently (random phases), output is continuous wave (CW) with fluctuations
• Synchronizing phases of modes through mode-locking mechanism generates short pulses
  • Modes interfere constructively at one point, resulting in high-intensity peak
  • Modes interfere destructively elsewhere, suppressing background
• Enables generation of pulses much shorter than cavity round-trip time pulse duration inversely proportional to bandwidth of locked modes

Types of mode-locking techniques

• Active mode-locking:
  • Utilizes external modulator to synchronize phases of modes
  • Amplitude modulation (AM) mode-locking uses electro-optic or acousto-optic modulator to modulate cavity losses at cavity round-trip frequency
  • Frequency modulation (FM) mode-locking employs electro-optic phase modulator to modulate phase of light at cavity round-trip frequency
• Passive mode-locking:
  • Relies on nonlinear optical element (saturable absorber) to self-modulate light
  • Kerr-lens mode-locking (KLM) exploits intensity-dependent refractive index (Kerr effect) of gain medium, leading to self-focusing and self-amplitude modulation
  • Semiconductor saturable absorber mirror (SESAM) mode-locking uses semiconductor saturable absorber mirror to introduce intensity-dependent losses, favoring formation of short pulses
• Passive mode-locking techniques generally produce shorter pulses than active methods not limited by modulation speed of external devices

Factors in pulse characteristics

• Q-switched lasers:
  • Pulse duration determined by cavity round-trip time and switching speed of Q-switch faster switching and shorter cavity lengths lead to shorter pulses
  • Peak power depends on energy stored in gain medium before switching and pulse duration higher stored energy and shorter pulses result in higher peak power
  • Repetition rate limited by time required to replenish population inversion after each pulse determined by pump power and upper-state lifetime of gain medium
• Mode-locked lasers:
  • Pulse duration inversely proportional to bandwidth of locked modes broader bandwidth supports shorter pulses
  • Dispersion management crucial to maintain short pulses by compensating for pulse broadening effects
  • Peak power depends on average power and pulse duration shorter pulses and higher average power lead to higher peak power
  • Repetition rate determined by cavity round-trip time (cavity length) shorter cavities result in higher repetition rates
• Choice of gain medium, cavity design, and operating parameters influence achievable pulse characteristics in both Q-switched and mode-locked lasers