Beam Compression
Beam compression in lasers and photonics most often means spatial reduction of a laser beam’s transverse size (diameter or width) while preserving collimation as much as possible. It is the inverse of beam expansion. In high-power ultrashort-pulse systems the same phrase is sometimes used more loosely for temporal pulse compression, because the “beam” includes both spatial and temporal profiles.
Spatial beam compression:
A collimated input beam of larger diameter is transformed into a collimated output beam of smaller diameter. Because the beam-parameter product (BPP = w0θ, or equivalently étendue) is conserved in a lossless optical system, shrinking the waist increases far-field divergence. Intensity scales as 1/w2, so compression raises peak intensity.
Common implementations:
Reversed Galilean or Keplerian telescope (two lenses or mirrors whose focal lengths set the magnification M=f2/f1<1).
Single-element “beam compressors” (meniscus or cylindrical-lens singlets) used especially with diode-laser bars and stacks to squeeze the fast-axis spacing of multiple emitters.
Anamorphic prism pairs or cylindrical optics for one-dimensional compression.
Gradient-index (GRIN) photonic-crystal or parabolic-index structures for compact spatial compression.
Typical compression ratios range from ~0.25–0.9 depending on the application. Care must be taken with aberrations, wavefront quality (M2), and damage thresholds when high peak powers are involved.
Temporal (pulse) compression:
In chirped-pulse amplification (CPA) and post-compression schemes the term “compressor” almost always denotes a device that shortens pulse duration. A positively chirped, stretched pulse is sent through a dispersive delay line (Treacy grating pair, prism pair, or chirped mirrors) that applies the opposite group-delay dispersion, restoring a short pulse. Nonlinear methods first broaden the spectrum (self-phase modulation in fiber, bulk, or gas) and then compress the new bandwidth.
Peak power rises because energy is packed into a shorter time window. Grating compressors for petawatt-class lasers are large, vacuum-housed systems whose size and damage threshold currently limit maximum energy.
Applications:
Spatial compression:
Raising intensity for laser machining, engraving, nonlinear frequency conversion, or high-harmonic generation.
Matching a large-aperture beam to a smaller optic, fiber, or waveguide.
Increasing brightness of diode-laser arrays by reducing emitter spacing.
Preparing a beam for tight focusing.
Temporal compression:
Generating the highest peak powers in CPA / OPCPA systems (TW–PW class).
Producing few-cycle or sub-10-fs pulses for attosecond science and strong-field physics.
Post-compression of high-repetition-rate Yb lasers to reach GW–TW peak powers at high average power.
Both forms of compression are limited by optical damage, residual spatial or temporal aberrations, and conservation laws (BPP for space, time–bandwidth product for pulses).
In practice they are frequently combined: a spatially compressed beam is sent into a pulse compressor, or a pulse compressor is followed by a focusing optic that further concentrates energy in space.