Lecture 9 and 10: Pulsed Lasers and Mode-Locking

Lecture 9 and 10: Pulsed Lasers and Mode-Locking

Lecture 9 and 10: Pulsed lasers and mode-locking 1. Ultrashort pulses Ultrashort pulse shorter than 1 ps (10-12 s) Shortest pulses generated from lasers: Type of laser Pulse duration Fiber laser ~35 - 50 fs Solid-state laser (e.g. Yb) ~40-50 fs Ti:Sapphire laser ~ 3 fs – 7 fs The shortest possible pulse of a given wavelength is one light cycle long: 1550 nm 4.3 fs 800 nm 2.7 fs Shorter wavelength shorter pulses 2. Measurement of ultrashort pulses Techniques: • Optical autocorrelation • FROG (Frequency Resolved Optical Gating) Improved version: GRENOULLIE • SPIDER (Spectral Phase Interferometry for Direct Electric-field Reconstruction) Autocorrelator Interferometric autocorrelation trace: FROG Source: Rick Trebino, Frequency Resolved Optical Gating SPIDER 3. Mode-locked lasers a) Difference between a CW and mode-locked laser: A mode-locked laser: Emits a train of equally-spaced, ultrashort pulses (femtosecond) - down to almost single- cycle Output spectrum is broad (tens-hundreds of nanometers)/ Broader spectrum shorter pulses In the frequency domain, the output consists of thousands/millions of narrow lines (comb- like structure) b) Relation between the number of synchronized modes and the pulse duration: More synchronized modes shorter pulse. c) Passive mode-locking and saturable absorber In order to achieve passive mode-locking we need a device called saturable absorber. The saturable absorber: - Blocks any CW radiation in the cavity - It forces the laser to synchronize the modes and generate pulses - Only high-intensity pulse can pass the saturable absorber Fig. Transmission of the saturable absorber vs intensity d) Active mode-locking In active mode-locking: • We need a modulator driven with an external signal • We can tune the repetition rate (multiplication by „N”) • The pulses are longer than with passive mode-locking e) Saturable absorbers: f) Artificial saturable absorbers: • It is a nonlinear optical effect acts as a saturable absorber • In general, nonlinear effects are power-dependent • Short pulses have a high peak power, so they can induce nonlinear effects Examples: • Nonlinear polarization rotation/evolution (NPR/NPE) • Nonlinear loop mirror (NOLM), nonlinear amplifying loop mirror (NALM) • Kerr-lens mode-locking (KLM) NPR/NPE: • The polarization state of light changes during propagation through a fiber • If the pulse is intense enough, different parts of the pulse might have different polarizations • Very fast mode-locking mechanism NOLM/NALM Fig. NOLM (a) and NALM (b) setup Laser with NOLM: KLM • Sometimes called „magic mode-locking” • The refractive index depends on the intensity: n(I) = n0 + n2(I) • „Kerr-lensing” – due to nonlinearities, the high intensity pulse is better focused than CW (so called „self-focusing”) • Only the pulsed beam can pass through the aperture g) Real saturable absorbers SESAM • Semiconductor Saturable Absorber Mirror (SESAM) • Mirror structure with GaAs quantum wells Fig. SESAM structure • Variety of SESAMs available commercially for different wavelengths • Flexible design: modulation depth, non-saturable losses and saturation intensity can be designed • Narrow bandwidth • Not always resistant to high optical powers • Degradation over time Fiber lasers with SESAM: Nanomaterial-based SAs • Main drawback of SESAMs: narrow-band operation (you need to have specific SESAMs for each laser) • Is it possible to have a broadband SA which could work in any laser, at any wavelength? • … novel low-dimensional (or two-dimensional nanomaterials) • Materials which can be exfoliated to single atomic layers (nanometer thickness) Graphene – single layer of carbon atoms, ordered in a „honeycomb” hexagonal lattice Optical properties of graphene: • Absorption of a single graphene layer: 2.3% (scales with the number of layers), • The absorption is wavelength-independent, • Due to the 3rd order nonlinearities, it shows saturable absorption at very low optical intensities Fig. Saturable absorption of single layer graphene and multilayer graphene Other SA materials: • Topological insulators (TI) – Sb2Te3, Bi2Te3, Bi2Se3 • Black Phosphorus (phosphorene, BP) • Transition metal dichalcogenides (TMD) – MoS2, MoSe2, WS2, etc. • Carbon Nanotubes (CNT) Integration of SA with fiber laser Saturable absorption measurement: Free-space setup for bulk samples (so called Z-scan) Fiber-based setup for fiber SA samples 4. The role of dispersion in mode-locked lasers • Net dispersion of the resonator determines the mode-locking regime • Generally we distinguish three basic regimes – All-anomalous – Near-zero (balanced) – Normal • The mode-locking regime influences the laser performance (pulse duration, spectral width, output power) Yb Er Tm Fig. dispersion of SMF-28e fiber with dashed spectral regions of typical lasers a) Anomalous dispersion regime • Net GDD < 0 • Typical for 1.55 and 2 µm lasers • Usually transform-limited pulse duration, but >100 fs long • Limited bandwidth and energy • Sech2 pulse shape with Kelly’s sidebands (optical soliton) Fig. typical soliton-like spectrum b) Normal dispersion regime • Net GDD > 0 • Typical for 1.06 µm lasers • Output pulses are chirped • Spectrum with steep edges and flat top (dissipative soliton) Fig. typical spectrum of all-normal dispersion laser (so called dissipative soliton) c) Balanced dispersion regime • Net GDD = 0 • Output pulses are chirped • Very broad and smooth spectra • So called „stretched-pulse” Fig. typical spectrum of balanced dispersion laser (so called stretched pulse regime) Fig. Soliton vs. stretched pulse – comparison of spectra generated in two regimes 5. Modeling of mode-locked fiber lasers The simulation takes info account all important factors: • Dispersion • Nonlinear effects • Saturable absorber • Loss (e.g. the output coupler) • Gain Fig. A numerical simulation enables optimization of lasers and better understanding of pulse dynamics inside the resonator 6. Polarization maintaining fibers “In fiber optics, polarization-maintaining optical fiber (PMF or PM fiber) is a single-mode optical fiber in which linearly polarized light, if properly launched into the fiber, maintains a linear polarization during propagation, exiting the fiber in a specific linear polarization state” (source: Wikipedia ) (Fiberlabs.com) If the mode-locked laser is made only of PM fibers and components: • One polarization propagating in the cavity • Only way to achieve robust, stable, and alignment-free operation • Almost totally resistant to any external disturbances, temperature changes, humidty, vibrations, etc. • Turn-key operation • Defined output polarization • No polarization controller needed .

View Full Text

Details

  • File Type
    pdf
  • Upload Time
    -
  • Content Languages
    English
  • Upload User
    Anonymous/Not logged-in
  • File Pages
    14 Page
  • File Size
    -

Download

Channel Download Status
Express Download Enable

Copyright

We respect the copyrights and intellectual property rights of all users. All uploaded documents are either original works of the uploader or authorized works of the rightful owners.

  • Not to be reproduced or distributed without explicit permission.
  • Not used for commercial purposes outside of approved use cases.
  • Not used to infringe on the rights of the original creators.
  • If you believe any content infringes your copyright, please contact us immediately.

Support

For help with questions, suggestions, or problems, please contact us