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 .