Nonlinear Mode-Locking PDF

Non-linear Mode-locking By : Prof. Walid Tawfik NILES, Cairo University 3/24/2020 Prof. Walid Tawfik 1  Cavity modes The spatial and temporal variations of field distributions at particular frequency are called cavity mode. A standing wave pattern is set up. many longitudinal modes...

Non-linear Mode-locking By : Prof. Walid Tawfik NILES, Cairo University 3/24/2020 Prof. Walid Tawfik 1  Cavity modes The spatial and temporal variations of field distributions at particular frequency are called cavity mode. A standing wave pattern is set up. many longitudinal modes can oscillate simultaneously 3/24/2020 Prof. Walid Tawfik 2 Cavity modes ln = 2 L/n Δν= c/2 L 3/24/2020 Prof. Walid Tawfik 3 3/24/2020 Prof. Walid Tawfik 4 This value ofΔν is much less than freq. gain profile so many of these modes can oscillate simultaneously. 3/24/2020 Prof. Walid Tawfik 5 Concepts of Mode Locking Mode-locking is a technique in optics by which a laser can be made to produce pulses of light of extremely short duration, on the order of picoseconds (10−12 s) or femtoseconds (10−15 s(. RANDOM LOCKED phases phase for forall allthe thelaser lasermodes modes Out of phase Irradiance vs. Time In phase Out of phase Out of phase Time Time 3/24/2020 Prof. Walid Tawfik 6 Mode locking 3/24/2020 Prof. Walid Tawfik 7 Mode locking 3/24/2020 Prof. Walid Tawfik 8 Mode locking 3/24/2020 Prof. Walid Tawfik 9  CW MODE LOCKED 3/24/2020 Prof. Walid Tawfik 10  is repetition time The time duration 3/24/2020 Prof. Walid Tawfik 11 If we want to have very brief flashes we must use a large value of N i.e. we want to large number of modes oscillate simultaneously. The minimum pulse width of a laser system is given by A wide bandwidth material should be used to get short pulses. 3/24/2020 Prof. Walid Tawfik 12  Bandwidth vs Pulsewidth Basic principles of ultrafast lasers broadest broader spectrum spectrum DnDt = const. narrow Dn spectrum bandwidth continuous Dt (CW) wave duration pulses shortest (mode-locked) pulses 3/24/2020 Prof. Walid Tawfik 13  Techniques for mode locking 1) characterized by many spikes with relative high peak intensity. 3/24/2020 Prof. Walid Tawfik 14 2) 3/24/2020 Prof. Walid Tawfik 15  3) 3/24/2020 Prof. Walid Tawfik 16 4) Pulses with short duration. 3/24/2020 Prof. Walid Tawfik 17 3/24/2020 Prof. Walid Tawfik 18  Mode-locking Mechanisms: Active mode-locking – Acousto-optic modulator Passive mode-locking – Saturable absorber (dye, solid state) – Optical Kerr effect 3/24/2020 Prof. Walid Tawfik 19 Active mode-locking - Acousto-optic modulator The most common active mode-locking technique. places a standing wave acousto-optic modulator into the laser cavity. Amplitude modulator is in the form of acousto optics modulator 3/24/2020 Prof. Walid Tawfik 20  When driven with an electrical signal (e.g. 1-10 kilovolts), this produces a sinusoidal amplitude modulation of the light in the cavity. 3/24/2020 Prof. Walid Tawfik 21  The modulation frequency f is such that 1/f is the round trip time or multiples of round trip time. The output laser beam from a mode-locked laser with short pulses. 3/24/2020 Prof. Walid Tawfik 22  Mode-lock pulse train Often the cavity is partially closed during pumping pulse is generated within cavity and is allowed to bounce back and forth in cavity for many times (~ few hundred times) i.e. a firm mode-locking and extremely short pulse is achieved 3/24/2020 Prof. Walid Tawfik 23  Active Mode locking For mode-locking, make sure that wM = mode spacing. 3/24/2020 Prof. Walid Tawfik 24 This means that: wM = 2L/cavity round-trip time Active Mode locking 3/24/2020 Prof. Walid Tawfik 25 Active Mode locking 3/24/2020 Prof. Walid Tawfik 26 Active Mode locking The superposition of these N phase-locked modes results in the total amplitude the total time-dependent intensity 3/24/2020 Prof. Walid Tawfik 27  THE ADVANTAGES: AOMs offer more deflection range Simple design, and Low power consumption (less than 3 watts). 3/24/2020 Prof. Walid Tawfik 28  Passive mode-locking - Saturable absorber a saturable absorber placed in a laser cavity. Only intense spike will be transmitted preferentially by the saturable absorber. Pump Saturable beam absorber Gain medium 3/24/2020 Prof. Walid Tawfik 29 Saturable absorbers light absorbers with a degree of absorption which is decreases at high optical intensities At sufficiently high incident light intensity, atoms in the ground state of a saturable absorber material become excited into an upper energy state at such a rate that there is insufficient time for them to decay back to the ground state before the ground state becomes depleted, and the absorption subsequently saturates. 3/24/2020 Prof. Walid Tawfik 30 As intensity goes higher the bleaching effect makes the dye transparent 3/24/2020 Prof. Walid Tawfik 31 3/24/2020 Prof. Walid Tawfik 32 After many round trips, this leads to a train of pulses and mode-locking of the laser. 3/24/2020 Prof. Walid Tawfik 33  Types of Saturable Absorbers Saturable absorbers used in the past were typically dyes Semiconductor saturable absorber mirrors. Quantum dots semiconductor saturable absorbers. Thin layers of carbon nanotubes. In rare cases, saturable absorber materials are used in the form of optical fibers. 3/24/2020 Prof. Walid Tawfik 34 Some common dyes and their corresponding saturable absorbers 3/24/2020 Prof. Walid Tawfik 35  Kerr-lens mode-locking Self-focussing can be used to suppress cw operation A medium’s refractive index depends on the intensity. n(I) = n0 + n2I 3/24/2020 Prof. Walid Tawfik 36 Kerr-Lensing Kerr medium (n = n0 + n2I) Low-intensity beam High-intensity ultrashort pulse Focused pulse 3/24/2020 Prof. Walid Tawfik 37 Kerr-lens mode-locking Placing an aperture at the focus favors a short pulse. 3/24/2020 Prof. Walid Tawfik 38 Kerr-lens mode-locking High-intensity pulse Mirror Additional focusing optics can arrange for perfect overlap of the high-intensity Ti:Sapph beam back in the Ti:Sapphire crystal. Low-intensity pulse But not the low- intensity beam! 3/24/2020 Prof. Walid Tawfik 39  Kerr-lens mode-locking It becomes energetically favourable for the light to be compressed into a short intense pulse. The pulse bounces back and forth through the cavity generating a train of output pulses. 3/24/2020 Prof. Walid Tawfik 40 Comparison Active with Passive Mode Locking Compared with passive mode locking, active mode locking typically generates longer pulses. A disadvantage is the need for an optical modulator, the electronic driver and (in most cases) means for synchronization. On the other hand, active mode locking can be the natural solution when pulse trains are required which are synchronized with some electronic signal, or when many lasers need to be operated in synchronism. Therefore, active mode locking is often used in the context of optical fiber communications. 3/24/2020 Prof. Walid Tawfik 41  Synchronous pumping *the mode-locked pump laser L1, which delivers short pulses with the time separation T = 2d1/c, is employed to pump another laser L2 *The optimum gain for the dye-laser pulses is achieved if they arrive in the active medium (dye jet) at the time of maximum inversion ΔN(t) 3/24/2020 Prof. Walid Tawfik 42 Synchronous pumping *If the optical cavity length d2 of the dye laser is properly matched to the length d1 of the pump laser resonator, the round-trip times of the pulses in both lasers become equal and the arrival times of the two pulses in the amplifying dye jet are synchronized. *requires an actively mode-locked laser to pump your laser *requires the two cavity lengths to be accurately matched *useful for converting long pulses into short pulses (e.g., 150 p sec argon-ion pulses  sub-p sec dye laser pulses) 3/24/2020 Prof. Walid Tawfik 43 Cavity dumping Definition: a method for extracting intense short or ultrashort pulses from a laser, using an optical switch in the laser resonator Cavity dumping is a technique for pulse generation which can be combined either with Q switching or with mode locking, or sometimes even with both techniques at the same time. In any case, the basic idea is to keep the optical losses of the laser resonator as low as possible for some time, so that an intense pulse builds up in the resonator, and then to extract this pulse within about one cavity round-trip time using a kind of optical switch (“cavity dumper”), such as an acousto-optic modulator or a Pockels cell 3/24/2020 Prof. Walid Tawfik 44 Cavity dumping 3/24/2020 Prof. Walid Tawfik 45 Cavity dumping 3/24/2020 Prof. Walid Tawfik 46