Tommi Hakulinen

Carbon nanotube films for ultrafast broadband technology

Mode-locked sub-picosecond operation of Yb-, Er- and Tm:Hodoped fiber lasers operating at 1.05 microm, 1.56 microm and 1.99 microm, respectively, is demonstrated using the same sample carbon nanotube-based saturable absorber mirror. A mesh of single-walled carbon nanotubes was deposited on an Ag-mirror using a one-step dry-transfer contact press method to combine broadband saturable absorption and high reflectance properties. The novel fabrication method of the polymer-free absorber and device parameters determined using nonlinear reflectivity measurement are described in detail. To our knowledge the observed operation bandwidth of approximately 1 microm is the broadest reported to date for a single carbon nanotube-based saturable absorber.

Tunable Raman Soliton Source Using Mode-Locked Tm–Ho Fiber Laser

We report a femtosecond pulse source that uses a mode-locked Tm-Ho oscillator and a self-frequency shift of Raman solitons in Tm-Ho power amplifier. The master oscillator mode-locked by an antimonide-based saturable absorber mirror produces 750-fs transform-limited soliton pulses over the tuning range from 1912 to 1972 nm. The soliton self-frequency shift in the amplifier resulted in transform-limited pulses with the wavelength adjusted by varying the amplifier pump power. We obtain ~150-fs soliton pulses at the wavelength of 2150 nm with average power up to 230 mW corresponding to the peak power of 27 kW. The efficiency of Raman conversion ranges from 47% to 62% over the tuning range

8 ns fiber laser Q switched by the resonant saturable absorber mirror

We demonstrate that resonant high-modulation-depth saturable absorbers allow efficient pulse shortening in Q-switched lasers. Using a 70% modulation depth resonant saturable absorber mirror we achieved 8 ns pulses that are close to the limit set by the cavity length and are, to our knowledge, the shortest pulses demonstrated to date from passively Q-switched fiber lasers.

Harmonically mode-locked VECSELs for multi-GHz pulse train generation

We report on optically-pumped vertical-external-cavity surface-emitting lasers passively mode-locked with a semiconductor saturable-absorber mirror. The potential of harmonic mode-locking in producing pulse trains at multigigahertz repetition rates has been explored. The results present first systematic study of multiple pulse formation in passively mode-locked VECSELs.

Long-wavelength fast semiconductor saturable absorber mirrors using metamorphic growth on GaAs substrates

Metamorphic growth of InP on GaAs has been used to decrease the absorption recovery time of 1.55μm semiconductor saturable absorber mirrors. We show that the recovery time can be reliably controlled by changing the thickness of an InP “lattice reformation layer” grown between the GaAs-based distributed Bragg reflector and the active region. Semiconductor saturable absorber mirrors with a thickness of the InP reformation layer around 200 nm or smaller exhibit a recovery time short enough to reliably mode-lock fiber lasers.

Low jitter Q-switched fiber laser using optically driven surface-normal saturable absorber modulator

A technique for stabilizing the repetition frequency of a passively Q-switched laser is presented using an optically driven surface-normal semiconductor modulator. A method is capable of significant reduction of the timing jitter in a passively Q-switched laser by optical triggering the saturable absorber semiconductor reflector. The experimental demonstration using passively Q-switched ytterbium-doped fiber laser shows the jitter reduction by factor of 1.66??10(3) from 50 mus down to 30 ns.

2 watt 2 μm Tm/Ho fiber laser system passively Q-switched by antimonide semiconductor saturable absorber

We report tunable thulium/holmium-doped single mode fiber laser passively Q-switched by an antimonide-based semiconductor saturable absorber mirror (SESAM) and boosted in Tm/Ho amplifier up to 2 W of average power. Pulse operation tunable from 1960 nm to 1990 nm with pulse energies up to 30 μJ has been achieved. The study presents the first demonstration of 2 μm Q-switched fiber laser using antimonide semiconductor technology.

1 μm saturable absorber with recovery time reduced by lattice mismatch

Metamorphic growth of lattice mismatched InGaP on GaAs has been used to fabricate a fast semiconductor saturable absorber mirror operating at the 1060 nm wavelength range. The absorption recovery time could be reduced to ∼ 5 ps without deteriorating the nonlinear absorption properties. The device was used to demonstrate self-starting operation of a mode-locked Yb-doped fiber laser and obtain high quality picosecond pulses.

Nonlinear Response of Saturable Absorber Mirrors for Different Operation Regimes

In this study, we show how the nonlinear response of a saturable absorber depends on the parameters of the optical pump pulses. The result suggests that it should always be specified at what conditions the semiconductor saturable absorber mirror (SESAM) reflectivity has been measured. Another important consequence resulted from this analysis is that the saturation fluence of SESAM could be very different for different operation regimes, for example, for mode-locking and Q-switching. We propose a simple method of testing the absorption recovery time by using optical pulses with different pulsewidths

Cavity-enhanced saturable and two-photon absorption in semiconductors

Two-photon and saturable absorption are studied by placing GaAs and InGaAs materials in a microcavity. We show that field enhancement occurring due to the cavity influences the threshold and dynamic range of rollover in the nonlinear response. This approach can be used in semiconductor laser mirrors with improved capabilities for self-starting passive mode locking with suppressed tendency to Q-switching instability.