Oleg G. Okhotnikov

Ultra-fast fibre laser systems based on SESAM technology: new horizons and applications

A promising route to manufacturing portable (sub-)picosecond fibre lasers is to use a semiconductor saturable absorber mirror (SESAM). With SESAMs, the mode-locked regime can be achieved for different values of cavity dispersion for a broad spectrum ranging from 0.8 to 1.6 μm. The fibre lasers, characterized by a high efficiency and reliability and a small footprint, are very attractive for applications traditionally occupied by solid-state lasers. The broad fluorescence spectrum makes different fibre gain media attractive for tuneable and ultra-short-pulse sources. In this paper, we discuss recent advances in ultra-fast fibre lasers. We study the fundamental properties and technical challenges of mode-locked fibre lasers operating in the 0.9–1.6 μm range, and the methods to achieve high peak-powers from all-fibre devices. The key component is the SESAM, notably, a dilute nitride SESAM. The SESAM supplies a strong mechanism for picosecond pulse generation that is entirely self-starting for a wide range of cavity dispersion and ensures stability against Q-switched mode-locking. In particular, compact mode-locked lasers stabilized by near-resonant SESAMs can be realized in a short fibre cavity free from any dispersion compensator. An appropriate dispersion delay line at the output of the master source may be used for pulse clean-up. The high-quality pulses obtained can then be compressed using traditional methods, when the pulse first undergoes spectral enrichment via self-phase modulation in an auxiliary fibre and then gets compressed in a grating pair. The fibre laser is capable of efficient wavelength conversion via second harmonic generation in non-linear crystals. Using a periodically poled LiNbO3 crystal for frequency doubling, we produce sub-100 fs pulses at 0.8 μm with a 50% conversion efficiency.

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.

Multifunctional Free-Standing Single-Walled Carbon Nanotube Films

We report a simple and rapid method to prepare multifunctional free-standing single-walled carbon nanotube (SWCNT) films with variable thicknesses ranging from a submonolayer to a few micrometers having outstanding properties for a broad range of exceptionally performing devices. We have fabricated state-of-the-art key components from the same single component multifunctional SWCNT material for several high-impact application areas: high efficiency nanoparticle filters with a figure of merit of 147 Pa(-1), transparent and conductive electrodes with a sheet resistance of 84 Ω/◻ and a transmittance of 90%, electrochemical sensors with extremely low detection limits below 100 nM, and polymer-free saturable absorbers for ultrafast femtosecond lasers. Furthermore, the films are demonstrated as the main components in gas flowmeters, gas heaters, and transparent thermoacoustic loudspeakers.

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

Dissipative dispersion-managed soliton 2 μm thulium/holmium fiber laser

We report a first dissipative dispersive-managed soliton fiber laser operating at 2 μm. The cavity comprised of all-anomalous-dispersion fiber employs chirped fiber Bragg grating, which ensures net-normal cavity dispersion and semiconductor saturable absorber for mode-locking.

Femtosecond soliton mode-locked laser based on ytterbium-doped photonic bandgap fiber.

We demonstrate a solid-core ytterbium-doped photonic bandgap fiber laser passively mode-locked with a semiconductor saturable absorber. Gain and anomalous dispersion simultaneously provided by the photonic crystal fiber allow for a compact subpicosecond soliton oscillator. We also discuss the effect of higher-order dispersion in photonic bandgap fiber on laser performance.

1.5-µm monolithic GaInNAs semiconductor saturable-absorber mode locking of an erbium fiber laser

We present a new monolithic GaAs-based semiconductor saturable absorber operating at 1.55 microm. An epitaxially grown absorber mirror in a GaInNAs/GaAs material system was successfully used to mode lock an erbium-doped fiber laser. The GaInNAs material system possesses intriguing physical properties and provides great potential for lasers and nonlinear optical devices operating at the 1.3-1.55-microm wavelength range.

Dispersion compensation-free fiber laser mode-locked and stabilized by high-contrast saturable absorber mirror

We report here a compact diode-pumped fiber laser that represents a promising route to designing a portable and rugged picosecond light source. The laser presented in this paper is based on a high-contrast semiconductor saturable absorber mirror (SESAM) and targets reliable picosecond-range sources. The cavity is simple since no dispersion compensators are used, and the SESAM-based mode locking mechanism is robust and self-starting, resulting in low-maintenance turn-key operation. We investigated pulse formation in a short-length fiber cavity and found that nonlinear effects in a near-resonant SESAM in combination with large-cavity dispersion provide the predominant mechanism that causes pulse shaping. The role of a resonant high-contrast SESAM in preventing low-frequency Q-switching instability has been elucidated. The effect of the recovery time of the SESAM on the stretched pulse width and spectrum for resonant-type absorber mirrors was also studied.

Double clad tapered fiber for high power applications.

We report a novel type of active fiber - tapered double clad fiber suitable for pumping by low brightness sources with large beam parameter product of 50/300 mm x mrad. Ytterbium double clad all-silica fiber (core/1(st) clad/2(nd) clad diameters 27/834/890 mum, NA(core)=0.11, NA(clad)=0.21), tapered down by a factor 4.8 for a length of 10.5 m was drawn from a preform fabricated by plasma chemical technologies. At a moderate Yb-ion concentration and 1:31 core/cladding ratio, the tapered double clad fiber demonstrates 0.9 dB/m pump absorption at 976 nm and excellent lasing slope efficiency. An ytterbium fiber laser with 84 W of output power and 92% slope efficiency, a 74 W superfluorescent source with 85% slope efficiency and amplifiers operating both in CW and pulsed regimes have been realized. All devices demonstrated robust single mode operation with a beam quality factor of M(2)=1.07.

All-fiber ytterbium soliton mode-locked laser with dispersion control by solid-core photonic bandgap fiber

We exploit an anomalous dispersion generated by a solid-core photonic bandgap fiber for dispersion compensation in an ytterbium fiber laser passively mode-locked with a semiconductor saturable absorber. The bandgap-guiding fiber, adequately compatible with standard fiber based on guiding via total internal reflection, allows for an environmentally robust all-fiber subpicosecond soliton oscillator at 1 mum.