Ultrafast and intense optics research

The Ultrafast and Intense Optics (UIO) group of ORC studies ultrafast optical phenomena, nonlinear optics, fibre lasers, and semiconductor lasers for rapidly developing applications, such as materials processing, medicine, biology and optical communications. This work is best described as interplay between fibre optics and semiconductor physics.

The group, headed by Prof. Oleg Okhotnikov, is equipped with a broad selection of top-notch optical, optoelectronic and measurement equipment, including commercial and in-house-built femtosecond and tunable laser sources. The research activities are backed up by worldwide collaboration with academic and industrial partners.

Contents

• 
  Recent research areas
  • Ultrashort pulse generation and manipulation
  • Nonlinear optics research
  • Dispersion compensation
  • High-power fibre lasers and amplifiers
  • Passive fibre components and thin-film coatings
  • Semiconductor disk lasers
• Future research directions
• Impact - from fundamental research to applications
• Researchers

Ultrashort pulse generation and manipulation

The group has an expertise in optical pulse generation based on MBE-grown semiconductor saturable absorber mirrors (SESAMs) used as end-mirrors of all-fibre laser cavities. By combining various dispersion compensation methods with careful laser cavity design and special optical fibre components, our group has studied a broad range of mode-locked lasers with normal and anomalous net cavity dispersion.

Typically, the pulse widths of our passively mode-locked lasers range from sub-100 femtoseconds to a few picoseconds, and our SESAMs cover a wide range of wavelengths in the near-IR regime. The SESAMs are easy to implement, scalable in power, and enable self-starting mode-locked operation. The figure on the left illustrates a typical mode-locked fibre laser based on SESAM technology. In addition to the simple architecture, the advantages of passively mode-locked fibre sources include facile alignment, self-starting capability, and the generation of high-quality ultrashort pulse trains.

Nonlinear optics research

Very high optical peak powers generated by intense laser pulses of short duration make it possible to explore various nonlinear optical phenomena such as high harmonic generation, supercontinuum generation, and Raman amplification. The UIO group holds a selection of state-of-the-art optical sources suitable for nonlinear optics research. Recently, we have demonstrated a broadband supercontinuum generated with a Q-switched tapered fibre laser in a photonic crystal fibre (PCF) with the highest pulse energy reported to date [Photon. Tech. Lett. 2011].

The UIO group develops fibre laser technology by characterizing a novel type of microstructured optical fibres – suspended-core fibres, developed in cooperation with Kotel’nikov Institute of Radio-Engineering and Electronics, Moscow. This type of fibres combines flexible nonlinear parameters with relatively simple geometrical structure. Using Yb-doped suspended-core fibre, 95 fs passively mode-locked fibre laser has been developed with pulse quality close to transform limit [Photon. Tech. Lett. 2010]. This novel type of fibre is a cost-effective and efficient alternative to conventional microstructured fibres.

Stimulated Raman scattering (SRS) is an attractive gain mechanism compared to rare-earth doped fibres due to availability at virtually any wavelength. SRS may produce high power output at wavelengths unavailable with other types of lasers. ORC develops ultrafast Raman fibre lasers pumped by semiconductor disk lasers. Oscillators with ps-scale pulse duration have been demonstrated both in normal [Opt. Express 2010] and anomalous dispersion regime [Opt. Lett. 2010].

Dispersion compensation

Dispersive optical effects are studied in semiconductors, microstructured fibres, and advanced fibre Bragg gratings (FBGs) [Appl. Opt. 2011]. The issues addressed include modeling, fabrication and characterization of semiconductor Gires-Tournois dispersion compensators, exploitation of intensity-dependent absorption in multiple quantum-wells (MQWs), the use of MQWs simultaneously as a dispersion compensator and as an absorber element, as well as the evaluation of the potential of PC fibres for dispersion compensation, pulse stretching and compression. The group has established a complete chromatic dispersion measurement system, capable of measuring reflection and transmission optics, as well as active and passive fibre components. The figures below illustrate a typical dispersion measurement of a photonic bandgap fibre.

High-power fibre lasers and amplifiers

High-power (kilowatt-class) fibre lasers are becoming increasingly viable substitutes for carbon dioxide lasers in industry, thanks to their remarkable wall-plug efficiency, compactness and perfect beam quality. Recently we have experimentally demonstrated a new type of high power fibre laser – a laser with a tapered double-clad active fibre (T-DCF) as a gain medium [Opt. Express 2008]. Tapered fibre, having a large diameter at the wide side (up to 1 mm), can be pumped by high-power laser diode bars with low brightness, which are the most cost-effective pump sources at the moment, while maintaining superb output beam quality [Opt. Express 2010]. In the pulsed regime, tapered fibre lasers enable high-energy Q-switching at arbitrarily low repetition rates due to their inherent amplified spontaneous emission (ASE) suppression mechanism [Opt. Express 2010]. At ORC, we are currently developing turn-key tapered fibre laser prototypes, as well as studying potential applications of the novel gain medium. In the future, our aim is to develop powerful master oscillator – power amplifier (MOPA) systems based on tapered fibre technology, mainly for materials processing and light detection and ranging (LIDAR) applications at the wavelengths of 1 µm and 1.5 µm, respectively.

Passive fibre components and thin-film coatings

The versatile glass processing equipment at ORC allows researchers to manufacture a wide range of passive optical fibre components such as fibre couplers, wavelength division multiplexers, fibre tapers, and pump combiners. The ability to fabricate such advanced components in-house significantly facilitates the top-notch research work at ORC. In addition, we have an advanced FBG inscription station utilizing a UV excimer laser and phase mask technology to produce high-quality FBGs with uniform, chirped, or even tapered design [J. Quantum Electron. 2010].

Furthermore, ORC is equipped with an electron beam evaporator for production of high-quality optical thin-films. The e-beam evaporator is typically utilized by manufacturing coatings for the various custom laser setups, e.g. anti-reflection coatings for semiconductor disk lasers with an intra-cavity diamond as heat spreader. Moreover, the flexibility of the evaporator system together with the availability of several dielectric materials allows manufacturing a wide variety of coatings for a range of applications.

Semiconductor disk lasers

Optically-pumped semiconductor disk lasers (OP-SDLs), also known as vertical external cavity surface emitting lasers (VECSELs) combine high average output power with circularly symmetric beam profile, which makes them attractive for numerous applications. The study of SDLs at ORC is focused on wavelength scaling of the lasers from visible to mid-IR wavelengths by exploiting wafer fusion, quantum dot based gain media and nonlinear frequency conversion. In addition, the research concentrates on short pulse generation via passive modelocking.

Semiconductor disk lasers can cover a wide spectral range by exploiting semiconductor bandgap engineering. However, the important 1.3 µm – 1.7 µm emission band is difficult to obtain with monolithically grown structures. Wafer fusion is an alternative technique that eliminates the requirement of lattice matching, thus allowing the usage of high quality Al(Ga)As/GaAs DBRs and InP based active regions with different lattice constants. In 2008 we reported the first 1.57 µm optically pumped wafer fused semiconductor disk laser [Opt. Express 2008]. The study was carried out in collaboration with EPFL in Switzerland. This technology has enabled CW operation from 1.2 µm – 1.57 µm SDLs [Opt. Express 2009].

The wavelength range of semiconductor disk lasers can be further extended by using a quantum dot based gain mirror which alleviates the requirement for lattice matching. The carrier confinement in three dimensions offers additional attractive features such as wide gain bandwidth, low threshold and temperature insensitive operation. We have demonstrated InAs/GaAs submonolayer and InGaAs Stranski-Krastanov grown quantum dot SDLs operating at 950 nm – 1260 nm wavelength band with low temperature dependency and wide wavelength tuneability [J. Cryst Growth 2008, Opt. Lett. 2010, Opt. Lett. 2009]. The emission wavelength has also been widened to visible by using second-harmonic generation [Opt. Lett. 2010].

The availability of diode-pumped, long-lifetime SDLs emitting directly at visible wavelengths with multi-watt output powers is very limited. The only viable option to achieve such performance is through frequency doubling, which can be accomplished by placing a second-harmonic crystal inside the SDL cavity [Opt. Express 2010]. In particular, SDLs are expected to have a huge impact on display and laser projection technology due to their capability of producing multi-watt output powers at visible wavelengths with excellent beam quality.

Semiconductor disk lasers can be reliably mode-locked with a semiconductor saturable absorber mirror to produce ultrashort optical pulses at multigigahertz repetition rates. The general dynamics of pulse formation in these ultrafast oscillators have been studied in detail [Opt. Express 2007, Phys. Rev. E 2008]. The discovered physical phenomena, such as bistability and harmonic mode-locking, have been proposed to lead to applications in high-speed communications and optical memory development. The spectral coverage of mode-locked SDLs has been extended to the telecom windows at 1.3 µm and 1.55 µm by the wafer fusion technique [Opt. Lett. 2009, Photon. Tech. Lett. 2010]. Novel methods for pulse generation from SDLs including the demonstration of an untraditional sliding frequency cavity have also been investigated [Photon. Tech. Lett. 2011].

Future research directions

We will continue to enhance our activity in the disk laser research and fibre optics by taking advantage of the state-of-the-art environment available at ORC, thanks to number of completed projects and, certainly, high-level expertise in this field. The research directions that we consider include the following areas:

1. Short pulse generation and manipulation using novel approaches: dissipative solitons, pulse energy scaling, advanced saturable absorbers; dispersion management using photonic crystal fibres and chirped FBGs.
2. Extending the spectral range of fibre oscillators by using novel gain media (Bi-fibres), Raman scattering in highly-nonlinear fibres, supercontinuum generation in suspended-core fibres with tailored dispersion map.
3. High-power fibre lasers in both CW and pulsed regimes using proprietary technology based on tapered active fibres. This technology will be extended to new spectral ranges by using thulium and erbium fibres in addition to ytterbium sources built recently at ORC.
4. Extending our SDL research to compact electrically pumped SDLs from 1.2 µm to 1.5X µm using wafer fusion technology.

Impact - from fundamental research to applications

The primary impact on scientific community, society and industry resulting from the research is expected in the following areas:

1. Micromachining and material modification using ultrashort pulse high-energy fibre lasers.
2. Long- and medium-range eye-safe LIDARs based on tapered fibre technology. Diffraction-limited beam quality and intrinsic suppression of Brillouin scattering of this concept, which allows building high-power single-frequency laser, will be implemented to develop efficient phase-sensitive LIDAR systems.
3. Ultra-broadband communication enabled by radically new fibre systems based on Raman fibre lasers pumped by SDLs.

Personnel

Valery Filippov, Regina Gumenyuk, Juuso Heikkinen, Juho Kerttula, Antti Rantamäki, Jussi Rautiainen, Esa Saarinen.