Riku Koskinen

Passively Q-switched Tm 3+ , Ho 3+ -doped silica fiber laser using a highly nonlinear saturable absorber and dynamic gain pulse compression

We demonstrate a compact core-pumped 2 microm Tm(3+), Ho(3+)- doped all-fiber laser passively Q-switched with an antimony-based saturable absorber. The 20 ns pulses are the shortest Q-switched pulses from a fiber laser operating beyond 1850 nm and were produced at a repetition rate of 57 kHz and pulse energy of 15 microJ using a short-length (4 ns) cavity. The large absorber modulation depth of approximately 70% together with transient gain compression is shown to provide an efficient mechanism for Q-switched pulse shortening.

Picosecond passively mode-locked GaSb-based semiconductor disk laser operating at 2μm

We report on a passively mode-locked optically pumped GaSb-based semiconductor disk laser producing stable picosecond optical pulses at a 1.95 μm wavelength. The gain mirror was comprised of a 15 quantum well InGaSb/GaSb structure. A fast semiconductor saturable absorber mirror with three InGaSb/GaSb quantum wells was used to attain self-starting mode-locked operation at a fundamental repetition rate of 881.2 MHz. The laser produced pulses with 30 pJ energy and a duration of 1.1 ps within a factor of 2 of the Fourier limit.

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.

Te-doping of self-catalyzed GaAs nanowires

Tellurium (Te)-doping of self-catalyzed GaAs nanowires (NWs) grown by molecular beam epitaxy is reported. The effect of Te-doping on the morphological and crystal structure of the NWs is investigated by scanning electron microscopy and high-resolution transmission electron microscopy. The study reveals that the lateral growth rate increases and axial growth rate decreases with increasing Te doping level. The changes in the NW morphology can be reverted to some extent by changing the growth temperature. At high doping levels, formation of twinning superlattice is observed alongside with the {111}-facetted sidewalls. Finally, the incorporation of Te is confirmed by Raman spectroscopy.

GaSb-Based Semiconductor Disk Laser With 130-nm Tuning Range at 2.5

We demonstrate a GaSb-based semiconductor disk laser emitting 0.6 W of output power at 2.5 μm in a fundamental mode regime (M2 <; 1.6). A gain structure grown by molecular beam epitaxy and assembled with an intracavity diamond heat spreader demonstrates a promising potential for power scaling and broad wavelength tuning. A tuning range of 130 nm with output power up to 310 mW has been achieved which represents the largest spectral coverage reported to date for disk lasers at this wavelength.

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The characteristics and the fabrication of a 1.9 μm superluminescent diode utilizing a cavity suppression element are reported. The strong suppression of reflections allows the device to reach high gain without any sign of lasing modes. The high gain enables strong amplified spontaneous emission and output power up to 60 mW in a single transverse mode. At high gain, the spectrum is centered around 1.9 μm and the full width at half maximum is as large as 60 nm. The power and spectral characteristics pave the way for demonstrating compact and efficient light sources for spectroscopy. In particular, the light source meets requirements for coupling to silicon waveguides and fills a need for leveraging to mid-IR applications photonics integration circuit concepts exploiting hybrid integration to silicon technology.

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We present a gallium antimonide-based semiconductor saturable absorber mirror (SESAM) operating at 2 μm wavelength region. GaSb-based material system is the preferred choice for fabricating surface-normal devices operating beyond 2 μm because it enables the use of highly reflective semiconductor reflectors and quantum wells for wide wavelength range. For the purpose of generating short laser pules, the SESAM was carefully designed to attain a large modulation depth. The device was utilised successfully to passively Q-switch a 2 μm Tm3+-/Ho3+ -doped fiber laser, demonstrating record-short Q-switch pulses of about 20 ns.

High power (60 mW) GaSb-based 1.9 μm superluminescent diode with cavity suppression element

The development of single-mode distributed feedback (DFB) lasers emitting high output powers in a broad wavelength range from 1980 to 2035 nm is reported. A unique feature of the development is the fabrication of lateral feedback gratings by nanoimprint lithography. We have varied a wide range of design parameters and studied their effect on the performance of the laser. The best uncoated devices exhibited a side-mode suppression ratio as high as >50 dB at output powers in excess of 14 mW. Moreover, a tuning range of over 12 nm was measured. After coating the facets with dielectric mirrors, the laser diodes could deliver an output power of more than 25 mW. In this letter, we prove the suitability of nanoimprint lithography to the fabrication of GaSb-DFB laser diodes by demonstrating the state-of-the-art devices made using imprint lithography.

Highly nonlinear GaSb-based saturable absorber mirrors

We review recent results concerning the development of GaSb-based heterostructures for semiconductor disk lasers. We focus on fabrication and design details of gain and semiconductor saturable absorber mirrors used to demonstrate disk lasers exhibiting high output power, broad tunability, and short pulse generation. We demonstrate a 2 μm gain structure with 15 InGaSb quantum wells emitting more than 4 W of output power at 15°C. Almost 1W output power was measured at an elevated temperature of 50°C. A tuning range of more than 150 nm was achieved by employing a gain mirror comprising quantum wells with different widths to provide broadband gain. Ultra-short pulse generation based on synchronous mode-locking and a preliminary demonstration of passively mode-locked semiconductor disk lasers based on GaSb saturable absorber mirrors are also discussed.

High Spectral Purity High-Power GaSb-Based DFB Laser Fabricated by Nanoimprint Lithography

Tunable mid-infrared semiconductor light sources are of particular interest for molecular spectroscopy; for example, CO2 and other atmospheric gases have strong absorption lines in this wavelength range. Currently, to detect multiple gases simultaneously one needs several continuous-wave (CW) operated mid-infrared laser diodes (LDs) with wide tuning range. For a miniaturized gas sensing system, a single high power and broadband light source is desired. To this end, we have proposed a novel, programmable/tunable light source for gas detection utilizing broadband superluminescent diode (SLD) and Si-based photonic integrated components [1]. When it comes to light source development, the preferred material system for reaching the 2–3 μm wavelength range is GaSb. Even though the performance of GaSb-based devices [2] are superior to InP-based emitters [3] in this wavelength range, the development of GaSb-based SLD have not been notable until recently [4].