We present an amplifier system for 2 µm ultrafast laser pulses for potential material processing applications. The amplifier gain material is a 0.75 at.% doped Ho:YAG slab crystal measuring 10 mm x 1.5 mm x 55 mm. The pump source is an in-house developed continuous wave Tm:YLF slab laser which produces a maximum output power of 340 W, centred at the 1908 nm Ho:YAG absorption peak. The pump beam full widths were 0.2 mm by 5.3 mm in the slab. The seed for the experiment was a mode-locked Tm:LuScO3 laser that produced 200 fs pulses (~23.6 nm spectral bandwidth) centred at 2094 nm. The spectral peak of the seed laser was chosen so as to spectrally overlap both the 2090 and 2097 nm emission peaks of Ho:YAG. The pulse repetition frequency of the seed laser was 115 MHz, and the average power as measured after an optical isolator was ~57 mW. In the initial experiment the seed was focused into the slab using a spherical doublet lens pair to a beam diameter of 0.2 mm. The measured single pass gain was ~10 (up to 0.54 W) when pumped with 280 W. The effective pump power (disregarding transmitted pump light) in the gain volume used for amplification was estimated to be 8.3 W. The spectral bandwidth of the output signal was measured at several output powers and shown to converge to ~11.8 nm. Based on these results and in-house simulations we will implement a pre-amplifier and scale 2 µm ultrashort pulses to >100 W average power at MHz PRFs.
The development of efficient, low-cost and robust high peak power pulsed lasers in the ~ 2 – 2.1 µm spectral region is required for many application areas in the mid-infrared (mid-IR) photonics sector. In particular, such laser sources can be used to efficiently access the deeper mid-IR region through optical parametric frequency conversion techniques, utilizing nonlinear crystals such as ZGP and OP-GaAs, or supercontinuum gener¬ation in highly nonlinear fibers. Such mid-IR frequency comb systems are of particular interest for laser countermeasures, remote sensing, high precision spec¬troscopy, and environmental monitoring. Compact and efficient ultrafast 2 µm lasers can also be used as seed sources for developing high energy amplifier systems operating in the 2 – 7 µm region which will benefit many applications from the areas of laser material pro-cessing, strong-field phys¬ics, as well as the development of tabletop X-ray coherent sources.
So far, the work on the development of pulsed lasers that operate in the 2-2.1 µm spectral region is rather limited and based predominantly on Ho-doped gain media which require relatively expensive and bulky Tm-laser pump sources.
Tm3+-doped cubic sesquioxides RE2O3 (RE=Lu, Sc, and Y) occupy a prominent position amongst other Tm3+-doped gain media. They possess advanta¬geous thermo-mechanical properties and spectroscopic features that make them ideal for high power lasers development in the 2 – 2.1 µm region using a low-cost laser diode pump platform around 800 nm. In particular, their thermal conductivities are in range of 13-17 W/m·K (compared to that of YAG which is 11 W/m·K) and in the case of Lu2O3 it decreases negligibly when the rare-earth-ion doping concentration is increased allowing high-power operation under direct diode pumping. In contrast to most Tm3+-doped crystalline and amorphous gain media, their broadband emission spectra extend well beyond 2 µm allowing efficient operation close to 2.1 µm reaching atmospheric transparency window. The attractive characteristics of rare-earth ion doped crystalline sesquioxides gain media have also led them to being studied extensively as ceramics. Currently, high optical quality sesquioxide ceramics can be produced by nanocrystalline and vacuum-sintering technologies. Such ceramic gain media possess stronger fracture toughness than single crystals and afford a high potential for size scalability thereby offering practical advantages in high-power laser implementations.
Here we report on our recent progress in the development of a diode-pumped Tm-doped sesquioxide class ceramic ultrafast lasers operating around 2.1 µm region. In particular, a diode-pumped femtosecond Tm:Lu2O3 laser is demonstrated generating directly transform-limited <500 fs pulses with an average output power in excess of 1 W and a peak power of >30 kW at a center wavelength of 2070 nm. Both semiconductor saturable absorber mirror and Ker-lens mode-locking techniques were investigated. The perspectives for further power scaling during ultrashort pulse generation under direct diode pumping around 800 nm and in-band pumping at 1.6 µm region will be discussed.
In this paper we present on-chip mode-locked waveguide lasers fabricated in Yb-doped phosphate glass and Er, Ybdoped phosphate glass. At 1 micron wavelength, pulse repetition rates of up to 15 GHz with pulses ~800 fs were demonstrated and at 1.5 micron, picosecond pulses with a repetition rate up to 7 GHz were demonstrated. Dispersion was controlled in the cavity by varying the spacing between the waveguide and the SESAM, while the repetition rate could be controlled by varying the optical power. The average power can also be scaled using an integrated optical amplifier and on-chip gain of up to 10 dB was demonstrated. All these individual components can be integrated in a single platform to achieve a high-power on-chip multi-GHz optical frequency comb. Furthermore, we discuss an application of such laser sources in high-capacity telecommunications applications.
In this work, we discuss mode-locking results obtained with low-loss, ion-exchanged waveguide lasers. With Yb3+-doped phosphate glass waveguide lasers, a repetition rate of up to 15.2 GHz was achieved at a wavelength of 1047 nm with an average power of 27 mW and pulse duration of 811 fs. The gap between the waveguide and the SESAM introduced negative group velocity dispersion via the Gires Tournois Interferometer (GTI) effect which allowed the soliton mode-locking of the device. A novel quantum dot SESAM was used to mode-lock Er3+, Yb3+-doped phosphate glass waveguide lasers around 1500 nm. Picosecond pulses were achieved at a maximum repetition rate of 6.8 GHz and an average output power of 30 mW. The repetition rate was tuned by more than 1 MHz by varying the pump power.
We report the first use of a Semiconductor Disk Laser (SDL) as a pump source for ~2μm-emitting Tm3+ (,Ho3+)-doped
dielectric lasers. The ~1213nm GaInNAs/GaAs SDL produces >1W of CW output power, a maximum power transfer net
slope efficiency of 18.5%, and a full width half maximum wavelength tuning range of ~24nm. Free-running operation of
a Tm3+-doped tellurite glass laser under 1213nm SDL pumping generated up to 60mW output power with 22.4% slope
efficiency. Wavelength tunable output is also obtained from 1845 to 2043nm. Improved performance with output powers
of ~200mW and a slope efficiency of ~35% are achieved by replacing the Tm3+-doped glass with a Tm3+-doped KYW
active medium. Emission of a Tm3+,Ho3+-codoped tellurite glass laser is also reported with maximum output power of
~12mW and a ~7% slope efficiency. Finally, preliminary investigations of 1213nm-pumping of a Tm3+,Ho3+-codoped
silica fibre laser lead to 36mW output power with ~19.3% slope efficiency.
The development of femtosecond lasers has continued rapidly over the past decade from laboratory systems to an
impressive range of commercial devices. Novel materials, notably quantum-dot semiconductor structures, have enhanced
the characteristics of such lasers and opened up new possibilities in ultrafast science and technology. In our most recent
work, it has been demonstrated that quantum-dot structures can be designed to provide an efficient means for the
generation and amplification of ultrashort optical pulses at high repetition rates. The work also confirms that quantum
dot based semiconductor saturable absorber mirrors exhibit a degree of flexibility which allows control and tuning of the
ultrashort pulse laser systems. Further developments in ultrashort-pulse solid-state, fibre and semiconductor external
cavity lasers, by means of both active and passive semiconductor quantum dot components are also presented.
Progress in the development of efficient and reliable diode-pumped ytterbium femtosecond laser systems based on Kerr-lens
mode locking effect is reported. Average output power of up to 1 W is demonstrated in a Kerr-lens mode locked
Yb:YVO4 laser with pulse durations as short as 80 fs at a pulse repetition frequency of 79 MHz. Measurements of the
nonlinear refractive indexes of the Yb3+:YVO4 crystal, n2, were performed and were determined to be 39×10-16 cm2/W
and 49×10-16 cm2/W for E||c and E⊥c polarizations, respectively. These results were found to be in a good agreement
with those calculated using both the Kramers-Krönig relation and Boling, Glass and Owyoung formula.
Keywords: Mode-locked lasers, diode-pumped lasers
Progress in the development of efficient and reliable diode-pumped femtosecond laser systems based on both Kerr-lens
and saturable-absorber mode locking is reported. Efficient Kerr-lens mode locking in diode pumped ytterbium-doped
lasers, namely Yb:KY(WO4)2 (Yb:KYW) and Yb:YVO4, is demonstrated with average output powers in excess of 1W, pulse durations around 100fs and electrical-to-optical efficiencies that exceed 15%. Novel semiconductor saturable
absorbers based on InAs/InGaAs quantum dots are described and their applicability for efficient femtosecond pulse
generation from near-infrared solid-state lasers is discussed. Efficient passive mode locking in the spectral regions
around 1.3&mgr;m and 1.55&mgr;m in Cr:forsterite and the more recently developed Er, Yb:YAl3(BO4)3 crystalline lasers has been demonstrated using low-loss InGaNAs saturable absorbers. A few examples of applications for this category of robust and efficient femtosecond lasers have been included. Specifically, the characteristics of a femtosecond visible
light source producing pulses as short as 200fs at 520nm are outlined.
Saturable absorbers on the base of lead sulfide QDs for lasers emitting at 1, 1.3, 1.5, 2.1 microns are introduced and characterized. It is demonstrated that these SAs can be used both for mode-locking and Q-switching of near IR lasers.
The development of femtosecond (fs) lasers has continued rapidly since the demonstration of fs Ti:Sapphire systems in 1989. Recent research has yielded lasers which offer greatly enhanced performance in all areas. In this document we describe the development of femtosecond lasers with electrical to optical efficiency > 14%, pulse repetition frequencies > 4GHz and compact and stable cavities. We further outline the use of such lasers for the generation of high power visible femtosecond pulses and their application within systems environments for ultrahigh speed data communications, ultrafast optical switching and optical analogue to digital conversion. We also describe progress in the development of femtosecond lasers based on both active and passive semiconductor quantum dot components.
We report a highly efficient diode-pumped femtosecond Yb:KYW laser having a compact three-element resonator that incorporates a prismatic output coupler. Near-transform limited pulses of 107fs duration at a centre wavelength of 1056nm are produced at repetition pulse frequency of 294MHz by utilising soft-aperture Kerr-lens mode locking. The femtosecond operation had a mode-locking threshold at a pump power of 250mW and the laser was tunable from 1042nm to 1075nm. The optical-to-optical conversion efficiency exceeded 50% in this femtosecond-pulse regime.
We demonstrate a highly efficient and low threshold passively mode-locked femtosecond Yb:KYW laser pumped by an InGaAs narrow-stripe laser diode and which incorporates a semiconductor saturable absorber mirror. Near-transform limited pulses of 123fs at 1047nm were produced at an average mode-locked power of 107mW for only 308mW of incident pump power. An optical-to-optical conversion efficiency of 35% was achieved and the corresponding electrical-to-optical efficiency exceeded 14%.