Generation of 70-fs pulses at 286  μm from a mid-infrared fiber laser

Abstract

We propose and demonstrate a simple route to fewoptical-cycle pulse generation from a mid-infrared fiber laser through nonlinear compression of pulses from a holmium-doped fiber oscillator using a short length of chalcogenide fiber and a grating pair. Pulses from the oscillator with 265 fs duration at 2.86 µm are spectrally broadened through self-phase modulation in step-index As 2 S 3 fiber to 140 nm bandwidth, and then re-compressed to 70 fs (7.3 optical cycles). These are the shortest pulses from a mid-infrared fiber system to date, and we note that our system is compact, robust and uses only commercially available components. The scalability of this approach is also discussed, supported by numerical modeling. The generation of laser pulses comprising only a few cycles of the electric and magnetic fields creates substantial scientific and technological opportunities. For example, such ultrashort pulses are enabling new time-resolved studies of atomic and molecular processes on unprecedented timescales and driving the development of tabletop extreme UV and attosecond pulse sources through high-harmonic generation (HHG) [1]. After significant progress in the near-infrared region, there is currently strong demand to push the wavelength of few-optical-cycle pulse sources to beyond 2 µm, into the mid-infrared (mid-IR): these wavelengths correspond to strong absorption resonances in many important organic materials, enabling further investigations and processing of new materials, and also offering advantages for HHG where the highest possible generated photon energy scales with the driving laser wavelength [1].At present, a widely used approach to mid-IR few-cycle pulse generation is parametric wavelength conversion (e.g. optical parametric chirp pulse amplification) of ultrafast near-IR sources, often including a subsequent compression stage [2][3][4][5][6]. This route has yielded mid-IR few-cycle lasers with remarkable performance, but their significant complexity and cost limit widespread practical applications. An alternative simpler approach is the direct development of mid-IR ultrafast oscillators. Bulk transition-metal doped II-VI semiconductors such as chromium-and iron-doped sulfide and selenide offer direct access to the 2-6 µm region, but while 46 fs pulses have been reported from mode-locked Cr:ZnS systems, such pulse generation has only been demonstrated thus far up to ∼2.4 µm [7].One highly promising route is the recent emergence of ultrafast rare-earth-doped fluoride fiber lasers, which bring the benefits of fiber laser technology (compact setups, simple thermal management, high beam quality etc.) to the mid-IR region. Mode-locked holmium-and erbium-based fiber lasers have been demonstrated at 2.7-2.9 µm wavelengths [8-12] (including subsequent extension to 3.6 µm using Raman soliton self-frequency shift [13]), producing tens of nanojoule energy pulses as short as 160 fs. Due to the limited gain bandwidths of holmium and erbium ions (which set the minimum mode-locked pulse width via the transform lim...

Direct nanoimprinting of moth-eye structures in chalcogenide glass for broadband antireflection in the mid-infrared

“…With the smallest height of just 815 25 nm, surface relief #1 is tailored to generate a considerable peak transmittance improvement of ΔT 12.2% at 3.3 μm; however, it is too small to support substantial broadband antireflection. With an optimum efficiency spectrum spanning 2.8 to 4.1 μm, this surface relief is ideal for handling the emission from a high-power mid-IR holmium-or erbium-doped ZBLAN (ZrF 4 -20BaF 2 -4LaF 3 -3AlF 3 -20NaF)-based fiber laser system, which operates in the 2.7-2.9 μm spectral range [37,38]. Surface relief #4 has the tallest fabricated moth-eye protrusions, which measure 1550 25 nm in height, and as a consequence, the relief features extremely broadband antireflection and achieves ΔT > 7.7% between 3.2 and 12 μm.…”

Ultrafast Dy3+:fluoride fiber laser beyond 3 μm

“…First, experimental works based on Er-and Hodoped fluoride fibers have demonstrated mode-locking operation with pulse durations at a picosecond level, employing fast saturable absorbers such as a semiconductor saturable absorber mirror [7,8] and graphene [9]. More sophisticated Er-and Ho-doped fluoride fiber systems based on the nonlinear polarization evolution (NPE) technique were been demonstrated later on, shortening the pulse duration down to the femtosecond regime [10,11].…”

Mid-IR Supercontinuum Generation

“…Step-index As2S3 chalcogenide fiber is an excellent candidate for this task. We show that a very short length of this fiber is sufficient to broaden the laser spectrum to levels that support record-low pulse durations in the mid-infrared and that a simple grating pair compressor can be utilized to re-compress the pulses to durations approaching the single-cycle regime [5].…”