Charles W. Rudy

Mid-infrared supercontinuum generation in tapered chalcogenide fiber for producing octave-spanning frequency comb around 3 μm.

We demonstrate mid-infrared (mid-IR) supercontinuum generation (SCG) with instantaneous bandwidth from 2.2 to 5 μm at 40 dB below the peak, covering the wavelength range desirable for molecular spectroscopy and numerous other applications. The SCG occurs in a tapered As(2)S(3) fiber prepared by in-situ tapering and is pumped by femtosecond pulses from the subharmonic of a mode-locked Er-doped fiber laser. Interference with a narrow linewidth c.w. laser verifies that the coherence properties of the near-IR frequency comb have been preserved through these cascaded nonlinear processes. With this approach stable broad mid-IR frequency combs can be derived from commercially available near-IR frequency combs without an extra stabilization mechanism.

Amplified 2-

We report the first mode-locked, thulium-doped fiber figure-eight laser. The mode-locked oscillator produces 1.5-ps pulses with 63 pJ of pulse energy at a 10.4-MHz repetition rate with a 3-nm bandwidth at a center wavelength of 2034 nm. After amplification, the pulses are compressed to 370 fs with ~ 50 nJ of pulse energy. The oscillator can also operate in a square pulse regime, yielding stable pulses from ~ 100 ps to 20 ns long with ~ 100 nJ per pulse after amplification.

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We report generation of 48 fs pulses at a center wavelength of 2070 nm using a degenerate optical parametric oscillator (OPO) synchronously-pumped with a commercially available 36-MHz, femtosecond, mode-locked, Yb-doped fiber laser. The spectral bandwidth of the output is ~137 nm, corresponding to a theoretical, transform-limited pulse width of 33 fs. The threshold of the OPO is less than 10 mW of average pump power. By tuning the cavity length, the output spectrum covers a spectral width of more than 400 nm, limited only by the bandwidth of the cavity mirrors.

Thulium-Doped All-Fiber Mode-Locked Figure-Eight Laser

We demonstrate a mid-IR frequency comb centered at 3120 nm with 650-nm (20-THz) bandwidth at a comb-teeth spacing of 500 MHz. The generated comb is based on a compact ring-type synchronously pumped optical parametric oscillator (SPOPO) operating at degeneracy and pumped by a mode-locked Er-doped 1560 nm fiber laser at a repetition rate of 100 MHz. We achieve high-repetition rate by using a fractional-length cavity with a roundtrip length of 60 cm, which is one-fifth of the length dictated by conventional synchronous pumping.

Sub-50 fs pulses around 2070 nm from a synchronously-pumped, degenerate OPO

Supercontinuum generation (SCG) in a tapered chalcogenide fiber is desirable for broadening mid-infrared (or mid-IR, roughly the 2-20 μm wavelength range) frequency combs(1, 2) for applications such as molecular fingerprinting, (3) trace gas detection, (4) laser-driven particle acceleration, (5) and x-ray production via high harmonic generation. (6) Achieving efficient SCG in a tapered optical fiber requires precise control of the group velocity dispersion (GVD) and the temporal properties of the optical pulses at the beginning of the fiber, (7) which depend strongly on the geometry of the taper. (8) Due to variations in the tapering setup and procedure for successive SCG experiments-such as fiber length, tapering environment temperature, or power coupled into the fiber, in-situ spectral monitoring of the SCG is necessary to optimize the output spectrum for a single experiment. In-situ fiber tapering for SCG consists of coupling the pump source through the fiber to be tapered to a spectral measurement device. The fiber is then tapered while the spectral measurement signal is observed in real-time. When the signal reaches its peak, the tapering is stopped. The in-situ tapering procedure allows for generation of a stable, octave-spanning, mid-IR frequency comb from the sub harmonic of a commercially available near-IR frequency comb. (9) This method lowers cost due to the reduction in time and materials required to fabricate an optimal taper with a waist length of only 2 mm. The in-situ tapering technique can be extended to optimizing microstructured optical fiber (MOF) for SCG(10) or tuning of the passband of MOFs, (11) optimizing tapered fiber pairs for fused fiber couplers(12) and wavelength division multiplexers (WDMs), (13) or modifying dispersion compensation for compression or stretching of optical pulses.(14-16.)

Fractional-length sync-pumped degenerate optical parametric oscillator for 500-MHz 3-μm mid-infrared frequency comb generation

We report a broad frequency comb centered at 2.09 μm produced by a degenerate OPO. We achieve 0.6 W of 94-fs transform limited pulses at 250 MHz and a conversion efficiency of 64%.

In-situ tapering of chalcogenide fiber for mid-infrared supercontinuum generation.

We demonstrate a degenerate mid-infrared frequency comb OPO with a fractional cavity length pumped by an ultrafast 100-MHz Er-nber laser. This produces a 600-nm wide output near 3μπι with a repetition rate of 500 MHz.