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PUBLISHED ONLINE: 24 NOVEMBER 2014 | http://www.nature.com/doifinder/10.1038/nphoton.2014.279
Web End =DOI: 10.1038/NPHOTON.2014.279
Octave-spanning semiconductor laser
Markus Rsch*, Giacomo Scalari*, Mattias Beck and Jrme Faist
We present a semiconductor injection laser operating in continuous wave with emission covering more than one octave in frequency and displaying homogeneous power distribution among the lasing modes. The gain medium is based on a heterogeneous quantum cascade structure operating in the terahertz range. Laser emission in continuous wave takes place from 1.64 THz to 3.35 THz with optical powers in the milliwatt range and more than 80 modes above threshold. For narrow waveguides, a collapse of the free-running beatnote to linewidths of 980 Hz, limited by jitter, indicate frequency comb operation on a spectral bandwidth as wide as 624 GHz, making such devices ideal candidates for octave-spanning semiconductor-laser-based terahertz frequency combs.
Abroadband gain medium is desirable for a wide range of applications in laser science, including widely tunable sources employing on-chip tuning13, an external cavity46 or ultrashort
pulse lasers7,8. Broad gain is especially interesting when combined with locking techniques that enable access to comb operation. Frequency combs act as rulers in the frequency domain and are realized starting from a short-pulse mode-locked laser9 or via nonlinear processes10,11.Combs have been demonstrated in the visible12, mid-infrared (mid-IR)11,13,14 and terahertz15,16 regions of the electromagnetic
spectrum. The laser emission from a comb can be stabilized and locked in frequency to highly stable microwave oscillators and effectively used in metrology and high-precision spectroscopy9,1719.
The most used and also most efcient way to stabilize a frequency comb is based on the self-referencing f2f method20, which requires octave-spanning laser emission. This allows also the determination of the carrier-envelope offset frequency of the comb. Achieving an octave-spanning spectrum is therefore a milestone for any broadband laser. To date, octave-spanning laser emission has been obtained by means of nonlinear optics (that is, broadening the laser spectra in a suitable medium21,22), by using parametric frequency
conversion in ultrahigh-Q-factor microresonators23 or by directly
integrating self-phase modulation into Ti:sapphire oscillators24,25. An
octave-spanning semiconductor injection laser, as presented in this Article, represents a new attractive step towards achieving compact, on-chip frequency combs. The semiconductor quantum cascade laser (QCL) is especially suitable for direct comb operation, without requiring an additional locking mechanism11,16. Comb formation...