Abstract
Dispersion compensation is vital for the generation of ultrashort and single cycle pulses from modelocked lasers across the electromagnetic spectrum. It is typically based on addition of an extra dispersive element to the laser cavity that introduces a chromatic dispersion opposite to that of the gain medium. To date, however, no dispersion compensation schemes have been successfully applied to terahertz (THz) quantum cascade lasers for short and stable pulse generation in the THz range. In this work, a monolithic on‐chip compensation scheme is realized for a modelocked QCL, permitting THz pulses to be considerably shortened from 16ps to 4ps. This is based on the realization of a small coupled cavity resonator that acts as an ‘off resonance’ Gires‐Tournois interferometer (GTI), permitting large THz spectral bandwidths to be compensated. This novel application of a GTI opens up a direct and simple route to sub‐picosecond and single cycle pulses in the THz range from a compact semiconductor source.
Highlights
In the terahertz (THz) frequency range (~ 0.5 - 5 THz), [1] with its proven applications in imaging, [2,3] metrology, [4,5] and non-destructive testing, [6,7] a semiconductor based technology platform for intense and ultrashort pulse generation has yet to be realized. This is in contrast to the optical and near-infrared (NIR) domain where ultrashort pulse generation can be readily achieved in devices such as mode locked semiconductor diodes and vertical external cavity surface emitting lasers (VECSELS). [8,9,10] THz quantum cascade lasers (QCLs) are a foundational semiconductor laser in the THz range, to date, the generation of stable and ultrashort pulses from QCLs has proven to be difficult
[14] Active mode locking, where the device is electrically modulated at its’ roundtrip, has been extensively applied but the pulses generated so far have been limited to the range of 10ps to 20ps, despite several years of research effort. [15,16] THz QCLs with extremely large gain bandwidths have been realized leading to impressive developments in frequency comb generation, [17,18] this has not translated directly into the formation of stable short pulses in the THz range (In general the pulse width is inversely proportional to the spectral bandwidth). [19]
THz dispersion compensation has been considered in the case of frequency comb generation via four wave mixing, no studies have been applied to active mode locked QCLs for stable short pulse generation. [17,18]
Summary
In the terahertz (THz) frequency range (~ 0.5 - 5 THz), [1] with its proven applications in imaging, [2,3] metrology, [4,5] and non-destructive testing, [6,7] a semiconductor based technology platform for intense and ultrashort pulse generation has yet to be realized. THz dispersion compensation has been considered in the case of frequency comb generation via four wave mixing, no studies have been applied to active mode locked QCLs for stable short pulse generation. By judiciously designing the length of the integrated GTI, applying the GTI ‘off-resonance’, and exploiting the high reflectivity of the metal-metal QCL waveguide, [12] significant compensation of the QCL’s inherent GDD can be realized This directly results in pulse durations as shorts as 4 ps, from 16 ps with a standard QCL geometry, with a continuous Gaussian spectral range extending from 2.3 to 2.9 THz. The dispersive effect of the GTI mirror is clearly demonstrated by characterizing a GTI of a length that results in zero dispersion compared with one that introduces too much dispersion. A broad bandwidth of the GDD compensation is achieved by exploiting the both the positive and the negative GDD regime of the ‘off-resonance’ GTI, compensating positive and negative gain GDD simultaneously
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
