Abstract
Quantum cascade lasers are proving to be instrumental in the development of compact frequency comb sources at mid-infrared and terahertz frequencies. Here we demonstrate a heterogeneous terahertz quantum cascade laser with two active regions spaced exactly by one octave. Both active regions are based on a four-quantum well laser design and they emit a combined 3 mW peak power at 15 K in pulsed mode. The two central frequencies are 2.3 THz (bandwidth 300 GHz) and 4.6 THz (bandwidth 270 GHz). The structure is engineered in a way that allows simultaneous operation of the two active regions in the comb regime, serving as a double comb source as well as a test bench structure for all waveguide internal self-referencing techniques. Narrow RF beatnotes (∼ 15 kHz) are recorded showing the simultaneous operation of the two combs, whose free-running coherence properties are investigated by means of beatnote spectroscopy performed both with an external detector and via self-mixing. Comb operation in a highly dispersive region (4.6 THz) relying only on gain bandwidth engineering shows the potential for broad spectral coverage with compact comb sources.
Highlights
Frequency combs [1] (FCs) have revolutionized many fields in physics such as metrology, spectroscopy and astronomy [1,2,3,4]
We show the simultaneous lasing of two octave-spaced FCs in a monolithic quantum cascade lasers (QCLs) operated in a μs pulsed regime
By considering the cavity free spectral range of multiple devices at these two operating frequencies, taking into account the material dispersion of GaAs, we are able to predict and attribute the observed BNs frequencies to the 2.3 and 4.6 THz combs. This we experimentally confirm for a 2.4 mm device by intermode beatnote spectroscopy, both conventionally using a Schottky diode, and using a self-mixing technique
Summary
Frequency combs [1] (FCs) have revolutionized many fields in physics such as metrology, spectroscopy and astronomy [1,2,3,4]. We employ our 4-well active region [22] by rescaling and centering the two emission frequencies at 2.3 THz and 4.6 THz, in order to have high intensities and stable phase coherent operation independently at the octave spacing. This way to operate is different from our previous works where we achieved octave-spaced laser lines [10, 19], not in a comb regime, from the low intensity modes on the sides of the emission of a three or four stack heterogeneous laser
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