Laser Frequency Combs with Fast Gain Recovery: Physics and Applications

Abstract

The dynamics of semiconductor lasers with fast gain is generally considered to suppress amplitude modulation, resulting in a frequency‐modulated output. Since their first demonstration of frequency comb operation, quantum cascade lasers—ideal representatives of this class of lasers with fast gain dynamics—were indeed shown to emit frequency‐modulated combs. The understanding of these frequency comb regimes has progressively evolved, being refined from simpler arguments to more advanced theories, considering the role of Kerr nonlinearity and linewidth enhancement factor. By now, the traditional frequency‐modulated combs are rather well understood in quantum cascade lasers, but new types of frequency combs have emerged, reopening interesting physics questions in these lasers. Recent advancements on two open frontiers will be discussed: harmonic frequency combs, characterized by mode skipping of many free spectral ranges, and spatiotemporal structures with solitonic features, recently reported in unidirectional ring cavities. These new types of frequency combs open technological and fundamental perspectives, sprouting new interest in quantum cascade lasers.

Self-Pulsations in Terahertz Quantum Cascade Lasers under Strong Optical Feedback: The Effect of Multiple Reflections in the External Cavity

“…It was noted that the optical feedback dynamics are dependent on the value of the linewidth enhancement factor ( ) of the laser [ 9 ]. For this particular design of the THz QCL used in our system, we extracted experimentally and it was [ 23 ], which agrees well with that reported by [ 5 , 33 , 34 ]. Although for THz QCLs varies from to , depending on the driving current and the aperture within the external beam path [ 33 ], the effects of the multiple ECRTs on the self-pulsation dynamics demonstrated here would not change qualitatively as long as the value of is within the reported range for the THz QCLs.…”

“…Terahertz (THz) quantum cascade lasers (QCLs) have undergone rapid development since their demonstration in 2002 [ 1 ], showing high emission power [ 2 ], quantum-limited linewidth [ 3 ], ultra-broadband gain [ 4 , 5 ], and being able to operate at high temperatures of up to 250 K in pulse mode [ 6 ]. Due to the absence of the relaxation oscillations and the small linewidth enhancement factor in the THz QCLs, it was widely accepted that THz QCLs are ultra-stable against optical feedback [ 7 , 8 ].…”

Efficient computation of coherent multimode instabilities in lasers using a spectral approach

“…For the case of a ring laser, these matrix elements have a simple physical interpretation. For a spatially-uniform inversion field, D(x, t) = D(t), all off-diagonal inversion matrix elements vanish, as is seen from equation ( 12) and the orthogonality relationship, equation (8). This case represents the single-mode lasing regime for ring lasers.…”

“…We are now able to make use of the generalized orthogonality relation, equation (8). Multiplying through by φ * m (x) and integrating over the spatial domain of the cavity, we find:…”

“…Even in such high-performance regimes, nonlinear, multimode effects emerge via mode competition and spatial hole burning, which must be understood and suppressed [1,2] to maximize laser power and efficiency. More recently, however, rather than an undesired effect, coherent multimode lasing phenomena have been the focus of many studies [3][4][5][6][7][8]. Such phenomena include ultrashort (sub-ps) pulse formation [9,10] useful for precision machining [11] and probing of ultrafast processes [12,13].…”