Microwave Frequency Comb Generation by Gain-Switching Versus Relaxation Oscillations

Abstract

Optoelectronic feedback on a laser diode is demonstrated to generate two distinct modes of pulse-train formation depending on the injection current J of the laser. For J close to the threshold current J th , the pulse repetition rate is the inverse of the loop delay. This behavior is attributed to feedback-induced gain-switching. In contrast, for J J th , the repetition rate is observed to be related to the relaxation-oscillation frequency f RO . The potential to generate pulse trains and associated microwave combs may find use in metrology, optical communications, sampling, and spectroscopy.

Effects of Timing Noise on Square-Wave Optoelectronic Oscillators

“…One finds [45] exp[inω 0 Φ(τ)] = e −nω 0 Λ|τ| where Λ determines the rate at which ϕ(t) wanders. The integration in Equation ( 6) is easily carried out, (10) The PSD line shapes centered at the modulation harmonics nω 0 are Lorentzians [45] with width linear in n. As for exp[inω 0 ϕ(τ)] = 0, this implies f (t) = 0, which is a consequence of the fact that the absolute timing over times t Λ −1 wanders uniformly over the period ω −1 0 . (B)…”

“…For example, we have shown that a laser diode with time-delayed optical feedback [6] or with combined optical and optoelectronic feedback [7] can result in output of a narrow-band microwave-modulated optical signal. OEO's spontaneously producing other types of amplitude modulation [8], such as square waves (SQ) [9] and pulses trains [10], have recently been demonstrated. Phase noise is an essential figure of merit defining sinusoidal OEO performance [2,4,6,7,[11][12][13][14][15][16].…”

Doublet structure in the optical spectrum of a semiconductor laser with optoelectronic feedback in a square-wave regime

“…6 When the injection current J is sufficiently above its threshold level J th , SWs originate from a Hopf bifurcation scenario similar to that observed in gain-switched optical-pulse generation near J th . 8 The SW repetition rate f rep was found to be a harmonic of the feedback rate, i.e., f rep = nτ −1 . τ is the time delay of the OE feedback loop and n a small positive integer.…”