Generation of optical comb frequency signal with high spectral flatness using two cascaded optical modulators

Optical sinc-shaped Nyquist pulses of exceptional quality

“…For instance, they can be obtained from conventional femtosecond lasers, such as Er-fibre3839, Yb-fibre40 and Ti:sapphire41 mode-locked lasers, or from a continuous wave optical source exploiting Kerr-nonlinearities in an optical resonator42434445, or employing a combination of strong intensity and phase modulation464748 together with chirped Bragg gratings49, dispersive medium50 or highly nonlinear fibres515253. However, every comb does not necessarily result in a sequence of Nyquist pulses, since a sinc-pulse sequence can only be obtained under specific conditions, requiring that the produced comb has to show line amplitudes as equal as possible, linear phase dependence through all lines and a strong suppression of out-of-band lines.…”

“…However, every comb does not necessarily result in a sequence of Nyquist pulses, since a sinc-pulse sequence can only be obtained under specific conditions, requiring that the produced comb has to show line amplitudes as equal as possible, linear phase dependence through all lines and a strong suppression of out-of-band lines. Thus, although flat frequency combs can be obtained using different methods, as for instance through phase modulation484950, the phase difference between lines and the existing out-of-band components make phase modulators improper for clean generation of sinc-shaped pulses.…”

“…In general, a close-to-ideal rectangular-shaped optical frequency comb can be produced using various implementations; for instance, a non-optimal frequency comb38394041424344454647484950515253 can be used in combination with a spectral line-by-line manipulation of the optical Fourier components5455 to control the amplitude and phase of each spectral line. It turns out that the complexity of this kind of pulse shapers significantly increases with the number of spectral lines, and in general pulse shapers are unable to manipulate a frequency comb having spectrally spaced lines below 1 GHz5455.…”

“…If the DC bias and the RF signal voltages applied to a single modulator are V B and ν s  cos(ω s t ), respectively, its normalized output optical field is given by the expression4856:…”

Variable and Flexible Optical Frequency Comb Source using Dual Mach Zehnder Modulator and Phase Modulator

“…As in case of microwave photonic filters (MWPF), it is required to shape the comb spectrum in the Gaussian profile right in the generation process to achieve optimal filter characteristics [9]. The PM is driven using first and second harmonics of the sinusoidal drive signal The output of DPMZM can be written as [13].…”