Architectures for enhancing bandwidth and power scalability of optical frequency combs
Vikram, Bhagavatula Shiva
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Optical frequency combs (OFCs) have revolutionized high bandwidth communications, spectroscopy, arbitrary RF waveform generation, LIDAR (Light Detection and Ranging), and astronomical spectrograph calibration. OFCs have seen rapid development due to their significant role in sustaining and achieving progress in various research and industrial applications. Wide bandwidth, tunable repetition rate, and power scalability are the desired characteristics of versatile OFCs. However, the existing OFC architectures do not utilize the resources efficiently and cannot adapt to varying conditions, leading to limited bandwidth enhancement and power scalability. This thesis attempts to tackle the two main challenges of bandwidth enhancement and power scalability of optical frequency combs by developing innovative architectures using electro-optic modulation and nonlinear fiber optics. We initially demonstrate the platform of electro-optic modulator-based frequency comb generation. The platform allows tunable repetition rate and central wavelength, making it an optimal stage for controlling the comb properties. OFC spectrum equalization and extraction of arbitrary carriers are demonstrated. A back-to-back DWDM transmission experiment demonstrated a capacity of more than 0.5 Tbps over 7 channels. However, the limited bandwidth restricts the applications of the source. To enhance the bandwidth, we developed an architecture to manipulate the comb generator's temporal output using two frequency combs. The modified pulse shape enhances self-phase modulation-based bandwidth enhancement in nonlinear fiber and is verified through simulations and experiments. With a bandwidth enhancement of more than 17 times, the realized multiwavelength source acts as a complete carrier source for DWDM communications across the entire C-band without any missing carriers and is also suitable for spectroscopy. Though bandwidth enhancement is realized, the carriers are not phase-locked, limiting the applications of the source. Further, the generated pulse profile need not always be the optimal profile for bandwidth enhancement. To realize robustness in bandwidth enhancement of phase-locked carriers, we adaptively optimize the spectral phase of the EO frequency comb to generate the optimal pulse profile. The adaptive optimization is performed using computer-controlled closed-loop automation. The control loop monitors the bandwidth enhancement after the nonlinear fiber and perturbs the EO comb pulse shape through a Fourier pulse shaper. The pulse shape optimization is tested under various perturbations to realize robust bandwidth scaling, with a record figure of merit of 4.86. The pulse profiles were analyzed using an in-house built zero-delay spectral shearing interferometer. Further, sub-picosecond pulses (0.73 ps) at a very high repetition rate (25 GHz) are generated from the optimized spectrum. The flexible, adaptable, robust OFC source is an optimal solution for high bandwidth optical communications through super-channels, optical sampling, low phase noise arbitrary RF waveform generation, and LIDAR apart from DWDM and spectroscopy. The OFCs still lack significant power scalability, mainly limited by stimulated Brillouin scattering (SBS). The linewidth of the source laser is broadened to realize this. Continuous linewidth tuning over an extensive range is required to balance the required power scaling and coherence degradation. Initially, we demonstrated continuous linewidth tuning to >10 GHz at 1064 nm, through phase modulation with a white noise source, whose power and bandwidth are controlled. The upper limit on the tuning range is due to the phase modulator's EO bandwidth and power handling limitations. Next, we further scale the noise phase modulation (and thus the tuning range) through an architecture based on four-wave mixing of noise broadened pumps. Linewidth scaling of up to 40 GHz (10 times the pump linewidth) is demonstrated in C-band. Though line broadening improves power scalability, the improvement is subnormal. The subnormal improvement is due to the significant power present at the Stokes frequency in the line broadened laser. The power at the Stokes frequency seeds the SBS process through reflections and lowers power scalability. Power at the Stokes frequency in the line-broadened spectrum is reduced with a novel modulation technique combining sinusoidal and noise phase modulation. This technique shapes the line broadened spectrum to have a rapid roll-off slope away from the carrier, with improved flatness near the carrier. The modulation scheme allows improved power scalability in comparison to noise phase modulation for a given linewidth. The line broadening architectures can be cascaded with the comb synthesis architectures to generate many seed lasers for spectral beam combining. All the architectures demonstrated in this work can be translated seamlessly across various wavelength regimes with equivalent components. This work is envisioned to direct innovative techniques in the development of robust optical communications and beam combining systems apart from strongly influencing the fields of LIDAR and arbitrary RF waveform synthesis in existing and emerging wavelength regimes.