KEYWORDS: Ultrafast phenomena, Reconstruction algorithms, Second harmonic generation, Frequency combs, Pulse signals, Electric fields, Spectroscopy, Autocorrelation, Signal intensity, Phase interferometry
High repetition frequency near-infrared ultra-short lasers are crucial for efficient generation of low-noise mid-infrared frequency combs due to their broad in-phase spectrum and high peak power. In this paper, we present an improved structure of second harmonic generation-frequency resolved optical gating (SHG-FROG) for characterizing few-cycle pulses by retrieving their spectral power, temporal shape, and phase using a ptychographic reconstruction algorithm. We managed to characterize frequency combs with repetition rate from 100 MHz to 1 GHz; and measured pulses as short as sub-4 cycle pulses (< 20 fs) using this FROG.
The development of broadband frequency combs has drawn consistent interest in the optics community. Among all the approaches, coherent Supercontinuum Generation (SCG) is one of the most promising techniques for applications such as molecular spectroscopy, fundamental chemistry and combustion analysis. A well-selected combination of pump properties and nonlinear medium is crucial in optimizing the spectrum coverage, power density and coherence of SCG. Coherent SCG can be achieved with a variety of pump sources, with pulse durations ranging from quasi-continuous wave to femtosecond. However, a minimum in-phase frequency component must be maintained to truthfully transfer the seed frequency combs to other frequencies. Nonlinear optical fibers for SCG include photonic crystal fiber, and step-indexed fibers. Being the most conventional fiber platform, step indexed Highly Nonlinear Fibers (HNLF) provide unprecedented environmental robustness with ease of handling and all-fiber capability. In this work, we explore the coherent SCG using a GHz repetition rate frequency comb. The output is dual stage amplified to facilitate the self-phase modulation and coherent soliton fission. To drive an octave-spanning coherent SCG, the amplified pulses were compressed to provide high gain power and in-phase spectrum. By carefully optimizing the pump parameters and the fiber properties, we generate an octave supercontinuum spectrum spanning the wavelength range from 1200 nm to 2400 nm, limited by the measurement range of spectrometer.
This study presents the development of a digital locking system for a mode-locked laser with a 50MHz repetition rate. Compared to traditional analog circuit locking systems, our digital approach provides a routine to significantly reduce the system footprint and complexity while achieving similar locking performance. Our digital laser servo provides a two- order-of-magnitude reduction in Allan deviation compared to the free-running state of the laser.
In order to bring frequency combs out of well-controlled laboratory environments, a robust mode-locked fiber laser is preferable with enhanced self-starting capability, and reduced size, weight, and power (SWaP). We try to reduce cavity loss as much as possible to lower the self-starting threshold and operation power. A lower concentration doped-fiber could restrain non-radiative transitions and scattering from color centers, which are two main loss sources of active fibers. We performed comparison experiments using two fibers with different rare-earth doping concentrations for better SWaP performance. Experimental results match with our predictions. Pumped by a 980nm semiconductor external-cavity stabilized diode laser, a mode-locking self-starts at about 60mW. The net-dispersion is managed by balancing the lengths of the positively dispersive Er-doped fiber with the negatively dispersive PM1550 fiber. It is worth noting that the passive mode-locking can be maintained with pump power as low as 30mW. The laser central wavelength is 1560nm and 3db bandwidth is about 42nm, corresponding to a transform-limited pulse duration of 61.43fs assuming sech2 pulse shape. The repetition frequency is 47.41MHz with more than 90dB signal-to-noise ration at 1KHz resolution bandwidth. At the minimum pump power, the laser output is at 4.2mW average power.
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