The implementation of wavelength conversion around 1.55 μm in quasi-phase-matched (QPM) periodically poled LiNbO3
(PPLN) waveguides is mostly based on the cascaded second-harmonic generation/difference-frequency generation
(SHG/DFG) process. As usual, a continuous wave (CW) and a pulsed wave are injected into a PPLN waveguide. Of them,
the pulsed wave is regarded as the information carrier, and the CW is taken as the control. To transfer the information of
optical codes from one wavelength to another, the codes can be applied to either the signal (CW-pumped scheme) or the
pump (pulse-pumped scheme). In this work, the temporal and spectral properties of wavelength conversions during pulse
propagation as well as the conversion efficiency in the two pumping schemes were compared experimentally and
theoretically under different conditions of input pulse width, pump power and pump central wavelength. In the
experiments, we adopted an MgO-doped PPLN waveguide, and a 40-GHz tunable picosecond-pulse source. The
conversion characteristics were systematically investigated when the CW and the pulsed wave were alternatively taken as
the pump at the quasi-phase-matching wavelength of the device. In the theory, we solved the coupled-mode equations and
explained the physical insights for the numerical results and experimental observations. The conversion properties of the
two pumping schemes were quantitatively compared. The simulated results agree well with the experimental data, and the
obtained results provide some guidelines for the design and application of QPM waveguides in wavelength conversion.
By using the phase modulated optical fiber loop mirror, repetition-rate-doubled or -tripled output is realized in the FM mode-locking fiber laser. 80 and 120GHz transform-limited pulses are experimentally demonstrated.
Second harmonic generation (SHG) in a periodically poled MgO-doped lithium niobate (PPMGLN) waveguide is studied using a tunable pulsed pump source composed of a mode-locked fiber ring laser and two tunable filters. In the experiment, the lasing wavelength can be tuned from 1530 to 1579 nm, and the pulse width can be tuned from 2 to 7 picoseconds at 40 GHz. Second-harmonic pulses are generated when the picosecond pump pulses pass through the PPMGLN waveguide. SHG conversion efficiency versus pump pulse width, pump power, and pump wavelength is investigated experimentally. Propagation behaviors of both pump and SHG pulses are then numerically simulated. Based on the temporal and spectral characteristics of conversion, a quantitative analysis on SHG efficiency is presented. The simulation results are in good agreement with the experimental data.
Active FM harmonic mode-locking of a fiber laser by intracavity phase modulation allows obtaining of stable laser pulses with a high repetition rate. Driving the phase modulator by an external RF synthesizer has an advantage of constantly applying only one modulation frequency to the phase modulator. That greatly facilitates the generation of stable laser pulses with very small noise. However, this approach requires constant frequency tuning of the synthesizer to compensate for small temperature fluctuations causing changes in the fundamental frequency of the laser cavity. In order to control the modulation frequency we mixed the RF signal from the laser output detected by a fast photodiode with the signal from the synthesizer. The amplitude of the measured DC component of the mixed signal depends on the phase difference of the two signals. The phase difference varies approximately linear with the laser detuning near the mode locking resonance. We develop software that performs constant measurement of the mixed signal and tuning the modulation frequency in order to keep the DC component of the mixed signal at a preset value. The program performed approximately two auto-tuning steps per second. The presented method allows very simple and reliable obtaining of stable computer controlled harmonic mode-locking of a fiber laser at 40 GHz repetition rate frequency.
A wide band FM light source is useful for many applications in the areas of communication, spectroscopy and sensing. Chirped light can be generated by modulating the laser output externally with a phase modulator. But the spectrum is usually not wide enough due to the modest modulation depth of available commercial modulator. A more efficient method is to perform phase modulation of light inside the laser cavity, which under certain cavity detuning conditions can perform deeper FM modulation through cavity mode coupling effect. The output spectrum is much wider than that obtained external modulation alone.
In this paper, we show the theoretical and experimental result of a 40GHz FM oscillation fiber laser. The detuning frequency and single-pass modulation depth can affect the final spectrum width. The wide spectrum of 1.42THz around 1560nm is obtained in the experiment. By using single mode fiber as dispersion medium only, we can compress the constant intensity light with 1.42THz bandwidth to 821fs pulse. This laser appears to be of great interest for study of high-speed optical communication systems for the impairments and their impacts at wide frequency range. Or, its ultra-wide, chirp output with high sweep rate will be useful for rapid and extremely sensitive interferometric measurements.
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