Harmonically mode-locked (HML) fiber lasers delivering pulses with pulse repetition rate (PRR) in the GHz range have become a valuable alternative to semiconductor and solid-state lasers ensuring high beam quality, reliability, userfriendly light output, inherent to laser configurations in all-fiber format. The main drawback of HML laser technology is the noise-induced irregularities of the time interval between pulses known as the HML timing jitter. Ensuring low-level supermode noise and precise pulse repetition rate tuneability in all-fiber-integrated harmonically mode-locked laser sources establishes a new level of their versatility and extends areas of their applications. We report on new techniques enabling both the mitigation of supermode laser noise and highly precise setting of the PRR in a soliton fiber laser harmonically mode-locked by nonlinear polarization evolution. The principle of operation relies on resonant interaction between the soliton pulses and a narrow-band continuous wave (CW) component cooperatively generated within the same laser cavity.
We report on linewidth narrowing and stabilization of semiconductor DFB laser implemented through its self-injection locking to an external fiber ring cavity in conjunction with an active optoelectronic feedback circuit controlled by a simple low-cost USB-DAQ card. The system enables narrowing of the DFB laser linewidth below ~0.5 kHz and drastically reduced the laser phase noise. Specifically, the laser configuration is fully spliced from the polarization maintaining (PM) single-mode optical fiber that exhibits significantly improved stability against the environment noise. Drastic narrowing of the DFB laser linewidth down to ~310 Hz and a phase noise less than –100 dBc/Hz (<30 kHz) are achieved with the PM fiber ring cavity built from a single fiber coupler. The reported PM laser configuration is of great interest for many laser applications where a narrow sub-kHz linewidth, simple design and low cost are important.
In addition to the well-known problem related to the growing demands in telecom fiber optical data transmission lines with extended bandwidth in recent years the researchers developed an interest in mode-locked fiber lasers of 1600-1700 nm spectral region in a number of biomedical applications. The known approach to get the generation in this range uses the Er-doped fiber mode-locked seed source of telecom range. The output ultrashort pulse then propagates in nonlinear optical fiber with anomalous dispersion undergoing the Raman shift to the longer wavelengths. We propose a method to control the characteristics of the output Raman soliton spectrum adjusting the polarization of the pump pulse at the input of the nonlinear fiber. We have shown that this method allows to tune the wavelength of the output spectrum maximum in the whole range (1600-1700 nm) while the output power remains constant. Our simulation results agree with experimental observations.
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