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.
We report on suppression of supermode noise in harmonically mode-locked fiber laser achieved through the injection of continuous wave (CW) into the laser cavity. Implementing this method, we have shown experimentally that supermode suppression level of harmonically mode-locked laser could be increased by 20-30 dB depending on the pulse energy of the mode-locked laser. Experiments have refined the requirements to the positions inside the laser spectrum assigned to the injected CW component, a Kelly soliton sideband, and the transparency peaks of the birefringent filter formed in the ring fiber cavity. The effect takes places at proximity of the injected CW to the Kelly sideband leading to phase-locking between the CW and the solitons.
Self-injection locking to an external fiber cavity is an efficient technique enabling drastic linewidth narrowing of semiconductor lasers. Recently, we have introduced a simple dual-frequency laser source that employs self-injection locking of a DFB laser in the external ring fiber cavity and Brillouin lasing in the same cavity. The laser performance characteristics are on the level of the laser modules commonly used with Brillouin Optical Time Domain Analysis (BOTDA). The use of a laser source operating two frequencies strongly locked through the Brillouin resonance simplifies the BOTDA system, avoiding the use of a broadband electrooptical modulator (EOM) and high-frequency electronics. In this work, in a direct comparison with the commercial BOTDA, we explore the capacity of our low-cost solution for BOTDA sensing, demonstrating distributed measurements of the Brillouin frequency shift in a 10 km sensing fiber with a 1.5 m spatial resolution.
We report on a new method for fabrication of saturable absorbers for operation in ring fiber lasers as a mode-locking element taking advantages of the manufacturing technologies of fiber couplers and microfiber tapers coated with carbon nanotubes. It is demonstrated in the experiment that the transmittance of the fabricated saturable absorber increases with an increase of the input radiation power. Also, we have shown that the fabricated saturable absorber enables generation of picosecond soliton pulses in ring fiber laser configuration.
Soliton fiber lasers with passive harmonic mode-locking (HML) have been known for several decades as reliable sources of pulse trains with a high repetition rate. They are commonly employed as frequency comb generators in a wide list of applications in spectroscopy, telecommunications, microwave photonics. Besides that exhibiting advantageous consumer properties, such as compactness, reliability, low cost and convenience of beam delivery approach these sources are among the most attractive alternatives for material processing, medicine lasers and other areas. In the first group of these lasers the HML occurs due to special intra-cavity periodic filter with a free spectral range (FSR) which is a multiple of the main cavity FSR and equals to the pulse repetition rate. The second group of HML lasers in this classification is attractive for the scientific community due to the automatic arrangement of strongly periodic pulse pattern in the laser cavity through pulse repulsion. However, it is difficult to specify the mechanism of pulse repulsion for each case. It can be based on interaction through saturating and relaxing dissipative parameters, continuous-wave (CW) radiation component, dispersion waves, or acoustic waves induced by electrostriction, etc. A common feature of all interaction induced effects is low intensity, in many cases only slightly exceeding the noise level. The noise-induced fluctuations in the time interval of the HML pulse train are known as HML timing jitter, and its value is significantly higher than the timing jitter in lasers operating at fundamental frequency. It is a major drawback of the HML laser technology. We report the stabilizing frequency shift effect in harmonic mode-locking ring soliton fiber laser that is studied theoretically and numerically. We demonstrate that the frequency shift contributes to an increase in the hardness of interpulse interactions and can led to stabilization of the periodic arrangement of pulses. The experiment carried out confirms the theoretical predictions and the results of numerical simulation.
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