A single-cavity triple-comb all-fiber laser is proposed by wavelength/polarization multiplexing. A variable optical attenuator is introduced to equalize the 1530-nm and 1550-nm gain profile of erbium-doped fiber for dual-wavelength pulses. Their repetition rate difference reach kHz level. Meanwhile, by further adjusting the intracavity polarization state, polarization-multiplexed dual-comb pulses with tens-of-Hz repetition rate difference in the 1550-nm gain region are obtained. The more than one-order-of-magnitude difference between the maximum and minimum repetition frequency difference and qualified passive mutual coherence of triple-frequency pulses is highlighted. These results indicate a highly potential triple-comb source for multiple-comb metrology such as triple-comb ranging and frequency measurement and so on.
We demonstrate a Q-switched mode-locked Er-doped fiber laser using an all-fiber grade-index multimode fiber-based modulator which generates dark-bright pair between bright pulse sequences and alternate bright and dark pulses. A section of dispersion compensation fiber (Nufern UHNA4) considered as a candidate normal group victory dispersion fiber is used to adjust the net dispersion of cavity. At a pump power of 410 mW, evident Q-switched instability modulating mode-locked bright pulses are observed, and the duration of Q-switched envelope changes from 1.8 μs to 8 μs along with the variation of power. Changing the state of polarization controller, the mode-locked bright pulse train is tuned to dark pulse train with reducing the duration of Q-switched envelope to 1.2 μs. What’s more, dark-bright pair between bright pulses train and alternate bright and dark pulses are also observed under second harmonic operations with suitable PC states. Coupled complex Ginzburg-Landau equation, field coupling model for propagation in multimode fiber, and fiber nonlinear effects are provided to reveal the underlying principles of the transition of these pulse trains. Because of the principal modes and filtering effect in multimode fibers, the formation and stable propagation of the dark-bright pair are precisely achieved. At the same time, the physical mechanism behind the unusual pairing of dark and bright pulses is that under certain conditions, cross-phase modulation can counteract the time extension of optical pulses caused by the combination of self-phase modulation and normal dispersion. Thus, the cross-phase modulation induced chirping on dark solitons enables dark-bright pair between bright pulse sequences to coexist.
In this work, we demonstrate a single-walled carbon nanotubes-based wavelength multiplexed fiber laser, which generates dual-comb pulse in the train of soliton rain. The fiber laser cavity is manipulated in repetition frequency of 16.58 MHz, 3 dB spectral bandwidth of 8.4 nm. Two asynchronous pulses constitute the soliton rain pulse sequences, which the intensity difference is about 5.72 dB between the dual frequencies. A piece of graded-index multi-mode fiber as a filter based on the multi-mode interference effect is introduced into cavity to improving the signal to noise ratio to ~62 dB, and locate the central wavelength of the dual-comb at 1556.7 nm and 1561.5 nm. The repetition rate difference of the dual-frequency is about 169 Hz with the resolution bandwidth of 1 Hz. The time delay of the dual-frequency pulse detected by cross-correlation method is 5.78 ms, which is well matched with the results in radio frequency spectrum. Different from the stable period of the general cross-correlation signal, our experimental results show several different sub-periods due to the existence of the drifting solitons in the soliton rain sequences. Meanwhile, the number of different sub-periods in the correlation decreases from six to three as the pump power reduced from 100 mA to 97.3 mA. Our work provides a new sight into the quasi-steady multi-soliton dynamics process in fiber lasers, and will be promising solutions for interference ranging, and synchronization and timing.
In our work, we experimentally demonstrate wavelength multiplexed dual-comb pulses based on multi-modal interference effect in a passively single-walled carbon nanotube mode-locking all fiber ring laser. The laser cavity achieves a variety of dual-wavelength mode-locked states by switching the polarization controller in the laser cavity. A piece of 25 cm long graded-index multi-mode fiber as a filter based on the multi-mode interference effect is introduced into cavity to fixing wavelength and to improving the signal to noise ratio. With optimized length of multi-mode fiber, we observed the two different filter state which located at 1559 nm and 1562 nm, 1561 nm and 1563 nm respectively in the different polarization dual-comb states. With suitable filtering state by stretching the multi-mode fiber, the two asynchronous pulse sequences coexist with diverse operation, which propagate with singlet and double pulses, respectively. The repetition rate of the laser is 16.59 MHz and the time period corresponding to the asynchronous pulse is ~60 ns. The repetition rate difference of dual-wavelength states reaches 100 Hz. In addition, we recorded the output modulation state of the laser cavity. Our research provides experimental basis for optical fiber sensing, wavelength division multiplexing communication system and high resolution spectroscopy.
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