Line-scanning technique using a PDMS grating in a microscope configuration.

We report the concept of a 1550 nm laser line scanning microscope based on a polydimethylsiloxane (PDMS) grating with scanning by stretching the PDMS grating to improve the scanning speed and enable low-cost scanning. Zemax is used to verify the possibility of realizing the system by simulating the illumination light path and the emission light path. The scanning field of view is 0.11m m×0.11m m, and the modulation transfer function (MTF) data of the 0th, ±1st, and ±2 nd diffraction orders in the illumination light path and the emission light path, respectively, meet the requirements of the diffraction limit resolution at the cutoff frequencies.

Generation of multiple solitons with controllable separation assisted by PM fiber.

A method of generating multiple solitons with controllable separation is proposed by injecting the dissipative soliton (DS) and conventional soliton (CS) into the polarization-maintaining fiber (PMF), respectively. The soliton separation can be adjusted subjectively from 12 to 28 ps, corresponding to PMF lengths from 10 to 24 m. Multiple solitons consisting of three, four, five, and six solitons can be observed at pump powers of 480, 550, 610, and 700 mW. DS's average single-pulse energy can increase from 20.4 to 40.7 pJ, while the average single-pulse energy of CS decreases from 29.3 to 13.7 pJ. The experimental results can contribute to the further understanding of the dynamics of multiple solitons.

Enhanced four-wave mixing in borophene-microfiber waveguides at telecom C-band.

All-optical wavelength conversion technology based on two-dimensional (2D) materials has lately received keen interest. As a new 2D material, borophene displays acceptable photoelectric properties. We demonstrate the all-optical wavelength conversion through four-wave mixing (FWM) in borophene-microfiber hybrid waveguides. Borophene is deposited at the thinnest part of the tapered fiber and enhanced FWM occurs in this photonic device. By optimizing the effect of nonlinear polarization, wavelength tuning, and power variation, the conversion efficiency increases to -19.1dB, corresponding to 3 dB conversion bandwidth in a range of 7.1 nm. In addition, this photonic device is employed to achieve all-optical wavelength conversion of 10 Gb/s non-return-to-zero digital sequence. The signal quality of converted light such as optical signal-to-noise ratio, bit-error-rate, and eye diagram are investigated, which indicates that the proposed wavelength converter has high conversion efficiency and remarkable stability. This study shows that the borophene-microfiber waveguide has potential application prospects in all-optical signal processing.

Combined influence of the gain and dispersion of a mode-locked fiber laser on a time-stretching analog-to-digital conversion link.

Using theory and experiments, we demonstrated the combined influence of the spectral gain and dispersion of a dissipative soliton mode-locked fiber laser on a time-stretching analog-to-digital conversion link without an optical amplifier. The theoretical and experimental results indicate the following: first, the amplitude and envelope shape of the stretched signal are mainly affected by the spectral gain of the dissipative soliton at different central wavelengths under a radio frequency signal of 10 GHz. Second, at the higher frequency of 25 GHz, the influence of the phase shift induced by the dispersion of different spectral ranges on the amplitude of the stretched signal becomes clearer. The amplitude of the stretched signal across all spectral ranges decrease, and the envelope shape differs from that at 10 GHz. Moreover, the wavelength at the maximum amplitude of the stretched signal changes, for which the influence of the spectral dispersion is greater than that of the spectral gain. Finally, the ratio of the amplitude at 25 GHz to that at 10 GHz at different spectral ranges are different, which indicates that the amplitude of the stretched signal at different spectral ranges is affected by the phase shift by different degrees.

Transmission performance of a 1.96 µm active mode-locked laser in smoke channel.

We demonstrate the data transmission characteristics of a 1.96 µm laser in an indoor simulated smoke channel. An active mode-locked thulium-doped fiber laser at 1961.63 nm can be generated, and the repetition rate is 2.11 GHz corresponding pulse duration of 12.47 ps. Therefore, the pulse can be modulated by a 2.11 Gb/s digital sequence to act as a carrier light source. The transmission characteristics of signal light under different visibility conditions of 0.5, 0.05, and 0.005 km are studied. Compared with the back-to-back conditions, the signal-to-noise ratios of the carrier can decrease after passing through the smoke channels, which are 3.93, 7.1, and 9.08 dB, respectively. At the same time, the received power jitter can increase from ±0.16 to ±1.14dB. The sensitivity can be -19.52dBm at the visibility of 0.005 km. The experimental results show that thulium-doped mode-locked laser has an excellent performance in a smoke channel.

1.9 µm all-optical wavelength converter based on a graphene oxide coated microfiber.

An all-optical wavelength converter based on graphene oxide (GO) is proposed at the 1.9 µm band. The homemade GO-coated microfiber is acquired through the optical deposition method, which shows a remarkable nonlinear optical response. Stable conversion efficiency up to -45.52 dB is obtained with 1 nm wavelength interval, and the wavelength tuning range can reach 6 nm (1969-1975 nm). With fixed wavelength interval, the conversion efficiency can increase with the increase of pump power. Simultaneously, the fluctuation of conversion efficiency is ±0.41 dB within 2 hours. The demonstrated all-optical wavelength converter based on GO can play an outstanding role in the future of all-optical communications and networks.

Low threshold soliton and a noise-like pulse conversion in an all-polarization-maintaining figure-eight cavity.

We experimentally report a low threshold soliton and a noise-like mode-locked fiber laser using an all-polarization-maintaining figure-eight cavity. We built a bidirectional pump structure without a phase shifter at the beginning of the experiment. The resonator has a high mode-locking threshold of 620 mW. Afterwards, we used a phase shifter in the resonator, and the laser can self-start in a conventional soliton (CS) mode-locked state when pump1 reaches the threshold of only 70 mW. The CS pulse with a duration of 863.8 fs can be observed at 1560 nm. When the two pump powers increase to 350 mW and 50 mW, the conventional soliton can convert to noise-like pulses. The central wavelength and pulse duration of noise-like mode-locked pulse are 1560.4 nm and 417.9 fs, respectively. The laser can realize conversion between ultrafast pulses and high-energy pulses, and have a low threshold that can be used for nonlinear frequency conversion, supercontinuum generation, sensing, etc.

Tunable peak power square-wave pulse based on an all-PM thulium-doped mode-locked fiber laser.

Nanosecond dissipative soliton resonance pulse is a demonstration of an all polarization-maintaining (PM) thulium-doped fiber laser in a nonlinear amplifying loop mirror (NALM)-based figure-eight configuration. Each loop of the apparatus includes a controllable power amplifier. With increased amplifier power, pulse width broadens linearly from 3.6 to 13.5 ns, and maximum single pulse energy can reach 27.5 nJ. Interestingly, the output peak power presents two completely opposite proportional effects in terms of the variation of settings for two amplifiers, respectively. The experimental results show that the NALM loop plays an important role for tunable pulse duration, and the unidirectional ring part makes a significant contribution for power scaling.

Transmission characteristics of 1.55 and 2.04 µm laser carriers in a simulated smoke channel based on an actively mode-locked fiber laser.

We experimentally demonstrate a free-space data transmission system in an indoor simulated smoke chamber with a laser carrier of an erbium-doped actively mode-locked fiber laser and a holmium-doped actively mode-locked fiber laser. Two additional semiconductor lasers operating at 0.85 and 1.06 µm are used to calibrate the visibility of a smoke channel using the Ijaz model and compare smoke attenuation with 1.55 and 2.04 µm lasers. The eye patterns and bit error rates of 1.55 and 2.04 µm laser carriers with a data rate of 4.04 Gbps are investigated experimentally at 0.5, 0.05, and 0.005 km visibilities. The experimental results show that the smoke attenuation is wavelength dependent for V < 0.5 km. As the visibility decreases, the long wavelength laser is less affected by the attenuation and power fluctuation caused by Mie scattering. The measured optical signal-to-noise ratios of the 1.55 and 2.04 µm laser carriers for V = 0.005 km are 4.83 and 8.62 dB, respectively. The corresponding link sensitivities are -14.59 and -17.74 dBm, respectively, indicating that the 2.04 µm data transmission system is more reliable under an extremely dense smoke condition.

Switchable generation of a sub-200 fs dissipative soliton and a noise-like pulse in a normal-dispersion Tm-doped mode-locked fiber laser.

We report on the switchable generation of a dissipative soliton (DS) pulse and a noise-like pulse (NLP) in an all-fiberized Tm-doped fiber laser in the normal-dispersion region. Mode-locking operation is achieved through a nonlinear polarization rotation component, and the cavity dispersion is compensated using ultra-high numerical aperture (UHNA4) fiber that is easy to integrate and low in cost. At a pump threshold of 510 mW, DS operation can first be achieved without additional filter. The 3 dB spectrum bandwidth of the DS pulse is greater than 50 nm, and the duration of the de-chirped pulse is 193 fs. By increasing the pump power to 880 mW, the mode-locking state can evolve into NLP operation with proper cavity polarization state. The 3 dB spectrum bandwidth and duration of de-chirped coherence spike are 105.6 nm and 121 fs, respectively. Meanwhile, ultra-broadband NLP (over 150 nm considering 3 dB spectrum width) can also be observed with the appropriate cavity parameters. All the proposed pulse patterns present good capacity for achieving narrow pulse width and withstanding high pulse energy.

Low SNR difference Nyquist-WDM channels with optical sinc-shaped pulses based on flat electro-optic frequency combs.

We experimentally demonstrate Nyquist wavelength-division-multiplexed (WDM) channels with a low signal-to-noise ratio (SNR) difference based on flat electro-optic combs (EOCs), which reduce the interchannel crosstalk penalty in Nyquist-WDM transmission with no guard band. The five Nyquist-WDM channels are generated through the insertion of uniform and coherent lines around each line of the EOCs from a dual-parallel Mach-Zehnder modulator. For the five channels, the normalized root-mean-square error of optical sinc-shaped pulses at a repetition rate of 9 GHz is between 1.23% and 2.04%. The SNRs of the Nyquist signal can be better than 30 dB by using flat EOCs with a narrow linewidth as WDM sources, and the difference in SNR is less than 0.6 dB for the WDM channels. The transmission performance of five Nyquist-WDM channels with no guard band is compared in a 56 km fiber link. The results show that our scheme provides a minimum interchannel sensitivity penalty of 0.7 dB at the forward-error-correction limit. The Nyquist-WDM channels with low SNR difference can effectively improve the communication performance of the Nyquist-WDM system.

1.9 µm square-wave passively Q-witched mode-locked fiber laser.

We propose and demonstrate the operation of Q-switched mode-locked square-wave pulses in a thulium-holmium co-doped fiber laser. By using a nonlinear amplifying loop mirror, continuous square-wave dissipative soliton resonance pulse is obtained with 4.4 MHz repetition rate. With the increasing pump power, square-wave pulse duration can be broadened from 1.7 ns to 3.2 ns. On such basis Q-switched mode-locked operation is achieved by properly setting the pump power and the polarization controllers. The internal mode-locked pulses in Q-switched envelope still keep square-wave type. The Q-switched repetition rate can be varied from 41.6 kHz to 74 kHz by increasing pump power. The corresponding average single-pulse energy increases from 2.67 nJ to 5.2 nJ. The average peak power is also improved from 0.6 W to 1.1 W when continuous square-wave operation is changed into Q-switched mode-locked operation. It indicates that Q-switched mode-locked operation is an effective method to increase the square-wave pulse energy and peak power.

Dual square-wave pulse passively mode-locked fiber laser.

We study a passively mode-locked square-wave pulse (SWP) fiber laser with a nonlinear amplifying loop mirror in the cavity. The net dispersion of the cavity is about 0.68 ps2 and the SWP mode-locked fiber laser can be realized. The peak power of the SWP hardly varies and the pulse duration gets expanded with the increasing pump power. SWPs breaking in the low nonlinear cavity can be observed and the stable dual SWP can be achieved in the experiment. When the total pump power stays at 800 mW, the interval of dual pulses is 41 ns. The widths of dual SWPs are both 1.5 ns. The output power rises linearly with the increasing of the pump power, while the interval of dual SWPs is almost constant. Then, the physical mechanism of the SWP breaking and vector nature of the pulse are analyzed.

Triple Brillouin frequency spacing multiwavelength fiber laser with double Brillouin cavities and its application in microwave signal generation.

We propose and experimentally investigate a multiwavelength Brillouin fiber laser (MBFL) with triple frequency spacing by employing a modular structure. In this scheme, we obtain nine channels optimized with frequency spacing of 0.259 nm. The single, double, and triple Brillouin frequency spacing for the MBFL can be easily realized by utilizing this modular structure. The impact of Brillouin pump (BP) power, BP wavelength, and erbium-ytterbium-doped fiber amplifier (EYDFA) output power on the performance of the MBFL is investigated, respectively. We also study the generation of beating frequency microwave signals based on single, double, and triple frequency spacing MBFL. 10.5 GHz, 21.48 GHz, and 31.77 GHz microwave signals with 3 dB linewidth of 16.4 MHz, 15.2 MHz, and 12.8 MHz are generated, respectively.

Widely tunable 2 µm continuous-wave and mode-locked fiber laser.

We propose and experimentally demonstrate a widely tunable mode-locked thulium-doped fiber laser. The laser can operate at both continuous-wave and mode-locked states at different pump power levels. A classic nonlinear polarization rotation structure is employed to obtain the passive mode-locked laser. A birefringence Lyot filter as a fiber comb filter is used to expand the tuning range. Thanks to the filtering component, the tuning range of the continuous-wave laser can reach 127 nm (1823-1950 nm). The tuning ranges of the single-wavelength and dual-wavelength mode-locked lasers are 94 nm (1835-1929 nm) and 87 nm (1831-1918 nm), respectively, with a 3.085 MHz repetition rate and 75 ps pulse width.

Tunable multiwavelength Tm-doped fiber laser based on the multimode interference effect.

A simple multiwavelength Tm-doped fiber laser at the 2 µm band based on multimode interference (MMI) is proposed and experimentally demonstrated. In this scheme, a 4 m Tm-doped single-mode fiber is pumped by a 1568 nm laser, and a single-mode-multimode-single-mode (SMS) fiber structure is used as an MMI filter in which the multimode fiber is used to tune the laser. Laser operation of up to three wavelengths is obtained based on the MMI filter. The wavelengths can be tuned by adjusting the polarization controller and rotating the multimode fiber in the SMS structure, and the tuning region is about 24 nm, i.e., 1892-1916 nm. The side-mode suppression ratio of the laser is about 54 dB. The 3 dB linewidth is less than 0.04 nm. Peak fluctuation at each wavelength is analyzed, and the results show that the power fluctuation is less than 3 dB around the average power.

Generation of mode-locked states of conventional solitons and bright-dark solitons in graphene mode-locked fiber laser.

This paper proposes a mode-locked fiber laser based on graphene-coated microfiber. The total length of the fiber laser resonant cavity is 31.34 m. Under the condition of stable output of bright-dark soliton pairs from the fiber laser, dual-wavelength tuning is realized by adjusting the polarization controller (PC), and the wavelength tuning range is 11 nm. Furthermore, the effects of polarization states on bright-dark solitons are studied. It is demonstrated that the mode-locking state can be switched between conventional solitons and bright-dark solitons in the graphene mode-locked fiber laser. Bright-dark soliton pairs with different shapes and nanosecond pulse width can be obtained by adjusting the PC and pump power.