Entire Wavelength Tuning Range Research Articles

Broadband tunable single-frequency Yb-doped fiber laser with dual ring subcavity and Sagnac ring filter

We demonstrate a broadband tunable ultra-narrow linewidth single-frequency (SF) Yb-doped fiber laser (YDFL). A dual ring subcavity (DRS) as a filtering component to eliminate the dense longitudinal mode inherent in YDFL and theoretically analyzed its filtering characteristics. By inserting a 0.8 m unpumped polarization-maintaining Yb-doped fiber in the Sagnac ring, a dynamic narrowband grating (DNG) based on the saturable absorption (SA) effect is constructed to ensure SF laser output over the entire wavelength tuning range. We achieve a wide wavelength tunable SF laser output in the range of ∼74 nm, covering 1021.06 nm to 1094.98 nm. The laser has an optical signal-to-noise ratio (OSNR) of more than 62.5 dB, a slope efficiency of 3.57 %, and an ultra-narrow linewidth of 622 Hz over the entire tunable range. In addition, the stability of the wavelength and power fluctuations below 0.018 nm and 0.67 dB, respectively. This tunable SF fiber laser has a compact structure, lower cost, and excellent performance, and is expected to be used in the fields of coherent communications, precision metrology, and so on.

Low spatial frequency laser-induced periodic surface structures in fused silica inscribed by widely tunable femtosecond laser pulses

The formation and evolution of laser-induced periodic surface structures in fused silica under irradiation of widely tunable (in the 1–3 upmum range) linearly polarized femtosecond (200 fs) pulses was studied experimentally. The structures were inscribed in high fluence regime (exceeding the surface ablation threshold for a single pulse) and characterized by using scanning electron microscopy and two dimensional Fourier transform. The results revealed rapid (after irradiation with a few successive pulses) formation of periodic laser-induced periodic surface structures aligned parallel to laser polarization, whose period increases with increasing the inscription wavelength, obeying the lambda /n law. With further increase of number of pulses, the generated structures gradually reorganize into laser polarization-independent low spatial frequency annular structures associated with formation of the damage crater, which fully established after irradiation with a few tens of successive laser pulses. This particular evolution scenario was observed over the entire wavelength tuning range of incident pulses.

Widely wavelength-tunable mode locked Er-doped fiber oscillator from 1532 nm to 1594 nm with high signal-to-noise ratio

A widely wavelength tunable mode locked Er-doped fiber laser oscillator based on nonlinear polarization rotation (NPR) effect was demonstrated. By intentionally introducing a position-and-width adjustable slit and a pair of reflective gratings into the cavity, which worked as a wavelength and bandwidth tunable spectral filter, continuous wavelength tunable mode locked operation from 1532 nm to 1594 nm was achieved, corresponding to a tuning range of 62 nm, which is a new record for mode locked continuously wavelength-tunable single mode Er-doped fiber oscillators based on nonlinear polarization rotation, to the best of our knowledge. During the wavelength tuning, the pulse durations for different wavelengths changed slightly and closed to the Fourier-transform limit. The signal-to-noise ratio (SNR) of the radio frequency (RF) spectrum of the output pulses is higher than 89 dB over the entire wavelength tuning range, superior to the typical SNR of 50–70 dB reported in most papers. In addition, the pulse duration could be tuned from 904 fs to 1339 fs by adjusting the width of the slit, for which the corresponding spectral bandwidth changed from 3.4 nm to 1.7 nm.

Polymeric tunable wavelength filter with two-stage cascaded tilted Bragg gratings.

Wavelength-division multiplexed optical communication systems used in 5G networks require tunable wavelength filters with narrow bandwidth for 100 GHz channel spacing, wide wavelength range to cover 16 channels, and a side mode suppression ratio (SMSR) exceeding 30 dB. To fabricate wavelength filters satisfying these specifications, tunable Bragg grating filters based on polymeric optical waveguides are proposed. The combination of mode-sorting waveguide and tilted Bragg grating enables the extraction of Bragg reflected signals to another path, without using an external circulator. Moreover, the double reflection by the two-stage cascaded structure produces narrower reflection bandwidth, improved SMSR characteristics, and reduced adjacent-channel crosstalk through the suppression of undesired mode coupling. The proposed device exhibits a 20 dB bandwidth of 1.0 nm and SMSR of 35 dB, over the entire wavelength-tuning range.

Femtosecond 78-nm Tunable Er:Fibre Laser Based on Drop-Shaped Resonator Topology

We report for the first time on study of a passively mode-locked Er-doped fibre laser with ultra-wide tuning (from 1524 to 1602 nm) of the central wavelength of its femtosecond output (503–940 fs). The recently proposed drop-shaped cavity topology for fibre lasers enabled a compact widely tuneable femtosecond laser with a single discrete element, a reflective diffraction grating used for wavelength tuning of the output radiation. The key features of the proposed laser are: spectral tuning does not require adjustment of any other cavity elements; stable generation with a single pulse per round trip is provided over the entire wavelength tuning range; and the output pulses are close to transform limited and feature relatively high (>58 dB) signal-to-noise ratio of laser inter-mode beats indicating high quality of mode locking across the entire wavelength tuning range. The proposed laser configuration relying on the new drop-shaped cavity topology simultaneously delivers stability and reliability of mode locking, possibility of broad-range wavelength tunability, and adjustability of the output pulse repetition rate of the laser (which is important in various metrological and other applications).

Transmission of 20 Gb/s PAM-4 signal over 20 km optical fiber using a directly modulated tunable DBR laser

We report 20 Gb/s transmission of four-level pulse amplitude modulation (PAM) signal using a directly modulated tunable distributed Bragg reflector (DBR) laser. Transmission distance over 20 km was achieved without using optical amplifiers and optical dispersion compensation modules. A wavelength tuning range of 11.5 nm and a 3 dB bandwidth greater than 10 GHz over the entire wavelength tuning range were obtained.

High-speed directly modulated widely tunable two-section InGaAlAs DBR lasers.

We report widely tunable two-section distributed Bragg reflector (DBR) lasers, which have InGaAlAs multiple quantum wells (MQWs) as the gain material. By butt-jointing InGaAsP, which has a photoluminescence wavelength of 1.4 μm as the material of the DBR section, a wavelength tuning range of 12 nm can be obtained by current injection into the DBR section. The direct modulation bandwidth of the lasers is greater than 10 GHz over the entire wavelength tuning range up to 40°C. Compared with InGaAsP DBR lasers having the same structure, the InGaAlAs lasers have smaller variations in both the threshold current and slope efficiency with the temperature because of the better electron confinement in the InGaAlAs MQWs. Moreover, the DBR-current-induced decreases in the modulation bandwidth and side mode suppression ratio (SMSR) of the optical spectra are notably smaller for the InGaAlAs lasers than for the InGaAsP lasers.

Extra-broadband wavelength-tunable actively mode-locked short-cavity fiber ring laser using a bismuth-based highly nonlinear erbium-doped fiber

We demonstrate an ultra-wideband wavelength-tunable actively mode-locked short-cavity laser employing a 151-cm-long bismuth-based highly nonlinear erbium-doped fiber (Bi-HNL-EDF). A wavelength tuning range of 87nm from 1533nm to 1620nm can be achieved because the Bi-HNL-EDF has an ultra-wide gain bandwidth. High nonlinearity of the Bi-HNL-EDF also collaborates with spectral filtering by an optical bandpass filter to suppress the supermode noise quite effectively. Total length of the fiber ring cavity is as short as 16m. Thus, stable and clean 5.6–6.1ps pulses with a repetition rate of 10GHz are successfully obtained over the wavelength tuning range almost completely covering both the conventional wavelength band (1530–1565nm) and the longer wavelength band (1565–1625nm). The bismuth-based short-cavity fiber laser also shows good performance in the back-to-back bit-error-rate measurements, and maintains bit-error-free mode-locking operation throughout the entire wavelength tuning range.

Characteristics of Wavelength-Tunable Harmonically Mode-Locked Short-Cavity Fiber Ring Laser Using a Bismuth-Oxide-Based Erbium-Doped Fiber and a Bismuth-Oxide-Based Highly Nonlinear Fiber

We propose and experimentally demonstrate a wavelength-tunable harmonically mode-locked short-cavity fiber ring laser, employing a 151-cm-long bismuth-oxide-based erbium-doped fiber (Bi-EDF) and a 414-cm-long bismuth-oxide-based highly nonlinear fiber (Bi-HNLF). A 75-nm continuous wavelength tuning from 1535 to 1610 nm can be achieved by utilizing the wide gain bandwidth of the Bi-EDF. Ultra-high nonlinearity of the Bi-HNLF collaborates with spectral filtering by an optical bandpass filter to suppress the supermode noise quite effectively. Total length of the ring cavity including all optical components is as short as 10 m. Back-to-back bit-error-rate measurements also confirm bit-error-free operation throughout the entire wavelength tuning range.

Characteristics of rational harmonic mode‑locked short‑cavity fiber ring laser using a bismuth‑oxide-based erbium‑doped fiber and a bismuth‑oxide‑based highly nonlinear fiber

We demonstrate a rational harmonic mode-locked fiber ring laser employing a 151-cm-long bismuth-oxide-based erbium-doped fiber (Bi-EDF) and a 250-cm-long bismuth-oxide-based highly nonlinear fiber (Bi-HNLF). Continuous wavelength tuning covering both the conventional wavelength band and the longer wavelength band can be achieved by utilizing the wide gain bandwidth of the Bi-EDF. The pulse amplitude can also be equalized by adjusting the modulation parameters of the intracavity modulator. Ultra-high nonlinearity of the Bi-HNLF collaborates with spectral filtering by an optical bandpass filter to suppress the supermode noise quite effectively. The total cavity length is as short as 10 m. Stable and amplitude equalized pulses up to 40 GHz can be successfully generated over the entire wavelength tuning range.

Generation of continuously wavelength-tunable optical short pulses by use of two self-seeded Fabry–Perot laser diodes and an optical switch

Generation of wavelength-tunable optical short pulses by use of two self-seeded Fabry-Perot laser diodes in a parallel configuration is described. The system supports continuous wavelength tuning in a relatively wide range of 42 nm. The side-mode suppression ratio achieved is 35 dB across the entire wavelength tuning range. The system is convenient for continuous wavelength tuning.

Cascaded strongly gain-coupled (SGC) DFB lasers with 15-nm continuous-wavelength tuning

More than 15 nm of continuous wavelength tuning range has been obtained by means of temperature tuning and current setting in three-section grating-cascaded strongly gain-coupled distributed-feedback lasers. Within the entire wavelength tuning range, a sidemode-suppression ratio beyond 50 dB, a linewidth of less than 10 MHz, and a fiber-coupled output power of more than 5 dBm were achieved.

Generation of wavelength tunable gain-switched pulses from FP MQW lasers with external injection seeding

High-repetition rate (20 GHz), wavelength tunable (40 nm) optical pulses have been generated by gain switching of 1.55-/spl mu/m Fabry-Perot (FP) multiple-quantum-well (MQW) lasers using continuous-wave (CW) light injection for longitudinal mode selection. After fiber compression, a pulse duration shorter than 5.8 ps was obtained over the entire wavelength tuning range. The pulsewidth, chirping characteristics and side-mode suppression ratio were investigated as functions of the seeding power and wavelength. In addition, we report on the observation of chaos and period doubling for a gain-switched laser without injection seeding, and show that irregular behavior can be suppressed by injection seeding of the laser.