Mode-locked Erbium Fiber Laser Research Articles

Dual-wavelength mode-locked erbium fiber laser utilizing a Ge-PCF saturable absorber

In this work, we demonstrate a mode-locked erbium-doped fiber laser (EDFL) based on germanium-core doped photonic crystal fiber (Ge-PCF) as a passive saturable absorber (SA). The proposed SA was incorporated inside the PCF core during the manufacturing process. The short length of the proposed high nonlinear fiber can achieve a stable mode-locked pulse by controlling the fiber and collapsing lengths. The proposed Ge-PCF has been integrated into an erbium-doped fiber ring laser to establish the desired ultrashort pulse with the merits of a short interaction length (<3cm), enhanced damage threshold, environmental robustness, ease of manufacture, and suitable mass production. The obtained output pulses at FWHM have a pulse width of 800 ps and a repetition rate of ∼19MHz. Furthermore, the EDFL based on Ge-PCF achieved a dual-wavelength output spectrum with narrow bandwidths centered at 1557.27 and 1558.81 nm. The short-pulse capabilities of the proposed fiber laser make it highly suitable for a wide range of photonic applications.

Generation of 583 fs optical pulses at 10 GHz from a regeneratively mode-locked fiber laser combining nonlinear polarization evolution

A regeneratively mode-locked erbium fiber laser was numerically investigated and experimentally demonstrated, which was able to generate a 583 fs pulse train at 10 GHz via intracavity pulse compression with nonlinear polarization evolution (NPE). To excite the NPE at such a high repetition rate, a dispersion map was intentionally introduced to obtain short pulses accompanied by high peaks through soliton-like pulse shaping. Numerical simulations indicated that steady-state oscillation with pulses below 1 ps can be successfully established using this laser configuration. Experimentally, we obtained a pulse duration of 583 fs and a 3 dB spectral width of 4.5 nm at an average output power of 15.6 mW. Simultaneously, supermode suppression of more than 80 dB was also obtained from the appropriate biased NPE.

Design and performance of a repetition rate controllable and wavelength tunable L + U-band actively mode-locked erbium fiber laser

Mode-locked lasers draw great interest in the laser research field due to their diverse applications and easy achievement of ultra-short and high-intensity optical pulse trains. Actively mode-locked (AML) fiber lasers are more flexible compared to passively mode-locked fiber lasers owing to their ability to control repetition rates and pulse intervals electronically. The design and development of wavelength tunable AML fiber lasers operating beyond the C-band have attracted great attention from the research community. In this paper, a repetition rate controllable and wavelength tunable L+U-band AML erbium fiber laser (AML-EFL) based on a single standard 1480 nm pump laser and Mach–Zehnder modulator (MZM) as an intra-cavity intensity modulator is demonstrated using numerical simulation. The AML-EFL is created using a MZM driven by electrical Gaussian pulses and a tunable optical bandpass filter that is used to tune the laser cavity at any wavelength in the 1565–1645 nm range. The repetition rate of the AML-EFL is controlled from 500 kHz to 1 GHz. Trains of pulses with pulse widths of 103 ns and 50 ps and pulse intervals of 2 µs and 1 ns are successfully generated at repetition rates of 500 kHz and 1 GHz, respectively. A pulse energy of 626 nJ is obtained for a 500 kHz repetition rate. Signal-to-noise ratios of 30 and 32 dB are observed for trains of mode-locked pulses with repetition rates of 500 kHz and 1 GHz, respectively. In addition, slope efficiencies of around 66% for an L-band wavelength of 1582.1 nm and 30% for a U-band wavelength of 1633.6 nm are obtained considering the optimized parameters.

Experiments on an AM Mode-Locked Laser as an Arbitrary Optical Function Generator

We show experimentally that various optical pulses can be generated from an AM mode-locked laser by employing a 10 GHz harmonically and regeneratively mode-locked erbium fiber laser. A software-controlled liquid crystal on silicon (LCoS) optical filter device, which can control the amplitude and phase of the input signal as a function of optical frequency, was employed to implement a specific filter function. The filter was characterized by the Fourier transformed spectral profile <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$A(\omega )$ </tex-math></inline-formula> of the output pulse <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$a(t)$ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$A(\omega +\Omega _{m}), $ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$A(\omega -\Omega _{m})$ </tex-math></inline-formula> . Here, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Omega _{m}$ </tex-math></inline-formula> is the AM modulation angular frequency. The filter resolution was set at 10 GHz. By installing a specific filter in the laser cavity, we succeeded in generating various pulses such as Gaussian, secant hyperbolic (sech), double-sided and single-sided exponential, parabolic, triangular, and Nyquist pulses with an arbitrary roll-off factor as with an optical function generator. It is interesting to note that we could experimentally generate very precise transform-limited (TL) optical pulses, which were originally designed by calculating the filter function. The generated spectral profile agreed well with the numerical results even below −40 dB, which also agreed with the theory. As we have shown theoretically, we could generate sech and parabolic pulses without fiber nonlinearity. An asymmetric single-sided pulse can also be generated by incorporating a complex filter function with phase information, where a small amount of phase rotation of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim \pi $ </tex-math></inline-formula> /60 in each round trip played an important role in generating the asymmetric pulse. Without the phase change, the pulse became symmetric and was no longer the designed pulse. A clean triangular pulse was difficult to generate since parabolic pulse shaping on the top of a sinusoidal intensity modulation is ineffective in creating a sharp peak.

Experiments on an FM Mode-Locked Laser as an Arbitrary Optical Function Generator

We describe experimentally that an FM mode-locked laser can generate various optical pulses such as Gaussian, secant hyperbolic (sech), double-sided exponential, rectangular, parabolic, triangular, and Nyquist pulses like an optical function generator. The experiments were carried out by employing a 10 GHz actively FM mode-locked erbium fiber laser, where a specific filter was installed in the laser cavity. The specific filter was characterized by the Fourier transformed spectrum <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$A(\omega )$ </tex-math></inline-formula> of an output pulse <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$a(t)$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$A(\omega $ </tex-math></inline-formula> + <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n\Omega _{m})$ </tex-math></inline-formula> with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n=- \infty \sim + \infty $ </tex-math></inline-formula> . Here, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Omega _{m}$ </tex-math></inline-formula> is the angular modulation frequency. We employed a software-controlled liquid crystal on silicon (LCoS) optical filter, which can control the amplitude and the phase of the input signal as a function of optical frequency. Therefore, we were able to generate various optical pulses by simply changing the filter shape using the software. The filter resolution was 10 GHz. The generated optical pulses agreed well with the theory, where the spectral profile even below −40 dB coincided well with the theoretical profile. We proved experimentally from the product of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta \nu \Delta \tau $ </tex-math></inline-formula> at −3 dB that the output pulses have none of the frequency chirp that would inevitably occur with conventional FM mode-locking. It was not possible to generate an asymmetric single-sided pulse as we pointed out theoretically. We were also able to generate parabolic and triangular intensity pulses as we predicted in a previous paper. Finally, we compared the possibilities of generating various pulses with AM and FM mode-locking techniques.

Ultrafast optics with a mode-locked erbium fiber laser in the undergraduate laboratory

We describe an ultrafast optics laboratory comprising a mode-locked erbium fiber laser, autocorrelation measurements, and a free-space parallel grating dispersion compensation apparatus. The gain spectrum of Er fiber provides a broad bandwidth capable of supporting sub-100 fs pulses centered near a wavelength of 1550 nm. The fiber laser design used here produces a train of pulses at a repetition rate of 55 MHz with pulse duration as short as 108 fs. The pulse duration is measured with a homebuilt autocorrelator using a simple Michelson interferometer that takes advantage of the two-photon nonlinear response of a common silicon photodiode. To compensate for temporal stretching of the short pulse due to group velocity dispersion in the fiber, an apparatus based on a pair of parallel gratings is used for pulse compression. A detailed part that lists in the supplementary material includes previously owned and common parts used by the telecommunications industry, which helps decrease costs of the laboratory. This provides a cost-effective way to introduce the principles of ultrafast optics to undergraduate laboratories.

Passively mode-locked composite erbium fiber laser with a pulse repetition rate of 150 MHz

We demonstrate a short cavity passively mode-locked composite erbium-doped fiber laser with saturable absorber based on aerosol-synthesized single-walled carbon nanotubes. Optical pulses had a duration of 973 fs while an average output power – 6 mW. Using a shortened laser cavity, a stable soliton generation with a pulse repetition rate of 150 MHz at the output of the all-fiber linear cavity was obtained.

Energy scaling of an erbium-doped mode-locked fiber laser oscillator

We investigate a promising approach to the energy scaling of mode-locked erbium fiber lasers. It consists in a special cavity design, comprised of a really long polarization maintaining (PM) part and a short non-PM part. Two separate fiber parts ensure the principle of space division for highly chirped dissipative soliton formation and NPE-based self-amplitude modulation effects. As a result, the pulse energy was increased up to 4 nJ with an estimated transform-limited pulse duration of 150 fs. It was also found that further energy scaling is limited by transition to a noise-like pulse generation regime. Numerical simulations revealed that this transition is associated with a contrast of the employed spectral filter and the amplifier noise level, which agrees well with the experiment.

10GHz regeneratively mode-locked erbium-doped fiber laser combined with nonlinear amplifying loop mirror

Mode-locked fiber lasers can generate short pulses with a high repetition rate. It can be used in nonlinear optical imaging, photo-communication and other fields. Obtaining narrow pulses with high repetition frequency has become a hot research topic. In this article, we demonstrate a hybrid mode-locked fiber laser, which is a regeneratively mode-locked erbium fiber laser operating at 10 GHz, incorporating a nonlinear amplifying loop mirror to generate a 1.9 ps pulse sequence. This all-fiber laser with a high-repetition-rate and short pulses has the advantages of high stability and self-starting, which has a wide range of applications.

Pump- and temperature-induced repetition frequency response study in hybrid mode-locked erbium fiber laser with distributed polarizer

We report a study on pump- and temperature-induced repetition frequency response in hybrid mode-locked stretched-pulse erbium fiber laser with distributed polarizer. For the first time to the best of our knowledge, we have shown frequency-dependent loss (FDL) contribution to pump-induced repetition frequency response behavior. FDL originates from wavelength-dependent saturation of nonlinear polarization evolution (NPE) mechanism which is inherent to stretched-pulse regime of operation. We have proposed an analytical model adequately describing NPE influence on pump-induced repetition frequency response. As it is observed, NPE-induced contribution substantially dominates over other physical perturbations leading to strictly different pump-induced repetition frequency response behavior compared to erbium fiber soliton laser.

Two-octave spanning supercontinuum generation in stoichiometric silicon nitride waveguides pumped at telecom wavelengths.

We demonstrate supercontinuum generation in stoichiometric silicon nitride (Si3N4 in SiO2) integrated optical waveguides, pumped at telecommunication wavelengths. The pump laser is a mode-locked erbium fiber laser at a wavelength of 1.56 µm with a pulse duration of 120 fs. With a waveguide-internal pulse energy of 1.4 nJ and a waveguide with 1.0 µm × 0.9 µm cross section, designed for anomalous dispersion across the 1500 nm telecommunication range, the output spectrum extends from the visible, at around 526 nm, up to the mid-infrared, at least to 2.6 µm, the instrumental limit of our detection. This output spans more than 2.2 octaves (454 THz at the -30 dB level). The measured output spectra agree well with theoretical modeling based on the generalized nonlinear Schrödinger equation. The infrared part of the supercontinuum spectra shifts progressively towards the mid-infrared, well beyond 2.6 µm, by increasing the width of the waveguides.

A 40 GHz, 770 fs regeneratively mode-locked erbium fiber laser operating at 1.6 µm

A 40 GHz, 770 fs regeneratively mode-locked erbium fiber laser operating at 1.6 µm

440 fs, 9.2 GHz regeneratively mode-locked erbium fiber laser with a combination of higher-order solitons and a SESAM saturable absorber.

We report a regeneratively FM mode-locked erbium fiber laser operating at 9.2 GHz, in which a SESAM saturable absorber and a higher-order soliton effect are combined to generate a 440 fs pulse train. Higher-order solitons have a narrow center peak while accompanied by pedestals on the wing, but such components can be well suppressed by the saturable absorber, which enables very efficient pulse narrowing. As a result, 440 and 480 fs transform-limited sech pulses were successfully obtained at output powers of 15 and 45 mW, respectively.

Large anomalous-dispersion mode-locked fiber laser based on a chirped fiber Bragg grating pair

A carbon-nanotube mode-locked erbium fiber laser with large net anomalous dispersion is presented. A chirped fiber Bragg grating (CFBG) pair is incorporated to increase the net-cavity anomalous dispersion and soliton splitting threshold. Self-started mode-locked laser produces stable pulses with repetition rate of 9.26 MHz. Laser spectrum is centered at ~1560 nm with 3 dB bandwidth of 0.43 nm. The typical output pulse energy and duration is 0.21 nJ and 8.05 ps, respectively.

Coherent pulse interrogation system for fiber Bragg grating sensing of strain and pressure in dynamic extremes of materials.

A 100 MHz fiber Bragg grating (FBG) interrogation system is described and applied to strain and pressure sensing. The approach relies on coherent pulse illumination of the FBG sensor with a broadband short pulse from a femtosecond modelocked erbium fiber laser. After interrogation of the FBG sensor, a long multi-kilometer run of single mode fiber is used for chromatic dispersion to temporally stretch the spectral components of the reflected pulse from the FBG sensor. Dynamic strain or pressure induced spectral shifts in the FBG sensor are detected as a pulsed time domain waveform shift after encoding by the chromatic dispersive line. Signals are recorded using a single 35 GHz photodetector and a 50 G Samples per second, 25 GHz bandwidth, digitizing oscilloscope. Application of this approach to high-speed strain sensing in magnetic materials in pulsed magnetic fields to ~150 T is demonstrated. The FBG wavelength shifts are used to study magnetic field driven magnetostriction effects in LaCoO3. A sub-microsecond temporal shift in the FBG sensor wavelength attached to the sample under first order phase change appears as a fractional length change (strain: ΔL/L<10-4) in the material. A second application used FBG sensing of pressure dynamics to nearly 2 GPa in the thermal ignition of the high explosive PBX-9501 is also demonstrated. Both applications demonstrate the use of this FBG interrogation system in dynamical extreme conditions that would otherwise not be possible using traditional FBG interrogation approaches that are deemed too slow to resolve such events.

Identifying the mechanisms of pulse formation and evolution in actively mode-locked Erbium fiber lasers with meters and kilometers-long

Abstract We investigated the dynamics of pulse evolution in Erbium-doped fiber ring lasers with cavity lengths varying from 16.4 m to 100.8 km actively mode-locked at repetition rate of 1 GHz. The novelty of this work is to explore the limits of Kuizenga–Siegman theory in ultralong fiber laser and to demonstrate the dynamics of pulse generation and propagation separately. When we vary the length of the Erbium-doped fiber lasers from meters to kilometers long, three operation regimes were identified: mode-locking regime (for cavity lengths with 16.4 m to 1 km), nonlinearity-dominant regime (1 to 10 km) and dispersion and nonlinearity regime that locked the ratio between soliton period and cavity length: Z S / L cav = 1.35 for cavities with 10 to 100 km in a soliton intracavity condition. The variation of pulse widths and the peak powers are analyzed to define the propagation regimes inside the cavities, depending on the cavity length ( L cav ) , dispersion length ( L D ) and nonlinear length ( L NL ) . When Lcav is shorter than LD and LNL, there is neither dispersive nor nonlinear effect during pulse evolution (pulse has duration of approximately 30 ps). In this regime, its final duration is determined by the standard theory of active mode-locking. For Lcav shorter than LD but ~ L NL , the pulse evolution is in nonlinearity-dominant regime where soliton propagation provides a sech2 profile with a TBP transform limited of 0.315. In addition, for cavities longer than 10 km, L cav ~ L D and longer (or much longer) than LNL, the pulse evolution is in the dispersion and nonlinearity dominant regime with its duration depending on the accumulated dispersion. In this regime the soliton effect takes place and the final pulse duration is defined by the cavity length which is approximately the soliton period.

The Nyquist laser

Nyquist pulses, which are defined as impulse responses of a Nyquist filter, can be used to simultaneously achieve an ultrahigh data rate and spectral efficiency (SE). Coherent Nyquist optical time-division multiplexing transmission increases SE, but the optical signal-to-noise ratio (OSNR) is limited by the amplitude of the original CW beam. To further improve transmission performance, here we describe a new pulsed laser that can emit an optical Nyquist pulse train at a repetition rate of 40 GHz. The Nyquist laser is based on a regeneratively and harmonically mode-locked erbium fiber laser that has a special spectral filter to generate a Nyquist pulse as the output pulse. The pulse width was approximately 3 ps, and the oscillation wavelength was 1.55 μm. The spectral profile of the Nyquist pulse can be changed by changing the spectral curvature of the filter with a roll-off factor, α, between 0 and 1. A Fabry–Perot etalon was also installed in the laser cavity to select longitudinal modes with a free spectral range of 40 GHz, resulting in the suppression of the mode hopping in the regenerative mode locking. A numerical analysis is also presented to explain the generation of a stable Nyquist pulse from the laser. The Nyquist laser is important not only for the direct generation of high-OSNR pulses but also for scientific advances, proving that pulse shapes that differ significantly from the conventional hyperbolic-secant and Gaussian pulse shapes can exist stably in a cavity.

Sub-100 fs and Passive Harmonic Mode-Locking of Dispersion-Managed Dissipative Fiber Laser With Carbon Nanotubes

We reported numerically and experimentally the dissipative soliton operation from a dispersion-managed passively mode-locked erbium fiber laser based on carbon nanotubes (CNTs). Outside compressing the dissipative soliton pulses with spectral width of 35 nm, pulses with 95 fs duration have been obtained. Harmonic mode-locking of the dissipative solitons has also been observed. It suggests that the formation of harmonic mode-locking in mode-locked fiber lasers is independent of the concrete features of the solitons. Additionally, we numerically and experimentally showed that the net dispersion value play a crucial role on the operation regimes of the dispersion-managed fiber laser and the characteristics of the output pulses.

Broadband infrared supercontinuum generation in a soft-glass photonic crystal fiber pumped with a sub-picosecond Er-doped fiber laser mode-locked by a graphene saturable absorber

A fiber-based supercontinuum source, comprising a graphene mode-locked erbium fiber laser and a highly nonlinear photonic crystal fiber (PCF), is reported. The nonlinear fiber has zero-dispersion wavelength shifted towards 1500 nm specifically for pumping with compact femtosecond and sub-picosecond fiber lasers operating in this spectral area. A chirped pulse amplification system seeded by a graphene mode-locked laser, generating linearly polarized 850 fs pulses and a pulse energy of 20 nJ at a repetition rate of 50 MHz, was used as the pump source. A 6 cm long, soft-glass PCF sample enabled generation of a supercontinuum spanning over an octave from 1000 to over 2300 nm in a 20 dB dynamic range. The measured results are interpreted numerically, based on a solution to the nonlinear Schrödinger equation using the split-step Fourier method; assignment of the nonlinear processes taking part in the observed broadening is proposed. The developed model is then used to estimate supercontinuum performance in the presented fiber with improved experimental conditions.

Experimental investigation of the cavity modulation frequency detuning effect in an active harmonically mode-locked fiber laser

We report the results of our recent experimental investigation of the cavity modulation frequency detuning effect on the output pulse dynamics in an active harmonically mode-locked erbium fiber laser. Our experimental study, which was carried out over a wide frequency detuning range, shows the existence of three different operating modes that depend on the degree of frequency detuning: (i)&#xA0;harmonic mode-locking, (ii)&#xA0;Q-switched harmonic/fundamental mode-locking, and (iii)&#xA0;a sinusoidal wave modulation mode. For this particular experimental laser configuration, the harmonic mode-locking mode is maintained over the frequency detuning range within &#xB1;15&#x2009;&#x2009;kHz, and then the Q-switched harmonic/fundamental mode-locking mode appears over the frequency detuning range from &#xB1;16 to &#xB1;36&#x2009;&#x2009;kHz. It is found that the Q-switched harmonic/fundamental mode-locking mode always produces both harmonic and fundamental repetition rate mode-locked pulses within a Q-switching envelope. A detailed experimental characterization of the output pulses in each operating mode is performed in terms of average power, pulse peak power, pulse width, and temporal width and repetition rate of the Q-switching envelope.