Bidirectional Pumping Scheme Research Articles

3.5 kW bidirectionally pumped narrow-linewidth fiber amplifier seeded by white-noise-source phase-modulated laser

We demonstrate an all-fiber narrow-linewidth amplifier employing a bidirectional pump scheme and cascaded white-noise-source phase-modulated seed laser. The stimulated Raman scattering effect in the amplifier is investigated by substituting different types of seed lasers. The influence of pump distributions and seed injection power on mode instability (MI) in the amplifier is also experimentally investigated. As a result, a 3 dB linewidth of 0.175 nm and a beam quality of M2 ≈ 1.5 are obtained at the output of ~3 kW, without observation of MI and stimulated Brillouin scattering effect. With the further increase of pump power, MI occurs as the output exceeds 3.17 kW, along with beam quality degradation. Optical efficiency decreases to 71.5% at the ultimate output of 3.5 kW. Therefore MI becomes the main limitation to further power scaling.

Transverse mode instability threshold enhancement in Yb-doped fiber lasers by cavity modification.

To enhance the transverse mode instability (TMI) threshold of a fiber oscillator, a novel configuration is presented. In this configuration the oscillator cavity length is considerably reduced and the remaining active fiber is released out of the cavity to absorb the rest of the pump power and amplify the output signal of the cavity. In fiber oscillators, the index gratings generated by both forward and backward propagating signals can interact with light propagating in the backward and forward directions and degrade the output beam quality. In the proposed modified configuration, due to lower intra-cavity forward and backward signal power the index grating is smoothed and the TMI threshold is increased. Experimental results indicate that this modified configuration has a higher TMI threshold than a conventional fiber oscillator. Finally a higher TMI threshold is achieved in the bidirectional pumping scheme of the modified configuration.

Study on the adiabaticity criterion of the thermally-guided very-large-mode-area fiber.

The adiabaticity criterion of the thermally-guided very-large-mode-area (TG VLMA) fiber is presented based on the mode-coupling theory firstly, to the best of our knowledge. The requirement for the adiabatic propagation of fundamental mode is discussed systematically. It is revealed that the pump absorption plays the most important role and the adiabaticity criterion can be met as long as it is small enough. Then, the effects of the configuration parameters of TG VLMA fiber on the up-limitation of pump absorption for the adiabaticity criterion are investigated. It is found that for the straight TG VLMA fiber, reducing the initial refraction index and inner-cladding diameter and utilizing the bi-directional pumping scheme are beneficial to the adiabatic propagation of fundamental mode. The bent TG VLMA fiber is also studied. It is found that the bent fiber is much more difficult to meet the adiabaticity criterion than the straight one. The results show that even with the 100-cm bend radius, the pump absorption should be smaller than 1 dB/m to meet the adiabaticity criterion. It is suggested that enlarging the core-to-cladding ratio can be helpful for loosening the adiabaticity criterion of bent TG VLMA fiber. These pertinent results can provide significant guidance for understanding and designing the TG VLMA fiber and pertinent lasers and amplifiers.

Numerical study on a single-mode continuous-wave thermally guiding very-large-mode-area fiber amplifier

The characteristics of a single-mode continuous-wave thermally guiding very-large-mode-area fiber amplifier are investigated numerically using the rate-equation model while taking thermal transfer into account. It is revealed that the seed power should play an important role in the fiber amplifier and should be large enough to ensure high output efficiency. The effects of three pumping schemes (i.e. the co-, counter- and bi-directional pumping schemes) and the initial refraction index difference are also studied. It is revealed that the optimum fiber length changes with the pumping scheme, and the initial refraction index difference should be lower than 10−4 in order to ensure the linear increment of the output signal power with the pump power. Furthermore, a brief comparison between the thermally induced waveguides in the fiber amplifiers for three pumping schemes is also made.

3.05 kW monolithic fiber laser oscillator with simultaneous optimizations of stimulated Raman scattering and transverse mode instability

We report a high power monolithic ytterbium-doped fiber laser oscillator with an output power of 3.05 kW, which is achieved by simultaneous optimizations of the stimulated Raman scattering (SRS) and transverse mode instability (TMI). The optimizations of the SRS are designed and utilized in the construction of the fiber laser oscillator, while the TMI threshold is optimized with the study of the dependence of TMI threshold on the pump distribution. In the fiber laser oscillator, the TMI threshold is enhanced by ∼30% when the counter-pump scheme is employed instead of the co-pump scheme. By applying bidirectional-pump scheme and appropriately distributing the pump power, the TMI threshold is further enhanced and the monolithic fiber laser oscillator achieves an output power of 3.05 kW with near diffraction limited beam quality.

2 kW single-mode fiber laser employing bidirectional-pump scheme

A 2 kW single-mode fiber laser with two cascade home-made cladding light strippers (CLSs) has been demonstrated by employing bidirectional-pump scheme. 2.009 kW signal power is obtained when pump power is 2.63 kW and the slope efficiency is 76.6%. Raman Stokes light is less than -47 dB at 2.009 kW even with a 10-m delivery fiber with core/inner cladding diameter of 20 μm/400 μm. The beam quality M2≤1.2 and the spectral FWHM bandwidth is 4.34 nm. There is no transverse mode instability and the output power stability of ±0.14% is achieved by special thermal management for a more uniform temperature distribution on the Yb-doped gain fiber.

Enhancement on the generation of amplified spontaneous emission in thulium-doped silica fiber at 2 [formula omitted

This paper investigates the generation of the amplified spontaneous emission (ASE) from thulium-doped silica fiber pumped at 1570 nm and 793 nm. The developed model provides the ASE spectral power as functions of the fiber length and the pump power under single-pass forward (SPF) and double-pass bi-directional (DPB) pumping schemes. A broadband ASE source generated by solving a set of rate and propagation equations for 1570 nm and 793 nm pumping transitions, and taking into account the influences of cross relaxation (CR) in both pump schemes. Our findings reveal that for 1570 nm pumping scheme, CR transition reduces the ASE generation. In addition, longer fluorescent lifetime increases the ASE power and reduces the pump power threshold. We numerically enhanced the generation of ASE broadband source from 1570 nm/1570 nm DPB pumping scheme. Our results show that 1570 nm/1570 nm DPB produces broadband ASE source with high slope efficiency and broader spectral bandwidth than that in SPF configuration. As a result, a 1570 nm/1570 nm DPB pump scheme is a suitable configuration to obtain higher power efficiency and a wider broadband source for the chosen thulium-doped silica fiber characteristics.

Tunable differential modal gain in FM-EDFA system using dual pumping scheme at 100 Gbps system capacity

In this manuscript, we demonstrate a few-mode system by tailoring of ring-doped EDFA index profile to mitigate the effect of differential modal gain under the presence of ten spatial linearly polarized modes. The FM-EDFA system also gives better output power spectra, gain spectra and optical signal-to- noise ratio of an individual mode for wide range of wavelength window (C-band) and BER below $$10^{-10}$$10-10 at 100 Gbps. Moreover, the better performance in FM-EDFA system is achieved by selecting mode bidirectional pumping scheme.

Mitigating transverse mode instability in all-fiber laser oscillator and scaling power up to 2.5 kW employing bidirectional-pump scheme.

Transverse mode instability (TMI) is one of the main limiting factors in kW-level fiber lasers. Unlike fiber amplifiers, TMI in fiber laser oscillators attracts less attention from researchers. In this work, we construct an all-fiber ytterbium-doped laser oscillator and investigate the performance in co-pumping and bidirectional-pumping configurations, respectively. In the co-pumping scheme, TMI occurs at ~1.6kW and restricts further output power scaling. Different from the characteristic of dynamic TMI in fiber amplifiers, quasi-static TMI is observed in the laser oscillator. Details of the temporal characteristic around the TMI threshold are provided. In the bidirectional-pumping scheme, experimental results validate that the TMI is mitigated notably by employing bidirectional-pumping instead of co-pumping. The output laser power is further scaled to 2.5kW with a slope efficiency of 74.5% and good beam quality (M2~1.3). At the maximum power, the FWHM bandwidth of optical spectra is 5.2nm, and the Raman stokes light is ~20dB below the signal.

High-power widely tunable all-fiber thulium-assisted optical parametric oscillator at SWIR band.

A novel short-wave infrared (SWIR) all-fiber thulium-assisted optical parametric oscillator (TAOPO) that exploits jointly optical parametric conversion and thulium amplification in a highly nonlinear fiber (HNLF) and thulium-doped fiber (TDF) is demonstrated. This is implemented through constructing a joint fiber line by directly fusion splicing 50 m HNLF with 1.5 m TDF. Incorporating a bidirectional-pumping scheme, i.e., forward-pumped by a step-tuned C-band pulsed laser, and simultaneously backward-pumped by an L-band continuous-wave laser, this TAOPO produces a pulsed SWIR laser at output power higher than 200 mW, signal-to-noise ratio over 40 dB, and wavelength tuning range beyond 150 nm from 1815 to 1968 nm. Via separate characterization of the HNLF and TDF joint fiber line, the tunability of the current TAOPO to shorter wavelength is only limited by the employed fiber components, while higher power could be realized by increasing the backward pump power. This TAOPO could be a promising platform for the generation of a highly functional SWIR source that facilitates applications such as bond-selective imaging of deep tissue.

1.55 μm hydrogen cyanide optical frequency-stabilized and 10 GHz repetition-rate-stabilized mode-locked fiber laser.

We describe a 1.55 μm hydrogen cyanide (HCN) optical frequency and repetition rate stabilized mode-locked fiber laser, where the optical frequency was locked to the P(10) HCN absorption line and the repetition rate was locked to 9.95328 GHz by using a microwave phase-locked loop. The optical frequency stability of the laser reached 5 x 10-11 with an integration time τ of 1 s. With a bidirectional pumping scheme, the laser output power reached 64.6 mW. To obtain a short pulse train, the average dispersion in the cavity was managed so that it was zero around 1.55 μm, resulting in a 0.95 ps pulse train. In addition, the stabilization of the optical frequency and the repetition rate, meant that the entire spectral profile remained the same for 24 hours.

Design and Performance Analysis of Raman Amplified XG-PON System

This paper describes the design considerations and performance analysis for an XG-PON Raman amplification scheme. The experimental results and theoretical calculations presented in this paper demonstrate the feasibility of Raman amplification in O-band access systems; thus, enhancing the performance of existing access installations while still preserving the passive nature of these networks. With the availability of cost-effective quantum dot laser diodes as high-power pumps, Raman amplification is proving to be an attractive candidate for access applications. We experimentally demonstrate that it is possible to achieve a total pump power of 759 mW using polarization multiplexed quantum dot laser diodes (QD-LDs), and using a backward pumping scheme we can support a purely passive XG-PON system with 50 km reach and 64 splits. With system level numerical simulations, we illustrate the use of forward or bi-directional pumping scheme to increase the splits further. We also confirm that the Raman pump output in the experiments, despite being below the single mode cut off for the standard SMF, was efficiently coupled into the trunk fibre using off-the-shelf components. We also show using numerical simulations that a flat gain spectrum can be obtained with less than 0.7 dB gain variation across the XG-PON upstream wavelength band (1260–1280 nm) using a multi-wavelength pumping scheme with just two pump wavelengths. From a detailed system analysis, we discuss the impact of broadband filtering (20 nm) at the receiver adopted by the XG-PON standards, and show that lower class (N1) receivers can be adopted for Raman gains higher than 16 dB.

160 W 800 fs Yb:YAG single crystal fiber amplifier without CPA

We demonstrate a compact and simple two-stage Yb:YAG single crystal fiber amplifier which delivers 160 W average power, 800 fs pulses without chirped pulse amplification. This is the highest average power of femtosecond laser based on SCF. Additionally, we demonstrate the highest small signal gain of 32.5 dB from the SCF in the first stage and the highest extraction efficiency of 42% in the second stage. The excellent performance of the second stage was obtained using the bidirectional pumping scheme, which is applied to SCF for the first time.

Evaluation of 100G DP-QPSK long-haul transmission performance using second order co-pumped Raman laser based amplification.

We present, for the first time, a detailed investigation of the impact of second order co-propagating Raman pumping on long-haul 100G WDM DP-QPSK coherent transmission of up to 7082 km using Raman fibre laser based configurations. Signal power and noise distributions along the fibre for each pumping scheme were characterised both numerically and experimentally. Based on these pumping schemes, the Q factor penalties versus co-pump power ratios were experimentally measured and quantified. A significant Q factor penalty of up to 4.15 dB was observed after 1666 km using symmetric bidirectional pumping, compared with counter-pumping only. Our results show that whilst using co-pumping minimises the intra-cavity signal power variation and amplification noise, the Q factor penalty with co-pumping was too great for any advantage to be seen. The relative intensity noise (RIN) characteristics of the induced fibre laser and the output signal, and the intra-cavity RF spectra of the fibre laser are also presented. We attribute the Q factor degradation to RIN induced penalty due to RIN being transferred from the first order fibre laser and second order co-pump to the signal. More importantly, there were two different fibre lasing regimes contributing to the amplification. It was random distributed feedback lasing when using counter-pumping only and conventional Fabry-Perot cavity lasing when using all bidirectional pumping schemes. This also results in significantly different performances due to different laser cavity lengths for these two classes of laser.

Influences of pumping schemes on the properties of dissipative soliton in fiber lasers

Based on the extended scalar nonlinear Schrödinger equation, the influences of pumping schemes on the properties of dissipative soliton are investigated theoretically in passively mode-locked ytterbium-doped fiber laser with positive dispersion cavity. Results show that, under the conditions of the same initial total gain, the dissipative solitons obtained in the backward pumping fiber laser have the narrowest spectral width, shortest pulse duration and highest pulse peak power for the forward, backward and bidirectional pumping schemes, moreover, the linear fitting degree of pulse chirp is lowest. With the same gain saturation energy, the pulse energies of the dissipative solitons are equal for three pumping schemes, but the differences of output spectra and temporal waveforms increase as the gain saturation energy increases.

Comparative Study of the Birefringence in Photonic Crystal Fiber Lasers

In this paper, we study the birefringence in photonic crystal fiber lasers PCFs and in conventional fiber lasers in the bi-directional pump scheme in the linear cavity laser. We show that the value of birefringence in photonic crystal fibers is smaller than that of conventional fiber lasers [1].

Dual-band wavelength tunable nonlinear polarization rotation mode-locked Erbium-doped fiber lasers induced by birefringence variation and gain curvature alteration.

With the combining effects of the fiber birefringence induced round-trip phase variation and the gain profile reshaping induced spectral filtering in the Erbium-doped fiber laser (EDFL) cavity, the mechanism corresponding to the central wavelength tunability of the EDFL passively mode-locked by nonlinear polarization rotation is explored. Bending the intracavity fiber induces the refractive index difference between orthogonal axes, which enables the dual-band central wavelength shift of 2.9 nm at 1570 nm region and up to 10.2 nm at 1600 nm region. The difference between the wavelength shifts at two bands is attributed to the gain dispersion decided by the gain spectral curvature of the EDFA, and the spacing between two switchable bands is provided by the birefringence induced variation on phase delay which causes transmittance variation. In addition, the central wavelength shift can also be controlled by varying the pumping geometry. At 1570 nm regime, an offset of up to 5.9 nm between the central wavelengths obtained under solely forward or backward pumping condition is observed, whereas the bidirectional pumping scheme effectively compensates the gain spectral reshaping effects to minimize the central wavelength shift. In contrast, the wavelength offset shrinks to only 1.1 nm when mode-locking at 1600 nm under single-sided pumping, as the gain profile strongly depends on the spatial distribution of the excited erbium ions under different pumping schemes. Except the birefringence variation and the gain spectral filtering phenomena, the gain-saturation mechanism induced refractive index change and its influence to the dual-band central wavelength tunability are also observed and analyzed.

Improved shooting algorithm using answer ranges definition to design doped optical fiber laser

Shooting method is a simple and powerful method to solve boundary value problems. By using shooting method and appropriate definition of comparative function with a proper guess for the initial value of pump and signal the computation process has short time and quick converge to correct answers. In this paper by definition of the answer ranges and proper comparative function, the requirement of proper initial guesses for forward signal and backward pump to solve rate equation is resolved. By putting initial and end of answer ranges as two set guesses, the signal and pump initial values were achieved fast and easily using the second method. In other words using answer ranges, the shooting algorithm was equipped with a process of response screening. This computational procedure, without requirement of experimental lookout can be used for fiber lasers and amplifiers. We also show that the former optimum length definition in strongly pumped power lasers cannot be used in lasers with conventional pump power and must be replaced with suitable one. Forward, backward and bi-directional pump scheme have been investigated using the proposed calculation procedure. An effect of background loss on optimum fiber laser length is also verified and the effects of input and output mirrors reflectivity, dopant concentration and fiber length in laser output power is investigated.

Theoretical and Numerical Study of Stress and Thermo-Optic in Photonic Crystal Fiber Laser in Different Pump Schemes

In this paper, we present a study of thermal, average power scaling, change in index of refraction and stress in photonic crystal fiber lasers with different pump schemes: forward pump scheme, backward pump scheme, forward pump scheme with reflection of 98%, backward pump scheme with reflection of 98% and bi-directional pump scheme. We show that management of thermal effects in fiber lasers will determine the efficiency and success of scaling-up efforts. In addition, we show that the most suitable scheme is the bi-directional.

Bidirectional multiplexing of heralded single photons from a silicon chip

We demonstrate integrated spatial multiplexing of heralded single photons generated from a single 96 μm long silicon photonic crystal waveguide in a bidirectional pump configuration. By using a low-loss fiber-coupled opto-ceramic switch, the multiplexing technique enhances the brightness of the single photon source by 51.2±4.0% while maintaining the coincidence-to-accidental ratio. Compared with the demonstration of multiplexing two individual sources, the bidirectional pump scheme represents a twofold reduction in the footprint of nonlinear devices for future large-scale integration of on-chip single photon sources. The 51.2±4.0% gain will make any quantum operation requiring n photons 1.5(n) times faster.