Yb-fiber Oscillator Research Articles

Robust all-polarization maintaining Yb fiber oscillator with a nonlinear optical loop mirror

We demonstrate an environmentally stable femtosecond Yb-doped all-polarizing maintained (PM) fiber laser with a nonlinear optical loop mirror (NOLM). While applying an unbalance coupling ratio coupler of 70/30 in a NOLM loop, the pulse spectral bandwidth was 20 nm with an output power of 8.95 mW at the repetition rate of 10.7 MHz. The recompressed pulse duration was 220 fs by using a pulse compressor based on pair of gratings. Due to the all-PM architecture, the root means square of output power fluctuation over 3 h was 0.4%. We expect that the developed all-PM Yb fiber oscillator will provide ultrafast broadband time-stretch spectroscopy in the future.

Impact of dispersion and intracavity polarization state on pump power fixed point in a Yb-fiber frequency comb

We experimentally explored the relationship between the pump power fixed point and the net-cavity dispersion in a Yb-fiber optical frequency comb. By continuously adjusting the distance of the grating pair in the Yb-fiber oscillator, we measured the pump power fixed point frequency in different dispersion regimes and different intracavity polarization states. We find that the fixed point frequency for pump power is not always near the carrier frequency but changes significantly with the net-cavity dispersion and polarization. Especially at the near zero-dispersion point, the fixed point has a local minimum, which is less than tens of THz and far lower than the carrier frequency. This is the first time to completely reveal the influence of net-cavity dispersion and intracavity polarization state on the fixed point in the experiment.

Multi-wavelength, nano-second actively mode-locked Yb-fiber oscillator with 100 nm wide Raman broadened spectrum

An all-normal dispersion, actively mode-locked Yb-fiber oscillator has been demonstrated which delivers multi-pulsing output with Raman broadened spectrum spanning more than 100 nm. Pulses of nano-second duration at 953.81 kHz repetition rate were obtained due to an elongated cavity consisting of 200 m long single mode fiber introducing high nonlinearity. The individual pulses of the multi-pulsing output corresponded to different wavelengths in the output spectrum and concurrently generated multiple first order Raman Stokes sidebands at close wavelength intervals. This resulted in a smoother broadband spectrum compared to usually obtained Raman broadened spectrum in normal dispersion regime which contains multiple higher order Stokes lines at large wavelength intervals when pumped by a single nano-second pulse. Analysis of the spectral power content revealed that nearly 50% of the output power contained in the Raman band around the 1120 nm region. By amplification of the generated pulses and further non-linear broadening through an ultra-high numerical aperture (UHNA) fiber, a broad spectrum having a wavelength span of nearly 260 nm with 1.5 Watt average power has been demonstrated showing application capability of the designed source.

Two-Dimensional White-Light Spectroscopy Using Supercontinuum from an All-Normal Dispersion Photonic Crystal Fiber Pumped by a 70 MHz Yb Fiber Oscillator.

We report on a new broadband, ultrafast two-dimensional white-light (2DWL) spectrometer that utilizes a supercontinuum pump and a supercontinuum probe generated with a ytterbium fiber oscillator and an all-normal dispersion photonic crystal fiber (ANDi PCF). We demonstrate compression of the supercontinuum to sub-20 fs and the ability to collect high quality 2D spectra on films of single-walled carbon nanotubes. Two spectrometer designs are investigated. Supercontinuum from ANDi PCF provides a means to generate broadband pulse sequences for multidimensional spectroscopy without the need for an optical parametric amplifier.

Multicolor Stimulated Raman Scattering Microscopy With Fast Wavelength-Tunable Yb Fiber Laser

Imaging of biological cells and tissues with subcellular spatial resolution is important in biology and medicine because it allows us to explore the dynamics of cells and to diagnose the structure of tissues. Among various optical imaging modalities, laser microscopy with fluorescent staining is a powerful method for this purpose. However, it still suffers from the limited applicability and cytotoxicity of a staining process. Stimulated Raman scattering (SRS) microscopy is an emerging technique of biological imaging based on molecular vibrational contrast, offering new opportunities of biomedical microscopy. In this paper, we introduce the principle and applications of SRS microscopy and discuss various methods of spectral imaging with SRS microscopy. Then, we present our multicolor SRS microscope, which employs a fast wavelength-tunable picosecond Yb fiber laser and a picosecond Ti:sapphire laser. In particular, we describe the technical details of the Yb fiber oscillator, synchronization electronics, high-speed wavelength-tunable filter, video-rate microscope setup and signal detection electronics including the photodetector and the lock-in amplifier. We hope that this paper will be informative for those interested in the application of ultrafast lasers to coherent Raman microscopy including SRS microscopy.

High-power self-similar amplification seeded by a 1 GHz harmonically mode-locked Yb-fiber laser

We demonstrate 1 GHz, 75 W, 65 fs pulse generation through chirped-pulse and self-similar amplification of a second-harmonic mode-locked Yb fiber oscillator. To confirm the experimental results of a chirped-pulse pre-amplifier, a theoretically calculative model is designed to simulate gain narrowing in the amplification. Specifically, the Kelly sidebands generated by a seed laser experience similar evolution under both conditions. The grism-based self-similar amplifier together with a high-efficiency grating compressor contribute to high-power ultrashort pulses whose spectra are efficiently broadened to a maximum 10 dB bandwidth of 56 nm with a center wavelength of 1032.2 nm owing to self-phase modulation in a gain fiber.

Simulations and experiments showing the origin of multiwavelength mode locking in femtosecond, Yb-fiber lasers

A stable and self-starting femtosecond breathing-pulse Yb-fiber oscillator is reported, modelocked using the nonlinear polarisation evolution mechanism. A bifurcation between two distinct modes of operation is demonstrated experimentally, producing pulses with a single central wavelength in one state, or following adjustment of the intra-cavity waveplates, the emission of pulses with three distinct central wavelengths. The maximum bandwidth was 72 nm at the -10 dB level and the pulses were compressible externally to 70 fs with energies of 0.75 nJ. The multi-wavelength pulses reported here are significantly shorter than the pico-second pulses previously observed from similar modelocked multi-wavelength sources. Vector simulations based on the nonlinear Schrodinger equation show that the multi-wavelength behaviour is produced by overdriving the nonlinear polarisation evolution based saturable absorber at the peak of the pulse, leading to transmission of the two wings of the strongly chirped pulse. This new insight shows clearly that the three pulses output in the multi-wavelength state are coherent. The agreement between simulation and experimental data shows nonlinear polarisation evolution based modelocked fiber lasers are a suitable platform for studying the nonlinear dynamics underlying the bifurcation of the output.

Sub-100 fs pulses from an all-polarization maintaining Yb-fiber oscillator with an anomalous dispersion higher-order-mode fiber.

We present an Yb-fiber oscillator with an all-polarization-maintaining cavity with a higher-order-mode fiber for dispersion compensation. The polarization maintaining higher order mode fiber introduces not only negative second order dispersion but also negative third order dispersion in the cavity, in contrast to dispersion compensation schemes used in previous demonstrations of all-polarization maintaining Yb-fiber oscillators. The performance of the saturable absorber mirror modelocked oscillator, that employs a free space scheme for coupling onto the saturable absorber mirror and output coupling, was investigated for different settings of the intracavity dispersion. When the cavity is operated with close to zero net dispersion, highly stable 0.5-nJ pulses externally compressed to sub-100-fs are generated. These are to our knowledge the shortest pulses generated from an all-polarization-maintaining Yb-fiber oscillator. The spectral phase of the output pulses is well behaved and can be compensated such that wing-free Fourier transform limited pulses can be obtained. Further reduction of the net intracavity third order dispersion will allow generating broader output spectra and consequently shorter pulses, without sacrificing pulse fidelity.

Efficient picosecond traveling-wave Raman conversion in a SrWO4 crystal pumped by multi-Watt MOPA lasers at 1064 nm

Raman conversion with a 50-mm-long SrWO4 crystal in a single-pass, traveling-wave setup has been investigated in both purely steady-state and transient stimulated Raman scattering (SRS) regimes. For steady-state SRS experiment, we employed as a pump source a Q-Switched master oscillator power amplifier (MOPA) laser system at 1064 nm, delivering 325 μJ, 550-ps-long pulses with diffraction limited beam quality and high spectral purity. At 2-kHz repetition rate, we obtained up to 90 μJ pulse energy and 250 ps pulse duration at 1180 nm, with a conversion slope efficiency close to quantum limit. To approach the transient SRS regime, we pumped the same crystal with 16-ps-long pulses from a hybrid MOPA laser system based on a mode-locked Yb-fiber oscillator followed by a diode-pumped bulk Nd:YVO4 power amplifier. At the maximum incident pump average power of 3.75 W, we obtained 1.4 W at the first Stokes Raman-shifted wavelength of 1180 nm (37 % optical-to-optical conversion efficiency), with 15 ps pulse duration and 70 % conversion slope efficiency.

Tunable, high-power, continuous-wave dual-polarization Yb-fiber oscillator.

We demonstrate a high-power, dual-polarization Yb-fiber oscillator, by separately locking the two linear polarization states defined by slow and fast axis of a polarization-maintaining gain fiber with volume Bragg gratings. Dual-line lasing is achieved with a tunable wavelength separation from 0.03 to 2 THz, while exceeding output powers of 78 W over the entire tuning range, maintaining a high beam-quality with M(2)<1.2. With this laser configuration we achieve a peak-to-peak power variation of <1% for the dual-line signal and <3% for the individual signals.

Laser-induced Breakdown Spectroscopy and ablation threshold analysis using a megahertz Yb fiber laser oscillator

A LIBS system is demonstrated using a 100m cavity Yb fiber oscillator producing ~70ps, 320nJ clusters of 50–100fs sub-pulses at 2MHz. A new empirical model for femtosecond ablation is presented to explain the LIBS signal intensity's non-linear dependence on pulse fluence by accounting for the Gaussian beam's spatial distribution. This model is compared to experimental data and found to be superior to linear threshold fits. This model is then used to measure the ablation threshold of Cu using a typical amplified Ti:sapphire system, and found to reproduce previously reported values to within ~20%. The ablation threshold of Cu using the Yb fiber oscillator system was measured to be five times lower than on the amplified Ti:sapphire system. This effect is attributed to the formation of nanostructures on the surface, which have previously been shown to decrease the ablation threshold. The plasma lifetime is found to be ~1ns, much shorter than that of nanosecond ablation, further indicating that the decreased threshold results from surface effects rather than laser–plasma interaction. The low threshold and high pulse energy of the Yb fiber oscillator allows the acquisition of LIBS spectra at megahertz repetition rates. This system could potentially be developed into a compact, fiber-based portable LIBS device taking advantage of the benefits of ultrafast pulses and high repetition rates.

Femtosecond fiber laser for high-stability and high-quality pulse compression

We demonstrate a femtosecond fiber master oscillator power amplifier system of high-stability and high-quality pulse compression, which is applied to corneal refractive surgery. The nonlinear-polarization-evolution mode-locking in a hybrid-cavity Yb-fiber oscillator consisting of both polarization-maintaining (PM) and non-PM fibers is demonstrated. In this paper, the PM-fibers introduced into the mode-locked fiber oscillator partially replace non-PM fibers. This alternative approach is conducive to reducing the adverse effect of uncontrolled fiber birefringence, which originates from the on-PM fiber suffering environmental temperature fluctuation and mechanical vibration. Once the length of non-PM fiber is comparable to the fiber beat length, the uncontrolled fiber birefringence caused by environment starts to abate the laser robustness, repeatability, and reliability of mode-locking. The stability becomes notoriously worse for long-cavity Yb-fiber oscillators. The PM-fibers adopted in the mode-locked fiber oscillator could improve the mode-locked stability of nonlinear-polarization-evolution self-started with long-cavity Yb-fiber oscillators. We study the dependence of the compressed pulse quality on the parameters of input pulse pre-chirp injected into the Yb-doped fiber amplifier. Due to the nonlinear-chirp and third order dispersion, the mode-locked pulse shape in time-domain will produce distortion during power being amplified in the Yb-doped fiber amplifier. A diffraction grating to adjust the pre-chirp of the input pulse from positive value to negative value launched into the Yb-doped fiber amplifier is placed between mode-locked Yb-fiber oscillator and Yb-doped fiber amplifier. We vary the pre-chirp by changing the distance between the diffraction grating and triangular prism and then adjust the second grating pair to compress the amplified pulses into its shortest pulse duration of full-width at half maximum measured by an autocorrelator. The experimental results show that the best compression quality of mode-locked pulse occurs at the negative pre-chirp with a measured pulse width of 183 fs. Deviation from this optimum pre-chirp degrades the compressed-pulse quality and features an increased temporal pedestal. The fiber laser produces self-started mode-locking at a repetition rate of 19.4 MHz, an average power of 1.2 W, a pulse width of 183 fs. The homemade femtosecond fiber laser is used to perform surgery on ex vivo pig corneas. The surgey shows that the light burst of femtosecond laser in corneal tissues could cut pig corneas, which proves that the femtosecond fiber laser satisfies the surgical operation requirements of animal corneas.

Energy scaling of Yb fiber oscillator producing clusters of femtosecond pulses

Bai NieMichigan State UniversityDepartment of Physics and AstronomyEast Lansing, Michigan 48824Greg ParkerVadim Vadimovich LozovoyMichigan State UniversityDepartment of ChemistryEast Lansing, Michigan 48824Marcos DantusMichigan State UniversityDepartment of Physics and AstronomyEast Lansing, Michigan 48824andMichigan State UniversityDepartment of ChemistryEast Lansing, Michigan 48824E-mail: dantus@msu.eduAbstract. A Yb fiber oscillator producing high-energy femtosecond pulseclusters is reported. Visualized by averaging autocorrelation, the outputpulses consist of femtosecond pulse clusters that appear as a picosecondenvelope with a ∼100-fs pulse in its center. Using more than 200-m fiber,the pulse energy is scaled up to 450 nJ. This high energy in a cluster offemtosecond pulses enables an important application—laser-inducedbreakdown spectroscopy.

Frequency comb based on a narrowband Yb-fiber oscillator: pre-chirp management for self-referenced carrier envelope offset frequency stabilization

Laser frequency combs are normally based on mode-locked oscillators emitting ultrashort pulses of ~100-fs or shorter. In this paper, we present a self-referenced frequency comb based on a narrowband (5-nm bandwidth corresponding to 415-fs transform-limited pulses) Yb-fiber oscillator with a repetition rate of 280 MHz. We employ a nonlinear Yb-fiber amplifier to both amplify the narrowband pulses and broaden their optical spectrum. To optimize the carrier envelope offset frequency (fCEO), we optimize the nonlinear pulse amplification by pre-chirping the pulses at the amplifier input. An optimum negative pre-chirp exists, which produces a signal-to-noise ratio of 35 dB (100 kHz resolution bandwidth) for the detected fCEO. We phase stabilize the fCEO using a feed-forward method, resulting in 0.64-rad (integrated from 1 Hz to 10 MHz) phase noise for the in-loop error signal. This work demonstrates the feasibility of implementing frequency combs from a narrowband oscillator, which is of particular importance for realizing large line-spacing frequency combs based on multi-GHz oscillators usually emitting long (>200 fs) pulses.

Self-compensation of third-order dispersion for ultrashort pulse generation demonstrated in an Yb fiber oscillator

Compensation of the intracavity dispersion in the mode-locked oscillator is known to be one of the most important factors for ultrashort pulse generation. However, recent investigations of a Yb-doped fiber mode-locked oscillator revealed that precise third-order dispersion (TOD) compensation is not always necessary for ultrashort pulse generation, owing to the strong nonlinearity that compensates residual TOD without reducing its spectral bandwidth. The origin of the nonlinear TOD compensation has remained unclear. To investigate the process in detail, we studied the pulse evolution inside a 30 fs Yb-doped fiber mode-locked oscillator both experimentally and numerically, and we found that the nonlinear phase shift with a temporally asymmetric pulse shape introduces an appropriate amount of TOD that exactly cancels the residual cavity dispersion.

All-fiber all-normal dispersion laser with a fiber-based Lyot filter

We propose the use of a short section of polarization-maintaining fiber as a birefringent medium to construct an all-fiber Lyot filter inside the cavity of a fiber laser. This allows mode-locked operation of an all-fiber all-normal dispersion Yb-fiber oscillator without the use of a bulk bandpass filter and using standard components. Moreover, filter bandwidth and modulation depth is easily controlled by changing the length and splice angle of the polarization-maintaining-fiber section, leading to an adjustable filter. At mode-locked operation, the 30% output fiber port delivers 1 nJ pulses that are dechirped to 230 fs duration.

Self referenced Yb-fiber-laser frequency comb using a dispersion micromanaged tapered holey fiber

We demonstrate a fully stabilized frequency comb in the 1mum spectral region based on an Yb-fiber oscillator and a cladding pumped chirped pulse Yb-fiber amplifier whose output is spectrally broadened in a dispersion micromanaged holey fiber. The dispersion micromanaged fiber is used to generate efficient, low noise spectral components at 523nm which are heterodyned with the second harmonic of the amplifier output for standard f-to-2f self-referenced carrier envelope offset frequency detection. For comb stabilization we phase-lock this offset frequency and the oscillator repetition frequency simultaneously to an RF reference by feedback controlling the oscillator pump diode current and the driving voltage of an intracavity piezo-electric fiber stretcher respectively.

Generation of Optional Pulse Shape by Yb Doped Fiber Laser System

We have demonstrated the generation of optional pulse shape by Yb doped fiber laser system. Yb fiber laser system operated the polarization-maintained pulsed at 1064 nm for single-transverse and -longitudinal mode using fiber Bragg grating (FBG), and this system generates optional pulse shapes that sliced up the Yb fiber oscillator by a LN (LiNbO3) intense modulator with 12.5-GHz bandwidth. The developed electronic-pulseshape generator supplies an arbitrary-voltage pulse to an integrated optic LN amplifier modulator which modulates the laser pulse. The output laser waveforms can obtain corresponding to the electronic waveforms such as rectangular, Gaussian and multi pulses with a 300-ps resolution.