Pulsed Fiber Oscillator

High Energy Pulse Generation

Approaching microjoule-level pulse energy with mode-locked femtosecond fiber lasers

B. Ortaç, M. Baumgartl, J. Limpert and A. TünnermannOptics Letters, vol. 34, pp. 1585-1587 (2009)

The development of powerful ultrashort laser pulse sources as versatile tools for ultrafast scientific and industrial applications has attracted considerable attention. Among all the solid-state lasers, ultrashort pulse generation in rare-earth-doped fibers is widely regarded as the most promising approach due to the waveguide geometry that confers them unique properties such as outstanding thermo-optical characteristics, or high single-pass and broadband gain. In spite of this, achieving high pulse energies in soliton mode-locked fiber lasers using conventional single-mode fiber is not as straightforward as in their bulky counterparts. The development of normal- dispersion mode-locked fiber lasers has been demonstrated to obviate the physical limits to pulse energy. However, the use of the new generation of low- nonlinearity Yb-doped large-mode-area fibers opens the door to power and energy scaling. We have demonstrated the generation of femtosecond pulses with energies close to the microjoule barrier directly from a passively mode-locked fiber oscillator. The optimization of the high single-pass gain in a low-nonlinearity microstructured fiber resulted in the extraction of the highest average output power and the highest pulse energy reported so far for high-power ultrashort fiber lasers. The self-starting fiber laser directly generates positively chirped picosecond pulses at a pulse repetition rate of 9.7 MHz with diffraction-limited beam quality. The presented fiber oscillator delivers 9 W of average output power, which corresponds to a pulse energy of 927 nJ. These pulses can be compressed down to 711 fs with up to megawatt peak power levels. To our knowledge, this is the first time that a mode- locked fiber oscillator has generated femtosecond pulses with pulse energies approaching the microjoule level in combination with high average output power.

High-Power Pulse Generation

High average and peak power femtosecond large pitch photonic crystal fiber laser
M. Baumgartl, F. Jansen, F. Stutzki, C. Jauregui, B. Ortaç, J. Limpert and A. Tünnermann
Optics Letters, vol. 36, pp. 244-246 (2011)

We have developed high power femtosecond large-mode-area ytterbium-doped large-pitch photonic crystal fiber. The combination of an instantaneous, non-absorptive mode-locking mechanism [nonlinear polarization rotation technique] with a novel PCF design in a spectral-filtering effect assisted chirped-pulse oscillator configuration enables a significant peak power enhancement with the generation of multi-megawatt pulses from a fiber laser. The laser reaches 27 W of average power at a 50.57 MHz repetition rate, corresponding to 534 nJ pulses. The output pulses are extra-cavity dechirped down to 100 fs with 3.2 MW peak power. To the best of our knowledge, these are the highest average and peak powers ever reached by a mode-locked fiber laser.

Ultra-Short Pulse Generation

In the experiment reported in this case we used a well-controlled stretched-pulse configuration to achieve a passively mode-locked Yb-doped double-clad fiber laser that produced 4.8-ps pulses with an average output power of 100 mW. These pulses were extra-cavity compressed to 90 fs. To our knowledge this is the first time that pulses so short and so energetic have been generated directly from an Yb-doped double-clad fiber laser operating at 1050 nm.

90 fs stretched-pulse ytterbium-doped double-clad fiber laser
B. Ortaç, A. Hideur, T. Chartier, M. Brunel, C. Özkul and F. Sanchez
Optics Letters, vol. 28, pp. 1305-1307 (2003)

In this case we report the generation of sub-80 fs pulses from a passively mode-locked all-normal-dispersion laser featuring a large-mode-area photonic crystal fiber. By exploiting the combined action of an saturable absorber mirror (SAM) and nonlinear polarization evolution (NPE) for pulse shaping, the laser directly generates 1 ps chirped pulses with pulse energies above 160 nJ at watt-level average power. These pulses could be compressed down to 77 fs outside the cavity, reaching ∼1.2 MW at the peak, which, to our knowledge, represents the highest peak power generated from a fiber oscillator. This performance competes with state-of-the-art femtosecond solid-state lasers. Numerical simulations show that pulse shaping in this laser is governed by the strong self-amplitude modulation induced by the SAM and NPE actions in combination with gain filtering.

Sub-80 fs dissipative soliton large-mode-area fiber laser
M. Baumgartl, B. Ortaç, C. Lecaplain, A. Hideur, J. Limpert and A. Tünnermann
Optics Letters, vol. 35, pp. 2311-2313 (2010)

Ultra-Long Pulse Generation

Energy scaling of femtosecond and picosecond fiber oscillators beyond the microjoule level
B. Ortaç, M. Baumgartl, O. Schmidt, A. Hideur, I. Sagnes, A. Granache, J. Limpert and A. Tünnermann
Photonics West, San Jose, California, USA, 24-29 January 2009. Proc. SPIE – The-International Society for Optical-Engineering, vol. 7203, 72030W (Feb. 24, 2009)

In the long pulse (picosecond regime), passive mode-locking is achieved employing a SAM based on a molecular beam epitaxial growth of InGaAs multi-quantum wells structure on a multi-layer GaAs/AlAs Bragg mirror. The low-intensity absorption of the SAM at around 1060 nm is ~85 %, the modulation depth ~10 % and the saturation fluence as high as 100 μJ/cm². The SAM presents a slow relaxation time of more than 100 ps. An optimization of the saturation criteria of the SAM resulted in stable mode-locked operation. When the mode-locking threshold is reached (at about 5 W average output power), the laser delivers a single-pulse train with a repetition rate of 10 MHz, as shown in Figure a. The operation of mode-locking is very stable and self-starting. The highest possible average output power is as high as 11 W, corresponding to a pulse energy of 1.1 μJ. The narrow line emission spectrum with the spectral bandwidth (3 dB) of 0.4 nm at the center wavelength of 1036.36 nm is shown in Figure. The laser generates clean picosecond pulses with a pulse width of 310 ps.

Low-Noise Performance of High-Power Mode-Locked Fiber Laser

Self-similar low-noise femtosecond ytterbium-doped double-clad fiber laser
B. Ortaç, A. Hideur, C. Chédot, M. Brunel, G. Martel and J. Limpert
Applied Physics B: Lasers and Optics, vol. 85, pp. 63-67 (2006)

Amplitude fluctuation is the common figure of merit used to evaluate the quality of a mode-locked pulse train. This quantity is derived from the pulse-train power spectrum obtained using a high-speed photodetector and a microwave spectrum analyzer. Figure shows an example of such a radio-frequency spectrum recorded at the fundamental frequency. The resolution bandwidth of the scan shown in Figure is 300 Hz on a span of 200 kHz. The different spectra do not present any noise substructure, which highlights the low amplitude noise of the laser. The amplitude noise is below the detection limit. To estimate an upper bound on the amplitude noise, we consider the width at the base of the peak of Figure as the noise structure width. This is justified by the experimental observations which show that by slightly biasing the polarization additive-pulse mode-locking action or increasing the pump power, we observe the emergence of an amplitude-noise substructure only in this zone. We calculate a noise level of about 0.048%. It is comparable to that measured in low-power stretched-pulse fiber lasers pumped with single-mode grating-stabilized telecommunication-qualified diodes. Finally, we report the generation of self-similar highly stable femtosecond pulses from an ytterbium-doped double-clad fiber laser. Positively chirped parabolic pulses with 6.4-ps duration and more than 3.2-nJ energy are obtained. These pulses are extra-cavity compressed to 140 fs.

Environmentally-Stable Fiber Laser

Experimental and numerical study of pulse dynamics in positive net-cavity dispersion mode-locked Yb-doped fiber lasers
B. Ortaç, M. Plötner, T. Schreiber, J. Limpert and A. Tünnermann
Optics Express, vol. 15, pp. 15595-15602 (2007)

We report the both numerically and experimentally generation of wave-breaking-free and stretched-pulses from an environmentally stable Yb-doped all-fiber laser. The linear cavity is constructed with polarization-maintaining single-mode fiber allowing environmentally-stable configuration. Nonlinear mode-locking mechanism is obtained by a directly glued SAM and dispersion management of the cavity is assured by chirped fiber Bragg grating (CFBG). Positively-chirped picosecond pulses with parabolic spectra are compressed to femtosecond range by using hollow-core photonic bandgap fiber (HC-PBG). The generation and intra-cavity evolution of wave-breaking-free and stretched-pulses are confirmed by a numerical analysis. To our knowledge, this is the first report of an all-fiber wave-breaking-free Yb-doped fiber oscillator and the generation of pulses with a parabolic spectral shaped in the stretched-pulse regime.

Cavity Designs

Linear Cavity

Self-starting self-similar all-polarization maintaining Yb-doped fiber laser
C. K. Nielsen, B. Ortaç, T. Schreiber, J. Limpert, R. Hohmuth, W. Richter and A. Tünnermann
Optics Express, vol. 13, pp. 9346-9351 (2005)

We have demonstrated, for the first time to our knowledge that a self-starting self-similar oscillator can be obtained with a linear cavity where a SAM is used as the nonlinear mode-locking mechanism. Further, the oscillator was intrinsically environmentally stable, as only PM fibers were used. Pulses with an energy of 1 nJ at a repetition rate of 17 MHz are obtained, where the pulse duration is characterized to be 7.2 ps. The self-similar spectral profile is centered at 1035 nm with a width of 11.3 nm. These pulses have been externally compressed to a pulse duration of 280 fs autocorrelation width.

Ring Cavity

Characterization of an ytterbium-doped double-clad fiber laser passively mode-locked by nonlinear polarization rotation
B. Ortaç, A. Hideur, M. Brunel, T. Chartier, M. Salhi, H. Leblond and F. Sanchez
Applied Physics B: Lasers and Optics, vol. 77, pp. 589-594 (2003)

The properties of an ytterbium-doped double-clad unidirectional fiber ring cavity laser, passively mode-locked by nonlinear polarization rotation are investigated in this work (see Figure). Cartographies of mode-locking regime versus half-wave plates orientations are presented for several values of the total cavity dispersion and for different pump powers. The experimental data concerning the passive mode-locking properties versus the total cavity dispersion have been compared with theoretical results obtained from a master equation which explicitly involves the orientation of the two phase plates.

Sigma Cavity

High-energy femtosecond Yb-doped fiber laser operating in the anomalous dispersion regime
B. Ortaç, J. Limpert and A. Tünnermann
Optics Letters, vol. 32, pp. 2149-2151 (2007)

The experimental setup of the passively mode-locked Yb-doped large-mode-area photonic crystal fiber laser is shown in Figure. The employed configuration is a sigma cavity. The Yb-doped fiber is mounted in the ring section of the cavity. The optical isolator ensures the unidirectional propagation of the laser light. The reflected polarization of the polarization beam splitter of the isolator serves as the output coupler. A grating pair (600 lines/mm) is inserted into the linear section of the cavity to control the total cavity GVD. A half-wave plate introduced between the isolator and the grating pair allows for controlling an additional loss in the grating pair system. Passive mode-locking is achieved through the saturable absorber mirror (SAM) placed at the end of the linear section of the cavity. For the first time to our knowledge, we have demonstrated the generation of high-energy ultrashort pulses in a Yb-doped large-mode-area photonic crystal fiber laser operating in the anomalous dispersion regime. The fiber laser directly generates stable and clean sub-500 fs pulses at a repetition rate of 53.3 MHz. In the single-pulse regime, the laser delivers 880 mW of average power corresponding to a pulse energy of more than 16.5 nJ with diffraction-limited quality determined by the intrinsically single-transverse-mode LMA fiber. The pulse energy is considerably higher than in conventional single-mode fiber oscillators operating in the anomalous dispersion regime that typically generate few 100 pJ before pulse breakup occurs.

Mode-Locked Regimes

Soliton Regime

High-energy soliton pulse generation with a passively mode-locked Er/Yb-doped multifilament-core fiber laser
B. Ortaç, J. Limpert, S. Jetschke, S. Unger, V. Reichel, J. Kirchhof and A. Tünnermann
Applied Physics B: Lasers and Optics, vol. 98, pp. 27-31 (2010)

For the first time to our knowledge, we have demonstrated the generation of high-energy short pulses in an Er/Yb-doped multifilament-core large-mode- area fiber laser operating in the purely anomalous dispersion regime. The fiber laser directly generates stable and clean 1.61 ps pulses at a repetition rate of 44 MHz. In the single- pulse regime, the laser delivers 400 mW of average power corresponding to pulse energy of more than 9 nJ with excellent beam quality determined by the intrinsically single- transverse-mode low-NA LMA fiber. The pulse energy is two orders of magnitude higher than so far reported for conventional single-mode fiber oscillators operating in the purely anomalous dispersion regime that typically generate few 100 pJ before pulse breakup occurs.

Stretched Pulse Regime

90 fs stretched-pulse ytterbium-doped double-clad fiber laser
B. Ortaç, A. Hideur, T. Chartier, M. Brunel, C. Özkul and F. Sanchez
Optics Letters, vol. 28, pp. 1305-1307 (2003)

The experimental setup of stretched pulse generation is shown in Figure. A diode-pumped Yb-doped double-clad (DC) fiber was used as the amplifying medium. Undoped single-mode fibers were spliced at both ends of the DC fiber, leading to a total cavity length of 9 m. For stretched-pulse operation we used an intra-cavity dispersive delay line composed of a 1200-line mm grating pair to partially compensate for the high, positive group-velocity dispersion of the fiber. Finally, we have developed a passively mode-locked Yb-doped double-clad fiber laser operating in the stretched-pulse regime. We have investigated the laser’s performance in pulse duration and energy as a function of pump power. The laser produces 4.8-ps Gaussian pulses with 85 mW of average power at an 18-MHz repetition rate. This corresponds to an energy per pulse of 4.5 nJ. These pulses are extra-cavity compressed to near-bandwidth- limited 90-fs pulses

Wave-Breaking-Free Regime

In the wave-breaking-free mode-locked fiber laser, the net-cavity dispersion is positive and the nonlinear pulse evolution in the normal group-velocity-dispersion (GVD) fiber segment is monotonic. The pulse accumulates a linear chirp, which is partially compensated at points in the cavity using a linear process (e.g. diffraction gratings). The pulse is always positively chirped inside the cavity with a minimum (but not transform limited) at the entrance of the normal GVD fiber. Our wave-breaking-free operation, the total fiber length inside the cavity is 4.9 m allowing for a second-order cavity dispersion of 0.055 ps². Hence, the laser operates in the normal dispersion regime. In this operation, the laser started CW operation at lower pump power, by increasing the pump power, the laser started to operate in a Q-switched mode-locking state. At launched pump power of 38 mW, the stable mode-locking threshold is reached and the laser delivers single-pulse trains at about 20.30 MHz. The mode-locking regime is very stable and self-starting with the same characteristic of the operation (spectrally and temporally) for equal pump power. Furthermore, we also confirmed the wave-breaking-free regime by numerical simulations.

Dispersion Compensation Free Regime

In the dispersion compensation free regime, there is no dispersion compensation element and no additional spectral filter introduced into the setup. The experimental setup of the passively mode-locked large-mode-area (LMA) fiber laser is shown in Figure. The sigma cavity is constructed with a saturable absorber mirror and short-length large-mode-area photonic crystal fiber. The laser generates positively-chirped pulses with an energy of 265 nJ at a repetition rate of 10.18 MHz in a stable and self-starting operation. The pulses are compressible down to 400 fs leading to a peak power of 500 kW. Numerical simulations accurately reflect the experimental results and reveal the mechanisms for self-consistent intra- cavity pulse evolution (see Figure). The pulse duration and spectral bandwidth changes only slightly during the intra-cavity propagation. This behavior is in contrast to other operation regimes, well know from dispersion-managed mode-locked fiber lasers (e. g. stretched-pulse regime) where temporal and spectral breathing is significantly more pronounced. The minimum pulse duration occurs directly after the SAM resulting from the nonlinear absorbing mechanism. The pulse duration monotonically increases during the amplification in the gain fiber. As revealed by the simulation the pulses are always positively chirped inside the cavity with only one minimum per round trip located at the entrance of the fiber.

Chirped-Pulse Regime

Self-starting passively mode-locked chirped-pulse fiber laser
B. Ortaç, M. Plötner, J. Limpert and A. Tünnermann
Optics Express, vol. 15, pp. 16794-16799 (2007)

Pulse dynamics in a passively mode-locked chirped-pulse fiber laser
B. Ortaç, M. Plötner, J. Limpert and A. Tünnermann
Applied Physics B: Lasers and Optics, vol. 99, pp. 79-82 (2010)

Impact of dispersion on pulse dynamics in chirped-pulse fiber lasers
M. Baumgartl, B. Ortaç, J. Limpert and A. Tünnermann
Applied Physics B: Lasers and Optics, vol. 107, pp. 263-274 (2012)

We develop a new approach of self-starting passively mode-locked fiber laser operating in the chirped-pulse regime for the first time. A chirped fiber Bragg grating in the cavity provides positive dispersion with negligible nonlinearity. The laser generates positively-chirped pulses with a pulse duration of 22 ps at a repetition rate of 44 MHz, which are compressible down to 1.5 ps. We believe that the presented approach reveals a pulse energy scaling potential of mode-locked fiber lasers as nonlinear effects are significantly diminished compared the other known operation regimes. Further studies, we also report on a systematic study of an environmentally stable mode-locked Yb-doped fiber laser operating in the chirped-pulse regime. The linear cavity chirped-pulse fiber laser is constructed with a saturable absorber mirror as nonlinear mode-locking mechanism and a nonlinearity-free transmission-grating-based stretcher/compressor for dispersion management. Mode-locked operation and pulse dynamics from strong normal to strong anomalous total cavity dispersion in the range of +2.5 to −1.6 ps² is experimentally studied. Strongly positively chirped pulses from 4.3 ps (0.01 ps²) to 39 ps (2.5 ps²) are obtained at normal net-cavity dispersion. In the anomalous dispersion regime, the laser generates average soliton feature negatively chirped pulses with autocorrelation pulse durations from 0.8 ps (−0.07 ps²) to 3.9 ps (−1.6 ps²). The lowered peak power due to the pulse stretching allows one to increase the double pulse threshold. Based on the numerical simulation, different regimes of mode locking are obtained by varying the intra-cavity dispersion, and the characteristics of average soliton, stretched-pulse, wave-breaking-free and chirped- pulse regimes are discussed

Saturable Absorbers

Nonlinear Polarization Rotation

The properties of an ytterbium-doped double-clad fiber laser, passively mode-locked by nonlinear polarization rotation are investigated in this work. Cartographies of mode-locking regime versus half-wave plates orientations are presented for several values of the total cavity dispersion and for different pump powers. Bistability between the continuous and the mode-locking regimes is pointed out. The effect of the total group velocity dispersion is described with a master mode-locking equation.

Semiconductor Saturable Absorber Mirror

Approaching microjoule-level pulse energy with mode-locked femtosecond fiber lasers
B. Ortaç, M. Baumgartl, J. Limpert and A. Tünnermann
Optics Letters, vol. 34, pp. 1585-1587 (2009)

Dissipative solitons in a passively mode-locked Er-doped fiber laser with strong normal dispersion
A. Cabasse, B. Ortaç, G. Martel, A. Hideur and J. Limpert
Optics Express, vol. 16, pp. 19322-19329 (2008)

Highly Fe-doped InGaAs/InP saturable absorber mode-locking of an erbium fiber laser
J-B. Lecourt, B. Ortaç, M. Guézo, C. Labbé, H. Folliot, S. Loualiche, A. Hideur and G. Martel
Journal of Nonlinear Optical Physics & Materials, vol. 14, pp. 427-437 (2005)

Passive mode-locking is achieved through the saturable absorber mirror (SAM) placed at the end of the linear section of the cavity. The SAM is based on a multilayer GaAs / AlAs Bragg mirror and a low- temperature molecular beam epitaxy grown InGaAs quantum well structure in front of the mirror. The low-intensity absorption of the SAM at approximately 1035 nm is 45%, the modulation depth 30%, and the saturation fluence as high as 100 J/cm2. The bi-temporal impulse response has a short relaxation time of 200 fs and a slower part of 500 fs. The saturation threshold is achieved by focusing the output of the dispersion compensation system onto the structure.

The semiconductor saturable absorber mirror (SAM) presents a low intensity reflectivity of 77%, modulation depth of 14% and saturation fluence of 25 μJ/cm². Temporal relaxation of the SAM is roughly estimated around 2 ps but clearly present a bi-temporal response time with a fast sub-picosecond component and a slower part approaching 10 ps at high fluence. One should note that up to a fluence of 458 μJ/cm², no reverse saturable absorption (free carrier absorption-based, FCA) has been observed for this SAM structure. A negligible positive dispersion (+ 1000 fs²) has been evaluated for the SAM at the laser central operating wavelength of 1560 nm.

Erbium-doped unidirectional fiber ring laser has been passively mode-locked with a novel ultra-fast semiconductor saturable absorber used in transmission. The structure is based on InGaAs/InP multiple quantum wells grown on an InP substrate. Different structures with recovery time down to picosecond have been tested. Relaxation time reduction has been obtained by iron doping. With such structures, detrimental regimes such as Q-switch and Q-switch/Mode-Lock regimes have been avoided. Solitonic and slightly stretched-pulse dispersion regimes have been tested. The laser delivers for both, completely self-starting picosecond pulses of 8 mW maximum average output power at a repetition rate around 25 MHz.

Polymer-Based Saturable Absorber

Dynamics of a vectorial neodymium-doped fiber laser passively Q-switched by a polymer-based saturable absorber
G. Martel, M. Bennoud, B. Ortaç, T. Chartier, J-M. Nunzi, G. Boudebs and F. Sanchez
Journal of Modern Optics, vol. 51, pp. 85-95 (2004)

The Polymer-Based Saturable Absorber mounted on a manual z-translation stage is made up of a thin slice (about 3 – 5 μm) of organic dye (BDeN molecules from Kodak) deposited on a microscope glass plate and is inserted at the focal plane of the unity-magnified intra-cavity telescope (see Figure mo3 and mo4). Another simpler configuration has also been tested and allows self-Q-switching. It consists of inserting the Polymer-Based Saturable Absorber between one end of the fibre and its collimating microscope objectives (mol and mo2). Although this configuration eliminates the intra-cavity telescope, it appeared less stable and less easy to use. We report the dynamic regimes obtained in a bipolarized neodymium-doped fibre laser self Q-switched by a thin slice of a polymer-based saturable absorber. We demonstrate the influence of the total losses and of the anisotropic losses induced by defocusing the saturable absorber and by tilting the cavity mirror, respectively.

Multiple Pulsing Regimes

Bound State Generation

Ultra-short bound states generation with a passively mode-locked high-power Yb-doped double-clad fiber laser
A. Hideur, B. Ortaç, T. Chartier, M. Brunel, H. Leblond and F. Sanchez
Optics Communications, vol. 225, pp. 71-78 (2003)

Generation of bound states of three ultrashort pulses with a passively mode-locked high-power Yb-doped double-clad fiber laser
B. Ortaç, A. Hideur, T. Chartier, M. Brunel, Ph. Grelu, H. Leblond and F. Sanchez
IEEE Photonics Technology Letters, vol. 16, pp. 1274-1276 (2004)

Binding widely-separated pulses with a passively mode-locked Yb-doped double-clad fiber laser
B. Ortaç, A. Hideur and M. Brunel
Applied Physics B: Lasers and Optics, vol. 79, pp. 185-192 (2004)

Generation of parabolic bound pulses from a Yb-fiber laser
B. Ortaç, A. Hideur, M. Brunel, C. Chédot, J. Limpert, A. Tünnermann and F. Ö. Ilday
Optics Express, vol. 14, pp. 6075-6083 (2006)

Observation of soliton molecules with independently evolving phase in mode-locked fiber laser
B. Ortaç, A. Zaviyalov, C. K. Nielsen, O. Egorov, R. Iliew, J. Limpert, F. Lederer and A. Tünnermann
Optics Letters, vol. 35, pp. 1578-1580 (2010)

We have reported for the first time to our best knowledge the experimental observation of bound states in high-energy passively mode-locked fiber laser operating in the normal dispersion regime at 1.05 μm. Stable bound states of two and three pulses have been obtained with a high-power ytterbium-doped double-clad fiber laser. These nanojoules pulses could be compressed extra- cavity to 100 fs. The laser has been modeled with a cubic–quintic CGL equation. In the region where localized solutions are stable, we have demonstrated that bound states of two pulses are stable. Experimentally, the selection of either single- and double-pulse operation is related to the pumping power. It is taken into account in the theoretical model through the total energy of the considered solution. A more complex model needs now to be elaborated to explicit the influence of the experimental parameters. Further studies, we have also reported on the emission of bound pulses of 5 ps whose separation can exceed 180 ps. Pulses are further compressed extra-cavity to 140 fs, leading to pulse separations that can reach approximately 1300 pulse widths, while pulses remain bound. In addition, we have been reported the experimental observation of ultra-short bound states in an ytterbium-doped double-clad fiber laser operating in the self-similar regime. Along with the previously reported results on bound pulse formation in the soliton and the stretched-pulse regimes, these results constitute the observation of the last member of the family of bound pulses; bound pulses exist for all common mode-locking regimes. Later studies, we have been demonstrated the experimental generation of two-soliton molecules in an all-polarization-maintaining ytterbium-doped fiber laser operating in the normal dispersion regime. These molecules exhibit an independently evolving phase and are characterized by a regular spectral modulation pattern with a modulation depth of 80% measured as an averaged value. Moreover, the numerical modeling confirms that the limited modulation depth of the spectrum is caused by the evolution of the phase difference between the pulses.

Harmonic Mode-Locking

Passive harmonic mode locking with a high-power ytterbium-doped double-clad fiber laser
B. Ortaç, A. Hideur and M. Brunel
Optics Letters, vol. 29, pp. 1995-1997 (2004)

We report passive harmonic mode locking of a high-power Yb-doped double-clad fiber laser operating in both the normal- and the anomalous-dispersion regimes with a fundamental repetition rate of 20.4 MHz. In the anomalous-dispersion regime, a bunch of 20 pulses is created from the initial single pulse. The creation of this bunch from the single-pulse regime takes only a few seconds. Slowly, the pulses rearrange themselves to occupy uniformly the whole cavity round trip. After a few minutes, harmonic mode locking is achieved. Because the free spectral range of the cavity is 20.4 MHz, the repetition rate of the laser in this case is 408 MHz. In the anomalous-dispersion regime, 1-ps, 125-pJ pulses are emitted at a higher repetition rate of 408 MHz. In the normal-dispersion regime, 116-fs, 1.7-nJ pulses are emitted at a repetition rate of 102 MHz. The results indicate a supermode suppression of more than 60 dB.

2 GHz passive harmonically mode-locked Yb-doped double-clad fiber laser
B. Ortaç, A. Hideur, G. Martel and M. Brunel
Applied Physics B: Lasers and Optics, vol. 81, pp. 507-509 (2005)

The main modification in the GHz repetition rate harmonic mode-locking configuration is the low total cavity dispersion value that is close to zero. In the case of our laser operating in the anomalous dispersion regime, pulses can be described in terms of “average solitons”. The soliton area theorem states that the product of the peak amplitude A0 and the pulse width τ is fixed by the averaged dispersion and non-linearity. This means that any modification of the total cavity dispersion will induce a modification of the pulse characteristics. The total cavity dispersion is presently reduced by more than an order of magnitude to −0.004 ps², while the pulse width is modified by only 30% (to 680 fs). This can explain to a certain extent the 91 pulses per round trip obtained in the present configuration. In this study, we have reported for the first time to our best knowledge passive harmonic mode locking of a high-power Yb-doped double-clad fiber laser operating near 1050 nm with repetition rate above 2 GHz. 680-fs, 48-pJ pulses are emitted.

Er, Er/Yb and Nd-Doped Fiber Lasers

The experimental set-up of the laser is drawn on Figure. It consists in a short heavily doped erbium fiber (HDEF) laser presenting high normal GVD. The net cavity dispersion is highly positive thanks to the minimization in length of all other passive fibered components which are restricted, in our case, solely to the pigtails of a wavelength division multiplexer (WDM). The laser cavity is mounted in a sigma configuration using a polarization-sensitive optical isolator. The 1.15 m HDEF presents an unpumped absorption of 80 dB/m at 1530 nm. Its GVD has been estimated at -48 (ps/nm)/km at 1550 nm. Pigtails of the WDM are made up of Hi1060 fibers with a measured GVD of 8.7 (ps/nm)/km. The net positive cavity dispersion is β2 ≈ + 0.063 ps² around 1550 nm. Rest of the cavity comprises bulk isolator, wave-plates, coupling lenses and commercial SAM (Batop 1550-23). The total optical cavity length is about 3.26 m leading to a pulse repetition rate of 92 MHz. We have been developed a passively mode-locked dispersion-managed erbium- fiber laser with large net cavity dispersion. The laser generates positively chirped 5.3 ps output pulses that are compressed down to 757 fs. The optical spectrum is characterized by steep edges and a narrow width of 8.3 nm, which is far below the gain-bandwidth of erbium. Numerical simulations show that in addition to the SAM nonlinearity, the gain and gain dispersion play a key role in the formation of these pulses. Then our results confirm the tendency that gain-guided solitons belong to a more general class of so-called “dissipative solitons”.

The complete experimental setup of the passively mode- locked Er/Yb-doped LMA fiber laser in sigma cavity configuration is presented in Figure. One key cavity element in this laser configuration is the erbium/ytterbium-doped multifilament-core LMA fiber. The new design of the LMA fiber open the door for high-energy pulse generation in the 1.5-μm wavelength region. In the single- pulse regime, the laser delivers 400 mW of average power corresponding to pulse energy of more than 9 nJ with excellent beam quality determined by the intrinsically single- transverse-mode low-NA LMA fiber. The pulse energy is two orders of magnitude higher than so far reported for conventional single-mode fiber oscillators operating in the purely anomalous dispersion regime that typically generate few 100 pJ before pulse breakup occurs.

We used active fibre having the following characteristics: core diameter 2.7 pm (cut-off wavelength = λ_c = 0.86 pm), numerical aperture (NA) 0.29; length 14m; doped with 500 parts in 10⁶ Nd³⁺ by weight (i.e. the pump power is totally absorbed along the fibre). We have experimentally studied the various dynamic regimes of a vectorial neodymium-doped fibre laser passively Q-switched by a polymer-based saturable absorber operating at λ = 1.08 μm. We have observed that, for a good cavity configuration, the dynamics are essentially chaotic and can present some antiphase behavior. For increasing values of the cavity losses, a route to chaos appears. The laser can operate spontaneously under a chaotic, a 4T-periodic, a 2T-periodic, and then a T-periodic mode. The role of the anisotropic losses has also been pointed out through the appearance of other dynamic regimes such as low-frequency modulated periodic regimes or intermittencies. In the chaotic regions the antiphase dynamics can be partially or totally lost.