Adaptive soliton laser

Abstract

A method of mode-locking utilizes passive frequency modulation of frequency comb induced stimulated Brillouin scattering resonance (SBSR) in a waveguide. It is discovered by the inventor that the SBSR effect creates a feedback control in forming down-chirped sech2-shaped bright or dark solitons in waveguides. These genuine solitons do not suffer from Cherenkov (or dispersive) radiation in transmission and adapt to different waveguides if peak intensity requirements are met. The mode-locking of an adaptive soliton laser with all passive fibers having the same type of dispersion is self-started and sustained by SBSR effect. This nonlinear optical effect suppresses Kelly-sidebands and broadens the spectrum for chirped femtosecond soliton generation. Adaptive soliton lasers of normal dispersion are capable of generating high energy solitons.

Description

[0001]ADAPTIVE SOLITON LASER FIELD OF THE INVENTION The present invention relates in general to a method of passive mode-locking and passively mode-locked lasers capable of producing stable chirped femtosecond solitons that do not suffer from dispersive radiation in transmission fibers and other waveguides. BACKGROUND OF THE INVENTION The performance and reliability of a mode-locked laser depends on the method of mode-locking used. All current passive mode-locking approaches, including saturable absorber, Kerr-lens, nonlinear polarization evaluation (NPE), and nonlinear optical loop mirror (NOLM), as well as its modification, nonlinear amplifying loop mirror (NALM), are based on intensity modulation which has a lower loss for higher intensity radiation (pulsed operation) compared to lower intensity radiation (continuous wave (CW) operation), resulting in the mode-locking operation for pulse generation being the dominant preference. The most popular passive mode-locking approach is using the switching effect of a saturable absorber to start and maintain mode-locking. The main switching effect with sufficient modulation depth for stable mode-locking is interband recombination which has a decay time >500 fs for all current saturable absorbers. Some saturable absorbers have intraband thermalization times as short as <100 fs, but their modulations are not deep enough to sustain stable pulses shorter than 200 fs on their own. Nevertheless, they are helpful for shorter pulse generation in starting mode-locking based on other faster nonlinear optical effects. For Fourier transform limit <200 fs generation, the Kerr effect and nonlinear polarization evolution (NPE) are typically utilized in solid-state and fiber lasers, respectively, to modulate the cavity loss or gain of the laser pulse. Unlike switching effects, they subserve mode- locking only at a small peak power range, if no adjustments are performed accordingly. Therefore, current mode-locking lasers with Fourier transform limit <100 fs are sensitive to vibrations and the variations of working environments. To achieve sufficient reliability for industry applications, special feedback controls must be applied with them. According to the evolution properties of the laser pulse inside the cavity, mode-locked lasers can be categorized as three types: stretched-pulse (dispersion-managed soliton), soliton, and quasi- soliton. A stretched-pulse laser is designed with dispersion compensation to optimize self-phase modulation (SPM) caused by the Kerr effect for spectrum broadening. The laser pulse in a stretched-pulse laser is alternately stretched and compressed more than one order of magnitude. In contrast, a soliton laser is mainly constructed with components of the same sign of dispersion. The laser pulse in a soliton laser keeps its pulse duration by balancing the dispersions and the nonlinear optical effects. Present solitons, including chirp-free soliton and up-chirped dissipative soliton, are based on the balance of group velocity dispersion (GVD) and the Kerr effect. They suffer from Cherenkov (dispersive) radiation, due to unbalanced third order dispersion. This is one of main reasons why soliton communication was predicted in 1973 and demonstrated in 1980 but has not had any significant progress in commercialization yet. Besides, these solitons are not able to easily obtain high energy and Fourier transform limit <200 fs because their soliton area theorems narrow their pulse durations as their pulse energy increases, which will increase their peak powers, but their peak powers are limited by stimulated Raman scattering. Once their area theorems are broken by strong amplification, the solitons become stretched-pulses without a chirp state balancing GVD and the Kerr effect. This happens in quasi-soliton lasers with large gains and spectrum filtering, such as regular all-normal-dispersion femtosecond fiber lasers and self-similar lasers. In these lasers, the laser pulses become stretched-pulses with significant SPM after the gain media and tend to be solitons after output coupling and spectrum filtering. Self- similar lasers are quasi-soliton lasers with smaller net dispersion and smaller SPM, but their pulse energies are also significantly smaller. Large SPM in higher energy quasi-soliton lasers generates distinct spectral peaks. Similarly, large net cavity dispersion in current soliton lasers causes Kelly-sidebands. Both correspond to significant picosecond floors. Moreover, due to gain narrowing, a typical fiber amplifier will limit the stretched-pulses to ~200 fs. Above drawbacks limit the usages of current femtosecond fiber laser systems to only refractive surgery and some micromachining. SUMMARY OF THE INVENTION Brillouin scattering is a basic nonlinear effect in optical fibers and other waveguides. Up to this day, stimulated Brillouin scattering (SBS) induced by the frequency comb of a laser pulse has never been seriously considered in laser pulse generation and propagation because without proper chirp and / or correct mode separation, SBS induced by different comb lines will mostly cancel out and only causes noise to traditional mode-locked lasers. However, when the Stokes line and anti-Stokes line of Brillouin scattering excited by each comb line are also the comb lines of the laser pulse, all the SBS induced by the frequency comb will resonate if the laser pulse is properly chirped for phase matching. This resonance effect will be referred to as stimulated Brillouin scattering resonance (SBSR). By introducing the SBSR effect into the nonlinear Schrödinger equation (NLSE), the inventor discovers that the SBSR effect has a feedback control effect in forming down-chirped sech2-shaped bright solitons in waveguides. Abnormal dispersion waveguides support low energy operation and normal dispersion waveguides support high energy operation. The SBSR effect adaptively broadens or narrows the spectrum of the soliton and suppresses Kelly-sidebands generation via acoustic wave generation and absorption. The solitons formed with the SBSR effect are adaptive to any waveguides if the peak intensity requirements are met. The SBSR effect balances not only GVD and the Kerr effect, but also the third order dispersion of the waveguides, and thus these genuine solitons do not suffer from dispersive radiation in transmission. These solitons will be referred to as adaptive solitons. The basis of the invention is a method of mode-locking based on passive frequency modulation of the SBSR effect which broadens the radiation band in a laser cavity, resulting in mode-locking (broadband) operation suppressing CW (narrowband) operation. This method consists of using a Brillouin scattering element to generate SBS signals significantly stronger than the noise of the laser in the cavity and a broadband gain medium to amplify the SBS signals, along with designing the laser cavity with the mode separation meeting the SBSR condition for the SBS signals to overlap with the laser modes, and utilizing polarization control elements to make the polarization of the laser pulse self-consistent and minimize the intensity modulation caused by the NPE effect in the laser cavity. The mode-locking is started and sustained by the SBSR effect when the polarization control is optimized. If the mode-locked lasers based on the present invention are built as soliton lasers with all passive fibers having the same type (positive or negative) of dispersion, their outputs will be down-chirped adaptive solitons. These kinds of lasers will be referred to as adaptive soliton lasers. In the preferred embodiment of the invention, the adaptive soliton laser is a fiber laser with the designed mode separation smaller than the minimal SBS linewidths of all the fibers in the laser cavity. In this case, the SBSR condition is intrinsically satisfied and the SBS threshold is low. Mode-locking is started and sustained by the SBSR effect. Polarization control elements, such as polarizers, waveplates, or inline polarization controllers, are then employed to make the polarization self-consistent and to minimize the intensity modulation caused by NPE. Being a femtosecond laser, it also includes a laser gain media with gain spectrum supporting femtosecond laser pulse generation, which may be a rare-earth doped fiber, a waveguide (other than fiber) gain medium, or a solid-state gain medium. In experiments conducted on a normal dispersion fiber laser built in accordance with the preferred embodiment, the mode separation (repetition rate) of the laser is 5.26 MHz, and mode- locking is self-started when the polarization control is optimized and the pump power is larger than a threshold. The output spectrum bandwidth is ~3.35THz and the pulse energy is >5 nJ. There are no Kelly-sidebands caused by dispersion or spectrum modulation caused by SPM shown on the spectrum, due to the suppression of the SBSR effect. The transform-limited pulse of the spectrum is well fitted to a 95 fs sech2-shape pulse. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings schematically illustrate the principle of the present invention and an exemplary adaptive soliton laser in accordance with a preferred embodiment of the present invention. Fig.1 is a schematic diagram of the working principle of the invention, mode-locking based on the SBSR effect. Fig.2 is a schematic diagram of the construction of an adaptive soliton laser based on the invention. Fig.3 is a schematic diagram of an adaptive soliton laser that is configured in accordance with a preferred embodiment of the present invention. Fig.4 is the measured spectrum output from the exemplary adaptive soliton laser with normal dispersion DETAILED DESCRIPTION OF THE INVENTION Different from the traditional passive mode-locking methods based on laser intensity modulations, the present invention of mode-locking method is based on frequency modulation caused by the SBSR effect. How to achieve SBSR is the key for the success of the invention. Fig.1 shows the frequency modulation of the SBSR effect 10. A laser pulse is a frequency comb 11 with mode separation ^^^^ which is generally corresponding to a repetition rate of a laser cavity. In a Brillouin scattering element, the Brillouin scattering with frequency Ω will be excited by each comb line ^^0of the frequency comb 11 and then generates Stokes line 12 of frequency^^0 − ^^ and anti-Stokes line 13 of frequency ^^0 + ^^. In general, the frequencies of the Stokesline 12 and anti-Stokes line 13 are not overlapping with any comb line of the frequency comb 11,and thus SBSR does not happen. Because generally ^^ >> ^^^^, to achieve SBSR effect, a lasershould be designed or controlled with at least one integer ^^^^meeting the following inequality: Ω− ΔΩ⁄ 2 < NB ⋅ δω < Ω + ΔΩ⁄ 2 (1)where ^^^^ is the bandwidth of Brillouin scattering. This SBSR condition, with at least two comblines of the frequency comb 11 with frequency ^^0 − ^^^^ ⋅ ^^^^ and ^^0 + ^^^^ ⋅ ^^^^, respectively,located in the vicinity of the peaks of Stokes line 12 and anti-Stokes line 13, will result in SBS occurring between the laser modes with mode separation ^^^^^^^^, which is the frequency of SBSR. The acoustic wave generated by one comb line will either be absorbed by other comb lines or will stimulate Brillouin scattering of other comb lines, resulting in all the stimulated Brillouin scatterings caused by the frequency comb to resonate. Consequently, all the comb lines of the frequency comb 11 will intrinsically be locked together by the SBSR frequency via the acoustic wave generation and absorption. By introducing the SBSR effect into the nonlinear Schrödinger equation (NLSE), the inventor discovers that the SBSR effect has a feed-back control effect in forming bright and dark adaptive solitons in waveguides at different peak power conditions. The bright adaptive solitons are down-chirped and sech2-shaped in both abnormal and normal dispersion waveguides. A sech2- shaped pulse is generally described with a time scale parameter ^^, and the pulse duration is ~1.763^^. In a waveguide, without taking the high order dispersion into account, ^^ can be approximately expressed as:^^2 = (^^2 − 2) ^^2⁄ (2^^0^^) (2)where ^^ = 0 or ^^ = 8^^^^^^0⁄ (3^^2) , ^^^^ is SBSR coefficient, ^^0 is the peak power of the which is positive for bright soliton and negative for dark soliton, ^^2is the group velocitydispersion and ^^ is Kerr nonlinearity. SBSR coefficient ^^^^ = ^^0⁄ (2^^0^^0) , where ^^0 is thepower gain coefficient of Brillouin scattering in units of m-1W-1, ^^0and ^^0are the center frequencies of the laser pulse and the acoustic wave, respectively. To obtain bright solitons, Eq.(2) requires ^^2 > 2 for ^^2 > 0 (normal dispersion) and ^^ = 0 for ^^2 < 0 (abnormal dispersion).To obtain dark solitons, Eq. (2) requires ^^2 > 2 for ^^2 < 0 and ^^ = 0 for ^^2 > 0 . In anabnormal dispersion waveguide, the expression of τ for a bright soliton is the same as that for a chirp-free soliton, meaning that the soliton is suitable for low energy operation. However, it has significantly lower transmission loss than a chirp-free soliton, due to the SBSR effect balancing out the third order dispersion of the delivery waveguide. It also has a significantly broader spectrum, due to chirp which is described by another chirp parameter different from ^^. The peak power of a bright adaptive soliton in a normal dispersion waveguide or a dark soliton laser in an abnormal dispersion waveguide must be larger than a threshold ^^^^ℎto meet the requirement ^^2> 2which can be rewritten as ^^0 > ^^^^ℎ = 3√2^^2⁄ (8^^^^) . More importantly, Eq. (2) shows that,different from chip-free and dissipative solitons, the pulse duration of a bright adaptive soliton in a normal dispersion waveguide broadens with the peak power increasing, supporting high energy operation. The stability of the mode-locking based on the SBSR effect depends on the bandwidth ^ω of the frequency comb 11. With the bandwidth reducing, the intensity of the comb lines at the Stokes line 12 and anti-Stokes line 13 will decrease, which weakens the SBSR effect. To take advantage of the SBSR effect, the bandwidth ^^^ of the frequency comb 11 should be larger than 2^^. This also requires the bandwidth of the gain spectrum 14 in a laser to be significantly larger than 2^^. There is no spectrum upper limit for the SBSR effect. Therefore, the present invention of passive mode-locking method using the frequency modulation of the SBSR effect is significantly more stable than the existing approaches, including saturable absorber, Kerr-lens, NPE, NOLM, and NALM, in generating <200 fs laser pulses. For 200 fs to 1ps pulse generation, its stability is also competitive with all other existing mode-locking methods. The general construction of an adaptive soliton laser 20 with the present invention of mode- locking method is illustrated in Fig.2. It consists of a broadband gain medium 21, a Brillouin scattering waveguide 22, a set of polarization control elements 23, and a mode separation controller 24 if necessary. The broadband gain medium 21 will generate the gain spectrum 14. The bandwidth of the gain spectrum limits the shortest Fourier transform limit of the laser pulse that can be generated from the laser. The Brillouin scattering waveguide 22 is the basic component determining the Brillouin scattering frequency ^^ and linewidth ^^^ of the Stokes line 12 and anti-Stokes line 13, and the strength of the SBSR effect. The Brillouin scattering waveguide 22 will also cause NPE which is peak power sensitive. To optimize the stability of the mode-locking based on the SBSR effect, the intensity modulation caused by NPE should be minimized. For this purpose, a set of polarization control elements 23 is used to control the polarization of the laser in the cavity. It can be constructed with polarizers, waveplates, or in-line polarization controllers. A mode separation controller 24 is required for an adaptive soliton laserwith mode separation ^^^^ > ^^^^ to get Eq. (1) satisfied for stable SBSR effect achievement.However, if ^^^^ ≤ ^^^^ , Eq. (1) is intrinsically satisfied, so the mode separation controller 24 isnot needed. locked laser becomes easier to construct and is extremely stable. As a laser, these above components should be built together as a ring or linear laser cavity 20, and a pump laser 25 with sufficient power is needed to pump the gain medium 21 to obtain the gain spectrum 14 with a bandwidth significantly broader than 2^^ and a power larger than the threshold if the laser is a normal dispersion soliton laser. For pure soliton operation, the maximal pump power should be limited to avoid stimulated Raman scattering from happening. Also, the laser cavity 20 should have an output coupling 26, which may be a fiber coupler, an end mirror, or other similar designs. For a soliton laser, the order of the components is not important for mode-locking achievement, though it will change the output power of the laser, due to different loss or gain of each component. With reference to Fig.3A, a preferred embodiment of a soliton fiber laser 30 illustrated was constructed to verify the operational theory of the present invention. The laser 30 is constructed as a ring cavity consisting of a fiber section 32 and a free space section 33. They are linked together with two similar fiber collimators 331 and 336, respectively, at the two ends of the fiber section and aligned being collinear. In addition to the two collimators 331 and 336, the free space section 33 consists of a half waveplate 332, two quarter waveplates 333 and 336, a polarizing beamsplitter (PBS) 341, and a polarization sensitive isolator 334. The fiber section 32 includes a fiber wavelength multiplexer (WDM) coupler 321, a rare-earth doped fiber 322, and a long single mode fiber 323. The gain medium 21 is the rare-earth doped fiber 322. The Brillouin scattering waveguide 22 is the long single mode fiber 323. The set of polarization control elements 23 is contained in the free space section 33. It is a typical NPE control waveplate group constructed with the half waveplate 332 and the two quarter waveplates 333 and 335, and the PBS 341. The laser 30 is designed with the total optical path length of the laser cavity long enough for the achievement of^^^^ < ^^^^⁄ 2 , and thus a mode separation controller 24 is not needed, meanwhile, at least twolongitudinal modes can be excited by each of the Stokes line 12 and anti-Stokes line 13 of every mode ^^0so that a chain reaction will happen and then mode-locking can be easily started by SBSR. The polarization sensitive isolator 334 is used to make the lasing unidirectional and to ensure the polarization in the laser is self-consistent. The rare-earth doped fiber 322 is pumped by the pump laser 31 via the WDM coupler 321. The laser output 34 via the PBS 341 is linearly polarized. The ratio of the output power is controlled by the half waveplate 332 and the quarter waveplate 333. The free space section 33 can be replaced by a replacement 35 shown in Fig.3B which is constructed with fiber components. Fiber fusion splicing will take the place of the two collimators 331 and 336. Two in-line polarization controllers 351 and 352 play the role of the three waveplates 332, 333, and 335 in NPE effect control. A polarization-maintaining (PM) fiber coupler 342 replaces the PBS 341 for the laser output 34. The main difference between the free space section 33 and the replacement 35 is that the laser output ratio of the free space section 33 is adjustable by tuning the half waveplate 332 and the quarter waveplate 333 but the replacement 35 is fixed to the coupling ratio of the PM fiber coupler 342. The fiber laser 30 is polarization sensitive. The waveplates 332 and 333 in the free space section or the in-line polarizer 351 in the replacement must be adjusted to get the laser lasing when the power of the pump 31 is significantly larger than the expected pump threshold. The laser output 34 should be detected by a fast photodetector and checked by an oscilloscope. The detectionbandwidth should be >> ^^^^ for sensing of the pulse train which could be random pulsing,regular mode-locking or multi-pulsing operation. The mode-locking of the adaptive soliton laser 30 is to be started by SBSR, which requires the laser power in the cavity to be larger than the threshold of SBS in the fiber, about 10 mW for a single mode fiber which is readily obtained. The laser spectrum broadens with the power of the pump 31 increasing. Once the laser spectrum is broader than 2^^, random pulsing will appear. By adjusting the polarization control elements 332, 333, and 335, or 351 and 352 to maximize the peak intensity, Q-switched mode-locking for a normal dispersion fiber laser or mode-locking for an abnormal dispersion fiber laser will be achieved, which can be checked from the oscilloscope. Use a spectrometer or optical spectrum analyzer to check the spectrum of the output 34. If there are no Kelly sidebands shown on the spectrum, you are on the right track to the adaptive soliton mode-locking in accordance with the present invention. For a normal dispersion fiber, keep increasing the pump power, the operation will switch from Q-switched mode-locking to adaptive soliton mode-locking. For an abnormal dispersion fiber laser, if there are Kelly sidebands on the spectrum, the mode-locking is NPE mode-locking, and the output 34 are chirp-free solitons. Readjust the polarization control elements to search other mode-locking status without Kelly sidebands. The stable mode-locking range of the present invention is a few times larger than that of NPE mode-locking. Its stability increases as the bandwidth ^^^ of the frequency comb 11 broadens. Increase the power of the pump 31 until multi-pulses or stimulated Raman scattering is observed, and then reduce the power of the pump 31 to get the operation back to normal mode- locking without multi-pulses or stimulated Raman scattering. The most stable pure adaptive soliton operation is then obtained. In experiments on the adaptive soliton laser 30 of Fig.3A, all the optical components in the cavity are normal dispersion. The gain medium 322 is a Yb doped fiber. The single mode fiber323 is ~35 meters HI-1060 with ^^ = 15.76 ^^^^^^ and ^^^^ = 19.5 ^^^^^^. The repetition rate of thecavity ^^^^ is 5.26 MHz, significantly smaller than the stimulated Brillouin linewidths ^^^^ of the single mode fiber 323. The basic conditions for mode-locking based on SBSR effect are ready. With the power of the pump 31 increasing, the laser is first running at CW and then switches to random pulsing when the laser power in the cavity is significantly higher than the threshold of stimulated Brillouin scattering 10 mW. Adjusting the polarization control elements 332, 333, and 335 in the free space section 33 (or 351 and 352 in the fiber replacement 35) to maximize the peak intensity, Q-switched mode-locking will be achieved. With the power of the pump 31 increasing, the repetition rate of the Q-switched mode-locking gets higher and higher until the operation switches to CW mode-locking when SBSR overcomes GVD, and then the laser pulse is down-chirped and quickly stretched to the stable state balancing the effects of SBSR, GVD and SMP. The power of the output 34 was then measured as ~20 mW. Further increase the power of the pump 31 until the maximal pure soliton operation is obtained. A stable and powerful adaptive soliton operation is then obtained. The output power was about 29 mW, corresponding to pulse energy of 5.5 nJ. The stability of the mode-locking is checked by only tuning the quarter waveplate 333. The tolerance is more than ±10o. The spectrum of the soliton was very close to the typical emission spectrum of Yb doped fiber. The spectrum of the output 34 shown in Fig.4 is already converted from wavelength to frequency. Its bandwidth ^^^ is about 3.35THz. The inverse Fourier transform of the spectrum ^^^ is well fitted to a 95 fs sech2-shape pulse. The time-bandwidth product of the adaptive soliton is ^^^^^^ = 0.318, very close to0.315 for sech2-shaped pulses.

Claims

1 CLAIMS:

1. A method of mode-locking creating frequency comb induced stimulated Brillouin scattering resonance (SBSR) in a laser cavity for passive frequency modulation, said laser cavity comprises: One or more Brillouin scattering elements for generating stimulated Brillouin scattering (SBS) Stokes lines and anti-Stokes lines excited by lasing lines in said laser cavity prior to mode-locking stronger than the noise of said lasing lines at the wavelength positions of said Stokes lines and anti-Stokes lines; a broadband gain medium with gain bandwidth larger than double of the maximal SBS frequency of said Brillouin scattering elements to amplify the SBS signals; and a control of mode separation of said laser cavity for each spectrum linewidth of Stokes lines and anti-Stokes lines in said Brillouin scattering elements excited by any longitudinal mode of said laser cavity covering at least one longitudinal mode of said laser cavity (SBSR condition).

2. A method of mode-locking of claim 1, wherein said laser cavity further includes one or more polarization control elements to make laser polarization self-consistent and minimize the intensity modulation caused by nonlinear polarization evolution (NPE) in said cavity.

3. A method of mode-locking of claim 2, wherein said one or more elements for laser polarization control is selected from the group comprising polarizers, waveplates, and in-line polarization controllers.

4. A method of mode-locking of claim 1, wherein said laser cavity is selected from the group comprising linear cavities constructed with two end mirrors, and ring cavities including an isolator for unidirectional lasing.

5. A method of mode-locking of claim 1, wherein said broadband gain medium is selected from the group comprising rare-earth doped gain fibers, waveguide (other than fiber) gain media, and solid-state gain media for femtosecond (<1ps) pulse generation.2 6. A method of mode-locking of claim 1, wherein said Brillouin scattering element is selected from the group comprising fibers, waveguides other than fibers, free space Brillouin scattering elements, and said broadband gain medium with sufficient SBR effect.

7. A method of mode-locking of claim 1, wherein said laser cavity is simplified by selecting a singular broadband gain medium with sufficient SBS effect as both said broadband gain medium and said Brillouin scattering element for a single element with both functions of providing broadband gain and generating SBS signals.

8. A method of mode-locking of claim 1, wherein said control of mode separation of said laser cavity is selected from the group comprising: a design of said mode separation of said laser cavity, said mode separation is equal to or smaller than the minimal SBS linewidth of said Brillouin scattering elements for said mode separation to intrinsically meet SBSR condition; a control of the optical path length of said laser cavity with mode separation larger than the minimal SBS linewidth of said Brillouin scattering elements for said mode separation to meet SBSR condition.

9. A method of mode-locking of claim 8, wherein said control of the optical path length of said laser cavity is selected from the group comprising micrometer heads, piezoelectric transducers, electro-optic (EO) modulators, and temperature or electro-thermal control to the position or the optical path length of one or more elements in said laser cavity.

10. A soliton laser mode-locked with a method of mode-locking according to any one of preceding claims, said soliton laser comprises: a broadband gain medium pumped by one or more pump lasers for a gain spectrum supporting femtosecond soliton pulse generation; and a segment of fiber as a Brillouin scattering element in a laser cavity to generate SBS signals stronger than the noise of lasing lines in said laser cavity, to make mode separation of said laser cavity equal to or smaller than the SBS linewidth of said segment of fiber for SBSR condition to be intrinsically satisfied, and to dominate dispersion of said laser cavity.

11. A soliton laser of claim 10, wherein said laser cavity further comprises:3 one or more pump laser coupling elements to couple said pump laser for pumping said broadband gain medium; one or more polarization control elements to make laser polarization self-consistent, and to minimize the intensity modulation caused by NPE; one or more laser output coupling elements for laser output; and one or more coupling fibers of said pump laser coupling elements, of said polarization control elements, and of said laser output coupling elements.

12. A soliton laser of claim 11, wherein said coupling fibers are selected from the group comprising the same fiber type as said segment of fiber, and fibers with the same sign of dispersion of said segment of fiber.

13. A soliton laser of claim 10, wherein said segment of fiber is selected from the group comprising polarization maintaining (PM) fibers, and single mode (SM) fibers.

14. A soliton laser of claim 10, wherein said segment of fiber is selected with normal dispersion for bright soliton generation with peak power higher than a threshold determined by the SBSR coefficient and dispersion of said segment of fiber, or for dark soliton generation with peak power lower than said threshold 15. A soliton laser of claim 10, wherein said segment of fiber is selected with abnormal dispersion for bright soliton generation with peak power lower than a threshold determined by the SBSR coefficient and dispersion of said segment of fiber, or for dark soliton generation with peak power higher than said threshold.

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