JP2006332666A - Short pulse amplification in 1 micron based on all fibers - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fiber laser amplifier which is fit to amplify ultrashort femtosecond laser pulses to give them millijoule (mJ) of energy output. <P>SOLUTION: A fiber laser cavity system 100', having a laser gain medium for receiving optical input projection from a laser pump 101, includes a positive diffusion fiber segment and a negative diffusion fiber segment for the generation of net negative diffusion so that self-phase modulation (SPM) and the expansion/compression of diffusion induction pulses are balanced in a fiber laser cavity, to generate output laser in the form of transform-limited pulse. In addition, the laser gain medium has a double-cladded ytterbium-doped photonic crystal fiber (DCYDPCF) 105, which amplifies and compresses laser pulses. The fiber laser cavity system 100' further includes an isolator 135', having polarization sensitivity, and polarization controllers 140-1, 140-2 for further shaping of the output laser. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  This application is a priority application based on US Application No. 11 / 136,040 filed on May 23, 2005 and a US Partial Continuation Application (CIP) based on that application. The present invention relates generally to an apparatus and method for providing a short-pulsed mode-locked fiber laser. More particularly, the present invention relates to a new configuration and method for providing a nonlinear polarization pulse-shaping mode-locked fiber laser having an improved and better controllable pulse shape.

  Prior art techniques for generating short pulse mode-locked fiber lasers face the technical difficulties and limitations of amplifying short pulses such as picosecond to 100 femtosecond pulses to high energy levels such as millijoule levels. ing. Furthermore, the practical usefulness of ultrashort high power lasers is often hampered by pulse shape distortion. In addition, such laser systems are often large, difficult to maintain optical alignment, and lack robustness. All of these difficulties prevent practical applications of ultrashort high power lasers.

  Historically, the generation of mode-locked lasers with pulse widths down to the femtosecond level is a difficult technique due to the limited resources of the saturated absorber and the anomalous dispersion of the fiber. Traditionally, a particular difficulty with short pulse mode-locked fiber lasers operating at wavelengths shorter than 1.3 μm is that there is no simple all-fiber based solution for dispersion compensation in this wavelength regime. (For wavelengths above 1.3 μm, there are several types of fibers that exhibit either normal or anomalous dispersion, and by splicing together different length fibers together, a cavity with tunable dispersion can be obtained. Thus, conventional researchers have used bulk devices such as grating pairs and prisms to provide a tunable amount of dispersion to the cavity. Unfortunately, these devices require fiber coupling to the bulk device, which results in the laser becoming very sensitive to alignment, ie the environment.

  Some prior art discloses various semiconductor saturated absorbers for constructing ultrashort high power laser systems. However, such an arrangement has resulted in the development of systems that are often large and less robust due to the implementation of free space optics. For example, see Non-Patent Documents 1 and 2.

Subsequently, a stretch mode-locked fiber laser is disclosed that further improves the production of short pulse high power lasers. However, even in stretch mode-locked fiber lasers, free-space optical components for collimating and coupling such as quarter-wave retarders and splitters are implemented. For example, see Non-Patent Documents 3 and 4.
Appl. Phys. B, 70, 375-378 (2000), S.A. N. Bagaiev, S.M. V. Cheprof, V.C. M.M. Klemenchev, S.C. A. Kuznetsov, V. S. Pivtsov, V. V. Pokasov, V. F. Zakuharash, “A femtosecond self-mode-locked Ti: sapphire laser with high stability of pulsation repetition and frequencies applications” Science 288, pp. 635-639 (2000), 70, 375-378 (2000), Jones D.C. J, Diddams S. A. Ranka J .; K. Stenz A. Windra R. S. Hall J. L. Kundi (R) S. T. T. et al. , "Carrierenvelopse phase control of femtosecond mode-locked laser and direct optical frequency synthesis" IEEE JOURNAL OF QUANTUM ELECTRONICS Vol. 37, No. 12, December 2001, John L. Hall, Jun Ye, Scott A. Diddams, Ron Shen Ma, Steve T. Kundi (R), David J. Jones, “Ultrasensitive Spectroscopy, the Ultrastable Lasers, the Ultrafast Lasers, and the Serially Nonlinear Fibers: A New Alliance Alliance.” IEEE J.I. Quant. Electon. 37, 1502 (2001), L.L. Holberg, C.I. W. Oates, E.C. A. Curtis, E.C. N. Ivanov, S. A. Diddams, Th. Udem, H.H. G. Robinson, J.H. C. Bergquist, R.D. J. et al. Lafuck, W. M.M. Itano, R.D. E. Dorlinger, D.C. J. et al. Wineland, "Optical frequency standards and measurements"

  The limits for practical applications of this type of laser system become more apparent when pulse width is further reduced due to worsening pulse shape distortion with high power fiber amplification requirements. As the pulse width is reduced to femtosecond levels, the peak power rises to over 10 kW, and strong nonlinear effects such as self-phase modulation (SPM) and XPM can lead to more severe spectral and temporal broadening. become. These non-linear effects and spectral and temporal broadening further introduce a greater degree of distortion into the laser pulse. This technical difficulty is not easily solved even if large mode area (LMA) fiber can be used to suppress SBS and SRS and increase saturation power. However, large mode area fiber also results in suppression of peak power when it is embodied, with undesirable results due to reduced efficiency.

  There is an urgent need to solve these technical difficulties as the broader application and utility of short pulse mode synchronization has been demonstrated for ultrafast phenomena measurement, micromachining, and biomedical applications. . Various techniques have been disclosed that attempt to solve such difficulties. This type of technology includes application of nonlinear polarization rotation (NLPR) or the stretch mode-locked fiber laser described above. Because NLPR handles time domain intensity dependent polarization rotation, pulse shape distortion cannot be prevented due to polarization rotation in both the time domain and the spectral domain. For these reasons, the prior art does not provide an effective system configuration and method that provides an effective ultra-short pulse high power laser system for generating high power laser pulses of acceptable pulse shape.

  In addition to the above difficulties, these laser systems require a grating pair in the laser cavity for dispersion control. Maintaining alignment in such systems is a time consuming task and thus impedes practical applications of systems embodied using free space optics and grating pairs. Lattice pairs also increase the size and weight of the laser device and hinder efforts to miniaturize devices that are implemented using such laser sources.

  Therefore, in the field of fiber laser design and manufacturing, a new and improved providing ultra-short high power mode-locked fiber laser with better controllable pulse shape that can solve the aforementioned difficulties There still exists a need to provide a structured and method. Furthermore, there is a need to amplify short laser pulses to a higher energy level to expand the practicality of this type of laser system.

Accordingly, one aspect of the present invention provides a fiber laser amplifier suitable for amplifying ultrashort femtosecond laser pulses to millijoule (mJ) energy output.
Another aspect of the present invention uses a nonlinear polarization rotation (NPE) and a dispersion managed fiber cavity to manipulate the pulse propagation in that cavity to balance self phase modulation (SPM) and dispersion induced pulse spreading / compression. Provide a way to make it happen. This polarization pulse shaping method can generate the transform limited pulse shape by the synergistic effect of fiber length, nonlinear effect and dispersion to solve the above difficulties faced by the prior art.

  Briefly, in a preferred embodiment, the present invention discloses a fiber laser cavity, the fiber laser cavity including a laser gain medium for receiving an optical input projection from a laser pump. In addition, this fiber laser cavity provides a net negative dispersion to balance self-phase modulation (SPM) and dispersion-induced pulse broadening-compression within the fiber laser cavity to produce a transform limited pulse-shaped output laser. A double-clad-ytterbium-doped photonic crystal that includes a positive dispersion fiber segment and a negative dispersion fiber segment that generate lasers, where the laser gain medium amplifies and compresses the laser pulses Further included is a fiber (DCYDPCF). In a preferred embodiment, the fiber laser cavity has polarization sensitivity to transmit a portion of the laser pulse to a pair of gratings for transmitting optical projections with anomalous dispersion to further shape the output laser. It includes a beam splitter that functions as an isolator. In another preferred embodiment, the fiber laser cavity includes a Faraday rotating mirror to reverse the polarization of the laser from the grating pair. In another preferred embodiment, the fiber laser cavity further includes a polarization sensitive isolator and a polarization controller to further shape the output laser. In another preferred embodiment, the gain medium further includes DCYDPCF having positive dispersion. In another preferred embodiment, the laser cavity is a ring laser cavity. In another preferred embodiment, the gain medium includes DCYDPCF that constitutes a positive dispersion fiber segment having a dispersion of about -55 ps / nm / km. In another preferred embodiment, the fiber laser cavity further includes an output coupler that transmits a portion of the laser as an output laser from the fiber laser cavity. In another preferred embodiment, the fiber laser cavity further includes a single mode fiber that constitutes a negative dispersion fiber segment connected to the gain medium. In another preferred embodiment, the output laser comprises a laser having a pulse width substantially less than 1 femtosecond. In another preferred embodiment, the output laser comprises a laser having a pulse width and Gaussian pulse shape of substantially less than 1 femtosecond. In another preferred embodiment, the output laser comprises a laser having a pulse width and soliton pulse shape of substantially less than 1 femtosecond. In another preferred embodiment, the output laser comprises a laser having a pulse width and hyperbolic pulse shape of substantially less than 1 femtosecond. In another preferred embodiment, the gain medium includes an LMA DCYDPCF further having a large mode area (LMA) in the range of 15 micrometers to 80 micrometers. In another preferred embodiment, the gain medium comprises high concentration LMA DCYDPCF having a ytterbium dopant concentration in the range of 10,000 ppm to 2000,000 ppm. In another preferred embodiment, the DCYDPCF has a single mode fiber large mode area (LMA) structure surrounded by an array of air gaps. In another preferred embodiment, the gain medium comprises an air core DCYDPCF bandgap fiber. In another preferred embodiment, it functions as a polarization sensitive isolator that further transmits the laser beam by further shaping the output laser by transmitting a part of the laser pulse to a roof mirror and a mirror having a reflection grating for transmitting light projection with anomalous dispersion A beam splitter.

  In a preferred embodiment, the present invention further discloses a method for generating a pulsed transform limited output laser from a laser cavity containing a laser gain medium. The method includes forming a laser cavity by employing a positive dispersion fiber segment and a negative dispersion fiber segment that produce a net negative dispersion. This method also amplifies and compresses laser pulses in a gain medium to balance dispersion-induced nonlinearity and self-phase modulation (SPM) in a fiber laser cavity to produce an output laser in a transform limited pulse shape. To do so, further includes projecting the input laser from the laser pump to the fiber laser cavity. In a preferred embodiment, the method further includes a polarization sensitive isolator for transmitting a portion of the laser pulse to a pair of gratings for transmitting optical projections with anomalous dispersion to further shape the output laser. Including a step of employing a beam splitter. In a preferred embodiment, the method further includes employing a Faraday rotating mirror to reverse the polarization of the laser from the pair of gratings. In a preferred embodiment, the method further includes employing ytterbium-doped fiber as a gain medium for laser pulse amplification and compression.

  These and other objects and advantages of the invention will be apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiment, which is illustrated in the accompanying drawings.

  Referring to FIG. 1, a schematic diagram of a nonlinear polarization pulse shaping mode-locked fiber laser 100 of the present invention is shown. The fiber laser is a ring structure laser, ytterbium-doped fiber (YDF) 105 as a gain medium, first and second collimators 135-1 and 135-2, and first and second polarization controllers 140-1 and 140-. 2, 980/1550 wavelength division multiplexing (WDM) coupler 110 and output beam splitter 130 are included. Output beam splitter 130 is coupled to a pair of gratings 145-1 and 145-2 coupled to Faraday rotator mirror 150. 0.5 meters of YDF105 is used as a gain medium in the fiber laser and is used for pulse width amplification and compression. This fiber has a high doping concentration, eg 600 dB / m at 976 nm, with a dispersion of -55 ps / nm / km. A 980 nm high power pump laser diode 101 coupled via a wavelength division multiplexer 110 is used to pump the YDF 105 and amplifies the pulses circulating in the cavity. The remaining part of the cavity is a single mode (SM) fiber, for example having a length of about 3 meters—a fiber 115 of 20 ps / nm / km, and a length of about 0.5 meters, at 1060 nm— Includes HI 1060 fiber 120 commercially available from Corning as a standard fiber with a dispersion of 20 ps / nm / km. The polarization splitter 130 serves as an isolator and is used to couple a portion of light in a predetermined polarization state outside the cavity. The gain medium YDF 105 has a normal dispersion fiber (β ″> 0), the remaining part of the fiber is a negative dispersion fiber (β ″ <0), and the average dispersion of the entire cavity is anomalous dispersion (β ″ Designed to operate at <0) The present invention embodies either lattice pairs or PGB fibers to achieve anomalous dispersion for stable transform limited pulses. Designed to operate in anomalous dispersion (β ″ <0).

  The fiber laser 100 of the present invention achieves a short pulse mode-locked fiber laser in the 1 micron region. N. , Chebotayev V. P. “Frequency Stability and Reproducibility of the 3.391m He-Ne Laser Stabilized on the Methane Line” (Appl. Phys. 1975, Vol. 7, p71). M.M. Jennings D. A. Peterson F. R. This is different from the conventional laser disclosed in “Laser Frequency Measurements: A Review, Limitations, Extension to 197 Thz” (Springer Ser. Opt. Sci., 1977, Vol. 7, p56). In particular, FIG. 1 discloses a laser cavity with a special sigma configuration. The sigma configuration provides the advantage of managing pulse propagation in the cavity while balancing self phase modulation (SPM) and dispersion to reduce saturation effects in the amplification region. On the other hand, NPE induced by nonlinear phase change of SPM makes the polarization in a single pulse intensity dependent. When a pulse passes through a polarization-sensitive splitter, the highest intensity that aligns with the splitter (by adjusting the polarization controller) passes and the lower part of the pulse is filtered, resulting in the shaping of the pulse. This acts as a saturated absorber (SA) and reduces the pulse width. In the 1 micron region, due to the fact that the fiber works only with positive dispersion, negative dispersion is achieved using a pair of gratings 145-1 and 145-2, the value of which is It can be adjusted by changing the separation distance between the two gratings. A Faraday rotator mirror 150 is used to reverse the polarization state to produce a reflected pulse with orthogonal polarization, so that it is propagated in another direction.

  If a laser system configured as shown in FIG. 1 is used with a polarization-sensitive beam splitter 130 coupled with a pair of gratings 145-1 and 145-2, a polarized output laser beam is obtained. Generated. The coupling ratio is adjustable between about 10 +/− 5% for mode-locked fiber lasers. Further, by utilizing dispersion matching and nonlinear polarization rotation, the illustrated laser system is self-starting, thereby greatly simplifying the operation process.

  The polarization shaping mode synchronization technique as disclosed in the present invention includes nonlinear polarization rotation (NLPR) or stretching mode approach as disclosed in Non-Patent Document 2, Non-Patent Document 3, and Non-Patent Document 4 described above. Different from the traditional approach. There are at least three major differences:

  (1) The conventional NLPR technology only considers time domain intensity dependent polarization rotation. The present invention applies polarization rotation of optical transmission and considers variables in both the time domain (intensity dependent) and the spectral domain (wavelength dependent). This is accomplished by selecting a polarizer and a quarter wave plate and a half wave plate (QWR / HWR). Basically, the bandwidth of the retarder is proportional to the index difference Δn of the birefringent material, phase = NΔn / λ, where λ is the wavelength and N is the order of the retarder or birefringent material, for example a fiber. In the differentiation of this equation, it can be seen that the bandwidth Δλ is inversely proportional to the product NΔn. This indicates that the laser system of the present invention can achieve greater bandwidth operation by using low order retarders, eg N = 1, and low birefringent materials. Therefore, the retarder is adjusted so that a larger bandwidth passes through the polarizer or polarization sensitive isolator.

  (2) Although the prior art considers only dispersion matching, the pulse shaping function of the present invention takes into account not only dispersion matching but also dispersion gradient matching, so that dispersion matching is managed over a larger spectral bandwidth. Guarantee. This can be done by using a combination of two or more fibers with different dispersions and gradients, for example, fiber 1 has different dispersion and dispersion gradients and Combining in a length ratio allows the total dispersion to reach zero over a large range in the wavelength region of interest, as shown in FIG. 1A. Accordingly, the present invention provides a laser system that can maximize the gain bandwidth and correspondingly push the pulse width to a minimum because the bandwidth is inversely proportional to the pulse width.

  (3) Conventional laser systems have been implemented with bulk free space optics in the laser system for either dispersion compensation or polarization control. As shown in FIG. 1 and described in further detail below, the present invention is implemented using all fiber based components, eliminating any free space components. Thus, the system as disclosed in the present invention provides the ultimate method in compaction for nano-processing system applications, and an ultrashort pulse laser module.

  FIG. 2 is a schematic diagram illustrating an ultra-compact and low cost all-fiber based high power femtosecond fiber laser system of the present invention. The laser system is constructed using all fiber based components. The fiber laser has a ring configuration that receives laser input via a 980 or 1550 nm WDM 110. The all-fiber-based laser cavity system 100 'is similar in structure to that shown in FIG. 1 and involves a Yb-doped fiber 105 as a gain medium for pulse width amplification and compression. For amplification of intracavity pulses, a telecom grade 980 nm pump laser is used to pump Yb ions. Instead of using grating pairs or prisms, another photonic crystal (PC) fiber 125 is employed to compensate for dispersion and dispersion gradient in the fiber laser cavity. Because the PC fiber 125 can provide both normal and anomalous dispersion in the 1060 nm range with uniquely configured properties, and can manipulate their dispersion gradients, the fiber laser cavity can be used for both dispersion and dispersion gradient matching. It is possible to design with it, which makes it possible to narrow the pulse to the maximum. In contrast to the prior art, the system shown in FIG. 2 provides polarization rotation in both the time domain (intensity dependent) and the spectral domain (wavelength dependent) in achieving ultrashort pulses of less than 50 femtoseconds. Consider. Polarization filtering is achieved by managing both dispersion and dispersion gradient, and by using a fiber-based inline polarization isolator and polarization controller. The all fiber based laser cavity system 100 'employs inline polarization controllers 140-1' and 140-2 'before and after an inline polarization sensitive isolator 135' embodied using a single mode (SM) fiber pigtail. is doing. The control with in-line polarization sensitivity can be one of the products commercially available from General Photonics, for example the PolarRite family of products. Polarization isolator 135 'has a high extinction ratio and allows only one linearly polarized light to pass over a broad spectrum. Due to the non-linear effects of SPM, the refractive index depends on the output intensity, so that in each individual pulse, the high intensity peak experiences different intensity induced birefringence than that experienced by the low intensity wing. become. When the peak polarization and the polarization isolator are aligned, only the peak portion of the pulse can be transmitted and the wing portion is blocked. Thus, by combining polarization shaping and dispersion management, it is possible to mode-lock pulses to femtosecond levels.

  Similar to the laser system shown in FIG. 1, amplification uses a small piece of heavily double clad Yd doped fiber (DCYDF) with large mode area (LMA) 105 shown in FIG. Achieved by: The DCYDF LMA combined with a short length helps balance the dispersion and non-linear effects such as SPM and XPM so that the pulse width does not widen after amplification. DCYDF can be used as a PC fiber, and a balance between dispersion and SPM can be obtained similarly. The laser system shown in FIGS. 2 and 3 has the advantage of being free from alignment and maintenance. It is much easier to handle all-fiber based fiber lasers and amplifiers than conventional mode-locked solid state and / or fiber lasers. There are no alignment and realignment issues related to this. Once the fiber and components are spliced and packaged together, no specially trained technicians are needed for operation and maintenance, significantly reducing costs and risks in field applications. It is also easy to incorporate into another module such as a telescope / focus system without extra optical alignment effort due to the flexibility of the optical fiber. In addition, this laser system utilizes the full spectrum of YDF gain to provide a high quality laser suitable for processing nanomaterials. The laser system is implemented using an all-photonic crystal fiber for both the gain medium and the transmission fiber in the cavity to compensate for both dispersion and dispersion gradient. The illustrated photonic crystal (PC) fiber exhibits new properties in the manipulation of its structure, such as a hollow lattice shape, and in filling factors to obtain normal and anomalous dispersion in the range below 1300 nm. PC fibers are used to compensate for both dispersion and gradient in the cavity by selecting various PC fibers to produce a short pulse fiber laser. In addition, due to one of its unique features of an effective area that is smaller than a regular single mode fiber, a stronger non-linear effect can be produced in the fiber and its impact on SPM can be used to The choice of PC fiber makes it possible to achieve shorter cavities. On the other hand, larger pulse energy can be extracted by using air core PC fiber features.

  As shown in FIG. 3, the high power amplifier YDF 105 is configured to convert the seed pulse input from the high power pump 101 through the pump coupling optical system 110 ′ to an average output of 10 W with the femtosecond ultrashort pulse amplification. Used to boot. The laser pump 101, which can be a 915 or 980 nm pump laser, amplifies very short pulses (picoseconds and femtoseconds) via a pump coupling optics 110 'and a fiber pump coupler (OFS, Somerset, NJ). Used to pump ytterbium ions. This is different from CW (continuous wave) and nanosecond (NS) pulses. Special consideration is required for the effects of SPM, XPM, and FWM. Dispersion must be carefully chosen to match and balance all effects to avoid pulse broadening and distortion in nonlinear short pulse fiber transmission modes.

  Pulse amplification is achieved by using short pieces of heavily double clad-ytterbium-doped photonic crystal fiber (DCYDPCF) 105 having a large mode area (LMA) as shown in FIG. The photonic crystal fiber (PCF) is designed to have a double clad structure to couple other pump outputs to the fiber to produce a high average power fiber laser. It has been shown that the air cladding region can be created by being surrounded by an inner cladding having a silica bridge web that is substantially narrower than the wavelength of the guided radiation. FIG. 4 shows a typical structure of a double clad PCF. According to this structure, a numerical aperture (NA) of about 0.8, which is considerably increased as compared with a general DCLMA fiber, can be obtained. The reduction of the inner cladding means that the overlapping ratio between the effective core and the inner cladding increases. This allows the use of short length fibers to achieve high power amplification.

  The large mode area (LMA) of DCYDF combined with a short length assists the balance between nonlinear effects such as SPM and XMP and dispersion, so that the pulse width does not widen after amplification. DCYDPCF may be a PC fiber, and similarly, a balance between dispersion and SPM can be obtained. The extra chirp of the pulse can be further dechirped by a piece of air-core photonic bandgap (PGB) fiber as shown in FIG. This PGB fiber gives a large anomalous dispersion (120 ps / nm / km) and is available, for example, as model number HC-1060-02 from Danish Crystal Fiber. This PGB fiber has anomalous dispersion, as opposed to a normal fiber at 1 micrometer. It can be used for pulse compensation (dechirping). By manipulating the number and structure of the holes, various dispersions can be obtained. To further improve surface damage, an end cap made of a piece of coreless fiber or glass can be attached to the PBG fiber to increase the mode area of the output beam at the end facet. This makes it possible to amplify picosecond to 100 femtosecond pulses to the millijoule level, as shown in FIG.

  To further improve the performance of distributed control, SPIE 4974, 148 (2003), V.C. As disclosed in Reikel et al., “Applications of pump multiplexed Yb-doped fiber lasers”, special fiber is embodied by manipulating the fill factor of the air hole. This structure is formed by stacking silica capillaries into a hexagonal dense structure and replacing the capillaries with solid silica rods at the center of the stack to form a solid fiber core. The air core is formed in a similar manner, thus forming a fiber commonly known as a photonic bandgap PBG fiber. Figures 5 and 6 show SEM photographs of PCF and changes in dispersion and slope versus hole size, respectively. From these figures it is clear that optimizing the photonic crystal fiber (PCF) structure can flatten the dispersion over a spectral range in excess of 200 nm. There is no possibility of achieving such performance with conventional fibers.

  With the laser system shown in FIG. 2, high power over 10 watts is achieved. Furthermore, the pump output conversion efficiency is as high as 85% with minimal nonlinear effects by using commercially available DCYDF. The beam output of the fiber laser has excellent output beam quality with single mode diffraction limited quality properties where M2 = 1. The gain fiber can be coiled to a small size and packaged with other components of a compact size. A standard product of a 15W fiber laser constructed according to FIG. 2 has all components packaged in a housing housing with dimensions approximately 130 mm × 70 mm × 35 mm. Thus, a compact size laser system is provided that can be conveniently customized.

  The fiber exhibits nonlinear birefringence that depends on the local intensity of the two orthogonally polarized field components. As a result, an elliptically polarized pulse will have two orthogonal components, an x component and a y component. These two components undergo different phase shifts, so that the polarization ellipse rotates. Since this phase shift is an intensity dependent process, it rotates the polarization of the pulse by different amounts depending on the local intensity of the pulse. Figures 7A and 7B show the physical effect of polarization on the pulse. Assuming that the non-linear effect is negligible, if FIG. 7A represents a uniformly polarized pulse launched into an isotropic optical fiber, it is uniformly polarized as illustrated in FIG. 7B. Output pulses are obtained. Thus, by firing the same pulse in the same fiber embodied with the effects of self phase modulation (SPM) and cross phase modulation (XPM), an output similar to FIG. 7B can be generated. When examining FIG. 7B, the low-intensity wings are not affected and a rotation of the polarization ellipse is observed as the pulse intensity increases. Therefore, nonlinear polarization evolution (NPE) induced by nonlinear phase change of self-phase modulation (SPM) causes polarization rotation since the polarization at this time is dependent on the pulse intensity. Thus, a mode-locking mechanism is provided by SPM induced NPE. If this pulse is passed through an isolator with polarization sensitivity controlled and adjusted by the polarization controller, only the highest intensity that aligns with that isolator will pass. The lower intensity part of the pulse is filtered out. As a result, the pulse is well shaped and thus acts as a saturated absorber (SA) that reduces the pulse width. Polarization controller 140 can be fiber-based, or bulk optical quarter-wave / half-wave retarder, or a combination of both. “Polarization sensitive isolators and polarization controllers” serve to select polarization for pulses having different polarization states in the time domain.

  As the pulse circulates in the fiber laser cavity, the laser pulse self-phases in both the negative anomalous single-mode fiber and the positive normal dispersion fiber region due to high peak power and short pulse width (<picoseconds) Experience the pulse spreading effect induced by modulation (SPM). Furthermore, in the positive dispersion region in YDF 105, ie in the region of β ″> 0, the peak power is very high (greater than 200 W for a 200 femtosecond pulse), so the nonlinear length and dispersion in the YDF 105 segment. The length is equivalent, i.e. about 1 m, the pulse can be compressed by using both self-phase modulation (SPM) and dispersion effects on a fiber with a mode field diameter of 10 microns (SPM). An analysis was performed to quantify the non-linear effects of phase change, and Figure 8 shows the results of the analysis, according to which the phase of the light (corresponding to the state of polarization) depends on the light intensity and At a given wavelength, a 3 dB output change can produce a 50% phase change at a given wavelength. In can change 10% of the wavelength produces a phase change of the same amount.

  This gain medium allows the generation of very short pulses of less than 50 femtoseconds, since the gain of Yb doped fibers can cover over 100 nm from 1000 nm to 1100 nm. However, because the polarization state is a function of wavelength (proportional to Δλ / λ and 10% for Yb fiber lasers), different wavelengths will experience different polarization states in the spectral domain. This also affects the pulse width and quality. Furthermore, even though dispersion management can be performed in a specific bandwidth, it is impossible to cover the entire 100 nm bandwidth of this gain medium by using conventional fibers. For the purpose of generating ultrashort laser pulses, dispersion gradient compensation must be taken into account. In conclusion, in order to use the maximum gain spectrum of YDF, dispersion gradient compensation is definitely required along with polarization compensation in the spectral domain. Simulation analysis was performed on fiber with a 10 micron mode field diameter to quantify nonlinear effects on (SPM) phase change. FIG. 8 is a simulation result with the phase of light corresponding to the polarization state, which has a strong functional relationship to the intensity and wavelength of light and a strong dependence on it. At a given wavelength, a 3 dB output change can produce a 50% phase change. At a given power level, a 10% change in wavelength can produce the same amount of phase change.

  By using heavily doped fibers with appropriate dispersion, the system shown in FIGS. 1 and 2 can amplify 1 mW, 100 femtosecond pulses to 100 mW while suppressing the pulse spreading effect. . FIG. 9 shows pulse traces obtained from the autocorrelator before and after amplification. Here it is shown that there is little distortion or spreading effect. If a highly doped double clad fiber is implemented with appropriately selected dispersion, the fiber will allow more pump output to be launched into the fiber, thus amplifying the output to 1 W further. be able to.

  FIG. 10 illustrates another preferred embodiment of the present invention. Instead of using a grating pair as shown in FIG. 11, a roof mirror 145-1 'as shown in FIG. 10 is used, and a grating pair embodied in a conventional laser system. Replace configuration. This roof mirror is used for replacement and to reflect light back onto the grating, which makes it possible to achieve pulse stretching and compression with one grating.

  Although the presently preferred embodiments have been described above, the present invention is not limited to the above disclosure. Those skilled in the art will appreciate that the above disclosure is susceptible to various changes and modifications. Accordingly, the appended claims are to be construed as including all variations and modifications that fall within the true spirit of the invention.

It is a functional block diagram about the short pulse mode synchronous fiber laser of this invention. It is a functional block diagram about the all-fiber short pulse mode-locking fiber laser of this invention. FIG. 2 is a functional block diagram for illustrating a high power amplifier for providing femtosecond laser pulses. It is sectional drawing of the double clad ytterbium dope photonic crystal fiber employ | adopted for FIG. It is SEM imaging sectional drawing of an air core photonic band gap fiber. FIG. 6 is a graph showing the increase in the mode area of the output beam at the end facet face with an end cap made of a piece of coreless fiber or glass attached to a photonic band gap (PBG) fiber. (A) and (B) are waveform diagrams showing changes in polarization when a laser pulse is transmitted across a laser cavity. It is a graph for showing the analysis result of the phase change shown as a function of the optical output. It is a waveform diagram of high-power ultrashort pulse amplification without significant distortion. It is a schematic diagram which shows the lattice structure of this invention. It is the schematic of the usage example of the grating | lattice of a pair structure.

Claims (35)

A fiber laser cavity comprising a laser gain medium that receives an optical input projection from a laser pump,
A positive dispersion fiber that generates a net negative dispersion to balance self-phase modulation (SPM) and dispersion-induced pulse broadening / compression within the fiber laser cavity to produce a transform limited pulse shaped output laser A fiber laser cavity further comprising a segment and a negative dispersion fiber segment, wherein the laser gain medium further comprises a double clad-ytterbium doped photonic crystal fiber (DCYDPCF) for amplifying and compressing laser pulses. 2. The fiber laser of claim 1, further comprising a beam splitter functioning as a polarization sensitive isolator that transmits a portion of a laser pulse to a pair of gratings for transmitting optical projections with anomalous dispersion to further shape the output laser. cavity. 3. The fiber laser cavity of claim 2, wherein the grating pair is optically connected to a Faraday rotating mirror that reverses the polarization of the laser from the grating pair. The fiber laser cavity of claim 1, wherein the output laser is further shaped by a polarization sensitive isolator and a polarization controller. The fiber laser cavity of claim 1, wherein the gain medium comprises the DCYDPCF having positive dispersion. The fiber laser cavity of claim 1, wherein the laser cavity is a ring laser cavity. The fiber laser cavity of claim 1, wherein the gain medium comprises the DCYDPCF comprising a positive dispersion fiber segment with a dispersion of about -55 ps / nm / km. The fiber laser cavity of claim 1, further comprising an output coupler that transmits a portion of the laser as the output laser from the fiber laser cavity. The fiber laser cavity of claim 1, further comprising a single mode fiber comprising a negative dispersion fiber segment connected to the gain medium. The fiber laser cavity of claim 1, wherein the output laser comprises a laser having a pulse width substantially less than 1 femtosecond. The fiber laser cavity of claim 1, wherein the output laser comprises a laser having a pulse width substantially less than 1 femtosecond and a Gaussian pulse shape. The fiber laser cavity of claim 1, wherein the output laser comprises a laser having a pulse width of substantially less than 1 femtosecond and a soliton pulse shape. The fiber laser cavity of claim 1, wherein the output laser comprises a laser having a pulse width of substantially less than 1 femtosecond and a hyperbolic pulse shape. The fiber laser cavity of claim 1, wherein the gain medium comprises the DCYDPCF further having a large mode area (LMA) in the range of 15 micrometers to 80 micrometers. The fiber laser cavity of claim 1, wherein the gain medium comprises a high concentration LMA DCYDPCF having a ytterbium dopant concentration in the range of 10,000 ppm to 2000,000 ppm. The fiber laser cavity of claim 1, wherein the DCYDPCF has a single mode fiber large mode area (LMA) structure surrounded by an array of air gaps. The fiber laser cavity of claim 1, wherein the gain medium further comprises an air-core DCYDPCF bandgap fiber. A beam splitter functioning as a polarization-sensitive isolator for further shaping the output laser by transmitting a part of the laser pulse to a roof mirror and a mirror having a reflection grating for transmitting light projection with anomalous dispersion. Item 1. The fiber laser cavity of Item 1. A method of generating a pulse-shaped transform limited output laser from a laser cavity comprising a laser gain medium, comprising:
Forming the laser cavity by employing a positive dispersion fiber segment and a negative dispersion fiber segment that produce a net negative dispersion;
Projecting an input laser from a laser pump to the fiber laser cavity to which a double clad-ytterbium-doped photonic crystal fiber (DCYDPCF) is applied as the gain medium, and amplifying and compressing laser pulses in the gain medium The method comprising: projecting to balance the dispersion-induced nonlinearity and self-phase modulation (SPM) in the fiber laser cavity to produce an output laser in a transform limited pulse shape. In order to further shape the output laser, the method further comprises adopting a beam splitter as a polarization sensitive isolator for transmitting a portion of the laser pulse to a pair of gratings for transmitting optical projections with anomalous dispersion. The method of claim 19. 21. The method of claim 20, further comprising employing a Faraday rotating mirror that reverses the polarization of the laser from the pair of gratings. 20. The method of claim 19, further comprising employing the DCYDPCF having a large mode area (LMA) in the range of 15 micrometers to 80 micrometers for laser pulse amplification and compression. 20. The method of claim 19, further comprising employing a polarization sensitive isolator and a polarization controller to further shape the output laser. 20. The method of claim 19, further comprising employing the DCYDPCF with positive dispersion as the gain medium for laser pulse amplification and compression. 20. The method of claim 19, further comprising employing the DCYDPCF with positive dispersion having a dispersion of about -55 ps / nm / km as the gain medium for laser pulse amplification and compression. 20. The method of claim 19, further comprising employing an output coupler for transmitting a portion of a laser as the output laser from the fiber laser cavity. 20. The method of claim 19, further comprising connecting a single mode fiber comprising a negative dispersion fiber segment to the gain medium. 20. The method of claim 19, wherein the output laser comprises a laser having a pulse width substantially less than 1 femtosecond. 20. The method of claim 19, further comprising generating the output laser having a pulse width substantially less than 1 femtosecond and a Gaussian pulse shape. 20. The method of claim 19, further comprising generating the output laser having a pulse width and soliton pulse shape that is substantially less than 1 femtosecond. 20. The method of claim 19, further comprising generating the output laser having a pulse width and a hyperbolic pulse shape substantially less than 1 femtosecond. 20. The method of claim 19, further comprising employing a high concentration LMA DCYDPCF having a ytterbium dopant concentration in the range of 10,000 ppm to 2000,000 ppm. 20. The method of claim 19, further comprising employing the DCYDPCF having a single mode fiber large mode area (LMA) structure surrounded by an array of air gaps. 20. The method of claim 19, further comprising employing the gain medium further comprising an air core DCYDPCF band gap fiber. In order to further shape the output laser, a beam splitter that functions as a polarization-sensitive isolator that transmits part of the laser pulse to a mirror that has a roof mirror and a reflection grating for transmitting light projection with anomalous dispersion is adopted. 20. The method of claim 19, further comprising:

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