KR20060121735A - All fiber based short pulse amplification at one micron - Google Patents

Abstract

An all fiber based short pulse amplification apparatus in a micro unit and a method thereof are provided to output high energy of milli-joule energy level by amplifying an ultra short femto-second laser pulse. A laser gain medium accommodates optical input emitted from a laser pump, and includes a double cladding Ytterbium doped photonic crystalline fiber(DC YDPCF)(105) amplifying and compressing a laser pulse. A positive dispersion fiber part and a negative dispersion part generate net negative dispersion for balance of expansion/reduction of self phase modulation(SPM) and dispersion induced pulse, in order to generate an output laser having a deformation-restricted pulse shape.

Description

Fiber based short pulse amplification device and amplification method in micro unit {ALL FIBER BASED SHORT PULSE AMPLIFICATION AT ONE MICRON}

1 is a functional block diagram of a short-pulse mode-locked fiber laser system in accordance with one embodiment of the present invention.

2 is a functional block diagram of a fiber based short pulse mode locked fiber laser according to one embodiment of the invention.

3 is a functional block diagram illustrating a high power amplifier for providing laser pulses in femtoseconds.

FIG. 4A is a cross-sectional view of the ytterbium doped photonic crystal fiber of the double coated optical mode region employed in FIG. 3.

4B is a cross-sectional view of a photon bandgap fiber with an air core.

4C is a graph showing the upper limit when a seamless fiber or glass attached to the photon band gap fiber is used to increase the mode area of the output beam at the end face.

5A and 5B are waveform graphs of polarization change that appear as a laser pulse is delivered to a laser resonator.

6 is a graph illustrating an analysis result of phase change according to light output.

7 is a graph showing waveforms of high power ultrashort pulse amplification without large distortion.

8 is a schematic cross-sectional view of a grating structure according to an embodiment of the present invention.

9 is a functional block diagram illustrating a grid pair.

<Explanation of symbols for the main parts of the drawings>

100, 100 ': Laser system 110: Wavelength division multiplexing coupler

130: Output beam scattering machine 140-1, 140-2: Polarization regulator

145-1, 145-2: Grid pair 150: Faraday rotating mirror

The present invention relates to a device for supplying a short-pulse mode-locked fiber laser and a method for supplying the short-pulse mode-locked fiber laser. More specifically, the present invention relates to apparatus construction and methods for supplying nonlinear polarization pulse-shaping mode-locked fiber lasers with improved and more adjustable pulse shapes.

Techniques for generating conventional short pulse mode locked fiber lasers amplify short pulses, for example from picoseconds (ps) to about 100 femtoseconds (fs), resulting in high energy levels, for example, milli Joules. In this regard, there are still technical difficulties and limitations. Moreover, practical applications of lasers with ultra-short pulses and high voltages are difficult to achieve. Specifically, the ultra-high power lasers are often hampered by their pulse shape distortion. In addition, such laser systems are sometimes bulky, difficult to maintain an aligned state of light, and are difficult to make sufficiently robust. These problems result in the practical application of the ultra-high power laser.

Historically, the generation of mode locked lasers with small pulse widths at the femtosecond level has been a difficult task due to the limitation of resources to be used as saturated absorbers and the irregular dispersion of fibers. Conventionally, short pulse mode locked fiber lasers operating at wavelengths of about 1.3 μm or less have been a particular challenge in that no solution to the simple dispersion compensation applied for all fibers in this wavelength range is available (about At wavelengths above 1.3 μm, there are several types of fibers that exhibit either normal or irregular dispersion, so that fibers with different wavelengths can be combined to obtain a cavity with an applicable dispersion). Previous researchers have therefore used bulky devices such as grating pairs or prisms that provide an amount of dispersion applicable to the resonator. Unfortunately, however, such devices need to couple the fibers to the bulky device, which results in the laser being very sensitive to the alignment of the fibers and thus the environment.

Several prior arts disclose various semiconductor saturable absorbers that make up the ultra-high power laser system. However, the configurations of the ultra-high voltage laser systems have often been developed into bulky and not robust systems using free space optics. s. yen. Femtosecond Self-Mode Locking Titanium Sapphire Laser with High Stability Pulse Repetition Frequency and Its Applications (Baggaev et al. A femtosecond self-mode-locked Ti: sapphire laser with high stability of pulserepetition frequency and its applications, Appl. Phys. B, 70, 375-378 (2000).) Or Jones D. second. Phase adjustment and optical frequency synthesis (Carrierenvelope of carrier-envelope of femtosecond-mode locked lasers by Jones DJ, Diddams SA, Ranka JK, Stentz A., Windeler RS, Hall JL, Cundi® ST). phase control of femtosecond mode-locked laser and direct optical frequency synthesis, Science, vol. 288, pp. 635-639, 2000.

In addition, stretched mode locked fiber lasers are disclosed to improve the method of producing the short pulse high power laser. However, even in the extended mode locked fiber lasers, free space optical components such as a quarter wave retarder or a spectroscope for spectroscopy or coupling are used. Examples of the extended mode locking fiber laser system are described in John El. Written by John L. Hall, Jun Ye, Scott A. Diddams, Long-Sheng Ma, Steven t. Cundi®, and David J. Jones. Ultrasensitive Spectroscopy, the Ultrastable Lasers, the Ultrafast Lasers, and the Seriously Nonlinear Fiber: A New Alliance for Physics and Metrology, IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 37, NO. 12, DECEMBER 2001 ) "Or El. "Optical frequency standards and measurements by L. Hollberg, CW Oates, EA Curtis, EN Ivanov, SA Diddams, Th. Udem, HG Robinson, JC Bergquist, RJ Rafac, WM Itano, RE Drullinger and DJ Wineland (Optical frequency standards and measurements, IEEE U. Quant. Electron. 37,1502 (2001).).

The limitations of the practicality of the laser systems are more and more revealed in order to meet the requirement of high power fiber amplification, as the pulse shape distortion appears when the pulse width is further reduced. When the pulse width is reduced to the femtosecond level and the peak output is increased to more than 10 Hz, strong nonlinear effects such as Self Phase Modulation (SPM) and Cross Phase Modulation (XPM) are introduced. It makes the broadening of spectrum and time even worse. These nonlinear effects and widening of spectrum and time further exacerbate the degree of distortion for the laser pulse. The technical problems described above include: Large Mode Area (Stimulated Brillouin Scattering (SBS)) and promoted Raman Scattering (SRS) to reduce saturation power and increase the saturation power; Although LMA) fibers can be used, they are not easily resolved. In addition, the use of the optical mode region fiber results in a reduction in the peak power and undesirable effects due to a decrease in efficiency.

As the short pulse mode locked fiber lasers have been found to have wider applications and their usefulness in the measurement of the field of rapid flux, microfabrication and physiology, it is an urgent task to solve the above-mentioned technical problems. . Several other techniques have been disclosed to solve the above problems, including the use of NonLinear Polarization Rotation (NLPR) or the already mentioned extended mode locking fiber lasers. Since the nonlinear polarization rotation deals with the polarization rotation depending on the intensity of the time domain, the distortion of the pulse shape due to the polarization development in both the time domain and the spectral domain cannot be prevented. For this reason, the prior art does not properly devise a configuration and method for providing efficient ultrashort pulsed high power laser systems that produce high power laser pulses with an appropriate pulse shape.

In addition to the problems mentioned above, these laser systems require grating pairs for dispersion control in the laser resonator. Maintaining alignment in these systems is becoming a time consuming task, thus preventing systems using free space optics and grating pairs from becoming practical. In addition, the grating pairs further increase the size and weight of the laser devices, hindering efforts to miniaturize devices using such a laser source.

Therefore, there is a need in the fiber laser design and manufacturing field to provide a new and improved configuration and method for producing ultra-high power mod lock fiber lasers with a more easily adjustable pulse shape to address the above mentioned problems. It remains entirely.

It is therefore an object of the present invention to provide a fiber laser amplifier that amplifies an ultrashort femtosecond laser pulse and outputs high energy at the milli Joule energy level.

Another object of the present invention is to provide a nonlinear polarization evolution (NPE) and dispersion adjustment fiber resonator to adjust the pulse propagation in the resonator and to balance self phase modulation (SPM) and dispersion induced pulse zooming. To provide a way to use it.

According to one embodiment of the invention, the present invention discloses a fiber laser resonator comprising a laser gain medium for receiving optical input projection from a laser pump. The fiber laser resonator generates net negative variances to balance magnetic phase modulation (SPM) and zooming of dispersion induced pulses internally to produce an output laser having a pulse shape with limited strain. A positively dispersed fiber portion and a negatively dispersed fiber portion. Here, the laser gain medium further comprises double coated, Yttb-doped photonic crystal fibers (DC YDPCF; hereinafter referred to as double coated ytterbium doped fibers) to amplify and compress the laser pulses. do.

According to an embodiment of the present invention, the fiber laser resonator functions as a polarization high sensitivity insulator for transmitting a portion of the laser pulse to a pair of gratings that transmits light output having irregular dispersion to form the shape of the output laser. It further comprises a beam scatterer.

According to another embodiment of the present invention, the fiber laser resonator further includes a Faraday rotating mirror that reverses the polarization of the laser emitted from the grating pair.

According to another embodiment of the present invention, the fiber laser resonator further includes a polarization high sensitivity insulator and a polarization controller to form another shape of the output laser.

According to another embodiment of the invention, the gain medium further comprises double coated ytterbium doped fibers (DC YDPCF) with positive dispersion.

According to another embodiment of the invention, the laser resonator is a ring-shaped laser resonator.

According to another embodiment of the invention, the gain medium comprises double coated ytterbium doped fibers (DC YDPCF) which constitute a positively dispersed fiber portion with a dispersion of 55 ps / nm / km.

According to another embodiment of the present invention, the fiber laser resonator further includes an output coupler for transmitting a portion of the laser from the fiber laser resonator as the output laser.

According to yet another embodiment of the invention, the fiber laser resonator further comprises single mode fibers connected to the gain medium to form a fiber portion with negative dispersion.

According to another embodiment of the present invention, the output laser comprises a laser having a pulse width of about 1 femtosecond or less.

According to another embodiment of the invention, the output laser comprises a laser having a pulse width of about 1 femtosecond or less and a Gaussian pulse shape.

According to another embodiment of the invention, the output laser comprises a laser having a pulse width of less than about 1 femtosecond and a soliton pulse shape.

According to another embodiment of the invention, the output laser comprises a laser having a pulse width of less than about 1 femtosecond and a hyperbolic pulse shape.

According to another embodiment of the present invention, the gain medium further comprises double coated ytterbium doped fibers having a light mode region (LMA) in the range of about 15 μm to about 80 μm.

According to another embodiment of the invention, the gain medium further comprises double coated ytterbium doped fibers having a light mode region (LMA).

According to another embodiment of the present invention, the gain medium comprises a high concentration double coated ytterbium doped fiber having a concentration of ytterbium (Yb) in the range of 10,000 ppm to 2,000,000 ppm (ppm).

According to an embodiment according to another aspect of the present invention, a method of generating an output laser with limited pulse shape deformation from a laser resonator comprising a laser gain medium is disclosed. The method includes forming the laser resonator by employing a positively dispersed fiber portion and a negatively dispersed fiber portion that produces a net negative dispersion. The method further includes emitting an input laser from a laser pump to a fiber laser resonator to amplify and compress a laser pulse in the gain medium. In this step, the dispersion induced nonlinearity with self phase modulation (SPM) is balanced in the fiber laser resonator to produce an output laser having a limited pulse shape.

According to one embodiment of the present invention, the method comprises a beam scatterer functioning as a polarization high sensitivity insulator for transferring a portion of the laser pulses to a grating pair that transmits light output with irregular dispersion for shaping the output laser. And further employing.

According to one embodiment of the invention, the method further comprises employing a Faraday rotating mirror that reverses the polarization of the laser emitted from the grating pair.

According to one embodiment of the invention, the method further comprises employing ytterbium (Yb) doped fibers as a gain medium for the amplification and compression of the laser pulses.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings, but the present invention is not limited to the following embodiments, and those skilled in the art will appreciate the technical features of the present invention. The present invention may be embodied in various other forms without departing from the spirit thereof.

1 is a functional block diagram of a short-pulse mode-locked fiber laser system in accordance with one embodiment of the present invention.

Referring to FIG. 1, the configuration of a nonlinear polarized pulse mode locked fiber laser system 100 in accordance with one embodiment of the present invention is schematically illustrated. The fiber laser system 100 has an annular structure, Ytterbium Doped Fiber (YDF) 105, first and second spectrometers 135-1 doped with ytterbium (Yb) used as a gain medium. 135-2), first and second polarization regulators 140-1 and 140-2, 980/1550 Wavelength Division Multiplexing (WDM) coupler 110 and output beam scatterer 130 do. The output beam scatterer 130 is used with grating pairs 145-1 and 145-2 used in conjunction with the Faraday rotating mirror 150. Ytterbium doped fiber (YDF) 105 of about half a meter is used as a gain medium in the fiber laser system and is used to increase or decrease the pulse width. Ytterbium doped fiber (YDF) 105 has a high concentration of doping concentration, for example, about 600 mW / m at a wavelength of about 976 nm and a dispersion of about 55 ps / nm / km. The high power pump laser diode 101 of about 980 nm combined with the wavelength division multiplexing coupler 110 is used to pump the ytterbium doped fiber (YDF) 105 to amplify the pulses circulating the resonator. The resonator may be, for example, a single mode fiber (SM) 115 having a dispersion amount of about 20 ps / nm / km and a length of about 3 m and a 20 ps / nm at a wavelength of about 1060 nm. A standard fiber having a dispersion of about / km, manufactured and sold by Corning, Inc., and includes HI fiber 120 having a length of about half a meter (m). The beam scatterer 130 provides the function as an insulator used for combining with a portion of the light exiting the resonator in a given polarization state. The gain medium 105 is composed of fibers having a normal amount of dispersion (β> 0) and fibers having a negative amount of dispersion (β <0), and the average amount of dispersion of the entire resonator is an irregular amount of negative dispersion. It is designed to have (β <0). The present invention employs the grating pairs or Photonoc Band Gap (PBG) fibers to obtain the irregular amount of dispersion that produces a pulse that is stably limited in conversion. The average dispersion amount of the entire resonator is designed to have an irregular negative dispersion amount β <0.

Fiber laser system 100 according to the present invention is Bagayev S. yen. "Frequency Stability and Reproducibility of the 3.39m He-Ne Laser Stabilized on the Methane Line, Appl.", By Bagayev SN, Chebotaiev V, P. Phys., 1975, v.7, p.56) or "Laser Frequency Measurement: 197 Terahertz Review, Limitations," by Ebenson KM, Jennings DA, Peterson FR et al. Short pulse mode locked fiber lasers in the 1 micron region as described in “Laser Frequency Measurements: A Review, Limitations, Extension to 197 Thz, Springer Ser. Opt. Sci., 1997, V.7, p.56”. It is different from conventional lasers in that it can be obtained. In particular, FIG. 1 discloses a special laser resonator such as a sigma configuration. The sigma configuration has advantages in terms of adjusting the pulse propagation in the resonator and in balancing the dispersion with the magnetic phase modulation (SPM) to reduce saturation effects in the amplification region. On the other hand, Nonlinear Phase Evolution (NPE) induced by the nonlinear phase change of the magnetic phase modulation (SPM) causes the polarizations to depend on a single pulse intensity. When the pulse passes through the polarization high sensitivity scatterer (beam scatterer) 130, only the highest intensity aligned with the polarization high sensitivity scatterer 130 passes (adjust the polarization adjusters 140-1, 140-2). By this), the portion having the low intensity among the pulses is filtered to form the pulse. It functions as a Saturation Absorber (SA) and reduces the pulse width. Due to the fact that in about 1 micro area the fiber operates with only positive dispersion, grating pairs 145-1 and 145-2 are used to obtain negative dispersion and grating pairs 145-1 and 145 The dispersion amount can be adjusted by changing the distance between -2). The Faraday rotating mirror 150 may be used to invert the polarization state so that the reflected pulses are in an orthogonal state and travel in different directions.

In the laser system configuration shown in FIG. 1, a polarized output laser beam is generated through polarization high sensitivity beam splitter 130 coupled with grating pairs 145-1 and 145-2. The mode locked fiber lasers are adjusted at a coupling ratio of about 10 W 5%. In addition, using dispersion matching and nonlinear polarization evolution, the laser system enables self-starting with a very simplified operation process.

Polarization mode locking techniques in accordance with one embodiment of the present invention, as already mentioned John L. John L. Hall et al., L. Holberg et al. And S. a. It differs from conventional approaches such as nonlinear polarization rotation (NLPR) or extended mode approach disclosed by S. A. Didamms et al., With at least three main differences as follows.

First, conventional nonlinear polarization rotation (NLPR) techniques only consider polarization rotation that depends on the time domain intensity. However, according to one embodiment of the invention, the polarization evolution of the optical transmission takes into account not only the change in the time domain (intensity dependent) but also the change in the spectral domain (wavelength dependent). This can be done by selecting a polarizer and a quarter wave plate and a half wave plate (HWP). Basically, the bandwidth of the phase retarder is proportional to the refractive index difference Δn of the birefringent material and satisfies the equation phase = NΔn / λ. Where? Is the wavelength and N is the coefficient of the birefringent material such as the phase retarder or fiber. By differentiating the above equation, it can be seen that the bandwidth Δλ is inversely proportional to the NΔn. This indicates that the laser system according to the invention can obtain a larger bandwidth by using a lower coefficient, for example N = 1, in the phase retarder or birefringent material. Thus, the phase retarders have a larger bandwidth and are adjusted to pass through the polarizer or polarization high sensitivity isolator.

Second, while the prior arts only consider dispersion matching, according to one embodiment of the present invention, not only the dispersion matching but also the dispersion gradient matching, so that the matching of the dispersion is adjustable over a larger spectral bandwidth. This can be done using a combination of two or more fibers with different dispersion amounts and dispersion slopes. For example, fibers having different amounts of dispersion and dispersion slopes can be combined at an appropriate length ratio so that the overall amount of dispersion in the desired wide area is near zero. Therefore, according to an embodiment of the present invention, the bandwidth is inversely proportional to the pulse width, thereby providing a laser system that minimizes the pulse width while maximizing the gain bandwidth.

Third, conventional laser systems employ bulky free space optics for dispersion compensation or polarization adjustment. However, according to one embodiment of the present invention, all are driven by fiber-based components and the free space optics components are excluded, resulting in the ultimate method of making dense ultra-short pulsed laser modules that can be applied to nanoscale process systems. This is provided.

2 is a functional block diagram of a fiber based short pulse mode locked fiber laser according to one embodiment of the invention.

2, the fiber laser system 100 'according to one embodiment of the present invention is all fiber based components. The fiber laser system 100 ′ has a ring structure for receiving an input laser through a wavelength division multiplexing coupler 110 having a wavelength of about 980 nm or about 1,550 nm. The fiber laser system 100 'has a structure similar to that of the fiber laser system 100 having the ytterbium (Yb) doped fiber (YDF) 105 shown in FIG. 1 as a gain medium for amplifying or reducing the pulse width. A telecom grade 980 nm pump laser is used to pump ytterbium (Yb) for the amplification of pulses inside the resonator. To compensate for dispersion and dispersion slope in the resonator, photonic crystal (PC) fibers 125 are used instead of grating pairs or prisms. Since the photonic crystal fiber 125 has a unique structural characteristic in the wavelength range of about 1,060 nm, not only can provide both regular or irregular dispersion amount, but also the dispersion slope can be adjusted, so that both dispersion amount and dispersion slope match. The fiber laser resonator can be designed so that the pulse width can be narrowed to the maximum. In contrast to conventional prior arts, the system shown in FIG. 2 is dependent on wavelength in the spectral domain as well as polarization evolution in the time domain (intensity dependent) to obtain ultrashort pulses of less than about 50 femtoseconds. Consider polarization development. This polarization filtering is performed by adjusting the amount of dispersion and the slope of the dispersion or further using fiber based inline polarization isolators and polarization regulators. The fiber based laser system 100 ′ employs inline polarization regulators 140-1 and 140-2 before and after the inline polarization high sensitivity insulator 135 ′ that operates with single mode fiber connection wires. . In-line polarization high sensitivity regulators 140-1 and 140-2 can be selected from General Photonics' polarite family. The polarization isolator 135 'has a high extinction ratio and allows only one linear polarization to pass through a wide spectral range. Because of the nonlinear effects of magnetic phase modulation (SPM), the refractive index depends on the output intensity, so within each pulse the high intensity peaks experience different birefringence than the low intensity wing portion suffers. When aligning the peak polarization with the polarization isolator 135 ', only the peak portion of the pulse can be delivered and the wing portion is blocked. Therefore, the pulse can be mode locked in femtosecond units through polarization formation and dispersion adjustment.

3 is a functional block diagram illustrating a high power amplifier for providing femtosecond laser pulses according to an embodiment of the present invention.

2 and 3, similar to the fiber laser system 100 shown in FIG. 1, a high density double cladding Yb Doped Fiber (DCYDF) having a light mode region (LMA) 105 ( Using the short section of 105), the amplification is performed. The optical mode region (LMA) of the short-length double coated ytterbium doped fiber (DCYDF) 105 allows to balance the dispersion with nonlinear effects such as magnetic phase modulation (SPM) or cross phase modulation (XPM). This prevents the pulse width from widening after amplification. The double coated ytterbium doped fiber (DCYDF) 105 may use photonic crystal (PC) fibers to balance the dispersion and nonlinear effects. The fiber laser system 100 ′ shown in FIGS. 2 and 3 has the advantage that there is no problem in alignment or maintenance. It is much easier to handle fiber based fiber lasers and amplifiers according to the present invention than to handle conventional mode locked solid state or fiber lasers. There is no sorting or reordering problem here. After the fibers and components are bonded together and packaged together, there is no need for specially trained technicians to operate or maintain them, thus having the effect of significantly reducing costs and risks in practical applications. In addition, the resiliency of the optical fibers allows for easy coupling with other modules such as telephoto systems or focus systems without special optical alignment efforts. In addition, the laser system 100 'takes full advantage of the gain spectrum of the ytterbium doped fiber (YDF) 105 to provide a high quality laser suitable for processing on nanoscale materials. The laser system 100 'operates using photonic crystal (PC) fibers for both the gain medium and the transfer fiber in the resonator to compensate for both the dispersion and dispersion slope. The photonic crystal (PC) fibers show new properties in controlling structures such as hollow lattice shapes and filling factors in order to obtain both normal or irregular dispersion in the range of about 1,300 nm or less. The photonic crystal (PC) fibers are used to select both various photonic crystal (PC) fibers, thereby compensating both the amount of dispersion and the slope of the dispersion in the resonator and to make a short pulse fiber laser. In addition, stronger nonlinear effects may occur within the fiber due to the unique property of having smaller effective area than typical single mode fibers, and the nonlinear effects on magnetic phase modulation (SPM) select appropriate photonic crystal (PC) fibers. This is used to obtain a shorter resonator. On the other hand, by utilizing the properties of the air core photonic crystal (PC) fibers, a larger pulse energy can be extracted.

As shown in FIG. 3, the high power amplifier ytterbium-doped fiber (YDF) 105 boots the seed pulses introduced from the high power pump 101 through the pump coupling optical system 110 ′ to produce ultra-high femtosecond units. However, pulse amplification allows the average power to reach 10W. The laser pump 101 may be a laser pump of about 915 nm or about 980 nm, and the amplification of ultrashort pulses in picoseconds or femtoseconds may be performed through coupling optics 110 'or a fiber pump coupler. In order to do this, it is used to pump ytterbium ions. This is different from pulses in Continuous Wave (CW) and NanoSecond (NS) units. Here, the effects of magnetic phase modulation (SPM), cross phase modulation (XPM), and Four Wave Mixing (FWM) must be specifically considered. In order to avoid widening and distortion of the pulse in the delivery modes of the nonlinear short pulse fiber, the dispersion must be chosen carefully so that all effects are matched and balanced.

Amplification of the pulses is accomplished using a fragment of a high concentration double coated ytterbium doped photonic crystal fiber (DC YDFCF) 105 having a light mode region (LMA), as disclosed in FIG. 4A. Photonic crystal fibers (PCFs) are designed to have double coating properties in order to add a separate pumping power to the fiber to produce a fiber laser with a higher average power. It has been known that air coverage areas can be created by enclosing the interior coating with a network of silica bridges that are narrower than the wavelength of the induced radiation. 4A shows a design of a typical double coated photonic crystal fiber. In the present invention, the Numerical Aperture (NA), which is significantly increased by about 0.8, may be comparable to existing double coated optical mode region fibers. Reducing the inner sheath means increasing the overlap ratio for the inner sheath of the active core. This allows the use of shorter lengths in the fiber for higher power amplification.

The optical mode region of the short coated ytterbium doped photonic crystal fiber (DC YDPCF) coupled to short lengths helps to balance the dispersion with nonlinear effects such as self phase modulation (SPM) and cross phase modulation (XPM) Do not allow the width to expand after amplification. The double coated ytterbium doped photonic crystal fiber (DC YDPCF) may be a common double coated fiber in that it balances the dispersion and self phase modulation (SPM). The extra chirp of the pulses can be further eliminated by fragments of air core photon band gap (PBG) fibers, as shown in FIG. 4B, which can be, for example, Crystalline fibers of the Danish part # HC-1060-02 model provide large irregular dispersion up to 120 ps / nm / ks. The concentric photon band gap (PBG) fibers have irregular dispersion. This is the opposite of the general fiber when about 1㎛. The concentric photon band gap (PBG) fibers can be used to compensate for (pulling) the pulses. By controlling the number and structure of air holes, various dispersions can be obtained. To further improve surface damage, the ends of the seam-free fibers or pieces of glass can be attached to the co-concentric photon band gap (PBG) fibers, thereby increasing the mode area of the output beam at the ends. This makes it possible to amplify pulses at picoseconds to about 100 femtoseconds to milli Joules, as shown in FIG. 4C.

In order to further improve the dispersion adjustment performance, As disclosed in "Applications of pump multiplexed Yb-doped fiber lasers (SPIE 4974, 148 (2003))" by V. Reichel, et al. Special fibers work by adjusting the filling factor that fills the air holes. The structure of the fiber is formed by stacking siliconized capillaries to form a hexagonal close packed structure and replacing the central portion of the stacked capillary with solid silicon rods to form a solid fiber core. Thus, the air core is formed by a method similar to the method of forming fibers, commonly known as photon band gap (PBG) fibers. 10A and 10B show scanning electron microscopy (SEM) images of photonic crystal fibers and show the dispersion and tilt changes with respect to the pore size. This clearly shows an optimized photonic crystal fiber structure, where the dispersion can be uniform in the region of the spectrum exceeding 200 μm. There is no possibility of obtaining these results with conventional fibers.

In the fiber laser system 100 ′ shown in FIG. 2, a high power of 10 W or more is produced. Using the double coated ytterbium doped fibers (DCYDF), the pump output conversion efficiency is about 85% by minimizing nonlinear effects. The beam power of the fiber laser has excellent beam quality in the nature of single mode diffraction, where the beam quality coefficient M satisfies M ² = 1. The gain fibers can be packaged in small dimensions with other components of compact size. In a standard fiber laser product of about 15 W having the configuration shown in FIG. 2, all components are packaged and housed in a container measuring approximately 130 mm 70 mm 35 mm. Thus, a compact laser system can be provided which can be conveniently changed in settings.

5A and 5B are waveform graphs of polarization changes that appear as a laser pulse is delivered to a laser resonator.

The fiber exhibits a nonlinear birefringence that depends on the local strength of the two orthogonally polarized field components. As a result, an elliptically polarized pulse has two orthogonal components, two components x and y. These two components experience different phase shifts, thus rotating the polarization ellipsoid. Since the phase shift is a process that depends on the intensity, the phase shift rotates the polarization of the pulse in different amounts depending on the local intensity of the pulses.

5A and 5B, the physical effects of polarization on pulses can be seen. Neglecting nonlinear effects, FIG. 5A shows uniformly polarized pulses oscillating with isotropic optical fibers and FIG. 5B shows uniformly polarized output pulses. Thus, by oscillating the same pulse to the same fiber on which the effects of self phase modulation (SPM) and cross phase modulation (XPM) work, a pulse similar to the output pulse shown in FIG. 5B is generated. Referring to FIG. 5B, the wing portion of low intensity is not affected, but as the intensity of the pulse increases, the rotation of the polarized ellipse is observed. Thus, since the polarization depends on the pulse intensity, nonlinear phase evolution (NPE) induced by nonlinear phase change of self phase modulation (SPM) causes polarization rotation. That is, the mode locking mechanism is caused by nonlinear phase evolution (NPE) induced by the self phase modulation (SPM).

Referring again to FIG. 2, when the pulse passes through the polarization high sensitivity insulator 135 ′ that is adjusted by the polarization regulators 140-1, 140-2, the polarization high sensitivity isolator 135 ′ and Only pulses of the highest intensity aligned are passed. The low intensity portion of the pulse is filtered. Thus, the pulse has a desirable shape and serves as a saturable absorber SA that reduces the pulse width. The polarization regulators 140-1 and 140-2 may be fiber based, bulky optical quarter or half wave phase retarders, or a combination of both. The polarization high sensitivity insulator 135 'and the polarization adjusters 140-1 and 140-2 serve to select polarizations of the pulses having different polarization states in the time domain.

When the pulse circulates through the pulsed laser resonator, the laser pulse is self phase modulated (SPM) in both the negative irregular single mode fiber region and the positive normally distributed fiber region because of the high power peak and short pulse width of less than picoseconds. Suffer from the pulse broadening effect induced by And in the ytterbium doped fiber (YDF) 105, in the positive dispersion region, i.e., the region of β > 0, because the peak power is very large (greater than about 200 W for a 200 femtosecond pulse), the nonlinear length and dispersion length Is approximately equal to about 1 m. The pulse can be compressed using both magnetic phase modulation (SPM) and dispersion effects.

6 is a graph illustrating an analysis result of phase change according to light output. In FIG. 6, the results of quantitative analysis for the effect of nonlinear change of phase by the self phase modulation (SPM) of a fiber having a mode region diameter of 10 μm are shown.

Referring to FIG. 6, it can be seen that the phase of light (corresponding to the polarization state) is highly dependent on the intensity and wavelength of the light. At a given wavelength, an output change of about 3 Hz can cause about 50% of the phase change. At a given output level, a wavelength change of about 10% can cause the same amount of phase change.

Since the gain of the ytterbium doped fibers can cover more than 100 nm from 1,000 nm to 1,100 nm, the gain medium allows the generation of very short pulses of less than approximately 50 femtoseconds. However, since the polarization state is a function of wavelength (which is about 10% for ytterbium fiber lasers proportional to Δλ / λ), in the spectral region, pulses of different wavelengths will experience different polarization states. This in turn affects the pulse width and the quality of the pulse. In addition, dispersion adjustments can also be made at certain bandwidths, so conventional fibers cannot be used to cover the entire 100 nm bandwidth of the gain medium. In order to generate ultrashort laser pulses, dispersion slope compensation must also be considered. In conclusion, in order to utilize the maximum gain spectrum of the ytterbium doped fiber (YDF), the dispersion slope compensation is necessary in combination with the polarization compensation in the spectral region. Simulation analysis on fibers with a mode region diameter on the order of 10 μm was performed to quantify the nonlinear effects on the (self phase modulation) phase change. In FIG. 6, simulation results are shown for the phase of light according to the polarization state which has a strong functional relationship to the intensity of the light and the wavelength and which is highly dependent thereon. An output change of 3 dB at a given wavelength can produce as much as 50% of the phase change. At a given output level, a change of about 10% of the wavelength can cause the same amount of phase change.

7 is a graph showing waveforms of high power ultrashort pulse amplification without large distortion.

Referring to FIG. 7, using heavily doped fibers with proper dispersion, the laser systems shown in FIGS. 1 and 2 amplify 1 ㎽ 100 femtosecond pulses and convert them into pulses of 100 갖는 with a small broadening effect. Can be. Specifically, the pulse traces before and after the amplification extracted from the autocorrelator are shown in FIG. 7, and it can be seen that there is little distortion or wide area effect. Since the fiber allows more pump output oscillation to the fiber, it is possible to use double coated high concentration doped fibers with a properly chosen dispersion to further amplify the output to 1W.

8 is a schematic cross-sectional view of a grating structure according to an embodiment of the present invention, and FIG. 9 is a functional block diagram showing a grating pair.

8 and 9, in the present invention, instead of using the grating pair as shown in FIG. 9, the roof mirror 145-1 'is used in place of the grating pair used in the conventional laser system. The roof mirror 145-1 ′ is used to transfer or reflect light to the grating 145-2, thereby allowing pulse extension or reduction even with only one grating.

According to the present invention, a nonlinear polarization expansion (NPE) and a resonator that can adjust dispersion are used to adjust the progress of the pulse within the cavity resonator and to balance the balance between self phase modulation (SPM) and expansion / contraction of the dispersion induced pulses. A method of maintaining is provided. When forming a polarized laser pulse according to the method, it is possible to generate a laser pulse shape with limited deformation through the combined effect of fiber length, nonlinear effects and dispersion. This solves the distortion problem of the laser pulses of the prior arts, the problems of the system generating the laser pulses, for example, the difficulty of maintaining the aligned state, the volume is too large, not robust, etc. Can be.

As described above, the present invention has been described with reference to the preferred embodiments of the present invention, but a person of ordinary skill in the art does not depart from the spirit and scope of the present invention as set forth in the claims below. It will be understood that various modifications and changes can be made.

Claims (35)

A laser gain medium comprising double coated ytterbium doped photonic crystal fibers (DC YDPCF) that receive optical input emitted from the laser pump and amplify and compress the laser pulses; And Positive to generate net negative variance to balance internal phase modulation (SPM) and zoom of variance induced pulses internally to produce an output laser with limited pulse shape. A fiber laser resonator comprising a dispersed fiber portion of a and a negative dispersed fiber portion of a. 2. The apparatus of claim 1, further comprising a beam scatterer that functions as a polarization high sensitivity insulator that sends a portion of the laser pulses to the grating pair to deliver light output with irregular dispersion for shaping the output laser. Fiber laser resonator. 3. The fiber laser resonator of claim 2, further comprising a Faraday rotating mirror that reverses the polarization of the laser emitted from the grating pair. The fiber laser cavity resonator of claim 1, further comprising a polarization high sensitivity insulator and a polarization adjuster for shaping the output laser. The fiber laser cavity resonator of claim 1, wherein the gain medium further comprises double coated ytterbium doped photonic crystal fibers (DC YDPCF) having a positive amount of dispersion. The fiber laser resonator of claim 1, wherein the fiber laser resonator is annular. The fiber laser cavity of claim 1, wherein said gain medium comprises said double coated ytterbium doped photonic crystal fibers (DC YDPCF) constituting a positively dispersed fiber portion with a dispersion of 55 ps / nm / km. Resonator. The fiber laser resonator of claim 1, further comprising an output coupler for sending a portion of the laser to the output laser. The fiber laser resonator of claim 1, further comprising single mode fibers connected to said gain medium to form a fiber portion having a negative dispersion amount. The fiber laser resonator of claim 1, wherein said output laser comprises a laser having a pulse width of less than 1 femtosecond. The fiber laser resonator of claim 1, wherein the output laser comprises a laser having a pulse width of less than 1 femtosecond and a Gaussian pulse shape. The fiber laser resonator of claim 1, wherein the output laser comprises a laser having a pulse width of less than 1 femtosecond and a soliton pulse shape. The fiber laser resonator of claim 1, wherein the output laser comprises a laser having a pulse width of less than 1 femtosecond and a hyperbolic pulse shape. 2. The fiber laser resonator of claim 1, wherein said gain medium comprises said double coated ytterbium doped photonic crystal fibers having a light mode region (LMA) in the range of 15 microns to 80 microns. The fiber laser resonator of claim 1, wherein said gain medium comprises a high concentration of double coated ytterbium doped photonic crystal fibers (DC YDPCF) having a ytterbium dopant concentration in the range of 10,000 ppm to 2,000,000 ppm. 2. The fiber laser resonator of claim 1, wherein said double coated ytterbium doped photonic crystal fibers (DC YDPCF) have a single mode optical mode region (LMA) structure surrounded by an array of air gaps. 2. The fiber laser resonator of claim 1, wherein said gain medium further comprises band gap fibers which are air gap double coated ytterbium doped photonic crystal fibers (DC YDPCF). The beam scatterer of claim 1, wherein the beam scatterer functions as a polarization high sensitivity insulator that delivers a portion of the laser pulse to a roof mirror and a mirror with reflective gratings to deliver light output with irregular dispersion for shaping the output laser. Fiber laser resonator, characterized in that it further comprises. Forming a laser resonator by employing a positive dispersed fiber portion and a negative dispersed fiber portion that produce a net negative dispersion; And The laser input from the laser pump is double coated as a gain medium that balances dispersion induced nonlinearity and self phase modulation in a fiber laser resonator to amplify / compress the laser pulses and produce an output laser with a strain-limited pulse shape. Outputting to a fiber laser resonator employing ytterbium doped photon fibers (DC YDPCF). 20. The method of claim 19, further comprising employing a beam scatterer that functions as a polarization high sensitivity insulator that delivers a portion of the laser pulses to a grating pair to deliver light output with irregular dispersion for shaping the output laser. And generating a pulse-shaped strain limiting output laser from the laser resonator. 21. The method of claim 20, further comprising employing a Faraday rotating mirror that inverts the polarization of the laser emitted from the grating pair. 20. The method of claim 19, further comprising employing the double coated ytterbium doped photonic crystal fibers having a light mode region (LMA) in the range of 15 microns to 80 microns for amplification and compression of laser pulses. How to generate pulse-shaped strain limiting output laser from laser resonator 20. The method of claim 19, further comprising employing a polarization high sensitivity ablation and polarization regulator for shaping the output laser. . 20. The laser cavity of claim 19, further comprising employing a double coated ytterbium doped fiber (DC YDPCF) having a positive dispersion amount as a gain medium for amplification and compression of the laser pulses. A method of generating a pulse shaped strain limiting output laser from a resonator. 20. The method of claim 19, further comprising employing a double coated ytterbium doped fiber (DC YDPCF) having a dispersion amount in an amount of 55 ps / nm / km as said gain medium for amplification and compression of laser pulses. Generating a pulse shaped strain limited output laser from said laser cavity resonator. 20. The method of claim 19, further comprising employing an output coupler for transferring a portion of the laser as said output laser from said fiber laser resonator. How to. 20. The method of claim 19, further comprising coupling a single mode fiber constituting a fiber portion with a negative dispersion to the gain medium, the pulsed strain limiting output laser from the laser resonator. How to. 20. The method of claim 19, wherein the output laser comprises a laser having a pulse width of less than 1 femtosecond. 20. The method of claim 19, further comprising generating the output laser having a pulse width of less than 1 femtosecond and a Gaussian pulse shape. Way. 20. The method of claim 19, further comprising generating the output laser having a pulse width of less than one femtosecond and a soliton pulse shape. Way. 20. The method of claim 19, further comprising generating the output laser having a pulse width of less than 1 femtosecond and a hyperbolic pulse shape. 20. The method of claim 19, further comprising employing a high concentration of double coated ytterbium doped photonic crystal fibers having a ytterbium doping concentration in the range of 10,000 ppm to 2,000,000 ppm. How to create it. 20. The method of claim 19, further comprising employing the double coated ytterbium doped fiber (DCYDF) having a single mode optical mode region (LMA) structure surrounded by an array of air gaps. Generating a pulse shaped strain limited output laser from the laser resonator. 20. The pulsed strain limiting output from the laser resonator of claim 19, wherein said gain medium further comprises a band gap fiber which is an air gap double coated ytterbium doped photonic crystal fiber (DC YDPCF). How to generate a laser. 20. The beam of claim 19, wherein the beam functions as a polarization high sensitivity insulator that delivers a portion of the laser pulse to a roof mirror and a mirror with reflective gratings to deliver light output with irregular dispersion for shaping the output laser. And employing a scatterer to produce a pulse-shaped strain limiting output laser from the laser resonator.

Images