Patent Application
20140376576
1. Field of the Invention
Embodiments of the present invention generally relate to high energy, ultrashort pulses from a net normal dispersion ytterbium fiber laser with an anomalous dispersion higher-order mode fiber. More specifically, embodiments of the present invention relate to a fiber oscillator with all-fiber dispersion compensation delivering pulse parameters comparable to solid-state oscillators having good compensation of higher order dispersion and intracavity nonlinearities.
2. Description of the Related Art
All integrated mode-locked Ytterbium-doped fiber lasers delivering high fidelity pulses or Ytterbium-solid state lasers delivering sub-200 fs pulses are very attractive as seed sources for Ytterbium fiber amplifier systems. The main demands for seed sources of such systems are good pulse quality and compressibility, and enough seed energy. The requirement for pulse energy and pulse compressibility becomes even more demanding for phase stabilized amplifier systems. In order to achieve a reliable phase lock, sub-100 fs pulses with a pulse-energy of several nJ are required. While in many applications solid-state oscillators are used to achieve this, the robustness and stability of all-fiber oscillators offer an interesting alternative.
Recently, many different approaches to push pulse compressibility and energy of fiber oscillators have been explored. In general, two different operating regimes can be distinguished: (1) fiber oscillators with net anomalous intracavity dispersion, and (2) fiber oscillators with net normal intracavity dispersion. Fiber oscillators operating in the first regime can produce highly compressible pulses, down to below 30 fs, but with only very limited pulse energy, much less than one nJ. In fact, in the net anomalous dispersion regime, the shortest pulse duration from an Yb-doped fiber oscillator was achieved at about 30 fs, with very limited pulse energy.
Fiber oscillators operating with normal intracavity dispersion can produce pulses with much higher pulse energy (i.e., several nJ), but the pulse fidelity from such oscillators is much lower. In the net normal dispersion regime, higher pulse energies can be achieved, but generally such systems yield a reduced pulse fidelity compared to oscillators operating with net anomalous intracavity dispersion. The shortest pulse duration from an Yb-doped fiber oscillator operating in the net normal dispersion regime is 50 fs, with 5 nJ pulse energy, using free space components (e.g., gratings) to introduce intracavity dispersion compensation. However, the pulse fidelity and the pulse duration were measured only with a second order autocorrelation, which does not provide reliable information of the pulse quality.
One of the main limitations in achieving better pulse quality is poor intracavity dispersion control. Most fiber oscillators with intracavity dispersion compensation use intracavity gratings. While the use of gratings sacrifices the robustness and stability of the fiber oscillator, the main limitation posed by intracavity gratings is that no compensation of higher order dispersion can be obtained. For more stable operation and better pulse fidelity, it is important to realize good compensation of higher order dispersion and intracavity nonlinearities. Hence, in order to develop a serious fiber-based alternative to the solid-state seed oscillators, fiber dispersion compensation is needed.
One approach for fiber based dispersion compensation is based on the design of waveguide dispersion in photonic crystal fibers. However, in order to achieve the required dispersion compensation, photonic crystal fibers with very small core diameters are needed, which leads to a large increase of the intracavity nonlinearities. Moreover, up to now, monolithic integration of photonic crystal fibers in fiber oscillators is problematic, since fusion splicing will destroy the fiber structure.
Thus, there is a need for high energy, ultrashort pulses from a net normal dispersion ytterbium fiber laser with a solid silica anomalous dispersion higher-order mode fiber.
Embodiments of the present invention generally relate to high energy, ultrashort pulses from a net normal dispersion ytterbium fiber laser with a solid silica anomalous dispersion higher-order mode fiber. More specifically, embodiments of the present invention relate to a fiber oscillator with all-fiber dispersion compensation delivering pulse parameters comparable to solid-state oscillators having good compensation of higher order dispersion and intracavity nonlinearities.
In one embodiment of the present invention, an oscillator comprises a length of single mode fiber and a length of higher-order mode fiber, where the group delay dispersion of the higher-order mode fiber is chosen to match 50% or more of the group delay dispersion of the single mode fiber, wherein a third-order dispersion of the oscillator matches a nonlinear phase buildup in the cavity of the oscillator, and the nonlinear phase buildup is dependent upon the pulse energy of the oscillator.
In another embodiment of the present invention, a method of matching third order dispersion in a high pulse energy ytterbium fiber laser oscillator for compensating nonlinear phase buildup comprises the steps of providing a high pulse energy ytterbium-fiber laser oscillator, selecting a length of higher-order mode fiber to maintain a net group delay dispersion and third order dispersion, wherein the third order dispersion matches a nonlinear phase buildup in the cavity of the oscillator, and adding the higher-order mode fiber to the ytterbium-fiber laser oscillator.
In yet another embodiment of the present invention, a high pulse energy ytterbium fiber laser comprises a length of single mode fiber and a length of higher-order mode fiber, where the group delay dispersion of the higher-order mode fiber is chosen to match 50% or more of the group delay dispersion of the single mode fiber; wherein a third-order dispersion of the laser matches a nonlinear phase buildup in a cavity of the laser, and the nonlinear phase buildup is dependent upon the pulse energy of the laser.
So the manner in which the above-recited features of the present invention can be understood in detail, a more particular description of embodiments of the present invention, briefly summarized above, may be had by reference to embodiments, which are illustrated in the appended drawings. It is to be noted, however, the appended drawings illustrate only typical embodiments of embodiments encompassed within the scope of the present invention, and, therefore, are not to be considered limiting, for the present invention may admit to other equally effective embodiments, wherein:
The headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description or the claims. As used throughout this application, the word “may” is used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). Similarly, the words “include”, “including”, and “includes” mean including but not limited to. To facilitate understanding, like reference numerals have been used, where possible, to designate like elements common to the figures.
Embodiments of the present invention generally relate to high energy, ultrashort pulses from a net normal dispersion ytterbium fiber laser with an anomalous dispersion higher-order mode fiber. More specifically, embodiments of the present invention relate to a fiber oscillator with all-fiber dispersion compensation delivering pulse parameters comparable to solid-state oscillators having good compensation of higher order dispersion and intracavity nonlinearities.
As used herein, the term “about” or “approximately,” or derivatives thereof, when referencing a numerical value, should be deemed to include within ten percent of such numerical value in either direction. In addition, when such terms are utilized to described absolutes (e.g., zero), the absolute should be deemed to include within one unit of reasonable measurement in either direction, as would ordinarily be used by those of ordinary skill in the art.
Many embodiments of the present invention seek to utilize the anomalous dispersion of a higher order mode (HOM) in a fiber to ensure stable modelocked operation with high pulse energy as set forth herein. HOM fibers are suitable for embodiments of the present invention because mode conversion can be achieved reliably, and integration of such HOM fibers is fairly straightforward. Additionally, the nonlinearities in the HOM module are reduced compared to those in standard single mode fiber (SMF), since the mode area in the HOM is larger. Furthermore, HOM fibers offer a possibility to compensate higher order dispersion terms in the cavity, which reduces the buildup of unwanted nonlinear phase on the pulses in the cavity.
In accordance with many embodiments, however, introducing anomalous dispersion alone is not enough to ensure stable modelocked operation with high pulse energy. As will be explained, when the total intracavity dispersion is anomalous, the pulse energy that can be extracted is very limited. Alternatively, when the total intracavity dispersion is normal, it is possible to extract higher energy pulses in a stable modelocked regime, but the pulse fidelity is compromised by the presence of significant amounts of higher order dispersion. For example, third order dispersion tends to destabilize the pulsed operation, i.e. it limits the pulse energy and pulse fidelity, and fourth order dispersion limits the operation bandwidth and compressed pulse duration. Thus, embodiments of the present invention seek to provide improved control of the intracavity dispersion in order to extract the highest pulse energy with the best compressible output pulses, up to the limits set by the gain bandwidth of the active medium, or even beyond.
In many embodiments, the first output comprises a polarization beamsplitter for pulse cleaning, and works with the non-linear polarization rotation and the spectral filter to maintain modelocked operation of the system. The optional second output may comprise a similar type of apparatus as the first output, and the output ratio between the first and second output may be controlled, for example, via a half-wave plate. In many embodiments, the fiber ring oscillator works in a net normal dispersion regime.
In many embodiments, applicable power ranges for the laser pump range from near 0 Watts to several Watts, depending on the embodiment and type of active fiber used. In accordance with one embodiment of the present invention, approximately 500-700 mW of pump power is provided. The ytterbium-doped fiber may generally comprise a highly doped fiber, including highly doped fibers that are photo darkening resistant.
In one embodiment, at least one polarization beam splitter is utilized to facilitate the modelocking mechanism. The second output port, which may be optional, may comprise a fixed-ratio (fiber) beam splitter. In certain embodiments, it is estimated that the optimum output coupling ratio at the second output port to be about 75 percent, but stable operation may be obtained up to 90 percent. With higher available pump power, this number can be increased.
The role of the spectral filter may be taken over by selecting a WDM or fiber isolator with a sufficiently narrow operation bandwidth. The filter may comprise a full-width-at-half-maximum transmission bandwidth up to 40 nm.
In many embodiments, the HOM fiber should be designed such that it compensates for 50% or more of the group delay dispersion and third order dispersion of the single mode fiber used in the oscillator.
A type of oscillator, such as the one depicted in
As shown in the Figure, the solid lines show the intracavity dispersion of the first embodiment of the oscillator, which had a repetition rate of 24 MHz. The bottom solid line shows the total dispersion of the normal dispersion components in that embodiment, mainly introduced by the single mode fiber and ytterbium doped active fiber. The top solid line shows the anomalous dispersion introduced by the HOM module. The middle solid line is the result of the addition of the two curves, which yields the total dispersion seen upon one roundtrip through the ring cavity. The dashed lines show the intracavity dispersion of the second embodiment of the oscillator, which had a repetition rate of 20 MHz. The bottom dashed line shows the total normal dispersion in the second embodiment, the difference between the normal dispersion in the first and second embodiments is due to the different length of single mode fiber used.
The top dashed line shows the anomalous dispersion introduced by the HOM module used in the second embodiment, which, as can be seen in
From the experimental measurements, it was determined that modelocked operation in the oscillator is based on nonlinear polarization rotation in combination with spectral filtering. Stable operation is ensured using fiber polarization controllers before and after the HOM module, as shown in
During the experimentation, a complete characterization of the oscillator output pulses is made for the all-normal dispersion oscillator having 2.4 nJ output energy, the oscillator with the first HOM module having 2.4 nJ output energy, and for the oscillator with the second HOM module having 6 nJ output energy. The different output energies for the different oscillators was to ensure stable modelocked operation and good pulse quality, which in the case of the all-normal dispersion oscillator and the oscillator with the first HOM module, was known to be not possible at higher output energies.
Using a high bandwidth oscilloscope and matching photodiode, and a long range Second-Harmonic Generation (SHG) Frequency-Resolved Optical Gating (FROG) scan, it was determined that the oscillators work in a single pulse mode. The results of a finer, short range, SHG FROG scan after compression with a transmission grating pair are presented in
The advantage of FROG characterization over other characterization methods is that it reliably shows whether a clean temporal pulse profile is obtained, or that a long-range pedestal is present. The disadvantage of many fiber oscillators is that they exhibit a large long-range pedestal, which limits their usefulness for further amplification for example. Since the long range pedestal can contain a large portion of the out-coupled energy, even as large as 50 percent, the pulse energy contained in the short compressed pulse may be largely over-estimated. Therefore, FROG characterization facilitates the most complete assessment of the pulse fidelity of ultrafast lasers.
As a result of the testing, the oscillator without intracavity dispersion compensation delivers the longest pulses and the smallest bandwidth; the oscillator with the first HOM module delivered pulses with a larger bandwidth, that consequently could be compressed to a pulse duration as short as 150 fs; and the oscillator with the second HOM module delivered the broadest spectrum, and the pulses could be compressed to almost their Fourier transform limited duration. The slight difference between the Fourier limited duration of 61 fs and the retrieved duration of 62 fs and the stronger satellite compared to the Fourier limited pulse is likely attributed to the residual TOD after the grating compressor.
As a result of the testing, embodiments of the present invention comprise a fiber oscillator with all-fiber dispersion compensation, delivering pulse parameters comparable to solid-state oscillators, and a precise way forward to further improve the performance of such fiber oscillators. As supported by the experimental results presented herein, use of improved dispersion compensation in the oscillator cavity directly leads to better pulse compressibility and higher attainable output pulse energy.
It should be appreciated, the physical mechanism underlying the stable modelocked operation of the above described oscillators is understood to be based on nonlinear polarization evolution. While the nonlinear polarization evolution, in conjunction with the polarization beamsplitter output coupler acts as an artificial saturable absorber, the action of self phase modulation, dispersion and the spectral filter ensure that the temporal characteristics of the pulse are reproduced after each roundtrip. While in an oscillator with no, or poor dispersion compensation the balance of dispersive stretching of the pulse during one roundtrip of the oscillator and the action of self phase modulation is only achieved at the cost of the generation of a substantial pulse pedestal, well designed dispersion compensation, as is achieved with HOM fibers, allows for better balancing of the actions of dispersive stretching, self phase modulation and spectral filtering during stationary operation of the oscillator.
Also, third order dispersion stretches pulses asymmetrically, i.e. the red and blue wings of the spectrum. The compensation of third order dispersion is especially critical in this sense, because it causes the pulse to broaden asymmetrically, which causes an uncompressible pulse pedestal at the laser output. While this effect can be reduced by using two output couplers, in which case the first output coupler acts as a pulse cleaner, and cleaner pulses are obtained at the second output coupler, numerical simulations have lead to the believe that excessive third order dispersion limits the pulse stability, operation bandwidth and achievable pulse energy.
The compensation of third order dispersion with a nonlinear phase shift has been observed in other optical systems external to a laser oscillator. One example that is related to the embodiments of the present invention is chirped pulse amplification. Compensation of nonlinear phase shift with third order dispersion or vice versa has been demonstrated in so called chirped pulse amplification of short-pulse fiber lasers. Chirped pulse amplification is a well-known scheme for amplifying short laser pulses. The chirped pulse amplification scheme involves pulse stretching in time domain to reduce peak power, amplification, and re-compression to gain high peak power and sub-ps pulse duration. The amplifying system requires careful management of the dispersion to have zero net group delay dispersion and preferably zero net higher order dispersion. For some CPA systems it is observed that a nonzero net third order dispersion can be compensated with a sufficient amount of nonlinear phase that is build-up as the pulse is propagating through the stretcher, amplifier, and compressor stages.
It is observed both, when solving the nonlinear schrödinger equation and in experimental demonstrations that for a given third order dispersion there is an optimum nonlinear phase shift. The total phase shift is flattened as a function of wavelength and potential secondary structure or sometimes referred to as a pedestal in time domain is significantly reduced. The effect is due to the combined action of third order dispersion and self-phase modulation. Furthermore, studies in the literature show that compensating third order dispersion with a nonlinear phase shift produces the same result even if the sign of the third order dispersion is reversed. For zero nonlinear phase shift or nonlinear phase shifts away from the optimal value the pulse will exhibit secondary structure on the pulse edge resulting from un-compensated third order dispersion.
While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. It is also understood that various embodiments described herein may be utilized in combination with any other embodiment described, without departing from the scope contained herein. In addition, embodiments of the present invention may be further scalable, as particular applications may require.
The present application claims priority to U.S. Provisional Patent Application No. 61/567,570, entitled “High-Fidelity, High-Energy Ultrashort Pulses at 1035 nm from a Net Normal-Dispersion Yb-Fiber Laser with Anomalous Dispersion Higher-Order-Mode Fiber,” filed Dec. 6, 2011, the disclosure of which is incorporated by reference herein in its entirety.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US12/68262 | 12/6/2012 | WO | 00 | 6/4/2014 |
| Number | Date | Country | |
|---|---|---|---|
| 61567570 | Dec 2011 | US |
