Gain flattening filter for preserving the spectral bandwidth in cryogenic ultrashort-pulse laser amplifiers
A gain flattening filter preserves very broad bandwidth, and therefore very short-duration pulses in ultrafast cryogenically-cooled laser amplifier systems, while also maintaining good overall efficiency. The filter is optimized for pulses amplified in a cryogenically cooled material.
This application claims the benefit of U.S. Provisional Patent Application No. 60/706,389, filed Aug. 8, 2005.
BACKGROUND OF THE INVENTION1. Field of the Invention
This invention relates to gain flattening filters for use in laser amplifiers. In particular, this invention relates to gain flattening filters optimized for use in cryogenic ultrashort-pulse laser amplifiers.
2. Description of Related Art
Gain narrowing has long been recognized as a problem in amplifiers where the beam passes through the amplifying medium multiple times (e.g. multipass configurations and regenerative configurations). Both multipass and regenerative amplifiers pass the beam being amplified through the gain material a number of times, in order to achieve sufficient amplification. Since gain follows a generally bell-shaped curve, the repeated passes result not just in increased power, but also in narrowed gain profile. Since the pulse duration and spectral bandwidth have an inverse relationship, spectral narrowing corresponds to longer-duration pulses, which is generally undesirable.
Gain-flattening etalons and other types of filters have been used in the past in room temperature regenerative [4] and multipass amplifiers [3, 5]. The resulting gain shape achieved is not ideal however, as it is not flat but rather a double peak shape or else exhibits “hard edges” on the spectrum. See, for example,
In past work, the present inventors have demonstrated the utility of cryogenic cooling for improving the average power available from an ultrafast laser amplifier system. [1, 2] However, in this work one difficulty is that when the ti:sapphire or the like is cryogenically cooled, changes in the gain spectrum of the material result in a further narrowing of the output spectral bandwidth from the amplifier, beyond the inevitable narrowing in multipass and regenerative configurations discussed above. The spectrum also shifts at cryogenic temperatures.
Hence a problem with cryogenically cooled ultrafast laser amplifier systems, especially multipass amplifiers, is a shift in the gain spectrum of ti:sapphire at cryogenic temperatures, along with a narrowing of the gain profile above and beyond the narrowing that occurs in room temperature systems. The conventional gain-flattening filter designs implemented for room temperature operation of ti:sapphire amplifiers do not work well at cryogenic temperatures. While the present inventors are experts in the field of cryogenically-cooled amplifiers, they were surprised by the poor performance of conventional filters in cryogenically-cooled amplifiers. Even though the shift in gain cross section of ti:sapphire with temperature is a rather small effect, and it was not expected that changes to the filter would be required to effectively use the filter at cryo temperatures, it turns out to be quite critical. Optimizing filters (and other elements of the amplifier) for use in cryogenically-cooled amplifiers was not intuitive and required a difficult iterative process, and even adjusting the temperature of the laser medium in order to match the profile obtained for the optimized filter.
A need remains in the art for a gain-flattening filter optimized for use in cryogenically-cooled ultrafast laser amplifier systems.
BIBLIOGRAPHY
- [1] S. Backus, R. Bartels, S. Thompson, R. Dollinger, H. C. Kapteyn, and M. M. Murnane, “High-efficiency, single-stage 7-kHz high-average-power ultrafast laser system,” Optics Letters, vol. 26, pp. 465-467, 2001.
- [2] S. J. Backus, H. C. Kapteyn, and M. M. Murnane, “Ultrashort pulse amplification in cryogenically cooled amplifiers.” U.S. Pat. No. 6,804,287: Regents of the University of Colorado, 2004.
- [3] E. Zeek, R. Bartels, M. M. Murnane, H. C. Kapteyn, S. Backus, and G. Vdovin, “Adaptive pulse compression for transform-limited 15-fs high-energy pulse generation,” Optics Letters, vol. 25, pp. 587-589, 2000.
- [4] C. Barty, T. Guo, C. Le Blanc, F. Raksi, C. Rose-Petruck, J. Squier, K. Wilson, V. Yakovlev, and K. Yamakawa, “Generation of 18-fs, multiterawatt pulses by regenerative pulse shaping and chirped-pulse amplification,” Optics Letters, vol. 21, pp. 668-70, 1996.
- [5] Z. Cheng, F. Krausz, and C. Spielmann, “Compression of 2 mJ kilohertz laser pulses to 17.5 fs by pairing double-prism compressor: analysis and performance,” Optics Communications, vol. 201, pp. 145-155, 2002.
- [6] Takada et al, “Broadband Regenerative Amplifier Using a Gain-Narrowing Compensator with Multiple Dielectric Layers,” Japanese Journal of Applied Optics, vol. 43, no. 11 B, pp. L1485-L1487.
It is an object of the present invention to provide a gain-flattening filter for use in cryogenically-cooled ultrafast laser amplifier systems.
The present invention solves this problem (wherein changes in the gain spectrum of the material at cryogenic temperatures result in a shifting, reshaping, and narrowing of the output spectral bandwidth from the amplifier, and a corresponding longer-duration pulse) using a specially designed spectral “gain flattening” filter, optimized specifically for the case of the cryogenically-cooled crystal, to maintain spectral bandwidth in the laser amplifier system.
In the case of a custom-designed filter, the shift in and narrowing of the gain cross section of ti:sapphire with temperature is a rather small effect, but it turns out to be quite critical. The conventional gain-flattening filter design implemented for room temperature operation of ti:sapphire amplifiers does not work well at cryogenic temperatures, while the filter designed for cryogenic operation according to the present invention does not work at room temperatures. Overall the gain-flattening filter works much better and with much more repeatability in the cryo-cooled situation.
The resulting optimum design, in the case of ti:sapphire cooled to cryogenic temperatures, corresponds to a spectral filter that transmits 40-50% of the light at the peak attenuation wavelength of 770-785 nm, and a FWHM of the gain-flattening filter of ˜75 nm
A preferred embodiment of the present invention uses a “multipass amplifier” configuration, wherein the pulses do not retrace exactly the same path through the laser crystal for many passes, but rather shift slightly for each pass until the final pass intersects a mirror, which removes the pulse from the amplifier for use as an output pulse. As a feature, the ultimate system output can be further optimized in a multipass configuration by inserting the spectral filter into the beam path for a variable number of passes. Causing the filter to intersect the beam for the first several passes but bypass it for the final several passes increases system gain and improves pulse shape.
The present invention finds its immediate usefulness in ultrashort-pulse ti:sapphire laser amplifiers. The typical operating parameters require on the order of 10 passes through the crystal in a multipass amplifier. An ultrafast laser amplifier system takes an ultrashort light pulse, “stretches” it in time, then puts the beam through an amplifier that typically requires 8-20 passes through a laser crystal to increase the pulse energy from ˜10−9 Joules to ˜10−3 J or higher and then is recompressed in a pulse compressor.
BRIEF DESCRIPTION OF THE DRAWINGS
Ultrafast laser amplifier system 100 takes an ultrashort light pulse 102, “stretches” it in time, then puts the beam through an amplification process that typically requires 8-20 passes through a laser crystal 114 to increase the output pulse 134 energy from ˜10−9 Joules to ˜10−3 J or higher. Pump beam 101 enters the system at the right in
Input beam 102 reflects off of flat mirror 104 and into the amplifier. The beam rings around the amplifier as half flat mirrors 106 and 116 send some of the light through the laser material 114, and allow some to reflect off flat mirrors 108 and 118 (and flat mirror 130) and curved mirrors 110 and 120. Optional mask 124 prevents ASE from forming in the amplifier.
In the multipass configuration of
Conventional gain flattening filters include a mounting frame that would obscure the beam if someone attempted to use them in this way. In addition, in conventional filters the layer structure does not extend to the edge. Therefore, if filter 122 is to be used in this manner as shown in
The physical reason for having filter 122 intersect only the earlier passes is 1) loss in the gain flattening filter, and 2) gain saturation. The gain-flattening filter is lossy, reflecting some of the light out of the path of the multipass amplifier.
This affects the overall efficiency of the amplifier; however, this loss is most significant in the final passes through the amplifier, when the pulse energy is highest. The amount of energy lost from the amplifier with each pass through the filter depends on the spectral shape, but is somewhere in the range of ˜20% in the case of the filter design of
However, at some point the amplifier pulse energy begins to reach saturation, where the pulse amplification process is extracting a significant fraction of the energy stored in the laser crystal. For example, in our typical configuration we “pump” the ti:sapphire laser crystal with 10 mJ of absorbed energy, with the pump laser wavelength of 532 nm. The total exactable energy is 10 mJ*(532 nm)/(800 nm)≈6-7 mJ. Once the pulse has extracted a significant fraction of this stored energy, the gain per pass begins to decrease. If the pulse then also passes through the gain-flattening filter, there are two problems: 1) with a lower overall gain in the laser crystal, the spectral filter overcompensates for the gain curve, making a broad but non-ideal spectral shape; and 2) the loss from the filter becomes significant in terms of overall energy loss from the pulse; i.e. when the pulse has ˜5 mJ energy a 20% loss is ˜1 mJ, and this loss is unrecoverable since the gain in the amplifier has become depleted.
The optimum solution is to insert the gain flattening filter for the initial passes through the multipass amplifier, while the final passes, once the pulse energy is reaching saturation, do not make use of the filter. In this way, the gain flattening filter can be matched to the small signal gain characteristics of the amplifier, to maintain or moderately broaden the spectrum of the pulse in these initial passes. In passes that are nearing saturation, the spectral filter is not there, and does not result in any loss of pulse energy and amplifier efficiency. Furthermore, some gain-narrowing can serve to spectrally-reshape the “square” pulse spectrum resulting from the initial passes, resulting in a more nearly ideal spectrum (which in the case where a short pulse duration is required, corresponds to a approximately Gaussian-shape spectrum). Specifically, varying the number of passes allows for the filter to start to broaden the spectrum as gain saturation causes the filter to overcompensate and start to form a broad “double peak” spectrum. Subsequent passes that do not go through the filter can then reshape the double peak to fill-in the hole, creating a more nearly ideal spectrum. Adjustment of the number of passes is critical to getting the optimum spectral shape, and this has not been done before. Changing the number of passes in which the spectral filter is used is simply a matter of translating the spectral filter in the case of
The blue shifted spectrum is caused by a blue shift in the cryogenically cooled amplifier, and a gain spectral narrowing is also observed. This leads to a narrower output spectrum compared to the room temperature case. However, with the cryo cooled amplifier, the thermal lensing is greatly reduced (1 cm @ 300 K, to 5 m @ 70 K), allowing multiple passes through the amplifier without distortion of the beam. This required fine tuning and many runs to adapt the filter design for cryo-cooled amplifiers. The critical parameters are depth, width, and spectral center wavelength. Also the wings of the filter must be under 5% modulation. See
Note that an iterative, trial and error process was needed to produce filters that have the desired gain broadening effect. Relatively small errors can make the filter useless, or even cause gain narrowing. For example, filter wings must be within about 5% modulation or less for good performance.
An actual produced filter 122 was formed in layers on a fused silica substrate. The heavy line is the filter actually made, and the line formed of x's denotes the theoretical desired filter profile. This filter was used to produce the data in
While the exemplary preferred embodiments of the present invention are described herein with particularity, those skilled in the art will appreciate various changes, additions, and applications other than those specifically mentioned, which are within the spirit of this invention. For example, variations in the design of the gain flattening filter can produce a similar transmission (or reflection) characteristics. Filters could also be positioned so that for each pass through the laser medium, the beam passes through the filter more than once, with the cumulative effect of the filter reflections (or transmissions) obtaining a similar gain-flattening effect.
Claims
1. A gain-flattening filter for maintaining spectral bandwidth in laser amplifier systems utilizing cryogenically cooled gain material, the filter comprising:
- a substrate; and
- a plurality of approximately quarter-wave alternating layers of high index and low index materials formed on the substrate;
- wherein the thickness of the layers is optimized to maintain spectral bandwidth of laser pulses amplified at cryogenic temperatures.
2. The filter of claim 1 wherein the layers comprise SiO2 and HfO2.
3. The filter of claim 1 wherein the layers comprises SiO2 and TiO2.
4. The filter of claim 1 wherein the alternating layers comprise approximately 9 layers of high index and low index materials each.
5. The filter of claim 1 wherein the gain attenuation spectrum is centered at about 770 nm.
6. The filter of claim 5 wherein the thickness of the alternating layers is optimized to provide 5% modulation or less outside the central gain flattening peak.
7. The filter of claim 6 wherein at the center of the attenuation spectrum, approximately 40% of light is transmitted.
8. The filter of claim 5 wherein the gain material is Ti:sapphire.
9. The filter of claim 1 wherein the thickness of the alternating layers is optimized to provide 5% modulation or less outside the central gain flattening peak.
10. The filter of claim 1 wherein at the center of the attenuation spectrum, approximately 40% of light is transmitted.
11. The filter of claim 1 having a spectral width at half the maximum attenuation of around 75 nm
12. The filter of claim 1 wherein the gain material is Ti:sapphire.
13. A multipass laser amplifier system of the type comprising means for introducing a pump laser beam, a gain material, means for energizing the gain material, mirrors for circulating the pulses around the amplifier several times, and means for removing a laser pulse from the amplifier, the improvements comprising:
- (a) a cryogenic chamber for maintaining the gain material at a cryogenic temperature; and
- (b) a gain flattening filter for maintaining the spectral bandwidth of the laser pulses, the filter optimized to maintain spectral bandwidth of laser pulses amplified by the cryogenically cooled gain material.
14. The amplifier of claim 13 wherein the cryogenic chamber is adjusted to maintain the laser material at a temperature which improves the performance of the optimized filter.
15. The amplifier of claim 13 wherein the mirrors and the filter are constructed and arranged to allow the laser pulse to pass through the filter for several passes and then to bypass the filter for several passes.
16. The amplifier of claim 13 wherein the filter comprises:
- a substrate; and
- a plurality of approximately quarter-wave alternating layers of high index and low index materials formed on the substrate;
- wherein the thickness of the layers is optimized to maintain spectral bandwidth of laser pulses amplified at cryogenic temperatures.
17. The amplifier of claim 16 wherein the gain material is Ti:sapphire.
18. The amplifier of claim 17 wherein the filter gain attenuation spectrum is centered at about 770 nm.
19. The method of preserving spectral bandwidth in a cryogenic ultrashort-pulse laser amplifier having a gain material comprising the steps of:
- energizing the gain material;
- cryogenically cooling the gain material;
- introducing an input pulse beam into the amplifier;
- optimizing a gain flattening filter to maintain the spectral bandwidth of the pulse beam at cryogenic temperatures;
- placing the gain flattening filter into at least a portion of the pulse beam;
- amplifying the pulse beam; and
- extracting the amplified pulse beam from the amplifier.
20. The method of claim 19, further including the step of adjusting the gain material temperature to improve the performance of the optimized filter.
21. The method of claim 19, further including the step of configuring the amplifier such that the filter intersects several early passes of the pulse beam in the amplifier and bypasses several later passes.
22. The method of claim 19 wherein the gain material is Ti:sapphire and wherein the optimization step centers the gain attenuation spectrum at about 770 nm.
Type: Application
Filed: Aug 8, 2006
Publication Date: Feb 8, 2007
Inventors: Sterling Backus (Erie, CO), Henry Kapteyn (Boulder, CO)
Application Number: 11/501,210
International Classification: H01S 3/00 (20060101); H04B 10/12 (20060101);