Patent Application
20170059711
The present invention relates to a laser system for producing high power laser pulses.
In the NRC Decadal Survey on Earth Science and Applications, LIDARs are identified as the primary instrumentation for several missions NASA plans to launch between 2013 and 2020. These missions will use LIDAR to measure global wind fields, aerosol profiles, clouds, and ocean color. The 2008 International Geoscience & Remote Sensing Symposium (IGARSS) called for NASA proposals addressing the need for more powerful, more energy-efficient and more reliable lasers in order to realize the full potential of tropospheric wind LIDARs for scientific research and applications.
NASA's Decadal Survey for Earth Science identifies the 3D Tropospheric Winds (3D Winds) mission and the Aerosols, Clouds, and Ecosystems (ACE) mission as critical missions for NASA's near future. The missions have in common the dependence on high power, pulsed laser transmitters. The ACE instrument will require stabilized backscatter wavelengths of 1064 nm, 532 nm, and 355 nm for its aerosol-tracking scheme. Also, due to the fact that the scattering efficiency off of atmospheric molecules is proportional to (1/λ)4, the 3D Winds instrument will require an output of up to 400 mJ per pulse in the ultraviolet region at 355 nm.
3D Winds measurement of global tropospheric wind profiles will advance the ability to predict and understand weather phenomena from the tropics to the poles. Except for the unevenly distributed single-level measurements at the surface by buoys, aircraft, and ships and the very few profile measurements from radiosonde stations primarily over land, the vertical structure of the horizontal wind field profile measurements are lacking. Satellite-based cloud drift inferred wind measurements are available but have fairly large errors in the wind measurement itself and the height allocation of the measurements is suspect. A capability to observe directly the global wind fields would be extremely valuable for numerical weather prediction, as well as scientific diagnostics of large-scale atmospheric transport, weather systems, and boundary layer dynamics in Earth's atmosphere.
According to various embodiments of the present invention, the laser system provided is comprised of a master oscillator, a pre-amplifier, a power amplifier, a frequency doubler, and a frequency tripler. The laser system is configured to generate three different wavelengths, for example, wavelengths of 1064 nm, 532 nm, and 355 nm. The pre-amplifier can be optically aligned along a beam path exiting the master oscillator, to receive and pre-amplify a laser beam generated by the master oscillator. The power amplifier can be optically aligned along a beam path exiting the pre-amplifier, and can be configured to receive a pre-amplified laser beam generated by the pre-amplifier, and further amplify it. The beam doubler and beam tripler can be optically aligned along a beam path exiting the power amplifier and can be configured to double and triple, respectively, an amplified laser beam. The laser system is configured to produce pulse energies at 1064 nm of about 1 J, and can double or triple the pulse energies to instead produce wavelengths of 532 nm and 355 nm, respectively.
The present invention provides a stable, high power laser oscillator and amplifier with a low part count and high efficiency. The High Efficiency Amplified Twin Head (HEATH) laser system of the present invention (the HEATH laser) is shown in
The ACE mission will provide simultaneous measurements of aerosol backscatter radiation and ocean color biosphere measurements by employing a High Spectral Resolution Lidar (HSRL) scheme that requires a laser capable of 3-wavelength production. Within each laser footprint, the data will link the entire size spectrum untangling their impacts and constrain their interaction in order to reduce uncertainties in climate forcing models. By quantifying the biological pump component of carbon sequestration, greater knowledge of the ocean biospheric responses to natural and manmade climate change can result. The ACE aerosol data combined with the 3D Winds data sets will generate powerful tools in predicting the global climate as well as closing the planetary energy budget.
The HEATH system meets the demands of both the ACE and 3D Winds missions in wavelength, power, stability, and lifetime According to the present invention, these two interdependent global climate change missions can greatly benefit from a single technology.
The present invention can be even more fully understood with the reference to the accompanying drawings which are intended to illustrate, not limit, the present invention.
According to various embodiments of the present invention, the laser system comprises a master oscillator, a pre-amplifier, a power amplifier, a frequency doubler, and a frequency tripler, for example, as shown and arranged in
The master oscillator can further comprise an enclosure that has a single aperture, and the single aperture can be configured to transmit a polarized, pulsed and polarized laser beam from the graded reflectivity mirror along the optical path to be output from the enclosure, and directed to the pre-amplifier. Other components, arrangements, configurations, and methods using the same, that can be implemented in the laser system include those described, for example, in Coyle et al., High Efficiency, 100 mJ per pulse, Nd: YAG oscillator optimized for space-based earth and planetary remote sensing, Optics & Laser Technology, 63 (2014) 13-18, Elsevier Ltd., which is incorporated herein in its entirety by reference. Other components, arrangements, configurations, and methods using the same, which can be implemented in the laser systems, spacecraft, and missions of the present invention include those as disclosed in U.S. Pat. No. 8,958,452 B2 to Coyle at el., which is incorporated herein in its entirety by reference, and as disclosed in concurrently filed U.S. patent application Ser. No. ______ to Coyle et al. entitled “Ground-based Laser Ranging System for Identification and Tracking of Orbital Debris” (Attorney Docket No. GSC-17295-1), which is incorporated herein in its entirety by reference.
The pre-amplifier of the laser system can comprise a twin head gain configuration that is adapted to or capable of amplifying, by four times, energy produced by the master oscillator. The master oscillator can be configured to produce 120 mJ single frequency laser pulse signals and the pre-amplifier can be configured to amplify the laser pulse signals, for example, by two times, four times, eight times, or the like. The pre-amplifier can be configured to amplify the laser pulse signals by four times to produce amplified signals of about 500 mJ. The pre-amplifier can comprise Nd:YAG slabs, and the Nd:YAG slabs of the pre-amplifier can have Brewster-cut end faces coated with an anti-reflection coating.
The power amplifier, also referred to herein as the amplifier, can be configured to amplify by two times, four times, eight times, or the like, the pre-amplified signals produced by the pre-amplifier. The amplifier can comprise one or more ceramic zigzag slabs. The amplifier gain module, or laser head, consists of the orthogonal units just as in the HEATH laser cavity. This provides both high gain typical of zigzag slabs, but with symmetrical beam quality effects of cylindrical laser rods. The amplifier can comprise a configuration that provides pumping from opposite sides of the ceramic zigzag slab.
According to various embodiments of the present invention, the laser system can be mounted in or on a spacecraft. The present invention also provides a combination of the laser system and a spacecraft. The laser system can be mounted in or on the spacecraft and can be configured to direct laser beam pulses of about 1 J toward the Earth, for example, to irradiate aerosols in the atmosphere above the Earth's surface, or to irradiate a water-covered or ocean-covered portion of the Earth's surface. The laser system can be configured to direct the laser beam pulses at a 532 nm wavelength toward the Earth, or at a 355 nm wavelength toward the Earth. The laser system can be configured to generate laser beams of three different wavelengths, independently, for example, the laser system can be configured to independently generate laser beams at a wavelength of 1064 nm, laser beams at a wavelength of 532 nm, and laser beams at a wavelength of 355 nm. The laser system can be configured to switch between generating the laser beams at the three different wavelengths, for example, to intermittently or periodically provide laser beam pulses at each of the different wavelengths.
The present invention also provides methods of forming about 1 J laser pulses, by forming laser pulses with the laser system described herein. A method is also provided for analyzing aerosol backscatter radiation, and comprises generating pulsed laser beams with a laser system as described herein, directing such pulsed laser beams from a spacecraft toward an atmosphere over a surface of the Earth, detecting reflected laser light returning to the spacecraft after reflecting off of the atmosphere, and analyzing aerosol backscatter radiation caused by the atmosphere, based on the detected reflected laser light. The generating and directing can comprise producing 1 J TEM00 laser pulses at 1064 nm, at 532 nm, and at 355 nm, independently, with the laser system. A schematic of the mission concept drawing for the ACE mission is depicted in
In yet another embodiment of the present invention, a method of analyzing ocean color biosphere measurements is provided. The method comprises generating pulsed laser beams with a laser system as described herein, directing the pulsed laser beams from a spacecraft toward a water-covered surface of the Earth, detecting reflected laser light returning to the spacecraft after reflecting off of the water-covered surface, and analyzing ocean color biosphere measurements based on the detected reflected laser light. In some embodiments, the generating and directing comprises producing 1 J TEM00 laser pulses at 1064 nm, at 532 nm, and at 355 nm, independently, with the laser system.
High Efficiency Amplified Twin Head (HEATH) laser system of the present invention is a next-generation, Joule-class, single-frequency stabilized, diode-pumped solid state laser (DPSSL) transmitter. Joule-level output production is provided through the configuration of the present invention, which accounts for the myriad of issues associated with the high nonlinear and self-damaging effects of these energies. It is also critical that effort be performed with an architecture designed to eventually meet the stringent thermal, power, reliability, and cost restrictions for flight. Leveraging research performed on both the oscillator and amplifier stages from the Laser Risk Reduction Program (LRRP), viable laser technology can be produced with proven high-power scalability that will satisfy the current performance requirements for both the 3D Winds and ACE missions.
A method to configure and optimize a laser system according to various embodiments of the present invention, includes the following procedures:
1. Seed, stabilize, and lock the THEO cavity for single frequency operation;
2. Amplify THEO's 1064 nm output to 500 mJ by a twin head preamplifier stage, for example, by using a large version of the THEO gain assembly;
3. Frequency double the output to 532 nm and record the ACE data set;
4. Replace the doubler with a larger, optimized power amplifier to achieve 1 Joule/pulse operation; and
5. Install the frequency doubling and tripling optics to achieve 3D Winds transmitter requirements.
The spectral stability, the pointing sensitivity, and TEM00 beam quality can be defined by the oscillator portion of any MOPA system, however, an advantage of the laser system of the present invention is the high power capability and excellent beam quality. The HEATH architecture requires a minimum number of components, support optics, and hardware. These component savings greatly contribute to a reduced thermal load, associated costs, complexity, and minimized spatial noise and diffraction effects in the beam. Every optical component in a flight laser increases the overall cost in manpower and material, especially when considering the number of spare parts required. Furthermore, it is standard practice to minimize the total length of high index glass in a pulsed laser beam's path to reduce nonlinear, dispersive, and diffraction effects. The HEATH laser system of the present invention can have an equivalent number of slabs as a similarly powered MOPA, but the total slab gain path is far shorter and its reduced optical component count greatly contributes to this advantage as well.
According to various embodiments of the present invention, the laser system comprises a twin head efficient oscillator, for example, as shown in the embodiments depicted in
A first example of the twin head efficient oscillator is shown in
Master Oscillator and Enclosure
An automated testing installation was developed to operate the THEO system over the course of several months. The long term testing layout is detailed in
The THEO laser operated at a significantly derated specification in this experiment, and yet produced >100 mJ per pulse at 100 Hz repetition rate for over 2×109 shots with zero degradation in pulse energy.
A similar but slightly different master oscillator in an enclosure is depicted in
The laser shown in
Cylindrical laser rods create cleaner pumping distributions, and typically better beam qualities, over zigzag slabs. However, rods almost always require direct liquid-contact cooling surrounding the crystal, and are limited in extraction efficiency. The zigzag slabs used in accordance with the present invention provide much easier diode pumping parameters, simpler thermal and mechanical engineering, and better extraction efficiency. Two major issues are always present with DPSSL architectures based on trapezoidal, or “zigzag”, laser slabs. These are (1) thermal lens compensation and (2) heat removal from, or thermal control of, the laser head. These thermal effects are controlled, however, according to various embodiments of the present invention.
Typically, high power laser diode arrays are aligned along one long (pump) face of the slab, and the slab is thermally bonded to a thermally conductive “heat sink” on the opposing or long face. The end faces are the optical faces through which the evolved laser cavity beam will travel. The steady state thermal properties of the slab created by the heat production (diode pumping), the heat removal (heat sink), and the radiative cooling (optical energy extraction), within the slab create optical distortions and must be carefully corrected for good performance. These distortions increase exponentially as the laser fluence ramps up, as does the difficulty in their compensation. Zigzag slabs produce asymmetrical thermal lens effects requiring passive optic compensation, and create poor multimode beam quality, which combine to severely limit the average power and/or repetition rates. Longer, higher aspect ratio slabs for higher pulse energies will begin to bend or “potato chip” due to the asymmetric heating, as well.
The THEO-derived twin-head gain module of the present invention greatly reduces thermal effects, requires no passive optics, and requires no diffractive hard apertures. This technology offers unmatched performance for high power, zigzag slab-based cavities and higher power amplification.
The 1 Joule HEATH laser can use on a Master Oscillator Power Amplifier (MOPA), but with the THEO pumping incorporated throughout. This approach raises the bar for system efficiencies, produces a transmitter with a low number of components, and creates an excellent beam.
A schematic of a stabilization scheme that can be employed is shown in
The THEO master oscillator produces 120 mJ, single frequency laser pulses that can be sent into the amplifier stages of the architecture where precision defocusing of the beam expander after the THEO master oscillator produces a small divergence that can compensate for larger slabs and maintain the fluence at a safe level. As can be seen in
The beam size can be carefully monitored and modeled throughout this stage for fluence damage mitigation. As the beam is amplified the HR mirror's prescription can be changed to allow the beam to grow to maintain a fluence level below damage thresholds. Another advantage of the double-pass configuration is that it allows for slightly lower pump powers than those needed over equivalent single-pass amplification, improved LDA derating, and the ability to fine tune its performance with greater accuracy. The natural polarized performance of zigzag slab end faces, however, naturally rejects one polarization over the other, and thus prevents use of a 2-pass polarization-coupled amplifier configuration. The preamp slabs' Brewster-cut end faces can be coated with a high damage threshold anti-reflection (AR) coating that is polarization independent. The AR coating performs identically with any polarization and is specially made to resist high field intensity laser fluencies.
The 120 mJ Q-switched laser pulses can be scaled up by a total factor of about 8.5, while maintaining excellent beam quality and high pointing stability for final frequency doubling and tripling. The 500 mJ beam from the pre-amp stage can be carefully monitored in order to assure that fluence levels stay under control before it is steadily amplified another 2× to achieve about 1 J, or greater, energy out of the final stage. Herein, “about 1 J” refers to pulse energies at 1064 nm of from 750 mJ to 1.25 J. The power amplifier ceramic zigzag slab, unlike smaller twin head preamp slabs, can be pumped from opposite sides with thermal conduction surfaces on both un-pumped surfaces.
According to various embodiments, 18-bar LDAs and large zigzag slab material can be used. For side pumped zigzag slabs greater than 5 mm in width, pumping can be provided from opposing faces, and each LDA can be placed at each zigzag bounce point. There is a crossover point as slab thickness grows where all the pump absorption is biased toward the pump face and no gain exists on the opposing face, and thus too much slab warping occurs. The power amp can implement this opposing face, bounce-point pumping technique and the slab can be thermally mounted on the top and bottom faces. Initial analysis can be performed to see if the X and Y thermal lensing effects (pump axis and normal axis, respectively) are within reason and allows for safe, reliable, optical correction. These results are seen in
Once 1 Joule pulse energies are achieved, the total system can be optimized for stable operation. In some embodiments the pulse energy achieved can be from about 750 mJ to about 1.25 J. Once the best possible TEM00 beam quality is consistent, intensity calculations can be checked to configure the best beam size for a certain operation. Great care can be taken to determine the precise beam setting for optimum performance. If the beam's cross section is too large and there is a reduced intensity within the nonlinear crystal, then the conversion efficiency will be low. The performance ramps up quickly with intensity, care can be taken not to induce damage in the pursuit of higher powers of 532 nm and 355 nm.
When considering space flight transmitters, higher energy space-borne LIDARs such as the Geoscience Laser Altimeter System (GLAS), based on a master oscillator power amplifier (MOPA) architecture, have not fared well in terms of operational lifetime. All three laser transmitters aboard the instrument showed a high rate of decay in both the 1064 nm and 532 nm channels with respect to the required 3-year mission lifetime. Given that both ACE and 3D-Winds will require between 5 and 10 times greater pulse energies at 1 μm with more than twice the repetition rate, the significant advances in reliability, lifetime, and developmental cost reductions achieved by the present invention become even more surprising and unexpected. As diode pumped solid-state laser (DPSSL) powers increase, significant improvements in optical efficiency are needed and are provided according to the present invention.
The European Space Agency's ADM-AEOLUS wind LIDAR transmitter produces 1064 nm pulses of 500 mJ at 1064 nm prior to doubling and tripling. Even though the laser is state of the art, its high thermal load and bus power requirements demand that it must operate in a periodic burst mode. Its final output will be a 12 second run of 130 mJ at 355 nm, which includes a 5 second warm-up and 7 seconds of data, and an “off” cycle time of 16 seconds, to increase lifetime and reduce power consumption. Thus, the reduction of system size, mass, and part count achieved by the present invention can be critical in the development of next-generation high energy DPSSLs at NASA to capitalize on a 3D Winds and ACE launch opportunity in order to gather the most data possible for the valuable committed resources.
Table 1 below shows a comparative summary of various high power remote sensing lasers, including the HEATH laser system of the present invention.
As detailed in Table 1 above, the HEATH laser system of the present invention meets or exceeds virtually all the critical specifications for a number of comparable selected laser transmitters. For example, the present HEATH laser system surpasses the performance of oscillator-only architectures such as TWiLiTE and CALIPSO in pulse energy and average power while maintaining considerable advantages in efficiency and lifetime.
In addition, the HEATH laser system of the present invention compares quite favorably against high-power MOPAs such as the Space Winds Laser Transmitter and the current commercial GLOW laser for a number of reasons. First, the pulse energy of the enhanced THEO master oscillator is four times greater than that used in its closest competitor, the Space Winds Laser Transmitter (200 mJ vs. 50 mJ), reducing the pump power load of the amplifier slabs and allowing for outstanding energy extraction efficiency.
These improvements enable an efficiency gain of 2× that of GLOW, a leap in average output power more than 2× that of the Space Winds Laser Transmitter, and 10× the pulse energy of TWiLiTE, but with a single amplifier stage instead of three.
Each of the following references is incorporated herein in its entirety by reference.
The entire contents of all references cited in this disclosure are incorporated herein in their entireties, by reference. Further, when an amount, concentration, or other value or parameter is given as either a range, preferred range, or a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, regardless of whether such ranges are separately disclosed. Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range. It is not intended that the scope of the invention be limited to the specific values recited when defining a range.
Other embodiments of the present invention will be apparent to those skilled in the art from consideration of the present specification and practice of the present invention disclosed herein. It is intended that the present specification and examples be considered as exemplary only with a true scope and spirit of the invention being indicated by the following claims and equivalents thereof.
The invention described herein was made in part by employees of the United States Government and may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefore.
