Method of inducing fluorescence using high-efficiency narrow-bandwidth potassium titanyl phosphate optical parametric oscillator

Abstract

A method for generating a pulsed high-repetition-rate narrow spectral bandwidth light to excite chemical species in a defined space includes inputting light from a first laser into an optical cavity having an input coupling mirror and an output coupling mirror disposed at a predetermined distance away from each other and a first crystal disposed therebetween, the first crystal being defined by i) a first cut angle, and ii) a first rotational disposition which together change the first wavelength of the input first laser to a second wavelength, mixing a mixing light at a mixing wavelength with the light at the second wavelength, thus generating an output light, and outputting light at the output wavelength out of the optical cavity at a predetermined power level to a defined space having chemical species or particles therein.

Description

METHOD OF INDUCING FLUORESCENCE USING HIGH-EFFICIENCY NARROWBANDWIDTH POTASSIUM TITANYL PHOSPHATE OPTICAL PARAMETRIC OSCILLATORCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present non-provisional patent application is related to and claims the priority benefit of U.S. Provisional Patent Application Serial 63 / 622,414, filed January 18, 2024, the contents of which are hereby incorporated by reference in its entirety into the present disclosure.STATEMENT REGARDING GOVERNMENT FUNDING

[0002] None.TECHNICAL FIELD

[0003] The present disclosure generally relates to a system and method of exciting gaseous chemical species in a space to thereby fluoresce, and in particular, to a system and method of using a high-efficiency narrow-bandwidth potassium titanyl phosphate (KTP) optical parametric oscillator to excite said chemical species.BACKGROUND

[0004] This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior ait.

[0005] There are numerous applications in hypersonic and turbulent flow fields that need spatially and temporally resolved diagnostics to better understand the physics of the flame and detonation-wave structure, detonation- wave interactions with gaseous or liquid reactants, turbulent boundary layers, and fuel / oxidizer mixing. Additionally, there are many applications in which chemical species within a predefined space need to be identified. Planar laser induced fluorescence (PLIF) is a non-intrusive laser diagnostic technique that can provide information on a two-dimensional distribution of key species in a flow stream or in said predefined space. In PLIF, typically a laser pulse excites a chemical species which in response thereto fluoresce. Early implementations of PLIF utilized nanosecond (ns) solid-state lasers to pump a dye laser and generate a tunable visible wavelength. The tunable output was then frequency doubled ormixed in a nonlinear crystal to generate a high-energy, tunable, narrowband UV beam to excite the species of interest. However, these measurements were typically limited in repetition rate, pulse energy, or both. High-speed continuously pulsed dye-pumped systems have been used to reach repetition rates of about 50 kHz, albeit with very low pulse energies of 140 pJ / pulse.

[0006] To achieve higher narrow-band repetition rates at relevant energies, the injection- seeded burst-mode parametric oscillator (OPO) was developed and used for PLIF of NO, OH, and CH after frequency mixing to UV wavelengths.

[0007] Table 1 below provides a list of prior art systems in which PLIF has been utilized each using an OPO and their associated burst-mode OPO UV energy generation efficiency.Table 1 - Prior Art systems and their associated burst-mode OPO UV energy generation efficiency* Efficiency calculated from fundamental 1064 nm.** 70% conversion to 532 nm and 40% to 355 nm are assumed from fundamental 1064 nm. OPO pump was used for the estimation.

[0008] In Table 1 efficiency is defined as the amount of fundamental energy from the laser to the amount of ultraviolet (UV) energy provided to the chemical species. For example, in the firstcolumn, the UV energy generated from the OPO is 200 pj from the energy provided by the laser (from the fundamental 1064 nm laser) which is 100 mJ, thus resulting in an efficiency of 0.2%. Table 1 provides efficiencies for all the prior art OPO systems which range from 0.17% to 0.87%.

[0009] There is an unmet need for a novel method that can improve efficiency of exciting chemical species in a space to thereby cause said chemical species to fluoresce using an OPO at high repetition rates in the UV wavelength range.SUMMARY

[0010] A method for generating a pulsed high-repetition-rate narrow spectral bandwidth light to excite chemical species in a defined space is disclosed. The method includes inputting light from a first laser having a first wavelength at a predetermined burst frequency into an optical cavity, the optical cavity having an input coupling mirror and an output coupling mirror disposed at a predetermined distance away from each other and a first crystal disposed therebetween, the first crystal being defined by i) a first cut angle, and ii) a first rotational disposition which together change the first wavelength of the input first laser to a second wavelength, the input coupling mirror configured to allow light at the first wavelength into the optical cavity but reflect light at the second wavelength, and the output coupling mirror configured to reflect light at the first wavelength. The method further includes mixing a mixing light at a mixing wavelength with the light at the second wavelength, thus generating an output light. Additionally, the method includes outputting light at the output wavelength out of the optical cavity at a predetermined power level to a defined space having chemical species or particles therein.

[0011] In the above method, the first crystal material is potassium titanyl phosphate (KTP).

[0012] The above method, further includes optically coupling light out of the first crystal with a second crystal defined by i) a second cut angle, and ii) a second rotational disposition.

[0013] In the above method, the first crystal has the first cut angle of about 59° with the first rotational disposition of between about 55° and about 64°.

[0014] In the above method, the first wavelength is about 532 nm.

[0015] In the above method, the second wavelength is between about 1410 nm and about 1710 nm, based on the first rotational disposition of the first crystal.

[0016] In the above method, the predetermined power level is between about 20% and about 60%.

[0017] In the above method, a mixing light with a mixing wavelength of about 266 nm, about 355 nm, or about 532 nm is mixed with light of the second wavelength to generate and output light with an output wavelength.

[0018] In the above method, the second rotational disposition of the second crystal includes an angle between about 26° and about 48°.

[0019] In the above method, the first cut angle is the same as the second cut angle.

[0020] In the above method, the first rotational disposition is opposite the second rotational disposition.

[0021] A system for generating a pulsed high-repetition-rate narrow spectral bandwidth light to excite chemical species in a defined space is also disclosed. The system includes an optical cavity. The system also includes a first laser outputting light at a first wavelength at a predetermined burst frequency into an optical cavity. Additionally, the system includes an input coupling mirror and an output coupling mirror disposed at a predetermined distance away from each other within the optical cavity. Furthermore, the system includes a first crystal disposed between the input and output coupling mirrors, the first crystal being defined by i) a first cut angle, and ii) a first rotational disposition which together change the first wavelength of the first laser to a second wavelength, the input coupling mirror configured to allow light at the first wavelength into and out of the optical cavity but reflect light at the second wavelength back into the cavity, and the output coupling mirror configured to reflect light at the first wavelength and output a predetermined power level the of light at the second wavelength out of the optical cavity. The system also includes a mixer configured to mix light at the second wavelength with a mixing light having a mixing wavelength and output an output light out of the optical cavity to a defined space having chemical species or particles therein.

[0022] In the above system, the first crystal material is potassium titanyl phosphate (KTP).

[0023] The above system further includes a second crystal defined by i) a second cut angle, and ii) a second rotational disposition, wherein light out of the first crystal is optically coupled with the second crystal.

[0024] In the above system, the first crystal has the first cut angle of about 59° with the first rotational disposition of between about 55° and about 64°.

[0025] In the above system, the first wavelength is about 532 nm.

[0026] In the above system, the second wavelength is between about 1410 nm and about 1710 nm, based on the first rotational disposition of the first crystal.

[0027] In the above system, the predetermined power level is between about 20% and about 60%.

[0028] In the above system, the mixing light has a mixing wavelength of about 266 nm, about 355 nm, or about 532 nm.

[0029] In the above system, the second rotational disposition of the second crystal includes an angle between about 26° and about 48°.

[0030] In the above system, the first cut angle is the same as the second cut angle.

[0031] In the above system, the first rotational disposition is opposite to the second rotational disposition.BRIEF DESCRIPTION OF DRAWINGS

[0032] FIG. la is a simplified schematic which depicts the basic operation of a system of the present disclosure.

[0033] FIG. lb is a schematic of a non-limiting example optical circuit according to the present disclosure.

[0034] FIG. 2a is a graph of Signal + Idler efficiency vs. pump energy in mJ / pulse.

[0035] FIG. 2b is a plot of the same parameters for the idler only vs. pump energy in mJ / pulse.

[0036] FIG. 2c is a graph of normalized intensity vs. time in ns.

[0037] FIG. 2d is a graph of beam diameter in mm vs. the Z direction in mm for beam quality of the 532 nm pump and 824 nm signal.

[0038] FIG. 2e is a graph of intensity vs. wavenumber in cm'1, with a 50-shot average and standard deviation.

[0039] FIG. 3a provides plots of energy in mJ vs. time in ns, representing profile plots at 1 MHz for 250 pulses of the 532 nm and 355 nm inputs, idler output, as well as UV outputs of the OPO generating about 350-400 pJ / pulse with a 1.2% efficiency.

[0040] FIG. 3b provides zoomed-in aspects of FIG. 3a to show the pulse-to-pulse variation and spacing of 1 ps.

[0041] FIG. 4a provides images of detonation waves based on OH planar laser induced fluorescence (PLIF).

[0042] FIG. 4b provides images of detonation waves, which show the radial fuel injection based on NO PLIF.DETAILED DESCRIPTION

[0043] For the purposes of promoting an understanding of the principles of the present disclosure, reference will now be made to the embodiments illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of this disclosure is thereby intended.

[0044] In the present disclosure, the term “about” can allow for a degree of variability in a value or range, for example, within 15%, within 10%, within 5%, or within 1% of a stated value or of a stated limit of a range.

[0045] In the present disclosure, the term “substantially” can allow for a degree of variability in a value or range, for example, within 85%, within 90%, within 95%, or within 99% of a stated value or of a stated limit of a range.

[0046] A novel method is disclosed herein which can improve efficiency of exciting chemical species in a space to thereby cause said chemical species to fluoresce using an optical parametric oscillator (OPO) operating in the ultraviolet (UV) wavelength. Towards this end, to improve the efficiency of conversion from the fundamental 1064 nm output (Fundamental) to the UV for high-speed planar laser induced fluorescence (PLIF), and to reduce the cost and complexity of the OPO providing narrowband excitation without use of an external seed laser, a burst-mode parametric oscillator (OPO) based on a potassium titanyl phosphate (KTP) Type-II crystal is disclosed herein. The developed KTP OPO could achieve a 1.8-1.9% efficiency from the Fundamental to the UV, which is a 3-4x increase from previously reported injection seeded burst-mode Type-I barium beta borate (BBO) OPOs within the last decade (see Table 1, above). Additionally, the linewidth of the KTP OPO is 1.5 cm-1, which is 20 times lower than unseeded BBO Type-I OPOs used for many burst-mode applications.

[0047] Referring to FIG. l a, a simplified schematic is provided which depicts the basic operation of a system 100 of the present disclosure. The system 100 includes a fundamental laser 102 (c.g., a 1064 nm burst- mode laser). The fundamental laser 102 may be frequency doubled by a frequency doubler internally (not shown) to thereby cut the wavelength to half (e.g., 532 nm + / - 1 nm). Alternatively, the fundamental laser may have the desired wavelength, and frequency doubler is added externally to fundamental laser 102 (not shown). Light 104 from the frequency doubler (not shown), or alternatively from the fundamental laser with internal doubler, is provided to a cavity 106 which outputs light 108 with the desired wavelength and optionally light 110. The cavity 106 includes an input coupling optical element 106i, an output coupling optical element IO62 disposed at a predetermined distance away from each other and at least one crystal IO63 disposed therebetween. The at least one crystal IO63 is configured to have an angular disposition, both in terms of cut angle as well as rotational position, which changes wavelength of light 104 to the wavelength of light 108 and optionally wavelength of the light 110. The input coupling optical element IO61 is configured to allow light 104 at the first wavelength into the optical cavity 106 but reflect light at the second and optionally third wavelengths of light 108 and 110 back into the cavity 106, and the output coupling optical element IO62 is configured to reflect light 104 having the first wavelength but allow at least a part of light at the second and optionally third wavelengths 108 and 110 out of the optical cavity 106.

[0048] The at least one crystal IO63, according to one embodiment is a KTP crystal with a cut angle of about 59 degrees to be tuned from about 55 degrees to about 64 degrees. Wavelength of light 104 is about 532 nm. According to one embodiment, wavelength of light 108 is based on two collimated beams, the first is between about 770 nm and about 855 nm and the second is between about 1410 and 1710 nm. To correct for refractive changes out of a single crystal of the at least one crystal IO63, two KTP crystals may be used back-to-back, allowing light to be refracted at the first of the two KTP crystals and re-refracted at the second of the two KTP crystals, as known to a person having ordinary skill in the art (walk-off compensation). The second KTP crystal has a cut angle of about 59 degrees to be tuned from about 55 degrees and 63 degrees. Each of the two KTP crystals may be positioned based on any combination of stationary or articulable disposition. If articulable, the angle of articulation of the KTP crystal may be basedon a predefined angle or a dynamically-defined angle. While not shown in FIG. 1 a, a mixing laser (not shown) may be added after the output coupling optical clement IO63 to further modulate the wavelength of light 108 to a different wavelength. A mixing crystal may be used in the mixing process as is known to a person having ordinary skill in the art and as shown in FIG. lb.

[0049] Referring to FIG. lb, a non-limiting example optical circuit is provided according to the present disclosure. As discussed with respect to FIG. la, a laser (e.g., having a fundamental wavelength of 1064 nm and about 10 ns pulse width) is initially frequency doubled to a generate a pump laser having half the wavelength of the fundamental wavelength. However, as mentioned above, a laser with the desired fundamental frequency may be used without a frequency doubler. The frequency -doubled pump laser is provided to the optical cavity including a first mirror for redirecting light into the cavity, an input mirror and an output coupler and one or two KTP crystals, wherein the input mirror allows light with the frequency-doubled wavelength into the cavity, the frequency-doubled light is refracted through the first KTP crystal, during such refraction, some of the photons of the frequency-doubled light are converted to a second wavelength (signal) and third wavelength (idler) photons simultaneously, and re-refracted through the second KTP crystal, and then onto the output coupler which reflects the frequency- doubled light, thus directing the frequency-doubled light back through the cavity for increased efficiency, but allows a fraction of the light with the modified wavelength out of the cavity. The second KTP crystal orientation in the cavity is also different compared to the first crystal. For example, the second crystal is rotated 180 deg around the axis parallel to the beam propagation relative to the first crystal. The input mirror prevents light with the modified wavelength from exiting the cavity by reflecting such lights. The cavity according to one non-limiting embodiment has a length of about 4.5 cm. The two KTP crystals are provided in a non-limiting configuration as two 8x8x15 mm Type-II KTP crystals (purchased from EKSMA OPTICS) cut at q =59 degrees and / = 0 degrees with a 532 / 1410-1550 nm AR coating. In FIG. lb, the frequency- doubled light polarization is optionally changed from a vertical polarization to a horizontal polarization since the two KTP crystals are configured to be rotated around a horizontal axis. This change of polarization is accomplished by a half- wave plate (HWP). However, if thecrystals are configured to be rotated vertically, this change in polarization may be avoided. In FIG. lb, the frcqucncy-doublcd light is shown with a solid line and denoted as having a 532 nm wavelength. Light with wavelength modification within the cavity is shown as long-dashed line having a wavelength between about 823 nm and about 850 nm as a signal and also as long-dash- short-dash line having a wavelength of between about 1420 nm to about 1503 nm as the idler, at least part of both of which are allowed to exit the cavity by the output coupler and both of which are prevented to exit the cavity by the input mirror. The crystals are oriented opposite of each other for walk-off compensation. Each cavity crystal is mounted to rotate about the horizontal axis on stepper motors with 0.007 degree resolution. The 532 beam enters the cavity through the input mirror, which is coated to transmit the pump (i.e., the 532 nm light) on both sides and reflect signal (i.e., light with wavelength between about 770 to 855 nm) and idler (i.e., light with wavelength between about 1410 nm and about 1710 nm) on the inside. The beam passes through the two crystals, and the output coupler reflects 20% of the signal and idler and completely reflects the 532 nm. The idler and signal are getting amplified by the crystals. The energy transfers from 532 nm pump beam into idler and signal each round trip until the end of the pump pulse. The cavity produces a vertically polarized signal and a horizontal idler, which is subject to change with the crystal orientation. The signal or idler is then mixed with the second, third, or fourth harmonic of the Nd:YAG pump laser in a Type-I BBO crystal to generate UV output targeting OH, NO, CH, O and CO chemical species, as described below.

[0050] Also shown in FIG. lb, there is a sum frequency generation (SFG) stage. A Type-I BBO crystal is rotated on a stepper motor with 0.007 degree resolution that allows a mixing beam input to be added to the signal to thereby generate the UV output. In the non-limiting example shown in FIG. lb, the mixing bean input is a laser shown as dashed line with a wavelength of 266, 355, or 532 nm. The polarization of the mixing beam input is optionally changed by utilizing another half wave plate and combined with the signal via a network of mirrors prior to reaching the BBO SFG crystal at which point the signal or idler and the mixing beam input are combined to generate the UV output shown as long-dash-dot line and having a tunable wavelength about 226, 230, 248, 284, and 390 nm.

[0051] Referring to FIG. 2a, a graph of Signal + Idler efficiency is provided vs. pump energy in mJ / pulsc and FIG. 2b plots the same parameters for the idler only, as only the idler is used for mixing in the OH and NO configurations. It is noted that the OPO cavity performs similarly over the 1410-1710 nm idler tuning range corresponding to 55-64° internal crystal angle range. The lasing threshold of the OPO cavity was measured to be at about 1 mJ / pulse of pump, annotated as a green circle in FIGs. 2a and 2b, which is 10-12 times lower than that for burst-mode BBO OPOs. The energy study shows the cavity energy output linearly increases as a function of pump energy. When observing the efficiency, there is a steep increase from the lasing threshold to about 12 mJ / pulse of pump pulse energy, where it stabilizes around 44% (annotated by a grey dashed lines in FIGs. 2a and 2b), reaching a maximum of 48% at 25-40 mJ. The crystals and cavity optics did not show any damage from this energy range at the repetition rate of 10 Hz and 8 ns pulse width.

[0052] Another quantification of cavity efficiency is the build-up time. It was measured to be 1.6 ns (see FIG. 2c, which is a graph of normalized intensity vs. time in ns), which equates to about 3.5 round trips when accounting for a cavity length of 4.5 cm with 30 mm of KTP medium (n = 1.78).

[0053] It should be appreciated that a Type-II crystal has an advantage over Type-I, as it produces superior beam quality due to the lower acceptance angle and narrower bandwidth. The beam quality of the 532 nm pump and 824 nm signal were measured with a Beam Squared Machine (SPIRICON BSQ-SP920) and results are plotted in FIG. 2d. The 10 Hz laser has a measured beam quality M2of 1.29 in the X and 1.43 in the Y directions, respectively. The signal beam quality deteriorated in both axes to M2of 8.43 in the X (non-phase matching) and 3.58 in the Y (phase matching) directions, respectively. The beam quality of the OPO cavity output is critical for mixing efficiency, and the KTP OPO has a good beam quality in the phase matching direction.

[0054] For the fundamental to UV efficiency measurements in the OH configuration, the 10 Hz Nd;YAG laser produced 9.2 mJ at 532 nm to pump the OPO cavity to generate idler at 1420 nm, which was then mixed with 4.75 mJ at 355 in a Type-I BBO at 34.2°. This generated a total of 500 pJ at 284.005 nm to excite the A-X (0,0) Qi(9) and Q2(8) transition pair, chosen because ofthe strength and temperature insensitivity. A 1 .9% OPO efficiency from the fundamental was calculated for the OH configuration as provided in Table 2 below. For the NO configuration, the 10 Hz Nd:YAG laser produced 15 mJ of 532 nm, which then was doubled to generate 5 mJ of 266 nm using a 5 mm long BBO crystal. The remaining 10 mJ at 532 pumped the cavity to produce an idler at 1503 nm, which was then mixed with the 266 in a Type-I BBO at 40.8°. This generated 400 pJ at 226.03 nm to excite a combination of A-X (0,0) Qi+Pi(14), Qi+Ri2(21 ) and other lines. A 1.8% OPO efficiency from the fundamental was calculated for the NO configuration (See Table 2, below), which represent a significant increase in efficiency as compared to the prior art systems provided in Table 1, above.Table 2 - System of the present disclosure and the associated burst-mode OPO UV energy generation efficiency for OH (284 nm excitation wavelength) and NO (226 nm excitation wavelength)* Efficiency calculated from fundamental 1064 nm.** 70% conversion to 532 nm and 40% to 355 nm are assumed from fundamental 1064 nm. OPO pump was used for the estimation.

[0055] Since the OPO cavity is unseeded, it is necessary to characterize the OPO output linewidth and its suitability for the excitation of selected energy transitions. The OPO cavity signal spectrum is plotted in FIG. 2e, which is a graph of intensity vs. wavenumber in cm’1, with a 50-shot average and standard deviation. The output was dispersed with a 0.75-meter spectrometer (instrument function of 0.42 cm"1) and was measured to have a FWHM bandwidth of 1.48 ± 0.19 cm"1, as annotated with a black dashed arrow in FIG. 2e. Assuming the idler has the same linewidth as the signal, and using the linewidth of the burst- mode laser of <0.05 cm"1, the deep UV output is estimated to have a bandwidth of about 1.48 cm"1(0.0086 nm) after mixing. For reference, the simulated FWHM of OH excitation of the Qi(9) and Q2(8) transition pair is 0.8 cm"1, and the NO excitation of the Qi+P2i(14), Q2+Ri2(21), Pi(23) is 1 cm"1(LIFBASE, 2000 K, 1 atm). From this analysis, it can be shown that the available bandwidth is sufficiently narrow to make LIF measurements of both species. To corroborate, two experimental sweeps of the cavity output central wavelengths were performed. The first was over an NO line in a 300 ppm NO cell at 1.28 atm and 300 K.

[0056] The burst-mode laser was used to visualize the variation in pulse intensity throughout the burst. A digital oscilloscope recorded the simultaneous profiles for the 532, 355, 1420, and 284 nm pulses from the OPO. Referring to FIG. 3a, which depicts plots of energy in ml vs. time in ns, profile plots at 1 MHz for 250 pulses of the OPO generating about 350-400 pl / pulse with a 1.2% efficiency are provided, due to non-optimal mixing. Referring to FIG. 3b, zoomed- in aspects of FIG. 3a are provided to show the pulse-to-pulse variation and spacing of 1 ps. The change in energy through the 284 nm burst train is from the BBO mixing crystal absorbing heat and thus changing the nonlinear properties. The maximum tested burst was 500 pulses at 14 mJ / pulse of 532 nm pump at 1 MHz. Increase in number of pulses above 250 resulted in damage to the KTP crystal, from charge defects in the crystal induced by gray track susceptibility. Though the KTP has a lower damage threshold than the BBO, maximum pulse trains of 200-250 were achieved, similar to prior burst-mode BBO OPOs discussed above.

[0057] The burst-mode KTP OPO was applied in the optically accessible RDC, to visualize a spatially resolved structure of the detonative and dcflagrativc combustion (OH) processes, and to visualize the NO seeded fuel injector response on the imposing detonation wave (NO). The experimental setup of the PLIF in the RDC demonstration was similar to that described in Hsu et al (P. S. Hsu, M. N. Slipchenko, N. Jiang, et al., Opt. Lett. 45, 5776 (2020). In short, the premixed RDC was operated at an air mass flow of about 1 Ibm / s and an equivalence ratio of about 1 resulting in a detonation wave frequency of 4 kHz (1550 m / s). The burst-mode laser pumped the OPO at 200 kHz to generate 284.005 nm (OH) and at 100 kHz to generate 226.03 nm (NO). The repetition rate of 100-200 kHz was used to record a sequence of images over several detonation wave passes with high resolution from the 200-pulse burst. The 5 out of 150 selected OH PLIF frames at 200 kHz are shown in FIG. 4a, which provides images of detonation wave. The images show combustion products from the previous cycle exiting the combustor channel to the right. At 360 ps, the detonation wave comes into plane anchored on the outer wall and is observed to be strongest axially about 15 mm into the combustor. Previous OH-PLIF has shown the presence of OH radicals near the injector, which could be a deflagrative burning zone, thus leading to wasted fuel and detonation wave inefficiencies.

[0058] The OPO of the present disclosure was tuned to generate 226 nm and a gaseous mixture of 1% NO in 99% N2 (10,000 ppm) was injected into the channel to visualize the dynamic response of a gas reacting stream to the imposing detonation wave for injector optimization. For stable RDC operation, it is necessary that fuel is constantly supplied to the combustor. This diagnostic can be used to quantify the fuel refill time and distance the fuel propagates into the combustor. Referring to FIG. 4b, in which images of detonation wave are provided, shows the radial fuel injection. The NO PLIF at 100 kHz demonstrates the injected gas filling of the detonation channel at 140 ps. The detonation wave passes at 160 ps displacing the gas, and the injector remains supplying fuel. Combining the two PLIF measurements can provide simultaneous tracking of reactants and product gases, necessary for combustor optimization.

[0059] Those having ordinary skill in the art will recognize that numerous modifications can be made to the specific implementations described above. The implementations should not be limited to the particular limitations described. Other implementations may be possible.

Claims

Claims:

1. A method for generating a pulsed high-rcpctition-ratc narrow spectral bandwidth light to excite chemical species in a defined space, comprising: inputting light from a first laser having a first wavelength at a predetermined burst frequency into an optical cavity, the optical cavity having an input coupling mirror and an output coupling mirror disposed at a predetermined distance away from each other and a first crystal disposed therebetween, the first crystal being defined by i) a first cut angle, and ii) a first rotational disposition which together change the first wavelength of the input first laser to a second wavelength, the input coupling mirror configured to allow light at the first wavelength into the optical cavity but reflect light at the second wavelength, and the output coupling mirror configured to reflect light at the first wavelength; mixing a mixing light at a mixing wavelength with the light at the second wavelength, thus generating an output light; outputting light at the output wavelength out of the optical cavity at a predetermined power level to a defined space having chemical species or particles therein.

2. The method of claim 1, wherein the first crystal material is potassium titanyl phosphate (KTP).

3. The method of claim 1, further comprising optically coupling light out of the first crystal with a second crystal defined by i) a second cut angle, and ii) a second rotational disposition.

4. The method of claim 1, wherein the first crystal has the first cut angle of about 59° with the first rotational disposition of between about 55° and about 64°.

5. The method of claim 1, wherein the first wavelength is about 532 nm.

6. The method of claim 4, wherein the second wavelength is between about 1410 nm and about 1710 nm, based on the first rotational disposition of the first crystal.

7. The method of claim 1, wherein the predetermined power level is between about 20% and about 60%.

8. The method of claim 3, wherein a mixing light with a mixing wavelength of about 266 nm, about 355 nm, or about 532 nm is mixed with light of the second wavelength to generate and output light with an output wavelength.

9. The method of claim 3, wherein the second rotational disposition of the second crystal includes an angle between about 26° and about 48°.

10. The method of claim 3, wherein the first cut angle is the same as the second cut angle.

11. The method of claim 3, wherein the first rotational disposition is opposite the second rotational disposition.

12. A system for generating a pulsed high-repetition-rate narrow spectral bandwidth light to excite chemical species in a defined space, comprising: an optical cavity; and a first laser outputting light at a first wavelength at a predetermined burst frequency into an optical cavity; an input coupling mirror and an output coupling mirror disposed at a predetermined distance away from each other within the optical cavity; a first crystal disposed between the input and output coupling mirrors, the first crystal being defined by i) a first cut angle, and ii) a first rotational disposition which together change the first wavelength of the first laser to a second wavelength, the input coupling miiTor configured to allow light at the first wavelength into and out of the optical cavity but reflect light at the second wavelength back into the cavity, and the output coupling mirror configured to reflect light at the first wavelength and output a predetermined power level the of light at the second wavelength out of the optical cavity; and a mixer configured to mix light at the second wavelength with a mixing light having a mixing wavelength and output an output light out of the optical cavity to a defined space having chemical species or particles therein.

13. The system of claim 12, wherein the first crystal material is potassium titanyl phosphate (KTP).

14. The system of claim 12, further comprising a second crystal defined by i) a second cut angle, and ii) a second rotational disposition, wherein light out of the first crystal is optically coupled with the second crystal.

15. The system of claim 12, wherein the first crystal has the first cut angle of about 59° with the first rotational disposition of between about 55° and about 64°.

16. The system of claim 12, wherein the first wavelength is about 532 nm.

17. The system of claim 16, wherein the second wavelength is between about 1410 nm and about 1710 nm, based on the first rotational disposition of the first crystal.

18. The system of claim 12, wherein the predetermined power level is between about 20% and about 60%.

19. The system of claim 16, wherein the mixing light has a mixing wavelength of about 266 nm, about 355 nm, or about 532 nm.

20. The system of claim 14, wherein the second rotational disposition of the second crystal includes an angle between about 26° and about 48°.

21. The system of claim 14, wherein the first cut angle is the same as the second cut angle.

22. The system of claim 14, wherein the first rotational disposition is opposite to the second rotational disposition.

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