Summary
This recently completed Phase II program resulted in the development of a new, laser-based, non-intrusive, diagnostic instrument for use in the study of supersonic and hypersonic flows and, in particular, reactive flows. The instrument is comprised of a single-frequency, high-energy, Ti:sapphire laser and high-resolution, atomic-resonance filter (ARF). The Ti:sapphire laser, via harmonic generation, provides a broadly tunable, high-brightness, high-spectral-purity source of ultraviolet radiation for non-intrusive probing of hypersonic flows. The optical ARF provides an incoherent, high-resolution, detection function for the instrument. The new instrumentation was developed in collaboration with the Princeton University Lasers & Applied Physics Group. Ultimately this innovation will allow accurate 2-D and 3-D measurements of scalar fields such as temperature, density, pressure and species concentrations (atomic, molecular, and radical), and vector fields such as velocity and vorticity. These measurement capabilities are critical to the future development of sophisticated supersonic and hypersonic aircraft.
The Ti:sapphire laser development work culminated in a highly reliable and stable single-frequency oscillator with efficient UV conversion to yield up to 40-mJ pulse energies at 254 nm. The laser pulsewidths were transform limited with typical spectral widths of 28 MHz. Details of the laser system are available at this site. The overall system has demonstrated two-dimensional flow visualization with spatial resolutions of approximately 3.4 line-pairs/mm at flow rates up to 800 ms-1 with 1% accuracy. To date the system has been in daily use for most of the past year and functions as a reliable laboratory diagnostic tool.
Background
In the past few years the challenge of high-speed flight has stimulated a great deal of research and development in the U.S. and elsewhere. In the U.S., the National Aerospace Plane (NASP) concept has served as a major force in reviving interest in hypersonic aerodynamics and propulsion. Similar efforts in Europe and Japan have resulted in a worldwide increase in activity in basic research, concept development, and design efforts.
Flight technology in the supersonic and hypersonic regime is far from being understood, and design optimization will require new understanding and new predictive capabilities developed as a result of fundamental research. The aerodynamic design of aircraft and the design of their propulsion systems will both require fundamental improvements in our understanding of high-speed flows.
In particular, the propulsion system research will require an understanding of the chemical kinetics, large and small scale mixing processes and heat transfer dynamics that take place under supersonic and hypersonic conditions. Gaining this understanding will necessarily involve the development of non-intrusive diagnostic instruments that can measure such flow field parameters as temperature, pressure, density, velocity and vorticity. Furthermore, it will be essential, in some cases, to measure these parameters for specific atomic, molecular and radical species in a flow that is comprised of a diverse mixture of species.
This need for sophisticated analysis has generated a great deal of interest in laser-based, high-resolution, spectroscopy techniques because single-frequency lasers combined with appropriate filter/detector technology can provide a non-intrusive, extremely selective mechanism for measuring the parameters mentioned above. The focus of this program was the development of a filtered Rayleigh scattering (FRS) technique using UV laser excitation.
Filtered Rayleigh Scattering
The FRS technique uses a spectrally narrow laser source and a sharp-cut-off optical filter to separate Doppler-shifted Rayleigh scatter coming from the flow field from the unshifted scatter that comes from stationary surfaces in the instrument's field of view. This technique has been shown to work with a frequency-doubled, Nd:YAG laser operated at 532 nm and an iodine (I) ARF. During this SBIR effort, we developed a FRS instrument that uses a mercury (Hg) ARF employing a strong absorption line at around 253 nm. In this system the illumination of the flow field is provided by a Ti:sapphire laser that operates at a fundamental wavelength of 761 nm and is frequency tripled, using nonlinear optical crystals, to produce the required 253-nm source.
The Hg /Ti:sapphire instrument provides a significant improvement over the I/Nd:YAG system simply by operating in the UV as a consequence of the well-known, fourth-power frequency dependence for the Rayleigh-scattering cross section. In addition, there is less "flare" scattering from metal surfaces, which leads to much better image contrast.
The overall objective of the SBIR project was to develop an instrument suitable for use in hypersonic wind tunnel facilities. This involved the development of some critical electro-optic components and, subsequently, integration of these new components into a complete diagnostic instrument. Specifically, one critical development task was to develop a robust, wide-bandwidth, control system capable of maintaining continuous, single-frequency operation of the Ti:sapphire laser even in the presence of high levels of ambient acoustic noise. We discuss this in more detail below. Other tasks included improved UV generation via third harmonic conversion in nonlinear crystals and continued development of the ARF detection system.
Development of the Injection-Locking Instrumentation
We needed to develop electronic control circuitry that would enable the reliable injection seeding of a pulsed Ti:sapphire laser with a continuously tunable, cw Ti:sapphire laser. The design goals for the system were that the injection seeding should be automatic and be able to accommodate variations in seed signal power and pulsed laser cavity finesse, as the two lasers are wavelength tuned, i.e. the seeding process should be wavelength invariant. Additionally, the system should be able to work in a high-acoustic-noise environment, typical of the lab environments where hypersonic gas flows are monitored. It was felt for an injection seeding system to work in these demanding conditions, a custom designed, high bandwidth PZT would be required in addition to a unique form of control architecture.
We have developed what we call a ramp and lock control system that adjusts the pulsed laser resonator modal frequencies to match the seed laser frequency for every laser pulse. If correctly implemented, such a system can nulls out the effects of temperature, vibration, and pump induced chirp in the relative frequencies of the two lasers and ensure successful single frequency operation for each laser pulse. A schematic of the complete system is shown in Figure 1.
Figure 1. General schematic of the ramp and lock injection-seeding control system.
The pulsed Ti:sapphire laser is seeded by a cw continuously scannable Ti:sapphire laser, which is coupled into the cavity via a fused-silica uncoated substrate operated a few degrees away from Brewster’s angle. By this means it provides both an entrance and exit port for the seed light. The seed light exiting from this port is filtered by the Fabry-Perot etalon formed by the pulsed laser resonator and is monitored with a Si-PIN photodiode. The signal from this photodiode provides the input to the ramp and lock control system that provides an accurate measure of the relative frequency of the two laser resonators.
The electronic instrumentation that has been developed consists of four major functional components:
1) the ramp and lock control electronics,
2) a high bandwidth, high voltage amplifier,
3) a high gain, low noise photodiode, and
4) a high bandwidth PZT actuator.
Figure 2 shows a schematic of the functional blocks of the control electronics.
Figure 2. Schematic of ramp and lock control circuitry.
The injection seeding system operates as follows. Prior to the firing of the pump laser, a voltage ramp is applied to the mirror-mounted PZT in the pulsed Ti:sapphire laser. As the high reflector is displaced, the amplified photodetector monitoring the cw seed beam circulating in the pulsed laser cavity detects fringes in time with a modulation depth of approximately 50%. The temporal fringes occur since the pulsed cavity acts as a low finesse scanning Fabry-Perot etalon. Each new fringe corresponds to an increment of the pulsed cavity length by one half the seed laser wavelength. As the cavity length is varied through l/2 the detected signal will move through a maximum, i.e. cavity resonant with injected wavelength, to a minimum, i.e. off resonance. Maximal transmission occurs when the pulsed resonator meets the cavity condition L = ml /2, where L is the length of the pulsed cavity; m is an integer, l is the wavelength of the injected seed laser.
Figure 3 illustrates the photodiode voltage as a function of time. Labeled on this diagram are temporal phases of the ramp and lock scheme, including the ramp initiation, locked region, and the arrival of the Q-switched pump pulse which is essentially coincident with the generation of the Ti:sapphire laser pulse. The signal from the photodiode is amplified and passes through a sample and hold circuit. During the ramp phase, a minimum and maximum level detector to determine the fringe peak height and depth monitors this signal. A potentiometer is used to vary the locking setpoint as a fraction of the difference between these levels. During the lock phase of the cycle, the photodiode signal is summed with an inverted lock setpoint value, giving the difference between the actual and desired voltages on the photodiode as the control loop error signal. A variable gain proportional-integral control loop, varies the voltage applied to the PZT stack to null this error signal. Depending upon the operating environment, additional filtering mechanisms may be added to the lock-loop as required. To eliminate loss of loop lock due to electrical transient noise pick-up when the pump laser is fired, the voltage applied to the PZT stack is clamped and the photodiode bias is gated off for a predetermined amount of time on the order of several 100ns prior to the laser firing.
Figure 3. Monitor photodiode signal during one cycle of loop lock.
Following the laser firing, the cavity operates without locking until the 20 ms prior to the laser firing again. During this unlocked period the photodiode signal will follow the perturbation of the cavity frequency relative to the seed laser. This perturbation is primarily from acoustic noise coupled to the cavity. The unlocked region continues until the next ramp is initiated and the whole process is repeated. Hence, the ramp and lock technique acquires lock before every pulse and as a result, regardless of how the cw frequency changes, the pulsed system tracks the seed laser and operates on a single frequency.
For single-frequency operation, the optimum lock set point is not at the peak of the fringe (the resonance condition), because of cavity length chirping induced by the pump laser as it thermally expands the length of the Ti:sapphire crystal and possibly changes its refractive index by nonlinear effects. This lock point offset compensates for the pump-induced change in the pulsed cavity's effective length. The pump induced frequency chirp is negative and opposite to the increasing frequency chirp caused by the PZT expansion decreasing the laser cavity length.
Since we have the option of locking to either side of the cavity fringe; there are two possible modes of operation, as illustrated in Figure 4. If the set point is on the negative side of the fringe maximum, then on arrival of the pump pulse, the cavity will be driven into resonance with the seed laser and the pulsed laser will operate on a single longitudinal mode. Conversely if the set point is on the negative side of the resonance point, the pump pulse negative frequency chirp will drive the cavity into anti-resonance. In this regime the seed laser can seed its two nearest neighbor modes, and the pulsed Ti:sapphire laser will operate on two longitudinal modes.
Figure 4. Fringe set points for two modes of operation, SLM and dual mode.
Figure 5 shows the resulting intensity profiles of the Ti:sapphire laser pulses for these two cases. For the SLM case, the output pulse profile is characteristically smooth and has a slightly shorter build up time in contrast to the modulated profile (due to mode beating) of the laser pulse in dual mode operation. Typically the Ti:sapphire laser will operate with 10-15% higher energy output if operating on two longitudinal modes as a result of reduced spatial hole burning in the gain medium. In some applications, where single frequency operation is not a concern this mode of operation may be useful for improved harmonic generation and increased pulse energy.
The temporal width does not necessarily dictate the spectral profile of the laser, though it does dictate the minimum spectral width as determined by the Fourier transform relationship. In order to assess the spectral width we used a 2-GHz-free-spectral-range, confocal Fabry-Perot instrument with a finesse of 100. With the etalon fixed and the laser scanning we obtained a 28 MHz linewidth (fwhm). The measured pulsewidth at that time was 22.5 ns, or the transform limit for the 28 MHz linewidth.
Figure 5. Pulse intensity profiles for single- and dual-mode Ti:sapphire laser operation.
The ramp and lock electronics and associated high-voltage operational amplifier to drive the PZT cavity control were packaged into rack-mounted components suitable for use in commercial products. Figure 6 is a photograph of the components, with the Ramp and Lock Injection Seeding Controller on top and the High Voltage Op Amp (Model HVA 375) on the bottom.
Figure 6. Packaged, rack-mountable Ramp and Lock Electronics.
Reference:
N.D. Finkelstein, W. R. Lempert, R.B. Miles, A Finch and G.A. Rines, "Cavity-locked, injection-seeded, titanium:sapphire laser and application to ultraviolet flow diagnostics," American Institute of Aeronautics and Astronautics, 34th Aerospace Sciences Meeting, Reno, NV, January 15-18, 1996 (AIAA 96-0177).
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