In this program we developed a solid-state laser source to provide tunable output through three spectral bands located between 1.5 and 2.5 m m. The core technology was a diode-laser-pumped Nd:YLF laser that was frequency-doubled to pump a titanium-doped-sapphire (Ti:S) laser. The Ti:S laser provided tunable pump radiation for two optical parametric oscillators (OPOs).
The output wavelengths of the OPOs were tuned by changing the pump Ti:S wavelength. This technique of tuning the OPO wavelength is mechanically simpler than the more common technique of angle-tuning the OPO crystal (which requires a physical rotation of the OPO crystal). In addition, it avoids the redirection of the output beam due to crystal rotation. Another advantage of pump-wavelength tuning is that it is possible to achieve rapid tuning with no moving parts by using an electronically tunable Ti:S laser. For demonstration purposes in this program we tuned the Ti:S laser with a conventional multi-plate birefringent filter, but tuners based on electro- or acoust-optics could provide considerably faster tuning.
The system requirements were for a 10-kHz repetition rate, which results in modest energies per pulse. In order to achieve efficient wavelength conversion using OPOs and sub-mJ pulses, it is desirable to use non-critical phase matching (NCPM). Non-critical phase matching allows tight focussing of the pump beam and long interaction lengths, resulting in low OPO thresholds. In this work rubidium titanyl arsenate (RTA) and cesium titanyl arsenate (CTA) were non-critially phase matched to produce the desired wavelength bands when pumped with a Ti:S laser.
The system we designed is shown schematically in Fig. 1. A diode-pumped Nd:YLF laser was frequency doubled using non-critically phase-matched lithium triborate (LBO). Approximately 6 W of 523-nm radiation was used to pump a three-mirror Ti:S laser to produce as much as 2.6 W of radiation tunable from 750 - 840 nm. Tuning of the Ti:S laser was accomplished by the computer-controlled, stepper-motor rotation of a birefringent filter. This tunable radiation was subsequently used to pump one of two optical parametric oscillators to produce tunable mid-IR radiation. Selection of the particular OPO was accomplished with motorized flip-mirror mounts.
The entire system was packaged in a custom enclosure and delivered to at the conclusion of the program to Eglin AFB.
Figure 1. A block diagram of the tunable source. Approximate average power levels (at a 10-kHz rate) and wavelengths are indicated for each stage.
In the course of the program, we achieved the following:
Figure 2 plots the output energy and pulsewidth of the Ti:S laser as a function of doubled Nd:YLF pulse energy, at a pulse rate of 10 kHz. The Ti:S laser for this data operated without the birefringent tuner installed. Figure 3 is tuning data on the RTA and CTA OPOs, plotted as a function of pump wavelength, while Figure 4 presents the RTA OPO idler energy output as a function of wavelength and Figure 5 plots the pulse shapes of the pump and OPO. Finally, Figures 6 and 7 show a plan view and photograph of the packaged system, respectively.
Figure 2. Ti:S laser energy (yellow circles) and pulsewidth (blue squares) vs. pump energy.
Figure 3. OPO wavelengths as a function of pump wavelength. The RTA OPO data points are circles, while the CTA OPO points are squares. Signal wavelengths are in blue, while the idler wavelengths are in green.
Figure 4. RTA OPO idler energy output as a function of wavelength
.
Figure 5. The time dependence of the idler (top trace) and pump (lower traces) for the RTA OPO. The pump signals are with the OPO off (black) and on (gray).
Figure 6. A schematic of the packaged laser system.
Figure 7. A photograph of the completed system.
Return to Recently Completed SBIR Programs or to Tunable Solid State Lasers