Dual Wavelength Injection-Locked Pulsed Ring Laser with Improved Noise Immunity, 18-R8168
Principal Investigators
Thomas Moore
F. Scott Anderson
Joseph Mitchell
Inclusive Dates: 07/01/10 – 12/01/11
Background — Interest in developing tunable laser systems that have the capability to produce two wavelengths of light simultaneously has intensified over the past several years. The motivation for developing dual-wavelength laser systems includes differential absorption lidar (DIAL), non-linear frequency mixing, pump-probe detection, and laser resonance ionization. Work on developing a portable Laser Desorption Resonance Ionization Mass Spectrometer (LDRIMS) instrument by SwRI space researchers led to a need for several specialized laser systems. The LDRIMS instrument performs geochronology and geochemistry measurements based on the ratios of certain rubidium (Rb) and strontium (Sr) isotopes. The resonance ionization technique selectively ionizes specific atoms or molecules by exciting them with specific wavelengths of laser light, simultaneously identifying the atom or molecule. The LDRIMS system, as it exists today, utilizes seven lasers that require a large amount of space and power. Developing a tunable laser with the capability to deliver two wavelengths simultaneously will significantly reduce the space and power requirements, avoid timing jitter issues among multiple systems, and decrease the burden of maintaining multiple laser systems. All these increase the possibility of using this technique in a space mission.
Approach — SwRI developed a unique approach for generating two wavelengths simultaneously. In addition, the laser output operates near the Fourier transform limit and provides stable output with immunity from mechanical vibration throughout the acoustic range. The system has the potential to reduce the number of lasers needed by the LDRIMS system by half and provides the capability to be ruggedized in a small portable package. To achieve these objectives, a Ti:sapphire ring laser design with a total path length of 50 cm is used to form a traveling wave oscillator. The traveling wave provides the capability to utilize the entire length of the Ti:sapphire laser crystal while eliminating standing modes. The ability to independently tune the two center wavelengths of the laser system is provided by two independent seed lasers, which are injected into the cavity via the output coupling mirror and propagate within the ring laser cavity. Alternatively, intra-cavity frequency selection optics such as angle-tuned interference filters, birefringence filters, prisms, or etalons may be used in lieu of the seed lasers when laser line width is not a critical factor. Enhanced noise immunity and the ability to achieve Fourier transform limited output are accomplished with the use of a nonlinear electro-optic crystal and a Ramp-Hold-Fire (RHF) cavity control technique. The RHF technique utilizes a KD*P crystal to modify the phase of the seed light propagating within the ring cavity until the light is resonant within the cavity. Once the resonance condition is detected, the resonance is held for a short period of time until the laser is fired, resulting in seeded Fourier transform limited laser output. If the time between the cavity resonance detection and laser fire is less than 30 μs, the laser output will be immune to acoustic noise. The laser system and techniques developed at SwRI represent a substantial step forward in multiple wavelength pulsed laser design.
Accomplishments — SwRI has successfully developed a Ti:sapphire ring laser with the capability to produce injection seeded laser output at two discrete wavelengths. Furthermore, SwRI has shown that this method can be extended to produce laser output at multiple discrete wavelengths with four or more seed lasers. Feedback control of the slave ring laser cavity using a KD*P crystal to phase modulate the seed light propagating within the slave oscillator has also been demonstrated. SwRI used a RHF technique for seeding the dual-wavelength laser system to generate 20 ns output pulses. Immunity to noise throughout the acoustic range can be achieved with the RHF technique provided the laser is fired within 30 μs of the resonance detection. The RHF technique may also be adapted to many other types of injection-seeded solid state laser systems to produce Fourier transform limited output. Laser performance measurements indicate that the slave oscillator is properly seeded and producing Fourier-transform limited pulsed laser output. SwRI has also demonstrated the capability of the seed lasers to drive the D2 transition of Rubidium 87 and the ability to continuously tune the laser wavelength from 776 nm to 790 nm.
