Measurement Technique for Continuous Wave, Modulated and Pulsed Monochromatic Radiation

NASA's CW Laser Source for Injection Seeding uses a single laser diode (LD) to produce multiple wavelengths. Depending on the application, the seed laser may or may not be locked to a wavelength reference. For example, in atmospheric differential absorption lidar (DIAL) active remote sensing applications, the seed laser has to be locked and referenced to the species of interest using gas cells. In this context, the seed laser source is first locked to an absorption feature and the generated wavelength is used as a reference from which other offset wavelengths are generated. However, if the requirement calls only to avoid atmospheric absorption then locking may not be required. Using this new technology, an airborne 2-micron triple pulse integrated path differential absorption (IPDA) LIDAR instrument has been developed at NASA Langley Research Center to measure the column content of atmospheric H2O and CO2 simultaneously and independently. This is achieved by transmitting three successive high-energy pulses, seeded at three different wavelengths, through the atmosphere. The three pulses are emitted 200 microseconds apart and repeated at 50 Hz. The seeding wavelengths were selected to achieve minimum measurement interference from one molecule to the other. Typically, this requires four different CW lasers for seeding. A part of that effort focused on adaptive targeting, which is based on the tuning capability of the on-line wavelength to meet a certain measurement objective depending on observational time and location. The off-line wavelength was assumed constant. The tuning capability can be achieved using the claimed seeding technique using a voltage-controlled oscillator for the on-line and fixed oscillator for the off-line.

The new NASA LaRC Pulsed 2-Micron Laser Transmitter for Coherent 3-D Doppler Wind Lidar Systems is an innovative concept and architecture based on a Tm:Fiber laser end-pumped Ho:YAG laser transmitter. This transmitter meets the requirements for space-based coherent Doppler wind lidar while reducing the mission failure risks. A key advantage of this YAG based transmitter technology includes the fact that the design is based on mature and low-risk space-qualified YAG host crystal. The transmitter operates at a 2096 nm wavelength using Ho:YAG, resulting in high atmospheric transmission (>99%), versus a transmitter operating at 2053 nm using co- doped Tm:Ho:LuLiF, which suffers limited transmission (90%) due to water vapor interference. In-band pumping through Tm:Fiber pump Ho:YAG architecture offers lower quantum defect from 1908 to 2096 nm (9.1%) compared to traditionally used co-doped Tm:Ho:LuLiF of 792 to 2051 nm (61%). The transmitter has an efficient pump compared to LuLF, since YAG has 27% higher pump absorption and 52% lower reabsorption of the emitted 2-micron, resulting in higher efficiency and lower heat load. Being isotropic, YAG is amenable for spatial-hole burning mitigation which supports linear cavity architecture without compromising injection seeding quality. This attribute is important in designing a compact, stable, high seeding efficiency laser. A folded linear cavity for long pulse (>200 ns), transform limited line-width (2.2 MHz) and high beam quality (M2 = 1.04) - the most critical parameters for coherent detection - are easier to achieve using YAG compared to LuLF. Lower heat load results in high repetition rate (>300 Hz) operation, which allows higher probability of wind measurements through broken clouds, off clouds, and below clouds, thus reducing errors and increasing science data product quantity and quality.

Advanced Coupled-cavity Etalon (ACE) significantly improves both in-band transmission and out-of-band rejection. In some cases, 12% more light is transmitted inside the passband and >3x more light is rejected outside the passband. Incorporating ACE into the recirculating etalon receiver (RER) improves performance significantly. ACE increases the wavelength resolution and enables closer channel spacing resulting in a very efficient, high resolution spectrometer. RACER has both high resolution and a high photon efficiency which allows flexibility for trading different combinations of reduced cross-talk and closer channel spacing.

This NASA technology transforms a conventional infrared (IR) imaging system into a multi-wavelength imaging pyrometer using a tunable optical filter. The actively tunable optical filter is based on an exotic phase-change material (PCM) which exhibits a large reversible refractive index shift through an applied energetic stimulus. This change is non-volatile, and no additional energy is required to maintain its state once set. The filter is placed between the scene and the imaging sensor and switched between user selected center-wavelengths to create a series of single-wavelength, monochromatic, two-dimensional images. At the pixel level, the intensity values of these monochromatic images represent the wavelength-dependent, blackbody energy emitted by the object due to its temperature. Ratioing the measured spectral irradiance for each wavelength yields emissivity-independent temperature data at each pixel. The filter’s Center Wavelength (CWL) and Full Width Half Maximum (FWHM), which are related to the quality factor (Q) of the filter, are actively tunable on the order of nanoseconds-microseconds (GHz-MHz). This behavior is electronically controlled and can be operated time-sequentially (on a nanosecond time scale) in the control electronics, a capability not possible with conventional optical filtering technologies.

Conventional scintillation radiation detectors make use of materials that emit light when hit by ionizing radiation. These large, bulky scintillators range anywhere from 6 inches to 6 feet in size. They are attached to glass photomultiplier tubes (PMTs), which are fragile, require high voltages, and are extremely sensitive to temperature changes. Conventional detectors also include a wave shifter (a material with a dopant to re-emit the scintillator light to match the sensitivity of the PMT or photodiode) which reduces efficiency and introduces unwanted weight and bulk. Glenn's particle counter overcomes these challenges. A prototype was constructed from off-the-shelf components, and features a small scintillator (less than six inches long) and a low-voltage, UV-sensitive, wide-bandgap photodiode as the detector. Careful matching of the properties of the light emitted from the scintillator, and tailoring the sensitivity of the photodiode eliminates the need for a PMT and a wave shifter, not only saving space, weight, and power but also eliminating potential failure modes. By using wide-bandgap detectors, it can operate in changing temperature, vibration, pressure, and gravity conditions without need for a temperature compensation system. Built from solid-state components, Glenn's device is compact and robust.