Continuous Wave Laser Source for Injection Seeding

instrumentation
Continuous Wave Laser Source for Injection Seeding (LAR-TOPS-331)
Precise, Fast Tuning, Frequency Agile, Offset Wavelength Control
Overview
Innovators at the NASA Langley Research Center have developed a new technique for generating a continuous wave (CW) laser source with agile wavelength switching capabilities that is suitable for injection seeding high-energy pulsed lasers. Laser output radiation spectral linewidth control is an essential feature for scientific applications, such as atmospheric active remote sensing. Generally, high energy lasers do not readily produce spectrally narrow linewidth output. In order to achieve a high energy output radiation that matches the spectroscopic features of desired measurement objectives, this CW laser source was developed to injection seed and control the wavelength of a high energy laser. NASA's CW laser source can provide multiple wavelengths suitable for single and multi-pulsed lasers using a low-power radiation source. This results in a compact, lightweight, and low power consumption injection seeder that is suitable for airborne and space-borne applications including, but not limited to Lidar applications, atmospheric observation instruments, and optical signal generators.

The Technology
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.
A block diagram of the main scheme used for generating the CW laser seed source.
Benefits
  • Increased reliability compared to competing systems: existing systems rely on optical switching and are incapable of operation at high frequencies while providing a consistent, reliable, spectrally pure wavelength like NASA's technology (the RF switches work at GHz rates)
  • Requires only one seed laser for multiple frequencies: the CW laser source can operate across up to four frequencies with narrow bandwidth within 200 microseconds using a single laser, saving on size, complexity, and system costs
  • Lower power requirements: the system does not require high power, only needing 0.5 to 1 watt at most
  • Flight proven technology: this seed laser has flown successfully in an aircraft

Applications
  • Aerospace: Lidar for spacecraft navigation
  • Automotive: Lidar for autonomous vehicles
  • Communications: optical signal generator for discrete and precise frequencies
  • Environmental monitoring: atmospheric observation instruments that require precise laser frequencies and terrestrial (geology, seismology, forestry) mapping
  • Marine: oceanic mapping applications
  • Remote sensing: Lidar applications requiring multiple narrow linewidth lasers with rapid switching
Technology Details

instrumentation
LAR-TOPS-331
LAR-19257-1
11,251,582
"Wavelength Locking to CO2 Absorption Line-Center for 2-Micron Pulsed IPDA Lidar Application," Refaat, Tamer F. et al., April 04, 2016,
https://ntrs.nasa.gov/search.jsp?R=20160009160

"Evaluation of 2-µm Pulsed Integrated Path Differential Absorption Lidar for Carbon Dioxide Measurement - Technology Developments, Measurements, and Path to Space," Singh, Upendra N. et al., January 23, 2018, https://ntrs.nasa.gov/search.jsp?R=20190026469.
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