Improved Laser performance through Planar Waveguide Technology Development

We propose a laser technology development to improve efficiency and performance for a variety of science applications including: Lunar Ice, 2-Step Laser Tandem Mass Spectrometer (L2MS) and trace gas measurements (CH₄, CO₂, H₂O, etc.).   We will investigate the development of a single-frequency, 1-µm laser as the pump source for optical parametric frequency conversion.  This pump laser source will be based on planar waveguide (PWG) technology.

The objective of this proposal is to design and build a 1- µm, single frequency master oscillator (MO) using PWG technology that can enable a variety of applications for Earth and Planetary Science.  Currently, there are a number of laser instrument concepts that need a 1-µm laser with very similar specifications, but there are no commercially available solutions.  With FY15 IRAD funds we will design and build the MO, and couple that to a power amplifier (PA) acquired from the ICESat-2/ATLAS laser risk reduction effort to demonstrate the power scaling capability.

We will utilize PWG technology for the MO design.   PWGs are a cutting edge laser technology that has superior efficiency, thermal management and performance.  Current solid-state laser technologies generally use one of three structures:  bulk crystals, fibers or PWGs.  Bulk crystals have the advantage of being simple and robust but are generally less efficient and have difficult thermal management due to a low surface area to volume ratio.  The efficiency issues with bulk crystals come from the low overlap between the pump beam and the signal beam – which means the pump wavelength is not efficiently converted into signal energy and therefore further contributes to crystal heating.  Fiber laser structures are waveguided in two dimensions and address both limitations of bulk crystals; 1) fiber can dissipate heat along its length and has a higher surface to volume ratio and 2) the waveguide can guide both the pump and the signal and optimize the overlap enabling very efficient operation.  However the fiber structure introduces a new limitation.  Because the light is confined in two dimensions, the energy density can get very high and this limits the peak powers achievable in fiber.  PWGs have the advantages of both structures.  First, it maintains a high surface area to volume ratio for excellent thermal control.  Second, the 1-D waveguiding achieves high overlap between pump and signal beams which enables very high efficiency operation.  Because the light can spread in one dimension, the energy confinement issue present in fibers disappears. 

In addition to the physical advantages of the PWG technology, we can leverage components left over from the ICESat-2/ATLAS laser risk reduction effort (>$80K worth of parts) to build the MO.   Two of the four vendors (Raytheon and Lockheed Martin) who competed for the ICESat-2/ATLAS Laser Transmitter Contract proposed a PWG design.  ATLAS funded (~$5M per vendor) an internal risk reduction effort to better understand the technologies being proposed to help evaluate and down-select the flight laser vendor.   With that, each vendor supplied a prototype laser to GSFC for testing and evaluation. We will use the laser components left over from this risk reduction effort to build the MO, and the power amplifier subsystem of one of the ATLAS laser deliverables to complete a MOPA prototype.   Although both the Raytheon and Lockheed Martin lasers had excellent performance, and were more efficient than the awarded Fibertek laser, they were not selected due to a lower technology readiness level.   These valuable assets, from ~$10M investment, are available to us for this IRAD effort, which will allow us to mature this state-of-the-art PWG technology for future laser instruments. 

This technology could enable a more functional, flexible and lower cost laser technology that will give GSFC a competitive advantage in winning future laser work and could enable future missions like Lunar Ice, 2-Step Laser Tandem Mass Spectrometer and trace gas (CH₄, CO2, H₂O, etc.) measurements for planetary and Earth exploration.  Earth science opportunities include the suite of Earth Venture opportunities.  Planetary science opportunities would include a Discovery-class lander on a primitive or icy body (such as a comet or carbonaceous asteroid), or future in situ investigations at Europa, Enceladus, or Titan.  Many laser missions are cost driven and a substantial improvement in efficiency will result in lower power requirements, less heat dissipation and overall lower mission cost.  This work fits extremely well into GSFC’s expertise and lines of business and builds on its leadership position in laser-based spaceflight instruments. 

As a first order demonstration, we will develop a 1-µm MO designed to pump optical parametric oscillators and amplifiers (OPOs and OPAs).  OPOs and OPAs are the most versatile technologies for achieving wavelengths from 1.4-3.5 µm.  These wavelengths are of interest for many science applications, in particular, gas spectroscopy for both Earth and planetary science.

The OPO/OPA technology is already being developed at GSFC by Haris Riris (694), Kenji Numata and Stewart Wu (554).  We will focus our efforts on the MO development to be integrated after successful prototype demonstration.  Currently, there are no commercially available lasers that meet all the requirements necessary for science measurements with an OPO/OPA system.  The proposed PWG technology will address current limitations and provide the needed performance to enable a space-borne OPO/OPA-based laser transmitter.   FY15 IRAD funding will allow us to mature this state-of-the-art PWG MO technology so that, once coupled with the OPO/OPA system, it could fly on an airplane mission in the FY16 timeframe.  We will evaluate different PWG laser resonator designs for the master oscillator to meet the requirements for a variety of science applications including Lunar Ice, 2-Step Laser Tandem Mass Spectrometer (L2MS) and Methane and other Trace Gas measurements.

The repetition rate requirement for Lunar Ice and L2MS is very different than that of the CH₄ and trace gas sensing, but the pulse energies are similar.  The laser designs for these are thus very different to meet these requirements.  For a low repetition rate laser (10’s Hz), the laser is peak power limited, thus it is possible to obtain the required energy (few mJ) in a single master oscillator (MO), Figure 2a.  Indeed, as shown in the master oscillators for GLAS, LOLA, MLA, etc. the MO can generate 2-3 mJ at 10’s Hz under quasi-continuous wave (QCW) pumping.  So for this application, an amplifier is not necessary to meet the energy requirement.  We will, for these applications, concentrate on obtaining single frequency and the necessary pulse width.  Timing control is also required for some of the applications, so we will also investigate active Q-switching.

For the high repetition rate laser (few kHz), the laser is average power limited, mostly due to the fact that the pump diodes typically operate in continuous mode and the thermal load on the gain medium is much higher than that of the low repetition rate case.  In this case, the maximum pulse energy typically obtained from these MOs is on the order of 100’s µJ.  To achieve the mJ energy level, a high power amplifier is usually needed in master oscillator power amplifier (MOPA) architecture.  In this case, we will design a MO that generates the required pulse width with corresponding transform-limited line width.  We will then leverage the use of power amplifiers from ATLAS to achieve milli-Joule level pulse energy requirement.