Final Report Summary - ASEL-MID-IR (Compact, high-energy, and wavelength-diverse coherent mid-infrared source)
As a host institute, ASELSAN has overseen the integration of talented researchers and engineers within numerous Career Integration Grants provided by the Research Executive Agency in the past. Among these grants, ASEL-MID-IR project has brought along a unique capability in exploring the mid-infrared coherent sources, which had been a previously non-existent technology for ASELSAN. As a company that specializes in developing electro-optics technology for vision systems (e.g. thermal cameras that can utilize the 3-5 μm and 8-12 μm spectral windows), the application areas were limited to the scenarios where only passive illumination plays a role. With an active coherent source, such as a mid-infrared laser, the capabilities of the existing imaging systems can be extended to the actively illuminated imaging sensors. This capability opens up new application fronts such as molecular gas spectroscopy, trace gas detection for infrastructure security, active imaging, and even diagnostic medicine (diagnosing pathological conditions through breath analysis).
Hence, with the integration of an experienced researcher, who has specialized in the field of nonlinear optics, solid-state lasers and nanophotonics, a broad and ambitious project proposal for a CIG grant was submitted. Here, the main objective was to develop a compact and portable source for mid-infrared wavelength (2-5 μm) that can deliver high average power, energetic pulses (>0.5 mJ) with high repetition rates (multi-kHz). The main source architecture used the near-infrared diode-pumped solid-state lasers and nonlinear frequency conversion media, utilizing χ(2) and χ(3) nonlinearities.
The project staff has opted for using diode-pumped Thulium (Tm) and Holmium (Ho) solid-state and fiber lasers near 2 μm center wavelength as the main pump source, which has become an active field of study over the last few years. These sources can be efficiently operated as pulsed lasers and display better thermal management and power scaling capabilities than the conventional 1-μm solid-state lasers. Afterwards, it was envisaged to use mid-infrared-transparent nonlinear crystals to convert the 2-μm output to 3-5 μm spectral window using an optical parametric oscillator (OPO). The most suitable conversion medium was identified as the ZnGeP2 (ZGP) crystal which displayed excellent thermal and mechanical properties and a χ(2) nonlinear coefficient that is almost an order of magnitude greater than its near-infrared counterparts. The feasibility of this architecture is verified through the in-house numerical simulations developed by the project staff.
With a commercial-off-the-shelf (COTS) Tm-doped fiber laser, the Ho-doped lasers have achieved more than 25-W of pulsed output power at 2.1 μm, with an adjustable pulse repetition rate (10-200 kHz) and pulse durations lower than 15 ns. Using this pump source, conversion to mid-IR with ZGP crystal generated more than 5 W of average power (signal+idler) for only 13.5 W input pump power. The corresponding conversion efficiency was ~40% which surpassed the initial 30% estimate. The spectral measurements revealed a broadband generation of mid-IR signal, covering the 3.5-4.8 μm interval. The output spectrum was also tunable with proper orientation of the ZGP crystal. Alternative OPO configurations, such as an intracavity OPO, have revealed the cascaded conversion dynamics where mid-IR OPO spectrum can backconvert into the pump baseband to effectively mimic a χ(3) nonlinear medium. Furthermore, to provide a compact packaging for the prototype an in-house Tm:YLF diode-pumped solid-state laser was developed to replace the COTS Tm:fiber laser.
In order to utilize the χ(3) nonlinearity, a mid-infrared stimulated Raman scattering (SRS) experiment was planned. This was a rather unconventional wavelength range to utilize SRS, because the Raman gain coefficients of known Raman crystals rapidly diminish towards longer wavelengths. For the case of a pulsed Ho:YAG laser pumping a BaWO4 external cavity Raman laser, the numerical simulations indicated that 2.6 μm first Stokes output can be generated from 2.1-μm excitation. Indeed the experimental demonstrations verified this finding, where more than 1.35 W average power is generated at 2.6 μm, with a repetition rate of 5 kHz. This had been the longest wavelength generated from a solid-state Raman laser source that displayed Watt-level output power scaling. The spectral range of the source also overlapped with the water vapor absorption window and can be useful in infrared hygrometry and other trace gas absorption studies.
The mid-infrared OPO source demonstration is then scaled down for the prototype integration. A comprehensive mechanical design was utilized to develop a self-sufficient mid-infrared laser unit that can deliver the demonstrated output power levels with dedicated electronics, thermal management and power circuitry. Through manufacturing of the optomechanical parts and acquisition of the OEM components, the prototype unit was completed and the device integration was finished in 2 weeks after the arrival of all the necessary components. The end result is a portable device that can be used in studying mid-infrared imaging and sensing applications.
The researcher has been instrumental in establishing the mid-infrared laboratory infrastructure and educating younger engineers on the laser-related subjects. He has honed his skills through part-time teaching of a graduate lasers course at a local university and he has either co-supervised the thesis work (Mr. Mert Baltacioglu's Master's Thesis) or served as a thesis committee member for ASELSAN engineers. Furthermore, he has served as a reviewer for several scientific journals, as a panel member for Scientific and Technological Research Council of Turkey’s project review boards and assisted other project support activities for H2020. He has participated in the scientific meetings, gave invited talks, presented posters, and co-authored several journal publications during his tenure. In short, the fellow was active within the scientific community as well as fulfilling his duties as an ASELSAN employee. The researcher was also promoted to a more senior position (Lead Design Engineer) and he is on a stable career path in ASELSAN.
The fellow has also continued his collaboration with his former research group at Cornell University. This collaboration focused on utilizing χ(3) nonlinearity in silicon waveguides for all-optical switching and investigating the nonlinear mechanisms for optical cross-talk such waveguide structures. Another collaborative effort was focused on the broadband generation of frequency combs on silicon waveguides. These efforts resulted on several journal publications and conference proceedings.
As a conclusion, ASEL-MID-IR CIG provided a fruitful working environment between the researcher and ASELSAN and both sides have benefited immensely from the project activities. The project goals were successfully realized and a functional and portable prototype has been developed. The researcher’s career prospects in the host institute and his home country have significantly improved and the project activities have contributed to his recognition. ASELSAN has obtained a significant know-how from the project activities and this has laid the groundwork for future mid-infrared-related applications.
Hence, with the integration of an experienced researcher, who has specialized in the field of nonlinear optics, solid-state lasers and nanophotonics, a broad and ambitious project proposal for a CIG grant was submitted. Here, the main objective was to develop a compact and portable source for mid-infrared wavelength (2-5 μm) that can deliver high average power, energetic pulses (>0.5 mJ) with high repetition rates (multi-kHz). The main source architecture used the near-infrared diode-pumped solid-state lasers and nonlinear frequency conversion media, utilizing χ(2) and χ(3) nonlinearities.
The project staff has opted for using diode-pumped Thulium (Tm) and Holmium (Ho) solid-state and fiber lasers near 2 μm center wavelength as the main pump source, which has become an active field of study over the last few years. These sources can be efficiently operated as pulsed lasers and display better thermal management and power scaling capabilities than the conventional 1-μm solid-state lasers. Afterwards, it was envisaged to use mid-infrared-transparent nonlinear crystals to convert the 2-μm output to 3-5 μm spectral window using an optical parametric oscillator (OPO). The most suitable conversion medium was identified as the ZnGeP2 (ZGP) crystal which displayed excellent thermal and mechanical properties and a χ(2) nonlinear coefficient that is almost an order of magnitude greater than its near-infrared counterparts. The feasibility of this architecture is verified through the in-house numerical simulations developed by the project staff.
With a commercial-off-the-shelf (COTS) Tm-doped fiber laser, the Ho-doped lasers have achieved more than 25-W of pulsed output power at 2.1 μm, with an adjustable pulse repetition rate (10-200 kHz) and pulse durations lower than 15 ns. Using this pump source, conversion to mid-IR with ZGP crystal generated more than 5 W of average power (signal+idler) for only 13.5 W input pump power. The corresponding conversion efficiency was ~40% which surpassed the initial 30% estimate. The spectral measurements revealed a broadband generation of mid-IR signal, covering the 3.5-4.8 μm interval. The output spectrum was also tunable with proper orientation of the ZGP crystal. Alternative OPO configurations, such as an intracavity OPO, have revealed the cascaded conversion dynamics where mid-IR OPO spectrum can backconvert into the pump baseband to effectively mimic a χ(3) nonlinear medium. Furthermore, to provide a compact packaging for the prototype an in-house Tm:YLF diode-pumped solid-state laser was developed to replace the COTS Tm:fiber laser.
In order to utilize the χ(3) nonlinearity, a mid-infrared stimulated Raman scattering (SRS) experiment was planned. This was a rather unconventional wavelength range to utilize SRS, because the Raman gain coefficients of known Raman crystals rapidly diminish towards longer wavelengths. For the case of a pulsed Ho:YAG laser pumping a BaWO4 external cavity Raman laser, the numerical simulations indicated that 2.6 μm first Stokes output can be generated from 2.1-μm excitation. Indeed the experimental demonstrations verified this finding, where more than 1.35 W average power is generated at 2.6 μm, with a repetition rate of 5 kHz. This had been the longest wavelength generated from a solid-state Raman laser source that displayed Watt-level output power scaling. The spectral range of the source also overlapped with the water vapor absorption window and can be useful in infrared hygrometry and other trace gas absorption studies.
The mid-infrared OPO source demonstration is then scaled down for the prototype integration. A comprehensive mechanical design was utilized to develop a self-sufficient mid-infrared laser unit that can deliver the demonstrated output power levels with dedicated electronics, thermal management and power circuitry. Through manufacturing of the optomechanical parts and acquisition of the OEM components, the prototype unit was completed and the device integration was finished in 2 weeks after the arrival of all the necessary components. The end result is a portable device that can be used in studying mid-infrared imaging and sensing applications.
The researcher has been instrumental in establishing the mid-infrared laboratory infrastructure and educating younger engineers on the laser-related subjects. He has honed his skills through part-time teaching of a graduate lasers course at a local university and he has either co-supervised the thesis work (Mr. Mert Baltacioglu's Master's Thesis) or served as a thesis committee member for ASELSAN engineers. Furthermore, he has served as a reviewer for several scientific journals, as a panel member for Scientific and Technological Research Council of Turkey’s project review boards and assisted other project support activities for H2020. He has participated in the scientific meetings, gave invited talks, presented posters, and co-authored several journal publications during his tenure. In short, the fellow was active within the scientific community as well as fulfilling his duties as an ASELSAN employee. The researcher was also promoted to a more senior position (Lead Design Engineer) and he is on a stable career path in ASELSAN.
The fellow has also continued his collaboration with his former research group at Cornell University. This collaboration focused on utilizing χ(3) nonlinearity in silicon waveguides for all-optical switching and investigating the nonlinear mechanisms for optical cross-talk such waveguide structures. Another collaborative effort was focused on the broadband generation of frequency combs on silicon waveguides. These efforts resulted on several journal publications and conference proceedings.
As a conclusion, ASEL-MID-IR CIG provided a fruitful working environment between the researcher and ASELSAN and both sides have benefited immensely from the project activities. The project goals were successfully realized and a functional and portable prototype has been developed. The researcher’s career prospects in the host institute and his home country have significantly improved and the project activities have contributed to his recognition. ASELSAN has obtained a significant know-how from the project activities and this has laid the groundwork for future mid-infrared-related applications.