Cadmium silicon phosphide, CdSiP2 (CSP), crystals have good nonlinear optical properties resulting in their use in optical parametric generation (OPO and OPA) of mid-infrared light. One common limitation on the performance of OPO materials is residual optical absorption which often results from point defects formed during crystal growth. Electron paramagnetic resonance (EPR) is a powerful technique for identifying and tracking point defects in materials. By correlating behaviors of native point defects exposed to 1064 nm light using EPR with changes in optical absorption bands, models are proposed for three of the observed broad optical absorption bands.
Cadmium silicon phosphide, CdSiP2 (CSP), exhibits the highest d-coefficient (d36 = 85 pm/V) among all practical nonlinear optical crystals. Its large band gap of 2.45 eV allows for 1-micron pumping with widely-available Nd- and Yb-based laser sources, and its dispersion properties are such that a 1-um pump yields non-critically phase-matched temperature-tunable output between 6.2-6.5 um (an attractive range for minimally-invasive laser surgery). However, residual 1-um absorption losses in CSP are not insignificant (0.16-0.2 cm-1). In this work we focused on identifying, and ultimately minimizing, the point defects responsible for these losses by correlating EPR spectra with polarized absorption near 1-um.
CdSiP2 (CSP) is a nonlinear optical material used for mid-infrared generation. For nonlinear optical materials, absorption bands associated with point defects often limit output power. We use electron paramagnetic resonance (EPR) to monitor paramagnetic charge states of defects. In CSP crystals, EPR shows singly ionized silicon vacancies (VSi-) initially present are eliminated by exposure to 1064 nm light. Our results suggest that 1064 nm light converts VSi- acceptors to nonparamagnetic doubly ionized (VSi2-) and neutral (VSi0) charge states. A thermal activation energy of 0.23 eV describes the recovery of the VSi- signal including at room temperature.
Rare-earth-doped fibers with single-crystal cores have the potential for 10x higher TMI threshold than their glass counterparts and are a promising candidate for use as gain media in high-power laser systems. Their utility has been limited by parasitic optical losses and difficulty in fabrication. This paper explores methods to reduce the losses in these fibers in the core, in the cladding and at the core-cladding interface. Fabrication methods are also discussed.
Doped single-crystal YAG fibers used as single-mode lasers require claddings with precise refractive index and high thermal conductivity. Three cladding materials that use coextrusion of green cladding on fiber cores as an initial processing step are described: 1. Undoped YAG cladding, followed by sintering or hot isostatic pressing. 2. Ca3Ga2Ge3O12 garnet cladding that melts beneath 1400°C. 3. LiCa2Mg2As3xV3-3xO12 garnet cladding that melts beneath 1100°C. Microstructures are characterized by TEM. Equipment and procedures are described. Garnet refractive index models are developed and validated to predict cladding refractive index. Advantages and disadvantages of the different claddings are compared.
Rare-earth-doped fibers with single-crystal cores have the potential for 10x higher TMI threshold than their glass counterparts and are a promising candidate for use as gain media in high-power laser systems. Their utility has been limited by parasitic optical losses and difficulty in fabrication. This paper explores methods to reduce the losses in these fibers in the core, in the cladding and at the core-cladding interface. Fabrication methods are also discussed.
Barium gallium selenide (BaGa4Se7) is a recently developed nonlinear optical material with a transmission window extending from 470 nm to 17 μm. A primary application of these crystals is production of tunable mid-infrared laser beams via optical parametric oscillation. Unintentional point defects, such as selenium vacancies, cation vacancies (barium and/or gallium), and trace amounts of transition-metal ions, are present in BaGa4Se7 crystals and may adversely affect device performance. Electron paramagnetic resonance (EPR) and optical absorption are used to identify and characterize active defects in BaGa4Se7 crystals grown at BAE Systems. Five distinct defects, each representing an electron trapped at a selenium vacancy, are observed with EPR (there are seven crystallographically inequivalent selenium sites in this monoclinic crystal). One defect is seen at room temperature before illumination. The other four are seen at lower temperature after exposure to 532 nm laser light. Each singly ionized selenium vacancy has a large, nearly isotropic, hyperfine interaction with 69Ga and 71Ga nuclei at one neighboring Ga site, which indicates a significant portion of the unpaired spin resides in a 4s orbital on this adjacent Ga ion. Optical absorption bands peaking between 430 and 750 nm are produced by the 532 nm light. These photoinduced bands are assigned to the selenium vacancies.
CdSiP2 (CSP) is a nonlinear optical crystal developed as a wider-band-gap analog of ZnGeP2 (ZGP) to enable mid-infrared generation. A direct comparison of the performance of ZGP and CSP crystals in mid-IR generating OPOs was performed with a 4 W Tm:YAP pump laser. CSP was shown to outperform ZGP in this configuration. A ring OPO using CSP with a 2.09 micron pump and 80W of power was used to generate 27 W of mid-IR light demonstrating CSP’s viability for high average power generation. An OPO seeded OPA was then used to directly compare CSP and ZGP with this same source as well as with an upgraded 140W source.
CdSiP2 (CSP) is a non-linear optical material for mid-infrared optical parametric oscillators. Previous work showed that an intrinsic acceptor (Si vacancy) produced unwanted absorption in the near-IR. The VSi concentrations are much reduced in recent growths. Other compensating defects now play an important role: iron impurities, an intrinsic donor (Si-on-Cd antisite), and a second intrinsic acceptor (Cd vacancy). We present photoinduced electron paramagnetic resonance (EPR) spectra to identify these defects. Illumination using light sources (lasers, LEDs) in the 500nm to 1064nm range can “reveal” these defects by converting them to their paramagnetic charge states. We present the wavelength dependence and thermal stability of these defects. Thermal decay data allow us to determine activation energies for various defect charge state transitions which allows us to predict decay times at room temperature of defect charge states and related absorption bands that can impact laser devices.
Temperature- and wavelength-dependent values of the ordinary (no) and extra-ordinary refractive index (ne) of GaN and 4H-SiC were measured over wavelength ranges of 1.9 to 7 μm and 1.9 – 5.5 μm, respectively, and over a temperature range of 79 to 400 K. Temperature-dependent Sellmeier equations for both GaN and SiC were obtained and thermooptic coefficients determined.
CdSiP2 (CSP) is a nonlinear optical chalcopyrite semiconductor developed as a wider-band-gap analog of ZnGeP2 (ZGP) to enable mid-infrared generation. Two laser architectures were explored to pump CSP crystals at 2 microns. The first was a ring OPO with two CSP crystals that produced 27 W of average power, demonstrating the viability of CSP as a material capable of producing high average power output. The second architecture was an OPO seeded OPA train that was used to directly compare the thermal lenses generated by pumping either CSP or ZGP with high average power 2 micron light. The CSP crystals demonstrated significantly less thermal lensing than the ZGP crystals.
Many optical metrology applications require light that is both coherent and broadband. Supercontinuum (SC) spanning several wavelength octaves is an obvious candidate for such applications. Optical fibers are a natural platform for SC generation due to the long interaction length of light within the fiber which allows for broad SC which can ultimately be used as a tunable narrowband source. For tunability in the mid-IR regime, YAG fibers are an excellent candidate due to their high transparency, Kerr nonlinearity, and damage threshold. In our work, we study SC in undoped crystalline YAG fibers produced via laser-heated pedestal growth. We use femtosecond pulses to generate SC in fiber, pumping at several wavelengths ranging out to the mid-IR. Studying the power-dependence of SC generation, we use SC width and shape as indicators of mechanisms that generate SC at each pump wavelength.
Single crystal fibers doped with Er or Tm and clad with a sapphire sol-gel were tested for both laser performance and super-continuum generation. Laser performance was explored for multiple sol-gel cladding deposition cycles (0 to 5) in addition to variable concentration (0.25% to 3%). A single sample showed exemplary performance (2% Tm with 3 deposition cycles) achieving 44.5% slope-efficiency. Super-continuum generation was compared in pure and doped fibers of both 150μm and 50μm diameters at five different pump wavelengths with an 80 fs source. Super-continuum was generated covering 1.5 octaves (790 nm pump) and <2.5 octaves (1645 nm pump) with a threshold pulse energy of 0.4 μJ (5 MW peak power).
Laser sources operating near a wavelength of four microns are important for a broad range of applications that require power scaling beyond the state-of-the-art. The highest power demonstrated in the spectral region from a solid-state laser source is based upon nonlinear optical (NLO) conversion using the NLO crystal ZnGeP2 (ZGP). High-power operation in ZGP is known to be limited by thermal lensing. By comparing the figure of merit for thermal lensing in ZGP with other NLO crystal candidates, CdSiP2 (CSP) particularly offers significant advantages. However as was the case with ZGP during its early development, the physics of observed crystal defects, and their relevance to power scaling, was not at first sufficiently understood to improve the crystal’s characteristics as a NLO wavelength conversion element. During the past decade, significant progress has been made (1) with the first reported growth of a large CSP crystals, (2) in understanding the crystal’s characteristics and its native defects, (3) in improving growth and processing techniques for producing large, low-loss crystals, and (4) in demonstrating CSP’s potential for generating high-power mid-infrared laser light. The paper will summarize this progress.
The need for high power, physically robust infrared laser systems that are capable of functioning in extreme environments has fueled the need to look for alternatives to the current state-of-the-art glass fiber sources. In particular, improvements to thermal management and a low stimulated Brillion scattering threshold are needed to increase the average output power of glass fiber systems. Rare earth (RE) doped single crystal fiber lasers have been proposed as a potential alternative with improved thermal management issues and a decreased SBS threshold. Recently, high-quality single crystal RE doped YAG fibers grown using laser heated pedestal growth (LHPG) have become commercially available [1]. LHPG has the potential to deliver flexible fiber sources that have the advantages of both single crystals and fibers, at a fraction of the cost of current bulk growth methods. Although LHPG single crystal fibers have demonstrated lasing, significant optimization of the fiber parameters must be done before they are suitable replacements for state-of-the-art laser fibers. In this study, the lasing properties of LHPG single crystal RE doped YAG fibers will be investigated to determine the efficiencies, loss mechanisms, and optimal doping levels for maximum output. The results will be discussed and possible design improvements will be proposed for future work.
[1] G. Maxwell et al., Proc. SPIE 8733, 1-8 2013.
Laser sources operating near a wavelength of four microns are important for a broad range of applications that require power scaling beyond the state-of-the-art. The highest power demonstrated in the spectral region from a solid-state laser source is based upon nonlinear optical (NLO) conversion using the NLO crystal ZnGeP2 (ZGP). High-power operation in ZGP is known to be limited by thermal lensing. By comparing the figure of merit for thermal lensing in ZGP with other NLO crystal candidates, CdSiP2 (CSP) particularly offers significant advantages. However as was the case with ZGP during its early development, the physics of observed crystal defects, and their relevance to power scaling, was not at first sufficiently understood to improve the crystal’s characteristics as a NLO wavelength conversion element. During the past decade, significant progress has been made (1) with the first reported growth of a large CSP crystals, (2) in understanding the crystal’s characteristics and its native defects, (3) in improving growth and processing techniques for producing large, low-loss crystals, and (4) in demonstrating CSP’s potential for generating high-power mid-infrared laser light. The paper will summarize this progress.
World-wide interest in germanium-on-silicon photonics has grown enormously during the past few years. We report on our study of germanium deposition for which we found that there is potential to engineer films with significant increases in hole mobility. In addition, we report on our development of wet-etch techniques to pattern thin films and to form tapered regions of Ge, both important for the fabrication of Ge photonic devices.
Electronic circuits alone cannot fully meet future requirements for speed, size, and weight of many sensor systems, such as digital radar technology and as a result, interest in integrated photonic circuits (IPCs) and the hybridization of electronics with photonics is growing. However, many IPC components such as photodetectors are not presently ideal, but germanium has many advantages to enable higher performance designs that can be better incorporated into an IPC. For example, Ge photodetectors offer an enormous responsivity to laser wavelengths near 1.55μm at high frequencies to 40GHz, and they can be easily fabricated as part of a planar silicon processing schedule. At the same time, germanium has enormous potential for enabling 1.55 micron lasers on silicon and for enhancing the performance of silicon modulators. Our new effort has begun by studying the deposition of germanium on silicon and beginning to develop methods for processing these films. In initial experiments comparing several common chemical solutions for selective etching under patterned positive photoresist, it was found that hydrogen peroxide (H2O2) at or below room temperature (20 C) produced the sharpest patterns in the Ge films; H2O2 at a higher temperature (50 C) resulted in the greatest lateral etching.
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