We demonstrate a C-band gain-switched seed laser intended for a EDFA-based fiber laser meeting the performance, footprint, robustness, and cost targets for volume time-of-flight LiDAR systems. The technology reported here leverages Freedom Photonics high-power DFBs coupled with a Black Forest Engineering control ASIC in a low-inductance package. As a result, the overall package is a compact form factor that can fit within a 16-pin butterfly package. To date, our 1550 nm seed technology delivers more than 2.5 nJ pulse energy for a 480KHz repetition rate on a 4 ns pulse, which is 30 times higher than conventional seed lasers. This technology is the first of its kind to realize a 1550 nm high-pulse energy seed laser for volume deployment of time-of-flight fiber-laser-based LiDAR systems.
We present a high-power DFB technology that meets the performance and volume demands of consumer automotive applications. Our DFB design is hardened and intended for use at extreme environmental conditions and operates at peak current density of 8 kA/cm2 - approximately 4 times higher than more conventional DFB lasers intended for use in telecommunication and sensing applications. We demonstrate that the risks associated with placing these components into high volume production with high yield can be managed through careful control of the laser design and manufacturing processes. To date, we show >90% of our DFB lasers fall within our control limits as defined by three sigma of the mean. This is the first high-power DFB laser suitable for widespread deployment into the consumer automotive market space.
In this work, we present a compact size and highly efficient nanosecond pulsed 1550nm single mode fiber laser that can operate from -40C to +95C. The laser generates 2 to 10 ns pulses at a repetition rate of hundreds kHz to a few MHz with hundreds to kilowatt peak power. The design of this laser is optimized to achieve over 10% wall-plug efficiency at room temperature with an ultra-low ASE noise less than 1%. The performance is also well maintained with less than 30% EO (electrical-optical) efficiency degradation at extreme temperatures and demonstrates high reliability consistent with deployment into harsh environments.The robust performance makes the laser an ideal source for lidar and sensing applications, along with other medical, scientific, and industrial applications.
Fiber lasers have experienced an explosive growth in output power over the last decade, enabling multi-kW output powers with nearly diffraction-limited beam quality. Transverse Mode Instability (TMI) is currently the ultimate limitation in average power scaling of CW fiber lasers. Therefore, new fiber designs and amplifier geometries will be required in the future to overcome TMI-induced limitations. To explore the parameter space, we have constructed several kW-class fiber amplifiers to examine the effects of different fiber designs and amplifier geometries using both commercially available fiber as well as custom, in-house drawn Yb-doped fiber. Investigations are ongoing which explore the effects of variations in the cladding diameter, core NA and fiber length. Results are presented which investigate the roles the cladding diameter and core NA play in system design with regard to thermal management and beam quality management, respectively. Finally, we will present our current progress towards long range, outdoor laser propagation at our TISTEF laser facility.
High power thulium-doped fibers rely on 793 nm pumping and cross-relaxation. While this approach has been historically successful, low optical-to-optical efficiencies and high thermal loads impede multi-kW power scaling. Another option is to in-band pump the final amplifier. In-band pumped thulium-doped fibers enable >80% efficiencies and low thermal loads. Design concepts and simulations for scaling thulium-doped fibers >1 kW with in-band pumping are discussed. Developing the high power 1.9 µm pump units, the incoherent fiber combiner, and the specially designed final amplifier are detailed. Requirements on the seed source (wavelength, power, etc.) are also described.
Thulium fiber lasers emit light of wavelengths spanning as low as 1650nm to 2200nm. This broad emission band is in the “eye-safety” wavelength regime and intersects with the IR atmospheric transmission window with its opacity subsiding past 1900nm wavelength. Consequently, a high power, single frequency, tunable thulium fiber laser with its tuning range from 1900nm to 2000nm has the unique capability of studying high power beam propagation through the atmosphere in regions of both weak and strong transmission. Moreover, such lasers can be made to tune across individual molecular absorption lines due to chemical species present in the atmosphere. This enables a detailed investigation on how individual molecular absorption lines affect the transmission of high power laser beams. In this paper, a 100kHz linewidth, near diffraction limited, 100W class, widely tunable CW thulium fiber laser system is described for atmospheric propagation studies. The fiber laser is of master oscillator power amplifier(MOPA) architecture with one pre-amp and a final power amplifier. The master oscillator is a 5mW class tunable external cavity diode and is tunable from 1900nm to 2000nm. The pre-amp amplifies the seed to 2-3W level, which is then further amplified to 100W by the final amplifier made from thulium doped 25um core 250um cladding 0.09NA fiber from Nufern. All fiber architecture allows efficient lasing at the lossy molecular absorption wavelengths.
Beam confinement or “no free-space optics” via fiber transmission can achieve improved reliability, lower cost, and reduced component count for active sensing systems. For midinfrared delivery, mechanically robust chalcogenide (arsenic sulfide) single-mode fibers are of interest. A 12-μm core diameter fiber is shown to transport >10 W at 2053 nm, and a 25-μm core diameter fiber enables single-mode beam transport from a 4550-nm quantum cascade laser. As midinfrared sources continue to increase their output power capabilities, chalcogenide fibers will eventually be limited in their power-handling capacity due to optical nonlinearities or thermal failure. These limitations are discussed and analyzed in the context of single-mode chalcogenide fibers in order to provide a framework for power transmission limitations in various operating regimes.
Defense sensing systems must be both productive and robust to accomplish their mission. Active infrared sensing devices consist of many components such as the active medium, mirrors, beam-splitters, modulators, gratings, detectors, etc. Each of these components is subject to damage by the laser beam itself or environmental factors. Misalignment of these components due to vibration and temperatures changes can also reduce performance. The result is a complex and expensive system subject to multiple points of degradation or complete failure. However, beam confinement or “no free-space optics” via fiber transmission and even component assembly within the fiber itself can achieve reliability and low cost for sensing systems with reduced component count and less susceptibility to misalignment.
We present measurements of high-power infrared laser beam transmission in chalcogenide fibers. The fiber compositions were As39S61 for the core and As38.5S61:5 for the cladding, resulting in a numerical aperture of 0.2. A polyetherimide jacket provided structural support. Multiwatt CW transmission was demonstrated in near single-mode 12 micron core fiber. Efficient coupling of quantum cascade lasing into anti-reflection coated chalcogenide fiber was also demonstrated. Efficient beam transport without damage to the fiber required careful coupling only into core modes. Beams with M2 ≥ 1.4 and powers higher than 1 W produced damage at the fiber entrance face. This was most likely due to heating of the highly absorptive polymer jacket by power not coupled into core modes. We will discuss current power limitations of chalcogenide fiber and schemes for significantly increasing power handling capabilities.
We report on a 2 μm master oscillator power amplifier (MOPA) fiber laser system capable of producing 700 μJ pulse energies from a single 1.5 m long amplifier. The oscillator is a single-mode, thulium-doped fiber that is Q-switched by an acousto-optic modulator. The oscillator seeds the amplifier with 1 W average power at 20 kHz repetition rate. The power amplifier is a polarization-maintaining, large mode area thulium-doped fiber cladding pumped by a 793 nm fiber-coupled diode. The fiber length is minimized to avoid nonlinearities during amplification while simultaneously enabling high energy extraction. The system delivers 700 μJ pulse energies with 114 ns pulse duration and 14 W average power at 1977 nm center wavelength.
To advance the science of high power fiber lasers, in-house drawn specialty optical fibers are investigated. Ongoing research involves the fabrication and testing of Yb- and Tm-doped fibers at 1μm and 2μm. Using specialized fiber and pump mixing geometries, dopant profiles and system configurations, the performance of our in-house drawn active fibers has been examined. Results on a highly multi-mode, high average power pulsed Raman fiber amplifier pumped by a thin disc laser are presented. The Raman fiber is a large mode-area graded index fiber, also drawn in house. Finally, the development of capabilities for kilometer range propagation experiments of kW-level CW and TW-level pulsed lasers at the TISTEF laser range is reported.
Delivering high peak powers from fiber lasers is limited by the accumulation of nonlinear effects due to the high optical intensities and the long interaction lengths of fibers. Peak power scaling at 2 μm is limited by modulation instability (MI), which is not found for 1 μm sources. This work investigates the performance of a spectrally broadband, nanosecond pulsed thulium-doped fiber laser. The average power and pulse energy performance of the single-mode amplifier delivers >20 W and ~280 μJ. A variable spectral filter is incorporated to study the onset of MI and subsequent spectral broadening as a function of seed linewidth. It is observed that MI-induced spectral broadening is enhanced for larger linewidths. However, when the seed linewidth is increased beyond >10 nm, this trend is reversed. A fiber amplifier model including MI (treated as degenerate four-wave mixing) simulates a parametric gain bandwidth of ~900 GHz for this amplifier configuration, which is equivalent to ~11.5 nm at the 1960 nm center wavelength. Therefore, the decrease in spectral broadening for seed linewidths <10 nm is due to a reduced overlap with the MI gain bandwidth. The capability to scale 2 μm fiber lasers to high powers is strongly dependent on the spectral quality of the seed. Any power initially located within the MI gain bandwidth will degrade performance, and must be considered for power scaling.
Due to the intrinsic absorption edge in silica near 2.4 μm, more exotic materials are required to transmit laser power in the IR such as fluoride or chalcogenide glasses (ChGs). In particular, ChG fibers offer broad IR transmission with low losses < 1 dB/m. Here, we report on the performance of in-house drawn multi-material chalcogenide fibers at four different infrared wavelengths: 2053 nm, 2520 nm and 4550 nm. Polymer clad ChG fibers were drawn with 12.3 μm and 25 μm core diameters. Testing at 2053 nm was accomplished using a > 15 W, CW Tm:fiber laser. Power handling up to 10.2 W with single mode beam quality has been demonstrated, limited only by the available Tm:fiber output power. Anti-reflective coatings were successfully deposited on the ChG fiber facets, allowing up to 90.6% transmission with 12.2 MW/cm2 intensity on the facet. Single mode guidance at 4550 nm was also demonstrated using a quantum cascade laser (QCL). A custom optical system was constructed to efficiently couple the 0.8 NA QCL radiation into the 0.2 NA ChG fiber, allowing for a maximum of 78% overlap between the QCL radiation and fundamental mode of the fiber. With an AR-coated, 25 μm core diameter fiber, >50 mW transmission was demonstrated with > 87% transmission. Finally, we present results on fiber coupling from a free space Cr:ZnSe resonator at 2520 nm.
This work studies the accumulated nonlinearities when amplifying a narrow linewidth 2053 nm seed in a single mode Tm:fiber amplifier. A <2 MHz linewidth CW diode seed is externally modulated using a fiberized acousto-optic modulator. This enables independent control of repetition rate and pulse duration (>30 ns). The pulses are subsequently amplified and the repetition rate is further reduced using a second acousto-optic modulator. It is well known that spectral degradation occurs in such fibers for peak powers over 100's of watts due to self-phase modulation, four-wave mixing, and stimulated Raman scattering. In addition to enabling a thorough test bed to study such spectral broadening, this system will also enable the investigation of stimulated Brillouin scattering thresholds in the same system. This detailed study of the nonlinearities encountered in 2 μm fiber amplifiers is important in a range of applications from telecommunications to the amplification of ultrashort laser pulses.
Thulium and holmium have become the rare earth dopants of choice for generating 2 micron laser light in silica fiber. The majority of Tm:fiber lasers are pumped with high power diodes at 790nm and rely upon cross-relaxation processes to achieve optical-to-optical efficiencies of 55-65%. Tm:fiber lasers can also be pumped at <1900nm by another Tm:fiber laser to minimize quantum defect, reaching efficiencies >90%. Ho:fiber lasers are similarly pumped by Tm:fiber lasers at 1900-1950nm, with <70% typical efficiency. In this work, Tm:fiber and Ho:fiber lasers are in-band pumped using the same experimental setup to directly compare their performance as 2 micron sources.
Ultra-large mode area thulium-doped photonic crystal fibers (Tm:PCF) have enabled the highest peak powers in 2
micron fiber laser systems to date. However, Tm:PCFs are limited by slope efficiencies of <50% when pumped with 790
nm laser diodes. A well-known alternative is pumping at 1550 nm with erbium/ytterbium-doped fiber (Er/Yb:fiber)
lasers for efficiencies approaching ~70%. However, these 1550 nm pump lasers are also relatively inefficient
themselves. A recently demonstrated and more attractive approach to enable slope efficiencies over 90% in thuliumdoped
step-index fibers is resonant pumping (or in-band pumping). This utilizes a high power thulium fiber laser
operating at a shorter wavelength as the pump. In this manuscript, we describe an initial demonstration of resonant
pumping in Tm:PCF. While the extracted power was still in the exponential regime due to pump power limitations, slope
efficiencies in excess of ~64 have been observed, and there is still room for improvement. These initial results show
promise for applying resonant pumping in Tm:PCF to improve efficiencies and facilitate high power scaling in ultralarge
mode area systems.
By utilizing photon energies considerably smaller than the semiconductors’ energy band gap, space-selective modifications can be induced in semiconductors beyond the laser-incident surface. Previously, we demonstrated that back surface modifications could be produced in 500-600 μm thin Si and GaAs wafers independently without affecting the front surface. In this paper, we present our latest studies on trans-wafer processing of semiconductors using a self-developed nanosecond-pulsed thulium fiber laser operating at the wavelength 2 μm. A qualitative study of underlying physical mechanisms responsible for material modification was performed. We explored experimental conditions that will enable many potential applications such as trans-wafer metallization removal for PV cell edge isolation, selective surface annealing and wafer scribing. These processes were investigated by studying the influence of process parameters on the resulting surface morphology, microstructure and electric properties.
Within the past 10 years, thulium (Tm)-doped fiber lasers have emerged as a flexible platform offering high average power as well as high peak power. Many of the benefits and limitations of Tm:fiber lasers are similar to those for ytterbium (Yb)-doped fiber lasers, however the ~2 µm emission wavelength posses unique challenges in terms of laser development as well as several benefits for applications. In this presentation, we will review the progress of laser development in CW, nanosecond, picosecond, and femtosecond regimes. As a review of our efforts in the development of power amplifiers, we will compare large mode area (LMA) stepindex and photonic crystal fiber (PCF) architectures. In our research, we have found Tm-doped step index LMA fibers to offer relatively high efficiency and average powers at the expense of fundamental mode quality. By comparison, Tm-doped PCFs provide the largest mode area and quasi diffraction-limited beam quality however they are approximately half as efficient as step-index fibers. In terms of defense related applications, the most prominent use of Tm:fiber lasers is to pump nonlinear conversion to the mid-IR such as supercontinuum generation and optical parametric oscillators/amplifiers (OPO/A). We have recently demonstrated Tm:fiber pumped OPOs which generate ~28 kW peak power in the mid-IR. In addition, we will show that Tm:fiber lasers also offer interesting capabilities in the processing of semiconductors.
We report on the performance of a prototype pump combiner for use with thulium-doped photonic crystal fiber (PCF). This platform is attractive for “all-fiber” high energy and high peak power laser sources at 2 μm. We will report on the performance of this integrated amplifier in comparison to free space amplification in Tm:PCF. In particular, we carefully look for spectral/temporal modulation resulting from multimode interference between fundamental and higher order transverse modes in the amplifier to evaluate this for ultrashort chirped pulse amplification. The slope efficiency for the all-fiber amplifier is 22.1 %, indicating the need for further improvement. However, an M2 < 1.07 demonstrates excellent beam quality, as well as amplified polarization extinction ratios of ~25 dB.
Optical trapping of single biological cells has become an established technique for controlling and studying
fundamental behavior of single cells with their environment without having "many-body" interference. The development
of such an instrument for optical diagnostics (including Raman and fluorescence for molecular diagnostics) via laser
spectroscopy with either the "trapping" beam or secondary beams is still in progress. This paper shows the development
of modular multi-spectral imaging optical tweezers combining Raman and Fluorescence diagnostics of biological cells.
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