A four-element fiber array has been constructed to yield 8 watts of coherently phased, linearly polarized light energy in a single far field spot. Each element consists of a 2-W single-mode fiber-amplifier chain. Phase control of each element is achieved with a lithium-niobate phase modulator. A master laser provides a linearly polarized, narrow linewidth signal that is split into five channels. Four channels are individually amplified using polarization maintaining fiber power amplifiers. Frequency broadening of the signal is necessary to avoid stimulated Brillouin scattering. The fifth channel is used as a reference arm. It is frequency shifted and then combined interferometrically with a portion of each channel's signal. Detectors sense the heterodyne modulation signal, and an electronics circuit measures the relative phase for each channel. Compensating adjustments are then made to each channel's phase modulator. The stability of the optical train is an essential contributor to its success. A state-of-the-art interferometer was built with mountless optics. A lens array was constructed using nano-positioning tolerances, where each lens was individually aligned to its respective fiber to collimate its output and point it at a common far field spot. This system proved to be highly robust and handled any acoustic perturbations.
We report on the frist experimental demonstration of a scalable fiber laser approach based on phase-locking multiple gain cores in an antiguided structure. A novel fabrication technology is used with soft glass components to construct the multie core fiber used in our experiments. The waveguide region is rectangular in shape and comprised of a periodic sequence of gain and no-gain segments having nearly uniform refractive index. The rectangular waveguide is itself embedded in a lower refractive index cladding region. Experimental resutls confirm taht our five-core Nd doped glass prototyep structure runs predominately in two spatial antiguided modes as predicted by our modeling.
We consider modelocking in an optical fiber laser cavity using a passive long period fiber grating. The grating as a passive modelocking element arises from the the nonlinear mode-coupling which occurs between co-propagating core and cladding modes. The underlying concept is as follows: a resonant and linear mode-coupling interaction transfers energy periodically between the core and cladding modes. Nonlinearity, however, can be used to detune the resonant interaction by shifting the propagation constant of each mode via self-phase and cross-phase modulation. Thus the low intensity parts of a pulse which propagate through the grating can be coupled out to the cladding and attenuated while high intensity portions are detuned and transmitted through the grating with minimal loss. This intensity discrimination when acting in combination with chromatic dispersion, self-phase modulation, and a bandwidth limited gain, can lead to stable modelocking operation in a optical fiber laser cavity. Stable pulses are generated for a wide variety of nonlinear coupling strengths between core and cladding modes. Further, dispersive radiation can be completely attenuated while generating the stable pulse trains. Self-starting in the cavity along with stability of the pulse trains under perturbation are considered. In conclusion, the long period fiber grating provides a simple, compact passive component for a modelocked laser source which is robust and efficient.
In figure-eight lasers (F8L) mode locking is achieved through a nonlinear fiber amplifier loop mirror (NALM) or an asymmetrical nonlinear optical loop mirror (NOLM). Recently, we have theoretically shown that the symmetrical NOLM with a twisted fiber is useful for passive mode locking of fiber lasers. In this work we experimentally demonstrate the operation of a F8L based on the symmetrical NOLM with a twisted low-birefringence fiber in the loop. The modelocking operation is achieved by nonlinear polarization rotation. We found that the counter-propagating beams accumulate a differential nonlinear phase shift when they have different As (where As is the Stokes parameter). At the input NOLM, we used a polarizer controller to adjust the clockwise beam to be circularly polarized, As=1. In the loop of the NOLM, we used a quarter-wave retarder to transform the counter-clockwise beam to linear polarization, As=0. The quarter-wave retarder was the only element that we adjust to achieve modelocking. The pulse repetition frequency was 0.8 MHz. The FWHM of the autocorrelation function was 0.7 ps. We used a pump power of 80 mW to get the modelocking operation. The modelocked laser ran in stable operation for hours. Even in this first experiments the laser shown several advantages. The adjustment procedure was straightforward. The laser shows stable operation and exhibits high pulse energy. We achieved stable generation of subpicosecond pulses with milliwatts of average output power.
We report a dual-wavelength actively mode-locked erbium doped fiber laser constructed using one amplitude modulator as mode locker, two fiber Bragg gratings (FBGs) as wavelength filters and two separate parts of gain mediums in order to minimize gain cross-saturation effects. Dual-wavelength pulses with a wavelength spacing of 0.8nm, 1.6nm, 2.4nm and 3.2nm were simultaneously obtained when the modulation frequency was about 2.5GHz in the experiment, respectively. By adjusting the cavity length of one wavelength with a delay line, the repetition rate of this channel can be changed to as two times as the other channel without changing the modulation frequency.
A new method for the monitoring of interferometer fiber optic sensors which utilizes a frequency-scanned fiber laser is investigated. The interrogation technique is based on the principle that if the laser frequency varies linearly with time, the optical signal reflected or transmitted is intensity-modulated at a frequency proportional to the optical path difference (OPD) in the interferometer. Fourier components in the detected optical output signal then correspond to the OPDs of any interferometers which have contributed to this modulation. The position of a peak in the power spectrum of this signal is proportional to the OPD of the interferometer responsible for that peak. Fine tuning of the OPD value is determined from the phase of the corresponding Fourier component. Experimentally, an Er:fiber laser scanned over a 46 nm range centered at 1545 nm was used to monitor intrinsic fiber Fabry-Perot interferometers (FFPIs). Variations in the laser scan rate were compensated using the optical signal modulated by a reference FFPI held at constant temperature. For three multiplexed sensors arranged in series, temperature was measured from 20°C to 610°C with a 0.02°C resolution.
We assess different power limits of cladding-pumped fiber lasers. Despite recent advances in pump sources, these are still primarily limited by available pump power. We find that it should be possible to reach output powers beyond 1 kW in single-mode ytterbium doped fiber lasers. Experimentally, we have realized an ytterbium-doped fiber laser with 272 W of output power at 1080 nm, with an M2-value of 3.2, as well as an erbium-ytterbium co-doped fiber laser with 103 W of output power at 1565 nm, with an M2-value of 2.0. We believe these are the highest-power ytterbium and erbium-ytterbium fiber lasers ever reported.
Rare earth doped chalcogenide glass fibers have been developed at NRL. Spectroscopic investigation of these glasses and fibers show that they possess strong and efficient mid-IR and long-wave IR emissions suitable for fiber lasers in these wavelength regions. Small fiber sources based on these emission lines have been developed and can be used as bright IR sources for characterizing focal planes.
Glass fiber lasers were invented in the 60's by Elias Snitzer at Americal Optical, soon after the invention of the first solid-state glass laser. However, it was not until the 80's when these waveguide devices were deployed in industrial applications, driven largely by the technological success of the semiconductor laser diode, which provided practical and efficient pumps, and by the advent of low loss rare-earth-doped optical fiber.
We have developed a Nd:doped cladding pumped fiber amplifier, which operates at 938nm with greater than 2W of output power. The core co-dopants were specifically chosen to enhance emission at 938nm. The fiber was liquid nitrogen cooled in order to achieve four-level laser operation on a laser transition that is normally three level at room temperature, thus permitting efficient cladding pumping of the amplifier. Wavelength selective attenuation was induced by bending the fiber around a mandrel, which permitted near complete suppression of amplified spontaneous emission at 1088nm. We are presently seeking to scale the output of this laser to 10W. We will discuss the fiber and laser design issues involved in scaling the laser to the 10W power level and present our most recent results.
The combination of wavelength-scale features and design flexibility offered by holey fibers leads to a significanlty broader range of optical properties than is possible in conventional optical fibers. Of particular interest, holey fibers offer the combination of broadband single mode guidance and large mode areas, and such fibers are promising for high power delivery applications such as including laser welding and machining, and for fiber lasers and amplifiers. Holey fiber technology has now reached the point that km-lengths of polymer-coated fiber with less than 1 dB/km loss at 1550nm are possible. As well as being of fundamental scientific interest, the novel guidance properties of holey fibers can be exploited to develop technologically important devices. Here recent advances in holey fibers will be presented, with a particular focus on recent results in developing holey fiber-based lasers.
Phase-shifted DFB fiber lasers (DFB-FL) can have two orthogonally polarized laser modes with a frequency offset proportional to the fiber birefringence. In order to achieve true single frequency operation one polarization mode must be extinguished. On the other hand, the existence of a polarization beat frequency (PBF) between the two states of polarization in a dual-mode DFB-FL can be utilized in a frequency-based fiber sensor. The underlying mechanisms leading to the quenching or preservation of the PBF must be controlled in either case. Twisting the DFB-FL to induce circular birefringence has been used to achieve single polarization-mode operation, though an explanation has been lacking. We present results of the reflection and transmission spectra, and the PBF, as a function of twist angle for two phase-shifted fiber gratings, and compare the results with recent theoretical calculations. We have also measured the PBF as a function of twist angle for the active case of a phase-shifted DFB-FL. Our results show that the PBF decreases monotonically as a function of twist angle for angles less than 700 degrees (over a 20 cm length), where the magnitude of the PBF signal can be suppressed by 20 dB or more. For larger twist angles, the magnitude of the PBF signal was not in general extinguished, in contrast to previously reported work. A possible explanation of such behavior will be presented in addition to a discussion of the applications of DFB-FLs as sensors.
A new series of fluorophosphate glass is developed which can be doped with an extremely high concentration of Nd3+. The dependence of the optical absorption and emission properties on dopant concentration is reported here for the concentration range 2.5×1020 to 1.25×1021 ions/cm3. Absorption and emission measurements are performed in order to evaluate the spontaneous emission probability, absorption cross-section, emission cross-section, and laser performance parameters. We have synthesized two glass systems: MBBA/NdI with an Nd3+ concentration of 2.50×1020 cm3 and MBBA/NdII with an Nd3+ concentration 6.26×1020 cm3. The stimulated emission cross sections are 1.14 and 1.64×104 cm2 for the 4F3/2→4I13/2 transition and 3.68 and 6.68×104 cm2 for the 4F3/2→4I13/2 transition in MBBA/NdI and MBBA/NdII, respectively. Similarly, the extraction efficiencies are measured to be 1.91, 2.31 (4FI3/2→4I13/2) and 6.18, 9.41 (4F3/2→4I11/2) in MBBA/NdI and the MBBA/NdII, respectively. This new (Mg, Ba)F2-based fluorophosphate glass (MBBA system) is promising for broadband compact optical fiber and waveguide amplifier applications.
In contrast to the traditional spectroscopic techniques, the picosecond stimulated Raman scattering (SRS) in fibers permits one to determine the anharmonicity constants for silica tetrahedron vibrations and local dopant vibrations. The results of new experiments using the technique of picosecond SRS excitation in pure and doped silica fibers are presented. Apart the fundamental Raman-active silica and dopant vibrations and their overtones the intense combination bands were detected. The comparative studies of undoped and erbium-doped silica fibers were made. When the photoinduced changes took place in a silica fiber core the distinct combination SRS bands were observed, whereas the presence of an erbium ions in silica network didn't disturb the SRS bands originated from silica tetrahedron vibrations.
Judd-Ofelt parameters for Nd3+ ions in a new series of (Mg, Ba)F2-based fluorophosphate glass (the MBBA system) are determined from the intensities of the integrated absorption bands of the Nd3+ ion in the MBBA system. The intensity parameters, Ω2, Ω6, and Ω4 for f-f transitions of Nd3+ ions are found to be -1.02, 10.82, and 4.27 (×1020 cm2), and 1.19, 3.95 and 3.11 in the MBBA/NdI and MBBA/NdII systems, respectively. The measured lifetime τf is 168 μs for the MBBA/NdI system, while the Judd-Ofelt analysis expects a radiative transition lifetime Arad for 4F3/2 to be 316 μs, resulting in a fluorescence quantum efficiency of 47%. The results are compared with those reported in the literature for other fluorophosphate glasses and show that 4F3/2 to 4I11/2 transition has the most potential for laser application with a peak fluorescence at 1056nm.
We report on a technique for obtaining adhesive-free optical bonds between optical fibers and a wide range of optical elements, including other optical fibers. This technique overcomes problems associated with epoxies or inorganic adhesives at the bond interface. It consists of optically contacting a precision polished flat fiber-ferrule assembly against a precision polished mating surface. The optical fibers and ferrule both are of the same or similar material, e.g. fused silica, and the optical fiber inside the ferrule has been stripped of its organic polymer coating. The optical bond is then heat treated to produce a strong permanent bond with negligible loss at the interface. Applications include the attachment of a fused silica optical fiber/ferrule assembly to an optical element of fused silica with negligible Fresnel reflection loss. This type of connection is capable of transmitting high-power laser radiation to a site for materials processing without laser damage at the interface. Another important area of use for optically bonded interfaces is the delivery of high-power diode laser pump radiation to a clad laser fiber.
We review our recent work on fiber based laser systems in the continuous wave and nanosecond pulse regime. A significant power and energy scaling was possible by applying low-numerical aperture large-mode-area ytterbium-doped double-clad fibers which emit an excellent beam quality and possess a reduced nonlinearity due to a mode field diameter of >20 μm. We report on a continuous wave fiber laser with an output power approaching 500 W from a single fiber and the amplification of ultrastable, narrow linewidth single-frequency radiation to the 100 W level with diffraction-limited beam quality without the limitation of nonlinear effects. Furthermore the amplification of nanosecond pulses to millijoule pulse energy and average powers up to 100 W is demonstrated.
We report on applications of Yb3+-doped fiber-laser systems in the micro-processing and the markign of different materials. We have demonstrated the drilling of small holes in glass slides with a thickness of 1 mm using a fiber-laser system wtih an otuput-power of about 35 watts. Furthermore, we have used a narrowband fiber-laser system with an output-power of a few watts for the marking of thin plastic films and samples of anodized aluminum. These applications are based on the experience of more than 10 years in the design, development and preparation of rare-earth doped double-clad silica fibers at the IPHT e.V. Jena. We systematically have optimized the composition and the geometry of these specialty fibers. So we could manufacture very efficient samples which were tested in several fiber-laser setups with output-powers of up to almost 200 watts. Especialy, in the high-power region we could solve some thermal problems using new concepts for the double-clad fiber cross section. We successfully designed, prepared and tested so-called Large-Mode-Area fibers as well as fibers with two or mroe refractive indices in the pump-core. Additionally, we increased the available pump-power of the one-end-pumped fiber-laser systems using a wavelength-multiplexed diode-laser fiber-coupled system.
Ytterbium (Tb) doped double-cladding fiber (DCF) lasers and amplifiers are being developed for a number of industrial and military applications. There are several key factors for maximizing the output power of these devices. Lambda Istruments is concentrating on two areas: component development and optical fiber development. The component development effort has focused on grating devices and pump couplers. Stable, highly reflective short-period fiber Bragg gratings are produced in DCF rapidly, have low insertion loss and can be customized for many different laser/amplifier applications. Long-period gratings are also being developed for possible novel use in fiber laser and amplifier applications. A proprietary fiber coupler under development is currently capable of an 85% coupling efficiency. The second focus for Lambda is the development of polarization maintaining Yb-doped DCF. Recent efforts have shifted towards making large mode area versions of these fibers to reduce nonlinear effects at high powers.
An Eye-safe Laser Radar has been developed under White Sands Missile Range sponsorship. The SEAL system, the Self-contained Eyesafe Autonomous Laser system, is designed to measure target position within a 0.5 meter box. Targets are augmented with Scotchlite for ranging out to 6 km and augmented with a retroreflector for targets out to 20 km. The data latency is less than 1.5 ms, and the position update rate is 1 kHz. The system is air-cooled, contained in a single 200-lb, 6-cubic-foot box, and uses less than 600 watts of prime power. The angle-angle-range data will be used to measure target dynamics and to control a tracking mount.
The optical system is built around a diode-pumped, erbium-doped fiber laser rated at 1.5 watts average power at 10 kHz repetition rate with 25 nsec pulse duration. An 8 inch-diameter, F/2.84 telescope is relayed to a quadrant detector at F/0.85 giving a 5 mrad field of view. Two detectors have been evaluated, a Germanium PIN diode and an Intevac TE-IPD. The receiver electronics uses a DSP network of 6 SHARC processors to implement ranging and angle error algorithms along with an Optical AGC, including beam divergence/FOV control loops.Laboratory measurements of the laser characteristics, and system range and angle accuracies will be compared to simulations. Field measurements against actual targets will be presented.
Kigre is developing new rare-earth-doped glass fiber laser materials specifically for use in multiple clad and multiple core LMA and super mode (guided wave) fiber laser constructs. In this work we describe new end-pump double clad fiber laser designs fabricated from high performance phosphate laser glass compositions. One DC LMA fiber is doped with erbium/ytterbium for 1.54μm laser emission. Another DC LMA fiber is doped with ytterbium for 1.03μm laser emission. A third DC multiple core "supermode" fiber is doped with neodymium for 1.053μm laser emission. Initial fiber laser performance data is presented. The rebium/ytterbium & ytterbium only doped fibers are end-pumped at 940/975nm with 40-Watt fiber coupled laser diodes. The neodymium-doped fiber is end-pumped with an 808nm 40-Watt fiber coupled laser diodes. Design and performance data for new side-pumped, highly doped phosphate DC LMA fiber laser architectures are presented.
The fiber laser concept is proven technology for telecom applications where its single mode performance and reliability is essential. Newer generation diode-pumped fiber lasers are using pump diodes especially developed for telecom applications and therefore offer excellent controllability of the laser pulse length and pulse frequency. In the past years the output power of fiber laser increased steadily. This high power performance enables the fiber laser to serve applications like industrial marking. High power fiber lasers are now approaching power levels sufficient for micro cutting applications.
Yb doped fiber lasers are of importance due to their potential applications as efficient pumps for erbium doped fiber amplifiers, as well as for achieving the argon ion-laser line of 488nm through frequency doubling. An efficient high power fiber laser operating at 980nm is presented. The fiber laser consists of an all-silica air clad fiber with an Yb doped core. The double-clad fiber configuration has a single-mode (SM) core and a small diameter inner silica clad. The inner clad is surrounded by air filled capillaries within a standard sized outer silica cladding. The air cladding provides the fiber with high numerical aperture (NA), greater than 0.6. This high NA is essential for efficient pump coupling into the small inner clad, as well as for the fiber laser saturation and its resulted efficient operation at a three level scheme. Fiber Bragg gratings were written on the core, thus forming an all-fiber cavity. We have achieved over 1W of 980nm laser emission in SM, with a threshold of 285mW and a slope-efficiency of 30%. Furthermore, a very narrow line-width of the laser emission enabled its efficient frequency doubling using an extra-cavity doubling configuration with PPKTP crystals, developed by Soreq NRC. Output power of 19mW at 490nm in transverse SM has been achieved using a single polarization component of the fiber laser emission, with a conversion efficiency of 14%/W. All fiber cavity design within an all-silica air-clad fiber, and emission suitable for efficient frequency doubling makes this source highly suitable and cost-effective for various applications, such as telecommunication and diagnostics.
Today’s industrial fiber lasers mean hands-off, maintenance free operation for more than 15,000h beam-on time in 24/7 high speed, high precision marking. High conversion efficiency allows for strictly air-cooled, extremely compact and easy to integrate laser-marking systems. Latest marking software in conjunction with dual head systems for enlarged marking fields offers much higher productivity than conventional dual head YAG laser marking technology. Fast marking of metals and ceramics is now also possible with today’s power levels.
The wavelength characteristics of Yb3+-doped double cladding fiber are investigated pumping by 915nm and 975nm high power laser diode respectively. The relationship between the pumping power and pumping wavelength on laser output are measured and discussed in detail.
Volume diffractive gratings (Bragg gratings) in photo-thermo-refractive (PTR) inorganic glass are proposed for incoherent laser beam combining because they have narrow spectral selectivity and diffraction efficiency greater than 95% from visible to near IR regions. They showed no laser-induced damage, no thermal lens, and no Bragg angle shift under CW Yb-fiber laser (1096 nm) irradiation at 100 kW/cm2. It opens the way to rugged, low-cost, efficient optics for high-power laser systems. Based on theoretical modeling of PTR Bragg gratings, we have designed a high-efficient technology for incoherent combining of two or several laser beams with certain wavelength shift. Two 100 W beams of Yb-fiber lasers in the range of 1080-1100 nm with the wavelength separation of 11 nm were combined with efficiency exceeding 75% while material losses did not exceed 2-4%. No fading or parameter change of PTR Bragg grating working in two 100 W beams were found. It was found that the process limiting efficiency of incoherent beam combining is the spectral widening of radiation of Yb-doped fiber lasers. At high power, their spectral width exceeds spectral selectivity of Bragg grating and causes a decrease of diffraction efficiency.
Fibers for high-power laser and amplifier applications require large claddings with high numerical apertures for efficiently coupling pump energy. In addition, such fibers should have high rare-earth dopant concentrations in relatively large cores, with low numerical apertures, to reduce non-linearities. Furthermore, polarization maintaining double-clad fibers (PM-DCF) are needed for coherently combining the outputs of several lasers/amplifiers to achieve output powers in excess of 100 kW for military and industrial laser applications. In this paper, we report the progress made towards fabricating PM double-clad fibers, with a variety of fiber characteristics, to facilitate development and production of high-power lasers and amplifiers. In particular, a Panda-type PM-DCF with a 0.06 NA, 30 micron diameter, Yb-doped core is reported. We also discuss various criteria that are critical for designing these PM double clad fibers.
We have generated the second, third, fourth, and fifth harmonics of the output of a Yb-doped fiber amplifier seeded by a passively Q-switched Nd:YAG microchip laser. The fiber amplifier employed multimode fiber (25 μm core diameter, V ~ 7.4) to provide high-peak-power pulses, but diffraction-limited beam quality was obtained by use of bend-loss-induced mode filtering. The amplifier output had a pulse duration of 0.97 ns and smooth, transform-limited temporal and spectral profiles (~500 MHz linewidth). We obtained high nonlinear conversion efficiencies using a simple optical arrangement and critically phase-matched crystals. Starting with 320 mW of average power at 1064 nm (86 µJ per pulse at a 3.7 kHz repetition rate), we generated 160 mW at 532 nm, 38 mW at 355 nm, 69 mW at 266 nm, and 18 mW at 213 nm. The experimental results are in excellent agreement with calculations. Significantly higher visible and UV powers will be possible by operating the fiber amplifier at higher repetition rates and pulse energies and by further optimizing the nonlinear conversion scheme.
High energy laser systems, both pulsd and CW, have become of significant interst in the recent past. To achieve higher powers in a laser system, it is often necessary to consider means by which individual lasers can be made coherent with one another. This can be achieved through the use of a master oscillator concept, which can have problems with overall stability, or by monitoring the phases of each individual laser and using feedback technique that can be used to combine individual pumped fiber gain sources into a cavity with a single output and a single set of longitudinal modes. We discuss the advantages of end pumping of double clad fiber lasers and speculate on means by which an all-glass double clad fiber laser may be developed.