We report the results from Raman fiber lasers with CW output powers of 224 Watts at 1.48 μm and 168 Watts at 1.7 μm, both of which are to the best of our knowledge the highest output powers from cascaded Raman resonators at these wavelengths. The Raman fiber lasers use unique Raman filter fibers to prevent threshold at the next Stokes wavelength, allowing for high spectral purity in the desired Stokes. In addition, the 1.7 μm output is pulsed to achieve 1.4 mJ in an 8.5 µs FWHM pulse.
An all-fiber time-stretched supercontinuum light source is presented. Time-stretching is obtained by dispersing the light in a specially designed highly dispersive fiber with a wide transmission bandwidth. The light source is based on a polarization-maintaining, self-starting femtosecond oscillator with repetition rate in the 1-10 MHz range. The supercontinuum stretches from 950 – 1700 nm and exhibits a high level of stability – both in terms of amplitude and repetition rate. The stretched light source is contained in a box with dimensions 25x25x5 cm3 with a single mode output fiber, making it suitable for a wide range of spectroscopy applications and swept source OCT.
An all-fiber supercontinuum light source based on a polarization-maintaining, self-starting femtosecond oscillator with repetition rate in the MHz range is presented. The supercontinuum stretches from 950 – 2100 nm and exhibits a high level of stability – both in terms of amplitude and repetition rate. The supercontinuum source displays an rms intensity noise of 0.05 % when integrated from 10 Hz to 1 MHz. The light source is contained in box with dimensions 25×25×5 cm3
with a single mode output fiber. This together with the high level of stability makes this light source well suited for applications in, e.g., spectroscopy and OCT.
We demonstrate 104 Watts of in-band output power from a cascaded Raman fiber laser operating around 1.7 μm with a spectral purity of over 90% operating in both continuous wave and pulsed regimes. Through the use of a filter fiber with its cutoff wavelength designed between the 6th and 7th Stokes orders, output above 1.8 μm is suppressed below threshold values. In the pulsed regime the laser produces output pulses ranging from 11.5 mJ pulses with 100 μs pulse width to 10 J pulses with 100 ms pulse width.
We report a 1kW, 14 μm-MFD, Yb fiber laser operating at 1117nm, without any parasitic lasing at shorter wavelengths, and negligible intra-cavity Raman gain at longer wavelengths (>50dB extinction at 1175nm). To the best of our knowledge, this is the first experimental demonstration of 1kW output at 1117nm from an Yb fiber laser. In contrast to the 14 μm-MFD of the gain fiber in this report, our previous records at 1117nm of 400W for an Yb oscillator and 450W for an Yb MOPA, were limited primarily by a 6 μm-MFD gain fiber.
Higher-order mode (HOM) fibers have been engineered to allow propagation of linearly polarized symmetric modes LP0,N in a robust way. Compared with the fundamental mode LP(0,1), HOMs exhibits an effective area that can be larger by over two order magnitude, and thus propagating light in these modes could greatly suppress the effect of nonlinear effects. HOM fibers could also be doped with rare earth ions in order to amplify light propagating in these modes, which offers the enormous potential for generating high-intensity pulses. Excitation of HOM gain fiber using cladding pumping with multimode pump source is attractive for ytterbium based amplifiers, because of the availability of low-cost multimode pump diodes in the 975nm wavelength range. One problem associated with cladding pumping which leads to excitation of the large doped core (over 100 μm diameter) is that it could result in a large amount of amplifiedspontaneous- emission (ASE) noise, particularly when the input signal is weak. Optimization of amplifier design is critical in order to suppress ASE and achieve high gain and pump-to-signal conversion efficiency. We conducted numerical modeling of a cladding pumped HOM-amplifier, which revealed that this problem could be mitigated by using a relatively long gain-fiber that allowed reabsorption of the forward propagating ASE resulting in a further amplification of the signal. We demonstrate efficient amplification of a LP0,10 mode with an effective area 3140μm2 in an Yb-doped HOM amplifier cladding pumped at 975nm. We have successfully obtained a 20.2dB gain for 0.95 W 1064 nm input seed signal to more than 105W.
Scaling the power-level of fiber sources has many practical advantages, while also enabling fundamental studies on the light-matter interaction in amorphous guiding media. In order to scale the power-level of fiber-sources without encountering nonlinear impairments, a strategy is to increase the effective-area of the guided optical-mode. Increasing the effective-area of the fundamental mode in a fiber, however, presents the challenges of increased susceptibility to mode-distortion and effective-area-reduction under the influence of bends. Therefore, higher-order-mode (HOM) fibers, which guide light in large effective-area (Aeff) Bessel-like modes, are a good candidate for scaling the power-level of robust fiber-sources. Many applications of high-power fiber-sources also demand a deterministic control on the polarization-state of light. Furthermore, a polarization-maintaining (PM)-type HOM fiber can afford the added possibility of coherent-beam combination and polarization multiplexing of high-power fiber-lasers. Previously, we reported polarization-maintaining operation in a 1.3 m length of PM-HOM fiber that was held straight. The PM-HOM fiber guided Bessel-like modes with Aeff ranging from 1200-2800 μm2. In this work, we report, for the first time, that the polarization-extinction-ratio (PER) of the HOM exceeds 10 dB in an 8 m long fiber that is coiled down to a diameter of 40 cm. This opens a path towards compact and polarization-controlled high-power fiber-systems.
NASA’s Goddard Space Flight Center has been developing lidar to remotely measure CO2 and CH4 in the Earth’s atmosphere. The ultimate goal is to make space-based satellite measurements with global coverage. We are working on maturing the technology readiness of a fiber-based, 1.57-micron wavelength laser transmitter designed for use in atmospheric CO2 remote-sensing. To this end, we are building a ruggedized prototype to demonstrate the required power and performance and survive the required environment.
We are building a fiber-based master oscillator power amplifier (MOPA) laser transmitter architecture. The laser is a wavelength-locked, single frequency, externally modulated DBR operating at 1.57-micron followed by erbium-doped fiber amplifiers. The last amplifier stage is a polarization-maintaining, very-large-mode-area fiber with ~1000 μm2 effective area pumped by a Raman fiber laser. The optical output is single-frequency, one microsecond pulses with >450 μJ pulse energy, 7.5 KHz repetition rate, single spatial mode, and < 20 dB polarization extinction.
We demonstrate high average power, high peak power amplification of 10 GHz, picosecond and femtosecond pulses in a Very-Large-Mode Area (VLMA), Er-doped fiber with an effective area of ~1050 μm2. A high power, singlemode Raman fiber laser with up to 183 W of power at 1480 nm served as a pump source. 130 femtosecond pulses with an average power of 115 W, peak power of 88 kW, and M2 of 1.18 were achieved. Simulations that take into account pair-induced quenching give excellent agreement with measurements.
We developed a core pumped, 3-color pulsed fiber laser with 40W average output at 1550 nm with a pulse duration of 5 ns at pulse-repetition frequency (PRF) of 1 MHz corresponding to 8kW peak power. The enabling technology for this high power laser was 1480 nm core pumping of a polarization maintaining, very large mode area (PM-VLMA) Er-doped fiber amplifier with ~37 μm mode field diameter. Optical pulses from modulated seed sources were pre-amplified to 1W average power and injected into the PM-VLMA core pumped with a 100W, 1480 nm Raman fiber laser. The VLMA amplifier showed no sign of non-linearity due to Stimulated Brillion Scattering or Four-wave mixing.
NASA’s Goddard Space Flight Center (GSFC) is working on maturing the technology readiness of a laser transmitter designed for use in atmospheric CO2 remote-sensing. GSFC has been developing an airplane-based CO2 lidar instrument over several years to demonstrate the efficacy of the instrumentation and measurement technique and to link the science models to the instrument performance. The ultimate goal is to make space-based satellite measurements with global coverage. In order to accomplish this, we must demonstrate the technology readiness and performance of the components as well as demonstrate the required power-scaling to make the link with the required signal-to-noise-ratio (SNR). To date, all the instrument components have been shown to have the required performance with the exception of the laser transmitter. In this program we are working on a fiber-based master oscillator power amplifier (MOPA) laser transmitter architecture where we will develop a ruggedized package and perform the relevant environmental tests to demonstrate TRL-6. In this paper we will review our transmitter architecture and progress on the performance and packaging of the laser transmitter.
Fiber designs are proposed that allow distributed wavelength filtering far more selective than conventional designs, and which is consistent with conventional fiber fabrication. By including a gradient that pre-compensates the bend perturbation in the cladding, the proposed designs overcome the usual tradeoff between mode area and wavelength selectivity. Simulations shows that the resulting fiber performance enables delivery of multi-kW signals over long distances with modest net Raman gain, using bend-resistant fibers of convenient core size.
We demonstrate a 1480 nm cascaded Raman fiber laser with a new high efficiency architecture providing a record output power of 204 W. We achieve this through multiple Raman shifts of a high power 1117nm Yb-doped fiber laser in a single pass configuration mediated at all intermediate wavelengths using a seed source comprising a low power conventional 1480nm Raman laser. The conversion efficiency from 1117nm to 1480nm is ~65% (for a quantum limited efficiency of 75%). Enhancement in efficiency is achieved by elimination of excess optical loss present in the conventional cascaded Raman resonator based architecture.
We demonstrate a cascaded Raman fiber laser with a record output power of 104 W at 1480 nm. We achieve
this with an 1117 nm Yb-doped fiber laser pumping a cascaded Raman conversion to 1480nm. Enhanced efficiency is
achieved in the Raman cavity using a fiber with a long wavelength cut-off which prevents further Raman conversion of
1480 nm light and thus allowing for longer cavity lengths. The output is single mode making it a bright source for core
or cladding pumping of erbium-doped fiber lasers.
We present for the first time a cascaded Raman fiber laser where the Yb-doped fiber laser and Raman fiber are combined
into a single fiber. We achieve 42.6 % slope efficiency at 1236 nm with respect to launched pump power.
We present a high power Er-doped fiber laser operating at 1555 nm, core pumped by a Raman fiber laser operating at
1480 nm. We achieve 55 W total output power for 67 W of pump power with a slope efficiency of 81%. To the best of
our knowledge, this is the highest output power achieved from a core-pumped Er-doped (Yb free) fiber laser to date.
We present the first direct measurements of enhanced nonlinearities in large-mode-area fibers due to
bend induced reductions in effective area. Both Raman scattering and self-phase modulation are
observed to increase in tightly coiled fibers. The measured increase in nonlinearity compares well with
predictions from simulations of the modal effective area.
We present a source of high power femtosecond pulses at 1550 nm generating compressed pulses at the end of a single mode fiber pigtail. The system generates sub 35 femtosecond pulses at a repetition rate of 50 MHz, with average powers greater than 400 mW. The pulses are generated in a passively modelocked, erbium doped fiber laser, and amplified in a short, erbium doped amplifier. The output of the fiber amplifier consists of highly chirped picosecond pulses. These picosecond pulses are then compressed in standard single mode fiber. While the compressed pulses in the SMF pigtail do show a low pedestal that could be avoided with the use of bulk-optic compression the desire to compress the pulses in SMF is motivated by the ability to splice the single mode fiber to a nonlinear fiber, for continuum generation applications. We demonstrate that with highly nonlinear dispersion shifted fiber (HNLF) fusion spliced directly to the amplifier output, we generate a supercontinuum spectrum that spans more than an octave, with an average power 350 mW. Such a high power, all-fiber supercontinuum source has many important applications including frequency metrology and biomedical imaging.
Due to a narrow window of high atmospheric transmission near 4 microns, there is a great deal of interest for a scalable laser energy source in this spectral region. We propose a concept that combines the advantages of solid-state and gas laser technology. A Nd:YAG laser is tuned to 1.3391 microns by inserting an intracavity etalon and raising the operating temperature of the laser rod to 85 degree(s)C. This allows us to excite the v (0 →3), J (4 → 5) vibrational-rotational transition of HBr. To stabilize the frequency, a diode laser locked to this HBr transition seeds the Nd:YAG laser. Once excited, HBr can potentially lase in three subsequent steps to the ground state, emitting three photons in the 4-micron region. We present theoretical and experimental results demonstrating the operational principle of this laser system.
We present a Raman laser based on rotational scattering in H2, synchronously pumped by a Q-switched, mode locked Nd:YAG laser. Appropriate choice of optics allows us to operate as either a first or second Stokes laser, while remaining below the threshold for extra-cavity scattering. We model the Raman scattering using a computer simulation based on higher order Stokes and anti-Stokes waves in the transient regime.