Watt-level, high-repetition-rate, mid-infrared pulses generated by wavelength conversion of an eye-safe fiber source

We report on a mid-infrared (mid-IR) source consisting of an approximately 10 W average-power, linearly polarized 1.54 microm wavelength pulsed fiber source pumping an optical parametric oscillator. From this source, we obtained average power in excess of 1 W in the 3.8-4.0 microm wavelength range at a pulse repetition frequency of 100 kHz. With a slightly different setup, we also achieved an average power of 0.25 W at 4.5 microm wavelength. To our knowledge, these values represent the highest mid-IR power obtained through wavelength conversion of an eye-safe fiber source.     

Ultrahigh-resolution optical coherence tomography with a fiber laser source at 1 microm

We report a compact, high-power, fiber-based source for ultrahigh-resolution optical coherence tomography (OCT) near 1 microm. The practical source is based on a short-pulse, ytterbium-doped fiber laser and on generation of a continuum spectrum in a photonic crystal fiber. The broadband emission has an average power of 140 mW and offers an axial resolution of 2.1 microm in air (<1.6 microm in biological tissue). The generation of a broad bandwidth is robust and efficient. We demonstrate ultrahigh-resolution, time-domain OCT imaging of in vitro and in vivo biological tissues.     

Multistage Yb-doped fiber amplifier generating megawatt peak-power, subnanosecond pulses

A Q-switched microchip laser generating 1064 nm wavelength, subnanosecond pulses at a 13.4 kHz repetition rate was used to seed a dual-stage amplifier featuring a 40 microm core Yb-doped photonic-crystal fiber (PCF) as the power amplifier. From this source, we obtained diffraction-limited (M2 = 1.05), approximately 450 ps pulses of energy > 0.7 mJ, peak power in excess of 1.5 MW, and an average power of approximately 9.5 W. By further amplifying the PCF output in a multimode 140 microm core Yb-doped fiber, we generated a peak power in excess of 4.5 MW, the highest obtained in a fiber source to our knowledge.     

Broadly tunable single-frequency cw mid-infrared source with milliwatt-level output based on difference-frequency generation in orientation-patterned GaAs

A narrow-linewidth mid-IR source based on difference-frequency generation of an amplified 1.5 microm diode laser and a cw Tm-doped fiber laser in orientation-patterned (OP) GaAs has been developed and evaluated for spectroscopic applications. The source can be tuned to any frequency in the 7.6-8.2 microm range with an output power of 0.5 mW. The measured characteristics of the OP-GaAs sample demonstrate a high quality of the material.     

Efficient approximately 2 microm Tm3+ -doped tellurite fiber laser

Laser emission in the range of 1.88-1.99 micrim from a Tm3+ -doped tellurite fiber is demonstrated when pumped with a diode-pumped Er3+/Yb3+-doped silica fiber laser operating at 1.57-1.61 microm. This pump source excites the Tm3+ ions directly into the F43 upper laser level and yields an output power of 280 mW with a slope efficiency of 76% in a 99%-12% laser cavity arrangement and a 32 cm long fiber. This result is very close to the Stokes efficiency limit of approximately 80%. This is, to the authors' knowledge, the first demonstration of high efficiency lasing in a tellurite fiber at wavelengths longer than 1.56 microm.     

Wavelength-versatile subpicosecond pulsed lasers using Raman gain in figure-of-eight fiber geometries

A family of compact pulsed fiber lasers is described that employs broadband, wavelength-flexible Raman scattering in passively mode-locked figure-of-eight fiber laser geometries. Specifically, sources at 1.57, 1.33, and 1.41 microm with respective soliton durations of 440, 500, and 860 fs are reported. Operation is possible at other wavelengths with a suitable pump source, gain fiber, and components.     

Synchronously pumped optical parametric oscillator driven by a femtosecond mode-locked fiber laser

A femtosecond all-fiber laser source incorporating a cw mode-locked Yb-doped silica fiber oscillator and amplifier has been used to synchronously pump an optical parametric oscillator based on periodically poled lithium niobate. The signal output, consisting of 330-fs pulses at a 54-MHz repetition rate and average powers up to 90 mW, was tuned from 1.55 to 1.95microm , with a corresponding idler range of 2.30-3.31microm .     

Ultrahigh-resolution optical coherence tomography using continuum generation in an air-silica microstructure optical fiber

We demonstrate ultrahigh-resolution optical coherence tomography (OCT) using continuum generation in an air-silica microstructure fiber as a low-coherence light source. A broadband OCT system was developed and imaging was performed with a bandwidth of 370 nm at a 1.3-mu;m center wavelength. Longitudinal resolutions of 2.5 microm in air and ~2 microm in tissue were achieved. Ultrahigh-resolution imaging in biological tissue in vivo was demonstrated.     

Mid-infrared difference-frequency generation of ultrashort pulses tunable between 3.2 and 4.8 microm from a compact fiber source

We report single-pass difference-frequency generation of mid-infrared femtosecond pulses tunable in the 3.2-4.8 microm range from a two-branch mode-locked erbium-doped fiber source. Average power levels of up to 1.1 mW at a repetition rate of 82 MHz are obtained in the mid infrared. This is achieved via nonlinear mixing of 170 mW, 65 fs pump pulses at a fixed wavelength of 1.58 microm, with 11.5 mW, 40 fs pulses tunable in the near-infrared range between 1.05 and 1.18 microm. These values indicate that the tunable near-infrared input component is downconverted with a quantum efficiency that exceeds 30%.     

Sensing of multimode-fiber strain by a dynamic photorefractive hologram

We present a strain sensor in which a multimode fiber is used as a sensitive element. High sensitivity to dynamic strains is achieved by means of vectorial wave mixing in a photorefractive CdTe:V crystal. It was found that the largest source of noise in our sensor is related to the instability of the polarization state of speckles emerging from the fiber. This noise is significantly diminished in fiber with a core of large diameter (550 microm).     

High-power femtosecond fiber-feedback optical parametric oscillator based on periodically poled stoichiometric LiTaO3

We demonstrate a synchronously pumped high-gain optical parametric oscillator with feedback through a fiber, using a passively mode-locked Yb:YAG thin-disk laser as a pump source. We obtain as much as 19-W average signal power at a wavelength of 1.45 microm in 840-fs pulses and 7.8 W of idler power at 3.57 microm. The repetition rate of the pulses is 56 MHz, and the transverse beam quality of the generated signal is M2 < 1.6.     

High-power optical parametric chirped-pulse amplifier system with a 1.55 microm signal and a 1.064 microm pump

We have designed and built a chirped-pulse parametric-amplifier system that utilizes a 10 Hz, 300 ps, Nd:YAG pump laser system; a 1.575 microm fiber oscillator and amplifier as the signal source; and rubidium titanyl phosphate and potassium titanyl arsenate nonlinear crystals. We obtained 260 fs, 30 mJ pulses centered at 1.550 microm, representing a gain of > 10(9) and a peak power of 100 GW. To our knowledge, these are the highest energy and peak power pulses ever produced in the 1.5-2.0 microm region     

Superbroadband fluorescence fiber fabricated with granulated oxides

We demonstrate a single-core multiply doped fiber that, when pumped with a single pump source of approximately 800 nm, emits a more than two-octaves-spanning fluorescence spectrum ranging from 365 to 2300 nm. The fiber preform is manufactured from granulated oxides, and the core is doped with five different rare earths. At a pump power of 250 mW the total emitted power is 34 microW; given a core diameter of 6.5 microm and a numerical aperture of 0.1, the radiance exceeds 3 kW sr(-1) cm(-2). We also demonstrate direct diode pumping of the fiber.     

Bending-insensitive single-mode hole-assisted fibers with reduced splice loss

We propose and demonstrate a novel type of bending-insensitive single-mode hole-assisted fiber that has a doped core and two layers of holes with two different airhole diameters. The fiber has a 9.3 microm mode field diameter, a bending loss of 0.011 dB/turn at 1.55 microm for a bending diameter of 10 mm, and a cutoff wavelength below 1.1 microm. The fiber can be fusion spliced to a conventional single-mode fiber with low loss.     

Ytterbium gain band self-induced modulation instability laser

We demonstrate an ytterbium gain band self-induced modulation instability laser. A highly nonlinear holey fiber is used to provide the anomalous dispersion required for bright soliton generation at 1 microm. The all-fiber integrated source yields a 40 GHz train of 4 ps pulses at a wavelength of 1064 nm.     

Single-transverse-mode 2.5-W holmium-doped fluoride fiber laser operating at 2.86 microm

A high-power tandem-pumped Ho3+, Pr3+-doped ZBLAN fiber laser is demonstrated. Using the free-running 1100-nm output from a diode-cladding-pumped Yb3+-doped silica fiber laser as the pump source, a maximum output power of 2.5 W was generated at a slope efficiency of 29% after the threshold of approximately 30 mW was reached. Saturation of the output is avoided with Pr3+ codoping, which allows single-transition output. The center wavelength of the output was 2.86 microm and the bandwidth at maximum power was approximately 15 nm.     

Optical guiding of atoms through a hollow-core photonic band-gap fiber

We demonstrate the first guiding of atoms through a hollow-core photonic band-gap fiber. Rb atoms from a thermal oven source travel through 6.1 and 4.0 cm long fibers with a guiding potential 1/e(2) radius of 3.0 microm. When the photon scattering rate is small, the observed guiding efficiency is greater than 70%. An unexpected low-speed cutoff is attributed to inefficient optical coupling.     

Broad-spectrum frequency comb generation and carrier-envelope offset frequency measurement by second-harmonic generation of a mode-locked fiber laser

A frequency comb spanning more than one octave has been achieved by injecting the second-harmonic generation (780 nm) of a mode-locked fiber laser (1.56 microm) into a photonic crystal fiber. We propose and realize a novel interferometric scheme for observing the carrier-envelope offset frequency of the frequency comb. Frequency noise has been observed on the measured carrier-envelope offset frequency, which has been confirmed to be generated in the photonic crystal fiber by comparing the measured beat frequencies between cw lasers and frequency combs before and after the photonic crystal fiber. The mode-locked fiber laser is considered to be an important candidate for the light source used in realizing a compact optical frequency measurement system including applications in the telecommunication bands.     

Single-mode operation of a coiled multimode fiber amplifier

We report a new approach to obtaining single-transverse-mode operation of a multimode fiber amplifier in which the gain fiber is coiled to induce significant bend loss for all but the lowest-order mode. We demonstrated this method by constructing a coiled amplifier using Yb-doped, double-clad fiber with a core diameter of 25 microm and a numerical aperture of ~0.1 (V approximately 7.4) . When the amplifier was operated as an amplified-spontaneous-emission source, the output beam had an M(2) value of 1.09 +/- 0.09 ; when seeded at 1064 nm, the slope efficiency was similar to that of an uncoiled amplifier. This technique will permit scaling of pulsed fiber lasers and amplifiers to significantly higher pulse energies and peak powers and cw fiber sources to higher average powers while maintaining excellent beam quality.     

Frequency-resolved coherent lidar using a femtosecond fiber laser

We demonstrate a coherent lidar that uses a broadband femtosecond fiber laser as a source and resolves the returning heterodyne signal into N spectral channels by using an arrayed-waveguide grating. The data are processed incoherently to yield an N-times improvement in the Doppler measurement of a surface vibration. For N=6, we achieve a sensitivity of 153 Hz, corresponding to a 0.12 mm/s motion, in 10 ms despite a signal that is speckle broadened to 14 kHz. Alternatively, the data are processed coherently to form a range image. For a flat target, we achieve a 60 microm range resolution, limited mainly by the source bandwidth, despite the dispersion of 1 km of optical fiber in the signal path.