Small-core As-Se fiber for Raman amplification

We have demonstrated Raman small-core As-Se fiber. More than 20-dB of gain was observed in a 1.1-m length of fiber pumped by a nanosecond pulse of approximately 10.8-W peak power at 1.50 microm. The peak of the Raman gain occurred at a shift of approximately 240 cm(-1). The Raman gain coefficient is estimated to be approximately 2.3 x 10(-11) m/W, which is more than 300 times greater than that of silica. The large Raman gain coefficient coupled with the large IR transparency window of these fibers shows promise for development of As-Se Raman fiber lasers and amplifiers in the near-, mid-, and long-IR spectral regions.     

Nonlinear-optical modulator for high-power fiber lasers

We propose a novel nonlinear-optical fiber modulator for high-power (>1-W) single-mode fiber lasers. The device is based on transferring the amplitude modulation from a low-power signal at the Stokes frequency onto a high-power beam through stimulated Raman scattering. The efficiency and limitations of the Raman modulator when highly Ge-doped fiber is used is considered. An insertion loss of less than 0.4 dB and an extinction ratio of 20 dB are predicted for modulation of a 10-W single-mode Nd-doped fiber laser.     

Co(2+)-doped flatband optical fiber attenuator

The attenuation properties of Co(2+) -doped single-mode optical fibers were experimentally and analytically investigated for preparation of a stable flatband attenuator. By controlling the Co(2+) -doped region in a fiber core, we could prepare a flatband attenuator in which the attenuation in wavelengths 1530-1610 nm was held to within 9.9-10.2 dB (3% variation) and the fluctuation of attenuation was less than 0.1 dB with 1-W input power for 100 h.     

Compact low-threshold Q-switched intracavity optical parametric oscillator

We report a singly resonant pulsed intracavity KTiOPO>(4) optical parametric oscillator that uses a semi-monolithic microchip laser design. The compact (50-mm-long), low-threshold (1.3-W) cavity uses a novel quadrupole deflector Q switch to give 4-microJ pulses at 1.064 microm and 0.4-microJ signal pulses of 5.6-ns duration at 1.53 microm with a repetition frequency of 5 kHz when it is pumped with a 2-W laser diode. The signal pulses are diffraction limited and single frequency.     

Compact,continuous-wave,singly resonant optical parametric oscillator based on periodically poled RbTiOAsO4 in a Nd:YVO4 laser

We describe a compact all-solid-state continuous-wave, singly resonant optical parametric oscillator (SRO) based on periodically poled RbTiOAsO4. The SRO is pumped at 1.064 microm by a Nd:YVO4 laser, which is itself pumped by a 3-W diode laser. Using the intracavity technique produced an oscillation threshold for the SRO of only 1.6 W (diode-laser power). For 3 W of diode pump power some 65 mW was obtained in the (nonresonant) idler (wavelength 3.52 microm). Temperature tuning over the range 10-100 degrees C resulted in tuning ranges of 1.52-1.54 and 3.41-3.54 microm for the signal and the idler waves, respectively. Importantly, relaxation oscillations were absent.     

Continuous-wave,high-power,Raman continuum generation in holey fibers

The possibility of using low pump power for cw Raman continuum generation is demonstrated by optimization of the pump peak power and by accounting for the loss-related reduction of the effective length of Raman interaction in holey fibers. A 3.8-W, 324-nm-wide cw Raman continuum with a spectral power density higher than 10 mW/nm is generated in a completely fiber-integrated, single-mode format.     

Low-pump-threshold continuous-wave singly resonant optical parametric oscillator

We describe a compact all-solid-state continuous-wave singly resonant optical parametric oscillator (SRO) with a minimal pump-power requirement. The SRO is based on periodically poled LiNbO(3) as the nonlinear material and is pumped by a 1-W diode-pumped Nd:YVO(4) minilaser at 1.064 microm . By exploiting the intracavity pumping technique in a 50-mm crystal, we have achieved SRO operation threshold at a diode pump power of only 310 mW.At 1 W of input diode power, the SRO delivers 70 mW of output power in the nonresonant idler at 3.66 microm , at a photon conversion efficiency of 55%. Multiparameter tuning of the SRO yields a signal wavelength range from 1.45 to 1.60 microm and an idler wavelength range from 3.16 to 4.02 microm in the mid infrared. The device is characterized by robust turnkey operation and long-term amplitude-stable performance.     

High-gain Tm-doped fluoride fiber amplifier

A diode-pumped Tm-doped fiber-optic amplifier that has a small-signal gain of >30 dB at 1870 nm is reported. Output pulses of up to 3-W peak power at a 1-60-kHz repetition rate can be generated by amplification of 20-40-ns laser diode pulses of up to 2-mW launched peak power. The output signal quality, i.e., the ratio of the output pulse energy and the total amplified spontaneous emission (ASE) output energy between two pulses, depends on the relative propagation direction of pump and signal and can be dramatically increased by choice of the correct propagation scheme. In the optimum pump geometry the pulse energy can be raised to as much as 20 times the ASE energy. This is the first report to the author's knowledge of fiber-optic amplification of short diode laser pulses near 1.9 microm with high repetition rates in Tm-doped fibers.     

Homodyne en face optical coherence tomography

We demonstrate, for what we believe to be the first time, the use of a 3 x 3 fiber-optic coupler to realize a homodyne optical coherence tomography (OCT) system for en face imaging of highly scattering tissues and turbid media. The homodyne OCT setup exploits the inherent phase shifts between different output ports of a 3 x 3 fiber-optic coupler to extract amplitude information of a sample. Our homodyne en face OCT system features a measured resolution of 14 microm axially and 9.4 microm laterally with a 90 dB signal-to-noise ratio at 10 micros integration time. En face OCT imaging of a stage 52 Xenopus laevis was successfully demonstrated at a depth of 600 microm within the sample.     

Dispersion management with microstructured optical fibers: ultraflattened chromatic dispersion with low losses

We numerically demonstrate ultraflattened chromatic dispersion with low losses in microstructured optical fibers (MOFs). We propose using two different MOF structures to get this result. Both structures are based on a subset of a triangular array of cylindrical air holes; the cross sections of these inclusions are circular, and a missing hole in the fiber's middle forms the core. In this MOF structure the diameters of the inclusions increase with distance from the fiber axis until the diameters reach a maximum. With this new design and with three different hole diameters, it requires only seven rings to reach the 0.2-dB/km level at lambda = 1.55 microm with a variation amplitude of dispersion below 3.0 x 10(-2) ps nm(-1) km(-1) of lambda = 1.5-1.6 microm. With the usual MOF (made from holes of identical diameter), we show that at least 18 hole rings are required for losses to decrease to < 1 dB/km at lambda = 1.55 microm.     

Tuning of group delay with stimulated Raman scattering-induced dispersion in gas-filled optical fiber

We report the first demonstration of group delay tuning with stimulated Raman scattering-induced dispersion in a hydrogen-filled hollow-core optical fiber. A pump laser induces a sharp refractive index change near the S_0(0)Raman transition of hydrogen molecules, enabling the control of the group velocity of signal pulses around the Stokes wavelength. Experiments with an 80-m-long hollow-core fiber filled with 2.5 bar hydrogen achieved continuous tuning of the pulse delay up to 1.42 ns by varying the Raman amplification from 0 to 10 dB. The tunable pulse delay is realized by changing the pump power as well as the hydrogen pressure. This work provides a new technique for controlling the pulse propagation in optical fibers with high flexibility.     

The on-off contrast in an all optical switch based on stimulated Raman scattering in optical fibers

We present the investigation of the ON-OFF contrast in an optical switch using stimulated Raman Scattering in optical fibers. The setup consists of a Raman circuit of two fiber stages connected in series with a spectral filter rejecting the signal inserted between them. The stage 1 works as saturated amplifier, in this stage the pump pulses are saturated when pump and signal are launched to the input or travel through the fiber without saturation when pump only is launched at the input. The stage 2 works as a Raman amplifier with amplification depending on the pump power entering from the first stage. When pump only is launched at the input enter to the second stage without saturation and amplifies the signal entering this stage, strong signal pulses appear at the output; when pump and signal are launched to the input the pump is saturated in the first stage and the filter rejected the amplified signal, so that only low power pump enters the second stage and no signal pulses appear at the output. We use 2 ns pump pulses at 1528 nm and continuous-wave signal at 1620 nm. In the first stage of Raman circuit, we use both fibers with normal and anomalous dispersion. In fibers with anomalous dispersion, pump saturation is affected by modulation instability. We find that the contrast may be improved using fibers with normal and anomalous dispersion connected in series in the first stage, provided there is appropriate selection of their lengths. The best achieved contrast was 15 dB at 6 W pump peak power.     

Stroboscopic ultrahigh-resolution full-field optical coherence tomography

We present a new technique that produces en face tomographic images with a 10-micros acquisition time per image. The setup consists of an interference microscope with stroboscopic illumination provided by a xenon arc flash lamp (10-micros flashes at 15 Hz). The tomographic images are obtained from two phase-opposed interferometric images recorded simultaneously by two synchronized CCD cameras. Transverse resolution better than 1.0 microm is achieved by use of high-numerical-aperture microscope objectives. The short coherence length of the source yields an axial resolution of 0.9 microm. 3 x 3 pixel binning leads to a detection sensitivity of 71 dB. Our system is suitable for various applications, particularly in biology for in vivo cellular-level imaging.     

Single-mode low-loss optical fibers for long-wave infrared transmission

In this Letter, we report single-mode fibers made of chalcogenide glasses with low loss in the 5-12μm range. Glasses from the Ge-As-Te-Se system were optimized to prevent nucleation and to exhibit low density of charge carriers. Single-mode fibers were obtained through the rod-in-tube method by substituting 2% Te/Se between the core and cladding glasses. The resulting single-mode fibers had a core diameter of 30μm and exhibited losses of ~6 dB/m at 10.6μm, and as low as 3-4dB/m in the 6-10μm range.     

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.     

Experimental evidence for Raman-induced limits to efficient squeezing in optical fibers

We report new experiments on polarization squeezing using ultrashort photonic pulses in a single pass of a birefringent fiber. We measure what is to our knowledge a record squeezing of -6.8+/-0.3 dB in optical fibers, which when corrected for linear losses is -10.4+/-0.8 dB. The measured polarization squeezing as a function of optical pulse energy, which spans a wide range from 3.5-178.8 pJ, shows a very good agreement with the quantum simulations, and for the first time we see the proof experimentally that Raman effects limit and reduce squeezing at high pulse energy.     

Raman amplification characteristics of As2Se3 photonic crystal fibers

We present the dispersion and Raman amplification characteristics of As2Se3 photonic crystal fibers (PCFs). We compare the gain characteristics with conventional As2Se3 fibers and find that the Raman gain efficiency in PCFs can be improved by a factor of more than 4. This allows us to either use a small length of the fiber or to use the low pump power to attain similar gain characteristics. Numerical simulations reveal that a peak gain of 10 dB can be achieved in a 1.1 m long PCF when it is pumped at 1.5 microm in wavelength with an input power of 500 mW.     

Generation of multiwatt, broadband continua in holey fibers

We demonstrate that 2-W average-fpower 310-nm-wide continua can be generated in holey fibers by use of a novel 6-W pump source at 770 nm. The pump source is demonstrated by use of the 64% efficient quasi-phase-matched second-harmonic generation of a seeded 10-W erbium fiber amplifier in periodically poled KTP. Nonlinear contributions to generation of high-power continua in holey fibers are identified and analyzed.     

Adaptive-optics ultrahigh-resolution optical coherence tomography

Merging of ultrahigh-resolution optical coherence tomography (UHR OCT) and adaptive optics (AO), resulting in high axial (3 microm) and improved transverse resolution (5-10 microm) is demonstrated for the first time to our knowledge in in vivo retinal imaging. A compact (300 mm x 300 mm) closed-loop AO system, based on a real-time Hartmann-Shack wave-front sensor operating at 30 Hz and a 37-actuator membrane deformable mirror, is interfaced to an UHR OCT system, based on a commercial OCT instrument, employing a compact Ti:sapphire laser with 130-nm bandwidth. Closed-loop correction of both ocular and system aberrations results in a residual uncorrected wave-front rms of 0.1 microm for a 3.68-mm pupil diameter. When this level of correction is achieved, OCT images are obtained under a static mirror configuration. By use of AO, an improvement of the transverse resolution of two to three times, compared with UHR OCT systems used so far, is obtained. A significant signal-to-noise ratio improvement of up to 9 dB in corrected compared with uncorrected OCT tomograms is also achieved.     

Low-loss polymeric optical waveguides with large cores fabricated by hot embossing

A simple low-cost method of fabricating polymeric optical waveguides with large core sizes for plastic optical fibers is presented. The waveguides are fabricated by hot embossing with an ultraviolet-cured epoxy resin stamper. The stamper is fabricated by replication of a rectangular groove mold that is made from silicone rubber replicated from a ridged original silicon master. The master is fabricated by anisotropic etching of (110) single-crystal silicon. Optical waveguides with large core sizes of 100-500 microm have been fabricated, and a low propagation loss of 0.19 dB/cm at 650 nm was achieved.