Birefringent in-phase supermode operation of a multicore microstructured fiber laser

We report the first observation of birefringent in-phase supermode operation of a phase-locked multicore fiber laser. The in-phase mode operation of our 12-core rectangular-array microstructured fiber laser was confirmed by the near-field distribution, the far-field diffraction pattern, and the optical spectrum. The birefringence of the in-phase mode in propagation constant Deltay was measured as ~ 4 x 10(-6) 1/mum. The break of the polarization degeneracy indicates the possibility of single polarization operation of phase-locked multicore fiber lasers and amplifiers.

Highly sensitive spectroscopic detection of heme-protein submonolayer films by channel integrated optical waveguide

A highly sensitive technique based on optical absorption using a single-mode, channel integrated optical waveguide is described for broad spectral band detection and analysis of heme-containing protein films at a glass/water interface. Fabrication steps and device characteristics of optical waveguides suitable for operation in the wavelength range of 400 - 650 nm are described. Experimental results reported here show a limit of detection smaller than 100 pg/cm(2) for a submonolayer of ferricytochrome c by probing the Soret transition band with a 406-nm semiconductor diode laser propagating in a single-mode, ion-exchanged channel waveguide. By taking advantage of the exceptionally low limit of detection, we examined the adsorption isotherm of cytochrome c on a glass surface with unprecedented detail. Unlike other surface-specific techniques (e.g. SPR, integrated optic Mach-Zehnder interferometer) that probe local refractive-index changes and therefore are very susceptible to temperature fluctuations, the integrated optical waveguide absorption technique probes molecular-specific transition bands and is expected to be less vulnerable to environmental perturbations.

Compact, single-frequency all-fiber Q-switched laser at 1 microm

We demonstrate a unique, all-fiber, actively Q-switched laser operating in the 1 microm region. The laser is compact, single mode, single frequency, highly polarized, and exhibits high peak power. The laser cavity is constructed without external coupling, utilizing fiber Bragg gratings that permit feedback at only a single polarization. By using a piezoelectric to press the fiber and modulate the fiber birefringence, the cavity is switched between high and low loss states, permitting Q-switching. We demonstrate this Q-switching at repetition rates up to 700 KHz.

Image amplifier based on Yb(3+)-doped multi-core phosphate optical fiber

We demonstrate image amplification with a 19-pixel optical image amplifier array based on high gain per unit length Yb(3+)-doped phosphate glass optical fiber. The 19 pixels of the image amplifier provide spatially uniform image amplification whose gain can reach 30 dB/pixel or more with a fiber length of 10 cm. This image amplifier responds quickly to changes in the image position - with potential for GHz-level frame rates. This unique approach for image amplification offers low noise, high gain, and wide field of view in a compact fiber-based device.

Phase locking and in-phase supermode selection in monolithic multicore fiber lasers

We report a compact multicore fiber laser that utilizes an all-fiber approach for phase locking and in-phase supermode selection. By splicing passive coreless fibers of controlled lengths to both ends of an active 19-core fiber, we demonstrate that the fundamental in-phase supermode can be selectively excited with a completely monolithic fiber device, instead of conventional free-space and bulk optics, to achieve phase-locked operation for a multiemitter laser device.

Single-frequency fiber oscillator with watt-level output power using photonic crystal phosphate glass fiber

Utilizing phosphate glass fiber with photonic crystal cladding and highly doped, large area core a cladding-pumped, single-frequency fiber oscillator is demonstrated. The fiber oscillator contains only 3.8 cm of active fiber in a linear cavity and operates in the 1.5 micron region. Spectrally broad, multimode pump light from semiconductor laser diodes is converted into a single-mode, single-frequency light beam with an efficiency of about 12% and the oscillator output power reached 2.3 W.

Photorefractive polymer device with video-rate response time operating at low voltages

The high-voltage bias required for video-rate compatible, efficient operation of a photorefractive polymer composite is reduced from 6-8 to 1.3 kV. At this low voltage, the device can hold erasable Bragg holograms with 80% efficiency in addition to having a video-rate response time. The transition of the hologram's state from thick to thin is analyzed in detail.

All-fiber passively mode-locked laser oscillator at 1.5 microm with watts-level average output power and high repetition rate

We report on a passively mode-locked all-fiber laser oscillator at 1.5 microm based on heavily doped phosphate-glass active fiber. An active fiber only 20 cm long is sufficient to produce as much as 2.4 W of average output power directly from the oscillator. The width of the mode-locked pulses varies from 8 ps at the lowest output power in the mode-locked state to 44 ps at the highest power. Our picosecond laser oscillator features a high repetition rate of 95 MHz and high peak pulse power of approximately 540 W. The oscillator combines the convenience of all-fiber construction with power performance that was previously achievable only with mode-locked bulk-optic laser oscillators or more complex systems involving fiber amplifiers.

Investigation of modal properties of microstructured optical fibers with large depressed-index cores

We present what is, to the best of our knowledge, the first systematic study on how negative core-cladding index difference influences microstructured optical fiber's modal behavior. Single-mode lasing has been realized for short-length cladding-pumped phosphate glass microstructured fibers with large depressed-index Er(3+)-Yb(3+)-codoped cores.

1.5-microm-band thulium-doped microsphere laser originating from self-terminating transition

Continuous-wave 1.5-microm-band laser is demonstrated in thulium doped tellurite glass microsphere by single laser pumping. 1.9-microm laser from the lower transition (3F4->3H6) of Tm3+ is generated to depopulate the lower level (3F4) of 1.5-microm transition (3H4->3F4) in order to achieve the population inversion. Laser wavelength of 3H4->3F4 transition is shifted by 30 nm from the emission peak. Slope efficiency of 1.9-microm laser is improved after the1.5-ìm laser starts to lase.

Single-frequency laser oscillator with watts-level output power at 1.5 microm by use of a twisted-mode technique

We report an all-fiber laser oscillator producing as much as 1.9 W of single-frequency direct output at 1.5 microm. Spatial gain hole burning in the active fiber has been eliminated by use of a twisted-mode cavity approach. The two short pieces of a polarization-maintaining fiber that were spliced to the ends of the active fiber served as ultracompact quarter-wave plates. To our knowledge, the use of such a wave plate to manipulate the polarization state of light inside a fiber laser cavity is reported here for the first time. The laser output is linearly polarized and delivered through a polarization-maintaining fiber pigtail. We believe that the output power of our laser is the highest among all single-frequency fiber laser oscillators reported to date.

Generation of watt-level single-longitudinal-mode output from cladding-pumped short fiber lasers

We generate as much as 1.6 W of continuous-wave 1550 nm single-longitudinal-mode output from a cladding pumped Er-Yb codoped phosphate fiber laser. This power is to our knowledge among the highest in single-longitudinal-mode fiber lasers. The narrowband fiber Bragg grating output coupler is demonstrated to be an effective element for providing the single-longitudinal-mode selection.

Evanescent field-based optical fiber sensing device for measuring the refractive index of liquids in microfluidic channels

We report a simple optical sensing device capable of measuring the refractive index of liquids propagating in microfluidic channels. The sensor is based on a single-mode optical fiber that is tapered to submicrometer dimensions and immersed in a transparent curable soft polymer. A channel for liquid analyte is created in the immediate vicinity of the taper waist. Light propagating through the tapered section of the fiber extends into the channel, making the optical loss in the system sensitive to the refractive-index difference between the polymer and the liquid. The fabrication process and testing of the prototype sensing devices are described. The sensor can operate both as a highly responsive on-off device and in the continuous measurement mode, with an estimated accuracy of refractive-index measurement of approximately 5 x 10(-4).

Short-length microstructured phosphate glass fiber lasers with large mode areas

We report fabrication and testing of the first phosphate glass microstructured fiber lasers with large Er-Yb-codoped cores. For an 11-cm-long cladding-pumped fiber laser, more than 3 W of continuous wave output power is demonstrated, and near single-mode beam quality is obtained for an active core area larger than 400 microm2.

3-Dimensional thermal analysis and active cooling of short-length high-power fiber lasers

A fully 3-dimensional finite element model has been developed that simulates the internal temperature distribution of short-length high-power fiber lasers. We have validated the numerical model by building a short, cladding-pumped, Er-Yb-codoped fiber laser and measuring the core temperature during laser operation. A dual-end-pumped, actively cooled, fiber laser has generated >11 W CW output power at 1535 nm from only 11.9 cm of active fiber. Simulations indicate power-scaling possibilities with improved fiber and cooling designs.

Single-frequency fiber ring laser with 1W output power at 1.5 microm

We report a single-frequency fiber laser with 1W output power at 1.5 mum which is to our knowledge, five times the highest power from a single-frequency fiber laser reported to-date. The short unidirectional ring cavity approach is used to eliminate the spatial gain hole-burning associated with the standing-wave laser designs. A heavily-doped phosphate fiber inside the ring resonator serves as the active medium of the laser. Up to 700mW of output power, the longitudinal mode hops have been completely eliminated by using the adjustable coupled-cavity approach. At higher power levels, the laser still oscillates at a single longitudinal mode, but with infrequent mode hops that occur at a rate of few hops per minute. Compared to the Watt-level single-frequency amplified sources, our approach is simpler and offers better noise performance.

Watts-level, short all-fiber laser at 1.5 microm with a large core and diffraction-limited output via intracavity spatial-mode filtering

We report over 2 W of single spatial-mode output power at 1.5 microm from an 8-cm-long, large-core phosphate fiber laser. The fiber has a numerical aperture of approximately equal to 0.17 and a 25-microm-wide core, heavily doped with 1% Er(+3) and 8% Yb(+3). The laser utilizes a scalable evanescent-field-based pumping scheme and can be pumped by as many as eight individual multimode pigtailed diode laser sources at a wavelength of 975 nm. Nearly diffraction-limited laser output with a beam quality factor M2 approximately equal to 1.1 is achieved by use of a simple intracavity all-fiber spatial-mode filter. Both spectrally broadband and narrowband operation of the laser are demonstrated.

Buried ion-exchanged glass waveguides: burial-depth dependence on waveguide width

A detailed theoretical and experimental study of the depth dependence of buried ion-exchanged waveguides on waveguide width is reported. Modeling, which includes the effect of nonhomogeneous time-dependent electric field distribution, agrees well with our experiments showing that burial depth increases linearly with waveguide width. These results may be used in the proper design of integrated optical circuits that need waveguides of different widths at different sections, such as arrayed waveguide gratings.

Photorefractive polymers sensitized by two-photon absorption

We demonstrate the recording of holograms and their nondestructive readout in a photorefractive polymer, using two-photon absorption. Sensitivity is provided by the excitation of the electroactive chromophore with femtosecond pulses, followed by charge injection into the photoconducting poly(N -vinylcarbazole) matrix. The holograms can be fully erased with a pulsed laser source but are insensitive to cw laser beams with the same wavelength. Studies of the field and intensity dependence of the diffraction efficiency indicate that the holograms are formed through the photorefractive effect.

Evidence for intervalence band coherences in semiconductor quantum wells via coherently coupled optical Stark shifts

We report the experimental observation of coherently coupled heavy-hole-light-hole Stark shifts, i.e., light-hole exciton shifts under heavy-hole exciton pumping conditions, in InGaAs quantum wells. The theoretical analysis of the data is based on a full many-body approach (dynamics-controlled truncation formalism) in the third-order nonlinear optical regime. It is shown that the Stark shift data can be interpreted as strong evidence of suitably defined nonradiative intervalence band coherences in a semiconductor quantum well. Hence, the observations establish a semiconductor analog of Raman coherences in three-level atoms.

Two-photon resonant third-harmonic generation in La2CuO4

Combining linear absorption and nonlinear third harmonic generation (THG) experiments, we investigate details of the electronic structure of the highly correlated electronic system in La2CuO4. We demonstrate strong THG mainly due to the charge transfer excitation from O (2p(sigma)) to Cu (3d(x2-y2)). The THG spectrum shows pronounced features due to three-photon and two-photon resonance enhancement as well as quantum interference effects. We obtain excellent agreement with a THG spectrum calculated in terms of the excitonic cluster model and can identify both odd and even symmetry excitation modes.