Raman Kashyap

Comblike dispersion-profiled fiber for soliton pulse train generation.

A novel optical fiber [comblike dispersion-profiled fiber (CDPF)] that consists of a chain of alternating segments of standard telecommunication fiber and dispersion-shifted fiber is proposed for the generation of a soliton pulse train based on nonlinear transformation of an optical beat signal. A totally integrated all-optical fiber source of a 59.1-GHz train of 2.2-ps solitons is demonstrated with a CDPF. For a beat signal generator we use a dual-frequency erbium fiber laser incorporating fiber grating reflectors that provides 16-kHz linewidths and a low phase noise of optical beating (< 5 x 10(-5)). Significant suppression of stimulated Brillouin scattering, which is essential for this technique, is achieved in the CDPF.

Fiber-Bragg-grating-assisted surface plasmon-polariton sensor

A theoretical scheme for a new surface plasmon-polariton (SPP) fiber sensor with a fiber Bragg grating imprinted into the fiber core for SPP excitation is presented for the first time to our knowledge. In our scheme the energy in the fiber core mode can be transferred to a SPP with high efficiency by means of a properly designed short-period fiber Bragg grating (SPG). Developed for the cylindrical (fiber) geometry, our scheme without loss of generality can be applied to a planar geometry. Our simulations are based on the coupled-mode method and are performed at telecommunications wavelengths.

Demonstration of a 3 mW threshold Er-doped random fiber laser based on a unique fiber Bragg grating

We demonstrate a novel random laser based on a single fiber Bragg grating. A long fiber Bragg grating fabrication technique allows the insertion of a large number of randomly distributed phase errors in the structure of the grating which induces light localization. By writing such a grating in a polarisation maintaining Er-doped fiber, a random laser is demonstrated by pumping the fiber with 976 and 1480 nm pump lasers. The number of emitted modes is observed to be a function of the length of the grating and of the pump power and single-mode operation is shown to be possible. The random fiber laser shows low-threshold (approximately 3 mW) and measured approximately 0.5 pm emission linewidth at a wavelength of around 1534 nm.

A novel millimeter-wave-band radio-over-fiber system with dense wavelength-division multiplexing bus architecture

In this paper, we propose a novel millimeter-wave-band radio-over-fiber (RoF) system with a dense wavelength-division multiplexing (DWDM) bus architecture. Two lasers with a small wavelength difference, phase locked and polarization aligned, are allocated at a central station (CS) for connecting the CS and each base station (BS), one laser for transmitting and the other for detection (the remote local oscillator). For the conceptual illustration, we consider a DWDM RoF system with a channel spacing of 12.5 GHz and RF of /spl sim/30-GHz millimeter-wave band. In the downlink system, a single-sideband (SSB) subcarrier is used with low RF imposed on an optical carrier at the CS, and an millimeter-wave-band RF signal is obtained at each BS using direct photodetection by the SSB subcarrier beat with the remote oscillator. In the uplink system, the received millimeter-wave-band RF signal at each BS is imposed on the two optical carriers simultaneously, one optical carrier with the closest SSB subcarrier is optically filtered out and fed into in the uplink transmission fiber without frequency interleaving; the electrical signal with a low IF can be photodetected directly at the CS. Such an RoF system has simple, cost-effective, and maintenance-reduced BSs, and is immune to laser phase noise in principle.

COUPLED-CAVITY ERBIUM FIBER LASERS INCORPORATING FIBER GRATING REFLECTORS

Narrow-linewidth erbium-doped fiber lasers of monolithic multiple linear cavity configurations that use intracore fiber grating reflectors are demonstrated. Robust single-frequency operation of a miniature coupled-cavity fiber laser with a linewidth of 30 kHz is reported. A novel multiple-cavity erbium fiber laser with simultaneous dual single-frequency lasing is also demonstrated. A 16-kHz linewidth and a frequency separation of 59 GHz with a stability of better than 3 MHz were achieved in this dual-frequency laser.

Laser cooling of solids

Laser cooling of solids, sometimes also known as optical refrigeration, is a fast developing area of optical science, investigating the interaction of light with condensed matter. Apart from being of fundamental scientific interest, this topic addresses a very important practical issue: design and construction of laser pumped solid-state cryocoolers, which are compact, free from mechanical vibrations, moving parts, fluids and can cause only low electromagnetic interference in the cooled area. The optical cryocooler has a broad area of applications such as in the development of magnetometers for geophysical sensors, in biomedical sensing and can be beneficial for satellite instrumentations and small sensors, where compactness and the lack of vibrations are very important. Simply, a laser cooler works on the conversion of low energy pump photons into high-energy anti-Stokes fluorescence photons by extracting some of the phonons (heat energy) in a material. That is, the process of laser cooling of solids is based on anti-Stokes fluorescence also known as luminescence upconversion, when light quanta in the red tail of the absorption spectrum are absorbed from a pump laser, and blue-shifted photons are spontaneously emitted. The extra energy extracted from the solid-state lattice in the form of the phonons is the quanta of vibrational energy which generates heat. The idea to cool solids with anti-Stokes fluorescence was proposed in 1929 by Peter Pringsheim and first demonstrated experimentally by Epsteins research team in 1995. In 1999, Steven Bowman proposed to use the optical refrigeration by anti-Stokes fluorescence within the laser medium to balance the heat generated by the Stokes shifted stimulated emission in a high-power solid-state bulk laser. Such a laser without internal heating named radiation-balanced or athermal laser was experimentally demonstrated for the first time in 2002. At the present time laser cooling of solids can be largely divided into three main areas: laser cooling of rare-earth doped solids, laser cooling in semiconductors and radiation-balanced lasers. All three areas are very interesting and important and will be considered in this paper.

Independent and Simultaneous Monitoring of Chromatic and Polarization-Mode Dispersion in OOK and DPSK Transmission

We propose and demonstrate a novel technique for a simultaneous chromatic and first-order polarization-mode-dispersion (PMD) monitoring method using a partial bit delay Mach-Zehnder interferometer (MZI) with radio-frequency (RF) clock tone monitoring. RF clock tones at the output of the two branches of the MZI behave oppositely with increasing chromatic dispersion (CD) which improves the sensitivity of the measurement. The technique increases CD monitoring sensitivity over standard clock tone methods by a factor of two for a nonreturn-to-zero intensity modulation format and a factor of five for a differential-phase-shift-keying modulation format. The accuracy of PMD monitoring is also enhanced. Moreover, the partial bit delay allows the signal to pass through the constructive branch of the MZI with no observable degradation of the signal quality, allowing it to be normally detected by a receiver

Design of Novel Unapodized and Apodized Step-Chirped Quasi-Phase Matched Gratings for Broadband Frequency Converters Based on Second-Harmonic Generation

The unapodized and apodized step-chirped gratings (SCGs) for broadband frequency converters based on quasi-phase-matched second-harmonic generation with pump depletion in lithium niobate waveguides have been theoretically modeled and simulated as a function of the number of sections, and compared with the linearly chirped gratings (LCGs) for the first time to our knowledge. It is shown that for the same length, using fewer sections with more segments and larger amounts of chirp, the efficiency and bandwidth of an SCG approach over that of an LCG and can be extensively improved with apodization. Moreover, the increasing chirp period and duty cycle for the SCG structure may provide a more convenient method for fabrication and poling. In addition, we present useful relations for the band-widths that help to find the appropriate number of segments in the proposed SCGs of a given length.

Phase-Noise Analysis of Optically Generated Millimeter-Wave Signals With External Optical Modulation Techniques

In this paper, the phase-noise performance of optically generated electrical signals based on external optical modulation techniques is investigated theoretically and experimentally. Mathematical models are developed to represent perturbations on the transmitted optical signal caused by the phase fluctuations of the electrical drive signal applied to the external modulator and the optical carrier that feeds the external modulator. Closed-form expressions of the power spectral density (PSD) for the electrical signals, generated both locally and remotely, are derived. The calculated PSD of the locally generated electrical signal indicates that its phase noise is determined only by the phase noise of the electrical drive signal. The PSD of the remotely generated signal shows that its spectral quality is also affected by the chromatic dispersion of the fiber and the optical carrier linewidth. An experimental setup that can generate a millimeter-wave (mm-wave) signal, continuously tunable from 32 to 60 GHz using an electrical drive signal tunable from 8 to 15 GHz, is built. The spectra of the generated millimeter-wave signal are measured for both locally and remotely generated electrical signals, with optical carriers of different linewidths. The theoretical results agree with the experimental measurements

Phase‐matched second‐harmonic generation by periodic poling of fused silica

90° phase matching in periodically poled fused silica is achieved over approximately 2 mm at a wavelength of 1064 nm. Electrical poling was done at 5 kV for 120 min using photolithographed periodic electrodes on fused silica glass at 250 °C. A nonlinearity equivalent to d 11/200 of quartz is estimated from measurements. Several phase‐matched interactions are reported and the implications for device applications discussed.