Alok Mehta

Wavelength tunable fiber lens based on multimode interference

A new concept for a fiber-based wavelength-tunable condensing lens is theoretically and experimentally investigated in terms of its ability to shift the longitudinal focus position as a function of wavelength. By exploiting multimode interference effects with reimaging theory, a compact and robust device, completely contained within a standard FC connector, can be constructed by simply splicing a single-mode fiber (SMF) to a finite length section of multimode fiber (MMF). This paper summarizes the theoretical and experimental results of such a device, with additional results for wavelength sensitivity.

Multimode interference-based fiber-optic displacement sensor

A new concept for a multimode interference-based fiber-optic displacement sensor is investigated both theoretically and experimentally. By applying multimode interference and reimaging theory to optical fibers, a displacement sensor consisting of only a section of multimode fiber fusion spliced to a single-mode fiber can be readily fabricated. Observation and analysis of the power reflected off of a surface and back through the device over a finite wavelength range provides enough information to use the device as a calibrated displacement sensor over a large displacement range.

Wavelength tuning of fiber lasers using multimode interference effects

We report on a novel scheme to fabricate a simple, cheap, and compact tunable fiber laser. The tuning is realized by splicing a piece of single-mode fiber to one end of an active double-clad fiber, while the other end of the single-mode fiber is spliced to a 15 mm long section of 105/125 multimode fiber. The fluorescence signal entering into the multimode fiber will be reproduced as single images at periodic intervals along the propagation direction of the fiber. The length of the multimode fiber is chosen to be slightly shorter than the first re-imaging point, such that the signal coming out from the single mode fiber is obtained in free space, where a broadband mirror retroreflects the fluorescence signal. Since the position of the re-imaging point is wavelength dependent, different wavelengths will be imaged at different positions. Therefore, wavelength tuning is easily obtained by adjusting the distance between the broadband mirror and the multimode fiber facet end. Using this principle, the tunable fiber laser revealed a tunability of 8 nm, ranging from 1088-1097 nm, and an output power of 500 mW. The simplicity of the setup makes this a very cost-effective tunable fiber laser.

Selective excitation of the TE/sub 01/ mode in hollow-glass waveguide using a subwavelength grating

In this work we demonstrate, for the first time to our knowledge, a novel subwavelength optical element for the selective excitation of the TE/sub 01/ mode inside a 300-/spl mu/m bore diameter hollow-glass waveguide (HGW) in order to reduce the transmission loss. The device consists of form birefringent gratings, where each section is oriented in a different direction so as to convert a linearly polarized input light to a rotating polarized one matching the TE/sub 01/ mode. Using this polarization converting element, we measured a 1.65-dB decrease in the transmission loss in the HGW at a wavelength of 1.55 /spl mu/m.

Guided Mode Resonance Filter as a Spectrally Selective Feedback Element in a Double-Cladding Optical Fiber Laser

In this work, a spectrally selective optical element is introduced based on a 2-D guided mode resonance filter (GMRF) as an external feedback element. The GMRF was designed to provide a highly efficient narrow linewidth reflection within the gain bandwidth of the fiber laser, while transmitting the pump beam. These features enabled the fiber laser to operate in an external cavity configuration to provide a wavelength-stabilized and narrow linewidth output within the optical -band.

Selective excitation of the LP11 mode in step index fiber using a phase mask

We present a novel mode selective coupling technique for step index fiber. This technique utilizes phase matching for excitation of higher-order modes while suppressing the fundamental mode. Using this technique, a phase element is fabricated and tested to demonstrate the high coupling efficiency to the LP11 mode. In addition, we derive an analytical expression of the coupling efficiency of the LP11 using a single

High efficiency surface-emitting laser with subwavelength antireflection structure

We report on a high efficiency tapered grating surface-emitting laser with an antireflection-structured (ARS) substrate. A 64% improvement of the device efficiency is obtained by monolithic integration of a sawtooth-shaped ARS on the GaAs substrate. Slope efficiencies of 0.82 W/A were measured at 975 nm in pulse pumping and are mainly limited by free-carrier absorption in the n-doped GaAs substrate. A maximum peak power of 25 W was obtained without coating the devices cleaved facet. The symmetry of the near-field intensity profile along the grating coupler is improved by varying the grating duty cycle from 20% to 55%.

Fiber-optic beam shaper based on multimode interference

A new method of fiber-optic based beam shaping is investigated both numerically and experimentally. A cylindrically symmetric method of lines (MoLs) is developed to simulate the device. The device is fabricated by fusion splicing a predetermined length of multimode fiber (MMF) to a single-mode fiber. The multimode interference (MMI) effects create ring-shaped field profiles at certain positions inside the MMF. The shaped beam can be used in medical applications requiring particular irradiation patterns.

Nanofabrication of a space-variant optical transmission filter

A space-variant optical transmission filter is demonstrated for which a simplified process is used to tailor the spatial response of the filter across the surface of a single wafer. A multilayer stack, of alternating high or low refractive index dielectric materials, was used to produce a narrow transmission notch in the center of a wide stop band. Subsequent patterning and etching of arrays of holes through the volume of the dielectric stack was performed to control the fill factor of the dielectric in the layers. The position of the transmission notch within the reflection spectrum was varied across the device surface by adjusting the hole diameter of the hole arrays. Experimental and numerical simulation were used to confirm the space-variant transmission characteristics of a single-wafer sample with two zones of different hole diameter arrays in the 1550 nm wavelength regime.

Spatially polarizing autocloned elements

A space-variant polarization converting element is introduced that utilizes an autocloning effect to produce high aspect ratio from birefringent gratings. This method utilizes a multilayer deposition process on a template to convert a linearly polarized incident beam to an azimuthally polarized output at a wavelength of 1.55 microm with more than 90% efficiency.