Tae-Jung Eom

Novel all-fiber bandpass filter based on hollow optical fiber

We report an in-line all-fiber bandpass filter based on a hollow optical fiber (HOF) core mode blocker along with a long-period fiber grating (LPG) pair. It is theoretically and experimentally confirmed that selective core mode blocking at the HOF region located in the middle of the LPG pair could provide efficient passband channels corresponding to the resonant wavelengths of the LPGs. The filter characteristics in passbands and the nearfield patterns of guided modes in the proposed device are discussed.

Broad-band tunable all-fiber bandpass filter based on hollow optical fiber and long-period grating pair

We report a tunable all-fiber bandpass filter based on a short hollow optical fiber serially concatenated between a pair of long-period fiber gratings. With novel core mode blocking in the hollow core fiber and its optimal design, the device showed a low insertion loss of 1.5 dB and broad-band passband tuning range of 84.3 nm covering both S- and C-band.

Optical temporal encoding/decoding of short pulses using cascaded long-period fiber gratings

A novel, optical temporal encoding/decoding method is proposed and demonstrated. This can be accomplished by passing a short optical pulse through cascaded long-period fiber gratings. It has the advantages of constructing ultrafast codes and developing resistance to interferometric perturbations among the coded pulses. To verify the feasibility as a code generator, two types of codes are generated and compared with the predicted code patterns. In addition, to show an application for an optical code-division-multiplexing system, decoding performances with matched and unmatched decoders are compared.

Waveguide-Type Wavelength-Tunable Šolc Filter in a Periodically Poled Ti : LiNbO

We have demonstrated the waveguide-type tunable wavelength filtering (Sole filter) characteristics in an 80-mm-long periodically poled Ti: LiNbO3 (Ti: PPLN) channel waveguide which has a domain period of 16.6 mum. At room temperature (25degC), the center wavelength and the full-width at half-maximum of the filter were about 1273.3 and 0.21 nm, respectively. By changing the temperature of the Ti: PPLN waveguide, we have tuned the center wavelength of the filter with a wavelength tuning rate of -0.683 nm/degC. Also, the characteristics of the waveguide-type Sole filter based on a Ti: PPLN waveguide were compared with those of a bulk PPLN device.

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We report an all-fiber bandpass filter (BPF) based on a null core hollow optical fiber serially concatenated between a pair of helicoidal long-period fiber gratings (HLPFGs) with flexible transmission control capability. With novel spectral functionality of HLPFGs that can be varied by the torsional stress to the helix, and core mode blocking in a hollow core fiber, the proposed device showed unique BPF characteristics whose transmission can be flexible and altered along with a low polarization-dependent loss and low thermal variation, which have not observed in prior photorefractive grating devices.

Waveguide

We have analyzed the Solc filtering characteristics in a periodically poled Ti:LiNbO3 (Ti:PPLN) multimode waveguide. The single- and dual-wavelength filtering were achieved under the optimized guiding condition for the TEM(00)-like mode and two mode (TEM(00)- and TEM(01)-like mode), respectively. The full width at half-maximum of the filter was about 0.21 nm at both guiding conditions. We found that the origin of two peaks of the dual-wavelength Solc filter in the two-mode guiding condition is the different effective refractive index between the TEM(00)- and TEM(01)-like modes. The wavelength difference of two peaks is about 0.8 nm at room temperature.

All-Fiber Bandpass Filter Based on Helicoidal Long-Period Grating Pair and Null Core Hollow Optical Fiber With Flexible Transmission Control

We have analyzed the multi-mode interference effect depending on the wavelength and the polarization states of input beam in a multi-mode Ti:LiNbO(3) waveguide at about 1300 nm region. The transmitted optical signal of a Ti:LiNbO(3) waveguide shows the periodic oscillation as a function of input wavelength. The measured average periodicity of the oscillation in TM and TE polarization beams were about 18 nm and 48 nm, respectively. Actually, the periodicity is determined by the refractive index difference between the two modes (fundamental and first modes). Therefore, we have explained the experimental results with the theoretical calculations which are derived from a quasi-analytical technique based on the effective-refractive- index method and the equation of coupling length determined by the mode phase factor in the multi-mode waveguide.

Wavelength filtering characteristics of Solc filter based on Ti:PPLN channel waveguide.

A temperature-insensitive dual-comb filter has been demonstrated for the first time by multimode interference based on a Ti:LiNbO3 channel waveguide. The phase difference between comb filters was about 180deg. We only observed less than plusmn0.125-nm variation of the center wavelength of the filter during temperature change from 20degC to 50degC. The measured extinction ratio and channel spacing of the comb filter were about -25 dB and 3.2 THz, respectively.

Characteristics of a multi-mode interference device based on Ti:LiNbO 3 channel waveguide

We report the formation of inner cladding modes in the optical fiber having an inner cladding structure. The inner cladding layer located between the core- and the cladding- layers of a conventional fiber might have, so called, inner cladding mode(s). The brief history of the inner cladding fiber and the spectral properties of the inner cladding mode are presented. By utilizing fiber gratings, the spectral properties of the inner cladding mode formed in Dispersion Compensating Fiber (DCF) are discussed. It was observed that one resonant peak of a long-period fiber grating was not sensitive to the variation on the cladding surface. With a fiber Bragg grating, a small group of unusual resonant Peaks was observed between the main Bragg Peak and the series of usual Peaks resulted from the mode coupling to counter-propagating cladding modes. Within the DCF by using fiber gratings, it is noted, at least one mode can be coupled to the inner cladding mode and a few outer cladding modes are severely affected by the inner cladding of the fiber.

Temperature-Insensitive Dual-Comb Filter Based on Multimode Interference Ti : LiNbO

A novel scheme of an optical fiber connector is proposed, which enables coupling between a passive device and two optical fibers without using any bulk optics. By using long-period fiber gratings (LPGs), appreciable coupling was obtained without the help of expensive and alignment-sensitive optics such as lenses and collimators. The incident beam that has been propagated along the core of a fiber is coupled to a cladding mode with a help of a LPG at first. The beam coupling between two fiber-ends occurs in the free-space through the claddings of both fibers. The coupled beam in the cladding of the second fiber is re-coupled to the core mode by the second LPG. The effective beam coupling is possible in this scheme due to the large diameter of the fiber cladding compared with the core. The large beam size reduces the divergence angle in the free space, which reduces the insertion loss. A passive device such as a DWDM filter can be inserted in the free space between the fiber ends. The working distance giving a 3 dB relative loss of the proposed connector was measured to be about 500 micrometers , which was 4-times longer than the case of the core-to-core coupling.