Hojoon Lee

Nonlinear switching of optical pulses in fiber Bragg gratings

We study numerically the nonlinear switching characteristics of optical pulses transmitted though fiber Bragg gratings. We consider both the uniform and phase-shifted gratings and compare their performance as a nonlinear switch. The nonlinear coupled-mode equations were solved numerically to obtain the pulse-switching characteristics. The steady-state behavior known to occur for continuous-wave optical beams is realized only for pulses wider than 10 ns with long tails. For pulsewidths in the range 0.1-1 ns, the use of phase-shifted gratings reduces the switching threshold, but the on-off contrast is generally better for uniform gratings. We also quantify the effects of rise and fall times associated with an optical pulse on nonlinear switching by considering the Gaussian pulses with smooth tails and nearly rectangular pulses with sharp leading and trailing edges.

Purely phase-sampled fiber Bragg gratings for broad-band dispersion and dispersion slope compensation

We demonstrated numerically that both the chromatic dispersion and the dispersion slope can be compensated by using purely phase-sampled superstructure fiber Bragg gratings provided both the grating period and the sampling period are chirped linearly along the grating. Adjusting the refractive index modulation and the chirp of sampling function, they can be designed to compensate dispersion of a large number of wavelength-division-multiplexing channels.

Suppression of stimulated Brillouin scattering in optical fibers using fiber Bragg gratings

We propose a novel scheme for suppressing stimulated Brillouin scattering in optical fibers. The scheme makes use of a single or a sampled Bragg grating fabricated within the fiber used for transmitting intense Q-switched pulses. The grating is designed such that the spectrum of the Stokes pulse generated through stimulated Brillouin scattering falls entirely within its stop band. We show numerically that 15-ns pulses with 2-kW peak power can be transmitted though a 1-m-long fiber with little energy loss using this scheme. A sampled grating can be used for longer fibers but its coupling coefficient should be higher. The proposed scheme should prove useful for double-clad fiber lasers and amplifiers.

Bandwidth equalization of purely phase-sampled fiber Bragg gratings for broadband dispersion and dispersion slope compensation

We propose a purely phase-sampled Bragg grating for dispersion and dispersion slope compensation by introducing a chirp in the grating period and coupling coefficient. The bandwidth of all reflected channels can be equalized by chirping the sampling period at the same time. We show that a trade-off exists between the linearity of dispersion slope and the equalization of reflection channel bandwidths and discuss how the values of the coupling coefficients can be optimized in practice to improve the device performance.

Multiple fiber Bragg grating sensor system using code-division multiple access.

We propose and demonstrate experimentally a multiple fiber Bragg grating sensor system that uses a LED as a light source and adapts the code-division multiple access method to separate individual sensor signals. We measured static and dynamic strains and cross-talk levels among sensor signals. With our proposed scheme, a low-cost sensor system can be designed with a fast response speed. Cross-talk levels between sensor signals are found to be below the -30-dB level.

The resonance wavelength-tuning characteristic of the arc-induced LPFGs by diameter modulation

In this paper, we fabricated the arc-induced long period fiber grating (LPFG) by core and cladding diameter modulation. The torsion experimental results demonstrate wavelength tunability by torsion directions. The resonance wavelength of the LPFG shifts toward the longer wavelength by about 8 nm with counterclockwise torsion and toward the shorter wavelength by about 10 nm with clockwise torsion. The LPFGs obtained by this fabrication method can be used as optical devices for tunable filtering or sensor applications.

Optical true time-delay beam-forming for phased array antenna using a dispersion compensating fiber and a multi-wavelength laser

We have proposed optical true-time delay beam-forming system using a DCF and a multi-wavelength laser for phase array antenna. Total time delay for each wavelength band originated from group delay of a DCF and the fixed delay line. As a result, a DCF for 80 km NZ-DSF could be used to obtain the radiation angle of 180° by tuning the multi-wavelengths of laser in the proposed optical true-time delay.

Photonic True-Time Delay for Phased-Array Antenna System using Dispersion Compensating Module and a Multiwavelength Fiber Laser

An optical true-time delay beam-forming system using a tunable dispersion compensating module (DCM) for dense-wavelength division modulation (DWDM) and a multiwavelength fiber ring laser for a phased array antenna is proposed. The multiwavelength fiber ring laser has one output that includes four wavelengths; and four outputs that include only single-wavelength. The advantage of such a multiwavelength fiber ring laser is that it minimizes the number of devices in the phased array antenna system. The time delays according to wavelengths, which are assigned for each antenna element, are obtained from the tunable DCM. The tunable DCM based on a temperature adjustable Fabry-Perot etalon is used. As an experimental result, a DCM could be used to obtain the change of the beam angle by adjusting the dispersion value of the DCM at the fixed lasing wavelengths of the fiber ring laser in the proposed optical true-time delay.

Multiwavelength Erbium-doped Fiber Ring Laser with Adjustable Line Power using Variable Attenuators and DWDM

The homogeneous line broadening in EDF leads to unstable lasing and strong mode competition. In this paper a multiwavelength fiber ring laser that overcomes these effects by using variable attenuators between the DWDM and 1×N coupler, has been proposed. Fiber ring lasers with four and eight wavelength outputs are implemented, and we show that the output powers can be adjusted with stable operation. The EDF ring laser has one four-wavelength output and four of single wavelengths, with OSNR of 50 and 60 dB respectively.

An economical and multiple fibre grating sensor system with a rapid response using code division multiple access

The performance of the ordinary fibre Bragg grating sensor strain measurement system, which uses a Fabry-Perot filter for scanning wavelength, has limitations in application due to the hysteresis characteristics of the PZT element in the filter, the slow scanning rate of the filter and the high cost of the system. The system proposed here uses a LED as a light source and adapts the method of code division multiple access (CDMA). Without a tunable laser source or Fabry-Perot filter, the price of the system is very low and the speed of response of the system is very fast. CDMA is based on correlation techniques implemented in order to separate out individual reflected sensor signals from a multiplexed signal.