Seung Kwan Kim

Threshold features of a Brillouin Stokes comb generated in a distributed fiber Raman amplifier.

We study threshold features of a Brillouin-shifted Stokes comb generated in a distributed fiber Raman amplifier. When the input power of Brillouin pump is linearly increased in high Raman gain, the first Stokes wave grows exponentially at much lower threshold power and then experiences an appreciable power decrease in the vicinity of the Brillouin combs threshold. This power reduction of the first Brillouin Stokes, which we did not see mentioned in previous reports, was caused by a power transfer to higher-order lines and initiated Brillouin comb generation. Moreover, the effects of Raman pump power and pumping direction on the threshold of a Brillouin comb are investigated.

Continuous-wave 532 nm pumped MgO:PPLN optical parametric oscillator with external power regulation and spatial mode filtering

We report a continuous-wave (CW) optical parametric oscillator (OPO) based on a MgO-doped periodically poled LiNbO(3) (MgO:PPLN) crystal. The 532 nm pump generates coherent radiation that is tunable from 800 to 920 nm for the signal and from 1250 to 1580 nm for the idler, respectively. The OPO output power exhibits a slowly varying instability that we attribute to a thermal effect induced by the pump. This instability is truncated by means of a low-pass servo that includes a single-mode fiber that filters the beam into a single spatial mode. The resulting output characteristics are promising for radiometric applications in the near infrared including most fiber-optic communication bands.

Spatial uniformity inspection apparatus for solar cells using a projection display

We demonstrate a measurement apparatus to inspect spatial uniformity of quantum efficiency of solar cells using a beam projector. Deviation of irradiance from the used beam projector over the area of 1.5×0.8 m on the cell plane was flattened within ±2.6% through gray scale adjustment, which was originally about 200%. Scanning a small square image with an area of 3×3 mm over a square-shaped photovoltaic cell with an area of 15.6×15.6 cm, we could identify the locations according to efficiency level and showed that the cell had quantum efficiency deviation of more than 10%. Utilizing the advantageous feature of a projection display, we also demonstrated that this apparatus can inspect the spatial uniformity of solar modules and panels consisting of multiple solar cells.

Analysis on Transition between Index- and Bandgap-guided Modes in Photonic Crystal Fiber

We calculate optical properties of guided modes of a hybrid-guiding photonic crystal fiber. The design and modeling of such hybrid-guiding PCF is made by replacing air holes with inserts of high refractive index material layer by layer in order. The optical properties such as mode intensity profile, mode dispersion, optical birefringence, confinement loss, and chromatic dispersion during transition of the guiding mechanism are analyzed and discussed. The guided modes in the hybrid-guiding region are also compared with those of reference index-guiding and bandgap-guiding photonic crystal fibers.

Tunable single longitudinal mode Tm-doped fiber ring laser

We present a tunable single longitudinal mode Tm-doped fiber ring laser incorporating a self-constructed saturable absorption grating and a blazed grating with wavelength tunability of over 100 nm. The spectral and temporal characteristics of the laser output are described.

Coherence effect on the measurement of optical fiber nonlinear coefficient

We investigated a source coherence effect on the measurement of the optical fiber nonlinear coefficient by use of dual optical frequencies. We produced coherent and incoherent dual optical frequencies with a self-heterodyne Mach–Zehnder interferometer with and without a delay line, respectively. Our measurement results included both an electrostrictive and a Kerr nonlinear coefficient because of the low modulation frequency. Comparing the coherent case with an incoherent case, we obtained a nonlinear coefficient that was more than 3% higher in the coherent case.

Study of methodology for gain-switched holmium-doped fiber laser combined with Q-switched Thulium-doped fiber laser

Pulsed fiber laser operating at 2000 nm is of great interest in applications such as LIDAR system using very low atmospheric absorption bands, material processing for transparent plastics, etc. Thulium- and holmium-doped silicate glass fibers have been the best choice as a gain medium for the fiber lasers. In particular, a holmium-doped fiber laser (HDFL) has drawn much attention because it opened up the possibility to extend the operating wavelength beyond 2100 nm. The pump source of the HDFL has been typically a thulium-doped fiber laser (TDFL) due to the availability of high power at 1950 nm where the HDF has the maximum absorption. A gain-switched HDFL is the pulsed fiber laser built on the same architecture. The pump source, in this case, has been either a Q-switched TDFL pump by a continuous wave (CW) laser at 793 nm or another gain-switched TDFL pumped by a pulsed laser at 1550 nm. Such cascaded pumping scheme leads the laser configuration to be complicated having redundant optical components [1, 2]. We proposed a couple of methodologies for integrating the gain-switched HDFL and the Q-switched TDFL on a single laser cavity. The main idea here is to let the HDF, the saturable absorber of the Q-switched TDFL, be the gain at the same time in the cavity, thereby being a gain-switched HDFL. One of proposed methods to realize the idea appears in Fig. 1(a). The laser consists of a ring cavity with a linear section. The TDF and HDF are in the ring cavity, and a fiber Bragg grating (FBG, reflection at 1950 nm), a variable optical attenuator and a fiber mirror are in the linear section. In the Q-switched TDFL configuration, the HDF functions as a saturable absorber. The FBG determines the lasing wavelength of 1950 nm at the highest level of attenuation (no reflection from the fiber mirror), which is for case (i) in Fig. 1(b). Very stable output pulses are observed in this case. The lasing wavelength jumps up to the wavelength of 2008 nm at the middle level of attenuation, which corresponds to case (ii) in Fig. 2(b). This is because the effective thulium gain at 2008 nm gets higher than that at 1950 nm. In the meantime, output pulses become unstable due to the low modulation depth of the HDF at the wavelength. At the lowest level of attenuation, the laser operates at two different wavelengths as shown in case (iii) in Fig. 2(b): one is at 1987 nm and the other is at 2013 nm. The attenuator can control the distance of separation, which can be understood by considering the spectral emission and absorption profiles of TDF and HDF. The shorter wavelength (1987 nm) is expected to be from the Q-switched TDFL and the longer wavelength (2013 nm) the gain-switched HDFL. In this presentation, we will introduce two methods for integrating gain-switched holmium-doped fiber laser and Q-switched thulium-doped fiber laser and describe their experimental results.

Characterization of electromagnetically induced transparency in Rydberg state of 87 Rb

We present the direct measurement of highly excited Rydberg state using a ladder-type electromagnetically induced transparency in the 5S_1/2-5P_3/2-nL transition of ⁸⁷Rb atoms in a room-temperature vapor cell.

Mode analysis of tunable single longitudinal mode Tm-doped fiber ring laser

Longitudinal modes of a tunable Tm-doped fiber ring laser with/without an unpumped Tm-doped fiber as a saturable absorption grating are analyzed using an all-fiber scanning ring resonator. Temporal characteristics of the laser are also discussed.

Optical injection locking of a singly resonantcontinuous-wave optical parametric oscillator

We report on experimental realization of optical injection locking of a singly resonant continuous-wave optical parametric oscillator (OPO) based on a thermally induced waveguide in a magnesium-oxide doped periodically poled lithium niobate. An external cavity diode laser is used to control the frequency of the resonant signal output of the OPO at 795 nm. The key to successful injection locking was the improvement of the OPO spatial modes by a special operating condition with a thermally induced waveguide. The phase coherence of injection locking is confirmed by recording a high-contrast interference fringe between the injection laser and the OPO output. High-resolution measurement of the frequency spectrum of the nonresonant idler output by the delayed self-heterodyne technique shows that the spectral linewidth of the OPO is reduced from 4.5 to 0.4 MHz by injection locking. The full locking range is assumed to be less than 1.7 MHz.