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We evaluate possibility of utilizing the mode-field matched center-launching (MCL) technique in the 850-nm band. For this purpose, we realize the MCL technique by fusion-splicing the HI-780 pigtail fiber of the transmitter to the OM2-type transmission multi-mode fiber (MMF). The results show that we can achieve the high coupling efficiency of >92% by optimizing the splicing conditions such as the arc-discharge duration and fiber-push distance. We confirm that, in the case of utilizing the MCL technique, the frequency-response characteristics of the MMF link becomes almost identical to that of the single-mode fiber (SMF) link. As a result, we could demonstrate the successful transmission of 112-Gb/s four-level pulse amplitude modulation (PAM4) signal operating at 852 nm over 1 km of the OM2-type MMF by utilizing the MCL technique. The performance of this link is measured to be very stable even in the presence of the mechanical perturbations such as fiber bending and fiber shaking.

We evaluate the effects of the external optical feedback on the on-off keying (OOK) and four-level pulse amplitude modulation (PAM4) signals in the single-mode fiber (SMF) link implemented by using a commercial 10G-class 1.55- $\mu \text{m}$ vertical-cavity surface-emitting laser (VCSEL). The results show that the PAM4 signal is much more sensitive to the external optical feedback than the OOK signal. For example, in the back-to-back condition, an error floor is observed at the bit-error rate (BER) of $\sim 8\times 10 ^{\mathbf {-3}}$ for the 10-Gbaud PAM4 signal when we set the optical feedback ratio to be −14 dB. In comparison, the 10-Gbaud OOK signal has a power penalty of only 0.4 dB (@BER $= 5\times 10 ^{-5}$ ) under the same conditions. The results also show that the PAM4 signal becomes more sensitive to the optical feedback after the SMF transmission due to the shallowed BER curve by the fiber dispersion. In addition, we confirm that the effects of the optical feedback are independent of the baud rate. From these results, we conclude that the error-free transmission of the high-speed PAM4 signal cannot be achieved if there is even one bad fiber connector having >2.5% reflection in the VCSEL-based SMF link.

We propose to implement a 2-channel optical-time-division-multiplexed (OTDM) system for short-reach optical interconnects by using a sinusoidally modulated light source instead of a complicated mode-locked laser as an input pulse source. In this system, the OTDM signal is obtained by bit-interleaving two optical return-to-zero (RZ) signals generated by using the sinusoidally modulated light. We operate these RZ signals in the orthogonal in-phase and quadrature domains to avoid the unwanted beat components. After the transmission, the OTDM signal is detected by using single photodetector, and then processed by a 2 × 2 multiple-input multiple-output equalizer. For a demonstration, we generate 150-Gb/s OTDM signal operating in the 8-level pulse-amplitude modulated (PAM-8) format by using commercial LiNbO3 Mach-Zehnder modulators and transmit this OTDM signal over 1.9 km of the standard single-mode fiber (SSMF). In addition, we fabricate the proposed OTDM transmitter in an integrated silicon-photonics chip and use it to demonstrate the transmission of the 64-Gb/s OTDM PAM-4 signal over 2.2 km of SSMF.

We propose to utilize the filtered Gaussian phase dither technique for the effective suppression of the multipath- interference (MPI) noises caused by multiple bad fiber connectors in the mobile fronthaul network (MFN) based on the radio-over- fiber (RoF) technology. We first evaluate the viable performances of the RoF-based MFN in the 5G wireless communication systems by using this technique and deduce the required bandwidth and phase modulation depth of the filtered Gaussian dither signal for the suppression of the MPI noises by numerical simulations. The results show that, by using the proposed technique, it is possible to suppress the MPI noises by ∼10 dB regardless of the number of bad fiber connectors. These simulation results are experimentally verified in the RoF link capable of transporting the 5G wireless signals (i.e., thirty-two 100-MHz orthogonal-frequency-division- multiplexed (OFDM) signals modulated in 64QAM or 256QAM format). We also evaluate the effectiveness of the proposed dither technique in a high-capacity RoF link implemented by using four wavelength-division-multiplexed channels and demonstrate the successful transmission of 128 100-MHz OFDM signals modulated in 256QAM format in the presence of multiple bad connectors.

We proposed to implement a 2-channel OTDM system by using a sinusoidally modulated light source instead of a mode-locked laser. For a demonstration, we transmit 150-Gb/s OTDM-PAM8 signal over 1.9-km long SSMF by using the proposed system.

We demonstrate the transmission of the 84-Gb/s DMT signal over 5 km of SSMF by using an injection-locked 10G-class 1.55-µm VCSEL having a 3-dB bandwidth of only 7.4 GHz.

We demonstrate the transmission of 2×28-Gb/s MGDM OOK signals operating at 852.6 nm over 2.2-km long SSMF. Only two photodetectors are used for the detection of these two MGDM signals with the help of a multiple-input single-output equalizer.

We report on the practical multiplexing techniques applicable to the next-generation ultrahigh-speed data center networks. In particular, we evaluate the feasibility of doubling the transmission capacity of the short-haul direct-detection system cost-effectively by utilizing the optical-time-division-multiplexing (OTDM) and mode-group-division-multiplexing (MGDM) techniques.

We propose to implement a simple 2-channel optical-time-division multiplexed (OTDM) system based on a phase-alternating pulse generator for a short-reach optical interconnects. The inserted pulse generator consisting of a phase modulator and an optical periodic filter can generate the optical pulse of which the repetition rate is double the frequency of the input electrical clock. The phase-alternating property of the pulse makes the OTDM signal robust to the chromatic dispersion. In the experiment, we optimize the optical pulse, use it for generating the 200-Gb/s OTDM signal operating in the 4-level pulse-amplitude modulated format, and transmit the signal over the 1.9-km standard single-mode fiber.

We experimentally demonstrate the transmission of 107-Gb/s PAM-4 signal over 25-km long standard single-mode fiber using a commercially available 1.31-μ m directly modulated laser. The limited bandwidth of laser is compensated bv using an electrical equalizer.

We demonstrate the transmission of 230-Gb/s DMT signal over 1.9 km of SSMF in the 1.5-μm IM/DD system by using a pre-chirp Mach-Zehnder modulator having a 3-dB bandwidth of only 22 GHz.

We propose a simple two-channel mode-group-division-multiplexing (MGDM) system operating in the 0.8 µm region over the standard single-mode fiber (SSMF). For the cost-effectiveness, we implement its receiver by using only two photodetectors (PDs) [instead of three PDs required for the detection of the L P 01 , L P 11 a , and L P 11 b modes]. We then detect the signal carried by the L P 01 mode by using a PD and a mode filter. On the other hand, the other signal carried by the L P 11 mode group is detected by using another PD and a multiple-input single-output equalizer (i.e., by subtracting the signal carried by the L P 01 mode from the multiplexed signal). For a demonstration, we transmit 2 × 28 G b / s MGDM on-off keying signal operating at 852.6 nm over 2.2 km of the SSMF by using the proposed technique. The results show that we can achieve the bit-error rate of < 3.8 × 10 − 3 for both the L P 01 and L P 11 modes.

We design a heterogeneous 12-core 4-LP-mode fiber having two different types of cores and trench-assisted graded-index profiles. For the optimum design of the index profiles of these two heterogeneous cores, we first investigate the impacts of various geometric parameters on the effective refractive index of each LP mode, maximum differential modal delay (DMD), and bending losses of the guiding and higher order modes. We then design a heterogeneous 12-core 4-LP mode fiber having a cladding diameter of 242 μm. The numerical analyses based on the coupled-mode equation show that the inter-core crosstalk levels of the proposed fiber are lower than -55 dB/km, regardless of modes, as long as the fiber is not bent tightly with a bending radius of -3 μm-2 and 42.2, respectively.

We explore the possibility of utilizing 1.55- $\mu \text{m}$ directly modulated lasers (DMLs) for optical access networks operating at 28 Gb/s per wavelength. For this purpose, we attempt to maximize the power budgets of 28-Gb/s duobinary and four-level pulse amplitude modulation (PAM-4) signals generated from a DML without using optical amplifiers or optical dispersion compensation modules. For experimental investigation, we set the bias current of the DML far above the threshold current, optimize the extinction ratio of the signals, and utilize the electrical equalization at the receiver. The experimental results show that we can secure power margins of 9.6 and 10 dB after transmission over 40-km long standard single-mode fiber for the duobinary and PAM-4 signals, respectively.

We develop a space-efficient single-mode fiber (SMF) having a cladding diameter of only 82 μm. This SMF has the depressed-cladding index profile and its mode-field diameter, cutoff wavelength, and macro bending loss are designed to be similar to those of the conventional step-index SMF. We fabricate this reduced-cladding SMF and measure its optical and mechanical characteristics. The results show that this fiber satisfies major specifications of the ITU-T G.654 recommendations. We also fabricate a fiber ribbon by using twelve of these reduced-cladding SMFs. Compared to a commercial fiber ribbon made of twelve standard SMFs having 125-μm cladding diameter, this fiber ribbon can improve the spatial efficiency by ∼75%.

We demonstrate a 4-channel mode-division-multiplexed (MDM) system by compensating the mode crosstalk using the digital optical phase conjugation technique.

We propose and demonstrate a common public radio interface (CPRI)-compliant 25.2923-Gb/s mobile fronthaul network for long term evolution-advanced (LTE-A) systems. In order to meet the CPRI specifications and the stringent latency requirement of the LTE-A fronthaul network cost-effectively, we use the synchronous time-division-multiplexing, forward-error correction, and electrical equalization techniques. To enhance the cost-effectiveness, we also use an electroabsorption-modulated laser and exploit the possibility of using simple modulation formats such as the ON–OFF keying, electrical duobinary, and four-level pulse amplitude modulation. The performance of the proposed network is evaluated experimentally in terms of the tolerances to fiber dispersion and optical reflection.

We propose a DSP-based nonlinearity compensation technique for RoF links which does not require any prior knowledge about the links. By using this technique, we successfully transport twelve 198-MHz-bandwidth 64-QAM f-OFDM signals over 20 km of SSMF.

We propose and demonstrate the compensation of mode coupling in MDM systems by using the digital optical phase conjugation technique. We achieve BERs better than 10-4 without electrical MIMO processing in a FMF link having large mode coupling.

We demonstrate the transmission of twelve 360-MHz filtered-OFDM signals in 64-QAM format over 20 km of SSMF by using a 1.55-µm directly modulated laser and a DSP-based composite second-order cancellation technique.

We examine the feasibility of utilizing RoF-based fronthaul network for the next-generation mobile communication systems. For this purpose, we evaluate the effects of the dispersion-induced RF power fading, CSO distortions induced by the interplay between the laser chirp and fiber dispersion, and MPI noises induced by optical reflections, etc.

We propose a novel carrier phase estimation method for the high-speed coherent WDM PON based on the reflective semiconductor optical amplifier (RSOA). By using this method, we can utilize a cost-effective DFB laser as the seed light in such a network due to its fast trackability. We experimentally achieve an excellent receiver sensitivity of −30.8 dBm even when a DFB laser (linewidth: 3 MHz) is used as a seed light source in the 25.78-Gb/s upstream QPSK transmission over 20 km of SSMF.

Objective Recently, many studies have reported that cervical alignment is related to clinical outcomes. However, poor visibility of anatomical structures during X-ray (XR) imaging limits accurate measurements. In supine magnetic resonance (MR) imaging, the boundary of the anatomical structure is clear, but the correlation to XR images taken in a standing position is problematic. In this study, we evaluated the agreement of sagittal alignment parameters between MR and XR measurements. Methods We retrospectively reviewed 268 patients. Cervical sagittal parameters were measured using XR and MR images, and their relationships were evaluated using Pearson's correlation, paired t-tests, and 2-way random, single score intraclass correlation coefficient (ICCs) (2,1). Using simple linear regression analysis, MR results were converted to the expected value (MR-E). The subsequent comparison of MR-Es with XRs was used to examine whether MR-Es could replace XRs when the measurement difference was less than 2 mm or 2°. Results The correlation between the MR and XR measurements was high, but ICCs showed low reliability. All parameters were significantly different between XR and MR measurements in paired t-tests. Converting the MR values eliminated the t-test differences between MR-Es and XRs, but did not affect correlations and ICCs. The replacement ratio included the Cobb angle: 20.3%, T1: 27.1%, the sagittal vertical axis: 17.6%, C1–2: 29.7%, and C2: 16.0%. Conclusion These results indicate that supine MR measurements could not replace upright XR measurements.

We demonstrate the transmission of 56-Gb/s PAM-4 signal over 20-km long SSMF by using a 1.55-μm DML without optical dispersion compensation. Instead, a linear electric equalizer is used for the compensation of dispersion-induced waveform distortions.

We propose and demonstrate a simple composite second-order (CSO) cancellation technique based on the digital signal processing (DSP) for the radio-over-fiber (RoF) transmission system implemented by using directly modulated lasers (DMLs). When the RoF transmission system is implemented by using DMLs, its performance could be limited by the CSO distortions caused by the interplay between the DML's chirp and fiber's chromatic dispersion. We present the theoretical analysis of these nonlinear distortions and show that they can be suppressed at the receiver by using a simple DSP. To verify the effectiveness of the proposed technique, we demonstrate the transmission of twenty-four 100-MHz filtered orthogonal frequency-division multiplexing (f-OFDM) signals in 64 quadrature amplitude modulation (QAM) format over 20 km of the standard single-mode fiber (SSMF). The results show that, by using the proposed technique, we can suppress the CSO distortion components by > 10 dB and achieve the error-vector magnitude performance better than 6% even after the 20-km long SSMF transmission. (C) 2017 Optical Society of America

There have been substantial efforts to implement high-speed (> 10 Gb/s) upstream transmission using reflective semiconductor optical amplifiers (RSOAs) in a coherent wavelength-division-multiplexed (WDM) passive optical network (PON). In such a network, it is necessary to estimate the carrier phase of upstream optical signal to retrieve the phasemodulated information created by RSOA. However, due to the severe waveform distortions caused by the limited modulation bandwidth of RSOA (typically less than 3 GHz), previously reported carrier phase estimation (CPE) algorithms cannot accurately estimate the carrier phase of high-speed quadrature phase-shift keying (QPSK) signal generated from the RSOA seeded by a distributed-feedback (DFB) laser. We propose a novel CPE method capable of tracking the carrier phase rapidly by using a small number of symbols (e.g., 15 symbols) even when the waveforms are severely distorted by the limited modulation bandwidth of RSOA. The proposed CPE method utilizes the linear relationship between the intensity modulation and phase modulation indices inherent in the semiconductor opto-electronic device. By using the proposed method, we demonstrate the transmission of 25.78-Gb/s QPSK signal in a 20km long loopback fiber link. In this experiment, a commercial DFB laser (linewidth: 3 MHz) is used as the seed light instead of an expensive narrow-linewidth laser. Also shown through the experiment is that the proposed CPE method is highly unsusceptible to variations of parameters required in the proposed method, such as the number of test phases, the accuracy of linewidth enhancement factor, and the accuracy of the normalized amplitude of DC component. (C) 2017 Optical Society of America

We report on technical challenges associated with the use of directly modulated lasers in the mobile fronthaul networks based on radio-over-fiber (RoF) technology. In particular, the effects of dispersion-induced RF power fading and multi-path interference are investigated through theoretical and experimental analyses for the RoF-based transmission system implemented by using directly modulated lasers.

We demonstrate the transmission of 51.56-Gb/s OOK signals over 15 km of SSMF using a 1.55-μm DML. A receiver-side duobinary filter is used together with FFE for the compensation of the bandwidth limitation and fiber dispersion.

We demonstrate the transmission of 51.56-Gb/s on-off keying (OOK) signals generated by using a 1.55-mu m directly modulated laser (DML) over 15-km long standard single-mode fiber. In this experiment, a duobinary electrical equalizer based on a finite-impulse-response filter is used at the receiver to increase the dispersion-limited transmission distance. We evaluate the performances of the 51.56-Gb/s OOK signals with respect to the transmission distance by using the frequency response analysis of the proposed system. This result is used to explain why it is effective to utilize the duobinary equalization (instead of binary equalization) for increasing the transmission distance. (C) 2016 Optical Society of America

We design and fabricate a space-efficient single-mode fiber (SMF) having a diameter of only 82 µm. The results show that this fiber satisfies the optical and mechanical specifications of the ITU-T G.654 recommendation. In comparison to the conventional SMF having a diameter of 125 µm, this fiber can be used to double the spatial efficiency in the space-sensitive system.

We analyze the inter-core crosstalk in homogeneous multi-core two-mode fibers (MC-TMFs) under bent condition by using the coupled-mode equations. In particular, we investigate the effects of the intra-core mode coupling on the inter-core crosstalk for two different types of MC-TMFs at various bending radii. The results show that the inter-core homo-mode crosstalk of LP(11) mode is dominant under the gentle fiber bending condition due to its large effective area. However, as the fiber bending becomes tight, the intra-core mode coupling is significantly enhanced and consequently makes all the inter-core crosstalk levels comparable to each other regardless of the mode. A similar tendency is observed at a reduced bending radius when the difference in the propagation constants between modes is large and core pitch is small.

In patients with tumors and spinal cord lesions, inflammation and tissue infection can result in mass effect detection on imaging. As a result, surgical biopsy procedures are often performed on the lesions. We report a rare case in which the thickening ligamentum flavum (LF) appeared to be a tumor in the epidural space of the cervical spine based on imaging findings. A 52-year-old man visited our outpatient clinic with severe shoulder pain and radicular pain in his right arm that had developed gradually after a traffic accident two months earlier. Magnetic resonance imaging of the cervical spine revealed an extradural mass at the cervicothoracic junction level. Suspecting a tumor, spinal decompression surgery was performed and a biopsy of the mass was obtained. At the time of surgery, the LF was thick and compressed the spinal cord. After successful removal of the LF, the spinal cord appeared normal. Histopathological examination confirmed the mass as the LF. The patient was discharged without pain or weakness two weeks postoperatively. This case demonstrated that when the LF of the cervicothoracic junction is thickened, it may be misdiagnosed as a cervical spine tumor compressing the spinal cord.

We analyze the inter-core crosstalk in a homogeneous multi-core two-mode fiber under bent condition by using coupled-mode equations. The effects of the intra-core mode coupling on the inter-core crosstalk are investigated for various bending radii.

함치성 낭종(dentigerous cyst)은 미맹출치의 치관(crown) 에서 발생하는 병변으로, 치관과 이를 둘러싸는 상피부 사이 에 체액이 축적된 결과로 치아 여포가 확장되어 발생하는 가 장 흔한 발육성 치성 낭종이다. 치성 낭종 중에 치근낭(radicular cyst) 다음으로 두 번째로 흔하며, 상악의 송곳니와 하악의 세 번째 대구치에서 흔히 발생한다. 미맹출치를 둘러싼 함치성 낭종은 흔히 미맹출치를 정상 위치에서 비정상적인 위치로 전 위시키는데, 특히 상악골에서 발생한 경우에는 미맹출치가 상 악동으로 전위되는 경우가 흔하다. 연관된 미맹출치와 함께 낭종을 완전히 제거하는 것이 표준 치료법으로, 상악동에서 발생한 함치성 낭종은 Caldwell-Luc 접근법을 통한 제거법이 현재까지 널리 사용되고 있다. 최근 에는 비강 내 접근법을 통한 내시경적 제거술로 치료한 증례 들이 외국 문헌에 소수 보고되었고, 국내에서도 중비도 상 악동 개방술(middle meatal antrostomy)을 통해 내시경적으 로 제거한 증례가 1예 보고되었다. 저자들은 비강 내 내시경을 이용한 하비도 상악동 개방술로 완전 절제한 상악동 내 함치성 낭종 증례를 경험하였고, 이에 대 한 장기 결과 보고와 더불어 내시경적 비강 내 제거술에 유용한 다양한 비강 내 접근법을 문헌고찰과 함께 보고하고자 한다.

Received October 28, 2010 Revised January 11, 2011 Accepted January 8, 2011 Address for correspondence Young-Jun Chung, MD Department of OtorhinolaryngologyHead & Neck Surgery, Dankook University College of Medicine, 359 Manghyang-ro, Dongnam-gu, Cheonan 330-715, Korea Tel +82-041-550-3974 Fax +82-041-556-1090 E-mail docjung@paran.com Background and ObjectivesZZThere have been some studies performed to evaluate subjective and objective changes after septoplasty and of their correlation, but have reported contradicting results. In this light, this study attempted to evaluate subjective and objective postoperative changes and investigate whether there is any correlation between them. Subjects and MethodZZThis study was performed prospectively in 48 patients who underwent septoplasty and outfracturing of both turbinates. The parameters of subjective nasal patency and acoustic rhinometry (AR) were measured before and after surgery. We evaluated these changes and determined if there exists a correlation between the changes of AR parameters and improvement of subjective nasal patency. ResultsZZMinimal cross-sectional area (MCA) and nasal volume in the convex side of nasal cavity were significantly improved after the surgery (p