Jae-Sik Sim

Monolithic integration of a multiwavelength laser array associated with asymmetric sampled grating lasers

We present a novel approach for a monolithically integrated multiwavelength laser array based on asymmetric sampled grating lasers. The asymmetric sampled grating laser combines sampled gratings of different periods with an index shifter to utilize the first-order reflection for lasing operation. With this structure, a simple fabrication procedure as well as high yield could be achieved without using complex and time-consuming e-beam lithography for multiperiod gratings. With numerical analysis based on a transfer matrix method, the effect of grating strength and mirror coating was analyzed to improve single-mode and power extraction performance. By using high-reflection/antireflection coatings on both facets, it was shown theoretically that high-power extraction efficiency as well as high single-mode yield was achieved. A four-channel laser array with 400-GHz wavelength spacing was fabricated and its operation at designed wavelengths was demonstrated. The individual laser showed a threshold current of 9-13 mA and a slope efficiency of around 0.21 W/A. A high sidemode suppression ratio over 44 dB was observed as well.

A 60-GHz-Band Analog Optical System-on-Package Transmitter for Fiber-Radio Communications

We developed an analog optical system-on-package (SoP) transmitter for a 60-GHz-band radio-over-fiber (RoF) link. The SoP transmitter consisted of an electroabsorption modulator, radio frequency amplifiers, and a bandpass filter. The 60-GHz RoF wireless link was prepared to measure the performance of the SoP transmitter. The transmission characteristics of 64-quadrature amplitude modulation (64-QAM) data of the 60-GHz RoF wireless link, including the SoP transmitter, were investigated by measuring the error vector magnitude (EVM) and signal-to-noise ratio (SNR) with a baseband frequency. The EVM of the 60-GHz RoF wireless link was between 2.25% and 2.80%, and the SNR was between 27.36 and 29.31 dB from 140 and 770 MHz, at input baseband power of -9 dBm. The noise figure had the minimum of 8.44 dB at 500 MHz. We successfully transmitted digital community antenna television (CATV) system signals through the 60-GHz RoF wireless link, including the SoP transmitter. Digital CATV signals of 86 channels could be transmitted through the 60-GHz RoF wireless link, and the total throughput was found to be 2.61 Gb/s.

1.55-μm spot-size converter integrated laser diode with conventional buried-heterostructure laser process

A 1.55-/spl mu/m spot-size converter integrated laser diode is demonstrated with conventional buried-heterostructure laser process. For the spot-size converter, we employed a double-waveguide structure in which a ridge-based passive waveguide was incorporated. The passive waveguide was optically combined with a laterally tapered active waveguide to control mode size. The threshold current was measured to be 5 mA together with high slope efficiency of 0.45 W/A. The beam divergence angles in the horizontal and vertical directions were as small as 9.0/spl deg/ and 7.8/spl deg/, respectively.

Asymmetric sampled grating laser array for a multiwavelength WDM source

We propose a multiwavelength laser array associated with asymmetric sampled grating lasers. The laser was designed to operate at the first-order reflection of a sampled grating with the aid of an index shifter. A four-channel laser array with 400-GHz wavelength spacing was fabricated and its operation at designed wavelengths was confirmed. Individual lasers showed a threshold current of 11-17 mA and slope efficiency of around 0.18 W/A. A high side-mode suppression ratio over 39 dB was observed as well.

40 Gbps electroabsorption modulated DFB laser with tilted facet formed by dry etching

We fabricated electroabsorption modulated distributed feedback (DFB) laser diodes for 40 Gbps application through selective-area growth method. The facet reflection was not sufficiently removed by just depositing anti-reflection coating on the as-cleaved facet and low frequency resonance occurred by optical feedback from facet to DFB laser. Tilted facet formed by dry etching processes successfully reduced the optical feedback and the resonance in E/O response decreased to as small as 2.8 dB.

Ring-resonator-integrated tunable external cavity laser employing EAM and SOA.

We propose and demonstrate a tunable external cavity laser (ECL) composed of a polymer Bragg reflector (PBR) and integrated gain chip with gain, a ring resonator, an electro-absorption modulator (EAM), and a semiconductor optical amplifier (SOA). The cavity of the laser is composed of the PBR, gain, and ring resonator. The ring resonator reflects the predetermined wavelengths into the gain region and transmits the output signal into integrated devices such as the EAM and SOA. The output wavelength of the tunable laser is discretely tuned in steps of about 0.8 nm through the thermal-optic effect of the PBR and predetermined mode spacing of the ring resonator.

10 Gbps transmission of electroabsorption modulators integrated with a 4-channel distributed Bragg reflector laser array

10 Gbps data transmission was demonstrated with electroabsorption modulators integrated with a 4-channel wavelength division multiplexing transmitter composed of distributed Bragg reflector (DBR) lasers. A selective area growth technique was employed for the control of bandgap energy in the gain, modulator and passive sections. Then the laser and modulator stripes were fabricated by a buried heterostructure process. The DBR lasers showed uniform threshold currents between 5 and 10 mA, while the slope efficiency was around 0.11 W A−1. By controlling the DBR currents, a 4-channel transmitter was achieved for the channel spacing of 100 GHz (0.8 nm). The electroabsorption modulator showed clear eye openings under 10 Gbps non-return-to-zero modulation. It allowed 10 Gbps error-free operation over a 25 km long single-mode fiber.

10 Gbit s−1 electroabsorption modulator integrated with a DBR laser using the selective area growth technique

We report 10 Gbit s−1 operation of an electroabsorption modulator (EAM) integrated with a distributed Bragg reflector (DBR) laser. The selective area growth technique was first employed in the 10 Gbit s−1 EAM fabrications to grow an active layer in the regions of the laser and the modulator. High side mode suppression ratios (SMSRs) over 45 dB and a 3 dB bandwidth of 12 GHz were observed. A clearly opened eye-diagram was also observed with 10 Gbit s−1 non-return to zero (NRZ) modulation signals.

10-Gb/s Tunable External Cavity Laser Using an Asymmetric Ring Resonator Reflector

We demonstrate a 10-Gb/s tunable external cavity laser with a polymer Bragg reflector and a highly integrated semiconductor gain chip including an asymmetric ring resonator reflector. The output power of the tunable laser is increased by adjusting the coupling strengths of two multimode interferences (MMIs) of the ring resonator reflector. The output power of the tunable laser is 6 mW at gain and semiconductor optical amplifier currents of 100 mA, and at a temperature of 25°C when the cross-coupling strengths ratios of two MMI couplers are 0.36 and 0.5. The tunable laser successfully provides 10-Gb/s transmissions through 20-km standard single mode fiber (SMF). The power penalty of the tunable laser is less than 2 dB after a 20-km SMF transmission for a 16-nm tuning range. The power penalty variation is less than 2 dB during the blue-shifted wavelength tuning.

Improving 60-GHz band radio-frequency with radio-over-fiber link characteristics of optical transmitter system-on-packaging

An optical transmitter system-on-packaging TxSoP with an electro-absorption modulator EAM was developed for 60-GHz band rf with radio-over-fiber rf/RoF uplink. An analysis to develop the TxSoP with a small noise figure was performed so that the noise figure of the modified TxSoP decreased by 15 dB. The rf/RoF uplink with the modified TxSoP was established, and the error vector magnitude EVM and the signal-to-noise ratio SNR of the link were measured between 2.2% and 3.2% and between 26 dB and 29 dB from 140 MHz to 800 MHz, respec- tively. Digital CATV signals corresponding to 86 channels were transmit- ted through the link, and total throughput of the link was 2.6 Gbps.