We present a terahertz (THz) radiation pumped by a passively mode-locked Yb-doped fiber laser using two fiberpigtailed log-spiral-based low-temperature-grown (LTG) InGaAs photoconductive antenna (PCA) modules. The modelocked fiber laser produces over 220 mW of the average optical power with positively chirped of 1.49 ps pulses. In order to generate THz radiation using the fiber-pigtailed PCA modules, the mode-locked optical pulses are pre-chirped with 538 fs using two diffraction gratings. We successfully achieved THz radiation over 2.0 THz using the pre-chirped pulses. We successfully observed the various absorption lines of water vapor dips in the free space of 120 mm.
We successfully demonstrate a THz generation using an ytterbium (Yb)-doped mode-locked femtosecond fiber laser and a home-made low-temperature grown (LTG) InGaAs Photoconductive antenna (PCA) module for THz Time-domain spectroscopy (TDS) systems. The Yb-doped fiber ring laser consists of a pump laser diode (PLD), a wavelength division multiplexer (WDM) coupler, a single-mode fiber (SMF), a 25 cm-long highly Yb-doped fiber, two collimators, two quarter wave plates (QWPs), a half-wave plate (HWP), a 10 nm broadband band pass filter, an isolator, and a polarizing beam splitter (PBS). In order to achieve the passively mode-locked optical short pulse, the nonlinear polarization rotation (NPR) effect is used. The achieved center wavelength and the 3 dB bandwidth of the modelocked fiber laser are 1.03 μm and ~ 15.6 nm, respectively. It has 175 fs duration after pulse compression with 66.2 MHz repetition rate. The average output power of mode-locked laser has more than 275 mW. The LTG-InGaAs PCA modules are used as the emitter and receiver in order to achieve the THz radiation. The PCA modules comprise a hyper-hemispherical Si lens and a log-spiral antenna-integrated LTG-InGaAs PCA chip electronically contacted on a printed circuit board (PCB). An excitation optical average pumping and probing power were ~ 6.3 mW and 5 mW, respectively. The free-space distance between the emitter and the receiver in the THz-TDS system was 70 mm. The spectrum of the THz radiation is achieved higher than 1.5 THz.
Our recent studies in regards of developing portable THz scanner for imaging and spectroscopy systems are presented. In the course, high power tunable continuous wave (CW) THz emitter and high sensitivity THz receiver platforms are presented. Those platforms can be realized with tunable optical beating source, broadband photomixer, arrayed photomixer and Schottky barrier diode, evanescently-coupled photodiodes with high saturation current, and semiconductor optical amplifier (SOA) integrated optical beating source. On the system level, our recent THz thickness measurement systems and the THz line scanner imaging system are presented.
A novel type of semiconductor beating source, a monolithically integrated dual-mode laser, and continuous-wave
terahertz (THz) system adopting it will be investigated. The combined system of the beating source with broadbandantenna-
integrated low-temperature-grown semiconductor photomixers shows the possibility of the realization of the
cost-effective and compact continuous-wave THz systems. Such a system is highly-demanded to examine the THz finger
prints of specimens without limitations. Since the optimized performance depends not only on the characteristics of
functional devices but also module configurations, various approaches such as traveling-wave photomixers, Schottky
barrier diodes, and nano-structure contained photomixers have been investigated to implement high-performance THz
platforms as the main building blocks of a THz system. Semiconductor-based compact and cost-effective photonics
technologies will envisage the bright future of THz systems.
We demonstrate the tunable continuous-wave (CW) terahertz generator based on the λ/4 phase-shifted 1.3 μm dual-mode laser diode (DML) and travelling-wave photodiode (TWPD). The DML and TWPD operate as an optical beat source and terahertz photomixer, respectively. The laser diodes (LDs) operating at the 1.3 μm have more suitable characteristics as optical beat sources than the LDs operating at 1.55 μm because of their high efficiency and better thermal stability. The micro-heaters are integrated on top of each DFB LD for mode beat frequency tuning. The fabricated DML was continuously tuned from 230 GHz to 1485 GHz by increasing the temperature of each DFB section independently via integrated micro-heaters. The high-speed TWPD with an InGaAs absorber was designed and fabricated to efficiently generate the photomixing terahertz CW. A complementary log-periodic antenna was integrated with the TWPD to radiate the generated terahertz wave with minimum reflection in the wide frequency range. The terahertz characteristics of the tunable CW terahertz generator based on the DML and TWPD were measured in a fiber-coupled, homodyne terahertz photomixing system. Our results of the tunable CW terahertz generator show the feasibility of a compact and highly efficient CW terahertz spectrometer and imager.
We demonstrate several optical beating sources based on 1.55 μm photonic devices. Broadband antenna-integrated,
low-temperature-grown (LTG) InGaAs photomixers for widely tunable continuous-wave THz generation and detection
are also verified. The novel optical beat sources show a beat frequency tuning range from 0.3THz to over 1.34 THz. The
dual-mode laser diode (DML) consists of one phase and two active sections. Micro-heaters are used to independently
tune the wavelengths of the two DML laser modes. Broadband antenna-integrated, LTG InGaAs photomixers are used as
THz wave generators and detectors. This use of 1.55 μm photonic devices could connect current THz and InP-based
communication technologies because the well-developed InP-based optoelectronic technologies are already expected to
enable the integration of tunable LD sources with other optical components such as semiconductor optical amplifiers
(SOAs), electro-absorption modulators, and waveguide-type THz photomixers. As well as realizing an optical fibercoupled
THz time-domain spectroscopy (TDS) system, we also successfully achieved continuous frequency tuning of the
CW THz emissions. Our results show that photomixing using the photonic devices is a promising approach to realize
compact, cost-effective, and portable THz spectrometer.
The cost-effective and repeatable technology for integration of polymer multimode waveguide and out-of-plane 45° reflector mirrors is developed. This method is cost-effective, repeatable, robust, and fully compatible with the standard manufacturing processes for a 90° optical bending structure.
The basic concept of the technology for integration of waveguide and out-of-plane 45° reflector mirrors is as follows; 1) The positively patterned master in order to mold waveguides is manufactured by using photolithography and Deep RIE (Reactive Ion Etching). And the master is polished to obtain 45°-inclined plane. 2) Both sides of the positively patterned master are divided into three parts by using a sawing machine. One is a center master (main-master) with a positively patterned waveguide and the others are side masters (sub-master) without a pattered waveguide. The main master and sub-master turned over get back together again. 3) The negatively patterned PDMS master to be able to mold simultaneously both waveguide and out-of-plane 45° reflector mirrors is manufactured through pouring PDMS gel into master and thermally curing the PDMS master. 4) The multimode tapered waveguides with out-of-plane 45° reflector mirrors are simultaneously embossed by using PDMS master. The UV (Ultraviolet) curable material is organic-inorganic hybrid material (HYBRIMER, core index: 1.51, clad index: 1.48).
The transmitter module is constructed on a MOB. The MOB was employed for several purposes; to align optical module passively, to use as heat sinker and also to support the boards. On this MOB, 1×4 arrays of vertical-cavity surface-emitting laser (VCSEL) and Tapered Waveguide with 45° reflector mirrors are integrated. The height and width of waveguide's core are 100 μm, 60 μm respectively and the pitch is 250 μm. The transmission access lines in transmitter are designed considering differential impedance matching for high-speed operation. We measured the insertion loss of this transmitter module using a 62.5 μm graded index fiber. The average insertion loss value is roughly about 7dB.
In this paper, we describe the cost-effective and simplified fabrication of an index modulation type buried waveguide using laser direct writing. Our studies have a potential of manufacturing waveguides on an uneven surface and a large area because there is no need for photo-mask, etching and development processes. We used organic-inorganic hybrid materials (HYBRIMER) for the fabrication of the waveguides, which have a high transparency from a visible region to an infrared region. We exposed the core layer (HYBRIMER) to a focused laser beam after a one-step spin coating process on a buffer layer. The silicon oxide was used as a buffer layer. The refractive index of the HYBRIMER film is increased by exposure from a laser beam. Therefore, the refractive index of the exposed region is higher than that of the unexposed region, which forms the index modulation type waveguide without an etching process. The fabricated waveguide channels were baked at 120°C during 3hrs for stabilization of the organic and inorganic networks. The laser direct writing apparatus was used to produce the pattern of waveguide channels. This system consists of a He-Cd laser radiating 325nm beam, high-resolution computer-controlled translation stages and a video camera that images the sample onto a monitor. The pattern of the waveguide channel was written using various writing speeds to optimize the writing condition. The core section of optimized waveguides was a rectangular shape and the core dimension was 7μm wide and 8μm high. The refractive index is increased from 1.495 to 1.5 after exposure. The difference of the refractive index between the core and cladding was approximately 0.33%. The insertion loss of the waveguides was measured by cut-back method using a single-mode fiber as an input tip, a multimode fiber (50 μm GI) as an output tip, and a 1310nm wavelength laser light source. The insertion loss shows a linear relationship with the length of the waveguide. The propagation loss of the buried waveguide was approximately 0.3dB/cm at a wavelength of 1310nm.
The system performance of data- and telecommunication equipment must keep up with the increasing network speed. Optical interconnections technology is a promising alternative for high-throughput systems. We demonstrate the optical backplane system using a waveguide-embedded optical backplane and two processing boards. The transmitter and receiver modules were prepared for optical printed circuit boards (PCBs), which consists of the metal optical bench, the driver chips, vertical-cavity surface-emitting lasers (VCSELs), photodiodes, and a tapered polymeric waveguide. We report high-speed transmission of 27–1 pseudorandom bit sequence (PRBS) nonreturn to zero (NRZ) data up to 10 Gbits/s through the optical backplane system. The results demonstrate that the optical backplane system can be practical and valuable for the future high-throughput systems by using metal optical bench and precisely machined optical plug-adaptor structure to achieve stable board-to-board interconnection.
The performance of data and telecommunication equipment must keep up with the increasing network speed. Optical interconnection technology is a promising alternative for high throughput systems. The Optical backplane system was demonstrated with waveguide-embedded optical backplane, transmitter board and receiver board. The transmitter and receiver module were prepared for optical PCB, which consists of the metal optical bench, the driver chips, VCSELs, photodiodes and a tapered polymeric waveguide. And parallel optical transmitter and a receiver module were attached onto the processing boards for the interconnection with optical backplane board. The tapered polymeric waveguides are fabricated using the hot embossing technique. And the propagation loss of the waveguide was approximately 0.1dB/cm at 850nm. The waveguide-embedded optical backplane boards were fabricated by using conventional PCB lamination process. The data transmission characteristics of the processing board have been investigated. In our optical backplane system, we demonstrated up to 10Gb/s 2<sup>7</sup>-1 PRBS NRZ data transmission from the transmitter board to the receiver board through optical backplane. The BERs were less than 10<sup>-12</sup> under 8Gb/s data rate, which is sufficient level for telecommunications.
A fully optical PCB with transmitter/receiver system boards and optical bakcplane was prepared, which is board-to-board interconnection by an optical slot. We report a 10 Gb/s PRBS NRZ data transmission between transmitter system board and optical backplane embedded multimode polymeric waveguide arrays. The basic concept of the optical PCB is as follows; 1) Metal optical bench is integrated with optoelectronic devices, driver and receiver circuits, polymeric waveguide and access line PCB module. 2) Multimode polymeric waveguide inside an optical backplane, which is embedded into PCB, 3) Optical slot and plug for high-density (channel pitch : 500 um) board-to-board interconnection. The polymeric waveguide technology can be used for transmission of data between transmitter/receiver processing boards and backplane boards. The main components are low-loss tapered polymeric waveguides and a novel optical plug and slot for board-to-board interconnections, respectively. The transmitter/receiver processing boards are designed as plug types, and can be easily plugged-in and -out at an optical backplane board. The optical backplane boards are prepared by employing the lamination processes for conventional electrical PCBs. A practical optical backplane system was implemented with two processing boards and an optical backplane. As connection components between the transmitter/receiver processing boards and backplane board, optical slots made of a 90°-bending structure-embedded optical plug was used. A 10 Gb/s data link was successfully demonstrated. The bit error rate (BER) was determined and
is 5.6×10<sup>-9</sup>(@10Gb/s) and the BER of 8 Gb/s is < 10<sup>-12</sup>.
Polymer waveguides have attracted a great deal of attention for their potential applications as optical components in optical communications, optical interconnections and optical sensors because they are easy to manufacture at a low temperature, and they have a low processing cost. Hot embossing is powerful and effective tools to produce a large volume of waveguides and structure high-precision micro/nano patterns of thin polymer films using a stamp for optical applications. In this work, fabrication techniques of hot embossed polymeric optical waveguides for parallel optical interconnection module, multi-channel variable optical attenuator and optical printed circuit boards are demonstrated. The single- and multi-mode waveguides are produced by core filling and UV curing processes. New approaches to fabricating single-mode polymeric waveguides with the high thermal stability in thermosetting polymers and two-dimensional multi-mode polymeric waveguides for high-density parallel optical interconnections as well as a simultaneous fabrication of single-mode polymeric waveguides with micro pedestals for passive fiber alignment are also reported.