Terahertz time-domain spectroscopy (THz-TDS) is one of the main research platforms for terahertz (THz) science and technology. However, a traditional system is not only bulky but also has some limitations in detecting micro and liquid samples because of the large THz beam spot dimensions and the strong THz-wave absorption by water. We have developed a THz-TDS measurement chip by directly fabricating THz transmission lines and photoconductive antenna electrodes on a low-temperature GaAs epitaxial wafer. This chip integrates THz-waves generation device, detection device, and the transmission line in the size of 1 mm × 20 μm and uses the between the THz evanescent field sample interaction near the transmission line to obtain the spectral information from the trace or liquid samples. Using a self-built measurement beam path, we tested the chip with 1550-nm femtosecond laser and obtained THz spectroscopy in the 0.1 to 0.7 THz range, thereby verifying the feasibility of the chip.
A simple system based on two cascaded Mach-Zehnder modulators, which can generate 160GHz optical terahertz waves from 40GHz microwave sources, is simulated and tested in this paper. Fiber grating filter is used in the system to filter out optical carrier. By properly adjusting the modulator DC bias voltages and the signal voltages and phases, 4-tupling optical terahertz wave can be generated with fiber grating. This notch fiber grating filter is greatly suitable for terahertz over fiber (TOF) communication system. This scheme greatly reduces the cost of long-distance terahertz communication. Furthermore, 10Gbps digital signal is modulated in the 160GHz optical terahertz wave.
Terahertz time domain spectroscopy system (THz-TDS) is the most commonly means of measuring terahertz time-domain spectroscopy. The time delay between the pump and probe laser is an important technology to realize THz time domain spectrum measurement. The translation platform with two mirrors and the mechanical structure is the popular means to adjust the optical path difference between the pump and probe laser to get the time delay of femtosecond pulse. Because of the limit of the mechanical structure and the phase-locked amplifier, this technique can’t scan spectrum fast. In order to obtain high quality signal, a long time will be taken to scan spectrum. So a more rapid and convenient time delay technology is required to Instead of the machine translation platform and accomplish the Rapid spectral measurement. Asynchronous optical sampling technique is a way to get the time delay by producing a very small frequency difference between the repetition frequency of two femtosecond lasers. The scanner time will be reduced, because of there is no waste of time, due to mechanical inertia, not only by using the asynchronous optical sampling method to replace the mechanical structure without the influence of vibration. It will greatly increase the degree of integration by using the fiber femtosecond laser and highly integrated circuit to realize optical asynchronous sampling.
To solve the problem above, a terahertz time-domain spectroscopy system based on asynchronous sampling is designed in this thesis. The system is based of two femtosecond laser whose repetition frequency is 100MHz.In order to realize asynchronous sampling, the control circuit of the two lasers is the most important. This thesis focuses on the researching, designing and experiment of this circuit. Firstly, the circuit is designed overall. Then the selection of the key device and the designing of the circuit principle is done by myself. Secondly, the test of the circuit to phase locked the master and slave fiber femtosecond lasers has been done. As a result, the two lasers can phase locked on two repetition frequencies with a small frequency difference calculated by the circuit.
This paper proposed a polarization independent terahertz modulator based on gold-Si metamaterials with symmetric structure, and aimed to modulate terahertz wave in communication systems. The transmission properties have been investigated in terahertz regime. We find that the resonance frequency of this device can be actively controlled by pump laser. The numerical simulations and experiments with OPTP system show that this device acts as a modulator with intensity modulation depth of 70% by gold structure.
The fundamental parameters of THz modulators based on the SRR structure metamaterial have been researched and found that the SRR size and opening width have a significant effect on the resonant frequency, meanwhile the thickness and width of this structure are very weak influence on the resonant frequency. The resonant frequency and response time have been calculated theoretically. The resonant frequency fits the simulation results well, and response time is mainly determined by the carrier lifetime which is in nanosecond order. We also proposed a program of experimental measurements of the SRR modulation speed. Finally we simulated a special SRR structure with metal strips on the back of wafer to increase modulation depth significantly.
We have designed a novel uncooled Terahertz (THz) imaging array based microbolometer. The Micro Electro Mechanical System (MEMS) technology is used to fabricate the imaging array which comprise bolometer layer, THz absorption layer, supporting layer (silicon nitride (SiNx)), resonant optical cavity and electrode. The vanadium oxide thin films is selected for the bolometer temperature sensing material because it has a high temperature coefficient of resistance (TCR) in the range 2%/K and 3%/K at room temperature and suitable square resistance 18.40kΩ/□, a small 1/f noise constant and can be integrated with signal read-out electronics in a cost efficient way. In order to provide a high absorption of the radiation in the bolometer membrane, a resonant optical cavity structure which has a reflection layer formed at the bottom of air gap is adopted for the targeted wavelength. The best air gap of the optical cavity structure is quarter of wavelength of interest, for example, 25μm for 3 THz radiation. The absorption mechanism is that the two reflected THz radiations coming from the absorption layer and reflection layer interfere destructively at THz absorption layer and free carriers in THz absorption layer absorb THz radiation, the absorbed THz radiation heats the imaging array so that the resistance values of bolometer change. The microbolometer need to be packaged in vacuum for best performance, so a cylindrical vacuum chamber which is sealed with polyethylene lamina for the THz radiation is fabricated. In order to maintain the vacuum performance of the chamber (conventional bolometers operate with vacuum levels <0.01mbar), the vacuum pump and molecular pump are adopted; furthermore the packaging technique of vacuum chamber is introduced in detail. Finally, because of its uncooled property of the microbolometer, it will have a low cost and be easy for fabrication of large array.
Terahertz modulation techniques are an indispensable key technique to achieve the future wireless terahertz wave high-speed communication systems. Metamaterials is artificial materials that geometric structure units are embedded in the traditional medium materials. In this letter we will introduce metamaterials terahertz modulation techniques and analyze the advantage and disadvantage of recent research progress in the world, including electrically controlled THz metamaterials modulators, optical controlled THz metamaterials modulators and some new modulators based on material-graphene
We have demonstrated a light control terahertz modulation system based on the semiconductor material, the system is composed of a commercial 850nm 10Gbps light source and a continuous THz source. The semiconductor device is the epitaxial growth Si layer on Si substrate wafer. When the 850nm light and continuous terahertz waves incident simultaneously to this semiconductor wafer, the modulated light can excite photogenerated carriers. And the carrier determines the absorption of terahertz waves, so the amplitude of output terahertz waves will be modulated. The tested modulation depth is 35%. The semiconductor material carriers lifetime determines both the modulation depth and modulation speed. So the carrier lifetime should be trade off considered. The modulation speed has been theoretical calculated and experimental tested. The carriers lifetime is tested about 2ns, and the modulation speed is calculated beyond gigabit per second.
We demonstrated a tunable long wavelength photodetector by using heteroepitaxy growth of InP-based
In0.53Ga0.47As-InP p-i-n structure on GaAs based GaAs/AlAs Fabry-Perot filter structure. High quality heterepitaxy was
realized by employing a thin low-temperature buffer layer. A wavelength tuning range of 10.0 nm, a quantum efficiency
of 23%, a spectral linewidth of 0.8 nm and a 3-dB bandwidth of 6.2 GHz were simultaneously obtained in the device.
Moreover, a Si based long wavelength photodetector with the same device structure was also fabricated successfully by
using Si/GaAs and GaAs/InP heteroepitaxy. Crack-free GaAs on Si and high-quality epilayer with area of
800μm×700μm was obtained by using mid-patterned growth and thermal-cycle annealing. The Si-based photodetector
with spectral linewidth of 1.1nm (FWHM) and quantum efficiency of 9.0% was demonstrated.
Abstract: We have explored the shared-layer integration fabrication of an resonant-cavity-enhanced
p-i-n photodector (RCE- p-i-n-PD) and a single heterojunction bipolar transistor (SHBT) with the
same epitaxy grown layer structure. MOCVD growth of the different layer structure for the GaAs
based RCE- p-i-n-PD/SHBT require compromises to obtain the best performance of the integrated
devices. The SHBT is proposed with super-lattice in the collector, and the structure of the base
and the collector of the SHBT is used for the RCE. Up to now, the DC characteristics of the
integrated device have been obtained.
Optical receiver of high speed and high sensitivity is indispensable for long distance fiber-optic communication systems
of transmitting rate up to Gb/s. In recent years, OEIC (Optoelectronic Integrated Circuits) optical receiver has attracted
more and more attention for its advantages over conventional optical receiver such as minimized parasitics, lower cost,
higher reliability and compact size. In this paper, design of preamplifier for InP-based PIN/HBT OEIC optical receiver
with share layer structure was presented. As a basis for design of preamplifier, HBT large signal model (GP model)
was investigated and model parameters were extracted and optimized. The extracted GP model shows a good agreement
with measured characteristics of HBT fabricated. Based on this GP model, the preamplifier was designed and fabricated
which exhibits good high-frequency characteristics of −3dB bandwidth is up to 2.0 GHz.
Integrated optical demultiplexing and receiving device based on one-mirror-inclined three-mirror cavity (OMITMiC) structure, or OMITMiC wavelength-selective photodetector, is a kind of novel integrated multifunction optoelectronic device which was proposed in 1996 and first realized with GaAs-based materials for short wavelength (less than 1μm) operation in 2001. Recently, after great efforts on developing controllable self-retreating dynamic mask (CSRDM) wet etching method for InP-based epitaxial layer and low temperature InP/GaAs wafer bonding technique, such a device operating at long wavelength region (1550nm) had also been successfully demonstrated and the measurement results shown that it features high-speed (12GHz with a mesa area of 40×36 μm2), high quantum efficiency (66%~78.4%), ultra-narrow spectral linewidth (0.6 nm) and wide range tuning (more than 10 nm ) simultaneously. In addition, a long wavelength monolithic OMITMiC photodetector with GaInNAs absorption layer has also been demonstrated. These achievements could have a significant impact on wavelength-division-multiplexed (WDM) optical fiber transmission systems and networks.
We demonstrate a wavelength-selective photodetector that combines a Fabry-Perot filtering-cavity (FPC) with a taper absorption-cavity (TAC). The taper cavity shows non-resonant effect but exhibits absorption enhancement effect, so that high-speed, high quantum efficiency, wide tuning range and ultra-narrow spectral linewidth can be achieved simultaneously. Device performance was theoretically investigated by including key factors such as taper angle, finite-size diffracting-beam input, and lateral walk-off in the taper cavity. The device was fabricated by bonding a GaAs-based FPC, which can be tuned via thermal-optic effect, with an InP-based TAC. The experiment results of the devices were reported in another paper.
Adding resonant cavity to increase quantum efficiency of the monolithically integrated PIN/HBT-Receiver is described. Between the InP buffer and device epitaxial structure, InP/InGaAsP quarter wavelength stack (QWS) are used to form DBR (Distributed Bragg Reflectors) mirror. The PIN-PD is integrated within a Fabry-Perot cavity and the incident light is reflected many times by the Fabry-Perot cavity and consequently absorbed many times by the absorption layer. Therefore, the quantum efficiency of this detector is enlarged, meanwhile other performances such as frequency response are not influenced. We discuss the method to fabricate the resonance cavity, make theory simulation, optimize design on it, and analyze the advantage of this device.
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