We introduce our developing photonics-based technologies for THz-wave generation/combination by arrayed photomixers. We demonstrated THz-wave power combination and beam steering at 300 GHz with phase tuning at optical delay lines. In addition, we made electrically-controlled arrayed phase shifters on a silica-based planar lightwave circuit, which showed a high-speed beam steering with 1-kHz repetition. For a future integrated THz-wave source, we also designed and fabricated arrayed lightwave sources to be coupled to the arrayed photomixers. Combination of these two arrays would enable to generate THz wave only with DC currents or voltages.
Terahertz (THz) range holds between infrared light and millimeter wave or microwave radiation. Moreover, THz waves is highly attenuated by the metal object or sensitive to an inter-molecular binding force. Therefore, imaging using THz range is attracted much attention for security, manufacturing, chemical imaging, and so on. In our research, the THz detector composed of Indium arsenide (InAs) high electron mobility transistor (HEMT) and one-sided directional slot antenna on a chip will be developed. In this paper, we focused on the antenna on a chip. The proposed antenna has three layers, namely, top antenna metal, dielectric substrate (BCB, benzocyclobutene) and bottom floating metal layer. There are a coplanar (CPW) feed lines and slots on the top antenna metal. By optimizing the size of the bottom floating metal layer, the radiation toward the back side is suppressed. The CPW feed line is connected the gate electrode on the InAs HEMT. In order to maximize the receiving THz signal form the antenna to InAs HEMT, antenna and gate input impedance is characterized by using the 3D electromagnetic simulator. It has been found that when the input impedance of the gate electrode changes from 10 ohms to 50 ohms, the voltage generated at the gate electrodes is tripled. The antenna was fabricated by the conventional photolithography process. The size of the radiation metal is 290 μm x 210 μm on the top metal with probe pads. The measured antenna gain is 5.57 dBi at 0.93 THz compared with the 5.96 dBi antenna gain at 1 THz from the simulation.
The application using the frequency range from 100 GHz to 10 THz has attracted much attention, especially in broadband and higher data-rate wireless communication. In the THz broadband wireless devices, photo mixing by using the uni-traveling carrier photodiode (UTCPD) on the indium phosphide (InP) substrate is a crucial component. UTCPD can down-convert the optical signal to THz wave. To reduce the loss of the connection area between the optical section and the THz section, THz-band antennas and transition lines should be fabricated on the same substrate as the optical section. In our previous research, 1 x 4 and 4 x 4 planar array antennas using one-sided directional slot dipole antenna elements and branched coplanar waveguide (CPW) are connected to the output of UTCPD on the InP substrate for the 300 GHz application. In this presentation, wideband 600 GHz one-sided directional slot antenna was designed. The antenna is based on the slot antenna on the top with the bottom floating metal layer. To enhance the bandwidth, round shape of the edge of the top metal layer was introduced. Moreover, 2 kinds of the antenna element with different resonance frequencies are designed. Antenna 1 (Ant1) has a center frequency = 600 GHz and gain = 2.23dBi. Antenna 2 (Ant2) has a center frequency = 650 GHz and gain = 3.28dBi. The whole size of the antenna elements is 290 um x 230 um and 280 um x 290 um, respectively. Each antenna element is connected to the UTCPD and optical waveguide through a coplanar waveguide (CPW) feed line. Next, we designed a 2-dimensional antenna array with 12 antenna elements. To enhance the bandwidth 4 Ant1s and 8 Ant2s are combined on the InP substrate. From the electromagnetic simulation, this array antenna has antenna gain = 11.89 dBi, 3-dB bandwidth =130 GHz and front-to-back ratio = 15.73 dB. The array size is 1,500 um x 1,500 um. The relative bandwidth can be enhanced from 5 % (reference array antenna) to 20 %. Moreover, by changing the delay line attached to the optical fiber, it is easy to obtain the phase difference of each antenna element. From the results, our proposed phased array antenna has a wideband, high gain and beam tilt characteristics.
The terahertz (THz) wave applications at frequencies from 100 GHz to 10 THz has attracted much attention, especially in a broadband wireless communication. In the THz broadband wireless devices, photo mixing by using the unitraveling- carrier photodiode (UTC-PD) on the InP substrate is a critical issue, which is down-converted to the optical signal to THz wave. In this situation, the loss in the THz section is a serious problem. Therefore, antennas and transition lines should be fabricated on the same substrate. In our previous research, 1 x 4 and 4 x 4 planar array antennas using one-sided directional slot dipole antenna elements and branched coplanar waveguide (CPW) connected to the output of UTC-PD on the InP substrate is designed. In this paper, 4 x 4 phased array antenna on InP substrate for 300 GHz broadband telecommunication is demonstrated. The total antenna size is 1,930 μm x 2,000 μm x 18 μm. Four 1 x 4 subarray antenna are stacked planarly, and each subarray antenna is connected to the UTC-PD through the CPW. Each antenna element is arranged at the distance of half wavelength in order to sharpen the directivity. By changing the delay line attached to the optical fiber, the phase difference of each subarray can be obtained. From the phase difference between each antenna elements, our proposed phased array antenna has a sharp beam and beam steering characteristics.
This paper presents a design and fabrication of 4 × 4 one-sided directional slot array antenna on indium phosphide (InP) substrate for 0.3 THz (300 GHz) wireless link. The antenna has top antenna metal layer and bottom floating metal layer. Polyimide dielectric layer is stacked between each metal layer. The antenna is placed on the deep etched InP substrate. By optimizing the length of the bottom floating metal layer, one-sided directional radiation can be realized. The branched coplanar wave guide (CPW) transmission line is connected to each antenna element with the same electrical length. The size of the 4 × 4 array antenna is 2,730 μm x 3,000 μm with uni-traveling-carrier photodiodes, DC bias and ground lines. Simulated realized gain in peak direction of the proposed antenna is 11.7 dBi. The transmission measurement is carried and measured received power.
For high-power THz wave generation by photomixing of two lightwaves, we proposed the synchronous power combiner which consists of eight-arrayed photomixers/antennas and the THz phase control system. We experimentally confirmed the effectiveness of the power combination by synchronizing the phases of the THz wave by the mechanical optical delay lines and also demonstrated the same functionality at the lightwave-circuit-based optical phase control system. We found that the directional gain is increasing with increasing the number of photomixers from two to three and it reached up to 4.5 dB.
This paper presents a design and fabrication of 1 x 4 one-sided directional slot array antenna with director metal layer on indium phosphide (InP) substrate for 300 GHz wireless link. The floating metal and polyimide dielectric layer are stacked on InP. Antenna is designed on the top metal layer. By optimizing the length of the bottom floating metal layer, one-sided directional radiation can be realized. The branched coplanar wave guide (CPW) transmission line is connected to each antenna element with the same electrical length. The size of the 1 x 4 array antenna is 2,550 µm x 1,217 µm x 18 µm. In order to enhance the gain of forward direction, director metal layer is placed over 83 µm from top metal layer. Simulated realized gain in peak direction of our antenna is 9.23 dBi. The measured center frequency is almost the same as that of the simulation results.
We are developing a new light source for swept-source OCT, namely, an external-cavity LD equipped with a KTN
electro-optic deflector. Being free from mechanical resonance, our 1.3-μm laser exhibits scanning range of almost 100
nm up to 200-kHz under a ±300 V deflector driving voltage. Using a semi-empirically derived equation, we find that
KTN's convex lens power degrades the coherence length, and this can be compensated with a cylindrical concave lens.
Such compensation was experimentally confirmed by observing reduction of elliptical beam divergence. OCT images of
a human fingernail are obtained using the swept source.
We present a new light source for the swept-source OCT, that is, an external-cavity LD incorporating an electro-optic
deflector. We use a KTN deflector that is unique in being very fast and simultaneously providing an appreciable
deflection caused by injected carriers. Particularly, high-speed and nearly linear to the applied voltage operation is
attained when KTN crystal is pre-charged. Our 1.3-μm Littman-Metcalf external-cavity laser exhibits static linewidth <
0.1 nm, and a 110-nm scanning range up to 150-kHz under a ±200 V sinusoidal driving voltage to the deflector. Being
free of mechanical resonance, the laser would hopefully realize a faster (in a separate study, deflector itself worked up to
400 kHz) and wavenumber-linear scan that is ideal for the swept-source OCT by designing the waveform of driving
voltage. And as for the resolving power of deflector, while our KTN deflector has only 35 spatial resolvable points, the
number of wavelength points for the swept source clearly exceeds to this limit, which we attribute to line narrowing
effect accompanied by the laser operation. Preliminary OCT images taken using the swept source are also presented.
We demonstrate a single-mode and fast wavelength swept light source by using Superestrucuture grating
distributed Bragg reflector (SSG-DBR) lasers for use in optical frequency-domain reflectometry optical coherence
tomography. The SSG-DBR lasers provide single-mode operation resulting in high coherency. Response of the
wavelength tuning is very fast; several nanoseconds, but there was an unintentional wavelength drift resulting from a
thermal drift due to injecting tuning current. The dri1ft unfortunately requires long time to converge; more than a few
milliseconds. For suppressing the wavelength drift, we introduced Thermal Drift Compensation mesa (TDC) parallel to
the laser mesa with the spacing of 20 μm. By controlling TDC current to satisfy the total electric power injected into
both the laser mesa and the TDC mesa, the thermal drift can be suppressed.
In the present work, we fabricated 4 wavelength's kinds of SSG-DBR laser, which covers respective
wavelength band; S-band (1496-1529 nm), C-band (1529-1564 nm), L--band (1564-1601 nm), and L+-band (1601-1639).
We set the frequency channel of each laser with the spacing 6.25 GHz and 700 channels. The total frequency channel
number is 2800 channels (700 ch × 4 lasers). We simultaneously operated the 4 lasers with a time interval of 500
ns/channel. A wavelength tuning range of more than 140 nm was achieved within 350 μs. The output power was
controlled to be 10 mW for all channels. A single-mode, accurate, wide, and fast wavelength sweep was demonstrated
with the SSG-DBR lasers having TDC mesa structure for the first time.
Waveguide p-i-n photodiodes are theoretically revealed to have a great advantage over conventional surface-illuminated p-i-n photodiodes and metal-semiconductor-metal photodiodes in the performance limit, the product of the bandwidth and the external quantum efficiency. Experimental results show a bandwidth of over 75 GHz and a high efficiency with a mushroom-mesa multimode waveguide p-i-n photodiode.