Development of Terahertz-wave Photomixer Module Using a Uni-traveling-carrier Photodiode

3.1 Features

Although the most widely used PD is the conventional pin-PD, there are two limitations regarding its performance (pin: positive, intrinsic, negative layers). One concerns the maximum operating speed. When lights are input into the pin-PD’s absorption layer, electron-hole pairs, which are the electric current carriers, are generated (Fig. 4(a)). In a pin-PD, both electrons and holes contribute to its O/E-conversion mechanism. Because a hole’s effective mass is larger than an electron’s and its velocity is much lower, the maximum operating speed of the pin-PD is limited by the hole velocity. The other limitation concerns the maximum output current. When the optical input power becomes higher, the band in the undoped absorption layer is bent by the space charge due to carrier accumulation. This band bending causes a drastic reduction in carrier velocity, which results in saturation of the output current. To overcome these limitations of the pin-PD, we studied the UTC-PD.

Fig. 4. Differences between band structures of conventional pin-PD and UTC-PD.

In the UTC-PD, a p-doped absorption layer is separated from a wide-gap undoped carrier collection layer (Fig. 4(b)), which is in clear contrast to the pin-PD. This structure means that only electrons are the active carriers running through the depleted undoped carrier collection layer. Most of the photogenerated holes relax extremely quickly into the absorption layer where holes are the major carrier. Consequently, the maximum operating speed of the UTC-PD is determined only by the electron velocity, which is much higher than the hole velocity. The space charge effect is also much smaller than in the pin-PD because carrier accumulation is extremely small in principle compared with the pin-PD. For these reasons, the UTC-PD can exhibit high performance, that is, ultrahigh-speed and high saturation output current simultaneously.

We have achieved a 3-dB-down bandwidth of 310 GHz and a 10-dB-down bandwidth of 750 GHz as the operational frequency bandwidth [4]. We have also obtained a maximum output current of 30 mA for high optical input power.

3.2 Optimization for THz-wave photomixer module

It is essential to optimize the UTC-PD’s band structure in order to obtain an output current with a terahertz frequency. We designed a thinner absorption layer than the one in the UTC-PD for the previous millimeter-wave photomixer; this reduced the UTC-PD’s carrier transit time when it operates at higher frequencies. The thicknesses and materials of the other layers were also optimized for compatibility with the thin absorption layer.

Besides the band structure, we also improved the physical structure of the UTC-PC chip. The fabricated UTC-PD chip designed to operate in the terahertz frequency range is shown in Fig. 5. The chip is 300 × 450 μm².

Fig. 5. Photograph of the fabricated UTC-PD chip.

The frequency response of the PD output current shows a generally broadband-shape caused by the resistor-capacitor time constant. The response decreases with increasing operating frequency. For the THz-wave photomixer, a stub-line was integrated on the chip to increase the output current. It acts as a resonant circuit for the UTC-PD to compensate for the roll-off of the frequency response. The output power of the THz-waves is designed to be as high as possible at around the designed center frequency. In this development, the center frequency was set to 0.35 THz. The output current is expected to increase at around the center frequency through use of the resonant effect. We measured the pulse photoresponse to evaluate the chip’s resonant design. From the measured pulse waveform oscillating at about 0.35 THz, we ascertained that the stub-line acted properly as a resonant circuit.

4.1 Optimization

For practical use, in addition to being high-performance, the photomixer module is required to be compact, lightweight, and easy to handle. Moreover, since the module will be applied to various high-frequency systems, it should be easy to assemble on a board or in a package. As a result of our investigation, we determined that a butterfly-package would be an excellent form for the module. It is a generic package with a good track record for many optical devices, so it is cheap and readily available.

For the module structure investigation, there are two main issues: they concern optical implementation and electrical implementation. The former covers how efficiently light is input to the UTC-PD, and the latter covers how efficiently ultrahigh-frequency current from the UTC-PD is converted into THz-waves.

Today’s high-frequency coaxial connectors have pronounced propagation loss for current with a frequency range above 0.1 THz, so it is essential to chose either the waveguide technique or direct radiation technique (quasi-optical technique) to output the current from the module. We developed a waveguide-based module structure during our development of the previous millimeter-wave photomixer module and confirmed its excellent characteristics. The THz-wave photomixer module is based on an extension of that technique.

The cross-section of the internal structure is schematically shown in Fig. 6. The optical part consists of an optical fiber and collimation lenses. The collimation lens system has good versatility for an optical design, and it also has excellent optical alignment stability among the optical fiber, lenses, and UTC-PD. The UTC-PD chip is mounted on a metal block with a rectangular waveguide whose aperture size is 0.864 mm × 0.432 mm. The UTC-PD chip and the waveguide are connected by a quartz coupler consisting of an impedance converter, a micro-strip line, and a radiator. The module is made by assembling these components in the butterfly-package with the same waveguide.

Fig. 6. Schematic cross-section of the module's internal structure.

The design of the quartz coupler is extremely important for high emission efficiency of the ultrahigh-frequency current as THz-waves into the waveguide. The design policies were as follows:

The quartz coupler was designed using a three-dimensional electromagnetic wave simulator to obtain flattened characteristics from 0.2 THz to 0.65 THz. As a result of simulation for various designs, the optimized one achieved reflection power of less than -30 dB and transmission power loss of less than 3 dB.

4.2 Performance

We fabricated a THz-wave photomixer module (Fig. 7) with the optimized design described in the previous section. The module is 3 × 1.65 × 1 cm³.

Fig. 7. Photograph of the fabricated THz-wave photomixer module.

The output power dependence on frequency when the optical beat signal’s frequency was swept from 0.2 THz to 0.65 THz is shown in Fig. 8. The output power was clearly enhanced at around 0.35 THz (designed center frequency of the stub-line) with the maximum value being over 0.5 mW [5]. This is the highest value ever reported for a photomixer with a single PD operating in this frequency range. The 3- and 10-dB-down bandwidths were as wide as 120 and 260 GHz, respectively, showing extremely wideband performance.

Fig. 8. Fabricated module's output power dependence on frequency.

The main feature of this module is that it simultaneously achieves both high-output power and extremely wide bandwidth. In addition to this high performance, it is very easy to use because it operates in continuous-wave operation at room temperature without any cooling.