170 GHz uni-traveling carrier photodiodes for
InP-based photonic integrated circuits

1. Introduction

Wireless millimetre-wave photonic technology capable of operating at carrier frequencies around 60 GHz has been successfully demonstrated [1]. The ever-increasing demand for high data rates will eventually push carrier frequencies above 100 GHz where limited solutions are currently available [2–4]. Photomixing in Uni-Traveling Carrier Photodiodes (UTC-PDs) has been proposed and demonstrated as a promising technique for frequency tunable millimetre-wave and sub millimetre-wave photonically enabled systems [5–9]. This generation scheme in combination with Photonic Integrated Circuit (PIC) technology can offer compact, tunable and highly efficient transmitters where the photodetector can be integrated with tunable lasers, amplifiers and modulators on the same photonic chip. The field of PIC technology has recently seen tremendous evolution. However, active-passive integration that allows Distributed Feedback (DFB) lasers, Electro-Absorption Modulators (EAMs), Semiconductor Optical Amplifiers (SOA) and Multimode Interference (MMI) couplers on the same photonic chip has so far delivered PICs with low-bandwidth photodiodes [10], [11]. The main limitation for the bandwidth of these photodiodes has been the common MQW epitaxy used in active sections. An early demonstration of the integration of UTC-PDs on a MQW platform produced photodiodes capable of detecting 40 Gb/s signals [12]. Recently, we demonstrated Coplanar Waveguide (CPW)-integrated photodiodes fabricated using techniques compatible with active-passive integration that were optimized for generation at 120 GHz [13]. These devices demonstrated a bandwidth of up to 110 GHz and a generated output power of more than 1 mW at 120 GHz together with a flat frequency response in the F-Band (90-140 GHz).

In this paper we demonstrate extremely high-bandwidth, CPW-integrated photodiodes fabricated using the same growth and fabrication steps as in [13]. The devices achieved a responsivity of 0.27 A/W at a wavelength of 1.55 μm, a 3-dB bandwidth of 170 GHz, and a generated output power of up to −5 dBm at 170 GHz and −9 dBm at 200 GHz. Although high bandwidth InP photodiodes have been previously demonstrated [7], [14], to the best of our knowledge, this is the first time that photodiodes fabricated using active-passive PIC technology have demonstrated such a high 3-dB bandwidth.

2. Growth and fabrication

Compared to stand-alone devices, a thick (> 1 μm) cap layer must be present at the top of the UTC-PD that needs also to act as the p-contact. The thickness of this layer is critical in order to achieve good confinement in the active sections and low propagation losses in the passive sections of the PIC. However, this thick layer imposes a high series resistance for the photodiode that results in a reduced 3-dB RC-limited bandwidth and more severe thermal effects. This trade-off was taken into consideration for the design of photodetectors such as those presented in [13] where the target was the optimisation of the device as a 120 GHz emitter. The same simulation procedure that was followed in [13] resulted in a photodiode active area that is only limited in dimensions from the fabrication procedure. Devices with an active area of 2 × 10 μm² and the same epitaxy as in [13] were fabricated for 3-dB bandwidth maximisation.

The waveguide CPW-integrated UTC-PDs, as shown in Fig. 1 , are fabricated on a semi-insulating InP substrate using gas source molecular beam epitaxy. The fabrication of these devices was implemented by monolithically integrating passive waveguides with photodiode active sections in two growth steps. Initially, passive waveguide sections and the PD absorber and collector layers were grown. In the second step a regrowth of the top cladding and p-contact layers was performed. The fabrication process is described in more detail in [13] and used techniques that are compatible with the integration of MQW active sections and the realization of shallow ridge laser and SOA waveguides.

 

Fig. 1 Image of Coplanar Waveguide (CPW)-integrated UTC-PD chip with 2 × 10 μm² active area and a 70 μm long optical input waveguide.

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3. Experimental arrangement

In order to assess the frequency response of the CPW-integrated photodiodes, an experimental arrangement using various CPW-type Ground-Signal-Ground (GSG) probes and downconverting mixers was used. A heterodyne signal generated from two lasers was amplified using an Erbium Doped Fibre Amplifier (EDFA) and fed into the photodiode through a lensed fiber with a 3 μm spot size. The signal was measured directly with a power meter for frequencies up to 90 GHz using DC–50 GHz, V-band (50-75 GHz) and W-band (75-110 GHz) GSG probes. Above 90 GHz, F-band (90-140 GHz) and G-band (140-220 GHz) probes and characterised sub-harmonic mixers were used and the Intermediate Frequency (IF) signal level was measured with a spectrum analyser. The experimental arrangement for these measurements is given in Fig. 2 .

 

Fig. 2 Experimental arrangement used for measurements in different frequency bands ranging from 90 to 220 GHz. Measurements up to 50 GHz were performed without a sub-harmonic down-converting mixer. For measurements up to 90 GHz, a calibrated power meter was used instead of the sub-harmonic mixer.

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4. Results and discussion

UTC-PDs were mounted on a thermally controlled stage for experimental characterisation from DC to 220 GHz. The responsivity of the devices was measured using the experimental arrangement described in the previous section and a reverse bias voltage of 3 V, which was found to be the optimum level of reverse bias voltage, was applied to the terminals of the device. The optical input power was swept by adjustment of the EDFA and a correction was applied to the optical power to account for losses in the optical path after the amplifier. The results are shown in Fig. 3 ; the resulting responsivity was 0.27 A/W at 1.55 μm while a Polarisation Dependent Loss (PDL) of 2 dB was also found. It is noteworthy that this device had no anti-reflection coating.

 

Fig. 3 Responsivity measurement from a UTC-PD with 2 × 10 μm² active area dimensions. The measurement was taken at a reverse bias voltage of 3 V.

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The frequency response of miniaturised UTC-PDs from the same fabrication run was measured using GSG probes at a photocurrent of 5 mA and a 3 V reverse bias voltage. The measurements were corrected for the insertion loss of the probes (up to 3 dB) and the conversion loss of the mixers (up to 9 dB). The power level at the edges of the measured bands (50, 75, 90 and 140 GHz) was confirmed with both sets of GSG probes and mixers. The total frequency response normalized to the power level of the lowest frequency (1 GHz) is plotted in Fig. 4 . These devices showed a 3 dB bandwidth of approximately 170 GHz. This was also confirmed with series resistance (~40 Ω) and capacitance (~9 fF) simulations where an RC limited bandwidth of approximately 200 GHz was calculated. Although previous work from other authors presented UTC-PDs with a 3-dB bandwidth approaching 310 GHz [15], these measurements used short pulses and devices were terminated with an effective 12.5 Ω load.

 

Fig. 4 Frequency response up to 200 GHz from UTC-PDs with 2 × 10 μm² active area dimensions. The measurements were taken at a photocurrent of 5 mA and a reverse bias of 3 V.

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Saturation measurements were also performed in the G-Band using the same experimental arrangement. The generated power at 150, 170, 200 and 220 GHz was measured as a function of the optical input power. The results, corrected for the probe insertion loss and the mixer conversion loss are given in Fig. 5 . To make sure that the highest generated power level was not limited by the mixer saturation level, the millimetre-wave power at 14 dBm optical input power was also confirmed with a measurement using a free space Terahertz power meter, an experimental arrangement similar to the one used in [13]. A good agreement was found for the maximum millimetre-wave output power levels between the two different experimental arrangements.

 

Fig. 5 Power saturation in the G-Band (140-220 GHz) from UTC-PDs with 2 × 10 μm² active area dimensions. Reverse bias voltage was 3 V for all measurements.

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The device achieved substantial output power levels in the millimetre-wave range with −2 dBm at 150 GHz, −5.5 dBm at 170 GHz, −9 dBm at 200 GHz and even −11 dBm at 220 GHz. As the optical input power increases from 7 dBm to 14 dBm, a decreasing power difference was observed between the curves that correspond to points within the bandwidth of the device (150 and 170 GHz) and those above the 3-dB bandwidth (200 and 220 GHz). This can be attributed to a further improvement of the transit-time limited bandwidth by the introduction of a quasi-field that further accelerates electrons as previously explained in [16]. Although the power levels presented here are lower than current state-of-the-art high bandwidth photodiodes measured with a 50 Ω load [8], [14], to the best of our knowledge, these are the first 170 GHz photodiodes fabricated using techniques that are suitable for monolithic integration with active and passive MQW-based devices.

6. Conclusions

We have demonstrated extremely high-bandwidth photodiodes using a technology compatible with active-passive monolithic integration with MQW-based photonic devices. UTC-PDs were designed for high bandwidth despite the high series resistance from the re-grown thick InP cap layer that should be present to reduce propagation losses in passive sections. The fabricated devices achieved a responsivity of 0.27 A/W, a 3-dB bandwidth of 170 GHz and a generated output power of −9 dBm at 200 GHz. In our previous work, we demonstrated output power of about 200 μW at 300 GHz from antenna-integrated devices [8]. By improving the saturation performance, we anticipate these devices to generate similar levels of power at 300 GHz and beyond. Future work will produce photomixing PICs for wireless communications in the millimetre-wave range. However, this work can provide a generic technology platform for PICs requiring extremely high-bandwidth photodiodes.