# THz photonics-based transmitter using multicore fiber combined with UTC-PD array for high data transmission in 300 GHz band

Bewindin Alfred Sawadogo, Aritrio Bandyopadhyay, Malek Zegaoui,

Mohammed Zaknoune, Pascal Szriftgiser, Karen Baudelle, Monika

Cieslikiewicz-Bouet, Géraud Bouwmans, Davy Gaillot, Esben Ravn Andresen,
et al.

► To cite this version:

Bewindin Alfred Sawadogo, Aritrio Bandyopadhyay, Malek Zegaoui, Mohammed Zaknoune, Pascal Szriftgiser, et al. THz photonics-based transmitter using multicore fiber combined with UTC-PD array for high data transmission in 300 GHz band. SPIE Photonics Europe 2022, Terahertz Photonics II, Apr 2022, Strasbourg, France. 18, 5 p., 10.1117/12.2621614 . hal-03741396

HAL Id: hal-03741396

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# THz photonics-based transmitter using multicore fiber combined with UTC-PD array for high data rate transmission in 300 GHz band

Bewindin A. Sawadogo<sup>a,b</sup>, Aritrio Bandyopadhyay<sup>a</sup>, Malek Zegaoui<sup>a</sup>, Mohammed Zaknoune<sup>a</sup>, Pascal Szriftgiser<sup>b</sup>, Karen Baudelle<sup>b</sup>, Monika Bouet<sup>b</sup>, Géraud Bouwmans<sup>b</sup>, Davy P. Gaillot<sup>a</sup>, Esben Andresen<sup>b</sup>, Guillaume Ducournau<sup>a</sup>, Laurent Bigot<sup>b</sup>

<sup>a</sup>IEMN Institut d'Electronique, de Microélectronique et de Nanotechnologies, 59650 Villeneuve d'Ascq, France

<sup>b</sup>Univ. Lille, CNRS, UMR 8523 - PhLAM - Physique des Lasers, Atomes et Molécules, F-59000 Lille, France

bewindin-alfred.sawadogo@univ-lille.fr, guillaume.ducournau@univ-lille.fr

## ABSTRACT

This work aims at illuminating an UTC-PD array with a multicore fiber towards multiple THz carrier generation. This photonics-based transmitter is expected to increase either the data rate via spatial multiplexing on different carriers or the emitted THz-power and therefore the transmission reach of THz systems operating in 300 GHz band. Preliminary results of the characterization of the sub-systems of the transmitter that is under development are presented here.

**Keywords:** UTC-PD, multicore fiber, photomixing, THz communications, 300 GHz band, transmitter array

## 1. INTRODUCTION

CISCO forecasts that the total number of mobile users will grow from 5.1 billion in 2018 to 5.7 billion by 2023 [1]. Thus, it is of paramount importance to increase the capacity of wireless transmission systems in order to cope with the increasing demand. Different solutions are envisioned for this purpose and one of them is the development of telecommunications systems based on THz technologies. Ranging from 100 GHz to 10 THz, the THz band has attracted increasing interest mainly because of the large bandwidth it offers, making it possible to target Tbit/s data rates. Such high rates are necessary for applications like holograms, augmented reality and vehicle-to-vehicle communications [2]. The so-called "300 GHz band" is a portion of the THz band between 252 and 321 GHz that is regulated by the IEEE 802.15.3d standard approved in 2017. This THz band is a good compromise between atmospheric absorption, technology performance and available bandwidth.

A few decades ago, the THz region, that lies between the microwaves and the optical domain in the electromagnetic spectrum, was designated as "THz gap" because there were no reliable and efficient transceivers capable of generating and detecting signals at these frequencies [3]. However, in the recent years, several THz link demonstrations have been realized. For the receiver side, an electronic approach has been mostly used (Schottky barrier diodes for direct detection, sub harmonic mixer for heterodyne detection). However, for the transmitter side, either a photonics-based (photomixers) or an electronics-based (III-V mixers, SiGe transistors, ...) approach has been considered [4]. Photomixing technique consists in the beating of two lightwaves $f_1$ and $f_2$ in a photodiode in order to generate a carrier wave $f_{THz} = |f_1 - f_2|$. Generally, UTC-PD (UniTraveling Carrier PhotoDiode) are preferred to PIN photodiodes as they provide more output power and wider bandwidth [5]. Moreover, photomixing is advantageous as it preserves the linearity of the signals over a large bandwidth and allows frequency agility (possibility to control the number of THz carriers by changing the number of laser sources). On the other hand, UTC-PD are still limited in terms of output power, which is crucial for long distance wireless transmission. Therefore, there are increasing investigations about techniques to increase THz power in photonics-based wireless links.

In this paper, an innovative transmitter that consists in the combination of a multicore fiber (MCF) with an UTC-PD array is proposed. This approach is expected to either increase 1) the generated THz power by N; N being the number of UTC-PD or 2) the number of optical channels which can be multiplexed over the same number of standard single-mode fiber (SSMF). Some of the components of the transmitter, namely an UTC-PD and a multicore fiber, were developed and characterized. The electrical performances of a single UTC-PD operating in the 290-350 GHz range are presented. Then, on-wafer data transmissions at data rates up to 100 Gbit/s were performed together with their respective BER curves.
Moreover, the characterization of an MCF adapted to the proper excitation of the UTC-PD array is presented as well as the cross-talk (XT) performances of the MCF equipped with fan-in fan-out devices.

## 2. SUB-SYSTEMS CHARACTERIZATION

### 2.1. Single UTC-PD

The fabricated UTC-PD is an InP/InGaAs-based device with an area of 4x4 µm² and a square shape mesa. Figure 1 shows a SEM view of the device. After the alignment marks definition, the photodiode mesa etching process that concerns part of the device topology (size) enables to access the top-contact layer (P). Two wet chemical selective etching solutions are used for InP and InGaAs. The mesa is protected during the wet etching by a SiO₂ hard mask. Using the same configuration process, SiO₂ and wet etching, is used for the mesa in order to isolate the devices from the InP substrate. The ohmic contacts, P and N, are done via the use of an e-beam bilayer resist and a lift-off process. The bottom contact (N) consists of Pd/Ti/Pt/Au and the top electrode (P) consists of 300 nm Au, both realized by e-beam evaporation. Coplanar waveguide (CPW) accesses are connected by air-bridges to the photodiode. A tri-layer resist scheme using selective resist development is used to define jointly the accesses and air-bridge in order to connect the photodiode to coplanar lines. Ti/Au (100-600 nm) metallization is deposited and lastly a lift-off process is used to realize the coplanar accesses and air-bridge.

<figure><img src="image_2.png" /><figcaption>Figure 1: Top of view of the complete device with the top electrode square configuration.</figcaption></figure>

The block diagram of the experimental setup used to characterize the UTC-PD is illustrated in Figure 2(a). Two Tunable Laser Sources (TLS) are combined and modulated by an I/Q modulator. Afterwards, two Erbium-Doped Fiber Amplifiers (EDFA) with an EVOA (Electronic Variable Optical Attenuator) in between make it possible to control the optical power illuminating the on-wafer UTC-PD which is coupled to a 300 GHz band CPW (CoPlanar Waveguide) probe. On the first hand, this setup enables to monitor the 300 GHz power level with respect to the photocurrent and the UTC-PD frequency response. In order to perform these measurements, the waveguide probe output is directly feeding a waveguide power-meter (PM5 Virginia Diodes). On the second hand, for communications measurements, the signal that is generated from the UTC-PD is processed at the receiver-end by integrating a Sub-Harmonic Mixer (SHM, 8-10 dB conversion losses) pumped by a multiplication chain and an amplifier (30 dB gain). The mixing of the modulated THz signal with the signal generated by the multiplication chain results in an Intermediate Frequency (IF) of 20 GHz which is driving a real-time oscilloscope. Offline processing is applied in order to display I/Q constellations and compute both Error Vector Magnitude (EVM) and Bit Error Rate (BER). All these measurements are performed in back-to-back configuration, i.e. the output of the probe is directly connected to the 300 GHz band receiver. Regarding the electrical characterization, the optimal reverse bias voltage for the UTC-PD operation is -1 V. For this value, the UTC-PD operates linearly over the power-range used, as illustrated by Figure 2(b).
<figure><img src="image_3.png" /><figcaption>Figure 2: (a) Block diagram of the experimental setup. (b) THz power versus photocurrent at 320 GHz and -1 V bias. (c) UTC-PD's frequency response between 290 and 350 GHz at 8 mA and -1 V bias.</figcaption></figure>

<figure><img src="image_4.png" /></figure>

As is illustrated in Figure 2(b) for a THz frequency of 320 GHz, the THz power (dBm) is linearly dependent on the photocurrent, confirming that UTC-PD is a square-law operation device, i.e. the output THz power is linearly proportional to $I^2$. The saturation occurs at $I=10.4$ mA (-1 V bias and 120.4 mW optical power) and results in a maximum THz power of -10.3 dBm. This saturation in UTC-PD is due to space charge effect. Figure 2(c) illustrates the frequency response of the UTC-PD measured by sweeping the carrier frequency from 290 GHz to 350 GHz with an 5 GHz step, an 8 mA photocurrent and -1 V bias. As expected, the power decreases when the frequency is increased like a low-pass filter response. This observed frequency dependence may cause linear distortions in the I/Q signal which can be corrected by a linear equalization. When the optical power is about 82.2 mW, the photocurrent is about 7.1 mA which implies that the sensitivity of the UTC-PD is about 0.086 A/W, close to the already-reported values of about 0.1 A/W [6]. Constellation diagrams and BER curves were measured for up to 100 Gbits/s data rate. Figure 3(b-d) present the constellations at 100, 80 and 50 Gbit/s for 16 QAM modulation format. Figure 3(a) shows the BER curve with respect to the photocurrent for each data rate. The results are promising because they are below FEC threshold ($\sim 4.10^{-3}$).

<figure><img src="image_5.png" /><figcaption>Figure 3: (a) BER curves obtained at 100, 80 and 50 Gbits/s. (b), (c) and (d) Constellations obtained at 320 GHz for a photocurrent of 8, 7 and 4 mA, respectively.</figcaption></figure>

As expected, for a given photocurrent, the higher the data rate, the higher the BER. Moreover, the BER decreases when the photocurrent is increased. However, for photocurrent values larger than 6 mA, the BER no longer improves because the overall system saturates. The fact that the BER curves recorded at different data rates do not have the same slope is attributed to the bandwidth limitations of the SHM for which the dispersion is larger when baud-rate (and associated bandwidth) is higher.

## 2.2. Multicore fiber

In this work, an array of 4 UTC-PD is proposed to be illuminated to demonstrate the THz multipath approach. To this end, two square arrays with a pitch of 15 µm and 20 µm haven been investigated, respectively. In order to reach an efficient optical feeding of all the UTC-PD of the array at the same time, two 4-core multicore fibers (MCF) were specifically manufactured with the same array configuration.

The fibers were obtained from a single preform made by MCVD (Modified Chemical Vapor Deposition) process and drawn into canes that were used for stack and draw, as is conventionally done to manufacture Photonics Crystal Fibers (PCF) [7]. The pitch between two cores was selected to match that of the UTC-PD array. For Fiber A, it is observed to be
about 15 µm as shown in the Scanning Electron Microscope (SEM) cross-section image in Figure 4(a). A 20 µm pitch was obtained for Fiber B. Each core of both fibers is single-mode at 1550 nm with a mode field diameter close to 4 µm at this wavelength. Fan-in fan-out fiber devices were manufactured and spliced on both sides of the MCF in order to selectively address each core using an SSMF. Besides, such a device also enables to address the THz band with optical signals coming from conventional fiber network. Because of the use of an MCF, it is necessary to pay attention to inter-cores coupling. Therefore, XT matrices were measured. Characterization was performed using a filtered ASE (Amplified Spontaneous Emission) source (wavelength 1550±1 nm) successively coupled, thanks to an optical switch, to the different SSMF input ports of the fan-in device and a power-meter that successively addresses the 4 output ports of the fan-out. During the measurements, the fibers were used with a bend radius of 6.5 cm. The fiber length was about 10 m. The results are reported in Figure 4(b). The fiber with the largest pitch (Fiber B) presents the smallest XT values. It can also be noticed that the lowest coupling rates in Figure 4(b) correspond to those between diagonal cores. The average insertion losses are about -2.1 dB and -1.1 dB for Fiber A and B, respectively.

a)

<figure><img src="image_6.png" /></figure>

b)

<table><caption>Figure 4. (a) SEM view of fiber A. (b) Table of inter-core coupling (extra diagonal elements) and insertion loss (diagonal elements) of fibers A and B spliced with fan-in/fan-out devices on both sides (values in dB).</caption><thead><tr><th rowspan="2">Input / Output</th><th colspan="4">Fiber A</th><th colspan="4">Fiber B</th></tr><tr><th>1</th><th>2</th><th>3</th><th>4</th><th>1</th><th>2</th><th>3</th><th>4</th></tr></thead><tbody><tr><th>1</th><td>-2.5</td><td>-10.3</td><td>-6.1</td><td>-17.6</td><td>-0.9</td><td>-60.3</td><td>-47.0</td><td>-72.3</td></tr><tr><th>2</th><td>-14.7</td><td>-1.8</td><td>-16.9</td><td>-8.8</td><td>-62.3</td><td>-1.1</td><td>-69.6</td><td>-29.5</td></tr><tr><th>3</th><td>-12.9</td><td>-17.8</td><td>-1.9</td><td>-7.7</td><td>-47.5</td><td>-67.4</td><td>-0.8</td><td>-35.5</td></tr><tr><th>4</th><td>-22.5</td><td>-11.1</td><td>-9.6</td><td>-2.1</td><td>-74.9</td><td>-31.9</td><td>-43.7</td><td>-1.4</td></tr></tbody></table>

### 3. CONCLUSION

Targeting future multi 100 Gbit/s THz links, a single UTC-PD does not provide enough power. Thus, the proposed approach tackles this issue with an innovative technique based on the power combining principle that consists in exciting at the same time an array of N UTC-PD by an N-core MCF. The total THz power is expected to increase by a factor of N if the same modulated signal is transmitted in all the cores, which would increase THz transmissions range. Alternatively, this configuration enables spatial multiplexing of data on N THz carriers to increase the data rate beyond 100 Gbit/s. Here, it was considered as a proof-of-concept to work with N = 4 with two UTC-PD array setups. The performances of a single UTC-PD operating at 320 GHz for up to 100 Gbit/s data rates with acceptable BERs have been reported with a maximum generated THz power of about -10.3 dBm. The crosstalk of the manufactured 4-core MCF is low enough over 1 m, typically less than -20 dB. The next step is the fabrication and characterization of the UTC-PD array for which these MCF were designed. This innovative approach consisting in combining an UTC-PD array and an MCF will enable to increase either the data rate up to 400 Gbit/s or the THz emission power four times compared to the state-of-the-art for extended
ranges of free space THz transmissions in the 300 GHz band. Also, using this approach, beam steering functionality can
be added to the system if suitable antenna structures are combined with the arrays.
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