PHOTONIC GENERATION OF TERAHERTZ WAVES

FOR COMMUNICATIONS AND SENSING

Tadao Nagatsuma

Graduate School of Engineering Science, Osaka University,

1-3 Machikaneyma, Toyonaka 560-8531, Japan

nagatuma@ee.es.osaka-u.ac.jp

Keywords: Terahertz, Photonics, signal generation, communication, sensing

Abstract: This paper describes how effectively the photonic signal generation schemes are employed to enhance the

performance of terahertz-wave systems such as wireless communication, spectroscopy, and tomographic

imaging.

1 INTRODUCTION

Research on exploring terahertz (THz) waves, which

cover the frequency range from 100 GHz to 10 THz,

have lately increased since the nature of these

electromagnetic waves is suited to spectroscopic

sensing as well as to ultra-broadband wireless

communications (Nagatsuma, 2009, 2011). One of

the obstacles to developing applications of THz

waves is a lack of solid-state signal sources.

For the generation of THz waves, photonic

techniques are considered to be superior to

conventional techniques based on electronic devices

with respect to wide frequency bandwidth,

tunability, and stability. Moreover, the use of optical

fiber cables enables us to distribute high-frequency

(RF) signals over long distances instead of metallic

transmission media such as coaxial cables and

hollow waveguides.

In this scheme, optical-to-electrical (O-E)

converters, or “photodiodes”, which operate at long

optical wavelengths (1.3-1.55 μm), play a key role,

and high output current is required in addition to

large bandwidth for practical applications. Recent

progress of high-power photodiode technology has

accelerated the development of THz-wave system

applications (Nagatsuma et al., 2009).

In this paper, we will describe how the photonics

technologies are employed in THz-wave systems,

showing some of our recent applications, in

particular based on based on “continuous wave

(CW)” signals, such as wireless communications,

spectroscopy and imaging.

Optical Signal

Source

(Pulse/CW)

Optical-

to-THz

Convertor

Antenna,

Lens,

Prism

Nonlinear Optical Material

Photoconductor

Photodiode

CW (λ

, λ

)

Figure 1: Schematic diagram of photonic continuous-wave

THz signal generation.

2 GENERATION AND

DETECTION OF CONTINUOUS

WAVES

In this section, we briefly review schemes for

photonic generation and detection of continuous-

wave (CW) THz waves.

2.1 Signal Generation

Figure 1 shows a schematic of CW THz-signal

generation based on optical-to-terahertz conversion.

First, intensity-modulated optical signals, whose

envelope is sinusoidal at a designated THz

frequency, are generated with use of light waves at

different wavelengths, λ

and λ

. Then, these two-

wavelength of lights are injected to conversion

media such as nonlinear optical materials (EO),

photoconductors (PC), and photodiodes (PD), which

Nagatsuma T.

PHOTONIC GENERATION OF TERAHERTZ WAVES FOR COMMUNICATIONS AND SENSING.

DOI: 10.5220/0004785000430048

In Proceedings of the Second International Conference on Telecommunications and Remote Sensing (ICTRS 2013), pages 43-48

ISBN: 978-989-8565-57-0

leads to the generation of THz waves at a frequency

given by

= cΔλ/λ

(1)

where Δλ is a difference in wavelength of lights, and

c is a velocity of light. The converted signals are

finally radiated into free space by an antenna, a lens,

etc.

Laser Diode

Coupler

(Combiner)

)

-f

Wavelength

Optical Frequency

Comb Generator

Optical

Filter

(a)

(b)

Wavelength

Optical Noise Source

(ASE Noise)

Optical

Filter

Optical

Filter

(c)

Figure 2: Schematic diagram of optical signal sources.

Typical optical signal sources are depicted in

Figs. 2(a) and 2(b); an optical heterodyning

technique using two frequency-tunable laser diodes,

and using the combination of an optical frequency

comb generator and filters, respectively. In the latter

case, two wavelength lights are filtered from the

optical filters, and this offers us with both wide-

band frequency tunability and excellent stability.

When so narrow linewidth or frequency resolution in

the spectroscopy and/or imaging is not required, we

can use two wavelength light filtered from the

optical noise source such as ASE noise in optical

amplifiers as shown in Fig. 2(c).

p-doped

Absorption Layer

Un-doped

Collection

Layer

n-contact

Layer

p-contact

Layer

Diffusion Block Layer

(C.B.)

(V.B.)

Un-doped

Absorption Layer

(a)

100

120

260 300 340 380 420

6 mA

10 mA

Frequency (GHz)

Detected Power (μW)

140 GHz

270

410

(b)

Figure 3: (a) Block diagram of the modified UTC-PD. (b)

Output power characteristics.

Among above-memtioned three types of optical-

to-electrical conversion media, the photodiode is

highly advantageous with respect the conversion

efficiency. In addition to the operation at long

optical wavelengths (1.3-1.55 μm), large bandwidth

and high output current are required for practical

applications. Among various types of long-

wavelength photodiode technologies, a uni-

traveling-carrier photodiode (UTC-PD) and its

derivatives have exhibited the highest output powers

at frequencies from 100 GHz to 1 THz, with

improvement in layer and device structures

(Nagatsuma et al., 2009).

Figure 3(a) shows the band diagram of the

photodiode optimized for the operation at 300-400

GHz, which is a modification of the UTC-PD. The

photodiode chip was packaged into the module with

a rectangular waveguide (WR-3) output port

(Wakatsuki et al., 2008). Figure 3(b) shows the

frequency dependence of the output power generated

from the module. The 3-dB bandwidth is 140 GHz

(from 270 to 410 GHz), which corresponds to the

maximum bit rate of 90 Gbit/s in the case of ASK

modulation. The peak output power was 110 μW at

380 GHz for a photocurrent of 10 mA with a bias

voltage of 1.1 V. The output power could be further

increased to over 500 μW by increasing the

photocurrent up to 20 mA with responsivity of 0.22

A/W.

Second International Conference on Telecommunications and Remote Sensing

To increase the output power to more than 1 mW,

one of the practical approaches is a power-

combining technique using an array of photodiodes.

With two photodiodes, the output power of >1 mW

has been obtained at the photocurrent of 18 mA per

photodiode at 300 GHz (Song et al., 2012).

At frequencies of over 300 GHz extending to 1

THz or higher, an antenna-integrated photodiode is

more efficient, and semi-spherical silicon lens is

often used to collimate a beam radiated from a

planar antenna such as bow-tie, log-periodic and

dipole antennas (Ito et al., 2005).

2.2 Signal Detection

As for detectors, there are several choices in the THz

regions, such as “direct detection” using Schottky

barrier diodes (SBDs) or bolometers and

“heterodyne detection” by combining mixers and

local oscillators (Fig. 4). There are electronic and

photonic mixers as well as electronic and photonic

local oscillators. We choose the best one depending

on required performance in each application.

Antenna

Diode

Bolometer

Antenna

Electronic

Mixer

LO Signal

Antenna

Electronic

Mixer

Antenna

(Lens)

Photonic

Mixer

Optical

LO Signal

SBD/Bolometer

SBD/SIS

PC/EO/PD

(a)

(b)

O-E

Converter

Optical

LO Signal

SBD/Bolometer

Electrical (E)

Optical(O)

E, O

Figure 4: Configurations of THz detection system. O:

optical signal, E: electrical signal.

3 APPLICATIONS TO

COMMUNICATIONS AND

SENSING

In this section, we present our recent applications

based on based on CW THz signals, such as wireless

communications, spectroscopy and imaging.

3.1 Wireless Communication

Figure 5 illustrates an example of the application

schemes with photonics-based approach, showing

how the photonic RF signal generation can be

employed together with fiber-optic links (Kleine-

Ostmann and Nagatsuma, 2011; H.-J. Song and

Nagatsuma, 2011). In addition to the wired link

using the light wave at a wavelength of λ

, we can

transmit the same data with the wireless link by

introducing another wavelength (λ

) of light wave

and mixing the two wavelengths of light in the RF

photodiode. The photodiode generates RF signals,

whose frequency can be determined the difference in

the wavelength of the two light waves, which is

given by eq. (1).

Laser

(1)

Laser

(2)

Optical

Modulator

Fiber-optic Link

Photodiode

Receiver

Base-band

Photodiode

Data

ｆ

Data

+ λ

Wireless Link

Figure 5: System concept of wired and wireless

convergence.

Figure 6: Photo of an experimemtal setup for the wireless

link.

Figure 7: Eye diagrams at 30 Gbit/s and 40 Gbit/s (b), and

bit error rate (BER) characteristics at 30 Gbit/s with a

carrier frequency of 300 GHz. Photocurrent (horizontal

axis of (c)) is proportional to the square root of the

transmitted power.

Photonic Generation of Terahertz Waves for Communications and Sensing

As for the receiver, we can use a simple diode

such as a Schottky-barrier diode (SBD) for the

demodulation based on the square-law detection in

the case of the ASK (OOK) data format. Thus, the

receiver becomes more cost-effective and energy-

efficient, if we can make use of a wide bandwidth

lying over the THz frequency region.

Figure 7 shows transmission characteristics at

300 GHz. For the generation of 300-GHz THz

waves, the wavelength difference in two lasers, Δλ,

was set to be 2.4 nm, and the optical signal is

converted to the RF (THz) signals by the UTC-PD.

THz waves are radiated from the horn antenna, and

dielectric lenses (2-inch diameter) are used to

collimate THz waves for both the transmitter and

receiver. The total antenna gain is about 40 dBi.

Transmission distance without significant decrease

in the received power was typically 0.5 – 1 m.

The performance limitation with respect to the

data rate is determined mainly by the bandwidth of

the UTC-PD in the transmitter and that of the SBD

detector in the receiver. The 3-dB bandwidth of the

UTC-PD is 140 GHz (from 270 to 410 GHz), which

corresponds to the maximum bit rate of 90 Gbit/s in

the case of ASK modulation. While the RF

bandwidth of the SBD detector also exceeds 100

GHz, IF (baseband) bandwidth for demodulated

signals in the receiver is about 20 GHz, which limits

the maximum bit rates. Figures 7(a) and 7(b) shows

eye diagrams demodulated by the receiver at 30

Gbit/s and 40 Gbit/s, respectively. From the bit error

rate (BER) characteristics, error-free (BER<10

-11

)

transmission has been achieved at 30 Gbit/s, which

is the highest data rate ever reported for “error-free”

wireless links.

Figure 8: Eye diagrams at the 600-GHz band receiver

obtained by changing the carrier frequency or the optical

wavelength difference Δλ from 450 GHz to 720 GHz at a

bit rate of 1.6 Gbit/s.

Much larger bandwidth can be ensured when the

carrier frequency can be shifted higher. By using

antenna-integrated THz UTC-PD module (Ito et al.,

2005) together with the 600-GHz band SBD detector

(WR-1.5 waveguide), we have increased the

available bandwidth of more than 250 GHz with a

single transmitter/receiver pair. Figure 8 shows

received and demodulated waveforms at carrier

frequencies from 450 GHz to 720 GHz. Clear eye

diagrams at 1.6 Gbit/s has been obtained, which

show error-free transmission over the extremely

large bandwidth of 270 GHz.

3.2 Spectroscopy

Recently, THz spectroscopy systems based on CW

technology, which use monochromatic sources with

an accurate frequency control capability, have

attracted great interest (Hisatake et al., 2013). The

CW source-based systems, referred to as frequency-

domain spectroscopy (FDS), provide a higher signal-

to-noise ratio (SNR) and spectral resolution. When

the frequency band of interest is targeted for the

specific absorption line of the objects being tested,

FDS systems with the selected frequency-scan

length and resolution are more practical in terms of

data acquisition time as well as system cost.

Figure 9: Experimental setup for the frequency-domain

spectroscopy. LD: laser diode, EDFA: Erbium-doped fiber

amplifier, FS: electro-optic frequency shifter.

THz-FDS system with photonic emitters and

detectors is frequently called a homodyne or self-

heterodyne system. Figure 9 shows a setup for the

THz-FDS system using the UTC-PD for the emitter

and a low-temperature-grown (LTG) GaAs

photoconductor for the detector. The optical

frequency/phase shifter (FS) enables us to accurately

measure both the amplitude and phase. Since two

Second International Conference on Telecommunications and Remote Sensing

laser diodes (LDs) are free-running, we monitor

each wavelength by the optical spectrum analyzer or

the wave meter. Figure 10 shows a photo of the

experimental setup for the spectroscpoy.

In order to check the frequency accuracy of the

system, we measured the absorption line of the water

vapor at 557 GHz as shown in Fig. 11. Currently, the

experimental standard deviation of the mean for the

absorption frequency is about 70 MHz.

Figure 10: Photo of an experimemtal setup for the

spectroscopy.

Figure 11: Measured absorption line of the water vapor at

557 GHz.

Optical

Amplifier

Optical

Modulator

UTC‐PD

Object

Pre‐

Amplifier

Lock‐in‐

Amplifier

BeamSplitter

ReferenceMirror

Movable

Transmitter Receiver

SBD

Terah er tzWaves

Figure 12: Block diagram of the tomography system using

the broadband THz noise signals.

Optical

Amplifier

UTC-PD

Wavelength

Tunable Laser

Wavelength

Fixed Laser

Optical

Modulator

(a)

(b)

(c)

Signal Power (a.u.)

0 5 10 15

Optical Path Length (mm)

Incident

Reflected

Figure 13: (a) Frequency-swept signal source. (b)

Reflection of THz waves at front-side (A) and back-side

(B). (c) Point spread function measured with the 600-GHz

band system showing the reflection points A and B of a

0.5-mm thick plastic plate.

3.3 Imaging

Figure 12 shows a block diagram of the tomographic

imaging system using a broadband terahertz noise

(incoherent signal) sources and a Mickelson

interferometer (Isogawa et al., 2012). This

configuration is similar to that of the optical

coherence tomography. The broadband noise signals

with sufficient power are generated by feeding the

amplified spontaneous emission (ASE) noise from

the Er-doped fiber amplifier to the photodiode.

In the setup, first, a THz wave travels to the

beam splitter after being collimated by a dielectric

lens. The beam splitter divides the THz wave into

two directions with a power ratio of 50/50. One goes

to the reference mirror and the other to an object

after being focused. Both reflected waves travel

back to the beam splitter again and go to the SBD as

a power detector. Finally, detected signals are

amplified by a preamplifier and a lock-in amplifier.

A personal computer (PC) controls positions of both

the reference mirror and the objective lens. The

noise bandwidth of 120 GHz centered at 350 GHz

determines the depth resolution of 1.2 mm (Isogawa

et al., 2012).

In place of the optical noise sources, the

combination of a fixed wavelength laser and a

wavelength tunable laser (Fig. 13(a)) enables us to

modify the tomography system from time-domain to

frequency-domain one (Ikeou et al., 2012). In this

scheme, depth-position information can be obtained

by Fourier-transforming the frequency-

(wavelength-) swept interference signals. Figure

Photonic Generation of Terahertz Waves for Communications and Sensing

13(c) shows the point spread function, which

corresponds to the position of the reflection points at

A (front-side) and B (back-side) for a plastic plate

with 0.5-mm thickness, when the 600-GHz band

system was used.

4 CONCLUSIONS

We have described system applications, which

efficiently take advantages of photonics-based ultra-

broadband signal generation techniques at over 100-

GHz frequencies. Use of optical fiber cables also

makes it easy to handle high-frequency signal

distribution or cabling in the instrumentation. These

features will not be replaced with electronic systems,

even though the operation frequency of electronic

devices is increasing up to the THz region.

ACKNOWLEDGEMENTS

The author wish to thank Drs H. -J. Song, K. Ajito,

N. Kukutsu, S. Kuwano, J. Terada, N. Yoshimoto,

and T. Ishibashi with NTT, S. Hisatake, M. Fujita, K.

S. Horiguchi, Y. Minamikata, T. Ikeou, H. Nishii

with Osaka University for their collaboration and

support. This work was supported in part by the

JST-ANR WITH program and by the Ministry of

Education, Science, Sports and Culture, Grant-in-

Aid for Scientific Research (A), 23246067, 2011.

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Second International Conference on Telecommunications and Remote Sensing