Continuous Tunable Terahertz Wave Generation via a Novel CW

Optical Beat Laser Source

Muhammad A. Ummy

, Simeon Bikorimana

, Abdullah Hossain

and Roger Dorsinville

Department of Electrical & Telecommunications Engineering Technology, New York City College of Technology,

City University of New York, 300 Jay St, Brooklyn, NY 11201, U.S.A.

Department of Electrical Engineering, City College of New York, City University of New York,

160 Convent Avenue New York, NY 10031, U.S.A.

Keywords: Dual-wavelength, Beam Combining, Power Scalability, Sagnac Interferometer, Laser Tuning, Ring Laser,

Semiconductor Optical Amplifiers.

Abstract: A novel technique of generating two colors or dual-wavelength in a fiber hybrid compound-ring resonator is

discussed. Generation of continuous-wave terahertz radiation is demonstrated by using a dual-wavelength

widely tunable C-band SOA-based fiber compound-ring laser as a light source, which excites a continuous-

wave terahertz photomixer operating at 1.55 µm telecom optical wavelengths. The proposed dual-

wavelength fiber laser has a hybrid compound-ring resonator structure and external reflectors that allow

output power upscaling and single or dual-output port operation, respectively. Wavelength selection and

continuous tunability are achieved by a widely tunable optical filter sandwiched between two fiber-Bragg

grating filters of similar Bragg center wavelength. The difference wavelength tuning range of 20.42 nm (i.e.,

2.51 THz) is demonstrated in the C-band. Continuous-wave terahertz radiation with continuous tunability

between 0.8 and 2.51 THz at room temperature using only a fiber laser source is achieved via photomixing.

1 INTRODUCTION

The development of efficient terahertz (THz)

systems using a combination of electronic and

optical technologies is an ongoing and important

research topic. THz waves have potential

applications in sensing and imaging because

different materials have highly distinguishable

spectral fingerprints due to their enhanced molecular

and atomic rotational and vibrational resonances in

the terahertz frequency band. Because the THz band

has low photon energy levels of 0.41 and 41 meV at

frequencies of 0.1 THz and 10 THz, respectively, it

is not as harmful as X-ray. Furthermore, compared

to X-ray radiation, terahertz waves are more suited

for high-resolution sensing and imaging. Due to its

non-ionizing nature, it can be exploited in various

fields and applications, such as biomedical (Pickwell

et al., 2006) and security screenings (Karpowicz et

Al., 2005). The THz band can penetrate different

materials (Tonouchi, 2007) and hence the widely

ranging applications in food, drug and quality

control of semiconductor (Hu et al., 1995);

(Yamashita et al., 2005).

Generation of THz radiation via laser sources

can be classified into two categories: pulsed and

CW-THz systems. Pulsed THz systems are more

complex than that of their CW THz counterparts.

Agility, high scanning speed, high resolution, and

simplicity of data processing (Karpowicz et al.,

2005; Mickan et al., 2000) are other attributes of the

CW terahertz generating system. Typically, the CW-

THz system uses two CW laser sources; one with a

fixed wavelength and another with a tunable

wavelength. In such systems, CW-THz radiation is

emitted when the two laser sources with slightly

different wavelengths (i.e., beat signal) combine

either in a THz- photomixer or a nonlinear optical

crystal. The difference wavelength can be tuned

over a range of several nanometers that covers a

range of frequencies in the terahertz gap (Preu et al.,

2011). Some of the CW laser sources used for

driving the THz-photomixer are distributed feedback

(DFB) laser diodes (Kim et al., 2009), QCLs

(Kumar, 2011), and group III-V lasers (Fischer et

al., 2009). Because such systems require at least two

laser sources, the experimental setups are relatively

costly.

Although the approach of combining two

independent CW laser sources has facilitated

significant advancements of CW-terahertz systems,

they still suffer from the negative effects of noise

mixing from both CW laser sources and frequency

instability which affects the beat frequency signal

generated by the photomixing of two slightly

different wavelengths. One of the noise reduction

Ummy, M., Bikorimana, S., Hossain, A. and Dorsinville, R.

Continuous Tunable Terahertz Wave Generation via a Novel CW Optical Beat Laser Source .

DOI: 10.5220/0006622800670073

In Proceedings of the 6th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2018), pages 67-73

ISBN: 978-989-758-286-8

techniques is to employ external modulators

(Frankel et al., 1998) which mitigates the

uncorrelated noises from mixing two independent

CW lasers. Hence, a single laser source with dual-

wavelength, dual-frequency, and dual-multimode

operation has been the best solution to avoid the

implementation of external systems that are used for

reducing noise and frequency drift in CW-terahertz

systems (Gu et al., 1999; Kim et al., 2011;

Morikawa et al., 1999; Belkin et al., 2008).

Most of the current fiber-based laser systems for

CW-terahertz photomixing remain large, inefficient,

and expensive. To achieve sufficient pump power to

excite a photomixer, large erbium-doped fiber

amplifiers (EDFAs) are typically used in linear

cavities or single-ring cavities. However,

semiconductor optical amplifier (SOA)-based laser

systems are more attractive than rare-earth-doped

fiber laser systems for several reasons; the main one

is the SOA’s low cost, footprint, and high efficiency.

In addition to high resolution and high scanning

speed properties of CW-terahertz systems,

continuous-wave laser sources typically are smaller

in size and less expensive (Shibuya et al., 2007).

High availability of fiber-based optical components

in the telecom-based wavelengths of fiber

communications has led to cost and size reductions,

as well as increased efficiency of CW-terahertz

systems (Carpintero et al., 2015). Current CW fiber

laser systems consist of a single output port with

fixed optical power. However, the output power

scalability and widely continuous wavelength

tunability properties of a dual-wavelength fiber laser

source are of great interest in developing user-

friendly and efficient CW THz systems.

We propose using the passive beam combining

method in compound-ring resonators as

demonstrated recently (Ummy et al., 2017) with

dual-wavelength instead of single wavelength

operation. The use of nested compound-ring cavities

to split circulating beams equally into N-number of

low power beams for amplification by N-number of

low power SOAs leads to a highly efficient and

high-power laser system that eliminates extra pump

lasers, and other expensive external high power

optical components. Typically, the generation of

tunable dual-wavelengths at telecom optical

wavelengths in a single laser source is achieved by

using a photonic crystal fiber (Soltanian et al.,

2015), FBG filters (Dong et al., 2016), a Fabry-Perot

filter in conjunction with an optical band-pass filter

(Pan et al., 2008), a fiber double-ring filter (Fan et

al., 2016) and an array waveguide grating (Ahmad et

al., 2012). Most of the aforementioned methods

require the use of circulators. Moreover, AWGs are

not continuously tunable (i.e., fixed wavelength

channel separation) and their tuning range is limited

to around 12 nm range. Thus, in the C-band, three

different laser sources are used to achieve a wide

tuning range of CW-THz radiation without any gaps

(Deninger et al., 2014).

In this work, we demonstrate a novel technique

of dual-wavelength selection with continuous

tunability over the C-band of 20.42 nm at room

temperature, which has the potential of generating

widely tunable CW-THz radiation via photomixing.

Furthermore, we explore the coherent beam

combining method based on the passive phase-

locking mechanism (Bruesselbach et al., 2005) of

two C-band low power SOAs-based all-single-mode

fiber hybrid compound-ring resonator by exploiting

beam combining (i.e., interference) at 3dB fiber

couplers that connect two parallel nested ring

cavities. As opposed to using multiple laser sources

(i.e., three CW laser sources) we achieve a wide

tuning range of CW-THz radiation by using single

laser source. The proposed dual-wavelength fiber

laser source is used as a single source to excite a

CW-THz photomixer where CW-THz radiation is

generated and detected using a CW-THz photomixer

and pyroelectric based THz sensor. We had

successfully generated THz radiation from 0.8 and

2.51 THz at room temperature.

2 EXPERIMENTAL SETUP

Fig. 1 illustrates the experimental setup of the C-

band SOA-based tunable fiber laser with two nested

ring cavities (i.e., hybrid compound-ring resonator)

and two broadband SLMs that can serve as either

dual-output ports or a single output port depending

on the reflectivity of each SLM. Each ring cavity is

comprised of two branches: I-II and I-III, for the

inner and the outer ring cavity, respectively. Both

ring cavities share a common branch, I, which

contains an SOA, SOA

(Kamelian, OPA-20-N-C-

SU), a tunable optical filter (TF-11-11-1520/1570)

sandwiched between two similar FBGs, and a

polarization controller, PC

. Branch II contains

SOA

(Thorlabs, S1013S), and a polarization

controller, PC2. Due to the lack of availability of a

third SOA, branch III only contains a polarization

controller, PC

. As Fig. 1 portrays, all branches are

connected by two 3dB fiber couplers, C

and C

which are connected to SLM

and SLM

correspondingly. Each SLM, SLM

and SLM

, in

conjunction with a PC (PC

and PC

, respectively)

PHOTOPTICS 2018 - 6th International Conference on Photonics, Optics and Laser Technology

acts as a variable optical reflector. By adjusting PC

or PC

, one can manipulate the reflectivity of SLM

and SLM

, respectively, and switch between single

and dual-output port configurations.

Figure 1: Experimental setup of the dual Sagnac loop

mirror dual-wavelength SOA-based tunable fiber hybrid

compound-ring laser.

Sandwiching the low power tunable optical filter

(TF) in the common branch, I, between two similar

FGBs centered at 1551.98 nm, one of which is fixed

at 1551.98 nm and the other which is tunable up to

1572.42 nm, allows for dual wavelength selection.

The three PCs (PC

, PC

and PC

) control the state

of polarization of the light circulating within the

compound ring cavity.

3 PRINCIPLE OF OPERATION

Generation of a continuously tunable dual

wavelength (i.e., two colors) was achieved in a

hybrid compound-ring resonator using two fiber

Bragg grating filters, FBG

and FBG

of the same

Bragg wavelength (i.e., λ

FBG1

=λ

FBG2

) and a widely

tunable optical filter, TF, of transmittance spectra.

The fixed wavelength is selected by the FBGs, and

the tunable wavelength, λ

, is selected by the TF.

The wavelength selection is performed in the

common branch, I, of the hybrid compound-ring

cavity.

The principle of operation of the proposed fiber

laser is as follows: assume that both semiconductor

optical amplifiers (SOAs) are driven above the

threshold bias current level, and the reflectivity of

each output coupler formed by a Sagnac loop mirror

(i.e., SLM

and SLM

) is adjusted to ≤0.1%. When

the pump level (i.e., bias current level) of either

SOA is more than the total fiber compound-ring

cavity losses, amplified spontaneous emission (ASE)

emitted from the SOAs propagates in the forward

and backward directions. For instance, when a bias

current, I

, of approximately 75 mA is injected into

SOA

(branch I), the emitted ASE emitted by SOA

circulates in a clockwise (cw) direction. The

clockwise propagating ASE reaches the FBG

filter,

which reflects a fixed wavelength, λ

FBG1

, back into

SOA

while the remaining ASE signal propagates

through the tunable filter, TF, which selects a

tunable wavelength λ

and rejects the rest of the

ASE spectrum. The selected tunable wavelength,

, is different from the Bragg wavelength of the

FBGs. Thus, the selected beam with tunable

wavelength, λ

, passes through the FBG

filter and

polarization controller PC

before it reaches port 1

of the 3-dB fiber coupler, C

, where it is equally

split (i.e., 50% goes to port 2 and port 3,

respectively) and is coupled into branches II and III

of the fiber compound-ring cavity. Half of the

selected beam that propagates into branch II passes

through polarization controller PC

and is amplified

by SOA

(i.e., when its bias current level is above

180 mA) before it arrives at port 2 of the 3-dB fiber

coupler C

, where the amplified signal is also

equally split between port 1 and 4 after being

combined with the beam at port 3 that propagates

through branch III. Half of the selected beam at port

4 of the 3-dB fiber coupler C

is fed into the output

coupler, SLM

. As the reflectivity of SLM

is set at

≤0.1%, the selected beam with λ

exits at port 1

(i.e., OUT1) of the 3-dB fiber coupler, C

. The other

50% of the selected beam coupled into port 1 of the

3-dB fiber coupler C

is further amplified by SOA

Therefore, this closes the ring structure, completes a

round trip in the clockwise direction and allows

lasing to occur at tunable wavelength λ

As mentioned earlier, the wavelength, λ

FBG1

selected by the FBG

is reflected back and

propagates in the counter-clockwise direction

through SOA

, where it is amplified and is equally

split by the 3-dB fiber coupler C

and is coupled

(i.e., 50% each) into branches II and III of the fiber

compound-ring cavity. 50% of the selected beam

with fixed wavelength, λ

FBG1

, propagates in branch

II and is further amplified by SOA

, while the other

50% propagates through branch III. Both light

beams propagate through polarization controllers,

, and PC

, respectively, before being combined

and equally split at ports 1 and 4 of the 3-dB fiber

coupler C

. Half of the selected beam at port 4 of the

3-dB fiber coupler C

is fed into the other output

coupler, SLM

, which then exits from port 1 (i.e.,

OUT2) of the 3-dB fiber coupler, C

. The other 50%

of the beam with the selected wavelength, λ

FBG1

, that

is coupled into branch I reaches the other fiber

Bragg grating filter, FBG

, and reflected back

toward the 3-dB fiber coupler, C

. There, it is

Continuous Tunable Terahertz Wave Generation via a Novel CW Optical Beat Laser Source

equally split and is respectively coupled into

branches II and III for further amplification which

leads to lasing of the fixed Bragg wavelength, λ

FBG,

after it traces its round trip back to the FBG

filter

while going through further amplification by SOA

and SOA

Note that as there is no optical isolator, the same

wavelength selection of λ

and λ

FBG1

occurs from

the counter-clockwise propagating ASE, where the

selected wavelengths λ

and λ

FBG1

circulate in the

counter-clockwise and clockwise directions and exit

at the output couplers, SLM

and SLM

respectively. Thus, two lasing wavelengths (i.e.,

tunable λ

and fixed λ

FBG

) coexist in the fiber

hybrid compound-ring cavity, and they are extracted

at both output couplers, OUT1 and OUT2. If the

reflectivity of output coupler SLM

is set to

maximum (i.e., ≥ 99.9%), then the light beam of

dual wavelength exits from the output coupler

SLM

, or vice versa. The wavelength separation

(i.e., Δλ

THz

) is controlled by continuously adjusting

the tunable filter, TF. An optical spectrum analyzer

(OSA), variable optical attenuator (VOA) and

optical power meter (PM) were used to characterize

the proposed fiber hybrid compound-ring laser. Note

that the path lengths of both loops are almost the

same since all branches have identical length and all

fiber connections utilize FC/APC connectors.

4 DUAL-WAVELENGTH

TUNABILITY AND POWER

STABILITY

We first set the bias currents for SOA

and SOA

200 and 500 mA, respectively. The reflectivity of

SLM

and SLM

were set and kept constant at ≤

0.1% and ≥ 99.9%, respectively. Then, the dual-

wavelength signal of the output light beam was

measured with an OSA. The wavelength separation

(i.e., wavelength beat signal) was tuned by manually

adjusting the tunable filter, from 1554.98 nm to

1572.33nm while simultaneously optimizing the

polarization controllers, PC

, PC

, and PC

, at each

wavelength. Moreover, as mentioned, the fixed

wavelength was selected by two FBG filters, which

are centered at 1551.98 nm. Fig. 2 shows

wavelength separation (i.e., beat signal: Δλ

THz

= λ

) of 3 nm, and 20.35 nm, which corresponds to

CW-THz beat frequencies (i.e., Δʋ

THz

= c*(Δλ

THz

(λ

*λ

))) of 0.37 and 2.5 THz, respectively.

Figure 2: Illustrates the wavelength spectrum of the dual-

wavelength fiber hybrid compound-ring laser. (a) and (b)

show wavelength separation between the fixed and tunable

wavelength of 3 nm and 20.35 nm, respectively.

The peak signals deducted from the measured

output wavelength spectra by using an OSA (e.g.,

Fig.2) were used to determine the optical signal-to-

noise ratio (OSNR). We subtracted the peak power

value at each center wavelength from the

background noise level of each wavelength

spectrum. The OSNR for the fixed wavelength and

tunable wavelength remained well above +50 dB

and +45 dB over the whole wavelength tuning range,

respectively.

We performed a short-term optical power

stability test at room temperature with SOA

, and

SOA

set at the standard bias current levels of 200

and 500 mA, respectively. The optical power

stability test was carried out over a total duration of

180 minutes with three-minute intervals and an OSA

resolution bandwidth of 0.01 nm. Fig.3 demonstrates

that the proposed fiber hybrid compound-ring laser

whose power fluctuations were within ±0.025 dB

and could have been further reduced by the proper

packaging of the system to make it more stable.

PHOTOPTICS 2018 - 6th International Conference on Photonics, Optics and Laser Technology

Figure 3: Shows the output power short-term fluctuations

measured over three hours at room temperature.

4.1 Generation and Detection of

CW-THz Radiation

The proposed tunable fiber laser was used as a light

source to excite an off-the-shelf photo-mixer (CW-

TH based Indium Gallium Arsenide (InGaAs)) as an

emitter and a pyroelectric-based terahertz sensor as a

detector. The experimental setup, which is used for

generating and detecting CW-terahertz radiation is

shown in Fig.4. The bias current of SOA

and SOA

was kept constant at 250 and 500 mA, respectively.

The output power of the laser source was controlled

by adjusting polarization controller PC

which was

placed in SLM

. We adjusted the PC so that the

output power does not exceed 10 mW. A wideband

1x2 single-mode fiber optic coupler with split ratio

of 95% and 5% was connected to the output coupler,

OUT1, of the laser system. The 95% output port was

connected to the CW-THz InGaAs emitter (TX).

The 5% output port was connected to an inline

power meter, PM, which was used to monitor the

excitation power. An optical spectrum analyzer was

employed to monitor the output spectrum of the

dual-wavelength laser. The tunable filter was

manually adjusted in steps of 0.1 nm. Note that an

automated continuously tunable filter can be used to

achieve an even higher wavelength tuning

resolution. The optical beat signal from the laser

source was delivered on the terahertz InGaAs-

photomixer through a single-mode fiber and

FC/APC.

Figure 4: (a) Pictorial view of the experimental setup for

CW-terahertz radiation measurements (TX: InGaAs-

photomixer and RX: pyroelectric THz sensor) and (b) its

schematic diagram.

Most of the terahertz radiation generated by the

antenna was radiated through the Indium Phosphide

(InP) substrate. Thus, a silicon lens was used to

couple the radiation into free space. An off-axis

parabolic mirror, OPM, was used to collect and

collimate the CW-terahertz signal from the THz

emitter. A polytetrafluoroethylene (i.e., Teflon) lens

was used to focus the CW-terahertz signal onto the

pyroelectric-based terahertz detector, which was

calibrated from 0.8 to 30 THz (Gentec-EO, Inc). An

optical chopper system, which provided a reference

signal of 25.7 Hz chopping frequency to the detector

module, was employed to increase the signal-to-

noise ratio.

A DC voltage source was used to bias the

terahertz InGaAs-photomixer at -1.4 V to increase

the terahertz radiation emitted by the photomixer.

The utilized CW-THz photomixer has a carrier

lifetime of about 0.3 ps and bandwidth of around 3

Continuous Tunable Terahertz Wave Generation via a Novel CW Optical Beat Laser Source

THz (Globisch et al., 2016). The theoretical

bandwidth curve was obtained by using Eq.1 below

(Carpintero et al., 2015),

 

()

THz









(1)

where A is a constant and τ is the photo-carrier

lifetime of photo-induced free-charges.

The CW-terahertz generation ranges from 0.875

to 2.51 THz when the tunable filter is tuned from

1554.98 nm to 1572.33 nm with a filter step size of

0.1 nm. This corresponds to around 1.2 GHz in the

C-band.

Figure 5: Frequency spectrum of CW-terahertz radiation

measured by a pyroelectric terahertz detector and

theoretical curve fit with A = 2 μW and τ = 0.3 ps.

The maximum measured average power was

around 350 nW around 1 THz as shown in Fig.5.

The measured power dropped around 80 nW above

1.5 THz, which is in agreement with the

specifications of the utilized CW-terahertz

photomixer (TOPTICA Photonics, Inc).

5 CONCLUSIONS

We successfully demonstrated the application of the

proposed dual-wavelength C-band SOA- based fiber

hybrid compound-ring laser for the generation of

continuous-wave terahertz radiation. The proposed

fiber laser was used to excite a terahertz InGaAs

photomixer. The CW-THz radiation emitted by the

InGaAs-photomixer was detected by a pyroelectric

terahertz sensor. The largest tuning range, Δλ

THz

, of

20.42 nm, which corresponds to a CW-THz beat

signal (i.e., Δʋ

THz

) of 2.51 THz, was achieved from

the proposed fiber laser source. Continuous

wavelength tuning was achieved at room

temperature from a single light source, unlike laser

systems that require several light sources to achieve

the same wavelength tuning range. By using an NxN

hybrid compound-ring structure with multiple low

power N-number of SOAs or the proposed fiber

laser source to provide a seed signal to a high gain

semiconductor optical amplifier and using different

pairs of FBG filters centered at approximately 1520

nm, one can achieve approximately a 50-nm

wavelength tuning range. Such a type of fiber laser

system can excite an electro-optic crystal (Soltanian

et al., 2015) to generate beat signals, Δʋ

THz

, that

can reach approximately 6THz. To the best of our

knowledge, this level of tuning range is far beyond

the range of the current commercially available CW

THz sources and uses low power optical

components.

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