Received: 22 August 2022

Revised: 7 October 2022

The Journal of Engineering

Accepted: 23 November 2022

DOI: 10.1049/tje2.12228

LETTER

Optical switch by direct modulated laser diode using fiber laser forpulse picking
Satoru Kaneko1
Yuna Hagiwara2

Manabu Yasui1
Masahito Kurouchi1
Masaru Nakamura2

1

Kanagawa Institute of Industrial Science andTechnology (KISTEC), Ebina, Kanagawa, Japan

2

Quark Technology Corp., Ibara, Okayama, Japan

Correspondence
Satoru Kaneko, Kanagawa Institute of Industrial
Science and Technology (KISTEC), Ebina,Kanagawa 243-0435 Japan.
Email: satoru@kistec.jpRobert York2Abstract
A new method of direct modulating a pre-amplifier laser diode as an optical switch isproposed. To modulate the laser diode with lower repetition rate, the repetition rate in thekilohertz region can be achieved using a source laser of 100 MHz without electro-opticalmodulators (EOM) or acousto-optical modulators (AOM). Using a direct modulated laser
diode, an ON/OFF ratio (extinction ratio) of 25 dB was demonstrated in this study.

Funding information
Amada Foundation, Grant/Award Number:
AF-2020227-B3; JKA Foundation, Grant/Award
Number: 2019M-1041INTRODUCTION

Ultrashort pulses can be advantageous for high efficiency andflexibility for laser system. A duration of 200 fs with peak powerof ∼2.5 kW has been reported [1, 2]. Since mode locking isused for the laser system, the repetition rate is given by thecavity length. The repetition rate of 100 MHz can be easily
obtained; however, it is difficult to produce the repetition rate
below 100 MHz with extra cavity length. A repetition rate in
the kilohertz region is required for a regenerative amplifier, or
fluorescence lifetime measurements, for example. In order toproduce the repetition rate in the kilohertz region, pulse pickers
such as electro-optical modulators (EO), acousto-optical (AO)
modulators or Mach–Zehnder modulators are often used [3–5].
However they require large size and high cost, and restrict thechoice of repetition rate.
Q-switch lasers, for example, have shown great successes
due to their simple and stable schemes. We previously proposed slower Q-switched YAG method to reduce its repetition
rate and to increase its power. The system was used as a laser
source for a pulsed laser deposition (PLD) system [6]. Only theQ-switch is modulated with lower repetition rate than the original repetition rate of flash lamp (slower Q-switched system)in order to obtain high energy fluency. Although the duration time became slightly wider (from 10 ns to 13 ns), thefluency becomes enough to produce an ablation on the target
in the system, and a variety of high quality epitaxial films [7–10]
including graphene [11, 12] and even nanoparticle [13] havebeen reported. In this presentation, we propose a new method
for pulse picking using a fiber laser with directly modulated
pre-amplifier laser diode as an optical switch. For underwatercommunication, for an example, the wavelength of 400–550 nm
is used; however, low power of laser and slow optical switching remains the optical communication speed less than a few
MHz. With the direct modulation method, optical communication of ∼100 MHz can be realized. An optical switch formode-locked operation uses a saturable absorber mirror (SAM)with a lifespan, and the reflectance of the element varies widely.
The direct modulation method can offer a long life and stabilityof performance.

2EXPERIMENTAL

An all polarization-maintaining (PM) fiber mode-locked laser
contains a passively mode-locked oscillator and a fiber preamplifer. The laser oscillator is based on a Fabry Perot (FP)cavity configuration with wavelength 1030 nm at repetition rateof 98 MHz. And an extended cavity length of approximately
210 cm produced 35 MHz from the oscillator. A PM fiber Bragg

This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided theoriginal work is properly cited, the use is non-commercial and no modifications or adaptations are made.
© 2023 The Authors. The Journal of Engineering published by John Wiley & Sons Ltd on behalf of The Institution of Engineering and Technology.
J. Eng. 2023;2023:e12228.
https://doi.org/10.1049/tje2.12228

wileyonlinelibrary.com/iet-joe

1 of 3

KANEKO ET AL.

2 of 3

FIGURE 1 Schematic of pulse picking laser system. (a) Regular pulse
picking system with optical switch and (b) direct modulating pumping LDsystem

grating (FBG) is used as the output mirror of the FP cavity,
and the FBG has a reflection peak wavelength of 1030 nm withbandwidth (FWHM, full width at half maximum) of 0.8 nm. The
1030 nm laser oscillator (OSC) used a 30 cm PM Yb-doped gainfiber powered by a 980 nm laser diode (LD) via a PM wavelength
division multiplexer (WDM). As an end reflector of the FP cavity, a saturable absorber mirror (SAM) directly butt-coupled wasused for self-started passive mode-locking of the fiber laser.
In order to reduce the repetition rate, some ratio of the laser
is directed out by optical PM coupler for producing a delayedgate signal for pulse picking. A fiber-based acoustic opticalmodulator (AOM), for example, is typically used for a pulse
picker with the delayed gate signal (Figure 1a). In this study,
the pumping LD was modulated by a controller with 50% duration of pulse width at 1 MHz without any optical switch such as
EO (Electro-Optical) or AO (Acousto Optical) modulator, as
shown in Figure 1b.

3

FIGURE 2 Detail of the pulse picking. A PIN photodiode (PIN-PD) wasused to detect laser pulse from Mode-locked Fiber Oscillator (MLFO, 300mW@100MHz) via beam splitter followed by a limiting amplifier (LIM-AMP)
for waveform shaping. The LIM-AMP was required for use of the frequency
divider comprised of Field-Programmable Gate Array (FPGA). The
Wavelength Division Multiplexer (WDM) multiplexed the original and thereduced carriers onto single fiber (3 mW@1MHz) followed by another AMP(300 mW@1MHz)RESULTS AND DISCUSSION

The 1030 nm laser generated by the oscillator showed a powerof 10 mW with a pulse width of 6 ps and with a frequency of98 MHz, pumped by a 980 nm laser diode (LD2 in Figure 1). In
this study, the modulated LD2 was used as an optical switch, asshown in Figure 1b.
Figure 2 shows the detail of the pulse picking system. A PIN
photodiode (PIN-PD) was used to detect a laser pulse from
the Mode-locked Fiber Oscillator (MLFO, 300 mW@100MHz)
via a beam splitter followed by a limiting amplifier (LIM-AMP)
for waveform shaping. The LIM-AMP was required for use of
the frequency divider comprised of Field-Programmable Gate
Array (FPGA). The FPGA generated a frequency of 1 MHz outof the original 98 MHz. With the optical delay adjustment, thelaser diode (LD) was stimulated by a LD driver with the thereduced source of 1 MHz, and the Wavelength Division Multiplexer (WDM) multiplexed the original and the reduced carriers
onto single fiber (3 mW@1MHz) followed by another AMP(300 mW@1MHz).

FIGURE 3 Extinction and amplified ratio with 1030 nm laser at 98 MHz.
The extinction ratio of 25 dB was achieved

One important factor for an optical switch is the ON/OFF
ratio (extinction ratio), which is expressed as the ratio of two
optical power levels of a digital signal generated by an opticalsource, for example, a laser diode. Typical extinction ratio forcommercial products using AO or EO modulations are ranging
from 15 to 45 dB [14, 15]. Figure 3 shows the extinction ratio
as a function of applied LD (LD2 in Figure 1b) current in thisstudy. Without using typical AO or EO modulator, an extinction
ratio of 25 dB was achieved by the direct modulated LD system.
The direct modulation method showed the practical extinction
rate could be produced without using AO or EO modulators.

KANEKO ET AL.

3 of 3and KISTEC (Kanagawa) Collaborative Research Program for
R&D Acceleration.
CONFLICT OF INTEREST
The authors have declared no conflict of interest.
DATA AVAILABILITY STATEMENT
Data available on request from the authors.
ORCID
Satoru Kaneko

https://orcid.org/0000-0003-0823-1666

REFERENCESFIGURE 4 Reduced repetition rate with an extra length cavity. The
repetition rate changed from 98 MHz to 35 MHz

With 98 MHz repetition rate, the 1030 nm laser modulated
by pre-amplifier (LD2) showed a slow rise profile and did notreach its maximum power of the source laser. In order to properly modulate the LD, the repetition rate of the source laser
was reduced with an extra cavity length employed in the OSC.
Figure 4 shows a reduced source laser repetition rate of 35 MHz
instead of 98 MHz with an additional 70 cm to the cavity length.
Using the reduced repetition rate of the source laser, LD modulated by pulse width can generate pulse width modulation withthe further low frequency. With an additional pre-amplifier, theoriginal power of 5 mW was amplified to 450 mW. Pulse widthmodulation as a burst mode is in progress.
In order to generate a short pulse, the mode-locked technique
is usually used for femto-second and pico-second pulsed laser,
and to pick up favorable repetition rate, changes of cavity lengthor pulse picking by EO or AO switch is usually used. The direct
picking can be a simple and low cost method to change therepetition rate without EO or AO switch.

4SUMMARY

We proposed a new method of direct modulated pre-amplifierlaser diode as an optical switch. The method directly modulated pre-amplifier with feedback of photodiode output fromsource laser, and showed the capability of signal generation withextinction ratio more than 25 dB.
AUTHOR CONTRIBUTIONS
Satoru Kaneko: Writing - original draft. Manabu Yasui: Methodology. Masahito Kurouchi: Methodology. Robert York: Investigation, methodology. Yuna Hagiwara: Methodology. Masaru
Nakamura: Conceptualization.
ACKNOWLEDGEMENTS
This study is partially supported by JKA founds (2019M-104)
from KEIRIN RACE, Amada foundation (AF-2020227-B3)

1. Schlauch, T., et al.: High peak power femtosecond pulses from modelocked
semiconductor laser in external cavity. Electronics Lett. 44(11), 678–679(2008)
2. Schlauch, T., et al.: Femtosecond passively modelocked diode laser withintracavity dispersion management. Opt. Expess 18(23), 24316–24324(2010)
3. van Hoek, A., Visser, A.J.W.G.: Pulse selection system with electro-optic
modulators applied to mode-locked cw lasers and time-resolved single
photon counting. Rev. Sci. Instrum. 52, 1199 (1981)
4. Balzer, J.C., et al.: Modelocked semiconductor laser system with pulse
picking for variable repetition rate. Electronics Lett. 47(25), 1387–1388(2011)
5. de Vries, O., et al.: Acousto-optic pulse picking scheme with carrierfrequency-to-pulse-repetition-rate synchronization. Opt. Expess 23(15),19586 (2015)
6. Kaneko, S., Shimizu, Y., Ohya, S.: Preparation of bisrcacuo multilayers byuse of slower q-switched 266 nm yag laser. Jpn. J. Appl. Phys. 40, 4870–4873 (2001)
7. Kaneko, S., et al.: Structural modulation on multilayered bismuth cuprate
observed by x-ray reciprocal space mapping. J. Appl. Phys. 97, 103904(2005)
8. Kaneko, S., et al.: Strain-amplified structural modulation in bi-cuprate hightc super superconductor. Phys. Rev. B 74, 054503 (2006)
9. Kaneko, S., et al.: Large constriction of lattice constant in epitaxial magnesium oxide thin film: Effect of point defects on lattice constant. J. Appl.
Phys. 107(7), 073523.1–073523.3 (2010)
10. Kaneko, S., et al.: Crystal orientation of epitaxial oxide film on siliconsubstrate. Appl. Surf. Sci. 586, 152776–1–5 (2022)
11. Kaneko, S., et al.: Layer-by-layer growth of graphene on insulator in co2oxidizing environment. ACS Omega 2, 1523–1528 (2017)
12. Kaneko, S., et al.: Carbon clusters on substrate surface for graphene
growth - theoretical and experimental approach. Sci. Rep. 12, 15809 (2022)
13. Kaneko, S., et al.: Optimizing coverage of metal oxide nanoparticle prepared by pulsed laser deposition on nonenzymatic glucose detection.
Talanta 84, 579–582 (2011)
14. Wan, P., et al.: Pulse shaping fiber lasers for free-space and lidar application.
Proc. SPIE 7817, 7810K–1 (2010)
15. Ruske, J.P., et al.: Integrated optical ultrashort-pulse picker with highextinction ratio. Electronics Lett. 39(20), 1442 (2003)

How to cite this article: Kaneko, S., Yasui, M.,
Kurouchi, M., York, R., Hagiwara, Y., Nakamura, M.:
Optical switch by direct modulated laser diode using
fiber laser for pulse picking. J. Eng. 2023, e12228 (2023).
https://doi.org/10.1049/tje2.12228