Multi-wavelength Erbium-doped Fiber Laser with Tunable

Wavelength Spacing

Xing Luo, Tong Hoang Tuan, Than Singh Saini, Hoa Phuoc Trung Nguyen,

Takenobu Suzuki and Yasutake Ohishi

Research Center for Advanced Photon Technology, Toyota Technological Institute,

2-12-1 Hisakata, Tempaku, Nagoya 468-8511, Japan

Keywords: Multi-wavelength, Erbium-doped Fiber Laser, Michelson Interferometer, Tunable Wavelength Spacing.

Abstract: We demonstrate a stable multi-wavelength ring cavity erbium-doped fiber laser in this paper. A fiber

Michelson interferometer is inserted into the ring laser cavity acting as a multi-wavelength filter to realize

multi-wavelength operation. The optical path difference of the two arms of the fiber Michelson

interferometer is tunable by changing the optical delay line. Thus, the wavelength spacing of the multi-

wavelength laser is tunable by tuning the optical delay line in the Michelson interferometer. Eventually, the

tunable range of the wavelength spacing is realized from 0.045 nm to 0.7 nm in our experiments. More than

60 wavelengths within 3 dB flatness with wavelength spacing of 0.0474 nm can be achieved.

1 INTRODUCTION

Multi-wavelength laser has found a lot of

applications in many filed, such as optical

communication, optical fiber sensing, laser

measurement, optical component testing and so on

(Liu et al., 2011; Salvadé et al., 2008), and many

efforts have been made to obtain high-quality multi-

wavelength laser. There are two issues to deal with

to obtain stable multi-wavelength laser with erbium-

doped fiber as the gain medium. Firstly, the erbium-

doped fiber laser suffers from homogenous line

broadening and cross-saturation in the room

temperature which could lead the unstability of the

multi-wavelength laser (Yao et al., 2004). Many

approaches have been developed to suppress the

homogenous line broadening in erbium-doped fiber

to obtain stable multi-wavelength laser. For

example, cooling the erbium-doped fiber in the

liquid nitrogen was proved to be an effective method

(Yamashita and Hotate, 1996); Utilizing the

nonlinear effects, like stimulated Brillouin scattering

or four wave mixing in the fiber, has achieved good

results in obtaining multi-wavelength erbium-doped

fiber laser (Al-Mansoori et al., 2005, Pan et al.,2006;

Xu et al., 2008). Secondly, in the multi-wavelength

erbium-doped fiber laser, the multi-wavelength filter

is a very important component. Many kinds of multi-

wavelength filters are adopted in the multi-

wavelength lasers, such as multi fiber Bragg grating

(Han et al.,2006), Fabry-Perot cavity (Al-Alimi et

al., 2018; Qin et al.,2006), programmable optical

filter (DeMiguel-Soto et al.,2014), Lyot-Saganc loop

with polarization maintaining fiber (Kim and Kang,

2004; Sugavanam et al., 2014), chirped fiber Bragg

grating (Dong et al., 2006) and so on. In some

applications, like upgradable optical communication

systems, tunability of multi-wavelength laser is very

important. For the most widely studied multi-

wavelength fiber laser using Lyot-Saganc loop filter

consisting of a piece of birefringence fiber, the

wavelength spacing is fixed once the length of the

birefringence fiber is selected (Wang et al., 2013,

Zhang et al., 2008). For the Brillouin-erbium multi-

wavelength laser, the wavelength spacing is still

fixed by the Brillouin shift and the narrow Brillouin

gain bandwidth. Although a wavelength spacing

switchable multi-wavelength Brillouin erbium fiber

laser was realized utilizing cascaded Brillouin gain

fibers, the wavelength spacing is limited by the

narrow bandwidth of the stimulated Brillouin

scattering (Wang, et al., 2016). A wavelength

spacing tunable multi-wavelength laser was

demonstrated by using superimposed chirped fiber

Bragg grating (Dong et al., 2006). Mach-Zehnder

optical fiber interferometer is also demonstrated to

be useful in a wavelength spacing tunable multi-

wavelength laser (Chen et al., 2007).

Luo, X., Tuan, T., Saini, T., Nguyen, H., Suzuki, T. and Ohishi, Y.

Multi-wavelength Erbium-doped Fiber Laser with Tunable Wavelength Spacing.

DOI: 10.5220/0006886302410245

In Proceedings of the 15th International Joint Conference on e-Business and Telecommunications (ICETE 2018) - Volume 1: DCNET, ICE-B, OPTICS, SIGMAP and WINSYS, pages 241-245

ISBN: 978-989-758-319-3

241

In this paper, we demonstrate a new multi-

wavelength erbium-doped fiber laser with tunable

wavelength spacing. A tunable Michelson fiber

interferometer consisting of an optical delay line is

inserted into the ring laser cavity to act as a

continuous tunable multi-wavelength filter.

Meanwhile, a nonlinear polarization rotation (NPR)

structure combined with a piece of long dispersion

shift fiber is introduced into this laser to suppress

longitudinal mode competition based on its

intensity-dependent loss feature. By tuning the

optical delay line, the tunability of the wavelength

spacing can be realized from 0.045 nm to 0.7 nm

continuously. More than 60 wavelengths within 3

dB flatness with wavelength spacing of 0.0474 nm

can be achieved.

2 EXPERIMENTAL SETUP

Figure 1(a) shows the schematic configurations of

the spacing-adjustable multi-wavelength erbium-

doped fiber laser. A 1480 nm laser diode (LD) is

connected to a wavelength division multiplexer

(WDM) to pump a piece of 2.1 m erbium-doped

fiber. The maximum output power of the 1480 nm

LD is ~283 mW. A piece of 5 km dispersion shift

fiber (DSF) is inserted into the laser cavity to

enhance the nonlinear effect. The 10% arm of a 10

dB coupler is used as the output port of the multi-

wavelength laser. A polarization dependent isolator

and two polarization controllers (PCs) constitute a

Nonlinear Polarization Rotation structure (NPR). A

fiber Michelson interferometer is inserted into the

laser cavity to act as the multi-wavelength filter and

realize multi-wavelength operation of the laser. The

schematic configuration of the spacing-adjustable

Michelson interferometer is shown in the red dashed

box in Fig.1 (a). The Michelson interferometer

consists of a 3 dB coupler, a tunable optical delay

line (ODL) and two wide band optical reflectors

(ORs). The mechanism of the Michelson

interferometer can be understood as following: The

input laser splits into two parts with the same

amplitude in the 3dB coupler; The two laser beams

propagate forward and are reflected back by the

wideband optical reflectors and then interfere with

each other in the 3 dB coupler. The interference

results in the comb filtering effect due to the

different optical path of the two arms. There is a

tunable ODL inserted into one arm of the fiber

Michelson interferometer, which makes the optical

path difference tunable. The Schematic

diagram of

the optical delay line is shown in Fig.1 (b). The ruler

of the ODL marks the position of the corner cube

mirror in the ODL. When changing the ODL ruler

reading to a position that makes the two arms of the

Michelson interferometer have the same length, the

optical path difference is zero. The wavelength

spacing of the transmission peaks of the Michelson

interferometer can be calculated by:

1480 nmLase

EDF

ISO

PD-ISO

ODL

1480/1550 WDM

10%

90%

10dB coupler

DSF

3dB coupler

(a)

cube mirror

Ruler

Fiber

cube mirror position

(b)

Figure 1: (a) Schematic diagram of the tunable multi-

wavelength laser with Michelson interferometer. EDF:

erbium-doped fiber; DSF: dispersion shift fiber; ODL:

optical delay line; OR: optical reflector; PD-ISO:

polarization dependent isolator; PC: polarization

controller. (b) Schematic diagram of the optical delay line.

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Wavelength,nm

Figure 2: A typical measured transmission spectrum of the

fiber Michelson interferometer.

OPTICS 2018 - International Conference on Optical Communication Systems

242

nL nL

 





 













(1)

Where λ is the central wavelength, n is the refractive

index of air which is near to 1 in our case; ΔL is the

optical path difference introduced by the ODL. As

the laser propagates through the ODL twice, there is

a coefficient 2 in the formula. A typical measured

transmission spectral of the Michelson

interferometer using an ASE source is shown in Fig.

2. The value of the transmission peak is -0.9 dB

which can be attributed to the loss of the two arms.

The transmission spectrum of the Michelson

interferometer indicates its great potential to be used

as a multi-wavelength filter. The optical path

difference of the two arms can be continuously

adjusted by tuning the ODL. Therefore, we can

realize wide range tuning of the wavelength spacing

of multi-wavelength fiber laser by changing the

ODL in the Michelson interferometer. The ruler of

the ODL marks the position of the corner cube

mirror in the ODL. The changing of the optical path

introduced by the ODL can be calculated by the

reading of the ruler of the ODL. In the measurement

of the transmission spectra of the Michelson

interferometer, it was found that when the ODL ruler

reading is set to ~30 mm the transmission peak

spacing become very wide. It means that in this case

the optical path difference of the two arms is about

zero. When changing the ODL ruler reading to a

position that makes the two arms of the Michelson

interferometer have the same length, the optical path

difference is zero. Moving the ODL ruler reading

away from 30 mm leads to the increasing of the

optical path difference and the narrowing of the

transmission peak spacing. So the ODL ruler reading

30 mm can be deemed as the zero point. As the ruler

reading is the position of the corner cube mirror in

the ODL, the optical path difference ΔL introduced

by the ODL is twice of the difference between the

ruler reading and the zero point.

3 RESULTS

In the experiments, wideband, flat and stable multi-

wavelength laser can be obtained with correct setting

of the polarization controllers and suitable pump

power. Once the pump power reaches the threshold,

the multi-wavelength output can be achieved by

correct setting of polarization controllers.

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28mm

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18mm

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20mm

Figure 3: Output spectra of the multi-wavelength laser

with different ODL ruler reading when the pump power is

fixed at 280 mW.

Multi-wavelength Erbium-doped Fiber Laser with Tunable Wavelength Spacing

243

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17mm

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17mm

Figure 4: Output spectra of the multi-wavelength laser

with ODL ruler reading of 17 mm. (a) Overall spectrum

measured by the OSA. (b) and (c) Detailed spectra

measured by the HRS.

As we mentioned above, the wavelength spacing

of the multi-wavelength laser can be tuned

continuously by adjusting the ODL according to

formula (1). Figure 3 shows the output spectra of the

multi-wavelength laser with different ODL ruler

reading. For instance, as shown in Fig. 3(e), the

multi-wavelength spacing is ~0.0607 nm. In this

case, the ODL ruler reading is 20 mm, the optical

path difference ΔL equal to 2*(30mm-20mm)

=20mm. When the central wavelength of the laser is

about 1560 nm and the optical path difference is 20

mm, from formula (1), the calculated wavelength

spacing is ~0.061 nm. The experiment results are in

good agreement with the theoretical values. As

shown in Fig.3, with different ODL ruler reading,

the wavelength spacing of the multi-wavelength

laser is different. The larger the ODL ruler reading

deviates from 30 mm, the narrower the wavelength

spacing is. It can be found that the contrast of the

multi-wavelength peaks to the background decreases

with the narrowing of the wavelength spacing. We

believe it is due to the limited resolution (0.05 nm)

of the optical spectral analyzer (OSA, Yokogawa

AQ6375B). When the ODL ruler reading is 18 mm,

the measured output spectrum of the multi-

wavelength laser is shown in Fig. 3(f). In this case,

the output spectrum seems very flat and the spectral

peaks of the multi-wavelength almost cannot be

recognized. When the ODL ruler reading is 17 mm,

the measured output spectrum is shown in Fig 4.(a)

and the theoretical wavelength spacing calculated

with formula (1) is ~0.047 nm. In order to obtain

more information about the output spectrum, we

used a high resolution spectrometer (HRS, Agilent

83453B) to measure the output of the multi-

wavelength laser. The measured spectra are shown

in Fig. 4(b) and Fig. 4(c). Obvious spectral peaks

with wavelength spacing 0.0474 nm can be observed.

About 60 wavelengths are obtained with 3 dB

flatness. We believe that increasing the deviation of

the ODL ruler reading of the ODL further, the

wavelength spacing will become narrower and more

wavelengths can be obtained. By tuning the ODL

carefully, the wavelength spacing can be adjusted

continuously and we can obtain multi-wavelength

laser output with any wavelength spacing within the

range. We believe that with proper ODL setting and

pump power, it is feasible to realize multi-

wavelength output with spacing of 0.8 nm which is

standard in telecommunication. In the experiment,

no obvious power and wavelength fluctuation were

observed when all the fibers are fixed well on the

optical table. On the other hand, as the NPR

structure is sensitive to the polarization status, we

found that the central wavelength of the laser would

change if we bend the fibers in the laser.

4 CONCLUSION

A multi-wavelength fiber laser with tunable

wavelength spacing is demonstrated. A Michelson

interferometer consist of an optical delay line was

used as a tunable multi-wavelength filter. A NPR

structure with a long piece of dispersion shift fiber

was used to the equalize multi-wavelength and

suppress wavelength competition. Finally, a multi-

wavelength ring fiber laser with flat spectra was

obtained with continuously tunable wavelength

spacing. The wavelength spacing can be tune

continuously from 0.045 nm to 0.7 nm. More than

60 wavelengths within 3 dB flatness with spacing of

0.0474 nm can be achieved.

OPTICS 2018 - International Conference on Optical Communication Systems

244

ACKNOWLEDGEMENTS

This work was supported by the JSPS KAKENHI,

grant No.15H02250, 17K18891 &18H01504 and by

JSPS and CERN under the JSPS-CERN joint

research program.

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