Nanosecond Pulse Generation near 1.55 Micron in the All-Fiber

Figure-Eight Mode-Lock Laser with Passive Nonlinear Loop Mirror

Svetlana S. Aleshkina, Mikhail M. Bubnov and Mikhail E. Likhachev

Fiber Optics Research Center of the Russian Academy of Sciences, 38 Vavilov Street, Moscow, Russia

Keywords: Fiber Laser, Nanosecond Laser, Figure-Eight Mode-lock Laser, Passive Nonlinear Loop Mirror.

Abstract: Figure-eight mode-lock all-fiber laser based on a passive nonlinear loop mirror emitting nanosecond pulses

at wavelength near 1.55 μm has been realized for the first time. Influence of the total laser dispersion on

output characteristics of the laser has been studied. It is revealed that the main problem of utilization fibers

with anomalous dispersion inside the passive nonlinear loop mirror is generation of low-energy optical

solitons accompanied with nanosecond pulse break up. Solutions of this problem are discussed and stable

laser schemes (i.e. all-polarization maintaining) are realized.

1 INTRODUCTION

Laser sources delivering nanosecond pulses are

widely used in the industry of micromachining,

LIDARs, systems of frequency conversion, and

supercontinuum generation. However simple and

stable scheme of sub-ns and few ns pulses generation

is still a challenge. Direct formation of such pulses by

electrical modulation of low-power laser diodes is

possible (Myrén and Margulis, 2005; Villegas et al.,

2011) but it requires complicated electronics control

and a multi-stage amplification with suppression of

amplified spontaneous emission. Also it is possible to

use alternative schemes based on mode–locking

(Kelleher et al., 2009) or passive Q-switching (Fotiadi

et al., 2007; Kurkov et al., 2009; Kurkov et al., 2010;

Dvoyrin et al., 2007; Dvoyrin, 2012; Jin et al., 2013).

However all these schemes suffer from long-term or

short-term instabilities (jitter, damage of elements

due to abnormal Q-switch pulse generation, absence

of polarization control and etc).

Recently a new all-fiber master-oscillator laser

scheme based on mode locking in a passive nonlinear

loop mirror (PNLM) has been proposed (Likhachev

et al., 2014). The main advantages of the proposed

laser design are the absence of adjustment elements

(monolithic all-polarization-maintaining fiber laser

scheme was realized), reliability (stable long-time

operation for more than 1000 hours was

demonstrated) and simplicity (just small sets of

standard components was used). Moreover small

modulation depth of PNLM provided a low lasing

threshold (twice lower compared to the figure-eight

scheme based on nonlinear amplifying loop mirror)

and "safe" start (the scheme start to operate from CW

regime instead of Q-switch regime for “classic”

figure-eight schemes). Stable pulse generation with a

rectangular temporal profile and spectral bandwidth

of less than 0.02 nm (less then spectrum analyzer

resolution) was demonstrated (Likhachev et al.,

2014).

It should be noted that nanosecond pulses in the

proposed scheme has been demonstrated only for 1

μm spectral range. At the same time numerous

applications (first of all – different types of LIDAR)

requires an eye-safe operation and therefore

wavelengths region near 1.55 microns is of great

interest. It is important to emphasize that the

fundamental difference between two spectral regions

(1 μm and 1.55 μm) is that optical fibers have

opposite dispersion sign (normal dispersion at 1 μm

and anomalous dispersion at 1.55 μm) that can

dramatically change the mechanism of pulses

formation, and even prevent correct operation of the

proposed scheme.

The purpose of the present work was realization

of stable nanosecond master-oscillator for the 1.55

μm spectral region as well as analysis of the

dispersion impact on the properties of the figure-eight

mode-locked lasers based on PNLM.

Aleshkina, S., Bubnov, M. and Likhachev, M.

Nanosecond Pulse Generation near 1.55 Micron in the All-Fiber Figure-Eight Mode-Lock Laser with Passive Nonlinear Loop Mirror.

DOI: 10.5220/0005687403050310

In Proceedings of the 4th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2016), pages 307-312

ISBN: 978-989-758-174-8

307

2 EXPERIMENTAL SETUP

The scheme of the all-fiber figure-eight laser is

depicted in Figure 1. Commercially available 210

mW semiconductor diode at 1460 nm pigtailed with

singlemode optical fiber was used to pump Er-doped

fiber. The Er-doped fiber had absorption of about 2

dB/m at wavelength of 1460 nm and 11 m of the fiber

was used in the scheme. The fiber dispersion was

about -30ps/(nm km). An isolator was placed between

Er-doped fiber and PNLM to ensure the

unidirectional operation. The PNLM was built by X-

type 30:70 fiber coupler and a passive single-mode

fiber. A 4.5% fiber coupler was used for signal

output. It is worth to note, that unabsorbed 1460 nm

pump could pass through the isolator and coupler, so

it was suppressed by a fiber 1460/1550 nm filter

WDM placed after 4.5% coupler port. The Yokogawa

AQ6370B optical spectrum analyzer (OSA) and a 500

MHz Tektronix TDS 3054C oscilloscope were used

to monitor the output spectrum and the pulse train,

correspondingly.

2.1 All-anomalous Dispersion Scheme

The laser scheme based on polarization-insensitive

components was tested at first. To adjust the

polarization state of the system a fiber polarization

controller was added to the scheme. Standard single

mode fiber (Corning SMF28) with length of about

160 m was used to form the PNLM. The fiber

dispersion was about 20 ps/(nm km). Passive

elements used inside the laser were also pigtailed by

SMF28 fiber. As a result the net cavity dispersion was

anomalous (β

~-4 ps

/km).

Figure 1: Laser scheme.

Three main regimes of pulse generation were

observed at the output of this laser scheme (Figure 2

and Figure 3). The most reproducible one was a set of

irregular low intensity pulses with a typical soliton

spectrum (Figure 2a and Figure 3a). It worth noting

that the regime was not observed early in the laser

scheme operated near 1µm. The reason is that in the

Figure 2: Regimes of pulse generation in all-anomalous

dispersion non-PM scheme. a – solitons regime; b – regime

of cooperative solitons and nanosecond pulse propagation;

c – regime of nanosecond pulse propagation.

1 µm spectral range standard fibers have normal

dispersion and formation of solitons propagating over

hundred of meters is not possible. It is completely

different from the case of pulse formation in the 1.55

µm spectral range, where standard fibers have

anomalous dispersion and could propagate solitons

over a long fiber distances. The second regime of

pulse generation (also easily reproducible) is

connected with existence in the cavity nanosecond

pulses and numerous low-intensity solitons,

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Figure 3: Spectrum of the pulses in all-anomalous

dispersion non-PM scheme. a – solitons regime; b – regime

of cooperative solitons and nanosecond pulse propagation;

c – regime of nanosecond pulse propagation.

simultaneously (Figure 2b and Figure 3b). The shape

of the nanosecond pulses varies quickly in this case.

Only by careful adjusting of the polarization

controller a stable self-started (at pump power more

than 60 mW) rectangle pulses was generated (Figure

2c and Figure 3c). It is significant that mode-locked

pulses were generated even when pump power was

decreased down to 20 mW that is known as pump

hysteresis effect (Nakazawa et al., 1991).

Dependences of the pulse duration on pump power

and estimated average power inside the cavity,

correspondingly, was linear (Figure 4 and Figure 5)

and pulses peak power did not change with the pump

power. The same behavior was also observed for the

pulse generation in the scheme with PNLM in 1 μm

spectral range and all-normal dispersion (Likhachev

et al., 2014) and corresponded to the peak-power-

locking inside PNLM (the PNLM has maximum

transition at fixed signal peak power). It is interesting

to note an important difference compared to the 1 µm

laser scheme in this case - the spectral bandwidth was

about of 8 nm (Figure 3c) as compared to less than

0.02 nm bandwidth (limited by spectrum analyzer

resolution) in the case of 1 µm laser.

Measured pulse repetition rate was 1.12 MHz.

Part of the power propagating between the pulses was

measured using integrating photodetector scheme

(Kotov et al., 2015) and it was found that 99% of the

signal located inside the pulse and less than 1%

(inaccuracy and systematic error of measurements) of

output power was concentrated between them. Thus

no low-energy solitons was generated in this case and

clear nanosecond pulses were generated.

Figure 4: Dependence of pulse duration from pump power;

time traces of the output pulses vs pump power on the inset.

Figure 5: Dependence of pulse duration and average power

from pump power after WDM.

Nanosecond Pulse Generation near 1.55 Micron in the All-Fiber Figure-Eight Mode-Lock Laser with Passive Nonlinear Loop Mirror

309

2.2 PM-Scheme for Pulse Generation

As could be seen from previous paragraph, regime of

generation was strongly changed by adjustment of

polarization controller in the case of non-

polarization-maintaining laser scheme. As a result

this scheme will suffer from a long-term instability

due to small variation of external stress and

temperature changes. PM-scheme is required for

practical applications.

PM laser scheme was realized by replacement of

standard components based on SMF 28 fiber on a

similar one with ability to maintain polarization

(PM1550 PANDA type optical fiber was used as

pigtail). The PNLM was made by 20:80 X-type PM

fiber coupler and standard PM 9/125 µm fiber. The

isolator was designed to pass only slow polarization

state (the fast axis was blocked) to achieve single-

polarization operation.

At first a 160 m of passive PM 9/125 µm fiber was

used inside the PNLM. In this case strong solitons

oscillations together with nanosecond pulses were

observed. Typical spectrum and time trace are shown

in Figure 6 and Figure 7.

Figure 6: Measured spectrum of the soliton and nanosecond

pulse generation.

Figure 7: Measured time trace of the generation regime.

It is interesting to note that estimated peak power

of second order solution was about 30 W that is close

to the peak power of maximum transmission of the

PNLM. Suppression of solitons generation could be

enhanced by increasing of the maximum-transition-

peak-power of PNLM (it will increase power loss of

solitons propagating through PNLM). To check this

idea we have reduced PNLM length down to 80m,

and thus have increased power of maximum

transmission through PNLM up to 70 W. A stable

nanosecond pulse generation was observed in this

case (see Figure 8). Long-term stability was tested for

operation at more than 100 hours and we did not

observe any variations of output power and pulse

form.

It worth noting that decreasing of nonlinearity

inside the cavity resulted in growth of the threshold

of pulse generation up to 140 mW. Repetition rate

was 2.216 MHz. Similar to the case of non-PM laser

scheme the spectral bandwidth was very wide - about

12 nm on the level of 3 dB (see Figure 9).

Figure 8: Time trace of the pulses in the PM scheme.

Figure 9: Measured spectrum of the pulses oscillating in the

PM scheme.

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2.3 All-normal Dispersion Scheme of

Nanosecond Pulse Generation at

1.55 Nm

In order to determine the impact of the dispersion on

the output characteristics of the laser system a special

home-made single-mode fiber with negative

dispersion at operating wavelength (D=-18 ns/(nm

km)) was used to form the PNLM. The fiber had

mode filed diameter of about 4.3 µm and cladding

diameter of 125 µm. Splicing loss with SMF28 fiber

after optimization splicing regimes was less than 0.5

dB. The length of the fiber inside PNLM was about

100 m. Except of the change of fiber inside PNLM

the laser scheme was identical to polarization-

insensitive scheme used in paragraph of 2.1.

Stable regime generation was achieved by the

appropriate adjustment of polarization controller. No

low-energy solitons generation was observed at all

positions of polarization controller that is due to

impossibility of soliton propagation over a long

length of fiber with normal dispersion. Self-starting

mode-lock regime was observed after pump power

turning on. Output laser characteristics were similar

to those of 1 μm laser (Likhachev et al., 2014) (Figure

10 and Figure 11). The main feature is that contrary

to anomalous dispersion regime the output spectrum

with a narrow peak (width is less than 0.02 nm) was

obtained at the pulse duration of 1 ns. As in the case

of (Likhachev et al., 2014) pulse duration increased

with pump power growth from 1 ns to 8 ns, pulse

amplitude did not change. It should be noted that

increasing of the pump power led to increasing of the

intense peaks number in the spectrum. Apparently it

was due to the incorrect configuration of the

polarization controller (the polarization state is not

fully reproduced in the round-trip cavity): small

instability was able to induce additional generation

Figure 10: Measured spectrum of the pulses oscillating in

all-normal dispersion scheme.

at neighbour wavelengths. It is important to say that

according to data of temporal trace pulses propagated

with the same velocity and composed the only one

pulse. Similar behaviour was observed for non-PM

laser scheme operating at 1 μm (Likhachev et al.,

2014). Suppression of the additional spectral

components generation could be achieved using

polarization-maintaining laser scheme.

Additional results will be presented at the

conference.

Figure 11: Dependence of pulse duration on pump power.

3 CONCLUSIONS

In summary, investigation of nanosecond pulse

generation regimes in the figure-8 laser scheme with

PNLM was carried out for the 1.55 μm spectral range.

It was shown that stable self-starting nanosecond

rectangular pulses can be obtained in the scheme both

with net-normal and net-anomalous dispersion.

However output laser spectrum is strongly depends

on the cavity dispersion: narrow-width spectrum can

be realized in the scheme with all-normal dispersion

only. It is experimentally demonstrated that the major

problem of nanosecond pulse generation in all-

anomalous dispersion scheme is appearance of low-

energy solitons. Such a regime can be efficiently

suppressed by appropriate polarization controller

adjustment in non-PM laser scheme or by decreasing

of PNLM length in the case of PM scheme. All-PM

scheme was created to exclude the degree of freedom

coursed by the possibility of adjusting the

polarization controller. A stable in the time

nanosecond pulses (similar to the case of pulse

generation in non-PM scheme) were observed in this

case.

Nanosecond Pulse Generation near 1.55 Micron in the All-Fiber Figure-Eight Mode-Lock Laser with Passive Nonlinear Loop Mirror

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ACKNOWLEDGEMENTS

This work was supported with a grant 14-19-01572

from the Russian Science Foundation. The authors

are grateful to E.M. Dianov, scientific director of the

Fiber Optics Research Center, and S.L. Semionov,

director of the Fiber Optics Research Center, for their

continuous interest in and support of this work.

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