Generation of Femtosecond Pulses in 1 µm Spectral Range by

Dispersion Managment with Asymptotically Single-mode Hybrid

Fiber

S. S. Aleshkina

, M. V. Yashkov

, A. K. Senatorov

, L. D. Iskhakova

, M. M. Bubnov

A. N. Guryanov

and M. E. Likhachev

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

Institute of High Purity Substances of Russian Academy of Sciences, 49 Tropinin Street, Nizhny Novgorod, Russia

Keywords: Hybrid Fiber, Femtosecond Pulse, Asymptotically-singlemode Fiber, Pulse Compression, Dispersion

Compensation, Fiber Laser, Solitons, Dispersion Shifted Fiber.

Abstract: A novel technique for suppression of unwanted modes in the hybrid fiber with anomalous dispersion near 1

µm has been proposed and realized. For this aim a special absorbing layer was placed to the position of the

hybrid mode electric field minimum. As a result all other mode has excessive loss in the spectral region near

1.06 µm. On the contrary the optical loss of the hybrid mode almost does not change. Realized fiber was used

for high quality pulse compression down to duration of 440 fs and energy of about of 20 pJ. Intra-cavity

dispersion management has allowed us to realize a master-oscillator with output pulse energy up to 0.55 nJ

and pulse duration below 700 fs.

1 INTRODUCTION

Intra- or extra-cavity dispersion management is

essential demand for femtosecond fiber lasers. The

lack of commercially available fibers with anomalous

dispersion in the 1 µm spectral region requires

utilization of bulk elements (diffraction grating (for

example, Okhotnikov, 2003)) for dispersion

compensation, which degrade system reliability. To

keep monolithic laser design specialty fibers with

anomalous dispersion in this spectral region has been

intensively developed. However by now all the

proposed fiber designs have significant drawbacks:

high nonlinearity (in the case of Photonic Cristal

Fibers (Monro, 1999; Akowuah, 2009; Herda, 2008;

Knight, 2000; Lim, 2002)), few-mode propagation

regime (in the case of hollow-core fibers (Lim, 2004;

Kolyadin, 2015; Saitoh, 2009; Bouwmans, 2003)),

sensitivity to temperature and fiber tension (in the

case of high order modes excitation by long-period

gratings (Nicholson, 2007; Ramachandran, 2006)).

Sophisticated production technology for all these

types of fibers, difficulties with splicing to ordinary

fibers (Herda, 2008; Knight, 2000; Lim, 2002; Lim,

2004) and limited operation bandwidth (Likhachev,

2007; Luan, 2004; Isomäki, 2006; Z. Várallyay, 2009;

Kibler, 2009) prevent wide spreading of above

mentioned fiber designs. In the current

communication we have proposed a new type of fiber

for dispersion management that is free from above

mention drawbacks. Utilization of this fiber allowed

us to generate high energy femtosecond pulses near

1.06 µm in different laser schemes.

In our previous work a novel hybrid fiber design

was proposed (Aleshkina, 2013). Refractive index

profile (RIP) of the structure consists of the core with

refractive index higher than that of undoped silica. It

is surrounded by one or few high-index ring layers,

thin depressed layer and undoped silica outer

cladding. From general point of view such fiber is a

multimode. However, the majority of the fiber modes

(including the fundamental one) are localized inside

the high index ring layers. There is only one mode,

which we call hybrid, located in the core region. The

mode electrical field distribution is similar to that of

the Bragg fiber mode (Likhachev, 2007). Similar to

the Bragg fiber the hybrid mode propagates inside the

depressed (relative to the high-index layers) core due

to coherent Fresnel reflections from high index ring

layers. However total internal reflection mechanism

allows propagation of the hybrid mode with a low loss

(effective refractive index of the hybrid mode is

Aleshkina S., Yashkov M., Senatorov A., Iskhakova L., Bubnov M., Guryanov A. and Likhachev M.

Generation of Femtosecond Pulses in 1 Îijm Spectral Range by Dispersion Managment with Asymptotically Single-mode Hybrid Fiber.

DOI: 10.5220/0006144200700075

In Proceedings of the 5th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2017), pages 70-75

ISBN: 978-989-758-223-3

higher than that of undoped silica cladding). The main

advantage of the hybrid mode is that it has anomalous

dispersion in the wavelength region of about 1 m

(Aleshkina, 2013; Aleshkina, 2015). Utilization of the

hybrid fiber as a chirped pulse compressor has

allowed us to decrease pulse duration from 8 ps to 330

fs, energy of the pulses was about 0.005 nJ

(Aleshkina, 2013).

An additional advantage of the hybrid fiber is

possibility of its splicing with a conventional step-

index single-mode fiber having a similar mode field

diameter. However, a small fraction (~ 10-20%) of

the modes localized in the high index ring layers is

also excited in this case. The propagation of several

modes in the core has not allowed us to realize the

truly single mode operation regime of the fiber used

as chirped pulse compressor. The autocorrelation

traces had several peaks with different intensity,

moving away from the central peak when fiber length

was increased. Thus an important step for

development of lasers based on hybrid fiber design is

the removal of unwanted modes from the hybrid fiber.

In the present paper, the method of unwanted

modes suppression in the hybrid fiber is

experimentally demonstrated. The realized optical

fiber was used as a chirped pulse fiber compressor, as

well as intracavity dispersion compensation element

of ring soliton laser. In both cases subpicosecond

pulses with Gauss shape were demonstrated. Study of

autocorrelation traces confirms single-mode

propagation regime of the hybrid fiber.

2 DESIGN OF THE HYBRID

FIBER, FABRICATION AND

CHARACTERISTICS

The main idea of the proposed method is based on

introduction of a highly absorbing cylindrical layer to

the position of operating mode minimum. In this case,

the optical loss of the hybrid mode is kept almost

unchanged, while the optical loss of the all other

(unwanted) modes can be made very high. The reason

is that the fraction of unwanted modes power

confined in the absorbing layer is orders of magnitude

higher as compared to that of the hybrid mode. Figure

1a shows a designed RIP of the hybrid fiber.

Calculated dispersion at a wavelength of 1.06 µm is

about 100 ps/(nm km) (Figure 1b). The electrical field

intensity distributions over the radius for the hybrid

mode LP

and modes LP

and LP

located in the

high index ring layers are shows in Figure 1a as well.

It can be seen that the hybrid mode LP

has two

intensity minima, one located inside the fiber core (on

the boundary with high-index ring layer), and the

second one located between the high-index ring

layers (Figure 2a). According to our calculations

optical minimum located between the high index ring

layers is less sensitive to bending, as well as to shift

the operating wavelength. Therefore, from a practical

point of view, introduction of the absorbing layer to

this position is more preferable. To achieve high

optical loss we used doping of the layer with Sm

ion

due to its intense absorption bands in the spectral

region near 1 micron.

Figure 1: a – Designed refractive index profile of the hybrid

fiber and calculated mode field intensity distribution; b –

calculated dispersion of the fiber.

The fiber preform was fabricated by the Modified

Chemical Vapour deposition (MCVD) technique.

The RIP was formed by geranium oxide doping of

silica glass. The absorbing layer is implemented by

solution doping method. Fiber with an external

diameter of 125 m was drawn from this perform and

coated by high refractive-index (n~1.52) acrylate

coating. Measured RIP and image of the fiber end are

shown in Figure 2. Estimated from prefom analysis

Sm ions distribution across the fiber cross section is

shown in Figure 2 as well.

Generation of Femtosecond Pulses in 1 Îijm Spectral Range by Dispersion Managment with Asymptotically Single-mode Hybrid Fiber

Investigation of modal content in the realized

hybrid fiber has showed that after 5 m the only one

hybrid LP

mode can propagates. Unwanted modes

were detected only at fiber lengths shorter than 5m by

displacement of the excitation beam from the fiber

axis. Even in this case the preferable excitation of the

hybrid mode is possible by co-axial splicing. The

estimated hybrid mode field diameter was 5.4

microns. The loss spectrum of the hybrid mode is

shown in Figure 3. Estimation loss of unwanted

modes exceed 14 dB/m. Splicing losses of the hybrid

fiber and standard singlemode step index fiber (core

and cladding diameter of 6 and 125 microns,

correspondingly) at a wavelength of 1.06 m were 2.5

dB. The dispersion measured in 0.5 m of the hybrid

fiber (Hen-Tai Shang, 1981) is shown in Figure 4.

Inaccuracy of measurements is associated with a

small operating spectral range of the hybrid fiber (~

100 nm). This spectral region is limited by

appearance of high order hybrid modes at short

wavelength and cut-off of the hybrid mode in the

longer wavelength.

Figure 2: Measured RIP of the hybrid fiber, evaluated mode

field distribution of the hybrid mode and distribution of the

ions across the fiber cross-section. On the inset the

image of the fiber end obtained by Scanning Electron

Microscopy.

Figure 3: Measured optical loss of the hybrid fiber.

Figure 4: Measured dispersion of the hybrid fiber.

3 COMPRESSION OF CHIRPED

PULSES WITH THE HYBRID

FIBER

First of all, the fabricated hybrid fiber was tested as a

chirped pulse compressor (similar to our previous

work (Aleshkina, 2013)). For this aim the master

oscillator was realized on the base of Semiconductor

Saturable Absorbing Mirror (SESAM) and passive

nonlinear optical loop mirror (scheme is depicted in

Figure 5). Multimode semiconductor pump diode

emitting at a wavelength of 976 nm was used as the

pump source. The pump radiation was launched in the

scheme with a help of 2+1-to-1 pump and signal

combiner. The Yb-doped fiber with cladding

absorption at pump wavelength about 8 dB/m was

used as the active fiber. The length of the active fiber

in the system was 3 m. The Yb-doped fiber core was

approximately 10 m, so a special mode field adapter

was fabricated to match the active fiber and other

passive fibers (core D~ 6 µm) used in the laser

scheme. The amplified signal from the active fiber

was couples to a passive nonlinear optical loop mirror

(NOLM) formed by 3 m of standard passive fiber

(similar to Corning HI1064) together with 30/70

coupler. A similar coupler was used to extract 70% of

the propagated signal outside the cavity and reflect

30% back with a help of SESAM. To ensure

unidirectional propagation inside the laser system,

each tail of the couplers were spliced with isolator.

Polarization controller was used to adjust polarization

state of irradiation inside the laser. The total cavity

length was about 10 m.

PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology

Figure 5: Chirped pulse oscillator scheme.

Mode-locking regime was achieved at pump

power of about 450 mW. The average output signal

power was 10 mW, the repetition frequency was

20.85 MHz. Typical dissipative solitons with spectral

width of about 11.8 nm was observed in this case

(Figure 6). Femtochrome Research Inc FR-103 mn

autocorrelator with fiber input was used to measure

the pulse duration in our experiments. The laser

system was polarization-insensitive. By this reason an

additional polarization controller based on a standard

optical step index fiber with length of 0.4 m was

places between the laser and autocorrelator. Pulse

width after the master oscillator was estimated to be

about 7 ps (inset of Figure 6).

Figure 6: The measured spectrum and time trace of the

master oscillator.

To carry out the pulse compression, the realized

hybrid fiber was placed between the output end of the

laser and input fiber end of polarization controller

delivering laser emittion to autocorrelator. The Figure

7 and Figure 8 show dependence of the pulse duration

on hybrid fiber length. It is important to emphasize

that even when hybrid fiber length was about 3 m the

autocorrelation traces had Gauss shape. No additional

intensity peaks were observed in autocorrelation trace

that indicated absence of the unwanted modes in the

system. Only with 1 m of the hybrid fiber a slight

broadening of the autocorrelation function was

observed, that we associate with excitation of

undesirable ring modes.

Figure 7: Dependence of pulse duration on length of the

hybrid fiber.

Figure 8: Measured time traces of the pulses after hybrid

fiber.

When length of the hybrid fiber was equal to 10.5

m the maximum pulse compression down to duration

of 440 fs was observed. The average output power

was 430 W. The signal was decreased by an order

of magnitude due to two splices of hybrid fiber with

standard single-mode fiber (~ 5 dB net) and intrinsic

hybrid mode loss (8 dB for the 10.5 m fiber length).

The energy in the pulse was 0.021 nJ. Taking into

consideration additional standard fiber at the output

of the laser (used in the polarization controller, input

of autocorrelator, etc) the hybrid fiber dispersion at

the wavelength of 1.06 m was estimated to be 63

ps/(nm km). No nonlinear effects were registered in

the hybrid optical fiber. We suggest that it is coursed

Generation of Femtosecond Pulses in 1 Îijm Spectral Range by Dispersion Managment with Asymptotically Single-mode Hybrid Fiber

by relatively large mode field diameter of the realized

hybrid fiber and relatively low electrical field

intensity.

The hybrid fiber was also used for intracavity

dispersion compensation in soliton laser. Laser

scheme is shown in Figure 9. The length of the hybrid

fiber inside the cavity was 6.5 m. Solitons with pulse

duration as short as 700 fs were obtained at the output

of the source. Typical spectra with Kelly’s peaks and

the autocorrelation function of the output signal are

shown in Figure 10 and Figure 11. Time-bandwidth

product of Gaussian shaped pulses was equal to 0.48.

Output power was 8.6 mW, the repetition frequency

was 15.75 MHz, the pulse energy was 0.55 nJ.

Figure 9: Schematic of the mode-locked soliton laser

cavity.

Figure 10: Registered spectrum on the output of the soliton

laser.

Figure 11: Registered time trace of the soliton laser.

4 CONCLUSIONS

Single-mode propagation regime of the hybrid fiber

with selective mode suppression was demonstrated

on the example of all-fiber chirped pulse compressor.

The usage of the hybrid fiber with length of 10.5 m

allowed us to compress pulses with duration of 7 ps

to 440 fs without deterioration of pulse shape and

quality. The use of the same hybrid fiber in the soliton

laser scheme has allowed us to realize stable pulse

lasing with soliton energy of 0.55 nJ and a peak power

of 850 watts

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 for his continuous

interest in and support of this work.

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