Wide Broadband ASE Source based on Thulium-doped Fibre

for 2 μm Wavelength Region

M. A. Khamis and K. Ennser

College of Engineering, Swansea University, Swansea, U.K.

Keywords: Thulium-doped Fibre, Amplified Spontaneous Emission, Silica Glass Material, Numerical Modelling.

Abstract: This paper investigates the generation of the amplified spontaneous emission (ASE) from thulium-doped

silica fibre pumped at 1570 nm. The developed model provides the ASE spectral power at short and long

wavelength bands by using two different thulium doped fibre types with optimized fibre length. Shorter

wavelengths in the emission band can be accessed with a short thulium fibre, whereas longer wavelengths

can be obtained using a long thulium fibre. Our findings reveal that, in contrast to a 100 nm (1800nm-

1900nm) and 70 nm (1900nm-1970nm) broadband source at short and long wavelength bands, a broader

spectrum source can be achieved at about 170 nm (1800nm-1970nm) by a combined of the two ASE spectra

via a wideband 50:50 coupler. As a result, the proposed ASE source configuration doubles the bandwidth of

the conventional single fibre based light source.

1 INTRODUCTION

Over the last few years, broadband sources near 2

μm have attracted the attention of many researchers.

Hu et al. (2015) point out a broadband source at 2

μm have many useful characteristics, including high

output power, high light brightness, good beam

quality, compact structure and excellent spatial

coherence. These characteristics make broadband

sources at 2 μm suitable for use in a number of

significant applications, such as remote sensing (Li

et al., 2014), gas sensing (Hsu et al., 2008), medical

surgery (Morse et al. 1995), materials processing

(Jackson Sabella and Lancaster, 2007) and

atmospheric lidar measurement (Sugimoto et

al.,1990). In addition, according to Halder et al.

(2012) and Cheung et al. (2015), high power and

wideband sources are required in optical coherence

tomography and fibre optic gyroscopes.

One effective way to generate this broadband

source is by using the process of amplified

spontaneous emission between

transition in

thulium-doped fibre. High broadband source

efficiency can be obtained via diode pumping on the

transition

→

in combination with high

concentration of thulium-doped silica fibre in order

to take into account the cross-relaxation process

(Oh, 1994; Shen et al., 2008). Alternatively, the need

for high doping concentrations can be avoided by

using an in-band pumping scheme, which directly

excites the upper laser level on the transition

→

. Nevertheless, Tsang et al. (2005) explain that

high efficiencies can be achieved because of the

much lower quanta defect associated with this

pumping scheme. However, in spite of the progress

in the performance of thulium broadband sources the

efficiency and bandwidth are still below those

routinely provided from conventional thulium fibre

laser oscillators.

To better optimize the broadband source

performance near 2 μm. It is necessary to develop a

theoretical modeling and perform simulations. There

are relatively few theoretical studies on ASE sources

based on thulium-doped fibre (TDF) (Gorjan et al.,

2012; Yu et al., 2010). On the other hand, various

experimental configurations for broadening the

spectral bandwidth of ASE source have been

presented in the literature. Broader bandwidths up to

72 nm have been demonstrated using Tm: Ho-doped

silica fibre (Tsang et al., 2005). Up to 100 nm of

broadband spectra has been achieved using single

end operation (Shen et al., 2008). However, these

configurations have limited spectra bandwidth. This

paper proposes a new thulium ASE configuration

that doubles the bandwidth of the conventional

configuration.

In this paper, a broadening ASE source can be

generated from combining the ASE output source of

Khamis M. and Ennser K.

Wide Broadband ASE Source based on Thulium-doped Fibre for 2 Îijm Wavelength Region.

DOI: 10.5220/0006101801410146

In Proceedings of the 5th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2017), pages 141-146

ISBN: 978-989-758-223-3

141

TDF1

TDF2

1570 WDM

1570 Pump

50:50 Coupler

ISO

ASE O/P

two different TDFs as shown in fig.(1). The first

fibre TDF

is a commercial thulium fibre (TmDF200

from OFS) with short fibre length. Agger and

Povlsen (2006) demonstrated that the center

emission spectrum of this fibre is at the wavelength

1800 nm. The second fibre TDF

is a commercial

thulium fibre (SM-TSF-9/125 from Nufern) with

long fibre length. Jackson (2009) shows that this

fibre produces an emission spectrum with 1900 nm

center wavelength. Long TDF leads to shift the

output spectrum to ward long wavelength bands (Li

et al. 2013). Thus, TDF

generated ASE1 source

with short wavelength bands which is centered at

1840nm and TDF

produced ASE2 source with long

wavelength bands which is centered at 1950nm. The

two ASE sources were combined together into a

single ASE source via wideband 50:50 coupler. This

type of coupler is already applied to design a widely

tunable thulium laser (Stevens and Legg 2015).

Notes that in our simulation, we choose couplers and

combiners have flatting coupling response over the

wavelengths range in order to allow broadband ASE

source.

Figure 1: A schematic diagram of proposed ASE source.

ISO is an optical isolator at 2μm, ASE o/p is the combined

ASE sources of TDF

and TDF

ASE output. Note that all

fibre ends are angle polished.

2 NUMERICAL MODEL OF ASE

A theoretical model of ASE generation are

presented. The investigation into ASE spectral

power is achieved by solving the rate and

propagation equations. The model takes into account

the wavelength-dependent absorption and emission

cross-sections. Based on Jackson and King (1999)

and including ASE, the rate equations of thulium

energy levels are established as follows:

12 1 21 2

21 2 12 1

(,) (,)

( , ) ( , )

dN N

w N zt w N zt

=−−

−+

(1)

(,) (,)

Nzt N Nzt=− (2)

Here N

is the Tm

concentration and set to be a

constant. τ

is the spontaneous lifetime of the

level N

and N

are the population densities of the

and

levels, respectively, w

p12

is the pumping

rate from

and w

p21

represents the de-

excitation of the

level; w

s21

is the stimulated

emission rate from

, and w

s12

is the

stimulated absorption rate from

. The

expressions of w

p12

, w

p21

, w

s21

and w

s12

can be

obtained from:

()(() ())

pappp

core

wpzpz

hcA

σλ

−+

(3)

()(() ())

peppp

core

wpzpz

hcA

σλ

−+

(4)

()[ () ()]

sasfb

core

w ASE z ASE z

hcA

σλ

(5)

()[ () ()]

sesfb

core

wASEzASEz

hcA

σλ

(6)

Here λ

is the wavelength of the pump light and λ

the signal light in vacuum; h is the Planck constant;

c is the light speed in vacuum; A

core

is the cross-

section area of the fibre core; σ

(λ

) and σ

(λ

) are

the absorption cross-sections of the pump light and

the signal light, respectively; σ

(λ

) and σ

(λ

) are the

emission cross-sections of the pump light and the

signal light, respectively; P

(z) is the pump

(corresponding to forward and backward) at position

z; and





() and



()are the forward and

backward amplified spontaneous emission powers at

position z; Γ

and Γ

are the confinement factors for

the pump and the signal, respectively which are

given by Eq. 7

(Whitley and Wyatt, 1993).

() 1 exp



Γ=− −





(7)

Where w is the mode-width parameter of the fibre

and a is the radius of the fibre. The normalized

frequency V of the fibre is given by:

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

142

2.aNA

(8)

As V>1.5the mode-width parameter w of a step-

index fibre can be determined by the normalized

frequency V of the fibre as:

3/2 6

0.632 1.478 4.76

−−

=+ +

(9)

Meanwhile, the pump power distribution along

the fibre length can be expressed by the following

propagation equation:

()[ ( ( ) () ( ) ())

- ]

ppep ap

zNzNz

σλ σλ

=± Γ −

(10)

The positive sign in (10) relates to the forward

direction and the negative sign to the reverse

direction. The distribution of the ASE forward and

backward powers along the fibre length can be

established as follows

(Hu et al., 2015; Yu et al., 2010):

()[ ( ( ) () ( ) ())

- ] 2 ( ) ( ) (11)

fses as

ses

dASE

SE z N z N z

σλ σλ

ασλ λ

=Γ −

+Δ

()[ ( ( ) () ( ) ())

- ] 2 ( ) ( ) (12)

bses as

ses

dASE

SE z N z N z

σλ σλ

ασλ λ

=− Γ −

−Δ

Where α

and α

are the intrinsic absorption at the

pump and signal wavelength for the Thulium-doped

fibre, respectively, ∆λ is the bandwidth of the

amplified spontaneous emission (ASE) around 2μm.

3 RESULTS AND DISCUSSION

To solve the thulium rate equations for the ASE

model in steady state condition, the time derivatives

of eq. (1) to (2) for in-band pumping are set to zero.

The fourth-order Runge-Kutta method is applied to

solve the differential equations of the pump and the

amplified spontaneous emission ASE signals. Table

1 and 2 summarizes all parameters values used in the

numerical simulations of the TDF

(Agger and

Povlsen 2006) and TDF

(Jackson 2009),

respectively. Initially, the entire population is

assumed to be at the ground level

in the

numerical calculations.

Table 1: Values of numerical parameters for TDF

Symbol Quantity Value

Thulium concentration 8.4×10

-3

Lifetime of level

650µs

In-band pump wavelength

1570nm

(λ

) Laser absorption cross

section at 1570nm.

2.1×10

-25

(λ

) Laser emission cross section

at 1570nm.

(λ

) Laser absorption cross

section at signal wavelength

See (Agger and

Povlsen, 2006)

(λ

)

Laser emission cross section

at signal wavelength

See (Agger and

Povlsen, 2006)

NA Numerical aperture 0.26

A Area cross section of the core 3.1×10

-11

-2

Intrinsic absorption at the

pump wavelength

1.07×10

-3

-1

Intrinsic absorption at the

signal wavelength

1.15×10

-2

-1

Table 2: Values of numerical parameters for TDF2.

Symbol Quantity Value

Thulium concentration 1.37×10

-3

Lifetime of level

250µs

In-band pump wavelength

1570nm

(λ

) Laser absorption cross

section at 1570nm.

2×10

-25

(λ

) Laser emission cross section

at 1570nm.

(λ

) Laser absorption cross

section at signal wavelength

See (Jackson

2009)

(λ

)

Laser emission cross section

at signal wavelength

See (Jackson

2009)

NA Numerical aperture 0.15

A Area cross section of the

core

6.36×10

-11

-2

Intrinsic absorption at the

pump wavelength

1.07×10

-3

-1

Intrinsic absorption at the

signal wavelength

1.15×10

-2

-1

Table 3 explains the initial conditions for the

pump power and the ASE spectrum in forward and

backward directions. The thulium-doped fibre with

length L is divided into N segments along the z-

direction. The solution is applied for the pump and

the ASE power propagating in the first segment

(segment 0) by using the initial conditions in Table

3. For the following segments (segment 1 to N-1),

the power for the pump and the ASE at one end of a

segment is applied as the input for the next segment.

Relaxation method is used to solve the differential

equations of the pump and the ASE powers (Emami

2011). Using the data of Table 1, 2 and the values of

the emission and the absorption cross-section, we

solve numerically the rate equations of the pump and

ASE power distribution.

Wide Broadband ASE Source based on Thulium-doped Fibre for 2 Îijm Wavelength Region

143

Table 3: Initial conditions.

Initial condition Explanation

(z=0) = Forward

launched pumppower

Initial condition for 1558nm

and 793nm pumps at z=0.

(z=L) =Backward

launched pump power

Initial condition for 1558nm

and 793nm pumps at z=L.

(z=0)= seed power

Initial condition for seed power

at z=0.

ASE

(z=0)=0

Initial condition for forward

amplified spontaneous

emission at z=0.

ASE

( z=L)=0

Initial condition for backward

amplified spontaneous

emission at z=L.

A MATLAB program is developed to evaluate

the optimum thulium-doped fibre length for each of

the two TDFs of fig. 1. Note that in our simulation

the feedbacks of ends’ reflection are set to zero and

it is only applied forward pump configuration.

Figure2 and 3 illustrate the theoretical prediction of

the output ASE forward power and the residual

pump power for TDF

and TDF

, respectively. The

launched pump power is equal 27dBm (0.55 W) at

case TDF

and 31.7dBm (1.5W) at TDF2. We

clearly notice that the optimum fibre length of TDF1

is 1.2 m at 1840nm output ASE as shown in fig. 2.

In contrast to TDF2, the optimum fibre length is 5m

at 1950nm output ASE as illustrated in fig. 3. In our

configuration, 1.2m of TDF

is a suitable length to

obtain short wavelength bands of ASE source which

is centred at 1840nm, whereas 8m of TDF

is more

suitable to access long wavelength bands which is

centred at 1950nm.

The next step is to investigate the output power

spectra of ASE at short and long wavelength bands.

Figure 4 shows the ASE spectrum of TDF

at 1.2m

Figure 2: Pump power distribution along TDF

length and

the forward ASE at 1840nm when the launched pump

power is 0.55W.

Figure 3: Pump power distribution along TDF

length and

the forward ASE at 1950nm when the launched pump

power is 1.5W.

fibre length and 27dBm launched pump power. The

bandwidths of forward ASE has full-width half

maximums (FWHM) of approximately 100 nm

between 1800nm and 1900nm compared to only

68nm and 88nm in forward and backward ASE

obtained by

Gorjan et al. (2012). The discrepancies are

due to the difference in thulium fibre characteristics.

Fig. 5 shows the ASE spectrum of TDF

at 8m fibre

length and 31.7dBm is the launched pump power.

The bandwidth of forward ASE at FWHM is

approximately 70 nm between 1900nm and 1970nm.

Finally, fig. 6 illustrates the combined ASE spectra

of the TDF

and TDF

spectra. Notes that a wide

50:50 coupler is used and assumed to be flat over the

range (1800nm-2000nm). It is clearly seen that the

combined ASE provides approximately 170 nm

bandwidth at FWHM between 1800nm and 1970nm.

Thus, this is the first time to our knowledge that over

170nm bandwidth ASE source based only on

thulium-doped fibre is reported.

Figure 4: Power spectrum of the forward ASE at 1.2m of

TDF

when 27dBm is the total input pump power.

0 0.2 0.4 0.6 0.8 1 1.2 1.4

0.05

Fiber length (m)

ASE Power (W)

0 0.2 0.4 0.6 0.8 1 1.2 1.4

0.5

Pump power (W)

0 1 2 3 4 5 6 7

0.05

0.1

Fiber length (m)

ASE Power (W)

0 1 2 3 4 5 6 7

Pump power (W)

1650 1700 1750 1800 1850 1900 1950 2000 2050

0.01

0.02

0.03

0.04

0.05

0.06

0.07

Wavelength (nm)

ASE peak power (W)

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

144

Figure 5: Power spectrum of the forward ASE at 8m of

TDF

when 31.7dBm is the total input pump power.

Figure 6: Combined power spectrum of the TDF

and

TDF

ASE spectra by using flat wide 50:50 coupler.

4 CONCLUSION

A theoretical model of ASE generation around 2µm

is built up by solving a set of rate and propagation

equations. A MATLAB program is developed using

the Runge-Kutta method to investigate the behaviour

of the ASE generation at 2µm from two different

thulium fibres types at 1570nm.

We chose two different fibre characteristics with

optimised fibre length in order to generate ASE

source for short and long wavelength bands. Wide

band ASE source can be generated by combining the

two wavelength bands. Thus, the main scope of this

study is to generate broad band ASE source at 2 µm

for applications that require broader ASE bandwidth

such as optical coherence tomography.

Our simulation results show that short wavelength

bands (1800nm-1900nm) with 100nm FWHM

bandwidth can be generated from the TDF

. In

contrast to long wavelength bands (1900nm-

1970nm) with 70nm FWHM bandwidth can be

generated from TDF

. More than 170nm (1800nm-

1970nm) should be produced from combining the

two above ASE spectra. Note that we choose

couplers and combiners have flatting coupling

response over the wavelengths range in order to

allow broadband ASE source. Hence, our suggested

configuration is a suitable arrangement to obtain

over 170nm wider broadband source at 2 μm from

thulium doped fibre.

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