Multi-mode capacity enhancement with PBG fibremodegap.eu/wp-content/uploads/2015/06/p14-mode-gap-few...Multi-mode capacity enhancement with PBG fibre Public Deliverable P14 January

Project: FP7-258033 MODE-GAP

Multi-mode capacity enhancement with PBG fibre

Project start date: 1st October 2010 Project completion date: 31st March 2015

Deliverable P14

Few Mode Fiber Amplifier Development

January 2015

Dissemination Level

PU Public X

PP Restricted to other programme participants (including the Commission Services)

RE Restricted to a group specified by the consortium (including the Commission Services)

CO Confidential, only for members of the consortium (including the Commission Services)

Multi-mode capacity enhancement with PBG fibre Public Deliverable P14 January 2015: Few Mode Fiber Amplifier Development

Page 2 of 20

Contents 1. Introduction .................................................................................................................................... 3

2. Cladding Pumped EDFA Design Supporting 4 and 6 Mode Group .................................................. 5

2.1. Introduction ............................................................................................................................ 5

2.2. Simulation ............................................................................................................................... 5

2.3. Conclusion ............................................................................................................................... 7

2.4. Key Results .............................................................................................................................. 7

2.5. References .............................................................................................................................. 7

3. Cladding Pumped 6-mode EDFA for Mode Division Multiplexed Transmission ............................. 8

3.1. Motivation and objectives ...................................................................................................... 8

3.2. Experimental setup for the cladding pumped 6M-EDFA ........................................................ 9

3.3. Measured gain and noise figure performance of the 6M-EDFA ........................................... 10

3.4. Conclusion ............................................................................................................................. 12

3.5. Key Results ............................................................................................................................ 12

3.6. References ............................................................................................................................ 12

4. Core Pumped Wideband Single Mode TDFA at 2m .................................................................... 14

4.1. Introduction .......................................................................................................................... 14

4.2. Experimental setup ............................................................................................................... 14

4.3. Results and discussion .......................................................................................................... 15

4.4. Conclusions ........................................................................................................................... 16

4.5. Key Results ............................................................................................................................ 16

4.6. References ............................................................................................................................ 16

5. Cladding Pumped 3M-TDFA for Mode Division Multiplexing at 2m .......................................... 18

5.1. Introduction .......................................................................................................................... 18

5.2. Experimental setup for the TM-TDFA ................................................................................... 18

5.3. Gain and noise figure performance ...................................................................................... 19

5.4. Conclusion ............................................................................................................................. 20

5.5. Key Results ............................................................................................................................ 20

5.6. References ............................................................................................................................ 20


Page 3 of 20

1. Introduction

This activities on few mode fibre amplifier development aims to design, implement and characterize a novel

platform of optical amplifiers operating at 1.55m and 2m wavelength regions for mode division multiplexed

(MDM) transmission.

In the 1.55m wavelength range, first few-mode EDFA supporting 2 mode group (3 spatial modes) has been

demonstrated in 2011 and significant progress has been made over the past few years to reduce differential

modal gain (DMG) and to scale up the number of guided modes. This year we have successfully demonstrated a

fully functional 4 mode group (6 spatial modes) EDFA with more than 20dB gain and a moderate DMG (< 2dB)

using a ring-shaped erbium doped fiber combined with bi-directional higher-order mode (LP21) core-pumping.

These 4M-EDFA platforms were delivered to Coriant in Germany and conducted a WDM-MDM transmission

experiment with few-mode inline fiber amplifier. As an alternative pumping scheme, cladding pumping was

employed in our 4M-EDFA to generate sufficient pump power for higher mode count EDFA and to reduce the

cost and/or energy saving of MDM systems. A small signal gain of >20dB was achieved across the C-band with

a differential modal gain of ~3dB amongst the mode groups. A comprehensive theoretical study shows that it

may be possible to achieve low DMG over the full C-band of EDFAs supporting 4 and 6 mode groups by

optimizing erbium dopant distribution of the active fiber with a cladding pumped configuration.

In the 2m wavelength range, we first investigate various optical architectures to enhance the gain of the

single-mode Tm3+

-doped fibre amplifier and a wideband TDFA operation covering from 1650nm to 2050nm

was successfully realized. These first prototype single-mode TDFAs have been delivered to Tyndall in Ireland

and conducted CWDM and DWDM 2m transmission experiment. The amplifier should allow for mode

division multiplexed data transmission in a potential new transmission window around 2m. Two mode group

TDFA was also investigated with cladding pumped configuration.

The present section addresses selected topics related to the development and characterization of both few-mode

EDFA as well as single mode and few-mode TDFAs for MDM transmission. The summary of this activity were:

Er3+

-doped fibre amplifier at 1.55m

- Cladding-pumped 4M and 6M-EDFA design

- Cladding-pumped 4M-EDFA

Tm3+

-doped fibre amplifier at 2m

- Core-pumped single-mode TDFA

- Cladding-pumped 2M-TDFA

In section 2 we present the cladding pumped four-mode-group and six-mode group EDFA designs for mode

division multiplexed transmission systems. A Genetic Algorithm was employed to minimize the differential

modal gain (DMG) over all the supported signal modes (at the same wavelength). The optimal EDFA designs

found through the algorithm provided less than 1 dB DMG across the C-band (1530-1565 nm) whilst achieving

more than 20 dB gain per mode.

In section 3 we experimentally demonstrate for the first time a 4 mode group (6 spatial modes) EDFA with a

cladding pumped architecture. Average modal gains are measured to be >20dB between 1534nm-1565nm with a

differential modal gain of ~3dB among the mode groups and noise figures of 6-7dB. The cladding pumped FM-

EDFA offers a cost effective alternative to core-pumped variant as low cost, high power multimode pumps can

be used, and offers performance, scalability and simplicity to FM-EDFA design.

In section 4, we report the first experimental realization and detailed characterization of single mode TDFAs

specifically designed for optical communications around 2 µm. The amplifier provides high gain (>35 dB), low

noise figure (< 5 dB), and wide bandwidth (>100 nm). To the best of our knowledge, this is the first time a

TDFA has been systematically evaluated for 2 µm optical fiber communications, which from the amplifier

perspective confirms the practicality of such a radical solution should future potential solutions to the “capacity

crunch” problem currently under investigation require it.

In section 5, we report the first demonstration of two-mode group (3 spatial modes) TDFA operating at 2µm for


Page 4 of 20

MDM transmission. Similar gain and noise figure performance for both the LP01 and LP11 modes are obtained in

a cladding pumped 2-mode group TDFA. A maximum gain of 18.3dB was measured at 1970nm with a 3dB gain

bandwidth of 75nm while the average noise figure was measured to be between 7-8dB for wavelengths longer

than 1970nm. We consider this to be an important step in extending MDM transmission to the new 2µm

transmission window providing new options for fiber capacity scaling.


Page 5 of 20

2. Cladding Pumped EDFA Design Supporting 4 and 6

Mode Group

2.1. Introduction

Space Division Multiplexing (SDM) has attracted considerable attention in high-capacity fiber-optic

communication systems as a radical approach to increase the capacity per-fiber [1]. One form of SDM uses Few

Mode Fibers (FMFs) which guide a restricted number of modes to define the independent spatial channels. So

far, both 2 and 4- mode group systems have been reported, with inline Few-Mode Erbium Doped Fiber

Amplifier (FM-EDFA) [2-4]. The majority of the demonstrated FM-EDFAs have been core-pumped with either

one or two single-mode pump diodes [4-5]. Obviously, as the number of modes supported by a single fiber is

increased, the total signal output power (and hence the required pump power needed for high gain, low noise

amplification) increases significantly, dictating the use of multiple, expensive single mode pumps. Reduction in

the cost per transmitted bit is essential for SDM to be seriously considered for commercial deployment and

cladding-pumping using high-power multimode pump diodes (which has already been demonstrated for multi-

core EDFAs [6]) provides a more practical and potentially much cheaper way to provide the pump radiation.

The first demonstration of cladding pumped EDFA supporting four mode groups reported a DMG (i.e. the

maximum gain difference among all the signal modes at the same wavelength) of around 4 dB [7]. When

developing FM EDFAs, minimizing DMG is of paramount importance in ensuring the best overall system

performance. In the modeling of cladding-pumped amplifiers, a simple but effective way to simulate the pump

is to assume that the pump intensity profile is uniform across the doped core [8- 9]. So careful tailoring of the

erbium doping profile in an active multimode fiber is therefore central to minimizing the DMG in cladding-

pumped FM-EDFAs. In this work, we employ a Genetic Algorithm (GA) [10] to help optimize the rare earth

doping profile of a cladding-pumped EDFA, supporting more than two propagating mode groups, in order to

reduce the DMG. The optimum fiber designs provide less than 1 dB DMG across the wavelength range (1530-

1600nm).

2.2. Simulation

The FM-EDFA was simulated using the simulation model described in our previous work [11]. For the noise

calculation, we split the wavelength band from 1490nm to 1630nm into 140 equal-width (1nm) wavelength

slots. In order to manipulate the erbium ion distribution, we chose to divide the erbium-doped core into several

layers whilst keeping the FRIP unchanged, so that the optimal dopant concentration in each core layer can be

numerically investigated for minimization of the DMG. For the purpose of reducing fabrication complexity, the

number of erbium-doped layers should be as few as possible. Through trials, we have found that at least three-

layer and a four layer doping structures (shown in Fig. 1) are required in order to achieve a DMG of less than 1

dB in the C- band (1530-1565 nm) for the 4M-EDFA and 6M-EDFA respectively. Thus for the 4M-EDFA,

there are two structural parameters (i.e. a1, a2 shown in Fig. 1a) determining the dimensions of the three-core

layers and three doping concentration parameters (i.e. ρ1, ρ2, ρ3) describing the erbium ion concentration of

each core layer to be optimized. Similarly, there are three structural parameters (i.e. a1, a2, a3 shown in Fig. 1b)

and four doping concentration parameters (i.e. ρ1, ρ2, ρ3, ρ4) to be optimized for the 6M-EDFA. Due to the

involvement of more than 5 free parameters, we implemented a GA to establish the optimum choice of the

dopant concentration and the dimension of each core layer for the minimization of the DMG. The GA is a

generic optimization tool for optimizing multiple parameters simultaneously based on a natural selection process

that mimics biological evolution. The GA has been successfully applied to several fiber designs, for example

ref. [12]. In the GA implemented here, the fitness function, “F”, that is used to evaluate the ‘quality’ of a given

structure and minimized by the algorithm, is defined as: 1565

1530

( )

( )i

nmi

ave inm

DMGF

G

(1)

Where DMG(i) is the differential modal gain calculated at wavelength i and the sum is performed over 5

uniformly spaced points in the interval. For all modeling work in this paper, a total input signal power of 0 dBm

at a single wavelength, co-propagating with a pump input power of 2.5W are assumed. It is to be noted here that

the total input signal power is equally split into the 6 spatially distinct signal modes in the case of 4M-EDFA

(i.e. LP01, LP11a, LP11b, LP21a, LP21b and LP02), i.e. -7.78 dBm per mode; and 10 spatially distinct signal modes in

the case of the 6MEDFA (i.e. LP01, LP11a, LP11b, LP21a, LP21b, LP02, LP31a, LP31b, LP12a, LP12b), i.e. -10 dBm per

mode. The amplifier model is run five times (to obtain the DMG values needed at different signal wavelengths)

to calculate the fitness function ‘F’ for a given fiber structure. The free parameters (or the ‘genes’ of the

algorithm) are the structural parameters ai (i.e. a1, a2, …) and the doping concentration ρi (i.e. ρ1, ρ2, …) of the


Page 6 of 20

i-th core layer.

Fig. 1: The fiber refractive index profile, the signal mode intensity distributions, and the doping profile

of (a) 4M-EDFA, denoted as ‘F1’ and (b) 6M- EDFA, denoted as ‘F2’, to be optimized through the GA;

where ρi (m-3) is the doping concentration of the i-th core layer.

Fig. 2: The modal gain (continuous line), noise figure (dashed line) and DMG (red dotted line)

characteristics of the best (a) 4-mode-group EDFA and (b) 6-mode-group EDFA calculated by the GA.

2.2.1. Four-mode-group EDFA

We have chosen to study a double-clad, step-index erbium doped fiber (EDF) with a core-to- inner clad

refractive index difference of 0.004 and a core diameter of 23μm. We assume that the fiber is weakly guiding

and supports the lowest four transverse mode groups from 1530 nm to 1565 nm, whose modal intensity profiles

are shown in Fig. 1a. The diameter of the inner cladding was chosen to be 100μm, compatible with our preferred

choice of pigtailed pump diode, and results in a core-to-cladding ratio of about 1: 4.3. The best fiber design (a1 =

3.074 μm, a2 = 8.730 μm, ρ1 = 1.00e+25 m-3

, ρ2 = 5.71e+24 m-3

, ρ3 = 9.98e+24 m-3

) found through the GA is

shown in Fig. 1a, here denoted as ‘F1’. This ‘W’ shaped erbium dopant distribution provides similar amounts of

overlap between the dopants and the signal modes. The gain and noise characteristics of the best fiber design are

plotted in Fig. 2a. A fiber length of 13 m is used to ensure minimal gain tilt across the C-band, as indicated by

the blue-shaded region in Fig. 2a. The gain and noise properties in the L-band is also plotted in the pink-shaded

region as shown in Fig. 2a. The DMG (shown in red dotted line in Fig. 2a) is controlled to better than 1 dB from

1530 nm to 1600 nm, with a minimum of 0.26dB at 1553 nm and maximum of 0.90 dB at 1587 nm. In the C

band, the modal gains are found to be more than 20 dB with a maximum gain of 23.37 dB for the LP01 mode at

around 1565 nm and a minimal gain of 21.51 dB for the LP11 mode at around 1538 nm, for the input

signal/pump power conditions described in section 2. The NFs shown in Fig. 2a are relatively high (i.e. between

5 to 7 dB) at the short wavelength region (1530-1542 nm) where the gain is relatively low. When the gain is

relatively high, in the wavelength region of 1545 nm - 1565 nm, the NFs are found to be below 5 dB. In the L-

band, the modal gains are much lower than those in the C-band, because the amplifier length is not optimized to

achieve high gain in the L-band.

2.2.2. Six-mode-group EDFA

In this instance the core diameter of the EDF is scaled up to 29 μm so as to support six-mode-groups while

keeping the core-to-inner clad refractive index difference and the inner cladding diameter exactly the same as in

the previous case. The FRIP and signal intensity profiles of the 6M-EDFA (denoted as ‘F2’) and its optimal

0 5 10 15 20 250.0

0.5

1.0

1.5

2.0

2.5

0.000

0.002

0.004

0.006

0.008

0.010

Er

ion

co

nce

ntr

ati

on

(1

025m

-3)

Radial position (m)

Er ion distribution

Ref

ract

ive

ind

ex d

iffe

ren

ce

FRIP

0 5 10 15 20 250.0

0.5

1.0

1.5

2.0

2.5

0.000

0.002

0.004

0.006

0.008

0.010

Er

ion

co

nce

ntr

ati

on

(1

025m

-3)

Radial position (m)

Er ion distributionR

efra

ctiv

e in

dex

dif

fere

nce

FRIP a2 a3

ρi

a1

a2

ρi

0

5

10

15

20

25 LP01

LP11

LP21

LP02

a1

0

5

10

15

20

25

LP01

LP11

LP21

LP02

LP31

LP12

(a) (b)

1530 1540 1550 15600.0

0.5

1.0

1.5

0

5

10

15

20

25

30

DM

G (

dB

)

Wavelength (nm)

LP01

LP11

LP21

LP02

LP31

LP12

Ga

in,

NF

(d

B)

1530 1540 1550 15600.0

0.5

1.0

1.5

0

5

10

15

20

25

30

DM

G (

dB

)

Wavelength (nm)

LP01

LP11

LP21

LP02

Ga

in,

NF

(d

B)

DMG

DMG

(a) (b)


Page 7 of 20

erbium dopant profile (a1 = 3.030 μm, a2 = 7.059 μm, a3 = 11.703 μm, ρ1 = 3.09e+24 m-3

, ρ2 = 7.94e+24 m-3

, ρ3 =

4.15e+24 m-3

, ρ4 = 9.85e+24 m-3

) obtained from the GA are shown in Fig. 1b. A fiber length of 12 m is used to

ensure minimal gain tilt across the C band, as shown in the blue-shaded region in Fig. 2b. The gain and noise

properties in the L-band are also plotted in the pink-shaded region in Fig. 2b. As shown in Fig. 2b, the DMGs

(the red dotted line) are found to be below 1 dB with a lowest value of 0.31 dB at 1561 nm and a highest value

of 0.91 dB at 1530 nm. The gain and noise characteristics of F2 are similar to that of F1. In the C-band, the

modal gains are well above 20 dB with a maximum of 23.47 dB for the LP31 mode at around 1557 nm and a

minimum of 21.38 dB for the LP01 mode at around 1538 nm. The NFs of the LP01, LP11 and LP21 modes are

relatively high (larger than 5dB) at the short wavelength region (i.e. 1530 - 1540 nm) but again become less than

5dB for wavelengths longer than 1545 nm. Again, the modal gains in the L-band are not optimum. However, the

optimization of gains in the L-band is beyond the scope of this paper. From more extensive simulations, we

have also found that the changes in length or pump powers (in order to optimize gain, NF at certain signal

wavelengths) of the amplifiers based on fibers F1 or F2 in general only result in a very small variation of the

DMGs (less than 5%).

2.3. Conclusion

We applied a Genetic Algorithm to minimize the differential modal gain in cladding pumped EDFAs supporting

four and six-mode-groups. The optimum 4M-EDFA and 6M-EDFA designs, exhibiting three-layer and four-

layer core structures (denoted as ‘F1’ and ‘F2) respectively, provide less than 1 dB DMG across the C- and L-

band. Over 20 dB gain across the C-band is obtained for both F1 and F2 using a forward pump power of 2.5W

and EDF lengths of 13m and 12m respectively. The amplifier performance in the L-band can be further

improved by optimizing the fiber length and pump power.

2.4. Key Results

• Optimization of the Er doping profile of cladding pumped 4M-EDFA and 6M-EDFA using a Genetic

Algorithm in order to reduce the differential modal gain.

• Good balanced modal gain performance achieved in both 4M-EDFA and 6M-EDFA (Over 20dB gain

and less than 1dB DMG over the C-band while NF varies between 4dB and 5dB).

2.5. References

[1] D. J. Richardson, et al. , “Space-division multiplexing in optical fibers”, Nat. Photonics 7, 354-362 (2013).

[2] E. Ip, et al. , “146λ×6×19-Gbaud Wavelength- and Mode-Division Multiplexed Transmission over 10×50-

km Spans of Few-Mode Fiber with a Gain-Equalized Few-Mode EDFA”, Proc. OFC’13, Paper PDP5A.2

(2013).

[3] V. A. J. M. Sleiffer, et al. , “73.7 Tb/s (96 x 3 x 256-Gb/s) mode-division-multiplexed DP-16QAM

transmission with inline MM-EDFA”, Opt. Express 20(26), B428-B438 (2012)

[4] M. Salsi, et al. , “A six mode erbium-doped fiber amplifier”, Proc. ECOC’ 12, Paper Th.3.A (2012).

[5] Y. Jung, et al., “Few-mode EDFA supporting 5 spatial modes with reconfigurable differential modal gain

control”, Proc. ECOC’13, Paper We.4.A.2 (2013).

[6] K. S. Abedin, et al., “Cladding pumped erbium doped multicore fiber amplifier”, Opt. Express 20(18),

20191-20200 (2012).

[7] E. Lim, et al. , “First Demonstration of Cladding Pumped Few-moded EDFA for Mode Division Multiplexed

Transmission”, Proc. OFC’14, Paper M2J.2 (2014).

[8] D. J. Richardson, et al., "High power fiber lasers: current status and future perspectives," J. Opt. Soc. Am. B:

Opt. Phys. 27, B63-B92 (2010).

[9] A. Hardy, "Signal amplification in strongly pumped fiber amplifiers," IEEE J. of Quantum Electron. 33,

307-313 (1997).

[10] D.E. Goldberg, Genetic algorithms in search, optimization and machine learning, (Addison-Wesley, New

York, 1989).

[11] Q. Kang, et al., “Accurate modal gain control in a multimode erbium doped fiber amplifier incorporating

ring doping and a simple LP01 pump configuration ”, Opt. Express 20 (19), 20835-20843 (2012).

[12] F. Poletti, et al. , “Inverse design and fabrication tolerances of ultra-flattened dispersion holey fibers”, Opt.

Express 13(10), 3728-3736 (2005).


Page 8 of 20

3. Cladding Pumped 6-mode EDFA for Mode Division

Multiplexed Transmission

3.1. Motivation and objectives

Mode division multiplexing (MDM), utilizing few-mode fibers supporting multiple spatial modes, is currently

under intense investigation as an efficient approach to overcome the current capacity limitations (~100Tbit/s per

fiber) of high-speed long-haul transmission systems based on single mode optical fiber. In order to realize the

potential energy and cost savings offered by MDM systems, the individual guided modes should be

simultaneously amplified within a few-mode erbium doped fiber amplifier (FM-EDFA) [1-3] and switched

within a few-mode compatible reconfigurable optical add-drop multiplexer (FM-ROADM). In FM-EDFAs,

mode dependent gain directly affects the system outage probability and minimizing the gain difference between

the guided spatial modes is essential to guarantee robust performance of the overall MDM system. Two main

strategies have been used to equalize the mode dependent gain: i) tailoring the radial erbium-doping

concentration profile of the erbium doped fiber [4] and ii) controlling the pump field intensity distribution [3]. In

practice, a combination of both strategies is generally required for higher mode count amplifiers. To date, FM-

EDFAs that simultaneously amplify up to 4 linearly polarized (LP) mode groups (i.e. 6 spatial modes) [5, 6]

have been demonstrated and a differential modal gain (DMG) of less than 2dB has been achieved using a ring-

doped erbium doped fiber combined with bi-directional higher-order mode (LP21) pumping [6]. However, as the

number of modes is scaled up to 10 and beyond, it will become increasingly challenging to meet the associated

pump power requirements with single-mode pump diodes. Moreover, even if this is technically possible,

multiplexing a number of single-mode pump diodes to generate sufficient pump power is an expensive way of

pumping such an amplifier. Cladding pumping represents a promising way to address these issues. High power,

low cost (in terms of $/W) multimode pump diodes operating in the multi-watt regime are now readily available

given the emergence and commercial success of the high power cladding pumped fiber laser. Cladding pumping

has previously been shown as a viable approach to pumping both multi-core [7] and multi-element fiber

amplifiers [8], but as of yet has not been fully demonstrated for a few-mode fiber amplifier. Herein, we

demonstrate for the first time the feasibility and detailed characterization of a cladding pumped FM-EDFA

amplifying all 6 spatial modes of an erbium doped fiber, note provisional results were reported in [9]. The fiber

was fabricated with a low-index cladding material to ensure compatibility with direct-diode cladding pumping

through one end of the gain fiber while a phase plate based mode multiplexer/demultiplexer was employed to

characterize the amplifier performance. The shape of the gain spectra of the 6M-EDFA was similar to that of the

conventional cladding pumped single-mode EDFA and an average signal gain of 20dB and DMG of ~3dB were

achieved across the C-band at an input signal power of -7.5dBm per mode. The detailed WDM gain spectra for

various fiber lengths, pump powers and input signal powers was also investigated.


Page 9 of 20

3.2. Experimental setup for the cladding pumped 6M-EDFA

Fig. 3: (a) Schematic diagram of the cladding pumped 6M-EDFA, (b) mode multiplexer based on binary phase plates and (c)

signal mode profiles before (top) and after (bottom) amplification. BS: non-polarizing beam splitter, P1-P3: phase plates,

DM: dichroic mirror, SMF: single mode fiber, MMF: multimode fiber, 6MF: passive 6-mode fiber, 6M-EDF: 6-mode

erbium doped fiber, AMP: amplifier.

Figure 3(a) shows the schematic diagram of our cladding pumped 6-mode EDFA (6M-EDFA) that

simultaneously amplifies the LP01, LP11a, LP11b, LP21a, LP21b and LP02 signal modes. To evaluate the gain

performance of the 6M-EDFA under a wavelength division multiplexing (WDM) configuration, 9 external

cavity lasers at distinct wavelengths across the C-band (1534-1560nm) were multiplexed to provide probe

signals. As the shortest available wavelength was 1534nm we couldn’t measure the amplifier performance over

the full C-band. An additional L-band tunable laser source (1565-1575nm) was combined to characterize the

short wavelength L-band edge of the amplifier. The 10 combined lasers across the C+L band were split into four

equally powered signal tributaries and fed to the mode multiplexer. The mode multiplexer was based on an

arrangement of phase plates and beam splitters in order to selectively excite the individual transverse modes in a

step-index 6-mode passive fiber (6MF, OFS Denmark) as depicted in Fig. 3(b). The 6MF was then spliced

directly to a double clad 6-mode erbium doped fiber (6M-EDF), in which the erbium ion density was raised at

the edges of the fiber core relative to the center of the core to help mitigate the DMG. The fiber was drawn from

the same preform as in [1], but with the key difference that the current fiber has a low refractive index (n~1.375)

polymer coating to guide pump light in the glass cladding. A detailed refractive index profile of the fiber is

presented in [1]. The splice losses between the 6MF and 6M-EDF for the different modes were measured as

0.5±0.2dB for all guided modes. The 6M-EDF has an outer cladding diameter of 97µm and a core diameter of

~26µm. The estimated effective NA of the core is ~0.10. The estimated absorption for 975nm pump light

launched into the cladding is ~1.08dB/m. The fiber was pumped using a counter-directional pumping scheme

incorporating a dichroic mirror which is highly reflective (>99%) at the pump wavelength and highly

transmissive (~98%) at the signal wavelength. The multimode pump module can deliver an optical power of up

to ~10W and was wavelength-stabilized at 975nm with a volume Bragg grating. The coupling efficiency of the

pump power into the inner cladding was measured to be ~80%. To suppress any Fresnel reflections of the signal

retro-reflecting back into the core, coreless end caps (diameter=200µm, length~600µm) were spliced at both

ends of the spliced active/passive fiber assembly. A polarization insensitive free-space optical isolator was used

to further prevent unwanted reflections. The amplified output was de-multiplexed by a simplified phased plate

WDM (9 ch)

1534-1560nm

50/50 AMP

TLS (1 ch)

1565-1575nm

MU

X Double clad

6M-EDF 6MF OSA

DM

SMF

SMF

LP01

SMF

phase

plate

LP11

LP21

f1 f2

6MF

BS

Isolator f1

SMF

LP02

p 0

p 0 p

0

p 0

0 p

P1, P2, P3

SMF

Mode MUX

(Input signal mode profile)

(Amplified signal mode profile)

LP01 LP11 LP21 LP02

f1

f1=4.5mm

f2=125mm

f2 f1

(a)

(b) (c)


Page 10 of 20

based de-multiplexer, which is composed of three phase plates (i.e. LP21a, LP21b, LP02 phase plates). By placing

the amplified signal beam at different spatial positions of the binary LP21a phase plate (e.g. uniform sector for

LP01 excitation, vertical half-sector for LP11a, horizontal half-sector for LP11b, and central quarter-sector for

LP21a) the desired mode can be converted into an LP01 mode and coupled efficiently into a single mode fiber.

Note that the two (nominally degenerate) orthogonal modes of the LP11 and LP21 mode groups experience

continuous mode-mixing along the length of the fiber and the spatial orientation of the lobes can also be altered

by external perturbations of the fiber. Therefore in order to minimize the uncertainty, the gain of LP11 mode

should be measured in two directions, perpendicular to each other, and the average of the two measurements

then used to estimate the average modal gain. In a similar way, the gain of LP21 mode can be estimated from

two spatial orientations of the LP21 phase plate, this time however separated by an angle of 45 degrees. To

confirm clean amplification of the input signals, transverse mode images were taken by a CCD before and after

amplification when the individual modes are amplified separately by the 6M-EDFA. As shown in Fig. 3(c) top,

the mode profiles of the four input signals are well defined after propagating through 10m of passive 6MF. The

quality of the individual input modes was largely preserved during amplification although some small

degradation can be noticed by careful comparison of the images in Fig. 3(c) (top versus bottom).

3.3. Measured gain and noise figure performance of the 6M-EDFA

The left hand plots of Fig. 4 show the measured gain spectra of the guided modes for three different lengths of

6M-EDF, i.e. 6m, 3.5m and 2m respectively, at a constant input signal power of -7.5dBm per mode. For the 6m

length of EDF (Fig. 4a), the gain peaked at ~1565 nm and exhibited a relatively narrow 3dB gain compression

bandwidth of ~10nm spanning from 1560nm to 1570nm. In order to shift the gain peak towards 1550nm to

better align with traditional C-band operation, we reduced the EDF length to 3.5m. As shown in Fig. 4b, this

resulted in a flatter gain spectrum with a 3dB gain compression bandwidth increased to ~20nm spanning from

1545nm to 1565nm. When the fiber length was further reduced to 2m (Fig. 4c) the gain was flattened over a

30nm bandwidth. Although the reduction in the EDF length reduced the overall pump absorption to ~2dB, the

FM-EDFA still exhibited respectable modal gains of ≥20dB between 1534nm to 1565nm. Note that the

maximum injected pump powers were different for different fiber lengths. For example, for L=6m, the

maximum injection pump power was about 1.3W but it was 3.1W for L=2m. This behavior can be easily

understood from the fact that input signal wavelengths are mostly situated in the C-band while the gain peak for

the L=6m EDF is located at 1565nm with a severe gain tilt, resulting in parasitic lasing when the amplifier was

pumped harder. The variation of noise figure (NF) as a function of fiber length is also shown in the right hand

plots of Fig. 4. Figure 4(a) shows that the NF rises sharply for wavelengths shorter than 1565nm while it tends

to decrease at longer wavelengths. The high NF at short wavelengths is mainly due to insufficient population

inversion within the active medium due to the limitations on the maximum allowable pump power set by

parasitic lasing and is clearly evident from the sharp drop in signal gain. The situation improved dramatically

when the EDF length was shortened to 2m. The NF was improved from 13-14dB to 6-7dB for all guided modes

due to the gain enhancement at the short wavelengths. The flattened gain spectra in Fig. 4(c) lead us to believe

that the amplifier will have similar gain/NF performance around the short wavelength edge of the C-band. The

NF of our cladding pumped 6M-EDFA is slightly higher compared to that of typical core pumped EDFAs but

the values are still reasonable when compared to cladding pumped single mode EDFAs [15-17]. We estimate a

measurement error of ±0.5dB in all of our gain and NF measurements.


Page 11 of 20

Fig. 4: Measured gain spectra (left) and noise figure (right) of the individual guided modes for three

different fiber lengths; (a) 6m, (b) 3.5m and (c) 2m.

We further investigated the performance of our cladding pumped 6M-EDFA as a function of the pump

power and input signal power. As shown in Fig. 4, the optimum fiber length was 2m in terms of the flattened

gain spectra over the C-band however we chose a 3.5m length of fiber for the detailed power performance

characterization because it improved the overall pump absorption and thereby the signal gains. Firstly, Fig. 5

depicts the modal gain and noise figure plots for different pump powers at a fixed input signal power of -

7.5dBm per mode. For low pump power (~1.1W), the amplifier gain spectrum experiences a severe gain tilt

where the gain peak located at ~1565nm with very little gain available at short wavelengths due to the low

inversion level. As the pump power was increased from 1.1W to 2.4W, the signal gains of each mode increased

due to the increased population inversion level however the gain at the short wavelengths grows more quickly.

The average DMG was measured to be ~3dB across the C-band. It is also evident that the noise figure of our

cladding pumped 6M-EDFA improves with increasing pump power (inversion) and the measured NF was below

6~7dB for wavelengths longer than 1540nm.

Secondly, the mode dependent signal gain and NF were measured as a function of input signal power at a

fixed pump power of 1.6W as shown in Fig. 6. The signal gain decreases with an increase in input signal power

for all modes due to the amplifier being operated in the saturated gain regime. The DMG did not show much

dependence on the input signal power and remains at ~3dB for all the input signal powers investigated. The

measured NF remains almost constant, regardless of the changes in input signal powers.

Fig. 5. Measured modal gain (a) and noise figure (b) as a function of launched pump power. The input

signal power is -7.5dBm per mode.

1530 1540 1550 1560 1570 15800

10

20

30

1530 1540 1550 1560 1570 15800

10

20

1530 1540 1550 1560 1570 15800

10

20

30

1530 1540 1550 1560 1570 15800

10

20

1530 1540 1550 1560 1570 15800

10

20

30

1530 1540 1550 1560 1570 15800

10

20L=2m, Pp=3.1W

L=3.5m, Pp=2.4W

Ga

in [d

B]

LP01

LP11

LP21

LP02

L=6m, Pp=1.3W

No

ise

fig

ure

[d

B] LP01

LP11

LP21

LP02

Ga

in [d

B]

LP01

LP11

LP21

LP02

No

ise

fig

ure

[d

B] LP01

LP11

LP21

LP02

Ga

in [d

B]

Wavelength [nm]

LP01

LP11

LP21

LP02

(c)

(b)

No

ise

fig

ure

[dB

]Wavelength [nm]

LP01

LP11

LP21

LP02

(a)

1530 1540 1550 1560 1570 15800

5

10

15

20

25

Ga

in [

dB

]

Wavelength [nm]

1.1W: LP01, LP11, LP21, LP02

2.4W: LP01, LP11, LP21, LP02

1530 1540 1550 1560 1570 15800

5

10

15

20

No

ise

fig

ure

[d

B]

Wavelength [nm]

1.1W: LP01, LP11, LP21, LP02

2.4W: LP01, LP11, LP21, LP02

(for different pump power)

Ps=-7.5dBm (a) (b)


Page 12 of 20

Fig. 6. (a) Measured modal gain and (b) noise figure as a function of input signal power per mode at a

fixed pump power of ~1.6W.

3.4. Conclusion

We have demonstrated for the first time a cladding pumped few-mode EDFA supporting 6 spatial modes. A

small signal gain of >20dB was achieved across the C-band with a differential modal gain of ~3dB amongst the

mode groups while the average noise figure was measured to be between 6-7dB. Reduction in the EDF length

from 6m to 2m widens the operating window of the FM-EDFA from 10nm to 30nm and ensures near full C-

band operation. The amplifier performance could be further improved by optimizing the core dopant distribution

and by reducing the core-to-clad area ratio. We consider this to be an important step in increasing the mode

scalability of the few-mode EDFA, offering cost-effective and efficient amplification of a large number of

spatial data channels in a single device.

3.5. Key Results

First cladding pumped few-mode EDFA supporting 6 spatial modes.

Achieved modal gains >20dB between 1534-1565nm with differential modal gain <3B

NF <6-7dB for wavelengths longer than 1540nm.

3.6. References

[1] Y. Jung, S. Alam, Z. Li, A. Dhar, D. Giles, I. Giles, J. Sahu, F. Poletti, L. Grüner-Nielsen, and D.

Richardson, "First demonstration and detailed characterization of a multimode amplifier for space division

multiplexed transmission systems," Opt. Express 19, B952-B957 (2011).

[2] Y. Jung, Q. Kang, V. A. J. M. Sleiffer, B. Inan, M. Kuschnerov, V. Veljanovski, B. Corbett, R. Winfield, Z.

Li, P. S. Teh, A. Dhar, J. Sahu, F. Poletti, S. -U. Alam, and D. J. Richardson, "Three mode Er3+ ring-doped

fiber amplifier for mode-division multiplexed transmission," Opt. Exp. 21, 10383-10392 (2013).

[3] N. Bai, E. Ip, T. Wang, and G. Li, “Multimode fiber amplifier with tunable modal gain using a

reconfigurable multimode pump,” Opt. Express 19, 16601-16611 (2011).

[4] Q. Kang, E. L. Lim, Y. Jung, J. K. Sahu, F. Poletti, C. Baskiotis, S. U. Alam, and D. J. Richardson,

“Accurate modal gain control in a multimode erbium doped fiber amplifier incorporating ring doping and a

simple LP01 pump configuration,” Opt. Express 20, 20835-20843 (2012).

[5] M. Salsi, D. Peyrot, G. Charlet, S. Bigo, R. Ryf, N. Fontaine, M. Mestre, S. Randel, X. Palou, C. Bolle, B.

Guan, G. Le Cocq, L. Bigot, and Y. Quiquempois, "A Six-Mode Erbium-Doped Fiber Amplifier," in 38th

European Conference and Exhibition on Optical Communication, paper Th.3.A.6 (2012).

[6] Y. Jung, Q. Kan, J. K. Sahu, B. Corbett, J. O’Callagham, F. Poletti, S. U. Alam, and D. J. Richardson,

“Reconfigurable modal gain control of a few-mode EDFA supporting six spatial modes,” IEEE Photon.

Technol. Lett. 26, 1100-1103 (2014).

[7] K. S. Abedin, T. F. Taunay, M. Fishteyn, D. J. DiGiovanni, V.R. Supradeepa, J. M. Fini, M. F. Yan, B. Zhu,

E. M. Monberg, and F.V. Dimarcello, "Cladding-pumped erbium-doped multicore fiber amplifier," Opt.

Exp. 20, 20191-20200 (2012).

[8] S. Jain, V. J. F. Rancaño, T. C. May-Smith, P. Petropoulos, J. K. Sahu, and D. J. Richardson, "Multi-element

fiber technology for space-division multiplexing applications," Opt. Express 22, 3787-3796 (2014).

1530 1540 1550 1560 1570 15800

5

10

15

20

25

Ga

in [

dB

]

Wavelength [nm]

-12.5dBm: LP01, LP11, LP21, LP02

-7.5dBm: LP01, LP11, LP21, LP02

(for different input signal power)

Pp=1.6W

1530 1540 1550 1560 1570 15800

5

10

15

20

No

ise

fig

ure

[d

B]

Wavelength [nm]

-12.5dBm: LP01, LP11, LP21, LP02

-7.5dBm: LP01, LP11, LP21, LP02

(a) (b)


Page 13 of 20

[9] E. L. Lim, Y. Jung, Q. Kang, T. C. May-Smith, N. H. L. Wong, R. Standish, F. Poletti, J. K. Sahu, S. Alam,

and D. J. Richardson, "First Demonstration of Cladding Pumped Few-moded EDFA for Mode Division

Multiplexed Transmission," in Optical Fiber Communication Conference, OSA Technical Digest (Optical

Society of America, 2014), paper M


Page 14 of 20

4. Core Pumped Wideband Single Mode TDFA at 2m

4.1. Introduction

As a result of the exponentially increasing volume of internet traffic, today’s telecom networks are rapidly being

driven towards their capacity limits. The quest for increasing transmission capacity has stimulated interest in

radical approaches, which may eventually justify a shift away from the traditional operating wavelengths around

1.55 µm [1]. Thulium-doped fiber amplifiers (TDFAs) operating around 2 µm offer the broadest gain spectrum

of all rare-earth doped fiber amplifiers and represent an attractive route towards significantly enhanced

transmission bandwidths [2]. Additionally, their operating region overlaps with the predicted minimum loss

window of hollow-core photonic bandgap fibers (HC-PBGFs), which hold great promise as a transmission

medium due to their ultra-low nonlinearity and faster transmission speed as compared to conventional solid core

fibers [3]. Recently, the first amplified data transmission system at 2 µm using TDFAs and HC-PBGFs has been

demonstrated, confirming the viability/practicability of this approach [4]. TDFAs have recently been

characterized extensively in an optical communications context, demonstrating high gain, low noise

amplification over more than 100 nm bandwidth around 2 µm 5. The current implementations are typically in-

band fiber laser pumped, which would limit their applicability in real-life transmission systems. Ultimately,

TDFAs will have to reach the same level of compactness, reliability and efficiency as current Erbium-based

systems in order to be considered as a true alternative solution, i.e. laser diode pumping is indispensable. Diode-

pumped TDFAs have been developed for emission in the S-band at 1470 – 1500 nm 6, but to date no such

pumping scheme has been demonstrated suitable for optical communications in the new waveband of interest at

2 µm. In this contribution we present the first implementations of TDFAs operating in the 2 µm wavelength

region, in-band pumped by laser diodes at 1550 nm. We demonstrate amplification over a 240 nm wide window

with up to 36 dB gain, noise figure as low as 4.5 dB and up to 100 mW saturated output power by combining

three TDFA designs optimized for short, central and long wavelength operation, respectively. These results

represent a major stepping stone in the assessment of such a radically new solution for next generation

transmission systems.

4.2. Experimental setup

Fig. 7: Experimental setup. TLS: tunable laser source; LD: laser diode; WDM: wavelength division multiplexer; TDF:

thulium-doped fiber.

The experimental setup of the diode-pumped TDFA is shown in Fig. 7. We employed a commercially available

single-mode Tm3+-doped fiber (OFS TmDF200) with ~6.2 µm mode-field diameter at 2000 nm and core

absorption of ~20 dB/m at 1565 nm. Three TDFA implemen-tations were investigated using 2 m, 4 m, and 8 m

of gain fiber, denoted TDFA-S/C/L, which optimize the amplifier performance at short, central, and long

wavelength bands, respectively. The fiber was core pumped in a bidirectional configuration by two Fabry–Pérot

(FP) laser diodes (Princeton Lightwave) operating at 1550 nm with 3 dB bandwidth of ~4 nm, each delivering

up to 210 mW (23.2 dBm) of pump power. Pump and signal wavelengths were combined using two 1570 / 2000

nm WDM couplers. Isolators were placed both at the input and output ends to prevent parasitic lasing. For

characterization, the TDFAs were seeded by an in-house built tunable laser source (TLS) providing narrow

linewidth operation in the range 1790 – 1990 nm. Additionally, we used three discrete-mode (DM) laser diodes

(Eblana Photonics) emitting at 2008 nm, 2025 nm, and 2045 nm to evaluate the TDFA performance at the long

http://en.wikipedia.org/wiki/Fabry%E2%80%93P%C3%A9rot_interferometer


Page 15 of 20

wavelength edge of the amplification bandwidth. A power meter and an optical spectrum analyzer were used to

measure gain and noise figure (NF) of the amplifiers.

4.3. Results and discussion

Fig. 8: Detailed wideband performance of TDFA-C using 4 m of gain fiber. (a) Wavelength dependence of small

signal gain (seed power: -20 dBm), saturated gain (seed power: 0 dBm), and noise figure (NF) for both gain curves. (b)

Output spectra over the tested wavelength band for 0 dBm seed power.

Fig. 8(a) shows the detailed characterization of the TDFA-C implementation, which uses 4 m of gain fiber. The

figure presents the wavelength dependence of the small-signal gain (measured with an input seed power of -20

dBm), the saturated gain (measured with an input seed power of 0 dBm), as well as the external NF for both

gain curves. The total pump power delivered by both pump diodes together is 26.2 dBm in all cases. The

amplifier achieves a small-signal peak gain of 36 dB at 1900 nm and provides gain over a more than 215 nm

wide window in the range 1830 – 2045 nm. Note that we could not perform measurements at longer

wavelengths due to the lack of a suitable seed source. The saturated gain curve is flat and only varies between

18 – 20 dB in the 1840 – 2010 nm waveband. We could not perform saturated gain measurements using the DM

diodes at 2025 nm and 2045 nm, because their output power level is below 0 dBm. There is no significant

difference in NF between small and saturated input signal powers, varying between 5 and 7 dB over the entire

tested spectral range. A single, compact and diode-pumped TDFA is therefore able to deliver more than 20 dB

small-signal gain and less than 7 dB NF over a nearly 200 nm wide transmission window in the 2 µm

wavelength region. The resulting amplified spectra have more than 30 dB optical signal to noise ratio (OSNR),

as shown in Fig. 8(b). The amplifier performance at the short and long wavelength edge of the amplification

band can be improved by varying the length of employed gain fiber. Fig. 9(a) compares the external small-signal

gain and NF performance of all three different amplifier configurations. As the gain fiber length is increased, the

maximum of the gain curve shifts from 1880 nm for TDFA-S (2 m of TDF) to 1950 nm for TDFA-L (8 m of

TDF). This behavior is due to the fact that at room temperature Tm3+ is effectively a three level laser system

resulting in the reabsorption of short wavelength components with the increase in fiber length. In comparison to

the performance of TDFA-C discussed above, TDFA-S provides up to 10 dB gain enhancement and up to 3 dB

improvement in NF for wavelengths below 1860 nm. Similarly, TDFA-L provides up to 4 dB higher gain and

up to 1 dB lower NF than TDFA-C for wavelengths beyond 1970 nm. TDFA-L also exhibits the lowest overall

measured NF of 4.5 dB at 2025 nm. However, in this amplifier configuration the NF rises sharply for

wavelengths below 1900 nm. We found that increasing the length of gain fiber above 8 m does not provide any

further performance advantage at long wavelengths. The above discussion highlights the possibility that high

gain, low noise amplification over the entire 240 nm band investigated can be achieved by combining all three

amplifier configurations in a transmission system, where short wavelengths up to 1860 nm are amplified by

TDFA-S, the central waveband 1860 – 1970 nm by TDFA-C, and TDFA-L provides amplification for longer

wavelengths. The combined amplifier system would provide more than 20 dB gain in the waveband 1810 –

2025 nm, and more than 16 dB at up to 2050 nm. These gain figures can be improved in the future using higher

pump power. The NF for the combined system is flat at ~5 dB over almost the entire waveband tested, but rising

rapidly below 1840 nm. The degradation of gain and NF below 1840 nm in both TDFA-S and TDFA-C is

caused by the exponentially increasing insertion loss of the WDM couplers, as shown in the inset of Fig. 9(a).


Page 16 of 20

We therefore stress that the reported performance, especially the NF, is mainly limited by the insertion loss and

operating bandwidth of the first generation fiber components at 2 µm. There is no fundamental reason why

TDFAs should be noisier than erbium based systems, and we expect the NF to approach the values known for

EDFAs once a new generation of low-loss components becomes available for the 2 µm region. Therefore, we

expect the true operating window of TDFAs to be even broader than physically demonstrated here, especially on

the short wavelength side. Seeding of TDFA-C at 1930 nm was chosen to demonstrate the power scaling

capability of the amplifier. Fig. 9(b) shows the variation of output power with increasing pump power of the

backward-pumping LD-2, while the forward-pumping LD-1 was fully powered up. We obtained more than 100

mW saturated output power at a total pump power of 420 mW with a slope efficiency of 40%.

Fig. 9: (a) Small-signal gain and noise figure (NF) of the TDFA incorporating different lengths of fibers. The

inset shows the measured signal loss of the WDM coupler. (b) TDFA-C efficiency at 1930 nm.

4.4. Conclusions

We presented the first in-band diode-pumped TDFAs operating in the 2 µm region and tested their application

as high performance amplifiers in potential future telecommunication networks. The TDFAs are analogous in

implementation and function to current EDFAs, but offer a far more extended bandwidth in this new waveband

of interest. By combining three different designs optimized for short, central and long wavelength operation,

respectively, we demonstrated amplification over a 240 nm wide window in the range 1810 – 2050 nm with up

to 36 dB gain, NF as low as 4.5 dB, and up to 100 mW saturated output power. Bandwidth and performance

were limited by the insertion loss of the employed components. Our results confirm the practicality of 2 µm

optical fiber communications from an amplifier perspective, and represent a significant advancement in terms of

compactness, robustness, controllability and power consumption of high performance TDFAs compared to

earlier fiber-laser-pumped systems.

4.5. Key Results

• First systematic evaluation of a TDFA for 2 m optical fiber communications.

• TDFA-C and TDFA-L by choosing two different designs, optimized for short and long wavelength

operation.

• Small signal gains of more than 30dB and NFs lower than 6.3dB over a 110nm bandwidth.

4.6. References

[1] T. Morioka et al., “Enhancing optical communications with brand new fibers," IEEE Commun. Mag. 50,

s31-s42 (2012).

[2] S. D. Jackson, "Theoretical modeling of Tm-doped silica fiber lasers," J. Lightw. Technol. 17, 948-956

(1999).

[3] F. Poletti et al., "Towards high-capacity fibre-optic communications at the speed of light in vacuum," Nature

Photon. 7, 279-284 (2013).

(a) (b)


Page 17 of 20

[4] M. Petrovich et al., "First demonstration of 2µm data transmission in a low-loss hollow core photonic

bandgap fiber," in ECOC (2012), paper Th.3.A.5.

[5] Z. Li et al., "Thulium-doped fiber amplifier for optical communications at 2 µm," Opt. Express 21, 9289-

9297 (2013)

[6] T. Kasamatsu et al., “Lser-diode pumping 1.4 and 1.56um) of gain-shifted thulium-doped fibre amplifier,”

Electron. Lett. 36, 1607-1609 (2000).


Page 18 of 20

5. Cladding Pumped 3M-TDFA for Mode Division

Multiplexing at 2m

5.1. Introduction

Space division multiplexing (SDM) [1] has attracted considerable attention amongst the high-capacity fiber-

optic communications community as a radical approach to increase data capacity by employing multiple

distinguishable spatial information channels through the same fiber. Several different transmission fibers (e.g.

multicore fibers and multimode fibers) are being intensively investigated around the globe and a 10-fold

increase in overall fiber capacity has already been achieved in little more than 2 years [2]. However the

nonlinearity of these solid-core silica fibers still remains at a similar level to that of SMF and this will ultimately

limit the maximum power that can be used. Moreover, the gain bandwidth of the erbium doped fiber amplifier

(EDFA) will further constrain the overall maximum achievable per-fiber capacity. In recent years, there has

been emerging research activity looking at overcoming the aforementioned limitations by moving to hollow

core photonic bandgap fiber (HC-PBGF) which offers the potential for ultralow nonlinearity, lower loss and

lower latency than conventional silica glass fibers [3]. Furthermore, the anticipated low loss spectral window of

HC-PBGFs is around 2 m, which overlaps with the gain bandwidth of the thulium doped fiber amplifier

(TDFA) [4], which offers the broadest gain bandwidth (extending potentially from 1700 to 2100nm) amongst all

rare earth doped fiber amplifiers. To this end, in ref. [5], the first 2μm data transmission over a HC-PBGF was

accomplished using an in-line single-mode TDFA. This transmission experiment demonstrated the technical

viability of using a new broadband optical communication window around 2μm. Moreover, low loss HC-PBGFs

are inherently multimode, raising the intriguing prospect of exploiting SDM to further enhance fiber capacity.

In this paper we present the first demonstration of a multimode (two mode-group, namely LP01 and LP11) TDFA

for SDM transmission around 2μm.

5.2. Experimental setup for the TM-TDFA

Fig. 10. Schematic diagram of the TMG-TDFA using backward cladding pumping at 790nm.

Fig. 10 shows a schematic diagram of our TMG-TDFA and associated setup for gain characterization. To

emulate the amplifier performance under a WDM configuration, the gain of the amplifier was measured under

constant saturation conditions by saturating the amplifier with one strong signal and measuring the gain at other

wavelengths with a much weaker probe. To provide a saturating signal, the output from a discrete-mode (DM)

laser diode (Eblana Photonics) operating at 2000nm was pre-amplified using an in-house built single-mode

TDFA and spectrally filtered by a narrow optical bandpass filter (FWHM=3nm) to eliminate amplified

spontaneous emission (ASE) noise. As a probe signal, an in-house built, narrow-linewidth tunable laser source

(TLS) operating over the range 1930-2020nm was used. Additionally, a second DM laser diode at 2045nm was

used to evaluate the TMG-TDFA performance at the long wavelength edge of the amplifier. A phase-plate based

mode multiplexer (MUX) was used to selectively excite the pure LP01 and LP11 signal modes of a 10m long

passive 2-mode group fiber (TMF) via a telecentric lens arrangement. The passive TMF (a graded index fiber

with 20 -long double-clad TMG-TDF

with a circular core to guide the signal and a D-shaped silica inner cladding with a low-index polymer outer

cladding to guide the heavily multimode 790nm pump beam. The refractive index profile and fiber geometry are

shown in Fig. 11a. The in-house fabricated active TMG-TDF has a high thulium concentration of approximately

SMF TMF

SM

F

BS LD(2000nm) +

AMP+NBPF

Double clad

TM-TDF Pump LD (p=790nm)

SM

F

OSA

MMF (105/125m)

DM

Mode MUX

multimode-AMP

Mode DEMUX

Saturating signal, ss

Probe signal, ps

TLS (1930~2020nm) + LD (2045nm)

f1 f2 f1

f1=4.5mm

f2=125mm

f3=8mm

f1

f3 f3

f3

1 2

3

p 0

p

PP 1 2

p 0


Page 19 of 20

3.2±0.3% by weight. The core diameter of the fiber is 12.6µm with a NA of 0.22 and the inner cladding

diameter is 125µm with an NA of 0.45. The large mode field diameter mismatch between active and passive

fibers results in a ~3dB splice loss for both LP01 and LP11 modes. The measured cladding pump absorption was

found to be ~12.1dB/m at 790nm. The 790nm multimode semiconductor pump laser has a maximum output

power of 3.2W and the pump light was free-space coupled into the inner cladding of the TMG-TDF in a

backward pumping configuration with an estimated coupling efficiency of ~80%. The amplified output was

separated from the incoming pump light by a dichroic mirror (high reflection at 2000nm and high transmission

at 790nm) and de-multiplexed by a simplified phase plate based de-multiplexer (DEMUX). By placing the

amplified signal beam at different spatial positions on the binary LP21 phase plate, the desired modes can be

converted into LP01 and coupled efficiently into a single mode fiber. The beam quality of the individual input

signals was largely preserved during amplification as shown in the CCD images in Fig. 11b.

Fig. 11. (a) Refractive index and mode profiles of the (in-house fabricated) TM-TDF, and (b) signal mode

profiles after passive TMF (top row), across the splice (middle row) and after amplification (bottom row).

5.3. Gain and noise figure performance

Firstly, we assigned the saturating signal to the LP01 spatial mode while the probe signal was assigned to either

the LP01 or LP11 mode. Secondly the saturating signal was assigned to the LP11 mode while the probe signal

was assigned to either the LP01 or LP11 mode. In both cases similar performance in terms of gain and noise

figure were observed. Fig. 12a shows the amplifier performance for the first case (i.e. ss=LP01 and ps=LP01

or LP11). A total output signal power of 15.6dBm was achieved for a cladding pump power of 3.2W which is

limited by our pump diode. The saturating and small probe input signal powers in the gain measurement were

0dBm and -15dBm, respectively. The maximum signal gain was found to be about 18.3dB for the LP11 and

17dB for the LP01 modes at 1970nm for the maximum available pump power of 3.2W. The 3dB gain bandwidth

was measured to be ~ 75nm for both modes. Fig. 12b shows that the NF rises sharply for wavelengths shorter

than 1960nm while it tends to decrease at longer wavelengths. The high NF at short wavelengths is mainly due

to insufficient population inversion (and hence effective signal loss) at the amplifier input and which is clearly

evident from the sharp drop in signal gain. The NF could be improved by increasing the pump power or

core/inner cladding diameter ratio so as to increase the average population inversion. The average NF was about

7-8dB for wavelengths longer than 1970nm for both modes. The NF values are relatively high compared to

those of typical core pumped EDFAs but they are still reasonable when compared to cladding pumped EDFAs.

from passive TMF (10m)

from TMF+active TDF (15cm)

After amplification

-10 0 10 20

0.000

0.005

0.010

0.015

0.020

n

Radius [m]

RIP

LP01

LP11

(a) (b)

1920 1940 1960 1980 2000 2020 20400

5

10

15

20

Ga

in [

dB

]

Wavelength [nm]

LP01

LP11

1920 1940 1960 1980 2000 2020 20400

5

10

15

20

NF

[d

B]

Wavelength [nm]

LP01

LP11

1950 2000 2050

-40

-20

0

Ou

tpu

t p

ow

er

[dB

m]

Wavelength [nm]

ss

(a) (b)

ps

75nm


Page 20 of 20

Fig. 12. Measured modal gain (a) and noise figure (b) as a function of wavelength at a fixed input probe

signal power of -15dBm and pump power of 3.2W. Inset: output spectra of the fiber amplifier.

5.4. Conclusion

A two-mode group thulium-doped fiber amplifier has been successfully demonstrated for the first time and

exhibits relatively good performance. We consider this to be an important step in extending SDM transmission

to the new 2m transmission window providing new options for fiber capacity scaling.

5.5. Key Results

• First two-mode group TDFA for MDM transmission at 2m.

• A maximum signal gain ~18.3dB at 1970nm and a 3dB gain bandwith of 75nm.

• Average noise figure~ 7-8dB for wavelengths longer than 1970nm.

5.6. References

[1] D. J. Richardson et al., “Space-division multiplexing in optical fibres,” Nat. Photonics 7, 354-362 (2013).

[2] V.A.J.M. Sleiffer et. al., “73.7Tb/s (96x3x256-Gb/s) mode-division-multiplexed DP-16QAM transmission

with inline MM-EDFA,” Opt. Exp. 20, B428-438 (2012).

[3] F. Poletti et al., “Towards high-capacity fibre optic communications at the speed of light in vacuum,” Nat.

Photonics 7, 279–284 (2013).

[4] Z. Li et al., “Thulium-doped fiber amplifier for optical communications at 2 µm,” Opt. Exp. 21, 9289–9297

(2013).

[5] M. N. Petrovich et al., "Demonstration of amplified data transmission at 2 µm in a low-loss wide bandwidth

hollow core photonic bandgap fiber," Opt. Exp. 21, 28559-28569 (2013).

http://www.opticsexpress.org/abstract.cfm?&uri=oe-21-8-9289

http://www.opticsexpress.org/abstract.cfm?&uri=oe-21-8-9289

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