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doi:10.1016/j.op

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Optics & Laser Technology 39 (2007) 1111–1114

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A four-passed ytterbium-doped fiber amplifier

Liu Yan�, Wang Chunyu, Lu Yutian

Shanghai Institute of Optic and Fine Mechanic, The Chinese Academy of Sciences, Shanghai, 201800, China

Received 20 January 2006; received in revised form 12 September 2006; accepted 22 September 2006

Available online 1 November 2006

Abstract

In this paper, a four-passed ytterbium-doped fiber amplifier (YDFA) is discussed. The gain and the pump and the signal light

propagation characteristics of the four-passed YDFA are described. It is found that, while using a shorter length of the fiber, a four-

passed fiber amplifier can realize the same output power as a single-pass fiber amplifier, and, for the same fiber lengths, a four-passed

fiber amplifier offers a significantly higher power than its single-pass counterpart.

r 2006 Elsevier Ltd. All rights reserved.

Keyword: Four passed YDFA; Power amplifier

1. Introduction

Fiber lasers and amplifiers developed rapidly in the lastfew years, because they offer a number of practicaladvantages over bulk solid-state lasers and amplifiers,including compact size, better stability and their out-standing thermo-optical properties. Ytterbium (Yb)-dopedfiber has attracted great interest because it does not havesome of the drawbacks associated with other rare-dopedfibers. Apart from the simple energy level structure, theYb-doped fiber amplifier (YDFA) offers a broad gainbandwidth (from 975 to 1200 nm) and an excellent powerconversion efficiency (80%). The YDFA does not exhibitthe excited state absorption phenomenon that can reducethe pump efficiency and lacks the concentration quenchingphenomenon. Thus, it offers high output power (or gain)with a smaller fiber length. YFDAs offer attractiveprospects for many applications, including power ampli-fier, sensing application and deep-space optical commu-nication [1].

However, there is a choice between Yb and Nd fibers toamplify light at 1064 nm. In fact, for some low-powerapplications, Nd-doped fiber is better than Yb-doped fiberbecause it is a pure four-level laser system. However, ingeneral, Yb fiber is more popular than Nd fiber for high-

e front matter r 2006 Elsevier Ltd. All rights reserved.

tlastec.2006.09.010

ing author. Tel.:+86 21 69918605; fax:+8621 69918507.

ess: liuyan703@163.com (L. Yan).

power fiber laser/amplifier application because it is moreefficient.Apart from the one-stage one-pass fiber amplifier

configuration, other fiber amplifier structures have beenadopted: the first is the multi-stage fiber amplifier that canachieve high output through several amplifier stages [2,3],the other is the one-stage two-pass fiber amplifier that canimprove the efficiency of utilization of the pump light [4,5].In 1994, a four-pass amplifier was demonstrated byAndreyev and Matveyev [6], who passed the light beamback and forth four times on a single axis through a gainmedium. In 1996, Lee and Halm [7] demonstrated a newtype of four-pass dye laser amplifier for the pulsedamplification of a CW narrow-bandwidth dye laser. Themain improvement of this system was to add a crystalpolarizer after gain medium, which reduces the effects ofdepolarization and reduces the possibility of parasiticoscillation between optical components used in theamplifier.

2. Configuration

In this paper, we discuss a four-pass yb fiber amplifiertheoretically, shown in Fig. 1. The seed source is anND:YAG laser operating at 1064 nm, M1 and M3 aredichroics that have high reflectivity for the signal wave-length and high transmission at the pump wavelength.

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Seed source

LD

P1

FR1

M1

M3

M2 FR2P2

Yb-doped fiber

isolator

Fig. 1. The schematic of four-pass yb-doped fiber amplifier. P1, P2: polarizer; FR1, FR2: faraday rotator; M1, M2, M3:dichroic mirror.

L. Yan et al. / Optics & Laser Technology 39 (2007) 1111–11141112

In contrast, M2 has high reflectivity for the pumpwavelength and high transmission at the signal wavelength.

The main operating principle is given as follows. Theincident signal propagates through a polarizing beamsplitter, P1, a 451 Faraday rotator, FR1 and a polarizingbeam splitter, P2, before being coupled into the YB-dopedfiber. The light beam passing through FR2 (451) is reflectedat M3, and its net polarization states rotation is 901 afterpassing through FR2 again. The light beam can then bereflected at P2 to M1. After passing through the fiber forthe second time, the reflecting beam can follow the samepath as before with the polarization state restored to theoriginal state. After the fourth pass through the fiber,the beam passes through P2, FR1 and the polarizationof the beam is rotated by 901. The signal is then reflected atP1 and leaves the optical system. In this process, the signalpasses through the Yb-doped fiber four times, increasingthe amplification.

3. Theoretical model

We used standard rate equations for two-level systems todescribe the gain and propagation characteristics of thefour-pass Yb-doped fiber power amplifier operating at1064 nm. Because the ASE power is negligible for a high-power amplifier with sufficient input signal (about 1mW),the effect of ASE was not taken into account. After theoverlap factors are introduced and the fiber loss ignored,the simplified two-level rate equations and propagationequations are given as follows [8]:

dN2ðz; tÞ

dt¼

GsPsðzÞ

AhvsN1ðz; tÞssa �N2ðz; tÞsse½ � þ

GpPpðzÞ

Ahvp

� N1ðz; tÞspa �N2ðz; tÞspe� �

�N2ðz; tÞ

t, ð1Þ

N1ðz; tÞ þN2ðz; tÞ ¼ N0, (2)

dP1ðzÞ

dz¼ P1ðzÞGs N2ðz; tÞsse �N1ðz; tÞssa½ �, (3)

dP2ðzÞ

dz¼ �P2ðzÞGs N2ðz; tÞsse �N1ðz; tÞssa½ �, (4)

dP3ðzÞ

dz¼ P3ðzÞGs N2ðz; tÞsse �N1ðz; tÞssa½ �, (5)

dP4ðzÞ

dz¼ �P4ðzÞGs N2ðz; tÞsse �N1ðz; tÞssa½ �, (6)

dPpðzÞ

dz¼ PpðzÞGp N2ðz; tÞspe �N1ðz; tÞspa

� �. (7)

Here, N0 is the yb-dopant concentration, N1 and N2 arethe ground and upper-level populations. Pp(z,t) is thepump power, Ps(z,t) is the signal power. spa,e are the pumpabsorption (emission) cross sections, ssa,e are the signalabsorption (emission) cross sections. Vp and Vs are thefrequencies of pump and signal light, respectively. A is thedoped area of the fiber and Gp (Gs) is the overlapping factorbetween the pump (signal) and the fiber-doped area. t is theupper state lifetime. P1,2,3,4 are signal power . Thesubscripts express the signal on the 1st, 2nd, 3rd and 4thpasses through the fiber.The boundary conditions associated with the above

differential equations was written as:

P1ð0Þ ¼ Psin; P1ðLÞ ¼ P2ðLÞ; P2ð0Þ ¼ P3ð0Þ,

P3ðLÞ ¼ P4ðLÞ; Ppð0Þ ¼ Ppin.

Psin is the input signal power and Ppin is the input pumppower.

4. Analysis

By solving a set of rate equations associated with theboundary conditions, the gain and propagation character-istics (such as the pumps, the signal) of single-modedouble-clad YDFA at 1064 nm by the 915 nm pump wasanalyzed.The main parameters used in the simulations are given as

follows:vp¼3.279�10

14Hz (lp¼ 915nm); spa¼ 8�10�25m2; spe ¼

5� 10�26m2;vs¼ 2.819� 1014Hz (ls ¼ 1064nm); ssa ¼ 3.4�10�25m2; sse¼ 5.0� 10�27m2; A ¼ 7.8� 10�11m2; N0 ¼ 2�1025m�3; t ¼ 0.84ms; Gp ¼ 0:01; Gs ¼ 0:6.Fig. 2 shows the propagation characteristics of the pump

power and signal power when the signal power is 1mWand pump power is 1W. The reflectance of the signal beamat the two dichroic mirrors is 1. The pump power is almostabsorbed with the fiber of length 15m.Figs. 3 and 4 illustrate the relationship between the

output signal power (gain) and the fiber position under forthe one-pass and four-pass configurations when the inputsignal power is 1mW and the pump power is 1W.

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0 3 6 9 12 15

0

1

P4

P3P2

P1

pow

er (

w )

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

-0.1

Pp

Z (m)

Fig. 2. Pump power and amplified signal power along the active fiber.

0 10 20 300

outp

ut p

ower

(w

)

fourpass

0.7

0.6

0.5

0.4

0.3

0.2

0.1

Z (m)

onepass

Fig. 3. Output signal power versus fiber length.

0 10 20 300

5

10

15

20

25

30

Z (m)

gain

(dB

)

onepass

fourpass

Fig. 4. Signal gain versus fiber length.

10

5

10

15

20

25

30

gain

(dB

)

Psin=1mwPsin=2mwPsin=3mw

0.2 0.4

pump power ( w )

0.6 0.8

Fig. 5. Gain versus the pump power.

L. Yan et al. / Optics & Laser Technology 39 (2007) 1111–1114 1113

To evaluate the four-passed fiber amplifier, we alsoconsider the one-passed fiber amplifier in which the signaland the pump light propagates along the fiber in the samedirection. The gain and output powers of two structuresincrease up to a certain length of the fiber, and then beginto decrease after a maximum point. The reason for thedecrease in output power and gain is insufficient popula-tion inversion due to excessive pump depletion and lossesexceeding the available gain at the signal wavelength. Butthere are some important differences. It is found that theoutput and gain curves of the four-passed fiber amplifierare sharper than the one-passed fiber amplifier. This givesthe four-passed fiber amplifier an advantage over the one-

passed fiber amplifier. For example, the output power ofthe four-passed fiber amplifier is 0.3W when the fiberlength is 5m, but the output power of the one-passed fiberamplifier is only 0.25W when the fiber length is 20m.Correspondingly, the gain of the four-passed amplifier is24.8 dB, while the one-passed amplifier gain is only 24 dB.It is demonstrated that a four-passed fiber amplifier using aquarter of the length of the fiber used in a one-passedamplifier, can achieve a larger signal output power andgain than the simple one-passed fiber amplifier. This is tosay, the single-stage four-passed fiber amplifier has twoprincipal advantages: on the one hand, it can achieve

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010

15

20

25

30

gain

(dB

)

0.005

signal power (w)

0.01 0.015 0.02

Ppin=0.8wPpin=1wPpin=1.2w

Fig. 6. Gain versus the input signal power.

L. Yan et al. / Optics & Laser Technology 39 (2007) 1111–11141114

higher output power and gain. On the other using only aquarter of the length of the fiber, it can achieve an outputpower equivalent to a one-passed amplifier. For the samepump power, it can achieve a higher output power by usinga shorter length of the fiber. It also avoids the requirementto adopt a multi-stage structure to amplify the signal. Thus,it is more economical.

Fig. 5 shows the variation of gain with pump power fordifferent input signal powers when the fiber is 15m long. Itis seen that the gain of the amplifier increases sharply withthe increasing pump power and then saturates after acertain level of pump power. For constant pump power,the smaller the input signal power, the higher the gain.Fig. 6 shows the variation of gain with the input signal

power for pump powers of 0.8, 1 and 1.2W when the fiberis 15m long. It is shown that the gain decreases withincreasing input signal power. The reason being that theamplifier easily gets saturated at higher signal powers for aconstant pump power. To reduce the ASE from a four-passed fiber amplifier, one method is to insert a filter in thebeam path of the amplifier [9].

5. Conclusion

In this paper, a single-stage four-passed YDFA isdemonstrated. The gain and the pump and the signal lightpropagation characteristics of four-passed YDFA weredescribed. It was found that the four-passed fiber amplifiershave advantages over the traditional one-passed fiberamplifier: it only requires a smaller fiber to realize theoutput that one-passed fiber amplifier can achieve, togetherwith a higher gain (output power). This structure is alsoadapted to the other rare-doped fiber amplifiers.

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