High Peak Power, Single-polarized, Sub-nanoseconds Pulses Generation of a Yb-doped Rod-type Photonic Crystal Fiber Amplifier

doi:10.4236/opj.2013.32B017

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Optics and Photonics Journal, 2013, 3, 69-72

doi:10.4236/opj.2013.32B017 Published Online June 2013 (http://www.scirp.org/journal/opj)

High Peak Power, Single-polarized, Sub-nanoseconds

Pulses Generation of a Yb-doped Rod-type Photonic

Crystal Fiber Amplifier*

Ziwei Wang1,2, Songtao Du1, Zhaokun Wang1,2, Jing He1,2, Yunrong Wei1, Qihong Lou1, Jun Zhou1

1Shanghai Key Laboratory of All Solid-St ate Lasers and Applied Techniques, Shang ha i Insti tute of Optic s and Fi ne Me c ha ni c s ,

Chinese Academy of Scien ces , Shanghai, China

2Universi ty of Chinese Academy of Scien ces , Beijing, China

Received 2013

ABSTRACT

We report a high-peak-power, single-polarized master oscillator power amplification system employing polarization-

maintaining Yb-doped rod-type photonic crystal fiber. The MOPA system comprises of a Q-switched microchip laser

generating ~630ps pulses at 8.6 kHz repetition-rate and two amplification stages employing double cladding fiber and

rod-type PCF respectively. The MOPA system obtains narrow spectral bandwidth, single-polarized pulses of 9W max-

imum output average power, corresponding to peak power of 1.7MW.

Keywords: Lasers; MOPA; Rod-type PCF; Single Poliriza tion; High-peak-power

1. Introduction

High-peak-power, single-polarized laser sources operat-

ing in sub-nanosecond regime are widely used in a range

of applications including frequency conversion [1], ma-

terial processing [2] and remote sensing[3]. Rare-earth-

doped fiber lasers and amplifiers with single-polarization,

narrow spectral linewidth, high signal-to-noise ratio and

excellent beam quality offer manifest advantages in the

applications above. In the past few years, large mode

area (LMA) double cladding fibers (DCFs) are widely

used in short-pulsed fiber amplifiers. In 2005, Fabio Di

Teodoro et al. [4] reported a three-stage amplifier gene-

rating pulses whose peak power in excess of 4.5MW

while pulse duration was 450ps and repetition rateswas

13.4 kHz. In 2010, Shaif-ul Alam et al. [5] introduced a

single-polarized master oscillator power amplification

(MOPA) system with output peak power of 39 kW at

20ps pulse duration 113.8MHz repetition rate. Although

LMA DCF can support high power for pulsed lasers, it

would become multiple transverse modes with beam

quality reduction [6]; moreover, there isn’t polariza-

tion-maintaining (PM) LMA DCF whose core diameter

is larger than 30 μm in the market. In 2007, a ~ 85ps

pulse duration, 105 kHz repetition-rate, 3MW peak pow-

er Yb-doped fiber amplifier exhibiting narrow spectrum

and diffraction-limited beam quality was reported by O.

Schmidt et al. [7]. T he significant component in the am-

plifier was a very-large-core, intrinsically single-mode,

rod-type photonic crystal fiber (PCF) [8] in power ampli-

fication stage. The rod-type fiber combines advantages of

fiber lasers and bulk solid lasers by particular micro-

structure design. Through enlarging core diameter and

reducing length, the peak power in core is lower. Fur-

thermore, it is designed as an extraordinary small refrac-

tive-index difference between core and cladding to be

intrinsically single mode. Although the rod- type fiber

cannot be coiled as ordinary fibers, it presents a range of

eminent features including single-mode operation, pump

acceptance angle and efficient pump absorption. The

paper discussed above reported a narrow pulse,

high-peak-power results, however, the damage of main

amplifier was observed at maximum power and pulse

distortion was also occurred when peak power was ~2MW.

The papers about rod-type PCF amplifier are mainly fo-

cused on non-polarize d amplificatio n so far, neverthe less

high-peak-power PM amplifier is rarely reported.

In this paper, we present a single-polarized dual-stage

master oscillator power amplification (MOPA) system

for ge nera ting hi gh-peak-po wer and si ngle -polarized pulses.

We employ a low-repetition-rate passively Q-switched

microchip laser as seed, and introduce Yb-doped, PM,

LMA double cladding fiber and 80cm-long, 100μm-core,

PM, rod-type PCF as gain medium in pre-amplification

and power amplification stage respectively. From this

system, we obtained ~600ps, 8.6kHz repetition-rate, center

*The work has been sponsored by the national foundation of

China

( NO.60908011，NO.60907045)

Z. W. WANG ET AL.

wavelength 1064nm, and spectrum linewidth 0.46nm,

polarization extinction ratio (PER) 13dB laser output of

average power 9W, corresponding to a peak power ex-

ceeding 1.7MW and pulse energy exceeding 1mJ.

2. Experimental Setup

The experimental architecture is shown in Figure 1. A

fiber-pigtailed Nd:YAG/Cr4+:YAG, single-polarized, pas-

sively Q-switched microchip laser is used as the ma ster

oscillator( M O) coupling into amplifier. Due to the low

gain of short Yb-doped fibers in 1064nm，the MO o utput

is amplified in a dual - stage, PM fiber amplifier c hai n t ha t

affords effective suppression of nonlinearities such as

amplified spontaneous emission(ASE) and simulated

Raman scattering(SRS). Polarization independent optical

isolators are used between am plification stages to p revent

backward laser leakage. Cladding power stripper is also

introduced to protect MO against backward pump laser.

For the first amplification stage, we use a 3m-long PM

Yb-doped double cladding fiber(core/cladding diameter

25/250 μm, core/cladding NA 0.06/0.46 and pump ab-

sorption ~5.10dB/m @975nm) as gain medium. The fi-

ber is backward pumped by two fiber-pigtailed diode

lasers at 975nm through a (2+1) ×1 combiner whose end

is spliced with endcap and angle (6°) polished to prevent

facet damage and backward-reflected power from coupl-

ing into fiber core. We employ two half-wave plates to

control the polarization of pre-amplified output pulses.

To remove all remaining ASE and preventing backward

pump light of the second stage, an inter-stage band-pass

filter is specialized designed, emitting only a spectral

band of 10nm centered at 1064nm. The filtered pre-am-

plified pulses are coupled into a 80cm-long, PM Yb-

doped rod-type PCF (core/cladding diameter 100/285μm,

core/cladding NA 0.02/0.6, and pump absorption ~30dB/

m@975nm) which is free-space backward pumped by a

LD at 975nm through a set of lens and dichroic mirrors

(1064nm HT&975nm HR). The rod-t ype P CF is endcap-

ping with large AR-coating silica columns(length 8mm

and diameter 8.2mm) in both ends in order to provide

appropriate beam expansion，thus reduce end facet in-

tensity and avoid damage of end facet for a 0° angle facet.

The amplified beam output s th rough a collimating len s.

The output characterization of the fiber amplifier in-

cludes average power, spectrum and temporal pulse pro-

file. We measure average power by Thorlabs S370C

power meter. Spectrum and temporal profile are detected

by Yokogawa AQ6370 optical spectrum analyzer and

LeCroy Wavesurfer 104MXs 1GHz oscilloscope con-

nected to a high speed InGaAs photodiode respectively.

3. Results and Discussion

The spectrum (a), pulse temporal profile (b) of MO are

presented in Figure 2. The average output power of MO

is ~170 mW with center wavelength at 1064.8 nm, 3dB

spectral bandwidth of 0.29 nm, pulse duration of 628 ps

and 8.6 kHz repetition-rate. And the PER of the MO is

~13dB.

Figure 3 provides the average output power of the

MOPA system as the function of launched pump power.

After pre-ampli fication, there is average power ~500mW

filtered pulses coupled into rod-t ype P C F . The maximu m

output average power is 9W corresponding to peak pow-

er exceeding 1.7MW, when pump power is 31W. The

slope efficiency extracted form a linear fit is ~44%. It is

emerged that signal power increases along with pump

power increase; the pump threshold of rod-type fiber is

~5W. Moreover, the shift in wavelength of pump

Figure 1. experimental setup. MO: master-oscillator; ISO:

optical is olat or ; CPS ：claddi ng power stripper ; PM-YDCF:

polarization-maintaining Yb-doped double cladding fiber;

LD: laser diode; HWP: half-wave plate; BPF: band-pass

filter; ROD-PM-PCF: rod-type polarization-maintaining

photonic c rystal fi ber; DF: dichroic mirror.

Figure 2. Seed’s features of (a) spectrum and (b) temporal

profile.

Z. W. WANG ET AL.

would move towards the Yb-absorption peak at 975nm

with pump power increasing. Owing to the endcaps, the

maximum in-core peak intensity of 68 GW/cm2 is re-

duced to ~10MW/cm2, which is far less than the meas-

ured surface damage threshold for sub-nanosecond pulses

in silica [9]. We suggest that signi ficant higher amplified

signal power scaling would be possible through increas-

ing pump power as power saturation or deduction isn’t

occurred. T hrough i ncreasi ng the p ower of p re-amplified

pulses, both the output power and optical-to-optical effi-

ciency would increase notably . Furthermore, we meas-

ure the PER to be ~13dB at maximum output operation,

that means the amplifier maintaining the polarization of

MO.

In Figure 4, the typical spectrum of seed pulses (a) is

presented comparing with output pulses of average power

at 9W (b). The output center wavelength is 1064.9nm,

and 3dB spectral bandwidth is 0.46nm. The amplified

spectrum is relatively broader than seed’s, it is due to

self-phase modulation (SPM) [10] in high peak power.

Thanks to the effective management of continuous wave

ASE, the signal to peak(~1040nm) ASE ratio exceeds

Figure 3. Aver ag e o utp ut power of si gn al lase r v ersus p ump

power.

Figure 4. (a) seed spectrum and (b) amplified spectrum at

output power of 9W.

Figure 5. Amplified pulse (a) temporal profile and (b) se-

quence at output powe r of 9W.

40dB; SRS and four-wav e-mixing (FWM) are not en-

gendered in the measurement. Furthermore, because of

backward pump, residual pump is hardly appeared. In

general, the spectrum represents an excellent signal-to-

noise ratio o f t he syste m, and it d e monstrates the bene fits

of rod-type PCFs in minimizing nonline ar effects.

Figure 5 shows the temporal profile (a) and pulse se-

quence (b) of output pulses at maximum average power.

The output pulse duration is 632 ps while repetitio n ra te is

8.6 kHz. Comparing to the seed pulse profile, an irregu-

lar temporal pulse distortion is exhibited owing to gain

saturation and SPM corresponding to the spectrum broa-

dening we discussed above. SPM would become signifi-

cant constraint in further power amplification scaling.

4. Conclusions

In this paper, we report on a single-polarized, dual-stage

MOPA system combining Q-switched microchip laser

with Yb-doped very-large-mode-area rod-type PCF in

sub-nanosecond regime. The amplifier generates high

peak power in excess of 1.7MW (corresponding to aver-

age power 9W, pulse duration 638ps and repetition rate

8.6 kHz) pulses in single-polarized, spectrally narrow,

high sig nal-to-noise ratio beam, which haven’t been gen-

erated in Yb-doped double cladding fiber amplifiers.

This work represents the highest peak power obtained

from single-polarized rod-type PCF amplifiers. With the

excellent properties including high-peak-power and sin-

gle-polarization, the MOPA system is a promising can-

didate to be used in applications including frequency

conversion, material processing and LIDAR. In the fu-

ture work, we would donate attempts to increase pulse

Z. W. WANG ET AL.

energy before main amplification and improve the pola-

rization of the system while maintains the simplicity and

stability of the system.

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