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Optics Communications 279 (2007) 173–176

Tunable, ls-pulsed ytterbium fiber laser system with a linewidthbelow 2.7 GHz

M. Engelbrecht *, D. Wandt, D. Kracht

Laser Zentrum Hannover e.V., Hollerithallee 8, 30419 Hannover, Germany

Received 16 February 2007; received in revised form 15 June 2007; accepted 11 July 2007

Abstract

We report on a widely tunable, pulsed laser system with narrow spectral linewidth based on a continuous wave ytterbium fiber oscil-lator, a pulse shaper and a power amplifier stage. The system is tunable from 1055 nm to 1085 nm and provides a maximum pulse energyof 155 lJ with a pulse duration of 1–5 ls. The linewidth is less than 2.7 GHz over the whole tuning range.� 2007 Elsevier B.V. All rights reserved.

PACS: 42.55.Wd

Keywords: Tunable laser; Fiber laser

1. Introduction

The sensitive detection of gaseous components by usinglaser spectroscopy is generally performed in the ‘‘finger-print’’ region of the appropriate molecules with theirstrong absorption lines beyond 3 lm. Unfortunately, nosimple and reliable primary laser source is currently avail-able in that wavelength range and with the small linewidthnecessary. However, a nonlinear frequency conversionstage, e.g. an optical parametric oscillator, pumped by ahigh-power near-infrared laser can generate tunable radia-tion within the whole above mentioned wavelength range.Systems pumped with fixed wavelength Nd-doped solidstate lasers have been demonstrated. They transfer the line-width of the pump laser into the mid infrared and they arewidely tunable by a change of the parameters of the nonlin-ear crystal or variation of the crystal temperature [1,2]. Adisadvantage, especially of temperature tuning, is the slowtuning speed.

0030-4018/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.optcom.2007.07.018

* Corresponding author. Tel.: +49 511 2788 239; fax: +49 511 2788 100.E-mail address: m.engelbrecht@lzh.de (M. Engelbrecht).

Ytterbium-doped fiber lasers are a good alternative aspump sources for frequency conversion as they offer a highefficiency and allow the generation of high peak power incombination with excellent spatial and spectral beam qual-ity [3,4]. Moreover, due to the amorphous structure of theirglass host the gain profile of the active ions is significantlybroadened [5]. This makes fiber lasers very attractive forthe construction of tunable laser sources. Tunable lasersbased on ytterbium-doped fibers are very common andthe full gain spectrum of this material has been exploredmore than 15 years ago [6]. Since then, the continuouswave output power was increased strongly. As well, tun-able and pulsed laser systems have been demonstrated[7,8]. Their fast tunability can be directly transformed intothe mid infrared region by parametric frequency conver-sion [9]. Unfortunately, laser oscillators are either limitedin the output power, or the high output power owing tothe high gain allows a higher number of longitudinal modesto reach threshold, leading to a linewidth of at least several10 GHz. A reduced linewidth increases the resolution ofspectroscopic measurements, whereas a high output powerensures a good conversion efficiency in the frequency con-version and a higher detection sensitivity.

174 M. Engelbrecht et al. / Optics Communications 279 (2007) 173–176

In this paper we report on a fast tunable, pulsed ytter-bium-doped fiber laser system, delivering linewidth below3 GHz. High peak output power is realized by a pulsedoperation, suitable for efficient nonlinear frequency conver-sion. In combination, this laser system builds a powerfulbasis for trace gas detection systems.

2. Experimental set-up

The experimental set-up is shown in Fig. 1. The laserconsisted of a master-oscillator, power amplifier scheme,which separates the spectral properties from the powergeneration.

The tunable master cw-oscillator consisted of a unidirec-tional ring resonator. The 25 m long active fiber was anytterbium-doped double-clad large-mode-area fiber (Insti-tut fur Hochtechnologie, Jena, Germany). It was pumpedin propagation direction through the end facets into theouter pump core (400 lm, NA: 0.38) by a fiber coupleddiode module, delivering an output power of 15 W at976 nm out of a fiber with a diameter of 400 lm and aNA of 0.22. The active fiber had an Yb2O3 doping concen-tration of 1000 mol parts in 106, a core diameter of 10 lm,a NA of 0.07, ensuring more than 95% pumplight absorp-tion and single transverse mode operation. Both fiber endswere polished with an angle of 7� to eliminate signal feed-back. The propagation direction was defined by an intra-cavity Faraday isolator. A double grating arrangement inLittman–Littrow configuration in combination with a tele-scope with a magnification of 5 defined the linewidth of thelaser [10,11]. The advantage of this tuning mechanism is thecombination of a small spectral filter with a wide tuningrange, which can not be provided by other tuning mecha-nisms, like Lyot filters or etalons. Recently available fiberFabry–Perot filters have the drawback of the very lowpower handling capability, but may become an attractivealternative in the future. In our setup wavelength tuningwas provided by rotation of the Littrow grating, whichwas mounted on a motorized rotation stage, allowing afull spectral scan in 2 s. The holographic gratings had

Tuning grating

Grating

Pump-diode

Yb-fiber

Yb-fiber

Power -Amplifier

EOM

Pulse- ShaperMaster -OscillatorPump-diode

Telescope

2

2 2

2

4

Mode-matching

Fig. 1. Setup of the laser system. The three modules of the laser system areseparated by dashed lines.

1200 lines/mm and the incidence angle on the Littman grat-ing was 80�. The polarization at the output of the fiber seg-ment was adjusted by means of a quarter and a halfwaveplate towards the input polarizing beam splitter cube(PBS) of the isolator. Additionally the half waveplate wasused to define the output coupling ratio through thisPBS. With an external half waveplate the seeding powerwas reduced for the following modules. In the second stage,a Pockels cell was used for pulse shaping. It provided anextinction ratio of 35 dB at the center wavelength. In orderto reduce the driving voltage of the Pockels cell, it wasoperated in a double pass arrangement. The beam was sep-arated by a Faraday isolator, which also prevented theoscillator from back-reflections of the following amplifica-tion stage. To adjust the polarization between the isolatorand the Pockels cell, an additional half waveplate wasplaced in between. The power amplifier stage was builtup by the same ytterbium fiber used in the oscillator, withthe same length. The fiber was pumped contra directionalto the laser propagation with a diode module similar tothe one used in the oscillator. To match the beam parame-ters between the pulse shaper and the amplifier, an addi-tional telescope was placed in between.

3. Experimental results

The laser system was continuously tunable from1055 nm to 1085 nm. The tuning range was limited by thesize and the efficiency of the gratings, which is reduced atthe edges of the tuning range. A wider tuning range, possi-ble for example by omitting the telescope, would lead to abroader linewidth. The oscillator was driven at a low pumppower and delivered 580 mW of output power with anabsorbed pump power of 5.1 W. This operation ensuredsaturated operation of the amplifier as well as a small line-width of the oscillator, which increased with the outputpower. The pulses generated from the Pockels cell wererectangular shaped with a length of 1 ls and 5 ls, respec-tively. The repetition rate was set to 20 kHz and the pulseshad a peak power of 500 mW at the entrance side of theamplifier due to some losses from the Pockels cell. Theamplification stage was pumped with a pumping powerof 14 W resulting in a maximum output power of 3.1 Wat 1070 nm, independently from the pulse duration gener-ated, as shown in Fig. 2. This corresponded to pulse energyof 156 lJ. The wavelength dependency of the average out-put power and the pulse energy of the laser system at20 kHz repetition rate and 1 ls pulse duration is shownin Fig. 3. To measure the pulse energy Ep, the time signalwas measured with a photodiode and the ratio A betweenthe area under the pulse and under the time signal inbetween the pulses was calculated. To exclude the cw compo-nent, the pulse energy was then calculated to Ep = A Æ P/R,where P is the average power and R is the repetition rate ofthe amplifier.

During the tuning process, the driving voltage for thePockels cell was not adjusted, but optimized for about

4 6 8 10 12 14

Aver

age

pow

er [W

]

Pump power [W]

1070 nm20 kHzPulse duration:

5 µs 1 µs

0

25

50

75

100

125

150

Puls

e en

ergy

[µJ]

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Fig. 2. Output power and pulse energy of the laser system versus pumppower at 1070 nm.

1060 1070 10802.0

2.5

3.0

3.5

4.0

Aver

age

pow

er [W

]

Wavelength [nm]

Average power

0

25

50

75

100

125

150

175

200

Pulse energy

Puls

e en

ergy

[µJ]

Fig. 3. Wavelength dependency of the average output power and the pulseenergy at 14 W pump power. The pulse duration was 1 ls and therepetition rate 20 kHz. The Pockels cell was optimized for 1070 nm.

—20

—30

—40

—50

—60

—701040 1060 1080 1100

Wavelength (nm)

Inte

nsity

(dB)

Fig. 4. Output spectra of the laser at different operating wavelength at1 ls pulse duration and 20 kHz repetition rate.

0.0 0.2 0.4 0.6 0.8 1.0 1.20

50

100

150

200

250

300

Pow

er [W

]

Time [µs]

1070 nmAverage power

0.24 W 0.72 W 1.72 W 3.13 W

Fig. 5. Pulse shape of the output pulses. For increasing average power, thepulse front gets more strongly amplified than the pulse tail.

M. Engelbrecht et al. / Optics Communications 279 (2007) 173–176 175

1070 nm wavelength. Therefore, the polarization rotationprovided by the Pockels cell derived from the optimumof 45� for each pass to the edges of the tuning range,decreasing the signal suppression inbetween the pulses bythe pulse shaping unit. This led to a continuous wave back-ground, which is strongly amplified. However, a couplingbetween the driving voltage of the Pockels cell and thewavelength setting would avoid this problem.

In Fig. 4 the optical spectra of the laser are shown for1 ls pulse duration and 20 kHz repetition rate for the cen-ter wavelength and for 1050 nm and 1080 nm. A good ratiobetween the laser line and amplified spontaneous emission(ASE) for the center wavelength can be observed. Here,more than 98% of the power were inside the laser line.However, to the edges of the tuning range, the power con-tent inside the ASE increased. For 1055 nm and 1085 nmthe power inside the laserline was still above 84%.

In Fig. 5 the pulse shape of the amplified pulses is shownat 1070 nm. It strongly deviated from the rectangular shapegiven by the input pulse. For the maximum pump power

and 1 ls pulse duration a peak power of 300 W wasachieved in the leading edge of the pulse. The powerdecreased over the pulse duration to 90 W at the end ofthe pulse. This pulse shape is caused to the fact, that thestored energy is reduced during the pulse duration. It isin agreement with the theory of pulse propagation in satu-rated laser amplifiers as described by Frantz and Nodvik[12]. The strength of the decrease depends on the inversiongenerated in the active medium, which corresponds to thepump power. Therefore, the decrease is strongest for high-est pump power. The power decrease during the pulse cannot be avoided as long as the amplifier is seeded with rect-angular pulses. However, recent works have shown that ashaping of the seed pulses can result in rectangular outputpulses [13], albeit a loss of pulse energy can be expected dueto a non saturated operation during such shaping pro-cesses. The pulse shaping was beyond the scope of thispaper. The same qualitative behavior of the pulse shapewas observed for 5 ls pulse duration with a maximum peakpower of 63 W. The small ripples on top of the pulse shape

-2.5 0.0 2.5 5.0 7.5 10.0 12.5

0.0

0.2

0.4

0.6

0.8

1.0

Inte

nsity

[a.u

.]

Frequency [GHz]

Fig. 6. Linewidth measurement at 1070 nm using a scanning Fabry–Perotinterferometer with 10 GHz free spectral range.

176 M. Engelbrecht et al. / Optics Communications 279 (2007) 173–176

were caused by a small backcoupling into the oscillator dueto the not sufficient suppression from the isolator inbe-tween the oscillator and the amplification stage.

The linewidth of our laser system using the double grat-ing arrangement was as narrow as 2.7 GHz FWHM at acenter wavelength of 1055 nm and decreased to 2.2 GHzat 1085 nm. It was measured by a scanning Fabry–Perotcavity with a free spectral range of 10 GHz and a finesseof 100 and is shown in Fig. 6. This linewidth is the smallestlinewidth observed to the best of our knowledge for pulsedtunable ytterbium fiber lasers with multi Watt level peakpower. However, the linewidth is broad enough that nosigns of Brillouin scattering were visible, neither in the laseroutput, nor in the backward direction.

4. Summary

In summary, we have demonstrated a pulsed, tunablefiber laser system that delivers output power sufficient for

efficient frequency conversion. With a linewidth of2.5 GHz, the system is a promising base for trace gas anal-ysis systems. Overall, a peak power of 300 W was shownfor pulses with a repetition rate of 20 kHz and a pulse dura-tion of 1 ls. The fast tunability of the system over the rangefrom 1055 nm to 1085 nm is an important feature for allkind of frequency scanning applications.

Acknowledgement

We gratefully thank the European Commission for sup-porting this research under contract number 2504 (the opti-cal nose).

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