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Optics Communications 282 (2009) 2199–2203
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Optics Communications
journal homepage: www.elsevier .com/locate /optcom
Efficient generation of 200 mJ nanosecond pulses at 100 Hz repetition ratefrom a cryogenic cooled Yb:YAG MOPA system
Stuart Pearce a,*, Ryo Yasuhara a,b, Akira Yoshida a, Junji Kawanaka a, Toshiyuki Kawashima a,b,Hirofumi Kan a,b
a Institute of Laser Engineering, Osaka University, 2-6 Yamada-oka, Suita, Osaka 565-0871, Japanb Hamamatsu Photonics K. K., 5000 Hirakuchi, Hamakita-ku, Hamamatsu, Shizuoka 434-8601, Japan
a r t i c l e i n f o
Article history:Received 14 January 2009Received in revised form 9 February 2009Accepted 13 February 2009
PACS:42.55.Xi42.60.By42.60.Lh
Keywords:High repetition rateYb:YAGYb:YAG ceramicCryogenic coolingDiode-pumpedSolid-state laserMOPARegenerative amplifier
0030-4018/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.optcom.2009.02.051
* Corresponding author.E-mail addresses: [email protected]
(S. Pearce).
a b s t r a c t
A diode-pumped Master Oscillator Power Amplifier (MOPA) laser system based on cryogenic cooledYb:YAG has been designed, developed and its output performance characterised. The laser system con-sists of a fibre oscillator, an active mirror regenerative amplifier and a four pass main amplifier. 2.4 mJ,10 ns, 100 Hz seed pulses from the fibre oscillator/regenerative amplifier arrangement were amplifiedup to pulse energies of over 200 mJ by using the four pass main amplifier arrangement. As a further studywe have obtained an increased slope efficiency of 40% and an optical-to-optical efficiency of 30% using apinhole vacuum spatial filter/image relay for laser mode control. With 1.8 mJ input seed pulses, outputpulse energies of around 150 mJ were achieved.
� 2009 Elsevier B.V. All rights reserved.
1. Introduction
Ultra-short laser pulses with both high pulse energy and highpeak powers are desired for many industrial applications includinghigh energy particle generation [1], hard X-ray generation [2] andinertial fusion energy [3]. For these applications, diode-pumped so-lid-state lasers are a promising source due to their high overall effi-ciency, high average power and their ability to produce a goodbeam quality [4]. A good example of a solid state ultra-short pulselaser is a Ti:sapphire laser, which has already opened up manyareas of high field science [5]. However, further increases to theintensity leads to new interesting areas of scientific research suchas cancer therapy [6] and laser driven gamma ray generation [7,8].To achieve the necessary high intensity for these applications thelasers require modification. One method to increase the intensityof the pulse is to shorten its duration. Recently, optical parametric
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chirped pulse amplifiers (OPCPA) have been extensively researchedand by combining them with high energy picosecond pump pulsesa promising method to obtain high pulse energy ultra-short pulselasers can be achieved [9].
For joule pulse energies and higher both Nd:YAG and Nd:Glassare popular laser materials to utilise. These commercial neodym-ium lasers are flash-lamp pumped systems with a typical repeti-tion rate of 10 Hz or single shot. Several diode-pumped high-power laser systems have been developed to produce 100 J pulseenergy in nanosecond pulse durations. These include the MercuryLaser (active material: Yb:S-FAP) [10] at Lawrence Livermore Na-tional Laboratory, Polaris (active material: Yb:Glass) [11] at Jenaand HALNA (active material: Nd:Glass) [12,13] at Osaka University.
To achieve both high pulse energies and high repetition rates asuitable laser gain material capable of high energy storage, highthermal toughness and strong absorption of the diode pump en-ergy is required. Cryogenic cooled Yb:YAG has many desirableproperties which include an increased thermal conductivity andemission cross section, and a decreased lower level re-absorptionrate [14] allowing for higher gain at lower pump intensities. At
2200 S. Pearce et al. / Optics Communications 282 (2009) 2199–2203
room temperature Yb:YAG is a quasi-three level laser but at 77 K, itbecomes a true four level laser, achieving a higher slope efficiencyand a lower threshold [15,16]. These benefits are ideal for a systemwith the goal of high pulse energies. However, an interesting pointto note is that at room temperature Yb:YAG has a broad gain band-width of 10 nm (full width half maximum) allowing for picosecondpulse durations, but at cryogenic temperature the bandwidth re-duces to �1.5 nm [17]. This gain narrowing can lead to difficultiesin the amplification of ultra-short pulses as it can reduce theamplified spectral bandwidth which in turn broadens the temporalpulse duration [18] and increases the complexity of the stretcher/compressor arrangement for chirped pulse amplification (CPA) la-ser systems.
Recently, Wandt et al. [9] generated 220 mJ in nanosecondpulses at 10 Hz repetition rate using a 6 mm diameter, 8 mm long3 at.% doped Yb:YAG rod in a four pass amplifier arrangement.With 2 mJ seed input energies, high output energy of over200 mJ was achieved. However, the input pump energy requiredfor this output energy was higher than 2.5 J. By maintaining con-stant diode pump energy of 2.2 J, they achieved close to 200 mJfor seed input energies of 3.0 mJ.
In this paper we investigated a cryogenic cooled Yb:YAG MOPAlaser as a feasibility study into a high energy ultra-short pulse, so-lid state laser system called GENBU [19]. The GENBU system hasbeen conceptually designed to satisfy the requirements of obtain-ing high pulse energies with ultra-short pulse durations. The sys-tem consists of two different laser arrangements, a main lasergenerating 2 kJ, 50–100 ps pulses and an OPCPA laser generating30 J, 5–10 fs pulses. Both sections of the laser system operate uptoa repetition rate of 100 Hz. The main laser is a diode-pumped solid-state laser used due to its compact system size, high efficiency,long operation lifetime, low maintenance and ease of operation.Furthermore, the main laser is used as the pump source for theOPCPA section. To show the viability of the GENBU laser projecta cryogenic cooled Yb:YAG MOPA was designed and developedfor upto sub-joule pulse energy levels. The MOPA consists of a fibrelaser, a regenerative amplifier and a main amplifier.
Fig. 1. Schematic layout of the regenerative amplifier based on an active mirror cyrogeniDM, dichroic flat mirror (AR 750–1000 nm, HR 990–1170 nm); Pockels cell, Lasermetric
2. Experimental set-up
2.1. Front end
For generation of the initial seed input, a single pulse is slicedfrom the CW output of a fibre laser using a pulse selector configu-ration consisting of a KD*P Pockels cell (Lasermetrics, 5057E) and acrossed pair of Glan-Thompson prisms. This generates 10 ns(FWHM) pulses with a repetition rate of up to 100 Hz. The selectedpulse train then acts as the seed for the Yb:YAG regenerativeamplifier. The configuration of the regenerative amplifier used inthe experiments is shown in Fig. 1. For the laser material we useda 9.8 at.% doped Yb:YAG ceramic with a size of 10 � 10 � 2 mm inan active mirror arrangement. One surface was HR-coated for1030 nm and 940 nm and the other was uncoated. The seed pulsesenter the gain material from the non coated surface at Brewster an-gle, with a TEM00 mode size on the Yb:YAG of 1.8 mm (1/e2).
The Yb:YAG ceramic was mounted onto a copper holder with100 lm thick indium foil between the holder and the laser crystalto improve the thermal contact. The copper holder was cooledusing liquid nitrogen and the whole assembly was held in a vac-uum to avoid condensation of the sensitive surfaces. To preventpulse overlap on the Yb:YAG active mirror the folded cavity lengthof the regenerative amplifier was 4.9 m long. A 600 lm core fibre-coupled laser diode with a central emission wavelength of 938 nmand a maximum peak power of 140 W was used as a Q-CW pumpsource and was focused onto the Yb:YAG. The pump duration waschosen to be 1000 ls as it is close to the upperstate lifetime forYb:YAG [20]. During the experiment the spatial profile of thepump beam on the laser material was nearly flat-topped with adiameter of 1.8 mm. The pump pulses were focused on theYb:YAG collinearly with the seed and amplified pulses at Brew-ster’s angle.
With no voltage on the Pockels cell, the polarization of the seedpulse train is rotated by 90� due to the double pass through thequarter-wave plate. This allows the seed pulse train to circulatethe regenerative amplifier cavity once. When a quarter-wave volt-
c cooled Yb:YAG ceramic. TFP(1) and TFP(2), thin film polarizers; FR, Faraday rotator;s 5046E.
Fig. 3. Output performance of the main amplifier, repetition rate 100 Hz; Outputpulse energy and optical-to-optical efficiency vs. diode pump energy, input seedpulse energy of 2.4 mJ.
S. Pearce et al. / Optics Communications 282 (2009) 2199–2203 2201
age is applied to the KD*P Pockels cell, the pulses circulating thecavity are trapped. Whereas incoming seed pulses are rejecteddue to the combination of the quarter-wave plate and the activePockels cell. The evolution of the pulse amplification in the cavitywas monitored by detecting the small leakage using a photodiodebehind M(END) After the period of amplification (about 30 roundtrips) a half-wave voltage is applied to the Pockels cell and thetrapped pulses are reflected out of the cavity by TFP(2). For theamplified pulses, the rotation by Faraday rotator, FR is cancelledby the half-wave plate, so they are reflected by TFP(1).
2.2. Main amplifier
The amplified pulses from the regenerative amplifier were col-limated, rotated to ‘‘p” polarization and used as the seed input forthe four pass main amplifier arrangement. Fig. 2 shows the set-upof the four pass amplifier designed for high output energies. Forthe gain material we used a 12 mm diameter, 6.6 mm long,5.8 at.% doped Yb:YAG rod. The rod was mounted between twocopper plates, held in vacuum and cooled using liquid nitrogen.Each side of the gain material was pumped by a 2.5 kW (maxi-mum peak power) fibre-coupled laser diode operating at 100 Hzrepetition rate. The pump duration of the laser diodes can be var-ied from 50 ls up to a maximum of 2000 ls. Two dichroic mir-rors (DM(1) and DM(2)) were used either side of the cryostat toallow the laser pulses to travel collinearly with the diode emis-sion through the laser material. Custom designed focusing opticswere used to maintain a constant top hat beam size of 4 mmthrough the Yb:YAG rod.
The ‘‘p” polarized pulses from the regenerative amplifier passthrough the first thin film polarizer, TFP(1). The subsequent rotationby the half-wave plate is cancelled by the Faraday rotator, FR(1)
allowing the pulses to pass through the second thin film polarizer,TFP(2) and enter the main amplifier layout.
After the initial pass of the cryogenic cooled Yb:YAG rod, thepolarization of the laser pulses are rotated by 90� using a doublepass of a Faraday rotator, FR(2). After which, the laser pulses passthrough the Yb:YAG for a second time. After the second pass, thepulses are reflected by TFP(2) to end mirror, M(5). After which, theyare reflected back on the same axis and pass through the Yb:YAGfor a third time. After this pass the polarization rotates by 90�again and is reflected back for a fourth and final time throughthe laser material. The amplified pulses now pass through TFP(2),however, FR(1) and the half-wave plate both provide a 45� rotationallowing the pulses to be reflected by TFP(1) towards an energymeter.
Fig. 2. Schematic layout of the four pass main amplifier based on cryogenic Yb:YAG, seedfilm polarizers; FR(1) and FR(2), Faraday rotator; DM(1) and DM(2), dichroic flat mirror (ARshows the top hat pump distribution of the laser diodes.
3. Results and discussion
For seed input pulse energies of <15 pJ the maximum outputenergy of the regenerative amplifier at 100 Hz was measured tobe �4.6 mJ. Thus, the regenerative amplifier provided an overallgain of �3 � 108. Using the amplified output from the regenerativeamplifier as the seed for the main amplifier, the pulse energy afterfour passes was measured for 700 ls pump duration as shown inFig. 3. For our initial experiment we used seed input energy of2.4 mJ.
The results show that the pulse energy increased linearly up to140 mJ with pump energies between 0.3 J and 0.7 J at 100 Hz. Thiscorresponds to slope efficiency, gs for the four pass amplifier of 30%and a maximum optical-to-optical efficiency, go-o of 19%, despitepoor spatial mode coupling between the pump beam and the laserbeam.
However, after 0.7 J input pump energy the output energyshows saturation effects, attributed to amplified spontaneousemission (ASE) and parasitic oscillations caused by a combinationof ASE and weak unintentional reflections either off the rod endsor off other optical components. Further increases to the inputpump energy showed that the output energy also increased. Thisincrease was credited to the improvement of the spatial mode cou-pling between the pump beam area and the amplified beam by athermal lensing effect. At further diode input energies, the spatialmode coupling moved away from the optimum and the output en-ergy decreased. The results shown in Fig. 3 suggest that with an
ed by the fibre oscillator/regenerative amplifier arrangement; TFP(1) and TFP(2), thin750–1000 nm, HR 990–1170 nm); M(1)–M(5) (HR 0�, 1030–1064 nm). The subfigure
Fig. 5. Output performance of the main amplifier with image relay optics,repetition rate 10 Hz, single pass small signal gain for four different pumpdurations vs. laser diode peak power. Input seed pulse energy: 0.36 mJ.
Fig. 6. Output performance of the main amplifier with image relay optics,repetition rate 10 Hz; Output energy vs. laser diode peak power for different pumpdurations.
Fig. 7. Output performance of the main amplifier with image relay optics,repetition rate 10 Hz. FWHM output pulse duration after four pass amplification.
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improvement to the spatial mode coupling, it may be possible toachieve higher output energies.
To improve the mode coupling we used a pinhole vacuum spa-tial filter/image relay between the second and third pass of thegain material. Vacuum spatial filters are useful in high pulse energylaser systems. They relay image the beam profile and remove un-wanted spatial noise, thus maintaining the beam quality throughthe complete laser system. Fig. 4 shows the new layout of the mainamplifier with the vacuum spatial filter/image relay. The mode sizeof the seed pulses from the regenerative amplifier were matched tothe pump area and kept constant using an iris aperture (imagepoint), two 1000 mm focal length lenses and the vacuum spatialfilter with a pinhole (�f/235). In addition the repetition rate wasreduced to 10 Hz to lower the thermal lensing effects.
The single pass small signal gain and the output energy of themain amplifier were measured. The seed input energy was reducedusing a separate aperture and measured to be 0.36 mJ. The outputenergy was measured for different laser diode peak powers up to amaximum of 3.7 kW and at four laser diode pump durations,700 ls, 300 ls, 200 ls and 100 ls. The small signal gain at the fourdifferent pump durations is shown in Fig. 5. At a peak power of�2.6 kW (pumped with both laser diodes) and pump duration of200 ls, the maximum achieved small signal gain was over 12 be-fore saturation occurred. At higher pulse durations, the ASE buildsup more rapidly and reduces the population inversion, subse-quently reducing the effective gain. Whereas at shorter pumppulse durations the build up of ASE is slower allowing higher gainto be achieved before saturation.
Fig. 6 shows the output pulse energy as a function of the laserdiode peak power for the same pulse durations used in the smallsignal gain measurements. For a seed input energy of 1.8 mJ, themaximum pulse energy obtained was �150 mJ between 200 lsand 700 ls pump duration. The output pulse width was 10 ns(FWHM) as shown in Fig. 7. Fig. 8 shows that the output beam pro-file after four passes is good. A maximum slope efficiency gs, of 44%and optical-to-optical conversion efficiency, go-o, of 30% was ob-tained for 700 ls pump duration despite a large transmission lossof around 30% for each pass.
As can be seen in Fig. 6, the output energy for 10 Hz repetitionrate reaches a maximum value. Further increases to the diode peakpower had no effect on its performance. As in the 100 Hz experi-ment the cause of this saturation was attributed to amplified spon-taneous emission (ASE) and parasitic oscillations, which agreeswith the trend of the measured small signal gain. The increase inthe output pulse energy observed for higher pump energies at100 Hz was not observed. This was attributed to the combinationof the vacuum spatial filter and the lower repetition rate reducingthe thermal lensing effects, which caused the improvement to thespatial mode overlap.
Increasing the pump energy increased the ASE and the parasiticoscillations became stronger. These unwanted oscillations entered
Fig. 4. Schematic layout of the four pass main amplifier with image relay to improve the spatial beam overlap; TFP(1) and TFP(2), thin film polarizers; FR(1) and FR(2), Faradayrotator; DM(1) and DM(2), dichroic flat mirror (AR 750–1000 nm, HR 990–1170 nm); M(1)–M(3) (HR 45�, 1030–1064 nm); M(4)–M(6) (HR 0�, 1030–1064 nm).
Fig. 8. Output performance of the main amplifier with image relay optics,repetition rate 10 Hz. Near field output beam profile after four pass amplification.Pulse energy: 146 mJ, Fluence: �1.35 J/cm2.
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the regenerative amplifier causing its output performance to dete-riorate and subsequently the main amplifiers performance. To helpreduce this deteriorating effect, two optical isolators were used be-tween the regenerative amplifier and the main amplifier, and theend mirror, M(6) was adjusted so that the third and fourth passwere slightly misaligned from the first and second.
4. Conclusion
In conclusion, we have developed an efficient sub-joule, 100 Hzdiode-pumped MOPA with cryogenic Yb:YAG to show initial viabil-ity of the joule-class laser system. At input energies of 2.4 mJ, pulseenergies of 214 mJ were obtained with an optical-to-optical effi-ciency, go-o of 19% and a slope efficiency, gs of 30%. The results sug-gested that with an improvement to the spatial beam coupling, theefficiency of the laser system might be improved. A further exper-iment was completed using image relay optics to improve the spa-tial mode coupling. With this optical set-up and a repetition rate of10 Hz, the efficiencies of the main amplifier were improved togo-o = 30% and gs = 44%.
At 10 Hz repetition rate, the pulse energy saturated at �150 mJdue to ASE and parasitic oscillations. At high pump energies, theoscillations became stronger and entered the regenerative ampli-fier lowering its output performance causing the performance ofthe main amplifier to deteriorate. To reduce the back-reflections,the end mirror M(6) was adjusted to misalign the third and fourthpass. In addition, two optical isolators were used between theregenerative amplifier and the main amplifier.
In the future to continue our investigation into the joule-classlaser system, we plan to investigate methods to effectively sup-press the ASE in the present optical layout to reduce its detrimentaleffect on the system performance. In addition we will also attemptto lower the transmittance loss of each pass and improve the ther-mal management by using a direct liquid nitrogen cooling designin both the regenerative amplifier and the main amplifier. We alsoplan to investigate the use of a novel gain material design allowingfor high damage threshold and lower temperature rise. With thesenew features to the laser system we should be able to operate athigher repetition rates without any deterioration of the output per-formance due to thermal effects and achieve much higher pulseenergies at much higher repetition rates.
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