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Optics & Laser Technology 35 (2003) 425–429

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Broadband spectral shaping in a Ti:sapphire regenerative ampli%erYuxin Leng∗, Lihuang Lin, Wenyao Wang, Yunhua Jiang, Bin Tang, Zhizhan XuShanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. Box 800-211, Shanghai 201800, China

Received 24 October 2001; received in revised form 28 January 2003; accepted 11 February 2003

Abstract

A birefringent crystal quartz plate of known thickness has been used as a spectral %lter for spectral shaping in a Ti:sapphire regenerativeampli%er. The spectral pro%le of the ampli%ed pulse ejected from the regenerative ampli%er was observed while adjusting the birefringentcrystal plate in the cavity. By altering the gain spectrum, the bandwidth of the regeneratively ampli%ed pulse was increased from ∼18 to∼35 nm by using a 0.34-mm thick birefringent plate. The output pulse spectrum from the regenerative ampli%er neared the bandwidth ofthe seed pulse. As a comparison, we used a coated %lter outside the regenerative ampli%er cavity, and the bandwidth of the regenerativelyampli%ed pulse was stretched to ∼28 nm. When the bandwidth was stretched to ∼35 nm, the pulse was compressed to ∼35 fs.? 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Ti:sapphire regenerative ampli%er; Spectral shaping; Birefringent plate

1. Introduction

The regenerative ampli%er has been used widely in achirped pulse ampli%cation (CPA) laser system to achievehigh intensity and short fs pulses [1]. In the CPA process, thecompressed pulse width is determined mainly by the band-width and the high order phase errors of the chirped pulse.Recently, a dispersion optical system has been designed tocompensate for the dispersive phase errors up to %fth order[2,3]. The compressed pulse width is then primarily aAectedby the bandwidth of the ampli%ed pulse. For the broadbandCPA, the gain narrowing eAect during pulse ampli%cationcan result in the bandwidth of the ampli%ed pulse narrowing.In a CPA laser system, a pulse is ampli%ed with high gain inthe regenerative ampli%er, so that the gain narrowing eAectin the regenerative ampli%er mainly limits the bandwidthof the ampli%ed pulse. In order to obtain broadband pulseampli%cation, spectral shaping technique is used widely inCPA laser systems to compensate for the gain narrowingeAect [4–6].

There are two classes of spectral shaping techniques,intra-cavity spectral shaping [4] and external-cavity spectralshaping [5,6]. Generally, the intra-cavity spectral shaping isthat a band pass %lter is used in the regenerative ampli%ercavity. When the peak of the gain spectrum is attenuated,a wider gain bandwidth can be achieved [4]. A thin etalon

∗ Corresponding author. Fax: +86-21-59528812.E-mail address: lengyuxin cn@hotmail.com (Y. Leng).

has been used as the band pass %lter to produce the mod-ulation in gain spectrum to attenuate the peak of the gainspectrum to achieve the wider bandwidth [7,8]. Because thethickness of the etalon is solid, it is diGcult to adjust thepro%le and the position of the modulation in gain spectrumconveniently to compensate the gain narrowing optimally.

In this paper, we use a birefringent quartz plate in aTi:sapphire regenerative ampli%er for intra-cavity spectralshaping. By theoretical calculation and experiment, thebandwidth that the regenerative ampli%er can support isstretched eGciently with the intra-cavity spectral shaping.A ∼35-nm bandwidth (FWHM) of the ampli%ed pulse isachieved, against the original ∼18-nm bandwidth (FWHM)without the spectral shaping. With the ∼35-nm band-width and intra-cavity spectral shaping, a 35-fs compressedpulse is subsequently achieved. The time–bandwidth prod-uct was 0.59. As a comparison, in the experiment withexternal-cavity spectral shaping, a ∼28-nm bandwidth ofthe ampli%ed pulse was obtained.

2. Theory

For intra-cavity spectral shaping, a birefringent plate canbe used in cavity to attenuate the peak of the gain spec-trum of a regenerative ampli%er. The birefringent plate canchange the polarization of the pulse oscillating in the regen-erative ampli%er cavity. For a broadband pulse, the state ofthe changed polarization is dependent on the spectrum of the

0030-3992/03/$ - see front matter ? 2003 Elsevier Science Ltd. All rights reserved.doi:10.1016/S0030-3992(03)00038-0

426 Y. Leng et al. / Optics & Laser Technology 35 (2003) 425–429

Fig. 1. Theoretic single-pass transmission curves for a birefringent quartz plate with diAerent parameters in the regenerative ampli%er cavity, accordingto Eq. (1): (a) � = 55◦, �= 82◦, the thickness t = 0:32 mm (1), 0:34 mm (2), 0:36 mm (3), and 0:5342 mm (4); (b) � = 55◦, �= 80◦ (1); �= 82◦(2), and � = 84◦ (3), the thickness t = 0:34 mm; and (c) � = 82◦, � = 53◦ (1); � = 55◦ (2), and � = 57◦ (3), the thickness t = 0:34 mm.

pulse. So the birefringent plate and the polarizer in the cav-ity can compose a band pass %lter, which can attenuate theintensity of the certain band of the spectrum of the ampli-%ed pulse. Thus a wider bandwidth gain of the regenerativeampli%er can be obtained.

Assuming there is only one polarizer in the cavity and onebirefringent quartz plate is used, a pulse in the cavity willpass the polarizer and the birefringent quartz plate twice ona single round trip. Assuming the optic axis of the crystalis parallel to the surface of the quartz crystal plate, and thebirefringent quartz plate is the uniaxial crystal, the singlepass transmission T for the regenerative ampli%er cavity canbe described as follows [9]:

T (�) = 1 − sin2(2�)n4

o − n2o cos2 �

(n2o − cos2 � cos2 �)2

×sin2{�tne[1 + cos2 � cos2 �=n2

e − cos2 � cos2 �=n2o]

�[1 − cos2 � sin2 �=n2e − cos2 � cos2 �=n2

o]1=2

− �tno

�[1 − cos2 �=n2o]1=2

}: (1)

Here, no and ne are the ordinary and the extraordinary re-fractive index of quartz crystal for the pulse with wave-length � respectively. � is the angle between the optic axis

and the incident face, and � is the angle between the crystalface and the incident light. It is indicated from Eq. (1) thata certain range wavelength can be attenuated by using thebirefringent quartz plate with certain thickness t and opticaxis orientation � and � (shown in Fig. 1a). For a birefrin-gent quartz plate with thickness t, the attenuation curve canbe adjusted by rotating the birefringent plate, i.e. changing� and � (shown in Fig. 1b and c).

It is shown in Fig. 1a, with a certain optic axis orien-tation � and �, the central wavelength of the attenuationcurve moves and the bandwidth of the attenuation curvedecreases with increasing crystal thickness (t). The centralwavelength of the attenuation curve can be same using bire-fringent plates with the diAerent thickness t. But the band-width of the attenuation curve achieved by using the thinnercrystal plate is narrower than the bandwidth achieved by us-ing a thicker crystal plate. Using the birefringent plate withcertain thickness, the angle � aAects the attenuation depth ofthe attenuation curve (shown in Fig. 1b), the angle � aAectsthe central wavelength of the attenuation curve (shown inFig. 1c). Therefore, according to the calculation, the neededattenuation curve can be obtained by selecting the birefrin-gent plate with certain thickness with a certain orientationof the optic axis in cavity. Then in experiment, the cen-tral wavelength of the attenuation curve can be adjusted to

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compensate for the gain narrowing eAect optimally by sim-ply rotating the optic axis.

In Eq. (1), the F–P eAect is neglected for the ideal bire-fringent plate. But in practical experiments, the F–P etaloneAect exists due to a level of surface reMectivity R for abirefringent plate. The F–P eAect will generate many littlepeaks on the pulse spectrum. However, with the low mirrorreMectivity R (¡ 0:1), the F–P eAect can be neglected [10].If the birefringent plate could be coated with AR coating, orthe incident angle is near the Brewster angle, the F–P eAectcan be neglected. In Eq. (1), the optical rotator eAect of thebirefringent plate is neglected too, because the birefringentplate is very thin.

3. Experiment

The experiment has been carried out in our 5.4TWTi:sapphire laser system [1], and the experimental schemeis shown in Fig. 2. The birefringent plate is inserted into theTi:sapphire regenerative ampli%er operating at 10 Hz. Theplate is thin enough, so that its eAect on the regenerativecavity can be neglected. The birefringent quartz crystal platewas x-axis cut, and its optic axis is parallel to the plate sur-face. A 76 MHz mode-locking pulse train was used as theseed pulse from a commercial self-mode-locked Ti:sapphireoscillator (B.M.I Corp.). The seed pulse is stretched from∼26 fs to ∼220 ps by a grating stretcher, and injected intothe regenerative ampli%er. The energy of a single stretchedseed pulse was about 4 nJ. The stretched pulse bandwidthwas ∼36 nm, and the central wavelength of the seed pulsewas at ∼790 nm (shown in Fig. 3). A %ber spectrum me-ter (SD2000, Ocean Optic Inc.) was used to measure thespectrum. A PIN photo-detector and an oscilloscope wereused behind the cavity mirror M2 to monitor the amplifyingprocess.

Without the seed pulse being injected into the regen-erative ampli%er, the bandwidth of ASE spectrum of thefree-running regenerative ampli%er was ∼30 nm (shown inFig. 4a). Therefore, the bandwidth of the ampli%ed pulsethat regenerative ampli%er could support was¡ 30 nm. Thebandwidth of the output pulse from the regenerative am-pli%ed was ∼18 nm (shown in Fig. 5a) for the seed pulsewith∼36 nm bandwidth. The central wavelength of the ASEspectrum was ∼785 nm, so that the central wavelength shiftexisted between the central wavelengths of the ASE spec-trum and the seed pulse spectrum. The output pulse energyfrom the regenerative ampli%er was ∼1 mJ for the ∼4 nJseed pulse. The pump energy was kept at ∼20 mJ.

Then a birefringent quartz plate with 0:34 mm thicknesswas inserted into the cavity. The thickness was determinedby Eq. (1) and the pro%le of the gain spectrum of the regen-erative ampli%er. When the plate was inserted into the cav-ity, the modulation in the ASE spectrum of the free-runningregenerative ampli%er was obtained. By adjusting the opticaxis orientation according to Eq. (1), the central wavelength

Stretcher

F

Ti:S

PCP

Regenerative amplifier

BF

Seed pulse

Output pulse

M1

M2

PIN

Oscilloscope

Fig. 2. Scheme of experiment. P—polarizer; PC—Pockels cell; M1,M2—plane mirrors; BF—birefringent quartz plate %lter; and F—Faradayisolator.

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Fig. 3. Spectrum of the seed pulse before being injected into the regen-erative ampli%er.

of the attenuation curve was located at the gain peak ofthe regenerative ampli%er, and the pro%le of the attenuationcurve was controlled to obtain the widest bandwidth. So theASE spectrum could be stretched eGciently (¿ 70 nm) tosupport the wider bandwidth pulse to be ampli%ed (shownin Fig. 4b).

Then the seed pulse was injected into the regenerativeampli%er with the birefringent plate in cavity. According tothe stretched ASE spectrum, the gain narrowing eAect in theregenerative ampli%er was controlled eGciently. The bire-fringent plate in cavity needed to be rotated a little to obtainthe optimum pulse shaping spectrum. For comparison, threeplates with 0.34-, 0.5342- and 0.93-mm thickness were usedseparately. The thickness of the plates was determined bythe above theory. The spectral shaping pulse spectrums withthe diAerent plates were shown in Fig. 5.

It is indicated in Fig. 5 that the bandwidth of the ampli-%ed pulse without the birefringent quartz plate in the regen-erative ampli%er was ∼18 nm, the central wavelength was∼785 nm (Fig. 5a). With the 0:34 mm birefringent quartzplate in cavity, a ∼35 nm bandwidth of the ampli%ed pulsewas obtained (Fig. 5b). The central wavelength was movedto ∼790 nm, which is matched to the seed pulse. The band-width of the ampli%ed pulse was stretched around two times,and the spectrum shift was compensated. By changing the

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Wavelengthn (nm)(a) (b)

Fig. 4. (a) ASE spectrum of the regenerative ampli%er without the birefringent quartz plate in cavity. The bandwidth is ∼30 nm and (b) ASE spectrumof the regenerative ampli%er with the birefringent quartz plat (0:34 mm thickness) in cavity. The bandwidth is ¿ 70 nm.

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(a) (b)

(c) (d)

Fig. 5. Experimental result of the spectral shaping in cavity: (a) spectrum of the output pulse from the regenerative ampli%er without the birefringentquartz plate, the bandwidth is ∼18 nm (FWHM); (b) spectrum of the output pulse from the regenerative ampli%er with the birefringent quartz plate(the thickness is 0:34 mm), the bandwidth is ∼35 nm (FWHM); (c) spectrum of the output pulse from the regenerative ampli%er with the birefringentquartz plate (the thickness is 0:5342 mm), the bandwidth is ∼30 nm (FWHM); and (d) spectrum of the output pulse from the regenerative ampli%erwith the birefringent quartz plate (the thickness is 0:93 mm).

birefringent plate with diAerent thickness (0:5342 mm thick-ness), diAerent attenuations curve and the stretched spec-trum of the ampli%ed pulse were achieved (Fig. 5c). If theattenuation depth was too large as with very thick birefrin-gent plate (0:93 mm thickness), the spectrum of the ampli-%ed pulse was split into two parts (Fig. 5d).

When the pump energy kept unvaried, because thebirefringent plate as the attenuator was used in regenera-tive ampli%er, it is necessary for the seed pulse to pass theregenerative ampli%er cavity with more times to depletethe gain of the regenerative ampli%er. Consequently, theenergy of the pulse output from the regenerative ampli%erdecreased. With the 0.34-mm thick birefringent plate in

cavity, the ∼24 round trips was needed instead of ∼14round trips without the plate in cavity, and the output pulseenergy from the regenerative ampli%er was decreased from∼1 to ∼0:4 mJ. The energy attenuation could be compen-sated for by the saturable ampli%cation in the subsequentmain ampli%er. The synchronization of the whole lasersystem should be adjusted for the delay.

In the experiment, the crystal plates were not AR coated,and the incident angle is close to the Brewster angle of thequartz crystal. Therefore, the surface reMectivity resulted inthe F–P etalon eAect according to the theory, and the pro%leof the pulse spectrum was not smooth (Fig. 4b). Becausethe birefringent plate is very thin, the change of the spatial

Y. Leng et al. / Optics & Laser Technology 35 (2003) 425–429 429

Fig. 6. ∼35 fs (FWHM) compressed pulse with ∼35 nm bandwidth using0:34 mm birefringent plate in cavity.

mode of the output from the regenerative ampli%er could beneglected.

When the ampli%ed bandwidth was stretched to ∼35 nmwith 0.34-mm thick birefringent quartz plate in the regenera-tive ampli%er, the pulse was compressed to ∼35 fs (FWHM)by a subsequent grating pair (shown in Fig. 6). The time–bandwidth product was 0.59. The pulse width was mea-sured by a single shot autocorrelator (Positive Light ModelSSA). Because the high order dispersion was not compen-sated completely, there were a little wing in the pro%le ofthe compressed pulse, and the time–bandwidth product wasbigger.

As a comparison, an external-cavity spectral shaping wasalso examined. A coated %lter, whose transmission is ∼40%near 730–830 nm, was inserted into the grating stretcher.The seed pulse was stretched in space according to the fre-quency in the grating stretcher. The %lter was inserted intothe grating stretcher to attenuate the partial spectrum of theseed pulse. The position of the %lter in the grating stretcherwas corresponding to the gain peak of the ASE spectrumof the regenerative ampli%er. The attenuated seed pulse wasampli%ed in the regenerative ampli%er subsequently. So thespectral attenuation in the seed pulse could compensate forthe gain narrowing eAect. For the ∼36-nm bandwidth of theseed pulse, a ∼28-nm bandwidth of the output pulse fromthe regenerative ampli%er was achieved (shown in Fig. 7).The stretched bandwidth was narrower with the extra-cavityspectral shaping, comparing with the intra-cavity spectralshaping using the birefringent quartz plate in cavity.

4. Conclusion

As a conclusion, the intra-cavity spectral shaping with thebirefringent quartz plate in the regenerative ampli%er cavitywas studied theoretically and experimentally. By adjustingthe orientation of the optic axis of the birefringent plate witha certain thickness, the gain narrowing eAect was compen-sated eGciently. The ASE spectrum of the regenerative am-pli%er was stretched, which could support the ampli%cation

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Fig. 7. Experimental result of the spectral shaping outside the cavity.The bandwidth of the output pulse from the regenerative ampli%er is∼28 nm (FWHM).

of the pulse with a wider spectrum. For the ∼36-nm seedpulse, the bandwidth of the ampli%ed pulse was stretchedfrom ∼18 to ∼35 nm by simply inserting a 0.34-mm thickbirefringent plate into the cavity, then the ampli%ed pulsewas compressed to ∼35 fs, and the time–bandwidth productwas 0.59. As a comparison, the ampli%ed pulse bandwidthwas stretched to ∼28 nm using the %lter with external-cavityspectral shaping.

The spectral shaping technique is simple without changingthe regenerative ampli%er cavity by using birefringent quartzplate. Compared to use an etalon in the cavity, it is moreconvenient to obtain the needed pulse spectrum curve bysimply rotating the birefringent plate in space.

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