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Optical performance of the CLIO infrared FEL
Nuclear Instruments and Methods in Physics Research A 331 (1993) 15-19 North-Holland
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
Optical performance of the CLIO infrared FEL R. Prazeres, J.M. Berset, F. Glotin, D. Jaroszynski and J.M. Ortega LURE, bat. 209d, Universit£ de Paris-Sud, 91405 Orsay cedex, France
First laser oscillation on the CLIO infrared FEL was obtained in January 1992. This paper describes the layouts of the optical devices used for CLIO, and discusses the optical performances. This machine consists of an rf linear accelerator, described in a companion paper, providing a 30/70 MeV electron beam through a 48 period planar undulator (K = 0 to 2). The optical cavity is 4.8 m long and uses broadband metal mirrors. The optical beam is extracted with an intracavity CaF 2 or ZnSe plate. Laser oscillation has been obtained thus far in the range of A = 2.5 to 15 ~m at accelerator energies of 32, 40 and 50 MeV. The average power of the laser is about 65 mW for the low duty cycle (6.25 Hz/32 ns) and up to 0.5 W for a duty cycle of 50 H z / 3 2 ns and should be 5-10 W at maximum repetition rate. The peak power extracted for 8 ps micropulses is 2.5 MW corresponding to 0.4% efficiency. Laser oscillation on the third harmonic of 10 Ixm has also been achieved (at A = 3.3 ~m). Application experiments have already been done with the CLIO infrared laser (companion paper on nonlinear absorption in InSb), showing the good reliability and overall quality of the laser. The programme now is to operate the accelerator at other energies so as to cover the rest of the designed wavelength range (2-20 p~m).
1. Introduction A s has b e e n d e m o n s t r a t e d now by several examples in the world [1], the free e l e c t r o n laser ( F E L ) can b e actually c o n s i d e r e d as a real user facility laser source in t h e far-infrared to visible r a n g e (at least for t h e so-called C o m p t o n type of FEL). A n o t h e r m a c h i n e has now j o i n e d the g r o u p of " u s e r facilities": t h e C L I O i n f r a r e d F E L of Orsay. T h e first lasing of C L I O was o b t a i n e d in J a n u a r y 1992, at a w a v e l e n g t h of 8 Ixm [2]. This F E L uses a 3 2 / 5 0 M e V rf linear accelerator, a n d a 48 p e r i o d p l a n a r u n d u l a t o r ( K = 0 to 2). T h e laser w a v e l e n g t h is p r e s e n t l y in t h e r a n g e of 3 to 15 txm. T h e F E L optical cavity is 4.8 m long a n d uses b r o a d b a n d m e t a l mirrors. Fig. 1 shows a general layout of t h e FEL. T h e laser b e a m p e r f o r m a n c e a n d stability have b e e n m e a s u r e d , a n d they have b e e n f o u n d to b e in good a g r e e m e n t with t h e expected values. T h e C L I O F E L has b e e n built principally as a user facility, a n d some application e x p e r i m e n t s have already b e e n done, such as n o n l i n e a r a b s o r p t i o n in InSb, a n d sum frequency g e n e r a t i o n o n surfaces. T h e C L I O F E L l a b o r a t o r y involves four user r o o m s a n d o n e F E L diagnostics room. T h e laser b e a m can b e sent to any o n e of t h e s e rooms, a n d a substantial p a r t of t h e laser b e a m time is n o w assigned to users.
2. The accelerator T h e e l e c t r o n b e a m is p r o d u c e d by a 3 2 - 5 0 M e V rf linear accelerator. T h e injector is a 90 k e V t h e r m i o n i c
gun, followed by a s u b h a r m o n i c p r e b u n c h e r at 0.5 GHz, a n d a b u n c h e r at 3 GHz. T h e e l e c t r o n b e a m at 3.6 M e V is t h e n a c c e l e r a t e d in a 4.5 m long travelling wave accelerating section, to the n o m i n a l energy. T h e energy should b e easily e x t e n d e d to 3 0 - 7 0 MeV. T a b l e 1 displays t h e m a i n a c c e l e r a t o r characteristics a n d the time s t r u c t u r e possibilities [3]. All t h e p a r a m e t e r s of t h e m a c h i n e a n d the e l e c t r o n b e a m diagnostics are m o n i t o r e d by computer. I n o r d e r to analyse t h e transfer of energy f r o m electrons to the optical b e a m , a real time s p e c t r u m analyser of the e l e c t r o n b e a m energy has b e e n installed after t h e u n d u l a t o r [4]. T h e e l e c t r o n b e a m is s c a t t e r e d according to energy by a dipole placed after t h e u n d u l a t o r , a n d a 32 e l e m e n t d e t e c t o r m e a s u r e s t h e s p e c t r u m in real time d u r i n g t h e macropulse. This allows a direct m e a s u r e m e n t of the e l e c t r o n energy loss in t h e lasing process.
3. FEL design T h e u n d u l a t o r is of t h e p l a n a r type with SmCo p e r m a n e n t m a g n e t s r e p r e s e n t i n g 48 periods of A0 = 4 cm. T h e deflection p a r a m e t e r can b e adjusted from K = 0 to K = 2. T h e o u t p u t r a d i a t i o n is linearly polarized. In reality, t h e u n d u l a t o r is c o m p o s e d of two half u n d u l a t o r s of 24 periods, for which the gap can b e i n d e p e n e n t l y adjusted, a n d t h e last half u n d u l a t o r may b e t a p p e r e d . However, presently all t h e e x p e r i m e n t s o n C L I O r e p o r t e d h e r e have b e e n d o n e with the same gap for b o t h half undulators, a n d w i t h o u t tapering. Fig. 2 shows a s p o n t a n e o u s s p e c t r u m with a 40 M e V elect r o n b e a m , a n d with a n u n d u l a t o r gap of 21 mm,
0168-9002/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
II. FIRST LASING
16
R. Prazeres et al. / Optical performance of the CLIO infrared FEL
e-
FEL mirror Brewster extraction pnate
beam dump
,
=.
,
5,111 ,
FEL mirror
,
,
I
~
z
x
~
Fig. 1. General layout of the CLiO infrared free electron laser.
Table 1 General characteristics of the linear accelerator Energy range Emittance 4v/3y~rc~' Energy spread Peak current Macropulse repetition rate Macropulse length Micropulse repetition rate Micropulse length
32-50 MeV 80,n- mmmrd 0.7% 40 A 6.25-50 Hz 10 ~,s 8, 16 or 32 ns 5-10 ps
representing a resonant wavelength, a R = 4.7 b~ml The experimental linewidth is A A / A = 3%, larger than the theoretical value A A / A = 2%. The laser cavity mirrors are fabricated with protected silver coatings on silicon substrates of 25 m m diameter, with a curvature radius of 3 m. The reflection coefficient is about 0.995 in the range 3 - 1 0 txm. The mirrors are separated by 4.8 m, and the Rayleigh length of the cavity is Z R = 1.2 m. The light is partially extracted from the cavity by a Z n S e or CaF 2 plate, which is set at 60 ° of incidence, near the Brewster angle. A mechanical device allows one to choose among
four different plates for the extraction. The Z n S e allows coverage of the range A = 0.6 to 18 I,m, with an extraction coefficient of 1.5%, representing losses of 6% for one cavity round trip. The CaF 2 allows coverage of the range A = 0.1 to 10 Izm, with an extraction coefficient of 0.2%, representing losses of 0.8%. A plate of KRS5 has been tested for extraction, but it did not permit lasing because of the poor optical quality of this material (large amount of optical scattering). The configuration of extraction with the Brewster plate creates four exit laser beams as shown in fig. 3. For each side of the plate, the two reflections of the b e a m are parallel and separated transversely by 2 mm (for ZnSe). The time distance between the two parallel laser pulses is about 50 ps (for ZnSe), which must be compared to the real laser pulse length of about 10 ps. Presently, only the double beam reflected by one side of the plate is sent to the diagnostics r o o m . The reflection on the other side is not used. After extraction, the laser beam passes through a telescope and is sent to the diagnostics r o o m through a 15 m pipe line. This line is extended to. the four user rooms. In order to avoid infrared absorption in the air (by H 2 0 , CO2, etc.) the whole infrared pipe line may be filled with dry air or a neutral gas such as nitrogen. The alignment is done with a H e N e laser, which is sent onto the Brewster plate inside the F E L cavity, and
4
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== 4.5
i 4.6
4.7
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i
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5
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.1
Gtm)
Fig. 2. S p o n t a n e o u s e m i s s i o n s p e c t r u m w i t h a 40 M e V d e c t r o n b e a m , a n d w i t h a n u n d u l a t o r g a p o f 21 m m , r e p r e s e n t i n g a r e s o n a n c e w a v e l e n g t h X R = 4.7 ~xm.
FEL / / 2
mirrm
.... l a s e r h e a n/
Fig. 3. Configuration of extraction with the Brewster plate, showing the four exit laser beams.
R. Prazeres et al. / Optical performance of the CLIO infrared FEL Table 2 Summary of the laser characteristics measured for the beam extracted out of one of the faces of the extraction plate. The maximum performances are obtained with the ZnSe extraction plate, at A = 8 p~m, for ymc 2= 40 MeV and gap= 14 mm. The 3rd harmonic is obtained with the CaF 2 extraction plate. Lasing on the fundamental Total spectral range Maximum peak power Maximum average power Maximum net gain Spectral width 21A/A Wavelength stability (jitter) Pulse length
3-15 p~m 10 MW in 2 ps 65 mW at 6.25 Hz/32 ns 140% 0.4% 0.2% 2-10 ps
Lasing on the 3rd harmonic Resonance wavelength Laser wavelength Net gain
10 Ixm 3.3 pxm 10%
then follows the complete laser trajectory, through the infrared line, up to the experimental rooms. The alignment of the C L I O laser cavity is first roughly done with this H e N e laser, and is then optimised by looking at the laser signal. The cavity length, as well as the mirror tilts, are adjusted by stepping motors, monitored from the diagnostics r o o m by computer. This computer also allows one to control the undulator gap. W e can observe a small drift of the cavity length after a few hours, but this can be easily corrected from the diagnostics room.
4. Experimental results The first laser b e a m has been obtained with the CaF 2 extraction plate, for which the losses are lower than with the ZnSe, and allows easier lasing. Nevertheless, the results that are presented here have been obtained with the Z n S e plate, for which the higher extraction coefficient leads to extraction of a larger amount of laser energy. The general optical characteristics are summarised in table 2. The numerical values shown here are only for one side extraction of the Brewster plate; the reflection on the other side is lost. A spectral range, between 3 and 15 p.m, is presently covered using three electron energy values: 32, 40 and 50 MeV. Fig. 4 shows the laser power, measured in the diagnostics room, as a function of the laser wavelength, which is modified by changing the undulator gap. The time delay to modify the undulator gap is about a few seconds, and the delay n e e d e d to change the electron energy is about 20 min because of the cycling of the accelerator magnets. This makes a rather flexible tunability of the laser. The small residual amount of water
17
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5~ t.
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s~
~.
0
2
4
6
8
10
12
14
16
laser wavelength (microns) Fig. 4. Laser power, measured in the diagnostics room, as a function of the laser wavelength, for 32, 40 and 50 MeV electron energy.
on the optics and the CO 2 in the line create absorption for two wavelength domains near 6 txm and 15 ~m. The laser power and gain displayed in table 2 are for the wavelength A = 8 ~m, which has the best performance. The electron energy is 40 MeV, and the undulator gap is 14 mm. In this case, a peak power of 2.5 M W has been obtained for a 8 ps pulse length) and a net gain of 140% has been observed at 32 MeV, gap at 14 mm. The peak power is deduced from the average power measurement, by taking into account the time structure of the laser and the fact that two parallel laser beams are extracted from each side of the intracavity Brewster plate (see fig. 3). The gain has been deduced from the rise time (25 ns for doubling of the signal strength) of the laser pulse shape, which is shown in fig. 5. The detector used is of HgCdTe. The vertical scale of this figure has been chosen in order to show the laser saturation, after about 2 txs; it does not allow one to see the first part of the exponential growth of the signal, from which the laser gain is I
I
I
I
I
I
[
I
I
Fig. 5. Laser pulse shape, at A = 8 I-cm, and current macropulse. II. FIRST LASING
18
R. Prazeres et al. / Optical performance of the CLIO infrared FEL stable~ A~/L :0.4%
8.05
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.~
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- 6 0 - 5 0 -40 - 3 0 - 2 0 -10 0 1 cavity length variation (~tm)
macropuise current : 101as
Fig. 6. FEL cavity detuning curve at 3. = 8 txm. deduced. The current pulse is 10 p~s long, and the saturation duration is about 7 ixs, corresponding to 220 laser micropulses of 10 ps. The cavity losses of 8% are deduced from the fall time of the laser pulse, which is 400 ns. This is the expected value, corresponding to 6% from the Z n S e extraction plate and 1% for each mirror. The F E L cavity detuning curve is shown in fig. 6 for A = 8 txm and Tmc 2 = 40 MeV. The tuning range is about Az = 40 txm. W e have verified that for the cavity length corresponding to the maximum laser power, the laser wavelength is unstable and wide: A A / A = 1.5%. For a shorter cavity length, there is a rather flat range where the laser wavelength is stable, about 0.2% jitter, and the width is only AA/A = 0.4%. The stable tuning range width is about 20 txm. In this range, the laser spectrum appears as shown in fig. 7. Let us point out that this spectrum is the temporal integration along the laser macroputse. A real time contour plotting spectrum is shown in fig. 8, where the ordinates are wavelength (txm) and time (ixs). This figure shows that the central wavelength shifts along the macropulse, and reaches an equilibrium value for saturation. The transverse profile of the laser b e a m at 8 txm, taken in the diagnostics room, is shown in fig. 9. A 32 element area pyroelectric detector has been used here (as well as for the laser spectrum in fig. 7). The profile
I I I I I time (~ts) 5 10 Fig. 8. Real time contour plotting spectrum, near Z = 8/~m, as a function of the time during the macropulse. 7.85
I I [ I
is well fitted by a Gaussian curve for which the waist is w = 3.9 m m in the diagnostics room. Lasing on the third harmonic has been observed with y m c 2 = 4 0 M e V and fundamental resonance wavelength AR = 10 txm. The extraction plate was the CaF2, which is strongly absorbing at this wavelength, and does not allow lasing on the fundamental. Nevertheless, the laser can operate on the third harmonic at A = 3.3 ~xm. The laser pulse is shown in fig. 10; the saturation is just reached during the last part of the macropulse. The gain has been measured to about 10%, and the average measured power was less than 1 m W for a 6.25 Hz maeropulse repetition rate. A laser pulse length m e a s u r e m e n t has been done with a Michelson interferometer and an InSb nonlinear detector [5]. The autocorrelafion of the pulse in the interferometer produces interference fringes, which are related to the coherence length of the laser pulse. The nonlinear property of the InSb detector permits detection of the coincidence of the two pulses of the inter-
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.5
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transverse wavelength Fig. 7. Laser spectrum at A = 8 ixm.
at
waist=3.9
--
I
I
mm [
t
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10
axis
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(mm)
Fig. 9. Transverse profile of laser beam at 8 ~m, taken in the diagnostics room:
R. Prazeres et aL / Opticalperformance of the CLIO infrared FEL I
i
I
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I
I
3rd i1)
laser
m
I
I
I _~
harmonic
pulse
__~
q
-- macro-pulse r
i ..... I
I
lOgs time (2~s/div) Fig. 10. Laser pulse when operating on third harmonic at A = 3.3 ~Lm,and current macropulse. ferometer. The integration of the fringes gives a signal which is proportional to the real length of the laser pulse, independently of its coherence length. The pulse length has b e e n found presently to be between 2 and 12 ps. A complete analysis of the pulse length as a function of the laser operating conditions is planned for the next F E L experiments.
19
doubled N d - Y A G laser (pump) has b e e n done successfully [6]. The mixing of the C L I O infrared wavelength (A = 5 Ixm) and the auxiliary laser (A = 0.532 ixm) is in the visible range, where the detectors are more sensitive than for infrared. The tunability of the F E L allows one to scan the molecular spectrum. The four rooms for the users are now available for laser experiments; and we have planned to dedicate a large amount of laser b e a m time to new application experiments. The rest of the time will be used for intrinsic F E L studies: for example improvement of the wavelength range (up to 20 Ixm) by decreasing the electron b e a m energy, or operating the F E L with undulator tapering. It will be also interesting to study of the effect on the laser of dephasing of the electron bunch on the accelerator. Operating the laser at a higher repetition rate of the time structure (50 H z / 4 ns) is also planned for the coming months. Laser extraction from the F E L cavity through a hole in the exit mirror is also planned for the near future. This will be useful for wavelengths larger than 15 p,m, for which the absorption in the Z n S e extraction plate forbids lasing.
References 5.
Conclusion
The C L I O infrared laser is already able to give a stable and tunable laser b e a m to users. The laser wavelength stability is about 0.2%, and the intensity stability is less than 10%. The time delay to modify the undulator gap is about a few seconds, and the delay n e e d e d to change the electron energy is about 20 min. This makes tunability of the laser rather flexible. A spectral range between 3 lxm and 15 ~ m is presently covered using three electron energy values: 32, 40 and 50 MeV. Application experiments have already started. A two photon absorption effect in InSb has been studied for different wavelengths, and this nonlinear property is used for laser pulse measurements [5]. Infrared microscopy, by pickup of an evanescent wave of the laser on a Z n S e medium, is now in progress. High energy laser pulses are necessary because of the p o o r efficiency of the energy pickup by the optical fiber. A sum frequency generation on molecular surfaces by combination o f the C L I O laser (test) and a
[1] S. Benson et al., presented at this Conference (14th Int. Free Electron Laser Conf., Kobe, Japan, 1992). P.W. van Amersfoort et al., Nucl. Instr. and Meth. A318 (1992) 42; C.A. Brau, Nucl. Instr. and Meth. A318 (1992) 38. [2] F. Glotin, J.M. Berset, R. Chaput, B. Kergosien, G. Humbert, D. Jaroszynski, J.M. Ortega, R. Prazeres, M. Velghe, J.C. Bourdon, M. Bernard, M. Deharnme, T. Garvey, M. Mencik, B. Mouton, M. Orneich, J. Roudier and P. R0udier, Proc. 3rd EPAC Conf., Berlin, 1992. [3] R. Chaput et al., these Proceedings (14th Int. Free Electron Laser Conf., Kobe, Japan, 1992) Nucl. Instr. and Meth. A331 (1993) 267. [4] M. Bergher, E. Jules and A. Louis-Joseph, IEEE Particle Accelerator Conf., San Francisco, 1991, vol. 2, p. 1225. [5] D. Jaroszynski et al., ref. [3], p. 640. [6] A. Peremans, P. Guyot-Sionnest, A. Tadjeddine, F. Glotin, J.M. Ortega and R. Prazeres, ref. [3], p. ABS 28. [7] A. Piednoir, F. Creuzet, C. Lieoppe and F. de Fornel, First specifications of the PSTM working in the infrared, Proc. NFO Conf., Besan~on, France, 1992, to be published.
II. FIRST LASING
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
Optical performance of the CLIO infrared FEL R. Prazeres, J.M. Berset, F. Glotin, D. Jaroszynski and J.M. Ortega LURE, bat. 209d, Universit£ de Paris-Sud, 91405 Orsay cedex, France
First laser oscillation on the CLIO infrared FEL was obtained in January 1992. This paper describes the layouts of the optical devices used for CLIO, and discusses the optical performances. This machine consists of an rf linear accelerator, described in a companion paper, providing a 30/70 MeV electron beam through a 48 period planar undulator (K = 0 to 2). The optical cavity is 4.8 m long and uses broadband metal mirrors. The optical beam is extracted with an intracavity CaF 2 or ZnSe plate. Laser oscillation has been obtained thus far in the range of A = 2.5 to 15 ~m at accelerator energies of 32, 40 and 50 MeV. The average power of the laser is about 65 mW for the low duty cycle (6.25 Hz/32 ns) and up to 0.5 W for a duty cycle of 50 H z / 3 2 ns and should be 5-10 W at maximum repetition rate. The peak power extracted for 8 ps micropulses is 2.5 MW corresponding to 0.4% efficiency. Laser oscillation on the third harmonic of 10 Ixm has also been achieved (at A = 3.3 ~m). Application experiments have already been done with the CLIO infrared laser (companion paper on nonlinear absorption in InSb), showing the good reliability and overall quality of the laser. The programme now is to operate the accelerator at other energies so as to cover the rest of the designed wavelength range (2-20 p~m).
1. Introduction A s has b e e n d e m o n s t r a t e d now by several examples in the world [1], the free e l e c t r o n laser ( F E L ) can b e actually c o n s i d e r e d as a real user facility laser source in t h e far-infrared to visible r a n g e (at least for t h e so-called C o m p t o n type of FEL). A n o t h e r m a c h i n e has now j o i n e d the g r o u p of " u s e r facilities": t h e C L I O i n f r a r e d F E L of Orsay. T h e first lasing of C L I O was o b t a i n e d in J a n u a r y 1992, at a w a v e l e n g t h of 8 Ixm [2]. This F E L uses a 3 2 / 5 0 M e V rf linear accelerator, a n d a 48 p e r i o d p l a n a r u n d u l a t o r ( K = 0 to 2). T h e laser w a v e l e n g t h is p r e s e n t l y in t h e r a n g e of 3 to 15 txm. T h e F E L optical cavity is 4.8 m long a n d uses b r o a d b a n d m e t a l mirrors. Fig. 1 shows a general layout of t h e FEL. T h e laser b e a m p e r f o r m a n c e a n d stability have b e e n m e a s u r e d , a n d they have b e e n f o u n d to b e in good a g r e e m e n t with t h e expected values. T h e C L I O F E L has b e e n built principally as a user facility, a n d some application e x p e r i m e n t s have already b e e n done, such as n o n l i n e a r a b s o r p t i o n in InSb, a n d sum frequency g e n e r a t i o n o n surfaces. T h e C L I O F E L l a b o r a t o r y involves four user r o o m s a n d o n e F E L diagnostics room. T h e laser b e a m can b e sent to any o n e of t h e s e rooms, a n d a substantial p a r t of t h e laser b e a m time is n o w assigned to users.
2. The accelerator T h e e l e c t r o n b e a m is p r o d u c e d by a 3 2 - 5 0 M e V rf linear accelerator. T h e injector is a 90 k e V t h e r m i o n i c
gun, followed by a s u b h a r m o n i c p r e b u n c h e r at 0.5 GHz, a n d a b u n c h e r at 3 GHz. T h e e l e c t r o n b e a m at 3.6 M e V is t h e n a c c e l e r a t e d in a 4.5 m long travelling wave accelerating section, to the n o m i n a l energy. T h e energy should b e easily e x t e n d e d to 3 0 - 7 0 MeV. T a b l e 1 displays t h e m a i n a c c e l e r a t o r characteristics a n d the time s t r u c t u r e possibilities [3]. All t h e p a r a m e t e r s of t h e m a c h i n e a n d the e l e c t r o n b e a m diagnostics are m o n i t o r e d by computer. I n o r d e r to analyse t h e transfer of energy f r o m electrons to the optical b e a m , a real time s p e c t r u m analyser of the e l e c t r o n b e a m energy has b e e n installed after t h e u n d u l a t o r [4]. T h e e l e c t r o n b e a m is s c a t t e r e d according to energy by a dipole placed after t h e u n d u l a t o r , a n d a 32 e l e m e n t d e t e c t o r m e a s u r e s t h e s p e c t r u m in real time d u r i n g t h e macropulse. This allows a direct m e a s u r e m e n t of the e l e c t r o n energy loss in t h e lasing process.
3. FEL design T h e u n d u l a t o r is of t h e p l a n a r type with SmCo p e r m a n e n t m a g n e t s r e p r e s e n t i n g 48 periods of A0 = 4 cm. T h e deflection p a r a m e t e r can b e adjusted from K = 0 to K = 2. T h e o u t p u t r a d i a t i o n is linearly polarized. In reality, t h e u n d u l a t o r is c o m p o s e d of two half u n d u l a t o r s of 24 periods, for which the gap can b e i n d e p e n e n t l y adjusted, a n d t h e last half u n d u l a t o r may b e t a p p e r e d . However, presently all t h e e x p e r i m e n t s o n C L I O r e p o r t e d h e r e have b e e n d o n e with the same gap for b o t h half undulators, a n d w i t h o u t tapering. Fig. 2 shows a s p o n t a n e o u s s p e c t r u m with a 40 M e V elect r o n b e a m , a n d with a n u n d u l a t o r gap of 21 mm,
0168-9002/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
II. FIRST LASING
16
R. Prazeres et al. / Optical performance of the CLIO infrared FEL
e-
FEL mirror Brewster extraction pnate
beam dump
,
=.
,
5,111 ,
FEL mirror
,
,
I
~
z
x
~
Fig. 1. General layout of the CLiO infrared free electron laser.
Table 1 General characteristics of the linear accelerator Energy range Emittance 4v/3y~rc~' Energy spread Peak current Macropulse repetition rate Macropulse length Micropulse repetition rate Micropulse length
32-50 MeV 80,n- mmmrd 0.7% 40 A 6.25-50 Hz 10 ~,s 8, 16 or 32 ns 5-10 ps
representing a resonant wavelength, a R = 4.7 b~ml The experimental linewidth is A A / A = 3%, larger than the theoretical value A A / A = 2%. The laser cavity mirrors are fabricated with protected silver coatings on silicon substrates of 25 m m diameter, with a curvature radius of 3 m. The reflection coefficient is about 0.995 in the range 3 - 1 0 txm. The mirrors are separated by 4.8 m, and the Rayleigh length of the cavity is Z R = 1.2 m. The light is partially extracted from the cavity by a Z n S e or CaF 2 plate, which is set at 60 ° of incidence, near the Brewster angle. A mechanical device allows one to choose among
four different plates for the extraction. The Z n S e allows coverage of the range A = 0.6 to 18 I,m, with an extraction coefficient of 1.5%, representing losses of 6% for one cavity round trip. The CaF 2 allows coverage of the range A = 0.1 to 10 Izm, with an extraction coefficient of 0.2%, representing losses of 0.8%. A plate of KRS5 has been tested for extraction, but it did not permit lasing because of the poor optical quality of this material (large amount of optical scattering). The configuration of extraction with the Brewster plate creates four exit laser beams as shown in fig. 3. For each side of the plate, the two reflections of the b e a m are parallel and separated transversely by 2 mm (for ZnSe). The time distance between the two parallel laser pulses is about 50 ps (for ZnSe), which must be compared to the real laser pulse length of about 10 ps. Presently, only the double beam reflected by one side of the plate is sent to the diagnostics r o o m . The reflection on the other side is not used. After extraction, the laser beam passes through a telescope and is sent to the diagnostics r o o m through a 15 m pipe line. This line is extended to. the four user rooms. In order to avoid infrared absorption in the air (by H 2 0 , CO2, etc.) the whole infrared pipe line may be filled with dry air or a neutral gas such as nitrogen. The alignment is done with a H e N e laser, which is sent onto the Brewster plate inside the F E L cavity, and
4
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5
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Fig. 2. S p o n t a n e o u s e m i s s i o n s p e c t r u m w i t h a 40 M e V d e c t r o n b e a m , a n d w i t h a n u n d u l a t o r g a p o f 21 m m , r e p r e s e n t i n g a r e s o n a n c e w a v e l e n g t h X R = 4.7 ~xm.
FEL / / 2
mirrm
.... l a s e r h e a n/
Fig. 3. Configuration of extraction with the Brewster plate, showing the four exit laser beams.
R. Prazeres et al. / Optical performance of the CLIO infrared FEL Table 2 Summary of the laser characteristics measured for the beam extracted out of one of the faces of the extraction plate. The maximum performances are obtained with the ZnSe extraction plate, at A = 8 p~m, for ymc 2= 40 MeV and gap= 14 mm. The 3rd harmonic is obtained with the CaF 2 extraction plate. Lasing on the fundamental Total spectral range Maximum peak power Maximum average power Maximum net gain Spectral width 21A/A Wavelength stability (jitter) Pulse length
3-15 p~m 10 MW in 2 ps 65 mW at 6.25 Hz/32 ns 140% 0.4% 0.2% 2-10 ps
Lasing on the 3rd harmonic Resonance wavelength Laser wavelength Net gain
10 Ixm 3.3 pxm 10%
then follows the complete laser trajectory, through the infrared line, up to the experimental rooms. The alignment of the C L I O laser cavity is first roughly done with this H e N e laser, and is then optimised by looking at the laser signal. The cavity length, as well as the mirror tilts, are adjusted by stepping motors, monitored from the diagnostics r o o m by computer. This computer also allows one to control the undulator gap. W e can observe a small drift of the cavity length after a few hours, but this can be easily corrected from the diagnostics room.
4. Experimental results The first laser b e a m has been obtained with the CaF 2 extraction plate, for which the losses are lower than with the ZnSe, and allows easier lasing. Nevertheless, the results that are presented here have been obtained with the Z n S e plate, for which the higher extraction coefficient leads to extraction of a larger amount of laser energy. The general optical characteristics are summarised in table 2. The numerical values shown here are only for one side extraction of the Brewster plate; the reflection on the other side is lost. A spectral range, between 3 and 15 p.m, is presently covered using three electron energy values: 32, 40 and 50 MeV. Fig. 4 shows the laser power, measured in the diagnostics room, as a function of the laser wavelength, which is modified by changing the undulator gap. The time delay to modify the undulator gap is about a few seconds, and the delay n e e d e d to change the electron energy is about 20 min because of the cycling of the accelerator magnets. This makes a rather flexible tunability of the laser. The small residual amount of water
17
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6
8
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laser wavelength (microns) Fig. 4. Laser power, measured in the diagnostics room, as a function of the laser wavelength, for 32, 40 and 50 MeV electron energy.
on the optics and the CO 2 in the line create absorption for two wavelength domains near 6 txm and 15 ~m. The laser power and gain displayed in table 2 are for the wavelength A = 8 ~m, which has the best performance. The electron energy is 40 MeV, and the undulator gap is 14 mm. In this case, a peak power of 2.5 M W has been obtained for a 8 ps pulse length) and a net gain of 140% has been observed at 32 MeV, gap at 14 mm. The peak power is deduced from the average power measurement, by taking into account the time structure of the laser and the fact that two parallel laser beams are extracted from each side of the intracavity Brewster plate (see fig. 3). The gain has been deduced from the rise time (25 ns for doubling of the signal strength) of the laser pulse shape, which is shown in fig. 5. The detector used is of HgCdTe. The vertical scale of this figure has been chosen in order to show the laser saturation, after about 2 txs; it does not allow one to see the first part of the exponential growth of the signal, from which the laser gain is I
I
I
I
I
I
[
I
I
Fig. 5. Laser pulse shape, at A = 8 I-cm, and current macropulse. II. FIRST LASING
18
R. Prazeres et al. / Optical performance of the CLIO infrared FEL stable~ A~/L :0.4%
8.05
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- 6 0 - 5 0 -40 - 3 0 - 2 0 -10 0 1 cavity length variation (~tm)
macropuise current : 101as
Fig. 6. FEL cavity detuning curve at 3. = 8 txm. deduced. The current pulse is 10 p~s long, and the saturation duration is about 7 ixs, corresponding to 220 laser micropulses of 10 ps. The cavity losses of 8% are deduced from the fall time of the laser pulse, which is 400 ns. This is the expected value, corresponding to 6% from the Z n S e extraction plate and 1% for each mirror. The F E L cavity detuning curve is shown in fig. 6 for A = 8 txm and Tmc 2 = 40 MeV. The tuning range is about Az = 40 txm. W e have verified that for the cavity length corresponding to the maximum laser power, the laser wavelength is unstable and wide: A A / A = 1.5%. For a shorter cavity length, there is a rather flat range where the laser wavelength is stable, about 0.2% jitter, and the width is only AA/A = 0.4%. The stable tuning range width is about 20 txm. In this range, the laser spectrum appears as shown in fig. 7. Let us point out that this spectrum is the temporal integration along the laser macroputse. A real time contour plotting spectrum is shown in fig. 8, where the ordinates are wavelength (txm) and time (ixs). This figure shows that the central wavelength shifts along the macropulse, and reaches an equilibrium value for saturation. The transverse profile of the laser b e a m at 8 txm, taken in the diagnostics room, is shown in fig. 9. A 32 element area pyroelectric detector has been used here (as well as for the laser spectrum in fig. 7). The profile
I I I I I time (~ts) 5 10 Fig. 8. Real time contour plotting spectrum, near Z = 8/~m, as a function of the time during the macropulse. 7.85
I I [ I
is well fitted by a Gaussian curve for which the waist is w = 3.9 m m in the diagnostics room. Lasing on the third harmonic has been observed with y m c 2 = 4 0 M e V and fundamental resonance wavelength AR = 10 txm. The extraction plate was the CaF2, which is strongly absorbing at this wavelength, and does not allow lasing on the fundamental. Nevertheless, the laser can operate on the third harmonic at A = 3.3 ~xm. The laser pulse is shown in fig. 10; the saturation is just reached during the last part of the macropulse. The gain has been measured to about 10%, and the average measured power was less than 1 m W for a 6.25 Hz maeropulse repetition rate. A laser pulse length m e a s u r e m e n t has been done with a Michelson interferometer and an InSb nonlinear detector [5]. The autocorrelafion of the pulse in the interferometer produces interference fringes, which are related to the coherence length of the laser pulse. The nonlinear property of the InSb detector permits detection of the coincidence of the two pulses of the inter-
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transverse wavelength Fig. 7. Laser spectrum at A = 8 ixm.
at
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t
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Fig. 9. Transverse profile of laser beam at 8 ~m, taken in the diagnostics room:
R. Prazeres et aL / Opticalperformance of the CLIO infrared FEL I
i
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3rd i1)
laser
m
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harmonic
pulse
__~
q
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I
lOgs time (2~s/div) Fig. 10. Laser pulse when operating on third harmonic at A = 3.3 ~Lm,and current macropulse. ferometer. The integration of the fringes gives a signal which is proportional to the real length of the laser pulse, independently of its coherence length. The pulse length has b e e n found presently to be between 2 and 12 ps. A complete analysis of the pulse length as a function of the laser operating conditions is planned for the next F E L experiments.
19
doubled N d - Y A G laser (pump) has b e e n done successfully [6]. The mixing of the C L I O infrared wavelength (A = 5 Ixm) and the auxiliary laser (A = 0.532 ixm) is in the visible range, where the detectors are more sensitive than for infrared. The tunability of the F E L allows one to scan the molecular spectrum. The four rooms for the users are now available for laser experiments; and we have planned to dedicate a large amount of laser b e a m time to new application experiments. The rest of the time will be used for intrinsic F E L studies: for example improvement of the wavelength range (up to 20 Ixm) by decreasing the electron b e a m energy, or operating the F E L with undulator tapering. It will be also interesting to study of the effect on the laser of dephasing of the electron bunch on the accelerator. Operating the laser at a higher repetition rate of the time structure (50 H z / 4 ns) is also planned for the coming months. Laser extraction from the F E L cavity through a hole in the exit mirror is also planned for the near future. This will be useful for wavelengths larger than 15 p,m, for which the absorption in the Z n S e extraction plate forbids lasing.
References 5.
Conclusion
The C L I O infrared laser is already able to give a stable and tunable laser b e a m to users. The laser wavelength stability is about 0.2%, and the intensity stability is less than 10%. The time delay to modify the undulator gap is about a few seconds, and the delay n e e d e d to change the electron energy is about 20 min. This makes tunability of the laser rather flexible. A spectral range between 3 lxm and 15 ~ m is presently covered using three electron energy values: 32, 40 and 50 MeV. Application experiments have already started. A two photon absorption effect in InSb has been studied for different wavelengths, and this nonlinear property is used for laser pulse measurements [5]. Infrared microscopy, by pickup of an evanescent wave of the laser on a Z n S e medium, is now in progress. High energy laser pulses are necessary because of the p o o r efficiency of the energy pickup by the optical fiber. A sum frequency generation on molecular surfaces by combination o f the C L I O laser (test) and a
[1] S. Benson et al., presented at this Conference (14th Int. Free Electron Laser Conf., Kobe, Japan, 1992). P.W. van Amersfoort et al., Nucl. Instr. and Meth. A318 (1992) 42; C.A. Brau, Nucl. Instr. and Meth. A318 (1992) 38. [2] F. Glotin, J.M. Berset, R. Chaput, B. Kergosien, G. Humbert, D. Jaroszynski, J.M. Ortega, R. Prazeres, M. Velghe, J.C. Bourdon, M. Bernard, M. Deharnme, T. Garvey, M. Mencik, B. Mouton, M. Orneich, J. Roudier and P. R0udier, Proc. 3rd EPAC Conf., Berlin, 1992. [3] R. Chaput et al., these Proceedings (14th Int. Free Electron Laser Conf., Kobe, Japan, 1992) Nucl. Instr. and Meth. A331 (1993) 267. [4] M. Bergher, E. Jules and A. Louis-Joseph, IEEE Particle Accelerator Conf., San Francisco, 1991, vol. 2, p. 1225. [5] D. Jaroszynski et al., ref. [3], p. 640. [6] A. Peremans, P. Guyot-Sionnest, A. Tadjeddine, F. Glotin, J.M. Ortega and R. Prazeres, ref. [3], p. ABS 28. [7] A. Piednoir, F. Creuzet, C. Lieoppe and F. de Fornel, First specifications of the PSTM working in the infrared, Proc. NFO Conf., Besan~on, France, 1992, to be published.
II. FIRST LASING