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Compact ultra-high gain multi-pass Nd:YAG amplifier with a low passive reflection phase conjugate mirror
15 July 1999
Optics Communications 165 Ž1999. 237–244 www.elsevier.comrlocateroptcom
Compact ultra-high gain multi-pass Nd:YAG amplifier with a low passive reflection phase conjugate mirror Yitshak Tzuk ) , Yaakov Glick, Michael M. Tilleman Soreq NRC, Electro-Optics DiÕision, YaÕne 81800, Israel Received 21 December 1998; received in revised form 3 May 1999; accepted 4 May 1999
Abstract We show a compact multi-pass amplifier, based on a single dual-rod laser-head which produces ultra-high gain. A double-pass produced a maximum small signal gain of 4 = 10 8. Another double-pass was permitted by including a specially designed Brillouin phase conjugate mirror ŽPCM.. This enabled a total gain of 7.7 = 10 10 , which raised an input signal of 10 pJ to 770 mJ output signal. To the best of our knowledge this is the highest gain reported to date from any type of laser amplifier scheme. The amplification system is fairly simple in that it consists of only one dual-rod laser head and hence only a single power supply. We show that this system can be utilized for producing high-energy long temporally-smooth narrow linewidth pulses, as well as high power controllable, temporally-modulated pulses. q 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: High-gain; Multi-pass amplifier; Nd:YAG amplifier; PCM; Brillouin
1. Introduction Single pass gains of 100 from flashlamp pumped Nd:YAG amplifiers are routinely achieved w1x. Generally, single rod gain is limited mainly by oscillation caused by internal scattering due to defects or whispering modes. When going to multiple pass amplifiers or to serial amplifiers, these problems are usually avoided, thus higher gain can be achieved. In these cases, gain can be increased until either strong amplified spontaneous emission ŽASE. becomes sig-
)
Corresponding author. Fax: q972-8-9434401.
nificant or parasitic oscillation due to residuary reflection from optical components or diffused light scattered from defects or dust begins. Increasing the pumping beyond this point will result in depletion of the amplifying medium’s stored energy which will inhibit the increase in the gain. The major problem of reaching high gain in a multi-pass amplifier is parasitic lasing while the ASE is relatively weaker. A few works with multi-pass high-gain amplifiers have been conducted in the past, in a variety of gain mediums w1–5x. In these studies, in order to avoid potential round trip oscillations, the beams were set up so that each pass travelled through a different path in the amplifier. Thus a separation of each pass after it has exited from the amplifier medium is
0030-4018r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 Ž 9 9 . 0 0 2 3 0 - 8
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enabled, and there are no full round trips which can cause lasing. In Ref. w1x a zigzag geometry was employed in a Nd:YAG slab, and a gain of 10 6 was achieved by passing through the amplifier four times. In Ref. w2x a single Ti:sapphire amplifier was used with four concave mirrors which repeatedly returned the beam back into the amplifier. Each pass was slightly displaced compared with the previous one, and the sixth pass was clipped off. In that work a gain of 2 = 10 6 was achieved. A similar type of setup was used in Ref. w3x producing 10 4 gain in Nd:YAG and 10 5 in Nd:YVO4 . In Ref. w4x a four-pass and then an eight-pass amplifier were combined to get a gain of 10 8 with Ti:sapphire. In Ref. w5x a dye amplifier is combined with Ti:sapphire amplifiers and a gain of 3 = 10 7 was achieved. In that work a stimulated Brillouin scattering ŽSBS. cell is utilized to suppress the ASE of the Ti:sapphire amplifier in a manner similar to that used in this work. All of these setups are characterized by the need for tight and accurate alignment of narrow beams and a resulting system that is highly sensitive to misalignment and has a small field of view for amplification. In this work we show that a compact multi-pass amplifier w6x, based on a single dual-rod laser-head, can be configured so that a wide beam travels back and forth through the full clear aperture of the amplifier separated by polarization schemes. A double-pass through the two rods Ž‘four-pass’. gave a maximum small signal gain of 4 = 10 8 and was limited by parasitic lasing. Another double-pass Ž‘eight-pass’. was permitted by including a specially designed Brillouin phase conjugate mirror ŽPCM.. This enabled a total gain of 7.7 = 10 10 , which raised an input signal of 10 pJ to 770 mJ output signal. There is a commercial laser ŽInfinity made by Coherent. which uses a similar setup with a dual rod head and a Brillouin phase conjugate mirror, but that setup allows for only a double pass through the two rods and the overall gain in that system is ; 10 4 . To the best of our knowledge this is the highest gain reported to date from any type of laser amplifier scheme. In addition, here the high gain is accomplished without the necessity of complex tight geometrical configurations. Furthermore, the amplification system is fairly simple in that it consists of only one dual-rod laser head and hence only a single power supply.
2. Experimental A schematic of the system is shown in Fig. 1. The input signal enters the amplifying system through two Faraday isolators ŽFI.. The lr2 plate near the seeder was set so that with no voltage on the Pockels cell the seeder was blocked Žwith 30 dB extinction ratio.. It is then directed towards the dual-rod laser head with a plate polarizer. After passing through the two rods it is reflected back for a second pass through the two rods Ž‘4-pass’. by mirror M1. The polarization has been shifted by the two passes through the lr4, and the beam which has been amplified by four passes through a rod can now transmit through the plate polarizer towards the PCM. The PCM reflects the beam back for four more passes or a total of eight passes. In the four pass setup the PCM is blocked by M2. FI2 is to enable output coupling of the eight-pass amplified signal. FI1 is intended to increase the isolation of the seeder from the system and to prevent parasitic oscillation caused by optical surfaces of the seeder. FI3 is used to isolate the detection system from the amplifying system in the 4-pass case. A PCM cell has high reflection Ž) 90%. when coherent energy above its threshold Žtypically ; 1 mJ in Q-switched pulses. is incident on it, and low reflection when the incident energy is below the threshold. This characteristic can be utilized to suppress the parasitic lasing on the one hand while allowing the amplified signal to be reflected and to proceed through the system. By the time the beam has reached the PCM it has been amplified due to four passes through a rod. If the beam power is above the threshold of the PCM it will be reflected and retrace its path back to FI2 for a total of eight passes through a rod. If the power at the PCM is below the PCM threshold, the beam will not be reflected. The working point of the system is therefore determined by the requirement that the ASE due the four single passes be below the PCM threshold and not open the PCM parasitically, while the amplified input signal be above that threshold. PCMs have been used in the past in a similar manner to suppress noise from Brillouin amplifiers w7,8x. Care has to be employed with a system with gain this high, since even very weak reflections can cause parasitic oscillation and subsequent depletion of the
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Fig. 1. FL, flashlamp; FI, Faraday isolator; FR, Faraday rotator; M, mirror; Pl. Pol, plate polarizing beamsplitter; Pol, cube polarizing beamsplitter; Pockel, Pockels cell; PCM, Brillouin phase conjugate mirror; Tel, =3 expanding telescope. M2 is removed for the eight-pass case.
available energy. This applies to the PCM cell as well. Although the nonlinear reflection of the PCM cell is effectively zero below threshold, one has to consider the passive linear backscatter from the cell. This scatter can be a reflection from the AR coating of the focusing lens or diffuse scatter from the walls of the cell. If this scatter is above 10y7 into the field of view of the amplifier, an oscillation can begin due to the very high gain. This oscillation occurs between M1 and this passive scatter from the PCM cell and leakage through the plate polarizer ŽPl. Pol... A PCM cell was designed to have an extremely low backscatter of - 10y8 towards the system, thus ensuring that oscillation could not begin without an input signal. The dual-rod amplifier head and power supply are from a Continuum made laser Žmodel NY61. with a capacitor bank of 30 mF and a maximal voltage of 1800 V. The head has a working temperature of 110 F and is cooled by distilled water. The rods manufactured by Litton are 6 = 115 mm, have parallel grooves running along their barrel to prevent whispering modes, and their end surfaces are tilted by 28 to prevent parasitic oscillations.
The input signal is generated by a CW Nd:YAG single longitudinal mode ŽSLM. laser manufactured by Lightwave with a narrow linewidth of ; 10 kHz and a output power of 10 mW. The PCM consists of a 25 cm long stainless steel cell filled with CS 2 , a well-known liquid with a high Brillouin constant w9x, and a plano-convex lens with an 80 mm focal length Žin air. on its entrance face. The cell was designed and positioned with the lens off axis and tilted, in order to ensure that a minimal amount of reflection is returned towards the amplifier. The design took into account specular reflections from the two surfaces of the lens as well as diffuse reflections from the inner walls of the cell.
3. Results 3.1. Ultra-high gain Gain was measured as a function of the flashlamp energy with the CW laser used as a seeder. In these measurements Ž1-, 2- and 4-pass. no voltage was
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applied on the Pockels cell, but a lr4 plate was inserted after the Pockels cell to allow partial transmission of the seeder. The input power of the seeder after the two Faraday isolators was 3.9 mW, and the passive transmission of the system till the PCM was 77%. The PCM was blocked with a mirror ŽM2., and a Faraday isolator ŽFI3. was placed between the system and the detector in order to avoid parasitic oscillation with the photodetector, as shown in Fig. 1. The measured gain results are shown in Fig. 2, for a single pass through one of the YAG rods Ž1-pass., a single pass through the two rods Ž2-pass. and then for a double pass through the two rods Ž4-pass.. The maximum small signal gain of the first rod was 100, while the second rod produced 200. A maximum gain of 4 = 10 8 was achieved in the four-pass case. In the four-pass case, calibrated neutral density ŽND. filters were placed in front of the seeder in order to insure that the rods were not saturated. As can be seen by the linear behavior of these three curves Žon the logarithmic scale. in Fig. 2, we stayed within the small signal gain regime for these measurements. A further indication that the gain is in the small signal gain regime, is that the curve of the 4-pass result is almost exactly the square of the 2-pass curve. The output signal from these gain measurements was a smooth pulse ; 50–100 ms long Žnarrower for higher energy output., determined by the ; 150 ms long flashlamp pulse. In the case of 4-pass gain, we did not go above a flashlamp energy of 43 J since parasitic lasing began at that point. It is
Fig. 2. Gain as a function of the input pump energy on the flashlamps, for the cases of a single pass through the first rod Ž‘1-pass’., a single pass through the two rods Ž‘2-pass’., two passes through the two rods Ž‘4-pass’. and four passes through the two rods Ž‘8-pass’..
Fig. 3. Measured amplified spontaneous emission ŽASE. as a function of input pump energy on the flashlamps.
very simple to differentiate between ASE and parasitic lasing. The ASE grows monotonously with the increase in pump energy, and has a smooth temporal profile. On the other hand, parasitic lasing begins at a certain pump energy level and is characterized by fluctuating temporal spikes. At that point the spontaneous extraction energy Žwithout input signal. grows dramatically. In all the gain measurements we subtracted the ASE from the amplified signal in order to calculate the gain. The ASE was measured when the seeder was blocked. The energy in the ASE for the case of four passes, is shown in Fig. 3. This graph represents the measured ASE, multiplied by 2 to account for the ASE exiting in the opposite direction from the system Žoff of FI2., which was not measured, but is assumed to be approximately equal to the measured ASE. For flashlamp energy of above 40 J, we used ND filters with OD of 4.2 Žtransmissions 6 = 10y5 . in front of the seeder. This resulted in a signal to noise ratio ŽSNR. of 1 Ži.e., the output energy with an input signal is twice the output energy without it.. According to this result the initial spontaneous emission power can be estimated to be equal to 3.9 mW = 6 = 10y5 s 0.23 mW. The stored energy Est is calculated using the following equation w10x, based on the experimental measurements: Est f
hÕA ln Ž G .
s
Ž 3.
and is shown in Fig. 4. The stored energy of the second rod was calculated by measuring the gain of
Y. Tzuk et al.r Optics Communications 165 (1999) 237–244
Fig. 4. Stored energy as a function of the input pumping energy on the flashlamp. The two top curves represent the energy stored in the two rods as calculated from the results of 4 passes and from the result of two passes respectively. The bottom curve is the first rod and the curve above it is the energy in the second rod as calculated from the 2-pass gain.
the two rods together and subtracting the already known gain of the first rod. Unblocking the PCM, enables the amplified signal of four passes to be reflected and return along the same path, thus undergoing a total of eight passes and consequently depleting the amplifiers. This could occur only for the cases in which the energy reaching the PCM after four passes is above the threshold of the PCM for reflection. This threshold with CS 2 as the active Brillouin medium in the PCM is about 200 mJ in a 17 ns pulse w11x, or 12 kW of peak power. In order to assist in reaching this peak power it was necessary to block the CW radiation until the peak of the gain was reached. This was accomplished with the help of a fast rise time Ž; 20 ns. Pockels cell ŽKDU P. which was opened at the peak of the gain and closed at a later time Ž) 500 ns later.. Without this procedure, the CW radiation would extract the energy from the rods before the peak of the gain, in a long pulse, thus with a peak power lower than the PCM threshold. When the Pockels cell is opened there is a transmission of 33% through the first polarizer which yielded a 2 mW seeding power after the two Faraday isolators. Eight passes were only achievable for pumping energies above 33 J. At that point there was a four-pass gain of 10 7. The seeder power of 2 mW resulted in an output of 20 kW which is above the threshold for reflection from the PCM. On the other hand, going above a pumping energy of 36 J allowed parasitic lasing and subsequent opening of the PCM
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without an input signal, which resulted in depletion of the amplifiers. At the highest gain, eight passes with 36 J input energy, the measured output energy was 770 mJ. The pulsewidth was 5 ns and in this case there was a pulse to pulse jitter of about "10 ns. The pulse shortening is a result of the efficient energy extraction by the leading part of the pulse w12x. When the Pockels cell was opened, the input signal was 2 mW or 10 pJ in 5 ns. This represents a gain of 7.7 = 10 10 as shown in Fig. 2. At that point the stored energy is 1.6 J, so that 48% of the stored energy is transferred to the output beam. This was achieved at a pulse repetition rate of 2 Hz. The beam diameter was about 5 mm. With the seeder blocked Žno voltage on the Pockels cell. the output ASE energy was - 1 mJ. The parasitic oscillation which occurred for an input energy of above 36 J is apparently a result of oscillation between the mirror M1 and the passive linear backscatter from the PCM cell due to the thermally-induced birefringence in the laser rods. At rep-rates of above 2 Hz the threshold for parasitic oscillation was lower than 36 J, this prevented us from working at higher rep-rates. The beam quality after eight passes was between 5–8 times diffraction limited. Working with a higher input-power seeder would have resulted in a better beam quality, since the input power into the PCM would have been higher above the PCM threshold. It has been shown that just above the PCM threshold, the fidelity of the process is low w13x. With the present seeder the maximum pump input energy of 36 J, limited the input energy into the PCM to about 1–2 times its threshold. Another factor which limited the beam quality was the birefringence in the laser rods which was responsible for the fact that some of the energy did not reach the PCM but rather was reflected by the plate polarizer. As a result, some of the spatial information needed for reconstructing the beam which had been aberrated by the rods was not available to the PCM. In the above setup the seeder filled the YAG rods. The diameter of the seeder beam at 1re 2 of the peak intensity at the PCM Žmeasured without gain. was 4.8 mm. When the seeder beam was slightly focused with the expanding telescope, so that the beam diameter at the PCM was 3.2 mm, we obtained different
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results. The final energy was the same, but the pulsewidth was much longer: it had a fast rise time of - 5 ns with a long tail of ) 100 ns, and a FWHM of approximately 50 ns. This is apparently due to the fact that in this setup the seeder did not extract from the perimeter of the rod efficiently so the gain remains in that region for a long time. There was another difference when this narrower seeder beam was used; the jitter was considerably lower, "1 ns and the beam energy was steadier. We are uncertain about the reason for this effect. 3.2. Long temporal pulse In a different experiment, the energy was extracted from the rods in four passes Žthe PCM was blocked with M2.. As in the 1-, 2- and 4-pass cases, the input signal was 3.9 mW from the CW-seeder, which was open during the whole 150 ms long flashlamp pulse. The output amplified signal was a long ; 50–65 ms pulse with a smooth Gaussian-like shape. The energy measured in this pulse is shown in Fig. 5 as a function of the input pumping energy. The maximum energy extracted in this method was 210 mJ, for an pumping energy of 38 J. This is a unique result, since such long laser pulses are usually achieved in a free-running laser. The pulse of a free-running laser is characterized by many temporal spikes caused by the many oscillations in the laser, while here the 50 ms pulse is temporally smooth. Above this pumping energy, pre-lasing began without the input seeder. However, when the seeder was injected, it suppressed the pre-lasing. The bandwidth
Fig. 5. The energy extracted when the input signal was CW as a function of the input pump energy on the flashlamp.
of this pulse was not measured, however since it originated from the narrow bandwidth seeder laser it is assumed to have a narrow bandwidth. 3.3. Pulse shaping The high gain system was used for pulse shaping, where we produced intense temporally flat-top pulses with controllable pulse-widths. This was accomplished, by slicing a temporal pulse from the CW seeder by using the Pockels cell. The pulse which was synchronized to be near the gain peak was amplified by four passes through the system resulting in a flat-top pulse as shown in Fig. 6, with energies as high as 10 mJ. The pulse width was controllable in the range of 5 ns to 2 ms. This was achieved by operating the Pockels cell in the differential method which enabled opening and closing it with a fast rise time of 20 ns ŽLasermetrics driver Model GS8.. The KDU P Pockels cell caused a problematic side effect. When the pulsewidth is above 700 ns secondary pulses appeared after the closing of the Pockels cell, created by mechanical oscillations of the Pockels cell caused by the Piezo-electric effect, which produced an induced voltage on the Pockels cell. A BBO Pockels cell or any other type of modulator with a weaker Piezo-electric effect would reduce this problem. The pulse modulation is limited by the saturation of the gain caused by the depletion of the stored energy. Due to this, the pulse shape did not stay flat-top when more than about 10 mJ was extracted from the rods. When that occurred either due to high gain or to long pulsewidths, the temporal shape started out flat but after some time it started to decay. This occurred since at those extracting energies the gain is no longer in the small signal gain regime. The same principle of modulating a weak input signal and amplifying it can be applied to achieve any sort of desired pulse shape with powers of 10’s of kW, limited by saturation of the gain caused by the depletion of the stored energy. In general when one desires to amplify a modulated weak signal, the output amplified signal will follow the exact temporal profile of the input signal only up to saturation. Beyond that point there will be a deviation from the input profile. High energy long smooth pulses may find application in various fields where it is desired to deliver
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Fig. 6. Temporal flat-top pulse, with a controllable pulsewidth, measured with a gain of 2 = 10 6 . Ža. t p s 250 ns, Žb. t p s 650 ns, Žc. t p s 1670 ns.
high energy into a material while avoiding high intensity and the creation of plasma. For example, in medical or material processing applications. Long pulses are also desired for intra-cavity spectroscopy applications where it is desirable to achieve long propagation paths in order to enhance the sensitivity. An important example of this application, is intracavity OPO’s where its tunability makes it suitable for spectroscopy. The conventional method for achieving long pulses is with a free-running laser
which suffers from temporal spikes, while here the long pulse is temporally smooth.
4. Conclusion In conclusion we show a compact ultra-high gain multi-pass amplifier, with a maximum gain of 7.7 = 10 10 . This high gain system can also be used for forming high energy long temporally smooth pulses
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with narrow bandwidth. We also showed that these long pulses can be manipulated for temporal pulse shaping.
w6x
w7x
References w8x w1x T.J. Kane, W.J. Kozlovsky, R.L. Byer, 62-dB-gain multiplepass slab geometry Nd:YAG amplifier, Opt. Lett. 11 Ž1986. 216–218. w2x P. Georges, F. Estable, F. Salin, J.P. Poizat, P. Grangier, A. Brun, High-efficiency multipass Ti:sapphire amplifiers for a continuous-wave single-mode laser, Opt. Lett. 16 Ž1991. 144–146. w3x H. Plaessmann, S.A. Re, J.J. Alonis, D.L. Vecht, W.M. Grossman, Multipass diode-pumped solid-state optical amplifier, Opt. Lett. 18 Ž1993. 1420–1422. w4x C. Le Blanc, G. Grillon, J.P. Chambaret, A. Migus, A. Antonetti, Compact and efficient multipass Ti:sapphire system for femtosecond chirped-pulse amplification at the terawatt level, Opt. Lett. 18 Ž1993. 140–142. w5x C.-K. Ni, A.H. Kung, Amplified spontaneous emission re-
w9x w10x w11x
w12x w13x
duction by use of stimulated Brillouin scattering: 2-ns pulses from a Ti:Al 2 O 3 amplifier chain, Appl. Opt. 37 Ž1998. 530. Y. Tzuk, Y. Glick, M.M. Tilleman, A. Kaufman, Compact ultra-high gain multi-pass Nd:YAG amplifier with a low passive reflection phase conjugate mirror, Proc. SPIE 3263 Ž1998. 134–142. Y. Glick, S. Sternklar, Reducing the noise in Brillouin amplification by mode selective phase conjugation, Opt. Lett. 17 Ž1992. 862–864. Y. Glick, S. Sternklar, 10 10 amplification and phase conjugation with high efficiency achieved by overcoming noise limitations in Brillouin two-beam coupling, J. Opt. Soc. Am. B 12 Ž1995. 1074–1082. B.Y. Zeldovich, N.F. Pilipetsky, V.V. Shkunov, Principles of Phase Conjugation, Springer Verlag, Berlin, 1985. W. Koechner, Solid-State Laser Engineering, SpringerVerlag, Berlin, 1996. S. Jackel, R. Lallouz, A. Ludmirsky, Phase-conjugated multiple-pass amplifiers for low repetition rate, high energy, pulsed Nd:glass lasers, Proc. SPIE 1038 Ž1989. 521. M.M. Tilleman, Temporal distortion of a short laser pulse in an absorbing amplifier, J. Appl. Phys. 66 Ž1989. 3814. J.J. Ottusch, D.A. Rockwell, Stimulated Brillouin scattering phase-conjugation fidelity fluctuations, Opt. Lett. 16 Ž1991. 369.
Optics Communications 165 Ž1999. 237–244 www.elsevier.comrlocateroptcom
Compact ultra-high gain multi-pass Nd:YAG amplifier with a low passive reflection phase conjugate mirror Yitshak Tzuk ) , Yaakov Glick, Michael M. Tilleman Soreq NRC, Electro-Optics DiÕision, YaÕne 81800, Israel Received 21 December 1998; received in revised form 3 May 1999; accepted 4 May 1999
Abstract We show a compact multi-pass amplifier, based on a single dual-rod laser-head which produces ultra-high gain. A double-pass produced a maximum small signal gain of 4 = 10 8. Another double-pass was permitted by including a specially designed Brillouin phase conjugate mirror ŽPCM.. This enabled a total gain of 7.7 = 10 10 , which raised an input signal of 10 pJ to 770 mJ output signal. To the best of our knowledge this is the highest gain reported to date from any type of laser amplifier scheme. The amplification system is fairly simple in that it consists of only one dual-rod laser head and hence only a single power supply. We show that this system can be utilized for producing high-energy long temporally-smooth narrow linewidth pulses, as well as high power controllable, temporally-modulated pulses. q 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: High-gain; Multi-pass amplifier; Nd:YAG amplifier; PCM; Brillouin
1. Introduction Single pass gains of 100 from flashlamp pumped Nd:YAG amplifiers are routinely achieved w1x. Generally, single rod gain is limited mainly by oscillation caused by internal scattering due to defects or whispering modes. When going to multiple pass amplifiers or to serial amplifiers, these problems are usually avoided, thus higher gain can be achieved. In these cases, gain can be increased until either strong amplified spontaneous emission ŽASE. becomes sig-
)
Corresponding author. Fax: q972-8-9434401.
nificant or parasitic oscillation due to residuary reflection from optical components or diffused light scattered from defects or dust begins. Increasing the pumping beyond this point will result in depletion of the amplifying medium’s stored energy which will inhibit the increase in the gain. The major problem of reaching high gain in a multi-pass amplifier is parasitic lasing while the ASE is relatively weaker. A few works with multi-pass high-gain amplifiers have been conducted in the past, in a variety of gain mediums w1–5x. In these studies, in order to avoid potential round trip oscillations, the beams were set up so that each pass travelled through a different path in the amplifier. Thus a separation of each pass after it has exited from the amplifier medium is
0030-4018r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 Ž 9 9 . 0 0 2 3 0 - 8
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enabled, and there are no full round trips which can cause lasing. In Ref. w1x a zigzag geometry was employed in a Nd:YAG slab, and a gain of 10 6 was achieved by passing through the amplifier four times. In Ref. w2x a single Ti:sapphire amplifier was used with four concave mirrors which repeatedly returned the beam back into the amplifier. Each pass was slightly displaced compared with the previous one, and the sixth pass was clipped off. In that work a gain of 2 = 10 6 was achieved. A similar type of setup was used in Ref. w3x producing 10 4 gain in Nd:YAG and 10 5 in Nd:YVO4 . In Ref. w4x a four-pass and then an eight-pass amplifier were combined to get a gain of 10 8 with Ti:sapphire. In Ref. w5x a dye amplifier is combined with Ti:sapphire amplifiers and a gain of 3 = 10 7 was achieved. In that work a stimulated Brillouin scattering ŽSBS. cell is utilized to suppress the ASE of the Ti:sapphire amplifier in a manner similar to that used in this work. All of these setups are characterized by the need for tight and accurate alignment of narrow beams and a resulting system that is highly sensitive to misalignment and has a small field of view for amplification. In this work we show that a compact multi-pass amplifier w6x, based on a single dual-rod laser-head, can be configured so that a wide beam travels back and forth through the full clear aperture of the amplifier separated by polarization schemes. A double-pass through the two rods Ž‘four-pass’. gave a maximum small signal gain of 4 = 10 8 and was limited by parasitic lasing. Another double-pass Ž‘eight-pass’. was permitted by including a specially designed Brillouin phase conjugate mirror ŽPCM.. This enabled a total gain of 7.7 = 10 10 , which raised an input signal of 10 pJ to 770 mJ output signal. There is a commercial laser ŽInfinity made by Coherent. which uses a similar setup with a dual rod head and a Brillouin phase conjugate mirror, but that setup allows for only a double pass through the two rods and the overall gain in that system is ; 10 4 . To the best of our knowledge this is the highest gain reported to date from any type of laser amplifier scheme. In addition, here the high gain is accomplished without the necessity of complex tight geometrical configurations. Furthermore, the amplification system is fairly simple in that it consists of only one dual-rod laser head and hence only a single power supply.
2. Experimental A schematic of the system is shown in Fig. 1. The input signal enters the amplifying system through two Faraday isolators ŽFI.. The lr2 plate near the seeder was set so that with no voltage on the Pockels cell the seeder was blocked Žwith 30 dB extinction ratio.. It is then directed towards the dual-rod laser head with a plate polarizer. After passing through the two rods it is reflected back for a second pass through the two rods Ž‘4-pass’. by mirror M1. The polarization has been shifted by the two passes through the lr4, and the beam which has been amplified by four passes through a rod can now transmit through the plate polarizer towards the PCM. The PCM reflects the beam back for four more passes or a total of eight passes. In the four pass setup the PCM is blocked by M2. FI2 is to enable output coupling of the eight-pass amplified signal. FI1 is intended to increase the isolation of the seeder from the system and to prevent parasitic oscillation caused by optical surfaces of the seeder. FI3 is used to isolate the detection system from the amplifying system in the 4-pass case. A PCM cell has high reflection Ž) 90%. when coherent energy above its threshold Žtypically ; 1 mJ in Q-switched pulses. is incident on it, and low reflection when the incident energy is below the threshold. This characteristic can be utilized to suppress the parasitic lasing on the one hand while allowing the amplified signal to be reflected and to proceed through the system. By the time the beam has reached the PCM it has been amplified due to four passes through a rod. If the beam power is above the threshold of the PCM it will be reflected and retrace its path back to FI2 for a total of eight passes through a rod. If the power at the PCM is below the PCM threshold, the beam will not be reflected. The working point of the system is therefore determined by the requirement that the ASE due the four single passes be below the PCM threshold and not open the PCM parasitically, while the amplified input signal be above that threshold. PCMs have been used in the past in a similar manner to suppress noise from Brillouin amplifiers w7,8x. Care has to be employed with a system with gain this high, since even very weak reflections can cause parasitic oscillation and subsequent depletion of the
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Fig. 1. FL, flashlamp; FI, Faraday isolator; FR, Faraday rotator; M, mirror; Pl. Pol, plate polarizing beamsplitter; Pol, cube polarizing beamsplitter; Pockel, Pockels cell; PCM, Brillouin phase conjugate mirror; Tel, =3 expanding telescope. M2 is removed for the eight-pass case.
available energy. This applies to the PCM cell as well. Although the nonlinear reflection of the PCM cell is effectively zero below threshold, one has to consider the passive linear backscatter from the cell. This scatter can be a reflection from the AR coating of the focusing lens or diffuse scatter from the walls of the cell. If this scatter is above 10y7 into the field of view of the amplifier, an oscillation can begin due to the very high gain. This oscillation occurs between M1 and this passive scatter from the PCM cell and leakage through the plate polarizer ŽPl. Pol... A PCM cell was designed to have an extremely low backscatter of - 10y8 towards the system, thus ensuring that oscillation could not begin without an input signal. The dual-rod amplifier head and power supply are from a Continuum made laser Žmodel NY61. with a capacitor bank of 30 mF and a maximal voltage of 1800 V. The head has a working temperature of 110 F and is cooled by distilled water. The rods manufactured by Litton are 6 = 115 mm, have parallel grooves running along their barrel to prevent whispering modes, and their end surfaces are tilted by 28 to prevent parasitic oscillations.
The input signal is generated by a CW Nd:YAG single longitudinal mode ŽSLM. laser manufactured by Lightwave with a narrow linewidth of ; 10 kHz and a output power of 10 mW. The PCM consists of a 25 cm long stainless steel cell filled with CS 2 , a well-known liquid with a high Brillouin constant w9x, and a plano-convex lens with an 80 mm focal length Žin air. on its entrance face. The cell was designed and positioned with the lens off axis and tilted, in order to ensure that a minimal amount of reflection is returned towards the amplifier. The design took into account specular reflections from the two surfaces of the lens as well as diffuse reflections from the inner walls of the cell.
3. Results 3.1. Ultra-high gain Gain was measured as a function of the flashlamp energy with the CW laser used as a seeder. In these measurements Ž1-, 2- and 4-pass. no voltage was
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applied on the Pockels cell, but a lr4 plate was inserted after the Pockels cell to allow partial transmission of the seeder. The input power of the seeder after the two Faraday isolators was 3.9 mW, and the passive transmission of the system till the PCM was 77%. The PCM was blocked with a mirror ŽM2., and a Faraday isolator ŽFI3. was placed between the system and the detector in order to avoid parasitic oscillation with the photodetector, as shown in Fig. 1. The measured gain results are shown in Fig. 2, for a single pass through one of the YAG rods Ž1-pass., a single pass through the two rods Ž2-pass. and then for a double pass through the two rods Ž4-pass.. The maximum small signal gain of the first rod was 100, while the second rod produced 200. A maximum gain of 4 = 10 8 was achieved in the four-pass case. In the four-pass case, calibrated neutral density ŽND. filters were placed in front of the seeder in order to insure that the rods were not saturated. As can be seen by the linear behavior of these three curves Žon the logarithmic scale. in Fig. 2, we stayed within the small signal gain regime for these measurements. A further indication that the gain is in the small signal gain regime, is that the curve of the 4-pass result is almost exactly the square of the 2-pass curve. The output signal from these gain measurements was a smooth pulse ; 50–100 ms long Žnarrower for higher energy output., determined by the ; 150 ms long flashlamp pulse. In the case of 4-pass gain, we did not go above a flashlamp energy of 43 J since parasitic lasing began at that point. It is
Fig. 2. Gain as a function of the input pump energy on the flashlamps, for the cases of a single pass through the first rod Ž‘1-pass’., a single pass through the two rods Ž‘2-pass’., two passes through the two rods Ž‘4-pass’. and four passes through the two rods Ž‘8-pass’..
Fig. 3. Measured amplified spontaneous emission ŽASE. as a function of input pump energy on the flashlamps.
very simple to differentiate between ASE and parasitic lasing. The ASE grows monotonously with the increase in pump energy, and has a smooth temporal profile. On the other hand, parasitic lasing begins at a certain pump energy level and is characterized by fluctuating temporal spikes. At that point the spontaneous extraction energy Žwithout input signal. grows dramatically. In all the gain measurements we subtracted the ASE from the amplified signal in order to calculate the gain. The ASE was measured when the seeder was blocked. The energy in the ASE for the case of four passes, is shown in Fig. 3. This graph represents the measured ASE, multiplied by 2 to account for the ASE exiting in the opposite direction from the system Žoff of FI2., which was not measured, but is assumed to be approximately equal to the measured ASE. For flashlamp energy of above 40 J, we used ND filters with OD of 4.2 Žtransmissions 6 = 10y5 . in front of the seeder. This resulted in a signal to noise ratio ŽSNR. of 1 Ži.e., the output energy with an input signal is twice the output energy without it.. According to this result the initial spontaneous emission power can be estimated to be equal to 3.9 mW = 6 = 10y5 s 0.23 mW. The stored energy Est is calculated using the following equation w10x, based on the experimental measurements: Est f
hÕA ln Ž G .
s
Ž 3.
and is shown in Fig. 4. The stored energy of the second rod was calculated by measuring the gain of
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Fig. 4. Stored energy as a function of the input pumping energy on the flashlamp. The two top curves represent the energy stored in the two rods as calculated from the results of 4 passes and from the result of two passes respectively. The bottom curve is the first rod and the curve above it is the energy in the second rod as calculated from the 2-pass gain.
the two rods together and subtracting the already known gain of the first rod. Unblocking the PCM, enables the amplified signal of four passes to be reflected and return along the same path, thus undergoing a total of eight passes and consequently depleting the amplifiers. This could occur only for the cases in which the energy reaching the PCM after four passes is above the threshold of the PCM for reflection. This threshold with CS 2 as the active Brillouin medium in the PCM is about 200 mJ in a 17 ns pulse w11x, or 12 kW of peak power. In order to assist in reaching this peak power it was necessary to block the CW radiation until the peak of the gain was reached. This was accomplished with the help of a fast rise time Ž; 20 ns. Pockels cell ŽKDU P. which was opened at the peak of the gain and closed at a later time Ž) 500 ns later.. Without this procedure, the CW radiation would extract the energy from the rods before the peak of the gain, in a long pulse, thus with a peak power lower than the PCM threshold. When the Pockels cell is opened there is a transmission of 33% through the first polarizer which yielded a 2 mW seeding power after the two Faraday isolators. Eight passes were only achievable for pumping energies above 33 J. At that point there was a four-pass gain of 10 7. The seeder power of 2 mW resulted in an output of 20 kW which is above the threshold for reflection from the PCM. On the other hand, going above a pumping energy of 36 J allowed parasitic lasing and subsequent opening of the PCM
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without an input signal, which resulted in depletion of the amplifiers. At the highest gain, eight passes with 36 J input energy, the measured output energy was 770 mJ. The pulsewidth was 5 ns and in this case there was a pulse to pulse jitter of about "10 ns. The pulse shortening is a result of the efficient energy extraction by the leading part of the pulse w12x. When the Pockels cell was opened, the input signal was 2 mW or 10 pJ in 5 ns. This represents a gain of 7.7 = 10 10 as shown in Fig. 2. At that point the stored energy is 1.6 J, so that 48% of the stored energy is transferred to the output beam. This was achieved at a pulse repetition rate of 2 Hz. The beam diameter was about 5 mm. With the seeder blocked Žno voltage on the Pockels cell. the output ASE energy was - 1 mJ. The parasitic oscillation which occurred for an input energy of above 36 J is apparently a result of oscillation between the mirror M1 and the passive linear backscatter from the PCM cell due to the thermally-induced birefringence in the laser rods. At rep-rates of above 2 Hz the threshold for parasitic oscillation was lower than 36 J, this prevented us from working at higher rep-rates. The beam quality after eight passes was between 5–8 times diffraction limited. Working with a higher input-power seeder would have resulted in a better beam quality, since the input power into the PCM would have been higher above the PCM threshold. It has been shown that just above the PCM threshold, the fidelity of the process is low w13x. With the present seeder the maximum pump input energy of 36 J, limited the input energy into the PCM to about 1–2 times its threshold. Another factor which limited the beam quality was the birefringence in the laser rods which was responsible for the fact that some of the energy did not reach the PCM but rather was reflected by the plate polarizer. As a result, some of the spatial information needed for reconstructing the beam which had been aberrated by the rods was not available to the PCM. In the above setup the seeder filled the YAG rods. The diameter of the seeder beam at 1re 2 of the peak intensity at the PCM Žmeasured without gain. was 4.8 mm. When the seeder beam was slightly focused with the expanding telescope, so that the beam diameter at the PCM was 3.2 mm, we obtained different
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results. The final energy was the same, but the pulsewidth was much longer: it had a fast rise time of - 5 ns with a long tail of ) 100 ns, and a FWHM of approximately 50 ns. This is apparently due to the fact that in this setup the seeder did not extract from the perimeter of the rod efficiently so the gain remains in that region for a long time. There was another difference when this narrower seeder beam was used; the jitter was considerably lower, "1 ns and the beam energy was steadier. We are uncertain about the reason for this effect. 3.2. Long temporal pulse In a different experiment, the energy was extracted from the rods in four passes Žthe PCM was blocked with M2.. As in the 1-, 2- and 4-pass cases, the input signal was 3.9 mW from the CW-seeder, which was open during the whole 150 ms long flashlamp pulse. The output amplified signal was a long ; 50–65 ms pulse with a smooth Gaussian-like shape. The energy measured in this pulse is shown in Fig. 5 as a function of the input pumping energy. The maximum energy extracted in this method was 210 mJ, for an pumping energy of 38 J. This is a unique result, since such long laser pulses are usually achieved in a free-running laser. The pulse of a free-running laser is characterized by many temporal spikes caused by the many oscillations in the laser, while here the 50 ms pulse is temporally smooth. Above this pumping energy, pre-lasing began without the input seeder. However, when the seeder was injected, it suppressed the pre-lasing. The bandwidth
Fig. 5. The energy extracted when the input signal was CW as a function of the input pump energy on the flashlamp.
of this pulse was not measured, however since it originated from the narrow bandwidth seeder laser it is assumed to have a narrow bandwidth. 3.3. Pulse shaping The high gain system was used for pulse shaping, where we produced intense temporally flat-top pulses with controllable pulse-widths. This was accomplished, by slicing a temporal pulse from the CW seeder by using the Pockels cell. The pulse which was synchronized to be near the gain peak was amplified by four passes through the system resulting in a flat-top pulse as shown in Fig. 6, with energies as high as 10 mJ. The pulse width was controllable in the range of 5 ns to 2 ms. This was achieved by operating the Pockels cell in the differential method which enabled opening and closing it with a fast rise time of 20 ns ŽLasermetrics driver Model GS8.. The KDU P Pockels cell caused a problematic side effect. When the pulsewidth is above 700 ns secondary pulses appeared after the closing of the Pockels cell, created by mechanical oscillations of the Pockels cell caused by the Piezo-electric effect, which produced an induced voltage on the Pockels cell. A BBO Pockels cell or any other type of modulator with a weaker Piezo-electric effect would reduce this problem. The pulse modulation is limited by the saturation of the gain caused by the depletion of the stored energy. Due to this, the pulse shape did not stay flat-top when more than about 10 mJ was extracted from the rods. When that occurred either due to high gain or to long pulsewidths, the temporal shape started out flat but after some time it started to decay. This occurred since at those extracting energies the gain is no longer in the small signal gain regime. The same principle of modulating a weak input signal and amplifying it can be applied to achieve any sort of desired pulse shape with powers of 10’s of kW, limited by saturation of the gain caused by the depletion of the stored energy. In general when one desires to amplify a modulated weak signal, the output amplified signal will follow the exact temporal profile of the input signal only up to saturation. Beyond that point there will be a deviation from the input profile. High energy long smooth pulses may find application in various fields where it is desired to deliver
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Fig. 6. Temporal flat-top pulse, with a controllable pulsewidth, measured with a gain of 2 = 10 6 . Ža. t p s 250 ns, Žb. t p s 650 ns, Žc. t p s 1670 ns.
high energy into a material while avoiding high intensity and the creation of plasma. For example, in medical or material processing applications. Long pulses are also desired for intra-cavity spectroscopy applications where it is desirable to achieve long propagation paths in order to enhance the sensitivity. An important example of this application, is intracavity OPO’s where its tunability makes it suitable for spectroscopy. The conventional method for achieving long pulses is with a free-running laser
which suffers from temporal spikes, while here the long pulse is temporally smooth.
4. Conclusion In conclusion we show a compact ultra-high gain multi-pass amplifier, with a maximum gain of 7.7 = 10 10 . This high gain system can also be used for forming high energy long temporally smooth pulses
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with narrow bandwidth. We also showed that these long pulses can be manipulated for temporal pulse shaping.
w6x
w7x
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