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RETRACTED — Studies on the effect of instability of divergence, pointing and amplitude of green and yellow radiation pulses of copper vapour laser in second harmonic and sum frequency conversion
Optics & Laser Technology 45 (2013) 428–434
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Studies on the effect of instability of divergence, pointing and amplitude of green and yellow radiation pulses of copper vapour laser in second harmonic and sum frequency conversion Om Prakash n, Ramakanta Mahakud, Shankar V. Nakhe, Sudhir K. Dixit
R&D C-3 Block, Laser System Engineering Section, Raja Ramanna Centre for Advanced Technology, Indore 452013 (M.P.), India
a b s t r a c t
Article history: Received 25 April 2012 Received in revised form 11 June 2012 Accepted 12 June 2012 Available online 30 June 2012
This paper presents the effect of single pulse stability of divergence angle, beam pointing angle and amplitude of green and yellow radiation pulses of an unstable resonator copper vapour laser (CVL) oscillator in the sum frequency mixing and second harmonic. The conversion efficiency of sum frequency generation was lower compared to second harmonic processes despite larger fundamental power being used in sum frequency experiments. However the net UV power obtained at the sum frequency was higher than both of the second harmonic UV frequencies. Lower SFG conversion efficiency (12.4%—271 nm) compared to SHG (16.7%—255 nm, 14.5%—289 nm) of individual CVL radiations is attributed to difference in single pulse stability of beam pointing, divergence and amplitude fluctuation of both CVL radiations in addition to commonly known fact of spatio-temporal mis-match. At the same fundamental input power (2.7 W), higher SH conversion efficiency of yellow (12.7%) compared to green (11.0%) is attributed to its better single pulse stability of beam pointing and divergence. & 2012 Elsevier Ltd. All rights reserved.
1. Introduction
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Keywords: Sum frequency mixing Second harmonic of green and yellow radiation Divergence and beam pointing stability
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a r t i c l e i n f o
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The high repetition rate, narrow line-width, high average power UV radiations (255 nm, 271 nm, 289 nm) generated from the second harmonic [1–3] or the sum frequency mixing [2,4] of CVL visible radiations (510 nm, 578 nm) are the potential sources for high speed manufacturing of photonics components such as Fiber Bragg Grating (FBG) [5], precision micro-machining, cutting and etching of semiconductors [6] and for optical pumping of various cerium-doped laser [7] to obtain tunable UV radiations (280–315 nm). The CVL based UV radiations (255 nm, 271 nm, 289 nm) are also the potential sources for high speed all-optical switching based on fiber Bragg grating (FBG) [8–10]. These optical switches are key components in optical communications and in optical bistability systems. In reported literature, the CVL to UV frequency conversion efficiencies were compared for all the three UV wavelengths [1–4]. The relative sum and second harmonic efficiencies depended on the green and yellow power ratio, their temporal mismatch, details of focusing geometry and the choice of non-linear crystal. In particular, in a spherical focusing geometry with BBO crystal, the UV conversion efficiencies were 9.6%, 5.5% and 6.4% for the second harmonic generation (SHG) of green,
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Corresponding author. Tel.: þ91 731 2442472; fax: þ91 731 2442400. E-mail address: [email protected] (O. Prakash).
0030-3992/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2012.06.013
sum frequency generation (SFG) of green and yellow and SHG of yellow at the average power of 4.8 W, 8.4 W and 3.6 W, respectively [11]. The UV output power level scaled to more than 1.0 W from a single CVL oscillator using the cylindrical focusing geometry in BBO, as it is a good candidate for high power operation in view of its high damage threshold and large acceptance angle along the azimuthal direction [2]. The conversion efficiencies of 34%, 19% and 17% were obtained at the SH of green, SF of green and yellow and SH of amplified yellow, respectively [2]. Recently in the BBO crystal, the conversion efficiency of 28%, 23%, 17.5% at the 255 nm, 271 nm and 289 nm, respectively, are reported from a single CVL system using tunable acoustic optical filter [12]. In this set-up, the different CVL radiations and amplitudes are controlled selectively using electronically tunable acousto-optical filter. Although higher conversion efficiency of about 50% has been reported for picosecond and femtosecond pulses using BiB3O6 (BIBO) crystal in the blue region due to high damage threshold and high nonlinear coefficient of BIBO crystal as compared to BBO crystal. But the minimum transmission wavelength of BIBO crystal is limited to 280 nm [13–14]. However this crystal is not suitable for SFG and SHG of green CVL radiations. From the reported literature, it is seen that the SFG conversion efficiency was lower as compared to that of second harmonic (SH) of green CVL radiation [15]. It is also observed that from a single CVL oscillator, the conversion efficiency of SH of yellow is lower than the SH of green [2,12]. However, the average power of green
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micro-positioner and was suitably tilted/aligned for SH or SFM of 510 nm and/or 578 nm radiations. The exact phase matching cut angle for normal incidence is, 461 for SFM of 510 and 578 nm, 50.61 for SHG of 510 nm and 421 for SHG of 578 nm. The fundamental and SF/SH beams were re-collimated by a fused silica cylindrical lens L4 and separated by fused silica (Supracil) prism, P. Average visible/UV powers was measured by a power meter (Gentech, PS-310 WB). The green and yellow pulses for 1 s time duration were recorded by a digital storage oscilloscope (Tektronix, 540 TDS) in persistence mode using two bi-planar photo-diodes of sub-nano-second rise time (ITL, S-20). The divergence and pointing stability of the both green and yellow CVL radiation pulses were estimated by recording the spatial shift in the far-field intensity distribution about the mean position at the focal plane of a lens L1 of focal length 100 cm. A magnified image about 4-times of the far-field intensity distribution was placed onto the CCD camera (Pixelfly qe, PCO AG) by an additional imaging lens of focal length 100 cm. Suitable neutral density filters of known transmission are used to attenuate the intensity to prevent the saturation of CCD camera. This event is captured and analysed by specially developed beam analysis software [16]. Software records the events in subsequent time in the form of spatially stacked picture. An orthogonal pair of cursors is centred on this spot. This provides the reference position to save line profile of the subsequently acquired images in a dynamically allocated memory of the PC with the help of a PCI bus based FG card. For present set-up, the CCD acquiring time was set to 174 ms, slightly less than the pulse separation of 181 ms at 5.5 kHz laser repetition rate. This ensured the single pulse recording. The subsequent images were acquired after time interval of about 1 s at 1 Hz rate.
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and yellow were not the same and the conversion efficiency for both the radiations is not compared at the same input power. Also a little attention is paid on understanding the reason of the low conversion efficiency in SFG as compared to SHG of CVL radiations except the some commonly known facts of spatio-temporal mismatch between the green and yellow radiations and green/ yellow average power ratio. It is well known that the non-linear crystals have limited angular acceptance. Hence any pulse to pulse variation of CVL beam divergence, pointing that exceeds the acceptance angle of crystal will affect the frequency conversion process by deteriorates the phase matching conditions. In a study on beam pointing stability on CVL, it was demonstrated that the beam pointing depends on the resonator characteristics [16]. Therefore, it is expected that the single pulse divergence angle and pointing angle of green and yellow radiation will be different. It is worth investigating the role of CVL beam divergence and pointing angle of green and yellow in non-linear frequency conversion. Another issue that affects the UV conversion efficiency is the single pulse fluctuation in the amplitude of green and yellow radiation. So far in CVL, inadequate attention is paid on the role of stability of divergence, beam pointing angle and the amplitude fluctuations in sum frequency conversion of CVL radiations. This paper presents the studies on the effect of single pulse stability of beam pointing angle, divergence angle and the amplitude of CVL radiations on the SFG of green and yellow. Lower SFM conversion efficiency compared to SH of either CVL radiations is explained in terms of difference in green and yellow beam pointing stability, divergence angles and amplitude fluctuations in addition to power ratio of green and yellow, temporal mismatch between both the green and yellow radiations. Higher SH conversion efficiency of yellow compared to the green at the same input power is addressed in relevance of single pulse stability of beam pointing angle and divergence angle.
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3. Results and discussion
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2. Experimental set-up
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The CVL used in this experiment [a homemade device of 18 W (5.5 kHz), 28 mm beam diameter, 150 cm tube length] employed a confocal positive branch unstable resonator (PBUR) of magnification M ¼100. The resonator consisted of a concave mirror M1 (F1 ¼250 cm), a convex mirror M2 (F2 ¼ 20 cm), an intracavity cube polarizer Z1 placed closed to M1, and a scraper mirror, SC, to take the beam out (Fig. 1). The resonator length, L, was kept equal to F1–F2 i.e. 242.5 cm to satisfy confocal conditions. The cube polarizer, Z1, polarizes both the radiations used in SFM. The scraper mirror out coupled CVL beam was telescopically compressed from 28 mm to 2.8 mm by using a combination of two achromatic lenses L1 and L2 (f1 ¼100 cm and f2 ¼10 cm). The amplified spontaneous emission (ASE) was removed by placing an aperture, A1, at the common focal plane. The collimated beam was line focused by a cylindrical lens, L3, in a 6 4 7 mm3 BBO crystal (M/s Casix) cut at 511. The crystal was mounted on a 5-axis
Fig. 1. Experimental set-up for sum frequency generation.
Fig. 2a and b shows the typical composite pictures of the farfield intensity distribution of the green and yellow radiation of URCVL of magnification 100, for the 500 pulses. The zigzag shape of the stacked picture is an indicative of position jitter of the farfield pattern. A careful look at Fig. 2 shows that the positional jitter of the far-field intensity distribution for the green is higher compared to yellow. Fig. 2 shows the composite data for the horizontal stack. The vertical stack was similar to horizontal stack for both the green and the yellow beams but not shown here. Fig. 3 shows the far-field intensity distribution of green and yellow radiation, respectively, recorded for 500 pulses. This reveals the stability in the position of the far-field intensity distribution and the variation in the magnitude of the intensity. It is derived from Fig. 2. The divergence angle for the green and yellow beams was estimated from Fig. 3 as follows:
y ¼ Do=d
ð1Þ
where d is the distance from the plane of minimum laser spot size from the lens L1, and Do is the width of far-field intensity distribution at the 1/e2 point of the maximum intensity. The estimated the average divergence angle of the green and yellow radiations were 122 mrad and 108 mrad, respectively. From Fig. 3, we see that there is significant spread in the single pulse far-field intensity distribution over 500 pulses. Variation in the single pulse divergence angle for 500 pulses were estimated from Fig. 2 and shown in Fig. 4. The single pulse divergence angle varies between 110 mrad and 140 mrad for the green beam. While for yellow beam, it was in between 95 mrad and 115 mrad. The variation in single pulse divergence angle for green beam was higher as compared to the yellow. This can be explained as follows. The CVL is a super-radiant laser with significant amplified
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spontaneous emission (ASE) right from the moment the gain is switched on. This optical noise is highly random in phase and amplitude distribution across the beam. Due to highly random nature, the starting optical noise is also different from pulse to pulse. In an unstable resonator, these seed radiations are magnified by the magnification factor M in the each round trip. Hence the divergence is reduced by a factor M in successive roundtrips which ultimately get clamped to diffraction limited value. The average divergence value achieved depends on the gain of individual CVL radiations and the type of optical resonator. In CVL, it is well known that the gain of green radiation is higher than the yellow radiation [17]. Therefore for the given resonator, the net divergence value and its fluctuations are controlled by the gain of radiation. Hence the divergence value and its fluctuation are higher for the green beam as compared to yellow beam (Fig. 4). The pointing instability of the green and yellow radiation were also estimated from Fig. 3 as
d ¼ Ds=d
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where Ds is the maximum wandering of the peak of the far-field intensity distribution from the mean position of the peak. The zero position is designated where the beam hits maximum number of times. Here the mean position of the peak is calculated from the mode of the peak position data over 500 pulses. The beam pointing stability of different pulses for green and yellow radiations is shown in Fig. 5. The pointing instability for the green and yellow radiation was 15 mrad and 10 mrad, respectively. The pointing stability of the yellow radiation was better than the green radiation. The behaviour can be explained as follows. In copper vapour laser, the beam pointing instability arises due to multiple factors such as random fluctuations in laser beam axis, resonator mode evolution, mechanical vibrations of mirror mounts and supporting structure, random refractive index fluctuation of the highly heated pulsed CVL plasma, varying temperature gradient in discharge sealing optical windows, ambient temperature fluctuations, air currents, etc. Since CVL is a high gain laser and mode build up from the noise with random phase. The extent to which the random phase distribution of the noise pulses affect the single pulse laser pointing stability, is closely linked to the fact as to how close is a single starting noise pulse in reaching the steady state in the resonator. The steady state depends on the gain of individual CVL radiations. The pointing stability of yellow radiation is better than the green most probably due to lower gain of yellow radiation. Fig. 6a and b shows the variations of UV power and conversion efficiency vs fundamental input power. The conversion efficiency for SFG is calculated as ZSF ¼PSFG/(Pg þPy) 100%, where Pg, Py and PSFG are average power of fundamental green, yellow and UV sum frequency, respectively. It is seen that the UV power increases as fundamental input power of both the beam increases. The maximum sum UV (271 nm) power of 1.21 W is observed with the conversion efficiency of 12.4%. As the fundamental power increases the conversion efficiency increases steeply and reaches maximum and then saturates. This saturation may be due to thermal de-phasing as observed in high power second harmonic generation of CVL [18,19]. The UV power and conversion efficiency at the second harmonic of green (255 nm, solid line) and yellow (289 nm, dash-dot line) were also measured by orienting the BBO crystal suitable phase matching angles for green and yellow radiations (Fig. 6b). The second harmonic conversion efficiency is estimated at ZSH ¼PSH/Pg/y 100%, where PSH is the second harmonic UV power and Pg/y is the average power of green or yellow. The maximum UV power of 940 mW at 255 nm is obtained with the conversion efficiency of 16.7%. About 608 mW UV power at 289 nm with conversion efficiency of 14.5% is
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Fig. 2. Composite pictures of the far-field intensity distribution of URCVL of magnification 100 for CVL beam: (a) green and (b) yellow.
Fig. 3. Variation of far-field intensity distribution recorded for 500 pulses for CVL beam: (a) green and (b) yellow.
ð2Þ
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green
Divergence (µrad)
150 140 130 120 110 100 90 100
200
300 Nos. of pulses
150 Divergence (µrad)
400
500
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0
140
yellow
130 120 110 100 90 100
200
300 Nos. of pulses
400
500
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0
Fig. 4. Variation of single pulse divergence of different pulses for green and yellow CVL beam.
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15
5 0
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Instability (µrad)
10
-5
green
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-15
0
100
200
300 Nos. of pulses
400
500
15
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Instability (µrad)
10 5 0
-5 -10
yellow
-15 0
100
200
300
400
500
Nos. of pulses Fig. 5. Beam pointing stability of different pulses for green and yellow CVL beam.
observed from second harmonic generation of yellow beam. From Fig. 6b it is clear that at the same fundamental input power, the UV power and conversion efficiencies are higher for yellow beam compared to green beam. At the fundamental input power of 2.7 W, conversion efficiency for the green and yellow beam were 11.0% and 12.7%, respectively. Table 1 summarizes the UV and
fundamental beam power at different power level, average and peak power of fundamental beam, pulse duration, divergence angle for the green and yellow wavelengths and the conversion efficiency. The average power is the average of pulses measured for 1 s time duration. Hence we recorded the CVL and the corresponding
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UV pulses for the 1 s time duration by operating oscilloscope in persistence mode. Fig. 7a–c shows the pulse shapes of the CVL (upper trace) and UV radiation (lower trace) corresponding to total beam (greenþyellow), green and yellow, respectively. The fluctuation in the amplitude for the total beam at 10 ns and 25 ns from the start were about 16% and 40%, respectively. This time slot is chosen because the efficient frequency conversion starts after 10 ns from the start of fundamental pulse. In the fluctuation estimation, we have ignored the odd points. The fluctuation in UV beam amplitude (271 nm) was about 22% and 50% at the same
2.0
12
1.8
11
1.6
10 9
1.2
Efficiency (%)
UV power (W)
1.4
8
1.0 7
0.8
6
0.6 0.4
5
0.2
4
0.0 1
2
3 4 5 6 7 8 9 Fundamental G+Y power (W)
10
11
3 12
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0
time. The amplitude variation for the green wavelength at the 10 ns and 25 ns from the start were about 20% and 45%, respectively. The corresponding fluctuations in the UV beam (255 nm) were 30% and 58%, respectively. The amplitude variations for the yellow component at the same time from the start of the pulse were about 24% and 50% and corresponding the UV beam (289 nm) were 30% and 60%, respectively. From Fig. 7b and c, it is noticed that the fluctuation in the yellow beam is higher than the green beam. This may be due to the fact that for the radiation circulating inside the optical resonator, the green beam saturates early as compared to yellow due to higher gain. This results into lower amplitude fluctuations for green component as compared to yellow. The higher amplitude fluctuations were observed in the UV beam as compared to fundamental beam for all the cases due to non-linear nature of frequency conversion. From Table 1, it is clear that the net UV power (1210 mW) at the sum frequency was higher than separately at both the second harmonic UV frequencies. However, the conversion efficiency of SFM (12.4%) was lower than the second harmonic processes (16.7% and 14.5%) despite larger fundamental power (9.8 W) being used in sum frequency experiments. In an ideal case of sum frequency mixing of two different wavelength laser pulses in a non-linear crystal, both the radiations pulses must exactly match in space, time and energy domain. This includes intensity distribution, divergence angle, spot size in the crystal and pulse shape/pulse width. Any mismatch will lead to reduced SF conversion efficiencies, ZSFG. Under assumption of constant pumping fields with constant spatial and temporal distribution of fundamental pulses and also under small depletion regime, ZSFG is given by [20]
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1.0
17 16
Efficiency (%)
UV power (W)
14 13
g
12
0.6
11
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y
9
0.4
8 7
0.2
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g 2.5
3.0
3.5 4.0 4.5 5.0 Fundamental G/Y power (W)
5.5
6
5 6.0
Fig. 6. Variations of UV power and conversion efficiency vs fundamental input power: (a) sum frequency (efficiency-dotted line; UV power-solid line) and (b) green (solid line) and yellow (dash-dot line).
2
2:97 104 def f L2 /U 1 S/U 2 S sinc2 ðDkL=2Þ /U 3 S ¼g 2 /U 1 þ U 2 S no no ne l A/U 1 þU 2 S
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y
0.8
ZSFG ¼
15
1 2 3 3
ð3Þ
where /U1S, /U2S and /U3S are the peak powers of 510 nm, 578 nm and 271 nm pulses, respectively, n01 and n02 are refractive index ordinary beams for 510 nm and 578 nm, ne3 is the refractive index of the extraordinary beam at 271 nm, deff and L are the effective non-linear coefficient and length of the crystal, A[Qfhdy ] is the area of the pump beam, f is the focal length of the cylindrical lens L3, h is the width of line focus, dy is the pump beam divergence, Dk is the phase mismatch. The average power is related with the peak power as /U i S ¼ P oi =ðr ti Þ for i ¼ 1,2,3 where r is the repetition rate, t is the pulse width. Constant g is a representative of mismatch between green and yellow components of CVL radiations in space as well as in time. For ideal case of perfect matching of radiations, g ¼1, otherwise g o1.
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Table 1 Fundamental and UV beam parameters. Fundamental beam
UV beam
Wavelength (nm)
Average Pulse Divergence power, Pav (W) width (ns) Dy (mrad)
Peak power, Ppk (kW)
Factor Ppk/Dy (kW/mrad)
Amplitude fluctuations (%)
Wave UV power length (nm) (mW)
Efficiency, Z (%)
Amplitude fluctuations (%)
510
5.6 3.3 2.7
32
122
31.3 17.7 15.1
0.257 0.146 0.124
t10,25a ¼ 20,45
255
940 363 297
16.7 11.4 11.0
t10,25 ¼30,58
578
4.2 2.7
31
108
24.2 15.5
0.224 0.143
t10,25 ¼ 24,50
289
608 341
14.5 12.7
t10,25 ¼30,60
510 þ 578
9.8 (5.6 g þ4.2 y) 6.0 (3.3 g þ 2.7 y)
33
–
53.0
–
t10,25 ¼ 16,40
271
1210
12.4
t10,25 ¼22,50
671
11.1
a
32.5
The amplitude fluctuations are measured at 10 ns and 25 ns from the start of fundamental pulse.
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Fig. 7. Pulse shapes of the CVL (upper trace) and UV radiation (lower trace) for (a) total beam (green þyellow), (b) green and (c) yellow.
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From Fig. 7, it is clear that the pulses of green and yellow radiations are very different. The green pulse is of modulated nature with fast rise and fall time while yellow pulse is of reasonably smooth nature with comparatively slower rise and fall time. The yellow pulse is also delayed about 4 ns with respect to green pulse. The differing values of divergence angles for both green and yellow radiations dictated that the spot sizes of both the focused radiations, on the crystal be different. Moreover, the variation in the intensity across the line focused spot is also different for 510 nm and 578 nm radiations as the amplitude recorded for 1 s, fluctuates significantly (Fig. 7). In addition to different divergence value, the spatial matching of both beams further worsened by the single pulse pointing stability of green and yellow. The maximum shift in the focal spot position for green and yellow due to the pointing instability will vary between 0 mrad and 25 mrad. The maximum change in the pointing angle will be sum of instability of green (15 mrad) and yellow (10 mrad). Thus in the SFM of CVL radiations, considerable mismatch in the space exist between green and yellow pulses due to divergence angle and pointing angle. This leads to variation in the intensity distribution of the line focused spot. Hence on one hand, high average/peak power of both the combined CVL radiation and their matching peak intensities lead to high average power SFM, however the mismatch in other spatial and temporal characteristics leads to sacrifice in SF conversion efficiency as compared to individual SH processes. The maximum conversion efficiency for the green and yellow were 15.7% and 14.5% at the input power of 5.6 W and 4.2 W, respectively. The higher efficiency of green beam compared to that of yellow beam is attributed to the higher input power. But at
the same average power level (2.7 W each) the efficiency of the yellow beam (12.7%) was higher than the green (11%). From Eq. (3), the second harmonic conversion efficiency in an ideal SH process (phase matched, Dk Q0) is given by
ZSH ¼ a/U i S=A ¼ aPoi =½ðrt i ÞA
ð4Þ
where,a is a constant, P oi is the average power of green/yellow beam. As the focal length of cylindrical lens and width of the line focus is the same for the both green and yellow, then from eq. (4), the efficiency is controlled by the factor Po/dy. From Table 1, it is noticed that at same Po/dy factor (0.143 kW/mrad), the efficiency for the yellow (12.7%) was higher than the green (11.4%). The BBO acceptance angle for 7 mm long crystal along the critical plane is calculated as 0.26 mrad, 0.26 mrad and 0.36 mrad at the wavelength 255 nm, 271 nm and 289 nm, respectively [15]. While in other plane it was higher than 1.0 mrad. As the CVL beam is demagnified about 10 times before focusing on the crystal, the divergence is increased by the factor 10. Hence the part of the CVL radiation will be out of the acceptance angle of BBO crystal. Hence any variations in the single pulse divergence angle will reduce the conversion efficiency. From Fig. 3, it is seen that the divergence variation is higher for the green beam as compared to yellow beam. Another parameter is the pointing stability, which also plays the crucial role in the SH process. The pointing stability of the green beam is poor than the yellow beam. This led to improvement in the SH conversion efficiency of the yellow beam as compared to green. However, the intensity uniformity across the line focus beam is also an important aspect to achieve the higher UV power [2]. Average power is measured as the average of the pulses of 1 s. The fluctuation (Fig. 7) in the amplitude of
O. Prakash et al. / Optics & Laser Technology 45 (2013) 428–434
yellow beam is higher than the green beam. This will tend to reduce the conversion efficiency. The net effect led to increase in the SH efficiency of the yellow component as compared to green component at the same factor of Po/dy. The difference in conversion efficiency will be more prominent especially at higher input power as indeed observed by performing the experiment at higher input power level from CVL MOPA beam [15]. It was shown that the SH conversion efficiency of yellow was higher ( 44%) as compared to the SH of green ( 30%) at the same fundamental input power in BBO. It is expected that by using the crystal of larger acceptance angle such as C-LBO, the effect of divergence angle fluctuation and beam pointing stability can be reduced and higher UV power can be generation. About 4.7 W UV power from a single oscillator and 15 W from a master oscillator power amplifier (MOPA), at 255 nm from kinetically enhanced CVL systems is accomplished using the C-LBO crystal [21,22].
4. Conclusions
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In conclusion, the studies on the effect of single pulse stability of divergence angle, beam pointing angle and the amplitude of green and yellow radiations of copper vapour laser (CVL) in the frequency conversion using line focussing geometry in a b-BBO crystal from a single oscillator are studied. At the same fundamental input, the SH conversion efficiency yellow radiation (12.7%) was higher than green (11.0%). The is attributed to better single divergence angle (108 mrad) and pointing instability (10 mrad) of yellow compared to green divergence angle (122 mrad) and pointing instability (15 mrad). Higher yellow conversion efficiency at the same fundamental input, attributed to better single pulse stability of beam pointing and divergence. About 1.2 W at 271 nm is generated at the fundamental input of 9.8 W (5.6 W greenþ4.2 W yellow) with conversion efficiency of 12.4%. Despite the higher fundamental input for the sum frequency generation, the conversion efficiency for the sum frequency of 12.4% was less than the second harmonic generation of green (16.7% at input of 5.6 W) and yellow (14.5% at input of 4.2 W). Lower SFG conversion efficiency compared to SHG of individual CVL radiation is attributed to mismatch in single pulse stability of beam pointing, divergence and amplitude fluctuation of CVL radiations in addition to poor overlap between both the CVL radiations in space and time domain.
References
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Acknowledgements
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Authors wish to acknowledge the assistance of Shri Jagdish Kumar for power supply maintenance and Shri H.S. Vora for software support.
Contents lists available at SciVerse ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
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Studies on the effect of instability of divergence, pointing and amplitude of green and yellow radiation pulses of copper vapour laser in second harmonic and sum frequency conversion Om Prakash n, Ramakanta Mahakud, Shankar V. Nakhe, Sudhir K. Dixit
R&D C-3 Block, Laser System Engineering Section, Raja Ramanna Centre for Advanced Technology, Indore 452013 (M.P.), India
a b s t r a c t
Article history: Received 25 April 2012 Received in revised form 11 June 2012 Accepted 12 June 2012 Available online 30 June 2012
This paper presents the effect of single pulse stability of divergence angle, beam pointing angle and amplitude of green and yellow radiation pulses of an unstable resonator copper vapour laser (CVL) oscillator in the sum frequency mixing and second harmonic. The conversion efficiency of sum frequency generation was lower compared to second harmonic processes despite larger fundamental power being used in sum frequency experiments. However the net UV power obtained at the sum frequency was higher than both of the second harmonic UV frequencies. Lower SFG conversion efficiency (12.4%—271 nm) compared to SHG (16.7%—255 nm, 14.5%—289 nm) of individual CVL radiations is attributed to difference in single pulse stability of beam pointing, divergence and amplitude fluctuation of both CVL radiations in addition to commonly known fact of spatio-temporal mis-match. At the same fundamental input power (2.7 W), higher SH conversion efficiency of yellow (12.7%) compared to green (11.0%) is attributed to its better single pulse stability of beam pointing and divergence. & 2012 Elsevier Ltd. All rights reserved.
1. Introduction
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Keywords: Sum frequency mixing Second harmonic of green and yellow radiation Divergence and beam pointing stability
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a r t i c l e i n f o
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The high repetition rate, narrow line-width, high average power UV radiations (255 nm, 271 nm, 289 nm) generated from the second harmonic [1–3] or the sum frequency mixing [2,4] of CVL visible radiations (510 nm, 578 nm) are the potential sources for high speed manufacturing of photonics components such as Fiber Bragg Grating (FBG) [5], precision micro-machining, cutting and etching of semiconductors [6] and for optical pumping of various cerium-doped laser [7] to obtain tunable UV radiations (280–315 nm). The CVL based UV radiations (255 nm, 271 nm, 289 nm) are also the potential sources for high speed all-optical switching based on fiber Bragg grating (FBG) [8–10]. These optical switches are key components in optical communications and in optical bistability systems. In reported literature, the CVL to UV frequency conversion efficiencies were compared for all the three UV wavelengths [1–4]. The relative sum and second harmonic efficiencies depended on the green and yellow power ratio, their temporal mismatch, details of focusing geometry and the choice of non-linear crystal. In particular, in a spherical focusing geometry with BBO crystal, the UV conversion efficiencies were 9.6%, 5.5% and 6.4% for the second harmonic generation (SHG) of green,
n
Corresponding author. Tel.: þ91 731 2442472; fax: þ91 731 2442400. E-mail address: [email protected] (O. Prakash).
0030-3992/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2012.06.013
sum frequency generation (SFG) of green and yellow and SHG of yellow at the average power of 4.8 W, 8.4 W and 3.6 W, respectively [11]. The UV output power level scaled to more than 1.0 W from a single CVL oscillator using the cylindrical focusing geometry in BBO, as it is a good candidate for high power operation in view of its high damage threshold and large acceptance angle along the azimuthal direction [2]. The conversion efficiencies of 34%, 19% and 17% were obtained at the SH of green, SF of green and yellow and SH of amplified yellow, respectively [2]. Recently in the BBO crystal, the conversion efficiency of 28%, 23%, 17.5% at the 255 nm, 271 nm and 289 nm, respectively, are reported from a single CVL system using tunable acoustic optical filter [12]. In this set-up, the different CVL radiations and amplitudes are controlled selectively using electronically tunable acousto-optical filter. Although higher conversion efficiency of about 50% has been reported for picosecond and femtosecond pulses using BiB3O6 (BIBO) crystal in the blue region due to high damage threshold and high nonlinear coefficient of BIBO crystal as compared to BBO crystal. But the minimum transmission wavelength of BIBO crystal is limited to 280 nm [13–14]. However this crystal is not suitable for SFG and SHG of green CVL radiations. From the reported literature, it is seen that the SFG conversion efficiency was lower as compared to that of second harmonic (SH) of green CVL radiation [15]. It is also observed that from a single CVL oscillator, the conversion efficiency of SH of yellow is lower than the SH of green [2,12]. However, the average power of green
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micro-positioner and was suitably tilted/aligned for SH or SFM of 510 nm and/or 578 nm radiations. The exact phase matching cut angle for normal incidence is, 461 for SFM of 510 and 578 nm, 50.61 for SHG of 510 nm and 421 for SHG of 578 nm. The fundamental and SF/SH beams were re-collimated by a fused silica cylindrical lens L4 and separated by fused silica (Supracil) prism, P. Average visible/UV powers was measured by a power meter (Gentech, PS-310 WB). The green and yellow pulses for 1 s time duration were recorded by a digital storage oscilloscope (Tektronix, 540 TDS) in persistence mode using two bi-planar photo-diodes of sub-nano-second rise time (ITL, S-20). The divergence and pointing stability of the both green and yellow CVL radiation pulses were estimated by recording the spatial shift in the far-field intensity distribution about the mean position at the focal plane of a lens L1 of focal length 100 cm. A magnified image about 4-times of the far-field intensity distribution was placed onto the CCD camera (Pixelfly qe, PCO AG) by an additional imaging lens of focal length 100 cm. Suitable neutral density filters of known transmission are used to attenuate the intensity to prevent the saturation of CCD camera. This event is captured and analysed by specially developed beam analysis software [16]. Software records the events in subsequent time in the form of spatially stacked picture. An orthogonal pair of cursors is centred on this spot. This provides the reference position to save line profile of the subsequently acquired images in a dynamically allocated memory of the PC with the help of a PCI bus based FG card. For present set-up, the CCD acquiring time was set to 174 ms, slightly less than the pulse separation of 181 ms at 5.5 kHz laser repetition rate. This ensured the single pulse recording. The subsequent images were acquired after time interval of about 1 s at 1 Hz rate.
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and yellow were not the same and the conversion efficiency for both the radiations is not compared at the same input power. Also a little attention is paid on understanding the reason of the low conversion efficiency in SFG as compared to SHG of CVL radiations except the some commonly known facts of spatio-temporal mismatch between the green and yellow radiations and green/ yellow average power ratio. It is well known that the non-linear crystals have limited angular acceptance. Hence any pulse to pulse variation of CVL beam divergence, pointing that exceeds the acceptance angle of crystal will affect the frequency conversion process by deteriorates the phase matching conditions. In a study on beam pointing stability on CVL, it was demonstrated that the beam pointing depends on the resonator characteristics [16]. Therefore, it is expected that the single pulse divergence angle and pointing angle of green and yellow radiation will be different. It is worth investigating the role of CVL beam divergence and pointing angle of green and yellow in non-linear frequency conversion. Another issue that affects the UV conversion efficiency is the single pulse fluctuation in the amplitude of green and yellow radiation. So far in CVL, inadequate attention is paid on the role of stability of divergence, beam pointing angle and the amplitude fluctuations in sum frequency conversion of CVL radiations. This paper presents the studies on the effect of single pulse stability of beam pointing angle, divergence angle and the amplitude of CVL radiations on the SFG of green and yellow. Lower SFM conversion efficiency compared to SH of either CVL radiations is explained in terms of difference in green and yellow beam pointing stability, divergence angles and amplitude fluctuations in addition to power ratio of green and yellow, temporal mismatch between both the green and yellow radiations. Higher SH conversion efficiency of yellow compared to the green at the same input power is addressed in relevance of single pulse stability of beam pointing angle and divergence angle.
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3. Results and discussion
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2. Experimental set-up
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The CVL used in this experiment [a homemade device of 18 W (5.5 kHz), 28 mm beam diameter, 150 cm tube length] employed a confocal positive branch unstable resonator (PBUR) of magnification M ¼100. The resonator consisted of a concave mirror M1 (F1 ¼250 cm), a convex mirror M2 (F2 ¼ 20 cm), an intracavity cube polarizer Z1 placed closed to M1, and a scraper mirror, SC, to take the beam out (Fig. 1). The resonator length, L, was kept equal to F1–F2 i.e. 242.5 cm to satisfy confocal conditions. The cube polarizer, Z1, polarizes both the radiations used in SFM. The scraper mirror out coupled CVL beam was telescopically compressed from 28 mm to 2.8 mm by using a combination of two achromatic lenses L1 and L2 (f1 ¼100 cm and f2 ¼10 cm). The amplified spontaneous emission (ASE) was removed by placing an aperture, A1, at the common focal plane. The collimated beam was line focused by a cylindrical lens, L3, in a 6 4 7 mm3 BBO crystal (M/s Casix) cut at 511. The crystal was mounted on a 5-axis
Fig. 1. Experimental set-up for sum frequency generation.
Fig. 2a and b shows the typical composite pictures of the farfield intensity distribution of the green and yellow radiation of URCVL of magnification 100, for the 500 pulses. The zigzag shape of the stacked picture is an indicative of position jitter of the farfield pattern. A careful look at Fig. 2 shows that the positional jitter of the far-field intensity distribution for the green is higher compared to yellow. Fig. 2 shows the composite data for the horizontal stack. The vertical stack was similar to horizontal stack for both the green and the yellow beams but not shown here. Fig. 3 shows the far-field intensity distribution of green and yellow radiation, respectively, recorded for 500 pulses. This reveals the stability in the position of the far-field intensity distribution and the variation in the magnitude of the intensity. It is derived from Fig. 2. The divergence angle for the green and yellow beams was estimated from Fig. 3 as follows:
y ¼ Do=d
ð1Þ
where d is the distance from the plane of minimum laser spot size from the lens L1, and Do is the width of far-field intensity distribution at the 1/e2 point of the maximum intensity. The estimated the average divergence angle of the green and yellow radiations were 122 mrad and 108 mrad, respectively. From Fig. 3, we see that there is significant spread in the single pulse far-field intensity distribution over 500 pulses. Variation in the single pulse divergence angle for 500 pulses were estimated from Fig. 2 and shown in Fig. 4. The single pulse divergence angle varies between 110 mrad and 140 mrad for the green beam. While for yellow beam, it was in between 95 mrad and 115 mrad. The variation in single pulse divergence angle for green beam was higher as compared to the yellow. This can be explained as follows. The CVL is a super-radiant laser with significant amplified
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spontaneous emission (ASE) right from the moment the gain is switched on. This optical noise is highly random in phase and amplitude distribution across the beam. Due to highly random nature, the starting optical noise is also different from pulse to pulse. In an unstable resonator, these seed radiations are magnified by the magnification factor M in the each round trip. Hence the divergence is reduced by a factor M in successive roundtrips which ultimately get clamped to diffraction limited value. The average divergence value achieved depends on the gain of individual CVL radiations and the type of optical resonator. In CVL, it is well known that the gain of green radiation is higher than the yellow radiation [17]. Therefore for the given resonator, the net divergence value and its fluctuations are controlled by the gain of radiation. Hence the divergence value and its fluctuation are higher for the green beam as compared to yellow beam (Fig. 4). The pointing instability of the green and yellow radiation were also estimated from Fig. 3 as
d ¼ Ds=d
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where Ds is the maximum wandering of the peak of the far-field intensity distribution from the mean position of the peak. The zero position is designated where the beam hits maximum number of times. Here the mean position of the peak is calculated from the mode of the peak position data over 500 pulses. The beam pointing stability of different pulses for green and yellow radiations is shown in Fig. 5. The pointing instability for the green and yellow radiation was 15 mrad and 10 mrad, respectively. The pointing stability of the yellow radiation was better than the green radiation. The behaviour can be explained as follows. In copper vapour laser, the beam pointing instability arises due to multiple factors such as random fluctuations in laser beam axis, resonator mode evolution, mechanical vibrations of mirror mounts and supporting structure, random refractive index fluctuation of the highly heated pulsed CVL plasma, varying temperature gradient in discharge sealing optical windows, ambient temperature fluctuations, air currents, etc. Since CVL is a high gain laser and mode build up from the noise with random phase. The extent to which the random phase distribution of the noise pulses affect the single pulse laser pointing stability, is closely linked to the fact as to how close is a single starting noise pulse in reaching the steady state in the resonator. The steady state depends on the gain of individual CVL radiations. The pointing stability of yellow radiation is better than the green most probably due to lower gain of yellow radiation. Fig. 6a and b shows the variations of UV power and conversion efficiency vs fundamental input power. The conversion efficiency for SFG is calculated as ZSF ¼PSFG/(Pg þPy) 100%, where Pg, Py and PSFG are average power of fundamental green, yellow and UV sum frequency, respectively. It is seen that the UV power increases as fundamental input power of both the beam increases. The maximum sum UV (271 nm) power of 1.21 W is observed with the conversion efficiency of 12.4%. As the fundamental power increases the conversion efficiency increases steeply and reaches maximum and then saturates. This saturation may be due to thermal de-phasing as observed in high power second harmonic generation of CVL [18,19]. The UV power and conversion efficiency at the second harmonic of green (255 nm, solid line) and yellow (289 nm, dash-dot line) were also measured by orienting the BBO crystal suitable phase matching angles for green and yellow radiations (Fig. 6b). The second harmonic conversion efficiency is estimated at ZSH ¼PSH/Pg/y 100%, where PSH is the second harmonic UV power and Pg/y is the average power of green or yellow. The maximum UV power of 940 mW at 255 nm is obtained with the conversion efficiency of 16.7%. About 608 mW UV power at 289 nm with conversion efficiency of 14.5% is
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Fig. 2. Composite pictures of the far-field intensity distribution of URCVL of magnification 100 for CVL beam: (a) green and (b) yellow.
Fig. 3. Variation of far-field intensity distribution recorded for 500 pulses for CVL beam: (a) green and (b) yellow.
ð2Þ
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green
Divergence (µrad)
150 140 130 120 110 100 90 100
200
300 Nos. of pulses
150 Divergence (µrad)
400
500
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0
140
yellow
130 120 110 100 90 100
200
300 Nos. of pulses
400
500
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0
Fig. 4. Variation of single pulse divergence of different pulses for green and yellow CVL beam.
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15
5 0
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Instability (µrad)
10
-5
green
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-15
0
100
200
300 Nos. of pulses
400
500
15
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Instability (µrad)
10 5 0
-5 -10
yellow
-15 0
100
200
300
400
500
Nos. of pulses Fig. 5. Beam pointing stability of different pulses for green and yellow CVL beam.
observed from second harmonic generation of yellow beam. From Fig. 6b it is clear that at the same fundamental input power, the UV power and conversion efficiencies are higher for yellow beam compared to green beam. At the fundamental input power of 2.7 W, conversion efficiency for the green and yellow beam were 11.0% and 12.7%, respectively. Table 1 summarizes the UV and
fundamental beam power at different power level, average and peak power of fundamental beam, pulse duration, divergence angle for the green and yellow wavelengths and the conversion efficiency. The average power is the average of pulses measured for 1 s time duration. Hence we recorded the CVL and the corresponding
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UV pulses for the 1 s time duration by operating oscilloscope in persistence mode. Fig. 7a–c shows the pulse shapes of the CVL (upper trace) and UV radiation (lower trace) corresponding to total beam (greenþyellow), green and yellow, respectively. The fluctuation in the amplitude for the total beam at 10 ns and 25 ns from the start were about 16% and 40%, respectively. This time slot is chosen because the efficient frequency conversion starts after 10 ns from the start of fundamental pulse. In the fluctuation estimation, we have ignored the odd points. The fluctuation in UV beam amplitude (271 nm) was about 22% and 50% at the same
2.0
12
1.8
11
1.6
10 9
1.2
Efficiency (%)
UV power (W)
1.4
8
1.0 7
0.8
6
0.6 0.4
5
0.2
4
0.0 1
2
3 4 5 6 7 8 9 Fundamental G+Y power (W)
10
11
3 12
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0
time. The amplitude variation for the green wavelength at the 10 ns and 25 ns from the start were about 20% and 45%, respectively. The corresponding fluctuations in the UV beam (255 nm) were 30% and 58%, respectively. The amplitude variations for the yellow component at the same time from the start of the pulse were about 24% and 50% and corresponding the UV beam (289 nm) were 30% and 60%, respectively. From Fig. 7b and c, it is noticed that the fluctuation in the yellow beam is higher than the green beam. This may be due to the fact that for the radiation circulating inside the optical resonator, the green beam saturates early as compared to yellow due to higher gain. This results into lower amplitude fluctuations for green component as compared to yellow. The higher amplitude fluctuations were observed in the UV beam as compared to fundamental beam for all the cases due to non-linear nature of frequency conversion. From Table 1, it is clear that the net UV power (1210 mW) at the sum frequency was higher than separately at both the second harmonic UV frequencies. However, the conversion efficiency of SFM (12.4%) was lower than the second harmonic processes (16.7% and 14.5%) despite larger fundamental power (9.8 W) being used in sum frequency experiments. In an ideal case of sum frequency mixing of two different wavelength laser pulses in a non-linear crystal, both the radiations pulses must exactly match in space, time and energy domain. This includes intensity distribution, divergence angle, spot size in the crystal and pulse shape/pulse width. Any mismatch will lead to reduced SF conversion efficiencies, ZSFG. Under assumption of constant pumping fields with constant spatial and temporal distribution of fundamental pulses and also under small depletion regime, ZSFG is given by [20]
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1.0
17 16
Efficiency (%)
UV power (W)
14 13
g
12
0.6
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y
9
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g 2.5
3.0
3.5 4.0 4.5 5.0 Fundamental G/Y power (W)
5.5
6
5 6.0
Fig. 6. Variations of UV power and conversion efficiency vs fundamental input power: (a) sum frequency (efficiency-dotted line; UV power-solid line) and (b) green (solid line) and yellow (dash-dot line).
2
2:97 104 def f L2 /U 1 S/U 2 S sinc2 ðDkL=2Þ /U 3 S ¼g 2 /U 1 þ U 2 S no no ne l A/U 1 þU 2 S
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y
0.8
ZSFG ¼
15
1 2 3 3
ð3Þ
where /U1S, /U2S and /U3S are the peak powers of 510 nm, 578 nm and 271 nm pulses, respectively, n01 and n02 are refractive index ordinary beams for 510 nm and 578 nm, ne3 is the refractive index of the extraordinary beam at 271 nm, deff and L are the effective non-linear coefficient and length of the crystal, A[Qfhdy ] is the area of the pump beam, f is the focal length of the cylindrical lens L3, h is the width of line focus, dy is the pump beam divergence, Dk is the phase mismatch. The average power is related with the peak power as /U i S ¼ P oi =ðr ti Þ for i ¼ 1,2,3 where r is the repetition rate, t is the pulse width. Constant g is a representative of mismatch between green and yellow components of CVL radiations in space as well as in time. For ideal case of perfect matching of radiations, g ¼1, otherwise g o1.
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Table 1 Fundamental and UV beam parameters. Fundamental beam
UV beam
Wavelength (nm)
Average Pulse Divergence power, Pav (W) width (ns) Dy (mrad)
Peak power, Ppk (kW)
Factor Ppk/Dy (kW/mrad)
Amplitude fluctuations (%)
Wave UV power length (nm) (mW)
Efficiency, Z (%)
Amplitude fluctuations (%)
510
5.6 3.3 2.7
32
122
31.3 17.7 15.1
0.257 0.146 0.124
t10,25a ¼ 20,45
255
940 363 297
16.7 11.4 11.0
t10,25 ¼30,58
578
4.2 2.7
31
108
24.2 15.5
0.224 0.143
t10,25 ¼ 24,50
289
608 341
14.5 12.7
t10,25 ¼30,60
510 þ 578
9.8 (5.6 g þ4.2 y) 6.0 (3.3 g þ 2.7 y)
33
–
53.0
–
t10,25 ¼ 16,40
271
1210
12.4
t10,25 ¼22,50
671
11.1
a
32.5
The amplitude fluctuations are measured at 10 ns and 25 ns from the start of fundamental pulse.
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Fig. 7. Pulse shapes of the CVL (upper trace) and UV radiation (lower trace) for (a) total beam (green þyellow), (b) green and (c) yellow.
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From Fig. 7, it is clear that the pulses of green and yellow radiations are very different. The green pulse is of modulated nature with fast rise and fall time while yellow pulse is of reasonably smooth nature with comparatively slower rise and fall time. The yellow pulse is also delayed about 4 ns with respect to green pulse. The differing values of divergence angles for both green and yellow radiations dictated that the spot sizes of both the focused radiations, on the crystal be different. Moreover, the variation in the intensity across the line focused spot is also different for 510 nm and 578 nm radiations as the amplitude recorded for 1 s, fluctuates significantly (Fig. 7). In addition to different divergence value, the spatial matching of both beams further worsened by the single pulse pointing stability of green and yellow. The maximum shift in the focal spot position for green and yellow due to the pointing instability will vary between 0 mrad and 25 mrad. The maximum change in the pointing angle will be sum of instability of green (15 mrad) and yellow (10 mrad). Thus in the SFM of CVL radiations, considerable mismatch in the space exist between green and yellow pulses due to divergence angle and pointing angle. This leads to variation in the intensity distribution of the line focused spot. Hence on one hand, high average/peak power of both the combined CVL radiation and their matching peak intensities lead to high average power SFM, however the mismatch in other spatial and temporal characteristics leads to sacrifice in SF conversion efficiency as compared to individual SH processes. The maximum conversion efficiency for the green and yellow were 15.7% and 14.5% at the input power of 5.6 W and 4.2 W, respectively. The higher efficiency of green beam compared to that of yellow beam is attributed to the higher input power. But at
the same average power level (2.7 W each) the efficiency of the yellow beam (12.7%) was higher than the green (11%). From Eq. (3), the second harmonic conversion efficiency in an ideal SH process (phase matched, Dk Q0) is given by
ZSH ¼ a/U i S=A ¼ aPoi =½ðrt i ÞA
ð4Þ
where,a is a constant, P oi is the average power of green/yellow beam. As the focal length of cylindrical lens and width of the line focus is the same for the both green and yellow, then from eq. (4), the efficiency is controlled by the factor Po/dy. From Table 1, it is noticed that at same Po/dy factor (0.143 kW/mrad), the efficiency for the yellow (12.7%) was higher than the green (11.4%). The BBO acceptance angle for 7 mm long crystal along the critical plane is calculated as 0.26 mrad, 0.26 mrad and 0.36 mrad at the wavelength 255 nm, 271 nm and 289 nm, respectively [15]. While in other plane it was higher than 1.0 mrad. As the CVL beam is demagnified about 10 times before focusing on the crystal, the divergence is increased by the factor 10. Hence the part of the CVL radiation will be out of the acceptance angle of BBO crystal. Hence any variations in the single pulse divergence angle will reduce the conversion efficiency. From Fig. 3, it is seen that the divergence variation is higher for the green beam as compared to yellow beam. Another parameter is the pointing stability, which also plays the crucial role in the SH process. The pointing stability of the green beam is poor than the yellow beam. This led to improvement in the SH conversion efficiency of the yellow beam as compared to green. However, the intensity uniformity across the line focus beam is also an important aspect to achieve the higher UV power [2]. Average power is measured as the average of the pulses of 1 s. The fluctuation (Fig. 7) in the amplitude of
O. Prakash et al. / Optics & Laser Technology 45 (2013) 428–434
yellow beam is higher than the green beam. This will tend to reduce the conversion efficiency. The net effect led to increase in the SH efficiency of the yellow component as compared to green component at the same factor of Po/dy. The difference in conversion efficiency will be more prominent especially at higher input power as indeed observed by performing the experiment at higher input power level from CVL MOPA beam [15]. It was shown that the SH conversion efficiency of yellow was higher ( 44%) as compared to the SH of green ( 30%) at the same fundamental input power in BBO. It is expected that by using the crystal of larger acceptance angle such as C-LBO, the effect of divergence angle fluctuation and beam pointing stability can be reduced and higher UV power can be generation. About 4.7 W UV power from a single oscillator and 15 W from a master oscillator power amplifier (MOPA), at 255 nm from kinetically enhanced CVL systems is accomplished using the C-LBO crystal [21,22].
4. Conclusions
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In conclusion, the studies on the effect of single pulse stability of divergence angle, beam pointing angle and the amplitude of green and yellow radiations of copper vapour laser (CVL) in the frequency conversion using line focussing geometry in a b-BBO crystal from a single oscillator are studied. At the same fundamental input, the SH conversion efficiency yellow radiation (12.7%) was higher than green (11.0%). The is attributed to better single divergence angle (108 mrad) and pointing instability (10 mrad) of yellow compared to green divergence angle (122 mrad) and pointing instability (15 mrad). Higher yellow conversion efficiency at the same fundamental input, attributed to better single pulse stability of beam pointing and divergence. About 1.2 W at 271 nm is generated at the fundamental input of 9.8 W (5.6 W greenþ4.2 W yellow) with conversion efficiency of 12.4%. Despite the higher fundamental input for the sum frequency generation, the conversion efficiency for the sum frequency of 12.4% was less than the second harmonic generation of green (16.7% at input of 5.6 W) and yellow (14.5% at input of 4.2 W). Lower SFG conversion efficiency compared to SHG of individual CVL radiation is attributed to mismatch in single pulse stability of beam pointing, divergence and amplitude fluctuation of CVL radiations in addition to poor overlap between both the CVL radiations in space and time domain.
References
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Acknowledgements
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Authors wish to acknowledge the assistance of Shri Jagdish Kumar for power supply maintenance and Shri H.S. Vora for software support.