Operation of a CVL pumped dye laser in the single longitudinal mode and its parametric characterization

Optik 124 (2013) 2837–2843

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Operation of a CVL pumped dye laser in the single longitudinal mode and its parametric characterization V.S. Rawat a,∗ , G. Sridhar a , Sunita Singh a , L.M. Gantayet b a b

Laser and Plasma Technology Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India Beam Technology Development Group, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India

a r t i c l e

i n f o

Article history: Received 10 April 2012 Accepted 16 August 2012

Keywords: Single mode laser Dye lasers Buildup time

a b s t r a c t The parametric characterization of a high repetition rate (∼9 kHz), indigenously developed, single longitudinal mode (SLM) dye laser based on a short Littman cavity and pumped by a copper vapor laser (CVL) is presented. The average bandwidth and the single pulse bandwidth have been measured to be ∼400 MHz and ∼315 MHz respectively for the SLM dye laser. Tunability of 12 nm over 554–566 nm has been achieved. The optimum dye concentration for minimum amplified spontaneous emission (ASE) has been determined experimentally for this SLM dye laser. The effect of pump power, grating diffraction efficiency, dye concentration on the buildup time of SLM and laser pulse duration has also been studied. The maximum buildup time was 15.5 ns for SLM dye laser pumped by green CVL. The buildup time is reduced by nearly 25% with increase of pump power from 1 W to 3 W. The minimum build up time of 6 ns was obtained with SLM dye laser pumped by second harmonic of Nd:YAG laser. © 2012 Elsevier GmbH. All rights reserved.

1. Introduction Tunable, pulsed, single longitudinal mode (SLM) dye laser are widely used in laser spectroscopy such as coherent anti strokes Raman spectroscopy, resonance ionization spectroscopy, etc. [1,2]. In these applications the transition line widths are very small. For variety of spectroscopic applications a large number of laser systems, each with special properties have been used. Narrow line width is achieved optimally by operating the laser in a single longitudinal mode (SLM) and high power is obtained by pulsed operation usually by optical pumping with high power fixed frequency lasers [3]. The copper vapor lasers (CVL) with in built high pulse repetition rate operation capability and emitting short duration pulses of few tens of ns duration are regarded as an ideal source for pumping dye lasers in the visible range. Because of the large emission bandwidth of fluorescence, high excitation flux is required for achieving laser emission from the organic dyes. To effect lasing in a highly dispersive and lossy cavity, high gain of dye is prerequisite [4–6]. Various methods of achieving tunable narrow band operation have been reported in literature such as CW single mode and pulsed dye laser oscillators, externally filtered multimode laser, pulsed amplification of CW single mode laser [8], single mode seeded power oscillator. To get single mode laser various combinations are reported in literature [7,8,13,16] such as hybrid multiple prism-grazing incidence (HMPGI), grazing incidence (GI), multiple

∗ Corresponding author. Tel.: +91 22 25590204; fax: +91 22 25505151. E-mail address: [email protected] (V.S. Rawat). 0030-4026/$ – see front matter © 2012 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2012.08.057

prism littrow (MPL), Hansch cavity with telescopic beam expender and Littrow cavity [10,17,19–21]. Pulsed single mode dye laser was achieved by Littman using grazing incidence type cavity [6]. Although the combination of beam expander and diffraction grating reduces the band width of the dye laser, it introduces more losses to the cavity causing reduction in overall conversion efficiency. Another common approach for spectral narrowing of the dye laser emission is the utilization of full grating length with unexpanded beam at grazing incidence [4,11,12]. The efficiencies of such narrow band dye laser have been less than 3% due to smaller diffraction efficiency of grating at high angle of incidence [18]. In the output characteristics of a SLM dye lasers apart from tuning range, bandwidth and fluence, the spectral purity of laser (the percentage of signal radiation in the total output) is also very important for spectroscopic applications. The main source of spectral impurity is amplified spontaneous emission (ASE) of the oscillator itself. The contrast ratio of narrow band laser output with respect to ASE for conventional transversely pumped dye laser is as high as 100 at the peak of lasing wavelength and falls to unity at the edges of the tuning range [3]. The high gain in the pulsed dye laser results in higher ASE, which may be comparable to the output energy at the edge of the tuning range. In this paper, we report parametric characterization of indigenously developed SLM dye laser based on Littman’s configuration. We have designed, developed and engineered a short cavity grazing incidence SLM dye laser in our lab [9]. The short cavity length provides more number of cavity round trips within the time period of inverted population of the dye [6,23]. The longitudinal pumping of dye spatially restricts the active gain volume within the laser cavity

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V.S. Rawat et al. / Optik 124 (2013) 2837–2843 Focusing Lens

Pump Beam

DBS

Str ip M irro r

PZT End Mirror Pivot Point

Grating SLM

Wavemeter WS - 7 L

PZT Dye Cell

FPE

CCD Monitor

Fig. 1. Schematic of the GIG SLM dye laser.

for TEMOO mode [14]. In a SLM dye laser the laser line widths are narrower than the longitudinal mode spacing of the laser and the laser signal is dominated by the mode nearest to the transition line of the dye [10,11,15]. We report the effect of pump power, grating efficiency and wavelength on the SLM pulse buildup time. We have optimized the dye concentration for minimum ASE and studied the effect of pump power on the SLM pulse duration. The buildup time of SLM dye laser plays an important role in determining the optical delays required in the master oscillator and power amplifier (MOPA) configurations. The buildup time is the inherent delay generated at the oscillator. In an oscillator amplifier configuration the optical delay including buildup time between the dye laser signal beam and amplifier pump beam must be same for maximizing the extraction efficiency of amplifier with minimum ASE. 2. Experimental design A schematic of the grazing incidence grating (GIG) SLM pulsed dye laser is shown in Fig. 1. It is based on Littman type configuration utilizing a cavity of length about 50 mm. The laser cavity consists of an end mirror (R > 99%), a strip-tuning mirror (R > 99%), a holographic grating (2400 l/mm) placed in grazing incidence configuration. Rhodamine 6G dye dissolved in ethanol was circulated through an indigenously designed metallic dye cell. The metallic dye cell is made of stainless steel having two replaceable optical quality quartz windows of 2 mm thickness with 1 mm dye flow channel: the total physical thickness of the dye cell is 5 mm. The 10 mm diameter 1 mm thick quartz windows are fixed with vitone O-rings to the dye cell. [9]. All the components of the SLM dye laser were mounted on a two stage differential rotary table. The first stage provides coarse movement with a minimum resolution of 25.92 arc-secs (∼46 pm) with a stepper motor of 50,000 microsteps per revolution. The second stage is used for the fine motion gives a minimum resolution of 0.0014 arc-secs (∼4 fm) with 80 mm arm length and utilizes a 20 ␮m PZT with drive voltage of 1 kV. A piezoelectric transducer (PZT) stack was fixed to the end mirror with an epoxy adhesive, which provides a maximum displacement of 10 ␮m at a drive voltage of 1 kV. The three optical components (grating, tuning mirror and end mirror) are positioned precisely with the help of linear motion tables. This precise positioning is necessary to match the cavity pivot point for mode hop free tuning over a wide range of wavelengths. The dye cell of internal dimensions 5 × 1 × 70 mm was mounted in the center of the rotation table and was tilted with respect to vertical axis by nearly 5◦ to prevent the unwanted reflections from the windows. The dye cell windows are anti reflection coated to avoid the sub cavity resonances inside the dye cell. The short cavity length (∼50 mm) of resonator provides wide axial mode separation (∼3 GHz), which is larger than the single pass width (∼2 GHz) of the dye laser. This makes it possible to obtain lasing of dye laser in single longitudinal mode. The angle of incidence on the grating was kept larger than 89◦ in order

to achieve single mode oscillation. Longitudinal pumping and tight focusing of the pump laser provide control over the transverse modes. Longitudinal pumping is favorable for the single longitudinal mode selection because a virtual aperture is created by pump beam within the gain medium [12]. The green beam (510.6 nm) of copper vapor laser operating at 9 kHz repetition rate was focused into the dye cell with a plano convex lens of focal length 200 mm. The focusing lens was mounted on a linear translational stage for precise control over the focal spot. The 0.2 mM rhodamine 6G in ethanol was circulated in the dye cell with a gear pump and the flow velocity was controlled by variable frequency drive (VFD). The flow velocity was measured to be 3.6 m/s, which provides a clearance ratio of two for 9 kHz pulse repetition rate and gain width of nearly 200 ␮m. The exact focal spot was located in front of the dye cell, which avoids both the damage to the dye cell windows and the thermal distortion in the dye active medium. The diameter of the focal spot in the gain medium is ∼200 ␮m. The pump beam diameter must be approximately matched with the mode diameter of the dye laser for maximum utilization of pump power in the single mode dye lasers. The laser operating wavelength 0 is governed by the wellknown diffraction grating formula 0 = d(sin  + sin ϕ)

(1)

where d is the grating period  and ϕ is the grating incidence and diffraction angles respectively. The tuning of the GIG laser wavelength was achieved by varying the diffraction angle (ϕ). The wavelength L supported by the SLM resonator cavity is given by equation 2L = NL

(2)

where L is cavity length, N is mode index number and L is laser wavelength supported by the cavity. For mode hop free tuning the wavelength selected by grating equation (say G ) and wavelength supported by cavity length (say L ) should always be the same (0 = G = L ). When the cavity is tuned the comb of cavity modes (L ) shifts proportional to the change in resonator length in the direction of tuning. The dispersion bandwidth (G ) also shifts in the direction of tuning but proportional to the angle of the tuning mirror (ϕ). Single mode tuning without mode hop is achieved when the cavity modes and the dispersion bandwidth shift together such that only one mode operates in the optimum gain for all wavelengths throughout the tuning range [5]. This implies that the cavity length and the angle are coupled in tuning process and must be controlled simultaneously. A large angle of incidence for GIG makes it possible to obtain a larger grating dispersion without a beam expander. The cavity spectral selectivity increases rapidly with grating incidence angle. For an incidence angle of 89.5◦ , the spectral width of the cavity transmission becomes comparable to cavity mode spacing. While increasing the incidence angle of grating, the SLM efficiency reduces, primarily

V.S. Rawat et al. / Optik 124 (2013) 2837–2843

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Geomatrical Spot size in micron

1200

1000

800

600

400

200

0 89.0

89.2

89.4

89.6

89.8

90.0

Grazing Incidence Angle Fig. 2. Variation of grating input aperture with grating incidence angle for fixed grating length of 62.5 mm.

due to a decrease in the grating diffraction efficiency. Further at higher incidence angle, the grating input aperture becomes smaller than the gain size in the resonator leading to an additional loss. The smaller input aperture at higher incidence angle limits the transverse (spot) size of the laser active region and, in turn, the maximum output power of the laser. In the GIG configuration, the grating aperture limits the diameter of the active region. The grating input aperture (W) is defined by [22,24] W = Lg cos 

(3)

where W is the input aperture diameter, Lg is the grating length and  is incidence angle. For example in the SLM dye laser for the 62.5 mm long grating with incidence angle of 89.5◦ ; the grating aperture is nearly 545 ␮m, which limits the maximum pump pulse energy to ∼2 mJ to avoid damage of dye cell windows. The grating aperture of ∼545 ␮m is two time greater the gain aperture inside the gain medium created by the focusing of pump beam on the dye cell. Fig. 2 shows the variation in grating aperture (W) using Eq. (3) with grating incidence angle () for a fixed grating length (Lg ) of 62.5 mm. The cavity spectral selectivity is defined by equation d n = d d cos 

(4)

where  is incidence angle to the grating, d is groove density, n is the order of the grating. The cavity spectral selectivity increases with increasing incidence angle as seen from Eq. (4), while the grating input aperture decreases with increasing incidence angle. Further reduction of grating aperture leads to overfilling of grating length which resulted to additional losses to the cavity for fixed grating length. 3. Results and discussion We have measured the time averaged bandwidth and single pulse bandwidth of the SLM dye laser. The SLM operation of the dye laser was verified by monitoring the spectral output of Fabry–Perot (FP) etalon of 7.5 GHz FSR and reflective finesse of 30. A small portion of SLM dye laser was coupled to a 62.5 ␮m core diameter optical fiber and a diverging output from the optical fiber was directed on to a FP etalon (FSR ∼7.5 GHz). The circular fringes at the output of FP etalon were focused with spherical lens of 500 mm focal length on to a CCD camera. The output of CCD camera was interfaced to a personal computer (PC) with a frame grabber card. Fig. 3a shows the FP etalon fringes of SLM dye laser. From the F P fringes the time averaged bandwidth measured to be ∼400 MHz.

Fig. 3. (a) Typical Faby Perot fringe of single mode dye laser with FSR of etalon 7.5 GHz. (b) Intensity pattern of SLM FP etalon fringe of SLM dye laser, show time average bandwidth of 400 MHz.

The laser is found to oscillate on a single frequency. The time averaged bandwidth of SLM dye laser was estimated from the FP etalon fringes. The FP etalon FSR of 7.5 GHz was multiplied by the ratio of peak width (FWHM) of second fringe to the separation in between the first and second fringe (Fig. 3b). The spectral bandwidth of the laser was measured by a Fabry Perot (FP) etalon of 7.5 GHz in conjunction with a wavelength meter (Angstrom WS7L). A fast CCD camera (Pixelfly qe, PCO AG) was externally triggered to measure the single shot single pulse bandwidth of the SLM dye laser using a FP etalon of FSR 7.5 GHz. The single pulse bandwidth for Rhodamine 6G dye was measured to be 315 MHz. Fig. 4a and b shows the FP etalon fringes and intensity variation along the fringe diameter respectively obtained with external triggering of fast CCD camera. Typical SLM pulse duration was 5 ns for a pump power of 1 W. An experimentally observed pump pulse and SLM pulses were monitored using photo diode (FND 100) and oscilloscope (Tektronix TDS 724D, 500 MHz). The pump pulse duration was ∼33 ns, while the SLM pulse duration was measured to be ∼5 ns with pump power of 1 W as shown in Fig. 5. It was experimentally observed that the SLM pulse duration is strongly dependent on the pump power. As can be seen from Fig. 6 the SLM pulse duration increases with increasing pump power. At higher pump power (3.5 W) the SLM pulse duration is 17 ns as shown if Fig. 7 while at lower pump power of 1 W it is 5 ns (Fig. 5). When the pump power is increased beyond 3.5 W, the laser begins to oscillate on two modes. Fig. 8 shows the two mode oscillation of SLM dye laser with higher pump power of 4 W. The SLM dye laser wavelength was measured using laser wavelength meter (Angstrom WS 7L) with an absolute accuracy of 60 MHz. Tuning range of SLM dye laser was from 554 nm to 566 nm (∼12 nm) by rotating the tuning mirror. The tuning range of SLM at two different pump powers is shown in Fig. 9. At higher pump power the tuning range can be seen to increase by 5 nm and increase in the tuning range is more toward the red region.

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Fig. 7. The SLM dye laser pulse CVL pump power ∼3.5 W.

Fig. 4. (a) Typical Faby Perot fringe of single pulse single mode dye laser with FSR of etalon 7.5 GHz. (b) Intensity pattern of SLM FP etalon fringe of single pulse SLM dye Laser, shows bandwidth of 315 MHz.

Fig. 8. Two mode oscillation for CVL pump power higher than 3.5 W.

Fig. 5. The SLM dye laser pulse and CVL pump pulse with pump power of ∼1 W. (a) SLM pulse and (b) pump pulse.

The SLM laser was so developed that, the axis of rotation of the tuning mirror passed through a geometrically located point known as pivot point. The surface planes of the tuning mirror, end mirror and grating intersect on this pivot point. The three optical components (grating, tuning mirror and end mirror) are positioned precisely with the help of linear motion tables. This precise positioning is necessary to match the cavity pivot point for mode hop free tuning over a wide range of wavelengths. The mode hop free tuning over 70 GHz was achieved by rotating the tuning mirror with help of 20 ␮m PZT about this common pivot point. The SLM wavelength and FP fringes were monitored simultaneously so the sudden jump in the SLM wavelength by one cavity FSR could not detected for mod hop free scanning. The jump in the SLM wavelength by cavity FSR which corresponds to the mode hop was not observed over 70 GHz scanning.

9

15.5 Buildup Time Pulse Duration

15.0

35

14.5 7

14.0 13.5

6

13.0 5

12.5 12.0

4

11.5

SLM output in mW

30

Pulse Duration (ns)

BuildupTime (ns)

At pump power 2.25 Watts At Pump power 1.14 Watts

8

(a)

25

(b)

20

15

10 1000

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PumpPower (mWatts)

5 552

Fig. 6. Pulse buildup time and pulse duration with pump power of SLM at peak wavelength for Rhodamine 6G.

554

556

558

560

562

564

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568

Wavelength in nm Fig. 9. Laser tuning range at two pump powers (a) at 2.25 W (b) at 1.14 W.

V.S. Rawat et al. / Optik 124 (2013) 2837–2843

It was experimentally observed that the peak wavelength for rhodamine 6G is blue shifted in the SLM GIG configuration in comparison to the lower loss cavities such as Littrow configuration due to higher laser threshold. The peak wavelength of 570–573 nm was observed in the low loss dye laser cavities as compared to peak wavelength of 560 nm in SLM GIG dye laser cavity. At larger diffraction angle, the diffraction efficiency of grating is reduced hence the loss line for the resonator is higher. To compensate for the higher losses higher gain is required for the laser oscillation to build inside the cavity. The high gain leads to the onset of the super radiance in a pulsed dye laser [3]. There is competition between the onset of lasing and super radiance inside the laser cavity. At lower wavelength (∼554 nm) intra cavity photon flux grows faster at the expense of the gain while at higher wavelength (∼564 nm) the super radiance dominates and lasing reduces to a bare minimum. Hence larger incidence angle of grating (higher threshold) resulted in increased blue shift from the peak wavelength leading to decreased range of tuning. In the SLM dye laser the laser pulse is delayed with respect to pump pulse because several round trips are required for building the laser radiation inside the resonator cavity. This delay in between pump pulse and SLM dye laser pulse is known as the buildup time [25]. In order to measure the buildup time the SLM dye laser and CVL pump laser beams were fed to two photodiodes. The output of lasers was observed on photodiodes (FND 100) in conjunction with a four channel oscilloscope (Tektronix TDS 724D, 500 MHz). The synchronization of the two signals was ensured by appropriately locating the photodiodes for pump and SLM beams and using the same length of BNC cable for both the photodiodes. The SLM pulse buildup time was experimentally observed to be strongly dependent on the pump power, resonator parameters such as grating diffraction efficiency and wavelength of dye laser in the tuning range. Fig. 6 shows the dependence of SLM buildup time on pump power for a dye concentration of 0.2 mM in ethanol. It can be seen that the laser buildup time is 11.7 ns for pump power of 3 W, while the buildup time is 15 ns for pump power of 1 W. The buildup time decreases as the input pump power increases; this is due to higher gain available for laser signal to grow faster inside the fixed loss cavity. While lower pump powers result in lower gain consequently increases the laser buildup time. At higher pump powers, the available gain above threshold lasts for a longer, which results in laser emissions for longer time duration. Further, in this case it has been experimentally observed that the two peaks in dye laser pulse can develop successively for a single pump pulse as shown in Fig. 10. The laser pulse begins to grow during the initial part of the pump pulse and since the gain is still enough after delivering the first pulse (peak a) the population inversion builds again giving rise to the onset of the second pulse (peak b). Measurement of temporal separation of the two pulses (∼7.7 ns the separation between peak (a) and peak (b)) led to the confirmation that they did not originate from the roundtrip peaks of CVL pump pulse. The time interval between the two successive peaks (a) and (c) corresponds to the round trip time of pump pulse for the CVL resonator length of 2.4 meters is 16 ns. At still higher pump power these two peaks merged together resulting in a single pulse of correspondingly longer duration. It was experimentally observed that the SLM pulse buildup time is dependent on the lasing wavelength of the dye laser. At the peak wavelength, the buildup time is small (∼12 ns) and as the lasing mode is tuned on either side of the line center the buildup time increases. The longer buildup time at either end of tuning curve is due to smaller gain available to the SLM laser on both edges of tuning curve. An alternate study of buildup time was done by pumping the SLM dye laser with second harmonic of Nd:YAG laser. The buildup time was 5–6 ns when the rhodamine 6G dye was pumped by the

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Fig. 10. SLM dye laser pulse at the lower pump power of ∼800 mW. Peaks (a) and (b) with separation of 7.7 ns. While separation of peaks (a) and (c) is 16 ns, which round trip time for CVL pump laser.

second harmonic of Nd:YAG laser as shown in Fig. 11. The buildup time was observed to be shorter ∼6 ns as compared to ∼12 ns for CVL pumped SLM dye laser. A different experiment was done with rhodamine 101 dye pumped by the yellow (∼578 nm) beam of CVL peak wavelength of rhodamine 101 is 600 nm. The buildup time for rhodamine 101 SLM dye was measured to be ∼6 ns as shown in Fig. 12. It can be concluded that for smaller Stokes shift (difference between the pump wavelength and lasing wavelength) the buildup time was smaller. Most of the grating based laser cavities have a rather high lasing threshold due to lower value of diffraction efficiency at higher angle of incidence. Hence the grating diffraction efficiency plays an important role in the laser buildup time of the laser. We have observed that the buildup time of SLM dye laser is reduced to 8 ns from 12 ns for holographic grating of 5% diffraction efficiency in comparison to a conventional grating with diffraction efficiency of 2% at same incidence angle. With higher diffraction efficiency of 5% the threshold line for the SLM dye laser is reduced for the same gain with corresponding reduction of the buildup time to 8 ns from12 ns. We have measured the ASE in the SLM signal at various dye concentration ranging from 0.15 mM to 0.5 mM for rhodamine 6G dye in the ethanol. It has been seen that the contribution of ASE is always more toward the edges of the tuning curve for each concentration. The contribution of ASE in the SLM signal was also found to increase with increasing dye concentration (Fig. 13). The ASE was measured by feeding the SLM dye laser to a monochromater (Applied Photophysics Ltd, London; model No. F 3.4) that spatially separates the ASE from the signal. The ASE and signal were separately measured

Fig. 11. Buildup time of 6 ns the Rhodamine 6G pumped with second harmonic of Nd:YAG pumped: (a) dye laser pulse and (b) Nd:YAG pump laser pulse.

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a : Dye Conc 0.2 mM b : Dye Conc 0.25 mM c : Dye Conc 0.4 mM d : Dye Conc 0.5 mM

2.2 2.0

d

1.8

Efficiency %

1.6 1.4 1.2

c

1.0 0.8

b

0.6

a

0.4 552

with a photodiode. As shown in Fig. 15 the ASE is about 20–30% at the edges of the tuning range and ∼2% at the center wavelength for higher dye concentration. At lower dye concentration the ASE is nearly 0.33% at the center wavelength and about 10–15% at the edges of the tuning range. This is due to low effective gain available for the SLM dye laser in the edges. We have observed that the tuning range and the peak wavelength of the SLM dye laser were dependent on the dye concentration for rhodamine 6G in ethanol pumped by CVL. The peak wavelength is red shifted by 4 nm as the dye concentration is increased from 0.15 mM to 0.5 mM. The tuning range is measured to be 10 nm (553–562 nm) for 0.15 mM dye concentration, while for 0.5 mM dye concentration the tuning range increased to 14 nm (554–568 nm) for the same configuration of the SLM. It was observed that the increase in the tuning range was toward red region as shown in Fig. 14. The pump power for higher dye concentration was smaller than the lower dye concentration due to higher availability of gain. The SLM output power is increased by ∼45% as the dye concentration is increased by factor of 3. It was also observed that the buildup time is reduced by 3 ns as the dye concentration is increased from 0.15 mM to 0.5 mM of Rhodamine 6G in ethanol as shown in Fig. 15. This reduction in the buildup time is due to higher gain available with increased dye concentration.

30 25 20

% ASE

15

a b c d e

: Dye Conc 0.15 mM : Dye Conc 0.2 mM : Dye Conc 0.25 mM : Dye Conc 0.3 mM : Dye Cons 0.35 mM

10 5 0 -5 552

556

558

560

562

564

566

568

570

Wavelength (nm) Fig. 14. Wavelength tuning ranges for different dye concentration of Rhodamine 6G in ethanol for: (a) dye concentration 0.20 mM, (b) dye concentration 0.25 mM, (c) dye concentration 0.40 mM, (d) dye concentration 0.50 mM.

Buildup Time 14.5 14.0

Buildup Time (ns)

Fig. 12. Buildup time of 6 ns for the Rhodamine 101 dye pumped by the yellow CVL beam: (a) SLM dye laser pulse and (b) CVL pump pulse. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

554

13.5 13.0 12.5 12.0 11.5 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55

Dye Concentration (mM) Fig. 15. Effect of dye concentration on the buildup time of Rhodamine 6G dye pumped with CVL.

4. Conclusions A single longitudinal mode pulsed GIG dye laser pumped by 9 kHz pulse repetition rate copper vapor laser has been indigenously developed and characterized. The laser single pulse bandwidth and time averaged bandwidth was measured to be 315 MHz and 400 MHz respectively. The SLM output wavelength is tunable over 12 nm. This tuning range was observed to be a function of the dye concentration and pump power. The mode hop free tuning of 70 GHz was achieved. The amplified spontaneous emission (ASE) was minimum at the peak of the tuning range and increased in both directions in the tuning curve. The ASE increased from 0.33% to 2%, buildup time reduced by 3 ns and the peak wavelength of tuning curve was red shifted by 4 nm with increase in dye concentration from 0.15 to 0.5 mM of Rhodamine 6G in ethanol for SLM dye laser. The effect of pump power, grating efficiency and dye concentration on the buildup time was studied. The buildup time is decreased from 15 ns to 12 ns by increasing pump power from 1 W to 3 W. The effect of pump power on the SLM pulse duration was studied. The pulse duration was increased from 3.5 ns to 8.5 ns as pump power was increased to 3 W from 1 W. Acknowledgements

554

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Wavelength (nm) Fig. 13. ASE present in the SLM signal for different dye concentration for Rhodamine 6G dye: (a) dye concentration 0.15 mM, (b) 0.20 mM dye concentration, (c) 0.25 mM dye concentration, (d) 0.30 mM dye concentration, (e) 0.35 mM dye concentration.

The author is indebted to Dr. A.K. Das Head, Laser and Plasma Technology Division, Bhabha Atomic Research Centre and Dr. D.J. Biswas, Laser and Plasma Technology Division for their invariable scientific encouragement throughout the course of this work.

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