Effect of Flow Induced Parameters on the Output of A High Repetition Rate Dye Laser

: The factors influencing the optical path stability in a dye laser flow cell are studied numerically and experimentally. A specially designed curved metallic dye flow cell providing a gain medium of 25 mm x 0.5 mm x 0.2 mm along with a compact resonator mechanical assembly is used in the study. The same configuration with gain medium of 15 mm x 0.5 mm x 0.2 mm is successfully used for single mode dye laser. The effects of flow induced vibrations on dye flow cell are studied with and without mechanically coupling it with the resonator structure for flow speeds varying from 1.33 m/s to 6.67 m/s at laser pump position. The effect of the mechanical instability, velocity fluctuation and temperature fluctuations in flowing dye solution on dye laser performance are studied at different flow speeds. These results are compared with the dye laser output parameters and found to be in good agreement. This study is useful in designing a high stability narrowband dye laser.


Introduction:
Tuneable dye laser with narrow frequency line width pumped by high repetition rate high power copper vapour laser (CVL) or its advanced variants are mostly used in material processing, laser isotope separation [1,2], medicine [3,4], high resolution spectroscopy [5,6] and many other applications. A dye laser is an integrated assembly of pump laser, resonator and dye flow cell. Among these the dye flow cell mostly governs the associated functional parameters with a dynamic system that can be are often reduced by mounting the system on shock absorbing materials and passive masses [11]. Vibration isolation table generally take care of low frequency vibrations from ground noise and building vibrations. 3 The pulsed CVL beam deposits the heat on dye laser window which is absorbed by flowing dye medium. This leads to change in refractive index due to temperature gradient which finally results in variation of wave length in dye laser [12,13]. To take off the gain medium heated by every CVL pulse, the speed of the fluid across the axis of the dye laser is often magnified by narrowing the flow channel. This results in the reduction of the geometrical hydraulic parameters of dye flow cell at CVL pump region [1,14,15]. As a result the non uniform thermal and flow eddies/ inhomogeneities are created. This result in non-uniformities in refractive index of the flowing dye gain medium and unfavourably influence the output of the dye laser.
Moreover the high solvent flow velocity leads to the scattering centres due to cavitations depending on dye flow cell geometry and thermal properties of solvent being used [16,17]. Absence of stream wise recirculation zone allows very few distortions of the wave front of the dye laser beam [18]. A careful design of dye flow cell and predicting its hydrodynamic performance along with dye laser parameters is therefore useful. When the flow field and temperature field are homogeneous the entire gain medium can put in power to the oscillating laser mode. With inhomogeneous flow and temperature field the emission band of a specific group of excited molecules is different from the emission spectrum of the system as a whole.
There exists temperature gradient/difference between downstream and upstream relative to pump laser CVL impact position in a dye cell due to the heat deposited by each pulse. This was studied by Amit et al by analyzing He Ne beam deflection and spread [8]. They used dye solution having 10% ethylene glycol in methanol and through a flow passage of 0.5 mm for the flow rate between 0.3 lit/min to 5 lit/min. A dye cell having 20 mm gain length was used by Maruyama et al and they found that flow speed of dye solution in the dye cell to be about 5 m/s is adequate for downright re substitution of exited dye solution between consecutive pulses of pump laser [5].
They also discussed a dye laser with gain length of 8 mm and with flow speed between 0.6 m/s to 8 m/s and ascertained frequency jitter because of turbulence at higher flow [19].
In this article we present our experimental and analytical study on the essential 4 parameters that influence the dye laser stability. A specifically designed dye laser set up was successfully used in a control experiment to relate the parameters influencing the dye laser performance. The main conclusion in this work is that the flow induced vibrations and associated mechanical instabilities, flow field behaviour and temperature field behaviour are strongly correlated to the dye laser performance. We examine each of these critical parameters independently in our specially designed dye laser set up and discuss the improvements associated to achieve high quality performance of dye laser collectively.

Dye Laser setup:
We have used an inexpensive in-house designed and fabricated arrangement to accommodate optical components and dye cell in a single mechanical structure that may compensate for the environmental effects [9]. The dye flow cell providing dimension of dye gain medium to be 0.2 mm x 0.5 mm x 25 mm was united in a mechanical assembly having provision to accommodate the required optics. In front of the dye cell, a cylindrical lens holder was mounted. This holder has a feature to rotate the cylindrical lens which helps in aligning the pump beam with the dye laser axis. Copper vapour laser beam was focused on to the dye medium using a cylindrical lens of focal length 55 mm. The optical layout of dye laser used is shown in figure 1.
It is fundamentally a traditional grazing incidence grating (GIG) arrangement [20,21]. Dye laser optical configuration is housed in a single mechanical structure which has been in house fabricated [9]. In this mechanical structure the laser axis was folded at right angle with the help of a beam expander prism of magnification of about 8.5 which is anti reflection coated for 570-590 nm. The dye laser beam was expanded by the prism and folded by 90 0 before incidence on a nearly placed grating with 2400 lines opposite to a tuning mirror and an output coupler having 20 % reflectivity. The dye laser beam output parameters were analyzed using a Fabry-Perot etalon of finesse = 25 and FSR = 5 GHz. A CCD camera was also linked to a computer. As a reference laser a He-Ne laser beam ( l = 632.816 nm) with frequency stabilization was arranged to travel through the optical path of the dye laser beam. As the complete dye laser set up is accommodated in a single mechanical structure, the 5 effect of environmental thermal effects and induced vibration and are significantly decreased due to high stiffness of this mechanical structure. The dye flow solution was prepared of a mixture of 30%, ethanol and 70% glycol with 1 mM concentration of Rhodamine 6G in. The dye flow cell was connected to a Stainless Steel (SS) pump, valves, a large SS reservoir, flow meter, micron filter and a compacted chiller unit of rating 1.5 TR.
To house the entire resonator optical layout a specially designed single monoblock metallic structure was employed [9]. In this monoblock resonator structure the grating mount has arrangements for changing the grating angle and alignment of the grating grooves at right angles to the incident beam. In order to accurately position it at necessary angle on the dye cell-grating axis, the beam expander prism could be translated and rotated. The tuning mirror has provision for tilting it in the vertical plane. The cylindrical lens for focusing the pump beam was placed in a mount that has arrangement for linearly moving it with respect to the dye cell for optimising the focusing conditions. It could also be rotated for angular adjustment of the pump region. All the individual optical mounts, made of stainless steel 304, were supported on two guide rods and bolted on to the two sides of the dye cell to form a single monolithic block. The self-weight of this monoblock makes it immune to the external vibrations caused by flow. Such a high mass also ensures that changes in resonator length, because of expansion of optical mounts due to the environmental temperature changes, will be on longer time scale.

Experimental set up for estimation of flow induced mechanical instabilities (figure 2):
In order to conduct experiments to evaluate flow induced mechanical instabilities a prototype dye cell with all the dimensions similar to actual dye cell was fabricated.
This test cell differs with actual dye flow cell only in side flat walls which are made of transparent acrylic glass in place of SS material. An observational experimental set up is briefly described as follows. A test cell with geometry similar to the dye laser mechanical structure but instead of SS for end flat walls transparent acrylic glass was used for this experiment. This was done to perform experiments at different flow 6 positions. A mixture of 30% ethanol and 70% glycol which is the solvent used in dye laser experiments was made to circulate in test dye flow cell. In order to ensure temperature variation in extended time scale, the test cell was coupled to a large capacity SS reservoir with high pressure SS pump, flow regulating valves, micron sieve, magnetic flow meter ( + / -2% accuracy of full flow), gauge to measure pressure difference, compacted chiller unit 1.5 TR ( + / -0.1 0 C stability) and a temperature monitor ( + / -0.1 0 C accuracy).
A 2 mW He-Ne laser beam (632.8 nm) with about 2.25 m rad. divergence was made to fall on the test flow cell using appropriate natural density optical filters as attenuators at 180 0 location which is laser pump position. The He-Ne laser light beam coming out from test flow cell was arranged to fall on a CCD window (PCO Pixel Fly camera PCO AG). This camera has facility to select acquiring time and pixel size. A frame grabber interface was used to connect CCD to a computer. The beam passed on through the flowing medium resulted in intensity distribution and spatial shift which were captured in the computer. Each consecutive image of line profile of intensity distribution and spatial shift was automatically stored in a computer and compound images were analyzed. Image processing software which was developed in-house was used in this experiment [22]. This software automatically obtains the intensity profile with spatial shift and constitutes pictures of the data composed from all frames over a given time period. The greatest eddy turn over time for the given flow conditions was calculated and the CCD acquiring time was set accordingly. It was estimated that 150 frames each were adequate to analyze results [17].

Numerical scheme for evaluation of flow inhomogenieties and temperature inhomogenieties:
All numerical Fluid Structure Interaction (FSI) modeling packages use Reynolds Averaged Navier-Stokes (RANS) to resolve a flow field. Though these methods are economical, they can't convey the time-coherent pressure variations at the wall of the apparatus that are the driving mechanism of turbulence-induced vibration. For a fully developed turbulent flow in a closed channel the velocity distribution is closely approximated by seventh root low [23]. When the amplitude of flow induced 7 vibration increases the mean velocity profile in flow duct gets modified and is different from those approximated by seventh root low or obtained by solving RANS [24]. Large Eddy Simulation (LES) solves for a local-averaged velocity rather than a time-averaged velocity to produce the pressure variations resulting the flow vibrations. Therefore, coupling an LES-based fluid model with a structural model that provides the capability necessary to analyze the turbulent-induced phenomenon has been used. To record and capture the fringes a frame grabber card with CCD camera was used.
Additionally software that estimates variation of the bandwidth and wavelength from the record of fringes was used [22]. As may be seen from figure (3) Where

Comparison of flow induced parameters with dye laser output:
In a CVL the pulse to pulse divergence and hence energy per pulse does not change noticeably because of fixed distance between resonator mirrors and fixed discharge tube diameter. Thus the CVL power pumped to dye laser will not change from pulse to pulse. Therefore for the dye laser set up presented here, the dye laser output parameters will be solely dependent on the mechanical instability, fluctuations in velocity and dye solution temperature. These 3 parameters are directly linked with the flow induced vibrations in the dye flow cell.      Optical layout of dye laser system Experimental set up for comparing turbulence in dye laser test cell at different ow rates Single mode dye laser variation of wavelength and bandwidth with time.

Figure 5
Symbolic mechanical assembly of resonator with dye ow cell housed in it.

Figure 6
Turbulence pattern at laser pump position in test dye ow cell as intensity variation of He Ne beam transmitted through prototype test cell and radial shifts in peak positions of intensity for sum ow velocities and Reynolds number 1.33 m/s and 6.67m/s when test dye ow cell is not coupled to the resonator structure. Turbulence pattern at laser pump position in test dye ow cell as intensity variation of He Ne beam transmitted through prototype test cell and radial shifts in peak positions of intensity for sum ow velocities and Reynolds number 1.33 m/s and 6.67m/s when test dye ow cell is coupled to the resonator structure.   Variation of different parameters from test dye ow cell and comparison with dye laser output at different ow velocities at laser pump region.