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A dye laser with a transverse-flow cuvette
A dye laser with a transverse-flow cuvette H. GERLACH,
H. SCHARF
A new type of transverse-flow dye laser is described which features a number of technical advantages; among these are a beam divergence of less than 6 mrad and stable operational characteristics even at 50 Hz. The low Fresnel number of the cavity results in low-order emission modes.
Since the time when stimulated emission in organic dyes first came into use, there have been numerous types of laser cuvettes, most of which were designed to meet specific needs.1-4 In principle, the flow direction of the active fluid may be chosen to be either parallel or transverse to the direction of the beam. If the laser geometry is small, the correspondingly small cross-section of the cuvette limits the flow of the active medium: a handicap that is particularly disturbing during repetitive pulsing (50 Hz) when the lasing fluid must be pumped rapidly to reduce schlieren. A transverse-flow cuvette with its larger crosssection is therefore preferable. This point is demonstrated by the following numerical example (in which identical active volumes are assumed). In a 1 in laser with a paralle&flow cuvette, the fluid must pass through a crosssection of 0.8 mmL, whereas in a transverse-flow cuvette the cross-section is 25 mm * . In longer lasers the ratio increases proportionally. This means that transverse flow becomes more favourable the longer the laser.
into the glass of the cap and separates the inflow and the outflow. The glass of the cap has a thickness of 1 mm, the inner width between the glass walls is 3 mm, and the tongue is 1 mm thick. This leaves a uniform clearing of 1 mm on each side between tongue and glass wall in which the fluid can circulate, giving an active zone that has a width of 1 mm. Two platelets of optical glass soldered to the cap, seal off the cuvette at both ends. Although the steel tongue does not touch the platelets, the quantity of fluid passing through this bypass is negligible if all parts are accurately manufactured.
Cuvette construction If a cross-sectional view of the new transverse-flow cuvette is considered: assuming the elliptical pumping light reflector to stand upright (ie with its larger diameter vertical), then the dye solution enters the cuvette from the top, turns around by 180” at the second focal line, and leaves the elliptical cylinder again at the top (Fig.1). The metallic part of the cuvette consists of two symmetrical halves joined by electron beam welding. In the example shown, the cavity of the cuvette is 1 in long and 1 mm high. The upper part is made of stainless steel; a glass cap, cemented in place to seal in the fluid, forms the lower part. A steel tongue whose base is welded to the cavity extends down The authors are with Carl Zeiss, 7082 Received 7 March 1972.
Oberkochen,
Germany. Fig.1
Oblique
view of the transverse-flow
cuvette
OPTICS AND LASER TECHNOLOGY.
AUGUST
1973
Advantages of the new folded laser cuvette can be summarized as follows: 1. The plug-in design of the cuvette permits fast and clean exchange of the active laser fluid without disturbing the adjustment of the resonator mirrors. 2. The small cross-section of the active cuvette region allows a high dye concentration and thus results in a proportionally longer lifetime of the dye solution. 3. The relatively large cross-section of the cuvette requires only a small pump for the solution. 4. When inserted in the elliptical reflector, the cuvette may be adjusted so that the active region is located where the energy density of the light is greatest. 5. The two glass platelets at the ends of the cuvette may be replaced by Brewster angle windows or mirrors. The elliptical cylinder reflector in which the plug-in cuvette is inserted for measurements, is made of polished aluminium and has a major diameter a of 2.5 mm and a minor diameter of 43a/2. Both end plates of the reflector carry adjustable mirrors spaced 50 mm apart, one of which has 100% reflectivity, the other 81%. The reflector is filled with circulating water for cooling.
Results and discussion With the dye concentration optimized and with mirror reflectivities of 99.7% and 81% respectively, the laser
Fig.3
Laser pulse.
The time deflection
factor
is 0.5 MS per division
will yield a time-averaged output of 0.5 W in 50 Hz operation, given an input of 550 W (11 J per pulse) and a pulse width of 1.5 /.Ls. A centrifugal pump maintaining a flow velocity of 0.7 m s-l is sufficient for circulating the dye solution. Parallel-flow lasers of comparable dimensions require a flow velocity of 3.5 m s-l. In order to test the new laser head, the following measurements were made. Output power as a function of the input energy
A graph of output power as a function of the input energy is shown in Fig.2. The electrical arrangement consisted of an MP capacitor of C = 1 PF and L = 150 nH, a triggered spark gap, a flash lamp, and the power supply. The connexions were kept as short as possible in order to keep the circuit inductance to 150 nH and to achieve a minimum discharge time. The rise time of the pumping light was about 0.8 ps, while the pulse tail had the exact shape of an exponential function (1 /e after 6 ps).
6
The optimum dye concentration turned out to be 5.6 x 10e4 moll‘l. This agrees with values measured on parallel-flow dye lasers of similar dimensions. With the pumping energy constant, the output power decreased more rapidly when the dye concentration was lowered rather than raised. It is therefore possible to use concentrations higher than 5.6 x 1OA mol 1-l without incurring a strong reduction in output power. At 8 x IO4 mol l-l, the output power dropped by 5%, and at 10” molV1 it dropped by,20%. Shape of the pulses
I
2
3
4
5
6
Ein
IJI
Output power as a function Fig.2 parameter is the dye concentration
7
8
9
10
of the input energy.
OPTICS AND LASER TECHNOLOGY.
AUGUST
II
The
1973
Fig.3 shows the shape of a laser pulse, where the time deflection factor is 0.5 ps per division. The data given previously was obtained from one-shot measurements, and, as during 50 Hz operation, the pulses have a symmetrical appearance. With increasing pumping energy, the pulse will become asymmetrical: the rise continues at the same rate, while the drop rate is slowed down. The laser
163
pulse comes progressively pulse.
closer to matching
the pumping
Pulse widths range from 0.3 ps at the threshold (1.25 J), to 1.5 /ASwhen the maximum pumping energy of 11 J is applied. Bandwidth of the emission
Depending on the excitation energy, the transverseflow laser emits in a bandwidth range from 3-5 nm. The emissions consist of equidistant single lines at 1.2 nm intervals rather than being continuous. This narrowing is due to the construction of the cuvette. The two side windows of the cuvette in each case form a Fabry-Perot etalon having a difference in thickness of 0.1 mm, which agrees with the observed line interval. The line width was found to be less than 60 pm. By varying the thickness difference, it is possible to construct laser cuvettes with other line intervals. The line structure can be completely avoided by using wedgeshaped windows and resonator mirrors. Wavelength of the laser pulses The wavelength depends on several parameters (Fig.4) among them being the dye concentration, reflectivity of the resonator mirrors, and length of the cuvette. Using these parameters together with the cross-sections of a dye molecule for absorption and induced emission (all of which are measurable), and neglecting triplet losses, it is possible to formulate an equation that permits a rough estimate of the emission wavelength5 :
daA /dh ----(UA d(oA + os)/dh
+u+u*
1 = __
1 In --
2md
RF2
1 t In ___ 1-v
uA
- cross-section
of the molecule for absorption
os
- cross-section emission
of the molecule for induced
m
_ number of dye molecules cm3
d
- active length of the laser
R,,R,
- reflectivity
V
- losses such as those introduced
distribution of the values of refractive index will not develop symmetrically within the cuvette, and therefore the quality of the resonator can only decrease. Divergence of the beam
Even though the cuvette is very narrow, the laser beam can be expected to have a narrow beam divergence because of the relatively long resonator (made up of two planeparallel mirrors). We have measured the angular aperture of the laser beam as a function of the output power and dye concentration c. The full angle is 4.5 mrad at c = 10m4molll; it increases with concentration to 6 mrad at c = 5.6 x 10m4molll, and thereafter decreases again steadily to 5.3 mrad at c = 10e3 mol ll. With the concentration constant, the angle of divergence will remain constant within the accuracy of the measurement independent of the laser output. The modes
Because of the low Fresnel number, only low order modes can oscillate in the Fabry-Perot resonator 6 of the transverseflow laser. If it is assumed that the amplification of the active medium can compensate for diffraction losses up to 60%, the highest attainable mode would be TEM,,. Contrary to this assumption however, the cross-section of the cuvette is not circular. Referring to Fig. 1, the section of the cuvette viewed perpendicular to the direction (taken to be the z direction) of the laser beam has a clearance in the y direction of 1 mm (F = 6) between steel tongue and glass cap. The maximum width in the x direction on the other hand is 2 mm (F = 30). The larger Fresnel number means that transverse modes of a higher order can develop in the x direction. These conditions are clearly shown in Fig.5. The modes are without exception of the type TEM,u, since the type TEM, l already shows 9% loss per passage
of the cavity mirrors by schlieren and
diffraction. This equation is valid only at the threshold, although experimental results have shown that it may also be applied with sufficient accuracy in cases of higher pumping energies. The equation has been used to calculate the wavelength as a function of concentration with V = 0. Fig.4 shows the results.6 Because of such effects as schlieren and diffraction, real cuvettes have losses other than zero. This is why the measured curve is displaced with respect to the theoretical curve in the direction of the positive abscissa. The shapes of both curves are in good agreement. Resonator losses can be estimated from the displacement of the curve; they are on the average around 60%. Schlieren, resulting from increased light absorption mainly due to high dye concentration, are primarily responsible for the losses. Since the laser is not pumped symmetrically, the
164
‘-i h 4
E
[mol I’]
Fig.4 Dependence of the wavelength of the laser pulse on the dye concentration. The curve A = f(c) was theoretically calculated from the formula given in the text. Ein = 6.5 J, RI = 99.7%. R2 = 81 .O%
OPTICS AND LASER TECHNOLOGY.
AUGUST
1973
(at F = 6). Thus, all modes TEM,, (F = 30) whose losses are less than 9% will begin to oscillate. This will be the case up to YE= 9, and only after that can modes of the type TEMnl occur if the total of the diffraction losses is not too large. The mode patterns reflect the asymmetry of the cuvette. The intensity maxima are stretched according to the stronger diffraction in they direction. Setting the modes in Fig.5 was possible only through mirror adjustment. The diffraction losses form only a small part of the total, which mainly consist of refraction losses (schlieren). Because the latter do not work selectively relative to the modes, the emission mode is determined by the resonator geometry. Acknowledgements This work was partially supported by the Federal Ministry for Education and Science (Code NT 6 1). The Minister for Education and Science cannot vouch for the validity, accuracy, and completeness of the information contained in this article and will not be held responsible for the rights of third parties. We wish to thank Dr W. Schmidt for many stimulating discussions and the Hellma company of Mullheim for the precise soldering of the glass cuvettes. References
F ig.5
Sorokin, P. P., Lankard, J. R. IBM J of Research and Development 11 (1967) 148 Furumoto, H. W., Ceccon, H. L. IEEE J Quant Electr 6 (1970) 262 Christov, A. V., Kosiowskij, D. A., Cherkasov, A. S. Opt Spectry 29 (1970) 221 Boiteux, M., de Witte, 0. Appl Opt 9 (1970) 514 Schmidt, W. Laser 4 (1970) 47 Schmidt, W. private information Kleen-Miiller. Laser (Springer-Verlag 1969) 54
The mode patterns of the dye laser
OPTICS AND LASER TECHNOLOGY.
AUGUST
1973
165
H. SCHARF
A new type of transverse-flow dye laser is described which features a number of technical advantages; among these are a beam divergence of less than 6 mrad and stable operational characteristics even at 50 Hz. The low Fresnel number of the cavity results in low-order emission modes.
Since the time when stimulated emission in organic dyes first came into use, there have been numerous types of laser cuvettes, most of which were designed to meet specific needs.1-4 In principle, the flow direction of the active fluid may be chosen to be either parallel or transverse to the direction of the beam. If the laser geometry is small, the correspondingly small cross-section of the cuvette limits the flow of the active medium: a handicap that is particularly disturbing during repetitive pulsing (50 Hz) when the lasing fluid must be pumped rapidly to reduce schlieren. A transverse-flow cuvette with its larger crosssection is therefore preferable. This point is demonstrated by the following numerical example (in which identical active volumes are assumed). In a 1 in laser with a paralle&flow cuvette, the fluid must pass through a crosssection of 0.8 mmL, whereas in a transverse-flow cuvette the cross-section is 25 mm * . In longer lasers the ratio increases proportionally. This means that transverse flow becomes more favourable the longer the laser.
into the glass of the cap and separates the inflow and the outflow. The glass of the cap has a thickness of 1 mm, the inner width between the glass walls is 3 mm, and the tongue is 1 mm thick. This leaves a uniform clearing of 1 mm on each side between tongue and glass wall in which the fluid can circulate, giving an active zone that has a width of 1 mm. Two platelets of optical glass soldered to the cap, seal off the cuvette at both ends. Although the steel tongue does not touch the platelets, the quantity of fluid passing through this bypass is negligible if all parts are accurately manufactured.
Cuvette construction If a cross-sectional view of the new transverse-flow cuvette is considered: assuming the elliptical pumping light reflector to stand upright (ie with its larger diameter vertical), then the dye solution enters the cuvette from the top, turns around by 180” at the second focal line, and leaves the elliptical cylinder again at the top (Fig.1). The metallic part of the cuvette consists of two symmetrical halves joined by electron beam welding. In the example shown, the cavity of the cuvette is 1 in long and 1 mm high. The upper part is made of stainless steel; a glass cap, cemented in place to seal in the fluid, forms the lower part. A steel tongue whose base is welded to the cavity extends down The authors are with Carl Zeiss, 7082 Received 7 March 1972.
Oberkochen,
Germany. Fig.1
Oblique
view of the transverse-flow
cuvette
OPTICS AND LASER TECHNOLOGY.
AUGUST
1973
Advantages of the new folded laser cuvette can be summarized as follows: 1. The plug-in design of the cuvette permits fast and clean exchange of the active laser fluid without disturbing the adjustment of the resonator mirrors. 2. The small cross-section of the active cuvette region allows a high dye concentration and thus results in a proportionally longer lifetime of the dye solution. 3. The relatively large cross-section of the cuvette requires only a small pump for the solution. 4. When inserted in the elliptical reflector, the cuvette may be adjusted so that the active region is located where the energy density of the light is greatest. 5. The two glass platelets at the ends of the cuvette may be replaced by Brewster angle windows or mirrors. The elliptical cylinder reflector in which the plug-in cuvette is inserted for measurements, is made of polished aluminium and has a major diameter a of 2.5 mm and a minor diameter of 43a/2. Both end plates of the reflector carry adjustable mirrors spaced 50 mm apart, one of which has 100% reflectivity, the other 81%. The reflector is filled with circulating water for cooling.
Results and discussion With the dye concentration optimized and with mirror reflectivities of 99.7% and 81% respectively, the laser
Fig.3
Laser pulse.
The time deflection
factor
is 0.5 MS per division
will yield a time-averaged output of 0.5 W in 50 Hz operation, given an input of 550 W (11 J per pulse) and a pulse width of 1.5 /.Ls. A centrifugal pump maintaining a flow velocity of 0.7 m s-l is sufficient for circulating the dye solution. Parallel-flow lasers of comparable dimensions require a flow velocity of 3.5 m s-l. In order to test the new laser head, the following measurements were made. Output power as a function of the input energy
A graph of output power as a function of the input energy is shown in Fig.2. The electrical arrangement consisted of an MP capacitor of C = 1 PF and L = 150 nH, a triggered spark gap, a flash lamp, and the power supply. The connexions were kept as short as possible in order to keep the circuit inductance to 150 nH and to achieve a minimum discharge time. The rise time of the pumping light was about 0.8 ps, while the pulse tail had the exact shape of an exponential function (1 /e after 6 ps).
6
The optimum dye concentration turned out to be 5.6 x 10e4 moll‘l. This agrees with values measured on parallel-flow dye lasers of similar dimensions. With the pumping energy constant, the output power decreased more rapidly when the dye concentration was lowered rather than raised. It is therefore possible to use concentrations higher than 5.6 x 1OA mol 1-l without incurring a strong reduction in output power. At 8 x IO4 mol l-l, the output power dropped by 5%, and at 10” molV1 it dropped by,20%. Shape of the pulses
I
2
3
4
5
6
Ein
IJI
Output power as a function Fig.2 parameter is the dye concentration
7
8
9
10
of the input energy.
OPTICS AND LASER TECHNOLOGY.
AUGUST
II
The
1973
Fig.3 shows the shape of a laser pulse, where the time deflection factor is 0.5 ps per division. The data given previously was obtained from one-shot measurements, and, as during 50 Hz operation, the pulses have a symmetrical appearance. With increasing pumping energy, the pulse will become asymmetrical: the rise continues at the same rate, while the drop rate is slowed down. The laser
163
pulse comes progressively pulse.
closer to matching
the pumping
Pulse widths range from 0.3 ps at the threshold (1.25 J), to 1.5 /ASwhen the maximum pumping energy of 11 J is applied. Bandwidth of the emission
Depending on the excitation energy, the transverseflow laser emits in a bandwidth range from 3-5 nm. The emissions consist of equidistant single lines at 1.2 nm intervals rather than being continuous. This narrowing is due to the construction of the cuvette. The two side windows of the cuvette in each case form a Fabry-Perot etalon having a difference in thickness of 0.1 mm, which agrees with the observed line interval. The line width was found to be less than 60 pm. By varying the thickness difference, it is possible to construct laser cuvettes with other line intervals. The line structure can be completely avoided by using wedgeshaped windows and resonator mirrors. Wavelength of the laser pulses The wavelength depends on several parameters (Fig.4) among them being the dye concentration, reflectivity of the resonator mirrors, and length of the cuvette. Using these parameters together with the cross-sections of a dye molecule for absorption and induced emission (all of which are measurable), and neglecting triplet losses, it is possible to formulate an equation that permits a rough estimate of the emission wavelength5 :
daA /dh ----(UA d(oA + os)/dh
+u+u*
1 = __
1 In --
2md
RF2
1 t In ___ 1-v
uA
- cross-section
of the molecule for absorption
os
- cross-section emission
of the molecule for induced
m
_ number of dye molecules cm3
d
- active length of the laser
R,,R,
- reflectivity
V
- losses such as those introduced
distribution of the values of refractive index will not develop symmetrically within the cuvette, and therefore the quality of the resonator can only decrease. Divergence of the beam
Even though the cuvette is very narrow, the laser beam can be expected to have a narrow beam divergence because of the relatively long resonator (made up of two planeparallel mirrors). We have measured the angular aperture of the laser beam as a function of the output power and dye concentration c. The full angle is 4.5 mrad at c = 10m4molll; it increases with concentration to 6 mrad at c = 5.6 x 10m4molll, and thereafter decreases again steadily to 5.3 mrad at c = 10e3 mol ll. With the concentration constant, the angle of divergence will remain constant within the accuracy of the measurement independent of the laser output. The modes
Because of the low Fresnel number, only low order modes can oscillate in the Fabry-Perot resonator 6 of the transverseflow laser. If it is assumed that the amplification of the active medium can compensate for diffraction losses up to 60%, the highest attainable mode would be TEM,,. Contrary to this assumption however, the cross-section of the cuvette is not circular. Referring to Fig. 1, the section of the cuvette viewed perpendicular to the direction (taken to be the z direction) of the laser beam has a clearance in the y direction of 1 mm (F = 6) between steel tongue and glass cap. The maximum width in the x direction on the other hand is 2 mm (F = 30). The larger Fresnel number means that transverse modes of a higher order can develop in the x direction. These conditions are clearly shown in Fig.5. The modes are without exception of the type TEM,u, since the type TEM, l already shows 9% loss per passage
of the cavity mirrors by schlieren and
diffraction. This equation is valid only at the threshold, although experimental results have shown that it may also be applied with sufficient accuracy in cases of higher pumping energies. The equation has been used to calculate the wavelength as a function of concentration with V = 0. Fig.4 shows the results.6 Because of such effects as schlieren and diffraction, real cuvettes have losses other than zero. This is why the measured curve is displaced with respect to the theoretical curve in the direction of the positive abscissa. The shapes of both curves are in good agreement. Resonator losses can be estimated from the displacement of the curve; they are on the average around 60%. Schlieren, resulting from increased light absorption mainly due to high dye concentration, are primarily responsible for the losses. Since the laser is not pumped symmetrically, the
164
‘-i h 4
E
[mol I’]
Fig.4 Dependence of the wavelength of the laser pulse on the dye concentration. The curve A = f(c) was theoretically calculated from the formula given in the text. Ein = 6.5 J, RI = 99.7%. R2 = 81 .O%
OPTICS AND LASER TECHNOLOGY.
AUGUST
1973
(at F = 6). Thus, all modes TEM,, (F = 30) whose losses are less than 9% will begin to oscillate. This will be the case up to YE= 9, and only after that can modes of the type TEMnl occur if the total of the diffraction losses is not too large. The mode patterns reflect the asymmetry of the cuvette. The intensity maxima are stretched according to the stronger diffraction in they direction. Setting the modes in Fig.5 was possible only through mirror adjustment. The diffraction losses form only a small part of the total, which mainly consist of refraction losses (schlieren). Because the latter do not work selectively relative to the modes, the emission mode is determined by the resonator geometry. Acknowledgements This work was partially supported by the Federal Ministry for Education and Science (Code NT 6 1). The Minister for Education and Science cannot vouch for the validity, accuracy, and completeness of the information contained in this article and will not be held responsible for the rights of third parties. We wish to thank Dr W. Schmidt for many stimulating discussions and the Hellma company of Mullheim for the precise soldering of the glass cuvettes. References
F ig.5
Sorokin, P. P., Lankard, J. R. IBM J of Research and Development 11 (1967) 148 Furumoto, H. W., Ceccon, H. L. IEEE J Quant Electr 6 (1970) 262 Christov, A. V., Kosiowskij, D. A., Cherkasov, A. S. Opt Spectry 29 (1970) 221 Boiteux, M., de Witte, 0. Appl Opt 9 (1970) 514 Schmidt, W. Laser 4 (1970) 47 Schmidt, W. private information Kleen-Miiller. Laser (Springer-Verlag 1969) 54
The mode patterns of the dye laser
OPTICS AND LASER TECHNOLOGY.
AUGUST
1973
165