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Single mode operation of a narrow bandwidth dye laser using a single prism, grazing incidence grating long cavity
ARTICLE IN PRESS
Optics & Laser Technology 39 (2007) 1140–1143 www.elsevier.com/locate/optlastec
Single mode operation of a narrow bandwidth dye laser using a single prism, grazing incidence grating long cavity Nageshwar Singh Laser Systems Engineering Division, Department of Atomic Energy, Raja Ramanna Centre for Advanced Technology, Indore 452013, MP, India Received 4 January 2006; received in revised form 2 September 2006; accepted 4 September 2006 Available online 16 October 2006
Abstract The single mode pulsed dye laser is an attractive tool for many spectroscopic applications. Long cavity tunable dye lasers generally operate in multi-longitudinal modes within the bandwidth of gain profile. Single longitudinal mode oscillation can be obtained by either making the cavity short enough or introducing an additional loss mechanism, in which all modes but one have a gain less than their loss. A new technique to achieve single mode operation in a long cavity dye laser, based on Rhodamine 6G dye in ethanol and ethylene glycol solution, pumped by a high repetition rate copper vapor laser, is reported. This laser, which operates in three modes in grazing incidence grating configuration (cavity length of 16 cm), has been made to lase in single mode by increasing the loss in the resonator through beam walk-off. r 2006 Elsevier Ltd. All rights reserved. Keywords: Single longitudinal mode; Dye laser; Beam walk-off
1. Introduction Single mode narrow bandwidth pulsed dye lasers are widely used in high-resolution optical spectroscopy [1] as well as in the efficient optical pumping of the excited species, where the spectral lines are very close to each other. Narrow bandwidth dye lasers have been realized in Littrow, multiple prism Littrow (MPL), grazing incidence grating (GIG), multiple prism grazing incidence grating (MPGIG), or hybrid multiple prism grazing incidence grating (HMPGIG) dispersive cavities [2], which usually operate with a number of axial modes. A number of approaches to demonstrate a tunable narrow band pulsed single mode dye laser have been reported, such as pulse amplification of cw single mode [3], external filtering of a multimode laser oscillator [4], a very short cavity oscillator incorporating frequency selective elements [5], etc. Various authors [5–10] have reported the single mode operation of pulsed dye lasers using different techniques such as the usage of intra-cavity etalons [11,12], modified interferometers [13], and short cavities [5], etc. Single mode operation Tel.: +91 731 2442448; fax: +91 731 2442400.
E-mail address: [email protected]. 0030-3992/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2006.09.004
by using an intra-cavity etalon can be achieved by varying the optical path length of the etalon (by adjusting its tilt or temperature) such that one of its transmission maxima coincides with the desired mode and no other transmission maximum falls in the gain bandwidth. The later condition requires that the free spectral range (FSR) of the etalon should be greater than half the gain bandwidth if the desired mode coincides with the center frequency, or in general, greater than the gain bandwidth. This requires precise selection of the FSR and finesse of the etalon to force the lasing in single mode. One way to ensure single mode operation is to use a short resonator length such that the axial spacing (c/2l) be greater than the gain bandwidth of the transition. But in practice it is very difficult to minimize the resonator length below a certain limit determined by the component sizes. It is a common practice to use a long cavity and to introduce additional optics (to suppress all modes except one below the loss line) to get single mode. Another approach to ensure single longitudinal mode operation is to increase the effective gain of the desired mode by injecting radiation seed at that mode so that it builds up faster and dominates. However, the power needed to lase a particular mode will increase rapidly as the desired mode moves farther from the line
ARTICLE IN PRESS N. Singh / Optics & Laser Technology 39 (2007) 1140–1143
center. Further, the injected radiation must be an allowed mode of the resonator, which requires a careful control of the resonator length. An important factor responsible for multimode or single mode is gain/loss line within the gain profile. Laser oscillation on a single mode can be achieved if the losses for all but the desired mode are increased to such an extent that they do not reach oscillation threshold. All these approaches have their own technological challenges. In this paper, a very simple and novel approach to obtain the single mode operation of a narrow bandwidth dye laser without using any additional optics or modifications in single prism GIG long cavity by introducing losses in the resonator through beam walk-off, is reported. 2. Experimental details Fig. 1 shows the schematic diagram of the dye laser oscillator. The dye laser is based on the Littman and Metcalf [14] and Duarte and Piper [15] scheme, which makes use of a single diffraction grating used in grazing incidence to provide wavelength selectivity. The construction and design of the dye laser was such that external factors like mechanical vibration do not significantly affect the output characteristics [16]. The cavity consists of an output coupler mirror (20% reflectivity), a dye cell, a single prism beam expander (magnification 8), and a grating (2400 lines/mm) in grazing incidence with a tuning mirror. The tuning mirror is rotated about an axis perpendicular to the plane of incidence to tune the dye laser. The overall cavity length was 16 cm. Rhodamine 6G (1.0 mM) dye in 30% ethanol and 70% ethylene glycol was flown through the dye cell. The flow was so rapid that the dye molecules irradiated by a pump laser pulse are replaced 2–3 times before arrival of the next pumping pulse. This was done using a dye circulation system. The Reynolds number of the flow of the dye solution was 736. A copper vapor laser (l ¼ 510:6 nm, 5 W, 5.6 kHz, plane parallel resonator) was used as pump source. The pump beam was transversely line focused onto the dye cell through a cylindrical lens of 6 cm focal length. The longitudinal modes of the output of the dye laser were analyzed using a Fabry–Perot etalon-based
Fig. 1. The schematic diagram of the dye laser cavity.
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set-up as shown in Fig. 2. This figure shows the optical arrangement for producing the Fabry–Perot interference (FPI) fringes of the dye laser. A beam expander and a scatterer make the beam diverging and uniformly illuminate the etalon having 5 GHz FSR and a finesse 25. A plano-convex lens of 20 cm focal length was used to image the fringe pattern onto a CCD camera connected to a frame grabber card inside a computer. 3. Results and discussion The GIG configuration with prism beam expander gives a narrow bandwidth output. However, single axial mode operation requires a very short cavity length of only a few centimeters. In practice it is difficult to minimize the resonator length below a certain limit due to the size of the individual components. In a GIG and beam expander cavity, all axial modes of spacing (c/2l) with gain exceeding the loss will oscillate. By increasing the resonator losses, the number of axial modes for which gain exceeds losses can be reduced. This principle has been used from a long time in single mode operation of dye lasers. Shoshan et al. [17] have used a 100% reflecting mirror at one end of the cavity, grating in grazing incidence, and tuning mirror at the other end. The laser output was taken directly from the grating in the form of the zero-order beam and they have achieved narrow linewidth of 2.4 GHz. In this case, as the output was taken from the grating zeroorder, it contains a large fraction of amplified spontaneous emission (ASE). ASE has several undesirable effects on the performance of a narrow band pulsed dye laser [18] like reduction of the laser efficiency, restriction of the tuning range, and formation of a broad band spectral background superimposed on the narrow band laser output. Littman and Metcalf [14] have used partially reflecting output mirror, grating in grazing incidence with tuning mirror and have achieved linewidth of 1.25 GHz. Littman [19] has used another grating in Littrow configuration in place of tuning mirror and achieved a single mode linewidth of 750 MHz.
Fig. 2. Optical arrangement for producing and analyzing the FPI fringe pattern of the dye laser.
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N. Singh / Optics & Laser Technology 39 (2007) 1140–1143
Duarte and Piper [15] have used additional prism beam expander in configuration used by Littman and Metcalf [14] to reduce the bandwidth of 1 GHz with improved efficiency. Maruyama et al. [11] have used double prism beam expander in Littrow configuration with intra-cavity etalon of FSR 30 GHz and a finesse of around 20 and achieved a bandwidth of 62 MHz. Bernhardt and Rasmussen [12] have used intra-cavity etalon (FSR 20 GHz, finesse 13) in addition to a multiple prism beam expander (magnification 40) in Littrow configuration to force the oscillations into a single mode and have achieved 60 MHz linewidth. In all these cases, narrow bandwidth of dye laser has been achieved by effectively increasing the dispersion in the cavity and single mode by increasing the losses for all except one mode. Apart from these, adjustment of gain/ loss by reducing the pump beam power near the threshold bring the oscillation in single mode. In such conditions instability of dye laser increases and require careful design of resonator structure (passive stabilization) and an active control of resonator length (active stabilization). Hung and Brechignac [20] have reported a novel simple cavity for grazing incidence pulsed dye laser in which the tuning mirror is rotated from Littman’s working position so that the beam diffracted at an angle is reflected back to the grating at the Littrow incidence angle. This modification is equivalent to the use of the second grating in Littman’s double grating design. Thus, single mode operation with a spectral bandwidth of 385 MHz was achieved using only one dispersive element (grating) in the linear cavity. In the same cavity, multimode operation was shown with improved efficiency by adjusting the position of tuning mirror. However, in this technique the tuning mirror is rotated about the same axis for single mode operation and tuning of the dye laser. Hence adjustment of angle of tuning mirror for single mode operation may also result in tuning of wavelength of the dye laser.In the present set-up the output was taken from the 20% reflectivity output coupler mirror and hence ASE is minimized. The dye laser (shown in Fig. 1) operates in three axial modes successfully and the more details about its performance have been reported elsewhere [16]. Fig. 3 shows the FPI fringe pattern of three axial modes of the dye laser. In normal operation of the dye laser, without any additional loss in the resonator, the observed separation of the axial modes is 990 MHz, close to the value of 937 MHz estimated from the resonator length. The number of axial modes was reduced by introducing losses in the cavity through rotating the tuning mirror about an axis parallel to the plane of incidence of diffracted beam from the grating. Fig. 4 shows the FPI pattern of two axial modes of the dye laser. Fig. 5 shows the FPI pattern of single mode of the dye laser. When the tuning mirror is aligned in such a way that the first-order diffracted beam from grating is incident normally on the tuning mirror, three axial modes lase. It is observed that the rotation of the tuning mirror about an axis parallel to the plane of incidence, rather than perpendicular, discriminates the
Fig. 3. Fabry–Perot interference pattern of three axial modes of dye laser.
Fig. 4. Fabry–Perot interference pattern of two axial modes of dye laser.
number of modes. The fraction of the reflected beam from the tuning mirror is walk away from the initial trajectory; hence beam walk-off takes place to increase the losses in the resonator. Fig. 6 shows the schematic of the rotated positions and direction of the tuning mirror. Dye lasers have different horizontal and vertical divergence and hence the beam size is elliptical. The maximum angle of rotation of tuning mirror in one direction depends on the beam size perpendicular to the plane of incidence and distance of tuning mirror from the grating. If d is the distance between grating and tuning mirror and w is the beam size (along minor axis), then the maximum angle of rotation y in either direction is y w=2 d. The angles of rotation y are very small and require precise control on the rotation to discriminate the number of modes. Precise rotation in either direction resulted in two axial modes (as shown in Fig. 4). Further rotation forces the lasing in single mode (as shown in Fig. 5). Once the tuning mirror is aligned in such a way that single mode oscillates, then the degree of freedom of
ARTICLE IN PRESS N. Singh / Optics & Laser Technology 39 (2007) 1140–1143
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any additional optics, can be made to operates in three, two and single axial mode just by controlling the rotation of tuning mirror along an axis parallel to the plane of incidence of the diffracted beam from the grating. The fraction of the reflected beam from the tuning mirror is walk away from the initial trajectory, to increase the losses in the resonator, hence beam walk-off takes place. This novel technique gives single mode of 360 MHz linewidth from a dye laser which normally (i.e. in symmetrically aligned condition) operating in three axial modes. As this configuration employs minimum number of optical components, hence misalignment and instability due to vibrations are greatly reduced. 4. Conclusions Fig. 5. Fabry–Perot interference pattern of single mode of dye laser.
In conclusion, a cavity with single prism beam expander and grating in grazing incidence with tuning mirror operating in three axial modes is shown to work in double and single mode operation, without using any additional optics, simply by rotating the tuning mirror about an axis parallel to the plane of incidence. This new technique is very simple to use and reduces the number of cavity modes. References
Fig. 6. Schematic of rotated positions and direction of tuning mirror.
rotation, about the axis parallel to the plane of incidence, is locked. The tuning of dye laser is achieved by rotating the tuning mirror about an axis perpendicular to the plane of incidence (as shown in Fig. 6). The precise rotation about this axis provides the tuning of dye laser. Hence, the tilting of tuning mirror for single mode oscillation does not affect the tuning of dye laser. Therefore, the position and rotation of tuning mirror plays significant role on the performance of the dye laser. As a result, the same cavity, without using
[1] Cagnac B, Grynberg G, Biraben F. J Phys 1973;34:845. [2] Duarte FJ, Hillman LW. Dye Laser Principle. New York: Academic Press; 1990. [3] Trehin F, Biraben F, Cagnac B, Grynberg G. Opt Commun 1979;31:76. [4] Rebhan U, Hildebrandt J. Opt Commun 1979;31:69. [5] Koprinkov IG, Stamenov KV, Stankov KA. Opt Commun 1982;42:264. [6] Saikan S. Appl Phys 1978;17:41. [7] Dinev SG, Koprinkov IG, Stamenov KV, Stankov KA. Appl Phys 1980;22:287. [8] Duarte FJ, Conrad RW. Appl Optics 1986;25:663. [9] McKinnie IT, Ahmad HB, Berry AJ, King TA. J Phys D: Appl Phys 1992;25:1687. [10] Schro¨der HW, Welling H, Wellegehausen B. Appl Phys 1973;1:343. [11] Maruyama Y, Kato M, Sugiyama A, Arisawa T. Opt Commun 1991;81:67. [12] Bernhardt AF, Rasmussen P. Appl Phys B 1981;26:141. [13] Pinard M, Leduc M, Trenec G, Minoff CGA, Laloe¨ F. Appl Phys 1979;19:399. [14] Littman MG, Metcalf HJ. Appl Opt 1978;17:2224. [15] Duarte FJ, Piper JA. Appl Opt 1981;20:2113. [16] Bhatnagar R, Singh N, Chaube R, Vora HS. Rev Sci Instrum 2004;75:5126. [17] Shoshan I, Danon NN, Oppenheim UP. J Appl Phys 1977;48:4495. [18] Nair LG, Dasgupta K. IEEE J Quantum Electronics 1985;21:1782. [19] Littman MG. Opt Lett 1978;3:138. [20] Hung ND, Brechignac Ph. Opt Commun 1985;54:151.
Optics & Laser Technology 39 (2007) 1140–1143 www.elsevier.com/locate/optlastec
Single mode operation of a narrow bandwidth dye laser using a single prism, grazing incidence grating long cavity Nageshwar Singh Laser Systems Engineering Division, Department of Atomic Energy, Raja Ramanna Centre for Advanced Technology, Indore 452013, MP, India Received 4 January 2006; received in revised form 2 September 2006; accepted 4 September 2006 Available online 16 October 2006
Abstract The single mode pulsed dye laser is an attractive tool for many spectroscopic applications. Long cavity tunable dye lasers generally operate in multi-longitudinal modes within the bandwidth of gain profile. Single longitudinal mode oscillation can be obtained by either making the cavity short enough or introducing an additional loss mechanism, in which all modes but one have a gain less than their loss. A new technique to achieve single mode operation in a long cavity dye laser, based on Rhodamine 6G dye in ethanol and ethylene glycol solution, pumped by a high repetition rate copper vapor laser, is reported. This laser, which operates in three modes in grazing incidence grating configuration (cavity length of 16 cm), has been made to lase in single mode by increasing the loss in the resonator through beam walk-off. r 2006 Elsevier Ltd. All rights reserved. Keywords: Single longitudinal mode; Dye laser; Beam walk-off
1. Introduction Single mode narrow bandwidth pulsed dye lasers are widely used in high-resolution optical spectroscopy [1] as well as in the efficient optical pumping of the excited species, where the spectral lines are very close to each other. Narrow bandwidth dye lasers have been realized in Littrow, multiple prism Littrow (MPL), grazing incidence grating (GIG), multiple prism grazing incidence grating (MPGIG), or hybrid multiple prism grazing incidence grating (HMPGIG) dispersive cavities [2], which usually operate with a number of axial modes. A number of approaches to demonstrate a tunable narrow band pulsed single mode dye laser have been reported, such as pulse amplification of cw single mode [3], external filtering of a multimode laser oscillator [4], a very short cavity oscillator incorporating frequency selective elements [5], etc. Various authors [5–10] have reported the single mode operation of pulsed dye lasers using different techniques such as the usage of intra-cavity etalons [11,12], modified interferometers [13], and short cavities [5], etc. Single mode operation Tel.: +91 731 2442448; fax: +91 731 2442400.
E-mail address: [email protected]. 0030-3992/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2006.09.004
by using an intra-cavity etalon can be achieved by varying the optical path length of the etalon (by adjusting its tilt or temperature) such that one of its transmission maxima coincides with the desired mode and no other transmission maximum falls in the gain bandwidth. The later condition requires that the free spectral range (FSR) of the etalon should be greater than half the gain bandwidth if the desired mode coincides with the center frequency, or in general, greater than the gain bandwidth. This requires precise selection of the FSR and finesse of the etalon to force the lasing in single mode. One way to ensure single mode operation is to use a short resonator length such that the axial spacing (c/2l) be greater than the gain bandwidth of the transition. But in practice it is very difficult to minimize the resonator length below a certain limit determined by the component sizes. It is a common practice to use a long cavity and to introduce additional optics (to suppress all modes except one below the loss line) to get single mode. Another approach to ensure single longitudinal mode operation is to increase the effective gain of the desired mode by injecting radiation seed at that mode so that it builds up faster and dominates. However, the power needed to lase a particular mode will increase rapidly as the desired mode moves farther from the line
ARTICLE IN PRESS N. Singh / Optics & Laser Technology 39 (2007) 1140–1143
center. Further, the injected radiation must be an allowed mode of the resonator, which requires a careful control of the resonator length. An important factor responsible for multimode or single mode is gain/loss line within the gain profile. Laser oscillation on a single mode can be achieved if the losses for all but the desired mode are increased to such an extent that they do not reach oscillation threshold. All these approaches have their own technological challenges. In this paper, a very simple and novel approach to obtain the single mode operation of a narrow bandwidth dye laser without using any additional optics or modifications in single prism GIG long cavity by introducing losses in the resonator through beam walk-off, is reported. 2. Experimental details Fig. 1 shows the schematic diagram of the dye laser oscillator. The dye laser is based on the Littman and Metcalf [14] and Duarte and Piper [15] scheme, which makes use of a single diffraction grating used in grazing incidence to provide wavelength selectivity. The construction and design of the dye laser was such that external factors like mechanical vibration do not significantly affect the output characteristics [16]. The cavity consists of an output coupler mirror (20% reflectivity), a dye cell, a single prism beam expander (magnification 8), and a grating (2400 lines/mm) in grazing incidence with a tuning mirror. The tuning mirror is rotated about an axis perpendicular to the plane of incidence to tune the dye laser. The overall cavity length was 16 cm. Rhodamine 6G (1.0 mM) dye in 30% ethanol and 70% ethylene glycol was flown through the dye cell. The flow was so rapid that the dye molecules irradiated by a pump laser pulse are replaced 2–3 times before arrival of the next pumping pulse. This was done using a dye circulation system. The Reynolds number of the flow of the dye solution was 736. A copper vapor laser (l ¼ 510:6 nm, 5 W, 5.6 kHz, plane parallel resonator) was used as pump source. The pump beam was transversely line focused onto the dye cell through a cylindrical lens of 6 cm focal length. The longitudinal modes of the output of the dye laser were analyzed using a Fabry–Perot etalon-based
Fig. 1. The schematic diagram of the dye laser cavity.
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set-up as shown in Fig. 2. This figure shows the optical arrangement for producing the Fabry–Perot interference (FPI) fringes of the dye laser. A beam expander and a scatterer make the beam diverging and uniformly illuminate the etalon having 5 GHz FSR and a finesse 25. A plano-convex lens of 20 cm focal length was used to image the fringe pattern onto a CCD camera connected to a frame grabber card inside a computer. 3. Results and discussion The GIG configuration with prism beam expander gives a narrow bandwidth output. However, single axial mode operation requires a very short cavity length of only a few centimeters. In practice it is difficult to minimize the resonator length below a certain limit due to the size of the individual components. In a GIG and beam expander cavity, all axial modes of spacing (c/2l) with gain exceeding the loss will oscillate. By increasing the resonator losses, the number of axial modes for which gain exceeds losses can be reduced. This principle has been used from a long time in single mode operation of dye lasers. Shoshan et al. [17] have used a 100% reflecting mirror at one end of the cavity, grating in grazing incidence, and tuning mirror at the other end. The laser output was taken directly from the grating in the form of the zero-order beam and they have achieved narrow linewidth of 2.4 GHz. In this case, as the output was taken from the grating zeroorder, it contains a large fraction of amplified spontaneous emission (ASE). ASE has several undesirable effects on the performance of a narrow band pulsed dye laser [18] like reduction of the laser efficiency, restriction of the tuning range, and formation of a broad band spectral background superimposed on the narrow band laser output. Littman and Metcalf [14] have used partially reflecting output mirror, grating in grazing incidence with tuning mirror and have achieved linewidth of 1.25 GHz. Littman [19] has used another grating in Littrow configuration in place of tuning mirror and achieved a single mode linewidth of 750 MHz.
Fig. 2. Optical arrangement for producing and analyzing the FPI fringe pattern of the dye laser.
ARTICLE IN PRESS 1142
N. Singh / Optics & Laser Technology 39 (2007) 1140–1143
Duarte and Piper [15] have used additional prism beam expander in configuration used by Littman and Metcalf [14] to reduce the bandwidth of 1 GHz with improved efficiency. Maruyama et al. [11] have used double prism beam expander in Littrow configuration with intra-cavity etalon of FSR 30 GHz and a finesse of around 20 and achieved a bandwidth of 62 MHz. Bernhardt and Rasmussen [12] have used intra-cavity etalon (FSR 20 GHz, finesse 13) in addition to a multiple prism beam expander (magnification 40) in Littrow configuration to force the oscillations into a single mode and have achieved 60 MHz linewidth. In all these cases, narrow bandwidth of dye laser has been achieved by effectively increasing the dispersion in the cavity and single mode by increasing the losses for all except one mode. Apart from these, adjustment of gain/ loss by reducing the pump beam power near the threshold bring the oscillation in single mode. In such conditions instability of dye laser increases and require careful design of resonator structure (passive stabilization) and an active control of resonator length (active stabilization). Hung and Brechignac [20] have reported a novel simple cavity for grazing incidence pulsed dye laser in which the tuning mirror is rotated from Littman’s working position so that the beam diffracted at an angle is reflected back to the grating at the Littrow incidence angle. This modification is equivalent to the use of the second grating in Littman’s double grating design. Thus, single mode operation with a spectral bandwidth of 385 MHz was achieved using only one dispersive element (grating) in the linear cavity. In the same cavity, multimode operation was shown with improved efficiency by adjusting the position of tuning mirror. However, in this technique the tuning mirror is rotated about the same axis for single mode operation and tuning of the dye laser. Hence adjustment of angle of tuning mirror for single mode operation may also result in tuning of wavelength of the dye laser.In the present set-up the output was taken from the 20% reflectivity output coupler mirror and hence ASE is minimized. The dye laser (shown in Fig. 1) operates in three axial modes successfully and the more details about its performance have been reported elsewhere [16]. Fig. 3 shows the FPI fringe pattern of three axial modes of the dye laser. In normal operation of the dye laser, without any additional loss in the resonator, the observed separation of the axial modes is 990 MHz, close to the value of 937 MHz estimated from the resonator length. The number of axial modes was reduced by introducing losses in the cavity through rotating the tuning mirror about an axis parallel to the plane of incidence of diffracted beam from the grating. Fig. 4 shows the FPI pattern of two axial modes of the dye laser. Fig. 5 shows the FPI pattern of single mode of the dye laser. When the tuning mirror is aligned in such a way that the first-order diffracted beam from grating is incident normally on the tuning mirror, three axial modes lase. It is observed that the rotation of the tuning mirror about an axis parallel to the plane of incidence, rather than perpendicular, discriminates the
Fig. 3. Fabry–Perot interference pattern of three axial modes of dye laser.
Fig. 4. Fabry–Perot interference pattern of two axial modes of dye laser.
number of modes. The fraction of the reflected beam from the tuning mirror is walk away from the initial trajectory; hence beam walk-off takes place to increase the losses in the resonator. Fig. 6 shows the schematic of the rotated positions and direction of the tuning mirror. Dye lasers have different horizontal and vertical divergence and hence the beam size is elliptical. The maximum angle of rotation of tuning mirror in one direction depends on the beam size perpendicular to the plane of incidence and distance of tuning mirror from the grating. If d is the distance between grating and tuning mirror and w is the beam size (along minor axis), then the maximum angle of rotation y in either direction is y w=2 d. The angles of rotation y are very small and require precise control on the rotation to discriminate the number of modes. Precise rotation in either direction resulted in two axial modes (as shown in Fig. 4). Further rotation forces the lasing in single mode (as shown in Fig. 5). Once the tuning mirror is aligned in such a way that single mode oscillates, then the degree of freedom of
ARTICLE IN PRESS N. Singh / Optics & Laser Technology 39 (2007) 1140–1143
1143
any additional optics, can be made to operates in three, two and single axial mode just by controlling the rotation of tuning mirror along an axis parallel to the plane of incidence of the diffracted beam from the grating. The fraction of the reflected beam from the tuning mirror is walk away from the initial trajectory, to increase the losses in the resonator, hence beam walk-off takes place. This novel technique gives single mode of 360 MHz linewidth from a dye laser which normally (i.e. in symmetrically aligned condition) operating in three axial modes. As this configuration employs minimum number of optical components, hence misalignment and instability due to vibrations are greatly reduced. 4. Conclusions Fig. 5. Fabry–Perot interference pattern of single mode of dye laser.
In conclusion, a cavity with single prism beam expander and grating in grazing incidence with tuning mirror operating in three axial modes is shown to work in double and single mode operation, without using any additional optics, simply by rotating the tuning mirror about an axis parallel to the plane of incidence. This new technique is very simple to use and reduces the number of cavity modes. References
Fig. 6. Schematic of rotated positions and direction of tuning mirror.
rotation, about the axis parallel to the plane of incidence, is locked. The tuning of dye laser is achieved by rotating the tuning mirror about an axis perpendicular to the plane of incidence (as shown in Fig. 6). The precise rotation about this axis provides the tuning of dye laser. Hence, the tilting of tuning mirror for single mode oscillation does not affect the tuning of dye laser. Therefore, the position and rotation of tuning mirror plays significant role on the performance of the dye laser. As a result, the same cavity, without using
[1] Cagnac B, Grynberg G, Biraben F. J Phys 1973;34:845. [2] Duarte FJ, Hillman LW. Dye Laser Principle. New York: Academic Press; 1990. [3] Trehin F, Biraben F, Cagnac B, Grynberg G. Opt Commun 1979;31:76. [4] Rebhan U, Hildebrandt J. Opt Commun 1979;31:69. [5] Koprinkov IG, Stamenov KV, Stankov KA. Opt Commun 1982;42:264. [6] Saikan S. Appl Phys 1978;17:41. [7] Dinev SG, Koprinkov IG, Stamenov KV, Stankov KA. Appl Phys 1980;22:287. [8] Duarte FJ, Conrad RW. Appl Optics 1986;25:663. [9] McKinnie IT, Ahmad HB, Berry AJ, King TA. J Phys D: Appl Phys 1992;25:1687. [10] Schro¨der HW, Welling H, Wellegehausen B. Appl Phys 1973;1:343. [11] Maruyama Y, Kato M, Sugiyama A, Arisawa T. Opt Commun 1991;81:67. [12] Bernhardt AF, Rasmussen P. Appl Phys B 1981;26:141. [13] Pinard M, Leduc M, Trenec G, Minoff CGA, Laloe¨ F. Appl Phys 1979;19:399. [14] Littman MG, Metcalf HJ. Appl Opt 1978;17:2224. [15] Duarte FJ, Piper JA. Appl Opt 1981;20:2113. [16] Bhatnagar R, Singh N, Chaube R, Vora HS. Rev Sci Instrum 2004;75:5126. [17] Shoshan I, Danon NN, Oppenheim UP. J Appl Phys 1977;48:4495. [18] Nair LG, Dasgupta K. IEEE J Quantum Electronics 1985;21:1782. [19] Littman MG. Opt Lett 1978;3:138. [20] Hung ND, Brechignac Ph. Opt Commun 1985;54:151.