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A Nd:Glass sweep laser for spectroscopic research
Volume
12, number
3
OPTICS COMMUNICATIONS
A Nd:GLASS
November
SWEEP LASER FOR SPECTROSCOPIC V.I. KRAVCHENKO
19 74
RESEARCH
and S.P. ANOHOV
Institute of Physics. Ukrainian Academy’ of Sciences, Kiev, USSR
Received
6 August
1974
The results of investigations of a Nd:glass sweep-laser with a tunable dispersion shown that the sweep laser may be used as a source in high-speed laser spectrometry
The progress of modern laser spectroscopy is closely connected with the development of stimulated emission sources distinguished by stepless frequency tuning over a wide range. For express precise measurements lasers with dynamic frequency shifting during the generation process are promised. In the region of 21.1 and above a certain success has been reached by using semiconductor lasers, gas lasers, parametric generators as well as spin-flip lasers [l-5], but there are no laser spectrometers with satisfactory characteristics in the more short wave region. The latter is connected with the difficulty to control the dynamic oscillation spectrum of lasers available in this region. In the present paper a simple Nd:glass sweep laser based on the use of a tunable dispersion resonator is described [6] and some questions of the application of such sources in spectroscopic research are discussed. The optical scheme of the laser is shown in fig. 1. As an active medium we use a sample of silicate glass 300 mm in length and 15 mm in diameter doped with Nd3+ (1 weight % of the activator). The optical resonator is formed by a flat mirror (R ~0.9) and a Perot-prism of total internal reflection adjusted on the shaft of an electric motor with the rotation speed changed from II =0.4 to 100 revolutions per second. Four superheavy flint glass prisms provided a dispersion of dp/du = 14.5 angl.sec/cm-1 at X= 1.06~. Two slits of 1 mm width were introduced into the resonator for increasing the frequency selectivity. In this case the half width of the Q-quality frequency dependence amounted to 5 cm-l. All refractive faces of prisms. sample-ends and the input face of the Perot-prism were cut at the Brewster’s angle to the axis of the resonator. The oscillation frequency is determined by 248
Dispersion
resonator are presented. of high resolution.
It is
prism
Neodymium
glass
s,it
Output mirror 1
Rotated
prism
Fig. 1. Optical scheme of sweep-laser. the angle position of the Perot-prism while the relationship between the sweeping rate I’and the rotation velocity of this prism n is given by the simple expression T/-
2nn dp/du
Typical pictures of sweep-laser kinetics and oscillation spectra are shown in fig. 2. At i/ 5 5 cm-l/ysec oscillograms of the radiation intensity represent the ordered sequence of spikes which follow with an interval of 2-20 psec depending on the sweeping rate and the pumping level. As far as every spike radiates on the new frequency, the generation spectrum consists of a regular kit of narrow lines which come in consecutive order. Fig. 2 shows the essential dependence of a single line-width 6v, the spectral interval between neighbouring lines Av and the overall spectrum width S on the sweeping rate I’. The highest possible spectrum width during one flash, Smax, is limited by the radiation line width. If sweeping rates are low (fig.2b,c)
Volume 12, number 3
OPTICS COMMUNICATIONS
November 1974
(a) 200 psec __.t
(d)
(4
0.420
(0
0.960
(9)
2.040
I
100 cm-l
,
,v
I 9434 cm-l
Fig. 2. Oscilloscope trace of tynical pulse and output spectrum of the sweep-laser at various sweeping rates.
the magnitude of S is determined only by the pumping pulse duration At,: S - VAt,. The increase of the sweeping rate at first leads to an extension of the tuning range and then to its diminution because of increasing losses due to the shortening of the time formation 6f a mode at the tuning frequency [7]. We obtained S,, = 240 cm-l, at V = 0.5 cm-l/psec. Intervals between the lines in the sweep-laser spectrum as a function of the sweeping rate and the pump power are given in fig. 3. It is evident that the value Au may be easily changed in the limits 0.1-10 cm-l. The separate linewidth in this case also changes and the narrowest one is &urn, y 0.05 cm-l. We would like to emphasize that the spectral-kinetic characteristics of the neodymium-glass sweep laser are determin-
ed not only by processes of mode formation in the tunable resonator but also by the spectro&opic qualities of the active material, such as follows: the ratio between the homogeneous and the inhomogeneous spectral-line broadening, the rate of energy migration along the spectrum, etc. This allows one to define the parameters mentioned above by comparing oscillation spectra produced under controlled conditions. At V> 5 cm-l/E.csec the output characteristics of the sweep laser are qualitatively changed : instead of regular spikes, monopulses of a complicated form are radiated and wide quasi-continuous bands appear in the oscillation spectrum (fig. 4). This laser operation is connected with the arising of the Q-switching effect which is due to the increase of rotation speed of the 249
Volume
12, number
OPTICS COMMUNICATIONS
(a)
10
7
3
________------
t
0
Y
1
3
2
Sweeping speed (cm-‘/P& (b) z-
v = 0.06 cm-'iwx
I
i
4..
’ 1
2 ‘Pump energy/Thresholdenergy
3
Fig. 3. Variation of intervals between the lines of spectrum against the sweeping rate and the pump energy.
Perot-prism. At high sweeping rates the operating conditions are interesting from the physical point of view because they allow to experimentally investigate the time formation of an oscillation mode in the tunable resonator. In addition they allow to control the
November
duration of the generation pulse over the range 1~ 10 /*sec. For the purposes of high-speed laser spectroscopy the low sweeping rates are most preferable because in this case there is an unequivocal connection between the single intensity bumps and the generation frequencies. Due to such correlation any influence on the sweep-laser generation spectrum causes conformable changes in the time development of the radiation intensity. Thus in high-speed laser spectrometry methods the information about the frequency dependence of the absorption, transmission, gain factor and so on is obtained from a comparison of the incident spike amplitudes and of the transmitted ones. The optical resolution using this method is determined by the choice of the values of Au and 6v, and the recording time is not longer than the duration of the laser oscillation (- 10e3 see). Experiments shown in the case of the sweep laser in travelling-wave operation that the frequency shift from spike to spike is equal to the intermode spacing and the optical resolution of the laser spectrometry may reach 5 X 10m3 cm -l. The above described sweep laser steadily operates within the range from 1.04 ,u to 1 .lO ~1,and within the range from 0.525 /J to 0.547 p at frequency doubling. This is useful for solving different problems in the spectroscopy of condensed and gaseous media. In particular this laser was used to investigate the gain profiles of some crystals and glasses doped with Nd”+. In the case of Nd:glasses operated at fixed frequencies complex deformation of the gain profile of the transition
(a)
10 psec
,
IOOcm~’ I
Fig. 4. Oscilloscope
250
trace and output
1974
spectrum
9434 cm-l WV
at high sweeping
rate.
Volume
12, number
3
OPTICS
COMMUNICATIONS
4F 3,2-41,,,2 is discovered and investigated [8], as it is known that the above deformation is connected with a Stark structure of the single spectrum radiation centre [9]. For the first time three absorption maxima were discovered in the vibronic band of th,e absorption spectrum of crystalline o-oxygen [lo]. They have not been observed earlier because of the weak resolution and the small aperture ratio of the conventional spectral devices. A high output power - a characteristic of Nd:glass lasers - ensures the high spectral brightness of the above sweep laser. This important property may be useful for the spectroscopic zoning of samples placed far from the source of radiation.
November
1974
References [ 1] K.W. Nill, F.A. Blum, A.R. Calawa and T.C. Harman, Appl. Phys. Lett. 19 (1971) 79. [2] Yu.A. Bykovsky, V.L. Velichansky, I.G. Goncharov, V.A. Maslov and V.V. Nikitin, Optika i Spektrosk. 30 (1971) 508. [3] E.D. Hinkley, Opto-electron. 4 (1972) 69. [4] W. Lange, J. Luther and B. Nottbeck, Opt. Commun. 8 (1973) 157. [S] H.R. Schlossberg and P.L. Kelly, Electronics 46 (1973) 55. [6] V.I. Kravchenko, M.S. Soskin and V.V. Tarabrov, Zh. Eksper. Teor. Fiz., Pisma 5 (1967) 355. [7] S.P. Anohov, V.I. Kravchenko and M.S. Soskin, Ukr. Fiz. Zh. 15 (1970) 1342. [8] S.P. Anohov, V.I. Kravchenko and A.I. Khizniak, Rep. at the VI All-Union Conf. Nonlinear Optics, USSR, Minsk, 1972. [9] M.M. Mann and L.G. De Shazer, J. Appl. Phys. 41 (1970) 2951. [lo] S.P. Anohov, V.I. Kravchenko, A.F. Prikhotko, M.S. Soskin, A.S. Ulitskii and L.I. Shanskii, Phys. Stat. Sol. (a) 22 (1974) K121.
251
12, number
3
OPTICS COMMUNICATIONS
A Nd:GLASS
November
SWEEP LASER FOR SPECTROSCOPIC V.I. KRAVCHENKO
19 74
RESEARCH
and S.P. ANOHOV
Institute of Physics. Ukrainian Academy’ of Sciences, Kiev, USSR
Received
6 August
1974
The results of investigations of a Nd:glass sweep-laser with a tunable dispersion shown that the sweep laser may be used as a source in high-speed laser spectrometry
The progress of modern laser spectroscopy is closely connected with the development of stimulated emission sources distinguished by stepless frequency tuning over a wide range. For express precise measurements lasers with dynamic frequency shifting during the generation process are promised. In the region of 21.1 and above a certain success has been reached by using semiconductor lasers, gas lasers, parametric generators as well as spin-flip lasers [l-5], but there are no laser spectrometers with satisfactory characteristics in the more short wave region. The latter is connected with the difficulty to control the dynamic oscillation spectrum of lasers available in this region. In the present paper a simple Nd:glass sweep laser based on the use of a tunable dispersion resonator is described [6] and some questions of the application of such sources in spectroscopic research are discussed. The optical scheme of the laser is shown in fig. 1. As an active medium we use a sample of silicate glass 300 mm in length and 15 mm in diameter doped with Nd3+ (1 weight % of the activator). The optical resonator is formed by a flat mirror (R ~0.9) and a Perot-prism of total internal reflection adjusted on the shaft of an electric motor with the rotation speed changed from II =0.4 to 100 revolutions per second. Four superheavy flint glass prisms provided a dispersion of dp/du = 14.5 angl.sec/cm-1 at X= 1.06~. Two slits of 1 mm width were introduced into the resonator for increasing the frequency selectivity. In this case the half width of the Q-quality frequency dependence amounted to 5 cm-l. All refractive faces of prisms. sample-ends and the input face of the Perot-prism were cut at the Brewster’s angle to the axis of the resonator. The oscillation frequency is determined by 248
Dispersion
resonator are presented. of high resolution.
It is
prism
Neodymium
glass
s,it
Output mirror 1
Rotated
prism
Fig. 1. Optical scheme of sweep-laser. the angle position of the Perot-prism while the relationship between the sweeping rate I’and the rotation velocity of this prism n is given by the simple expression T/-
2nn dp/du
Typical pictures of sweep-laser kinetics and oscillation spectra are shown in fig. 2. At i/ 5 5 cm-l/ysec oscillograms of the radiation intensity represent the ordered sequence of spikes which follow with an interval of 2-20 psec depending on the sweeping rate and the pumping level. As far as every spike radiates on the new frequency, the generation spectrum consists of a regular kit of narrow lines which come in consecutive order. Fig. 2 shows the essential dependence of a single line-width 6v, the spectral interval between neighbouring lines Av and the overall spectrum width S on the sweeping rate I’. The highest possible spectrum width during one flash, Smax, is limited by the radiation line width. If sweeping rates are low (fig.2b,c)
Volume 12, number 3
OPTICS COMMUNICATIONS
November 1974
(a) 200 psec __.t
(d)
(4
0.420
(0
0.960
(9)
2.040
I
100 cm-l
,
,v
I 9434 cm-l
Fig. 2. Oscilloscope trace of tynical pulse and output spectrum of the sweep-laser at various sweeping rates.
the magnitude of S is determined only by the pumping pulse duration At,: S - VAt,. The increase of the sweeping rate at first leads to an extension of the tuning range and then to its diminution because of increasing losses due to the shortening of the time formation 6f a mode at the tuning frequency [7]. We obtained S,, = 240 cm-l, at V = 0.5 cm-l/psec. Intervals between the lines in the sweep-laser spectrum as a function of the sweeping rate and the pump power are given in fig. 3. It is evident that the value Au may be easily changed in the limits 0.1-10 cm-l. The separate linewidth in this case also changes and the narrowest one is &urn, y 0.05 cm-l. We would like to emphasize that the spectral-kinetic characteristics of the neodymium-glass sweep laser are determin-
ed not only by processes of mode formation in the tunable resonator but also by the spectro&opic qualities of the active material, such as follows: the ratio between the homogeneous and the inhomogeneous spectral-line broadening, the rate of energy migration along the spectrum, etc. This allows one to define the parameters mentioned above by comparing oscillation spectra produced under controlled conditions. At V> 5 cm-l/E.csec the output characteristics of the sweep laser are qualitatively changed : instead of regular spikes, monopulses of a complicated form are radiated and wide quasi-continuous bands appear in the oscillation spectrum (fig. 4). This laser operation is connected with the arising of the Q-switching effect which is due to the increase of rotation speed of the 249
Volume
12, number
OPTICS COMMUNICATIONS
(a)
10
7
3
________------
t
0
Y
1
3
2
Sweeping speed (cm-‘/P& (b) z-
v = 0.06 cm-'iwx
I
i
4..
’ 1
2 ‘Pump energy/Thresholdenergy
3
Fig. 3. Variation of intervals between the lines of spectrum against the sweeping rate and the pump energy.
Perot-prism. At high sweeping rates the operating conditions are interesting from the physical point of view because they allow to experimentally investigate the time formation of an oscillation mode in the tunable resonator. In addition they allow to control the
November
duration of the generation pulse over the range 1~ 10 /*sec. For the purposes of high-speed laser spectroscopy the low sweeping rates are most preferable because in this case there is an unequivocal connection between the single intensity bumps and the generation frequencies. Due to such correlation any influence on the sweep-laser generation spectrum causes conformable changes in the time development of the radiation intensity. Thus in high-speed laser spectrometry methods the information about the frequency dependence of the absorption, transmission, gain factor and so on is obtained from a comparison of the incident spike amplitudes and of the transmitted ones. The optical resolution using this method is determined by the choice of the values of Au and 6v, and the recording time is not longer than the duration of the laser oscillation (- 10e3 see). Experiments shown in the case of the sweep laser in travelling-wave operation that the frequency shift from spike to spike is equal to the intermode spacing and the optical resolution of the laser spectrometry may reach 5 X 10m3 cm -l. The above described sweep laser steadily operates within the range from 1.04 ,u to 1 .lO ~1,and within the range from 0.525 /J to 0.547 p at frequency doubling. This is useful for solving different problems in the spectroscopy of condensed and gaseous media. In particular this laser was used to investigate the gain profiles of some crystals and glasses doped with Nd”+. In the case of Nd:glasses operated at fixed frequencies complex deformation of the gain profile of the transition
(a)
10 psec
,
IOOcm~’ I
Fig. 4. Oscilloscope
250
trace and output
1974
spectrum
9434 cm-l WV
at high sweeping
rate.
Volume
12, number
3
OPTICS
COMMUNICATIONS
4F 3,2-41,,,2 is discovered and investigated [8], as it is known that the above deformation is connected with a Stark structure of the single spectrum radiation centre [9]. For the first time three absorption maxima were discovered in the vibronic band of th,e absorption spectrum of crystalline o-oxygen [lo]. They have not been observed earlier because of the weak resolution and the small aperture ratio of the conventional spectral devices. A high output power - a characteristic of Nd:glass lasers - ensures the high spectral brightness of the above sweep laser. This important property may be useful for the spectroscopic zoning of samples placed far from the source of radiation.
November
1974
References [ 1] K.W. Nill, F.A. Blum, A.R. Calawa and T.C. Harman, Appl. Phys. Lett. 19 (1971) 79. [2] Yu.A. Bykovsky, V.L. Velichansky, I.G. Goncharov, V.A. Maslov and V.V. Nikitin, Optika i Spektrosk. 30 (1971) 508. [3] E.D. Hinkley, Opto-electron. 4 (1972) 69. [4] W. Lange, J. Luther and B. Nottbeck, Opt. Commun. 8 (1973) 157. [S] H.R. Schlossberg and P.L. Kelly, Electronics 46 (1973) 55. [6] V.I. Kravchenko, M.S. Soskin and V.V. Tarabrov, Zh. Eksper. Teor. Fiz., Pisma 5 (1967) 355. [7] S.P. Anohov, V.I. Kravchenko and M.S. Soskin, Ukr. Fiz. Zh. 15 (1970) 1342. [8] S.P. Anohov, V.I. Kravchenko and A.I. Khizniak, Rep. at the VI All-Union Conf. Nonlinear Optics, USSR, Minsk, 1972. [9] M.M. Mann and L.G. De Shazer, J. Appl. Phys. 41 (1970) 2951. [lo] S.P. Anohov, V.I. Kravchenko, A.F. Prikhotko, M.S. Soskin, A.S. Ulitskii and L.I. Shanskii, Phys. Stat. Sol. (a) 22 (1974) K121.
251