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Simultaneous oscillation of several independently tunable spectral lines in a distributed feedback dye laser
Volume 20, number 1
OPTICS COMMUNICATIONS
January 1977
SIMULTANEOUS OSCILLATION OF SEVERAL INDEPENDENTLY TUNABLE SPECTRAL LINES IN A DISTRIBUTED FEEDBACK DYE LASER A.N. RUBINOV, T.Sh. EFENDIEV, A.V. ADAMUSHKO Institute of Physics o f Byelorussian Academy o f Sciences, Minsk, USSR and
J. BOR Institute of Experimental Physics, A ttila Jozsef University, Szeged, Hungary Received 3 November 1976
A simple method is proposed which allows obtaining distributed-feedback laser action simultaneously at several spectral lines of any wavelength with independent tuning within the gain profile of the dye.
As a rule in distributed-feedback (DFB) lasers one narrow line with the wavelength corresponding to the period of a spatial modulation of the medium is generated [1]. However, in [2] it was shown that under formation in a dye solution of the complex spatially varying gratings with some modulation periods, a simultaneous oscillation of two or more radiation frequencies might take place. This was done by pumping a dye with radiation composed of several discrete spectral components. In the present work a new simple technique for simultaneous generation in a monochromatically pumped DFB dye laser of two or more narrow spectral lines with a variable separation between them and independent tuning over the spectrum is developed. The idea o f the method is based on the strong dependence of the DFB laser wavelength on the incidence angle of the pumping beam and the possibility of the use of more than two pumping beams interfering in the active medium at different angles. In fig. 1 two pairs of coherent pumping beams (I' I" and I I ' - I I " ) incident upon the dye cell are shown as an example. After penetrating into the cell the four pumping beams form in the solution a complex system of standing waves. It is easy to see that six amplitude-phase gratings correspond to the system with the following periods:
, g'
t"
e"
~
r"
"T'-k-
t
z
i
Fig. 1. Setup of the pump beams trace.
Xp dl-
kp
2n s sin ot1 '
d2 - 2n s sin a 2 ' kp
d 3 =d 4 -
(1) 2n s sin ½ (a I + or2)' ~kp
d5 = d 6 2n s sin ½ (a 1 - a 2 ) ' 159
Volume 20, number 1
CYLINDRICAL LENS Sl2nm / PUMP
OPTICS COMMUNICATIONS
B,EAMSPLITTER5
January 1977
DYECELL
O0T~O T~//
0
L IqHT
SPHERICAL LENS
Fig. 2. Schematic diagram of the experimental setup. The pump light is polarized normally to the plane of incidence.
where Xp = pumping wavelength (in vacuo), and'n s = refractive index of the dye solution. It is easy to see from formulas (1) that if the incident angles are chosen properly, a simultaneous laser action at two independent wavelengths, corresponding to the periods d 1 and d 2, will occur in the direction along the liquid-glass interface. At the same time the intermediate wavelength corresponding to the interference period d3, d 4 should be observed under angles to the interface -+{ (a 1 - a2). If one of the four pumping beams is shut off a laser action should be observed only at two lines; each line could be tuned separately by varying the appropriate angle of interference. The above considerations were checked experimentally. The experimental setup is given in fig. 2. As is seen, the pumping beam is split here into two beams. The angle between them can be changed by rotation of the mirror M, and in addition the prism P can also be rotated. The 300 kW 2 d harmonic of a YAG:Nd 3+ laser was used for pumping. The dye used was 3 × 10 - 3 M Rhodamine 6G in ethanol. The experiments were carried out at a pulse repetition rate of 25 pps. The spectra of the DFB dye laser under separate excitation by the pumping beams I and |I are represented in fig. 3a, b. It is seen that a certain emission line corresponds to each of the pumping beams. In the case of simultaneous excitation of the solution by beams I and II both lines XI and ~kli are observed and in addition the third line XlI I between them appears (fig. 3c). The last one is caused by the interference of the pumping beams I and II. The emission at the wavelength XI and XII was gen160
a)
b)
ii
c)
d) Fig. 3. Spectra of the DFB dye laser under separate (a, b) and simultaneous (c, d) excitation by the pumping beams I and II. erated along the solution-prism interface, while the emission at the intermediate wavelength )tii 1 was propagated at the angle +-71 (a 1 - a2) to it, accurately corresponding to the calculated value. When varying a spectral distance between lines XI and ~kli from some to hundreds Angstr6ms this angle varies from several rranutes to several degrees. If one of the beams (I or II) falls only on one of the side surfaces of the prism P, then three pumping beams rather than four reach the solution-prism interface. In this case only two lines are observed in the laser spectrum (fig. 3d). Fig. 4a demonstrates the case when varying the intersection angle of the beams I and II by means of rotation of the mirror M, tuning of two emission lines with an unchangeable third one was observed. In fig. 4b a simultaneous tuning of the three emission
Volume 20, number 1
OPTICS COMMUNICATIONS
January 1977
b i
t
o
(b)
Ca)
l:ig. 4. Independent (a) and simultaneous (b) tuning of spectral lines of the DFB dye laser.
lines, achieved by rotation o f the prism P around its axis is shown. Thus a simple method is proposed which allows obtaining a DFB laser action simultaneously at several spectral lines of any wavelength with independent tuning within the gain profile of the dye.
References [1] C.V. Shank, J.F.. Bjorkholm and H. Kogelnik, Appl. Phys. Letters 18 (1971) 395. [2] T.Sh. Efendiev and A.N. Rubinov, Sov. J. Quantum Electron. 2 (1975) 858 (in Russian).
161
OPTICS COMMUNICATIONS
January 1977
SIMULTANEOUS OSCILLATION OF SEVERAL INDEPENDENTLY TUNABLE SPECTRAL LINES IN A DISTRIBUTED FEEDBACK DYE LASER A.N. RUBINOV, T.Sh. EFENDIEV, A.V. ADAMUSHKO Institute of Physics o f Byelorussian Academy o f Sciences, Minsk, USSR and
J. BOR Institute of Experimental Physics, A ttila Jozsef University, Szeged, Hungary Received 3 November 1976
A simple method is proposed which allows obtaining distributed-feedback laser action simultaneously at several spectral lines of any wavelength with independent tuning within the gain profile of the dye.
As a rule in distributed-feedback (DFB) lasers one narrow line with the wavelength corresponding to the period of a spatial modulation of the medium is generated [1]. However, in [2] it was shown that under formation in a dye solution of the complex spatially varying gratings with some modulation periods, a simultaneous oscillation of two or more radiation frequencies might take place. This was done by pumping a dye with radiation composed of several discrete spectral components. In the present work a new simple technique for simultaneous generation in a monochromatically pumped DFB dye laser of two or more narrow spectral lines with a variable separation between them and independent tuning over the spectrum is developed. The idea o f the method is based on the strong dependence of the DFB laser wavelength on the incidence angle of the pumping beam and the possibility of the use of more than two pumping beams interfering in the active medium at different angles. In fig. 1 two pairs of coherent pumping beams (I' I" and I I ' - I I " ) incident upon the dye cell are shown as an example. After penetrating into the cell the four pumping beams form in the solution a complex system of standing waves. It is easy to see that six amplitude-phase gratings correspond to the system with the following periods:
, g'
t"
e"
~
r"
"T'-k-
t
z
i
Fig. 1. Setup of the pump beams trace.
Xp dl-
kp
2n s sin ot1 '
d2 - 2n s sin a 2 ' kp
d 3 =d 4 -
(1) 2n s sin ½ (a I + or2)' ~kp
d5 = d 6 2n s sin ½ (a 1 - a 2 ) ' 159
Volume 20, number 1
CYLINDRICAL LENS Sl2nm / PUMP
OPTICS COMMUNICATIONS
B,EAMSPLITTER5
January 1977
DYECELL
O0T~O T~//
0
L IqHT
SPHERICAL LENS
Fig. 2. Schematic diagram of the experimental setup. The pump light is polarized normally to the plane of incidence.
where Xp = pumping wavelength (in vacuo), and'n s = refractive index of the dye solution. It is easy to see from formulas (1) that if the incident angles are chosen properly, a simultaneous laser action at two independent wavelengths, corresponding to the periods d 1 and d 2, will occur in the direction along the liquid-glass interface. At the same time the intermediate wavelength corresponding to the interference period d3, d 4 should be observed under angles to the interface -+{ (a 1 - a2). If one of the four pumping beams is shut off a laser action should be observed only at two lines; each line could be tuned separately by varying the appropriate angle of interference. The above considerations were checked experimentally. The experimental setup is given in fig. 2. As is seen, the pumping beam is split here into two beams. The angle between them can be changed by rotation of the mirror M, and in addition the prism P can also be rotated. The 300 kW 2 d harmonic of a YAG:Nd 3+ laser was used for pumping. The dye used was 3 × 10 - 3 M Rhodamine 6G in ethanol. The experiments were carried out at a pulse repetition rate of 25 pps. The spectra of the DFB dye laser under separate excitation by the pumping beams I and |I are represented in fig. 3a, b. It is seen that a certain emission line corresponds to each of the pumping beams. In the case of simultaneous excitation of the solution by beams I and II both lines XI and ~kli are observed and in addition the third line XlI I between them appears (fig. 3c). The last one is caused by the interference of the pumping beams I and II. The emission at the wavelength XI and XII was gen160
a)
b)
ii
c)
d) Fig. 3. Spectra of the DFB dye laser under separate (a, b) and simultaneous (c, d) excitation by the pumping beams I and II. erated along the solution-prism interface, while the emission at the intermediate wavelength )tii 1 was propagated at the angle +-71 (a 1 - a2) to it, accurately corresponding to the calculated value. When varying a spectral distance between lines XI and ~kli from some to hundreds Angstr6ms this angle varies from several rranutes to several degrees. If one of the beams (I or II) falls only on one of the side surfaces of the prism P, then three pumping beams rather than four reach the solution-prism interface. In this case only two lines are observed in the laser spectrum (fig. 3d). Fig. 4a demonstrates the case when varying the intersection angle of the beams I and II by means of rotation of the mirror M, tuning of two emission lines with an unchangeable third one was observed. In fig. 4b a simultaneous tuning of the three emission
Volume 20, number 1
OPTICS COMMUNICATIONS
January 1977
b i
t
o
(b)
Ca)
l:ig. 4. Independent (a) and simultaneous (b) tuning of spectral lines of the DFB dye laser.
lines, achieved by rotation o f the prism P around its axis is shown. Thus a simple method is proposed which allows obtaining a DFB laser action simultaneously at several spectral lines of any wavelength with independent tuning within the gain profile of the dye.
References [1] C.V. Shank, J.F.. Bjorkholm and H. Kogelnik, Appl. Phys. Letters 18 (1971) 395. [2] T.Sh. Efendiev and A.N. Rubinov, Sov. J. Quantum Electron. 2 (1975) 858 (in Russian).
161