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A simple technique for separation of laser beams in collinear frequency mixing
infrared Phys. Vol. 28, No. 3, pp. 163-164, 1988 Printed in Great Britain. All rights reserved
0020-0891/88 $3.00 + 0.00 Copyright 0 1988 Pergamon Press plc
A SIMPLE TECHNIQUE FOR SEPARATION BEAMS IN COLLINEAR FREQUENCY G.
C.
BHAR,
N.
P.
GHOSH
Burdwan University, Physics Department,
OF LASER MIXING
and S. DAS Burdwan-713104,
India
(Received I7 November 1987)
Abstract-Ingenious selection of an appropriate dispersive prism has enabled us to separate physically the strong pump laser beam from the closely spaced nonlinear generated frequencies, thereby avoiding use of delicate interference filters. The principle has been successfully tested in nonlinear experiments.
In nonlinear frequency conversion experiments like second harmonic generation, upconversion and sum-frequency mixing, the unconverted strong input pump laser beam is required to be separated from the generated weak radiation for useful applications. This is often awkward and requires a polariser and interference filter (edge or band-pass) set to reduce the strong pump beam to the lowest acceptable level. In so doing the useful generated radiation is also attenuated appreciably. The problem becomes severe in upconversion where the rather closely spaced weak upconverted signal has to be separated from the strong pump laser beam. (‘7’)For example in upconversion of a 10 pm signal with a Nd laser as pump, the up-converted signal at 0.97 pm must be separated from the pump. Polarisation discrimination can not eliminate the strong pump to the required extent. It is also often difficult to realise the desired discrimination by a set of narrow-band interference filters and trading off much of upconverted signal. Although all these could be avoided in noncolinear interaction, the problem of proper beam overlap and limitation of interaction length make collinear interaction versatile.(3) A proposal is made to use a dispersive prism of appropriate material and angle so as to obtain reasonabie separation by the deviation angle between these two wavelengths. Ordinary glass prisms offer too little differential deviation between the rather closely spaced wavelengths (1.064 and 0.97 pm). Extra dense flint (EDF) glass prisms, due to their high dispersion, produce reasonable differential deviation. Table 1 shows calculated values taking some arbitrary prism angles which would need to be chosen properly to avoid total internal reflection. Since reflection loss is minimum at normal incidence, this implies that the angle of the prism should be below a critical value. Also shown in the table are the values of differential deviation for separation of the second harmonic of 1.064 pm laser radiation. The appropriate prism material for separation in case of such upconversion is AgGaS,, since the wavelengths fell on the steep part of its dispersion curve and a large differential deviation is obtained. Table 1. Differential deviation by prism in nonlinear devices Devices
Material of prism
Angle of prism
UC UC UC UC SHG
AgGaS, AgGaS, Ag, SbS, EDF EDF
SHG
DEDF
12.5” 40” 15” 30” 30 60” 30”
Angle of incidence 0” 45” 0 0 0” 50 0”
Separation in degree 0.61” 1.6” 2.4 0.14” 1.6” 5” 2.7”
This technique of separation has been applied to the case second harmonic generation of Nd:YAG laser radiation in a KDP crystal. One 30” EDF glass prism was used under normal incidence to separate the harmonic from the fundamental. The measured separations are found to agree with the calculated values. Measurements were also made with angles of incidences other than normal. Although the differential separation rises with increased angle of incidence, the increased reflection loss causes reduced separated output power. 163
164 Acknowledgement-The Government of India
G. C. BHAR ef al authors acknowledge the Department for partial financial support.
of Science & Technology
REFERENCES 1. G. C. Bhar, N. P. Ghosh and S. Das, Infrared Phys. 27, 245 (1987). 2. P. Koidl and W. Jantz, Appl. Phys. Left. 31, 99 (1977). 3. W. Lahmann, K. Tibulski and H. Welling, Opt. Commun. 17, 18 (1976).
and Directorate
of Aeronautics,
0020-0891/88 $3.00 + 0.00 Copyright 0 1988 Pergamon Press plc
A SIMPLE TECHNIQUE FOR SEPARATION BEAMS IN COLLINEAR FREQUENCY G.
C.
BHAR,
N.
P.
GHOSH
Burdwan University, Physics Department,
OF LASER MIXING
and S. DAS Burdwan-713104,
India
(Received I7 November 1987)
Abstract-Ingenious selection of an appropriate dispersive prism has enabled us to separate physically the strong pump laser beam from the closely spaced nonlinear generated frequencies, thereby avoiding use of delicate interference filters. The principle has been successfully tested in nonlinear experiments.
In nonlinear frequency conversion experiments like second harmonic generation, upconversion and sum-frequency mixing, the unconverted strong input pump laser beam is required to be separated from the generated weak radiation for useful applications. This is often awkward and requires a polariser and interference filter (edge or band-pass) set to reduce the strong pump beam to the lowest acceptable level. In so doing the useful generated radiation is also attenuated appreciably. The problem becomes severe in upconversion where the rather closely spaced weak upconverted signal has to be separated from the strong pump laser beam. (‘7’)For example in upconversion of a 10 pm signal with a Nd laser as pump, the up-converted signal at 0.97 pm must be separated from the pump. Polarisation discrimination can not eliminate the strong pump to the required extent. It is also often difficult to realise the desired discrimination by a set of narrow-band interference filters and trading off much of upconverted signal. Although all these could be avoided in noncolinear interaction, the problem of proper beam overlap and limitation of interaction length make collinear interaction versatile.(3) A proposal is made to use a dispersive prism of appropriate material and angle so as to obtain reasonabie separation by the deviation angle between these two wavelengths. Ordinary glass prisms offer too little differential deviation between the rather closely spaced wavelengths (1.064 and 0.97 pm). Extra dense flint (EDF) glass prisms, due to their high dispersion, produce reasonable differential deviation. Table 1 shows calculated values taking some arbitrary prism angles which would need to be chosen properly to avoid total internal reflection. Since reflection loss is minimum at normal incidence, this implies that the angle of the prism should be below a critical value. Also shown in the table are the values of differential deviation for separation of the second harmonic of 1.064 pm laser radiation. The appropriate prism material for separation in case of such upconversion is AgGaS,, since the wavelengths fell on the steep part of its dispersion curve and a large differential deviation is obtained. Table 1. Differential deviation by prism in nonlinear devices Devices
Material of prism
Angle of prism
UC UC UC UC SHG
AgGaS, AgGaS, Ag, SbS, EDF EDF
SHG
DEDF
12.5” 40” 15” 30” 30 60” 30”
Angle of incidence 0” 45” 0 0 0” 50 0”
Separation in degree 0.61” 1.6” 2.4 0.14” 1.6” 5” 2.7”
This technique of separation has been applied to the case second harmonic generation of Nd:YAG laser radiation in a KDP crystal. One 30” EDF glass prism was used under normal incidence to separate the harmonic from the fundamental. The measured separations are found to agree with the calculated values. Measurements were also made with angles of incidences other than normal. Although the differential separation rises with increased angle of incidence, the increased reflection loss causes reduced separated output power. 163
164 Acknowledgement-The Government of India
G. C. BHAR ef al authors acknowledge the Department for partial financial support.
of Science & Technology
REFERENCES 1. G. C. Bhar, N. P. Ghosh and S. Das, Infrared Phys. 27, 245 (1987). 2. P. Koidl and W. Jantz, Appl. Phys. Left. 31, 99 (1977). 3. W. Lahmann, K. Tibulski and H. Welling, Opt. Commun. 17, 18 (1976).
and Directorate
of Aeronautics,