- Email: [email protected]
Pulsed neon laser at 5401Å with subnanosecond emission
Volume 1, number 5
PULSED
January 1972
OPTICS COMMUNICATIONS
NEON
LASER
AT
54OlA
WITH
SUBNANOSECOND
EMISSION*
M. CORTI C.I.S.E.,
&grate,
Milan,
Italy’
Received 5 November 1971
A neon laser generating subnanosecond light pulses on the 5401-A line by means of amplified spontaneous emission is described. Peak power is 8 kW. Relevant features of the laser optical pulse are re-
ported.
Generation of short light pulses by means of amplified spontaneous emission has been extensively described in the literature [l-11]. In this paper we present a crossed field laser which has been designed to give subnanosecond light pulse? of several kilowatts of peak power on the 5401-A line of neon. The laser is operated by a sparkgap and the excitation is of the travelling wave type [5]. A pulse duration of 0.8 nsec has been achieved, with a peak power of 8 kW and a repetition rate of 50 Hz. The simultaneous presence of such characteristics in pulsed neon lasers has not been previously reported in the literature. Higher peak power was indeed obtained at the expense either of a reduction in the repetition rate [5] or of an increase in pulse duration [4]. Some relevant features of the laser pulse have also been measured such as the optical spectrum, the wave-front spatial homogeneity and the output energy as a function of the laser discharge length. The laser tube construction (see fig. 1) is similar to that described by Leonard (33. The discharge channel has a rectangular cross section 3 cm high and 0.3 cm wide and is 110 cm long. The upper and lower electrodes are made of aluminum. The lower electrode is itself the mechanical sustaining block of the laser. The discharge channel walls consist of two sheets of glass 0.5 cm thick. The optical windows at the ends of the tube are made of quartz. The system is kept together by epoxy resins. The geometry of the laser is such that the electrical inductance is kept at reasonably low values. The energy storage capacitance is given by a set of 36 coaxial cables 2 m long (Amphenol RGU 8). A spark-gap switch connects the charging cables to a second set of 36 transmitting cables which are terminated on the laser. The
transmitting cables are sequentially scaled in length in such a way as to generate an excitation travelling along the tube with the same speed as the light pulse. The spark-gap is filled with nitrogen at a pressure of 4 atm and has an interelectrode distance of 2.5 mm. Its geometry is carefully studied to minimize the inductance. Spark-gap trigger pulses are generated by a * Partially
supported of Research).
by C.X.R.
Fig. 1. View of the laser
(Italian National Council
with transverse
electrodes.
discharge
373
Ol’TICS
January 1972
CokIMLNlCATloNS
thyratron circuit and a step-up transformer. The output power is an increasing function of the operating voltage. The upper limit is set by breakdown in the insulating material of the charging cables. It was found that for the RGU 8 cables 25 kV was a rather safe operating voltage. At that voltage and at the optimum neon pressure (35 torr) in the discbarge channel, the peak power output for the 5401-A line is of 8 kW’. The width at half-maximum of the light pulse is about 0.8 nsec. The observation is accomplished by means of a travrlling leave oscilloscope (Tektronix 519)
and a TRG 105 B photocell with Sl photosurfacet. The overall response time of the observation system is 0.65 nsec. This value has been obtained by calibrating the system with a modelocked ruby laser emitting picosecond pulses. Fig. 2 shows subsequent light pulses for the same operating conditions. The pulse reproducibility is within 10”;. The laser can be operated up to 50 Hz. Higher repetition rates have not been tried since no cooling system is provided. The neon gas consumption may be kept lower than 0.5 litre ‘hour in the continuous flow operation. The laser works without mirrors. In fact the addition of a reflector at one end does not change appreciably the output peak power and shape of the light pulse. This is due to the fact that the travelling wave excitation provides a highly ? Absolute power values are obtained from the spectral sensitivity curve of the Sl type photosurface together aith the calibration data at 6’943 .8 given by the manufacturer.
8.2 M'peak
power
‘. /.
A
4
“‘I?,
,5~‘,
,
I,-? j”
0 /
-00
50 Fig. 2. Photograph of subsequent pulses at the 5401-A line of neon taken uith a Tektronix 519 oscilloscope and ‘I’RG 105 13 photocell. Horizontal 2 nsec/cm.
374
time scale
is
A
o , 100
LASERLENGTH,
CM
Fig. 3. Output energy of the optical pulse for the X01-hi
line of neon as a function of the laser length. The graph at the right is a logarithmic expansion of the lower part of the curve.
Volume 4, number
5
OPTICS COMMUNICATIONS
January
1972
Fig. 4. Densitometer trace of the plane-mirror tainty and plate nonlinearity
Fabry-PBrot picture of 200 Ne laser pulses. The zero level uncergive the experimental error in the linewidth determination quoted in the text; the lree spectral range is 1500 MHz.
asymmetrical light output: approximately an 8:l ratio in the power emitted from the two ends. The output energy has been measured as a function of the laser length. The curve shows first an exponential rise, then a linear saturation. This behaviour can be described rather well by the theory of the optical pulse amplifier in the rate equation regime [12]. The curve is shown in fig. 3; the exponential part is redrawn in a logarithmic scale and corresponds to a gain per unit length of 6 m-1. In the saturation regime the observed shape of the optical pulse was practically independent of the laser discharge length. Therefore the shape of the linear part of the curve in fig. 3 corresponds to an output peak power increase per unit laser volume of 500 W/cm3. The optical spectrum of the laser pulse has been investigated by means of a Fabry-P&rot interferometer. The instrumental width of the plane mirror Fabry-P6rot was about 150 MHz and its free spectral range 1500 MHz. Fig. 4 shows the densitometer trace of the Fabry-P6rot picture of 200 laser shots. Laser light has been previously attenuated in order to avoid plate saturation. The line width is about 700 MHz, in fair agreement with the width of the Fourier transform of the light intensity pulse envelope approximated by a gaussian shape of fwhm = 0.8 nsec. The line width is determined with an experimental error of about 30%, due to difficulties in photographic plate absolute calibration. We have also investigated the wave-front uniformity of the light pulse by means of photographic analysis. A deep striation in the direction
Fig. 5. Enlarged picture of the far field of the laser. 375
Volume
-4. number
5
OPTICS
COMMUNICATIONS
perpendicular to the larger dimension of the spot size is observed*. Fig. 5 shows a magnified portion of it. Striations have been okserved also in the far field pattern of the 3371-A group of lines emitted by the nitrogen molecules. A way to obtain a homogeneous wave front is to use a mode selected oscillator followed by an amplifier. like the MOPA system advertised by AVCO. Thanks are due to F. T. Arecchi, for proposing this research and continuous discussions; V. Degiorgio and M. Giglio, for helpful advice; A. Vendramini and M. Malvezzi, for help in the experimental
*
work.
same observation has already been performed by A. S7iil,e (private corrrniunication) and interpreted as n superposition of randomly phased plane WXWS. The
376
January
1972
REFERENCES [l] H.Heard. Nature 200 (1963) 667. [2] D.M. Clunie, R. S.A.Thorn and K. E.Threzise, l’hys. Letters II (1965) 28. [3] D. A. Leonard, Appl. f’hys. Letters i (1965) I. [AI D.A. Leonard, IEEE J.Quantum Electron. QE-:I (1967) 133. [31 J. D. Shipmnn Jr.. Appl. Whys. Letters 10 (1967) 3. [ti] G. Ericsson and R. Lidholt, Arlriv Fysilc 37 [l!J68) 537. [ ‘71hI. Geller, D. E. Altmnn and T. A. DeTemplc. Appl. opt. 7 (1968) 2232. [Y/ S. R. Nilsson. 0. SteinvaIl, C. K. Subramanian and L. IIiigberg. Physica Scripta 1 (1970) 153. [t)] 1~. 1.. Hod&on. i’hys. Rev.-Letters 25 (1970) 49-i. [ 101It. Lb’.Wnynant. J. 1). Shipman Jr. Ii. C. Elton and A. W. Ali. Proc. IEEE 59 (1971) 679. [ll] J. Goldhar, R. M. Osgood Jr. and A. Javan. Appl. I’hys. Letters 18 (1971) 167. 1121 L. M. Fran& and J. S. Nodvik, J. Appl. I’hys. ::4 (IYB3) 2346.
PULSED
January 1972
OPTICS COMMUNICATIONS
NEON
LASER
AT
54OlA
WITH
SUBNANOSECOND
EMISSION*
M. CORTI C.I.S.E.,
&grate,
Milan,
Italy’
Received 5 November 1971
A neon laser generating subnanosecond light pulses on the 5401-A line by means of amplified spontaneous emission is described. Peak power is 8 kW. Relevant features of the laser optical pulse are re-
ported.
Generation of short light pulses by means of amplified spontaneous emission has been extensively described in the literature [l-11]. In this paper we present a crossed field laser which has been designed to give subnanosecond light pulse? of several kilowatts of peak power on the 5401-A line of neon. The laser is operated by a sparkgap and the excitation is of the travelling wave type [5]. A pulse duration of 0.8 nsec has been achieved, with a peak power of 8 kW and a repetition rate of 50 Hz. The simultaneous presence of such characteristics in pulsed neon lasers has not been previously reported in the literature. Higher peak power was indeed obtained at the expense either of a reduction in the repetition rate [5] or of an increase in pulse duration [4]. Some relevant features of the laser pulse have also been measured such as the optical spectrum, the wave-front spatial homogeneity and the output energy as a function of the laser discharge length. The laser tube construction (see fig. 1) is similar to that described by Leonard (33. The discharge channel has a rectangular cross section 3 cm high and 0.3 cm wide and is 110 cm long. The upper and lower electrodes are made of aluminum. The lower electrode is itself the mechanical sustaining block of the laser. The discharge channel walls consist of two sheets of glass 0.5 cm thick. The optical windows at the ends of the tube are made of quartz. The system is kept together by epoxy resins. The geometry of the laser is such that the electrical inductance is kept at reasonably low values. The energy storage capacitance is given by a set of 36 coaxial cables 2 m long (Amphenol RGU 8). A spark-gap switch connects the charging cables to a second set of 36 transmitting cables which are terminated on the laser. The
transmitting cables are sequentially scaled in length in such a way as to generate an excitation travelling along the tube with the same speed as the light pulse. The spark-gap is filled with nitrogen at a pressure of 4 atm and has an interelectrode distance of 2.5 mm. Its geometry is carefully studied to minimize the inductance. Spark-gap trigger pulses are generated by a * Partially
supported of Research).
by C.X.R.
Fig. 1. View of the laser
(Italian National Council
with transverse
electrodes.
discharge
373
Ol’TICS
January 1972
CokIMLNlCATloNS
thyratron circuit and a step-up transformer. The output power is an increasing function of the operating voltage. The upper limit is set by breakdown in the insulating material of the charging cables. It was found that for the RGU 8 cables 25 kV was a rather safe operating voltage. At that voltage and at the optimum neon pressure (35 torr) in the discbarge channel, the peak power output for the 5401-A line is of 8 kW’. The width at half-maximum of the light pulse is about 0.8 nsec. The observation is accomplished by means of a travrlling leave oscilloscope (Tektronix 519)
and a TRG 105 B photocell with Sl photosurfacet. The overall response time of the observation system is 0.65 nsec. This value has been obtained by calibrating the system with a modelocked ruby laser emitting picosecond pulses. Fig. 2 shows subsequent light pulses for the same operating conditions. The pulse reproducibility is within 10”;. The laser can be operated up to 50 Hz. Higher repetition rates have not been tried since no cooling system is provided. The neon gas consumption may be kept lower than 0.5 litre ‘hour in the continuous flow operation. The laser works without mirrors. In fact the addition of a reflector at one end does not change appreciably the output peak power and shape of the light pulse. This is due to the fact that the travelling wave excitation provides a highly ? Absolute power values are obtained from the spectral sensitivity curve of the Sl type photosurface together aith the calibration data at 6’943 .8 given by the manufacturer.
8.2 M'peak
power
‘. /.
A
4
“‘I?,
,5~‘,
,
I,-? j”
0 /
-00
50 Fig. 2. Photograph of subsequent pulses at the 5401-A line of neon taken uith a Tektronix 519 oscilloscope and ‘I’RG 105 13 photocell. Horizontal 2 nsec/cm.
374
time scale
is
A
o , 100
LASERLENGTH,
CM
Fig. 3. Output energy of the optical pulse for the X01-hi
line of neon as a function of the laser length. The graph at the right is a logarithmic expansion of the lower part of the curve.
Volume 4, number
5
OPTICS COMMUNICATIONS
January
1972
Fig. 4. Densitometer trace of the plane-mirror tainty and plate nonlinearity
Fabry-PBrot picture of 200 Ne laser pulses. The zero level uncergive the experimental error in the linewidth determination quoted in the text; the lree spectral range is 1500 MHz.
asymmetrical light output: approximately an 8:l ratio in the power emitted from the two ends. The output energy has been measured as a function of the laser length. The curve shows first an exponential rise, then a linear saturation. This behaviour can be described rather well by the theory of the optical pulse amplifier in the rate equation regime [12]. The curve is shown in fig. 3; the exponential part is redrawn in a logarithmic scale and corresponds to a gain per unit length of 6 m-1. In the saturation regime the observed shape of the optical pulse was practically independent of the laser discharge length. Therefore the shape of the linear part of the curve in fig. 3 corresponds to an output peak power increase per unit laser volume of 500 W/cm3. The optical spectrum of the laser pulse has been investigated by means of a Fabry-P&rot interferometer. The instrumental width of the plane mirror Fabry-P6rot was about 150 MHz and its free spectral range 1500 MHz. Fig. 4 shows the densitometer trace of the Fabry-P6rot picture of 200 laser shots. Laser light has been previously attenuated in order to avoid plate saturation. The line width is about 700 MHz, in fair agreement with the width of the Fourier transform of the light intensity pulse envelope approximated by a gaussian shape of fwhm = 0.8 nsec. The line width is determined with an experimental error of about 30%, due to difficulties in photographic plate absolute calibration. We have also investigated the wave-front uniformity of the light pulse by means of photographic analysis. A deep striation in the direction
Fig. 5. Enlarged picture of the far field of the laser. 375
Volume
-4. number
5
OPTICS
COMMUNICATIONS
perpendicular to the larger dimension of the spot size is observed*. Fig. 5 shows a magnified portion of it. Striations have been okserved also in the far field pattern of the 3371-A group of lines emitted by the nitrogen molecules. A way to obtain a homogeneous wave front is to use a mode selected oscillator followed by an amplifier. like the MOPA system advertised by AVCO. Thanks are due to F. T. Arecchi, for proposing this research and continuous discussions; V. Degiorgio and M. Giglio, for helpful advice; A. Vendramini and M. Malvezzi, for help in the experimental
*
work.
same observation has already been performed by A. S7iil,e (private corrrniunication) and interpreted as n superposition of randomly phased plane WXWS. The
376
January
1972
REFERENCES [l] H.Heard. Nature 200 (1963) 667. [2] D.M. Clunie, R. S.A.Thorn and K. E.Threzise, l’hys. Letters II (1965) 28. [3] D. A. Leonard, Appl. f’hys. Letters i (1965) I. [AI D.A. Leonard, IEEE J.Quantum Electron. QE-:I (1967) 133. [31 J. D. Shipmnn Jr.. Appl. Whys. Letters 10 (1967) 3. [ti] G. Ericsson and R. Lidholt, Arlriv Fysilc 37 [l!J68) 537. [ ‘71hI. Geller, D. E. Altmnn and T. A. DeTemplc. Appl. opt. 7 (1968) 2232. [Y/ S. R. Nilsson. 0. SteinvaIl, C. K. Subramanian and L. IIiigberg. Physica Scripta 1 (1970) 153. [t)] 1~. 1.. Hod&on. i’hys. Rev.-Letters 25 (1970) 49-i. [ 101It. Lb’.Wnynant. J. 1). Shipman Jr. Ii. C. Elton and A. W. Ali. Proc. IEEE 59 (1971) 679. [ll] J. Goldhar, R. M. Osgood Jr. and A. Javan. Appl. I’hys. Letters 18 (1971) 167. 1121 L. M. Fran& and J. S. Nodvik, J. Appl. I’hys. ::4 (IYB3) 2346.