Selfmodelocking of a transversely excited N2 laser in the first positive system at1.048 μmT.J. Gleason, C.S. Willett, R.M. Curnutt, and J.S. Kruger Citation: Applied Physics Letters 21, 276 (1972); doi: 10.1063/1.1654376 View online: http://dx.doi.org/10.1063/1.1654376 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/21/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Selfmodelocking suppression with intracavity fullerenes Appl. Phys. Lett. 65, 667 (1994); 10.1063/1.112953 Novel selfmodelocking mechanism in narrowband lasers Appl. Phys. Lett. 63, 2597 (1993); 10.1063/1.110442 Selfmodelocking of a semiconductor laser using positive feedback Appl. Phys. Lett. 56, 7 (1990); 10.1063/1.102658 Selfmodelocking of an iodine photodissociation laser J. Appl. Phys. 46, 4083 (1975); 10.1063/1.322118 CONTINUOUS SELFMODELOCKING IN A CO–He–O2 LASER Appl. Phys. Lett. 18, 424 (1971); 10.1063/1.1653479

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Self-mode-Iocking of a transversely excited N2 laser in the first positive system at 1.048 J-Lm

T.J. Gleason, C.S. Willett, R.M. Curnutt, and J.S. Kruger Harry Diamond Laboratories, Department of the Army, Washington, D. C. 20438

(Received 5 May 1972)

An investigation has been made of the spontaneous self-mode-locking in a transient trans­versely excited N2 laser operating in the first positive system (B 37[,-A 3~:) in molecular nitrogen on the 0-0 band at 1. 048 /-Lm. The laser emission, which had a typical pulse length of 60 nsec, consisted of a train of mode-locked pulses of about 6-nsec duration with a peak power of about 0.5 kW.

Laser oscillation in the first positive system (B 31T,-

A 3~=) in N2 was first reported by Mathias and Parker. 1

They observed pulsed oscillation on a number of vibra­tional bands in the first positive system using an axially excited discharge in a O.4-m-Iong 7-mm-i,d. tube at a N2 pressure of 4 Torr. The peak power obtained was less than 0.5 W on all the vibrational bands. Only low output power in the first positive system had been re­ported until recently, when Wood and his co-workers2

reported use of the pulsed transverse-excitation resis­tor-loaded technique developed by Beaulieu3 for use in atmospheric-pressure CO2 lasers to excite a N2 -He mix­ture and obtain a peak output of 0.2 kW.

In this letter we report observations of higher peak out­put powers and spontaneous mode locking in a small transversely excited N2 laser that utilizes the preioniza­tion technique of Dumanchin. 4 Our laser, which has an active plasma length of 29 cm, operates in pure nitrogen over a pressure range of 20-90 Torr, with an optimum pressure of 45 Torr, and in a helium -nitrogen mixture over the range 20-110 Torr, without the development of hot low -voltage high -current arc discharges. We be­lieve that this is the first report of spontaneous mode locking on transitions in molecular nitrogen in a laser system that is inherently transient in operation. 5,6

Whereas transient high -power laser oscillation in the second positive system (C 31Tu -B 31T,) in the ultraviolet has been studied extensively, 7-10 little attention has been paid to high-power transient oscillations that can be ob­tained on transitions in the first positive system, whose upper vibrational levels are the lower vibrational levels of the second positive system (Fig. 1). In our laser, oscillation was deliberately restricted to the 0-0 band of the B 31TI' -A 3 L: transition at 1. 048 Mm by the use of resonator mirrors with reflectivity peaked at 1. 06 Mm, which suppressed oscillation on other (0-1, 1-0, 2-1, 2-0, 3-1, and 4-2) bands ll in the near infrared.

The laser used here consisted of a 5 -cm -i, d. cast lucite tube sealed with borosilicate glass windows. The trans­verse excitation was produced by means of a segmented­cathode structure, 29 cm long and 2 cm wide, and a plate anode with its edges shaped to apprOximate Rogowski profiles, 12 separated from the cathode by 2 cm. The cathode consisted of four hacksaw blades with 10 teeth/cm, internally connected to a common point. They were sandwiched between five insulated preioniza­tion rods. The diameter of the brass preionization rods was 2.4 mm, and the insulation consisted of Teflon tub­ing 1. 0 mm thick in intimate contact with the rods. The preionizer rods shared a single common connection and

Appl. Phys. Lett., Vol. 21, No.6, 15 September 1972 276

were connected externally to ground by means of a 30-kV 500-pF doorknob-type coupling capacitor, as in the method described by Dumanchin. 4 Current pulses of 125-nsec duration with a rise time of 75 nsec were obtained by discharging a O. 02-MF capaCitor at 10-25 kV through low-inductance circuitry by means of a simple single­channel untriggered adjustable spark gap. The laser would operate for about 5 min in a sealed -off configura -tion, but was normally operated with a slow gas flow to overcome gas contamination caused by minor leaks in the laser channel.

The plane-parallel optical resonator consisted of a high­reflecti vity dielectric -coated borosilicate -glass flat with a reflection coefficient of 99.4% at 1. 05 Mm and another flat with a reflectivity of 82.2% at 1. 05 Mm, spaced apart at distances between 65 and 104 cm. The beam divergence half-angle was 1. 25 mrad for the 65-cm cavity. Oscillation could be achieved over a mirror-

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FIG. 1. Partial potential energy curves of molecular nitrogen, showing near-infrared laser transitions in the first positive (B 37[,-A 1~~ system and the 0-0 band, and the laser transition in the second positive (C 37[u - B 37[,) system at 0.3371 !-1m.

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SELF-MODE-LOCKING OF AN N2 LASER 277

c d

VV\rv \f'v\ --I \+-2nS --I 1-2nS

FIG. 2. Waveforms of (a) laser output (upper trace) and dis­charge current (inverted) (lower trace); (h) mode-locked laser pulse, 103. 5-cm cavity; (c) mode-locked laser pulse, 65-cm cavity (expanded scale); (d) mode-locked laser pulse, 103.5-cm cavity (expanded scale).

adjustment range of ± 12 arc sec using the 82.2% output­coupling mirror. Laser oscillation was detected by means of a vacuum planar photodiode with an S -1 photo­cathode, and the signal was observed on a fast oscillo­scope. This detection system had a rise time of about 2.5 nsec.

Oscillation on the 0-0 band at 1. 048 JJ.m occurred on the trailing edge of the current pulse with the peak power occurring about 100 nsec after the peak of the pulse. We believe that the time relationship between the current pulse and the laser pulse can be attributed more to gain switching than to excitation processes involving the upper laser level. Time-resolved spontaneous line in­tensity measurements are being conducted to clarify this. The duration of the laser pulse is approximately 60 nsec FWHM under optimum discharge conditions. Fig­ure 2(a) illustrates the time relationship between a laser pulse and the discharge current. Figure 2(b) shows a typical laser pulse that is exhibiting partial mode lock­ing with a cavity length of 103.5 cm. The extent of the mode locking was found to be insensitive to the pulse current, gas pressure, and flow rate. The mode-locked pulses, which have a modulation depth of 500/0, have an

interval of 6.9 nsec corresponding to the round-trip transit time (2L/c) of the cavity [Fig. 2(d)). The mode locking was reproducible from pulse to pulse, but the laser pulse amplitude varied due to pulse-to-pulse dif­ferences in the breakdown voltage of the spark gap that resulted in variations in the voltage and energy applied to the laser channel. Decreasing the cavity length to 65 cm leads to a decrease in the round-trip transit time and to a corresponding reduction in the pulse-to-pulse spacing of the mode-locked pulses to 4.5 nsec [Fig. 2(c)). LaSing with the cavity described could not be ob­tained beyond a mirror separation of 104 cm because of a difficulty in aligning the flat mirrors. Mode locking could be observed over a range of reflectivities of the output mirror of 99-75%. Mode locking here appears to be unlike self-mode-locking in a high-pressure CO2 lasers with transverse pulse excitation in which experi­mental parameters are important. 13,H Since the lower laser level is approximately 6 eV above the ground state, the mode locking cannot be attributed to a ther­mal saturation effect.

The authors wish to acknowledge unpublished informa­tion relevant to the 1. 048 - JJ.m N2 laser supplied by O. R. Wood and M.A. Pollack of the Bell Telephone Labora­tories, and express considerable thanks to D. Chambers for his expertise in the construction of the laser tube.

IL.E.S. Mathias andJ.T. Parker, Appl. Phys. Letters 3,16 (1963).

20.R. Wood, E.G. Burkhardt, M.A. Pollack, and T.J. Bridges, Appl. Phys. Letters 18, 261 (1971).

3A.J. Beaulieu, Appl. Phys. Letters 16, 504 (1970). 4R. Dumanchin, Quantum Electronics Conference, Kyoto, 1970 (unpublished); Laser Focus 8, 32 (1971).

5W.R. Bennett, Jr., Appl. opt. Suppl. 2,3 (1965). BW. T. Walter, M. Piltch, N. Solimene, and G. Gould, IEEE J. Quantum Electron. QE-2, 474 (1966).

7D.A. Leonard, Appl. Phys. Letters 7,4 (1965). 8M. Geller, D.E. Altman, and T.A. DeTemple, Appl. Opt. 7, 2232 (1968).

9J.D. Shipman, Jr., Appl. Phys. Letters 10,3 (1967). lOA. W. Ali, Appl. Opt. 8, 993 (1969). 11M• A. Pollack in Handbook of Lasers, edited by R.J. Pressley

(Chemical Rubber Co. , Cleveland, OhiO, 1971), pp. 298-349. 12J. Cobine, Gaseous Conductors (Dover, New York, 1958),

pp. 177-181. 13J. Gilbert and J. L. Lachambre, Appl. Phys. Letters 18, 187

(1971) • 14D.L. Lyon, E.V. George, and H.A. Haus, Appl. Phys. Let­

ters 17,474 (1970).

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