By injection of manosecond pulses from an AM mode-locked TEA CO2 laser in a passive mode-locked multi-atmosphere TE CO2 laser the shot-to-shot reproducibility of the generated subnanosecond pulses was increased to almost 100%

Goor, F.A. van

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van Goor, F.A.

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Increased reliability of passive mode-locking a multi-atmosphere TE CO2 laser by injection mode-locking

University of Twente Research Information

Volume 57, number 4 OPTICS COMMUNICATIONS 15 March 1986 I N C R E A S E D  RELIABILITY O F  P A S S I V E  M O D E - L O C K I N G  A M U L T I - A T M O S P H E R E  TE CO 2 L A S E R  BY I N J E C T I O N  M O D E - L O C K I N G  F.A. VAN G O O R  Twente Unicersio of Technology, Enschede, The Nether/ands' Received 20 May 1985: revised manuscript received 18 November 1985 By injection of nanosecond pulses from an AM mode-locked TEA CO: laser in a passive mode-locked multi-atmosphere TE ('()~ laser the shot-to-shot reproducibility of the generated subnanosecond pulses was increased to almost 100%. Passive mode-locking of  the TE CO 2 laser has been demonstrated by a number o f  authors [ 1 - 3 ]  using sev- eral types o f  saturable absorbers. The shortest pulses have been generated using p-type germanium yielding pulses as short as 400 ps at one atmosphere gas pres- sure [4] and 150 ps at ten atmospheres [3]. Reprodu- cible passive mode-locking can only be achieved when the laser parameters are carefully adjusted as shown for example by Haus [5] and Taylor et al. [6]. The most important  conditions required for stable passive mode-locking are the location of  the saturable absor- ber, it must be placed at one end or in the middle of  the cavi ty ; the  laser should operate just above thresh- old with only axial modes,  the linear losses of  the cav- i ty must be dominated by the small signal loss of  the saturable absorber. As the intensity of  the biggest fluc- tuat ion has to saturate the absorber before the gain saturates and as the saturation intensity of  p-type ger- manium is usually of  the same order of  magnitude as the saturation intensity o f  the gain medium [7], the beam diameter at the absorber must be smaller than in the amplifier. Since the number of  fluctuations with comparable intensity in the time window given by the roundtrip time increases with pressure [6] and the gain period decreases with pressure it is more difficult to ob tain reproducible pulses with passive mode-locking at higher pressure. In this contribution we describe a method to in- crease the reproducibil i ty of  passive mode-locking in a multi-atmosphere TE CO 2 laser. The method is es- 1250mm / | /  Au TEA-C02 2000mm 93.8cm HP TE-EO 2 pGe 1Ge flaf BS \\\1oo. 187-5 cm \ \ AMmode rocker Ge f[a 40 HHz [80%__ PD2 , , I  I / Fig. 1. Experimental configuration. 254 0 030-4018/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) Volume 57, number 4 OPTICS COMMUNICATIONS 15 March 1986 sentially injection mode-locking [8] with a p-type ger- manium saturable absorber placed in the resonator of  the regenerative amplifier. The experimental  configuration is shown in fig. 1. The master oscillator was a 1 X 1 × 20 cm 3 UV pre- ionized TEA CO 2 discharge operating with a 1:1:3 = CO 2 : N 2 : He mixture at one atmosphere total  pressure. A germanium acousto-optic modulator  driven by a 40 MHz lithium niobate crystal caused amplitude modula- tion. As a result the laser produced a train o f  one nano- second duration pulses on the 10.6 P(20)  transition with a total  energy of  about 100 mJ. The pulses, coming from the master oscillator were injected in a multi  atmosphere TE CO 2 slave oscillator through the outcoupling mirror. This laser consisted of  a 0.5 × 0.5 × 30 cm 3 UV preionized TE CO 2 dis- charge [9] operating with a 1 : 1 : 10 = CO 2 : N 2 : He gas mixture at total  pressure ranging from one to ten at- mospheres. The p-type germanium saturable absorber used was a 1.2 mm thick plan parallel plate placed at the Brewster angle as close as possible (~7.5 cm) to the outcoupling mirror. The measured small-signal absorp- t ion of  the 1 g2 cm p-germanium was 0.27 mm -1 and the saturation intensity was about 10 MW cm - 2  [7]. In the case of  no-injection the system with satur- c ,4 V ' I  Z ~ J  02: N2: He =1:1:10 1 arm. I I I I l I I I I 5ns/div Fig. 2. Part of the pulse train from the passive model locked TE CO 2 laser operating at one atmosphere gas-pressure with- out injection of an external pulse train. c ,-4 Z 6.3ns =- "~ 5.5 arm. [02: N2:He:I:I :10 I 1 I I 1 I I I I TIME 2nsldiv. Fig. 3. Pulses from the passive mode locked laser with injec- tion of one-nanosecond pulses from the AM mode locked TE CO 2 laser. ble absorber operates just above threshold at one atmo- sphere gas pressure and produces pulses shorter than one nanosecond. However, about 30% of  the pulse trains contained more than one pulse of  comparable intensity in the roundtrip time window. Furthermore the reproducibil i ty was poor. Fig. 2 shows an example of  one of  the best shots at one atmosphere. At in- creased gas pressures there was no reliable mode-lock- ing possible with this configuration. However, when the pulse train from the master oscillator was injected and the delay between the two discharges was properly chosen we observed for almost all shots reliable pulses. In fig. 3 we display some of  the strongest pulses in the train with widths shorter than 760 ps, the response time of  our photondrag detector and Tektronix 7912 AD transient digitizer detection system. Reliable pulses were observed at all gas pressures of  the slave oscillator ranging from one to ten atmospheres However at high pressure operation the p-type german- ium absorber as well as to the germanium outcoupling mirror were damaged. In conclusion we have demonstrated a method to enhance the reproducibil i ty of  generating subnanosec- ond pulses with a combination of  passive- and injec- tion mode-locking. We found that,  using injection mode-locking, the conditions for good passive mode- locking are no longer stringent so that probably a larger beam diameter at the p-germanium can be chosen to avoid damage. 255 Volume 57, number 4 OPTICS COMMUNICATIONS 15 March 1986 This work  was sponsored by the organizat ion for fundamenta l  research o f  mat te r  (FOM),  the Netherlands.  References [ 1 ] A.F. Gibson, M.F. Kimmitt and B. Norris. Appl. Phys Lett. 24 (1974) 306. [2] R. Fortin, F. Rheault, J. Gilbert, M. Blanchard and J.L. Lachambre, Can. J. Phys. 51 (1973) 414. [3] A.J. Alcock and A.C. Walker, Appl. Phys Lett 25 (1974) 299. [4] B.J. Feldman and J.F. Figueira, Appl. Phys Lett. 25 (1974) 301. [5] H.A. Haus, J. Appl. Phys. 46 (1975) 3049. [6] R.S. Taylor, B.K. Garside and E.A. Ballik, IEEE J. Quant. Electron. QE-14 (1978) 532, [7] A.F. Gibson, C.A. Rosito, C.A. Raffo and M.F. Kimmitt, Appl. Phys. Lett. 21 (1972) 356. [8] P.A. B61anger and J. Boivin, Can. J. Phys. 54 (1976) 720. [9] A.J. Alcock, K. Leopold and M.C. Richardson, Appl. Phys. Lett. 23 (1973) 562. 256 

Increased reliability of passive mode-locking a multi-atmosphere TE CO2 laser by injection mode-locking

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