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A combined actively and passively Q-switched microchip laser

Magnus Arvidsson, Bjöm Hansson, Martin Holmgrena and Carsten Lindströma

Department of Physics - Optics, Royal Institute of Technology, 100 44 Stockholm, SwedenaSpectra Precision AB, Box 64, 182 1 1 Danderyd, Sweden

ABSTRACT

Until now active or passive Q-switchmg have been used to achieve highly energetic laser pulses. Both these pulsing techniqueshave advantages as well as drawbacks. Passive Q-switching is extremely simple and need no electronic driving, but the emitted

pulse has a large time jitter ( 100 ns to 1 ms) between the emitted pulses, which is detrimental in many applications. Active Q-switching on the other hand require advanced high voltage drive electronics which consume a substantial amount of electricalpower, but the emitted pulse train is normally clean and well-behaved. In this paper we propose and demonstrate a novel Q-switched diode-pumped solid-state laser design, which combines the advantages of active and passive Q-switchmg, resulting in

low time jitter as well as simple drive electronics with low power consumption.

The plane-plane diode pumped laser consisted of a 0.5 mm long 3% Nd3:YVO4 crystal acting as the gain medium, with the firstmirror directly coated on the input face. The laser chip was followed by the active modulator, a 2 mm long z-cut LiMbO3crystal, which in turn was followed by the saturable absorber, a 0.6 mm thick Cr4:YAG crystal, and the output mirror. Allelements were optically bonded together and the total cavity length including mirrors was 3.5 mm. The monolithic laser has a

low jitter, here limited by our driving electronics to 85 ps, at a switching voltage of 300 V, compared to the V of 5.2kV. The

pulse length is only 3 ns when the laser operates in combined Q-switched mode and 12 ns when passively Q-switched. Inaddition, multiple pulses caused by the active modulator is suppressed by the saturable absorber.

Keywords: Q-switch, jitter, diode-pumped, solid-state-laser, microchip, electro-optic modulator.

SPIE Vol. 3265 • 0277-786X198/$1O.OO106 -

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1. INTRODUCTION

Diode pumped solid-state lasers and particularly microchip lasers belong to a rapidly emerging laser area, which combine acomparable simple fabrication with integration of various functions'2. Passively Q-switched microchip lasers can be made small

and provide short high power pulses for a wide range of applications. Zayhowski, have shown how these lasers can be frequency

doubled , quadrupled' and used to pump optical parametric amplifiers5. Rapidly growing fields of use for pulsed laser are range

finding and LIDAR. These applications would benefit from simple, inexpensive Q-switched lasers with low power consumption.

Pulsed microchip lasers are ideal for ranging since they provide short pulses with high peak power along with excellent beamquality. There are many approaches to measure distance with a pulsed laser. The simplest way is to trigger a time measurement

circuit directly with the signal from the detector. The accuracy in this approach depend on the rise-time of the pulse. Theamplitude variations is also a contributing source for error. In this case however the repetition rate of the laser does not affectthe result, hence a substantial jitter can be accepted. If the requirements on accuracy is higher, another approach needs to beconsidered. By integrating over a large number of pulses, and using other means of detecting the pulse than triggering on theslope of the pulse the accuracy can be improved radically. To be able to do this in a system the jitter needs to be inthe order of afew hundred picoseconds. The only way to achieve such a low jitter in a Q-switched laser has so far been to use active Q-switching. Zayhowski has reported on active Q-switchmg of microchip lasers in different configurations 6'7.The drawback ofactive Q-switching is however that they require high-speed, high-voltage electronics, and these usually consume a great deal ofpower. There has been work done on stabilismg passively Q-switched lasers, and progress has been reported 8,9 but the results

are not good enough integration in systems yet.In this paper we present a novel design that combine the two ways of Q-switching. This gives a laser with the low jitterassociated with active Q-switchmg, but with less demanding switching voltage requirements.

2. THE COMBINED Q-SWITCH

In a passively Q-switched laser the inversion density in the gain medium grows as the laser is pumped. The losses from thesaturable absorber prevents the onset of lasing until the gain is high enough. When the gain exceeds the losses the lasing startsand the saturable absorber bleaches, thus reducing the losses further. Once the saturable absorber is bleached the gain in thecavity becomes much larger than the losses. The intensity in the cavity rapidly builds up to a giant pulse containing the energystored in the gain medium. Smce the repetition rate of this process is only dependent on when the gain exceeds the losses it issensitive to fluctuations. Previous work show that the repetition rate can be synchronised with modulation of the pumping

thereby reducing the uncertainty in time for the emission of the laser pulse. Mandeville et. al. reports on a reduction of the jitter

from 30 .ts to 1 j.ts . The explanation proposed is that the threshold of the laser is not a fixed value, but rather a window since

the process is statistical. When the inversion reaches this window a pulse is emitted somewhere within this window. If theinversion passes this window rapidly the uncertainty in time when the pulse is emitted will be small. On the other hand if theinversion passes this windows slowly the time for the emission of a pulse will be more uncertain. In addition to creating theuncertainty in time the uncertainty in inversion density is contributing to the amplitude variations usually observed in passivelyQ-switched lasers .In an actively Q-switched laser the time for removal of the inserted losses is determined by the drive electronics. The jitter willtherefore be determined by the jitter in the drive electronics and the build-up time of the laser pulse. Since the laser build-up is astatistical process where a few photons starts the build-up of the pulse there will be an uncertainty in time when the pulse isemitted in the actively Q-switched laser. One way of minimising the jitter due to these statistical phenomena in the laser is to

107


z

zCrdD

z

shorten the pulse build-up time as much as possible. The build-up time Tb can easily be calculated for a given laser configuration

'°fromEq. 1.

XIfl(J (1)

Where 'r is the cavity lifetime, r is the ratio between the inversion in the laser at switching and the threshold inversion just after

switching. is the steady state photon number, which is the photon number that would be present in the cavity if it werecontinuously pumped r times above its threshold right after Q-switching. n1 is the photon number in the cavity when the pulsestarts to build up. Since the build-up time in an actively Q-switched microchip laser is in the range of a few nanoseconds itshould be possible to obtain a jitter in the range of a few hundred picoseconds due to the statistical variations of the start of the

build-up.

In our novel design we use a combined Q-switch, consisting of both a saturable absorber and an active modulator. Figure 1shows the operation principle. The curve I in Fig. 1 represents the inversion density. Due to the pumping of the laser, theinversion density increases and hence the gain increases. The window associated with passive Q-switching is shown in Fig. 1 as

A. When there are no losses in the cavity from the active modulator, then window A is the valid one. This includes outcoupling

losses and other losses in the cavity. In this case a pulse will most likely be emitted during the time shown as 6t in thefigure. In

this case the jitter or 3t is dependent on the slope of the inversion density as discussed above. When the losses from the active

modulator is present in the cavity along with the losses from the saturable absorber, B is the valid threshold window.

B

A

Figure 1. The relation between inversion density and time in the combined Q-switched laser

108

to & tL


When the laser is operated in combined mode the losses from the active modulator is inserted into the cavity directly after apulse has been emitted, at time to in the figure. During the build-up of the inversion density in the gain medium, the lasing issuppressed by the combined losses of the saturable absorber and the active modulator, and window B is the valid one, illustrated

in the figure by solid lines. When a pulse is due to be emitted, the losses from the active modulator is removed. Since the onlyexisting loss in the cavity corresponds to the saturable absorber, window A is the valid one. The inversion density and hence thegain is well above the threshold for the laser at this time. Because of the now high gain in the laser the build-up of thelaser pulsewill occur directly after the active modulator has been switched to transmission. The high gain in the cavity will also lead to afaster build-up of the laser pulse resulting in a shorter pulse compared to passive Q-switching. The saturable absorber also have

another effect, it suppresses extra pulses due to slow switching of the active modulator.

Losses from saturable absorber

Figure 2. At time tL the losses from the active modulator is removed, then the lasing starts and the saturable absorber is bleached rapidly. When

the lasing starts, the inversion density drops and the photon number increases. As the inversion density continue to decrease because

of the lasing that empties the cavity, the photon number decreases.

3. EXPERIMENTAL SETUP

The configuration sketched in Fig. 3 was used to realise the combined actively and passively Q-switched laser. It consisted of a0.5 mm thick flat-flat polished 3% Nd:YVO4 crystal with a laser mirror coating deposited on the input side. It was hightransmitting at the 808 nm pump wavelength and high reflecting at the 1064 nm laser wavelength. A 0.6 mm thick [1001-cutCr4:YAG crystal with an initial transmission of 85 % was used as saturable absorber. The crystallographic axes of saturableabsorber was aligned with respect to the polarised emission of the Nd:YVO4 crystal to obtain maximum efficiency

109

t


110

1.2mm

4I4

1064nm

y

Nd3:YVO4 LiNbO3 Cr:YAG YAG

OpticsLD (808 nm)

Figure.3 The monolithic laser design.

The strong electro-optic effect in LiNbO3 encouraged us to use this material as the active modulator. A 2 mm long z-cut crystal

with polished surfaces was placed between the Cr:YAG crystal and the Nd:YVO4 crystal. The lithium niobate crystal wasdesigned to be robust and at the same time have a large electro-optic effect. To achieve this two grooves were cut along the x-

axis, as shown in Fig. 4. These grooves were tilled with a conducting paste to make the electrodes. The distance between the

grooves was 1.2 mm resulting in a half wave voltage, V of 5.2 kV. The outcoupling mirror had a reflectance of 90 % and was

deposited on a 0.4 mm clear YAG support. All the crystals were optically bonded together in a monolithic design with a total

length of 3.5 mm. There were no need for a polarising component in the cavity since we utilise the strongly polarised emission

from the Nd:YVO4 crystal 12

x

Figure 4. The lithium niobate crystal with groove used in the monolithic laser.

The laser was pumped with a 500 mW multimode diode laser having a 50 pm emitting aperture. The pump light at 808 nm was

collimated and focused with two lenses, both with a focal length of 6.24 mm ( NA=0.4). An ILX 3744 diode laser controllerwas used to drive the pump laser, and a HTS 3 1-06 high voltage switch14 was used to switch the electro-optic modulator as

shown in Fig 5.


Pulse generator H-V power supply 500MHZ Iloscilloscope Frequency analyser

lOOns square pulses @ 25 kHzH-V Switch

Pump diode

IDiodecontroler

I I 1Ft—chip laser 1 .5 GHz detector

Figure 5. The complete set-up to operate the microchip laser and measure the time jitter.

4. EXPERIMENTAL RESULTS

The described laser design and fine tuning of the optical coupling of the pump beam made the laser operate with a repetition rate

of 50 kHz free running, i.e. with no voltage applied to the active modulator. The pulse width was then measured to 12 nsFWHN'I with a HP 54615B oscilloscope and a 1.5 GHz detector. When applying an electrode voltage of 300 V dc and bringing

it down to 0 V in a pulse with a fall time of 10 ns whenever a pulse was wanted, the frequency was tuned to a repetition rate of

25 kHz. The pulse length was then measured to 3 ns corresponding to an optical peak power of around 100 W correspondingin a pulse energy of 300 nJ. The output radiation at 1064 nm had a beam divergence of 6.2 mrad. We also noticed that theoutput pulses were free from after-pulses, indicating that the multiple pulse problem usually observed with this slow switchtimes in pure active Q switching are effectively eliminated 13 The jitter and pulse repetition rate were measured with a Fluke

PM668 1 frequency analyser. As illustrated in Fig. 6, the corresponding low jitter of 65 ps was achieved when the electrodevoltage was 600 V. Our calculations indicate a round trip loss of 12 % when an electrode voltage of 600 V is applied. At anelectrode voltage of 300 V the jitter was 85ps. It should be mentioned that the jitter from the drive electronics was in the meorder and therefore we fmd it difficult to measure the minimum jitter in this laser design. However, the low jitter in this laserdesign has encouraged us to further explore this new laser concept.

We should also mention that the pulse-to-pulse amplitude variations, with and without a voltage applied to the electrodes, were< 1%.For electrode voltages lower than 300 V the laser still emits pulses synchronised with the driving electronics but the jitter

increases dramatically. When no voltage was applied and the laser was passively Q-switched the jitter was about 300 ns.

'111


Applied voltage ( V)

Figure. 6 The variation of thejitter with respect to the applied voltage over the lithium niobate crystal.

5. CONCLUSION

In conclusion, we report on a combined actively and passively Q-switched laser with stable performance and time jitter lower

than 85 p5. This was achieved with a switching voltage of only 300 V, approximately one order of magnitude lower than for

conventional active Q-switching. We have also noticed spurious free output without double pulsing. The fundamentals of thecombined actively and passively Q-switched lasers function have been briefly discussed.

6. ACKNOWLEDGEMENTS

The work on this laser has been performed at the Diode Laboratory at Spectra Precision AB. We would like to thank MikaelFlerzman for solutions on the high voltage switching and Anders Josefsson for input on laser design. We are in dept to Fredrilc

Laureil at the Royal Institute of Technology for helpful discussions and corrections in the manuscript.

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