WP13

GENERATION AND DETECTION OF 13 ps Q-SWITCHED PULSES FROM A 1.3 pm SEMICONDUCTOR LASER CONTAINING A SATURABLE ABSORBER

Frances R. Laughton, David A. Barrow, John H. Marsh Department of Electronics and Electrical Engineering, University of Glasgow, Glasgow GI2 8QQ, Scotland, U.K.

Him L. Portnoi A.F. IoHe Physico-Technical Institute. Russian Academy of Sciences, Politekhnicheskaya 26, S t . Petersburg 194021, Russia.

The generation of high peak power picosecond pulses at 1.3 pm from a InGaAsPlInP semiconductor laser is reported for the first time. A novel sensitive two-photon absorption waveguide autocorrelator was used to codinn that the pulse length

was 13 ps. The pulse energy of the laser was 12 pJ. from which the peak power of the laser was calculated to be 1 W.

Introduction

One method of producing laser pulses with high peak powers is Q-switching. In a semiconductor laser this approach is very versatile as the self-pulsation frequency can be controlled by changing the drive current. We report Q-switching of a InGaAsPlInP laser in which a saturable absorber is formed by implantation of heavy ions into the facets. The width of the pulses was measured using a novel high-sensitivity autocorrelator, which uses two-photon absorption as its nonlinear mechanism.

10 pm * Implanted

absorber Q-switched Pulse Generation

Figure 1: Schematic of Q-switched InGaAsPlInF' laser. InGaAsPiInP Laser Fabrication

Stripe geometry InGaAsPlInP DH lasers with a cavity length of 200pm were produced as follows: firstly the structure was grown on a p-InP substrate by liquid phase epitaxy. All the layers were grown nominally undoped n-type. including the buffer layers. During growth, zinc diffused out of the substrate and through the buffer layer, so making the lower layers p-type. The stripe was formed as a shallow mesa about 6 pm wide with Si02 insulation. The optical mode was essentially gain-guided, and the emission wavelength was -1.3 pm. A schematic of the laser is shown in Fig. 1.

For Q-switched operation, a saturable absorber with an ultrafast recovery time is required. The saturable absorber is essentially an unpumped region of the cavity in which light is absorbed. creating electron-hole pairs. Due to Moss-Burstein filling at high optical intensities, the absorber becomes transparent. The recovery time of the saturable absorber should be faster than the recovery time of

the gain following emission of a pulse-in practice this implies that a recovery time of 10-20 ps is required. The recovery time can be r e d u d by partially amorphising the material using ion implantation (1) - in this case nitrogen ions of energy 18.7 MeV were implanted into both facets with a dose of about 1 x 1 0 l 2 cm-2, as shown schematically in Fig. 2. Such an implantation energy produces amorphous tracks to a depth of about 10pm. The criterion:

m. 1) -

active - a absorber

where y is the gain coefficient, Lacrive is the active gain length, a is the absorption coefficient, and L.&sorber is the saturable absorber length is therefore satisfied. The total dose was implanted in several steps, with annealing by quasi-cw lasing following each step. The behaviour of defects in InGaAsP is different to that in AlGaAs, so that a 5 to 10 times greater implantation dose is required in the f m e r case.

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lOOA

Figure 2: Schematic of the amorphisation of the laser facets to create the saturable absorber.

The advantage of this method of forming the saturable absorber is that most of the implanted material is unaffected by the implantation-amorphisation only OCCUTS

in a 100 radius around the tracks followed by the ions. The sharpness of the optical absorption edge is therefore essentially identical to that of the original bulk semiconductor but, on the other hand, free carriers generated by optical absorption will diffuse to the nearest amorphous track, where they will recombine rapidly. The carrier lifetime is therefore determined by the average separation of the amorphous tracks (2).

Q-Switched Laser Operation

The diodes were driven using a pulse generator which produced electrical pulses with a rise time of 150 ps, a pulse length between 0.4 and 2 ns and a repetition rate of 100 kHz. In a Q-switched laser, the recovery time of the gain medium is determined by the drive current - by using a low peak current of -2OOmA. it was possible to ensure that only one single Q-switched pulse was produced for every electrical pulse.

Pulsewidth Measurement Using A Two-Photon Absorption Waveguide Autocorrelator

The Two-Photon Absorption Autocorrelator

An autocorrelator determines laser pulse durations by measuring the autocorrelation function of the pulse with itself by means of a suitable nonlinear optical process.

Usually second-harmonic generation (SHG) is employed as the nonlinear mechanism (3), but we have recently been investigating a two-photon absorption (TPA) autocorrelator (4). and this is what was used to measure the pulsewidth of the Q-switched InGaAsPlW semiconductor laser.

Fig. 3 shows a schematic of the TPA autocorrelator. A Michelson interferometer splits the incident train of pulses into two orthogonally-polarised beams of equal intensity, which are then end-fire coupled into the reverse-biased p-i-n GaAslAlGaAs waveguide. (The two pulses are orthogonally polarised in order to reduce interference effects within the waveguide.)

Because the Carrier density generated by means of TPA is a quadratic function of the light intensity. the photocurrent generated as a result of TPA varies as a function of the instantaneous peak intensity in the waveguide, and hence the delay time between the two pulse trains, b. By measuring the photocurrent as a function of td, the laser pulsewidth can therefore be obtained.

"r.

-1 Modelockedlasa

waveguide

V (to lock-in amplifier)

.pulses

Figure 3: Experimental arrangement of P A autoconelator.

TPA offers significant advantages over SHG as the nonlinear mechanism in an autocorrelator. The TPA autocorrelator is relatively wavelength-insensitive, as it can be used to measure pulsewidths of any laser whose photon energy is less than the euergy gap Eg of the autocorrelator's guiding layer, but greater than Eg/2. The SHG autocorrelator, in contrast, can only be used over a much smaller range of wavelengths. due to restrictive phase-matching conditions in the SHG crystal. The other major advantages of the TPA autocorrelator are its greater possibilities for integration and that it is very efficient, because the effect of TPA is enhanced by the greater interaction length of the waveguide structure. This high efficiency is vital if semiconductor laser pulsewidths are to be measured.

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TPA waveguide autocorrelator fabrication

The MBE-grown p-i-n waveguide structure consisted of a GaAs guide region surrounded by Alo.pjGa0.85As cladding regions. Those parts of the cladding regions in which there was expected to be significant overlap of the optical mode were left undoped, in order to d u c e losses due to Eree carrier absorption.

Ridge waveguides 3 pm wide, 1 mm long and separated by about 100 pm were fabricated in this material, and wet etching was used to mesa between the ribs to isolate individual devices. The reverse breakdown voltage for the material was at least -30 V, and the dark current at -5 V was about -90 PA. (It is important that this dark current is as small as possible in order to increase the autocorrelator's sensitivity.)

Laser pulsewidth measurement

The pulsewidth of the Q-switched InGaAsPflnP laser was found by measuring the photocurrent due to TPA as a function of the time delay between the two pulse trains in the reverse-biased waveguide. A 1 0 R resistor was placed in series with the waveguide, and the photocurrent was calculated by measuring the voltage across the resistor using a lock-in amplifier. The peak optical power from the laser was about 1 W and, immediately before entering the waveguide, the average power in each beam was about 2 pw.

The autocorrelation trace thus obtained is shown in Fig. 4. It should be noted that a previous attempt to obtain a conventional autocorrelation trace for this laser using a SHG crystal had been unsuccessful, because the laser power was too low. This suggests that the TPA autocomlator is at least as sensitive as a conventional auto"-.

Experimental data "1 Gaussian fit , (FWH E O 7 2 4 0 -30 -20 -10 0 10 20 30 40

Delay time between two beams @s)

M I

Figure 4: Autocorrelation trace of pulses from the Q-switched 1.3 pm GaInAsPflnP semiconductor laser.

We have previously described a simple theoretical model (4) which assumes pulses with a Gaussian intensity profile of the form

From this the average autocorrelation photocurrent can be written as a function of b:

0%: 3) where e is the electronic charge, W is the volume in which the photogenerated carriers are created, h v is the photon energy, Ime is the average intensity of each of the beams, t,, is the time between laser pulses, /3 is the P A coefficient and a is the one-photon absorption coefficient (which is that fraction of the total waveguide propagation loss resulting from the generation of electron-hole pairs by single photons).

From Eq. 3, we find that the width of the autocorrelation trace should be divided by in order to obtain the true laser pulsewidth. In this way, the FWHM of the laser pulse was calculated to be 13 ps, which agrees very closely with a streak camera measurement of the pulsewidth. This demonstrates the high sensitivity of the TPA autocorrelator in characterising short optical pulses in the 1.3- 1.5 pm spectral range.

References

1. E.L. Portnoi, N.M. Stel'makh and A.V. Chelnokov, "Characteristics of heterolasers with a saturating absorber obtained by the deep ionic implantation", Pisma v Z h d Tekhnicheskoi Fiziki, Vol. 15, pp. 4448, 1989.

2. E.A. Avrutin and M.E. Portnoi, "Estimate of the lifetimes of nonequilibrium carriers in a semiconductor irradiated with heavy-ions", Sov. Wys. Semicond., Vol. 22. pp. 968-970, 1988.

3. E. Mathieu and H.J. Keller, "Intensity correlation functions of a non-Q-switched laser, measured by second- harmonic generation", J. Appl. Phys., Vol. 41, pp. 1560- 1567, 1970.

4. F.R. Laughton, J.H. Marsh and A.H. Kean, "Very sensitive two-photon absorption GWAlGaAs waveguide detector for an autocorrelator", Electron. Lett., Vol. 28, pp. 1663-1665, 1992.

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