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5.4.1 Introduction

Ultrafast all-optical switching has been demonstrated using AlGaAs based semiconductor waveguide structures at photon energies corresponding to approximately half the bandgap energy.^^’^^At these wavelengths, nonlinear phase shifts in excess of

2n

radians have been directly observed in straight guides^^, due mainly to the minimal competition from nonlinear absorption, but also with the assistance of a localised enhancement in the nonlinear refractive index related to two- photon transitions (real or virtual, depending on w avelength).N onlinear index coefficients of

ri

2

^ + l x

cm^W'l have been estimated for this material which is more than 1 0 0 times stronger than that observed for fused-silica-based optical fibres. This large nonlinearity, together with their potential for inducing nonlinear phase shifts of the order of

n

radians, make these semiconductor guides attractive and compact replacements for optical fibres in coupled-cavity mode-locked lasers.^® In this section, the mode locking of a coupled-cavity KCl:TlO(l) colour-centre laser is described where a passive AlGaAs waveguide was incorporated in the control cavity.

In previous experiments where a near travelling-wave semiconductor optical amplifier was used as a nonlinear element in a coupled-cavity laser^®»^^, considerable care had to be taken to minimise parasitic reflections from the facets. The anti­ reflection coatings on the device were found to be inadequate, and a complex approach involving a ring cavity geometry containing two optical isolators was necessary to obtain stable mode-locked operation^^.

5.4.2 Experiment

The coupled-cavity laser was constructed in the conventional Fabry-Perot configuration as shown in Figure 5.14, with mirrors MO to M l forming the master cavity and M l to M2 the control cavity. The KCl:TlO(l) colour-centre crystal was pumped at average powers of up to 2 W by an acousto-optically mode-locked

Chapter 5 Applications 125

Nd:YAG laser operating at 1064 nm. The useful output from the colour-centre laser

was obtained by inserting a 50/50 beamsplitter (BS) in the control cavity as illustrated in Figure 5.14. In the experiments described here, the passive AlGaAs waveguides were fabricated at an angle to the cleaved facets, such that light entered and exited the guide at Brewster's angle thus reducing parasitic reflections from the facets and rendering anti-reflection coatings unnecessary.

AlGaAs waveguides Nd:YAG pump

MAIN

OUTPUT

Figure 5.14. Schematic of the coupled-cavity mode-locked laser incorporating a passive waveguide in the control cavity.

The waveguides were grown in AlGaAs onto a GaAs substrate using molecular beam epitaxy at the Department of Electrical and Electronic Engineering at Glasgow University. The composition of the waveguide is the same as that of the integrated Mach-Zehnder interferometer described in the previous section. A 1.5 |im thick Alo.i8Gao.8 2As guiding layer was sandwiched between buffer and upper cladding layers, both having 24% A1 composition. Twelve parallel guiding ribs with widths varying from 3.0 to 5.5 jam were revealed in a 1.5 jam thick upper cladding layer to a height of approximately 1.3 jam using reactive ion etching. Using an estimate for the modal refractive index, the Brewster's angle was calculated to be 73.3 deg for these waveguides. The substrate (see inset of Fig. 5.14) had dimensions (1X w) = 4.15 X 5.35 mm2, and the waveguides were 4.33 mm long. This relatively large substrate presented at the Brewster angle required coupling lenses with working distances greater than 2 mm, and hence light was coupled into the waveguides using a X 10 microscope objective (OBJ). The exit beam was collected using a Melles-Griot diode collimating lens (DCL) having a numerical aperture of 0.276 and this beam was focussed onto the highly reflecting mirror M2 and retroreflected.

Chapters Applications 126

The throughput of the guide was typically 15% for the initial pass, and the effective reflection coefficient for the combined waveguide and mirror M2 was estimated to be just less than 2%. (Typically, we would have an equivalent reflection coefficient of ai'ound 50% when using an optical fibre in the control cavity). The poor efficiency is primarily related to modal mismatches caused by the Brewster-angling of the substrate, and by the use of long-working-distance lenses, although the linear loss of the guide also contributes. The parasitic reflectivity of the device as seen by the master cavity (mirror M2 blocked) was approximately a factor of 500 smaller, and so this did not prevent mode locking. The most stable mode-locking condition was observed when the transmission of the common mirror M l was increased to 22%, which resulted in average output powers of around 50-70 mW in the main output (see Fig. 5.14).

As is normal with coupled-cavity lasers, the optical path lengths of the master and control cavities were required to be matched to within a fraction of a wavelength. This was achieved using stabilisation electronics to control the position of mirror M2, which was mounted on a piezo-electric translator (PZT)^^, Normally the error signal would be derived by monitoring the average output power, but the low level of feedback in the laser described here was insufficient to induce significant interferometric fluctuations of the power levels in either of the two output beams. Long term stabilisation was achieved, however, by deriving an error signal from the second-harmonic of the main output power. By contrast, this latter, more complicated approach was not required in the previous experiments where the semiconductor optical amplifier was employed, even though the feedback level was less. The active waveguide^^ differs in that it suffered from significant nonlinear attenuation, which in turn, caused a large modulation of the output power of the secondary output as the laser switched alternately from mode-locked to continuous operation with changes in the length of the control cavity.

The colour-centre laser was mode-locked by synchronous pumping in order to initiate the coupled-cavity mode locking. Self-starting of the mode locking was not observed during these experiments, as result of the ultrafast recovery time of the nonlinearity of the AlGaAs waveguides. With the active waveguide^^, start-up from mode-beating was possible since it relied on self-phase modulation (SPM) primarily associated with the much stronger resonant nonlinearity arising from carrier density changes.

Chapter 5 Applications 111

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