Objective A randomised controlled trial to determine the acceptability to dentists and patients of cavity preparation with an Erbium:YAG laser as compared with conventional handpieces. Methods Fifteen dentists (9 GDPs, 1 community dentist and 5 hospital dentists) treated 77 patients (age range 3.5-68 years old) who had two matched cavities, in a split mouth, randomised trial. One cavity was prepared conventionally, the other with the laser, with dentist and patient preference determined by questionnaire. Results In the majority of cases, where dentists expressed a preference, it was for conventional cavity preparation, and this was significant (P < 0.001). In more than half of the laser appointments, dentists had to use conventional handpieces to complete the cavity. Principle difficulties reported with the laser were access (25 cases) and slow speed of cutting (11 cases). Patients aged ≥10 years who expressed a preference, preferred laser treatment, and this was significant (P < 0.001). Patients aged <10 years, assessed using a simplified pictorial questionnaire, did not show a significant preference for either technique. Conclusions Dentists preferred conventional handpieces for cavity preparation while patients aged ≥10 years old preferred laser treatment. Patients <10 years old did not express a preference.

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Fax: (310) 317-5679The loop phase conjugate mirror (PCM) has attracted much of attention recently for application to high power solid state lasers, because of its ability to work with a unique combination of laser radiation parameters, such as high aberrations, Q-switched through CW pulse formats, low peak and high average powers, broad spectral bandwidths [ 1,2]. Another challenge for any kind of PCM in general is to accommodate depolarized laser beams, especially, that the commonly used technique, first shown for SBS [3], and based on the transformation of depolarized light before entering a PCM into linear polarized component, is very difficult to apply for a loop type of PCM.In [4] we suggested an idea how to modify the basic loop PCM optical scheme in order to work with depolarized radiation. In this paper we present experimental results showing near 100% polarization conjugation fidelity, in combination with compensation of phase distortions and high reflectivity.Basically, loop PCM is a laser that is seeded and triggered by an external signal beam, which we want to be phase conjugated. The optical scheme is shown in Fig.1. The ring resonator of this laser contains a nonlinear medium, which acts as one of the resonator mirrors after the signal beam writes a hologram (or grating) in it. The signal beam E, enters the loop, passes through the nonlinear medium, reflects from mirrors, and then as beam E, it intersects with E, inside the nonlinear medium. The interference pattern of these two beams records a hologram via a nonlinear mechanism. When the hologram reflectivity, combined with the gain of laser amplifier fulfills a threshold condition, spontaneous emission builds up, giving rise of a strong return E,, which, under certain conditions, may be close to a phase conjugate replica of the signal beam E,. A 'leaky" optical diode (which usually consists of a Faraday rotator, polarizers, and other optical elements) inside the resonator serves to prevent saturation of the amplifier by the input signal E,, and to support only unidirectional generation of the return beam E,, providing controllable attenuation for the input signal and insignificant losses for the generated return E,.For an arbitrary polarized signal beam, we actually have two loop resonators operating in parallel: one for the S and another for the P polarization components, which are wraped up together in one optical scheme. To accommodate the loop scheme with this situation, we first modified the optical diode to allow it to work with two independent S and P polarization components of the input and return beams. In addition, we placed a half-wave plate inside the loop resonator to serve as a polarization mixing element. This Diode \3 Figure 1 0-7803-4950-4/98/$10.00 1998 IEEE 288

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