Amplification of near-infrared light in a photorefractive ring resonator

We have demonstrated efficient amplification of near-infrared, 0.83-μm and 1.06-μm light, in a photorefractive ring resonator using Rh:BaTiO3. The optical power oscillating inside the ring exceeded the pump power by a factor of up to 2.34. The sensitivity of a ring resonator to nanometer changes in its length was characterised using a piezo-mirror. PACS: 42.65. Hw; 42.70.Nq; 42.60.Da Photorefractive resonators have been the subject of several investigations over the past few years. In particular, theoretical analysis of their spatio-temporal [1] and nonlinear [2] dynamics, stability [3], transverse [4] and coherence [5] effects has been extensively researched [6]. One of the resonator’s geometries – a unidirectional ring resonator – has proved particularly interesting because of its unique features. Its geometry is often very tolerant to the position of a photorefractive crystal in the cavity and the incidence angle of the pump beam, as the gratings that develop are self-adaptive and will grow from small seed beams, such as scattered light, if the process is favourable. The resonating beam accumulates energy from successive two-beam coupling amplification, on each round trip, through the photorefractive material until saturation sets in, but loses energy from absorption and other losses such as Fresnel reflections from the crystal, and imperfect mirrors. The resonating beam builds up providing the coupling efficiency exceeds the combined absorption and resonator losses. Such a ring resonator has the capability of providing large amplification of weak signals and being very sensitive to minute changes in its length. The first successful demonstrations of unidirectional ring resonators were carried out by White et al. [7], Kwong et al. [8] and Ewbank and Yeh [9] in which it was discovered that small frequency detuning effects exist between the pump beam and the oscillation beam in photorefractive oscillators. They investigated threshold conditions for oscillations and the maximum permissible cavity-length detuning. A ring resonator can support oscillations over a wide range of cavity ∗Corresponding author. (Fax: +44-23/8059-3910, E-mail: [email protected]) detuning in spite of a narrow, typically few Hz, photorefractive gain. As has been shown, this is due to photorefractive phase shift, which can compensate for cavity detuning and thus help to fulfil the round-trip resonator condition [10]. In characterising the performance of a photorefractive ring resonator, it is useful to consider two parameters. One of them – the conversion efficiency ξ – is the power conversion from the external pump beam into the resonating beam inside the cavity, defined as the ratio of the resonating beam power Pext to the power of the external pump beam Pint, which is given by [11] ξ = Pres Ppump = Rout − e (αeff−Γ )L eαeffL − Rout (1) where Rout is the out-coupling mirror reflectivity, Γ is the two-beam coupling coefficient, L is the length of the interaction region inside the photorefractive material and αeff is the effective absorption coefficient that includes combined losses from the crystal, absorption, mirror and Fresnel reflection losses. This equation is valid above the threshold of oscillations. The other parameter that characterises photorefractive ring resonators is the efficiency of extracting energy from the resonator. It is defined as the ratio of the power of light outcoupled from the resonator (Pout) to the power of the external pump beam (Ppump) [11]: η= Pout Ppump = 1− Rout Rout Rout − e(αeff−Γ )L eαL − Rout . (2) As can be seen from (1), the conversion factor ξ can be larger than 1 if the coupling coefficient significantly exceeds absorption and reflection losses. However, in all previously published papers on photorefractive ring resonators, they have been pumped by visible light, i.e. in the regimes where absorption was high and therefore no net amplification was observed. In the work presented here, we have successfully demonstrated the build up and amplification of power in a ring resonator using a near to optimum wavelength, using material and optical component parameters established earlier [12]. We report on efficiently working resonators pumped by near-infrared, 830-nm and 1.06-μm lasers. The power os-