Temperature tuning of two-photon absorption microcavity photodetectors for wavelength selective pulse monitoring

Introduction: Two-photon absorption (TPA) has recently received attention with optical performance monitoring of group velocity dispersion, polarisation mode dispersion and optical signal-to-noise ratio having been demonstrated [1, 2]. TPA is a weak, nonlinear process and as such is inefficient. A number of different detectors have previously been used to overcome the low level of TPA in semiconductors such as silicon APDs and detectors with long absorption lengths [3, 4]. The detector used here is a TPA GaAs/AlAs microcavity which has been previously shown to increase TPA by .10 000 times [5]. These cavities consist of an absorber layer sandwiched between two highly reflective Bragg stacks. The detector intrinsically has a built-in filter with a spectral full-width at half-maximum (FWHM) of less than 1.5 nm. To use one detector to monitor several channels in a wavelength division multiplexed (WDM) system requires these detectors to be tunable. Previously, it has been reported that these cavities can be tuned by 55 nm using angle tuning, thereby allowing one device to cover the entire C-band [6]. Angle tuning, however, will cause the detector response to change, owing to changes in the TPA enhancement factor and the polarisation sensitivity of the photodetector response [6]. In this Letter, we investigate the use of temperature based tuning of the TPA microcavity as a robust, cost-effective means of tuning [7]. As temperature change has been shown to influence other TPA detectors [8] it is necessary to fully understand the temperature dependence of the response of a TPA microcavity. This investigation demonstrates how practical such tuning would be for optical performance monitoring in a telecommunications network.