Abstract
Precise knowledge of an optical device's frequency response is crucial for it to be useful in most applications. Traditional methods for determining the frequency response of an optical system (e.g. optical cavity or waveguide modulator) usually rely on calibrated broadband photo-detectors or complicated RF mixdown operations. As the bandwidths of these devices continue to increase, there is a growing need for a characterization method that does not have bandwidth limitations, or require a previously calibrated device. We demonstrate a new calibration technique on an optical system (consisting of an optical cavity and a high-speed waveguide modulator) that is free from limitations imposed by detector bandwidth, and does not require a calibrated photo-detector or modulator. We use a low-frequency (DC) photo-detector to monitor the cavity's optical response as a function of modulation frequency, which is also used to determine the modulator's frequency response. Knowledge of the frequency-dependent modulation depth allows us to more precisely determine the cavity's characteristics (free spectral range and linewidth). The precision and repeatability of our technique is demonstrated by measuring the different resonant frequencies of orthogonal polarization cavity modes caused by the presence of a non-linear crystal. Once the modulator has been characterized using this simple method, the frequency response of any passive optical element can be determined to a fine resolution (e.g. kilohertz) over several gigahertz.
Highlights
Several methods exist for measuring the frequency response of an optical cavity or modulator
We demonstrate a new calibration technique on an optical system that is free from limitations imposed by detector bandwidth, and does not require a calibrated photo-detector or modulator
The precision and repeatability of our technique is demonstrated by measuring the different resonant frequencies of orthogonal polarization cavity modes caused by the presence of a non-linear crystal
Summary
Several methods exist for measuring the frequency response of an optical cavity or modulator. Techniques based on frequency modulation are effective in determining a cavity’s FSR17–21 These methods require both a calibrated modulator and a fast photo-detector with a known frequency response. Several techniques exist that can determine the response of a modulator that do not require a fast photodetector, such as optical heterodyning[22–25] or sweptfrequency techniques[26]. As optical device bandwidths continue to increase, it is necessary to develop a measurement technique that determines their frequency response without depending on the precise calibration of a high-speed photo-detector, modulator, or optical cavity. We use a recursive method to obtain the frequency response of an optical system This system consists of both an optical cavity and a high-speed (uncalibrated) amplitude modulator, which can be simultaneously characterized from the same measurement configuration. Once the modulator has been calibrated, this measurement technique can be used to characterize the frequency response of any passive optical element, and is not limited to cavities
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