New method for in-depth characterization of GaAs-based electro-optic phase modulators
Fig. 1: Schematic description of measurement method. DUT: device under test; S: frequency shifter, UM: modulation signal applied to the DUT, PR: photo-receiver; I&Q: signal sampling and I&Q demodulation.
Fig. 2: Spectrum of phase modulation (left) with corresponding RAM (right) for modulation of a TE mode optical signal. Modulation frequency and resolution bandwidth correspond to 500 kHz and 3.8 kHz, respectively.
Chip-based modulators provide the means to micro-integrate phase control and modulation into hybrid laser and spectroscopy modules in a very compact and robust way. GaAs-based phase modulators are attractive due to unique integration possibilities with existing semiconductor electronics [1]. In electrical terms, GaAs chip-based modulators behave like electrical diodes operated in reverse direction, and hence the modulation efficiency and residual amplitude modulation (RAM) as well as the non-linearities are expected to depend on the operating parameters. These are offset bias voltage, modulation voltage amplitude and modulation frequency.
We have developed and successfully implemented a new method to accurately determine optical phase shifts of less than a degree so that, for example, the modulation efficiency can be determined as a function of bias and modulation voltage. Since this method determines phase modulation and RAM as a function of time, it also allows analyzing non-linearities (signal distortion) of the device. As modulation analysis is carried out in real-time, the technique further provides the means to optimize coupling of the optical field to the GaAs chip waveguide structure with respect to modulation efficiency and minimum RAM.
The measurement setup is based on a heterodyne interferometer (see Fig. 1). The output of the device under test (DUT) interferes with the reference beam on a fast photo-receiver. The corresponding RF beat note signal contains the full information on the modulation behavior of the DUT. To extract this information, the quadrature components (I&Q) of the beat note signal are recorded as a function of time, and amplitude and phase modulation are determined in real-time by analog demodulation. Finally, spectra for amplitude and phase modulation are determined by FFT in real-time.
Fig. 2 shows the spectrum of phase and amplitude modulation of a TE mode optical signal at the wavelength of 1064 nm. The modulation was applied to a 4 mm long GaAs/AlGaAs P-p-p-n-n-N double heterostructure phase modulator operated at a bias voltage of -1.5 VDC and driven by a sinusoidal modulation signal (frequency fM = 500 kHz) with an amplitude of 1.4 V. The Fourier components of the phase modulation spectrum at the modulation frequency fM and its 2nd harmonic correspond to the phase shift caused by linear effects (LEO and carrier density-related effects) and quadratic effects (e.g. the quadratic electro-optic effect), respectively. An optical phase shift smaller than 0.1 degrees can be measured, which demonstrates the sensitivity of the measurement method. The linear modulation efficiency is given by the Fourier component at the modulation frequency divided by the modulation voltage amplitude. Determination of the linear modulation efficiency for TE and TM modes delivers the linear electro-optic coefficient of the GaAs-based phase modulator waveguide. The quadratic electro-optic coefficients for the TE and TM modes of the waveguide structure are determined by dividing the respective Fourier components at the 2nd harmonic of the modulation frequency by the square of the modulation voltage amplitude.
Similar to the phase shift measurements, the required information of the amplitude modulation can be directly extracted from the Fourier components of the amplitude modulation, see Fig. 2. Application of this method is not limited to GaAs-based modulators, but can also be applied to any kind of EO phase modulators.
Analysis of the GaAs/AlGaAs P-p-p-n-n-N double heterostructure phase modulator delivers the following results. The linear phase modulation efficiency corresponds to 60.33°/V and 25.25°/V for TE and TM modes, respectively. The corresponding LEO coefficient is found to be -1.79 × 10-10 cm/V. The quadratic phase modulation efficiency is determined to 0.69°/V2 for the TE and 0.59°/V2 for TM modes. The corresponding QEO coefficients are -2.84 × 10-17 cm2/V2 and -2.41 × 1 0-17 cm2/V2 for the TE and TM modes, respectively. The related RAM level for a modulation amplitude less than 100 degrees is below 10-3.
Publication
[1] B. Arar, H. Wenzel, R. Güther, O. Brox, A. Maaßdorf, A. Wicht, G. Erbert, M. Weyers, G: Tränkle, H.N.J. Fernando, A. Peters, "Double-heterostructure ridge-waveguide GaAs/AlGaAs phase modulator for 780 nm lasers", Appl. Phys. B, vol. 116, no. 1, pp. 175-181 (2014).