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Description
In this work, we present a customized pn depletion type Mach-Zehnder modulator (MZM) as well as a fully integrated wavelength division multiplexing (WDM) transmitter design with the merits of high bandwidth and radiation hardness, aiming to upgrade the optical data transmission of future detector systems. A detailed characterization of the modulators on modulation efficiency and RF response was carried out. Based on this work, the first optical link with a data rate of 11.3 Gb/s was set up transmitting 6x10$^{12}$ bits error free.
Summary
The number of detector read-out channels as well as the data rate of each individual channel are rapidly increasing in detector instrumentation. Optical links exploited in current large-scale detector systems use directly modulated laser diode based transmitter units. In this scheme, each transmitter is connected by an individual optical fiber with an off-detector receiver. The most viable way to cope with ever increasing data throughput is adding more and more fibers at the price of increased mass and space requirements. In contrast we propose an integrated WDM transmission system with which the data transmission bandwidth of each fiber can be greatly enhanced. The system also addresses the problem of limited radiation hardness of the laser diodes as they will be placed outside of the radiation area.
According to the radiation hardness study of CERN [1], we fabricated radiation-hardened MZMs with customized etch depth and doping concentration. From steady state measurements, we found the driving voltage on one 3 mm long arm to get a phase shift of $\pi$ is about 15.3 V, resulting in the figure of merit $V_\pi*L$ = 4.6 Vcm. This is a significant improvement compared to our previous device. Also, we fabricated another set of modulators with different lengths and etch depth for comparison on the same chip. Extensive characterizations are in progress and a detailed comparison will be presented.
Considering the DC measurement results, RF measurements were carried out by applying a reverse bias voltage of 2.5 V and an RF amplitude of 2 $V_{pp}$ to the two phase shifters. The modulated optical signal was measured with an electrical spectrum analyzer via a fast photodiode. From the measurements we derived a nearly linear decrease of the signal with frequency at a rate of 1.1 dB per GHz.
Furthermore a complete transmission experiment was set up based on an FPGA as high-speed data source, a two-stage amplifier to drive the modulator with an amplitude of up to 7 $V_{pp}$, the modulator, an optical amplifier and a commercial SFP+ receiver. The receiver output was connected to the FPGA for bit error rate measurements. We conducted successfully error free transmissions of 6x10$^{12}$ bits at a rate of 11.3 Gb/s with a driving amplitude down to 3.1 $V_{pp}$. A thorough characterization to sound the limits of error free transmission and transmission with acceptable error rates for different scenarios is ongoing and will be presented.
[1] M. Zeiler, S. Seif El Nasr-Storey, S. Detraz, A. Kraxner, L. Olantera, C. Scarcella, Ch. Sigaud, C. Soos, J. Troska, F. Vasey, “Radiation Damage in Silicon PhotonicMach–Zehnder Modulators and Photodiodes”, IEEE Trans. on Nuclear Science, vol. 64, no. 11, Nov. 2017, pp. 2794
