1 X 18 Array Of Low Voltage, Asymmetric Fabry-Perot Modulators For Gigabit Data Transmission Applications

Asymmetric Fabry-Perot modulators (AFPMs) possess many qualities which make them excellent candidates for use in smart pixel applications [l]. They inherently operate in a surface normal configuration and are polarization independent. Devices have been designed to achieve low insertion loss, high contrast ratio, low voltage operation and high electrical bandwidth [2-31. Recently, we have extended this work in the direction of packaged arrays with low voltage (0-5V) and high bandwidth (10 GHz) operation. The design includes intracavity contacts for low series resistance and coplanar transmission lines on a semi-insulating substrate for impedance matching. The package includes standard microwave (SMA) connectors and a planar microlens array. The AFPM consists of a multiple quantum well (MQW) p-i-n diode between two DBR mirrors of unequal reflectivity which form a resonant cavity. In the zero-bias, low absorption state, the reflectivity is high (typically > 50%). Applying an electric field across the p-i-n diode increases the cavity absorption via the quantum confined Stark effect (QCSE), decreasing the effective back mirror reflectivity to match that of the front mirror and nulling out the reflected signal. The device design parameters include the front mirror reflectivity, the number of quantum wells, the quantum well and barrier thicknesses and compositions, thickness of the contact regions, operating voltage swing and the operating wavelength. Since the QCSE can be treated as an instantaneous process on picosecond time scales, the modulation speed is limited in practice only by the speed with which the field applied across the MQW region can be modulated. Quantum well material (1 OOA GaAs wells with 45A A10.3Ga0.7As barriers) was characterized in terms of absorption as a function of wavelength for different applied voltages (or equivalently, electric fields) using photocurrent measurements. Using this data, a modulator was designed to have €3 dB insertion loss and >lo GHz bandwidth (with a 50Q parallel termination resistor) for a 4V modulation swing and operate at a wavelength of about 853 nm to be compatible with GaAs QW laser sources. Typical DC characteristics are shown in Figure 1. Growth non-uniformity across the array adversely affects performance. We fabricated a I x 18 array of AFPMs with a 250 pm pitch. We observed approximately a 1 nm variation in the optimum wavelength of operation across the array. This translates into a maximum insertion loss of 4 dB and minimum contrast ratio of 8 dB across the entire array for a fixed wavelength of operation. Location and orientation of an array on the wafer in addition to the optical bandwidth of the device determine these values and so improvements are possible i n the future. Microwave measurements were performed using an HP85 1 OC network analyzer. The transmission line and device were characterized by measuring S i 1 at a bias of 2V. The equivalent circuit model is shown in the inset of Figure 2. The parallel RC equivalent circuit represents the ohmic contacts whose resistivity was characterized using TLM patterns and found to be high. The series RC equivalent circuit values are in good agreement with the expected values for a 40 p m diameter device with a 0.47 pm thick intrinsic region. The light reflected by the modulator was focused into a multimode fiber and converted to an electrical signal by a 25 GHz detector. Two 18 dB, 20 GHz microwave gain stages were used to amplify the signal before being measured by the spectrum analyzer. A simulation of the S ~ I measurement using the S 1 1 model was performed using Microwave Spice. The simulated and experimental data are in good agreement and are shown in Figure 2. The 3 dB bandwidth is approximately 4 GHz. When a 50R termination resistor is included in the simulation, a 3 dB bandwidth of 7.3 GHz is achieved. The device was incorporated into a gigabit optical data transmission testbed and it’s bit error rate (BER) performance evaluated at 1 Gbit/s. The BER curve is shown in Figure 3. The receiver sensitivity is limited to -19 dBm at a BER of 10-9 due to the thermal noise (NEP = 1 pW optical) A Ti:Sapphire laser was used as the optical source for the S2i measurements.