A High Reliability High Sensitivity Lightwave Receiver for the SL Undersea Lightwave System

Undersea lightwave systems need receivers with the utmost in both reliability and sensitivity. Our design has evolved from the type which we provided for an earlier deep-sea trial. The primary features of our present receiver are an InGaAs planar p-i-n photodiode with passivation and antireflection coating and a Si bipolar integrated circuit, both encapsulated in a special chip carrier. Both the photodiode and the amplifier are placed in the same carrier so that a direct chip-to-chip wire bond can minimize stray capacitance, which helps to maximize sensitivity. The back-illuminated structure of the photodiode allows the InP substrate to serve as a window in the hermetically sealed carrier. Since both active devices are in a separate package from the passive components, they can undergo the high temperature bake-out and hermetic seal followed by the high temperature high bias. purge necessary to assure the reliability of the semiconductors. This optical chip carrier is then mounted inside another hermetic package which contains the passive components, including the optical fiber pigtail, which must undergo bake-out and seal at more moderate temperatures. Within the constraints of reliability, bandwidth, and dynamic range, the physical structure of the integrated circuit and its bias conditions have been chosen to maximize sensitivity. The design of the circuit enables us to select those devices from a production lot which will achieve the highest sensitivity in a completed receiver. This receiver is used in systems operating with NRZ data from 140 to 440 Mbit/s. At 1.3 μm and 296 Mbit/s the mean sensitivity achieved was an average optical power
\bar{P} = -35.6
dBm for a 10
^-9
bit error rate (BER), with a best value of -36.3 dBm. This receiver is physically robust, being able to withstand stresses well in excess of those encountered in actual use. Preliminary aging data indicate that a long-term failure rate of less than 2.5 in 10
⁹
device hours can be achieved.