There are two types of optical transceivers available on the market: coherent and non-coherent. Typically, non-coherent transceivers use simple modulation techniques such as optical intensity modulation (amplitude control) and direct detection. As a result, the signal could be either NRZ or PAM4. Due to their low cost and low power consumption, they are preferred over coherent transceivers. Their main disadvantage is that they can only be used for short distances.

Figure.1: Optical Transceivers

Data-center line transmissions, 40G data-center line transmissions, and short-range 100G metro transmissions use non-coherent transceivers.

In this blog, we will discuss the TX side of non-coherent optical transceivers. The TX consists of a PCB section and a package section. It is necessary for the package section to be hermetic since it contains all electrical-to-optical components. In addition to the outer electrical interface, the PCB section contains four parallel differential laser drivers, which are connected to the package using wirebonds or flexible cables.

Figure.2: Non-coherent optical transceivers

https://www.qsfptek.com/qt-news/the-internal-components-and-structure-of-the-optical-transceiver.html

The TOSA section is contained within the package. TOSA is an acronym for transmit optical sub-assembly. It consists of four electro-optic channels controlled by a PLC. A PLC is an acronym for a Planar Lightwave Circuit. A passive light device that combines or splits Lightwave signals of different wavelengths. There is an EML chip, a termination, a DC block, and a lens in each of the four channels. EML is an acronym for electro absorption modulated laser.

Figure.3: Inside TOSA

An EML consists of two sections. The first is a DC laser that generates continuous laser light at a specific wavelength. The end section of the EML is composed of a PIN diode, called an EAM (Electro-Absorption Modulator). PIN diodes serve as intensity modulators. Through the lenses, the laser signals coming from the EMLs, each at a specific wavelength, are directed to the PLC. By using the DWDM (Wavelength division multiplexing) technique, all four signals are combined into one optical signal.

Figure.4: EML structure

Source: Researchgate.net

RF signals are transmitted from the EML sub-mount to the EAM via an RF line. DC and RF components are present in the signal. In response to the voltage on the top pad, EAM changes its characteristics. As soon as the signal passes through the EAM, it passes through a termination, followed by a DC block. It is important to terminate the cable in order to avoid AC reflection. DC blocks prevent the DC part of the signal from shorting to ground. Only the AC portion of the signal is allowed to pass.

Figure.5: Wirebonds in the EML submount

Designing optical transceivers using HFSS.

By utilizing HFSS, designers are able to:

Design and optimize the PCB external transitions in accordance with IEEE 801.3.
Design and optimize the internal RF routing on the PCB.
Design and optimize the wirebond/flexible cable transition between the PCB and the package.
Four (4) design and optimize the EML section: package RF line + wirebond + sub-mount RF line + wirebond + EMA diode + wirebond + sheet resistance (50Ohm) + capacitor.
The equivalent model of the diode can be added using circuit and a full path analysis can be performed.
Analyze and minimize crosstalk between RF lines on the PCB and in the TOSA package.
Minimize crosstalk between Tx RF lines and Rx RF lines.

Current design challenges:

EML design is challenging, and maintaining a low cost is challenging as well. The most challenging part of TOSA is controlling the ground current so that it follows the main RF line signal. In general, ground currents follow short paths and avoid crossing under the EAM, resulting in an uncontrollable voltage under the EAM. In order for AEM to be reliable, particularly for PAM4 applications, smart design is necessary.

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