In this example, we will demonstrate how INTERCONNECT can be used to design and simulate an optical transceiver.

## Problem definition: More details

An optical transceiver is a single, packaged device that works as a transmitter and receiver. It is an important part of optical network equipment that converts electrical signals to optical signals and vice versa. The data flow simulator of INTERCONNECT allows for time domain simulations, where each data is a time stamped signal sample. Now you can simulate time domain impairments such as intersymbol interference. The circuit below shows an externally modulated NRZ transmitter and a direct detection receiver using a PIN photodiode:

To simulate a transceiver in INTERCONNECT we can start by creating the transmitter and receiver modules using the elements from the Element Library (connecting the elements and setting their properties). INTERCONNECT allows you to perform signal integrity analysis in the transmitted and received time domain waveforms using different types of analyzers, such as oscilloscopes and spectrum analyzers.

## Discussion

Using INTERCONNECT, it is trivial to carry out time domain simulations of externally modulated laser transmitters or direct detection lightwave receivers using different modulation formats. For time domain simulations the user can set the time window and the number of samples, which will define the signal sampling rate used in the simulation. In order to simplify the process of estimating these parameters, one can simply define the transmitter bitrate, the number of samples per bit and the bit sequence length. INTERCONNECT will then calculate the time window, sample rate and the number of samples.

The simulation time window is:

$$

t_{w}=L_{B} \cdot T_{B}=\frac{L_{B}}{\mathrm{B}_{\mathrm{R}}}

$$

Where L_{B} is the sequence length, T_{B} is the bit period and B_{R} is the bit rate. The simulation sample rate is:

$$

f_{S}=\frac{N_{S / B}}{T_{B}}=N_{S / B} B_{R}

$$

Where N_{S/B} is the number of samples per bit. The number of samples is:

$$

N_{S}=L_{B} \cdot N_{\mathrm{S} / \mathrm{B}}=t_{w} f_{S}

$$

For analog signals, we can simply define the simulation time window as:

$$

t_{w}=N_{S} T_{S}=\frac{N_{S}}{f_{S}}

$$

Where T_{s} is the sampling period. For instance, for a bit rate of 10 GBits/s, 16 bits and 64 samples per bit, the number of samples is 1024, the time window is 1.6 ns, and the sample rate is 640 GHz. The time domain waveform will have the following properties:

The frequency domain information is:

Depending on the simulation input property, these parameters can be calculated automatically by INTERCONNECT.

## Results

Signal integrity analysis is done by special elements, the analyzers. Analyzers allows for post-processing of data stored in monitors. The Eye Diagram analyzer creates eye plots from the signal at the Bessel filter output port. It uses the original signal from the NRZ Pulse Generator as a reference signal to estimate and compensate for propagation delays (clock recovery) between the transmitter output and receiver input. The attenuator element simulates channel absorption. As attenuation increases the effects of thermal noise increases at the receiver, causing the eye diagram to close:

Analyzers can be inserted at different points of the circuit for detailed analysis of the signal evolution from the transmitter to the receiver:

### Related publications

J. Senior, Optical Fiber Communications: Principles and Practice, 3rd ed. (Pearson Education Limited 2009)