Embodiments are directed to photonic integrated circuits that include compact arrangements of Mach-Zehnder interferometers. Specifically, a Mach-Zehnder interferometer may include an input beam splitter and an output beam splitter that are configured to introduce light to and receive light from, respectively, a pair of intermediate waveguides. The Mach-Zehnder interferometer may be configured such that light enters and exits the pair of intermediate waveguides in different directions. Multiple Mach-Zehnder interferometers may be configured in this way and nested such that a pair of intermediate waveguides of one Mach-Zehnder interferometer may at least partially wrap around the intermediate waveguides of another Mach-Zehnder interferometer.

A system including an optical transmitter. In some embodiments, the system includes: a first array of lasers; a first wavelength multiplexer, connected to the first array of lasers; a first coupler, connected to the wavelength multiplexer; and a first wavelength meter, connected to a first output of the first coupler.

According to an aspect there is provided an optical monitoring device comprising: a first input for receiving a portion of a first optical signal coupled from a first waveguide into the first input; a second input for receiving at least a portion of a second optical signal coupled into the second input; a mixing unit for controlling combining of the portion of the first optical signal with the at least a portion of the second optical signal into a combined signal at an output from the mixing unit; and at least one photodetector for detecting the combined signal. The optical monitoring device is configured to apply a modulation signal to modulate at least one of a phase of the portion of the first and/or second optical signal, a coupling of the portion of the first and/or second optical signal into the respective input, or an amplitude of the portion of the first and/or second optical signal being transferred into the combined signal.