Now, a team of researchers led by electrical engineer Marko Lončar at SEAS has developed a method for building a highly efficient integrated isolator that’s seamlessly incorporated into an optical chip made of lithium niobate. But conventional isolators have been relatively bulky in size and require more than one type of material to be joined together, creating a roadblock to achieving enhanced performance.

Isolators also help cut down on signal noise by preventing light from bouncing around unfettered. To protect the lasers from damage and instability, these systems also require isolators, components that prevent light from traveling in undesired directions. Paulson School of Engineering and Applied Sciences (SEAS) could drastically improve optical systems for many practical applications.Īll optical systems - used for telecommunications, microscopy, imaging, quantum photonics, and more - rely on a laser to generate photons and beams of light. (Image: Loncar Lab/Harvard SEAS)Īn optical isolator developed at the Harvard John A. Paulson School of Engineering and Applied Sciences, Cambridge, MA Optical micrograph of the electro-optic isolator chip on thin-film lithium niobate, comprising four devices with varying modulation length. Effectively, this results in a 3-port device, hence the utility of the second symbol for directional couplers in figure 1. This termination can be internal to the device and port 4 is not accessible to the user. However, the device is not normally used in this mode and port 4 is usually terminated with a matched load (typically 50 ohms). A portion of the power applied to port 2 will be coupled to port 4. Directional couplers are frequently symmetrical so there also exists port 4, the isolated port. Port 2 is the transmitted port where the power from port 1 is outputted, less the portion that went to port 3. Port 3 is the coupled port where a portion of the power applied to port 1 appears. Port 1 is the input port where power is applied. The symbol may have the coupling factor in dB marked on it. The symbols most often used for directional couplers are shown in figure 1. Two symbols used for directional couplers These include providing a signal sample for measurement or monitoring, feedback, combining feeds to and from antennas, antenna beam forming, providing taps for cable distributed systems such as cable TV, and separating transmitted and received signals on telephone lines. Many of these waveguide couplers correspond to one of the conducting transmission line designs, but there are also types that are unique to waveguide.ĭirectional couplers and power dividers have many applications. Also at microwave frequencies, particularly the higher bands, waveguide designs can be used.

However, lumped component devices are also possible at lower frequencies, such as the audio frequencies encountered in telephony. This technique is favoured at the microwave frequencies where transmission line designs are commonly used to implement many circuit elements. A directional coupler designed to split power equally between two ports is called a hybrid coupler.ĭirectional couplers are most frequently constructed from two coupled transmission lines set close enough together such that energy passing through one is coupled to the other. Power entering the output port is coupled to the isolated port but not to the coupled port. An essential feature of directional couplers is that they only couple power flowing in one direction. They couple a defined amount of the electromagnetic power in a transmission line to a port enabling the signal to be used in another circuit. Power dividers (also power splitters and, when used in reverse, power combiners) and directional couplers are passive devices used mostly in the field of radio technology.

A 3 dB 2.0–4.2 GHz power divider/combiner.

From left to right: input, coupled, isolated (terminated with a load), and transmitted port.

Active rf isolator