What is an Optical to Electrical Converter?

An optical-to-electrical converter is the main component for designing optical instruments. As the name suggests it is a modulating device that converts incoming optical signals from a laser source to electrical signals, in data communication systems. This device usually consists of a photodetector and an amplifier (a device that amplifies the electrical signal). The main component of an optical to electrical converter is the photodetector. It is a semiconductor device that is sensitive to light energy and it converts photons (or light) into electrical current. In a photodetector, when the energy of the photon is greater than the energy band gap (ie., hν > E_g), it excites the electrons at the valence band to the conduction band. This vacancy of electrons creates a hole resulting in a flow of current that is known as the photocurrent. The magnitude of the photocurrent can be expressed by the following equation:

Where I_p is the photocurrent produced by the photodiode, P_in = total optical power entering the photodetector, R = fresnel reflection coefficient at the air-semiconductor, e is the electron charge, h = Planck’s constant, v  = frequency of the incoming light, α = absorption coefficient of the semiconductor at the incident wavelength, and d is the width of the absorption region.

A photodetector can operate in two modes: photovoltaic (zero-bias) or photoconductive (reverse bias). The photovoltaic mode of optical to electrical converter is shown in the above figure, where photovoltaic means the direct conversion of light into electric power using semiconducting materials. The characteristic of a current source is that its voltage must be determined by other elements in the circuit. If the photodetector is connected to a load resistance of R_L, the output voltage of the circuit is given by:

Where V is the output voltage, ρ is the responsitivity of the photodetector in the selected wavelength of the optical source, and R_L is the resistance of load in this circuit. The first figure shows the simplest way to bias a PIN diode to convert the optical signal into a voltage signal.

The second figure shows photoconductive mode using an operational amplifier for the optical to electrical conversion. The measured output current is linearly proportional to the input optical power. In photoconductive mode, the photodetector is reverse biased i.e., the cathode is attached to the positive side of a battery with respect to the anode. 

Here the incident photon in the active region PIN junction generates electron-hole pair. And in this biased region, there is a built-in potential (diffusion potential), which leads to the movement of electrons towards the n side and holes towards the p side and therefore the carriers are swept apart which gives the reverse photocurrent.

This reverse bias reduces the response time because the width of the depletion layer (the intrinsic layer) increases, which decreases the junction's capacitance and increases the region with an electric field that will cause electrons to be quickly collected. Also, after the separation of electrons and holes, they do not recombine further as because of the highly generated electric field. So, the relation between photocurrent & illuminance is linearly proportional. 

Applications of Optical to Electrical Converters

Optical-to-electrical converters are designed for measuring optical communications signals. Their broad wavelength range and multi-mode input optics make these devices ideal for applications including Ethernet, Fibre Channel, and ITU telecom standards.  One of the primary applications of an optical-to-electrical converter is enabling electrical-to-electrical test equipment to characterize optical-to-electrical devices. Each converter's telecom-grade components include a lithium niobate (LiNbO₃) modulator stabilized by a fully automatic bias controller and a tunable or fixed-wavelength laser source. It is extremely useful in a variety of laboratory, factory, and field service situations where a quick check of the operation of a laser source, optical transmitter, or the throughput of a fiber optic communications link is required. Other applications involve general laboratory testing of optical components, field service testing and troubleshooting, laser alignment and tuning, and plasma physics measurements.