Fiber-optic cables offer the ability to move vast amounts of data over inexpensive and low-loss glass fibers. Converting a light signal into an electric signal is usually accomplished with a photo diode connected to a transimpedance amplifier. This article will give a brief summary of key diode parameters and discuss how they impact performance in a typical application (shown in Figure 1).
A photodiode connected to the ONET2804T transimpedance amplifier.
Photodiodes are designed to operate at a specific optical wavelength. Fiber optics can operate at a wide variety of wavelengths, ranging from 650nm to 1550nm. Plastic and multimode fiber operate at the shorter wavelengths, but will typically have low data rates. Most high-speed data links use 1310nm or 1550nm. These two wavelengths have good loss and dispersion in single-mode fiber.
Make sure that your photodiode is designed for the correct wavelength. Be aware that the optical wavelength will drive the diode chemistry. You will not find a silicon diode that operates at 1310nm; diodes at this wavelength are typically Indium gallium aluminum arsenic (InGaAlAs) or indium gallium arsenic (InGaAs).
There are two main types of photodiodes.
P-intrinsic-N (PIN) diodes can be very fast and require very little reverse bias to operate. The drawback is that in order to operate at high speeds with a low reverse bias voltage, the diode’s active area is very small. This means that alignment with your light source will be very critical. A typical reverse bias voltage for a PIN diode is 2.5V.
The second main photo-diode architecture is the avalanche photodiode, or APD. This diode has a conventional P-N junction and will require a fairly large reverse bias voltage to keep junction capacitance to a minimum. Because the APD uses such a large reverse bias to reduce junction capacitance, it is possible to use a larger active region and the diode will be more sensitive. One of the drawbacks of an APD is its large reverse bias voltage, which is typically15V or higher. This can present some challenges with respect to power-supply configurations and board layout. Take care to prevent the diode voltage from damaging the receiving amplifier.
Diode sensitivity will be specified as amps per watt. This ratio is the amps of photo current per watt of optical illumination. A typical number is 0.5 to 0.9A/W of illumination power and is based on a specified reverse bias voltage. The optical power must be focused on the diode’s active area. Diode active areas are usually specified in square microns, where a micron is one millionth of a meter. In general, higher-speed diodes will have lower sensitivity.
As I mentioned above, photodiodes nearly always operate with a reverse bias voltage. This helps minimize junction capacitance. With both PIN and APD diodes, the junction capacitance will be a function of the diode’s reverse bias voltage. Within the diode’s operating range, junction capacitance will decrease as the reverse bias voltage increases. The diode data sheet will specify the recommended operating voltage. For optical communication applications, a 1pF diode junction capacitance is very large. For data rates 25GBps or higher, a diode with 100fF (0.1pF) or less of capacitance will be best. Even for lower-signal-bandwidth applications, keeping the diode capacitance low is very important.
When a diode is reverse-biased, there will be a very small current associated with the applied voltage. In the case of a photodiode, this current will be specified as a dark current, which means that with no illumination, and at the specified temperature and reverse bias voltage, there will be a small leakage current. The current should be much smaller than the current produced by your lowest level of illumination. Typical values of dark current are 1nA or less.
Packaging can be very important for a photodiode. Diodes are often sold in bare die form, where the die is epoxied into place and then bond wires are attached to the bare die. This is the best form if you are building a complete module and want to design the entire optical and electrical interface. For other applications, you can purchase the diode in a package with electrical leads as well as an optical fiber “pigtail,” as shown in Figure 2. In this case, the optical and electrical interface to the diode is already finished. You can even terminate the optical fiber in your choice of standardized connector.
Evaluation board with photodiode connected to LMH5401 transimpedance amplifier.
Photodiodes are an important part of a high-speed signal path, which makes careful selection central to the success of your circuit design. The transimpedance amplifier that connects to the diode is another important step in the optical system design process, helping to convert a light signal into an electric signal without impacting performance.