Glossary

Bit Rate: The actual rate at which bits are sent over a communications link. This can often be significantly higher than the actual Data Rate because of overhead needed to ensure good transmission (e.g. forward error correction schemes can have up to 25% overhead).

Conversion Gain, CG: The gain of a detector or receiver specified in units of Volts/Watt. CG takes into account the load impedance to tell the user how much voltage will actually be seen at the load for a given amount of optical input in Watts.

Dark Current: The DC current that flows through a detector when there is no light present. Usually measured in the nanoamp range.

Data Rate: The actual rate at which “desired information” is sent over a communications link.

dB: Logarithmic unit for specifying relative power ratios, i.e. ratio (in dB) = 10 log(P1/P2) e.g. 20 dB = 10 log(100), -20 db = 10 log(0.01)

dBm: Logarithmic unit for specifying power ratios relative to 1mW, i.e. ratio (in dBm) = 10 log(P1/1 mW) e.g. 3dBm = 2 mW, -3 dBm = 0.5 mW

Electrical Bandwidth, -3dBe: The frequency at which the electrical output power of the detector or receiver falls to 50% of its value at DC. The product of electrical bandwidth and pulsewidth for a normal “Gaussian” pulse shape is 310 ps-GHz.

NEP: The amount of optical input power that produces the same output level as the inherent noise level of the detector / receiver, i.e. a signal-to-noise ratio of one. Usually given in picowatts per root bandwidth. Total noise level is calculated by multiplying the NEP by the square root of the full bandwidth.

Optical Return Loss, ORL: The amount of light reflected (lost) back out of the detector towards the light source. Measured in dB relative to the input power level. For commercial single-mode systems, typical ORL values for a detector must be better than -27dB.

For multimode systems, -14dB is usually the maximum tolerable value.

Optical Bandwidth, -3dBo: The frequency at which the output current or voltage of the detector/receiver falls to 50% of its value at DC. Mathematically, it yields the same bandwidth as the –6dB point of the electrical response, -6dBe, because of the squared relationship between voltage and power. The product of optical bandwidth and pulsewidth for a normal “Gaussian” pulse shape is 440 ps-GHz.

Output Termination: The electrical resistance seen looking back into the output of a detector or receiver. This value affects the conversion gain of a detector by dictating how much current actually makes it to the final load.

Pulsewidth (Impulse Response): The full duration at half the maximum value (FDHM) of the output electrical pulse when the detector is illuminated by a negligibly short optical pulse.

Responsivity: The sensitivity of a detector element to light in amps/watt. It tells you how much current is produced for a given amount of optical input power.

Risetime: The 10-90% risetime of the output signal step when the detector is illuminated by a negligibly short optical step function. This is difficult to do in practice, so the measurement is simulated mathematically by integrating the pulsewidth (see above).

Sensitivity: The optical input power (in dBm) required to achieve a particular Bit Error Ratio, BER (or signal to noise ratio) at the output of the receiver. Usually specified for BER’s of 10 -12 (or a S/N of 7). Sensitivity is a function of many parameters that affect signal integrity within a system, and so is commonly used as a figure of merit for combined effects.