Mumbai University > Electronics Engineering > Sem7 > Optical Fiber Communication

Marks: 5M

Year: Dec 2014

• The two common terms used to describe bandwidth of a photodetector are electrical and optical bandwidth.

• The BW of the detector however is measured by its response in frequency-domain.

• The frequency response can be determined from the impulse response by mathematically transforming it into the frequency domain (via Fourier Transform) and then squaring the result to yield electrical power as a function of frequency. The electrical bandwidth, or -3dBe of the detector is then defined as the frequency at which the electrical power spectrum drops to 50% (or -3 dB) of its value at DC.

• Consider the spectrum of the impulse itself without squaring it to get power, since the output current or voltage is proportional to input optical power, the point at which the voltage spectrum falls to 50% of its DC value is known as the optical bandwidth, or -3dBo.

• The relationship between optical and electrical spectra can be seen in the figure below. The electrical spectrum simply goes as the square of the optical spectrum, because power is proportional to the square of the voltage. On the log scale, this squared relationship appears as a factor of two differences in decibels (dB). Therefore, when the optical power spectrum has dropped to its -3dB point, the corresponding electrical power spectrum has dropped to its -6dB point at exactly the same frequency.

• Electrical bandwidth = -3 dB electrical

  Optical bandwidth = -3 dB optical = -6 dB electrical