KD7064 Optical Communications System

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A)In order for the light sources to function properly and find practical use, there are a number of requirements that they must satisfied. Name four of them.

B)Outline three advantages and three disadvantages of the laser diodes compared with the light emitting diodes when used as a light source in optical fibre communications.

D)An optical fibre link has the following specifications:

Rise time of the source (LED):

Rise time of fibre - Intermodal:

Rise time of the photodetector (PIN):

 

From the system rise time considerations:

(i)Estimate the maximum bit rate that can be achieved on the link when using a non-return to zero data format.

(ii) The minimum electrical bandwidth of the link.

D)Using only diagrams illustrates the three steps that are required to generate a laser beam.

A)There are two types of photodetectors that are widely used in optical fibre communications. Name them and identify three key features for each.

B)Write down expressions for the responsivity of the photodetectors identified in Q3 (A).

If the active area of a photodiode is reduced, do you expect the followings:

(i)Responsivity to increase or decrease? Explain.

(ii)Bandwidth to decrease/increase? Explain.

C) Consider a fibre optic communication link, which uses a light emitting diode and a PIN photodetector as a source and a receiver, respectively. The light launched into the fibre is 50 mW, the fibre link is 25 km, fibre attenuation is 2 dB/km, and the photodetector responsivity of 0.6 A/W. The load resistance at the receiver is 1.5 k and the overall system bandwidth is 60 MHz. The temperature is 40o C. Determine:

(i)The received optical power at the photodetector

(ii)The photo current generated by the photodetector

(iii)The thermal noise

(iv)The power signal-to-noise ratio (in dB) at receiver.

You are asked to design a transmitter for visible light communications (VLC) system using a single LED. In Figure Q5 the nominal output light power of LED is as a function of the DC driving current.

(The LED can provide a maximum 200 mW optical power (at the maximum driving current 350 mA).  LED (without lens) has its field of view (FOV) of 120o. Suppose the received power is uniformly distributed in the receiving plane.

(a)Draw the VLC system diagram and explain the function of the system.

(b)Identify the key features of visible light communications

(c)Determine the minimum number of LEDs that are required for a VLC system that has a range of 1.5 m with the given FOV=120o and using maximum modulation depth (the ratio between peak-to-peak current of data signal and the DC (average) current)?

The receiver has the sensitivity of -30 dBm (average power) and its collection area is 50 mm2.

Question 4

 

A free space optical communications link (FSO) has a link range L of 2 km.  The transmitter uses a laser with very narrow full-angle beam divergence α of 0.2º and an emitted power Pt of 100 mW.  The laser beam is modulated by a Non-to-Return-Zero (NRZ) signal at the data rate Rb of 1.25 Gbit/s.  At the receiver a concentration lens with a radius of 20 cm is used to focus the light to the photodetector.  The receiver sensitivity is -30 dBm.  The system experiences a noise source with a spectral power density of 0.05 fW/Hz and the bandwidth of the electrical low-pass filter is BW = 0.75*Rb.

In clear weather the attenuation coefficient of the atmospheric channel  is 0.34 km-1.

(a)Discuss the atmospheric channel and calculate the atmospheric loss.

(b)Determine the FSO link budget.

(c)Use Figure Q6 to calculate the atmospheric power loss under light fog conditions.  Does the system have a sufficient link power margin? Comment on the results.

(d)Calculate the noise power present at the receiver and the link signal-to-noise ratio (SNR) under clear weather condition.

(e)To ensure the FSO link works properly, in light fog conditions, determine the required increase in transmit power. Comment on the result.