Optical Receivers Abdul Rehman

Receiver Components Most lightwave systems employ the digital format. Figure below shows a digital optical receiver Its components are arranged into three groups, the front end, the linear channel and data-recovery section

Front End It consists of photodiode (modeled as current source) followed by a preamplifier Photodiode converts the optical bit stream into an electrical time-varying signal The role of the preamplifier is to amplify the electrical signal for further processing

∆f∆f The design of the front end requires trade-off between speed and sensitivity A high impedance front end with large load resistance R L Increases input voltage Reduces thermal noise Reduces bandwidth ∆ f  12πRLCT12πRLCT can’t be considerably less than the bit rate Equalizer is used to increase the bandwidth, that attenuates low-frequency components of the signal more than the high-frequency components, thus increasing the effective bandwidth

ver sensitivity is not of concern, R L can simply be decreased to increase the bandwidth thus aking low-impedance front ends Transimpedancefrontendprovideshigh sensitivity together with large bandwidth Its dynamic range is also improved compared with high-impedance front end These are often used in optical receivers because of their improved characteristics

as Theloadresistanceis connectedfeedback resistancearoundan inverting amplifier Thusthebandwidthis enhanced by a factor of G comparedwithhigh- impedance front ends Stabilityoffeedbackis major design issue

Linear Channel Linear channel in optical receivers consists of high-gain amp and low pass filter. An equalizer is sometimes added to just before the amplifier to correct for the limited bandwidth of the front end It has AGC to limit the average output voltage to a fixed level irrespective of the incident average optical power at the receiver The low pass filter shapes the voltage pulse and reduces noise without intersymbol interference (ISI). The low pass filter has the smallest bandwidth so the

The combination of preamplifier, main amplifier, and the filter acts as a linear system hence given the name linear channel Equalizer I p (t) Preamp Amp Filter Z in (ω) G p (ω) G A (ω) H F (ω) Z T (ω) V out (t)

% I % The output voltage can be written as ∞ V out  t   z T  t   I p  t    z T  t −  t '  I p  t '  dt ' (i) −∞ Where I p  t  is the photocurrent generated in response to the incident optical power (I p  RP in ) In freq domain V out (ω) = Z T (ω) ⋅ % p (ω) (ii) V out (ω) = V in  ω ) ⋅ G p (ω) ⋅ G A (ω) ⋅ H F (ω) Where Z T is the total impedance at the frequency ω and tilde represents the Fourier transform

~ ~ ~ ~ ~ Z T ( ω ) = ~  V iin  ω ) = Z iin  ω ) ⋅ I p  ω ) = I p  ω  Y in  ω  Where Y in  ω  is the input admittance V out ( ω ) = I p ( ω ) Y in ( ω )  G p ( ω ) ⋅ G A ( ω ) ⋅ H F ( ω ) Where G p ( ω ), G A ( ω ) and H F (  ) are transfer functions of the preamplifier, the main amplifier and the filter V out ( ω ) G p ( ω ) ⋅ G A ( ω ) ⋅ H F ( ω ) I p ( ω ) Y in ( ω ) (iii)

~   ~ ~ V out  0   Z T  0   I p  0  V out (  ) Z T ( ω ) I p ( ω ) V out (0) Z T (0) I p (0) H out (ω) H T (ω) H p (ω) H out ( ω )  H T ( ω )  H p ( ω ) (iv) Where H T ( ω ) is the total transfer function of the linear channel

Data Recovery The data-recovery section of optical receivers consists of a decision circuit and a clock-recovery circuit The purpose of clock-recovery circuit is to isolate a spectral component at f=B from the received signal This component provides information about the bit slot TBTB  1 B  to the decision circuit and helps to synchronize the decision process Clock recovery is more difficult in the case of NRZ format because the signal received lacks a spectral component at f=B (In RZ format a spectral component at f=B is present)

The decision circuit compares the output from the linear channel to threshold level, at sampling times determined by the clock-recovery circuit and decides whether the signal corresponds to to bit 1 or bit 0 Eyediagramisformedbysuperposing2-3bit-long electrical sequences in the bit stream on top of each other. It is called because of its appearance Noise and the timing jitter causes closing of the eye. The best sampling time corresponds to maximum opening of the eye. Eye diagram provides a visual way of monitoring the receiver performance

 data B 1 B 2 Input data Clock recovering input f  Decision circuit Clock, f=B RZ format, NRZ- format Regenerate d  Re −  timed  Bit slot T B  component at f=B present SAW filter no component at f=B

Circuit Input Ref DFF Data out Decision comparator clock Eye diagram Superimposing in same time slot Ideal Eye diagram for NRZ format

Degraded Eye diagram for NRZ format Noise and time jitter close the eye Best sampling time for maximum opening BER related to eye closure