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Paper Review Optics Express, 2007 I. Review of Analog-to-Digital Converters II. Motivations III. Photonic Assisted ADCs IV. Photonic Sampled ADCs V. Conclusion Special Topics in Optical Engineering II (15/1) Minkyu Kim Analog-to-Digital

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Special Topics in Optical Engineering II (15/1) Minkyu Kim

Paper Review

Optics Express, 2007 I. Review of Analog-to-Digital Converters II. Motivations

  • III. Photonic Assisted ADCs
  • IV. Photonic Sampled ADCs

V. Conclusion

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Special Topics in Optical Engineering II (15/1) Minkyu Kim

Analog-to-Digital Converter

  • Analog-to-digital converter
  • Sampling(S/H or T/H) + Quantization(Quantizer)
  • Sampling frequency, resolution(N bits) are important specs ex)6-bit 500MS/sec ADC
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Special Topics in Optical Engineering II (15/1) Minkyu Kim

ADC Basics

  • SNR Calculation
  • Quantization error energy =

1 π‘Šπ‘€π‘‡πΆ βˆ’

π‘Šπ‘€π‘‡πΆ 2 π‘Šπ‘€π‘‡πΆ 2

π‘Š

𝑝𝑣𝑒 2 π‘’π‘Š = π‘Š

𝑀𝑇𝐢 2

12

  • Signal energy =

1 π‘ˆ 𝑒=0 𝑒=π‘ˆ π΅π‘‘π‘—π‘œ2 πœ•π‘’ 𝑒𝑒 = 𝐡2 2 = 22π‘‚π‘Š

𝑀𝑇𝐢 2

8

<Vin vs Digital Output & Quantization error> <Sampling & Quantization>

  • SNR(Signal-to-Noise Ratio) =

3 2 22𝑂

SNR(dB) = 1.76 + 𝑂 Γ— 6.02 N = [SNR(dB)-1.76]/6.02

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Special Topics in Optical Engineering II (15/1) Minkyu Kim

Resolution Degradation

  • Factors inducing resolution degradation

(1) Timing errors : random jitter, broadening of the sampling time (2) Amplitude errors : Random noise, nonlinearities

  • ENOB(Effective Number Of Bits)
  • Effective resolution from N
  • ENOB = [SINAD(dB) – 1.76]/6.02, SINAD(SIgnal-to-Noise And Distortion)

<Actual quantization with noise and nonlinearities>

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Special Topics in Optical Engineering II (15/1) Minkyu Kim

Motivation for Photonic ADCs

Limited by many factors(Thermal noise, jitter, etc)  Photonic ADC push performance to limitations

<State-of-art in electronic ADCs>

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Special Topics in Optical Engineering II (15/1) Minkyu Kim

Classes of Photonic ADCs

  • Photonic Assisted ADCs
  • Electronic ADCs that use photonics to improve limiting properties
  • Photonic Sampled ADCs
  • Only sampling is performed in the optical domain
  • Photonic Quantized ADCs
  • Only quantization is performed in the optical domain
  • Photonic Sampled & Quantized ADCs
  • Both sampling & quantization is performed in optical domain
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Special Topics in Optical Engineering II (15/1) Minkyu Kim

Optical Link & Photonic ADCs

<Generic analog optical link>

  • CNR(Carrier-to-Noise Ratio)
  • 𝐷𝑂𝑆 =

𝑛𝑆𝑄 2/2 (πœπ‘‘

2+πœπ‘’β„Ž 2 +πœπ‘†π½π‘‚ 2

)

  • CNR is used in place of SNR in analog link
  • 𝐹𝑂𝑃𝐢 = 𝐷𝑂𝑆 𝑒𝐢 βˆ’ 1.76 /6.02
  • SFDR(Spur-Free Dynamic Range)
  • Nonlinearities in an analog optical link
  • Caused by both optical devices and RF

spectrum

  • No general formulation for arbitrary input

spectra

𝑛 ∢ π‘π‘π‘’π‘£π‘šπ‘π‘’π‘—π‘π‘œ π‘’π‘“π‘žπ‘’β„Ž 𝑆 ∢ π‘†π‘“π‘‘π‘žπ‘π‘œπ‘‘π‘—π‘€π‘—π‘’π‘§ 𝑝𝑔 𝑄𝐸 𝑄 ∢ π‘ƒπ‘žπ‘’π‘—π‘‘π‘π‘š π‘žπ‘π‘₯𝑓𝑠 πœπ‘‘: π‘‡β„Žπ‘π‘’ π‘œπ‘π‘—π‘‘π‘“ πœπ‘’β„Ž: π‘ˆβ„Žπ‘“π‘ π‘›π‘π‘š π‘œπ‘π‘—π‘‘π‘“ πœπ‘†π½π‘‚: 𝑆𝐽𝑂 π‘œπ‘π‘—π‘‘π‘“

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Special Topics in Optical Engineering II (15/1) Minkyu Kim

Optical Link & Photonic ADCs

<ENOB as a function of link BW for an analog optical link> Assume RIN = 0, no path loss, linear

High optical power is required for high resolution(>10 ENOB)  Moderate resolution & 10s of bandwidth ADC may be a better target for photonic ADCs

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Special Topics in Optical Engineering II (15/1) Minkyu Kim

Photonic Assisted ADCs

I. Sample using mode-locked laser

Ο‰ Ξ” Ο‰ t Ξ” T Ο‰ t Ο‰ t Ξ” T Modes of cavity Periodic impulse train Gain curve Short pulse Mode locked source Short pulse train

Γ— = = βˆ—

FFT

  • Advantages
  • Faster rise time and lower pulse-to-pulse jitter
  • Can remove clock from ADC circuit with fiber
  • Address multiple points from one optical source
  • Disadvantages
  • Integrating mode locked laser into commercial

product is hard

<General schematic of photonic assisted ADC>

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Special Topics in Optical Engineering II (15/1) Minkyu Kim

Photonic Assisted ADCs

II. Optical track & hold

  • Limitation of direct illumination of a single optoelectronic switch

(1) Capacitor is charged by weak input signal(Track) (2) Semiconductor lifetime is not enough short(Hold)

a. Diode bridge

  • No optical pulse : diode bridge works(Track)
  • Optical pulse : diode bridge off(Hold)
  • Advantages
  • Reduced aperture time
  • Low clock jitter
  • Clock isolation  No clock/signal interference
  • 1 GS/s, 9.6 SNR bits achieved

<Photonic assisted ADC using diode bridge circuit>

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Special Topics in Optical Engineering II (15/1) Minkyu Kim

Photonic Assisted ADCs

II. Optical track & hold b. Photonic-assisted time-interleaved ADC

<Photonic assisted time-interleaved ADC> <Optically triggered differential S/H circuit>

  • 4 ENOB for input bandwidth up to 40 GHz achieved

approximately 6 times bandwidth of electronic ADCs

<Electronic time-interleaved ADC>

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Special Topics in Optical Engineering II (15/1) Minkyu Kim

Photonic Assisted ADCs

  • III. Optically triggered electron beam ADC

<Optically triggered e-beam ADC> <Basic cathode ray tube ADC>

  • Using mode-locked laser  low-jitter high repetition rate train of light pulse
  • ~4 ENOB for input bandwidth up to 100 GS/s achieved
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Special Topics in Optical Engineering II (15/1) Minkyu Kim

Photonic Sampled ADCs

I. Photonic sampled ADC without DeMUX

  • All of the timing characteristics are controlled by the low-noise optical clock
  • Sampling time is set by the pulse width, the bandwidth of modulator
  • Timing jitter is set by jitter of the laser
  • Several issues exist
  • Modulator linearity
  • PD responsivity
  • PD recovery time  DeMUX data

<Photonic sampled and electronically quantized ADC>

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Special Topics in Optical Engineering II (15/1) Minkyu Kim

Photonic Sampled ADCs

II. Photonic sampled and demultiplexed ADC

<Photonic sampled and demultiplexed ADC>

  • Achieved 505MS/s, ENOB = 9.8
  • Two major sources of error : pulse-to-pulse amplitude fluctuation, timing jitter
  • Path matching is hard, calibration is needed
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Special Topics in Optical Engineering II (15/1) Minkyu Kim

Conclusion

  • Review of electronic ADCs
  • Overcome limitation of electronic ADCs with photonic ADCs
  • Photonic assisted ADCs
  • Photonic sampled ADCs