All PON systems and most optical transport systems use direct detection optics, that is the optical signal from the transmitter traverses the optical fiber and lands directly on the photodiode which converts the optical signal to electrical current.
With the introduction of APDs (avalanche photodiodes) the sensitivity of direct detection is enhanced by using the avalanche effect to boost the current for a given optical signal power. The APD has been very successful in GPON and the smaller link budgets in 10G EPON, XG(S)-PON and NG-PON2. XG(S)-PON and NG-PON2 also have larger link budgets (to increase the PON split ratio) that can surpass the ability of APDs to receive the signals without errors.
Both XG(S)-PON and NG-PON2 have the same link budget options:
- N1 = 29dB
- N2 = 31dB
- E1 = 33dB
- E2 = 35dB
Focusing on NG-PON2, current APD technology is just able to meet the smallest budget, N1. For other larger budgets an optical receiver pre-amplifier has been considered to boost the signal and achieve the higher budgets by increasing the receiver sensitivity. Work is still being done on receiver pre-amplifiers for this application, and the jury is still out on the feasibility. Another optical technology capable of achieving large optical link budgets is coherent optics.
Coherent optics is a technology that can drastically increase optical receiver sensitivity and easily meet and surpass the highest link budgets defined in the standards. However coherent optics has typically been considered prohibitively expensive for use in consumer grade optics due to the large number of precision optical components required for the receiver and the ultra-stability required for the laser transmitter.
In the upcoming series of blogs, we will examine coherent optics and consider possible applications for high-speed PON systems, focusing particularly on NG-PON2. There will be three additional installments:
Installment #2: What is coherent optics and why should I care?
In this installment, we will examine the history and principals behind coherent optics as well as the applications for them.
Installment #3: Have coherent optics been considered for PON and consumer level optics? If not, why not?
In this installment, we will look at the complexities of coherent optics and why they have not yet found traction in practical PON systems.
Installment #4: What are Quasi-Coherent Optics and do they change the equation on Coherent Optics and PON?
In the final installment, we introduce the concept of "Quasi-Coherent Optics," report on the state-of-the-art in this arena and re-examine the possible application for PON systems.