This teaching material is a part of e-Photon/ONe Master study in Optical Communications and Networks Course and module: Author(s): No part of this presentation can be reused without the permission of author(s). Users are requested to ask for permission by specifying the purpose of the usage. one.org Optical coherent detection and coherent transmission systems Michel Morvan

Veronique Moeyaert Optical Transmission, VI – Coherent detection Revision : Photodetection is a quadratic (envelope) type detection Incoming light PIN: several volts PDA: several tens of volts Bias voltage RcRc Low noise preamp. Photocurrent

Veronique Moeyaert Optical Transmission, VI – Coherent detection Revision : Characteristics of direct detection Direct detection means that the optical signal is directly detected by a photodiode without any other processing except prior optical filtering. The photocurrent is proportional to the detected optical power : photodetection is an envelope detection. The phase and frequency information contained in the optical signal cannot be recovered in the photocurrent : only amplitude (or power) modulation can be received using direct detection.

Veronique Moeyaert Optical Transmission, VI – Coherent detection Revision : Signal noise ratio for direct detection The detected electrical signal is baseband In the case of low level signal detection, the receiver’s thermal noise is dominant. Hence, the post detection electrical SNR can be derived as: Where S is the photodiode’s responsivity, d is the SQRT of the thermal noise current PSD and B bb the equivalent noise band of the receiver’s baseband filter.

Veronique Moeyaert Optical Transmission, VI – Coherent detection Revision : Principle of optical coherent detection  Among the optical transmission community, coherent detection means heterodyne or homodyne detection. Local Oscillator Laser Incoming signal

Veronique Moeyaert Optical Transmission, VI – Coherent detection Revision : Balanced coherent receiver Local Oscillator Laser Incoming signal 3 dB The two beat currents I het1 and I het2 are out of phase due to the 90° hybrid optical coupler, hence they add together at the preamp input The common baseband direct detection signals (e.g. LO laser amplitude noise) are in phase and hence are rejected.

Veronique Moeyaert Optical Transmission, VI – Coherent detection Revision : Beat signal of the two optical fields Assumingwe find after calculations : Intermediate Frequency IF=  Scalar product of the unitary field vectors

Veronique Moeyaert Optical Transmission, VI – Coherent detection Revision : s(t) m(t) LO Laser amplifier f IF IF Filter demodulator Architecture of an optical coherent receiver baseband Demodulation   Baseband fSfS   Optical carrier f f OL  f S : heterodyne f OL =f S : homodyne  IF=0 Photodetection f IF   Intermediate frequency

Veronique Moeyaert Optical Transmission, VI – Coherent detection Revision : Basic architecture of a radio receiver f0f0 f  baseband demodulation   Baseband with IF: heterodyne without IF: homodyne demodulator IF filter s(t) amplifier filter f FI Frequency downshift  Carrier f 0 m(t) frequency downshift f FI   Intermediate frequency

Veronique Moeyaert Optical Transmission, VI – Coherent detection Revision : Requirements for a correct coherent detection The SOP (States Of Polarization) of both the LO and signal fields must be identical to maximize the IF signal level : the IF signal is null if the fields are orthogonal. The photodiode’s electrical bandwidth must be wide enough to detect the modulated IF beat signal. The frequency difference (IF) between the signal lasers and LO must be kept constant to ensure a correct demodulation In the case of homodyne detection (IF=0), the phase difference between the LO and signal lasers must be locked.

Veronique Moeyaert Optical Transmission, VI – Coherent detection Revision : Signal noise ratio for heterodyne detection The detected electrical signal is centered on IF The signal is proportional to the optical signal field. In the case of high local oscillator power with P LO >>P S, shot noise from the LO detected power is predominant over the receiver’s thermal noise. Hence, the post detection electrical CNR can be derived as: Where B IF is the equivalent noise band of the IF filter.

Veronique Moeyaert Optical Transmission, VI – Coherent detection Revision : Sensitivity : Direct Vs coherent detection Example : 2,5 Gbit/s ASK modulated optical signal  Direct detection receiver with : 0.9 A/W PIN photodiode sensitivity 7 pA/  Hz noise current 2 GHz baseband filter  Heterodyne receiver with : 0.9 A/W PIN photodiode sensitivity 7 pA/  Hz noise current 4 GHz wide IF filter 5 mW local oscillator power Noiseless demodulator  CNR=SNR

Veronique Moeyaert Optical Transmission, VI – Coherent detection Revision : Detection of an ASK modulated signal 13 dB

Veronique Moeyaert Optical Transmission, VI – Coherent detection Revision : Pros and cons of optical coherent detection Better sensitivity than non- amplified direct detection. Very high selectivity (IF filtering in the electrical domain) Possibility of using any modulation format incl. PSK and QAM. Possibility of fine IF post- detection signal processing such as chromatic dispersion compensation Needs Polarisation Control IF lock needed More complex than IM/DD receivers. Needs higher bandwidth receivers than for IM/DD except for homodyne detection.  Ultra high Bit-rate demands homodyne detection. Optical Phase Lock Loop needed for homodyne detection ProsCons

Veronique Moeyaert Optical Transmission, VI – Coherent detection Revision : Quantum limit sensitivity performance of major coherent detection schemes heterodyne ASK and FSK asynchronous demodulation 40 photons / bit at Heterodyne ASK and FSK synchronous demodulation 36 photons / bit at heterodyne DPSK asynchronous demodulation 20 photons / bit at homodyne ASK and synchronous heterodyne FSK 18 photons / bit at homodyne PSK 9 photons / bit at dB 0,45 dB OOK/DD 10 photons / bit at 10 -9

Veronique Moeyaert Optical Transmission, VI – Coherent detection Revision : Spectral linewidth requirements Laser phase noise leads to non zero linewidth with Lorentzian shape. Heterodyning (or homodyning) transposes phase both signal and LO laser phase noise onto the IF carrier : The required laser linewidth ranges from several kHz to several MHz depending on modulation format (PSK, FSK,…) and bit rate Source: Agilent/ Test & Measurement World, 1/1/2001 example : IF spectrum obtained using self heterodyning of a DFB laser.

Veronique Moeyaert Optical Transmission, VI – Coherent detection Revision : f opt The ultimate selectivity : the optical « tuner » A coherent optical receiver with tuneable local oscillator laser is an optical tuner. Selectivity is determined by the IF filter. Image Frequency rejection receivers can be built. IF LO IF LO

Veronique Moeyaert Optical Transmission, VI – Coherent detection Revision : Current status on optical coherent systems Coherent detection is still a very efficient and promising technique When looking at the current actual needs in optical transmission, coherent detection is « excessively good » and not economically competitive :  It’s more complex  It’s more expensive Needs low cost integrated or hybrid receivers to be competitive. Optically preamplified direct detection has about the same sensitivity. Homodyne detection very delicate. Can be used to generate millimetre waves.