ECE 228A Fall 2008 Daniel J. Blumenthal! 11.1!Lecture 11 -Optical Sources and Transmitters!ECE 228A Fall 2008 Daniel J. Blumenthal! 11.2!Table 1 - MSA Standards Overview Multi -Source Agreement Synopsis Serial Transceivers SFP “Small Form Factor Pluggable” popular mainstream pluggable for data rates to 2.5 Gb/s, up to 4 Gb/s Fiber Channel SFP+ being promoted for 10 Gb/s QSFP for 4 channel implementations DWDM versions avail able for Metro telecom systems www.sffcommittee.org XFP “10G Small Form Factor Pluggable” 10GbEthernet, OC192, (10G FC anticipated) Available in non -WDM, CWDM and DWDM versions Available to 80 km reach www.xfpmsa.org 10 Gbit Ethernet Transponders Xenpak XAUI electrical interface Hot pluggable Large legacy market www.xenpak.org Xpak / X2 Functionally equivalent to Xenpak, but XPAK and X2 are two rival mechanical designs in a smaller form factor. Designs do not require card cutout. Www.x2msa.org www.xpak.org Other Selected Transponder Module MSAs 300pin Fixed pluggable transponder SFF (Small Form Factor) version more space efficient Large power dissipation budget allows advanced (high power) functions to be implemented 10GbE and OC192 Sonet/SDH Tunable Laser versions available. www.300pinmsa.org Parallel Optics SNAP12, POP4, Quadlink 12 channel and 4 channel parallel optics modules for very short reach, ultra high bandwidth applications. www.snapoptics.org www.popoptics.orgECE 228A Fall 2008 Daniel J. Blumenthal! 11.3!Photon Energy and Bandgap!Energy of Photon! Ea− Eb= hω= Eg+h2k22mc+h2k22mvFor an electron in upper state a and potential lower state b, the downward rate of transition is!Ra→b∝ fc(Ea) 1 − fvEb( )Effective inversion due to electrons and holes within dk!N2− N1→ρ(k)dkVfc(Ea) 1 − fv(Eb)[ ]− fv(Eb) 1 − fc(Ea)[ ]{ }=ρ(k)dkVfc(Ea) − fv(Eb)[ ]ECE 228A Fall 2008 Daniel J. Blumenthal! 11.4!Forward biased p-n junction!Spontaneous Emission!Electroluminescence!P-type!N-type!• Light is emitted from forward biased junction in all directions!Light Emitting Diodes (LEDs)!ECE 228A Fall 2008 Daniel J. Blumenthal! 11.5!➱ Internal optical power! Pint=ηintIq     (hω) =# photonssec    energyphoton     LED Optical Characteristics!➱ Output optical Power!➱ Important: the output optical power is (to a first instance) proportional to the driving current:!ωηηηhqIPPintextintextout⋅==Internal quantum efficiency!Output coupling efficiency!)()(outtIktP ⋅≅ECE 228A Fall 2008 Daniel J. Blumenthal! 11.6!➱ Full width half maximum spectral width: it is roughly given by:!➱ Note that it is directly proportional to the device temperature T!chTKhTKBB28.18.1λλν≈Δ≈ΔEx : !GaAs LEDs (830 nm, 300 K) ⇒ Δλ≈ 30 nm!!InGaAsP LEDs (1310 nm, 300 K) ⇒ Δλ≈ 60 nm!LED Spectral Width!λ#λ0#Δλ!)( fPout➱ The output optical power (coupled in the output fiber pigtail) for most commercial LED is in the order of !! !–20 dBm to –10 dBm !ECE 228A Fall 2008 Daniel J. Blumenthal! 11.7!LED Modulation Bandwidth!The average time it takes for an electron to arrive in the active region due to Ibias and for an electron to disappear from the active region due spontaneous recombination is called the carrier lifetime (τc)!LED Optical Bandwidth! LED Electrical Bandwidth!f3dB,opt=32πτcf3dB,ele=12πτcExample: τc is on the order of 1-5ns. Therefore the electrical bandwidth is on the order of !ECE 228A Fall 2008 Daniel J. Blumenthal! 11.8!Important Laser Diode Parameters! Light current curve:! Threshold current (typically Ith=1-20 mA)! Temperature dependence of threshold (T0=100K)! Slope efficiency (typically R=0.2 mW/mA)! Quantum efficiency (fraction of input electrons that produce useful light) η=R e/hν=R/(1.24W/A/λ)' Near field and far field characteristics! Fiber coupling! Spectra ! Dispersion!ECE 228A Fall 2008 Daniel J. Blumenthal! 11.9!Lasing Threshold!Ibias!Poptical!Ith!Pth!LED Region!Lasing Region!➱ Power-Current (P-I) Characteristics!Slope = ΔP/ΔI = efficiency !ECE 228A Fall 2008 Daniel J. Blumenthal! 11.10! p209ECE 228A Fall 2008 Daniel J. Blumenthal! 11.11!p248 Spectra The spectra is current dependent. As you modulate from off to on, the spectra changes. For long distance, single mode is essential, and what matters is single mode under modulationECE 228A Fall 2008 Daniel J. Blumenthal! 11.12!Semiconductor Emission Wavelength!ECE 228A Fall 2008 Daniel J. Blumenthal! 11.13!Double Heterostructure Lasers (Kroemer)! Carriers diffuse away so it is difficult to get high gain! A method of confining the carriers to a region in space is necessary! Double heterostructure (proposed in 1964 but not implemented until 1968, which led to the first cw lasers).!- -!++!ECE 228A Fall 2008 Daniel J. Blumenthal! 11.14!➯ A heterostructure is a p-n junction between materials with dissimilar bandgaps!➯ Used to confine: Carriers (efficiency) and Photons (waveguide)!Optical and quantum confinement!Electron Energy!P-type!N-type!Injected electrons!Injected holes!------!+++++++!Refractive Index!n1!n2!n2!Optical Mode!Optical gain!Optical waveguiding!ECE 228A Fall 2008 Daniel J. Blumenthal! 11.15!Fabry-Perot Cavities!➱ The equivalent of an electronic comb filter, but for optical frequencies, the Fabry-Perot (FP) cavity is used for feedback in lasers and as optical filters!Partially reflecting mirror (R)!Partially reflecting mirror (R)!Cavity material with index of refraction n!L!Pin(λ)!Pout(λ)!1.545! 1.55! 1.555!-50!-45!-40!-35!-30!-25!-20!-15!-10!-5!0!Wavelength (µm)!Power Transfer Function (dB)!R=0.99!R=0.95!R=0.75!l = 100µm, n = 3.511!δλ = c2/2nlν2!ECE 228A Fall 2008 Daniel J. Blumenthal! 11.16!Light!Light!n≈3.5!Multimode Fabry-Perot SC Lasers!Ibias!l!1.535 1.54 1.545 1.55 1.555 1.56 1.565x 10-6-50-45-40-35-30-25-20-15-10-50wavelength (µm)FP cavity modes!SC optical gain!1.535 1.54 1.545 1.55 1.555 1.56 1.565x 10-6-50-45-40-35-30-25-20-15-10-50wavelength (µm)Relative Output Power!ECE 228A Fall 2008 Daniel J. Blumenthal! 11.17!FP SC Laser Output Characteristics!• 1.3 µm multimode lasers are good for bit rates < 2Gbs and distances up to 100 km.!ECE 228A Fall 2008 Daniel J. Blumenthal! 11.18!Carrier Density Modulation! Plotting the magnitude change in n1 as a function of ωm, we see the