We wish to design an opto-isolator OIC consisting of a laser, waveguide, and detector monolithically coupled as shown above. The following assumptions apply: (1) All coupling of energy from input to output is via photons emitted by the laser and detected by the detector. Electrical leakage currents are negligible. (2) Neglect optical coupling losses between the laser/waveguide and waveguide/detector. (3) The current source driving the laser has a maximum current = 5 Amps.
(4) It is required that the current transfer ratio be Iout Iin ≥ 0.1. Question Which combinations(s) of lasers (a, b, c), waveguides (d, e), and detectors (f, g) described in the following paragraphs would be suitable? For each possible combination calculate Iout/Iin. Show all work (calculations) to indicate why certain combinations are possible or not. Possible lasers (a) A DFB GaAs/(GaAl)As laser with grating spacing Λ = 3400 A, index ˚ in the light emitting layer n = 3.6, operating in the third-order reflection mode. (b) An unconfined field GaAs laser with Fabry-Perot type endface reflectors. The following parameters have been either measured or established. (1) Half-power points of the emission peak for spontaneous emission have been measured for this material at room temperature to be at 9200 A and 8800 ˚ A. ˚ (2) Index of refaction = 3.3 (3) Thicknes of light emitting layer = 10 microns (4) Thickness of active (inverted pop.) layer = 1 micron (5) Internal quantum efficiency = 0.7 (above threshold) (6) Average absorption coefficient = 30 cm−1 (7) Width = 300μm (8) Length = 935μm (9) Carrier energy distribution factor ζ = 1 (10) Reflectivity of Fabry-Perot surfaces = 0.4 (11) Negligible series I 2 R loss (12) Emission wavelength λ0 = 9000 A˚ (c) A confined field GaAs laser with all parameters the same as in (b) except for the following: 3,4) Thickness of the light emitting layer = thickness of active layer = 1μm 6) Average absorption coefficient = 10 cm−1 Possible Waveguides (d) Ga0.9Al0.1As on Ga0.8Al0.2As (heteroepitaxial type) thickness of the Ga0.9Al0.1As waveguiding layer = 1.0μm, length = 5 mm (e) Carrier concentration reduction type in GaAs substrate n = 2 × 1018/cm3 waveguide n = 1 × 1015/cm3 thickness of the waveguiding layer = 3μm length = 2 mm Note: Use graphs given in Chap. 4 for determining waveguide index of refraction and attenuation (absorption loss).
Possible Detectors
(f) GaAs schottky barrier avalanche type
depletion width = 3μm
length = 0.1 mm
photomultiplication factor Mph = 2
(g) GaAs Franz-Keldysh type
depletion width = 3μm
length = 0.1 mm
applied electric field in depletion layer = 2 × 107 V/m
Note: Assume all absorption in the detectors results in carrier generation,
i.e. the quantum efficiency equals 1.