4500HWM02

# 4500HWM02 - Mode-Lacked Laser A semiconductor laser...

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Unformatted text preview: Mode-Lacked Laser A semiconductor laser diode (L.D.) is to be mode-locked at a repetition rate of 40 GH z. Thus a stream of short pulses is to be produced by the laser at this repetition rate. The iaser cavity consists of a high-reﬂection coating for one reﬂector and a fiber Bragg grating for the other reﬂector. An anti-reﬂection (A.R.) coating is to be used on the other face of the diode laser. The laser is schematically shown in the drawing. I L1 L2 I Ida—A L @ => L H II_ ‘3' FIBER BRAGG REFLECTOR The RF sinusoidal frequency that drives the laser correSponds to the pulse repetition rate. The round—trip optical path length of the cavity must be adjusted so that in one period of the drive signal, a pulse travels one complete round—trip in the laser cavity. The round—trip optical path length of the cavity is = n1L1 + 71ng + 71,313 + H3L3 + TI,ng + nlLl The laser diode has a refractive index of 711 : 3.35 and a length of L1 = 500nm. The air gap has a refractive index of n2 2 1 and a length of L2 = 200nm. The optical ﬁber has a refractive index of ns = 1.50. The freespace wavelength of the light is 850 nm. Calculate, showing all work, the location L3 of the ﬁber grating to produce a 40G'Hz repetition rate of laser pulses. Express your answers in mm accurately to four signiﬁcant figures. Put your ﬁnal answer in the space provided below. location of ﬁber grating, L3 = mm. Mode-Locked Laser m = 3.35, L1 = 500pm = 0.5mm 112 = 1, L2 = 200pm = 0.2mm n3 2 f = g: T : 31" = '4"o"G'1 Hz : 251” T = O.P.L. C O.P.L. = n1L1 + 71ng + n3L3 + n3L3 + n,ng + n1L1 O.P.L. = 2 (n1L1 + 71sz + n3L3) 2 (n1L1 + 11ng + 71ng) :2 Tc 2n3L3 = To — 2 (n1L1 + nng) L3 2 [Tc — 2 (n1L1 + n2L2)]/2n3 L3 = [7.495mm — 2 (1.675mm + 0.2mm)]/3 L3 = 1.2483mm ...
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4500HWM02 - Mode-Lacked Laser A semiconductor laser...

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