This preview shows pages 1–2. Sign up to view the full content.
This preview has intentionally blurred sections. Sign up to view the full version.View Full Document
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 ...
View Full Document
- Spring '08