are promising for development of small-size delay lines. Application of special or
nano-dimension optical fibers (Fig. 2.13d, e) with low losses at small bend radii
(1–3 mm) (0.001 dB/one bend) allows creation of miniature delay lines (1–50 μs)
with overall sizes from 10 to 30 mm [49, 50].
At present, in many countries, commercially available photodiodes with the
bandwidth of 12, 18, and 60 GHz are manufactured [51–55].
Their natural output photocurrent phase noises are ultralow and are less than
À120 to À130 dB/Hz at offsets 1–10 kHz from the nominal frequency in the
microwave range [52–56]. The photodiode construction allows its mating with the
optical fiber. A coefficient of optoelectronic conversion of photodiodes is up to
0.5 A/W. One of features of modern photodiodes of microwave range is the small
laser emission
optical lens
optical fiber
electrodes of DC voltage
Signal electrode
Ground
LiNbO 3
optical lens
optical fiber
laser emission
a)
b)
c)
0
–2
–4
AFR |K
MZ | (dB)
–6
–8
–10
0 5 10 15
1
2
3
20
Frequency f (GHZ)
25 30 35 40
optical waveguide
Fig. 2.11 (a) Planar implementation of the electro-optical Mach–Zehnder modulator (MZ section
length is 10 mm), (b) the external view of the MZ modulator, (c) its experimental AFC for MZ
modulator with the RF modulation bandwidth 12 GHz (curve 1), 21 GHz (2), 27 GHz (3); the laser
wavelength is λ ¼ 1550 nm
2.7 Modern Elements of OEO: A Laser, the Optical Fiber, and a Photodetector
45