4.2. COMMON PHOTODETECTORS 141
Figure 4.6: (a) Schematic cross section of a mushroom-mesa waveguide photodiode and (b) its
measured frequency response. (After Ref. [17];©c1994 IEEE; reprinted with permission.)
structure, a bandwidth of 120 GHz has been realized [14]. The use of such a structure
within a FP cavity should provide ap–i–nphotodiode with a high bandwidth and high
efficiency.
Another approach to realize efficient high-speed photodiodes makes use of an opti-
cal waveguide into which the optical signal is edge coupled [16]–[20]. Such a structure
resembles an unpumped semiconductor laser except that various epitaxial layers are
optimized differently. In contrast with a semiconductor laser, the waveguide can be
made wide to support multiple transverse modes in order to improve the coupling ef-
ficiency [16]. Since absorption takes place along the length of the optical waveguide
(∼ 10 μm), the quantum efficiency can be nearly 100% even for an ultrathin absorption
layer. The bandwidth of suchwaveguide photodiodesis limited byτRCin Eq. (4.1.9),
which can be decreased by controlling the waveguide cross-section-area. Indeed, a
50-GHz bandwidth was realized in 1992 for a waveguide photodiode [16].
The bandwidth of waveguide photodiodes can be increased to 110 GHz by adopting
a mushroom-mesa waveguide structure [17]. Such a device is shown schematically in
Fig. 4.6. In this structure, the width of thei-type absorbing layer was reduced to 1.5μm
while thep- andn-type cladding layers were made 6μm wide. In this way, both the
parasitic capacitance and the internal series resistance were minimized, reducingτRC
to about 1 ps. The frequency response of such a device at the 1.55-μm wavelength
is also shown in Fig. 4.6. It was measured by using a spectrum analyzer (circles) as
well as taking the Fourier transform of the short-pulse response (solid curve). Clearly,
waveguidep–i–nphotodiodes can provide both a high responsivity and a large band-
width. Waveguide photodiodes have been used for 40-Gb/s optical receivers [19] and
have the potential for operating at bit rates as high as 100 Gb/s [20].
The performance of waveguide photodiodes can be improved further by adopting
an electrode structure designed to support traveling electrical waves with matching
impedance to avoid reflections. Such photodiodes are calledtraveling-wave photode-
tectors. In a GaAs-based implementation of this idea, a bandwidth of 172 GHz with
45% quantum efficiency was realized in a traveling-wave photodetector designed with
a1-μm-wide waveguide [21]. By 2000, such an InP/InGaAs photodetector exhibited a
bandwidth of 310 GHz in the 1.55-μm spectral region [22].