154 CHAPTER 4. OPTICAL RECEIVERS
Figure 4.14: Flip-chip OEIC technology for integrated receivers. The InGaAs photodiode is
fabricated on an InP substrate and then bonded to the GaAs chip through common electrical
contacts. (After Ref. [57];©c1988 IEE; reprinted with permission.)
p–i–nphotodiode has also been integrated with HEMTs to develop a two-channel OEIC
receiver.
In another approach [64]–[69], the heterojunction-bipolar transistor (HBT) technol-
ogy is used to fabricate thep–i–nphotodiode within the HBT structure itself through a
common-collector configuration. Such transistors are often calledheterojunction pho-
totransistors. OEIC receivers operating at 5 Gb/s (bandwidth∆f=3 GHz) were made
by 1993 [64]. By 1995, OEIC receivers making use of the HBT technology exhib-
ited a bandwidth of up to 16 GHz, together with a high gain [66]. Such receivers can
be used at bit rates of more than 20 Gb/s. Indeed, a high-sensitivity OEIC receiver
module was used in 1995 at a bit rate of 20 Gb/s in a 1.55-μm lightwave system [67].
Even a decision circuit can be integrated within the OEIC receiver by using the HBT
technology [68].
A third approach to InP-based OEIC receivers integrates a MSM or a waveguide
photodetector with an HEMT amplifier [70]–[73]. By 1995, a bandwidth of 15 GHz
was realized for such an OEIC by using modulation-doped FETs [71]. By 2000, such
receivers exhibited bandwidths of more than 45 GHz with the use of waveguide photo-
diodes [73]. Figure 4.15 shows the frequency response together with the epitaxial-layer
structure of such an OEIC receiver. This receiver had a bandwidth of 46.5 GHz and
exhibited a responsivity of 0.62 A/W in the 1.55-μm wavelength region. It had a clear
eye opening at bit rates of up to 50 Gb/s.
Similar to the case of optical transmitters (Section 3.4), packaging of optical re-
ceivers is also an important issue [75]–[79]. The fiber–detector coupling issue is quite
critical since only a small amount of optical power is typically available at the pho-
todetector. The optical-feedback issue is also important since unintentional reflections
fed back into the transmission fiber can affect system performance and should be mini-
mized. In practice, the fiber tip is cut at an angle to reduce the optical feedback. Several
different techniques have been used to produce packaged optical receivers capable of
operating at bit rates as high as 10 Gb/s. In one approach, an InGaAs APD was bonded
to the Si-based IC by using the flip-chip technique [75]. Efficient fiber–APD coupling
was realized by using aslant-ended fiberand a microlens monolithically fabricated on