ASK Multiport Optical Homodyne Receivers

Several types of ASK multiport homodyne receivers are investigated, and the impact of the phase noise and of the shot noise on these receivers is analyzed. The simplest structure is the conventional multiport receiver with a matched filter in each branch. This structure can tolerate AvT (Av is the laser linewidth and T is the bit duration) of several percent with a small power penalty (3.6 percent for 1-dB penalty and 9.2 percent for 2-dB penalty). Optimization of branch filters of conventional multiport receivers does not help when the linewidth (and the penalty) is small but does improve the receiver performance for larger linewidths. The most important point of the paper is the novel wide-band filter-rectifier-narrow-band filter (WIRNA) structure, proposed and investigated here for the first time for optical communication systems. . I t is shown that the optimized WIRNA homodyne receivers are extremely robust with respect to the phase noise: the WIRNA tolerable value of AvTis 3.6 percent for 1-dB penalty and more than 50 percent for 2-dB penalty. Thus, the WIRNA structure opens, for the first time, the possibility of constructing homodyne receivers operating at several hundred megabits per second with conventional DFB lasers without complicated external cavities. Under no-phase-noise conditions, all the multiport receivers investigated here have the same performance, which is identical to that of heterodyne ASK receivers. In addition, the optimized WIRNA receivers can tolerate (approximately) the same laser linewidth as the heterodyne ASK receivers. Thus, the main difference between the WIRNA multiport homodyne and heterodyne receivers is that the former shifts the processing to a lower frequency range, in return for a more complicated implementation. This difference makes the WIRNA multiport homodyne receivers particularly attractive at high (say, several gigabit per second) hit rates. I I. INTRODUCTMN NTENSIVE research in coherent optical communications [1]-[lo] showed that the coherent detection may offer several important advantages with respect to the conventional combination intensity modulation/direct detection (IM/DD). These advantages include improved receiver sensitivity, greatly enhanced frequency selectivity, conveniently tunable optical receivers, and the possibility of using alternative modulation formats (FSK and/or PSK). With the present day trend toward higher bit rates (Rb) , coherent receivers operating at several hundred megabit-or several gigabit-per second appear to be particularly attractive. A designer of a heterodyne receiver faces several difficult problems at high bit rates. First, extremely large bandwidth optical detectors are required since the IF frequency is typically (but not always) equal Manuscript received January 29, 1986; revised October 6, 1986. This work was partially supported by the Bondesministerium fur Forschung und Technologie of the Federal Republic of Germany. L. Kazovsky is with Bell Communications Research, Navesink Research and Engineering Center, Red Bank, NJ 07701. P. Meissner and E. Patzak are with Heinrich-Hertz-Institut fur Nachrichtentechnik Berlin, D-1000 West Berlin 10, West Germany. IEEE Log Number 8613081. to 3-5 times the bit rate R,. Second, semiconductor lasers frequently exhibit a peak in both amplitudeand phasenoise spectra located at a frequency of several ( 110) GHz. If the IF spectrum happens to overlap this noise peak, then the system performance can deteriorate; to the best of the authors’ knowledge this effect has not been investigated until now. Homodyne receivers can, in principle, alleviate both problems since they only require the baseband bandwidth. Unfortunately, a conventional synchronous homodyne receiver requires phase-locking between the transmitter and the LO laser. The phase-locking is difficult to achieve, and leads to extremely stringent requirements on the laser linewidth (around 3 X times Rb, see [7], [SI). Thus, an asynchronous homodyne receiver, i.e., a homodyne receiver without phase-locking, appears to be desirable. The difference between a synchronous and an asynchronous homodyne receiver can be explained as follows. The signal current produced by a photodetector of a homodyne receiver is equal to B, cos 4, where B,y is the ignal amplitude and # is the random phase. With the ASK modulation format, the information is carried by the value of B,; a receiver produces an estimate of B,-say B,-and compares it with a threshold. Different receivers use different techniques to evaluate B, . A synchronous receiver attempts to keep q5 close to zero; if q5 << 1, then the signal current is a close estimate of B,. This approach has the highest possible sensitivity. Unfortunately, the circuitry needed to keep C#I << 1 imposes extremely stringent requirements on the laser linewidth [SI, [9]. An asynchronous receiver makes no attempt to maintain # << 1 via phase-locking. Instead, it uses several detectors with a fixed phase shift between them. Receivers of this type are referred to as multiport receivers [ 111-[ 131. For example, a three-port receiver uses three detectors with the output signal currents equal to B, cos q5, B, cos ( 4 + l2Oo), and B, cos (q5 + 240°), respectively. Simple trigonometry shows that if these currents are squared and then added together, then the result is equal to 1.5B:, irrespectively of the value of #. Thus, B, can-be evaluated without phase locking. Since the multiport asynchronous receiver does not use phase locking, it is extremely tolerant to phase noise (see Section VII). As a result, laser linewidth requirements of a multiport homodyne receiver are greatly relaxed as compared to those of a synchronous homodyne receiver (see Section VIII). Thus, the multiport receivers seem to offer the best of both worlds-they use only a baseband part of the frequency spectrum, but do not require phase locking. These advantages are 0733-8724/87/0600-0770$01 .OO