Gain equalization of EDFA cascades

Investigates the impact of wavelength-dependent erbium-doped fiber amplifier (EDFA) gain spectrum on multichannel direct-detection lightwave transmission systems employing multiple amplifiers. An analysis is presented which quantifies the constraints imposed by received power imbalance, signal-to-noise ratio (SNR), and receiver sensitivity on an EDFA cascade. Expressions are derived which relate the system constraints to the EDFA gain imbalance, bit rate, number of channels, and receiver dynamic range. Results demonstrate that when four-wave mixing (FWM) is compensated in an 11-channel system, received power imbalance can impose a significant constraint on transmission distance when the EDFA gain imbalance is greater than 1 dB or when bit rate is less than 1.8 Gb/s. In addition, performance of the preemphasis gain equalization technique is studied for multichannel systems employing APD or p-i-n/FET direct-detection optical receivers. Simple expressions are derived which can be used to quantify the increase in transmission distance obtained when employing preemphasis equalization. Results indicate that equalization of the received power spectrum can provide a two- to four-fold increase in the transmission distance when using APD receivers, compared to a one- to two-fold improvement with p-i-n/FET receivers. Analytic results are compared with results obtained by proven simulation methods and found to be in good agreement     

Gain-bandwidth-limited response in long-wavelength avalanche photodiodes

The gain-bandwidth(GB)-limited response of In0.53Ga0.47As/ InP heterostructure avalanche photodiodes (APD's) and related devices used in long-wavelength digital optical receivers is calculated. We find that these diodes, as currently designed, are useful at bit ratesB lsim 2Gbit/s when employed in conjunction with high-sensitivity optical receivers. Response at higher bit rates may be obtained depending on the details of device design. On the other hand, use of poor-quality receivers that require moderate-to-high values of optimum gain can significantly degrade the performance of heterostructure APD's at high bit rates due to GB limitations. We also show that APD receiver bandwidth can be expressed in terms of the sensitivity obtained using the receiver in conjunction with a p-i-n photodiode. It is found that the response speed of optimized receivers is lowest for an APD effective ionization rate ratio ofk = 0.5.     

Chirping-induced waveform distortion in 2.4-Gb/s lightwavetransmission systems

The performance of high-speed lightwave transmission systems operating in the 1.55-μm-wavelength region with conventional single-mode optical fiber can be influenced by the dynamic wavelength chirping exhibited by directly modulated semiconductor lasers. In a recent paper (1989), the authors presented the first detailed analysis of the bit error ratio performance of lightwave systems influenced by chirping-induced waveform distortion. Here, additional numerical results which address the performance implications of the waveform distortion by considering the dependence of the receiver sensitivity on the decision time and decision threshold, the eye pattern, and bit error rate ratio pattern are presented. It is shown that the response of the linear receiver filter can influence the dependence of the receiver sensitivity on the decision time     

The sensitivity of photoconductor receivers for long-wavelength optical communications

We calculate the sensitivity of In0.53Ga0.47As photoconductor receivers for use in moderate to high bit-rate lightwave transmission applications. It is found that the noise of photoconductive receivers is dominated at all bit ratesB < 4Gbit/s by Johnson noise in the conductive channel. Nevertheless, the total noise current decreases approximately linearly with photoconductive gain, and therefore the sensitivity of photoconductive receivers can be comparable to high-sensitivity p-i-n photodiode receivers. The sensitivity of practical photoconductive receivers compares most favorably with p-i-n receivers in the bit-rate range of 500-2 Gbit/s. However, receivers employing high-speed In0.53Ga0.47As/InP avalanche photodiodes are expected to be more sensitive than photoconductive receivers over the entire bit-rate range considered. In this analysis, we consider the effects of slow photoconductor response on receiver sensitivity, and find that the limited gain-bandwidth product of practical photoconductors increases the complexity of the receiver circuit by necessitating equalization, resulting in a decrease in receiver sensitivity and dynamic range.     

Photoconductor receiver sensitivity

The various contributions to photoconductor (PC's) noise are calculated and are used to determine the sensitivity of digital photoconductor receivers for use in lightwave communication systems. We find that Johnson noise is the most significant source of noise current up to bit rates as high as 4 Gbit/s, above which FET channel noise becomes dominant. In comparing the results obtained for ideal photoconductive receivers with receivers employing p-i-n photodetectors, we find that the sensitivities of both circuits are comparable, provided that low-capacitance p-i-n receivers are employed. In contrast, we find that avalanche photodiode receivers have higher sensitivities than either photoconductor or p-i-n receivers over the entire bit-rate range considered. It is concluded that equalization necessary for photoconductor receiver operation at high bit rates due to a limited gain-bandwidth product significantly degrades the sensitivity of the receiver.     

Comparison of available detectors for digital optical fiber systems for the 1.2-1.55 µm wavelength range

Receiver sensitivity is estimated at 1.3 and 1.5 μm for commercial Ge APD's for bit rates between 8 and 1200 MBd, for a variety of APD diameters and operating temperatures. Although both holes and electrons are injected into the depletion region at these wavelengths, the measured photocurrent excess multiplication noise is found empirically to be well described by the simple expression for unilateral carrier injection into the depletion region, while the measured noise on the bulk leakage current can be characterized by the photocurrent parameters for wavelengthssim1.8 mum. Measurements at BTRL on a 140 Mbit/s system receiver using an Optitron GA-1 Ge APD at temperatures in the range20-60degC agree within 1 dB with the theoretical model using these data. The performance of Ge APD-based receivers is strongly influenced by noise on the leakage current and is therefore susceptible to temperature fluctuations. The ionization rates for holes and electrons are comparable in Ge, resulting in a high excess noise factor and a strong dependence of multiplication factor on bias voltage. Thus, APD's for long-wavelength digital optical receivers operating below ∼1 GBd require a bulk leakage current densityll 10^{-4}A/cm²and markedly different ionization rates for holes and electrons, in order to match the otherwise superior performance of the present-day high-impedance p-i-n/FET hybrid.     

Integrated optical receivers using MSM detectors

The performance of a metal-semiconductor-metal (MSM) detector when combined with preamplifier electronics is discussed, and it is noted that certain advantages due to its horizontal surface can result in receivers with fundamentally higher performance than that possible with vertical p-i-n detector designs. The sensitivity of optical receivers using a MSM detector is analyzed, and a set of scaling rules is used to compare a MSM receiver's sensitivity with that of a similar receiver using a vertical p-i-n photodiode structure. Assuming identical preamplifier designs it is shown that the MSM-based receiver, except at very high bit rates, can in principle consistently outperform its p-i-n counterpart with as much as a 6-dB improvement in sensitivity     

Optical preamplifier receiver for spectrum-sliced WDM

Spectrum-slicing provides a low-cost alternative to the use of multiple coherent lasers for wavelength division multiplexing (WDM) applications by utilizing spectral slices of a single broadband noise source for creating the multichannel system. In this paper we analyze the performance of both p-i-n and optical preamplifier receivers for spectrum-sliced WDM using actual noise distributions, and the results are compared with those using the Gaussian approximation. This extends prior results of Marcuse for the detection of deterministic signals in the presence of optical amplifier and receiver noise. Although the methodology is similar, the results are considerably different when the signal is itself noise-like. For the case of noise-like signals, it is shown that when an optical preamplifier receiver is used, there exists an optimum filter bandwidth which minimizes the detection sensitivity for a given error probability. Moreover the evaluated detection sensitivity, in photons/bit, represents an order of magnitude (>10 dB) improvement over conventional detection techniques that employ p-i-n receivers. The Gaussian approximation is shown to be overly conservative when dealing with small ratios of the receiver optical to electrical bandwidth, for both p-i-n and preamplifier receivers     

Low-noise receivers for fiber-optic microwave signal transmission

The problem of low-noise reception of high-frequency narrowband modulated optical radiation is discussed. Relatively simple p-i-n diode/FET based optical receiver structures which achieve good narrowband optical sensitivity are proposed. These receivers achieve good gain and noise performance due to a lossless parallel resonance circuit which shunts the input to the detector and FET. Design characteristics are presented for S-band receivers constructed using low-capacitance photodetectors, and GaAs MESFETs. Experimental realizations of these receiver designs, which achieve conversion efficiencies on the order of 250 V/W and optical noise equivalent power of less than 10 pW/√Hz for 0.8-μm radiation, are reported. Scaling rules which should be of use in optimizing the performance of more advanced designs are stated and other types of receiver structures which can be expected to yield high narrowband optical sensitivity are discussed     

High-sensitivity and wide-dynamic-range 10 Gbit/s APD/preamplifieroptical receiver module

InAlAs avalanche photodiodes (APD) and SiGe-HBT preamplifier 10 Gbit/s optical receiver modules have been developed. The measured back-to-back minimum sensitivity and the optical overload with a pseudorandom binary sequence of 2³¹ - 1 at a bit error rate of 10^-9 are -29.5 and +0.4 dBm, respectively. The dynamic range is 29.9 dB. These results show the highest sensitivity and the widest dynamic range yet reported for 10 Gbit/s APD receivers     

Germanium reachthrough avalanche photodiodes for optical communication systems at 1.55-µm wavelength region

Germanium reachthrough avalanche photodiodes (Ge RAPD's) with high-frequency response have been designed, fabricated, and tested. In the calculation of frequency response, optimum depletion layer width of 21 µm has been found for 1.55-µm wavelength with the highest cutoff frequency of 830 MHz. The diodes fabricated by this design showed frequency degradation of less than 2 dB at 500 MHz and at 1.55 µm. This response has been unchanged up to 1.58 µm, indicating useful spectral limit lies at more than 1.58 µm. The diodes exhibited quantum efficiency of 80 percent and excess noise factor of 6.1 at a multiplication of 10 both for 1.55 µm. The breakdown voltage was 60- 90 v. The sensitivity of the diodes was measured at 100 Mb/s and 1.55 µm. The minimum detectable power of -44.3 dBm which is by 5.2 dB better than the conventional p⁺-n Ge APD has been achieved for 10^-11error rate. Comparison with InGaAs APD and p-i-n/FET receiver has been made by calculating minimum detectable power of RAPD at 500 Mb/s. Calculated sensitivity of RAPD is 1-2 dB worse than InGaAS APD and comparable to that of InGaAs p-i-n/FET receiver estimated from the reported experimental results.     

Analysis of sensitivity degradation caused by the flicker noise ofGaAs-MESFETs in fiber-optic receivers

The total input noise current and sensitivity of the fiber-optic receiver was calculated. The flicker noise source was included by adopting a pertinent flicker noise model. Power penalties caused by the flicker noise were calculated for various fiber-optic receivers using the calculated noise current. It has been found that the flicker noise affects the sensitivity over the whole range of the bit rates, and that the total input capacitance is an important parameter affecting the power penalty which is serious in the case of a high-impedance-type p-i-n FET receiver. The optimum feedback resistance for practical p-i-n FET receiver design is also suggested     

Multigigabit-per-second avalanche photodiode lightwave receivers

High-speed avalanche photodiodes and high-sensitivity receivers are vital components for future multigigabit-per-second lightwave transmission systems. We review theoretical and experimental performance of high-speed III-V avalanche photodiodes, and also that of multigigabit-per-second lightwave receivers using FET and bi-polar amplifiers. Particular attention is given to APD gain-bandwidth product, and to its effect on high-speed receiver sensitivity. Comparisons between measured receiver sensitivities and calculated performance are presented for bit rates up to 8 Gbit/s.     

Ge-on-SOI-Detector/Si-CMOS-Amplifier Receivers for High-Performance Optical-Communication Applications

In this paper, an overview and assessment of high-performance receivers based upon Ge-on-silicon-on-insulator (Ge-on-SOI) photodiodes and Si CMOS amplifier ICs is provided. Receivers utilizing Ge-on-SOI lateral p-i-n photodiodes paired with high-gain CMOS amplifiers are shown to operate at 15 Gb/s with a sensitivity of -7.4 dBm (BER=10^-12) while utilizing a single supply voltage of only 2.4 V. The 5-Gb/s sensitivity of similar receivers is constant up to 93 degC, and 10-Gb/s operation is demonstrated at 85 degC. Error-free (BER<10^-12) operation of receivers combining a Ge-on-SOI photodiode with a single-ended high-speed receiver front end is demonstrated at 19 Gb/s, using a supply voltage of 1.8 V. In addition, receivers utilizing Ge-on-SOI photodiodes integrated with a low-power CMOS IC are shown to operate at 10 Gb/s using a single 1.1-V supply while consuming only 11 mW of power. A perspective on the future technological capabilities and applications of Ge-detector/Si-CMOS receivers is also provided     

16 Gbit/s transmission experiments using a directly modulated 1.3μm DFB laser

A 16 Gb/s electrically time-division-multiplexed lightwave link is discussed. The 16 Gb/s electronic signal was generated by multiplexing together eight copies of the 2-Gb/s pseudorandom sequence (length 2¹⁵-1) produced by a commercial BER test set. A 22-km transmission distance was achieved using a directly modulated, 1.3-μm wavelength DFB laser and a 50-Ω p-i-n receiver. Receiver sensitivity for a BER of 10^-9 was -2.0 dBm. The addition of an optical preamplifier required a more sensitive receiver to avoid saturation-induced distortion in the preamplifier. This was accomplished by reducing the 2-Gb/s word length to 24 b, thereby lowering the intersymbol interference penalty and effectively increasing the receiver sensitivity. Under these conditions, the optical preamplifier receiver sensitivity was -19 dBm, and a 64.5-km transmission was demonstrated     

Determination of bit-rate and sensitivity limits of an optimizedp-i-n/HBT OEIC receiver using SPICE simulations

The sensitivity of an OEIC receiver depends essentially on the physical sources of device and circuit noise referred to its input, provided that the inter-symbol interference (ISI) makes no significant contribution. For well designed receivers, the latter situation can be realized only at an optimum bandwidth (f_{3 dB.opt}) for a given bit rate (B) or vice versa. In this paper, we have determined the relationship between the bit rate and the 3-dB bandwidth for negligible and pre-set levels of ISI for an optimized p-i-n/HBT transimpedance receiver with adjustable bandwidth. We have used SPICE simulations in the frequency domain to determine the effect of device and circuit noise, and SPICE transient analysis to determine the effect of ISI on the sensitivity. The ratio f_{3 dB.opt}/B has been found to vary from 0.65 to 0.45 when B changes from 10 to 20 Gbps for the OEIC receiver used     

Sensitivity penalty calculation for burst-mode receivers using avalanche photodiodes

This paper presents the sensitivity penalty for burst-mode receivers using avalanche photodiodes. The analysis takes into account detailed avalanche photodiode statistics, additive Gaussian noise, intersymbol interference and dc offsets in the receiver channel. The penalty has been calculated via comparison of bit-error rates (BERs), obtained using numerical integration, both in continuous- and burst-mode operation. Sensitivity penalties for burst-mode operation as a function of the mean avalanche gain are presented. The Gaussian approximation systematically underestimates the burst-mode penalty. It is shown that the penalty depends upon both the type of avalanche photodiode (APD) and the required BER. Optimum avalanche gains maximizing the sensitivity of the receiver are given. The influence of dc-offsets upon the sensitivity is studied. Furthermore, it is shown that the impulse response of the filters used to extract the decision threshold profoundly impacts the receiver performance. Finally, some important guidelines for the design of high sensitivity and wide dynamic range burst-mode receivers are given.     

Performance of In0.53Ga0.47As and InPjunction field-effect transistors for optoelectronic integratedcircuits. II. Optical receiver analysis

For pt.I see ibid., p.957-65. The receiver under study consists of an In_0.53Ga_0.47As p-i-n photodiode and an In_0.53Ga_0.47As or InP JFET transimpedance preamplifier. For this study, the two-region JFET model developed in pt.I is extended to include the dependence of receiver noise on transistor design. The authors find that the channel doping should be small enough to avoid shot noise due to the onset of tunneling current between the gate and drain, where as it must be large enough to provide adequate gain. Also, they show that the receiver sensitivity is not a strong function of input FET gate width. Hence, for circuits with high device density, the gate width and the FET power dissipation can be an order of magnitude less than for those structures currently investigated, thereby incurring a sensitivity penalty of only 1 dB as against wide-gate transistors. Optimized receivers using either InP or In_0.53Ga_0.47As JFETs are found to have comparable sensitivities     

High-speed monolithic silicon photoreceivers on high resistivityand SOI substrates

We compare monolithic silicon optical receivers fabricated on high resistivity and silicon-on-insulator (SOI) substrates. Each receiver consisted of a lateral p-i-n photodiode and an NMOS transimpedance preamplifier. At a drain voltage (V_DD) of 3.5 V, a photodiode voltage (V_PD) of 30 V, and a wavelength of 850 nm, the high resistivity receiver exhibited sensitivities of -31.9 dBm at 622 Mb/s and -23.2 dBm at the maximum operating speed of 1.0 Gb/s. At V_DD =5 V and V_PD=20 V, the sensitivity of the SOI receiver was -26.1 dBm at 622 Mb/s, -20.2 dBm at 1.0 Gb/s and -12.2 dBm at the maximum speed of 2.0 Gb/s. Single supply operation at 5 V and 3 V was also demonstrated for the SOI receiver. Methods for extending the speed and improving the sensitivity characteristics in more advanced technologies with lower supply voltages are discussed     

Performance bounds on chip-matched-filter receivers for bandlimited DS/SSMA communications

Performance bounds on chip-matched-filter (CMF) receivers for bandlimited direct-sequence spread-spectrum multiple-access (BL-DS/SSMA) systems with aperiodic random spreading sequences are obtained. First, the optimum transmit-receive chip waveform pairs that maximize the conditional signal-to-interference ratio are derived. This leads to performance bounds on CMF receivers when the conditional Gaussian approximation for cyclostationary multiple-access interference (MAI) is exploited. The bounds are used to examine the dependence of the MAI suppression capability of the CMF receivers on the excess bandwidth of the system and the delay profile of multiple-access users. The system employing the flat-spectrum chip waveform pair is shown to have near-optimum average bit-error rate performance among the fixed CMF (FCMF) receiver systems. Numerical results are provided for an adaptive CMF receiver and for FCMF receivers employing several different fixed chip waveforms.