WO1994006223A1

WO1994006223A1 - A noise matching network

Info

Publication number: WO1994006223A1
Application number: PCT/AU1993/000449
Authority: WO
Inventors: Robert Aram Minasian; Moon Soo Park
Original assignee: The University Of Sydney
Priority date: 1992-09-04
Filing date: 1993-09-01
Publication date: 1994-03-17
Also published as: AU4935393A; GB9504229D0; GB2286494A; JPH08506219A; KR0144068B1; GB2286494B

Abstract

A noise matching network for an optical receiver, comprising a T-network having an inductive element (3) in the series arm or arms, and a capacitive element (8) in the shunt arm or arms, wherein the network also comprises a resistive element (9) in at least one of the arms.

Description

"A NOISE MATCHING NETWORK"

TECHNICAL FIELD

This invention concerns a noise matching network for an optical receiver. In a further aspect the invention concerns an optical receiver including the noise matching network.

BACKGROUND ART

Optical receivers commonly employ either a PIN photodiode, or an avalanche photodiode (APD) connected to a preamplifier. The preamplifier will commonly employ gallium arsenide (GaAs) MESFETs, or HEMTs. In the past, see B.L. Kasper, J.C. Campbell, J.R. Talman, A.H. Gnauck, J.E. Bowers and .S. Holden "An APD/FET optical receiver operation at 8 Gbits/s, " ^". Lightwave Technol . vol LT-5, no. 3, pp. 344-347, March 1987, the photodiodes have been directly coupled to the preamplifier in order to reduce stray capacitance to ground and thereby improve the preamplifier bandwidth.

More recently, see G. Jacobsen,--J.X. Kan and I.G. Garrett, "Tuned front - end design for heterodyne optical receivers, "J. Lightwave Technol . vol LT-7, no. 1, pp.105-114, Jan. 1989, it has been shown that the inclusion of tuning between the photodiode and the preamplifier can improve thermal noise performance. The use of serial and parallel tuning inductors and transformer tuning has been proposed. In addition, in J. . Gimlett, "A new low noise 16 GHz PIN/HEMT optical receiver, "Fourteenth European Conference on Optical Communication (ecoc ' 88) , Post Deadline papers, pp. 13 - 16, Sept. 1988, an inductive/resistive T-network has been proposed having inductive components in the series arms, and inductive and resistive components in the parallel arm.

Earlier work by the present inventors, M.S. Park, R.A. Minasian "High-Speed Optoelectric Integrated

Receiver Design for Fibre-Optic Communications" JIREE has proposed the use of a third order T-network having inductive components in the series arms and a capacitive component in the parallel arm.

SUMMARY OF THE INVENTION The present invention is an extension of the inventors' earlier work and relies upon the realisation by the inventors that the use of resistance with the inductors will improve noise performance. The inventors have also extended their work to identify several classes of noise matching networks.

According to the present invention, there is provided a noise matching network for an optical receiver, comprising a T-network having an inductive element in the series arm or arms, and a capacitive element in the shunt arm or arms, wherein the network also comprises a resistive element in at least one of the arms.

The resistance in the series arms decreases the network Q factor, while also degrading noise performance due to the thermal noise from the resistors. However, an improved wide band frequency response can be achieved with only slight degradation in noise performance.

Low pass filter type matching networks may utilise inductive and resistive elements in series in at least one of the series arms, and a capacitive element in at least one of the parallel arms.

A first order filter comprises an inductive and resistive element in series in the series arm.

A second order filter comprises an inductive element in series with a resistive element in the series arm, and a capacitive element in the parallel arm. A resistive element may be included in series with the capacitor in the parallel arm, as may an inductive element either in addition to or as an alternative to the resistive element.

A third order filter comprises two series arms and a single parallel arm all connected to a common node. A third order filter may comprise an inductive element in series with a resistive element in at least one of the series arms, and a capacitor in the parallel arm. A resistive element may be included with the capacitor in the parallel arm, as may an inductive element either in addition to or as an alternative to the resistive element.

Band pass filter type noise matching networks include inductive and resistive elements in series in at least one of the series arms, optionally with a capacitive element arranged in series in at least one of the series arms. At least one of the parallel arms comprises a parallel combination of a capacitive element, or a capacitive element in series with a resistive element, and an inductive element, or an inductive element in series with a resistive element. Additional parallel arms may comprise a capacitive element or an inductive element in series with a resistive element, or both in parallel. Higher order filters comprise two or more parallel arms and two or more series arms.

Where an inductive element is in series with a resistive element, this may be implemented by employing the internal resistance of the inductive element provided a lossy inductor is selected.

The network elements may be implemented in either hybrid or optoelectronic integrated technologies. For instance, the inductive elements may be implemented using small diameter bond wire, or high impedance microstrip line. The capacitive elements may be implemented using microstrip line.

According to a further aspect of the present invention, there is provided an optical receiver comprising a photodetector coupled to a preamplification stage via a noise matching circuit embodying to the first aspect of the invention.

Optical receivers embodying the invention enjoy superior noise performance, both low pass and band pass, with good frequency response flatness and excellent optical sensitivity over a wide, multi-gigahertz, band response. The noise matching networks provide the required impedances to the photodetector and the preamplifier so that the lowest noise can be achieved. The invention may be applied to high capacity optical communications system which employ either direct or heterodyne detection methods for optical signal to multi-gigahertz frequencies. The invention enables optical receivers to achieve ultra-low noise conversion of the optical signals to electrical signals. The enhanced sensitivity of the optical receiver results in increased transmission distance or an increased power budget of an optical network.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: figure 1 is a first order low-pass filter type matching network embodying the present invention; figure 2a is a third order low-pass filter type matching network embodying the present invention, figures 2b to 2f show variations of this third order filter; figure 3a is an alternative third order low pass filter type matching network embodying the present invention, figures 3b to 3g show variations of this third order filter; figure 4 is a fifth order low pass filter type matching network embodying the present invention; figure 5 is an alternative fifth order low pass filter type matching network embodying the present invention; figure 6 is a seventh order low pass filter type matching network embodying the present invention; figure 7 is an alternative seventh order low pass filter type matching network embodying the present invention; figure 8a is a third order band pass filter type matching network embodying the present invention, figures 8b to 8f show variations of this third order filter; figure 9 is a fourth order band pass filter type noise matching network embodying the present invention; figure 10 is an alternative fourth order band pass filter type noise matching network embodying the present invention; figure 11 is another alternative fourth order band pass filter type noise matching network embodying the present invention; figure 12 is a further alternative fourth order band pass filter type noise matching network embodying the present invention; figure 13 is a fifth order band pass filter type noise matching network embodying the present invention; figure 14 is an alternative fifth order band pass filter type noise matching network embodying the present invention; and figure 15 is a schematic diagram of an optical receiver embodying the further aspect of the present invention.

The same reference numerals have been used throughout the drawings to refer to corresponding elements.

BEST MODES FOR CARRYING OUT THE INVENTION

In the figures the noise matching networks, indicated generally by the numeral 100, are shown to be two-port networks having an input port 1 and an output port 2.

In the first order low pass filter type matching network shown in figure 1, the network comprises a single series arm 3, having a lossy inductor represented as an inductive element 4 and a resistive element 5. In figure 2a, a third order network comprises two series arms 3 and 6 having lossy inductors, and in the parallel arm 7 there is a capacitive element 8 and a resistive element 9 in series.

One or more of the resistive elements may be omitted. For instance in figure 2b the resistive element in the second series arm 6 is omitted; in figure 2c the resistive element in the first series arm 3 is omitted and in figure 2d the resistive element in the parallel arm 7 is omitted; in figure 2e the resistive elements in both the second series arm 6 and the parallel arm 7 are omitted; and in figure 2f the resistive elements in the first series arm 3 and the parallel arm 7 are omitted.

In an alternative third order network shown in figure 3a the same elements are used in the series arms 3 and 6, but the parallel arm includes a capacitive element 8 and an inductive element 10 in series. One or more of the resistive elements may be omitted. For instance, the resistive element in the second series arm 6 may be omitted as in figure 3b, or the resistive element in the first series arm 3 may be omitted as in figure 3c. Alternatively an additional resistive element 9 may be included in the parallel arm 7 in series with the capacitor 8 and inductor 10, as shown in figure 3d. In this case a resistive element 5 may be included in both series arms, as shown in figure 3e, or in only the first series arm 3 or second series arm 6 as shown in figure 3f and figure 3g respectively.

In a fifth order network shown in figure 4 there are three similar lossy inductors in the respective series arms 3, 6 and 11, and two parallel arms 7 and 12 each comprising a capacitor 8 connected at respective junctions between adjacent series arms.

In figure 5 an alternative fourth order network has the same three series arms 3, 6 and 11, but the parallel arms 7 and 12 now comprise capacitive elements 8 in series with resistive elements 9.

In figure 6 a seventh order network employs four lossy inductors in the series arms 3, 6, 11 and 13, and three parallel arms comprising capacitors 8 connected to respective junctions between adjacent series arms.

In figure 7 an alternative seventh order network employs the same series arms 3, 6, 11 and 13, but the parallel arms 12 and 14 now comprise capacitive elements 8 in series with resistive elements 9.

In figure 8a a third order band pass filter type matching network comprises two series arms 3 and 6, each having a lossy inductor represented by an inductive element 4 and a resistive element 5 in series. In the parallel arm 7 there is a parallel combination of a lossy capacitor represented as capacitive element 8 in series with resistive element 9, and a lossy inductance represented as inductive element 15 and resistive element 16. The resistive element 5 in either the second series arm 6 or first series arm 3 may be omitted, as shown in figures 8b and 8c respectively. Alternatively the resistive element 9 in series with capacitor 8 in one limb of the parallel arm 7 could be omitted as shown in figure 8d. In addition the resistive element in either the second series arm 6 or first series arm 3 may be omitted as shown in figures 8e and 8f respectively .

In a fourth order network shown in figure 9 there are two lossy inductive elements in the series arms 3 and 6, and two parallel arms 7 and 17. The first parallel arm 7 comprises a capacitance 8 in parallel with a lossy inductance represented by inductive element 15 and resistive element 16. In the second parallel arm there is merely a lossy inductance represented by inductive element 15 and resistive element 16. In the alternative fourth order network shown in figure 10 there are the same series arms 3 and 6, but in the first parallel arm 7 there is a parallel arrangement of a lossy capacitance represented by capacitive element 8 and resistive element 9 and a lossy inductance represented by inductive element 15 and resistive element 16; the second parallel arm 17 is the same as before. A further fourth order network, shown in figure 11, is the same as the network shown in figure 9 except for the inclusion of a capacitor 18 in the second series arm 6, connected in series with the lossy inductor. The figure 12 embodiment is similar to the figure

10 embodiment with the inclusion of capacitor 18 in the second series arm 6.

In figure 13 a fifth order network is shown having three series arms 3, 6, and 11 containing lossy inductors, and two parallel arms 7 and 17 both comprising a capacitor 8 in parallel with a lossy inductor represented by inductive element 15 and resistive element 16. In the second series arm 6 a capacitor 18 has been added in series with lossy inductor. In the alternative fifth order network shown in figure 14 the same lossy inductive elements and the same capacitor 18 are used in the series arms 3, 6 and 11, but the parallel arms 7 and 17 now each comprise a lossy capacitor represented by capacitive element 8 and resistive element 9, in parallel with a lossy inductor.

The low pass filter topologies, seen in figures 1 to 7, perform over a frequency range from near DC to microwave, multi-gigahertz, frequencies. The band pass filter topologies, seen in figures 8 to 14, operate over a wide pass band at microwave frequencies.

In figure 15 the construction of a optical receiver employing a noise matching network is shown. The receiver 101 comprises four stages: a detector 102; a noise matching network 100; a preamplifier 103; and a load 104.

The detector comprises a photodiode 105 and has an impedance Z _h (ω) which comprises a junction capacitance C_d and a series resistance R_s.

The noise matching network is any network within the scope of the first aspect of the invention, and will typically comprise one of the networks shown in figures 1 to 14. The output impedance of the matching network is Z_s (ω) , including the effect of the photodiode impedance.

By means of the general noise analysis method based on the noise figure concept it is possible to establish the general noise matching network requirements for minimum equivalent input noise current. This directly utilises the available noise parameters such as minimum noise figure F_min (ω) , noise resistance R_n (ω) , and optimum source admittance Y_opt (ω) which are specified in the preamplifier transistor data sheets.

Preamplifier 103 comprises HEMT or MESFET transistors 106 and 107, resistors 108, 109 and 110, and a capacitor 111. Resistor 110 and capacitor 111 form a conventional RC differentiator at the output to equalise the low frequency response of the high impedance preamplifier.

The total equivalent input noise current density appearing across the photodiode junction capacitance is given by

ϊ? (ω) = 4kT (ωC_D)²G_s(ω)F(ω) |D(ω)₊Z_ph(ω) ₊ B(ω)|²

where

G_s (ω) is the conductance of the matching network impedance Z_s (ω) ,

F(ω) is the noise figure, and

D(ω) and B(ω) are the 2-port transmission parameters for the matching network N.

The requirements of the noise matching network for minimising in the input noise i_n (ω) have been determined to be that the matching network output admittance approaches Y_opt (ω) and that the transducer power gain is maximised. From these requirements noise matching networks can be determined for any order, and although the invention has been described with reference to particular examples of matching networks it should be appreciated that it is not limited to those particular networks.

Claims

THE CLAIMS

1. A noise matching network for an optical receiver, comprising a T-network having an inductive element in the series arm or arms, and a capacitive element in the shunt arm or arms, wherein the network also comprises a resistive element in at least one of the arms.

2. A noise matching network according to claim 1, comprising an inductive and a resistive element in series in at least one of the series arms, and a capacitive element in a parallel arm.

3. A noise matching network according to claim 2, further comprising a resistive element in series with the capacitive element in the parallel arm.

4. A noise matching network according to claim 2, further comprising an inductive element in series with the capacitive element in the parallel arm.

5. A noise matching network according to claim 4, further comprising a resistive element in series with the capacitive and inductive elements in the parallel arm.

6. A noise matching network according to claim 1, comprising an inductive and a resistive element in series in at least one of the series arms, and a capacitive element in parallel with a series combination of an inductive element and a resistive element in the parallel arm.

7. A noise matching network according to claim 6, further comprising a resistive element in series with the capacitive element.

8. A noise matching network according to any preceding claim, further comprising at least one additional parallel arm.

9. A noise matching network according to claim 8, wherein the additional parallel arm includes a capacitive element.

10. A noise matching network according to claim 8, wherein the additional parallel arm includes a capacitive element in series with a resistor.

11. A noise matching network according to claim 8, wherein the additional parallel arm comprises an inductive element in series with a resistive element.

12. A noise matching network according to claim 8, wherein the additional parallel arm comprises a capacitive element in parallel with a series combination of an inductive element and a resistive element.

13. A noise matching network according to claim 8, further comprising an additional series arm comprising an inductive element in series with a resistive element.

14. A noise matching network according to claim 8, comprising an capacitive element in series with the inductive and the resistive elements in the additional series arm.

15. A noise matching network according to any preceding claim and of higher order.