Integral differential optical signal receiver

What is claimed is:

1. An optical signal receiver for reception of an optical signal and conversion of that signal to an electrical signal, the receiver comprising:

an optical amplifier capable of receiving the optical signal;

an optical splitter optically coupled to the optical amplifier and having two optical outputs;

an optical sensor coupled to the optical splitter, the optical sensor having:

a first output terminal;

a second output terminal;

a first photo detector which produces an electrical signal in response to a light input, coupled between the first and second output terminals, the first photo detector element being optically coupled to the first output of the optical splitter; and

a second photo detector which produces an electrical signal in response to a light input, coupled between the first and second output terminals and in parallel with the first photo detector, the second photo detector element being optically coupled to the second output of the optical splitter, wherein the signal from the second output of the optical splitter is delayed relative to the signal from the first output; and wherein the electrical signal of the second photo detector is opposite in polarity to that of the electrical signal of the first photo detector canceling out at least some of first signal.

2. The receiver of claim 1 wherein the first and second photo detectors are PiN type photodiodes.

3. The receiver of claim 2 wherein the first and second photodiodes are Indium Gallium Arsenide.

4. The receiver of claim 1 wherein the optical sensor further includes a first voltage source coupled in series with the first photo detector and a second voltage source coupled in series with the second photo detector.

5. The receiver of claim 4 wherein the first and second voltage sources are transformer isolated DC power supplies.

6. The receiver of claim 4 wherein the first and second voltage sources are photovoltaic elements and the receiver further includes a light source coupled to the photovoltaic element to produce a selected voltage output.

7. The receiver of claim 1 further comprising:

a resistor coupled to the first output terminal;

a bias power source coupled to the resistor; and

wherein the second output terminal is grounded.

8. The receiver of claim 1 wherein the optical amplifier is an erbium doped fiber amplifier.

9. The receiver of claim 1 wherein the optical amplifier is a semiconductor optical amplifier.

10. The receiver of claim 1 wherein the light signals on the light input represent bits of data and the second output is delayed by one bit length.

11. The receiver of claim 1 wherein the optical splitter is an evanescent coupler.

12. The receiver of claim 1 wherein the optical splitter includes:

a substrate;

a collimator lens optically coupled to the optical signal;

an astigmatic lens optically coupled to the collimator lens;

a knife edge beam splitter which splits the optical signal into two signals on the first and second outputs;

a micro lens optically coupled to the first and second outputs;

a corner reflector optically coupled to the second output which elongates the path of one of the light signals.

13. The receiver of claim 12 further comprising an actuator coupled to the corner reflector to vary the path of one of the light signals.

14. The receiver of claim 1 wherein the optical sensor is fabricated on an active substrate.

15. The receiver of claim 14 wherein the substrate is lithium-niobate.

16. The receiver of claim 14 wherein the substrate is indium-phosphide.

17. The receiver of claim 14 further comprising:

a first waveguide fabricated on the substrate which optically couples the first output terminal to the first photo detector;

a second waveguide fabricated on the substrate which optically couples the second output terminal to the second photo detector, the second waveguide being less than or equal to a bit length longer than the first waveguide; and

wherein the splitter is fabricated on the substrate.

18. The receiver of claim 14 wherein the outputs of the detectors are coupled to amplifier electronics fabricated on the substrate.

19. A method of receiving an optical signal and converting the signal to an electrical signal, the method comprising:

amplifying the light signal;

splitting the light signal into a first and second segment;

delaying the first segment;

converting the delayed first segment of the light signal using a first photo detector into a first electrical signal and converting the second segment of the light signal using a second photo detector into a second electrical signal;

wherein the second photo detector is coupled in parallel with the first photo detector and the second electrical signal is opposite in polarity from the first electrical signal;

measuring the second electrical signal to generate an electrical signal representative of the optical signal; and

combining the second electrical signal with the first electrical signal through the outputs of the first and second photo detectors to nullify at least part of the second electrical signal to allow further detection of additional light signals.

20. The method of claim 19 further comprising:

reverse biasing the electrical signal representing the first segment by connecting a power supply; and

reverse biasing the electrical signal representing the second segment by connecting a second power supply.

21. The method of claim 19 further comprising adding a resistor and a voltage source to the electrical output.

22. The method of claim 19 further comprising selecting a delay for the first input signal for optimal detection of the optical signal.

23. An optical receiver for converting an amplified optical signal on an optical fiber to an electrical signal, the receiver comprising:

an optical connector connected to the optical fiber; a passive substrate;

an active substrate mounted on the passive substrate;

a splitter fabricated on the active substrate and coupled to the optical connector, the splitter having two outputs for splitting the optical signal;

a first and second waveguide coupled to the two outputs of the splitter respectively, the first waveguide being longer than the second waveguide;

a first photo detector optically coupled to the first waveguide, having an anode and a cathode;

a second photo detector optically coupled to the second waveguide, having a cathode coupled to the anode of the first photo detector and an anode coupled to the cathode of the first photo detector; and

an output node coupled to the anode of the first photo detector and the cathode of the second photo detector.

24. The receiver of claim 23 wherein the photo detectors are PiN photodiodes.

25. The receiver of claim 24 wherein the photodiodes are InGaAs photodiodes.

26. The receiver of claim 23 wherein the passive substrate is silicon and the active substrate is indium phosphide.

27. The receiver of claim 23 wherein amplifier electronics are fabricated on the active substrate.

28. The receiver of claim 23 wherein the first and second photo detectors are die chips mounted in trench cavities formed in the active substrate.

29. The receiver of claim 23 further comprising:

a first photovoltaic element coupled in series with the first photo detector;

a second photovoltaic element coupled in series with the second photo detector; and a light source coupled to photovoltaic elements to produce a selected voltage output.

30. The receiver of claim 29 further comprising:

an alignment block with a groove which aligns the optical fiber; and

wherein the passive substrate further includes a corresponding groove and the optical fiber is located between the groove of the alignment block and the groove in the passive substrate.

31. An optical signal receiver for reception of an optical signal and conversion of that signal to an electrical signal, the receiver comprising:

an optical splitter for receiving the optical signal, the splitter having two optical outputs;

an optical sensor coupled to the optical splitter, the optical sensor having:

a first output terminal;

a second output terminal;

a first photo detector which produces an electrical signal in response to a light input, coupled between the first and second output terminals, the first photo detector element being optically coupled to the first output of the optical splitter;

a second photo detector which produces an electrical signal in response to a light input, coupled between the first and second output terminals and in parallel with the first photo detector, the second photo detector element being optically coupled to the second output of the optical amplifier, wherein the signal from the second output of the optical splitter is delayed relative to the signal from the first output; and

a load element coupled in series to a bias voltage source from the first output terminal to a ground point allowing the first and second output terminals to float relative to the ground point.

Description Submit all comments and votes

FIELD OF INVENTION

This invention relates to a high sensitivity optical signal receiver. More particularly, the invention relates to a method and system for receiving and converting optical signals with a high signal to noise ratio.

BACKGROUND OF INVENTION

Optical receivers are used in fiber optical networks such as those for telecommunication networks in order to detect light signals. All optical receivers currently function as a single-ended threshold optical signal detector which uses a photo detector and a DC reference to produce a digital signal in response to an optical input signal. Input light pulses are sensed by a single photodetector that converts light energy into an electrical current. The current pulse is then sensed by either a transimpedance or high-impedance amplifier and converted into a voltage signal. The output of the amplifier is further filtered electrically into an output signal which enters a voltage comparator for logic level conversion.

The output of the comparator is a digital bit equivalent to the bit data represented by the input light signal. A comparator logic ONE output value equates to the presence of a light pulse while a logic ZERO equates to the absence of a light pulse. The output of the comparator represents the separation point between analog processing for the comparator input and digital processing of the output.

A typical optical telecommunications link consists of a transmitter light source, an optical fiber span, interconnecting optical elements and the receiver. The success of the receiver to determine the presence of light pulse depends on the available signal-to-noise ratio. In an optical transmission system, there are many variables that distort and contaminate light signals traveling in the fiber as well as noise levels at the receiver. Common optical signal degradation factors are laser output power limitations, fiber attenuations, splitter losses, excess termination losses, laser extinction ratio, in-line optical amplifier gain and detector quantum efficiency. Factors that will increase the noise factor are dark current noise, amplified spontaneous emission noise, crosstalk noise, modal noise, phase noise, laser noise, Johnson thermal noise, shot noise and electronic amplifier noise. In particular, Johnson thermal noise, shot noise and electronic amplifier noise are of the most concern for optical receivers.

Typically PiN photodiodes in conjunction with a load resistor are used for optical receivers because they are the only electrical circuit stable enough to run at multi-gigabit rates. The load resistor functions to quickly discharge the photodiode after the detection of a light pulse. However at high frequencies above 1 Ghz, Johnson noise from the load resistor is predominant. This noise may be 1,000 times higher than amplifier electronics noise and 10,000 times higher than shot noise. The load resistor value must be low to achieve a high bandwidth by having a short RC time constant which is governed by the resistor value and the internal photodiode capacitance. As the RC value decreases, the bandwidth of operation will increase. However, a low resistor value also generates higher Johnson noise resulting in a tradeoff between noise and discharge time.

Signal levels are always positive in polarity with respect to signal ground in optical signal detection. This method of detection is highly efficient when signal levels are strong since a signal pulse can easily be discerned using a DC threshold reference level that is substantially above background noise. The DC threshold reference level is ideally set at the mid-point between detection probability functions for a ONE and a ZERO. With weaker signals, setting the DC threshold level becomes increasingly difficult. This problem may be minimized by using automatic gain control or AGC. However, AGC requires an error signal before a correction shift may be made. The elapsed time between a transient error and the AGC response is a major limiting factor as a fast AGC response leads to instability problems while a slow AGC response limits its effectiveness.

To address these shortcomings, an approach to optical telecommunications transmission technology, that was patterned after superheterodyne radio receivers. This optical format required special modulation of the transmitted signal that altered either the amplitude, phase, frequency or polarization of the carrier light frequency. Data was not transmitted as simple on and off pulses but as continuous light. At the receiver, a strong monochromatic local laser at a specific wavelength is mixed with the weak input signal to produce an intermediate or IF frequency similar to a radio receiver. The IF frequency is then processed through IF filters to demodulate the encoded information into an amplitude signal. It finally enters a threshold circuit that converts the signal back to the original ONE and ZERO data stream. This method of data extraction is commonly known in linear circuits as phase lock loop demodulation. To accomplish the mixing in the optical domain, an evanescent coupler is used to mix the two signals (the local oscillator and the input light signal) to form two copies of the signal. Each copy of the light signal is sensed by separate photodiodes connected in a balanced detector arrangement that parallels a "Wheaton-bridge" circuit. The teaching from this balanced detector arrangement was limited to common mode cancellation of local oscillator noise (laser spontaneous emission noise). Coherent detection has been replaced by simple direct-detection because of its complexity and incompatibility with dense wavelength division multiplexing ("DWDM") solutions.

Thus, a need exists for an optical receiver which allows high bandwidth without significant delays due to high resistance. There is a further need for an optical receiver which allows both differentiation and integration of an optical signal conversion. There is also a need for an optical receiver with an efficient signal to noise ratio. There is also a need for an optical receiver with common mode rejection to allow improved dynamic range. There is additionally a need for an optical receiver which may be integrated with other processing electronics. There is also a need for an optical receiver which allows flexibility in components for biasing the electrical output.

SUMMARY OF THE INVENTION

These needs may be addressed by the present invention which is embodied in an optical signal receiver for reception of an optical signal and conversion of that signal to an electrical signal. The receiver has an optical amplifier capable of receiving the optical signal. An optical splitter is optically coupled to the optical amplifier and has two optical outputs. An optical sensor is coupled to the optical splitter and has a first output terminal and a second output terminal. A first photo detector which produces an electrical signal in response to a light input is coupled between the first and second output terminals. The first photo detector element is exposed to the first output of the optical splitter. A second photo detector which produces an electrical signal in response to a light input is coupled between the first and second output terminals and in parallel with the first photo detector. The second photo detector element is exposed to the second output of the optical amplifier. The signal from the second output of the optical splitter is delayed relative to the signal from the first output.

The invention may also be embodied in a method of receiving an optical signal and converting the signal to an electrical signal. The light signal is amplified and then split into a first and second segment. The first segment is delayed and the first segment of the light signal and the second segment of the light signal are converted into electrical signals. The electrical signals are compared to generate an electrical signal representative of the optical signal.

The invention may also be embodied in an optical receiver for converting an amplified optical signal on an optical fiber to an electrical signal. The receiver has an optical connector connected to the optical fiber and a passive substrate. An active substrate is mounted on the passive substrate. A splitter is fabricated on the active substrate and coupled to the optical connector, the splitter has two outputs for splitting the optical signal. A first and second waveguide are coupled to the two outputs of the splitter respectively, the first waveguide being longer than the second waveguide. A first photo detector is optically coupled to the first waveguide and has an anode and a cathode. A second photo detector is optically coupled to the second waveguide and has a cathode coupled to the anode of the first photo detector and an anode coupled to the cathode of the first photo detector. An output node is coupled to the anode of the first photo detector and the cathode of the second photo detector.

It is to be understood that both the foregoing general description and the following detailed description are not limiting but are intended to provide further explanation of the invention claimed. The accompanying drawings, which are incorporated in and constitute part of this specification, are included to illustrate and provide a further understanding of the method and system of the invention. Together with the description, the drawings serve to explain the principles of the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of an optical receiver according to one embodiment of the present invention;

FIG. 2 is a block diagram of the photo detector unit of the optical receiver in FIG. 1;

FIGS. 3A-3D are different variations of the photo detector unit in FIG. 2 for achieving balanced biphasic mode operation with a quiescent voltage equal to ground;

FIGS. 4A-4C are different variations of the photo detector unit in FIG. 2 for achieving unipolar mode operation with a quiescent voltage above ground;

FIGS. 5A-5C are timing diagrams of the input and output signals of the receiver in FIG. 1

FIG. 6 is a top view of a fabrication assembly using an active substrate for the optical receiver in FIG. 1;

FIG. 7 is a side view of the fabrication assembly in FIG. 6;

FIG. 8 is a cross section view of the fabrication assembly taken along the line 8--8' in FIG. 6;

FIG. 9 is a top view of the fabrication assembly for an optical receiver using a dual fiber splitter design; and

FIG. 10 is a top view of a bench mirror configuration for an optical receiver.

DESCRIPTION OF THE PREFERRED EMBODIMENT

While the present invention is capable of embodiment in various forms, there is shown in the drawings and will hereinafter be described a presently preferred embodiment with the understanding that the present disclosure is to be considered as an exemplification of the invention, and is not intended to limit the invention to the specific embodiment illustrated.

FIG. 1 shows an integral-differential sensor optical receiver 10 which receives a signal from an input light source 12. The input light source 12 is a fiber optic cable in this example but may be from the output of a DWDM demultiplexer or the final output stage of a multi-link long haul span. The input light signal from the input light source 12 is first amplified by an optical amplifier 14. The optical amplifier 14 in this example is an Erbium Doped Fiber Amplifier (EDFA) but may be any suitable optical amplifier. The optical signal from the optical amplifier 14 is then coupled into an evanescent fiber splitter 16. The fiber splitter 16 has a first output 18 and a second output 20. The light signal is divided by the splitter 16 between the outputs 18 and 20. The second delayed output 20 is designed to have a longer optical path length as compared to the first output 18 and thus delays that light signal. The two light segments from the outputs 18 and 20 are input to a photo detector unit 22. The photo detector unit 22 has a first photo detector which is a photodiode 24 which senses the light from the output 18 and a second photo detector which is a photodiode 26 which senses the light from the delayed output 20. The photo detector unit 22 has two electrical output nodes 28 and 30 which are coupled to the photodiodes 24 and 26. The signal output of the receiver 10 is generated across the two output nodes 28 and 30 and are electrically connected to preamplifier electronics 32 for further signal processing.

FIG. 2 is a block diagram of the photo detector unit 22 in FIG. 1. The photodiodes 24 and 26 are PiN photodiodes in this example, but may also be Indium-Gallium-Arsenide (InGaAs) or any Ill-V compound material detector. The photo detector unit 22 also has two floating DC power supplies 36 and 38 and a bias resistor 40 connected to a DC bias potential source 42. Both of the photodiodes 24 and 26 operate in the reverse bias mode.

The first photodiode 24 has an anode 44 and a cathode 46. Similarly, the second photodiode 26 has an anode 48 and a cathode 50. The anode 44 of the first photodiode 24 is serially connected to the negative side of the first floating DC reverse bias power supply 36. The anode 48 of the second photodiode 26 is similarly connected to the negative side of a second floating DC reverse bias power supply 38. The cathode 46 of the photodiode 24 is connected to the positive side of bias power supply 38 to form the output node 28. The cathode 50 of the photodiode 24 is connected to the positive side of bias power supply 36 to form the output node 30.

As may be realized, the photodiodes 22 and 24 and the bias sources 36 and 38 of the optical sensor unit 22 form a complete electrical floating closed loop circuit. Any point in this circuit loop can be used as a return or signal ground reference to any other node in the same circuit. A series of four nodes 28, 30, 52 and 54 may be connected in the circuit loop. As an optical receiver, either the node 28 or node 54 is used as the circuit ground reference point. If the node 28 is grounded, then the node 30 becomes the output signal. The bias point of operation for the node 30 is set by a high value resistor for the resistor 40 connected between the node 30 and the desired bias voltage of voltage source 42. For 0 volt bias operation, the bias voltage is simply set to 0 volts, effectively grounding the resistor 40 to the same ground point as the node 28.

This first node arrangement is preferred when using a high impedance amplifier in single mode (signals always greater or equal to 0 volts) operation or when using a transimpedance amplifier in tristate (signals can go positive or negative) detection. For positive bias operation, the bias voltage of the voltage source 42 may be set to a positive voltage level such as 5 volts DC. This positive bias operation is preferred when signals must always be above 0 volts for high impedance or transimpedance amplification. The choice in bias voltage is determined by the end application and the interface to other logic circuits. The floating characteristics of the photo detector unit 22 permits this flexibility.

The receiver 10 is capable of integrating and differentiating the input signal. The integral function of the receiver 10 is derived inherently from the parasitic capacitance of the PiN photodiodes 24 and 26. The differential function is derived from the common mode property of the photodiodes 24 and 26. The voltage-phase output from the output nodes 28 and 30 is therefore a combination of integration and differentiation. The integration mode is dominant whenever one side of the optical signal on the photodiodes 24 and 26 is stronger than the other. This initiates a charge or discharge action on the output node or sense node 30. Since the sense node 30 is dominated by pure capacitance (the load resistor value is very high), the resultant node voltage will rise or fall linearly under steady state input currents of like polarity. If equal light intensities were present on the photodiodes 24 and 26, the sense node 30 is operating in the differential mode where current balance keeps the sense node voltage unchanged. This is similar to the "hold" function of a sample-and-hold memory cell. In the "hold" mode, only leakage currents can alter the sense node charge. The only leakage path on the sense node 30 is through the high value bias resistor 40 under a long time constant. This leakage has virtually no effect over short durations of 20 clock cycles.

On each side of the circuit loop, the photodiodes 24 and 26 and respective power supplies 36 and 38 can be transposed in position (without rotation) and not affect the basic electrical loop function. For example, the components in FIG. 2 may be rearranged in FIGS. 3A-3D which use like element numbers as those in FIG. 2 without altering the functionality of the optical photo detector 22. In all of these variations, the node 28 is coupled to a ground reference. This orientation flexibility permits two loop arrangements in either a diode--diode-supply--supply arrangement as shown in FIGS. 3A and 3D or the diode-supply-diode-supply configurations shown in FIGS. 3B and 3C. Depending on the circuit application and fabrication layout factors, each arrangement will have its own merits. In the circuits shown in FIGS. 3A-3D, the output has identical function but the flexibility in component location permits efficient two dimensional or three dimensional layout options during fabrication and packaging. The configurations in FIGS. 3A-3D are biphasic circuits where the quiescent operating point is ground. The reverse bias supplies 36 and 38 in FIGS. 3A-3D will drop the voltages across the two photodiodes 24 and 26 relative to th