Control of linear trans-impedance amplifier (TIA) during settling after recovering from loss of signal in receiver

Abstract

To provide a system and a method for accurately detecting recovery from the loss of a signal in an optical receiver.SOLUTION: An optical device, which is part of an optical receiver 310, comprises: a photodiode 311 configured to receive optical signals; and a linear trans-impedance amplifier (TIA) 312 having an input stage 313, an output stage 315, and at least one variable gain amplifier (VGA) 314 provided between the input stage and the output stage. The optical device also comprises: an automatic gain control loop 331 configured to rectify an output of the at least one VGA and compare the rectified output with a threshold gain setting to generate an automatic gain control voltage; and a detection circuitry 330 being configured to detect a rate of change in the automatic gain control voltage and being configured to determine a first state indicative of an absence of the optical signals at the photodiode. At least in response to the determined first state, the detection circuitry is configured to disable the output stage of the linear TIA.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The subject matter of this disclosure relates generally to optical receivers, and in some non-limiting embodiments, to systems and methods for accurately detecting when a linear transimpedance amplifier (TIA) of an optical receiver settles after a signal resumption event (e.g., after a loss of signal) occurs at the input of the TIA. [Background technology]

[0002] For the purposes of this disclosure, consider the physical interface of a fiber optic cable. As shown in Figure 1A, data is transmitted over an optical fiber 102 from a laser transmitter 100 driven by a laser driver. An optical signal 130 from the optical fiber 102 is received at a receiver 104 via a combination of a photodetector 110 followed by a linear transimpedance amplifier (TIA) 120, the output of which is electrical differential data 140 as shown in Figure 1B.

[0003] In a data communications receiver 104, such as a small form-factor pluggable (SFP) module, one requirement is to squelch the output of the receiver 104 when an input optical signal loss event is detected. The loss of signal may result from unplugging the receiver, reconfiguring the system, or other reasons. For example, FIG. 1B shows an input loss 132 of the optical signal 130 and also shows a squelch 142 of the TIA output data 140. For PAM4 data, the TIA must be linear in its amplification and is called a linear TIA. When the input optical signal 130 resumes, the receiver's TIA 120 needs time to readapt to the change in input optical power level, during which time the TIA 120 will not operate correctly. The TIA 120 preferably does not send out invalid data while the adaptation is still in progress. Therefore, there is a demand to stop the output of data 140 from TIA 120 until it becomes possible to send valid data, and there is a demand to start sending valid data 140 from TIA 120 as soon as possible once normal operation is achieved.

[0004] Some existing solutions immediately turn on the TIA output data 140 upon optical signal resumption and send invalid data D11 as shown in FIG. 1B until the TIA 120 settles again to a valid operating state. Other solutions avoid sending such invalid data D11 by keeping the TIA output squelched 142 using fixed delays. Because the TIA settling time depends on the input optical signal level, these fixed delays may be shorter or longer than the actual settling time required for the TIA 120. If the delay is shorter than the actual TIA settling time, invalid data D11 will be sent. If the delay is longer than the TIA settling time, the response time to optical signal resumption will be unnecessarily long. Not only do existing solutions not properly time the resumption of the TIA 120 output of data 140, but existing technology does not have a mechanism to reduce the time to output valid data (140) during an optical input signal resumption event to meet the desired shorter settling time. Although U.S. Patent No. 11,381,318, entitled "Control of Trans-Impedance Amplifier (TIA) during Settling after Recovery from Loss of Signal in Receiver," assigned to the assignee of the present application and incorporated herein by reference in its entirety, discloses a system and method for accurately detecting the settling of a TIA after a signal restart event to avoid sending invalid data, this patent does not take into account the peculiarities of such a system when utilizing a linear TIA. For example, when the input signal to a linear TIA restarts, the automatic gain control (AGC) loop of the linear TIA takes time to settle, and the settling time is proportional to the input step. Until the linear TIA control loop settles to the transient step input, the linear TIA cannot send valid data. Currently, there is no way to accurately detect the settling instant of the AGC feedback loop of the linear TIA to avoid sending invalid data. Summary of the Invention [Problem to be solved by the invention]

[0005] The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.

[0006] It is therefore an object of the subject matter of the present disclosure to provide a system that overcomes some or all of the shortcomings of the prior art. [Means for solving the problem]

[0007] According to a non-limiting embodiment, an optical device for an optical signal is provided. The optical device comprises a photodiode configured to receive an optical signal, a linear transimpedance amplifier (TIA) having an input stage, an output stage, and at least one variable gain amplifier (VGA) disposed between the input stage and the output stage. The input stage is coupled to the photodiode, and the output stage is coupled to the input stage and is controllable to enable and disable an output of the linear TIA output. The optical device also comprises an automatic gain control loop configured to rectify an output of the at least one VGA and compare the rectified output with a threshold gain setting to generate an automatic gain control voltage, and a detection circuit configured to detect a rate of change of the automatic gain control voltage and configured to determine a first condition indicative of an absence of an optical signal at the photodiode. In response to at least the determined first condition, the detection circuit is configured to disable the output stage of the linear TIA.

[0008] The detection circuit may be configured to determine a second state indicative of the presence of the optical signal at the photodiode, and in response to the second state determined after at least the first state and the detected rate of change, the detection circuit may be configured to disable an output stage of the linear TIA. The detection circuit may comprise a received signal strength indicator coupled to the photodiode and configured to indicate the first and second states of the optical signal at the photodiode. In response to at least the second state and the absence of a rate of change of the feedback signal of the feedback amplifier, the detection circuit may be configured to enable an output stage of the linear TIA. The detection circuit may comprise a digital logic circuit configured to process a logical combination of the first state, the second state, the presence of the rate of change, and the absence of the rate of change, and to provide enable and disable signals to the output stage of the linear TIA based on the processed logical combination.

[0009] The automatic gain control loop may include a rectifier connected to an output of the at least one VGA and a threshold circuit configured to generate a threshold gain setting. The detection circuit may include a slope detection circuit configured to detect a rate of change of the automatic gain control voltage. In one non-limiting embodiment, the slope detection circuit includes an operational amplifier, a first comparator configured to compare an output of the operational amplifier to a positive automatic gain control voltage and provide a first comparison state from the comparison, a second comparator configured to compare an output of the operational amplifier to a negative automatic gain control voltage and provide a second comparison state from the comparison, and an OR logic gate configured to receive the first and second comparison states and output a signal indicative of a rate of change of the automatic gain control voltage. The optical device may be at least a part of an optical receiver or an optical transceiver.

[0010] According to a non-limiting embodiment, an optical receiver is provided that includes a linear transimpedance amplifier (TIA) including a photodiode, an input stage, an output stage, and at least one variable gain amplifier (VGA) disposed between the input stage and the output stage, the linear TIA coupled to the photodiode, an automatic gain control loop configured to rectify an output of the at least one VGA and compare the rectified output with a threshold gain setting to generate an automatic gain control voltage signal, a slope detection circuit configured to receive the automatic gain control voltage signal and configured to monitor the automatic gain control voltage signal from the automatic gain control loop, the slope detection circuit configured to provide a first slope status signal in response to detecting a slope in the automatic gain control voltage signal indicating that a slope has been detected, and a logic circuit coupled to the slope detection circuit and coupled to an output stage of the linear TIA, the logic circuit configured to squelch the output stage of the linear TIA in response to the first slope status signal.

[0011] The optical receiver may further comprise a received-signal-strength indicator (RSSI) configured to provide a first indication signal asserting a loss of signal (LOS) at the photodiode and a second indication signal deasserting the LOS at the photodiode. The logic circuit may be configured to output a squelch signal from the logic circuit to an output stage of the linear TIA in response to the first indication signal asserting the loss of signal (LOS). The logic circuit may be configured to output a squelch signal from the logic circuit to an output stage of the linear TIA in response to the second indication signal deasserting the loss of signal (LOS) and the first slope status signal indicating that a slope has been detected. The slope detection circuit may be further configured to provide a second slope status signal indicating that a slope has not been detected to the logic circuit in response to a slope not being detected in the automatic gain control voltage signal. The logic circuit may be configured to output a squelch deassertion signal from the logic circuit to an output stage of the linear TIA in response to a second indication signal deasserting loss of signal (LOS) and a second slope status signal indicating that no slope is detected. In response to receiving the squelch deassertion signal, the output stage of the linear TIA may be activated.

[0012] The automatic gain control loop may comprise a rectifier connected to an output of the at least one VGA and a threshold circuit configured to generate a threshold gain setting. The slope detection circuit may comprise an operational amplifier, a first comparator configured to compare an output of the operational amplifier to a positive automatic gain control voltage and provide a first comparison state from the comparison, a second comparator configured to compare an output of the operational amplifier to a negative automatic gain control voltage and provide a second comparison state from the comparison, and an OR logic gate configured to receive the first and second comparison states and output a signal indicative of the slope of the automatic gain control voltage.

[0013] According to a non-limiting embodiment, a method for controlling an output of a linear transimpedance amplifier (TIA) is provided, the method including: in response to a first indication signal asserting a loss of signal (LOS), sending a squelch signal to an output buffer of the linear TIA to squelch an output voltage of the linear TIA, rectifying an output of at least one variable gain amplifier VGA of the linear TIA and comparing the rectified output to a threshold gain setting to generate an automatic gain control voltage signal, monitoring the automatic gain control voltage signal to determine whether a slope has been detected in the automatic gain control voltage, in response to detecting a slope in the automatic gain control voltage signal, sending a first slope status signal indicating that a slope has been detected, and in response to a second indication signal deasserting LOS and the first slope status signal, sending a squelch signal to an output buffer of the linear TIA to continue squelching the output voltage of the linear TIA.

[0014] The method may further include, in response to no tilt being detected in the automatic gain control voltage signal, sending a second slope status signal indicating no tilt was detected, and in response to the second indication signal to deassert LOS and the second slope status signal, sending an unsquelch signal to an output buffer of the linear TIA to unsquelch an output voltage of the linear TIA. The first indication signal to assert LOS and the second indication signal to assert LOS may be provided by a received signal strength indicator (RSSI) at a photodiode of the optical receiver. The automatic gain control voltage signal may be monitored by a slope detection circuit to determine whether a tilt is detected in the automatic gain control voltage.

[0015] Further embodiments are described in the following numbered clauses.

[0016] Clause 1: An optical device for an optical signal, comprising: a linear transimpedance amplifier (TIA) having a photodiode configured to receive the optical signal; an input stage, an output stage, and at least one variable gain amplifier (VGA) disposed between the input stage and the output stage, the input stage coupled to the photodiode and the output stage coupled to the input stage and controllable to enable and disable an output of the linear transimpedance amplifier output; an automatic gain control loop configured to rectify the output of the at least one VGA and compare the rectified output with a threshold gain setting to generate an automatic gain control voltage; and a detection circuit configured to detect a rate of change of the automatic gain control voltage and configured to determine a first state indicative of the absence of an optical signal at the photodiode, wherein in response to at least the determined first state, the detection circuit is configured to disable the output stage of the linear transimpedance amplifier.

[0017] Clause 2: An optical device as described in clause 1, wherein the detection circuit is configured to determine a second state indicating the presence of an optical signal in the photodiode, and in response to the second state determined at least after the first state and the detected rate of change, the detection circuit is configured to disable an output stage of the linear TIA.

[0018] Clause 3: An optical device as described in clause 1 or 2, wherein the detection circuit includes a received signal strength indicator coupled to the photodiode and configured to indicate first and second states of the optical signal at the photodiode.

[0019] Clause 4: An optical device as described in clause 1 or 2, wherein the detection circuit is configured to enable an output stage of the linear TIA in response to at least a second state and the absence of a rate of change of the feedback signal of the feedback amplifier.

[0020] Clause 5: The optical device described in clause 4, wherein the detection circuit comprises a digital logic circuit configured to process a logical combination of the first state, the second state, the presence of the rate of change, and the absence of the rate of change, and provides enable and disable signals to an output stage of the linear TIA based on the processed logical combination.

[0021] Clause 6: An optical device described in any of clauses 1 to 5, wherein the automatic gain control loop comprises a rectifier connected to the output of the at least one VGA and a threshold circuit configured to generate a threshold gain setting.

[0022] Clause 7: An optical device described in any of clauses 1 to 6, wherein the detection circuit comprises a slope detection circuit configured to detect a rate of change of the automatic gain control voltage, the slope detection circuit comprising an operational amplifier, a first comparator configured to compare an output of the operational amplifier with a positive automatic gain control voltage and provide a first comparison state from the comparison, a second comparator configured to compare an output of the operational amplifier with a negative automatic gain control voltage and provide a second comparison state from the comparison, and an OR logic gate configured to receive the first and second comparison states and output a signal indicative of the rate of change of the automatic gain control voltage.

[0023] Clause 8: An optical device according to any one of clauses 1 to 7, wherein the optical device is at least a part of an optical receiver or an optical transceiver.

[0024] Clause 9: An optical receiver comprising: a linear transimpedance amplifier (TIA) having a photodiode, an input stage, an output stage, and at least one variable gain amplifier (VGA) disposed between the input stage and the output stage, the linear TIA coupled to the photodiode; an automatic gain control loop configured to rectify an output of the at least one VGA and compare the rectified output with a threshold gain setting to generate an automatic gain control voltage signal; a slope detection circuit configured to receive the automatic gain control voltage signal and configured to monitor the automatic gain control voltage signal from the automatic gain control loop, the slope detection circuit configured to provide a first slope status signal indicating that a slope has been detected in response to detecting a slope in the automatic gain control voltage signal; and a logic circuit coupled to the slope detection circuit and coupled to an output stage of the linear TIA, the logic circuit configured to squelch the output stage of the linear TIA in response to the first slope status signal.

[0025] Clause 10: The optical receiver of clause 9, further comprising a received signal strength indicator (RSSI) configured to provide a first indication signal to assert a loss of signal (LOS) at the photodiode and a second indication signal to deassert the LOS at the photodiode.

[0026] Clause 11: An optical receiver as described in clause 9 or 10, wherein the logic circuit is configured to output a squelch signal from the logic circuit to an output stage of the linear TIA in response to a first instruction signal asserting loss of signal (LOS).

[0027] Clause 12: An optical receiver described in any of clauses 9 to 11, wherein the logic circuit is configured to output a squelch signal from the logic circuit to an output stage of the linear TIA in response to a second indication signal deasserting loss of signal (LOS) and a first tilt status signal indicating that tilt has been detected.

[0028] Clause 13: An optical receiver described in any of clauses 9 to 11, wherein the slope detection circuit is further configured to provide a second slope status signal to the logic circuit indicating that no slope is detected in response to no slope being detected in the automatic gain control voltage signal.

[0029] Clause 14: An optical receiver as described in clause 13, wherein the logic circuit is configured to output a squelch deassert signal from the logic circuit to an output stage of the linear TIA in response to a second instruction signal deasserting loss of signal (LOS) and a second slope status signal indicating that no slope is detected, and the output stage of the linear TIA is activated in response to receiving the squelch deassert signal.

[0030] Clause 15: An optical receiver described in any of clauses 9 to 14, wherein the automatic gain control loop comprises a rectifier connected to the output of the at least one VGA and a threshold circuit configured to generate a threshold gain setting.

[0031] Clause 16: An optical receiver described in any of clauses 9 to 15, wherein the slope detection circuit comprises an operational amplifier, a first comparator configured to compare the output of the operational amplifier with a positive automatic gain control voltage and provide a first comparison state from the comparison, a second comparator configured to compare the output of the operational amplifier with a negative automatic gain control voltage and provide a second comparison state from the comparison, and an OR logic gate configured to receive the first and second comparison states and output a signal indicative of the slope of the automatic gain control voltage.

[0032] Clause 17: A method for controlling an output of a linear transimpedance amplifier (TIA), comprising: in response to a first instruction signal asserting a loss of signal (LOS), sending a squelch signal to an output buffer of the linear TIA to squelch an output voltage of the linear TIA; rectifying an output of at least one variable gain amplifier VGA of the linear TIA and comparing the rectified output with a threshold gain setting to generate an automatic gain control voltage signal; monitoring the automatic gain control voltage signal to determine whether a slope has been detected in the automatic gain control voltage; in response to a slope being detected in the automatic gain control voltage signal, sending a first slope status signal indicating that a slope has been detected; and in response to a second instruction signal deasserting LOS and the first slope status signal, sending a squelch signal to the output buffer of the linear TIA to continue squelching the output voltage of the linear TIA.

[0033] Clause 18: The method of clause 17, further comprising: in response to no slope being detected in the automatic gain control voltage signal, sending a second slope status signal indicating that no slope has been detected; and in response to the second indication signal deasserting LOS and the second slope status signal, sending an unsquelch signal to an output buffer of the linear TIA to unsquelch an output voltage of the linear TIA.

[0034] Clause 19: The method of clause 17 or 18, wherein the first indication signal to assert LOS and the second indication signal to assert LOS are provided by a received signal strength indicator (RSSI) at a photodiode of the optical receiver.

[0035] Clause 20: The method of any of clauses 17-19, wherein the automatic gain control voltage signal is monitored by a slope detection circuit to determine whether a slope is detected in the automatic gain control voltage.

[0036] These and other features and characteristics of the subject matter of the present disclosure, the method of operation and function of the associated elements of construction, and the combination of parts and economies of manufacture will become more apparent upon consideration of the following description and the appended claims, with reference to the accompanying drawings, all of which form a part hereof, and in which like reference numerals indicate corresponding parts in the various views. It is to be expressly understood, however, that the drawings are for purposes of illustration and description only and are not intended as a definition of the limits of the subject matter of the present disclosure. As used in this specification and claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.

[0037] Further advantages and details of the subject matter of the present disclosure are explained in more detail below with reference to the exemplary embodiments shown in the accompanying drawings. [Brief description of the drawings]

[0038] [Figure 1] FIG. 1A illustrates a conventional optical communication system having a transmitter that transmits an optical signal over an optical fiber to a receiver.

[0039] FIG. 1B illustrates a comparison of an optical signal received at a receiver and an invalid output signal output by the receiver due to recovery from a loss of signal (LOS) condition.

[0040] [Diagram 2] FIG. 2 illustrates an example of a desired output of a linear transimpedance amplifier (TIA) operated in accordance with the present disclosure.

[0041] [Diagram 3] FIG. 3 is a simplified block diagram of an optical receiver having a linear TIA in accordance with the present disclosure.

[0042] [Figure 4] FIG. 4 illustrates a non-limiting embodiment of a slope detection circuit and a non-limiting embodiment of a digital logic circuit.

[0043] [Diagram 5] FIG. 5 illustrates the use of a fixed delay from the time of LOS deassertion to turn the output of the linear TIA "on."

[0044] [Figure 6] FIG. 6 illustrates turning "on" the output of a linear TIA using dynamic delay determined by a slope detection circuit in accordance with a non-limiting embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] For purposes of the following description, the terms "end", "upper", "lower", "right", "left", "vertical", "horizontal", "top", "bottom", "lateral", "longitudinal" and their derivatives shall refer to the subject matter of the present disclosure when oriented in the drawings. However, it should be understood that the subject matter of the present disclosure may assume various alternative modifications and step sequences unless expressly stated otherwise. It should also be understood that the specific devices and processes illustrated in the accompanying drawings and described in the following specification are merely exemplary embodiments of the subject matter of the present disclosure. Thus, specific dimensions and other physical characteristics related to the embodiments disclosed herein are not to be construed as limiting, unless expressly stated otherwise.

[0046] As used herein, aspects, components, elements, structures, acts, steps, functions, instructions, and the like should not be construed as critical or essential unless expressly stated to be so. Also, as used herein, the articles "a" and "an" are intended to include one or more and may be used interchangeably with "one or more" and "at least one." When only one is intended, the term "one" or similar language is used. Also, as used herein, terms such as "has," "have," "having," and the like are intended to be open-ended terms. Also, the phrase "based on" is intended to mean "based at least in part on," unless expressly stated otherwise.

[0047] Some non-limiting embodiments are described herein in relation to a threshold value. As used herein, meeting a threshold may refer to a value being greater than the threshold, more than the threshold, higher than the threshold, equal to or greater than the threshold, less than the threshold, less than the threshold, lower than the threshold, less than or equal to the threshold, equal to the threshold, etc.

[0048] The present disclosure is directed to a slope detection circuit for accurately detecting when a linear transimpedance amplifier (TIA) in an optical receiver has settled after a signal resume event (e.g., after a signal loss) occurs at the input of the linear TIA. Accurate sensing of the settling of the linear TIA is used to prevent invalid data from being sent by squelching the output of the linear TIA until the TIA's control loop re-adapts and re-settles to the correct operating state.

[0049] Referring to FIG. 2, an example of a desired output of a linear TIA operating in accordance with the present disclosure is shown. Data 200 at the output of the linear TIA continues until a loss of signal is detected at the receiver. When an input optical signal loss event is detected and an LOS indication is asserted, the output of data at the linear TIA is squelched for a period P21. At time 202, the input optical signal resumes and the LOS indication is deasserted, but the squelch at the output of the TIA is not yet deasserted because the receiver (TIA) needs time to adapt or re-adapt. After time 202, there is a slope in the TIA's automatic gain control (AGC) loop voltage signal, which can be detected by a slope detection circuit, which will be described in more detail below. In response, the receiver output is further squelched for a period P22 until the linear TIA adapts. Finally, at time 204 when the adaptation or settling of the linear TIA of the receiver is completed after period P22, the slope of the AGC loop voltage signal of the linear TIA disappears and the squelch of the output of the linear TIA is deasserted. In response, the data 200 at the receiver output can continue without invalid data. Using the slope detection circuit disclosed herein, the instant of settling of the AGC loop of the linear TIA can be accurately sensed to avoid sending invalid data until the loop settles, and valid data at the linear TIA output can be quickly utilized. In one non-limiting configuration, the period P22 from when the input signal resumes to when the squelch of the output of the linear TIA ends can be within 80 μs of the transient step input.

[0050] With an understanding of how the slope detection circuit of the present disclosure is intended to control the output of the linear TIA of the receiver, the discussion will now turn to the circuitry and other features of the receiver and the method of controlling the output of the linear TIA of the receiver. Specifically, FIG. 3 shows a simplified block diagram of an optical receiver 310 according to the present disclosure. The optical receiver 310 includes a photodiode 311, a received signal strength indicator (RSSI) 317, a linear TIA 312, a DC cancellation feedback amplifier 316, a slope processing circuit 329, and an output buffer 315.

[0051] In general, as described below, optical receiver 310 receives an optical signal at photodiode 311, and linear TIA 312 converts (and amplifies) the varying input current from the photodiode to a voltage for output from output buffer 315 as a differential output voltage 680. In one non-limiting embodiment, linear TIA 312 includes a low noise input stage 313 configured to amplify the signal from photodiode 311, followed by a variable gain amplifier (VGA) stage 314 and an output buffer 315. Although only a single VGA stage 314 is shown in FIG. 3, multiple VGA stages may be present based on the application, so this should not be construed as limiting the disclosure. Based on the input signal amplitude, VGA stage 314 increases or decreases its gain to keep the output of TIA 312 linear. The VGA output is rectified by rectifier 318 and compared to a threshold gain setting generated by threshold circuit 320 to generate a control voltage (V agc ) is generated. In one non-limiting embodiment, the threshold circuit 320 provides a threshold current to achieve a particular VGA gain. agc ) is set by the AGC loop to make the current output of the rectifier equal to the threshold current.

[0052] The DC cancellation feedback amplifier 316 is used to reduce the DC component of the fluctuating input current using negative feedback. The DC cancellation feedback amplifier 316 is connected to the differential outputs 656, 658 from the VGA stage 314.

[0053] Briefly, a photodiode 311 has a cathode connected to a received signal strength indicator (RSSI) 317 and an anode connected to an input stage 313 of a linear TIA 312. The photodiode 311 is configured to receive an input optical signal and convert the input optical signal to an electrical signal 650 whose current is proportional to the optical power. The low noise input stage 313 of the linear TIA 312 is configured to receive the electrical signal 650 from the photodiode 311 and provide an electrical signal 652 to a VGA stage 314 of the linear TIA 312. The input stage 313 is connected to feedback from a DC cancellation feedback amplifier 316. That is, a feedback signal 670 from the feedback amplifier is connected to the input stage 313 to process DC offsets at the output of the VGA stage 314.

[0054] The VGA stage 314 is coupled to an output buffer 315 of the linear TIA 312, which provides differential outputs 656, 658 to the output buffer 315. In turn, the output buffer 315 provides a differential output voltage 680 for the receiver 310. However, the differential output voltage 680 of the output buffer 315 can be squelched and unsquelched in response to a control signal 672 (e.g., a squelch signal or an unsquelch signal) received at the output buffer 315 from circuitry described below.

[0055] Linear TIA 312 uses LOS status 600 to squelch differential output voltage 680 of output buffer 315 in response to a signal loss. TIA 312 uses LOS status 600 and slope processing circuit 329 to unsquelch differential output voltage 680 of output buffer 315 in response to a signal resumption.

[0056] At the cathode side of the photodiode 311, a received signal strength indicator (RSSI) 317 is configured to indicate the received signal strength and provide, via an output terminal of the RSSI 317, a LOS status signal 600 indicating a loss of signal (LOS) in response to the input optical signal being lost. That is, in response to the input optical signal being lost, the LOS is asserted. In response, the output stage 315 is switched off in a manner discussed below.

[0057] When the input optical signal resumes, the LOS is deasserted. That is, a LOS status signal 600 is generated or provided indicating that the optical signal is not lost (i.e., not LOS or the LOS is deasserted). In response, the linear TIA 312 adapts or readapts to such changes using an automatic gain control (AGC) loop 331 and a DC cancellation loop 333. The AGC loop 331 is configured to rectify the output of the VGA stage 314 and compare the rectified output with a threshold gain setting to generate an automatic gain control voltage signal 675. The linear TIA 312 requires a certain settling time to adapt or readapt. During the settling time, the output data from the linear TIA 312 is invalid.

[0058] To facilitate restarting and sending valid data from the output buffer 315, the circuitry of the optical receiver 310 (FIG. 3) includes a slope processing circuit 329. As shown generally here and discussed in more detail below, the slope processing circuit 329 includes a slope detection circuit 330 and a digital logic circuit 350. The slope processing circuit 329 receives the LOS status signal 600 from the RSSI 317 and an automatic gain control voltage signal (V agc ) 675 from the automatic gain control loop 331. agc ) 675 may be configured to receive and monitor.

[0059] In response to the LOS status signal 600 indicating a loss of signal (LOS), the slope processing circuit 329 squelches the differential output voltage 680 by providing or sending a squelch signal 672 to the output buffer 315. For example, the squelch signal 672 is used to disable the output buffer 315. If the LOS status signal 600 indicates that the optical signal is not lost, the control signal 672 from the slope processing circuit 329 to the output buffer 315 is the automatic gain control voltage signal (V agc ) 675 depending on whether tilt is detected.

[0060] The slope detection circuit 330 is configured to monitor the settling of the linear TIA 312 after the resumption of the input optical signal indicated by the LOS status signal 600. In some examples, the slope processing circuit 329 switches the linear TIA's output buffer 315 "on" after the linear TIA 312 has settled by sending an unsquelch signal 672 (e.g., a squelch deasserted signal) to the output buffer 315. That is, in response to the linear TIA 312 having settled after the signal loss, the slope processing circuit 329 turns the linear TIA's output buffer 315 "on" by sending an unsquelch signal 672 (e.g., a squelch deasserted signal) from the slope processing circuit 329 to the output buffer 315. In a particular example, the slope processing circuit 329 turns the TIA's output buffer 315 "off" (e.g., squelches) by sending the squelch signal 672 to the output buffer 315. Shown here only diagrammatically, signal 672 is shown as enabling and disabling output buffer 315 by operating a switch to a current source 335 to output buffer 315 (see FIG. 4).

[0061] The automatic gain control loop 331 generates the automatic gain control voltage signal (V agcIn response to the automatic gain control loop 331 being in a settling mode with a slope at input 675, the output from the slope detection circuit 330 is asserted, e.g., at a high voltage level. In response to the automatic gain control loop 331 having settled, e.g., the settling of the automatic gain control loop 331 being completed, the output from the slope detection circuit 330 is deasserted, e.g., at a low voltage level. Thus, the output of the slope detection circuit 330 may be used to prevent invalid data at the differential output voltage 680 of the output buffer 315 by keeping the output of the linear TIA 312 squelched with a squelch signal 672 until the automatic gain control loop 331 of the linear TIA has settled. Using the slope detection circuit 330 to so precisely determine the settling instant of the automatic gain control loop 331 of the linear TIA may prevent invalid data from being sent from the optical receiver 310. Further details of this slope detection circuit 330 are discussed below with reference to FIG. 4.

[0062] With an understanding of the circuitry of the receiver 310 having the slope processing circuit 329 (slope detection circuit 330 and digital logic circuit 350) used with the linear TIA 312, the discussion now turns to further details of the slope detection circuit 330. With reference to Figure 4, further details of an example of a slope detection circuit 330 and an example of a digital logic circuit 350 for the slope processing circuit 329 are shown. In this configuration, the slope detection circuit 330 and the digital logic circuit 350 are configured to output a squelch / unsquelch signal 672.

[0063] The slope detection circuit 330 detects the automatic gain control voltage signal (V agc ) 675 and has an output terminal for outputting a slope status signal 671. The slope detection circuit 330 receives an automatic gain control voltage signal (V agc ) 675. By processing that signal 675, the slope detection circuit 330 determines the received automatic gain control voltage signal (V agc ) 675. The automatic gain control voltage signal (Vagc ) 675 indicates that the signal is changing with time and has not yet settled.

[0064] Digital logic circuit 350 has a first input terminal for receiving LOS status signal 600 and has a second input terminal for receiving slope status signal 671 from slope detection circuit 330. Digital logic circuit 350, which may include any suitable digital logic, provides a squelch signal 672. In response to receiving LOS status signal 600 indicating a loss of signal (LOS), digital logic circuit 350 is configured to send squelch signal 672 to output buffer 315. In response to output buffer 315 receiving squelch signal 672, the output of output buffer 315 is squelched as described above.

[0065] The slope detection circuit 330 generates an automatic gain control voltage signal (V agc In response to detecting a slope in the slope detection circuit 330, the slope detection circuit 330 generates an automatic gain control voltage signal (V agc ) 675 is detected. agc In response to no tilt being detected in the automatic gain control voltage signal (V agc ) 675 provides or outputs a second tilt status signal 671 (eg, a deasserted signal having a different predetermined voltage level) indicating that no tilt has been detected.

[0066] Accordingly, the output of the tilt detection circuit 330 is coupled to a digital logic circuit 350 configured to receive a tilt status signal 671, which is coupled to an automatic gain control voltage signal (V agc )675 is the slope of the automatic gain control voltage signal (V agc ) 675 can indicate that it has been detected or not detected.

[0067] In some examples, a first input terminal of the digital logic circuit 350 receives a LOS status signal 600 indicating that the optical signal is no longer lost (i.e., has resumed after being lost), and the digital logic circuit 350 generates an automatic gain control voltage signal (V agc In response to receiving a first slope status signal 671 indicating that a slope in slope detector 675 has been detected, it still sends a squelch signal 672 to output buffer 315 which causes differential output voltage 680 to remain squelched.

[0068] In another example, a first input terminal of the digital logic circuit 350 receives a LOS status signal 600 indicating that the optical signal is no longer lost (i.e., has resumed after being lost), and the digital logic circuit 350 generates an automatic gain control voltage signal (V agc In response to receiving a second slope status signal 671 indicating no slope is detected at output buffer 315 (slope detection signal 675), output buffer 315 sends an unsquelch (squelch deassert) signal 672 to output buffer 315. In response, differential output voltage 680 of output buffer 315 is unsquelched.

[0069] Looking more closely at the slope detection circuit 330, the slope detection circuit 330 generates an automatic gain control voltage signal (V agc ) 675 as an input. The operational amplifier 337 is configured to receive an automatic gain control voltage signal (V agc ) 675 to generate an output voltage. For example, an arrangement of resistors and capacitors creates a time delay in the voltage at a first input terminal of the operational amplifier 337 compared to a second input terminal of the operational amplifier 337. In response, the automatic gain control voltage signal (V agc ) 675, the voltage difference between the input terminals of the operational amplifier 337 is agc ) 675.

[0070] The output terminal of the operational amplifier 337 is coupled to the base of a transistor 339. The emitter of the transistor 339 is coupled to a first comparator 341 and a second comparator 342 of a window comparator 343. The first comparator 341 and the second comparator 342 have outputs connected to an OR gate 345 from which a slope status signal 671 is available. The first comparator 341 receives an automatic gain control voltage signal (V agc ) 675 and receives a signal from the emitter of transistor 339 at its non-inverting terminal. The output terminal of first comparator 341 is configured to (a) output a high voltage level in response to the non-inverting terminal of first comparator 341 having a higher voltage than the inverting terminal of first comparator 341, and (b) output a low voltage level in response to the voltage at the non-inverting terminal being less than or equal to the voltage at the inverting terminal of first comparator 341.

[0071] In that regard, the second comparator 342 outputs an automatic gain control voltage signal (V agc ) 675 and receives a signal from the emitter of transistor 339 at its inverting terminal. The output terminal of second comparator 342 is configured to (a) output a high voltage level in response to the inverting terminal of second comparator 342 having a lower voltage than the non-inverting terminal of second comparator 342, and (b) output a low voltage level in response to the voltage at the inverting terminal being greater than or equal to the voltage at the non-inverting terminal of second comparator 342.

[0072] Using the outputs from the comparators 341, 342, the OR gate 345 is configured to output a high voltage level in response to the first comparator 341 or the second comparator 342 having a high voltage level, and the OR gate 345 is configured to output a low voltage level in response to both comparators 341, 342 providing a low voltage level. Although the OR gate can output a high voltage level in response to both of its inputs receiving a high voltage level, the comparators 341, 342 of FIG. 4 do not both output high voltage levels at the same time. That is, if comparator 341 outputs a high voltage level, then comparator 342 can only output a low voltage level, and if comparator 341 outputs a high voltage level, then comparator 342 can only output a low voltage level.

[0073] The first comparator 341 outputs an automatic gain control voltage signal (V agc ) 675, the second comparator 342 detects the positive slope of the change in the automatic gain control voltage signal (V agc ) 675 received at an input terminal of the slope detection circuit 330. agc In response to a positive slope in the slope detection circuit 330, the output of the first comparator 341 goes to a high voltage level and the output of the OR gate 345 goes to a high voltage level as the first slope status signal 671 indicating that a slope has been detected. agc ) 675 and output a first slope status signal 671 at a high voltage level to indicate that a slope has been detected.

[0074] In another example, the automatic gain control voltage signal (V agcIn response to a negative slope in the slope detection circuit 330, the output of the second comparator 342 goes to a high voltage level and the output of the OR gate 345 goes to a high voltage level as the first slope status signal 671 indicating that a slope has been detected. agc ) 675 and output a first slope status signal 671 at a high voltage level to indicate that a slope has been detected.

[0075] In another example, the automatic gain control voltage signal (V agc In response to the absence of a slope in the automatic gain control voltage signal (V) 675, the output of the first comparator 341 goes to a low voltage level and the output of the second comparator 342 goes to a low voltage level. This causes the output of the OR gate 345 to go to a low voltage level as the second slope status signal 671 indicating that no slope has been detected. In this manner, the slope detection circuit 330 controls the automatic gain control voltage signal (V agc ) 675, and the automatic gain control voltage signal (V agc ) 675 may output a second tilt status signal 671 at a low voltage level to indicate no tilt.

[0076] 3, the tilt detection circuit 330 and the digital logic circuit 350 may be coupled to the linear TIA 312. In other examples, the tilt detection circuit 330 and the digital logic circuit 350 may be integrated into or considered part of the linear TIA 312. For example, the integrated circuit of the linear TIA 312 may include components of the tilt detection circuit 330 and the digital logic circuit 350.

[0077] To further illustrate the teachings of the present disclosure, schematic examples of measurements of unsquelching of the output of a linear TIA in response to an input light step are shown in Figures 5 and 6. Figure 5 plots the output of an unsquelching operation using a fixed delay according to the prior art, and Figure 6 plots the output of an unsquelching operation using the slope detection circuit of the present disclosure.

[0078] 5, curve 501 shows the TIA input voltage at different times. The TIA input voltage 501 can be measured and is equivalent to the electrical signal 650 from the photodiode 311. Curve 502 shows the automatic gain control voltage signal (V agc 5 shows the linear TIA automatic gain control loop 331 settling can be interpreted from the slope of curve 502. Curve 503 shows the LOS status at different times. The LOS status of curve 503 switches from LOS asserted to LOS deasserted at time T53.

[0079] Curve 504 shows a conventional squelch signal 672 sent to output buffer 315 that deasserts at time T53 with a fixed delay after the LOS asserted status, and curve 505 shows the output of linear TIA 312 from output buffer 315. Curves 504 and 505 of FIG. 5 show that the output of the linear TIA is turned "on" at time T53 with a fixed delay D51 from time T51 when LOS status curve 503 switches from LOS asserted status to LOS deasserted status. Settling of the automatic gain control loop 331 of the linear TIA is controlled by the automatic gain control voltage signal (V agc5. The output of the linear TIA shown in curve 505 remains squelched between times T52 and T53, even after the automatic gain control loop of the linear TIA has settled from T52, as can be interpreted based on curve 502. This fixed delay D51 therefore results in undesirable operation, as the linear TIA does not resume outputting data after it has already produced valid data. Although not shown in the figure, a fixed delay D51 could similarly cause the linear TIA to prematurely resume outputting data before its automatic gain control loop has settled, resulting in the linear TIA outputting invalid data.

[0080] In contrast, in FIG. 6, the output of the TIA is turned "on" using a dynamic time span (between T61 and T62) determined by a slope detection circuit according to the present disclosure. Referring to FIG. 6, curve 601 shows the TIA input voltage at different times. The TIA input voltage 601 can be measured and is equivalent to the electrical signal 650 from the photodiode 311. Curve 602 shows the automatic gain control voltage signal (V agc 6 shows a time t610 of the LOS status of the linear TIA 331. The settling of the automatic gain control loop 331 of the linear TIA can be interpreted from the slope of the curve 602. Curve 604 shows the LOS status at different times. The LOS status of curve 603 switches from LOS asserted to LOS deasserted at time T61.

[0081] Curve 604 shows the squelch signal 672 from the slope processing circuit 329 to the output buffer 315, and curve 605 shows the output of the linear TIA 312 from the output buffer 315. Curve 606 shows the slope status signal 671 over time. The automatic gain control voltage signal (V agc In response to detecting a slope at V 675, the slope detection circuit 330 at time T61 adjusts the automatic gain control voltage signal (V agc ) 675 is detected. agc In response to no tilt being detected in the automatic gain control voltage signal (V) 675, the tilt detection circuit 330 at time T62 agc ) 675 provides or outputs a second tilt status signal 671 (eg, a deasserted signal having a different predetermined voltage level) indicating that no tilt has been detected.

[0082] Curves 604 and 605 of Figure 6 show the use of a dynamic delay determined by a slope detection circuit, such as circuit 330 of Figure 3, to turn the output of a linear TIA "on" at time T62. The settling of the automatic gain control loop 331 of the linear TIA is dependent on the automatic gain control voltage signal (V agc6. The linear TIA can be interpreted from the settling of the slope detection circuit 675. Between times T61 and T62, the linear TIA (e.g., the automatic gain control loop of the linear TIA) readapts as indicated by the slope between T61 and T62 in the curve 602. From time T62, the automatic gain control loop of the linear TIA settles. That is, from time T62, the linear TIA enters or switches into an active or normal operating mode. In response to the linear TIA entering into the normal operating mode, the output of the TIA shown in the curve 605 is quickly switched "on" due to the use of the slope detection circuit. In response, the output of the linear TIA is turned "on" (time T62) as soon as the linear TIA has completed its adaptation to the input optical signal resumes and entered into the normal operating mode (time T62).

[0083] While the subject matter of the present disclosure has been described in detail for purposes of illustration, based on what are presently considered to be the most practical and preferred embodiments, it should be understood that such details are for illustrative purposes only, and that the subject matter of the present disclosure is not limited to the disclosed embodiments, but on the contrary, is intended to cover modifications and equivalent arrangements within the spirit and scope of the appended claims. For example, it should be understood that the subject matter of the present disclosure contemplates that, to the extent possible, one or more features of any embodiment can be combined with one or more features of any other embodiment. [Explanation of symbols]

[0084] 100 Laser transmitter 102 Optical Fiber 104 Receiver 110 Photodetector 120 Linear Transimpedance Amplifier, TIA, Linear TIA 130 Optical Signal 132 Input Lost 140 TIA output data 142 Squelch 200 Data As of 202 As of 204 310 Optical Receiver 311 Photodiode 312 Linear TIA 313 Low Noise Input Stage 314 Variable Gain Amplifier (VGA) Stage 315 Output Buffer 316 DC Cancellation Feedback Amplifier 317 Received Signal Strength Indicator (RSSI) 318 Rectifier 320 Threshold Circuit 329 Tilt Processing Circuit 330 Tilt detection circuit, tilt detector 331 Automatic Gain Control Loop, AGC Loop 333 DC Cancellation Loop 335 Current Source 337 Operational Amplifier 339 Transistor 341 First Comparator 342 Second Comparator 343 Window Comparator 345 OR Gate 350 Digital Logic Circuits 600 LOS Status Signal 650 Electrical Signals 652 Electrical Signals 656 Differential Output 658 Differential Output 670 Return Signal 671 Tilt Status Signal 672 Control signals, squelch signals, unsquelch signals 675 Automatic Gain Control Voltage Signal 680 Differential Output Voltage D11 Invalid Data P21 Period P22 Period

Claims

1. An optical device for optical signals, comprising: a photodiode configured to receive the optical signal; a linear transimpedance amplifier (TIA) having an input stage, an output stage, and at least one variable gain amplifier (VGA) disposed between the input stage and the output stage, the input stage coupled to the photodiode and the output stage coupled to the input stage, the linear transimpedance amplifier being controllable to enable and disable output of the linear transimpedance amplifier output; an automatic gain control loop configured to rectify an output of the at least one VGA and compare the rectified output with a threshold gain setting to generate an automatic gain control voltage; a detection circuit configured to detect a rate of change of the automatic gain control voltage and configured to determine a first state indicative of an absence of the optical signal at the photodiode; Equipped with and in response to at least the determined first condition, the detection circuit is configured to disable the output stage of the linear transimpedance amplifier. Optical devices.

2. 2. The optical device of claim 1, wherein the detection circuit is configured to determine a second state indicative of the presence of the optical signal at the photodiode, and in response to the second state determined after at least the first state and the detected rate of change, the detection circuit is configured to disable the output stage of the linear TIA.

3. The optical device of claim 2 , wherein the detection circuit comprises a received signal strength indicator coupled to the photodiode and configured to indicate the first and second states of the optical signal at the photodiode.

4. 3. The optical device of claim 2, wherein the detection circuit is configured to enable the output stage of the linear TIA in response to at least the second condition and an absence of the rate of change of a feedback signal of a feedback amplifier.

5. 5. The optical device of claim 4, wherein the detection circuit comprises a digital logic circuit configured to process a logical combination of the first state, the second state, the presence of the rate of change, and the absence of the rate of change, and to provide enable and disable signals to the output stage of the linear TIA based on the processed logical combination.

6. 2. The optical device of claim 1, wherein the automatic gain control loop comprises a rectifier connected to the output of the at least one VGA and a threshold circuit configured to generate the threshold gain setting.

7. The detection circuitry comprises a slope detection circuit configured to detect the rate of change of the automatic gain control voltage, the slope detection circuit comprising: An operational amplifier, a first comparator configured to compare an output of the operational amplifier to a positive automatic gain control voltage and provide a first comparison state from the comparison; a second comparator configured to compare the output of the operational amplifier to a negative automatic gain control voltage and provide a second comparison state from the comparison; an OR logic gate configured to receive the first and second comparison states and output a signal indicative of the rate of change of the automatic gain control voltage; The optical device of claim 1 , comprising:

8. The optical device of claim 1 , wherein the optical device is at least a part of an optical receiver or an optical transceiver.

9. A photodiode; a linear transimpedance amplifier (TIA) comprising an input stage, an output stage, and at least one variable gain amplifier (VGA) disposed between the input stage and the output stage, the linear TIA being coupled to the photodiode; an automatic gain control loop configured to rectify an output of the at least one VGA and compare the rectified output with a threshold gain setting to generate an automatic gain control voltage signal; a slope detection circuit configured to receive the automatic gain control voltage signal and configured to monitor the automatic gain control voltage signal from the automatic gain control loop, the slope detection circuit configured to, in response to detecting a slope in the automatic gain control voltage signal, provide a first slope status signal indicating that the slope has been detected; a logic circuit coupled to the slope detection circuit and coupled to the output stage of the linear TIA, the logic circuit configured to squelch the output stage of the linear TIA in response to the first slope status signal; An optical receiver comprising:

10. a received signal strength indicator (RSSI) configured to provide a first indication signal to assert a loss of signal (LOS) at the photodiode and a second indication signal to deassert the LOS at the photodiode; The optical receiver of claim 9 further comprising:

11. 11. The optical receiver of claim 10, wherein the logic circuit is configured to output a squelch signal from the logic circuit to the output stage of the linear TIA in response to the first indication signal asserting a loss of signal (LOS).

12. 12. The optical receiver of claim 11, wherein the logic circuit is configured to output a squelch signal from the logic circuit to the output stage of the linear TIA in response to the second indication signal deasserting loss of signal (LOS) and the first slope status signal indicating that the slope has been detected.

13. 12. The optical receiver of claim 11, wherein the slope detection circuit is further configured, in response to no slope being detected in the automatic gain control voltage signal, to provide a second slope status signal to the logic circuit indicating that no slope has been detected.

14. 14. The optical receiver of claim 13, wherein the logic circuit is configured to output a squelch deassert signal from the logic circuit to the output stage of the linear TIA in response to the second indication signal deasserting loss of signal (LOS) and the second slope status signal indicating that the slope has not been detected, and the output stage of the linear TIA is activated in response to receiving the squelch deassert signal.

15. 10. The optical receiver of claim 9, wherein the automatic gain control loop comprises a rectifier connected to the output of the at least one VGA and a threshold circuit configured to generate the threshold gain setting.

16. The tilt detection circuit includes: An operational amplifier, a first comparator configured to compare an output of the operational amplifier to a positive automatic gain control voltage and provide a first comparison state from the comparison; a second comparator configured to compare the output of the operational amplifier to a negative automatic gain control voltage and provide a second comparison state from the comparison; an OR logic gate configured to receive the first and second comparison states and output a signal indicative of the slope of the automatic gain control voltage; 10. The optical receiver of claim 9, comprising:

17. 1. A method for controlling an output of a linear transimpedance amplifier (TIA), comprising: in response to a first indication signal asserting a loss of signal (LOS), sending a squelch signal to an output buffer of the linear TIA to squelch an output voltage of the linear TIA; rectifying an output of at least one variable gain amplifier VGA of the linear TIA and comparing the rectified output with a threshold gain setting to generate an automatic gain control voltage signal; monitoring the automatic gain control voltage signal to determine whether a slope is detected in the automatic gain control voltage; in response to detecting a slope in the automatic gain control voltage signal, sending a first slope status signal indicating that the slope has been detected; in response to a second indication signal deasserting LOS and the first slope status signal, sending a squelch signal to the output buffer of the linear TIA to continue squelching the output voltage of the linear TIA; A method comprising:

18. in response to no tilt being detected in the automatic gain control voltage signal, sending a second tilt status signal indicating that no tilt was detected; in response to the second indication signal deasserting LOS and the second slope status signal, sending an unsquelch signal to the output buffer of the linear TIA to unsquelch the output voltage of the linear TIA; 20. The method of claim 17, further comprising:

19. 20. The method of claim 17, wherein the first indication signal to assert LOS and the second indication signal to assert LOS are provided by a received signal strength indicator (RSSI) at a photodiode of an optical receiver.

20. 20. The method of claim 17, wherein the automatic gain control voltage signal is monitored by a slope detection circuit to determine if a slope is detected in the automatic gain control voltage.

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