Resonant quantum well modulator driver

A drive circuit and method of controlling a quantum well modulator are disclosed. The drive circuit can be disposed in an optical transceiver having a quantum well modulator configured to retro-modulate an incoming optical signal. The drive circuit can include separate modulating and bias voltage sources. A level of the modulating voltage and the bias voltage can be determined based on an ambient temperature of the optical transceiver and can be adjusted to compensate for variations in the optical performance of the quantum well modulator. The quantum well modulator can be controlled in intervals. The modulating voltage can be applied to the quantum well modulator during a first interval. A current associated with the modulating voltage can be returned to the modulation voltage source during a second interval. A timing of the first and second intervals can be based on electrical properties of the quantum well modulator.

BACKGROUND

The present invention relates to optical communications and, more specifically, to a retro-modulating optical transceiver.

In combat identification systems, an optical transceiver can employ modulators and/or reflectors to respond to an interrogating beam of light. The modulators control the incoming beam and the reflectors direct it back toward the source. Modulating retro-reflectors, in general, are becoming smaller, lighter, and faster. Also, modulating retro-reflectors are becoming increasingly portable and their use on the battlefield and in combat training is growing.

BRIEF SUMMARY

Techniques for controlling the operation of a quantum well modulator are disclosed. In one embodiment, a drive circuit is disclosed. The drive circuit includes a bias voltage source coupled to the quantum well modulator for supplying a bias voltage to the quantum well modulator. A modulation voltage source is also coupled to the quantum well modulator for supplying a modulating voltage. The bias voltage and the modulating voltage operate to control the optical properties of the quantum well modulator. A first switch is coupled to the modulation voltage source and to the quantum well modulator. Based on its state, the first switch delivers the modulating voltage to the quantum well modulator via a first conduction path. A second switch is coupled to the first switch and to the quantum well modulator. The second switch returns a current associated with the modulating voltage to the modulation voltage source via a second conduction path according to it state. A processor varies the state of the first switch and the second switch such that the modulating voltage is alternately supplied from the modulation voltage source and the current is returned to the modulation voltage source. Optionally, the processor adjusts a level of the modulating voltage and the bias voltage based on an ambient temperature of the drive circuit.

In another embodiment, an communication device is disclosed. The communication device includes a modulator, a retro-reflector, and a processor. The modulator includes a quantum well modulator which modulates an incoming optical signal in response to a modulation voltage. A first switch of the modulator selectively delivers the modulating voltage from a power supply to the quantum well modulator based on a first control signal. A second switch of the modulator selectively returns a current associated with the modulating voltage to the power supply based on a second control signal. The processor generates a timing of the first and second control signals such that pulses of the modulation voltage are delivered to the quantum well modulator during a first interval and current is returned to the power supply during a second interval. The retro reflector is coupled to the quantum well modulator and reflects the modulated optical signal away from the communication device.

In one embodiment, a drive circuit is disclosed. The drive circuit is coupled to a first voltage source and a second voltage source. The first voltage source establishes a bias level of the quantum well modulator. The second voltage source supplies a modulating voltage to the quantum well modulator for changing its optical properties when the quantum well modulator is biased by the first voltage source. A temperature detector detects an ambient temperature of the drive circuit. A processor coupled to the temperature detector controls delivery of the modulating voltage in response to an incoming optical signal and dynamically adjusts a voltage level of the first voltage source and the second voltage source based on the ambient temperature.

In one embodiment, a method of controlling a quantum well modulator is disclosed. The method includes detecting an ambient temperature of the quantum well modulator, determining a bias voltage level based on the ambient temperature, and delivering the bias voltage to the quantum well modulator. The method includes determining a modulating voltage level based on the ambient temperature and delivering pulses of the modulating voltage to the quantum well modulator in response to an incoming optical signal. The method also includes determining a timing of the pulses based on electrical characteristics of the drive circuit and the quantum well modulator so as to minimize a power consumption of the drive circuit.

DETAILED DESCRIPTION OF EMBODIMENTS

The ensuing description provides preferred exemplary embodiment(s) only, and is not intended to limit the scope or applicability of the disclosure. Rather, the ensuing description of preferred embodiment(s) will provide those skilled in the art with an enabling description for implementing the preferred exemplary embodiment. It is understood that various changes may be made in the function and arrangement of the elements without departing from the spirit and scope of the disclosure.

A drive circuit and method of controlling a quantum well modulator are disclosed. The drive circuit can be disposed in an optical transceiver having a quantum well modulator configured to retro-modulate an incoming optical signal. The drive circuit can include separate modulating and bias voltage sources. A level of the modulating voltage and the bias voltage can be determined based on an ambient temperature of the optical transceiver and can be adjusted to compensate for variations in the optical performance of the quantum well modulator. The quantum well modulator can be controlled in intervals. The modulating voltage can be applied to the quantum well modulator during a first interval. A current associated with the modulating voltage can be returned to the modulation voltage source during a second interval. A timing of the first and second intervals can be based on electrical properties of the quantum well modulator.

FIG. 1is a functional block diagram of a free space optical system100according to one embodiment of the present invention. Optical communication system100can include a first transceiver110that is configured to generate a modulated optical signal. The modulated optical signal can be transmitted to a second transceiver150, for example, via a free space communication channel. The second transceiver150can be configured to receive the modulated optical signal and to generate a return coded optical signal.

First transceiver110can include an optical transmitter120for generating an outgoing optical signal and an optical receiver130configured to receive a retro-modulated optical signal, or some other received optical signal. The optical transmitter120can include an optical source122such as a laser. The output of optical source122can be controlled by a transmit driver124which can, for example, modulate the optical signal by modulating the laser drive current.

In some embodiments, transmit driver124can be configured to pulse the current to the optical source122to create a pulsed optical output signal. Transmit driver124can control the timing and duration of the pulses according to a data source, such as a data and control module140. The optical signal can be coupled from optical source122to an optical amplifier126which can be configured to amplify the modulated optical signal before coupling it to the communication channel.

Second transceiver150can be configured to receive the modulated optical signal over the communication channel. As shown inFIG. 1, second transceiver150includes an optical receiver160and a quantum well modulator controller165(also referred to herein as “QWM controller” or “controller”). Optical receiver160can be configured to receive the modulated optical signal from the communication channel and to recover the modulation data.

Controller165can determine, for example, if at least a portion of the modulation data corresponds to a predetermined signal or sequence. If it is determined that the modulation data recovered from the incoming optical signal corresponds to the predetermined signal or sequence, controller165can control retro-modulation of the incoming optical signal by quantum well modulator195and retro-reflector190. For example, by adjusting a voltage applied to quantum well modulator195, an on-off keying of the incoming optical signal can be performed.

Second transceiver150can also include a modulation data source170. When the predetermined signal or sequence is detected, controller165can drive quantum well modulator195with a modulation voltage as determined by modulation data source170. Quantum well modulator195can be positioned on the front surface of retro-reflector190to retro-modulate the incident optical signal. Based on the modulation voltage, quantum well modulator195can re-modulate the incoming optical signal. Retro-reflector190can reflect the incoming optical signal away from second transceiver150along the direction of the incident optical signal. In this manner, the second transceiver150is not required to include an optical signal source.

The electronics of second transceiver150can be powered by a battery180. For example, second transceiver150can be embodied in an optical tag such an used with a combat identification (CID) system. In CID systems, optical tags are portable devices which can be carried or worn on a garment and retro-reflect optical signals as a means of communication. Optical tags, for example, can detect a challenge code as part of an incoming optical signal and retro-modulate the incoming signal with an appropriate response code. In this way, a person or object can be identified as friendly (or unknown) in a battlefield environment or as part of a combat training exercise.

QWM controller165can include a high-efficiency drive circuit for controlling quantum well modulator195. The drive circuit can be configured to deliver a modulating voltage to quantum well modulator195from a power source of optical transceiver150and to return a current associated with the modulating voltage back to the power source. In one embodiment, controller165includes a processor which implements resonant switching of the modulating voltage so as to exploit electrical characteristics of quantum well modulator195. In additional embodiments, controller165compensates for environmental conditions by detecting an ambient temperature of second transceiver150and dynamically adjusting voltage levels according to the optical performance characteristics of quantum well modulator195.

FIG. 2is a functional block diagram of a retro-modulating optical transceiver200according to one embodiment of the present invention. As shown, optical transceiver200includes a window215through which an incoming signal can pass into housing205. In a combat identification system, for example, window215may pass portions of the infrared spectrum into housing205. These signals can have wavelengths ranging from about 700 nm up to approximately 1 mm. In some embodiments, the incoming signals may have other wavelengths, including portions of the visible spectrum (380 nm-750 nm). For convenience, the incoming signals are referred to as “optical signals” regardless of whether they are visible to the human eye.

An incoming optical signal passing through window215can be coupled to a filter210. As shown, filter210is coupled to optical receiver160and can limit its exposure to environmental radiation not used for communication. For example, filter210can be matched to the optical properties of receiver160to limit the effect of solar loading when optical transceiver200is used in daylight conditions. Optical receiver160can include a photodiode such as a silicon (Si), germanium (Gr), indium gallium arsenide (InGaAs), or like photo detector that is sensitive to wavelengths of the optical communication signal. Optical receiver160converts the incoming optical signal into an electrical signal.

The incoming optical signal can also pass through window215to quantum well modulator195. Quantum well modulator195can perform optical switching based on electroabsorption. The electroabsorption effect operates on very small time scales, for example, on the order of picoseconds or less. When an electric field is applied across an active region of quantum well modulator195, absorption for photon energies increases just below the band gap by the Stark effect. As the electric field is increased further, the band edge shifts to lower photon energies. By controlling a voltage applied to quantum well modulator195and therefore the electric field, its optical properties can be rapidly changed.

As shown, quantum well modulator195is coupled to a front surface of a retro-reflector190. Retro-reflector190can be a corner-cube reflector (CCR) or like device configured to direct the incoming optical signal away from optical transceiver200. Corner cube reflectors (CCRs) can be pyramids with three internal reflective surfaces and a front entrance base. The reflective surfaces can be joined with 90 degree angles at the apex of the pyramid. The base may have different shapes, for example a triangle, a square, a hexagon, a circle, and is referred to as a front surface. Under the control of QWM driver230, quantum well modulator195and retro-reflector190can re-modulate the incoming optical signal. For example, optical transceiver200can perform a kind of on-off keying of the incoming optical signal such that it is re-modulated according to signals from QWM driver230.

Processor220is coupled to an output of optical receiver160for receiving electrical signals representative of the incoming optical signal. Processor220can be a field programmable gate array (FPGA), microprocessor, microcontroller, application-specific integrated circuit (ASIC), or like processing device.

In a combat identification system, the optical signal can comprise a coded message having, for example, a transmit code of the day (TCOD). The coded message can be arranged with a frame-synchronization preamble, followed by the TCOD, and then by an interrogation pulse stream. Processor220can be configured to detect the TCOD portion of the coded message and verify that it is valid. When the TCOD has been verified, processor220can be configured to output control signals to QWM driver230with which to re-modulate one or more portions of the coded optical message. For example, when a valid TCOD is detected, processor220may cause the interrogation pulse stream to be re-modulated with an identification code or other data.

QWM driver230can be configured to deliver a modulating voltage to quantum well modulator195in response to signals from processor220. As shown, QWM driver230includes bias voltage source235and modulation voltage source240both of which can be powered by battery180. In other embodiments, bias voltage source235and modulation voltage source240can be separate from QWM driver230such that each is coupled to QWM driver230for supplying its respective voltage.

QWM driver230is a high-efficiency driver which extends the operating life of battery180while maintaining an optimal modulation depth of the retro-modulated signal. In one embodiment, QWM driver230controls delivery of the modulating voltage to quantum well modulator195such that current is sourced from and returned to modulation voltage source240with minimal loss. Also, a level of the bias voltage source235and the modulating voltage source240can be varied to compensate for changes in the optical properties of quantum well modulator195due to ambient temperature.

As shown, processor220is coupled to temperature sensor250. In operation, temperature sensor250can generate a signal representative of the ambient temperature of optical transceiver200. Processor220can separately control the output of bias voltage source235and modulation voltage source240based on the ambient temperature. In other words, processor220can be configured to compensate for variations in ambient temperature by adjusting a level of the bias voltage and the modulating voltage applied to quantum well modulator195.

In some embodiments, processor220retrieves temperature compensation values from a memory260or other storage device. Memory260can include volatile and/or non-volatile elements for storing program instructions and data used by processor220. In one embodiment, memory260includes a lookup table. The lookup table can include values of ambient temperature and associated bias voltage levels and modulating voltage levels considered to be optimum for re-modulating the incoming optical signal. An exemplary temperature table is provided below.

As the exemplary table illustrates, bias voltage and modulating voltage vary based on ambient temperature. In some embodiments, modulating voltage can represent the amplitude of an alternating voltage waveform such as a ½ sinusoidal waveform. Bias voltage can be a DC value. It will be understood that these values are illustrative only and that other voltage levels and waveforms are specifically contemplated for use with the present invention. For example, bias and modulating voltage levels may be selected to achieve an optimum modulation depth based on a breakdown voltage or other characteristics of quantum well modulator195.

Processor220can supply one or more control signals to QWM driver230for controlling the timing and delivery of the modulating voltage to quantum well modulator195. Processor220can time delivery of the control signals to QWM modulator230in relation to the incoming optical signal. For example, if a valid TCOD is detected, processor220can time-align the control signals with a portion of the incoming optical signal such as the interrogation pulse stream. The control signals from processor220can also establish a duration and pulse rate at which the modulating voltage is supplied from modulation voltage source240to quantum well modulator195.

In one embodiment, processor220and QWM driver230perform a controlled switching (also “resonant switching”) of the modulating voltage based on electrical properties of the quantum well modulator195. During a first switching cycle, for example, QWM driver230can make a connection between modulation voltage source240and quantum well modulator195such that the modulating voltage is applied to quantum well modulator195. During a second switching cycle, QWM driver230can interrupt the connection and return current associated with the modulating voltage to the modulation voltage source240. QWM driver230can include an inductor and the switching cycles can be determined based on a resonance between the inductor and a capacitance of quantum well modulator195. By delivering current from and returning current to modulation voltage source240, the power requirements of optical transceiver200can be reduced and the life of battery180can be extended.

FIG. 3shows portions of a quantum well modulator drive circuit300according to embodiments of the present invention. Drive circuit300can be used with QWM driver230and, more generally, as part of optical transceivers150,200. As shown, drive circuit300is configured to superimpose a voltage from modulation voltage source240and bias voltage source235upon quantum well modulator195.

Processor220is coupled to modulation voltage source240and bias voltage source235. Processor220can be configured to set a level of the bias and modulating voltages based on ambient temperature. For example, processor220may calculate a level of the bias and modulating voltages based on the ambient temperature or it may retrieve predetermined voltage values from memory260.

By employing separate modulation and bias voltage sources, drive circuit300avoids losses and can extend the life of battery180. For example, a full amplitude drive voltage for use with quantum well modulator can exceed 90V. Driving quantum well modulator195with a full amplitude voltage can result in losses from an excessive charge transfer. However, with drive circuit300, losses associated with the bias voltage are due mainly to leakage currents and can be considered minimal. Also, as described below, drive circuit300can minimize losses due to the modulating voltage by delivering the modulating voltage and subsequently returning a current associated with the modulating voltage to modulation voltage source240.

Processor220can be configured to generate upper drive signal310and lower drive signal320. Drive signals310,320can be pulses characterized by a duty cycle and repetition rate or they can be other types of signals. As shown, upper drive signal310is coupled to an upper switch330for controlling a modulating voltage from modulation voltage source240. Upper switch330can be a p-channel transistor and upper drive signal310can be coupled to its gate terminal through capacitor C1. When upper switch330is conducting, the modulating voltage causes a current flow along a first conduction path P1.

Lower drive signal320can be coupled to a lower switch340for controlling a second conduction path P2. As shown, lower drive signal320is connected to the gate terminal of an n-channel transistor. When conducting, lower switch340allows a current to flow from inductor350back to the modulation voltage source240. Thus, as shown, upper drive signal330controls delivery of the modulating voltage whereas lower drive signal320controls return of a current associated with the modulating voltage to modulation voltage source240.

Persons of skill in the art will recognize that other switching arrangements are possible within the scope of the present invention. In particular, upper switch330and lower switch340are not limited to transistors or to specific types of transistors but can include other switching devices or circuits for controlling a current flow to and from quantum well modulator195.

Processor220can be configured to generate upper and lower drive signals310,320to perform a controlled switching of the modulating voltage.FIG. 4shows a pair of exemplary drive signals such as can be generated by processor220. For purposes of illustration, upper drive signal310and lower drive signal320are shown as square waves having a 3V amplitude such as can be used to interface with CMOS devices. Also, an exemplary 16 MHz clock rate is selected for defining clock cycles.

As shown, both upper drive signal310and lower drive signal320are initially in a logical high state. Lower drive signal320then transitions to a logical low state at the start of the interval labeled T1. At that point, switches330and340can be in a non-conducting state representing the start of a switching cycle. Interval T1is shown as having a period of 62.5 ns or one exemplary clock cycle. At the beginning of interval T2, upper drive signal310transitions to a logical low state. This can represent a time when upper switch330begins conducting and current flows along path P1from modulation voltage source240.

During interval T2, the modulating voltage is applied to quantum well modulator195. As shown, T2lasts for 437.5 ns (seven exemplary clock cycles) and is terminated when upper drive signal310transitions from the logical low state to a logical high state. Interval T3defines a dead time lasting for 62.5 ns after which lower drive signal320transitions from the logical low to the logical high state. Interval T4represents a return interval during which a current can flow along conduction path P2back to the modulation voltage source240. By controlling the timing of the drive signals, processor220causes the modulating voltage to be delivered to quantum well modulator195during a first interval and a current associated with the modulating voltage to be returned to the modulation voltage source during a second interval.

In some embodiments, the timing and duration of drive signals310,320are based on electrical properties of quantum well modulator195. As illustrated, quantum well modulator195can be represented by an equivalent series resistance360and a capacitance370. A value of inductor350can be selected to create a resonance with the depleted capacitance of quantum well modulator195. Processor220can be configured to generate the drive signals to exploit the resonance. For example, by driving quantum well modulator195at approximately a resonant frequency of the quantum well modulator195and inductor350elements, it is possible to achieve a voltage doubling effect and to improve overall efficiency of drive circuit300.

FIG. 5illustrates a relationship between current flow and an inductance value of inductor350for an exemplary drive circuit such as drive circuit300. Two data series A, B are depicted. Series A corresponds to a switching rate of approximately 1.5 MHz. Series B values are based on a switching rate of approximately 751 kHz. For a given switching rate, resonant characteristics of the drive circuit can be exploited by selecting a value of inductor350which minimizes current flow. As illustrated, an inductance of approximately 7.9 μH minimizes current flow in the exemplary drive circuit for both data series.

FIG. 6shows another embodiment of a drive circuit600such as can be used with QWM controller230. Drive circuit600is similar to drive circuit300with the exception that processor220supplies only upper drive signal310. Diode D1is also added between the source and drain terminals of upper switch330. Drive circuit600eliminates the transition (dead time) periods separating the modulating interval and return interval.

FIG. 7illustrates a method700of controlling a quantum well modulator according to one embodiment of the present invention. The method700can be used with an optical transceiver such as optical transceiver200and, in particular, with a drive circuit such as drive circuits300,600.

At block710, an ambient temperature of the optical transceiver is detected. Ambient temperature can be an indicator of the optical performance of a quantum well modulator. Based on the ambient temperature, at block720a bias voltage level and a modulating voltage level are determined. For example, predetermined values of bias voltage and modulation voltage may be retrieved from a memory of the optical transceiver or calculated to achieve an optimum modulation depth for retro-modulating an incoming optical signal.

At block730, an incoming optical signal is detected. The incoming optical signal can include an identification code used to trigger retro-modulation by the optical transceiver. For example, the optical transceiver may be configured to recognize a challenge code as part of an optical message and to retro-modulating the incoming optical signal with an appropriate response code. The response code can be time-aligned with the optical message and retro-modulation can accomplished by selectively applying a modulating voltage to a quantum well modulator of the optical transceiver.

During a first switching interval, block740, the modulating voltage is delivered from a power source of the optical transceiver to the quantum well modulator. The modulating voltage can change the optical properties of the quantum well modulator causing it to selectively pass or attenuate portions incoming optical signal. In some embodiments, the optical transceiver is a portable device and the power source can include one or more batteries.

The modulating voltage can cause a current to flow from the power source. For example, current may flow in a drive circuit of the optical transceiver. During a second switching interval, block750, current associated with the modulating voltage is returned to the power source. At block760, the optical transceiver alternates between delivering the modulating voltage to the quantum well modulator and returning current to the power source. The timing and duration of the first and second switching intervals can be determined based on electrical properties of the drive circuit and the quantum well modulator. For example, the timing of the intervals may be based on a resonance such that current flow associated with the modulating voltage is reduced.

Specific details are given in the above description to provide a thorough understanding of the embodiments. However, it is understood that the embodiments may be practiced without these specific details. For example, circuits may be shown in block diagrams to avoid unnecessary detail. In other instances, well-known circuits, processes, algorithms, structures, and techniques may be shown without unnecessary detail to avoid obscuring the embodiments.

While the principles of the disclosure have been described above in connection with the specific apparatuses and methods, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the disclosure.