Trw Limited

Northrop Grumman Systems Corporation

Terabeam Corporation

David Lawrence Rollins

Lawrence J Lembo

Richard A Fields

Mark Kintis

Eric L Upton

Elizabeth Twyford Kunkee

Stephen R Perkins

Bruce A Ferguson

Juan C Carillo, Jr

Robert M Pierce

John C Brock

Gerald Edward Tourgee

James E Leight

Timothy Alan Sochor

Pierre Robert Barbier

Thomas J Wiltsey

John Walker Robinson, Iii

An improved analog optical system which provides improved dynamic range as well as sensitivity relative to known analog optical systems. The analog optical system includes a Mach-Zehnder modulator (MZM) operated with a low bias to improve sensitivity. In accordance with an important aspect of the invention, the optical system utilizes two optical wavelengths with two effective bias points to cancel even ordered distortion associated with low biasing. Two lasers having different wavelengths are applied to the Mach-Zehnder modulator by way of a wavelength division multiplexer (WDM). Alternately, a single laser producing two optical carriers having different wavelengths could be used in place of the two single wavelength lasers and the WDM. The modulator bias control circuit forces two optical carriers to two bias points on opposite sides of the minimum bias point thus, providing equal modulation depth with opposite sign on each of the two optical carriers. The frequency separation of the two optical carriers, together with the nominal optical path length difference between the two arms of the Mach-Zehnder modulator determines the degree of low biasing achieved and thus the resulting potential for improvement in sensitivity and third-order linearity. The receiver or demodulator also includes a WDM to separate the two wavelengths which, in turn, are applied to two separate photodetectors allowing each wavelength to be detected separately. The complementary photocurrents are subtracted with a balanced photodetector pair to provide a linearized RF output.

Linearized optical link using a single Mach-Zehnder modulator and two optical carriers

An apparatus and method for combining multiple optical beams into mode-scrambled optical signals. In one embodiment, the apparatus includes a plurality of laser beam sources, each to produce a modulated optical beam. A plurality of input fiber segments, each comprising a multimode optical fiber core, are operatively coupled at one end to a respective laser source to receive a respective modulated optical beam. A multimode optical beam combiner is used to operatively couple the output ends of the input fiber segments to the input end of an output fiber segment having a multimode optical fiber core. The modulated optical beams produced by the laser beam sources are combined into a single mode-scrambled optical signal. The apparatus enables multiple optical beams having similar wavelengths to be combined to increase optical signal strength. It also enables multiple optical beams having different wavelengths to be combined to form a wavelength division multiplexed signal.

Apparatus and method for combining multiple optical beams in a free-space optical communications system

An analog optical link ( ) that provides high-fidelity over bandwidths greater than 1 GHz is presented. The analog optical link ( ) includes a transmitter ( ) having an optical modulator ( ) and a receiver ( ) having an optical demodulator ( ). In one embodiment, the modulator ( ) is a phase modulator ( ) and particularly a pre-emphasis phase modulator ( ) that operates as a frequency modulator at low frequencies. The demodulator ( ) is a frequency demodulator ( ) that includes a feed-forward function for cancelling noise.

Wideband, high-fidelity analog optical link design

An RF combiner ( ) that combines a plurality of RF signals ( ) in the optical domain. The combiner ( ) includes a single optical source ( ) that generates an optical beam ( ). The optical beam ( ) is directed through a series of optical modulators ( ), such as optical phase modulators. Each modulator ( ) is responsive to an RF signal ( ) that is to be combined with the other RF signals ( ). Each modulator ( ) modulates the optical signal ( ) with the RF signal ( ) so that the modulations combine in an additive manner. A single optical phase demodulator ( ) is used to demodulate the composite phase modulated optical beam ( ) to generate the combined RF signal ( ). Suitable delay devices ( ) can be used between the optical modulators ( ), or the RF signals can be matched so that the RF signals combine in phase.

RF combiner based on cascaded optical phase modulation

An apparatus and method implements a first plurality of single-mode optical fibers into which to inject wavelength division multiplexed (WDM) optical signals. A tapered fiber bundle (TFB) is coupled to the first plurality of single-mode optical fibers, with the TFB including a second plurality of fused single-mode optical fibers correspondingly coupled to the first plurality of single-mode optical fibers at a first end of the TFB. The second plurality of fused single-mode optical fibers are capable to receive the WDM optical signals from the first plurality of single-mode optical fibers at the first end and to provide the WDM optical signals to a free space optical communication path at a second end.

Apparatus and method to use a tapered fiber bundle in a free space optical communication system

An optical device for use with an optical input beam comprising an optical thresholding device positioned along an optical path defined by the propagation direction of the optical input beam. If the combined intensity of the optical input beam and a control beam exceeds a threshold level, the optical beam passes through the thresholding device. Preferably, the optical thresholding device is a saturable absorber. When the device is configured as an optical comparator, the intensity of the optical input beam exceeds the threshold level and the thresholding device saturates and turns transparent so that the control beam passes through the thresholding device as an optical indicator beam. When the device is configured as an optical signal attenuator and the intensity of the optical input signal is negligible compared to that of the control beam, the combined intensity of the beams does not saturate the thresholding device.

Optical devices employing an optical thresholder

An optical modulator ( ) that operates as a phase modulator at high frequency and as a frequency modulator at low frequencies to provide a suitable signal-to-noise ratio over a wide frequency band. The modulator ( ) includes an optical waveguide and opposing RF electrodes ( ) formed in the waveguide ( ). An RF input signal is applied to the electrodes ( ) to create an electric field that changes the index of refraction of the waveguide ( ) to affect the propagation speed of an optical carrier signal ( ), and thus provide the modulation. The electrodes ( ) are long velocity-matched electrodes that provide the RF loss versus RF frequency needed to produce the desired V&pgr; versus RF frequency over the entire bandwidth. The lower V&pgr; at lower RF frequencies will emphasize the lower frequencies and improve the low-frequency fidelity of the optical link by boosting the low-frequency signals above the excess link noise generated in the demodulator ( ).

Optical phase modulator design incorporating pre-emphasis

An optical inverting system employs a first optical structure having an index of refraction that varies with the intensity of an incident beam and a second optical structure having a constant index of refraction, and forming an interface therebetween. An optical pulse stream is combined with a laser beam and the combined beam is applied to the first optical structure, impinging the interface at a predetermined angle of incidence. If the angle of incidence is less than a critical angle, the beam is refracted into the second optical structure. If the angle of incidence is greater than the critical angle, the beam is completely reflected at the interface. Thus the output of the second optical structure is an inversion, and the output of the first optical structure is a level shifted replica, of the optical digital pulse stream.

Refractive index manipulating optical inverter

A demodulation system used in connection with an analog optical link. An optical carrier signal modulated with an RF signal is split into two portions. One carrier signal portion is applied to a coarse demodulator that generates a demodulated signal representative of the RF signal and the additive inverse of an error signal. The coarse demodulator output is inverted and applied to a phase modulator along with the second carder signal portion. The phase modulator modulates the optical carrier signal with the additive inverse of the demodulated signal from the coarse demodulator, and the RF signal components of the carrier signal and the demodulated signal cancel, leaving the carrier signal modulated with the error signal. The modulated carrier signal is filtered, then demodulated to regenerate the error signal, which is combined with the demodulated signal from the coarse demodulator to recreate the RF signal with minimal excess noise and distortion.

Enhanced feed forward optical frequency/phase demodulator

An optical device for use with an optical input beam comprises and optical thresholding device having a predetermined threshold level, and is positioned along an optical path defined by the propagation direction of the optical input beam. A source generates a control beam through the optical thresholding device, wherein if the combined intensity of the optical input beam and the control beam is large enough to exceed the threshold level of the thresholding device, the optical beam passes through he thresholding device. The thresholding device attenuates the optical beam as it passes therethrough. In a preferred embodiment, the optical thresholding device is a saturable absorber. When the device is configured as an optical comparator, the intensity of the optical input beam is large enough to exceed the threshold level of the thresholding device, the thresholding device saturates and turns transparent so that the control beam passes through the thresholding device as an optical indicator beam and the optical input beam passes through the thresholding device. When configured as an optical signal attenuator and the intensity of the optical input signal is negligible compared to that of the control beam the combined intensity of the optical input signal and the control beam do not saturate the thresholding device.

An optical analog-to-digital converter (,) that makes use of a downward-folding successive approximation conversion scheme that employs subtraction of optical signals. A pulsed optical signal (,) to be converted is applied as an input to each of a plurality of converter channels (,), where each channel (,) outputs one of the bits of the digital output of the converter (,). The input signal (,) to each channel (,) is sent to a thresholding device (,) that determines whether the intensity of the signal is greater than or less than a predetermined threshold value. The first channel thresholding device (,) compares the input signal (,) to a threshold value that is one-half of a known maximum intensity. Subsequent channel thresholding devices (,) compare the input signal to a threshold value that is one-half of the intensity used in the previous channel in a downward-folding scheme. If the intensity of the input signal (,) is greater than the threshold value in a particular channel (,), then that threshold value is subtracted from the input signal (,) in each of the successive channels before being applied to the thresholding device (,) in that channel (,). The subtraction elements (,) can be saturable absorbers or electro-absorption modules in a particular design. Additionally, the control signals from a particular channel (,) to each of the successive channels to determine whether subtraction will occur can be provided by an optically controlled switch (,), such as a saturable absorber, or an electrically controlled switch, such as an electro-absorption module.

Delayed pulse saturable absorber-based downward-folding optical A/D

A demodulation system used in connection with an analog optical link that provides a wide dynamic range and SNR potential for large RF signal bandwidths. An optical carrier signal modulated with an RF signal is applied to an optical splitter in the demodulation system that splits the signal into first and second carrier signals. One of the carrier signals is applied to a coarse demodulator that provides either PM or FM demodulation to generate a demodulated signal representative of the signal that includes the RF signal and the additive inverse of an error signal. The output from the coarse demodulator is inverted, and integrated in the FM case, and then applied to a phase modulator along with the second optical carrier signal from the optical splitter. The phase modulator modulates the optical carrier signal with the additive inverse of the demodulated signal so that the portion of the RF signal occurring in both the optical carrier signal and the demodulated signal are cancelled, and the optical carrier signal is modulated with the error signal. The modulated carrier signal is applied to a fine demodulator that demodulates the signal to generate the error signal. The error signal and the demodulated signal are combined, and what remains is a substantial copy of the RF signal with minimal excess noise and distortion. Unbalanced Mach-Zehnder interferometers and photodetectors can be provided in the coarse demodulator and the fine demodulator to provide the demodulation.

Feed forward optical frequency/phase demodulator

The optical hold unit (100) of the present invention includes an optical modulator (108) that has an electrical input, an optical input, and an optical output. A 1.times.N optical splitter (106) is also provided that has an optical input and N optical outputs. In addition, N optical paths (112) are individually coupled to the N optical outputs and carry one of the N output signals. Each optical path has an associated propagation delay. Optical delay elements may be located in any of the N optical paths that carry the output signals. The optical delay elements serve to lengthen the propagation delay (114a-e) of the optical path (112a-e) in which the optical delay element is located. In an alternative embodiment, the optical hold unit (200) includes an optical modulator (108) that has an electrical input, an optical input, and an optical output. An optical resonator (202) is also provided and connected to the optical output of the modulator (108). The optical resonator (202) also includes a partially transmissive output (222) to which an optical path is connected. The optical resonator (202) may also include a gain medium (208) or an optical switch (210).

Optical sample and hold architecture

An optical quantizer (10) that employs a chain of optical thresholding devices (16) positioned in the propagation path of an optical input beam (12) to be quantized. Each optical thresholding device (16) saturates and turns transparent if the intensity of the optical beam (12) that impinges it is above a predetermined threshold level designed into the device (16). If the input beam (12) saturates the optical thresholding device (16), the device (16) outputs an indicator signal (22) identifying the saturation. The input beam (12) propagates through the optical thresholding device (16) with some attenuation caused by the saturation, and impinges subsequent optical thresholding devices (16) in the chain. Eventually, the attenuation of the input beam (12) caused by the multiple saturations will decrease the beam intensity below the threshold level of the next optical thresholding device (16). The number of indicator signals (22) gives an indication of the intensity of the input beam (12). The optical thresholding devices (16) can provide optical or electrical indicator signals (22) depending on the type of thresholding device (16) used.

Repetitive absorptive thresholding optical quantizer

An optical analog-to-digital converter (10) which fully operates in the optical domain and utilizes an upward-folding successive approximation approach for conversion. The converter (10) includes a plurality of optical stages (14, 16, 18) where each stage (14, 16, 18) generates a digital bit. Each stage (14, 16, 18) includes an optical threshold switch (30, 56, 78) that sets the bit high when the switch (30, 56, 78) is closed. When a sample amplitude of the analog signal is compared to a threshold value and found to exceed the threshold value, the bit is set to 'high' and the sample is passed directly onto the next stage (14, 16, 18). If the sample amplitude is found to be less than the threshold value, the bit is set to 'low' and an intensity equal to the maximum signal intensity minus the threshold intensity is added to the sample amplitude. Each successive stage (14, 16, 18) compares the normalized signal sample to thresholds growing closer and closer to the maximum signal intensity. Multiple bits can be obtained by cascading stages.

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Hawthorne, CA, United States of America

David Lawrence Rollins