Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/50—Transmitters
  • H04B10/501—Structural aspects
  • H04B10/503—Laser transmitters
  • H04B10/505—Laser transmitters using external modulation
• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
  • G02F1/21—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  by interference
  • G02F1/225—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  by interference in an optical waveguide structure
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/50—Transmitters
  • H04B10/501—Structural aspects
  • H04B10/503—Laser transmitters
  • H04B10/505—Laser transmitters using external modulation
  • H04B10/5051—Laser transmitters using external modulation using a series, i.e. cascade, combination of modulators
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/50—Transmitters
  • H04B10/501—Structural aspects
  • H04B10/503—Laser transmitters
  • H04B10/505—Laser transmitters using external modulation
  • H04B10/5053—Laser transmitters using external modulation using a parallel, i.e. shunt, combination of modulators
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/50—Transmitters
  • H04B10/501—Structural aspects
  • H04B10/503—Laser transmitters
  • H04B10/505—Laser transmitters using external modulation
  • H04B10/5057—Laser transmitters using external modulation using a feedback signal generated by analysing the optical output
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/50—Transmitters
  • H04B10/501—Structural aspects
  • H04B10/503—Laser transmitters
  • H04B10/505—Laser transmitters using external modulation
  • H04B10/5057—Laser transmitters using external modulation using a feedback signal generated by analysing the optical output
  • H04B10/50577—Laser transmitters using external modulation using a feedback signal generated by analysing the optical output to control the phase of the modulating signal
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/50—Transmitters
  • H04B10/516—Details of coding or modulation
  • H04B10/532—Polarisation modulation
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/50—Transmitters
  • H04B10/516—Details of coding or modulation
  • H04B10/54—Intensity modulation
  • H04B10/541—Digital intensity or amplitude modulation
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/50—Transmitters
  • H04B10/516—Details of coding or modulation
  • H04B10/548—Phase or frequency modulation
  • H04B10/556—Digital modulation, e.g. differential phase shift keying [DPSK] or frequency shift keying [FSK]
  • H04B10/5561—Digital phase modulation
• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F2203/00—Function characteristic
  • G02F2203/06—Polarisation independent

Definitions

• This invention relates generally to electro-optical modulation devices and their methods and use, and more particularly to electro-optical modulation devices formed on a single chip, and their methods of use.
• Lasers are widely used today in fiber and free space segments for high data rate communication links, remote sensing applications (LIDAR) and more.
• LIDAR remote sensing applications
• the light signal is modulated, usually using electro-optical modulators.
• the modulation scheme commonly used is On-Off Keying (OOK), see Figure 2a, where only the power of the light is modulated.
• Alternative modulation schemes include Phase Shift Keying (PSK), where the data is encoded in the phase of the signal.
• PSK Phase Shift Keying
• RF communications more advanced modulation schemes are used, such as QPSK (quadrature phase shift keying) and QAM (quadrature amplitude modulation) see Figure 2b.
• QPSK quadratture phase shift keying
• QAM quadrature amplitude modulation
• the light should be modulated both in amplitude and phase, essentially with a complex modulation signal.
• the present invention is an integrated electro- optical modulator capable of modulating a light signal with an arbitrary complex signal.
• Such modulating formats as PSK for example, BPSK and QPSK
• PSK for example, BPSK and QPSK
• T. G. Hodgkinson "Demodulation of Optical DPSK using in-phase and quadrature detection", Electronics Letters, Vol. 21, No 19, pp. 867-868, 1985.
• the majority of the work in this field was made by implementing non-integrated solutions, i.e. various optical components such as amplitude and phase modulators connected by optical fibers.
• Such communication schemes were abandoned in the late 1980's and are still not implemented due to their complexity and high cost.
• Optical devices including X-cut LiNbO 3 have been described in, for example, US published application no. 2001/0007601, filed July 12, 2001, and U.S. Patent No. 5,416,859, issued May, 16, 1995.
• the US patent No.5526448, filed June 11, 1996 discloses the optical waveguide modulator with a reduced DC drift.
• the foregoing published application and patent are incorporated by reference to the extent necessary to understand the present invention.
• Optical devices currently available are based on non-integrated and/or semi-integrated solutions, i.e. optical fibers or optical fiber-based components were used for connecting of various electro- optical components and/or splitting/combining the optical signals.
• an object of the present invention is to provide an optical device, its methods of use, that modulates an input signal in phase and/or amplitude domain.
• Another object of the present invention is to provide integrated single monolithic devices, and their methods of use, for arbitrary generation of optical signals by changing phase and/or amplitude.
• Yet another object of the present invention is to provide integrated, single monolithic devices for optical data communication applications.
• a further object of the present invention is to provide integrated, single monolithic devices for optical data communication applications.
• Another object of the present invention is to provide integrated, single monolithic devices for remote sensing applications.
• a further object of the present invention is to provide integrated, single monolithic devices for quadrature phase shift keying (BPSK and/or QPSK).
• BPSK and/or QPSK quadrature phase shift keying
• Yet another object of the present invention is to provide integrated, single monolithic devices for quadrature amplitude modulation (QAM) applications
• Still a further object of the present invention is to provide integrated, single monolithic devices for controlled chirp or Barker coding applications for LADAR as well as other remote sensing applications.
• an optical device with a first Mach-Zehnder modulator that produces a first output, and a second Mach-Zehnder modulator that produces a second output.
• a splitter is coupled to the first and second Mach-Zehnder modulators.
• a combiner combines the first and second outputs.
• a phase shifter is coupled to the first and second Mach-Zehnder modulators.
• the first Mach-Zehnder modulator, the second Mach-Zehnder modulator, the splitter, the combiner and the phase shifter are formed as part of a single planar chip made of electro-optical material.
• an optical device in another embodiment, includes a first Mach-Zehnder modulator producing a first output, a second Mach-Zehnder modulator producing a second output, a third Mach-Zehnder modulator producing a third output and a fourth Mach-Zehnder modulator producing a fourth output.
• a first input splitter is coupled to the first and second Mach-Zehnder modulators.
• a first phase shifter is coupled to the first and second outputs.
• a first output combiner is positioned to combine the first and second outputs from the first and second Mach-Zehnder modulators.
• a second input splitter is coupled to the third and fourth Mach-Zehnder modulators.
• a second phase shifter is coupled to the third and fourth outputs.
• a second output combiner is positioned to combine the third and fourth outputs.
• a method for producing an optical output.
• An optical device is provided with first and second Mach-Zehnder modulators formed as part of a single planar chip made of electro-optical material.
• a first output is produced from the first Mach-Zehnder modulator.
• a second output is produced from the second Mach-Zehnder modulator.
• the first and second outputs are combined to produce a combined output.
• a method for producing a dual polarization transmission.
• a device is provided that includes a first optical device with first and second Mach-Zehnder modulators, and a second optical device with third and fourth Mach- Zehnder modulators.
• the first and second optical devices are formed as part of a single planar chip made of an electro-optical material.
• a first output with a first polarization is produced from the first optical device.
• a second output with a second polarization is produced from the second optical device.
• the first and second outputs are combined to produce a beam with two orthogonal polarization signals.
• Figure 1 The preferred design of the quadrature modulator on X cut (a) and Z cut (b) of LiNbO 3 .
• Figure 3 (a) A block diagram of the quadrature modulator, (b) A block diagram of the quadrature modulator operating in two (orthogonal) polarization states of light.
• Figure 4 is a block diagram illustrating a calibration scheme of the Figure 1(a) device.
• Figure 5 shows the QPSK constellation without bias (in black) and with bias (in gray).
• Figure 6 is a flow chart of a search algorithm for bias control which can be used with the present invention
• Figure 7 is a flow chart of an algorithm that can be used for the power equalization using driver controller.
• Figure 8 illustrates QPSK constellation with 90 degrees phase difference and with a small phase offset.
• Figure 9 is a flow chart of a search algorithm that can be used to keep changing the phase (while other parameters are kept constant) to make the power variance minimal.
• Figure 10 illustrates an experimental setup for testing pulses and modulation time alignment of the Figure 1(a) device.
• Figure 11 is flow chart that illustrates an algorithm for optimizing the timing alignment by maximizing the optical power for one embodiment of the present invention.
• Figure 12 (a) is an eye diagram of a 12.5 Gb/s RZ-OOK signal with perfect synchronization of the pulse and data for the Figure 1(a) device.
• Figure 12(b) is an eye-diagram of a 12.5 Gb/s RZ-OOK signal with worst case misalignment of half a bit-period delay for one embodiment of the present invention.
• Figure 13(a) is an eye diagram of a 12.5 Gb/s RZ-PSK signal with perfect synchronization of the pulse and data for one embodiment of the present invention
• Figure 13 (b) is an eye diagram of a 12.5 Gb/s RZ-PSK signal with worst case timing misalignment of half a bit-period delay for one embodiment of the present invention.
• an optical device in one embodiment, includes, a first Mach-Zehnder modulator that produces a first output, and a second Mach-Zehnder modulator which produces a second output.
• the first and second Mach-Zehnder modulators are coupled to an input splitter.
• a combiner combines the first and second outputs from first and second Mach- Zehnder modulators.
• a phase shifter is coupled to the first and second Mach-Zehnder modulators.
• the first Mach-Zehnder modulator, second Mach-Zehnder modulator, input splitter, combiner and the phase shifter are each formed as part of a single chip made of electro-optical material.
• the optical device of the present invention is an integrated optical device that is formed on a single chip, single piece of crystal including but not limited to a monolithic piece of a crystal wafer, that can be made of an electrooptical crystal including but not limited to LiNBO 3 .
• different cuts of the LiNbO 3 crystal are utilized including but not limited to X, Y, or Z.
• the present invention can utilize but not limited to Metal In-Diffusion and/or (Annealed) Protonic-Exchange technology, Wet Etching, Reactive Ion (Beam) etching, Plasma etching, and others.
• the optical device of the present invention integrated on a single chip can be used for any combination of quadrature (phase/amplitude) modulation such as quadrature amplitude modulation (QAM).
• QAM quadrature amplitude modulation
• the optical device of the present invention is formed as integrated on a single chip, the process steps utilized are partly disclosed in
• the optical output of the optical device consists of an input signal that is modulated in phase/amplitude, such as by way of illustration quadrature phase shift keying (QPSK) or quadrature amplitude modulation (QAM) for communications, or controlled chirp or Barker coding for LADAR applications.
• QPSK quadrature phase shift keying
• QAM quadrature amplitude modulation
• the preferred design of the transmitter chip is shown in Figure 1.
• the two possible embodiments are shown on: X and Z cut of the LiNbO 3 crystal, see, Figure la and Figure lb, respectively.
• the light 10 impinges the device through the input 11, and split into two branches (In-phase and Quadrature-phase) 20, 21.
• the light in each branch is directed into its own Mach-Zehnder interferometer (MZI) at 22 and 23.
• MZI Mach-Zehnder interferometer
• the data is introduced into the light beam by the phase shift in the MZI. This phase shift is controlled by two RF signals applied to 18 and 19.
• the ground electrodes are marked as 12.
• Bias voltage 16 and 19 control the operating point of the MZIs.
• the voltage applied to the electrode 15 introduces a phase shift of 90 ° between the MZI outputs 24 and 25 to create a quadrature modulation.
• Ground electrode is marked as 14.
• the modulated light 13 comes out through output 26.
• a block diagram of one embodiment of the optical device of the present invention, hereafter the "quadrature modulator" 100 is shown in Figure 3(a).
• a passive Y-junction 102 divides the light coming from input 101 into two preferably equal beams 103 and 104. These beams 103 and 104 are directed to two Mach-Zehnder electro-optical modulators 105 and 106 driven by data RF signals, 110 and 111 respectively. Bias voltage 112 and 113 are applied to each Mach-Zehnder modulator to set the DC bias point.
• An additional phase shifter 107 obtains 90° phase difference between the two Mach-Zehnder modulators.
• Branches 114 and 115 are combined together using a Y-junction 108. The light comes out of the output 109 modulated by the quadrature data.
• FIG. 3(b) A general scheme for modulation on two (orthogonal) polarization states is shown in Figure 3(b).
• the light coming from input 210 is divided into two branches 101 and 201 by outermost splitter or outermost polarization-splitter 212 (in the case of polarization splitter the incoming laser can be polarized at 45 deg).
• the light propagating in waveguides 100 and 200 is in accordance with direction of electro-optical operation of the modulator(s).
• the quadrature modulation of the polarized light is described in the previous paragraph.
• the light with each orthogonal polarization state is quadrature modulated by its own quadrature modulator 221 and 222, respectively (shown in Figure 3(b)).
• the light coming from outputs 109 and 209 is combined together by outermost polarization combiner 213.
• Resulting output signal 211 consists two quadrature-modulated signals, each of different (orthogonal) polarization.
• the quadrature modulator calibration could be carried out first at the manufacturing facility, then during the system setup, and then continuously during the operation.
• Equalization of the light contribution from 114, 115; 214, 215 in the overall output 211 is equalization of the light contribution from 114, 115; 214, 215 in the overall output 211.
• the calibration is done by tapping the power of the branch marked as 211.
• the block diagram of the quadrature modulator calibration scheme is shown in Figure 4.
• the calibration block is marked as 300.
• the tapped signal from branch 211 is detected by a photodiode 302 followed by an A/D converter 312. After digitization the digital signal is directed into a processor 303.
• the processor controls the 90° phase shift through phase controller 304, which supplies the voltage to the phase shifters 107,207.
• the bias control for the four MZFs is carried out by the bias controller 305 which supplies the DC bias voltage to 105,106,205,206.
• Gain controller 306 controls the amplitude (power) of the MZI driver 307 that receives RF signals 308,309,310,311.
• Each of quadrature modulator 100 and 200 has two bias inputs (112 and 113 for 100; 212 and 213 for 200), the bias should put the MZI at extinction point (where the MZI output power is zero).
• Figure 5 shows the QPSK constellation without bias (in black) and with bias (in gray), the average photocurrent (which is proportional to the power) is proportional to
• the bias is controlled by employing a search algorithm, such as shown in Figure 6, that keeps changing the bias (112, 113, 212 and 213 separately) to make the average power minimal.
• the bias minimization may be done on payload (provided that all constellation points have equal probability) or on training sequences.
• Algorithm implementations are not limited to the mentioned below example, a variety of other algorithms can be used.
• the goal of the power equalization calibration is that the four signals coming from channels 114, 115, 214, 215 have equal contributions (one quarter each) to the power of output signal 211. This can be done in two ways:
• Amplitude modulate one of the channels at a time with low frequency and small index of modulation and detect the power at the modulation frequency. If the amplitude of the channel to be measured is A' A(l + ms ( ⁇ w t)), with A as the nominal amplitude, m the index of modulation and ⁇ m the modulation angular frequency, then the photocurrent is proportional to
• Yet another method is to tap the power of outputs 114, 115, 214 and 215 and measure them with the same diode (using a switch) or different diodes with uniform parameters.
• Fig 8 shows the QPSK constellation with 90 degrees phase difference (in blue) and with a small phase offset.
• the phase is controlled by employing a search algorithm, like the one shown in Figure 9 that keeps changing the phase (while other parameters are kept constant) to make the power variance minimal. This may be done on payload or during training.
• Applicable algorithms are not limited to the mentioned below example, a variety of the other algorithms can be used.
• Precise alignment can be achieved by optical delay of the pulse train or electronic delay of the sinusoidal wave applied to the optical pulse generator.
• Optical delay is an expensive solution, however, due to optical coupling and limited range of delay.
• Electronic delay can be implemented with a mechanical-based microwave adjustable delay line that is an acceptable solution for laboratory use but not practical for commercial deployment.
• active closed-loop control of the timing alignment is required to minimize mistiming due to short and long term drift of the group delays of the on-board electronic components such as driver amplifiers, serializers/multiplexers, phase drift of VCO, etc. caused by the effect of environment such as temperature change.
• Optimal time alignment can be achieved by maximizing the average optical power of the signal.
• the electrical signal that drives the data modulator has finite rise and fall times. This fact coupled with the typical fifty percent duty cycle of RZ pulse provides an adequate sensitivity for this technique using only the average optical power as a feedback signal.
• Only a low speed (kHz range) optical power detector for momtoring, a low speed analog-to-digital and digital-to-analog converters (ADC and DAC) and a microprocessor for signal processing are required. These are low cost commercial off-the-shelf components and no other special components are required. The specifications of these components do not depend on the data rate which makes the present technique scalable to high bit rates.
• FIG. 10 shows the test setup.
• MZM Mach-Zehnder modulators
• a CW laser 260 is launched to a LiNbO 3 MZM 261 driven by a 12.5 GHz sinusoidal wave from a clock source.
• the MZ modulator was biased at quadrature to produce a near 50% duty cycle pulse train.
• a clock driver amplifier with a built-in voltage-controlled phase shifter 269 from Multilink (MTC5531) was inserted between the 12.5 GHz clock source 271 and the RZ modulator 261.
• the phase shift versus voltage of the MTC5531 is close to linear with a slope of approximately 45°/V. This corresponds to approximately 10 ps/V for 12.5 GHz clock rate.
• the optical pulse was directed to a second push-pull MZM 263 driven by a 12.5 Gb/s electrical binary NRZ data pulse from a pattern generator 270, which was synchronized with the clock source 271.
• the data MZM can be biased at quadrature to produce an OOK signal or at null to produce a PSK signal.
• the output of the data MZM is directed to an oscilloscope 264 for momtoring. A portion of the output was tapped using a tap coupler 272 and sent to an optical power meter 265 with an analog voltage output proportional to the average optical power.
• the voltage output is connected to a 12-bit ADC (National Instrument) 266, which is connected to a desktop computer 267.
• the computer runs a program with an algorithm similar to the one shown in Figure 11.
• the output of a 16-bit DAC (National Instrument) 268 from the computer is connected to the phase shifter voltage input of the Multilink clock driver 269.
• a variable optical delay line 262 was inserted between the pulse and data MZMs (261 and 263) to test the automatic synchronization setup by introducing arbitrary timing misalignment.
• Figures 12 (a) and 13 (a) show the eye diagrams of the 12.5 Gb/s RZ-OOK and RZ-PSK signals with the computer program activated and timing alignment optimized. With the computer control loop deactivated, the optical delay line was adjusted to produce the worst-case timing misalignment such that the transitions of the NRZ data signal were aligned to the peaks of the RZ pulses as shown in Figures 12 (b) and 13 (b) for RZ-OOK and RZ-PSK.
• the transmitted signal is described as:
• phase ⁇ should alternate between two values: + k and -f- + y& + #- , where k is 0 or 1 and constant during the integration, so that p(t) is not constant.
• Quadrature modulator For closed loop control the signal at the output of Quadrature modulator is tapped and detected by a low BW PIN diode optical detector.
• the first and second terms of the equation produce signals which are independent of the timing offset ⁇ .
• the calibration process can be executed in two steps:

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