Radio frequency signal generator and optical intensity modulator

Abstract

1. A radio frequency (RF) signal generator (100), comprising: an arbitrary waveform generator (AWG) (102) operative to generate an RF signal (110) including one or more pulse pairs (112), each pulse pair including a first pulse (114) and a second pulse (116), each pulse pair having an RF signal amplitude envelope (118) configured to compensate for a non-uniform gain response of an optical amplifier in the time domain.

Description

[Technical Field]

[0001] The present invention relates to a radio frequency (RF) signal generator. The present invention further relates to an optical intensity modulation device. [Background technology]

[0002] Optical systems for light-matter interactions, such as quantum computing ion or atom traps, atomic clocks, atomic interferometers, and quantum gravimeters, require optical pulses with precisely controlled pulse parameters. Optical pulses can be formed using directly modulated laser diodes or by intensity modulation of continuous-wave (cw) optical signals using optical modulators such as acousto-optic modulators (AOMs), Mach-Zehnder electro-optic modulators (MZM-EOMs), or semiconductor optical amplifiers (SOAs). An RF signal is provided as a drive signal to the optical modulator, which in turn drives the optical modulator to intensity-modulate the cw optical signal and form the optical pulses. Summary of the Invention [Problem to be solved by the invention]

[0003] It is an object to provide an improved radio frequency (RF) signal generator. A further object is to provide an improved optical intensity modulation device. [Means for solving the problem]

[0004]

[0006] One aspect provides a radio frequency (RF) signal generator comprising an arbitrary waveform generator (AWG) operative to generate an RF signal including one or more pulse pairs, the pulse pairs including a first RF signal pulse and a second RF signal pulse, the pulse pairs having an RF signal amplitude envelope configured to compensate for a non-uniform gain response of an optical amplifier in the time domain.

[0005] The RF signal generator advantageously generates an RF signal that can be used as a drive signal for the optical modulator to generate corresponding optical pulses, which can then be amplified by an optical amplifier. Optical amplifiers typically have a non-uniform gain response in the time domain caused by inverse depletion in the optical amplifier gain medium that occurs as the pulses propagate through the gain medium. The RF signal amplitude envelope of the RF signal pulse pair causes the optical modulator to generate optical pulses with a corresponding intensity / power envelope to compensate for the non-uniform gain response of the optical amplifier experienced by the optical pulses propagating through the gain medium.

[0006] In one embodiment, the RF signal amplitude envelope has a leading edge and a trailing edge, and the amplitude of the RF signal increases between the leading edge and the trailing edge. The RF signal may be advantageously used as a drive signal for an optical modulator to generate corresponding optical pulses that are amplified by an optical amplifier having a non-uniform gain response in the time domain.

[0007] In one embodiment, the amplitude of the RF signal increases nonlinearly over at least a portion of the RF signal amplitude envelope between the leading edge and the trailing edge. The RF signal may be advantageously used as a drive signal for an optical modulator to generate corresponding optical pulses that are amplified by an optical amplifier having a non-uniform gain response in the time domain.

[0008] In one embodiment, the amplitude of the RF signal increases approximately exponentially over at least a portion of the RF signal amplitude envelope between the leading edge and the trailing edge, and the RF signal may be used as a drive signal for an optical modulator to generate corresponding optical pulses that are advantageously amplified by an optical amplifier having a non-uniform gain response in the time domain.

[0009] In one embodiment, each RF signal pulse has a leading edge and a trailing edge, and the amplitude of the RF signal increases between the leading edge and the trailing edge of each RF signal pulse. The RF signal may be used as a drive signal for an optical modulator to generate a corresponding optical pulse that is advantageously amplified by an optical amplifier having a non-uniform gain response in the time domain.

[0010] In one embodiment, the RF signal amplitude of the first RF signal pulse increases from a first amplitude to a second, larger amplitude, and the amplitude of the second RF signal pulse increases from a third amplitude to a fourth, larger amplitude, the third amplitude and the fourth amplitude being greater than the first amplitude and the second amplitude. The RF signal may be advantageously used as a drive signal for an optical modulator to generate corresponding optical pulses that are amplified by an optical amplifier having a non-uniform gain response in the time domain.

[0011] In one embodiment, in a first RF signal pulse, the RF signal has a first phase, and in a second RF signal pulse, the RF signal has a second phase. There is a phase difference between the first and second phases. The RF signal generator advantageously generates an RF signal that can be used as a drive signal for an optical modulator to generate pairs of optical pulses having a phase difference between the optical pulses. The phase difference between the optical pulses may be the same as the phase difference between the RF signal pulses. In general, there may be a known correlation between the phase of the RF signal pulse and the phase of the optical pulse. As an example, for a first-order sideband, the phase of the optical signal may have a 1:1 correlation with the phase of the RF signal. For a second-order sideband, the phase of the RF signal may be 2:1 that of the optical signal. Thus, the phase difference imposed between the optical signal pulses can be controlled by the RF signal. Similarly, the amplitude of the RF signal may have a linear correlation with the amplitude of the resulting optical signal.

[0012] In one embodiment, the AWG is operative to generate the RF signal having an instantaneous phase change between the first RF signal pulse and the second RF signal pulse of the pulse pair.

[0013] In one embodiment, the first and second RF signal pulses of a pulse pair have a time interval between the first and second RF signal pulses, and the AWG is operative to generate the RF signal such that the phase change between the first and second RF signal pulses of the pulse pair occurs during the time interval.

[0014] In one embodiment, the AWG comprises an interface circuit, one or more processors, and a memory including instructions executable by the processors, whereby the AWG operates to determine an RF signal amplitude envelope and generate an RF signal using the determined RF signal amplitude envelope.

[0015] In one embodiment, the AWG is further operative to receive an input signal including an RF signal amplitude envelope and to store the RF signal amplitude envelope in a memory. The AWG is operative to determine the RF signal amplitude envelope by retrieving the RF signal amplitude envelope from the memory. The AWG may advantageously be provided with the RF signal amplitude envelope resulting from modeling the optical amplifier gain response to be compensated.

[0016] In an embodiment, the AWG is further operative to receive an input signal including a gain response of the optical amplifier in the time domain. The AWG is operative to determine an RF signal amplitude envelope for a target optical pulse shape output from the optical amplifier having the optical amplifier gain response to compensate for the optical amplifier gain response. The AWG is operative to generate an RF signal using the determined RF signal amplitude envelope. This advantageously enables the AWG to determine the RF signal amplitude envelope to compensate for a modeled or measured gain response of the optical amplifier.

[0017] In one embodiment, the AWG is further operative to receive an input signal including a detected optical pulse shape output from the optical amplifier. The AWG is operative to determine a difference between the detected pulse shape and a target pulse shape. The AWG is operative to determine an RF signal amplitude envelope to at least partially compensate for the difference. The AWG is operative to generate an RF signal using the determined RF signal amplitude envelope. This advantageously enables the AWG to determine the RF signal amplitude envelope to iteratively compensate for a non-uniform gain response of the optical amplifier in real time.

[0018] In one embodiment, the RF signal generator further comprises an RF frequency multiplier for frequency multiplying the RF signal from the AWG. In one embodiment, the RF signal comprises a train of pulse pairs, which may advantageously be used as a drive signal for an optical modulator to generate a corresponding train of optical pulse pairs.

[0019] In one embodiment, each pulse pair has a first duration, and successive pulse pairs of the pulse train are separated by a time interval of a second duration longer than the first duration. The RF signal may advantageously be used as a drive signal for an optical modulator to generate a corresponding train of optical pulse pairs with a dark time between the pulse pairs longer than the duration of the pulse pairs. In some embodiments, the first duration is between 100 ns and 1000 ns, such as between 250 ns and 450 ns, or between 200 ns and 500 ns. Such durations have been found to be particularly useful in quantum applications.

[0020] In one embodiment, the second duration is 10 times or more than the first duration, such as approximately 100 times or more than the first duration, such as approximately 1000 times or more than the first duration. An RF signal may advantageously be used as a drive signal for an optical modulator to generate a corresponding train of pairs of optical pulses with long dark periods between the pulse pairs. In some embodiments, the second duration is between 0.1 ms and 1000 ms, such as between 1 ms and 50 ms, or between 0.5 ms and 100 ms.

[0021] In one embodiment, the AWG is operative to generate the RF signal in response to receiving a trigger signal, the RF signal including a single pulse pair, which may be advantageously used as a drive signal for an optical modulator to generate a corresponding single optical pulse pair.

[0022] Corresponding embodiments and advantages also apply to the optical intensity modulation devices described below. One aspect of the present invention provides an optical intensity modulation device comprising an RF signal generator, an optical modulator, and an optical amplifier. The RF signal generator may include an arbitrary waveform generator (AWG) operative to generate an RF signal including one or more pulse pairs, the pulse pairs including a first RF signal pulse and a second RF signal pulse. The one or more pulse pairs may be provided to the same RF signal for driving an optical modulator. The pulse pairs have an RF signal amplitude envelope configured to compensate for a non-uniform gain response of an optical amplifier in the time domain. The optical modulator is configured to receive the RF signal from the RF signal generator as a drive signal and is operative to intensity-modulate an optical signal to form one or more optical pulse pairs including a first optical pulse and a second optical pulse, the optical pulse pairs having an intensity envelope corresponding to the RF signal amplitude envelope of each pulse pair of the RF signal. The optical amplifier is configured to amplify the one or more optical pulse pairs output from the optical modulator to form one or more output optical pulse pairs. The optical amplifier has the non-uniform gain response in the time domain. The optical intensity modulator may be configured to modulate the phase and amplitude independently of each other. As an example, the RF signal may be amplitude modulated to control the shape of the optical pulse. The RF signal may further be frequency modulated to control the optical frequency of the optical pulse.

[0023] The optical intensity modulator advantageously operates to output amplified optical pulse pairs in which the non-uniform gain response of the optical amplifier has been pre-compensated by the shape of the optical pulse pairs generated by the optical modulator driven by the RF signal generated by the RF signal generator. Forming optical pulse pairs with compensated intensity / power envelopes can enable the optical modulator to compensate for pulse shape perturbations introduced by the non-uniform gain response of the optical amplifier, which advantageously enables the optical modulator to form output optical pulse pairs, each pulse of the pair having a target pulse shape independent of such pulse shape perturbations.

[0024] One aspect of the present invention provides an optical pulse source comprising a laser configured to provide a continuous wave (cw) optical signal and the aforementioned optical intensity modulation device arranged to receive the cw optical signal from the laser and modulate the intensity of the received cw optical signal to generate one or more optical pulses. Generating optical pulses by modulating a cw optical signal has several advantages. One is that cw lasers, particularly cw fiber lasers, generally have superior optical properties compared to, for example, laser diodes. Another is that high-speed modulators, such as electro-optic modulators, can provide much faster and more accurate modulation of the cw optical signal than direct modulation of a laser diode without introducing significant noise into the generated optical pulses.

[0025] The disclosed RF signal generator and optical intensity modulation device may be configured for use in quantum applications. In general, the disclosed device is particularly suited to applications in atomic, molecular, and optical physics (AMO). By way of example, applications may include quantum computing, cryptography, quantum gyroscopes, gravity detection systems, and atomic clocks. In general, the present invention is particularly suited to applications requiring pulsed laser applications in which the pulses have well-defined shapes, energies, and phases. This is typically the case in quantum mechanical ensemble state control, e.g., controlling the quantum state of a quantum mechanical ensemble.

[0026] One aspect of the present invention provides a laser system comprising a light source, such as a cw laser, and an optical intensity modulation device as disclosed herein, the device comprising an RF signal generator as disclosed herein. The light source may be a single-frequency fiber laser.

[0027] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0028] [Figure 1] FIG. 2 is a block diagram illustrating an embodiment of an RF signal generator. [Figure 2]2 is a diagram showing an RF signal pulse train generated by the RF signal generator of FIG. 1; [Figure 3] FIG. 2 is a block diagram illustrating an embodiment of an RF signal generator. [Figure 4] 4 shows a pair of RF signal pulses generated by the RF signal generator of FIG. 3; [Figure 5] FIG. 2 is a block diagram illustrating an embodiment of an RF signal generator. [Figure 6] FIG. 2 is a block diagram illustrating an embodiment of an RF signal generator. [Figure 7] FIG. 2 is a block diagram illustrating an embodiment of an RF signal generator. [Figure 8] FIG. 1 is a block diagram illustrating an embodiment of an optical modulation device. [Figure 9] 9A is a diagram showing an optical pulse pair output from an optical modulator of the optical modulation device of FIG. 8 (input to an optical amplifier), and FIG. 9B is a diagram showing an optical pulse pair output from the optical amplifier of the optical modulation device of FIG. 8. [Figure 10] 1A and 1B show (a) a pair of rectangular optical pulses input to an optical amplifier having a non-uniform gain response, and (b) the resulting output optical pulse pair from the optical amplifier. [Figure 11] FIG. 1 is a block diagram illustrating an embodiment of an optical modulation device. [Figure 12] 10 is a graph showing experimental data (fall time) regarding the optical intensity modulation device according to the present disclosure. [Figure 13] 10 is a graph showing experimental data (rise time) regarding the optical intensity modulation device according to the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0029] 1 and 2, one embodiment provides a radio frequency (RF) signal generator 100 comprising an arbitrary waveform generator (AWG) 102. The AWG operates to generate an RF signal 104 including a train 110 of pulse pairs 112. Each pulse pair 112 includes a first RF signal pulse 114 and a second RF signal pulse 116. Each pulse pair has an RF signal amplitude envelope 118 configured to compensate for the non-uniform gain response of an optical amplifier in the time domain.

[0030] The RF signal amplitude envelope describes how the RF signal amplitude of a pulse pair changes over the duration of the pulse pair, from the leading edge of the first RF signal pulse (forming the leading edge of the RF signal amplitude envelope) to the trailing edge of the second RF signal pulse (forming the trailing edge of the RF signal amplitude envelope). The RF signal amplitude envelope bridges the RF signal amplitude change between pulses, i.e., the RF signal amplitude decreases at the trailing edge of the first RF signal pulse and increases at the leading edge of the second RF signal pulse. The RF signal may be amplitude modulated such that the amplitude of a given RF signal pulse varies over time. The RF signal may oscillate at a given predetermined frequency. A technical effect of the present invention is that upon receiving an RF signal, one or more frequency sidebands can be generated by an optical modulator, such as an electro-optic modulator. The sidebands may be spaced apart from the carrier signal by a frequency f, where f corresponds to the frequency of the RF signal. In some embodiments, the predetermined frequency is selected from a range of 1 GHz to 20 GHz, such as 1 GHz to 10 GHz. The amplitude modulation of the RF signal may determine the shape of the optical pulses generated by the optical modulators disclosed herein.

[0031] In some embodiments, the RF signal oscillation is sinusoidal. Alternatively, the RF signal waveform may be a non-sinusoidal waveform, such as a sawtooth waveform. This may also be referred to as optical serrodyne modulation.

[0032] Each pulse pair 112 has a first duration τ, and successive pulse pairs in the pulse train 110 are separated by a time interval T of a second duration. The time interval is the length of time between the trailing edge of the second RF signal pulse of one pulse pair and the leading edge of the first RF signal pulse of the next pulse pair. In some embodiments, the first duration τ is between 10 ns and 1000 ns, such as between 200 ns and 500 ns, or between 100 ns and 750 ns. In some embodiments, the second duration T is between 0.1 ms and 1000 ms, such as between 0.5 ms and 100 ms, or between 1 ms and 50 ms.

[0033] In one embodiment, the time interval T is longer than the pulse pair duration τ, and thus the pulse train has a dark time between pulse pairs that is longer than the pulse pair duration. In one embodiment, the time interval T between pulse pairs is greater than or equal to 10 times the pulse pair duration τ, such that the pulse train has a long dark period between pulse pairs.

[0034] In one embodiment, the time interval T between pulse pairs is greater than or equal to 100 times the pulse-pair duration τ. For example, the time interval T between pulse pairs may be approximately 1000 times the pulse-pair duration τ.

[0035] For example, for a pulse pair duration τ of about 100 ns and a time interval T between pulse pairs of 1 ms, the pulse train has an on:off (ie, RF signal pulses:non-RF signal pulses) ratio of 1:10000.

[0036] As can be seen in FIG. 2, the RF signal amplitude envelope 118 of each pulse pair 112 has a leading edge (formed by the leading edge of the first pulse 114) and a trailing edge (formed by the trailing edge of the second pulse 116).

[0037] In one embodiment, the amplitude of the RF signal 104 increases between the leading and trailing edges of the RF signal amplitude envelope. The RF signal may oscillate at a predetermined frequency between the leading edge and the training edge.

[0038] In one embodiment, the amplitude of the RF signal increases non-linearly across at least a portion of the RF signal amplitude envelope between a leading edge and a trailing edge of the RF signal amplitude envelope. In one embodiment shown in FIG. 2, the amplitude of the RF signal 104 increases approximately exponentially over at least a portion of the RF signal amplitude envelope between the leading and trailing edges of the RF signal amplitude envelope.

[0039] 2, each RF signal pulse 114, 116 has a leading edge and a trailing edge, and the amplitude of the RF signal 104 increases between the leading and trailing edges of each RF signal pulse.

[0040] In one embodiment, the RF signal amplitude of the first RF signal pulse 114 increases between its leading edge and its trailing edge from a first amplitude to a second, larger amplitude, and the RF signal amplitude of the second RF signal pulse 116 increases between its leading edge and its trailing edge from a third amplitude to a fourth, larger amplitude, the third and fourth amplitudes being greater than the first and second amplitudes.

[0041] In one embodiment shown in FIG. 3, the RF signal amplitude of the first RF signal pulse 114 increases from 0.22V to 0.43V, and the RF signal amplitude of the second RF signal pulse 116 increases from 0.47V to 1.00V.

[0042] In one embodiment, in a first RF signal pulse, the RF signal has a first phase, and in a second RF signal pulse, the RF signal has a second phase. There is a phase difference between the first and second phases, e.g., a phase difference of π or a phase difference of π / 2. The implemented phase difference may be selected depending on the required phase difference between the optical pulses of the optical pulse pair generated using the RF signal. Therefore, the resulting phase difference between the two optical pulses in the optical signal may be controlled by controlling and selecting the phase of the RF signal, e.g., the phase of the RF signal pulse. The phase difference may be achieved independently of the selection of the frequency of the RF signal. For many applications, such as quantum applications, it is important to be able to precisely control the phase of the optical pulses. In particular, it may be desirable to impose a fast phase change between the two pulses in a given pulse pair.

[0043] In one embodiment, the AWG 102 operates to generate an RF signal 104 having an instantaneous phase change between the first RF signal pulse 114 and the second RF signal pulse 116 of each pulse pair 112 .

[0044] In one embodiment, the first RF signal pulse 114 and the second RF signal pulse 116 of each pulse pair 112 have a time interval between them, and the AWG 102 operates to generate the RF signal 104 such that a phase change between the first RF signal pulse and the second RF signal pulse of each pulse pair occurs during the respective time interval.

[0045] For example, the time interval may be in the range of 10-20 ns, or may be a fraction, such as 1 / 10, of the RF signal pulse width. 3 and 4 , one embodiment provides an RF signal generator 200 comprising an AWG 202. The AWG operates to generate an RF signal 204 including a single pulse pair 212. For example, the AWG may operate at 12.5 gigasamples per second to generate a sinusoidal RF signal 204 having a frequency of 2.5 GHz. In some embodiments, the AWG is configured to generate an RF signal having a frequency between 1 GHz and 20 GHz, such as between 2 GHz and 6 GHz, or between 1 GHz and 10 GHz. The RF signal may oscillate around a zero value with a given selected frequency in the aforementioned range.

[0046] The AWG 202 operates to generate an RF signal in response to receiving a trigger signal 206 . The trigger signal may be generated internally by the AWG 202 or may be received from an external optical system. For example, the trigger signal may be received from an external optical system, such as a light-matter interaction system, to which light pulses generated using an RF signal are delivered. A series of time-separated trigger signals may be received by the AWG, and the AWG is caused to generate a respective RF signal single pulse pair in response to each trigger signal.

[0047] Pulse pair 212 includes a first RF signal pulse 214 and a second RF signal pulse 216. The pulse pair has an RF signal amplitude envelope 218 that is configured to compensate for the non-uniform gain response of the optical amplifier in the time domain.

[0048] As can be seen in FIG. 4, the RF signal amplitude envelope 218 of the pulse pair 212 has a leading edge (formed by the leading edge of the first pulse 214) and a trailing edge (formed by the trailing edge of the second pulse 216).

[0049] In one embodiment, the amplitude of the RF signal 204 increases between the leading and trailing edges of the RF signal amplitude envelope. In one embodiment, the amplitude of the RF signal increases non-linearly across at least a portion of the RF signal amplitude envelope between a leading edge and a trailing edge of the RF signal amplitude envelope.

[0050] 4, the amplitude of the RF signal 204 increases approximately exponentially over at least a portion of the RF signal amplitude envelope between the leading and trailing edges. In addition to the exponential increase, the amplitude may have a sinusoidal contribution, such that the amplitude may be described as the sum of an exponential term and a sinusoidal term.

[0051] 4, each RF signal pulse 214, 216 has a leading edge and a trailing edge. The amplitude of the RF signal 204 increases between the leading and trailing edges of each RF signal pulse. The rise time of the signal may be very fast, such that the leading and / or trailing edges of the RF signal pulse have very steep slopes. This may be desired to generate an approximately square optical pulse.

[0052] In one embodiment, the RF signal amplitude of the first RF signal pulse 214 increases between its leading edge and its trailing edge from a first amplitude to a second, larger amplitude, and the RF signal amplitude of the second RF signal pulse 216 increases between its leading edge and its trailing edge from a third amplitude to a fourth, larger amplitude, the third and fourth amplitudes being greater than the first and second amplitudes.

[0053] In one embodiment shown in FIG. 4, the RF signal amplitude of the first RF signal pulse 214 increases from 0.22V to 0.43V, and the RF signal amplitude of the second RF signal pulse 216 increases from 0.47V to 1.0V.

[0054] In one embodiment, during a first RF signal pulse, the RF signal has a first phase, and during a second RF signal pulse, the RF signal has a second phase. There may be a phase difference between the first and second phases. This can be seen, for example, in FIG. 4 in the change in RF signal 204 between the trailing edge of first RF signal pulse 214 and the leading edge of second RF signal pulse 216. For example, a phase difference of π or a phase difference of π / 2. The implemented phase difference may be selected depending on the required phase difference between the optical pulses of the optical pulse pair generated using the RF signal. The technical effect of implementing a phase difference in the RF signal pulse and the corresponding optical pulse is that the pulses can be easily distinguished, making the optical pulses suitable for quantum applications, such as for controlling quantum states in an ensemble of quantum states.

[0055] In one embodiment, the AWG 202 operates to generate an RF signal 204 having an instantaneous phase change between the first RF signal pulse 214 and the second RF signal pulse 216 of each pulse pair 212 .

[0056] In one embodiment, the first RF signal pulse 214 and the second RF signal pulse 216 of each pulse pair 212 have a time interval between them. The AWG 202 operates to generate the RF signal 204 such that a phase change between the first RF signal pulse and the second RF signal pulse of the pulse pair occurs during the time interval.

[0057] For example, the time interval may be in the range of 1 to 100 ns, such as 10 to 20 ns, or may be a fraction, such as 1 / 10, of the RF signal pulse width. The time interval between two different pulse pairs may be significantly longer than the time interval between two pulses within a given pulse pair. For example, the time between two pulse pairs may be greater than 0.1 ms, such as greater than 1 ms, such as about 100 ms. The pulse repetition rate of the RF signal may be selected in the range of 10 Hz to 10 kHz, such as from 1 kHz to 3 kHz, or from 1 kHz to 5 kHz.

[0058] One embodiment provides an RF signal generator 300 comprising an AWG 302. The AWG operates to generate an RF signal including one or more pulse pairs 112, 212, as described above.

[0059] The AWG may include an interface circuit 306, one or more processors 308, and a memory 310 containing instructions 312. The instructions are executable by the processor to cause the AWG to operate to determine an RF signal amplitude envelope and to generate an RF signal using the determined RF signal amplitude envelope.

[0060] In one embodiment, the AWG 302 is further operative to receive an input signal including an RF signal amplitude envelope. The AWG is operative to store the RF signal amplitude envelope in the memory 310. The AWG is operative to determine the RF signal amplitude envelope by retrieving the RF signal amplitude envelope from the memory.

[0061] In an alternative embodiment, the AWG 302 is further operable to receive an input signal including a non-uniform gain response of the optical amplifier in the time domain. The AWG is operable to determine an RF signal amplitude envelope to compensate for the non-uniform gain response of the optical amplifier relative to a target optical pulse shape output from the optical amplifier having the non-uniform gain response of the optical amplifier. The AWG is operable to generate an RF signal using the determined RF signal amplitude envelope.

[0062] In an alternative embodiment, the AWG 302 is further operative to receive an input signal including detected optical pulse shapes of a first optical pulse and a second optical pulse of an optical pulse pair output from the optical amplifier. The AWG is operative to determine a difference between the detected pulse shape and a target pulse shape and to determine an RF signal amplitude envelope to at least partially compensate for the difference. The AWG is operative to generate the RF signal using the determined RF signal amplitude envelope. In some embodiments, the target pulse shape is approximately rectangular or square in the time domain. By having a well-defined pulse shape, the energy in the pulse is likewise well-defined and replicable.

[0063] 6, the RF signal generator 400 further includes an RF frequency multiplier 402. The frequency multiplier is for multiplying the frequency of the RF signal output from the AWG. For example, the frequency multiplier may be a frequency doubler for doubling the frequency of the RF signal output from the AWG.

[0064] In one embodiment shown in FIG. 7, the RF signal generator 500 further comprises an RF amplifier 502 before the RF frequency multiplier 402 for amplifying the RF signal output from the AWG 202 .

[0065] 8 and 9, one embodiment provides an optical intensity modulation device 600 comprising an RF signal generator 100 as described above, an optical modulator 610, and an optical amplifier 620. The optical modulator 610 receives the RF signal output from the RF signal generator 100 as a drive signal. The optical modulator operates to modulate the optical signal to form a train of optical pulse pairs corresponding to the train of RF signal pulse pairs output by the RF signal generator 100. The optical pulses in the train of optical pulses may have approximately the same shape and / or the same energy in the time domain. The optical modulator 610 may be configured to modulate a continuous wave (cw) optical carrier signal with a carrier wavelength to generate one or more sidebands on the optical carrier signal. The one or more sidebands may be spaced apart from the carrier signal by a frequency f, where f corresponds to the frequency of the RF signal. In some embodiments, the modulator is configured to generate multiple sidebands on the optical carrier signal, where the sidebands are spaced apart by integer multiples of the frequency f. The modulator may further be configured to transmit as much light as possible from the carrier wavelength to the wavelength given by the generated sidebands, such as from the carrier wavelength to the wavelength of the first sideband. The optical modulator 610 may be an electro-optic modulator.

[0066] The optical amplifier 620 is configured to amplify the optical pulse pairs output from the optical modulator to form output optical pulse pairs. The optical amplifier has a non-uniform gain response in the time domain, and the RF signal amplitude envelope is configured to compensate for this.

[0067] In an alternative embodiment, the optical intensity modulation device 600 comprises an RF signal generator 200 as described above, an optical modulator 610, and an optical amplifier 620. The optical modulator 610 receives the RF signal output from the RF signal generator 200 as a drive signal. The optical modulator operates to modulate the optical signal to form an optical pulse pair 612 including a first optical pulse 614 and a second optical pulse 616, as shown in FIG. 9( a) (the input to the optical amplifier 620 is the same as the output from the optical modulator 610). The phase of the optical pulses can be controlled by controlling the phase of the RF signal. In some embodiments, there is a phase difference between the two pulses in a given optical pulse pair 612. The optical intensity modulation device 600 may be configured to modulate the optical signal with respect to both phase and amplitude. The optical pulse pair 612 output from the optical modulator corresponds to the RF signal pulse pair 212 output by the RF signal generator 200. That is, the shape of the optical power envelope of the optical pulse pair 612 corresponds to the shape of the RF signal amplitude envelope of the RF signal pulse pair, and the optical power of the first and second optical pulses 614, 616 increases in the same manner as the RF signal amplitude of the first and second RF signal pulses. The shape of the optical power envelope of the optical pulse pair may be controlled by amplitude modulation of the RF signal.

[0068] Optical amplifier 620 is configured to amplify optical pulse pair 612 to form output optical pulse pair 622, as shown in FIG. 9(b). Optical amplifiers have non-uniform gain responses in the time domain, and the RF signal amplitude envelope is configured to compensate for this. Because optical pulse pair 612 has corresponding optical power envelopes, the optical pulse pair is correspondingly configured to compensate for the non-uniform gain response of the optical amplifier. The resulting output optical pulses 624, 626 have approximately constant optical power and may be described as "square" or "top-hat" optical pulses.

[0069] In contrast, Figure 10(b) shows a pair of optical output pulses 710 output from the optical amplifier 620 when a pair 700 of rectangular optical pulses 702, 704, as shown in Figure 10(a), is input to the optical amplifier. That is, when the drive signal of the optical modulator 610 includes a pair of rectangular RF pulses that do not have an RF signal amplitude envelope configured to compensate for the non-uniform gain response of the optical amplifier. The non-uniform gain response of the optical amplifier results in these "uncompensated" optical pulses 712, 714 having non-uniform, decreasing optical power envelopes.

[0070] In one embodiment shown in FIG. 11, an optical intensity modulation device 800 comprises an RF signal generator 300 as described above, an optical modulator 610, an optical amplifier 620, and an optical detector 802. The photodetector 802 is configured to detect the pulse shapes of the first and second optical pulses of the optical pulse pair output by the optical amplifier. The photodetector is further configured to generate an output signal indicative of the detected optical pulse shapes. The AWG 302 is operative to receive as its input the output signal from the photodetector.

[0071] Referring to Figures 12 and 13, these show two graphs with experimental data for an optical intensity modulation device according to the present disclosure. The data are presented as two graphs, each showing the fall time and rise time of the optical intensity modulation device. Both graphs show the photodetector, PD, signal (V) versus time (ns). It can be seen that the optical intensity modulation device provides very fast fall and rise times, less than 1 ns or even less than 900 ps. It can even be faster than this, somewhat limited by the choice of detector. Here, the rise time is defined as the time between 10% and 90% PD signals, as measured by the photodetector.

Claims

1. A radio frequency (RF) signal generator (100, 200, 300, 400, 500) comprising: an arbitrary waveform generator (AWG) (102, 202, 302) operative to generate an RF signal (104, 204) including one or more pulse pairs (112, 212), the pulse pairs including a first RF signal pulse (114, 214) and a second RF signal pulse (116, 216); the pulse pair having an RF signal amplitude envelope (118, 218) configured to compensate for a non-uniform gain response of an optical amplifier in the time domain; The first RF signal pulse (114, 214) has a first phase, the second RF signal pulse (116, 216) has a second phase, and there is a phase difference between the first phase and the second phase.

2. 2. The RF signal generator of claim 1, wherein the RF signal amplitude envelope has a leading edge and a trailing edge, and the amplitude of the RF signal increases between the leading edge and the trailing edge.

3. 3. The RF signal generator of claim 2, wherein the amplitude of the RF signal increases nonlinearly over at least a portion of the RF signal amplitude envelope between the leading edge and the trailing edge.

4. 4. The RF signal generator of claim 2, wherein the amplitude of the RF signal increases approximately exponentially over at least a portion of the RF signal amplitude envelope between the leading edge and the trailing edge.

5. 5. The RF signal generator of claim 1, wherein the RF signal oscillates at a predetermined frequency, and the oscillation is a sine wave.

6. 6. The RF signal generator of claim 5, wherein the predetermined frequency is selected from the range of 1 GHz to 10 GHz.

7. 7. The RF signal generator of claim 1, wherein each RF signal pulse has a leading edge and a trailing edge, and wherein the amplitude of the RF signal increases between the leading edge and the trailing edge of each RF signal pulse.

8. the RF signal amplitude of the first RF signal pulse (114, 214, 614) increases from a first amplitude to a second, greater amplitude; 8. The RF signal generator of claim 1, wherein an RF signal amplitude of the second RF signal pulse (116, 216, 616) increases from a third amplitude to a fourth, larger amplitude, the third amplitude and the fourth amplitude being greater than the first amplitude and the second amplitude.

9. An RF signal generator according to any preceding claim, wherein the RF signal is amplitude modulated such that the amplitude of a given RF signal pulse varies with time.

10. The RF signal generator of any one of claims 1 to 9, wherein the phase difference between the first phase and the second phase is π or π / 2.

11. 11. The RF signal generator of claim 1, wherein the AWG operates to generate the RF signal having an instantaneous phase change between the first RF signal pulse and the second RF signal pulse of the pulse pair.

12. 12. The RF signal generator of claim 11, wherein the first RF signal pulse (114, 214) and the second RF signal pulse (116, 216) of each pulse pair (112, 212) have a time interval between the first RF signal pulse (114, 214) and the second RF signal pulse (116, 216), and the AWG (102, 202, 302) operates to generate the RF signal such that the phase change between the first RF signal pulse and the second RF signal pulse of the pulse pair occurs during the time interval.

13. 13. The RF signal generator of claim 12, wherein the time interval is between 1 ns and 200 ns, such as between 10 ns and 100 ns.

14. 14. The RF signal generator of claim 1, wherein one or both of the first RF signal pulse and the second RF signal pulse have a pulse duration between 50 ns and 300 ns, such as between 100 ns and 250 ns.

15. The RF signal generator of any preceding claim, wherein the pulse durations of the first and second RF signal pulses are approximately the same within a given pulse pair.

16. The RF signal generator of any one of claims 1 to 15, wherein the pulse energies of the first RF signal pulse and the second RF signal pulse are approximately the same within a given pulse pair.

17. The AWG (102, 202, 302) is receiving an input signal comprising a gain response of an optical amplifier in the time domain; determining an RF signal amplitude envelope (118, 218) for a target optical pulse shape output from an optical amplifier having the gain response of the optical amplifier to compensate for the gain response of the optical amplifier; The RF signal generator of any one of claims 1 to 16, further operative to: generate said RF signal (104, 204).

18. The AWG (102, 202, 302) is receiving an input signal including detected optical pulse shapes of the first optical pulse and the second optical pulse of an optical pulse pair output from an optical amplifier; determining a difference between the detected pulse shape and a target pulse shape; determining an RF signal amplitude envelope (118, 218) to at least partially compensate for the difference; The RF signal generator of any one of claims 1 to 17, further operative to: generate said RF signal (104, 204).

19. The RF signal generator of any one of claims 1 to 18, further comprising an RF frequency multiplier (402) for frequency multiplying the RF signal from the AWG (102, 202, 302).

20. The RF signal generator of any one of claims 1 to 19, wherein the RF signal (104) comprises a train of pulse pairs (112).

21. 21. The RF signal generator of claim 20, wherein each pulse pair (112) has a first duration, and successive pulse pairs of the pulse train are separated by a time interval of a second duration that is longer than the first duration.

22. 22. The RF signal generator of claim 21, wherein the second duration is 10 times or more than the first duration, such as approximately 100 times or more than the first duration, such as approximately 1000 times or more than the first duration.

23. 23. The RF signal generator of claim 21 or 22, wherein the first duration is between 100 ns and 500 ns and the second duration is between 0.5 ms and 1.5 ms.

24. An RF signal generator according to any preceding claim, wherein the pulse repetition rate is between 0.1 kHz and 10 kHz, such as about 1 kHz.

25. 25. The RF signal generator of claim 1, wherein the AWG (102, 202, 302) is operative to generate the RF signal (204) comprising a single pulse pair (212) in response to receiving a trigger signal.

26. An optical intensity modulation device (600, 800), An RF signal generator (100, 200, 300, 400, 500) according to any one of claims 1 to 25, an optical modulator (610) configured to receive the RF signal from the RF signal generator as a drive signal, and operative to intensity modulate an optical signal to form one or more optical pulse pairs (616) including a first optical pulse (614) and a second optical pulse (616), the optical pulse pairs having intensity envelopes corresponding to the RF signal amplitude envelopes (118, 218) of respective pulse pairs (112, 212) of the RF signal; an optical amplifier (620) configured to amplify the one or more optical pulse pairs output from the optical modulator to form one or more output optical pulse pairs (622), the optical amplifier having the non-uniform gain response in the time domain.

27. 27. An optical intensity modulation device according to claim 26, configured to modulate the optical signal with respect to both phase modulation and amplitude modulation.

28. 28. The optical intensity modulation device of claim 26 or 27, wherein there is a phase difference between a first optical pulse and the second optical pulse, and the output optical pulse pair is suitable for quantum applications such as for controlling quantum states.

29. 29. An optical intensity modulation device according to any one of claims 26 to 28, wherein the optical modulator is configured to generate one or more sidebands on the optical signal.

30. 30. An optical intensity modulation device according to any one of claims 26 to 29, wherein the sidebands are spaced apart from the carrier signal by a frequency f, where f corresponds to the frequency of the RF signal.

Images