WO2025202033A1

WO2025202033A1 - Optical locking unit

Info

Publication number: WO2025202033A1
Application number: PCT/EP2025/057707
Authority: WO
Inventors: Zhixin Liu; Kassem AMANY
Original assignee: UCL Business Ltd
Current assignee: UCL Business Ltd
Priority date: 2024-03-25
Filing date: 2025-03-20
Publication date: 2025-10-02
Anticipated expiration: 2026-09-25
Also published as: GB2639861A; GB202404252D0

Abstract

Embodiments of the present disclosure provide methods and apparatus for a harmonic locking unit for facilitating the generation of low noise RF and/or mm-wave generation The locking unit comprises a harmonic generator configured to generate a plurality of harmonics from the plurality of laser beams and select one or more overlapping harmonics between one or more pairs of laser beams from among the plurality of laser beams. The locking unit further comprises a photodiode configured to convert the overlapping harmonics to an electrical signal, and a phase frequency detector configured to generate a correction signal based on the electrical signal. The locking unit additionally comprises a feedback controller configured to control the plurality of laser beams based on the correction signal.

Description

OPTICAL LOCKING UNIT

Technical Field

Embodiments of the present disclosure relate to methods and apparatus for optical locking, and in particular relate to harmonic locking units for a plurality of laser beams.

Low noise microwave signals with high timing stability may be used in applications such as navigation and positioning systems, communication technologies, radar and sensing mechanisms, and scientific instruments. These applications may heavily rely on microwave signals with minimal noise to provide high quality performance. As these technologies develop, the impacts of noise on the performance of these technologies may become increasingly significant. Accordingly, there may be an escalating need for microwave sources with reduced noise. In addition, such systems and applications may have increasing constraints on system size and/or power efficiency that render conventional all-electronic frequency synthesisers unsuitable.

Conventional approaches to generating low noise microwave signals include electronic frequency synthesisers and optical-based frequency synthesisers. Optical-based radio frequency (RF) synthesisers are frequency synthesis systems that utilize photonic technologies to generate stable and precise RF signals. In particular, optical-based RF synthesisers may provide improved stability and precision of RF signals which in turn may provide performance improvements in applications over electronic approaches for generating low-noise microwaves. Optical-based RF synthesizers may be used, for example in high frequency applications, such as high frequency radar, sensing, and wireless communication systems. High frequency applications may be applications that require frequencies of over 10GHz.

The phase noise in electronic RF frequency synthesisers may exhibit a quadratic increase with signal frequency. This behaviour may become a limiting factor for performance in high frequency applications. More specifically, phase noise may be a significant limiting factor for mm-wave communication in 3^rd Generation Partnership Project (3GPP) 6^th Generation (6G) communication networks, at least for the reason that it precludes the utilisation of high bandwidth signals and may further limit the wireless ranging resolution of 6G system to the order of 10s of centimetres. In contrast, optical-based RF synthesisers use photonics for signal generation, which may exhibit superior phase noise performance at high frequencies. For example, optical-based RF synthesisers may generate signals with noise levels that are more than two orders of magnitude lower than signals produced by equivalent pure-electronic synthesisers. Consequently, the integration of optical techniques in frequency synthesis may provide more robust and efficient communication systems, and may further provide precise sensing technologies operation in the millimetre-wavelength region.

Summary

It is desirable to provide methods and apparatus for optical locking that facilitate the generation of low noise RF and/or mm-wave generation. Such locking mechanisms may address noise limitations in conventional electronics, including one or more of the issues discussed above. Such methods and apparatus for optical locking may provide performance improvements in applications including wireless radio systems, precision radar systems, electronic instruments, wireless communication equipment, and/or defence devices.

The summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. The summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. For the avoidance of doubt, the scope of the claimed subject matter is defined by the appended claims.

Embodiments of the present disclosure provide a harmonic locking unit for a plurality of laser beams. The locking unit comprises a harmonic generator configured to generate a plurality of harmonics from the plurality of laser beams and select one or more overlapping harmonics between one or more pairs of laser beams from among the plurality of laser beams. The locking unit further comprises a photodiode configured to convert the overlapping harmonics to an electrical signal, and a phase frequency detector (PFD) configured to generate a correction signal based on the electrical signal. The locking unit additionally comprises a feedback controller configured to control the plurality of laser beams based on the correction signal. Brief Description of Drawings

For a better understanding of the present disclosure, and to show how it may be put into effect, reference will now be made, by way of example only, to the accompanying drawings, in which:

Figure 1 is a diagram of a harmonic locking unit in accordance with embodiments;

Figure 2A, Figure 2B, and Figure 2C (collectively referred to as Figure 2) are diagrams of a signal generator in accordance with embodiments;

Figure 3 is a diagram of laser tones processed by the harmonic locking unit in accordance with embodiments;

Figure 4 is a diagram of a control unit in accordance with embodiments;

Figure 5A, Figure 5B and Figure 5C (collectively referred to as Figure 5) are diagrams of a further signal generator in accordance with embodiments; and

Figure 6A and Figure 6B (collectively referred to as Figure 6) are diagrams of a further signal generator in accordance with embodiments.

Detailed Description

Embodiments disclosed herein provide a harmonic locking unit for a plurality of laser beams. Such harmonic locking units may be provided within a signal generator to lock two or more lasers within the signal generator. The harmonic locking unit may implement an optical phase lock loop method. Furthermore, the harmonic locking unit may improve on conventional optical phase lock loop methods (which may require a photodiode bandwidth which is the same as or similar to the RF/mm-wave signals to be generated) by adding a harmonic generator, acting as a harmonic generation stage in the signal processing implemented by the apparatus. The harmonic generation stage may accordingly translate the phase noise of the source lasers to high order harmonics. This may in turn alter the phase and/or frequency relationship of two largely spaced lasers to instead be considered as closely spaced harmonic tones which may be approximated as a single harmonic tone of a particular frequency value. By detecting the single harmonic tone, a PFD may be used to relate the largely spaced lasers to one another and thus may provide a method to lock the lasers using feedback control methods. The optical frequency of a generated signal may be locked to yield a reference frequency feeding a reference block to the PFD. Alternatively, the lasers may be locked to one another to provide a single value frequency that is not locked to a reference value. In some examples, one of the plurality of lasers may be tuneable to match the frequency and/or phase of another of the plurality of lasers. Alternatively, each of the plurality of lasers may be tuneable to one or more specific values. In such a case, phase noise may be determined by the lasers of the signal generator and thus phase noise may not be dependent on generated frequency.

Signals generated by methods and apparatus disclosed herein may have frequencies of over 100GHz, including in the THz range; experimental results discussed herein include frequencies of 170GHz. In contrast, signals generated by conventional methods and apparatus are typically of the order of 10-100GHz, for example 60GHz. In addition, the spacing of two lasers within a signal generator in accordance with embodiments may be greater than or equal to 10GHz.

Figure 1 depicts a harmonic locking unit 100 in accordance with embodiments. The harmonic locking unit 100 comprises a harmonic generator 102, a photodiode 104, a phase frequency detector (PFD) 106, and a feedback controller 108. The harmonic generator 102 may be considered as forming the harmonic generation stage of the harmonic locking unit. In specific embodiments, the harmonic generator 102 may comprise a frequency comb generator and/or a non-linear element. The harmonic locking unit 100 may receive a plurality of laser beams from an external source. For example, the harmonic locking unit 100 may be configured to receive a plurality of laser beams from a plurality of lasers. In specific embodiments, one or more of the plurality of lasers may be a continuous-wave (CW) laser. For example, each of the plurality of lasers may be a CW laser. Alternatively or additionally, one or more of the plurality of lasers may be narrowlinewidth lasers. For example, one or more of the plurality of lasers may have a fundamental linewidth in the order of kHz or sub-kHz. The wavelength of the lasers may be tuneable by adjusting the laser temperature, laser current, or variables associated with the external cavity housing the lasers (e.g. cavity temperature). The plurality of lasers may comprise N lasers, where N is a positive natural number. In specific embodiments, N may be less than or equal to 10. In more specific embodiments, N may be equal to 2. Accordingly, in present embodiments the plurality of laser beams may comprise between two and ten laser beams.

Accordingly, the plurality of lasers may be used as a seed for the harmonic locking unit 100. Once the laser beams have entered the harmonic locking unit 100, the beams may first be received and processed by a harmonic generator 102. The harmonic generator 102 may be configured to generate a plurality of harmonics from the plurality of laser beams. That is, the harmonic generator 102 may be configured to expand the wavelength of each laser into multiple optical tones or harmonics, wherein each optical harmonic has a deterministic relationship with its associated seeded laser. For example, experimental results show that the phase of high order harmonic tones generated using a driving phase modulator or optoelectronic comb with low noise RF signals may be identical to that of the seeded CW signals. Accordingly, the harmonic generator 102 may be one or more of the following: an electronic frequency comb, a non-linear frequency comb generator, and/or a phase modulator driven by an external RF source. In specific examples, the harmonic generator may comprise one or more of the following: a Kerr comb; a Fabry-Perot (FP) modulator, a mode-locked laser, one or more intensity modulators, one or more phase modulators, and/or any combination thereof. A combination of intensity and/or phase modulators may be preferred, in order to provide more precise tuning of harmonic orders and frequencies that are utilized by the harmonic locking unit 100, thus improving the tunability of the harmonic locking unit 100 as a whole.

The harmonic generator 102 may be further configured to select one or more overlapping harmonics between one or more pairs of laser beams from among the plurality of laser beams. The selected harmonics may then be transmitted from the harmonic generator 102 to a photodiode 104. The photodiode 104 may be configured to convert the overlapping harmonics to an electrical signal. Once the electric signal has been generated by the photodiode 104, the signal may be transmitted to a phase frequency detector (PFD) 106. The PFD 106 may be configured to generate a correction signal based on the electrical signal. That is, the harmonic tones or laser harmonics that closely align with each other in the optical domain may be detected (and prefiltered as appropriate) and used as an error signal or correction signal to control the wavelength of the plurality of lasers entering the harmonic locking unit 100. The correction signal may then be fed to a feedback controller 108. The feedback controller 108 may be configured to control the plurality of laser beams based on the correction signal.

A harmonic locking unit 100 as described herein may be implemented in a signal generator. Accordingly, a signal generator comprising a harmonic locking unit 100 as described herein may further comprise a plurality of lasers. The system may comprise a coupler and/or wavelength multiplexer, configured to combine the outputs of the lasers.

Figure 2 depicts a signal generator 200 comprising a harmonic locking unit in accordance with embodiments. As shown in Figure 2A, the signal generator 200 may comprise a harmonic generator 202, an optical filter 214 (which may in turn form a part of the harmonic generator 202 and/or the harmonic locking unit of the signal generator), a broadband photodiode 220, and a plurality of lasers 216. The signal generator 200 may further comprise a control unit 212, wherein the control unit 212 comprises a photodetector, PFD, and feedback controller. The signal generator 200 may comprise a reference clock 210 configured to provide a signal to the control unit 212 and the harmonic generator 202. In the specific example of Figure 2, the signal generator 200 comprises two lasers, a first CW laser 216 and a second CW laser 216, which act as seeds. That is, the specific example of Figure 2 is a dual seed signal generator. However, other arrangements of lasers may also be used.

As shown in Figure 2, the outputs of the first laser 216 and the second laser 216 are combined using a coupler 222. The coupler 222 may be a 50-50 coupler and/or a wavelength multiplexer. Alternatively or additionally, the coupler 222 may be configured to provide two combined outputs, wherein a first combined output is fed to the harmonic locking unit (for example, by being fed directly into the harmonic generator 202 of the harmonic locking unit) and the second combined output is fed towards the broadband photodiode 220. The broadband photodiode may accordingly be configured to beat the discrete wavelength of the second combined output to generate a RF/mm-wave signal. The signal generator 200 may therefore comprise an output photodiode, configured to convert the combined output of the lasers into a combined electrical signal. Further, the output photodiode may be one or more of: an intrinsic broadband photodiode, a positive-intrinsic-negative (PIN) photodiode, a uni-traveling-carrier (UTC) photodiode, an avalanche photodiode, and/or a terahertz (THz) photodiode. In specific embodiments, the output photodiode may comprise graphene.

Figure 2B depicts the harmonics or tones generated by the harmonic generator 202 in relation to each laser input, with reference to harmonic frequency (f) and amplitude (A). The harmonics generated from the input of laser 1 are shown in unbroken lines, and the harmonics generated from the input of laser 2 are shown in dashed lines. Each of the first laser 216 and the second laser 216 produce a plurality of harmonics. The plurality of harmonics generated by a single laser or seed may be considered to be a frequency comb. As shown in Figure 2B, the frequency comb generated by the first laser (or first seed) and the frequency comb generated by the second laser (or second seed) have a number of overlapping frequencies. These overlapping frequencies are indicated in Figure 2B using a further dotted-and-dashed line. These frequencies may then be selected by the harmonic generator 202.

In specific examples such as Figure 2, the harmonic generator 202 may comprise an optical filter 214 configured to select the overlapping harmonics and thus improve the accuracy of the generated correction signals. That is, the harmonic locking unit of present embodiments may comprise an optical filter configured to filter the overlapping harmonics. For example, the optical filter 214 may comprise one or more of the following: a grating, and/or a thin-film filter. In specific examples, the optical filter 214 may comprise one or more of the following: a fibre or waveguide grating based optical filter, a liquid crystal based filter, an optical interferometer based filter, and/or an acousto-optic filter. Figure 2C depicts an example of the two frequency combs of Figure 2B after being filtered by an optical filter 214 to select the overlapping harmonics; in consistency with Figure 2B, the harmonics generated from the input of laser 1 are shown in unbroken lines, and the harmonics generated from the input of laser 2 are shown in dashed lines. The signal as shown in Figure 2C may then be transmitted to the control unit 212.

An example of a control unit 312 in accordance with present embodiments is shown in Figure 3. Accordingly, the control unit 312 may comprise the photodiode 304, PFD 306, and feedback controller 308 of the harmonic locking unit. The control unit 312 may further comprise an electronic filter 320. Accordingly, the harmonic locking unit 100 of Figure 1 may be considered as comprising a harmonic generator 102 and a control unit.

The control unit 312 may comprise a reference clock 310, configured to provide a signal to the PFD 306 and feedback controller 308. Reference clock 310 may be the same clock as reference clock 210 of the signal generator 200, or a separate clock. For example, in a case where the harmonic generator 202 requires a reference clock in order to generate harmonics (e.g. wherein the harmonic generator 202 comprises a modulator) the reference clock 310 may be connected to the feedback controller 308, PFD 306, and harmonic generator 202. However, in cases where the harmonic generator does not require a reference clock to generator harmonics, no connection between a reference clock and the harmonic generator may be provided. That is, a connection between a reference clock and the harmonic generator may be present. Accordingly, as shown in Figure 3, the harmonic locking unit in present embodiments may comprise a reference clock configured to provide a signal to the feedback controller and the PFD. Further, the reference clock may be configured to provide a signal to the harmonic generator.

As shown in Figure 2C, once a pair of frequency combs have been filtered by an optical filter 214 the resulting extracted tones may not be formed of a single pair. This may for example be due to the limited bandwidth of practical optical filters. However, each pair of filtered tones will have a frequency difference with each other pair of filtered tones (Af). For example, in the specific embodiment of Figure 2C, the extracted tones form three pairs of filtered tones (Pair 1 , Pair 2, and Pair 3). The frequency difference between Pair 1 and Pair 2 is denoted by Af1 , the frequency difference between Pair 2 and Pair 3 is denoted by Af2, and the frequency difference between Pair 1 and Pair 3 is denoted by Af3. As shown, the main overlapping harmonics of interest may be Pair 2, due to having a larger amplitude. However, the beat frequency detected by a photodiode processing the signal shown in Figure 2C may not provide an accurate frequency for processing by the PFD (for example, the beat frequency generated by the photodiode may be located at a frequency that is not equal to or approximately that of Pair 2, or the photodiode may generate a plurality of signal frequencies).

Accordingly, in specific embodiments an electronic or electrical filter 324 may be used to further extract one or more pairs of filtered tones and remove unwanted harmonics. That is, the harmonic locking unit of present embodiments may comprise an electrical filter 324 configured to filter the electrical signal of photodiode 304. This may further improve the accuracy of correction signals generated by the harmonic locking unit. That is, the electrical filter 324 may be configured to extract one beat note for feedback control. The electrical filter 324 may extract the pair of filtered harmonics with the highest amplitude or highest power (e.g. Pair 2 of Figure 2C). In specific examples, the electrical filter 324 may comprise one or more of: a low pass, high pass, and/or a bandpass filter. In further specific examples, the electrical filter may be one or more of: an electrical Resistor-Capacitor (RC) low pass filter and/or an electrical resonator based bandpass filter. The electric filter 324 may therefore extract a pair of filtered harmonics, for example the pair with highest amplitude and/or highest power. Alternatively or additionally, the extracted pair of harmonics may have a frequency spacing between the harmonics that is different to the frequency spacing between other present harmonic pairs. In the specific example of Figure 2C, the frequency spacing between the harmonics of Pair 2 may be unique whereas the frequency spacing between the harmonics of Pair 1 may be the same as the frequency spacing between the pair of harmonics of Pair 3. The electric filter 324 may therefore select and filter out pairs of harmonics based on the frequency spacing between the harmonic pairs. The frequency spacing between the extracted pair of filtered harmonics accordingly forms a beat note, which is used to generate a correction signal.

The beat note selected by the electrical filter 324 may carry the frequency and/or phase difference of the two frequency combs and may then be detected by the PFD 306. In specific examples, the PFD 306 may be one or more of the following: a phase lock loop, an/or a linear electronic frequency discriminator. Accordingly, the PFD 306 may generate a correction signal or error signal corresponding to the phase and frequency variation of the beat note signal. This correction signal may be used by the feedback controller 308 to control the plurality of laser beams based on the correction signal. For example, the feedback controller 308 may output control signals to adjust the frequency and/or the phase of either laser 1 and/or laser 2. A slow loop that controls the long-term frequency deviation (for example, caused by temperature drift) may additionally be used. Accordingly, in specific examples the feedback controller comprises one or more Proportional-lntegral-Derivative (PID) controllers.

Therefore, in present embodiments the feedback controller 308 is configured to track the beat of overlapping harmonics, in comparison to the beat of the plurality of lasers directly (as in conventional systems). It will be appreciated that in specific cases the beat note frequency of the overlapping harmonics may be zero, representing the case that the edge tones of the two neighbouring combs are perfectly overlapped in frequency with a constant phase.

Figure 4 depicts various example harmonic generators, that may be used to generate frequency combs from laser inputs. These example harmonic generators include Kerr combs, FP-modulators, Mode-locked lasers, intensity modulators (IMs) and phase modulators (PMs). Accordingly, the harmonic generator may comprise one or more of the following: a Kerr comb; a Fabry-Perot (FP) modulator, a mode-locked laser, one or more intensity modulators, one or more phase modulators, and/or any combination thereof. In a specific case shown in Figure 4, the harmonic generator may comprise an intensity modulator and a phase modulator. Figure 5A depicts a further example of a signal generator 500 that is a multiple seed signal generator. Accordingly, the signal generator 500 comprises N lasers where N is a positive natural number. In specific embodiments, N may be less than or equal to 10. In more specific embodiments, N may be equal to 2. The lasers may for example be CW lasers.

It will be appreciated that the signal generator 500 of Figure 5 is an expansion of the signal generator 200 of Figure 2, and accordingly may comprise any of the units discussed with reference to Figure 2 that are not explicitly depicted or referenced by Figure 5. That is, the method discussed with reference to Figure 2 may be extended to multiple seed lasers as shown in Figure 5A, where the output of N lasers are combined. Accordingly, multiple lasers can be locked using a number of parallel optical harmonic or optical phase lock loops.

The signal generator 500 of Figure 5A comprises at least two optical phase shifters 526. The feedback controller of the signal generator (which may form a part of control array 512) may be configured to control the plurality of laser beams based on the correction signal using the optical phase shifters 526. That is, the optical phase shifters 526 may be configured to receive signalling from the feedback controller and control the plurality of laser beams based on the received signalling. Figure 5A depicts two optical phase shifters 526, with a first optical phase shifter 526 connected to and controlling Laser 1 and Laser 2 and a second optical phase shifter 526 connected to and controlling at least Laser N. Accordingly, it will be appreciated that a single optical phase shifter 526 may be configured to control any number of laser beams. That is, an optical phase shifter 526 may be configured to control a single laser beam. Alternatively, an optical phase shifter 526 may be configured to control a plurality of laser beams. Thus, a signal generator 500 may comprise one or more optical phase shifters 516. In specific examples, a signal generator may comprise only one optical phase shifter. Alternatively, a signal generator may comprise N optical phase shifters, wherein each optical phase shifter corresponds to one of the N lasers 516, such that each laser has a corresponding optical phase shifter.

As shown in Figure 5A, the frequency combs generated by the harmonic generator 502 may be directly output from the signal generator 500. Accordingly, the signal generator 500 may generate a wide-band comb signal, wherein the wide-band comb signal is locked in both frequency and phase.

Figure 5B depicts an alternative arrangement of a multiple seed signal generator 500. In contrast to the signal generator of Figure 5A, signal generator 500 of Figure 5B comprises a coupler 552. The coupler 522 may be a 50-50 coupler, however other couplers may be used for example in systems comprising more than two inputs or outputs. Alternatively or additionally, the coupler 522 may be configured to provide two combined outputs, wherein a first combined output is fed to the harmonic locking unit (for example, by being fed directly into the harmonic generator 502 of the harmonic locking unit) and the second combined output is fed towards a broadband photodiode 520. The broadband photodiode may accordingly be configured to beat the discrete wavelength of the second combined output to generate a RF/mm-wave signal.

As shown in Figure 5, signal generator 500 comprises N lasers 516 labelled Laser 1 through to Laser N, a harmonic generator 502, N optical filters 514 labelled Optical Filter 1 through to Optical Filter N, and a control array 512. The N optical filters 514 may correspond to one of the N lasers 516, such that each laser has a corresponding optical filter. The control array 512 may be considered to be equivalent to the control unit 312 of Figure 3 and may accordingly comprise any of the components described with reference to Figure 3. Furthermore, the control array 512 may comprise one or more control units. For example, the control array 512 may comprise N control units as depicted in Figure 3. Each control unit may correspond to one of the N lasers 516, such that each laser has a corresponding control unit.

The signals being fed into each component of signal generator 500 are depicted in Figure 5C. As shown in Figure 5A and Figure 5B, each laser 516 of the signal generator 500 may provide an input into the harmonic generator 502. The harmonic generator 502 may then be configured to provide multiple outputs to the optical filter 514. Each output provided by the harmonic generator 502 may correspond to one of the N lasers 516, such that each output may be used to generate a correction signal for said one of the N lasers 516. Each output signal may comprise a plurality of frequency combs, each comb associated with one of the N lasers 516. Example outputs comprising at least three combs are depicted in Figure 5C; frequency combs associated with Laser 1 are depicted in unbroken lines, frequency combs associated with Laser 2 are depicted in dashed lines, and frequency combs associated with Laser N are depicted in dotted lines.

Each frequency comb generated by the harmonic generator 502 will have a limited bandwidth, and will overlap with either one or two further frequency combs having either a range of frequencies that has a greater or lesser average frequency. Accordingly, the harmonic generator 502 may comprise an optical filter 514 configured to select a pair of overlapping harmonics for each output signal generated by the harmonic generator 502. Alternatively, the optical filter 514 may be separate from the harmonic generator 502. In specific embodiments, the signal generator 500 may comprise a plurality of optical filters 514. For example, the signal generator may comprise N optical filters 514, each optical filter configured to receive one of the output signals generated by the harmonic generator 502. Accordingly, the optical harmonic lock loop employed in the specific example of Figure 5 for each pair of neighbouring combs may be a corresponding and expanded method to that discussed with reference to Figure 2, wherein a bank of optical filters that centre at different wavelength are used to extract the overlapped tones for different comb pairs.

The optical filter(s) 514 may be configured to select a pair of frequencies from a pair of overlapping frequency combs, as shown in Figure 5C. That is, a number of frequency combs signals may be fed into the optical filter 514 and a relevant pair of frequencies (denoted by a dotted circle) may be selected by the optical filter 514 and may further be fed into the control array 512. The control array 512 may comprise a PFD, which may be configured to generate a plurality of correction signals wherein each correction signal corresponds to an input signal from the optical filter 514. Accordingly, a pair of overlapping frequencies selected from the frequency combs may be used to generate a correction signal for a laser of the plurality of lasers. Accordingly, and as depicted in the specific example of Figure 5C, the optical filter may receive N sets of frequency combs from the harmonic generator 502 and may select N pairs of frequencies from the received combs. The control array may then generate N correction signals, each correction signal corresponding to a pair of selected frequencies.

A feedback controller may then control the plurality of laser beams based on the correction signals, for example by using one or more optical phase shifters. The use of an optical harmonic lock loop in this way may allow for flexible control of frequency and/or phase of each individual laser. Furthermore, the use of a optical harmonic lock loop may enable the generation of arbitrary optical waveforms at a tuneable repetition wave.

The one or more optical phase shifters 526 in Figure 5 may be replaced by one or more optical frequency shifters 526. That is, either optical frequency shifters or optical phase shifters may be used to control the output of the plurality of lasers by the feedback controller. It will therefore be understood that the harmonic locking unit may comprise one or more optical phase shifters and/or one or more optical frequency shifters configured to shift the phase and/or frequency of one or more of the plurality of laser beams. Any suitable combination of optical phase shifters and/or optical frequency shifters may be used.

The harmonic generator 502 may be any suitable comb generator, for example an external seeded comb generator. In specific examples, the harmonic generator 502 may comprise one or more of: a cascaded intensity modulator, a cascaded phase modulator, a Kerr frequency comb generator, a FP modulator, and/or a mode-locked laser under injection locking.

Accordingly, the harmonic locking unit of the signal generator 500 may be configured to combine frequency combs from N lasers with a constant frequency and phase difference. The frequency and phase of the lasers may thus be stabilised using a feedback look, resulting in the output of a flat wideband comb (as shown in Figure 5A) or a RF/mm-wave signal (as shown in Figure 5B). That is, by detecting the waveform using a broadband photodiode (for example, as shown in Figure 5B), an arbitrary RF signal may be generated with low phase noise. For example, experimental results show that signal generators in accordance with embodiments may generate phase noise of less than -145 dBc/Hz at >300kHz frequency offset for 170GHz signal; this may be particularly suitable for use in applications such as ultra-fast optical instruments, radar signals, and for integrated and sensing communication functions such as in 6G systems.

For example, in radar applications a low phase noise source may be needed in order to obtain high precision ranging measurements with large dynamic ranges (on the order of 100s of metres to kilometres). Further, in wireless communications 5G signals may use mm-waves at 50GHz for transmissions. It is expected that 5G/6G developments will exploit carriers of >100GHz, and the phase noise associated with conventional all-electronic signal generation may limit system performance. Accordingly, signal generation methods with reduced phase noise may be desirable in order to improve the performance of wireless data transmission in high frequency (>70GHz) data transmission.

A further signal generator arrangement is disclosed in Figure 6A and Figure 6B. In a manner corresponding with the signal generator 500 of Figure 5, the signal generator 600 comprises a plurality of lasers 616, a plurality of optical shifters 626, a coupler 622, a harmonic generator 602, one or more optical filters 614, and a control array 612. It may be understood that any of the above discussion relating to these elements of Figure 5 may be additionally applicable to these elements in Figure 6.

The signal generator 600 differs from the signal generator depicted in Figure 5 in that it comprises an optical amplifier 630 and nonlinear system 632. A first coupler 622 accordingly may provide a signal to the harmonic generator 602 and to the optical amplifier 630. The optical amplifier 630 may be configured to amplify the optical signal received from the coupler 622. The optical amplifier 630 may then form a part of and/or be connected to the nonlinear fibre/waveguide system 632, which may in turn be further connected to a second coupler 634. Accordingly, the signal generator 600 may comprise an optical amplifier configured to amplify the combined output of the lasers of the signal generator.

The nonlinear system 632 may be dispersion-engineered, and may be configured to form an ultra-wide band frequency comb 628. In order to achieve this comb expansion, the nonlinear system 632 may comprise a combination of different fibres to form a fibre system and/or a corresponding combination of waveguides of different geometries and/or structures to form a waveguide system. Thus, the signal generator 600 may comprise a nonlinear system 632 configured to expand the combined output of the lasers into a wide-band optical comb 628. Further, the nonlinear system may comprise one or more of: a nonlinear fibre, and/or a nonlinear waveguide.

In the signal generator 600, the phase locked laser arrays entering the first coupler 622 may form an optical pulse which may be tuneable in both repetition rate and shape (for example, using the harmonic locking unit of the present embodiments). Such pulses may then be amplified by the optical amplifier 630 and launched into the nonlinear system 632 (formed by one or more fibres, one or more waveguides, and/or a combination thereof) in order to expand the optical lines in the frequency domain and generate a wide-band optical comb 628. The optical bandwidth of the expanded ultra-wide ban optical comb 628 may be on the order of 100nm, and accordingly may be used by ultra-high frequency photodiode 620 to convert the optical signals into THz signals for various THz applications (including for example sensing applications and communications applications). Accordingly, the signal generator 600 may directly output the wide-band optical comb 628 or a THz signal generated by a broadband or THz photodiode connected to the nonlinear system 632. A second coupler 634 may be used in order to provide multiple outputs/photodiodes 620.

Embodiments of the present disclosure facilitate the generation of low noise RF and/or mm- wave generation. That is, embodiments of the present disclosure provide improved methods for locking wavelengths of a plurality of lasers, for example by locking each laser wavelength to a pre-configured value or by locking the wavelength of a second laser to a wavelength of a first laser. Harmonic locking units of the present disclosure therefore provide a laser signal that may be fed into a photodiode to produce a signal with reduced phase noise. Such a signal may accordingly provide improved performance in high frequency applications, such as high frequency radar, sensing, and wireless communication systems.

Embodiments of the present disclosure may therefore provide signal generation systems and methods with lower phase noise, improved frequency tuning, and/or multi-tone phase coherent generation. These aspects may be desirable for optical communications and microwave photonic systems. Existing photonic-assisted signal synthesizers (including optoelectronic oscillator (OEO) synthesizers, injection locking of lasers, and/or electronic locking of lasers to the same ultra-stable cavity) may be limited in terms of tuning range, phase noise, and robustness. These limitations are compared and contrasted with experimental results obtained from present embodiments in Table 1 below:

Table 1 : comparison of experimental results between present embodiments and existing technologies

Accordingly, present embodiments may allow for a wide tuning range/improved tuneability and small tuning steps/continuous tuning, whilst maintaining low phase noise that does not scale with frequency (compared to an all-electric approach). Furthermore, present embodiments may provide low cost and low power consumption compared to conventional electronic and optical phase lock loop based approaches, as only MHz-level electronics may be used. Present embodiments may therefore include systems wherein the system is any one of: a communications system, a sensing system, an imaging system, a distance measurement system and a spectroscopy system. Sensing systems may include for example Radio Detection and Ranging (RADAR) systems, Light Detection and Ranging (LIDAR) systems, precise distance measurement systems, and/or medical imaging systems.

References in the present disclosure to “one embodiment”, “an embodiment” and so on, indicate that the embodiment described may include a particular feature, structure, or characteristic, but it is not necessary that every embodiment includes the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to implement such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

It should be understood that, although the terms “first”, “second” and so on may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and similarly, a second element could be termed a first element, without departing from the scope of the disclosure. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed terms.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the present disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “has”, “having”, “includes” and/or “including”, when used herein, specify the presence of stated features, elements, and/or components, but do not preclude the presence or addition of one or more other features, elements, components and/ or combinations thereof. The terms “connect”, “connects”, “connecting” and/or “connected” used herein cover the direct and/or indirect connection between two elements.

The present disclosure includes any novel feature or combination of features disclosed herein either explicitly or any generalization thereof. Various modifications and adaptations to the foregoing exemplary embodiments of this disclosure may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings. However, any and all modifications will still fall within the scope of the non-limiting and exemplary embodiments of this disclosure. For the avoidance of doubt, the scope of the disclosure is defined by the claims.

Claims

1. A harmonic locking unit for a plurality of laser beams, the locking unit comprising: a harmonic generator configured to generate a plurality of harmonics from the plurality of laser beams and select one or more overlapping harmonics between one or more pairs of laser beams from among the plurality of laser beams; a photodiode configured to convert the overlapping harmonics to an electrical signal; a phase frequency detector, PFD, configured to generate a correction signal based on the electrical signal; and a feedback controller configured to control the plurality of laser beams based on the correction signal.

2. A harmonic locking unit as claimed in Claim 1 , wherein the harmonic locking unit further comprises: an optical filter configured to filter the overlapping harmonics.

3. A harmonic locking unit as claimed in Claim 2, wherein the optical filter is one or more of: a fibre or waveguide grating based optical filter, a liquid crystal based filter, an optical interferometer based filter, and/or an acousto-optic filter.

4. A harmonic locking unit as claimed in any preceding claim, wherein the harmonic locking unit further comprises: an electrical filter configured to filter the electrical signal.

5. A harmonic locking unit as claimed in Claim 4, wherein the electrical filter is one or more of: an electrical Resistor-Capacitor, RC, low pass filter and/or an electrical resonator based bandpass filter.

6. A harmonic locking unit as claimed in any preceding claim, wherein the harmonic locking unit further comprises: a reference clock configured to provide a signal to the feedback controller and the PFD.

7. A harmonic locking unit as claimed in Claim 6, wherein the reference clock is further configured to provide a signal to the harmonic generator.

8. A harmonic locking unit as claimed in any preceding claim, wherein the harmonic generator is one or more of: a Kerr comb; a Fabry-Perot, FP, modulator; a mode- locked laser; one or more intensity modulators; one or more phase modulators; and any combination thereof.

9. A harmonic locking unit as claimed in any preceding claim, wherein the feedback controller comprises one or more Proportional-lntegral-Derivative, PID, controllers.

10. A harmonic locking unit as claimed in any preceding claim, wherein the harmonic locking unit comprises one or more optical phase shifters and/or one or more optical frequency shifters configured to shift the phase and/or frequency of one or more of the plurality of laser beams.

11. A harmonic locking unit as claimed in any preceding claim, wherein the plurality of laser beams comprises between two and ten laser beams.

12. A signal generator comprising: the plurality of lasers; and the harmonic locking unit as claimed in any preceding claim.

13. A signal generator as claimed in Claim 12, wherein the signal generator further comprises: an output photodiode, configured to convert the combined output of the lasers into a combined electrical signal.

14. A signal generator as claimed in Claim 13, wherein the output photodiode is one or more of:, an intrinsic broadband photodiode, , a positive-intrinsic-negative, PIN, photodiode, a uni-traveling-carrier, UTC, photodiode, an avalanche photodiode, and/or a terahertz, THz, photodiode.

15. A signal generator as claimed in any of Claims 12 and 13, wherein the signal generator further comprises: a nonlinear system configured to expand the combined output of the lasers into a wide-band optical comb.

16. A signal generator as claimed in Claim 15, wherein the nonlinear system comprises one or more of: a nonlinear fibre, and/or a nonlinear waveguide.

17. A signal generator as claimed in any of Claims 15 and 16, wherein the nonlinear system further comprises an optical amplifier configured to amplify the combined output of the lasers.

18. A signal generator as claimed in any of Claims 12 to 17, wherein the signal generator further comprises a coupler and/or wavelength multiplexer, configured to combine the outputs of the lasers.

19. A system comprising the signal generator of any one of Claims 12 to 18, wherein the system is any one of: a communications system, a sensing system, an imaging system, a distance measurement system and a spectroscopy system.