Optical frequency comb generation device and method

Abstract

An optical comb generator receives light from at least two continuous wave laser sources, generates a different frequency comb with each laser source, and generates an output that is a simultaneous combination of the different frequency combs. Different frequency combs span different frequency bands, but at least partially frequency overlap with adjacent combs in an overlap region. The spectral filter generates a filtered output having at least one pair of hues from the overlap region, the pair of hues including one hue from each of two adjacent combs. Feedback control based on the filtered output is applied to adjust the light entering the comb generator such that the frequencies and phases of the pair of hues from the overlap region are aligned. In this manner, a plurality of different overlapping combs are combined together to form a single seamless comb across a wider frequency range.

Description

Technical Field

[0001] The present invention relates to optical frequency comb generation. Background Art

[0002] Frequency combs have applications in a variety of fields, such as metrology, spectroscopy, microwave electronics, sensing, medical imaging, instrumentation, wireless communications, and optical communications. For example, in the field of optical communications, replacing an array of N lasers (e.g., N=64, but N can be in the hundreds) with a single frequency comb can result in huge cost and energy savings. The coherent nature of the comb lines (the phases of the comb tones are correlated) and the equal frequency spacing of the comb tones offer the prospect of ultra-high spectral efficiency (and therefore high-capacity, fast networks) and the generation of high-purity electronic RF carriers for connecting optical systems to wireless systems. In many applications, the comb source needs to have sufficiently high optical power, low noise, and a flat spectrum (i.e., similar power for all tones) to realize these benefits.

[0003] There are problems in generating frequency combs with these properties, such as having a large number of tones over a wide frequency band, each with sufficient and similar optical power.

[0004] Generating combs that are tunable in wavelength, bandwidth, and tone spacing is also problematic. Summary of the invention

[0005] The present invention is designed in view of the above problems.

[0006] Accordingly, the present invention provides an optical frequency comb generating device, comprising:

[0007] a comb generator configured to simultaneously receive light from a plurality of continuous wave laser sources of different frequencies and to generate a combined output, the combined output comprising a combination of a plurality of frequency combs, each comb from a respective one of the laser sources, wherein the plurality of frequency combs span different frequency bands, and wherein a comb from a first laser source at least partially overlaps in frequency with an adjacent comb from a second laser source in an overlap region;

[0008] a spectral filter configured to receive a portion of the combined output and selectively produce a filtered output comprising at least one pair of shades from the overlap region, the pair of shades comprising one shade from the comb of the first laser source and one shade from the comb of the second laser source;

[0009] A feedback control arrangement is arranged to receive the filtered output of the spectral filter and to generate at least one control signal based thereon, the control signal being arranged to adjust the light entering the comb generator so that the frequency and phase of the pair of tones from the overlapping region are substantially aligned.

[0010] Another aspect of the invention provides a method of generating an optical frequency comb, comprising:

[0011] transmitting light from a plurality of continuous wave laser sources of different frequencies through a comb generator simultaneously to produce a combined output comprising a combination of a plurality of frequency combs, each comb from a respective one of the laser sources, wherein the plurality of frequency combs span different frequency bands, and wherein a comb from a first laser source at least partially overlaps in frequency with an adjacent comb from a second laser source in an overlap region;

[0012] spectrally filtering a portion of the combined output to produce a filtered output comprising at least one pair of shades from the overlapping region, the pair of shades comprising one shade from the comb of the first laser source and one shade from the comb of the second laser source;

[0013] Based on the filtered output of the spectral filter, feedback control is applied to generate at least one control signal arranged to adjust the light entering the comb generator so that the frequency and phase of the pair of tones from the overlapping region are substantially aligned.

[0014] Yet another aspect of the present invention provides an optical frequency comb generator comprising:

[0015] A series of cascaded optical modulators, comprising at least one phase modulator and at least one light intensity modulator or a laser source whose light intensity can be modulated;

[0016] a generator for generating an oscillating electrical signal for driving the modulator; and

[0017] Adjustment means are arranged to adjust the oscillating electrical signal applied to at least one of the modulators.

[0018] Further optional aspects of the invention are defined in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

[0020] Figure 1 is a schematic diagram of an apparatus for generating an optical frequency comb using two laser sources as seeds according to an embodiment of the present invention;

[0021] Figure 2 is used in Figure 1 A schematic diagram of an embodiment of a feedback control device used in an embodiment of;

[0022] Figure 3 is a device for generating an optical frequency comb using multiple laser sources as seeds according to an embodiment of the present invention (as Figure 1 Schematic diagram of the expansion of

[0023] Figure 4is a schematic diagram of an optical frequency comb generator according to a further embodiment of the present invention; and

[0024] Figure 5 is a schematic diagram of an optical frequency comb generator according to a further embodiment of the present invention. DETAILED DESCRIPTION

[0025] In the drawings, similar components have similar reference numerals, and their repeated description is omitted. Terms such as "photon", "optical" and "light" used herein do not in any way limit the subject matter to visible light, but rather encompass any suitable region of the electromagnetic spectrum, including at least infrared (IR), visible light and ultraviolet (UV).

[0026] Figure 1 A first embodiment of the invention is shown. A first continuous wave laser source CW1 is used as a seed for a frequency comb generator 10, which produces a first frequency comb 12, which includes a number of separate and evenly spaced frequencies (f) called "tones" (or comb lines). For simplicity, the term "frequency comb" will sometimes be referred to as "comb" in this article. The comb generator 10 can be any conventional or novel comb generator, for example: an optoelectronic frequency comb generator composed of a cascaded optical intensity modulator and a phase modulator (discussed further below); a Kerr frequency comb generator; a comb generator based on an extremely nonlinear fiber; an optoelectronic comb generator based on a FP cavity; a mode-locked laser under injection locking.

[0027] The second continuous wave laser source CW2 is also used as a seed for the comb generator 10. The outputs of the laser sources CW1 and CW2 are combined using a coupler or wavelength multiplexer (not shown) and input to the comb generator 10. The comb generator 10 outputs a second frequency comb 14 based on the seeding of the second laser source CW2. The laser sources CW1 and CW2 have different frequencies (i.e., different wavelengths) (i.e., peak frequencies or center frequencies), so the first comb 12 and the second comb 14 are shifted so that the high frequency tones of the first comb 12 and the low frequency tones of the second comb 14 overlap in the frequency domain in the overlap region 16.

[0028] A portion of the output light of the comb generator 10 is split to a filter 20 (e.g. using a splitter or a tap connector, not shown), which transmits light only in the overlap region 16 (i.e., the filter 20 is a spectral filter that spectrally filters the light); in fact, the filter 20 may pass only light within a sub-frequency range throughout the overlap region 16. Any suitable filter may be used, such as a grating filter or a thin film filter. The filtered light may include one or more pairs of tones, each pair of tones consisting of one tone originating from the first laser source CW1 and one tone originating from the second laser source CW2. In this context, the tones from each of the two laser sources are considered to be a "pair" when they are closest in frequency to each other. A pair of tones is schematically shown in a frequency diagram 22. The filtered light is transmitted to a feedback control device 24 where it is detected, analyzed and used to generate one or more error signals which are used to control either or both of the laser sources CW1, CW2 used as seeds for the comb generator 10 and / or to control optical components in the optical path between either or both laser sources and the comb generator 10. In the embodiment shown, feedback is used to control the wavelength (frequency) of CW2 and to adjust an optical phase shifter 26 to control the phase of the light from CW2. By using the feedback control device 24, the frequency and phase of a pair of tones in the overlap region 16 can be locked so that the frequencies and phases are substantially aligned (preferably the pair of tones are arranged to constructively combine with zero phase difference, but they may also be locked or aligned at some other constant non-zero phase angle). In this way, the phase and frequency of the first comb 12 and the second comb 14 become coherent, effectively forming a single coherent wide bandwidth comb 28.

[0029] Now refer to Figure 2 The feedback control device 24 is described. The light transmitted by the filter 20 from the overlapping region 16 of the comb is detected by a photodiode (PD) or other suitable photodetector. Depending on the design of the filter 20, the filtered light may not only be one pair of tones, but multiple pairs of tones, as shown in the spectrum 30 at the input of the photodiode. The electrical signal from the photodiode includes the beat frequency of each pair of tones, and different pairs generate different beat frequencies, as shown in the spectrum 32. An optional signal filter 34 (low pass filter or band pass filter) such as an electronic filter or a digital filter can be used to extract one beat note (e.g., the highest power beat note, see spectrum 36) for use in feedback. If the filter 20 extracts a pair of tones, the signal filter 34 can be omitted.

[0030] The beat note carries the frequency and phase differences of the two combs 12, 14. The beat note is detected electronically by a phase frequency detector (PFD) 38 (e.g., a phase locked loop or a linear electronic frequency detector), which generates an error signal corresponding to the phase and frequency variations of the beat note signal. The error signal is transmitted to a feedback controller 40, such as a proportional-integral-derivative (PID) controller, which outputs control signals to adjust the frequency and phase of the light received by the comb generator 10 from the second laser source CW2 (it may similarly control the first laser source CW1 or both). Various control signals may be output by the feedback controller 40, including: a fast loop phase control signal 42 for adjusting the optical phase shifter 26 (or phase modulator); a control signal 44 for adjusting the frequency of the continuous wave laser source CW2; and optionally, there may be a slow feedback loop signal 46 for controlling long term frequency deviations of the laser source, typically caused by thermal drift.

[0031] When the pair of comb tones in the overlap region 16 are perfectly aligned in frequency with a constant phase, then the beat tone frequency becomes zero, as does the error signal, and the two combs 12 , 14 are combined together to form a wider coherent comb 28 .

[0032] Figure 3 An embodiment of the present invention is shown, wherein Figure 1 The dual-seed laser is extended to multiple seed lasers. N continuous wave lasers CW1, CW2...CWN (in principle, N can be any natural number greater than 2; Figure 1 The outputs of the two optical filters 20 (covering the case where N=2) are combined and transmitted simultaneously through the comb generator 10 to produce N combs, each of which has a limited bandwidth, but overlaps and is distributed in the frequency space. There is a row of filters 20, each centered on a different wavelength. The first filter 20.1 extracts the hue in the overlapping region 50 between the first comb and the second comb; the second filter 20.2 extracts the hue in the overlapping region 52 between the second comb and the third comb, and so on, until the N-1th filter 20.N-1 extracts the hue in the overlapping region between the N-1th comb and the Nth comb. The filtered light is then processed by the feedback control device 24, which is now N-1 feedback control devices 24 as described above (for example, reference Figure 2 ). In this way, multiple combs overlap and combine at a constant frequency and phase difference to form a flat broadband comb 28. Figure 2 The feedback loop described stabilizes the frequency and phase. In an alternative embodiment, the feedback control device 24 may include: N-1 separate feedback control devices (e.g. Figure 2 each feedback control device of the type shown); or may include a single feedback control device that detects and processes N-1 input signals in parallel or sequentially; or intermediate combinations of these alternatives.

[0033] An example of a comb generator 10 will now be described, which may be used in the above-described embodiments of an optical frequency comb generating apparatus, or may be used as a standalone embodiment of a comb generator.

[0034] Figure 4 An optical frequency comb generator is shown, which uses a continuous wave source CW as a seed light source. The light is modulated by an optical intensity modulator IM (e.g., a Mach-Zehnder modulator MZM) and a series of phase modulators PM, all of which are driven by a sinusoidal signal from the same RF source 60 of frequency fc. Preferably, the seed light source CW is a wavelength tunable laser with a narrow linewidth (e.g., a linewidth of <1kHz). The optical intensity modulator IM shapes the pulses, and the first phase modulator PM strongly modulates them (e.g., 6-8×π phase modulation) to generate a strong chirp, thereby generating a flat frequency comb at the output 62. By adding more phase modulator PM stages, the number of optical tones is increased. The RF signals driving the phase modulator PM and the optical intensity modulator IM need to have the same phase to generate a flat spectrum, so a phase shifter 64 is included in the electronic signal path for fine tuning the phase. The spacing (frequency difference) between adjacent comb tones can be adjusted by changing the frequency of the driving RF signal fc.

[0035] Adjusting the power driving the phase modulator PM makes the bandwidth (number of tones) of the comb adjustable. This is achieved by an adjustment device, which in this embodiment includes a tunable RF attenuator 66 for each phase modulator PM for adjusting the RF power applied to the phase modulator. Optionally, in this embodiment, an RF amplifier 68 is provided in the electrical signal path before each tunable RF attenuator to enhance the RF signal before attenuation. These can be fixed gain RF amplifiers 68. In alternative embodiments, a tunable gain RF amplifier (with or without attenuator 66) can be used to adjust the RF power driving the phase modulator.

[0036] Therefore, a bandwidth-tunable optical frequency comb generator is realized, and the optical frequency comb generator can generate a flat low-noise spectrum.

[0037] Figure 5 Another embodiment of an optical frequency comb generator is shown. Figure 4 This is an extension of the embodiment of the present invention, and therefore the description of the corresponding parts will be omitted to avoid repetition.

[0038] Figure 5The comb generation device also includes a driver 70 for controlling the electrical signal driving the optical intensity modulator IM. Suitable drivers include electronic pulse shapers (e.g. nonlinear electronic devices) or digital-to-analog converters. The driver 70 can be used to pre-shape (or pre-distort) the optical pulses (including pulse width and peak power) (by driving the optical intensity modulator); and by controlling the pulse shape, the bandwidth and comb spacing of the resulting optical frequency comb can be controlled. This eliminates the need to use other optical components for pulse shaping (e.g. optical fibers with nonlinear ring mirrors, or waveform shapers), which can be expensive and bulky.

[0039] The output of the cascaded modulators can optionally be passed through a comb expansion stage, via Figure 5 The components on the right are shown. They include: a fixed passive pulse compressor 72 (e.g., a fixed grating or a fixed length of optical fiber); a first optical amplifier 74 (e.g., an erbium-doped fiber amplifier EDFA); a comb filter 76; a second optical amplifier 78 acting as an enhancer; and an optical nonlinear medium 80 (e.g., a highly nonlinear optical fiber or waveguide). Using such an extension stage, the comb bandwidth can be extended to hundreds of nanometers, for example.

[0040] The comb filter 76 is an optical filter that transmits tones but filters out noise between tones to improve the optical signal-to-noise ratio (OSNR) before pumping the nonlinear medium 80. In one example, the comb filter is an FP filter with high finesse (e.g., finesse greater than 1000) and a free spectral range (FSR) equal to the comb spacing (fc). In other examples, the comb filter 76 is a high-resolution waveform shaper, a ring filter, or a cascaded fiber Bragg grating. Depending on the maximum power handling capability, the comb filter 76 can be included between the first optical amplifier 74 and the second optical amplifier 78, as shown, or can be after the second optical amplifier 78 (power amplifier).

[0041] The use of a driver 70 to shape the waveform, thereby driving the optical intensity modulator IM to shape the optical pulse, means that a fixed passive compressor 72 can be used to achieve the optimal pulse shape to pump the nonlinear medium 80 without the need for an active tunable optical pulse shaper. The desired optical pulse shape can be achieved using digital signal processing methods or machine learning methods, such as using feedback from the comb output 62 to control the driver 70. In this way, the driver 70 generates the desired waveform to drive the optical intensity modulator IM, and the light from the optical intensity modulator generates the desired pulse after passing through all components in the optical signal chain, which pumps the nonlinear medium 80 to generate the desired comb spectrum (e.g., flat, wide bandwidth, low noise). The comb is tunable by changing the signal applied by the driver 70 to the optical intensity modulator IM.

[0042] In addition, the use of the above scheme to generate asymmetrically shaped pulses can achieve tailored comb spectra for specific wavelength regions of interest. The shape of the optical pulse strongly affects the shape of the comb spectrum generated after the modulator IM, PM. In addition, the symmetry of the pulse entering the nonlinear medium 80 determines the overall flatness and bandwidth extension of the comb. The use of customized asymmetrical pulses formed by pre-distorting the waveform enables the formation of tilted or asymmetrical comb broadening, which can be targeted at, for example, longer or shorter wavelengths.

[0043] In an alternative embodiment, Figure 5 Each of the additional components, including the driver 70, the pulse compressor 72 and the comb filter 76, may be optionally omitted or used separately, alone or in any combination. Figure 4 and Figure 5 A single light intensity modulator and several phase modulators are shown, but this is just an example. Embodiments have in common the feature of having a series of one or more cascaded modulators, which may include more or less of each type of modulator, and arranged in any order. It is also contemplated that Figure 4 and Figure 5 The continuous wave laser source CW and the optical intensity modulator IM are replaced by a laser source whose light intensity can be modulated, such as a mode-locked laser or other pulsed laser source or a time-modulated laser source, followed by at least one phase modulator and any number of optical intensity modulators (including zero, one or more).

Claims

1. An optical frequency comb generating device, comprising: a comb generator configured to simultaneously receive light from a plurality of continuous wave laser sources of different frequencies and to generate a combined output, the combined output comprising a combination of a plurality of frequency combs, each comb from a respective one of the laser sources, wherein the plurality of frequency combs span different frequency bands, and wherein a comb from a first laser source at least partially overlaps in frequency with an adjacent comb from a second laser source in an overlap region; a spectral filter configured to receive a portion of the combined output and selectively produce a filtered output, the filtered output comprising at least a pair of shades from the overlap region, the pair of shades comprising one shade from the comb of the first laser source and one shade from the comb of the second laser source; A feedback control device is arranged to receive the filtered output of the spectral filter and generate at least one control signal based on the filtered output, the control signal being arranged to adjust the light entering the comb generator so that the frequency and phase of the pair of tones from the overlapping region are substantially aligned.

2. An apparatus according to claim 1, wherein the combined output comprises a synthetic single comb, the synthetic single comb comprising a plurality of evenly spaced tones, the evenly spaced plurality of tones spanning a frequency band wider than the frequency band of a comb generated from a single laser source of the plurality of laser sources via the comb generator.

3. The device according to claim 1 or 2, wherein the feedback control device comprises: a photodetector for converting the filtered output into an electrical signal containing the beat notes of the pair of tones from the overlap region; and A phase frequency detector is used to detect the phase difference and frequency difference of the pair of tones from the electrical signal.

4. The apparatus of claim 3, wherein the at least one control signal generated by the feedback control means is arranged to minimize the phase difference and the frequency difference of the pair of tones.

5. The apparatus according to any one of the preceding claims, further comprising an optical phase shifter, wherein the control signal from the feedback control device is used to adjust the phase of light received by the comb generator from at least one of the plurality of laser sources by adjusting the optical phase shifter.

6. An apparatus according to any one of the preceding claims, wherein a control signal from the feedback control means is used to adjust the frequency of light output by one of the plurality of said laser sources and received by the comb generator.

7. The device according to any of the preceding claims, wherein the number of said laser sources is N, wherein N>2, and wherein the device comprises at most N-1 said feedback control devices and N-1 corresponding spectral filters.

8. The device according to any one of the preceding claims, wherein the comb generator comprises: a series of cascaded optical modulators including at least one light intensity modulator and at least one phase modulator; a generator for generating an oscillating electrical signal for driving the modulator; and Adjustment means are arranged to adjust the amplitude of the oscillating electrical signal applied to at least one of the modulators.

9. An apparatus according to claim 8, comprising a plurality of phase modulators, wherein the adjustment means is arranged to adjust the amplitude of the oscillating electrical signal applied to each phase modulator.

10. An apparatus as claimed in claim 9, wherein the adjustment means is arranged to make the electrical signal applied to each phase modulator individually adjustable.

11. The device according to claim 9 or 10, wherein the adjustment device comprises at least one of: a variable attenuator; an amplifier with variable gain.

12. Apparatus according to claim 8, 9, 10 or 11, wherein the adjustment means comprises a controllable electronic pulse shaper arranged to shape the oscillating electrical signal subsequently applied to the light intensity modulator.

13. A method of generating an optical frequency comb, comprising: transmitting light from a plurality of continuous wave laser sources of different frequencies simultaneously through a comb generator to produce a combined output, the combined output comprising a combination of a plurality of frequency combs, each comb from a respective one of the laser sources, wherein the plurality of frequency combs span different frequency bands, and wherein a comb from a first laser source at least partially overlaps in frequency with an adjacent comb from a second laser source in an overlap region; spectrally filtering a portion of the combined output to produce a filtered output, the filtered output comprising at least a pair of shades from the overlap region, the pair of shades comprising one shade from the comb of the first laser source and one shade from the comb of the second laser source; Based on the filtered output of the spectral filter, feedback control is applied to generate at least one control signal arranged to adjust light entering the comb generator so that the frequency and phase of the pair of tones from the overlap region are substantially aligned.

14. An optical frequency comb generator comprising: A series of cascaded optical modulators, comprising at least one phase modulator and at least one light intensity modulator or a laser source whose light intensity can be modulated; a generator for generating an oscillating electrical signal for driving the modulator; and Adjustment means are arranged to adjust the oscillating electrical signal applied to at least one of the modulators.

15. Apparatus according to claim 14, comprising a plurality of phase modulators, wherein the adjustment means is arranged to adjust the amplitude of the oscillating electrical signal applied to each phase modulator.

16. Apparatus according to claim 15, wherein the adjustment means is arranged to render the electrical signal applied to each phase modulator individually adjustable.

17. The device according to claim 15 or 16, wherein the adjustment device comprises at least one of: a variable attenuator; an amplifier with variable gain.

18. The apparatus of claim 17, wherein each variable attenuator is a tunable RF attenuator.

19. An apparatus according to any one of claims 14 to 18, wherein the adjustment means comprises a controllable electronic pulse shaper arranged to shape the oscillating electrical signal subsequently applied to the light intensity modulator.

Images