Dual frequency comb source, integrated system using same, and method for performing dual frequency comb spectroscopy

Abstract

A dual frequency comb source (100) based on an optical integrated circuit, an integrated system (300) for dual comb spectroscopy, and a corresponding method are disclosed. The dual frequency comb source (100) includes first and second semiconductor integrated mode-locked lasers (110, 120), a master laser (130), and a connection device (102) between the master laser and each of the first and second mode-locked lasers, all located on the same substrate (101) as the optical integrated circuit. The master laser is configured to generate lasing lines for simultaneous optical injection locking of the first and second mode-locked lasers, and the first and second mode-locked lasers are configured to generate the first and second frequency combs, respectively. The connection device is further adapted to coherently transmit lasing light from the master laser to each mode-locked laser. The mode-locked lasers include a gain section and a saturable absorber section that provide mode-locking, and extended optical cavities (113, 123) formed in the substrate.

Description

[Technical Field]

[0001] The present invention relates to optical frequency comb generators and frequency comb spectroscopy, and more particularly to optical dual frequency comb generators based on integrated semiconductor mode-locked lasers. [Background technology]

[0002] The generation of frequency combs based on mode-locked semiconductor lasers has long been known in the art. However, the short cavity of integrated semiconductors effectively limits the pulse repetition rate of the laser to a few tens of gigahertz, which is too broad for high-density spectroscopy applications such as gas absorption spectroscopy. To accurately resolve spectral absorption features at atmospheric pressure, sub-gigahertz repetition rates are required.

[0003] Silica wedge microresonators on silicon chips have been proposed by Suh et al. as one possible source of dual frequency combs based on soliton generation in a Kerr cavity in their paper "Microresonator soliton dual-comb spectroscopy," Science (6312), 600-603, November 2016. However, these microresonators require precise alignment to the fiber taper to couple the intense light from two external CW fiber lasers. This makes this approach difficult to operate and requires hundreds of milliwatts of on-chip optical power. The comb line spacing of approximately 22 GHz is still too wide for high-density spectroscopy applications, such as gas absorption spectroscopy, which require sub-gigahertz repetition rates. Dual-comb spectroscopy measurements based on silica wedge microresonators are limited in their frequency resolution by the limited mutual coherence of the two external fiber lasers.

[0004] Therefore, there remains a need for a mutually coherent dual frequency comb source that can be integrated on a single substrate. Summary of the Invention

[0005] It is an object of embodiments of the present invention to provide a mutually coherent dual frequency comb source that can be integrated on a single substrate.

[0006] It is a further object of embodiments of the present invention to provide a mutually coherent dual frequency comb source that does not require phase adjustment of the optical resonator to achieve mutual coherence.

[0007] The above object is achieved by a method and device according to the present invention.

[0008] In a first aspect, the present invention relates to a mutually coherent dual frequency comb source based on an optical integrated circuit, the dual frequency comb source comprising, on the same substrate of the optical integrated circuit, first and second semiconductor integrated mode-locked lasers for generating first and second frequency combs, respectively; a master laser for generating a continuous-wave (CW) lasing line for simultaneous optical injection locking of the first and second mode-locked lasers; and a connecting device interposed between an output port of the master laser and a respective input port of each of the first and second mode-locked lasers for coherently transmitting lasing light generated by the master laser to each mode-locked laser. Each of the first and second mode-locked lasers includes a gain section and a saturable absorber section for providing mode-locking. At least the gain section is provided as a hetero-integrated III-V material layer stack bonded to the substrate. Furthermore, each of the first and second mode-locked lasers includes an extended optical cavity including a passive waveguide structure formed in a waveguide layer of the substrate. At least the resonant mode of each extended optical cavity that is frequency-locked to the continuous wave laser line has a spectral bandwidth of less than 5 MHz at a 5 μs integration time, preferably less than 3 MHz at a 5 μs integration time (e.g., less than 1 MHz at a 5 μs integration time) upon injection into that extended optical cavity, and the master laser is configured to generate a CW lasing line having a linewidth narrower than the frequency spacing between the modes of the first and second frequency combs, thereby enabling reliable and mutually phase-coherent injection locking of the first and second frequency combs to the CW lasing line emitted by the master laser.

[0009] By optically injection locking each comb line of the first and second generated frequency combs to a master lasing line, the lasing line generated by the master laser efficiently stabilizes the carrier offset frequencies of both the first and second mode-locked lasers. As a result of the optical injection locking and the coherence properties between the comb lines of the frequency combs, the low noise and stable phase coherence properties of the master laser are mutually transferred to the multiple comb lines across the first and second generated frequency combs. The coherent transfer of lasing light generated by the master laser to each mode-locked laser preserves phase coherence between the two light waves injected into the two mode-locked lasers for injection locking. As a result, the first and second generated frequency combs are mutually coherent, i.e., each comb line of the first frequency comb is coherent with each other comb line of the first frequency comb and each comb line of the second frequency comb, and vice versa. Furthermore, the comb lines of the first and second frequency combs are coherent with the continuous-wave lasing line generated by the master laser, and are specifically tunable via the CW lasing line of the master laser. Providing a dual frequency comb source as an integrated optical circuit ensures a high degree of phase coherence and long-term stability necessary for dual-comb spectroscopy applications. For example, even in the presence of external mechanical disturbances (e.g., vibrations), thermal disturbances (e.g., temperature fluctuations), etc., the mutual coherence of the two frequency combs is maintained for a time long enough to perform dual-frequency comb spectroscopy. Therefore, in embodiments of the present invention, a coherence time of approximately 100 μs or longer can be ensured.

[0010] According to a preferred embodiment of the present invention, the passive waveguide structure of the mode-locked laser extended optical cavity comprises a wide bandgap material that transmits light over a wide spectral bandwidth. This has the advantage that low intracavity losses, including linear and nonlinear absorption losses, can be obtained in the wide spectral range encompassed by the generated frequency comb. Therefore, higher pulse energies can be generated by the mode-locked laser, and shorter gain sections can be implemented in the mode-locked laser, which reduces the noise contribution of the amplified spontaneous emission. The low chromatic dispersion of the wide bandgap material allows for reduced pulse broadening in the optical cavity, which is beneficial for achieving very short optical pulses and wide frequency combs. The wide bandgap material may be a dielectric material.

[0011] According to an embodiment of the present invention, the extended optical cavity of each of the first and second mode-locked lasers may be configured as a linear optical resonator or as a ring resonator. Ring resonators have the advantage that they do not require mirrors for optical feedback, and therefore the dispersion contribution of such mirrors is not present in ring resonators. In contrast, linear resonators are not subject to bidirectional pulse lasing, which requires higher-performance laser implementations, and end mirrors in linear resonators can be used to limit the optical bandwidth.

[0012] According to an embodiment of the present invention, the connecting device may comprise a coupling element in optical communication with the output port of the master laser, and at least two delivery waveguides optically connected between the coupling element and the input ports of the first and second mode-locked lasers. Coupling to at least two separate delivery waveguides allows additional optical components to be inserted into each delivery waveguide, which has the advantage that the lasing lines for injection locking can be operated separately for each of the first and second mode-locked lasers.

[0013] According to an embodiment of the present invention, each of the first and second mode-locked lasers may comprise a gain section and a saturable absorber section adapted for electrical pumping. The gain section and the saturable absorber section of at least one of the first and second mode-locked lasers may be made from different semiconductor materials or from the same semiconductor material. If made from the same semiconductor material, the arrangement and / or size of the layers within the stack may be different or the same for the gain section and the saturable absorber section.

[0014] According to an embodiment of the present invention, the gain sections of the first and second mode-locked lasers may be hetero-integrated on a substrate. In particular, III-V on silicon-on-insulator (SOI) or insulator-on-insulator hetero-integration is an established and versatile photonics platform for which the advantages of combining low-loss waveguide capabilities with highly efficient gain materials bonded to photonic integrated circuits (PICs) have been experimentally verified.

[0015] According to an embodiment of the present invention, the connection device may further include a frequency shifter disposed between the output port of the master laser and the input port of only one of the first and second mode-locked lasers. This has the advantage that aliasing effects in the beat notes of the acquired interferogram may be reduced and / or two mutually coherent frequency combs may be locked onto comb lines in the central part of the comb. Aliasing is generally caused by residual comb lines spaced to the left or right of the optically injection-locked comb lines located at the short-wavelength or long-wavelength ends of the frequency comb, respectively. These residual comb lines generally lie outside the usable optical bandwidth (e.g., a 3 dB bandwidth) but may still have non-negligible intensity.

[0016] According to embodiments of the present invention, the connection device may not have any optical isolator, including an optical isolator that is part of the master laser (e.g., located at the output port). This has the advantage of eliminating the need for an optical isolator, which is difficult to implement on-chip in a CMOS-compatible manufacturing process. Therefore, embodiments of the present invention enable large-scale manufacturing of compact, cost-effective, and mechanically stable PICs with no moving parts, for example, using a wafer-scale process such as a CMOS-compatible process.

[0017] According to an embodiment of the present invention, the connecting device may further comprise at least one optical attenuator for controlling the amount of optical power injected into each of the first and second mode-locked lasers. The presence of the optical attenuator is beneficial because it allows for injection of a lower optical power into the optical cavities of the first and second mode-locked lasers for injection locking relative to the optical power of the lasing line as emitted by the master laser. Operating the master laser at a higher output power has the advantage that a stable, low-noise, and narrow linewidth can be obtained for reliable injection locking. Furthermore, the attenuator effectively protects the master laser from interference from optical feedback.

[0018] According to embodiments of the present invention, at least one of the first and second mode-locked lasers can be configured to generate a frequency comb with a comb spacing of less than 10 GHz (e.g., a frequency comb mode spacing of 100 MHz to 10 GHz). This has the advantage that dual frequency comb spectroscopy can be performed in the gas phase at atmospheric pressure, where collisional broadening of vibrational absorption lines is in the gigahertz range and is not limited by undersampling constraints (ambiguities caused by undersampling). Compared to approaches using interleaved spectra, embodiments of the present invention can more accurately determine absorption line parameters, enabling applications in time-resolved spectroscopy. The RF linewidth associated with the comb lines in the RF domain can be less than 500 kHz across the entire frequency comb.

[0019] In a further aspect, the present invention relates to an integrated system for dual frequency comb spectroscopy, comprising a dual frequency comb source based on an optical integrated circuit according to an embodiment of the first aspect, and a photodetector configured to receive the combined frequency combs of the first and second mode-locked lasers. Furthermore, a sensing region is provided in an optical path extending between only one of the first and second mode-locked lasers and the photodetector. The sensing region is configured to allow light-matter interaction between the generated frequency comb and a species to be sensed, such that a difference in comb spacing between the generated first and second frequency combs is within a detectable radio frequency range of the photodetector.

[0020] The sensing region may include a waveguide section configured to detect evanescent waves between the frequency comb and the species to be sensed when guided by the waveguide section. A curvilinear waveguide for evanescent sensing may be tightly convoluted to use only a limited amount of surface area of ​​the optical integrated circuit, yet still provide a relatively long interaction length. The dual frequency comb spectroscopy system may further comprise processing means for detecting and analyzing beat lines in an interferogram generated by the photodetector in response to interference of the received first and second frequency combs. Furthermore, the system may be provided as an optical integrated circuit.

[0021] In yet another aspect, the present invention relates to a method for performing dual frequency comb spectroscopy on a sample comprising a species to be detected. The method includes providing a dual frequency comb light source based on an optical integrated circuit according to an embodiment of the first aspect, wherein a repetition rate of a first generated frequency comb differs from a repetition rate of a second generated frequency comb. In another method step, lasing light generated by a master laser is coherently transmitted from an output port of the master laser to respective input ports of first and second mode-locked lasers, thereby injection-locking the first and second mode-locked lasers to the wavelength of the lasing light. In a further step, the first generated frequency comb is directed through a sensing region configured to receive the species to be detected and to enable photo-matter interaction between the frequency comb and the species. The method also includes contacting the sample with the sensing region and detecting a beat note in an interferogram generated when the first and second generated frequency combs are superimposed on a photodetector, where the first generated frequency comb is interacting with the species in the sample.

[0022] A further advantage of some embodiments of the present invention is that mode-locked lasers can be passively mode-locked, thereby eliminating the need for active RF modulation, thereby simplifying laser design and operation. This allows for power savings. In other embodiments of the present invention, mode-locked lasers are adapted for hybrid mode-locking, which has the added advantage that the repetition rates of the two generated frequency combs can be stabilized even at low RF power. Hybrid mode-locking of mode-locked lasers has the further advantage that the repetition rates of the first and second generated frequency combs can be referenced to a high-quality frequency standard, such as a maser or cesium clock transition, making it possible to perform dual-frequency comb spectroscopy with very high frequency resolution.

[0023] A further advantage of some embodiments of the present invention is that broad and dense comb frequency spectra with over 1000 lines can be obtained with a relatively short comb line spacing of less than 10 GHz (e.g., about 1 GHz). The frequency comb can have an optical bandwidth of over 1 THz in the near infrared. This enables accurate and high-resolution broadband gas-phase spectroscopy.

[0024] A further advantage of some embodiments of the present invention is that a mutually coherent dual-frequency comb source for spectroscopy does not rely on frequency tuning or multiple scans of comb lines to obtain a complete acquisition of a spectral line or band. Continuous scanning may be required in other dual-comb approaches that provide only a small number of comb lines (e.g., tens to hundreds of comb lines) and / or have too large a repetition rate difference and / or too large a repetition rate. Therefore, a further advantage of some embodiments of the present invention is that dual-comb spectroscopy can be performed without a scanning element, enabling real-time multispectroscopy. Thus, a single acquisition of a broadband spectral line or band can be achieved in microseconds, and measurements can be accumulated and averaged over multiple scans over long time spans, on the order of milliseconds, to make the measurement more accurate.

[0025] A further advantage of some embodiments of the present invention is that the optical gain provided on-chip provides sufficiently strong pulse peak power to allow for long interaction lengths in the gas sample (e.g., absorption paths tens to hundreds of meters long in the gas sample).

[0026] A further advantage of some embodiments of the present invention is that injection lock is obtained on-chip without the need for any external feedback loop for stabilization.

[0027] A further advantage of some embodiments of the present invention is that the generation of two mutually coherent frequency combs does not rely on external high-power (e.g., on the order of watts) lasers that are not suitable for optical integration.

[0028] A further advantage of some embodiments of the present invention is that generating two mutually coherent frequency combs requires only a small amount of power, e.g., a few hundred milliwatts, e.g., on the order of 100 mW, making the dual-comb source battery-powered.

[0029] A further advantage of some embodiments of the present invention is that optically injection locking the generated frequency comb to a narrow-linewidth master laser ensures good mutual coherence of the two combs, and also provides good thermal and mechanical stability, since the absolute frequency position of the generated and locked comb drifts according to the frequency shift experienced by the injection-locked lasing line generated by the master laser on the same substrate.

[0030] A further advantage of some embodiments of the present invention is that they provide a flat-peaked comb spectrum and good sensitivity (e.g., good signal-to-noise ratio of the RF fundamental beat note) compared to frequency comb generators that have no comb lines, too few or sparsely distributed lines, or poor signal-to-noise ratios.

[0031] A further advantage of some embodiments of the present invention is that they do not require separate components such as external cavity mirrors or external saturable absorber mirrors, thus avoiding alignment and coupling issues for those components. In particular, to obtain narrow comb line spacing, e.g., less than 10 GHz, no external high-quality optical cavity is required.

[0032] A further advantage of some embodiments of the present invention is that only a single photodetector is required to perform dual-comb spectroscopy.

[0033] A further advantage of some embodiments of the present invention is that the extended optical cavity of the mode-locked laser is arranged according to an anti-collision pulse configuration. The anti-collision pulse configuration allows for higher pulse peak powers to be obtained at the output of the light source and / or allows for reduced timing jitter in the pulse train generated by the mode-locked laser. The anti-collision pulse configuration or topology means that there is only one pulse circulating within the mode-locked laser cavity, passing through the cavity twice in one round trip. Furthermore, the anti-collision pulse configuration allows for amplification of the lasing line for output locking when injected into the optical cavity via the first (back) reflector instead of the second (front) reflector.

[0034] Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.

[0035] Certain objects and advantages of the present invention have been described hereinabove for purposes of summarizing the invention and the advantages achieved over the prior art. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, it will be recognized by those skilled in the art that the present invention may be practiced in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages that may be taught herein.

[0036] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. [Brief explanation of the drawings]

[0037] The invention will now be further described, by way of example, with reference to the accompanying drawings, in which:

[0038] [Figure 1]1 shows a dual frequency comb source provided as an optical integrated circuit according to a first embodiment of the present invention, where a combination of two mutually coherent frequency combs is provided by the output of the source. [Figure 2] 1 shows a dual frequency comb light source based on an integrated optical circuit according to a second embodiment of the present invention, in which two mutually coherent frequency combs are each provided by two different outputs of the light source. [Figure 3] 1 illustrates an optical integrated circuit implementing a system for dual frequency comb spectroscopy according to an embodiment of the present invention, the system including an integrated dual frequency comb source according to the first embodiment of FIG. [Figure 4] 2 is a modification of the integrated optical circuit-based dual frequency comb source associated with FIG. 1, in which the variable optical attenuator on the optical path of the connecting device is replaced. [Figure 5] 1 is a recorded spectrum corresponding to one of the frequency combs generated by an integrated dual frequency comb source according to an embodiment of the present invention. [Figure 6] 1 is a recorded dual frequency comb absorption spectrum obtained by transmitting one of the generated frequency combs of a light source through a sample gas cell using a dual frequency comb spectroscopy system according to an embodiment of the present invention.

[0039] The drawings are only schematic and non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn to scale for illustrative purposes. The dimensions and relative dimensions do not necessarily correspond to actual reductions to practice of the invention.

[0040] Any reference signs in the claims shall not be construed as limiting the scope.

[0041] In the different drawings, the same reference signs refer to the same or similar elements. DETAILED DESCRIPTION OF THE INVENTION

[0042] The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims.

[0043] The terms first, second, and the like in this description and claims are used to distinguish between similar elements and are not necessarily used to describe order, whether temporal, spatial, sequential, or in any other way. The terms so used are interchangeable under appropriate circumstances, and it is understood that the embodiments of the invention described herein are capable of operating in sequences other than those described or illustrated herein.

[0044] It should be noted that the term "comprising" used in the claims should not be interpreted as being limited to the means recited thereafter, but does not exclude other elements or steps. Thus, it should be interpreted as specifying the presence of the stated features, integers, steps, or components as referred to, but without excluding the presence or addition of one or more other features, integers, steps, or components, or groups thereof. Thus, the scope of the expression "a device comprising means A and B" should not be limited to a device consisting only of components A and B. This means that, in the context of the present invention, the relevant components of the device are only A and B.

[0045] References throughout this specification to "one embodiment" or "an embodiment" mean that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrase "in one embodiment" or "in an embodiment" in various places throughout this specification do not necessarily all refer to the same embodiment, but may. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments, as would be apparent to one of ordinary skill in the art from this disclosure.

[0046] Similarly, in the description of exemplary embodiments of the invention, it will be understood that various features of the invention may be grouped together in a single embodiment, drawing, or description for the purpose of streamlining the disclosure and aiding in understanding one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the Detailed Description are expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of this invention.

[0047] Furthermore, some embodiments described herein include some features included in other embodiments but not other features, while as will be understood by those skilled in the art, combinations of features from different embodiments are intended to be within the scope of the present invention and form different embodiments.

[0048] In the description provided herein, numerous specific details are set forth. However, it will be understood that embodiments of the present invention may be practiced without these specific details. In other instances, well-known methods, structures, and techniques have not been shown in detail in order not to obscure an understanding of this description.

[0049] definition When referring to a mode-locked laser in this disclosure, it is meant that the laser is operated in the mode-locked regime to emit a regular pulse train at the pulse repetition rate of the laser. The optical output spectrum of a pulse train produced by a mode-locked laser consists of a plurality of equally spaced lines, i.e., a pulse train at a repetition rate of f repFor a mode-locked laser, a fixed phase relationship is established between the equidistant lines of the frequency comb associated with the longitudinal modes of the laser cavity; therefore, a mode-locked laser generates a frequency comb whose lines are mutually coherent. Furthermore, subsequent pulses in the generated pulse train also have a constant phase shift (carrier-envelope offset frequency f CEO ), are mutually coherent. As a result, a frequency comb has spectral positions that satisfy the equation f = f CEO +n×f rep (n is a positive integer). Therefore, the frequency comb has two independent degrees of freedom, two frequencies f CEO , and f rep If is known, then all frequencies of the comb are well defined.

[0050] When passive mode-locking is referred to in this disclosure, it refers to a mode-locking technique applied to a laser that includes an intensity-dependent transmitting element, such as a saturable absorber in a semiconductor laser. The intensity-dependent transmitting element can rapidly modulate the intracavity loss on the timescale of a single circulating pulse without the need for an externally supplied modulation signal. This contrasts with active mode-locking techniques, which rely on an actively driven optical modulator placed in the laser cavity.

[0051] When hybrid mode-locking is referred to in this disclosure, it refers to a stabilization technique applied to passively mode-locked semiconductor lasers that operates by electrical injection of a radio frequency (RF) signal modulated at the pulse repetition rate of the mode-locked semiconductor laser. The gain and / or saturable absorber of the semiconductor laser may be modulated by such an RF signal. Hybrid mode-locking stabilizes the repetition rate of the semiconductor laser and often reduces phase noise and timing jitter.

[0052] When heterogeneous integration is referred to in this disclosure, it relates to the process of assembling separately manufactured components into higher level assemblies with increased functionality and improved operating characteristics.

[0053] In this disclosure, when referring to an integrated dual frequency comb source, it means a dual frequency comb source provided as an optical integrated circuit, i.e., a dual frequency comb source based on an optical integrated circuit. The optical integrated circuit is typically provided as an optical chip, and then the dual frequency comb source based on the optical integrated circuit constitutes an on-chip dual frequency comb.

[0054] FIG. 1 shows an integrated dual frequency comb source 100 according to a first embodiment of the present invention, which can be provided as a photonic integrated circuit (PIC). The dual frequency comb source 100 includes a first integrated semiconductor mode-locked laser 110, a second integrated semiconductor mode-locked laser 120, and a master laser 130 on the same substrate 101. Each of the first and second mode-locked lasers 110 and 120 includes a gain section 111 and 121 and a saturable absorber section 112 and 122 electrically isolated from the gain section, for example, by a deeply etched isolation trench. The saturable absorber section can also be electrically isolated from the gain section by providing a saturable absorber section that is mechanically isolated from the gain section. Furthermore, the extended optical cavity 113 and 123 of each mode-locked laser is optically coupled to the corresponding gain section 111 and 121 and the corresponding saturable absorber section 112 and 122. The first and second semiconductor mode-locked lasers 110, 120 preferably have similar designs so that, after fabrication, their characteristics, such as noise characteristics, thermal response, and lasing threshold, closely match. Similar designs may relate to, but are not limited to, similar geometric configurations of the extended optical cavities, similar or identical materials used to implement the extended optical cavities, similar or identical materials used to implement the gain sections, similar or identical materials used to implement the absorber sections, and similar or identical materials used to attach the gain and / or absorber sections to the same substrate (e.g., adhesive bonding). This design similarity is also practical from a manufacturing standpoint, as it allows the two mode-locked lasers to be processed simultaneously, or at least quickly sequentially, in a single processing step, which may be wafer-scale processing. For example, adhesive die wafer bonding or transfer printing can be used to attach the gain and / or absorber sections to the same substrate. However, the lengths of the extended optical cavities 113, 123 of each mode-locked laser 110, 120 can be made to differ by only a few percent, thus resulting in two mode-locked lasers with slightly different frequency comb line spacings (repetition rates).The master laser 130 is optically coupled to both the first mode-locked laser 110 and the second mode-locked laser 120, such that light generated by the master laser 130 is delivered and injected into each of the two mode-locked lasers 110, 120. More specifically, the light generated by the master laser 130 is injected into the respective optical cavities (standing wave resonators) 113, 123 of each of the first and second mode-locked lasers. In this embodiment, an output port 136 of the master laser 130 is in optical communication with the corresponding input ports 117, 127 of the first and second mode-locked lasers 110, 120, for example, via a connecting device 102 on a substrate 101 that includes an optical path extending between the output port 136 of the master laser 130 and the input ports 117, 127 of the mode-locked lasers 110, 120. Such a connecting device 102 preferably comprises waveguides 103, 104 for directing light from the output ports of the master laser to the corresponding input ports of the mode-locked laser.

[0055] Each of the gain sections 111, 121 comprises a semiconductor gain medium and is adapted for electrical pumping by forward current injection. The gain sections 111, 121 may comprise an epitaxial III-V material layer stack bonded to the substrate 101, for example, by bonding the material layer stack with a benzocyclobutene (BCB) adhesive layer in a mold wafer bonding process or by molecularly bonding the material layer stack to the substrate. The substrate 101 may be a silicon-on-insulator (SOI) substrate, e.g., a semiconductor substrate such as silicon, germanium, or silicon carbide, or an insulator-on-insulator substrate such as silicon nitride on silica. As a result, a dual frequency comb source based on an optical integrated circuit is provided as a hetero-integrated PIC. A typical III-V material layer stack may be fabricated in InP technology and may comprise layers of InP ternary and quaternary compositions for vertical optical mode confinement and charge carrier confinement. A ridge may be formed in the III-V material layer stack by etching the upper layers to achieve lateral optical mode confinement. For example, an active layer for light emission by carrier recombination may include one or more InGaAsP quantum wells adjacent to an InP barrier. Alternatively, the active layer of the III-V material layer stack may comprise quantum dots or quantum dash mesons. The gain sections 111, 121 may have longitudinal lengths of several hundred micrometers, e.g., 200 μm to 4000 μm, e.g., 400 μm to 1600 μm, depending on the material gain and the forward current used for electrical pumping.

[0056] Each of the saturable absorber sections 112, 122 may comprise a nonlinear absorbing medium (e.g., a semiconductor absorbing medium) or an artificial saturable absorber (e.g., a nonlinear waveguide interferometer or a nonlinear waveguide loop mirror). The semiconductor absorbing medium may be identical to the unpumped gain medium of the gain section, and may comprise the same epitaxial III-V material layer stack as the gain section heterogeneously integrated on the substrate 101, for example, by adhesive bonding, molecular bonding, or transfer printing. Furthermore, each of the saturable absorber sections 112, 122 comprising a semiconductor medium is adapted to apply a reverse DC bias voltage across the entire saturable absorber section, or a reverse DC bias voltage combined with an RF signal modulated at the pulse repetition rate of the respective mode-locked laser 110, 120, for example, to obtain faster recovery and / or repetition rate stabilization by hybrid mode-locking. The longitudinal length of the semiconductor-based saturable absorber sections 112, 122 is typically a small fraction of the longitudinal length of the respective gain sections 111, 121, e.g., a factor of 10 or less, e.g., 10 μm to 400 μm, e.g., 10 μm to 100 μm. However, embodiments are not limited to the same epitaxial layer stack and common absorption medium for the gain and saturable absorber sections in each mode-locked laser. For example, the saturable absorber section may comprise a portion of an extended optical cavity implemented as a waveguide with cladding layers containing diffused absorbing dopant ions or quantum dot material. To further improve pulse slope and / or reduce cavity losses compared to the gain section, a medium or epitaxial layer stack with lower saturation fluence and / or shorter recovery time and / or reduced multiphoton absorption loss may be selected for the saturable absorber section. An advantage of embodiments of the present invention in which the gain and saturable absorber sections of a mode-locked laser are formed in the same substrate-bonded epitaxial III-V material layer stack is that a continuous and uniform active waveguide can be defined within the active region of the layer stack, thereby providing good modal gain and reduced cavity losses in the absence of an additional spot transformer / vertical taper.Furthermore, when fabricated in a multi-step epitaxial growth process, the substrate-bonded epitaxial III-V material layer stack can contain different epitaxial layer structures on the same die, with the advantage that different active waveguide cross sections can be defined for the gain section and the saturable absorber section, respectively. For example, the active waveguide cross section in the saturable absorber section can be reduced compared to the waveguide cross section in the gain section to obtain a higher index setting of the guided optical mode in the active region, which can also result in a lower saturation power. Another possibility is to provide a saturable absorber section corresponding to a substrate-bonded film-type III-V material layer stack, similar to those known in the field of uni-traveling carrier (UTC) waveguide photodetectors (e.g., III-V material layer stacks having a cross-section as described in section two of L. Shen, Y. Jiao, W. Yao, Z. Cao, J. van Engelen, G. Roelkens, M. Smit, and J. van der Tol, "High-bandwidth uni-traveling carrier waveguide photodetector on an InP-membrane-on-silicon platform," Opt. Express 24, 8290 -8301 (2016)).

[0057] The extended optical cavities (resonators) 113, 123 of each of the first and second mode-locked lasers 110, 120 comprise passive waveguide structures for confining the transverse components of the optical beam traveling back and forth within the optical cavity and for longitudinally guiding the optical beam along the extended optical cavity. The passive waveguide structures thus define the transverse modes of the optical beam within the optical cavity. Typically, the waveguide structures are configured to be single-mode, for example, but not limited to, by combining suitable waveguide geometries, materials, and cladding. For example, transverse multimode waveguide geometries are possible, and may be combined with mode converters, mode adapters, and / or mode filters disposed within the optical cavity to advantageously restore the single-mode regime. Care is generally taken to excite only the fundamental mode in the multimode waveguide geometries, for example, by suitable tapers. For example, it is typically beneficial to arrange a transverse multimode waveguide geometry in which the gain section 111, 121 and / or the saturable absorber section 112, 122 are coupled to the respective optical cavities 113, 123. The excited fundamental mode propagating along the transverse multimode waveguide in the gain section allows for an increased effective mode area of ​​the fundamental mode, thereby lowering the pulse peak fluence and increasing the maximum pulse peak power before reaching gain saturation. Expanding the transverse multimode waveguide in the gain section and / or the saturable absorber section also allows for better alignment tolerances with respect to hetero-integrated epitaxial layer stacks. Furthermore, widening the transverse multimode waveguide in the saturable absorber section typically allows for more efficient and rapid removal of photogenerated charge carriers. The majority of the waveguide structure is single-mode, and excited higher-order modes can again be efficiently filtered. However, a multimode waveguiding regime may be beneficial in at least other parts of the waveguide structure if it further reduces nonlinear absorption losses (e.g., multiphoton absorption effects, free carrier absorption) or allows a more flexible approach to controlling the intracavity dispersion characteristics.

[0058] In embodiments of the present invention, a waveguide structure is generally formed in a waveguide layer of the substrate 101, for example, using pulsed laser writing or photolithography and etching techniques known in the art. Such a waveguide layer of the substrate 101 is generally supported by an optically transparent lower cladding layer (e.g., silica or sapphire) and can have air as an upper cladding material or can comprise an upper cladding layer (e.g., silica). Furthermore, an optically transparent insulating cladding layer, for example, comprising a low-loss dielectric material compared to doped semiconductor materials, has the advantage of further reducing propagation losses of the waveguide. Thus, a waveguide structure may be provided on the substrate 101 as a waveguide, which supports and guides multiple longitudinal cavity modes within an extended spectral range (e.g., more than 100 longitudinal modes, e.g., more than 1000 longitudinal modes, spanning an optical range of more than 3 nm, e.g., more than 10 nm). It is preferable to do this with as little optical loss as possible. As a result, it is preferable to form the waveguide in a material that has a wide bandgap (e.g., greater than 2 eV at the center φ of the Brillouin zone) and low optical absorption losses, both linear and nonlinear, in the spectral region corresponding to the frequency comb generated by the mode-locked laser. Non-limiting examples of waveguide materials include silicon, silicon nitride, silicon carbide, gallium nitride, lithium niobium, tantalum oxide, ion-diffused glass, and chalcogenide glass, and waveguides formed in these materials have optical propagation losses of less than 3 dBcm. -1 Less than 1 dB cm of optical propagation loss, for example -1Wide-bandgap dielectric waveguide materials, such as silicon nitride or silicon, have the advantage of significantly less nonlinear multiphoton absorption and less free-carrier-related dispersion or absorption compared to narrow (direct) bandgap waveguide materials, such as InP. Therefore, circulating intracavity pulses are less distorted and absorbed in these preferred waveguide materials. As a result, the gain section of the corresponding mode-locked laser can be shortened in length, which is beneficial in terms of the amount of amplified spontaneous emission (ASE) noise that adversely affects the spectral purity of the generated frequency comb. Furthermore, extended optical cavities using wide-bandgap dielectric waveguide materials enable the implementation of hetero-integrated narrow-linewidth master lasers on the same substrate, achieving optical linewidths of, for example, 1 kHz, whereas the optical linewidths of lasers based on narrow (direct) bandgap waveguide materials, such as InP, are much larger, for example, on the order of at least several hundred kilohertz.

[0059] The waveguide structure may include a waveguide that may have straight and / or curved sections. In practice, long optical cavities 113, 123, e.g., greater than 1 cm, e.g., 1 cm to 3 cm, or even greater than 3 cm, can be obtained in a compact manner (e.g., small footprint on available substrate surface area) by folded or spiraled waveguide configurations (e.g., spiral-formed waveguide sections). Various waveguide types exist to define passive waveguide structures, such as, but not limited to, ridge waveguides, rib waveguides, and buried waveguides. Depending on the waveguide material and geometry, the passive waveguide structure of one or both extended optical cavities of a mode-locked laser may have a physical length greater than 1.0 mm. For example, it may be greater than 3.0 mm, e.g., greater than 1 cm, e.g., 1 cm to 3 cm, or even tens of centimeters long. In comparison, the total physical length of the active waveguide in the gain and saturable absorber sections of a mode-locked laser is typically about 1 mm.

[0060] Considering the example of an extended optical cavity based on a 400-nm-thick, 37.0-mm-long silicon waveguide (with a group index of approximately 3.90) and a 1.0-mm-long gain and saturable absorber section in III-V material (with a group index of approximately 3.95, which matches well with that of the silicon waveguide for efficient coupling) in a mode-locked laser using anticollision pulse topology, the total optical path length of one complete round trip of the pulse in the extended optical cavity is approximately 2 × (37.0 mm × 3.90 + 1.0 mm × 3.95) = 296.50 mm, corresponding to a comb spacing of approximately 1 GHz. In another example, the extended optical cavity of each ring-resonator-type mode-locked laser with a symmetric configuration (colliding pulse topology) is based on a 200-mm-long silicon nitride waveguide (with a group index of approximately 2.00), but the total length of the gain and saturable absorber section in III-V material (with a group index of approximately 3.95) is much shorter, at only 1.2 mm. This results in a total optical path length of approximately 404.74 mm for one complete pulse round trip in the extended optical cavity, corresponding to a theoretical comb spacing of approximately 742 MHz (cavity round trip time ca 1.35 ns). Experiments have measured a comb spacing equal to 755 MHz in this particular example. The discrepancy with the theoretical value is caused by a number of effects, including gain saturation, processing variations, and temperature effects.

[0061] The waveguide structure of the optical cavity does not need to be uniformly distributed longitudinally, but may include tapers and wider bends between different waveguide widths. Furthermore, passive optical elements such as spot-size converters, mode filters, and / or adapters, polarization rotators, etc. may be included. In this embodiment, for example, the waveguide of the extended optical cavity may include interrupted and / or tapered sections of smaller width where the gain section and / or saturable absorber section are provided. As a result, the guided optical mode associated with the longitudinal cavity mode is no longer supported and guided by the waveguide itself, but instead is coupled and guided into the additional waveguide structure provided in the gain section and / or absorber section. These tapered sections are preferably adapted for good mode coupling into the additional waveguide structure of the gain section and / or absorber section, thereby reducing coupling losses as part of the optical cavity losses. Furthermore, the tapered cross section is preferably adapted for good mode coupling over a wide spectral range, e.g., the spectral width of the frequency comb emitted by each mode-locked laser. In embodiments of the present invention, mode transitions between the passive and active waveguide structures of the extended optical cavity in the gain section, or in the gain and saturable absorber sections, may be achieved by vertical tapers, e.g., single-, double-, or multi-step vertical taper structures. For example, mode transitions from a silicon nitride passive waveguide of the extended optical cavity to an active waveguide in the gain region of a heterogeneously integrated layer stack of III-V materials may involve a two-step taper structure, in which the mode first transitions from the silicon nitride waveguide to a further waveguide patterned in an intermediate layer of hydrogenated amorphous silicon, and then from the further waveguide back to the active waveguide in the gain region.

[0062] Optical feedback is provided by reflector arrangements at both ends of the extended optical cavities 113, 123. The first reflectors 114, 124 of the corresponding extended optical cavities 113, 123 are arranged so that their transmissive apertures serve as the corresponding input ports 117, 127 of the mode-locked lasers 110, 120 and their reflective apertures are optically connected to the waveguide structures of the extended optical cavities. For example, the first reflectors 114, 124 may be provided as distributed Bragg reflectors (DBRs), e.g., as diffractive waveguide gratings formed in the waveguide structure, e.g., the waveguide core itself and / or the waveguide cladding adjacent to the core. The reflective and transmissive apertures of the first reflectors then correspond to the cross-section of the waveguide structure at the start and end points of the grating, respectively, with the start points located at the sides of the extended optical cavities. Alternatively, a waveguide loop mirror or a partially reflective multimode interferometer (MMI) may be provided as the first reflector 114 or 124. Similarly, a second reflector 115, 125 is provided at the opposite end of each extended optical cavity 113, 123 such that the reflective opening of the extended optical cavity 113, 123 is optically connected to its waveguide structure. As described for the first reflectors 114, 124, the second reflectors 115, 125 may also be provided as a DBR, e.g., a diffractive waveguide grating, a partially reflective MMI, or a waveguide loop mirror, respectively. Both the first reflectors 114, 124 and the second reflectors 115, 125 are broadband reflectors, e.g., their optical passbands extend over at least 3 nm (e.g., greater than 5 nm, e.g., greater than 10 nm) and spectrally overlap with the corresponding mode-locked longitudinal cavity modes of the first mode-locked laser 110 and the second mode-locked laser 120, respectively. The reflectivity of the first reflectors 114, 124 may be approximately 90% (e.g., 85%-95%), which is a good tradeoff between cavity mirror losses on the one hand and sufficient coupling efficiency for the injection-locked laser line on the other hand. Experimental verification has shown that only 1-5 microwatts of optical power injected into the extended cavities of the mode-locked lasers 110, 120 is sufficient to establish wavelength locking of the generated frequency comb.Higher injection power of the injection-locked laser line can be used to extend the locking range. For example, an extended locking range of up to 14 MHz has been demonstrated with 10 μW of intracavity injection power. Regarding the second reflectors 115, 125, their reflectivity may be in the range of 30% to 70%, e.g., approximately 50%. This lower reflectivity compared to the first reflector ensures that the pulse trains emitted by the mode-locked lasers 110, 120 have good peak power and / or pulse energy (e.g., pulses with pulse energies of up to 1 pJ for picosecond-long pulses and / or peak powers of up to 1 W), which can be further increased by subsequent pulse compression.

[0063] At least one (e.g., both) of the extended optical cavities 113, 123 may include a phase shifter (not shown) for actively adjusting the optical path length of the optical cavities 113, 123 upon application of an electrical signal. For example, heaters based on dissipative metals or doped trace silicon, or electrorefractive phase shifters based on free carrier dispersion effects, constitute suitable options for implementing intracavity phase shifters. Such phase shifters may be provided along the waveguide structure of the extended optical cavities and / or on the first or second reflectors. Fine-tuning the optical path length of the extended optical cavities by a phase shifter has the additional advantage of being able to correct deviations from the intended difference in optical path length between the extended optical cavities 113 of the first mode-locked laser 110 and the extended optical cavities 123 of the second mode-locked laser 120, which are typically caused by fabrication variability. This difference in optical path length necessarily translates into a corresponding difference in pulse repetition rate between the frequency combs generated by the first and second mode-locked lasers. Intentional differences in optical path length and corresponding repetition rate can be beneficial in dual-comb spectroscopy applications, where a small repetition rate offset is required to obtain multiple beat notes upon combining the two frequency combs on a photodetector. As an example, a pulse repetition rate tuning range of approximately 50 kHz / K can be obtained using heaters as phase shifters, and typical relative changes in each cavity length caused by fabrication variability can be approximately ±0.05%. Therefore, correction of the pulse repetition rate of at least one mode-locked laser 110, 120 within a range of approximately 1 MHz is feasible due to heating of the waveguide structure to a temperature difference of approximately 20 K. This corresponds to a maximum expected offset in the repetition rates of the two generated frequency combs (e.g., 2 × 0.05% × 1 GHz = 1 MHz) for a design target nominal repetition rate of 1 GHz.The intentional relative change in the cavity length of the first and second mode-locked lasers for the purpose of performing dual-comb spectroscopy measurements is preferably selected to be larger than the unintentional deviation caused by processing; for example, the intentionally introduced relative change in cavity length may be 0.2%.

[0064] The optical coupling element 105 may be part of the connecting device 102 for distributing the light provided by the master laser 130 to each of the first and second mode-locked lasers 110, 120, for example, by branching a single output waveguide into at least two different delivery waveguides 103 and 104. The coupling element 105 may be configured to equally split the optical output power provided by the master laser 130 into the separate delivery waveguides 103, 104. However, different coupling ratios may be provided for the delivery waveguides coupled to each input port of the mode-locked lasers. For example, a directional coupler (DC), an interference-based coupler (multimode interferometer MMI, Mach-Zehnder interferometer MZ), or a combination thereof may be used as the coupling element 105. The coupling element 105 may comprise two or more optical components and may be implemented, for example, by a sequence or cascade of optical components suitable for achieving optical coupling (e.g., MMI, MZ, DC, Y-junction, X-junction). The coupling ratio between the light provided by the master laser 130 and the coupled light sent to the mode-locked lasers 110, 120 may be adjustable by, for example, but not limited to, electro-optic phase adjustment of the waveguide portion of a coupling element, such as the outgoing waveguide portion of an X-junction or Y-junction, or the waveguide portion of a directional coupler or interference component (interferometer arm). An adjustable coupling ratio has the advantage that better control of the injected optical power can be achieved for each mode-locked laser. This may be important, for example, when the propagation losses along the delivery waveguides 103, 104 differ by a predetermined amount due to design asymmetries and / or the presence of additional optical components along the waveguides, or when the two mode-locked lasers have different injection locking thresholds. Furthermore, the coupling element 105 may distribute the light generated by the master laser 130 to two or more delivery waveguides in communication with the mode-locked lasers 110, 120. A further waveguide may receive the light coupled by the coupling element 105 to tap a portion of the light of the master laser 130 for monitoring, for example, via an internal or external photodetector coupled to the further waveguide.

[0065] Various other optical components may be optionally provided in the optical path of the connecting device 102. In particular, a variable optical attenuator 106 may be inserted in the optical path of the output waveguide connected to the master laser output port 136 to control the amount of optical output power delivered and injected into each mode-locked laser 110, 120. The variable optical attenuator may have an attenuation range between 20 dB and 50 dB. In combination with a 10% transmittance of the first reflectors 114, 124, at least 30 dB of extinction may be achieved for the narrow laser line provided by the master laser 130 for simultaneous injection locking of the two mode-locked lasers 110, 120. Thus, the master laser 130 may be advantageously operated in a regime providing an optical output power of 1 mW or more (e.g., approximately 10 mW) such that its RIN noise is strongly reduced and the associated laser linewidth is sufficiently narrow relative to the repetition rate of the generated frequency comb, e.g., at least 1 part per mille of the repetition rate, e.g., less than 0.1% of the repetition rate. At least 30 dB of attenuation then results in 1 μW to 10 μW within the extended optical cavity of each mode-locked laser, demonstrating that good optical injection locking and phase coherence transfer can be achieved between the injection line provided by the master laser 130 and the comb line of the generated frequency comb. Surprisingly, the inventors discovered that no optical isolator is required, either in the connecting device 102 (e.g., in the respective optical paths extending between the master laser output port 136 and each mode-locked laser 110, 120) or as part of the master laser 130 itself. While implementing such an optical isolator in a PIC is notoriously difficult, an optical isolator is usually strictly necessary to prevent partially reflected light from recoupling into the master laser 130 and disrupting its proper functioning by significantly broadening its narrow linewidth, resulting in loss of optical injection locking and phase coherence transfer from the master laser to the mode-locked laser.However, the inventors have realized that the strong optical attenuation experienced by the partially reflected light after two passes through attenuator 106, such as 40 dB or more, e.g., 60 dB or more, e.g., 60 dB to 120 dB, effectively protects master laser 130 from such disturbances and ensures stable emission without spectral broadening of the injection-locked narrow-linewidth laser line. Furthermore, the spectral filtering function inherent in the narrow-band cavity of master laser 130, or in a narrow-band external optical cavity coupled to master laser output port 136, effectively protects against interference with the narrow-linewidth operation of master laser 130 by pulses in the pulse train generated by the mode-locked laser and re-injected into master laser 130. One way to implement a variable optical attenuator is to provide a waveguide-based interferometer in which one interferometer arm is equipped with a phase shifter. Another way to implement a variable optical attenuator involves providing an electroabsorption device optically coupled to the output waveguide (e.g., a PIN junction or a PN junction formed across the output waveguide via doping) connected to the master laser output port 136. Furthermore, a frequency shifter 107 can be provided in the optical path of one of the delivery waveguides 103. This has the advantage that the injection-locked comb lines of the first and second mode-locked lasers 110, 120 can be offset by, for example, only a small fraction of the frequency comb's repetition rate, e.g., 1% to 1% of the repetition rate. Thus, injection locking of the frequency comb can be performed to comb lines located in the center instead of the sides of the frequency comb, and aliasing artifacts in the beat notes are eliminated or at least reduced. Possible frequency shifters may be implemented as integrated acousto-optic modulators (e.g., using surface acoustic waves) or phase shifters arranged in a single-sided band carrier suppression configuration.

[0066] 1, an optical combiner 108 is coupled to the second reflectors 115, 125 of each mode-locked laser 110, 120 to combine the two frequency combs generated by the first and second mode-locked lasers into the output 109 of the integrated dual frequency comb source 100. As the combiner, for example, a broadband directional coupler or a multimode interferometer can be used.

[0067] The master laser 130 is configured to generate a single continuous wave (CW) lasing line at its output port 136, whose wavelength is within the spectral range of the corresponding mode-locked longitudinal cavity modes of each of the two semiconductor mode-locked lasers 110, 120. The master laser 130 is further configured to generate a CW lasing line having a linewidth less than the frequency comb line spacing of both mode-locked lasers 110, 120, e.g., the linewidth of the CW lasing line is less than 1% of the repetition rate of the generated frequency comb, e.g., less than 1 MHz for a 1 GHz repetition rate of the generated frequency comb. The linewidth of the CW lasing line is less than the optical linewidth associated with the longitudinal cavity modes, e.g., preferably less than 100 kHz (e.g., about 10 kHz or less). In principle, any stable single-mode laser that meets the above-mentioned linewidth requirements and can deliver optical output power on the order of at least milliwatts or tens of milliwatts can be provided as the master laser 130 on the same substrate 101 as the first and second mode-locked lasers. However, in a preferred embodiment of the present invention, the master laser 130 comprises an extended optical cavity 133 formed in the same waveguide layer of the substrate 101 as the waveguide structures of the extended optical cavities 113, 123 of each of the mode-locked lasers 110, 120, and its optical path length is approximately equal to the length of the cavities of the first and second mode-locked lasers, e.g., differing by less than 1% therefrom. This has the advantage that the positions of the longitudinal cavity modes in the optical frequency space of the mode-locked laser closely match those of the master laser. As a result, the master laser 130 can provide a narrow-linewidth CW line for optical injection locking that is within the locking range even with low injected optical power (e.g., microwatts) in the optical cavities of each mode-locked laser. In addition, the frequency drift (particularly thermal drift) is approximately equal for the longitudinal cavity mode generated by the master laser 130 and for the locked longitudinal cavity modes of each of the first and second mode-locked lasers 110,120.

[0068] The master laser 130 is also preferably a semiconductor laser. In this embodiment, the master laser 130 comprises a gain section 131 and an extended optical cavity 133 having a highly reflective rear reflector 134 (e.g., a reflective loop mirror or DBR) and a partially reflective front reflector 135 (e.g., a partially reflective DBR or waveguide loop mirror). The gain section 131 may be an epitaxial material layer stack (e.g., an epitaxial III-V or II-VI layer stack, e.g., an epitaxial layer stack based on InP or CdSe / MgSe), which can be hetero-integrated in the PIC by, for example, adhesive bonding, molecular bonding, or transfer printing of the layer stack onto the substrate 101. Thus, the gain section 131 of the master laser 130 may comprise the same gain material as the gain sections 111, 121 of the mode-locked lasers 110, 120, but is not limited to this. Alternative embodiments may provide a hybrid integrated master laser (e.g., semiconductor laser) comprising or consisting of, for example, a monolithic epitaxial layer stack of III-V of II-VI composite materials die-bonded to a substrate. In yet another alternative embodiment, the master laser may be formed directly on the substrate and may comprise strained or strained and doped germanium. In contrast to the extended optical cavities 113, 123 of the mode-locked lasers 110, 120, the reflectors 134, 135 of the extended optical cavity 133 of the master laser preferably have a narrow optical bandwidth suitable for single-mode selection. If the reflector arrangement of the extended optical cavity 133 of the master laser 130 is not selective for single-mode operation, an external optical bandpass filter that allows selection of only one lasing mode of the master laser 130 may be provided as part of the connection device 102 and coupled to the master laser output port 136.

[0069] The first and second mode-locked lasers 110, 120 are configured for passive mode-locking, which has the advantage of eliminating the need for additional RF circuitry. This may be achieved by providing separate electrical contacts for the gain section and the saturable absorber section to supply the forward current and reverse bias voltage, respectively. An electrical isolation barrier is preferably provided between the contacts between the gain section and the absorber section to reduce crosstalk. However, some embodiments of the present invention may support hybrid mode-locking if the PIC is complemented with additional external RF circuitry to generate an oscillating electrical reference signal for repetition rate stabilization. For example, hybrid mode-locking may be supported by including RF traces to deliver the reference signal to the contact electrodes of the saturable absorber sections 112, 122.

[0070] Preferably, the mode-locked lasers 110, 120 are arranged according to an anti-collision pulse configuration. To this end, the saturable absorber sections 112, 122 are positioned relative to the extended optical cavities 113, 123 so that they are closer to or at least partially above the low-reflectivity second reflectors 115, 125 than the high-reflectivity first reflectors 114, 124. The anti-collision configuration has the advantage of achieving higher output power, lower timing jitter, and improved RF spectral purity compared to the anti-collision pulse configuration. Furthermore, the extended optical cavities arranged in the anti-collision pulse configuration ensure that a single circulating pulse is obtained within the cavity. A further advantage of the anti-collision pulse configuration is that the injection-locking laser line provided by the master laser is injected into the optical cavities 113, 123 on the side adjacent to the gain sections 111, 121 instead of the saturable absorber sections 112, 122. This has the effect that the laser line injected for locking is then amplified in the gain medium and modulates the gain more efficiently compared to injection on the side adjacent to the saturable absorber sections 112, 122. Therefore, lower optical powers for injection locking can be obtained and back reflections to the master laser 130 are less severe.

[0071] Nevertheless, other embodiments of the present invention may include a mode-locked laser arranged according to a colliding pulse configuration (e.g., a symmetric configuration) in which the saturable absorber section is centrally located with respect to each extended optical cavity. Furthermore, mode-locked lasers according to a symmetric configuration typically have a separate gain section on each side of the centrally located saturable absorber section. For example, a symmetrically configured mode-locked laser may display a three-section hetero-integrated III-V material-based active region, with the saturable absorber region forming a central section adjacent to two lateral electrically isolated gain sections. This symmetric configuration can be applied to extended optical cavities of not only ring resonator type but also linear type.

[0072] Performance characteristics of the frequency comb produced by the exemplary hetero-integrated III-V on SOI mode-locked laser according to embodiments of the present invention include a frequency comb spectral width of at least 12 nm (e.g., measured as a 10 dB width) and more than 1000 comb lines (e.g., more than 1400 comb lines) separated by 1 GHz line spacing. The comb lines have good spectral purity, a good peak signal-to-noise ratio (SNR) relative to the noise floor, and a low-frequency range in the RF spectrum associated with the comb lines below the fundamental. For example, the corresponding RF spectral fundamental of the comb may have a spectral 10 dB linewidth of less than 100 kHz (e.g., less than 10 kHz, e.g., less than 1 kHz) and a peak SNR of more than 50 dB, providing the basis for stable mode-locking with little pulse intensity noise and timing jitter. An exemplary hetero-integrated epitaxial InP-based layer stack on an SOI mode-locked laser can consume as little as 140 mW of power during operation. More generally, embodiments of the present invention may provide semiconductor mode-locked lasers 110, 120, with the length of the extended optical cavity 113, 123 corresponding to a pulse repetition rate of less than 20 GHz (e.g., less than 10 GHz, e.g., less than 5 GHz, e.g., about 1 GHz or less) in the generated frequency comb. The linewidth in the RF spectrum associated with the frequency comb, i.e., the detection of beat notes in the RF domain, is preferably less than 1 MHz (e.g., less than 100 kHz, e.g., less than 10 kHz, e.g., 1 kHz or less, e.g., in the sub-Hz region). The optical bandwidth of the generated frequency comb spectrum may be greater than 100 GHz (e.g., greater than 300 GHz, e.g., greater than 1 THz). The comb spectrum may be flat-peaked and may comprise 100 or more (e.g., 1000 or more, e.g., 1400 or more) spectroscopy comb lines. Here, a frequency comb spectrum is considered to be peak-flat across its optical bandwidth if the variation in the amplitude (e.g., peak spectral power density) of the comb lines across the comb does not vary by more than 3 dB with respect to a certain upper level.The optical linewidth of each comb line, when integrated over a 5 μs time interval, is generally less than 5 MHz (e.g., less than 1 MHz, e.g., less than 500 kHz, e.g., 100 kHz or less). However, to obtain mode-locking by optical injection locking, it is sufficient that only one comb line of each mode-locked laser locked to the CW lasing line of the master laser (regardless of whether additional frequency shifts are applied) has an optical linewidth of less than 5 MHz (e.g., less than 1 MHz, e.g., less than 500 kHz, e.g., 100 kHz or less). The one comb line of each locked mode-locked laser may correspond to the comb line with the smallest linewidth throughout the comb, e.g., the comb line closest to the frequency locking point associated with the dominant noise source. Generally, the optical linewidth associated with a comb line increases quadratically with mode number n, starting with the comb line with the smallest linewidth as n=0.

[0073] While the embodiment of FIG. 1 concerns a linear resonator for two mode-locked lasers, these may be implemented as ring lasers operating in the pulsed regime when mode-locked. In the ring laser configuration, the extended optical cavities of the first and second mode-locked lasers comprise a closed-loop waveguide structure without a reflector. The output waveguide is optically coupled to the closed-loop waveguide structure, for example, via a directional coupler or MMI disposed within the closed-loop waveguide structure to couple the generated frequency comb. The saturable absorber section of each mode-locked laser may then be interposed between the two gain sections according to a colliding pulse configuration, for example, by disposing the saturable absorber section midway within the extended optical cavity relative to the coupling section with the output waveguide. A master laser may be coupled to one end of each output waveguide of the first and second mode-locked lasers, while the opposite end of the same output waveguide directs the generated frequency comb to the output of the PIC, for example, a cleaved fiber that may be anti-reflection coated.

[0074] During operation, the gain sections 111, 121 of the first and second mode-locked lasers 110, 120 are each supplied with a forward current exceeding the laser threshold current. For example, a forward bias current of 60 to 120 mA may be supplied to each of the III-V group on SOI type mode-locked lasers. Furthermore, a reverse bias voltage is applied across the saturable absorber sections 112, 122 of each of the mode-locked lasers 110, 120. For example, a reverse bias voltage bias of -2 V to -3 V may be applied to each of the III-V group on SOI type (e.g., InP-based) mode-locked lasers. The selected forward current and reverse bias voltage are such that each of the two semiconductor mode-locked lasers 110, 120 spontaneously switches into the mode-locked regime. Because the supplied forward current and reverse bias voltage contain only DC components, passive mode-locking is achieved. However, in certain embodiments of the present invention, to enable hybrid mode-locking, an RF component may be added, for example, via a bias T, to the DC component of either the forward current (current modulation) or the reverse bias voltage (absorption modulation) of at least one of the mode-locked lasers 110, 120. Such an RF signal may be synthesized from a 10 MHz hydrogen maser reference signal to directly relate the repetition rate of the frequency combs generated by the first and second mode-locked lasers to the frequency reference. Hybrid mode-locking has the advantage of further stabilizing the repetition rate of the generated frequency combs and producing a narrower RF linewidth upon beat note detection. Furthermore, the master laser 130 is supplied with a current above the laser threshold and begins to emit a narrow CW laser line, which is then injected into the extended optical cavities 113, 123 of the two mode-locked lasers 110, 120 to achieve injection locking and therefore mutual coherence of the two mode-locked lasers 110, 120. The first and second mode-locked lasers are preferably first set in pulsed lasing mode before optical injection locking is performed. The phase coherence and wavelength of the CW lasing line generated by the master laser 130 is transferred to each mode-locked laser 110, 120, thereby simultaneously achieving carrier offset frequency stabilization and mutual coherence of the two generated frequency combs.At least one of the semiconductor mode-locked lasers 110, 120 may be locked to a higher harmonic, i.e., one or more circulating pulses are established in the corresponding extended optical cavity 113, 123, which leads to an increase in the pulse repetition rate of the emitted frequency comb. Higher repetition rates may be useful for optical communication applications or LIDAR. This can be achieved without compromising the good spectral purity of the beat tone in the RF spectrum associated with the frequency comb. For example, spectral linewidths of several kHz are possible.

[0075] Optionally, the optical power injected into each mode-locked laser can be adjusted using, for example, at least one variable optical attenuator, which may be part of the connecting device 102 described above with respect to embodiments of the present invention. Furthermore, the optical power injected into each mode-locked laser can be adjusted by adjusting the coupling ratio of an adjustable coupling element, which may be part of the connecting device 102 described above with respect to embodiments of the present invention. The wavelength of the narrow CW laser line for injection locking emitted by the master laser 130 can be frequency-shifted for only one of the two mode-locked lasers, for example, using a frequency shifter 107 in one of the delivery waveguides 103, 104. This has the advantage that the two comb lines in the generated mutually coherent frequency comb, which are injection-locked to the CW laser line of the master laser 130, do not coincide, i.e., do not have the same wavelength. As a result, the left-right degeneracy of the beat-note interferometer that exists for two mutually coherent, aligned frequency combs during heterodyne detection, is eliminated by intentionally introducing misalignment of the two injection-locked comb modes. Therefore, more beat notes can be clearly detected with dual-comb spectroscopy, which allows for a wider spectral detection range. Even with two frequency combs locked with comb lines located on either side of each comb, the amount of aliasing obtained when detecting beat notes can be further reduced if a frequency shifter is used.

[0076] In other embodiments of the present invention, separate frequency shifters may be disposed in each of the delivery waveguides 103, 104, or alternatively, in only one of the delivery waveguides and in the waveguide portion of the connecting device common to the delivery paths of the CW lasing light to the first and second mode-locked lasers (e.g., a single output waveguide connected between the output port 136 of the master laser and the coupling element 105). In such embodiments, one of the frequency shifters may be used to achieve better spectral alignment of the CW lasing line emitted by the master laser with the resonant mode of one of the mode-locked lasers, while the other frequency shifter serves the purpose of controlling the mistuning of the comb lines constituting the first frequency comb compared to the comb lines constituting the second frequency comb by injecting and frequency locking the resonant mode of the other of the mode-locked lasers. In still other embodiments, a single frequency shifter may be disposed in the first and second mode-locked lasers (e.g., a single output waveguide connected between the output port 136 of the master laser and the coupling element 105). This has the advantage that better spectral alignment of the CW lasing line emitted by the master laser with the resonant modes of one or both of the mode-locked lasers can be achieved, as discussed above. This may be particularly important in embodiments in which the CW lasing line emitted by the master laser is not tunable. Furthermore, an actuator may be provided as part of a feedback loop configured to receive information regarding drift of the generated frequency comb and adjust the control signal for setting the frequency shift of the frequency shifter accordingly. This can further improve the long-term stability of the dual frequency comb source.

[0077] FIG. 2 illustrates a second embodiment of an integrated dual frequency comb source 200. This embodiment differs from the previous embodiment in that the outputs of the first and second mode-locked lasers 110 and 120 are not coupled to each other via an output combiner. Instead, output waveguides 203 and 204 are connected between each of the two mode-locked lasers 110 and 120 and the corresponding outputs 209a and 209b of the dual frequency comb source 200. This has the advantage that the two generated frequency combs can be accessed independently of each other. The output waveguides 203 and 204 can be tilted with respect to the output facet or cut surface of the dual frequency comb source 200, which is implemented as a PIC. This has the advantage of reducing spurious reflections and preventing unwanted optical feedback by re-entering the extended optical cavities of the mode-locked lasers 110 and 120. Therefore, spurious pulse trains can be suppressed. As an alternative to this embodiment, a truncated facet of the PIC may be used as a second reflector for at least one extended cavity of the mode-locked lasers 110, 120, with this truncated facet also serving as the corresponding output 209a, 209b of the dual frequency comb source 200. Such an alternative output waveguide is not required between the mode-locked laser and each output of the dual frequency comb source, thereby reducing propagation losses and improving outcoupling efficiency. Furthermore, two individual variable optical attenuators 206a and 206b are inserted into each optical path connecting the master laser 130 with each one of the mode-locked lasers 110, 120. This has the advantage that differences in required optical injection power or locking range caused by processing variability can be compensated for individually for each mode-locked laser 110, 120. The additional attenuators 206a, 206b may be used to reduce the minimum extinction ratio of the attenuator 106 closest to the master laser 130, compared to having only a single attenuator 106, or to replace it with a non-variable attenuator. As an alternative to this embodiment, the common attenuator 106 closest to the master laser 130 may be omitted entirely.

[0078] The insertion of additional attenuators 206a, 206b is not limited to this embodiment, and the above-mentioned modifications may also be applied to other embodiments of the present invention. Although less preferred due to higher propagation losses, embodiments of the present invention may be implemented in common InP-based platforms, such as monolithically integrated PICs in InP-based materials, using active-passive integration.

[0079] FIG. 3 illustrates an integrated system 300 for dual frequency comb spectroscopy. Such a system can be used for gas absorption spectroscopy. The system 300 includes a dual frequency comb source as described above for the first embodiment of FIG. 1. As described above with respect to embodiments of the present invention, the first and second generated frequency combs have slightly different repetition rates (e.g., by intentionally differentiating the optical path lengths of the corresponding optical cavities 113, 123) (e.g., by selecting a relative difference of less than 1% between the optical cavity lengths of the first and second mode-locked lasers 110, 120). Additionally, the system 300 includes a sensing region 301 where one of the generated frequency combs interacts with surrounding gas molecules, and a balanced photodetector 302. For example, the sensing region 301 includes a spiral waveguide section along which surrounding gas molecules interact with the evanescent tail of the guided train of pulses. As a result of the interaction of the gas molecules with the frequency comb used for sensing, comb lines that overlap with the spectral absorption features of the gas molecules are reduced in intensity. The combiner 108 combines the corrected frequency comb for detection with a reference frequency comb that has been prevented from interacting with gas molecules. Detection on a balanced photodetector 302 connected to the output of the combiner 108 creates an interferogram of beat notes that can be recorded and analyzed in the RF domain, e.g., in an RF spectrum analyzer electrically connected to the output of the photodetector 302. Signal processing means (e.g., a microprocessor that may be integrated with the system 300) then performs a Fourier transform of the interferogram to reconstruct the absorption spectrum in the RF domain, which can then be referenced back to the optical domain. This referencing to the optical domain is achieved by comparing the injection-locked comb lines to a predetermined reference wavelength, e.g., obtained from a calibration measurement, and by controlling the temperature of the integrated spectroscopy system within a tight range (e.g., controlling the temperature within ±0.1 K) to avoid mode hops in the master laser. The predetermined reference wavelength may be the calibrated wavelength of the CW lasing line of the master laser 130 compared to a wavelength standard (e.g., a molecular absorption line of a gas such as acetylene).Alternatively, the CW lasing line produced by the master laser 130 can be tracked or stabilized relative to a wavelength reference (e.g., a molecular absorption line of a gas such as acetylene) by tapping a portion of the optical power of the CW lasing line produced by the master laser into a gas cell and detecting the relative change in transmittance, which represents wavelength drift. The gas cell may be on-chip, for example, integrated within a PIC, or off-chip.

[0080] The broad and dense frequency combs (e.g., 1000 or more comb lines spaced approximately 1 GHz apart) generated by the first and second mode-locked lasers 110, 120 allow for the resolution of vibrational absorption features impinging on and spreading out of gas at atmospheric pressure. Because the two frequency combs are mutually coherent due to injection locking and their comb lines drift by the same amount (corresponding to the drift of the injection lasing line of the master laser 130), no additional stabilization mechanism is required to enable high-speed parallel broadband molecular spectroscopy. Another advantage of embodiments of the present invention is that spectroscopic measurements can be performed without requiring scanning of either of the two generated frequency combs.

[0081] Other integrated dual-comb frequency spectroscopy systems are possible in which the sensing region 301 is not provided in the substrate 101 itself, but instead the optical path for sensing extends into the ambient space and / or into an external device (e.g., a single-pass or multi-pass gas cell) coupled to an input / output port of the system 300. Furthermore, the detection of beat notes and recording of the interferogram may be performed on an external photodetector, for example, using the complementary output of the combiner 108.

[0082] Referring to FIG. 4, a dual frequency comb source 400 based on an integrated optical circuit is shown. This is a variation of the dual frequency comb source 100 described with reference to FIG. 1. The dual frequency comb source 400 based on an integrated optical circuit differs from the dual frequency comb source 100 of FIG. 1 in that the variable optical attenuator 106 is replaced with a Vernier-type optical filter 406. This Vernier-type optical filter 406 may be implemented as two microring resonators connected in series, with the free spectral ranges of the two microring resonators slightly mismatched, resulting in a combined free spectral range of the Vernier-type filter 406 that is a multiple of the free spectral ranges of both microring resonators (the Vernier effect). The combined free spectral range of the Vernier-type filter 406 can exceed the individual free spectral ranges of the microring resonators by an order of magnitude, and values ​​for the combined free spectral range of 500 GHz up to several THz (e.g., 2-3 THz) are easily obtainable. Other implementations of the vernier optical filter 406 are also possible, for example, by a cascaded interferometer such as a cascaded Mach-Zehnder interferometer. During operation, the transmittance characteristics (e.g., peak) of the vernier optical filter 406, which may be wavelength tunable, are spectrally matched to the wavelength of the CW laser line emitted by the master laser 130. This can also be achieved by designing the vernier optical filter so that its transmittance characteristics (e.g., peak) match the wavelength of the emitted CW laser line, by wavelength tuning the master laser 130, by frequency shifting the CW laser line emitted by the master laser with a frequency shifter of a connecting device inserted between the master laser output port and the vernier optical filter, or a combination thereof. Similar to the variable optical attenuator 106, the vernier optical filter 406 can be inserted into the optical path of the output waveguide connected to the master laser output port 136 to obtain strong optical attenuation of the counter-propagating light re-injected into the master laser 130.The counterpropagating light to be attenuated primarily relates to the comb lines of the frequency combs generated by the first and second mode-locked lasers 110 and 120 and passing through their respective ports 117 and 127. However, in embodiments that provide one or more frequency shifters 107 as part of the connecting device 102, it may also include partially reflected and frequency-shifted CW laser light. This effectively protects the master laser 130 from disturbances caused by the operation of the mode-locked lasers 110 and 120, ensuring stable emission without spectral broadening of the narrow linewidth laser line due to injection locking. With respect to the vernier-type optical filter 406, strong optical attenuation can be defined as an extinction ratio measurable from its transmittance profile. A practically achievable extinction ratio may be at least 25 dB, e.g., 25 dB to 60 dB. While the optical attenuator is not part of the connecting device 102 in this embodiment, it is possible to combine the vernier-type optical filter 406 with one or more optical attenuators that can be variable in the optical path of the same connecting device, as described, for example, in the embodiments associated with FIG. 1 or FIG. 2.

[0083] Figure 5 shows a portion of a recorded spectrum (approximately 50%) of a frequency comb emitted by one of the semiconductor mode-locked lasers of an integrated dual frequency comb source according to one embodiment of the present invention. The semiconductor mode-locked laser is provided as a heterointegrated III-V on SOI laser (e.g., 400 nm thick silicon) and includes a 700 μm-long gain section with six InP / InGaAsP quantum wells in the active layer and an extended optical cavity based on passive silicon waveguides (e.g., 2 cm to 4 cm long) or silicon nitride waveguides (e.g., 4 cm to 8 cm long). The spectrum was obtained by measuring beat notes in the RF domain generated by optical heterodyne detection when the frequency comb emitted by the mode-locked laser was coupled to an externally generated electro-optic (EO) frequency comb on a balanced photodetector. Here, the EO frequency comb serves as a local oscillator array for downconversion to the RF domain. The same CW lasing line of the master laser used for injection locking of the mode-locked laser also serves as the carrier and seed line of an EO frequency comb generator (e.g., an intensity modulator coupled to a highly nonlinear optical fiber), with its line spacing set slightly different from the repetition rate of the semiconductor mode-locked laser frequency comb (e.g., 0.5 MHz difference in comb line spacing). This ensures that the two frequency combs, i.e., those generated by the semiconductor mode-locked laser and the EO comb generator, are mutually coherent. The measured beat notes (average interferograms acquired over 1200 consecutive acquisitions with a duration of 100 μs) are then Fourier transformed and converted back to optical frequencies to recover the spectrum. Here, the strong beat note between the injection-locked mode of the launch frequency comb coupled to the injection-locked lasing line as generated by the master laser and the carrier wavelength of the externally generated EO frequency comb is used as a landmark to convert the recorded RF spectrum back to the optical domain.Inspection of the recorded spectrum reveals that the frequency comb, limited by the spectral range of the externally generated EO frequency comb (e.g., 50% overlap with the frequency comb generated by the mode-locked laser), counts 660 comb lines spanning 666 GHz. The spectrum further demonstrates that the frequency comb generated by the mode-locked laser is smooth and flat-peaked. The pulse repetition rate, due to the comb line spacing, is approximately 1.01 GHz. The full spectrum of the frequency comb generated by the mode-locked laser is even broader, spanning 1.17 THz and containing 1154 comb lines.

[0084] The present invention also relates to a method for performing dual frequency comb spectroscopy on a sample containing a species to be detected. The species for detection may be gas molecules at atmospheric pressure or above atmospheric pressure. The method includes providing an integrated dual frequency comb source according to the above-described embodiment and a sensing region for light-matter interaction between the species to be detected and one of the frequency combs generated by the integrated dual frequency comb source. The sensing region may be constituted by the integrated dual frequency comb source (e.g., in the optical path of only one of the first and second generated frequency combs) or may be external to the integrated dual frequency comb source (e.g., an absorption cell or a free-space region). Next, first and second mode-locked lasers are injection-locked to the lasing wavelength provided by the master laser. In a further step, the sample is brought into contact with the sensing region to provide light-matter interaction between the species and the first of the two frequency combs generated by the integrated dual frequency comb source, the second of which serves as a reference frequency comb. An interferogram including multiple beat lines is then recorded upon detection of the combination of the first and second frequency combs.

[0085] Figure 6 shows the confirmed absorption features AF1-AF3 and the CW lock line in the dual-comb spectroscopy recording. A dual-frequency comb interferometer system for spectroscopy was used, consisting of a dual-frequency comb source based on an integrated optical circuit similar to that shown in Figure 2, an external detection region, and an external photodetector for recording the beat lines in the time-domain interferometer. The external detection region corresponds to the CO2-filled absorption gas cell. The spectral recording was generated by averaging 90 μs long-time slices over a total acquisition time of 17.5 ms, followed by inverse Fourier transformation and back conversion to the optical frequency domain. More than 700 spectral lines, e.g., up to 800, could be distinguished. In the dual-frequency comb spectroscopy experiment, the wavelength of the injection-locked CW laser line was 1598.00 nm, the repetition rate difference between the two mode-locked lasers was set to 253 kHz, and a 75 MHz frequency shift was applied to one branch of the CW laser line by a frequency shifter. The frequency spacing between adjacent comb lines in each of the two generated frequency combs was approximately 1.01 GHz, which is also equal to the resolution achieved with the dual-comb interferometer in that experiment, and a Lorentzian fit to the corresponding RF spectral lines demonstrated a FWHM of less than 20 Hz with a bandwidth of 100 Hz resolution.

[0086] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be interpreted as limiting the scope.

Claims

1. A mutually coherent dual frequency comb source (100) based on an integrated optical circuit, comprising, on the same substrate (101) of the integrated optical circuit: first and second mode-locked lasers (110, 120) for generating first and second frequency combs, respectively, each of the first and second mode-locked lasers comprising: a gain section and a saturable absorber section for providing mode-locking; and an extended optical cavity (113, 123) having a passive waveguide structure formed in a waveguide layer of the substrate (101), wherein at least one resonant mode of each extended optical cavity has a spectral bandwidth of less than 5 MHz with a 5 μs integration time; and at least the gain section is provided as a hetero-integrated III-V material layer stack bonded to the substrate (101); a master laser (130) that generates a continuous wave lasing line for simultaneous optical injection locking of the first and second mode-locked lasers, the master laser (130) being configured to generate the continuous wave lasing line having a linewidth narrower than a frequency spacing between modes of the first and second frequency combs; a connection device (102) interposed between an output port (136) of the master laser and respective input ports (117, 127) of each of the first and second mode-locked lasers to coherently transmit lasing light generated by the master laser to each mode-locked laser.

2. 2. The dual frequency comb source (100) of claim 1, wherein the extended optical cavity (113, 123) of each of the first and second mode-locked lasers (110, 120) is arranged as a linear optical resonator or as a ring resonator.

3. 3. The dual frequency comb source (100) of claim 1 or 2, wherein the master laser (130) comprises an extended optical cavity (133) formed in a waveguide layer of the substrate (101) that is identical to the passive waveguide structures of the extended optical cavities (113, 123) of the first mode-locked laser (110) and the second mode-locked laser (120).

4. 4. The dual frequency comb light source (100) of claim 3, wherein the optical path length of the extended optical cavity (133) of the master laser differs from the corresponding optical path length of any one of the extended optical cavities (113, 123) of the mode-locked laser by less than 1%.

5. 5. The dual frequency comb source (100) of claim 1, wherein the connecting device (102) comprises: a coupling element (105) in optical communication with the output port (136) of the master laser; and at least two delivery waveguides (103, 104) optically connected between the coupling element and the input ports (117, 127) of the first and second mode-locked lasers.

6. 6. The dual frequency comb source (100) of claim 1, wherein the gain section (111, 121) of each of the first and second mode-locked lasers (110, 120) is configured for injection of an electrical bias current, and / or the saturable absorber section (112, 122) of at least one of the first and second mode-locked lasers (110, 120) is configured to receive a radio frequency signal modulated at a rate corresponding to a frequency spacing between adjacent comb lines of a frequency comb generated by the mode-locked lasers.

7. 7. The dual frequency comb source (100) according to claim 1, wherein the gain section (111, 121) and the saturable absorber section (112, 122) of at least one of the first and second mode-locked lasers are made from different semiconductor materials, or the gain section (111, 121) and the saturable absorber section (112, 122) of at least one of the first and second mode-locked lasers are formed in the same hetero-integrated III-V material bonded to the substrate (101).

8. 8. The dual frequency comb source (100) of claim 1, wherein each of the first and second mode-locked lasers includes a corresponding first reflector (114, 124) disposed at the input port (117, 127), and the reflectivity value of the first reflector is between 85% and 95%.

9. 9. The dual frequency comb source (100) of claim 1, wherein the connecting device further comprises at least one frequency shifter (107) disposed between the output port (136) of the master laser and the input port (127) of at least one of the first and second mode-locked lasers.

10. The dual frequency comb source (100) according to any one of claims 1 to 9, characterized in that the connecting device (102) does not comprise any optical isolator.

11. 11. The dual frequency comb source (100) of claim 1, wherein the connecting device (102) further comprises at least one optical attenuator (106) for controlling the amount of optical power injected into each of the first and second mode-locked lasers.

12. 12. The dual frequency comb source (100) of claim 1, wherein at least one of the first and second mode-locked lasers is configured to generate a frequency comb with a comb spacing of less than 10 gigahertz, or the passive waveguide structure of the extended optical cavity of at least one of the first and second mode-locked lasers has a physical length of more than 1.0 mm.

13. The dual frequency comb source (100) of any one of claims 1 to 12, wherein the connecting device (102) further comprises a Vernier-type optical filter (406).

14. A dual frequency comb source (100) according to any one of claims 1 to 13, wherein each resonant mode of each extended optical cavity has a spectral bandwidth of less than 1 MHz with an integration time of 5 μs.

15. An integrated system (300) for dual frequency comb spectroscopy, comprising: A dual frequency comb light source (100) based on an integrated optical circuit according to any one of claims 1 to 14, a photodetector (302) positioned to receive the combined frequency comb of the first and second mode-locked lasers; a sensing region (301) in an optical path extending between only one of the first and second mode-locked lasers and the photodetector, the sensing region configured to allow photo-matter interaction between the generated frequency comb and a species to be sensed; An integrated system (300), wherein a difference in comb spacing between the generated first and second frequency combs is within a detectable radio frequency range of the photodetector.

16. 16. The integrated system of claim 15, wherein the sensing region comprises a waveguide portion configured for sensing evanescent waves between the generated frequency comb and the sensed species guided by the waveguide portion.

17. 17. The integrated system of claim 15 or 16, further comprising processing means for detecting and analyzing beat lines in an interferogram produced by the photodetector in response to interference of the received first and second frequency combs.

18. An integrated system according to any one of claims 15 to 17, characterized in that the integrated system is provided as an optical integrated circuit.

19. 1. A method of performing dual frequency comb spectroscopy on a sample containing a species to be detected, comprising: providing an integrated-optics-based dual frequency comb source (100) according to any one of claims 1 to 14, wherein a repetition rate of a first generated frequency comb is different from a repetition rate of a second generated frequency comb, and the first generated frequency comb and the second generated frequency comb are generated by a first mode-locked laser (110) and a second mode-locked laser (120) of the integrated-optics-based dual frequency comb source (100), respectively; optically injection locking the first and second mode-locked lasers to the wavelength of the continuous wave lasing light by coherently transmitting the continuous wave lasing light generated by the master laser from an output port (136) of the master laser to respective input ports (117, 127) of each of the first and second mode-locked lasers; directing the first generated frequency comb through a sensing region configured to receive the species to be sensed and to allow optical material interaction between the frequency comb and the species; contacting the sample with the sensing area; detecting a beat note in an interferogram produced when the first and second generated frequency combs are superimposed on a photodetector, wherein the first generated frequency comb is interacting with the species in the sample.

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