Optical circuit arrangement

Abstract

An optical circuit (100) for a laser device, the optical circuit (100) comprising: a first polarization-maintaining optical cavity (110a) comprising: a first gain medium (140a) excitable by a first pump light source (130a) to generate light in a first wavelength range; and a first saturable absorber 170a configured to perform passive mode locking of an optical pulse in the first polarization maintaining optical cavity (110a); and a second polarization-maintaining optical cavity (110b), the second polarization-maintaining optical cavity comprising: a second gain medium (140b) different from the first gain medium (140a), the second gain medium being excitable by a second pump light source (130b) to generate light in a second wavelength range; and a second saturable absorber (170b) configured to perform passive mode locking of an optical pulse in the second polarization-maintaining optical cavity (110b), where the first polarization-maintaining optical cavity (110a) and the second polarization-maintaining optical cavity (110b) share a common branch (120), and where the common branch (120) does not include a saturable absorber.

Description

Technical Field

[0001] The present invention relates to an optical circuit arrangement, and more particularly, to an optical circuit arrangement for a coherent Raman microscopy laser device. Background Art

[0002] Raman spectroscopy enables label-free chemical characterization of tissues and cells. It is based on the Raman scattering effect of molecules and uses a single continuous-wave laser. Such spontaneous Raman scattering is weak, so Raman spectroscopy is usually slow. Coherent Raman spectroscopy (CRS), including coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS), relies on the nonlinear excitation of molecules and can increase the Raman intensity by several orders of magnitude. In theory, this increase in Raman intensity allows measurements to be made at video-rate imaging speeds, which theoretically means that CRS can be used in a variety of applications in many different fields.

[0003] CRS can be implemented in a narrowband or broadband manner, where narrowband means exciting a single vibration frequency at a time, and broadband means exciting multiple vibration modes simultaneously. The narrowband method is usually referred to as "hyperspectral CRS" and is based on a narrowband tunable laser source that allows the reconstruction of the vibration spectrum by continuously acquiring the response of the system at different Raman modes. The broadband method is usually defined as "multiplex CRS" and relies on a combination of broadband and narrowband optical pulses to parallelize the Raman modes that can be simultaneously excited and detected. Multiplex CRS has the great advantage of single-shot spectral acquisition, enabling fast and chemically selective imaging to be achieved simultaneously. Since there is no non-resonant background, multiplex SRS imaging is a particularly beneficial technique.

[0004] CRS requires the use of synchronized ultrafast lasers (at least picosecond level) from two laser sources, where the pump pulse and the Stokes pulse are matched to the Raman frequency and bandwidth to establish and detect the vibrational coherence inside the sample. Currently, solid-state lasers pumped by optical parametric oscillators have been widely used as the laser source for CRS because these laser sources can cover the entire Raman spectrum (0 cm -1 -4000 cm -1 ). Such solid-state laser devices include large doped crystals or glasses as the gain medium and require the use of bulky optical devices. Therefore, they are not only vulnerable to alignment errors, prone to instability, but also incur high usage costs. In addition, their relatively large footprint hinders their effective deployment in a clinical environment. For example, they cannot be easily moved between different wards in a hospital and are not convenient to carry.

[0005] In recent years, the use of fiber-format lasers has become increasingly popular because such laser devices provide a simpler, more cost-effective, and smaller-footprint excitation source. They are also more reliable and do not require alignment compared to solid-state lasers that pump optical parametric oscillators.

[0006] U.S. Patent No. US7,372,880 discloses a pulsed fiber laser capable of generating ultrashort optical pulses. The pulsed fiber laser includes an optical ring resonator having a length of rare-earth doped fiber as a gain medium. In use, the gain medium responds to a pump light source to generate optical gain in the resonator. To facilitate pulse generation, carbon nanotubes (CNTs) are used as a nonlinear optical material or a saturable absorber material to convert continuous-wave laser into an ultrafast optical pulse train. A saturable absorber is an optical component having a certain optical loss that decreases at high light intensities. Each time the pulse hits the saturable absorber while circulating in the optical ring resonator, the absorption of the saturable absorber reaches saturation, thereby temporarily reducing the loss. In each resonator round-trip, the saturable absorber favors the light with a higher intensity because this light can saturate the absorption slightly more than the light with a lower intensity. After multiple round-trips, only a single pulse will remain.

[0007] Research in this field is dedicated to using passive mode-locking techniques to achieve synchronization of dual-wavelength ultrafast laser sources to generate synchronized optical pulses from two different laser sources. This technique requires the use of a common saturable absorber shared by the two laser sources, such as optically coupling the common saturable absorber to two fiber cavities doped with different rare-earth materials.

[0008] In the article "Passive synchronization of all-fiber lasers through a common saturable absorber" published by Zhang et al. in Optics Communications (2011) (Zhang), the synchronization of two all-fiber mode-locked lasers is disclosed. The two all-fiber mode-locked lasers operate at 1 μm and 1.54 μm and are coupled by using a shared single-wall carbon nanotube absorber. In addition, in the article "Ultrafast fibre laser sources: Example of recent developments" published by Zhang et al. in Fibre Optics Technology (2014), the recent developments in the field of ultrafast compact all-fiber lasers are summarized. More specifically, Zhang discloses the use of graphene and single-wall carbon nanotubes as passive elements to perform the synchronization and passive mode-locking of laser pulses in two coupled optical cavities. The optical cavities include a ytterbium- or erbium-doped fiber gain medium for generating dual-wavelength optical pulses for pump-probe spectroscopy techniques.

[0009] In the article "Passive synchronization of erbium and thulium doped fiber mode-locked lasers enhanced by common graphene saturable absorber" published by Sotor et al. in Optics Letters (2014) (Sotor), the synchronization of optical pulses from two loop resonators is disclosed by using a common graphene saturable absorber. Each loop resonator includes one of an erbium-doped and a thulium-doped fiber gain medium. In Sotor's article, a 1569 nm laser diode is used to pump the thulium gain medium to generate optical pulses at 2 μm. A wavelength division multiplexer (WDM) filter is provided to filter out any unabsorbed pump light at a wavelength of 1569 nm, and the unabsorbed pump light is removed to the erbium loop resonator and output together with the 1.5 μm optical pulses.

[0010] There is a desire to provide a more efficient and robust dual-cavity passive synchronization optical circuit for a laser device. SUMMARY OF THE INVENTION

[0011] Aspects of the present invention are defined in the following independent claims, and reference should now be made to these independent claims. Optional features are listed in the dependent claims.

[0012] In a first aspect, the present disclosure provides an optical circuit for a laser device, particularly for a laser device in a CRS. The optical circuit includes a first polarization-maintaining optical cavity and a second polarization-maintaining optical cavity. The first polarization-maintaining optical cavity includes a first gain medium and a first saturable absorber. The first gain medium can be excited by a first pump light source to generate light within a first wavelength range, and the first saturable absorber is configured to perform passive mode locking on optical pulses in the first polarization-maintaining optical cavity. The second polarization-maintaining optical cavity includes a second gain medium different from the first gain medium and a second saturable absorber. The second gain medium can be excited by a second pump light source to generate light within a second wavelength range, and the second saturable absorber is configured to perform passive mode locking on optical pulses in the second polarization-maintaining optical cavity. The first polarization-maintaining optical cavity and the second polarization-maintaining optical cavity share a common branch, where the common branch does not include a saturable absorber.

[0013] Saturable absorbers can be used to initiate and facilitate strong intracavity pulses through intensity-dependent loss, i.e., the pulse (probe) experiences reduced loss due to a higher energy pulse (pump). Different from the shared saturable absorber laser cavity configuration, the disclosed common branch optical circuit arrangement utilizes independent saturable absorbers in each optical cavity. Thus, as in the known shared saturable absorber laser cavity configuration, the operating life of the disclosed optical circuit arrangement can be extended due to reduced stress on the saturable absorber caused by excessive nonlinear saturable absorption / heating induced by simultaneous pulses from multiple optical cavities. Additionally, the inventors of the present disclosure have found that the characteristics of the saturable absorber may vary depending on the number of wavelengths transmitted through it. This crosstalk effect is unpredictable and is generally adverse to the mode locking and / or synchronization mechanisms of each individual wavelength in the common branch shared by two optical cavities. Therefore, providing two independent saturable absorbers (one for each optical cavity) enables each saturable absorber to be optimized for the wavelength associated with its respective optical cavity and avoids unwanted crosstalk effects.

[0014] Similar to the shared saturable absorber laser cavity configuration, the disclosed common branch optical circuit arrangement also exploits passive optical synchronization generated by cross-phase modulation (XPM) interactions in the common branch. The strength of the XPM interaction in the common branch is proportional to the peak intensity of the interacting pulses and the non-linearity of the medium. The interaction length of the common branch affects the XPM-induced synchronization range. Generally, the longer the common branch, the better the XPM interaction. However, due to the group velocity mismatch (GVM) phenomenon, an upper limit is imposed on the allowable length of the common branch. It has been found that, in terms of passive synchronization, the disclosed optical circuit including a relatively short common branch can be as effective as full cavity injection. Due to its slave-slave optical structure, the disclosed common branch optical circuit can exhibit better passive synchronization than the cavity injection arrangement, where the XPM-induced repetition rate variation feedback affects the pulses in both optical cavities, increasing the allowable cavity length mismatch in the optical circuit. Additionally, performing XPM in the common branch between the two optical cavities ensures sufficient XPM interaction for synchronization without the need for external amplification, thus enabling a more efficient optical circuit with a lower laser emission threshold.

[0015] The laser emission threshold is the lowest excitation level at which the output of the laser is dominated by stimulated emission rather than spontaneous emission. Below this threshold, the output power of the laser increases slowly with increasing excitation. Above the laser emission threshold, the slope of the power versus excitation increases by several orders of magnitude. The emission linewidth of the laser also decreases by several orders of magnitude above the laser emission threshold. Above the laser emission threshold, the laser is said to be lasing.

[0016] In the disclosed optical circuit, the configurations of the first optical cavity and the second optical cavity can take several different forms.

[0017] In a first embodiment, the first polarization-maintaining optical cavity can have a ring configuration, and the second polarization-maintaining optical cavity can have a ring configuration. In a ring configuration, the optical cavity follows a path that forms a complete / continuous optical loop. In a ring-ring configuration, a portion of the two optical cavities is joined by a common branch. Compared to a linear cavity configuration, when using a transmissive saturable absorber (CNT, graphene, semiconductor-based SA), the ring-ring cavity configuration experiences lower losses per round trip in the optical cavity because the pulses in each optical cavity pass through the saturable absorber only once. There are no restrictions on the possible location of the common branch within each optical cavity except between the pump WDM and the active fiber, as the presence of the common branch WDM will prevent the pump light from reaching the active fiber due to the presence of the optical filter.

[0018] In a second embodiment, the first polarization-maintaining optical cavity may have a linear configuration, and the second polarization-maintaining optical cavity may have a ring configuration. In the linear configuration, the optical cavity follows a linear path whose ends do not intersect. Compared with the ring-ring configuration, the linear-ring configuration has the advantage of being more compact because its total fiber length is half that of the ring cavity counterpart. In addition, for a dispersion compensation cavity using FBG and SESAM, the linear cavity arrangement always has lower losses relative to the ring counterpart, resulting in a lower laser emission threshold.

[0019] In a third embodiment, the first polarization-maintaining optical cavity may have a linear configuration, and the second polarization-maintaining optical cavity may have a linear configuration. Advantageously, the linear-linear optical cavity configuration effectively doubles the XPM interaction length, thereby improving the acceptable cavity mismatch in the optical circuit. This is because, in the linear-linear configuration, the cavity pulse passes through the common branch twice during each round trip in each cavity.

[0020] Optionally, the first saturable absorber is different from the second saturable absorber.

[0021] Optionally, at least one of the first saturable absorber or the second saturable absorber includes at least one of the following: graphene / carbon allotropes, single-walled carbon nanotubes (SWCNT), semiconductor saturable absorber mirrors (SESAM), or transmission semiconductor-based saturable absorbers. Advantageously, these saturable absorber (SA) types can reduce the footprint of the optical cavity arrangement because, compared with the nonlinear amplifying loop mirror and the nonlinear polarization rotation / evolution SA types, they can provide an effective mode-locking function using a shorter fiber length. This enables a larger proportion of the fiber to be used in the common branch, thereby obtaining a larger cavity mismatch tolerance.

[0022] Any of the SA types described can be used in any of the cavity configurations described. Transmission SAs, such as graphene / carbon allotropes, SWCNT, and transmission semiconductor-based SAs, are particularly suitable for ring cavities because they only experience linear losses once during each cavity round trip, thereby reducing the laser emission threshold. However, reflective SAs (such as SESAM) are particularly suitable for linear cavities. In the case of using a transmission SA in a linear cavity arrangement, a fiber coupling mirror must be provided at the output of the SA. Particularly advantageous SA combinations with a reduced laser emission threshold include: a first transmission SA and a second transmission SA for the ring-ring cavity configuration, a first transmission SA and a second SESAM SA for the ring-linear cavity configuration, and a first SESAM SA and a second SESAM SA for the linear-linear cavity configuration.

[0023] Optionally, at least one of the first saturable absorber and / or the second saturable absorber is mounted on a temperature control system. This ensures that the first and / or second saturable absorber operates in a consistent and predictable manner.

[0024] Optionally, the common branch includes a high nonlinearity device or material. The high nonlinearity device may include one or more of a photonic crystal fiber (PCF), a high nonlinear fiber (HNLF), a small mode area fiber, or a tapered fiber. The high nonlinearity material may include a two-dimensional material (TDM), polyethylene terephthalate (PET), polymethyl methacrylate (PMMA), and / or cellulose acetate (CA). The nonlinear index of a standard single-mode fiber at 1030 nm is about 2.7×10 -7 cm 2 / GW to 2.8×10 - 7 cm 2 / GW. The materials listed above have a higher nonlinear index or an effective nonlinear coefficient relative to the standard fiber, and thus have a higher nonlinearity. The high nonlinearity device generally has the same nonlinear index as the standard fiber, but achieves a higher nonlinearity due to a smaller mode field area. The high nonlinearity device or material can be spliced between two single-mode polarization-maintaining fibers.

[0025] Optionally, the common branch includes a single-mode polarization-maintaining fiber.

[0026] Optionally, the common branch includes a single-mode fiber.

[0027] Optionally, at least one of the first polarization-maintaining optical cavity or the second polarization-maintaining optical cavity includes a polarization isolator or a dispersion compensation device or a circulator with a dispersion compensation device.

[0028] Optionally, at least one of the first gain medium or the second gain medium includes a ytterbium-doped fiber, an erbium-doped fiber, a neodymium-doped fiber, or a thulium-doped fiber.

[0029] Optionally, at least one of the first polarization-maintaining optical cavity or the second polarization-maintaining optical cavity includes a polarization fiber coupler.

[0030] Optionally, at least one of the first polarization-maintaining optical cavity or the second polarization-maintaining optical cavity includes an optical delay line for matching the lengths of the first polarization-maintaining optical cavity and the second polarization-maintaining optical cavity. Optionally, the optical delay line includes a fiber pigtail optical delay line. Using a delay line in one or more optical cavities enables pairing of different optical cavities by equalizing the lengths of different optical cavities.

[0031] Passive synchronization of multiple mode-locked fiber oscillators solves many technical problems, including precise timing distribution between different wavelength regions and multi-wavelength synchronized mode-locked laser sources for microscopy. The key advantage of the shared common branch synchronization mechanism based on passive XPM is modularity, which can be extended to multiple optical cavities by adding a new common branch shared with adjacent cavities.

[0032] In a fourth embodiment, the optical circuit may further include a third polarization-maintaining optical cavity. The third polarization-maintaining optical cavity includes: a third gain medium capable of being excited by a third pump light source to generate light within a third wavelength range; and a third saturable absorber configured to perform passive mode-locking on optical pulses in the third polarization-maintaining optical cavity. In such an embodiment, the second polarization-maintaining optical cavity and the third polarization-maintaining optical cavity may share a common branch, where the common branch does not include a saturable absorber.

[0033] The modularity of the disclosed optical circuit helps to achieve transfer synchronization from the first and second optical cavities to a third optical cavity adjacent to the second optical cavity through XPM. Due to the interaction of two pulses in the common branch, this geometry enables the use of the lowest accessible order XPM (third-order nonlinearity), thus ensuring the strongest synchronization effect. The synchronization range of the resulting adjacent optical cavities is not affected by the presence of other adjacent optical cavities.

[0034] Synchronizing multiple oscillators together has advantages in broadband coherent Raman microscopy because it can simultaneously generate multiple pump / Stokes couplings, covering multiple Raman regions. One implementation of broadband SRS microscopy can use a combination of two narrowband pump / Stokes beams coupled to a broadband Stokes / pump beam: one pump / Stokes will probe a Raman-active region (i.e., CH stretch) through the broadband Stokes / pump, and the other pump / Stokes centered at a different wavelength will probe another Raman-active region (i.e., fingerprint) through the same broadband Stokes / pump.

[0035] Generating multiple pump / Stokes couplings directly from the oscillators enables each optical cavity to be optimized for the desired wavelength range without being affected by other optical cavities in the optical circuit. In addition, the proposed solution can provide a higher power spectral density at the detector for broadband SRS, thus achieving a higher signal-to-noise ratio within a constant total average power and Raman spectral coverage at the detector. Moreover, the proposed solution enables improved handling of non-ultra-wideband pulses in microscopy because using multiple narrowband pump / Stokes beams requires Stokes / pump pulses with a smaller width and thus less dispersion.

[0036] Optionally, the first wavelength range and the second wavelength range may not overlap, and the second wavelength range and the third wavelength range do not overlap.

[0037] Optionally, the first gain medium includes erbium-doped fiber, the second gain medium includes ytterbium-doped fiber, and the third gain medium includes neodymium-doped fiber.

[0038] Optionally, the first gain medium includes erbium-doped fiber, the second gain medium includes ytterbium-doped fiber, and the third gain medium includes thulium-doped fiber.

[0039] Optionally, each common branch in the optical circuit is enclosed between two wavelength division multiplexers. In some examples, each common branch in the optical circuit is enclosed between two micro-optical filter couplers or micro-optical filter splitters.

[0040] In one embodiment, each of the first polarization-maintaining optical cavity, the second polarization-maintaining optical cavity, and the third polarization-maintaining optical cavity has an annular configuration.

[0041] In one embodiment, the first polarization-maintaining optical cavity and the second polarization-maintaining optical cavity have an annular configuration, and the third polarization-maintaining optical cavity has a linear configuration.

[0042] In one embodiment, the first polarization-maintaining optical cavity and the third polarization-maintaining optical cavity have a linear configuration, and the second polarization-maintaining optical cavity has an annular configuration.

[0043] In an alternative embodiment, the optical circuit may further include a third polarization-maintaining optical cavity. The third polarization-maintaining optical cavity includes: a third gain medium that can be excited by a third pump light source to generate light within a third wavelength range; and a third saturable absorber configured to perform passive mode locking on optical pulses in the third polarization-maintaining optical cavity. The first polarization-maintaining optical cavity, the second polarization-maintaining optical cavity, and the third polarization-maintaining optical cavity may share the same common branch, where the common branch does not include a saturable absorber.

[0044] In a second aspect, the present disclosure provides a laser device for outputting filtered optical pulses to induce coherent Raman scattering (CRS) in a sample. The laser device may include any one of the optical circuit arrangements according to the first aspect above. The laser device may further include a first optical filter and a second optical filter, where the first optical filter and the second optical filter are configured to filter the light from the first polarization-maintaining optical cavity and the second polarization-maintaining optical cavity, respectively, so as to output a first filtered optical pulse within a first predetermined wavelength range and a second filtered optical pulse within a second predetermined wavelength range.

[0045] The disclosed laser device can achieve CRS at a fast enough speed for non-invasive imaging. That is, by measuring the detailed molecular composition through the vibration response of the tissue detected by CRS, objective and quantitative information of the tissue can be obtained. Examples of the laser device also provide a convenient tool for pump-probe experiments and a suitable pump source for parametric mixing and frequency up / down conversion.

[0046] Broadly, in the described laser device, each optical cavity of the laser device may include a gain element and a single-mode polarization-maintaining fiber. An optical delay line with a fiber pigtail inserted halfway can be used to match the length of the optical cavity. After the synchronous oscillator, a fiber amplifier can also be provided to increase the average power of the two branches to the hundreds of mW level required for the application. In other words, two independent laser media are mode-locked and synchronized to provide pump pulses and Stokes pulses for CRS. The two independent mode-locked optical cavities are synchronously locked via a common branch between the two optical cavities through the shared XPM interaction (i.e., the pulses have the same repetition rate, and the optical delay between the two optical pulse trains is constant). In a broadband configuration, frequency detuning can be achieved by using a narrowband (broadband) pump and a broadband (narrowband) Stokes; in a narrowband configuration, frequency detuning can be achieved by a tunable filtering stage located inside or outside the cavity.

[0047] Compared with the known implementations of CRS, in which one of the two required independent pulses of different frequencies is generated by parametric amplification, in the laser device described herein, different laser media emitting at different frequencies are passively synchronized, thus greatly simplifying the generation of the broadband or multi-color (multi-frequency) pulse sequences required for CRS.

[0048] In the described example, two independent mode-locked oscillators or optical cavities are provided, which are synchronized through the XPM interaction in the shared cavity section.

[0049] The laser device described herein enables the passive synchronization of fiber lasers, providing a very simple and low-cost laser source for CRS. Due to its simple, compact, and cost-effective design, and the alignment-free operation that does not require a large optical setup, fiber lasers can achieve a robust and stable source.

[0050] As will be described below, examples of the laser device described herein have been applied to coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS), thus demonstrating the concept. Due to their compactness and all-optical synchronization, the described examples are good sources for CRS in the high wavenumber region as well as the fingerprint region.

[0051] The arrangement is described in more detail below, and the arrangement takes the form of a laser device for outputting filtered optical pulses to induce coherent Raman scattering in a sample.

[0052] Optionally, both the first optical filter and the second optical filter may include a fiber Bragg grating (FBG) configured to output optical pulses within a first predetermined wavelength range and a second predetermined wavelength range. A fiber Bragg grating is a short section of optical fiber that reflects light of a specific wavelength and transmits all other light. This effect is obtained by creating a periodic variation in the refractive index of the fiber core, which creates a wavelength-selective mirror. The FBG can also be designed as a chirped mirror to introduce a predetermined dispersion in the reflected wavelength of the light.

[0053] Optionally, both the first optical filter and the second optical filter may include a fixed-wavelength optical filter configured to set the first predetermined wavelength range and the second predetermined wavelength range, respectively.

[0054] Optionally, both the first optical filter and the second optical filter may include a tunable optical filter and be configured to change the first predetermined wavelength range and the second predetermined wavelength range, respectively. A tunable optical filter allows a user to specify the wavelength range such that the wavelength ranges of the pump optical pulse and the Stokes optical pulse can be varied relative to the sample being measured.

[0055] Optionally, the tunable or fixed-wavelength optical filter may include an etalon-based fiber-optic tunable or fixed-wavelength filter. An etalon is a dielectric material in which its specific thickness and refractive index determine the bandwidth of each transmission peak, and only one wavelength is transmitted with maximum transmittance. The etalon-based fiber-optic tunable or fixed-wavelength filter works by selecting the refractive index of the material medium to select a specific resonant wavelength. The wavelength that resonates with the optical length of the cavity is transmitted, while other wavelengths are reflected.

[0056] Optionally, the first optical filter and the second optical filter are respectively positioned within the first polarization-maintaining optical cavity and the second polarization-maintaining optical cavity, and wherein the first polarization-maintaining optical cavity and the second polarization-maintaining optical cavity output the filtered optical pulses at a first optical output and a second optical output, respectively. Mounting the optical filter within the optical cavity ensures that optical pulses with an undesired wavelength range are quickly filtered after their generation.

[0057] Optionally, the first optical filter and the second optical filter are respectively positioned outside the first polarization-maintaining optical cavity and the second polarization-maintaining optical cavity, and wherein the first polarization-maintaining optical cavity and the second polarization-maintaining optical cavity output the optical pulses at a first optical output and a second optical output, respectively. Placing the optical filter outside the optical cavity eliminates the need to repeatedly filter recycled filtered optical pulses while also enabling the construction of a simple and compact optical cavity.

[0058] The optical pulse can be filtered so that only the optical pulses within a defined wavelength range are output to the coherent Raman spectroscopy, thereby producing a more accurate measurement. In addition, the use of two synchronized and mode-locked laser sources greatly reduces the influence of the optical filter on the optical power of the pump pulse and the Stokes pulse, making it a general choice for CRS.

[0059] Optionally, the laser device further includes a first fiber amplifier doped with a first gain medium at the first optical output and a second fiber amplifier doped with a second gain medium at the second optical output, for amplifying the optical pulse or the filtered optical pulse. This ensures that the amplified optical pulse is amplified within the correct wavelength range. The use of the amplifier can mitigate the reduction of the optical power when the optical filter is in place.

[0060] Optionally, the laser device further includes a second harmonic generation crystal, which is operatively coupled to the optical output of at least one of the first fiber amplifier or the second fiber amplifier. The second harmonic generation crystal can be formed of one or more of periodically poled lithium niobate (PPLN) or periodically poled potassium titanyl phosphate (PPKTP).

[0061] Optionally, the laser device further includes an acousto-optic modulator or an electro-optic modulator, which is operatively coupled to at least one of the first amplifier or the second amplifier.

[0062] Optionally, the laser device further includes an acousto-optic modulator or an electro-optic modulator, which is operatively coupled to the output of at least one of the first polarization-maintaining optical cavity or the second polarization-maintaining optical cavity.

[0063] Optionally, the spectra of the two optical pulses output from the first polarization-maintaining cavity or the second polarization-maintaining cavity can be broadened due to self-phase modulation in the fiber amplifier section, so as to increase the spectral bandwidth and shorten the compressed pulse duration.

[0064] Optionally, the laser device is a fiber laser. Optionally, the laser device is an all-fiber laser. Optionally, the first optical cavity and the second optical cavity include isotropic fibers. Optionally, each of the first optical cavity and the second optical cavity includes a single-mode fiber.

[0065] Optionally, the laser gain medium includes ytterbium or erbium, wherein optionally, the predetermined wavelength range generated by the laser gain medium corresponds to the full Raman spectrum of 0 cm -1 -4000 cm -1 of the full Raman spectrum.

[0066] Optionally, the predetermined wavelength range includes ranges of 1000 nm to 1100 nm and / or 1535 nm to 1600 nm and / or 910 nm to 950 nm and / or 1800 nm to 1900 nm.

[0067] Optionally, a second harmonic generation crystal (i.e., periodically poled lithium niobate - PPLN -, periodically poled potassium titanyl phosphate - PPKTP -) can be used to frequency convert the amplified optical pulses.

[0068] Optionally, an acousto - optic modulator (AOM) or an electro - optic modulator (EOM) can be placed outside both the first polarization - maintaining cavity and the second polarization - maintaining cavity.

[0069] In a third aspect, the present disclosure provides an optical device. The optical device can include any one of the above - mentioned laser devices. In addition, the optical device includes two collimators configured to collimate the filtered optical pulses. This limits the divergence of the filtered optical pulses. Optionally, one of the collimators includes a delay stage configured to achieve overlap on the measured sample.

[0070] Optionally, the optical device further includes a dichroic mirror configured to combine the collimated optical pulses from the two collimators.

[0071] Optionally, the laser device includes a band - pass or short - pass filter for removing a pair of filtered optical pulses before CARS detection.

[0072] Optionally, the laser device includes a band - pass or long - pass filter for removing pump optical pulses before SRG detection.

[0073] Optionally, the laser device includes a band - pass or short - pass filter for removing Stokes optical pulses before SRL detection.

[0074] In a fourth aspect, the present disclosure provides a method of outputting filtered optical pulses from a laser device to induce coherent Raman scattering in a sample. The method includes generating light in respective different wavelength ranges using a first polarization - maintaining optical cavity including a first gain medium and a second polarization - maintaining optical cavity including a second gain medium different from the first gain medium, wherein the first gain medium and the second gain medium are each capable of being excited by a pump light source. Mode - locking is performed via a first saturable absorber and a second saturable absorber optically coupled to the first polarization - maintaining optical cavity and the second polarization - maintaining optical cavity, respectively. The method further includes synchronizing light from the first polarization - maintaining optical cavity and the second polarization - maintaining optical cavity via a common branch between the first polarization - maintaining optical cavity and the second polarization - maintaining optical cavity, wherein the common branch does not include a saturable absorber.

[0075] In a fifth aspect, the present disclosure provides an optical circuit for a laser device. The optical circuit includes N polarization-maintaining optical cavities. Each of the N polarization-maintaining optical cavities includes: a gain medium that can be excited by a pump light source to generate light within a wavelength range; and a saturable absorber that is configured to perform passive mode locking on optical pulses in the polarization-maintaining optical cavity. Each of the N polarization-maintaining optical cavities shares a common branch with at least one other polarization-maintaining optical cavity among the N polarization-maintaining optical cavities. Among them, the common branch between the N polarization-maintaining optical cavities does not include a saturable absorber.

[0076] Optionally, the gain medium of each polarization-maintaining optical cavity can be excited by a pump light source to generate light within a wavelength range that does not overlap with the wavelength range of the light generated by exciting the gain medium of an adjacent polarization-maintaining optical cavity.

[0077] Optionally, at least N - 1 of the polarization-maintaining optical cavities include an optical delay line for matching the length of the cavity for synchronization purposes.

[0078] Optionally, the optical delay line includes a fiber pigtail optical delay line.

[0079] Optionally, each of the N polarization-maintaining optical cavities has an annular configuration.

[0080] Optionally, 1 ≤ M ≤ 2 of the N polarization-maintaining optical cavities have a linear configuration, and N - M of the N polarization-maintaining optical cavities have an annular configuration.

[0081] It should be understood that the features described with respect to the first, second, third, and fourth aspects of the present disclosure can also be applied to the fifth aspect. Description of the Drawings

[0082] The present invention will be described in more detail by way of example with reference to the accompanying drawings, in which:

[0083] Figure 1 is a schematic diagram of an optical circuit according to a first embodiment of the present invention;

[0084] Figure 2 is a schematic diagram of an optical circuit according to a second embodiment of the present invention;

[0085] Figure 3 is a schematic diagram of an optical circuit according to a third embodiment of the present invention; and

[0086] Figure 4 is a schematic diagram of an optical device according to the present invention;

[0087] Figure 5 is a schematic diagram of an optical circuit according to an embodiment of the present invention;

[0088] Figure 6 is a schematic diagram of an optical circuit according to an embodiment of the present invention;

[0089] Figure 7 is a schematic diagram of an optical circuit according to an embodiment of the present invention. Detailed implementation

[0090] The following refers to Figures 1 to 3 and Figures 5 to 7 Describe an optical circuit used in a laser device for causing coherent Raman scattering in a sample according to an example of the present invention.

[0091] Figure 1 An optical circuit 100 according to a first embodiment of the present invention is shown. The optical circuit 100 includes two independent mode-locked optical cavities, oscillators or resonators for generating two sets of picosecond-duration optical pulses in different wavelength ranges suitable for CRS. The two optical cavities 110a, 110b are joined together at a common branch 120.

[0092] More specifically, continuing to refer to Figure 1 the optical circuit 100 of Figure 1 each of the two optical cavities 110a, 110b of the optical circuit 100 of

[0093] is arranged in a loop form. The ends of the optical fibers constituting the optical cavities 110a, 110b are optically connected using any suitable coupler so that the optical pulses circulate in the loop until they are ejected from the optical cavity at their respective optical exits. In addition, each of the optical exits includes a fiber coupler to provide approximately 20 - 30% output for its respective cavity. The mode-locked optical cavities 110a, 110b each include a pump light source 130a, 130b to the respective optical cavities 110a, 110b to excite the gain elements 140a, 140b located or deposited inside the optical cavities 110a, 110b. In this example, the gain elements 140a, 140b are optical fibers doped with rare earth gain elements. Laser devices using such gain elements are generally referred to as fiber lasers. Figure 1The example shown uses two different optical fibers as gain elements. One of the optical fibers is doped with a rare-earth gain element in the form of ytterbium (Yb) 140a. The other optical fiber is doped with a rare-earth gain element in the form of erbium (Er) 140b. In this example, the pump light sources used to excite the gain elements are the 976 nm wavelength pump light source 130a for exciting the Yb-doped optical fiber, and the 976 nm wavelength pump light source 130b for exciting the Er-doped optical fiber. The optical pulses generated from the Yb and Er gain media are within the desired pump wavelength and Stokes wavelength ranges.

[0094] Optical isolators 150a, 150b are optically coupled in each of the optical cavities 110a, 110b to force the circulation direction in the common branch 120 to be the same. This ensures that the optical pulses generated by the gain media 140a, 140b in the optical cavities travel in a single direction in the loops forming the optical cavities 110a, 110b. That is, the optical pulses generated from the gain media 140a, 140b are directed towards the optical output. In this example, the optical isolators 150a, 150b are fiber-based Faraday isolators. In other examples, the optical isolators 150a, 150b may include polarization circulators with dispersion compensation devices and output couplers.

[0095] A pair of optical pulses generated in each of the optical cavities 110a, 110b are passively synchronized via the XPM interaction in the common branch 120, which is shared by the two loops forming the optical cavities 110a, 110b. In this example, the common branch 120 includes a high nonlinearity device or material 160 to enhance the XPM interaction strength for synchronization.

[0096] Each of the optical cavities 110a, 110b includes its respective saturable absorber 170a, 170b in addition to the common branch 120. The function of the saturable absorber has been described in the above Summary of the Invention section. A saturable absorber is a light absorber whose absorption degree decreases at high light intensities. In the optical circuit 100, this allows passive mode-locked pulses to circulate in each of the optical cavities 110a, 110b. That is, passive mode-locking allows the generation of femtosecond optical pulses. The saturable absorbers 170a, 170b have a short enough recovery time to enable fast loss modulation.

[0097] Figure 1The saturable absorbers 170a, 170b in can be graphene-based polymer-composite saturable absorbers, which have ultrafast recovery times and broadband operation capabilities. Graphene saturable absorbers can be prepared by gently sonicating bulk graphite to exfoliate it. First, a dispersion rich in the resulting single-layer graphene and few-layer graphene is mixed with an aqueous solution of polyvinyl alcohol to obtain a polymer composite. Alternatively, other saturable absorbers can be used to passively mode-lock optical pulses, such as saturable absorbers including single-walled carbon nanotubes (CNTs). However, any of the saturable absorbers described in the above Summary of the Invention section can also be used.

[0098] A pair of optical cavities 110a, 110b do not need to be exactly the same. The difference in cavity length between the two optical cavities 110a, 110b is compensated by adding optical delay lines 180a, 180b to either or both of the optical cavities. In this example, the optical delay lines 180a, 180b are located in the two optical cavities 110a, 110b after the isolator 150a and before the isolator 150b. In this example, the optical delay lines 180a, 180b include fiber pigtail delay lines. The fiber pigtail delay lines are optically coupled to the exit of the isolator 150a and the entrance of the isolator 150b in each of the optical cavities 110a, 110b. In other examples, the optical delay lines 180a, 180b can include series output couplers.

[0099] The wavelength range of the optical pulses generated at each of the optical cavities 110a, 110b is determined by the type of gain medium excited in the respective optical cavity.

[0100] Each of the optical cavities 110a, 110b has an exit to output together from the optical circuit 100 a first filtered optical pulse in a first predetermined wavelength range and a second filtered optical pulse in a second predetermined wavelength range. In one example, if the exit is a circulator with a dispersion compensation device and an output coupler, the exit can be located in 150a, 150b. The optical exits can each be coupled to different fiber optic amplifiers. The associated fiber optic amplifiers can be doped with gain elements corresponding to the gain media 140a, 140b responsible for optical pulse generation. In one example, ytterbium-doped and erbium-doped fiber optic amplifiers are provided for the optical cavities 110a, 110b respectively to amplify the optical pulses at the ytterbium and erbium wavelengths to an average power of 100 mW.

[0101] Figure 2 An optical circuit 200 according to a second embodiment of the present invention is shown. Compared with Figure 1Similar to the optical circuit 100 shown, the optical circuit 200 includes two independent mode-locked optical cavities, oscillators, or resonators for generating two sets of picosecond-duration optical pulses in different wavelength ranges suitable for CRS. The two optical cavities 210a, 210b are joined together at a common branch 220.

[0102] Unlike the optical circuit 100, in the optical circuit 200, only one of the two optical cavities of the optical circuit 200, i.e., the optical cavity 210b, is arranged in a loop form, while the other optical cavity 210a is arranged in a linear configuration. As in the optical circuit 100, the mode-locked optical cavities 210a, 210b of the optical circuit 200 each include a pump light source 230a, 230b into each of the optical cavities 210a, 210b to excite the gain elements 240a, 240b located or deposited inside the optical cavities 210a, 210b. In this example, the gain elements 240a, 240b are fibers doped with rare-earth gain elements.

[0103] An optical isolator 250 is optically coupled within the optical cavity 210b. This is to ensure that the optical pulses generated by the gain medium 240b travel in a single direction or in one and only one direction along the loop forming the optical cavity 210b. That is, the optical pulses generated from the gain medium 240b are directed towards the optical output. In this example, the optical isolator 250 may include a fiber-based Faraday isolator. In other examples, the optical isolator 250 may include a polarization circulator with a dispersion compensation device and an output coupler.

[0104] As described with respect to the optical circuit 100, in each of the optical cavities 210a, 210b, a pair of optical pulses generated are passively synchronized via XPM interaction in the common branch 220, which is shared by the two optical cavities 210a, 210b. In this example, the common branch 220 includes a high nonlinearity device or material 260 to enhance the XPM interaction strength for synchronization.

[0105] The optical cavities 210a, 210b each include their respective saturable absorbers 270a, 270b in addition to the common branch 220. As described with respect to the optical circuit 100, in the optical circuit 200, this allows passively mode-locked pulses to circulate in each of the optical cavities 210a, 210b. The saturable absorbers 270a, 270b have a recovery time short enough to enable fast loss modulation. Any of the saturable absorbers mentioned in the present disclosure can be used in the optical circuit 200. In the case where a transmissive saturable absorber is provided in a linear optical cavity, a fiber-coupled mirror must be provided in the optical cavity after the saturable absorber.

[0106] The difference in cavity length between the two optical cavities 210a, 210b is compensated by adding optical delay lines 280a, 280b to either of the optical cavities. In this example, the optical delay line 280a is located after the gain medium 240a in the optical cavity 210a, while the optical delay line 280b is located after the isolator 250 in the optical cavity 210b. In this example, the optical delay lines 280a, 280b comprise fiber pigtail delay lines. The fiber pigtail delay lines are optically coupled to the output of the isolator 250 in the optical cavity 210b. In other examples, the optical delay lines 280a, 280b may comprise output couplers in series.

[0107] The optical cavity 210a has a dispersion compensator and an output coupler (a chirped fiber Bragg grating in this example) 190 to output a first filtered optical pulse within a first predetermined wavelength range from the optical circuit 200. The optical cavity 210b has a corresponding output to output a second filtered optical pulse within a second predetermined wavelength range from the optical circuit 200. In one example, the outputs may be located in 190 and 280b. The optical outputs may each be coupled to different fiber amplifiers. The associated fiber amplifiers may be doped with gain elements corresponding to the gain media 240a, 240b responsible for optical pulse generation. In one example, ytterbium-doped and erbium-doped fiber amplifiers are provided for the optical cavities 210a, 210b respectively to amplify optical pulses at ytterbium and erbium wavelengths to an average power of 100 mW.

[0108] Figure 3 An optical circuit 300 according to a third embodiment of the present invention is shown. Similar to the optical circuits 100 and 200, the optical circuit 300 includes two independent mode-locked optical cavities, oscillators or resonators for generating two sets of picosecond-duration optical pulses in different wavelength ranges suitable for CRS. The two optical cavities 310a, 310b are joined together at a common branch 320.

[0109] Different from the optical circuits 100 and 200, in the optical circuit 300, the two optical cavities 310a, 310b of the optical circuit 300 are both arranged in a linear configuration. As in the optical circuits 100 and 200, the mode-locked optical cavities 310a, 310b of the optical circuit 300 each include pump light sources 330a, 330b to the respective ones of the optical cavities 310a, 310b to excite gain elements 340a, 340b located or deposited inside the optical cavities 310a, 310b. In this example, the gain elements 340a, 340b are fibers doped with rare-earth gain elements.

[0110] The optical circuit 300 may include polarization couplers 350a, 350b that are optically coupled to the exits of the gain media 340a, 340b in the optical cavities 310a, 310b, respectively. This is used to select the polarization state on the slow axis and can act as an optical exit.

[0111] As described with respect to optical circuits 100 and 200, a pair of optical pulses generated in each of the optical cavities 310a, 310b are passively synchronized via XPM interaction in the common branch 320, which is shared by the two optical cavities 310a, 310b. In this example, the common branch 320 includes a high nonlinearity device or material 360 to enhance the XPM interaction strength for synchronization. In this linear-linear optical cavity arrangement, since the optical pulses generated in each of the optical cavities 310a, 310b pass through the common branch 320 twice, the effective XPM interaction length is increased. This improves the cavity mismatch allowed in the optical circuit 300.

[0112] The optical cavities 310a, 310b each include their respective saturable absorbers 370a, 370b in addition to the common branch 320. As described with respect to optical circuits 100 and 200, in the optical circuit 300, this allows passively mode-locked pulses to circulate in each of the optical cavities 310a, 310b. The saturable absorbers 370a, 370b have a recovery time short enough to enable fast loss modulation. Any of the saturable absorbers mentioned in this disclosure can be used in the optical circuit 300.

[0113] The difference in cavity lengths between the two optical cavities 310a, 310b is compensated by adding optical delay lines 380a, 380b to either of the optical cavities. In this example, the optical delay line 380a is located after the gain medium 340a in the optical cavity 310a, while the optical delay line 380b is located after the gain medium 340b in the optical cavity 310b. In this example, the optical delay lines 380a, 380b may include fiber pigtail delay lines.

[0114] The optical cavities 310a, 310b have dispersion compensators and output couplers 390a, 390b to output a first filtered optical pulse in a first predetermined wavelength range and a second filtered optical pulse in a second predetermined wavelength range from the optical circuit 300 (a chirped fiber Bragg grating in this example). The optical exits can each be coupled to different fiber amplifiers. The associated fiber amplifiers can be doped with gain elements corresponding to the gain media 340a, 340b responsible for optical pulse generation. In one example, ytterbium-doped and erbium-doped fiber amplifiers are provided for the optical cavities 310a, 310b respectively to amplify the optical pulses at the ytterbium and erbium wavelengths to an average power of 100 mW.

[0115] In other embodiments, the dispersion compensators and output couplers 390a, 390b may be replaced with high-reflectivity mirrors, and the polarization couplers 350a, 350b are used as output couplers.

[0116] In each of the disclosed optical circuits 100, 200, and 300, the position of the SA and / or the common branch relative to the components in the circuit may be changed, provided that the common branch 320 and the dedicated WDM are positioned after the pump diodes 330a, 330b and the active fiber gain elements 340a, 340b in each cavity. The SA may be positioned before or after the common branch.

[0117] Figure 4 An optical device 400 according to the present invention is shown. The optical device 400 includes an optical circuit 410 and additional optical elements 420. The optical circuit 410 may include any one of the above-described optical circuits 100, 200, or 300. The optical elements 420 direct the optical pulses generated by the optical circuit 410 to irradiate a sample 450 on which coherent Raman scattering is occurring. The scattering of the sample is filtered via a short-pass, band-pass, or long-pass filter 460 before entering a multi-channel dispersion detector 470 (i.e., a spectrometer, a multi-channel lock-in amplifier).

[0118] Figure 4 The optical elements 420 of the illustrated arrangement or setup include collimators 430a, 430b. The optical circuit 410 outputs two filtered optical pulses, and each filtered optical pulse passes through a collimator. Thus, the two filtered optical pulses are collimated in their respective collimators 430a, 430b to limit the beam divergence of the optical pulses when they are combined by a dichroic mirror 440a in a subsequent combining step. In some cases, when overlap on the sample 450 is desired, one of the two collimators 430a, 430b may be placed on a delay stage 430c. Alternatively or additionally, a delay stage 430c may be provided before the collimators 430a, 430b.

[0119] Figure 4The optical element 420 of the arrangement or setup shown also includes a dichroic mirror 440a. A dichroic mirror is a mirror body that has different reflection and transmission characteristics at different wavelengths. The dichroic mirror 440a is used to combine two collimated light pulses from different cavities. Then they are focused into the sample 450. A short-pass or band-pass or long-pass filter 460 and a multi-channel dispersive detector 470 are sequentially located downstream of the sample. In a CARS detection configuration, the short-pass filter 460 is used to remove the pump light pulse and the Stokes light pulse from the sample. A short-pass filter is a filter that has a sharp transition from transmission to reflection. The resulting CARS spectrum is measured at the spectrometer 470. In an SRS configuration (SRG or SRL), the pump (SRG) light pulse or the Stokes (SRL) light pulse is removed via a long-pass (SRG) or short-pass (SRL) optical filter 460 after the sample. The resulting SRG or SRL spectrum is measured at the multi-channel lock-in amplifier 470.

[0120] Figure 5 An optical circuit 500 according to an embodiment of the present invention is shown, which includes three optical cavities 510a, 510b, 510c, and each optical cavity includes a pump light source 530a, 530b, 530c to each of the optical cavities 510a, 510b, 510c to excite gain elements 540a, 540b, 540c located or deposited inside the optical cavities 510a, 510b, 510c. In this example, the gain elements 540a, 540b, 540c are optical fibers doped with rare-earth gain elements.

[0121] The first optical cavity 510a and the second optical cavity 510b share a common branch in the same manner as Figure 2 the optical circuit 200. Different from the optical circuit 200, the second optical cavity 510b of the optical circuit 500 includes a second common branch shared with the third optical cavity 510c.

[0122] An optical isolator 550 is optically coupled inside the optical cavity 510b. This is to ensure that the light pulse generated by the gain medium 540b travels in a single direction or in one and only one direction in the loop forming the optical cavity 510b. That is, the light pulse generated from the gain medium 540b is guided towards the optical outlet. In this example, the optical isolator 550 may include a fiber-based Faraday isolator. In other examples, the optical isolator 550 may include a polarization circulator with a dispersion compensation device and an output coupler.

[0123] The optical pulses generated in each of the optical cavities 510a, 510b, 510c are passively synchronized via XPM interactions in a common branch that is shared between optical cavity 510a to optical cavity 510b and optical cavity 510b to optical cavity 510c. In this example, the common branch includes high nonlinearity devices or materials 560a, 560c to enhance the XPM interaction strength for synchronization.

[0124] Each of the optical cavities 510a, 510b, 510c includes its respective saturable absorber 570a, 570b, 570c in addition to the common branch. As described above, this allows passively mode-locked pulses to circulate in each of the optical cavities 510a, 510b, 510c. The saturable absorbers 570a, 570b, 570c have a recovery time short enough to enable fast loss modulation. Any of the saturable absorbers mentioned in this disclosure can be used in the optical circuit 500.

[0125] The differences in cavity lengths between optical cavity 510a to optical cavity 510b and between optical cavity 510b to optical cavity 510c are compensated by adding optical delay lines 580a, 580b, 580b to one or more of the optical cavities. In this example, the optical delay lines 580a and 580c are respectively located after the gain media 540a and 540c in the optical cavities 510a and 510c, while the optical delay line 580b is located after the isolator 550 in the optical cavity 510b. In this example, the optical delay lines 580a, 580b, 580c include fiber pigtail delay lines. The fiber pigtail delay lines are optically coupled to the output of the isolator 550 in the optical cavity 510b. In other examples, the optical delay lines 580a, 580b, 580c can include output couplers in series.

[0126] The optical cavities 510a and 510c have dispersion compensators and output couplers (chirped fiber Bragg gratings in this example) 590a, 590c to output first filtered optical pulses within a first predetermined wavelength range from the optical circuit 500. The optical cavity 510b has a corresponding outlet to output second filtered optical pulses within a second predetermined wavelength range from the optical circuit 500. In one example, the outlets can be located in 590a, 590c and 580b. The optical outlets can each be coupled to different fiber amplifiers. The associated fiber amplifiers can be doped with gain elements corresponding to the gain media 540a, 540b, 540c responsible for optical pulse generation. In one example, Er-doped, Yb-doped and Nd-doped fiber amplifiers are respectively provided for the optical cavities 510a, 510b, 510c to amplify the optical pulses at the Er, Yb and Nd wavelengths to an average power of 100 mW.

[0127] Figure 5An optical circuit 500 is shown, which has three optical cavities 510a, 510b, 510c arranged in a linear-ring-linear configuration. However, it should be understood that alternative arrangements are possible. For example, Figure 6 shows an optical circuit 600 similar to that shown in Figure 5 and has three optical cavities 610a, 610b, 610c. However, in the optical circuit 600, the optical cavity 610a is in a ring configuration, thus forming an optical circuit 600 with a ring-ring-linear configuration. Although the configurations are different, the optical circuit 600 can operate in a manner similar to the optical circuit 500.

[0128] Figure 7 An optical circuit is shown having three optical cavities 710a, 710b, 710c arranged in a ring-ring-ring configuration. As discussed with respect to the optical circuit 600, the features described with respect to the optical circuit 500 also apply to the optical circuit 700.

[0129] It should be understood that although not shown, the optical circuits according to the present invention may include more than three optical cavities and have various configurations. In one example, the optical circuit includes N optical cavities, each optical cavity sharing a common branch with at least one adjacent optical cavity. In some examples, all N optical cavities can be arranged in a ring / loop configuration. In other examples, 1 < M < 2 of the N optical cavities can be arranged in a linear configuration, while N - M optical cavities are arranged in a ring / loop configuration.

[0130] Embodiments of the present invention have been described. It should be understood that variations and modifications can be made to the described embodiments within the scope of the present invention.

Claims

1. An optical circuit for a laser device, the optical circuit comprising: A first polarization-maintaining optical cavity, the first polarization-maintaining optical cavity comprising: A first gain medium, the first gain medium being capable of being excited by a first pump light source to generate light within a first wavelength range; and A first saturable absorber, the first saturable absorber being configured to perform passive mode locking on optical pulses in the first polarization-maintaining optical cavity; and A second polarization-maintaining optical cavity, the second polarization-maintaining optical cavity comprising: A second gain medium, the second gain medium being different from the first gain medium, the second gain medium being capable of being excited by a second pump light source to generate light within a second wavelength range; and A second saturable absorber, the second saturable absorber being configured to perform passive mode locking on optical pulses in the second polarization-maintaining optical cavity, wherein the first polarization-maintaining optical cavity and the second polarization-maintaining optical cavity share a common branch, and wherein the common branch does not include a saturable absorber.

2. The optical circuit according to claim 1, wherein the first polarization-maintaining optical cavity has an annular configuration, and the second polarization-maintaining optical cavity has an annular configuration.

3. The optical circuit according to claim 1, wherein the first polarization-maintaining optical cavity has a linear configuration, and the second polarization-maintaining optical cavity has an annular configuration.

4. The optical circuit according to claim 1, wherein the first polarization-maintaining optical cavity has a linear configuration, and the second polarization-maintaining optical cavity has a linear configuration.

5. The optical circuit according to any one of the preceding claims, wherein the first saturable absorber is a saturable absorber in a different form from the second saturable absorber.

6. The optical circuit according to any one of the preceding claims, wherein at least one of the first saturable absorber or the second saturable absorber comprises at least one of the following: graphene / carbon allotropes, single-walled carbon nanotubes, semiconductor saturable absorption mirrors, or transmission semiconductor-based saturable absorbers.

7. The optical circuit according to claim 2, wherein the first saturable absorber is a transmission semiconductor-based saturable absorber, and the second saturable absorber is a transmission semiconductor-based saturable absorber.

8. The optical circuit according to claim 3, wherein the first saturable absorber is a semiconductor saturable absorption mirror, and the second saturable absorber is a transmission semiconductor-based saturable absorber.

9. The optical circuit according to claim 4, wherein the first saturable absorber is a semiconductor saturable absorption mirror, and the second saturable absorber is a semiconductor saturable absorption mirror.

10. The optical circuit according to any one of the preceding claims, wherein the common branch comprises a single-mode polarization-maintaining fiber or a single-mode fiber.

11. The optical circuit according to any one of the preceding claims, wherein the common branch comprises a high nonlinearity device or material.

12. The optical circuit according to any one of the preceding claims, wherein at least one of the first polarization-maintaining optical cavity or the second polarization-maintaining optical cavity comprises a polarization isolator or a dispersion compensation device or a circulator having a dispersion compensation device.

13. The optical circuit according to any one of the preceding claims, wherein at least one of the first gain medium or the second gain medium comprises ytterbium-doped fiber, erbium-doped fiber, neodymium-doped fiber or thulium-doped fiber.

14. The optical circuit according to any one of the preceding claims, wherein at least one of the first polarization-maintaining optical cavity or the second polarization-maintaining optical cavity comprises an optical delay line for matching the lengths of the first polarization-maintaining optical cavity and the second polarization-maintaining optical cavity, and optionally, wherein the optical delay line comprises a fiber pigtail optical delay line.

15. The optical circuit according to any one of the preceding claims, wherein each of the first polarization-maintaining optical cavity and the second polarization-maintaining optical cavity comprises at least one of isotropic fiber or single-mode fiber.

16. The optical circuit according to any one of the preceding claims, further comprising a third polarization-maintaining optical cavity, the third polarization-maintaining optical cavity comprising: a third gain medium capable of being excited by a third pump light source to generate light in a third wavelength range; and a third saturable absorber configured to perform passive mode locking on optical pulses in the third polarization-maintaining optical cavity, wherein the second polarization-maintaining optical cavity and the third polarization-maintaining optical cavity share a common branch, and wherein the common branch does not comprise a saturable absorber.

17. The optical circuit according to claim 16, wherein the first wavelength range and the second wavelength range do not overlap, and wherein the second wavelength range and the third wavelength range do not overlap.

18. The optical circuit according to any one of claims 16 or 17, wherein the first gain medium comprises erbium-doped fiber, the second gain medium comprises ytterbium-doped fiber, and the third gain medium comprises neodymium-doped fiber.

19. The optical circuit according to any one of claims 16 or 17, wherein the first gain medium comprises erbium-doped fiber, the second gain medium comprises ytterbium-doped fiber, and the third gain medium comprises thulium-doped fiber.

20. The optical circuit according to any one of claims 16 to 19, wherein each common branch in the optical circuit is enclosed between two wavelength division multiplexers.

21. The optical circuit according to any one of claims 16 to 20, wherein each common branch in the optical circuit is enclosed between two micro-optical filter couplers or micro-optical filter splitters.

22. The optical circuit according to any one of claims 16 to 21, wherein each of the first polarization-maintaining optical cavity, the second polarization-maintaining optical cavity and the third polarization-maintaining optical cavity has an annular configuration.

23. The optical circuit according to any one of claims 16 to 21, wherein the first polarization-maintaining optical cavity and the second polarization-maintaining optical cavity have an annular configuration, and wherein the third polarization-maintaining optical cavity has a linear configuration.

24. The optical circuit according to any one of claims 16 to 21, wherein the first polarization-maintaining optical cavity and the third polarization-maintaining optical cavity have a linear configuration, and wherein the second polarization-maintaining optical cavity has an annular configuration.

25. The optical circuit according to any one of the preceding claims, wherein the optical circuit comprises N polarization-maintaining optical cavities, the N polarization-maintaining optical cavities comprising the first polarization-maintaining optical cavity and the second polarization-maintaining optical cavity, and each of the N polarization-maintaining optical cavities shares a common branch with an adjacent polarization-maintaining optical cavity, wherein the common branch does not comprise a saturable absorber.

26. The optical circuit according to claim 25, wherein the gain medium of each polarization-maintaining optical cavity is capable of being excited by a pump light source to generate light within a wavelength range that does not overlap with the wavelength range of light generated by exciting the gain medium of an adjacent polarization-maintaining optical cavity.

27. The optical circuit according to any one of claims 25 to 26, wherein at least N - 1 of the polarization-maintaining optical cavities comprise optical delay lines for matching the length of the cavities for synchronization purposes.

28. The optical circuit according to claim 27, wherein the optical delay line comprises a fiber pigtail optical delay line.

29. The optical circuit according to any one of claims 25 to 28, wherein each of the N polarization-maintaining optical cavities has a ring configuration.

30. The optical circuit according to any one of claims 25 to 28, wherein 1 ≤ M ≤ 2 of the N polarization-maintaining optical cavities have a linear configuration, and N - M of the N polarization-maintaining optical cavities have a ring configuration.

31. A laser device for outputting filtered optical pulses to induce coherent Raman scattering in a sample, the laser device comprising: the optical circuit according to any one of the preceding claims; and a first optical filter and a second optical filter, wherein the first optical filter and the second optical filter are configured to filter light from the first polarization-maintaining optical cavity and the second polarization-maintaining optical cavity respectively, so as to output a first filtered optical pulse within a first predetermined wavelength range and a second filtered optical pulse within a second predetermined wavelength range.

32. The laser device according to claim 31, wherein at least one of the first optical filter or the second optical filter is a tunable optical filter and is configured to change the first predetermined wavelength range or the second predetermined wavelength range respectively, and optionally, wherein the tunable optical filter comprises a Fabry - Perot based fiber optic tunable filter.

33. The laser device according to any one of claims 31 to 32, wherein the first optical filter and the second optical filter are respectively positioned within the first polarization-maintaining optical cavity and the second polarization-maintaining optical cavity, and wherein the first polarization-maintaining optical cavity and the second polarization-maintaining optical cavity output the filtered optical pulses at a first optical output and a second optical output respectively.

34. The laser device according to any one of claims 31 to 33 further includes a first fiber amplifier doped with a first gain medium at the first optical output and a second fiber amplifier doped with a second gain medium at the second optical output, for amplifying the light or the filtered optical pulses.

35. The laser device according to claim 34, wherein the second harmonic generation crystal, the acousto-optic modulator or the electro-optic modulator is operatively coupled to an optical output of at least one of the first fiber amplifier or the second fiber amplifier.

36. The laser device according to any one of claims 31 to 34, wherein the acousto-optic modulator or the electro-optic modulator is operatively coupled to an output of at least one of the first polarization-maintaining optical cavity or the second polarization-maintaining optical cavity.

37. The laser device according to any one of claims 31 to 36, wherein the laser device is one of a fiber laser or an all-fiber laser.

38. The laser device according to any one of claims 31 to 37, wherein the predetermined wavelength range corresponds to the full Raman spectrum of 0 cm -1 - 4000 cm -1 and optionally, wherein the predetermined wavelength range includes at least one range of 1000 nm to 1100 nm and / or 1535 nm to 1600 nm and / or 910 nm to 950 nm and / or 780 nm to 800 nm and / or 1800 nm to 1900 nm.

39. An optical device, the optical device includes the laser device according to any one of claims 31 to 38 and two collimators, the two collimators are configured to collimate the filtered optical pulses, and optionally, the optical device further includes a dichroic mirror, the dichroic mirror is configured to combine the collimated optical pulses from the two collimators.

40. A method for outputting filtered optical pulses from a laser device to induce coherent Raman scattering in a sample, the method includes: generating lights in respective different wavelength ranges by using a first polarization-maintaining optical cavity including a first gain medium and a second polarization-maintaining optical cavity including a second gain medium different from the first gain medium, wherein the first gain medium and the second gain medium are each capable of being excited by a pump light source; mode-locking the lights from the first polarization-maintaining optical cavity and the second polarization-maintaining optical cavity by using a first saturable absorber and a second saturable absorber optically coupled to the first polarization-maintaining optical cavity and the second polarization-maintaining optical cavity respectively; filtering the lights from the first polarization-maintaining optical cavity and the second polarization-maintaining optical cavity by using a first optical filter and a second optical filter respectively; outputting a first filtered optical pulse in a first predetermined wavelength range from the first optical filter and outputting a second filtered optical pulse in a second predetermined wavelength range from the second optical filter; and synchronizing the lights from the first polarization-maintaining optical cavity and the second polarization-maintaining optical cavity via a common branch between the first polarization-maintaining optical cavity and the second polarization-maintaining optical cavity, wherein the common branch does not include a saturable absorber.

41. The method according to claim 40, wherein the first predetermined wavelength range does not overlap with the second predetermined wavelength range.

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