Optical circuit configuration

Abstract

1. An optical circuit for a laser apparatus 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 passively mode-lock optical pulses in the first polarization-maintaining optical cavity (110a); and a second polarization-maintaining optical cavity (110b) comprising a second gain medium (140b) different from the first gain medium (140a) excitable by a second pump light source (130b) to generate light in a second wavelength range and a second saturable absorber (170b) configured to passively mode-lock optical pulses in the second polarization-maintaining optical cavity (110b), wherein the first polarization-maintaining optical cavity (110a) and the second polarization-maintaining optical cavity (110b) share a common branch (120), the common branch (120) not including a saturable absorber.

Description

[Technical Field]

[0001] The present invention relates to optical circuitry, and more particularly to optical circuitry for use in coherent Raman microspectroscopy laser devices. [Background technology]

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

[0003] CRS can be implemented in narrowband or broadband ways. Narrowband means that a single vibrational frequency is excited at a time, while broadband means that multiple vibrational modes are excited simultaneously. The narrowband approach, commonly referred to as "hyperspectral CRS," is based on a narrowband tunable laser source, which allows for the reconstruction of a vibrational spectrum by serially acquiring the system's response in different Raman modes. The broadband approach, commonly defined as "multiplexed CRS," relies on the combination of broadband and narrowband optical pulses, thereby resulting in the parallelization of Raman modes that can be simultaneously excited and detected. Multiplexed CRS has the significant advantage of single-shot spectral acquisition, which allows for fast, chemically selective imaging at once. Multiplexed CRS imaging is a particularly useful technique due to the absence of nonresonant background.

[0004] CRS requires the use of ultrafast, picosecond or faster synchronized lasers from two laser sources, with pump and Stokes pulses matched in Raman frequency and bandwidth to establish and detect vibrational coherence within the sample. Currently, laser sources for CRS cover the entire Raman spectrum (0–4000 cm). -1 ) is widely used (0-4000 cm) -1 ). Such solid-state laser devices contain bulk pieces of doped crystal or glass as the gain medium and require the use of bulky optics. This makes them prone to misalignment and instability, as well as high capital costs. Their relatively large footprint also limits their effective deployment in clinical settings, as they cannot be easily moved between different wards within a hospital or conveniently handled.

[0005] The use of fiber-format lasers has gained popularity in recent years because such laser devices offer a simpler, more cost-effective pump source with a smaller footprint. They are also more reliable and do not require alignment compared to solid-state laser-pumped optical parametric oscillators.

[0006] U.S. Patent No. 7,372,880 discloses a pulsed fiber laser capable of generating ultrashort optical pulses. The pulsed fiber laser includes an optical ring resonator with a length of rare-earth-doped fiber as a gain medium. In use, the gain medium generates optical gain within the resonator in response to an excited light source. To facilitate pulse generation, carbon nanotubes (CNTs) are used as nonlinear optical or saturable absorber materials to convert a continuous-wave laser into an ultrafast train of optical pulses. A saturable absorber is an optical component that has a certain optical loss that is reduced at high optical intensities. Each time a pulse strikes the saturable absorber as it circulates through the optical ring resonator, the pulse saturates the absorber's absorption, temporarily reducing the loss. With each round trip through the resonator, the saturable absorber favors light with a certain high intensity because this light can saturate the absorption slightly less than light with a lower intensity. After many round trips, a single pulse remains.

[0007] In this field, research is being conducted towards synchronizing two-wavelength ultrafast laser sources using passive mode-locking techniques 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, for example, by optically coupling the common saturable absorber to two fiber cavities doped with different rare-earth materials.

[0008] Zhang et al., "Passive synchronization of all-fiber lasers through a common saturable absorber," Optics Letter, (2011) (Zhang) discloses the synchronization of two all-fiber mode-locked lasers operating at 1 μm and 1.54 μm, coupled using a common single-walled carbon nanotube absorber. Furthermore, Zhang et al., "Ultrafast fiber laser sources; Example of recent developments," Optical Fiber Technology, (2014) summarizes recent developments in the field of ultrafast, compact all-fiber lasers. More specifically, Zhang discloses the use of graphene and single-walled carbon nanotubes as passive elements to perform synchronization and passive mode-locking of laser pulses in two coupled optical cavities. The optical cavities include ytterbium- or erbium-doped fiber gain media to generate dual-wavelength optical pulses for pump-probe spectroscopy.

[0009] Sotor et al., "Passive synchronization of Erbium and Tulium doped fiber mode-locked lasers enhanced by a common graphene saturable absorber," Optics Express (2014) (Sotor) discloses the use of a common graphene saturable absorber to synchronize optical pulses from two loop resonators, each containing one of the erbium and thulium doped fiber gain media. In Sotor, a 1569 nm laser diode is used to pump the thulium gain medium to generate 2 μm optical pulses. A wavelength division multiplexer (WDM) filter is provided to remove unabsorbed pump light at 1569 nm, which is then purged into the erbium loop resonator, which outputs 1.5 μm optical pulses. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] U.S. Patent No. 7,372,880 [Non-patent literature]

[0011] [Non-Patent Document 1] Zhang et al., “Passive synchronization of all-fiber lasers through a common saturable absorber”, Optics Letter, (2011) [Non-patent document 2] Zhang et al., “Ultrafast fiber laser sources;Example of recent developments”, Optical Fiber Technology,(2014) [Non-patent document 3] Sotor et al., “Passive synchronization of Er-bium and Tulium doped fiber mode-locked lasers enhanced by common graphene saturable absorber”, Optics Express (2014) Summary of the Invention [Problem to be solved by the invention]

[0012] It would be desirable to provide a more efficient and robust dual-cavity passively synchronized optical circuit for use in laser devices.

[0013] Various aspects of the invention are defined in the following independent claims, to which reference should now be made. Optional features are set out in the dependent claims. [Means for solving the problem]

[0014] In a first aspect, the present disclosure provides an optical circuit for a laser device, particularly a laser device used 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 capable of being pumped by a first pump light source to generate light in a first wavelength range and a first saturable absorber configured to perform passive mode-locking of 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 capable of being pumped by a second pump light source to generate light in a second wavelength range and a second saturable absorber configured to perform passive mode-locking of 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, and the common branch does not include a saturable absorber.

[0015] Saturable absorbers can be used to initiate and enhance powerful intracavity pulses through intensity-dependent loss. That is, the pulse (probe) sees a reduction in loss induced by the higher-energy pulse (pump). Unlike shared saturable absorber laser cavity configurations, the disclosed common branch optical circuit configuration utilizes independent saturable absorbers in each optical cavity. As a result, the operational lifetime of the disclosed optical circuit configuration can be extended due to reduced stress on the saturable absorber resulting from excessive nonlinear saturable absorption / heating induced by simultaneous pulses from multiple optical cavities, as in known shared saturable absorber laser cavity configurations. Furthermore, the inventors of the present disclosure have found that the characteristics of the saturable absorber can vary depending on the number of wavelengths transmitted through the saturable absorber. This crosstalk effect is unpredictable and typically detrimental to the mode-locking and / or synchronization mechanisms of each individual wavelength in the common branch shared by the two optical cavities. Therefore, by providing two independent saturable absorbers, one for each optical cavity, each saturable absorber can be optimized for the wavelength associated with its respective optical cavity, avoiding undesirable crosstalk effects.

[0016] Similar to the shared saturable absorber laser cavity configuration, the disclosed common branch optical circuit configuration utilizes passive optical synchronization resulting from 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 nonlinearity of the medium. The interaction length of the common branch affects the locking range via XPM. Generally, the longer the common branch, the better the XPM interaction, but the group-velocity mismatch (GVM) phenomenon imposes an upper limit on the allowable length of the common branch. We found that the disclosed optical circuit, which includes a relatively short common branch, can be as effective as full cavity injection in terms of passive synchronization. Due to its slave-slave optical structure, the disclosed common branch optical circuit may exhibit improved passive synchronization over cavity injection configurations. In this configuration, repetition rate-varying feedback due to XPM affects the pulses in both optical cavities, increasing the allowable cavity-length mismatch in the optical circuit. Furthermore, performing XPM in the common branch between the two optical cavities ensures sufficient XPM interaction for synchronization without the need for external amplification, resulting in a more efficient optical circuit with a lower lasing threshold.

[0017] The lasing threshold is the lowest excitation level at which the output of a laser is dominated by stimulated emission rather than spontaneous emission. Below the threshold, the output power of a laser increases slowly with increasing excitation. Above the lasing threshold, the slope of power versus excitation increases by an order of magnitude. The linewidth of the laser's emission also decreases by an order of magnitude above the lasing threshold. Above the lasing threshold, the laser is said to be lasing.

[0018] The configuration of the first and second optical cavities in the disclosed optical circuits can take several different forms.

[0019] 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 cavities follow a path that forms a complete / continuous optical loop. In a ring-ring configuration, portions of the two optical cavities are coupled by a common branch. Compared to a linear cavity configuration, the pulse in each optical cavity passes through the saturable absorber only once, so when using a transmissive saturable absorber (CNT, graphene, or transmissive semiconductor-based SA), the ring-ring cavity configuration has a lower loss per optical cavity round trip. Because the presence of a common branch WDM prevents the pump light from reaching the active fiber due to the presence of an optical filter, 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.

[0020] 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 a linear configuration, the optical cavity follows a linear path without end-to-end intersections. Compared to a ring-ring configuration, the linear-ring configuration has the advantage of being more compact, since the total fiber length is half that of a ring-cavity pair. Furthermore, for dispersion-compensated cavities employing FBGs and SESAMs, the linear cavity configuration always results in lower loss and a lower lasing threshold than a ring-cavity pair.

[0021] In a third embodiment, the first polarization-maintaining optical cavity can have a linear configuration, and the second polarization-maintaining optical cavity can have a linear configuration. Advantageously, the linear-linear optical cavity configuration effectively doubles the XPM interaction length, thereby increasing the allowable cavity mismatch in the optical circuit. This occurs because in a linear-linear configuration, the cavity pulse passes through the common branch twice for each cavity round trip.

[0022] Optionally, the first saturable absorber is different from the second saturable absorber.

[0023] Optionally, at least one of the first saturable absorber or the second saturable absorber comprises at least one of graphene / carbon allotropes, single-walled carbon nanotubes (SWCNTs), semiconductor saturable absorber mirrors (SESAMs), or transmissive semiconductor-based saturable absorbers. Advantageously, these saturable absorber (SA) types can provide effective mode-locking functionality over shorter fiber lengths compared to nonlinear amplification loop mirrors and nonlinear polarization rotation / generation SA types, thereby reducing the footprint of the optical cavity arrangement. This allows a larger proportion of fiber to be used in the common branch, resulting in greater cavity mismatch tolerance.

[0024] Any of the described SA types can be used in any of the described cavity configurations. Transmissive SAs, such as graphene / carbon allotrope, SWCNT, and transmissive semiconductor-based SAs, are particularly well suited to ring cavities because linear losses occur once per cavity round trip, leading to a lower lasing threshold. However, reflective SAs, such as SESAMs, are particularly well suited to linear cavities. When a transmissive SA is used in a linear cavity configuration, a fiber-coupled mirror must be provided at the output of the SA. Particularly advantageous SA combinations with reduced lasing thresholds include a first transmissive SA and a second transmissive SA for a ring-ring cavity configuration, a first transmissive SA and a second SESAM SA for a ring-linear cavity configuration, and a first SESAM SA and a second SESAM SA for a linear-linear cavity configuration.

[0025] Optionally, at least one of the first saturable absorber and / or the second saturable absorber is mounted on a temperature control system to ensure that the first saturable absorber and / or the second saturable absorber operate in a consistent and predictable manner.

[0026] Optionally, the common branch includes a highly nonlinear device or material. The highly nonlinear device may include one or more of a photonic crystal fiber (PCF), a highly nonlinear fiber (HNLF), a small mode area fiber, or a tapered fiber. The highly nonlinear material may include two-dimensional materials (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 approximately 2.7-2.8×10 -7 cm 2 / GW. The above materials have a higher nonlinear index or effective nonlinear coefficient relative to standard fiber, and therefore higher nonlinearity. Highly nonlinear devices typically have the same nonlinear index as standard fiber, but achieve higher nonlinearity due to a smaller mode area. Highly nonlinear devices or materials may be spliced between two single-mode polarization-maintaining fibers.

[0027] Optionally, the common branch includes a single-mode polarization-maintaining fiber.

[0028] Optionally, the common branch comprises a single mode fiber.

[0029] 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.

[0030] Optionally, at least one of the first gain medium or the second gain medium comprises a fiber doped with ytterbium, erbium, neodymium, or thulium.

[0031] Optionally, at least one of the first polarization-maintaining optical cavity or the second polarization-maintaining optical cavity includes a polarizing fiber optic coupler.

[0032] 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. The use of a delay line in one or more optical cavities enables pairing of non-identical optical cavities by equalizing their lengths.

[0033] Passive synchronization of multiple mode-locked fiber oscillators addresses many technical challenges, including precise timing distribution between different wavelength regions and multi-wavelength synchronous mode-locked laser sources for microscopy. The main advantage of passive XPM-based shared common branch synchronization mechanisms is their modularity, allowing for scalability to multiple optical cavities by adding new common branches shared with neighboring cavities.

[0034] In a fourth embodiment, the optical circuit may further include a third polarization-maintaining optical cavity, the third polarization-maintaining optical cavity including a third gain medium excitable 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 of 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, the common branch not including the saturable absorber.

[0035] The modularity of the disclosed optical circuit facilitates transfer synchronization from the first and second optical cavities to a third optical cavity adjacent to the second optical cavity via XPM. This arrangement allows the adoption of the lowest accessible order of XPM (third-order nonlinearity) thanks to the interaction of two pulses in a common branch, ensuring the strongest synchronization effect. The resulting synchronization range of adjacent optical cavities remains unaffected by the presence of other adjacent optical cavities.

[0036] Synchronizing multiple oscillators together offers advantages in broadband coherent Raman microscopy by enabling the generation of multiple pump / Stokes pairs that simultaneously cover multiple Raman regions. One implementation for broadband SRS microscopy can use a combination of two narrowband pump / Stokes beams coupled to a broadband Stokes / pump beam. One pump / Stokes probes one Raman-active region (i.e., C-H stretching) via the broadband Stokes / pump, while the other pump / Stokes is centered at a different wavelength that probes another Raman-active region (i.e., fingerprint) via the same broadband Stokes / pump.

[0037] By generating multiple pump / Stokes pairs directly from the oscillator, each optical cavity can be optimized for a desired wavelength range independently of other optical cavities in the optical circuit. Furthermore, the proposed solution can provide increased power spectral density at the detector for broadband SRS, resulting in a higher signal-to-noise ratio at a given total average power at the detector and Raman spectral coverage. Furthermore, the proposed solution can enable improved handling of non-ultrabroadband pulses in the microscope, since the use of multiple narrowband pump / Stokes beams requires narrower Stokes / pump pulses with less dispersion.

[0038] Optionally, the first wavelength range and the second wavelength range may be non-overlapping, and the second wavelength range and the third wavelength range are non-overlapping.

[0039] Optionally, the first gain medium comprises an erbium-doped fiber, the second gain medium comprises an ytterbium-doped fiber, and the third gain medium comprises a neodymium-doped fiber.

[0040] Optionally, the first gain medium comprises an erbium-doped fiber, the second gain medium comprises an ytterbium-doped fiber, and the third gain medium comprises a thulium-doped fiber.

[0041] Optionally, each common branch of the optical circuit is enclosed between two wavelength division multiplexers. In some embodiments, each common branch of the optical circuit is enclosed between two micro-optical filter couplers or micro-optical filter splitters.

[0042] 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 a ring configuration.

[0043] In one embodiment, the first polarization-maintaining optical cavity and the second polarization-maintaining optical cavity have a ring configuration, and the third polarization-maintaining optical cavity has a linear configuration.

[0044] 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 a ring configuration.

[0045] In another embodiment, the optical circuit comprises a third polarization-maintaining optical cavity, the third polarization-maintaining optical cavity comprising a third gain medium excitable by a third pump light source to generate light at a third range of wavelengths, and a third saturable absorber configured to perform passive mode-locking of 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 can share the same common branch, the common branch not including the saturable absorber.

[0046] In a second aspect, the present disclosure provides a laser apparatus for outputting filtered optical pulses for inducing coherent Raman scattering (CRS) in a sample, the laser apparatus may include any optical circuit configuration according to the first aspect described above. The laser apparatus may also include a first optical filter and a second optical filter configured to filter light from the first polarization-maintaining optical cavity and the second polarization-maintaining optical cavity, respectively, to output first filtered optical pulses in a first predetermined wavelength range and second filtered optical pulses in a second predetermined wavelength range.

[0047] The disclosed laser device enables sufficiently fast CRS for noninvasive imaging, i.e., obtaining objective and quantitative information about tissue by measuring its detailed molecular composition through the vibrational response detected by CRS. The example laser device also provides a convenient tool for pump-probe experiments, providing a suitable pump source for parametric mixing and frequency up / down conversion.

[0048] Generally, in the described laser device, each optical cavity of the laser device can include a gain element and a single-mode polarization-maintaining fiber. The lengths of the optical cavities can be matched using a fiber pigtail optical delay line inserted in half. Following a synchronized oscillator, a fiber amplifier can be provided to increase the average power of the two branches to the hundreds of milliwatts required for the application. In other words, two independent laser media are mode-locked and synchronized to provide pump and Stokes pulses for CRS. The two independent mode-locked optical cavities are synchronously locked by a shared XPM interaction via the common branch between the two optical cavities (i.e., the pulses have the same repetition rate and a constant optical delay between the two optical pulse trains). Frequency detuning can be achieved in a broadband configuration by employing a narrowband (broadband) pump and a wideband (narrowband) Stokes, or in a narrowband configuration by a tunable filter stage located inside or outside the cavity.

[0049] In contrast to known implementations of CRS, in which one of the two required independent pulses at different frequencies is generated by parametric amplification, in the laser apparatus described herein, different laser media emitting at different frequencies are passively synchronized, greatly simplifying the generation of broadband or polychromatic (multi-frequency) pulse sequences required for CRS.

[0050] In the described embodiment, two independent mode-locked oscillators or optical cavities are provided that are synchronized by XPM interactions in a shared cavity segment.

[0051] The laser device described herein passively synchronizes fiber lasers, providing a very simple and low-cost laser source for CRS. Fiber lasers enable a robust and stable light source due to their simple, compact, and cost-effective design and alignment-free operation without the need for bulky optical setups.

[0052] As explained below, embodiments of the laser device described herein have been applied to Coherent Anti-Stokes Raman Scattering (CARS) and Stimulated Raman Scattering (SRS) as proof of concept. Due to their compactness and all-optical synchronization, the embodiments described herein are good sources for CRS in the high wavenumber and fingerprint regions.

[0053] The arrangement is described in more detail below and takes the form of a laser device that outputs filtered light pulses for inducing coherent Raman scattering in the sample.

[0054] Optionally, both the first and second optical filters may include fiber Bragg gratings (FBGs) configured to output optical pulses at first and second predetermined wavelength ranges. A fiber Bragg grating is a short segment of fiber that reflects specific wavelengths of light and transmits all other wavelengths. This effect is achieved by creating periodic variations in the refractive index of the fiber core, creating a wavelength-selective mirror. FBGs can be designed similarly to chirped mirrors, thus introducing a predetermined dispersion into the wavelengths of the reflected light.

[0055] Optionally, both the first and second optical filters may comprise fixed wavelength optical filters configured to set the first and second predetermined wavelength ranges, respectively.

[0056] Optionally, both the first and second optical filters may comprise tunable optical filters configured to vary the first and second predetermined wavelength ranges, respectively, where the wavelength ranges may be specified by a user such that the wavelength ranges of the pump and Stokes optical pulses may be varied with respect to the sample being measured.

[0057] Optionally, tunable or fixed wavelength optical filters can include etalon-based fiber optic tunable or fixed wavelength filters. An etalon is a dielectric material whose specific thickness and refractive index dictate the bandwidth of each transmission peak, allowing only one wavelength to be transmitted at maximum transmission. Etalon-based fiber optic tunable or fixed wavelength filters function by selecting the refractive index of the material medium to select a specific resonant wavelength. Wavelengths resonant with the optical length of the cavity are transmitted, while other wavelengths are reflected.

[0058] Optionally, a first optical filter and a second optical filter are disposed in the first polarization-maintaining optical cavity and the second polarization-maintaining optical cavity, respectively, and the first polarization-maintaining optical cavity and the second polarization-maintaining optical cavity output the filtered optical pulses at the first optical outlet and the second optical outlet, respectively. Mounting the optical filters in the optical cavities ensures that optical pulses in undesired wavelength ranges are filtered out quickly after generation.

[0059] Optionally, the first and second optical filters are located outside the first and second polarization-maintaining optical cavities, respectively, which output the optical pulses at the first and second optical outlets, respectively. Locating the optical filters outside the optical cavities eliminates the need for repeated filtering of the recirculating filtered optical pulses, allowing for the construction of a simple and compact optical cavity.

[0060] The optical pulses are filtered so that only optical pulses within a defined wavelength range are output for coherent Raman spectroscopy, resulting in more accurate measurements. Furthermore, the use of two synchronized mode-locked laser sources significantly reduces the effect of optical filters on the optical power of the pump and Stokes pulses, making it a versatile choice for CRS.

[0061] Optionally, the laser device further comprises 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 in the correct wavelength range. The use of amplifiers mitigates the optical power reduction that occurs when an optical filter is installed.

[0062] Optionally, the laser device further includes a second harmonic generating crystal operably coupled to the optical output of at least one of the first fiber amplifier or the second fiber amplifier, and the second harmonic generating crystal may be formed from one or more of periodically poled lithium niobate (PPLN) or periodically poled potassium titanyl phosphate (PPKTP).

[0063] Optionally, the laser apparatus further includes an acousto-optic or electro-optic modulator operably coupled to at least one of the first amplifier or the second amplifier.

[0064] Optionally, the laser device further comprises an acousto-optic or electro-optic modulator operably coupled to the outlet of at least one of the first polarization-maintaining optical cavity or the second polarization-maintaining optical cavity.

[0065] Optionally, the spectrum of both the optical pulses exiting the first polarization-maintaining cavity or the second polarization-maintaining cavity can be broadened in the fiber amplifier section by self-phase modulation to increase the spectral bandwidth and decrease the compressed pulse duration.

[0066] 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 comprise isotropic optical fibers. Optionally, the first optical cavity and the second optical cavity each comprise a single mode optical fiber.

[0067] Optionally, the laser gain medium comprises ytterbium or erbium, and optionally the predetermined wavelength range produced by said laser gain medium is between 0 and 4000 cm. -1 corresponds to the entire Raman spectrum of

[0068] Optionally, the predetermined wavelength range comprises the ranges 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.

[0069] Optionally, frequency conversion of the amplified optical pulses can be performed using second harmonic generation crystals (i.e., periodically poled lithium niobate - PPLN -, periodically poled potassium titanyl phosphate - PPKTP -).

[0070] Optionally, an Acousto-Optic Modulator (AOM) or an Electro-Optic Modulator (EOM) can be located outside both the first polarization-maintaining cavity and the second polarization-maintaining cavity.

[0071] In a third aspect, the present disclosure provides an optical device. The optical device may include any of the laser devices described above. The optical device further includes two collimators configured to collimate the filtered light pulses, which limits the divergence of the filtered light pulses. Optionally, one of the collimators includes a delay stage configured to achieve overlap on the measured sample.

[0072] Optionally, the optical device further comprises a dichroic mirror configured to combine the collimated light pulses from both of the two collimators.

[0073] Optionally, the laser device includes a bandpass or shortpass filter to remove the filtered light pulse pairs before CARS detection.

[0074] Optionally, the laser device includes a bandpass or longpass filter to remove the pump light pulses prior to SRG detection.

[0075] Optionally, the laser device includes a bandpass or shortpass filter to remove the Stokes optical pulses prior to SRL detection.

[0076] In a fourth aspect, the present disclosure provides a method for outputting filtered optical pulses from a laser device to induce coherent Raman scattering in a sample. The method includes generating light in 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, where the first gain medium and the second gain medium are excitable by a pump light source. Mode-locking is performed via first and second saturable absorbers optically coupled to the first and second polarization-maintaining optical cavities, respectively. The method further includes synchronizing the light from the first and second polarization-maintaining optical cavities via a common branch between the first and second polarization-maintaining optical cavities, where the common branch does not include a saturable absorber.

[0077] 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 excitable by a pump light source to generate light at a range of wavelengths and a saturable layer configured to perform passive mode-locking of optical pulses in the polarization-maintaining optical cavities. Each of the N polarization-maintaining optical cavities shares a common branch with at least one other of the N polarization-maintaining optical cavities. Here, the common branch between the N polarization-maintaining optical cavities does not include a saturable absorber.

[0078] Optionally, the gain medium of each polarization-maintaining optical cavity is excitable by a pump light source to produce light in a range of wavelengths that does not overlap with the wavelength range of light produced by pumping the gain medium of an adjacent polarization-maintaining optical cavity.

[0079] Optionally, at least N-1 of the polarization-maintaining optical cavities include an optical delay line for matching the length of the cavities for synchronization purposes.

[0080] Optionally, the optical delay line comprises a fiber pigtail optical delay line.

[0081] Optionally, each of the N polarization-maintaining optical cavities has a ring configuration.

[0082] Arbitrarily, N polarization-maintaining optical cavities TIFF2025527289000002.tif6150 cavities have a linear configuration and NM cavities of the N polarization-maintaining optical cavities have a ring configuration.

[0083] It will be appreciated that features described in relation to the first, second, third and fourth aspects of the present disclosure are also applicable to the fifth aspect. The invention will now be described in more detail with reference to the accompanying drawings. [Brief explanation of the drawings]

[0084] [Figure 1]1 is a schematic diagram of an optical circuit according to a first embodiment of the present invention. [Figure 2] FIG. 4 is a schematic diagram of an optical circuit according to a second embodiment of the present invention. [Figure 3] 1 is a schematic diagram of an optical circuit according to a third embodiment of the present invention; [Figure 4] 1 is a schematic diagram of an optical device of the present invention; [Figure 5] 1 is a schematic diagram of an optical circuit according to an embodiment of the present invention. [Figure 6] 1 is a schematic diagram of an optical circuit according to an embodiment of the present invention. [Figure 7] 1 is a schematic diagram of an optical circuit according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0085] An optical circuit for use in a laser apparatus for inducing coherent Raman scattering in a sample according to an embodiment of the present invention will now be described with reference to Figures 1 to 3 and 5 to 7.

[0086] 1 shows an optical circuit 100 according to a first embodiment of the present invention. The optical circuit 100 includes two independent mode-locked optical cavities, oscillators, or resonators for generating two sets of optical pulses on the order of picoseconds in different wavelength ranges suitable for CRS. The two optical cavities 110a, 110b are coupled together at a common branch 120.

[0087] Continuing to refer to the optical circuit 100 of FIG. 1 in more detail, each of the two optical cavities 110a, 110b of the optical circuit 100 of FIG. 1 is arranged in the form of a loop. The ends of the optical fibers constituting the optical cavities 110a, 110b are optically connected using any suitable coupler to circulate optical pulses in the loop until they are emitted from the optical cavities at their respective optical outlets. Furthermore, each optical outlet includes a fiber coupler that provides approximately 20-30% of the output power to the respective cavity. Each of the mode-locked optical cavities 110a, 110b includes a pump light source 130a, 130b for each of the optical cavities 110a, 110b, which excites a gain element 140a, 140b disposed or deposited within the optical cavity 110a, 110b. The gain elements 140a, 140b in this example are optical fibers doped with rare-earth gain elements. Laser devices using such gain elements are commonly referred to as fiber lasers.

[0088] The selection of pump light sources 130a, 130b and gain elements 140a, 140b depends on the optical spectrum required by the CRS. In the example shown in Figure 1, two different optical fibers are used as gain elements. One optical fiber 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 for pumping the gain elements are 976 nm wavelength pump light source 130a for pumping the Yb-doped fiber and 976 nm wavelength pump light source 130b for pumping the Er-doped fiber. The optical pulses generated from the Yb and Er gain media are within the desired pump and Stokes wavelength ranges.

[0089] The optical isolators 150a, 150b are optically coupled within each optical cavity 110a, 110b to force circulation in the same direction within the common branch 120. This ensures that the optical pulses generated by the gain media 140a, 140b in the optical cavities travel in a single direction in the loop forming the optical cavities 110a, 110b. That is, the optical pulses generated from the gain media 140a, 140b are directed toward the optical exit. In this example, the optical isolators 150a, 150b are fiber-based Faraday isolators. In another example, the optical isolators 150a, 150b can include polarization circulators with dispersion compensation devices and output couplers.

[0090] The pairs of optical pulses generated in each optical cavity 110 a, 110 b are passively synchronized via XPM interactions in a common branch 120 shared by both loops forming the optical cavities 110 a, 110 b. In this example, the common branch 120 includes a highly nonlinear device or material 160 to enhance the XPM interaction strength for synchronization.

[0091] Each optical cavity 110a, 110b includes its own saturable absorber 170a, 170b outside the common branch 120. The function of a saturable absorber is explained in the Summary of the Invention section above. A saturable absorber is an optical absorber whose degree of absorption decreases at high optical intensities. In the optical circuit 100, this allows passively mode-locked pulses to circulate in each of the optical cavities 110a, 110b. That is, passive mode-locking enables the generation of femtosecond optical pulses. The saturable absorbers 170a, 170b have a sufficiently short recovery time so that high-speed loss modulation can be achieved.

[0092] The saturable absorbers 170a and 170b in Figure 1 may be graphene-based polymer composite saturable absorbers with ultrafast recovery times and broadband operation. Graphene saturable absorbers can be prepared by exfoliating bulk graphite using mild sonication. The resulting dispersions, initially enriched with single-layer and few-layer graphene, are then mixed with an aqueous solution of polyvinyl alcohol to obtain a polymer composite. Other saturable absorbers, such as those containing single-walled carbon nanotubes (CNTs), can alternatively be used to passively mode-lock optical pulses, although any of the saturable absorbers described in the Summary of the Invention section above can be used.

[0093] The pair of optical cavities 110a, 110b need not be identical. The difference in cavity length between the two optical cavities 110a, 110b can be compensated for by adding optical delay lines 180a, 180b to one or both of the optical cavities. In this example, the optical delay lines 180a, 180b are positioned in both 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 for each optical cavity 110a, 110b. In another example, the optical delay lines 180a, 180b can include output couplers in series.

[0094] The wavelength range of the optical pulses generated in each optical cavity 110a, 110b is determined by the type of gain medium pumped in the respective optical cavity.

[0095] Each optical cavity 110a, 110b has an outlet for outputting both 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 100. In one example, in the case of a circulator with a dispersion compensation device and an output coupler, the outlets may be located at 150a, 150b. The optical outlets may be coupled to different fiber amplifiers. The associated fiber amplifiers may be doped with gain elements corresponding to the gain media 140a, 140b responsible for generating the optical pulses. In one example, Yb- and Er-doped fiber amplifiers are provided in the optical cavities 110a, 110b, respectively, to amplify optical pulses of Yb and Er wavelengths to an average power of 100 mW.

[0096] Figure 2 shows an optical circuit 200 according to a second embodiment of the present invention. Similar to the optical circuit 100 shown in Figure 1, the optical circuit 200 comprises two independent mode-locked optical cavities, oscillators, or resonators, which generate two sets of optical pulses on the order of picoseconds in different wavelength ranges suitable for CRS. The two optical cavities 210a, 210b are coupled together at a common branch 220.

[0097] Unlike the optical circuit 100, in the optical circuit 200, only one of the two optical cavities 210b of the optical circuit 200 is arranged in a loop, while the other optical cavity 210a is arranged in a linear configuration. Similar to the optical circuit 100, the mode-locked optical cavities 210a, 210b of the optical circuit 200 each include a pump light source 230a, 230b for each of the optical cavities 210a, 210b, respectively, to excite gain elements 240a, 240b disposed or deposited within the optical cavities 210a, 210b. The gain elements 240a, 240b are optical fibers doped with rare-earth gain elements in this example.

[0098] The optical isolator 250 is optically coupled into the optical cavity 210b to ensure that the optical pulses generated by the gain medium 240b travel in a single direction within the optical cavity 210b, forming a loop. That is, the optical pulses generated from the gain medium 240b are directed toward the optical exit. In this example, the optical isolator 250 can include a fiber-based Faraday isolator. In another example, the optical isolator 250 can include a polarization circulator with a dispersion compensation device and an output coupler.

[0099] As described in connection with the optical circuit 100, pairs of optical pulses generated in each of the optical cavities 210 a, 210 b are passively synchronized via XPM interactions in a common branch 220 shared by both optical cavities 210 a, 210 b. In this example, the common branch 220 includes a highly nonlinear device or material 260 to enhance the XPM interaction strength for synchronization.

[0100] Each of the optical cavities 210a, 210b includes its own saturable absorber 270a, 270b outside the common branch 220. As described in connection with the optical circuit 100, in the optical circuit 200, this allows passive mode-locked pulses to circulate within each of the optical cavities 210a, 210b. The saturable absorbers 270a, 270b have a sufficiently short recovery time so that fast loss modulation is achieved. Any of the saturable absorbers mentioned in this disclosure can be utilized in the optical circuit 200. If a transmissive saturable absorber is provided in the linear optical cavity, a fiber-coupled mirror must be provided in the optical cavity after the saturable absorber.

[0101] The difference in cavity length between the two optical cavities 210a, 210b is compensated for by adding an optical delay line 280a, 280b to one of the optical cavities. In this example, the optical delay line 280a is located in the optical cavity 210a after the gain medium 240a, while the optical delay line 280b is located in the optical cavity 210b after the isolator 250. In this example, the optical delay lines 280a, 280b include fiber pigtail delay lines. The fiber pigtail delay line is optically coupled to the exit of the isolator 250 in the optical cavity 210b. In another example, the optical delay lines 280a, 280b can include output couplers in series.

[0102] Optical cavity 210a includes a dispersion compensator and output coupler (a chirped fiber Bragg grating in this example) 190 and outputs first filtered optical pulses in a first predetermined wavelength range from optical circuit 200. Optical cavity 210b includes a corresponding outlet for outputting second filtered optical pulses in a second predetermined wavelength range from optical circuit 200. In one example, the outlets can be located at 190 and 280b. The optical outlets can each be connected to a different fiber amplifier. The associated fiber amplifiers can be doped with gain elements corresponding to the gain media 240a, 240b responsible for optical pulse generation. In one example, Yb- and Er-doped fiber amplifiers are provided in optical cavities 210a, 210b, respectively, to amplify optical pulses at Yb and Er wavelengths to an average power of 100 mW.

[0103] 3 shows an optical circuit 300 according to a third embodiment of the present invention. Similar to optical circuits 100 and 200, optical circuit 300 includes two independent mode-locked optical cavities, oscillators, or resonators for generating two sets of optical pulses on the order of picoseconds in different wavelength ranges suitable for CRS. The two optical cavities 310a, 310b are coupled together at a common branch 320.

[0104] Unlike the optical circuits 100 and 200, in the optical circuit 300, both of the two optical cavities 310a, 310b of the optical circuit 300 are arranged in a linear configuration. Similar to the optical circuits 100 and 200, the mode-locked optical cavities 310a, 310b of the optical circuit 300 each include a pump light source 330a, 330b for each of the optical cavities 310a, 310b, which excites a gain element 340a, 340b disposed or deposited inside the optical cavities 310a, 310b. In this example, the gain elements 340a, 340b are optical fibers doped with rare-earth gain elements.

[0105] The optical circuit 300 can include polarization couplers 350a and 350b optically coupled to the outputs of the gain media 340a and 340b in the optical cavities 310a and 310b, respectively, which can be used to select the polarization state on the slow axis and serve as optical outputs.

[0106] As described in connection with the optical circuits 100 and 200, pairs of optical pulses generated in each of the optical cavities 310 a, 310 b are passively synchronized via XPM interactions in the common branch 320 shared by both optical cavities 310 a, 310 b. In this example, the common branch 320 includes a highly nonlinear device or material 360 to enhance the XPM interaction strength for synchronization. In this linear-linear optical cavity configuration, each optical pulse generated in each of the optical cavities 310 a, 310 b passes through the common branch 320 twice, thereby increasing the effective XPM interaction length. This improves the tolerable cavity mismatch in the optical circuit 300.

[0107] Each optical cavity 310a, 310b includes its own saturable absorber 370a, 370b outside the common branch 320. As described in connection with optical circuits 100 and 200, in optical circuit 300, this allows passively mode-locked pulses to circulate through each optical cavity 310a, 310b. The saturable absorbers 370a, 370b have a sufficiently short recovery time so that fast loss modulation is achieved. Any of the saturable absorbers mentioned in this disclosure can be utilized in optical circuit 300.

[0108] The difference in cavity length between the two optical cavities 310a, 310b is compensated for by adding an optical delay line 380a, 380b to one of the optical cavities. In this example, optical delay line 380a is located in optical cavity 310a after gain medium 340a, and optical delay line 380b is located in optical cavity 310b after gain medium 340b. In this example, optical delay lines 380a, 380b can include fiber pigtail delay lines.

[0109] The optical cavities 310a, 310b include dispersion compensators and output couplers 390a, 390b, and output first filtered optical pulses in a first predetermined wavelength range and second filtered optical pulses in a second predetermined wavelength range from the optical circuit 300 (chirped fiber Bragg gratings in this example). The optical outputs may be coupled to different fiber amplifiers, respectively. The associated fiber amplifiers may be doped with gain elements corresponding to the gain media 340a, 340b responsible for generating the optical pulses. In one example, Yb- and Er-doped fiber amplifiers are provided in the optical cavities 310a, 310b, respectively, to amplify optical pulses at Yb and Er wavelengths to an average power of 100 mW.

[0110] In other embodiments, the dispersion compensators and output couplers 390a, 390b may be replaced with high reflectivity mirrors and polarization couplers 350a, 350b may be used as output couplers.

[0111] In each of the disclosed optical circuits 100, 200, and 300, the location of the SA and / or the common branch with respect to the components in the circuit can be varied, provided that the common branch 320 and dedicated WDM are located in each cavity after the pump diodes 330 a, 330 b and active fiber gain elements 340 a, 340 b. The SA can be located either before or after the common branch.

[0112] 4 shows an optical device 400 according to the present invention. The optical device 400 includes an optical circuit 410 together with an additional optical element 420. The optical circuit 410 may include any of the optical circuits 100, 200, or 300 described above. The optical element 420 directs the optical pulses generated by the optical circuit 410 to illuminate a sample 450 where coherent Raman scattering is taking place. The scattering from the sample is filtered by a short-pass, band-pass, or long-pass filter 460 before entering a multi-channel dispersive detector 470 (spectrometer, multi-channel lock-in amplifier).

[0113] The optical element 420 of the configuration or setup shown in FIG. 4 includes collimators 430a and 430b. The optical circuit 410 outputs two filtered light pulses through each collimator. Thus, the two filtered light pulses are collimated in the respective collimators 430a and 430b to limit the beam divergence of the light pulses when they are combined in a subsequent combining step by the dichroic mirror 440a. If it is necessary to achieve overlap on the sample 450, one of the two collimators 430a and 430b may be placed on a delay stage 430c. Alternatively or additionally, the delay stage 430c may be provided before the collimators 430a and 430b.

[0114] The optical element 420 in the arrangement or configuration shown in FIG. 4 also includes a dichroic mirror 440a. A dichroic mirror is a mirror with different reflection and transmission characteristics at different wavelengths. Two parallel light pulses from different cavities are combined using the dichroic mirror 440a. They are then focused onto the sample 450. A short-pass, band-pass, or long-pass filter 460 and a multichannel dispersive detector 470 are positioned downstream of the sample. In the CARS detection configuration, the pump and Stokes light pulses from the sample are filtered out using the short-pass filter 460. A short-pass filter is a filter with a very sharp transition from transmission to reflection. The resulting CARS spectrum is measured by the spectrometer 470. In the SRS configuration (either SRG or SRL), the pump (SRG) or Stokes (SRL) light pulse is filtered out after the sample by the long-pass (SRG) or short-pass (SRL) light filter 460. The resulting SRG or SRL spectrum is measured with a multi-channel lock-in amplifier 470 .

[0115] 5 shows an optical circuit 500 according to one embodiment of the present invention, comprising three optical cavities 510a, 510b, 510c, each comprising a pump light source 530a, 530b, 530c for each of the optical cavities 510a, 510b, 510c, which excites a gain element 540a, 540b, 540c disposed or deposited within the optical cavity 510a, 510b, 510c. The gain elements 540a, 540b, 540c in this example are optical fibers doped with rare earth gain elements.

[0116] The first optical cavity 510a shares a common branch with the second optical cavity 510b in a manner equivalent to the optical circuit 200 of Figure 2. Unlike the optical circuit 200, the second optical cavity 510b of the optical circuit 500 includes a second common branch that is shared with a third optical cavity 510c.

[0117] The optical isolator 550 is optically coupled into the optical cavity 510b to ensure that the optical pulses generated by the gain medium 540b travel in a single direction within the optical cavity 510b, forming a loop. That is, the optical pulses generated from the gain medium 540b are directed toward the optical exit. In this example, the optical isolator 550 can include a fiber-based Faraday isolator. In another example, the optical isolator 550 can include a polarization circulator with a dispersion compensation device and an output coupler.

[0118] The optical pulses generated in each of the optical cavities 510 a, 510 b, and 510 c are passively synchronized via XPM interactions in a common branch shared between the optical cavities 510 a-510 b and 510 b-510 c, which in this example includes highly nonlinear devices or materials 560 a, 560 c to enhance the XPM interaction strength for synchronization.

[0119] Each of the optical cavities 510a, 510b, and 510c includes its own saturable absorber 570a, 570b, and 570c outside the common branch. As described above, this allows passively mode-locked pulses to circulate in each of the optical cavities 510a, 510b, and 510c. The saturable absorbers 570a, 570b, and 570c have sufficiently short recovery times so that fast loss modulation is achieved. Any of the saturable absorbers mentioned in this disclosure can be utilized in the optical circuit 500.

[0120] Differences in cavity length between optical cavities 510a-510b and between optical cavities 510b-510c can be compensated for by adding optical delay lines 580a, 580b, and 580c to one or more of the optical cavities. In this example, optical delay lines 580a and 580c are disposed in optical cavities 510a and 510c after gain media 540a and 540c, respectively, and optical delay line 580b is disposed in optical cavity 510b after isolator 550. In this example, optical delay lines 580a, 580b, and 580c include fiber pigtail delay lines. The fiber pigtail delay line is optically coupled to the output of isolator 550 at optical cavity 510b. In another example, optical delay lines 580a, 580b, and 580c can include output couplers in series.

[0121] Optical cavities 510a and 510c include dispersion compensators and output couplers (chirped fiber Bragg gratings in this example) 590a and 590c, which output first filtered optical pulses in a first predetermined wavelength range from the optical circuit 500. Optical cavity 510b has corresponding outlets for outputting second filtered optical pulses in a second predetermined wavelength range from the optical circuit 500. In one example, the outlets can be located at 590a, 590c, and 580b. Each optical outlet can be connected to a different fiber amplifier. The associated fiber amplifiers can be doped with gain elements corresponding to the gain media 540a, 540b, and 540c responsible for generating the optical pulses. In one example, Er-, Yb-, and Nd-doped fiber amplifiers are provided for optical cavities 510a, 510b, and 510c, respectively, to amplify optical pulses at Er, Yb, and Nd wavelengths to an average power of 100 mW.

[0122] FIG. 5 shows an optical circuit 500 having three optical cavities 510a, 510b, 510c arranged in a linear-ring-linear configuration. However, it will be understood that alternative configurations are possible. For example, FIG. 6 shows an optical circuit 600 similar to that shown in FIG. 5, having three optical cavities 610a, 610b, 610c. However, in optical circuit 600, optical cavity 610a is in a ring configuration, thereby forming an optical circuit 600 in a ring-ring-linear configuration. Despite the difference in configuration, optical circuit 600 can function in the same manner as optical circuit 500.

[0123] FIG. 7 shows having three optical cavities 710a, 710b, 710c arranged in a ring-ring-ring configuration. As described in connection with optical circuit 600, the features described with respect to optical circuit 500 are equally applicable to optical circuit 700.

[0124] Although not shown, it will be understood that an optical circuit according to the present invention can include more than three optical cavities in various configurations. In one example, the optical circuit includes N optical cavities, and each optical cavity shares 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 can be arranged in a ring / loop configuration.

[0125] The embodiments of the present invention have been described above. It will be understood that changes and modifications can be made to the described embodiments within the scope of the present invention.

Claims

1. A first gain medium that can be excited by a first pump light source to generate light in a first wavelength range, A first polarization-maintaining optical cavity comprising: a first saturable absorber configured to perform passive mode-locking of optical pulses in the first polarization-maintaining optical cavity; A second gain medium, different from the first gain medium, which can be excited by a second pump light source to generate light in a second wavelength range, A second polarization-maintaining optical cavity comprising: a second saturable absorber configured to perform passive mode-locking of optical pulses in the second polarization-maintaining optical cavity; An optical circuit for a fiber laser apparatus, wherein the first polarization-maintaining optical cavity and the second polarization-maintaining optical cavity have a common branch for passive synchronization, the common branch is surrounded by two wavelength-division multiplexers, the common branch does not contain a saturable absorber, and the optical pulses generated by the second polarization-maintaining optical cavity have the same polarization axis in the common branch as the optical pulses generated by the first polarization-maintaining optical cavity.

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

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

5. The optical circuit according to claim 1, wherein the first saturable absorber is a saturable absorber of a different form from the second saturable absorber.

6. The optical circuit according to claim 1, wherein the common branch includes a single-mode polarization-maintaining fiber or a single-mode fiber.

7. The optical circuit according to claim 1, wherein the common branch includes a highly nonlinear device or material.

8. Furthermore, comprising a third polarization-maintaining light cavity, the third polarization-maintaining light cavity is A third gain medium that can be excited by a third pump light source to generate light in a third wavelength range, The third saturable absorber is configured to perform passive mode-synchronization of the optical pulse in the third polarization-maintaining optical cavity, The optical circuit according to claim 1, wherein the second polarization-maintaining optical cavity and the third polarization-maintaining optical cavity share a common branch, and the common branch does not contain a saturable absorber.

9. The optical circuit according to claim 8, wherein the first wavelength range and the second wavelength range do not overlap, and the second wavelength range and the third wavelength range do not overlap.

10. The optical circuit according to claim 1, comprising N polarization-maintaining light cavities including the first and second polarization-maintaining light cavities, each of the N polarization-maintaining light cavities sharing a common branch with an adjacent polarization-maintaining light cavity, and the common branch not containing a saturable absorber.

11. The optical circuit according to claim 10, wherein the gain medium of each polarization-maintaining light cavity can be excited by a pump light source, and generates light with wavelengths that do not overlap with the wavelength range of light generated by exciting the gain medium of an adjacent polarization-maintaining light cavity.

12. The optical circuit according to claim 10, wherein each of the N polarization-maintaining optical cavities has a ring configuration.

13. The N polarization-maintaining light cavities The optical circuit according to claim 10, wherein the cavity has a linear configuration, and the N-M cavities of the N polarization-maintaining optical cavities have a ring configuration.

14. An optical circuit according to any one of claims 1 to 13, It comprises a first optical filter and a second optical filter, A fiber laser apparatus that outputs filtered light pulses for inducing coherent Raman scattering in a sample, wherein the first optical filter and the second optical filter are configured to filter light from the first polarization-maintaining light cavity and the second polarization-maintaining light cavity, respectively, to output a first filtered light pulse in a first predetermined wavelength range and a second filtered light pulse in a second predetermined wavelength range.

15. A step of generating light in different wavelength ranges using a first polarization-maintaining light cavity containing a first gain medium and a second polarization-maintaining light cavity containing a second gain medium different from the first gain medium, wherein the first gain medium and the second gain medium are each excitable by a pump light source. A step of mode-synchronizing light from the first polarization-maintaining light cavity and the second polarization-maintaining light cavity using first and second saturable absorbers optically coupled to the first polarization-maintaining light cavity and the second polarization-maintaining light cavity, respectively. The steps include filtering the light from the first polarization-maintaining light cavity and the second polarization-maintaining light cavity using the first optical filter and the second optical filter, respectively. The steps include outputting a first filtered light pulse at a first predetermined wavelength range from the first optical filter, and outputting a second filtered light pulse at a second predetermined wavelength range from the second optical filter, A method for outputting filtered optical pulses from a fiber laser apparatus to induce coherent Raman scattering in a sample, comprising the steps of: passively synchronizing light from a first polarization-maintaining optical cavity and a 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 is surrounded by two wavelength-division multiplexers, does not contain a saturable absorber, and the optical pulses generated by the second polarization-maintaining optical cavity have the same polarization axis in the common branch as the optical pulses generated by the first polarization-maintaining optical cavity.

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