Laser device

Abstract

A laser device (10) for outputting filtered light pulses for inducing coherent Raman scattering in a sample (42). A laser device (10) includes: a first optical cavity (20a) including a first gain medium (24a); and a second optical cavity (20b) comprising a second gain medium (24b) different from the first gain medium. Each of the first gain medium (24a) and the second gain medium (24b) is excitable by a pump light source (22a, 22b) to produce light in respective different wavelength ranges. A synchronizer (30) is optically coupled to the first optical cavity (20a) and the second optical cavity (20b). The synchronizer (30) is configured to synchronize and mode-lock light from the first light cavity (20a) and the second light cavity (20b). The laser device (10) further includes a first optical filter (34a) and a second optical filter (34b). The first optical filter (34a) and the second optical filter (34b) are configured to filter light from the first optical cavity (20a) and the second optical cavity (20b), respectively, to output a first filtered light pulse at a first predetermined wavelength range and a second filtered light pulse at a second predetermined wavelength range.

Description

[0001] Divisional Application Statement

[0002] This application is a divisional application of a PCT international application with an international application date of May 11, 2018 and an international application number of PCT / GB2018 / 051283, which entered the Chinese national phase on January 9, 2020, and a Chinese invention patent application with an invention name of “Laser Device” and an application number of 201880046105.7. Technical Field

[0003] The present invention relates to laser devices, and in particular to laser devices for use in conjunction with coherent Raman spectroscopy. Background Art

[0004] 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. This spontaneous Raman scattering is weak, so Raman spectroscopy is usually slow. Coherent Raman spectroscopy (CRS) includes coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS), and relies on nonlinear excitation of molecules and can increase the Raman intensity by several orders of magnitude. In theory, this enhancement of Raman intensity allows measurements to be made at video-rate imaging speeds, which means that CRS can be applied to a variety of applications in a variety of different fields.

[0005] CRS requires the use of synchronized ultrafast lasers at least in the picosecond range from two laser sources, where pump and Stokes pulses matched to the Raman frequency and bandwidth are used to establish and detect vibrational coherence in the sample. Currently, solid-state laser-pumped optical parametric oscillators have been widely used as laser sources for CRS because these laser sources can access the full Raman spectrum (0-4000cm -1 ). Such solid-state laser devices include bulk doped crystals or glasses as gain media and require the use of bulky optics. As a result, these devices are not only prone to misalignment and instability, but also come with high costs. In addition, their relatively large footprint makes them inefficiently deployed in clinical settings; for example, they cannot be easily moved around different wards in a hospital or operated conveniently.

[0006] In recent years, fiber lasers have become increasingly popular because they provide a simpler, more economical excitation source with a smaller footprint, are more reliable and require no calibration compared to solid-state laser-pumped optical parametric oscillators.

[0007] US 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 having a section of rare earth-doped optical fiber as a gain medium. In use, the gain medium responds to a pumped light source to generate optical gain in the resonator. To facilitate the generation of pulses, carbon nanotubes (CNTs) are used as nonlinear optical or saturable absorber materials to convert continuous wave laser light into a sequence of ultrafast optical pulses. A saturable absorber is an optical element with a certain optical loss, wherein the optical loss decreases at high light intensities. As the pulse circulates in the optical ring resonator, each time the pulse hits the saturable absorber, it saturates the absorption of the saturable absorber, thereby temporarily reducing the loss. In each loop trip of the resonator, the saturable absorber will prefer light with a higher intensity because such light can saturate the absorption slightly more than light with a lower intensity. After multiple loop trips, a single pulse will be maintained.

[0008] In order to generate synchronized optical pulses from two different laser sources, the synchronization of dual-wavelength ultrafast laser sources using passive mode locking techniques has been studied. This technique requires the use of a common saturable absorber shared by the two laser sources, for example, optically coupling the common saturable absorber into two optical fiber cavities doped with different rare earth materials.

[0009] Zhang et al., in "Passive synchronization of all-fiber lasers through a common saturable absorber" in Optics Letter, 2011, disclosed synchronization of two all-fiber mode-locked lasers operating at 1 μm and 1.54 μm and coupled by using a common single-walled carbon nanotube absorber. In addition, Zhang et al., in "Ultrafast fibre laser sources: Example of recent developments" in Optical Fiber Technology, 2014, outlined current developments in the field of ultrafast compact all-fiber lasers. Specifically, Zhang disclosed the use of graphene and single-walled carbon nanotubes as passive elements to achieve synchronization and passive mode locking of laser pulses in two coupled optical cavities. The cavities included an optical fiber gain medium doped with ytterbium or erbium for generating dual-wavelength optical pulses for pump-probe spectroscopy techniques.

[0010] Sotor et al., in "Passive synchronization of erbium and thulium doped fiber mode-locked lasers enhanced by common graphene saturable absorber" in Optics Express, 2014, disclose the use of a common graphene saturable absorber to synchronize light pulses from two ring resonators, each containing one of the erbium- and thulium-doped fiber gain media. In Sotor's disclosure, to generate light pulses at 2 μm, a 1569 nm laser diode is used to excite the thulium gain medium. A wavelength division multiplexer (WDM) filter is provided to filter out any unabsorbed pump light at a wavelength of 1569 nm, where the unabsorbed pump light is removed to the erbium ring resonator that will output 1.5 μm light pulses. Summary of the invention

[0011] The use of fiber lasers, especially all-fiber lasers, leads to the miniaturization of laser sources for CRS, thus realizing mobile or portable measurement systems. Such lasers also do not require calibration. In particular, their potential uses are expanded to different fields. However, the synchronously mode-locked dual laser pulses generated in the fiber laser are typically distributed over a wide wavelength range, or in the case disclosed by Soto, the laser pulses include unabsorbed pump light, which peaks at undesired wavelengths. Therefore, the measurement accuracy is significantly reduced when such fiber lasers are used in CRS.

[0012] Su et al. disclosed a single laser light source for coherent Raman spectroscopy in "Single-fiber-laser-based wavelength tunable excitation for coherent Raman spectroscopy" in the Journal of the Optical Society of America, Series B, 2013. In what Su disclosed, the frequency difference between the pump and Stokes pulses is caused by the self-frequency shift of solitons in nonlinear optical fibers (solitons are wave packets that propagate at a constant speed while maintaining their shape). It is disclosed that another possible way to improve the spectral resolution of CRS is to apply narrowband optical filters to the pump and Stokes beams. However, it is also disclosed that this will significantly reduce the optical power of the pump and Stokes beams, and therefore chirping (the frequency of the signal changes over time) is considered a better choice than optical filtering.

[0013] The present applicant alleviates the above-mentioned problem of improving the accuracy of CRS measurement by providing a laser device for outputting filtered light pulses for inducing coherent Raman scattering in a sample, wherein the laser device comprises: a first optical cavity including a first gain medium; and a second optical cavity including a second gain medium different from the first gain medium. Each of the first gain medium and the second gain medium can be excited by a pump light source to generate light in respective different wavelength ranges. In other words, two optical cavities are provided. The laser device also includes a synchronizer, and a first optical filter and a second optical filter, the synchronizer optically coupled to the first optical cavity and the second optical cavity, wherein the synchronizer is configured to synchronize and mode-lock the light from the first optical cavity and the second optical cavity, and the first optical filter and the second optical filter are configured to filter the light from the first optical cavity and the second optical 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.

[0014] In this way, significantly, a laser device is provided that enables CRS to be imaged non-destructively at a sufficiently fast speed. That is, the detailed molecular composition of the tissue is measured by the vibration response detected by the CRS, thereby obtaining objective and quantitative information of the tissue. The example of the laser device also provides a convenient tool for pump-probe experiments and provides a suitable pump source for parametric mixing and frequency up / down conversion.

[0015] Typically, in the described laser devices, each cavity of the laser device comprises a gain element and an isotropic single-mode fiber. The cavity length is matched using a fiber-pigtailed optical delay line inserted in one of the cavities. Saturable absorbers are used to initiate and promote strong intracavity pulse generation through intensity-dependent losses and to coordinate synchronization through absorptive nonlinear coupling, i.e., the pulse (probe) observes a reduction in losses caused by a higher-energy pulse (pump). Frequency tuning is achieved by a tunable filter stage located inside the cavity or at its output. After the synchronized oscillator, a fiber amplifier increases the average power of the two branches to the hundreds of milliwatts required for the application. In other words, two independent laser media are synchronously mode-locked to provide multicolor pulse trains for CRS. Passive synchronization is achieved by sharing a saturable absorber using nanomaterials. Frequency detuning is achieved by a tunable filter stage located inside or outside the cavity.

[0016] Specifically, in the described examples, for CRS, the laser device uses ultrafast fiber lasers synchronized by nanomaterials. Two independent mode-locked fiber lasers are locked in synchronization (i.e., pulses with the same repetition rate) through a shared interaction of a saturable absorber or synchronizer based on nanomaterials, such as graphene, allotropes of carbon, layered crystals, and hybrid nanomaterials.

[0017] Compared to the known implementation of CRS, in which one of the two required independent picosecond pulses of different frequencies is produced by parametric amplification, in the laser device described in this article, different laser media emitting at different frequencies are passively synchronized using nanomaterials, thereby greatly simplifying the generation of the multi-color (multi-frequency) pulse sequence required for CRS.

[0018] In the described example, two independent mode-locked oscillators or optical cavities are provided, which are synchronized by the shared interaction of a saturable absorber in a common cavity portion, wherein the saturable absorber is in the form of a graphene-based polymer-composite saturable absorber (GSA). The GSA is prepared by exfoliating bulk graphite by mild ultrasonic treatment. The polymer composite is then obtained by mixing a dispersion enriched with single-layer graphene and few-layer graphene with an aqueous solution of polyvinyl alcohol. Each oscillator includes gain elements of Yb and Eb, an optical isolator and a tunable bandpass filter. A fused fiber coupler provides 30% of the output for each cavity independently. The optical cavity length is matched using a fiber-pigtailed optical delay line inserted into the erbium cavity, arm or loop of the laser device and corresponding to a pulse repetition rate of approximately 18 MHz in the synchronized state.

[0019] In the described example, the GSA is used both to initiate and facilitate mode-locked operation and to coordinate synchronization, taking advantage of the unique ultra-wideband nonlinear response of graphene. In ytterbium-doped and erbium-doped fiber amplifiers, the dual outputs of synchronized two-color lasers at ytterbium (pump) and erbium (Stokes) wavelengths are independently amplified to an average power of 100 mW. The two beams are then collimated, synchronized, and combined using a dichroic mirror arrangement before being focused onto a methanol test sample and bandpass filtered for CARS detection. The optical path of the intracavity delay line is tuned to convert the laser characteristics into passively synchronized mode-locking. The operating wavelength of the laser can be in the range of 1040 nm to 1080 nm (ytterbium) and 1535 nm to 1560 nm (erbium) (corresponding to approximately 2750 cm -1 ~3200cm -1 Frequency detuning) for tuning.

[0020] The laser device described herein passively synchronizes a fiber laser, providing a very simple and low-cost laser source for CRS. The fiber laser can provide a robust and stable source due to its simple, compact and cost-effective design, and enables alignment-free operation without the need for bulky optical setups.

[0021] As described below, examples of the laser devices described herein have been applied to CARS, demonstrating the above points. Due to their compactness and all-optical synchronization, the described examples are good sources of CRS in the high wavenumber range.

[0022] Various aspects of the invention are defined in the independent claims, to which reference will now be made. Optional features are defined in the dependent claims.

[0023] A variety of arrangements will be described in more detail below and take the form of a laser device for outputting filtered light pulses for inducing coherent Raman scattering in a sample. The laser includes: a first optical cavity including a first gain medium; and a second optical cavity including a second gain medium different from the first gain medium. Each of the first gain medium and the second gain medium can be excited by a pump light source to produce light in respective different wavelength ranges. A synchronizer is optically coupled to the first optical cavity and the second optical cavity. The synchronizer is configured to synchronize and mode-lock the light from the first optical cavity and the second optical cavity. The laser also includes a first optical filter and a second optical filter. The first optical filter and the second optical filter are configured to filter the light from the first optical cavity and the second optical 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.

[0024] Optionally, the laser device is a fiber laser. Optionally, the laser is an all-fiber laser. Optionally, the first optical cavity and the second optical cavity include isotropic optical fibers. Optionally, each of the first optical cavity and the second optical cavity includes a single-mode optical fiber.

[0025] The synchronizer allows synchronization and phase locking of ultrafast laser pulses, which allows two laser pulses from a fiber laser source to be combined, thereby reducing the total footprint required for coherent Raman spectroscopy. The optical pulses are filtered so that only light pulses within a limited wavelength range are output to the coherent Raman spectroscopy, resulting in more accurate measurements. In addition, the use of two synchronized and mode-locked laser sources significantly reduces the impact of optical filters on the optical power of the pump and Stokes pulses, making it a versatile choice for CRS.

[0026] In another embodiment, a laser device for outputting filtered light pulses that induce coherent Raman scattering in a sample is provided, the laser device comprising: a first optical cavity and a second optical cavity, the first optical cavity comprising a first gain medium, the second optical cavity comprising a second gain medium different from the first gain medium, wherein each of the first gain medium and the second gain medium can be excited by a pump light source to generate light in respective different wavelength ranges; a synchronizer, which is optically coupled to the first optical cavity and the second optical cavity, wherein the synchronizer is configured to synchronize and mode-lock the light from the first optical cavity and the second optical cavity; and a first optical filter and a second optical filter, wherein the first optical filter and the second optical filter are configured to filter the light from the first optical cavity and the second optical 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.

[0027] Optionally, 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.

[0028] Optionally, the first optical filter and the second optical filter are both tunable optical filters and are configured to change the first predetermined wavelength range or the second predetermined wavelength range, respectively. The tunable optical filter allows a user to specify a wavelength range so that the wavelength range of the pump and Stokes light pulses can be changed with the sample being measured.

[0029] Optionally, the tunable optical filter comprises an etalon based on a fiber tunable optical filter. An etalon is a dielectric material whose specific thickness and refractive index determine the bandwidth of each propagation peak and only one wavelength is transmitted with maximum transmission. The etalon based on a fiber tunable optical filter works by selecting the refractive index of the medium of the material to select a specific resonant wavelength. The wavelength that resonates with the optical length of the cavity is transmitted, while other wavelengths are reflected.

[0030] Optionally, the first optical filter and the second optical filter are disposed inside the first optical cavity and the second optical cavity, respectively, and the first optical cavity and the second optical cavity output the filtered optical pulses at a first optical outlet and a second optical outlet, respectively. Placing the optical filter inside the optical cavity ensures that optical pulses with an undesirable wavelength range are filtered immediately after the optical pulses are generated.

[0031] Optionally, the first optical filter and the second optical filter are respectively arranged outside the first optical cavity and the second optical cavity, and the first optical cavity and the second optical cavity output the optical pulse at a first optical outlet and a second optical outlet, respectively. The optical filter is arranged outside the optical cavity, eliminating the need to repeatedly filter the recycled filtered optical pulse, and making the structure of the optical cavity simple and compact.

[0032] Optionally, the laser device further comprises: a first fiber amplifier and a second fiber amplifier, the first fiber amplifier being at the first light outlet and doped with the first gain medium, the second fiber amplifier being at the second light outlet and doped with the second gain medium, the first fiber amplifier and the second fiber amplifier being used to amplify the light or the filtered light pulse. This ensures that the amplified light pulse is amplified at the correct wavelength. The use of the amplifier mitigates the reduction in optical power when an optical filter is installed.

[0033] Optionally, the synchronizer comprises graphene or an allotrope of carbon, such as carbon nanotubes. Optionally, the synchronizer comprises graphene, because graphene is able to synchronize lasers at virtually any wavelength in the infrared.

[0034] Optionally, the synchronizer comprises a saturable absorber and is configured to synchronize and passively mode-lock the optical pulses from the first optical cavity and the second optical cavity. The use of a saturable absorber enables the generation of synchronized ultrafast mode-locked laser pulses from different laser sources.

[0035] Optionally, one of the first optical cavity and the second optical cavity comprises an optical delay line for matching the length of the first optical cavity and the second optical cavity. Optionally, the optical delay line comprises a fiber pigtailed optical delay line. Using a delay line in one of the optical cavities can achieve pairing of non-identical optical cavities by making the optical cavity lengths equal.

[0036] Optionally, the laser gain medium comprises ytterbium or erbium, wherein optionally, the predetermined wavelength range generated by the laser gain medium corresponds to 0 to 4000 cm -1 Full Raman spectrum.

[0037] Optionally, the predetermined wavelength range includes: a range of 1040nm to 1080nm and / or a range of 1535nm to 1600nm.

[0038] In another embodiment, an optical device and two collimators are provided, the two collimators being configured to collimate the filtered light pulses. This limits the divergence of the filtered pulses. Optionally, one of the collimators includes a delay stage configured to achieve superposition on the measured sample.

[0039] Optionally, the optical device further comprises two dichroic mirrors configured to combine the collimated light pulses from the two collimators.

[0040] Optionally, the laser device comprises a bandpass or shortpass filter for removing the filtered light pulse pairs before CARS detection.

[0041] In another embodiment, a method for outputting filtered light pulses for inducing coherent Raman scattering in a sample from a laser device is provided, the method comprising: generating light in different wavelength ranges using a first optical cavity including a first gain medium and a second optical cavity including a second gain medium different from the first gain medium, wherein each of the first gain medium and the second gain medium can be excited by a pump light source. In other words, two optical cavities are required to output filtered light pulses for inducing coherent Raman scattering in a sample. The method further comprises: synchronizing and mode-locking the light from the first optical cavity and the second optical cavity using a synchronizer optically coupled to the first optical cavity and the second optical cavity; filtering the light from the first optical cavity and the second optical cavity respectively using a first optical filter and a second optical filter; outputting a first filtered light pulse in a first predetermined wavelength range from the first optical filter and outputting a second filtered light pulse in a second predetermined wavelength range from the second optical filter. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The present invention will now be described in more detail by way of example with reference to the accompanying drawings, in which:

[0043] Figure 1 is a schematic diagram of a fiber laser device embodying one aspect of the present invention;

[0044] Figure 2 It shows that from Figure 1 Spectrum and pulse duration of an unfiltered, genlocked erbium laser pulse output from a fiber laser device;

[0045] Figure 3 It shows that from Figure 1 A graph showing the spectrum of a synchronously locked ytterbium laser pulse output from a fiber laser device;

[0046] Figure 4 It shows that from Figure 1 A graph showing the spectrum of a synchronously locked erbium laser pulse output from a fiber laser device;

[0047] Figure 5 It shows the use of Figure 1A graph of the CARS spectrum of a methanol sample measured by a fiber laser apparatus is shown. DETAILED DESCRIPTION

[0048] Reference below Figures 1 to 5 A laser device 10 and method for outputting filtered light pulses for inducing coherent Raman scattering in a sample 42 according to an example of the present invention will be described.

[0049] First reference Figure 1 Typically, the laser device 10 has two independent mode-locked optical cavities, oscillators or resonators 20a, 20b for generating two sets of optical pulses with a duration of picoseconds in different wavelength ranges suitable for CRS. Figure 1 The arrangement 14 also includes an optical element 12 that directs light pulses from the laser device to illuminate a sample 42 where coherent Raman scattering occurs. Scattering from the sample is filtered by a short pass or band pass filter 44 before entering a spectrometer 46.

[0050] For more details, refer to Figure 1 The laser device 10, Figure 1 Each of the two optical cavities 20a, 20b of the laser device is arranged in the form of a loop. In order to make these light pulses circulate in the loop until they leave the optical cavity at their respective light outlets 26a, 26b, the ends of the optical fibers constituting the optical cavity are optically connected using any suitable coupler. In addition, each light outlet includes a fused fiber coupler to provide approximately 30% of the output for its respective cavity. The mode-locked optical cavity includes a pump light source 22a, 22b for each optical cavity 20a, 20b to excite the gain elements 24a, 24b located or arranged inside the optical cavity 20a, 20b. In this example, the gain elements 24a, 24b are optical fibers doped with rare earth gain elements. Lasers using such gain elements are generally referred to as fiber lasers.

[0051] The choice of pump light sources 22a, 22b and gain elements 24a, 24b depends on the spectrum required for the CRS. Figure 1 The example shown uses two different optical fibers as gain elements. One fiber is doped with a rare earth gain element 24a in the form of ytterbium (Yb), and the other fiber is doped with a rare earth gain element 24b in the form of erbium (Er). In this example, the pump light sources that excite the gain elements are a 976nm wavelength pump light source 22a for exciting the ytterbium-doped fiber, and a 980nm wavelength pump light source 22b for exciting the erbium-doped fiber. The optical pulses generated from the ytterbium and erbium gain media are within the desired pump wavelength and Stokes wavelength range.

[0052] The optical isolators 28a, 28b are optically coupled to the exit of the corresponding gain medium 22a, 22b in each optical cavity 20a, 20b. This is to ensure that the optical pulses generated by the gain medium 22a, 22b in the optical cavity propagate in a single direction or one direction and only in one direction in the loop forming the optical cavity. In other words, the optical pulses generated from the gain medium 22a, 22b are directed to the optical output end. In this example, the optical isolators 28a, 28b are fiber isolators based on Faraday isolators.

[0053] The optical pulse pairs generated in each optical cavity 20a, 20b are synchronized using a suitable synchronizer 30. The synchronizer is shared by the two loops forming the optical cavities 20a, 20b. The synchronizer forms a part of the two loops. In this example, the synchronizer takes the form of a saturable absorber 30. The function of the saturable absorber has been described in the background technology section above. A saturable absorber is a light absorber whose absorbance decreases at high optical intensities. In the laser device 10, this allows passive mode-locked pulses to circulate in each optical cavity. More specifically, the function of the saturable absorber is to start and promote the mode-locked operation and coordinate the synchronization of the optical pulses. In other words, passive mode locking allows the generation of femtosecond optical pulses. The saturated absorber has a sufficiently short recovery time, thereby achieving fast loss modulation.

[0054] Figure 1 The saturable absorber 30 in the present invention is a graphene saturable absorber based on a polymer composite material, which has an ultrafast recovery time and broadband operation. Graphene is used as a passive element to synchronize lasers of arbitrary wavelengths in the infrared, due to its linear electronic dispersion using a point band gap at the Dirac point. The graphene saturable absorber 30 used in this article is prepared by exfoliating bulk graphite using a mild ultrasonic treatment, wherein a dispersion enriched with the obtained single-layer graphene and few-layer graphene is first mixed with an aqueous solution of polyvinyl alcohol to obtain a polymer composite material. Other saturable absorbers can also be selectively used to achieve passive mode locking and synchronization of optical pulses, such as a saturable absorber including single-walled carbon nanotubes (CNTs).

[0055] This pair of optical cavities 20a, 20b does not need to be identical. The difference in cavity length between the two optical cavities 20a, 20b is compensated by adding an optical delay line 32 in any of the optical cavities. In this example, the optical delay line 32 is located in the loop or optical cavity 20b including the optical cavity using erbium as the gain medium and after the isolator 28b. The optical delay line is a fiber pigtail delay line 32. The fiber pigtail delay line is optically coupled to the outlet of the isolator in the optical cavity using erbium as the gain medium. This corresponds to a repetition rate of pulses of about 18MHz in the synchronized state.

[0056] The wavelength range of the optical pulses generated at each optical cavity 20a, 20b is determined by the type of gain medium excited in the respective optical cavity. Figure 2 As shown, the light pulses generated by erbium are distributed over a relatively wide spectrum between about 1500nm (nanometers) and 1650nm, which is useful for broadband CARS. That is, pulses of less than 100fs (femtoseconds) can be generated.

[0057] However, optical pulses with such a wide spectrum may affect the accuracy of CRS, which cannot meet the needs of broadband laser pulses. Figure 1 As shown, an optical filter 34a, 34b is installed in each optical cavity 20a, 20b to optically filter out optical pulses that fall outside the desired wavelength range. In this example, the optical filter is located downstream of the isolator 28a in the first (ytterbium) optical cavity 20a or directly after the isolator 28a, and the optical filter is also located downstream of the optical delay line 32 in the second (erbium) optical cavity 20b or directly after the optical delay line 32. In this example, the optical filter is a tunable optical filter, so that the desired wavelength range can be tuned and applied to the Raman spectroscopy technique. However, the optical filters 34a, 34b can be fixed or non-tunable optical filters, or passband filters, such as Lyot filters.

[0058] Each optical cavity 20a, 20b has an outlet 26a, 26b to output a first filtered pulse in a first predetermined wavelength range and a second filtered pulse in a second predetermined wavelength range from the laser device 10 together. Figure 1 In the example of FIG. 1 , the outlets 26a and 26b are located downstream of the optical filters 34a and 34b. The optical outlets are coupled to different optical fiber amplifiers 36a and 36b, respectively. The relevant optical fiber amplifiers are doped with gain elements corresponding to the gain media 24a and 24b, wherein the gain media 24a and 24b are used to generate optical pulses. Figure 1 In the example of FIG. 1 , in order to amplify optical pulses of ytterbium (pump) and erbium (Stokes) wavelengths to an average power of 100 mW (milliwatts), ytterbium and erbium doped fiber amplifiers 36a, 36b are provided to the optical cavities 20a, 20b, respectively.

[0059] Figure 1The optical element 12 of the arrangement or device 14 shown includes collimators 38a, 38b. The laser device 10 outputs two filtered light pulses, and each light pulse passes through a collimator. Therefore, in order to limit the divergence of the light pulse beam when the light pulses are combined by dichroic mirrors 40a and 40b as described below in the subsequent combination step, the two filtered light pulses are collimated in their respective collimators 38a, 38b. In some cases, if it is necessary to achieve superposition on the sample 42, one of the two collimators can be placed in the delay stage 38c.

[0060] Figure 1 The optical element 12 of the arrangement or device 14 shown also includes a dichroic mirror 40a, 40b. The dichroic mirror is located downstream of each collimator 38a, 38b. The dichroic mirror is a mirror with different reflection and transmission characteristics at different wavelengths. The dichroic mirror 40a, 40b is used to combine two collimated light pulses from different cavities. Then, they are focused into the sample 42. A short-pass filter 44 and a spectrometer 46 are located downstream of the sample. The short-pass filter 44 is used to remove the pump and Stokes light pulses from the sample. The short-pass filter is a filter that transitions sharply from transmission to reflection. The resulting CARS spectrum is measured at the spectrometer 46.

[0061] In relation to Figure 1 In an alternative laser device to the laser device shown, the synchronized and mode-locked optical pulses from each optical cavity 20a, 20b are not filtered in the respective optical cavity. More specifically, the unfiltered broadband optical pulses, such as Figure 2 The broadband optical pulses shown can be output from each optical cavity and then filtered using an optical filter located outside the optical cavity. For example, an optical filter can be arranged between the optical outlet 26a, 26b and the entrance of the amplifier 36a, 36b to filter the generated optical pulses before amplification, or an optical filter can be connected to the output of its respective amplifier to filter the amplified optical pulses. These arrangements make the structure simpler and / or make the optical cavity smaller because the optical filter is removed from the optical cavity.

[0062] exist Figure 1 In the example shown, as described above, the tunable optical filters 34a, 34b are configured to tune the wavelengths of the ytterbium and erbium optical pulses to the ranges of 1040nm to 1080nm and 1535nm to 1560nm, respectively, corresponding to approximately 2750cm -1 ~3200cm -1 The frequency is detuned. Figure 3 and Figure 4Measured spectra of filtered ytterbium and erbium pulses in the synchronously locked state are shown in FIG, where their full width at half maximum (FWHM) in the tuning range are approximately 1.2 nm and 2 nm, respectively. Figure 2 Compared to the spectrum of an unfiltered erbium pulse in Figure 4 The spectrum of the filtered erbium pulse in the laser is focused on the desired wavelength, thus improving the measurement accuracy of CARS. In addition, a background-free intensity autocorrelator is used to measure the time domain output of the two light pulses. The deconvoluted FWHM pulse durations measured at 1060nm and 1550nm for the ytterbium and erbium pulses, respectively, are approximately 5 picoseconds. This enables single-frequency CARS microscopy to cover the CH (carbon-hydrogen bond) stretching band.

[0063] Figure 5 The measured CARS spectrum of the test methanol sample is shown. 3 About 2840cm -1 The measurement shows that the laser device according to the example of the present invention is suitable for generating optical pulses for coherent Raman spectroscopy, thereby obtaining accurate CARS measurements.

[0064] Embodiments of the present invention have been described above. It should be appreciated that changes and modifications may be made to the described embodiments within the scope of the present invention.

Claims

1. A laser device (10) for outputting filtered light pulses for inducing coherent Raman scattering in a sample (42), the laser device (10) comprising: A first optical cavity (20a) comprising a first gain medium (24a); a second optical cavity (20b) comprising a second gain medium (24b) different from the first gain medium (24a), wherein each of the first gain medium (24a) and the second gain medium (24b) can be excited by a pump light source (22a, 22b) to generate light in respective different wavelength ranges; a synchronizer (30) optically coupled to the first optical cavity (20a) and the second optical cavity (20b), wherein the synchronizer (30) is configured to synchronize and mode-lock the light from the first optical cavity (20a) and the second optical cavity (20b); A first optical filter (34a) and a second optical filter (34b), wherein the first optical filter (34a) and the second optical filter (34b) are configured to: filter the light from the first optical cavity (20a) and the second optical cavity (20b) respectively to output a first filtered picosecond optical pulse in a first predetermined wavelength range and a second filtered picosecond optical pulse in a second predetermined wavelength range; and output the filtered picosecond optical pulses at a first optical outlet (26a) of the first optical cavity (20a) and a second optical outlet (26b) of the second optical cavity (20b) respectively; wherein at least one of the first optical outlet (26a) and the second optical outlet (26b) is coupled to an optical isolator (28a, 28b) respectively to output the filtered picosecond optical pulses in a directionally manner; a first optical fiber amplifier (36a), which is located at the first optical outlet (26a) and is doped with the first gain medium, and is used to amplify the picosecond optical pulse output by the first optical filter; and a second optical fiber amplifier (36b), which is located at the second optical outlet (26b) and is doped with the second gain medium, and is used to amplify the picosecond optical pulse output by the second optical filter; wherein at least one of the first optical filter (34a) and the second optical filter (34b) is a tunable optical filter and is configured to change the first predetermined wavelength range or the second predetermined wavelength range respectively, and the filtered picosecond optical pulse output at the first optical outlet (26a) has a half-maximum full-width spectrum width within the corresponding tuning range of 0.02nm to 1.5nm, and the filtered picosecond optical pulse output at the second optical outlet (26b) has a half-maximum full-width spectrum width within the corresponding tuning range of 0.05nm to 3.4nm; wherein the synchronizer (30) includes a saturable absorber and is configured to synchronize and passively mode-lock the picosecond optical pulses from the first optical cavity (20a) and the second optical cavity (20b); and Wherein, the laser device (10) is integrated in an optical device, and the optical device comprises: two collimators (38a, 38b) configured to collimate the filtered picosecond light pulses; Two dichroic mirrors (40a, 40b) configured to combine the collimated picosecond light pulses from the two collimators (38a, 38b).

2. The laser device according to claim 1, characterized in that: The tunable optical filter comprises an etalon based on a fiber tunable optical filter.

3. The laser device according to claim 1, characterized in that: The first optical filter (34a) and the second optical filter (34b) are disposed inside the first optical cavity (20a) and the second optical cavity (20b), respectively.

4. The laser device according to claim 1, characterized in that: The first optical filter (34a) and the second optical filter (34b) are disposed outside the first optical cavity (20a) and the second optical cavity (20b), respectively.

5. The laser device according to claim 1, characterized in that: The synchronizer (30) includes graphene or an allotrope of carbon.

6. The laser device according to claim 1, characterized in that: The laser device (10) is a fiber laser.

7. The laser device according to claim 6, characterized in that: The laser device (10) is an all-fiber laser.

8. The laser device according to claim 6, characterized in that: Each of the first optical cavity (20a) and the second optical cavity (20b) includes an isotropic optical fiber.

9. The laser device according to any one of claims 6 to 8, characterized in that: Each of the first optical cavity (20a) and the second optical cavity (20b) includes a single-mode optical fiber.

10. The laser device according to claim 1, characterized in that: One of the first optical cavity (20a) and the second optical cavity (20b) comprises an optical delay line (32), and the optical delay line (32) is used to match the lengths of the first optical cavity (20a) and the second optical cavity (20b).

11. The laser device according to claim 10, characterized in that: The optical delay line comprises an optical fiber pigtail type optical delay line.

12. The laser device according to claim 1, characterized in that: The gain medium includes either ytterbium or erbium.

13. The laser device according to claim 1, characterized in that: The first predetermined wavelength range and the second predetermined wavelength range include: a range of 1040 nm to 1080 nm and / or a range of 1535 nm to 1600 nm.

14. A method of outputting filtered light pulses from a laser device (10) for inducing coherent Raman scattering in a sample (42), the method comprising: A first optical cavity (20a) including a first gain medium (24a) and a second optical cavity (20b) including a second gain medium (24b) different from the first gain medium (24a) are used to generate light in different wavelength ranges, wherein each of the first gain medium (24a) and the second gain medium (24b) can be excited by a pump light source (22a, 22b); Using a synchronizer (30) coupled to the first optical cavity (20a) and the second optical cavity (20b), the light from the first optical cavity (20a) and the light from the second optical cavity (20b) are synchronized and mode-locked; Using a first optical filter (34a) and a second optical filter (34b) to filter the light from the first optical cavity (20a) and the second optical cavity (20b) respectively; outputting a first filtered picosecond light pulse in a first predetermined wavelength range from the first optical filter (34a); outputting a second filtered picosecond light pulse in a second predetermined wavelength range from the second optical filter (34b); amplifying the picosecond optical pulse output by the first optical filter at a first optical fiber amplifier (36a) doped with the first gain medium; amplifying the picosecond optical pulse outputted by the second optical filter at a second optical fiber amplifier (36b) doped with the second gain medium; and The synchronizer (30) is used to synchronize and passively mode-lock the light from the first optical cavity (20a) and the second optical cavity (20b), wherein the synchronizer (30) comprises a saturable absorber; The method further comprises: outputting the light from a first light outlet (26a) in the first light cavity (20a) and from a second light outlet (26b) in the second light cavity (20b); wherein at least one of the first light filter (34a) and the second light filter (34b) is a tunable light filter, and the filtered picosecond light pulse output at the first light outlet (26a) has a half-maximum full-width spectral width in a corresponding tuning range of 0.02nm to 1.5nm, and the filtered picosecond light pulse output at the second light outlet (26b) has a half-maximum full-width spectral width in a corresponding tuning range of 0.05nm to 3.4nm. wherein at least one of the first optical outlet (26a) and the second optical outlet (26b) is coupled to an optical isolator (28a, 28b) to directionally output the filtered picosecond optical pulses; wherein the filtering step comprises tuning the tunable optical filter to change the first predetermined wavelength range or the second predetermined wavelength range; Wherein, the method further comprises: collimating the first filtered picosecond light pulse and the second filtered picosecond light pulse using two collimators (38a, 38b), respectively; and The collimated picosecond light pulses from the two collimators (38a, 38b) are combined using two dichroic mirrors (40a, 40b).

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