Patent Description:
Light detecting systems are used to illuminate an object and to detect a return light from said object, for instance in Light detection and ranging (LiDAR), spectrometer or imaging applications, for instance microscopy imaging.

Light detecting systems are known from e.g. <CIT>, <CIT>, and <CIT>.

For such light detecting systems, there is a need to generate a sufficient light power in order to enable a part of the illumination light which is returned to be detected.

Such a constraint traditionally leads to bulky optical arrangements, because of the use of a number of optical components, such as amplification stages and optical isolators, on both an illumination optical path and a return optical path.

Compared to the state of the art, it is proposed an optical arrangement which overcome the deficiencies of the prior art.

The invention provides an optical arrangement for a light detecting system, the optical arrangement being configured:.

The term "illumination beam" shall be construed as the complete beam running along the complete illumination optical path from the laser source to the target, whatever the spectral content therein.

In the optical arrangement, the illumination optical path and the return optical path share a common optical path portion by use of the multi-clad fiber. In other words, the return path is monolithically integrated in the architecture.

The term "source beam" designates the part of the illumination beam which has not been modified by the core of the multi-clad optical fiber. The term "scanning beam" designates the part of the illumination beam which results from the modification of the source beam by the core of the multi-clad optical fiber. In other words, the core transforms the source beam into the scanning beam.

Such an architecture has various advantages.

Such an architecture may be advantageous for various applications, such as for medical or LiDAR applications.

Such an optical arrangement may comprise one or more of the features below described, or any combination thereof.

There are a variety of source beams features which may be selected for the optical arrangement. For instance, in embodiments, the source beam may be a continuous wave (CW) beam, as for instance a frequency modulation continuous wave (FMCW). By contrast, in embodiments, the source beam is a pulsed laser beam. For instance, the pulses of the source beam have a rate of <NUM>.

The optical arrangement may be provided in various manners. In embodiments, the optical arrangement may be provided uncoupled with a laser source and/ or to an optical detector. In embodiments, the optical arrangement comprises the laser source. In embodiments, the optical arrangement comprises the optical detector.

In embodiments, the source beam at the first end of an optical fiber arrangement is a monochromatic pulsed beam centered on the first wavelength, and having a full width at half maximum (FWHM) lower as <NUM>, preferably lower as <NUM>.

In embodiments, the at least second wavelength is shifted from the first wavelength by at least <NUM>.

In embodiments, the shifting is performed by Raman effect in the multi-clad fiber.

In embodiments, the first wavelength is <NUM>.

In embodiments, the dichroic beamsplitter has a high transmittance (HT) for wavelength higher as <NUM> and a high reflectivity for wavelengths lower as <NUM>.

In embodiments, the core is further configured to generate a supercontinuum in a range comprising the second wavelength.

In embodiments, the supercontinuum comprises at least the spectral range [<NUM>-<NUM>], preferably a larger spectral range, such as for instance [<NUM> - <NUM>].

Indeed, wavelengths below <NUM> may be dangerous for eye safety.

In embodiments, the multi-clad optical fiber is selected such that a ratio of a P x L / D is above a threshold equal to 250W, wherein P denotes for a power peak of light of the monochromatic pulsed beam at one end of the multi-clad fiber, L denotes for a length of the multi-clad optical fiber, and D denotes for the diameter of the core of the multi-clad optical fiber.

In embodiments, the optical arrangement further comprises a notch filter or a wavelength bandpass filter arranged on the return optical path.

In embodiments, the notch filter or the wavelength bandpass filter is wavelength-tunable.

In embodiments, the notch filter or the wavelength bandpass filter are further configured to successively tune the wavelength to different wavelength values selected in the range of the supercontinuum. Thanks to these features, the reflected beam can be filtered in a wavelength-selective manner, such that a spectral response of the target may be detected by a spectral scanning.

In embodiments, the optical detector is a unique broadband optical detector.

In embodiments, the optical fiber arrangement further comprises an amplifying optical fiber coupled to the multi-clad optical fiber, such that the laser beam propagates in the amplifying optical fiber, wherein the amplifying optical fiber is configured to optically amplify the laser beam propagating in said amplifying optical fiber such that a ratio between an unamplified power peak and an amplified power peak of the laser beam is higher than <NUM>, preferably higher than <NUM>.

In an embodiment, the unamplified power peak may be in the range [<NUM> mW - <NUM> mW], and is amplified up to a power peak in the range [1W - <NUM> kW], preferably [100W- 10kW].

In embodiments, the amplifying optical fiber and the multi-clad optical fiber are optically coupled by a welding of the first end of the multi-clad fiber with an end of the amplifying optical fiber, in order to form a single optical fiber.

In embodiments, the amplifying optical fiber may be a ytterbium-doped fiber. In embodiments, the amplifying optical fiber may be optically pumped, for instance by a 60mW optical power. In embodiments, the amplifying optical fiber may be <NUM> meters long. In embodiments, the amplifying optical fiber may be either a simple clad fiber, or preferably a double clad fiber. In embodiments, the amplifying optical fiber may be arranged between the transceiver arrangement and the multi-clad optical fiber. In embodiments, the amplifying optical fiber is configured to amplify the first wavelength.

In embodiments, the light detecting system is a light detecting and ranging (LiDAR) system. For instance, the scanning unit may be configured to spatially scan areas in a spatial range comprised between <NUM> and <NUM>.

The invention namely provides an optical arrangement for a LiDAR, the optical arrangement being configured:.

The invention also provides a vehicle comprising a LiDAR system comprising an optical arrangement as described hereinabove.

The invention further provides the optical arrangement hereinabove described, but used for another kind of light detecting system than a LiDAR.

For instance, the optical arrangement is used in a spectral analysis and/or imaging system, for instance for microscopy and/or medical imaging. In such arrangement, the scanning unit may comprise a microscope head. For instance, the microscope head may be configured to be displaced in a step-by-step manner in order to spatially scan a two or a three-dimensional area. For instance, the area has a length of an order of magnitude of <NUM>,<NUM>, <NUM> or <NUM>.

The optical arrangement may also be used in Optical Coherence Tomography (OCT) systems. For instance, the OCT system may use a mode-locked laser as a laser source. For instance, the pulse repetition rate is in the order of magnitude of <NUM>.

Figures and the following detailed description contain, essentially, some exact elements. They can be used to enhance understanding the invention and, also, to define the invention if necessary.

For the sake of conciseness, the elements which are similar or equivalent through the description will be described with reference to the same reference numbers.

<FIG> represents an optical arrangement <NUM> used in a LIDAR (Light Detection and Ranging System) for sensing a target <NUM>.

The LiDAR includes a laser source <NUM>, and an optical detector <NUM>, which may comprise an optical sensor or a plurality of optical sensors. The laser source <NUM> emits scanning pulses. When a scanning pulse is reflected by the target <NUM>, the LiDAR can determine the distance based on the time of flight of a return pulse received by the optical detector <NUM>.

The LiDAR namely comprises the optical arrangement <NUM> for transmitting the scanning pulse from the laser source <NUM> to the target <NUM>, and for transmitting the return pulse back from the target <NUM> to the detector <NUM>.

In the following, the optical arrangement <NUM> will be described in more details.

The optical arrangement <NUM> comprises at least a transceiver arrangement <NUM>, an optical fiber arrangement <NUM>, and the scanning unit <NUM> as pictured on <FIG>.

The scanning unit <NUM> is configured to steer the scanning pulse <NUM> in an orientable direction to the target <NUM> and to receive the return pulse <NUM> from the target <NUM> from said orientable direction.

The optical fiber arrangement <NUM> is configured to route the scanning pulse <NUM> from the laser source <NUM> to the scanning unit <NUM>, and to route a return pulse <NUM> back through the double-clad fiber <NUM> to the detector <NUM>. In other words, the scanning pulse <NUM> and the return pulse <NUM> share a same portion of optical path in two opposite directions, which will be further referred to as a scanning direction <NUM>, and a sensing direction <NUM>.

The transceiver arrangement <NUM> is configured to split in free space the optical paths of the scanning pulse <NUM> and the return pulse <NUM> in two non colinear optical paths corresponding to the optical path from the laser source <NUM> and the optical path to the optical detector <NUM>. Such a splitting is performed by means of a dichroic beamsplitter <NUM>, which is highly reflective (HR) for the wavelength of the scanning pulse 11and highly transmittive (HT) for the return pulse <NUM>.

On the example pictured, the dichroic beamsplitter <NUM> is disposed with an angle of <NUM>° by contrast with an output of the laser source <NUM>, such that the scanning pulse <NUM> is deviated by an angle of <NUM>° up to the optical fiber arrangement <NUM>, whereas the return pulse <NUM> is not deviated on its travel to the optical detector <NUM>, such that the optical paths are split in perpendicular optical paths 11a and 12c. One can see on <FIG> that in a variant, the configuration may be exactly inversed by selecting an inverse beamsplitter <NUM> in order that the scanning pulse <NUM> is not deviated, whereas the return pulse <NUM> is.

One can provide a filter <NUM> between the beamsplitter <NUM> and the detector <NUM> in order to improve the spectral sensing of the return light.

A coupling lens <NUM> is arranged on the optical path 11b between the dichroic beam splitter <NUM> and the optical fiber arrangement <NUM>, in order to collimate the laser light in both the scanning direction <NUM> and the sensing direction <NUM> from the free space to the optical fiber arrangement <NUM>.

The optical fiber arrangement <NUM> comprises a double-clad optical fiber (DCF) <NUM> which will be described more in references with <FIG>, and an amplifying fiber <NUM>.

A DCF is an optical fiber with a structure consisting of three layers of optical material instead of the usual two. As one can see on <FIG>, the inner-most layer is a core <NUM>. The core <NUM> is surrounded by an inner cladding <NUM>, which is surrounded by an outer cladding <NUM>. The three layers are made of materials with different refractive indices.

As one can see on <FIG>, the spectrum shape of the scanning pulse <NUM> is transformed while traveling along the scanning direction <NUM>, at different stages of propagation.

One can see on the figure different optical paths portions 11a, 11b and 11c for the scanning direction <NUM>, and different optical paths portions 12a and 12b for the sensing direction <NUM>.

The optical path portion 11a is the portion of the optical path comprised between the laser source <NUM> and the beamsplitter <NUM>. The optical path portion 11b is the portion of the optical path comprised between the beamsplitter <NUM> and the optical fiber arrangement <NUM>. The optical path portion 11c is the portion of the optical path comprised between the optical fiber arrangement <NUM> and the output of the scanning unit <NUM>.

The optical path portion 12c is the portion of the optical path comprised between the optical fiber arrangement <NUM> and the optical detector <NUM>.

One can see schemes <NUM>, <NUM> and <NUM> of the spectrum of the scanning beam <NUM> for each stage of propagation through the optical path in the transmitting direction <NUM>.

The laser source <NUM> is configured such that the scanning pulse <NUM> is a monochromatic laser beam on the optical path portion 11a, as represented by the spectrum <NUM>. The narrow line is centered on <NUM> and has a FWHM of <NUM>,<NUM>.

The core <NUM> has nonlinear properties, such that the DCF <NUM> is a supercontinuum (SC) fiber. In other words, the scanning pulse <NUM> propagates through the core <NUM> of the double-clad optical fiber <NUM> which is configured to spread the spectrum of said scanning pulse <NUM> such that it is transformed from monochromatic to supercontinuum, as represented by the spectrum <NUM>. The supercontinuum extends at least from <NUM> to <NUM>.

The length of the DCF <NUM> may be preferably <NUM> meters. Indeed, the length of the DCF <NUM> should be selected such that nonlinear effects in the DCF <NUM> are sufficient to get a supercontinuum from a monochromatic laser pulse.

One can get such a result by selecting a DCF <NUM> such that a ratio of a P x L / D is above a threshold equal to 250W, wherein P denotes for a power peak at the entry of the multi-clad fiber, L denotes for a length of the multi-clad optical fiber, and D denotes for the diameter of the core <NUM>.

The DCF <NUM> may be selected for instance in a set of Passive fibers referred on the optical component manufacturer Thorlabs ® by the references P-<NUM>/125DC, P-<NUM>/125DC, P-<NUM>/400DC or P-<NUM>/250DC, which are optimized for coupling to active doped fibers for amplification. For instance, the diameters of the core <NUM>, inner cladding <NUM> and outer cladding <NUM> may be selected in the following triplet values, in micrometers (µm): [diameter of (core, inner cladding, outer cladding)] = [(<NUM>, <NUM>, <NUM>), or (<NUM>,<NUM>, <NUM>) or (<NUM>, <NUM>, <NUM>) or (<NUM>, <NUM>, <NUM>)], wherein the core diameter specification refers to the far-field mode field diameter at <NUM>. For instance, the NA of the core <NUM> is selected in [<NUM>, <NUM>, <NUM>].

In general, the DCF <NUM> may present a cladding geometry of the DCF <NUM> which is round. The DCF <NUM> may present a numerical aperture (NA) for the inner cladding <NUM> above or equal to <NUM>. The DCF <NUM> may present a coating material is a low-index acrylate. The diameter of the inner cladding <NUM> may be equal to or higher as ten times the diameter of the core <NUM>. The NA of the core <NUM> may be lower as or equal to <NUM>/<NUM> of the NA of the inner cladding <NUM>.

Prior to travel through the DCF <NUM>, the scanning pulse <NUM> is amplified by the amplifying fiber <NUM>, as represented by spectrum <NUM>. Indeed, the scanning pulse <NUM> is required to have a sufficient power peak to enable the DCF <NUM> to apply nonlinear effects on the scanning pulse <NUM>.

As one can see, the spectrum is more or less the same as the spectrum <NUM>, but with an amplified power. For instance, the power peak is amplified by a factor <NUM>.

The amplifying fiber <NUM> is an Ytterbium-doped (Yb-doped) fiber which is optically pumped as depicted by the symbol <NUM>. The optical amplifying fiber <NUM> is fiber-coupled with the double-clad optical fiber <NUM>. More precisely, one end of both the optical amplifying fiber <NUM> and the DCF <NUM> is meld with each other, such that the two optical fibers form a single long optical fiber.

The amplifying optical fiber is configured to amplify the optical power of the scanning beam <NUM>, when optically pumped by an optical pump arrangement <NUM>.

The amplifying fiber <NUM> may be preferably <NUM> meters long. Such a length enables the scanning pulse <NUM> to be enough pumped, from an unpumped peak power value around 250mW up to a pumped peak power value higher than 100W.

The amplifying fiber <NUM> may be selected in matching active fibers referred on the products of Thorlabs ® by the references YB1200-<NUM>/125DC, YB1200-<NUM>/125DC, YB1200-<NUM>/400DC and respectively YB1200-<NUM>/250DC, which are compatible with the passive fibers described hereinabove (for instance YB1200-<NUM>/250DC is compatible with P-<NUM>/250DC as the core diameter and the inner cladding diameter have the same values).

In general, the amplifying fiber <NUM> is preferably selected in double-clad fibers, such that the amplifying fiber <NUM> has the same core diameter as the diameter of the core <NUM> of the DCF <NUM>, and the same inner cladding diameter as the diameter of the inner cladding <NUM> of the DCF31. For instance, the inner cladding geometry is octagonal.

For instance, the diameters of the core, inner cladding and outer cladding may be selected in the following triplet values, in micrometers (µm): [diameter of (core, inner cladding, outer cladding)] = [(<NUM>, <NUM>, <NUM>), or (<NUM>,<NUM>, <NUM>) or (<NUM>, <NUM>, <NUM>) or (<NUM>, <NUM>, <NUM>)], for an octagonal cladding measured flat to flat, wherein the core diameter specification refers to the far-field mode field diameter at <NUM>.

For instance, the NA of the core <NUM> is selected in [<NUM>, <NUM>, <NUM>]. The coating material of the outer cladding <NUM> may be low-Index Acrylate. The cladding NA may be equal to or above <NUM>,<NUM> or <NUM>,<NUM>. The cladding absorption at the wavelength of pumping <NUM> may be comprised between <NUM>,<NUM> and <NUM>,<NUM>.

The scanning pulse <NUM> travel continues after the stage of the optical fiber arrangement <NUM>, to the scanning unit <NUM>. One can see schemes <NUM> and <NUM> of the spectrum of the return pulse <NUM> for each stage of propagation through the optical path in the sensing direction <NUM>.

The scanning unit <NUM> comprises a tunable notch <NUM>, in order to remove a narrow band centered on a selectable wavelength from the supercontinuum spectrum.

The optical path portion 12a is the portion of the optical path comprised between the scanning unit <NUM> and the notch <NUM> on the sensing direction <NUM>. The optical path portion 12b is the portion of the optical path comprised between the notch <NUM> and the optical fiber arrangement <NUM>.

As one can see, and for the only sake of illustration, the spectrum <NUM> of the return pulse <NUM> is represented with the assumption that the target <NUM> is a perfect plan mirror. The spectrum <NUM> is the same as the spectrum <NUM>, but with the narrow band removed.

By selecting successive different wavelength from the supercontinuum, on can spectrally scan the target <NUM>. In a variant, one can replace the notch <NUM> by a bandpass filter. However, advantageously, using a notch <NUM> instead of a bandpass filter enables to use a maximum of the return power from the target <NUM>.

The scanning unit <NUM> further comprises a f/<NUM> lens <NUM>, and a scanning header <NUM> which is spatially orientable, as pictured by the arrow <NUM>.

As one can see, the f/<NUM> lens <NUM> is used on the optical paths 11c et 12b, in order to collimate the return pulses <NUM> back into the optical fiber arrangement <NUM>. For instance, the f/<NUM> lens may be selected in the lenses referred by the reference AC127-<NUM>-C of the manufacturer Thorlabs ®. Such a lens is an achromatic doublet, anti-reflective coated in the range <NUM> -<NUM>. The focal distance may be <NUM> and the diameter <NUM>,<NUM>.

The scanning header <NUM> comprises optical components for steering the scanning pulse <NUM> in an orientable direction, in order to enable the LiDAR to spatially scan the target <NUM>.

The foregoing discussion disclosed and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.

Namely, a light detecting system for medical applications, comprising an optical arrangement <NUM> according to a variant is represented with reference to <FIG>. As one can see, the target <NUM> represented on <FIG> is a skin comprising a blood vein, which is imaged by the light detecting system.

Similarly to the example of <FIG>, the light detecting system comprises a laser source <NUM> and a optical detector <NUM> arranged with a dichroic beamsplitter <NUM> in front of the same optical fiber arrangement <NUM>. The laser source <NUM> and the optical detector <NUM> may operate for laser pulses in the short wave infrared (SWIR) range for instance.

As represented, the scanning unit may be different to the one of the optical arrangement <NUM> of the LiDAR of <FIG>. Indeed, the scanning unit <NUM> comprises a lens arrangement for collimating the light a short focal distance lower than <NUM> on the target <NUM>.

Claim 1:
An optical arrangement (<NUM>) for a light detecting system, the optical arrangement (<NUM>) being configured:
- to steer an illumination beam (<NUM>) along an illumination optical path from a laser source (<NUM>) to a target (<NUM>) to be sensed, in a transmitting direction (<NUM>), and
- to transmit a reflected beam (<NUM>) along a return optical path from the target (<NUM>) to an optical detector (<NUM>), in a receiving direction (<NUM>), wherein the reflected beam comprises a reflection of the illumination beam (<NUM>) on the target (<NUM>),
the optical arrangement (<NUM>) comprising:
- a transceiver arrangement (<NUM>) comprising a dichroic beamsplitter (<NUM>), the transceiver arrangement (<NUM>) being configured for:
* transmitting a source beam (11a) from the laser source (<NUM>) to a first end of an optical fiber arrangement (<NUM>), in one way selected among: through or by reflection on the dichroic beamsplitter (<NUM>), and
* transmitting the reflected beam (12a) from said first end of the optical fiber arrangement (<NUM>) to the optical detector (<NUM>) in the other way selected among: through or by reflection on the dichroic beamsplitter (<NUM>),
- the optical fiber arrangement (<NUM>) comprising a multi-clad optical fiber (<NUM>) which comprises a core (<NUM>), at least one inner cladding (<NUM>), and an outer cladding (<NUM>); wherein
said core (<NUM>) is arranged to receive the source beam having a first wavelength, such that the source beam propagates in the core (<NUM>) in said transmitting direction, and
the core is further configured to generate a scanning beam from the source beam (11a) propagating in the core, wherein the scanning beam comprises at least a second wavelength which is different from the first wavelength, and
said at least one inner cladding (<NUM>) is configured to receive the reflected beam (<NUM>), comprising the at least second wavelength, such that the reflected beam propagates in the inner cladding in said receiving direction (<NUM>),
- a scanning unit (<NUM>) optically coupled to a second end of the optical fiber arrangement, the scanning unit being configured to steer the scanning beam to said target (<NUM>), and to receive the reflected beam from the target (<NUM>).