As one can see, each FBG of rank k is tuned for a different wavelength λ_k, such as to reflect a narrow spectral band of light centered on the wavelength λ_kwithin the spectral range 2. On the example of the FIG. 2, only the gratings of the rank k=1, rank k=2 and rank k=N are represented.

At each FBG of rank k, only a pulse of rank k is reflected, and the rest of the light is transmitted to the FBG of next rank.

Therefore, the reflected narrowband light pulses will be separated in time, due to the time it takes for light to travel twice the distance between successive FBGs. Namely, the time interval Δt between two successive pulses is equal to twice the space interval ΔL, multiplied by n/c, wherein n is the index of the optical fiber, and c is the speed of the light in void. For instance, with an index of n=1.5 and a space interval ΔL equal to 30 cm, one get a time interval Δt equal to 3 ns.

Thus, the initial broadband laser beam pulse 1 is divided into the series of narrowband light pulses. This is advantageous because it allows spectral discrimination by resolving the pulses with a single fast detector.

Preferably, but optionally, the FBGs are arranged as on FIG. 2, i.e. such that the selected wavelength λ_kof a pulse of rank k is an available wavelength of the wavelength comb next to the pulse of immediate previous rank.

In this embodiment, the delay input 5 and the delay output 6 of the are the very same end of the superstructured FBG 14 optical fiber.

Examples of LiDAR Devices with a Spectral Delay Unit

As represented on FIGS. 3 to 8, it is provided examples of multispectral LiDAR devices taking advantage of the above-described superstructured FBG 14.

Each represented multispectral LiDAR device comprises:

- a supercontinuum laser source 17 for emitting the broadband laser beam pulse 1 into an optical input 8 of the LiDAR device,
- an optical transmitter 11 configured for outputting light on an obstacle 102 to detect, and
- an optical receiver 9 configured to receive a reflection of the outputted light from the obstacle 102, and
- an optical detector 16 configured to detect a time-of-flight and an optical power of the reflection,
- an superstructured FBG 14 line for transforming the broadband laser beam pulse 1 into the pulse-train as explained with reference to FIGS. 1 and 2,
- an optical circulator 13 having at least two ports in order to connect at least some of the previous elements through optical fiber.

More specifically, on FIG. 3, the superstructured FBG 14 line is arranged on the emitting side of the represented multispectral LiDAR device 10.

A first port of the optical circulator 13 is fiber-coupled with the optical input 8, a second port is fiber-coupled to the superstructured FBG 14 line, the third port is fiber-coupled with a scanning module 12, and the fourth port is fiber-coupled with the optical detector 16. The arrows 15 represent the propagation of light inside the optical circulator 13.

The scanning module 12 comprises both the optical transmitter 11 and the optical receiver 9.

Advantageously, introducing the delays in the signal prior that the signal arrives to the scanning module 12 for scanning the obstacle 102 is practical, because the supercontinuum laser source 17 is already emitting inside an optical fiber.

For instance, the supercontinuum laser source 17 can be a solid-state laser source.

FIG. 4 represents the same multispectral LiDAR device 10 as FIG. 3, additionally to the spectrum of the light at each step of the propagation from the supercontinuum source 17.

The spectrum 20 is a schematic of the train of monochromatic pulses generated and emitted by the multispectral LiDAR device 10 at the output of the scanning module 12. The spectrum 21 is a schematic of the reflected train of monochromatic pulses received by the multispectral LiDAR device 10.

Namely, one can compare the spectrum 20 of the pulse-train to the spectrum 21 of the pulse-train reflected by the obstacle 102: depending on the wavelength channel, the light of the corresponding pulse is more or less absorbed by the obstacle 102.

The optical detector 19 receives the reflected train of pulses and distinctly detects an optical power for each peak, therefore for each wavelength. Therefore, the single optical detector 19 enables acquiring spectral information about the obstacle 102.

For instance, the optical detector 19 is a single sensor comprising an avalanche photodiode (APD) electrically connected to a digitizer having a sample rate of 3 GS/s, and to Field Programmable Gate Arrays (FPGA).

Some computation may be programmed to recover the time-of-flight of a pulse (for instance a monochromatic pulse), and thus the distance of a part of the obstacle 102 which is reflective to the pulse (for instance the monochromatic pulse). For instance, the computation comprises Fast Fourier Transform (FFT) based cross correlation.

The amplitude of the optical power of each detected peak is compared with the spectrum of the emitted broadband laser pulse.

This step may require multiple stages and a demultiplexer. For instance, in a 1st stage, one can use a fast transimpedance amplifier, and in further stages, a demultiplexer into N slower (integrating) amplifiers.

In a variant represented on FIG. 5, the superstructured FBG 14 line is also arranged on the emitting side of the represented multispectral LiDAR device 110, for the same advantages as already stated.

A difference with the multispectral LiDAR device 10 of FIG. 3 is that the third port of the optical circulator 13 is directly fiber-coupled to the optical transmitter 11. In this embodiment, the optical receiver 9 may be a free optical space lens which focus the received light onto the optical detector 16.

In a variant represented on FIG. 6, by contrast with the two examples of multispectral LiDAR devices above-described, the superstructured FBG 14 line is arranged on the receiving side of the represented multispectral LiDAR device 111. Introducing the delays on the receiver side is equivalent from an optics perspective, since the delays are introduced by passive components.