Method for controlling the peak value of a pulsed fiber laser

Abstract

This invention provides a method for controlling the peak value of a pulsed fiber laser in a distance measurement method, which enables reliable distance measurement by maintaining a stable reach by keeping the peak value of the pulsed wave irradiated onto an object constant. [Solution] In a distance measurement method in which pulsed light from a pulsed fiber laser device 10 is irradiated onto an object 3, and the reflected pulsed light from the irradiated pulsed light reflected by the object is received, and the time from the emission of the irradiated pulsed light to the receipt of the reflected pulsed light is converted into distance to measure the distance, the irradiated pulsed light and the reflected pulsed light are detected by an avalanche photodiode 21, and the peak value of the irradiated pulsed light is calculated based on the detection of the irradiated pulsed light, and the generated irradiated pulsed light is controlled to keep the peak value constant based on this peak value.

Description

Technical Field

[0001] The present invention relates to a method for controlling the peak value of a pulsed fiber laser in distance measurement using a so-called time-of-flight method with a pulsed fiber laser. in distance measurement.

Background Art

[0002] Conventionally, as a distance measurement by the time-of-flight method in which pulsed light of a pulsed laser is irradiated onto an object, and the reflected pulsed light reflected by the object is received, and the time from the projection of the irradiation pulsed light to the reception of the reflected pulsed light is converted into a distance to measure the distance, various methods are known.

[0003] For example, in Patent Document 1, pulsed laser light is emitted by a semiconductor laser, branched by a beam splitter into an object wave (irradiation wave) irradiated onto an object and a reference wave, and the reflected wave of the object wave from the object and the reference wave are received by one photodiode. The signal component output from this photodiode is separated in a mask circuit into a reflection signal component corresponding to the reflected wave and a reference signal component corresponding to the reference wave. The operations of triggering the semiconductor laser by signals delayed by a delay circuit for the reflection signal component and the reference signal component are each repeated until the count-up of a counter, and the time is integrated. The time corresponding to the distance to the object is obtained by subtracting the time obtained for the reference signal component from the time obtained for the reflection signal component, and this time is converted into a distance. A distance measurement method is described.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] The amount of reflected light from an object changes depending on the reflectivity of the object's surface and the distance to the object. However, in the conventional distance measurement method described in Patent Document 1, this change is addressed by using a gain adjustment circuit and an APC circuit to keep the amplitude of the reflected signal component output from the amplification circuit constant. However, since a reflected wave pulse is an object wave pulse reflected by an object, if a stable object wave pulse is not emitted, the received reflected wave pulse will also be unstable. Furthermore, it is known that the reach of an object wave pulse is related to its peak output, not its average output.

[0006] When the peak value of an object wave pulse fluctuates and decreases due to changes in ambient temperature, the range of the object wave pulse shortens. This can lead to situations where the object wave cannot reach objects that were previously reachable, or where, even if the object wave reaches an object, the light intensity of the reflected wave pulse falls below a certain level, making it impossible to detect the reflected wave using a photodiode. This situation presents a problem because the conventional measurement method, which compensates for changes in the light intensity of the received reflected wave pulse by keeping the amplitude of the reflected signal component constant, cannot cope with this situation.

[0007] The present invention solves the above-mentioned problems and aims to provide a method for controlling the peak value of a pulsed wave in a distance measurement method using a pulsed fiber laser, which maintains a stable reach and enables reliable distance measurement by keeping the peak value of the pulsed wave irradiated onto an object constant. [Means for solving the problem]

[0008] To achieve the above objective, a first aspect of the pulse fiber laser peak value control method according to the present invention is a distance measurement method in which pulsed light from a pulse fiber laser is irradiated onto an object, reflected pulsed light is received from the object, and the time from the emission of the irradiated pulsed light to the receipt of the reflected pulsed light is converted into distance to measure the distance, wherein the irradiated pulsed light and the reflected pulsed light are detected by an avalanche photodiode (hereinafter referred to as "APD"), the peak value of the irradiated pulsed light is calculated based on this detection, and the peak value of the irradiated pulsed light generated based on this peak value is controlled to be constant.

[0009] To achieve the same objective, a second aspect of the pulsed fiber laser peak value control method according to the present invention is as follows: In the first aspect, the current value of the irradiation pulse light detected by the APD is converted to a voltage value, then A / D converted and input to a peak calculation means, for example, a field-programmable gate array (FPGA). The FPGA calculates the peak value of the irradiation pulse light, and based on this peak value, outputs a drive control signal to an excitation laser diode drive circuit (hereinafter referred to as "excitation LD drive circuit") that drives an excitation laser diode (hereinafter referred to as "excitation LD") that emits excitation light to an amplification fiber that generates the irradiation pulse light of the pulsed fiber laser, so that the peak value of the generated irradiation pulse light becomes constant. It also outputs a drive control signal to a pulsed laser diode drive circuit (hereinafter referred to as "pulse LD drive circuit") that drives a pulsed laser diode (hereinafter referred to as "pulse LD") that emits seed laser pulses to the amplification fiber.

[0010] Similarly, in order to achieve the above objective, a third aspect of the pulsed fiber laser peak value control method according to the present invention is that, in the first and second aspects, the peak value of the irradiated pulsed light is obtained by interpolating the sampled values.

[0011] Similarly, in order to achieve the above objective, a fourth aspect of the pulsed fiber laser peak value control method according to the present invention is that, in the first and second aspects, the calculated peak value of the irradiated pulsed light is the average value of a plurality of irradiated pulsed light peak values.

[0012] Furthermore, an example of an apparatus to which the pulsed fiber laser peak value control method according to the present invention is an apparatus for measuring distance, which irradiates an object with pulsed light from a pulsed fiber laser, receives the reflected pulsed light reflected by the object, and measures the distance by converting the time from the emission of the irradiation pulsed light to the receipt of the reflected pulsed light into distance, and includes an amplifying fiber that generates the irradiation pulsed light, an APD that detects the reflected pulsed light and the peak value of the irradiation pulsed light, a pulsed LD controlled by a pulsed LD drive circuit to emit a seed laser for generating the irradiation pulsed light to the amplifying fiber, an FPGA that outputs a drive current corresponding to the peak value of the irradiation pulsed light to an excitation LD drive circuit, and an excitation LD that is driven and controlled by the excitation LD drive circuit to emit excitation light to the amplifying fiber so as to generate pulsed light having the same peak value as the peak value of the irradiation pulsed light detected by the APD. [Effects of the Invention]

[0013] According to the present invention, by keeping the peak value of the pulsed light from the pulsed fiber laser irradiating an object constant, a stable range can be maintained, and distance measurement can be reliably performed. [Brief explanation of the drawing]

[0014] [Figure 1] A block diagram showing one embodiment of a distance measuring device to which the pulse fiber laser peak value control method according to the present invention is applied. [Figure 2] A schematic diagram showing the waveforms of the irradiation pulse and reflected pulse detected by the APD. [Figure 3] A schematic diagram showing sampled data of reflected pulsed light detected by APD. [Figure 4] A schematic diagram showing a comparison of the peak and average values ​​of the irradiation pulse light detected by the APD. [Modes for carrying out the invention]

[0015] Hereinafter, an embodiment of a distance measuring device to which the pulse fiber laser peak value control method according to the present invention is applied will be described based on the attached drawings. As shown in Figure 1, the distance measuring device 1 uses the time-of-flight method to measure distance by irradiating an object 3 with pulsed light emitted from the output fiber 11 of the pulsed fiber laser device 10 via the light projection block 2, and then measuring the distance by converting the time from the emission of the pulsed light to the incidence of the reflected pulsed light into distance. The pulsed fiber laser device 10 is equipped with a monitor fiber 12 in addition to the output fiber 11, and pulsed light emitted from the monitor fiber 12 is incident on the light receiving block 4.

[0016] The pulsed fiber laser device 10 includes an amplification fiber 13 that generates irradiation pulse light. Seed laser pulses emitted from a pulse LD 17 controlled by a drive signal from a pulse LD drive circuit 16 are incident on this amplification fiber 13. In addition, excitation pulses emitted from an excitation LD 19 controlled by a drive signal from an excitation LD drive circuit 18 are incident on the amplification fiber 13. The excitation LD 19 is driven and controlled by the excitation LD drive circuit 18 to emit excitation light, which is a CW laser, into the amplification fiber 13 so as to generate irradiation pulse light having the same peak value as the irradiation pulse light detected by the APD 21 described later.

[0017] The irradiation pulsed light generated by the amplifier fiber 13 passes through the filter 14 in a desired wavelength band, for example, in the range of 1550 ± 1 nm, and the generated irradiation pulsed light is branched by the coupler 15 and sent to the output fiber 11 and the monitor fiber 12, respectively. The branching ratio is 99% on the output fiber 11 side, and the remaining 1% is on the monitor fiber 12 side. Then, as described above, from the output fiber 11, the irradiation light pulse is irradiated onto the object 3 through the light projecting block 2, and the reflected light pulse enters the light receiving block 4.

[0018] The reflected pulsed light incident on the light receiving block 4 and the irradiation pulsed light from the monitor fiber 12 enter the APD 21. This APD 21 detects the reflected pulsed light, detects the peak value of the irradiation pulsed light, and outputs it as a current value. The output current value is converted into a voltage value by the current-voltage conversion circuit 22, and further digitally converted by the A / D conversion circuit (ADC) 23 and input to the FPGA 24 which is the peak calculation means. This FPGA 24 calculates the peak value of the irradiation pulsed light and outputs a drive current control signal corresponding to this peak value to the excitation LD drive circuit 18, and outputs a drive current control signal corresponding to the calculated peak value to the pulsed LD drive circuit 16. Also, the FPGA 24 outputs the distance to the object 3 measured by the time-of-flight method as ranging value data.

[0019] Subsequently, the peak value control method of the pulsed fiber laser in the distance measuring device 1 described above will be described in more detail.

[0020] 99% of the irradiation pulse light emitted from the output fiber 11 is irradiated from the light projection block 2 onto the object 3, and the reflected pulse light reflected by the object 3 enters the light receiving block 4. On the other hand, simultaneously with the 99% irradiation pulse light emitted from the output fiber 11, 1% of the irradiation pulse light emitted from the monitor fiber 12 enters the light receiving block 4. And the time obtained by subtracting the time it takes for 1% of the irradiation pulse light to reach the light receiving block 4 from the monitor fiber 12 from the time it takes for 99% of the irradiation pulse light to be emitted from the output fiber 11 and reach the light receiving block 4 as the reflected pulse light is the time corresponding to the distance to the object 3. By converting this time into distance, the distance to the object 3 can be obtained.

[0021] [[ID=*3]] The incidence of 1% of the irradiation pulse light and the reflected pulse light onto the light receiving block 4 is detected by the APD 21. Since the APD 21 has ultra-high-speed responsiveness, as shown in FIG. 2, the peak value of each pulse can be detected. In the APD 21, first, 1% of the irradiation pulse light is detected as a current value, and then, after a delay corresponding to the time from the light projection block 2 to the object 3, the reflected pulse light is detected as a current value. These current values are converted into voltage by the current-voltage conversion circuit 22, and further A / D converted by the A / D converter (ADC) 23 and input to the FPGA 24.

[0022] The FPGA 24 calculates the peak value of the input irradiation pulse light, and based on the calculated peak value, outputs a drive control signal to the excitation LD drive circuit 18 that drives the excitation LD 19 that emits excitation light to the amplification fiber 13 so that the peak value of the generated irradiation pulse light becomes constant, to emit a CW laser, which is the excitation light. Also, the FPGA 24 outputs a drive control signal to the pulse LD drive circuit 16 to emit a seed laser pulse from the pulse LD 17 to the amplification fiber 13. The seed laser pulse is amplified by the amplification fiber 13 to generate irradiation pulse light.

[0023] In this way, FPGA24 controls the peak value of the irradiation pulse light detected by APD21 to be constant. That is, by controlling the peak value of the irradiation pulse light emitted from monitor fiber 12 to be constant, the peak value of the irradiation pulse light emitted from output fiber 11 becomes constant.

[0024] When calculating the peak value of the irradiation pulse light, as shown in Figure 3, it is good if the sampling point during A / D conversion by the A / D converter 23 coincides with the peak value (see Figure 3(a)), but there are also cases where they do not coincide (see Figure 3(b)). In these cases where they do not coincide, it is not possible to calculate an accurate peak value, so it is preferable to improve the accuracy of calculating the peak value of the irradiation pulse light by using an interpolation method such as spline interpolation. The dotted lines shown in Figures 3(a) and (b) represent the interpolated curves.

[0025] Alternatively, the average of multiple peak values ​​P1, P2, P3...Pn of the irradiation pulse light detected by the APD21 may be used as the peak value of the irradiation pulse light, and control may be performed based on this average peak value. As shown in Figure 4, the average value of the peak values ​​detected by the APD21 is significantly larger than the average output value of the irradiation pulse light detected by a conventional photodiode, thus improving the signal-to-noise ratio and robustness compared to conventional methods.

[0026] Furthermore, Amplified Spontaneous Emission (ASE) is CW light that passes through filter 14 with a wavelength width of 1 nm around 1550 nm and leaks out as part of the irradiation pulse, accounting for several percent. Since ASE varies depending on the pulse repetition frequency and ambient temperature, performing Automatic Power Control (APC) using the peak value of the irradiation pulse light can significantly reduce the proportion of ASE compared to performing APC using the average value of the irradiation pulse light shown in Figure 4. Also, since the peak value of the irradiation pulse light contributes to the reach of the irradiation pulse light, it is desirable to perform APC using peak value information with ASE removed as much as possible.

[0027] Furthermore, the present invention is not limited to the embodiments described above. For example, the wavelength band of the irradiation pulse light transmitted through the filter 14 is not limited to 1550±1nm. Also, the ratio of splitting the irradiation pulse light to the output fiber 11 and the monitor fiber 12 by the coupler 15 is not limited to 99% and 1%. Moreover, the peak calculation means is not limited to an FPGA, but may be an Application Specific Integrated Circuit (ASIC), for example. [Explanation of Symbols]

[0028] 1. Distance measuring device 2. Floodlight Block 3 objects 4. Light receiving block 10. Pulsed fiber bilaser device 11 Output Fiber 12 Monitor Fiber 13 Amplifying Fiber 14 filters 15 Couplers 16. Pulse LD driving circuit 17 Pulse LD 18 Excitation LD driving circuit 19 Excited LD 21 APD 22 Current-Voltage Conversion Circuit 23. Analog-to-digital converter (ADC) 24 FPGA

Claims

1. In a distance measurement method in which pulsed light from a pulsed fiber laser is shone onto an object, and the reflected pulsed light from the shone object is received, and the time from the emission of the shone pulsed light to the receipt of the reflected pulsed light is converted into distance to measure the distance, The irradiation pulse light and the reflected pulse light are detected by an avalanche photodiode, and the peak value of the irradiation pulse light is calculated based on the detection of the irradiation pulse light. Based on this peak value, the system controls the generated irradiation pulse light so that its peak value remains constant. A method for controlling the peak value of a pulsed fiber laser, characterized by the following features.

2. The current value of the irradiation pulse light detected by the avalanche photodiode is converted to a voltage value, then A / D converted and input to the peak calculation means. The peak calculation means calculates the peak value of the irradiation pulse light, and based on this peak value, outputs a drive control signal to the excitation laser diode drive circuit, which drives the excitation laser diode that emits excitation light to the amplification fiber that generates the irradiation pulse light of the pulse fiber laser, so that the peak value of the generated irradiation pulse light becomes constant, and also outputs a drive control signal to the pulse laser diode drive circuit, which drives the pulse laser diode that emits seed laser pulses to the amplification fiber. The method for controlling the peak value of a pulsed fiber laser according to feature 1.

3. The peak value of the aforementioned irradiation pulse light is calculated by interpolating the sampled values. A method for controlling the peak value of a pulsed fiber laser according to claim 1 or 2.

4. The detected peak value of the irradiation pulse light is the average value of multiple peak values ​​of the irradiation pulse light. A method for controlling the peak value of a pulsed fiber laser according to claim 1 or 2.

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