1. Introduction

Laser ranging is a practical technology that has widespread applications such as meteorological measurement, industrial production, aeronautics, and astronautics and so on [1–3]. The development of laser ranging adopts the time probe through light fly all the time [4,5]. A general approach is using a stable pulse laser to detect distances in long-range environments such as satellite laser ranging, meanwhile, and a timer is ready to obtain a precise time simultaneously. Pairing laser and timer ranging systems requires modular design method assemble many complicated facilities to ensure the ranging precision [6,7]. Another common way of ranging is to use laser phase for modulating the amplitude of a continuous wave laser, generating the sinusoidal wave and compared it with the reflected beam to draw the time by the phase difference, and further figure up the distance between laser and target [8]. For a short distance, the later method is generally chosen because the range finding is limited by the modulated frequency and is hard to distinguish in a large distance situation. In addition to modulating the amplitude of the laser, the researchers developed a promising frequency-modulated continuous-wave (CW) way by altering the frequency of the laser regularly [9–11]. By using a modulated laser sources where the central wavelength of this laser is varied linearly or triangularly to carry out the study. The reference light and the reflected light interact to generate the interference pattern and estimate the distance by the microwave signal acquired from RF spectrum [12,13]. However, one of the problems that restricts the development of laser ranging technology is that the use of semiconductor laser that has obstacles on the coherence length, modulated frequency, repetition rate, bandwidth, and so on.

Nowadays, numerous techniques are proposed and perfected to solve this laser source problem. A combination of optical recirculating frequency shifter loop and frequency linear sweeper is introduced to generate chirped light broadband with narrow linewidth [11]. By using a monolithic integrated two-section distributed feedback (DFB) laser with the sideband optical injection locking technique, accurate information of distance and velocity can be extracted [14]. Except for the improvements of semiconductor lasers, novel lasers are also being tried for laser ranging systems [15]. The soliton combs, not only can directly generate the linear frequency light, but also serves as a supplementary means to access the DFB laser, applying in laser ranging system to achieve a high performance [16,12]. Fiber laser is also an excellent candidate owing to the compact generation in sterling mode-locking, Q-switch, and continuous wave (CW) light, and has been applied in various ranging scenarios [17]. In recent years, the considerable merits of the self-sweeping fiber laser make it a typical sort and prolong its development [18–21]. The linear cavity with self-sweeping microsecond pulse shows promising potential for applications such as laser rangefinder and vibrometer [22]. Very recently, a team has demonstrated a novel method to detect the distance information. Specifically, they designed a fiber-based device that through the standing-wave field to generate interference pattern in virtue of unpumped gain fiber, and thus permitted a rough information extraction from a quite erratic pulsation [23]. The vague pulse apparently increases the difficulty of obtaining useful information. Fortunately, a quasi-CW state self-sweeping fiber laser is proposed based on an unidirectional cavity [24]. The simple configuration of the fiber cavity allows infrared light to pass through a series of fiber elements with trivial loss. The light then gets absorbed by a piece of ytterbium-doped fiber and the reflector renders light beam propagates reversely to form a dynamical grating, resulting in the stable self-sweeping effect that contains rich pulse information showing the unambiguous interference pattern [24].

In this work, we proposed and demonstrated an intracavity ranging system with the help of a self-sweeping fiber laser. Unlike the previous unidirectional cavity design, a ranging setup consisting of collimator and reflected mirror with varied distances is fused in the standing-wave field. We measure the behaviors of self-sweeping effect when this distance is set as ∼30 cm, in which a self-sweeping state with few-longitudinal-mode range is obtained. The stable quasi-CW state occurs at any distance we measured. Further, we measure the RF spectra at different space lengths to extract the distance information. The laser ranging based on the intracavity devices have potential to overcome limitations of traditional laser ranging system.

2. Experimental setup

Figure 1 details the experimental setup of intracavity ranging system based on self-sweeping fiber laser. A highly stable 976 nm laser diode (LD) with the largest output power of 650 mW and a length of 1.5 m (Coherent SM-YSF-HI-HP) ytterbium-doped single-mode fiber (YDF1) offer the pump power and gain medium, respectively. The rest of components totally form the unidirectional ring cavity. A 1 × 2 wavelength division multiplexer (WDM) is used to connect LD and YDF1. The amplified stimulated emission (ASE) from the pumped gain fiber will confront the port 1 of circulator (CIR). The CIR with three ports is a key element in this system, which ensure the unidirectional transmission of light because of the high isolation of 50 dB. The light through port 2 of CIR will enter in the YDF2, Col, and the mirror. YDF2 is also the same length as the YDF1, which has a core absorption at 915 nm of 75 dB/m and a numerical aperture of 0.11. Col has a broad band operated range from 700 nm to 1100 nm which links fiber by an APC connector, and the mirror can operate from 750 nm to 1100 nm with a reflectivity of 99%, whose diameter is 12.7 mm. Then the reflected light by the mirror will enter the Col, YDF2, and come back to CIR and go through the port 3 of CIR. At the same time, a polarization controller (PC) is assembled between CIR and YDF2 to adjust the interference state. Next, the light will arrive at the 1 × 2 20/80 coupler in which the 20% of light will be introduced to measure the performance, and the rest of 80% light will return into the WDM to fulfill the ring cavity. The WDM, CIR, coupler also are the broadband components to ensure a smooth laser output without reflection. We also measured the loss of air part (col + air + mirror) that is about 0.73 dB. In experiment, a fine adjustment of mirror is needed to ensure that the loss is within an acceptable range, and the mirror is in the best position we can achieve.

 

Fig. 1. The experimental setup of intracavity ranging system based on self-sweeping fiber laser. LD: laser diode; WDM: wavelength division multiplexer; YDF1 and 2: ytterbium-doped fiber; CIR: circulator; PC: polarization controller; Col: collimator.

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3. Laser performance

In the measurement, an optical spectrum analyzer (OSA, YOKOGAWA AQ6370D) is employed to observe the central wavelength and fluctuation, as shown in Fig. 2. While the laser pump power was set as 100 mW, a stable optical spectrum was observed through the OSA, whose central wavelength was located at 1062.792 nm. The wavelength fluctuation also is recorded in the inset of Fig. 2. We can see that the wavelength fluctuation is within 0.016 nm at a measured time of 15 minutes. This is a very small variation for central wavelength of a fiber laser so that we cannot find the obvious feature of self-sweeping effect.

 

Fig. 2. The optical spectrum of this fiber laser. Inset: the wavelength fluctuation at a time scale of 900 seconds.

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The strong evidence that proving the fiber laser being operating at the self-sweeping operation is observed by the intensity dynamics which are monitored by a fiber input InGaAs biased detector (Thorlabs DET08CFC/M) with a bandwidth of 5 GHz and the Oscilloscope with a bandwidth of 1 GHz (Rohde & Schwars RTE 1104). The typical self-sweeping pulse of quasi-CW state is depicted in Fig. 3. However, at a 5 ms measured time, there are two peak pulses that do not contain the interference pattern, which means the change of central wavelength is not continuous. In a larger scale of 10 ms, this signal occurs three times, which are shown in the inset of Fig. 3. We also can see that there are six interference patterns between the signals in red circles. This has displayed the stable self-sweeping operation along with the constant frequency interval. For the self-sweeping state, when another wavelength occurs gradually, the previous wavelength intensity decreased, but it doesn’t completely disappear. Therefore, both wavelengths will exist together on a short time scale. This overlapping region leads to the production of beating frequency [25]. According to the pulse number at different powers, there are seven longitudinal-modes operate in this cavity, which can be seen as the few-longitudinal-mode self-sweeping state. It can be attributed to the short length of YDF2 and the narrow reflector. A similar state was also reported on the microsecond pulse self-sweeping fiber laser by measuring the wavelength stopping state that is same spectral representation with Fig. 2 [26]. When a self-sweeping state is interrupted, the laser needs to find a mode that can oscillate by the support of high gain to generate the self-sweeping effect, and the YDF2 needs more time to form the single frequency laser output and initiate the self-sweeping state. Therefore, the signals in the red circles that do not exist the beating frequency represent the regeneration of self-sweeping. Actually, it maybe represents a natural wavelength fluctuation influenced by the internal gain and the external environment, which is a different mechanism with the self-sweeping operation.

 

Fig. 3. The few-longitudinal-mode pulse dynamics of this fiber laser. Inset: the screen of Oscilloscope.

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Figure 4 shows the quasi-CW intensity at a pump power of 80 mW and the average pulse repetition rate, respectively. One can see that the intensity contains three parts, interference pattern, relaxation oscillation and CW. The laser threshold is about 65 mW, and the self-sweeping operation lasts in a pump power range from 65 mW to 110 mW. The average repetition rate of interference signal is linearly increased from 1.256 ms to 0.667 ms, that is a range from 0.79 kHz to 1.5 kHz. The lower repetition rate is due to the lower pump power range ruled by self-sweeping effect.

 

Fig. 4. The pulse dynamics of the laser. (a) The observed quasi-continuous wave of self-sweeping at a pump power of 80 mW; (b) the average pulse repetition rate at different pump power.

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The radio spectra are measured by the signal and spectrum analyzer (SSA, Rohde & Schwars FPS) with a bandwidth of 160 MHz. Figure 5(a) describes the RF signal when the laser runs at self-sweeping regions. The observed RF spectrum ranges of 80 MHz when the laser pump power is 80 mW. We can see that there is a peak occurred at a value of about 16.81 MHz, and the corresponding beat frequency can be obtained by zooming the signal from the inset in which the period of beat frequency is about 59.50 ns. Note that the RF spectra are measured by stopping the screen of SSA because the beat frequency signal of self-sweeping operation appears in a relatively short period of time. The stability of RF signals is also measured within 30 mins, the value of peak has no obvious change, as shown in Fig. 5(b). Due to the existence of seven longitudinal modes in this cavity and the frequency interval is about 16.81 MHz, the operated bandwidth can be calculated as 117.67 MHz, which corresponds to the spectral bandwidth of ∼0.437 pm. This value is less than the spectral fluctuation we mentioned in Fig. 2.

 

Fig. 5. The observed RF spectrum of quasi-continuous wave of self-sweeping at a pump power of 75 mW. (a) The RF spectrum in a time scale of 80 MHz; Inset: the corresponding interference patter by zooming the pulse in Fig. 4(a); (b) The stability of RF signal within 30 mins.

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Expect for the observation at the high-frequency region of RF spectrum, we also monitor the low-frequency signal. The RF spectrum at low-frequency is exhibited in Fig. 6(a), the relaxation oscillation peak is about 32 kHz when the pump power is 100 mW at a frequency range of 500 kHz. When the laser operates at self-sweeping region, the black curve can be generated. The red line is the signal without any incident light. The obvious difference is the low-frequency signal within 100 kHz. There is no difference when the frequency exceeds 100 kHz, which means the fiber laser has a low noise. Furthermore, we record the relaxation oscillation peak at different pump power, and the data are displayed in Fig. 6(b). In fact, the relaxation oscillation peak can also be obtained by the measurement of intensity dynamics. The peak frequency is an increasing process with the ascending pump power, which can be confirmed by both RF signal and pulse signal. However, the narrow power range makes it difficult to obtain an overall view of variation.

 

Fig. 6. (a) The observed RF spectrum of quasi-continuous wave of self-sweeping at a pump power of 100 mW; (b) the relaxation oscillation peak at different pump power.

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4. Ranging results

The layout of a proof of concept of intracavity ranging is displayed in Fig. 7 that details the process of intracavity ranging when the position of mirror was changed four times. These positions 1, 2, 3, 4 are about 10.0 cm, 19.0 cm, 29.0 cm, 39.0 cm from the Col, which are measured by a ruler. In this demonstration, the measurement of laser performance is carried out when the mirror was located at position 3. Next, we place the mirror at different positions and record the RF spectrum, and the results are exhibited in Fig. 7(b). By acquiring the intensity and RF spectra, we can obtain the spacing of longitudinal-mode immediately. Based on this method, the intracavity ranging can be performed easily. Here, the spacing of longitudinal mode Δv can be expressed as

 

Fig. 7. The observed RF spectrum of at different space length at a pump power of 85 mW.

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5. Conclusion

In this work, we demonstrate a single-frequency self-sweeping fiber laser with a few-longitudinal-mode range and on this basis further propose an intracavity ranging system. Based on the clear beat frequency of self-sweeping pulse, we adopt a length of space path as measured distance and make it a part of laser cavity. By analyzing the RF spectra, the exact distance information can be extracted. When the space path was set as 30 cm, we observed the behaviors of self-sweeping state. The average pulse repetition rate increases from 0.796 kHz to 1.499 kHz, and the RF signal locates at 17.97 MHz when the two longitudinal mode exists in fiber laser. The relaxation oscillation peak also increases with an ascending pump power. This work extends the application of self-sweeping fiber laser a platform to carry out laser ranging. In practice, self-sweeping fiber laser with a larger sweeping range can also be applied to ranging scenarios. The proposed ranging system is not affected by the coherent length of the light source, the modulation frequency and other parameters, so as to obtain a higher accuracy. Although limited by the conditions, we only performed the ranging distance within 40 cm, but a larger distance is expected to be achieved.