1. Introduction

Recently, single-frequency fiber lasers (SFFLs) have attracted significant attention, thanks to their all-fiber compact configuration, high spectral purity, and convenient thermal management capabilities [1–4]. In particular, 1 µm SFFL, as a meaningful branch of the SFFL family, has made prominent progress and extensive development in fields like lidar, material forming, nonlinear frequency transformation, and other applications [5–8]. However, with the increasing development of science and technology, many precision laser application fields, represented by quantum optics, biophotonics, atom trapping, and space gravitational wave detection, have put forward more stringent technical requirements for the intensity noise performance of 1 µm SFFL. For instance, an intensity noise suppression of 15 dB in a laser source has increased the lifetime of atoms in the optical tweezer by an order of magnitude [9]. Furthermore, the space-based laser interferometer space antenna (LISA) mission not only requires that the RIN of the light source be less than -160 dB/Hz @ heterodyne frequency (5∼50 MHz) but also requires the RIN of the low-frequency band (<1 Hz) to be as small as possible [10–12]. In addition, an output power over 2 W is necessary for signal detection in the LISA system and laser transmission in other applications [13]. Hence, within these application tractions, the pursuit of wide-bandwidth low-RIN and high-power SFFL at 1 µm has emerged as a pivotal research focus.

With regard to the suppression of intensity noise, various techniques such as optoelectronic feedback, mode cleaning, and gain saturation methods have been extensively investigated. The optoelectronic feedback method typically involves photoelectric signal sampling and dynamically adjusting the power fluctuations of the final output laser [9,14–16]. Researchers at Harvard University have achieved a RIN suppression of 15 dB by employing an executor combination of an electro-optic modulator (EOM) and acousto-optic modulator (AOM) to suppress intensity noise in the frequency range from 100 Hz to 1 MHz [9]. However, limited by the bandwidth constraints of servo or feedback systems, a superior intensity noise suppression across a wide frequency range is challenging to achieve. Mode cleaning is a practical method to reduce intensity noise, especially when combined with optoelectronic feedback technology [17,18]. Nonetheless, the mode cleaner based on spatial optical cavity structure introduces partial volume optic elements, which increases the complexity of use and sensitivity of environmental interferences. The third technique has exploited the gain saturation effect of a semiconductor optical amplifier (SOA) or booster optical amplifier (BOA), achieving effective suppression of RIN in a wide frequency range [19,20]. However, due to semiconductor material properties and carrier dynamics, the working range is constrained to the high-frequency band, and the output power is limited to tens of mW. Previously, we demonstrated a 1064 nm SFFL with low RIN over a wide frequency range based on two-fold optical gain saturation effects from BOA and a reflective Yb-doped fiber amplifier [21]. Regrettably, the challenge of low-frequency-band RIN reduction also exists, and the output power level is still limited to hundreds of mW. Although an external fiber amplifier can effectively increase the output laser power, it is still challenging to maintain or suppress intensity noise during laser amplification, especially for ultra-low-RIN lasers.

In this article, we present a systematic approach that seamlessly integrates RIN suppression and power amplification within SFFLs, realizing significant advancement in performance and versatility. The functional framework has harnessed the dual gain saturation effect within the BOA and reflective Yb-doped fiber amplifier (RYDFA), combined with the analog-digital hybrid optoelectronic feedback technology. A significant RIN suppression spanning over 31 octaves from 0.1 Hz to 380 MHz is achieved, and the RIN beyond 1 Hz remains consistently below -150 dB/Hz. Remarkably, within the frequency range from 10 Hz to 10 GHz, the RIN levels have reached as low as -160 dB/Hz, nearly approaching the SNL over more than 29 octaves. Furthermore, an output power of 10.8 W and a linewidth of less than 0.8 kHz are obtained simultaneously. This accomplishment signifies the first realization of a high-power 1 µm SFFL demonstrating near-SNL RIN performance across such an extensive frequency span.

2. Experimental setup

Figure 1 depicts the experimental configuration of the 10 W super-wideband ultra-low-intensity-noise SFFL at 1 µm. This system occupies a space of approximately 178*79*60 (cm³) on the optical platform (including the complete circuitry and drivers) and comprises five modules: a single-frequency fiber seed source (Seed Module), an intensity-noise suppression stage based on a BOA (BOA Module), a pre-amplification stage (1st Amplifier), a main amplification stage with low-frequency intensity-noise suppression (2nd Amplifier), and an electric-loop feedback and final output stage (Output & Feedback). The laser seed is built with a distributed Bragg reflector (DBR) short-linear-cavity structure, which consists of a 1.3-cm-long highly-ytterbium-doped phosphate fiber (YDPF), a polarization-maintaining narrow-band fiber Bragg grating (PM-NB-FBG) and a high-reflection broadband FBG (BB-FBG) with a reflectivity of more than 99.9% [22]. The PM-NB-FBG, written in a single-mode PM fiber, has a 3-dB bandwidth of 0.06 nm and a reflectivity of 65% at the signal wavelength. Noted that, to diminish the frequency noise of the signal light caused by temperature fluctuations, the entire laser cavity is situated on an aluminum V-shaped heat sink with a temperature stability of 0.05 °C (Not marked on the experimental device diagram) [23]. The laser cavity is backward-pumped by a 980 nm single-mode LD (SM-LD 1) via a 980/1064 nm PM wavelength division multiplexer (PM-WDM 1) that also couples output the signal laser. To prevent any detrimental effects on the laser performance resulting from back reflections generated by subsequent components, a PM isolator (PM-ISO1) is seamlessly integrated directly behind the PM-WDM 1.

 

Fig. 1. Experimental setup of the 10 W super-wideband ultra-low-intensity-noise SFFL at 1 µm, which is composed of five modules. The seed module is the fiber oscillator, the BOA module for noise suppression, the 1st amplifier is the pre-amplifier for signal power, the 2nd amplifier for noise suppression and main amplification, and the output & feedback is the final output of laser and electrical feedback system. PM-WDM: polarization-maintaining wavelength division multiplexer; SM-LD: single-mode laser diode; ISO: isolator; VOA: variable optical attenuator; BOA: booster optical amplifier; BPF: band-pass filter; YSF: ytterbium-doped single-cladding fiber; CIR: circulator; MM-LD: multi-mode laser diode; PMC: polarization-maintaining coupler; DC-YDF: double-cladding ytterbium-doped fiber; PD: photodetector; PMTI: polarization-maintaining tap isolator; LPF: low-pass filter; PS: phase shifter; AMP: amplifier; ADC: analog-to-digital converter; MCU: microcontroller unit; DAC: digital-to- analog converter; ADD: adder.

Download Full Size | PDF

The signal laser generated from the seed module is directly transmitted to the BOA module, where the nonlinear amplification effect of the BOA is utilized for the first-step suppression of RIN in the middle-to-high frequency range. To achieve an optimized gain-saturated regime of the BOA, the laser power is carefully adjusted using a variable optical attenuator (VOA) before being introduced into the BOA module. A PM-ISO (PM-ISO2) is employed to prevent the detrimental impact of the backward reflected light on the stable operation of the BOA module. Additionally, a PM band pass filter (PM-BPF) with a 3 dB bandwidth of 0.4 nm is adopted to filter out the amplified spontaneous emission (ASE) that arises during the amplification process of the BOA.

The signal light from the BOA module is subsequently sent into the 1st amplifier module, which comprises an SM-LD 2, a PM-WDM 2, a PM ytterbium-doped single-cladding fiber (PM-YSF), a PM-ISO 3, and a PM circulator (PM-CIR). For boosting the laser power, SM-LD 2 with an output power of 460 mW is utilized to forward pump a 3-meter-long PM-YSF with an absorption coefficient of 250 dB/m at a pumping wavelength of 975 nm. This forward pumping is achieved through a PM-WDM (PM-WDM 2), and the PM-ISO 3 is adopted at the exit of the active fiber. A PM optical circulator (PM-CIR) is used to allow the light to couple in and out of the 2nd amplifier module.

Within the 2nd amplifier module, a 2-m-long PM double-cladding ytterbium-doped fiber (PM-DC-YDF) is pumped by a multi-mode laser diode (MM-LD) through a (2 + 1) × 1 PM-Combiner. A reflective loop solely composed of optical fibers is utilized to achieve a bidirectional configuration for the RYDFA structure, which can effectively suppress the intensity noise in the low-to-middle frequency range. The fiber loop, which connects two arms of a 2 × 1 PM coupler (PMC) with a 20/80 ratio, operates as a reflective end with an ideal reflectivity of approximately 64% [24]. Furthermore, an SM-LD 3 is seamlessly integrated into another pump input arm of the PM-Combiner to achieve a more significant reduction of intensity noise. This configuration has provided the pump power regulation function by external signals to build the photoelectric feedback loop.

Subsequently, the laser with high-power output from the 2nd amplifier module is directed into a PM tap isolator (PMTI) of the output & feedback module. The large port of the PMTI primarily transmits the primary laser signal of approximately 10 W, which will be used for the relevant experiments as the experimental light source. Meanwhile, the small port outputs a laser signal of around 10 mW, serving as the feedback light to further restrain the intensity noise. A low-noise photodetector (PD) with a bandwidth of 13 MHz converts the fluctuating optical intensity signal into the corresponding electrical signal. This electrical signal is then processed through a hybrid proportional-integral-derivative (PID) feedback loop, which combines analog PID and digital PID for effective feedback control. The analog PID section consists of a low-pass filter (LPF), a phase shifter (PS), and an electrical amplifier (AMP). The digital PID section includes an analog-to-digital converter (ADC), microcontroller unit (MCU), and a digital-to-analog converter (DAC). These two feedback signals are superimposed by the adder (ADD) and loaded into the modulated port of the driver of SM-LD 3 to form closed-loop feedback.

3. Results and discussion

The spectral characteristics of the super-wideband ultra-low-intensity-noise 1 µm SFFL at different testing ports are illustrated in Fig. 2(a). The resolution and scanning range of the optical spectrum analyzer (OSA) is set to 0.1 nm and 150 nm, respectively. Additionally, the input power of the signal laser sent into the OSA is uniformly adjusted to 0 dBm to ensure data consistency. As observed, the center wavelength of the seed laser is 1064.0 nm with an optical signal-to-noise ratio (OSNR) of ∼75 dB. However, the BOA is prone to generating ASE during the amplification of the signal laser, as illustrated by the blue line in Fig. 2. Meaningfully, the introduction of BPF has filtered most of these ASE components. After 1st amplifier module, the ASE is slightly increased. Then, some ASE in the 1035 nm band appears in the spectrum of the 2nd amplifier output, which is mainly attributed to the restricted choice of gain fiber length by considering later intensity noise suppression. Fortunately, the OSNR of the final output laser is still more than 60 dB. Under the condition of 315 mW output power of the 1st amplifier, the comparison between the pump power of the MM-LD in the 2nd amplifier module and the output power of the SFFL is depicted in Fig. 2(b). Notably, the output power of the signal laser increases almost linearly with the pump optical power, and the slope efficiency reaches 55.3%. Finally, the maximum output power of the laser is up to 10.8 W. While the MM-LD attains a total output power of 19.06 W at this juncture, the corresponding optical-to-optical efficiency ultimately reaches a level of approximately 56.7%. Considering no power saturation phenomenon in the amplification process, the system framework can further enhance the output power.

 

Fig. 2. (a) Measured optical spectra of the SFFL from the seed, after BOA module, after BPF, after 1st amplifier module, and after 2nd amplifier module. (b) Output power versus the pump power in the 2nd amplifier module.

Download Full Size | PDF

As depicted in Fig. 3, the measured results of RIN for various sections within this SFFL experimental apparatus are presented. The laser beam is transformed into an electric signal by a low-noise high-speed PD, and then the measurement of RIN is accomplished through the collaborative use of an electronic spectrum analyzer (ESA) for high-frequency testing and a dynamic signal analyzer (DSA) for low-frequency testing. Throughout the testing process, the power outputs from different sections directed into the PD are consistently adjusted at 8 mW to enhance comparability. The corresponding SNL (-163.3 dB/Hz) is also given in Fig. 3 for comparison.

 

Fig. 3. RIN of the SFFL in the seed module, after BOA module, after 1st amplifier module, after 2nd amplifier module without feedback, and after 2nd amplifier with feedback, along with the SNL for comparison.

Download Full Size | PDF

It can be observed that the performance of the free-running seed source is relatively poor in terms of intensity noise. Firstly, there is a relaxation oscillation peak with a RIN exceeding -95 dB/Hz at the frequency of about 2 MHz. Secondly, starting from 10 kHz, the RIN gradually worsens, increasing from -140 dB/Hz to -80 dB/Hz at the lowest detecting frequency of 0.1 Hz. These characteristics have profound implications for noise-sensitive applications, limiting the promotion of the SFFL in high-precision optical fields.

Subsequently, within the BOA module, the laser generated by the seed module undergoes the initial stage of intensity noise suppression. The RIN of the laser after traversing the BOA module is depicted by the blue line in Fig. 3. Benefiting from the gain saturation effect of the BOA, the RIN above the frequency of 200 kHz is effectively suppressed to -161 dB/Hz, which is only higher 2.3 dB than the SNL. Impressively, the relaxation oscillation noise originating from the seed module is also suppressed to -161 dB/Hz, representing a remarkable 66 dB suppressing amplitude. Constrained by the mismatching response between the carrier dynamics and noise-suppressing mechanism in the BOA within the low-frequency range, the RIN-suppressed capability of the BOA module gradually weakens as frequencies drop below 200 kHz. Around 10 kHz, the suppression effectiveness diminishes significantly. Additionally, the utilization of BOA resulted in a slight increase in RIN at frequencies within the range of 0.2 kHz to 7 kHz. This is attributed to the introduction of electrical noise during the driving of the BOA, including instability in power supplies, electromagnetic interference, etc. Another factor is the influence of the BOA's inherent properties, such as fluctuations in power stability caused by spontaneous emission during operation and output intensity fluctuations resulting from carrier relaxation processes in semiconductor materials [25,26].

The purpose of the 1st amplifier module is to pre-amplify the laser power before entering the 2nd amplifier stage. It can be observed that there is no significant change in the RIN of the signal light after passing through this module. Specifically, the RIN has slightly reduced in the frequency range of 100 Hz to 8 kHz, attributed to the mild gain saturation effect of the active fiber in this optical amplifier [27]. In addition, the RIN in the frequency band of less than 50 Hz has some deterioration, resulting from the noise transmission of the pump driver. Several discrete noise spikes come from power-frequency signals, electromagnetic interference signals, and cooling fan vibration.

Following, the output laser is injected into the 2nd amplifier module to experience the second suppression of the RIN. The green curve in Fig. 3 represents the measured RIN result of the SFFL after undergoing 2nd amplification without activating the feedback system. It is evident that the bidirectional configuration of the reflective loop mirror adeptly harnesses the gain saturation effect of the 2nd amplifier module twice, facilitating the nonlinear amplification of the laser signal to promote the inhibition of intensity noise [28,29]. Due to the longer upper-level particle lifetime of the YDF, the fiber-based gain saturation effect is mainly impactful in the low-frequency band below 10 kHz, which realizes the perfect frequency-band connection with the RIN-suppressing mechanism from the semiconductor material. Notably, the RIN level is suppressed to less than -110 dB/Hz throughout the frequency band above 1 Hz, and the maximum suppression amplitude exceeds 30 dB. Although this acquired RIN in the low-frequency range is still far away from the SNL, it should be noted that this stage of intensity noise suppression lays a solid foundation for the post-stage photoelectric feedback, and effectively reduces the gain control pressure of the feedback loop.

On the above basis, the optoelectronic feedback loop loaded into SM-LD 3 is activated to build the third stage of the RIN-reduced framework, which mainly targets the intensity noise in the low-frequency band that has not been well processed previously. To overcome the difficulty of the conventional photoelectric feedback loop to satisfy the amplitude-frequency function matching relation in the ultra-wide frequency band, a hybrid PID feedback system including analog loop and digital loop is elaborately designed. This link builds automatic feedback control with stability enhancing of the 2nd amplifier, and effectively reduces intensity noise in low-frequency bands. As indicated by the red line in Fig. 3, the RIN in the total range from 0.1 Hz to 100 kHz has been suppressed to different extents, and a maximum inhibition amplitude of approaching 40 dB is obtained. Moreover, adept utilization of the optoelectronic feedback mechanism and the gain saturation effect of the YDF ensures no adverse effects on RIN across other frequency ranges.

Ultimately, we have acquired an efficient reduction of RIN within a super-wide bandwidth from 0.1 Hz to 380 MHz, which overtakes a suppressing span of over 31 octaves. The RIN at the very-low frequency of 0.1 Hz is remarkably inhibited by 40 dB, and the obtained value is near -120 dB/Hz. Furthermore, the RIN values remain below -150 dB/Hz at the frequency range over 1 Hz, apart from the power-frequency signal with its harmonics and occasional pulse peaks caused by environmental electrical interference. More importantly, within the frequency range of 10 Hz to 10 GHz, the RIN of the fiber laser remains below -160 dB/Hz, which means the laser RIN approaches a level near the SNL across a frequency span exceeding 29 octaves. To our best understanding, this is the lowest RIN result of SFFL within such an extensive frequency range, and this is the widest working bandwidth of a near-SNL SFFL. This remarkable characteristic will enable the SFFL to be applied in various fields where higher demands are placed on laser RIN in different frequency ranges, for example, space-based gravitational wave detection, super-long-distance space coherent velocity measurement and ranging, demonstrating significant application value and extensive prospects.

To examine the effect of the super-wide-bandwidth RIN suppression process on the linewidth of SFFL, we have employed a delay self-heterodyne measured system based on a Mach-Zehnder interferometer. This system mainly includes a 6 km long Hi-1060 fiber and an 80 MHz fiber-coupled acoustic-optic modulator, and the obtained linewidth results at various positions of the experimental setup are presented in Fig. 4(a). It is important to note these five lines are almost overlapping, which means that the laser linewidth has nearly no change in the total experimental process. However, all these self-heterodyne line shapes have presented evident coherent envelopes, and all these main signals have exhibited spike morphology. This phenomenon is ascribed to the incomplete coherence elimination due to the linewidth of the measured SFFL being significantly narrower than the resolution of the testing system (∼40 kHz) [30]. Fortunately, the contrast difference between the second peak and second trough (CDSPST) within the coherent envelope can be applied to estimate the laser linewidth [31]. The CDSPST value of the final output laser is measured as 14.16 dB, which is revised to 13.94 dB according to the Ref. [32]. Consequently, the linewidth of the ultimate output from the SFFL is computed to be less than 0.8 kHz, which signifies a coherent length of more than 37.5 km.

 

Fig. 4. (a) Measured self-heterodyne spectra of the SFFL from the seed, after BOA, after 1st amplifier module, 2nd amplifier module without feedback, and after 2nd amplifier with feedback. (b) Measured frequency noise of the SFFL from the seed, after BOA, after 1st amplifier module, 2nd amplifier module without feedback, and after 2nd amplifier with feedback.

Download Full Size | PDF

Apart from the laser linewidth, the frequency noise of this SFFL is also evaluated through a fiber Michelson interferometer with a 100 m optical path difference and an optical phase demodulator based on phase-generated carrier (PGC) technology [33]. As depicted in Fig. 4(b), the frequency noise testing results of various states have exhibited a high degree of consistency, which further confirms that the RIN suppressing process has no impact on the frequency-domain characteristic of this SFFL, including frequency noise and linewidth. The frequency noise at 1 kHz is lower than 170 Hz²/Hz, corresponding to a phase noise result of -127.8 dB.rad/Hz^1/2/m. Utilizing the Beta-separation line for evaluation [34], we find that the linewidth of the SFFL is less than 0.75 kHz, in good agreement with the linewidth measurements obtained through CDSPST. Additionally, these noise spikes in the low-frequency range are mainly originated from electromagnetic interference and environmental disturbance.

To analyze the resonant modes of the final output laser and ensure the single-longitudinal-mode (SLM) characteristics for future applications, a scanning Fabry-Pérot interferometer with a spectral resolution of 7.5 MHz and a free spectral range of 1.5 GHz is employed. Within one scanning period of the interferometer, the signal trace recorded by the oscilloscope reveals the presence of only two peaks, as shown in the inset of Fig. 5(a). This result indicates that the laser operates in a stable single-longitudinal-mode status, without any mode hopping or risk of multi-longitudinal mode. Furthermore, the long-term stability of the final output power is carefully measured via a laser power meter with a resolution of 10 mW, as shown in Fig. 5(a). Thereinto, the fluctuation amplitude of the output power is 50 mW, and the average value of the output power is 10.8 W. Consequently, the power instability over a continuous operation spanning of 6 hours is less than ±0.46%, which expresses superior power constancy.

 

Fig. 5. (a) Six-hour long-term power stability of the final output. Inset: Single-longitudinal-mode status of the final output laser characterized by a scanning Fabry–Pérot interferometer. (b) Four-hour long-term polarization extinction ratio of the final output. Inset: Polarization state of the output laser.

Download Full Size | PDF

Then, the polarization state of the final output laser is meticulously measured. A spatial polarization beam splitter (PBS) is utilized to separate fast-axis and slow-axis light, which are detected by two laser power meters to assess the polarization extinction ratio (PER) of output laser. As depicted in Fig. 5(b), the PER of the final output laser consistently maintained a level exceeding 20 dB over a continuous runtime of 4 hours. Moreover, an optical polarization analyzer is employed to observe the polarization parameters of the laser, as shown in the inset of Fig. 5(b). Notably, within the inset, the red dot is positioned near the equator of the Poincaré sphere, and the measured degree of polarization (DOP) is 99%. These two test results collectively demonstrate the sustained polarized stability of the final output laser, which consistently operates in a near-linear polarization state. This characteristic holds substantial promise for applications in scenarios and fields where stringent polarization requirements of the laser are essential.

4. Conclusion

In summary, a 10 W super-wideband ultra-low-intensity-noise SFFL at 1 µm is developed experimentally. By skillfully exploiting the dual optical gain saturation effect in the BOA and RYDFA, not only the intensity noise of the ultra-wideband is significantly suppressed, but also the power scale of the output laser is effectively improved. Furthermore, the analog and digital hybrid PID feedback system integrated into the RYDFA has remarkably enhanced the RIN suppression capabilities in the low-frequency range. Importantly, these three RIN inhibition mechanisms work together and enhance suppression without interfering with each other, thus constructing a super-wideband RIN-reduced frequency range from 0.1 Hz to 380 MHz, spanning over 31 octaves. Eventually, the RIN at the very-low frequency of 0.1 Hz is remarkably inhibited to near -120 dB/Hz and remains consistently below -150 dB/Hz at the frequency range over 1 Hz. More significantly, over the entire frequency span from 10 Hz to 10 GHz, the RIN of SFFL is maintained below -160 dB/Hz, nearing the SNL across a range that exceeds 29 octaves. To the best of our knowledge, this is the lowest RIN result of SFFL within such an extensive frequency range, and this is the highest output power of the near-SNL super-wideband SFFL. In addition, the SFFL system exhibits high-quality laser characteristics, including a linewidth within 0.8 kHz, a long-term stable PER of 20 dB, and an OSNR exceeding 60 dB. This groundbreaking technology, which facilitates simultaneous RIN suppression and power amplification, holds great promise for crucial applications in fields such as free-space laser communication, squeezed light generation, space-based gravitational wave detection, and super-long-distance space coherent velocity measurement and ranging.