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Abstract
In this study, we present an ultralow noise single-frequency fiber laser operating at 1550 nm, utilizing a traveling-wave ring cavity configuration. The frequency noise of the laser approaches the thermal noise limit, achieving a white noise level of 0.025 Hz2/Hz, resulting in an instantaneous linewidth of 0.08 Hz. After amplification, the output power reaches 4.94 W while maintaining the same low white noise level as the laser oscillator. The integration linewidths of the laser oscillator and amplifier are 221 Hz and 665 Hz, respectively, with both exhibiting relative intensity noises that approach the quantum shot noise limit. To the best of our knowledge, this work shows the lowest frequency noise combined with relatively high power for this type of ring cavity fiber laser.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Low-noise single-frequency fiber lasers (SFFLs) have garnered significant attention in advanced scientific applications, including gravitational wave detection, high-resolution spectroscopy, and optical frequency combs [1–4]. The narrow linewidth laser at ∼1550 nm serves as the primary light source for third-generation gravitational wave detectors: Einstein telescope [5]. Moreover, specific applications demand SFFLs with higher output powers, such as gravitational wave detection, which requires laser powers exceeding the watt level. Typically, combining low-noise laser oscillators with subsequent Master Oscillator Power Amplifier (MOPA) configurations is a common strategy to achieve low-noise, high-power lasers. Therefore, it is of great significance to investigate methods for reducing noise in laser oscillators and mitigating additional noise introduced by amplifiers.
Distributed Bragg Reflector (DBR) and Distributed Feedback (DFB) lasers are widely employed in SFFL oscillators. These lasers combine narrowband feedback with increased longitudinal mode spacing, enabling stable single-longitudinal-mode (SLM) operation that is less susceptible to external disturbances. The well-known Schawlow-Townes linewidth theory establishes a fundamental limit on laser noise arising from spontaneous emission [6]. However, practical laser linewidths often surpass this prediction by several orders of magnitude. In most SFFLs, 1/f noise, arising from thermal fluctuations, is a major limiting factor in noise performance, leading to laser frequency noise scaling inversely with the length of the laser cavity [7–12]. Typical DBR and DFB lasers feature relatively short cavity lengths, often only a few centimeters long, which constrains their ability to further reduce laser noise. To address this limitation, techniques such as frequency stabilization to the reference cavity [13], self-injection locking [14,15], and delayed line feedback [16] have been employed to mitigate laser noise. Undoubtedly, these methods also introduce complexity to the system.
Long travelling-wave ring cavity lasers hold inherent potential for achieving ultra-lower noise SFFLs by simultaneously increasing intracavity photon lifetime and effectively suppressing their fundamental thermal noise. However, fiber ring cavity lasers face challenges from mode competition due to densely spaced longitudinal modes. Random thermal exchanges with the surroundings, perturbations from vibrations and acoustic waves, all of which can lead to mode hopping. Methods like mode selection in coupled cavities and narrowband filtering have been employed to stabilize SLM oscillation in fiber ring lasers [17–20]. One effective method for stabilizing SLM oscillation is the use of an unpumped active fiber as a saturable absorber (SA) [21–24], which not only suppresses unwanted longitudinal modes but also acts as a dynamic grating, serving as a narrowband “filter” adaptable to the frequency of the lasing mode. Nevertheless, this configuration remains sensitive to environmental noise. Most published papers have reported laser linewidths for such fiber ring lasers around kilohertz, such as 1.5kHz [21], 0.95kHz [22], 1.3kHz [25], 0.7kHz [26], 9.07kHz [27], 2kHz [28], 1.09kHz [29], 16kHz [30], indicating that the laser noise is far above their fundamental thermal noise limit. Meanwhile, another technique for achieving low noise, self-injection locking, has seen significant progress on integrated photonics platforms in recent years. The current state of the art in this technology has achieved a white noise level of 0.2 Hz2/Hz, corresponding to a laser instantaneous linewidth at the Hz level [31].
On the other hand, in most studies, amplified spontaneous emission (ASE) is considered the primary noise source in MOPA setups [32–34]. However, for the signal with ultralow white noise, high signal-to-noise ratio amplification processes, the additional noise introduced by ASE can be negligible [32,35]. In practice, non-ASE noise sources such as pump LD current noise, thermal fluctuations, and vibrations within the amplifier often dominate the noise contribution. These noise sources typically manifest themselves in the low-frequency range, which is frequently hided by the thermal noise of the laser seed. Therefore, these noise sources can only be highlighted, studied, and distinguished during the amplification process using an ultra-low noise laser seed.
In this paper, we employ a ring cavity to achieve an ultralow noise single-frequency laser oscillator and the power boosting by the subsequent MOPA. In comparison to state-of-the-art DFB SFFLs, it exhibits notable ultra-low-noise. Benefiting from such a low-noise laser experimental platform, we can study the impact of various noise sources on both the laser oscillator and the amplifier, enabling us to develop strategies to suppress their influence effectively. A white noise of 0.025Hz2/Hz (instantaneous linewidth of 0.08 Hz) and an integrated linewidth of 221 Hz are achieved for the laser oscillator. After amplification, the laser achieves a power output of 4.97W while maintaining the same white noise level as the oscillator. The relative intensity noise (RIN) of both the oscillator and amplifier approach the shot noise limit. The findings of this study on the noise characteristics of the laser contribute to the realization of high-power, ultralow noise SFFLs.
2. Experimental setup
Figure 1 illustrates the experimental setup of our SFFL oscillator. A 976 nm laser diode (LD), delivered via a single-mode fiber pigtail, is coupled through a wavelength-division multiplexer (WDM) to pump a ∼50 cm-long erbium-doped fiber (EDF, Nufern, SM-GDF-6/125). The resulting laser emission is guided through a circulator, subsequently entering an unpumped EDF of ∼10 cm in length. Subsequently, it is reflected back into the fiber ring resonator via a fiber Bragg grating (FBG) with a reflectivity of 99.5%. This unpumped EDF functions as a saturable absorber, ensuring the stability of SLM oscillations. Additionally, an inline fiber polarizer is inserted between the unpumped EDF and the circulator to suppress polarization-induced fluctuations and enhance the stability of the SLM. A 20/80 optical coupler uses the 20% output port for laser extraction, the isolator obstructs the influence of backward light on the modes gain in the resonator. The total length of the fiber cavity is ∼3 meters, which, in comparison to the traditional ring cavities with lengths exceeding 10 meters [21,22], can be considered relatively “short”. However, it is still longer than the SSFLs of DFB or DBR configuration by several orders of magnitude. It allows for a substantial enhancement in the photon lifetime within the cavity, thereby achieving ultralow laser noise. Furthermore, this cavity is advantageous for subsequent packaging of the entire laser setup, effectively mitigating the influence of external environmental disturbances. Replacing the conventional polarization controller with a polarizer allows for controlling the polarization state within the cavity while also considering the convenience of packaging the entire cavity. We employed a scanning Fabry-Pérot (FP) interferometer (Thorlabs, SA200-12B) to monitor the operation status of the laser longitudinal modes. Within a single free spectral range (FSR) of the interferometer (1.5 GHz), only a solitary transmission peak was observed, validating the laser operation in a SLM status, as depicted in the inset of Fig. 1. We have also observed a long-term redshift in the laser frequency, and after drifting ∼200 MHz in about 10 minutes, mode hopping occurred. The observed frequency drift is attributed to the temperature increase within the laser cavity, and precise temperature control can eliminate this phenomenon. Due to precise tailoring of the unpumped EDF length and the utilization of the polarizer, no adjustments to the laser cavity are required upon each startup. This renders the laser oscillator insensitive to external environmental perturbations, thus showing a remarkable robustness.
Fig. 1. Experimental setup for single frequency fiber laser oscillator. OC: output coupler, WDM: wavelength-division multiplexer, ISO: isolator, EDF: erbium-doped fiber, PD: photodiode
3. Results and discussion
The laser frequency noise was measured using the self-homodyne method based on an unbalanced Mach-Zehnder interferometer with a delay line of 100 m [36]. The meticulous packaging of the 100 m optical fiber delay line, combined with the use of low-noise photodetectors and data acquisition cards, minimizes the noise floor of our measurement system. This ensures accurate determination of the laser frequency noise. Firstly, we measured the frequency noise of the unpackaged laser, as illustrated by the black curve in Fig. 2. The results demonstrate excellent noise characteristics in the frequency range exceeding 1 kHz: ∼100 Hz2/Hz@1kHz, ∼10 Hz2/Hz@10kHz, and white noise of 0.025 Hz2/Hz@1 MHz, corresponding to the instantaneous (Lorentzian) linewidth of 0.08 Hz. In the frequency range below 1 kHz, multiple peaks emerge, attributed to environmental noise sources, including acoustic waves, vibrations, and thermal fluctuations in the surrounding air. Simultaneously, a pronounced noise at 75.5kHz from relaxation oscillation was observed, reaching a peak value of 3300 Hz2/Hz.
Fig. 2. Frequency noises for the single frequency laser oscillator under packaged, unpackaged and active power stabilization states. The purple dot line is the frequency noise of the commercial fiber laser (NKT, Koheras E15).
In order to mitigate frequency noise below 1 kHz, we packaged the laser cavity. Fibers and fiber components were affixed onto an aluminum plate using silicone thermal paste, and the aluminum plate was precisely temperature-controlled via a thermoelectric cooler (TEC). The temperature fluctuations of the laser cavity are stabilized at 0.002°C, thereby eliminating temperature-induced frequency drift and mode hopping. Through temperature control, the laser frequency drift is kept below 10 MHz after one hour. The four corners of the aluminum plate were supported by air springs, and the entire setup was enclosed within an aluminum box, with the inner walls adorned with foam. The frequency noise of the packaged laser, depicted by the blue curve in Fig. 3, exhibits a complete suppression of the noise peaks in the frequency range below 1 kHz. Low-frequency noise is commonly associated with the laser integration linewidth. Therefore, laser packaging proves effective in reducing the integrated linewidth. Using the β-line method [37], we calculated integration linewidths of 4.2kHz and 221 Hz (0.1s integration time) for the unpackaged and packaged laser, respectively. Comparing the frequency noise performance of our laser with that of a commercially available high-performance DFB laser (NKT, Koheras E15), while disregarding the relaxation oscillation, we observe an approximate 10 dB reduction in frequency noise from 100 Hz to 1 MHz offset frequency.
Fig. 3. (a) RINs for the single frequency laser seed under packaged, unpackaged and active power stabilization states. (b) The RINs within low-frequency range
To suppress relaxation oscillation noise peak, we employed an active feedback loop to stabilize the laser output power. As depicted in Fig. 1, we utilized 10% of the laser power as a feedback signal, which, upon photodetection, was converted into an electrical signal and input to a servo controller. This controller modulated the current of the pump LD, thereby stabilizing the laser output power. The frequency noise of the laser after active control is depicted by the red curve in Fig. 2, with the relaxation oscillation noise peak reduced to 100 Hz2/Hz. However, some additional noise peaks emerge. Due to control bandwidth limitations in the current driver for the 976 nm LD, the peak around 35 kHz is attributed to the gain introduced by the servo controller, while the series of sharp spikes in the range of 10∼1000 Hz are attributed to the 50 Hz electrical power line and its harmonics stemming from the control circuits. Thermal noise originates from temperature-induced fluctuations in the length and refractive index of the fiber. Therefore, it can be transformed into the laser frequency noise PSD by calculating the PSD of the temperature fluctuations [7,8]. The black dashed line in Fig. 2 represents the calculated fundamental thermal noise of the laser cavity. It is worth noting that, for the single-frequency laser oscillator depicted in Fig. 1, the mode filtering mechanism is provided by the dynamic grating formed between the circulator and FBG, which can be regarded as a standing-wave sub-cavity inserted into the fiber ring. Its cavity length and the refractive fluctuations contribute to laser frequency noise. Therefore, we chose a cavity length of 25 cm for this sub-cavity to calculate the fundamental thermal noise. Neglecting the relaxation oscillation noise peak, when we combine the laser frequency noise after packaging and power stabilization (depicted by the blue and red curves in Fig. 2), we observe that at offset frequencies greater than 1 kHz, the calculated fundamental thermal noise agrees well with the measured laser frequency noise. The excess noise below 1 kHz originates from nonequilibrium thermal fluctuations within the gain fiber [9] and thermomechanical noise [10].
The laser relative intensity noise (RIN) was measured using an electrical spectrum analyzer (ESA, Tektronix, RSA507A), as depicted in Fig. 3. Given the lower measurement limit of 9 kHz for the ESA, we employed a 16-bit data acquisition card (NI, USB 6361) to record laser power variations and conduct spectral analysis, thereby obtaining the laser RINs below 10 kHz offset frequency. The results are depicted in the inset of Fig. 3(b). In all three laser states: unpackaged, packaged, and under active power stabilization, the laser RINs tend to the noise floor of -146 dBc/Hz at offset frequencies exceeding 1 MHz. Considering the saturation power of the low-noise photodetector (MenloSystems, FPD510) used in the experiment, we attenuated the laser power to 0.2 mW before it enters the photodetector, leading to an estimated shot noise level of -149 dBc/Hz. Around the offset frequency of 75 kHz, the RIN peak induced by relaxation oscillations is -98 dBc/Hz and reduced to -115 dBc/Hz after power stabilization. In the frequency range below 1 kHz, the unpackaged laser shows no environmental noise-induced RIN peaks. It maintains a higher overall RIN compared to the packaged laser. After active power stabilization, the laser overall RIN decreases; however, it still exhibits harmonic sharp spikes from the electrical power line. External environmental noise sources, stemming from acoustic waves and vibrations, modify the refractive index of fiber in the resonator via stress, thereby coupling to the laser frequency noise. However, the gain fiber and FBG length comprise a relatively small portion of the entire cavity, consequently their refractive index variations contribute insignificantly to the power fluctuations. In the case of the unpackaged laser cavity without temperature stabilization control, increased thermal noise originating from the fiber cavity and fiber components themselves leads to an increase in RIN due to the self-heating mechanisms [7]. This correlation between RIN and frequency noise is frequency independent for the low frequency range. The active feedback loop directly modulates the current of the pump LD, thereby allowing direct control and reduction of laser RIN. Active power stabilization not only reduces the relaxation oscillation peak but also mitigates noise in other frequency ranges. Specifically, in the range from 1 kHz to 10 kHz, the reduction in RIN brings the frequency noise closer to the fundamental thermal noise limit, implying that the oscillator is also subject to additional frequency noise caused by pump fluctuations [38]. Although this approach mitigates the laser noise to a certain extent, it introduces additional noise in the low-frequency region due to the feedback loop circuits, thereby contributing to an increase in the laser integrated linewidth. An integrated linewidth of 1.54 kHz is obtained under this power feedback loop. However, we believe that by choosing a feedback loop with low electronic noise, it will be possible to avoid the introduction of additional electrical power line harmonic noise. In the following laser amplification experiments, we did not employ this power feedback loop for this laser oscillator. Nonetheless, this feedback loop can be conveniently activated if required in practical applications.
Figure 4 illustrates the experimental setup for the laser amplifier. The seed delivers an output power of 6.5 mW at pump power of 80 mW, which is then pre-amplified using a commercially available erbium-doped fiber amplifier (Connet, MFAS-Er-C-B-BA). Most amplified spontaneous emission (ASE) generated therein is eliminated via a tunable bandpass filter (BPF). In order to prevent the backward light, a signal power of 0.6 W from the pre-amplifier is launched into the power amplifier through an isolator (ISO). The gain fiber in the power amplifier is a 3 m-long erbium-ytterbium co-doped double-clad fiber (EYDF, Nufern, SM-EYDF-10P/125), which is pumped by a 18 W, 976 nm LD through a (2 + 1) × 1 combiner. The cladding absorption coefficient at the pump wavelength is measured to be 9.6 dB/m by cutback method. The idler port of the combiner is utilized for monitoring the backward light within the amplifier. The 1% port of a 99/1 coupler at the output end is employed for spectral and noise measurements. Low and high refractive index UV-curable adhesives are applied at the fusion points on both ends of the EYDF, facilitating light guidance and stripping of the pump light, respectively. To remove the excess heat generated in the active fiber, it is spooled on an aluminum heatsink cooled to 15 °C. Output power of the final amplifier is depicted in Fig. 5(a), a maximum output power is 4.94 W is obtained when the incident pump power increases to 16 W without any rollover. The slope efficiency versus incident pump power measured after the output port of the combiner is 26.3%. At an output power of 4W, we observed a power instability of <0.5% within one hour, which is shown in Fig. 5(b). The laser spectra at three stages: seed, pre- and power amplifier are given in Fig. 5(c). The laser central wavelength is 1550.3 nm, determined by the FBG in the laser seed. Most of the ASE generated in the pre-amplifier of the seed is eliminated by the filter. In our optimized power amplifier, the length of the EYDF was adjusted to align its emission peak with the central wavelength of the laser signal. As a result, the final amplifier output spectrum features a 64 dB signal-to-noise ratio. It was observed that some residual ASE remained in the longer wavelength region of the spectra for both the pre- and power amplifiers. Utilizing higher-performance filters can result in a higher signal-to-noise ratio exceeding 70 dB, as estimated from the noise floor in the shorter wavelength region of the spectrum. Throughout the amplification process, the backward power remains consistently in the range of a few milliwatts, and no Stimulated Brillouin Scattering signals were detected.
Fig. 4. Experimental setup for laser amplifier. EYDF: erbium-ytterbium co-doped double-clad fiber, OC: output coupler, BPF: bandpass filter, ISO: isolator.
Fig. 5. (a) Output power versus pump power of the power amplifier. (b) The stability for the laser power. (c)Laser spectra for the seed, pre-amplifier and power amplifier.
The comparison of frequency noise among the laser seed, pre- and power amplifier is depicted in Fig. 6. In the frequency region from 30 kHz to 1 MHz, the laser frequency noise at the three stages overlap, all exhibiting extremely low white noise levels. This suggests that the amplification process introduces the ASE noise at a very low level, which does not significantly broaden the laser instantaneous (Lorentzian) linewidth. In the frequency range from 10 Hz to 40 kHz, both the pre- and power amplifiers display numerous spikes and overall increase in the frequency noise spectra. During our amplification process, the primary source of noise is the fluctuation in the pump intensity of the amplifier. According to the β-line method, the laser integration linewidth is estimated as 665 Hz for the final power amplifier output. As illustrated by the red curve in Fig. 6, the increase in integration linewidth after amplification is primarily attributed to noise spikes in the 10 Hz to 100 Hz frequency range. These noise spikes originate from table vibrations and current noise in the pump LD. With the isolation packaging of amplifier and the use of lower noise LDs, we believe that the final linewidth of the amplifier can be kept as low as the laser seed. As shown in Fig. 7, the RINs of the pre- and power amplifiers also exhibit numerous noise spikes in the low frequency range while they all approach quantum shot noise levels beyond 1 MHz offset frequency. This further confirms that the additional frequency noise in the low-frequency range within the amplifier, as indicated in Fig. 6, is indeed attributed to power fluctuations.
Fig. 7. (a) The RINs for the laser seed, pre-amplifier, and power amplifier, (b) The RINs within low-frequency range
4. Conclusion
In conclusion, we have demonstrated an ultralow-noise single-frequency fiber ring laser with an output power of 4.94 W. We have measured the frequency noises and RINs of both the laser seed and the amplifier. Technical noises are mitigated through careful packaging and power stabilization feedback, thereby bringing the frequency noise close to the thermal noise limit. The laser white noise is as low as 0.025 Hz2/Hz, corresponding to an instantaneous linewidth of 0.08 Hz. The integrated linewidths of the laser oscillator and the final amplifier are 221 Hz and 665 Hz, respectively, with RINs of approximately -146 dBc/Hz, approaching quantum shot noise levels. The increase in the integrated linewidth introduced by the amplifier is attributed to fluctuations in its pump intensity. Therefore, we believe that by utilizing LDs with lower current noise, it is possible to maintain its integrated linewidth at the level of the oscillator.
Funding
Natural Science Foundation of Xuzhou Municipality (KC22296); National Natural Science Foundation of China (61805112, 62035007).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but maybe obtained from the authors upon reasonable request.
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