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Abstract
An optimized bidirectional pumping fiber amplifier is demonstrated to achieve low-frequency intensity noise suppression and effective power enhancement simultaneously. Based on the concept analysis of the gain saturation effect, the influence of input signal power and pump power on intensity noise suppression is investigated and optimized systematically. Further combining with the optimization of the pumping configuration to achieve the even-distribution gain, the relative intensity noise (RIN) of 1083 nm single-frequency fiber laser (SFFL) is suppressed with 9.1 dB in the frequency range below 10 kHz. Additionally, the laser power is boosted from 10.97 dBm to 25.02 dBm, and a power instability of ±0.31% is realized. This technology has contributed to simultaneously improving the power and noise performance of the 1083 nm SFFL, which can be applied to a multi-channel helium (He) optically pumping magnetometer. Furthermore, this technique has broken the mindset that power amplification of the conventional fiber amplifiers will inevitably cause the degradation of intensity noise property, and provided a valuable guidance for the development of high-performance SFFLs.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With properties of compact structure, high optical signal-to-noise ratio (OSNR), narrow linewidth, and stable single-longitudinal-mode operation, single-frequency fiber lasers (SFFLs) have been broadly applied in many fields, such as nonlinear conversion, optical spectroscopy, magnetic field detection, gravitational wave detection and so on [1–4]. Specifically, for multi-channel helium (He) optically pumping magnetometer (OPM), stringent requirements will be imposed on 1083 nm SFFL, particularly the promoted power with low intensity noise [5,6]. Especially, the relative intensity noise (RIN) at the frequency of 1 kHz is required to be lower than -135 dB/Hz to achieve the Swarm’s goal resolution of 1 pT/Hz [7]. However, in most case, the power improvement in fiber laser is primary at a certain cost of RIN degradation [8–10]. Therefore, it is of vital value to simultaneously achieve RIN suppression and power amplification.
In the past decades, several techniques have been developed to suppress RIN. Optoelectronic feedback is a typical technique to suppress the low-frequency RIN by compensating the output power fluctuation [11]. However, optoelectronic feedback itself does not provide laser gain, but rather introduces optical loss during the light extraction detection process. Therefore, this method needs to be combined with an optical amplifier, but this operation will increase the system complexity. Besides, semiconductor optical amplifier (SOA) and booster optical amplifier (BOA) are demonstrated to suppress RIN by virtue of the gain saturation effect from the fast carrier dynamics [12,13]. Regrettably, the low-frequency RIN is difficult to be managed by SOA/BOA, which is attributed to the mismatch between the fast carrier action and low- frequency power fluctuations from the nonlinear dynamics in itself [14,15]. In addition, the output power of the amplified laser is generally about tens of mW, which is limited by the reflection of the active layer facets conductive to forming the self-excited oscillation [16]. Except for BOA/SOA, fiber amplifier also can provide optical gain and inherently embrace the gain saturation effect. Recently, some pervious researches have presented a specific fiber amplifier including a partial reflection round-trip structure as a strategy to suppress the low-frequency intensity noise [17–19], while the system needs strict and careful parameter control in order to achieve an effective work. Therefore, whether it is possible to develop a universal fiber amplifier with RIN suppression and power amplification simultaneously is still an open question.
In this article, we present an optimized bidirectional pumping fiber amplifier to achieve the performance improvement of low-frequency intensity noise suppression and power simultaneously. Through systematically optimizing the input signal power and the pump power, the gain saturation effect is strengthened to provide a superior RIN suppression. Further combining with the regulation of the pumping configuration to achieve the even-distribution gain, the power enhancement capacity and the RIN inhibition is further improved. Ultimately, a 9.1 dB RIN suppression of the 1083 nm SFFL is achieved in the frequency below 10 kHz. Besides, the output power of 25.02 dBm with a power instability of ±0.31% is yielded.
2. Experimental setup
Figure 1 shows the experimental setup of power boost and RIN suppression based on the bidirectional pumping configuration for 1083 nm SFFL. This system mainly consists of a laser seed and a fiber amplifier two parts. As the core of this laser seed, the distributed Bragg reflector (DBR) resonant cavity is composed of a 9.5 mm Yb3+-doped glass fiber and a pair of mutually matched fiber Bragg gratings (FBGs). More details of the glass fiber and the FBGs can be found in our previous works [20–23]. A 974 nm single-mode laser diode (LD) is used as pump source, and a 974/1083 nm polarization-maintaining wavelength division multiplexer (PM-WDM1) is utilized to filtering out the residual pump light. Then 1083 nm laser travels through a polarization-maintaining isolator (PM-ISO1), which prevents the degradation of laser performance from the return light. The DBR resonant cavity is packaged in an aluminum V-shaped groove tube thermal-controlled by a thermoelectric cooler (TEC). The advanced temperature accuracy of 0.01°C can promote the stability of the power, wavelength, and the single-longitudinal-mode property. To avoid the different working condition of pump source influencing intensity noise of the seed, the driving current of LD1 is constantly set as 400 mA, and the seed power is controlled by VOA1 to achieve different input signal power for subsequent amplifier [24].
Fig. 1. Experimental setup of power boost and RIN suppression based on bidirectional pumping fiber amplifier configuration for 1083 nm SFFL.
In the fiber amplifier, a bidirectional pumping configuration is designedly introduced. The polarization-maintaining Yb3+-doped single-cladding fiber (YSF) which has a core absorption of 250 dB/m at 975 nm is applied as gain medium. The active fiber length of 7 m is pre-optimized, which avoids the generation of self-excited laser by reflection between the gratings of LD2 and LD3 and induces a superior power output with low RIN. Two single-mode LDs (LD2 and LD3) pump the active fiber forward and backward through PM-WDM1 and PM-WDM2, respectively. Particularly, the driving currents of these two LDs are fixed as 1040 mA to reduce the optical property difference of LDs at different working current conditions. Thus, two variable optical attenuators (VOA2 and VOA3) are leaded into the experimental framework to finely adjust the pump power from 0 to 500 mW. In addition, two 1/699 couplers (C1) and (C2) are applied to monitor the characteristics of the pump light from $C_O^1$ and $C_O^2$ port. A polarization-maintaining tap-isolator (PMTI) is arranged to prevent the return light and the 1% tap port is used to monitor the optical property. Regarding the amplified spontaneous emission (ASE) produced from the laser amplifying process, a polarization-maintaining circulator (PM-CIR) and a high-reflective FBG (HR-FBG, reflectivity of 99.99% and 3 dB bandwidth of 0.45 nm) are combined to filter out this undesired light.
3. Concept analysis of gain saturation
In a conventional fiber amplifier, the relation between the input signal power Pin and the output power Pout can be expressed as [17]
where G is the linear optical gain, and α is the linear loss factor. The optical gain achieved in the fiber amplifier depends on the population densities in different electronic levels, which themselves depend on the optical intensities. When the incident light intensity is weak, the optical fiber amplifier is in a small signal operation state, and provides a relatively fixed small signal gain which is usually proportional to the absorbed pump power and to the stored energy. However, a fiber amplifier device cannot maintain a fixed gain for arbitrarily input signal powers, because this would require adding arbitrary amounts of power to the amplified signal. Therefore, the gain must be reduced for high input signal powers, and this phenomenon is called gain saturation. Under this condition, in order to analyze the dynamic relationship between the input signal power and the output power, the derivative of Eq. (1) can be written as When the derivative has an analytical solution which can make dPout /dPin close to 0, it indicates that the output power can be stabilized and will not change with the input signal power anymore. Solving this derivative equation dPout /dPin = 0, the relationship between G and Pin can be expressed as where C is the constant. As Eq. (3) expressed, G is inversely proportional to Pin, meaning that gain decreases with input signal power increase, i.e., gain saturation which exhibits gain reduction for high input signal power. Under this condition, RIN is suppressed which is based on the nonlinear interaction between the input signal and noise [14,15,25,26].Specifically, for a given amplifier, there is the appropriate input signal power making the amplifier enter a gain-saturated operating state. Considering that Pin carries a small perturbation between the minimum power Pin-min and maximum power Pin-max, the low Pin-min will obtain a large gain while the high Pin-max will embrace a small gain, thus reducing the fluctuation of output laser. Based on the gain saturation effect, it is apparent that the power fluctuation of Pin is weakened during the amplification process, thus suppressing RIN of Pout.
In order to further explain the laser dynamic process in the fiber amplifier, the active fiber length L can be regarded as the superposition of N micro length l along signal laser propagation direction (z axis). In each l unit length, the corresponding gain g is small and the power amplification coefficient is exp(g) (which is approximately equal to 1 + g for small g). That is, the contribution of G can be regarded as the integrated effect of g. In the state of constant pump power and propagation losses, the gain coefficient g(z) can be expressed as
where gss(z) is the small-signal gain at the given pump power, Pin(z) is the input signal power for l length unit at the z propagation position, and Psat is the saturation power. The small-signal gain is the amplification coefficient working in the linear amplification regime, which is related to the pump power Pp. The saturation power can be written as Psat = Ahv/((σem+σabs)τ), where A is the effective mode area, hν is the photon energy at the signal wavelength, σem and σabs are the emission and absorption cross sections at the emission wavelength, τ is the upper-state lifetime. For a pre-designed fiber amplifier with fixed structure, the saturation power is constant.Therefore, in order to satisfy the gain saturation effect, the optical gain G must be closely related to the input signal power Pin and the pump power Pp, which are the focus of subsequent RIN suppression experiments.
4. Results and discussion of RIN suppression
After the analysis of the gain saturation concept, the RIN suppression experiment is executed. With 7 m active fiber, different input signal power is successively introduced to the amplifier under each LD pumping power of 250 mW, which aims to ensure that the input signal power makes RIN maximum suppressed from gain saturation effect. Output power and gain versus input signal power shown in Fig. 2(a). Under the constant 400 mA driving current of LD1, the input signal power has been adjusted from -13.01 dBm to 12.46 dBm. Taking the gain drop of 3 dB as the threshold value (corresponding to the input signal power of -1.03 dBm), the linear and the nonlinear amplification regimes are distinguished. In the former, the amplifier provides relatively stable optical gain, and the output power almost increases linearly. More significantly, in the nonlinear regime, the gain has gradually decreased and the output power has slowly boosted to become saturated, which has verified the gain saturation effect discussed above. Furthermore, the RINs of 1083 nm laser output are measured by a spectrum analyzer with a resolution bandwidth of 1 Hz, as Fig. 2(b) shown. Considering the ∼ms scale recovery time of the inversion population in the Yb3+-doped fiber, the measured bandwidth of RIN focus on the low-frequency range below 20 kHz. The laser power is attenuated to 1 mW before being input to a low-noise InGaAs photoelectric detector (PD) to ensure consistency in the entire measurement. Under the low input signal power condition, the noise is amplified as equal scale as power, showing RIN consistence with that of the seed. With the input signal power increasing, the noise suppression effect caused by the gain saturation begins to appear, and it gradually strengthens along with the input signal power enhancing from 3.62 dBm to 8.75 dBm. Nevertheless, the further boosting of the input signal power does not improve the suppression effect of RIN, and a maximum suppressing amplitude of 5.9 dB with an effective bandwidth of 4 kHz is acquired. The saturation of the RIN suppression is ascribed to the saturated interaction between intensity noise and input signal under high input signal power condition [14]. The noise spikes at the frequency of 50 Hz and 150 Hz are originated from the power frequency signal. Therefore, regarding the RIN suppression effect and the output power, and considering the principle of universality, the input signal power of 10.97 dBm is applied in the subsequent experiment.
Fig. 2. Laser characteristics of the fiber amplifier output with different input signal powers. (a) Gains and output powers after PM-CIR. (b) Measured RIN spectra of amplified laser.
Following, the experimental research focuses on the effect of pump power on RIN suppression. Firstly, to reduce the experimental error from different intensity noise of LDs [24], two desired LDs with no obvious RIN difference are selected to apply, as shown in Fig. 3(a). Meanwhile, the pump powers of two LDs are controlled by two VOAs rather than the pumping current, which can maintain a stable characteristic of the intensity noise for two LDs. Subsequently, the RIN levels of the amplified laser at the frequency of 300 Hz are measured as Fig. 3(b) shown, which aims to vividly demonstrate RIN suppression evolution under different pump powers of two LDs. Thereinto, the black solid line is equi-RIN line, on which RIN levels are identical. The purple dotted line is equi-pump-power line, on which the sum of co-pumping and counter-pumping power is same. It is absolutely evident that RIN deceases both in bidirectional and unidirectional pumping configuration with the pump power increasing, resulting from the stronger gain saturation effect as well as stronger nonlinear interaction between input signal and noise in power amplification process [14]. More significantly, RIN of the bidirectional pumping configuration is always superior than that of unidirectional pumping under the same total pump power, which demonstrates the contribution of even-distribution gain to RIN suppression [18]. In co-pumping or counter-pumping configuration, the gain coefficient is maximum at the pump power injection port and then has become dissipated due to the consumption of pump power, showing the obvious gain gradient along the active fiber length. Therefore, the relatively lower RIN in bidirectional pumping configuration results from the travelling of a longer non-linear amplification regime due to the lower gain gradient along the active fiber length. In addition, with total pump power increasing, the suppressing amplitude of RIN is larger in bidirectional pumping configuration, indicating that more prominent advantage of RIN suppression in bidirectional pumping configuration can be obtained at high pump power. Meaningfully, this result has provided significant guidance for higher power fiber amplifier with low intensity noise.
Fig. 3. (a) RIN spectral results of LD2 and LD3. (b) RIN levels of amplified laser at the frequency of 300 Hz under different pump powers of LD2 and LD3. (c) RIN spectra of different pumping configuration. (d) Output power and gain versus input signal power in bidirectional pumping fiber amplifier under both LD2 and LD3 with a pump power of 500 mW.
Figure 3(c) shows RIN spectra of amplified laser under different pumping amplification configuration. In the co-pumping or counter-pumping configuration with the pump power of 500 mW, the tested RIN results are almost coincident, which has a suppressing amplitude of 4.7 dB and a working range of below 4 kHz. The fundament may be that the two pumping configurations have provided mirror-like gain distribution characteristics along the active fiber length, and both of which are comparably uneven. Meaningfully, the RIN of bidirectional pumping configuration is further suppressed by 1.2 dB, benefitting by the uniform gain distribution mentioned above. Furthermore, the optimized suppression of RIN is expectedly obtained under both LD2 and LD3 with a pump power of 500 mW. The suppressing amplitude is boosted to 9.1 dB, i.e., from -128.6 dB/Hz to -137.7 dB/Hz, which mainly results from the positive contribution of the increasing of pump power for the gain saturation effect. The effective bandwidth is broadened to 10 kHz, which is limited by the ∼ms scale recovery time of the inversion population in the Yb3+-doped fiber. Simultaneously, the RIN at the frequency of 1 kHz is suppressed to -136.5 dB/Hz, which has satisfied the intensity noise requirement of the laser source to achieve the Swarm’s goal resolution of 1 pT/Hz.
As mentioned above, the suppression of RIN results from the gain saturation effect, thus, the distinguishing of the linear and nonlinear amplification regime is significant to further understand the relationship between power evolution and noise suppression. The output power and gain versus the input signal power are characterized under synchronously fixing both LD2 and LD3 pumping power of 500 mW, as Fig. 3(d) shown. A minimum input signal power of -6.41 dBm is set to avoid excess residual pumping laser, which may make LD damaged. The linear and the nonlinear amplification region are distinguished by the input signal power (3.10 dBm). This result demonstrates that the input signal power of 10.97 dBm exactly belongs to nonlinear regime, indicating the appropriate strategy of the input signal power chosen to suppress RIN. In addition, it is worth mentioning that the increasing pump power results in higher linear gain and the positive shift of the input signal power threshold value, which can be attributed to the higher pump power can provide more upper-level particles.
In addition, the optical spectra of the seed and the amplifier are analyzed, as shown in Fig. 4(a). The optical spectrum resolution and span bandwidth are 0.05 nm and 200 nm, respectively. In addition, the laser power is attenuated to about 3 mW to obtain the normalized power peak before being input to the optical spectrum analyzer. The effective working of the HR-FBG and PM-CIR has filtered the ASE constituent, and the OSNR of the amplified laser is up to 75 dB. Also, the linewidth is investigated via the recirculating delayed self-heterodyne method with 6 km delayed fiber and 80 MHz fiber-coupled acoustic optical modulator. The spectrum resolution and span bandwidth are 1 kHz and 300 kHz, respectively. The input power injected to the system is about 10 mW and the 2nd beat signal (corresponding to 12 km delayed fiber) is utilized to analyze the laser linewidth. The linewidth curves before HR-FBG and after HR-FBG are almost coincident with the seed, which indicates that the bidirectional pumping power amplification will not obviously degrade the property of linewidth. The 20-dB spectrum width with Lorentz fitting is estimated as 146.7 kHz, which corresponds to the 3-dB linewidth of 7.3 kHz, as shown in the inset of Fig. 4(a). Additionally, the laser longitudinal-mode operation is analyzed by a Fabry-Perot scanning interferometer with a free spectral range (FSR) of 1.5 GHz and resolution of 7.5 MHz, as shown Fig. 4(b). A normative single-longitudinal-mode operation is confirmed, and the mode competition and multi-longitudinal mode are not detected. Besides, the evolution of the polarization state of the laser before and after amplification is analyzed using a polarization analyzer, as the inset of Fig. 4(c) shown. It is clearly that the both red points are located on the equator of the Poincaré sphere, which demonstrates that the lasers before and after amplification have nice linear polarization performance. The degree of polarization before and after amplification is measured to be 99.6% and 99.3%, respectively, corresponding to polarization-extinction ratio of 23.96 dB and 21.52 dB. The slight deterioration of the polarization state results from the fusion between optical fibers, the coil of fiber and the transmission of light field. Moreover, for the power scaled up to 25.02 dBm, a power instability of ±0.31% is yielded, as Fig. 4(c) shown.
Fig. 4. (a) Optical spectra of the seed and the amplifier; inset: Beat signal of the seed and the laser before and after HR-FBG; (b) longitudinal-mode performance of the amplified output laser. (c) Power instability of the amplified output laser; inset: Degree of polarization of the laser before (the left) and after (the right) amplification.
5. Conclusion
In summary, we present an optimized bidirectional pumping fiber amplifier to achieve the performance improvement of low-frequency intensity noise suppression and output power simultaneously. The concept of the gain saturation effect is briefly analyzed to provide the specific parameter direction of the noise suppression. Then, the input signal power and the pump power investigated and optimized systematically to realize a superior suppression of intensity noise. Besides, combining with the regulation of the pumping configuration to achieve the even-distribution gain, the power enhancement capacity and the RIN inhibition is further improved. Consequently, the RIN of the 1083 nm SFFL is suppressed with 9.1 dB in the frequency below 10 kHz, and the output power is scaled up to 25.02 dBm with a power instability of ±0.31%. Moreover, an OSNR of 75 dB and a laser linewidth of 7.3 kHz are achieved simultaneously in the final output laser. This fiber amplifier with concurrently promoting performance of RIN and power is potential to be applied in the multi-channel He OPM. Furthermore, this technique has broken the mindset that power amplification of the conventional fiber amplifiers will inevitably cause the degradation of intensity noise property, and provided a valuable guidance for the development of high-performance SFFLs, which is conductive to the applications such as frequency metrology, cold atom optical lattice, quantum key distribution, and gravitational wave detection.
Funding
National Key Research and Development Program of China (2022YFB3606400); the Key-Area Research and Development Program of Guangdong Province (2020B090922006); Major Program of the National Natural Science Foundation of China (61790582); National Natural Science Foundation of China (12204180, 62035015, 62275082, U22A6003); Fundamental Research Funds for the Central Universities (D6223090); Leading talents of science and technology innovation of Guangdong Special Support Plan Program (2019TX05Z344); China Postdoctoral Science Foundation (2021M701256); Guangdong Basic and Applied Basic Research Foundation (2022A1515012594); Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01X137); Guangzhou Basic and Applied Basic Research Foundation (202201010003); Independent Research Project of State Key Lab of Luminescent Materials and Devices, South China University of Technology (Skllmd-2022-13); Open Project Program of Shanxi Key Laboratory of Advanced Semiconductor Optoelectronic Devices and Integrated Systems (2022SZKF02).
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 may be obtained from the authors upon reasonable request.
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