An all-fiber high-power and broad-frequency-band near-shot-noise-limited kHz-linewidth (Δν ~1.7 kHz) single-frequency master-oscillator power amplifier (MOPA) laser at 1.5 μm is demonstrated. To significantly suppress the intensity noise of seed laser and mitigate the detrimental effects of amplified spontaneous emission and stimulated Brillouin scattering in fiber amplifiers, more than 23 W of a stable low noise single-frequency laser output is achieved with a relative intensity noise of < –150 dB/Hz @0.5 mW (near to the shot-noise limit: –152.9 dB/Hz) in the frequency band from 0.1 to 50 MHz. It is believed that the achieved laser performance of ultra-low intensity noise and high-power output make the laser source become a promising candidate in further applications, such as cold atom optical lattice, quantum key distribution, and gravitational wave detection.
© 2017 Optical Society of America
High-power single-frequency fiber lasers (SFFLs) operating at 1.5 μm have attracted much attention due to their promising merits of abroad gain window, low noise, narrow spectral linewidth, good beam quality, and robust all-fiber design [1–3]. Especially, the single-frequency operating simultaneously accompanies with high output power (tens of watts), extremely low intensity noise and kHz-linewidth characteristic is mandatory for demanding fundamental research fields, such as frequency metrology , cold atom optical lattice , squeezed light generating [6, 7], quantum key distribution [8, 9], and interferometric gravitational wave detection . High-power SFFLs based on the all-fiber master-oscillator power amplifier (MOPA) scheme have already been demonstrated [3, 11–15]. In spite of the impressive improvements of laser performance (e.g., power scaling, spectral linewidth, linearly polarized, beam quality, etc) in these reported works, the noise properties have been less investigated and still need to be further optimized.
For single-frequency MOPA lasers, it is very challenging to realize high output power as well as shot-noise-limited intensity noise in a broad frequency band. There are several limitation factors or noise sources that severely hamper further power scaling and ultra-low noise operation. For instance, the commonly used seed laser (e.g., nonplanar ring oscillator laser or fiber oscillator) generally possesses a strong inherent resonance relaxation oscillation at the medium frequencies even higher 30~50 dB than the noise floor [3, 12, 14]. Though the relaxation oscillation intensity noises can be sufficiently reduced by using the feedback units or the nonlinear amplification effect of the saturated optical amplifiers [16–19], the output power is still limited to several tens of milliwatts. Recently, a 2.5 W low noise clad-pumped Yb-fiber MOPA with a relative intensity noise (RIN) of −152 dB/Hz @6 mW was demonstrated through seed laser and pump laser intensity feedback controls. The RIN was still above 10 dB of the theoretical shot-noise limit (−162.1 dB/Hz @6 mW) and this approach requires a complex feedback setup . Moreover, a significant amount of excess intensity noise is usually introduced in MOPA fiber laser due to the effects of stimulated Brillouin scattering (SBS), Rayleigh scattering, and amplified spontaneous emission (ASE) during the amplification process [21–26].
In our previous work, a 17.5 mW intensity noise suppressed distributed Bragg reflector (DBR) single-frequency Er3+/Yb3+-codoped phosphate fiber laser was developed with a RIN of < –150 dB/Hz in the frequency band from 0.8 kHz to 50 MHz . Here we report, a 23 W kHz-linewidth all-fiber single-frequency MOPA laser at 1.5 μm with a RIN of < –150 dB/Hz @0.5 mW (near-shot-noise-limited) in a broad frequency band.
2. Experimental setup
The experimental setup of the high-power ultra-low intensity noise single-frequency MOPA laser is depicted in Fig. 1. It consists of an intensity-noise-reduced single-frequency seed laser and two-stage fiber amplifiers. The seed laser implements a DBR short-cavity, which is constructed by using a high-reflection wide-band fiber Bragg grating (HR-FBG), a narrow-band polarization-maintaining FBG (PM-FBG), and a 15-mm-long highly Er3+/Yb3+-codoped phosphate fiber (EYPF) . The laser cavity is backward pumped by a 980 nm laser diode (LD) via a 980/1550 nm PM wavelength division multiplexer (PM-WDM), which also couples out the signal laser. After a PM isolator (PM-ISO), the signal laser is then launched into the noise-suppression unit, which is composed of a variable optical attenuator (VOA), a polarization controller (PC), a semiconductor optical amplifier (SOA), and a band-pass filter (BPF, 3 dB bandwidth: 1 nm). For suppressing the intensity noise of seed laser, the laser power feeding into the SOA is adjusted to ~7 mW by the VOA, and the laser polarization state is tuned to align with the main axis of SOA by the PC, respectively. The driving current of the SOA is set to 350 mA. A BPF is inserted to filter out the ASE brought in by the SOA due to its nonlinear amplification effect. After the noise-suppression unit, an output power of ~10 mW single-frequency signal laser at 1550 nm with a signal-to-noise ratio (SNR) of > 72 dB is yielded. More details of the seed lasers can be found in our previous works [19, 25–27].
The signal from the noise-suppression seed laser is first boosted through a pre-amplifier, which is backward pumped by a multimode 915 nm LD with the pump power of 9 W via a (2 + 1) × 1 combiner. A 5-m-long Er3+/Yb3+-codoped double cladding silica fiber (EYDF 1) with a 10 μm/0.20 NA core and a 128 μm/0.45 NA cladding is utilized. A 1064/1550 nm WDM is installed to filter out the 1.0 μm-band ASE from the excited Yb3+. The boosted signal with an output power of ~2.3 W is then launched into the power amplifier after a high-power isolator (HISO) and a dense wavelength division multiplexer (DWDM, 3 dB bandwidth: 0.5 nm).
In the power amplifier, a 1/99 fused coupler with 2 × 2 port is used to monitor the backward propagating lights for evaluating of SBS effect. A 2.5-m-long EYDF 2 with the same structure to that used in pre-amplifier is employed. The cladding absorption coefficient of EYDF 2 is ~6 dB/m at 976 nm. The power amplifier is backward pumped by two multimode 976 nm LDs with the total pump power of 49.1 W via another (2 + 1) × 1 combiner. After each amplifier, the residual pump lights in the cladding are removed by cladding-mode strippers (CMS 1 and CMS 2) comprised of high-refractive index gel. A 0.2-m-long matched single-mode fiber (Corning, SMF-28e) with the end-cap design is used as the output delivery fiber. All the components are placed on an optical table and the two EYDFs are covered by thermal grease for active cooling during the experiments.
3. Results and discussion
In order to realize ultra-low noise intensity as well as high output power in single-frequency MOPA, the seed laser with an active noise-suppression based on the gain saturation effect of SOA is needed. In general, the intensity noise measurement with an attenuated operation to reduce the laser power (to several milliwatts level) for detector is necessary and widespread used [14, 28, 29]. The RIN is characterized by an InGaAs photodetector (PD) with the bandwidth of 150 MHz and an electrical spectrum analyzer with the bandwidth resolution of 1 kHz. All the laser powers are attenuated to ~0.5 mW before injected into PD to ensure the consistency in the whole measurements. The measured RINs of the seed laser with and without noise suppression, as well as the theoretical shot-noise limit (SNL, –152.9 dB/Hz @0.5 mW) in the frequency band of 0–50 MHz, as shown in Fig. 2(a) and the inset of Fig. 2(a). Here, the SNL that is defined as 2hν/P, where h is the Planck constant, ν is lasing frequency, and P is the laser power . With an active intensity-noise-suppression, it can be clearly seen that around the relaxation oscillation frequencies of 1.65 MHz, a maximum noise suppression ratio ~55 dB of the intensity noise peak is achieved from –95 to –150 dB/Hz. Subsequently, the whole dominant peak of the laser relaxation oscillation completely disappeared. After suppression, the seed laser shows very low intensity noise of RIN < –150 dB/Hz @0.5 mW at the frequency band up to 50 MHz.
During the development, to carefully investigate and optimize each part of MOPA fiber laser for minimizing the intensity noise, the noise properties of pre-amplifier with the noise-suppression seed laser based on different pumping methods is first carried out. Figure 2(b) shows the measured RINs of the pre-amplifier with forward pumping and backward pumping, and the SNL are also shown for comparison. Under the same pump power in pre-amplification process, it is found that the case of backward pumping shows a slightly better noise performance due to the relative high efficiency, which is about 3 dB lower than that of the forward one. With the backward pumping, a RIN level of < −150 dB/Hz is rapidly reached at 0.1 MHz, which is found to be the same as that of the noise-suppression seed laser.
The generation of C-band ASE from fiber amplifier is harmful to the noise performance and can potentially lead to unstable operation [26, 27]. It is thus interesting to investigate and optimize the output spectrum performance. Figure 3(a) presents the output spectra of the noise-suppression seed laser and the pre-amplifier without or with a DWDM, which is recorded by an optical spectrum analyzer (OSA) with a resolution bandwidth of 0.1 nm. It is noted that, the ASE around 1543 nm appears in the spectrum, which would be further boosted in the power amplifier. While a high-power DWDM is installed to strip the ASE in pre-amplifier, the SNR significantly improved from 58 to 72 dB. It is also seen that when compared to the noise-suppression seed laser, the SNR of the pre-amplifier is 72 dB, which has no obvious deterioration. Thus, an output power of about 2.3 W with relatively pure spectrum is obtained from the pre-amplifier.
For the power amplifier, due to a high cladding absorption of the EYDF 2, only a short fiber length is needed to balance the requirement for sufficient pump absorption and SBS suppression. Based on the simulation model, the calculated optimization length of EYDF 2 is ~2.5 m when an input signal power of 2 W and a pump power of 50 W are provided. At the beginning of the power amplification, the nonlinear effect of SBS is observed while the total length of power amplifier is ~7.3 m including another HISO (not shown in the Fig. 1), the delivery fiber, and the combiner. By monitoring RINs of the power amplifier with different output powers, which is close to SBS operation can be detected in the frequency range of 0–50 MHz, as shown in Fig. 3(b). At a relatively low power of 11.6 W, it is observed that a constant RIN around –150 dB/Hz is detected in the whole frequency band. In the regime of SBS with a frequency band of 0–15 MHz, the intensity noise started to increase exponentially with threshold power levels of around 12.7 W, to a maximum RIN of ~–120 dB/Hz. Subsequently, the low RIN is recovered at the frequency beyond 15 MHz. The intensity coupling mechanisms that origin of SBS induced broadband intensity noise is attributed to Stokes and anti-Stokes Brillouin scattering .
Several approaches such as specially designed gain-tailored fiber, standard large-mode-area fiber, strain and temperature gradients, have successfully applied to suppress the SBS [11–13, 15]. The simplest suppression method is to reasonably cut the total fiber length of amplification system as short as possible. The total length is shortened to ∼3.1 m in such single-mode fiber power amplifier, and the calculated SBS threshold is ~21 W according to the theoretical formula. Thus, to reduce the detrimental effect of SBS for minimizing the excess RIN is focused in our amplification system.
The output power properties of the MOPA laser are measured. Figure 4(a) shows the laser output power versus the pump power of MOPA laser. When the total pump power is 49.1 W, the maximal output power reached 23 W without any roll-over, and the corresponding optical-to-optical efficiency is ~46.8%. The power stability of the laser output power is investigated, as shown in the inset of Fig. 4(b). It is found that the MOPA laser operates continuously within 2 hours and the output power instability of < 1% is observed. The SBS effect is also investigated by monitoring the backward propagating power versus the pump power, as illustrated by Fig. 4(a). With output power increasing, the backward propagating power increased approximately linearly. When the maximal output power is reached, the power level of backward power is only ~170 mW and did not exhibit any sudden increase, indicating that it is still below the SBS threshold. However, it is also noted that the increase slope of the backward propagating power turns bigger when the output power exceeds 12.7 W. This can be attributed to the rise of 1.0 µm-band ASE from the excited Yb3+ at this power level, the output spectra of the backward propagating light with different output powers are shown in the inset of Fig. 4(a). It is known that the backward ASE is generaly stronger than the forward one, so the rise of ASE can be easily reflected by the increase of backward power. In addition, the normal intensity noise spectrum with an output power of 23 W (in Fig. 5(a)) also indicates that the SBS did not occur. It is deduced that the temperature and strain gradients also exist in the amplifier system that will further improve the SBS threshold [12, 13, 15].
The output spectrum of MOPA laser at the maximum output power is recorded by an OSA, as plotted in Fig. 4(b). It can be seen that the signal at 1550 nm is clearly observed in the spectrum region, and there is no obvious ASE and residual pump light. Eventually, the obtained SNR of the MOPA at this power levels is higher than 70 dB. The single-frequency characteristic of MOPA laser is also confirmed by a scanning Fabry-Pérot interferometer with a finesse of 200 and a free spectral range of 1.5 GHz. There is only one longitudinal mode operation is testified.
Figure 5(a) shows the RINs of the MOPA laser (blue line), the free running (the MOPA laser with the seed without noise suppression used, gray line), and the SNL (green line) are also shown for comparison in the frequency band of 0–50 MHz. When the noise-suppression seed laser is employed and then amplified, it is observed that a high power and an ultra-low intensity noise output at the frequency band up to 50 MHz is obtained from the MOPA. At low frequency region (< 0.1 MHz), there are different noise sources that generated several spurious spikes can be able to identify in spectrum, as depicted in the inset of Fig. 5(a). Obviously, the amplification process added excess noises at frequencies between 10 and 100 kHz while compared with that of the seed laser (see the inset of Fig. 2(a)). The spurious spikes around 50 kHz due to electrical supplies and external acoustics can be easily filtered by further acoustics isolation measures. The pump noise of multimode LD can affect the spectrum up to 100 kHz. Beyond the frequency of 100 kHz, where the impact of technical noise is strongly reduced and the spectrum will be dominated by the fundamental effects such as shot-noise, ASE, and SBS. It is noted that the RIN in the frequency range from 0.1 to 50 MHz reaches to −150 dB/Hz @0.5 mW, which approaches the SNL. There is no obvious degradation of intensity noise in other frequencies after power amplification is observed.
To further investigate the noise characteristics, the linewidths of the noise-suppression seed laser and the MOPA laser at a maximum output power are measured. The self-heterodyne method involving a 50-km-long fiber delayed Mach-Zehnder interferometer and a 40 MHz acoustic optical modulator is employed. The measurement results are given in Fig. 5(b), it can be revealed that both the laser linewidth curves are almost coincident, and the 20 dB spectrum widths are estimated to be 34 kHz, indicating the laser Lorentz linewidth are 1.7 kHz. So it is concluded that the laser linewidths are not deteriorated during the power amplification, owing to the effective management of ASE and SBS in the low noise amplification system.
In conclusion, a high-power broad-frequency-band near-shot-noise-limited all-fiber single-frequency 1.5 μm MOPA laser is developed. By suppressing the intensity noise of seed laser and mitigating the detrimental effects of ASE and SBS in Er3+/Yb3+-codoped fiber amplifiers, a stable output power of more than 23 W single-frequency laser is obtained with a laser linewidth of 1.7 kHz and a SNR of > 70 dB. The measured RIN of MOPA laser is < –150 dB/Hz in the frequency band of 0.1–50 MHz, which approaches the shot-noise limit. The results show that the high-power ultra-low intensity noise MOPA laser would be a promising candidate for fundamental research applications, such as cold atom optical lattice, quantum key distribution, and gravitational wave detection.
National Key Research and Development Program of China (2016YFB0402204), NSFC (11674103, 61635004, 61535014, 51132004, and 51302086), the Fundamental Research Funds for Central Universities (2015ZM091), China National Funds for Distinguished Young Scientists (61325024), Guangdong Natural Science Foundation (2016A030310410), and the Science and Technology Project of Guangdong (2013B090500028, 2014B050505007, and 2016B090925004).
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