1. Introduction

VCSELs have been applied in a wide array of fields, such as the atomic optical clock, optical communication, optical frequency comb, photonic microwave generation, light detection and ranging (LiDAR), and high-speed image acquisition [1–10], because they have the features of single-longitudinal mode, wide frequency tuning range, circularly symmetrical beam profile, low-power consumption, high-speed modulation, and convenience of two-dimensional array [11–13]. The performances of these applications greatly depend on the linewidth of the VCSELs. For example, the transmission penalty due to fiber chromatic dispersion in data center communication system can be reduced by employing a VCSEL with narrower linewidth as the transceiver [3]. Narrowing the linewidth of the VCSEL to be less than 10% of the absorption dip can minimize the amplitude noise on the transmitted optical signal [14].

At present, many methods have been developed to compress the linewidth of VCSEL. Reducing the aperture of the VCSEL can effectively narrow the linewidth at the expense of lowering the output power [3,15]. A VCSEL with linewidth of 0.03 nm can be achieved when the aperture is 3 µm. Optimizing the cavity length of the VCSEL can compress the linewidth to tens of megahertz or megahertz at the expense of increasing the fabrication difficulty [16,17]. Using optical feedback from both free space and fiber loop mirror, the free running VCSEL with original linewidth of ∼65 MHz can be compressed to hundreds of kilohertz or less, depending on the bias current to the VCSEL [8]. Employing resonant optical feedback from an on-chip microring add drop filter, a single longitudinal mode VCSEL with a linewidth as narrow as 32.6 kHz and a contrast of 47 dB can be achieved recently [18]. Due to the limitation of compression mechanism, the narrowest linewidth of VCSEL is in the order of tens of kilohertz based on the above methods. However, extensive research fields require the laser linewidth in the order of sub-kilohertz, such as, high-resolution optical sensing [19–21], high speed optical coherent communication [22], high-resolution spectroscopy [23,24]. In our previous research, using the RBS as the compression mechanism, the linewidth of distributed feedback (DFB) laser can be compressed to tens of Hertz [25], because the RBS can provide a weak distributed feedback signal with the continuous phase perturbation effect, which can effectively enhance the photon lifetime without inducing multi-longitudinal modes [25,26]. Fortunately, VCSELs have shown similar sensitivity to external optical feedbacks as DFB lasers [27]. Thus, the technique used for deeply compressing the linewidth of DFB lasers can be adopted for VCSELs.

In this work, we demonstrate an ultra-narrow-linewidth VCSEL based on external-cavity weak distributed feedback from the RBS in optical fiber. Herein, a VCSEL with the linewidth as narrow as 435 Hz and a contrast of 55 dB are experimentally achieved by RBS fiber with a feedback level of RBS signal of -27.6 dB. Before compression, the linewidth of the free-running VCSEL is 2 MHz. Then, we explore the evolution of laser linewidth under different driving current. With the increase of driving current, the linewidth of the laser can maintain compressing in this process. By adjusting the thermal resistance of the laser from 4.5 kΩ to 7.0 kΩ, the output-wavelength can be tuned from 1543.324 nm to 1542.06 nm with a tuning rate of -0.506 nm/kΩ, and the linewidth fluctuates in the range of 553-419 Hz. To the best of our knowledge, this is the first experimental demonstration that the linewidth of VCSEL can be compressed to hundreds of Hertz. Our proposed method can be utilized for narrowing the linewidth of VCSELs operating in other bands or VCSEL arrays, which will further improve the performance of VCSEL-based applications and some cutting-edge potential applications, such as the precision soliton comb formation and control [28], frequency dependent sensing [29], and low noise signal processing [30].

2. Configuration and experimental results

The ultra-narrow linewidth VCSEL setup is schematically shown in Fig. 1. The VCSEL is driven by a Model 6100 laser diode current and temperature controller (Newport Corp.). Then, the output part of the VCSEL is injected into the RBS fiber which is used to generate RBS signal. Subsequently, the compressed laser signal transmits through an isolator and a 3-dB coupler (C₁). The first output port of the 3-dB coupler is sent to an optical spectrum analyzer (OSA, AQ6370D, YOKOGAWA) to detect the laser output optical spectrum. At the same time, the second output port is launched into a delayed self-heterodyne interferometry (DSHI) based linewidth measurement system, which is consisted of two 3-dB couplers (C₂ and C₃), an acousto-optic modulator (AOM) with a frequency shift of 100 MHz, a 50 km single-mode fiber based delay line, a photodetector (PD, PDB450C, Thorlabs), and an electrical spectrum analyzer (ESA, FSV30, ROHDE&SCHWARZ).

 

Fig. 1. Schematic of the experimental setup.

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The current applied to the VCSEL with a threshold current of 2 mA is 4 mA. Figure 2(a) shows the electrical spectra of the VCSEL at different feedback level of RBS signal, where the feedback level is set to be from -34.9 dB to -27.6 dB, which is obtained by controlling the length of the RBS fiber. Inhere, a stabilized laser source, a 3-dB coupler and power meter are used to calibrate the feedback amount of the RBS fiber with different lengths before the laser experiment. It can be seen from Fig. 2(a) that the output spectra become narrower with the feedback level of RBS signal increasing. The evolution of linewidth with different feedback level of RBS signal is shown in Fig. 2(b). It reveals that the linewidth quickly reduces from 2.675 kHz to 0.630 kHz when the feedback level of RBS signal increases from -34.9 dB to -28.8 dB. Subsequently, the linewidth slowly decreases from 0.543 kHz to 0.435 kHz, because the feedback level of the RBS signal changes little. According to the Ref. [25], the laser linewidth can be continually compressed by increasing the feedback amount, because it can continue to improve the photon lifetime in the cavity. However, increasing the feedback by changing the fiber length in our experiment will lead to the formation of the stimulated Brillouin backscattering. There is a frequency difference between the Stokes optical feedback and the main cavity laser, which is harmful to further compress the linewidth. Therefore, the narrowest linewidth is achieved with the feedback level of RBS signal of -27.6 dB, as shown in Fig. 2(c). The 20 dB bandwidth of the electrical spectrum is 8.7 kHz, which is 20 times the laser linewidth. The laser linewidth can be calculated as 435 Hz, accordingly. The inset of Fig. 2(c) shows the linewidth of free-running VCSEL, which is 2 MHz with a scanning range of 80 MHz. The optical spectrum of the VCSEL with the feedback level of RBS signal of -27.6 dB is shown in Fig. 2(d). Two lasing wavelengths with a power difference of ∼34 dB represents the two polarization states, and the central wavelength of the main lasing polarization is 1542.3 nm. The polarization state in the main cavity of VCSEL with distributed feedback can still be considered to be linearly polarized, because the relatively large power difference of the two polarizations state.

 

Fig. 2. (a) Electrical spectra measured with different feedback level of RBS signal. (b) The evolution of laser linewidth with different feedback level of RBS signal. (c) Electrical spectrum measured with the feedback level of RBS signal of -27.6 dB. The 20 dB bandwidth is 20 times the laser linewidth, which is calculated to be 435 Hz. (d) Optical spectrum measurement result of the VCSEL with the feedback level of RBS signal of -27.6 dB.

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The electrical spectra of the free-running VCSEL at different driving current are very broad, and the spectra width under different driving current are nearly unchanged, as shown in Fig. 3(a). Then, the comparison result of the VCSEL with feedback level of the RBS signal of -27.6 dB is revealed in Fig. 3(b). The electrical spectra are obviously narrowed under the same driving current. The values of the linewidth with the different driving current are shown in Fig. 3(c). The black square represents the linewidth of the free-running laser, which is in the range of 2.03 MHz-2.1 MHz with the driving current varying from 3.5 mA to 6 mA. The red dot is the linewidth of the VCSEL with the feedback level of the RBS signal of -27.6 dB under same driving current. Compared with free-running VCSEL, the linewidth becomes narrower, and the corresponding linewidth fluctuates in the range of 464 Hz-430 Hz.

 

Fig. 3. (a) Electrical spectra of the free-running VCSEL at different driving current. (b) Electrical spectra of the VCSEL with the feedback at different driving current. (c) Relationship between the laser linewidth and the driving current.

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The RBS belongs to elastic scattering which is effective for all wavelengths within the gain spectrum. When the laser wavelength of the main cavity is tuned, the feedback signal can adapt this change and still narrow the laser linewidth. Figure 4(a) shows the tunability of the VCSEL with the feedback level of RBS signal of -27.6 dB. The lasing wavelength will be blue shifted with the thermal resistance increasing, and the corresponding relationship between central wavelength and thermal resistance is shown in Fig. 4(b). The laser can be tuned at a linear slope of -0.506 nm/kΩ, from 1543.324 nm to 1542.06 nm with the thermal resistance varying from 4.5 kΩ to 7.0 kΩ. In the tuning process, the electrical spectra of the VCSEL are revealed in Fig. 4(c), and the corresponding relationship between laser linewidth and thermal resistance is shown in Fig. 4(d). It shows a decreasing trend of the linewidth from 553 Hz to 419 Hz with the thermal resistance varying from 4.5 kΩ to 7.0 kΩ. The operating state of laser switches from heating mode to the cooling mode with the increasing of the thermal resistance, which can improve output power and stability of the laser. Therefore, the linewidth of the VCSEL tends to decrease during the tuning process.

 

Fig. 4. (a) Optical spectra of the VCSEL at different thermal resistance with RBS feedback level of -27.6 dB. (b) Relationship between the central wavelength and the thermal resistance. (c) Electrical spectra of the VCSEL at different thermal resistances with a feedback level of RBS signal of -27.6 dB. (d) Relationship between the laser linewidth and the thermal resistance.

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The frequency noises of the VCSEL with and without feedback are shown in Fig. 5. Because VCSEL has poor thermal stability, when frequency noise is measured at room temperature without any acoustic insulation or vibration isolation, the laser frequency cannot maintain stable in the time window width larger than 10 ms. Therefore, we only give the frequency noise in the span larger than 1 kHz, which mainly shows white noise part. The frequency noise decreases from 5.9 × 10¹¹ Hz²/Hz to 10⁴ Hz²/Hz at 10³Hz with the feedback level of RBS signal of -27.6 dB, and the frequency noise is compressed to be below 10³ Hz²/Hz for the Fourier frequencies from 10 kHz to 1 MHz. The linewidth from the white noise floor is estimated to be 578 Hz, which is roughly consistent with the measured results by DSHI. Due to the fluctuation of ambient temperature and random vibration, the laser has 1/f frequency noise in the low frequency span, which causes the integral linewidth to be slightly larger than the intrinsic linewidth determined by the frequency white noise. It can be seen from Fig. 5 that the frequency noise power spectrum density in the low frequency band is higher than that in the high frequency band. We firmly believe that the integral linewidth can approach the intrinsic noise by precise temperature control, acoustic insulation, vibration isolation, and improving the thermal stability of the VCSEL in the future work.

 

Fig. 5. The frequency noise spectra of free-running laser and the VCSEL with the feedback level of RBS signal of -27.6 dB.

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3. Conclusion

In conclusion, we have demonstrated an ultra-narrow linewidth VCSEL based on external-cavity weak distributed feedback from the RBS in one-dimensional optical waveguide. A linewidth as narrow as 435 Hz has been experimentally achieved in a VCSEL with the feedback level of RBS signal of -27.6 dB, which basically coincides with the result deduced from the frequency noise spectrum. With the increase of driving current, the linewidth of the VCSEL can maintain compressing in this process. By adjusting the thermal resistance of the laser from 4.5 kΩ to 7.0 kΩ, the output-wavelength can be tuned from 1543.324 nm to 1542.06 nm with a tuning rate of -0.506 nm/ kΩ, and the linewidth fluctuates in the range of 553-419 Hz. Such an ultra-narrow linewidth VCSEL will further promote the performance of single- and array-VCSEL-based applications, such as sensing, communication, and imaging.