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Abstract
Simultaneous self-injection locking of two vertical-cavity surface-emitting lasers (VCSELs) to a single whispering-gallery-mode (WGM) microcavity is experimentally demonstrated. The linewidths of the two VCSELs are compressed from 3.5 MHz and 5 MHz to 20.9 kHz and 24.1 kHz, which is on the same order of magnitude as that of locking each VCSEL to the microcavity separately. Moreover, the frequency noises of the two simultaneously locked VCSELs are suppressed by more than 60 dB below the offset frequency of 100 kHz compared to that of the free-running VCSELs. The method demonstrated here might be used in the multi-wavelength laser array with low phase and frequency noises, especially the VCSELs with the unique architecture of a two-dimensional array.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Lasers with narrow linewidth and high frequency stability are crucial in various applications ranging from optical coherent communication and gravitation wave detection to high precision spectroscopy and metrology [1–5]. Semiconductor lasers including edge-emitting and VCSELs have attracted considerable attention due to their inherent advantages of small size and less power consumption over conventional lasers. Nonetheless, the linewidth of a commercial semiconductor laser usually ranges from hundreds of kilohertz to a few megahertz, which strictly limits the performances of the applications mentioned above [6]. Self-injection locking in an external cavity configuration has proved to be an effective technique to deeply compress the laser linewidth and suppress the high-frequency noise [7–11]. The noise suppression ratio is proportional to the square of the quality factor (Q-factor) of the external cavity [12]. Employing mirrors, gratings, or Fabry-Perot cavities as the external feedback element in a semiconductor laser with an anti-reflection coating facet usually compresses the linewidth from the original megahertz to tens of kilohertz [13–15]. While the linewidth of the distributed feedback (DFB) semiconductor laser coupled to a WGM or a spiral microcavity with a higher Q-factor can be compressed to the level of sub-kilohertz or even sub-hertz [16–20]. A DFB semiconductor laser with sub-Hz instantaneous linewidth is achieved by feeding Rayleigh scattering in a prism coupled MgF2 WGM resonator back into the lasing cavity, where the loaded Q-factor of the millimeter-sized crystalline WGM resonator is 600 million [17]. Feeding the Rayleigh scattering from a CMOS-foundry-fabricated Si3N4 resonator with a Q-factor of 260 million back into the cavity of a DFB laser leads to a short-term linewidth as narrow as 1.2 Hz, corresponding to a five-orders-of magnitude noise reduction [19].
WGM cavity has been used to lock various lasers operating at different wavelengths [21,22]. However, a WGM resonator is only used to lock a single-longitudianl mode laser except the work reported in [23], where multiple frequencies of a Farby-Perot diode laser are simultaneously locked to different modes of a single WGM cavity. Multi-wavelength laser array with narrow linewidth and low frequency noise is in high demand in wavelength division multiplexing (WDM) systems and integrated microwave photonics (IMWP) [24,25]. Herein, we take the vertical-cavity surface-emitting lasers (VCSELs) as an example to experimentally demonstrate the self-injection locking of two lasers to a single WGM cavity with a Q-factor of one million simultaneously. The linewidths of the two VCSELs are compressed from 3.5 MHz and 5 MHz to 20.9 kHz and 24.1 kHz, respectively, both of which correspond to a two-order of magnitude compression ratio. Compared to the free-running VCSELs, the frequency noises are suppressed by more than 60 dB below the offset frequency of 100 kHz. The method demonstrated here may provide a solution for a two-dimensional VCSEL array with both narrow linewidth and low frequency noise.
2. Principle and experimental setup
Figure 1(a) shows the experimental setup used for simultaneously locking two VCSELs to a single WGM microcavity. Two VCSELs are combined by a 3 dB coupler (C1), by which the input is equally divided into two parts. One part of the 3 dB coupler (C1) connects with the WGM cavity in an add-drop configuration through a circulator. The outputs of the VCSELs are coupled into the WGM cavity by passing from the second port to the third port of the circulator. After one round trip, part of the WGM is coupled back to the through port connecting with an optical spectrum analyzer (OSA, YOKOGAWA, AQ6370D). The WGM field collected by the drop port connecting with the first port of the circulator is fed back into the lasers after passing through the 3 dB coupler (C1) again. A variable optical attenuator (VOA) and a 1:9 coupler (C2) are inserted between the first port of the circulator and the drop port of the microcavity to study the effect of the feedback level on the linewidth reduction.
Fig. 1. (a) Schematic diagram of the two VCSELs simultaneously locked to a single WGM microcavity, (b) Optical spectrum at the drop port of the WGM cavity.
Figure 1(b) shows the spectrum of the WGM cavity measured at the drop port with a broadband light source ranging from 1530 nm to 1565 nm and the OSA. The free spectrum range (FSR) of the WGM cavity is approximately 0.4 nm around 1557 nm. The coupling efficiency between the input port and drop port is 0.316%. Both the coupling losses between the bus/dropping waveguides and the WGM cavity and the losses induced by the mode mismatch between the single-mode fiber and the waveguide ports decrease the coupling efficiency. The WGM resonator made of Hydex glass is packaged with four ports coupled with single-mode fibers. The connection between the WGM resonator and the circulator is realized by an optical fiber flange. There is no heater integrated into the WGM resonator. The WGM microcavity with a Q-factor on the order of 106 is housed in a chamber with a temperature of 25°C during the whole process. We tune the lasing wavelength by adjusting the thermal resistance to actively match the WGM resonance.
According to the model originally developed in [23], the optical field of one VCSEL interacts with only one mode of the resonator since the wavelength spacing between the two locked VCSELs is twice the FSR of the resonator. Therefore, two VCSELs can be locked to different modes of a single WGM resonator simultaneously. Once the VCSEL is locked to the WGM resonator, its linewidth is significantly reduced due to the resonant feedback. The linewidth reduction mainly depends on the quality factors of the laser cavity and the WGM resonator, the reflection amount of the WGM resonator, and the linewidth enhancement factor [12,23]. The ratio between the linewidth of the locked laser and the free-running laser is given by [12]
To measure the linewidth and frequency noise of the lasers, the other part of the 3 dB coupler (C1) connects to an isolator accompanied by a dense wavelength division multiplexer (DWDM), and a delayed self-heterodyne interferometer (DSHI) based measurement system [27]. The DSHI system consists of two 3 dB couplers (C3 and C4), an acoustic-optical modulator (AOM) with a frequency shift of 100 MHz, a piece of single-mode fiber-based delay line (Delay), a photodetector (PD, THORLABS, PDB 450C), and a signal analyzer (ESA, ROHDE&SCHWARZ, FSV30). The lengths of the delay fiber used for the linewidth and the frequency noise measurements are 50 km and 10 m, respectively.
3. Experimental results and discussions
In order to demonstrate the simultaneous locking of the two VCSELs to a single WGM cavity and compare the performances, the linewidth and the frequency noise of the VCSELs are measured under the circumstances of free-running, independent locking of each VCSEL, and simultaneous locking of two VCSELs. When the pump current is set to be 6 mA, the optical spectrum of the first VCSEL (VCSEL#1) with a threshold current of 2 mA under the free-running state is shown in Fig. 2(a). The dominant lasing wavelength is 1556.654 nm. The wavelength spacing and power contrast between the dominant polarization mode and its orthogonal counterpart are 0.17 nm and 31 dB, respectively. The original linewidth of VCSEL#1 measured by the DSHI system is 3.5 MHz, as shown in Fig. 2(b), where the blue dots and red solid line are the experimental results and its Lorentzian fit.
Fig. 2. (a) Optical spectrum of VCSEL#1 under free-running state, (b) Beat spectrum of the free-running VCSEL#1, (c) Optical spectrum of VCSEL#1 separately locked to the WGM cavity, (d) Beat spectrum of the locked VCSEL#1, (e) Frequency noises of the free-running and separately locked VCSEL#1.
Then the lasing wavelength of the VCSEL#1 is precisely tuned by adjusting the thermal resistance of the driver (Newport, Model 6100) to match with the WGM resonance. Once the VCSEL#1 is locked to the WGM cavity, the ripples on the spectrum are removed, obtaining a more purified spectrum, as shown in Fig. 2(c). The lasing wavelength of the locked VCSEL#1 is 1556.66 nm, corresponding to a wavelength shift of 0.006 nm resulted from the pulling effect of optical feedback. As shown in Fig. 2(d), the locked linewidth is narrowed to 17.5 kHz according to the Lorentzian fit, which is two orders of magnitude narrower than that of the free-running VCSEL#1. Moreover, the frequency noise of VCSEL#1 under the locked state is suppressed by more than 60 dB below the offset frequency of 100 kHz, as shown in Fig. 2(e). The white noise on the 3973 Hz2/Hz level corresponds to a Lorentzian linewidth of ∼12.5 kHz, which is on the same order measured by the SDHI system [6]. The frequency noise spectrum of a commercially available narrow linewidth laser (NKT, Koheras BasiK E15) measured by the same system also demonstrates noise at high frequency. Therefore, the noise at the high offset frequency mainly results from the background noise of the measurement system.
The performance of the second VCSEL (VCSEL#2) is measured separately according to the same procedure, and the experimental results are shown in Fig. 3. Like VCSEL#1, there are also some ripples on the optical spectrum of the free-running VCSEL#2 lasing at 1557.396 nm when the pump current is 6 mA, as shown in Fig. 3(a). The original linewidth of VCSEL#2 is 5 MHz, as shown by the experimental dots in blue and the Lorentzian fit in red in Fig. 3(b). After locking VCSEL#2 to the other order WGM of the cavity, purified optical spectrum is obtained with feedback, as shown in Fig. 3(c), in which a slight wavelength shift of 0.006 nm occurs between the free-running and locked states. The linewidth is reduced to 26.6 kHz due to the optical feedback provided by the add-drop cavity, as shown in Fig. 3(d). Figure 3(e) shows that the frequency noise is suppressed by more than 60 dB below the offset frequency of 100 kHz. And the white noise on the level of 6895 Hz2/Hz indicates that the linewidth of the locked VCSEL#2 is 21.7 kHz, which is two orders of magnitude narrower than the free-running linewidth.
Fig. 3. (a) Optical spectrum of VCSEL#2 under free-running state, (b) Beat spectrum of the free-running VCSEL#2, (c) Optical spectrum of VCSEL#2 separately locked to the WGM cavity, (d) Beat spectrum of the locked VCSEL#2, (e) Frequency noises of the free-running and separately locked VCSEL#2.
To lock VCSEL#1 and VCSEL#2 simultaneously, the two VCSELs are turned on and pumped by a current of 6 mA, which is the same as that used for pumping the two VCSELs separately. The lasing wavelength of VCSEL#2 is firstly tuned toward the longer wavelength by adjusting the thermal resistance until it enters the locking state, whereas VCSEL#1 is still off the resonance of the WGM cavity. Then the lasing wavelength of VCSEL#1 is slowly tuned toward the longer wavelength by adjusting the thermal resistance of the driver until it is locked to another resonance of the WGM cavity. It is worth noting that redshift happens to the WGM resonance due to the thermal effect induced by VCSEL#1, which slightly deteriorates the locking state of VCSEL#2. Precisely tuning the thermal resistance of VCSEL#2 again is required to simultaneously lock the two VCSELs to different resonances of a single WGM cavity. Figure 4(a) shows the optical spectrum of the locked VCSELs, where the lasing wavelengths are respectively 1556.66 nm and 1557.402 nm. As shown in Fig. 4(b) and Fig. 4(c), the linewidths of the simultaneously locked VCSELs are 20.9 kHz and 24.1 kHz, respectively, which are approximately one order of magnitude higher than the theoretical predictions. The difference in linewidth between the two circumstances of locking the two VCSELs simultaneously and separately is no more than 3.5 kHz. The white frequency noises of the two VCSELs under simultaneous locking state are nearly the same as that of locking VCSEL#1 or VCSEL#2 separately, as shown in Fig. 2(e). Figure 3(e), and Fig. 4(d). The linewidths of the two simultaneously locked VCSELs within two hours are shown in Fig. 4(e), which are recorded every 5 minutes. The maximum linewidth variation for the two VCSELs is no more than 5 kHz.
Fig. 4. (a) Optical spectrum of the simultaneously locked VCSEL#1 and VCSEL#2, Beat spectra of the simultaneously locked (b) VCSEL#1 and (c) VCSEL#2, (d) Frequency noises of the simultaneously locked VCSEL#1 and VCSEL#2, (e) Linewidths of two simultaneously locked VCSELs within two hours, (f) Dependences of the linewidths of the locked VCSELs on the feedback level.
The effect of the feedback level on the linewidths of the locked VCSELs is studied by adjusting the feedback power through the VOA. The feedback level here is defined as the ratio between the optical power fed back into the laser cavity and the laser output power. The total loss of the feedback power includes the loss caused by the reflection of the VCSEL output mirror with a reflectivity of ∼99%, the insertion losses of the circulator and the WGM microresonator, and the coupling losses of the couplers (C1 and C2). The feedback power from the microresonator is monitored by a power meter (PM). The linewidths of the two VCSELs as a function of the feedback level are shown in Fig. 4(f), which indicates that the linewidth decrease with the increase of the feedback power. Minimum linewidths are achieved when the feedback level is −52.2 dB, corresponding to the scenario wherein the loss between the first port of the circulator and the drop port of the microcavity is zero.
4. Conclusion
In summary, simultaneous locking of two VCSELs to two different resonances of a single WGM cavity is experimentally demonstrated. Under the simultaneous locking state, the two VCSELs demonstrate similar performances with that of a separate locking. The linewidths of the two VCSELs are narrowed by two orders of magnitude, and the frequency noises are suppressed by more than 60 dB below the offset frequency of 100 kHz. The approach presented here may provide a way for an on-chip multiple wavelength semiconductor laser array with narrow linewidth, which is in high demand in the wavelength division multiplexing system and other relative applications.
Funding
National Natural Science Foundation of China (61635004, 61905028, 61933004); China Postdoctoral Science Foundation (2018M643406); Chongqing Postdoctoral Program for Innovative Talents (CQBX201902); Exchange Project for Key Lab of Optical Fiber Sensing and Communications, Ministry of Education of China (ZYGX2021K010); Fundamental Research Funds for the Central Universities (2019CDCGGD323, 2020CDCGJ036); Innovative Research Groups of Chongqing (cstc2020jcyj-cxttX0005); National Science Fund for Distinguished Young Scholars (61825501).
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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