Self-Injection Locking of a Distributed Feedback Laser Diode Using a High-Finesse Fabry-Perot Microcavity

Abstract

Optical microcavities have been widely used in nonlinear optics, quantum optics, and laser technologies. Here we demonstrate the self-injection locking of a distributed feedback diode laser using home-made high-finesse Fabry-Perot microcavity. The Fabry-Perot microcavity is fabricated from an x-cut lithium niobate crystal with highly reflective coatings. Frequency pulling effect can be observed for a successful locking, and results in a single-longitudinal mode lasing with narrow linewidth. The lasing wavelength and output power are found robust to the laser-diode current and temperature variations, in comparison to the free-running case. We further characterize the laser linewidth with beat note measurement with a high-performance external cavity diode laser, with beat-note linewidth of 601.85 kHz. This results shows a new method for laser frequency stabilization in a simple setup, and may find applications in telecommunication and spectroscopy.

1. Introduction

Optical microcavities are important for optical integration in practical applications [1,2,3,4,5,6,7,8,9]. Among them, Fabry-Perot microcavities (FPMC) are relatively simple to fabricate with high finesse [10,11,12], which is important for nonlinear optics applications, e.g., second-harmonic generation [12,13,14,15,16], parametric amplification [17,18,19], frequency-comb generation [20], and entangled photon generation [21]. Efficient nonlinear processes can be achieved due the cavity enhancements, with high-reflective coatings over nonlinear optical material like LiNbO₃ (LN) and KTiOPO₄ (KTP) [22,23,24,25].

On the other hand, semiconductor lasers are key components in coherent telecommunication and dense wavelength-multiplexing transmission systems, due to their convenience and low cost. However, the above applications require narrow linewidth and high wavelength stability, which can be challenging for normal diode lasers. Even with narrow-linewidth designs, as in distributed feedback lasers or distributed Bragg reflector laser, their output wavelength can be easily affected by the current and temperature fluctuations, and relatively high sidebands, which exist in their spectra. Several approaches have been developed to stabilize the diode lasers [26,27,28,29,30], including the external feedback from the high-finisse cavity.

In this work, we fabricate a FPMC, with high-finesse of up to 55.5. This FPMC is used for the self-injection locking of a semiconductor distributed feedback (DFB) laser. The locked DFB laser shows low wavelength drift, which is free from current and temperature variation. A 402 kHz beat-note linewidth can be observed, showing a good and stable locking.

2. The Fabry-Perot Microcavity

The FPMC we used is made from an x-cut lithium niobate crystal sheet, and high-reflection coated on both facets. The lithium niobate crystal is, first, finely-polished down to the thickness of ~100 μm, with sub-micron surface roughness. The thickness of each coating layer is optimized for the desired reflectivity, which is 94.5% at 1550 nm wavelength. The total thickness of Ta₂O₅ and SiO₂ layers were 1179.9 nm, and 1801.2 nm, respectively for both surfaces, and 15 periods are used to guarantee a sufficient reflection [31]. The exact thickness of each layer is listed in the Appendix A (

Table A1. Thickness of each coated layer in FPMC.

Layer	Material	Thickness (nm)
1	SiO2	84.90
2	Ta2O5	79.82
3	SiO2	289.63
4	Ta2O5	189.36
5	SiO2	269.93
6	Ta2O5	187.44
7	SiO2	269.10
8	Ta2O5	187.15
9	SiO2	268.84
10	Ta2O5	187.12
11	SiO2	268.80
12	Ta2O5	187.11
13	SiO2	268.96
14	Ta2O5	161.89
15	SiO2	81.04

).

The surface roughness of the Ta₂O₅ and SiO₂ coating (via evaporation) is essential in achieving a high finesse for FPMC. Figure 1a,c show the atomic-force microscope (AFM) image of the front, and rear surfaces of the coated crystal, respectively. Figure 1b,d shows zoom-in of the central parts in Figure 1a,c. It can be seen that for both surfaces, the roughness fluctuates within 3 nm, which is key to suppressing the light scattering and to achieve the high finesse. Figure 1e plots the measured cavity resonance by sweeping the frequency of a tunable laser, indicating the full-width half-maxima (FWHM) linewidth of ~12 GHz. The theoretical linewidth of FPMC is estimated as $\Delta \nu =\frac{c}{2\pi nd}\frac{1-r}{\sqrt{r}}$, where n is refractive index, d is cavity length, r is surface reflectivity and c is vacuum speed of light. This gives 12.204 GHz, which is quite close to measured value. The free-spectral range (FSR) of the cavity is measured as ~678.7 GHz, giving the finesse of ~55.5 and the Q (central frequency over linewidth) of 1.62 × 10⁴. One typical sample of the FPMC is shown in Figure 1e (The inset picture).

3. Self-injection Locking of a DFB Laser

The experimental set-up for the injection locking of a DFB laser (model number LSDLD155-5-S-0-2-SMFA with current driver LDTC0520) is sketched in Figure 2. The output beam of a DFB laser passes through an optical circulator, and collimated into free-space to pass he FPMC. The transmission is then sent back into the laser via a 90:10 fiber beam splitter. The transmission into the FPMC is measured to be ~86%. A series of half-wave plates (HWP) and quarter-wave plates (QWP) are applied to ensure that the light beam is injected back to the DFB laser with the same polarization output. An optical spectrum analyzer (OSA) is used to characterize the laser spectrum.

Figure 3a plots the measured laser wavelength as a function of the driving current before (black) and after (red) the injection locking. Before locking, the laser output is directed to OSA without the feedback loop. The threshold current of the free-running DFB laser is ~11.3 mA. When the laser is injection-locked, the threshold (10 mA) is lower than the free-running case, due to the extra external feedback. It can be seen that, before the injection locking, the laser wavelength increases with the driving current, while after locking, the output wavelength is stabilized to the FPMC resonance (1550.24 nm). This indicates that the output power of the locked laser can be modulated without varying its wavelength, which is beneficial for telecommunication applications. Figure 3b compares the lasing wavelength (red) and the FPMC resonance (black) at different FPMC temperature. To control the temperature of the FPMC, it is put inside an oven with temperature-control accuracy of 0.01 °C. It can be seen that, after the locking, the laser wavelength follows the variation of the change of FPMC resonance. In this measurement, the current is set 24.8 mA, which is far above the threshold (5 mA). Figure 3c compares the measured laser spectrum before and after injection locking, and the result shows that the sidebands of the free-running laser can be suppressed by ~9 dB. Figure 3d displays the laser output wavelength, measured after the 10% port of the 90:10 fiber beam splitter, as a function of laser temperature. In the free-running case, the laser wavelength increased as the temperature of the laser diode (black line), mainly due to the refractive index dependence of the temperature. However, when the laser is locked, the output wavelength is stabilized.

The free-running DFB laser we used has a relatively large linewidth, which can be reduced by the self-injection locking with the FPMC. We used heterodyne detection [32,33] to measure the beat note between the locked laser and the high-performance reference external cavity diode laser (Santec TSL-710, 200 kHz linewidth). For the free-running case shown in Figure 4a (blue curve), the captured oscilloscope trace of the beating signal is a distorted sinusoidal function. Figure 4b shows the measured beat spectrum using an electric spectrum analyzer (resolution bandwidth 50 Hz). A sharp peak at 102.5 MHz is clearly visible, corresponding the beat frequency between the free-running laser and Laser 1 (zoom-in of the marked regime shown in inset picture). The measured FWHM linewidth is 7.1 MHz, indicating the free-running laser linewidth at this level. When the laser is locked, the temporal waveform appears more regularly, as shown in Figure 4c. Figure 4d shows the measured beat spectrum using the same electric spectrum analyzer setting. The beat-note linewidth is fit to be 402 kHz FWHM (assuming Lorentzian shape), which is orders of magnitude narrower than the free-running case. In this measurement, the temperature was 23.997 °C and the current was 24.8 mA.

4. Conclusions

We fabricated a FPMC with high-finesse of 55.5, for the self-injection locking of a semiconductor DFB laser. High-quality injection locking has been presented in this paper with clear frequency-pulling effects, which stabilizes the laser output to the FPMX resonance. Narrow linewidth is confirmed with significantly reduced beat-note linewidth of 402 Hz FWHM, in comparison to 7.1 MHz FWHM in the free-running case. Here, we focus on the locking of a DFB laser, similar setup can be used to lock laser diodes of other types. This result may find broad application prospects in the fields of remote optical communication networks, optical sensing, and quantum information technologies, with a simple and compact setup.