Self-Injection Locking of a Gain-Switched Laser Diode

We experimentally observe a self-injection locking regime of the gain-switched laser to a high-$Q$ optical microresonator. We reveal that the comb generated by the gain-switched laser experiences a dramatic reduction of comb-teeth linewidths in this regime. We demonstrate the Lorentzian linewidth of the comb teeth of sub-kHz scale as narrow as for a nonswitched self-injection locked laser. Such a setup allows generation of high-contrast electrically tunable optical frequency combs with tunable comb-line spacing in a wide range from 10 kHz up to 10 GHz. The characteristics of the generated combs are studied for various modulation parameters—modulation frequency and amplitude, and for parameters, defining the efficiency of the self-injection locking—locking phase, coupling efficiency, and pump frequency detuning.

Figure 1

Experimental setup. WGM modes excited with a coupling prism by DFB 1550-nm laser without an isolator. Supply current is modulated with a rf generator through a bias tee. Transmitted light is analyzed on the oscilloscope, optical spectrum analyzer (OSA) and electrical spectrum analyzer (ESA).

Figure 2

(a)–(f) Comparison of the nonswitched (left panels) and GS laser diode (right panels) in the SIL regime. (a),(b) The transmission spectra of the nonswitched (a) and gain-switched (b) laser in the SIL regime. (c)–(f) The spectrograms of (a),(b), where (e),(f) are enlarged area of the locking state. (g) Evolution of the output spectrum of the SIL GS laser with the variation of the detuning of the GS laser central frequency from the microresonator eigenfrequency. High-contrast lines near 1.8 GHz is the 5-MHz spaced comb (also presented in the insets), solid blue is a noisy part of the spectrum.

Figure 3

Spectra of the 100-MHz gain-switched comb generated by the free-running (top panel) and self-injection locked (bottom panel) laser. SIL leads to noise suppression and higher power level. The lower spectrum is noticeably spectrally purified.

Figure 4

Analysis of the teeth linewidth of the GS combs in the SIL regime for high (3 GHz) and low (100 MHz) modulation frequency. We fit the beat-note signal with the Voigt profile. The central line and sideband both have narrow kHz-scale linewidth.

Figure 5

The rf spectrum evolution with variation of the coupling efficiency.

Figure 6

The frequency combs observed in the SIL regime with gain-switched laser. Spectra are obtained by means of the heterodyne method. Comb teeth are marked with red circles and red arrows, and green arrows correspond to the interference lines. (a) The 40-kHz spaced comb. The spacing between adjacent lines is presented on the inset. (b) The 10-MHz spaced comb with width wider than 1 GHz. (c). The 0.3-GHz spaced comb. (d) The 0.5-GHz spaced comb. (e) The 1-GHz spaced comb. (f) The 10-GHz spaced comb.

Figure 7

The dependence of the GS comb full width at $-45$-dBm level on the switching frequency in the SIL regime. The GS spectrum width without SIL is presented with yellow dots.

Figure 8

The spectrum broadening with the modulation voltage increase in the SIL regime. The top panel corresponds to the voltage of 1.4 V, the bottom panel to 8.0 V at 5 MHz frequency.

Figure 9

Single sideband spectral density of the phase noise of the beat-note signal of the reference laser and the laser under study in different regimes. Gray line is an unlocked DFB laser. Red line is a SIL laser without modulation applied. The rest is the center line and two sidebands on each side of SIL GS laser. Black lines are the asymptotes corresponding to linewidths of 0.1, 1, 10, 100 kHz, and 1 MHz. It can be seen that for frequencies above ${10}^3$, the values are below 1-kHz asymptote. SIL itself stabilizes at short averaging times and above ${10}^3$ the fluctuations due to temperature instability are dominant. The lower the line power, the higher the plateau for frequencies closer to MHz.