Semiconductor external cavity laser with integrated planar waveguide bragg grating and wide-bandwidth frequency modulation
The present disclosure describes semiconductor external cavity laser with wide bandwidth frequency modulation capabilities. The laser is preferably packaged in a standard form-factor package, such as a 14-pin butterfly package. The front end of the cavity comprises an integrated planar circuit (e.g., silica-on-silicon planar lightwave circuit with Bragg gratings), and the “back facet” of the laser is implemented as a high-reflection (HR) coated LiNbO 3 phase tuning section in the double pass configuration. AC-voltage signal applied to the electrodes of phase tuning section modulates a refractive index of the propagating TE-polarization mode of external cavity and produces frequency modulation. Such frequency modulation is not associated with any thermal behavior of the gain element included in the external cavity laser, and has a negligible phase delay over a wide bandwidth.
1. A semiconductor external cavity laser (ECL) with narrow line width, low noise and wide bandwidth uniform frequency modulation capabilities in both magnitude and phase, the semiconductor ECL comprising:
a silica-on-silicon planar light wave circuit (PLC) comprising a semiconductor laser waveguide with embedded Bragg grating;
a semiconductor gain chip butt coupled to the PLC, and
a lithium niobate phase tuning element is optically coupled to the semiconductor gain chip;
wherein anti-reflection (AR) coating cover a front facet of the lithium niobate phase tuning element, a front facet of the semiconductor gain chip that butt coupled to the PLC, and a front facet of the PLC; and high-reflection (HR) coating covers a back facet of the lithium niobate phase tuning element defining a back facet of an external cavity of the semiconductor ECL;
wherein an alternating-current (AC)-voltage applied to electrodes of the lithium niobate phase tuning element generates an electric field, and the semiconductor gain chip acting as a polarizer allows to propagate in the external cavity only a transverse electric (TE) mode aligned perpendicular to a direction of the electric field in the lithium niobate phase tuning element; where a length of the lithium niobate phase tuning element is tailored to lasing characteristics of the PLC to achieve uniform frequency modulation response in phase and magnitude.
2. The semiconductor ECL of claim 1 , wherein the lithium niobate phase tuning element utilizes a double-pass configuration.
3. The semiconductor ECL of claim 1 , wherein the AC-voltage signal applied to the electrodes of the lithium niobate phase tuning element modulates a refractive index of the TE-mode propagating in the external cavity and produces uniform frequency modulation in both magnitude and phase.
4. The semiconductor ECL of claim 1 , wherein the PLC, the semiconductor gain chip and the lithium niobate phase tuning element are assembled on an extended thermoelectric cooler (TEC) chip.
5. The semiconductor ECL of claim 4 , wherein thermal effect of the semiconductor gain chip is isolated from the frequency modulation by the use of the TEC chip.
6. The semiconductor ECL of claim 4 , wherein the semiconductor ECL on top of the TEC chip is enclosed in a standard 14-pin butterfly package.
7. The semiconductor ECL of claim 1 , wherein the narrow linewidth of the ECL is below 10 kHz.
8. The semiconductor ECL of claim 1 , wherein bandwidth of the uniform frequency modulation is up to 100 MHz.
9. The semiconductor ECL of claim 1 , wherein the lithium niobate phase tuning element comprises bulk lithium niobate crystal.
10. The semiconductor ECL of claim 9 , wherein the lithium niobate phase tuning element is coupled in free space with the semiconductor gain chip using an AR-coated coupling lens.
11. The semiconductor ECL of claim 10 , wherein the coupling lens is mounted on a V-grooved submount for proper alignment in free space, thereby increasing optical coupling efficiency between the semiconductor gain chip and the lithium niobate phase tuning element.
12. The semiconductor ECL of claim 1 , wherein the lithium niobate phase tuning element comprises:
a lithium niobate waveguide with X-cut crystal orientation, and
the electrodes of the lithium niobate phase tuning element comprise a first pair and a second pair of electrodes on both sides of the lithium niobate waveguide.
13. The semiconductor ECL of claim 12 , wherein the first pair of electrodes is used for frequency modulation using AC voltage, and a second pair of electrodes is used for fine adjustment of direct current (DC) voltage used for phase control of the TE mode propagating in the external cavity independent of a bias current applied to the semiconductor gain chip.
14. The semiconductor ECL of claim 12 , wherein the lithium niobate phase tuning element is butt coupled with the semiconductor gain chip.
15. The semiconductor ECL of claim 14 , wherein the semiconductor gain chip has a tapered termination at its AR-coated back facet for mode matching with the lithium niobate waveguide, thereby increasing coupling efficiency between the semiconductor gain chip and the lithium niobate phase tuning element.
16. The semiconductor ECL of claim 15 , wherein 20-50V AC voltage is used for a lithium niobate waveguide length of 2-5 mm.