Laser including adjustable reflector

Abstract

To provide a semiconductor laser device that outputs light radiation in a wavelength range, and a method of manufacturing and operating such a device.SOLUTION: A multiwavelength laser device 20 includes an active optical chip 22 on a Group-III-V semiconductor substrate 26 such as GaAs or InP, and a PIC 24 on an optical substrate 40 such as a silicon-on-insulator (SOI) substrate. In the chip 22, a light gain medium 30 includes a reflection type semiconductor light amplifier 28 and a reflector 36 formed at an inner edge of the gain medium 30. The PIC 24 includes an external laser cavity 42, is optically coupled to a waveguide 38, and is terminated at an adjustable reflector 50. The reflector 50 is adjusted so that its reflectivity is reduced at a peak of the gain band of the gain medium 30. The cavity 42 includes a phase tuner 48 for adjusting the phase of the cavity and a comb filter 44 and a band pass filter 46 in series.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] [CROSS REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 538,487, filed Sep. 14, 2023, which is incorporated herein by reference.

[0002] The present invention relates generally to optoelectronic devices, and more particularly to semiconductor lasers. [Background technology]

[0003] Wavelength domain multiplexing (WDM) is used in high-speed optical communications to allow multiple communication signals with different carrier wavelengths to be transmitted over the same optical fiber. Some WDM devices use a single laser to emit radiation in a comb of wavelengths to provide carriers at multiple wavelengths simultaneously. As used herein and in the claims, the term "comb" refers to a set of distinct spectral lines, the distinct spectral lines being spaced apart by equal steps in wavelength. The term "comb filter" refers to an optical filter having a passband that includes such a comb, such that the comb filter passes light having wavelengths in the set of distinct spectral lines while blocking wavelengths between the lines of the set. The terms "light" and "optical radiation" are used interchangeably herein and refer to electromagnetic radiation in any of the infrared, visible, and ultraviolet spectral ranges.

[0004] Mach-Zehnder interferometers (MZIs) are used in a variety of silicon photonics applications. For example, El Shamy et al. describe a tunable filter based on MZIs in "Modelling, characterization, and applications of silicon on insulator loop terminated asymmetric Mach Zehnder interferometer," Scientific Reports 12:3598 (2022). The authors describe a loop-terminated MZI whose spectrum is tunable not only by controlling the phase difference between the interferometer arms, but also by using the coefficients of its directional coupler to form a spectrally tunable filter. Summary of the Invention

[0005] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention described below provide semiconductor laser devices that output optical radiation over a range of wavelengths, as well as methods for fabricating and operating such devices.

[0006] Thus, according to one embodiment of the present invention, there is provided an optoelectronic device including a gain medium having first and second ends and configured to amplify laser radiation within a gain band having a peak at a given wavelength. A laser cavity includes the gain medium and includes a first reflector disposed on a first side of the gain medium; and a second reflector disposed on a second side of the gain medium opposite the first side. The second reflector has a reflectivity as a function of wavelength that is tunable to reduce the reflectivity of the second reflector at the peak of the gain band, thereby broadening the spectrum of the laser radiation emitted from the gain medium through the second reflector.

[0007] In a disclosed embodiment, the device comprises a bandpass filter disposed within the laser cavity and having a passband that encompasses the peak of the gain band.

[0008] Additionally or alternatively, the device comprises a comb filter disposed within the laser cavity and configured to pass a set of distinct wavelength sub-bands within the gain band, the set of distinct wavelength sub-bands defining a comb, wherein tuning the second reflector to reduce the reflectivity increases the number of the wavelength sub-bands in the laser radiation emitted from the gain medium. In disclosed embodiments, the comb filter comprises an optical ring resonator.

[0009] In some embodiments, the second reflector includes a tunable Mach-Zehnder Interferometer (MZI). In disclosed embodiments, the tunable MZI includes a pair of waveguides disposed on a substrate and a heater disposed on the substrate and coupled to vary a temperature of at least one of the waveguides.

[0010] Additionally or alternatively, the gain medium and the first reflector are both included in a reflective semiconductor optical amplifier (RSOA), and the second reflector is included in an external laser cavity disposed on an optical substrate and optically coupled to the RSOA. In one embodiment, the RSOA includes III-V semiconductor material, and the external laser cavity includes a silicon photonic integrated circuit (PIC).

[0011] There is also provided a method for generating radiation according to an embodiment of the present invention. The method includes providing a gain medium having first and second ends and configured to amplify laser radiation within a gain band having a peak at a given wavelength. The gain medium is inserted into a laser cavity including a first reflector disposed on a first side of the gain medium and a second reflector disposed on a second side of the gain medium opposite the first side, the second reflector having an adjustable reflectivity as a function of wavelength. The second reflector is adjusted to reduce the reflectivity at the peak of the gain band, thereby broadening the spectrum of the laser radiation emitted from the gain medium through the second reflector.

[0012] The present invention will be more fully understood from the following detailed description of the embodiments thereof taken in conjunction with the drawings. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a block diagram that illustrates a schematic representation of a multi-wavelength laser device, in accordance with an embodiment of the present invention.

[0014] [Figure 2A] 2 is a schematic detail diagram of an adjustable reflector used in the device of FIG. 1, according to one embodiment of the present invention.

[0015] [Figure 2B] 2B is a schematic plot of the reflectivity of the adjustable reflector of FIG. 2A.

[0016] [Diagram 3] 2 is a plot that generally illustrates the spectral characteristics of the laser device of FIG. 1, in accordance with one embodiment of the present invention.

[0017] [Figure 4] 2 is a plot that illustrates a schematic diagram of tailoring the spectral characteristics of the laser device of FIG. 1 in accordance with an embodiment of the present invention. [Diagram 5] 2 is a plot that illustrates a schematic diagram of tailoring the spectral characteristics of the laser device of FIG. 1 in accordance with an embodiment of the present invention.

[0018] [Figure 6] 2 is a plot that illustrates a schematic output spectrum of the laser device of FIG. 1, in accordance with one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Some multi-wavelength semiconductor lasers used in WDM applications emit a comb of wavelength sub-bands that extend over a particular spectral range. The density of the comb (i.e., the spacing between the central wavelengths of the sub-bands) is determined by the length of the laser cavity, while the spectral range is primarily limited by the width of the gain band of the optical gain medium in the laser. The gain band typically has a peak at a particular wavelength and drops off sharply on either side of the peak. The gain medium is contained within the laser cavity with reflectors on opposite sides of the gain medium, along with a comb filter within or outside the laser cavity.

[0020] To increase the information carrying capacity of a WDM communication link, it is desirable for the laser driving the link to emit a broad comb of multiple wavelengths and have approximately equal output power at all wavelengths of the comb. To this end, in an embodiment of the invention, the output reflector of the laser cavity containing the gain medium has a tunable spectral reflectivity (i.e., a reflectivity that varies as a function of wavelength). The output reflector is tuned to reduce its reflectivity at the peak of the gain band, thereby broadening the spectrum of the laser radiation emitted from the gain medium via the second reflector.

[0021] The broadened spectrum allows the laser device to output a comb containing a larger number of wavelength subbands for a given spacing between the subbands. Alternatively, the principles of the invention may be applied to broaden the spectral output band of other types of lasers, not just comb lasers. In either case, a broad, flat laser output spectrum is produced without compromising the efficiency of the laser by tailoring the spectral reflectivity of the output reflector, since the power output capability of the gain medium is fully utilized, i.e., additional losses at the peak of the gain band are converted into increased laser power output at the wings of the gain band.

[0022] In some embodiments, the output reflector comprises a tunable Mach-Zehnder interferometer (MZI) that comprises a pair of waveguides disposed on a substrate, such as a photonic integrated circuit (PIC). A heater on the substrate is actuated to vary the temperature of one or both of the waveguides, thus adjusting their relative optical path lengths. The resulting phase difference of the radiation propagating in the two waveguides determines the fraction of energy output through the reflector as a function of wavelength. The spectral reflectance is tuned by controlling the temperature of the heater. Alternatively, other types of broadband tunable optical elements can be used to control the spectral reflectance of the output reflector.

[0023] 1 is a block diagram that illustrates a schematic of a multi-wavelength laser device 20, according to one embodiment of the present invention. The device 20 comprises an active optical chip 22 on a III-V semiconductor substrate 26, such as GaAs or InP, and a PIC 24 on an optical substrate 40, e.g., a silicon-on-insulator (SOI) substrate. For mechanical stability and compact packaging, the chip 22 may be bonded to the optical substrate 40, e.g., using a suitable adhesive. Alternatively, the chip 22 and the PIC 24 may be mounted separately and connected, e.g., by a suitable optical fiber.

[0024] The chip 22 includes a reflective semiconductor optical amplifier (RSOA) 28 with an optical gain medium 30 that amplifies laser radiation within a particular gain band, and a reflector 36 formed at the inner end of the gain medium 30. Application of a drive voltage between electrodes 32 and 34 on the chip 22 produces optical gain in the medium 30 across the gain band, e.g., over a band of tens of nanometers centered around a peak at about 1310 nm or 1550 nm depending on the type of gain medium used. The gain medium 30 may include, for example, GaInAsP, AlGaInAs quantum wells, or InAs quantum dots that emit light in these gain bands. The reflector 36 may include, for example, a distributed Bragg reflector (DBR) or a reflective coating on the rear facet of the RSOA. A waveguide 38 on the substrate 26 conveys the laser radiation to or from the RSOA at the end opposite the reflector 36.

[0025] The PIC 24 comprises an external laser cavity 42, which is optically coupled to the waveguide 38 and terminates in an adjustable reflector 50. The reflector 50 is partially reflective to allow the multi-wavelength laser beam generated by the device 20 to exit the cavity, and has an adjustable reflectivity as a function of wavelength. Specifically, in this embodiment, the reflector 50 is tuned to reduce its reflectivity at the peak of the gain band of the gain medium 30. This function of the reflector 50 is described in more detail below.

[0026] The cavity 42 includes a comb filter 44 and a bandpass filter 46 in series, along with a phase tuner 48 for adjusting the phase of the cavity. In this example, the comb filter 44 comprises an optical ring resonator, with the ring length selected such that the filter 44 passes a comb of wavelength sub-bands within the gain band of the RSOA 28. Thus, the multi-wavelength laser device 20 can function as a beam source for WDM communications applications, for example, with the wavelength spacing between the sub-bands in the comb set equal to the spacing between the WDM channels. The bandpass filter 46 has a passband that encompasses the peaks of the gain band of the gain medium 30. The structure and operation of the comb filter 44 and the bandpass filter 46 may be based, for example, on the components of the comb laser described in U.S. Patent Application No. 18 / 366,705, filed August 8, 2023, the disclosure of which is incorporated herein by reference. Alternatively, tunable output reflectors of the type described herein may be used to broaden the spectral range of other types of semiconductor lasers and cavity designs known in the art.

[0027] 2A is a schematic detail diagram of a tunable reflector 50 according to one embodiment of the present invention. In this embodiment, the reflector 50 comprises a thermally tunable MZI comprising a pair of waveguides 52, 54 on a substrate 40, with respective heaters 62, 64 comprising resistive traces on the substrate 40. The waveguides 52 and 54 are coupled together at their ends by directional couplers 56 and 58, and at the inner end of the MZI by a termination loop 60. The coupling coefficients of the directional couplers 56 and 58 determine the relative amount of laser energy split between the waveguides 52 and 54, thus setting the basic reflectivity of the reflector 50 (e.g., as described in the above-mentioned El Shamy et al. publication). At the outer end of the MZI, the waveguide 52 is coupled to the cavity 42, while the waveguide 54 outputs the laser beam from the cavity.

[0028] To control the spectral reflectance of reflector 50, different currents are applied to heaters 62 and 64 (or current is applied to only one of the heaters) via electrical terminals 66 on substrate 40. The temperature difference between waveguides 52 and 54 creates a difference in the effective optical path length of the waveguides, which in turn adjusts the phase difference between the light waves joining at directional couplers 56 and 58. This phase difference, which varies as a function of wavelength, controls the spectral reflectance of reflector 50 at each wavelength within the gain band of laser device 20, i.e., the ratio between the energy output from waveguide 54 and the energy reflected back into cavity 42 via waveguide 52. This spectral reflectance can be adjusted in amplitude and wavelength by varying the currents applied to heaters 62 and / or 64.

[0029] 2B is a schematic plot showing reflectivity 70 as a function of wavelength for adjustable reflector 50, in accordance with one embodiment of the present invention. In this example, the currents applied to heaters 62 and 64 are selected to minimize the reflectivity at about 1320 nm, near the peak of the gain band of gain medium 30. The reflectivity is about 0.05 at the gain band peak and increases over a 50 nm range on either side of the peak.

[0030] 3 is a plot that generally illustrates the spectral characteristics of laser device 20, according to one embodiment of the present invention. Curve 80 shows the gain of gain medium 30 as a function of wavelength (λ), in arbitrary units of inverse centimeters (cm-1). Curve 82 shows the net loss of the laser cavity as a function of wavelength, in the same units as the gain. The net loss includes a fixed loss, which is typically roughly constant as a function of wavelength, and the transmittance (inverse of reflectance) of reflector 50, which varies as a function of wavelength, as shown in FIG. 2B.

[0031] The condition for lasing at a given wavelength λ is that gain (λ)≈net loss (λ), meaning that the net gain (the difference between gain and net loss) is approximately zero. Curve 84 shows the net gain as a function of wavelength. At the peak of the gain band (shown by curve 80), the reflectivity of reflector 50 (shown by curve 82) is reduced so that the net gain has a broad, flat peak with a value of 0 cm-1 that extends over a range of approximately 1300-1320 nm.

[0032] Figures 4 and 5 are plots that show schematic adjustment of the spectral characteristics of the laser device 20 according to an embodiment of the present invention. In both plots, curve 86 shows the additional loss due to the bandpass filter 46, which is adjusted to select the operating wavelength range of the laser device. In Figure 4, the bandpass filter 46 and reflector 50 are adjusted to shift the output wavelength range of the laser device 20 to shorter wavelengths in a range centered around 1309 nm, while in Figure 5, the output wavelength range is centered around 1322 nm.

[0033] FIG. 6 is a plot that shows a schematic of the output spectrum of the laser device 20 according to an embodiment of the present invention. Curve 84 shows the net gain of the laser as a function of wavelength based on the curves shown in FIG. 3, while curve 94 shows the passband of the comb filter 44 (FIG. 1). Curve 92 shows the resulting output spectrum of the laser device, producing a wide comb of separate wavelength sub-bands 94 spaced approximately 1 nm apart in wavelength and approximately equal in output power level. As mentioned above, the range of comb wavelengths may be shifted by appropriate adjustment of the bandpass filter 46 and reflector 50. Alternatively, the output characteristics of the laser device, such as the width of the emission band and the number of sub-bands in the comb, may be varied by appropriate modification of the laser cavity parameters.

[0034] The above-described embodiments are cited as examples, and the present invention is not limited to what has been particularly shown and described above. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof not disclosed in the prior art that would occur to one skilled in the art upon reading the foregoing description.

Claims

1. a gain medium having a first end and a second end and configured to amplify laser radiation within a gain band having a peak at a given wavelength; and a laser cavity including the gain medium, the laser cavity comprising: a first reflector disposed on a first side of the gain medium; and an optoelectronic device comprising a second reflector disposed on a second side of the gain medium opposite the first side, the second reflector having a reflectivity as a function of wavelength tunable to reduce the reflectivity of the second reflector at the peak of the gain band, thereby broadening the spectrum of the laser radiation emitted from the gain medium through the second reflector.

2. 10. The optoelectronic device of claim 1, comprising a bandpass filter disposed within said laser cavity and having a passband that encompasses said peak of said gain band.

3. 2. The optoelectronic device of claim 1, comprising a comb filter disposed within the laser cavity and configured to pass a set of distinct wavelength sub-bands within the gain band, the set of distinct wavelength sub-bands defining a comb, and wherein adjusting the second reflector to reduce the reflectivity increases a number of the wavelength sub-bands in the laser radiation emitted from the gain medium.

4. The optoelectronic device of claim 3 , wherein the comb filter comprises an optical ring resonator.

5. The optoelectronic device of claim 1 , wherein the second reflector comprises a tunable Mach-Zehnder interferometer (MZI).

6. 6. The optoelectronic device of claim 5, wherein the tunable MZI includes a pair of waveguides disposed on a substrate, and a heater disposed on the substrate and coupled to vary a temperature of at least one of the waveguides.

7. 7. The optoelectronic device of claim 1, wherein the gain medium and the first reflector are both included in a reflective semiconductor optical amplifier (RSOA), and the second reflector is included in an external laser cavity disposed on an optical substrate and optically coupled to the RSOA.

8. The optoelectronic device of claim 7 , wherein the RSOA comprises a III-V semiconductor material and the external laser cavity comprises a silicon photonic integrated circuit (PIC).

9. 1. A method for generating radiation, comprising: providing a gain medium having a first end and a second end, the gain medium being configured to amplify laser radiation within a gain band having a peak at a given wavelength; inserting the gain medium into a laser cavity having a first reflector disposed on a first side of the gain medium and a second reflector disposed on a second side of the gain medium opposite the first side, the second reflector having a tunable reflectivity as a function of wavelength; and adjusting the second reflector to reduce the reflectivity at the peak of the gain band, thereby broadening the spectrum of the laser radiation emitted from the gain medium through the second reflector. A method for providing the above.

10. 10. The method of claim 9, comprising inserting a bandpass filter into the laser cavity having a passband that encompasses the peak of the gain band.

11. 10. The method of claim 9, further comprising inserting a comb filter into the laser cavity, the comb filter configured to pass a set of distinct wavelength sub-bands within the gain band, the set of distinct wavelength sub-bands defining a comb, and adjusting the second reflector to reduce the reflectivity thereby increasing the number of the wavelength sub-bands in the laser radiation emitted from the gain medium.

12. The method of claim 11 , wherein inserting the comb filter comprises coupling an optical ring resonator into the laser cavity.

13. 10. The method of claim 9, wherein inserting the gain medium comprises coupling a tunable Mach-Zehnder Interferometer (MZI) to function as the second reflector.

14. 14. The method of claim 13, wherein coupling the tunable MZI comprises providing a pair of waveguides and a heater on a substrate, and adjusting the second reflector comprises controlling the heater to vary a temperature of at least one of the waveguides.

15. 15. The method of claim 9, wherein inserting the gain medium comprises providing a reflective semiconductor optical amplifier (RSOA) including the gain medium and the first reflector, and optically coupling an external laser cavity to the RSOA, the external laser cavity being disposed on an optical substrate and including the second reflector.

16. 16. The method of claim 15, wherein providing the RSOA comprises forming the RSOA on a III-V semiconductor material, and optically coupling the external laser cavity to the RSOA comprises coupling the RSOA to a silicon photonic integrated circuit (PIC) via a waveguide.

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