MONOLITHICALLY INTEGRATED INP ELECTRO-OPTICALLY TUNEABLE RING LASER, A LASER DEVICE AS WELL AS A CORRESPONDING METHOD

Abstract

A tuneable ring laser having a ring cavity, wherein the ring cavity comprises at least one ring resonator having a waveguide for guiding waves, a phase modulator having a waveguide for guiding waves, one or more power couplers for coupling the waves in, and out of, the at least one ring resonator, wherein a cross section of the waveguides of the at least one ring resonator and the phase modulator is configured as PIN diodes and act as an electro-refractive modulator such that the tuneable ring laser is tuneable by applying a reverse bias voltage.

Claims

1-15. (canceled)

16. A tunable ring laser having a ring cavity, wherein the ring cavity comprises: a ring resonator having a waveguide; a phase modulator having a waveguide; and a power coupler coupled to the ring resonator, wherein a cross section of the waveguide of at least one of the ring resonator or the phase modulator is configured as an electro-refractive modulator such that the tunable ring laser is tunable by applying a reverse bias voltage to at least one of the waveguide of the ring resonator or the waveguide of the phase modulator respectively.

17. The tunable ring laser of claim 16, wherein the tunable ring laser is a monolithically integrated, indium phosphide (InP), tunable ring laser.

18. The tunable ring laser of claim 17, wherein the power coupler is a multimode interference coupler.

19. The tunable ring laser of claim 17, wherein the ring resonator is a first ring resonator and the ring cavity comprises a second ring resonator.

20. The tunable ring laser of claim 17, comprising a broadband reflector arranged to couple a first propagation mode with a second propagation mode, the second propagation mode opposite the first propagation mode.

21. The tunable ring laser of claim 17, wherein the ring cavity further comprises a semiconductor optical amplifier arranged for amplifying a light wave in the ring cavity.

22. The tunable ring laser of claim 21, wherein the semiconductor optical amplifier comprises indium gallium arsenide phosphide (InGaAsP) multi-quantum-well based material.

23. The tunable ring laser of claim 17, wherein at least one of the waveguide of the phase modulator or the waveguide of the ring resonator is an etched ridge waveguide, wherein a cross-section of the etched ridge waveguide is a vertical PIN diode and acts as an electro-refractive modulator.

24. The tunable ring laser of claim 19, wherein a circumference of the first ring resonator is different to a circumference of the second ring resonator.

25. A method of operating a tunable ring laser, wherein the method comprises: coupling, by a power coupler, a first light wave and a second light wave in a ring resonator; guiding, by the ring resonator and a phase modulator, a first light wave and a second light wave via at least one of a waveguide of the ring resonator or a waveguide of the phase modulator respectively, wherein a cross section of the waveguide of at least one of the ring resonator or the phase modulator respectively is configured as an electro-refractive modulator, applying, to at least one of the waveguide of the ring resonator or the waveguide of the phase modulator respectively, a reverse bias voltage to tune the tunable ring laser.

26. The method of claim 25, wherein the ring resonator is a first ring resonator and the ring cavity comprises a second ring resonator.

27. The method of claim 25, wherein the cavity comprises a semiconductor optical amplifier, and wherein the method further comprises: amplifying, by the semiconductor optical amplifier, a light wave in the cavity.

28. The method of claim 27, wherein the semiconductor optical amplifier comprises indium gallium arsenide phosphide (InGaAsP) multi-quantum-well based material.

29. The method of claim 25, wherein the tunable ring laser further comprises a broadband reflector, and wherein the method comprises: coupling, by the broadband reflector, a first propagation mode, with a second propagation mode, the second propagation mode opposite the first propagation mode.

30. A method of fabricating a tunable ring laser having a ring cavity, the method comprising: at least partly providing a ring resonator having a waveguide; at least partly providing a phase modulator having a waveguide; and at least partly providing a power coupler coupled to the resonator, wherein a cross section of the waveguide of at least one of the ring resonator or the phase modulator is configured as an electro-refractive modulator such that the tunable ring laser is tunable by applying a reverse bias voltage to at least one of the waveguide of the ring resonator or the waveguide of the phase modulator respectively.

31. The method of claim 30, comprising at least one of: using an indium phosphide (InP) integration technology, or using a silicon technology.

32. The method of claim 30, wherein the tunable ring laser is a monolithically integrated, indium phosphide (InP), tunable ring laser.

33. The method of claim 32, comprising at least partly providing a broadband reflector arranged to couple a first propagation mode with a second propagation mode, the second propagation mode opposite the first propagation mode.

34. The method of claim 32, wherein at least one of the waveguide of the phase modulator or the waveguide of the ring resonator is an etched ridge waveguide, wherein a cross-section of the etched ridge waveguide is a vertical PIN diode and acts as an electro-refractive modulator.

35. The method of claim 32, wherein at least one of: the one or more power coupler is a multimode interference coupler; the ring resonator is a first ring resonator, and the method comprises at least partly providing a second ring resonator; the ring resonator is a first ring resonator, and the method comprises at least partly providing a second ring resonator with a circumference different to a circumference of the first ring resonator; the method comprises at least partly providing a semiconductor optical amplifier arranged for amplifying a light wave in the ring cavity; or the method comprises at least partly providing a semiconductor optical amplifier arranged for amplifying a light wave in the ring cavity, the semiconductor optical amplifier comprising indium gallium arsenide phosphide (InGaAsP) multi-quantum-well based material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] FIG. 1 shows a ring laser in accordance with the present disclosure;

[0045] FIG. 2 shows a Microscope image of the ring laser in accordance with the present disclosure.

DETAILED DESCRIPTION

[0046] FIG. 1 shows a ring laser 1 in accordance with the present disclosure.

[0047] The tuneable ring laser 1 is a tunable ring laser, monolithically integrated on Indium phosphide, InP, InP is a binary semiconductor composed of indium and phosphorus. It has a zincblende crystal structure, comparable to that of GaAs and most of the III-V semiconductors. The inventors have found that a semiconductor double heterostructure InP (p-doped)-InGaAsP (intrinsic)-InP (n-doped) is especially suitable to be used in case a reverse bias is to be applied for tuning the ring laser.

[0048] The tuneable ring laser 1 has a ring cavity which comprises two ring resonators 3 for guiding waves. As shown in FIG. 1, the tuneable ring laser 1 has two horizontally oriented waveguides which are substantially parallel to each other. These horizontally oriented waveguides are connected to each other via the ring resonators 3.

[0049] The ring resonators 3 resemble PIN diodes and act as an electro-refractive modulator such that the tuneable ring laser can be tuned, in frequency, by applying a reverse bias voltage.

[0050] In the context of the present disclosure, a PIN diode is a diode with a wide, undoped intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region.

[0051] The wide intrinsic region, i.e. the “i”-region, is in contrast to an ordinary p-n diode. The wide intrinsic region makes the PIN diode suitable for guiding light. The “i”-region makes it possible to tune the ring laser by applying a reverse bias voltage. The reverse bias voltage amends, amongst other, the amount of free carriers in the PIN medium, thereby effectively changing the frequency properties of the ring laser 1.

[0052] One of the reasons for using a PIN diode is that it may be undesirable that the optical mode has a large overlap with the p-doped region. Using a PN diode, the overlap will be unavoidably large. Furthermore, if there is an intrinsic region in which the optical mode is guided, then the optical mode may have a large overlap with the region in which electro-optic effects happen therefore increasing its efficiency.

[0053] The monolithically integrated InP tuneable ring laser 1 further comprises a phase modulator 6. The phase modulator may also be controlled by applying a reverse bias voltage.

[0054] One or more power couplers 2 may be provided for coupling the waves in, and out of, the at least one ring resonator 3. Further, the cavity further comprises a semiconductor optical amplifier, SOA, 3 arranged for amplifying the waves in the cavity.

[0055] Even further, the ring laser further comprises a broadband reflector 4 which is arranged to couple emission in a first propagation mode, for example clock-wise propagation mode, to a second propagation mode configured opposite the first propagation mode, for example anti clock-wise propagation mode. The output of the laser is indicated with reference numeral 5.

[0056] The phase modulator tuning mechanism that is proposed in this disclosure may be based on a reverse biased PIN structure in the corresponding waveguide. It differs from the usual electronic current injection based or thermal heating in the prior art. The presented ring laser avoids significant on-chip heat dissipation. It is shown that reverse biasing the phase sections results in values for the heat dissipation <100 μW at 8 V bias. This is a significantly lower power level than that required for thermal tuning mechanisms for a filter, e.g. DBR section, which typically need several 10s of mW. The origin of the heat dissipation the phase modulators used is the reverse bias current due to the carrier depletion in the intrinsic region which increases at higher voltages.

[0057] The proposed tuning mechanism can also enable faster tuning compared to thermal tuning due to the weaker slow transient thermal effects which are involved.

[0058] Furthermore, contrary to current injection tuning no significant propagation losses due to free-carrier absorption takes place. Low additional propagation losses occur only at high voltages due to electro-absorption. In fact, at low voltages (<5 V) losses are even slightly reduced due to free-carriers depletion.

[0059] The disclosure is directed to an unidirectional single mode ring laser monolithically integrated on InP. The laser deploys the Vernier effect from, for example, two periodical spectral filters for its single-mode operation and it is tuned using voltage controlled electro-optic effects.

[0060] In a specific embodiment of the ring laser, two ring resonators with slightly different circumference are utilized to increase the Free Spectral Range, FSR, of the filter and to select a single lasing mode within the modal gain bandwidth. The enhanced FSR from the Vernier effect may be calculated by:

[00001]${\mathrm{\Delta \lambda }}_{\mathrm{FSR},\mathrm{Vernier}}=\frac{{\mathrm{\Delta \lambda }}_{\mathrm{FSB}1}{\mathrm{\Delta \lambda }}_{\mathrm{FSR}2}}{{\mathrm{\Delta \lambda }}_{\mathrm{FSR}1}-{\mathrm{\Delta \lambda }}_{\mathrm{FSR}2}}$

[0061] Here, Δλ.sub.FSR1 and Δλ.sub.FSR2 are the FSR of the individual ring resonators. The FSR of an individual ring resonator is given by Δλ.sub.FSR=x.sup.2/n.sub.gL, where n.sub.g is the group index, L the circumference of a single ring and λ the wavelength. The detuning of the two rings may be large enough such that the tuning range is limited by the modal gain bandwidth of the SOA and not the FSR of the Vernier intra-cavity filter.

[0062] The laser was designed using a component library of a commercially available active-passive InP-based integration technology. The ring cavity may include a 1 mm long SOA with InGaAs multi-quantum-well based material. The SOA is based on a shallow etched ridge waveguide. The radii of the two rings may be 120 μm and 123 μm. The resulting difference in path length is 18.85 μm. The two ring resonators are implemented using 2×1 multimode interference (MMI) couplers with 50% splitting ratio. The total circumference of one ring resonator, including the length of the MMIs, is 1.400 mm and the second is 18.85 μm longer.

[0063] This configuration yields a Vernier FSR of about 35 nm. This FSR ensures that the laser tuning range is larger than 30 nm therefore covering the span of a single band and does not compromise single mode operation. A 0.4 mm long electro-optic phase modulator section is also included in the laser cavity to facilitate independent tuning the cavity modes. The phase section can be used to keep the lasing mode aligned with the transmission maximum of the two ring filters to prevent mode-hopping of the laser.

[0064] The two ring resonators and the phase section can be tuned by applying a reverse bias voltage. They are all deeply etched ridge waveguides, their cross-section is a vertical PIN diode and act as electro-refractive modulators (ERM). The non-intentionally doped guiding layer is bulk InGaAsP quaternary material with bandgap at 1.25 μm. The effective refractive index changes are a result of both field (Pockels and Kerr) and carrier (plasma/carrier depletion and band-filling) effects. The addition of these effects results in a modulator efficiency of about 15°/Vmm for TE polarized light.

[0065] The power out-coupling is implemented by a 2×2 MMI with 85-15 splitting ratio. The out-coupling power percentage is 15%. The unidirectional operation of the ring laser is ensured by an extra-cavity broadband reflector which couples the amplified spontaneous emission (ASE) in the anti-clock-wise propagation mode to the clock-wise mode. The broadband reflector is a multi-mode interference reflector.

[0066] The total length of the cavity is 5.9 mm corresponding to a cavity mode free-spectral range (FSR) of 13.5 GHz (0.108 nm).

[0067] The spacing of the cavity modes and the ring filter modes is such that a cavity mode falls within a ring filter transmission peak every 4×Δλ.sub.FSR. Effectively the cavity modes within this 4×Δλ.sub.FSR range are suppressed by the ring filters. The gain difference between the lasing mode and the neighbouring cavity mode falling within a ring resonator transmission peak (which is 4×Δλ.sub.FSR away) is about 8%. This transmission difference is adequate to ensure single mode operation of the laser.

[0068] FIG. 2 shows a Microscope image 11 of the ring laser in accordance with the present disclosure.

[0069] The ring cavity may include a 1 mm long SOA with InGaAsP multi-quantum-well based material. The SOA may be based on a shallow etched ridge waveguide. The two ring resonators may have a radius of ˜200 μm and the power coupling is implemented using 2×1 MultiMode Interference, (MMI) couplers with 50% splitting ratio.

[0070] The total circumference of a single ring resonator, including the length of the MMIs, is 1.4 mm and 1.419 mm. A 0.4 mm long electro-optic phase modulator section is also included in the laser cavity to facilitate independent tuning the cavity modes.

[0071] The two ring resonators and the phase section can be tuned by applying a reverse bias voltage. They are both deeply etched ridge waveguides, their cross-section is a vertical PIN diode and act as electro-refractive modulators, ERM.

[0072] The non-intentionally-doped guiding layer is bulk InGaAsP quaternary material with bandgap at 1.25 μm. The effective index changes are a result of both field, Pockels and Kerr, and carrier, plasma/carrier depletion and band-filling, effects. The addition of these effects results in a modulator efficiency of about 15°/Vmm for TE polarized light.

[0073] The power out-coupling is implemented by a 2×2 MMI with 85-15 splitting ratio. The out-coupling power percentage is 15%. The unidirectional operation of the ring laser is ensured by an extra-cavity broadband reflector which couples the amplified spontaneous emission, ASE, in the anti-clock-wise propagation mode to the clock-wise mode. The broadband reflector is a multi-mode interference reflector. The total length of the cavity is 5.9 mm corresponding to a cavity mode free-spectral range of 13.5 GHz, i.e. corresponding to 0.108 nm.

[0074] The laser shown in FIG. 2 was fabricated using a commercially available active-passive InP-based integration technology. A microscopic image of the laser is shown in FIG. 2. Its footprint is 2.17×0.56 mm2. The laser was characterized at 18° C. using a water-cooler based temperature stabilized mount. The waveguide from the laser output is angled with respect to the chip facet to suppress back-reflections to the laser cavity.

[0075] The chip facet was also coated with anti-reflection coating to further suppress back-reflections. The laser output light was coupled out of the chip using a single-mode lensed fibre. Typical coupling losses between the chip facet and the lensed fibre due to the mode mismatch are ˜4 dB.

[0076] It is noted that, at low tuning voltages the dissipated tuning power is 2-3 orders of magnitude lower than other tuneable lasers which typically use thermo-optic tuning in order not to disturb the laser linewidth. On average the dissipated tuning power is at least an order of magnitude lower. Furthermore, the intra-cavity ring resonators help in decreasing the laser linewidth which may be important in many applications, i.e. coherent communications, sensing etc. The laser may be fabricated using a generic and commercially available Indium Phosphide, InP, photonic integration technology.

[0077] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims, In the claims, the word “Comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope thereof.