Monolithically integrated inp electro-optic tunable ring laser, laser device and corresponding method

Abstract

A tunable ring laser with a ring cavity, wherein the ring cavity comprises: at least one ring resonator having a waveguide for guiding a wave; a phase modulator having a waveguide for guiding a wave; and one or more power couplers for coupling the wave in and out of the at least one ring resonator, wherein the cross-section of the waveguide of the at least one ring resonator and the phase modulator is configured as a PIN diode and acts as an electrorefractive modulator, such that the tunable ring laser can be tuned by applying a reverse bias voltage.

Description

Background Technology

[0001] Widely tunable laser sources with narrow linewidths play a crucial role in high-speed communications, sensing, spectroscopy, quantum optics, and optical detection and ranging applications. For example, in coherent optical communication systems, 16-QAM (quadrature amplitude modulation) requires a linewidth of 120 kHz at 40 Gb / s, while upgrading to 64QAM places a hundred times more stringent requirements on the laser linewidth.

[0002] Active stabilization schemes have been proposed to improve the linewidth and frequency noise characteristics of free-running lasers. This approach is more common in spectroscopy and sensing applications. The most widely used and powerful technique is Pound-Drever-Hall (PDH) frequency locking. In this technique, the laser is locked to an external optical cavity using negative feedback. In the case of diode lasers, two control signals are typically used: one for the semiconductor optical amplifier (SOA) and one for the external acousto-optic modulator. Among known methods that allow PDH locking using a single control loop, this approach demonstrates limitation only by loop delay. This method has deployed bulk lithium niobate single-sideband modulators with bandwidths up to 5 MHz.

[0003] In recent years, considerable effort has been made to reduce the linewidth of chip-based integrated lasers from several MHz to 100 kHz and below. Different integration strategies have been followed to achieve this goal. For example, monolithically integrated tunable distributed Bragg reflector (DBR) lasers with intrinsic linewidths of 50–100 kHz have been demonstrated. This involves sampling the mirrors and thermally tuning the supermode DBR grating. Tunable lasers based on current injection tuning have also been reported in the past. However, due to the known linewidth broadening mechanism of current injection, such DBRs are not suitable for applications requiring very low linewidths.

[0004] In summary, this disclosure aims to realize a single-mode longitudinal-wave unidirectional ring laser that provides tunability across the entire frequency band, generally used in telecommunications but not limited to the aforementioned specific fields. Tuning the proposed laser does not significantly interfere with / broaden the intrinsic linewidth of the laser emission mode, thereby making the linewidth performance independent of the laser emission wavelength within the tuning range. Summary of the Invention

[0005] It would be advantageous to realize a ring laser that can be tunable over a particularly wide frequency band, wherein the tuning has little or no effect on the intrinsic linewidth of the laser emission mode.

[0006] In a first aspect of the disclosure, a tunable ring laser having a ring cavity is provided, such as a monolithically integrated InP tunable ring laser, wherein the ring cavity comprises:

[0007] - At least one ring resonator, said at least one ring resonator having a waveguide for guiding the wave,

[0008] - A phase modulator, the phase modulator having a waveguide for guiding a wave,

[0009] - A power coupler, the power coupler being used to couple the wave in and out of the at least one ring resonator.

[0010] The cross section of the waveguide of the at least one ring resonator and the phase modulator is configured as a PIN diode and acts as an electrorefractive modulator, such that the tunable ring laser can be tuned by applying a reverse bias voltage.

[0011] The proposed ring laser can operate, for example, at wavelengths between approximately 1500 nm and 1650 nm (e.g., 1550 nm). Such wavelengths are particularly suitable for telecommunications applications. However, according to this disclosure, the laser is not limited to this specific technical field.

[0012] Using the presented ring laser, the laser emission peak can be tuned in a range exceeding, for example, 30 nm. This single peak can be achieved by superimposing three filters (i.e., two ring resonators and the ring cavity itself). The combination of these filters suppresses the peak, i.e., modes other than the laser emission mode.

[0013] The tuning of the laser according to this disclosure is based on applying a negative voltage, i.e., reverse bias, to the tuning section of the laser (i.e., the resonator and phase modulator). This differs from other tuning principles such as thermal tuning or forward bias (i.e., positive voltage) because it results in reduced power consumption, does not interfere with the width of the laser emission peak, and can be faster because there are no slow transient effects like those from heat dissipation. Furthermore, the use of a ring resonator further reduces the width of the laser emission peak.

[0014] In the prior art, a thermal element can be used to perform tuning. The thermal element causes the optical path length of the resonator to increase or decrease. According to this disclosure, at least one ring resonator is tuned by applying a reverse bias voltage. Applying a reverse bias voltage can change the path length and the intrinsic properties of the material itself. This is caused by several factors, one of which is the linear electro-optic effect and the second-order electro-optic effect of the Pockel effect and Kerr effect, respectively.

[0015] Power couplers are used to couple waves in and out of at least one ring resonator. A power coupler can be a multimode interference coupler with, for example, a 50% splitting ratio. Another option is to place the ring resonator physically close to the ring cavity itself, allowing coupling effects based on the cavity / ring resonator geometry.

[0016] Preferably, the monolithically integrated ring cavity includes two ring resonators.

[0017] In another example, the ring laser also includes a broadband reflector arranged to couple the emission in a first propagation mode (e.g., clockwise propagation mode) to a second propagation mode (e.g., counterclockwise propagation mode) configured opposite to the first propagation mode.

[0018] The above ensures that the wave propagation direction in the ring laser is unidirectional.

[0019] In yet another example, the cavity further includes a semiconductor optical amplifier (SOA) arranged to amplify the wave in the cavity.

[0020] A semiconductor optical amplifier is, for example, an optical amplifier based on a semiconductor gain medium. The optical amplifier may resemble a laser diode, with its end mirror replaced by an anti-reflective coating.

[0021] It should be noted that semiconductor optical amplifiers (SOAs) may include InGaAsP multi-quantum-well based materials.

[0022] Based on the foregoing, this disclosure relates to a monolithically integrated unidirectional single-mode ring laser. Within a multi-project wafer operation framework, the laser can be fabricated using commercially available InP-based integration technology. The vernier effect of the transmission spectra of the two ring resonators can be used with slightly different perimeters to select the laser emission mode. The two ring resonators can be tuned in reverse-biased operation by primarily deploying a voltage-controlled electro-optic effect.

[0023] The tunable laser according to this disclosure can also be fabricated using silicon technology, wherein the modulator and semiconductor optical amplifier can be integrated on top of the silicon waveguide using a bonding technique. In such an implementation, the mode can travel from the silicon waveguide to the InP waveguide, which acts as a phase modulator, and then back to silicon.

[0024] Unlike thermal tuning, this tuning mechanism avoids significant on-chip heat dissipation due to reverse bias operation, and does not increase the laser's linewidth. This contrasts with current injection tuning, which is known to cause significant linewidth widening and significant heat dissipation. Ring resonators also help reduce linewidth because they increase the effective cavity length. Generally, ring lasers suffer from directionality problems. However, here, unidirectional operation of the laser can be ensured by a broadband reflector that couples the counterclockwise propagation mode to the clockwise mode (and vice versa).

[0025] This mirror does not need to have a high-reflectivity coated facet; instead, it can be suitably placed at one of the two output ports of the laser for multimode interference reflection. Effectively, the unidirectional laser can then be placed anywhere within the die / chip without having to be connected to the reflective facet.

[0026] In another example, one of the waveguides is an etched ridge waveguide, wherein the cross-section of the etched ridge waveguide is a vertical PIN diode and acts as an electrical refractive modulator (ERM).

[0027] In another example, the ring cavity comprises two ring resonators with different radii.

[0028] The radii of the two ring resonators can be, for example, 120 μm and 123 μm long, or any equivalent length. The resulting difference in path length can then be approximately 18.85 μm. The two ring resonators can be implemented using a 2×1 multimode interference MMI coupler with, for example, a 50% splitting ratio. The total perimeter of one ring resonator (including the length of the MMI) can be 1.400 mm, while the second ring resonator can be 18.85 μm longer. This configuration produces a vernier FSR of approximately 35 nm.

[0029] In a second aspect of this disclosure, a laser device for emitting a laser beam is provided, wherein the laser device includes a tunable ring laser according to this disclosure.

[0030] Laser devices can be, for example, telecommunications equipment equipped with lasers, i.e., telecommunications equipment used for optical communication using optical fibers. Telecommunications equipment can be, for example, a modem.

[0031] It should be further noted that the advantages explained with respect to the first aspect of this disclosure (i.e., the tunable ring laser) also apply to the second aspect of this disclosure (i.e., the laser device).

[0032] In a third aspect of this disclosure, a method for operating a tunable ring laser according to any one of the preceding claims is provided, wherein the method comprises the following steps:

[0033] - The wave is coupled into the at least one ring resonator by the one or more power couplers;

[0034] - Waves are guided by the at least one ring resonator and the phase modulator through their respective waveguides.

[0035] - Apply the reverse bias voltage to the PIN diode of any of the waveguides to tune the tunable ring laser.

[0036] It should be noted that a voltage-controlled device can be used to apply a reverse bias voltage to either of the PIN diodes in the waveguide. The voltage-controlled device may have two output terminals, the first of which is connected to the P-type semiconductor region of the PIN diode, and the second of which is connected to the N-type semiconductor region of the PIN diode.

[0037] It should be further noted that the advantages disclosed with respect to the first aspect of this disclosure (i.e., the tunable ring laser) also apply to the third aspect of this disclosure (i.e., the method of operating the tunable ring laser).

[0038] In one example of the above method, the ring cavity includes two ring resonators.

[0039] In another example, the cavity includes a semiconductor optical amplifier (SOA), and the method further includes the following steps:

[0040] - The wave in the cavity is amplified by the SOA.

[0041] In another example, the SOA comprises InGaAsP multi-quantum-well based materials.

[0042] These and other aspects of the invention will become clear and understood by referring to one or more embodiments described below. Attached Figure Description

[0043] Figure 1 A ring laser according to this disclosure is shown;

[0044] Figure 2 A microscope image of a ring laser according to the present disclosure is shown. Detailed Implementation

[0045] Figure 1 A ring laser 1 according to the present disclosure is shown.

[0046] Tunable ring laser 1 is a monolithically integrated tunable ring laser on indium phosphide (InP), a binary semiconductor composed of indium and phosphorus. InP has a zincblende crystal structure, comparable to that of GaAs and most III-V semiconductors. The inventors have discovered that the semiconductor dual heterostructure InP(p-doped)-InGaAsP(intrinsic)-InP(n-doped) is particularly suitable for applications requiring a reverse bias to tune the ring laser.

[0047] The tunable ring laser 1 has a ring cavity, which includes two ring resonators 3 for guiding waves. For example... Figure 1As shown, the tunable ring laser 1 has two horizontally oriented waveguides that are substantially parallel to each other. These horizontally oriented waveguides are connected to each other via a ring resonator 3.

[0048] The ring resonator 3 is similar to a PIN diode and acts as an electrorefractive modulator, allowing the tunable ring laser to be tuned in frequency by applying a reverse bias voltage.

[0049] In the context of this disclosure, a PIN diode is a diode having a wide, undoped intrinsic semiconductor region located between a p-type semiconductor region and an n-type semiconductor region.

[0050] The wide intrinsic region, or "i" region, contrasts with that of a typical PIN diode. This wide intrinsic region makes the PIN diode suitable for guiding light. The "i" region allows the ring laser to be tuned by applying a reverse bias voltage. Among other things, the reverse bias voltage modulates the amount of free carriers in the PIN dielectric, thereby effectively altering the frequency characteristics of the ring laser 1.

[0051] One reason for using PIN diodes is that a large overlap between the optical mode and the p-doped region is undesirable. With PN diodes, a larger overlap is unavoidable. Furthermore, if an intrinsic region exists within which the optical mode is guided, the optical mode can have a large overlap with the region where the electro-optic effect occurs, thus improving its efficiency.

[0052] The monolithically integrated InP tunable ring laser 1 also includes a phase modulator 6. The phase modulator can also be controlled by applying a reverse bias voltage.

[0053] One or more power couplers 2 may be provided for coupling waves in and out of at least one ring resonator 3. Further, the cavity also includes a semiconductor optical amplifier (SOA) 3, which is arranged to amplify the waves in the cavity.

[0054] Furthermore, the ring laser also includes a broadband reflector 4, which is arranged to couple the emission in a first propagation mode (e.g., clockwise propagation mode) to a second propagation mode (e.g., counterclockwise propagation mode) configured opposite to the first propagation mode. The output of the laser is indicated by reference numeral 5.

[0055] The phase modulator tuning mechanism proposed in this disclosure is based on a reverse-biased PIN structure in the corresponding waveguide. This differs from the conventional electron current injection or thermal heating methods used in the prior art. The proposed ring laser avoids significant on-chip heat dissipation. It has been shown that reverse biasing the phase segment results in a heat dissipation of <100 μW at an 8V bias. This is significantly lower than the power level required by thermal tuning mechanisms that typically require tens of mW of filters (e.g., DBR segments). The heat dissipation of the phase modulator used originates from the reverse bias current caused by carrier depletion in the intrinsic region, which increases at higher voltages.

[0056] Because it involves a weaker, slower transient thermal effect, the proposed tuning mechanism can achieve faster tuning compared to thermal tuning.

[0057] Furthermore, unlike current-injection tuning, significant propagation losses attributable to free carrier absorption do not occur. The low additional propagation losses due to electroabsorption occur only at high voltages. In fact, at low voltages (<5V), the losses are even slightly reduced due to free carrier depletion.

[0058] This disclosure relates to a unidirectional, single-mode ring laser monolithically integrated on an InP. The laser utilizes a vernier effect from, for example, two periodic spectral filters for its single-mode operation, and it is tuned using a voltage-controlled electro-optic effect.

[0059] In a specific implementation of the ring laser, two ring resonators with slightly different perimeters are used to increase the free spectral range (FSR) of the filter, and a single laser emission mode is selected within the mode gain bandwidth. The enhanced FSR from the vernier effect can be calculated as follows:

[0060]

[0061] Here, Δλ FSR1 and Δλ FSR2 This refers to the FSR of each ring resonator. The FSR of each ring resonator is determined by Δλ. FSR =λ 2 / n g L is given, where n g Here, L is a set of exponents, L is the circumference of a single ring, and λ is the wavelength. The detuning of the two rings can be large enough that the tuning range is limited by the modal gain bandwidth of the SOA rather than by the FSR of the vernier cavity filter.

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

[0063] This configuration produces a vernier FSR of approximately 35 nm. This FSR ensures a laser tuning range greater than 30 nm, thus covering the span of a single frequency band without affecting single-mode operation. A 0.4 mm long electro-optic phase modulator segment is also included in the laser cavity to facilitate independent tuning of the cavity modes. The phase segment is used to keep the laser emission mode aligned with the transmission maxima of the two ring filters to prevent mode hopping in the laser.

[0064] The two ring resonators and the phase segment can be tuned by applying a reverse bias voltage. Both ring resonators and the phase segment are deep-etched ridge waveguides with a cross-section of a vertical PIN diode, acting as an electro-refractive modulator (ERM). The undoped guide layer is a bulk InGaAsP quaternary material with a bandgap of 1.25 μm. The effective refractive index variation is a result of both field (Pockell and Kerr) effects and carrier (plasma / carrier depletion and band filling) effects. The addition of these effects results in a modulator efficiency of approximately 15° / Vmm for TE-polarized light.

[0065] Power output coupling is achieved via a 2×2 MMI with a splitting ratio of 85–15. The output coupling power percentage is 15%. Unidirectional operation of the ring laser is ensured by an external broadband reflector that couples amplified spontaneous emission (ASE) from the counterclockwise propagation mode to the clockwise mode. The broadband reflector is a multimode interferometer 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 between the cavity mode and the ring filter mode is such that the cavity mode is spaced 4×Δλ apart. FSR The signal falls within the transmission peak of the loop filter. The loop filter effectively suppresses this 4×Δλ. FSR Cavity modes within the range. The laser emission mode and the adjacent cavity modes falling within the transmission peak of the ring resonator (distance 4×Δλ). FSR The gain difference between the two modes is approximately 8%. This transmission difference is sufficient to ensure single-mode operation of the laser.

[0068] Figure 2 A microscope image 11 of a ring laser according to the present disclosure is shown.

[0069] The ring cavity may include a 1 mm long SOA made of InGaAsP multi-quantum-well substrate material. The SOA may be based on a shallowly etched ridge waveguide. The two ring resonators may have a radius of ~200 μm, and power coupling is achieved using a 2×1 multimode interference (MMI) coupler with a 50% splitting ratio.

[0070] The total perimeter of a single ring resonator (including the length of the MMI) is 1.4 mm and 1.419 mm. A 0.4 mm long electro-optic phase modulator segment is also included in the laser cavity to facilitate independent tuning of the cavity mode.

[0071] The two ring resonators and the phase segment can be tuned by applying a reverse bias voltage. Both ring resonators and the phase segment are deeply etched ridge waveguides with a cross-section of a vertical PIN diode, acting as an electro-refractive modulator (ERM). The undoped guide layer is a bulk InGaAsP quaternary material with a bandgap of 1.25 μm. The effective refractive index variation is a result of both field (Pockell and Kerr) effects and carrier (plasma / carrier depletion and band filling) effects. The addition of these effects results in a modulator efficiency of approximately 15° / Vmm for TE-polarized light.

[0072] Power output coupling is achieved via a 2×2 MMI with a splitting ratio of 85–15. The output coupling power percentage is 15%. Unidirectional operation of the ring laser is ensured by an external broadband reflector that couples amplified spontaneous emission (ASE) from the counterclockwise propagation mode to the clockwise mode. The broadband reflector is a multimode interferometer. The total cavity length is 5.9 mm, corresponding to a cavity mode free spectral range of 13.5 GHz, i.e., 0.108 nm.

[0073] Figure 2 The laser shown was manufactured using a library of commercially available InP-based active-passive integration components. Microscopic images of the laser are available in [location missing]. Figure 2 The image is shown in the figure. The area occupied by the microscope image is 2.17 × 0.56 mm². The laser was characterized at 18°C ​​using a water-cooled temperature stabilization mount. The waveguide from the laser output is angled relative to the chip facets to suppress back reflections to the laser cavity.

[0074] The chip facets are also coated with an anti-reflective coating to further suppress back reflection. A single-mode lens fiber is used to couple the laser output light out of the chip. Typical coupling loss due to mode mismatch between the chip facets and the lens fiber is ~4 dB.

[0075] It is important to note that at low tuning voltages, the dissipated tuning power is 2-3 orders of magnitude lower compared to other tunable lasers that typically use thermo-optical tuning, so as not to interfere with the laser linewidth. On average, the dissipated tuning power is at least one order of magnitude lower. Furthermore, the intracavity ring resonator helps reduce the laser linewidth, which can be important in many applications (i.e., coherent communication, sensing, etc.). The laser can be fabricated using general and commercially available indium phosphide (InP) photonic integration technology.

[0076] By studying the accompanying drawings, the disclosure, and the appended claims, those skilled in the art can understand and implement other variations of the disclosed embodiments in practicing the claimed invention. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite articles "a" or "an" do not exclude a plural. The mere fact that certain measures are recited in mutually different dependent claims does not imply that a combination of these measures cannot be used for benefit. Any reference numerals in the claims should not be considered as limiting their scope.

Claims

1. 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; A power coupler coupled to the ring resonator; as well as A broadband reflector, arranged to couple emission in a first propagation mode to a second propagation mode opposite to the first propagation mode. The waveguides of the ring resonator and the phase modulator are both PIN diodes in cross-section and are configured as electrorefractive modulators, enabling the tunable ring laser to be tuned by applying a reverse bias voltage.

2. The tunable ring laser according to claim 1, wherein the tunable ring laser is a monolithically integrated indium phosphide tunable ring laser.

3. The tunable ring laser according to claim 1, wherein at least one of the following is satisfied: The power coupler is a multimode interference coupler; or The ring resonator is a first ring resonator, and the ring cavity includes a second ring resonator.

4. The tunable ring laser according to claim 1, wherein the broadband reflector is a multimode interference reflector.

5. The tunable ring laser of claim 1, wherein the ring cavity further comprises a semiconductor optical amplifier arranged to amplify light in the ring cavity.

6. The tunable ring laser of claim 5, wherein the semiconductor optical amplifier comprises an indium gallium arsenide phosphide (InGaAsP) multi-quantum-well material.

7. The tunable ring laser of claim 1, wherein at least one of the waveguide of the ring resonator and the waveguide of the phase modulator is an etched ridge waveguide, wherein the cross-section of the etched ridge waveguide is a vertical PIN diode and acts as an electrorefractive modulator.

8. A method of operating a tunable ring laser having a ring cavity, wherein the method comprises the following steps: The first and second optical waves are coupled into the ring resonator by a power coupler; The first optical wave and the second optical wave are guided by the ring resonator and the phase modulator respectively through at least one of the waveguides of the ring resonator and the phase modulator, wherein the cross-section of the waveguide of the ring resonator and the waveguide of the phase modulator are both PIN diodes and both are configured as electrorefractive modulators; A reverse bias voltage is applied to at least one of the waveguides of the ring resonator and the phase modulator to tune the tunable ring laser. The transmission in the first propagation mode is coupled to the second propagation mode by a broadband reflector, the second propagation mode being the opposite of the first propagation mode.

9. The method according to claim 8, wherein the tunable ring laser is a monolithically integrated indium phosphide tunable ring laser.

10. The method of claim 8, wherein the ring resonator is a first ring resonator, and the ring cavity includes a second ring resonator.

11. The method of claim 8, wherein the annular cavity comprises a semiconductor optical amplifier, and wherein the method further comprises: The light in the annular cavity is amplified by the semiconductor optical amplifier.

12. The method of claim 11, wherein the semiconductor optical amplifier comprises an indium gallium arsenide phosphide (InGaAsP) multi-quantum-well material.

13. The method of claim 8, wherein the broadband reflector is a multimode interference reflector.

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