Monolithically integrated InP electro-optically tunable ring laser, laser device, and corresponding method

Abstract

A tunable ring laser having a ring cavity, the ring cavity comprising at least one ring resonator having a waveguide for guiding waves, a phase modulator having a waveguide for guiding waves, and one or more power couplers for coupling waves into and out of the at least one ring resonator, wherein the cross sections of the waveguides of at least one ring resonator and phase modulator are configured as PIN diodes and act as electrorefractive modulators such that the tunable ring laser is tunable by applying a reverse bias voltage.

Description

[Background technology]

[0001] Widely tunable laser sources with narrow linewidths play an important role in applications in high-speed communications, sensing, spectroscopy, quantum optics, and photodetection, as well as lasing. For example, coherent optical communication systems require 16 quadrature amplitude modulation, QAM, at 40 Gb / s with a linewidth of 120 kHz. On the other hand, extending to 64 QAM places a 100 times more stringent requirement 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 for spectroscopy and sensing applications. The most widely used and powerful technique is the Pound-Drever-Hall (PDH) frequency locking. In this technique, the laser is locked to an external optical cavity by using negative electrical feedback. In the case of diode lasers, two control signals are usually used: a control signal of a semiconductor optical amplifier, SOA, and an external acousto-optical modulator. Known approaches that allow the implementation of PDH locking using a single control loop show limitations only due to the loop delay. This approach deploys bulk lithium niobate single-sideband modulators with a bandwidth of less than 5 MHz.

[0003] Considerable efforts have been made in recent years to reduce the linewidth of chip-based integrated lasers from a few MHz to the 100 kHz range or below. To achieve this goal, various integration approaches have been taken. For example, monolithically integrated tunable distributed Bragg reflector (DBR) lasers with intrinsic linewidths of 50 kHz to 100 kHz have been demonstrated. Mirrors were sampled and supermode DBR gratings were thermally tuned. 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 accordance with the above, the present disclosure is directed to the realization of a single-mode longitudinal wave unidirectional ring laser that is tunable over the entire band, typically used for long distance communications, but not limited to this particular field. The tuning of the proposed laser does not significantly disturb / broaden the inherent lasing linewidth of the lasing mode, making the lasing linewidth performance independent of the lasing wavelength within the tuning range. Summary of the Invention

[0005] In particular, it can be tuned over a wide frequency band, and tuning has little or no effect on the intrinsic linewidth of the lasing mode; It is advantageous to achieve a ring laser.

[0006] In a first aspect of the disclosure, a tunable ring laser, for example a monolithically integrated InP tunable ring laser having a ring cavity, the ring cavity comprising: at least one ring resonator having a waveguide for guiding a wave; a phase modulator having a waveguide for guiding a wave; a power coupler for coupling waves into and out of the at least one ring resonator; Equipped with At least one of the ring resonators and the phase modulator has a waveguide cross section configured as a PIN diode and acting as an electrorefractive modulator such that the tunable ring laser is tunable by applying a reverse bias voltage. A tunable ring laser is provided.

[0007] The provided ring lasers can operate, for example, at wavelengths between 1500 nm and 1650 nm, for example around 1550 nanometers. Such wavelengths are particularly suitable for telecommunications applications. However, the lasers according to the present disclosure are not limited to such a particular technology field.

[0008] With the proposed ring laser it is possible to tune the lasing peak over a range of, for example, more than 30 nm. Such a single peak may be achieved by the superposition of three filters, namely, two ring resonators and the ring cavity itself. The combination of these filters may suppress the peak, i.e. the mode, except for the lasing mode.

[0009] Tuning of the laser according to the present disclosure is based on applying a negative voltage, i.e. a reverse bias, to the tuning section of the laser, i.e. the resonator and the phase modulator. This is different compared to other principles of tuning such as thermal tuning or forward bias, i.e. a positive voltage, because it leads to less power consumption, does not hinder the width of the lasing peak, and can be faster due to the absence of slow transient effects such as thermal dissipation. Furthermore, the use of a ring resonator further reduces the width of the lasing peak.

[0010] In the prior art, tuning may be performed using a heating element, which increases or decreases the optical path length of the resonator. According to the present disclosure, at least one ring resonator is tuned by applying a reverse bias voltage. Applying a reverse bias voltage may change the path length and the intrinsic properties of the material itself. This occurs due to several factors. One of these factors is the linear electro-optic effect and the four-element electro-optic effect, the Pockels effect and the Kerr effect, respectively.

[0011] A power coupler is used to couple the waves into and out of at least one ring resonator. The power coupler may be a multimode interference coupler with, for example, a 50% splitting ratio. Another option is that the ring resonator is physically placed close to the ring cavity itself, so that the coupling effect occurs based on the cavity / ring resonator geometry.

[0012] Preferably, the monolithically integrated ring cavity comprises two ring resonators.

[0013] In a further embodiment, the ring laser further comprises a broadband reflector arranged to couple radiation in a first propagation mode, e.g., a clockwise propagation mode, into a second propagation mode, e.g., a counterclockwise propagation mode, configured opposite to the first propagation mode.

[0014] The above ensures that the wave propagation direction within the ring laser is unidirectional.

[0015] In yet another embodiment, the cavity further comprises a Semiconductor Optical Amplifier (SOA) arranged to amplify the wave within the cavity.

[0016] A semiconductor optical amplifier is, for example, an optical amplifier based on a semiconductor gain medium. It may look like a laser diode where the end mirrors are replaced with anti-reflective coatings.

[0017] It should be noted that semiconductor optical amplifiers, SOAs, may comprise InGaAsP multiple quantum well based materials.

[0018] Continuing from above, the present disclosure may be directed to a monolithically integrated unidirectional single mode ring laser. The laser may be fabricated using commercially available InP-based integration technology in the framework of a multi-project wafer run. The Vernier effect of the transmission spectra of two ring resonators may be used with slightly different circumferences to select the lasing mode. The two ring resonators may be tuned in reverse bias operation by deploying a mostly voltage-controlled electro-optic effect.

[0019] The tunable laser according to the present disclosure may also be fabricated using silicon technology, where the modulator and semiconductor optical amplifier may be integrated on top of a silicon waveguide using bonding techniques. In such an embodiment, the mode may travel from the silicon waveguide to an InP waveguide, a phase modulator, and back to silicon again.

[0020] Due to the reverse bias operation, this tuning mechanism avoids significant heat dissipation on the chip, unlike thermal tuning, and at the same time does not increase the linewidth of the laser. This is in contrast to current injection tuning, which is well known to cause significant linewidth broadening and significant heat dissipation. Ring resonators also help to reduce the linewidth, since they increase the effective length of the cavity. In general, ring lasers may suffer from directivity problems. However, here the unidirectional operation of the laser may be ensured by a broadband reflector that couples the counterclockwise propagation mode to the clockwise propagation mode or vice versa.

[0021] This mirror may have a multi-mode interference reflection that can be placed just at one of the two exit ports of the laser, rather than a highly reflective coated surface, so that effectively a unidirectional laser can be placed somewhere on the die / chip that is not necessarily connected to a reflective surface.

[0022] In a further embodiment, any of the waveguides is an etched ridge waveguide, the cross section of which is a vertical PIN diode, acting as an Electro-Refractive Modulator (ERM).

[0023] In a further embodiment, the ring cavity comprises two ring resonators of different radii.

[0024] The radii of the two ring resonators may be, for example, 120 μm and 123 μm long or comparable. Thus, the resulting difference in path length may be equal to about 18.85 μm. The two ring resonators may be implemented using a 2×1 multimode interference, MMI coupler, for example, with a split ratio of 50%. The total circumference of one ring resonator, including the MMI length, may be 1.400 mm, and the total circumference of the second ring may be 18.85 μm. This configuration gives a Vernier FSR of about 35 nm.

[0025] In a second aspect of the present disclosure, there is provided a laser device for emitting a laser beam, the laser device comprising a tunable ring laser according to the present disclosure.

[0026] The laser device may be, for example, a telecommunications device equipped with a laser, i.e., a telecommunications device used for optical communication using optical fibers. The telecommunications device may be, for example, a modem, etc.

[0027] It is further noted that the advantages discussed with respect to the first aspect of the present disclosure, which is a tunable ring laser, are also applicable to the second aspect of the present disclosure, which is a laser device.

[0028] In a third aspect of the present disclosure there is provided a method of operating a tunable ring laser according to any one of the preceding claims, the method comprising the steps of: coupling waves in at least one ring resonator with one or more power couples; guiding, by the at least one ring resonator and the phase modulator, a wave through a respective waveguide of the at least one ring resonator and the phase modulator; applying a reverse bias voltage to one of the PIN diodes in the waveguide, thereby tuning the tunable ring laser; A method is provided, comprising:

[0029] It should be noted that a reverse bias voltage may be applied to either PIN diode of the waveguide using a voltage controlled device. The voltage controlled device may have two output terminals, a first of which is connected to the P-type semiconductor region of the PIN diode and a second of which is connected to the N-type semiconductor region of the PIN diode.

[0030] It is further noted that the advantages disclosed with respect to the first aspect of the present disclosure, which is a tunable ring laser, are also applicable to the third aspect of the present disclosure, which is a method of operating a tunable ring laser.

[0031] In an embodiment of the method described above, the ring cavity comprises two ring resonators.

[0032] In a further embodiment, the cavity comprises a semiconductor optical amplifier (SOA), and the method further comprises: The SOA amplifies the waves in the cavity. Further includes:

[0033] In another embodiment, the SOA comprises an InGaAsP multiple quantum well based material.

[0034] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. [Brief description of the drawings]

[0035] [Figure 1] FIG. 1 illustrates a ring laser in accordance with the present disclosure. [Diagram 2] FIG. 1 illustrates a microscope image of a ring laser according to the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] FIG. 1 is a diagram illustrating a ring laser 1 according to the present disclosure.

[0037] The tunable ring laser 1 is a tunable ring laser monolithically integrated on indium phosphide (InP). InP is a binary semiconductor composed of indium and phosphorus. InP has a zinc-blende crystal structure, which is comparable to that of GaAs and most of the III-V semiconductors. The inventors have discovered that the semiconductor double heterostructure InP (p-doped)-InGaAsP (intrinsic)-InP (n-doped) is particularly suitable for use in the case where a reverse bias is applied to tune the ring laser.

[0038] The tunable ring laser 1 has a ring cavity with two ring resonators 3 for guiding waves. As shown in Fig. 1, the tunable ring laser 1 has two horizontally oriented waveguides, which are substantially parallel to each other. The horizontally oriented waveguides are connected to each other via the ring resonators 3.

[0039] The ring resonator 3 resembles a PIN diode and acts as an electrorefractive modulator such that the tunable ring laser can be tuned in frequency by applying a reverse bias voltage.

[0040] In the context of this disclosure, a PIN diode is a diode that has a large, undoped intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region.

[0041] The wide intrinsic area, or "i" region, contrasts with normal pn diodes. The wide intrinsic area makes PIN diodes suitable for guiding light. The "i" region allows the ring laser to be tuned by applying a reverse bias voltage. The reverse bias voltage modifies, among other things, the amount of free carriers in the PIN medium, thereby effectively changing the frequency characteristic of the ring laser 1.

[0042] One of the reasons for using PIN diodes is that it may not be desirable for the optical mode to overlap significantly with the p-doped regions. With PN diodes, the overlap is unavoidably large. Furthermore, if there is an intrinsic region in which the optical mode is guided, the optical mode may overlap significantly with the region where the electro-optic effect occurs, thus improving its efficiency.

[0043] The monolithically integrated InP tunable ring laser 1 further comprises a phase modulator 6. The phase modulator may also be controlled by applying a reverse bias voltage.

[0044] One or more power couplers 2 may be provided for coupling the waves into and out of the at least one ring resonator 3. Furthermore, the cavity further comprises a semiconductor optical amplifier (SOA) 3 arranged to amplify the waves within the cavity.

[0045] Furthermore, the ring laser further comprises a broadband reflector 4 arranged to couple radiation in a first propagation mode, e.g. clockwise propagation mode, into a second propagation mode arranged opposite to the first propagation mode, e.g. counterclockwise propagation mode. The output of the laser is indicated by reference 5.

[0046] The phase modulator tuning mechanism proposed in this disclosure may be based on a reverse-biased PIN structure in a corresponding waveguide. This mechanism is different from the usual current injection based or thermal heating of the prior art. The provided ring laser avoids significant heat dissipation on the chip. It has been shown that reverse biasing the phase section leads to a value of heat dissipation of less than 100 μW at a bias of 8 V. This is a much lower power level than is required for thermal tuning mechanisms for filters, e.g. DBR sections, which usually require tens of mW. The source of heat dissipation used by the phase modulator is the reverse bias current due to the depletion of carriers in the intrinsic region, which increases at higher voltages.

[0047] The proposed tuning mechanism may also allow faster tuning compared to thermal tuning due to the weaker and slower transient thermal effects involved.

[0048] Furthermore, in contrast to current injection tuning, no significant propagation losses occur due to free carrier absorption. A small amount of additional propagation loss occurs only at high voltages due to electroabsorption. In fact, at low voltages (below 5V), the losses are even slightly reduced due to free carrier depletion.

[0049] The present disclosure is directed to a monolithically integrated, unidirectional, single-mode ring laser on InP that utilizes the Vernier effect, e.g., from two regular spectral filters for its single-mode operation, and is tuned using a voltage-controlled electro-optic effect.

[0050] In a particular embodiment of a ring laser, two ring resonators with slightly different circumferences are utilized to increase the Free Spectral Range (FSR) of the filter and select a single lasing mode within the modal gain bandwidth. The enhanced FSR from the Vernier effect may be calculated by:

number

[0051] Here, Δλ FSR1 and Δλ FSR2 is the FSR of an individual ring resonator. The FSR of an individual ring resonator is Δλ FSR = Δλ 2 / n g L, where n g is the group index, L is the circumference of a single ring, and λ is the wavelength. The detuning of the two rings can be large enough so that the tuning range is limited by the geometric gain bandwidth of the SOA, not the FSR of the Vernier intracavity filter.

[0052] The laser was designed using a building block library of commercially available active-passive InP-based integration technologies. The ring cavity may contain a 1 mm long SOA with InGaAs multiple 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 a 2×1 multimode interference (MMI) coupler with a splitting ratio of 50%. The total circumference of one ring resonator, including the length of the MMI, is 1.400 mm and the total circumference of the second ring is 18.85 μm.

[0053] This configuration gives a vernier FSR of approximately 35 nm. This FSR ensures that the tuning range of the laser is greater than 30 nm, thus covering the span of a single band, and does not interfere with single mode operation. A 0.4 mm long electro-optic phase modulator section is also included in the laser cavity to facilitate independent tuning of the cavity modes. The phase section can be used to keep the lasing modes tuned to the maximum transmission of the two ring filters to prevent mode hopping of the laser.

[0054] The two ring resonators and the phase section can be tuned by applying a reverse bias voltage. These are all deeply etched ridge waveguides, whose cross sections are vertical PIN diodes, acting as electrorefractive modulators (ERMs). The unintentionally doped guiding layers are bulk InGaAsP quaternary material with a bandgap at 1.25 μm. The effective refractive index change is the result of both field (Pockels and Kerr) and carrier (plasma / carrier depletion and band filling) effects. The addition of these effects leads to a modulator efficiency of about 15° / Vmm for TE polarized light.

[0055] The power outcoupling is performed by a 2x2 MMI with a splitting ratio of 85-15. The percentage of outcoupling power is 15%. The unidirectional operation of the ring laser is ensured by an extra-cavity broadband reflector, which couples the Amplified Spontaneous Emission (ASE) of the counterclockwise propagation mode into the clockwise mode. The broadband reflector is a multimode interference reflector.

[0056] The total cavity length is 5.9 mm, covering a cavity mode free spectral range of 13.5 GHz (0.108 nm).

[0057] The spacing between the cavity modes and the ring filter modes is 4×Δλ FSR Effectively, this 4×Δλ FSR The cavity modes within the range are suppressed by the ring filter. The lasing mode and the adjacent cavity modes (4×Δλ) that are the transmission peaks of the ring resonator are suppressed. FSR The difference in gain between the two (above 100 nm and below 100 nm) is approximately 8%. This difference in transmission is sufficient to ensure single-mode operation of the laser.

[0058] FIG. 2 shows a microscope image 11 of a ring laser according to the present disclosure.

[0059] The ring cavity may contain a 1 mm long SOA with InGaAsP multiple quantum well based material. The SOA may be based on a shallow etched ridge waveguide. The two ring resonators may have a radius of about 200 μm, and power coupling is performed using a 2 × 1 multimode interference (MMI) coupler with a splitting ratio of 50%.

[0060] The total circumference of the 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 section is also included in the laser cavity to facilitate independent tuning of the cavity modes.

[0061] The two ring resonators and the phase section can be tuned by applying a reverse bias voltage. Both are deeply etched ridge waveguides, whose cross sections are vertical PIN diodes, acting as electrorefractive modulators, ERMs. The unintentionally doped guiding layers are bulk InGaAsP quaternary material with a bandgap at 1.25 μm. The effective index change is the result of both field (Pockels and Kerr) and carrier, plasma / carrier depletion and band filling effects. The addition of these effects leads to a modulator efficiency of about 15° / Vmm for TE polarized light.

[0062] The power outcoupling is performed by a 2x2 MMI with a splitting ratio of 85-15. The percentage of outcoupling power is 15%. The unidirectional operation of the ring laser is ensured by an extra-cavity broadband reflector, which couples the amplified spontaneous emission (ASE) of the counterclockwise propagation mode into the clockwise mode. The broadband reflector is a multimode interference reflector. The total cavity length is 5.9 mm, corresponding to a free spectral range of 13.5 GHz of the cavity modes, i.e. 0.108 nm.

[0063] The laser shown in Figure 2 was fabricated using commercially available active-passive InP-based integration technology. A microscope image of the laser is shown in Figure 2. The footprint of the laser is 2.17 × 0.56 mm 2 The laser was characterized at 18 °C using a water-cooled based temperature-stabilized mount. The waveguide from the laser output is angled with respect to the chip facet to suppress back reflections into the laser cavity.

[0064] The chip facet was also coated with an anti-reflection coating to further suppress back reflections. The laser output light was coupled out of the chip using a single-mode lensed fiber. Typical coupling loss between the chip facet and the lensed fiber due to mode mismatch is about 4 dB.

[0065] It is noted that at low tuning voltages, the scattered tuning power is two to three orders of magnitude lower than other tunable lasers that typically use thermo-optic tuning to avoid perturbing the laser's oscillation linewidth. On average, the scattered tuning power is at least an order of magnitude lower. Additionally, the ring resonator in the cavity helps to reduce the laser's oscillation linewidth, which may be important for many applications, i.e., coherent communication, sensing, etc. The laser may be fabricated using common, commercially available indium phosphide, InP, photonic integration techniques.

[0066] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in the practice of the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the term "comprising" does not exclude other elements or steps, and the indefinite articles "a" or "an" do 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 understood as limiting the scope of the claims.

Claims

1. 1. A tunable ring laser having a ring cavity, the ring cavity comprising: a ring resonator having a waveguide; a phase modulator having a waveguide; a power coupler coupled to the ring resonator; a broadband reflector arranged to couple a first propagation mode to a second propagation mode opposite to the first propagation mode; Equipped with A tunable ring laser, wherein a cross section of the waveguide of the ring resonator or the waveguide of the phase modulator acts as an electrorefractive modulator, which acts such that the tunable ring laser is tunable by applying a reverse bias voltage to the waveguide of the ring resonator or the waveguide of the phase modulator.

2. 10. The tunable ring laser of claim 1, wherein the tunable ring laser is a monolithically integrated Indium Phosphide (InP) tunable ring laser.

3. the power coupler being a multimode interference coupler; the ring resonator is a first ring resonator and the ring cavity comprises a second ring resonator; or the ring resonator is a first ring resonator, the ring cavity comprises a second ring resonator, and a circumference of the first ring resonator is different from a circumference of the second ring resonator; 3. The tunable ring laser of claim 2, wherein at least one of the following is satisfied:

4. 4. A tunable ring laser as claimed in claim 2 or 3, wherein the ring cavity further comprises a semiconductor optical amplifier arranged to amplify the light wave within the ring cavity.

5. 5. The tunable ring laser of claim 4, wherein the semiconductor optical amplifier comprises an indium gallium arsenide phosphide (InGaAsP) multiple quantum well based material.

6. 6. A tunable ring laser according to claim 2, wherein the waveguide of the ring resonator or the waveguide of the phase modulator is an etched ridge waveguide, the cross section of which is a vertical PIN diode and acts as an electrorefractive modulator.

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

8. 8. A laser device for emitting a laser beam, said laser device comprising a tunable ring laser according to any one of claims 1 to 7.

9. 1. A method of operating a tunable ring laser, comprising: coupling a first light wave and a second light wave in the ring resonator with a power coupler; guiding the first and second light waves through at least one of the waveguide of the ring resonator or the waveguide of the phase modulator by the ring resonator and the phase modulator, the cross section of the waveguide of the ring resonator or the waveguide of the phase modulator being configured to act as an electrorefractive modulator; applying a reverse bias voltage to at least one of the waveguides of the ring resonator or the waveguides of the phase modulator to tune the tunable ring laser; coupling the first propagation mode to a second propagation mode opposite the first propagation mode with a broadband reflector; A method comprising:

10. 10. The method of claim 9, wherein the tunable ring laser is a monolithically integrated Indium Phosphide (InP) tunable ring laser.

11. the ring resonator is a first ring resonator and the ring cavity of the tunable ring laser comprises a second ring resonator; or the ring resonator is a first ring resonator, the ring cavity comprises a second ring resonator, and a circumference of the first ring resonator is different from a circumference of the second ring resonator; The method of claim 10, comprising:

12. the ring cavity of the tunable ring laser further comprising a semiconductor optical amplifier positioned to amplify the light wave in the ring cavity; The method comprises:

12. The method of claim 9, further comprising amplifying the light wave in the ring cavity with the semiconductor optical amplifier.

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

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