Monolithically integrated inp electro-optically tunable ring laser, laser device as well as corresponding method

Abstract

To provide a tunable ring laser.SOLUTION: Disclosed is a tunable ring laser having a ring cavity. In the tunable ring laser, the ring cavity comprises 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 the waves in, and out of, the at least one ring resonator. A cross section of the waveguides of the at least one ring resonator and the phase modulator is configured as PIN diodes and acts as an electro-refractive modulator such that the tunable ring laser is tunable by applying a reverse bias voltage.SELECTED DRAWING: Figure 1

Description

Background Art

[0001] A widely tunable laser source with a narrow oscillation linewidth plays an important role in high-speed communication, sensing, spectroscopy, quantum optics, and optical detection, as well as in ranging applications. For example, in a coherent optical communication system, 16 quadrature phase amplitude modulation, QAM, at an oscillation linewidth of 40 Gb / s and 120 kHz is required. On the other hand, expanding to 64QAM imposes a 100-fold stricter requirement on the laser oscillation linewidth.

[0002] Schemes for actively stabilizing have been proposed to improve the oscillation 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 by using negative electrical feedback. In the case of diode lasers, two control signals are usually used: the control signal of a semiconductor optical amplifier, SOA, and an external acousto-optic modulator. Known approaches enabling an implementation of PDH locking using a single control loop have shown limitations due to only the loop delay. This approach deploys a bulk lithium niobate single-sideband wave modulator in a band of 5 MHz or less.

[0003] Considerable efforts have been made in recent years to reduce the oscillation linewidth of chip-based integrated lasers from several MHz to below the range of 100 kHz. To achieve this goal, various integration methods have been adopted. For example, a monolithically integrated, tunable distributed Bragg reflector (DBR) laser with a natural oscillation linewidth of 50 kHz to 100 kHz has been demonstrated. The mirrors were sampled and the supermode DBR gratings were thermally tuned. Tunable lasers based on current injection have also been reported in the past. However, due to the known oscillation linewidth broadening mechanism of current injection, such DBRs are not suitable for applications that require a very low oscillation linewidth.

[0004] Accordingly, the present disclosure is directed to the realization of a single-mode longitudinal-wave unidirectional ring laser that is tunable over the entire band. This is typically used for long-distance communication but is not limited to this particular field. The tuning of the proposed laser does not significantly interfere with / broaden the natural oscillation linewidth of the lasing mode and makes the oscillation linewidth performance independent of the lasing wavelength within the tuning range. SUMMARY OF THE INVENTION

[0005] It is advantageous to achieve a ring laser that can be tuned over a particularly wide frequency band and the tuning does not affect, or hardly affects, the natural oscillation linewidth of the lasing mode.

[0006] In a first aspect of the present disclosure, a tunable ring laser, for example, a tunable ring laser having a ring cavity of InP that is monolithically integrated, wherein the ring cavity has at least one ring resonator having a waveguide for guiding waves, a phase modulator having a waveguide for guiding waves, ​A power coupler for coupling waves inside and outside at least one ring resonator, comprising wherein the cross-section of the waveguide of at least one ring resonator and the phase modulator is configured as a PIN diode, and acts as an electro-optic 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 laser can operate, for example, in the range from 1500 nm to 1650 nm, for example, near a wavelength of 1550 nanometers. Such wavelengths are particularly suitable for long-distance communication applications. However, the lasers according to the present disclosure are not limited to such specific technical fields.

[0008] Using the proposed ring laser, it is possible to tune the lasing peak, for example, over a range exceeding 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 peaks, i.e., modes, except for the lasing mode.

[0009] The tuning of the lasers 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 from other tuning principles such as thermal tuning or forward bias, i.e., a positive voltage. The reason is that this leads to less power consumption, does not interfere with the width of the lasing peak, and can be faster because there are no slow transient effects such as heat dissipation. Furthermore, the use of the ring resonator further reduces the width of the lasing peak.

[0010] In the prior art, tuning may be performed using a heating element. The heating element increases or decreases the length of the optical path 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 length of the path and the inherent properties of the material itself. This is caused by a plurality of factors. One of these factors is the linear electro-optic effect and the four-element electro-optic effect, namely the Pockels effect and the Kerr effect, respectively.

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

[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, for example a clockwise propagation mode, to a second propagation mode configured in the opposite direction to the first propagation mode, for example a counterclockwise propagation mode.

[0014] This ensures that the propagation direction of the wave in the ring laser is unidirectional.

[0015] In yet another embodiment, the cavity further comprises a semiconductor optical amplifier (SOA) arranged to amplify the wave in the cavity.

[0016] A semiconductor optical amplifier is, for example, an optical amplifier based on a semiconductor gain medium. This may appear as a laser diode, where the end mirrors are replaced by an antireflection coating.

[0017] Note that the semiconductor optical amplifier, SOA, may comprise InGaAsP multiple quantum well-based materials.

[0018] Subsequently, the present disclosure may be directed to a monolithically integrated unidirectional single-mode ring laser. The laser may be fabricated using InP-based integration technology commercially available 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 outer perimeters to select the lasing mode. The two ring resonators may be tuned in reverse bias operation by exploiting an electro-optic effect that is mostly voltage-controlled.

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

[0020] Due to the reverse bias operation, this tuning mechanism, unlike thermal tuning, avoids significant dissipation of heat on the chip and at the same time does not increase the oscillation linewidth of the laser. This is in contrast to current injection tuning, which is well known to cause significant broadening of the oscillation linewidth and significant dissipation of heat. The ring resonators also assist in reducing the oscillation linewidth since they increase the effective length of the cavity. Generally, 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 multimode interference reflection that can be placed exactly at one of the two exit ports of the laser, rather than a highly reflective coating surface. Therefore, effectively, the one-way laser can be placed anywhere on the die / chip that is not necessarily connected to the reflective surface.

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

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

[0024] The radii of the two ring resonators may be, for example, lengths of 120 μm and 123 μm, or comparable ones. Therefore, 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 with a splitting ratio of, for example, 50%. The total circumference of one ring resonator including the length of the MMI may be 1.400 mm, and the total circumference of the second ring may be 18.85 μm. This configuration provides 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 an adjustable ring laser according to the present disclosure.

[0026] This laser device may be, for example, a long-distance communication device equipped with a laser, that is, a long-distance communication device used for optical communication using an optical fiber. The long-distance communication device may be, for example, a modem or the like.

[0027] It should be further noted that the advantages described with respect to the first aspect of the present disclosure, which is an adjustable 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 an adjustable ring laser according to any one of the preceding claims, the method comprising: coupling waves in at least one ring resonator by one or more power couplings; guiding waves through respective waveguides of at least one ring resonator and a phase modulator by the at least one ring resonator and the phase modulator; applying a reverse bias voltage to a PIN diode in any of the waveguides to thereby tune the adjustable ring laser. A method is provided that includes these steps.

[0029] It should be noted that the reverse bias voltage may be applied to a PIN diode in any of the waveguides using a voltage control device. The voltage control device may have two output terminals, with a first output terminal of the two output terminals connected to a P-type semiconductor region of the PIN diode and a second output terminal of the two output terminals connected to an N-type semiconductor region of the PIN diode.

[0030] It should be further noted that the advantages disclosed with respect to the first aspect of the present disclosure, which is an adjustable ring laser, are also applicable to the third aspect of the present disclosure, which is a method of operating an adjustable 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 comprises: amplifying waves in the cavity by the SOA. Further includes

[0033] In another embodiment, the SOA comprises a material based on multiple quantum wells of InGaAsP.

[0034] These and other aspects of the present invention will become apparent from and will be elucidated with reference to the embodiments described hereinafter.

Brief Description of the Drawings

[0035]

Figure 1

Figure 2

Mode for Carrying Out the Invention

[0036] FIG. 1 is a diagram showing 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 two-element semiconductor composed of indium and phosphorus. InP has a zinc blende crystal structure and is comparable to most crystal structures of GaA and III-V semiconductors. The inventor has found that the semiconductor double heterostructure InP (p-doped)-InGaAsP (intrinsic)-InP (n-doped) is particularly suitable for use in cases 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. These two waveguides are substantially parallel to each other. These horizontally oriented waveguides are connected to each other via the ring resonator 3.

[0039] The ring resonator 3 is similar to a PIN diode and acts as an electro-optic modulator so that an adjustable ring laser can be tuned in frequency by applying a reverse bias voltage.

[0040] In the context of the present disclosure, a PIN diode is a diode having a wide, undoped intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region.

[0041] The wide intrinsic region, i.e., the "i" region, is in contrast to a normal p-n diode. The wide intrinsic region makes the PIN diode suitable for guiding light. The "i" region enables tuning of the ring laser by applying a reverse bias voltage. The reverse bias voltage modifies, among other things, the amount of free carriers in the PIN medium, thereby efficiently changing the frequency characteristics of the ring laser 1.

[0042] One reason for using a PIN diode is that it may be undesirable for the optical mode to overlap significantly with the p-doped region. When using a PN diode, the overlap is inevitably large. Further, 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-optical effect occurs and thus improve its efficiency.

[0043] The monolithically integrated tunable ring laser 1 of InP further includes 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 waves inside and outside at least one ring resonator 3. Further, the cavity further includes a semiconductor optical amplifier (SOA) 3 arranged to amplify the waves in the cavity.

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

[0046] The phase modulator tuning mechanism proposed in the present disclosure may be based on a reverse-biased PIN structure in a corresponding waveguide. This mechanism is based on normal current injection and is different from the thermal heating of the prior art. The provided ring laser avoids significant heat dissipation on the chip. It has been shown that applying a reverse bias to the phase section leads to a heat dissipation value of less than 100 μW at a bias of 8V. This is a significantly lower output level than that required for the thermal tuning mechanism for filters, such as DBR sections, which usually require several tens of mW. The cause of the 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 can enable 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 loss occurs 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 (less than 5V), the loss is even slightly reduced due to free carrier depletion.

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

[0050] In certain embodiments of the ring laser, two resonators of rings with slightly different outer circumferences are utilized to increase the free spectral range (FSR) of the filter and select a single lasing mode within the morphological gain bandwidth. The improved FSR from the Vernier effect may be calculated as follows.

Equation

[0051] Here, Δλ FSR1 and Δλ FSR2 are the FSRs of the individual ring resonators. The FSR of the individual ring resonator is given by Δλ FSR = Δλ 2 / n g L. Here, n g is the group index, L is the outer circumference of the single ring, and λ is the wavelength. The detuning of the two rings may be large enough such that the tuning range is limited by the morphological gain bandwidth of the SOA rather than the FSR of the Vernier cavity filter.

[0052] The laser was designed using a component library of commercially available active-passive InP-based integration technologies. The ring cavity may include a 1 mm long SOA with InGaAs multiple quantum well-based materials. 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 resonators of the two rings are implemented using a 2×1 multimode interference (MMI) coupler with a 50% splitting ratio. 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] With this configuration, an FSR of about 35 nm is provided for the vernier. This FSR ensures that the tuning range of the laser is greater than 30 nm, thus covering the span of a single band and not interfering with single-mode operation. An electro-optic phase modulator section with a length of 0.4 mm is also included in the laser cavity to facilitate independent tuning of the cavity modes. The phase section can be used to maintain the lasing mode in a state adjusted to the maximum transmission of two ring filters in order 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, and their cross-sections are vertical PIN diodes, which act as electro-optic modulators (ERMs). The intentionally undoped guide layer is a four-element material of bulk InGaAsP with a bandgap at 1.25 μm. The change in the effective refractive index is the result of both the effects of the fields (Pockels and Kerr) and the carriers (plasma / carrier depletion and band filling). The addition of these effects leads to a modulator efficiency of about 15° / Vmm for light with TE polarization.

[0055] Power out-coupling is implemented by a 2×2 MMI with a splitting ratio of 85-15. The percentage of the out-coupled power is 15%. The single-direction operation of the ring laser is ensured by a broadband reflector outside the cavity. This reflector couples the amplified spontaneous emission (ASE) of the counterclockwise propagation mode to the clockwise mode. The broadband reflector is a multimode interference reflector.

[0056] The total length of the cavity is 5.9 mm, corresponding to a free spectral range of 13.5 GHz (0.108 nm) for the cavity modes.

[0057] The spacing between the cavity mode and the ring filter mode is such that the cavity mode becomes the ring filter transmission peak every 4×Δλ FSR Within this range, the cavity modes are suppressed by the ring filter. The gain difference between the lasing mode and the adjacent cavity mode (separated by 4×Δλ FSR which becomes the transmission peak of the ring resonator) is approximately 8%. This difference in transmission is sufficient to ensure single-mode operation of the laser. FSR away) is about 8%. This difference in transmission is sufficient to ensure single-mode operation of the laser.

[0058] FIG. 2 is a diagram showing a microscopic image 11 of a ring laser according to the present disclosure.

[0059] The ring cavity may include a 1-mm-long SOA with an 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 the power coupling is implemented using a 2×1 multimode interference (MMI) coupler with a 50% splitting ratio.

[0060] The total circumference of a single ring resonator including the length of the MMI is 1.4 mm and 1.419 mm. An electro-optic phase modulator section with a length of 0.4 mm is also included in the laser cavity to facilitate independent tuning of the cavity mode.

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

[0062] Power out-coupling is performed by a 2×2 MMI with a splitting ratio of 85-15. The percentage of the out-coupled power is 15%. The unidirectional operation of the ring laser is ensured by a broadband reflector outside the cavity. This reflector couples the amplified spontaneous emission (ASE) of the counterclockwise propagating mode to the clockwise mode. The broadband reflector is a multimode interference reflector. The total length of the cavity is 5.9 mm, corresponding to a free spectral range of 13.5 GHz for the cavity mode, i.e., corresponding to 0.108 nm.

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

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

[0065] At low tuning voltages, it should be noted that the scattered tuning power is two to three orders of magnitude lower than that of other tunable lasers that typically use thermo-optic tuning in order not to disturb the laser oscillation linewidth. On average, the scattered tuning power is at least one order of magnitude lower. Furthermore, the ring resonator in the cavity helps to reduce the laser oscillation linewidth. This may be important for many applications, namely coherent communication, sensing, etc. The laser may be manufactured using common, commercially available indium phosphide, InP, photonic integration technology.

[0066] 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 terms "comprising" do 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 construed as limiting the scope of the claims.

Claims

1. An adjustable ring laser having a ring cavity, said ring cavity comprising: at least one ring resonator having a waveguide; a phase modulator having a waveguide; one or more power couplers for coupling waves inside and outside said at least one ring resonator; wherein the cross sections of the waveguides of said at least one ring resonator and said phase modulator are configured as PIN diodes, and said adjustable ring laser acts as an electro-optic modulator so as to be adjustable by applying a reverse bias voltage.

2. The adjustable ring laser according to claim 1, wherein said one or more power couplers are multimode interference (MMI) couplers.

3. The adjustable ring laser according to claim 1 or 2, wherein said ring cavity comprises two ring resonators.

4. The adjustable ring laser according to any one of claims 1 to 3, further comprising a broadband reflector arranged to couple radiation in a first propagation mode, for example a clockwise propagation mode, to a second propagation mode configured in the opposite direction to said first propagation mode, for example a counterclockwise propagation mode.

5. The adjustable ring laser according to any one of claims 1 to 4, further comprising a semiconductor optical amplifier (SOA) arranged to amplify the wave within said cavity.

6. The adjustable ring laser according to claim 5, wherein said SOA comprises a material based on InGaAsP multiple quantum wells.

7. The adjustable ring laser according to any one of claims 1 to 6, wherein any one of said waveguides is an etched ridge waveguide, the cross section of said etched ridge waveguide is a vertical PIN diode, and acts as an electro-optic refractive modulator (ERM).

8. The adjustable ring laser according to any one of claims 1 to 7, wherein said adjustable ring laser is a monolithically integrated adjustable ring laser based on InP.

9. The adjustable ring laser according to any one of claims 1 to 8, wherein said ring cavity comprises two ring resonators with different radii.

10. A laser device for emitting a laser beam, wherein the laser device comprises an adjustable ring laser according to any one of claims 1 to 9, said laser device.

11. A method of operating an adjustable ring laser according to any one of claims 1 to 10, said method comprising: coupling waves in the at least one ring resonator by means of said one or more power couplers; guiding waves through respective waveguides of the at least one ring resonator and the phase modulator by means of the at least one ring resonator and the phase modulator; applying said reverse bias voltage to any of said PIN diodes of the waveguide, thereby tuning said adjustable ring laser; said method.

12. The method according to claim 11, wherein said ring cavity comprises two ring resonators.

13. Said cavity comprises a semiconductor optical amplifier (SOA), and said method comprises: amplifying said wave in said cavity by means of said SOA The method according to claim 11 or claim 12, further comprising.

14. The method according to claim 13, wherein said SOA comprises a material based on InGaAsP multiple quantum wells.

15. Said ring laser further comprises a broadband reflector, and said method comprises: coupling radiation in a first propagation mode, for example a clockwise propagation mode, to a second propagation mode, for example a counterclockwise propagation mode, by means of said broadband reflector The method according to any one of claims 11 to 14, comprising.

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