EXTERNAL CAVITY LASER COMPRISING A PHOTONIC CRYSTAL RESONATOR

Abstract

A laser comprising: at least one wavelength selective reflector that comprises a waveguide vertically coupled to at least one photonic crystal resonator, the waveguide and photonic crystal resonator being arranged to provide wave-vector matching between at least one mode of the photonic crystal resonator and at least one mode of the waveguide; an optical gain medium for generating light for coupling into the waveguide, and a reflector at an end of the optical gain medium, the reflector and the photonic crystal resonator defining a laser cavity. Light generated by the optical gain medium is coupled into the waveguide and coupled into the photonic crystal resonator, and partially reflected back to the optical gain medium.

Claims

1. A laser comprising: at least one wavelength selective reflector that comprises a waveguide vertically coupled to at least one photonic crystal resonator, the waveguide and photonic crystal resonator being arranged to provide wave-vector matching between at least one mode of the photonic crystal resonator and at least one mode of the waveguide; an optical gain medium for generating light for coupling into the waveguide, and a reflector at an end of the optical gain medium, the reflector and the photonic crystal resonator defining a laser cavity, wherein light generated by the optical gain medium is coupled into the waveguide and coupled into the photonic crystal resonator, and partially reflected back to the optical gain medium.

2. A laser as claimed in claim 1, wherein the at least one photonic crystal resonator is of a material of different refractive index to that of the waveguide

3. A laser as claimed in claim 2 wherein the at least one photonic crystal resonator is in a layer of refractive index nb, and the device further comprises; a barrier layer of refractive index n.sub.c, whereby n.sub.c<n.sub.a and n.sub.c<n.sub.b, and a lower cladding of refractive index n.sub.d<n.sub.b, wherein the resonator layer is between the barrier layer and the lower cladding, and the waveguide is on top of the barrier layer and aligned with the at least one resonator.

4. A laser as claimed in claim 1, wherein the waveguide carries a single mode or the waveguide is multimode.

5. A laser as claimed in claim 1, wherein the at least one resonator carries a plurality of modes, among which at least one cavity-mode overlaps spatially with at least one mode propagating through the waveguide, thereby allowing for coupling of light from the waveguide to the resonator.

6. A laser as claimed in claim 1 comprising multiple resonators each operable at a different wavelength, so as to provide multiple different output wavelengths.

7. A laser as claimed in claim 6 wherein the multiple resonators are coupled so as to allow four wave mixing or another parametric process, thereby providing a mode phase locking mechanism.

8. A laser as claimed in claim 1 wherein the waveguide is a glass, a TRIPLEX or a HYDEX waveguide, or a polymer waveguide or a waveguide made of Silicon Oxynitride or a dielectric waveguide.

9. A laser as claimed in claim 1, wherein the photonic crystal has a network of holes forming a regular lattice defined by a set of parameters and wherein the parameters are selected to provide wave-vector matching between at least one mode of the resonator and at least one mode of the waveguide.

10. A laser as claimed in claim 1, wherein the photonic crystal is made of Silicon Nitride or of a III-V semiconductor material such as Indium Phosphide, Gallium Arsenide, Gallium Nitride or Indium Gallium Phosphide.

11. A laser as claimed in claim 1, wherein a modulator is associated with each resonator, the modulator being operable to change the resonant wavelength of its associated resonator.

12. A laser as defined in claim 11 where the modulator is operable to modulate an output of the laser at a modulation frequency in the 1-100 Gigahertz range.

13. A laser as claimed in claim 11 comprising a wavelength filter for converting the wavelength/frequency modulation to amplitude modulation.

14. A laser as claimed in claim 1 wherein the optical gain medium has a broadband wavelength output, for example 50 nm or more, such as 100 nm or more.

15. A laser as claimed in claim 1 wherein the optical gain medium is operable to be electrically stimulated to cause light emission.

16. A laser as claimed in claim 15 wherein the optical gain medium comprises a semiconductor optical amplifier.

17. A laser as claimed in claim 1 wherein the waveguide and the at least one photonic crystal resonator of the wavelength selector are monolithically integrated.

18. A laser as claimed in claim 1 comprising an array of photonic crystal reflectors, each photonic crystal reflector being such that its reflectivity reduces with increasing incident power.

19. A laser as claimed in claim 1 wherein the output is taken from a rear facet of the semiconductor optical amplifier.

20. A method for assembling a laser of claim 1 comprising flip-chip bonding the optical gain medium to the waveguide of the wavelength selector element.

21. A method for assembling a laser of claim 1, comprising flip-chip bonding the waveguide of the wavelength selector element to the at least one photonic crystal resonator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Various aspects of the invention will now be described by way of example only, and with reference to the following drawings, of which:

[0021] FIG. 1(a) is a schematic view of a laser built around a narrow linewidth photonic crystal reflector and semiconductor optical amplifier;

[0022] FIG. 1(b) is a cross section through the laser of FIG. 1(a);

[0023] FIG. 1(c) is a schematic view of the laser of Figure (a) but with a modulator for allowing modulation of the laser output wavelength;

[0024] FIG. 2 shows normalised spectral power versus resonant wavelength for various different photonic crystal hole positions;

[0025] FIG. 3 is a plot of intensity versus wavelength for a photonic crystal laser;

[0026] FIG. 4(a) is a schematic diagram showing a multi-wavelength laser source consisting of three cavities each with different resonance wavelengths;

[0027] FIG. 4(b) is a cross section through the laser of FIG. 4(a);

[0028] FIG. 5 is a plot of photonic crystal reflectivity vs incident power for the laser of FIG. 4;

[0029] FIG. 6(a) shows an asymmetric MZI with arms of different lengths;

[0030] FIG. 6(b) is a plot of transmission at two different wavelengths as a function of difference in length between the arms of the MZI;

[0031] FIG. 7 is a schematic diagram showing another laser built around a reflective semiconductor optical amplifier and a narrow linewidth photonic crystal reflector, and

[0032] FIG. 8 is a schematic of a laser based on a narrow linewidth photonic crystal reflector and semiconductor optical amplifier that has been assembled using flip-chip techniques.

DESCRIPTION OF THE INVENTION

[0033] FIGS. 1(a) and (b) show a laser that has a wavelength selective component with a gain section, which combine to make an external cavity laser. The wavelength selective component has waveguide 3 with material refractive index n.sub.a, which may have a large mode area, integrated with and positioned directly above a photonic crystal resonator 2. Modes in the waveguide and the photonic crystal resonator are vertically coupled and overlap. The gain section has an electrically pumped semiconductor optical amplifier 1 that generates relatively broadband light (for example having a bandwidth of 50-100 nm) that is coupled directly into the waveguide of the wavelength selective component using, for example butt coupling. A reflector 6 is provided on a rear surface of the semiconductor optical amplifier 1. Optionally, an anti-reflection layer is provided on the front surface of the semiconductor optical amplifier 1. The reflector 6 and the photonic crystal resonator 2 define an external laser cavity.

[0034] The photonic crystal resonator 2 is in a material of refractive index n.sub.b arranged between a barrier layer 4 of refractive index n.sub.c and a cladding dielectric layer 5 of refractive index n.sub.d. The cladding layer 5 is on a substrate layer 7, with index n.sub.a. In most cases, the material used has n.sub.b >1.8. Also, in practice, n.sub.b is not equal to n.sub.a. Typically, the barrier layer is 100-200 nm thick. In some cases, a gap may be provided between the waveguide and the photonic crystal resonator, in which case the barrier layer would typically comprise air.

[0035] The photonic crystal resonator 2 is created by a structural defect or cavity in a photonic crystal that forms an optical mode localized to the defect region. The mode volume of the cavity mode is small enough to generate broad wave-vector distributions, resulting in increased coupling with the waveguide. In particular, the mode volume of the cavity is less than ten cubic wavelengths, where the wavelength is the resonant wavelength of the cavity. The resonator has a unique resonant wavelength. Only the waveguide modes at the resonant frequency couple to the cavity.

[0036] The waveguide 3 extends along the longitudinal axis of the photonic crystal and is placed on top of the barrier layer so as to align vertically with the resonator 2. The thickness profile of the barrier layer 5 may vary to promote or repress coupling between the waveguide and the defect-cavity in different parts of the device. The relative values of the refractive indices n.sub.a, n.sub.b, n.sub.c and n.sub.d are chosen to promote vertical evanescent coupling between the at least one mode propagating through the waveguide 3 and the mode propagating through the resonant cavity. This is typically achieved when the following conditions are met:

{n.sub.c<n.sub.a,n.sub.c<n.sub.b,n.sub.b≠n.sub.a,n.sub.d<n.sub.b,n.sub.d<n.sub.a}

[0037] In use, light generated by the semiconductor optical amplifier 1 is at least partially reflected by photonic crystal resonator 2. The lasing wavelength is set by that of the photonic crystal resonator 2. When the incident light is resonant with the photonic crystal cavity, a portion is coupled into the cavity and experiences a π/2 phase shift. When this is coupled from the cavity, it experiences an additional π/2 phase shift. In the forward direction, some of the light from the cavity destructively interferes with a portion of the forward propagating light carried by the waveguide, and some propagates forward. In the backward direction, light that is coupled out of the cavity provides feedback to the laser cavity/resonator. This causes light at the resonant wavelength of the photonic crystal cavity to become dominant in the laser cavity. Hence, the wavelength of the light output from the laser is determined by the resonant wavelength of the photonic crystal cavity. The photonic crystal resonator transmits a portion of incident light. The levels of transmitted and reflected light can be controlled by suitable design of the photonic crystal resonator and waveguide.

[0038] The photonic crystal resonator has a small mode size. Because of this, it has an expanded wave vector space allowing phase matching with the waveguide, which is typically made of a low modal index polymer. On resonance, light is coupled in the photonic crystal resonator and out-coupled in the opposite direction. Reflectivities in the range 10-90% and 0.1 nm linewidth or better can be achieved with a suitable design of the waveguide-resonator system and an ideal laser mirror.

[0039] As noted above, the photonic crystal reflector controls the lasing wavelength. In practice, the wavelength is defined by the position of the holes in the crystal. Small changes in the position can cause significant changes in the resonant wavelength, and so the lasing wavelength. FIG. 2 shows resonant wavelength as a function of hole position.

[0040] The laser of the invention uses a photonic crystal reflector. This has a number of advantages with respect to a Distributed Bragg Reflector. The reflection spectrum does not exhibit the side lobes typical of the DBR, enabling the realisation of high side mode suppression ratio lasing. Linewidths are also much narrower (<0.1 nm) adding in the selection of a single longitudinal mode. This is shown in FIG. 3. Using a photonic crystal reflector is a cheaper means of achieving precise control of the lasing wavelength as lasing wavelength is defined only by the photonic crystal cavity and is independent of the materials used to provide gain. In contrast, for DFB or DBR lasers, the same material system, typically III-V semiconductor, for both the gain and the reflector making precise control more difficult.

[0041] The laser of FIGS. 1(a) and (b) may be adapted to realise a modulated laser, for example a frequency modulated laser. This can be done by combining a tuning element such as a pin or pn diode with the photonic crystal resonator(s). The lasing wavlength will track the resonance wavelength of the photonic crystal cavity providing a laser whose wavelength varies in time. As an example, the resonator may be embedded between a P doped region 10 and an N doped region 11 of a photonic crystal slab to form a resonator-modulator, as shown in FIG. 1(c). The resonance wavelength can be controlled individually by varying a voltage applied to the associated P and N doped regions 10 and 11 respectively.

[0042] FIGS. 4(a) and (b) show a laser that has multiple wavelength selective components with an off-chip gain section, which combine to make multiple external laser cavities. In this case, the basic layer structure of the wavelength selector is the same as for FIG. 1. However, multiple photonic crystal resonators are provided in series, each being vertically coupled to the waveguide. Each resonator is arranged to have a different resonant wavelength. The off chip gain section has a reflective semiconductor optical amplifier that has a broadband (100 nm+) reflector at one end. The multiple photonic crystal resonators and the mirror combine to make multiple external laser cavities. Within each cavity a different wavelength of light circulates, the wavelength being defined by the photonic crystal resonator.

[0043] Whilst not shown, it will be appreciated that each of the photonic crystal resonators of FIG. 4 could be associated with a modulator, so that its output can be modulated. For example, each could have the P and N doped regions described with reference to FIG. 1(c). The resonance wavelength can be controlled individually by varying a voltage applied to the associated P and N doped regions. Each of the photonic crystal resonators could be controlled individually or together.

[0044] The laser of FIG. 4 has an optical output that can provide multiple different wavelengths simultaneously, while maintaining stable continuous wave operation. Due to the nature of the photonic crystal reflectors and the cavity-waveguide vertically coupled structure multiple cavities can be cascaded together. Here, each cavity has a slightly different resonance wavelength and hence reflects different wavelengths. Each cavity along with the mirror 6 on the other side of the semiconductor optical amplifier 1 forms a different laser cavity with distinct wavelengths. Minimising competition between lasing modes is crucial to minimising relative intensity noise. The effects of two photon absorption, the reflectivity of each photonic crystal reflector reduces with increasing coupled power, see FIG. 5. This equalizes the power of each lasing mode and stablizes the multi-wavelength output. This effect may be realised by striking a balance between waveguide cavity coupling and the loss introduced by two photon absorption. The cavity Q-factor is given by the following equation:

[00001]$\frac{1}{Q_{\mathrm{total}}}=\frac{1}{Q_{\mathrm{intrinsic}}}+\frac{1}{Q_{\mathrm{coupling}}}+\frac{1}{Q_{\mathrm{TPA}}}$

[0045] Where Q.sub.total is the overall Q-factor, Q.sub.intrinsic the unloaded Q-factor (in the absence of losses due to TPA), Q.sub.coupling is the Q-factor associated with coupling between the resonator and the waveguide, and Q.sub.TPA is associated with losses due to two photon absorption, which is a function of the incident power.

[0046] The reflectivity (R) of the photonic crystal reflector is given by:

[00002]$R={\left(\frac{Q_{\mathrm{coupling}}}{Q_{\mathrm{total}}}\right)}^2$

[0047] By varying Q.sub.coupling appropriately, the reflectivity can be made more or less sensitive to the effects of two photon absorption. To provide a mode equalizing reflectivity, Q.sub.coupling (controlled via design) and Q.sub.TPA at the desired power (determined through a detailed study of effects of power on the PhC reflector) should be chosen such that a change in the incident power changes the reflectivity significantly (e.g. from more than 20% to less than 10%) see FIG. 5.

[0048] In a further embodiment, an array of identical photonic crystal cavities are coupled together and used to provide multi-wavelength feedback into the gain medium through four wave mixing. Photonic crystals are highly effective at increasing the efficiency of otherwise weak nonlinear phenomena [Optics Express 18, 26613-26624 (2010), Optics Express 20, 17474-17479 (2012)] giving rise to significant frequency conversion in silicon at milliwatt and microwatt power levels. With a suitable set of coupled silicon photonic cavities, each resonant at the same wavelength, mode splitting takes place and a system with multiple resonances evenly spaced in frequency can be realised. The initially single wavelength lasing will match one of the central resonances and light will build up in the coupled cavity system, acting as the pump. Parametric processes occur in the coupled photonic crystal cavity system to generate signal and idler waves, seeded by spontaneous emission from the semiconductor optical amplifier coupled into the coupled cavity system via the split resonances. The signal and idler waves will be phase matched to the pump and will be coupled back into the semiconductor optical amplifier giving rise to lasing on additional longitudinal modes, which have a fixed phase relationship to the initial line. This provides a phase-locking mechanism that minimises mode competition and provides low modal relative intensity noise.

[0049] Furthermore, with a suitably designed system, cascaded four wave mixing can take place providing lasing at a large number of wavelengths.

[0050] In all of the above examples, a frequency modulated laser may be realised by combining a tuning element, such as a pin or pn diode, with the photonic crystal resonator(s), the tuning element being operable to tune (vary) the resonant wavelength of the photonic crystal resonator(s). The lasing wavlength will track the resonance wavelength of the photonic crystal cavity proving a laser whose wavelength varies in time.

[0051] Instead of wavelength modulation, intensity or amplitude modulation can also be realised. This can be done using an asymmetric Mach Zehnder interferometer, as shown in FIG. 6. The asymmetric Mach Zehnder interferometer has an arm, in this case a loop of material, which extends from a first location on the waveguide to a second downstream location in the direction of light transmission. The arm is made of the same material as that of the waveguide. Some of the light transmitted along the waveguide passes into the arm and some continues along the length of the waveguide. Due to the difference in arm lengths, when light in the arm re-joins the main waveguide, the transmission is wavelength dependent. For example, at around ΔL=558.8 μm, light at wavelength 1550 nm is completely transmitted due to the constructive interference between the two arms, whereas at 1551 nm almost no transmission is possible due to destructive interference between the two arms. By choosing an appropriate length of the arm, amplitude modulation can be realised.

[0052] FIG. 7 is a schematic diagram showing the laser built around a reflective semiconductor optical amplifier and a narrow linewidth photonic crystal reflector. A passive ring resonator is added on the output waveguide to enable frequency modulation to amplitude modulation conversion. In this case, an additional waveguide may be provided parallel to the first waveguide that is coupled to the ring resonator. This additional waveguide can be used as the laser output.

[0053] In a further embodiment, the reflectivity of the reflector on the rear facet of the semiconductor optical amplifer is reduced, producing an output beam. Depending on the application the output may be taken from either side of the laser.

[0054] The commercial application of the invention is in low-energy optical links, in particular the need for on computer chip optical networks such as those developed by Intel and IBM to reduce the use of electrical connections. A key advantage arises from the small capacitance of the photonic crystal reflector. Conventionally lasers or external modulators have capacitances in the hundreds of picofarad range resulting high power consumption during modulation. Here, only the photonic crystal reflector is modulated and femtojoule switching energies have already been experimentally demonstrated. Therefore, the power consumption of the laser is primarily that of the gain element which may be less than 20 mW for state of the art devices. At bit rates of 10-20 Gbit/s the energy per bit approaches 1 pJ, a ground breaking number. Furthermore, the electronic circuits required to drive a femtofarad modulator are much less expensive and consumes less power than those required for higher capacitance components.

[0055] This laser of the invention is not limited to applications in optical communications. It could also be used in optical sensing (for example remote optical sensing) in which the resonant frequency of the photonic crystal cavity changes in response to some stimulus. Such changes can, be detected using a filter combined with a photodiode.

[0056] The laser of the invention can be made using flip chip bonding. This type of bonding provides chip placement with better than 1 micron precision. When combined with large mode area waveguides, this allows the photonic crystal and gain chip to be assembled in a low cost manner yet giving very high coupling efficiencies. This combines favourably with wafer bonding based approaches, in which a III-V wafer or III-V dies are attached to a silicon wafer and then patterned, which invariably wastes a significant proportion of the III-V material. The flip-chip bonding based approach, on the contrary makes efficient use of expensive III-V material.

[0057] In another embodiment, a silicon chip containing the photonic crystal, and a chip containing the glass or polymer based waveguides are fabricated independently and assembled using flip-chip bonding and the gain chip subsequently attached, as shown in FIG. 8. In this case, each chip may be tested prior to assembly, improving yields. Solder pads 9 are used to connect the chips together

[0058] A number of materials may be used for the construction of the wavelength selective device. The waveguide may be of a polymer or Silicon oxynitride or of more complex composite structures such as TriPleX™ or HYDEX®. The barrier layer may be a dielectric material such as silica, deposited using chemical vapour deposition techniques or spin-on glass. The photonic crystal slab may be manufactured in Silicon, Silicon Nitride or in a III-V semiconductor material such as Indium Phosphide, Gallium Arsenide, Indium Gallium Phosphide or Gallium Nitride. It could also be made of a Silicon/Germanium multilayer. The structure of the photonic crystal lattice may vary according to specifications as well as the number and design of defect-cavity resonators and resonator-modulators. The lower cladding is typically made of silica, though air is possible in some instances. The substrate is silicon or a III-V semiconductor.

[0059] A skilled person will appreciate that variations of the disclosed arrangements are possible without departing from the invention. For example, number of optional functions may be incorporated into the device. For example, where multiple resonators are provided, each may perform a different signal processing function, including that of filter, ON/OFF switch, amplitude modulator and dispersion compensator. Equally, rather than being monolithically integrated, the RSOA and silicon chip could be combined on a MEMS-based platform containing movable microlenses. In this case, the optical components are mounted onto the MEMS assembly using conventional assembly tools with relatively low precision. Parts can be off optimum position by tens of microns, with no optical connection to each other. The microlenses are movable to direct the optical components achieving efficient coupling. Once optimal alignment is achieved the microlenses are locked down, see U.S. Pat. No. 8,346,037. By using different materials, (e.g. GaAs for the gain chip and silicon carbide for the photonic crystal), narrow linewidths, high side mode suppression laser can be realised at other wavelengths. Accordingly the above description of the specific embodiment is made by way of example only and not for the purposes of limitation. It will be clear to the skilled person that minor modifications may be made without significant changes to the operation described.